Processing of UPVC in single and twin screw extruders
The processing characteristics of PVC compounds play a major role on the ability of the equipment to control the final properties of the product. Therefore, information on the effect of processing conditions on such characteristics. and its influence on the final properties can lead to an optimization of the extrusion process. In this work. a statistically based experimental design was used in order to investigate the role of the operating conditions on the extrusion of a simple lead based PVC formulation. Both a single and a twin screw extruder were used - they were instrumented, and data collected by computer. A die was designed for the formulation used. with both shear and extensional flows being taken into account. These experiments produced both core samples along the screw(s) and final products with varying states of fusion. The fusion mechanism on processing was monitored with the help of electron and light microscopy techniques. thermal analysis and density measurements. In both types of extruders the primary particles were found to fuse well before the grains, but the overall mechanism differs for each type of machine. An experimental technique was developed for the study of the Residence Time (RT) and Residence Time Distribution (RTD) of the material in the processing equipment. These functions were related to the operating conditions and compared with the available models for extrusion. The degree of fusion of the processed products was assessed by rheology and thermal analysis. A range of mechanical properties, relevant to the general end - use of rigid PVC formulations. was analysed: flexural. tensile and impact properties were found to be directly related with fusion level. Hardness, measured using a newly developed apparatus. proved to be a potential technique for the degree of gelation of PVC formulations.