U.S. patent application number 11/276068 was filed with the patent office on 2007-08-16 for curable compositions for optical articles.
This patent application is currently assigned to 3M Innovative Properties Company. Invention is credited to Ming Cheng, Mark F. Ellis, Babu N. Gaddam, Ying-Yuh Lu, Peter M. Olofson, Jianhui Xia.
Application Number | 20070191506 11/276068 |
Document ID | / |
Family ID | 38369529 |
Filed Date | 2007-08-16 |
United States Patent
Application |
20070191506 |
Kind Code |
A1 |
Lu; Ying-Yuh ; et
al. |
August 16, 2007 |
CURABLE COMPOSITIONS FOR OPTICAL ARTICLES
Abstract
A curable composition is provided comprising an oligomer having
a plurality of pendent and/or terminal ethylenically unsaturated,
free-radically polymerizable functional groups, a free-radically
polymerizable crosslinking agent, and/or a diluent monomer, and a
photoinitiator. The composition, when cured, is non-yellowing,
exhibits low shrinkage and low birefringence making it suitable for
many optical applications such as optical lenses, optical fibers,
prisms, light guides, optical adhesives, and optical films.
Inventors: |
Lu; Ying-Yuh; (Woodbury,
MN) ; Xia; Jianhui; (Woodbury, MN) ; Olofson;
Peter M.; (Oakdale, MN) ; Cheng; Ming;
(Woodbury, MN) ; Ellis; Mark F.; (St. Paul,
MN) ; Gaddam; Babu N.; (Woodbury, MN) |
Correspondence
Address: |
3M INNOVATIVE PROPERTIES COMPANY
PO BOX 33427
ST. PAUL
MN
55133-3427
US
|
Assignee: |
3M Innovative Properties
Company
|
Family ID: |
38369529 |
Appl. No.: |
11/276068 |
Filed: |
February 13, 2006 |
Current U.S.
Class: |
522/178 |
Current CPC
Class: |
Y10T 428/10 20150115;
C09K 2323/00 20200801; C08L 2666/02 20130101; C09J 151/003
20130101; C08F 265/04 20130101; C08F 265/06 20130101; C08F 290/06
20130101; C09D 151/003 20130101; G02B 1/041 20130101; C08F 2/48
20130101; C08F 290/061 20130101; C08L 51/003 20130101; G02B 1/041
20130101; C08L 33/00 20130101; C08L 51/003 20130101; C08L 2666/02
20130101; C09D 151/003 20130101; C08L 2666/02 20130101; C09J
151/003 20130101; C08L 2666/02 20130101 |
Class at
Publication: |
522/178 |
International
Class: |
B29D 11/00 20060101
B29D011/00 |
Claims
1. A curable composition comprising: a) 50 to 99 parts by weight of
a (meth)acryloyl oligomer having a plurality of pendent,
free-radically polymerizable functional groups and a T.sub.g of
.gtoreq.20.degree. C.; b) 1 to 50 parts by weight of a
free-radically polymerizable crosslinking agent and/or a diluent
monomer, and c) 0.001 to 5 parts by weight of a photoinitiator,
based on 100 parts by weight of a) and b).
2. The curable composition of claim 1 comprising less than 25 parts
by weight of said diluent monomer.
3. The curable composition of claim 1 comprising less than 1 to 40
parts by weight of said crosslinking agent.
4. The composition of claim 1 comprising 1 to 30 parts by weight of
said crosslinking agent.
5. The composition of claim 1 wherein said oligomer comprises of
polymerized monomer units comprising: a) 50 to 99 parts by weight
of(meth)acrylate esters monomer units homopolymerizable to a
polymer having a glass transition temperature .gtoreq.20.degree.
C., b) 1 to 50 parts by weight of monomer units having a pendent,
free-radically polymerizable functional group c) less than 40 parts
by weight of monomer units homopolymerizable to a polymer having a
glass transition temperature <20.degree. C., based on 100 parts
by weight of a) and b).
6. The composition of claim 5 wherein said (meth)acrylate esters
are homopolymerizable to a polymer having a glass transition
temperature .gtoreq.50.degree. C.
7. The composition of claim 5 comprising 60 to 97 parts by weight
of said (meth)acrylate esters monomer units, and 3 to 40 parts by
weight of monomer units having a pendent, free-radically
polymerizable functional group.
8. The composition of claim 5 wherein said monomer units having a
pendent, free-radically polymerizable functional group are prepared
by reacting monomer units having a pendent reactive functional
group with a mono ethylenically unsaturated compound having a
coreactive functional group.
9. The composition of claim 8 wherein said pendent reactive
functional groups are selected from hydroxyl, secondary amino,
oxazolinyl, oxazolonyl, acetyl acetonyl, carboxyl, isocyanato,
epoxy, aziridinyl, acyloyl halide, and cyclic anhydride groups.
10. The composition of claim 1, wherein the degree of
polymerization of said oligomer is controlled by a chain transfer
agent.
11. The composition of claim 1 wherein said free-radically
polymerizable crosslinking agent has a functionality of at least 2,
and is of the formula R-(Z).sub.n, where Z comprises a
free-radically polymerizable functional group such as a
carbon-carbon double bond, n is greater than 1 and R is an organic
radical having a valency of n.
12. The composition of claim 1 wherein the sum of said crosslinking
agent and said diluent monomer is less than 40 parts by weight.
13. The composition of claim 1 exhibiting shrinkage of less than 5%
by volume when cured.
14. The composition of claim 1 exhibiting a birefringence of less
than 1.times.10.sup.-6 when cured.
15. The composition of claim 1 having a T.sub.g of >50.degree.
C. when cured.
16. The composition of claim 1 having a CIELAB b* value of less
than 1.5 when cured.
17. The composition of claim 1 having an index of refraction
greater than about 1.45 and less than about 1.75 when cured.
18. The composition of claim 1 having light transmission greater
than about 85% when cured.
19. The composition of claim 1 wherein said oligomer is prepared by
an adiabatic polymerization process.
20. The composition of claim 19, wherein said process comprises:
(a) providing the oligomer composition of the invention in a batch
reactor; (b) deoxygenating the mixture, wherein step (b) can
optionally at least partially overlap with step (c); (c) heating
the mixture to a sufficient temperature to generate sufficient
initiator free radicals from at least one thermal free radical
initiator so as to initiate polymerization; (d) allowing the
mixture to polymerize under essentially adiabatic conditions to
yield an at least partially polymerized mixture; (e) optionally
heating the mixture to generate free radicals from some or all of
any initiator that has not generated initiator free radicals,
followed by allowing the mixture to polymerize under essentially
adiabatic conditions to yield a further polymerized mixture; (f)
optionally repeating step (e) one or more times (g) optionally
repeating steps (a) to (e) one or more times with cooling between
repeats.
21. The composition of claim 20 wherein said oligomer composition
further comprises a chain transfer agent to control the molecular
weight of the oligomer.
22. The cured composition of claim 1.
23. A shaped article comprising the cured composition of claim
1.
24. An optical coating comprising the cured composition of claim 1.
Description
FIELD OF THE INVENTION
[0001] The present invention provides curable compositions
containing (meth)acryloyl oligomers that are readily polymerized to
produce optical articles and coatings.
BACKGROUND OF THE INVENTION
[0002] Optical materials and optical products are useful to control
the flow and intensity of light. Examples of useful optical
products include optical lenses such as Fresnel lenses, prisms,
optical light fibers, light pipes, optical films including totally
internal reflecting films, retroreflective sheeting, and
microreplicated products such as brightness enhancement films and
security products. Examples of some of these products are described
in U.S. Pat. Nos. 4,542,449, 5,175,030, 5,591,527, 5,394,255, among
others.
[0003] Polymeric materials have found a variety of uses in optical
articles and are widely used in place of such articles made from
ground glass because the former are light in weight and inexpensive
to produce. Polycarbonates, for example, are characterized by
excellent clarity, resistance to discoloration, high strength, and
high impact resistance. However, thermal polymerization of monomers
to form polymers is generally accompanied by high shrinkage during
cure (e.g., from 11 to 20%) and extended curing time (e.g., from 5
to 16 hours or more). The high shrinkage levels create difficulties
in the production of precision optics (such as lenses or prisms)
from this material, particularly in the production of articles
having larger thicknesses or large differences in thickness between
the center and edges of the article. The extended cure times tie up
production facilities and lead to inefficient utilization of the
dies in which the articles are molded. Also, the thermal cure cycle
used to polymerize the monomer consumes large amounts of energy and
undesirably thermally stresses the dies.
[0004] Optical products can be prepared from high index of
refraction materials, including monomers such as high index of
refraction (meth)acrylate monomers, halogenated monomers, etc., and
other such high index of refraction monomers that are known in the
optical product art. See, e.g., U.S. Pat. Nos. 4,568,445,
4,721,377, 4,812,032, and 5,424,339. Some of these polymers may be
advantageously injection molded, but such molding operations lead
to high birefringence in the resulting article, and a subsequent
annealing step may be required. Further, poly(methyl methacrylate)
polymers tend to be moisture sensitive, and will swell on exposure
to moisture or humidity, further leading to birefringence.
[0005] Several disclosures are related to optical coatings, which
are generally less than two mils (50.8 micrometers) thick. They
fail to describe if those compositions would have a desired balance
of useful properties such as low polymerization shrinkage, low
viscosity, absence of coloration, high hardness, resistance to
stress cracking, moisture or humidity sensitivity and low
birefringence necessary in the production of precision optical
components such as lenses, including Fresnel lenses, and prisms.
Additionally, they fail to teach how to obtain resins providing the
desired balance of properties that are useful for providing cast
precision optical articles. Moreover, many of the polymeric
compositions generally have too high a viscosity to be useful for
optical casting purposes.
SUMMARY OF THE INVENTION
[0006] The present invention includes a curable composition
comprising a (meth)acryloyl oligomer having a plurality of pendent,
ethylenically unsaturated, free-radically polymerizable functional
groups, and having a T.sub.g.gtoreq.20.degree. C. (preferably
having a T.sub.g.gtoreq.50.degree. C.); a free-radically
polymerizable crosslinking agent and/or a diluent monomer; and a
photoinitiator. The composition, when cured, is non-yellowing,
exhibits low shrinkage and low birefringence and low sensitivity to
moisture, making it suitable for many optical applications
including, but not limited to optical lenses, optical fibers,
prisms, diffractive lenses, microlenses, microlens arrays, Fresnel
lenses, light guides, and optical films and coatings. The
composition is low viscosity so that it may be used as an optical
adhesive and in conventional molding operations, and build
molecular weight by a chain-growth addition process. Further,
articles may be prepared by cast and cured processes and thereby
avoids birefringence induced by injection molding processes.
[0007] Generally, curable systems containing a significant amount
of solvent, monomers and reactive diluents can give rise to a
significant increase in density when transformed from the uncured
to the cured state causing a net shrinkage in volume. As is well
known, shrinkage can cause unpredictable registration in precise
molding operations such as those required in manufacture of optical
elements such as lenses. Shrinkage can also create residual stress
in such optical articles, which can subsequently lead to optical
defects, including high birefringence.
[0008] The present invention also provides shaped articles,
including optical articles, and a method for preparing the same
comprising, in one embodiment, the steps of:
[0009] (1) mixing the components to form an optical casting
composition,
[0010] (2) optionally degassing the composition,
[0011] (3) optionally heating the composition,
[0012] (4) introducing the composition into a suitable mold,
and
[0013] (5) effecting polymerization, preferably
photopolymerization, of the composition.
[0014] The present invention addresses the needs of the industry by
providing a rapid cure, solvent free, curable composition, to
produce thick precision optics such as optical lenses, light
guides, prisms, etc., with low birefringence for applications in
electronic displays, cameras, binoculars, fax machines, bar code
scanners, and optical communication devices. The present invention
is especially useful in preparing prisms such as those used in
polarizing beam splitters (PBS's) used in optical imager systems
and optical reader systems. The term "optical imager system" as
used herein is meant to include a wide variety of optical systems
that produce an image for a viewer to view. Optical imager systems
of the present invention may be used, for example, in front and
rear projection systems, projection displays, head-mounted
displays, virtual viewers, heads-up displays, optical computing
systems, optical correlation systems, and other optical viewing and
display systems.
[0015] A PBS is an optical component that splits incident light
rays into a first polarization component and a second polarization
component. Traditional PBS's function based on the plane of
incidence of the light, that is, a plane defined by the incident
light ray and a normal to the polarizing surface. The plane of
incidence also is referred to as the reflection plane, defined by
the reflected light ray and a normal to the reflecting surface.
Based on the operation of traditional polarizers, light has been
described as having two polarization components, a p- and an
s-component. The p-component corresponds to light polarized in the
plane of incidence. The s-component corresponds to light polarized
perpendicular to the plane of incidence.
[0016] To achieve the maximum possible efficiency in an optical
imaging system, a low f/# system is desirable (see, F. E. Doany et
al., Projection display throughput; Efficiency of optical
transmission and light-source collection, IBM J. Res. Develop. V42,
May/July 1998, pp. 387-398). The f/# measures the light gathering
ability of an optical lens and is defined as: f/#=f(focal
length)/D(diameter or clear aperture of the lens). The f/# (or F)
measures the size of the cone of light that may be used to
illuminate an optical element. The lower the f/#, the faster the
lens and the larger the cone of light that may be used with that
optical element. A larger cone of light generally translates to
higher light throughput. Accordingly, a faster (lower f/#)
illumination system requires a PBS able to accept light rays having
a wider range of incident angles. The maximum incident angle
.THETA..sub.max (the outer rays of the cone of light) may be
mathematically derived from the f/#;
.THETA..sub.max=tan.sup.-1((2F).sup.-1)
[0017] Traditional folded light path optical imaging systems have
employed an optical element know as a MacNeille polarizer.
MacNeille polarizers take advantage of the fact that an angle
exists, called Brewster's angle, at which no p-polarized light is
reflected from an interface between two media of differing
refractive index (n). Brewster's angle is given by:
.THETA..sub.B=tan.sup.-1(n.sub.1/n.sub.0), where n.sub.0 is the
refractive index of one medium, and n.sub.1 is the refractive index
of the other. When the angle of incidence of an incident light ray
reaches the Brewster angle, the reflected beam portion is polarized
in the plane perpendicular to the plane of incidence. The
transmitted beam portion becomes preferentially (but not
completely) polarized in the plane parallel to the plane of
incidence. In order to achieve efficient reflection of s-polarized
light, a MacNeille polarizer is constructed from multiple layers of
thin films of materials meeting the Brewster angle condition for
the desired angle. The film thicknesses are chosen such that the
film layer pairs form a quarter wave stack.
[0018] There is an advantage to this construction in that the
Brewster angle condition is not dependent on wavelength (except for
dispersion in the materials). However, MacNeille polarizers have
difficulty achieving wide-angle performance due to the fact that
the Brewster angle condition for a pair of materials is strictly
met at only one angle of incidence. As the angle of incidence
deviates from this angle a spectrally non-uniform leak develops.
This leak becomes especially severe as the angle of incidence on
the film stack becomes more normal than the Brewster's angle. As
will be explained below, there are also contrast disadvantages for
a folded light path projector associated with the use of p and
s-polarization, referenced to the plane of reflection for each
ray.
[0019] Typically, MacNeille PBS's are contained in glass cubes,
wherein a PBS thin-film stack is applied along a diagonal plane of
the cube. By suitably selecting the index of the glass in the cube,
the PBS may be constructed so that light incident normal to the
face of the cube is incident at the Brewster angle of the PBS.
However, the use of cubes gives rise to certain disadvantages,
principally associated with the generation of thermal
stress-induced birefringence that degrades the polarization
performance of the component. Even expensive pre-annealed cubes may
suffer from this difficulty. Also cubes add significant weight to a
compact system.
[0020] MacNeille-type PBS's reportedly have been developed capable
of discrimination between s- and p-polarized light at f/#'s as low
as f/2.5, while providing extinction levels in excess of 100:1
between incident beams of pure s- or pure p- polarization.
Unfortunately, as explained below, when MacNeille-type PBS's are
used in a folded light path with reflective imagers, the contrast
is degraded due to depolarization of rays of light having a
reflection plane rotated relative to the reflection plane of the
principal ray. As used below, the term "depolarization" is meant to
describe the deviation of the polarization state of a light ray
from that of the principal light ray. As light in a projection
system generally is projected as a cone, most of the rays of light
are not perfectly parallel to the principal light ray. The
depolarization increases as the f/# decreases, and is magnified in
subsequent reflections from color selective films. This
"depolarization cascade" has been calculated by some optical
imaging system designers to effectively limit the f/# of MacNeille
PBS based projectors to about 3.3, thereby limiting the light
throughput efficiency of these systems. See, A. E. Rosenbluth et
al., Contrast properties of reflective liquid crystal light valves
in projection displays, IBM J. Res. Develop. V42, May/July 1998,
pp. 359-386.
As used herein:
[0021] "Actinic radiation" means photochemically active radiation
and particle beams. Actinic radiation includes, but is not limited
to, accelerated particles, for example, electron beams; and
electromagnetic radiation; for example, microwaves, infrared
radiation, visible light, ultraviolet light, X-rays, and
gamma-rays. The radiation can be monochromatic or polychromatic,
coherent or incoherent, and should be sufficiently intense to
generate substantial numbers of free radicals in the actinic
radiation curable compositions.
[0022] "(Meth)acryloyl groups" means both acryloyl and methacryloyl
groups, and includes acrylate, methacrylate, acrylamide and
methacrylamide groups.
[0023] "Ethylenically unsaturated groups" include, but are not
limited to, vinyl, vinyloxy, (meth)acryloyl and the like.
[0024] "Melt processible" is used to refer to oligomer compositions
that possess or achieve a suitable low viscosity for coating or
molding at temperatures less than or equal to 100.degree. C., using
conventional molding or coating equipment.
[0025] "Photocuring" and "photopolymerization" are used
interchangeably in this application to indicate an actinic
radiation induced chemical reaction in which relatively simple
molecules combine to form a chain or net-like macromolecule.
[0026] "100% solids" means a composition free of unreactive
species, such as solvents.
[0027] "Transmittance" of radiant energy refers to the passage of
radiant energy through a material.
[0028] "Transparency" may be considered as a degree of regular
transmission, and thus the property of a material by which objects
may be seen through a sheet thereof. A transparent material
transmits light without significant diffusion or scattering.
BRIEF DESCRIPTION OF THE FIGURE
[0029] FIGS. 1 and 2 are schematics of a process of the
invention.
DETAILED DESCRIPTION
[0030] The present invention provides curable materials comprising
one or more (meth)acryloyl oligomers having a plurality of pendent,
free-radically polymerizable functional groups, and having a
T.sub.g.gtoreq.20.degree. C. (preferably a
T.sub.g.gtoreq.50.degree. C.); a free-radically polymerizable
crosslinking agent and/or a diluent monomer, and a photoinitiator.
In many embodiments, the present invention provides curable
materials with low shrinkage, residual stress and birefringence
that is optically clear and non-yellowing for applications in
precision optics and electronic displays.
[0031] The composition of the present invention minimizes shrinkage
and birefringence due to optimum molecular weight of the
(meth)acryloyl oligomer and loading of the crosslinker and/or
reactive diluent. The low shrinkage compositions of this invention
are particularly useful in molding applications or in any
applications where accurate molding and/or registration is
required. The present invention provides new compositions that may
be formulated as 100% solids, cured by free-radical means and that
exhibit properties that meet or exceed those of the art. The
present invention provides compositions that exhibit less than 5%
shrinkage, and preferably less than 3%. The compositions are low in
viscosity and suitable for molding processes, including precision
molding processes. The compositions generally have a viscosity less
than 20,000 centipoise, less than 15,000 centipoise, or less than
10,000 centipoise at application temperatures of 100.degree. C. or
less. The compositions generally have a viscosity of at least 100
centipoise, or at least 500 centipoise at temperatures of
100.degree. C. or less.
[0032] The articles of the invention may have a thickness greater
than about 0.5 millimeters, generally a birefringence (absolute) of
less than 1.times.10.sup.-6, light transmission greater than about
85%, preferably greater than 90%, and a CIELAB b* less than about
1.5 units, preferably less than about 1.0 unit for samples with
thickness of 4.8 millimeters.
[0033] The composition generally comprises:
[0034] 50 to 99 parts by weight, preferably 60 to 95 parts and most
preferably 70 to 95 parts of an oligomer having a plurality of
pendent free-radically polymerizable functional groups and having a
T.sub.g.gtoreq.20.degree. C., preferably .gtoreq.50.degree. C.;
[0035] 1 to 50 parts by weight, preferably 5 to 40 parts, and most
preferably 5 to 30 parts of a free-radically polymerizable
crosslinking agent and/or a diluent monomer;
[0036] and 0.001 to 5 parts be weight, preferably 0. 001 to 1, most
preferably 0.01 to 0.1 parts of a photoinitiator, based on 100
parts by weight of oligomer and crosslinking agent and/or reactive
diluent monomer.
[0037] In some preferred embodiments, the crosslinking agent
comprises 1 to 40 parts by weight, preferably 1 to 30 parts by
weight, and most preferably 1 to 20 parts by weight. In some
embodiments, the reactive diluent comprises less than 25 parts by
weight, preferably less than 15 parts by weight and most preferably
less than 10 parts by weight.
[0038] The oligomer generally comprises polymerized monomer units
comprising: [0039] a) 50 to 99 parts by weight, preferably 60 to 97
parts by weight, most preferably 80 to 95 parts by weight of
(meth)acryloyl monomer units homopolymerizable to a polymer having
a glass transition temperature >20.degree. C., preferably
.gtoreq.50.degree. C., preferably the (meth)acryloyl monomer units
are (meth)acrylate monomer units [0040] b) 1 to 50 parts by weight,
preferably 3 to 40 parts by weight, most preferably 5 to 20 parts
by weight, of monomer units having a pendent, free-radically
polymerizable functional groups, [0041] c) less than 40 parts by
weight, preferably less than 30 parts by weight, most preferably
less than 20 parts by weight, of monomer units homopolymerizable to
a polymer having a glass transition temperature less than
20.degree. C., based on 100 parts by weight of a) and b).
[0042] The first component oligomer comprises one or more high
T.sub.g monomers, which if homopolymerized, yield a polymer having
a T.sub.g>20.degree. C., preferably >50.degree. C. Preferred
high T.sub.g monomers are monofunctional (meth)acrylate esters of
mono- and bicyclic aliphatic alcohols having at least 6 carbon
atoms, and of aromatic alcohols. Both the cycloaliphatic and
aromatic groups may be substituted, for example, by C.sub.1-6
alkyl, halogen, sulfur, cyano, and the like. Especially preferred
high T.sub.g monomers include 3,5-dimethyladamantyl(meth)acrylate;
isobornyl(meth)acrylate; 4-biphenyl(meth)acrylate;
phenyl(meth)acrylate; benzyl methacrylate; and
2-naphthyl(meth)acrylate; dicyclopentadienyl (meth)acrylate.
Mixtures of high T.sub.g monomers may also be used. Providing the
monomer can be polymerized with the rest of the monomers that
comprise the (meth)acrylate monomers, any high T.sub.g monomer
including styrene, vinylesters and the like, can be used. However,
the high T.sub.g monomer is typically an acrylate or methacrylate
ester.
[0043] Other high T.sub.g monomers include C.sub.1-C.sub.20
alkyl(meth)acrylates such as methyl methacrylate, ethyl
methacrylate, isopropyl methacrylate, n-butyl methacrylate,
isobutyl methacrylate, t-butyl(meth)acrylate, stearyl methacrylate,
cyclohexyl methacrylate, 3,3,5-trimethylcyclohexyl methacrylate,
tetrahydrofurfuryl methacrylate, allyl methacrylate, bromoethyl
methacrylate; styrene; vinyl toluene; vinyl esters such as vinyl
propionate, vinyl acetate, vinyl pivalate, and vinyl neononanoate;
acrylamides such as N,N-dimethyl acrylamide, N,N-diethyl
acrylamide, N-isopropyl acrylamide, N-octyl acrylamide, and
N-t-butyl acrylamide, and (meth)acrylonitrile. Blends of high
T.sub.g monomers may be used.
[0044] Most preferred high T.sub.g monomers are selected from
linear, branched, cyclo, and bridged cycloaliphatic(meth)acrylates,
such as isobornyl(meth)acrylate, cyclohexyl methacrylate,
3,3,5-trimethylcyclohexyl methacrylate, methyl methacrylate, ethyl
methacrylate, isopropyl methacrylate, n-butyl methacrylate,
isobutyl methacrylate, t-butyl(meth)acrylate, stearyl methacrylate,
and mixtures thereof, for their environmental (heat and light)
stability.
[0045] The first component oligomer of the composition comprises
one or more pendent groups that include free-radically
polymerizable unsaturation. Preferred pendent unsaturated groups
include (meth)acryloyl, including (meth)acryloxy, and
(meth)acrylamido. Such pendent groups can be incorporated into the
polymer in at least two ways. The most direct method is to include
among the monomer units of ethylene di(meth)acrylate,
1,6-hexanediol diacrylate (HDDA), or bisphenol-A di(meth)acrylate.
Useful polyunsaturated monomers include allyl, propargyl, and
crotyl(meth)acrylates, trimethylolpropane triacrylate,
pentaerythritol triacrylate, and allyl
2-acrylamido-2,2-dimethylacetate.
[0046] Using the "direct method" of incorporating the pendent,
free-radically polymerizable functional group, useful functional
monomers include those unsaturated aliphatic, cycloaliphatic, and
aromatic compounds having up to about 36 carbon atoms that include
a functional group capable of free radical addition such as those
groups containing a carbon-carbon double bond including vinyl,
vinyloxy, (meth)acrylate, (meth)acrylamido, and acetylenic
functional groups.
[0047] Examples of polyethylenically unsaturated monomers that can
be used include, but are not limited to, polyacrylic-functional
monomers such as ethylene glycol diacrylate, propylene glycol
dimethacrylate, trimethylolpropane triacrylate,
1,6-hexamethylenedioldiacrylate, pentaerythritol di-, tri-, and
tetraacrylate, and 1,12-dodecanedioldiacrylate;
olefinic-acrylic-functional monomers such as allyl methacrylate,
2-allyloxycarbonylamidoethyl methacrylate, and 2-allylaminoethyl
acrylate; allyl 2-acrylamido-2,2-dimethylacetate; divinylbenzene;
vinyloxy group-substituted functional monomers such as
2-(ethenyloxy)ethyl(meth)acrylate, 3-(ethynyloxy)-1-propene,
4-(ethynyloxy)-1-butene, and
4-(ethenyloxy)butyl-2-acrylamido-2,2-dimethylacetate, and the like.
Useful polyunsaturated monomers, and useful reactive/co-reactive
compounds that may be used to prepare a polymer having pendent
unsaturation are described in greater detail in U.S. Pat. No.
5,741,543 (Winslow et al.).
[0048] Preferred polyunsaturated monomers are those where the
unsaturated groups are of unequal reactivity. Those skilled in the
art recognize that the particular moieties attached to the
unsaturated groups affect the relative reactivities of those
unsaturated groups. For example, where a polyunsaturated monomer
having unsaturated groups of equal reactivity (e.g., HDDA) is used,
premature gellation of the composition must be guarded against by,
for example, the presence of oxygen, which acts as a radical
scavenger. Conversely, where a polyunsaturated monomer having
unsaturated groups of differing reactivities is used, the more
reactive group (such as (meth)acrylate as (meth)acrylamido)
preferentially is incorporated into the polymer backbone before the
less reactive unsaturated group (such as vinyl, allyl, vinyloxy, or
acetylenic) reacts to crosslink the composition. The direct method
is generally not preferred due to difficulty in control of
branching and premature gellation.
[0049] An indirect, but preferred, method of incorporating pendent
groups that comprise polymerizable unsaturation into the first
polymer is to include among the monomer units of the polymer some
that comprise a reactive functional group. Useful reactive
functional groups include, but are not limited to, hydroxyl, amino
(especially secondary amino), oxazolonyl, oxazolinyl, acetoacetyl,
carboxyl, isocyanato, epoxy, aziridinyl, acyl halide, and cyclic
anhydride groups. Preferred among these are carboxyl, hydroxyl and
aziridinyl groups. These pendent reactive functional groups are
reacted with unsaturated compounds that comprise functional groups
that are co-reactive with the reactive pendent functional group.
When the two functional groups react, an oligomer with pendent
unsaturation results.
[0050] Using the "indirect method" of incorporating the pendent,
free-radically polymerizable functional groups, useful reactive
functional groups include hydroxyl, secondary amino, oxazolinyl,
oxazolonyl, acetyl, acetonyl, carboxyl, isocyanato, epoxy,
aziridinyl, acyl halide, vinyloxy, and cyclic anhydride groups.
Where the pendent reactive functional group is an isocyanato
functional group, the co-reactive functional group preferably
comprises a secondary amino or hydroxyl group. Where the pendent
reactive functional group comprises a hydroxyl group, the
co-reactive functional group preferably comprises a carboxyl,
isocyanato, epoxy, anhydride, or oxazolinyl group. Where the
pendent reactive functional group comprises a carboxyl group, the
co-reactive functional group preferably comprises a hydroxyl,
amino, epoxy, isocyanate, or oxazolinyl group. Most generally, the
reaction is between nucleophilic and electrophilic functional
groups that react by a displacement or condensation mechanism.
[0051] Representative examples of useful co-reactive compounds
include hydroxyalkyl(meth)acrylates such as
2-hydroxyethyl(meth)acrylate, 4-hydroxybutyl(meth)acrylate, and
2-(2-hydroxyethoxy)ethyl(meth)acrylate; aminoalkyl(meth)acrylates
such as 3-aminopropyl(meth)acrylate and 4-aminostyrene; oxazolinyl
compounds such as 2-ethenyl-1,3-oxazolin-5-one and
2-propenyl-4,4-dimethyl-1,3-oxazolin-5-one; carboxy-substituted
compounds such as (meth)acrylic acid and
4-carboxybenzyl(meth)acrylate; isocyanato-substituted compounds
such as isocyanatoethyl(meth)acrylate and
4-isocyanatocyclohexyl(meth)acrylate; epoxy-substituted compounds
such as glycidyl(meth)acrylate; aziridinyl-substituted compounds
such as N-acryloylaziridine and 1-(2-propenyl)-aziridine; and
acryloyl halides such as (meth)acryloyl chloride.
[0052] Preferred functional monomers have the general formula
##STR1## wherein R.sup.1 is hydrogen, a C.sub.1 to C.sub.4 alkyl
group, or a phenyl group, preferably hydrogen or a methyl group;
R.sup.2 is a single bond or a divalent linking group that joins an
ethylenically unsaturated group to polymerizable or reactive
functional group A and preferably contains up to 34, preferably up
to 18, more preferably up to 10, carbon and, optionally, oxygen and
nitrogen atoms and, when R.sup.2 is not a single bond, is
preferably selected from ##STR2## wherein R.sup.3 is an alkylene
group having 1 to 6 carbon atoms, a 5- or 6-membered cycloalkylene
group having 5 to 10 carbon atoms, or an alkylene-oxyalkylene in
which each alkylene includes 1 to 6 carbon atoms or is a divalent
aromatic group having 6 to 16 carbon atoms; and A is a functional
group, capable of free-radical addition to carbon-carbon double
bonds, or a reactive functional group capable of reacting with a
co-reactive functional group for the incorporation of a
free-radically polymerizable functional group.
[0053] It will be understood, in the context of the above
description of the first component oligomer, that the
ethylenically-unsaturated monomer possessing a free-radically
polymerizable group is chosen such that it is free-radically
polymerizable with the crosslinking agent and reactive diluent. The
reactions between functional groups provide a crosslink by forming
a covalent bond by free-radical addition reactions of
ethylenically-unsaturated groups between components. In the present
invention the pendent functional groups react by an addition
reaction in which no by-product molecules are created, and the
exemplified reaction partners react by this preferred mode.
[0054] Where the curable composition is to be processed using high
temperatures and the direct method of including pendent
unsaturation has been used, care must be taken not to activate
those pendent groups and cause premature gelation. For example,
hot-melt processing temperatures can be kept relatively low and
polymerization inhibitors can be added to the mixture. Accordingly,
where heat is to be used to process the composition, the
above-described indirect method is the preferred way of
incorporating the pendent unsaturated groups.
[0055] The oligomer may optionally further comprise lower T.sub.g
alkyl(meth)acrylate esters or amides that may be homopolymerized to
polymers having a T.sub.g of less than 20.degree. C.
Alkyl(meth)acrylate ester monomers useful in the invention include
straight-chain, cyclic, and branched-chain isomers of alkyl esters
containing C.sub.1-C.sub.20 alkyl groups. Due to T.sub.g and side
chain crystallinity considerations, preferred lower T.sub.g
alkyl(meth)acrylate esters are those having from C.sub.1-C.sub.8
alkyl groups. Useful specific examples of alkyl(meth)acrylate
esters include: methyl acrylate, ethyl acrylate, n-propyl acrylate,
butyl acrylate, iso-amyl(meth)acrylate, n-hexyl(meth)acrylate,
n-heptyl(meth)acrylate, n-octyl(meth)acrylate,
iso-octyl(meth)acrylate, 2-ethylhexyl(meth)acrylate,
iso-nonyl(meth)acrylate, and decyl(meth)acrylate. Most preferred
(meth)acrylate esters include methyl acrylate, ethyl acrylate,
butyl acrylate, isooctyl(meth)acrylate, 2-ethylhexyl(meth)acrylate,
cyclohexyl acrylate. The lower T.sub.g alkyl(meth)acrylate esters
are added in such an amount such that the resulting oligomer has a
T.sub.g of 20.degree. C. or greater. In general, such low T.sub.g
monomers are used in amounts of 40 parts by weight or less,
preferably 30 parts by weight or less, most preferable 20 parts by
weight or less.
[0056] The theoretical T.sub.g of an oligomer may be calculated,
for example, using the Fox equation,
1/T.sub.g=(w.sup.1/T.sub.g.sup.1+w.sup.2/T.sub.g.sup.2), where
w.sup.1 and w.sup.2 refer to the weight fraction of the two
components and T.sub.g.sup.1 and T.sub.g.sup.2 refer to the glass
transition temperature of the two components, as described for
example in L. H. Sperling, "Introduction to Physical Polymer
Science", 2.sup.nd Edition, John Wiley & Sons, New York, p. 357
(1992) and T. G. Fox, Bull. Am. Phys. Soc., 1, 123 (1956), which
are incorporated herein by reference. Using the T.sub.g of the
component monomers, and an estimate of the weight fractions thereof
in the oligomer, one may calculate the T.sub.g of the resulting
oligomer. As understood by one skilled in the art, the Fox equation
may be used for a system with more than two components.
[0057] The oligomer may be prepared using radical polymerization
techniques by combining an initiator and monomers in the presence
of a chain transfer agent. In this reaction, a chain transfer agent
transfers the active site on one growing chain to another molecule
that can then start a new chain so the degree of polymerization may
be controlled. The degree of polymerization of the resulting
oligomer may be 10 to 300, preferably 15 to 200, more preferably 20
to 200. It has been found if the degree of polymerization is too
high, the composition is too high in viscosity, and not easily melt
processible. Conversely, if the degree of polymerization is too
low, the shrinkage of the cured composition is excessive and leads
to high birefringence in the cured composition.
[0058] Chain transfer agents may be used when polymerizing the
monomers described herein to control the molecular weight of the
resulting oligomer. Suitable chain transfer agents include
halogenated hydrocarbons (e.g., carbon tetrabromide) and sulfur
compounds (e.g., lauryl mercaptan, butyl mercaptan, ethanethiol,
and 2-mercaptoethyl ether, isooctyl thioglycolate,
t-dodecylmercaptan, 3-mercapto-1,2-propanediol). The amount of
chain transfer agent that is useful depends upon the desired
molecular weight of the oligomer and the type of chain transfer
agent. The chain transfer agent is typically used in amounts from
about 0.1 parts to about 10 parts; preferably 0.1 to about 8 parts;
and more preferably from about 0.5 parts to about 6 parts based on
total weight of the monomers.
[0059] Suitable initiators for this oligomerization reaction
include, for example, thermal and photo initiators. Useful thermal
initiators include azo compounds and peroxides. Examples of useful
azo compounds include 2,2'-azobis(2,4-dimethylpentanenitrile),
(Vazo 52, commercially available from E. I. duPont de Nemours &
Co.); 2,2'-azobis(isobutyronitrile), (Vazo 64, commercially
available from E. I. duPont de Nemours & Co.);
2,2'-azobis(2-methylbutyronitrile), (Vazo 67, commercially
available from E. I. duPont de Nemours & Co.);
1,1'-azobis(cyanocyclohexane), (Vazo 88, commercially available
from E. I. duPont de Nemours & Co.);
1,1'-azobis(1-cyclohexane-1-carbonitrile), (V-40, commercially
available from Wako Pure Chemical Industries, Ltd.); and dimethyl
2,2'-azobis(isobutyrate), (V-601, commercially available from Wako
Pure Chemical Industries, Ltd.). Examples of useful peroxides
include benzoyl peroxide; di-t-amyl peroxide, t-butyl peroxy
benzoate, 2,5-dimethyl-2,5 Di-(t-butylperoxy)hexane,
2,5-dimethyl-2,5-Di-(t-butylperoxy)hexyne-3, lauroyl peroxide, and
t-butyl peroxy pivalate. Useful organic hydroperoxides include but
are not limited to compounds such as t-amyl hydroperoxide and
t-butyl hydroperoxide.
[0060] Useful photoinitiators include benzoin ethers such as
benzoin methyl ether and benzoin butyl ether; acetophenone
derivatives such as 2,2-dimethoxy-2-phenyl-acetophenone and
2,2-diethoxy acetophenone; and acylphosphine oxide derivatives and
acylphosphonate derivatives such as
diphenyl-2,4,6-trimethylbenzoylphosphine oxide,
isopropoxy(phenyl)-2,4,6-trimethylbenzoylphosphine oxide, and
dimethyl pivaloylphosphonate. Of these,
2,2-dimethoxy-2-phenyl-acetophenone is preferred. The initiator is
typically used at a level of 0.001 to 5 parts by weight per 100
parts by weight monomer(s).
[0061] The composition further comprises a crosslinking agent
having a plurality of pendent, ethylenically unsaturated,
free-radically polymerizable functional groups. Useful crosslinking
agents have an average functionality (average number of
ethylenically unsaturated, free-radically polymerizable functional
groups per molecule) of greater than one, and preferably greater
than or equal to two. The functional groups are chosen to be
copolymerizable with the pendent ethylenically unsaturated,
free-radically polymerizable functional groups on the first
component oligomer. Useful functional groups include those
described for the first component oligomer and include, but are not
limited to vinyl, vinyloxy, (meth)acryloyl and acetylenic
functional groups.
[0062] Useful crosslinking agents have the general formula:
R-(Z).sub.n
[0063] where Z is a free-radically polymerizable functional group
such as a carbon-carbon double bond, n is greater than 1 and R is
an organic radical having a valency of n. Preferably R is an
aliphatic alkyl radical of valency n which may be linear or
branched.
[0064] Examples of such crosslinking agents include:
C.sub.2-C.sub.18 alkylenediol di(meth)acrylates, C.sub.3-C.sub.18
alkylenetriol tri(meth)acrylates, such as 1,6-hexanediol
di(meth)acrylate, trimethylolpropane tri(meth)acrylate,
propoxylated trimethylolpropane triacrylate such as CD501 from
Sratomer Co., Exton, Pa., triethyleneglycol di(meth)acrylate,
pentaeritritol tri(meth)acrylate, and tripropyleneglycol
di(meth)acrylate, and di-trimethylolpropane tetraacrylate,
polyalkyleneglycol dimethacrylate such as Bisomer.TM. EP 100DMA
from Cognis Co. For ease of mixing, the preferred crosslinking
agent is not a solid material at application temperatures.
[0065] The composition according to the invention may comprise at
least one reactive diluent. The reactive diluents can be used to
adjust the viscosity of the composition. Thus, the reactive
diluents can each be a low viscosity monomer containing at least
one functional group capable of polymerization when exposed to
actinic radiation. For example, vinyl reactive diluents and
(meth)acrylate monomer diluents may be used.
[0066] The functional group present on the reactive diluents may be
the same as that used in the curable (meth)acrylate oligomer.
Preferably, the radiation-curable functional group present in the
reactive diluent is capable of copolymerizing with the
radiation-curable functional group present on the radiation-curable
oligomer. The reactive diluents generally have a molecular weight
of not more than about 550 or a viscosity at room temperature of
less than about 500 mPascal.sec (measured as 100% diluent).
[0067] The reactive diluent may comprise monomers having a
(meth)acryloyl or vinyl functionality and a C.sub.1-C.sub.20 alkyl
moiety. Examples of such reactive diluents are ethyl(meth)acrylate,
isopropyl(meth)acrylate, t-butyl(meth)acrylate,
n-butyl(meth)acrylate, cyclohexyl(meth)acrylate,
isobornyl(meth)acrylate, isooctyl(meth)acrylate,
2-ethylhexyl(meth)acrylate, stearyl(meth)acrylate,
phenoxyethyl(meth)acrylate, benzyl(meth)acrylate and the like. Low
volatile alkyl (meth)acrylates such as isobornyl(meth)acrylate,
2-ethylhexyl(meth)acrylate, cyclohexyl(meth)acrylate,
isooctyl(meth)acrylate, stearyl(meth)acrylate,
phenoxyethyl(meth)acrylate, benzyl(meth)acrylate are preferred
reactive diluents.
[0068] The reactive diluent is preferably added in such an amount
that the shrinkage of the cured compositions does not exceed around
5%, preferably not above around 3%, as measured by the test method
described herein. Suitable amounts of the reactive diluents have
been found to be less than about 25 parts by weight, preferably
about 0 to about 15 parts by weight, and more preferably about 0 to
about 10 parts by weight. Preferably, the sum of the amounts of the
reactive diluent and the crosslinking agent is less than 40 parts
by weight.
[0069] The components of the composition may be combined and cured
with a photoinitiator. The photoinitiator improves the rate of cure
and percent conversion of the curable compositions, but the depth
of cure (of thicker coatings or shaped articles) may be
deleteriously affected as the photoinitiator may attenuate the
transmitted light that penetrates the thickness of the sample. The
photoinitiator is used in an amount of less than 1.0 weight %,
preferably less than 0.1 weight %, most preferably less than 0.05
weight %.
[0070] Conventional photoinitiators can be used. Examples include
benzophenones, acetophenone derivatives, such as
a-hydroxyalkylphenylketones, benzoin alkyl ethers and benzil
ketals, monoacylphosphine oxides, and bis-acylphosphine oxides.
Preferred photoinitiators are ethyl 2,4,6-trimethylbenzoylphenyl
phosphinate (Lucirin.TM. TPO-L) available from BASF, Mt. Olive,
N.J., 2-hydroxy-2-methyl-1-phenyl-propan-1-one (IRGACURE 1173.TM.,
Ciba Specialties), 2,2-dimethoxy-2-phenyl acetophenone (IRGACURE
651.TM., Ciba Specialties), phenyl bis-(2,4,6-trimethyl
benzoyl)phosphine oxide (IRGACURE 819, Ciba Specialities). Other
suitable photoinitiators include mercaptobenzothiazoles,
mercaptobenzooxazoles and hexaryl bisimidazole. Often, mixtures of
photoinitiators provide a suitable balance of properties.
[0071] The compositions can then be applied to the desired
substrate or added to a mold and exposed to actinic radiation such
as UV light. The composition may be exposed to any form of actinic
radiation, such as visible light or UV radiation, but is preferably
exposed to UVA (320 to 390 nm) or UVB (395 to 445 nm) radiation.
Generally, the amount of actinic radiation should be sufficient to
form a non-tacky, dimensionally stable solid mass. Generally, the
amount of energy required for curing the compositions of the
invention ranges from about 0.2 to 20.0 J/cm.sup.2.
[0072] The photopolymerization may be effected by any suitable
light source including carbon arc lights, low, medium, or high
pressure mercury vapor lamps, swirl-flow plasma arc lamps, xenon
flash lamps, ultraviolet light emitting diodes, and ultraviolet
light emitting lasers For many applications it may be desirable to
use an LED light source or array to effect the curing. Such LED
sources may effect a faster cure and provide less heat to the
composition during cure. One suitable LED source is the Norlux
large area array, series 808 (available from Norlux, Carol Stream,
Ill.).
[0073] A preferred method of making the oligomer is through an
adiabatic polymerization method (see for example, U.S. Pat. No.
5,986,011 (Ellis) or U.S. Pat. No. 5,753,768 (Ellis), incorporated
herein by reference). In such a polymerization, the polymerization
initiator(s) may be used at a low level, to reduce color due to the
initiator fragments incorporated into the polymer. Further, during
an adiabatic polymerization, partly because of low initiator
levels, and partly due to the increasing temperature profile that
accompanies polymerization, conditions can be selected such that
the initiator is essentially completely consumed during the
polymerization or at the end of the polymerization. Having all
thermal polymerization initiator consumed advantageously prevents
or reduces unwanted polymerization and crosslinking during the
functionalization step of the oligomer using the "indirect method"
of incorporating the pendent, free-radically polymerizable
functional groups (described herein). Further, having no
significant traces of thermal initiator present beneficially
improve the stability of the functionalized oligomer during storage
and transport, prior to molding and further curing.
[0074] The adiabatic polymerization process comprises the steps of:
[0075] (a) providing the oligomer composition of the invention in a
batch reactor; [0076] (b) deoxygenating the mixture, wherein step
(b) can optionally at least partially overlap with step (c); [0077]
(c) heating the mixture to a sufficient temperature to generate
sufficient initiator free radicals from at least one thermal free
radical initiator so as to initiate polymerization; [0078] (d)
allowing the mixture to polymerize under essentially adiabatic
conditions to yield an at least partially polymerized mixture;
[0079] (e) optionally heating the mixture to generate free radicals
from some or all of any initiator that has not generated initiator
free radicals, followed by allowing the mixture to polymerize under
essentially adiabatic conditions to yield a further polymerized
mixture; and [0080] (f) optionally repeating step (e) one or more
times. [0081] (g) optionally repeating steps (a) to (e) one or more
times with cooling between repeats. Step (g) is useful if the
monomers have a heat of reaction such that it is difficult to
achieve the desired conversion to oligomer in one adiabatic
polymerization step without getting too hot. Multiple repeats of
steps (a) to (e) with cooling between repeats to the proper
temperature(s) and then polymerizing adiabatically further in the
one or more repeats can be beneficial to control the final
polymerization temperature to a desired level. This may prevent
yellowing due to polymer degradation as result of the heat of
polymerization.
[0082] By appropriately selecting initiator(s) and amounts in step
(a) and optional use of step (g), the conversion to polymer can be
advantageously controlled to be sufficiently high so as to provide
curable materials with low shrinkage, residual stress and
birefringence. Further, in some instances, the functionalization
and addition of reactive diluents can then be performed while in
the same reaction equipment minimizing contamination and oxidation
of the final curable formulation.
[0083] The composition and process for making optical products of
the present invention are applicable to a variety of applications
needing optical elements including, for example, optical lenses
such as Fresnel lenses, prisms, optical films, such as high index
of refraction films, non-warping and low birefringence film e.g.,
microreplicated films such as totally internal reflecting films, or
brightness enhancing films, flat films, multilayer films,
retroreflective sheeting, optical light fibers or tubes, and
others. Such optical products are useful in optical assemblies,
optical projection systems, such as projection televisions, as well
as displays and other devices containing the optical assemblies.
The optical products of this invention include articles that are
currently prepared from ground glass, or injection molded
plastic.
[0084] Such articles may have a thickness of about 0.5 mm or
greater, and can be prepared from a curable composition of this
invention which is made by mixing in a suitable vessel, in any
convenient order, the oligomer, crosslinking agent and/or reactive
diluent, and a photoinitiator. Mixing is continued until the
components of the composition are in a single phase. Thicknesses of
25 mm articles have been achieved using the composition and curing
process of this invention.
[0085] At the time of use, the composition is preferably degassed
using a vacuum of less than about 25 Torr or by flowing the
composition in a thin film past a suitable boundary. The degassed
composition is introduced, optionally using a pressure of about 2
to 400 Kg/cm.sup.2, into a mold corresponding to the shape of the
article that is desired to be prepared. Such molds are generally
made of plastic, glass or metal, or combinations thereof.
[0086] In one embodiment, the curable composition may be applied to
the surface of the mold having the requisite shape or to mold
elements corresponding to the desired optical article, such as a
lens. The volume of curable composition that enters the mold or
mold elements can be controlled by sliding a squeegee across the
surface of the mold. The amount of curable composition can also be
applied by other known coating techniques, such as by the use of a
roller. If desired, heating may be used to reduce the viscosity of
the composition and provide more efficient molding. As described,
many embodiments of the invention are melt-processible, i.e.
possess or achieve a suitable low viscosity for coating or molding
at temperatures less than or equal to 100.degree. C.
[0087] The mold elements may be completely filled or may be
partially filled. If the photopolymerizable composition is a 100%
solids, non-shrinking, curable material, then the shape of the
cured composition will remain the same as that of the mold
elements. However, if the photopolymerizable composition shrinks as
it hardens, then the liquid will contract, creating unreliable
registration, and introducing optical defects. Preferably, the
photopolymerizable composition includes materials that shrink by
less than about 5% by volume, and preferably less than about 3%,
during curing.
[0088] To initiate photopolymerization, the molds are filled,
placed under a source of actinic radiation such as a high-energy
ultraviolet source having a duration and intensity of such exposure
to provide for essentially complete (greater than 80%)
polymerization of the composition contained in the molds. If
desired, filters may be employed to exclude wavelengths that may
deleteriously affect the reactive components or the
photopolymerization. Photopolymerization may be effected via an
exposed surface of the curable composition, or "through-mold" by
appropriate selection of a mold material having the requisite
transmission at the wavelengths necessary to effect
polymerization.
[0089] Photoinitiation energy sources emit actinic radiation, i.e.,
radiation having a wavelength of 700 nanometers or less which is
capable of producing, either directly or indirectly, free radicals
capable of initiating addition polymerization and chain-growth
polymerization of the optical casting resins of this invention.
Preferred photoinitiation energy sources emit ultraviolet
radiation, i.e., radiation having a wavelength between about 180
and 460 nanometers, including photoinitiation energy sources such
as carbon arc lights, low, medium, or high pressure mercury vapor
lamps, swirl-flow plasma arc lamps, xenon flash lamps, ultraviolet
light emitting diodes, and ultraviolet light emitting lasers.
Particularly preferred ultraviolet light sources are xenon flash
lamps available from Xenon Corp, Wilburn, Mass., such as models
RC-600, RC-700 and RC-747 pulsed UV-Vis curing systems and LED
sources such as Norlux Series 808 large area array, (available from
Norlux, Carol Stream, Ill.). Although not preferred, the curable
composition may also use convention thermal initiators, previously
described.
[0090] In some embodiments, the optical product can contain one or
more features, such as flat or curved surfaces (including convex
and concave surfaces), or replicated or microreplicated surfaces
(such as Fresnel lenses), either of which can be derived from the
composition of the invention and a suitable mold. Structure-bearing
articles can be constructed in a variety of forms, including those
including plurality of linear prismatic structures having a series
of alternating tips and grooves. An example of such a film is BEF,
having regular repeating pattern of symmetrical tips and grooves.
Other examples include patterns in which the tips and grooves are
not symmetrical and in which the size, orientation, or distance
between the tips and grooves is not uniform. Several examples of
surface structure bearing articles useful as brightness enhancing
films are described in U.S. Pat. No. 5,175,030, (Lu et al.) and
U.S. Pat. No. 5,183,597, (Lu) said descriptions being incorporated
herein by reference.
[0091] According to the descriptions of Lu and Lu et al., a
structure-bearing optical product can be prepared by a method
including the steps of (a) preparing a polymerizable composition;
(b) depositing the polymerizable composition onto a master negative
microstructured molding surface in an amount barely sufficient to
fill the cavities of the master; (c) filling the cavities by moving
a bead of the polymerizable composition between a preformed base
and the master, at least one of which is flexible; and (d) curing
the composition. The master can be metallic, such as nickel,
nickel-plated copper or brass, or can be a thermoplastic material
that is stable under the polymerization conditions, and that
preferably has a surface energy that allows clean removal of the
polymerized material from the master.
[0092] In a preferred embodiment, the optical article comprises a
polarizing beam splitter, wherein incident light is split into
first and second substantially polarized beam states that may be
used in an image display system. As shown in FIG. 1, the beam
splitter comprises a first prism (60a), a second prism (60b) and a
polarizing layer having a pass axis disposed therebetween (20). At
least one prism comprises the instant cured composition. Each of
the prisms has a first surface coincident with the polarizing
layer, and two or more outer surfaces. As used herein, the term
"prism" refers to an optical element that controls the angular
transmission of incident light through the polarizing layer, and
the angular character of light exiting the article. The prisms may
be regular polygons, such as triangular prisms, or may have one or
more features that confer optical power to the article, such as
curved faces, or microreplicated features, such as microlenses (and
arrays thereof) or Fresnel lenses. Further, the prisms may further
comprise mirrored elements, such as a vapor deposited metal coating
on or more surfaces.
[0093] Although depicted as including two triangular prisms (see
FIGS. 1 and 2), the prisms may be any suitable shape disposed on
one or both sides of the polarizing layer to achieve the desired
purpose of the PBS. In some embodiments, one or more of the outer
surfaces of the first and second prisms, i.e. one of the surfaces
not adjacent the polarizing layer, may be curved, either convex or
concave, or may comprise a structured surface, such as a Fresnel
lens surface. Such curved surfaces provide optical power to the
polarizing beam splitter; i.e., they converge or diverge light
passing therethrough. The degree to which a lens or mirror
converges or diverges light usually is equal to the reciprocal of
the focal length of the device.
[0094] Further, one or more of the first surfaces (i.e. the surface
adjacent to the polarizing layer) may be curved or microreplicated.
For example, a first prism may have a convex first surface, and a
second prism may have a mating concave first surface, with a
polarizing layer disposed therebetween. Further, one or more of the
outer surfaces of the first and second prisms, (i.e. one of the
surfaces not adjacent the polarizing layer), may be fully or
partially reflective; i.e. comprises a vapor-deposited metal
coating.
[0095] Reflective polarizing layers in exemplary PBS's constructed
according to the present disclosure include linear reflective
polarizers having a pass axis. In one embodiment, the polarizing
layer may be a wire grid polarizer, such as those described in
Schnabel et al., "Study on Polarizing Visible Light by
Subwavelength-Period Metal-Stripe Gratings", Optical Engineering
38(2), pp. 220-226, February 1999, relevant portions of which are
hereby included by reference. A wire grid polarizer consists of an
array of very fine parallel lines or ribbons of metal coated on
glass or other transparent substrates. The wire array efficiently
polarizes the incident light when the width and spacing are small
compared to the incident wavelength(s). Common metals for the wire
grid array include gold, silver, and aluminum among others known in
the art.
[0096] In one embodiment the polarizing beam splitter may comprise
a first prism having a first surface and at least two outer
surfaces, a second prism having a first surface and least two outer
surfaces, and a wire grid polarizer disposed between the first
surfaces of the first and second prisms. Preferably, a wire grid
polarizer comprising a substrate, such as glass, is bonded to the
first surfaces by means of an optical adhesive. Less preferably,
the wire grid is deposited, such as by vapor deposition techniques,
on one of said first surfaces, and the second prism bonded
thereto.
[0097] Wire-grid polarizers absorb small portions of the received
light. This generates heat in the substrates and is therefore not
preferred. For example, 5-10% of the light is absorbed by aluminum
stripes in the same manner as an aluminum mirror surface. Since the
performance of the wire-grid polarizer is sensitive to the
geometric stability of the metal stripes, a small change in the
substrates due to thermal expansion can degrade the polarizer's
performance.
[0098] In another embodiment, the polarizing layer may comprise
alternating repeating layers of a pair of inorganic thin film
materials deposited on the first surface of one or both prisms. The
pair of thin film materials comprises one low refractive index
material and one high refractive index material. The indices,
called a MacNeille pair, are chosen such that, for a given angle of
incidence of a light beam, the reflection coefficient for
p-polarized light (rd is essentially zero at each thin film
interface. The angle at which r.sub.p is zero is called the
Brewster angle, and the formula relating the Brewster angle to the
numerical values of the indices is called the MacNeille condition.
The reflection coefficient for s-polarized light (r.sub.s) is
non-zero at each thin film interface. Therefore, as more thin film
layers are added, the total reflectivity for s-polarized light
increases while the reflectivity for p-polarized light remains
essentially zero. Thus, an unpolarized beam of light, incident upon
the thin film stack, has some or all of the s-polarized components
reflected while essentially all of the p-polarized component is
transmitted.
[0099] In one embodiment, the repeating layers of a pair of
inorganic thin film materials (the optical stack) is deposited on
the first surface of a prism and the bonded to the first surface of
a second prism, such as with an optical adhesive to form a
polarizing beam splitter. The polarizing beam splitter comprises at
least one set of pairs of alternating layers of materials having
low and high indices of refraction compared to each other. The
thicknesses of the layers are chosen such that the quarterwave
criterion is met for the wavelength of the incident collimated
light beam by each of layers of low and high refractive index
material. The optical properties of the prism material, and the
properties of the composite optical stack, all combine to divide
the incident light beam into two polarization components.
[0100] Suitable materials for the thin films include any materials
that are transparent (exhibit low absorption) in the spectrum of
interest. For broadband visible light, suitable thin film materials
are silicon dioxide (n=1.45), amorphous hydrogenated silicon
nitride (n=1.68-2.0); titanium dioxide (n=2.2-2.5); magnesium
fluoride) (n=1.38); cryolite (Na.sub.3AlF.sub.6, n=1.35); zinc
sulphide (n=2.1-2.4); zirconium oxide (n=2.05); haffiium oxide
(n=2.0); and aluminum nitride (n=2.2).
[0101] Several thin film deposition techniques can be used to
deposit the composite optical stack on the prisms, including
thermal and electron beam evaporation, and ion beam sputtering and
magnetron sputtering. However, thermal and electron beam
evaporation should provide good thin film qualities and
sufficiently high deposition rates for acceptable manufacturing
rates. More importantly, low index films such as magnesium fluoride
and cryolite can be deposited by this method. Electron beam
deposition is regularly used in the coatings industry for high
index materials such as titanium dioxide, zirconium oxide, hafnium
oxide, and aluminum nitride.
[0102] Preferably, the polarizing layer may be a multilayer optical
film. Examples of reflective polarizing films suitable for use as
polarizing film in the embodiments of the present disclosure
include reflective polarizers including a birefringent material,
manufactured by 3M Corporation, St. Paul, Minn., such as those
described in U.S. Pat. No. 5,882,774, (Jonza et al.); U.S. Pat. No.
6,609,795(Weber et al.); and U.S. Pat. No. 6,719,426 (Magarill et
al.), the disclosures of which are hereby incorporated by reference
herein. Suitable reflective polarizing films for polarizing film 22
also include polymeric reflective polarizing films that include
multiple layers of different polymeric materials. For example,
polarizing film may include a first layer and a second layer, where
the polymeric materials of the first and second layer are different
and at least one of the first and second layers is birefringent. In
one embodiment of the present disclosure, the polarizing film may
include a multi-layer stack of first and second alternating layers
of different polymer materials, as disclosed in U.S. Pat. No.
6,609,795 (Weber et al.). Other materials suitable for making
multilayer reflective polarizers are listed, for example in Jonza
et al., U.S. Pat. No. 5,882,774; Weber et al., U.S. Pat. No.
6,609,795. In another embodiment of the present disclosure,
multiple reflective polarizing films may be used.
[0103] Suitable reflective polarizing films are typically
characterized by a large refractive index difference between first
and second polymeric materials along a first direction in the plane
of the film and a small refractive index difference between first
and second polymeric materials along a second direction in the
plane of the film, orthogonal to the first direction. In some
exemplary embodiments, reflective polarizing films are also
characterized by a small refractive index difference between the
first and second polymeric materials along the thickness direction
of the film (e.g., between the first and second layers of different
polymeric materials). Examples of suitable refractive index
differences between the first and second polymeric materials in the
stretched direction (i.e., x-direction) range from about 0.15 to
about 0.20. The refractive indices in the non-stretched directions
(i.e., the y-direction and the z-direction) are desirably within
about 5% of one another for a given material or layer, and within
about 5% of the corresponding non-stretched directions of a
different material or an adjacent layer.
[0104] The polymeric materials selected for the layers of an
exemplary multilayer polarizing film may include materials that
exhibit low levels of light absorption. For example, polyethylene
terephthalate (PET) exhibits an absorption coefficient of less than
1.0.times.10.sup.-5 centimeter.sup.-1. Accordingly, for a
reflective polarizer film that includes PET and has a thickness of
about 125 micrometers, the calculated absorption is about
0.000023%, which is about 1/200,000 of an absorption of a
comparable wire-grid polarizer.
[0105] Low absorptions are desirable because polarizers used in
PBS's are exposed to very high light density, which can lead to the
failure of the polarizers. For example, absorptive-type polarizer
films absorb all the light with unwanted polarization. This
generates significant heat. Substrates with high thermal
conductivity, such as sapphire, are therefore needed to conduct the
heat away from the polarizer films. Moreover, the substrates are
exposed to high heat loads, which correspondingly generate thermal
birefringence in the substrates. Thermal birefringence in the
substrates degrades the contrast and contrast uniformity of the
optical system, such as an image display system. As a result, only
few materials can be qualified for the substrates with traditional
PBS's (e.g., sapphire, quartz, leads content glass, and
ceramics).
[0106] The present invention provides a multilayer article
comprising a multilayer optical film and a cured optical coating on
one or both major surfaces of the optical film. Providing such a
coating protects the multilayer optical film from environmental
stresses and adds strength and rigidity thereto. The multilayer
article may be prepared by providing a multilayer optical film,
coating at least one major surface of the multilayer optical film
with the curable composition, and curing. In another embodiment,
separately prepared films comprising the cured composition may be
adhered to one or both major surfaces of the multilayer optical
film by means of an optical adhesive, described further herein.
[0107] The present invention provides a method of making a
polarizing beam splitter. The method comprises introducing the
curable composition into a suitable mold, and curing the
composition to form a prism. The mold may be of any suitable
configuration, one or more surfaces of which may be curved. The
polarizing layer may then be bonded, adhered, or otherwise affixed
to the resulting prism(s) by any optical adhesive, such as known in
the art. In one embodiment, a first prism may be bonded to a first
surface of the polarizing layer, the second prism bonded
sequentially to the exposed surface of the polarizing layer. In
another embodiment, the first and second prisms are concurrently
bonded to opposite surfaces of the polarizing layer.
[0108] Useful optical adhesives are substantially free of
UV-absorbing chromophores such as extended aromatic structures or
conjugated double bonds. Useful adhesives include, for example:
NOA61, a UV cured thiol-ene based adhesive available from the
Norland Company (Cranbury, N.J.); Loctite series (e.g., 3492, 3175)
UV cured acrylic adhesives available from Henkel Loctite Corp.,
1001 Trout Brook Crossing, Rocky Hill, Conn. 06067
(www.loctite.com). OP series (e.g., 21, 4-20632, 54, 44) UV cured
acrylic adhesives available from Dymax Corporation, Torrington,
Conn.
[0109] One useful adhesive include those compositions described in
U.S. Published Appln. No. 20040202879 (Xia et al.), incorporated
herein by reference, which comprise at least one polymer with
either an acid or base functionality that forms a pressure
sensitive adhesive, a high T.sub.g polymer with an weight average
molecular weight greater than 100,000 with an acid or base
functionality, and a crosslinker, wherein the functionality on the
pressure sensitive adhesive and the high T.sub.g polymer cause an
acid-base interaction that forms a compatibilized blend. After
accelerated aging of the adhesive composition at 80.degree. C. and
90% relative humidity for approximately 500 hours in an oven, the
adhesive mixture is translucent or optically clear.
[0110] Another useful adhesive includes microstructured adhesive,
which comprise a continuous layer of a pressure-sensitive adhesive
having a microstructured surface, wherein the microstructured
surface comprises a series of features and wherein the lateral
aspect ratio of the features range from about 0.1 to about 10,
wherein the spacing aspect ratio of the features range from about 1
to about 1.9, and wherein each feature has a height of about 2.5 to
about 375 micrometers. Such adhesives are described in U.S. Pat.
Nos. 5,650,215, 6,123,890, 6,315,851, 6,440,880 and 6,838,150 (each
Benson et al.) incorporated herein by reference.
[0111] Other useful adhesives include Soken.TM. 1885 PSA
(commercially available from Soken Chemical & Engineering Co.,
Ltd, Japan), NEA PSA (as described in the Example 1 of published
U.S. 20040202879 (Lu et al.)), Lens Bond.TM. Type C59 (a thermally
cured styrene based adhesive available from Summers Optical,
Hatfield, Pa., a division of EMS Acquisition Corp., and NOA61.TM.
(a UV cured thiol-ene based adhesive, available from Norland
Company, Cranbury, N.J.).
[0112] In another embodiment, the polarizer may be prepared as
shown schematically in FIG. 1. Here, a prism mold 10a, having an
open first surface or face, and optional tabs 11a and b, is
combined with a polarizing layer 20 and rigid side plate 30. The
angles between the mold faces may be varied as desired, and either
or both outer faces 12a/b may be curved or have any desired pattern
imparted thereto, such as a diffracting pattern, including a
Fresnel lens may be integrally molded. The respective first
surfaces (those coincident with the polarizing layer 20) of the
first and second prisms may also be curved, or have an integral
replicated pattern. Advantageously, the curved first surfaces of
the first and second prisms may configured so they may be mated,
such as with a concave and convex surface, with the polarizing
layer 20 disposed therebetween.
[0113] The parts 10a, 20 and 30 may be secured via clamps on tabs
11a/b, or by other suitable means. A tensioning means (not shown)
may be used to maintain the polarizing layer 20 flat. The assembled
mold may rest on a smooth, rigid surface 15, such as glass, or an
integral bottom (not shown) may be provided to the mold 10a. This
assembly defines a prism shaped cavity 40a, into which the curable
composition may be introduced, and cured by application of UV
energy. If desired, a second rigid surface (not shown) may cover
the top of the mold to protect it from atmospheric oxygen.
Desirably, either rigid surface 15 or second rigid surface is made
of glass or other suitable material which is transparent to the
incident light source used for curing. Alternatively to the second
rigid surface, the mold assembly may be blanketed with an inert
glass to exclude oxygen.
[0114] Upon cure, the rigid side plate 30 may be removed, and a
second prism mold 10b secured thereto, forming a second chamber
40b, with the polarizing layer 20 forming the common faces between
molds 10a and 10b, so that the first surfaces of the first and
second prisms will each be adjacent the polarizing layer. As result
of the curing the polarizing layer is now integral to the first
surface of the first prism-shaped cured composition. The second
mold may also have curved outer faces or other desired molded
shapes (not shown). This second chamber 40b may be filled with the
curable composition, cured, the mold assembly removed to provide a
polarizing beam splitter 60a having two prisms, and an integral
polarizing layer disposed therebetween, on the respective first
surfaces of the prisms. If desired, the respective prisms may be
provided, by a suitably configured mold, with integral interlocking
tabs for securing the first and second prisms together, or for
securing the beam splitter into a mount in a display device.
Further, the first and second prisms may be providing with
alignment means, such as tabs or indicators, for aligning the first
and second prisms with respect to each other, the polarizing layer,
or in a mount in a display device.
[0115] The alignment means may comprise corresponding male and
female portions that interconnect. The polarizing beams splitter
may comprise a first prism and second prism, where first prism
includes male portion, and second prism that includes female
portion. Male portion may be a rectangular surface that encompasses
a portion of the surface of first prism adjacent to the reflective
polarizing film, and which projects therefrom. Similarly, female
portion may be a rectangular depression that is disposed within the
majority of the surface of second prism adjacent to the reflective
polarizing film.
[0116] The male members and female portions may be substituted with
other engagement mechanisms such that one prism includes at least
one male member that is configured to engage with a respective
female portion located in the opposing prism. Those of ordinary
skill in the art will also readily appreciate that different
numbers of the male members and the female portions than those
exemplified herein may be used in accordance with the present
disclosure. For example, an exemplary PBS may include three or more
male members received within three or more female portions.
[0117] The male members and the female portions discussed above may
be molded with the respective first and second prisms. The first
and second prisms may then be secured together with the assistance
of the male portions and the female portions to form polarizing
beam splitters. This technique may involve placing the reflective
polarizing film between the first prism and the second prism. The
first prism may then be oriented relative to the second prism such
that the male portion(s) are aligned with the corresponding female
portion(s). This alignment is beneficial for ensuring that the
first prism is accurately positioned relative to the second prism.
The first prism may then engage second prism by concurrently
inserting male portions into the corresponding female portions.
This compresses the reflective polarizing film between the incident
surfaces of the first prism and the second prism to provide a
smooth, planar interface. The male portion(s) may be secured to the
corresponding female portions with an adhesive. Additionally, the
first prism may be secured to the second prism by fitting and/or
welding the male members to the corresponding female portions
(e.g., ultrasonic, infrared, heat staking, snap fits, press fits,
and chemical welding).
[0118] An alternate process is shown schematically in FIG. 2. Here,
two prism molds having open faces, 110a and 110b (corresponding to
the first surfaces of the resultant prisms) are secured together
via optional tabs 111a and b(or any suitable means), with the
polarizing layer 120 disposed between on the common first faces of
molds 110a and b. If desired, and tensioning means may be used to
maintain the polarizing layer 120 flat. This creates two chambers
140a/b that may be filled with the curable composition, and cured
to produce a polarizing beam splitter, with an integral polarizing
layer, in a single step. Again, the molds may be of any suitable
shape and size, and the exterior faces may be curved.
EXAMPLES
[0119] These examples are merely for illustrative purposes only and
are not meant to be limiting on the scope of the appended claims.
All parts, percentages, ratios, etc. in the examples and the rest
of the specification are by weight, unless noted otherwise.
Solvents and other reagents used were obtained from Sigma-Aldrich
Chemical Company; Milwaukee, Wis. unless otherwise noted.
TABLE-US-00001 Table of Abbreviations Abbreviation or Trade
Designation Description IBOA Isobornyl acrylate, available from
Sartomer Company Inc, Exton, PA MMA Methyl methacrylate HEA
Hydroxyethyl acrylate HEMA Hydroxyethyl methacrylate IOTG Isooctyl
thioglycolate, available from TCI America, Portland, OR MCE
Mercaptoethanol IEM Isocyanatoethyl methacrylate, available from
Showa Denka, Japan MAnh Methacrylic anhydride HDDMA 1,6 Hexanediol
dimethacrylate, SR239, available from Sartomer Company Inc, Exton,
PA Vazo 52 2,2'-Azobis(2,4-dimethylvaleronitrile), available from
DuPont Company, Wilmington, DE Vazo 88
1,1'-Azobis(cyanocyclohexane), available from DuPont Company,
Wilmington, DE DBDL Dibutyltin dilaurate A31 Release A Silicone
liner from DuPont TeiJin Films U.S. Liner Limited Partnership,
Wilmington, DE Lucirin TPO-L Ethyl 2,4,6-trimethylbenzoyl phenyl
phosphinate available from BASF, Mt. Olive, NJ CN945A60
Trifunctional aliphatic urethane acrylate blended with SR306,
(tripropyleneglycol diacrylate), in an approximate 60:40 ratio
available from Sartomer Company Inc, Exton, PA CN1963 Aliphatic
urethane dimethacrylate blended with TMPTMA (trimethylolpropane
trimethacrylate), in an approximate 75:25 ratio available from
Sartomer Company Inc, Exton, PA Ebecryl 600 Bisphenol-A epoxy
diacrylate available from Surface Specialties UCB, Smyrna, GA
Ebecryl 830 Polyester hexaacrylate available from Surface
Specialties UCB, Smyrna, GA PBS FILM multilayer reflective
polarizing film manufactured by 3M Corporation, St. Paul, MN,
described in U.S. Pat. No. 5,882,774, (Jonza et al.); U.S. Pat. No.
6,609,795 (Weber et al.); and U.S. Pat. No. 6,719,426 (Magarill et
al.).
Test Methods
Molecular Weight Measurement by SEC
[0120] Size exclusion chromatography (SEC) for molecular weight and
molecular weight distribution was performed using a Waters 717Plus
autosampler, 1515 HPLC pump, 2410 differential detector, and the
following Waters columns: Styragel HR 5E, Styragel HR 1. All
samples were run in THF at 35.degree. C. with a flow rate of 1.0
mL/min. Linear polystyrene standards were used for calibration.
Dynamic Mechanical Analysis (DMA) Measurement
[0121] DMA for T.sub.g and modulus determination of cured
compositions was performed using a LC-ARES Test Station (Rheometric
Scientific, Piscataway, N.J.) in a torsion mode. The sample size
was approximately 25 millimeters by 10 millimeters by Imillimeter.
The length of the sample was measured by the test station and the
width and thickness of the sample were measured with a caliper. The
test was performed by ramping the temperature from 25.degree. C. to
180.degree. C. at 5.degree. C. per minute. The frequency used was 1
Hertz.
Yellowing Resistance Test
[0122] The % Transmittance (% T) at a wavelength of 420 nanometers
of 3.2 centimeter (1.25 inches) diameter by 0.5 centimeter (0.19
inch) thick cured samples was measured before and after 7 days
aging in a 120.degree. C. oven. The % T was measured using a TCS
Plus Spectrophotometer (BYK-Gardner USA, Silver Spring, Mo.).
Generally, samples with % T at 420 nanometers of less than 85%
display a yellow color. A sample is considered to have good
yellowing resistance if the % T at 420 nanometers after aging is
greater than 85%.
Volume Shrinkage Determination
[0123] Density of the curable compositions and the cured materials
were measured by a pycnometer. % volume shrinkage was calculated
based on the density change during cure of the curable materials. %
volume shrinkage=100.times.(density of cured material--density of
curable material before cure)/density of curable material before
cure.
Water Absorption Measurement
[0124] A weighed 3.2 centimeter (1.25 inches) diameter by 0.5
centimeter (0.19 inch) thick cured disk sample is placed in water
at 23.degree. C. for 14 days. % water absorption=100.times.(sample
weight after 14 days in water-sample weight before water
soaking)/sample weight before water soaking.
Birefringence Determination
[0125] Transmission Spectral Ellipsometry (TSE) was used to measure
retardance of the sample. Birefringence of the sample was
determined by dividing the retardance by sample thickness. The
sample is a round disc, 3.2 centimeter (1.25 inches) diameter by
0.5 centimeter (0.19 inch) thick. The sample was mounted on a
rotating stage, and the TSE retardance data was measured at a
series of positions using a J. A. Woollam M2000 Variable Angle
Spectral Ellipsometer. In-plane measurements were taken at 4
locations 6 millimeters apart in two orthogonal directions, for a
total of 8 in-plane measurements. The measured retardances were
averaged in the wavelength range between .lamda.=545-555
nanometers.
EXAMPLES 1-7
Preparation of Oligomer Syrups:
[0126] In Examples 1-7, IBOA, HEA, chain transfer agent IOTG or
MCE, and the 1.sup.st charge of thermal initiators Vazo 52 and 88,
according to Table 1, were added to a four neck flask equipped with
a reflux condenser, thermometer, mechanical stirrer, and nitrogen
gas inlet. The mixture was stirred and heated to 60.degree. C.
under nitrogen. The temperature of the reaction mixture peaked at
around 150.degree. C. during the polymerization. After the reaction
peak, the batch was further polymerized at 140.degree. C. for 30
minutes with the addition of the 2.sup.nd initiator, Vazo 88, to
reduce residual monomers and eliminate initiator. A sample was
taken at the end of this reaction period for oligomer molecular
weight determination by SEC. After that, the batch was cooled to
100.degree. C. The HDDMA reactive diluent was added to the reactor
to reduce viscosity of the batch. A solution of the DBDL catalyst
in IEM was then added to the batch to react with the hydroxyls on
the IBOA/HEA polymer chains, incorporating methacrylate functional
groups to the polymer. The reaction was complete in 2 hours.
Preparation of Cured Samples:
[0127] After completion of the reaction, the reactive oligomer
syrups of Table 1 were formulated with 0.02 weight % TPOL
photoinitiator and cured by a xenon flash lamp according to the
procedures described in the section Preparation of Test Samples.
The cured samples were tested for % volume shrinkage,
birefringence, Tg, water absorption, % transmittance, and aging
stability, using the methods described in the above Test Method
section. TABLE-US-00002 TABLE 1 Example 1 2 3 4 5 6 7 IBOA* (g)
190.0 190.0 190.0 190.0 180.0 180.0 190.0 HEA (g) 10.0 10.0 10.0
10.0 20.0 20.0 10.0 IOTG (g) 1.0 2.0 4.0 16.0 16.0 -- 4.0 MCE (g)
-- -- -- -- -- 20.0 -- Vazo 52/(g) 0.025/ 0.025/ 0.025/ 0.025/
0.025/ 0.025/ 0.025/ Vazo 88 (g) 0.025 0.025 0.025 0.025 0.025
0.025 0.025 Vazo 88 (g) 0.050 0.050 0.050 0.050 0.050 0.050 0.050
HDDMA* (g) 66.7 50.0 50.0 22.2 22.2 22.2 22.2 IEM* (g) 13.36 13.36
13.36 13.36 26.7 53.4 13.36 DBDL (g) 0.0467 0.0467 0.0467 0.0467
0.093 0.18 0.0467 *The IBOA monomer and HDDMA reactive diluent in
Examples 3, 4 and 5 were purified by passing through a column of
activated basic aluminum oxide powder, Brokmann1, .about.150 mesh,
58 A.degree., 155 m.sup.2/g, from Aldrich to remove inhibitors. The
IEM monomer in the same examples was distilled to remove the
inhibitor. The other ingredients in Examples 3, 4, and 5, and all
ingredients in Examples 1, 2, 6, and 7 were used as received from
the vendors without purification.
EXAMPLES 8-9 AND COMPARATIVE EXAMPLE C1
[0128] To prepare the reactive oligomer syrups of Examples 8-9 and
Comparative Example C1, HDDMA reactive diluent, according to Table
2, was added to samples of the reactive oligomer syrup prepared in
Example 7. The reactive oligomer syrups were formulated with 0.02
weight% TPOL photoinitiator and cured by a xenon flash lamp at
80.degree. C. for 5 minutes. The cured samples were tested for %
volume shrinkage, birefringence, Tg, water absorption, %
transmittance, and aging stability, using the methods described in
the above Test Method section. TABLE-US-00003 TABLE 2 Example 8 9
C1 Oligomer/HDDMA Ratio 80/20 70/30 60/40 Example 7 (grams) 20.0
20.0 20.0 HDDMA (grams) 2.50 5.71 10.0 Lucirin TPOL (grams) 0.0045
0.0051 0.0060
EXAMPLES 10-11
[0129] To prepare reactive oligomer syrups of Example 10 and 11 in
Table 3, the same polymerization procedure described in the
Examples 1-7 above was followed, except 30 minutes after the
addition of the 2.sup.nd initiator at 140.degree. C., MAnh was
added and the mixture was reacted at 140.degree. C. for another 3
hours with efficient stirring. After that, the batch was cooled
down to 110.degree. C. and HDDMA reactive diluent was added to the
reactor to further reduce the viscosity of the batch.
TABLE-US-00004 TABLE 3 Example 10 11 IBOA (g) 190 160 MMA (g) 0 26
HEA (g) 10 10 HEMA (g) 0 4 IOTG (g) 4 8 Vazo 52/(g) 0.025/ 0.025/
Vazo 88 (g) 0.025 0.025 Vazo 88 (g) 0.050 0.050 MAnh (g) 14.12
19.16 HDDMA (g) 50 35.3
COMPARATIVE EXAMPLES C2-C5
[0130] The reactive oligomer or oligomer and reactive diluent
mixtures, CN945A60, CN1963, Ebecryl 600 and Ebecryl 830, were
formulated with 0.02% TPOL photoinitiator and cured by a Xenon
Flash lamp at 80.degree. C. for 5 minutes according to procedures
described in the preparation of test sample. TABLE-US-00005 TABLE 4
Comparative Example C2 C3 C4 C5 CN945A60 (g) 100 -- -- -- CN1963
(g) -- 100 -- Ebecryl 600 (g) -- -- 100 -- Ebecryl 830 (g) -- -- --
100 Lucirin TPOL (g) 0.02 0.02 0.02 0.02
PREPARATION OF TEST SAMPLES OF EXAMPES 1-11 AND COMPARATIVE
EXAMPLES C1-C5
[0131] Curable compositions with photoinitiator and other additives
were prepared by preheating the oligomer syrups described in the
Examples and Comparative Examples with a desired photoinitiator and
other additives (if used) at 80.degree. C. and mixing in a white
disposable cup by a DAC-100 mixer (both cup and mixer are available
from Flack Tek Inc, Landrum, New Jersey). The compositions were
degassed in a vacuum chamber and then allowed to cool to room
temperature before use.
[0132] Curing of the above curable materials was carried out by the
following steps: 1) Onto a Pyrex glass plate approximately 15
centimeters (6 inches) by 15 centimeters (6 inches) by 0.5
centimeter (0.19 inch) was placed an approximately 15 centimeters
(6 inches) by 15 centimeters (6 inches) piece of 51 micrometers (2
mils) A31 release liner, 2) on top of the release liner was placed
an approximate same size glass or silicone rubber mold with a 3
centimeter (1.25 inches) diameter opening at the center, 3) then
the mold was filled with the curable compositions taking care to
avoid bubbles, 4) then a second piece of approximately 15
centimeters (6 inches) by 15 centimeters (6 inches) of 51
micrometers (2 mils) A3 1 release liner was placed on top of the
filled mold, 5) another Pyrex glass plate approximately 15
centimeters (6 inches) by 15 centimeters (6 inches) by 0.5
centimeter (0.19 inch) was placed on top of the release liner, and
6) finally, the filled mold was placed onto a heating station at
80.degree. C. in a chamber and allowed to equilibrate. The curable
compositions were cured by a Xenon flash lamp (Model #4.2 Lamp Hsg,
pulse rate of 8 Hz) with RC-747 Pulsed UV Visible System (Xenon
Corporation, Woburn, Mass.) for 5 minutes.
[0133] The weight average molecular weight which was measured by
SEC, the Tg which was measured by Dynamic Mechanical Analysis
(DMA), the % volume shrinkage which was determined by a pycnometer,
and the birefringence which was measured by ellipsometry are shown
in Table 5. Visibly noticeable color and % transmittance measured
by the TCS Plus Spectrophotometer after 7 days aging in a
120.degree. C. oven are also shown in Table 5. Water absorption
data obtained as described in the Test Method section above are
shown in Table 6. TABLE-US-00006 TABLE 5 Sample Color % T at 420 nm
Volume after 120.degree. C. after 120.degree. C. Oligo. Oligo/ Tg
Shrinkage aging for 7 aging for 7 Example MW HDDMA (%) (.degree.
C.) (%) Birefringence days days 1 30K 75/25 -- -- 2.59 .times.
10.sup.-7 None 87.3 2 18K 80/20 -- -- 1.51 .times. 10.sup.-7 None
89.3 3 9.0K 80/20 109 2.8 5.07 .times. 10.sup.-7 -- -- 4 3.0K 90/10
-- 2.1 1.32 .times. 10.sup.-7 -- -- 5 3.2K 90/10 88 2.4 4.03
.times. 10.sup.-7 -- -- 6 1.4K 90/10 -- -- 6.33 .times. 10.sup.-7
-- -- 7 9.0K 90/10 94 0.9 1.82 .times. 10.sup.-7 None 89.9 8 9.0K
80/20 100 2.7 1.47 .times. 10.sup.-7 None 90.2 9 9.0K 70/30 94 4.7
4.39 .times. 10.sup.-7 None 90.1 C1 9.0K 60/40 79 5.5 1.24 .times.
10.sup.-6 None 89.4 10 9.0K 80/20 -- -- -- None 91.3 11 6.5K 85/15
-- -- -- None 90.5 C2 -- -- 53 -- -- Yellow 81.0 C3 1.2K -- 97 6.8
3.88 .times. 10.sup.-6 Yellow 74.3 (3 days) C4 -- -- 67 -- 7.06
.times. 10.sup.-5 Yellow 80.7 C5 -- -- 60 -- 7.81 .times. 10.sup.-6
yellow 82.8
[0134] TABLE-US-00007 TABLE 6 Water Absorption Example (14 days in
23.degree. C. water) 8 0.17% 10 0.22% C3 0.87%
EXAMPLE 12
[0135] In Example 12, IBOA, HEA, chain transfer agent IOTG, and the
1.sup.st charge of thermal initiators Vazo 52 and 88, according to
Table 7, were added to a four neck flask equipped with a reflux
condenser, thermometer, mechanical stirrer, and nitrogen gas inlet.
The mixture was stirred and heated to 60.degree. C. under nitrogen.
The temperature of the reaction mixture peaked at around
180.degree. C. during the polymerization. After the reaction peak,
the batch was further polymerized at 140.degree. C. for 30 minutes
with the addition of the 2.sup.nd initiator, Vazo 88, to reduce
residual monomers and eliminate initiator. After that, the batch
was cooled to 120.degree. C. and the IBOA reactive diluent was
added to the reactor to reduce viscosity of the batch. MAnh was
then added to the batch to react with the hydroxyl groups on the
oligomer chains to incorporate methacrylate groups. The reaction
time was approximately 6 hours. The HDDMA was then added and the
solution was cooled to ambient. The reactive oligomer syrups of
Table 7 were formulated with 0.02 weight % Lucirin TPOL
photoinitiator to make the curable material composition to prepare
plastic prisms and PBS prisms. TABLE-US-00008 TABLE 7 Example 12
IBOA (g) 185 HEA (g) 15 IOTG (g) 8 Vazo 52 (g)/Vazo 88 (g)
0.025/0.025 Vazo 88 (g) 0.050 IBOA (g) 10 MAnh (g) 21.18 HDDMA (g)
35.3 Lucirin TPOL (g) 0.055
EXAMPLE 13
Preparation of a Plastic Prism
[0136] For Example 13 the prism mold described in FIG. 1 was used.
Component 10a was made of stainless steel with component 15 being a
glass plate, component 20 was not used and component 30 was a glass
microscope slide. The volume 40a was filled with the formulated
reactive oligomer syrup prepared in Example 12 and another glass
plate was placed on top of filled volume 40a. The assembly was
cured with a Xenon flash lamp at room temperature for 5
minutes.
EXAMPLE 14
Preparation of a Plastic PBS Prism
[0137] For Example 14, the prism mold described in FIG. 1 was used.
Component 10a was made of stainless steel with component 15 being a
glass plate, component 20 was PBS Film and component 30 was a glass
microscope slide. The volume 40a was filled with the formulated
reactive oligomer syrup prepared in Example 12 and another glass
plate was placed on top of filled volume 40a. The assembly was
cured with a Xenon flash lamp at room temperature for 5 minutes.
The glass slide was removed and a plastic prism such as prepared in
Example 13 was attached to the PBS Film surface with an Optical
Adhesive to generate a PBS prism.
EXAMPLE 15
Preparation of a Plastic PBS Prism
[0138] For Example 15, the prism molds described in FIG. 1 were
used. Component 10a was made of stainless steel with component 15
being a glass plate, component 20 was PBS film and component 30 was
a glass microscope slide. The volume 40a was filled with the
formulated reactive oligomer syrup prepared in Example 12 and
another glass plate was placed on top of filled volume 40a. The
assembly was cured with a Xenon flash lamp at room temperature for
5 minutes. The component 30 was then removed and a second mold 10b
was placed adjacent to the PBS Film surface of the cured mold. The
volume 40b was filled with the formulated reactive oligomer syrup
prepared in Example 12 and a glass plate was placed on top of
filled volume 40b. The assembly was cured with a Xenon flash lamp
at room temperature for 5 minutes.
EXAMPLE 16
Preparation of a Plastic PBS Prism
[0139] For Example 16, the prism molds described in FIG. 2 were
used. Components 110a and 110b were made of stainless steel with
component 115 being a glass plate, component 120 was PBS Film film.
The volumes on either side of 120, 140a and 140b, were filled with
the formulated reactive oligomer syrup prepared in Example 12 and
another glass plate was placed on top of the filled volumes. The
assembly was cured with a Xenon flash lamp at room temperature for
5 minutes.
* * * * *