U.S. patent application number 12/391738 was filed with the patent office on 2009-06-18 for post-formable multilayer optical films and methods of forming.
This patent application is currently assigned to 3M Innovative Properties Company. Invention is credited to Olester Benson, JR., James M. Jouza, Janet T. Keller, William W. Merrill, Andrew J. Ouderkirk, Roger J. Stumo, Michael F. Weber.
Application Number | 20090155540 12/391738 |
Document ID | / |
Family ID | 27358036 |
Filed Date | 2009-06-18 |
United States Patent
Application |
20090155540 |
Kind Code |
A1 |
Merrill; William W. ; et
al. |
June 18, 2009 |
POST-FORMABLE MULTILAYER OPTICAL FILMS AND METHODS OF FORMING
Abstract
Articles including post-formed multilayer optical films with
layers of at least one strain-induced birefringent material,
methods of manufacturing such articles by post-forming multilayer
optical films, and multilayer optical films that are particularly
well-suited to post-forming operations are disclosed. The articles,
methods and multilayer optical films of the present invention allow
for post-forming of multilayer optical films including
strain-induced index of refraction differentials while retaining
the desired optical properties of the multilayer optical films.
Inventors: |
Merrill; William W.; (White
Bear Lake, MN) ; Jouza; James M.; (Woodbury, MN)
; Benson, JR.; Olester; (Woodbury, MN) ;
Ouderkirk; Andrew J.; (Woodbury, MN) ; Weber; Michael
F.; (Shoreview, MN) ; Keller; Janet T.;
(Bagan, MN) ; Stumo; Roger J.; (Shoreview,
MN) |
Correspondence
Address: |
3M INNOVATIVE PROPERTIES COMPANY
PO BOX 33427
ST. PAUL
MN
55133-3427
US
|
Assignee: |
3M Innovative Properties
Company
|
Family ID: |
27358036 |
Appl. No.: |
12/391738 |
Filed: |
February 24, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10883059 |
Jun 30, 2004 |
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12391738 |
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|
10115559 |
Apr 3, 2002 |
6788463 |
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10883059 |
|
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|
|
09126917 |
Jul 31, 1998 |
|
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|
10115559 |
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09006591 |
Jan 13, 1998 |
6531230 |
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09126917 |
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09006086 |
Jan 13, 1998 |
6045894 |
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09006591 |
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Current U.S.
Class: |
428/172 |
Current CPC
Class: |
B32B 27/36 20130101;
Y10T 428/24628 20150115; G02B 5/3083 20130101; Y10T 428/24479
20150115; B32B 2307/42 20130101; G02B 5/3033 20130101; Y10S 428/91
20130101; B32B 27/08 20130101; G02B 5/305 20130101; Y10T 428/31855
20150401; B41M 3/14 20130101; B44F 1/14 20130101; G02B 5/0841
20130101; B32B 3/28 20130101; B42D 25/29 20141001; Y10T 428/24694
20150115; G02B 5/287 20130101; B32B 2307/418 20130101; B32B 7/02
20130101; G02B 5/32 20130101; Y10T 428/24612 20150115; Y10T
428/31504 20150401 |
Class at
Publication: |
428/172 |
International
Class: |
B32B 3/30 20060101
B32B003/30 |
Claims
1. An article comprising a portion formed from a moldable material
and a multilayer optical film having a first major surface attached
to the portion and a second major surface disposed generally
opposite the first major surface, the multilayer optical film
comprising an optical stack having a plurality of layers, the
layers comprising at least one birefringent polymer and at least
one second polymer, wherein the optical stack comprises a first
index of refraction differential between layers in the optical
stack along a first axis and a second index of refraction
differential along a second axis that is perpendicular to the first
axis, wherein the second index of refraction differential is
different than the first index of refraction differential, and
wherein at least the second major surface comprises a
three-dimensional permanent deformation.
2. The article of claim 1, wherein the three-dimensional permanent
deformation has a curved shape.
3. The article of claim 1, wherein the three-dimensional permanent
deformation comprises a depressed area.
4. The article of claim 1, wherein the three-dimensional permanent
deformation comprises a raised area.
5. The article of claim 1, wherein the three-dimensional permanent
deformation comprises a raised area in the second major surface and
a depressed area in the first major surface, wherein the raised
area is disposed opposite the depressed area.
6. An article comprising a multilayer optical film comprising an
optical stack having a plurality of layers, the layers comprising
at least one birefringent polymer and at least one second polymer,
wherein the optical stack comprises a first index of refraction
differential between layers in the optical stack along a first axis
and a second index of refraction differential along a second axis
that is perpendicular to the first axis, wherein the second index
of refraction differential is different than the first index of
refraction differential, and wherein the first and second major
surfaces each comprise a plurality of undulations comprising
arcuate portions, valley portions, and intermediate portions
connecting the arcuate portions and the valley portions.
7. The article of claim 6, wherein the undulations have sinusoidal
shapes.
8. The article of claim 6, wherein the arcuate portions and the
valley portions are generally flat.
9. The article of claim 6, wherein the multilayer optical film
comprises a reflective polarizer.
10. The article of claim 6, wherein the multilayer optical film
comprises a mirror.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 10/883,059, filed Jun. 30, 2004, which is a
continuation of U.S. Pat. No. 6,788,463, filed Apr. 3, 2002, which
is a continuation of U.S. patent application Ser. No. 09/126,917,
filed Jul. 31, 1998, now abandoned, which is a continuation-in-part
of U.S. Pat. No. 6,531,230, filed Jan. 13, 1998 and a
continuation-in-part of U.S. Pat. No. 6,045,894 filed Jan. 13,
1998, and incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to the field of birefringent
multilayer optical films. More particularly, the present invention
relates to post-formable multilayer optical films including at
least one birefringent material and methods of manufacturing
post-formed articles from multilayer optical films.
BACKGROUND OF THE INVENTION
[0003] Conventional methods of providing reflective objects
typically include the use of metal or substrates coated with thin
layers of metals. Forming the articles completely of metal is
typically expensive and may also suffer from other disadvantages
such as increased weight, etc. Metal coated articles are typically
plastic substrates coated with a reflective metallic layer by
vacuum, vapor or chemical deposition. These coatings suffer from a
number of problems including chipping or flaking of the metallic
coating, as well as corrosion of the metallic layer.
[0004] One approach to addressing the need for reflective objects
has been the use of multilayer articles of polymers such as those
discussed in U.S. Pat. Nos. 5,103,337 (Schrenk et al.); 5,217,794
(Schrenk); 5,684,633 (Lutz et al.). These patents describe
articles, typically films or sheets, that include multiple layers
of polymers having different indices of refraction and, as a
result, reflect light incident on the films. Although most of the
listed patents recite that the articles are post-formable, only a
few of them actually address the modifications needed to ensure
that the articles retain their optical properties after forming.
Among those modifications are the use of discontinuous layers (U.S.
Pat. No. 5,217,794) and increasing the number of layers in the
article or film (U.S. Pat. No. 5,448,404).
[0005] Multilayer articles including layers of birefringent
materials, their optical properties and methods of manufacturing
them are disclosed in, e.g., PCT Publication Nos. WO 97/01774 and
WO 95/17303. This class of articles includes alternating layers of
a birefringent material and a different material in which the
refractive index differential between the alternating layers is
caused, at least in part, by drawing of the article, typically
provided in the form of a film. That drawing causes the refractive
index of the birefringent material to change, thereby causing the
inter-layer refractive index differential to change. Those
strain-induced refractive index differentials provide a number of
desirable optical properties including the ability to reflect light
incident on the films from a wide range of angles, high
reflectivity over broad ranges of wavelengths, the ability to
control the reflected and transmitted wavelengths, etc. For
simplicity, multilayer articles including one or more layers of
birefringent materials will be referred to below as "multilayer
optical films."
[0006] None of the known multilayer articles and multilayer optical
films and the patents/publications describing them, however,
address the problems associated with post-forming multilayer
optical films. As discussed above, multilayer optical films
including alternating layers of materials including at least one
birefringent material rely on strain-induced refractive index
differentials.
[0007] Because multilayer optical films rely on refractive index
differentials developed by drawing, post-forming of multilayer
optical films poses a number of problems. The additional strain
caused during the post-forming processes can affect the refractive
index differentials in the multilayer optical films, thereby
affecting the optical properties of the multilayer optical films.
For example, a multilayer optical film designed to reflect light of
one polarization orientation and transmit light of the orthogonal
polarization orientation may be altered during post-forming such
that it reflects light with both polarization orientations. In
addition, many post-forming processes involve the use of heat
during forming, and that heat may alter the strain-induced
crystallization that serves as the basis for the refractive index
differentials in many multilayer optical films. As a result, the
multilayer optical film may exhibit altered optical characteristics
due to the changed refractive index differentials. Furthermore,
some multilayer optical films including strain-induced birefringent
layers may be stretched to levels at or near their rupture or
breaking points during manufacturing. As a result, any further
processing that introduces additional strain may well result in
rupture of the multilayer optical films.
SUMMARY OF THE INVENTION
[0008] The present invention provides articles including
post-formed multilayer optical films including layers of at least
one strain-induced birefringent material, methods of manufacturing
such articles by post-forming multilayer optical films, and
multilayer optical films that are particularly well-suited to
post-forming operations. The articles, methods and multilayer
optical films of the present invention allow for post-forming of
multilayer optical films including strain-induced index of
refraction differentials while retaining the desired optical
properties of the multilayer optical films.
[0009] In one aspect, the present invention provides an article
including multilayer optical film having an optical stack including
a plurality of layers, the layers comprising at least one
birefringent polymer and at least one different polymer, wherein
the optical stack includes a strain-induced index of refraction
differential along at least a first in-plane axis, and further
wherein the thickness of the optical stack varies non-uniformly
over the optical stack.
[0010] In another aspect, the present invention provides an article
including multilayer optical film having an optical stack including
a plurality of layers, the layers including at least one
birefringent polymer and at least one different polymer, wherein
the optical stack includes a strain-induced index of refraction
differential along a first in-plane axis and substantially the
entire optical stack reflects at least about 85% of light of
desired wavelengths that is polarized along the first in-plane
axis, and further wherein the thickness of the optical stack varies
by at least about 10% or more.
[0011] In another aspect, the present invention provides an article
including multilayer optical film having an optical stack including
a plurality of layers, the layers including at least one
birefringent polymer and at least one different polymer, wherein
the optical stack includes a strain-induced index of refraction
differential along a first in-plane axis, and further wherein the
optical stack defines first and second major surfaces, the first
major surface including at least one depressed area formed
therein.
[0012] In another aspect, the present invention provides an article
including multilayer optical film having an optical stack including
a plurality of layers, the layers including at least one
birefringent polymer and at least one different polymer, wherein
the optical stack includes a strain-induced index of refraction
differential along a first in-plane axis, wherein the thickness of
the optical stack varies; and a substrate attached to the
multilayer optical film.
[0013] In another aspect, the present invention provides a method
of manufacturing an article including a multilayer optical film by
providing a multilayer optical film having an optical stack
including a plurality of layers, the layers including at least one
birefringent polymer and at least one different polymer, wherein
the optical stack exhibits a strain-induced index of refraction
differential along a first in-plane axis, and further wherein the
optical stack has a first thickness; and permanently deforming the
optical stack from the first thickness to a second thickness,
wherein the optical stack exhibits a post-formed strain-induced
index of refraction differential along the first in-plane axis
after deformation.
[0014] In another aspect, the present invention provides a
multilayer optical film having a sequence of alternating layers of
a birefringent polymer and a different polymer, the birefringent
polymer including PEN, wherein the birefringent polymer exhibits a
total polarizability difference in a range of from at least about
0.002 up to about 0.018, and further wherein the birefringent
polymer exhibits a maximum in-plane birefringence of about 0.17 or
less.
[0015] In another aspect, the present invention provides a
multilayer optical film having a sequence of alternating layers of
a birefringent polymer and a different polymer, the birefringent
polymer including PET, wherein the birefringent polymer exhibits a
total polarizability difference in a range of from at least about
0.002 up to about 0.030, and further wherein the birefringent
polymer exhibits a maximum in-plane birefringence of about 0.11 or
less.
[0016] In another aspect, the present invention provides a method
of manufacturing an article including a multilayer optical film by
providing a multilayer optical film with an optical stack that
includes a plurality of layers, the layers including at least one
birefringent polymer and at least one different polymer, wherein
the optical stack includes a strain-induced index of refraction
differential along at least a first in-plane axis; and corrugating
the optical stack to cause a change in its visual appearance.
[0017] In another aspect the present invention provides an article
including a multilayer optical film having an optical stack that
includes a plurality of layers, the layers including at least one
birefringent polymer and at least one different polymer, wherein
the optical stack includes a strain-induced index of refraction
differential along at least a first in-plane axis, and further
wherein the optical stack has a corrugated configuration.
[0018] These and other features and advantages of the present
invention are discussed below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a schematic diagram of one multilayer optical film
according to the present invention.
[0020] FIG. 2 is a plan view of a portion of one post-formed
multilayer optical film according to the present invention
including areas deformed along two in-plane directions.
[0021] FIG. 2A is an enlarged partial cross-sectional view of the
post-formed multilayer optical film of FIG. 2 taken along line
2A-2A.
[0022] FIGS. 2B and 2C are enlarged partial cross-sectional views
of alternative post-formed multilayer optical films deformed along
two in-plane directions.
[0023] FIG. 3 is a plan view of a portion of one post-formed
multilayer optical film according to the present invention
including areas deformed along one in-plane direction.
[0024] FIG. 3A is an enlarged partial cross-sectional view of the
post-formed multilayer optical film of FIG. 3 taken along line
3A-3A.
[0025] FIGS. 3B and 3C are enlarged partial cross-sectional views
of alternative post-formed multilayer optical films deformed along
one in-plane direction.
[0026] FIG. 4 is a perspective view of a portion of one post-formed
multilayer optical film according to the present invention.
[0027] FIG. 5 is an enlarged partial cross-sectional view of the
multilayer optical film of FIG. 4 taken along line 5-5 in FIG.
4.
[0028] FIG. 6 is a partial cross-sectional view of another
post-formed multilayer optical film according to the present
invention.
[0029] FIG. 7 is a partial cross-sectional view of a headlight
assembly including post-formed multilayer optical film according to
the present invention.
[0030] FIG. 8 is an enlarged cross-sectional view of one portion of
the headlight assembly of FIG. 7 taken along line 8-8.
[0031] FIG. 9 is an enlarged cross-sectional view of one portion of
the headlight assembly of FIG. 7 taken along line 9-9.
[0032] FIG. 10 is a plan view of one light guide including
post-formed multilayer optical film according to the present
invention.
[0033] FIG. 11 is an enlarged partial cross-sectional view of the
light guide of FIG. 10 taken along line 11-11.
[0034] FIG. 12 is a graph illustrating the relationship between
draw ratio (horizontal axis) and crystallinity (vertical axis) in
the birefringent materials of a multilayer optical film.
[0035] FIG. 12A illustrates the index of refraction in the
direction of drawing (vertical axis) as a function of the draw
ratio (horizontal axis) for one uniaxially drawn PEN film in which
the orthogonal in-plane axis dimension is held generally
constant.
[0036] FIG. 13 is a graph illustrating temperature (horizontal
axis) versus crystallization rate (vertical axis) for an exemplary
birefringent material.
[0037] FIG. 14 is a perspective view of an article including
post-formed multilayer optical film with selected areas having
different optical properties.
[0038] FIG. 15 is a cross-sectional view of a composite including
an multilayer optical film and a substrate.
[0039] FIG. 16 is a plan view of the composite of FIG. 15
illustrating that the substrate may be provided in selected
areas.
[0040] FIGS. 17 and 18 present the measured transmissions of light
polarized in the MD and TD directions, respectively, as discussed
in Example 2.
[0041] FIG. 19 compares the spectra of cases 2, 5 and 6 as
discussed in Example 6.
[0042] FIG. 20 presents the block fractional transmissions for the
three cases discussed in Example 7.
[0043] FIG. 21 is a partial schematic diagram of a corrugating
apparatus used in connection with Example 12.
[0044] FIG. 22 is a perspective view of the corrugated multilayer
optical film discussed in Example 12.
[0045] FIG. 23 is a perspective view of the corrugated multilayer
optical film discussed in Example 12 with undulations configured
differently from those shown in FIG. 22.
[0046] FIG. 24 shows a plan view of a portion of a multilayer
optical film after it has undergone a corrugation process such as
discussed in Example 12.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS OF THE
INVENTION
[0047] The present invention is directed at articles including
post-formed multilayer optical films including layers of at least
one strain-induced birefringent material, methods of manufacturing
such articles by post-forming multilayer optical films, and
multilayer optical films that are particularly well-suited to
post-forming operations. Post-forming of multilayer optical films
presents problems because most, if not all, post-forming processes
result in deformation of the film from its manufactured state.
Those deformations can adversely affect the optical and mechanical
properties of the multilayer optical film.
[0048] While the present invention is frequently described herein
with reference to the visible region of the spectrum, various
embodiments of the present invention can be used to operate at
different wavelengths (and thus frequencies) of electromagnetic
radiation. For simplicity, the term "light" will be used herein to
refer to any electromagnetic radiation (regardless of the
wavelength/frequency of the electromagnetic radiation) capable of
being reflected by the multilayer optical films of the present
invention. For example, the multilayer optical films may be capable
of reflecting very high, ultrahigh, microwave and millimeter wave
frequencies of electromagnetic radiation. More preferably, the term
"light" will refer to electromagnetic radiation including the
ultraviolet through the infrared spectrum (including the visible
spectrum). Even more preferably, "light" as used in connection with
the present invention can be defined as electromagnetic radiation
in the visible spectrum.
[0049] Furthermore, the multilayer optical films and processes of
post-forming multilayer optical films according to the present
invention rely on strain-induced index of refraction differentials
between layers in the films. Typically, those differentials will
not be expressed herein numerically. Where they are discussed with
reference to specific indices of refraction, however, it should be
understood that the values used are determined using light having a
wavelength of 632.8 nanometers.
[0050] As used herein, the terms "reflection" and "reflectance" and
variations thereof refer to the reflectance of light rays from a
surface. Similarly, the terms "transmission" and "transmittance"
and variations thereof are used herein in reference to the
transmission of light through a surface, optical stack, film, etc.
Except where dyes or colorants are intentionally added, the optical
stacks of the present invention preferably exhibit low or minimal
absorption losses (typically less than 1% of incident light), and
substantially all of the incident light that is not reflected from
the surface of an optical stack will be transmitted
therethrough.
[0051] As used herein, the term "extinction ratio" is defined to
mean the ratio of total light transmitted in one polarization to
the light transmitted in an orthogonal polarization.
Multilayer Optical Films
[0052] Many multilayer optical films used in connection with the
present invention and methods of manufacturing them are described
in U.S. Pat. Nos. 5,882,774 (Jonza et al.); 6,101,032 (Ouderkirk);
6,157,490 (Wheatley et al.); 6,207,260 (Wheatley et al.); U.S. Ser.
No. 09/006,288 (filed on Jan. 13, 1998, now abandoned); U.S. Pat.
No. 6,179,948 (Merrill et al.); and U.S. Ser. No. 09/006,591 (filed
on Jan. 13, 1998); as well as in various other patents and patent
applications referred to herein. Briefly, however, multilayer
optical films as used herein refers to optical films including at
least one birefringent material provided in contiguous layers with
at least one other material such that desired strain-induced
refractive index differentials are provided between the layers
making up the films. The multilayer optical films preferably
exhibit relatively low absorption of incident light, as well as
high reflectivity for both off-axis and normal light rays.
[0053] The reflective properties generally hold whether the films
are used for pure reflection or reflective polarization of light.
The unique properties and advantages of multilayer optical films
provides an opportunity to design highly reflective post-formed
articles that exhibit low absorption losses. One multilayer optical
film used in the methods and articles of the present invention is
illustrated in FIG. 1 and includes a multilayer stack 10 having
alternating layers of at least two materials 12 and 14.
[0054] The multilayer optical films according to the present
invention all include an optically active portion that will be
referred to herein as the "optical stack," i.e., those layers that
provide the desired reflective properties of the multilayer optical
films by virtue of the refractive index differentials within the
optical stack. Other layers and/or materials may be provided in
addition to the optical stack. For example, skin layers may be
provided on the outside of the optical stack to improve the
mechanical properties of the films or provide some other desired
property or properties including secondary optical effects such as
retardation or polarization conversion, but the bulk of the
reflective optical characteristics of the films are determined by
the properties of the optical stacks.
[0055] Although only two layers 12 and 14 are illustrated, it will
be understood that the optical stack of the multilayer optical film
10 can include tens, hundreds or thousands of layers, and each
layer can be made from any of a number of different materials,
provided that at least one of the materials is birefringent. The
characteristics which determine the choice of materials for a
particular optical stack depend upon the desired optical
performance of the film. The optical stack may contain as many
materials as there are layers in the stack. For ease of
manufacture, however, preferred optical thin film stacks contain
only a few different materials. Some considerations relating to the
selection of materials for the optical stacks of multilayer optical
films of the present invention are discussed below in the section
entitled "Materials Selection."
[0056] The boundaries between the materials, or chemically
identical materials with different physical properties, within the
stack can be abrupt or gradual. Except for some simple cases with
analytical solutions, analysis of the latter type of stratified
media with continuously varying index is usually treated as a much
larger number of thinner uniform layers having abrupt boundaries
but with only a small change in properties between adjacent
layers.
[0057] Further considerations relating to the selection of
materials and manufacturing of optical films can be obtained with
reference to U.S. Pat. No. 5,882,774 (Jonza et al.); U.S. Pat. No.
6,157,490 (Wheatley et al.); U.S. Pat. No. 6,207,260 (Wheatley et
al.); U.S. Ser. No. 09/006,288 (filed on Jan. 13, 1998, now
abandoned); U.S. Pat. No. 6,179,948 (Merrill et al.); and U.S. Ser.
No. 09/006,591 (filed on Jan. 13, 1998).
[0058] The preferred optical stack is comprised of low/high index
pairs of film layers, wherein each low/high index pair of layers
has a combined optical thickness of 1/2 the center wavelength of
the band it is designed to reflect at normal incidence. The optical
thickness is the physical layer thickness multiplied by the index
of refraction of the material in the layer for a given wavelength
and polarization plane cross-section. Stacks of such films are
commonly referred to as quarterwave stacks.
[0059] As indicated above, at least one of the materials is
birefringent, such that the index of refraction (n) of the material
along one direction is affected by stretching the material along
that direction. The indices of refraction for each layer are n1x,
n1y, and n1z for layer 12, and n2x, n2y, and n2z for layer 14. For
the purposes of the present invention, the x and y axes will
generally be considered to lie within the plane of the film and be
perpendicular to each other. The z axis will be perpendicular to
both the x and y axes and will generally be normal to the plane of
the film.
[0060] The stack 10 can be stretched in two (typically)
perpendicular in-plane directions to biaxially orient the
birefringent material in the layer 14, or the stack 10 may be
stretched in only one in-plane direction (uniaxially oriented). By
stretching the multilayer stack over a range of uniaxial to biaxial
orientation, a film can be created with a range of reflectivities
for differently oriented incident light. The multilayer stack can
thus be made useful as reflective polarizers or mirrors.
[0061] If the stack 10 is stretched in the x and y directions, each
adjacent pair of layers 12 and 14 exhibit refractive index
differentials between layers in each of the two mutually
perpendicular in-plane directions (x & y). The values of the
refractive index differentials can be represented by .DELTA.x
(which is equal to (n1x-n2x) where n1x is greater than n2x) and
.DELTA.y (where .DELTA.y=n1y-n2y). It will be understood that a
reflective polarizer will preferably exhibit a .DELTA.x in stack 10
that is sufficiently high to achieve the desired reflectivity and,
further, that the stack 10 will exhibit a .DELTA.y that is
sufficiently low such that a substantial percentage of light with
coincident polarization is transmitted.
[0062] An important parameter for improving the reflectivity of
multilayer optical films at oblique angles of incidence is the
control of n1z and n2z in relation to the other indices. First
assume that n1x is the larger of n1x and n2x such that .DELTA.x is
positive and |.DELTA.x|>|.DELTA.y|. To increase the reflectivity
of the multilayer optical stack at oblique angles of incidence
compared to normal incidence, it may be preferred that
.DELTA.z<.DELTA.x. More preferably, .DELTA.z.apprxeq.0, and even
more preferably .DELTA.z<0.
[0063] For reflective mirror films, the desired average
transmission for light of each polarization and plane of incidence
generally depends upon the intended use of the reflective film. The
average transmission at normal incidence for any polarization
direction for a narrow bandwidth reflective film, e.g., a 100
nanometer bandwidth within the visible spectrum is desirably less
than 30%, preferably less than 20% and more preferably less than
10%. A desirable average transmission along each polarization
direction at normal incidence for a partial reflective film ranges
anywhere from, for example, 10% to 50%, and can cover a bandwidth
of anywhere between, for example, 100 nanometers and 450
nanometers, depending upon the particular application.
[0064] For a high efficiency reflective mirror film, average
transmission at normal incidence for any polarization direction
over the visible spectrum (400-700 nm) is desirably less than 10%,
preferably less than 5%, more preferably less than 2%, and even
more preferably less than 1%. The average transmission at 60
degrees from the normal axis for any plane of incidence and
polarization direction for a high efficiency reflective film from
400-700 nanometers is desirably less than 10%, preferably less than
5%, more preferably less than 2%, and even more preferably less
than 1%.
[0065] In addition, asymmetric reflective films may be desirable
for certain applications. In that case, average transmission for
one polarization direction may be desirably less than, for example,
50%, while the average transmission along another polarization
direction may be desirably less than, for example 20%, over a
bandwidth of, for example, the visible spectrum (400-700
nanometers), or over the visible spectrum and into the near
infrared (e.g., 400-850 nanometers).
[0066] In summary, multilayer optical films used in the methods and
articles of the present invention include a multilayer stack 10
having alternating layers of at least two diverse polymeric
materials 12 and 14, at least one of which preferably exhibits
birefringence, such that the index of refraction of the
birefringent material is affected by stretching. The adjacent pairs
of alternating layers preferably exhibit at least one
strain-induced refractive index differential (.DELTA.x, .DELTA.y)
along at least one of two perpendicular in-plane axes as discussed
briefly below. The selection of materials and/or the orientation
process conditions can be used to control the value of .DELTA.z in
relation to the values of .DELTA.x and .DELTA.y.
[0067] By stretching the multilayer stack over a range of uniaxial
to biaxial orientation, a multilayer optical film can be created
with a range of reflectivities for differently oriented plane
polarized light along with the plane of incidence or polarization
parallel to various film axes (typically corresponding to the
stretch directions) based on the values of .DELTA.x, .DELTA.y, and
.DELTA.z. Preferably, those refractive index differentials are
generally uniform throughout the film to provide uniform optical
properties throughout the film. Variations in those refractive
index differentials that fall below desired minimum values for the
desired optical characteristics may cause undesirable variations in
the optical properties of the films.
[0068] Although the articles including post-formed multilayer
optical film, the methods of producing those articles, and the
post-formable multilayer optical films are often described or
explained below with reference to multilayer optical films designed
to exhibit broadband reflectance over the visible spectrum, it will
be understood that the same concepts could apply to articles,
methods and films that exhibit reflectance of light having any
desired range or ranges of wavelengths and any desired polarizing
qualities. In other words, the present invention is useful with
both polarizing multilayer optical films (that preferentially
reflect light of one polarization orientation while transmitting
light with the orthogonal polarization orientation), as well as
multilayer optical films that provide uniform properties for light
having any polarization orientation.
[0069] Other optical films suitable for use in the post-forming
process of the present invention include, for example, multilayer
films and films comprised of a blend of immiscible materials having
differing indices of refraction. Examples of suitable multilayer
films include polarizers, visible and infrared mirrors, and color
films such as those described in Patent Publications WO 95/17303,
WO 96/19347, and WO 97/01440; filed applications having U.S. Pat.
Nos.; 6,531,230 (Weber et al.), 6,045,894 (Jonza et al.), 5,103,337
(Schrenk), 5,122,905 (Wheatley et al), 5,122,906 (Wheatley),
5,126,880 (Wheatley), 5,217,794 (Schrenk), 5,233,465 (Schrenk),
5,262,894 (Wheatley), 5,278,694 (Wheatley), 5,339,198 (Wheatley),
5,360,659 (Arends), 5,448,404 (Schrenk), 5,486,949 (Schrenk)
4,162,343 (Wilcox), 5,089,318 (Shetty), 5,154,765 (Armanini),
3,711,176 (Alfrey, Jr. et al.); and Reissued U.S. Patents RE 31,780
(Cooper) and RE 34,605 (Schrenk), the contents of which are
incorporated herein by reference. Examples of optical films
comprising immiscible blends of two or more polymeric materials
include blend constructions wherein the reflective and transmissive
properties are obtained from the presence of discontinuous
polymeric regions, such as the blend mirrors and polarizers as
described in Patent Publication WO 97/32224, the contents of which
is incorporated herein by reference. Preferred films are multilayer
films having alternating layers of a birefringent material and a
different material such that there is a refractive differential
between the alternating layers. Especially preferred are multilayer
films wherein the birefringent material is capable of
stress-induced birefringence, wherein the refractive index
differential between the alternating layers is caused, at least in
part, by drawing the film. The drawing or similar forming process
causes the refractive index of the birefringent material to change,
thereby causing the inter-layer refractive index differential to
change. Those strain-induced refractive index differentials provide
a number of desirable optical properties, including the ability to
reflect light incident on the films from a wide range of angles,
high reflectivity over broad ranges of wavelengths, the ability to
control the reflected and transmitted wavelengths, etc.
Post-Forming of Optical Films
[0070] As used in connection with the present invention,
post-forming can include a variety of processes designed to produce
articles having a variety of shapes different from the smooth,
planar-surfaced film shape of the multilayer optical film as
manufactured. Preferred manufacturing processes involve casting or
otherwise forming the film, followed by stretching the film in one
direction for a uniaxially stretched film. If the film is to be
biaxially stretched, it is typically stretched in both the
longitudinal (i.e., machine) direction and in the cross-web
direction although any two directions may be used (preferably two
generally perpendicular directions). Both uniaxially and biaxially
stretched multilayer optical films are manufactured as generally
smooth, planar films with caliper or thickness variations of about
.+-.5% or less as manufactured.
[0071] Post-forming, as discussed with respect to the present
invention, involves further processing of the optical stacks in the
multilayer optical films to obtain some permanent deformation in
the optical stack. The deformation will preferably involve thinning
of the optical stack and it may also involve deforming at least one
surface of the film from the uniformly smooth, planar-surfaced film
shape in which it is manufactured.
[0072] Because the deformations may cause the planarity of the
optical stack to be disrupted, it should be understood that, where
discussed, the in-plane directions are considered to be relative to
a localized area of the optical stack or a point on the optical
stack. For a curved optical stack, the in-plane axes can be
considered to lie in a plane defined by the tangent lines formed at
a particular point on the optical stack. The z-axis would then be
perpendicular to that plane.
[0073] Post-forming may also include embossing in which the optical
layers of the multilayer optical film, i.e., those layers
responsible for the reflective properties of the multilayer optical
film, are deformed to produce a change in the optical properties of
the film. Embossing that provides a textured surface to a skin
layer without significantly affecting the optical properties of the
optical layers within the multilayer optical film is not considered
post-forming within the meaning of that term as used herein.
Embossing of a multilayer colored mirror films has been discussed
in, e.g., U.S. patent application Ser. No. 08/999,624 (now
abandoned) and U.S. Pat. No. 6,045,894 (Jonza et al.).
[0074] As can be seen in the embodiments discussed below,
post-formed articles are produced by deforming a generally smooth,
planar-surfaced film or sheet material to an article having
three-dimensional characteristics. Articles including post-formed
multilayer optical film can include post-formed multilayer optical
film having relatively small deformations such as those experienced
as a result of embossing the optical layers of the multilayer
optical film, up to larger scale deformations such as thermoformed
multilayer optical film used in, e.g., a deep lamp cavity, having a
high aspect ratio (i.e., depth to width ratio).
[0075] Post-forming operations will typically, but not necessarily,
employ heat to improve the working qualities of the multilayer
optical film. The post-forming processes may also employ pressure,
vacuum, molds, etc. to further improve the working qualities of the
multilayer optical film, as well as increase the throughput of the
process. For example, one typical post-forming method is
thermoforming, including the various forms of vacuum or pressure
molding/forming, plug molding, etc. Post-forming may also include
re-drawing or stretching films or portions/areas of films in planar
directions or stretching the films into non-planar or curved
shapes.
[0076] It may be helpful to further describe post-forming in terms
of the amount of draw induced in the optical stack. In general,
post-forming can involve a texturing of the optical stack, shallow
drawing of the optical stack, and deep drawing of the optical
stack. In the cases where the post-forming involves texturing
and/or shallow drawing, it may be possible to use both fully drawn
and underdrawn multilayer optical films (as described below) to
perform the methods because the draw ratios to be experienced may
be relatively small. When performing deep draws, however, it may be
advantageous to use underdrawn optical stacks because of their
increased extensibility as compared to fully-drawn multilayer
optical films. Some exemplary post-forming processes and the
articles manufactured thereby are presented below.
[0077] One approach to characterizing deformation of the optical
stack in a post-formed multilayer optical film according to the
present invention is depicted in FIGS. 2 and 2A-2C. The optical
stack 20 includes a first major side 24 and a second major side 26
(see FIG. 2A). Also illustrated are selected areas 22 in which the
optical stack 20 has been deformed. The selected areas 22 are
depicted as being substantially uniform in size and arranged in
regular, repeating pattern. It will however, be understood that the
selected areas 22 may be non-uniform and/or provided in pattern
that irregular/non-repeating.
[0078] One of the selected areas 22 and the surrounding optical
stack 20 is seen in the enlarged, partial cross-sectional view of
FIG. 2A. The result of the post-forming is that the thickness of
the optical stack 20 varies. One of the ways in which that
variation can manifest itself is that each of the selected areas 22
can form a depression in the otherwise generally smooth, planar
first major side 24 of the optical stack 20. This post-forming may
be considered as one example of texturing, i.e., causing
deformations in one surface 24 of the optical stack 20 that do not
necessarily find any corresponding deformation on the opposite
surface 26 of the optical stack 20. Texturing does, however, differ
from embossing of skin layers in that the optical stack 20 is
itself deformed.
[0079] Another manifestation of the thickness variations in an
optical stack 120 is illustrated in FIG. 2B where both the first
and second major sides 124 and 126 are deformed in selected areas
122 and 128. Like selected area 122 on the first major side 124,
selected area 128 on the second major side 126 is also formed as a
depression in the otherwise generally smooth planar second major
side 126. This is one example of a shallow draw that could be
caused by pressure or by strain.
[0080] Yet another manifestation of the thickness variations in an
optical stack 220 is illustrated in FIG. 2C where both the first
and second major sides 224 and 226 are deformed in selected areas
222 and 228. While selected areas 222 are formed as depressions on
the first major side 224, the selected area 227 on the second major
side 226 is formed as a raised area extending outwards from the
otherwise generally smooth, planar second major side 226. As
depicted, it may be preferred that the raised area 228 on the
second major side 226 be located opposite the depressed area 222 on
the first major side 224.
[0081] The post-forming result depicted in FIG. 2C is another
example of what could be considered a shallow draw, i.e.,
deformation of the optical stack 220 in the opposing sides 224 and
226 of the optical body 220.
[0082] FIG. 3 and cross-sectional views 3A-3C illustrate an
alternative embodiment of a post-formed multilayer optical film
according to the present invention. The optical stack 20' includes
a first major side 24' and a second major side 26' (see FIG. 3A).
Also illustrated are selected areas 22' in which the optical stack
20' has been deformed. The selected areas 22' are depicted as being
substantially uniform in size. It will however, be understood that
the selected areas 22' may be non-uniform.
[0083] Referring back to FIG. 2, the selected areas 22 of optical
stack 20 are deformed along both in-plane axes (x & y). In
contrast, the selected areas 22' of optical stack 20' are
preferably deformed along only one in-plane axis (the x axis in
FIG. 3). If the optical stack 20' is designed to operate as a
reflective polarizer in the deformed areas 22', it may be desirable
to deform those areas in the direction of maximum index difference.
That should reduce post-forming extension in the matched refractive
index direction. As a result, the reflective performance of the
polarizing optical stack 20' may be better maintained and, in some
cases, increased extension along the proper direction may increase
the desired reflectivity of the optical stack 20'.
[0084] One of the selected areas 22' and the surrounding optical
stack 20' is seen in the enlarged, partial cross-sectional view of
FIG. 3A. The result of the post-forming is that the thickness of
the optical stack 20' varies. One of the ways in which that
variation can manifest itself is that each of the selected areas
22' can form a depression in the otherwise generally smooth, planar
first major side 24' of the optical stack 20'.
[0085] Another manifestation of the thickness variations in an
optical stack 120' is illustrated in FIG. 3B where both the first
and second major sides 124' and 126' are deformed in selected areas
122' and 128'. Like selected area 122' on the first major side
124', selected area 128' on the second major side 126' is also
formed as a depression in the otherwise generally smooth, planar
second major side 126'.
[0086] Yet another manifestation of the thickness variations in an
optical stack 220' is illustrated in FIG. 3C where both the first
and second major sides 224' and 226' are deformed in selected areas
222' and 228'. While selected areas 222' are formed as depressions
on the first major side 224', the selected area 227' on the second
major side 226' is formed as a raised area extending outwards from
the otherwise generally smooth, planar second major side 226'. As
depicted, it may be preferred that the raised area 227' on the
second major side 226' be located opposite the depressed area 222'
on the first major side 224'.
[0087] The deformations illustrated in FIGS. 2A-2C and 3A-3C can be
characterized by the ratio of the thickness t.sub.o in the
undeformed portions of the optical stacks to the thickness t.sub.f
of the deformed portions of the optical stacks. Both of those
thicknesses are preferably measured between the major surfaces of
the optical stacks, i.e., the thickness of any skin layers is not
considered. Typically, it may be desirable that the ratio
t.sub.o:t.sub.f be at least about 1.1:1 or greater. In some cases,
it is desirable that the ratio t.sub.o:t.sub.f be at least about
1.5:1 or greater, more preferably at least about 1.75:1 or greater,
and even more preferably at least about 2:1 or greater.
[0088] FIGS. 4 & 5 illustrate a more extreme example of the
post-formed optical stack 220 illustrated in FIG. 2C. The
post-formed optical stack 30 illustrated in FIGS. 4 & 5 can be
considered an example of a deep draw post-forming process. The
optical stack 30 of FIG. 4 includes a first major side 34 (see FIG.
5) and a second major side 36 along with a plurality of selected
areas 32 in which the optical stack 30 has been post-formed to
provide depressed areas 32 formed on the first major side 34 of the
optical stack and raised areas 37 formed on the second major side
36 of the optical stack 30.
[0089] The deformed areas of the deeply drawn optical stack can be
characterized by the aspect ratio of the width (w) of the depressed
areas 32 as measured across the opening 33 of the depressed area 32
to the depth (d) of the depressed areas 32 as measured from the
first major side 34 of the optical stack 30. It is preferred that
the width of the depressed area 32 be measured across its narrowest
dimension. It may be desirable that the depressed areas 32 have an
aspect ratio w:d of about 10:1 or less, more desirably 2:1 or less,
even more desirably about 1:1 or less, and still more desirably
about 0.5:1 or less.
[0090] Alternatively, the deformation in the optical stack 30 can
be measured in absolute terms. For example, it may be preferred
that the depth d be at least about 0.1 millimeter or more; more
preferably at least about 1 millimeter or more; and even more
preferably at least about 10 millimeters or more. It will be
understood that where the depth d of the depressed areas 32
approaches or exceeds the thickness of the optical stack 30, the
more likely it is that a raised area 37 will be formed on the
second major side 36 of the optical stack.
[0091] The measurement of the depth d of the depressed areas 32
formed on the first major side 34 of the optical stack 30 is not
limited to those instances in which the first major side is planar.
Turning now to FIG. 6, where the optical stack 130 of a multilayer
optical film is depicted in a curved configuration. The optical
stack 130 includes a depressed area 132 formed on the first major
side 134 of the optical stack 130 and a corresponding raised area
137 on the second major side 136 of the optical stack 130. The
depth d of the depressed area 132 will preferably be measured from
the geometric surface defined by the first major side 134 of the
optical stack 130 and will typically be the largest depth from that
geometric surface.
[0092] FIGS. 7-9 depict another illustrative article including
post-formed multilayer optical film. FIG. 7 is a cross-sectional
view of a headlight assembly 40 for, e.g., an automobile or truck.
The headlight assembly 40 includes a lens 42, a lamp cavity 44
having a reflective inner surface 46, and a light source 48 mounted
within the lamp cavity 44.
[0093] It is preferred that the reflective inner surface 46 of the
lamp cavity 44 include post-formed multilayer optical mirror film
manufactured according to the principles of the present invention.
In this embodiment, it is preferred that the multilayer optical
film used be highly reflective for visible light and it may also be
helpful if the multilayer optical film is also reflective for light
into the infrared spectrum to limit heat build-up of the lamp
cavity 44 due to absorption of infrared energy by the substrate on
which the reflective inner surface 46 is located. Alternatively, if
the multilayer optical film has sufficient structural integrity
such that entire lamp cavity 44 is constructed of the multilayer
optical film, it may be preferable that the multilayer optical film
be transmissive for infrared energy to limit heat build-up within
the headlight assembly 40.
[0094] FIG. 8 is an enlarged cross-sectional view of the lamp
cavity 44 taken along line 8-8 in FIG. 7, and FIG. 9 is an enlarged
cross-sectional view of the lamp cavity 40 taken along line 9-9 in
FIG. 7. Both of the views depict a layer of post-formed multilayer
optical film 50 on the inner surface 46 of the lamp cavity 44.
Because the multilayer optical film 50 typically lacks sufficient
structural rigidity alone, it may be preferred to mount the
multilayer optical film 50 on a substrate 52 or some other form of
structural support, e.g. a frame, etc., by any suitable technique.
Alternatively, the multilayer optical film can be laminated to or
coextruded with a thicker layer that provides structural rigidity
either before or after post-forming operations.
[0095] Post-forming processes do not typically deform a multilayer
optical film uniformly and, as a result, the thickness of the
optical stacks in post-formed multilayer optical films according to
the present invention vary. The variations in thickness of the
post-formed multilayer optical film are in direct contrast with the
controlled uniform thickness of the multilayer optical film as
manufactured. That uniform thickness is desired because the
thickness of the optical layers within the multilayer optical film
define, in part, its optical properties. As a result, variations in
the multilayer optical film as manufactured are not desired because
they can adversely impact the uniform optical properties of the
film. For example, non-uniformities in the optical stack of
multilayer optical film as manufactured can result in iridescence
or other optical artifacts.
[0096] Thickness variations in the optical stack of post-formed
multilayer optical film are, in large part, caused by variations in
the strain experienced in different areas of the multilayer optical
film during post-forming. In other words, some areas of the
post-formed multilayer optical film may experience significant
deformation (strain) while other areas may experience little or no
deformation during post-forming.
[0097] The optical stacks of post-formed multilayer optical film in
articles will, as a result, often include variations in thickness
as illustrated in FIGS. 3A-3C, 8 and 9. For example, the thickness
of the multilayer optical film 50 varies between the two points in
the lamp cavity 44. The thickness t.sub.1 of the optical stack of
the post-formed multilayer optical film seen in FIG. 8 is thicker
than the thickness t.sub.2 of the optical stack of the post-formed
multilayer optical film depicted in FIG. 9. In both areas, however,
it is preferred that the reflectivity of the multilayer optical
film 50 for the desired range of wavelengths remain high for
normal, as well as off-axis, light. The importance of off-axis
reflectivity can be seen in FIG. 7 where light from the light
source 48 may approach portions of the light cavity 44 at high
angles off of normal.
[0098] The thickness variations in the optical stack can cause what
is commonly referred to as band shifting. In other words, the range
of wavelengths of which any multilayer optical film is reflective
is, in part, a function of the physical thickness of the layers in
the multilayer optical film. Varying the physical thickness of the
layers can cause the range of wavelengths over which the film is
reflective to change. Because changes in thickness typically
involve thinning of the multilayer optical film from its
manufactured thickness, band shifting is usually downward. For
example, a multilayer optical film that exhibits broadband
reflectance of light with wavelengths over the range of 400-900
nanometers and is thinned by a factor of 2 during post-forming
will, after thinning, typically exhibit broadband reflectance for
light with wavelengths in the range of 200-450 nanometers.
[0099] One approach to compensate for the effects of thinning
multilayer optical films (or any multilayer article exhibiting
reflectivity as a result of refractive index differentials, is
discussed in U.S. Pat. No. 5,448,404 (Schrenk et al.). Essentially,
the thinning effect and corresponding band shift can be compensated
for by adjusting the bandwidth of the multilayer optical film as
manufactured such that, after post-forming, the multilayer optical
film has layers with the appropriate optical thickness to reflect
light with the desired wavelengths.
[0100] Although both the upper and lower band edges may be adjusted
to compensate for thinning, for broadband mirrors it may be
preferable to adjust only the upper edge of the range of reflected
wavelengths upward by a factor that is at least as large as the
expected maximum factor by which the multilayer optical film will
be thinned during post-forming. By increasing the upper limit of
the range of wavelengths over which the multilayer optical film
reflects light before post-forming or drawing, the portions of the
post-formed multilayer optical film that are thinned during
post-forming will maintain their reflectivity over the desired
range of wavelengths (assuming the maximum factor by which the
multilayer optical film is thinned during post-forming does not
exceed the factor by which the upper limit of the wavelength range
has been adjusted to account for thinning during post-forming).
[0101] For broad band mirrors, it is typically not preferred to
adjust the lower limit in the reflected wavelength range because
some areas of the multilayer optical film may experience little or
no deformation or thinning during post-forming. By supplying a
multilayer optical film that, before post-forming, already reflects
light at the lower end of the desired range of wavelengths,
reflectivity of the entire post-formed multilayer optical film at
the lower end of the desired range of wavelengths can be retained
after post-forming.
[0102] For example, if the post-formed multilayer optical film in
the article is to reflect substantially all visible light (i.e.,
400-700 nanometer light), then before post-forming the multilayer
optical film should reflect normal incident light in at least the
wavelength range of from about 400 nanometers to about 900
nanometers multiplied by the expected thinning factor (the increase
in the upper edge bandwidth from 700 to 900 nanometers is provided
to compensate for light approaching at angles off of the normal
axis). If the maximum factor by which the post-formed multilayer
optical film is expected to be thinned during post-forming is 2,
then the multilayer optical film will preferably reflect normal
incident light in at least the wavelength range of from about 400
nanometers to about 1800 nanometers. If the maximum factor by which
the post-formed multilayer optical film is expected to be thinned
during post-forming is 3, then the multilayer optical film will
preferably reflect normal incident light in at least the wavelength
range of from about 400 nanometers to about 2700 nanometers.
[0103] If the optical stack of a multilayer optical film is
designed to compensate for thinning, variations in the thickness of
the post-formed multilayer optical film can be allowed without
significantly affecting reflectivity of the optical stack over the
desired wavelengths. For example, the ratio t.sub.1:t.sub.2 in the
post-formed multilayer optical film article 50 illustrated in FIGS.
7-9 may be at least about 2:1 or more without significantly
affecting the reflective properties of the multilayer optical film.
In some cases, it may be possible to provide multilayer optical
films that can support thickness ratios of 3:1 or more without
significant degradation of the optical properties of the
post-formed multilayer optical film over desired wavelengths.
[0104] FIGS. 10 & 11 illustrate another post-formed article
according to the present invention. The article 70 is a light guide
that can distribute light from a single source 72 to a plurality of
distribution points 74a, 74b and 74c (collectively referred to as
distribution points 74). Light guide 70 could be used in, e.g.,
lighting an instrument panel in an automobile or the like.
[0105] As seen best in the cross-sectional view of FIG. 11, the
light guide 70 can be formed from film 76 that has been post-formed
into the desired shape. Bonded over the post-formed film 76 is a
cover film 78 that, in the depicted embodiment, is a substantially
planar sheet of film 78. It will, however, be understood that the
cover film 78 could also be post-formed if desired. Different areas
of the post-formed film 76 and/or the cover film 78 can be
post-formed to varying thicknesses to allow for the transmission of
light of different wavelengths (e.g., visible light with different
colors). The two multilayer optical films 76 and 78 can be bonded
using a variety of techniques. In the depicted embodiment, the
films 76 and 78 are adhesively bonded using an adhesive 77. Other
techniques for bonding include mechanical fasteners or clamps,
welding, etc.
[0106] Although some specific examples of articles including
post-formed multilayer optical film have been described above, it
will be understood that post-formed multilayer optical film may be
included in any article in which it is desired to take advantage of
the unique optical properties of multilayer optical films. For
example, articles including post-formed multilayer optical film may
find use in the automotive area for headlights, taillights, and
other areas where the reflective properties of the post-formed
articles according to the present invention would be advantageous.
In addition, post-formed articles could also be used in the
automotive industry as trim pieces for head lamps, bezels, knobs,
automotive trim, and the like. The articles may also find
application in trim articles such as the light work for consumer
appliances including refrigerators, dishwashers, washers, dryers,
radios, and the like. They may also find use as toys or novelty
items. Other applications for post-formed articles according to the
present invention include light guides and/or pipes, shaped
reflectors for exterior lighting applications, bulb reflectors for
use in, e.g., backlit computer displays, medical/dental instruments
other than those described herein (e.g., disposable laparoscopic
mirrors), etc. In still other applications, the post-formed
articles may provide colored mirrors or filters for use in, e.g.,
center high mount stop lamps, decals, hood ornaments, etc. Other
uses include jewelry, seasonal ornaments (e.g., Christmas tree
ornaments), graphics, textured coatings, etc.
[0107] The post-formed articles of the present invention may also
be used as decorative items. Decorative items that may be formed
from the corrugated films include ribbons, bows, wrapping paper,
gift bags, garlands, streamers, centerpieces, and ornaments. The
post-formed articles may also be employed in a gift box or other
decorative packaging (e.g., cosmetic or food packaging), yarns, or
they may be arranged as a window in a gift bag. These examples of
decorative items are presented for illustrative purposes only and
should not be construed as a limitation on the variety of
decorative items in which the post-formed articles of the present
invention may be employed.
[0108] Furthermore, the articles according to the present invention
may be constructed entirely of post-formed multilayer optical film
or they may only include multilayer optical film in their
construction. If the post-formed multilayer optical film
constitutes only a portion of the article, it will be understood
that the post-formed multilayer optical film could be integrated
into larger assemblies by any suitable techniques, such as insert
injection molding, ultrasonic welding, adhesive bonding, and other
techniques.
Underdrawn Multilayer Optical Films
[0109] Of the multilayer optical films described in U.S. Pat. No.
5,882,774 (Jonza et al.), the mirror constructions of such films
are typically optimized for a high index differential. The films
typically have low extensibility limits (i.e., those limits beyond
which the films typically deform without fracture or tear during
deformation) because they are stretched during manufacturing to
levels that provide the desired high index of refraction
differential. In addition, some of the multilayer optical films may
be heat-set during manufacturing. Heat setting induces further
crystallization within the film and that increased crystallization
will typically further reduce the extensibility limits of the
films.
[0110] As a result of their relatively low extensibility limits,
known multilayer optical films such as those described in U.S. Pat.
No. 5,882,774 (Jonza et al.) may be difficult to post-form without
resulting in significant negative effects on the optical properties
of the resulting post-formed multilayer optical film. Although the
methods described above may be helpful in providing articles
including post-formed multilayer optical film and methods of
forming the articles, another approach to providing articles
including post-formed multilayer optical films can be pursued.
[0111] That other approach involves using multilayer optical films
in which the extensibility limits of the film are increased for
post-forming by deliberate underdrawing of the film during its
manufacture to produce what will be described with respect to the
present invention as an "underdrawn multilayer optical film" or
"underdrawn film". Such underdrawn multilayer optical film can then
be provided in a rolls or sheets for use in a subsequent
post-forming process or it may be directed into an in-line
post-forming process.
[0112] Multilayer optical film including layers of one or more
birefringent materials alternating with another material may be
characterized according to the strain-induced orientation and/or
crystallinity of the birefringent materials in the films. In fully
drawn films, or at least films considered to be fully drawn for the
purposes of the present invention, the birefringent materials will
typically exhibit higher levels of orientation and/or crystallinity
than a corresponding multilayer optical film constructed of the
same materials that is underdrawn.
[0113] The higher level of crystallinity in the fully drawn films
is, in large part, the result of the increased effective strain to
which the multilayer optical film is subjected during
manufacturing. As discussed above, fully drawn films are typically
drawn to higher levels to improve their reflective properties.
Those reflective properties are largely based on the orientation
and/or crystallinity of the birefringent materials in the film,
which can be correlated to the index of refraction of the
birefringent materials. As a result, orientation and/or
crystallinity are also related to the refractive index
differentials (.DELTA.x, .DELTA.y) in any multilayer optical
film.
[0114] Because an underdrawn multilayer optical film is not
subjected to the same level of effective strain as is a fully drawn
multilayer optical film with the same construction, the
birefringent material in the underdrawn multilayer optical film
will typically exhibit reduced crystallinity or at least one
reduced in-plane refractive index differential (.DELTA.x or
.DELTA.y) as compared to a fully drawn multilayer optical film
manufactured with the same materials, layer thicknesses, numbers of
layers, etc.
[0115] The reduced orientation and/or crystallinity may also
typically result in reduced refractive index differentials in the
underdrawn multilayer optical films as compared to the same
construction in a fully drawn state. As a result, it may be helpful
to increase the number of layers usually required to cover a given
wavelength range with a given reflectance. Second order peaks from
the thicker layers of the broader band may reduce the actual need
for an increase in the layer numbers. Such considerations can,
however, be determined based on the discussions in U.S. Pat. No.
5,882,774 (Jonza et al.).
[0116] It is important to note that, in addition to an upper limit
on crystallinity for an underdrawn multilayer optical film, there
is also preferably a lower limit as well. In other words, an
underdrawn multilayer optical film including birefringent materials
in its layers will include at least some level of strain-induced
crystallinity. By providing underdrawn multilayer optical films
with at least some level of strain-induced crystallinity, the
post-forming of the underdrawn multilayer optical films will
typically be more predictable as compared to a film in which no
strain-induced crystallization is found in the birefringent
materials.
[0117] The importance of providing an underdrawn multilayer optical
film with at least some strain-induced crystallinity is illustrated
in FIG. 12, an idealized graph of draw ratio (horizontal axis)
versus crystallinity (vertical axis) for multilayer optical films
including layers of at least one birefringent material alternating
with another material. The behavior illustrated in FIG. 12 is
typical of polyesters such as PEN, PET or co-polymers comprising
them which can develop birefringence and which can be cast from a
die and quenched efficiently resulting in an initial cast web or
film with very little crystallinity. FIG. 12 may also characterize
other quenchable, birefringent polymeric materials that are
susceptible to strain-induced crystallization. Again, such quenched
films would preferably exhibit only low levels of crystallinity
caused by crystallization during quenching prior to drawing. As
drawing of the film is begun, the crystallinity of the birefringent
materials in the multilayer optical film may begin to increase, but
the increases are at relatively low initial rates. Those draw
ratios at which the strain-induced crystallinity increases at a
relatively low initial rate are included in what will be defined as
Regime I for the purposes of the present invention. As the draw
ratio increases past Regime I into what will be referred to as
Regime II, the crystallinity of the birefringent material in the
multilayer optical film as a function of the draw ratio increases
at a significantly faster rate than in Regime I.
[0118] In Regime I of FIG. 12, the effect of drawing is
approximately reversible in as much as cessation of drawing and
continued heating allows for the relaxation of orientation (i.e. a
reduction in the index of refraction differences in the three
principal material directions) with minimal crystallization. The
reversibility is not necessarily complete because Regime I
typically appears in a temperature region of large supercooling.
Thus crystallization is thermodynamically favored but kinetically
hampered. Accumulated time during drawing and relaxation at these
temperatures (e.g. via cycling) may eventually bring the material
into Regime II via the relatively slow accumulation of
crystallinity. Nevertheless, it is this approximate reversibility
that distinguishes Regime I from Regime II. In general, the degree
of crystallinity (or total polarizability as described later)
tolerable in this regime depends on the particular polymer, its
quenching conditions and its pre-drawing post process
conditions.
[0119] The draw ratio at which the rate of crystallization of the
birefringent material in the multilayer optical film begins to
increase significantly and move into Regime II can be influenced by
a number of factors including draw rate, temperature, etc. After
the birefringent material has experienced sufficient strain-induced
crystallization to enter Regime II, however, it will typically
follow the crystallization curve defined by that initial drawing.
In other words, the film cannot continue to be drawn without
inducing crystallization in the birefringent materials at the
increased rates associated with Regime II in the graph of FIG. 12.
As a result, the characteristics of the film will be subject to
less variability when drawn further in post-forming processes
because the crystallization rate of the birefringent materials is,
in large part, set by the pre-stretching required to put the film
into Regime II.
[0120] For a multilayer optical film including birefringent
materials that have not experienced sufficient strain-induced
crystallization to enter Regime II, further stretching or drawing
during post-forming will not be as predictable because the point at
which the crystallization rate starts to increase significantly is
subject to the factors listed above, e.g., temperature and draw
rate. As a result, the film could experience small increases in the
draw ratio that result in significant increases in the rate of
crystallization of the birefringent materials or it could
experience large draw ratios with a relatively small increase in
the rate of crystallization of the birefringent materials. In
either case, the level of predictability is reduced as compared to
a film that includes sufficient strain-induced crystallization such
that its rate of crystallization is largely set, i.e., the
birefringent materials in the multilayer optical film have entered
Regime II.
[0121] In the case of many polymers, especially the polyesters
including PEN, PET and copolymers including PEN and/or PET, a third
regime develops in which the index of refraction increases at a
much slower rate with respect to the draw ratio. Often the total
polarizability will also change at a much slower rate as well. FIG.
12A illustrates the index of refraction in the direction of drawing
(vertical axis) as a function of the measured draw ratio
(horizontal axis) for one uniaxially drawn PEN film in which the
orthogonal in-plane axis dimension is held generally constant. The
PEN used for this illustrative case had an intrinsic viscosity of
0.48 and was drawn according to a linear draw profile of 20% per
second initial draw rate at 130 degrees Celsius.
[0122] For the illustrated case, Regime II begins at a draw ratio
of about two (2) and Regime III begins at a draw ratio of about
three (3). The onset of these regimes depends on process and
material conditions including, for example, raising the strain
rate, raising the intrinsic viscosity, lowering the temperature,
and/or lowering the glass transition temperature (e.g., by lowering
the moisture and/or plasticizer content) may all lower the draw
ratio at onset for Regimes II and III from those illustrated in
FIG. 12A. The molecular weight distribution, rather than just an
intrinsic viscosity may also alter the regime onsets. Analogous
results can be expected for biaxially drawn films.
[0123] In view of the above discussion, one difference between a
fully drawn multilayer optical film and an underdrawn multilayer
optical film of the same construction is that the fully drawn
multilayer optical film includes birefringent materials in which
the crystallinity is higher than the crystallinity of the
birefringent materials in the underdrawn multilayer optical films.
Where the birefringent material in the multilayer optical film is a
polyester, it may be preferred that the crystallinity of the
birefringent polymer is about 18% or less, more preferably about
15% or less. In comparison, the crystallinity of the same
birefringent polyesters in the fully drawn multilayer optical films
will be at least about 20% or more, more typically about 25% or
more.
[0124] In addition to an upper limit for crystallinity, underdrawn
films can also be characterized by a lower limit for the
crystallinity of the birefringent materials in the underdrawn
multilayer optical film, because the birefringent materials in the
films do preferably exhibit some level of strain-induced
crystallinity. In other words, it is preferred that the
birefringent materials in the multilayer optical films have entered
Regime II as discussed above. For multilayer optical films
including polyesters as the birefringent materials, it may be
preferred that the lower limit of crystallinity of the birefringent
materials in the multilayer optical film be at least about 3% or
more, in some instances more preferably at least about 5% or more,
and in other instances even more preferably at least about 10% or
more. Higher levels of crystallinity typically provide higher
levels of birefringence in the underdrawn state and reflect the
degree of underdrawing. Higher birefringence can improve the
performance of the initial underdrawn state in a finished
post-formed article.
[0125] Although we do not wish to be limited by any particular
theory, it is believed that the lowest level of crystallinity
provides a minimum level of connectivity between the
micro-crystalline domains, e.g. via tie chains, which substantially
reduces the propensity for large scale relaxation of the developing
morphology. In many instances, crystallization at these levels will
move the birefringent materials in the multilayer optical film into
Regime II. The exact threshold of lower crystallinity depends upon
the chemical nature of the material including the composition and
molecular weight as well as upon the process conditions such as
temperature, rate and duration of draw and heating
[0126] Although crystallinity may be used to characterize
underdrawn multilayer optical films, underdrawn multilayer optical
films may alternatively be characterized using what will be
referred to herein as "total polarizability" of the layers
including birefringent materials. Determination of total
polarizability is based on the refractive indices of the layer or
layers including birefringent materials within the multilayer
optical film.
[0127] The "total polarizability difference" will be defined as the
difference between the total polarizability of the drawn material
and that of the quenched amorphous state of the same material. Any
given material is expected to possess a maximum total
polarizability difference in a certain maximal fully drawn state.
Where the multilayer optical film includes two or more different
layers with different compositions of birefringent materials, total
polarizability difference will preferably be measured for the
layers including birefringent materials with the largest total
polarizability difference relative to its maximum total
polarizability difference as determined by the methods discussed
below.
[0128] Refractive indices may be measured by a variety of standard
methods using, e.g., an Abbe refractometer or a prism coupling
device (e.g. as available from Metricon, Piscataway, N.J.).
Although it is difficult to directly measure the refractive indices
of the materials in the individual layers of the optical stack of
the multilayer optical film, the refractive indices of the optical
stack as a whole can be reliably measured. Furthermore, the
refractive indices of the optical stack as a whole are weighted
averages of the refractive indices of the materials in each of the
individual layers making up the optical stack.
[0129] If, for example, the optical stack is constructed of two or
more materials, the interdiffusional effects between layers are
small, and the refractive indices of only one of the materials
changes significantly in response to drawing, then the refractive
indices of the individual layers can be estimated based on the
refractive indices of the optical stack as a whole. These estimates
are based on the typically accepted assumption that the refractive
indices of the optical stack as a whole are the
optical-thickness-weighted averages of the refractive indices of
the materials in the various layers of the optical stack.
[0130] In another variation, in those films in which one or more of
the materials making up the layers of the optical stack are also
present in thicker skin layers and/or internal protective boundary
layers, then it can typically be assumed that the refractive
indices are the same for the same material, whether that material
is found in the layers of the optical stack or elsewhere in the
multilayer optical film. As a result, if the refractive indices of
only one of the materials making up the optical stack is unknown
and the refractive indices of the other materials in the optical
stack are known, then measurement of the refractive indices of the
optical stack will allow for calculation of the refractive indices
of the unknown material. In some instances, measurement of the
refractive indices may require destructive peeling or other known
techniques of isolating the various layers of the multilayer
optical films.
[0131] Typically, the refractive indices of the birefringent
materials in the multilayer optical film will be determined based
on the above techniques because it is the refractive indices of the
birefringent materials that change in response to drawing or
deformation. Assuming conservation of molecular polarizability
within the birefringent materials of the optical stack (an
assumption that is typically considered a reasonable approximation
for many semi-crystalline polymers, including the polyesters used
in preferred underdrawn multilayer optical films, e.g., PEN, PET
and copolymers of PET and PEN), an anisotropic analogue of the
Clausius-Mossetti equation using a Lorenz-Lorentz local field
yields the following equation which results in a number referred to
above as the total polarizability of the birefringent
materials:
(n.sub.1.sup.2-1)/(n.sub.1.sup.2+2)+(n.sub.2.sup.2-1)/(n.sub.2.sup.2+2)+-
(n.sub.3.sup.2-1)/(n.sub.3.sup.2+2)=.rho.K=Total polarizability
where n.sub.1, n.sub.2 and n.sub.3 are the refractive indices in
the principal directions of a given layer within the multilayer
optical film, .rho. is the density of the materials in that layer,
and K is a volume polarizability per unit mass for the materials in
that layer. Total polarizability is a function of wavelength due to
the wavelength dependence of the indices of refraction. As a
result, when referred to numerically herein, total polarizability
will be determined with respect to light having a wavelength of
632.8 nanometers (e.g., as provided by a helium-neon laser light
source).
[0132] It should be noted that an alternative to the total
polarizability equation can also be used. In this alternative, each
of the three principal indices in the equation is set equal to the
simple average of the three measured principal indices. The total
polarizability is then called a refractivity and an analogous
refractivity difference may be defined. Likewise, density and
crystallinity may be calculated. These may vary from that
calculated using the total polarizability. For discussion purposes,
the total polarizability calculation is used in the examples that
follow.
[0133] Many semi-crystalline polymers, such as isotactic
polypropylene and polybutylene terephthalate, are difficult to
quench in the amorphous state; or if quenched, are difficult to
re-heat fast enough or process cold enough to prevent significant
quiescent crystallization prior to drawing. Such polymers may not
exhibit Regime I under typical process conditions. Rather, the
connectivity in the morphology means that all subsequent drawing is
at least partially effective and the material essentially begins in
Regime II after casting and quenching. As with materials that
exhibit Regime I behavior, these materials can still be drawn and
oriented. Moreover, the higher the degree of underdrawing (i.e. the
lower the degree of drawing), the higher the level of residual
extensibility available during the post processing (e.g.
thermoforming).
[0134] From a functional standpoint, the onset of Regime II sets a
certain level of extensibility related to the ultimate
extensibility. This ultimate extensibility will vary somewhat with
draw conditions. The amount of underdrawing is relative to this
ultimate extensibility. Fully drawn films are drawn near to this
limit. Underdrawn films are drawn below this amount, but preferably
have been drawn past the onset of Regime II. The level of
underdrawing desired may be a function of the level of
extensibility desired for the subsequent post forming process.
[0135] The level of underdrawing is also a function of direction.
Upon onset of Regime II, a certain level of drawing is locked in.
This amount may vary in direction depending upon the process
conditions at the time of onset. For example, a uniaxially drawn
film will have a higher degree of underdrawing in the non-drawn
direction at the point of Regime II onset. In the case of mirror
films, equal underdrawing in both directions may be preferred. This
may be achieved by minimizing the in-plane birefringence. As used
here, the in-plane birefringence is simply defined as the absolute
value or magnitude of the difference between the maximum and
minimum refractive index values in the plane on the film. In the
case of a uniaxially drawn film, this is typically the difference
between the indices of refraction in the draw and non-drawn
directions. In the case of polarizing films, a large in-plane
birefringence is desired within the constraints of the underdrawing
required to obtain a desired level of extensibility in the post
process.
[0136] As can be seen by the directional nature of underdrawing,
crystallinity or total polarizability alone does not fully
characterize the level of underdrawing, although it sets useful
limits with regards to the transition between Regime I and II and
between underdrawn and fully drawn films. It should be understood
that a certain level of extensibility reflects a corresponding
level of underdrawing. For example, films drawn quickly in Regime
II may not achieve the same level of crystallinity as those drawn
slowly or those that continue to be heated at the draw temperature
after drawing to heat set the films. The latter may be less
extensible than the former; however, they may still be more
extensible than other films slightly more drawn but less heat set.
Thus maximum and minimum levels of crystallinity and/or total
polarizability difference are most applicable in delineating the
bounds of what is meant as an underdrawn film and not necessarily a
sole measure of the relative performance among that class of
films.
[0137] The total polarizability difference of the birefringent
materials in underdrawn multilayer optical films including PEN
(and, by the definitions provided below in the section regarding
materials selection, predominantly PEN copolymers) as measured in
the birefringent layers is preferably within a range of from about
0.002 up to about 0.018, more preferably within a range of from
about 0.002 up to about 0.016. Within either range, it may be
desirable that the maximum in-plane birefringence of reflective
polarizing multilayer optical films is less than about 0.22, more
preferably less than about 0.17, and, in some cases, still more
preferably less than about 0.15. In the case of underdrawn mirror
films, a maximum in-plane birefringence of less than about 0.14 is
preferred in combination with either of the ranges for the total
polarizability difference in the birefringent materials.
[0138] Total polarizability difference of the birefringent
materials in underdrawn multilayer optical films including PET
(and, by the definitions provided below in the section regarding
materials selection, predominantly PET copolymers) as the measured
birefringent layer is preferably within a range of from about 0.002
up to about 0.030, more preferably within a range of from about
0.002 up to about 0.0024. In the case of mirror films, these ranges
are preferably coupled with a maximum in-plane birefringence of
less than about 0.11, more preferably less than about 0.04.
[0139] The differences between the preferred levels of total
polarizability and birefringence for the various polymers reflects
the differences in the amorphous and crystalline densities of the
different materials. The differences also reflect the intrinsic
maximum birefringence of the different polymers, as well as the
limits of extensibility after the onset of Regime II as discussed
above.
[0140] In addition to the total polarizability and maximum in-plane
birefringence, underdrawn multilayer optical films can also be
characterized by reflectivity. For example, where the total
polarizability difference of the measured birefringent materials is
within the various ranges discussed above, it may be preferred that
the multilayer optical film reflect at least about 85% of normal
incident light of desired wavelengths that is polarized along at
least one in-plane axis, more preferably the film may reflect at
least about 90% of normal incident light of desired wavelengths
that is polarized along at least one in-plane axis. If the
multilayer optical film is intended to be a mirror film, i.e., not
a reflective polarizer, it may be preferred that the reflective
performance of the film in terms of percent reflectance hold for at
least one of and more preferably two generally perpendicular
in-plane axes.
[0141] As indicated in the equation presented above, total
polarizability of the material(s) in a given layer of the optical
stack of the multilayer optical film represents the product of
density and the volume polarizability per unit mass of the
material(s) in that layer. The volume polarizability per unit mass
(K) is typically considered an invariant material property under
draw according to the conservation of molecular polarizability
assumption discussed above. Drawing of birefringent materials
causes strain-induced crystallization as discussed above and, in
most birefringent materials, the density of the material varies
based on whether the material is crystallized or amorphous.
[0142] As a result, the density of the birefringent materials in
the multilayer optical films changes based on the amount of
strain-induced crystallization in the birefringent materials. Those
changes in density can be used to estimate the level of
strain-induced crystallization in the underdrawn multilayer optical
films according to the present invention. This method of
determining the level of strain-induced crystallization is not,
however, without its limits.
[0143] One class or type of preferred birefringent materials used
in the multilayer optical films according to the present invention
are semi-crystalline. If the crystals in the semi-crystalline
birefringent materials are relatively small, an effective
refractive index for the semi-crystalline aggregate may be
measured. This is often the case in polymers, such as polyesters
(e.g., PEN and PET), that are drawn from a relatively amorphous
state to a state of semi-crystallinity. In such cases, the density
of the birefringent material (based on the refractive indices) may
be estimated from the total polarizability and used to determine
the level of crystallinity in the birefringent materials using a
standard correlation between crystallinity and density.
[0144] In either case, the above discussions set out different
approaches to characterizing underdrawn films according to the
present invention. In the first, the strain-induced crystallinity
of the birefringent materials is measured and used to define
underdrawn multilayer optical films. In the second, the refractive
indices of the birefringent materials can be used to determine the
total polarizability of the birefringent materials which can also
be used to define underdrawn multilayer optical films. In still
another manner, the strain-induced crystallinity can be determined
based, at least in part, on the refractive indices used to
determine total polarizability.
[0145] For example, the total polarizabilities of amorphous cast
webs of PET and PEN are found to be about 0.989 and 1.083,
respectively, and the densities of the amorphous materials are
measured using a standard density gradient column at about 1.336
and 1.329 grams per cubic centimeter, respectively. The resulting
volume polarizabilities can be calculated at about 0.740 and 0.815
cubic centimeters per gram for PET and PEN, respectively. Densities
of drawn films of PET and PEN may now be calculated by dividing the
total polarizabilities by the respective volume polarizabilities.
Moreover, the crystallinity may be estimated given the density of
the pure crystalline phase, estimated as 1.407 grams per cubic
centimeter for the typical crystalline phase of PEN and 1.455 grams
per cubic centimeter for the crystalline PET.
[0146] The crystallinity can be estimated via a linear
interpolation of the actual density between the amorphous density
(zero crystallinity) and the pure crystalline density. Such
crystalline estimates may vary from other measures as it neglects
densification of the non-crystalline phase due to orientation and
rarefication of the crystalline phase due to imperfections and
defects. Other methods for determining crystallinity include
Differential Scanning Calorimetry and X-ray Scattering.
Measurements obtained by these methods may be correlated to the
density or total polarizability methods described herein through
the use of suitable drawn film standards. It can typically be
assumed that copolymers will have volume polarizabilities that are
weight averages of their components, so that similar calculations
can be made on co-polymers, if the type of crystals are known.
Usually, this is the crystal corresponding to the predominant
crystallizing monomer or subunit. Total polarizability can be used
to characterize the underdrawn state of many systems. However, lack
of a definitive total polarizability measurement in no way limits
the utility of the invention. In some cases, the extensibility of a
non-birefringent layer may be limiting. For example, a
non-birefringent semi-crystalline second material layer may still
become drawn during film processing. Under drawing to suit this
layer would be desirable When the material has very low or no
inherent birefringence, as is the case with a few polymers such as
poly methyl methacrylate, then little or no orientational
information can be derived. Nevertheless, the extensibility of such
a non-birefringent non-crystalline second material may also be
limiting. In the case of non-crystalline materials, the orientation
may be relaxed and thus the extensibility recovered by pre-heating
prior to draw. Optimizing the conditions of such pre-heating must
balance the recovered extensibility of the amorphous material
against any lost extensibility by the birefringent semi-crystalline
first material. In the examples that follow below, it is believed
that the birefringent strain-hardening layers (e.g., PEN or 90/10
coPEN layers) are the limiting layers for extensibility, whereas
the second material layers (e.g., PMMA, PETG, or 70/0/30 coPEN) are
believed to be nearly isotropic for the conditions used to
manufacture the optical stacks. Finally, in a semi-crystalline
material, if the crystals are relatively large, haze and scattering
may obscure index measurements.
Process Considerations for Post-Forming Multilayer Optical
Films
[0147] Because the post-formed multilayer optical films used in
connection with the present invention rely on birefringent
materials that provide strain-induced refractive index
differentials to obtain the desired optical properties, variations
in deformation of the multilayer optical film during post-forming
can be particularly problematic.
[0148] As discussed above, the index of refraction differentials
(.DELTA.x, .DELTA.y) in the multilayer optical film as manufactured
are, in large part, the result of drawing of the multilayer optical
film during manufacturing which causes the indices of refraction of
the birefringent materials to change. Those changes cause
refractive index differentials large enough to provide the desired
reflective properties. Because the strain in the multilayer optical
film during manufacturing is largely uniform, the strain-induced
index of refraction differentials are also largely uniform over the
film, and the resulting reflective properties are also largely
uniform.
[0149] In post-forming processes the birefringent layers in the
multilayer optical film are subjected to additional strain. One
difference from manufacturing of the multilayer optical film is,
however, that the strain induced during post-forming is not uniform
over the film. The variations in thickness of the optical stack in
a post-formed multilayer optical film as discussed above are, in
part, indicative of the variations in strain over the post-formed
multilayer optical film.
[0150] As a result, if the birefringent materials in the multilayer
optical film are capable of further strain-induced index of
refraction changes, the index of refraction differentials in the
multilayer optical film may be changed as a result of post-forming.
Furthermore, if the strain induced during post-forming is not
uniform, the index of refraction changes in the post-formed
multilayer optical film may also be non-uniform and may result in
non-uniform optical properties in the post-formed multilayer
optical film.
[0151] In addition to non-uniform post-forming strain-induced
changes, another difficulty associated with post-forming multilayer
optical films including strain-induced refractive index
differentials in connection with birefringent materials is that
many post-forming processes employ heat to improve the working
properties of the multilayer optical film during deformation. The
strain-induced changes in the refractive indices of the
birefringent materials in the multilayer optical film are typically
the result of strain-induced crystallization of the birefringent
materials. The strain-induced crystallization and corresponding
refractive indices can, however, be changed when the birefringent
materials are subjected to heat during post-forming.
[0152] For example, heating may result in increased crystallization
due to the heat during post-forming or decreased crystallization as
a result of melting or relaxation during post-forming. In either
case, changes in the crystallization level of the birefringent
materials can result in a change in the refractive index
differentials in the film. The potential crystallization changes in
the birefringent materials may be further exacerbated by the
simultaneous post-forming deformation and heating of the film
which, in combination, may cause greater changes in the
recrystallization/refractive index of the birefringent materials
than either action alone.
[0153] The present invention, however, overcomes these difficulties
to provide articles including post-formed multilayer optical film
and methods of producing those articles. These results are achieved
even though all of the multilayer optical films referred to in
connection with the present invention include birefringent
materials and rely on strain-induced refractive index differentials
to obtain the desired optical properties.
[0154] Although post-forming may be most advantageously pursued
using the "underdrawn" multilayer optical films described above, it
may also be possible to obtain desirable post-forming results using
multilayer optical films including a birefringent material and
other materials that do not meet the definitions of underdrawn
multilayer optical films, e.g., constructed according to U.S.
patent Ser. No. 08/472,241.
[0155] In the post-forming methods of the present invention, it may
be desirable to heat the multilayer optical films to forming
temperatures that are near to, but below, the peak crystalline
melting temperatures of the birefringent materials. Such heating
can improve the extensibility of multilayer optical films during
post-forming processing. By heating the multilayer optical film to
those levels, the tendency of the multilayer optical film to
fracture or tear at a given draw ratio during post-forming may be
decreased. In addition, the forces required to post-form the films
may be reduced as a result of the increased forming
temperature.
[0156] Underdrawn multilayer optical films may also have increased
extensibility under these process conditions. Because processing
under these conditions is in the melting regime, precise
temperature control is desirable to ensure uniform drawing and
reduce or prevent damage to the post-formed multilayer optical film
in the article. Such damage could take the form of complete
melting, with concomitant loss of birefringence and/or hole
formation in the multilayer optical film.
[0157] Reducing the stress required for a given amount of
deformation during post-forming may reduce the tendency of the
materials in the film to fracture, thereby enhancing extensibility.
Heating the multilayer optical film to a forming temperature near
the peak crystalline melting temperature of the birefringent
material in the film may also enhance extensibility by melting less
perfect crystals, thereby loosening the morphological
microstructure in the birefringent material layers.
[0158] For example, one material used in some preferred multilayer
optical films according to the present invention is polyethylene
naphthalate (PEN), which has a peak melting point of about 270
degrees Celsius (520 degrees Fahrenheit) using standard
differential scanning calorimetry (DSC). The onset of melting is,
however, often seen at about 255 degrees Celsius (490 degrees
Fahrenheit) or below. This onset of melting may be attributable to
the melting of less well-developed crystals within the PEN with the
peak melting temperature being that point at which all or nearly
all of the crystals in the material have melted. Heating the
birefringent materials in the multilayer optical film may also
increase mobility within the microstructure, thereby activating
crystal slip and other deformation mechanisms that could enhance
extensibility of the multilayer optical film.
[0159] The extent to which heating may improve extensibility of the
multilayer optical films according to the present invention will,
at least in part, vary based on the materials used in the films.
Some materials may exhibit larger increases in extensibility when
heated as compared to others. Furthermore, the combination of
materials within each of the multilayer optical films according to
the present invention can also affect improvements in extensibility
of the film as a whole.
[0160] For example, to improve the extensibility of the multilayer
optical films, it may be preferred to heat the multilayer optical
films to a forming temperature in the range of from about 30
degrees Celsius (about 55 degrees Fahrenheit) below the peak
crystalline melting temperature of the birefringent material up to
about the peak crystalline melting temperature of the birefringent
material during post-forming. It may be more preferred to heat the
film to a forming temperature in the range of from about 15 degrees
Celsius (about 30 degrees Fahrenheit) below the peak crystalline
melting temperature of the birefringent material up to about the
peak crystalline melting temperature of the birefringent material
during post-forming. These forming temperatures may increase
extensibility and reduce the likelihood of fracture of multilayer
optical films during post-forming processing.
[0161] A way to improve uniformity in the multilayer optical film
during post-forming is to include materials in the multilayer
optical film that are subject to strain hardening during
deformation. Strain hardening is a property of materials in which
the stress required to achieve a particular level of strain
increases as the material is strained (i.e., stretched).
Essentially, strain hardening materials may provide self-regulation
of the thinning process due to post-forming.
[0162] In terms of molding, as the multilayer optical film is
stretched during post-forming, unquenched sections of the film that
have not yet made contact with a mold surface will tend to draw
more uniformly after the onset of strain hardening. As a result,
those portions of the film that have been stretched to the point at
which strain hardening occurs will progressively stretch less while
those portions of the film that have not experienced strain
hardening will continue to stretch at faster rates. The end result
is that the thinner (i.e., strain hardened) portions of the film
will thin to a certain point after which the thicker portions of
the film will continue to stretch and become thinner, effectively
evening out the stretching or thinning of layers in the multilayer
optical film during post-forming processing. This reinforcement
effect of strain hardening is also operative in post-forming
processes in which no mold is present to provide quenching of the
film during post-forming. One material that provides strain
hardening properties in a multilayer optical film is PEN. In
general, strain-hardening is typically observed in many
semi-crystalline polymers at high enough levels of strain.
[0163] Strain-hardening can help to regulate the uniformity of the
drawing process, thus potentially reducing variations in the amount
of deformation experienced by the film during post-forming. If the
bandwidth of the multilayer optical film as manufactured is
specifically designed to the final biaxial draw ratio of the
post-forming process, rather than the draw ratio at tear or
fracture as discussed above, then strain hardening can allow the
design of a multilayer optical film with a narrower, more
reflective band for use in the post-forming process.
[0164] The effect of strain hardening may also influence the degree
to which vacuum-forming as one post-forming process will allow for
adequate or desirable mold replication. Pressurized or plug
assisted molding techniques may be needed for accurate post-forming
processing of materials in which strain hardening potentially
increases the resistance of the film to stretching during the
molding process. The effect of strain hardening may be influenced
by both the post-forming draw conditions and the degree of draw
(strain-hardening) before post-forming is initiated.
[0165] In addition to the above, one further consideration in
developing an appropriate post-forming process may include an
analysis of the rate of crystallization for the given materials as
a function of temperature. Referring now to FIG. 13, an idealized
graph of rate of crystallization (vertical axis) as a function of
temperature (horizontal axis), it can be seen that crystallization
rate increases with temperature to a certain point, referred to as
the peak crystallization rate temperature T.sub.max, after which
the rate of crystallization tends to fall again as the temperature
moves towards the peak crystalline melting temperature T.sub.m of
the material. Differential scanning calorimetry may be used to
estimate T.sub.max. For PEN, T.sub.max has been estimated at about
220 degrees Celsius (about 430 degrees Fahrenheit) using
differential scanning calorimetry upon heating at 20.degree.
C./min., and T.sub.max has been estimated at about 208 degrees
Celsius (about 406 degrees Fahrenheit) using differential scanning
calorimetry upon cooling at 5.degree. C./min. Although we do not
wish to be held to any theory, it is thought that the extensibility
of multilayer optical films during post-forming can be improved in
many cases if the forming temperatures used are not the same as the
peak crystallization rate temperature of the birefringent material
or materials in the film. This may be particularly applicable to
films that have not already been heat set, and especially
underdrawn films. Nevertheless, if the film is sufficiently
underdrawn, extensibility and thus post-processability may still be
acceptable after heating at these temperatures. The following
discussion elucidates the effects of post forming near T.sub.max
for some cases; e.g. certain underdrawn, non-heatset films
comprising certain polyesters. It should be understood that
multilayer optical films comprising materials other than polyesters
may behave differently in their relation between peak
crystallization temperature and optimal forming temperatures.
[0166] Further crystallization and morphological changes during
pre-heating before post-forming may reduce extensibility and
post-formability. In one aspect, it may be preferred that the
forming temperature of the film during post forming be lower than
the peak crystallization rate temperature of the birefringent
material with the lowest peak crystallization rate temperature in
the film, more preferably more than about 10 degrees Celsius below
the peak crystallization rate temperature of the birefringent
material with the lowest peak crystallization rate temperature in
the film, and even more preferably more than about 20 degrees
Celsius below the peak crystallization rate temperature of the
birefringent material with the lowest peak crystallization rate
temperature in the film. It may also be preferred that the forming
temperature be greater than the peak crystallization rate
temperature of the birefringent material with the highest peak
crystallization rate temperature in the film, more preferably more
than about 10 degrees Celsius above the peak crystallization rate
temperature of the birefringent material with the highest peak
crystallization rate temperature in the film, and even more
preferably about 20 degrees Celsius above the peak crystallization
rate temperature of the birefringent material with the highest peak
crystallization rate temperature in the film.
[0167] These forming temperature limitations may be combined as
desired. For example, it may be preferred that the forming
temperature be more than about 10 degrees Celsius below the peak
crystallization rate temperature of the birefringent material with
the lowest peak crystallization rate temperature in the film or
more than about 20 degrees Celsius above the peak crystallization
rate temperature of the birefringent material with the highest peak
crystallization rate temperature in the film. In another
alternative, it may be desired that the forming temperature be more
than about 20 degrees Celsius below the peak crystallization rate
temperature of the birefringent material with the lowest peak
crystallization rate temperature in the film or greater than the
peak crystallization rate temperature of the birefringent material
with the highest peak crystallization rate temperature in the film.
Other combinations of these different limitations will also be
apparent upon further analysis.
[0168] Where only one birefringent material is present in the
multilayer optical film, the forming temperature limitations can be
more simply expressed. It may be preferred that the forming
temperature of the film be different than the peak crystallization
rate temperature of the birefringent material in the film.
Alternatively, it may be preferred to define the forming
temperature in terms of ranges, e.g., it may be preferred that the
forming temperature of the film be more than about 10 degrees
Celsius below the peak crystallization rate temperature of the
birefringent material, more preferably more than about 20 degrees
Celsius below the peak crystallization rate temperature of the
birefringent material in the film. It may also be preferred that
the forming temperature be more than about 10 degrees Celsius above
the peak crystallization rate temperature of the birefringent
material film, more preferably about 20 degrees Celsius above the
peak crystallization rate temperature of the birefringent material
in the film.
[0169] After post-forming draw, it may be desirable to deliberately
heat set the formed article to improve its reflectivity. This heat
setting preferably follows the last post-forming drawing step;
e.g., further crystallization can now be encouraged with attendant
refractive index difference increases without consideration of
further extensibility after the final post-forming draw step.
[0170] Although the methods of post-forming multilayer optical
films in general are discussed above, the post-forming of
underdrawn multilayer optical films may be varied while still
providing desirable post-forming results. One significant variation
is that the forming temperature of the underdrawn multilayer
optical films may lie well below the peak crystallization rate
temperatures of the birefringent materials within the films. Heat
setting following the final post-forming draw step may also be
desirable for articles manufactured from underdrawn multilayer
optical films. For example, the crystallinity (and, as a result,
the reflectance) of portions of the underdrawn films that have not
been drawn during post-forming can be increased by heat-setting
following the final post-forming draw steps. In addition, those
portions of the underdrawn film that were drawn during post-forming
can also experience increased crystallinity and the attendant
reflectance.
[0171] The underdrawn multilayer optical films can be provided with
and post-formed according to all of the variations described above
with respect to multilayer optical films in general. In other
words, they can be provided as highly reflective films that retain
their reflectivity after post-forming, etc. Furthermore, the
modifications discussed above for thinning effects should also be
considered when manufacturing and processing underdrawn multilayer
optical films as well.
Post-Forming Selected Areas of Multilayer Optical Films
[0172] The articles including post-formed multilayer optical film
and the methods of post-forming multilayer optical film described
thus far have focused on articles and methods in which the
post-formed multilayer optical film exhibits uniform optical
properties. There are, however other articles and methods according
to the present invention in which it may be desirable to provide
post-formed multilayer optical film with non-uniform appearance.
For example, it may be desired to provide post-formed multilayer
optical film in which selected areas of the multilayer optical film
are reflective for light of desired wavelengths while other
selected areas of the post-formed multilayer optical film transmit
light with the same or other desired wavelengths.
[0173] It may also be desirable to provide an article including
post-formed multilayer optical film in which selected areas in the
post-formed multilayer optical film are transmissive for visible
wavelengths while the remainder of the post-formed multilayer
optical film is reflective for visible wavelengths. To accomplish
that result using a multilayer optical film that is, as
manufactured, reflective for visible light, the multilayer optical
film in the selected areas could be stretched or thinned during the
post-forming process such that all of the tuned bandwidths of the
layers in the multilayer optical film stack in the selected
transmissive areas are less than 400 nanometers after post-forming.
The result of such a process would be an article including
post-formed multilayer optical film that is highly reflective in
the areas in which the reflective bandwidth remains in the visible
spectrum, while the article would exhibit transmission in those
areas in which the post-formed multilayer optical film has been
thinned to allow transmission in the visible spectrum.
[0174] As an alternative to the previously described process,
multilayer optical films could be provided and post-formed in
methods that result in selected transmissive and reflective areas
within the post-formed multilayer optical film in the same article,
but in which the unthinned layers remain transparent while those
selected areas that are thinned during post-forming become
reflective. For example, the multilayer optical film as
manufactured could be tuned to be reflective for wavelengths from
about 900 to about 2025 nanometers, i.e., above the visible
spectrum. Films designed to reduce higher order harmonics that give
perceptible color in the visible region of the spectrum may be
preferred. Some suitable films are described in U.S. Pat. Nos. Re.
34,605 and 5,360,659, and in U.S. Pat. No. 6,207,260 (Wheatley et
al.).
[0175] If such a multilayer optical film were post-formed, the
selected areas of the multilayer optical film that are to be
reflective would be deliberately thinned during post-forming by an
appropriate factor, e.g., 2.25, to retune the multilayer optical
film in those selected areas such that visible wavelengths, i.e.,
those between about 400 to about 900 nanometers, were substantially
reflected. The remaining portions or areas of the multilayer
optical film and the article that are not thinned sufficiently to
reflect light in the visible spectrum would remain transmissive to
visible light.
[0176] Many variations on these concepts can be envisioned. For
example, the multilayer optical films could be post-formed in
methods such that the selected areas are sharply defined resulting
in short transition zones between reflective/transparent areas, or
they could be deliberately designed with long transition zones in
which the post-formed multilayer optical film would exhibit
iridescence as various wavelengths of light were reflected or
transmitted. In another variation, different selected areas could
be thinned to reflect different selected wavelengths. In that
manner, the selected areas could exhibit, e.g., different colors.
The end result of applying the principles of multilayer optical
films and methods of post-forming multilayer optical films
according to the present invention is that desired combinations of
optical effects can be obtained by selecting films with the desired
optical and post-forming properties and processing the films to
obtain post-formed articles with the desired optical
properties.
[0177] One example of an article including post-formed multilayer
optical film that is deformed in selected areas is depicted in FIG.
14. The article 90 is a light box including a cover 92 that
includes selected areas 94 in the shape of indicia, in this case
alphanumeric characters. In one embodiment, the post-formed
multilayer optical film of the cover 92 can be formed from a
multilayer optical film that is substantially reflective over the
visible spectrum as manufactured. The multilayer optical film can
be post-formed in manners such as those described above such that
the multilayer optical film in the background area 96 surrounding
the selected areas 94 is thinned during post-forming such that the
multilayer optical film in the background area 96 is transparent to
at least a portion of the visible spectrum while the selected areas
94 are substantially unchanged.
[0178] In another embodiment, the background areas 96 may be
maintained as reflective to the visible spectrum while the selected
areas 94 are deformed or thinned to provide a different optical
effect from the background area 96. For example, the selected areas
94 may be embossed or blow molded or otherwise post-formed to thin
the film in selected areas 94 sufficiently that they become
transmissive to at least a portion of the visible spectrum. Other
variations on the construction and manufacture of articles
including post-formed multilayer optical film in which selected
areas are post-formed can also be envisioned based on the examples
discussed above.
Post-Forming Multilayer Optical Films with Substrates
[0179] FIG. 15 illustrates another feature of multilayer optical
films and articles including post-formed multilayer optical films
according to the present invention. In some instances the
post-formed multilayer optical films alone may lack sufficient body
or rigidity to provide the desired mechanical properties. For
example, the multilayer optical films may lack sufficient
structural strength and/or stiffness to hold a desired shape. FIG.
15 illustrates one solution to that problem in that the multilayer
optical film 102 may be laminated to or otherwise attached to a
substrate 104 to provide a composite 100 with the desired
mechanical properties. In some instances, the substrate 104 may be
manufactured integrally with the multilayer optical film 102, and
in other cases the multilayer optical film 102 may be manufactured
independently and later attached to the substrate 104 to form the
composite 100. If the substrate 104 is manufactured integrally with
the multilayer optical film 102, it may be a thicker layer of one
of the materials provided in the multilayer optical film 102 or it
may be provided of another material that can be coextruded, cast,
or otherwise formed with the multilayer optical film 102. In
another alternative, the substrate 104 may be provided as a coating
on the multilayer optical film 102.
[0180] Furthermore, although a substrate 104 is shown only one side
of the multilayer optical film 102, it will be understood that the
substrate 104 could be provided on both sides of the multilayer
optical film 102. In addition, although the substrate 104 is
depicted as a single layer, it will be understood that it could be
a composite of different layers of the same or different materials
based on the desired characteristics of the substrate 104
[0181] In some cases, the materials selected for the substrate 104
may have little, if any, effect on the optical properties of the
multilayer optical film 102 but will provide a post-formable layer
that is otherwise compatible with the multilayer optical film 102.
In one aspect, the substrate 104 may simply supply desired
structural stiffness/rigidity to the post-formed article, thereby
reducing the need to laminate the post-formed multilayer optical
film to another structure. Examples of suitable materials for the
substrate 104 include, but are not limited to polycarbonates,
polyvinyl chlorides, PETG, acrylics, methacrylics, nylons,
polyolefin, polypropylene, etc.
[0182] Another mechanical property that may be supplied by the
substrate 104 is strain-hardening during deformation as discussed
above with respect to the multilayer optical film. That
strain-hardening property may be used to limit the stresses placed
on the attached multilayer optical film 102, thereby acting to
distribute the stresses over the multilayer optical film 102 in a
way that improves the post-formability of the composite 100 over
the post-formability of the multilayer optical film 102 alone.
[0183] The materials selected for substrate 104 may provide desired
optical properties instead of, or in addition to, desired
mechanical properties. For example, the substrate 104 may function
as a mirror for selected wavelengths of light such as infrared
radiation, the substrate 104 may include colorants or otherwise
introduce color into the composite 100, the substrate 104 may
provide diffusing properties in either or both transmittance or
reflectance (to, e.g., reduce iridescence).
[0184] One class of films that may be particularly useful in
connection with post-forming of multilayer optical films is
described in U.S. Pat. No. 6,256,002.
[0185] Although in many instances the substrate 104 will be
coextensive with the multilayer optical film 102, it is also
envisioned that the substrate may be attached only on selected
areas of the multilayer optical film as depicted in FIG. 16 where
the substrate 114 is provided in selected areas on the multilayer
optical film 112. It will also be understood that the substrate 114
may be provided in the form of a grid, mesh or other discontinuous
form on the multilayer optical film 112 to improve its
post-formability. It may, for example, be advantageous to provide
the substrate 114 discontinuously in manners that assist in
defining the selected areas of the post-formed multilayer optical
film as described above with respect to FIG. 14. In such an
application, the substrate 114 may prevent or reduce drawing of the
multilayer optical film 112 during post-forming in manners that are
difficult or impossible to achieve through the use of post-forming
techniques alone.
[0186] Regardless of whether the multilayer optical films used in
connection with the present invention are included with substrates,
underdrawn or fully drawn, etc. the selection of the materials in
the films is discussed below.
Materials Selection
[0187] A variety of polymer materials suitable for use in the
present invention have been taught for use in making coextruded
multilayer optical films. For example, the polymer materials listed
and described in U.S. Pat. Nos. 4,937,134, 5,103,337,
5,1225,448,404, 5,540,978, and 5,568,316 to Schrenk et al., and in
5,122,905, 5,122,906, and 5,126,880 to Wheatley and Schrenk are
useful for making multilayer optical films according to the present
invention. Of special interest are birefringent polymers such as
those described in 5,486,949 and 5,612,820 to Schrenk et al, U.S.
Pat. No. 5,882,774 (Jonza et al.), and U.S. application Ser. No.
09/006,601 (filed Jan. 13, 1998, now abandoned). Regarding the
preferred materials from which the films are to be made, there are
several conditions which should be met to make the multilayer
optical films of this invention. First, these films should consist
of at least two distinguishable polymers; the number is not
limited, and three or more polymers may be advantageously used in
particular films. Second, at least one of the two required
polymers, referred to below as the first polymer, preferably has a
stress optical coefficient having a large absolute value. In other
words, it preferably should be capable of developing a large
birefringence when stretched. Depending on the application, the
birefringence may be developed between two orthogonal directions in
the plane of the film, between one or more in-plane directions and
the direction perpendicular to the film plane, or a combination of
these. In the special case that the isotropic indices are widely
separated, the preference for large birefringence in the first
polymer may be relaxed, although at least some birefringence is
desired. Such special cases may arise in the selection of polymers
for mirror films and for polarizer films formed using a biaxial
process which draws the film in two orthogonal in-plane directions.
Third, the first polymer should be capable of maintaining
birefringence after stretching, so that the desired optical
properties are imparted to the finished film. Fourth, the other
required polymer, referred to as the "second polymer", should be
chosen so that in the finished film, its refractive index, in at
least one direction, differs significantly from the index of
refraction of the first polymer in the same direction. Because
polymeric materials are typically dispersive, that is, the
refractive indices vary with wavelength, these conditions must be
considered in terms of a particular spectral bandwidth of
interest.
[0188] Other aspects of polymer selection depend on specific
applications. For polarizing films, it is often advantageous for
the difference in the index of refraction of the first and second
polymers in one film-plane direction to differ significantly in the
finished film, while the difference in the orthogonal film-plane
index is minimized. If the first polymer has a large refractive
index when isotropic, and is positively birefringent (that is, its
refractive index increases in the direction of stretching), the
second polymer will typically be chosen to have a matching
refractive index, after processing, in the planar direction
orthogonal to the stretching direction, and a refractive index in
the direction of stretching which is as low as possible.
Conversely, if the first polymer has a small refractive index when
isotropic, and is negatively birefringent, the second polymer will
typically be chosen to have a matching refractive index, after
processing, in the planar direction orthogonal to the stretching
direction, and a refractive index in the direction of stretching
which is as high as possible.
[0189] Alternatively, it is possible to select a first polymer
which is positively birefringent and has an intermediate or low
refractive index when isotropic, or one which is negatively
birefringent and has an intermediate or high refractive index when
isotropic. In these cases, the second polymer may typically be
chosen so that, after processing, its refractive index will match
that of the first polymer in either the stretching direction or the
planar direction orthogonal to stretching. Further, the second
polymer will typically be chosen such that the difference in index
of refraction in the remaining planar direction is maximized,
regardless of whether this is best accomplished by a very low or
very high index of refraction in that direction.
[0190] One means of achieving this combination of planar index
matching in one direction and mismatching in the orthogonal
direction is to select a first polymer which develops significant
birefringence when stretched, and a second polymer which develops
little or no birefringence when stretched, and to stretch the
resulting film in only one planar direction. Alternatively, the
second polymer may be selected from among those which develop
birefringence in the sense opposite to that of the first polymer
(negative-positive or positive-negative). Another alternative
method is to select both first and second polymers which are
capable of developing birefringence when stretched, but to stretch
in two orthogonal planar directions, selecting process conditions,
such as temperatures, stretch rates, post-stretch relaxation, and
the like, which result in development of unequal levels of
orientation in the two stretching directions for the first polymer,
and/or for the second polymer such that one in-plane index is
approximately matched to that of the first polymer, and the
orthogonal in-plane index is significantly mismatched to that of
the first polymer. For example, conditions may be chosen such that
the first polymer has a biaxially oriented character in the
finished film, while the second polymer has a predominantly
uniaxially oriented character in the finished film.
[0191] The foregoing is meant to be exemplary, and it will be
understood that combinations of these and other techniques may be
employed to achieve the polarizing film goal of index mismatch in
one in-plane direction and relative index matching in the
orthogonal planar direction.
[0192] Different considerations apply to a reflective, or mirror,
film. Provided that the film is not meant to have some polarizing
properties as well, refractive index criteria apply equally to any
direction in the film plane, so it is typical for the indices for
any given layer in orthogonal in-plane directions to be equal or
nearly so. It is advantageous, however, for the film-plane indices
of the first polymer to differ as greatly as possible from the
film-plane indices of the second polymer. For this reason, if the
first polymer has a high index of refraction when isotropic, it is
advantageous that it also be positively birefringent. Likewise, if
the first polymer has a low index of refraction when isotropic, it
is advantageous that it also be negatively birefringent. The second
polymer advantageously develops little or no birefringence when
stretched, or develops birefringence of the opposite sense
(positive-negative or negative-positive), such that its film-plane
refractive indices differ as much as possible from those of the
first polymer in the finished film. These criteria may be combined
appropriately with those listed above for polarizing films if a
mirror film is meant to have some degree of polarizing properties
as well.
[0193] Colored films can be regarded as special cases of mirror and
polarizing films. Thus, the same criteria outlined above apply. The
perceived color is a result of reflection or polarization over one
or more specific bandwidths of the spectrum. The bandwidths over
which a multilayer film of the current invention is effective will
be determined primarily by the distribution of layer thicknesses
employed in the optical stack(s), but consideration must also be
given to the wavelength dependence, or dispersion, of the
refractive indices of the first and second polymers. It will be
understood that the same rules applied to the visible spectrum will
also generally be apply to the infrared and ultraviolet
wavelengths, as well as any other electromagnetic radiation for
which the films are designed.
[0194] Absorbance is another consideration. For most applications,
it is advantageous for neither the first polymer nor the second
polymer to have any absorbance bands within the bandwidth of
interest for the film in question. Thus, all incident light within
the bandwidth is either reflected or transmitted. However, for some
applications, it may be useful for one or both of the first and
second polymer to absorb specific wavelengths, either totally or in
part.
[0195] Although many polymers may be chosen as the first polymer,
certain of the polyesters have the capability for particularly
large birefringence. Among these, polyethylene 2,6-naphthalate
(PEN) is frequently chosen as a first polymer for films of the
present invention. It has a very large positive stress optical
coefficient, retains birefringence effectively after stretching,
and has little or no absorbance within the visible range. It also
has a large index of refraction in the isotropic state. Its
refractive index for polarized incident light of 550 nm wavelength
increases when the plane of polarization is parallel to the stretch
direction from about 1.64 to as high as about 1.9. Its
birefringence can be increased by increasing its molecular
orientation which, in turn, may be increased by stretching to
greater stretch ratios with other stretching conditions held
fixed.
[0196] Other semicrystalline naphthalene dicarboxylic polyesters
are also suitable as first polymers. Polybutylene 2,6-Naphthalate
(PBN) is an example. These polymers may be homopolymers or
copolymers, provided that the use of comonomers does not
substantially impair the stress optical coefficient or retention of
birefringence after stretching. The term "PEN" herein will be
understood to include copolymers of PEN meeting these restrictions.
In practice, these restrictions imposes an upper limit on the
comonomer content, the exact value of which will vary with the
choice of comonomer(s) employed. Some compromise in these
properties may be accepted, however, if comonomer incorporation
results in improvement of other properties. Such properties include
but are not limited to improved interlayer adhesion, lower melting
point (resulting in lower extrusion temperature), better
rheological matching to other polymers in the film, and
advantageous shifts in the process window for stretching due to
change in the glass transition temperature.
[0197] Suitable comonomers for use in PEN, PBN or the like may be
of the diol or dicarboxylic acid or ester type. Dicarboxylic acid
comonomers include but are not limited to terephthalic acid,
isophthalic acid, phthalic acid, all isomeric
naphthalenedicarboxylic acids (2,6-, 1,2-, 1,3-, 1,4-, 1,5-, 1,6-,
1,7-, 1,8-, 2,3-, 2,4-, 2,5-, 2,7-, and 2,8-), bibenzoic acids such
as 4,4'-biphenyl dicarboxylic acid and its isomers,
trans-4,4'-stilbene dicarboxylic acid and its isomers,
4,4'-diphenyl ether dicarboxylic acid and its isomers,
4,4'-diphenylsulfone dicarboxylic acid and its isomers,
4,4'-benzophenone dicarboxylic acid and its isomers, halogenated
aromatic dicarboxylic acids such as 2-chloroterephthalic acid and
2,5-dichloroterephthalic acid, other substituted aromatic
dicarboxylic acids such as tertiary butyl isophthalic acid and
sodium sulfonated isophthalic acid, cycloalkane dicarboxylic acids
such as 1,4-cyclohexanedicarboxylic acid and its isomers and
2,6-decahydronaphthalene dicarboxylic acid and its isomers, bi- or
multi-cyclic dicarboxylic acids (such as the various isomeric
norbornane and norbornene dicarboxylic acids, adamantane
dicarboxylic acids, and bicyclo-octane dicarboxylic acids), alkane
dicarboxylic acids (such as sebacic acid, adipic acid, oxalic acid,
malonic acid, succinic acid, glutaric acid, azelaic acid, and
dodecane dicarboxylic acid.), and any of the isomeric dicarboxylic
acids of the fused-ring aromatic hydrocarbons (such as indene,
anthracene, pheneanthrene, benzonaphthene, fluorene and the like).
Alternatively, alkyl esters of these monomers, such as dimethyl
terephthalate, may be used.
[0198] Suitable diol comonomers include but are not limited to
linear or branched alkane diols or glycols (such as ethylene
glycol, propanediols such as trimethylene glycol, butanediols such
as tetramethylene glycol, pentanediols such as neopentyl glycol,
hexanediols, 2,2,4-trimethyl-1,3-pentanediol and higher diols),
ether glycols (such as diethylene glycol, triethylene glycol, and
polyethylene glycol), chain-ester diols such as
3-hydroxy-2,2-dimethylpropyl-3-hydroxy-2,2-dimethyl propanoate,
cycloalkane glycols such as 1,4-cyclohexanedimethanol and its
isomers and 1,4-cyclohexanediol and its isomers, bi- or multicyclic
diols (such as the various isomeric tricyclodecane dimethanols,
norbornane dimethanols, norbornene dimethanols, and bicyclo-octane
dimethanols), aromatic glycols (such as 1,4-benzenedimethanol and
its isomers, 1,4-benzenediol and its isomers, bisphenols such as
bisphenol A, 2,2'-dihydroxy biphenyl and its isomers,
4,4'-dihydroxymethyl biphenyl and its isomers, and
1,3-bis(2-hydroxyethoxy)benzene and its isomers), and lower alkyl
ethers or diethers of these diols, such as dimethyl or diethyl
diols.
[0199] Tri- or polyfunctional comonomers, which can serve to impart
a branched structure to the polyester molecules, can also be used.
They may be of either the carboxylic acid, ester, hydroxy or ether
types. Examples include, but are not limited to, trimellitic acid
and its esters, trimethylol propane, and pentaerythritol.
[0200] Also suitable as comonomers are monomers of mixed
functionality, including hydroxycarboxylic acids such as
parahydroxybenzoic acid and 6-hydroxy-2-naphthalenecarboxylic acid,
and their isomers, and tri- or polyfunctional comonomers of mixed
functionality such as 5-hydroxyisophthalic acid and the like.
[0201] Polyethylene terephthalate (PET) is another material that
exhibits a significant positive stress optical coefficient, retains
birefringence effectively after stretching, and has little or no
absorbance within the visible range. Thus, it and its high
PET-content copolymers employing comonomers listed above may also
be used as first polymers in some applications of the current
invention. The term "PET" as used herein will be understood to
include PET and its high PET content copolymers that function
similarly to PET alone.
[0202] When a naphthalene dicarboxylic polyester such as PEN or PBN
is chosen as first polymer, there are several approaches which may
be taken to the selection of a second polymer. One preferred
approach for some applications is to select a naphthalene
dicarboxylic copolyester (coPEN) formulated so as to develop
significantly less or no birefringence when stretched. This can be
accomplished by choosing comonomers and their concentrations in the
copolymer such that crystallizability of the coPEN is eliminated or
greatly reduced. One typical formulation employs as the
dicarboxylic acid or ester components dimethyl naphthalate at from
about 20 mole percent to about 80 mole percent and dimethyl
terephthalate or dimethyl isophthalate at from about 20 mole
percent to about 80 mole percent, and employs ethylene glycol as
diol component. Of course, the corresponding dicarboxylic acids may
be used instead of the esters. The number of comonomers which can
be employed in the formulation of a coPEN second polymer is not
limited. Suitable comonomers for a coPEN second polymer include but
are not limited to all of the comonomers listed above as suitable
PEN comonomers, including the acid, ester, hydroxy, ether, tri- or
polyfunctional, and mixed functionality types.
[0203] Often it is useful to predict the isotropic refractive index
of a coPEN second polymer. A volume average of the refractive
indices of the monomers to be employed has been found to be a
suitable guide. Similar techniques well-known in the art can be
used to estimate glass transition temperatures for coPEN second
polymers from the glass transitions of the homopolymers of the
monomers to be employed.
[0204] In addition, polycarbonates having a glass transition
temperature compatible with that of PEN and having a refractive
index similar to the isotropic refractive index of PEN are also
useful as second polymers. Polyesters, copolyesters,
polycarbonates, and copolycarbonates may also be fed together to an
extruder and transesterified into new suitable copolymeric second
polymers.
[0205] It is not required that the second polymer be a copolyester
or copolycarbonate. Vinyl polymers and copolymers made from
monomers such as vinyl naphthalenes, styrenes, ethylene, maleic
anhydride, acrylates, acetates, and methacrylates may be employed.
Condensation polymers other than polyesters and polycarbonates may
also be used. Examples include: polysulfones, polyamides,
polyurethanes, polyamic acids, and polyimides. Naphthalene groups
and halogens such as chlorine, bromine and iodine are useful for
increasing the refractive index of the second polymer to a desired
level. Acrylate groups and fluorine are particularly useful in
decreasing refractive index when this is desired.
[0206] It will be understood from the foregoing discussion that the
choice of a second polymer is dependent not only on the intended
application of the multilayer optical film in question, but also on
the choice made for the first polymer, and the processing
conditions employed in stretching. Suitable second polymer
materials include but are not limited to polyethylene naphthalate
(PEN) and isomers thereof (such as 2,6-, 1,4-, 1,5-, 2,7-, and
2,3-PEN), polyalkylene terephthalates (such as polyethylene
terephthalate, polybutylene terephthalate, and
poly-1,4-cyclohexanedimethylene terephthalate), other polyesters,
polycarbonates, polyarylates, polyamides (such as nylon 6, nylon
11, nylon 12, nylon 4/6, nylon 6/6, nylon 6/9, nylon 6/10, nylon
6/12, and nylon 6/T), polyimides (including thermoplastic
polyimides and polyacrylic imides), polyamide-imides,
polyether-amides, polyetherimides, polyaryl ethers (such as
polyphenylene ether and the ring-substituted polyphenylene oxides),
polyarylether ketones such as polyetheretherketone ("PEEK"),
aliphatic polyketones (such as copolymers and terpolymers of
ethylene and/or propylene with carbon dioxide), polyphenylene
sulfide, polysulfones (including polyethersulfones and polyaryl
sulfones), atactic polystyrene, syndiotactic polystyrene ("sPS")
and its derivatives (such as syndiotactic poly-alpha-methyl styrene
and syndiotactic polydichlorostyrene), blends of any of these
polystyrenes (with each other or with other polymers, such as
polyphenylene oxides), copolymers of any of these polystyrenes
(such as styrene-butadiene copolymers, styrene-acrylonitrile
copolymers, and acrylonitrile-butadiene-styrene terpolymers),
polyacrylates (such as polymethyl acrylate, polyethyl acrylate, and
polybutyl acrylate), polymethacrylates (such as polymethyl
methacrylate, polyethyl methacrylate, polypropyl methacrylate, and
polyisobutyl methacrylate), cellulose derivatives (such as ethyl
cellulose, cellulose acetate, cellulose propionate, cellulose
acetate butyrate, and cellulose nitrate), polyalkylene polymers
(such as polyethylene, polypropylene, polybutylene,
polyisobutylene, and poly(4-methyl)pentene), fluorinated polymers
and copolymers (such as polytetrafluoroethylene,
polytrifluoroethylene, polyvinylidene fluoride, polyvinyl fluoride,
fluorinated ethylene-propylene copolymers, perfluoroalkoxy resins,
polychlorotrifluoroethylene, polyethylene-co-trifluoroethylene,
polyethylene-co-chlorotrifluoroethylene), chlorinated polymers
(such as polyvinylidene chloride and polyvinyl chloride),
polyacrylonitrile, polyvinylacetate, polyethers (such as
polyoxymethylene and polyethylene oxide), ionomeric resins,
elastomers (such as polybutadiene, polyisoprene, and neoprene),
silicone resins, epoxy resins, and polyurethanes.
[0207] Also suitable are copolymers, such as the copolymers of PEN
discussed above as well as any other non-naphthalene
group-containing copolyesters which may be formulated from the
above lists of suitable polyester comonomers for PEN. In some
applications, especially when PET serves as the first polymer,
copolyesters based on PET and comonomers from said lists above
(coPETs) are especially suitable. In addition, either first or
second polymers may consist of miscible or immiscible blends of two
or more of the above-described polymers or copolymers (such as
blends of sPS and atactic polystyrene, or of PEN and sPS). The
coPENs and coPETs described may be synthesized directly, or may be
formulated as a blend of pellets where at least one component is a
polymer based on naphthalene dicarboxylic acid or terephthalic acid
and other components are polycarbonates or other polyesters, such
as a PET, a PEN, a coPET, or a co-PEN.
[0208] Another preferred family of materials for the second polymer
for some applications are the syndiotactic vinyl aromatic polymers,
such as syndiotactic polystyrene. Syndiotactic vinyl aromatic
polymers useful in the current invention include poly(styrene),
poly(alkyl styrene)s, poly(aryl styrene)s, poly(styrene halide)s,
poly(alkoxy styrene)s, poly(vinyl ester benzoate), poly(vinyl
naphthalene), poly(vinylstyrene), and poly(acenaphthalene), as well
as the hydrogenated polymers and mixtures or copolymers containing
these structural units. Examples of poly(alkyl styrene)s include
the isomers of the following: poly(methyl styrene), poly(ethyl
styrene), poly(propyl styrene), and poly(butyl styrene). Examples
of poly(aryl styrene)s include the isomers of poly(phenyl styrene).
As for the poly(styrene halide)s, examples include the isomers of
the following: poly(chlorostyrene), poly(bromostyrene), and
poly(fluorostyrene). Examples of poly(alkoxy styrene)s include the
isomers of the following: poly(methoxy styrene) and poly(ethoxy
styrene). Among these examples, particularly preferable styrene
group polymers, are: polystyrene, poly(p-methyl styrene),
poly(m-methyl styrene), poly(p-tertiary butyl styrene),
poly(p-chlorostyrene), poly(m-chloro styrene), poly(p-fluoro
styrene), and copolymers of styrene and p-methyl styrene.
[0209] Furthermore, comonomers may be used to make syndiotactic
vinyl aromatic group copolymers. In addition to the monomers for
the homopolymers listed above in defining the syndiotactic vinyl
aromatic polymers group, suitable comonomers include olefin
monomers (such as ethylene, propylene, butenes, pentenes, hexenes,
octenes or decenes), diene monomers (such as butadiene and
isoprene), and polar vinyl monomers (such as cyclic diene monomers,
methyl methacrylate, maleic acid anhydride, or acrylonitrile).
[0210] The syndiotactic vinyl aromatic copolymers of the present
invention may be block copolymers, random copolymers, or
alternating copolymers.
[0211] The syndiotactic vinyl aromatic polymers and copolymers
referred to in this invention generally have syndiotacticity of
higher than 75% or more, as determined by carbon-13 nuclear
magnetic resonance. Preferably, the degree of syndiotacticity is
higher than 85% racemic diad, or higher than 30%, or more
preferably, higher than 50%, racemic pentad.
[0212] In addition, although there are no particular restrictions
regarding the molecular weight of these syndiotactic vinyl aromatic
polymers and copolymers, preferably, the weight average molecular
weight is greater than 10,000 and less than 1,000,000, and more
preferably, greater than 50,000 and less than 800,000.
[0213] The syndiotactic vinyl aromatic polymers and copolymers may
also be used in the form of polymer blends with, for instance,
vinyl aromatic group polymers with atactic structures, vinyl
aromatic group polymers with isotactic structures, and any other
polymers that are miscible with the vinyl aromatic polymers. For
example, polyphenylene ethers show good miscibility with many of
the previous described vinyl aromatic group polymers.
[0214] When a polarizing film is made using a process with
predominantly uniaxial stretching, particularly preferred
combinations of polymers for optical layers include PEN/coPEN,
PET/coPET, PEN/sPS, PET/sPS, PEN/Eastar.TM., and PET/Eastar.TM.,
where "coPEN" refers to a copolymer or blend based upon naphthalene
dicarboxylic acid (as described above) and Eastar.TM. is a
polyester or copolyester (believed to comprise
cyclohexanedimethylene diol units and terephthalate units)
commercially available from Eastman Chemical Co. When a polarizing
film is to be made by manipulating the process conditions of a
biaxial stretching process, particularly preferred combinations of
polymers for optical layers include PEN/coPEN, PEN/PET, PEN/PBT,
PEN/PETG and PEN/PETcoPBT, where "PBT" refers to polybutylene
terephthalate, "PETG" refers to a copolymer of PET employing a
second glycol (usually cyclohexanedimethanol), and "PETcoPBT"
refers to a copolyester of terephthalic acid or an ester thereof
with a mixture of ethylene glycol and 1,4-butanediol.
[0215] Particularly preferred combinations of polymers for optical
layers in the case of mirrors or colored films include PEN/PMMA,
PET/PMMA, PEN/Ecdel.TM., PET/Ecdel.TM., PEN/sPS, PET/sPS,
PEN/coPET, PEN/PETG, and PEN/THV.TM., where "PMMA" refers to
polymethyl methacrylate, Ecdel.TM. is a thermoplastic polyester or
copolyester (believed to comprise cyclohexanedicarboxylate units,
polytetramethylene ether glycol units, and cyclohexanedimethanol
units) commercially available from Eastman Chemical Co., "coPET"
refers to a copolymer or blend based upon terephthalic acid (as
described above), "PETG" refers to a copolymer of PET employing a
second glycol (usually cyclohexanedimethanol), and THV.TM. is a
fluoropolymer commercially available from 3M Co.
[0216] For mirror films, a match of the refractive indices of the
first polymer and second polymer in the direction normal to the
film plane is sometimes preferred, because it provides for constant
reflectance with respect to the angle of incident light (that is,
there is no Brewster's angle). For example, at a specific
wavelength, the in-plane refractive indices might be 1.76 for
biaxially oriented PEN, while the film plane-normal refractive
index might fall to 1.49. When PMMA is used as the second polymer
in the multilayer construction, its refractive index at the same
wavelength, in all three directions, might be 1.495. Another
example is the PET/Ecdel.TM. system, in which the analogous indices
might be 1.66 and 1.51 for PET, while the isotropic index of
Ecdel.TM. might be 1.52.
[0217] It is sometimes preferred for the multilayer optical films
of the current invention to consist of more than two
distinguishable polymers. A third or subsequent polymer might be
fruitfully employed as an adhesion-promoting layer between the
first polymer and the second polymer within an optical stack, as an
additional component in a stack for optical purposes, as a
protective boundary layer between optical stacks, as a skin layer,
as a functional coating, or for any other purpose. As such, the
composition of a third or subsequent polymer, if any, is not
limited. Some preferred multicomponent constructions are described
in U.S. Pat. No. 6,207,260 (Wheatley et al.).
[0218] The selection criteria for the materials of the optical
stack layers may also be useful in the selection of appropriate
materials for thick internal or external skin protective layers.
The criteria for the second polymer may be more desirable than
those for the first polymer. In some cases, however, the mechanical
properties of the birefringent first material, such as high glass
transition temperature to reduce sticking to rollers, low
coefficients of thermal expansion, mechanical stiffness, etc., may
be desirable. In the case of films designed for post-forming, it
may be desirable to use materials of lower draw stiffness to
improve formability at a given applied stress, e.g., vacuum
pressure, or otherwise improve extensibility.
EXAMPLES
[0219] Advantages of the invention are illustrated by the following
examples. However, the particular materials and amounts thereof
recited in these examples, as well as other conditions and details,
are to be interpreted to apply broadly in the art and should not be
construed to unduly limit the invention.
Example 1
Fully Drawn Mirror Film
[0220] A multilayer film of polyethylene 2,6-naphthalate (PEN) and
polymethylmethacrylate (PMMA) was co-extruded, cast and drawn to
make a fully drawn PEN:PMMA multilayer mirror film. A 0.48 IV PEN
(made by 3M Co., St. Paul Minn.) was dried at 135.degree. C. for 24
hours and then fed directly into a single screw extruder with an
exit temperature of about 285.degree. C. PMMA (CP-82 grade
available from Ashland Chemical) was dried by feeding into a twin
screw extruder equipped with a vacuum and with an exit temperature
of about 260.degree. C. The resin streams were co-extruded into a
224 multilayer feedblock set at 275.degree. C. and equipped with an
internal protective boundary layer (PBL). Pumping rates were
maintained so that the approximate optical thickness of each
PEN:PMMA layer pair was approximately equal in the optical stack,
i.e. an "f-ratio" of 0.5. The PBL was supplied with PEN at
approximately one-half the volume as that supplied to the sum of
all the PEN layers in the optical stack. The layer pairs in the
optical stack had an approximately linear gradient in optical
thickness. The multilayer stack including the PBL was split with an
asymmetric multiplier to form two streams in a width ratio of
1.55:1, spread to equivalent widths and re-stacked to form a two
packet multilayer stack of 448 layers separated by an internal
protective layer. An additional PEN (IV 0.48) skin was added to
each side of the multilayer stack, with each skin layer comprising
about 10% of the total volumetric flow. The total stream was cast
from a die at about 285 C onto a quench wheel set at 65.degree. C.
The PEN skins refractive indices were essentially isotropic after
casting with indices of 1.64 at 632.8 nm as measured by a Metricon
Prism Coupler, available from Metricon, Piscataway, N.J. The cast
thickness was approximately 0.07 cm.
[0221] The first draw process used a conventional length orienter
(LO). The film was preheated with hot rollers set at 125 C and fed
into a draw gap comprising a slow roll and fast roll and an
infra-red heater set at 80% power. The infrared heater consisted of
an assembly of IR heater elements (approximately 5000 watts per
element), each about 65 cm long. The elements were approximately 10
cm above the film. Residence time in the draw gap was about 4
seconds. The fast roll was set to accomplish a 3.3 times draw and
the drawn film was quenched. The average PEN indices were highly
oriented at about 1.79, 1.59 and 1.55 as measured by the Metricon
Prism Coupler) in the in-plane draw direction y-axis (MD), the
in-plane crossweb direction x-axis (TD) and thickness (z) (ND)
direction, respectively. The film was next drawn transversely using
a conventional tenter in a second draw step to a final transverse
draw ratio of about 4.0. The tenter was set at 132.degree. C. in
the preheat, 135.degree. C. in the draw zone, 249.degree. C. in the
heat set zone and 49.degree. C. in the quench zone. Preheating,
drawing and heat setting were accomplished over periods of
approximately 25, 5 and 40 seconds. The final PEN indices were
1.7284, 1.7585 and 1.5016 while the PMMA indices were approximately
isotropic at 1.49, all at 632.8 nm as measured by the Metricon
Prism Coupler. The measured reflectance band covered the spectrum
from 400 nm to 950 nm with over 95% average reflectivity. The total
polarizability was thus calculated as 1.1043 and the total
polarizability difference was 0.0215 for the birefringent PEN
layer. The density was estimated as 1.3549 g/cc as discussed above
and the fractional crystallinity was calculated as 0.33.
Example 2
Underdrawn Mirror Film
[0222] A multilayer film of PEN and PETG (a copolymer of PET
comprising some substitution of ethylene glycol with 1,4
cyclohexane diol during polymerization) was co-extruded, cast and
drawn to make an underdrawn PEN:PETG multilayer mirror film. A 0.48
IV PEN (made by 3M Co., St. Paul Minn.) was dried at 135.degree. C.
for 24 hours and then fed directly into a single screw extruder
with exit temperature about 285.degree. C. PETG (available from
Eastman Chemical, TN) was dried by feeding into a twin screw
extruder equipped with a vacuum and with an exit temperature of
about 285.degree. C. These resin streams were co-extruded into a
209-multilayer feedblock set at 285.degree. C. Pumping rates were
maintained so that the approximate optical thickness of each
PEN:PETG layer pair was approximately equal in the optical stack,
i.e. an "f-ratio" of 0.5. The layer pairs in the optical stack had
an approximately linear gradient in optical thickness. A PBL was
then supplied with PEN in an amount approximately 20% of the final
volumetric flow. The multilayer stack including the PBL was split
with an asymmetric multiplier to form two streams in width ratio of
1.55:1, spread to equivalent widths and re-stacked to form a two
packet multilayer stack of 418 optical layers separated by an
internal protective layer. An additional PEN (IV 0.48) skin was
added to each side of the multilayer stack, each skin layer
comprising about 12.5% of the total volumetric flow. The total
stream was cast from a die at about 285 C onto a quench wheel set
at 65.degree. C. The PEN skins refractive indices were essentially
isotropic after casting with indices of 1.64 at 632.8 nm as
measured by the Metricon Prism Coupler. The cast thickness was
approximately 0.07 cm.
[0223] The first draw process used a conventional length orienter
(LO). The film was preheated with hot rollers set at 120.degree. C.
and fed into a draw gap comprising a slow roll and fast roll and an
infrared heater set at 60% power. The infrared heater consisted of
an assembly of IR heater elements (approximately 5000 watts per
element), each about 65 cm long. The elements were approximately 10
cm above the film. Residence time in the draw gap was about 4
seconds. The fast roll was set to accomplish a 2.7 times draw and
the drawn film was quenched. The film was next drawn transversely
using a conventional tenter in a second draw step to a final
transverse draw ratio of about 3.3. The tenter was set at
132.degree. C. in the preheat zone, 135.degree. C. in the draw
zone, 135 vC in the heat set zone and 49 C in the quench zone.
Preheating, drawing and heat setting were accomplished over periods
of approximately 25, 5 and 40 seconds. The final PEN indices were
1.69, 1.72 and 1.53 while the PETG indices were approximately
isotropic at 1.56, all at 632.8 nm as measured by the Metricon
Prism Coupler. Note that PMMA could be substituted for the PETG in
this example with improved optical performance.
[0224] The film, made as described, is an underdrawn mirror film.
This film was re-drawn simultaneously at 135.degree. C. over 1
second to an additional measured true draw ratios of
1.27.times.1.22, with a biaxial draw ratio of about 1.55, as might
occur during a thermoforming process. The same film was then
further heat set for 4 minutes at 175.degree. C. to form a fully
drawn film. Shorter time periods, e.g. several seconds, could be
applied at higher temperatures, e.g. 220.degree. C., to accomplish
similar heat set results. The underdrawn film had high
extensibility. In another case, the underdrawn film was re-drawn
simultaneously at 135.degree. C. over 2.4 seconds to a measured
true draw ratios of 1.63.times.1.58, i.e. the biaxial draw ratio
during re-drawing was 2.6. The progress of index (n) development in
the MD, TD and ND directions (x,y,z directions) at 632.8 nm as well
as the calculated total polarizability (TP), total polarizability
difference (TPD), estimated density (in g/cc) and fractional
crystallinity (X) (calculated from the density) are presented in
the following table:
TABLE-US-00001 Case MD n TD n ND n TP TPD Density X Under-drawn
1.6949 1.7283 1.5275 1.0904 0.0077 1.3379 0.1113 Re- 1.7033 1.7331
1.5168 1.0908 0.0080 1.3383 0.1167 drawn 1.27 .times. 1.22 Re-
1.7124 1.7302 1.5081 1.0891 0.0064 1.3363 0.0907 drawn 1.63 .times.
1.58 Heat set 1.7188 1.7511 1.4995 1.0962 0.0135 1.345 0.2021
In this series of examples, re-drawing to the higher biaxial draw
ratio does not greatly alter the crystallinity or total
polarizability.
[0225] FIGS. 17 and 18 present the measured transmissions of light
polarized in the MD and TD directions using a Perkin-Elmer
Lambda-19. Outside of the multilayer reflection band, the
transmission is about 85% rather than 100% due to surface
reflections. The following table identifies some of the approximate
spectral features:
TABLE-US-00002 Approx. Ave. % % Min. Location Approx. Transmission
Transmission of Min. Case Band MD TD MD TD approx. Underdrawn
725-1425 nm 26.2 14.7 1.8 0.5 1390 nm Re-drawn 430-920 nm 19.0 12.0
3.1 2.2 890 nm 1.27 .times. 1.22 Heat set 420-950 nm 15.6 10.0 1.1
<0.3 855 nm
[0226] The band is primarily the first order reflection band,
although some second order reflections may also contribute to this
band. Higher order peaks are evident as well, such as the third
order peak at about 450 nm for the 1390 nm reflection peak (i.e.
transmission valley). The band shifts in proportion to the biaxial
draw ratio as expect between the underdrawn and re-drawn case. The
band transmission decreases, i.e. the band reflectivity increases,
after heat setting as a result of increased index differences
between the birefringent PEN layers and the approximately isotropic
PETG layers.
Example 3
Comparison of Fully Drawn, Underdrawn, and Cast Web Films
[0227] A fully drawn film made according to example 1, an
underdrawn film made according to example 2 and an undrawn cast web
made in a similar fashion to that in example 1 substituting a
copolymer of PEN for the PEN layers and using thinner skins and PBL
layers, were thermoformed into approximately spherical caps using
the process described below The fully drawn film was a multilayer
optical mirror film comprising approximately 400 optical layers
alternating in PEN and PMMA with thicker PEN skin layers and a
thick internal PEN layer, originally drawn 3.3.times.4.0. The
underdrawn film was a multilayer optical mirror comprising
approximately 400 optical layers alternating in PEN and PETG (a
copolymer of PET) with thicker PEN skin layers and a thick internal
PEN layer, originally drawn about 80% of the fully drawn film, i.e.
2.7.times.3.3, under similar process conditions of applied heating
and line speed (e.g. strain rate) on the same process line. The
cast web comprised approximately 400 layers alternating in a coPEN
consisting of 90% PEN and 10% PET subunits (i.e. a 90/10 coPEN) and
PMMA with thicker 90/10 coPEN skin layers and a thick internal
90/10 coPEN layer. The films were place over a circular aperture
about 3.3 cm in diameter. A vacuum of nearly one atmosphere was
applied and the films were heated for a few seconds using a heat
gun. The temperature was estimated at about 200.degree. C., using a
thermocouple placed in the air stream of the heat gun at the same
distance and residence time as the film.
[0228] The cast web drew the most but also drew the most unevenly,
forming an elongated, roughly hemispherical cap. The base of the
cap had an outer diameter of 3.2 cm. The height of the cap was
about 1.75 cm. The cast web was originally about 675 microns thick.
Near the top of the cap, the thickness varied between 140 and 225
microns. The biaxial draw ratio thus varied widely with a maximum
value of around 4.8. The initial refractive index in the 90/10
coPEN skin layer was nearly isotropic, with a value of 1.6355 at
632.8 nm. At the thinnest part, the indices in the three principal
directions in the final cap were approximately 1.6685, 1.6766 and
1.5784 at 632.8 nm.
[0229] The fully drawn mirror film and the underdrawn mirror films
drew much more uniformly with a spread in thickness of about 10% or
less across most of the approximately spherical cap, as would be
expected with strain-hardening films. The fully drawn film was
initially 68 microns and thinned to about 58 microns across the
cap, giving a biaxial draw ratio of about 1.17. The base of the cap
had an outer diameter of 3.25 cm. The height of the cap was about
0.55 cm. The indices of refraction in the birefringent PEN skin
layer, initially at 1.7276, 1.7693 and 1.5014, remained about the
same after thermoforming. The film remained highly reflective. The
underdrawn film was initially 105 microns and thinned to about 78
microns across the cap, giving a biaxial draw ratio of about 1.35.
The base of the cap had an outer diameter of 3.25 cm. The height of
the cap was about 0.65 cm. The indices of refraction in the
birefringent PEN skin layer, initially at 1.6939, 1.7367 and
1.5265, increased slightly in the originally in-plane directions to
1.7120 and 1.7467 while the thickness direction index decreased to
1.5081 after thermoforming. In this particular case, the initial
underdrawn film was transparent at the lower spectral end of the
visible wavelengths due to its increased thickness relative to the
fully drawn film. The reflectivity across the visible spectrum
increased in the spherical cap due to the band shifting to cover
these lower wavelengths as well as the increase in index difference
between the birefringent PEN and the nearly isotropic PETG
layers.
Comparative Example 1
Thermoformed Cast Web
[0230] A cast web was about 34.5 mils thick was made as described
in Example 1. The cast web as described in Example 3 was heated and
vacuum formed into a deep cylindrical mold. The resulting part
formed had a cylindrical shaft and a spherical end cap. The inner
diameter of the cylinder was about 2.1 cm. The depth of the
cylinder and spherical cap was about 1.9 cm. The deviation from the
straight sides of the cylinder into the cap occurs at about 1 cm,
so that the cap is nearly hemispherical. A grid was drawn on the
part before forming with each line separated by about 0.6 cm.
[0231] Large nonuniformities in draw conditions were observed
across the sample. Over the top of the cap, a grid segment was
stretched to about 2.8 cm, suggesting a nominal draw over the
hemisphere of about 4.7.times.4.7, resulting in a biaxial draw
ratio of 22. Uniform drawing across the entire shaped part above
the base would have required a biaxial draw ratio of about 4. There
were signs of severe delamination failure in the cast web. This
became a benefit for the analysis: to further analyze the part, the
skin layer interior to the part was stripped off with the remainder
of the piece remaining intact. Five samples were cut from the skin
as shown in the table below:
TABLE-US-00003 thickness thickness In-plane In-plane Z biax draw
Total Estimated Sample (min.) (max.) Index, nx index, ny Index, nz
ratio Polarizability Crystallinity 1-base 3.67 3.75 1.6435 1.6419
1.6429 1.0 1.330743 0.0186 2-top 0.12 0.14 1.7293 1.7067 1.5419
28.5 1.353209 0.3077 3-cyl 2.55 3.24 1.6572 1.6431 1.6275 1.45-1.15
1.3284 -0.012 4-cyl 1.25 1.76 1.6686 1.6395 1.6195 2.97-2.11
1.330097 0.0103 5-top 0.12 0.14 1.7190 1.7030 1.5557 28.5 1.354125
0.3195 Thickness are measured in mils (0.001 inches). All optical
measurements were taken at 632.8 nm using the Metricon Prism
Coupler.
[0232] Sample #1 shows that the undrawn skin layer is about 11% of
the total thickness of the cast web. Because of delamination, the
base was only measured where this was not present. The biaxial draw
ratio was then calculated using the ratio of this average base
thickness to the final sample thickness.
[0233] Samples #2 and #3 were essentially at the top of the
spherical cap. The true biaxial draw ratio is slightly higher than
that anticipated by the gridline expansion as would be expected for
a nonuniformly drawn piece: the cap is thinnest at the top. The
thickness was determined both using a caliper gauge and using the
thin film thickness calculation available on the Metricon. The
latter yielded a value of 3.5 microns, that is, about 0.14 mils, in
agreement with the caliper gauge. Note that the "in-plane" indices
are less than other fully drawn mirror films, the high total
polarizability resulting from the high z indices.
[0234] Sample #3 was taken from the bottom of the cylinder, from
about 0.2 to 0.7 cm above the base. The long direction was cut
around the circumference of the cylinder. This circumferential
direction is considered the x direction for purposes of the
preceding table. Sample #4 was cut directly above, from about 0.7
to 1.0 cm above the base. Apparently, the draw is more directed
around the hoop of the cylinder than towards the cap as indicated
by the indices of refraction. The low biaxial draw ratios lead to
very low deviation from isotropy in this sample.
[0235] The effectiveness of the orientation process can also be
seen by estimating the crystallinity using the concept of total
polarizability. Due to experimental error, the estimates are only
good to about +/-0.02 fractional crystallinity as defined here
using the total polarizability concept. From the values indicated
in the table, the base and cylinder wall sections were still
essentially amorphous: only the highly drawn spherical cap had
significant crystallinity. Besides the concomitant effects on
reflectivity via the index differences, this non-uniformity also
results in non-uniform mechanical properties of the formed
part.
Example 4
Relative Extensibility of Fully Drawn and Underdrawn Films
[0236] The relative extensibility of a fully drawn film made in
accordance with example 1 was compared to that of an underdrawn
film made in accordance with example WM2. The initial biaxial draw
ratio of the fully drawn film was 13.2 (3.3.times.4.0), while the
initial biaxial draw ratio of the underdrawn film was 8.9
(2.7.times.3.3). Again, the draw conditions used to make these
films were similar, except for the final draw ratios in each
direction. Several samples of each were drawn simultaneously
biaxially at an initial rate of 10%/second (e.g. 1.5.times.1.5 over
5 seconds) at 130.degree. C. and 160.degree. C. until breakage. A
biaxial laboratory film stretcher was used, in which the film is
gripped by pressure actuated clips. Because stress tends to
concentrate at the clips, the film tends to break near a clip first
and thus the reported elongation at break will tend to be slightly
lower than what may be achieved under a more uniform stress field.
The fully drawn samples tended to break at draw ratios of
1.3.times.1.3 or less, i.e. a biaxial draw ratio of about 1.7. The
underdrawn samples tended to strain harden around draw ratios of
1.5.times.1.5 and tended to break around 1.7.times.10.7, i.e. a
biaxial draw fully drawn film ratio of 2.9. A total biaxial draw
ratio to break for each film case may be constructed by multiplying
the initial biaxial draw ratio to form the film by the biaxial draw
ratio to break. The total biaxial draw ratio to break for the fully
drawn film is thus about 22.4 and for the underdrawn film about
25.9. The similarity might be expected given the similar process
conditions. For example, fully drawn films made at higher
temperatures or lower strain rates during the first drawing step,
e.g. LO step, often require a higher draw ratio to achieve the same
MD index level. Under these altered circumstances, the initial and
total biaxial draw ratios would be higher for the fully drawn film
than for the particular fully drawn film cited in this example. For
the fully drawn film of this example, the total biaxial draw ratio
may be slightly less than that of the underdrawn film of this
example because the fully drawn film was also heat set.
Example 5
Uniaxial Extensibility of a Fully Drawn Film at Various
Temperatures
[0237] The extensibility of a fully drawn film made in accordance
with Example 1 was measured in uniaxial mode for a variety of
temperatures using a standard Model #1122 Instron tensile tester
available from Instron Corp., Canton Mass. Strips 2.5 cm wide were
cut and mounted with an initial draw gap of 5 cm. Averages were
taken over 5 samples and the maximum elongation also noted among
the samples. The jaw up speed was set at 30 cm/second. The results
are provided in the following table:
TABLE-US-00004 St. Dev. Temperature Average Maximum % Of % Nominal
Peak .degree. C. Elongation % Elongation Elongation Stress (psi)
204 59.4 73.6 10.4 268 177 67.9 84.6 16.9 386 163 81.0 86.1 5.3 467
149 90.0 116.4 20.9 602 135 82.1 110.1 17.8 661 121 89.6 96.2 4.3
888
The draw ratio at break is the elongation at break plus unity, i.e.
1.82 for 135.degree. C. Notice that the elongation to break is
similar at 130.degree. C. and 160.degree. C. as in example 4. The
peak stress usually coincided with the break stress. This example
indicates the utility of elevating the post-forming temperature to
lower the nominal drawing stress, e.g. to obtain greater
formability for a given forming stress, e.g. a vacuum pressure.
Thus thermoforming at lower pressures to the same extent of final
biaxial draw can be achieved with higher forming temperatures under
the conditions of this example. This example also indicates a
reduction in extensibility as the post-forming temperature
approaches the peak crystallization rate temperature. The draw
ratio at break is reasonably constant at about 1.85 until the
temperature of peak crystallization is approached (220.degree.
C.).
[0238] The draw ratios in the preceding table are not the biaxial
draw ratios because the width is unconstrained and can neck down
during elongation. A purely elastic, incompressible neck down in a
true uniaxial draw to 1.85 draw ratio would result in a final neck
down draw ratio of about 0.74 across the sample width and a final
biaxial draw ratio of 1.36. The actual final draw ratio across the
sample width was intermediate between 1.0 and 0.74, thus the
biaxial draw ratio compares favorably with the reported
extensibility of the fully drawn film in biaxial mode of example 4.
Other factors that can effect the comparison include the less
concentrated stress at the clips which might raise the biaxial draw
ratio and the uni-directional nature of the extension which might
lower the biaxial draw ratio.
Example 6
Postforming an Underdrawn Reflective Polarizer Film
[0239] A multilayer film of PEN and coPEN was co-extruded, cast and
drawn to make a variety of PEN:coPEN multilayer reflective
polarizer films. A 0.48 IV PEN (made by 3M Co., St. Paul Minn.) was
dried at 135.degree. C. for 24 hours and then fed directly into a
single screw extruder with exit temperature about 285.degree. C. A
0.54 IV 70/0/30 coPEN (i.e. a copolymer of PEN formed from 70%
naphthalene dicarboxylic acid and 30 dimethyl isophthalate
proportions by weight, and ethylene glycol; also made by 3M Co.,
St. Paul) was dried by feeding into a twin screw extruder equipped
with a vacuum and with an exit temperature of about 285.degree. C.
The intrinsic viscosities (IV) were measured on resin pellets using
a 60/40 weight % phenol/o-dichlorobenzene solvent at 30.degree. C.
These resin streams were co-extruded into a 224 multilayer
feedblock set at 285.degree. C. and equipped with an internal
protective boundary layer (PBL). Pumping rates were maintained so
that the approximate optical thickness of each PEN:coPEN layer pair
was approximately equal in the optical stack, i.e. an "f-ratio" of
0.5. The PBL was supplied with coPEN in approximately one-half the
volume as that supplied to the sum of all the PEN layers in the
optical stack. The layer pairs in the optical stack had an
approximately linear gradient in optical thickness. The multilayer
stack including the PBL was split with an asymmetric multiplier to
form two streams in width ratio of 1.55:1, spread to equivalent
widths and re-stacked to form a two packet multilayer stack of 448
layers separated by an internal protective layer. The multilayer
stack including the PBL was split again with an asymmetric
multiplier to form two streams in width ratio of 1.25:1, spread to
equivalent widths and re-stacked to form a four packet multilayer
stack of 896 layers separated by an internal protective layer. An
additional coPEN (IV 0.54) skin was added to each side of the
multilayer stack with each skin layer comprising about 10% of the
total volumetric flow. The total stream was cast from a die at
about 285 C onto a quench wheel set at 65.degree. C. The coPEN
skins refractive indices were essentially isotropic after casting
with indices of 1.6225 at 632.8 nm as measured by the Metricon
Prism Coupler. The cast thickness was approximately 0.066 cm.
[0240] The film was drawn transversely using the laboratory biaxial
stretcher of example 2. In each case, the draw ratio in the second
in-plane direction was approximately unity. Case 1 was drawn at
130.degree. C. and an initial rate of 20%/second over 20 seconds to
a final measured draw ratio of 4.8 in a single draw step. Cases 2
and 3 were made using a very underdrawn intermediate. Cases 2 and 3
were drawn to approximately 3.5.times., at 130.degree. C., at an
initial rate of 20%/second and over a total of 10 seconds. These
Cases 2 and 3 were then re-heated for 44 seconds at the second draw
step process temperature, i.e. the post forming step temperature,
and post formed by drawing over 10 seconds in the same direction as
the first step to a final draw ratio of about 4.5. Case 2 was
re-heated and post formed at 130.degree. C. with a final measured
draw ratio of 4.6. Case 3 was re-heated and post formed at
175.degree. C. with a final measured draw ratio of 4.4. Case 4 was
made by a similar process to the first drawing step of Cases 2 and
3, i.e. drawn at 130.degree. C. over 13 seconds to a final measured
draw ratio of 3.8. Case 4 was then heated for 65 seconds at
130.degree. C. without re-drawing. Thus Case 4 is indicative of an
underdrawn portion of a final article that undergoes the
post-forming temperatures without additional draw or post-forming
heat set. Case 5 was drawn at 130.degree. C. and an initial rate of
20%/second over 25 seconds to a final measured draw ratio of 5.4 in
a single draw step. Case 6 was made by a similar process to the
first drawing step of Cases 2 and 3, i.e. drawn at 130.degree. C.
over 13 seconds to a final measured draw ratio of 3.8. Case 6 was
then heated for 65 seconds at 175.degree. C. without re-drawing.
The following table presents the final index values of the post
formed film as measured using the Metricon Prism Coupler at 632.8
nanometers. The draw direction is x, the non-drawn in-plane
direction is y, and the thickness direction is z. The calculated
total polarizability (TP) is estimated for the birefringent layer,
as are the total polarizability differences (TPD), the estimated
density (in g/cc) and the fractional crystallinity (X) calculated
based on the estimated density.
TABLE-US-00005 Case nx ny nz TP TPD Density X 1, skin 1.6426 1.6194
1.6110 1, stack 1.7067 1.6211 1.5871 1, est. PEN 1.7708 1.6228
1.5632 1.0925 0.0098 1.3405 0.1437 2, skin 1.6330 1.6228 1.6195 2,
stack 1.7053 1.6218 1.5933 2, est. PEN 1.7776 1.6208 1.5671 1.0969
0.0142 1.3459 0.2139 3, skin 1.6254 1.6251 1.6230 3, stack 1.7338
1.6258 1.5720 3, est. PEN 1.8422 1.6265 1.5210 1.1025 0.0198 1.3528
0.3019 4, skin 1.6315 1.6183 1.6188 4, stack 1.6859 1.6251 1.5948
4, est. PEN 1.7403 1.6282 1.5710 1.0870 0.0042 1.3337 0.0564 5,
skin 1.6424 1.6187 1.6142 5, stack 1.7251 1.6183 1.5789 5, est. PEN
1.8078 1.6185 1.5436 1.0966 0.01388 1.3455 0.2088 6, skin 1.6256
1.6225 1.6220 6, stack 1.7254 1.6227 1.5714 6, est. PEN 1.8252
1.6229 1.5208 1.0943 0.0115 1.3427 0.1719
Case 1 is thus an example of a single step process that makes an
underdrawn film. Cases 2 and 3 begin with an underdrawn
intermediary but finish as fully drawn. Case 4 is approximately
that underdrawn intermediary. It represents a low level of
effective drawing (e.g. Regime II). Case 5 is a single-step fully
drawn reflective polarizer. Case 6 is the underdrawn intermediary
re-heated as in a post forming step without further drawing with a
greatly enhanced level of effective drawing compared to Case 4
(e.g. Regime III).
[0241] The following table summarizes the optical performance of
the various Cases:
TABLE-US-00006 Ave. Location Blue Red Fractional Minimum of Case
Edge Edge Transmission Transmission Minimum 1 <400 nm 900 nm
0.117 0.003 852 nm 2 413 973 0.112 0.012 897 3 403 1012 0.115 0.003
941 4 480 1074 0.199 0.033 992 5 <400 885 0.063 0.002 810 6 470
1080 0.109 0.005 840
The blue edge is defined as the lower edge of the reflection band
where the fractional transmission is 0.5. The red edge is defined
as the upper edge of the reflection band where the fractional
transmission is 0.5. The average transmission is a flat average
across the reflection band from the blue edge plus 20 nm to the red
edge minus 20 nm. The minimum transmission is the lowest value
measured where the transmission measurement is smoothed over 3 nm,
and the location is the wavelength of this occurrence. The band
positions in part result from the different biaxial draw ratios and
in part from the varying initial stack thickness of the cast web.
The pass fractional transmissions were uniformly high across the
reflection bands for every case, with band averages of greater than
0.86. The difference between this result and unity is accounted for
the most part by surface reflections.
[0242] Cases 1, 2 and 3 are all films underdrawn to the final same
amount. These cases demonstrate the utility of making an underdrawn
film, e.g. Case 4, of low orientation and crystallinity (e.g. total
polarizability) which is then subsequently post formed (e.g. into a
shaped article). Case 4 underdrawn films can be further post formed
as described in example 7.
[0243] Case 6 demonstrates the utility of a post forming heat
setting step, e.g. after the shaping of an article by drawing
and/or molding. Case 6 demonstrates at least the same optical
performance as the re-drawn underdrawn cases. Thus a single article
formed from an initially underdrawn film could have both re-drawn
and non-drawn areas with similar optical performance. This
performance compares reasonably with a fully drawn film.
[0244] FIG. 19 compares the spectra of cases 2, 5 and 6, for the
block states of the reflective polarizer, i.e. the fractional
transmission of light polarized in the draw direction at normal
incidence. A typical pass state, i.e. the fractional transmission
of light polarized in the non-drawn in-plane direction at normal
incidence, is also presented.
[0245] It should be noted that a homogeneous undrawn cast web of
PEN was drawn according to the conditions of Cases 1 and 5 at
175.degree. C. The cast film drew non-uniformly and remained
essentially isotropic. This should be contrasted with Case 3, which
was underdrawn to about 3.5 at 130 C and then re-drawn at 175 C
with approximately the same optical effect as the underdrawn film
Case 2 and the single-step underdrawn film Case 1. According to the
index measurements, the higher post-forming temperature of Case 3
could improve the optical performance. Actual performance of these
cases is also affected by the band widths: wider bands tend to be
leakier than narrower bands using the same layer gradient.
Dispersion, i.e. the change in index with wavelength, is another
factor. The index difference between the PEN and coPEN layers in
this example tend to increase with decreasing wavelength. Thus the
same stack construction will have better optical performance as the
red edge shifts to lower wavelengths.
Example 7
Postforming an Underdrawn Film in Multiple Steps
[0246] An underdrawn reflective polarizer film may also be post
formed through multiple steps. In this example, an undrawn
multilayer cast web of PEN and coPEN was co-extruded and cast
according to example 6. The film was drawn transversely using the
laboratory biaxial stretcher of example 2. In each case, the draw
ratio in the second in-plane direction was approximately unity. In
case A, the cast web first was drawn at 135.degree. C. and an
initial rate of 20%/second over 10 seconds to a measured draw ratio
of 3.2 in a single draw step. The film of case A could not be
peeled apart using typical methods. The transmission spectra were
measured using a Perkin-Elmer Lambda-19 spectrophotometer and the
sample was preheated for 25 seconds at 135.degree. C., then further
preheated for 25 seconds at 160.degree. C. and re-drawn over
another 10 seconds to a final measured draw ratio of approximately
4.8. This is case B. A portion of the film was destructively peeled
and indices measured at 632.8 nm. Transmission spectra were
measured using the Perkin-Elmer Lambda-19 spectrophotometer.
Finally, the sample was again preheated for 25 seconds at
135.degree. C., then further preheated for 25 seconds at
160.degree. C. and re-drawn over another 4 seconds to a final
measured draw ratio of approximately 6.0. This is case C. A portion
of the film was destructively peeled and indices measured at 632.8
nm. Transmission spectra were measured using a Perkin-Elmer
Lambda-19 spectrophotometer. The following table presents the final
index values of the post formed film as measured using a
Perkin-Elmer Lambda-19 spectrophotometer. The draw direction is x,
the non-drawn in-plane direction is y, and the thickness direction
is z. The calculated total polarizability (TP) is estimated for the
birefringent layer, as are the total polarizability differences
(TPD), the density (in g/cc) and the fractional crystallinity
(X).
TABLE-US-00007 Sam- ple nx ny nz TP TPD Density X B, 1.6426 1.6194
1.6152 skin B, 1.7704 1.6185 1.5864 stack B, 1.7704 1.6176 1.5576
1.0908 0.0081 1.3384 0.1176 est. PEN C, 1.6330 1.6228 1.6195 skin
C, 1.7053 1.6218 1.5933 stack C, 1.7776 1.6208 1.5671 1.0969 0.0142
1.3459 0.2139 est. PEN
In these cases, the effect of the second re-drawing step was to
increase the total polarizability and the amount of effective draw
with only a modest effect on the index differences.
[0247] FIG. 20 presents the block fractional transmissions for the
three cases. The strength of the block reflectance band is similar
for cases B and C. The band is slightly improved in case C in part
due to an increase in the layer density due to thinning from case B
to C.
Example 8
Thermoformed Mirror Film Headlamp
[0248] A 35.6 cm..times.35.6 cm. (14 inch by 14 inch) sample of
polymeric multilayer mirror film made according to Example 1 was
thermoformed into the shape of a rectangular headlamp using a
Formech 450 Vacuum Forming Machine (obtained from 6 McKay Trading
Estate, Kensal Road, London). To start, the controls for heating
zones 1,2, and 3 of the vacuum former were set to level 3, and the
apparatus was allowed to equilibrate for at least 30 minutes to
ensure that the heating plate was at the correct temperature. A
room temperature silicone rubber mold in the shape of a rectangular
headlamp (Wagner's Halogen Headlamp H4701 High Beam) was placed in
the center of the movable platform on the vacuum former, with the
longest dimension pointing to the right and left with respect to
the operator. The frame of the vacuum former was unlocked and
lifted up, and the multilayer mirror film was taped over the open
cavity directly above the mold and movable platform. The entire
perimeter of the film was securely taped down using 5.08 cm (2
inch) wide Scotch.TM. brand 471 tape (available from 3M Company,
St. Paul, Minn.) to ensure a hermetic seal, which is needed to
maintain vacuum at a later step. It is important to ensure that
there are no wrinkles in the tape that may create channels through
which the vacuum might leak. The frame of the vacuum former was
then closed down and locked to ensure a tight closure.
[0249] Two 1.27 cm (1/2 inch) metal block spacers were placed on
the vacuum former frame's corners closest to the operator in order
to allow the heating plate to be raised sufficiently to allow room
for the mold. The heating plate was then slid onto the metal blocks
so that the rails of the hot plate would lie on the edge of these
blocks, and the heating plate was kept in position for 30 seconds
to soften the film. The movable platform containing the silicone
rubber mold was then raised all the way up so that the mold would
deform the multilayer mirror film. The vacuum was immediately
turned on and a vacuum pulled in order to stretch the film around
the mold.
[0250] After ten seconds, the heating plate was removed from the
sample by lifting a few inches and sliding it back into its
original position. Lifting the hot plate is important to avoid
burning the film. The film was then allowed to cool for about 10
seconds and the vacuum was turned off. After about 15 seconds, the
movable platform and mold were dropped away from the film and the
metal spacer blocks were removed from the vacuum former. The frame
of the vacuum former was then unlocked and lifted to allow removal
of the tape and film. This procedure resulted in a thermoformed
article with no significant wrinkles or color distortions when
viewed at a direction normal to the film.
Example 9
Embossed Color Shifting Security Film
[0251] A color shifting security film was made and embossed
according to Examples 1 and 4 in U.S. Pat. No. 6,045,894 (Jonza et
al.), which is herein incorporated by reference. A multilayer film
containing about 418 layers was made on a sequential flat-film
making line via a coextrusion process. This multilayer polymer film
was made PET and ECDEL.TM. 9967 where PET was the outer layer or
"skin" layer. A feedblock method (such as that described by U.S.
Pat. No. 3,801,429) was used to generate about 209 layers with an
approximately linear layer thickness gradient from layer to
layer.
[0252] The PET, with an intrinsic viscosity (IV) of 0.60 dl/g was
pumped to the feedblock at a rate of about 34.0 Kg/hr and the
ECDEL.TM. at about 32.8 Kg/hr. After the feedblock, the same PET
extruder delivered PET as protective boundary layers to both sides
of the extrudate at about 8 Kg/hr total flow. The material stream
then passed though an asymmetric double multiplier, as described in
U.S. Pat. Nos. 5,094,788 and 5,094,793, with a multiplier ratio of
about 1.40. The multiplier ratio is defined as the average layer
thickness of layers produced in the major conduit divided by the
average layer thickness of layers in the minor conduit. Each set of
209 layers has the approximate layer thickness profile created by
the feedblock, with overall thickness scale factors determined by
the multiplier and film extrusion rates.
[0253] The ECDEL.TM. melt process equipment was maintained at about
250.degree. C., the PET (optics layers) melt process equipment was
maintained at about 265.degree. C., and the multiplier, skin-layer
meltstream and die were maintained at about 274.degree. C. The
feedblock used to make the film for this example was designed to
give a linear layer thickness distribution with a 1.3:1 ratio of
thickest to thinnest layers under isothermal conditions. To achieve
a smaller ratio for this example, a thermal profile was applied to
the feedblock. The portion of the feedblock making the thinnest
layers was heated to 285.degree. C., while the portion making the
thickest layers was heated to 268.degree. C. In this manner the
thinnest layers are made thicker than with isothermal feedblock
operation, and the thickest layers are made thinner than under
isothermal operation. Portions intermediate were set to follow a
linear temperature profile between these two extremes. The overall
effect is a narrower layer thickness distribution which results in
a narrower reflectance spectrum. Some layer thickness errors are
introduced by the multiplier, and account for the minor differences
in the spectral features of each reflectance band. The casting
wheel speed was set at 6.5 m/min (21.2 ft/min).
[0254] After the multiplier, thick symmetric skin layers were added
at about 35.0 Kg/hour that was fed from a third extruder. Then the
material stream passed through a film die and onto a water cooled
casting wheel. The inlet water temperature on the casting wheel was
about 7.degree. C. A high voltage pinning system was used to pin
the extrudate to the casting wheel. The pinning wire was about 0.17
mm thick and a voltage of about 5.5 kV was applied. The pinning
wire was positioned manually by an operator about 3-5 mm from the
web at the point of contact to the casting wheel to obtain a smooth
appearance to the cast web. The cast web was continuously oriented
by conventional sequential length orienter (LO) and tenter
equipment. The web was length oriented to a draw ratio of about 2.5
at about 100.degree. C. The film was preheated to about 100.degree.
C. in about 22 seconds in the tenter and drawn in the transverse
direction to a draw ratio of about 3.3 at a rate of about 20% per
second. The film was heat set for about 20 seconds in an oven zone
set at 226.degree. C.
[0255] The finished film had a final thickness of about 0.08 mm.
The band edge at normal incidence was 720 nm, just beyond the
visible edge of 700 nm, so that the film looked clear. At 45
degrees, the band edge had shifted over to 640 nm, and the film
appeared cyan. At 60 degrees, the total lack of transmitted red
light made the film a brilliant cyan, due to the high reflectance
of the multilayer stack even at this angle of incidence. If this
film is viewed where there is only a single light source, the
specular reflection was evident (red) even with a white paper
background. When laminated to a black background (no transmitted
light), the red was easily visible. Although this film exhibited
the desired color change, a film of fewer layers and narrower
bandwidth would be more desirable.
[0256] The film was then embossed between a roll at 149.degree. C.
(300.degree. F.) and a pre-heated plate. The film thinned down from
3.4 mils to about 3.0 mils in the embossed regions. A surprising
result of this embossing was the how apparent a gold reflection
became. A bright gold was observed in the embossed region changing
to cyan or deeper blue as the viewing angle is made shallower. The
appearance was similar to gold leaf, yet (at least in this example)
is not as uniform. Bright red and green were also apparent. The
dramatic change from gold to blue while the unembossed areas change
from clear to cyan provided an overt verification feature that was
more dramatic than a transparent hologram.
Example 10
Vacuum Forming of a Trifurcated Light Guide
[0257] A trifurcated light guide was vacuum formed from a highly
reflective PEN/PMMA multilayer mirror that was made as described in
Example 2 of U.S. Pat. No. 6,080,467 (Weber et al.). A coextruded
film containing 601 layers was made on a sequential
flat-film-making line via a coextrusion process. Polyethylene
Naphthalate (PEN) with an Intrinsic Viscosity of 0.57 dl/g (60 wt.
% phenol/40 wt. % dichlorobenzene) was delivered by extruder A at a
rate of 114 pounds per hour with 64 pounds per hour going to the
feedblock and the rest going to skin layers described below. PMMA
(CP-82 from ICI of Americas) was delivered by extruder B at a rate
of 61 pounds per hour with all of it going to the feedblock. PEN
was on the skin layers of the feedblock. The feedblock method was
used to generate 151 layers using the feedblock such as those
described in U.S. Pat. No. 3,801,429, after the feedblock two
symmetric skin layers were coextruded using extruder C metering
about 30 pounds per hour of the same type of PEN delivered by
extruder A. This extrudate passed through two multipliers producing
an extrudate of about 601 layers. U.S. Pat. No. 3,565,985 describes
similar coextrusion multipliers. The extrudate passed through
another device that coextruded skin layers at a total rate of 50
pounds per hour of PEN from extruder A. The web was length oriented
to a draw ratio of about 3.2 with the web temperature at about
280.degree. F. The film was subsequently preheated to about
310.degree. F. in about 38 seconds and drawn in the transverse
direction to a draw ratio of about 4.5 at a rate of about 11% per
second. The film was then heat-set at 440.degree. F. with no
relaxation allowed. The finished film thickness was about 3 mil.
The bandwidth at normal incidence was about 350 nm with an average
in-band extinction of greater than 99%. The amount of optical
absorption was difficult to measure because of its low value, but
was less than 1%.
[0258] A 17.8 cm (7 inch) by 25.4 cm (10 inch) by 2.5 cm (1 inch)
block of wood was used to prepare a vacuum forming mold. A series
of small holes were drilled in the lowest part of grooves routed in
the wood as shown in FIG. 10. After removing the release liner from
one side of an acrylic foam double sided tape, the adhesive was
applied to the periphery on the non-routed side of the wood block
to form a chamber beneath the mold; the second release liner was
not removed from the other side of the adhesive tape. The mold was
then placed on the vacuum table of a vacuum forming apparatus. The
multilayer film was mounted in a heating frame, and the film was
heated for 4 minutes beneath an electrical heating element to
177.degree. C. (350.degree. C.). The film was then rapidly lowered
onto the evacuated mold, drawing the polymer film into the grooved
cavity. The film maintained its high reflectivity after the vacuum
forming operation.
[0259] While the formed film was still in the mold, double-sided
adhesive tape was applied to the portions of the film that were not
drawn into the mold. A second sheet of mirror film was then adhered
to the formed mirror film. The tips of the four termini were cut
off to form an inlet with three outlets as shown in FIG. 10. The
terminus of a fiber optic light fixture was inserted into the inlet
of the light guide, and when light was directed into the light
guide input, light emerged from each of the outlets.
Example 11
Structured Surfaced Multilayer Optical Film
[0260] A coextruded film containing 601 layers of PEN/coPEN was
made on a sequential flat-film-making line via a coextrusion
process as described in Example 10 of U.S. Pat. No. 5,882,774
(Jonza et al.). A Polyethylene naphthalate (PEN) with an intrinsic
viscosity of 0.54 dl/g (60 wt % Phenol plus 40 wt %
dichlorobenzene) was delivered by on extruder at a rate of 75
pounds per hour and the coPEN was delivered by another extruder at
65 pounds per hour. The coPEN was a copolymer of 70 mole % 2,6
naphthalene dicarboxylate methyl ester, 15% dimethyl isophthalate
and 15% dimethyl terephthalate with ethylene glycol. The feedblock
method was used to generate 151 layers. The feedblock was designed
to produce a gradient distribution of layers with a ration of
thickness of the optical layers of 1.22 for the PEN and 1.22 for
the coPEN. The PEN skin layers were coextruded on the outside of
the optical stack with a total thickness of 8% of the coextruded
layers. The optical stack was multiplied by two sequential
multipliers. The nominal multiplication ratio of the multipliers
were 1.2 and 1.27, respectively. The film was subsequently
preheated to 310.degree. F. in about 40 seconds and drawn in the
transverse direction to a draw ratio of about 5.0 at a rate of 6%
per second. The finished film thickness was about 2 mils. Samples
of the film were embossed using four different nickel electroformed
tools and a large hydraulic Wabash Press equipped with a 7.6 cm (3
inch) piston and a platens heated to 191.degree. C. (375.degree.
F.).
[0261] An X-cut fastener (negative) tool was placed on a 2.54 mm
(0.1 inch) thick sheet of aluminum. The mirror film was placed on
the tool and then covered with two sheets of 3 mil polyester
terephthalate and another sheet of 0.1 inch aluminum. The sandwich
was placed closed between the heated platens with minimal pressure
and the sandwich was heated for 60 seconds. A force of 6000 lbs was
applied to the sandwich for 60 seconds. After the force was
removed, the embossed film was removed from the tool. The
post-formed film showed altered colors in the square embossed areas
with both transmitted and reflected light due to thinning of the
multilayer optical stack.
[0262] A linear section of the X-cut fastener tool was placed on a
2.54 mm (0.1 inch) thick sheet of aluminum. The mirror film was
placed on the tool and then covered with two sheets of 3 mil
polyester terephthalate and another sheet of 0.1 inch aluminum. The
sandwich was placed closed between the heated platens with minimal
pressure and the sandwich was heated for 60 seconds. A force of
6000 lbs was applied to the sandwich for 60 seconds. After the
force was removed, the embossed film was removed from the tool. The
post-formed film showed altered colors in the linear embossed areas
with both transmitted and reflected light due to thinning of the
multilayer optical stack.
[0263] An X-cut flat top (positive) tool was placed on a stack of
16 sheets of notebook paper because of the rough back of the tool.
The tool and paper were placed on a 2.54 mm (0.1 inch) thick sheet
of aluminum. The mirror film was placed on the tool and then
covered with two sheets of 3 mil polyester terephthalate and
another sheet of 0.1 inch aluminum. The sandwich was placed closed
between the heated platens with minimal pressure and the sandwich
was heated for 90 seconds. A force of 6000 lbs was applied to the
sandwich for 60 seconds. After the force was removed, the embossed
film was removed from the tool. The post-formed film showed altered
colors in the pyramidal embossed areas with both transmitted and
reflected light due to thinning of the multilayer optical
stack.
[0264] A 21 mil cube corner tool was placed on a 2.54 mm (0.1 inch)
thick sheet of aluminum. The mirror film was placed on the tool and
covered with a sheet of 1/4 inch silicone rubber. The sandwich was
placed closed between the heated platens with minimal pressure and
the sandwich was heated for 30 seconds. A force of 2000 lbs was
applied to the sandwich for 60 seconds. After the force was
removed, the perforated film was removed from the tool. The
post-formed film showed altered colors in the hexagonal embossed
areas with both transmitted and reflected light due to thinning of
the multilayer optical stack.
[0265] The same 21 mil cube corner tool was also used to cold
emboss the multilayer optical film. The cube corner tool was
adhesively attached to a 0.25 inch sheet of polymethylmethacrylate.
The mirror film was placed on the tool and covered with a sheet of
1/4 inch silicone rubber. The sandwich was placed into the press
and a force of 2000 lbs was applied to the sandwich for 10 seconds.
After the force was removed, the embossed film was removed from the
tool. The post-formed film showed altered colors in the triangular
pyramidal embossed areas with both transmitted and reflected light
due to thinning of the multilayer optical stack.
[0266] The structured surfaced multilayered films of this example
are useful as optical filters, controlled transmission reflectors,
optical diodes, diffuse polarizing/depolarizing reflectors,
focussing reflectors, decorative films, and light guides. The thin
flexible films can be used in the same ways as a highly reflective
metallized film without worry of corrosion and cracking of the
metallic thin film upon severe/extreme deformation, embossing, or
perforation or the dangers associated with their conductivity.
Example 12
Corrugated Ribbons
[0267] A post-forming process that may be used to produce a
decorative item, such as any of the previously mentioned decorative
items, is a corrugation process. FIG. 21 shows an arrangement for
corrugating the films that includes first and second generally
cylindrical corrugating members or rollers 220 and 221 each having
an axis and a multiplicity of spaced ridges 219 defining the
periphery of the corrugating members 220 and 221. Each corrugating
member 220 and 221 is driven by its own drive mechanism. The spaces
between ridges 219 are adapted to receive ridges 219 of the other
corrugating member in meshing relationship with the multilayer
optical film 212 inserted therebetween. The arrangement also
includes means for rotating at least one of the corrugating members
220 or 221 so that when the film 212 is fed between the meshed
portions of the ridges the film 212 will be generally conformed to
the periphery of the first corrugating member 220.
[0268] Process parameters that influence the decorative appearance
of the resulting corrugated films include the temperatures of the
corrugating rollers, the nip pressure between the corrugating
rollers, the diameter of the corrugating rollers, the line speed,
the shape of ridges 219, and the number of corrugations per inch
that the rollers are designed to produce. The number of
corrugations per inch is determined by the spacing between ridges
219. Specifically, a pair of intermeshing ridges creates one
corrugate. As the examples presented below will illustrate, these
parameters may be adjusted to produce different decorative
effects.
[0269] The structure 210 that results from the previously described
corrugation process is shown in FIG. 22. The undulations may be
characterized by arcuate portions 213, valley portions 214, and
intermediate portions 215 and 216 which connect the arcuate
portions to the valley portions. While the undulations shown in
FIG. 22 are sinusoidal in shape, it should be recognized that the
corrugation process may create undulations of other shapes, such as
shown in FIG. 23, for example. In addition, the corrugates need not
extend along the width of the film. Rather, they may extend in any
direction in the plane of the film.
[0270] In accordance with one aspect of the present invention, in
additions to the undulations formed by the corrugation process, the
corrugation process also results in variations in the thickness of
the film layers. In particular, the ridges 219 of the corrugating
members stretch the intermediate portions 215 and 216 of corrugated
film 210 so that these portions are thinner than arcuate and valley
portions 213 and 214. Because of the variations in thickness of the
film, the different portions of the film will reflect light of
different wavelengths, producing a noticeable shift in color of the
intermediate portions compared to the arcuate and valley portions
213 and 214. This phenomenon, referred to as color or band
shifting, occurs because the range of wavelengths reflected by a
multilayer optical film is, in part, a function of the physical
thickness of the layers in the multilayer optical film.
Optical Characteristics of Corrugated Films
[0271] The pre-corrugated film was fabricated to have a uniform
thickness within a specified tolerance (typically about .+-.5%).
When held taut and viewed in normal transmission under fluorescent
room lighting, the pre-corrugated film appeared to exhibit
primarily a single color, for example, cyan. Flexure of the film
produced substantial changes in the film color so that a range of
colors were visible along the film. That is, the pre-corrugated
film exhibited angularly sensitive reflective color filtration.
This effect occurs because the film reflects incident light in one
wavelength range and transmits light in another wavelength range,
with the wavelength ranges of reflection and transmission varying
with changes in the angle of incidence of the light. Thus, the
particular color that is observed on a given portion of the film
may differ from the color observed on another portion of the film
because flexure of the film causes light to strike the different
portions of film at different angles of incidence. In other words,
the number of colors that are observed increases as the number of
different planes occupied by various portions of the film
increases.
[0272] FIG. 24 shows an exemplary pattern observed in normal
transmission after the film has undergone a corrugation process in
accordance with the method of the present invention to provide the
film with an undulating variation in thickness. The appearance of
the film has changed substantially in comparison to the
pre-corrugated film. In contrast to the primarily cyan appearance
of the pre-corrugated film (when it is tautly arranged without any
flexure so that the number of different planes which reflect light
is minimized), the corrugated film displays different colored bands
that extend in the cross-web direction. In particular, bands 320
and 322 of alternating color are formed, with bands 20 appearing in
one color (e.g., yellow) and bands 322 appearing in another color
(e.g., cyan). Bands 320 correspond to intermediate portions 215 and
216 shown in FIG. 22, which have a reduced layer thickness as a
result of the corrugation process, and bands 322 correspond to the
arcuate and valley portions 213 and 214. In other words, the
corrugated film has alternating bands or striations of different
colors along its length because of color shifting arising from the
thickness variations.
[0273] When observing light reflected from the corrugated film, the
corrugated film appears to have a greater brilliance in comparison
to the pre-corrugated film. This is caused by the increased
angularity of the film produced by the corrugation process. The
increased angularity increases the number of source locations from
which light is directed back to the viewer. In addition, the
different portions of the film extend in different planes and light
is reflected over a greater range of incident angles, which as
previously mentioned, results in different colors of light being
observed.
[0274] The corrugating process as employed in the present invention
will now be further described by the following specific
examples.
Example 12(a)
[0275] A decorative colored mirror film was made using the
corrugation process of the present invention. The pre-creped film
was prepared from a coextruded film containing 224 layers made on a
sequential flat-film making line by a coextrusion process. This
multilayer polymer film was made from polyethylene naphthalate
(PEN) (60 wt. % phenol/40 wt. dichlorobenzene) with an intrinsic
viscosity of 0.48 dl/g available from Eastman Chemical Company and
polymethyl methacrylate (PMMA) available from ICI Acrylics under
the designation CP82. PETG 6763 provided the outer or "skin"
layers. PETG 6763, believed to be a copolyester based on
terephthalate as the dicarboxylate and 1,4-cyclohexane dimethanol
and ethylene glycol as the diols, is commercially available from
Eastman Chemicals Co., Rochester, N.Y. A feedblock method (such as
that described by U.S. Pat. No. 3,801,429) was used to generate
about 224 layers which were coextruded onto a water chilled casting
wheel and continuously oriented by conventional sequential length
orienter (LO) and tenter equipment. PEN was delivered to the
feedblock by one extruder at a rate of 24.2 Kg/hr and the PMMA was
delivered by another extruder at a rate of 19.3 Kg/hr. These
meltstreams were directed to the feedblock to create the PEN and
PMMA optical layers. The feedblock created 224 alternating layers
of PEN and PMMA with the two outside layers of PEN serving as the
protective boundary layers (PBLs) through the feedblock. The PMMA
melt process equipment was maintained at about 274.degree. C.; the
PEN melt process equipment, feedblock, skin-layer modules were
maintained at about 274.degree. C.; and the die was maintained at
about 285.degree. C. A gradient in layer thickness was designed for
the feedblock for each material with the ratio of thickest to
thinnest layers being about 1.25.
[0276] After the feedblock, a third extruder delivered PETG as skin
layers (same thickness on both sides of the optical layer stream)
at about 25.8 Kg/hr. Then the material stream passed through a film
die and onto a water cooled casting wheel using an inlet water
temperature of about 24.degree. C. A high voltage pinning system
was used to pin the extrudate to the casting wheel at 3.1
meters/min. The pinning wire was about 0.17 mm thick and a voltage
of about 4.9 kV was applied. The pinning wire was positioned
manually by an operator about 3-5 mm from the web at the point of
contact to the casting wheel to obtain a smooth appearance to the
cast web.
[0277] The cast web was length oriented with a draw ratio of about
3.1:1 at about 130.degree. C. In the tenter, the film was preheated
before drawing to about 135.degree. C. in about 30.9 seconds and
then drawn in the transverse direction at about 140.degree. C. to a
draw ratio of about 4.5:1, at a rate of about 20% per second. The
finished pre-corrugated film had a final thickness of about 0.05
mm.
[0278] The pre-corrugated multilayer film was fed into the nip
between the corrugating rollers 220 and 221 shown in FIG. 21. The
corrugating members had a diameter of about 9.01-9.02 inches, with
ridges shaped to form about 71/2 corrugations per inch along the
length of the resultant corrugated film. Both corrugating members
were heated to 250.degree. F. The nip pressure applied between the
corrugating members was 50 pounds force per lineal inch (pli), and
the line speed was 5 feet per minute (fpm).
[0279] The precorrugated multilayer colored mirror film, as
observed in normal transmission under fluorescent room lighting,
exhibited randomly distributed areas of clear, cyan and blue
elongated in the crossweb direction. The resulting corrugated
colored mirror film had significantly changed in its visual
appearance. As observed in normal transmission under fluorescent
room lighting, both the peak and valley portions or regions of the
corrugated colored mirror film were cyan in color. The intermediate
portions or regions located between the peaks and valleys changed
to yellow in color in normal transmission as observed under
fluorescent room lighting. It is believed that this observed color
change in the connecting regions between the peaks and valleys was
due to film thinning during the corrugation process. The caliper of
the corrugated colored mirror film in the intermediate regions was
measured and found to be thinner than the caliper measured for the
peak and valley regions. The caliper of the intermediate regions
was also thinner than the caliper of the pre-corrugated multilayer
mirror film.
[0280] The caliper of the pre-corrugated colored mirror film and
the caliper of the intermediate regions between the peaks and
valleys of the corrugated colored mirror film were measured in a
conventional manner using a manual caliper instrument (Model #
293-761, manufactured by Mitutoyo Corporation, 31-19, Shiba5-chome,
Minato-ku, Tokyo 108, Japan). The caliper data was obtained by
averaging ten measurements randomly chosen from within each film
sample. The caliper data for this film is presented below:
TABLE-US-00008 Thickness of precorrugated colored mirror film: 1.54
mils (std dev 0.11) Thickness of intermediate region between the
1.17 mils (std dev 0.33) peaks and valleys of the corrugated
film:
Example 12(b)
[0281] A decorative colored mirror film was prepared in a manner
similar to that described for Example 12(a) above. The
pre-corrugated multilayer colored mirror film 12 was formed from a
coextruded film containing 224 layers made on a sequential
flat-film making line by a coextrusion process. This multilayer
polymer film was made from polyethylene naphthalate (PEN)(60 wt. %
phenol/40 wt. % dichlorobenzene)) with an intrinsic viscosity of
0.48 dl/g available from the Eastman Chemical Company and
polymethyl methacrylate (PMMA) available from ICI Acrylics under
the designation CP82, where PEN provided the outer or "skin"
layers. A feedblock method (such as that described by U.S. Pat. No.
3,801,429) was used to generate about 224 layers which were
coextruded onto a water chilled casting wheel and continuously
oriented by conventional sequential length orienter (LO) and tenter
equipment. PEN was delivered to the feedblock by one extruder at a
rate of 38.8 Kg/hr and the PMMA was delivered by another extruder
at a rate of 30.1 Kg/hr. These meltstreams were directed to the
feedblock to create the PEN and PMMA optical layers. The feedblock
created 224 alternating layers of PEN and PMMA with the two outside
layers of PEN serving as the protective boundary layers (PBL's)
through the feedblock. The PMMA melt process equipment was
maintained at about 274.degree. C.; the PEN melt process equipment,
feedblock, skin-layer modules were maintained at about 274.degree.
C.; and the die was maintained at about 285.degree. C. A gradient
in layer thickness was designed for the feedblock for each material
with the ratio of thickest to thinnest layers being about 1.31.
[0282] After the feedblock, a third extruder delivered 0.48 IV PEN
as skin layers (same thickness on both sides of the optical layer
stream) at about 23.9 Kg/hr. Then the material stream passed
through a film die and onto a water cooled casting wheel using an
inlet water temperature of about 29.degree. C. A high voltage
pinning system was used to pin the extrudate to the casting wheel
at 5.2 meters/min. The pinning wire was about 0.17 mm thick and a
voltage of about 6.2 kV was applied. The pinning wire was
positioned manually by an operator about 3-5 mm from the web at the
point of contact to the casting wheel to obtain a smooth appearance
to the cast web.
[0283] The cast web was length oriented with a draw ratio of about
3.1:1 at about 130.degree. C. In the tenter, the film was preheated
before drawing to about 140.degree. C. in about 18 seconds and then
drawn in the transverse direction at about 140.degree. C. to a draw
ratio of about 4.6:1, at a rate of about 15% per second. The
finished pre-corrugated film had a final thickness of about 0.05
mm.
[0284] The corrugating members of the corrugating arrangement were
shaped to form about 13 corrugations per inch along the length of
the corrugated film. Both corrugating members were heated to
250.degree. F., the nip pressure between the corrugating rollers
was 50 pli, and the line speed was 15 fpm.
[0285] The pre-corrugated film was cyan in color when observed in
normal transmission under fluorescent room lighting. The resulting
corrugated film had changed in visual appearance. As observed in
normal transmission under fluorescent room lighting, the peak and
valley regions and the intermediate regions between the peaks and
valleys all remained cyan in color, but the intermediate regions
exhibited a deeper shade of cyan. Moreover, when observing light
reflected from the film, the film appeared much more brilliant than
the film described in Example 1, giving the film a visual
appearance strikingly different from the film in Example 1. The
increased brilliance presumably occurred because of the increased
angularity in the film resulting from the formation of the peaks
and valleys.
Example 12(c)
[0286] The corrugated colored mirror film prepared in Example 12(a)
was cut into rolls of film 1/2 inch in width using a conventional
razor blade. A 47/8 inch diameter confetti bow having 31 loops was
then formed from the roll of film. The bow was prepared using a
Cambarloc bow machine available from Cambarloc Engineering, Inc.
Lebanon, Mo.
Example 12(d)
[0287] The corrugated colored mirror film prepared in Example 12(b)
was cut into 1/2 inch width rolls, from which confetti bows were
prepared, as described in Example 3.
Example 12(e)
[0288] A decorative color mirror film was prepared in a manner
similar to that described in Example 12(a). The pre-corrugated
multicolored mirror film was formed from a coextruded film
containing 224 layers made on a sequential flat-film making line by
a coextrusion process. This multilayer polymer film was made from
copolyethylene naphthalate (LMPP) comprised of 90 mol % naphthalate
and 10 mol % terephathalate as the dicarboxylates and 100% ethylene
glycol as the diol with an intrinsic viscosity of 0.48 dl/g and
polymethyl methacrylate (PMMA) available from ICI Acrylics under
the designation CP71, where LMPP provided the outer or skin layers.
A feedblock method (such as that described by U.S. Pat. No.
3,801,429) was used to generate about 224 layers which were
coextruded onto a water chilled casting wheel and continuously
oriented by conventional sequential length orienter (LO) and tenter
equipment. LMPP was delivered to the feedblock by one extruder at a
rate of 46.0 Kg/hr and the PMMA was delivered by another extruder
at a rate of 35.9 Kg/hr. These meltstreams were directed to the
feedblock to create the LMPP and PMMA optical layers.
[0289] The feedblock created 224 alternating layers of LMPP and
PMMA with the two outside layers of LMPP serving as the protective
boundary layers through the feedblock. The PMMA melt process
equipment was maintained at about 265.degree. C.; the PEN melt
process equipment, feedblock, skin-layer modules were maintained at
about 265.degree. C.; and the die was maintained at about
285.degree. C. A gradient in layer thickness was designed for the
feedblock for each material with the ratio of thickest to thinnest
layers being about 1:2. An axial rod, as described in filed patent
application U.S. Ser. No. 09/006,288 (now abandoned), was used to
narrow the bandwidth.
[0290] After the feedblock, a third extruder delivered 0.48 IV LMPP
as skin layers (same thickness on both sides of the optical layer
stream) at about 93.2 Kg/hr. Then the material stream passed
through a film die and onto a water cooled casting wheel using an
inlet water temperature of about 18.degree. C. A high voltage
pinning system was used to pin the extrudate to the casting wheel
at 6.6 meters/min. The pinning wire was about 0.17 mm thick and a
voltage of about 5.6 kV was applied. The pinning wire was
positioned manually by an operator about 3-5 mm from the web at the
point of contact to the casting wheel to obtain a smooth appearance
to the cast web.
[0291] The cast web was length oriented with a draw ratio of about
3:3:1 at about 120 C. In the tenter, the film was preheated before
drawing to about 125 C in about 14 seconds and then drawn in the
transverse direction at about 125 C to a draw ratio of about 4:3:1,
at a rate of about 20% per second. The finished pre-corrugated film
had a final thickness of about 0.05 mm.
[0292] The pre-corrugated film was cyan in color when observed in
normal transmission under fluorescent room lighting. The resulting
corrugated film when observed in normal transmission under
fluorescent lighting exhibited a magenta color at the outside edges
of the peaks and valleys while the remaining regions of the film
maintained the cyan color.
Example 13
Point Embossed Colored Mirror Film
[0293] A decorative colored mirror film was made by point embossing
a multilayer colored mirror film using conventional embossing
equipment. The input film used for the embossing was a coextruded
film containing 224 layers made on a sequential flat-film making
line by a coextrusion process. This multilayer polymer film was
made from polyethylene naphthalate (PEN)(60 wt. % phenol/40 wt. %
dichlorobenzene)) with an intrinsic viscosity of 0.48 dl/g
available from the Eastman Chemical Company and polymethyl
methacrylate (PMMA) available from ICI Acrylics under the
designation CP82. PETG 6763 provided the outer or "skin" layers.
PETG 6763, believed to be a copolyester based on terephthalate as
the dicarboxylate and 1,4-cyclohexane dimethanol and ethylene
glycol as the diols, is commercially available from Eastman
Chemicals Co., Rochester, N.Y. A feedblock method (such as that
described by U.S. Pat. No. 3,801,429) was used to generate about
224 layers which were coextruded onto a water chilled casting wheel
and continuously oriented by conventional sequential length
orienter (LO) and tenter equipment. PEN was delivered to the
feedblock by one extruder at a rate of 24.2 Kg/hr and the PMMA was
delivered by another extruder at a rate of 19.3 Kg/hr. These
meltstreams were directed to the feedblock to create the PEN and
PMMA optical layers. The feedblock created 224 alternating layers
of PEN and PMMA with the two outside layers of PEN serving as the
protective boundary layers (PBL's) through the feedblock. The PMMA
melt process equipment was maintained at about 274.degree. C.; the
PEN melt process equipment, feedblock, skin-layer modules were
maintained at about 274.degree. C.; and the die was maintained at
about 285.degree. C. A gradient in layer thickness was designed for
the feedblock for each material with the ratio of thickest to
thinnest layers being about 1.25.
[0294] After the feedblock, a third extruder delivered PETG as skin
layers (same thickness on both sides of the optical layer stream)
at about 25.8 Kg/hr. Then the material stream passed through a film
die and onto a water cooled casting wheel using an inlet water
temperature of about 24.degree. Celsius. A high voltage pinning
system was used to pin the extrudate to the casting wheel at 3.1
meters/min. The pinning wire was about 0.17 mm thick and a voltage
of about 4.9 kV was applied. The pinning wire was positioned
manually by an operator about 3-5 mm from the web at the point of
contact to the casting wheel to obtain a smooth appearance to the
cast web.
[0295] The cast web was length oriented with a draw ratio of about
3.1:1 at about 130.degree. C. In the tenter, the film was preheated
before drawing to about 135.degree. C. in about 30.9 seconds and
then drawn in the transverse direction at about 140.degree. C. to a
draw ratio of about 4.5:1, at a rate of about 20% per second. The
finished film had a final thickness of about 0.05 mm.
[0296] The film was passed between two nipped heated embossing
rollers. The top embossing roller, which was heated to 250 degrees
F., had a raised diamond shaped embossing pattern engraved on its
surface. The embossing pattern was designed so that 5% of the
surface area of the film would be embossed with the diamond
pattern. The bottom laminating roller had a smooth surface and was
heated to 250 degrees F. The nip pressure was 100 pounds force per
lineal inch (pli) and the line speed was 5 feet per minute
(fpm).
[0297] Prior to embossing, the multilayer colored mirror film
exhibited randomly distributed areas of clear, cyan, and blue
elongated in the crossweb direction, as observed in normal
transmission under fluorescent room lighting. The resulting
embossed colored mirror film had changed in its visual appearance.
As observed in normal transmission under fluorescent room lighting,
the embossed areas of the film were magenta in color, while the
film in the areas between the embossed regions remained similar in
appearance to the pre-embossed film, that is, exhibiting randomly
distributed areas of clear, cyan and blue elongated in the crossweb
direction. It is believed that this observed color change in the
embossed areas of the film compared to the non-embossed areas of
the film was due to film thinning that occurred as a result of the
embossing process. Cross sectional scanning electron
photomicrographs (SEMs) taken of the resulting embossed colored
mirror film showed that the thickness of the embossed areas of the
film were approximately 63% of the thickness of the non-embossed
areas of the film.
[0298] The embossed colored mirror film was then slit into 1/2 inch
width rolls using a conventional razor blade slitting method. A
4.875 inch diameter confetti bow having 31 loops was then formed
from the roll of film. The bow was prepared using a Cambarloc bow
machine (see U.S. Pat. No. 3,464,601) available from Cambarloc
Engineering, Lebanon, Mo.
[0299] The patents, patent applications, patent documents, and
publications cited herein are incorporated by reference in their
entirety, as if each were individually incorporated by reference.
Various modifications and alterations of this invention will become
apparent to those skilled in the art without departing from the
scope of this invention, and it should be understood that this
invention is not to be unduly limited to the illustrative
embodiments set forth herein.
* * * * *