Patent GB-1048891-A (GB)
Title: Improvements relating to methods of making microporous rubber sheet material
IPC: C08J9/26
Filed: 1964-02-03
Abstract: Microporous natural or synthetic rubber sheet material may be produced by a method which includes adding a dispersion of starch in water to a dispersion of uncured rubber particles in water, applying the rubber and starch dispersion and a coagulant to a surface and permitting the mixture to gel irreversibly to form a sheet having a skeleton of rubber, drying the sheet at a temperature below that at which the starch is gelatinized to avoid cracking of the skeleton of rubber gel, thereafter subjecting the sheet to a temperature adequate to cure the rubber, swelling the starch in the rubber sheet, and removing the starch and moisture to form pores.  Synthetic rubbers specified are styrene-butadiene copolymers and polychloroprene.  Two types of coagulant are mentioned (a) of the delayed action type, e.g. sodium or potassium silicofluoride which are mixed with the rubber and starch dispersion and thereafter spread on the surface, (b) of the instantaneous type, e.g. calcium nitrate, magnesium acetate, aluminium sulphate or zinc chloride which are applied to the surface and allowed to dry before the rubber and starch dispersion is added to it.  The starch may be of any vegetable source such as potato, maize, rice or wheat.  Dispersing agents may be added to the starch dispersion, e.g. compounds in the nature of sodium salts of polymerized alkyl naphthalene sulphonic acids having short alkyl chains and also polyelectrolytes.  The latex dispersion may include curing agents such as sulphur and zinc oxide and accelerators such as zinc mercaptobenzothiazole and zinc diethyl dithiocarbamate.  The drying temperature is in the range of 40 DEG  C. to 50 DEG  C. and the curing temperature in the range of 100 DEG  C. to 150 DEG  C. Swelling of the starch may be by mechanically rolling the sheet material and then immersing it in boiling water for two hours.  The starch is then removed by immersion in boiling dilute acid, e.g. 2% sulphuric acid for two hours or by enzymatic degradation, e.g. 24 hours at 50 DEG  C. in a 5% slurry of diastase trypsin or pepsin, followed by rinsing in water and drying at about 50 DEG  C.  Examples I and II describe the preparation of a rubber sheet and a rubber girdle respectively.ALSO:Fabrics may be coated by adding a dispersion of starch in water to a dispersion of uncured rubber particles in water, applying the dispersion and a coagulant to a fabric surface and permitting the mixture to gel irreversibly, drying the structure at a temperature below that at which the starch is gelatinized to avoid cracking of the skeleton of rubber gel, thereafter subjecting the structure to a temperature adequate to cure the rubber.  Swelling the starch in the rubber and removing the starch and moisture from the rubber to form pores in it.  The rubber may be natural rubber or synthetic rubber e.g. a butadiene-styrene copolymer or a polychloroprene.  Preferably, the coagulant is applied to the surface and allowed to dry to a film before the rubber and starch dispersion is applied to it.  The starch is suitably removed by first degrading it e.g. by an acid treatment or by enzymatic degradation, the structure is mirsed in water to remove the degradation products and then dried.