DETERMINATION OF STEADY-STATE ELONGATIONAL VISCOSITY FOR RUBBER COMPOUNDS USING BELL-MOUTHED DIES

A bell-mouthed die geometry was designed to cause convergent flow at a constant, uniform, elongational strain rate. An equation was derived, which showed that steady-state elongational viscosity could be calcul ated from a plot of pressure drop due to elongation against a simple f unction of die length. To obtain values of pressure drop due to elonga tion, it was necessary to correct the total pressure drop measured acr oss the bell-mouthed dies for the contribution from shear occurring ne ar the die wall. For this purpose, a simplified shape for the bell-mou thed dies was assumed, comprising several parallel sided segments. App lying a formula to pressure drop data measured across straight dies co rresponding to these segments gave an estimate of the pressure drop du e to shear across the bell-mouthed dies. Pressure drops due to elongat ion were determined by subtracting the pressure drop due to shear from the total pressure drop measured across the bell-mouthed dies. Measur ements were also carried out with lubrication to validate the shear co rrection method. The results indicate that for the compound used in th is study, a combination of bell-mouthed and straight-sided dies can be used in a conventional capillary rheometer to determine steady-state elongational viscosity. An elongational viscosity of 190 kPa s at 90 d egrees C and at a strain rate of 10 s(-1) was determined for a simple styrene-butadiene rubber compound. (C) 1997 John Wiley & Sons, Inc.