Patent GB-1349827-A (GB)
Title: Method of controlling the stability of metal coating compositions
IPC: B05D7/14
Filed: 1972-06-12
Abstract: 1349827 Coated metal AMCHEM PRODUCTS Inc 12 June 1972 [14 June 1971] 27320/72 Heading B2E [Also in Division C3] A coating of a film-forming material is applied to the surface of a metal susceptible to acid-etching by contacting the surface with a coating composition comprising the filmforming material dispersed, dissolved or emulsified in an aqueous phase containing a metaletching acid, an oxidising agent and a dispersing agent, for a period sufficient to enable acid attack upon the metal surface to generate metal ions which are oxidised by the oxidising agent as they enter the aqueous phase to form destabilizing ions therein which tend to render the coating composition unstable and thus cause the coating composition to coagulate, the stability of the coating composition being maintained by (a) adding dispersing agent to the coating composition continuously or intermittently so as to match the metal ion concentration build-up, and/or (b) by removing metal ion so as to prevent the concentration of metal ion from building up, the effective concentration of destabilizing metal ions being maintained at not more than 3 grams per litre when using technique (a) alone or in conjunction with technique (b) and at not more than 1.5 grams per litre when using technique (b) alone. Preferably the metal ions are removed from the coating composition by precipitation as a hydroxide, the preferred precipitating agent being calcium hydroxide. Preferred film-forming resins are polyethylene, polytetrafluoroethylene, polyacrylic resins, polyvinyl chloride, styrene-butadiene copolymers. The dispersing agents may be anionic or non-ionic surface active agents, e.g. sulphuric acid esters, sulphonic acids, surface active agents containing amine and carboxyl groups, esters and amides. The acid may be sulphuric, hydrochloric, hydrofluoric, nitric, phosphoric, hydrobromic or hydroiodic acid, or acetic, chloroacetic, trichloroacetic, lactic, tartaric, polyacrylic acid, fluoboric, fluotitanic and fluosilicic acid. An acid which contains an oxidizing anion can serve as the acid and the oxidizing agent, e.g. nitric acid. Preferably the pH of the coating composition is 1.6-3.8. The oxidizing agent may be perborate, bromate, permanganate; nitrite, nitrate, chlorate, preferably added as an alkali metal or ammonium salt. Preferred oxidizing agents are hydrogen peroxide and dichromate. Dichromate can be added as chromic acid, potassium dichromate, magnesium dichromate, potassium chromate, sodium chromate and preferably calcium dichromate or an aqueous solution containing chromic acid and a calcium salt, e.g. calcium carbonate by employing an aqueous solution made from potassium dichromate and calcium acetate. Preferably the composition also contains fluoride ion, e.g. by adding hydrofluoric acid. The preferred coating composition contains (a) 5-550 grams per litre of resin dispersed in the composition, the source of the resin being a latex, (b) 0.4-5 grams per litre of fluoride ion, (c) a peroxidizing agent in amount to provide 0.01-0.2 gram equivalent of oxidizing equivalent per litre of composition, and (d) acid in amount to give the composition pH 1.6-3.8. Other optional ingredients include coalescing agents, wetting agents (over and above any wetting agent which may be present in the source of the coating-forming material), water-dispersable pigment. The metal surface may be a ferrous metal or zinc surface (which may be either a simple zinc surface or a zinc-coated ferrous metal surface), or a metal surface having a chemical or electrochemical conversion coating or other non-siccative coating, e.g. phosphate, chromate, oxalate, or oxide (anodized or chemically converted) coatings. The coating composition may be applied by spraying, immersing and preferably relative motion is maintained between the coating composition and the metal surface during the coating process. Preferably resin coatings are subsequently fused.