Erasable recording medium comprising a dimer acid polyamide resin

Provided is an erasable recording medium and a method for recording and erasing same. The erasable recording medium comprises a thermally stable substrate and a polymeric layer deposited on the substrate comprised of a dimer acid polyamide. Such a recording medium has an excellent dimensional and mechanical stability on storage, readily and easily records information, and has an ability to be easily and completely erased when erasure is desired.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a novel optical information recording medium and 
the recording of information thereon. More particularly, the present 
invention relates to an information recording medium, preferably in the 
form of a disk, suitable for use with optical recording and playback 
apparatus, which recording medium is stable yet readily and accurately 
erasable. 
2. Description of the Prior Art 
Various optical recording media and methods for recording information 
thereon are known to the prior art. For example, the recording of 
information in the form of deformations or ripples in a thermoplastic film 
is known, with techniques for achieving such deformations involving the 
steps of (1) forming a charge pattern on the surface of the thermoplastic 
film in accordance with the information to be recorded, (2) heating the 
thermoplastic film to its melting point so as to permit the electrostatic 
forces produced by the charges to form a deformation pattern in the 
thermoplastic film corresponding to the charge pattern and thus to the 
information to be recorded, and (3) then cooling the thermoplastic film 
below its melting point to fix the thus formed deformation pattern in the 
film. Reading of the information represented by the deformation pattern in 
the thermoplastic film may be accomplished using well known optical 
techniques. See, e.g., U.S. Pat. No. 3,952,146. 
Techniques for erasing the deformation pattern involve reheating the 
thermoplastic film above its melting point to a significantly higher 
termperature than during recording in order to permit surface tension to 
smooth out the surface. Selective erasure may be accomplished by confining 
the heating to a selected area. After cooling, another deformation pattern 
may be recorded in the thermoplastic film. 
Clemens, in U.S. Pat. Nos. 3,842,194; 3,842,217; and 3,909,517; describes a 
conductive video disk comprising a molded plastic disk having video and 
audio information in the form of geometric variations in a spiral groove 
in the disk surface. These disks are coated first with a conductive 
material, such as a metal, which acts as a first electrode, and then with 
a dielectric layer, such as an inert polymer layer. A metal-tipped stylus 
acts as a second electrode of a capacitor and the information signals are 
monitored by the stylus which notes changes in capacitance between the 
stylus and the disk surface as the information, in the form of 
depressions, passes beneath the stylus when relative motion is established 
between the disk and the stylus. 
Optical recording methods in which light from a laser is focused upon the 
surface of a recording medium with sufficient intensity to cause ablation 
of surface material have also been proposed. In such methods, an 
information representative pattern of pits may be formed in the surface of 
the recording medium by suitably controlling the intensity of the focused 
light in accordance with the information to be recorded while relative 
motion is established between the recording medium and the focused light 
spot. 
For instance, in recent years, attention has been increasingly paid to the 
information recording method in which information is written in a thin 
film of metal or the like formed on a substrate by using a laser ray or 
beam. According to such a method, the information recording has been 
accomplished by forming holes or recesses in the metallic thin film under 
the action of a thermal energy beam such as a laser ray. See, e.g., U.S. 
Pat. No. 4,238,803. 
Spong, U.S. Pat. No. 4,097,895, describes a recording medium which 
comprises a light reflecting material, such as aluminum or gold, coated 
with a light absorbing layer, such as fluorescein, which is operative with 
an argon laser light source. The thickness of the light absorbing layer is 
chosen so that the structure has minimum reflectively. An incident light 
beam then ablates, vaporizes or melts the light absorbing layer, leaving a 
hole and exposing the light reflecting layer. After recording at the 
wavelength of the recording light, maximum contrast between the minimum 
reflectance of the light absorbing layer and the reflectance of the light 
reflecting layer exists. In this regard, note also U.S. Pat. No. 
4,219,826. 
Carlson, in U.S. Pat. No. 3,475,760, discloses a system for directly 
recording information in a thermoplastic film as a deformation by using a 
high energy laser scanning beam of small diameter. Erasure of the film 
deformation is accomplished by recording over the information to be erased 
using a similar laser beam but with a much smaller scan line spacing, 
preferably so as to provide overlap of the scan lines. Suitable 
thermoplastic films are disclosed as including vinyltoluenebutadiene, 
polystyrene ortho-terphenyl, polyethylene, and nitrocellulose. 
The recording medium, of course, is one of the key elements in any optical 
and/or audio recording system, and this is particularly true in the home 
entertainment market where erasable recording disks would be a very 
desirable product, provided the recording disks could match the commercial 
magnetic tapes with regard to the technical parameters and the economics 
of a recording medium. Although many different media have been 
investigated for possible use in the various aforediscussed recording 
systems, and in particular the laser systems, a suitable recording medium 
or disk which is stable and economical, yet readily erasable when desired, 
has heretofore not been provided. Such a recording medium, and in 
particular if in the form of a disk, would be readily accepted and would 
fill the void in the video recording marketplace. The search for improved, 
erasable optical recording media is continuously ongoing. 
Accordingly, it is a major object of the present invention to provide a 
novel optical recording medium which is stable, yet readily and accurately 
erasable when desired. 
It is yet another object of the present invention to provide a novel 
erasable recording medium upon which information is recorded as a 
deformation and wherein said information can easily be erased to thereby 
provide a blank recording medium upon which information can be recorded. 
It is still another object of the present invention to provide an erasable 
recording medium, particularly useful for video recordings, which 
comprises a polymeric layer exhibiting specifically selected properties to 
render the recording and erasing of information from the recording medium 
most effective and economic. 
It is still another object of the present invention to provide an erasable 
recording disk comprising a uniquely suited polymer layer which renders 
the recording and erasing of information technically accurate and 
economically expedient. 
These and other objects, as well as the scope, nature and utilization of 
the invention, will be apparent to those skilled in the art from the 
foregoing description and the appended claims. 
SUMMARY OF THE INVENTION 
In accordance with the foregoing objectives, provided hereby is such a 
novel information recording medium. The features of the present invention 
are in part derived from the discovery that dimer acid polyamides exhibit 
certain specified properties which make them most advantageously 
employable in erasable information recording media, particularly in disk 
form, wherein the information is recorded as deformations in a film layer 
comprising a dimer acid polyamide. Accordingly, the erasable information 
recording medium of the present invention comprises a multi-layer 
structure comprised of a relatively thick, thermally stable substrate, and 
a polymeric layer deposited on said substrate comprised of a dimer acid 
polyamide, e.g., having a weight average molecular weight in the range of 
from about 2,000 to 100,000, and most preferably in the range of from 
about 2,000 to about 20,000. Such a multi-layer structure comprising the 
said polymer layer has surprising applicability as an erasable information 
recording medium as it has excellent dimensional and mechanical stability 
on storage, readily and easily records information; and yet, has an 
ability to be readily and quickly erased when erasure is desired. 
It is also preferable that the dimer acid polyamide polymer employed in the 
erasable recording medium of the present invention be soluble in 
conventional organic solvents, yet be insoluble in water. The dimer acid 
polyamide polymer should also preferably be capable of forming a thin film 
and be optically homogeneous and non-scattering upon forming said film, 
and should adhere well to the substrate. 
In a preferred embodiment of the present invention, the information layer 
comprising the dimer acid polyamide further contains a dye which is 
absorptive at the light frequency of a predetermined recording light 
source.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The polymers employed in the erasable recording medium of the present 
invention are dimer acid polyamides. Such polyamide resins are 
manufactured conventionally by reacting dimer acids, or their esters, with 
diamines, and can be obtained commercially. Such polymers, however, have 
not heretofore been recognized as being suitable for use in erasable 
information recording media. 
The uniquely suitable dimer acid polyamides employed in the present 
invention are of a molecular weight so that the polyamide resin exhibits a 
melt viscosity (measured in poise) in the temperature range of from about 
110.degree. C. to about 200.degree. C. such that the ratio of surface 
tension of the resin (measured in dynes/cm) to melt viscosity (measured in 
poise) at some point in said temperature range is at least 1:100. In 
general, the suitable dimer acid polyamide resins exhibit a weight average 
molecular weight in the range of from about 2,000 to about 100,000, 
typically up to about 60,000 weight average molecular weight. However, it 
is preferable that the dimer acid polyamide resin employed be a low 
molecular weight dimer acid polyamide having a weight average molecular 
weight in the range of from about 2,000 to about 20,000, more preferably 
in the range of from about 2,500 to about 18,000 and most preferably of 
from about 3,000 to about 16,000. 
Such dimer acid polyamides are surprisingly well-suited for use in an 
erasable recording medium due to the polymers exhibiting the low melt 
viscosity, and hence, the combined properties of surface tension and 
viscosity such that the resin has a ratio of surface tension to viscosity 
of at least 0.01, preferably of at least 2, more preferably of at least 
five, and most preferably of at least eight, at some point in the 
temperature range of from about 110.degree. C. to about 200.degree. C. 
This ratio (in cm/sec) of surface tension (as measured in dynes/cm) to 
melt viscosity (as measured in poise) is believed to signify the 
capability of a polymer film comprised of the polymer to be erased, i.e., 
smoothed by levelling, as the rate of erasure is believed to be generally 
proportional to the surface tension of the polymer and inversely 
proportional to the melt viscosity of the polymer at the temperature of 
erasure. By exhibiting the aforementioned surface tension to viscosity 
ratio, an extremely fast erasure rate can be attained, e.g., up to 
10.sup.5 bits/sec or more. Such a high erasure rate is extremely important 
for commercial erasable recording media such as in disk form, used in 
situations where time is of the essence, e.g., in data processing, in 
order to insure that proper and complete erasure of the information is 
attained when desired in a very short time and in a facile manner. Without 
such a capability for a quick, easy and complete erasure, the technical 
aspects and hence commercial viability of the product for such application 
would be adversely effected. The use of dimer acid polyamides in 
accordance with the present invention, however, provides such a 
capability. 
Moreover, the ability of the dimer acid polyamides to have such a surface 
tension to melt viscosity ratio in the low temperature range of from 
110.degree. to 200.degree. C., more preferably from 140.degree. to 
200.degree. C., and most preferably from 150.degree. to about 180.degree. 
C., essentially precludes the possibility of writing during erasure since 
the erasure levelling occurs at a temperature well below the thermal 
degradation (or writing) temperature. Advantageously, the expenditure of 
energy for erasure is also of a relatively limited nature due to the 
capability of erasure of recorded information at such a low temperature. 
Yet, the erasure temperature for the recording medium of the present 
invention is still thereby sufficiently above normal ambient temperatures 
that recorded information is not inadvertently erased by levelling upon 
storage or upon scanning with a low intensity read laser beam. 
Generally, it is preferred that the dimer acid polyamide employed exhibit a 
melt viscosity of about 5 poise or less at some point within the 
temperature range of about 110.degree. C. to 200.degree. C., although any 
melt viscosity which coupled with the surface tension gives a ratio of 
0.01 would be suitable as discussed previously. Accordingly, the polymer 
film should preferably exhibit a surface tension of at least 10 dynes/cm, 
and more preferably at least 25 dynes/cm, in order to achieve the desired 
erasure and erasure rates, which would be achieved at the temperature(s) 
that the dimer acid polyamide film exhibits a viscosity of about 5 poise. 
It should be understood that the polymer film need not exhibit the 
aforementioned surface tension to viscosity ratio throughout the entire 
defined temperature range in order to realize the benefits thereof, but 
only at some point therein. 
The lower molecular weight dimer acid polyamides, i.e., having a weight 
average molecular weight of from 2,000 to 20,000, are most preferred since 
they generally exhibit combined properties of surface tension and 
viscosity such that the resin has a ratio of surface tension to viscosity 
in the preferred range of at least two within the specified temperature 
range of from about 110.degree. C. to 200.degree. C. Thus, fast and 
effective erasure rates are insured. However, when a high intensity light 
source, e.g., laser, is employed, the lower molecular weight dimer acid 
polyamides would have a greater tendency to splatter during the recording 
of information vis-a-vis the higher molecular weight dimer acid 
polyamides, e.g., up to 100,000 weight average molecular weight. Thus, 
depending upon the intensity of the light/writing source to be used, a 
higher molecular weight dimer acid polyamide may be more suitable to avoid 
any problems with splattering. 
While the dimer acid polyamides having a weight average molecular weight of 
greater than 20,000 and up to 100,000 exhibit generally lower ratios of 
surface tension to viscosity in the temperature range of from 110.degree. 
C. to about 200.degree. C., the ratio is at least 0.01, and thus does 
signify excellent erasure and levelling properties. The erasure rate may 
not accordingly be as high as for the lower molecular weight dimer acid 
polyamides, but their erasure capabilities are more than sufficient for 
applications where speed of erasure is not critical, e.g., for documents 
to be stored for any length of time, or for home use, where speed of 
erasure is not as important. Indeed, the erasure rate may still approach 
10.sup.3 bits/sec. Accordingly, due to the properties exhibited by the 
higher molecular weight dimer acid polyamides, they as well show 
surprising applicability in an erasable information recording medium, 
i.e., excellent dimensional and mechanical stability on storage, the ready 
and easy recordation of information, yet an ability to be readily and 
accurately erased when erasure is desired. 
The dimer acid polyamide resins employed in accordance with the present 
invention are also uniquely suited for use in the encoding layer for they 
generally exhibit a Vicat softening point in the range of from about 
30.degree. C. to about 120.degree. C., even more preferably in the range 
of from about 45.degree. C. to about 100.degree. C., and most preferably 
in the range of from about 50.degree. C. to about 80.degree. C., with the 
softening point being an important property in regard to the dimensional 
stability of the recording medium upon storage. The possession of such a 
Vicat softening point insures that the polymer layer will remain 
dimensionally stable for normal variations in ambient temperature, and 
hence provide a permanent record upon storage. The Vicat softening point 
of a polymer can be measured in accordance with standard defined test 
procedures, e.g., ASTM D1525-76. 
Further with regard to dimensional stability, the dimer acid polyamides of 
the present invention are also essentially insoluble in water so that the 
polymer layer does not swell in a humid environment. A recording disk of 
the present invention comprising a dimer acid polyamide should thus be 
unaffected by normal variations in humidity. 
The dimer acid polyamides employed in the present invention also generally 
exhibit a third physicochemical property which, in combination with the 
Vicat softening point and surface tension to melt viscosity ratio, make 
them so suitable for use in an erasable recording medium. The temperature 
of the onset of thermal degradation, for example, as measured by 
thermogravimetric analysis and defined for the purposes of the present 
invention as that temperature at which a sample loses 10 percent of its 
original weight, is generally at least 250.degree. C. for the dimer acid 
polyamides employed in the erasable recording medium of the present 
invention. More preferably, the temperature of thermal degradation is at 
least 300.degree. C., but generally no greater than about 500.degree. C. 
Accordingly, the thermal degradation temperature of the dimer acid 
polyamides sufficiently exceeds the erasure temperature range, e.g., up to 
about 200.degree. C., so that no writing occurs during the erasure of 
information. This also provides for an important advantage, for if the 
temperature at which erasure can occur is too close to the thermal 
degradation temperature, problems with respect to the ease, completeness 
and quality of the erasure would be encountered. 
The polymer film comprised of the dimer acid polyamides of the present 
invention also undergo thermal degradation with a minimal amount of 
charring and with a minimal amount of residue being left, e.g., about 2% 
or less. This allows the erasable recording medium surface to remain free 
of extraneous debris, etc. throughout the various erasure-recording 
sequences to which the recording medium may be subjected. Being free of 
debris thereby results in less scattering of light and less noise, i.e., a 
higher signal-to-noise ratio is achieved. The technical aspects of the 
recording medium are thereby more acceptable and commercially viable. 
The dimer acid polyamides are also optically homogeneous and do not scatter 
light. The more optically homogeneous the polymer of a recording layer, of 
course, the better the technical parameters, e.g., higher resolution 
and/or information density, of the recording medium. This advantageous 
optical homogeneity of the dimer acid polyamides is believed to be due to 
their lack of crystallinity. Linear chain polyamides, such as the nylons, 
are high crystalline and hence of an opaque nature. The dimer acid 
polyamides of the present invention, however, are not of a crystalline or 
ordered structure due to the particular predominant structure of the dimer 
acid polyamides, which comprises a cyclic ring with aliphatic side chains 
attached thereto. 
Preparation of the dimer acid polyamides employed in the invention can be 
carried out in the conventional manner by reacting substantially 
equivalent amounts of the diamines with the dimer acids or the 
amide-forming derivatives thereof, especially their esters, at 
condensation temperatures between about 160.degree. C. and 250.degree. C. 
The residual water of condensation can be advantageously removed by 
applying a vacuum, e.g., of 15-20 mm Hg for 1 to 2 hours. The 
polymerization is conducted until a dimer acid polyamide product of the 
desired (or suitable) molecular weight is attained. 
As mentioned, instead of the free acids it is also possible to use, in the 
conventional manner, their amide-forming derivatives, especially their 
acid esters, the natural choice being those that can readily be subjected 
to aminolysis, for example, methyl esters and ethyl esters. 
The dimeric carboxylic acids used in the manufacture of the polyamides of 
the present invention can be prepared in known manners. See, for example, 
Encyclopedia of Chemical Technology, Kirk-Othmer, 3rd ed., Vol. 7, pp. 768 
et seq. and articles referenced therein. Generally, the dimer acids can be 
prepared, for example, by radical or ionic polymerization or by thermal 
polymerization. The starting materials used in such polymerization 
processes are natural monoethylenic or polyethylenic carboxylic acids and 
possibly also saturated or monoacetylenic or polyacetylenic carboxylic 
acids. Polymerization is preferably carried out with the aid of a 
catalyst, e.g., alumina. In addition to dimeric carboxylic acid, the 
polymer obtained can also contain varying minor amounts of monomeric and 
trimeric carboxylic acids. Pure dimeric carboxylic acid is obtainable, 
however, by distillation. The dimeric carboxylic acid used for the 
preparation of the polyamides of the invention preferably has a dimeric 
carboxylic acid content of at least about 60%, and more preferably a 
dimeric carboxlic acid content of at least from 70 to 90% by weight. The 
remainder comprises trimeric and higher polymeric components and monomeric 
carboxylic acid. 
The diamines used in the manufacture of the polyamides employed in the 
present invention can be any suitable diamine which can provide a 
polyamide product of the desired molecular weight and/or physical and 
chemical properties. Preferred are the aliphatic diamines having from 2 to 
about 6 carbon atoms (i.e., ethylene diamine to hexamethylene diamine), 
with ethylene diamine in mixture with higher diamines being most 
preferred. When ethylene diamine is employed in a predominant amount, the 
predominant structure of the dimer acid polyamide resin would comprise 
recurring units of the structural formula 
##STR1## 
The cyclic ring and aliphatic side chains of the polyamide resin lead to 
the amorphous characteristic of the polymer which is believed to account 
for the polymer's excellent optical properties, i.e., optical homogeneity. 
Suitable dimer acid polyamide materials for use in the erasable recording 
medium of the present invention are commercially available, e.g., from 
Union Camp Corporation under the registered trademark Uni-Rez.RTM., and 
from Emery Industries, Inc. under the registered trademark Emerez.RTM.. 
Such low molecular weight dimer acid polyamides have heretofore been used 
as hot melt adhesives and in printing inks. 
The following table lists the physical properties of several such 
commercially available and suitable resins. The viscosity measurements 
were made by a rotational viscometry technique using a Brookfield 
Synchro-Lectric Viscometer manufactured by Brookfield Engineering 
Laboratories, Inc., Stoughton, Mass. The thermogravimetric analyses in the 
table were obtained with a DuPont Model 951 thermogravimetric analyzer, 
with the samples maintained under a nitrogen atmosphere at a heating rate 
of 15.degree. C./minute. 
TABLE 
__________________________________________________________________________ 
DIMER ACID POLYAMIDES 
M.P. 
VICAT** 
DSC VISCOSITY, (POISE) 
(LIT.) 
S.P. M.P. 
H MELT 
TGA BROOKFIELD 
* 
RESIN (.degree.C.) 
(.degree.C.) 
(.degree.C.) 
(CAL/G) 
T(10%).degree.C. 
LIT (TEMP) 
(150.degree.) 
--M.sub.w 
__________________________________________________________________________ 
UNIREZ .RTM. 
2220 110 40 107 0.6 386 1.7 
(160) 
-- -- 
2931 110 72 105 5.6 391 30 (160) 
88 13,600 
2940 110 70 107 4.8 385 15 (160) 
34 9,600 
2942 102 54 92 2.6 407 16 (160) 
24 -- 
2961 142 -- 104 3.8 382 1 (160) 
. . . 3,200 
EMEREZ .RTM. 
1530 110 57 108 5.8 398 24 (160) 
115 16,300 
1533 104 54 87 2.0 385 33 (160) 
220 7,600 
1540 110 78 110 6.2 394 16 (160) 
75 15,500 
1548 120 45 117 2.0 428 8 (160) 
25 3,100 
1549 120 -- 111 1.2 381 14 (160) 
38 -- 
1552 137 37 89,121 
4.0,1.0 
408 60 (190) 
-- 26,000 
1553 110 36 93 2.1 415 100 
(190) 
-- 58,000 
1565 95 -- 54 1.3 403 5 (190) 
40 -- 
1556 100 -- 99 5.9 394 8 (190) 
110 -- 
1566 165 43 109,135 
2.3,1.3 
413 80 (190) 
-- 33,000 
__________________________________________________________________________ 
*Weight average molecular weight (--M.sub.w) determined in reference to 
calibrated polystyrene samples. 
**Blanks indicate molded disks either too brittle or too soft for the 
Vicat measurement. 
As can be seen from the Table, the Vicat softening point for the materials 
examined fall within the range of from about 36.degree. to 78.degree. C. 
From the melt viscosity data obtained by measurement with a Brookfield 
viscometer at 150.degree. C. and the manufacturer's viscosity data, a 
temperature at which the viscosity of the polymer reaches 5 poise can be 
extrapolated using the relationship of the log of the viscosity vs 1/T. 
For the majority of Emerez materials and the Unirez 2931, 2940 and 2942 
materials, a viscosity of 5 poise is achieved in the range of 167.degree. 
to 197.degree. C. Such a melt viscosity in conjunction with the surface 
tension of the dimer acid polyamides employed in accordance with the 
present invention, which is generally at least 10 dynes/cm in the 
temperature range of from about 110.degree. to 200.degree. C., allows for 
complete and accurate erasure to be quickly attained. (The surface tension 
of a polymer material can be measured by conventional methods. Examples of 
such methods include the capillary rise method, the drop weight method, 
the Wilhelmy plate method and the du Nouy method. The surface tension is 
found to be largely independent of molecular weight for a class of 
polymers.) Even for the higher molecular weight materials, e.g., Emerez 
1552, 1553 and 1566, the melt viscosity in the temperature range of from 
110.degree. C. to 200.degree. C. is such as to allow for a ratio of 
surface tension/melt viscosity of 0.01, and at least 0.1. This as well 
signifies the suitability of the higher molecular weight dimer acid 
polyamides for use in an erasable recording medium. 
From the thermogravimetric analyses (TGA), it is seen that the thermal 
decomposition temperature of all the materials listed fall within the 
range of from about 380.degree. to 430.degree. C. The TGA scans also 
indicated that the residue after complete thermal degradation is around 2% 
or less. 
The substrate of the erasable recording medium of the present invention to 
which the dimer acid polyamide is applied is illustratively, and 
preferably, formed in the shape of a disk, a major surface of which is 
processed to be polished and flat. The substrate is made of a thermally 
stable material, which is also mechanically rigid and dimensionally stable 
and which ideally has an isotropically low coefficient of thermal 
expansion. Therefore, the recording medium does not warp during storage, 
writing, erasure or reading. 
Depending upon the particular, conventional read mechanism employed, the 
substrate may be opaque, transparent or highly reflective regarding the 
read laser beam. It is also preferred that the substrate material be 
compatible with the polymer employed and that the polymer adhere well to 
the substrate. Good adhesion, for example, may be defined operationally as 
the inability to remove the coating from the substrate by means of 
adhesive tape, e.g., Scotch.RTM. brand type. Examples of suitable 
materials for the substrate include glass, polycarbonate, and 
polymethylmethacrylate, as well as metals such as aluminum. 
If the polymer employed in the recording film is transparent at the 
predetermined wavelength of the laser output for writing, it is generally 
necessary to add a dye of appropriate absorption wavelength to the polymer 
in order to effect absorption of the incident radiation. If, for example, 
the writing is to be achieved with a helium-neon laser of 633 nm, then Oil 
Blue N dye, Colour Index 61555, .lambda..sub.max 637 nm, would be a 
suitable dye for incorporation into the polymer film. If, however, an 
argon laser is used which provides an output of a wavelength of about 458 
nm, an organic dye such as fluorescein, which is highly absorptive at the 
light frequency of the argon laser, can advantageously be used. Other dyes 
which may be used include certain nigrosine dyes. Of course, it is 
preferred to employ dyes of suitable properties so as to not detrimentally 
affect the recordability and erasability of the recording medium. The 
addition of a dye may change the physical properties, e.g., melt viscosity 
of the polymer layer, and this should be taken into account when choosing 
an appropriate polymer/dye system. Other additives, e.g., stabilizers, 
might also be desirably added to the polymer, and their affect, if any, on 
the properties should also be considered. 
The erasable recording medium of the present invention is prepared by 
coating the substrate with a polymer or polymerdye film. Since the read, 
write and erase steps all require operating within a very narrow depth of 
focus, the film, when applied, must provide a very flat surface in order 
to avoid errors and noise. In order to facilitate the coating procedure, 
it is also generally advantageous that the polymer (and dye) be soluble in 
an organic solvent, such as an alcohol or ketone. In this regard, when a 
dye is used, the polymer and dye should be compatible and mutually 
co-soluble. Also, upon evaporation of the solvent, the dye (or other 
additives if present) should not precipitate in a particulate form, which 
particulates would cause a scattering of light. 
Any suitable coating technique may be used to achieve such a flat surface, 
with a conventional technique such as spin coating, which allows for a 
high degree of control of film thickness and flatness, being preferred. It 
is, of course, important that the polymer form a thin membrane coating. 
For example, the dimer acid polyamide resins employed in accordance with 
the present invention may be readily cast from a toluene-propanol solution 
onto glass or polymethylmethacrylate to afford thin optically homogeneous 
(not cloudy) films. The films generally adhere well to glass and 
polymethylmethacrylate and cannot be removed via adhesive tape. 
In an illustrative recording system embodying the principles of the present 
invention, a record blank of the abovedescribed (disk) form is subject to 
rotation at a constant linear or constant angular velocity while a beam of 
light from a light source, e.g., a laser, is focused on the polymer 
surface of the disk. The intensity of the light beam is controlled in 
accordance with information to be recorded. Illustratively, the control is 
effected in accordance with carrier waves modulated in frequency by 
picture-representative video signals, with the light beam intensity 
varying as a result between a high level sufficient to effect ablation of 
the absorptive material and a low level insufficient to effect such 
ablation, the frequency of the level alternations varying as the video 
signal amplitude changes. Preferred writing speeds are in the range of 
from 10.sup.6 to 10.sup.7 bits per second. 
The relative diameter and depth of the holes or pits formed by ablation 
will, of course, depend not only on the optical and thermal properties of 
the polymer or polymer/dye layer, but also on the characteristics of the 
writing beam, i.e., focused spot diameter, depth of focus, intensity 
profile and intensity and duration of the writing pulse. Optimization of 
these parameters is familiar to those skilled in the art. It is preferred, 
however, that the ablation of the polymer layer does not proceed to the 
substrate layer. If ablation is to proceed to the substrate, the substrate 
material employed is preferably glass or a metallized substrate or some 
other high surface energy substrate material. 
As a result of the ablation of the polymeric material, an information track 
comprising a succession of spaced pits is formed in the polymer surface of 
the disk, the pits appearing in those surface regions exposed to the high 
intensity beam due to vaporization of the polymer layer material in 
response to the high intensity beam exposure. Variations in the length and 
separation of the pits are representative of the recorded information. 
Where a continuous sequence of pictures is to be recorded, a spiral 
information track may be formed by providing relative motion, in a radial 
direction and at a constant rate during the recording, between the 
recording beam and the rotating disk. Alternatively, in the absence of 
such relative motion during the recording, a circular information track 
may be formed appropriate for "slide" or freeze-frame recording purposes. 
The result of the above-described recording process is the formation of an 
information record of a form which facilitates recovery of the recorded 
information by optical playback processes. The information track of such 
an information record comprises (1) undisturbed surface regions 
alternating with (2) pit regions formed by the ablation process, due to 
complete, or at least partial, removal of the polymer layer coating of the 
substrate. This information track can be, for example, in either analog or 
digital form. 
In playback or read operations pursuant to the principles of the present 
invention, a light beam is focused upon the information track of a 
rotating information record of the above-described type. The playback beam 
has a constant intensity at a level insufficient to effect ablation of the 
polymer layer or erasure of the recorded information by levelling. A 
photodetector, positioned to receive light reflected from the successive 
regions of the information track as they pass through the path of the 
focused light, develops a signal representative of the recorded 
information. 
Several variations in the playback or reading system are possible with the 
erasable recording medium of the present invention. For example, the 
information can be read based upon the relative reflection from the 
surface of the polymer or polymer/dye layer of the disk. In this mode, 
when a beam from the read laser is focused on the surface of the disk a 
part of the light is absorbed, but a fraction of the light is reflected. 
The reflected rays retrace their path back through a lens and are directed 
by a beamsplitter to a photodetector. When the read beam encounters a hole 
or pit created during the writing step, however, the rays ae scattered and 
a lower intensity is reflected back toward the detector. Thus, a written 
bit is registered as a decrease in reflected intensity. Alternatively, the 
read beam can be focused at a level corresponding to the depth of the 
ablated hole (as predetermined), so that a bit is registered as an 
increase in reflected intensity. The chief function of the substrate in 
either of the aforediscussed modes is to provide mechanical support for 
the active layer since the substrate's reflectivity or transparency does 
not come into play. The substrate is preferably of a transparent nature 
when used in conjunction with such a playback system. 
Information can also be read via a system atuned to the relative 
reflectance from the surface of the substrate. In this mode, the substrate 
must be of a reflective nature in an appropriate wavelength range. For 
example, the substrate can have a metallized surface, e.g., of aluminum, 
upon which the polymer is deposited. In operation, the read laser is 
focused on the highly reflective surface. In the unwritten portions of the 
disk, i.e., having no pits, a part of the incident beam is absorbed upon 
going through the polymer/dye layer and that part which is reflected from 
the surface does not retrace its path to thereby be detected by a 
photodetector. In the recorded portions of the disk, however, i.e., having 
the pits formed via ablation, the beam goes through a shorter distance of 
absorbing layer, and is reflected from the substrate reflective surface, 
so that a bit is registered as an increase in reflected intensity. 
The read mode can also be adjusted to read in accordance with relative 
reflectance from the surface of the polymer layer and from the surface of 
the substrate. Again, the substrate, when employing this mode, should be 
reflective. This concept or mode is described by D. G. Howe and J. J. 
Wrobel, in J. Vac. Sci. Technol., 18, 92 (1981). See also U.S. Pat. No. 
4,097,895. 
When employing this mode, the recording medium should comprise a dyed 
polymer layer of a precise thickness which is coated onto a reflective, 
e.g., metallized, substrate. Writing is done at a wavelength where the dye 
absorbs and reading at a wavelength where the dye is transparent. In the 
read step, reflection occurs both from the surface of the dyed polymer and 
from the surface of the metallized substrate, and these reflections can 
combine either constructively or destructively at the surface of the 
polymer coating. If the thicknesses of the initial coating and of the 
coating remaining in the ablated hole are chosen correctly, then the 
combination is destructive in the unwritten areas and constructive in the 
ablated areas, so that a bit is registered as an increase in the intensity 
of reflected light. 
Information can also be read, for example, based upon light transmission 
through the disk. In such a playback system, it is necessary for the 
substrate to be transparent. A recorded bit is registered as an increase 
in intensity of the light transmitted through the disk since there is less 
absorbing dye in the path of the beam where the dye/polymer layer material 
has been ablated. 
Due to the particular selected properties exhibited by the polymers 
comprising the polymer layer of the recording medium of the present 
invention, complete and accurate erasure of recorded information can be 
most facilitously accomplished. Erasure of the recorded information is 
readily carried out by heating the disk to a sufficiently high temperature 
such that the dye/polymer layer becomes fluid and the surface is levelled 
by surface tension forces, i.e., in the temperature range of from about 
140.degree. to 200.degree. C. This may be done globally by heating the 
entire disk in an oven or some other suitable heating means, or by means 
of a laser beam whose intensity is intermediate between that of the write 
beam and read beam. It is generally necessary to heat an area considerably 
greater than that of a single bit (typically 1 .mu.m in diameter), so that 
addressable (bit-by-bit) erasure can be difficult. 
Although the invention has been described with preferred embodiments, it is 
to be understood that variations and modifications may be resorted to as 
will be apparent to those skilled in the art. Such variations and 
modifications are to be considered within the purview and the scope of the 
claims appended hereto.