US2992139A - Method for electrostatic spraying of non-conductors - Google Patents

Description

United States Patent Grant E. Arhart, Greensboro, N.C.,

a corporation Chemical Coatings, Inc., Chicago, 11].,

of Delaware N Drawing. Filed May 2, 1958, Ser. No. 732,429

7 Claims. (Cl. 117-215) My invention relates to methods of coating articles formed of non-conducting materials, such as wood, fibrous material, etc., by electrolytic spray means, whereby certain new and improved results are obtained.

I realize that, broadly considered, electrolytic spray coating has been practiced for some time and apparatus adaptable for this purpose has been and is being marketed by Ransburg Electro-Coating Corporation of Indianapolis, Indiana, certain of which equipment may be adapted for the practice of my invention. For example, Starkey United States Patent No. 2,723,921, dated November 15, 1955, describes one of such previously known methods of electrolytic or electrostatic coating, of which my invention may be considered an improvement.

I am also familiar with Cross Patent 2,281,558, dated May 5, 1942, which has to do with the manufacture of sandpaper by a method which utilizes deposition of material on a web of paper in an electrostatic field.

A particular object of my invention is the application of a coating to a base of non-conducting material by an electrostatic method whereby either a clear or pigmented coating may be applied economically and efliciently, utiliZing a minimum of coating material. An important advantage of my invention is that it makes possible the coating of wood or other non-conductive material having an attractive grain, color or texture with a clear coating whereby such features of the coated base material remain visible in the finished coated article. I believe this to be novel in,my invention.

Another important advantage of my invention is that it makes possible the coating of non-conductive material with a single relatively thin coat.

The above mentioned objectives are not possible in any of the previously known methods of this type with which I am familiar.

Broadly considered, in accordance with my invention, the non-conductive surface is first coated with a mixture or solution of resin, with or without pigment, and an aqueous solution of electrolyte; after drying, the article is then electrostatically coated with either a clear or pigmented organic coating.

More specifically, my invention is practiced by first applying to the non-conductive surface, as by dipping, brushing, spraying or equivalent method, a mixture or solution of resinous material in a suitable solvent or solvents with an aqueous solution of phosphoric acid, which has been found vastly superior to any other electrolyte which I have tried. After air drying, the article is then passed through a suitable Ransburg electrostatic sprayer, as for example a machine known as Ransburg No. 2, or other suitable equipment of this character, where it is spray coated with a desired organic coating, which coating is likewise air dried or forced dried. If desired, an additional coating may subsequently be applied electrostatically as a top coat following a graining operation.

Example No. 1

Percent Polyvinyl butyral resin 7.0 Isopropyl alcohol 73.5 Xylol 15.5 Phosphoric acid 2.0 Water 2.0

The xylol referred to in the formula is a commercial mixture of ortho, meta and para xylene and is employed for its solvent value to facilitate the application of the mixture to the base surface. It may be replaced by other suitable solvents and, in fact, is not an indispensable constituent.

The alcohol serves as a distributing agent for the resin and is of value in the formula in rendering the water miscible with the other constituents. Other suitable solvents known to the chemist skilled in this field may be substituted.

The resin solids included in the above formula serve as a sealer of the porous substrate and aid in holding or scaling in the electrolytic solution until such time as the article is electrostatically sprayed.

Other resins may be substituted for the one referred to above, the solvent balance being modified according to normal practice in this art, in order, to accommodate the difference in solubility characteristics of the various resins. As examples of other resins which may be employed I might mention the vinyl acetates, the acrylic resins, the alkyd resins, the cellulose resins, the epoxy resins, the hard resins such as ester resins, hydrocarbons, maleics, phenolics, and such natural resins as may have properties suitable for the purposes here described. Nitrogenous resins, oleo-resinous varnishes and polyesters having the desired properties, as known to those skilled in the art, may also be employed.

The mixture obtained by the above formula is applied as by spraying, to the surface of a non-conductive article such as wood, and after air drying for about 15 minutes the article so coated is placed on a conveyor and passed before a Ransburg No. 2 station or the like. At this point there is applied a clear coat such as the following:

Percent Half-second butyrate 11 Synthetic hard resin 13 Plasticizer 3 Thinners 73 Suitable examples of the constituents referred to above will be obvious to those skilled in the art.

Another suitable clear coating may be represented by the following formula:

Percent Synthetic alkyd resin solids 10 Hard resin solids 10 Chlorinated rubber 10 Solvents and driers 69 Plasticizer 1 Example N0. 2

Percent Titanium dioxide 1.6 Aluminum silicate 4.1 Polyvinyl butyral resin 5.6 Isopropyl alcohol 80.0 Xylene 1.5 Water 4.3 Phosphoric acid 2.9

The mixture obtained by the above formula is applied to the non-conductive article as by a spray gun, allowed to dry in the air and the article thus coated is passed before a Ransburg No. 2 station where a pigmented organic coating is applied as represented by the following formula:

Percent Color pigments 11 Extender pigments 9 Synthetic alkyd resin solids 15 Synthetic chlorinated rubber 14 Thinner and driers 51 Another example of a pigmented coating is:

Percent Titanium dioxide 30 Extender pigment 16 Synthetic alkyd resin solids 9 Chlorinated rubber 7 Solvents and driers 38 Example N0. 3

Percent Polyvinyl butyral resin 7.2 Zinc chromate 6.9 Magnesium silicate 1.0 Butyl alcohol 16.1 Ethyl alcohol 62.0 Phosphoric acid 3.6 Water 3.2

The mixture represented by the above formula is applied by spray gun or otherwise to the non-conductive article and allowed to dry in the air. The article is then hung on a conveyor line and passed before a Ransburg No. 2 station where it is coated with a pigmented coating such as specified above, or other suitable formula.

As stated above, the process constituting my invention has important advantages over previously known electrostatic coating methods. I find that satisfactory coatings and finishes may be applied with a minimum of materials, a second coat frequently being found unnecessary. Furthermore, clear coatings may be applied, permitting the color and grain of the base material to show through.

I am aware that carbon black and other solids have been used as conductors in electrostatic coating methods previously known and such materials have important disadvantages as compared with my invention. Such solids completely conceal the color, grain, etc., of the base material and furthermore require a heavier coating of the applied coating material to conceal the color of said conductive materials. Furthermore, I find that use of the electrolytic conductors according to my invention act more efficiently in attracting the coating material electrostatically applied, and thus effect economies.

Various other modifications of my invention may suggest themselves to those skilled in the art Without departing from the spirit of my invention. Hence, I do not wish to be limited .to the specific examples or illustrations described above, except to the extent indicated in the appended claims.

I claim:

1. A method of coating a normally non-conductive base article, comprising applying to the surface of said article a coating material distributed in a liquid medium containing in solution a relatively small portion of phosphoric acid, drying, and thereafter spraying said coated article in an electrostatic field with a second coating.

2. A process as in claim 1 wherein the second coat is sufliciently clear to permit substantial visibility therethrough.

3. A process as in claim 1 wherein both coats are sufi'iciently clear to permit visibility of the base material in the finished article.

4. A process as in claim 1 wherein the first coat comprises a substantially clear-drying composition of a resin dispersed in a solvent therefor, the phosphoric acid is in aqueous dilution, and an alcohol is present.

5. A process as in claim 1 wherein the second coat is pigmented.

6. A process as in claim 1 wherein both of the coatings are pigmented.

7. A method of coating wood, to render the grain of the wood visible in the finished article, comprising applying to the surface of the article a coating composition comprising substantially clear-drying resin dispersed in organic liquid solvent and distributing media, said coating composition including a minor proportion of phosphoric acid and water, drying the article so coated, and thereafter spraying said coated article in an electrostatic field with a second coating, the latter being likewise clear drying.

References Cited in the file of this patent UNITED STATES PATENTS 1,881,445 Flanzer et a1. Oct. 11, 1932 2,583,000 Lylte Jan. 22, 1952 2,617,742 Olson Nov. 11, 1952 2,698,814 Ransburg Ian. 4, 1955 2,719,820 Allen Oct. 4, 1955 2,723,921 Starkey Nov. 15, 1955 2,761,797 Young Sept. 4, 1956 2,762,726 Saunders Sept. 11, 1956 2,772,190 Haayman et a1. Nov. 27, 1956

Claims (1)

1. A METHOD OF COATING A NORMALLY NON-CONDUCTIVE BASE ARTICLE, COMPRISING APPLYING TO THE SURFACE OF SAID ARTICLE A COATING MATERIAL DISTRIBUTED IN A LIQUID MEDIUM CONTAINING IN SOLUTION A RELATIVELY SMALL PORTION OF PHOSPHORIC ACID, DRYING, AND THEREAFTER SPRAYING SAID COATED ARTICLE IN AN ELECTROSTATIC FIELD WITH A SECOND COATING.