Patent GB-808964-A (GB)
Title: Improvements in the production of articles of expanded rubber
IPC: C08L21/00
Filed: 1957-04-10
Abstract: A process for the production of expanded rubber articles by foaming up a rubber latex by stirring air or other gases into the latex, filling the resultant foam into a mould and then coagulating and vulcanizing, comprises using a latex which has been obtained by graft polymerization in aqueous emulsion by first polymerizing to a conversion of 5 to 20 per cent by weight a monomer mixture of 75 to 100 per cent by weight of styrene and 25 to 0 per cent by weight of butadiene in aqueous emulsion and then further polymerizing to a conversion of 60 to 85 per cent by weight after the addition of such an amount of monomeric butadiene that the weight ratio of the total amount of butadiene present to styrene amounts to 50:50 to 70:30. The latex may be used in admixture with natural rubber latex.  In the polymerization, emulsifying agents which are preferably employed are salts of carboxylic acids (especially resin acids such as dehydroabietic acid) and of sulphonic acids, and alkyl sulphuric acid esters. Suitable polymerization catalysts are watersoluble inorganic per-salts, e.g. potassium persulphate, which can be used in conjunction with reducing agents such as sodium bisulphite.  The latices obtained can be concentrated by creaming with alginate or polyacrylic acid salts or by evaporation.  Before or during foaming there are added coagulants, such as fluosilicci acid salts and vulcanizing agents such as sulphur, zinc oxide and ultra accelerator. Colour pigments, fillers and stabilizers can also be added.  In examples: (1) a mixture of 5 kg. styrene and 550 g. butadiene, emulsified in an aqueous solution of sodium dehydroabietate, caustic soda, sodium sulphate and potassium persulphate is polymerized at 45 DEG  to 15 per cent conversion, 4.45 kg. butadiene is added and the polymerization continued to 70 per cent conversion.  The latex obtained (which may be stabilized with distyryldiphenylamine, phenyl b -naphthylamine or 2,21-methylene-bis-(4-methyl-6-tertiary butyl phenol)) is mixed with natural rubber latex and sodium silicofluoride, sulphur, zinc oxide and tetramethylthiuram disulphide.  The mixture is foamed by stirring in air, the foam is put into a mould and after coagulation is vulcanized by heating.  (2) Different proportions of reactants are employed in a process similar to (1), the vulcanizing agents being employed as a paste with polyvinyl alcohol.ALSO:A process for the production of expanded rubber articles by foaming up a rubber latex by stirring air or other gases into the latex, filling the resultant foam into a mould and then coagulating and vulcanizing, comprises using a latex which has been obtained by graft polymerization in aqueous emulsion by first polymerizing to a conversion of 5 to 20 per cent. by weight a monomer mixture of 75 to 100 per cent. by weight of styrene and 25 to 0 per cent. by weight of butadiene in aqueous, emulsion and then further polymerizing to a conversion of 60 to 85 per cent. by weight after the addition of such an amount of monomeric butadiene that the weight ratios of the total amount of butadiene present to styrene amounts to 50 : 50 to 70 : 30.  The latex is preferably used in admixture with natural rubber latex.  Before or during foaming there are added coagulants such as fluosilicic acid salts and vulcanizing agents such as sulphur, zinc oxide and ultra accelerator (e.g. tetsamethyl thiuram disulphide) which may be added as a paste with polyvinyl alcohol.