About the determination of the steady state flow for polymer melts in capillary rheometers

The different parameters influencing the time required to reach steady state flow in polymer melts in a capillary rheometer were investigated. The parameters of the rheometer considered were the contraction ratio, the capillary diameter and its length to diameter ratio. The influence of polymer compressibility and viscosity were also taken into account. It was found that the time required to reach steady state flow for the studied polymers increased when the amount of material in the reservoir, the viscosity of the melt and the contraction ratio for a given L/D capillary ratio were increased and vice versa. It is shown that the main characteristic of the rheometer that determines the time to reach a steady flow is the contraction ratio and not the capillary diameter. This result is explained on the basis of polymer compressibility, its viscosity and the mechanical work done on the system, which depends on the amount of material in the barrel.