Influence of Carbon Black on Processability of Rubber Stocks III. Extensional Viscosity

Abstract

Abstract A new instrument for measurement of extensional viscosity was designed and employed in the evaluation of SBR compounds containing carbon black and other non-black fillers. It was shown that carbon black filled rubbers generally do not reach steady-state viscosity ; the stress continues to increase with increasing strain up to the point of rupture, due to rapid orientation of molecular chains during extensional flow. The data was evaluated in terms of the Denn-Marrucci equation, and the effect of carbon black on the two adjustable parameters was found. It was found that higher carbon black loading or structure increases only the viscosity coefficient, i.e., it changes the relative level of the flow curve without changing its shape. On the other hand, an increase in surface area increases both the viscosity coefficient and the relaxation time, i.e., it increases the upward curvature of the flow curve. A very good correlation was observed between Mooney viscosity values and calculated viscosity coefficients. Some agreement was found between calculated relaxation times and the rates of stress relaxation for rubbers containing nonblack fillers, while in carbon black loaded stocks the observed differences in stress relaxation rates were too small to draw definite conclusions. The use of high surface area, low structure carbon black is recommended for applications where stiffening of the SBR compounds during extension is desired.