U.S. patent application number 11/397992 was filed with the patent office on 2006-10-12 for heat setting optical films.
This patent application is currently assigned to 3M Innovative Properties Company. Invention is credited to Timothy J. Hebrink, William W. Merrill.
Application Number | 20060226561 11/397992 |
Document ID | / |
Family ID | 36778332 |
Filed Date | 2006-10-12 |
United States Patent
Application |
20060226561 |
Kind Code |
A1 |
Merrill; William W. ; et
al. |
October 12, 2006 |
Heat setting optical films
Abstract
A method of making an optical film includes providing a film,
substantially uniaxially orienting the film, and heat setting the
oriented film. The film includes a polymeric material capable of
developing birefringence.
Inventors: |
Merrill; William W.; (White
Bear Lake, MN) ; Hebrink; Timothy J.; (Scandia,
MN) |
Correspondence
Address: |
3M INNOVATIVE PROPERTIES COMPANY
PO BOX 33427
ST. PAUL
MN
55133-3427
US
|
Assignee: |
3M Innovative Properties
Company
|
Family ID: |
36778332 |
Appl. No.: |
11/397992 |
Filed: |
April 5, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60669865 |
Apr 8, 2005 |
|
|
|
Current U.S.
Class: |
264/1.34 ;
264/235.6 |
Current CPC
Class: |
C08J 2367/03 20130101;
C08J 5/18 20130101; B29K 2069/00 20130101; C08G 63/181 20130101;
C09K 2323/031 20200801; B29K 2995/0034 20130101; B29C 55/085
20130101; B29K 2995/0051 20130101; B29C 35/0277 20130101; B29D
11/00644 20130101; B29K 2067/003 20130101; G02B 5/3083 20130101;
B29C 55/08 20130101; B29C 55/06 20130101 |
Class at
Publication: |
264/001.34 ;
264/235.6 |
International
Class: |
B29D 7/01 20060101
B29D007/01 |
Claims
1. A method of making an optical film comprising: providing a film
comprising a polymeric material capable of developing
birefringence; substantially uniaxially orienting the film; and
heat setting the oriented film.
2. The method of claim 1 wherein the oriented film exhibits an
extent of uniaxial character U equal to or greater than about
0.2.
3. The method of claim 1 wherein the oriented film exhibits an
extent of uniaxial character U equal to or greater than about
0.7.
4. The method of claim 1 wherein the step of heat setting comprises
heating the film to a temperature above a glass transition
temperature of at least one polymeric material thereof and below a
melting point of that polymeric material.
5. The method of claim 1 wherein the step of heat setting comprises
heating the film for a period of time between about 1 second and
about 10 minutes.
6. The method of claim 1 wherein the step of heat setting comprises
heating the film to a temperature above a glass transition
temperature of each polymeric material thereof and below a melting
point of each polymeric material thereof.
7. The method of claim 1 wherein: the step of substantially
uniaxially orienting the film occurs at a first temperature; and
the step of heat setting comprises heating the film to a second
temperature higher than the first temperature.
8. The method of claim 1 wherein the polymeric material capable of
developing birefringence is selected from the group consisting of
polyethylene napthalate, polyethylene terephthalate, polypropylene
and syndiotactic polystyrene.
9. The method of claim 1 wherein the polymeric material capable of
developing birefringence comprises a polyester.
10. The method of claim 1 wherein the polymeric material capable of
developing birefringence comprises a semicrystalline polymer.
11. The method of claim 1 wherein the film further comprises a
polyester/polycarbonate blend.
12. An optical film made by the process of claim 1, wherein the
oriented optical film is a reflective polarizer.
13. An optical film made by the process of claim 12, wherein the
oriented optical film is a multilayer reflective polarizer or a
diffusely reflective polarizer.
14. An optical film made by the process of claim 1, wherein the
oriented optical film is a compensator.
15. The method of claim 1 wherein the step of providing a film
comprises providing a roll of film.
16. The method of claim 1 wherein the step of providing a film
comprises providing the film from an extruder.
17. The method of claim 1 wherein: the step of providing a film
comprises providing a film having an initial refractive index in a
stretch direction, an initial refractive index in an in-plane
direction orthogonal to the stretch direction, and an initial
refractive index in a thickness direction; and the step of heat
setting results in a final refractive index in the in-plane
direction orthogonal to the stretch direction being greater than
the initial refractive index in the in-plane direction orthogonal
to the stretch direction.
18. The method of claim 17 wherein the step of heat setting results
in a final refractive index in the thickness direction being
greater than the initial refractive index in the thickness
direction.
19. The method of claim 1 wherein: the step of providing a film
comprises providing a film having an initial refractive index in a
stretch direction, an initial refractive index in an in-plane
direction orthogonal to the stretch direction, and an initial
refractive index in a thickness direction; and the step of heat
setting results in a final refractive index in the thickness
direction being greater than the initial refractive index in the
thickness direction.
20. The method of claim 1 wherein: the step of providing a film
comprises providing a film having an initial difference between an
initial refractive index in an in-plane direction orthogonal to a
stretch direction and an initial refractive index in a thickness
direction; and the step of heat setting results in a final
difference between a final refractive index in the in-plane
direction orthogonal to the stretch direction and a final
refractive index in the thickness direction, wherein the final
difference is less than the initial difference.
21. The method of claim 1 wherein: the step of providing a film
comprises providing a film having an initial refractive index in a
stretch direction, an initial refractive index in an in-plane
direction orthogonal to the stretch direction, and an initial
refractive index in a thickness direction; and the step of heat
setting results in a final refractive index in the stretch
direction being greater than the initial refractive index in the
stretch direction.
22. The method of claim 1 wherein: the step of providing a film
comprises providing a film having an initial difference between an
initial refractive index in a stretch direction and an initial
refractive index in an in-plane direction orthogonal to a stretch
direction; and the step of heat setting results in a final
difference between a final refractive index in the stretch
direction and a final refractive index in the in-plane direction
orthogonal to a stretch direction, wherein the final difference is
less than the initial difference.
23. The method of claim 1 wherein the step of substantially
uniaxially orienting the film comprises stretching a non-continuous
portion of the film in a tenter apparatus.
24. The method of claim 1 wherein the step of substantially
uniaxially orienting the film comprises stretching the film in a
transverse direction.
25. The method of claim 1 further comprising: quenching the
oriented film.
26. The method of claim 1 wherein the film comprises a plurality of
layers; and the step of heat setting results in increased adhesion
between adjacent layers.
27. The method of claim 1 wherein: the film has a strain-induced
crystallization point, which is a draw ratio at which significant
strain-induced crystallization occurs; and the step of
substantially orienting the film comprises stretching the film at a
draw ratio greater than the strain-induced crystallization
point.
28. The method of claim 1 wherein the step of substantially
uniaxially orienting the film comprises stretching the film in a
stretch direction at a first draw ratio, and the step of heat
setting comprises stretching the film in the stretch direction at a
second draw ratio, the second draw ratio being greater than the
first draw ratio.
29. The method of claim 1 wherein the step of substantially
uniaxially orienting the film comprises stretching the film in a
stretch direction at a first draw ratio, and the step of heat
setting comprises stretching the film in the stretch direction at a
second draw ratio, the second draw ratio being less than the first
draw ratio.
30. The method of claim 1 wherein the step of substantially
uniaxially orienting the film comprises stretching the film in a
stretch direction at a first draw ratio, and the step of heat
setting comprises stretching the film in the stretch direction at a
second draw ratio, the second draw ratio being about equal to the
first draw ratio.
31. The method of claim 1 wherein the film comprises an initial
density; and the step of heat setting results in the film having a
final density greater than the initial density.
32. The method of claim 1 wherein: the step of providing a film
comprises providing a film having a plurality of layers, at least
one of the layers comprising an amorphous material; and the step of
heat setting comprises retaining an amorphous character of the
layer comprising the amorphous material.
33. The method of claim 32 wherein the amorphous material comprises
polycarbonate or a blend of polycarbonate and copolyester.
34. The method of claim 1 further comprising a second heat setting
step.
35. A method of processing a film, the method comprising: conveying
a film along a machine direction while holding opposing edge
portions of the film; stretching the film by moving the opposing
edge portions along diverging, curvilinear paths to form a
stretched film; and heat setting the stretched film.
36. The method of claim 35, wherein the diverging paths are
substantially parabolic.
37. The method of claim 35, wherein the step of heat setting
comprises heating the film to a temperature above a Tg of at least
one polymeric component thereof.
38. The method of claim 35, wherein the step of heat setting
comprises heating the film to a temperature above a Tg of all
polymeric components thereof.
39. The method of claim 35, wherein the step of heat setting
comprises heating the film to a temperature between a Tg and a
melting point of the film.
40. The method of claim 35, wherein stretching the film comprises
stretching the film to a draw ratio in excess of four.
41. A film made by the process of claim 35.
42. An optical film comprising polyethylene terephthalate, the
polyethylene terephthalate having a crystallinity greater than
33%.
43. The optical film of claim 42 comprising polyethylene
terephthalate, the polyethylene terephthalate having a
crystallinity greater than 36%.
44. The optical film of claim 42 comprising polyethylene
terephthalate, the polyethylene terephthalate having a
crystallinity greater than 39%.
45. An optical film comprising polyethylene napthalate, the
polyethylene napthalate having a crystallinity greater than
28%.
46. The optical film of claim 45 comprising polyethylene
napthalate, the polyethylene napthalate having a crystallinity
greater than 30%.
47. The optical film of claim 45 comprising polyethylene
napthalate, the polyethylene napthalate having a crystallinity
greater than 48%.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a non-provisional application claiming
the benefit of priority from U.S. Provisional Application No.
60/669,865, filed Apr. 8, 2005, and entitled, "Heat Setting Optical
Films."
TECHNICAL FIELD
[0002] The present disclosure relates to heat setting processes for
making optical films, as well as films made by these processes.
BACKGROUND
[0003] Typically, an optical film that functions as a linear
polarizer includes an in-plane block state ("x" direction) and
in-plane pass state ("y" direction) for light normally incident to
the plane of the film. Thus, light normally incident with a linear
polarization state aligned with the x direction is maximally
blocked (i.e. minimally transmitted) and light normally incident
with a linear polarization state aligned with the y direction is
minimally blocked (i.e. maximally transmitted). Light incident
off-normal has intermediate levels of transmittance as a function
of its alignment relative to the film. The axis normal to the plane
of the film is referred to as the "z" direction.
SUMMARY
[0004] The present disclosure describes a method of making an
optical film that includes providing a film, substantially
uniaxially orienting the film, and heat setting the oriented film.
The film includes a polymeric material capable of developing
birefringence.
[0005] The details of one or more embodiments of the present
disclosure are set forth in the accompanying drawings and the
description below. Other features, objects, and advantages of the
present disclosure will be apparent from the description and
drawings, and from the claims.
BRIEF DESCRIPTION OF DRAWINGS
[0006] FIG. 1 is a plot of principal refractive index trends for
PEN;
[0007] FIG. 2 is a schematic top view of a prior art tenter
apparatus used to stretch film;
[0008] FIG. 3 is a perspective view of a portion of film in the
prior art process depicted in FIG. 2 both before and after the
stretching process;
[0009] FIG. 4 is a plot of asymmetric index growth upon heat
setting in PEN;
[0010] FIG. 5 is a plot of principal refractive index trends for
CoPEN;
[0011] FIG. 6 is a plot of principal refractive index trends for
CoPEN;
[0012] FIG. 7 is a schematic illustration of a prior art batch
process for drawing a multilayer optical film showing the film both
before and after the stretch;
[0013] FIG. 8 is a schematic illustration of the stretching process
according to one embodiment of the present disclosure;
[0014] FIG. 9 is a block diagram showing process steps according to
an embodiment of the present disclosure;
[0015] FIG. 10 is a schematic top view of a portion of a stretching
apparatus;
[0016] FIG. 11 is a top view of a portion of the apparatus of FIG.
10;
[0017] FIG. 12 illustrates an end view of an arrangement of
gripping members that may be used in the apparatus of FIG. 10;
[0018] FIG. 13 is a schematic illustration of a portion of the
tracks illustrating one embodiment of a stretching apparatus;
[0019] FIG. 14 is a schematic illustration of one embodiment of
adjustable tracks for a primary stretching region of a stretching
apparatus;
[0020] FIG. 15 is a schematic side cross-sectional view of one
embodiment of tracks and a track shape control unit for a
stretching apparatus;
[0021] FIG. 16 is a schematic view of a portion of the track and
track shape control unit of one embodiment of FIG. 10;
[0022] FIG. 17 is a schematic view of another portion of the track
and track shape control unit of one embodiment of FIG. 10;
[0023] FIG. 18 is a schematic illustration of another embodiment of
tracks for a primary stretching region of a stretching
apparatus;
[0024] FIG. 19 is a graph of examples of suitable boundary
trajectories for a primary stretching region of a stretching
apparatus;
[0025] FIG. 20 is a graph of examples of suitable boundary
trajectories for a primary stretching region of a stretching
apparatus illustrating the use of different stretching regions with
different parabolic configurations;
[0026] FIG. 21 is a graph of examples of suitable boundary
trajectories for a primary stretching region of a stretching
apparatus including boundary trajectories that are linear
approximations to suitable parabolic or substantially parabolic
boundary trajectories;
[0027] FIG. 22 is a schematic illustration of one embodiment of a
take-away system for a stretching apparatus according to the
present disclosure;
[0028] FIG. 23 is a schematic illustration of another embodiment of
a take-away system for a stretching apparatus;
[0029] FIG. 24 is a schematic illustration of a third embodiment of
a take-away system for a stretching apparatus;
[0030] FIG. 25 is a schematic illustration of a fourth embodiment
of a take-away system for a stretching apparatus;
[0031] FIG. 26 is a schematic illustration of a fifth embodiment of
a take-away system for a stretching apparatus;
[0032] FIG. 27 is a schematic illustration of one embodiment of a
take-away system, for using in, for example, a conventional
stretching apparatus such as that illustrated in FIG. 2; and
[0033] FIG. 28 is a plot of refractive index trends with
composition.
[0034] While the above-identified drawing figures set forth several
exemplary embodiments of the disclosure, other embodiments are also
contemplated. This disclosure presents illustrative embodiments of
the present invention by way of representation and not limitation.
Numerous other modifications and embodiments can be devised by
those skilled in the art which fall within the scope and spirit of
the principles of the present disclosure. The drawing figures are
not drawn to scale.
[0035] Moreover, while embodiments and components are referred to
by the designations "first," "second," "third," etc., it is to be
understood that these descriptions are bestowed for convenience of
reference and do not imply an order of preference. The designations
are presented merely to distinguish between different embodiments
for purposes of clarity.
[0036] Unless otherwise indicated, all numbers expressing feature
sizes, amounts, and physical properties used in the specification
and claims are to be understood as being modified in all instances
by the term "about." Accordingly, unless indicated to the contrary,
the numbers set forth are approximations that can vary depending
upon the desired properties using the teachings disclosed
herein.
DETAILED DESCRIPTION
[0037] Many optical films used in polarizer or polarizing film
applications suffer from an asymmetry, e.g. a refractive index
difference, between "y" and "z" principal directions. For example,
off-axis color, that is, color variations in the pass state as a
function of off-normal angle of incidence, can be amplified or
created by a mismatch between the y and z refractive indices, ny
and nz, respectively. (Here, nx is the refractive index for light
polarized along the x direction.).
[0038] An illustration of the space of typical refractive index
sets (nx, ny, nz), for a film or a layer of film consisting of 2,6
polyethylene napthalate (PEN), is provided in FIG. 1. The
development of the refractive index set as a function of draw ratio
at an illustrative stretch temperature of 130.degree. C. is
exemplified by the data with the solid markings. The films were
stretched in a laboratory scale stretching device from initially
unoriented cast samples. The samples were stretched in one in-plane
direction while constraining the film in the other in-plane
direction at selected gripping points along the edge, with a
nominal initial rate of 20%/sec using a stretch profile that
increased the nominal draw ratio linearly in time. The true final
draw ratio was measured using fiducial line markings marked upon
the sample across the location where the refractive indices were
measured. For illustration purposes, the refractive index was
measured at 632.8 nm using a Metricon Prism Coupler (available from
Metricon, Picataway N.J.), the red light source provided by He--Ne
laser.
[0039] FIG. 2 illustrates a conventional tenter stretching process
10 that stretches film 12 transversely to the direction of film
travel 14. Film 12 may be continuously fed or introduced as a
non-continuous portion of film 12. The film travel direction is
referred to as the machine direction (MD), and the stretch
direction is referred to as the transverse or tenter direction
(TD). The film 12 is gripped at both edges 16 by some gripping
apparatus, typically an arrangement of tenter clips (not shown in
FIG. 2). The tenter clips can be connected to tenter chains that
ride along linearly diverging tenter tracks or rails. This
arrangement propels the film forward in a machine direction 14 of
film travel and stretches the film 12 in the transverse direction.
Thus an initial, unoriented portion 18 in the film may be stretched
into a final, oriented portion 20 in one example. As shown in FIG.
3, the unoriented portion 18 of the film shown in FIG. 2 may have
dimensions T (thickness), W (width) and L (length). After the film
is stretched by a factor of .lamda., the dimensions of that portion
of film have changed to those shown on portion 20.
[0040] While the data of FIG. 1 derive from a batch stretching
device with discontinuous edge grips both along the x and y
directions, these data exemplify a typical film stretched in the
conventional tenter process of FIG. 2: one-directional stretching,
in which the film 12 is stretched in a first in-plane principal
direction (x) while the second in-plane direction (y) is maintained
at constant or nearly constant draw ratio, e.g., the y direction
draw ratio is nearly unity. In one embodiment, the continuous film
is fed at a constant rate into a transversely stretching device and
exits the device at the same constant rate.
[0041] FIG. 1 demonstrates the asymmetry in the refractive indices,
e.g. the differences in the ny and nz, that develops due to the
asymmetric treatment of the stretch on the material in the y and z
directions. In this illustrated case, the draw ratio remains nearly
constant in y while the draw ratio in z decreases with increasing
draw ratio in x (e.g. as required by near volume conservation as
weakly adjusted due to densification with crystallization).
[0042] When the film is heat set following the stretching process,
the asymmetry is further increased. This situation is exemplified
by the open markers in FIG. 1. In this case, the film was heat set
for 2 minutes at 175.degree. C.
[0043] FIG. 1 shows that the nx and ny values tend to increase
under these heat setting conditions, while the nz values drop, for
samples stretched above a critical draw ratio level for the given
stretch temperature and rate (or equivalently, for a given level of
initial refractive index development).
[0044] FIG. 4 further illustrates how heat setting increases the
differences between the ny and nz refractive indices. The square
markers indicate the index difference after stretch without heat
setting whereas the triangular markers indicate the index
difference after the stretching and heat setting. The offset
increase in asymmetry is nearly constant, with a small decreasing
slope with increasing x draw ratio, as depicted by the diamond
markers. Therefore, when y/z "asymmetric" films, e.g. films with
significant differences in ny and nz immediately following stretch,
such as those stretched in a conventional tenter (FIGS. 2-3), are
heat set following the stretching step, the asymmetry in refractive
index increases. Such a difference in the refractive indices in the
y & z directions can lead to an undesirable color effect in
some applications.
[0045] The general trends shown in FIGS. 1 and 4 are applicable to
a variety of polyesters. Of particular interest are PEN,
polyethylene terephthalate (PET) and copolymers of intermediate
composition. FIG. 5 illustrates the case of 85/15 mole percent
mixture of PEN-like and PET-like moieties in the co-polymer, a
so-called "85/15 coPEN." The term "PEN-like" moiety includes block
copolymers of PEN. The term "PET-like" moiety includes block
copolymers of PET. The methods of stretch are nearly identical to
those of the PEN in FIG. 1, except the stretch temperature was set
at 120.degree. C.
[0046] As shown in FIG. 5, in some embodiments, heat setting allows
one to achieve a given refractive index at a different draw ratio
than without heat setting. For example, if one desires an nx of
about 1.8, one could either use a 4.5 draw ratio or use a lower 2.5
draw ratio and then heat set the film; both processes lead to nx
equal to about 1.8. As another example, if one desires an nz of
about 1.54, one could either use a 4.25 draw ratio or use a lower
2.5 draw ratio and then heat set the film; both processes lead to
nz equal to about 1.54.
[0047] Under the conditions of FIG. 5, the effective point of
significant strain-induced crystallization is at an x draw ratio of
about 2.2. FIG. 6 illustrates the sharpness of this transition,
with materials stretched to an x draw ratio of about 2.1 exhibiting
relaxation to isotropy after heat setting. Below an x draw ratio of
about 2.1, nx=ny=nz after heat setting. Above an x draw ratio of
about 2.2, index development leads to a result of nx>ny>nz
after heat setting. This point will shift in different examples,
depending on factors such as the selected materials and processing
conditions.
[0048] Use of a parabolic tenter (discussed with reference to FIG.
10, below) can lead to uniaxial stretching at relatively high draw
ratios in some embodiments. Use of other machines and processes can
lead to uniaxial stretching at lower draw ratios, but the resulting
film may lack the desired level of refractive index development.
Heat setting can be used to achieve the desired level of index
development, as discussed with reference to FIGS. 5 and 6. In some
cases, lower draw ratios are used with certain films, such as films
including microstructures, because higher draw ratios could damage
the microstructures. In these and other cases, heat setting can
also be used to promote refractive index development in the
stretched films.
[0049] Commonly owned U.S. Pat. Nos. 6,939,499; 6,916,440;
6,949,212; and 6,936,209; incorporated herein by reference,
describe continuous processes for processing optical films, such as
multilayer optical films. In such processes, the optical film is
oriented by stretching along a first in-plane axis of the film (x
direction) while allowing contraction of the film in the second
in-plane axis (y or machine direction (MD)) and in the thickness (z
or normal direction (ND)) of the film. The stretching is achieved
by grasping edge portions of the film and moving the edge portions
of the film along predetermined paths that diverge to create
substantially similar proportional dimensional changes in the
second in-plane axis of the film (y) and in the thickness direction
(z) of the film.
[0050] In exemplary embodiments, in contrast to the heat set
behavior of conventional one-direction stretched materials, which
have significant differences in ny and nz immediately following
stretching, the heat setting of substantially uniaxially stretched
films, in which contraction is allowed in the y and z directions to
minimize differences in ny and nz, has a completely different
effect. Heat setting following a substantially uniaxial stretching
process maintains or decreases any small existing refractive index
asymmetry of these films. Thus, where the refractive indices in the
y & z directions become more equal, fewer problems with
undesirable color effects arise.
[0051] The heat setting procedures described below may be applied
following any process that provides substantially uniaxial
stretching of an optical film such as, for example, a multilayer
optical film (MOF). The heat setting procedures described in this
disclosure are particularly useful for substantially uniaxially
stretched films including one or more polyester layers.
[0052] FIG. 7 illustrates a batch technique 22 for substantially
uniaxially stretching an optical film such as, for example, a
multilayer optical film, suitable for use as a component in an
optical body such as a polarizer. The film 24 is stretched in the
direction of the arrows 26, and the central portion 28 necks down
so that two edges 30 of the film 24' are no longer parallel after
the stretching process. The central portion 28 of the film 24'
provides the most useful optical properties because it is far
enough removed from the shear boundary conditions to experience low
levels of shear aberrations such as caliper variation.
[0053] While the batch process described in FIG. 7 may in some
cases provide suitable film properties, the substantially uniaxial
stretching processes described in commonly owned U.S. Pat. Nos.
6,939,499; 6,916,440; 6,949,212; and 6,936,209; all incorporated
herein by reference, are particularly suitable in some
embodiments.
[0054] In general, uniaxial orientation of a birefringent polymer
provides an optical film (or layer of a film) in which the index of
refraction in two of three orthogonal directions is substantially
the same (for example, the width (W) and thickness (T) direction of
a film, as illustrated in FIG. 8). The index of refraction in the
third direction (for example, along the length (L) direction of the
film) is different from the indices of refraction in the other two
directions. Typically, perfect uniaxial orientation is not required
and some degree of deviation from the optimal conditions can be
allowed depending on a variety of factors including the end-use
application of the optical film. Moreover, it should be understood
that while the present disclosure refers to three "orthogonal
directions," the corresponding directions may not be exactly
orthogonal due to non-uniformities in the film.
[0055] In general, the substantially uniaxial stretching process
includes stretching a film that can be described with reference to
three mutually orthogonal axes corresponding to the machine
direction (MD), the transverse direction (TD), and the normal
direction (ND). These axes correspond to the width, length, and
thickness of the film, as illustrated in FIG. 8. A substantially
uniaxial stretching process stretches film 32 from an initial
configuration 34 to a final configuration 36. The machine direction
(MD) is the general direction along which the film travels through
a stretching device, for example, the apparatus as illustrated in
FIG. 10. The transverse direction (TD) is the second axis within
the plane of the film and is orthogonal to the machine direction.
The normal direction (ND) is orthogonal to both MD and TD and
corresponds generally to the thickness dimension of the polymer
film.
[0056] FIG. 9 is a block diagram of a typical substantially
uniaxial stretching process 38 as described in this disclosure. In
step 40, a film is supplied or provided to a stretching apparatus.
The process optionally includes a preconditioning step 42. The film
is stretched in step 44. The film is post-conditioned in step 46.
The film is removed from the stretching apparatus in step 48.
[0057] FIG. 10 illustrates one embodiment of a substantially
uniaxial stretching process and an apparatus 50 for achieving the
substantially uniaxial stretch. This process may be used with the
heat setting procedures described in this disclosure. It will be
recognized that the process illustrated by FIG. 9 can be
accomplished using one or more apparatuses apart from a stretching
apparatus (which at a minimum performs step 44 of FIG. 9). These
one or more additional apparatuses may perform one or more of the
functions (for example, functions represented by steps 40, 42, 46
and 48 illustrated in FIG. 9.
[0058] In the illustrated embodiment of FIG. 10, the apparatus 50
includes a region 52 where the film 32 is introduced into the
stretching apparatus 50. The film 32 can be provided by any
desirable method. For example, the film 32 can be produced in a
roll or other form and then provided to stretching apparatus 50. As
another example, the stretching apparatus 50 can be configured to
receive the film 32 from an extruder (if, for example, the film 32
is generated by extrusion and ready for stretching after extrusion)
or a coater (if, for example, the film 32 is generated by coating
or is ready for stretching after receiving one or more coated
layers) or a laminator (if, for example the film 32 is generated by
lamination or is ready for stretching after receiving one or more
laminated layers).
[0059] Generally, the film 32 is presented in region 52 to one or
more gripping members that hold opposing edges of the film and
convey the film along opposing tracks 54 defining conveyance paths.
The gripping members (see FIG. 12, for example) typically hold the
film 32 at or near the edges of the film 32. The portions of the
film 32 held by the gripping members are often unsuitable for use
after stretching, so the position of the gripping members is
typically selected to provide sufficient grip on the film 32 to
permit stretching while controlling the amount of waste material
generated by the process.
[0060] Gripping members, such as clips, can be directed along the
track 54 by, for example, rollers 56 rotating a chain along the
track 54 with the gripping members coupled to the chain. The
rollers 56 are connected to a driver mechanism that controls the
speed and direction of the film 32 as it is conveyed through the
stretching apparatus 50. Rollers 56 can also be used to rotate and
control the speed of belt-type gripping members. The belts and
rollers 56 optionally include interlocking teeth to reduce or
prevent slippage between the belt and roller 56.
[0061] The apparatus 50 optionally includes a preconditioning
region 58 that in one embodiment is enclosed by an oven 60 or other
apparatus or arrangement to heat the film 32 in preparation for
stretching. The preconditioning region 58 can include a preheating
zone 62, a heat soak zone 64, or both. In at least some
embodiments, there may be a small amount of film stretching that
occurs in order to set the contact between the gripping members and
the film, as illustrated by the boundary trajectory of FIG. 13. In
at least some instances, there may not actually be any stretching
but the increase in separation between the opposing tracks may
account, at least in part, for thermal expansion of the film 32 as
the film 32 is heated.
[0062] The film 32 is stretched in the primary stretching region
66. Typically, within the primary stretching region 66 the film
zone undergoing stretching 68 is heated or maintained in a heated
environment above the glass transition of the polymer(s) of the
film 68. For polyesters, the temperature range is typically between
about 80.degree. C. and about 160.degree. C. Examples of suitable
heating elements include convective and radiative heating elements,
although other heating elements can also be used. In some
embodiments, the heating elements used to heat the film 32 can be
controlled individually or in groups to provide a variable amount
of heat. Such control can be maintained by a variety of processes
including those that allow for variability in the temperature of
the heating elements or in the direction or speed of air directed
from the heating element to the film 68. The control of the heating
elements can be used, if desired, to variably heat regions of the
film 68 to improve or otherwise alter uniformity of stretching
across the film 68. For example, areas of the film 68 that do not
stretch as much as other areas under uniform heating can be heated
more to allow easier stretching.
[0063] Within the primary stretching region 66, the gripping
members follow generally diverging tracks 54 to stretch the polymer
film 68 by a desired amount. The tracks 54 in the primary
stretching region 66 and in other regions of the apparatus 50 can
be formed using a variety of structures and materials. Outside of
the primary stretching region 66, the tracks 54 are typically
substantially linear. The opposing linear tracks 54 can be parallel
or can be arranged to be converging or diverging. Within the
primary stretching region 66, the tracks 54 are generally diverging
and are generally curvilinear. In some exemplary embodiments, the
generally curvilinear shapes of the tracks may be approximated
using linear track segments.
[0064] In one example, the apparatus 50 typically includes a
post-conditioning region 70. For example, the film 32 may be heat
set in zone 72 and quenched in zone 74. The film 32 is quenched
when all components reach a temperature level below their glass
transition temperatures. In some other embodiments, quenching is
performed outside the stretching apparatus 50.
[0065] In the embodiment illustrated in FIG. 10, a takeaway system
is used to remove the film 32 from the primary stretching region
66. In the illustrated embodiment, this takeaway system is
independent of (i.e., not directly connected to) the tracks 54 upon
which the film 32 was conveyed through the primary stretching
region 66.
[0066] For the purposes of this disclosure, the term heat set
refers to a heating protocol in which the film 32 is heated
following orientation to enhance film properties such as, for
example, crystal growth, dimensional stability, and/or overall
optical performance. The heat setting is a function of both
temperature and time, and factors must be considered such as, for
example, commercially useful line speed and heat transfer
properties of the film, as well as the optical clarity of the final
product. In an exemplary embodiment, the heat setting process
involves heating the film 32 to above the glass transition
temperature (Tg) of at least one polymeric component thereof, and
preferably above the Tg of all polymeric components thereof.
Exemplary polymeric materials include PEN, PET, coPENS,
polypropylene and syndiotactic polystyrene. In one embodiment of
the heat setting process, the film 32 is heated above the stretch
temperature of the film 32, although this is not required. In
another embodiment, in the heat setting process the film 32 is
heated to a temperature between the Tg and the melting point of the
film 32.
[0067] In general, there is an optimal temperature for the rate of
crystallization that results from a balance of the kinetic and
thermodynamics of the system. This temperature is useful when
minimization of the heat set time is a primary consideration. A
typical starting point for tuning the conditions to find the best
balance between the various product and process considerations is
about halfway between the Tg and the melting point of the film 32.
For example, the glass transition temperatures for PET and PEN are
approximately 80.degree. C. and 120.degree. C., respectively, under
dry conditions. The glass transition temperatures of copolymers of
intermediate compositions of PET and PEN (so-called "coPENs") are
intermediate between those of the homopolymers. The melting points
cover a range of temperatures due to the range of imperfections in
the physical crystals due to their size and constraints. A rough
estimate for the melting points of PET and PEN is about 260.degree.
C. for PET and about 270.degree. C. for PEN. The melting points of
the so-called coPENs are typically less than those of the
homopolymers and can be measured approximately, for example by
Differential Scanning Calorimetry (DSC).
[0068] Thus, the starting point range for heat setting in PET and
PEN is, for example, between about 170 and 195.degree. C. Actual
process setpoints depend on residence times and heat transfer
within a given process. Residence times may range from about 1
second to about 10 minutes and depend not only on process
conditions but also the desired final effect, for example, the
amount of crystallinity, the increase in delamination resistance,
and optimization of haze given other properties. Minimizing the
residence time is often useful for considerations such as
minimizing equipment size. Higher temperatures may reduce the
required time to attain a certain level of crystallinity. However,
higher temperatures also may cause melting of imperfect crystalline
structures that may then re-form into larger structures. This may
produce unwanted haze for some applications.
[0069] In one embodiment, the portions of the film that were held
by the gripping members through the primary stretching region 66
are removed. To maintain a substantially uniaxial stretch
throughout substantially all of the stretch history (as shown in
FIG. 10), at the end of the transverse stretch, the rapidly
diverging edge portions 76 are preferably severed from the
stretched film 68 at a slitting point 78. A cut can be made at 78
and flash or unusable portions 76 can be discarded.
[0070] In one embodiment, the process also includes a removal
region 80. Optionally a roller 82 is used to advance the film, but
this may be eliminated. In one embodiment, the roller 82 is not
used as it would contact the final film 84 with the attendant
potential to damage the final film 84. In one embodiment, another
cut 86 is made and unused portion 88 is discarded.
[0071] The removal region 80 may also include an optional isolation
zone (not shown in FIG. 10) in which the film temperature is
controlled to reduce and/or eliminate undesirable film properties
such as bowing. In the isolation zone, the film may be wound onto a
roll, but winding is not required. Following removal from the
optional isolation zone, the film may optionally be coated or
laminated, or subjected to processing to impart a surface texture
or surface structures.
[0072] In one embodiment, direct converting to a finished product
takes place after take-away. In another embodiment, film 84 leaving
the take-away system is typically wound on rolls for later use. In
one example, the film 84 may be unwound and transferred to an
optional second heating unit (not shown in FIG. 10). In the second
heating unit, the film 84 may be gripped and placed under tension
as needed to prevent wrinkling. This process typically takes place
at a temperature below the original stretch temperature applied in
the stretching zone 66. The second heating unit may simply be an
oven where the film 84 may be placed in roll or sheet form to
enhance its properties. For example, a second heat soak procedure
may be applied in the second heating zone in which the film 84 is
heated to a temperature below the Tg of at least one film
component, preferably below the Tg of all film components. Again,
the second heat soak is typically performed below the initial
stretch temperature applied to the film 84 in the stretching zone
66. The second heat soak may continue for an extended period such
as, for example, hours or days, until the desired film properties
such as shrinkage resistance, or creep resistance are achieved. For
example, heat soak for PET is typically performed at about
50-75.degree. C. for several hours to days, while heat soak for PEN
is typically performed at about 60-115.degree. C. for several hours
to days. Heat soaking can also be achieved in part under some
post-processing activities. For example, the film 84 may be coated
and dried or cured in an oven with some heat soaking effect.
[0073] Following the second heating zone, the film 84 may
optionally be transferred to a second quench and/or set zone (not
shown in FIG. 10). In the second quench and/or set zone, the film
84 may be placed under tension and/or toed-in along converging
rails to control shrinkage and warping. Following the optional
second quench and/or set zone, the film may be re-rolled.
[0074] FIGS. 11-12 illustrate one embodiment of the gripping
members and track. One example of suitable gripping members 90
includes a series of clips that sequentially grip the film 32
between opposing surfaces and then travel around a track 54. The
gripping members can nest or ride in a groove or a channel along
the track 54. Another example is a belt system that holds the film
32 between opposing belts or treads, or a series of belts or
treads, and directs the film 32 along the track 54. Belts and
treads can, if desired, provide a flexible and continuous, or
semi-continuous, film conveyance mechanism. A variety of opposing,
multiple belt methods are described, for example, in U.S. Pat. No.
5,517,737 and in European Patent Application Publication No.
0236171 A1 (the entire contents of each of which are herein
incorporated by reference). The tension of the belts is optionally
adjustable to obtain a desired level of gripping.
[0075] A belt or clip can be made of any material. For example, a
belt can be of composite construction. One example of a suitable
belt includes an inner layer made of metal, such as steel, to
support high tension and an outer layer of elastomer to provide
good gripping. Other belts can also be used. In some embodiments,
the belt includes discontinuous tread to provide good gripping.
[0076] Other methods of gripping and conveying the film through a
stretcher are known and may be used. In some embodiments, different
portions of the stretching apparatus can use different types of
gripping members 90.
[0077] The gripping members 90 of the embodiment illustrated in
FIGS. 11 and 12 are a series of tenter clips. These clips can
afford overall flexibility via segmentation. The discrete clips are
typically closely packed and attached to a flexible structure such
as a chain. The flexible structure rides along or in channels along
the track 54. Strategically placed cams and cam surfaces open and
close the tenter clips at desired points. The clip and chain
assembly optionally rides on wheels or bearings or the like. In one
example, the gripping members 90 are tenter clips mounted on top
and bottom bearings rolling between two pairs of inner and outer
rails. These rails form, at least in part, the track 54.
[0078] The edges of the gripping members 90 define a boundary edge
for the portion of the film 32 that will be stretched. The motion
of the gripping members 90 along the tracks 54 provides a boundary
trajectory that is, at least in part, responsible for the motion
and stretching of the film 32. Other effects (e.g., downweb tension
and take-up devices) may account for other portions of the motion
and stretching. The boundary trajectory is typically more easily
identified from the track 54 or rail along which the gripping
members 90 travel. For example, the effective edge of the center of
the gripping member 90, e.g. a tenter clip, can be aligned to trace
the same path as a surface of the track 54 or rail. This surface
then coincides with the boundary trajectory. In practice, the
effective edge of the gripping members 90 can be somewhat obscured
by slight film slippage from or flow out from under the gripping
members 90, but these deviations can be made small.
[0079] In addition, for gripping members 90 such as tenter clips
the length of the edge face can influence the actual boundary
trajectory. Smaller clips will in general provide better
approximations to the boundary trajectories and smaller stretching
fluctuations. In at least some embodiments, the length of a clip
face edge is no more than about one-half the total initial distance
between the opposing boundary trajectories or tracks. In a
particularly suitable embodiment, the length of a clip face edge is
no more than about 1/4 the total initial distance between the
opposing boundary trajectories or tracks.
[0080] The two opposing tracks 54 are optionally disposed on two
separate or separable platforms or are otherwise configured to
allow the distance between the opposing tracks 54 to be adjustable.
This can be particularly useful if different sizes of film 32 are
to be stretched by the apparatus 50 or if there is a desire to vary
the stretching configuration in the primary stretching region 66,
as discussed below. Separation or variation between the opposing
tracks 54 can be performed manually, mechanically (for example,
using a computer or other device to control a driver that can alter
the separation distance between the tracks 54), or both.
[0081] Since the film 32 is held by two sets of opposing gripping
members 90 mounted on opposing tracks 54, there are two opposing
boundary trajectories. In at least some embodiments, these
trajectories are mirror images about an MD center line of the
stretching film 32. In other embodiments, the opposing tracks 54
are not mirror images. Such a non-mirror image arrangement can be
useful in providing a variation (for example, a gradient or
rotation of principal axes) in one or more optical or physical
properties across the film 32.
[0082] FIG. 13 illustrates one embodiment of a supply region 52
followed by a preconditioning region 58 and primary stretching
region 66. Within the preconditioning region 58 (or optionally in
the supply region 52) a gripping member set zone 92 is provided in
which the tracks 54 diverge slightly to set the gripping members 90
(for example, tenter clips) on the film 32. The film 32 is
optionally heated within this zone 92. In one embodiment, this
initial TD stretch is no more than about 5% of the final TD stretch
and generally less than about 2% of the final TD stretch and often
less than about 1% of the final TD stretch. In some embodiments,
the zone 92 in which this initial stretch occurs is followed by a
zone 94 in which the tracks 54 are substantially parallel and the
film 32 is heated or maintained at an elevated temperature.
[0083] In all regions of the stretching apparatus 50, the tracks 54
can be formed using a series of linear or curvilinear segments that
are optionally coupled together. The tracks 54 can be made using
segments that allow two or more (or even all) of the individual
regions to be separated (for example, for maintenance or
construction). As an alternative or in particular regions or groups
of regions, the tracks 54 can be formed as a single continuous
construction. The tracks 54 can include a continuous construction
spanning one or more adjacent regions 52, 58, 66, 70, 80 of the
stretcher 50. The tracks 54 can have any combination of continuous
constructions and individual segments.
[0084] In some embodiments, the tracks 54 in the primary stretching
region 66 are coupled to, but separable from, the tracks 54 of the
preceding regions. In some embodiments, the tracks in the
succeeding post-conditioning or removal regions 70, 80 are
typically separated from the tracks 54 of the primary stretching
region 66, as illustrated, for example, in FIGS. 22-27.
[0085] Although the tracks in the primary stretching region 66 are
curvilinear in FIG. 10, linear track segments can also be used in
some embodiments. In one embodiment, these segments are aligned
(by, for example, pivoting individual linear segments about an
axis) with respect to each other to produce a linear approximation
to a desired curvilinear track configuration. Generally, the
shorter the linear segments are, the better the curvilinear
approximation can be made. In some embodiments, the positions of
one or more, and preferably all, of the linear segments are
adjustable (pivotable about an axis) so that the shape of the
tracks 54 can be adjusted if desired. Adjustment can be manual or
the adjustment can be performed mechanically, such as under control
of a computer or other device coupled to a driver. It will be
understood that curvilinear segments can be used instead of or in
addition to linear segments.
[0086] Continuous tracks 54 can also be used through each of the
regions 52, 58, 66, 70, 80. In particular, a continuous,
curvilinear track 54 can be used through the primary stretching
region 66. The continuous, curvilinear track 54 typically includes
at least one continuous rail that defines the track 54 along which
the gripping members 90 run. In one embodiment, the curvilinear
track 54 includes two pairs of inner and outer rails with tenter
clips mounted on top and bottom bearings rolling between the four
rails.
[0087] In some embodiments, the continuous track 54 is adjustable.
One method of making an adjustable continuous track 54 includes the
use of one or more track shape control units. These track shape
control units are coupled to a portion of the continuous track 54,
such as the continuous rail, and are configured to apply a force to
the track 54 as required to bend the track 54. FIG. 14
schematically illustrates one embodiment of such an arrangement
with the track shape control units 96 coupled to the track 54.
Generally, the track shape control units 96 have a range of forces
that the track shape control unit 96 can apply, although some
embodiments may be limited to control units 96 that are either on
or off.
[0088] The track shape control units 96 can typically apply a force
toward the center of the film 32 or apply a force away from the
center of the film 32 or, preferably, both. The track shape control
units 96 can be coupled to a particular point on the adjustable
continuous track 54 or the track shape control units 96 can be
configured so that the track 54 can slide laterally along the
control unit 96 while still maintaining coupling between the track
54 and control unit 96. This arrangement can facilitate a larger
range of motion because it allows the track 54 to more freely
adjust as the control units 96 are activated. Generally, the track
shape control units 96 allow the track 54 to move through a range
of shapes that deviate from the equilibrium shape of the track 54,
for example, shapes 54 and 54' of FIG. 14. The equilibrium and the
adjusted shapes of the tracks may be linear or curvilinear.
Typically, the track shape control unit 96 and the track 54 can
move along a line (or other geometric shape) of motion 98. When
more than one track shape control unit 96 is used, the track shape
control units 96 can have the same or similar lines of motion and
ranges of motion 98 or the lines and ranges of motions 98 for the
individual track shape control units 96 can be different.
[0089] In some embodiments, one or more points 100 of the track are
fixed. The fixed points 100 can be anywhere along the track 54
including at or near the start (as illustrated in FIG. 14) or end
of the primary stretching region 66. The fixed points 100 can also
be positioned at other points along the track 54 as illustrated in
FIG. 18.
[0090] One example of a suitable track shape control unit 96 and
track 54 is illustrated in FIG. 15. The track 54 in this embodiment
includes four rails 102 with tenter clips (not shown) mounted on
bearings (not shown) rolling between the four rails 102. The track
shape control unit 96 includes a base 104 that is coupled to a
driver (not shown), top and bottom inner contact members 106, and
top and bottom outer contact members 108. The inner and outer
contact members 106, 108 are coupled to the base 104 so that moving
the base 104 allows the contact members 106, 108 to apply a force
to inner and outer surfaces of the rails 102, respectively.
[0091] In exemplary embodiments, the inner contact members 106 have
a shape, when viewed from above or below, that provides only small
areas of contact between the contact members 106, 108 and the rails
102, as illustrated in FIGS. 16 and 17 (FIG. 16 shows the rails 102
and inner contact member 106). Examples of such shapes include
circular and ovoid, as well as diamond, hexagonal, or other similar
shapes where contact between the inner contact members 106 and the
rails 102 is made at the apex of these shapes. The outer contact
members 108 can be similarly fashioned so that the portion of the
outer contact member 108, when viewed from above or below, comes to
a point to make contact with the rails 102, as illustrated in FIG.
17 (FIG. 17 shows the rails 102 and the portion of the outer
contact member 108 that makes contact with the rails 102). Using
such shapes allows the track shape control unit 96 to exert a
force, if desired, to modify the track shape while allowing the
track 54 to slide laterally through the control unit 96 rather than
being fixed to the control unit 96. This configuration can also
allow the track 54 to adjust its instantaneous slope within the
control unit 96. For one or both of these reasons, the track 54 can
have a larger range of shape adjustment. In other embodiments,
there can be fewer or more contact members 106, 108 or there may be
only inner or only outer contact members 106, 108.
[0092] As further illustrated in FIG. 18, the tracks 54 can be
configured to provide zones 110, 112, 114 within the primary
stretching region 66 that have different stretching characteristics
or that can be described by different mathematical equations. In
some embodiments, the tracks 54 have a shape that defines these
different zones 110, 112, 114. In other embodiments, the tracks 54
can be adjusted, using for example the track shape control units 96
discussed above, to provide a variety of shapes 116, 118 beyond
simple, monofunctional arrangements. This can be advantageous
because it allows different portions of the primary stretching
region 66 to accomplish desired functions. For example, an initial
stretching zone may have a particular shape (for example, a
super-uniaxial shape with U>1 and F>1 as described below)
followed by one or more later zones with different shapes (for
example, a uniaxial shape). Optionally, intermediate zones can be
provided that transition from one shape to another. In some
embodiments, the individual zones 110, 112, 114 can be separated or
defined by points 100 of the track 54 that are fixed.
[0093] In some embodiments, the track 54 has a non-uniform
cross-sectional shape along the length of the track 54 to
facilitate bending and shaping of the track 54. For example, one or
more rails 102 used in the track 54 can have different
cross-sectional shapes. As an example, in the four-rail
construction described above, each of the rails 102, or a subset of
the rails 102, has a varied cross-section along the length of the
track 54. The cross-section can be varied by, for example, altering
either the height or thickness of the track 54 (or a component of
the track 54 such as one or more continuous rails 102) or both. As
an example, in one embodiment the thickness of the track 54 or one
or more rails 102 in the track 54 decreases or increases along the
length of the track 54 in the machine direction. These variations
can be used to support a particular track shape or a variation in
track shape adjustability. For example, as described above, the
track 54 may have several different zones 110, 112, 114, each zone
110, 112, 114 having a different track shape 54. The
cross-sectional variation of the track 54 or component of the track
54 can vary within each zone 110, 112, 114 to achieve or facilitate
a particular rail 102 shape and can vary between zones 110, 112,
114. As an example, a zone 112 with a relatively thick
cross-sectional shape can be disposed between two other zones 110,
114 to isolate or provide a transitional space between the two
zones 110, 114.
[0094] As an example of variation in track 54 or rail 102
cross-section, the arclength, s, can be used to represent a
position along the track 54 in the design of the thickness profile
of a track 54 or portion of a track, such as a rail 102. The
arclength, s, at the start of stretch is defined as zero and at the
other end of the stretch is defined as L with corresponding
thicknesses at the beginning and end of stretch being designated as
h(0) and h(L), respectively. The track 54 or track component (e.g.,
rail 102) in this particular embodiment has a taper over a portion
of the beam from L' to L'' between s=0 and s=L such that the
thickness h(L') at position L' is greater than the thickness h(L'')
at position L''. In this manner, either L' or L'' may be at the
higher arclength coordinate (i.e., L'>L'' or L'<L''). One
example of a useful thickness profile is a taper given by the
function for thickness, h(s), as a function of arclength s over the
rail 102 from L' to L'', provided by the equation:
h(s)=(h(L')-h(L''))(1-(s-L')/(L''-L')).sup..alpha.+h(L'') where
.alpha. is the positive rate of taper resulting in decreasing
thickness from L' to L''.
[0095] When L' is less than L'', this results in a decreasing
thickness with arclength. When L' is greater than L'' this results
in an increasing thickness with arclength. The track 54 can
optionally be apportioned into sections, each with its own local
L', L'' and rate of taper. The maximum thickness of the track 54 or
track component, such as a rail, depends on the amount of
flexibility desired at that point on the track 54. Using beam
theory as applied to a track or rail, it can be shown that in the
case of a straight beam with a taper, a value for .alpha. of one
third provides a beam that bends parabolically in response to a
load at one end. When the beam begins in a curved equilibrium
configuration or is loaded by several control points, other tapers
may be more desirable. For transformation across a variety of other
shapes, it may be useful to have both increasing and decreasing
thickness within a given track 54 or track component, or
numerically calculated forms of the taper over any of these
sections. The minimum thickness at any point along the track 54 or
track component depends on the amount of required strength of the
track 54 to support the stretching forces. The maximum thickness
can be a function of the level of needed flexibility. It is
typically beneficial to maintain the level of track adjustment
within the elastic range of the track 54 or track component, e.g.
to avoid the permanent yielding of the track 54 or track component
and loss of repeatable adjustment capability.
[0096] The paths defined by the opposing tracks 54 affect the
stretching of the film 32 in the MD, TD, and ND directions. The
stretching transformation can be described as a set of draw ratios:
the machine direction draw ratio (MDDR), the transverse direction
draw ratio (TDDR), and the normal direction draw ratio (NDDR). When
determined with respect to the film 32, the particular draw ratio
is generally defined as the ratio of the current size, (for
example, length, width, or thickness) of the film 32 in a desired
direction (for example, TD, MD, or ND) and the initial size (for
example, length, width, or thickness) of the film 32 in that same
direction. Although these draw ratios can be determined by
observation of the polymer film 32 as stretched, unless otherwise
indicated, reference to MDDR, TDDR, and NDDR refers to the draw
ratio determined by a track 54 used to stretch the polymer film
32.
[0097] At any given point in the stretching process, TDDR
corresponds to a ratio of the current separation distance of the
boundary trajectories, L, and the initial separation distance of
the boundary trajectories, L.sub.0, at the start of the stretch. In
other words, TDDR=L/L.sub.0. In some instances (as in FIGS. 2 and
9, for example), TDDR is represented by the symbol .lamda.. At any
given point in the stretching process, MDDR is the cosine of the
divergence angle, .theta., the positive included angle between MD
and the instantaneous tangent of the boundary trajectory, e.g.
track 54 or rail 102. It follows that cot(.theta.) is equal to the
instantaneous slope (i.e., first derivative) of the track 54 at
that point. Upon determination of TDDR and MDDR,
NDDR=1/[(TDDR)(MDDR)] provided that the density of the polymer film
is constant during the stretching process. If, however, the density
of the film changes by a factor of .rho..sub.f, where
.rho..sub.f=.rho..sub.0/.rho. with .rho. being the density at the
present point in the stretching process and .rho..sub.0 being the
initial density at the start of the stretch, then
NDDR=.rho..sub.f/[(TDDR)(MDDR)] as expected. A change in density of
the material can occur for a variety of reasons including, for
example, due to a phase change, such as crystallization or partial
crystallization, caused by stretching or other processing
conditions.
[0098] Perfect uniaxial stretching conditions, with an increase in
dimension in the transverse direction, result in TDDR, MDDR, and
NDDR of .lamda., (.lamda.).sup.-1/2, and (.lamda.).sup.-1/2,
respectively, as illustrated in FIG. 8 (assuming constant density
of the material). In other words, assuming uniform density during
the stretch, a uniaxially oriented film is one in which
MDDR=(TDDR).sup.-1/2 throughout the stretch. A useful measure of
the extent of uniaxial character, U, can be defined as: U = 1 MDDR
- 1 TDDR 1 / 2 - 1 ##EQU1##
[0099] For a perfect uniaxial stretch, U is one throughout the
stretch. When U is less than one, the stretching condition is
considered "subuniaxial". When U is greater than one, the
stretching condition is considered "super-uniaxial". In a
conventional tenter, the polymer film 12 is stretched linearly
along edges 16, as illustrated in FIG. 2, to stretch a region 18 of
the film to a stretched region 20. In this example, the divergence
angle is relatively small (e.g., about 3.degree. or less), MDDR is
approximately 1 and U is approximately zero. If the film 12 is
biaxially stretched so that MDDR is greater than unity, U becomes
negative. In some embodiments, U can have a value greater than one.
States of U greater than unity represent various levels of
over-relaxing. These over-relaxed states produce an MD compression
from the boundary edge. If the level of MD compression is
sufficient for the geometry and material stiffness, the film will
buckle or wrinkle.
[0100] As expected, U can be corrected for changes in density to
give U.sub.f according to the following formula: U f = 1 MDDR - 1 (
TDDR .rho. f ) 1 / 2 - 1 ##EQU2##
[0101] Preferably, the film is stretched in plane (i.e., the
boundary trajectories and tracks are coplanar) such as shown in
FIG. 10, although non-coplanar stretching trajectories are also
acceptable. The design of in-plane boundary trajectories is
simplified because the in-plane constraint reduces the number of
variables. The result for a perfect uniaxial orientation is a pair
of mirror symmetric, in-plane, parabolic trajectories diverging
away from the in-plane MD centerline. The parabola may be portrayed
by first defining TD as the "x" direction and MD as the "y"
direction. The MD centerline between the opposing bounding
parabolas may be taken as the y coordinate axis. The coordinate
origin may be chosen to be the beginning of the primary stretching
region 66 and corresponds to the initial centerpoint of the central
trace between the parabolic trajectories. The left and right
bounding parabolas are chosen to start (y=0) at minus and plus
x.sub.0, respectively. The right bounding parabolic trajectory, for
positive y values, that embodies this embodiment of the present
disclosure is: x/x.sub.0=(1/4)(y/x.sub.0).sup.2+1
[0102] The left bounding parabolic trajectory is obtained by
multiplying the left-hand side of the above equation by minus
unity. In the discussion below, descriptions of and methods for
determining the right bounded trajectory are presented. A left
bounded trajectory can then be obtained by taking a mirror image of
the right bounded trajectory over the centerline of the film.
[0103] For sub-uniaxial stretches, the final extent of truly
uniaxial character can be used to estimate the level of refractive
index matching between the y (MD) and z (ND) directions by the
equation: .DELTA.n.sub.yz=.DELTA.n.sub.yz(U=0).times.(1-U) where
.DELTA.n.sub.yz is the difference between the refractive index in
the MD direction (i.e., y-direction) and the ND direction (i.e.,
z-direction) for a value U and .DELTA.n.sub.yz(U=0) is that
refractive index difference in a film stretched identically except
that MDDR is held at unity throughout the stretch. This
relationship has been found to be reasonably predictive for
polyester systems (including PEN, PET, and copolymers of PEN or
PET) used in a variety of optical films. In these polyester
systems, .DELTA.n.sub.yz(U=0) is typically about one-half or more
of the difference .DELTA.n.sub.xy(U=0), which is the refractive
difference between the two in-plane directions MD (y-axis) and TD
(x-axis). Typical values for .DELTA.n.sub.xy(U=0) range up to about
0.26 at 633 nm. Typical values for .DELTA.n.sub.yz(U=0) range up to
about 0.15 at 633 nm. For example, a 90/10 coPEN, i.e. a
copolyester comprising about 90% PEN-like repeat units and 10%
PET-like repeat units, has a typical value at high extension of
about 0.14 at 633 nm. Films comprising this 90/10 coPEN with values
of U of 0.75, 0.88 and 0.97 as measured by actual film draw ratios
with corresponding values of .DELTA.n.sub.yz of 0.02, 0.01 and
0.003 at 633 nm have been made according to the methods described
herein.
[0104] A coplanar parabolic trajectory can provide uniaxial
orientation under ideal conditions. However, other factors can
affect the ability to achieve uniaxial orientation including, for
example, non-uniform thickness of the polymer film, non-uniform
heating of the polymer film during stretching, and the application
of additional tension (for example, machine direction tension)
from, for example, down-web regions of the apparatus. In addition,
in many instances it is not necessary to achieve perfect uniaxial
orientation. Instead, a minimum or threshold U value or an average
U value that is maintained throughout the stretch or during a
particular portion of the stretch can be defined. For example, an
acceptable minimum/threshold or average U value can be 0.2, 0.5,
0.7, 0.75, 0.8, 0.85, 0.9, or 0.95, as desired, or as needed for a
particular application. Generally, any minimum/threshold or average
U value that is more than 0 is suitable.
[0105] As an example of acceptable nearly uniaxial applications,
the off-angle characteristics of reflective polarizers used in
liquid crystalline display applications is strongly impacted by the
difference in the MD and ND indices of refraction when TD is the
principal stretch direction. An index difference in MD and ND of
0.08 is acceptable in some applications. A difference of 0.04 is
acceptable in others. In more stringent applications, a difference
of 0.02 or less is preferred. For example, the extent of uniaxial
character of 0.85 is sufficient in many cases to provide an index
of refraction difference between the MD and ND directions in
polyester systems containing polyethylene naphthalate (PEN) or
copolymers of PEN of 0.02 or less at 633 nm for single-direction
transversely stretched films. For some polyester systems, such as
polyethylene terephthalate (PET), a lower U value of 0.80 or even
0.75 may be acceptable because of lower intrinsic differences in
refractive indices in non-substantially uniaxially stretched
films.
[0106] Oriented optical films made by methods of the present
disclosure include reflective polarizers such as multilayer
reflective polarizers and diffusely reflective polarizers.
Descriptions of the latter can be found in commonly owned U.S.
Provisional Application Ser. No. 60/668,944, filed Apr. 6, 2005,
and U.S. application Ser. No. ______, filed ______, both entitled,
"Diffuse Reflective Polarizing Films with Orientable Polymer
Blends," and in U.S. Pat. Nos. 5,825,543, 6,057,961, 6,590,705, and
6,057,961, incorporated herein by reference. Such diffusely
reflective polarizers include a continuous phase of a first
thermoplastic polymer and a discontinuous or disperse phase of a
second thermoplastic polymer. Either or both of the first or second
polymers may be a birefringent material. In one embodiment, a
diffusely reflective polarizer includes more than one continuous
phase and/or more than one disperse phase.
[0107] In another embodiment, oriented optical films made by
methods of the present disclosure include compensators and
retarders. An exemplary embodiment is an "a-plate," which is a
birefringent optical element, such as, for example, a plate or
film, having its principle optical axis within the x-y plane of the
optical element. Positively birefringent a-plates can be fabricated
using, for example, uniaxially stretched films of polymers such as,
for example, polyvinyl alcohol, or uniaxially aligned films of
nematic positive optical anisotropy liquid crystal polymer (LCP)
materials. Negatively birefringent a-plates can be formed using
uniaxially aligned films of negative optical anisotropy nematic LCP
materials, including for example discotic compounds.
[0108] When the volume fraction for binary blends of high polymers
of roughly equivalent viscosity is greater than about 40% and
approaches 50%, the distinction between the disperse and continuous
phases becomes difficult, as each phase becomes continuous in
space. Depending upon the materials of choice, there may also be
regions where the first phase appears to be dispersed within the
second, and vice versa. For a description of a variety of
co-continuous morphologies and for methods of evaluating,
analyzing, and characterizing them, see Sperling and the references
cited therein (L. H. Sperling, "Microphase Structure," Encyclopedia
of Polymer Science and Engineering, 2nd Ed., Vol. 9, 760-788, and
L. H. Sperling, Chapter 1, "Interpenetrating Polymer Networks: An
Overview," Interpenetrating Polymer Networks, edited by D.
Klempner, L. H. Sperling, and L. A. Utracki, Advances in Chemistry
Series #239, 3-38, 1994).
[0109] One set of acceptable parabolic trajectories that is nearly
or substantially uniaxial in character can be determined by the
following method. This described method determines the "right"
boundary trajectory directly, and the "left" boundary trajectory is
taken as a mirror image. First, a condition is set by defining an
instantaneous functional relationship between TDDR measured between
the opposing boundary trajectories and MDDR defined as the cosine
of the non-negative divergence angle of those boundary
trajectories, over a chosen range of TDDR.
[0110] Next, the geometry of the problem is defined as described in
the discussion of the parabolic trajectories. x.sub.1 is defined as
the initial half distance between the boundary trajectories and a
ratio (x/x.sub.1) is identified as the instantaneous TDDR, where x
is the current x position of a point on the boundary trajectory.
Next, the instantaneous functional relationship between the TDDR
and MDDR is converted to a relationship between TDDR and the
divergence angle. When a specific value of U is chosen, the
equations above provide a specific relationship between MDDR and
TDDR which can then be used in the algorithm to specify the broader
class of boundary trajectories that also includes the parabolic
trajectories as a limiting case when U approaches unity. Next, the
boundary trajectory is constrained to satisfy the following
differential equation: d(x/x.sub.1)/d(y/x.sub.1)=tan(.theta.) where
tan(.theta.) is the tangent of the divergence angle .theta., and y
is the y coordinate of the current position of the opposing point
on the right boundary trajectory corresponding to the given x
coordinate. Next, the differential equation may be solved, e.g. by
integrating 1/tan(.theta.) along the history of TDDR from unity to
the maximum desired value to obtain the complete coordinate set
{(x,y)} of the right boundary trajectory, either analytically or
numerically.
[0111] As another example of acceptable trajectories, a class of
in-plane trajectories can be described in which the parabolic
trajectory is used with smaller or larger initial effective web TD
length. If x.sub.1 is half of the separation distance between the
two opposing boundary trajectories at the inlet to the primary
stretching region 66 (i.e. the initial film TD dimension minus the
selvages held by the grippers which is the initial half distance
between opposing boundary trajectories), then this class of
trajectories is described by the following equation:
.+-.(x)/(x.sub.1)=(1/4)(x.sub.1/x.sub.0)(y/x.sub.1).sup.2+1 where
x.sub.1/x.sub.0 is defined as a scaled inlet separation. The
quantity x.sub.0 corresponds to half of the separation distance
between two opposing tracks required if the equation above
described a parabolic track that provided a perfectly uniaxial
stretch. The scaled inlet separation, x.sub.1/x.sub.0, is an
indication of the deviation of the trajectory from the uniaxial
condition. In one embodiment, the distance between the two opposing
tracks in the primary stretching zone is adjustable, as described
above, allowing for the manipulation of the trajectory to provide
values of U and F (described below) different than unity. Other
methods of forming these trajectories can also be used including,
for example, manipulating the shape of the trajectories using track
shape control units or by selecting a fixed shape that has the
desired trajectory.
[0112] For super-uniaxial stretches, the severity of the wrinkling
can be quantified using the concept of overfeed. The overfeed, F,
can be defined as the uniaxial MDDR (which equals (TDDR).sup.-1/2)
divided by the actual MDDR. If the actual MDDR is less than the
uniaxial MDDR, the overfeed F is less than unity and the MDDR is
under-relaxed resulting in a U less than unity. If F is greater
than unity, the stretch is super-uniaxial and the MDDR is
over-relaxed relative to the uniaxial case. At least a portion of
the extra slack can be accommodated as a wrinkle because the
compressive buckling threshold is typically low for thin, compliant
films. When F is greater than unity, the overfeed corresponds at
least approximately to the ratio of the actual film contour length
in the wrinkles along MD to the in-plane contour length or
space.
[0113] Because of the relationship between TDDR and MDDR in the
case of constant density, F can be written as:
F=1/(MDDR.times.TDDR.sup.1/2)
[0114] Typically, F is taken as density independent for design
purposes. Large values of F anytime during the process can cause
large wrinkles that can fold over and stick to other parts of the
film, thereby causing defects. In at least some embodiments, the
overfeed, F, remains at 2 or less during the stretch to avoid or
reduce severe wrinkling or fold-over. In some embodiments, the
overfeed is 1.5 or less throughout the course of the stretch. For
some films, a maximum value of F of 1.2 or even 1.1 is allowed
throughout the stretch.
[0115] For at least some embodiments, particularly embodiments with
U>1 through the entire stretch, rearranging the definition of
overfeed provides a relative bound on a minimum MDDR given a
current TDDR: MDDR>1/(F.sub.max.times.TDDR.sup.1/2) where
F.sub.max can be chosen at any preferred level greater than unity.
For example, F can be selected to be 2, 1.5, 1.2, or 1.1, as
described above.
[0116] When the over-feed is less than unity, there is effectively
more in-plane space along MD than is desired for the truly uniaxial
stretch and the MDDR may be under-relaxed and causing MD tension.
The result can be a U value less than unity. Using the
relationships between U, F, MDDR and TDDR there is a corresponding
correlation between U and F which varies with TDDR. At a critical
draw ratio of 2, a minimum U value corresponds to a minimum
overfeed F of about 0.9. For at least some boundary trajectories
including boundary trajectories in which U>1 for the entire
stretch, MDDR can be selected to remain below a certain level
during a final portion of stretch, e.g.:
MDDR<1/(F.sub.min.times.TDDR.sup.1/2) where F.sub.min is 0.9 or
more for a final portion of stretch after a draw ratio of 2.
[0117] As an example, trajectories can be used in which
MDDR<(TDDR).sup.-1/2 (i.e., U>1) throughout the stretch,
F.sub.max is 2, and the film is stretched to a TDDR of at 4. If the
trajectories are coplanar, then the film is stretched to a TDDR of
at least 2.4 and often at least 5.3. If F.sub.max is 1.5, then the
film is stretched to a TDDR of at least 6.8. If the trajectories
are coplanar, then the film is stretched to a TDDR of at least 2.1
and often at least 4.7. If F.sub.max is 1.2, then the film is
stretched using coplanar trajectories to a TDDR of at least 1.8 and
often at least 4.0. For coplanar or non-coplanar boundary
trajectories, if no limit is placed on F, then the film is
stretched to a TDDR of greater than 4 and often at least 6.8. In
another example, coplanar trajectories can be used in which
(F.sub.min)(MDDR)<(TDDR).sup.-1/2 throughout the stretch,
F.sub.max is 2, F.sub.min is 0.9, and the film is stretched to a
TDDR of at least 4.6 and often at least 6.8. If F.sub.max is 1.5,
then the film is stretched to a TDDR of at least 4.2 and often at
least 6.1, If F.sub.max is 1.2, then the film is stretched to a
TDDR of at least 3.7 and often at least 5.4. If no limit is placed
on F, then the film is stretched to a TDDR of at least 8.4. A
boundary trajectory can also be used in which
(F.sub.min)(MDDR)<(TDDR).sup.-1/2 throughout the stretch,
F.sub.max is 1.5, F.sub.min is 0.9, and the film is stretched to a
TDDR of at least 6.8.
[0118] Other useful trajectories can be defined using F.sub.max.
Useful trajectories include coplanar trajectories where TDDR is at
least 5, U is at least 0.85 over a final portion of the stretch
after achieving a TDDR of 2.5, and F.sub.max is 2 during
stretching. Useful trajectories also include coplanar trajectories
where TDDR is at least 6, U is at least 0.7 over a final portion of
the stretch after achieving a TDDR of 2.5, and F.sub.max is 2
during stretching.
[0119] Yet other useful coplanar trajectories include those in
which MDDR<TDDR.sup.-1/2<(F.sub.max)(MDDR) during a final
portion of the stretch in which TDDR is greater than a critical
value TDDR'. The following provides minimum draw ratios that should
be achieved for the trajectory in some exemplary embodiments. When
TDDR' is 2 or less, then for F.sub.max=2, the minimum draw is 3.5;
for F.sub.max=1.5, the minimum draw is 3.2; and for F.sub.max=2,
the minimum draw is 2.7. When TDDR' is 4 or less, then for
F.sub.max=2, the minimum draw is 5.8; for F.sub.max=1.5, the
minimum draw is 5.3; and for F.sub.max=1.2, the minimum draw is
4.8. When TDDR' is 5 or less, then for F.sub.max=2, the minimum
draw is 7; for F.sub.max=1.5, the minimum draw is 6.4; and for
F.sub.max=1.2, the minimum draw is 5.8.
[0120] In general, a variety of acceptable trajectories can be
constructed using curvilinear and linear tracks so that the
overfeed remains below a critical maximum level throughout the
stretching to prevent fold-over defects while remaining above a
critical minimum level to allow the desired level of truly uniaxial
character with its resulting properties.
[0121] A variety of sub-uniaxial and super-uniaxial trajectories
may be formed using the parabolic shape. FIG. 19 illustrates
examples of different levels of minimum U after a critical TDDR and
demonstrate different maximum overfeeds up to a final desired TDDR.
The curves are represented by coordinates x and y as scaled by
x.sub.1, half the initial separation distance of the tracks. The
scaled x coordinate, the quantity (x/x1), is therefore equal to the
TDDR. Curve 120 is the ideal case with a value of x.sub.1/x.sub.0
of 1.0. Curve 122 is the parabolic case with a value of
x.sub.1/x.sub.0 of 0.653 in which U remains greater than 0.70 above
a draw ratio of 2.5. Curve 124 is the parabolic case with a value
of x.sub.1/x.sub.0 of 0.822 in which U remains above 0.85 after a
draw ratio of 2.5.
[0122] Curves 126. 128, and 130 illustrate various levels of
overfeed. The overfeed, TDDR and scaled inlet width are related by:
x.sub.1/x.sub.0=(F.sup.2(TDDR)-1)/(TDDR-1)
[0123] It follows directly that the overfeed increases with
increasing TDDR in the parabolic trajectories described here. Curve
126 is the parabolic case with a value of x.sub.1/x.sub.0 of 1.52
in which the overfeed remains below 1.2 up to a final draw ratio of
6.5. Curve 128 is the parabolic case with a value of
x.sub.1/x.sub.0 of 2.477 in which the overfeed remains below 1.5 up
to a final draw ratio of 6.5. Curve 130 is the parabolic case with
a value of x.sub.l/x.sub.0 of 4.545 in which the overfeed remains
below 2 up to a final draw ratio of 6.5. The level of overfeed is a
function of the final draw ratio in these cases. For example, using
a value of x.sub.1/x.sub.0 of only 4.333 rather than 4.545 allows
stretching to a final TDDR of 10 while keeping the overfeed under
2.
[0124] For the parabolic trajectories, a relationship allows the
direct calculation of MDDR at any given TDDR for a fixed scaled
inlet width:
MDDR=(TDDR(x.sub.1/x.sub.0)+(1-x.sub.1/x.sub.0)).sup.-1/2
[0125] One observation is that the relationship between MDDR and
TDDR is not an explicit function of the y position. This allows the
construction of composite hybrid curves comprising sections of
parabolic trajectories that are vertically shifted in y/x.sub.1.
FIG. 20 illustrates one method. A parabolic trajectory for the
initial portion of the stretch is chosen, curve 132, and a
parabolic trajectory is chosen for the final portion, curve 134.
The initial curve 132 is chosen to provide a super-uniaxial stretch
with a maximum overfeed of 2.0 at a draw ratio of 4.5. Curve 132
has a scaled inlet width of 4.857. The final curve 134 is chosen to
be a sub-uniaxial stretch with a minimum U of 0.9 at the 4.5 draw
ratio. Curve 134 has a scaled inlet width of 0.868. The actual
track or rail shape follows curve 132 up to TDDR of 4.5 and then
continues on curve 136 which is a vertically shifted version of
curve 134. In other words, a trajectory can have an initial
stretching zone with tracks having a functional form corresponding
to: .+-.(x)/(x.sub.1)=(1/4)(x.sub.1/x.sub.0)(y/x.sub.1).sup.2+1 and
then a later stretching zone with tracks having a functional form
corresponding to:
.+-.(x)/(x.sub.2)=(1/4)(x.sub.2/x.sub.0)((y-A)/x.sub.2).sup.2+1;
where x.sub.1 and x.sub.2 are different and A corresponds to the
vertical shift that permits coupling of the trajectories. Any
number of parabolic segments may be combined in this manner.
[0126] The parabolic trajectories, and their composite hybrids, can
be used to guide the construction of related trajectories. One
embodiment involves the use of linear segments to create
trajectories. These linear approximations can be constructed within
the confines of parabolic trajectories (or composite hybrids) of
maximum overfeed and minimum overfeed (or minimum U) at a chosen
TDDR' larger than a critical draw ratio, TDDR*. Values for TDDR*
can be selected which relate to the onset of strain-induced
crystallinity with examples of values of 1.5, 2, and 2.5 or may be
related to elastic strain yielding with lower values of 1.2 or even
1.1. The range of TDDR* generally falls between 1.05 and 3.
Portions of the rail or track below TDDR* may not have any
particular constraints on minimum overfeed or U and may fall
outside the confines of the constraining parabolic
trajectories.
[0127] In FIG. 21, curve 138 is chosen to be the constraining
parabolic trajectory of minimum overfeed at the chosen draw ratio,
TDDR', illustrated here at a value of 6.5. For illustration, the
minimum overfeed constraining parabolic trajectory has been chosen
as the ideal curve with a scaled inlet width of unity. Using the
relationship between overfeed, TDDR and scaled inlet width, curve
140 is identified as the constraining parabolic trajectory of
maximum overfeed where the maximum value of F is 2.0 at the TDDR
value of 6.5. Curve 140 is now vertically shifted to form curve 142
so that the two constraining parabolic trajectories meet at the
chosen TDDR' of 6.5. It should be remarked that curves 140 and 142
are equivalent with respect to stretching character. Curve 142
merely delays the stretch until a later spatial value of y/x.sub.1
of 2.489. An approximation of linear or non-parabolic curvilinear
segments will tend to lie between these constraining trajectories
above TDDR*.
[0128] Unlike parabolic trajectories that possess increasing
divergence angles with increasing TDDR, linear trajectories have a
fixed divergence angle. Thus the overfeed decreases with increasing
TDDR along a linear segment. A simple linear approximation can be
constructed by choosing a line with a divergence angle equal to the
desired minimum overfeed at the chosen TDDR. The line segment may
be extrapolated backwards in TDDR until the overfeed equals the
maximum allowed. A subsequent linear segment is started in similar
fashion. The procedure is repeated as often as necessary or
desired. As the maximum overfeed decreases, the number of segments
needed for the approximation increases.
[0129] When the TDDR drops below TDDR*, any number of methods may
be used to complete the track or rail as long as the constraint on
maximum overfeed is maintained. In FIG. 21, curve 144 is a linear
approximation constrained by a maximum overfeed of 2. Because of
this large maximum overfeed, it comprises only two linear sections.
The final linear segment extends all the way backwards from the
chosen TDDR of 6.5 to a lower TDDR of 1.65. In this case, TDDR* is
taken as 2. Without a constraint on U below a TDDR of 2, one method
of finishing the track is to extrapolate a second linear segment
from TDDR at 1.65 back to TDDR of unity at the y/x.sub.1 zero
point. Note that this causes the second segment to cross the lower
constraining parabola, since the constraint is not effective below
TDDR*.
[0130] In FIG. 21, curve 146 is the result of using a tighter value
for the maximum overfeed of 1.5. Here the constraining parabolic
trajectory of maximum overfeed is not shown. Three linear segments
are required. The first segment extends backwards from TDDR of 6.5
to TDDR of 2.9. The second segment assumes a divergence angle equal
to the constraining parabolic trajectory of minimum overfeed at
this TDDR value of 2.9 and extends backwards to a TDDR of 1.3. This
second segment ends below TDDR*. The final segment completes the
track or rail shape for curve 146 using a different method than
that used for curve 144. Here the same procedure for the last
segment is used as for the previous segments, resulting in a delay
of the onset of stretching with a higher y/x.sub.1 value. A third
method of completing the track is to set the overfeed to the
maximum at the initial TDDR of unity.
[0131] General, non-linear and non-parabolic trajectories fitting
the requirements of the present disclosure can be constructed using
the constraining parabolic trajectories. The maximum overfeed
constraining parabolic trajectory is the curve of minimum slope,
i.e. maximum divergence angle, as a function of TDDR. The minimum
overfeed constraining parabolic trajectory is the curve of maximum
slope, i.e. minimum divergence angle, as a function of TDDR. In
general, curves can be extrapolated backwards from the chosen TDDR'
using any function of slope that lies between the constraining
bounds.
[0132] A simple method for defining a function for the slope that
lies between these constraints is to take a simple linear
combination of known curves within the envelope. Curve 148 in FIG.
21 illustrates this simple method. In this example, curve 148 is
formed by a linear combination of the maximum overfeed constraining
parabolic trajectory, curve 142, and the linear approximation to
it, curve 144, with the linear weights of 0.7 and 0.3,
respectively. In general, functions that are not simple linear
combinations can also be used.
[0133] The aforementioned method for describing the various
non-parabolic trajectories of the present disclosure can be applied
over different sections of the track, e.g. the example of FIG. 21
for TDDR up to 6.5 may be combined with another section for TDDR
over 6.5 with different requirements and therefore different
maximum and minimum constraining trajectories over that higher
range of TDDR. In this case, the TDDR' of the previous section of
lower stretch takes on the role of TDDR*. In general, TDDR' may be
chosen across the range of desired stretching. Various sections may
be used to account for the various phenomenon of stretching, such
as yielding, strain-induced crystallization, onset of necking or
other stretch non-uniformity, onset of strain-hardening or to
account for the development of various properties within the film.
Typical break points include those for TDDR*, the range of 3 to 7
for strain-hardening in polyesters, and typical final draw values
in the range of 4 to 10 or more.
[0134] The procedures for determining boundary trajectories for the
present disclosure and the method of extrapolating backwards to
lower TDDR from a chosen TDDR' may be used in an analogous method
of extrapolating forward to higher TDDR from a chosen TDDR''.
Again, two constraining trajectories are formed, joined at the
lowest chosen TDDR''. A convenient value for TDDR'' is the initial
TDDR of unity. In this method, the constraining trajectory of
minimum overfeed or U lies above the maximum overfeed curve. FIG.
20 exhibits an example of this method in which the hybrid curve 136
lies between the minimum overfeed constraint, curve 134, and the
maximum overfeed constraint, curve 132.
[0135] Still another class of boundary trajectories can be defined
and may, in some embodiments, be useful in suppressing residual
wrinkles. Because the uniaxial condition in the absence of shear
provides a principal MD stress of zero, it is anticipated, using
finite strain analysis, that the principal MD stress will actually
go into slight compression under these conditions. Using finite
strain analysis and a Neo-Hookean elastic solid constitutive
equation, it is discovered that a suitable criterion for preventing
compressive stresses may optionally be given by the following
equation:
((TDDR)(MDDR)).sup.-4+((TDDR)(MDDR)).sup.2-(TDDR).sup.-2-(MDDR).sup.-2-si-
n.sup.2(.theta.)((TDDR)(MDDR)).sup.-2=0 where MDDR is the cosine of
the divergence angle. This optional method of the present
disclosure then specifies this class of boundary trajectories.
[0136] As indicated above, the film may be stretched out-of-plane
using out-of-plane boundary trajectories, i.e. boundary
trajectories that do not lie in a single Euclidean plane. There are
innumerable, but nevertheless particular, boundary trajectories
meeting relational requirements of this preferred embodiment of the
present disclosure, so that a substantially uniaxial stretch
history may be maintained using out-of-plane boundary trajectories.
The boundaries may be symmetrical, forming mirror images through a
central plane, e.g. a plane comprising the initial center point
between the boundary trajectories, the initial direction of film
travel and the initial normal to the unstretched film surface. In
this embodiment, the film may be stretched between the boundary
trajectories along a cylindrical space manifold formed by the set
of line segments of shortest distance between the two opposing
boundary trajectories as one travels along these boundary
trajectories at equal rates of speed from similar initial
positions, i.e., colinear with each other and the initial center
point.
[0137] The trace of this ideal manifold on the central plane thus
traces out the path of the film center for an ideal stretch. The
ratio of the distance along this manifold from the boundary
trajectory to this central trace on the central plane to the
original distance from the start of the boundary trajectory to the
initial center point is the instantaneous nominal TDDR across the
film spanning the boundary trajectories, i.e. the ratios of the
half-distances between the current opposing points on the boundary
trajectories and the half-distances between the initial positions
of the opposing points on the boundary trajectories. As two
opposing points move at constant and identical speeds along the
opposing boundary trajectories, the corresponding center point on
the central trace changes speed as measured along the arc of the
central trace, i.e. the curvilinear MD. In particular, the central
trace changes in proportion with the projection of the unit tangent
of the boundary trajectory on the unit tangent of the central
trace.
[0138] The classes of trajectories described above are illustrative
and should not be construed as limiting. A host of trajectory
classes are considered to lie within the scope of the present
disclosure. As indicated above, the primary stretching region can
contain two or more different zones with different stretching
conditions. For example, one trajectory from a first class of
trajectories can be selected for an initial stretching zone and
another trajectory from the same first class of trajectories or
from a different class of trajectories can be selected for each of
the subsequent stretching zones.
[0139] Although exemplary embodiments of the present disclosure
encompass all boundary trajectories comprising a minimum value of
U>0, typical embodiments of the present dislosure include all
nearly or substantially uniaxial boundary trajectories comprising a
minimum value of U of about 0.2, about 0.5, about 0.7, more
preferably approximately 0.75, still more preferably about 0.8 and
even more preferably about 0.85. The minimum U constraint may be
applied over a final portion of the stretch defined by a critical
TDDR preferably of about 2.5, still more preferably about 2.0 and
more preferably about 1.5. In some embodiments, the critical TDDR
can be about 4 or 5. Above a critical TDDR, certain materials, e.g.
certain monolithic and multilayer films comprising orientable and
birefringent polyesters, may begin to lose their elasticity or
capability of snap back because of the development of structure
such as strain-induced crystallinity. The critical TDDR may
coincide with a variety of material and process (e.g. temperature
and strain rate) specific events such as the critical TDDR for the
onset of strain-induced crystallization. The minimum value of U
above such a critical TDDR can relate to an amount of non-uniaxial
character set into the final film.
[0140] A variety of boundary trajectories are available when U is
subuniaxial at the end of the stretching period. In particular,
useful boundary trajectories include coplanar trajectories where
TDDR is at least 5, U is at least 0.7 over a final portion of the
stretch after achieving a TDDR of 2.5, and U is less than 1 at the
end of the stretch. Other useful trajectories include coplanar and
non-coplanar trajectories where TDDR is at least 7, U is at least
0.7 over a final portion of the stretch after achieving a TDDR of
2.5, and U is less than 1 at the end of the stretch. Useful
trajectories also include coplanar and non-coplanar trajectories
where TDDR is at least 6.5, U is at least 0.8 over a final portion
of the stretch after achieving a TDDR of 2.5, and U is less than 1
at the end of the stretch. Useful trajectories include coplanar and
non-coplanar trajectories where TDDR is at least 6, U is at least
0.9 over a final portion of the stretch after achieving a TDDR of
2.5, and U is less than 1 at the end of the stretch.
[0141] Useful trajectories also include coplanar and non-coplanar
trajectories where TDDR is at least 7 and U is at least 0.85 over a
final portion of the stretch after achieving a TDDR of 2.5.
[0142] In some embodiments, a small level of MD tension is
introduced into the stretching process to suppress wrinkling.
Generally, although not necessarily, the amount of such MD tension
increases with decreasing U. In some embodiments, it is useful to
increase the tension as the stretch proceeds. For example, a
smaller value of U earlier in the stretch may tend to set more
non-uniaxial character into the final film. Thus it may be
advantageous to combine the attributes of various trajectory
classes into composite trajectories. For example, a uniaxial
parabolic trajectory may be preferred in the earlier portions of
the stretch, while the later portions of the stretch may converge
on a different trajectory. In another arrangement, U may be taken
as a non-increasing function with TDDR. In still another
arrangement, the overfeed, F, may be a non-increasing function with
TDDR after a critical draw ratio of, for example, 1.5, 2, or
2.5.
[0143] The uniaxial parabolic trajectory assumes a uniform spatial
stretching of the film. Good spatial uniformity of the film can be
achieved with many polymer systems with careful control of the
crossweb and downweb caliper (thickness) distribution of the
initial, unstretched film or web, coupled with the careful control
of the temperature distribution at the start of and during the
stretch. For example, a uniform temperature distribution across the
film initially and during stretch on a film of initially uniform
caliper should suffice in most cases. Many polymer systems are
particularly sensitive to non-uniformities and will stretch in a
non-uniform fashion if caliper and temperature uniformity are
inadequate.
[0144] Non-uniform film stretching can occur for a variety of
reasons including, for example, non-uniform film thickness or other
properties, non-uniform heating, etc. In many of these instances,
portions of the film near the gripping members stretch faster than
those near the center. This creates an MD tension in the film that
can limit ability to achieve a final uniform MDDR. One compensation
for this problem is to modify the parabolic or other uniaxial
trajectory to present a lower MDDR. In other words,
MDDR<(TDDR).sup.-1/2 for all or a portion of the stretch.
[0145] In one embodiment, a modified parabolic or other uniaxial
trajectory is selected in which MDDR<(TDDR).sup.-1/2,
corresponding to a larger divergence angle, for all of the stretch.
In at least some instances, this condition can be relaxed because a
U value of less than unity is acceptable for the application. In
such instances, a modified parabolic or other uniaxial trajectory
is selected in which (0.9)MDDR<(TDDR).sup.-1/2.
[0146] In another embodiment, a modified parabolic or other
uniaxial trajectory is selected in which MDDR<(TDDR).sup.-1/2
for an initial stretching zone in which the TDDR is increased by at
least 0.5 or 1. A different trajectory is then maintained for the
remainder of the stretch. For example, a later stretching zone
(within the stretching region 34) would have a parabolic or other
uniaxial trajectory in which MDDR is equal to or approximately
equal to (within .+-.5% and, preferably, within .+-.3%)
(TDDR).sup.-1/2. As an example, the initial stretching zone can
accomplish a TDDR level up to a desired value. In one embodiment,
this desired value is typically no more than about 4 or 5. The
later stretching zone can then increase the TDDR from the desired
value of the initial stretching zone (or from a higher value if
there are intervening stretching zones). Generally, the later
stretching zone is selected to increase the TDDR value by 0.5 or 1
or more.
[0147] Again, in at least some instances, the MDDR and TDDR
relationship can be relaxed because a U value of less than unity is
acceptable for the application. In such instances, the modified
parabolic or other uniaxial trajectory of the initial stretching
zone is selected in which (0.9)MDDR<(TDDR).sup.-1/2.
[0148] The heat set procedure of the present disclosure may be
performed during various portions of the stretching process. In one
embodiment, the film 32 may be heat set following stretching and
hand-off to a takeaway system, i.e. in a heated takeaway zone. In
another embodiment, the film 32 may be heat set in an on-line zone
subsequent to initial quenching and setting of the film 32, e.g. in
a separate oven device that re-heats the film 32. In yet another
embodiment, the film 32 may be heat set after winding into a roll
after the initial process, e.g. in a separate oven device not
connected on-line to the stretching apparatus 50.
[0149] During heat setting, the draw ratio used for stretching the
film may be increased, maintained, or decreased compared to the
draw ratio used to induce the substantially uniaxial orientation.
In other words, the film 32 may be further stretched or the
stretching may be relaxed, e.g. with a toe-in (reduction in stretch
ratio) as provided by an edge gripping mechanism in any of these
steps. For example, the take-away can be toed-in or the film 32 may
be gripped in a clip system and conveyed by it through the separate
oven device with a variable width profile, e.g. a toe-in or an
increase in stretch perhaps also followed by a subsequent toe-in.
The heat set procedure may be performed with the film continuously
gripped and under tension, e.g. using an edge gripping profile of
increase or reduced tension or both as controlled by the separation
profile of the opposing grips, or gripped continuously or
discontinuously along a system of converging and diverging rails.
The film can also be unconstrained at the edges.
[0150] Heat setting can also be combined with other film
post-processing. For example, the film may be coated and dried or
cured in an oven with some heat setting effect.
[0151] In some embodiments, such as that illustrated in FIG. 22, a
takeaway system 150 can use any film conveyance structures such as
tracks 150, 152 with gripping members such as, for example,
opposing sets of belts or tenter clips. TD shrinkage control can be
accomplished using tracks 152, 154 which are angled (as compared to
parallel tracks 156, 158 that could be used in other embodiments of
a suitable take-away system 150). For example, the tracks 152, 154
of the take-away system 150 can be positioned to follow a slowly
converging path (in one embodiment, making an angle .theta. of no
more than about 5.degree.) through at least a portion of the post
conditioning region 70 to allow for TD shrinkage of the film 32
with cooling. The tracks 152, 154 in this configuration allow the
control of TD shrinkage to increase uniformity in the shrinkage. In
other embodiments, the two opposing tracks 152, 154 can be
diverging typically at an angle of no more than about 3.degree.
although wider angles can be used in some embodiments. This can be
useful to increase the MD tension of the film 32 in the primary
stretching region 66 to, for example, reduce property
non-uniformity such as the variation of principal axes of
refractive index across the film 32.
[0152] In some embodiments, the position of the take-away system
150 can be adjustable to vary the position along the stretching
apparatus 50 at which the take-away system 150 grips the film 32,
as illustrated in FIG. 23. This adjustability provides one way to
control the amount of stretching to which the film 32 is subjected.
Film 32 received by tracks 156', 158' of a take-away system earlier
in the stretch (shown by dotted lines in FIG. 23) will generally
have a smaller TDDR than would film received by a tracks 156, 158
of a take-away system 150 positioned later in the stretch (shown in
solid lines in FIG. 23). The take-away system 150 can also,
optionally, allow adjustment in the distance between the opposing
tracks 152, 154, 156, 158 of the take-away system 150. In addition,
the take-away system 150 can also, optionally, be configured to
allow adjustment in the length of the take-away system 150.
[0153] Another example of a possible take-away system 150,
illustrated in FIG. 25, includes at least two different regions
with separated tracks 152, 154, 156, 158. These regions can be
formed using two separate sets 152, 154 and 156, 158 of opposing
tracks as illustrated in FIG. 24. In one embodiment, illustrated in
FIG. 24, the first region can include tracks 152, 154 that are
disposed at a convergence angle to provide TD shrinkage control and
the tracks 156, 158 in the second regions can be parallel. In other
embodiments, the opposing tracks of the two different regions can
be set at two different convergence angles to provide TD shrinkage
control, as described above, or the first region can have parallel
tracks and the second region have tracks disposed at a convergence
angle to provide TD shrinkage control. Alternatively or
additionally, the two different tracks can be set at two different
takeaway speeds to decouple the primary stretching region 66 from a
takeaway region that applies tension to remove wrinkles.
[0154] In one embodiment of the take-away system 150 illustrated in
FIG. 24, the tracks 156', 158' are nested within the opposing
tracks 152, 154 prior to receiving the film 32. When the film 32 is
initially received by the opposing tracks 152, 154, the tracks
156', 158' move to the position 156, 158 illustrated in FIG. 24. In
other embodiments, the opposing tracks 152, 154, 156, 158 are
positioned as illustrated in FIG. 24 (i.e., not nested) in the
absence of any film 32. Another example of a take-away system is
illustrated in FIG. 25. In this example, the tracks 152, 154 of the
take-away system are angled with respect to the centerline of the
film 32 as the film 32 is conveyed through the tracks 54 of the
primary stretching region 66.
[0155] The angle of the two opposing conveyance mechanisms 152, 154
can be the same, for example, an angle .beta., or the angle can be
different and can be described as .beta.+.epsilon. for one track
152 and .epsilon.-.epsilon. for the other track 154. Typically,
.beta. is at least about 1.degree. and can be an angle of about
5.degree., 10.degree., or 20.degree. degrees or more. The angle
.epsilon. corresponds to the converging or diverging angle
described above to provide TD shrinkage control, for example. In
some embodiments, the tracks 54 in the primary stretching zone 66
can also be disposed at an angle .phi. and the tracks 152, 154 are
angled at .phi.+.beta.+.epsilon. and .phi.+.epsilon.-.epsilon. as
illustrated in FIG. 25. An angled take-away system 150, primary
stretching zone 66, or both can be useful to provide films 32 where
the principal axis or axes of a property of the film 32, such as
the refractive index axes or tear axis, is angled with respect to
the film 32. In some embodiments, the angle that the take-away
system 150 makes with respect to the primary stretching zone 66 is
adjustable manually or mechanically using a computer-controlled
driver or other control mechanism or both.
[0156] In some embodiments using an angled take-away system 150,
the two opposing tracks 152, 154 are positioned to receive film 32
having the same or substantially similar TDDR (where the dotted
line 160 indicates film 32 at the same TDDR), as illustrated in
FIG. 25. In other embodiments, the two opposing tracks 152, 154 are
positioned to receive the film 32 so that the TDDR is different for
the two opposing tracks 152, 154 (the dotted line 160 of FIG. 26
indicates film 32 at the same TDDR), as illustrated in FIG. 26.
This latter configuration can provide a film 32 with properties
that change over the TD dimension of the film 32.
[0157] Referring back to FIG. 10, release of the selvages from a
continuous gripping mechanism can be done continuously; however,
release from discrete gripping mechanisms, such as tenter clips,
should preferably be done so that all the material under any given
clip is released at once. Discrete release mechanisms may cause
larger upsets in stress that may be felt by the stretching web
upstream. In order to assist the action of the isolating takeaway
device, it is preferred in one embodiment to use a continuous
selvage separation mechanism in the device, such as, for example,
the "hot" slitting of the selvage 76 from the central portion of a
heated, stretched film.
[0158] In one embodiment, the slitting location 78 is preferably
located near enough to the "gripline," e.g. the isolating takeaway
point of first effective contact by the gripping members of the
take-away system, to minimize or reduce stress upsets upstream of
that point. If the film is slit before the film is gripped by the
take-away system, instable takeaway can result, for example, by
film "snapback" along TD. The film is thus preferably slit at or
downstream of the gripline. Slitting is a fracture process and, as
such, typically has a small but natural variation in spatial
location. Thus it may be preferred to slit slightly downstream of
the gripline to prevent any temporal variations in slitting from
occurring upstream of the gripline. If the film is slit
substantially downstream from the gripline, the film between the
takeaway and boundary trajectory will continue to stretch along TD.
Since only this portion of the film is now stretching, it now
stretches at an amplified draw ratio relative to the boundary
trajectory, creating further stress upsets that could propagate
upstream, for example, undesirable levels of machine direction
tension propagating upstream.
[0159] The slitting is preferably mobile and re-positionable so
that it can vary with the changes in takeaway positions needed to
accommodate variable final transverse draw direction ratio or
adjustment of the position of the take-away system. An advantage of
this type of slitting system is that the draw ratio can be adjusted
while maintaining the stretch profile simply by moving the
take-away slitting point 78.
[0160] A variety of slitting techniques can be used including a
heat razor, a hot wire, a laser, a focused beam of intense infrared
(IR) radiation or a focused jet of heated air. In the case of the
heated jet of air, the air may be sufficiently hotter in the jet to
blow a hole in the film, such as by heat softening, melting, or
controlled fracture under the jet. Alternatively, the heated jet
may merely soften a focused section of the film sufficiently to
localize further stretching imposed by the still diverging boundary
trajectories, thus causing eventual fracture downstream along this
heated line through the action of continued film extension. The
focused jet approach may be preferred in some cases, especially
when the exhaust air can be actively removed, e.g. by a vacuum
exhaust, in a controlled fashion to prevent stray temperature
currents from upsetting the uniformity of the stretching process.
For example, a concentric exhaust ring around the jet nozzle can be
used. Alternatively, an exhaust underneath the jet, e.g. on the
other side of the film, can be used. The exhaust may be further
offset or supplemented downstream to further reduce stray flows
upstream into the stretching zone.
[0161] Another attribute of one embodiment of the take-away system
is a method of speed and or MD tension control so that the film can
be removed in a manner compatible with the output speed. In one
embodiment, this take-away system is used to pull out any residual
wrinkles in the film. In one example, the wrinkles are initially
pulled out during start up by a temporary increase in the takeaway
speed above the output speed of the final, released portion of the
stretched film. In another example, the wrinkles are pulled out by
a constant speed above the output film MD speed during continuous
operation, such as in the case of a super-uniaxial stretch in the
final portion of stretch. In yet another example, the speed of the
takeaway is set above the MD velocity of the film along the
boundary trajectories at the gripline. This can also be used to
alter the properties of the film. This over-speed of the takeaway
can also reduce the final value of U; in some cases, this is a
consideration in the context of the final end use of the film.
[0162] The principles of MD and TD shrinkage control described
above can also be applied to other stretching apparatuses including
the conventional tenter configuration illustrated in FIG. 2. FIG.
27 illustrates an embodiment in which the tracks 54 from a primary
stretching region 66 (such as the linear diverging tracks
illustrated in FIG. 2) continue into or through a portion of a
post-conditioning region 70 (see FIG. 10). The film is then
optionally captured by an isolated takeaway system 156, 158, if
desired. The continuation of the tracks 54 can be used to cool the
film and allow for shrinkage of the film.
[0163] In some embodiments, the continued tracks 162 follow a
slowly converging path (making an angle .theta. of no more than
about 5.degree. in one embodiment) through at least a portion of
the post conditioning region 70 to allow for TD shrinkage of the
film with cooling. The tracks in this configuration allow the
control of TD shrinkage to increase uniformity in the shrinkage. In
some embodiments, the tracks 164 follow a more aggressively
converging path (making an angle .phi. of at least 15.degree. in
some embodiments, and typically in the range of 20.degree. and
30.degree.) through at least a portion of the post conditioning
region 70 to provide MD shrinkage control of the film with cooling.
In some embodiments as illustrated in FIG. 27, the post
conditioning region 70 includes both slowly converging tracks 162
and more aggressively converging tracks 164. In other embodiments,
only one set of tracks 162 or tracks 164 is used.
[0164] One useful measure of the uniaxial character of the film
made in accordance with a substantially uniaxial stretch process is
the "extent of unixial character" described e.g. in U.S. Pat. No.
6,939,499, incorporated herein by reference. The approximate
uniaxial character of the resulting film can be discerned by this
process measurement. In one measure, the extent of uniaxial
character is derived from the nominal draw ratios as set by the
bounding trajectories at the gripping edges of the device during
stretching, as further modified by the conditions of the take away
system. In another measure of the extent of uniaxial character, the
actual draw ratios of the film can be directly measured, e.g., by
physical marking of the initial input cast web or film with a grid
pattern of known size, and re-measurement after final film
formation, e.g. the factor .rho..sub.f.
[0165] The heat setting of the present disclosure allows for a
greater range of control on the allowable set of refractive
indices. In particular, higher values can be obtained at a fixed
level of optical power as measured by the difference between nx and
nu (discussed below), or a still higher nu value at lower levels of
optical power can be obtained.
[0166] The heat treatment allows an additional measure of control
on the set of principal refractive indices initially resulting from
the stretch and may impart additional advantages such as, for
example, improved dimensional stability including shrinkage
control, enhanced creep resistance, improved imprint resistance, as
well as enhanced tear resistance and other physical properties.
[0167] In some films comprising certain material systems, the heat
treatment maintains or even improves the extent of uniaxial
character in the resulting final film. In the case of optical
films, this can maintain or even improve performance in
applications using non-normal incident light. For example,
so-called off-angle color performance can be maintained or improved
in multilayer optical films (MOF) used for brightness enchancement.
MOF films used for polarizing beam splitting can also be enhanced.
The method can also be used to enhance orientation and performance
of microstructures formed on the surface of the film, e.g. in a
polarizing beam splitting application.
[0168] For multilayer reflective polarizing films with a high
degree of uniaxial orientation (e.g. as achieved by a truly
uniaxial stretching process), higher levels of contrast can be
achieved with a fixed material constructions, i.e. fixed low index
material. This can be applied generally in applications that use
these films, for example, in polarizing beam splitter applications,
e.g. as described in U.S. Pat. No. 6,609,795 and U.S. Patent
Application Publication No. 2004/0227994 using the films separately
or together in stacks of two of more such films.
[0169] Using a heat setting step, imprint resistance can be
achieved using a high index skin layer. In many systems, the
imprint resistance is increased through the increased crystallinity
in an oriented skin layer. The oriented skin layer may comprise a
material similar to a birefringent layer in the optical stack of an
MOF or it may include a different material suitably chosen to
co-extrude and orient in the film formation process.
[0170] Heat setting may also relieve the existence of "residual
stresses" often remaining in the film after stretch, depending for
example on the restraint conditions during or after heat setting.
Reduced restraint, achieved by toe-in, for example, can contribute
to stress reduction. This can lead to improved dimensional
stability including lower levels of shrinkage, lower levels of
thermal expansion, and improved resistance to warpage.
[0171] Other possible mechanical improvements upon heat setting may
be increased tear resistance or even increased inter-layer
delamination resistance. In some systems, high temperature heat
setting near the melting point improves the interlayer adhesion.
For example, it may improve the interfacial penetration between
layers that can be perturbed during the stretching process.
[0172] Further, when the method of the present disclosure is
applied to a film construction including a strain-induced
crystallized polyester skin layer, the imprint resistance of the
film is improved. Light levels of heat treatment do not
significantly change the result; however, a heavier level of heat
treatment creates a film with essentially no denting.
[0173] In one embodiment, one or more of the heat set film layers
remains amorphous, leading to improved web handling and mechanical
properties. In an exemplary embodiment, the amorphous layers
comprise polycarbonate or a blend of polycarbonate and
copolyester.
[0174] In certain polyester systems, heat setting provides higher
optical power or birefringence at a much lower draw ratio than is
otherwise typically achieved by stretching alone. For example,
polyesters such as PET, PEN and compositions including both PET and
PEN are typically stretched to draw ratios of 4, 5, 6 or higher.
These materials may be stretched to just above the strain-induced
crystallization point and then heat set to achieve index values
comparable to those higher draw ratios. As a further example, a
film with a microstructured surface may be stretched in the cross
direction, e.g. perpendicular to an elongate direction, at a
substantially reduced draw ratio that may not overly destroy the
shape of the intended final structures. See, for example,
copending, commonly assigned U.S. Provisional Application Ser. No.
60/638,732; U.S. application Ser. No. 11/184,027; filed Dec. 23,
2004, incorporated herein by reference. High levels of index can be
achieved throughout the microtextured structure along the cross
direction as long as the onset point of significant strain-induced
crystallization has been surpassed throughout the structure. This
is especially useful for making structures with "fiber symmetric"
index sets with high birefringence when the structures have a
height varying or "profile" direction perpendicular to the stretch
direction.
[0175] In the case of true or nearly uniaxially oriented films, the
ny and nz are nearly identical, e.g. within a few hundredths of an
index unit. An interesting and informative view of the space of
allowable index sets can be obtained with the data reduction
illustrated in FIG. 28.
[0176] To obtain the data plotted in FIG. 28, one first calculates
the average of the ny and nz indices actually obtained, regardless
of process conditions. The average value, defined here as nu for
"uniaxial index of refraction," is a measure of the expected ny/nz
value in a "virtual" truly uniaxial condition. In a multilayer
optical film (MOF) polarizer, which includes alternating layers of
birefringent and isotropic polymeric materials, nu is the target
pass state refractive index value of the birefringent material to
match to the isotropic index of the second, in some cases low
index, material layer.
[0177] Second, one takes the difference between nx and nu. This
difference is the block state index difference, a measure of the
reflective power or optical power of the MOF polarizer in the
virtual state.
[0178] Third, one plots the block state index difference versus the
virtual truly uniaxial pass state index nu.
[0179] FIG. 28 shows the resulting plot for a variety of stretch
conditions, both truly uniaxial as described above and simply
one-directional as performed in a conventional tenter apparatus
(FIGS. 2-3). The data cover a wide range of effective molecular
orientations as induced by stretch temperature, rate and draw
ratio, for polyesters spanning the range of homopolymer PEN,
through the various "coPENs" to the homopolymer PET. The coPENS are
expressed in terms of the ratio of mole percent PEN-like moiety to
mole percent PET-like moiety; for example, 85/15 co-polymer, a
so-called "85/15 coPEN," is a copolymer having 85 mole percent
PEN-like moiety and 15 mole percent PET-like moiety.
[0180] As a guide to the data, un-fitted, equally spaced, parallel
lines are arranged in 10 weight % intervals across the composition
range. So, the top line follows the trend for 100% PEN. The next
line represents 90% PEN and 10% PET; the following line represents
80% PEN and 20% PET, and so forth. The bottom line follows the
trend for 100% PET.
[0181] The data remarkably falls close to these lines across the
composition range. The effect of heat setting following a
substantially uniaxial orientation is shown for the examples of PET
and PEN and intermediate compositions including both PET and PEN.
With reference to PET, for example, it can be seen that heat
setting effectively moves the index set up a line. Thus, after 100%
PET is heat set, it optically behaves more like a coPEN of 10% PEN
and 90% PET. With reference to PEN, for example, it can be seen
that heat setting also effectively moves the index set up. Thus,
heat setting generally results in higher optical power (on the
y-axis) for a given material, particularly at a given matching
index (on the x-axis).
[0182] Moreover, for a given level of optical power (on the
y-axis), heat setting increases the matching index (on the x-axis)
by about 0.01 or more, compared to the untreated material. The
greater control in nu for a given high index, birefirengent
material, such as PET, for example, allows additional flexibility
in the choice of materials, especially the second, in some cases
low index, material in an optical film such as a MOF. Typically
this second material is chosen to match the ny index of the
oriented polyester in a polarizing film. Often this second material
is a copolyester chosen not only for its index matching but also
for its flow compatibility and mechanical attributes. Generally, a
higher index target allows for a higher glass transition of such
materials. Thus an additional advantage is the additional
dimensional stability obtained in MOF construction using a low
index material with higher glass transition temperature. Moreover,
the use of higher index materials allows for MOF construction with
thinner and/or fewer layers.
[0183] Remarkably, unlike the asymmetric case, it appears that the
heat setting of the present disclosure of substantially uniaxially
stretched films either maintains or actually increases the extent
of uniaxial character of the films.
[0184] Another measure of uniaxial character is the relative
birefringence, which compares the differences between the two
similar refractive indices, e.g. ny and nz, and between the
significantly different refractive index, e.g. nx along the main
stretch direction, and the average of the two similar indicies,
e.g. nu. More precisely, the relative birefringence is given by:
Relative birefringence=|ny-nz|/|nx-nu| where again nu is the
average of the two similar indices of refraction, ny and nz, and
the absolute values of the differences are taken. The relative
birefringence decreases as the uniaxial character of the films
increases.
[0185] In some exemplary embodiments, it appears that the heat
setting of the present invention either maintains or actually
decreases the relative birefringence, particularly in certain
polyester systems in which the relative birefringence before
heat-setting is 0.1 or less. In other exemplary embodiments, small
increases in the relative birefringence result. In many
embodiments, the final relative birefringence can be 0.1 or less,
even as an (absolute) in-plane birefringence (e.g. at 632.8 nm) of
0.1 or more is achieved. In other embodiments, the final relative
birefringence is 0.25, 0.2 or less.
[0186] The nature of the tension in the stretch direction (TD in
one example) during the heat setting of the present example is an
important factor in the control of the index set. In general, a
higher level of TD tension through the heat setting process tends
to increase nx more than ny/nz, while a lower or zero level of TD
tension tends to increase the ny/nz while the nx increases slightly
or even decreases in value. Thus, low tension is useful in
increasing the ny/nz value, while high tension is useful in
increasing optical power at a fixed nu level. Thus the processes
described herein provide a method for contrast improvement with a
fixed material construction.
EXAMPLES
General Notes on Examples:
[0187] Two polyester-based constructions using two methods for
making nearly truly uniaxial film are exemplified. The first set of
examples include multilayered optical films (MOF) with a PET outer
skin layer made through a batch tentering process as described
herein with reference to FIG. 7. The second set of examples
comprise MOF with a PEN outer skin made through a parabolic
tentering process such as that described in U.S. Pat. Nos.
6,939,499; 6, 916,440; 6,949,212; and 6,936,209.
[0188] Heat setting was performed in a batch stretching device in
which the film could be constrained in the x and or y directions
with edge grippers. The stress in these constrained directions was
also measured during the course of the heat setting. The films were
heat set at 175.degree. C. for three minutes, unless otherwise
noted.
[0189] In the examples, the x direction is associated with the
so-called transverse direction (TD) and the y direction is
associated with the machine direction (MD).
[0190] Indices of refraction were measured using a Metricon Prism
Coupler, available from Metricon, located in Piscataway, N.J. In
general, two modes can be measured with the device. The TE mode is
used to measure an in-plane index of refraction. The TM mode is
used to measure the through-thickness (for example, "z") index of
refraction. One may therefore measure in the TM mode for various
orientations of the in-plane states. For example, one may use the
TM mode when the film is oriented to measure the in-plane index in
the TD direction (noted as TD/z). As another example, the TM mode
may be used with the film rotated to measure the MD in-plane index
(noted as MD/z). In general, the through-thickness indices should
be about the same regardless of the in-plane orientation. However,
discrepancies may arise due to the sharpness of the signal as a
function of film orientation.
PET Examples:
[0191] MOF films with PET skin layers were highly extended using
the batch tentering process described in FIG. 7 (Examples 1-7). The
index development in the PET skins after the stretching step was
measured using an average of "top" and "bottom" sides using a
Metricon Prism coupler. Because of the extreme thinness of the
outer PET layer, wave coupling modes rather than a sharp knee were
observed in the reflected intensity vs. incidence angle plot. To
improve precision, the index was uniformly measured as the location
of the leading edge of the first observed mode. This reasonably
agrees with wave mode fitting under certain circumstances, but may
lead to a small understatement of nx in other circumstances. The ny
and nz modes typically have less sharp readings. Again the leading
edge of the intensity drop was used.
[0192] Using this method the initial indices averaged 1.699, 1.541
and 1.539 at 632.8 nm, for the nx, ny and nz respectively. The
initial relative birefringence using these index values is thus
0.013.
[0193] The overall results are presented in Table 1 below.
TABLE-US-00001 TABLE 1 Refrac- In-plane tive orien- Indices tation/
at TM mode Uniax 632.8 nm TD TD MD MD MD measured MD/z TD/z MD/z ND
limit TD- PET Examples TD top bot ave top bot ave TD/z top top bot
bot ave MD-Z MD Example 1: Slight Draw TD w heat Start, Before
1.6939 1.7035 1.6987 1.5407 1.5402 1.5405 1.5391 1.5392 1.5387
1.5385 1.5389 0.0016 0.1583 Heat Set 5% stretch& 1.7042 1.7106
1.7074 1.542 1.545 1.5435 1.5382 1.5385 1.5385 1.5402 1.5389 0.0046
0.1639 Heat Set Diff 1st-Start 0.0087 0.0031 0.0000 0.0031 0.0056
Example 2: Slight Draw TD w heat Start, Before 1.6936 1.7027 1.6982
1.5403 1.5403 1.5403 1.5391 1.5391 1.5378 1.5387 1.5387 0.0016
0.1579 Heat Set 5% stretch& 1.7012 1.7063 1.7038 1.5403 1.5403
1.5403 1.5374 1.5378 1.5371 1.5383 1.5377 0.0026 0.1635 Heat Set
Diff 1st-Start 0.0056 0.0000 -0.0010 0.0010 0.0056 Example 3:
Constrain TD/MD Start, Before 1.6962 1.7043 1.7003 1.5409 1.5416
1.5413 1.5391 1.5385 1.5363 1.5365 1.5376 0.0036 0.1590 Heat Set
After 1st 1.6995 1.7095 1.7045 1.5429 1.5421 1.5425 1.5418 1.5418
1.5421 1.5405 1.5416 0.0010 0.1620 Heat Set Diff 1st-Start 0.0042
0.0012 0.0039 -0.0027 0.0030 Example 4: Taut start MD free Start,
Before 1.6918 1.7029 1.6974 1.5412 1.5414 1.5413 1.5394 1.5396
1.5385 1.54 1.5394 0.0019 0.1561 Heat Set After 1st 1.7037 1.7124
1.7081 1.5438 1.5423 1.5431 1.5414 1.5421 1.5416 1.5432 1.5421
0.0010 0.1650 Heat Set Diff 1st-Start 0.0107 0.0017 0.0027 -0.0010
0.0090 Example 5: Slight Slack at start TD Tension dev. MD free
Start, Before 1.6933 1.7027 1.6980 1.5412 1.5403 1.5408 1.5389
1.5409 1.5382 1.5403 1.5396 0.0012 0.1573 Heat Set After 1st 1.6999
1.7080 1.7040 1.5441 1.5436 1.5439 1.544 1.5434 1.5418 1.5423
1.5429 0.0010 0.1601 Heat Set Diff End-Start 0.0059 0.0031 0.0033
-0.0002 0.0029 Example 6: TD no tension MD free Start, Before
1.6925 1.703 1.6978 1.5418 1.5418 1.5418 1.54 1.5392 1.5391 1.5391
1.5394 0.0025 0.1560 Heat Set After 1st 1.6921 1.7003 1.6962 1.5476
1.5454 1.5465 1.5421 1.5443 1.5446 1.5454 1.5441 0.0024 0.1497 Heat
Set Diff 1st-Start -0.0016 0.0047 0.0048 -0.0001 -0.0063 After 2nd
1.6928 1.703 1.6979 1.5472 1.5494 1.5483 1.5456 1.5458 1.5508
1.5501 1.5481 0.0002 0.1496 Heat Set Diff 2nd-start 0.0002 0.0065
0.0087 -0.0022 -0.0063 Example 7: No Tension MD free Start, Before
1.6957 1.7033 1.6995 1.5412 1.5398 1.5405 1.5392 1.5396 1.5398
1.5398 1.5396 0.0009 0.1590 Heat Set After 1st 1.6952 1.7029 1.6991
1.5449 1.5449 1.5449 1.5427 1.5447 1.5437 1.5461 1.5443 0.0006
0.1542 Heat Set Diff 1st-Start -0.0004 0.0044 0.0047 -0.0003
-0.0048
[0194] The first two PET examples demonstrate the use of heat
setting with a small continuing stretch ending at an addition 5%
draw ratio. In these examples, the films were mounted taut in both
x(TD) and y(MD) directions. Due to the discontinuous nature of the
edge gripping system, the MD constraint is less than constant
initial strain. The films demonstrate an increased nx and nearly
constant ny and nz. A very small increase in asymmetry is noted.
Removal of the MD constraint may reduce this asymmetry.
[0195] In the third PET example, the films were mounted taut in x
and y but no stretching took place during heat setting. Again, the
x index (TD index) increased while the ny held nearly constant.
Surprisingly, the nz increased upon heat setting, although some of
this effect may be a result of the measurement as the knee
sharpened after heat setting. Thus the asymmetry may have decreased
or at least maintained in this case.
[0196] In the fourth PET example, the film was mount taunt only in
the x direction. The largest increase in nx was observed here. The
ny and nz each increased marginally.
[0197] In the fifth PET example, the film was begun with a slight
slack built into the mounting. The 2.5 inch TD span was deflected
about 0.25 inch out-of-plane by this slack. In this case, the nx
increase was just slightly one-half that of the fourth PET example,
but the ny increase was nearly double. The nz only marginally
increased. The film appeared taut at the end of the heat
setting.
[0198] In the sixth and seventh PET examples, the films were
provided with double the initial slack of the fifth PET example. In
these replicate cases, the films retained a slight residual slack
after heat setting. The nx essentially remained constant in these
cases, even as the ny and nz increased in nearly identical amounts.
In the sixth case, the film was measured after the first heat
setting and re-mounted for a second step, again 3 minutes at
175.degree. C. Further increases in nx and ny were observed, again
at nearly constant nx.
[0199] The effect of heat setting on the level of crystallinity was
estimated using the estimated increase in density as inferred by
the increases in the indices of refraction in accord with an
anisotropic analogue of the Lorenz-Lorentz relationship as
described in U.S. Pat. No. 6,788,463, incorporated herein by
reference (see Lorentzian in Tables 2 and 4). The amorphous density
was taken as 1.335 g/cc and the fully crystalline density as 1.457
g/cc. The volumetric polarizability was taken as 0.73757 cc/g. As
shown in Table 2, the analysis indicates that the crystallinity
(e.g. a crystal fraction of 0.32 fraction equals a 32%
crystallinity) increased from just over 30% in these samples to 40%
in the case of the double treated sixth PET example. In an
exemplary embodiment, the PET has a crystallinity after heat
setting greater than 33% (e.g., Example 2); in another exemplary
embodiment, the PET has a crystallinity greater than 36% (e.g.,
Example 3 and Example 6 after 1.sup.st heat set); in another
exemplary embodiment, the PET has a crystallinity greater than 37%
(e.g., Example 1 and Example 7 after 1.sup.st heat set); in another
exemplary embodiment, the PET has a crystallinity greater than 38%
(e.g., Example 5); in another exemplary embodiment, the PET has a
crystallinity greater than 39% (e.g., Example 4); and in another
exemplary embodiment, the PET has a crystallinity greater than 40%
(e.g., Example 6 after 2.sup.nd heat set).
[0200] Higher extremes in time and temperature would be expected to
further increase the levels of crystallinity and index changes.
TABLE-US-00002 TABLE 2 TD MD ND Relative Density Crystal PET
Examples ave ave ave Birefringence Lorentzian est est. Example 1:
Slight Draw TD w heat Start, Before Heat Set 1.6987 1.5405 1.5389
0.0099 1.013126 1.3736004 0.316 5% stretch& Heat Set 1.7074
1.5435 1.5389 0.0280 1.018289 1.3805995 0.374 Diff 1st-Start 0.0087
0.0031 0.0000 0.057 Example 2: Slight Draw TD w heat Start, Before
Heat Set 1.6982 1.5403 1.5387 0.0102 1.012722 1.3730524 0.312 5%
stretch& Heat Set 1.7038 1.5403 1.5377 0.0161 1.014613
1.3756157 0.333 Diff 1st-Start 0.0056 0.0000 -0.0010 0.021 Example
3: Constrain TD/MD Start, Before Heat Set 1.7003 1.5413 1.5376
0.0227 1.013557 1.3741843 0.321 After 1st Heat Set 1.7045 1.5425
1.5416 0.0058 1.017881 1.380046 0.369 Diff 1st-Start 0.0042 0.0012
0.0039 0.048 Example 4: Taut start MD free Start, Before Heat Set
1.6974 1.5413 1.5394 0.0123 1.013202 1.3737032 0.317 After 1st Heat
Set 1.7081 1.5431 1.5421 0.0059 1.019906 1.3827922 0.392 Diff
1st-Start 0.0107 0.0017 0.0027 0.075 Example 5: Slight Slack at
start TD Tension dev. MD free Start, Before Heat Set 1.6980 1.5408
1.5396 0.0074 1.013311 1.3738507 0.318 After 1st Heat Set 1.7040
1.5439 1.5429 0.0061 1.018937 1.3814789 0.381 Diff End-Start 0.0059
0.0031 0.0033 0.063 Example 6: TD no tension MD free Start, Before
Heat Set 1.6978 1.5418 1.5394 0.0156 1.013603 1.3742459 0.322 After
1st Heat Set 1.6962 1.5465 1.5441 0.0159 1.017499 1.3795283 0.365
Diff 1st-Start -0.0016 0.0047 0.0048 0.043 After 2nd Heat Set
1.6979 1.5483 1.5481 0.0015 1.021004 1.3842807 0.404 Diff 2nd-start
0.0002 0.0065 0.0087 0.082 Example 7: No Tension MD free Start,
Before Heat Set 1.6995 1.5405 1.5396 0.0056 1.013843 1.3745717
0.324 After 1st Heat Set 1.6991 1.5449 1.5443 0.0039 1.018043
1.380266 0.371 Diff 1st-Start -0.0004 0.0044 0.0047
PEN Examples:
[0201] Multilayer optical films (MOF) with PEN skin layers were
highly extended using the parabolic tentering process. Film was
used from a single MD lane of the continuous final film to enhance
reproducibility of the initial state. The index development in the
PEN skin layers after the stretching step was measured using an
average of "top" and "bottom" sides using a Metricon Prism coupler.
The method of index measurements using the leading edge of the wave
modes and intensity knees, as per the PET examples, were again
used.
[0202] Two PEN skin replicate examples were made (Examples 8-9).
The films were begun with a slight slack built into the mounting.
The 2.5 inch TD span was deflected about 0.5 inch out-of-plane by
this slack. Heat setting conditions were applied for 3 minutes at
175.degree. C. The film retained residual slack after treatment.
The index changes with heat setting are presented in Table 3.
[0203] As indicated in Table 3 below, the leading edge method ("by
knee") compared very closely to the "offset" mode method provided
in the software accompanying the Metricon. (The leading edge method
was operator estimated, rather than using the knee estimation
software accompanying the Metricon.) Using these methods the
initial indices averaged 1.868, 1.569 and 1.553 at 632.8 nm, for
nx, ny and nz respectively. The initial relative birefringence
using these index values is thus 0.053. TABLE-US-00003 TABLE 3
Refrac- In-plane tive orien- Indices tation/ at TM mode Uniax 632.8
nm TD TD MD MD MD measured MD/z TD/z MD/z ND limit TD- PEN Examples
TD top bot ave top bot ave TD/z top top bot bot ave MD-Z MD Example
8: Meas. By Knee, 1.8690 1.8662 1.8676 1.5681 1.5699 1.5690 1.5526
1.5542 1.5514 1.5527 0.0163 0.2986 Start before Heat Set Meas. By
Offset, 1.8671 1.8677 1.8674 1.5677 1.5691 1.5684 1.5530 1.5523
1.5511 1.5526 1.5523 0.0162 0.2990 Start before Heat Set Averages
1.8675 1.5687 1.5525 0.2988 between measurement methods After 1st
1.8593 1.8602 1.8598 1.5702 1.5709 1.5706 1.5577 1.5582 1.5580
1.5577 1.5579 0.0126 0.2892 Heat Set Diff 1st-Start -0.0078 0.0015
0.0052 -0.0036 -0.0096 Example 9: Meas. By Knee 1.8671 1.8672
1.8672 1.5699 1.5704 1.5702 1.5506 1.5512 1.5496 1.5523 1.5509
0.0192 0.2970 After 1st 1.8588 1.8587 1.8588 1.5740 1.5706 1.5723
1.5609 1.5598 1.5613 1.5607 0.0116 0.2865 Heat Set Diff 1st-Start
-0.0084 0.0022 very poor 0.0097 -0.0076 -0.0106 knees here After
2nd 1.8577 1.8577 1.5825 1.5825 1.5738 1.5754 1.5746 0.0079 0.2752
Heat Set Diff 2nd-start -0.0095 0.0124 0.0237 -0.0113 -0.0218
[0204] As seen in the PET low/no tension cases, the ny and nz
increased. However, under these conditions, the nx actually dropped
significantly. One would therefore expect less subsequent film
shrinkage. One difference between these cases is that the current
PEN case does not have a large toe-in condition after stretch that
the PET cases have. Thus, some of this index drop is related to
residual stress relief and visco-elastic relaxation during the heat
setting. It is expected that the PEN skin films after heat setting
have significantly less high temperature shrinkage (for example, at
temperatures above the glass transition of the PEN, up to the heat
set temperature) than the untreated initial film. In the second
replicate, the film was measured after the first heat setting and
re-mounted for a second step, this time for 3 minutes at
190.degree. C. Significant increases in ny and nz were observed,
with only a slight drop in nx, in agreement with the trend of the
sixth and seventh PET skin cases.
[0205] The untreated PEN-skin film, the film after the first heat
set and the film after the more severe second heat set were all
tested for mechanical dent/imprint resistance. To test for imprint
resistance, a pressure sensitive adhesive was laminated onto one
surface of the film and that surface was then laminated onto a
glass slide. A piece of BEF.TM. brightness enhancement film,
available from 3M Company, St. Paul Minn. was placed with its
micro-textured surface against an exposed film surface with a
weight of 150 g on top to ensure intimate contact. The resulting
pressure was estimated as 200 g/sq. inch. The film was then tested
for 24 hours at 85.degree. C. The initial film and the film with
the light first heat setting were modestly imprint resistance, but
significant denting occurred. The film with the second, more severe
heat setting exhibited almost no dents.
[0206] Again, the effect of heat setting on the level of
crystallinity was estimated using the estimated increases in
density as inferred by the increases in the indices of refraction
in accord with an anisotropic analogue of the Lorenz-Lorentz
relationship as described in U.S. Pat. No. 6,788,463. The amorphous
density for PEN was taken as 1.329 g/cc and the fully crystalline
density as 1.407 g/cc. The volumetric polarizability was taken as
0.81501 cc/g. As shown in Table 4, the analysis indicates that the
crystallinity increased very little in the first heat step,
providing further evidence for the mechanism of index changes as
one of visco-elastic relaxation.
[0207] This also indicates that the level of crystallinity is a
major factor in imprint resistance. After the more extreme heat
setting, the film acquired a much higher level of crystallinity,
estimated at about 48%. This more crystalline final film exhibited
the highest level of imprint resistance among the example presented
here.
[0208] In an exemplary embodiment, the PEN has a crystallinity
after heat setting greater than 28% (e.g., Example 8); in another
exemplary embodiment, the PEN has a crystallinity greater than 30%
(e.g., Example 9 after 1.sup.st heat set); in another exemplary
embodiment, the PEN has a crystallinity greater than 48% (e.g.,
Example 9 after 2.sup.nd heat set).
[0209] As shown by the examples, second or subsequent heat setting
steps can be used to obtain desired film properties. TABLE-US-00004
TABLE 4 TD MD ND Relative Density Crystal PEN Examples ave ave ave
Birefringence Lorentzian est est. Example 8: Meas. By Knee, Start
1.8676 1.5690 1.5527 0.0530 1.100845 1.3507131 0.278 before Heat
Set Meas. By Offset, Start 1.8674 1.5684 1.5523 0.0526 1.100255
1.3499895 0.269 before Heat Set Averages between 1.8675 1.5687
1.5525 0.0528 0.274 measurement methods After 1st Heat Set 1.8598
1.5706 1.5579 0.0428 1.101116 1.3510461 0.282 Diff 1st-Start
-0.0078 0.0015 0.0052 0.009 Example 9: Meas. By Knee 1.8672 1.5702
1.5509 0.0627 1.100354 1.3501114 0.271 After 1st Heat Set 1.8588
1.5723 1.5607 0.0398 1.102886 1.3532172 0.310 Diff 1st-Start
-0.0084 0.0022 0.0097 0.040 After 2nd Heat Set 1.8577 1.5825 1.5746
0.0283 1.11388 1.3667074 0.483 Diff 2nd-start -0.0095 0.0124 0.0237
0.213
CoPEN Examples: Co-polyester Example 10:
[0210] A co-polyester, intermediate in composition between PEN and
PET, was formed by charging an extruder with a pellet mixture of 85
mol % PEN (with an approximate intrinsic viscosity (IV) of 0.5) and
15 mol % PET (with an approximate IV of 0.8). These transesterified
in-situ during extrusion and were cast to form a clear unoriented
cast web comprising a so-called 85/15 coPEN. A film comprising this
material can be used as a birefringent layer in a multilayer
optical film, e.g. a reflective polarizer film.
[0211] A strip 6 cm long by 2.5 cm wide was cut from the cast web
and drawn on a laboratory stretching apparatus. The strip was
pre-heated for 1 minute at 130 degrees C. and drawn along its
length without constraint in its width at a nominal draw rate of
20%/second to a final draw ratio of 5.5 as measured by fiducial
marks placed on the film before stretching.
[0212] After stretching, the film was quenched to room temperature
and the refractive indices were measured at 632.8 nm using the
Metricon Prism Coupler as 1.8436, 1.5668 and 1.5595 along the
length, width and thickness directions, respectively. Thus, a
relative birefringence of 0.061 was obtained after stretching.
[0213] The oriented film was then mounted with slight initial
tension along the length, and uncontrained in the width, and heat
at 170 degrees C. for two minutes. The film was quenched again, and
the refractive indices were measured at 632.8 nm using the Metricon
Prism Coupler as 1.8404, 1.5718 and 1.5492 along the length, width
and thickness directions, respectively. Thus, a relative
birefringence of 0.081 was obtained after stretching.
Co-polyester Example 11:
[0214] An 85/15 coPEN was formed and drawn in similar manner to of
example 10. A film comprising this material can be used as a
birefringent layer in a multilayer optical film, e.g. a reflective
polarizer film.
[0215] The oriented film was then mounted with slight initial
tension along the length, and uncontrained in the width, and heated
at 190 degrees C. for 30 seconds. The film was further heated for
90 more seconds at 190 degrees C. while the draw ratio was reduced
from its initial 5.5.times. after stretching to 4.7.times. after
heat setting. The film was quenched again, and the refractive
indices were measured at 632.8 nm using the Metricon Prism Coupler
as 1.8185, 1.5827 and 1.5576 along the length, width and thickness
directions, respectively. Thus, a relative birefringence of 0.101
was obtained after stretching.
Heat Set Uniaxially Oriented Multi-Layer Optical Films with
Polycarbonate/Copolyester Blend Isotropic Layers
Comparative Example 1
Multilayer Optical Film--PEN/CoPEN5545HD/CoPEN7525HD Reflective
Polarizer
[0216] A multilayer reflective polarizer film was constructed with
first optical layers created from a polyethylene naphthalate and
second optical layers created from
copolyethylenenaphthalate(CoPEN5545HD) and skin layers or
non-optical layers created from a higher Tg
copolyethylenenaphthalate(CoPEN7525HD).
[0217] The above described PEN and CoPEN5545HD were coextruded
through a multilayer melt manifold to create a multilayer optical
film with 275 alternating first and second optical layers. This 275
layer multi-layer stack was divided into 3 parts and stacked to
form 825 layers. The PEN layers were the first optical layers and
the CoPEN5545HD layers were the second optical layers. In addition
to the first and second optical layers, a set of non-optical
layers, also comprised of CoPEN5545HD were coextruded as PBL
(protective boundary layers) on either side of the optical layer
stack. Two sets of skin layers comprising CoPEN7525HD were also
coextruded on the outer side of the PBL non-optical layers through
additional melt ports. The multi-layer film construction was in
order of layers: CoPEN7525HD skin layer, a CoPen5545HD PBL, 825
alternating layers of optical layers PEN/CoPEN5545HD, a second
CoPen5545HD PBL, and a second skin layer CoPEN7525HD.
[0218] The multilayer extruded film was cast onto a chill roll at
15 meters per minute (45 feet per minute) and heated in an oven at
150.degree. C. (302.degree. F.) for 30 seconds, and then uniaxially
oriented at a 5.5:1 draw ratio. After orientation, the drawn
multi-layer film was passed through a heat set oven at 200.degree.
C. for 15 seconds. A reflective polarizer film of approximately 150
microns (6 mils) thickness was produced which was too mechanically
brittle for web handling, winding into a roll, or die-cutting into
film parts without breaking.
Comparative Example 2
Multilayer Optical Film--CoPEN90100/CoPEN-tbia/CoPEN-tbia
Reflective Polarizer
[0219] A multilayer reflective polarizer film was constructed with
first optical layers created from a copolyethylenenaphthalate
(CoPEN9010) and second optical layers created from
copolyethylenenaphthalate (CoPEN-tbia) and skin layers or
non-optical layers created from copolyethylenenaphthalate
(CoPEN-tbia).
[0220] The above described CoPEN9010 and CoPEN-tbia were coextruded
through a multilayer melt manifold to create a multilayer optical
film with 275 alternating first and second optical layers. The
CoPEN9010 layers were the first optical layers and the CoPEN-tbia
layers were the second optical layers. In addition to the first and
second optical layers, a set of non-optical layers, also comprised
of CoPEN-tbia, were coextruded as PBL (protective boundary layers)
on either side of the optical layer stack. Two sets of skin layers
comprising CoPEN-tbia were also coextruded on the outer side of the
PBL non-optical layers through additional melt ports. The
multi-layer film construction was in order of layers: CoPEN-tbia
skin and PBL layers, 275 alternating layers of optical layers
CoPEN9010/CoPEN-tbia, and a second set of skin and PBL layers of
CoPEN-tbia.
[0221] The multilayer extruded film was cast onto a chill roll at
15 meters per minute (45 feet per minute) and heated in an oven at
150.degree. C. (302.degree. F.) for 30 seconds, and then uniaxially
oriented at a 6.5:1 draw ratio. After orientation, the drawn
multi-layer film was passed through a heat set oven at 200.degree.
C. for 15 seconds. A reflective polarizer film of approximately 37
microns (1.5 mils) thickness was produced which was too
mechanically brittle for web handling, winding into a roll, or
die-cutting into film parts without breaking.
Example 3
Multilayer Optical Film--PEN/CoPEN5050HH/SA115 Reflective Polarizer
Film
[0222] A multilayer reflective polarizer film was constructed with
first optical layers created from a polyethylene naphthalate and
second optical layers created from copolyethylene naphthalate
(CoPEN5050HH) and skin layers or non-optical layers created from a
cycloaliphatic polyester/polycarbonate blend commercially available
from Eastman Chemical CO. under the tradename "SA115."
[0223] The above described PEN and CoPEN5050HH were coextruded
through a multilayer melt manifold to create a multilayer optical
film with 275 alternating first and second optical layers. This 275
layer multi-layer stack was divided into 3 parts and stacked to
form 825 layers. The PEN layers were the first optical layers and
the CoPEN-5050HH layers were the second optical layers. In addition
to the first and second optical layers, a set of non-optical
layers, also comprised of CoPEN5050HH were coextruded as PBL
(protective boundary layers) on either side of the optical layer
stack. Two sets of SA115 skin layers were also coextruded on the
outer side of the PBL non-optical layers through additional melt
ports. The construction was in order of layers: SA115 skin layer,
CoPEN5050HH PBL layer, 825 alternating layers of optical layers of
PEN and CoPEN-5050HH, a second CoPEN5050HH PBL layer, and a second
SA115 skin layer.
[0224] The multilayer extruded film was cast onto a chill roll at
15 meters per minute (45 feet per minute) and heated in an oven at
150.degree. C. (302.degree. F.) for 30 seconds, and then uniaxially
oriented at a 5.5:1 draw ratio. After orientation, the drawn
multi-layer film was passed through a heat set oven at 200.degree.
C. for 15 seconds. A reflective polarizer film of approximately 150
microns (6 mils) thickness was produced. This film was not
mechanically brittle, could easily be wound into a film roll, and
die cut into film parts without breaking.
Example 4
Multilayer Optical Film--CoPEN9010/SA115/SA115 Reflective Polarizer
Film
[0225] A multilayer reflective polarizer film was constructed with
first optical layers created from a polyethylene naphthalate
(CoPEN9010), and with second optical layers and skin layers created
from a cycloaliphatic polyester/polycarbonate blend commercially
available from Eastman Chemical under the series tradename
"SA115".
[0226] This CoPEN9010 was coextruded with SA115 through a
multilayer melt manifold to create a multilayer optical film with
275 alternating first and second optical layers. The CoPEN9010
layers were the first optical layers and the SA115 layers were the
second optical layers. In addition to the first and second optical
layers, a set of non-optical layers, also comprised of SA115, were
coextruded as PBL (protective boundary layers) on either side of
the optical layer stack. Two skin layers comprised of SA115 were
also coextruded on the outer side of the PBL non-optical layers
through additional melt ports. The construction was in order of
layers: SA115 outer skin and PBL layers, 275 alternating optical
layers of CoPEN9010 and SA115, and a further set of SA115 PBL and
outer skin layers.
[0227] The multilayer extruded film was cast onto a chill roll at
22 meters per minute (66 feet per minute) and heated in an oven at
139.degree. C. (283.degree. F.) for 30 seconds, and then nearly
truly uniaxially oriented at a 6:1 draw ratio. After orientation,
the drawn multi-layer film was passed through a heat set oven at
200.degree. C. for 15 seconds. A reflective polarizer film of
approximately 30 microns (1.2 mils) was produced which was not
mechanically brittle, could easily be wound into a film roll, and
die cut into film parts without breaking.
Description of polymer making for above examples.
Manufacture of PEN
[0228] The polyethylene naphthalate (PEN) used to form the first
optical layers was synthesized in a batch reactor with the
following raw material charge: dimethyl naphthalene dicarboxylate
(136 kg), ethylene glycol (73 kg), manganese (II) acetate (27 g),
cobalt (II) acetate (27 g) and antimony (III) acetate (48 g). Under
a pressure of 2 atmospheres (1520 torr or 2.times.105 N/m2), this
mixture was heated to 254.degree. C. while removing methanol (a
transesterification reaction by-product). After 35 kg of methanol
was removed, triethyl phosphonoacetate (49 g) was charged to the
reactor and the pressure was gradually reduced to 1 torr (131 N/m2)
while heating to 290.degree. C. The condensation reaction
by-product, ethylene glycol, was continuously removed until a
polymer with an intrinsic viscosity of 0.48 dL/g (as measured in
60/40 wt. % phenol/o-dichlorobenzene) was produced.
Manufacture of CoPEN9010
[0229] The copolyethylene naphthalate(CoPEN9010) used to form the
first optical layers was synthesized in a batch reactor with the
following raw material charge: 126 kg dimethyl naphthalene
dicarboxylate, 11 kg dimethyl terephthalate, 75 kg ethylene glycol,
27 g manganese acetate, 27 g cobalt acetate, and 48 g antimony
triacetate. Under pressure of 2 atm (2.times.105 N/m2), this
mixture was heated to 254.degree. C. while removing methanol. After
36 kg of methanol is removed, 49 g of triethyl phosphonoacetate was
charged to the reactor and than the pressure was gradually reduced
to 1 torr while heating to 290.degree. C. The condensation reaction
by-product, ethylene glycol, was continuously removed until a
polymer with an intrinsic viscosity of 0.50 dL/g, as measured in
60/40 wt. % phenol/o-dichlorobenzene, was produced.
Manufacture of CoPEN5545HD
[0230] The copolyethylene-hexamethylene naphthalate polymer
(CoPEN5545HD) used to form the second optical layers was
synthesized in a batch reactor with the following raw material
charge: dimethyl 2,6-naphthalenedicarboxylate (88.5 kg), dimethyl
terephthalate (57.5 kg), 1,6-hexane diol (4.7 kg), ethylene glycol
(81 kg), trimethylol propane (239 g), cobalt (II) acetate (15 g),
zinc acetate (22 g), and antimony (III) acetate (51 g). The mixture
was heated to a temperature of 254.degree. C. at a pressure of two
atmospheres (2.times.105 N/m2) and the mixture allowed to react
while removing the methanol reaction product. After completing the
reaction and removing the methanol (approximately 39.6 kg) the
reaction vessel was charged with triethyl phosphonoacetate (37 g)
and the pressure was reduced to one torr (263 N/m2) while heating
to 290.degree. C. The condensation by-product, ethylene glycol, was
continuously removed until a polymer with intrinsic viscosity 0.56
dl/g as measured in a 60/40 weight percent mixture of phenol and
o-dichlorobenzene was produced. The CoPEN5545HD polymer produced by
this method had a glass transition temperature (Tg) of 94.degree.
C. as measured by differential scanning calorimetry at a
temperature ramp rate of 20.degree. C. per minute. The CoPEN5050HH
polymer had a refractive index of 1.612 at 632 nm.
Manufacture of CoPEN7525HD
[0231] The copolyethylene-hexamethylene naphthalate polymer
(CoPEN7525HD) used to form the second optical layers was
synthesized in a batch reactor with the following raw material
charge: dimethyl 2,6-naphthalenedicarboxylate (114.8 kg), dimethyl
terephthalate (30.4 kg), 1,6-hexane diol (5.9 kg), ethylene glycol
(75 kg), trimethylol propane (200 g), cobalt (II) acetate (15 g),
zinc acetate (22 g), and antimony (III) acetate (51 g). The mixture
was heated to a temperature of 254.degree. C. at a pressure of two
atmospheres (2.times.105 N/m2) and the mixture allowed to react
while removing the methanol reaction product. After completing the
reaction and removing the methanol (approximately 39.6 kg) the
reaction vessel was charged with triethyl phosphonoacetate (37 g)
and the pressure was reduced to one torr (263 N/m2) while heating
to 290.degree. C. The condensation by-product, ethylene glycol, was
continuously removed until a polymer with intrinsic viscosity 0.52
dl/g as measured in a 60/40 weight percent mixture of phenol and
o-dichlorobenzene was produced. The CoPEN7525HD polymer produced by
this method had a glass transition temperature (Tg) of 102.degree.
C. as measured by differential scanning calorimetry at a
temperature ramp rate of 20.degree. C. per minute. The CoPEN7525HD
polymer had a refractive index of 1.624 at 632 nm.
Manufacture of CoPEN5050HH
[0232] The copolyethylene-hexamethylene naphthalate polymer
(CoPEN5050HH) used to form the second optical layers was
synthesized in a batch reactor with the following raw material
charge: dimethyl 2,6-naphthalenedicarboxylate (80.9 kg), dimethyl
terephthalate (64.1 kg), 1,6-hexane diol (15.45 kg), ethylene
glycol (75.4 kg), trimethylol propane (2 kg), cobalt (II) acetate
(25 g), zinc acetate (40 g), and antimony (III) acetate (60 g). The
mixture was heated to a temperature of 254.degree. C. at a pressure
of two atmospheres (2.times.105 N/m2) and the mixture allowed to
react while removing the methanol reaction product. After
completing the reaction and removing the methanol (approximately
42.4 kg), the reaction vessel was charged with triethyl
phosphonoacetate (55 g) and the pressure was reduced to one torr
(263 N/m2) while heating to 290.degree. C. The condensation
by-product, ethylene glycol, was continuously removed until a
polymer with intrinsic viscosity 0.55 dl/g as measured in a 60/40
weight percent mixture of phenol and o-dichlorobenzene was
produced. The CoPEN5050HH polymer produced by this method had a
glass transition temperature (Tg) of 85.degree. C. as measured by
differential scanning calorimetry at a temperature ramp rate of
20.degree. C. per minute. The CoPEN5050HH polymer had a refractive
index of 1.601 at 632 nm.
Manufacture of CoPEN-tbia
[0233] The synthesis of the CoPEN-tbia polymer was carried out in a
batch reactor which was charged with the following materials:
dimethyl 2,6-naphthalenedicarboxylate (47.3 kg), dimethyl
terephthalate (18.6 kg), 1,4-cyclohexane dimethanol (40.5 kg),
neopentyl glycol (15 kg), ethylene glycol (41.8 kg), trimethylol
propane (2 kg), cobalt (II) acetate (36.3 g), zinc acetate (50 g),
and antimony (III) acetate (65 g). The mixture was heated to a
temperature of 254.degree. C. at a pressure of two atmospheres
(2.times.105 N/m2) and the mixture allowed to react while removing
the methanol reaction product.
[0234] After completing the reaction and removing all of the
methanol (approximately 13.1 kg) the reaction vessel was charged
with tertiary-butyl isophthalate (43.2 kg). The reaction was
continued at 254.degree. C. until approximately 7.4 kg of water was
removed and the reaction was complete. The reaction vessel was
charged with triethyl phosphonoacetate (70 g) and the pressure was
reduced to one torr (263 N/m2) while heating to 290.degree. C. The
condensation by-product, ethylene glycol, was continuously removed
until a polymer with intrinsic viscosity 0.632 dl/g as measured in
a 60/40 weight percent mixture of phenol and o-dichlorobenzene was
produced. The CoPEN-tbia polymer produced by this method had a
glass transition temperature (Tg) of 102.degree. C. as measured by
differential scanning calorimetry at a temperature ramp rate of
20.degree. C. per minute. The CoPEN-tbia polymer had a refractive
index of 1.567.
[0235] All patents, patent applications, provisional applications,
and publications referred to or cited herein are incorporated by
reference in their entirety, including all figures and tables, to
the extent they are not inconsistent with the explicit teachings of
this specification.
[0236] It should be understood that the examples and embodiments
described herein are for illustrative purposes only and that
various modifications or changes in light thereof will be suggested
to persons skilled in the art and are to be included within the
spirit and purview of this application.
* * * * *