Patent GB-719729-A (GB)
Title: Improvements in the production of porous shaped articles true to shape and size fromsynthetic thermoplastic materials
IPC: B29C44/00
Filed: 1952-04-24
Abstract: Discrete particles of thermoplastic synthetic material which contain a gas that is under pressure are introduced into a perforated mould, and the particles are heated in the mould to a temperature above the softening point of the material, so that the particles are expanded by the said gas under pressure and sintered.  When the material is gas-containing polystyrene the expansion may be carried out in hot water or steam at 100-140 DEG  C., or in glycerine at 100-130 DEG C., or by heating with infra-red rays or hot air.  When the material is gas-containing polyamide the expansion is effected in hot hydrocarbon, e.g. paraffin oil.  The discrete particles may be made (a) by heating a mixture of the synthetic thermoplastic material and a raising agent in a closed mould, if desired under internal gas pressure, to a temperature above the softening point of the material, cooling the mixture and comminuting it; or (b) by treating melts of the materials with neutral gases (e.g. nitrogen) under pressure, cooling and comminuting; or (c) by mixing the material in comminuted form with liquid or solid gas, e.g. liquid air or solid carbon monoxide, heating the mixture under pressure, cooling and comminuting.  The synthetic thermoplastic material may be polystyrene, polyvinyl chloride, polyvinylidene chloride, polyvinyl carbazole, polyvinyl esters of organic acids, polyacrylic acid and polymethacrylic acid, polyamides or polyurethanes.  The raising agent may be bicarbonate, azo compound or hydrazine compound.  Dyestuffs, softeners, pigments and substances yielding non-combustible gases when they burn may be added.  Examples are given describing the making of lightweight dolls, balls and plates; and other articles are mentioned.ALSO:Discrete particles of thermoplastic synthetic material which contain a gas that is under pressure are introduced into a perforated mould, and the particles are heated in the mould to a temperature above the softening point of the material, so that the particles are expanded by the said gas under pressure and sintered. When the material is gas-containing polystyrene the expansion may be carried out in hot water or steam at 100-140 DEG  C., or in glycerine at 100-130 DEG  C., or by heating with infra-red rays or hot air. When the material is gas-containing polyamide the expansion is effected in hot hydrocarbon, e.g. paraffin oil.  The discrete particles may be made (a) by heating a mixture of the synthetic thermoplastic material and a raising agent in a closed mould, if desired under internal gas pressure, to a temperature above the softening point of the material, cooling the mixture and comminuting it; or (b) by treating melts of the materials with neutral gases (e.g. nitrogen) under pressure, cooling and comminuting; or (c) by mixing the material in comminuted form with liquid or solid gas, e.g. liquid air or solid carbon monoxide, heating the mixture under pressure, cooling and comminuting.  The synthetic thermoplastic material may be polystyrene, vinyl or acrylic resins, polyamides or polyurethanes.  The raising agent may be bicarbonate, azo compound or hydrazine compound. Dyestuffs, softeners, pigments and substances yielding non-combustible gases when they burn may be added.  Examples are given describing the making of lightweight dolls, balls and plates; and other articles are mentioned.