Sandwich-type capacitive electronic discs

At least one layer of a conductive organic polymer, formed by electropolymerization on the anode in a two electrode cell, is bonded to a plastic substrate by compression with sufficient heat and pressure to cause it to bond to the substrate and release from the anode. Improved sandwich-type capacitive electronic discs having an information pattern in ultra thin outer conductive layers which are homogeneous and which contain no conductive particles are formed by this process by providing as the anode a mastering disc containing an information pattern which is the negative of the desired information pattern.

This invention relates to improved high density information discs having a 
homogeneous conductive layer and a method for making them. 
BACKGROUND OF THE INVENTION 
Clemens, in U.S. Pat. No. 3,842,194, discloses a capacitive video disc 
playback system which includes a high density information record having a 
conductive surface. Initially, information records for this system were 
sandwich-type constructions, e.g. the record was formed of a plastic disc 
coated with a layer of conductive metal which was in turn coated with a 
dielectric material, such as polystyrene. Subsequently improved 
information discs for this system were homogeneous and comprised a 
thermoplastic matrix having embedded therein sufficient finely divided 
conductive carbon black particles to obtain capacitive playback. Suitable 
molding compositions for such discs are disclosed in Martin et al U.S. 
Pat. No. 4,228,050. 
Conductive molding compositions, such as described by Martin et al, are 
comparatively expensive due principally to the amount of high quality, low 
density, conductive carbon black contained therein. Additional various 
additives, such as stabilizers, lubricants and the like, in such 
compositions also contribute to the cost of a disc produced therefrom. 
Other workers have sought to reduce the cost of such a disc by bonding 
thin layers of the expensive conductive molding composition onto an 
inexpensive support or core disc to form a sandwich-type disc. 
O'Mara, in U.S. Pat. No. 4,390,487, discloses a method of forming a 
sandwich-type capacitive electronic disc ("CED") into which the conductive 
molding composition is injected into a compression mold and a 
nonconductive core material is injected inside of the conductive plastic 
to form a preform which is compression molded to form a CED. 
Ruda, in copending U.S. patent application Ser. No. 522,332, filed Aug. 11, 
1983, discloses a sandwich-type CED having comparatively thin conductive 
layers, i.e. about 2 to 30 mils thick, which are formed by passing the 
conductive molding composition, in molten form, between rollers in 
production calendering equipment or in a two-roll mill. The conductive 
composition is, in essence, that disclosed by Martin et al. 
Dixon et al, in application Ser. No. 556,354, filed Nov. 29, 1983, now 
abandoned, disclose preparation of a sandwich-type CED by forming a 
dispersion of conductive carbon black particles in a solution of a 
thermoplastic resin such as polyvinyl chloride, and certain additives, 
coating the dispersion onto a core disc and drying to form conductive 
layers which are then embossed with an information pattern. Di Marco, in 
application Ser. No. 556,355, filed Nov. 29, 1983, now U.S. Pat. No. 
4,515,830, discloses forming a similar dispersion from the heat-treated 
conductive molding composition disclosed by Martin et al and preparing 
sandwich-type CED's therefrom in a similar manner. The conductive layers 
of Dixon et al and DiMarco are substantially thinner than those of Ruda. 
The conductive layer in the CED's produced by each of the above methods 
contains conductive carbon black particles and additives for the 
processing and stabilization of the thermoplastic resin. In addition to 
the cost of the conductive carbon, the abrasiveness of molding 
compositions such as disclosed by Martin et al causes the stampers used to 
emboss the information pattern in the conductive layer or disc to wear out 
or score rather quickly and, thus, become unuseable. This adds another 
cost to the prouction of CED's. There is also to be considered in each 
instance possible reactions and/or degradations among the various 
components of the formulation during the heat and pressure of compression 
molding. 
In accordance with this invention, there is provided a method of preparing 
sandwich-type CED's which have conductive layers at least as thin as those 
of Dixon et al and DiMarco. The conductive layers of the subject CED's are 
homogeneous and contain no conductive carbon black or other conductive 
particles. 
SUMMARY OF THE INVENTION 
Sandwich-type capacitive electronic discs are prepared by 
electropolymerization of an organic monomer onto a conventional mastering 
disc having a negative form of the desired information pattern therein, 
thereby forming a positive patterned layer of an organic polymer having 
sufficient conductivity for capacitive playback. The polymer layer is then 
bonded onto a plastic core disc.

DETAILED DESCRIPTION OF THE INVENTION 
The method of this invention is applicable to any organic monomer which can 
be electropolymerized to form a polymer layer on a metal anode which layer 
possesses the requisite conductivity for capacitive playback. At the 
present time, it is required that the conductive layer have a bulk 
resistivity below about 500 ohm-cm at 900 megahertz. While polymers formed 
from a number of organic monomers, such as acetylene, benzene or 
quinoline, do have the requisite conductivity as deposited, only pyrrole 
has been found by us to be sufficiently stable in the electrically 
conductive polymerized form to be attractive for use in preparing 
sandwich-type CED's. Therefore, although the method of this invention is 
equally applicable to any organic monomer which can be electropolymerized 
to form a polymer having sufficient conductivity to provide capacitive 
playback, it will be described specifically with reference to pyrrole. 
Polypyrrole has long been recognized as a conductive organic polymer. 
Formation of conductive films of polypyrrole on the anode in a two 
electrode cell by electropolymerization is also known. There have been a 
number of recent studies on the reaction parameters of the formation of 
polypyrrole films, including free standing films, in this manner and the 
effect of changes in variable conditions of the reaction on the properties 
of the films. In addition, certain uses have been proposed for such 
conductive films in connection with semiconductor processing such as, for 
example, depositing them on n-type semiconductors as a protection against 
photoanodic decomposition, as Schottky type diodes and the like. There is 
nothing in the literature, however, which would suggest that polypyrrole, 
or any conductive organic polymer, might be useful as the conductive 
layers in a sandwich-type CED. 
There is further nothing in the literature which suggests that such 
ultrathin films of conductive organic polymer could be cleanly transferred 
from the anode where they were deposited to the surface of a plastic disc. 
We have found that this is possible and, further, that an information 
pattern in the film can be transferred cleanly and without distortion. 
That such transfer was possible could not have reasonably been predicted 
from the literature. 
In accordance with this invention, it is preferred that polypyrrole be 
formed from an aqueous medium containing a substantial percentage of an 
organic solvent. A number of such solvents have been reported in the 
literature. Acetonitrile, which is frequently mentioned, may be used alone 
or in combination with other solvents, for example, ethylene glycol and 
glycerol. It has been found that, for the process of this invention, an 
aqueous medium containing glycerol is most suitable. In general, the 
aqueous medium of this invention contains from about 5 to 50, preferably 
about 20, percent by volume of glycerol. The medium contains, the monomer, 
i.e. pyrrole, and a suitable electrolyte salt. Generally, the 
concentration of pyrrole in the medium is between about 0.1 and 1.0, 
preferably about 0.5 percent by volume. 
In general, the use of organic electrolyte salts is preferred for the 
subject process over inorganic salts such as cupric sulfate or ferrous 
sulfate. Suitable organic electrolyte salts include 
tetra-n-butylammonium-tetrafluoroborate (TBABF.sub.4) and 
tetramethylammonium-p-toluenesulfonate (TMATS), with the latter being 
preferred. The electrolyte salt is suitably present in from about 0.05 to 
0.5, preferably about 0.1, molar concentration. The use of these organic 
electrolyte salts produces a dramatic increase in the conductivity of the 
polypyrrole film in comparison to inorganic salts such as, cupric sulfate. 
In general, the bath temperature during deposition of the polypyrrole 
conductive layer according to this invention is suitably between about 
15.degree. and 25.degree. C. The cell configuration is not particularly 
critical to the present process and any conventional electrodeposition 
apparatus can be utilized. The distance between electrodes is suitably 
from about 30 to 60 mm. The cell current is from about 0.02 and 0.3 in 
mA/cm.sup.2. The higher current, which is preferred because the quality of 
the conductive layer is improved, is most readily attainable with the 
above-mentioned organic electrolyte salts. 
In accordance with this invention, a conventional metal master disc used to 
produce commercial CEDs is utilized as the anode in a two-electrode cell. 
While various metals may be used to prepare the master disc, it is 
preferred that it be of nickel or an alloy which is predominately nickel. 
Chromium plated nickel master discs are also particularly suitable for the 
subject process. By "master disc" is meant a disc having a negative, i.e. 
a mirror image, pattern of the desired information pattern. The remaining 
electrode is suitably copper. 
In accordance with this invention, the electrolyte solution is deoxygenated 
in the cell, preferably by nitrogen purge, before the pyrrole is added 
thereto. Generally, a thirty minute nitrogen flow is sufficient. The 
pyrrole is then added and current flow initiated. Deposition of black 
polypyrrole on the anode master disc is carried out for a time sufficient 
to deposit a layer at least about 0.5 micrometer thick and preferably 
between about 1.0 and 3 micrometers thick. Most suitably, the polypyrrole 
layer is about 1.5 micrometers thick. For a 1.5 micrometer thick layer of 
polypyrrole, a current flow of 50 mA for about one-half hour is generally 
sufficient where anode and cathode are 60 mm apart. After deposition is 
complete, the layer of polypyrrole is dried and washed in an organic 
solvent such as acetone. 
The plastic core disc for the subject sandwich-type CEDs is suitably 
comprised of a vinyl chloride-based formulation. Such compositions 
typically contain, in addition to the polymer, lubricants, plasticizers, 
stabilizers and the like. It is important that the core disc have 
acceptable surface quality and smoothness to preserve the integrity of the 
information pattern in the polypyrrole layers. 
The subject CEDs are prepared by placing a core disc, preferably having a 
thickness of about 2 mm, between two master discs, each of which has a 
coating of polypyrrole, bonding the polyrrole layers to the core disc, 
thereby releasing the polypyrrole from the master discs. Suitably, a 
temperature of from about 120.degree. to 160.degree. preferably about 
140.degree. C., and a pressure of from about 10 to 20 kg/cm.sup.2, 
preferably about 15 kg/cm.sup.2, are utilized. It is considered unexpected 
that the polypyrrole layers readily separate from the master discs without 
any distortion and adhere well to the plastic core disc. 
It is preferred in accordance with this invention to coat the side of the 
polypyrrole layer to be bonded to the plastic core disc with a thin layer 
of a conventional adhesive to enhance bonding. Generally, a latex adhesive 
such as a polyvinylidene acrylic latex preparation available as Daran 229 
from W. R. Grace & Co. is acceptable. The latex adhesive is suitably 
applied to the polypyrrole layers by spin-coating to achieve an even 
coating. The thus-formed CEDs have been found to separate readily from the 
master disc with no distortion of the information pattern. 
The polypyrrole conductive layers have a conductivity of between about 
1.times.10.sup.2 and 5.times.10.sup.3 S/m, which is equivalent to a bulk 
resistivity of between about 0.02 to 1 ohm/cm. This is well within the 
maximum requirement for playback, i.e. a bulk resistivity below about 500 
ohm-cm at 900 megahertz. The high conductivity of the conductive layers of 
the subject sandwich CEDs is such that some instability, e.g. a loss of up 
to 20 percent of conductivity, which has been observed upon storage under 
stressed conditions, can be tolerated. 
CEDs prepared in accordance with this invention will produce acceptable 
video and audio reproduction upon capacitive playback in the as-pressed 
condition. It is preferred, however, to coat the subject CEDs with a 
lubricant such as is utilized in conventional CEDs. Suitable lubricants 
include fractionated methylalkyl siloxane preparations such as disclosed 
in U.S. Pat. No. 4,277,101, issued June 23, 1981, preferably doped with an 
additive such as disclosed in U.S. Pat. No. 4,330,583, issued May 18, 
1982, or U.S. Pat. No. 4,355,062, issued Oct. 19, 1982. 
The following Examples further illustrate this invention, it being 
understood that the invention is in no way intended to be limited to the 
details described therein. In the Examples, all parts and percentages are 
on a weight basis and all temperatures are in degrees Celsius, unless 
otherwise stated. 
EXAMPLE 1 
A solution was formed by dissolving 2.15 g of cupric sulfate in 135 ml of 
distilled water at 20.degree.. To this solution was slowly added 0.56 ml 
of pyrrole and the resultant solution was charged to a suitable 
electrolysis cell having a copper plate as the cathode and a nickel plate 
as the anode. Each plate had an area of about 15 square centimeters. A 
voltage of 0.5 volt was applied across the cell. The current density was 
about 0.15 mA/cm.sup.2. These conditions were maintained for about two 
hours during which a film of polypyrrole approximately two micrometers 
thick deposited at the anode. The film, which separated readily from the 
plate, was washed in deionized water and dried under vacuum. 
Two films of polypyrrole were individually prepared as above with the 
exception that the anode was replaced with a nickel mastering disc of the 
type used to make capacitive electronic discs. Current flow was maintained 
for two hours during which the mastering disc received a layer of 
polypyrrole 2 micrometers thick. 
A nonconductive core disc, 2 mm thick, comprised of a poly(vinyl chloride) 
resin, plasticizers and stabilizers was placed between the polypyrrole 
sheets in a conventional compression molding press. In order to improve 
adhesion to the nonconductive core disc, each sheet was coated with the 
latex emulsion Daran 229, available from W. R. Grace & Co. The press, 
which was heated to 140.degree., was partially closed to soften the sheets 
and to allow air to escape. The mold was completely closed and held for 
four minutes at 140.degree.. The resultant capacitive electronic disc 
easily separated from the mastering discs upon cooling and had sufficient 
conductivity to be played on a conventional capacitive electronic disc 
player, i.e. its resistivity was less than 1.0 ohm-cm. 
EXAMPLE 2 
As an electrolyte salt, 98.14 g of tetramethylammonium-p-toluenesulfonate 
was dissolved in 3200 ml of distilled water at 25.degree.. A total of 800 
ml of glycerol was added thereto and the mixture deoxygenated by purging 
with a nitrogen stream for thirty minutes. The concentration of 
tetramethylammonium-p-toluenesulfonate in the solution was 0.1M. After 
deoxygenation was completed, 16.7 ml of pyrrole was added to the solution 
with stirring under a nitrogen atmosphere. 
The mixture was placed in an electrolysis cell as in Example 1. A nickel 
mastering disc of the type used to prepare capacitive electronic discs was 
utilized as the anode. The cathode was a copper plate as in Example 1. A 
constant current of 50 mA was passed through the cell for 45 minutes 
during which a 1.9 micrometers thick film of polypyrrole formed on the 
anode. The electrolyte solution was maintained under a nitrogen atmosphere 
during deposition of the film. The film was washed with acetone and dried. 
Two films formed as above were pressed onto a nonconductive core disc as 
described in Example 1. The resultant capacitive electronic disc, which 
separated readily from the mastering discs, was demonstrably superior in 
playback and other desirable properties to the disc formed in Example 1. 
The bulk resistivity of the polypyrrole layer after lamination was between 
0.02 and 0.10 ohm-cm. The disc was clearly superior to that formed in 
Example 1 in terms of stability and retention of conductivity. 
Discs prepared by this method produced an acceptable picture upon playback 
in the as-pressed state. However, in each instance, the quality of both 
the picture and sound reproduction was improved by coating the disc with a 
thin layer of a doped, fractionated methylalkylsiloxane lubricant.