(i) Inorganic Compounds (mainly metal oxides): Include zinc oxide, hydrated lime, litharge, red lead, white lead, magnesium oxide. Zinc oxide is the most common and is generally used in combination with a fatty acid to form a rubber soluble soap in the rubber matrix. The majority of metal oxide are used in coated or treated forms in order to disperse more readily in the rubber mixtures. Normal use level is 2 to 5 phr.
(ii) Organic acids: Normally used in combination with metal oxides such as zinc oxide; they are generally monobasic fatty acids or mixtures of the types, stearic, oleic, lauric, palmitic and myristic acids, and hydrogeneated oils from palm, castor, fish, and linseed oils. The usage of each particular type depends on the accelerator used and the amounts of compounding ingredients present. Normal use level is 1 to 3 phr.
It is believed that fatty acids with zinc oxide (or other metal oxides) give a salt that form intermediate complexes with the accelerators. The complex thus formed is more effective in activating the sulfur present in the mixture, thus increasing the reaction rate.