Note: Descriptions are shown in the official language in which they were submitted.
~150837
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LASER RECORDING MEDIUM
This invention relates generally to binary data information
storage systems and, in particular, to a data recording
medium responsive to energy from a focused laser beam.
The data processing industry has made rapid strides in
providing computers systems and related peripheral equipment
for manipulating binary encoded numeric and alphabetic data
at faster speeds and storing such data at higher densities
and lower costs. Large corporations and government bureaus
have placed increasing reliance on data processing equipment
in automating data collection, storage and processing to
improve the efficiency of handling business transactions,
accounting information, etc. Increases in computer operat-
ing speeds are largely the result of improvements in semi-
conductor technology which have produced large scale inte-
grated (LSI) circuits involving higher densities of binary
logic elements or gates operating at faster speeds. Sub-
stantial increases in memory densities have also been
achieved. In the semiconductor memory area, bit density
increases have resulted both from improved LSI technology
which enables a shrinking of the size of memory cell ele-
ments and from new LSI technology such as magnetic bubble
domain memories. In the magnetic memory area, density
improvements in hard and flexible disc systems have been
achieved by improvements in magnetic recording media and
reading and writing heads associated therewith.
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Despite the substantial increases in semiconductor and
magnetic memory system densities, the cost per bit of such
storage media together with encoding costs does not
justify the use of such technology for storing, on a routine
5 basis, 'arge volumes of traditional business records, such
as correspondence, reports, forms, legal documents, etc.
The storage and maintenance of both current working files of
these documents and archives of selected documents which
must be retained securely for long periods of time is still
10 largely a manual operation involving increasingly costly
personnel and storage space.
Digital laser recording technology has been developed in
recent years to provide high density binary data storage
15 which is readily integratable with both computer data
processing equipment and facsimile document scanning and
printing apparatus. This technology enables real time
optical recording of image data in a highly compressed
format and rapid opto-electronic access to recorded image
data and can thus provide the basic framework for computer
based document storage and retrieval and an overall record
management system. At the heart of this technology is a
laser beam writing and reading system which is capable of
storing binary digital information in the form of the
presence or absence of minute holes created in a thin film
recording medium as a highly focused, modulated laser beam
is scanned across the recording medium.
The basic principles of laser image recording are set forth
in Becker U.S. Patent 3,474,457. Becker et al. U.S. Patent
3,654,624 and McFarland et al. U.S. Patent 3,657,707 show a
laser recording system utilizing a rotating drum carrying a
laser recording medium comprising flexible strips of plastic
materials (such as Mylar) with a layer of energy absorbing
material thereon. Such a laser recording medium is more
fully described in Becker et al. U.S. Patent 3,665,483.
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However, the use of a rotating drum or other mechanical
scanning of the recording medium limits the record scanning
speed during both recording and retrieval of data and thus
artificially constrains the overall system to data writing
5 and reading speeds substantially less than those dictated by
available laser beam energies and recording media sensiti-
vities. In addition, the use of flexible recording media
limits the alignment precision which can be reproducibly
achieved between data tracks and the laser beam path and,
10 correspondingly, constrains the system to data bit densities
substantially lower than the minimum cell size dictated by
the system optics. Moreover, flexible recording media are
highly subject to contamination by dust particles which may
cause data writing and/or reading errors and thus require
15 special handling and storage in dust-free compartments
within the system. It is thus apparent that different
approaches to scanning the laser beam across the recording
medium and different structures for the recording medium
itself are required to provide a system that fully utilizes
20 the write/read speed and bit densities of which laser beam
recording technology is inherently capable and also simpli-
fies the recording media storage and handling requirements.
Becker et al. U.S. Patent 4,001,840 discloses a laser
25 recording system which utilizes a mirror assembly rotatable
on two orthogonal axes to deflect a laser beam in two
directions for writing data on a recording layer formed on
a rigid glass substrate. This mirror-beam deflection
system is capable of achieving faster beam scanning, and the
rigid glass substrate supporting the recording layer enables
more precise, reproducible alignment between the recording
medium and the scanned laser beam. However, it has been
found that the use of a layer of recording material directly
on a glass substrate results in a laser recording medium of
substantially less sensitivity than a corresponding laser
recording medium comprising a recording layer formed on a
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1150837
plastic substrate. In addition, the affinity between the metal recording
layer and a glass substrate may produce irregularities in the shapes and
sizes of holes burned into the recording layer. Use of a glass substrate
thus necessitates the forming of a more complex recording medium in order to
maintain overall sensitivity of the laser recording system and to achieve high
writing speeds with low error rates.
Artisans in this field have generally recognized the benefits of com-
bining a layer of plastic material intermediate the substrate and the record-
ing layer with a protective coating over the recording layer. However, while
plastics have been suggested for use as the protective layer, in practice
artisans have typically employed inorganic materials such as silicon dioxide
in the protective coating, because the solvent-based plastic materials of the
intermediate layer are dissolved or attacked when a protective layer of the
same or similar solvent-based plastic material is attempted to be applied as
the solvent utilized readily penetrates the thin layer of laser recording
material.
In a copending Canadian application by A. Forster and M. Ockers, Serial
No. 361,239, filed September 30, 1980, the use of a vapor deposited plastic
layer as a protective coating for a laser recording medium is disclosed. In
this application the method of depositing the protective plastic layer on
top of the recording layer of
A
i~S0837
the medium precludes any attacking of the intermediate layer
between the substrate and the recording layer, since no
solvent is present in the vapor deposition process. Accord-
ingly, the intermediate layer between the recording layer
and the substrate may be a layer of solvent-based plastic
material. Alternatively, Forster and Ockers disclose the
use of a vapor deposited layer of plastic material as the
intermediate layer between the substrate and the recording
layer. While the Forster and Ockers approach provides a
10 recording medium in which the recording layer is encased
between two plastic layers, it requires the use of special
vapor deposition apparatus to form the parylene layers
utilized in the recording medium.
15 A laser recording medium in accordance with this invention
comprises a substrate, a first layer of plastic material
formed on the substrate, a layer of optical energy absorbing
material (i.e. a recording layer) formed on the first layer
of plastic material, and a second layer of plastic material
20 formed on the recording layer to provide a protective
coating therefore, with the plastic material of the first
layer being characterized by substantial solvent resistance
and the plastic material of the second layer being a sol-
vent-based plastic material. In accordance with a further
25 aspect of this invention, the plastic material of the first
layer formed on the substrate is a crosslinked polymeric
material formed by reacting one or more components of a
class of materials comprising active polymers with one or
more components of a class of materials comprising cross-
30 linking organic moieties. Preferably the reaction formingthe crosslinked polymeric material is carried out at ele-
vated temperature and in the presence of a selected catalyst
to speed the formation of the crosslinked mater~`al. Alter-
natively, certain components of crosslinking organic moie-
35 ties may be reacted together in the presence of a selectedcatalyst to form self-condensation, crosslinked polymers.
.A-35140
11S0~3~
By appropriate selection, solvent-resistant plastic layers
which have all the necessary characteristics for serving as
an intermediate layer are formed.
In accordance with another aspect of this invention, the
plastic material of the first layer is a polymeric material
formed in a vapor deposition process wherein a hot reactive
monomer vapor is condensed as a polymeric coating on the
substrate. The polymeric material formed in this fashion
may comprise a parylene material.
By first forming a layer of solvent-resistant plastic
material on the substrate, a multi-layer laser recording
medium can be readily completed by next forming the thin
recording layer on the solvent-resistant intermediate layer
and then promptly coating the recording layer with a layer
of common, solvent-based plastic material to seal the
recording layer against any deterioration which may other-
wise be caused by abrasion or reaction with the ambient
environment to form metal oxides or contamination from the
ambient at~osphere. Thus, in accordance with this invention
solvent-resistant coating is formed on the substrate at a
less critical time in the process of forming a laser re-
cording medium, so that final protection of the recording
25 layer formed thereon can be simply and promptly provided by
a solvent-based plastic layer.
Other features and advantages of this invention will be
apparent from the consideration of the detailed description
30 given below in conjunction with the accompanying drawings.
Fig. 1 is a block diagram illustrating exemplary laser
recording apparatus utilizing a laser recording medium in
accordance with this invention.
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Fig. 2 is a fragmented cross-sectional view of the structure
of a laser recording medium in accordance with this inven-
tion.
Fig. 1 illustrates the apparatus utilized in a typical laser
beam recording system. This type of laser recording system
is now generally well known in the art and need not be
discussed in detail herein. Reference is made to the above
mentioned Becker U.S. Patent 3,474,457 and Becker et al.
U.S. Patent 4,001,840 for a more detailed discussion of the
principles of laser recording and exemplary apparatus
embodying these principles. Generally laser beam recording
involves the use of a laser 10 with its output coupled to a
beam modulator 20 which is driven by an input signal means
50 to produce a modulated laser beam output. In a binary
data writing mode the input signals means supplies a stream
of binary digits such that the modulator produces a binary
amplitude modulation of the laser beam. Focusing and
scanning apparatus 30 receives the modulated laser beam,
focuses it to a very small spot on recording medium 40 and
scans it in a predetermined pattern across recording medium
40. As the modulated laser beam strikes various sequential
cell locations of the recording layer in laser recording
medium 40, it burns a very small hole (0.5-1.0 microns in
diameter) therein if the modulated laser beam is on at that
time or leaves the recording layer undisturbed if the
modulated laser beam is off. The term "burn" is typically
used in the art to describe the hole formation in the
recording layer even though the recording layer is actually
melted or vaporized to create a hole rather than being
burned in the ordinary sense of the word. Accordingly, the
binary data input to the modulator 20 is reproduced on
recording medium 40 as the presence or absence of a hole at
each cell location in the recording medium. The bit pattern
written into recording medium 40 can be later read by again
scanning the recording medium with an unmodulated laser beam
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and detecting the presence or absence of a hole in each cell
location in terms of the amount of light reflected at each
cell location.
5 As generally discussed above, laser data recording apparatus
is inherently capable of recording binary data at very high
densities on the order of about 109 bits per square inch.
As previously noted to provide apparatus which enables a
laser recording system to achieve bit densities approaching
the inherent capability of the technology it places heavy
demands on all aspects of the laser recording system and
especially the laser recording medium. Since data is
recorded in the form of the presence or absence of minute
holes burned into the recording layer by highly focused
laser beam, the overall stability and durability of the
laser recording medium both during the recording process and
for a long time period thereafter is critical in determining
the ultimate bit density which can be utilized and still
achieve data writing and reading at low error rates over
long periods of time. Stability and durability are espec-
ially critical if the laser recording system is to be
utilized for archival storage of image data from documents
which are thereafter destroyed.
To provide a recording medium which can accurately and
reproducably be aligned with the scanning laser beam in a
laser recording system requires that the recording medium
utilize a dimensionally stable inflexible substrate such as
a thin glass slide of the type generally used by the semi-
conductor industry in forming highly accurate photomasksused in the production of large scale integrated circuits.
Such glass slides form the basis for a recording medium
which has excellent dimensional stability and can easily be
integrated into an overall data slide handling system for
reproducably positioning the recording medium with reference
to the scanning path of the laser beam. Further, it is
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1150837
necessary to form on the glass substrate a recording layer
of material which is sensitive to optical energy of the
wavelength of the laser beam in a manner which will provide
overall long term stability for the recording medium.
Fis. 2 illustrates the structure of a laser recording
medium in accordance with this invention as comprising a
transparent substrate 41 having formed thereon a first layer
of plastic material 42, a recording layer 43 and a second
10 layer of plastic material 44. Transparent substrate 41 is
preferably a glass slide. Conveniently, the glass slide may
be about four inches square and 60 mils thick. On one
surface of glass substrate 41 a first layer of plastic
material 42 is formed. Preferably the laser beam is incident
15 on recording layer 43 through the glass substrate 41 and
intermediate layer 42 since any dust particles which might
accumulate on the exposed substrate surface are then out-of-
focus during reading and writing of data in recording layer
43. In accordance with this -nvention the material of this
first layer is characterized by substantial solvent-resis-
tance. This characteristic may be achieved by utilizing a
crosslinked polymeric material which, although utilizing
solvent-based plastic materials in its formation, achieves
substantial solvent-resistance due to the cross-linking of
the polymers comprising the final material of the layer.
Alternatively, solvent-resistant plastic layer 42 can be
provided by utilizing a polymeric material such as parylene
which also has a high solvent resistance and is formed in a
vapor deposition process wherein a hot reactive monomer
vapor is condensed on substrate 41 as a polymeric coating.
This condensed polymeric coating is optionally formed only
on one surface of the glass substrate 41 if suitable masking
techniques are utilized on the other surface or may be
formed on all surfaces of substrate 41. Depending on the
process utilized in forming solvent-resistant plastic layer
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--10--
42 it may be formed to a thickness in the range of 0.05
microns to 10 microns. Thickness values throughout this
range are readily attainable utilizing a parylene vapor
deposition process. When utilizing a coating process
involving plastic material initially dissolved in a solvent,
thicknesses in the range from 0.5 to about 2 microns are
readily achievable.
The optical and other characteristics of the marerials of an
intermediate layer 42 of solvent-resistant plastic materials
are suited to a laser recording medium for use in a system
in which the recording layer is burned by a laser beam
transmitted through both the substrate and the intermediate
layer. Intermediate layers which have high optical clarity
are produced. The solvent resistant materials have an index
of refraction in the range of 1.3-1.7 and are thus suffi-
ciently closely matched to that of glass to minimize re-
flections. These materials also have a much lower thermal
conductivity than the glass substrate to provide a laser
recording medium of high sensitivity to laser beam energy.
The materials adhere well to the glass substrate and bond
well to a metal recording layer to produce a stable record-
ing medium.
A number of well-known materials may be utilized as the data
recording layer 43. Preferably, recording layer 43 is
formed with relatively low melting point metals such as
bismuth or tellurium. Recording layer 43 is preferably
formed to a thickness of about 50-200 Angstroms in order to
provide a high sensitivity to laser energy incident thereon.
Layer 44 of plastic material has the principal function to
protect the recording layer 43 from abrasion and contami-
nation by chemicals or other materials existing in the
ambient environment in which recording medium 40 will be
employed. Since intermediate layer 42 is formed of a
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solvent-resistant plastic material, protective layer 44 can
be formed in a solvent-plastic coating process using any of
the common plastics, including acrylic, polystyrene, poly-
urethene, polyethylene, epoxy, cellulose acetate materials
or mixtures thereof dissolved in a solvent such as toluene,
~etone or aromatic hydrocarbons. The solvents will not
themselves adversely affect the thin recording layer and the
solvent-resistance of the intermediate layer 42 maintains
the integrity of the bond of both recording layer 43 to
intermediate layer 42 and intermediate layer 42 with sub-
strate 41. To provide sufficient protection for recording
layer 43, the protective layer 44 is preferably formed to a
thickness of at least 0.5 microns.
In general the formation of a crosslinked polymeric material
to serve as intermediate layer 42 involves the selection of
one or more polymeric materials with active hydroxyl,
carboxylic or hydrogen (amide) groups to react with organic
moieties that condense on such active groups in the presence
of a catalyst and at an elevated temperature to speed the
crosslinking. ~lternatively, certain components of organic
moieties can be reacted together with certain catalysts to
form self-condensation polymers. Some general examples of
active polymers which may be utilized are cellulose esters,
polyvinyl acetals, polyester resins, acrylic resins, epoxy
resins, polyvinyl alcohol, polyvinyl acetate, and alkyd
resins. Some examples of crosslinking moieties are melamine
resins, isocyanates, acid anhydrides and formaldehyde
resins. Useful catalysts include a number of acids, bases
and organometallics.
The following specific examples are given to illustrate the
present invention in greater detail but are not to be
construed to limit the scope of the invention.
A-35140
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EXAMPLE 1
A clean glass slide was coated with a plastic having the
following formulation of components:
5 Components Parts by Weight
Polyester 4900 (DUPONT) 5
Methylene Chloride 86.75
Flow Control Agent 0.16
~ethyl Oxitol 7.8
Isocyanate Prepolymer (RC 803-DUPONT) 0.25
The slide coated with this formulation was baked for four
hours at a temperature of 150C to produce crosslinking of
the polyester resin and isocyanate prepolymer. This resulted
in a clear plastic coating about 0.5 microns thick with
excellent adhesion to the glass slide and good solvent
resistance. Adhesion was tested by cellophane tape on the
layer and pulling it off at right angles to the substrate.
Solvent resistance was tested by dropping methyl ethyl
ketone on the surface and rubbing the surface with a swab
having MEK thereon. Thereafter a layer of tellurium appro-
ximately 200 Angstroms thick was applied to the crosslinkedplastic layer by vacuum deposition. Next, a protective
coating of polymeric material was applied to the tellurium
recording layer utilizing the following components:
Components Parts by Weight
25 Cellulose Acetate Butyrate
(CAB381-20 - EASTMAN CHEMICALS)7.5
Methyl Ethyl Ketone 89.5
Flow Contro~ Agent 0.06
Methyl Oxitol 2.94
The protective coating was baked for about fifteen minutes
at a temperature of 110C. Thereafter inspection of the
three-layer structure showed that no dissolution of the
underlying crosslinked polymer layer had occurred and an
integral encapsulated metal recording layer was produced.
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EXAMPLE 2
A clean glass slide was coated with a plastic formulation
having the following components:
Components Parts by Weight
Oil-Free Polyester Alkyd Resin
(Aroplaz 6755-Al-80-ASHLAND CHEMICALS) 58
Methyl Ethyl Ketone 274
Hexamethoxy Methyl Melamine
(CYMEL 303-AMERICAN CYANAMID)22.5
Flow Control Agent 0.6
Cellulose Acetate Butyrate
(CAB 551-0.2 - EASTMAN CHEMICALS)1.6
Methyl Oxitol 235
P-Toluene Sulfonic Acid as a catalyst
(CYCAT 4040 - AMERICAN CYANAMID)0.7
Isopropanol 15.4
The plastic coating with the above formulation was baked for
fifteen minutes at a temperature of 150C to produce a
crosslinked melamine-polyester film. This plastic film was
the optically clear coating with excellent adhesion and
solvent resistance. The coating thickness was approximately
0.5 microns.
The next step was to apply a thin layer of tellurium to
serve as the recording layer. This was done by vacuum
deposition of a thin film about 200 Angstroms thick.
Thereafter the same protective coating as described in
Example 1 was applied. The resulting structure was an
integral encapsulated metal recording layer having no damage
to the intermediate layer caused during the formation of the
protective coating.
EXAMPLE 3
A clean glass slide was coated with a plastic material
having the following formulation:
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Components Parts by Weight
Hexamethoxy Methyl ~lelamine
(CYMEL 303 - AMERICAN CYANAMID) 17
Methyl Ethyl Ketone 423
Flow Control Agent 0-34
5 P-Toluene Sulfonic Acid as a catalyst
(CYCAT 4040 - AMERICAN CYANAMID) 0.8
Methyl Oxitol 16.7
Isopropanol 17.2
Cellulose Acetate Buryrate
(CAB 381-0.5 - EASTMAN CHEMICALS) 25
The plastic coating with this formulation was baked for
fifteen minutes at 150C to achieve crosslinking of the
constituent materials. The resulting clear coating had
excellent adhesion and good solvent resistance. The coating
thickness was approximately 0.5 microns.
Thereafter a recording layer of tellurium was deposited in a
vacuum deposition process to a thickness of approximately
200 Angstroms followed by application of a protective
coating as described above in Example 1. This resulted in an
integral encapsulated metal recording layer in which the
intermediate coating was not affected by the application of
the protective coating.
EXAMPLE 4
A clean glass slide was coated with a plastic material
having the following formulation:
Components Parts by Weight
Oil-free Polyester Alkyd Resin
(Aroplaz 6755-Al-80 - ASHLAND CHEMICALS) 49.2
Methyl Ethyl Ketone 232.6
Hexamethoxy Methyl Melamine
(CYMEL 303 - ASHLAND CHEMICALS)19.2
Flow Control Agent 0.5
Cellulose Acetate Butyrate
(CAB 551-0.2 - EASTMAN CHEMICALS)1.4
Methyl Oxitol 199.4
Isopropanol 13.1
Xylene 105
P-Toluene Sulfonic Acid as a catalyst
(CYCAT 4040 - AMERICAN CYANAMID)0.6
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1150837
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The plastic coating with this formulation was then baked for
fifteen minutes at a temperature of 150C to produce cross-
linking of the plastic constituents. An optically clear
coating with excellent adhesion and solvent resistance in
5 accordance with standard tests was obtained. The plastic
layer had a thickness of about 0.5 microns.
Thereafter a layer of tellurium about 200 Angstroms thick
was deposited on the intermediate plastic coating in a
vacuum deposition process. Thereafter a second plastic
layer was formed by applying a plastic material of the
following composition on the layer of tellurium:
Components Parts by Weight
Hexamethoxy Methyl Melamine
(CYMEL 303 - AMERICAN CYANAMID)8 5
Methyl Ethyl Ketone 423
Flow Control Agent 0.17
Cellulose Acetate Butyrate
(CAB 381-0.5 - EASTMAN CHEMICALS) 12.5
Methyl Oxitol 0.8
Isopropanol 8.6
P-Toluene Sulfonic Acid as a catalyst
(CYCAT 4040 - AMERICAN CYANAMID) 0.4
This second plastic coating was baked for fifteen minutes at
a temperature of 150C to produce crosslinking of the
plastic constituents. This resulted in a protective layer
over the tellurium recording layer with no disturbance of
either the tellurium layer or the underlying plastic layer.
Thereafter a layer of aluminum was applied to the second
plastic layer by vacuum deposition to a thickness of appro-
ximately 750 Angstroms. Finally, a protective coating ofpolymeric material was applied over the aluminum film
utilizing the protective layer composition set forth above
in Example 1.
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The laser recording medium produced in accordance with this
example utilizes the tellurium layer as the recording layer
with the aluminum layer acting as a reflective layer for
laser beam energy transmitted through the thin tellurium
layer. The resulting laser recording medium was charac-
terized by excellent durability and stability of the con-
stituents layers.
EXAMPLE 5
A clean glass slide was coated with a plastic material
having the following formulation:
Components Parts by Weight
Polyvinyl Butyral
(BUTVAR B-73 - MONSANTO) 11.2
Methyl Oxitol 924
15 Hexamethoxy Methyl Melamine
(CYMEL 303 - AIIERICAN CYANAMID) 7.4
Flow Control Agent 0.14
Isopropanol 10.7
P-Toluene Sulfonic Acid
(CYCAT 4040 - AMERICAN CYANAMID) 0.49
The plastic coating with this formulation was baked for
fifteen minutes at a temperature of 150C to produce cross-
linking of the constituent plastic material. An optically
clear coating with excellent adhesion and solvent resistance
was achieved. Formation of a complete recording medium
utilizing this intermediate layer can then be achieved using
any of the additional steps set forth in previous examples.
EXAMPLE 6
In this example a catalyzed one component plastic layer was
formed by coating a clean glass slide with a plastic mat-
erial of the following formulation:
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Components Parts by Weight
Hexamethoxy Methyl Melamine
(CYMEL 303 - AMERICAN CYANAMID)25
Methyl Ethyl Ketone 200
Flow Control Agent 0.2
5 Methyl Oxitol 9.8
Isopropanol 14.3
P-TolueneSolfonic Acid as a catalyst
(CYCAT 4040 - AMERICAN CYANAMID) 0.7
The coating with this formulation was baked for fifteen
minutes at a temperature of 150C to produce a self-con-
densation type crosslinking of the plastic material. Theresult was an optically clear coating with excellent adhe-
sion and solvent resistance. Completion of a laser record-
ing medium can be achieved as in any of the Examples 1-4 set
forth above.
EXAMPLE 7
A clean glass slide was provided with a layer of parylene C
deposited on both sides of th~ glass slide with a thickness
of about 10 microns. Thereafter a layer of tellurium
was vacuum deposited on the parylene layer to a thickness of
approximately 200 Angstroms. Next, a protective coating of
a plastic material having the following composition was
applied:
Components Parts by Weight
25 Cellulooe Acetate Butyrate
(CAB 381-20 - EASTMAN CHEMICALS)7.5
Methyl Ethyl Ketone 89.5
Flow Control Agent 0.06
Methyl Oxitol 2.94
This coating was baked for about fifteen minutes at a
temperature of 110C. The protective layer thusly formed
was approximately two microns thick. The solvent-based
protective coating produced no damage to the recording layer
or the intermediate layer of parylene and, accordingly, an
integral encapsulated recording layer was produced.
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37
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In each of the above examples the Flow Control Agent may
comprise one of the Union Carbide silicones marketed under
the trade names L4-500, L5310, and L6202. It will be appre-
ciated by those skilled in this art that other permutations
and combinations of the various examples set forth above
could be employed to achieve the same or similar results.
It will be apparent to those skilled in the art that the
structure of this invention could be adapted to form a more
complex laser recording medium involving one or more addi-
tional recording layers by utilizing successive layers of
solvent-resistant material with a final solvent-based
material utilized as the protective coating over the medium.
Furthermore, the invention is readily adaptable to recording
media structures involving a reflecting layer (not shown)
formed on top of protective layer 44 shown in Fig. 2 with
the thickness of the protective layer being selected in
conjunction with the optical characteristics of the reflect-
ing layer formed thereon to maximize the reflection of
optical energy transmitted through recording layer 43 back
to that recording layer, thereby to further increase the
sensitivity of the recording medium to laser beam energy.
Example 4 above comprises a recording medium structure
having such a reflecting layer and including a final pro-
tective coating formed over the aluminum reflecting layer tocompletely encapsulate it.
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