GB718161A - Improvements in or relating to electric heating elements - Google Patents

Abstract

An electric heating element comprises a base material carrying a hardened dimensionally stabilized thin electrically conductive film consisting of an initially fluid non-conductive organic binder cured to permanently solid form and containing flaky metal particles not above 40 microns in size and flaky graphite particles under 10 microns in size, the film resulting from curing the binder to non-meltable form with the conductor flakes maintained in contacting, interfitting relation in the binder. The binder comprises a thermosetting resinous plastic material, e.g. a silicone resin, or an alkyd, phenolic, furfural, or vinyl resin, where applied to a rigid member, or may be flexible where applied to a woven fabric or tape, and may comprise a silicone rubber, natural rubber, buna N or neoprene. The metal particles may be silver, nickel, zinc or an alloy, e.g. stainless steel and are preferably under 15 microns in size. In order to increase the resistance of the film, metal oxide particles 15 to 40 microns in size are uniformly distributed therein, e.g. antimony oxide, titanium dioxide, zinc oxide, chromic oxide or ferric oxide. The graphite particles are preferably of 1-5 microns. The graphite and the metal oxide have a negative temperature coefficient and the metal has a positive temperature coefficient, and the whole resistance has a small positive temperature coefficient, e.g. 0.004 per 1 DEG C. A fire retardant material normally stable but producing a flame quenching gas, may be distributed in the binder and may comprise a chlorinated hydrocarbon plasticizer, e.g. chlorinated polyphenyl to release chlorine at 600 DEG F. The metal oxide also acts as a fire retardant. Examples of proportions of the ingredients used are given. Thus the film may comprise 30 parts amorphous graphite; 30 parts antimony oxide, 40 parts silver, 8 parts chlorinated hydrocarbon and 80 parts alkyd resin. Another example is 11.5 parts nickel, 68 parts zinc, 10 parts graphite, 4 parts chlorinated hydrocarbon and 80 parts alkyd resin. The film is applied to a surface, e.g. a wall or an aircraft surface to be kept free of ice, by spraying or knife spreading, preferably after thinning with a solvent such as a hydrocarbon xylene, alcohol, ketone, ester or ether. The film is then heated to drive off the solvent and cure the resin and is preferably 0.001-0.025 ins. thick. If the surface to be coated is electrically conductive, it has first applied thereto an insulating layer comprising a binder as set out above mixed with mica and vermiculite. The layer is thermally cured and when cured some of the particles therein migrate into the lower layer.