GB2117672A - Coating process - Google Patents

Abstract

A process of coating a conductive substrate in which the substrate is first coated by electrodeposition and the initial coating is subsequently treated in a separate step with an aqueous dispersion of a slip-promoting additive.

Description

SPECIFICATION Coating process This invention relates to a process for applying a coating to a conductive substrate by electrodeposition.

The application of a coating to a conductive substrate by an electrodeposition process is advantageous, as compared with application by a conventional dip process using a more viscous and concentrated coating composition, since electrodeposition ensures that the final coating is of uniform thickness; there is good penetration into recesses; articles of complex shape can be coated; and there is little wastage due to drainage and rinsing of the more dilute coating composition.

In the electrodeposition process an adherent coating film, together with an outer layer of the coating composition, is present on the substrate immediately after the passage of electric current and removal of the substrate from a coating bath. The substrate may be, and is most usually, subjected to a subsequent rinsing treatment before the electrodeposited coating film is dried and/or stoved.

In the conventional dip process, however, only a layer of coating composition is present on the substrate and this must be freed from water or other solvent by evaporation or it must be otherwise treated before an adherent coating film is produced.

It is known to add to coating compositions used in a dip coating process certain additives which modify the surface properties of the final coating, for example additives which make the surface of the final coating more slippery so that it has improved resistance to scratching or abrasion. It has been believed to be necessary for success that the additive is already present in the composition before application to a substrate and the improved properties of the final coating are believed to be due to a high concentration of the additive at the surface of the coating film. In the case of an electrodeposition process it would seem reasonable to assume that when the initial adherent coating film is subjected to rinsing, the additive would be substantially removed from the film surface and that the potential for improvement of surface properties would be lost.However, we have found surprisingly that the surface properties of the final coating film resulting from an electrodeposition process are improved, even when no slip-promoting additive has been added to the coating composition used, if the initial electrodeposited film is treated in a separate step with an aqueous dispersion of a slip-promoting additive.

Therefore according to this invention we provide a process for applying to a conductive substrate by electrodeposition a coating having improved surface properties, wherein an electric current is passed between the substrate as one electrode and a counter-electrode when immersed in a coating composition comprising a dispersion of film-forming polymer in an aqueous medium, the coated substrate being subsequently treated in a separate step with an aqueous dispersion of a slip-promoting additive.

We also provide an aqueous dispersion of a slip-promoting additive for use in the present process.

We further provide a coated conductive substrate and a coated article which have been coated by the present process.

By a slip-promoting additive we mean any additive which when present in a chemically similar aqueous coating composition used in a conventional dip or spray coating process makes a significant improvement in the resistance to scratching or abrasion, i.e. the marresistance, of the final coating. Suitable chemical categories of slip-promoting additive include the water-dispersible silicone resins, and hydrocarbons such as polyethylene wax.

Particularly suitable additives are the waterdispersible silicones commercially available from Byk-Mallinckrodt under the trademark Byk, for example Byk 303 and 341, and from Dow Corning as Paint Additives 11 4 and 57; and polyethylene wax commercially available as Lubaprint 405/10X.

By an aqueous dispersion of the additive we mean that it is stably dispersed in particulate form or is dissolved in an aqueous medium such as rinse water during its use. Similarly the film-forming polymer used in the coating composition may be present as a solution or as a stable dispersion of particles.

Preferably the initial coating film electrodeposited on the substrate is allowed to remain wet until it is treated with the aqueous dispersion of slip-promoting additive. Preferably the treated coating film is subjected to a heat treatment, for example stoving, as an immediately following step. Preferably the slip-promoting additive is present in an aqueous rinse liquid and preferably the initial coating film is subjected to at least one rinse treatment before it is subjected to treatment with a rinse liquid containing the slip-promoting additive.

The general method of applying the electrodeposited coating film in the present process is conventional and the conductive substrate may be made an anode or a cathode. The process is particularly appropriate for the continuous coating of metal articles for example articles which comprise iron, steel, copper, aluminium, zinc, brass and other metals or alloys.

In one important embodiment of this invention a substantially transparent coating is first applied to a metal substrate having a surface appearance which it is desired to see and yet to protect from deterioration. A process of this type is described in our Spanish Patent No.

493712 wherein an article which has been previously plated with a decorative metal or which comprises a polished or bright metal surface is made a cathode and is coated by electrodeposition with a transparent coating of film-forming material from an aqueous medium. By a decorative metal we mean a metal which imparts, optionally together with the cathodic coating, a decoragive effect to the article for example high reflectivity, or a pleasing colour such as that of silver or gold. Such a metal may comprise for example a precious metal, for example gold, silver or platinum, or a non-precious metal such as copper, nickel or tin, or a mixture or separate layers of precious or non-precious metals for example brass, or separate successive layers of nickel and silver.

The invention is particularly appropriate to the high cost metals such as silver. In many cases the metal will possess both decorative and preservative properties.

Polished or bright metal surfaces, as distinct from plated metal surfaces, may be produced, for example, by chemical, electrochemical or mechanical polishing of metal such as brass, copper or a precious metal.

The initial, electrodeposited, transparent film is then subjected to at least one aqueous rinse treatment before it is subjected to treatment with an aqueous rinse liquid containing the slip-promoting additive according to this invention.

The transparent coating described above may be tinted, for example by the presence of a dyestuff, but in a further embodiment of the invention, when it is not desired to see the surface of the substrate but to obliterate it, the coating may be opaque due to the presence of pigment, extender and the like.

In both of the embodiments just described there is obtained a final coating which has improved surface properties, for example improved mar resistance.

Suitable film-forming polymers for use in this invention include addition polymers such as those prepared from acrylic monomers; epoxy resins, for example the epoxyamine adducts; polybutadiene resins; and alkyd resins.

The aqueous dispersion of slip-promoting agent may also contain other materials for example organic solvents and surface active agents.

Following the treatment with the aqueous dispersions of slip-promoting additive the substrate is preferably heated to an elevated temperature, for example in the range 1 20-220 C.

In a typical process according to the invention in which it was desired to obtain a marresistant transparent coating on a steel article electroplated with a thin coating of silver, the electroplated article was first coated by electrodeposition with an initial film of an epoxyamine resin as described in our Spanish Patent mentioned above. The article was then rinsed with de-ionised water and was then rinsed again with deionised water in which was dispersed the Dow Corning silicone slip-promoting additive PA57 reduced with water to an actual concentration of 0.5% silicone by weight in the rinse water. After stoving at 1 50 C the final coating on the article had improved mar resistance as compared with an article coated in a similar manner but not rinsed with water containing the slip-promoting additive.

Preferably the aqueous dispersion of the slip-promoting additive with which the coating film is treated, for example the rinse liquid just discussed, contains from 0.01 to 10% by weight of the additive based on the weight of the dispersion.

Claims (9)

1. A process for applying a coating to a conductive substrate by electrodeposition wherein an electric current is passed between the substrate as one electrode and a counter electrode when immersed in a composition comprising a dispersion of film-forming polymer in an aqueous medium, the coated substrate being subsequently treated in a separate step with an aqueous dispersion of a slippromoting additive as herein defined.

2. A process according to claim 1 wherein the coated substrate remains wet until treated with the aqueous dispersion of slip-promoting additive.

3. A process according to claim 1 or claim 2, wherein the coated substrate is subjected to at least one aqueous rinse before it is treated with the aqueous dispersion of slippromoting additive.

4. A process according to any one of claims 1 to 3, wherein the treated coated substrate is subjected to a heat treatment.

5. A process according to any one of claims 1 to 4, wherein the substrate to be coated has been previously plated with a decorative metal as herein defined or comprises a polished or bright metal surface.

6. A process according to claim 5, wherein the decorative metal is silver.

7. A process according to any one of claims 1 to 6, wherein the slip-promoting additive is selected from water-dispersible silicone resins and polyethylene waxes.

8. A process according to claim 1 and substantially as herein described.

9. An article which has been coated by a process according to any one of claims 1 to 8.