U.S. patent application number 11/326766 was filed with the patent office on 2006-06-01 for optical film with co-continuous phases.
This patent application is currently assigned to 3M Innovative Properties Company. Invention is credited to Richard C. Allen, Lockwood W. Carlson, Arthur L. Kotz, Biswaroop Majumdar, Timothy J. Nevitt, Andrew J. Ouderkirk, Carl A. Stover, Michael F. Weber.
Application Number | 20060114563 11/326766 |
Document ID | / |
Family ID | 24443701 |
Filed Date | 2006-06-01 |
United States Patent
Application |
20060114563 |
Kind Code |
A1 |
Allen; Richard C. ; et
al. |
June 1, 2006 |
OPTICAL FILM WITH CO-CONTINUOUS PHASES
Abstract
An optical film is provided which comprises a disperse phase of
polymeric particles disposed within a continuous birefringent
matrix. The film is oriented, typically by stretching, in one or
more directions. The size and shape of the disperse phase
particles, the volume fraction of the disperse phase, the film
thickness, and the amount of orientation are chosen to attain a
desired degree of diffuse reflection and total transmission of
electromagnetic radiation of a desired wavelength in the resulting
film.
Inventors: |
Allen; Richard C.; (Mendota
Heights, MN) ; Kotz; Arthur L.; (White Bear Lake,
MN) ; Carlson; Lockwood W.; (Stillwater, MN) ;
Nevitt; Timothy J.; (Red Wing, MN) ; Ouderkirk;
Andrew J.; (Woodbury, MN) ; Stover; Carl A.;
(St. Paul, MN) ; Weber; Michael F.; (Shoreview,
MN) ; Majumdar; Biswaroop; (Delmar, NY) |
Correspondence
Address: |
3M INNOVATIVE PROPERTIES COMPANY
PO BOX 33427
ST. PAUL
MN
55133-3427
US
|
Assignee: |
3M Innovative Properties
Company
|
Family ID: |
24443701 |
Appl. No.: |
11/326766 |
Filed: |
January 6, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11001971 |
Dec 2, 2004 |
6999233 |
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11326766 |
Jan 6, 2006 |
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10614775 |
Jul 7, 2003 |
6987612 |
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11001971 |
Dec 2, 2004 |
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08801329 |
Feb 18, 1997 |
6590705 |
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10614775 |
Jul 7, 2003 |
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08610110 |
Feb 29, 1996 |
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08801329 |
Feb 18, 1997 |
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Current U.S.
Class: |
359/487.01 |
Current CPC
Class: |
G02B 5/3008 20130101;
Y10T 428/249955 20150401; Y10T 428/249953 20150401; G02B 5/3083
20130101; G02B 1/04 20130101; G02B 5/305 20130101 |
Class at
Publication: |
359/490 ;
359/483 |
International
Class: |
G02B 5/30 20060101
G02B005/30; G02B 27/28 20060101 G02B027/28 |
Claims
1. An optical body, comprising: an open-celled polymeric
birefringent first phase; and a substantially nonbirefringent
second phase disposed within the cells of said first phase; wherein
the absolute value of the difference in index of refraction of said
first and second phases is .DELTA.n.sub.1 along a first axis and
.DELTA.n.sub.2 along a second axis orthogonal to said first axis,
and wherein the absolute value of the difference between
.DELTA.n.sub.1 and .DELTA.n.sub.2 is at least about 0.05, and
wherein the diffuse reflectivity of said first and second phases
taken together along at least one axis for at least one
polarization of electromagnetic radiation is at least about
30%.
2. An optical body, comprising: an open-celled polymeric
birefringent first phase; and a substantially nonbirefringent
second phase disposed within the cells of said first phase; wherein
the absolute value of the difference in index of refraction of said
first and second phases is .DELTA.n.sub.1 along a first axis and
.DELTA.n.sub.2 along a second axis orthogonal to said first axis,
and wherein the absolute value of the difference between
.DELTA.n.sub.1 and .DELTA.n.sub.2 is at least about 0.05, and
wherein said first phase has a larger birefringence than said
second phase.
3. The optical body of claim 2, wherein the birefringence of said
first phase is at least 0.02 greater than the birefringence of said
second phase.
4. The optical body of claim 2, wherein the birefringence of said
first phase is at least 0.05 greater than the birefringence of said
second phase.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This is a continuation application of U.S. application Ser.
No. 11/001,971, filed Dec. 2, 2004, allowed, which is a divisional
application of U.S. application Ser. No. 10/614,775, filed Jul. 7,
2003, now U.S. Pat. No. 6,987,612, which is a continuation
application of U.S. application Ser. No. 08/801,329, filed Feb. 18,
1997, now U.S. Pat. No. 6,590,705, which is a continuation-in-part
of U.S. application Ser. No. 08/610,110, filed Feb. 29, 1996, now
abandoned, the disclosure of which is herein incorporated by
reference.
FIELD OF THE INVENTION
[0002] This invention relates to optical materials which contain
structures suitable for controlling optical characteristics, such
as reflectance and transmission. In a further aspect, it relates to
control of specific polarizations of reflected or transmitted
light.
BACKGROUND
[0003] Optical films are known to the art which are constructed
from inclusions dispersed within a continuous matrix. The
characteristics of these inclusions can be manipulated to provide a
range of reflective and transmissive properties to the film. These
characteristics include inclusion size with respect to wavelength
within the film, inclusion shape and alignment, inclusion
volumetric fill factor and the degree of refractive index mismatch
with the continuous matrix along the film's three orthogonal
axes.
[0004] Conventional absorbing (dichroic) polarizers have, as their
inclusion phase, inorganic rod-like chains of light-absorbing
iodine which are aligned within a polymer matrix. Such a film will
tend to absorb light polarized with its electric field vector
aligned parallel to the rod-like iodine chains, and to transmit
light polarized perpendicular to the rods. Because the iodine
chains have two or more dimensions that are small compared to the
wavelength of visible light, and because the number of chains per
cubic wavelength of light is large, the optical properties of such
a film are predominately specular, with very little diffuse
transmission through the film or diffuse reflection from the film
surfaces. Like most other commercially available polarizers, these
polarizing films are based on polarization-selective
absorption.
[0005] Films filled with inorganic inclusions with different
characteristics can provide other optical transmission and
reflective properties. For example, coated mica flakes with two or
more dimensions that are large compared with visible wavelengths,
have been incorporated into polymeric films and into paints to
impart a metallic glitter. These flakes can be manipulated to lie
in the plane of the film, thereby imparting a strong directional
dependence to the reflective appearance. Such an effect can be used
to produce security screens that are highly reflective for certain
viewing angles, and transmissive for other viewing angles. Large
flakes having a coloration (specularly selective reflection) that
depends on alignment with respect to incident light, can be
incorporated into a film to provide evidence of tampering. In this
application, it is necessary that all the flakes in the film be
similarly aligned with respect to each other.
[0006] However, optical films made from polymers filled with
inorganic inclusions suffer from a variety of infirmities.
Typically, adhesion between the inorganic particles and the polymer
matrix is poor. Consequently, the optical properties of the film
decline when stress or strain is applied across the matrix, both
because the bond between the matrix and the inclusions is
compromised, and because the rigid inorganic inclusions may be
fractured. Furthermore, alignment of inorganic inclusions requires
process steps and considerations that complicate manufacturing.
[0007] Other films, such as that disclosed in U.S. Pat. No.
4,688,900 (Doane et. al.), consists of a clear light-transmitting
continuous polymer matrix, with droplets of light modulating liquid
crystals dispersed within. Stretching of the material reportedly
results in a distortion of the liquid crystal droplet from a
spherical to an ellipsoidal shape, with the long axis of the
ellipsoid parallel to the direction of stretch. U.S. Pat. No.
5,301,046 (Konuma et al.) make a similar disclosure, but achieve
the distortion of the liquid crystal droplet through the
application of pressure. A. Aphonin, "Optical Properties of
Stretched Polymer Dispersed Liquid Crystal Films: Angle-Dependent
Polarized Light Scattering, Liquid Crystals, Vol. 19, No. 4,
469-480 (1995), discusses the optical properties of stretched films
consisting of liquid crystal droplets disposed within a polymer
matrix. He reports that the elongation of the droplets into an
ellipsoidal shape, with their long axes parallel to the stretch
direction, imparts an oriented birefringence (refractive index
difference among the dimensional axes of the droplet) to-the
droplets, resulting in a relative refractive index mismatch between
the dispersed and continuous phases along certain film axes, and a
relative index match along the other film axes. Such liquid crystal
droplets are not small as compared to visible wavelengths in the
film, and thus the optical properties of such films have a
substantial diffuse component to their reflective and transmissive
properties. Aphonin suggests the use of these materials as a
polarizing diffuser for backlit twisted nematic LCDs. However,
optical films employing liquid crystals as the disperse phase are
substantially limited in the degree of refractive index mismatch
between the matrix phase and the dispersed phase. Furthermore, the
birefringence of the liquid crystal component of such films is
typically sensitive to temperature.
[0008] U.S. Pat. No. 5,268,225 (Isayev) discloses a composite
laminate made from thermotropic liquid crystal polymer blends. The
blend consists of two liquid crystal polymers which are immiscible
with each other. The blends may be cast into a film consisting of a
dispersed inclusion phase and a continuous phase. When the film is
stretched, the dispersed phase forms a series of fibers whose axes
are aligned in the direction of stretch. While the film is
described as having improved mechanical properties, no mention is
made of the optical properties of the film. However, due to their
liquid crystal nature, films of this type would suffer from the
infirmities of other liquid crystal materials discussed above.
[0009] Still other films have been made to exhibit desirable
optical properties through the application of electric or magnetic
fields. For example, U.S. Pat. No. 5,088,807 (Waters et al.)
describes a liquid crystal device which consists of a layer of
fibers permeated with liquid crystal material and disposed between
two electrodes. A voltage across the electrodes produces an
electric field which changes the birefringent properties of the
liquid crystal material, resulting in various degrees of mismatch
between the refractive indices of the fibers and the liquid
crystal. However, the requirement of an electric or magnetic field
is inconvenient and undesirable in many applications, particularly
those where existing fields might produce interference.
[0010] Other optical films have been made by incorporating a
dispersion of inclusions of a first polymer into a second polymer,
and then stretching the resulting composite in one or two
directions. U.S. Pat. No. 4,871,784 (Otonari et al. ) is
exemplative of this technology. The polymers are selected such that
there is low adhesion between the dispersed phase and the
surrounding matrix polymer, so that an elliptical void is formed
around each inclusion when the film is stretched. Such voids have
dimensions of the order of visible wavelengths. The refractive
index mismatch between the void and the polymer in these
"microvoided" films is typically quite large (about 0.5), causing
substantial diffuse reflection. However, the optical properties of
microvoided materials are difficult to control because of
variations of the geometry of the interfaces, and it is not
possible to produce a film axis for which refractive indices are
relatively matched, as would be useful for polarization-sensitive
optical properties. Furthermore, the voids in such material can be
easily collapsed through exposure to heat and pressure.
[0011] Optical films have also been made wherein a dispersed phase
is deterministically arranged in an ordered pattern within a
continuous matrix. U.S. Pat. No. 5,217,794 (Schrenk) is exemplative
of this technology. There, a lamellar polymeric film is disclosed
which is made of polymeric inclusions which are large compared with
wavelength on two axes, disposed within a continuous matrix of
another polymeric material. The refractive index of the dispersed
phase differs significantly from that of the continuous phase along
one or more of the laminate's axes, and is relatively well matched
along another. Because of the ordering of the dispersed phase,
films of this type exhibit strong iridescence (i.e.,
interference-based angle dependent coloring) for instances in which
they are substantially reflective. As a result, such films have
seen limited use for optical applications where optical diffusion
is desirable.
[0012] There thus remains a need in the art for an optical material
consisting of a continuous and a dispersed phase, wherein the
refractive index mismatch between the two phases along the
material's three dimensional axes can be conveniently and
permanently manipulated to achieve desirable degrees of diffuse and
specular reflection and transmission, wherein the optical material
is stable with respect to stress, strain, temperature differences,
and electric and magnetic fields, and wherein the optical material
has an insignificant level of iridescence. These and other needs
are met by the present invention, as hereinafter disclosed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a schematic drawing illustrating an optical body
made in accordance with the present invention, wherein the disperse
phase is arranged as a series of elongated masses having an
essentially circular cross-section;
[0014] FIG. 2 is a schematic drawing illustrating an optical body
made in accordance with the present invention, wherein the disperse
phase is arranged as a series of elongated masses having an
essentially elliptical cross-section;
[0015] FIGS. 3a-e are schematic drawings illustrating various
shapes of the disperse phase in an optical body made in accordance
with the present invention;
[0016] FIG. 4a is a graph of the bidirectional scatter distribution
as a function of scattered angle for an oriented film in accordance
with the present invention for light polarized perpendicular to
orientation direction;
[0017] FIG. 4b is a graph of the bidirectional scatter distribution
as a function of scattered angle for an oriented film in accordance
with the present invention for light polarized parallel to
orientation direction;
[0018] FIG. 5 is a schematic representation of a multilayer film
made in accordance with the present invention; and
[0019] FIGS. 6a and 6b are electron micrographs of optical films
made in accordance with the present invention.
[0020] FIG. 7 is a schematic diagram of an Interpenetrating Polymer
Network (IPN).
SUMMARY OF THE INVENTION
[0021] In one aspect, the present invention relates to a diffusely
reflective film or other optical body comprising a birefringent
continuous polymeric phase and a substantially nonbirefringent
disperse phase disposed within the continuous phase. The indices of
refraction of the continuous and disperse phases are substantially
mismatched (i.e., differ from one another by more than about 0.05)
along a first of three mutually orthogonal axes, and are
substantially matched (i.e., differ by less than about 0.05) along
a second of three mutually orthogonal axes. In some embodiments,
the indices of refraction of the continuous and disperse phases can
be substantially matched or mismatched along, or parallel to, a
third of three mutually orthogonal axes to produce a mirror or a
polarizer. Incident light polarized along, or parallel to, a
mismatched axis is scattered, resulting in significant diffuse
reflection. Incident light polarized along a matched axis is
scattered to a much lesser degree and is significantly spectrally
transmitted. These properties can be used to make optical films for
a variety of uses, including low loss (significantly nonabsorbing)
reflective polarizers for which polarizations of light that are not
significantly transmitted are diffusely reflected.
[0022] In a related aspect, the present invention relates to an
optical film or other optical body comprising a birefringent
continuous phase and a disperse phase, wherein the indices of
refraction of the continuous and disperse phases are substantially
matched (i.e., wherein the index difference between the continuous
and disperse phases is less than about 0.05) along an axis
perpendicular to a surface of the optical body.
[0023] In another aspect, the present invention relates to a
composite optical body comprising a polymeric continuous
birefringent first phase in which the disperse second phase may be
birefringent, but in which the degree of match and mismatch in at
least two orthogonal directions is primarily due to the
birefringence of the first phase.
[0024] In still another aspect, the present invention relates to a
method for obtaining a diffuse reflective polarizer, comprising the
steps of: providing a first resin, whose degree of birefringence
can be altered by application of a force field, as through
dimensional orientation or an applied electric field, such that the
resulting resin material has, for at least two orthogonal
directions, an index of refraction difference of more than about
0.05; providing a second resin, dispersed within the first resin;
and applying said force field to the composite of both resins such
that the indices of the two resins are approximately matched to
within less than about 0.05 in one of two directions, and the index
difference between first and second resins in the other of two
directions is greater than about 0.05. In a related embodiment, the
second resin is dispersed in the first resin after imposition of
the force field and subsequent alteration of the birefringence of
the first resin.
[0025] In yet another aspect, the present invention relates to an
optical body acting as a reflective polarizer with a high
extinction ratio. In this aspect, the index difference in the match
direction is chosen as small as possible and the difference in the
mismatch direction is maximized. The volume fraction, thickness,
and disperse phase particle size and shape can be chosen to
maximize the extinction ratio, although the relative importance of
optical transmission and reflection for the different polarizations
may vary for different applications.
[0026] In another aspect, the present invention relates to an
optical body comprising a continuous phase, a disperse phase whose
index of refraction differs from said continuous phase by greater
than about 0.05 along a first axis and by less than about 0.05
along a second axis orthogonal to said first axis, and a dichroic
dye. The optical body is preferably oriented along at least one
axis. The dichroic dye improves the extinction coefficient of the
optical body by absorbing, in addition to scattering, light
polarized parallel to the axis of orientation.
[0027] In another aspect of the present invention, an optical body
is provided which has at least first and second phases that are
co-continuous along at least one axis. The index of refraction of
the first phase differs from that of the second phase by greater
than about 0.05 along a first axis and by less than about 0.05
along a second axis orthogonal to said first axis. In other
embodiments, three or more co-continuous phases may be used to
achieve the same or similar matches and mismatches along mutually
perpendicular axes.
[0028] In the various aspects of the present invention, the
reflection and transmission properties for at least two orthogonal
polarizations of incident light are determined by the selection or
manipulation of various parameters, including the optical indices
of the continuous and disperse phases, the size and shape of the
disperse phase particles, the volume fraction of the disperse
phase, the thickness of the optical body through which some
fraction of the incident light is to pass, and the wavelength or
wavelength band of electromagnetic radiation of interest.
[0029] The magnitude of the index match or mismatch along a
particular axis will directly affect the degree of scattering of
light polarized along that axis. In general, scattering power
varies as the square of the index mismatch. Thus, the larger the
index mismatch along a particular axis, the stronger the scattering
of light polarized along that axis. Conversely, when the mismatch
along a particular axis is small, light polarized along that axis
is scattered to a lesser extent and is thereby transmitted
specularly through the volume of the body.
[0030] The size of the disperse phase also can have a significant
effect on scattering. If the disperse phase particles are too small
(i.e., less than about 1/30 the wavelength of light in the medium
of interest) and if there are many particles per cubic wavelength,
the optical body behaves as a medium with an effective index of
refraction somewhat between the indices of the two phases along any
given axis. In such a case, very little light is scattered. If the
particles are too large, the light is specularly reflected from the
particle surface, with very little diffusion into other directions.
When the particles are too large in at least two orthogonal
directions, undesirable iridescence effects can also occur.
Practical limits may also be reached when particles become large in
that the thickness of the optical body becomes greater and
desirable mechanical properties are compromised.
[0031] The shape of the particles of the disperse phase can also
have an effect on the scattering of light. The depolarization
factors of the particles for the electric field in the index of
refraction match and mismatch directions can reduce or enhance the
amount of scattering in a given direction. The effect can either
add or detract from the amount of scattering from the index
mismatch, but generally has a small influence on scattering in the
preferred range of properties in the present invention.
[0032] The shape of the particles can also influence the degree of
diffusion of light scattered from the particles. This shape effect
is generally small but increases as the aspect ratio of the
geometrical cross-section of the particle in the plane
perpendicular to the direction of incidence of the light increases
and as the particles get relatively larger. In general, in the
operation of this invention, the disperse phase particles should be
sized less than several wavelengths of light in one or two mutually
orthogonal dimensions if diffuse, rather than specular, reflection
is preferred.
[0033] Dimensional alignment is also found to have an effect on the
scattering behavior of the disperse phase. In particular, it has
been observed, in optical bodies made in accordance with the
present invention, that aligned scatterers will not scatter light
symmetrically about the directions of specular transmission or
reflection as randomly aligned scatterers would. In particular,
inclusions that have been elongated by orientation to resemble rods
scatter light primarily along (or near) a cone centered on the
orientation direction and having an edge along the specularly
transmitted direction. For example, for light incident on such an
elongated rod in a direction perpendicular to the orientation
direction, the scattered light appears as a band of light in the
plane perpendicular to the orientation direction with an intensity
that decreases with increasing angle away from the specular
directions. By tailoring the geometry of the inclusions, some
control over the distribution of scattered light can be achieved
both in the transmissive hemisphere and in the reflective
hemisphere.
[0034] The volume fraction of the disperse phase also affects the
scattering of light in the optical bodies of the present invention.
Within certain limits, increasing the volume fraction of the
disperse phase tends to increase the amount of scattering that a
light ray experiences after entering the body for both the match
and mismatch directions of polarized light. This factor is
important for controlling the reflection and transmission
properties for a given application. However, if the volume fraction
of the disperse phase becomes too large, light scattering
diminishes. Without wishing to be bound by theory, this appears to
be due to the fact that the disperse phase particles are closer
together, in terms of the wavelength of light, so that the
particles tend to act together as a smaller number of large
effective particles.
[0035] The thickness of the optical body is also an important
control parameter which can be manipulated to affect reflection and
transmission properties in the present invention. As the thickness
of the optical body increases, diffuse reflection also increases,
and transmission, both specular and diffuse, decreases.
[0036] While the present invention will often be described herein
with reference to the visible region of the spectrum, various
embodiments of the present invention can be used to operate at
different wavelengths (and thus frequencies) of electromagnetic
radiation through appropriate scaling of the components of the
optical body. Thus, as the wavelength increases, the linear size of
the components of the optical body are increased so that the
dimensions, measured in units of wavelength, remain approximately
constant. Another major effect of changing wavelength is that, for
most materials of interest, the index of refraction and the
absorption coefficient change. However, the principles of index
match and mismatch still apply at each wavelength of interest.
DETAILED DESCRIPTION OF THE INVENTION
Introduction
[0037] As used herein, the terms "specular reflection" and
"specular reflectance" refer to the reflectance of light rays into
an emergent cone with a vertex angle of 16 degrees centered around
the specular angle. The terms "diffuse reflection" or "diffuse
reflectance" refer to the reflection of rays that are outside the
specular cone defined above. The terms "total reflectance" or
"total reflection" refer to the combined reflectance of all light
from a surface. Thus, total reflection is the sum of specular and
diffuse reflection.
[0038] Similarly, the terms "specular transmission" and "specular
transmittance" are used herein in reference to the transmission of
rays into an emergent cone with a vertex angle of 16 degrees
centered around the specular direction. The terms "diffuse
transmission" and "diffuse transmittance" are used herein in
reference to the transmission of all rays that are outside the
specular cone defined above. The terms "total transmission" or
"total transmittance" refer to the combined transmission of all
light through an optical body. Thus, total transmission is the sum
of specular and diffuse transmission.
[0039] As used herein, the term "extinction ratio" is defined to
mean the ratio of total light transmitted in one polarization to
the light transmitted in an orthogonal polarization.
[0040] FIGS. 1-2 illustrate a first embodiment of the present
invention. In accordance with the invention, a diffusely reflective
optical film 10 or other optical body is produced which consists of
a birefringent matrix or continuous phase 12 and a discontinuous or
disperse phase 14. The birefringence of the continuous phase is
typically at least about 0.05, preferably at least about 0.1, more
preferably at least about 0.15, and most preferably at least about
0.2.
[0041] The indices of refraction of the continuous and disperse
phases are substantially matched (i.e., differ by less than about
0.05) along a first of three mutually orthogonal axes, and are
substantially mismatched (i.e., differ by more than about 0.05)
along a second of three mutually orthogonal axes. Preferably, the
indices of refraction of the continuous and disperse phases differ
by less than about 0.03 in the match direction, more preferably,
less than about 0.02, and most preferably, less than about 0.01.
The indices of refraction of the continuous and disperse phases
preferably differ in the mismatch direction by at least about 0.07,
more preferably, by at least about 0.1, and most preferably, by at
least about 0.2.
[0042] The mismatch in refractive indices along a particular axis
has the effect that incident light polarized along that axis will
be substantially scattered, resulting in a significant amount of
reflection. By contrast, incident light polarized along an axis in
which the refractive indices are matched will be spectrally
transmitted or reflected with a much lesser degree of scattering.
This effect can be utilized to make a variety of optical devices,
including reflective polarizers and mirrors.
[0043] The present invention provides a practical and simple
optical body and method for making a reflective polarizer, and also
provides a means of obtaining a continuous range of optical
properties according to the principles described herein. Also, very
efficient low loss polarizers can be obtained with high extinction
ratios. Other advantages are a wide range of practical materials
for the disperse phase and the continuous phase, and a high degree
of control in providing optical bodies of consistent and
predictable high quality performance.
Effect of Index Match/Mismatch
[0044] In the preferred embodiment, the materials of at least one
of the continuous and disperse phases are of a type that undergoes
a change in refractive index upon orientation. Consequently, as the
film is oriented in one or more directions, refractive index
matches or mismatches are produced along one or more axes. By
careful manipulation of orientation parameters and other processing
conditions, the positive or negative birefringence of the matrix
can be used to induce diffuse reflection or transmission of one or
both polarizations of light along a given axis. The relative ratio
between transmission and diffuse reflection is dependent on the
concentration of the disperse phase inclusions, the thickness of
the film, the square of the difference in the index of refraction
between the continuous and disperse phases, the size and geometry
of the disperse phase inclusions, and the wavelength or wavelength
band of the incident radiation.
[0045] The magnitude of the index match or mismatch along a
particular axis directly affects the degree of scattering of light
polarized along that axis. In general, scattering power varies as
the square of the index mismatch. Thus, the larger the index
mismatch along a particular axis, the stronger the scattering of
light polarized along that axis. Conversely, when the mismatch
along a particular axis is small, light polarized along that axis
is scattered to a lesser extent and is thereby transmitted
specularly through the volume of the body.
[0046] FIGS. 4a-b demonstrate this effect in oriented films made in
accordance with the present invention. There, a typical
Bidirectional Scatter Distribution Function (BSDF) measurement is
shown for normally incident light at 632.8 nm. The BSDF is
described in J. Stover, "Optical Scattering Measurement and
Analysis" (1990). The BSDF is shown as a function of scattered
angle for polarizations of light both perpendicular and parallel to
the axis of orientation. A scattered angle of zero corresponds to
unscattered (spectrally transmitted) light. For light polarized in
the index match direction (that is, perpendicular to the
orientation direction) as in FIG. 4a, there is a significant
specularly transmitted peak with a sizable component of diffusely
transmitted light (scattering angle between 8 and 80 degrees), and
a small component of diffusely reflected light (scattering angle
larger than 100 degrees). For light polarized in the index mismatch
direction (that is, parallel to the orientation direction) as in
FIG. 4b, there is negligible specularly transmitted light and a
greatly reduced component of diffusely transmitted light, and a
sizable diffusely reflected component. It should be noted that the
plane of scattering shown by these graphs is the plane
perpendicular to the orientation direction where most of the
scattered light exists for these elongated inclusions. Scattered
light contributions outside of this plane are greatly reduced.
[0047] If the index of refraction of the inclusions (i.e., the
disperse phase) matches that of the continuous host media along
some axis, then incident light polarized with electric fields
parallel to this axis will pass through unscattered regardless of
the size, shape, and density of inclusions. If the indices are not
matched along some axis, then the inclusions will scatter light
polarized along this axis. For scatterers of a given
cross-sectional area with dimensions larger than approximately
.lamda./30 (where .lamda. is the wavelength of light in the media),
the strength of the scattering is largely determined by the index
mismatch. The exact size, shape and alignment of a mismatched
inclusion play a role in determining how much light will be
scattered into various directions from that inclusion. If the
density and thickness of the scattering layer is sufficient,
according to multiple scattering theory, incident light will be
either reflected or absorbed, but not transmitted, regardless of
the details of the scatterer size and shape.
[0048] When the material is to be used as a polarizer, it is
preferably processed, as by stretching and allowing some
dimensional relaxation in the cross stretch in-plane direction, so
that the index of refraction difference between the continuous and
disperse phases is large along a first axis in a plane parallel to
a surface of the material and small along the other two orthogonal
axes. This results in a large optical anisotropy for
electromagnetic radiation of different polarizations.
[0049] Some of the polarizers within the scope of the present
invention are elliptical polarizers. In general, elliptical
polarizers will have a difference in index of refraction between
the disperse phase and the continuous phase for both the stretch
and cross-stretch directions. The ratio of forward to back
scattering is dependent on the difference in refractive index
between the disperse and continuous phases, the concentration of
the disperse phase, the size and shape of the disperse phase, and
the overall thickness of the film. In general, elliptical diffusers
have a relatively small difference in index of refraction between
the particles of the disperse and continuous phases. By using a
birefringent polymer-based diffuser, highly elliptical polarization
sensitivity (i.e., diffuse reflectivity depending on the
polarization of light) can be achieved. At an extreme, where the
index of refraction of the polymers match on one axis, the
elliptical polarizer will be a diffuse reflecting polarizer.
Methods of Obtaining Index Match/Mismatch
[0050] The materials selected for use in a polarizer in accordance
with the present invention, and the degree of orientation of these
materials, are preferably chosen so that the phases in the finished
polarizer have at least one axis for which the associated indices
of refraction are substantially equal. The match of refractive
indices associated with that axis, which typically, but not
necessarily, is an axis transverse to the direction of orientation,
results in substantially no reflection of light in that plane of
polarization.
[0051] The disperse phase may also exhibit a decrease in the
refractive index associated with the direction of orientation after
stretching. If the birefringence of the host is positive, a
negative strain induced birefringence of the disperse phase has the
advantage of increasing the difference between indices of
refraction of the adjoining phases associated with the orientation
axis while the reflection of light with its plane of polarization
perpendicular to the orientation direction is still negligible.
Differences between the indices of refraction of adjoining phases
in the direction orthogonal to the orientation direction should be
less than about 0.05 after orientation, and preferably, less than
about 0.02.
[0052] The disperse phase may also exhibit a positive strain
induced birefringence. However, this can be altered by means of
heat treatment to match the refractive index of the axis
perpendicular to the orientation direction of the continuous phase.
The temperature of the heat treatment should not be so high as to
relax the birefringence in the continuous phase.
Size of Disperse Phase
[0053] The size of the disperse phase also can have a significant
effect on scattering. If the disperse phase particles are too small
(i.e., less than about 1/30 the wavelength of light in the medium
of interest) and if there are many particles per cubic wavelength,
the optical body behaves as a medium with an effective index of
refraction somewhat between the indices of the two phases along any
given axis. In such a case, very little light is scattered. If the
particles are too large, the light is specularly reflected from the
surface of the particle, with very little diffusion into other
directions. When the particles are too large in at least two
orthogonal directions, undesirable iridescence effects can also
occur. Practical limits may also be reached when particles become
large in that the thickness of the optical body becomes greater and
desirable mechanical properties are compromised.
[0054] The dimensions of the particles of the disperse phase after
alignment can vary depending on the desired use of the optical
material. Thus, for example, the dimensions of the particles may
vary depending on the wavelength of electromagnetic radiation that
is of interest in a particular application, with different
dimensions required for reflecting or transmitting visible,
ultraviolet, infrared, and microwave radiation. Generally, however,
the length of the particles should be such that they are
approximately greater than the wavelength of electromagnetic
radiation of interest in the medium, divided by 30.
[0055] Preferably, in applications where the optical body is to be
used as a low loss reflective polarizer, the particles will have a
length that is greater than about 2 times the wavelength, of the
electromagnetic radiation over the wavelength range of interest,
and preferably over 4 times the wavelength. The average diameter of
the particles is preferably equal or less than the wavelength of
the electromagnetic radiation over the wavelength range of
interest, and preferably less than 0.5 of the desired wavelength.
While the dimensions of the disperse phase are a secondary
consideration in most applications, they become of greater
importance in thin film applications, where there is comparatively
little diffuse reflection.
Geometry of Disperse Phase
[0056] While the index mismatch is the predominant factor relied
upon to promote scattering in the films of the present invention
(i.e., a diffuse mirror or polarizer made in accordance with the
present invention has a substantial mismatch in the indices of
refraction of the continuous and disperse phases along at least one
axis), the geometry of the particles of the disperse phase can have
a secondary effect on scattering. Thus, the depolarization factors
of the particles for the electric field in the index of refraction
match and mismatch directions can reduce or enhance the amount of
scattering in a given direction. For example, when the disperse
phase is elliptical in a cross-section taken along a plane
perpendicular to the axis of orientation, the elliptical
cross-sectional shape of the disperse phase contributes to the
asymmetric diffusion in both back scattered light and forward
scattered light. The effect can either add or detract from the
amount of scattering from the index mismatch, but generally has a
small influence on scattering in the preferred range of properties
in the present invention.
[0057] The shape of the disperse phase particles can also influence
the degree of diffusion of light scattered from the particles. This
shape effect is generally small but increases as the aspect ratio
of the geometrical cross-section of the particle in the plane
perpendicular to the direction of incidence of the light increases
and as the particles get relatively larger. In general, in the
operation of this invention, the disperse phase particles should be
sized less than several wavelengths of light in one or two mutually
orthogonal dimensions if diffuse, rather than specular, reflection
is preferred.
[0058] Preferably, for a low loss reflective polarizer, the
preferred embodiment consists of a disperse phase disposed within
the continuous phase as a series of rod-like structures which, as a
consequence of orientation, have a high aspect ratio which can
enhance reflection for polarizations parallel to the orientation
direction by increasing the scattering strength and dispersion for
that polarization relative to polarizations perpendicular to the
orientation direction. However, as indicated in FIGS. 3a-e, the
disperse phase may be provided with many different geometries.
Thus, the disperse phase may be disk-shaped or elongated
disk-shaped, as in FIGS. 3a-c, rod-shaped, as in FIG. 3d-e, or
spherical. Other embodiments are contemplated wherein the disperse
phase has cross sections which are approximately elliptical
(including circular), polygonal, irregular, or a combination of one
or more of these shapes. The cross-sectional shape and size of the
particles of the disperse phase may also vary from one particle to
another, or from one region of the film to another (i.e., from the
surface to the core).
[0059] In some embodiments, the disperse phase may have a core and
shell construction, wherein the core and shell are made out of the
same or different materials, or wherein the core is hollow. Thus,
for example, the disperse phase may consist of hollow fibers of
equal or random lengths, and of uniform or non-uniform cross
section. The interior space of the fibers may be empty, or may be
occupied by a suitable medium which may be a solid, liquid, or gas,
and may be organic or inorganic. The refractive index of the medium
may be chosen in consideration of the refractive indices of the
disperse phase and the continuous phase so as to achieve a desired
optical effect (i.e., reflection or polarization along a given
axis).
[0060] The geometry of the disperse phase may be arrived at through
suitable orientation or processing of the optical material, through
the use of particles having a particular geometry, or through a
combination of the two. Thus, for example, a disperse phase having
a substantially rod-like structure can be produced by orienting a
film consisting of approximately spherical disperse phase particles
along a single axis. The rod-like structures can be given an
elliptical cross-section by orienting the film in a second
direction perpendicular to the first. As a further example, a
disperse phase having a substantially rod-like structure in which
the rods are rectangular in cross-section can be produced by
orienting in a single direction a film having a disperse phase
consisting of a series of essentially rectangular flakes.
[0061] Stretching is one convenient manner for arriving at a
desired geometry, since stretching can also be used to induce a
difference in indices of refraction within the material. As
indicated above, the orientation of films in accordance with the
invention may be in more than one direction, and may be sequential
or simultaneous.
[0062] In another example, the components of the continuous and
disperse phases may be extruded such that the disperse phase is
rod-like in one axis in the unoriented film. Rods with a high
aspect ratio may be generated by orienting in the direction of the
major axis of the rods in the extruded film. Plate-like structures
may be generated by orienting in an orthogonal direction to the
major axis of the rods in the extruded film.
[0063] The structure in FIG. 2 can be produced by asymmetric
biaxial orientation of a blend of essentially spherical particles
within a continuous matrix. Alternatively, the structure may be
obtained by incorporating a plurality of fibrous structures into
the matrix material, aligning the structures along a single axis,
and orienting the mixture in a direction transverse to that axis.
Still another method for obtaining this structure is by controlling
the relative viscosities, shear, or surface tension of the
components of a polymer blend so as to give rise to a fibrous
disperse phase when the blend is extruded into a film. In general,
it is found that the best results are obtained when the shear is
applied in the direction of extrusion.
Dimensional Alignment of Disperse Phase
[0064] Dimensional alignment is also found to have an effect on the
scattering behavior of the disperse phase. In particular, it has
been observed in optical bodies made in accordance with the present
invention that aligned scatterers will not scatter light
symmetrically about the directions of specular transmission or
reflection as randomly aligned scatterers would. In particular,
inclusions that have been elongated through orientation to resemble
rods scatter light primarily along (or near) the surface of a cone
centered on the orientation direction and along the specularly
transmitted direction. This may result in an anisotropic
distribution of scattered light about the specular reflection and
specular transmission directions. For example, for light incident
on such an elongated rod in a direction perpendicular to the
orientation direction, the scattered light appears as a band of
light in the plane perpendicular to the orientation direction with
an intensity that decreases with increasing angle away from the
specular directions. By tailoring the geometry of the inclusions,
some control over the distribution of scattered light can be
achieved both in the transmissive hemisphere and in the reflective
hemisphere.
Dimensions of Disperse Phase
[0065] In applications where the optical body is to be used as a
low loss reflective polarizer, the structures of the disperse phase
preferably have a high aspect ratio, i.e., the structures are
substantially larger in one dimension than in any other dimension.
The aspect ratio is preferably at least 2, and more preferably at
least 5. The largest dimension (i.e., the length) is preferably at
least 2 times the wavelength of the electromagnetic radiation over
the wavelength range of interest, and more preferably at least 4
times the desired wavelength. On the other hand, the smaller (i.e.,
cross-sectional) dimensions of the structures of the disperse phase
are preferably less than or equal to the wavelength of interest,
and more preferably less than 0.5 times the wavelength of
interest.
Volume Fraction of Disperse Phase
[0066] The volume fraction of the disperse phase also affects the
scattering of light in the optical bodies of the present invention.
Within certain limits, increasing the volume fraction of the
disperse phase tends to increase the amount of scattering that a
light ray experiences after entering the body for both the match
and mismatch directions of polarized light. This factor is
important for controlling the reflection and transmission
properties for a given application.
[0067] The desired volume fraction of the disperse phase will
depend on many factors, including the specific choice of materials
for the continuous and disperse phase. However, the volume fraction
of the disperse phase will typically be at least about 1% by volume
relative to the continuous phase, more preferably within the range
of about 5 to about 15%, and most preferably within the range of
about 15 to about 30%.
Co-Continuous Phases
[0068] When the volume fraction for binary blends of high polymers
of roughly equivalent viscosity 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
Engineerng, 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).
[0069] Materials having co-continuous phases may be made in
accordance with the present invention by a number of different
methods. Thus, for example, the polymeric first phase material may
be mechanically blended with the polymeric second phase material to
achieve a co-continuous system. Examples of co-continuous
morphologies achieved by blending are described, for example, in D.
Bourry and B. D. Favis, "Co-Continuity and Phase Inversion in
HDPE/PS Blends: The Role of Interfacial Modification", 1995 Annual
Technical Conference of the Society of Plastics Engineers ANTEC,
Vol. 53, No. 2, 2001-2009 (polystyrene/polyethylene blends), and in
A. Leclair and B. D. Favis, "The role of interfacial contact in
immiscible binary polymer blends and its influence on mechanical
properties", Polymer, Vol. 37, No. 21, 4723-4728, 1996
(polycarbonate/polyethylene blends).
[0070] Co-continuous phases may also be formed in accordance with
the present invention by first by dissolving them out of
supercritical fluid extractions, such as that disclosed for blends
of polystyrene and poly(methyl methacrylate) in U.S. Pat. No.
4,281,084, and then allowing them to phase separate following
exposure to heat and/or mechanical shear, as described by in N.
Mekhilef, B. D. Favis and P. J. Carreau, "Morphological Stability
of Polystyrene Polyethylene Blends", 1995 Annual Technical
Conference of the Society of Plastics Engineers ANTEC, Vol. 53, No.
2, 1572-1579).
[0071] A further method of producing co-continuous phases in
accordance with the present invention is through the creation of
interpenetrating polymer networks (IPNs). Some of the more
important IPNs include simultaneous IPNs, sequential IPNs, gradient
IPNs, latex IPNs, thermoplastic IPNs, and semi-IPNs. These and
other types of IPNs, their physical properties (e.g., phase
diagrams), and methods for their preparation and characterization,
are described, for example, in L. H. Sperling and V. Mishra,
"Current Status of Interpenetrating Polymer Networks", Polymers for
Advanced Technologies, Vol. 7, No. 4, 197-208, April 1996, and in
L. H. Sperling, "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). Some of the major methods for preparing these systems
are summarized below.
[0072] Simultaneous IPNs may be made by mixing together the
respective monomers or prepolymers, plus the crosslinkers and
activators, of two or more polymer networks. The respective
monomers or prepolymers are then reacted simultaneously, but in a
non-interfering manner. Thus, for example, one reaction may be made
to proceed by way of chain polymerization kinetics, and the other
reaction may be made to proceed through step polymerization
kinetics.
[0073] Sequential IPNs are made by first forming an initial polymer
network. Then, the monomers, crosslinkers, and activators of one or
more additional networks are swollen into the initial polymer
network, where they are reacted in situ to yield additional polymer
networks.
[0074] Gradient IPNs are synthesized in such a manner that the
overall composition or crosslink density of the IPN varies
macroscopically in the material from one location to another. Such
systems may be made, for example, by forming a first polymer
network predominantly on one surface of a film and a second polymer
network predominantly on another surface of the film, with a
gradient in composition throughout the interior of the film.
[0075] Latex IPNs are made in the form of latexes (e.g., with a
core and shell structure). In some variations, two or more latexes
may be mixed and formed into a film, which crosslinks the
polymers.
[0076] Thermoplastic IPNs are hybrids between polymer blends and
IPNs that involve physical crosslinks instead of chemical
crosslinks. As a result, these materials can be made to flow at
elevated temperatures in a manner similar to that of thermoplastic
elastomers, but are crosslinked and behave as IPNs at the
temperatures of normal use.
[0077] Semi-IPNs are compositions of two or more polymers in which
one or more of the polymers are crosslinked and one or more of the
polymers are linear or branched.
[0078] As indicated above, co-continuity can be achieved in
multicomponent systems as well as in binary systems. For example,
three or more materials may be used in combination to give desired
optical properties (e.g., transmission and reflectivity) and/or
improved physical properties. All components may be immiscible, or
two or more components may demonstrate miscibility. A number of
ternary systems exhibiting co-continuity are described, for
example, in 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).
[0079] Characteristic sizes of the phase structures, ranges of
volume fraction over which co-continuity may be observed, and
stability of the morphology may all be influenced by additives,
such as compatibilizers, graft or block copolymers, or reactive
components, such as maleic anhydride or glycidyl methacrylate. Such
effects are described, for example, for blends of polystyrene and
poly(ethylene terephthalate) in H. Y. Tsai and K. Min, "Reactive
Blends of Functionalized Polystyrene and Polyethylene
Terephthalate", 1995 Annual Technical Conference of the Society of
Plastics Engineers ANTEC, Vol. 53, No. 2, 1858-1865. However, for
particular systems, phase diagrams may be constructed through
routine experimentation and used to produce co-continuous systems
in accordance with the present invention.
[0080] The microscopic structure of co-continuous systems made in
accordance with the present invention can vary significantly,
depending on the method of preparation, the miscibility of the
phases, the presence of additives, and other factors as are known
to the art. Thus, for example, one or more of the phases in the
co-continuous system may be fibrillar (see, e.g., FIG. 7), with the
fibers either randomly oriented or oriented along a common axis.
Other co-continuous systems may comprise an open-celled matrix of a
first phase, with a second phase disposed in a co-continuous manner
within the cells of the matrix. The phases in these systems may be
co-continuous along a single axis, along two axes, or along three
axes.
[0081] Optical bodies made in accordance with the present invention
and having co-continuous phases (particularly IPNs) will, in
several instances, have properties that are advantageous over the
properties of similar optical bodies that are made with only a
single continuous phase, depending, of course, on the properties of
the individual polymers and the method by which they are combined.
Thus, for example, the co-continuous systems of the present
invention allow for the chemical and physical combination of
structurally dissimilar polymers, thereby providing a convenient
route by which the properties of the optical body may be modified
to meet specific needs. Furthermore, co-continuous systems will
frequently be easier to process, and may impart such properties as
weatherability, reduced flammability, greater impact resistance and
tensile strength, improved flexibility, and superior chemical
resistance. IPNs are particularly advantageous in certain
applications, since they typically swell (but do not dissolve) in
solvents, and exhibit suppressed creep and flow compared to
analogous non-IPN systems (see, e.g., D. Klempner and L. Berkowski,
"Interpenetrating Polymer Networks", Encyclopedia of Polymer
Science and Engineering, Vol. 8, 278-341.
[0082] One skilled in the art will appreciate that the principles
of co-continuous systems as are known to the art may be applied in
light of the teachings set forth herein to produce co-continuous
morphologies having unique optical properties. Thus, for example,
the refractive indices of known co-continuous morphologies may be
manipulated as taught herein to produce new optical films in
accordance with the present invention. Likewise, the principles
taught herein may be applied to known optical systems to produce
co-continuous morphologies.
Thickness of Optical Body
[0083] The thickness of the optical body is also an important
parameter which can be manipulated to affect reflection and
transmission properties in the present invention. As the thickness
of the optical body increases, diffuse reflection also increases,
and transmission, both specular and diffuse, decreases. Thus, while
the thickness of the optical body will typically be chosen to
achieve a desired degree of mechanical strength in the finished
product, it can also be used to directly to control reflection and
transmission properties.
[0084] Thickness can also be utilized to make final adjustments in
reflection and transmission properties of the optical body. Thus,
for example, in film applications, the device used to extrude the
film can be controlled by a downstream optical device which
measures transmission and reflection values in the extruded film,
and which varies the thickness of the film (i.e., by adjusting
extrusion rates or changing casting wheel speeds) so as to maintain
the reflection and transmission values within a predetermined
range.
Materials for Continuous/Disperse Phases
[0085] Many different materials may be used as the continuous or
disperse phases in the optical bodies of the present invention,
depending on the specific application to which the optical body is
directed. Such materials include inorganic materials such as
silica-based polymers, organic materials such as liquid crystals,
and polymeric materials, including monomers, copolymers, grafted
polymers, and mixtures or blends thereof. The exact choice of
materials for a given application will be driven by the desired
match and mismatch obtainable in the refractive indices of the
continuous and disperse phases along a particular axis, as well as
the desired physical properties in the resulting product. However,
the materials of the continuous phase will generally be
characterized by being substantially transparent in the region of
the spectrum desired.
[0086] A further consideration in the choice of materials is that
the resulting product must contain at least two distinct phases.
This may be accomplished by casting the optical material from two
or more materials which are immiscible with each other.
Alternatively, if it is desired to make an optical material with a
first and second material which are not immiscible with each other,
and if the first material has a higher melting point than the
second material, in some cases it may be possible to embed
particles of appropriate dimensions of the first material within a
molten matrix of the second material at a temperature below the
melting point of the first material. The resulting mixture can then
be cast into a film, with or without subsequent orientation, to
produce an optical device.
[0087] Suitable polymeric materials for use as the continuous or
disperse phase in the present invention may be amorphous,
semicrystalline, or crystalline polymeric materials, including
materials made from monomers based on carboxylic acids such as
isophthalic, azelaic, adipic, sebacic, dibenzoic, terephthalic,
2,7-naphthalene dicarboxylic, 2,6-naphthalene dicarboxylic,
cyclohexanedicarboxylic, and bibenzoic acids (including
4,4'-bibenzoic acid), or materials made from the corresponding
esters of the aforementioned acids (i.e., dimethylterephthalate).
Of these, 2,6-polyethylene naphthalate (PEN) is especially
preferred because of its strain induced birefringence, and because
of its ability to remain permanently birefringent after stretching.
PEN has a refractive index for polarized incident light of 550 nm
wavelength which increases after stretching when the plane of
polarization is parallel to the axis of stretch from about 1.64 to
as high as about 1.9, while the refractive index decreases for
light polarized perpendicular to the axis of stretch. PEN exhibits
a birefringence (in this case, the difference between the index of
refraction along the stretch direction and the index perpendicular
to the stretch direction) of 0.25 to 0.40 in the visible spectrum.
The birefringence can be increased by increasing the molecular
orientation. PEN may be substantially heat stable from about
155.degree. C. up to about 230.degree. C., depending upon the
processing conditions utilized during the manufacture of the
film.
[0088] Polybutylene naphthalate is also a suitable material as well
as other crystalline naphthalene dicarboxylic polyesters. The
crystalline naphthalene dicarboxylic polyesters exhibit a
difference in refractive indices associated with different in-plane
axes of at least 0.05 and preferably above 0.20.
[0089] When PEN is used as one phase in the optical material of the
present invention, the other phase is preferably
polymethylmethacrylate (PMMA) or a syndiotactic vinyl aromatic
polymer such as polystyrene (sPS). Other preferred polymers for use
with PEN are based on terephthalic, isophthalic, sebacic, azelaic
or cyclohexanedicarboxylic acid or the related alkyl esters of
these materials. Naphthalene dicarboxylic acid may also be employed
in minor amounts to improve adhesion between the phases. The diol
component may be ethylene glycol or a related diol. Preferably, the
index of refraction of the selected polymer is less than about
1.65, and more preferably, less than about 1.55, although a similar
result may be obtainable by using a polymer having a higher index
of refraction if the same index difference is achieved.
[0090] Syndiotactic-vinyl aromatic polymers useful in the current
invention include poly(styrene), poly(alkyl styrene), poly(styrene
halide), poly(alkyl styrene), poly(vinyl ester benzoate), and these
hydrogenated polymers and mixtures, or copolymers containing these
structural units. Examples of poly(alkyl styrenes) include:
poly(methyl styrene), poly(ethyl styrene), poly(propyl styrene),
poly(butyl styrene), poly(phenylstyrene), poly(vinyl naphthalene),
poly(vinylstyrene), and poly(acenaphthalene) may be mentioned. As
for the poly(styrene halides), examples include:
poly(chlorostyrene), poly(bromostyrene), and poly(fluorostyrene).
Examples of poly(alkoxy styrene) include: poly(methoxy styrene),
and poly(ethoxy styrene). Among these examples, as particularly
preferable styrene group polymers, are: polystyrene, poly(p-methyl
styrene), poly(m-methyl styrene), poly(p-tertiary butyl styrene),
poly(p-chlorostyrene), poly(m-chloro styrene), poly(p-fluoro
styrene), and copolymers of styrene and p-methyl styrene may be
mentioned.
[0091] Furthermore, as comonomers of syndiotactic vinyl-aromatic
group copolymers, besides monomers of above explained styrene group
polymer, olefin monomers such as ethylene, propylene, butene,
hexene, or octene; diene monomers such as butadiene, isoprene;
polar vinyl monomers such as cyclic diene monomer, methyl
methacrylate, maleic acid anhydride, or acrylonitrile may be
mentioned.
[0092] The syndiotactic-vinyl aromatic polymers of the present
invention may be block copolymers, random copolymers, or
alternating copolymers.
[0093] The vinyl aromatic polymer having high level syndiotactic
structure referred to in this invention generally includes
polystyrene having syndiotacticity of higher than 75% or more, as
determined by carbon-13 nuclear magnetic resonance. Preferably, the
degree of syndiotacticity is higher than 85% racemic diad, or
higher than 30%, or more preferably, higher than 50%, racemic
pentad.
[0094] In addition, although there are no particular restrictions
regarding the molecular weight of this syndiotactic-vinyl aromatic
group polymer, preferably, the weight average molecular weight is
greater than 10,000 and less than 1,000,000, and more preferably,
greater than 50,000 and less than 800,000.
[0095] As for said other resins, various types may be mentioned,
including, for instance, vinyl aromatic group polymers with atactic
structures, vinyl aromatic group polymers with isotactic
structures, and all polymers that are miscible. For example,
polyphenylene ethers show good miscibility with the previous
explained vinyl aromatic group polymers. Furthermore, the
composition of these miscible resin components is preferably
between 70 to 1 weight %, or more preferably, 50 to 2 weight %.
When composition of miscible resin component exceeds 70 weight %,
degradation on the heat resistance may occur, and is usually not
desirable.
[0096] It is not required that the selected polymer for a
particular phase be a copolyester or copolycarbonate. Vinyl
polymers and copolymers made from monomers such as vinyl
naphthalenes, styrenes, ethylene, maleic anhydride, acrylates, and
methacrylates may also be employed. Condensation polymers, other
than polyesters and polycarbonates, can also be utilized. Suitable
condensation polymers include polysulfones, polyamides,
polyurethanes, polyamic acids, and polyimides. Naphthalene groups
and halogens such as chlorine, bromine and iodine are useful in
increasing the refractive index of the selected polymer to the
desired level (1.59 to 1.69) if needed to substantially match the
refractive index if PEN is the host. Acrylate groups and fluorine
are particularly useful in decreasing the refractive index.
[0097] Minor amounts of comonomers may be substituted into the
naphthalene dicarboxylic acid polyester so long as the large
refractive index difference in the orientation direction(s) is not
substantially compromised. A smaller index difference (and
therefore decreased reflectivity) may be counterbalanced by
advantages in any of the following: improved adhesion between the
continuous and disperse phase, lowered temperature of extrusion,
and better match of melt viscosities.
Region of Spectrum
[0098] While the present invention is frequently described herein
with reference to the visible region of the spectrum, various
embodiments of the present invention can be used to operate at
different wavelengths (and thus frequencies) of electromagnetic
radiation through appropriate scaling of the components of the
optical body. Thus, as the wavelength increases, the linear size of
the components of the optical body may be increased so that the
dimensions of these components, measured in units of wavelength,
remain approximately constant.
[0099] Of course, one major effect of changing wavelength is that,
for most materials of interest, the index of refraction and the
absorption coefficient change. However, the principles of index
match and mismatch still apply at each wavelength of interest, and
may be utilized in the selection of materials for an optical device
that will operate over a specific region of the spectrum. Thus, for
example, proper scaling of dimensions will allow operation in the
infrared, near-ultraviolet, and ultra-violet regions of the
spectrum. In these cases, the indices of refraction refer to the
values at these wavelengths of operation, and the body thickness
and size of the disperse phase scattering components should also be
approximately scaled with wavelength. Even more of the
electromagnetic spectrum can be used, including very high,
ultrahigh, microwave and millimeter wave frequencies. Polarizing
and diffusing effects will be present with proper scaling to
wavelength and the indices of refraction can be obtained from the
square root of the dielectric function (including real and
imaginary parts). Useful products in these longer wavelength bands
can be diffuse reflective polarizers and partial polarizers.
[0100] In some embodiments of the present invention, the optical
properties of the optical body vary across the wavelength band of
interest. In these embodiments, materials may be utilized for the
continuous and/or disperse phases whose indices of refraction,
along one or more axes, varies from one wavelength region to
another. The choice of continuous and disperse phase materials, and
the optical properties (i.e., diffuse and disperse reflection or
specular transmission) resulting from a specific choice of
materials, will depend on the wavelength band of interest.
Skin Layers
[0101] A layer of material which is substantially free of a
disperse phase may be coextensively disposed on one or both major
surfaces of the film, i.e., the extruded blend of the disperse
phase and the continuous phase. The composition of the layer, also
called a skin layer, may be chosen, for example, to protect the
integrity of the disperse phase within the extruded blend, to add
mechanical or physical properties to the final film or to add
optical functionality to the final film. Suitable materials of
choice may include the material of the continuous phase or the
material of the disperse phase. Other materials with a melt
viscosity similar to the extruded blend may also be useful.
[0102] A skin layer or layers may reduce the wide range of shear
intensities the extruded blend might experience within the
extrusion process, particularly at the die. A high shear
environment may cause undesirable surface voiding and may result in
a textured surface. A broad range of shear values throughout the
thickness of the film may also prevent the disperse phase from
forming the desired particle size in the blend.
[0103] A skin layer or layers may also add physical strength to the
resulting composite or reduce problems during processing, such as,
for example, reducing the tendency for the film to split during the
orientation process. Skin layer materials which remain amorphous
may tend to make films with a higher toughness, while skin layer
materials which are semicrystalline may tend to make films with a
higher tensile modulus. Other functional components such as
antistatic additives, UV absorbers, dyes, antioxidants, and
pigments, may be added to the skin layer, provided they do not
substantially interfere with the desired optical properties of the
resulting product.
[0104] The skin layers may be applied to one or two sides of the
extruded blend at some point during the extrusion process, i.e.,
before the extruded blend and skin layer(s) exit the extrusion die.
This may be accomplished using conventional coextrusion technology,
which may include using a three-layer coextrusion die. Lamination
of skin layer(s) to a previously formed film of an extruded blend
is also possible. Total skin layer thicknesses may range from about
2% to about 50% of the total blend/skin layer thickness.
[0105] A wide range of polymers are suitable for skin layers.
Predominantly amorphous polymers include copolyesters based on one
or more of terephthalic acid, 2,6-naphthalene dicarboxylic acid,
isophthalic acid phthalic acid, or their alkyl ester counterparts,
and alkylene diols, such as ethylene glycol. Examples of
semicrystalline polymers are 2,6-polyethylene naphthalate,
polyethylene terephthalate, and nylon materials.
Antireflection Layers
[0106] The films and other optical devices made in accordance with
the invention may also include one or more anti-reflective layers.
Such layers, which may or may not be polarization sensitive, serve
to increase transmission and to reduce reflective glare. An
anti-reflective layer may be imparted to the films and optical
devices of the present invention through appropriate surface
treatment, such as coating or sputter etching.
[0107] In some embodiments of the present invention, it is desired
to maximize the transmission and/or minimize the specular
reflection for certain polarizations of light. In these
embodiments, the optical body may comprise two or more layers in
which at least one layer comprises an anti-reflection system in
close contact with a layer providing the continuous and disperse
phases. Such an anti-reflection system acts to reduce the specular
reflection of the incident light and to increase the amount of
incident light that enters the portion of the body comprising the
continuous and disperse layers. Such a function can be accomplished
by a variety of means well known in the art. Examples are quarter
wave anti-reflection layers, two or more layer anti-reflective
stack, graded index layers, and graded density layers. Such
anti-reflection functions can also be used on the transmitted light
side of the body to increase transmitted light if desired.
Microvoiding
[0108] In some embodiments, the materials of the continuous and
disperse phases may be chosen so that the interface between the two
phases will be sufficiently weak to result in voiding when the film
is oriented. The average dimensions of the voids may be controlled
through careful manipulation of processing parameters and stretch
ratios, or through selective use of compatibilizers. The voids may
be back-filled in the finished product with a liquid, gas, or
solid. Voiding may be used in conjunction with the aspect ratios
and refractive indices of the disperse and continuous phases to
produce desirable optical properties in the resulting film.
More Than Two Phases
[0109] The optical bodies made in accordance with the present
invention may also consist of more than two phases. Thus, for
example, an optical material made in accordance with the present
invention can consist of two different disperse phases within the
continuous phase. The second disperse phase could be randomly or
non-randomly dispersed throughout the continuous phase, and can be
randomly aligned or aligned along a common axis.
[0110] Optical bodies made in accordance with the present invention
may also consist of more than one continuous phase. Thus, in some
embodiments, the optical body may include, in addition to a first
continuous phase and a disperse phase, a second phase which is
co-continuous in at least one dimension with the first continuous
phase. In one particular embodiment, the second continuous phase is
a porous, sponge-like material which is coextensive with the first
continuous phase (i.e., the first continuous phase extends through
a network of channels or spaces extending through the second
continuous phase, much as water extends through a network of
channels in a wet sponge). In a related embodiment, the second
continuous phase is in the form of a dendritic structure which is
coextensive in at least one dimension with the first continuous
phase.
Multilayer Combinations
[0111] If desired, one or more sheets of a continuous/disperse
phase film made in accordance with the present invention may be
used in combination with, or as a component in, a multilayered film
(i.e., to increase reflectivity). Suitable multilayered films
include those of the type described in WO 95/17303 (Ouderkirk et
al.). In such a construction, the individual sheets may be
laminated or otherwise adhered together or may be spaced apart. If
the optical thicknesses of the phases within the sheets are
substantially equal (that is, if the two sheets present a
substantially equal and large number of scatterers to incident
light along a given axis), the composite will reflect, at somewhat
greater efficiency, substantially the same band width and spectral
range of reflectivity (i.e., "band") as the individual sheets. If
the optical thicknesses of phases within the sheets are not
substantially equal, the composite will reflect across a broader
band width than the individual phases. A composite combining mirror
sheets with polarizer sheets is useful for increasing total
reflectance while still polarizing transmitted light.
Alternatively, a single sheet may be asymmetrically and biaxially
oriented to produce a film having selective reflective and
polarizing properties.
[0112] FIG. 5 illustrates one example of this embodiment of the
present invention. There, the optical body consists of a multilayer
film 20 in which the layers alternate between layers of PEN 22 and
layers of co-PEN 24. Each PEN layer includes a disperse phase of
syndiotactic polystyrene (sPS) within a matrix of PEN. This type of
construction is desirable in that it promotes lower off-angle
color. Furthermore, since the layering or inclusion of scatterers
averages out light leakage, control over layer thickness is less
critical, allowing the film to be more tolerable of variations in
processing parameters.
[0113] Any of the materials previously noted may be used as any of
the layers in this embodiment, or as the continuous or disperse
phase within a particular layer. However, PEN and co-PEN are
particularly desirable as the major components of adjacent layers,
since these materials promote good laminar adhesion.
[0114] Also, a number of variations are possible in the arrangement
of the layers. Thus, for example, the layers can be made to follow
a repeating sequence through part or all of the structure. One
example of this is a construction having the layer pattern . . .
ABCABC . . . , wherein A, B, and C are distinct materials or
distinct blends or mixtures of the same or different materials, and
wherein one or more of A, B, or C contains at least one disperse
phase and at least one continuous phase. The skin layers are
preferably the same or chemically similar materials.
Additives
[0115] The optical materials of the present invention may also
comprise other materials or additives as are known to the art. Such
materials include pigments, dyes, binders, coatings, fillers,
compatibilizers, antioxidants (including sterically hindered
phenols), surfactants, antimicrobial agents, antistatic agents,
flame retardants, foaming agents, lubricants, reinforcers, light
stabilizers (including UV stabilizers or blockers), heat
stabilizers, impact modifiers, plasticizers, viscosity modifiers,
and other such materials. Furthermore, the films and other optical
devices made in accordance with the present invention may include
one or more outer layers which serve to protect the device from
abrasion, impact, or other damage, or which enhance the
processability or durability of the device.
[0116] Suitable lubricants for use in the present invention include
calcium sterate, zinc sterate, copper sterate, cobalt sterate,
molybdenum neodocanoate, and ruthenium (III) acetylacetonate.
[0117] Antioxidants useful in the present invention include
4,4'-thiobis-(6-t-butyl-m-cresol),
2,2'-methylenebis-(4-methyl-6-t-butyl-butylphenol),
octadecyl-3,5-di-t-butyl-4-hydroxyhydrocinnamate,
bis-(2,4-di-t-butylphenyl) pentaerythritol diphosphite, Irganox.TM.
1093
(1979)(((3,5-bis(1,1-dimethylethyl)-4-hydroxyphenyl)methyl)-dioctadecyl
ester phosphonic acid), Irganox.TM. 1098
(N,N'-1,6-hexanediylbis(3,5-bis(1,1-dimethyl)-4-hydroxy-benzenepropanamid-
e), Naugaard.TM. 445 (aryl amine), Irganox.TM. L 57 (alkylated
diphenylamine), Irganox.TM. L 115 (sulfur containing bisphenol),
Irganox.TM. LO 6 (alkylated phenyl-delta-napthylamine), Ethanox 398
(flourophosphonite), and
2,2'-ethylidenebis(4,6-di-t-butylphenyl)fluorophosnite.
[0118] A group of antioxidants that are especially preferred are
sterically hindered phenols, including butylated hydroxytoluene
(BHT), Vitamin E (dialpha-tocopherol), Irganox.TM. 1425 WL(calcium
bis-(O-ethyl(3,5-di-t-butyl-4-hydroxybenzyl))phosphonate),
Irganox.TM. 1010
(tetrakis(methylene(3,5,di-t-butyl-4-hydroxyhydrocinnamate))methane)-
, Irganox.TM. 1076 (octadecyl
3,5-di-tert-butyl-4-hydroxyhydrocinnamate), Ethanox.TM. 702
(hindered bis phenolic), Etanox 330 (high molecular weight hindered
phenolic), and Ethanox.TM. 703 (hindered phenolic amine).
[0119] Dichroic dyes are a particularly useful additive in some
applications to which the optical materials of the present
invention may be directed, due to their ability to absorb light of
a particular polarization when they are molecularly aligned within
the material. When used in a film or other material which
predominantly scatters only one polarization of light, the dichroic
dye causes the material to absorb one polarization of light more
than another. Suitable dichroic dyes for use in the present
invention include Congo Red (sodium
diphenyl-bis-.alpha.-naphthylamine sulfonate), methylene blue,
stilbene dye (Color Index (CI)=620), and 1,1'-diethyl-2,2'-cyanine
chloride (CI=374 (orange) or CI=518 (blue)). The properties of
these dyes, and methods of making them, are described in E. H.
Land, Colloid Chemistry (1946). These dyes have noticeable
dichroism in polyvinyl alcohol and a lesser dichroism in cellulose.
A slight dichroism is observed with Congo Red in PEN.
[0120] Other suitable dyes include the following materials:
##STR1## The properties of these dyes, and methods of making them,
are discussed in the Kirk Othmer Encyclopedia of Chemical
Technology, Vol. 8, pp. 652-661 (4th Ed. 1993), and in the
references cited therein.
[0121] When a dichroic dye is used in the optical bodies of the
present invention, it may be incorporated into either the
continuous or disperse phase. However, it is preferred that the
dichroic dye is incorporated into the disperse phase.
[0122] Dychroic dyes in combination with certain polymer systems
exhibit the ability to polarize light to varying degrees. Polyvinyl
alcohol and certain dichroic dyes may be used to make films with
the ability to polarize light. Other polymers, such as polyethylene
terephthalate or polyamides, such as nylon-6, do not exhibit as
strong an ability to polarize light when combined with a dichroic
dye. The polyvinyl alcohol and dichroic dye combination is said to
have a higher dichroism ratio than, for example, the same dye in
other film forming polymer systems. A higher dichroism ratio
indicates a higher ability to polarize light.
[0123] Molecular alignment of a dichroic dye within an optical body
made in accordance with the present invention is preferably
accomplished by stretching the optical body after the dye has been
incorporated into it. However, other methods may also be used to
achieve molecular alignment. Thus, in one method, the dichroic dye
is crystallized, as through sublimation or by crystallization from
solution, into a series of elongated notches that are cut, etched,
or otherwise formed in the surface of a film or other optical body,
either before or after the optical body has been oriented. The
treated surface may then be coated with one or more surface layers,
may be incorporated into a polymer matrix or used in a multilayer
structure, or may be utilized as a component of another optical
body. The notches may be created in accordance with a predetermined
pattern or diagram, and with a predetermined amount of spacing
between the notches, so as to achieve desirable optical
properties.
[0124] In a related embodiment, the dichroic dye may be disposed
within one or more hollow fibers or other conduits, either before
or after the hollow fibers or conduits are disposed within the
optical body. The hollow fibers or conduits may be constructed out
of a material that is the same or different from the surrounding
material of the optical body.
[0125] In yet another embodiment, the dichroic dye is disposed
along the layer interface of a multilayer construction, as by
sublimation onto the surface of a layer before it is incorporated
into the multilayer construction. In still other embodiments, the
dichroic dye is used to at least partially backfill the voids in a
microvoided film made in accordance with the present invention.
APPLICATIONS OF PRESENT INVENTION
[0126] The optical bodies of the present invention are particularly
useful as diffuse polarizers. However, optical bodies may also be
made in accordance with the invention which operate as reflective
polarizers or diffuse mirrors. In these applications, the
construction of the optical material is similar to that in the
diffuser applications described above. However, these reflectors
will generally have a much larger difference in the index of
refraction along at least one axis. This index difference is
typically at least about 0.1, more preferably about 0.15, and most
preferably about 0.2.
[0127] Reflective polarizers have a refractive index difference
along one axis, and substantially matched indices along another.
Reflective films, on the other hand, differ in refractive index
along at least two in-film plane orthogonal axes. However, the
reflective properties of these embodiments need not be attained
solely by reliance on refractive index mismatches. Thus, for
example, the thickness of the films could be adjusted to attain a
desired degree of reflection. In some cases, adjustment of the
thickness of the film may cause the film to go from being a
transmissive diffuser to a diffuse reflector.
[0128] The reflective polarizer of the present invention has many
different applications, and is particularly useful in liquid
crystal display panels. In addition, the polarizer can be
constructed out of PEN or similar materials which are good
ultraviolet filters and which absorb ultraviolet light efficiently
up to the edge of the visible spectrum. The reflective polarizer
can also be used as a thin infrared sheet polarizer.
OVERVIEW OF EXAMPLES
[0129] The following Examples illustrate the production of various
optical materials in accordance with the present invention, as well
as the spectral properties of these materials. Unless otherwise
indicated, percent composition refers to percent composition by
weight. The polyethylene naphthalate resin used was produced for
these samples using ethylene glycol and
dimethyl-2,6-naphthalenedicarboxylate, available from Amoco Corp.,
Chicago, Ill.
[0130] These reagents were polymerized to various intrinsic
viscosities (IV) using conventional polyester resin polymerization
techniques. Syndiotactic polystyrene (sPS) may be produced in
accordance with the method disclosed in U.S. Pat. No. 4,680,353
(Ishihara et al). The examples includes various polymer pairs,
various fractions of continuous and disperse phases and other
additives or process changes as discussed below.
[0131] Stretching or orienting of the samples was provided using
either conventional orientation equipment used for making polyester
film or a laboratory batch orienter. The laboratory batch orienter
used was designed to use a small piece of cast material (7.5 cm by
7.5 cm) cut from the extruded cast web and held by a square array
of 24 grippers (6 on each side). The orientation temperature of the
sample was controlled a hot air blower and the film sample was
oriented through a mechanical system that increased the distance
between the grippers in one or both directions at a controlled
rate. Samples stretched in both directions could be oriented
sequentially or simultaneously. For samples that were oriented in
the constrained mode (C), all grippers hold the web and the
grippers move only in one dimension. Whereas, in the unconstrained
mode (U), the grippers that hold the film at a fixed dimension
perpendicular to the direction of stretch are not engaged and the
film is allowed to relax or neckdown in that dimension.
[0132] Polarized diffuse transmission and reflection were measured
using a Perkin Elmer Lambda 19 ultraviolet/visible/near infrared
spectrophotometer equipped with a Perkin Elmer Labsphere S900-1000
150 millimeter integrating sphere accessory and a Glan-Thompson
cube polarizer. Parallel and crossed transmission and reflection
values were measured with the e-vector of the polarized light
parallel or perpendicular, respectively, to the stretch direction
of the film. All scans were continuous and were conducted with a
scan rate of 480 nanometers per minute and a slit width of 2
nanometers. Reflection was performed in the "V-reflection" mode.
Transmission and reflectance values are averages of all wavelengths
from 400 to 700 nanometers.
[0133] Transmission electron micrographs were taken of finished
film, cross-sectioned in a plan perpendicular to the machine
direction to determine the nature of the dispersed phase. The outer
layers of three-layer constructions were removed from oriented
film, leaving only the blend layer for embedding. Samples were
embedded in 3M Scotchcast.TM. 5 Electrical Resin which was cured at
room temperature. The embedded samples were microtomed using a
diamond knife, on a Reichert Ultracut.TM. S microtome at room
temperature, into thin sections of approximately 90 nm thickness,
using a cutting rate of 0.2 millimeters per second. The thin
sections were floated onto distilled, deionized water and collected
for transmission electron microscopic evaluation on a 200 mesh
copper grid reinforced with a carbon/formvor substrate.
Photomicrographs were taken using a JEOL 200CX Transmission
Electron Microscope.
[0134] Scanning electron microscopic evaluations were performed on
cast webs prior to film orientation to determine the nature of the
disperse phase. Pieces of web were fractured to expose a plane
perpendicular to the machine direction while immersed in liquid
nitrogen. Samples were then trimmed and mounted on aluminum stubs
prior to sputter coating with gold palladium. Photomicrographs were
taken using a Hitachi S530 Scanning Electron Microscope.
Example 1
[0135] In Example 1, an optical film was made in accordance with
the invention by extruding a blend of 75% polyethylene naphthalate
(PEN) as the continuous or major phase and 25% of
polymethylmethacrylate (PMMA) as the disperse or minor phase into a
cast film or sheet about 380 microns thick using conventional
extrusion and casting techniques. The PEN had an intrinsic
viscosity (IV) of 0.52 (measured in 60% phenol, 40%
dichlorobenzene). The PMMA was obtained from ICI Americas, Inc.,
Wilmington, Del., under the product designation CP82. The extruder
used was a 3.15 cm (1.24'') Brabender with a 1 tube 60 .mu.m Tegra
filter. The die was a 30.4 cm (12'') EDI Ultraflex.TM. 40.
[0136] About 24 hours after the film was extruded, the cast film
was oriented in the width or transverse direction (TD) on a
polyester film tentering device. The stretching was accomplished at
about 9.1 meters per minute (30 ft/min) with an output width of
about 140 cm (55 inches) and a stretching temperature of about
160.degree. C. (320.degree. F). The total reflectivity of the
stretched sample was measured with an integrating sphere attachment
on a Lambda 19 spectrophotometer with the sample beam polarized
with a Glan-Thompson cube polarizer. The sample had a 75% parallel
reflectivity (i.e., reflectivity was measured with the stretch
direction of the film parallel to the e-vector of the polarized
light), and 52% crossed reflectivity (i.e., reflectivity was
measured with the e-vector of the polarized light perpendicular to
the stretch direction).
Example 2
[0137] In Example 2, an optical film was made and evaluated in a
manner similar to Example 1 except using a blend of 75% PEN, 25%
syndiotactic polystyrene (sPS), 0.2% of a polystyrene glycidyl
methacrylate compatibilizer, and 0.25% each of Irganox.TM. 1010 and
Ultranox.TM. 626. The synthesis of polystyrene glycidyl
methacrylate is described in Polymer Processes, "Chemical
Technology of Plastics, Resins, Rubbers, Adhesives and Fibers",
Vol. 10, Chap. 3, pp. 69-109 (1956) (Ed. by Calvin E.
Schildknecht).
[0138] The PEN had an intrinsic viscosity of 0.52 measured in 60%
phenol, 40% dichlorobenzene. The sPS was obtained from Dow Chemical
Co. and had a weight average molecular weight of about 200,000,
designated subsequently as sPS-200-0. The parallel reflectivity on
the stretched film sample was determined to be 73.3%, and the
crossed reflectivity was determined to be 35%.
Example 3
[0139] In Example 3, an optical film was made and evaluated in a
manner similar to Example 2 except the compatibilizer level was
raised to 0.6%. The resulting parallel reflectivity was determined
to be 81% and the crossed reflectivity was determined to be
35.6%.
Example 4
[0140] In Example 4, an three layer optical film was made in
accordance with the present invention utilizing conventional three
layer coextrusion techniques. The film had a core layer and a skin
layer on each side of the core layer. The core layer consisted of a
blend of 75% PEN and 25% sPS 200-4 (the designation sPS-200-4
refers to a copolymer of syndiotactic-polystyrene containing 4 mole
% of para-methyl styrene), and each skin layer consisted of 100%
PEN having an intrinsic viscosity of 0.56 measured in 60% phenol,
40% dichlorobenzene.
[0141] The resulting three-layer cast film had a core layer
thickness of about 415 microns, and each skin layer was about 110
microns thick for a total thickness of about 635 microns. A
laboratory batch stretcher was used to stretch the resulting
three-layer cast film about 6 to 1 in the machine direction (MD) at
a temperature of about 129.degree. C. Because the edges of the film
sample parallel to the stretch direction were not gripped by the
lab stretcher, the sample was unconstrained in the transverse
direction (TD) and the sample necked-down in the TD about 50% as a
result of the stretch procedure.
[0142] Optical performance was evaluated in a manner similar to
Example 1. The parallel reflectivity was determined to be 80.1%,
and the crossed reflectivity was determined to be 15%. These
results demonstrate that the film performs as a low absorbing,
energy conserving system.
Examples 5-29
[0143] In Examples 5-29, a series of optical films were produced
and evaluated in a manner similar to Example 4, except the sPS
fraction in the core layer and the IV of the PEN resin used were
varied as shown in Table 1. The IV of the PEN resin in the core
layer and that in the skin layers was the same for a given sample.
The total thickness of the cast sheet was about 625 microns with
about two-thirds of this total in the core layer and the balance in
the skin layers which were approximately equal in thickness.
Various blends of PEN and sPS in the core layer were produced, as
indicated in Table 1. The films were stretched to a stretch ratio
of about 6:1 in either the machine direction (MD) or in the
transverse direction (TD) at various temperatures as indicated in
Table 1. Some of the samples were constrained (C) in the direction
perpendicular to the stretch direction to prevent the sample from
necking down during stretching. The samples labeled "U" in Table 1
were unconstrained and permitted to neckdown in the unconstrained
dimension. Certain optical properties of the stretched samples,
including percent transmission, reflection, and absorption, were
measured along axes both parallel and crossed or perpendicular to
the direction of stretch. The results are summarized in TABLE
1.
[0144] Heat setting, as indicated for Examples 24-27, was
accomplished by manually constraining the two edges of the
stretched sample which were perpendicular to the direction of
stretch by clamping to an appropriately sized rigid frame and
placing the clamped sample in an oven at the indicated temperature
for 1 minute. The two sides of the sample parallel to the direction
of stretch were unconstrained (U) or not clamped and allowed to
neckdown. The heatsetting of Example 29 was similar except all four
of the edges of the stretched sample were constrained (C) or
clamped. Example 28 was not heat set. TABLE-US-00001 TABLE 1
Stretch Stretch Stretch Example Temp. Direction Constrained PEN
Fraction Heat Set Constrained Trans. Trans. Reflec. Reflec. Number
(.degree. C.) (MD/TD) (C/U) IV (sPS) Temp. Heat Set (Perp.) (Para.)
(Perp.) (Para.) 5 135 TD C 0.53 0.25 76.2 20.4 22.6 75.3 6 135 TD C
0.47 0.75 80.2 58.4 19.4 40 7 142 TD C 0.53 0.25 74.2 21.8 25.3
77.3 8 142 TD C 0.47 0.75 76.0 41.0 23.8 55.6 9 129 TD C 0.53 0.25
71.2 21.2 26.5 76.2 10 129 TD C 0.47 0.75 76.8 48.9 22.4 49.6 11
129 MD U 0.53 0.25 81.5 27.6 17.2 67 12 129 TD U 0.53 0.25 66.8
22.1 25 71.9 13 129 MD U 0.47 0.25 79.5 20.3 19.3 73.7 14 129 TD U
0.47 0.25 66.3 26.2 32.5 69.4 15 129 TD U 0.47 0.5 73.0 26.2 24.7
68.7 16 129 MD U 0.47 0.5 75.4 20.6 23.2 76.1 17 129 MD U 0.47 0.1
82.1 27.3 16.9 67 18 129 MD U 0.56 0.25 80.1 15.0 18 80.3 19 129 TD
U 0.56 0.25 70.2 21.6 25.2 70.7 20 129 MD C 0.47 0.25 75.8 28.7
23.4 70.1 21 129 MD C 0.47 0.5 79.8 27.8 19.7 70.8 22 135 MD C 0.47
0.1 80.5 36.7 19.2 62.6 23 135 MD C 0.53 0.25 77.2 21.1 21.8 76.6
24 129 MD U 0.56 0.25 150 U 83.7 17.3 17.3 74 25 129 MD U 0.56 0.25
220 U 82.1 16 18 75.8 26 129 MD U 0.56 0.25 135 U 84.7 17 18 75.3
27 129 MD U 0.56 0.25 165 U 83 16 16.5 76.3 28 129 MD U 0.56 0.25
CNTRL 83.7 17 17.5 76 29 129 MD U 0.56 0.25 230 C 29 129 MD U 0.56
0.25 230 C
[0145] All of the above samples were observed to contain varying
shapes of the disperse phase depending on the location of the
disperse phase within the body of the film sample. The disperse
phase inclusions located nearer the surfaces of the samples were
observed to be of an elongated shape rather than more nearly
spherical. The inclusions which are more nearly centered between
the surfaces of the samples may be more nearly spherical. This is
true even for the samples with the skin layers, but the magnitude
of the effect is reduced with the skin layers. The addition of the
skin layers improves the processing of the films by reducing the
tendency for splitting during the stretching operation.
[0146] Without wishing to be bound by theory, the elongation of the
inclusions (disperse phase) in the core layer of the cast film is
thought to be the result of shear on the blend as it is transported
through the die. This elongation feature may be altered by varying
physical dimensions of the die, extrusion temperatures, flow rate
of the extrudate, as well as chemical aspects of the continuous and
disperse phase materials which would alter their relative melt
viscosities. Certain applications or uses may benefit from
providing some elongation to the disperse phase during extrusion.
For those applications which are subsequently stretched in the
machine direction, starting with a disperse phase elongated during
extrusion may allow a higher aspect ratio to be reached in the
resulting disperse phase.
[0147] Another notable feature is the fact that a noticeable
improvement in performance is observed when the same sample is
stretched unconstrained. Thus, in Example 9, the % transmission was
79.5% and 20.3% in the parallel and perpendicular directions,
respectively. By contrast, the transmission in Example 16 was only
75.8% and 28.7% in the parallel and perpendicular directions,
respectively. There is a thickness increase relative to constrained
stretching when samples are stretched unconstrained, but since both
transmission and extinction improve, the index match is probably
being improved.
[0148] An alternative way to provide refractive index control is to
modify the chemistry of the materials. For example, a copolymer of
30 wt % of interpolymerized units derived from terephthalic acid
and 70 wt % of units derived from 2,6-naphthalic acid has a
refractive index 0.02 units lower than a 100% PEN polymer. Other
monomers or ratios may have slightly different results. This type
of change may be used to more closely match the refractive indices
in one axis while only causing a slight reduction in the axis which
desires a large difference. In other words, the benefits attained
by more closely matching the index values in one axis more than
compensate for the reduction in an orthogonal axis in which a large
difference is desired. Secondly, a chemical change may be desirable
to alter the temperature range in which stretching occurs. A
copolymer of sPS and varying ratios of para methyl styrene monomer
will alter the optimum stretch-temperature range. A combination of
these techniques may be necessary to most effectively optimize the
total system for processing and resulting refractive index matches
and differences. Thus, an improved control of the final performance
may be attained by optimizing the process and chemistry in terms of
stretching conditions and further adjusting the chemistry of the
materials to maximize the difference in refractive index in at
least one axis and minimizing the difference at least one
orthogonal axis.
[0149] These samples displayed better optical performance if
oriented in the MD rather than TD direction (compare Examples
14-15). Without wishing to be bound by theory, it is believed that
different geometry inclusions are developed with an MD orientation
than with a TD orientation and that these inclusions have higher
aspect ratios, making non-ideal end effects less important. The
non-ideal end effects refers to the complex geometry/index of
refraction relationship at the tip of each end of the elongated
particles. The interior or non-end of the particles are thought to
have a uniform geometry and refractive index which is thought to be
desirable. Thus, the higher the percentage of the elongated
particle that is uniform, the better the optical performance.
[0150] The extinction ratio of these materials is the ratio of the
transmission for polarizations perpendicular to the stretch
direction to that parallel to the stretch direction. For the
examples cited in Table 1, the extinction ratio ranges between
about 2 and about 5, although extinction ratios up to 7 have been
observed in optical bodies made in accordance with the present
invention without any attempt to optimize the extinction ratio. It
is expected that even higher extinction ratios (e.g., greater than
100) can be achieved by adjusting film thickness, inclusion volume
fraction, particle size, and the degree of index match and
mismatch, or through the use of iodine or other dyes.
Example 30-100
[0151] In Examples 30-100, samples of the invention were made using
various materials as listed in Table 2. PEN 42, PEN 47, PEN 53, PEN
56, and PEN 60 refer to polyethylene naphthalate having an
intrinsic viscosity (IV) of 0.42, 0.47, 0.53, 0.56, and 0.60,
respectively, measured in 60% phenol, 40% dichlorobenzene. The
particular sPS-200-4 used was obtained from Dow Chemical Co.
Ecdel.TM. 9967 and Eastar.TM. are copolyesters which are available
commercially from Eastman Chemical Co., Rochester, N.Y. Surlyn.TM.
1706 is an ionomer resin available from E.I. du Pont de Nemours
& Co., Wilmington, Del. The materials listed as Additive 1 or 2
include polystyrene glycidyl methacrylate. The designations GMAPS2,
GMAPS5, and GMAPS8 refer to glycidyl methacrylate having 2, 5, and
8% by weight, respectively, of glycidyl methacrylate in the total
copolymer. ETPB refers to the crosslinking agent
ethyltriphenylphosphonium bromide. PMMA VO44 refers to a
polymethylmethacrylate available commercially from Atohaas North
America, Inc.
[0152] The optical film samples were produced in a manner similar
to Example 4 except for the differences noted in Table 2 and
discussed below. The continuous phase and its ratio of the total is
reported as major phase. The disperse phase and its ratio of the
total is reported as minor phase. The value reported for blend
thickness represents the approximate thickness of the core layer in
microns. The thickness of the skin layers varied when the core
layer thickness varied, but was kept to a constant ratio, i.e., the
skin layers were approximately equal and the total of the two skin
layers was about one-third of the total thickness. The size of the
disperse phase was determined for some samples by either scanning
electron microscope (SEM) or transmission electron microscope
(TEM). Those examples which were subsequently stretched using the
laboratory batch orienter are shown by an "X" in the column labeled
Batch Stretched. TABLE-US-00002 TABLE 2 Major Minor Example Major
Phase Minor Phase Core Layer TEM Batch Number Phase (%) Phase (%)
(microns) Additive 1 Additive 2 SEMs (microns) Stretched 30 PEN.42
75 sPS-200-4 25 9.8 -- -- -- -- -- 31 PEN.42 75 sPS-200-4 25 16.3
-- -- 10 -- -- 32 PEN.47 75 sPS-200-4 25 9.8 -- -- -- -- x 33
PEN.47 75 sPS-200-4 25 16.3 -- -- 8 -- x 34 PEN.47 50 sPS-200-4 50
9.8 -- -- -- -- -- 35 PEN.47 50 sPS-200-4 50 16.3 -- -- 5 -- x 36
PEN.47 90 sPS-200-4 10 9.8 -- -- -- -- -- 37 PEN.47 90 sPS-200-4 10
16.3 -- -- 3 -- x 38 PEN.53 75 sPS-200-4 25 9.8 -- -- -- -- -- 39
PEN.53 75 sPS-200-4 25 16.3 -- -- 7 -- x 40 PEN.56 75 sPS-200-4 25
9.8 -- -- -- -- -- 41 PEN.56 75 sPS-200-4 25 16.3 -- -- 6 -- x 42
sPS-200-4 75 PEN.42 25 9.8 -- -- -- -- -- 43 sPS-200-4 75 PEN.42 25
16.3 -- -- -- -- -- 44 sPS-200-4 75 PEN.47 25 9.8 -- -- -- -- -- 45
sPS-200-4 75 PEN.47 25 16.3 -- -- -- -- x 46 sPS-200-4 75 PEN.53 25
16.3 -- -- -- -- -- 47 sPS-200-4 75 PEN.53 25 9.8 -- -- -- -- -- 48
sPS-200-4 75 PEN.56 25 9.8 -- -- -- -- -- 49 sPS-200-4 75 PEN.56 25
16.3 -- -- -- -- -- 50 PET.60 75 Ecdel .TM. 25 16.3 -- -- -- -- --
9967 51 PET.60 75 Surlyn .TM. 25 16.3 -- -- 2 -- -- 1706 52 PEN.47
75 Ecdel .TM. 25 16.3 -- -- 2 -- x 9967 53 PEN.47 100 -- -- 16.3 --
-- -- -- -- 54 PEN.47 75 sPS-200 25 16.3 -- -- -- -- -- 55 PEN.47
75 sPS-200 25 9.8 -- -- 10 -- -- 56 PEN.47 75 sPS-320 25 9.8 -- --
12 -- -- 57 PEN.47 75 sPS-320 25 16.3 -- -- -- -- -- 58 PEN.47 95
sPS-320 5 9.8 -- -- -- -- -- 59 PEN.47 95 sPS-320 5 16.3 -- -- --
-- -- 60 PEN.56 100 -- -- 16.3, 9.8 -- -- -- -- x 61 PEN.56 75
sPS-200 25 9.8 -- -- 10 -- -- 62 PEN.56 75 sPS-200 25 16.3 -- -- --
-- x 63 PEN.56 95 sPS-200 5 9.8 -- -- -- -- -- 64 PEN.56 95 sPS-200
5 16.3 -- -- -- -- x 65 PEN.56 75 sPS-320 25 9.8 -- -- 10 -- -- 66
PEN.56 75 sPS-320 25 16.3 -- -- -- -- -- 67 PEN.47 95 sPS-200 5
16.3 2% 0.25% 1 0.3 x GMAPS2 ETPB 68 PEN.47 95 sPS-200 5 9.8 2%
0.25% -- -- -- GMAPS2 ETPB 69 PEN.56 75 sPS-200 25 9.8 6% 0.25% --
-- -- GMAPS2 ETPB 70 PEN.56 75 sPS-200 25 16.3 6% 0.25% 0.5 2.5 x
GMAPS2 ETPB 71 PEN.56 75 sPS-200 25 9.8 2% 0.25% -- 0.8 -- GMAPS2
ETPB 72 PEN.56 75 sPS-200 25 16.3 2% 0.25% 1 -- -- GMAPS2 ETPB 73
PEN.56 95 sPS-200 5 9.8 2% 0.25% -- -- -- GMAPS2 ETPB 74 PEN.56 95
sPS-200 5 16.3 2% 0.25% -- -- -- GMAPS2 ETPB 75 PEN.56 75 sPS-200
25 9.8 6% 0.25% -- -- -- GMAPS2 ETPB 76 PEN.56 75 sPS-200 25 16.3
6% 0.25% 0.8 1 x GMAPS2 ETPB 77 PEN.56 75 sPS-200 25 9.8 2% 0.25%
-- -- -- GMAPS2 ETPB 78 PEN.56 75 sPS-200 25 16.3 2% 0.25% -- -- --
GMAPS2 ETPB 79 PEN.56 75 sPS-200 25 9.8 6% 0.25% -- -- -- GMAPS2
ETPB 80 PEN.56 75 sPS-200 25 16.3 6% 0.25% -- -- x GMAPS2 ETPB 81
PEN.56 75 sPS-200 25 9.8 6% 0.25% -- -- -- GMAPS2 ETPB 82 PEN.56 75
sPS-200 25 16.3 6% 0.25% 0.5 -- -- GMAPS2 ETPB 83 PEN.56 95 sPS-200
5 9.8 2% 0.25% -- -- -- GMAPS2 ETPB 84 PEN.56 95 sPS-200 5 16.3 2%
0.25% -- -- -- GMAPS2 ETPB 85 PEN.56 75 sPS-200 25 9.8 0.5% 0.25%
-- -- -- GMAPS2 ETPB 86 PEN.56 75 sPS-200 25 9.8 0.5% 0.25% -- --
-- GMAPS2 ETPB 87 PEN.47 75 Eastar 25 16.3 -- -- -- -- x 88 PEN.47
75 Eastar 25 9.8 -- -- -- -- -- 89 PEN.47 75 Eastar 25 16.3 -- --
-- -- -- 90 PEN.47 75 Eastar 25 9.8 -- -- -- -- -- 91 PEN.47 75
PMMA 25 9.8 -- -- -- -- -- VO44 92 PEN.47 75 PMMA 25 16.3 -- -- 10
-- -- VO44 93 PEN.47 75 PMMA 25 16.3 6% -- -- 0.7 -- VO44 MMA/GMA
94 PEN.47 75 PMMA 25 9.8 6% -- -- -- -- VO44 MMA/GMA 95 PEN.47 75
PMMA 25 9.8 2% -- -- 1.2 -- VO44 MMA/GMA 96 PEN.47 75 PMMA 25 16.3
2% -- -- -- x MMA/GMA 97 PEN.47 75 sPS-200-4 25 916.3 0.5% Congo --
-- -- x VO44 Red 98 PEN.47 75 sPS-200-4 25 16.3 0.15% -- -- -- x
Congo Red 99 PEN.47 75 sPS-200-4 25 9.8 0.25% -- -- -- -- Methylene
Blue 100 PEN.47 75 sPS-200-4 25 9.8 0-0.25% -- -- -- -- Methylene
Blue
The presence of the various compatibilizers was found to reduce the
size of the included or disperse phase.
Example 101
[0153] In Example 101, an optical film was made in a manner similar
to Example 4 except the resulting core thickness was about 420
microns thick, and each skin layer was about 105 microns thick. The
PEN had a 0.56 IV. The cast film was oriented as in Example 1,
except the temperature of stretch was 165.degree. C. and there was
a 15 day delay between casting and stretching. The transmission was
87.1% and 39.7% for parallel and perpendicularly polarized light,
respectively.
Examples 102-121
[0154] In Examples 102-121, optical films were made as in Example
101, except that orientation conditions were varied and/or the
sPS-200-0 was replaced with either copolymers of sPS containing
either 4 or 8 mole % of para-methyl styrene or with an atactic-form
of styrene, Styron 663 (available from Dow Chemical Company,
Midland, Mich.) as listed in Table 3. Evaluations of transmission
properties are also reported. Transmission values are averaged over
all wavelengths between 450-700 nm. TABLE-US-00003 TABLE 3
Temperature Rail Perpendicular Parallel % of Draw Setting
Transmission Transmission Ex. sPS PS PEN IV (.degree. C.) (cm) (%)
(%) 101 25 200-0 0.56 165 152 87.1 39.7 102 35 200-0 0.56 165 152
87.8 44.4 103 15 200-4 0.56 165 152 86.1 43.5 104 25 200-4 0.56 165
152 86.5 43.6 105 35 200-4 0.56 165 152 88.2 50.7 106 15 200-8 0.56
165 152 89.3 40.7 107 25 200-8 0.56 165 152 88.5 42.8 108 35 200-8
0.56 165 152 88.6 43.3 109 15 Styron 0.56 165 152 89.3 45.7 663 110
25 Styron 0.56 165 152 87.8 41.6 663 111 35 Styron 0.56 165 152
88.8 48.2 663 112 15 Styron 0.48 165 152 88.5 62.8 663 113 25
Styron 0.48 165 152 87.1 59.6 663 114 35 Styron 0.48 165 152 86.8
59.6 663 115 15 200-0 0.48 165 152 88.0 58.3 116 25 200-0 0.48 165
152 88.0 58.7 117 35 200-0 0.48 165 152 88.5 60.6 118 15 200-4 0.48
165 152 89.0 57.4 119 35 200-4 0.48 165 152 87.3 64.0 120 35 200-0
0.56 171 127 86.5 65.1 121 35 200-0 0.56 171 152 88.1 61.5
[0155] These examples indicate that the particles of the included
phase are elongated more in achine direction in high IV PEN than in
low IV PEN. This is consistent with the vation that, in low IV PEN,
stretching occurs to a greater extent near the surface of the film
than at points interior to the film, with the result that fibrillar
structures are formed near the surface and spherical structures are
formed towards the center.
[0156] Some of these Examples suggest that the orientation
temperatures and degree of orientation are important variables in
achieving the desired effect. Examples 109 to 114 suggest that
quiescent crystallization need not be the only reason for the lack
of transmission of a preferred polarization of light.
Examples 122-124
[0157] In Example 122, a multilayer optical film was made in
accordance with the invention by means of a 209 layer feedblock.
The feedblock was fed with two materials: (1) PEN at 38.6 kg per
hour (intrinsic viscosity of 0.48); and (2) a blend of 95% CoPEN
and 5% by weight of sPS homopolymer (200,000 molecular weight). The
CoPEN was a copolymer based on 70 mole % naphthalene dicarboxylate
and 30 mole % dimethyl isophthalate polymerized with ethylene
glycol to an intrinsic viscosity of 0.59. The CoPEN/sPS blend was
fed into the feedblock at a rate of 34.1 kg per hour.
[0158] The CoPEN blend material was on the outside of the
extrudate, and the layer composition of the resulting stack of
layers alternated between the two materials. The thicknesses of the
layers was designed to result in a one-quarter wavelength stack
with a linear gradient of thicknesses, and having a 1.3 ratio from
the thinnest to the thickest layer. Then, a thicker skin layer of
CoPEN (made in accordance with the method described above to make
the CoPEN/sPS blend, except the molar ratios were 70/15/15
naphthalene dicarboxylate/dimethyl terephthalate/dimethyl
isophthalate) devoid of sPS was added to each side of the 209 layer
composite. The total skin layer was added at a rate of 29.5 kg per
hour, with about one-half of this quantity on each side or surface
of the stack.
[0159] The resulting skin layer clad multilayer composite was
extruded through a multiplier to achieve a multilayer composite of
421 layers. The resulting multilayer composite was then clad with
another skin layer of the 70/15/15 CoPEN on each surface at a total
rate of 29.5 kg per hour with about one-half of this quantity on
each side. Since this second skin layer may not be separately
detectable from the existing skin layer (as the material is the
same), for the purposes of this discussion, the resulting extra
thick skin layer will be counted as only one layer.
[0160] The resulting 421 layer composite was again extruded through
a 1.40 ratio asymmetric multiplier to achieve a 841 layer film
which was then cast into a sheet by extruding through a die and
quenching into a sheet about 30 mils thick. The resulting cast
sheet was then oriented in the width direction using a conventional
film making tentering device. The sheet was stretched at a
temperature of about 300.degree. F. (149.degree. C.) to a stretch
ratio of about 6:1 and at a stretch rate of about 20% per second.
The resulting stretched film was about 5 mils thick.
[0161] In Example 123, a multilayer optical film was made as in
Example 122, except that the amount of sPS in the CoPEN/sPS blend
was 20% instead of 5%.
[0162] In Example 124, a multilayer optical film was made as in
Example 122, except that no sPS was added to the film.
[0163] The results reported in Table 4 include a measure of the
optical gain of the film. The optical gain of a film is the ratio
of light transmitted through an LCD panel from a backlight with the
film inserted between the two to the light transmitted without the
film in place. The significance of optical gain in the context of
optical films is described in WO 95/17692 in relation to FIG. 2 of
that reference. A higher gain value is generally desirable. The
transmission values include values obtained when the light source
was polarized parallel to the stretch direction (T.smallcircle.)
and light polarized perpendicular to the stretch direction
(T.perp.). Off-angle-color ( OAC) was measured using an Oriel
spectrophotometer as the root mean square deviation of p-polarized
transmission at 50 degree incident light of wavelength between 400
and 700 nm. TABLE-US-00004 TABLE 4 Ex. mole % sPS Gain T.sub..perp.
(%) To (%) OAC (%) 122 5 1.5 83 2 1.5 123 20 1.45 81 1.5 1.2 124 0
1.6 87 5 3.5
[0164] The value of off-angle-color (OAC) demonstrates the
advantage of using a multilayer construction within the context of
the present invention. In particular, such a construction can be
used to substantially reduce OAC with only a modest reduction in
gain. This tradeoff may have advantages in some applications. The
values of T.smallcircle. for the examples of the invention may be
lower than expected because light scattered by the sPS dispersed
phase may not be received by the detector.
Example 125
[0165] A three layer film was made in accordance with Example 4.
The core layer consisted of 70% CoPEN whose intrinsic viscosity was
0.55 measured in 60% phenol, 40% dichlorobenzene, 70% sPS 200-7,
plus an additional 2% Dylark 332-80 (available from NOVA Chemical).
Each skin consisted of 100% CoPET having an intrinsic viscosity of
0.65 measured in methylene chloride.
[0166] The CoPEN was a copolymer based on 62 mole % naphthalene
dicarboxylate and 38 mole % dimethyl terephthalate. The CoPET was a
copolymer based on 80 mole % dimethyl carboxylate and 20 mole %
dimethyl isophthalate.
[0167] The cast film was oriented in a manner consistent with
Example 1. The stretching was accomplished at 5.8 meters per minute
(19 feet per minute) with an output width of 147 cm (58 inches).
The stretch temperature was 124.degree. C. The heat set temperature
was 163.degree. C. The perpendicular transmission was 85.3%, and
the parallel transmission was 21.7%.
Examples 126-130
[0168] The following examples illustrate the production of a
co-continuous morphology in an optical system of the present
invention.
[0169] In Examples 126 through 130, a series of optical films were
produced and evaluated in a manner similar to Example 125, except
the sPS fraction in the core layer and the stretch temperature were
varied as shown in Table 5. TABLE-US-00005 TABLE 5 Dispersed
Stretch Example Fraction or Co- Temperature Trans. Trans. Number
sPS continuous (.degree. C.) (Perp.) (Para.) 125 0.30 D 124 85.3
21.7 126 0.35 D 135 86.3 21.1 127 0.40 D 129 86.4 21.9 128 0.44 --
124 85.8 25.9 129 0.53 C 129 86.6 33.6 130 0.81 D 135 88.1 69
[0170] The parallel and perpendicular transmission values for
Examples 125 to 130 show good optical performance. The high value
for perpendicular transmission for Example 130 transmission
suggests an effective match in the refractive indices in both
phases for polarized light aligned in the direction perpendicular
to the stretch direction.
[0171] Scanning electron micrographs were taken of fracture
surfaces of cast web for Examples 126 and 127. As in Example 125,
there was clear evidence of spherical or elliptical particles
dispersed in an otherwise continuous matrix. Transmission electron
micrographs were taken for Examples 129 and 130; these are shown in
FIGS. 6a and 6b, respectively. FIG. 6a illustrates the morphology
of co-continuous phases. Inspection of the micrograph shows
inclusions of both the coPEN and the sPS phases, as well as regions
where each appears to be the continuous phase. By contrast, FIG. 6b
shows coPEN dispersed into an sPS matrix.
[0172] The preceding description of the present invention is merely
illustrative, and is not intended to be limiting. Therefore, the
scope of the present invention should be construed solely by
reference to the appended claims.
* * * * *