The interaction of fillers and lubricants in rigid PVC compositions

The physical properties of rigid PVC products can be related to formulation, preblending and processing characteristics. A fuller understanding between these interrelations involves investigation of a complete processing system from powder additives to end product. A knowledge of these relationships can lead to a optimisation of formulation aspects and processing conditions. Fillers and lubricants have often been incorporated into PVC compounds on a empirical basis, however in this investigation a wide range of compositions were carefully chosen and included a calcium carbonate filler at levels up to 40 phr. These were dry blended and then characterised by bulk density and filler content. The premix was processed using an instrumented twin screw extruder, a wide processing 'window' being chosen to provide a product range with large variations in fusion level. Further compounds were extruded, incorporating two different impact modifiers. The extrudates was assessed for degree of fusion by differential thermal analysis, solvent immersion and microscopy. The filler distribution, surface appearance and residual grain structure was observed using a range of microscopy techniques and the results related to operating conditions. A measure of fusion level was obtained from mast er curves of heat of fusion versus processing temperature'. The filler level did not influence the degree of fusion. Various mechanical properties of the pipe were assessed. Impact performance was measured using an instrumented falling weight impact tester and the subsequent fracture behaviour depended on the composition and fusion level. Impact properties were dramatically reduced at a critical filler content and ductile-brittle transition temperatures were obtained for the extrudates by testing at low and high temperatures. Tensile properties did not follow the trends observed during impact testing but indicated a progressive reduction in tensile properties with increasing filler content.