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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a novel method of recording information by
the use of a laser beam of highenergy density and an novel information
recording medium advantageously employed for the recording method. More
particularly, the invention relates to a method of recording information
based on change of phase condition (i.e., phase change) and an information
recording medium capable of recording and erasing information on the basis
of the phase change.
2. Description of Prior Art
Information recording media utilizing a beam of high energy density such as
laser beam have been developed in recent years and are put to practical
use. Such recording medium is called an optical disc and its practical
applications have been found, for example, as video disc and audio disc as
well as disc memory for large-capacity computer and large-capacity static
image file.
The optical disc basically comprises a disc-shaped transparent substrate of
a plastic or glass material and a recording layer provided on the
substrate. As a material of the recording layer, there are known a metal
such as Bi, Sn, In or Te, a semi-metal, and a dye such as cyanine dye,
metal complex dye or quinoline dye. Writing of information on the optical
disc can be conducted, for example, by irradiating the optical disc with
laser beam. Under irradiation with the laser beam, the irradiated area of
the recording layer of the optical disc absorbs energy of the beam and a
rise in temperature locally occurs, and as a result a chemical or physical
change is caused to alter (or change) optical characteristics of the
recording layer in the irradiated area, whereby the recording of
information can be made.
The optical characteristics of the recording layer is altered, for example,
by forming pits or protruded portions on the surface of the recording
layer or forming bubbles in the recording layer, and the recording of
information utilizing the formation of pits is widely employed. Otherwise,
a method of utilizing a reaction between two recording layers, a method of
utilizing a phase change and a method of utilizing magnetic inversion are
also known for recording information. As the method of utilizing a phase
change, there is known a process comprising elevating a temperature of a
non-crystalline recording layer of As-Te-Ge type to higher than a glass
transition point to cyrstallize the layer so as to perform recording of
information.
Reading of information from the optical disc is also conducted by
irradiating the optical disc with laser beam. The information can be
reproduced by detecting reflected light or transmitted light corresponding
to the change in the optical characteristics of the recording layer.
The optical disc includes two types, that is, one capable of recording (or
writing) information only once (i.e., read only type) and the other
capable of recording and erasing information repeatedly (i.e., erasable
type).
Erasure of information from the optical disc can be done only in the case
that recording of information is attained through reversible change of the
recording layer. Accordingly, an optical disc which records information
through change of shape (i.e., physical structure) of the recording layer
such as formation of pits, etc. is of a read only type.
As a material of the recording layer, an inorganic material (e.g., a metal
or a semi-metal) and a dye are heretofore used as described before, and
any organic polymer material which is excellent in various properties such
as workability, safety and cost had not been proposed up to recently.
Recently, however, there is proposed an information recording method
comprising using a polymer blend composed of a vinylidene fluoride polymer
and polymethyl methacrylate as a material of the recording layer and
utilizing change of phase condition of the polymer blend between a
homogeneous state (i.e., state of solution) and a phase-separated state,
as described by Miyata, et al. in "Optical Recording Material Using
Polymer Blend" in the Collected Papers for The First High
Science-Technology Exhibition, P. 31 (1986) published by Tokyo
Agricultural and Technological Univ., 1986. However, it is practically
impossible to record sufficient amount of information by using only the
polymer blend because the polymer blend itself has a low absorption for a
light such as a laser beam. Hence, the polymer blend appears to be hardly
put into practical use.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a novel method of
recording information using an organic polymer material as a recording
material and a novel information recording medium employed for the method.
There is provided by the present invention a method of recording
information comprising the steps of:
irradiating a light-absorbing material with a laser beam for causing said
light-absorbing material to absorb energy of the beam and generate a heat,
said light-absorbing material being arranged in contact with a polymer
blend which changes in its phase condition between a homogeneous state and
a phase-separated state in response to change of its temperature across
its cloud point, whereby elevating the temperature of said polymer blend
in the vicinity of said light-absorbing material to a temperature of not
lower than the cloud point of the polymer blend to change its phase
condition into a state which is different from the original state before
the elevation of the temperature; and
rapidly cooling the polymer blend to fix the polymer blend in its changed
phase condition.
The recording method of the invention can be advantageously performed by
using an information recording medium comprising a substrate and a
recording layer provided on one surface or both surfaces of the substrate
which contains the above-mentioned polymer blend and a light-absorbing
material.
The recording method of the invention can be also advantageously performed
by using an information recording medium comprising a substrate, a
recording layer which is provided on one surface or both surfaces of the
substrate and comprises the above-mentioned polymer blend, and a
light-absorbing layer which is arranged in contact with the recording
layer and comprises a light-absorbing material.
The information which has been recorded on the medium according to the
above-described method can be erased for the next recording of other
information. Erasure of the recorded information can be easily carried out
by the process comprising the steps of uniformly irradiating the
above-mentioned polymer blend with a light for causing the light-absorbing
material to generate a heat, elevating a temperature of the polymer blend
to a temperature of not lower than its cloud point between the homogeneous
state and the phase-separated state, and then gradually cooling the
polymer blend. By this process, the polymer blend can be returned to its
original state (i.e., un-recorded state). The polymer blend can be also
returned to its original state (i.e., un-recorded state) by directly
heating the polymer blend at a temperature of not lower than the cloud
point and then gradually cooling the polymer blend. Further, the recorded
information can be partially rewritten with ease by the following process.
The light-absorbing material which is in contact with the aimed portion of
the polymer blend is irradiated with a laser beam. Under irradiation with
the laser beam, the light-absorbing material absorbs energy of the beam to
generate a heat. By the generated heat, a temperature of the aimed portion
of the polymer blend is elevated to a temperature of above the cloud point
of the polymer blend so as to change or alter the phase condition of the
polymer blend, and the polymer blend is then gradually cooled. Thus, the
recorded information can be partially rewritten.
In the invention, the term "cloud point" means a temperature at which the
composition of the polymer blend gives a change or an alteration of phase
condition (i.e., phase change) between a homogeneous state and a
phase-separated state. In general, the change of the phase condition is
apparently observed as a change from a transparent state (i.e.,
homogeneous state) to an opaque state (or a translucent state).
The polymer blends employable in the invention can be a polymer blend of a
lower critical solution temperature (LCST) type and a polymer blend of an
upper critical solution temperature (UCST) type. The polymer blend of LCST
type shows a phase-separated state at a temperature of not lower than the
cloud point, while the polymer blend of UCST type shows a homogeneous
state at a temperature of not lower than the cloud point. Any of those
polymer blends gives a change of phase condition between the homogeneous
state and the phase-separated state in accordance with variation of
temperature across the cloud point. In more detail, any of those polymer
blends is transparent in the homogeneous state because the polymer
components are homogeneously mixed with each other, while it is opaque in
the phase-separated state because the phase-separated polymer components
differ from each other in the refractive index. Further, the polymer blend
has such characteristics that the polymer blend which has temporarily
turned opaque or transparent at a temperature of not lower than the cloud
point returns to its original (starting) state when gradually cooled, but
keeps the opaque or transparent state even at room temperature after it is
cooled rapidly for fixing.
In the method of the invention, a polymer blend which serves a recording
material is brought into contact with a light-absorbing material, and the
light-absorbing material is irradiated with a laser beam. Under
irradiation with the beam, the light-absorbing material absorbs energy of
the beam and generates a heat. Upon Receipt of the heat, the polymer blend
increases in its temperature to a temperature of above its cloud point to
change in the phase condition, whereby the polymer blend turns opaque or
transparent. Thereafter, the polymer blend is rapidly cooled to fix the
irradiated area of the polymer blend in the opaque or transparent state.
Thus, recording of information is performed. Reproduction of the recorded
information can be done by a conventional process of irradiating with a
laser beam of relatively low level. In more detail, the recorded
information can be read out based on the difference of reflectance or
transmittance based on the difference of transparency.
Moreover, the polymer blend which has been partially changed in the phase
condition can be returned to its original state in the irradiated area
(recorded area) by the process of again elevating a temperature of the
polymer blend to a temperature of above the cloud point and then gradually
cooling. By this operation, the whole polymer blend returns to its
original transparent or opaque state, and the recorded information is
erased. Partial erasure of the recorded information can be also performed
by subjecting the aimed area of the polymer blend to the same erasing
operation as described above.
Accordingly, the information recording medium of the present invention
comprising a recording layer containing the above-described polymer blend
and light-absorbing material is very useful as a recording medium of
E-DRAW (Erasable-Direct Read After Write) type in which information can be
recorded and erased at any time.
The present invention can be effectively applied to recording media of
various forms, for example, optical discs such as a video tape, an audio
tape, a flexible disc and a disc memory for large-capacity computer or
large-capacity static image file, as well as optical recording media of
other forms such as card, sheet and tape.
The recording layer of the recording medium according to the present
invention can be easily formed by a coating method, because the recording
material employed is a polymer. Accordingly, the process for the
preparation of the recording medium can be simplified, the time required
for the preparation of the medium can be very shortened, and the
manufacturing cost can be reduced.
BRIEF DESCRIPTION OF DRAWINGS
FIGS. 1 to 6 are sectional views showing examples of the constitution of an
information recording medium according to the present invention.
FIG. 7 is a graph showing phase condition of a LCST type polymer blend.
FIG. 8 is a graph showing an alteration of phase condition with time in the
recording procedure of information and the erasing procedure of
information.
DETAILED DESCRIPTION OF THE INVENTION
The information recording medium of the invention is now described by
referring to the attached drawings.
Each of FIGS. 1 to 6 is a sectional view showing an example of the
constitution of the information recording medium of the present invention.
In FIG. 1, the recording medium comprises a substrate 1 and a recording
layer 2 provided on one surface of the substrate 1. The recording layer 2
comprises, as essential components, a polymer blend and a light-absorbing
material.
In FIG. 2, the recording medium comprises a substrate 1 and recording
layers 2, 2' provided on both surfaces of the substrate.
The information recording medium of the invention may have a reflecting
layer and/or a protective layer. For example, in FIG. 3, the recording
medium comprises a substrate 1, a recording layer 2, a reflecting layer 3
and a protective layer 4, superposed in this order.
In FIG. 4, the recording medium comprises a substrate 11, a light-absorbing
layer 12 and a recording layer 13, in which the latter two layers are
provided on one surface of the substrate 11. The recording layer 13
comprises, as an essential component, a polymer blend.
In FIG. 5, the recording medium comprises a substrate 11, light-absorbing
layers 12, 12' and recording layers 13, 13', in which the latter two sets
of layers are provided on both surfaces of the substrate 11.
In FIG. 6, the recording medium comprises a substrate 11, a light-absorbing
layer 12, a recording layer 13, a reflecting layer 14 and a protective
layer 15, superposed in this order.
It should be understood that the above-described embodiments by no means
restrict the constitution of the information recording medium of the
invention. For example, the reflecting layer is provided between the
recording layer (or light-absorbing layer) and the substrate in the case
that a laser beam is irradiated from the opposite side of the substrate to
perform recording or reproducing of information. Further, a variety of
known intermediate layers such as an undercoating layer or a pre-groove
layer may be provided between the substrate and the recording layer (or
light-absorbing layer).
The information recording medium of the invention can be prepared, for
example, by the following process.
In one embodiment, the recording layer essentially comprises a polymer
blend and a light-absorbing material dispersed in the polymer blend. In
another embodiment, the recording layer essentially comprises a polymer
blend and a light-absorbing material is arranged in an independent layer
which is arranged in contact with the recording layer.
The polymer blend employable as a recording material in the invention is a
mixture of two or more polymers, and capable of bringing about change or
alteration of phase condition (i.e., phase change or phase alteration)
between the solution state and the phase-separated state. However, there
are some cases where the same phase change is brought about even in a
combination of a polymer and a monomer, and such combination is included
in the polymer blend employable in the invention.
The polymer blend is divided into two types, that is, a polymer blend of
lower critical solution temperature (LCST) type and a polymer blend of
upper critical solution temperature (UCST) type. The LCST type polymer
blend is transparent and in the homogeneous state at room temperature but
is phase-separated at a temperature of above the cloud point to turn
opaque, while the UCST type polymer blend is opaque and in the
phase-separated state at room temperature but is converted in the
homogeneous state at a temperature of above the cloud point to turn
transparent.
FIG. 7 is a graph showing an example of phase condition of the LCST type
polymer blend, in which a volume fraction (i.e., volume ratio) with
respect to one polymer of the employed two polymers is plotted on the
abscissa and a temperature is plotted on the ordinate.
In FIG. 7, each point on the curve is a cloud point of the polymer
composition. The region of lower temperature than the cloud point (i.e.,
region below the curve in FIG. 7) shows the homogeneous solution state
(transparent), and the region of higher temperature than the cloud point
(i.e., region above the curve in FIG. 7) shows the phase-separated state
(opaque). The polymer blend is homogeneously mixed at a lower temperature
than the lowest cloud point (Tc) regardless that the polymer blend has any
composition. From this viewpoint, the temperature Tc is called a lower
critical solution temperature.
On the contrary, the UCST type polymer blend shows a convex curve in its
graph of phase condition, and the upper critical solution temperature is
determined in the same manner.
The cloud point of the polymer blend varies depending on nature of the
employed polymers, composition thereof, and further a molecular-weight
distribution thereof and a molecular weight thereof in the case of an
amorphous polymer. The cloud point of the polymer blend preferably is in
the range of 60.degree. to 400.degree. C., more preferably in the range of
80.degree. to 300.degree. C.
Examples of the LCST type polymer blend are as follows:
(1) combination of amorphous polymers such as polystyrene and
polyvinylmethyl ether, styrene/acrylonitrile copolymer and
poly-.epsilon.-caprolactone, styrene/acrylonitrile copolymer and
polymethyl methacrylate, polyvinyl nitrate and polymethyl methacrylate,
ethylene/vinyl acetate copolymer and chlorinated rubber,
poly-.epsilon.-caprolactone and polycarbonate (bisphenol A type),
p-chlorostyrene/ochlorostyrene copolymer and
poly(2,6-dimethyl-1,4-phenyleneoxide), polycarbonate (bisphenol A type)
and ethylene oxide block copolymer, butylene terephthalate/tetrahydrofuran
block copolymer and polyvinyl chloride, and thermoplastic
polyurethane[poly-.epsilon.-caprolactone soft block] and polyvinyl
chloride;
(2) combination of a crystalline polymer and an amorphous polymer such as
polyvinylidene fluoride and polymethyl acrylate, polyvinylidene fluoride
and polyethyl acrylate, polyvinylidene fluoride and polymehtyl
methacrylate, polyvinylidene fluoride and polyethyl methacrylate, and
polyvinylidene fluoride and polyvinyl methyl ketone; and
(3) combination of a crystalline polymer and a crystalline monomer such as
polyethylene oxide and trioxane, and poly-.epsilon.-caprolactone and
trioxane.
Examples of the UCST type polymer blend include a combination of amorphous
polymers such as a combination of polystyrene and polyisoprene, a
combination of polystyrene and polyisobutene, a combination of
polypropylene oxide and polybutadiene, and a combination of polyisobutene
and polydimethyl siloxane.
The above-described polymers can be appropriately copolymerized with other
monomers to prepare copolymers, provided that the resulting copolymers is
of LCST or UCST.
The light-absorbing material employable in the invention has a high
absorption for a laser beam and generates a heat by the light-absorption.
A semiconductor laser giving near infrared rays has been put into practical
use as a laser for recording and reproducing information, so that
preferably employed as the light-absorbing material is a dye showing a
high absorption ability for the light in the near infrared region of
700-900 nm.
Examples of such dyes are given below.
##STR1##
in which n is 2 or 3.
##STR2##
in which R is hydrogen or N(CH.sub.3).sub.2.
##STR3##
in which R is an alkyl group; and X is a hologen atom.
##STR4##
in which R is any one of a substituted or unsubstituted alkyl group, an
alkoxy group, an aralkyl group and an alkenyl group; X is hydrogen or a
halogen atom; Y.sup.- is any one of halogen, perchlorate, substituted or
unsubstituted benzene sulfonate, p-toluene sulfonate, methylsulfonate,
ethylsufonate, benzene carboxylate, methylcarboxylate and trifluoromethyl
carboxylate; and n is an integer of 0 to 3.
.PHI.-L=.PSI.(X.sup.-).sub.m (6)
in which each of .PHI. and .PSI. is an indole ring residue or a benzoindole
ring residue; L is a connecting group for forming monocarbocyanine,
dicarbocyanine, tricarbocyanine or tetracarbocyanine; X.sup.- is an
anionic ion; and m is 0 or 1.
##STR5##
in which X is hydrogen, a chlorine atom, a bromine atom or a methyl group;
n is an integrer of 1 to 4; and A is a quaternary ammonium group.
##STR6##
in which R is hydrogen or OC.sub.2 H.sub.5.
##STR7##
in which R is OH, NH.sub.2, NHX or NX.sub.2 (X is an alkyl group, and X'
is the hydrogen, an alkyl group, an allyl group, an amino group or a
substituted amino group); and R' is OH, NH.sub.2, NHX, NX.sub.2 or
##STR8##
(X and X' have the same meanings as defined above).
##STR9##
There can be employed other dyes such as a triallyl methane dye, a
phthalocyanine dye, an inmonium dye and a nitoso compound than the
above-mentioned dyes.
In the case that the recording layer comprises the polymerblend and the
light-absorbing material, the recording layer can be formed by the process
comprising the steps of dissolving the polymer blend and the
light-absorbing material in an appropriate solvent to prepare a coating
solution, applying the coating solution over the surface of the substrate,
and drying the coated layer.
Examples of the solvent employable for the preparation of the coating
solution include polymer blends such as toluene, xylene, ethyl acetate,
butyl acetate, Cellosolve acetate, methyl ethyl ketone, dichloromethane,
1,2-dichloroethane, dimethylformamide, methylisobutyl ketone,
cyclohexanone, tetrahydrofuran, ethyl ether, dioxane, ethanol, n-propanol,
isopropanol and n-buthanol; solvents capable of dissolving or dispersing
the polymer blend and the light-absorbing material; and mixtures thereof.
The coating solution may further contain other additives such as an
antioxidant, an UV absorbent, a plasticizer and a lubricant according to
the purpose.
The coating procedure can be carried out by a conventional method such as
spray coating, spin coating, dip coating, roll coating, blade coating,
doctor roll coating and screen printing.
The ratio between the polymer blend and the light-absorbing material in the
coating solution varies depending on nature of the employed polymer blend
and light-absorbing material. The ratio therebetween is generally in the
range of 100:0.1 to 100:100 (polymer blend:light-absorbing material, by
weight), preferably in the range of 100:1 to 100:50. The recording layer
can be composed of a single layer or plural layers. The thickness of the
recording layer is generally in the range of 0.01 to 10 .mu.m, preferably
0.02 to 1 .mu.m, from the viewpoint of optical density required for the
optical recording. The recording layer can be provided on both surfaces of
the substrate (see: FIG. 2), as described hereinbefore.
The substrate material employed in the invention can be selected from any
materials which have been employed as the substrates of the conventional
recording media. From the viewpoint of optical characteristics,
smoothness, workability, handling properties, long-term stability and
manufacturing cost, preferred examples of the substrate material include
glass such as tempered glass, acrylic resins such as cell-cast polymethyl
methacrylate and injection-molded polymethyl methacrylate; vinyl chloride
resins such as polyvinyl chloride and vinyl chloride copolymer; epoxy
resins; polycarbonate resins; amorphous polyolefin; and polyesters.
Preferred are polymethyl methacrylate, polycarbonate resins, epoxy resins
amorphous polyolefin, polyesters and glass from the viewpoints of
dimensional stability, transparency, and plainness. These materials can be
employed in the form of a flexible film or a rigid sheet.
The surface of the substrate on which a recording layer is arranged may be
provided with an undercoating layer for the purpose of improving
smoothness, adhesion to the recording layer and sensitivity by heat
insulating and preventing the recording layer from being denatured.
Examples of materials for such undercoating layer include polymer
materials such as polymethyl methacrylate, acrylic acid/methacrylic acid
copolymer, styrene/maleic anhydride copolymer, polyvinyl alcohol,
N-methylolacrylamide, styrene/sulfonic acid copolymer,
styrene/vinyltoluene copolymer, chlorosulfonated polyethylene,
nitrocellulose, polyvinyl chloride, chlorinated polyolefin, polyesters,
polyimides, vinyl acetate/vinyl chloride copolymer, ethylene/vinyl acetate
copolymer, polyethylene, polypropylene and polycarbonate; organic
materials such as silane-coupling agents; and inorganic materials such as
inorganic oxides (e.g., SiO.sub.2, Al.sub.2 O.sub.3), and inorganic
fluorides (e.g., MgF.sub.2).
In the case using a glass plate as the substrate, an undercoating layer for
keeping the recording layer from alkali metal ions and alkalline earth
metal ions (which are leberated from the glass plate) is preferably
provided on the substrate. Such undercoating layer can be made of a
polymer having a hydrophilic group and/or maleic anhydride group in the
molecular structure, such as styrene/maleic anhydride copolymer.
The undercoating layer can be formed by dissolving or dispersing the
material of the layer in an appropriate solvent and coating the solution
or dispersion on a substrate through a known coating method such as spin
coating, dip coating, or extrusion coating. The thickness of the
undercoating layer is generally in the range of 0.005 to 20 .mu.m,
preferably in the range of 0.01 to 10 .mu.m.
On the substrate (or the undercoating layer) may be provided a pre-groove
layer for the purpose of forming tracking pre-grooves or protruded and
depressed portions indicating information such as address signals. The
pre-groove layer can be made of a monomer such as a monoester, diester,
triester or tetraester of acrylic acid, or its oligomer in combination
with a photopolymerization initiator.
The pre-groove layer can be formed on the substrate, for instance, by the
following process. A mixture liquid of the above-mentioned acrylic acid
ester and photopolymerization initiator is coated over a stamper
(precisely prepared mother plate). On the coated layer of the mixture
liquid is placed the substrate, and the layer is irradiated with an
ultraviolet rays through the stamper or the substrate to harden the liquid
phase of the layer, whereby the liquid phase is firmly fixed to the
substrate. Then the substrate is separated from the stamper to obtain a
substrate having a pre-groove layer. The pre-groove layer generally has a
thickness of 0.05 to 100 .mu.m, preferably 0.1 to 50 .mu.m. In the case of
using a plastic material as the substrate, the pre-groove can be formed
directly on the substrate by a molding process such as injection molding
or extrusion molding.
A reflecting layer may be provided on the recording layer for the purpose
of increasing S/N ratio in the reproduction procedure and enhancing
sensitivity in the recording procedure. In this case, recording of
information or reproduction of recorded information is carried out by
irradiating the recording layer with a laser beam from the substrate side.
The reflecting layer is not always provided on the recording layer, and
can be provided between the substrate (or undercoating layer) and the
recording layer. In this case, recording of information or reproduction of
recorded information is carried out by irradiating the recording layer
with a laser beam from the upper side of the medium (i.e., opposite side
of the substrate side.
The reflecting layer is essentially composed of a light-reflecting
material. The light-reflecting material has a high reflectance for the
laser beam. Examples of the light-reflecting material include metals such
as Mg, Se, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Co, Ni, Ru,
Rh, Pd, Ir, Pt, Cu, Ag, Au, Zn, Cd, Al, Ga, In, Si, Ge, Te, Pb, Po, Sn and
Bi, and semimetals. Preferred are Al, Cr and Ni. These materials can be
employed singly or in combination. Alloys thereof can be also employed in
the invention.
The reflecting layer can be formed on any layer of the recording layer,
substrate or the undercoating layer using the light-reflecting material
according to a known method such as deposition, spattering or ion plating.
The thickness of the reflecting layer is generally in the range of 100 to
3,000 angstroms.
In the case of providing the recording layer on only one surface of the
substrate and performing recording and reproduction of information from
the substrate side, the reflecting layer may be provided on the surface of
the recording layer not facing the substrate.
A protective layer may be further provided on the surface of the recording
layer or the reflecting layer not facing the substrate (i.e., exposed
surface side) to physically or chemically protect the recording layer. The
protective layer can be also provided on the surface of the substrate
where the recording layer is not provided to enhance a resistance to
damage or humidity. As a material of the protective layer, there can be
mentioned inorganic materials such as SiO, SiO.sub.2, MgF.sub.2 and
SnO.sub.2 ; and organic materials such as thermoplastic resins,
thermosetting resins and UV-curable resins.
The protective layer can be formed on the recording layer (or the
reflecting layer) and/or the substrate by laminating a plastic film having
been prepared by extrusion processing on any of those layers and/or the
substrate by way of an adhesive layer. Otherwise, a method of vacuum
deposition, spattering or coating can be also applied to form the
protective layer. In the case of using the thermoplastic resin or the
thermosetting resin, the resin is dissolved in an appropriate solvent to
prepare a coating solution, and the solution is coated over the recording
layer and/or the substrate. The coated layer is then dried to form a
protective layer. In the case of using the UV-curable resin, a solution of
the resin in an appropriate solvent is coated over the recording layer
and/or the substrate, and the coated layer of the solution is irradiated
with ultraviolet rays to cure the layer so as to form a protective layer.
In any case, the coating solution may further contain a variety of
additives such as an antistatic agent, an antioxidant and an UV-absorbent
according to the purpose. The thickness of the protective layer is
generally in the range of 0.1 to 100 .mu.m.
As described hereinbefore, the polymer blend and the light-absorbing
material can be incorporated into different layers. For instance, a
recording layer comprising the recording material and a light-absorbing
layer comprising the light-absorbing material can be arranged
independently and in contact with each other.
In this case, the light-absorbing material can be one of the dyes described
hereinbefore. Alternatively, the light-absorbing material can be a metal
or semimetal exemplified for the material of the light-reflecting layer.
There is no specific limitation with respect to the arrangement of the
recording layer and the light-absorbing layer, so long as both are
arranged in contact with each other. The formation of these layers can be
performed in manners similar to the formation of the recording layer
comprising both the polymer blend and the light-absorbing material.
The thickness of the light-absorbing layer is generally in the range of 100
to 3,000 angstroms, preferably in the range of 300 to 1,000 angstroms.
Processes of recording information and erasing information according to the
invention are described hereinafter referring to the information recording
medium comprising a substrate and a recording layer which is shown in FIG.
1.
Recording of information is carried out in the following manner. A
semiconductor laser giving near infrared rays such as Ga-As laser is
employed. A laser beam converged by a known method is irradiated on the
substrate side (or the recording layer side) of the information recording
medium. Under irradiation, the light-absorbing material (e.g., near
infrared rays-absorbing material) in the irradiated area of the recording
layer instantaneously absorbs the beam energy to generate a heat. Owing to
the generated heat, the polymer blend which is in contact with the
light-absorbing material in the recording layer is heated to have a
temperature of above its cloud point, whereby the phase condition of the
polymer blend is changed. For example, the LCST type polymer blend having
been transparent and in the homogeneous state is phase-separated to turn
opaque by the elevation of a temperature. The recording medium is then
rapidly cooled, and the polymer blend is kept in the opaque state even at
room temperature. Thus, only the irradiated area of the recording layer
turns opaque, and the information is recorded accordingly. In the case of
the UCST type polymer blend, only the irradiated area of the recording
layer is kept in the transparent homogeneous state.
Reproduction of the recorded information from the recording medium can be
carried out in the same manner as that conventionally used in the read-out
of the information. In detail, a laser beam for reproducing information is
irradiated on the recording layer side or the substrate side of the medium
to measure light reflectance of the reflected light. On the basis of the
difference of the reflectance between the opaque area and the transparent
area of the recording layer, the recorded information can be reproduced.
When the reflecting layer is provided on the surface of the recording
layer, the difference of the light reflectance becomes more prominent to
enhance the S/N ratio for reproduction. On the contrary, when the
reflecting layer is not provided on the recording layer, transmittance of
the transmitted light of the laser beam is measured. On the basis of the
difference of the transmittance, the recorded information can be
reproduced.
Erasing of information can be performed in the following manner. Using the
same semiconductor laser as described | | |
