Patent GB-1395998-A (GB)
Title: Process for finishing metal substrates
IPC: C25D13/06
Filed: 1972-09-28
Abstract: 1395998 Coating metals E I DU PONT DE NEMOURS & CO 28 Sept 1972 44870/72 Heading B2E A process for finishing a metal, e.g. steel) substrate comprises (1) electrodepositing a primer coating of a carboxylic polymer on the metal substrate by immersing the substrate in a coating bath of an electrocoating cell containing a cathode the substrate forming the anode, passing a direct current at 50 to 600 volts through the cell to deposit a continuous film on the substrate removing the coated substrate from the bath, then washing it with water and baking it at 150 to 300‹C; wherein the coating bath comprises an aqueous dispersion of a film-forming-constituency having a solids content of 3 to 30% and a pH of 7 to 9.5, the film forming constituent consisting of 50 to 100% by wt of a carboxylic polymer having an acid number of 5 to 200 and correspondingly from 0 to 50% by wt of a cross linking agent and the polymer being neutralised with a water soluble basic compound; (2) applying a coating of a sealer composition consisiting of 50-95 wt % epoxy resin (Mw 20,000-200,000) and 50-5% of an acrylic polymer; (3) applying a coating of an acrylic organosol laquer over the sealer coating and baking the coating to form a glossy, smooth finish 1 to 5 mils thick; wherein the acrylic organosol laquer comprises 10-60 wt % of film forming constituents dispersed in an organic liquid, the film forming constituents consisting of a graft copolymer comprising a backbone (A) and one or more side chains (B), the backbone (A) being a polymeric segment of units (a) and up to 15 wt % of units (b), said (b) units providing available active grafting sites for graft polymerisation, and each side chain (B) being a polymeric segment, (A) and (B) being different, and (a) and (B) being composed of at least 50 wt % of units of esters of acrylic, methacrylic and/or ethacrylic acid, provided 'that either (A) or (B) if prepared as a separate polymeric entity, is substantially soluble in said organic liquid, whereas the other of (A) or (B) if prepared as a separate polymeric entity is substantially insoluble in the same organic liquid. The metal substrate is preferably phosphatised before electrocoating Typical examples of carboxylic acid polymers for use in the primer coating are alkyd, epoxy ester, or acrylic resins, the reaction products of a dicarboxylic acid anhydride and a drying oil, epoxidised drying oils, and styrene/ allyl alcohol copolymers reacted with a maleinised drying oil fatty acid. Lists of examples of each kind of resin are given. As cross-linking agent for the acid polymer the following may be used: melamine formaldehyde, alkyloated melamine formaldehyde, tetrakisethoxymethylbis-methoxymethyl melamine, urea formaldehyde, benzoguaramine formaldehyde, melamine toluene sulphamide resins and mixtures thereof, or polyamines or polyhydroxyl terminated compounds. There may be present a pigment, such as carbon black, iron oxide, barium chromate, lead silicate, or strontium chromate, and a surfactant, particularly a non- ionic surfactant. The sealer composition may be applied by spraying, electrodeposition, dipping, brushing or flow-coating. The acrylic organosol laquer may be applied to a wet or dried sealer, by spraying, electrostatic spraying, dipping, brushing, or flow coating. Up to 50% of the (a) and (B) portions of the acrylic organosol laquer resin may consist of another monomer not contributing grafting sites, such as: acrylic acid; methacrylic acid and their amides and nitriles; vinyl acetate; styrene; α-methyl styrene; vinyl toluene; or 3-(2-methacryloxyethyl)-2, 2-spirocyclohexyloxazolidine. The (b) portion consists of monomer units providing grafting sites. Examples of such monomer units are those having the general formula, where X may be (where the C atom is attached to the backbone), Y may be -H, -CH 3  or -CH 2 CH 3 , and Z may be  where R 2  and R 3  may be Y, where R 4  and R 5  are: hydrogen; C 1- C 4  alkyl; -CH 2 -CH 2 -OR 6  where R 6  is C 1 -C 4  alkyl; provided that when X is  Z can be -H or -NH-CH 2 -CH=CH 2  and provided further that the combine -X-Z group may be, Allyl methacrylate and diethylaminoethyl methacrylate are preferred. The organosol may be pigmented with pigments such as metallic oxides (e.g. TiO 2 , ZnO), metal hydroxides, metal flakes, metal powders, chromates (e.g. lead chromate), sulphides, sulphates, carbonates, carbon black, silica, talc, china clay, iron blues, or organic reds or maroons. A list of suitable plasticisers is given. The resin may also be clear.