New processability concepts for rigid vinyls

The dynamic thermal stability of rigid PVC resin compositions as determined from Brabender torque rheometer tests has a linear relationship of stability time and torque from which a practical measure of processability at a fixed torque can be derived. The stability time at 1500 m‐g torque, t 1500 , has been used to characterize differences between PVC homopolymer and copolymer resins in tin, calcium‐zinc, lead, and bariumcadmium stabilizer systems. In every case, the vinyl chloride‐propylene copolymers are unique in giving substantial process‐ability advantages. The logarithm of stability time at a fixed torque, t 1500 , is a linear function of intrinsic viscosity, [η], for PVC homopolymers, giving a single line function corresponding to the expression log t 1500 = A + B [η]. The constants, A and B , vary with each of the four stabilizer systems, and are related to the effectiveness of the particular formulations used. The ratio of t 1500 for copolymers to the basic t 1500 value for PVC homopolymers at the same intrinsic viscosity (molecular weight) is a practical measure of relative processability, t rel , which is characteristic of PVC resin types and independent of stabilizer systems.