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Description  |
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FIELD OF THE INVENTION
The present invention relates to a method for molding a transparent plastic
substrate, and more particularly, to a method for molding a transparent
plastic substrate which is used as a disc substrate for discs (e.g.,
optical discs and magneto-optical discs) in an optical writing, reading
and erasing system.
BACKGROUND OF THE INVENTION
A transparent plastic substrate has an excellent low birefringence which is
the basic performance in optical properties, and a great attention is
recently paid thereto.
The transparent plastic substrate is generally produced by a cast molding
method comprising pouring a polymerizable liquid material such as
polyallyl carbonate, polyolpoly(meth)acrylate or epoxy acrylate into a
cavity of a mold and radical polymerizing the same in the mold to obtain a
transparent plastic substrate as disclosed in, for example, Japanese
Patent Application (OPI) Nos. 130450/83 and 137150/83 (the term "OPI" as
used herein means a "published unexamined Japanese patent application").
In addition, a method for conducting photopolymerization in a mold
constituted by glass plates at both sides (Japanese Patent Application No.
202557/85), a method of thermal polymerization after casting a liquid
resin into a mold under vacuum, a method of thermal polymerization under
pressure for a liquid resin casted (Japanese Patent Application (OPI) No.
203414/85), and so on are known.
Conventional cast molding method of the transparent plastic substrate
requires about 24 hours from the pouring of a reactive liquid material
into the mold used to completion of molding of the transparent plastic
substrate. Therefore, the efficiency of use of the mold is extremely low,
and the number of stampers to transfer signals, grooves for trucking and
so on is increased. This is a serious problem to the productivity of the
transparent plastic substrate.
Furthermore, as a method for increasing the efficiency of use of the mold,
it is sufficient to shorten the molding time. However, for example, if
radical polymerization is completed in the mold by light energy,
polyfunctional (meth)acrylate compounds produce a residual strain by the
shrinkage during polymerization or curing reaction.
In demolding the transparent plastic substrate from the mold constituted by
a glass, a metal, a rubber or the like after the completion of molding,
the transparent plastic substrate is broken or causes cracking, or the
mold is broken. Those are the practical problems. Accordingly, it has been
desired to improve the defects.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a practically
useful molding method for the plastic substrate which can increase the
efficiency of use of a mold used in the cast molding method of
polyfunctional (meth)acrylate compounds, and can minimize cracking or
breakage of the plastic substrate during radical polymerization and
demolding.
Another object of the present is to provide a method for obtaining a
transparent plastic substrate by radical polymerization of a
polyfunctional (meth)acrylate compound by a cast molding method, wherein
when the glass transition temperature (Tg) of the plastic substrate
obtained in the mold by radical polymerization reaches 10.degree. to
100.degree. C., the substrate is removed from the mold, and the substrate
is post-cured to obtain a substrate having a Tg of 110.degree. C. or more.
Further objects of the present invention is to provide a method for
obtaining a transparent plastic substrate by radical polymerization of a
polyfunctional (meth)acrylate by a cast molding method, wherein when a
glass transition temperature (Tg) of the plastic substrate obtained in the
mold by radical polymerization reaches 10.degree. to 100.degree. C., the
substrate is removed from the mold, the substrate is postcured to obtain a
substrate having a Tg of 110.degree. C. or more, and the substrate is
further post-cured at a temperature of 150.degree. C. or more to obtain a
plastic substrate having a small residual content of double bond, a low
water absorption and so little influence on the deterioration of a life of
recording film.
In accordance with the method of the present invention, the radical
polymerization time in the mold is about 0.5 to 1 hour and the pre-cured
substrate is post-cured outside the mold. Therefore, the amount of
substrates produced per mold can be increased to 20 to 40 times.
Further, since the substrate pre-cured under the condition that the glass
transition temperature is low is removed from the mold, the breakage of
the substrate does not substantially occur.
Furthermore, in accordance with the present invention, the plastic
substrate obtained is heated at least at 150.degree. C. as the post-curing
temperature, so that the residual content of double bond in the final
plastic substrate is decreased to 30% or less, and the degree of water
absorption of the substrate is decreased. As a result, the effect of
increasing optical disc substrate characteristics can be increased.
BRIEF DESCRIPTION OF THE DRAWING
The FIGURE is an illustrative drawing showing an experimental method to
examine the relationship between the deterioration of a recording film and
the degree of water absorption of a synthetic resin substrate.
1 . . . Synthetic resin substrate
2 . . . Recording material
3 . . . Quartz plate
4 . . . Teflon spacer
5 . . . Paraffin wax
6 . . . Clamp
7 . . . Thermostat
8 . . . Pure water
DETAILED DESCRIPTION OF THE INVENTION
The polyfunctional (meth)acrylate compound is a compound represented by the
formula (I):
##STR1##
wherein R.sub.1 is an alcohol residue having 2 to 50 carbon atoms, R.sub.2
is H or CH.sub.3, and n is an integer of 2 to 6, and which, when radical
polymerized, produces a cured product having a glass transition
temperature of 110.degree. C. or more.
Representative examples of the polyfunctional (meth)acrylate compound are
methacrylic or acrylic acid esters such as
2,2'-bis[4-(.beta.-methacryloyloxy)cyclohexyl]propane,
2,2'-bis[4-(.beta.-methacyloyloxydiethoxy)cyclohexyl) propane,
bis(oxymethyl)tricyclo[5,2,1,0.sup.2,6 ]decane dimethacrylate,
1,4-bis(methacyrloyloxymethyl)cyclohexane,
trimethylolpropanetri(meth)acrylate, neopentylglycol di(meth)acrylate,
1,6-hexanediol di(meth)acrylate, 1,3-butanediol di(meth)acrylate,
diethylene glycol di(meth)acrylate, and
2,2'-bis[4-(methacryloyldiethoxy)phenylpropane. These compounds are used
alone or as mixtures thereof.
2,2'-Bis[4-(.beta.-methacryloyloxyethoxy)cyclohexyl]propane,
bis(oxymethyl)tricyclo[5,2,1,0.sup.2,6 ]decane dimethacrylate, and
1,4-bis(methacryloyloxymethyl)cyclohexane are particularly preferred from
the standpoint of optical properties. The term "(meth)acrylate" as used
herein means both acrylate and methacrylate.
In addition to the compounds of the formula (I), radical polymerizable
monomers conventionally used as viscosity controlling agents can be used
in an amount of 10wt% or less. Examples of such polymerizable monomers are
vinyl compounds such as styrene, chlorostyrene, dichlorostyrene,
vinyltoluene, divinylbenzene, vinyl acetate and vinyl chloride;
(meth)acryl compounds such as methyl methacrylate, phenyl (meth)acrylate,
benzyl (meth)acrylate, 2-phenoxyethyl (meth)acrylate, cyclohexyl
(meth)acrylate, glycidyl (meth)acrylate, epoxy (meth)acrylate and urethane
(meth)acrylate; allyl compounds such as diethylene glycol
bisallylcarbonate and diallyl phthalate; and the like.
The radical initiator used in polymerization of the monomer is not
particularly limited. For example, peroxides such as benzoyl peroxide,
diisopropyl peroxy carbonate lauryoyl peroxide and tertiary butyl peroxy
pivalate, azo compounds such as azoisobutylonitrile, photosensitizers such
as benzophenone, benzoin ethyl ether, benzyl, acetophenone and
anthraquinone, and sulfur compounds such as diphenyl sulfide and
thiocarbamates, which are conventional radical initiator can be used.
The above radical polymerization can be carried out under conventional
radical polymerization conditions (e.g., heating, light irradiation and
electron beam irradiation). The amount of the radical initiator added is
0.01 to 10 parts by weight per 100 parts by weight of the polyfunctional
(meth)acrylate compounds. The polymerization temperature is 10.degree. to
200.degree. C. and preferably 30.degree. to 150.degree. C. The
polymerization can be carried out in an atmosphere of air or inert gas.
The transparent plastic substrate obtained by radical polymerizing the
above polyfunctional (meth)acrylate by the cast molding method has the
glass transition temperature (Tg) at the time of removing the substrate
from the mold of 10.degree. to 100.degree. C. If the Tg is less than
10.degree. C., curing is insufficient and, therefore, the transparent
plastic substrate is deformed by the force exerted in removing the
substrate from the mold. This is a great problem in obtaining a
transparent plastic substrate having substrate performance of, e.g., high
surface smoothness. On the other hand, if the Tg is more than 100.degree.
C., the cross-linking reaction of the polyfunctional (meth)acrylate
compound proceeds partially and the molded product becomes brittle.
Therefore, the substrate breaks in the mold due to the residual strain
such as cure shrinkage and causes cracking. Moreover, the substrate breaks
by the force exerted at the time of removing the substrate from the mold
and causes cracking. The Tg range of the substrate resin at the time of
removing from the mold is 10.degree. to 100.degree. C. and preferably
20.degree. to 80.degree. C. As a result, the radical polymerization time
of the polyfunctional (meth)acrylate in the mold can be controlled to be 2
hours or less and preferably 1 hour or less.
The plastic substrate removed from the mold is placed on a first place and
post-cured until its Tg reaches 110.degree. C. or more. This post-curing
enables to proceed the curing reaction to the extent that the performance
of the substrate is satisfactory, without causing the breakage and
cracking due to the curing in the mold. The post curing can be carried out
by tecnhiques such as heating, irradiation with ultraviolet rays and
irradiation with electron beams.
The post-curing is preferably carried out stepwise with respect to the
temperature and irradiation dose. The post-curing temperature is
preferably 150.degree. C. or more. It is also preferred that the
transparent plastic substrate is obtained by curing the same at
temperatures of 200.degree. C. or more in a vacuum, nitrogen or inert gas
atmosphere.
In the present invention, the pre-cured plastic substrate is removed from
the mold just in the incomplete radical polymerization state that the
glass transition temperature of the pre-cured substrate is 10.degree. to
100.degree. C., so that mechanical residual strain and optical residual
strain due to the shrinkage during polymerization can be minimized. By
carrying out the post-curing after removal from the mold, a difference in
mechanical residual strain between the surface and the inside of the
substrate, which is due to the restrain of mold can be removed. This
post-curing is such that the glass transition temperature of the plastic
substrate obtained by the polymerization in the mold is 10.degree. to
100.degree. C., and in order to maintain the shape, the temperature cannot
be rapidly increased to 150.degree. C. If the post-curing temperature is
low, the residual content of double bond is increased. However, by
post-curing at temperature of 150.degree. C. or more, the radical
polymerization of the residual portion of double bond proceeds, resulting
in decrease in the content of double bond in the substrate.
The present invention is explained in greater detail by reference to the
following Examples and Comparative Examples. Unless otherwise indicated,
all percents, parts, ratios and the like are by weight.
In the Examples and Comparative Examples, the glass transition temperature
(Tg), the residual content of double bond (%), and the influence onto a
recording film were measured by the following methods.
Glass Transition Temperature (Tg)
Measured with a differential scanning colorimeter (DSC) (Du Pont 990
System) at a rate of 10.degree. C./min. in air.
Residual Content of Double Bond (%)
Measured by the IR analytical method.
A base line is drawn in peaks of 1560 to 1660 cm.sup.-1 and 2600 to 3600
cm.sup.-1. The areas, S.sub.1 and S.sub.2, of the peaks of 1560 to 1660
cm.sup.1 and 2600 to 3600 cm.sup.-1 are measured, and n is determined by
the equation: n=S.sub.1 /S.sub.2. With n of a liquid resin as 100% and n
at S.sub.i =0 as 0%, a calibration line is determined. Each sample is
measured for n, and % is calculated from the calibration line.
Influences on Recording Film
Examination of the shape after soaking in water.
As shown in the drawing, a recording film 2 is formed on a substrate 1 by
sputtering, and sealed with a quartz plate 3 through a teflon spacer 4 and
a paraffin wax 5 and clamped with a clamp 6. This is dipped in a
thermostat 7 filled with pure water 8. When the degree of saturated water
absorption is reached, it is taken out and its shape is examined with a
microscope.
EXAMPLE 1
0.5 parts of benzoyl peroxide was added to 100 parts of
2,2'-bis[4-(.beta.-methacryloyloxyethoxy)cyclohexyl]propane to prepare a
liquid resin. This mixture was heated to 60.degree. C., uniformly stirred
and mixed, defoamed, poured into a cavity of a mold constituted by a glass
plate having a diameter of 120 mm, a silicone rubber and a metal stamper,
and cured at a polymerization temperature of 80.degree. C. After 30
minutes, the cured product was removed from the mold. This operation could
be conducted easily. A crack- and defect-free plastic disc substrate was
obtained. Tg of the plastic disc substrate was 38.degree. C.
The substrate was placed on a flat plate and post-cured at 80.degree. C.,
and a transparent plastic disc substrate having good transferring
properties of a stamper signal was obtained.
The substrate had not defect, i.e., the appearance was good, and the
substrate had no warp.
The residual content of double bond in the substrate post-cured at
150.degree. C. for 2 hours was 28%, and the degree of saturated water
absorption was 1.3%.
EXAMPLE 2
A polymerizable solution prepared by adding 0.5 part of benzoyl peroxide to
100 parts of bis(oxymethyl)tricyclo[5,2,1,0.sup.2,6 ]decane dimethacrylate
in place of 2,2'-bis[4-(.beta.-methacryloyloxyethoxy)cyclohexyl]propane in
Example 1 was subjected to the pre-curing in the same manner as in Example
1. After 35 minutes, the cured product was removed from the mold. This
operation could be carried out easily. A crack- and defect-free plastic
disc substrate was obtained.
Tg of the plastic disc substrate was 38.degree. C.
The substrate was placed on a flat plate and post-cured at 100.degree. C.,
and a plastic disc substrate having the final Tg of 251.degree. C. and
good transferring properties of a stamper signal was obtained.
EXAMPLE 3
A polymerizable solution prepared by adding 0.5 part of benzoyl peroxide to
60 parts of 2,2'-bis[4-(.beta.-methacryloyloxy)cyclohexyl]propane and 40
parts of trimethylolpropane trimethacrylate was subjected to pre-curing in
the same manner as in Example 1. After 35 minutes, the cured product was
removed from the mold. This operation could be carried out easily. A
crack- and defect-free plastic disc substrate was obtained. The Tg of the
plastic disc substrate was 25.degree. C.
The substrate was placed on a flat plate and post-cured at 80.degree. C.,
and a plastic disc substrate having the final Tg of 168.degree. C. and
good transferring properties of a stamper signal was obtained.
COMPARATIVE EXAMPLE 1
0.5 Part of benzoyl peroxide was added to 100 parts of
2,2'-bis[4-(.beta.-methacryloyloxyethoxy)cyclohexyl]propane to prepare a
polymerizable solution. This solution was subjected to the pre-curing
(polymerization temperature: 80.degree. C.; time: 4 hours) in a mold in
the same manner as in Example 1. The cured product was removed from the
mold. The curing reaction in the mold proceeded only partially, and the
cured product was brittle. During the operation, the plastic disc
substrate was cracked. Tg of the plastic disc substrate was 132.degree. C.
EXAMPLE 4
To 100 parts of 2,2'-bis[4-(.beta.-methacryloyloxyethoxy)cyclohexyl]propane
were added 0.5 part of a photosensitizer (Irgacure.RTM. 651, produced by
Ciba Geigy AG) and 0.5 part of benzoyl peroxide. The resulting mixture was
heated to 60.degree. C., uniformly stirred and mixed, and defoamed. This
liquid was poured into a cavity of a mold constituted by a glass plate
having a diameter of 120 mm, a silicone rubber and a metal stamper, and
irradiated with ultraviolet rays by passing 20 times under a 80 W/cm UV
lamp. The cured product was then removed from the mold to obtain a crack-
and defect-free plastic disc substrate. The Tg of this substrate was
25.degree. C.
The Tg of the substrate post-cured at 80.degree. C. for 2 hours was
148.degree. C., and the residual content of double bond was 38%.
The substrate prior to post-curing was post-cured at 150.degree. C. for 2
hours. The residual content of double bond in the substrate thus
post-cured was 32%, and the degree of saturated water absorption of the
substrate was 1.6%.
EXAMPLE 5
A crack- and defect-free plastic disc substrate was obtained in the same
manner as in Example 1 except that bis(oxymethyl)tricyclo[5,2,1,0.sup.2,6
]decane dimethacrylate was used in place of
2,2'-bis[4-(.beta.-methacryloyloxyethoxy)cyclohexyl]propane.
The Tg of the plastic disc substrate was 38.degree. C.
The substrate was placed on a flat plate and post-cured at 100.degree. C.
to obtain a plastic disc substrate having the final Tg of 251.degree. C.
and good transferring properties of stamper signal.
This substrate was post-cured at 150.degree. C. for 2 hours. The residual
content of double bond in the substrate thus obtained was 28%.
This substrate was further post-cured at 200.degree. C. for 2 hours. The
residual content of double bond in the substrate thus obtained was 17%.
This substrate was additionally post-cured in N.sub.2 at 300.degree. C. for
2 hours. The residual content of double bond in the substrate thus
obtained was 8%.
Each substrate was measured for the degree of water absorption and the
influences on a recording film.
The results obtained are shown in the Table below.
TABLE
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Post-Curing
Degree of Saturated
Temperature
Water Absorption Influences on
(.degree.C.)
(%) Recording Film
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100 1.9 Numerous pinholes
150 1.5 Several pinholes
200 0.9 Good
300 (in N.sub.2)
0.7 Good
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COMPARATIVE EXAMPLE 2
The procedure of Example 4 was repeated except that the post-curing was
carried out only by irradiation with ultraviolet rays. Even after UV lamp
50 time passages, the Tg was 120.degree. C. and did not reach to the final
Tg of 148.degree. C. The residual content of double bond was about 43 to
45%.
While the invention has been described in detail and with reference to
specific embodiments thereof, it will be apparent to one skilled in the
art that various changes and modifications can be made therein without
departing from the spirit and scope thereof.
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