Application of electroconductive polysalt complexes

Abstract

 A water-insoluble electroconductive coating, which will allow subsequent coatings such as ZnO or dielectric resins to be applied from an aqueous-based system, is applied to a substrate as a double coating; a strong cationic polymer followed by a strong anionic polymer or vice versa. This results in the formation of a polysalt at the interface that is both water-insoluble and electroconductive.

Description

[0001] This invention relates to water-insoluble electroconductive coatings, and is more particularly concerned with a process for preparing electroconductive coated substances by applying a polysalt to a substrate.

[0002] A common application for electroconductive polymers is in the manufacture of paper for electrographic reproduction. The polymer is utilized in a conductive coating formulation which is applied to a paper substrate and subsequently overcoated with an organic-solvent-based dielectric or photoconductive layer such as zinc oxide. This method has many disadvantages,such as the fire hazards associated with organic solvents and the high cost of solvent-recovery systems for pollution control.

[0003] It is well known that water-soluble linear conductive polymers such as poly(diallyldimethylammonium chloride) and poly-(vinylbenzyltrimethylammonium chloride) may be made less soluble or even totally insoluble via covalent cross-linking. However, as the degree of cross-linking is increased, the polymer solution becomes gel-like and unworkable. Thus, covalently cross-linked polymers are not convenient for electroconductive coatings.

[0004] An alternative approach to the formulation of water-insoluble conductive polymers is through the mechanism of ionic bonding. An electroconductive polysalt complex formed by the interaction of a polymer containing strong basic (cationic) residues and a polymer containing weak anionic (acidic) residues forms a clear stable homogeneous aqueous solution at low pH. This solution may be formed in a pigmented slurry of the type used in the manufacture of conductivized paper without causing any detrimental effect to either the polysalt or the conductivity of the paper. (See Sharpe, Jr. et al., U.S. Patent No. US-A-4,040,984).

[0005] Polysalt complexes of a strong acid and strong base polymeric combination are also known. These polysalts exhibit high d.c. resistivity, but following special treatment, i.e., equilibration with highly concentrated electrolyte solutions, they become effective d.c. conductors. Polysalts of this nature require special ternary solvent systems, which mitigate against their usefulness in electroconductive coating applications. The manufacture of electroconductive paper by standard procedures involves formulation of the conductive polymer into an aqueous slurry of a pigment such as clay or calcium carbonate and a binder system such as hydroxyethylated starch or polyvinyl alcohol. These polysalts are incompatible with these procedures since (I) variation in the ternary solvent system for the polysalt causes precipitation of the polysalt; (2) the electrolyte doping and the electrolyte used in the ternary solvent system for the polysalt are undesirable components in the coating formulation; and (3) the use of the requisite water-miscible organic solvent in the ternary system would require solvent recovery units for compliance with pollution control standards.

[0006] The present invention attempts to circumvent the problems associated with polysalt complexes of strong acid and strong base polymers for electroconductive coatings by forming the polysalt complex upon the substrate in a two-step process. This eliminates the need for either a ternary solvent system or electrolyte doping for conductivity. The use of the water-insoluble polysalt conductive coating of this invention allows for subsequent coatings to be applied from aqueous systems. This eliminates the hazards previously encountered with organic-based secondary coatings.

[0007] In accordance with the present invention, a water-insoluble electroconductive coating is applied to a substrate by a) applying an aqueous solution of a charged polymer as a primary coating to the substrate in the range of 0.1 to 3.0 pounds of primary coating per 3,000 sq. ft. (0.16 to 4.9 g/m²) of substrate; b) applying an aqueous solution of an oppositely charged polymer as a top coating by a spray technique to the coated substrate from step (a) in the range of 0.05 to 2.0 pounds of top coating per 3,000 sq. ft. (0.08 to 3.3 g/m2) of the substrate; c) allowing sufficient time for the primary coating and the top coating to react at their interface and form an ionic polysalt complex that is both water-insoluble and electroconductive.

[0008] The water-insoluble polysalt complex of this invention is formed on the substrate via the instantaneous interaction of the two oppositely charged polymers at their interface when one polymer is sprayed upon the coating of the other. Either charged polymer (anionic or cationic) may be applied as the primary coating, the remaining oppositely charged polymer serving as the top coating. As used herein, the term "charged polymer" refers to either an anionic or a cationic polymer.

[0009] The use of water-soluble charged polymers for both the top and primary coatings allows for convenient formation of the water-insoluble polysalt complex on the substrate. With respect to electroconductive paper manufacture, the use of water-soluble polymers is a standard procedure and as such, the process for water-insoluble polysalt formation of this invention is most applicable.

[0010] The electroconductive coatings of the present invention are thus formed of a polysalt complex between a water soluble-charged polymer with strong cationic residues such as poly(diallyldimethyl- ammonium chloride) and a water-soluble charged polymer with strong anionic residues such as polysodiumvinylsulphonate or polysodium- styrenesulphonate. Other strong cationic polymers useful in this invention include all of those water-soluble polymers that contain a substantial portion of strong cationic functional groups such as quaternary ammonium, quaternary phosphonium and quaternary sulphonium substituents. These polymers include poly(vinylbenzyl- trimethyl ammonium chloride), poly(methacryloloxyethyltrimethyl ammonium chloride) and copolymers, for example of diallyl dimethyl ammonium chloride and diacetone acrylamide.

[0011] Representative strong anionic polymers also include poly-(allylsulphonic)acid, sulphonated polymethylolacrylamide, and poly(sulphostyrene).

[0012] The process of the present invention can best be understood by reference to the following illustrative example, which demonstrates the interchangeability of the cationic and anionic polymers for use as either the primary coating or the top coating under conventional coating techniques. Although the present invention is described in connection with a coating for electrographic paper as a preferred embodiment, synthetic substrates such as Mylar (polyethyleneglycol terephthalate), nylon and polyethylene may be coated with the polysalt complex of the present invention without departing from the invention.

EXAMPLE

[0013] Barrier-coated paper is manually coated with the following formulations:

(a) primary coating - 1.06 1b/3000 sq. ft. (1.72 g/m²) of poly(diallyldimethylammonium chloride) and top coating - 0.45 1b./3000 sq. ft. (0.73 g/m²) of polysodiumvinyl- sulfonate.

(b) primary coating - 1.06 1b./3000 sq. ft. (1.72 g/m2) of poly(diallyldimethylammonium chloride) and top coating - 0.40 lb./3000 sq. ft. (0.65 g/m²) of polysodiumvinyl- sulfonate.

(c) primary coating - 1.63 1b./3000 sq. ft. (2.65 g/m²) of polysodiumvinylsulphonate and top coating - 0.23 lb./ 3000 sq. ft. (0.37 g/m2) of poly(diallyldimethylammonium chloride).

(d) primary coating - 1.50 lb./3000 sq. ft. (2.44 g/m2) of polysodiumvinylsulphonate and top coating - 0.09 lb./ 3000 sq. ft. (0.15 g/m²) of poly(diallyldimethylammonium chloride).

(e) primary coating - 1.30 lb./3000 sq. ft. (2.12 g/m²) of polydiallyldimethylammoniumchloride and no top coating.

The coated sheets are dried for 15 seconds on a print drier and for 15 minutes in a 135°C forced air oven. The coated sheets are conditioned overnight at 50% RH and 22°C after which they are weighed to obtain coat weight, then evaluated for conductivity.

[0014] Two circles of 3.375 inches (8.57 cm) diameter are cut from the conditioned coated sheets. Surface resistivity (I/conductivity) is measured by using a Keithley Resistivity Adapter and Keithley Electrometer. The test specimens are placed in the adapter, coated side down. A direct current of 100 volts is applied across the surface and the current (measured in amperes) is read directly from the electrometer. The surface resistivity in ohms/unit of area is calculated using the following equation:

Test specimens are then subjected to a water-soak test in which the test specimens are immersed for 15 seconds in a 1,000-ml beaker which contains 900 ml of mildly agitated water. The test specimens are then dried for ten minutes at 105°C in an oven and reconditioned at 50% RH and 22°C overnight. The test specimens are then lightly calendared at about 500 psi (3.5 MPa) and the surface resistivity again measured. The difference in the two readings, before and after the water soaked test, indicates the level of water resistance. In this regard smaller differences are indicative of a greater degree of water resistance.

[0015] Table 1 sets forth comparisons between the number of sheets coated with formulation A to E and shows that increased water insensitivity does occur regardless of whether the primary coating is the anionic or the cationic polymer.

Claims

1. A process for applying to a substrate a water-insoluble electroconductive coating comprising a) applying an aqueous solution of a charged polymer as a primary coating to the substrate in the range of 0.1 to 3.0 pounds of primary coating per 3,000 sq. ft. (0.16 to 4.9 g/m²) of substrate; b) applying an aqueous solution of an oppositely charged polymer as a top coating by a spray technique to the coated substrate from step (a) in the range of 0.05 to 2.0 pounds of top coating per 3,000 sq. ft. (0.08 to 3.3 g/m²) of the substrate; c) allowing sufficient time for the primary coating and the top coating to react at their interface and form an ionic polysalt complex that is both water-insoluble and electroconductive.

2. A process as claimed in Claim 1 in which the substrate is paper.

3. A process as claimed in Claim 1 or 2 in which the charged polymer of the primary coating is a polymer containing strong cationic residues.

4. A process as claimed in Claim 3 in which the oppositely charged polymer of the top coating is a polymer containing strong anionic residues.

5. A process as claimed in Claim 1 or 2 in which the charged polymer of the primary coating is a polymer containing strong anionic residues.

6. A process as claimed in Claim 5 in which the oppositely charged polymer of the top coating is a polymer containing strong cationic residues.

7. A process as claimed in Claim 3 or 6 in which the polymer with strong cationic residues is poly(diallyldimethylammonium chloride).

8. A process as claimed in Claims 4 or 5 in which the polymer with strong anionic residues is polysodiumvinylsulphonate.