Patent GB-693819-A (GB)
Title: Improvements in or relating to sealed electrical apparatus
IPC: H01F41/12
Filed: 1952-04-09
Abstract: A transformer is completely sealed by filling irregularities with a pasty mass which is hardened to a porous solid body.  It is then coated with a resinous material through which spaces are left, and finally impregnated with a polymerizable liquid resin. The pasty mass comprises a mixture with 1-3 parts by volume of a volatile solvent, of one part of fibrous insulating material such as shredded asbestos fibres, or chopped glass fibres, 1-5 parts of a finely-divided inorganic solid such as powdered silica, and 0.01 to 0.1 parts of a hardenable resinous binder.  Suitable materials are, cellulose acetate, cellulose acetate-butyrate, ethyl cellulose, cellulose propionate, polyvinyl acetate, polyvinyl butyral, glycerol phthalate and glycol maleate.  The external coating may comprise powdered mica, shredded asbestos fibres or chopped glass fibres, mixed with a thermosetting composition, e.g. one made by heating mixed castor oil and maleic anhydride to form castor-oil maleate which was dissolved in monomeric styrene with a small addition of hydroquinone, and powdered mica was added, followed by benzoyl peroxide catalyst. Drying oils such as linseed, peanut, perilla, soybean, cotton-seed, corn, and cashew-nut shell may be added to the original materials. The coating may comprise a single polymerizable component such as diallyl phthalate, diallyl succinate, diallyl maleate, diallyl adipate, allyl alcohol, methallyl acrylate, diallyl ether, allyl acrylate, or allyl crotonate, or a mixture of unsaturated polymerizable liquid monomer and an unsaturated alkyd resin such as the reaction product of an ethylenic dicarboxylic acid or anhydride thereof, e.g. maleic acid, fumaric acid, maleic anhydride, monochloromaleic acid, itaconic acid, itaconic anhydride, citraconic acid and citraconic anhydride with a polyhydric alcohol such as glycol, glycerol or pentaerythritol.  In preparing unsaturated alkyd esters, the unsaturated alpha-beta dicarboxylic acid or anhydride may be replaced with up to 90 per cent of its weight by saturated aliphatic dicarboxylic acid or aryl dicarboxylic acid or anhydride such as succinic acid, adipic acid, sebacic acid, phthalic acid, and anhydride.  Also mixtures of polyhydric alcohols may be employed. Epoxides may replace glycols.  The alkyd esters may be dissolved in a liquid unsaturated monomer having the group H2C=C<, such as monostyrene, alpha methyl styrene, 2,4-dichlorostyrene, paramethyl styrene, vinyl acetate, methyl methacrylate, ethyl acrylate, diallyl phthalate, diallyl succinate, diallyl maleate, allyl alcohol, methallyl alcohol, acrylonitrile, methyl vinyl ketone, diallyl ether, vinylidene chloride, butyl methacrylate, allyl acrylate, allyl crotonate, 1,3-chloroprene and divinyl benzene.  Catalysts such as benzoyl peroxide, lauroyl peroxide, acetyl peroxide, ascaridole, and tertiary-butyl hydroperoxide, and inhibitors such as hydroquinone, tannin phenolics, and benzaldehyde may be added.  The impregnating material may be that of the coating without the solid filling but with more polymerizable monomer.  To create the spaces through the coating needed for impregnation strips of neoprene rubber, silicone rubber, polytetrafluoroethylene or cellulose acetate may be used.