Patent GB-679539-A (GB)
Title: Improvements in electrostatic capacitors
IPC: H01G4/22
Filed: 1948-07-27
Abstract: An electrostatic capacitor comprises electrodes separated by an interspacer impregnated with the solid, substantially insoluble and infusible copolymerization product of a poly-unsaturated alkyd resin initially reacted to an acid member below 150, alcoholised to reduce the acid number below 75 and acylated to render it devoid of free hydroxyl radicals, with a liquid copolymerizable monomer in the proportion of 20-70 per cent by weight of the composition.  Saturated polyhydric alcohols which may be used for making the alkyd are ethylene glycol; 1,2-propylene glycol; isobutylene glycol; trimethylene glycol; 1,4-butylene glycol; phthalyl alcohol; glycerol; a -methyl-glycerol; pinacol; sorbitol; mannitol; erythritol; pentaerythritol; diethylene glycol; pentamethylene glycol; trethylene glycol; tetramethylene glycol; unsaturated polyhydric alcohols are vinyl glycol or the compound HOCH2CH=CHCH2OH.  Suitable unsaturated acids are maleic, fumaric, itaconic, citraconic, acetylene dicarboxylic, glutaconic, methylene malonic, or mesaconic.  Acids such as phthalic, succinic, adipic, azalic, sebacic or malonic may also be included.  Monohydric alcohols or acids, saturated or unsaturated may be incorporated in the reaction mixture as modifying ingredients.  Lists are given of suitable ingredients of this type. Acids suitable for the final acylation of the alcoholised alkyd are acetic, propionic, acrylic, methacrylic, oleic, cinnamic, benzoic or chloracetic.  Suitable copolymerizable monomers are styrene; vinyl acetate; acrylonitrile; methyl, butyl, allyl, crotyl or methallyl acrylate, a -chloracrylate or methacrylate; glycol diacrylate; diallyl phthalate; diallyl, dimethallyl, divinyl, di-isoprophenyl or dicrotyl carbonate or other unsaturated alcohol polyesters of polybasic acids.  The resin or resin mixture may be copolymerized by heat or light, preferably with peroxide catalysts, several of which are mentioned.  Rate of polymerization may be increased by addition of activators, e.g. cobalt linoleate, cobalt naphthenate, cuprous oxide, trithanolamine or stannous chloride.  An inhibitor such as tert.-butyl catechol, diphenylamine, or hydroquinone may be introduced to preclude premature polymerization.  In examples: (1) ethylene glycol and maleic anhydride are reacted, the product reacted with methanol, and, after removal of excess methanol with acetic anhydride; (2) diethylene glycol and maleic anhydride are reacted, the product heated with n-propanol, excess of which is removed by distillation and the product esterified with acetic anhydride; (3) the methylated ethylene glycol maleate of example (1) is esterified with oleic acid; (4) phthalic anhydride, maleic anhydride and ethylene glycol are reacted together, the product treated with n-propanol and then with acetic anhydride; (5) is similar to (4) with hydroquinone present during the initial reaction.  Capacitor sections may be impregnated, preferably after drying with the compositions of the invention.  Vacuum and pressure may be used to aid impregnation. Excess resin may be removed by washing the sections with styrene. The sections may be encased preferably in a casing moulded from a resin similar to that used as dielectric in which case both resins may be cured at the same time (see Group V).  The resin for the casing preferably contains no inhibitor and is mixed with a suitable filler, e.g. calcium carbonate, magnesium carbonate, silica or diatomaceous earth.  Specifications 600,917 and 644,287 are referred to.