ABSTRACT
Processing aids can be defined in various ways. No definite line can be drawn between process aids, for example, and the plasticizers and softeners discussed in the next chapter. Nevertheless process aids will be considered to be the materials included in a mix primarily to reduce the time and energy required in breaking down the polymer. Some idea of the range and complexity of processing aids was shown earlier by Klingensmith. Stearic acid as a processing aid has a low solubility in many polymers and is not usually used at over 3 phr. It helps prevent compound from sticking to mill and calendar rolls. Probably the most common fatty acid soap used in rubber, zinc stearate most often serves as a surface coating for raw stock but can be used as an internal release agent. Zinc salts of unsaturated fatty acids, such as zinc oleate, are physical peptizers for natural rubber.
