Process for curing rubber compositions

Hydroxylated carbon blacks suitable for use in rubber compositions and in which the number of hydroxyl groups attached to the carbon particles is less than the maximum possible but at least 0,5% of such maximum are prepared by reacting a carbon black, e.g. a furnace, thermal acetylene, channel or graphitized black, with just sufficient of an aqueous solution of an oxidizing agent, e.g. a persulphate, dichromate, permanganate, hypochlorite, peroxide, or oxyacid such a nitric acid, as to hydroxylate the black to the required degree.ALSO:Rubber compositions of improved abrasion and tear resistance are prepared by heating natural or synthetic rubber to curing temperature, e.g. 130 DEG to 160 DEG C., in the presence of a free-radical curing agent and a partially hydroxylated carbon black in which the number of hydroxyl groups attached to the carbon particles is less than the maximum possible but at least 0,5% of such maximum, preferably up to 60% thereof for natural rubber compositions and up to 80% for synthetic rubber mixes. The hydroxylated carbon blacks are prepared by reacting a carbon black, e.g. a furnace, thermal, channel, acetylene, or graphitized black, with an aqueous solution of an oxidizing agent, e.g. an alkali metal persulphate, or a hypochlorite, permanganate, dichromate, peroxide, or oxyacid. Preferably 10 to 100 parts by weight of hydroxylated black are added to 100 parts of rubber and mixed therewith by normal methods, e.g. milling, or alternatively the black may be mixed with rubber latex and the mix coagulated. Suitable free-radical curing agents are those having the general formula wherein R1 and R6 represent aryl groups, and R2, R3, R4, and R5 are hydrogen or alkyl groups with less than four carbon atoms. Dicumyl peroxide is the preferred such agent and preferably 1 to 5 parts by weight are incorporated per 100 parts by weight of the rubber. Specified synthetic rubbers are polyisoprenes, polychloroprenes, polybutadienes, butadiene-styrene copolymers, acrylonitrile, methyl isopropenyl ketone, butyl rubber and polyurethanes.