TOLE METALLIQUE PROTEGEE ET SON PROCEDE DE FABRICATION

1402941 Coated galvanised sheet steel: coat ing apparatus ROBERTSON BAUELEMENTE GmbH 25 April 1973 [1 May 1972] 19533/73 Headings B2E and B2L [Also in Division C3] A post-formable protected galvanised steel sheet comprises a galvanised steel core having a wash primer coat applied to at least one surface of the galvanised core and a weather resistant hot-melt alkyd resin coat applied to the wash primer. Preferably a roll 10 of galvanised steel sheet is unwound and cut by a guillotine 11 into sheets 12a which are heated to 60-100‹C, supported and impelled by an uprush of hot gases from burners 20 in a chamber 13. The sheets are coated on both sides with a wash primer in a volatile solvent by rollers 14, the thickness of the coating being governed by doctor rolls 22, 25, Fig 5, Flashing off of solvent is aided by passing the coated sheets through a first drier 15 wherein the sheets are supported, heated to above the solvent flash temperature and impelled by an uprush of warm recycled air which passes through a perforated plate 32 from plenum chamber 31. The primer is reheated to 60- 105‹C in a chamber 16 wherein heat is supplied by combustion gases passing through a perforated base then through a flue 34 to a pump 29 then a pipe 30 to be recycled through chamber 15. The heated sheet is hot-melt coated with the alkyd resin by rollers 17 before being quenched in a water tank at a temperature at least 50‹C below the softening point of the hot-melt coating. The hot melt alkyd may cross-link in air so that after a period of exposure, it becomes non-softenable, and the sheets non-postformable. The primer system preferably, includes polyvinyl butyral, zinc tetroxy chromate, solvents and aqueous phosphoric acid (see Division C3). The primer may include modifying resins such as melamine, phenolic resins, vinyl ethers, phenoxy resin, epoxy resin or alkyd resins. The primer is applied as a dispersion in an organic solvent. The alkyd resin is preferably formed from the esterification of unsaturated aliphatic acids (e.g. tall oil fatty acids, oleic acids, dehydrated castor oil, tung oil, linseed oil), alkyl (meth)- acrylates, dicarboxylic acids such as phthalic aci dor triols such as trimethylol ethane. The resin may also contain waxes, wetting agents, driers (e.g. cobalt naph'thenate), pigments, or inert fillers (e.g. crushed limestone, talc or short asbestos fibres).