Electrically Conductive Transparent Plastic Material

Abstract

An electrically conductive clear transparent self-supporting plastic sheet consisting of a homogeneous solid molecular mixture of a clear transparent conductive polymer and a film forming clear transparent polymer is obtained by combining a quaternary ammonium polyelectrolyte, such as polydimethyl diallyl ammonium chloride, with a film forming polymer, such as polyvinyl alcohol, and a mutual solvent, such as water. The mixture is then cast onto a smooth surface, the mutual solvent evaporated, and the resultant sheet material stripped from the casting surface to obtain a sheet of electrically conductive clear transparent plastic material useful as a conductive plastic or, after coating with a charge retentive film, as an electrographic recording medium.

Description

DESCRIPTION OF THE PRIOR ART

Heretofore, electrically conductive transparent films have been prepared by combining a film forming polymer, a plasticizer for the polymer, an electrolyte soluble in the plasticizer, and a solvent for the polymer and casting the resultant mixture on a casting surface to provide a self-supporting, transparent, electrically conductive film. An example of such prior art is disclosed in U. S. Pat. No. 3,357,930 issued Dec. 12, 1967. In this prior art disclosure it was contemplated that a relatively wide range of electrolytes, including acids, bases, or salts could be employed, provided that the electrolyte was soluble in the plasticizer. However, conductive films formed according to the aforecited prior art do not appear to have the requisite stability when employed as the conductive substrate for a clear transparent charge retentive film of an electrographic recording web.

SUMMARY OF THE PRESENT INVENTION

The principal object of the present invention is the provision of an improved electrically conductive transparent plastic material which is perfectly colorless or non-light absorbing.

One feature of the present invention is the provision of a clear transparent conductive plastic material having a conductive polymer combined with a film forming polymer, whereby the improved stability of the film forming polymer is imparted to the resultant composite polmeric plastic material.

Another feature of the present invention is the same as the preceding feature wherein the conductive polymer is a quaternary ammonium polyelectrolyte.

Another feature of the present invention is the same as the first feature wherein the conductive polymer is polypiperidinium chloride.

Another feature of the present invention is the same as any one or more of the preceding features including the provision of a clear non-conductive plastic charge retentive film disposed overlaying one major surface of a sheet of the conductive plastic material to form an electrographic recording web.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flow diagram, in block diagram form, depicting the method for making conductive plastic sheet according to the present invention, and

FIG. 2 is a fragmentary perspective view, partly in section, depicting an electrographic recording web employing features of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIGS. 1 and 2, there is shown in block diagram form the method for making the electrographic recording web 1 of FIG. 2 which comprises a clear, transparent, conductive sheet, as of 0.010 inch thick, having a clear, transparent, charge retentive film 3, as of 4 microns thick, adhered to the clear conductive sheet 2. Such an electrographic recording web is suitable for use in electrophotography to provide a transparency of photographic quality.

In the first step of the method, a film forming polymer, such as polyvinyl alcohol, is combined with a conductive polymer, such as polydimethyl diallyl ammonium chloride and a mutual solvent for both polymers to form a clear, non-light absorbing, colorless liquid comprising a homogenous mixture of the polymers in the solvent, such as water. The mixture, in the next step, is cast as a sheet onto a substrate to a uniform thickness, as of 0.040 inch thick. In the next step, the solvent is evaporated to leave a clear, non-light absorptive, transparent film, as of 0.010 inch thick on the substrate. The film is then peeled from the substrate to produce an electrically conductive, clear, non-light absorptive transparent film which may be utilized as the clear conductive sheet 2 of the electrographic recording web 1, or which may be utilized for other applications where a transparent conductive plastic sheet is desired. In the next step, the conductive sheet 2 is coated to a thickness, as of 4 microns, with a clear charge retentive film such as of polyvinyl chloride-acetate and allowed to dry, thereby producing the electrographic recording web 1 of FIG. 2.

The film forming polymeric material useful in the practice of this invention includes a wide variety of polymers which are transparent in sheets varying from thicknesses corresponding to 0.0001 inch to 1 inch. Aside from transparency, the polymer should be castable in the form of a gel, as opposed to a hard, highly crossed linked film, so as to allow ionic mobility, and its usefulness as a film should not be effected by the continued presence of the solvent, for example, water in the amounts required to effect ionization of the electrolyte.

Among the film forming polymers particularly preferred in the practice of this invention may be mentioned those which are polymers of hydroxyl containing monomers, esters, or acetals. Examples of such polymers include polyvinyl alcohol, agar, carboxymethylcellulose, polyvinyl acetate, polyvinyl acetate-alcohol, copolymers, polyvinyl acetal, polyvinyl formal, acrylic resins, such as polyacrylic acid, methyl cellulose, polyvinylpyrrolidone, CARBOSET 525 acrylic polymer and CARBOSET 514 acrylic polymer, the latter two being manufactured by B. F. Goodrich Chemical Company, a division of the B. F. Goodrich Company, 3135 Euclid Avenue, Cleveland 15, Ohio. It should be noted that in describing the polymers as those of named monomers, it is not implied that they are necessarily made by actual polymerization of such monomers. Thus, for example, polyvinyl alcohol may be described as a polymer of monomeric vinyl alcohol, but is invariably produced by indirect means, as vinyl alcohol is non-existent in fact.

The conductive polymer materials useful in the practice of this invention include polymers which are transparent or nearly so in thin films. The conductive polymers are capable of providing both a source of ions and the necessary water-holding capacity (hygroscopicity). Polyelectrolytes are particularly suited for this application because of their ionic nature and ability to hold water at low humidity. Particularly suitable polyelectrolytes include the quaternary ammonium polyelectrolytes. The polymers of this type include vinylbenzyl quaternary ammonium compounds and can be any water-soluble or water-dispersible homopolymer or copolymer of one or more vinylbenzyl quaternary ammonium compounds, or copolymers of a predominant amount of vinylbenzyl quaternary ammonium compound and a minor amount of a nonacidic copolymerizable compound such as acrylamide, or copolymers containing, in the polymer molecules, residues corresponding to from 95.0 to 99.99 percent by weight of one or more vinylbenzyl quaternary ammonium compounds and from 5.0 to 0.01 percent by weight of divinyl benzene. More specifically, the electroconductive substance can be a homopolymer or a copolymer of any two or more vinylbenzyl quaternary ammonium compounds having the general formula:

wherein R, R' and R" each represents individually a monovalent radical selected from the group consisting of CH.sub.2 OHCH.sub.2, CH.sub.3 CHOHCH.sub.2, CH.sub.2 OHCHOHCH.sub.2 and alkyl, aryl cycloalkyl and aralkyl hydrocarbon radicals, and R, R' and R" collectively represent the radicals of the formula:

wherein the three valences are attached to the nitrogen atom, the R groups containing a total of not more than 12 carbon atoms in the sum of the constituent radicals, or a copolymer of at least 65 percent by weight of any one or more of such vinylbenzyl quaternary ammonium compounds with not more than 35 percent by weight of acrylamide, or a copolymer containing, in the polymer molecules, residues corresponding to from 95.0 to 99.99, preferably from 99.0 to 99.99 percent by weight of any one or more of such vinylbenzyl quaternary ammonium compounds and from 5.0 to 0.01, preferably from 1.0 to 0.01, percent by weight, of divinylbenzene. Mixtures of any two or more of the homopolymers or copolymers can also be used. The polymers are of relatively high molecular weight and are characterized by a viscosity of at least 2 centipoises for a 0.5 percent by weight solution of the polymer in an aqueous 2 percent by weight solution of sodium chloride at 25.degree. C. as determined by an Ostwald viscosimeter.

In the preferred embodiment of the invention the electroconductive polymer is a homopolymer or copolymer of one or more vinylbenzyl quaternary ammonium compounds having the aforementioned general formula wherein R represents an alkyl radical containing from one to four carbon atoms, R' and R" each represents a radical of the group consisting or CH.sub.2 OHCH.sub.2, CH.sub.3 CHOHCH.sub.2, CH.sub.2 OHCHOHCH.sub.2 and lower alkyl radicals containing from 1 to 4 carbon atoms and Y is an anion, e.g., a sulfate, a chloride, a nitrate, or a hydroxyl ion, copolymers of at least 65 percent by weight of any two or more such vinylbenzyl quaternary ammonium compounds and not more than 35 percent by weight of acrylamide, or copolymers of from 99.0 to 99.99 percent by weight of at least one such vinylbenzyl quaternary ammonium compound and from 1.0 to 0.01 percent by weight of divinylbenzene. A particularly suitable quaternary ammonium polymer is polypiperidinium chloride having an integral heterocyclic ring as shown by the following general formula: ##SPC1##

wherein the anion is a chloride and the polymer comprises a homopolymer of disubstituted diallyl ammonium chloride monomers where R and R' are selected from the groups consisting of methyl, ethyl, and methyl, ethyl, butyl and octyl, respectively. A particular one of the aforementioned polypiperidinium chlorides is polydimethyl diallyl ammonium chloride marketed by Calgon Corporation of Pittsburgh, Pennsylvania, as conductive polymer No. 261.

Mutual solvents useful in the practice of this invention include water methanol-water mixtures.

In order to illustrate more fully the nature of the invention and the manner of practicing same, the following examples are presented.

EXAMPLES

A castable composition is prepared by dissolving 15 percent by weight polyvinyl alcohol in water. A suitable grade of polyvinyl alcohol is ELVANOL grade 70-05 available from E. I. DuPont de Nemours & Company, Inc., Organic Chemicals Department. This grade of polyvinyl alcohol is a hot-water-soluble grade and is slurried in cold water and then heated to complete solution. More particularly, add to the vessel the required amount of water at 75.degree. F. or below and start agitation. Sift the polyvinyl alcohol into the vortex such that it is rapidly worked out and dispersed and stir this way for 10 minutes. Raise the temperatures to 194.degree. F. or above and continue stirring at 194.degree.-203.degree. F. until the resin is completely dissolved, usually about 30-60 minutes. To the dissolved polyvinyl alcohol add polydimethyl diallyl ammonium chloride (conductive polymer 261 manufactured by Calgon Corporation) in the proportions 20 parts by weight to each 100 parts by weight of dry polyvinyl alcohol. The conductive polymer is preferably added in a 40 percent water solution, i.e., 40 percent by weight conductive polymer in water and stirred into the polyvinyl alcohol until dispersed uniformly therein. The solution is coated on a glass plate or other suitable substrate. The coating on the glass plate substrate is obtained by spreading a pool of the solution on the substrate with a glass rod to obtain a uniform thickness for the coating of approximately 0.040 inch. The coated substrate is then put in an oven at 170.degree.to 180.degree. F. and heated until dry, typically about 30 minutes. Upon drying, the coating reduces to a thickness of approximately 0.010 inch. When dry the coated substrate is removed from the oven and allowed to cool to room temperature. Upon cooling to room temperature, the coating is stripped or peeled from the substrate to obtain the clear, transparent, conductive plastic sheet of the present invention.

In the aforecited specific example, a 25 percent solution of QX 3521 polymeric quaternary ammonia salt made by Dow Chemical Company may be substituted for polydimethyl diallyl ammonium chloride. The sheet 2 is perfectly clear, sufficiently flexible for further processing, and surprisingly resistant to finger marks, moisture or stickiness. The bulk conductivity of the sheet 2 was approximately 10.sup.7 ohm centimeters when tested in ambient conditions (20.degree. C. and 50percent relative humidity).

The amount of conductive polymer to be employed will depend to some extent upon the degree of conductivity desired. As a general rule, however, it can be said that the amount of conductive polymer should be in the range between 5 percent and 50 percent by weight of the film forming polymer.

To make an electrographic recording web, the cast sheet, after drying on the substrate, is coated with a dielectric charge retentive film as of vinyl chloride-acetate to a thickness as of 4 microns. If desired, the sheet 2 may be then stripped from the substrate or prepared in rolls or the like after the manner of making cellophane and similar films to provide a self-supporting, transparent electrically conductive sheet with a charge retentive film adhered thereto.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claims

What is claimed is:

1. An electrically conductive clear transparent self-supporting plastic sheet comprising:

a homogenous solid molecular mixture of a clear transparent conductive polymer and a film forming clear transparent polymer;

the conductive polymer being a water-dispersible quaternary ammonium polyelectrolyte consisting of at least one member selected from the group consisting of

a. homopolymers of a vinylbenzyl quaternary ammonium compound having the general formula:

wherein R, R' and R" each represents individually a monovalent radical selected from the group consisting of CH.sub.2 OH--CH.sub.2, CH.sub.3 --CHOH--CH.sub.2, CH.sub.2 OH--CHOH--CH.sub.2 and alkyl, aryl, cycloalkyl and aralkyl hydrocarbon radicals, and R, R' and R" collectively represent the formula:

wherein the three valences are attached to the nitrogen atom, and Y is an anion, said R groups containing a total of not more than 12 carbon atoms in the substituent radicals,

b. copolymers of at least two such vinylbenzyl quaternary ammonium compounds,

c. copolymers of at least 65 percent by weight of at least one of such vinylbenzyl quaternary ammonium compounds and not more than 35 percent by weight of acrylamide, and

d. copolymers corresponding to from 95.0 to 99.99 percent by weight of at least one of such vinylbenzyl quaternary ammonium compounds and from 5.0 to 0.01 percent by weight of divinylbenzene; and

the film forming polymer is at least one material selected from the group consisting of, polyvinyl alcohol, agar, carboxymethylcellulose, polyvinyl acetate, polyvinyl acetate-alcohol copolymers, polyvinyl acetal, polyvinyl formal, polyacrylic acid, methyl cellulose, and polyvinylpyrrolidone.

2. The apparatus of claim 1 wherein the film forming polymer is polyvinyl alcohol.