PVC fusion (gelation) occurs with melting of crystallites followed by recrystallization. This recrystallization, as it occurs at the primary particle boundary. physically ties together the structure into a three dimensional network. One way of measuring the extent of this three dimensional network structure is to measure the force required to deform the material through a zero land length capillary. We examine this test method for various molecular weight PVC compounds, with and without plasticizer, and for various degrees of grinding the product.

PVC profile extrusion compounds have a unique morphology. While other polymers gradually decrease in extrusion die swell with increasing length/thickness (L/D) ratio, PVC profile extrusion compounds have a low die swell, quite independent of the die's L/D ratio in the range of 5 to 20. The fact that the die land length can be changed without changing the extrudate swell is an important consideration, which makes die design and balancing dies simpler and easier for PVC profile extrusion compounds. While other polymers substantially increase extrudate swell with increased shear rate, the swell of the PVC profile compounds is not much affected by shear or extrusion rate. This unique behavior allows wider processing latitude in profile extrusion and faster extrusion rates than with other polymers. Another unique factor in the rheology of PVC profile extrusion compounds is that extrusion die swell increases with increasing melt temperature, while other polymers have decreasing die swell with increasing melt temperature. The unusual rheology of PVC profile extrusion compounds is attributed to its unique melt morphology, where the melt flow units are 1 um bundles and molecules that have low surface to surface interaction and entanglement at low processing temperatures but increased melting and increased entanglement at higher processing temperatures. Other polymers, unlike PVC, have melt flow at the molecular level.

Extrusion products that to not meet specification are generally reground for extrusion reprocessing. This paper describes the effect of heat history of regrind materials on extrusion quality, including surface appearance (melt fracture), toughness, edge tear, and heat stability. It is hoped that the information presented in this paper will help processors optimize use of regrind for best performance.