U.S. patent application number 10/298903 was filed with the patent office on 2003-04-10 for pressure-sensitive adhesive for pressure-sensitive adhesive layer in volume hologram laminate.
This patent application is currently assigned to Dai Nippon Printing Co., Ltd.. Invention is credited to Kawasaki, Masahiro, Otaki, Hiroyuki, Shinga, Kazumasa, Ueda, Kenji.
Application Number | 20030068491 10/298903 |
Document ID | / |
Family ID | 17721645 |
Filed Date | 2003-04-10 |
United States Patent
Application |
20030068491 |
Kind Code |
A1 |
Otaki, Hiroyuki ; et
al. |
April 10, 2003 |
Pressure-sensitive adhesive for pressure-sensitive adhesive layer
in volume hologram laminate
Abstract
A pressure-sensitive adhesive for a pressure-sensitive adhesive
layer in a volume hologram laminate is provided which, when kept in
the pressed state, for example, during storage, is less likely to
cause spotty hologram defects in the volume hologram layer. The
pressure-sensitive adhesive for a pressure-sensitive adhesive layer
in a volume hologram laminate is a pressure-sensitive adhesive for
a pressure-sensitive adhesive layer in a volume hologram laminate
comprising a substrate 2 and, stacked on the substrate 2 in the
following order, a first pressure-sensitive adhesive layer 3, a
volume hologram layer 5, a second pressure-sensitive adhesive layer
4, and a surface protective film 6, the second pressure-sensitive
adhesive layer having been formed from an acrylic
pressure-sensitive adhesive comprising an acrylic copolymer resin,
composed mainly of an alkyl acrylate, and a crosslinking agent, the
second pressure-sensitive adhesive layer having a dynamic storage
modulus of not less than 2.5.times.105 Pa as measured at a
measuring frequency of 6.28 rad/sec and a temperature of 50.degree.
C. and a loss tangent (tan .delta.) defined as dynamic loss
modulus/dynamic storage modulus at 120.degree. C. of not more than
0.15.
Inventors: |
Otaki, Hiroyuki;
(Shinjuku-Ku, JP) ; Ueda, Kenji; (Shinjuku-Ku,
JP) ; Shinga, Kazumasa; (Itabashi-Ku, JP) ;
Kawasaki, Masahiro; (Itabashi-Ku, JP) |
Correspondence
Address: |
PARKHURST & WENDEL, L.L.P.
1421 PRINCE STREET
SUITE 210
ALEXANDRIA
VA
22314-2805
US
|
Assignee: |
Dai Nippon Printing Co.,
Ltd.
Shinjuku-Ku
JP
|
Family ID: |
17721645 |
Appl. No.: |
10/298903 |
Filed: |
November 19, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10298903 |
Nov 19, 2002 |
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09679537 |
Oct 6, 2000 |
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6509076 |
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Current U.S.
Class: |
428/343 ;
428/355AC |
Current CPC
Class: |
C08L 2666/14 20130101;
G03H 2240/50 20130101; G09F 3/02 20130101; G03H 2250/10 20130101;
C09J 133/062 20130101; Y10T 428/2848 20150115; Y10T 428/14
20150115; Y10T 428/2891 20150115; C08L 63/00 20130101; C08G 2170/40
20130101; C08L 75/00 20130101; G03H 1/0248 20130101; Y10T 428/28
20150115; C09J 2301/302 20200801; G09F 3/10 20130101; C08G 18/625
20130101; C09J 133/062 20130101; C08L 2666/14 20130101 |
Class at
Publication: |
428/343 ;
428/355.0AC |
International
Class: |
B32B 007/12; B32B
015/04 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 8, 1999 |
JP |
1999-287779 |
Claims
1. A pressure-sensitive adhesive for a pressure-sensitive adhesive
layer in a volume hologram laminate comprising a substrate and,
formed on the substrate in the following order, a first
pressure-sensitive adhesive layer, a volume hologram layer, a
second pressure-sensitive adhesive layer, and a surface protective
film, said second pressure-sensitive adhesive layer having been
formed from an acrylic pressure-sensitive adhesive comprising an
acrylic copolymer resin, composed mainly of an alkyl acrylate, and
a crosslinking agent, said second pressure-sensitive adhesive layer
having a dynamic storage modulus of not less than
2.5.times.10.sup.5 Pa as measured at a measuring frequency of 6.28
rad/sec and a temperature of 50.degree. C. and a loss tangent (tan
.delta.) defined as dynamic loss modulus/dynamic storage modulus at
120.degree. C. of not more than 0.15.
2. The pressure-sensitive adhesive according to claim 1, wherein
the crosslinking agent is a crosslinking agent for room temperature
crosslinking.
3. The pressure-sensitive adhesive according to claim 1, wherein
the crosslinking agent is a crosslinking agent for heat
crosslinking.
Description
TECHNICAL FIELD
[0001] The present invention relates to a pressure-sensitive
adhesive for a pressure-sensitive adhesive layer in a volume
hologram laminate which can reduce spotty hologram defects caused
in a stacked and/or pressed state in transit or during storage in
warehouses or the like.
BACKGROUND ART
[0002] Conventional volume hologram laminates generally have a
construction comprising a substrate and, stacked on the substrate
in the following order, a pressure-sensitive adhesive layer, a
volume hologram layer, a pressure-sensitive adhesive layer, and a
transparent protective film. Volume hologram laminates applied to
various forms of substrates, such as identification cards or
papers, admission tickets for examination, ID cards, booklets such
as passports, have been developed. However, volume hologram
laminates using, as a dry volume phase type hologram recording
material, a recording material comprising a matrix polymer and a
photopolymerizable compound pose a problem of the occurrence of
spotty hologram defects when the volume hologram materials applied,
for example, to books or the like are kept in a stacked and/or
pressed state in transit or during storage in warehouses or the
like.
[0003] The spotty hologram defects are found when the
hologram-recorded layer is observed. The spots are in the form of
ellipse having a size of 0.2 to 2 mm in major axis and about 0.1 to
1.5 mm in minor axis, or in the form of a circle having a radius of
about 0.1 to 2 mm, and occur in a regularly arranged state. The
hologram defects depend upon a light source or a viewing angle.
Therefore, the hologram defects are considered as holographic
defects rather than as defects attributable to the inclusion of air
bubbles or foreign matter in the volume hologram layer.
DISCLOSURE OF THE INVENTION
[0004] Accordingly, it is an object of the present invention to
provide a pressure-sensitive adhesive for a pressure-sensitive
adhesive layer in a volume hologram laminate which, when kept in
the pressed state, for example, during storage, is less likely to
cause spotty hologram defects in the volume hologram layer.
[0005] According to one aspect of the present invention, there is
provided a pressure-sensitive adhesive for a pressure-sensitive
adhesive layer in a volume hologram laminate comprising a substrate
and, stacked on the substrate in the following order, a first
pressure-sensitive adhesive layer, a volume hologram layer, a
second pressure-sensitive adhesive layer, and a surface protective
film, said second pressure-sensitive adhesive layer having been
formed from an acrylic pressure-sensitive adhesive comprising an
acrylic copolymer resin, composed mainly of an alkyl acrylate, and
a crosslinking agent, said second pressure-sensitive adhesive layer
having a dynamic storage modulus of not less than
2.5.times.10.sup.5 Pa as measured at a measuring frequency of 6.28
rad/sec and a temperature of 50.degree. C. and a loss tangent (tan
.delta.) defined as dynamic loss modulus/dynamic storage modulus at
120.degree. C. of not more than 0.15.
[0006] According to a preferred embodiment of the present
invention, the crosslinking agent is a crosslinking agent for room
temperature crosslinking.
[0007] According to another preferred embodiment of the present
invention, the crosslinking agent is a crosslinking agent for heat
crosslinking.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is an explanatory cross-sectional view of a volume
hologram laminate according to the present invention;
[0009] FIG. 2 is a diagram showing the temperature dependency of
dynamic storage modulus, dynamic loss modulus, and loss tangent
(tan .delta.) as measured at a measuring frequency of 6.28 rad/sec
for the second pressure-sensitive adhesive layer in Example 1;
[0010] FIG. 3 is a diagram showing the temperature dependency of
dynamic storage modulus, dynamic loss modulus, and loss tangent
(tan .delta.) as measured at a measuring frequency of 6.28 rad/sec
for the second pressure-sensitive adhesive layer in Example 2;
[0011] FIG. 4 is a diagram showing the temperature dependency of
dynamic storage modulus, dynamic loss modulus, and loss tangent
(tan .delta.) as measured at a measuring frequency of 6.28 rad/sec
for the second pressure-sensitive adhesive layer in Example 3;
[0012] FIG. 5 is a diagram showing the temperature dependency of
dynamic storage modulus, dynamic loss modulus, and loss tangent
(tan .delta.) as measured at a measuring frequency of 6.28 rad/sec
for the second pressure-sensitive adhesive layer in Comparative
Example 1; and
[0013] FIG. 6 is a diagram showing the temperature dependency of
dynamic storage modulus, dynamic loss modulus, and loss tangent
(tan .delta.) as measured at a measuring frequency of 6.28 rad/sec
for the second pressure-sensitive adhesive layer in Comparative
Example 2.
BEST MODE FOR CARRYING OUT THE INVENTION
[0014] The present inventor has found that hologram defects in a
volume hologram laminate are attributable to a correlation between
the hardness of the volume hologram layer and the hardness of a
pressure-sensitive adhesive layer provided between the volume
hologram layer and the surface protective film, which has led to
the completion of the present invention.
[0015] The volume hologram laminate according to the present
invention is shown in FIG. 1 (a cross-sectional view). In the
drawing, numeral 1 designates a volume hologram laminate, numeral 2
a substrate, numeral 3 a first pressure-sensitive adhesive layer,
numeral 4 a second pressure-sensitive adhesive layer, numeral 5 a
volume hologram layer, and numeral 6 a surface protective film.
[0016] Films or sheets of paper, synthetic paper, synthetic resins
or metals may be used as the substrate 2 in the volume hologram
laminate. The substrate 2 may take various forms, for example,
sheets such as admission tickets for examination, cards such as ID
cards, or booklets such as passports. In this case, a volume
hologram material with a photograph of a face, a scene or the like
being hologram recorded therein in a single color or a full color
is applied to the substrate. When the volume hologram laminate is
used as a color filter for liquid crystal display devices, the
glass substrate or electrode layer in the liquid crystal cell is
the substrate.
[0017] The volume hologram laminate shown in FIG. 1 has a
construction comprising a volume hologram layer 5 stacked on a
substrate 2 through a first pressure-sensitive adhesive layer 3.
Alternatively, the volume hologram laminate may have a structure
comprising the volume hologram layer 5 stacked on the substrate 2
through a pressure-sensitive adhesive double coated tape. In this
case, a first pressure-sensitive adhesive layer 3, a volume
hologram layer 5, a second pressure-sensitive adhesive layer 4, and
a surface protective film 6 may be stacked in that order on the
substrate 2 through a pressure-sensitive adhesive layer and a
plastic film, such as a transparent or colored polyethylene
terephthalate film.
[0018] The volume hologram layer 5 is such that, after coating of a
volume hologram recording material on a support film, interference
fringes corresponding to the wave surface of light from an object
have been recorded in the form of transmission modulation or
refractive index modulation within the layer and, also in the
duplication, can be easily prepared by bringing an original plate
of a volume hologram into close contact with the volume hologram
layer 5 and then performing exposure and development.
[0019] The volume hologram layer 5 is a photosensitive material for
dry volume phase type hologram recording, comprising a matrix
polymer, a photopolymerizable compound, a photopolymerization
initiator, a sensitizing dye, and an optionally added
plasticizer.
[0020] Photopolymerizable compounds include photopolymerizable,
photocrosslinkable monomers, oligomers, and prepolymers having at
least one ethylenically unsaturated bond per molecule which will be
described later, and mixtures of the above compounds, for example,
unsaturated carboxylic acids and salts thereof, esters of
unsaturated carboxylic acids with aliphatic polyhydric alcohol
compounds, and compounds in which unsaturated carboxylic acids are
linked to aliphatic polyamine compounds by an amide linkage.
[0021] Specific examples of monomers of unsaturated carboxylic
acids include acrylic acid, methacrylic acid, itaconic acid,
crotonic acid, isocrotonic acid, maleic acid, and the above
unsaturated carboxylic acids substituted with a halogen, for
example, chlorinated unsaturated carboxylic acids, brominated
unsaturated carboxylic acids, and fluorinated unsaturated
carboxylic acids. Salts of unsaturated carboxylic acids include
sodium and potassium salts of the above acids.
[0022] Regarding specific examples of monomers of esters of
aliphatic polyhydric alcohol compounds with unsaturated carboxylic
acids, acrylic esters include ethylene glycol diacrylate,
triethylene glycol diacrylate, 1,3-butanediol diacrylate,
tetramethylene glycol diacrylate, propylene glycol diacrylate,
neopentyl glycol diacrylate, trimethylolpropane triacrylate,
trimethylolpropane tri(acryloyloxypropyl) ether, trimethylolethane
triacrylate, hexanediol diacrylate, 1,4-cyclohexanediol diacrylate,
tetraethylene glycol diacrylate, pentaerythritol diacrylate,
pentaerythritol triacrylate, pentaerythritol tetraacrylate,
dipentaerythritol diacrylate, dipentaerythritol triacrylate,
dipentaerythritol tetraacrylate, dipentaerythritol hexaacrylate,
sorbitol triacrylate, sorbitol tetraacrylate, sorbitol
pentaacrylate, sorbitol hexaacrylate, tri(acryloyloxyethyl)
isocyanurate, polyester acrylate oligomer, 2-phenoxyethyl acrylate,
2-phenoxyethyl methacrylate, phenol ethoxylate monoacrylate,
2-(p-chlorophenoxy)ethyl acrylate, p-chlorophenyl acrylate, phenyl
acrylate, 2-phenylethyl acrylate, (2-acryloxyethyl) ether of
bisphenol A, ethoxylated bisphenol A diacrylate,
2-(1-naphthyloxy)ethyl acrylate, o-biphenyl methacrylate, and
o-biphenyl acrylate.
[0023] Methacrylic esters include tetramethylene glycol
dimethacrylate, triethylene glycol dimethacrylate, neopentyl glycol
dimethacrylate, trimethylolpropane trimethacrylate,
trimethylolethane trimethacrylate, ethylene glycol dimethacrylate,
1,3-butanediol dimethacrylate, hexanediol dimethacrylate,
pentaerythritol dimethacrylate, pentaerythritol trimethacrylate,
pentaerythritol tetramethacrylate, dipentaerythritol
dimethacrylate, dipentaerythritol hexamethacrylate, sorbitol
trimethacrylate, sorbitol tetramethacrylate,
bis-[p-(3-methacryloxy-2-hyd- roxypropoxy)phenyl]dimethylmethane,
bis-[p-(acryloxyethoxy)phenyl]dimethyl- methane,
2,2-bis(4-methacryloyloxyphenyl)propane, and 2-naphthyl
methacrylate.
[0024] Itaconic esters include ethylene glycol diitaconate,
propylene glycol diitaconate, 1,3-butanediol diitaconate,
1,4-butanediol diitaconate, tetramethylene glycol diitaconate,
pentaerythritol diitaconate, and sorbitol tetraitaconate.
[0025] Crotonic esters include ethylene glycol dicrotonate,
tetramethylene glycol dicrotonate, pentaerythritol dicrotonate, and
sorbitol tetracrotonate.
[0026] Isocrotonic esters include ethylene glycol diisocrotonate,
pentaerythritol diisocrotonate, and sorbitol tetraisocrotonate.
[0027] Maleic esters include ethylene glycol dimaleate, triethylene
glycol dimaleate, pentaerythritol dimaleate, and sorbitol
tetramaleate.
[0028] Halogenated unsaturated carboxylic acids and esters thereof
usable herein include 2,2,3,3-tetrafluoropropyl acrylate,
1H,1H,2H,2H-heptadecafluorodecyl acrylate,
2,2,3,3-tetrafluoropropyl methacrylate,
1H,1H,2H,2H-heptadecafluorodecyl methacrylate, 2,4,6-tribromophenyl
methacrylate, dibromoneopentyl dimethacrylate (tradename: NK Ester
DBN, manufactured by Shin-Nakamura Chemical Co., Ltd.),
dibromopropyl acrylate (tradename: NK Ester A-DBP, manufactured by
Shin-Nakamura Chemical Co., Ltd.), dibromopropyl methacrylate
(tradename: NK Ester DBP, manufactured by Shin-Nakamura Chemical
Co., Ltd.), methacrylic acid chloride, 2,4,6-trichlorophenyl
methacrylate, methyl 2-chloroacrylate, ethyl 2-chloroacrylate,
n-butyl-2-chloroacrylate, tribromophenol acrylate, and
tetrabromophenol acrylate.
[0029] Specific examples of monomers of compounds, in which
unsaturated carboxylic acids are linked to aliphatic polyamine
compounds by an amide linkage, include methylenebisacrylamide,
methylenebismethacryl-amide, 1,6-hexamethylenebisacrylamide,
1,6-hexamethylenebismethacrylamide, diethylenetriamine
trisacrylamide, xylylenebisacrylamide, xylylene-bismethacrylamide,
N-phenylmethacrylamide, and diacetoneacrylamide.
[0030] Other examples include polyisocyanate compounds having two
or more isocyanate groups per molecule described in Japanese Patent
Publication No. 41708/1973 and vinyl urethane compounds having two
or more polymerizable vinyl groups per molecule with a
hydroxyl-containing vinyl monomer represented by the following
general formula being added thereto.
CH.sub.2.dbd.C(R)COOCH.sub.2CH(R')OH
[0031] wherein R and R' each independently represent a hydrogen
atom or a methyl group.
[0032] Urethane acrylates described in Japanese Patent Laid-Open
No. 37193/1976 and polyester acrylates and polyfunctional acrylates
and methacrylates, prepared from epoxy resin and (meth)acrylic acid
or the like, described in Japanese Patent Laid-Open No. 64183/1973
and Japanese Patent Publication Nos. 43191/1974 and 30490/1977 may
also be used.
[0033] Further, compounds introduced as photocurable monomers and
oligomers in Journal of the Adhesion Society of Japan, Vol. 20, No.
7, 300-308 may also be used.
[0034] Regarding other examples, phosphorus-containing monomers
include: mono(2-acryloyloxyethyl) acid phosphate (tradename: Light
Ester PA, manufactured by Kyoeisha Chemical Co., Ltd.) and
mono(2-methacryloyloxyet- hyl) acid phosphate (tradename: Light
Ester PM, manufactured by Kyoeisha Chemical Co., Ltd.); and epoxy
acrylate monomers (tradename: Ripoxy VR-60, manufactured by Showa
High Polymer Co., Ltd.; and tradename: Ripoxy VR-90, manufactured
by Showa High Polymer Co., Ltd.).
[0035] NK Ester M-230G (tradename: manufactured by Shin-Nakamura
Chemical Co., Ltd.) and NK Ester 23G (tradename: manufactured by
Shin-Nakamura Chemical Co., Ltd.) are also usable.
[0036] Further examples of monomers include: triacrylates
represented by formulae: 1(tradename: Aronix M-315, manufactured by
Toa Gosei Chemical Industry Co., Ltd.) 2
[0037] (tradename: Aronix M-325, manufactured by Toa Gosei Chemical
Industry Co., Ltd.); 2,2'-bis(4-acryloxy-diethoxyphenyl)propane
(tradename: NK Ester A-BPE-4, manufactured by Shin-Nakamura
Chemical Co., Ltd.); and tetramethylolmethane tetraacrylate
(tradename: NK Ester A-TMMT, manufactured by Shin-Nakamura Chemical
Co., Ltd.).
[0038] Plasticizers, which may be optionally added, include:
polyhydric alcohols, such as ethylene glycol, diethylene glycol,
triethylene glycol, glycerin, and trimethylolpropane; phthalic
ester plasticizers, such as dimethyl phthalate (DMP), diethyl
phthalate (DEP), dibutyl phthalate (DBP), heptylnonyl phthalate
(HNP), di-2-ethylhexyl phthalate (DOP), di-n-octyl phthalate
(DNOP), di-i-octyl phthalate (DCapP), (79 alkyl) phthalate (D79P),
di-i-decyl phthalate (DIDP), ditridecyl phthalate (DTDP),
dicyclohexyl phthalate (DCHP), butylbenzyl phthalate (BDP),
ethylphthalylethyl glycolate (EPEG), and butylphthalylbutyl
glycolate (BPBG); aliphatic dibasic acid ester plasticizers, such
as di-2-ethylhexyl adipate (DOA), di-(methylcyclohexyl) adipate,
diisodecyl adipate (DIDA), di-n-hexyl azelate (DNHZ),
di-2-ethylhexyl azelate (DOZ), dibutyl sebacate (DBS), and
di-2-ethylhexyl sebacate (DOS); citric ester plasticizers, such as
triethyl citrate (TEC), tributyl citrate (TBC), triethyl
acetylcitrate (ATEC), and tributyl acetylcitrate (ATBC); epoxy
plasticizers, such as epoxidized soybean oil; and phosphoric ester
plasticizers, such as tributyl phosphate (TBP), triphenyl phosphate
(TPP), tricresyl phosphate (TCP), and tripropylene glycol
phosphate.
[0039] Regarding initiator systems, photopolymerization initiators
include, for example, 1,3-di(t-butyldioxycarbonyl)benzophenone,
3,3',4,4-tetrakis(t-butyldioxycarbonyl)benzophenone,
N-phenylglycine, 2,4,6-tris(trichloromethyl)-s-triazine,
3-phenyl-5-isoxazolone, 2-mercaptobenzimidazole, and dimers of
imidazole. Preferably, the photopolymerization initiator is
decomposed after recording of the hologram from the viewpoint of
stabilizing the recorded hologram. For example, an organic peroxide
system is preferred because it can be easily decomposed upon
ultraviolet irradiation.
[0040] Examples of sensitizing dyes include dyes having absorption
of light at 350 to 600 nm, such as thiopyrilium salt, merocyanine,
quinoline, styrylquinoline, ketocoumarin, thioxanthene, xanthene,
oxonol, cyanine, rhodamine, thiopyrilium salt, pyrilium ion, and
diphenyliodonium ion dyes. Sensitizing dyes having absorption of
light in a wavelength region of not more than 350 nm or not less
than 600 nm may also be used.
[0041] Matrix polymers usable herein include polymethacrylic esters
or partial hydrolyzates thereof, polyvinyl acetate or hydrolyzates
thereof, polyvinyl alcohol or partial acetalization products
thereof, triacetylcellulose, polyisoprene, polybutadiene,
polychloroprene, silicone rubber, polystyrene, polyvinyl butyral,
polychloroprene, polyvinyl chloride, chlorinated polyethylene,
chlorinated polypropylene, poly-N-vinylcarbazole or derivatives
thereof, poly-N-vinylpyrrolidone or derivatives thereof, copolymer
of styrene with maleic anhydride or semi-esters thereof, and
copolymers using, as a comonomer, at least one monomer selected
from copolymerizable monomers, such as acrylic acid, acrylic
esters, methacrylic acid, methacrylic esters, acrylamide,
acrylonitrile, ethylene, propylene, vinyl chloride, and vinyl
acetate, and mixtures of the above polymers. Preferred are
polyisoprene, polybutadiene, polychloroprene, polyvinyl alcohol or
partial acetalization products of polyvinyl alcohol, e.g.,
polyvinyl acetal, polyvinyl butyral, polyvinyl acetate,
ethylene/vinyl acetate copolymer, and vinyl chloride/vinyl acetate
copolymer, or mixtures of the above polymers.
[0042] The recorded hologram may be stabilized through monomer
transfer by heating. To this end, the matrix polymer preferably has
a relatively low glass transition temperature and can facilitate
the monomer transfer.
[0043] The amount of the photopolymerizable compound used is
generally 10 to 1,000 parts by weight, preferably 10 to 100 parts
by weight, based on 100 parts by weight of the binder resin as a
matrix polymer.
[0044] The amount of the photopolymerization initiator used is
generally 1 to 10 parts by weight, preferably 5 to 10 parts by
weight, based on 100 parts by weight of the binder resin (matrix
polymer).
[0045] The amount of the sensitizing dye used is generally 0.01 to
1 part by weight, preferably 0.01 to 0.5 part by weight, based on
100 parts by weight of the binder resin (matrix polymer).
[0046] Other photosensitive material components include various
nonionic surfactants, cationic surfactants, and anionic
surfactants.
[0047] The hologram-recording material is added, for example, to
acetone, methyl ethyl ketone, methyl isobutyl ketone,
cyclohexanone, benzene, toluene, xylene, chlorobenzene,
tetrahydrofuran, methyl cellosolve, ethyl cellosolve, methyl
cellosolve acetate, ethyl cellosolve acetate, ethyl acetate,
1,4-dioxane, 1,2-dichloroethane, dichloromethane, chloroform,
methanol, ethanol, isopropanol, or a mixed solvent composed of the
above solvents to prepare a coating liquid having a solid content
of 15 to 25% by weight. The thickness of the hologram-recording
layer is generally 0.1 to 50 g m, preferably 5 to 20 .mu.m.
[0048] Such hologram-recording materials include, for exmaple,
Omnidex 352 or 706 manufactured by E. I. du Pont de Nemours &
Co.
[0049] Examples of recording lights for the volume hologram layer
according to the present invention include: krypton laser (1.5 W)
(wavelength 337.5 nm, 350.7 nm, or 356.4 nm); argon laser (40 mW)
(wavelength 351.1 nm or 368.8 nm); neon laser (50 mW) (wavelength
332.4 nm); cadmium laser (15 mW) (wavelength 325.0 nm); and, in a
visible light region, argon laser (wavelength 514.5 nm, 488 nm, or
457.9 nm) and krypton laser (wavelength 647.1 nm, 568.2 nm, or
520.8 nm). One of these wavelengths is taken out, and applied to
excite the photopolymerization initiator to perform recording of a
monochromatic hologram or a color hologram.
[0050] The volume hologram layer according to the present invention
(when the volume hologram layer is specified by various properties,
the volume hologram layer hereinafter refers to a volume hologram
layer with a hologram recorded thereon) has a glass transition
point of 30 to 70.degree. C., preferably 35 to 60.degree. C.
[0051] For the volume hologram layer according to the present
invention, the dynamic storage modulus is one of the properties
which reflects the hardness of the volume hologram layer. The
dynamic storage modulus is measured by the following equipment and
method.
[0052] Sample: Hologram recording film: A volume hologram film
prepared by recording a hologram on a laminate film of polyethylene
terephthalate film (hereinafter referred to as "PET film")/volume
hologram layer/polyvinyl chloride film or PET film and then
pleeling off the PET film as the uppermost layer or the polyvinyl
chloride film or the PET film as the lowermost layer.
[0053] Equipment: Solid viscoelastic analyzer RSA-II (manufactured
by Rheometrics)
[0054] Attachment (mode): For pulling of film
[0055] Frequency: 6.28 rad/sec
[0056] Temp.: -50.degree. C. to 100.degree. C.
[0057] Method:
[0058] (1) The measuring sample is set to a fixture for tensile
measurement of a film.
[0059] (2) The temperature dependency at 6.28 rad/sec in a
temperature region of -50.degree. C. to 100.degree. C. is measured.
The dynamic storage modulus (E', Pa) at 50.degree. C. is determined
from data on the temperature dependency. The temperature 50.degree.
C. was selected from the viewpoint of comparison with the hardness
in the stored state in warehouses or the like in the summer period.
The volume hologram layer according to the present invention has a
dynamic storage modulus of 5.times.10.sup.5 Pa to 5.times.10.sup.7
Pa at 50.degree. C.
[0060] The half value width of diffraction light in the volume
hologram recorded on the volume hologram layer is not more than 30
nm. The volume hologram, which has been recorded using a color
tuning film and has a half value width of more than 30 nm in the
diffraction light thereof, is free from spotty hologram defects in
the pressed state, although the detailed reason is not known. The
present invention is particularly useful in color Lippmann
holograms or single color holograms having a half value width of
not more than 30 nm in the diffraction light of the volume
hologram.
[0061] The pressure-sensitive adhesive layers 3, 4 will be
described. An acrylic pressure-sensitive adhesive is used as the
pressure-sensitive adhesive layer. The acrylic pressure-sensitive
adhesive is composed mainly of an acrylic copolymer resin produced
by copolymerizing an alkyl acrylate, other monomer, and a
functional monomer.
[0062] The alkyl group in the alkyl acrylate has 4 to 15 carbon
atoms. Examples of alkyl acrylates usable herein include n-butyl
acrylate, 2-ethylhexyl acrylate, isooctyl acrylate, and isononyl
acrylate. They may be used alone or as a mixture of two or
more.
[0063] Other monomers include, for example, methyl acrylate, methyl
methacrylate, styrene, acrylonitrile, and vinyl acetate. They may
be used alone or as a mixture of two or more.
[0064] Functional monomers include, for example, acrylic acid,
methacrylic acid, itaconic acid, hydroxyethyl acrylate,
hydroxyethyl methacrylate, propylene glycol acrylate, acrylamide,
methacrylamide, glycidyl acrylate, glycidyl methacrylate,
dimethylaminoethyl methacrylate, and tert-butylaminoethyl
methacrylate. They may be used alone or as a mixture of two or
more.
[0065] The constituent ratio (% by weight) of the alkyl acrylate to
the other monomer to the functional monomer in the acrylic
copolymer resin is 70 to 99:0 to 20:0.01 to 20, preferably 80 to
95:0 to 10:0.1 to 15. The acrylic copolymer resin has a weight
average molecular weight of 700,000 to 1,200,000, preferably
800,000 to 1,000,000.
[0066] The acrylic pressure-sensitive adhesive contains the acrylic
copolymer resin, a crosslinking agent for room temperature
crosslinking or heat crosslinking, and optionally a tackifier in a
suitable solvent.
[0067] The crosslinking agent for room temperature closslinking is
one which enables crosslinking of the acrylic pressure-sensitive
adhesive by aging at room temperature. Specific examples of such
crosslinking agents include polyisocyanate compounds and trimers of
these polyisocyanate compounds, isocyanate-terminated urethane
prepolymers produced by reacting the polyisocyanate compounds with
polyol compounds, polyisocyanate compounds thereof, and trimers of
these polyisocyanate compounds.
[0068] Specific examples of polyisocyanates include 2,4-tolylene
diisocyanate, 2,5-tolylene diisocyanate, 1,3-xylylene diisocyanate,
1,4-xylylene diisocyanate, diphenylmethane-4,4'-diisocyanate,
3-methyldiphenylmethane diisocyanate, hexamethylene diisocyanate,
isophorone diisocyanate, dicyclohexylmethane-4,4'-diisocyanate,
dicyclohexylmethane-2,4'-diisocyanate, and lysine isocyanate. Metal
chelate compounds of aluminum, titanium or the like or
polyfunctional epoxy compounds may be used as a crosslinking
agent.
[0069] The crosslinking agent for room temperature crosslinking is
preferably added in an amount of 0.005 to 20 parts by weight,
particularly preferably 0.01 to 10 parts by weight, based on 100
parts by weight of the acrylic copolymer resin.
[0070] The crosslinking agent for heat crosslinking is such that
the function of crosslinking can be attained by heating the acrylic
pressure-sensitive adhesive at 100.degree. C. or above, preferably
130.degree. C. or above, for a heating time of one min to 30 min.
Specific examples of crosslinking agents for heat crosslinking
include methylol-containing compounds produced by reacting
formaldehyde with melamine, benzoguanamine, urea or the like, and
etherification products produced by partially or entirely
etherifying the methylol groups with an aliphatic alcohol.
[0071] The crosslinking agent for heat crosslinking is preferably
added in an amount of 0.01 to 25 parts by weight, particularly
preferably 0.1 to 20 parts by weight, based on 100 parts by weight
of the acrylic copolymer resin.
[0072] The tackifier is optionally added to the acrylic
pressure-sensitive adhesive from the viewpoint of improving the
tackiness. Tackifiers usable herein include, for example, rosin
resins, terpene resins, and xylene resins. The tackifier may be
added in an amount of 0 to 50% by weight, preferably 0 to 40% by
weight, based on the acrylic pressure-sensitive adhesive.
[0073] For the acrylic pressure-sensitive adhesive, the acrylic
copolymer resin, the crosslinking agent for room temperature
crosslinking or heat crosslinking and optionally the tackifier is
added to a solvent, such as methyl ethyl ketone, toluene, ethyl
acetate, ethanol, or isopropanol, to prepare a solvent-type
pressure-sensitive adhesive having a solid content of 15 to 50% by
weight, preferably 20 to 35% by weight. The solvent-type
pressure-sensitive adhesive is coated by means of a knife coater, a
gravure coater or the like to a coating thickness on a dry basis of
4 to 30 .mu.m, preferably 5 to 25 .mu.m. The coating is then
crosslinked by aging at room temperature in the case of the room
temperature crosslinking type or by heating at the above-described
temperature in the case of the heat crosslinking type to form first
and/or second pressure-sensitive adhesive layers.
[0074] The present inventors have found that the problem of
hologram defects can be solved by specifying the dynamic storage
modulus at 50.degree. C. and the loss tangent (tan .delta.) defined
by dynamic loss modulus/dynamic storage modulus at 120.degree. C.
for the second pressure-sensitive adhesive layer adjacent to the
hologram layer.
[0075] The dynamic storage modulus at 50.degree. C. and the loss
tangent (tan .delta.) defined by dynamic loss modulus/dynamic
storage modulus at 120.degree. C. for the second pressure-sensitive
adhesive layer provided between the volume hologram layer and the
surface protective film according to the present invention is one
of the properties which reflects the hardness of the
pressure-sensitive adhesive layer, and is measured by the following
equipment and method.
[0076] Sample: An acrylic pressure-sensitive adhesive was coated on
a release film A (SP-PET-05-50-BU, manufactured by Tohcello Co.,
Ltd.) to a coating thickness on a dry basis of 15 .mu.m. A release
film B (SP-PET-02-50-BU, manufactured by Tohcello Co., Ltd.) is
then laminated on the coating, followed by aging at room
temperature for one week in the case of room temperature
crosslinking type, or by heating at 140.degree. C. for 15 min in
the case of heat crosslinking type to form a pressure-sensitive
adhesive layer (a measuring sample). In the measurement, the
release film A was peeled off, and the pressure-sensitive adhesive
layer was wound around a spatula or the like from the release film
B with caution so as to prevent the inclusion of air bubbles,
thereby forming a roll which has a size suitable for housing in a
parallel plate (compression) (4.75 mm.O slashed.) as a measuring
attachment (mode) in the following measuring equipment and has a
given thickness (when set in the measuring equipment) of 2.5 to 3.5
mm.
[0077] Equipment: viscoelastic analyzer RSA-II, manufactured by
Rheometrics
[0078] Attachment (mode): Parallel plate (compression) 4.75 mm.O
slashed.
[0079] Frequency: 6.28 rad/sec
[0080] Temp.: -100.degree. C. to 200.degree. C.
[0081] Method:
[0082] (1) The measuring sample is sandwiched between parallel
plates, and set in the measuring equipment.
[0083] (2) The temperature dependency at a measuring frequency of
6.28 rad/sec in a temperature region of -100.degree. C. to
200.degree. C. is measured. The dynamic storage modulus (Pa) at
50.degree. C., the dynamic storage modulus (Pa) at 120.degree. C.,
and the dynamic loss modulus (Pa) are determined from data on the
temperature dependency, and the loss tangent (tan .delta.) at
120.degree. C. was determined as the ratio of the dynamic loss
modulus (Pa) at 120.degree. C. to the dynamic storage modulus (Pa)
at 120.degree. C.
[0084] One example of data on the temperature dependency of the
dynamic storage modulus {E'(.DELTA.)} dynamic loss modulus
{E"(.quadrature.)}, and loss tangent {tan .delta. (.diamond.)} in a
temperature region of -100.degree. C. to 200.degree. C. is shown in
FIG. 2. The example shown in the drawing is data for the second
pressure-sensitive adhesive layer prepared in Example 1 below.
[0085] The second pressure-sensitive adhesive layer in the volume
hologram laminate according to the present invention has a dynamic
storage modulus of not less than 2.5.times.10.sup.5 Pa, preferably
2.5.times.10.sup.5 Pa to 5.times.10.sup.7 Pa, as measured under
conditions of 6.28 rad/sec and 50.degree. C.
[0086] The present inventors have found that, even when the dynamic
storage modulus at 50.degree. C. is not less than
2.5.times.10.sup.5 Pa, a loss tangent (tan .delta.) value of more
than 0.15 at 120.degree. C. causes spotty hologram defects under
pressed conditions.
[0087] This type of pressure-sensitive adhesive layer generally
becomes rubbery at 100.degree. C. or above. Therefore, the value of
loss tangent (tan .delta.) at 120.degree. C., which brings about a
complete rubbery state, may be regarded as a value indicating one
property of the pressure-sensitive adhesive layer. In order to
prevent spotty hologram defects under pressed conditions, the
pressure-sensitive adhesive layer should satisfy a dynamic storage
modulus of not less than 2.5.times.10.sup.5 Pa at 50.degree. C.
and, at the same time, a loss tangent (tan .delta.) value of not
more than 0.15, preferably not more than 0.1. The lower limit of
the loss tangent (tan .delta.) is about 0.01, although the lower
limit varies depending upon the properties of the
pressure-sensitive adhesive.
[0088] Next, the surface protective film 6 in the volume hologram
laminate according to the present invention will be described. The
surface protective film 6 is transparent, and examples thereof
include films of resins, such as polyethylene, polypropylene,
polyethylene fluoride, polyvinylidene fluoride, polyvinyl chloride,
polyvinylidene chloride, ethylene-vinyl alcohol, polyvinyl alcohol,
polymethyl methacrylate, polyether sulfone, polyether ether ketone,
polyamide, tetrafluoroethylene-perfluoroalkyl vinyl ether
copolymer, PET, and polyimide. The film thickness is generally 2 to
200 .mu.m, preferably 10 to 50 .mu.m.
[0089] If necessary, a hardcoat (not shown) may be provided on the
surface protective film from the viewpoint of enhancing the
protective properties of the surface protective film in its
surface. The hardcoat may be formed, for example, by coating a
silicone, fluorosilicone, melamine alkyd, urethane acrylate
(ultraviolet-curable) or other system by dipping, spraying, or roll
coating to a thickness of 1 to 50 .mu.m, preferably 3 to 25
.mu.m.
[0090] Likewise, the surface of the surface protective film 7 or
the surface of the hardcoat may be subjected to release treatment
(not shown). The release treatment may be carried out by coating a
fluoro release agent, a silicone release agent, a stearate release
agent, a wax release agent or the like by dipping, spraying, or
roll coating.
EXAMPLES
[0091] The present invention will be described in more detail with
reference to the following examples.
Example 1
[0092] Preparation of Transparent Protective Film/Second
Pressure-Sensitive Adhesive Layer/Release Film
[0093] A pressure-sensitive adhesive solution having the following
composition was coated by means of a knife coater on a release film
(thickness 50 .mu.m, SP-PETO5-50-BU, manufactured by Tohcello Co.,
Ltd.) comprising a silicone resin coated on a PET film to a coating
thickness on a dry basis of 10 .mu.m to form a room
temperature-crosslinked pressure-sensitive adhesive layer.
[0094] Acrylic copolymer resin (n-butyl acrylate/methyl
methacrylate/acrylic acid copolymer, weight average molecular
weight 900,000) . . . 100 pts.wt.
[0095] Crosslinking agent for room temperature crosslinking
(polyisocyanate: Coronate L, manufactured by Nippon Polyurethane
Industry Co., Ltd.) . . . 1.44 pts.wt.
[0096] Solvent (mixed solvent of ethyl acetate/toluene/methyl ethyl
ketone=20/60/20(volume ratio)) . . . 200 pts.wt.
[0097] A PET film (thickness 50 .mu.m, Lumirror T-60, manufactured
by Toray Industries, Inc.) as a transparent protective film was
laminated on the room temperature-crosslinked pressure-sensitive
adhesive layer.
[0098] For this pressure-sensitive adhesive layer, the dynamic
storage modulus (E') and the dynamic loss modulus (E") were
measured at a measuring frequency of 6.28 rad/sec in the
temperature range of -100.degree. C. to 200.degree. C. in the same
manner as described above. The results of measurement and the
calculation results of loss tangent (tan .delta.) are shown in FIG.
2. As can be seen from FIG. 2, the dynamic storage modulus at
50.degree. C. was 3.3.times.10.sup.5 Pa, and the loss tangent (tan
.delta.) at 120.degree. C. was 0.11.
[0099] Preparation of Release Film A/First Pressure-Sensitive
Adhesive Layer/Release Film B
[0100] A pressure-sensitive adhesive solution having the same
composition as the second pressure-sensitive adhesive layer was
immediately coated to a thickness on a dry basis of 15 .mu.m by
means of a knife coater on a release film A (thickness 50 .mu.m,
SP-PETO5-50-BU, manufactured by Tohcello Co., Ltd.) comprising a
silicone resin coated on a PET film. A release film B (thickness 50
.mu.m, SP-PETO2-50-BU, manufactured by Tohcello Co., Ltd.)
comprising a silicone resin coated on a PET film was laminated on
the coating.
[0101] Preparation of Hologram Recording Film
[0102] A color Lippmann hologram was recorded by laser beams having
wavelengths of 458 nm, 532 nm, and 647 nm on a hologram recording
film (HRF 800.times.001, manufactured by E. I. du Pont de Nemours
& Co.) of a laminate composed of a PET film (thickness 50
.mu.m)/hologram recording material layer (thickness 15 .mu.m)/PET
film (thickness 50 .mu.m). This volume hologram layer had a glass
transition point of 46.degree. C., and a dynamic storage modulus of
6.28.times.10.sup.6 Pa at a measuring frequency of 6.28 rad/sec and
a temperature of 50.degree. C.
[0103] Next, one of the PET films in the hologram recording film
was peeled off. Separately, the release film was separated from the
transparent protective film/second pressure-sensitive adhesive
layer/release film prepared above. The surface of the second
pressure-sensitive adhesive layer was laminated on the hologram
recording film on its surface from which the PET film had been
removed, followed by heating at 140.degree. C. for 15 min. Thus, a
transparent protective film/second pressure-sensitive adhesive
layer/hologram recording layer/PET film was prepared.
[0104] Preparation of Hologram Laminate
[0105] The PET film was peeled from the hologram recording film
prepared above. Separately, the release film A was peeled from the
release film A/first pressure-sensitive adhesive layer/release film
B prepared above, followed by lamination of both the structures on
top of the other to prepare a transparent protective film/second
pressure-sensitive adhesive layer/hologram recording layer/first
pressure-sensitive adhesive layer/release film B. This hologram was
evaluated for spectral properties. As a result, the half value
width was 17 to 20 nm.
[0106] The release film B was then peeled off, and the structure
with the release film B removed therefrom was applied onto a paper
substrate. The laminate was stored for 3 days under conditions of
50.degree. C. and 0.12 kg/cm.sup.2. The hologram after the storage
was visually inspected for hologram defects at an angle, which
enables the defects to be seen most deeply, under a three
bright-line fluorescent tube (HI-LUMIC N FL4EX-N-PK, manufactured
by Hitachi, Ltd.). As a result, the defect level of the hologram
was 0.4, indicating that the storage stability was excellent.
[0107] In the determination of the defect level, the state of
freedom from spotty hologram defect was presumed to be 0 (zero),
and the formed spots were visually evaluated for the size (mm) and
the depth (relative value 1-10). The product of the size by the
depth was regarded as the defect level. When the defect level was
not more than 3, the hologram was regarded as having excellent
storage stability.
Example 2
[0108] A hologram laminate was prepared in the same manner as in
Example 1, except that, instead of the pressure-sensitive adhesive
solution in Example 1, a pressure-sensitive adhesive solution
having the following composition was used to form a room
temperature-crosslinked pressure-sensitive adhesive layer.
[0109] Acrylic copolymer resin (n-butyl acrylate/methyl
methacrylate/acrylic acid copolymer, weight average molecular
weight 1,000,000) . . . 100 pts.wt.
[0110] Crosslinking agent for room temperature crosslinking
(polyisocyanate: Coronate L, manufactured by Nippon Polyurethane
Industry Co., Ltd.) . . . 1.5 pts.wt.
[0111] Solvent (mixed solvent of ethyl acetate/toluene/methyl ethyl
ketone=20/60/20(volume ratio)) . . . 200 pts.wt.
[0112] The results of measurement at a frequency of 6.28 rad/sec
and the like in the second pressure-sensitive adhesive layer are
shown in FIG. 3. As can be seen from FIG. 3, the dynamic storage
modulus at 50.degree. C. was 3.5.times.10.sup.5 Pa, and the loss
tangent (tan 6) at 120.degree. C. was 0.07.
[0113] A storage test was carried out in the same manner as in
Example 1. As a result, the defect level of the hologram was 0.4,
indicating that the storage stability was excellent.
Example 3
[0114] Instead of the pressure-sensitive adhesive solution in
Example 1, a pressure-sensitive adhesive solution having the
following composition was immediately coated by means of a knife
coater to a thickness on a dry basis of 10 .mu.m.
[0115] Acrylic copolymer resin (n-butyl acrylate/methyl
methacrylate/acrylic acid copolymer, weight average molecular
weight 900,000) . . . 100 pts.wt.
[0116] Crosslinking agent for heat crosslinking (curing agent B,
methylol-containing compound, manufactured by Saiden Chemical
Industry Co., Ltd.) . . . 1.5 pts.wt.
[0117] Solvent (mixed solvent of ethyl acetate/toluene/methyl ethyl
ketone=20/60/20 (volume ratio)) . . . 200 pts.wt.
[0118] The pressure-sensitive adhesive layer was then heat treated
at 140.degree. C. for 15 min, thereby forming a heat-crosslinked
pressure-sensitive adhesive layer. A PET film (thickness 50 .mu.m,
Lumirror T-60, manufactured by Toray Industries, Inc.) was
laminated on the heat-crosslinked pressure-sensitive adhesive
layer.
[0119] The results of measurement at a frequency of 6.28 rad/sec
and the like in the pressure-sensitive adhesive layer are shown in
FIG. 4. As can be seen from FIG. 4, the dynamic storage modulus at
50.degree. C. was 8.9.times.10.sup.5 Pa, and the loss tangent (tan
.delta.) at 120.degree. C. was 0.03.
[0120] A storage test was carried out in the same manner as in
Example 1. As a result, the defect level of the hologram was 0.4,
indicating that the storage stability was excellent.
Comparative Example 1
[0121] A hologram laminate was prepared in the same manner as in
Example 1, except that, instead of the pressure-sensitive adhesive
solution in Example 1, a pressure-sensitive adhesive solution
having the following composition was used to form a room
temperature-crosslinked pressure-sensitive adhesive layer.
[0122] Acrylic copolymer resin (n-butyl acrylate/methyl
methacrylate/acrylic acid copolymer, weight average molecular
weight 900,000) . . . 100 pts.wt.
[0123] Crosslinking agent for room temperature crosslinking
(polyisocyanate: Coronate L, manufactured by Nippon Polyurethane
Industry Co., Ltd.) . . . 0.7 pt.wt.
[0124] Solvent (mixed solvent of ethyl acetate/toluene/methyl ethyl
ketone=20/60/20(volume ratio)) . . . 200 pts.wt.
[0125] The results of measurement at a measuring frequency of 6.28
rad/sec and the like in the second pressure-sensitive adhesive
layer are shown in FIG. 5. As can be seen from FIG. 5, the dynamic
storage modulus at 50.degree. C. was 3.2.times.10.sup.5 Pa, and the
loss tangent (tan .delta.) at 120.degree. C. was 0.29.
[0126] A storage test was carried out in the same manner as in
Example 1. As a result, it was found that spotty dot defects
occurred in the hologram. The defect level was 4.5, indicating that
the hologram had a problem of storage stability.
Comparative Example 2
[0127] A hologram laminate was prepared in the same manner as in
Example 1, except that, instead of the pressure-sensitive adhesive
solution in Example 1, a pressure-sensitive adhesive solution
having the following composition was used to form a room
temperature-crosslinked pressure-sensitive adhesive layer.
[0128] Acrylic copolymer resin (2-ethylhexyl acrylate/vinyl
acetate/acrylic acid copolymer, weight average molecular weight
600,000) . . . 100 pts.wt.
[0129] Crosslinking agent for room temperature crosslinking
(polyisocyanate: Coronate L, manufactured by Nippon Polyurethane
Industry Co., Ltd.) . . . 0.5 pt.wt.
[0130] Solvent (mixed solvent composed of ethyl
acetate/toluene=50/50 (volume ratio)) . . . 180 pts.wt.
[0131] The results of measurement at a frequency of 6.28 rad/sec
and the like in the second pressure-sensitive adhesive layer are
shown in FIG. 6. As can be seen from FIG. 6, the dynamic storage
modulus at 50.degree. C. was 2.times.10.sup.5 Pa, and the loss
tangent (tan .delta.) at 120.degree. C. was 0.28.
[0132] A storage test was carried out in the same manner as in
Example 1. As a result, it was found that spotty dot defects
occurred in the hologram. The defect level was 9, indicating that
the hologram had a problem of storage stability.
[0133] The pressure-sensitive adhesive for a pressure-sensitive
adhesive layer in the volume hologram laminate according to the
present invention, when kept in the pressed state, for example,
during storage, is less likely to cause spotty hologram defects in
the volume hologram layer.
* * * * *