The shear level was increased during twin screw extrusion of PVC at different melt temperatures by inserting a hole plate in front of the screw tips. The variation of shear level did not significantly affect the capillary pressure in capillary viscometry at 135°C. However, the falling weight impact strength was markedly influenced. In internal water pressure tests at 60°C premature failure was obtained for pipes extruded at 198°C at the highest shear level.

The effect of shear on fusion and mechanical properties of rigid PVC pipes

A 50 mm twin screw extruder was used to study the melting profiles of various PVC formulations. Melt temperature and degree of flux are measured axially along various mixing paddle arrangements.

The viscous work is correlated with the rheology of the PVC mass and effective shear rate of the intermeshing co-rotating screws. The results of this investigation are used to develop a conceptual model of compounding PVC formulations.

The radial mixing patterns in paddle segments down the barrel length are analyzed by introduction of trace quantities of pigment added to the dry blend. The resulting residence time distributions coupled with localized shear rate distributions give a numerical evaluation of shear strain required to flux PVC particles into a continuous fluid mass.

By proper agitator design, compounding of PVC formulations can be accomplished with minimal thermal history.

Co-rotating twin screw compounding studies of PVC formulations

The personal computer has started a revolution in the way data is collected and stored by bringing computer capabilities within the economic reach of even the smallest company. Computer data acquisition and manipulation expands the usefulness of the already valuable torque rheometer as a tool for both rheological studies and routine quality control testing. Analysis of more fundamental rheological properties than just the traditional torque vs. time curve is possible without the time consuming manual data manipulation. Data storage and recall of routine or Q.C. testing for accurate comparisons of results is made easier. Torque rheometer uses made easier by the use of a computer are reviewed. Also given are details of how to hook up a personal computer to an existing torque rheometer.

The assembly and use of a computerized torque rheometer

Standard PVC compounds containing various lubricant systems were fused in a Haake Torque Rheometer equipped with a torque integrator, holding the shear stress time value constant. The total work put into the system to fuse the compound was therefore held constant. The fused material was compression molded into sheeting for physical testing. The data will characterize the compounds for both rheological and physical properties at several processing conditions. Plotted curves will depict the lubricant systems and processing temperatures necessary to obtain maximum physical strength.

Malcolm H. Heffner

Papers

Temperature and shear history effects on fusion of PVC