Patent GB-860345-A (GB)
Title: Method of forming shaped articles by bonding inert particles
Filed: 1957-03-11
Abstract: Articles are made from inert particulate matter by coating with a binder to form a porous aggregate, moulding to the required shape under moderate pressure, impregnating with a hardenable liquid epoxy resin composition which will not readily dissolve the binder, and hardening the composition.  Of particular interest as impregnating materials, because of their low viscosities, are mixtures of epoxy resin and alkenyl succinic anhydrides, e.g. tetrapropenyl succinic anhydride.  The binders which may be used include hydrophilic and at least partly and preferably completely, water-soluble polymers of acrylic compounds, e.g. polyacrylic acid, ammonium polyacrylate, and polyacrylamide.  Of particular interest, the tertiary amine salt of polyacrylic acid acts both as a binder for the inert particulate matter, and as a chemical hardening agent for the epoxy resin, and thus no hardening agent has to be incorporated into the mouldable aggregate.  Some compositions harden to a resinous state before they can be made a homogenous liquid, in which case the hardening agent may be distributed throughout the porous shape and the resin alone used as the impregnant.  Melamine is referred to as the hardening agent in this connection.ALSO:Articles are made from inert particulate material by coating the particles with a binder to form a porous aggregate, moulding the aggregate to the required shape under moderate pressure, impregnating with a hardenable liquid synthetic resin composition which will not readily dissolve the binder, and hardening the composition.  The binders which may be used include hydrophilic and at least partly, and preferably completely, water-soluble polymers of acrylic compounds such as polyacrylic acid, ammonium polyacrylate, and polyacrylamide.  Of particular interest, the tertiary amine salt of polyacrylic acid acts both as a binder and as a chemical hardening agent for epoxy resin, and thus is hardening agent has to be incorporated into the mouldable aggregate.  Among the inert particulate materials which may be used as sand, glass beads, cullet, powdered iron, silicon carbide, aluminium, decomposed silicate, vermiculite, perlite, nylon flock, glass fibres, polystyrene beads, and powdered resins such as cured phenolic resins. The average particle size should not exceed 250 microns.  The mould used may be constructed of cheap, easily worked material such as wood.  The impregnating resins used are preferably of the epoxy type, the chemical hardening agents for which should be incorporated in the aggregate rather than in the impregnating liquid.  The impregnation may be accomplished by placing the shape in a shallow bowl of the liquid, whereby the liquid is taken up by capillary action, porous blocks which extend slightly above the resin surface being used to support the shape. One particular application is the encasement of an electrical component in a resinous, moisture-proof, insulating capsule which consists of heat-cured resin filled with minute siliceous particles to which a chemical hardening agent for the resin is bound prior to imprenation by means of a binder insoluble in the resin.  Leads extending through the surface are tightly sealed as the coefficient of thermal expansion of the capsule is of the order of that of metal by virtue of its large proportion of siliceous material.