U.S. patent application number 11/405056 was filed with the patent office on 2006-11-23 for optical polarizer.
This patent application is currently assigned to 3M Innovative Properties Company. Invention is credited to James M. Jonza, Andrew J. Ouderkirk, Carl A. Stover, Michael F. Weber.
Application Number | 20060262400 11/405056 |
Document ID | / |
Family ID | 46255387 |
Filed Date | 2006-11-23 |
United States Patent
Application |
20060262400 |
Kind Code |
A1 |
Ouderkirk; Andrew J. ; et
al. |
November 23, 2006 |
Optical polarizer
Abstract
A reflective polarizer and a dichroic polarizer are combined to
provide an improved optical polarizer. The dichroic and reflective
polarizers are typically in close proximity to each other, and are
preferably bonded together to eliminate the air gap between the
polarizers. The combination of the two polarizers provides a high
reflectivity of one polarization and high transmission for the
perpendicular polarization from the reflective polarizer side of
the combined polarizer, and high absorption and transmission for
light of orthogonal polarization from the dichroic polarizer side.
The combination also reduces iridescence as seen in transmission
and when viewed in reflection from the dichroic polarizer side. The
increased extinction ratio and low reflectivity of the optical
polarizer allows use of a lower extinction ratio dichroic polarizer
in applications requiring a given extinction ratio and high
transmission.
Inventors: |
Ouderkirk; Andrew J.;
(Woodbury, MN) ; Weber; Michael F.; (Shoreview,
MN) ; Jonza; James M.; (Woodbury, MN) ;
Stover; Carl A.; (St. Paul, MN) |
Correspondence
Address: |
3M INNOVATIVE PROPERTIES COMPANY
PO BOX 33427
ST. PAUL
MN
55133-3427
US
|
Assignee: |
3M Innovative Properties
Company
|
Family ID: |
46255387 |
Appl. No.: |
11/405056 |
Filed: |
April 17, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11012651 |
Dec 15, 2004 |
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11405056 |
Apr 17, 2006 |
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09013819 |
Jan 27, 1998 |
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11012651 |
Dec 15, 2004 |
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08402042 |
Mar 10, 1995 |
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09013819 |
Jan 27, 1998 |
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08171239 |
Dec 21, 1993 |
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08402042 |
Mar 10, 1995 |
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08171098 |
Dec 21, 1993 |
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08171239 |
Dec 21, 1993 |
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08359436 |
Dec 20, 1994 |
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11012651 |
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08360215 |
Dec 20, 1994 |
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11012651 |
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Current U.S.
Class: |
359/487.02 ;
359/487.05; 359/489.07 |
Current CPC
Class: |
B29D 11/00 20130101;
B32B 27/08 20130101; G02B 5/3083 20130101; G02B 5/305 20130101;
B32B 27/36 20130101; G02F 1/13362 20130101; G02F 1/133536 20130101;
B29K 2995/0034 20130101; B29K 2995/003 20130101; B29C 55/023
20130101; G02B 5/3025 20130101; G02F 1/133545 20210101; B32B 7/02
20130101; G02B 27/283 20130101 |
Class at
Publication: |
359/490 ;
359/483 |
International
Class: |
G02B 5/30 20060101
G02B005/30 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 20, 1994 |
WO |
PCT/US94/14323 |
Dec 21, 1994 |
IL |
112101 |
Jan 2, 1995 |
MX |
950077 |
Dec 20, 1994 |
WO |
PCT/US94/14324 |
Dec 20, 1994 |
IL |
112072 |
Jan 2, 1995 |
MX |
950078 |
Claims
1.-6. (canceled)
7. A display, comprising: a liquid crystal display module; a
backlight; a front display polarizer comprising a broadband
reflective polarizer having anisotropic refractive indices such
that light having a first polarization state is reflected and light
having a second polarization state is transmitted, the reflective
polarizer having a viewing side facing away from the liquid crystal
display module and an opposing side facing the liquid crystal
display module.
8. A display as recited in claim 1, further comprising a rear
display polarizer disposed between the backlight and the liquid
crystal display module.
9. A display as recited in claim 2, wherein the rear display
polarizer comprises a broadband reflective polarizer having
anisotropic refractive indices such that light having a first
polarization state is reflected and light having a second
polarization state is transmitted.
10. A display as recited in claim 3, wherein the rear display
polarizer further comprises an absorbing polarizer disposed at the
viewing side of the reflective polarizer and being aligned to
absorb light of the first polarization and to transmit light of the
second polarization.
11. A display as recited in claim 1, wherein the front display
polarizer further comprises an absorbing polarizer disposed at the
viewing side of the reflective polarizer and being aligned to
absorb light of the first polarization and to transmit light of the
second polarization.
12. A display as recited in claim 5, wherein the absorbing
polarizer is bonded to the viewing side of the reflective
polarizer.
13. A display as recited in claim 1, wherein the reflective
polarizer comprises polymeric material.
14. A display as recited in claim 1, wherein the reflective
polarizer comprises a multilayer stack of two different
materials.
15. A display as recited in claim 8, wherein the multilayer stack
comprises alternating layers of a polymer based on naphthalene
dicarboxylic acid and a polymer formed from at least one of the
following: naphthalene dicarboxylic acid, isopthalic acid,
terephthalic acid, cycloalkane dicarboxylic acids, alkane
dicarboxylic acids, ethylene glycol, alkane glycols, cycloalkane
glycols, bisphenol A and carbonic acid.
16. A display as recited in claim 1, wherein the reflective
polarizer exploits constructive interference to reflect light of
the first polarization.
17. A display as recited in claim 1, wherein the reflective
polarizer is a cholesteric reflective polarizer.
18. A display as recited in claim 11, wherein the front display
polarizer further comprises an optical retarder.
19. A display as recited in claim 1, further comprising an
antireflection coating disposed on the reflective polarizer.
20. A method of increasing brightness of a display comprising a
liquid crystal display module and a backlight, the method
comprising the step of: disposing a broadband reflective polarizer
with a viewing side of the reflective polarizer facing away from
the liquid crystal display module and an opposing side of the
reflective polarizer facing the liquid crystal display module;
wherein the reflective polarizer has anisotropic refractive indices
such that light having a first polarization state is reflected and
light having a second polarization state is transmitted and wherein
the reflective polarizer returns at least a portion of light of the
first polarization supplied by the backlight and passed by the
liquid crystal module back into the backlight.
21. A method as recited in claim 14, further comprising disposing
between the backlight and the liquid crystal display module an
additional broadband reflective polarizer having anisotropic
refractive indices such that light having a first polarization
state is reflected and light having a second polarization state is
transmitted.
22. A method as recited in claim 14, wherein the reflective
polarizer comprises a multilayer stack of two different
materials.
23. A method as recited in claim 16, wherein the reflective
polarizer exploits constructive interference to reflect light of
the first polarization.
24. A method as recited in claim 16, wherein the multilayer stack
comprises alternating layers of a polymer based on crystalline
naphthalene dicarboxylic acid and a copolyester of naphthalene
dicarboxylic acid and tetephthalic acid.
25. A method as recited in claim 14, wherein the reflective
polarizer is a cholesteric reflective polarizer.
26. A method as recited in claim 14, wherein the display further
comprises an antireflection coating disposed on the reflective
polarizer.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 09/013,819, filed Jan. 27, 1998, which is a
continuation of U.S. patent application Ser. No. 08/402,042, filed
Mar. 10, 1995, now abandoned, which is a continuation-in-part of
U.S. patent application Ser. No. 08/171,239, filed Dec. 21, 1993,
now abandoned, which is a continuation-in-part of application Ser.
No. 08/171,098, Dec. 21, 1993, now abandoned, and a
continuation-in-part of application Ser. No. 08/359,436, filed Dec.
20, 1994, now abandoned, and a continuation-in-part of application
Ser. No. 08/360,215, filed Dec. 20, 1994, now abandoned.
TECHNICAL FIELD
[0002] The invention is an improved optical polarizer.
BACKGROUND
[0003] Optical polarizing film is widely used for glare reducing
sunglasses, increasing optical contrast, and in Liquid Crystal
Displays (LCD). The most commonly used type of polarizer used in
these applications is a dichroic polarizer. Dichroic polarizers are
made by incorporating a dye into a polymer sheet that is stretched
in one direction. Dichroic polarizers can also be made by
uniaxially stretching a semicrystalline polymer such as polyvinyl
alcohol, then staining the polymer with an iodine complex or
dichroic dye, or by coating a polymer with an oriented dichroic
dye. These polarizers typically have an extinction ratio (the ratio
of transmission of light polarized perpendicular to the stretch
direction to the polarization parallel to the stretch direction) of
over 500:1. Dichroic polarizers normally have some absorption of
light polarized in the high transmission orientation. Losses in
this orientation are typically 10-20%
[0004] Commercial polarizers typically use polyvinyl alcohol as the
polymer matrix for the dye, however, other polymers can be used.
U.S. Pat. No. 4,756,953 describes the use of polyethylene
naphthalate as the polymer matrix.
[0005] Low profile, sheet form reflective polarizers are available
that reflect one polarization of light and transmit the other.
These polarizers tend to be more efficient in transmitting light of
the high transmission polarization. This is due to the use of a
non-absorbing dielectric stack for polarizing light. These
polarizers tend to have equal reflectivity for light irradiating
the sheet from either side. These types of polarizers also tend to
have some defects, such as leakage of light through localized areas
of the sheet, and incomplete reflectivity of the high extinction
polarization over the wavelength region of interest. This leakage
of light and incomplete reflectivity is often called
iridescence.
SUMMARY
[0006] A reflective polarizer and a dichroic polarizer are combined
to provide an improved optical polarizer. The dichroic and
reflective polarizers are typically in close proximity to each
other, and are preferably bonded together to eliminate the air gap
between the polarizers. The combination of the two polarizers
provides a high reflectivity for light of a first polarization and
high transmission for light of a second, perpendicular polarization
from the reflective polarizer side of the optical polarizer, and
high absorption for light of the first polarization and high
transmission for light of the second, perpendicular polarization
from the dichroic polarizer side. Iridescence as seen in
transmission and when viewed in reflection from the dichroic
polarizer side is also reduced as compared to the reflective
polarizer alone. This reduction in iridescence is useful in
improving the cosmetic appearance of optical displays, the
extinction ratio of optical polarizers, and the optical uniformity
of a display.
[0007] The increased extinction ratio and low reflectivity of the
present optical polarizer allows use of a lower extinction ratio
dichroic polarizer in applications requiring a given extinction
ratio. By lowering the extinction ratio required of dichroic
polarizer the absorptive losses in the dichroic polarizer for
transmitted rays can be reduced. Thus, the present optical
polarizer has improved transmissive extinction ratios for rays
entering from either side of the present optical polarizer, low
reflected intensity for rays partially transmitted by the dichroic
polarizer in the first polarization that are reflected by the
reflective polarizer, and lower absorptive losses as compared to a
dichroic polarizer alone.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] The various objects, features and advantages of the present
optical polarizer shall be better understood upon reading and
understanding the following detailed description and accompanying
drawings in which:
[0009] FIG. 1 shows the present optical polarizer, including a
reflective polarizer and a dichroic polarizer placed proximate
thereto;
[0010] FIG. 2 shows a preferred multilayer reflective polarizer
having a dichroic polarizer bonded thereto;
[0011] FIG. 3 shows an embodiment of a display incorporating a
reflective polarizer and dichroic polarizer;
[0012] FIG. 4 shows another embodiment of a display incorporating a
reflective polarizer and dichroic polarizer;
[0013] FIG. 5 shows another embodiment of a display incorporating
two combined reflective polarizers and dichroic polarizers;
[0014] FIG. 6 shows a liquid crystal display incorporating a
reflective polarizer and a dichroic polarizer;
[0015] FIG. 7 shows a two layer stack of films forming a single
interface.
[0016] FIGS. 8 and 9 show reflectivity versus angle curves for a
uniaxial birefringent system in a medium of index 1.60.
[0017] FIG. 10 shows reflectivity versus angle curves for a
uniaxial birefringent system in a medium of index 1.0.
[0018] FIGS. 11, 12 and 13 show various relationships between
in-plane indices and z-index for a uniaxial birefringent
system.
[0019] FIG. 14 shows off axis reflectivity versus wavelength for
two different biaxial birefringent systems.
[0020] FIG. 15 shows the effect of introducing a y-index difference
in a biaxial birefringent film with a large z-index difference.
[0021] FIG. 16 shows the effect of introducing a y-index difference
in a biaxial birefringent film with a smaller z-index
difference.
[0022] FIG. 17 shows a contour plot summarizing the information
from FIGS. 15 and 16;
[0023] FIGS. 18-23 show optical performance of multilayer mirrors
given in Examples 3-6;
[0024] FIGS. 24-28 show optical performance of multilayer
polarizers given in Examples 7-11;
[0025] FIG. 29 shows optical performance of the multilayer mirror
given in Example 12;
[0026] FIG. 30 shows optical performance of the AR coated polarizer
given in Example 13;
[0027] FIG. 31 shows optical performance of the polarizer given in
Example 14; and
[0028] FIGS. 32A-32C show optical performance of multilayer
polarizers given in Example 15.
DETAILED DESCRIPTION
[0029] FIG. 1 shows an optical polarizer 10 that has two primary
components. These are a dichroic polarizer 11 and a reflective
polarizer 12. The two polarizers are aligned to provide maximum
transmissivity. The combination of the two polarizers provides a
high reflectivity for light of a first polarization and high
transmission for light of a second, perpendicular polarization from
the reflective polarizer side of the optical polarizer, and high
absorption for light of the first polarization and high
transmission for light of the second, perpendicular polarization
from the dichroic polarizer side.
[0030] In use, the combined polarizers are illuminated on one or
both of the outside facing surfaces. Ray 13 is of a polarization
that is preferentially reflected by reflective polarizer 12 to form
ray 14. Light of ray 13 transmitted by reflective polarizer 12
forms ray 15 which is attenuated by dichroic polarizer 11. Light
ray 16 which is perpendicularly polarized to ray 13 is
preferentially transmitted by reflective polarizer 12 and is
slightly attenuated by dichroic polarizer 11. Ray 17 is of a
polarization that is preferentially absorbed by dichroic polarizer
11, and which is also preferably of the same polarization as ray
13. The portion of light of ray 17 which is transmitted by dichroic
polarizer 11 is further attenuated by reflection off reflective
polarizer 12 forming ray 18 which is further absorbed by dichroic
polarizer 11. Light ray 19 which is polarized perpendicular to ray
17, and which is of the same polarization as ray 16, is
preferentially transmitted by both dichroic and reflective
polarizers 11 and 12, respectively.
[0031] The dichroic polarizer 11 is typically in close proximity to
the reflective polarizer 12. Preferably they are bonded to each
other to eliminate the air gap between the polarizers, as shown in
FIG. 2.
[0032] The preferred and illustrative reflective polarizer body 12
shown in FIG. 2 is made of alternating layers (ABABA . . . ) of two
different polymeric materials. These are referred to as material
"(A)" and material "(B)" throughout the drawings and description.
The two materials are extruded together and the resulting multiple
layer (ABABA . . . ) material is stretched (5:1) along one axis (X)
and is not stretched appreciably (1:1) along the other axis (Y).
The X axis is referred to as the "stretched" direction while the Y
axis is referred to as the "transverse" direction.
[0033] The (B) material has a nominal index of refraction (1.64 for
example) which is not substantially altered by the stretching
process. The (A) material has the property of having the index of
refraction altered by the stretching process. For example, a
uniaxially stretched sheet of the (A) material will have one index
of refraction (1.88 for example) associated with the stretched
direction and a different index of refraction (1.64 for example)
associated with the transverse direction. By way of definition, the
index of refraction associated with an in-plane axis (an axis
parallel to the surface of the film) is the effective index of
refraction for plane-polarized incident light whose plane of
polarization is parallel to that axis.
[0034] Thus, after stretching, the multiple layer stack (ABABA . .
. ) of material shows a large refractive index difference between
layers (1.88 minus 1.64) associated with the stretched direction.
While in the transverse direction, the associated indices of
refraction between layers are essentially the same (1.64 and 1.64
in the example). These optical characteristics cause the multiple
layer laminate to act as a reflecting polarizer that will transmit
the polarization component of the incident light which is correctly
oriented with respect to the axis 22. Axis 22 is defined as the
transmission axis. The light which is transmitted by the reflective
polarizer body 12 is referred to as having a first polarization
orientation.
[0035] The light which does not pass through the reflective
polarizer body 12 has a polarization orientation orthogonal or
perpendicular to the first orientation. Light exhibiting this
polarization orientation will encounter the index difference which
results in reflection of this light. This defines a so-called
"extinction" axis 24. In this fashion the reflective polarizer body
12 transmits light having a selected polarization, and reflects
light having the other polarization.
[0036] The optical performance and uniformity of a reflective
polarizer can be improved by adding a dichroic polarizer proximate
to at least one side of the multilayer stack, or by incorporating a
dichroic polarizer into at least one of the layers in the
multilayer stack. In such a configuration, the transmission axis 27
of the dichroic polarizer 11 is preferably aligned with the
transmission axis 22 of the reflective polarizer 12. When the
dichroic polarizer 11 is on one side of reflective polarizer 12, as
shown in FIG. 1, the reflection of light ray 17 on the dichroic
polarizer side will be reduced due to attenuation of reflected ray
18 by dichroic polarizer 11 in comparison to the reflection of ray
17 off reflective polarizer 12 without dichroic polarizer 11. The
reflectivity of ray 13 off reflective polarizer 12 is not
substantially affected by dichroic polarizer 11. This produces an
optical polarizer 10 which is antireflective on at least one side.
Antireflection of one side of the optical polarizer 10 is useful in
displays, particularly in certain backlit displays where the
reflected polarization can be used to increase the display
brightness while the other side, the viewing side, of the polarizer
must not reflect light. Iridescence as seen in transmission through
either direction, and iridescence when viewed in reflection from
the dichroic polarizer side are reduced by the addition of dichroic
polarizer 11. This reduction in iridescence is useful in improving
the cosmetic appearance of the display, the extinction ratio of the
polarizer, and the optical uniformity of the display.
[0037] The configuration of dichroic and reflective polarizers
shown in FIG. 1 creates a high efficiency optical polarizer.
Combining dichroic polarizer 11 with reflective polarizer 12
results in an optical polarizer 10 which has a higher extinction
ratio for transmitted light than that which is achieved using the
dichroic polarizer alone. This configuration also produces low
reflectivity for ray 17 from the dichroic polarizer side due to
attenuation of reflected ray 18 by dichroic polarizer 11. For
applications requiring a given extinction ratio and high
transmissivity, the increased extinction ratio and low reflectivity
of optical polarizer 10 allows the use of a dichroic polarizer 11
which has a lower extinction of the first polarization than could
otherwise be used. By lowering the extinction required of dichroic
polarizer 11, the absorptive losses in polarizer 11 for transmitted
rays 16 and 19 can be reduced. Thus, the optical polarizer 10 has
improved transmissive extinction ratios for ray pair 17 and 19 and
ray pair 13 and 16, low reflected intensity for reflected ray 18
off of reflective polarizer 12, and lower absorptive losses than
could be achieved using a dichroic polarizer alone. The preferred
extinction for the dichroic polarizer 11 for use in liquid crystal
displays is 10 to 99.99%, more preferred is 50 to 99%, more
preferred is 60 to 97%, and most preferred is 70 to 95%. The
preferred extinction for the reflective polarizer is 20 to 99.99%,
more preferred is 50 to 99.9% and most preferred is 90 to 99%.
[0038] Reflective polarizers may have some dielectric interference
in the second polarization at either normal or off-normal angles,
or both. This reflection may present problems due to reflected
glare and attenuation of transmitted light of the second
polarization. An efficient dichroic polarizer aligned as shown in
FIG. 1 will only weakly attenuate this reflection. In certain
applications, this reflection will be acceptable. In general,
however, the reflective polarizer will preferably have minimal
reflection in the second polarization over the range of optical
angles used by the device (nominally .+-.45 degrees for a TFT or
STN liquid crystal display). In general it is preferred that the
reflection of the reflective polarizer of linearly polarized light
of the second polarization be less than 20%, more preferably less
than 10%, and most preferably less than 5%. This reflectivity is
the average for the wavelength range and use angle range of
interest for specific or general applications. It is also preferred
that the reflectivity of the reflective polarizer for linearly
polarized light of the first polarization be preserved over angles
orthogonal to the extinction axis relative to the normal direction.
Preferred is that the reflectivity of the first polarization is
reduced to no less than 30% at the maximum angle of interest, more
preferred is 60%, and most preferred is that the reflectivity be
maintained or increase at off-normal angles over the range of
angles of interest.
[0039] The reflective and dichroic polarizers may be various
combinations of broad band and narrow band polarizers. For example,
a narrow band reflective polarizer may be combined with a dichroic
polarizer with extinction over the same band range. This
combination can be used to produce polarizers in the red, green,
blue, cyan, magenta, or yellow bands with higher extinction and
superior color definition compared to a colored dichroic polarizer.
Other combinations include the use of a broad band reflective
polarizer with dichroic polarizers with nonuniform extinction in
the visible spectrum. For example, certain polyvinyl alcohol/iodine
polarizers have excellent extinction in the green and red portion
of the spectrum, and less in the blue. This polarizer can be
combined with a broad band reflective polarizer in order to provide
good extinction at blue wavelengths. Nonuniform optical extinction
may also be useful for increasing the optical performance of the
combined polarizers. For example, the maximum radiometric
transmission from the combination of reflective and dichroic
polarizers may be obtained with minimum luminous reflectivity by
using a dichroic polarizer with relatively high absorption in the
green and less absorption in the blue and red. Insufficient
extinction in the reflective polarizer at normal and off-normal
angles may also be compensated by increasing the extinction of the
dichroic polarizer in the necessary spectral regions. For example,
a reflective polarizer that has insufficient extinction for red
light of the second polarization at off-normal angles can be
compensated by using a dichroic polarizer with relatively red high
extinction.
[0040] Dichroic polarizer 11 can be incorporated into optical
polarizer 10 by placing the reflective and dichroic polarizers in
the same optical path or by laminating them together. Dichroic
polarizer 11 can be incorporated with reflective polarizer 12
before orientation by extruding or laminating at least one layer of
a mixture of dichroic dyestuff in polymer onto the multilayer cast
film, by a dichroic dyestuff added to the polymer resin of one or
more of the skin layers of the multilayer reflective polarizer, or
by adding dye to one or more layers in the multilayer stack.
Multilayer extrusion techniques also allow the ability to tailor
the distribution of dichroic dye within the individual layers
making up the optical stack. This may allow the dye to be located
in regions of greatest utility. For example, a dye may be
preferably concentrated in regions of maximum or minimum "E" field
intensity within the optical stack. By appropriate choice of the
dichroic dyestuff and polymer matrix, stretching the resulting film
will simultaneously produce the dichroic and reflective polarizers
in the proper orientation.
[0041] Athraquinone and azo dyes may be used as the dichroic dye,
as well as other dichroic dye materials. In some applications the
dye does not have to be highly dichroic when oriented. Applications
requiring relatively high absorption of both polarizations, for
example, sunglasses or in displays requiring reduced glare, can use
a less dichroic, or non-dichroic dye.
[0042] The dichroic polarizer 11 may be incorporated into one or
both sides of a reflective polarizer by coating a solution of
polyvinyl alcohol onto the cast (unoriented) multilayer film and
simultaneously forming the multilayer reflective polarizer and the
dichroic polarizer. The cast film can be primed for adhesion before
coating by solution coating on an inorganic or polymeric primer
layer, corona treatment, or by physical treatment. A suitable
solution based primer for this application are water soluble
copolyesters commonly used for priming polyethylene terephthlate
films such as described in U.S. Pat. No. 4,659,523. The polyvinyl
alcohol coating solution should contain between 2 and 20% polymer
in water based on weight, with the preferred concentration being
between 5 and 15%. The polyvinyl alcohol should have a degree of
hydrolysis of between 95 and 100%, preferably between 97 and 99.5%.
The dry coating weight should range from 2 to 80 grams per square
meter. The polyvinyl alcohol coated cast film is then stretched at
elevated temperatures to develop oriented polyvinyl alcohol and the
multilayer reflective polarizer. This temperature is preferably
above the glass transition temperature of least one of the
components of the multilayer reflective polarizer. In general, the
temperature should be between 80 and 160 C, preferably between 100
and 160 C. The film should be stretched from 2 to 10 times the
original dimension. Preferably, the film will be stretched from 3
to 6 times the original dimension. The film may be allowed to
dimensionally relax in the cross-stretch direction from the natural
reduction in cross-stretch direction (equal to the square root of
the stretch ratio) to being constrained (i.e. no substantial change
in cross-stretch dimensions). The film may be stretched in the
machine direction, as with a length orienter, or in width using a
tenter. The oriented polyvinyl alcohol coating is then stained with
either iodine based staining solutions, dye based staining
solutions, or combinations of the two solutions and stabilized if
necessary with suitable solutions such as boric acid and borax in
water. Such staining and fixing techniques are known in the art.
After drying the film, the dichroic polarizer can be protected by
laminating or coating on a protective film such as cellulose based
polymers, acrylate polymers, polycarbonate polymers, solution based
or radiation cured acrylate based adhesive or non-adhesive
coatings, polyethylene terephthalate or other polyester based
films, or an additional sheet of reflective polarizer film. In
cases where the state of polarized light rays entering or exiting
the polarizer 10 from the dichroic polarizer side is not critical,
birefringent polymers such as biaxially oriented polyethylene
terephthalate may be used as the protective layer.
[0043] A dichroic polarizer suitable for use in this invention is
described in U.S. Pat. Nos. 4,895,769 and 4,659,523. The polarizers
described in these patents may be combined with the reflective
polarizer preferably with one side of the polyvinyl alcohol
dichroic polarizer protected with a separate polymer and the other
side of the dichroic polarizer bonded to the reflective polarizer.
The dichroic polarizer may be made from relatively thin polyvinyl
alcohol coatings (i.e., preferably less than 40 g per square meter,
more preferably less than 10 g/m.sup.2, more preferably less than 4
g/m.sup.2, even more preferably less than 2 g/m.sup.2). Thin
coatings will have less absorption of the polarization
perpendicular to the stretch direction, yet still have good
extinction in first polarization when the high transmission axis is
aligned with the high transmission axis of a reflective polarizer.
Thin coatings are also faster to process.
[0044] The polarizer of this invention has at least one dichroic
polarizer and one reflective polarizer sections (as shown in FIG.
1). Other combinations are also suitable, including polarizers
having either dichroic/reflective/dichroic sections or
reflective/dichroic/reflective sections.
[0045] FIG. 3 shows the combined reflective polarizer 12 and
dichroic polarizer 11 as used in a transmissive display. Liquid
crystal module 52 switches the polarization of transmitted light
supplied by backlight 54 through a conventional dichroic polarizer
53. In this mode, the reflective polarizer returns at least a
portion of the light of the first polarization passed by the liquid
crystal module 52 back into the backlight. This light may be
recycled by the backlight and be used to increase the brightness of
the display.
[0046] FIG. 4 shows the use of the combined polarizers 11 and 12 as
the rear polarizer in a transmissive display. In this mode, the
reflective polarizer may enhance the brightness of a display by
returning the light of the first polarization that would ordinarily
be absorbed by the rear dichroic polarizer in a conventional
display.
[0047] FIG. 5 shows combined polarizers 11 and 12 used as both the
front and rear polarizers in a display. The displays shown in FIGS.
3, 4, and 5 can be used in a transflective mode by inserting a
partial reflector between the backlight and the rear polarizer, and
can be used as a reflective display by replacing the backlight with
a reflective film.
[0048] In the display configurations of FIGS. 4 and 5, it may be
desirable to laminate or otherwise similarly attach the optical
polarizer to the optical cavity. Laminating the optical polarizer
to the optical cavity eliminates the air gap between them and thus
reduces surface reflections which would otherwise occur at the
air/reflective polarizer element boundary. These reflections reduce
the total transmission of the desired polarization by the
reflective polarizing element. By attaching the reflective
polarizer side of the optical polarizer to the optical cavity,
these surface reflections are reduced and total transmission of the
desired polarization by the optical polarizer is increased. If the
optical polarizer is not so attached to the optical cavity, use of
an AR coated polarizer such as that described below in Example 13
may be desirable, particularly in the display configurations of
FIGS. 4 and 5.
[0049] Most liquid crystal modules 52 such as those shown in FIGS.
3, 4, and 5 generally include a thin layer of liquid crystal
material sandwiched between two glass layers. To minimize parallax,
the configuration shown in FIG. 6 can be used. There the combined
polarizers 11 and 12 are located between the liquid crystal 56 and
glass layers 58 and 59 of the liquid crystal module 52. By locating
the combined polarizers in this manner, parallax which may be
otherwise introduced in varying degrees depending upon the
thickness of the glass layers is eliminated.
[0050] A Polaroid Corporation model number HN-38 dichroic
polarizing film was placed against the multilayer reflective
polarizer formed as discussed herein. The polarizers were aligned
for maximum transmission of one polarization. The combination of
the dichroic and reflective polarizers eliminated visible
iridescence of the reflective polarizing film when viewed in
transmission in either direction. The dichroic polarizer also
eliminated reflected visible iridescence from the reflective
polarizer when viewed in reflection through the dichroic polarizer.
Thus, the combination of a dichroic polarizer with a reflective
polarizer improves the cosmetic uniformity of the reflective
polarizer.
[0051] The reflectivity and transmissivity of this optical
polarizer was measured with a Lambda 9 spectrophotometer at 550 nm
using a sample beam polarized with a Melles-Griot dichroic
polarizer model number 03-FPG-009. Reflectivity measurements were
made using an integrating sphere. Separate reflectivity
measurements were made with the samples backed first with a white
diffuse reflector and then with a black backing. The transmissivity
of the combined polarizers was 65.64% when aligned in the
spectrophotometer for maximum transmission, and 0.05% when aligned
for minimum transmission. When the dichroic polarizer was facing
the integrating sphere and an absorbing backing was used, the
reflectivity of the combined polarizers was 13.26% when aligned for
maximum reflectivity and 4.37% when aligned for minimum
reflectivity. The maximum and minimum reflectivity of the combined
polarizers when the reflective polarizer was facing the integrating
sphere was 99.22% and 16.58%, respectively. The above measurements
were repeated with a white reflection standard behind the sample.
The reflectivity of the combined polarizers with the dichroic
polarizer facing the integrating sphere was 47.47% when aligned for
maximum reflectivity, and 4.41% when aligned for minimum
reflectivity. The maximum and minimum reflectivity of the combined
polarizers when the reflective polarizer was facing the integrating
sphere was 99.32% and 36.73%, respectively. Thus, the combination
of the two polarizers effectively renders one side of the
reflective polarizer antireflected without substantially affecting
the reflectivity of the other side of the reflective polarizer
[0052] The transmission of Polaroid Corporation model HN-38
dichroic polarizing film and the reflective polarizer were measured
at 430 nm using the procedure described above. The transmission of
the dichroic polarizer with the sample cross polarized to the
sample beam was 0.63%. The transmission of the reflective polarizer
under the same conditions was 48%. The transmission of the
combination of the two polarizers aligned for minimum transmission
was 0.31%. Thus, the extinction of a dichroic polarizer can be
increased by including a reflective polarizer in the optical
path.
Optical Behavior and Design Considerations of Multilayer Stacks
[0053] The optical behavior of a multilayer stack 10 such as that
shown above in FIG. 2 will now be described in more general
terms.
[0054] The optical properties and design considerations of
multilayer stacks described below allow the construction of
multilayer stacks for which the Brewster angle (the angle at which
reflectance goes to zero) is very large or is nonexistant. This
allows for the construction of multilayer mirrors and polarizers
whose reflectivity for p polarized light decrease slowly with angle
of incidence, are independent of angle of incidence, or increase
with angle of incidence away from the normal. As a result,
multilayer stacks having high reflectivity for both s and p
polarized light over a wide bandwidth, and over a wide range of
angles can be achieved.
[0055] The average transmission at normal incidence for a
multilayer stack, (for light polarized in the plane of the
extinction axis in the case of polarizers, or for both
polarizations in the case of mirrors), is desirably less than 50%
(reflectivity of 0.5) over the intended bandwidth (It shall be
understood that for the purposes of the present application, all
transmission or reflection values given include front and back
surface reflections). Other multilayer stacks exhibit lower average
transmission and/or a larger intended bandwidth, and/or over a
larger range of angles from the normal. If the intended bandwidth
is to be centered around one color only, such as red, green or
blue, each of which has an effective bandwidth of about 100 nm
each, a multilayer stack with an average transmission of less than
50% is desirable. A multilayer stack having an average transmission
of less than 10% over a bandwidth of 100 nm is also preferred.
Other exemplary preferred mutlilayer stacks have an average
transmission of less than 30% over a bandwidth of 200 nm. Yet
another preferred multilayer stack exhibits an average transmission
of less than 10% over the bandwidth of the visible spectrum
(400-700 nm). Most preferred is a multilayer stack that exhibits an
average transmission of less than 10% over a bandwidth of 380 to
740 nm. The extended bandwidth is useful even in visible light
applications in order to accommodate spectral shifts with angle,
and variations in the multilayer stack and overall film
caliper.
[0056] The multilayer stack 10 can include tens, hundreds or
thousands of layers, and each layer can be made from any of a
number of different materials. The characteristics which determine
the choice of materials for a particular stack depend upon the
desired optical performance of the stack.
[0057] The stack can contain as many materials as there are layers
in the stack. For ease of manufacture, preferred optical thin film
stacks contain only a few different materials. For purposes of
illustration, the present discussion will describe multilayer
stacks including two materials.
[0058] The boundaries between the materials, or chemically
identical materials with different physical properties, can be
abrupt or gradual. Except for some simple cases with analytical
solutions, analysis of the latter type of stratified media with
continuously varying index is usually treated as a much larger
number of thinner uniform layers having abrupt boundaries but with
only a small change in properties between adjacent layers.
[0059] Several parameters may affect the maximum reflectivity
achievable in any multilayer stack. These include basic stack
design, optical absorption, layer thickness control and the
relationship between indices of refraction of the layers in the
stack. For high reflectivity and/or sharp bandedges, the basic
stack design should incorporate optical interference effects using
standard thin film optics design. This typically involves using
optically thin layers, meaning layers having an optical thickness
in the range of 0.1 to 1.0 times the wavelength of interest. The
basic building blocks for high reflectivity multilayer films are
low/high index pairs of film layers, wherein each low/high index
pair of layers has a combined optical thickness of 1/2 the center
wavelength of the band it is designed to reflect. Stacks of such
films are commonly referred to as quarterwave stacks.
[0060] To minimize optical absorption, the preferred multilayer
stack ensures that wavelengths that would be most strongly absorbed
by the stack are the first wavelengths reflected by the stack. For
most clear optical materials, including most polymers, absorption
increases toward the blue end of the visible spectrum. Thus, it is
preferred to tune the multilayer stack such that the "blue" layers
are on the incident side of the multilayer stack.
[0061] A multilayer construction of alternative low and high index
thick films, often referred to as a "pile of plates", has no tuned
wavelengths nor bandwidth constraints, and no wavelength is
selectively reflected at any particular layer in the stack. With
such a construction, the blue reflectivity suffers due to higher
penetration into the stack, resulting in higher absorption than for
the preferred quarterwave stack design. Arbitrarily increasing the
number of layers in a "pile of plates" will not always give high
reflectivity, even with zero absorption. Also, arbitrarily
increasing the number of layers in any stack may not give the
desired reflectivity, due to the increased absorption which would
occur.
[0062] The relationships between the indices of refraction in each
film layer to each other and to those of the other layers in the
film stack determine the reflectance behavior of the multilayer
stack at any angle of incidence, from any azimuthal direction.
Assuming that all layers of the same material have the same
indices, then a single interface of a two component quarterwave
stack can be analyzed to understand the behavior of the entire
stack as a function of angle.
[0063] For simplicity of discussion, therefore, the optical
behavior of a single interface will be described. It shall be
understood, however, that an actual multilayer stack according to
the principles described herein could be made of tens, hundreds or
thousands of layers. To describe the optical behavior of a single
interface, such as the one shown in FIG. 7, the reflectivity as a
function of angle of incidence for s and p polarized light for a
plane of incidence including the z-axis and one in-plane optic axis
will be plotted.
[0064] FIG. 7 shows two material film layers forming a single
interface, with both immersed in an isotropic medium of index no.
For simplicity of illustration, the present discussion will be
directed toward an orthogonal multilayer birefringent system with
the optical axes of the two materials aligned, and with one optic
axis (z) perpendicular to the film plane, and the other optic axes
along the x and y axis. It shall be understood, however, that the
optic axes need not be orthogonal, and that nonorthogonal systems
are well within the spirit and scope of the present invention. It
shall be further understood that the optic axes also need not be
aligned with the film axes to fall within the intended scope of the
present invention.
[0065] The reflectivity of a dielectric interface varies as a
function of angle of incidence, and for isotropic materials, is
different for p and s polarized light. The reflectivity minimum for
p polarized light is due to the so called Brewster effect, and the
angle at which the reflectance goes to zero is referred to as
Brewster's angle.
[0066] The reflectance behavior of any film stack, at any angle of
incidence, is determined by the dielectric tensors of all films
involved. A general theoretical treatment of this topic is given in
the text by R. M. A. Azzam and N. M. Bashara, "Ellipsometry and
Polarized Light", published by North-Holland, 1987.
[0067] The reflectivity for a single interface of a system is
calculated by squaring the absolute value of the reflection
coefficients for p and s polarized light, given by equations 1 and
2, respectively. Equations 1 and 2 are valid for uniaxial
orthogonal systems, with the axes of the two components aligned. 1
) .times. .times. r pp = n .times. .times. 2 .times. z * n .times.
.times. 2 .times. o .times. .times. O .function. ( n .times.
.times. 1 .times. z 2 - no 2 .times. sin .times. 2 .times. q ) - n
.times. .times. 1 .times. .times. z * n .times. .times. 1 .times.
.times. o .times. .times. O .function. ( n .times. .times. 2
.times. z 2 - no 2 .times. sin 2 .times. q ) n .times. .times. 2
.times. z * n .times. .times. 2 .times. o .times. .times. O
.function. ( n .times. .times. 1 .times. z 2 - no 2 .times. sin
.times. 2 .times. q ) + n .times. .times. 1 .times. .times. z * n
.times. .times. 1 .times. .times. o .times. .times. O .function. (
n .times. .times. 2 .times. z 2 - no 2 .times. sin 2 .times. q )
.times. .times. 2 ) .times. .times. r s = O .function. ( n .times.
.times. 1 .times. o 2 - no 2 .times. sin 2 .times. q ) - O
.function. ( n .times. .times. 2 .times. o 2 - no 2 .times. sin 2
.times. q ) O .function. ( n .times. .times. 1 .times. .times. o 2
- no 2 .times. sin 2 .times. q ) + O .function. ( n .times. .times.
2 .times. .times. o 2 - no 2 .times. sin 2 .times. q ) ##EQU1##
where q is measured in the isotropic medium.
[0068] In a uniaxial birefringent system, n1x=n1y=n1o, and
n2x=n2y=n2o.
[0069] For a biaxial birefringent system, equations 1 and 2 are
valid only for light with its plane of polarization parallel to the
x-z or y-z planes, as defined in FIG. 7. So, for a biaxial system,
for light incident in the x-z plane, n1o=n1x and n2o=n2x in
equation 1 (for p-polarized light), and n1o=n1y and n2o=n2y in
equation 2 (for s-polarized light). For light incident in the y-z
plane, n1o=n1y and n2o=n2y in equation 1 (for p-polarized light),
and n1o=n1x and n2o=n2x in equation 2 (for s-polarized light).
[0070] Equations 1 and 2 show that reflectivity depends upon the
indices of refraction in the x, y (in-plane) and z directions of
each material in the stack. In an isotropic material, all three
indices are equal, thus nx=ny=nz. The relationship between nx, ny
and nz determine the optical characteristics of the material.
Different relationships between the three indices lead to three
general categories of materials: isotropic, uniaxially
birefringent, and biaxially birefringent. Equations 1 and 2
describe biaxially birefringent cases only along the x or y axis,
and then only if considered separately for the x and y
directions.
[0071] A uniaxially birefringent material is defined as one in
which the index of refraction in one direction is different from
the indices in the other two directions. For purposes of the
present discussion, the convention for describing uniaxially
birefringent systems is for the condition nx=ny.sup.1 nz. The x and
y axes are defined as the in-plane axes and the respective indices,
nx and ny, will be referred to as the in-plane indices.
[0072] One method of creating a uniaxial birefringent system is to
biaxially stretch (e.g., stretch along two dimensions) a multilayer
stack in which at least one of the materials in the stack has its
index of refraction affected by the stretching process (e.g., the
index either increases or decreases). Biaxial stretching of the
multilayer stack may result in differences between refractive
indices of adjoining layers for planes parallel to both axes thus
resulting in reflection of light in both planes of
polarization.
[0073] A uniaxial birefringent material can have either positive or
negative uniaxial birefringence. Positive uniaxial birefringence
occurs when the z-index is greater than the in-plane indices
(nz>nx and ny). Negative uniaxial birefringence occurs when the
z-index is less than the in-plane indices (nz<nx and ny).
[0074] A biaxial birefringent material is defined as one in which
the indices of refraction in all three axes are different, e.g.,
nx.sup.1 ny.sup.1 nz. Again, the nx and ny indices will be referred
to as the in-plane indices. A biaxial birefringent system can be
made by stretching the multilayer stack in one direction. In other
words the stack is uniaxially stretched. For purposes of the
present discussion, the x direction will be referred to as the
stretch direction for biaxial birefringent stacks.
Uniaxial Birefringent Systems (Mirrors)
[0075] The optical properties and design considerations of uniaxial
birefringent systems will now be discussed. As discussed above, the
general conditions for a uniaxial birefringent material are
nx=ny.sup.1 nz. Thus if each layer 102 and 104 in FIG. 7 is
uniaxially birefringent, n1x=n1y and n2x=n2y. For purposes of the
present discussion, assume that layer 102 has larger in-plane
indices than layer 104, and that thus n1>n2 in both the x and y
directions. The optical behavior of a uniaxial birefringent
multilayer system can be adjusted by varying the values of n1z and
n2z to introduce different levels of positive or negative
birefringence. The relationship between the various indices of
refraction can be measured directly, or, the general relationship
may be indirectly observed by analysis of the spectra of the
resulting film as described herein.
[0076] In the case of mirrors, the desired average transmission for
light of each polarization and plane of incidence generally depends
upon the intended use of the mirror. The average transmission along
each stretch direction at normal incidence for a narrow bandwidth
mirror across a 100 nm bandwidth within the visible spectrum is
desirably less than 30%, preferably less than 20% and more
preferably less than 10%. A desirable average transmission along
each stretch direction at normal incidence for a partial mirror
ranges anywhere from, for example, 10% to 50%, and can cover a
bandwidth of anywhere between, for example, 100 nm and 450 nm,
depending upon the particular application. For a high efficiency
mirror, average transmission along each stretch direction at normal
incidence over the visible spectrum (400-700nm) is desirably less
than 10%, preferably less than 5%, more preferably less than 2%,
and even more preferably less than 1%. In addition, asymmetric
mirrors may be desirable for certain applications. In that case,
average transmission along one stretch direction may be desirably
less than, for example, 50%, while the average transmission along
the other stretch direction may be desirably less than, for example
20%, over a bandwidth of, for example, the visible spectrum
(400-700 nm), or over the visible spectrum and into the near
infrared (e.g, 400-850 nm).
[0077] Equation 1 described above can be used to determine the
reflectivity of a single interface in a uniaxial birefringent
system composed of two layers such as that shown in FIG. 7.
Equation 2, for s polarized light, is identical to that of the case
of isotropic films (nx=ny=nz), so only equation I need be examined.
For purposes of illustration, some specific, although generic,
values for the film indices will be assigned. Let n1x=n1y=1.75,
n1z=variable, n2x=n2y=1.50, and n2z=variable. In order to
illustrate various possible Brewster angles in this system, no=1.60
for the surrounding isotropic media.
[0078] FIG. 8 shows reflectivity versus angle curves for
p-polarized light incident from the isotropic medium to the
birefringent layers, for cases where n1z is numerically greater
than or equal to n2z (n1z.sup.3 n2z). The curves shown in FIG. 8
are for the following z-index values: a) n1z=1.75, n2z=1.50; b)
n1z=1.75, n2z=1.57; c) n1z=1.70, n2z=1.60; d) n1z=1.65, n2z=1.60;
e) n1z=1.61, n2z=1.60; and f) n1z=1.60=n2z. As n1z approaches n2z,
the Brewster angle, the angle at which reflectivity goes to zero,
increases. Curves a-e are strongly angular dependent. However, when
n1z=n2z (curve f), there is no angular dependence to reflectivity.
In other words, the reflectivity for curve f is constant for all
angles of incidence. At that point, equation 1 reduces to the
angular independent form: (n2o-n1o)/(n2o+n1o). When n1z=n2z, there
is no Brewster effect and there is constant reflectivity for all
angles of incidence.
[0079] FIG. 9 shows reflectivity versus angle of incidence curves
for cases where n1z is numerically less than or equal to n2z. Light
is incident from isotropic medium to the birefringent layers. For
these cases, the reflectivity monotonically increases with angle of
incidence. This is the behavior that would be observed for
s-polarized light. Curve a in FIG. 9 shows the single case for s
polarized light. Curves b-e show cases for p polarized light for
various values of nz, in the following order: b) n1z=1.50,
n2z=1.60; c) n1z=1.55, n2z=1.60; d) n1z=1.59, n2z=1.60; and e)
n1z=1.60=n2z. Again, when n1z=n2z (curve e), there is no Brewster
effect, and there is constant reflectivity for all angles of
incidence.
[0080] FIG. 10 shows the same cases as FIGS. 8 and 9 but for an
incident medium of index no=1.0 (air). The curves in FIG. 10 are
plotted for p polarized light at a single interface of a positive
uniaxial material of indices n2x=n2y=1.50, n2z=1.60, and a negative
uniaxially birefringent material with n1x=n1y=1.75, and values of
n1z, in the following order, from top to bottom, of: a) 1.50; b)
1.55; c) 1.59; d) 1.60; f) 1.61; g) 1.65; h) 1.70; and i) 1.75.
Again, as was shown in FIGS. 8 and 9, when the values of n1z and
n2z match (curve d), there is no angular dependence to
reflectivity.
[0081] FIGS. 8, 9 and 10 show that the cross-over from one type of
behavior to another occurs when the z-axis index of one film equals
the z-axis index of the other film. This is true for several
combinations of negative and positive uniaxially birefringent, and
isotropic materials. Other situations occur in which the Brewster
angle is shifted to larger or smaller angles.
[0082] Various possible relationships between in-plane indices and
z-axis indices are illustrated in FIGS. 11, 12 and 13. The vertical
axes indicate relative values of indices and the horizontal axes
are used to separate the various conditions. Each Figure begins at
the left with two isotropic films, where the z-index equals the
in-plane indices. As one proceeds to the right, the in-plane
indices are held constant and the various z-axis indices increase
or decrease, indicating the relative amount of positive or negative
birefringence.
[0083] The case described above with respect to FIGS. 8, 9, and 10
is illustrated in FIG. 11. The in-plane indices of material one are
greater than the in-plane indices of material two, material I has
negative birefringence (n1z less than in-plane indices), and
material two has positive birefringence (n2z greater than in-plane
indices). The point at which the Brewster angle disappears and
reflectivity is constant for all angles of incidence is where the
two z-axis indices are equal. This point corresponds to curve f in
FIG. 8, curve e in FIG. 9 or curve d in FIG. 10.
[0084] In FIG. 12, material one has higher in-plane indices than
material two, but material one has positive birefringence and
material two has negative birefringence. In this case, the Brewster
minimum can only shift to lower values of angle.
[0085] Both FIGS. 1 1 and 12 are valid for the limiting cases where
one of the two films is isotropic. The two cases are where material
one is isotropic and material two has positive birefringence, or
material two is isotropic and material one has negative
birefringence. The point at which there is no Brewster effect is
where the z-axis index of the birefringent material equals the
index of the isotropic film.
[0086] Another case is where both films are of the same type, i.e.,
both negative or both positive birefringent. FIG. 13 shows the case
where both films have negative birefringence. However, it shall be
understood that the case of two positive birefringent layers is
analogous to the case of two negative birefringent layers shown in
FIG. 13. As before, the Brewster minimum is eliminated only if one
z-axis index equals or crosses that of the other film.
[0087] Yet another case occurs where the in-plane indices of the
two materials are equal, but the z-axis indices differ. In this
case, which is a subset of all three cases shown in FIGS. 11-13, no
reflection occurs for s polarized light at any angle, and the
reflectivity for p polarized light increases monotonically with
increasing angle of incidence. This type of article has increasing
reflectivity for p-polarized light as angle of incidence increases,
and is transparent to s-polarized light. This article can be
referred to as a "p-polarizer".
[0088] The above described principles and design considerations
describing the behavior of uniaxially birefringent systems can be
applied to create multilayer stacks having the desired optical
effects for a wide variety of circumstances and applications. The
indices of refraction of the layers in the multilayer stack can be
manipulated and tailored to produce devices having the desired
optical properties. Many negative and positive uniaxial
birefringent systems can be created with a variety of in-plane and
z-axis indices, and many useful devices can be designed and
fabricated using the principles described here.
Biaxial Birefringent Systems (Polarizers)
[0089] Referring again to FIG. 7, two component orthogonal biaxial
birefringent systems and the design considerations affecting the
resultant optical properties will now be described. Again, the
system can have many layers, but an understanding of the optical
behavior of the stack is achieved by examining the optical behavior
at one interface.
[0090] A biaxial birefringent system can be designed to give high
reflectivity for light with its plane of polarization parallel to
one axis, for a broad range of angles of incidence, and
simultaneously have low reflectivity and high transmission for
light with its plane of polarization parallel to the other axis for
a broad range of angles of incidence. As a result, the biaxial
birefringent system acts as a polarizer, transmitting light of one
polarization and reflecting light of the other polarization. By
controlling the three indices of refraction of each film, nx, ny
and nz, the desired polarizer behavior can be obtained. Again, the
indices of refraction can be measured directly or can be indirectly
observed by analysis of the spectra of the resulting film, as
described herein.
[0091] Referring again to FIG. 7, the following values to the film
indices are assigned for purposes of illustration: n1x=1.88,
n1y=1.64, n1z=variable, n2x=1.65, n2y=variable, and n2z=variable.
The x direction is referred to as the extinction direction and the
y direction as the transmission direction.
[0092] Equation 1 can be used to predict the angular behavior of
the biaxial birefringent system for two important cases of light
with a plane of incidence in either the stretch (xz plane) or the
non-stretch (yz plane) directions. The polarizer is a mirror in one
polarization direction and a window in the other direction. In the
stretch direction, the large index differential of 1.88-1.65=0.23
in a multilayer stack with hundreds of layers will yield very high
reflectivities for s-polarized light. For p-polarized light the
reflectance at various angles depends on the n1z/n2z index
differential.
[0093] In many applications, the ideal reflecting polarizer has
high reflectance along one axis (the so-called extinction axis) and
zero reflectance along the other (the so-called transmission axis),
at all angles of incidence. For the transmission axis of a
polarizer, it generally desirable to maximize transmission of light
polarized in the direction of the transmission axis over the
bandwidth of interest and also over the range of angles of
interest. Average transmission at normal incidence for a colored
polarizer across a 100 nm bandwidth is desirably at least 50%,
preferably at least 70% and more preferably at least 90%. The
average transmission at 60 degreees from the normal for p-polarized
light (measured along the transmission axis) for a narrow band
polarizer across a 100 nm bandwidth is desirably at least 50%,
preferably at least 70% and more preferably at least 80%.
[0094] The average transmission at normal incidence for a polarizer
in the transmission axis across the visible spectrum (400-700 nm
for a bandwidth of 300 nm) is desirably at least 50%, preferably at
least 70%, more preferably at least 85%, and even more preferably
at least 90%. The average transmission at 60 degrees from the
normal (measured along the transmission axis) for a polarizer from
400-700 nm is desirably at least 50%, preferably at least 70%, more
preferably at least 80%, and even more preferably at least 90%.
[0095] For certain applications, high reflectivity in the
transmission axis at off-normal angles are preferred. The average
reflectivity for light polarized along the transmission axis should
be more than 20% at an angle of at least 20 degrees from the
normal.
[0096] If some reflectivity occurs along the transmission axis, the
efficiency of the polarizer at off-normal angles may be reduced. If
the reflectivity along the transmission axis is different for
various wavelengths, color may be introduced into the transmitted
light. One way to measure the color is to determine the root mean
square (RMS) value of the transmissivity at a selected angle or
angles over the wavelength range of interest. The % RMS color,
C.sub.RMS, can be determined according to the equation: C RMS =
.intg. .lamda. .times. .times. 1 .lamda. .times. .times. 2 .times.
( ( T - T _ ) 2 ) 1 / 2 .times. d .lamda. T _ ##EQU2## where the
range 11 to 12 is the wavelength range, or bandwidth, of interest,
T is the transmissivity along the transmission axis, and {overscore
(T)} is the average transmissivity along the transmission axis in
the wavelength range of interest.
[0097] For applications where a low color polarizer is desirable,
the % RMS color should be less than 10%, preferably less than 8%,
more preferably less than 3.5%, and even more preferably less than
2.1% at an angle of at least 30 degrees from the normal, preferably
at least 45 degrees from the normal, and even more preferably at
least 60 degrees from the normal.
[0098] Preferably, a reflective polarizer combines the desired %
RMS color along the transmission axis for the particular
application with the desired amount of reflectivity along the
extinction axis across the bandwidth of interest. For example, for
narrow band polarizers having a bandwidth of approximately 100 nm,
average transmission along the extinction axis at normal incidence
is desirably less than 50%, preferably less than 30%, more
preferably less than 10%, and even more preferably less than 3%.
For polarizers having a bandwidth in the visible range (400-700 nm,
or a bandwidth of 300 nm), average transmission along the
extinction axis at normal incidence is desirably less than 40%,
more desirably less than 25%, preferably less than 15%, more
preferably less than 5% and even more preferably less than 3%.
[0099] Reflectivity at off-normal angles, for light with its plane
of polarization parallel to the transmission axis may be caused by
a large z-index mismatch, even if the in-plane y indices are
matched. The resulting system thus has large reflectivity for p,
and is highly transparent to s polarized light. This case was
referred to above in the analysis of the mirror cases as a "p
polarizer".
[0100] For uniaxially stretched polarizers, performance depends
upon the relationships between the alternating layer indices for
all three (x, y, and z) directions. As described herein, it is
desirable to minimize the y and z index differentials for a high
efficiency polarizer. Introduction of a y-index mismatch is
describe to compensate for a z-index mismatch. Whether
intentionally added or naturally occurring, any index mismatch will
introduce some reflectivity. An important factor thus is making the
x-index differential larger than the y- and z-index differentials.
Since reflectivity increases rapidly as a function of index
differential in both the stretch and non-stretch directions, the
ratios Dny/Dnx and Dnz/Dnx should be minimized to obtain a
polarizer having high extinction along one axis across the
bandwidth of interest and also over a broad range of angles, while
preserving high transmission along the orthogonal axis. Ratios of
less than 0.05, 0.1 or 0.25 are acceptable. Ideally, the ratio
Dnz/Dnx is 0, but ratios of less than 0.25 or 0.5 also produce a
useable polarizer.
[0101] FIG. 14 shows the reflectivity (plotted as -Log[1-R]) at
75.degree. for p polarized light with its plane of incidence in the
non-stretch direction, for an 800 layer stack of PEN/coPEN. The
reflectivity is plotted as function of wavelength across the
visible spectrum (400-700 nm). The relevant indices for curve a at
550 nm are n1y=1.64, n1z=1.52, n2y=1.64 and n2z=1.63. The model
stack design is a linear thickness grade for quarterwave pairs,
where each pair thickness is given by
d.sub.n=d.sub.o+d.sub.o(0.003)n. All layers were assigned a random
thickness error with a gaussian distribution and a 5% standard
deviation.
[0102] Curve a shows high off-axis reflectivity across the visible
spectrum along the transmission axis (the y-axis) and that
different wavelengths experience different levels of reflectivity.
This is due to the large z-index mismatch (Dnz=0.11). Since the
spectrum is sensitive to layer thickness errors and spatial
nonuniformities, such as film caliper, this gives a biaxial
birefringent system with a very nonuniform and "colorful"
appearance. Although a high degree of color may be desirable for
certain applications, it is desirable to control the degree of
off-axis color, and minimize it for those applications requiring a
uniform, low color appearance, such as liquid crystal displays or
other types of displays.
[0103] Off-axis reflectivity, and off-axis color can be minimized
by introducing an index mismatch to the non-stretch in-plane
indices (n1y and n2y) that create a Brewster condition off axis,
while keeping the s-polarization reflectivity to a minimum.
[0104] FIG. 15 explores the effect of introducing a y-index
mismatch in reducing off-axis reflectivity along the transmission
axis of a biaxial birefringent system. With n1z=1.52 and n2z=1.63
(Dnz=0.11), the following conditions are plotted for p polarized
light: a) n1y=n2y=1.64; b) n1y=1.64, n2y=1.62; c) n1y=1.64,
n2y=1.66. Curve a shows the reflectivity where the in-plane indices
n1y and n2y are equal. Curve a has a reflectance minimum at
0.degree., but rises steeply after 20.degree.. For curve b,
n1y>n2y, and reflectivity increases rapidly. Curve c, where
n1y<n2y, has a reflectance minimum at 38.degree., but rises
steeply thereafter. Considerable reflection occurs as well for s
polarized light for n1y.sup.1 n2y, as shown by curve d. Curves a-d
of FIG. 15 indicate that the sign of the y-index mismatch (n1y-n2y)
should be the same as the z-index mismatch (n1z-n2z) for a Brewster
minimum to exist. For the case of n1y=n2y, reflectivity for s
polarized light is zero at all angles.
[0105] By reducing the z-axis index difference between layers, the
off axis reflectivity can be further reduced. If n1z is equal to
n2z, FIG. 10 indicates that the extinction axis will still have a
high reflectivity off-angle as it does at normal incidence, and no
reflection would occur along the nonstretch axis at any angle
because both indices are matched (e.g., n1y=n2y and n1z=n2z).
[0106] Exact matching of the two y indices and the two z indices
may not be possible in some multilayer systems. If the z-axis
indices are not matched in a polarizer construction, introduction
of a slight mismatch may be desired for in-plane indices n1y and
n2y. This can be done by blending additional components into one or
both of the material layers in order to increase or decrease the
respective y index as described below in Example 15. Blending a
second resin into either the polymer that forms the highly
birefringent layers or into the polymer that forms the selected
polymer layers may be done to modify reflection for the
transmission axis at normal and off-normal angles, or to modify the
extinction of the polarizer for light polarized in the extinction
axis. The second, blended resin may accomplish this by modifying
the crystallinity and the index of refraction of the polymer layers
after orientation.
[0107] Another example is plotted in FIG. 16, assuming n1z=1.56 and
n2z=1.60 (Dnz=0.04), with the following y indices a) n1y=1.64,
n2y=1.65; b) n1y=1.64, n2y=1.63. Curve c is for s-polarized light
for either case. Curve a, where the sign of the y-index mismatch is
the same as the z-index mismatch, results in the lowest off-angle
reflectivity.
[0108] The computed off-axis reflectance of an 800 layer stack of
films at 75.degree. angle of incidence with the conditions of curve
a in FIG. 16 is plotted as curve b in FIG. 14. Comparison of curve
b with curve a in FIG. 14 shows that there is far less off-axis
reflectivity, and therefore lower perceived color and better
uniformity, for the conditions plotted in curve b. The relevant
indices for curve b at 550 nm are n1y=1.64, n1z=1.56, n2y=1.65 and
n2z=1.60.
[0109] FIG. 17 shows a contour plot of equation I which summarizes
the off axis reflectivity discussed in relation to FIG. 7 for
p-polarized light. The four independent indices involved in the
non-stretch direction have been reduced to two index mismatches,
Dnz and Dny. The plot is an average of 6 plots at various angles of
incidence from 0.degree. to 75.degree. in 15 degree increments. The
reflectivity ranges from 0.4.times.10.sup.-4 for contour a, to
4.0.times.10.sup.-4 for contour j, in constant increments of
0.4.times.10.sup.-4. The plots indicate how high reflectivity
caused by an index mismatch along one optic axis can be offset by a
mismatch along the other axis.
[0110] Thus, by reducing the z-index mismatch between layers of a
biaxial birefringent systems, and/or by introducing a y-index
mismatch to produce a Brewster effect, off-axis reflectivity, and
therefore off-axis color, are minimized along the transmission axis
of a multilayer reflecting polarizer.
[0111] It should also be noted that narrow band polarizers
operating over a narrow wavelength range can also be designed using
the principles described herein: These can be made to produce
polarizers in the red, green, blue, cyan, magenta, or yellow bands,
for example.
[0112] An ideal reflecting polarizer should transmit all light of
one polarization, and reflect all light of the other polarization.
Unless laminated on both sides to glass or to another film with a
clear optical adhesive, surface reflections at the air/reflecting
polarizer interface will reduce the transmission of light of the
desired polarization. Thus, it may in some cases be useful to add
an antireflection (AR) coating to the reflecting polarizer. The AR
coating is preferably designed to dereflect a film of index 1.64
for PEN based polarizers in air, because that is the index of all
layers in the nonstretch (y) direction. The same coating will have
essentially no effect on the stretch direction because the
alternating index stack of the stretch direction has a very high
reflection coefficient irrespective of the presence or absence of
surface reflections. Any AR coating known in the art could be
applied, provided that the coating does not overheat or damage the
multilayer film being coated. An exemplary coating would be a
quarterwave thick coating of low index material, ideally with index
near the square root of 1.64 (for PEN based materials).
Materials Selection and Processing
[0113] With the above-described design considerations established,
one of ordinary skill will readily appreciate that a wide variety
of materials can be used to form multilayer mirrors or polarizers
according to the invention when processed under conditions selected
to yield the desired refractive index relationships. The desired
refractive index relationships can be achieved in a variety of
ways, including stretching during or after film formation (e.g., in
the case of organic polymers), extruding (e.g., in the case of
liquid crystalline materials), or coating. In addition, it is
preferred that the two materials have similar rheological
properties (e.g., melt viscosities) such that they can be
co-extruded.
[0114] In general, appropriate combinations may be achieved by
selecting, as the first material, a crystalline or semi-crystalline
material, preferably a polymer. The second material, in turn, may
be crystalline, semi-crystalline, or amorphous. The second material
may have a birefringence opposite to or the same as that of the
first material. Or, the second material may have no
birefringence.
[0115] Specific examples of suitable materials include polyethylene
naphthalate (PEN) and isomers thereof (e.g., 2,6-, 1,4-, 1,5-,
2,7-, and 2,3-PEN), polyalkylene terephthalates (e.g., polyethylene
terephthalate, polybutylene terephthalate, and
poly-1,4-cyclohexanedimethylene terephthalate), polyimides (e.g.,
polyacrylic imides), polyetherimides, atactic polystyrene,
polycarbonates, polymethacrylates (e.g., polyisobutyl methacrylate,
polypropylmethacrylate, polyethylmethacrylate, and
polymethylmethacrylate), polyacrylates (e.g., polybutylacrylate and
polymethylacrylate), syndiotactic polystyrene (sPS), syndiotactic
poly-alpha-methyl styrene, syndiotactic polydichlorostyrene,
copolymers and blends of any of these polystyrenes, cellulose
derivatives (e.g., ethyl cellulose, cellulose acetate, cellulose
propionate, cellulose acetate butyrate, and cellulose nitrate),
polyalkylene polymers (e.g., polyethylene, polypropylene,
polybutylene, polyisobutylene, and poly(4-methyl)pentene),
fluorinated polymers (e.g., perfluoroalkoxy resins,
polytetrafluoroethylene, fluorinated ethylene-propylene copolymers,
polyvinylidene fluoride, and polychlorotrifluoroethylene),
chlorinated polymers (e.g., polyvinylidene chloride and
polyvinylchloride), polysulfones, polyethersulfones,
polyacrylonitrile, polyamides, silicone resins, epoxy resins,
polyvinylacetate, polyether-amides, ionomeric resins, elastomers
(e.g., polybutadiene, polyisoprene, and neoprene), and
polyurethanes. Also suitable are copolymers, e.g., copolymers of
PEN (e.g., copolymers of 2,6-, 1,4-, 1,5-, 2,7-, and/or
2,3-naphthalene dicarboxylic acid, or esters thereof, with (a)
terephthalic acid, or esters thereof; (b) isophthalic acid, or
esters thereof; (c) phthalic acid, or esters thereof; (d) alkane
glycols; (e) cycloalkane glycols (e.g., cyclohexane dimethanol
diol); (f) alkane dicarboxylic acids; and/or (g) cycloalkane
dicarboxylic acids (e.g., cyclohexane dicarboxylic acid)),
copolymers of polyalkylene terephthalates (e.g., copolymers of
terephthalic acid, or esters thereof, with (a) naphthalene
dicarboxylic acid, or esters thereof; (b) isophthalic acid, or
esters thereof; (c) phthalic acid, or esters thereof; (d) alkane
glycols; (e) cycloalkane glycols (e.g., cyclohexane dimethanol
diol); (f) alkane dicarboxylic acids; and/or (g) cycloalkane
dicarboxylic acids (e.g., cyclohexane dicarboxylic acid)), and
styrene copolymers (e.g., styrene-butadiene copolymers and
styrene-acrylonitrile copolymers), 4,4'-bibenzoic acid and ethylene
glycol. In addition, each individual layer may include blends of
two or more of the above-described polymers or copolymers (e.g.,
blends of SPS and atactic polystyrene). The coPEN described may
also be a blend of pellets where at least one component is a
polymer based on naphthalene dicarboxylic acid and other components
are other polyesters or polycarbonates, such as a PET, a PEN or a
co-PEN.
[0116] Particularly preferred combinations of layers in the case of
polarizers include PEN/co-PEN, polyethylene terephthalate
(PET)/co-PEN, PEN/sPS, PET/sPS, PEN/Eastar, and PET/Eastar, where
"co-PEN" refers to a copolymer or blend based upon naphthalene
dicarboxylic acid (as described above) and Eastar is
polycyclohexanedimethylene terephthalate commercially available
from Eastman Chemical Co.
[0117] Particularly preferred combinations of layers in the case of
mirrors include PET/Ecdel, PEN/Ecdel, PEN/sPS, PEN/THV, PEN/co-PET,
and PET/sPS, where "co-PET" refers to a copolymer or blend based
upon terephthalic acid (as described above), Ecdel is a
thermoplastic polyester commercially available from Eastman
Chemical Co., and THV is a fluoropolymer commercially available
from 3M Co.
[0118] The number of layers in the device is selected to achieve
the desired optical properties using the minimum number of layers
for reasons of film thickness, flexibility and economy. In the case
of both polarizers and mirrors, the number of layers is preferably
less than 10,000, more preferably less than 5,000, and (even more
preferably) less than 2,000.
[0119] As discussed above, the ability to achieve the desired
relationships among the various indices of refraction (and thus the
optical properties of the multilayer device) is influenced by the
processing conditions used to prepare the multilayer device. In the
case of organic polymers which can be oriented by stretching, the
devices are generally prepared by co-extruding the individual
polymers to form a multilayer film and then orienting the film by
stretching at a selected temperature, optionally followed by
heat-setting at a selected temperature. Alternatively, the
extrusion and orientation steps may be performed simultaneously. In
the case of polarizers, the film is stretched substantially in one
direction (uniaxial orientation), while in the case of mirrors the
film is stretched substantially in two directions (biaxial
orientation).
[0120] The film may be allowed to dimensionally relax in the
cross-stretch direction from the natural reduction in cross-stretch
(equal to the square root of the stretch ratio) to being
constrained (i.e., no substantial change in cross-stretch
dimensions). The film may be stretched in the machine direction, as
with a length orienter, in width using a tenter.
[0121] The pre-stretch temperature, stretch temperature, stretch
rate, stretch ratio, heat set temperature, heat set time, heat set
relaxation, and cross-stretch relaxation are selected to yield a
multilayer device having the desired refractive index relationship.
These variables are inter-dependent; thus, for example, a
relatively low stretch rate could be used if coupled with, e.g., a
relatively low stretch temperature. It will be apparent to one of
ordinary skill how to select the appropriate combination of these
variables to achieve the desired multilayer device. In general,
however, a stretch ratios in the range from 1:2 to 1:10 (more
preferably 1:3 to 1:7) in the stretch direction and from 1:0.5 to
1:10 (more preferably from 1:0.5 to 1:7) orthogonal to the stretch
direction is preferred.
[0122] Suitable multilayer devices may also be prepared using
techniques such as spin coating (e.g., as described in Boese et
al., J. Polym. Sci.: Part B, 30:1321 (1992) for birefringent
polyimides, and vacuum deposition (e.g., as described by Zang et.
al., Appl. Phys. Letters, 59:823 (1991) for crystalline organic
compounds; the latter technique is particularly useful for certain
combinations of crystalline organic compounds and inorganic
materials.
[0123] The invention will now be described by way of the following
examples. In the examples, because optical absorption is
negligible, reflection equals 1 minus transmission (R=1-T).
EXAMPLE 1
Polarizer
[0124] PEN and a 70 naphthalate/30 terephthalate copolyester
(coPEN) were synthesized in a standard polyester resin kettle using
ethylene glycol as the diol. The intrinsic viscosity of both the
PEN and the coPEN was approximately 0.6 dl/g. Single layer films of
PEN and coPEN were extruded and then uniaxially stretched, with the
sides restrained, at approximately 150.degree. C. As extruded, the
PEN exhibited an isotropic refractive index of about 1.65, and the
coPEN was characterized by an isotropic refractive index of about
1.64. By isotropic is meant that the refractive indices associated
with all axes in the plane of the film are substantially equal.
Both refractive index values were observed at 550 nm. After
stretching at a 5:1 stretch ratio, the refractive index of the PEN
associated with the oriented axis increased to approximately 1.88.
The refractive index associated with the transverse axis dropped
slightly to 1.64. The refractive index of the coPEN film after
stretching at a 5:1 stretch ratio remained isotropic at
approximately 1.64.
[0125] A satisfactory multilayer polarizer was then made of
alternating layers of PEN and coPEN by coextrusion using a 51-slot
feed block which fed a standard extrusion die. The extrusion was
run at approximately 295.degree. C. The PEN was extruded at
approximately 23 lb/hr and the coPEN was extruded at approximately
22.3 lb/hr. The PEN skin layers were approximately three times as
thick as the layers within the extruded film stack. All internal
layers were designed to have an optical 1/4 wavelength thickness
for light of about 1300 nm. The 51-layer stack was extruded and
cast to a thickness of approximately 0.0029 inches, and then
uniaxially stretched with the sides restrained at approximately a
5:1 stretch ratio at approximately 150.degree. C. The stretched
film had a thickness of approximately 0.0005 inches.
[0126] The stretched film was then heat set for 30 seconds at
approximately 230.degree. C. in an air oven. The optical spectra
were essentially the same for film that was stretched and for film
that was subsequently heat set.
EXAMPLE 2
Polarizer
[0127] A satisfactory 204-layered polarizer was made by extruding
PEN and coPEN in the 51-slot feedblock as described in Example 1
and then employing two layer doubling multipliers in series in the
extrusion. The multipliers divide the extruded material exiting the
feed block into two half-width flow streams, then stack the
half-width flow streams on top of each other. U.S. Pat. No.
3,565,985 describes similar coextrusion multipliers. The extrusion
was performed at approximately 295.degree. C. using PEN at an
intrinsic viscosity of 0.50 dl/g at 22.5 lb/hr while the coPEN at
an intrinsic viscosity of 0.60 dl/g was run at 16.5 lb/hr. The cast
web was approximately 0.0038 inches in thickness and was uniaxially
stretched at a 5:1 ratio in a longitudinal direction with the sides
restrained at an air temperature of 140.degree. C. during
stretching. Except for skin layers, all pairs of layers were
designed to be 1/2 wavelength optical thickness for 550 nm
light.
[0128] Two 204-layer polarizers made as described above were then
hand-laminated using an optical adhesive to produce a 408-layered
film stack. Preferably the refractive index of the adhesive should
match the index of the isotropic coPEN layer.
EXAMPLE 3
PET:Ecdel, 601, Mirror
[0129] A coextruded film containing 601 layers was made on a
sequential flat-film-making line via a coextrusion process. A
Polyethylene terephthalate (PET) with an Intrinsic Viscosity of 0.6
dl/g (60 wt. % phenol/40 wt. % dichlorobenzene) was delivered by
one extruder at a rate of 75 pounds per hour and Ecdel 9966 (a
thermoplastic elastomer available from Eastman Chemical) was
delivered by another extruder at a rate of 65 pounds per hour. The
PET was on the skin layers. The feedblock method (such as that
described in U.S. Pat. No. 3,801,429) was used to generate 151
layers which was passed through two multipliers producing an
extrudate of 601 layers. U.S. Pat. No. 3,565,985 describes
exemplary coextrusion multipliers. The web was length oriented to a
draw ratio of about 3.6 with the web temperature at about
210.degree. F. The film was subsequently preheated to about
235.degree. F. in about 50 seconds and drawn in the transverse
direction to a draw ratio of about 4.0 at a rate of about 6% per
second. The film was then relaxed about 5% of its maximum width in
a heat-set oven set at 400.degree. F. The finished film thickness
was 2.5 mil.
[0130] The cast web produced was rough in texture on the air side,
and provided the transmission as shown in FIG. 18. The %
transmission for p-polarized light at a 60.degree. angle (curve b)
is similar the value at normal incidence (curve a) (with a
wavelength shift).
[0131] For comparison, film made by Mearl Corporation, presumably
of isotropic materials (see FIG. 19) shows a noticeable loss in
reflectivity for p-polarized light at a 60.degree. angle (curve b,
compared to curve a for normal incidence).
EXAMPLE 4
PET:Ecdel, 151, Mirror
[0132] A coextruded film containing 151 layers was made on a
sequential flat-film-making line via a coextrusion process. A
Polyethylene terephthalate (PET) with an Intrinsic Viscosity of 0.6
dl/g (60 wt phenol/40 wt. % dichlorobenzene) was delivered by one
extruder at a rate of 75 pounds per hour and Ecdel 9966 (a
thermoplastic elastomer available from Eastman Chemical) was
delivered by another extruder at a rate of 65 pounds per hour. The
PET was on the skin layers. The feedblock method was used to
generate 151 layers. The web was length oriented to a draw ratio of
about 3.5 with the web temperature at about 210.degree. F. The film
was subsequently preheated to about 215.degree. F. in about 12
seconds and drawn in the transverse direction to a draw ratio of
about 4.0 at a rate of about 25% per second. The film was then
relaxed about 5% of its maximum width in a heat-set oven set at
400.degree. F. in about 6 seconds. The finished film thickness was
about 0.6 mil.
[0133] The transmission of this film is shown in FIG. 20. The %
transmission for p-polarized light at a 60.degree. angle (curve b)
is similar the value at normal incidence (curve a) with a
wavelength shift. At the same extrusion conditions the web speed
was slowed down to make an infrared reflecting film with a
thickness of about 0.8 mils. The transmission is shown in FIG. 21
(curve a at normal incidence, curve b at 60 degrees).
EXAMPLE 5
PEN:Ecdel, 225, Mirror
[0134] A coextruded film containing 225 layers was made by
extruding the cast web in one operation and later orienting the
film in a laboratory film-stretching apparatus. A Polyethylene
naphthalate (PEN) with an Intrinsic Viscosity of 0.5 dl/g (60 wt. %
phenol/40 wt. % dichlorobenzene) was delivered by one extruder at a
rate of 18 pounds per hour and Ecdel 9966 (a thermoplastic
elastomer available from Eastman Chemical) was delivered by another
extruder at a rate of 17 pounds per hour. The PEN was on the skin
layers. The feedblock method was used to generate 57 layers which
was passed through two multipliers producing an extrudate of 225
layers. The cast web was 12 mils thick and 12 inches wide. The web
was later biaxially oriented using a laboratory stretching device
that uses a pantograph to grip a square section of film and
simultaneously stretch it in both directions at a uniform rate. A
7.46 cm square of web was loaded into the stretcher at about
100.degree. C. and heated to 130.degree. C. in 60 seconds.
Stretching then commenced at 100%/sec (based on original
dimensions) until the sample was stretched to about 3.5.times.3.5.
Immediately after the stretching the sample was cooled by blowing
room temperature air on it.
[0135] FIG. 22 shows the optical response of this multilayer film
(curve a at normal incidence, curve b at 60 degrees). Note that the
% transmission for p-polarized light at a 60.degree. angle is
similar to what it is at normal incidence (with some wavelength
shift).
EXAMPLE 6
PEN:THV 500, 449, Mirror
[0136] A coextruded film containing 449 layers was made by
extruding the cast web in one operation and later orienting the
film in a laboratory film-stretching apparatus. A Polyethylene
naphthalate (PEN) with an Intrinsic Viscosity of 0.53 dl/g (60 wt.
% phenol/40 wt. % dichlorobenzene) was delivered by one extruder at
a rate of 56 pounds per hour and THV 500 (a fluoropolymer available
from Minnesota Mining and Manufacturing Company) was delivered by
another extruder at a rate of 11 pounds per hour. The PEN was on
the skin layers and 50% of the PEN was present in the two skin
layers. The feedblock method was used to generate 57 layers which
was passed through three multipliers producing an extrudate of 449
layers. The cast web was 20 mils thick and 12 inches wide. The web
was later biaxially oriented using a laboratory stretching device
that uses a pantograph to grip a square section of film and
simultaneously stretch it in both directions at a uniform rate. A
7.46 cm square of web was loaded into the stretcher at about
100.degree. C. and heated to 140.degree. C. in 60 seconds.
Stretching then commenced at 10%/sec (based on original dimensions)
until the sample was stretched to about 3.5.times.3.5. Immediately
after the stretching the sample was cooled by blowing room
temperature air at it.
[0137] FIG. 23 shows the transmission of this multilayer film.
Again, curve a shows the response at normal incidence, while curve
b shows the response at 60 degrees.
EXAMPLE 7
PEN:CoPEN, 449--Low Color Polarizer
[0138] A coextruded film containing 449 layers was made by
extruding the cast web in one operation and later orienting the
film in a laboratory film-stretching apparatus. A Polyethylene
naphthalate (PEN) with an Intrinsic Viscosity of 0.56 dl/g (60 wt.
% phenol/40 wt. % dichlorobenzene) was delivered by one extruder at
a rate of 43 pounds per hour and a CoPEN (70 mol % 2,6 NDC and 30
mol % DMT) with an intrinsic viscosity of 0.52 (60 wt. % phenol/40
wt. % dichlorobenzene) was delivered by another extruder at a rate
of 25 pounds per hour. The PEN was on the skin layers and 40% of
the PEN was present in the two skin layers. The feedblock method
was used to generate 57 layers which was passed through three
multipliers producing an extrudate of 449 layers. The cast web was
10 mils thick and 12 inches wide. The web was later uniaxially
oriented using a laboratory stretching device that uses a
pantograph to grip a square section of film and stretch it in one
direction while it is constrained in the other at a uniform rate. A
7.46 cm square of web was loaded into the stretcher at about
100.degree. C. and heated to 140.degree. C. in 60 seconds.
Stretching then commenced at 10%/sec (based on original dimensions)
until the sample was stretched to about 5.5.times.1. Immediately
after the stretching the sample was cooled by blowing room
temperature air at it.
[0139] FIG. 24 shows the transmission of this multilayer film.
Curve a shows transmission of light polarized in the non-stretch
direction at normal incidence, curve b shows transmission of
p-polarized light at 60.degree. incidence, and curve c shows
transmission of light polarized in the stretch direction at normal
incidence. Note the very high transmission of light polarized in
the non-stretch direction at both normal and 60.degree. incidence.
Average transmission for curve a over 400-700 nm is 87.1%, while
average transmission for curve b over 400-700 nm is 97.1%.
Transmission is higher for p-polarized light at 60.degree.
incidence because the air/PEN interface has a Brewster angle near
60.degree., so the transmission at 60.degree. incidence is nearly
100%. Also note the high extinction of light polarized in the
stretched direction in the visible range (400-700nm) shown by curve
c, where the average transmission is 21.0%. The % RMS color for
curve a is 1.5%. The % RMS color for curve b is 1.4%.
EXAMPLE 8
PEN:CoPEN, 601--High Color Polarizer
[0140] A coextruded film containing 601 layers was produced by
extruding the web and two days later orienting the film on a
different tenter than described in all the other examples. A
Polyethylene Naphthalate (PEN) with an Intrinsic Viscosity of 0.5
dl/g (60 wt. % phenol/40 wt. % dichlorobenzene) was delivered by
one extruder at a rate of 75 pounds per hour and a CoPEN (70 mol %
2,6 NDC and 30 mol % DMT) with an IV of 0.55 dl/g (60 wt. %
phenol/40 wt. % dichlorobenzene) was delivered by another extruder
at a rate of 65 pounds per hour. The PEN was on the skin layers.
The feedblock method was used to generate 151 layers which was
passed through two multipliers producing an extrudate of 601
layers. U.S. Pat. No. 3,565,985 describes similar coextrusion
multipliers. All stretching was done in the tenter. The film was
preheated to about 280.degree. F. in about 20 seconds and drawn in
the transverse direction to a draw ratio of about 4.4 at a rate of
about 6% per second. The film was then relaxed about 2% of its
maximum width in a heat-set oven set at 460.degree. F. The finished
film thickness was 1.8 mil.
[0141] The transmission of the film is shown in FIG. 25. Curve a
shows transmission of light polarized in the non-stretch direction
at normal incidence, curve b shows transmission of p-polarized
light at 60.degree. incidence, and curve c shows transmission of
light polarized in the stretch direction at normal incidence. Note
the nonuniform transmission of p-polarized light at both normal and
60.degree. incidence. The average transmission for curve a over
400-700 nm is 84.1%, while the average transmission for curve b
over 400-700 nm is 68.2%. The average transmission for curve c is
9.1%. The % RMS color for curve a is 1.4%, and the % RMS color for
curve b is 11.2%.
EXAMPLE 9
PET:CoPEN, 449, Polarizer
[0142] A coextruded film containing 449 layers was made by
extruding the cast web in one operation and later orienting the
film in a laboratory film-stretching apparatus. A Polyethylene
Terephthalate (PET) with an Intrinsic Viscosity of 0.60 dl/g (60
wt. % phenol/40 wt. % dichlorobenzene) was delivered by one
extruder at a rate of 26 pounds per hour and a CoPEN (70 mol % 2,6
NDC and 30 mol % DMT) with an intrinsic viscosity of 0.53 (60 wt. %
phenol/40 wt. % dichlorobenzene) was delivered by another extruder
at a rate of 24 pounds per hour. The PET was on the skin layers.
The feedblock method was used to generate 57 layers which was
passed through three multipliers producing an extrudate of 449
layers. U.S. Pat. No. 3,565,985 describes similar coextrusion
multipliers. The cast web was 7.5 mils thick and 12 inches wide.
The web was later uniaxially oriented using a laboratory stretching
device that uses a pantograph to grip a square section of film and
stretch it in one direction while it is constrained in the other at
a uniform rate. A 7.46 cm square of web was loaded into the
stretcher at about 100.degree. C. and heated to 120.degree. C. in
60 seconds. Stretching then commenced at 10%/sec (based on original
dimensions) until the sample was stretched to about 5.0.times.1.
Immediately after the stretching the sample was cooled by blowing
room temperature air at it. The finished film thickness was about
1.4 mil. This film had sufficient adhesion to survive the
orientation process with no delamination.
[0143] FIG. 26 shows the transmission of this multilayer film.
Curve a shows transmission of light polarized in the non-stretch
direction at normal incidence, curve b shows transmission of
p-polarized light at 60.degree. incidence, and curve c shows
transmission of light polarized in the stretch direction at normal
incidence. Note the very high transmission of p-polarized light at
both normal and 60.degree. incidence. The average transmission for
curve a over 400-700 nm is 88.0%, and the average transmission for
curve b over 400-700 nm is 91.2%. The average transmission for
curve c over 400-700 nm is 27.9%. The % RMS color for curve a is
1.4%, and the % RMS color for curve b is 4.8%.
EXAMPLE 10
PEN:CoPEN, 601, Polarizer
[0144] A coextruded film containing 601 layers was made on a
sequential flat-film-making line via a coextrusion process. A
Polyethylene naphthalate (PEN) with an intrinsic viscosity of 0.54
dl/g (60 wt % Phenol plus 40 wt % dichlorobenzene) was delivered by
on extruder at a rate of 75 pounds per hour and the coPEN was
delivered by another extruder at 65 pounds per hour. The coPEN was
a copolymer of 70 mole % 2,6 naphthalene dicarboxylate methyl
ester, 15 % dimethyl isophthalate and 15% dimethyl terephthalate
with ethylene glycol. The feedblock method was used to generate 151
layers. The feedblock was designed to produce a gradient
distribution of layers with a ration of thickness of the optical
layers of 1.22 for the PEN and 1.22 for the coPEN. The PEN skin
layers were coextruded on the outside of the optical stack with a
total thickness of 8% of the coextruded layers. The optical stack
was multiplied by two sequential multipliers. The nominal
multiplication ratio of the multipliers were 1.2 and 1.27,
respectively. The film was subsequently preheated to 310.degree. F.
in about 40 seconds and drawn in the transverse direction to a draw
ratio of about 5.0 at a rate of 6% per second. The finished film
thickness was about 2 mils.
[0145] FIG. 27 shows the transmission for this multilayer film.
Curve a shows transmission of light polarized in the non-stretch
direction at normal incidence, curve b shows transmission of
p-polarized light at 60.degree. incidence, and curve c shows
transmission of light polarized in the stretch direction at normal
incidence. Note the very high transmission of p-polarized light at
both normal and 60.degree. incidence (80-100%). Also note the very
high extinction of light polarized in the stretched direction in
the visible range (400-700nm) shown by curve c. Extinction is
nearly 100% between 500 and 650 nm.
EXAMPLE 11
PEN:sPS, 481, Polarizer
[0146] A 481 layer multilayer film was made from a polyethylene
naphthalate (PEN) with an intrinsic viscosity of 0.56 dl/g measured
in 60 wt. % phenol and 40 wt % dichlorobenzene purchased from
Eastman Chemicals and a syndiotactic polystyrene (sPS) homopolymer
(weight average molecular weight=200,000 Daltons, sampled from Dow
Corporation). The PEN was on the outer layers and was extruded at
26 pounds per hour and the sPS at 23 pounds per hour. The feedblock
used produced 61 layers with each of the 61 being approximately the
same thickness. After the feedblock three (2.times.) multipliers
were used. Equal thickness skin layers containing the same PEN fed
to the feedblock were added after the final multiplier at a total
rate of 22 pounds per hour. The web was extruded through a 12''
wide die to a thickness or about 0.011 inches (0.276 mm). The
extrusion temperature was 290.degree. C.
[0147] This web was stored at ambient conditions for nine days and
then uniaxially oriented on a tenter. The film was preheated to
about 320.degree. F. (160.degree. C.) in about 25 seconds and drawn
in the transverse direction to a draw ratio of about 6:1 at a rate
of about 28% per second. No relaxation was allowed in the stretched
direction. The finished film thickness was about 0.0018 inches
(0.046 mm).
[0148] FIG. 28 shows the optical performance of this PEN:sPS
reflective polarizer containing 481 layers. Curve a shows
transmission of light polarized in the non-stretch direction at
normal incidence, curve b shows transmission of p-polarized light
at 60.degree. incidence, and curve c shows transmission of light
polarized in the stretch direction at normal incidence. Note the
very high transmission of p-polarized light at both normal and
60.degree. incidence. Average transmission for curve a over 400-700
nm is 86.2%, the average transmission for curve b over 400-700 nm
is 79.7%. Also note the very high extinction of light polarized in
the stretched direction in the visible range (400-700nm) shown by
curve c. The film has an average transmission of 1.6% for curve c
between 400 and 700 nm. The % RMS color for curve a is 3.2%, while
the % RMS color for curve b is 18.2%.
EXAMPLE 12
PET:Ecdel, 601, Mirror
[0149] A coextruded film containing 601 layers was made on a
sequential flat-film-making line via a coextrusion process. A
Polyethylene terephthalate (PET) with an Intrinsic Viscosity of 0.6
dl/g (60 wt. % phenol/40 wt. % dichlorobenzene) was delivered to
the feedblock at a rate of 75 pounds per hour and Ecdel 9967 (a.
thermoplastic elastomer available from Eastman Chemical) was
delivered at a rate of 60 pounds per hour. The PET was on the skin
layers. The feedblock method was used to generate 151 layers which
was passed through two multipliers producing an extrudate of 601
layers. The multipliers had a nominal multiplication ratio of 1.2
(next to feedblock) and 1.27. Two skin layers at a total throughput
of 24 pounds per hour were added symmetrically between the last
multiplier and the die. The skin layers were composed of PET and
were extruded by the same extruder supplying the PET to the
feedblock. The web was length oriented to a draw ratio of about 3.3
with the web temperature at about 205.degree. F. The film was
subsequently preheated to about 205.degree. F. in about 35 seconds
and drawn in the transverse direction to a draw ratio of about 3.3
at a rate of about 9% per second. The film was then relaxed about
3% of its maximum width in a heat-set oven set at 450.degree. F.
The finished film thickness was about 0.0027 inches.
[0150] The film provided the optical performance as shown in FIG.
29. Transmission is plotted as curve a and reflectivity is plotted
as curve b. The luminous reflectivity for curve b is 91.5%.
EXAMPLE 13
PEN:CoPEN, 601, Antireflected Polarizer
[0151] A coextruded film containing 601 layers was made on a
sequential flat-film-making line via a coextrusion process. A
Polyethylene naphthalate (PEN) with an intrinsic viscosity of 0.54
dl/g (60 wt % Phenol plus 40 wt % dichlorobenzene) was delivered by
on extruder at a rate of 75 pounds per hour and the coPEN was
delivered by another extruder at 65 pounds per hour. The coPEN was
a copolymer of 70 mole % 2,6 naphthalene dicarboxylate methyl
ester, 30% dimethyl terephthalate with ethylene glycol. The
feedblock method was used to generate 151 layers. The PEN skin
layers were coextruded on the outside of the optical stack with a
total thickness of 8% of the coextruded layers. The feedblock was
designed to make a linear gradient in layer thickness for a 149
layer optical stack with the thinnest layers on one side of the
stack. The individual layer thicknesses were designed in pairs to
make equal thickness layers of the PEN and coPEN for each pair.
Each pair thickness, d, was determined by the formula
d=d.sub.o+d.sub.o*0.003*n, where d.sub.o is the minimum pair
thickness, and n is the pair number between 1 and 75. The optical
stack was multiplied by two sequential multipliers. The nominal
multiplication ratio of the multipliers were 1.2 and 1.27,
respectively. The film was subsequently preheated to 320.degree. F.
in about 40 seconds and drawn in the transverse direction to a draw
ratio of about 5.0 at a rate of 6% per second. The finished film
thickness was about 2 mils.
[0152] A silical sol gel coating was then applied to one side of
the reflecting polarizer film. The index of refraction of this
coating was approximately 1.35. Two pieces of the AR coated
reflecting polarizer film were cut out and the two were laminated
to each other with the AR coatings on the outside. Transmission
spectra of polarized light in the crossed and parallel directions
were obtained. The sample was then rinsed with a 2% solution of
ammonium bifluoride (NH4HF2) in deonized water to remove the AR
coating. Spectra of the bare multilayer were then taken for
comparison to the coated sample.
[0153] FIG. 30 shows the spectra of the coated and uncoated
polarizer. Curves a and b show the transmission and extinction,
respectively, of the AR coated reflecting polarizer, and curves c
and d show the transmission and extinction, respectively, of the
uncoated reflecting polarizer. Note that the extinction spectrum is
essentially unchanged, but that the transmission values for the AR
coated polarizer are almost 10% higher. Peak gain was 9.9% at 565
nm, while the average gain from 425 to 700 nm was 9.1%. Peak
transmission of the AR coated polarizer was 97.0% at 675 nm.
Average transmissions for curve a over 400-700 nm was 95.33%, and
average transmission for curve d over 400-700 nm was 5.42%.
EXAMPLE 14
PET:Ecdel, 601, Polarizer
[0154] A coextruded film containing 601 layers was made on a
sequential flat-film-making line via a coextrusion process. A
polyethylene terephthalate (PET) with an Intrinsic Viscosity of 0.6
dl/g (60 wt. % phenol/40 wt. % dichlorobenzene) was delivered to a
feedblock by one extruder at a rate of 75 pounds per hour and Ecdel
9967 (a thermoplastic elastomer available from Eastman Chemical)
was delivered to the feedblock by another extruder at a rate of 60
pounds per hour. The PET was on the skin layers. The feedblock
method was used to generate 151 layers which passed through two
multipliers (2.times.) producing an extrudate of 6.01 layers. A
side stream with a throughput of 50 pounds per hour was taken from
the PET extruder and used to add two skin layers between the last
multiplier and the die. The web was length oriented to a draw ratio
of about 5.0 with the web temperature at about 210.degree. F. The
film was not tentered. The finished film thickness was about 2.7
mil.
[0155] FIG. 31 shows the transmission for this film. Curve a shows
the transmission of light polarized in the stretch direction, while
curve b shows the transmission of light polarized orthogonal to the
stretch direction. The average transmission from 400-700 nm for
curve a is 39.16%.
EXAMPLE 15
PEN:CoPEN, 449, Polarizers
[0156] A coextruded film containing 449 layers was made by
extruding the cast web in one operation and later orienting the
film in a laboratory film-stretching apparatus. A polyethylene
naphthalate (PEN) with an Intrinsic Viscosity of 0.53 dl/g (60 wt.
% phenol/40 wt. % dichlorobenzene) was delivered by one extruder at
a rate of 26.7 pounds per hour to the feedblock and a different
material was delivered by second extruder at a rate of 25 pounds
per hour to the feedblock. The PEN was the skin layers. The
feedblock method was used to generate 57 layers which passed
through three multipliers producing an extrudate of 449 layers. The
cast web was 0.0075 mils thick and 12 inches wide. The web was
later uniaxially oriented using a laboratory stretching device that
uses a pantograph to grip a square section of film and stretch it
in one direction at a uniform rate while it is constrained in the
other. A 7.46 cm square of web was loaded into the stretcher at
about 100.degree. C. and heated to 140.degree. C. for 60 seconds.
Stretching then commenced at 10%/sec (based on original dimensions)
until the sample was stretched to about 5.5.times.1. Immediately
after stretching, the sample was cooled by blowing room temperature
air at it.
[0157] The input to the second extruder was varied by blending
pellets of the following poly(ethylene esters) three materials: (i)
a CoPEN (70 mol % 2,6-napthalene dicarboxylate and 30 mol %
terephthalate) with an intrinsic viscosity of 0.52 (60 wt. %
phenol/40 wt. % dichlorobenzene); (ii) the PEN, same material as
input to first extruder; (iii) a PET, with an intrinsic viscosity
of 0.95 (60 wt. % phenol/40 wt. % dichlorobenzene). TTF 9506
purchased from Shell.
[0158] For the film shown in FIG. 32A the input to the second
extruder was 80 wt % of the CoPEN and 20 wt % of the PEN; for the
film shown in FIG. 32B the input to the second extruder was 80 wt %
of the CoPEN and 20 wt % of the PET; for the film shown in FIG. 32C
the input to the second extruder was CoPEN.
[0159] FIGS. 32A, 32B, and 32C show the transmission of these
multilayer films where curve a shows transmission of light
polarized in the non-stretch direction at normal incidence, curve b
shows transmission of p-polarized light polarized in the
non-stretched direction at 60.degree. incidence, and curve c shows
transmission of light polarized in the stretch direction at normal
incidence. Note that the optical response of these films is
sensitive to the chemical composition of the layers from the second
extruder. The average transmission for curve c in FIG. 32A is
43.89%, the average transmission for curve c in FIG. 32B is 21.52%,
and the average transmission for curve c in FIG. 32C is 12.48%.
Thus, extinction is increased from FIG. 32A to FIG. 32C.
[0160] For the examples using the 57 layer feedblock, all layers
were designed for on1y one optical thickness (1/4 of 550 nm), but
the extrusion equipment introduces deviations in the layer
thicknesses throughout the stack resulting in a fairly broadband
optical response. For examples made with the 151 layer feedblock,
the feedblock is designed to create a distribution of layer
thicknesses to cover a portion of the visible spectrum. Asymmetric
multipliers were then used to broaden the distribution of layer
thicknesses to cover most of the visible spectrum as described in
U.S. Pat. Nos. 5,094,788 and 5,094,793.
EXAMPLE 16
PEN:sPS, 481, Reflective/Dichroic Polarizer
[0161] A coextruded film containing 481 layers was made from
2,6-polyethlyne naphthalate purchased from Eastman Chemicals with
an intrinsic viscosity of 0.56 dl/g measured in 60 wt % phenol and
40 wt % dichlorobenzene, and a syndiotactic polystyrene homopolymer
(weight average molecular weight of 200,000 Daltons, sampled from
Dow Corporation. The PEN was the outer layers and was extruded at
26 pounds per hour and the sPS was extruded at 23 pounds per hour.
The feedblock used produced 61 layers with each of the 61 layers
being approximately the same thickness. After the feedblock, three,
2.times. multipliers were used. U.S. Pat. No. 3,565,985 describes
similar multipliers. Equal thickness skin layers containing the
same 2,6-polyethylene naphthalate fed to the feedblock were added
after the final multiplier at a total rate of 22 pounds per hour.
The web was extruded through a 12'' wide die to a thickness of
about 0.011 inches (0.276 mm). The extrusion temperature was 296
degrees C.
[0162] The cast web was coated at about 2 feet per minute using a
doctor blade with a 1 mil gap with a primer layer of 4.8%
sulfopolyester WB 50 and 0.1% Triton X100 in water. This was dried
for 2 minutes at 50 degrees C. in a forced air oven, then coated at
about 2 feet pre minute with a doctor blade with a 2 mil gap with a
solution of about 200 grams of Air Products #107 polyvinyl alcohol
in 1.1 liters of deionized water. The film was dried for 5 minutes
in a 50 degree C. forced air oven. The coated web was uniaxially
oriented using a laboratory stretching device that uses a
pantograph to grip a 7.46 cm square section of the coated cast
film. The sides of the film were constrained during stretching. The
sample was loaded into the stretcher at 100 degrees C. and heated
to 120 degrees C. in 60 seconds. The film was stretched in one
direction at about 10% per second (based on the original
dimension). The final stretch ratio was about 5.5 to 1. Immediately
after stretching the sample was cooled by blowing room temperature
air on it. The finished film thickness was about 2.0 mils for the
reflective polarizer, and 0.3 mil for the dichroic polarizer. The
multilayer film and the coating has sufficient adhesion to allow
orientation without delamination. The coated, oriented film was
stained for 20 seconds in a 35 degree C. solution of 0.4% iodine
and 21% potassium iodide in deionized water. The film was removed
from the solution, the excess allowed to drain, and then placed
into a 65 degree C. solution of 5% boric acid and 15% borax in
water for 60 seconds. The sample was then dried in air at room
temperature.
[0163] The optical characteristics of the sample was measured with
a Perkin-Elmer Lambda 19 using an integrating sphere attachment.
The reflectivity of the coated reflective polarizer from the
reflective polarizer side was (averaged from 400 to 700 nm) 8.2%
for the transmitted polarization and 98.5% for the reflected
polarization. Average reflectivity from the dichroic polarizer side
was 7.5% for the high absorption polarization and 4.7% for the
transmitted polarization. Average transmission for the high
transmission polarization was 74.6%, and for the high extinction
polarization, 0.58%.
[0164] Synthesis of Sulfopolyester WB 50: A one gallon polyester
kettle was charged with 111.9 g (5.5 mole %) sodiosulfoisophthalic
acid, 592.1 g (47.0 mole %) terephthalic acid, 598.4 g (47.5 mole
%) isophthalic acid, 705 g ethylene glycol, 59.9 g neopentyl
glycol, 0.7 g antimony oxide, and 2.5 g sodium acetate. The mixture
was heated with stirring to 230 C at 50 psi under nitrogen for 2
hours, during which time water evolution was observed. The
temperature was increased to 250 C and the pressure was then
reduced, vacuum applied (0.2 Torr), and the temperature increased
to 270 C. The viscosity of the material increased over a period of
45 minutes, after which time a high molecular weight, clear, visous
sulfopolyester was drained. This sulfopolyester was found by
differential scanning calorimetry to have a Tg of 70.3 C. The
theoretical sulfonate equivalent weight was 3847 g polymer per mole
of sulfonate. 500 g of polymer was dissolved in a mixture of 2000 g
water and 450 g isopropanol at 80 C. The temperature was then
raised to 95 C in order to remove the isopropanol (and a portion of
the water), yielding a 22% solids aqueous dispersion.
[0165] Although the preferred reflective polarizer body 12 has been
described as a multilayer stack of polymeric materials, it shall be
understood that other reflective polarizers could be substituted
therefore without departing from the scope of the present
invention. Other reflective polarizers include cholesteric liquid
crystal polarizers using an optical retarder placed between the
reflective polarizer and dichroic polarizer, tilted optic prismatic
and non-prismatic multilayer polarizers, and first-order
diffractive polarizers.
[0166] Thus, although the present optical polarizer has been
described with reference to the preferred embodiment, those skilled
in the art will readily appreciate that other embodiments may be
utilized and changes made in form and detail without departing from
the spirit and scope of the present invention.
* * * * *