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

The different parameters influencing the time required to reach steady stat e flow in polymer melts in a capillary rheometer were investigated. The par ameters of the rheometer considered were the contraction ratio, the capilla ry diameter and its length to diameter ratio. The influence of polymer comp ressibility and viscosity were also taken into account. It was found that t he time required to reach steady state flow for the studied polymers increa sed 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 a nd vice versa. It is shown that the main characteristic of the rheometer th at 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 polyme r compressibility, its viscosity and the mechanical work done on the system , which depends on the amount of material in the barrel, (C) 2001 Elsevier Science Ltd. All rights reserved.