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Studies of the rheological behavior upon milling and compounding of styrene-butadiene rubber (SBR) polymers having different molecular weight and molecular weight distribution using the Monsanto processibility tester show that polymers having a high molecular weight fraction have higher stress-relaxation time values, because of the higher degree of polymer entanglements. As temperature is increased, the high molecular friction has less effect on the stress-relaxation time. Polymers not having high molecular weight fractions show viscosity to be directly proportional to stress relaxation values tested on specimens before and after being submitted to polymer breakdown on a lab mill. Experiments measure the degree of high molecular weight breakdown and the chain extension and contraction that occurs during milling and time conditioning. Increasing test temperature shows a decrease in stress relaxation time. Compound viscosity values of polymers having high molecular weight fraction decrease less upon increasing temperatures than polymers having no high molecular weight fraction. The processibility tester can be used to characterize the rheological properties of an elastomer or its compounds. This information can be used to duplicate a “good” processing compound or to detect a “bad” one based on the particular processing requirements.
Group leader, Technical Service Applications Research, Rubber and Chemical Corporation, Baton Rouge, LA
Stock #: JTE10745J