U.S. patent application number 12/314987 was filed with the patent office on 2009-05-28 for method of manufacturing an intrinsic polarizer.
This patent application is currently assigned to SEIKO EPSON CORPORATION. Invention is credited to Michael K. Gerlach, Jonathan M. Mack, Pradnya V. Nagarkar, Gerald N. Nkwantah, Philip J. Ralli.
Application Number | 20090134535 12/314987 |
Document ID | / |
Family ID | 35115711 |
Filed Date | 2009-05-28 |
United States Patent
Application |
20090134535 |
Kind Code |
A1 |
Gerlach; Michael K. ; et
al. |
May 28, 2009 |
Method of manufacturing an intrinsic polarizer
Abstract
An improved method of forming an intrinsic polarizer, referred
to as a K-type polarizer, includes stretching a polymeric film a
first stretching step. The polymeric film comprises a hydroxylated
linear polymer which is converted after the first stretching step
to form dichroic, copolymer polyvinylene blocks aligned in the
polymeric film. The polymeric film is stretched in a second
stretching step while converting the hydroxylated linear polymer.
This method produces an improved K-type polarizer with excellent
polarizing and color characteristics. For example, the dichroic
ratio is higher than 100, the color shift for light passed through
the polarizer in a crossed configuration is low, and the absorption
of light in the blue region of the visible spectrum is more than
one half of the absorption for light in the middle of the visible
spectrum.
Inventors: |
Gerlach; Michael K.;
(Brookline, MA) ; Nkwantah; Gerald N.; (Brockton,
MA) ; Mack; Jonathan M.; (Boylston, MA) ;
Nagarkar; Pradnya V.; (Newton, MA) ; Ralli; Philip
J.; (Sudbury, MA) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
P.O. BOX 320850
ALEXANDRIA
VA
22320-4850
US
|
Assignee: |
SEIKO EPSON CORPORATION
TOKYO
JP
|
Family ID: |
35115711 |
Appl. No.: |
12/314987 |
Filed: |
December 19, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10910211 |
Aug 3, 2004 |
|
|
|
12314987 |
|
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Current U.S.
Class: |
264/1.34 |
Current CPC
Class: |
G02B 5/3033
20130101 |
Class at
Publication: |
264/1.34 |
International
Class: |
B29D 11/00 20060101
B29D011/00 |
Claims
1. A method for making a polarizer from a polymeric film having an
original length and comprising a hydroxylated linear polymer, the
method comprising: stretching the polymeric film a first stretching
step; converting the hydroxylated linear polymer, after the first
stretching step, to form dichiroic, copolymer polyvinylene blocks
aligned in the polymeric film; and stretching the polymeric film in
a second stretching step while converting the hydroxylated linear
polymer.
2. A method as recited in claim 1, wherein the hydroxylated linear
polymer is at least one of polyvinyl alcohol, polyvinyl acctal,
polyvinul ketal and polyvinyl ester.
3. A method as recited in claim 1, further comprising exposing the
polymeric film to a dehydration catalyst before stretching the
polymeric film in the second stretching step.
4. A method as recited in claim 3, wherein exposing the polymeric
film to the dehydration catalyst immersing the polymeric film in a
bath containing the dehydration catalyst.
5. A method as recited in claim 3, wherein the dehydration catalyst
is an aqueous catalyst comprising at least one of hydrochloric
acid, hydrobromic acid, hydroiodic acid, phosphoric acid, and
sulphuric acid in methanol.
6. A method as recited in claim 3, wherein dehydration catalyst
comprises hydrochloric acid with a concentration ranging from about
0.01 Normal to about 4.0 Normal.
7. A method as recited in claim 3, further comprising stretching
the polymeric film in the first stretching step while exposing the
polymeric film to the dehydration catalyst.
8. A method as recited in claim 7, further comprising immersing the
polymeric film in the dehydration catalyst and stretching the
polymeric film in the first stretching step while immersed in the
dehydration catalyst.
9. A method as recited in claim 3, further comprising stretching
the polymeric film in the first stretching step after exposing the
polymeric film to the dehydration catalyst.
10. A method as recited in claim 3, further comprising stretching
the polymeric film in the first stretching step before exposing the
polymeric film to the dehydration catalyst.
11. A method as recited in claim 3, wherein converting the
hydroxylated linear polymer comprises heating the polymeric film to
effect partial dehydration of the polymeric film, thereby forming
vinylene block segments.
12. A method as recited in claim 11, further comprising converting
the polymeric film by heating the polymeric film after exposing the
polymeric film to the dehydration catalyst.
13. A method as recited in claim 12, wherein heating the polymeric
film comprises exposing the polymeric film to infrared
radiation.
14. A method as recited in claim 12, wherein heating the polymeric
film comprises convectively heating the polymeric film.
15. A method as recited in claim 1, wherein the second stretching
step is bidirectional unrelaxed, unidirectional unrelaxed or
parabolic.
16. A method as recited in claim 1, wherein the first stretching
step is bidirectional unrelaxed, unidirectional unrelaxed or
parabolic.
17. A method as recited in claim 1, wherein the first stretching
step comprises stretching the polymeric film to an intermediate
length in the range from about 3.5 to about 7 times the original
length.
18. A method as recited in claim 17, wherein the second stretching
step comprises stretching the polymeric film to a stretched length
of about 1.1 to 2.5 times the length of the intermediate
length.
19. A method as recited in claim 1, wherein stretching in the first
and second stretching steps combines to stretching the polymeric
film from about 4 times to about 8.5 times the original length.
20. A method as recited in claim 1, further comprising borating the
polymeric film after second stretching step.
21. A method as recited in claim 20, wherein borating the polymeric
film comprises exposing the polymeric film to a borating solution
containing at least boric acid.
22. A method as recited in claim 21, wherein the solution contains
from about 5% to about 20% by weight boric acid.
23. A method as recited in claim 21, wherein the solution also
contains one of a sodium borate and a potassium borate.
24. A method as recited in claim 23, wherein the solution contains
from about 0% to about 7% by weight sodium borate decahydrate.
25. A method as recited in claim 21, wherein the borating solution
has a temperature in excess of 50.degree. C.
26. A method as recited in claim 25, wherein the borating solution
has a temperature in the range from about 70.degree. C. to about
110.degree. C.
27. A method as recited in claim 21, further comprising drying the
polymeric film after exposing the polymeric film to the borating
solution containing at least boric acid.
28. A method as recited in claim 20, further comprising permitting
the polymeric film to shrink to a borated length while borating the
polymeric film.
29. A method as recited in claim 28, further comprising stretching
the polymeric film in a third stretching step after permitting the
polymeric film to shrink while borating the polymeric film.
30. A method as recited in claim 29, wherein stretching the
polymeric film in the third stretching step comprises stretching
the polymeric film up to about 120% of the borated length.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This is a Division of application Ser. No. 10/910,211 filed
Aug. 3, 2004. The disclosure of the prior application is hereby
incorporated by reference herein in its entirety.
[0002] This application concerns the technical field of intrinsic
polarizes, which is also addressed by the following U.S. patent
applications: application Ser. No. 10/403,885, filed Mar. 31, 2003,
entitled "PROCESS FOR MAKING AN INTRINSIC POLARIZER," which is a
continuation-in-part of application Ser. No. 10/277,252 filed Oct.
20, 2002, entitled "ENHANCED INTRINSIC POLARIZER", which is a
continuation-in-part of pending application Ser. No. 10/118, 489,
filed Apr. 6, 2002, entitled "ENHANCED K-TYPE POLARIZER", the
relevant parts of which are incorporated by reference herein.
FIELD OF THE INVENTION
[0003] The invention relates to synthetic dichroic plane polarizers
based on molecularly oriented polyvinyl alcohol films and, in
particular, to a method of making a high efficiency intrinsic
polarizing sheet or film.
BACKGROUND
[0004] Normally, light waves vibrate in a large number of planes
about the axis of a light beam. If the waves vibrate in one plane
only, the light is said to be plane polarized. Several useful
optical systems can be implemented using polarized light. For
example, in the manufacture of electro-optical devices such as
liquid crystals display screens, cross polarizers are used in
conjunction with an addressable liquid crystal interlayer to
provide the basis for image formation. In the field of photography,
polarizing filters have been used to reduce the glare and the
brightness of specular reflection. Polarizing filters, circular
polarizers or other optical components have also been used for
glare reduction in display device screens.
[0005] Linear light polarizing films, in general, owe their
properties of selectively passing radiation vibrating along a given
electromagnetic radiation vector, and absorbing electromagnetic
radiation vibrating along a second given electromagnetic radiation
vector, to the anisotropic character of the transmitting film
medium. Dichroic polarizers are absorptive, linear polarizers
having a vectoral anisotropy in the absorption of incident light.
The term "dichroism" is used herein as meaning the property of
differential absorption and transmission of the components of an
incident beam of light depending on the polarization direction of
the incident light. Generally, a dichroic polarizer will transmit
radiant energy polarized along one electromagnetic vector and
absorb energy polarized along a perpendicular electromagnetic
vector. A beam of incident light, on entering a dichroic polarizer,
encounters two different absorption coefficients, one low and one
high, so that the emergent light vibrates substantially in the
direction of low absorption (high transmission).
SUMMARY OF THE INVENTION
[0006] Intrinsic polarizers are polarizers whose base material is
converted to a dichroic material, and so a polarizing effect is
produced without the need to adsorb a dichroic material, such as
iodine or dye, to the base material. Intrinsic polarizers,
therefore, have a simpler construction, provide the possibility of
being less expensive, are thinner and do not require the additional
cover layers required by iodine or dye polarizers.
[0007] One embodiment of the invention is directed to a method for
making a polarizer from a polymeric film having an original length
and comprising a hydroxylated linear polymer. The method comprises
stretching the polymeric film a first stretching step and
converting the hydroxylated linear polymer, after the first
stretching step, to form dichroic, copolymer polyvinylene blocks
aligned in the polymeric film. The polymeric film is stretched in a
second stretching step while converting the hydroxylated linear
polymer.
[0008] Another embodiment of the invention is directed to an
intrinsic polarizer that comprises a sheet of PVA-type matrix
containing vinylene polymer blocks. The sheet defines a pass
polarization axis and a block polarization axis perpendicular to
the pass polarization axis. Light having an electrical vector
parallel to the pass polarization axis is substantially transmitted
through the sheet and light having an electrical vector parallel to
the block polarization axis is substantially absorbed in the sheet.
The sheet exhibits a ratio, R, having a value of less than 2, where
R is the ratio of the intrinsic absorbance for light at 550 nm
polarized parallel to the block polarization axis over the
intrinsic absorbance for light at 400 nm polarized parallel to the
block polarization axis. The sheet also exhibits a polarization
efficiency ratio in excess of 99%.
[0009] Another embodiment of the invention is directed to an
intrinsic polarizer that comprises a sheet of PVA-type matrix
containing vinylene polymer blocks. The sheet defines a pass
polarization axis and a block polarization axis perpendicular to
the pass polarization axis. Light having an electrical vector
parallel to the pass polarization axis is substantially transmitted
through the sheet and light having an electrical vector parallel to
the block polarization axis is substantially absorbed by the
vinylene polymer blocks. The sheet has an intrinsic absorption
spectrum such that, when crossed with an identical sheet and
illuminated with a cold cathode fluorescent tube (CCFT) light
source, the sheet transmits light having an a* co-ordinate with a
magnitude of less than 2 and a b* co-ordinate with a magnitude of
less than 2.
[0010] Another embodiment of the invention is directed to an
intrinsic polarizer that comprises a sheet of PVA-type matrix
containing vinylene polymer blocks. The sheet defines a pass
polarization axis and a block polarization axis perpendicular to
the pass polarization axis. Light having an electrical vector
parallel to the pass polarization axis is substantially transmitted
through the sheet and light having an electrical vector parallel to
the block polarization axis is substantially absorbed by the
vinylene polymer blocks. The sheet exhibits a dichroic ratio of
more than 100.
[0011] The above summary of the present invention is not intended
to describe each illustrated embodiment or every implementation of
the present invention. The figures and the detailed description
which follow more particularly exemplify these embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The invention may be more completely understood in
consideration of the following detailed description of various
embodiments of the invention in connection with the accompanying
drawings, in which:
[0013] FIG. 1 schematically illustrates a method of manufacturing a
polarizer according to principles of the present invention;
[0014] FIG. 2 presents a graph showing the absorption spectra for
Samples 5-8 discussed in Examples 2 and 3;
[0015] FIG. 3 presents a graph showing the standard spectrum for a
cold cathode fluorescent tube (CCFT);
[0016] FIG. 4 presents a graph showing transmission of light as a
function of position across the polarizer for a sample polarizer
manufactured according to principles of the present invention;
[0017] FIG. 5 presents a graph showing absorption of light in a
polarizer as a function of wavelength for polarizers converted
under different conditions;
[0018] FIG. 6 presents a graph showing transmission of light as a
function of wavelength through a single polarizer layer for
different polarizers converted under different conditions;
[0019] FIG. 7 presents a graph showing transmission through a pair
of crossed polarizers for different polarizers converted under
different conditions;
[0020] FIG. 8 presents a graph showing the transmission through the
crossed polarizers of FIG. 7, for light emitted from a CCFT light
source having an emission spectrum as shown in FIG. 3, and
photopically corrected for the response of the human eye;
[0021] FIG. 9 presents a graph showing the value of peak absorption
and the value of the b* color co-ordinate for polarizers as a
function of conversion temperature
[0022] FIG. 10 presents a graph showing the absorbance in a single
polarizer sheet for four different types of polarizer sheets;
[0023] FIG. 11 presents a graph showing the absorbance in a pair of
crossed polarizer sheets for four different types of polarizer
sheets; and
[0024] FIG. 12 presents a graph showing the crossed transmission
through four different types of polarizers, for light emitted from
a CCFT light source having an emission spectrum as shown in FIG. 3,
and photopically corrected for the response of the human eye.
[0025] FIG. 13 schematically illustrates passage of light through
an exemplary intrinsic polarizer.
[0026] FIG. 14 schematically illustrates passage of light through a
pair of crossed exemplary intrinsic polarizers.
[0027] While the invention is amenable to various modifications and
alternative forms, specifics thereof have been shown by way of
example in the drawings and will be described in detail. It should
be understood, however, that the intention is not to limit the
invention to the particular embodiments described. On the contrary,
the intention is to cover all modifications, equivalents, and
alternatives falling within the spirit and scope of the invention
as defined by the appended claims.
DETAILED DESCRIPTION
[0028] The present invention is applicable to intrinsic polarizers,
and is more particularly applicable to a method of making an
intrinsic polarizer with improved optically properties.
[0029] Examples of intrinsic polarizers include, for example, a
polyvinylene-based polarizer such as a K-type polarizer. An
intrinsic polarizer derives its basic dichroism from the
light-absorbing properties of its matrix, rather than from the
light-absorbing properties of dye additives, stains, or suspended
crystalline material, although additives such as dyes may be used
to supplement the intrinsic dichroism. Typically, intrinsic
polarizers comprise a sheet or film of oriented poly(vinyl
alcohol)-type (PVA-type) material, having an oriented suspension of
a dehydration product of PVA, polyvinylene in a matrix of PVA.
Intrinsic polarizers of this kind are typically formed by
unidirectionally stretching the polymeric film to align the PVA
matrix and by heating the PVA-type polymeric film in the presence
of a dehydration catalyst, such as hydrochloric acid, to produce
conjugated polyvinylene blocks. The formation of conjugated
polyvinylene blocks from the polyvinyl alcohol is often referred to
as "conversion." The oriented and dehydrated film may be referred
to as "raw K". By orienting the PVA matrix unidirectionally, the
transition moments of the conjugated polyvinylene blocks are also
oriented, and the material becomes visibly dichroic. The conjugated
polyvinylene blocks may be referred to as dichroic chromophores. A
boration treatment may be employed after converting the polymeric
film, as described in U.S. Pat. No. 5,666,223, the relevant parts
of which are incorporated herein by reference.
[0030] The present invention relates to an enhanced intrinsic
polarizer and method of making same in which improved polarizing
properties are obtained. One embodiment of the method is directed
to stretching the PVA film in a first stretching step, and then
converting the film while stretching the film in a second
stretching step. The first stretching step may take place before,
during, or after the film has been exposed to the dehydration
catalyst.
[0031] The resulting polarizer comprises a composite of a
molecularly oriented film of PVA/polyvinylene block copolymer
material having the polyvinylene blocks thereof formed by molecular
dehydration of a film of polyvinylalcohol. The molecularly oriented
film of polyvinylalcohollpolyvinylene block copolymer material
comprises a uniform distribution of light-polarizing molecules of
polyvinylalcoholpolyvinylene block copolymer material varying in
the number (n) of the conjugated repeating vinylene units of the
polyvinylene block of the copolymer. The value of n ranges from 2
to about 25. The degree of orientation of the light polarizing
molecules increases throughout the range with increasing values of
n. The degree of orientation of the molecules in conjunction with
the concentration distribution of each polyvinylene block is
sufficient to impart to the polymeric sheet a photopic dichroic
ratio (R.sub.D), of at least 75.
[0032] Ignoring surface reflections, the photopic dichroic ratio,
D, is defined as:
D=Az/A.sub.y (1)
[0033] Where A.sub.2 and A.sub.y are determined in the following
manner. The sample polarizer is illuminated with the sample beam of
white light in a dual beam spectrophotometer. The sample beam is
pre-polarized using a high efficiency Glan-type polarizer. The
amount of light transmitted through the sample polarizer at a
particular wavelength is compared to the amount of light at the
sample wavelength in the reference beam, and the absolute
absorbance of the sample polarizer is calculated as a function of
wavelength from the ratio of the transmitted light in the sample
and reference beams. The absorbance is calculated over the range
380 nm-780 nm. The absorbance spectra are sample polarizer and for
light polarized perpendicular absorbance spectra are then
spectrally corrected for the spectrum of a particular light source
and the response of the human eye (photopic correction). The
integrated area under the corrected parallel absorbance spectrum
corresponds to the amount of spectrally corrected light in the
parallel polarization state that is absorbed in a single pass
through the sample polarizer, A.sub.y. The integrated area under
the corrected perpendicular absorbance spectrum corresponds to the
amount of spectrally corrected light, in the perpendicular
polarization state, that is absorbed in a single pas through the
sample polarizer, A.sub.z.
[0034] FIG. 13 schemtically illustrates passage of light through an
exemplary intrinsic polarizer disclosed herein. Intrinsic polarizer
1300 comprises polyvinylalcohol-type matrix containing vinylene
polymer 1301. The polymer blocks have been oriented by stretching.
The polarizer is in the form of a sheet which defines x, y, and z
axes, all of which are orthogonal. Cold cathode fluorescent tube
1302 emits light, depicted by single ray 1303, consisting of
unpolarized light represented by electrical vectors 1304 and 1305.
Electrical vector 1304 is parallel to the x axis also referred to
as the block polarization axis. Electrical vector 1305 is parallel
to the y axis also referred to as the pass polarization axis. The
intrinsic polarizer transmits light, depicted by single ray 1306,
which consists of polarized light represented by electrical vector
1305 and 1307. Ideally, no light is absorbed along the pass
polarization axis; electrical vector 1305 is shown unchanged after
passing through the polarizer. It is generally desirable for all
light to be blocked along the block polarization axis, however,
light typically leaks through the polarizer; thus, electrical
vector 1304 becomes reduced in magnitude to give electrical vector
1307 after passing through the polarizer.
[0035] FIG. 14 schematically illustrates passage of light through a
pair of crossed exemplary intrinsic polarizers. An identical sheet
of intrinsic polarizer 1300 is placed adjacent the polarizer such
that the orientations of the vinylene polymer blocks 1301 are
generally perpendicular to each other. Light depicted by single ray
1306 passes through the pair of crossed polarizers with transmitted
light represented by single ray 1308. Ideally, all light along the
x and y axes is blocked from being transmitted, however, light
along these axes typically leaks through the pair of crossed
polarizers; thus, electrical vectors 1304 and 1305 become reduced
in magnitude to give electrical vectors 1310 and 1309,
respectively. The transmitted light represented by single ray 1308
has an a* co-ordinate with a magnitude of less than 2 and a b*
co-ordinate with a magnitude of less than 2, wherein the a* and b*
co-ordinates are measured according to the CIELAB color system.
[0036] One method for producing an enhanced intrinsic polarizer of
the present invention is now described with respect to FIG. 1,
which generally shows different steps in the manufacturing process
of a K-type polarizer. The PVA-type film 100 is exposed to a
dehydration catalyst 102, such as an aqueous acid solution, and is
provided with a first stretch. The first stretch may take place
before, during or after the film is exposed to the dehydration
catalyst. The film 104 is then converted to produce the dichroic
chromophore and simultaneously stretched in a second stretching
step 106. After conversion, the film may be borated 108, for
example in a boration bath, and then dried and stretched in a third
stretching step.
[0037] A support layer 110 may optionally be added to the film
and/or stripped off from the intrinsic polarizer film at various
stages during the manufacturing process. In the illustrated
embodiment, a support layer 110 is optionally added either before
or after the boration step 108.
Intrinsic Polarizer Starting Material
[0038] K-type polarizers use polymeric films derived from
molecularly oriented polyvinyl alcohol. Vinylalcohol polymers
include any linear 1,3-polyhydroxylated polymer or copolymer, or
derivative thereof that may be dehydrated to a linear, conjugated
vinylic polymer. Useful vinylalcohol polymers include polymers and
copolymers of units having the formula:
##STR00001##
wherein R is H, a C.sub.1-C.sub.8 alkyl, or an aryl group; and R'
is H, or a hydrolysable functional group such as a C.sub.1-C.sub.8
acyl group. Preferably, R and R' are H. In addition to poly(vinyl
alcohol) polymers and copolymers, specifically contemplated are
polyvinyl acetals and ketals and esters as materials from which the
molecularly oriented sheet or film can be formed. These are
referred to a PVA-type materials. In the following discussion,
references to poly(vinyl alcohol) are understood to cover polyvinyl
acetals and ketals and esters, and references to vinylalcohol are
understood to cover vinyl acetals, vinyl ketals and vinyl
esters.
[0039] Useful co-monomers that may be polymerized with the
vinylalcohol monomers to produce vinylalcohol copolymers may
include any free-radically polymerizable monomers including
olefins, such as ethylene, propylene and butylene, acrylates,
acetylenes and methacrylates such as methyl (meth)acrylate, vinyl
acetates and styrenes. Specifically contemplated for use in the
present invention are copolymers of ethylene and vinylalcohol.
Generally, the amount of co-monomer is less than 30 mole % and is
preferably less than 10 mole %. Higher amounts may retard the
formation of conjugated vinylene blocks (poly(acetylene) blocks)
and deleteriously affect the performance of the polarizer.
[0040] The preferred vinylalcohol polymers are homo- and copolymers
of polyvinyl alcohol. Most preferred are polyvinyl alcohol
homopolymers. Commercially available polyvinyl alcohols, such as
those available from Celanese Chemicals, Inc., Dallas, Tex., under
the tradename CELVOL, are classified by viscosity and percent
hydrolysis. Polyvinyl alcohols having low viscosities are preferred
for ease of coating, while having a sufficiently high molecular
weight to provide adequate moisture resistance and good mechanical
properties.
[0041] Melt-processible polyvinyl alcohol may also be used in this
invention.
[0042] The melt processible vinylalcohol polymers are plasticized
to enhance their thermal stability and allow them to be extruded or
melt-processed. The plasticizer can be added externally or may be
part of the vinylalcohol polymer chain, in other words the
plasticizer is polymerized or grafted onto the vinylalcohol polymer
backbone.
[0043] Vinylalcohol polymers that can be externally plasticized
include commercially available products such as "Mowiol" 26-88 and
"Mowiol" 23-88 vinylalcohol polymer resin available from Clariant
Corp., Charlotte, N.C. These "Mowiol" vinylalcohol polymer resins
have a degree of hydrolysis of 88%. "Mowiol" 26-88 vinylalcohol
polymer resin has a degree polymerization of 2100 and a molecular
weight of about 103,000.
[0044] Plasticizers useful in externally plasticizing vinylalcohol
polymer include high boiling, water-soluble, organic compounds
having hydroxyl groups. Examples of such compounds include
glycerol, polyethylene glycols such as triethylene glycol and
diethylene glycol, trimethylol propane, and combinations thereof.
Water is also useful as a plasticizer. The amount of plasticizer to
be added varies with the molecular weight of the vinylalcohol
polymer. In general, the plasticizer will be added in amounts of
between about 5% to about 30%, and preferably between about 7% to
about 25%. Lower molecular weight vinylalcohol polymers typically
require less plasticizer than higher molecular weight vinylalcohol
polymers. Other additives for compounding externally plasticized
vinylalcohol polymers include processing aids, i.e. Mowilith DS
resin from Hoechst A. G., and anti-blocking agents, i.e., stearic
acid, hydrophobic silica, colorants, and the like.
[0045] Externally plasticized vinylalcohol polymers are compounded
by slowly adding the organic plasticizer and typically water to the
vinylalcohol polymer powder or pellets under constant mixing until
the plasticizer is incorporated into the vinylalcohol polymer,
which occurs when the batch reaches a temperature of from about
82.degree. C. (180.degree. F.) to about 121.degree. C. (250.degree.
F.). The lower the molecular weight of the vinylalcohol polymer
resin, the lower the maximum batch temperature required to
incorporate the plasticizer. The batch is held at that temperature
for about 5 to 6 minutes. The batch is then cooled to about between
71.degree. C. (160.degree. F.) and 93.degree. C. (200.degree. F.)
at which time an antiblocking agent can be added. The batch is
further cooled to about 66.degree. C. (150.degree. F.), at which
time the vinylalcohol polymer granulates can be removed from the
mixer and extruded.
[0046] The compounding steps used to externally plasticize the
vinylalcohol polymer can be eliminated when an internally
plasticized vinylalcohol polymer is made, except where it is
desirable to add colorants, etc. Useful internally plasticized
vinylalcohol polymers are commercially available. Such products
include "Vinex" 2034 and "Vinex" 2025, both available from Celanese
Chemicals and Vinylon VF-XS available from Kuraray (Japan).
[0047] Materials available from Celanese under the Vinex trademark
represents a unique family of thermoplastic, water-soluble,
polyvinylalcohol resins. Specifically, the "Vinex" 2000 series
including "Vinex" 2034 and "Vinex" 2025 represent internally
plasticized cold and hot water soluble polyvinylalcohol copolymer
resins. Such internally plasticized vinylalcohol copolymers are
described in U.S. Pat. No. 4,948,857, herein incorporated by
reference. Such copolymers have the following general formula:
##STR00002##
where R is hydrogen or methyl;
[0048] R' is a C.sub.6-C.sub.18 acyl group
[0049] y is 0 to 30 mole %;
[0050] z is 0.5 to 8 mole %; and
[0051] x is 70 to 99.5 mole %.
[0052] These copolymers retain the strength properties of
poly(vinylalcohol) while also exhibiting increased flexibility. The
acrylate monomer represented in the above formula gives the
copolymer its internal plasticization effect. The degree of
polymerization of the copolymers can range from about 100 up to
about 4000, preferably between about 2000 and 4000. The degree of
polymerization is defined as the ratio of molecular weight of the
total polymer to the molecular weight of the unit as referenced in
formula 2. Other internally plasticized poly(vinylalcohol)
copolymer resins and preparation of these resins are discussed in
U.S. Pat. No. 4,772,663. "VINEX" 2034 resin has a melt index
typically of about 8.0 g/10 mins. and a glass transition
temperature of about 30.degree. C. (86.degree. F.). "VINEX" 2025
resin has a melt index typically of 24 g/10 mins. and a glass
transition temperature of about 29.degree. C. (84.degree. F.).
[0053] Polyvinyl alcohols and copolymers thereof, are commercially
available with varying degrees of hydrolysis, e.g., from about 50%
to 99.5+%. Preferred polyvinyl alcohols have a degree of hydrolysis
from about 80% to 99.5%. In general, a higher degree of hydrolysis,
corresponds to better polarizer properties. Also, polyvinyl
alcohols with a higher degree of hydrolysis have better moisture
resistance. Higher molecular weight polyvinyl alcohols also have
better moisture resistance, but increased viscosity. In the context
of this invention, it is desirable to find a balance of properties
in which the polyvinyl alcohol has sufficient moisture resistance,
can be handled easily in a coating or casting process and can be
readily oriented. Most commercial grades of poly(vinylalcohol)
contain several percent residual water and unhydrolyzed poly(vinyl
acetate).
[0054] Coating of the dispersion/solution may be accomplished by a
variety of known methods, including, for example, coating the
substrate using techniques, such as shoe coating, extrusion
coating, roll coating, curtain coating, knife coating, die coating,
and the like, or any other coating method capable of providing a
uniform coating. The substrate may be coated with a primer or
treated with a corona discharge to help anchor the polyvinyl
alcohol film to the substrate. Suitable solution based primers are
water-soluble copolyesters commonly used for priming polyethylene
terephthalate films such as those described in U.S. Pat. No.
4,659,523. After coating, the polyvinyl alcohol film is dried at a
temperature typically from about 100.degree. C. to 150.degree. C.
The thickness of the dried coating may vary depending on the
optical characteristics desired, but is typically from about 25
.mu.m to 125 .mu.m (1-5 mils).
[0055] In another approach, the vinylalcohol polymer layer may be
melt-processed. As with solution coating, a melt comprising the
vinylalcohol may be cast onto a substrate such as a carrier web or
support layer. The vinylalcohol polymer film may also be
melt-blown. The vinylalcohol polymer melt may also be coextruded
with the substrate using a variety of equipment and a number of
melt-processing techniques, typically extrusion techniques, well
known in the art. For example, single- or multi-manifold dies, full
moon feedblocks, or other types of melt processing equipment can be
used, depending on the types of materials extruded.
Stretching Steps
[0056] The manufacture of an enhanced intrinsic polarizing sheet or
film typically begins with a polymeric film of a hydroxylated
linear high PVA-type polymer having an original length, and
generally having a thickness on the order of 0.001 inches (25
.mu.m) to 0.004 inches (100 .mu.m). A suitable stretching device or
other similar mechanism or system may be used to initially stretch
the polymeric film from about 3.5 times to about 7.0 times the
original length of the polymeric film or greater. The first
stretching step is typically conducted at a temperature above the
glass transition temperature of the polymeric material.
[0057] The film may be stretched in a gaseous medium, such as air,
or in a liquid medium, such as deionized water or an aqueous
dehydration catalyst. When stretching in a gaseous medium, the film
may be heated to temperatures, for example, in excess of
300.degree. F. When the film is stretched before being dipped into
a liquid medium, the stretching step may be referred to as a "dry
stretch."
[0058] When stretching in an aqueous medium, additional agents may
be added to aid in the process, such as organic or inorganic salts,
boric acid and/or borax, e.g., a surfactant, such as Triton X100
commercially available from Union Carbide, (Danbury, Conn.).
Stretching in an aqueous medium may also allow undesirable
elements, such as glycerin, to leach out of the polymer film.
[0059] When the film is stretched in a liquid medium, the
stretching step may be referred to as a "wet stretch." The film may
also be stretched after being removed from the liquid medium. The
film typically has absorbed some of the liquid and dehydration
catalyst, if present, and so the step of stretching after removing
the film from the liquid medium may still be referred to as a "wet
stretch."
[0060] Stretching may be effected by the provision of heat
generating elements, fast rollers, and slow rollers. For example,
the difference in the rotational rate between rollers may be
exploited to create corresponding tension in the area of the sheet
transported therebetween. When heat generating elements heat the
sheet, stretching is facilitated and more desirably effected.
Temperature control may be achieved by controlling the temperature
of heated rolls or by controlling the addition of radiant energy,
e.g., by infrared lamps. A combination of temperature control
methods may be utilized.
[0061] In unidirectional orientation, the film may be stretched
without lateral restraint from shrinking, or may be restrained from
shrinking in the lateral direction. Such restraint may impose a
small degree of bidirectional orientation to the film. For example,
a film may be stretched in a down-web direction and its lateral
width maintained constant using a tentering apparatus.
[0062] Stretching may be performed at various stages throughout the
film manufacturing process. Stretching that occurs before
conversion is referred to herein as a first stretching step, and
may occur before the film is exposed to the dehydration catalyst,
while the film is in the dehydration catalyst and/or after the film
has been removed from the dehydration catalyst. Stretching that
occurs simultaneously with conversion is referred to as a second
stretching step and stretching that occurs after conversion, for
example during or after a boration step, is referred to as a third
stretching step.
Support Layer
[0063] It may be desirable to cast, laminate or otherwise affix the
polymeric film onto a substrate such as a support film layer,
heated roller, or carrier web. A support layer, when bonded or
otherwise affixed to the polymer film provides mechanical strength
and support to the article so it may be more easily handled and
further processed. Some useful methods of using a support layer are
described in U.S. Pat. No. 5,973,834 (Kadaba et al.), U.S. Pat. No.
5,666,223 (Bennett et al.) and U.S. Pat. No. 4,895,769 (Land et
al.), the relevant portions of which are incorporated by
reference.
[0064] If desired, the optional support layer may be oriented in a
direction substantially transverse to the direction of orientation
of the vinylalcohol polymer film. By substantially transverse, it
is meant that the support layer may be oriented in a direction at
least .+-.45.degree. from the direction of orientation of the
vinylalcohol polymer film layer. Such orientation of the support
layer may provide greater strength in the transverse direction than
is provided by an unoriented support layer.
[0065] In practice, the support layer may be oriented before or
after attaching to the vinylalcohol polymer layer. In one
embodiment, the vinylalcohol polymer may be oriented substantially
uniaxially and bonded to an oriented support layer so that the
directions of the orientations of the two layers are substantially
transverse.
[0066] Any of a variety of materials can be used for the carrier
web or support layer. Suitable materials include known polymeric
sheet materials such as the cellulose esters, e.g., nitrocellulose,
cellulose acetate, cellulose acetate butyrate, polyesters,
polycarbonates, vinyl polymers such as the acrylics, and other
support materials that can be provided in a sheet-like form.
Polyesters are especially useful, depending on the particular
application and the requirements thereof. A preferred polyester is
polyethylene terephthalate, available under the Mylar and Estar
tradenames, although other polyethylene terephthalate materials can
be employed. In particular, one type of film that may be used as a
support layer is the Vikuiti.TM. brand DBEF type of reflective
polarizer film, available from 3M Company, St. Paul, Minn.
[0067] The thickness of the support material varies with the
particular application. In general, from the standpoint of
manufacturing considerations, supports having a thickness of about
0.5 mil (0.013 mm) to about 20 mils (0.51 mm) can be conveniently
employed.
[0068] Polarizing sheets or films made according to the present
invention may be laminated between or to supporting sheets or
films, such as sheets of glass or sheets of other organic plastic
materials, and that light polarizers of the present invention
either in laminated or unlaminated form may be employed wherever
other forms of light-polarizing plastic materials have been used,
for example, in connection with sunglasses, sun visors, window pane
glass, variable light transmission windows, glare masks, room
partitions, and display devices such as liquid crystal display
panels, emissive display devices, cathode ray tubes, or advertising
displays.
[0069] Any of a variety of adhesives can be used for laminating the
polarizing films onto other layers or substrates including
polyvinyl alcohol adhesives and polyurethane adhesive materials.
Inasmuch as the polarizer will normally be employed in optical
applications, an adhesive material which does not have an
unacceptable affect on the light transmission properties of the
polarizer will generally be employed. The adhesive may, on the
other hand, include a colorant to produce a desired color effect.
The thickness of the adhesive material varies with application. In
general, thicknesses of about 0.20 mil (0.005 mm) to about 1.0 mil
(0.025 mm) are satisfactory.
Exposing Film to Dehydration Catalyst
[0070] The PVA-type film is subjected to a conversion step, which
may take place before or after bonding the vinylalcohol polymer to
a support layer, or without any support layer. In the conversion
step, a portion of the vinyl alcohol polymer in the polymeric film
is converted to polarizing molecules of block copolymers of
poly(vinylene-co-vinyl alcohol). One approach to converting the
vinyl alcohol is first to expose the vinyl alcohol film to a
dehydration catalyst and then to heat the exposed film, thus
causing dehydration to take place.
[0071] The film may be exposed to the dehydration catalyst in
different ways. For example, the film may be dipped or immersed in
an aqueous dehydration catalyst with sufficient residence time to
allow the catalyst to diffuse into the film. Other methods might
include exposing the film to acidic fumes containing the
dehydration catalyst. Dipping the polymeric film potentially allows
higher processing speeds to be attained than with an acid fuming
process since diffusion of aqueous species is faster in solution
than in the gaseous state. In addition, the catalyst can be
introduced to both sides of the polymeric film when the film is
dipped in the catalyst. When exposing the film to acidic fumes, on
the other hand, the film is typically exposed only on one side.
Accordingly, the dipping approach potentially provides a more
uniform concentration of the catalyst in the polymeric film, which
may impact the cross-sectional distribution of dehydration chain
lengths in the resulting raw K film and provide a more balanced
distribution of chains.
[0072] The dehydration catalyst may be any acid or other agent
which is capable of effecting in the presence of heat or other
appropriate processing condition the removal of hydrogen and oxygen
atoms from the hydroxylated moieties of the linear polymer to leave
conjugated vinylene units. Typical acids include hydrochloric acid,
hydrobromic acid, hydroiodic acid, phosphoric acid, and sulphuric
acid in methanol. The desired degree of dehydration may vary,
depending on the desired contrast and the film thickness, but is
typically in the range of 0.1 to 10%, preferably 1 to 5% of the
available hydroxyl groups are converted to vinylene groups (i.e.,
--CH.sub.2--CHOH--.fwdarw.--CH.dbd.CH).
[0073] For example, the polymeric film may be immersed in an
aqueous hydrochloric acid solution for about one second to several
minutes. In another example, the polymeric film may be immersed in
deionized water for about one second to about five minutes and then
immersed in an aqueous hydrochloric acid solution for about one
second to several minutes. The concentration of the aqueous
hydrochloric acid solution is preferably about 0.01 Normal to about
4.0 Normal.
[0074] The dehydration step may also be achieved by other methods,
such as by coating the oriented sheet with an acid coating and then
subjecting it to a heating step to effect the dehydration of the
polymeric sheet, or by coating the oriented sheet with an acid
donor layer. In the latter example, a photoacid generator or a
thermal acid generator is dissolved or dispersed in the donor layer
and, upon irradiation with a radiant energy, the incipient acid
diffuses into the adjacent vinylalcohol polymer matrix to effect
partial dehydration of the vinylalcohol polymer to conjugated
vinylene [poly(acetylene)] segments. The radiant energy may be
thermal energy or ultraviolet light energy, depending on the type
of acid generator used.
[0075] Processing agents may be added to the acid to aid in the
process, such as organic or inorganic salts, boric acid and/or
borax, e.g., a surfactant, such as Triton X100 commercially
available from Union Carbide, (Danbury, Conn.).
Conversion
[0076] After exposing the film to the dehydration catalyst,
PVA-type film and the adsorbed catalyst may then be heated, whereby
the oriented film is converted into the desired dehydration
product, polyvinylene. The film may be heated through conduction
heating, convection heating, radiation heating, or a combination
thereof. The conversion process results in the converted film
giving up water and the acid catalyst in the form of vapor.
[0077] For example, the polymeric film and the catalyst may be
passed through a heating oven with a temperature range of from
about 88.degree. C. to about 205.degree. C. for about a few seconds
to about ten minutes. In another approach, the film and catalyst
may be exposed to microwave radiation heating or to laser
heating.
[0078] Another method of converting the film is to expose film and
catalyst to radiant infrared heating, for example generated using
an infrared heating lamp or lamps, from about one second to about
sixty seconds. Infrared heating potentially allows higher
processing speeds to be attained than with hot air impingement
methods. In addition, infrared heating allows for a rapid startup
and shutdown of the conversion process. Furthermore, when heating
is effected using a number of radiant heaters placed across the
film, it may be possible to achieve lanewise control of the
conversion process by individually controlling the amount of
radiation emitted from the different radiant heaters.
[0079] Variations in the temperature and duration of the
dehydration heating step may affect the optical properties of the
finished polarizer. Considerable latitude in process parameters
exists without detriment to the formation of the copolymer and its
concomitant polarization properties. There is a balance among time,
temperature and acid concentration for a given optical property.
For example, the extent of penetration of the acid into the film
may be controlled by altering the temperature of the acid solution,
altering the residence time of the film in the acid, and/or
altering the concentration of the acid. For example, a lower
transmission polarizer may be achieved at a given temperature by
using longer immersion times. At a given immersion time, lower
transmission may by achieved at higher temperatures. Generally, it
is preferred that the diffusion of dehydration catalyst within the
film reaches equilibrium. If a high transmission polarizer is
desired, lower acid concentrations are preferred. If a lower
transmission polarizer is desired then higher acid concentrations
may be used.
[0080] The film may be subjected to a second stretching step during
the conversion process. In other words, the film may be stretched a
second time while the conversion process is occurring. This second
stretching step may result in an increase in the film length by up
to about 2.5 times the intermediate length of the film obtained
after the first stretching step. Like the first stretching step,
the second stretching step occurs at a temperature above the glass
transition temperature of the polymeric material, and may be
effected by the provision of heat generating elements, fast
rollers, and slow rollers.
Boration
[0081] The polymeric film may also be subjected to a boration step
following conversion, in which the oriented film is borated, for
example by exposing the converted film to an aqueous boration
solution. The boration step effects relaxation and cross-linking. A
third stretching step may be carried out before, during, or after
the polymeric film is borated. For example, the polymeric film may
be submerged and allowed to soften and/or swell in an aqueous
boration solution. This often results in relaxation, or shrinkage,
of the film. The film is subsequently removed and dried. The film
may receive a third stretch during and/or after drying following
the boration step. In another approach, the polymeric film may be
stretched when still submerged into the boration solution.
[0082] The boration step may employ one or more baths. For example,
in a two-bath boration treatment, the first bath may contain water,
and the second, a boric ion contributing species. The order of the
baths may be reversed or both baths may contain varying
concentrations and/or mixtures of boric ion contributing species.
Stretching and/or relaxation of the polymeric film may be conducted
in any one or more of these baths.
[0083] The boration solution generally comprises boric acid. In
addition, the boration solution may comprise either sodium or
potassium hydroxide, or may include a substance from the class
consisting of the sodium and potassium borates, preferably borax.
The concentration of boric acid and borax or other borate in the
solution or solutions to which the oriented polarizing film is
subjected may vary. Preferably, the boric acid is present in a
higher concentration than the borax or other borate, and the
solutions may contain from about 5% to about 20% by weight of boric
acid and from 0% to about 7% by weight of borax. A preferred
concentration ranges from about 6%-16% by weight of boric acid and
from 0%-3% by weight of borax.
[0084] The polarizing sheets or films may be immersed in a boration
solution or solutions for a period of about one minute to about
thirty minutes and preferably maintained at about 50.degree. C. or
higher. A preferred boration temperature ranges from about
70.degree. C. to about 110.degree. C. Boration of the molecularly
oriented polymeric film is subject to considerable variation. For
example, the temperature of the boration solution may be varied,
and the concentration thereof may be increased at higher
temperatures. It is desirable that the solution be heated to at
least 50.degree. C. or greater in order to accomplish rapid
"swelling" and cross-linking of the sheet.
[0085] Following exposure to the boron-containing solution, the
polarizing sheet may be rinsed and dried. The sheet may be rinsed
using any suitable method, such as passing the sheet through a bath
of de-ionized water, or by spraying de-ionized water on the sheet.
The sheet may be dried by heating the sheet, for example through
convection or radiation heating. In one approach, the sheet may be
passed through a convection oven.
[0086] Processing agents may be added to the boration bath to aid
in the process, for example, a surfactant such as Triton X100
commercially available from Union Carbide, (Danbury, Conn.).
[0087] The polarizing sheet typically shrinks during the boration
step, if not left under tension. Allowing the polarizing sheet to
shrink permits the polarizer sheet to take up more boron-containing
solution, and thus leads to a higher degree of cross-linking, with
a concomitant increased environmental stability. The polarizing
sheet may be restretched after boration. For example, the sheet may
be stretched in a third stretching step up to about 120% of the
shrunk length. The restretching may be performed while the sheet is
still in the boration bath or after it has been removed from the
boration bath. For example, if the boration step is followed by
rinsing and drying, the restretch may take place in a deionized
rinsing bath or while being dried.
[0088] Subsequent to the second stretching step and/or boration
step, the resulting intrinsic polarizer may be bonded or laminated
to an optional support layer. The optional layer may be the same or
different from an optional support layer previously stripped
off.
[0089] The process of wet-stretching, conversion and boration can
be applied to the PVA-type film as a continuous, integrated
process. Such a continuous process is simpler than the multistep
processes that have been used for intrinsic polarizers in the past,
and leads to higher film yield and reduced polarizer cost.
[0090] To further illustrate the present invention, the following
Examples are provided, but the present invention is not to be
construed as being limited thereto. Unless otherwise indicated, all
parts, percents and ratios are by weight. In the Examples,
unpolarized light transmission was measured on the raw K samples by
passing a beam of white light through the sample, through a
photopic filter, and then through a photo-detector. Unpolarized
light transmission on raw K samples for an intrinsic polarizer
typically ranges from 15% to about 50%. The polarizing efficiency
was calculated according to the following equation by determining
the transmittance with axes parallel (T.sub.par) which was
determined by overlapping the sample polarizer with the high
efficiency polarization analyzer in such a manner as to make the
axes thereof parallel with each other, and the transmittance with
axes crossed (T.sub.perp), which was determined by overlapping the
same in such a manner as to make the axes at right angles to each
other:
Polarizing
efficiency,.eta.(%)=(T.sub.par-T.sub.perp)/(T.sub.par+T.sub.perp).times.1-
00 (3)
[0091] Unless otherwise indicated, all Examples used an aqueous
boration solution having a 9%-12% boric acid concentration and a 3%
borax concentration.
EXAMPLE 1
Birefringent Characteristics
[0092] Four samples of PVA-type film were stretched by different
amounts, as listed in Table I. These samples were prepared for
measuring the birefringence that results from the orientation of
the PVA molecules when the PVA film is stretched. As can be seen,
the amount of birefringence, which is related to the degree of
orientation, increases with increased stretching. Also, the film
become thinner with increased stretching.
TABLE-US-00001 TABLE I Sample Stretch (%) Birefringence Thickness
(.mu.m) 1 400 0.0337 31.4 2 650 0.0400 23.9 3 750 0.0418 21.1 4 850
0.0438 17.8
[0093] Each film was 2400 DP PVA, available from Kuraray Co. Ltd.,
Osaka, Japan., contained about 12% glycerin plasticizer and, before
stretching, had a thickness of 75 .mu.m and a width of 26'' (66
cm). All samples were stretched by 400% in a first, wet stretch
step, and were then subsequently stretched by different amounts in
a second stretch step under IR illumination. For Samples 1-4, the
wet stretch took place in deionized water, with no acid present.
Accordingly, there was no dehydration catalyst, and so no
conversion took place when the film was exposed to the IR lamp. In
all cases, the IR lamp was a Protherm Infrared Heater, FS Series,
medium wavelength, available from Process Thermal Dynamics, Inc.,
Brandon, Minn., unless indicated otherwise. The conditions under
which Samples 1-4 were made are listed in Table II.
TABLE-US-00002 TABLE II Sample Manufacturing Conditions Sample No.
1 2 3 4 Input line speed (cm/s) 0.5 0.25 0.25 0.25 Water bath
temperature (.degree. C.) 36.7 36.7 36.7 36.7 Wet Stretch (1.sup.st
step, %) 400 400 400 400 Water bath path length (cm) 86 86 86 86 IR
lamp temperature (.degree. C.) 445 550 563 568 IR filament,
distance from film (cm) 20 20 20 20 IR lamp length (cm) 25 25 25 25
Reflector distance from film (cm) 7.5 7.5 7.5 7.5 IR stretch
(2.sup.nd step, %) 100 163 187 213 Total stretch (%) 400 650 750
850
[0094] Apart from Sample 1, all samples had an input line speed of
0.25 cm/sec, corresponding to 0.5 feet/minute. Sample 1 was
transported twice as fast, with a line speed of 0.5 cm/sec (1
foot/minute). The film was passed into a water bath held at a
temperature of 36.7.degree. C. The path length in the water was 86
cm (34 in). The samples were each stretched by 400% in the water. A
value of stretch indicates the ratio of the length after stretching
to the length before stretching. Thus, a stretch of 400% indicates
that a length of 1 unit of film was stretched to a length of 4
units.
[0095] The films were then removed from the water tank and exposed
to IR light from an IR lamp whose temperature could be adjusted.
The IR lamp had a reflector behind the heating element to reflect
heat to the film. In all cases the distance between the reflector
and the film was 7.5 cm. The different samples were stretched by
different amounts when exposed to the IR lamp. Sample 1 was not
stretched when exposed to the IR lamp, as indicated by the stretch
amount of 100%. The films were unsupported during the wet stretch
and IR stretch steps.
EXAMPLE 2
Polarization Characteristics
[0096] Four more samples were prepared for measuring how the
polarization performance of the film is dependent on the amount of
stretching.
TABLE-US-00003 TABLE III Pol. eff. Sample Stretch (%) Kv (%) (%) D
5 400 42.4 98.13 51.6 6 650 42.3 99.88 87.5 7 750 42.5 99.96 107.4
8 850 42.5 99.94 101.7
[0097] Samples 5-8 were prepared like Samples 1-4 in Example 1
above, except that the water bath contained acid. Therefore, for
Samples 5-8, the exposure to the IR lamp resulted in conversion of
the film. The transmission for upolarized light (Kv) was calculated
from transmission measurements through the films made using a
spectrophotometer (Cary Model No. 5E). Kv is the average
transmission through the film of light polarized parallel to the
transmission axis (T.sub.par) and of light polarized perpendicular
to the transmission axis (T.sub.perp). The polarizing efficiency
was calculated using expression (3) provided above.
[0098] The dichroic ratio, D, was defined in expression (1) above.
The results listed in Table III show that the polarization
coefficient and the dichroic ratio generally increase for increased
amounts of stretching, while maintaining a substantially constant
value transmission for light in the pass polarization state. The
values of polarization coefficient and dichroic ratio for Sample 8,
however, are a little less than those for Sample 7. The conversion
conditions and boration step conditions affect the optical
characteristics of the resultant polarizer film. The conversion and
boration conditions for Sample 8 had not been optimized. It is
believed that the polarization performance of a film stretched by
850% may be increased with optimization of boration conditions.
[0099] The polarization efficiencies of Samples 7 and 8 are both
high, in excess of 99.94%, and the dichroic ratios are both higher
than 100. Thus the polarization properties of Samples 7 and 8 are
significantly improved over previously obtained values for KE
polarizers.
[0100] The conditions under which Samples 5-8 were made are listed
in Table IV.
TABLE-US-00004 TABLE IV Sample Manufacturing Conditions Sample No.
5 6 7 8 Input line speed (cm/s) 0.5 0.25 0.25 0.25 Water bath
temperature (.degree. C.) 31 37.8 35 35 Wet Stretch (1.sup.st step,
%) 400 400 400 400 Water bath path length (cm) 86 250 86 86 IR lamp
temperature (.degree. C.) 445 550 560 570 IR filament, distance
from film 20 20 20 20 (cm) IR lamp length (cm) 25 25 25 25
Reflector distance from film (cm) 7.5 7.5 7.5 7.5 IR stretch
(2.sup.nd step, %) 100 163 187 213 Boration tank temperature
(.degree. C.) 85 87.8 90.5 93.3 Boric acid conc. (%) 11.25 11.9
11.2 11.2 Sodium borate decahydrate 3.0 3.0 3.0 3.0 (borax) conc.
(%) Boration relax (%) 0 5 10 13 Restretch (3.sup.rd stretch)
during rinse 108 106 106 106 and dry (%) Total stretch (%) 430 655
720 790
[0101] After the second stretch step, during conversion, the films
were placed into a boration bath containing boric acid and sodium
borate decahydrate (Na.sub.2B.sub.4O.sub.7.10H.sub.2O, also known
as borax). The films were permitted to relax in the boration bath,
resulting in a shrinkage of as much as 13% in length. The films
were rinsed in water and dried in a convection oven after being
removed from the boration bath. The films were restretched
(3.sup.rd stretch step) during the rinse and dry process. The films
were unsupported by a support layer throughout the fabrication
process.
EXAMPLE 3
Color Characteristics
[0102] The color characteristics of Samples 5-8 were calculated for
light transmitted through a single layer and also for a light
transmitted through a pair of crossed layers (transmission axes
perpendicular), using the wavelength dependent absorbance
measurements made using the using the spectrophotometer. The
absorbance spectra for the different samples are shown in FIG. 2.
The absorbance spectra for Samples 5-8 are respectively labeled as
curves 205, 206, 207 and 208 in FIG. 2. The hue of the transmitted
light was subsequently calculated, and is listed in Table V. Unless
otherwise stated, the hue is calculated for illumination using a
cold cathode fluorescent tube (CCFT), as is commonly used in LCD
displays for e.g. laptop computers. The spectrum of the CCFT source
is presented in FIG. 3, normalized so that the total intensity
integrated over the wavelength range shown, the area under the
curve, is equal to one. Color, or hue, is presented according to
the CIELAB color system, which uses three co-ordinates: L*, a* and
b*. The L* co-ordinate is related to lightness, the a* co-ordinate
represents red/green color and the b* co-ordinate represents
yellow-blue color. A positive value of a* corresponds to red and a
negative value of a* corresponds to green. A positive value of b*
corresponds to yellow and a negative value of b* corresponds to
blue. The (a*,b*) co-ordinate of (0,0) represents a neutral hue,
black, grey or white, depending on the value of the L* co-ordinate.
Furthermore, a value of a* or b* whose magnitude is less than 1
results in a barely perceptible change in color from neutral. The
color characteristics are presented for a single sheet, for a pair
of sheets oriented with the transmission axes parallel and for a
pair of sheets with the transmission axes crossed.
TABLE-US-00005 TABLE V Color Characteristics Single Single Parallel
Parallel Crossed Crossed Sample a* b* a* b* a* b* 5 -0.891 8.593
-2.410 15.081 16.978 8.834 6 -0.160 4.720 -0.261 9.045 2.475 -2.252
7 -0.501 3.475 -0.853 6.544 0.556 -0.461 8 -0.113 3.219 -0.113
6.140 1.025 -1.074
[0103] Sample 5 has relatively poor color characteristics, since
the values for a* in the crossed configuration and for b* in both
configurations are so large. Samples 6-8, on the other hand have
significantly improved color characteristics. Both Samples 7 and 8
show values of a* and b* whose magnitudes are less than 2 and, in
fact, are less than 1, for Sample 7 in the crossed configuration.
Sample 7, in particular, shows a neutral hue in the crossed
configuration and a slightly blue hue in the single layer
configuration. In addition, Sample 8 shows a substantially neutral
hue in the crossed configuration and a more neutral hue in the
single layer configuration than Sample 7. In the parallel
configuration, both Samples 7 and 8 show a hue of less than one a*
unit, and show reasonable magnitudes of b* of less than 7. These
color characteristics are relatively neutral because the
concentration of conjugated vinylene blocks is relatively constant
over a large range of n, where n is the number of vinyl units
conjugated in the polyvinylene block, as is discussed in U.S.
patent application Ser. No. 10/277,252, incorporated herein by
reference.
[0104] Samples 7 and 8, therefore, provide excellent polarization
characteristics, as listed in Table III, while also providing
excellent color characteristics. Such color neutrality has not
previously been obtainable in intrinsic K-type polarizers of high
polarization quality without the aid of extrinsic chromophores such
as dyes.
EXAMPLE 4
Uniformity of Polarization and Color Characteristics
[0105] The uniformity of the optical characteristics across the
film (cross-web) was measured for a film made under the conditions
listed above for Sample 7, i.e. a total stretch of 720%. The
stretched film had a width of about 9 inches (23 cm). The
transmission, Kv, polarizing co-efficiency, dichroic ratio and
color were measured at steps of one inch (2.5 cm) from one of the
edges of the film. The results are shown in Tables VI and VII.
TABLE-US-00006 TABLE VI Polarization Characteristics vs. Film
Position Distance from Polarizing co- edge (cm) Kv (%) efficiency
(%) D 2.5 42.3 99.95 97.4 5 42.9 99.96 92.5 7.5 41.9 99.96 90.8 10
42.3 99.96 98.6 12.5 42.2 99.96 97.3 15 42.4 99.96 103.4 17.5 42.4
99.95 101.6 20 42.7 99.92 103.2
[0106] The values for Kv are also shown in the graph presented in
FIG. 4. Kv varies by about .+-.0.6% across the width of the film.
The polarizing efficiency varies by about .+-.0.02% across the film
and the dichroic ratio varies by about .+-.5 across the film. The
variation in the polarization characteristics across the film is
relatively small.
TABLE-US-00007 TABLE VII Color Characteristics vs. Film Position
Distance from Single Single Crossed Crossed edge (cm) a* b* a* b*
2.5 -0.188 2.603 0.918 -1.235 5 -0.252 3.233 0.438 -0.426 7.5
-0.465 3.499 0.425 -0.352 10 -0.433 3.417 0.594 -0.460 12.5 -0.404
3.655 0.537 -0.395 15 -0.518 3.854 0.555 -0.279 17.5 -0.466 4.073
0.565 -0.234 20 -0.4101 3.506 0.952 -0.680
[0107] The color characteristics vary by only small amounts over
the film. For example, other than one value close to the edge, all
the hues in the crossed configuration have a magnitude of less than
one, which is minimally perceptible to the human eye, if
perceptible at all. The only value greater than one, (b* at 2.5 cm
from the edge) is greater than one by only a small amount, and is
barely perceptible to the human eye. In the single layer
configuration, the magnitudes of a* all remain less than one, while
the magnitudes of b* vary between 2.603 and 4.073.
EXAMPLE 5
IR Lamp Temperature
[0108] The effect of the lamp temperature on polarization and color
characteristics was investigated. Samples 9-12 were fabricated
under the same conditions as listed above for Sample 7, except that
the lamp temperature was varied and the boration conditions were:
boric acid concentration .about.9.5% and sodium borate decahydrate
concentration .about.3%.
[0109] The absorbance (optical density) and transmittance of the
different polarizer sheets were calculated from measurements made
using the dual beam spectrophotometer. The resulting absorbance and
transmittance for single polarizer sheets are shown in FIGS. 5 and
6 respectively. The absorbance is the intrinsic absorbance, and
results from intrinsic absorption by the polyvinylene blocks in the
film itself, and not from absorption of species added to the film,
such as iodine or dye. The absorbance curves for samples 9-12 are
labeled in FIG. 5 as curves 509. 510, 511 and 512 respectively.
Also, the transmission curves in FIG. 6 for samples 9-12 are
labeled respectively as curves 609, 610, 611 and 612.
[0110] The absorbance for Samples 10 and 11 is particularly high in
the blue region of the spectrum, a region of the spectrum that has
previously seen relatively low absorbance for K-type polarizers
using previous methods of construction. In particular, the ratio,
R, of the absorbance at 550 nm over the absorbance at 400 nm is
around 1.54, showing that the absorbance at the blue end of the
spectrum is around two-thirds of the absorbance in the middle of
the spectrum. Thus, Samples 10 and 11 both show that the ratio, R,
is less than 2, and is less than 1.7.
[0111] The transmission through a pair of crossed polarizer sheets
is shown in FIG. 7 for the different polarizer samples. The
transmission curves for samples 9-12 are labeled respectively as
curves 709, 710, 711 and 712. The transmission plotted on the
y-axes in FIGS. 6 and 7 is the absolute value of transmittance.
Thus, a transmission value of 0.1 indicates that 10% of the
incident light is transmitted through the polarizers. When the data
of FIG. 7 are convoluted with a CCFT light source, whose spectrum
is given in FIG. 3, and photopically corrected for the response of
the human eye, the transmission spectrum of the crossed polarizers
is as shown in FIG. 8. The photopically corrected transmission
curves for samples 9-12 are respectively labeled as curves 809,
810, 811 and 812 in FIG. 8.
[0112] The polarization characteristics for Samples 9-12 are shown
in Table VIII, the transmission characteristics are given in Table
IX and the color characteristics are given in Table X.
TABLE-US-00008 TABLE VIII Polarization Characteristics Lamp
Polarizing Contrast Sample temp. (.degree. C.) co-eff. (%) D Ratio
9 550 99.79 98.6 463 10 560 99.89 115.7 914 11 570 99.89 116.0 873
12 580 98.94 76.6 93
[0113] The contrast ratio is defined as the ratio of the parallel
transmission over the crossed transmission, listed below in Table
IX. It should be noted that, concomitant with the relatively low
value of R, the polarizing efficiency exhibited by Samples 10 and
11 is greater than 99.8%, while the dichroic ratio, D, for both
samples was greater than 110.
TABLE-US-00009 TABLE IX Transmission Characteristics (%) Single
Sample (Kv) Parallel Crossed 9 43.0 36.9 0.0796 10 43.2 37.3 0.0408
11 43.2 37.3 0.0427 12 43.2 37.0 0.3946
[0114] The transmission characteristics are integrated over the
visible spectrum for the polarizers illuminated with the CCFT
standard light source and photopically corrected for the response
of the human eye. The values of crossed transmission correspond to
the areas under the curves shown in FIG. 8.
TABLE-US-00010 TABLE X Color Characteristics single single par.
par. crossed crossed Sample a* b* a* b* a* b* 9 0.117 1.311 -0.210
3.253 5.509 -8.814 10 -0.651 3.793 -1.206 7.200 1.602 -0.780 11
-0.720 4.022 -1.317 7.589 1.590 -0.374 12 -0.034 3.210 -0.771 6.127
10.207 -0.037
[0115] The color characteristics listed in Table X correspond to
the calculated hues produced under illumination by light from the
standard CCFT light source that has passed through the particular
polarizer configuration. Thus, "single" refers to the hue of the
light that is transmitted through a single layer of the polarizer
sample, "par." refers to the hue of the light that has passed
through a stack of two layers of the polarizer sample with the
transmission axes parallel, and "crossed" refers to the hue of the
light that has passed through a stack of two layers of the
polarizer sample whose transmission axes are perpendicular.
[0116] As can be seen from inspection of Tables VIII-X, and FIGS.
5-8, the performance of the polarizer peaks when the lamp
temperature used in this particular manufacturing process is in the
region of 560.degree. C.-570.degree. C. Of particular note is the
performance of the polarizer in the blue portion of the spectrum.
Previously, high values of A.sub.z and low values of T.sub.z in the
blue portion of the spectrum were not achieved with intrinsic
polarizers. The processes of wet stretching and simultaneous
stretching and conversion result in improved blue performance, with
low transmittance and high absorbance. The absorbance is greater
than 2 through the blue region (400 nm-500 nm), and is greater than
3 for some wavelengths in the range 400 nm-450 nm. Accordingly, the
change is color when used in a crossed configuration is relatively
small. In particular, Samples 10 and 11 both exhibit values of a*
and b* whose magnitudes are less than 2, and the value of b* is
less than 1.
[0117] Lamp temperatures outside of the range 560.degree.
C.-570.degree. C. resulted in increased blue transmission, for
example as is shown in FIG. 6 for the curves corresponding to a
lamp temperature of 550.degree. C. and 580.degree. C. This
increased transmission may indicate that the number of shorter
vinylene blocks (low n) being conjugated during the conversion
process is less than at temperatures around 560.degree.
C.-570.degree. C. Furthermore, at higher than optimal temperatures,
e.g. 580.degree. C., there is increased transmission of light in
the red region of the spectrum. This is shown in the departure of
the curves corresponding to 580.degree. C. from the other curves in
the region of 600 nm-700 nm. This may indicate that the number of
longer vinylene blocks (high n) being conjugated is reduced when
the temperature is higher than optimum.
EXAMPLE 6
Control of "Blue Leak"
[0118] Traditional methods of preparing K-type polarizers suffer
from a so-called "blue-leak", in the crossed state absorption
spectrum, where absorbance drops to relatively low values for
wavelengths below about 450 nm. The currently accepted way of
increasing the blue absorption is to add a blue-absorbing dye to
the intrinsic polarizing film. The data presented under Example 5,
however, suggest that the blue absorption can be controlled to some
extent by the temperature and power of the IR lamp used in the
conversion process. Thus, use of the manufacturing process
discussed above provides the ability to control the yellow-blue, or
b*-axis, color of the resulting polarizer film. This is due to the
modulation of the dehydration chain length distribution of the
chromophore in the film, with a larger relative ratio of short
chain lengths (low n) providing the increase in blue
absorption.
[0119] Since there is no requirement to add a blue absorbing dye,
it is easier to control the manufacture of the resulting polarizer,
as it is only the heating source that is controlled, and there is
no need to precisely control the adsorption of the blue-absorbing
dye. Additionally, the resulting polarizer provides improved
environmental stability since the absorbing chromophores are
intrinsic to the PVA matrix, and there is no dye adsorbed on the
polarizer film surface.
[0120] To explore the blue performance of the K-type polarizer
further, a number of samples were prepared using the following
method. A cast film of polyvinyl alcohol, 75 .mu.m thick,
containing .about.12% wt. glycerin plasticizer and with
approximately 2400 average degree of polymerization, was stretched
by a factor of 650% while being passed through a .about.0.05 Normal
aqueous solution of hydrochloric acid at a temperature of
52.degree. C. After stretching, the film was passed through an IR
heating zone where the film was heated, thus causing conversion of
some of the PVA to polyvinylene blocks. While the film was
continuously processed in this manner, the power applied to the IR
heating zone was increased, and the temperature of the heater was
monitored using a thermocouple. The geometry of the heater was
different from that used in Examples 1-5, so the temperatures of
the IR heater discussed in the previous examples do not necessarily
correspond to those of this example.
[0121] As the heating temperature was increased, the resulting,
un-borated polarizer was sampled. The un-borated polarizer was
measured spectrally in the two orthogonal absorption axes (y and z)
by orientation with a calcite crystal analyzer in a UV-visible
spectrophotometer. The resulting A.sub.z and A.sub.y spectral
curves were used to calculate the parallel and crossed state color
of the raw polarizer samples and wavelength of maximum absorption
(.lamda..sub.max) of the A.sub.z component as a function of IR
temperature.
[0122] The results are shown in the graph in FIG. 9. The value of
.lamda..sub.max (solid curve) reduces as the temperature increases,
indicating that the concentration of smaller conjugated vinylene
molecules (low n) increases with increasing temperature. Also shown
in FIG. 9 is a curve (dashed line) that shows the value of b* for
crossed polarizers as a function of IR temperature. The value of b*
is close to zero for this particular manufacturing process at a
temperature of about 645.degree. C.
EXAMPLE 7
Comparison with Different Types of Polarizers
[0123] The performance of a polarizer manufactured according to the
processes discussed here, and referred to as Wet KE, or wet-stretch
KE polarizer, was compared to that of other types of polarizers.
The wet-stretch KE polarizer was manufactured under conditions
similar to those for Sample 10.
[0124] The other types of polarizers included an iodine polarizer,
a dye stuff polarizer and a dry stretch KE polarizer (Dry KE). The
iodine polarizer had a layer of PVA with adsorbed iodine molecules,
sandwiched between two layers of cellulose triacetate (TAC), and
was taken from a Sharp model 13B2UA LCD television, supplied by
Sharp Electronics Corp, Mahwah, N.J. The dye stuff polarizer had a
layer of PVA with adsorbed dichroic dyes, sandwiched between two
layers of TAC, and was taken from a Philips active matrix display,
model no. LTE072T, supplied by Philips Consumer Electronics North
America, Atlanta, Ga.
[0125] The dry stretch polarizer was a K-type polarizer made using
a process that included a 7.times. dry stretching step at
182.degree. C. After dry stretching step, the film was exposed to
hydrochloric acid vapors and dehydrated by heating the fumed film
in an oven at a temperature in excess of 125.degree. C., for
example as discussed in U.S. Pat. No. 5,666,223.
[0126] The film was dipped into a first boration bath, held at a
temperature of 80.degree. C., that contained a solution of 7% boric
acid and 3% borax. The film relaxed in length by 10% when in the
first bath. The film was then dipped in a second boration bath, at
a temperature of 88.degree. C., that contained a solution of 9.5%
boric acid and 3% borax. The film was stretched to a ratio of 1.15
in the second boration bath and then stretched by another ratio of
1.06 after removal from the second boration bath, for an overall
stretch ratio of 7.7. The film was dried following the final
stretch.
[0127] The absorbance of each polarizer, as a function of
wavelength, was calculated from measurements made using the Cary
Model 5E spectrophotometer. The absorbance (optical density) for
each polarizer is shown in FIG. 10, as a function of wavelength,
for light polarized parallel to the transmission axis of each
polarizer. The absorbance curves for the iodine, dyestuff, dry
stretch and wet stretch polarizers are respectively numbered as
curves 1002, 1004, 1006 and 1008 in FIG. 10. The absorbance
(optical density) spectrum for each polarizer is shown in FIG. 11
for light polarized perpendicular to the transmission axis of each
polarizer. The absorbance curves for the iodine, dyestuff, dry
stretch and wet stretch polarizers are respectively numbered as
curves 1102, 1104, 1106 and 1108 in FIG. 11. Polarization and
transmission characteristics for the four different types of
polarizers are listed in Table XI, and the color characteristics
are listed in Table XIII.
TABLE-US-00011 TABLE XI Polarization and Transmission
Characteristics Polarizing Cont. Single Parallel Crossed Polarizer
co-eff. (%) D Ratio (Kv) (%) (%) (%) Iodine 99.98 120.0 4411 42.7
36.5 0.0083 Dye 99.85 37.4 686 37.8 28.6 0.0416 Dry KE 99.86 90.9
731 42.5 36.1 0.0494 Wet KE 99.96 110.9 2434 42.7 36.4 0.0149
TABLE-US-00012 TABLE XII Color Characteristics single single par.
par. crossed crossed Polarizer a* b* a* b* a* b* Iodine -2.054
4.356 -3.699 8.119 0.165 -0.480 Dye -0.948 3.484 -1.618 6.289
-0.063 -0.079 Dry KE 0.442 0.881 0.278 2.784 6.840 -12.767 Wet KE
-0.457 3.245 -0.806 6.160 0.778 -0.816
[0128] The dye stuff polarizer absorbs significantly more light in
the pass polarization state than for the other three types of
polarizer, shown by the high absorbance in FIG. 10. This gives rise
to the significantly lower values of Kv and parallel transmission
for the dye stuff polarizer listed in Table XI. Both the iodine and
dye stuff polarizer have layers of TAC, which absorbs light in the
blue: this explains the substantially identical absorbance in FIG.
10 for the iodine and dye stuff polarizers for wavelengths less
than about 430 nm. Both the wet-stretched and dry-stretched KE
polarizers, on the other hand, absorb less light in the wavelength
range below about 450 nm than the iodine and dye stuff polarizers.
It is believed that this relatively low value of blue light
absorption is due to the absence of TAC layers in the KE
polarizer.
[0129] Considering now the absorption curves presented in FIG. 11,
which show absorption of light polarized perpendicular to the
transmission axis, the iodine polarizer shows the highest value of
absorption (OD about 4.5) for wavelengths in the range 400 nm-700
nm. The wet-stretch KE polarizer, on the other hand, has a maximum
OD of about 4, and demonstrates levels of absorption similar to
that for the iodine polarizer for wavelengths between about 400 nm
and 550 nm. The absorption of the wet-stretch KE polarizer is
significantly higher in the range 400 nm-550 nm than for the
dry-stretch KE polarizer. This confirms that the process used to
manufacture the wet-stretch KE polarizer results in increased
numbers of short chain (low n) polyvinylene blocks.
[0130] Review of the polarization and transmission characteristics
listed in Table XI shows that the wet-stretch polarizer displays
better performance than the dyestuff polarizer and the dry-stretch
polarizer, and is comparable in most characteristics to the iodine
polarizer. Also, in terms of the color characteristics, listed in
Table XII, the wet stretch KE polarizer shows performance that is
as color neutral as, if not more neutral than, that of the iodine
polarizer. In particular, the hue of light transmitted through
parallel polarizers is more neutral with the wet-stretch KE
polarizer than the iodine polarizer, for both a* and b*. More
particularly, the magnitude of a* is less than one for the
wet-stretch KE polarizer, compared with a magnitude of more than 3
for the iodine polarizer. Also, the value of b* (6.1599) is less
for the wet-stretch KE polarizer than for the iodine polarizer
(8.1189).
[0131] For the crossed polarizer configuration, the magnitudes of
a* and b* are both slightly less for the iodine polarizer than for
the wet-stretch KE polarizer. However, the magnitudes of a* and b*
for the wet-stretch polarizer are both less than one, which means
that there is no perceptible hue, or only a barely perceptible hue,
for the wet-stretch polarizer. The color characteristics of the dye
stuff polarizer are similar to those of the wet-stretch polarizer
for parallel and crossed configurations, but the transmission and
polarization characteristics of the dye stuff polarizer are not as
good as those of the wet-stretch KE polarizer.
[0132] The absorption of light polarized perpendicular to the
transmission axis is higher in the red end of the spectrum for both
the iodine and dye stuff polarizers than for either the wet-stretch
KE polarizer or the dry-stretch polarizer, as is seen in FIG. 11.
This is not a significant advantage, however, for applications
where the light passing through the polarizer is to be viewed by
the human eye and/or where the light source illuminating the
polarizer has low output in the red portion of the spectrum.
[0133] When the wavelength dependent response of the human eye and
commonly used light sources are considered, the performance of the
wet-stretch KE polarizer is similar to that of the iodine
polarizer. FIG. 12 shows the photopically corrected transmission
for crossed polarizers for the four different types of polarizers,
where the illumination source is assumed to be the standard CCFT
source. The photopically corrected transmission curves for the
iodine, dyestuff, wet stretch and dry stretch polarizers are
respectively labeled 1202, 1204, 1206 and 1208. The area under each
curve corresponds to the photopically corrected transmission
through the crossed polarizer pair. Both the dye stuff and
dry-stretch polarizers allow significant leakage of light at the
550 nm peak, and the dry-stretch polarizer also transmits
significant amounts of light in the blue region of the spectrum.
The transmission through crossed wet-stretch polarizers is almost
exactly the same as through crossed iodine polarizers, except for
small differences at about 580 nm and 610 nm. These differences,
however, are small, as is evidenced by the fact that magnitudes of
a* and b* are both less than one for the crossed, wet-stretch
polarizer.
[0134] Thus, a KE polarizer manufactured in accordance with the
description herein shows transmission, polarization and color
properties that are substantially the same as iodine polarizers. KE
polarizers are intrinsic and, unlike iodine or dye stuff
polarizers, do not require the adsorption of dichroically absorbing
species, and do not require cover layers, such as TAC, for
environmental stability. Thus, a practical KE polarizer may be made
thinner than either iodine or dye stuff polarizers, has a less
complex structure than iodine or dye stuff polarizer, and is less
expensive to manufacture than iodine or dye stuff polarizers.
Furthermore, the wet-stretch polarizers are more able to withstand
conditions of high humidity than iodine or dye stuff
polarizers.
[0135] Intrinsic polarizers as described herein may be used with
other layers. For example, a polarizer may be used with a substrate
to provide structural support, or may be used with a liquid crystal
display. The other layers used with the polarizer may be isotropic
or may be birefringent.
[0136] The present invention should not be considered limited to
the particular examples described above, but rather should be
understood to cover all aspects of the invention as fairly set out
in the attached claims. Various modifications, equivalent
processes, as well as numerous structures to which the present
invention may be applicable will be readily apparent to those of
skill in the art to which the present invention is directed upon
review of the present specification. The claims are intended to
cover such modifications and devices.
* * * * *