Patent GB-821390-A (GB)
Title: Cyclic foaming method of making cellular plastic bodies
IPC: C08J9/14
Filed: 1957-09-10
Abstract: A method for increasing the foamed volume of a closed cell, foamed, thermoplastic resin mass which contains in its cell a gas which is a non-solvent or a poor solvent for such resin comprises heating such foamed mass to a heat plastifying temperature, for a time sufficient to plastify the resin but below a temperature at which the foam would be collapsed or the resin would be decomposed, to expand such foamed mass, and cooling and exposing such heat expanded mass to an atmosphere of a gas which has a permeability rate greater than the permeability rate of the primary foaming gas initially present through the cell walls of the foamed mass for a time sufficient to permit substantial diffusion inward of the latter, and repeating said cycles of heating and cooling with exposure to an atmosphere of said second more permeable gas for a plurality of cycles whereby the volume of the foamed mass is continually increased to a desired volume in excess of that obtainable from the heat expansion of the primary gas alone.  Suitable blowing-agents for producing the foamed product in the first stage are mono- or dichlorodifluoromethanes and symmetrical dichlorotetrafluoroethane.  For the second stage suitable gases are air, helium, nitrogen and carbon dioxide. Resins mentioned are polystyrene and its copolymers with vinyltoluene or divinylstyrene; polyvinyltoluene; polydichlorostyrene and polymethylmethacrylate.  The resin may be in the form of particles, sheets or moulded articles.  Examples illustrate the application of the process to foamed materials which include a copolymer of styrene, butadiene and divinylbenzene, and also the product obtained by polymerizing with a benzoyl peroxide catalyst methyl methacrylate in the presence of dichloro-difluoromethane at 80 DEG  C. and foaming the product at 125 DEG  C.ALSO:A method for increasing the foamed volume of a closed cell, foamed, thermoplastic resin mass which contains in its cell a gas which is a non-solvent or a poor solvent for such resin comprises heating such foamed mass to a heat plastifying temperature, for a time sufficient to plastify the resin but below a temperature at which the foam would be collapsed or the resin would be decomposed, to expand such foamed mass, and cooling and exposing such heat expanded mass to an atmosphere of a gas which has a permeability rate greater than the permeability rate of the primary foaming gas intially present through the cell walls of the foamed mass for a time sufficient to permit substantial diffusion inward of the latter, and repeating said cycles of heating and cooling with exposure to an atmosphere of said second more permeable gas for a plurality of cycles whereby the volume of the foamed mass is continually increased to a desired volume in excess of that obtainable from the heat expansion of the primary gas alone. Suitable blowing agents for producing the foamed product in the first stage are mono- or dichlorodifluoromethanes and symmetrical dichlorotetrafluoroethane.  For the second stage suitable gases are air, helium, nitrogen and carbon dioxide. Resins mentioned are polystyrene and its copolymers with vinyltoluene or divinylstyrene; polyvinyltoluene; polydichlorostyrene and polymethylmethacrylate. The resin may be in the form of particles, sheets or moulded articles.  Examples illustrate the application of the process to foamed materials which include a copolymer of styrene, butadiene and divinylbenzene, and also the product obtained by polymerising with a benzoyl peroxide catalyst methyl methacrylate in the presence of dichloro-difluoromethane at 80 DEG C. and foaming the product at 125 DEG C.