U.S. patent application number 12/443342 was filed with the patent office on 2010-02-11 for adhesives inhibiting formation of artifacts in polymer based optical elements.
This patent application is currently assigned to 3M INNOVATIVE PROPERTIES COMPANY. Invention is credited to Kathleen E. Allen, Beverly J. Blake, Charles L. Bruzzone, Erin L. Coleman, James P. DiZio, Maureen C. Nelson.
Application Number | 20100033816 12/443342 |
Document ID | / |
Family ID | 39268789 |
Filed Date | 2010-02-11 |
United States Patent
Application |
20100033816 |
Kind Code |
A1 |
DiZio; James P. ; et
al. |
February 11, 2010 |
ADHESIVES INHIBITING FORMATION OF ARTIFACTS IN POLYMER BASED
OPTICAL ELEMENTS
Abstract
Inhibiting formation of optical artifacts in a multi-layer film
polarizer of an optical imaging assembly that includes a polarizing
beam splitter. The beam splitter may include a multilayer
reflective polarizing film having at least two materials, one of
which may exhibit birefringence after uniaxial orientation; an
adhesive disposed on the multilayer reflective polarizing film; and
at least a first prism disposed on the adhesive. The adhesive may
include a plasticizer for inhibiting formation of optical artifacts
in the polarizing film.
Inventors: |
DiZio; James P.; (Saint
Paul, MN) ; Nelson; Maureen C.; (West Saint Paul,
MN) ; Allen; Kathleen E.; (Crescent Springs, KY)
; Blake; Beverly J.; (Loveland, OH) ; Coleman;
Erin L.; (Lebanon, OH) ; Bruzzone; Charles L.;
(Woodbury, MN) |
Correspondence
Address: |
3M INNOVATIVE PROPERTIES COMPANY
PO BOX 33427
ST. PAUL
MN
55133-3427
US
|
Assignee: |
3M INNOVATIVE PROPERTIES
COMPANY
Saint Paul
MN
|
Family ID: |
39268789 |
Appl. No.: |
12/443342 |
Filed: |
September 25, 2007 |
PCT Filed: |
September 25, 2007 |
PCT NO: |
PCT/US2007/079389 |
371 Date: |
March 27, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60827659 |
Sep 29, 2006 |
|
|
|
Current U.S.
Class: |
359/489.11 ;
427/532; 560/190; 568/852 |
Current CPC
Class: |
G02B 27/283 20130101;
B29C 65/483 20130101; C09J 11/06 20130101; G02B 5/04 20130101; B29D
11/00644 20130101; C08K 5/11 20130101; G02B 1/04 20130101; B29K
2995/0026 20130101; C09J 163/00 20130101; C08K 5/053 20130101; B29L
2011/0066 20130101; G02B 5/305 20130101 |
Class at
Publication: |
359/495 ;
359/485; 427/532; 568/852; 560/190 |
International
Class: |
G02B 27/28 20060101
G02B027/28; G02B 5/30 20060101 G02B005/30; B05D 3/06 20060101
B05D003/06; C07C 31/20 20060101 C07C031/20; C07C 69/347 20060101
C07C069/347 |
Claims
1. A polarizing beam splitter, comprising: a polymer based
polarizing film; an adhesive layer disposed on the polarizing film;
and a first optical element disposed on the adhesive layer, wherein
the adhesive layer comprises a crystallization inhibiting
plasticizer component in an amount effective for inhibiting
formation of (i) crystallized domains; (ii) particles; or (iii) a
combination thereof, that cause optical artifacts in the polarizing
film.
2. The polarizing beam splitter according to claim 1, further
comprising a second plasticizer component in the adhesive
layer.
3. The polarizing beam splitter according to claim 2, wherein the
first and second plasticizer components are from a group consisting
of: glycols, such as ethylene gycol, polypropylene glycol,
di(ethylene glycol) ethyl ether, and triethylene glycol; aromatic
or aliphatic esters, such as di(2-ethyhexyl) adipate and
di(ethylene glycol) benzoate; and water.
4. The polarizing beams splitter according to claim 1, wherein the
adhesive is from a group consisting of multifunctional or
monofunctional, aromatic or aliphatic epoxy resins along with
multifunctional or monofunctional aromatic or aliphatic alcohol or
amine containing curatives.
5. The polarizing beam splitter according to claim 1, wherein the
first plasticizer component amount included in the adhesive varies
from between about 1 and 30 weight percent of the adhesive
layer.
6. The polarizing beam splitter according to claim 5, wherein the
first plasticizer component amount included in the adhesive varies
from between about 1 and 5 weight percent of the adhesive
layer.
7. The polarizing beam splitter according to claim 1, wherein the
crystallization inhibiting plasticizer inhibits formation of (i)
crystallized domains; (ii) particles; or (iii) a combination
thereof, when subjected to light having wavelengths longer than
about 420 nm.
8. The polarizing beam splitter according to claim 1, wherein the
polarizing film is a multilayer polyester polarizing film.
9. An optical apparatus comprising: a polymer based optical
element; an adhesive disposed on the optical element; wherein the
adhesive comprises a crystallization inhibiting plasticizer
component in an amount effective for inhibiting formation of (i)
crystallized domains; (ii) particles; or (iii) a combination
thereof in the optical element that cause optical artifacts in the
optical element.
10. A transparent adhesive composition comprising a crystallization
inhibiting plasticizer component in an amount effective for
inhibiting formation of (i) crystallized domains; (ii) particles;
or (iii) a combination thereof, that cause optical artifacts in an
adjacent polymer based optical element.
11. The transparent adhesive composition according to claim 10,
wherein the crystallization inhibiting plasticizer component is
from a group consisting of: glycols, such as ethylene gycol,
polypropylene glycol, di(ethylene glycol) ethyl ether and
triethylene glycol; aromatic or aliphatic esters, such as
di(2-ethyhexyl) adipate and di(ethylene glycol) benzoate; and
water.
12. The transparent adhesive composition according to claim 10,
wherein the adhesive is from a group consisting of multifunctional
or monofunctional, aromatic or aliphatic epoxy resins along with
multifunctional or monofunctional aromatic or aliphatic alcohol or
amine containing curatives.
13. The transparent adhesive composition according to claim 11
wherein the plasticizer component amount included in the adhesive
varies from between about 1 and 30 weight percent of the adhesive
layer.
14. The transparent adhesive composition according to claim 13
wherein the plasticizer component amount included in the adhesive
varies from between about 1 and 5 weight percent of the adhesive
layer.
15. A polarizing beam splitter comprising: (a) a birefringent film
comprising a plurality of first material layers and a plurality of
second material layers, wherein the first material layers comprise
a polymer selected from a group consisting of polyethylene
terephthalate and copolymers of polyethylene terephthalate and
polyethylene naphthalate and the second material layers comprise a
copolyester; (b) at least one prism comprising a base adjacent a
first major surface of the birefringent film; and, (c) an adhesive
disposed between the first major surface and the birefringent film,
wherein the adhesive comprises at least a crystallization
inhibiting plasticizer in an amount effective for controlling the
migration of the plasticizer to at least an adjacent optical
polarizing film for inhibiting crystallization in the polarizing
film.
16. An optical assembly, comprising: (a) the polarizing beam
splitter of claim 1, a first path being defined through the
polarizing beam splitter for light in a first polarization state;
and (b) at least one imager disposed to reflect light back to the
polarizing beam splitter, portions of light received by the at
least one imager being polarization rotated, polarization rotated
light propagating along a second path from the imager and through
the polarizing beam splitter.
17. A projection system, comprising: (a) a light source to generate
light; (b) conditioning optics to condition the light from the
light source; (c) an imaging core to impose an image on conditioned
light from the conditioning optics to form image light, the imaging
core including at least one polarizing beam splitter of claim 1 and
at least one imager; and (d) a projection lens system to project
the image light from the imaging core.
18. A method of stabilizing a polymer based optical element, the
method comprising: disposing a material on a polymer based optical
element, the material comprising a crystallization inhibiting
plasticizer component in an amount effective for inhibiting
formation of (i) crystallized domains; (ii) particles; or (iii) a
combination thereof, that cause optical artifacts in an adjacent
polymer based optical element; and, passing radiation through the
material and the optical element.
19. The method of claim 18, wherein the material comprises an
adhesive.
20. The method of claim 19 wherein the disposing comprises an
adhesive layer wherein the crystallization inhibiting plasticizer
component is from a group consisting of: glycols, such as ethylene
glycol, polypropylene glycol, di(ethylene glycol) ethyl ether, and
triethylene glycol; aromatic or aliphatic esters, such as
di(2-ethyhexyl) adipate and di(ethylene glycol) benzoate; and
water.
21. The method of claim 20, wherein the disposing comprises having
the crystallization inhibiting plasticizer component included in
the adhesive by an amount that varies from between about 1 and 30
weight percent of the adhesive layer.
22. The method of claim 20 wherein the disposing comprises having
the crystallization inhibiting plasticizer component included in
the adhesive by an amount that varies from between about 1 and 5
weight percent of the adhesive layer.
23. The method of claim 21 wherein the disposing comprises having
adhesive from a group consisting of multifunctional or
monofunctional, aromatic or aliphatic expoxy resins along with
multifunctional or monofunctional aromatic or aliphatic alcohol- or
amine-containing curatives.
24. The method of claim 18, wherein the passing radiation has a
wavelength that is longer than about 420 nm.
Description
BACKGROUND
[0001] Polarizing beam splitter (PBS) assemblies are found in a
variety of optical imaging assemblies, such as front and rear
projection systems, projection displays, head-mounted displays,
virtual viewers, head up displays, optical computing, optical
correlation, and other similar optical viewing and display systems.
A PBS assembly may include at least one multilayer reflective
optical polarizing film (MOF). In general, a MOF is a multi-layer
polymer based film that functions as a polarizer and contains at
least two different materials, at least one of which exhibits
birefringence after uniaxial orientation. The MOF film is
sandwiched between two prisms and functions to reflect a particular
polarization of light while transmitting orthogonal polarizations.
An adhesive is disposed between the MOF and a surface of at least
one of the prisms to provide for structural integrity of the PBS
assembly as well as provide optical coupling.
[0002] One example of a PBS assembly use is in a liquid crystal on
silicon (i.e., LCOS) rear projection television system. LCOS rear
projection television systems generate relatively high amounts of
light energy and heat. Impingement onto the PBS assembly of such
light energy levels and heat, especially over time, can have
significant adverse effects on the average lifetimes of PBS
assemblies. More particularly, relatively high amounts of light
energy and heat can reduce transmissivity of the MOF over time.
SUMMARY
[0003] There is a need for continuing improvements, whereby the
stability and/or useful life of optical imaging assemblies,
particularly those utilizing polarizing elements, can be extended
in a manner that does not otherwise compromise their optical
benefits.
[0004] The present description includes a polarizing beam splitter,
comprising a polymer based polarizing film. An adhesive layer is
disposed on the polarizing film and a first optical element is
disposed on the adhesive layer. The adhesive layer includes at
least a crystallization inhibiting plasticizer component in an
amount effective for inhibiting formation of (i) crystallized
domains; (ii) particles; or (iii) a combination thereof that cause
optical artifacts in the polarizing film.
[0005] In another aspect, the present description provides an
optical apparatus comprising a polymer based optical element and an
adhesive disposed on the optical element. The adhesive comprises a
crystallization inhibiting plasticizer component in an amount
effective for inhibiting formation of (i) crystallized domains;
(ii) particles; or (iii) a combination thereof in the optical
element that cause optical artifacts in the optical element.
[0006] Yet another aspect of the present description provides a
polarizing beam splitter comprising a birefringent film having a
plurality of first material layers and a plurality of second
material layers. The first material layers include a polymer
selected from the group consisting of polyethylene terephthalate
and copolymers of polyethylene terephthalate and polyethylene
naphthalate. The second material layers include a copolyester. The
polarizing beam splitter further includes at least one prism
comprising a base adjacent a first major surface of the
birefringent film and an adhesive disposed between the first major
surface and the birefringent film. The adhesive comprises at least
a crystallization inhibiting plasticizer in an amount effective for
inhibiting crystallization and/or particle formation in the
polarizing film.
[0007] Another aspect of the description provides a method of
stabilizing a polymer based optical element comprising disposing an
adhesive on a polymer based optical element. The adhesive comprises
a crystallization inhibiting plasticizer component in an amount
effective for inhibiting formation of (i) crystallized domains;
(ii) particles; or (iii) combinations thereof, that cause optical
artifacts in an adjacent polymer based optical element. The method
further includes passing radiation through the adhesive and the
optical element wherein crystallized domains and/or particles are
inhibited from forming in the optical element.
BRIEF DESCRIPTION OF THE DRAWING
[0008] This description may be more completely understood in
consideration of the following detailed description of various
embodiments in connection with the accompanying drawings, in
which:
[0009] FIG. 1 schematically illustrates an embodiment of a PBS
having a multilayer reflective polarizing film;
[0010] FIG. 2 schematically illustrates an enlarged cross-section
of a multilayer reflective polarizing film and adhesive;
[0011] FIG. 3 schematically illustrates an enlarged perspective of
a portion the multilayer reflective polarizing film;
[0012] FIG. 4 schematically illustrates a projection system;
[0013] FIG. 5 schematically illustrates another projection system;
and,
[0014] FIG. 6 illustrates a graph comparing a PBS having an
adhesive(s) to a PBS that does not have such adhesive(s).
DETAILED DESCRIPTION
[0015] One aspect of this description relates to adhesives utilized
in combination with a polymer based polarizing film for avoiding
optical artifacts induced in such film when used, for example, in
an optical imaging system that includes a Cartesian polarizing beam
splitter ("PBS") of a kind used in projection systems. Particularly
useful embodiments of the invention address the use of PBS's that
are subject to continued exposures under relatively high light
energy and heat conditions which tend to reduce stability and/or
useful life of standard PBS assemblies. For example, after extended
periods of use some MOF films can yellow and char in a relatively
short time period following initiation of the formation of haze in
the MOF films. This rapid and pronounced failure is a special
concern when utilized in extremely expensive projection systems,
such as rear projection television systems.
[0016] It has been determined that the haze formed in polarizing
films of PBS assemblies may be influenced by humidity and that
average lifetimes are generally lower in dryer environments than in
wetter environments. Accordingly, the average lifetimes of such PBS
assemblies while operated under such relatively high light
irradiance and associated heat especially in dry conditions are
diminished significantly.
[0017] While the following illustrated embodiments are described in
the context of such a projection system, the principles and scope
of the present invention are not limited thereby. Rather, the
present invention is more broadly directed to adhesives
particularly adapted for use in combination with polymer based
optical elements, whereby optical artifacts, such as haze, can be
minimized or eliminated. Hence, optical imaging systems described
herein are exemplary of many different kinds in which the present
invention is useful.
[0018] FIG. 1 illustrates one embodiment of a polarizing beam
splitter (PBS) assembly 10. A PBS assembly is an optical component
that splits incident light rays into a first (transmitted)
polarization component and a second (reflected) polarization
component. PBS assembly 10 includes a thin film, polymer based,
multilayer reflective polarizing film 12, at least one adhesive
layer 14 adhered between mutually opposing surfaces of multilayer
reflective polarizing film 12 and an optical substrate, such as
prism 16; at least one adhesive layer 18 adhered between mutually
opposing surfaces of multilayer reflective polarizing film 12 and
an optical substrate, such as prism 20. PBS assembly 10 is similar
to one described in commonly assigned U.S. Pat. No. 6,486,997 and
to copending and commonly assigned US Patent Publication No.
2005/0168697A1.
[0019] While PBS assembly 10 illustrates multilayer reflective
polarizing film 12 sandwiched between prisms 16 and 20, it will be
appreciated that multilayer reflective polarizing film 12 may be
adhered to just one surface of one of the optical prisms.
Multilayer reflective polarizing film 12 may function as a
polarizer and may contain at least two different materials, at
least one of which may exhibit birefringence after uniaxial
orientation.
[0020] Although depicted as including two prisms 16 and 20, PBS
assembly 10 may include any suitable optical elements, such as
optical substrates or the like disposed on one or either side of
multilayer reflective polarizing film 12. Prisms 16 and 20 can be
constructed from any light transmissive material having a suitable
refractive index to achieve the desired purpose of the PBS
assembly. The prisms should have refractive indices less than that
which would create a total internal reflection condition for beams
of light with the intended f-number. Prisms 16 and 20 are typically
made of isotropic materials, although other materials can be used.
Typical materials for use as prisms include, but are not limited
to, ceramics, glass, and polymers. For environmental purposes, a
more particular category of glass includes glasses that do not
contain lead oxide, but can contain other metallic oxides, such as
boron oxide. A more typical example is a commercially available
glass N-SK-5, available from Schott, Corp. of Dureya, Pa., USA
having no lead content and an index of refraction of about
1.584.
[0021] More particularly, PBS assembly 10 can have prism 16 and
prism 20. Prism 16 is the prism that first accepts the incoming
light from a light source. The adhesive carries additives that
inhibit the formation of optical artifacts that diminish
transmissivity in polymer based optical elements while the PBS
assembly is subjected to high intensity light for significant
exposure times. Adhesives with such additives are preferably placed
at least between prism 16, which is closest to the source of light,
and multilayer reflective polarizing film 12.
Polarizing Element
[0022] As illustrated in FIGS. 2 and 3, multilayer reflective
polarizing film 12 is a polarizer such as described in commonly
assigned U.S. Pat. No. 6,609,795. It operates to discriminate
between nominally s- and p-polarized light. In particular,
multilayer reflective polarizing film 12 may be birefringent and
may have at least two materials of different refractive index, one
of which exhibits birefringence after uniaxial orientation. In one
embodiment, multilayer reflective polarizing film 12 is comprised
of a plurality of stacked alternating first and second
light-transmissive layers 22.sub.a-n (collectively 22) and
24.sub.a-n (collectively 24) disposed between the adhesive layers
14, 18. While multilayer reflective polarizing film 12 may
generally comprise hundreds of layers, this simplified illustration
exemplifies the principles of operation of the polarizer. Each of
first and second layers 22, 24 is provided with refractive indices
in the x, y, and z directions. For example, first layer 22 may have
a first set of refractive indices n.sub.22x, n.sub.22y, n.sub.22z
and second layer 24 may each have a second set of refractive
indices, n.sub.24x, n.sub.24y, and n.sub.24z.
[0023] Another aspect of multilayer reflective polarizing film 12
is that the materials of first and second layers 22, 24 begin as
isotropic materials (i.e., having substantially similar refractive
indices in the x, y, and z directions) and after uniaxial
orientation, at least one of the materials exhibits birefringence.
There are three possible combinations: (1) the first material
exhibits birefringence while the second material remains isotropic,
(2) the first material remains isotropic while the second material
exhibits birefringence, and (3) the first and second both exhibit
birefringence. In a preferred embodiment, after uniaxial
orientation, the first material is birefringent and experiences an
increase in refractive index along the stretched direction while
the second material remains isotropic and the refractive index
difference between the first and second material is typically
between specified value 0.15 and 0.25 in the stretch direction.
[0024] It will be appreciated that the composition of multilayer
reflective polarizing film 12 can vary considerably given the
optical properties desired for a particular optical system that is
to be constructed, such as the PBS assembly 10. As noted, the broad
scope of this invention is not limited to the particular
composition of a polymeric multilayer reflective polarizing film 12
or its method of fabrication. Therefore, the description of
multilayer reflective polarizing film 12 in this embodiment is for
use in an optical system. PBS assembly 10 is but one example of
many that can be used.
[0025] In a preferred embodiment, the polarizing film has first and
second layers 22, 24 made of first and second polymers;
respectively. For example, first layer 22 may be a relatively high
index layer while second layer 24 may be a low index layer. These
relative values refer to the indices observed along the x direction
of multilayer reflective polarizing film 12. Many relatively high
and low index polymeric materials may be used. In this embodiment,
the relatively high index polymer in first layer 22 may be a
polyethylene naphthalate (PEN) film. The low index second layer 24
may be a polyethylene terephthalate (PET) film. Other suitable
polymeric materials can serve as the birefringent/isotropic layers.
The indices of first and second layers 22, 24 can be suitably
composed and/or fabricated to exhibit the desired optical
properties necessary to achieve the kinds and degrees of optical
properties, such as polarizations, sought. For example, to modify
the optical properties at least one of first or second layers 22,
24, the layer is stretched uniaxially in specified directions by
known means, whereby for example, first layer 22 can have its
collective refractive index modified. Typically many low index
polymeric materials may be stretched. One typical low index polymer
film may a polyethylene terephthalate (PET) film. Reference is made
to commonly assigned U.S. Pat. No. 6,609,795 for a description of
examples of the materials that may be used for making multilayer
reflective polarizing film 12. Suitable polymers for use as the low
index layer when PET is used as the relatively high index layer are
as follows. It may be desirable for the low index polymers to
remain isotropic upon uniaxial orientation at typical PET draw
temperatures. Thus, the low index polymers may have a glass
transition temperature below that of PET (i.e., less than about
80.degree. degrees C.). The low refractive index polymers may have
one or more the following properties: (1) thermal stability at PET
melt processing temperatures, (2) UV stable or UVA protectable, (3)
relatively high clarity (i.e., relatively high transmission and low
absorption), (4) theological properties close enough to PET for
stable flow during coextrusion, (5) good interlayer adhesion with
PET, (6) low dispersion, and (7) drawability (i.e., the ability to
be oriented) without birefringence. The present invention
contemplates that other kinds of polymer based optical elements or
polarizers, such as retarder films, absorptive polarizes, PEN- or
polycarbonate (PC)-containing polarizers, wavelength plates, color
filters, mirrors, color mirrors, and other suitable types. Clearly,
other polymeric materials are possible and other polymer materials
can be used as first and second material layers so long as the
criteria discussed herein have been satisfied.
Adhesives
[0026] Reference is made also to FIG. 2 for illustrating one
embodiment of adhesive layer 14 made consistent with the teachings
of this description. Adhesive layer 14 may be any suitable optical
or transparent adhesive composition used for bonding optical
elements, such as PBS assemblies. As used in this regard,
"transparent" means that the transparent adhesive composition layer
must allow an effective proportion of incident radiation to pass
therethrough. The present description envisions that it is
applicable to more than visible light and is intended to embrace
inhibiting crystallization or particle formation that creates
artifacts for the transmission of wavelengths outside the visible
spectrum. The artifacts that are minimized or eliminated are
primarily haze that causes light scattering due to crystalline
domains and/or particles greater than about 100 nm in size.
Scattering will generally occur when the size of the crystalline
domains and/or particles are about 1/4 wavelength (e.g. 100
nanometers), and as such cause a whitish haze in the polarizing
film. As used in the present application, the term "crystallized
domains" means nodules that form in the polymer that are large
enough to form haze and includes nodules/particles that are a
compilation of crystallized polymer chains and/or fragments
thereof.
[0027] The adhesive composition of this description contains one or
more plasticizer components in an amount effective for inhibiting
the formation of optical artifacts primarily in polymer based
optical elements in PBS assemblies. While polymer based PBS optical
elements are discussed, it will be understood that the present
invention is directed to the use of one or more plasticizer
components in the adhesive that act as a crystallization inhibitor
for inhibiting formation of relatively large domains and/or
particles (e.g. 0.1-0.3 M or 100-300 nanometers in size) in any
polymer based optical element adhered to an adhesive. As such,
optical artifacts created by crystallization/or particle formation
due to exposures to specific radiation over certain periods of time
are inhibited. For example, such specific radiation may include
light having wavelengths in or above the blue range, (e.g.,
wavelengths longer than 420 nm.), with light intensity that is
significantly higher than light intensity that the polarizing film
would experience in a reference rear projection TV. As a result,
the diminishment of artifacts prolongs the stability and/or useful
life of the optical elements for their intended purposes.
[0028] The transparent adhesive composition resin composition may
be acrylic, vinyl, ether, epoxy, or urethane based. In this
embodiment, it may be a curable resin, such as a transparent or
optical adhesive resin composition. The transparent or optical
adhesive resin composition possesses high strength and
low-viscosity and contains additives that allow it to be cured
either by exposure to elevated temperatures, or upon exposure to UV
and visible light in a relatively short period of time. Other
suitable curatives are contemplated by this invention. The
transparent adhesive composition may also be a pressure sensitive
adhesive. The transparent adhesive resin composition may comprise
one or more epoxy resins in an amount that can vary depending on
the desired properties and uses of the resulting composition. In
this regard, typical suitable epoxy transparent resins include, but
are not limited to, epoxy resins from a group consisting of
multifunctional or monofunctional, aromatic or aliphatic epoxy
resins along with multifunctional or monofunctional aromatic or
aliphatic alcohol or amine containing curatives.
[0029] The transparent adhesive resin composition used in such PBS
systems also contains other additives besides the one or more
plasticizer components and additives noted above. These other
additives may include, for example, light stabilizers such as an
excited state quencher, an antioxidant, a UV absorber, and a
radical scavenger. The transparent adhesive resin composition may
also comprise one or more curatives in a suitable amount. In this
embodiment, the transparent adhesive resin composition is disposed
on a surface as a thin layer. It can be applied in any suitable
manner, such as bar coating or as metered drops that are allowed to
spread. Typically, the thickness of the applied transparent
adhesive resin composition may range from about 1 .mu.M to about
200 .mu.M. More typically, the thickness may range from about 10
.mu.M to about 60 .mu.M. Other thickness ranges of the transparent
adhesive resin composition can be used depending on the desired
properties and use of the composition as well as the ingredients of
the composition. If the thickness is too thin then not enough of
the plasticizer is able to migrate to the polarizing film so as to
inhibit formation of the optical artifacts therein.
[0030] While the exemplary embodiments herein may be described in
the context of an adhesive for delivering a plasticizer, the
principles and scope of the present disclosure are not limited
thereby. Rather, the present disclosure is more broadly directed to
compositions that may be particularly adapted for use in allowing
an effective amount of a plasticizer to migrate to delay formation
of the haze by thwarting or inhibiting crystallization. Such a
composition should also be transparent as that term is used in the
present application. It will be also be appreciated that the
plasticizer may be directly applied, such as by coating or other
suitable approach.
Plasticizers
[0031] According to one aspect of the present description one or
more plasticizer components may be included in the adhesive. It was
deduced by the present investigators that the optical artifacts,
such as haze, are induced in multilayer reflective polarizing film
12 when exposed to relatively high levels of incident radiant
energy and heat. These relatively high levels are experienced in,
for example, LCOS projection systems. It is believed that the noted
shortcomings of the polymer based optical elements are due to light
flux having caused multilayer reflective polarizing film 12 to
undergo chain scission due to, for example, known Norrish cleavage
and other reactions. As a result, it has been observed that this
tends to lower the molecular weight (MW), thereby creating cleaved
sections of the chain that are more mobile than the original
polymer chain. It was deduced that these lower MW sections tend to
concentrate or agglomerate over time in multilayer reflective
polarizing film 12. Such concentration or agglomeration tends to
create haze in multilayer reflective film 12 thereby diminishing
the latter's stability and transmissivity.
[0032] Slight haze formation also enables light scatter, allowing
the high intensity light to more effectively interact with the
polymer, which effectively increases the damage. The increased
mobility and growing concentrations of the lower MW species allows,
for example, PET and/or PEN moieties, to even further concentrate
to the point of crystallization or particle formation of such
moieties. It was further deduced that heat enables increased
movement of the lower MW species as well as the scission reactions,
thereby increasing the rate of crystallization. The resulting
particles eventually grow to sizes that create what is observed as
a `whitish` haze in multilayer reflective polarizing film 12.
[0033] This scenario is very different from typical degradation
that is experienced in polymers of this type, such as by
ultraviolet radiation. Typically, a yellowing is noticed as the
first optical sign of light/heat induced degradation and there are
specific strategies based on absorbing the offending light and
minimizing the resulting yellow in the film. For example, UV
inhibitors may inhibit photodegradation of optical polymer films.
However, the present description addresses a different strategy,
which is to thwart the non-typical haze (i.e., whitish haze) that
is generated in the PBS assembly by crystallization or particle
formation.
[0034] The crystallization inhibitors of the present invention are
plasticizers, which tend to inhibit the growth of such crystals or
particles in multilayer reflective polarizing film 12. According to
the present invention, the plasticizers are chosen from a general
group. Some examples of typical plasticizers consist of glycols,
such as ethylene glycol, polypropylene glycol, di(ethylene glycol)
ethyl ether and triethylene glycol, aromatic or aliphatic esters,
such as di(2-ethylhexyl) adipate, di(ethylene glycol) benzoate, and
water.
[0035] Such plasticizers tend to migrate between an adhesive layer
and multilayer reflective polarizing film 12 adhered thereto, and
thence into the film, whereby crystallization of, for example, PET
and/or PEN moieties, are diminished significantly in the adjacent
film layers. The amount of the plasticizer and migration ability
are important parameters in delaying formation of the haze.
[0036] It will be appreciated that more than one plasticizer can be
used according to the present description. In one embodiment, both
ethylene glycol and water can be used. Water is thought to prevent
the crystallization by physically inserting itself between two
potentially crystallizable materials, getting in the way by
association and steric hindrance. Other combinations of plasticizer
components can be used. Examples of such combinations include but
are not limited to: general glycols and water; glycols and esters;
esters and water; and aromatic or aliphatic moieties that interact
with the polymers making up the MOF.
[0037] In one illustrated embodiment, the transparent adhesive
resin composition for use in this application comprises one or more
epoxy resins in an amount of up to about 99.5 weight percent, based
on the total weight of the composition. The epoxy composition of
this description may comprise one or more epoxy resins in an amount
of from about 75 weight percent to about 98 weight percent, based
on the total weight of the composition. Further, the epoxy
composition of this invention may comprise one or more epoxy resins
in an amount of from about 94 weight percent to about 97 weight
percent, based on the total weight of the composition.
[0038] The plasticizer can be present in the transparent adhesive
resin composition in an amount that will obtain the desired degree
of prevention of formation of the artifacts. The amount of one
crystallization inhibiting plasticizer component employed in the
adhesive material may vary widely but it usually forms between
about 1 and 30 weight percent, or even between about 1 and about 5
weight percent.
[0039] The transparent adhesive resin composition of this invention
may contain up to about 3.5 weight percent, or even up to about 6
percent, of various additives such as fillers, stabilizers,
adhesion promoters (for example, silica, silanes, antioxidants,
radical scavengers, excited state quenchers, and UV absorbers, and
the like, so as to reduce the weight and/or cost of the epoxy
composition, adjust viscosity, provide additional reinforcement,
modify the transparency of the epoxy compositions and optical
assemblies, and/or to stabilize the PBS assembly from
degradation.
[0040] One embodiment of an optical imager system is illustrated in
FIG. 4, where system 410 includes light source 412, for example arc
lamp 414 with reflector 416 to direct light 418 in a forward
direction. Light source 412 may also be a solid state light source,
such as a light emitting diode or a laser light source. System 410
also includes PBS 420, e.g., the single or multi-film PBS described
herein. Light with x-polarization, i.e., polarized in a direction
parallel to the x-axis, is indicated by the circled x. Light with
y-polarization, i.e., polarized in a direction parallel to the
y-axis, is indicated by a solid arrow. Solid lines indicate
incident light, while dashed lines indicate light that has been
returned from reflective imager 426 with a changed polarization
state. Light provided by light source 412 can be conditioned by
conditioning optics 422 before illuminating PBS 420. Conditioning
optics 422 change the characteristics of the light emitted by
source 412 to characteristics that are desired by the projection
system. For example, conditioning optics 422 may alter any one or
more of the divergence of the light, the polarization state of the
light, the spectrum of the light. Conditioning optics 422 may
include, for example, one or more lenses, a polarization converter,
a pre-polarizer, and/or a filter to remove unwanted ultraviolet or
infrared light.
[0041] The x-polarized components of the light are reflected by PBS
420 to reflective imager 426. The liquid crystal mode of reflective
imager 426 may be smectic, nematic, or some other suitable type of
reflective imager. If reflective imager 426 is smectic, reflective
imager 426 may be a ferroelectric liquid crystal display (FLCD).
Reflective imager 426 reflects and modulates an image beam having
y-polarization. The reflected y-polarized light is transmitted
through PBS 420 and is projected by projection lens system 428, the
design of which is typically optimized for each particular optical
system, taking into account all the components between projection
lens system 428 and the imager(s). Controller 452 is coupled to
reflective imager 426 to control the operation of reflective imager
426. Typically, controller 452 activates the different pixels of
reflective imager 426 to create an image in the reflected
light.
[0042] An embodiment of multi-imager projection system 500 is
schematically illustrated in FIG. 5. Light 502 is emitted from
source 504. Source 504 may be an arc or filament lamp, or any other
suitable light source for generating light suitable for projecting
images. Source 504 may be surrounded by reflector 506, such as an
elliptic reflector (as shown), a parabolic reflector, or the like,
to increase the amount of light directed towards the projection
engine.
[0043] Light 502 is typically treated before being split into
different color bands. For example, the light 502 may be passed
through optional pre-polarizer 508, so that only light of a desired
polarization is directed towards the projection engine. The
pre-polarizer may be in the form of a reflective polarizer, so that
reflected light, in the unwanted polarization state, is redirected
to source 504 for re-cycling. Light 502 may also be homogenized so
that the imagers in the projection engine are uniformly
illuminated. One approach to homogenizing light 502 is to pass
light 502 through reflecting tunnel 510, although it will be
appreciated that other approaches to homogenizing the light may
also be employed.
[0044] In the illustrated embodiment, homogenized light 512 passes
through first lens 514 to reduce the divergence angle. Homogenized
light 512 is then incident on first color separator 516, which may
be, for example, a dielectric thin film filter. First color
separator 516 separates light 518 in a first color band from
remaining light 520.
[0045] Light 518 in the first color band may be passed through
second lens 522, and optionally third lens 523, to control the beam
size of light 518 in the first color band incident on first PBS
524. Light 518 passes from first PBS 524 to first imager 526.
Imager 526 reflects image light 528 in a polarization state that is
transmitted through PBS 524 to x-cube color combiner 530. Imager
526 may include one or more compensation elements, such as a
retarder element, to provide additional polarization rotation and
thus increase contrast in the image light.
[0046] Remaining light 520 may be passed through fourth lens 532.
Remaining light 520 is then incident on second color separator 534,
for example a thin film filter or the like, to produce light beam
536 in a second color band and light beam 538 in a third color
band. Light beam 536 in the second color band is directed to second
imager 540 via second PBS 542. Second imager 540 directs image
light 544 in the second color band to x-cube color combiner
530.
[0047] Light beam 538 in the third color band is directed to third
imager 546 via third PBS 548. Third imager 546 directs image light
550 in the third color band to x-cube color combiner 530.
[0048] Image light 528, 544 and 550 in the first, second and third
color bands is combined in x-cube color combiner 530 and directed
as a full color image beam to projection optics 552. Polarization
rotating optics 554, for example half-wave retardation plates or
the like, may be provided between PBS 524, 542 and 548 and x-cube
color combiner 530 to control the polarization of the light
combined in x-cube color combiner 530. In the illustrated
embodiment, polarization rotating optics 554 are disposed between
x-cube color combiner 530 and first PBS 524 and third PBS 548. Any
one, two, or all three of PBS 524, 542, and 548 may include one or
more multilayer reflective polarizing films as described
herein.
[0049] It will be appreciated that variations of the illustrated
embodiment may be used. For example, rather than reflect light to
the imagers and then transmit the image light, the PBS may transmit
light to the imagers and then reflect the image light. The above
described projection systems are only examples; a variety of
systems can be designed that utilize the multi-film PBS of the
present invention.
Examples
[0050] The polyester multi-layer reflective polarizing films used
were similar in construction. The adhesives were made in accordance
with general methods of making adhesives. Of course, each of the
adhesives represented a different example of at least one
crystallization inhibiting plasticizer component according to the
present invention.
Experimental Setup
General
[0051] PBS assemblies were tested with a light-irradiating device
that focused substantial light onto the multilayer optical film
(MOF) (inside the PBS). The PBSs' contained MOF film designed to
reflect a certain polarization of "blue light." The incident light
was filtered to deliver a band of light in the blue range, with
about a 434 nm low wavelength cutoff and about a 514 nm long
wavelength cutoff (50% transmission for the specified cuts). A
descriptive quantification of the light intensity incident on the
test samples was referred to as the intensity ratio. This ratio
compared the highest watts/mm.sup.2 experienced by the test PBS to
the watts/mm.sup.2 experienced by a PBS in a reference rear
projection television. This watts/mm.sup.2 "experience" was a
combination of actual light intensity delivered to the PBS by the
lamp/optics configuration, along with the amount of times that
light traveled to and from the PBS. In our typical accelerated
testing, the PBS test film experienced about 12 times the light
intensity as a reference rear projection TV's PBS; this was called
a "12.times." test, with 12.times. referring to this intensity
ratio between the tester and a reference TV. Further explanation of
the intensity ratio calculation can be found in the published paper
C. L. Bruzzone, J. J. Schneider, and S. K. Eckhardt, "6.1
Photostability of Polymeric Cartesian Polarizing Beam Splitters",
SID 04 Digest, pp. 60-63 (2004).
[0052] For all samples, the outer temperature of the PBS cube
assembly was artificially controlled to about 42.degree. C. The
PBS's were observed with the naked eye, and also monitored by
transmissive UV/visible spectra of the PBS assembly taken at
periodic times during the experiments. The spectra allowed for the
calculation of b*, a typical quantification of yellowness. Failure
was determined by an unacceptable change in color, reflected in a
b* color value of 3.75.
[0053] The adhesive was mixed in a similar manner for each sample,
excepting for exact formulation differences. Additives were first
dissolved into the amine based curative and then that mixture was
mixed with the epoxy resin. The adhesives were allowed to sit for 1
hour to remove bubbles and then used in the PBS construction. PBS
assemblies were constructed in the same manner for all samples. The
construction consists of applying metered drops of adhesive to the
outlet prism, laying the film on the adhesive, applying drops of
adhesive to the film, and covering with the inlet prism. The PBS
was cleaned by lightly wiping the surfaces with acetone and then
the assembly was cured in a 60.degree. C. oven for 24 hrs.
Samples 1-15
[0054] Samples 1-15 were prepared under estimated humidity of
10-20% RH. The experimental samples tested the use of water and
three other possible plasticizers. The plasticizers were mixed into
an epoxy-based adhesive (containing light stabilizers). The
plasticizer structures are shown below:
[0055] Plasticizers (other than water) used in experiments to
extend accelerated lifetime
##STR00001##
Results
[0056] The numerical data in the list below is separated into four
columns. "Actual time (hours) to failure (AHTF)" was the elapsed
time until the PBS assembly developed a b* color of 3.75.
"Acceleration intensity ratio ("X")" was the average light
intensity experienced by the test sample relative to that
experienced by a PBS in a reference rear projection television.
"(AHTF)*("X")" was the actual hours to failure multiplied by the
intensity ratio. "Estimated increase in lifetime as ratio of
control sample average." was the ratio of the sample (AHTF)*("X")
to the control value average of 11461. It was a measure of how many
times longer the sample lived vs. the controls. Control referred to
film and adhesive that was standard, with no added plasticizers or
treatments, see samples 1-5. Vacuum treated film, see samples 6 and
7, referred to film that was subjected to 0.1 Torr for 2 days, and
then immediately used in the construction of a PBS. This vacuum
treatment was meant to simulate a film that experiences very dry
conditions before testing.
[0057] The data in Table 1 show that plasticizers extend the
accelerated lifetime of the PBS assemblies. Plasticizer-containing
samples afforded lifetimes that ranged from about 1.2 to 1.6 times
the control lifetime. Adding water to the adhesive also afforded
increased lifetimes. In actual use, a PBS will need to function for
the life of the rear projection TV. It is felt that plasticizers
with lower volatility than water are important tools since water
concentrations may change based on the climate over the actual
usage timeframe. The vacuum treated film showed a lower lifetime
than the control. This helps to show that lack of water
(plasticizer) is detrimental to lifetime. Even though environmental
humidity can equilibrate the water concentration in a film, the
vacuum samples show that this equilibration is not so fast that the
effects of initial extreme dryness can be completely overcome
within the accelerated testing duration.
TABLE-US-00001 TABLE 1 Lifetimes afforded by various samples
Acceleration Actual Hours to Intensity Sample Sample Type Failure
(AHTF) Ratio ("X") (AHTF)*("X") 1 Control sample containing no
Plasicizer 1242 11.3 14035 2 Control sample containing no
Plasicizer 862 11.9 10258 3 Control sample containing no Plasicizer
836 11.8 9865 4 Control sample containing no Plasicizer 1311 104
13634 5 Control sample containing no Plasicizer 928 123 11414 6
Control sample containing no Plasicizer 765 125 9663 Control sample
average of samples 1-6 991 11.7 11461 7 Sample containing no
plasticizer, vacuum treated film 745 121 9015 8 Sample containing
no plasticizer, vacuum treated film 745 106 7897 9 Sample
containing 3% by wt water added to adhesive 1407 12 16884 10 Sample
containing 3% by wt water added to adhesive 1620 108 17496 11
Sample containing 10% by wt ethyleneglycol added to adhesive 1357
120 16284 12 Sample containing 10% by wt ethyleneglycol added to
adhesive 1315 106 13939 13 Sample containing 30% by wt
di(2-ethylhexyl)adipate added to 1315 121 15912 adhesive 14 Sample
containing 20% propyleneglycol by wt added to adhesive 1620 104
16848 15 Sample containing 20% propyleneglycol by wt added to
adhesive 1811 10 18110
[0058] Reference is made to FIG. 6 in which is illustrated a graph
of Lifetime v. Yellowness (comparing lifetime hours of PBS
assemblies vs. b* values for yellowness) for illustrating an
example of the dramatic increase in useful life afforded to PBS
assemblies using a multi-layer film and an adhesive made according
to the present invention. For a PBS assembly in which a multi-layer
film and the adhesive of the present invention was not used, the
commencement of the yellowing occurred at about 750 hours. This is
in sharp contrast to the yellowing for a PBS assembly in which an
adhesive that includes one of the crystallization inhibiting
plasticizers of the present invention (i.e., 10% ethylene glycol)
delayed commencement of yellowing to about 1300 hours.
[0059] This invention may take on various modifications and
alterations without departing from its spirit and scope.
Accordingly, this invention is not limited to the above-described
aspects thereof, but is to be controlled as set forth in the
following claims and any equivalents thereof.
* * * * *