TABLE OF CONTENTS Process aids are added primarily to PVC-U (including foam) to improve fusion characteristics and melt flow during the processing stage. They can also be added to improve the processability of semi-rigid PVC formulations. Addition levels are typically from 1 to 6 phr with resultant improvements in output and surface finish. Transparency can also be improved in clear formulations. Chemically, these products are either acrylic polymers (methyl methacrylate or acrylate) or styrenic copolymers (containing methacrylate). The molecular weight and T g control their efficiency. Having high compatibility with PVC, these high-molecular-weight materials entangle the shorter PVC chains and transfer the shear generated during processing to promote fusion. The effects of processing aids can be summarised as follows:
Melt homogeneity improvement by promoting the breakdown of PVC particles and ensuring that a homogeneous melt is obtained. Good melt homogeneity is essential for achieving good physical properties and surface finish.
Melt strength improvement as a result of their compatibility. Good melt strength is essential for blow moulding, high-speed take off from calenders or extrusion lines, and foam applications.
Melt extensibility and elasticity improvements to permit wider operating conditions and higher outputs.
Processing aids with a lubricating effect are made through variation of the chemical composition to give a less compatible material. This results in little effect on PVC gelation but helps metal release and reduces plate-out.
