Processing Aids - Processing behavior involves the gelation process of the polymer and the melt rheology of the polymer. Small quantities of processing aids can play a predominant role in improving both steps. Most commercial processing aids are copolymers of methyl methacrylates and alkyl acrylates, with MMA as the major component. The molecular weight and the glass transition temperature control their efficiency. The effects of the processing aid can be summarized as follows: • Acceleration of gelation • Control of melt rheology • Lubrication In order to obtain a homogeneous melt, PVC particles should be broken down to their primary particles under heat and shear in the processing equipment. Without the addition of processing aids, PVC particles have a low coefficient of friction with the hot metal surface of the processing equipment and also with other particles. Hence long processing times at high temperatures would be required to achieve complete gelation. The addition of the processing aid increases particle to particle friction, combined with enhanced heat and shear transfer leading to a faster breakdown of the particles and a faster gelation. The ability of processing aids to promote PVC gelation is greatly influenced by the processing aid’s chemical composition and molecular weight. Lower Tg processing aids are more effective in reducing the gelation time or reducing the processing temperature.