When developing or fine-tuning a PVC formulation for profile extrusion it is usual to pay close attention to the fusion characteristics, such as fusion time, fusion and equilibrium torque and temperature rise. However, very often little or no attention is paid to the pseudoplastic behaviour and visco-elastic properties of the formulation in the molten state. Understanding and characterizing the melt flow in profile dies provides a useful insight into the performance of a formulation in terms of post-extrusion shrinkage, surface gloss and mechanical properties. Robin Madgwick and Bernard Cora of Rohm and Haas Plastics Additives describe how a new impact modifier can act on the rheological characteristics of the melt to improve the mechanical and physical properties of vinyl profiles.
Various components of a formulation can affect the visco-elastic properties of a PVC melt, in particular due to their relatively high addition level impact modifiers can play an important role. The type of impact modifier can change the velocity profile of the melt in the extrusion die and affect the melt's elastic properties, relaxation time and recoverable shear compliance.