The Blooming of Sulfur and Other Ingredients from Compounded Stocks

Abstract

Abstract “Blooming” is the name given to the crystallization of compounding ingredients usually on the surface of a stock, cured or uncured, accompanied by the migration of the substance from the bulk to the surface. Almost all ingredients which are soluble in rubber possess the capacity to bloom: sulfur, selenium, many accelerators, antioxidants, organic pigments, fatty acids and so on. Blooming reduces the tackiness of the stock, which causes difficulties in forming multi-ply articles (tires, belts, etc.). To restore tackiness the bloomed substances are washed off with a solvent, and in cases of particularly intense blooming the surface is wet with rubber solvents. The need for these operations reduces labor productivity and hinders automation. In the production of molded articles blooming of ingredients causes nonuniform stresses to be set up. Blooming is a cause of rejection in production (stains on overcoat fabric, on the surface of colored rubbers and so on) and greatly harms production. However, in some case blooming proves useful; films formed as a result of blooming protect rubber from the action of light and air. Thus for instance Graffe notes that vulcanized rubber which is coated with bloomed sulfur proved more resistant to aging under service conditions than rubber without this coating. At the present time wide use is made of the capacity of antioxidants and waxes for giving coatings by blooming which protect vulcanized rubber from ozone aging. All this shows how important are the recognition and correct understanding of the causes of blooming.