Influence of Carbon Black on Processability of Rubber Stocks. IV. Kinetics of Extrusion Shrinkage

Abstract

Abstract A careful examination of post-extrusion relaxation rates of SBR compounds containing various carbon blacks has shown that two distinct relaxation processes are taking place. The fast recovery region observed at times of less than 0.1 s after exit from the die is thought to be due to solid-like elastic recovery of the polymer imposed in the extensional flow region at the entrance to the die. The magnitude of that recovery increases with increasing surface area and decreasing structure of carbon black. The slow recovery region is observed at times longer than 1 s and caused by molecular orientation that occurs mainly during the extensional flow at the die entrance and maintained during the viscous flow through the die. Carbon black surface area and structure do not significantly affect the magnitude of the shrinkage in the slow recovery region. The observed two relaxation mechanisms may be simulated mathematically by two modified Maxwell elements in parallel, whose spring constants (and relaxation times) differ by a large factor.