US1781507A

US1781507A - Method of preparing metal for painting

Info

Publication number: US1781507A
Application number: US309905A
Authority: US
Inventors: James H Gravell
Original assignee: Amchem Products Inc
Current assignee: Henkel Corp
Priority date: 1928-10-02
Filing date: 1928-10-02
Publication date: 1930-11-11
Anticipated expiration: 1947-11-11

Description

Patented Nov. 11, 1930 UNITED STATES PATENT OFFICE JAMES H. GRA'V'ELL, OF ELKINS PARK, PENNSYLVANIA, ASSIGNOR TO AMERICAN CHEMICAL PAINT COMPANY, OF AMBLER, PENNSYLVANIA, A CORPORATION OF DELAWARE Ho Drawing.

This invention relates to preparing metal for painting or other applied finish and, although it has various applications in the art, it is especially useful for preparing sheet steel I articles, such as automobile bodies, fenders and stampings, for receiving applied finishes,

such as paint, lacquer, apan, varnish, etc.

The objects of the invention are to clean the metal, to prepare it for painting or other applied finishes, to prevent the development of defects in the applied finish, to remove rust and scale from the metal, to retard the v rusting of steel, to provide an acid cleaning process that will not produce subsequent corrosion, to reduce the cost of cleaning metal to be painted and generally to improve the art of preparing metal for painting.

The term painting as used in the following specification and claims is intended to embrace any applied finish such as is employed for metals for the purpose of preservmg or ornamenting the surface thereof and any method of applying same.

Generally stated, my process consists in coating the metal with acid, usually phosphoric acid, and then treating the metal with a material adapted to neutralize the acid.

When phosphoric acid is applied to metal,

such as steel, a certain amount of the metal is dissolved in the acid and when the metal is then treated with a material adapted to neutralize the acid, the said metal in solution is precipitated as a water insoluble material with the result that the metal becomes covered with a film of this insoluble material, which tends to prevent it from rusting, and, at the same time, affords an excellent surface on which to apply various finishes, such as paint, lacquer, apan or varnish.

As phosphoric acid is a relatively expensive acid, I have found that considerable economy can be accomplished by selectively controlling the action of phosphoric acid so that, although its action in removing rust and scale is unhampered, its action on good metal is materially reduced, thus effecting a large saving in both metal and acid and providing a more uniform film on the metal. Any suitable control agent may be used, but I 50 prefer due to its effectiveness, to employ a METHOD OF PREPARING METAL FOR PAINTING Application filed October 2, 1928. Serial No. 309,905.

thio-amide, as for instance, thiourea. This I admix with the phosphoric acid in small amfiunts before coating the metal with the am In practice I have found that much better results are obtained if the neutralizing material is removed from the metal before the paint is applied and thus I deem it advisable to wash the metal with water after performing the initial steps of my process. This washing with water not only removes excessive neutralizing material but also removes all deleterious products of its reaction.

When there is danger of the washing operation not being performed with sufiicient care, it is well not to use strong caustics such as caustic soda, tri-sodium phosphate, etc., to neutralize the phosphoric acid coating as traces of such materials left behind on the metal, may adversely aflect the painted finishes. In order to overcome this possible defeet, I prefer to employ a material for neutralizing purposes that neutralizes the phosphoric acid to a hydrogen phosphate, and yet in itself is not caustic, and hence, if any of the neutralizing material or any of the neutral.- ized acid is left behind on the work after the washing operation, no harm is done. A neutralizing material well suited for the purpose is di-sodium phosphate (Na HPO which in itself is not a caustic material, yet will neutralize phosphoric acid to mono-sodium phosphate (NaH PO and precipitate the metal dissolved in the acid.

The di-sodium phosphate may be used dissolved in water and the same solution may be used and reused until it is exhausted. In order to determine when the solution is exhausted it is well to add a small amount of an acid indicator to the solution so that an indicative color change willdevelop as soon as any free acid remains un-neutralized. I have found in practice that methyl orange proves to be a very satisfactory indicator.

It often happens, either through design or accident, that the metal to be treated has a coating of oil on it, in which case it is difficult to coat the metal with phosphoric acid. It is accordingly advisable to remove this oil coating before trying to coat the metal with acid. This may be accomplished by treating the metal with a caustic soda solution before the acid is applied or by admixing oil removing solvents, such as alcohols, keto-nes, ethers, etc., with the phosphoric acid so as to cause it to coat the metal. However, I consider the most practical method to be to heat the metalto a temperature adapted to remove any coating of oil before applying the acid.

To carry out my process commercially, as for instance, on an automobile steel fender I may proceed as follows I first heat the metal to a temperature of'600 F., more or less, to burn off all oil and grease, and after it has cooled sufficiently, I immerse it in a. bath of phosphoric acidmade by admixing the following materials in substantially the proportions stated:

Water, 10,000 gallons; phosphoric acid (7 5%) 1,000 gallons; thiourea, 0.0036 pounds more or less,-' the purpose being to use sufii-- cient acid to properly clean the metal and sufiicient thiourea to selectively control the action of the acid.

I permit the metal to remain in the bath long enough to clean it to satisfaction and then remove it. The metal comes from the bath coated with phosphoric acid and ready to be treated with the neutralizing material. This is conveniently done by immersing the metal in. a bath having a proportional composition substantially of:

Di-sodium phosphate, 1 pound; water, 2 gallons; methyl orange, trace.

I permit the metal to remain in the neutralizing bath long enough to neutralize all freephosphoric acid on the metal, which usually takes a minute or two, more or less. I then remove 1t and lmmerse it in a bath of runnlng water to remove the material earned over from the neutralizing bath, after which I permit the metal to dry. It is then ready for receiving the applied painted finish.

If desired, all baths may be used hot, in order to speed up the operations, but I prefer to use only the acid bath heated, and to dry the metal which is leaving the water bath at an elevated temperature in an oven.

Metal treated according to my process is free from oil and grease and will not show a water-break if dipped into water; it has no tendency to rust from the after effects of my preparing process and, in fact, will remain for long periods without protection, if not exposed to the weather, without showing signs of rust. When painted finishes, such as paint, lacquer, japan or varnish is applied to metal prepared by my process, it is unusually permanent and does not show development of defects. As the process as described may be carried out with the use of baths, it is evident that the metal parts to be treated may be hung on a continuous chain and be made to pass automatically from bath to bath and thus eliminate much handling and the accompanying expense.

Having now described my invention, I claim.:

1. The method of preparing metal for painting, consisting in coating the metal with phosphoric acid and treating the metal with a material adapted to convert phosphoric acid into a hydrogen phosphate.

2. The method of preparing metal for painting, consisting in coating the metal with phosphoric acid and thereafter treating the metal with a non-caustic neutralizing material. p

3. The method of preparing metal for painting, consisting in coating the metal with phosphoric acid and treating the metal with a material adapted to convert phosphoric acid into a hydrogen sodium phosphate.

4. The method of preparing metal for painting, consisting in coating the metal with phosphoric acid and thereafter treating the metal with di-sodium phosphate.

5. The method of preparing metal for painting, consisting in first removing any oil from the surface thereof, then coating the .metal with phosphoric acid and thereafter neutralizing the phosphoric acid with a noncaustic material.

6. The method of preparing metal for painting, consisting in heating the metal to a temperature adapted'to remove oil from the surface thereof, coating the metal with phosphoric acid and thereafter treating the metal with a non-caustic material adapted to neutralize phosphoric acid.

7. The method of preparing metal for painting, consisting in heating the metal to a temperature adapted to remove oil from the surface. thereof, coating the metal with controlled phosphoric acid, treating the metal with di-sodium phosphate in the presence of an acid indicator and removing the di-sodium phosphate from the metal together with the deleterious products of its reaction.

7 Signed at New Yorkin the county of New York and State of New York this 1st day of October, A. D. 1928.-

JAMES H. GRAVELL.