Production of Industrial Sponge Rubber

Abstract

Abstract The production of sponge rubber for industrial use not only involves the blowing of a mass of plastic rubber compound to a given shape of predetermined density, with concurrent vulcanization, but also the production of a sponge product which will withstand the same severe conditions as will solid rubber produced by modern compounding. Until recent years, the aging and heat-resisting properties of sponge rubber left much to be desired, mainly because of the large surface area susceptible to oxidation, and the large amounts of oils and softeners used to plasticize the rubber. The oils themselves, far from beneficial to aging properties, necessitated in many cases high sulfur ratios to restore the nerve to the vulcanized sponge. It is, however, necessary to plasticize the raw rubber to obtain satisfactory blowing. Within limits, the pore size and density of a sponge from a given stock are governed to a greater extent by the plasticity of the mixed compound than by the amount of blowing agent contained in the stock. To obtain maximum physical properties, the method of plasticizing which has the least deleterious effects on the vulcanized product has to be chosen. This may be one, or a combination of, the following: (1) breakdown of the crude rubber on cool mixing rolls; (2) hot breakdown of the crude rubber at temperatures above 120° C on open rolls or in an enclosed mixer; (3) use of chemical plasticizers; (4) use of plantation softened rubber, such as that produced by the Socfin Co. of Malaya, or by the Schidrowitz-Ungar process, and (5) addition of large quantities of softening oils during mixing.