Capillary Flow Properties of PVC Melts
Keywords:
Capillary flow properties,
Flow curve,
End correction coefficient,
Melt fracture,
Die swell
1991 Volume 19 Issue 2 Pages 64-71
Details
1991 Volume 19 Issue 2 Pages 64-71
The effects of degree of polymerization and temperature on flow properties of PVC melts have been studied by using a Koka Flow Tester. Each flow curve is expressed by two straight lines of different slopes which meet at a breaking point. The critical shear rate γ′wc at the breaking point increases as the degree of polymerization decreases or the temperature increases. The critical shear stress τc at γ′wc increases as the degree of polymerization decreases, being almost independent of the temperature. The end correction coefficient ν shows a minimum or begins to increase around γ′wc. The coefficient ν increases as the degree of polymerization increases or the temperature decreases. The extrudate is soomth on the surface at low shear rates, shows a small wavy roughness (shark skin) at shear rates just below γ′wc, becomes fairly smooth again beyond γ′wc, and then shows intensive irregular roughness (melt fracture) at higher shear rates. The die swell becomes notable as degree of ploymerization decreases or the temperature increases. The die swell takes a maximum at a shear rate just below γ′wc, where a shark skin appears on the extrudate. Differing from usual thermpolastic resin melts, PVC melts shows a negative correlation between the die swell ratio and the end correction coefficient. This seems to be due to the following reason: PVC melts have high viscosities and have long relaxation times for strain recovery, and thus the extrudates solidify without being fully relaxed. This tendency of solidification without relaxation is more noticeable as the degree of polymerization increases or the temperature decreases.
