Patent GB-1355331-A (GB)
Title: Process for preparing insulating substrates
IPC: H05K3/18
Filed: 1970-02-18
Abstract: 1355331 Laminates FORMICA INTERNATIONAL Ltd 19 April 1971 [18 Feb 1970 17 Feb 1971] 7744/70 and 15562/71 Divided out of 1355331 Heading B5N [Also in Divisions B2 and D1] An insulating substrate is obtained by treating a fibrous sheet, mat or web which is a paper sheet or web, a woven cotton sheet or web or a glass fibre woven sheet or glass fibre mat with a synthetic rubber composition comprising a mixture of (1) a selectively or uniformly etchable rubber polymer and optionally a volatile medium and (2) a thermosettable resin which is a polyester, epoxy or methacrylic resin or a resin derived from formaldehyde and phenol, urea or melamine, and heating the treated sheet or web under presusre to cure the thermosettable resin and remove and residual volatile medium, so as to provide on the surface of the substrate a selectively etchable rubber composition. The product may be obtained by treating the sheet, mat or web with a synthetic rubber composition comprising a mixture of the thermosettable resin substantially in the uncured or A stage and a uniformly or selectively etchable rubber polymer and heating the treated substrate under pressure. Preferably, the mixture comprises a volatile solvent(s) and the treated product is dried to remove most of the solvent(s) so that the sheet is non-tacky, but without curing the resin substantially beyond the B stage prior to heating under pressure. In a second embodiment the fibrous sheet, mat or web is treated with a solution or dispersion of any thermosettable resin and dried to remove most of the solvent and without curing the resin substantially beyond P stage, one or both sides of the sheet, mat or web is provided with a layer of a mixture of a selectively or uniformly etchable rubber polymer and the thermosettable resin and the fibrous sheet, mat or web is then heated under pressure to consolidate the assembly and provide on one or both major surfaces a layer of selectively etchable rubber composition. In a third embodiment a fibrous web or sheet is treated with a thermosettable resin varnish and dried without curing the resin substantially beyond the B stage. A second fibrous sheet, mat or web as defined above is treated with a mixture of a selectively or uniformly etchable rubber polymer and the thermosettable resin and a volatile solvent and the second sheet is dried. The two dried sheets, mats or webs are superimposed and the assembly so produced is consolidated by heating under pressure. The pressing is preferably performed using planing press plates so as to produce planar surfaces in the laminate. A release sheet may be interposed between the surface of the lamina and the press plate or the latter may be coated with a release medium. Suitable synthetic rubber polymers are butadiene polymers and neoprene polymers such as acrylonitrile-butadiene-styrene terpolymers or acrylonitrile-butadiene or butadienestyrene copolymers. In the first embodiment the mixture used preferably contains 40-95% by weight of rubber solids and the amount applied to the sheet is preferably sufficient to give 20-70% by weight of mixture solids based on the weight of the dried sheet, mat or web. In the second embodiment the amount of resin applied is preferably sufficient to give 20-70% by weight of resin solids based on the weight of resin-treated fibrous material. The layer of rubber composition may be applied by roller coating, curtain coating or by the use of a transverse sheet process or by the use of a preformed film. Sufficient rubber composition should be applied to provide a thickness after pressing of 0À025-0À1 mm. In the third embodiment the resin-treated sheet perferably contains 30-55% by weight of resin when the fibrous material is cellulosic and 35-55% by weight of resin when it is glass. Preferably the rubber compositiontreated fibrous sheets contain 30-55% by weight of rubber composition solids based on the weight of the rubber solvent-free impregnated sheet. In a modification of the embodiments the curing stage is suitably performed on a stack of thermosettable resin-impregnated fibrous sheets, at least one of the surface sheets being impregnated or coated with the rubber composition. Consolidation of the stack by a single heating and pressing operation gives a laminate having on at least one of its major surfaces a selectively etchable rubber composition. Alternatively, a laminate may be prepared by heating under pressure a stack of fibrous sheets at least one of the outer sheets being of the type specified above impregnated and/or coated with a mixture comprising both a thermosettable resin as specified above and an etchable rubber polymer to give on at least one of the major laminated surfaces a selectively etchable rubber composition. Alternatively one or two webs, mats or sheets as specified above treated with the synthetic rubber composition are superimposed on respectively, one or both major surfaces of a further fibrous sheet or web or of a stack of further sheets or webs, the further sheet(s) or web(s) being impregnated with a thermosettable resin, and the assembly so produced is consolidated by heating under pressure, or wherein the web, mat or sheet as specified above treated with the synthetic rubber composition or a stack of such webs, mats or sheets is superimposed on a metal sheet and the assembly so produced is consolidated by heating under pressure. An insulating substrate comprising a metal sheet having an insulating coating thereon can be produced by a process in which a fibrous sheet, mat or web as specified above is treated with the synthetic rubber composition or a stack of said webs, mats or sheets, is superimposed on a metal sheet, and the assembly so produced is consolidated by heating under pressure. The heating under pressure may be carried out in a conventional press. A suitable cure cycle is 10-60 minutes at 120-180 C. and 35-63 kg/cm.<SP>2</SP>