Influence of Carbon Black on Processability of Rubber Stocks. II. Extrusion Shrinkage

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Abstract
Extrusion shrinkage is caused by molecular relaxation of polymer after exit from the die. The rate of molecular relaxation is extremely rapid and under the present experimental conditions, 80% of limiting extrusion shrinkage, L∞, is reached in less than 10 sec. The dependence of extrusion shrinkage L, on time t may be expressed by an empirical equation: L=L∞(1−kt−n) where k and n are constants. An increase in relaxation temperature increases values of L∞ and decreases n, while increase in shear rate decreases constant k. As found previously, extrusion shrinkage, L∞, is related to shear stress. Maintaining a good uniformity of product in factory extrusion may be accomplished by controlling extrusion pressure as well as by speeding up and controlling the process of molecular relaxation. The latter goal may be attained by maintaining the extrudate at an elevated temperature for about 2 min, and controlling that temperature within narrow limits. The results show the importance of defining extrusion conditions, including extrusion pressure and the temperature at which the extrudate is allowed to relax. The feasibility of rapid measurements (i.e., within ca. 1 min) of extrusion shrinkage using a laser beam detection system was demonstrated, and a good correlation was found with results obtained by conventional (weighing) method. The concept of occluded rubber and its effect on extrusion shrinkage was reexamined and found to provide a very satisfactory prediction of extrusion shrinkage of SBR compounds containing a wide selection of carbon blacks.

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