Comparison of superimposed effects in high-shear-rate capillary rheology of polystyrene, polypropylene, and linear low-density polyethylene melts

Abstract

Polymer melts exhibit unique rheological behaviors at high shear rate up to 106 s−1, which is a common phenomenon in micro‐injection molding. Both online and commercial capillary rheometers, which were modified to allow regulation of back pressure, were used for measuring the melt shear viscosities of polystyrene (PS), polypropylene (PP), and linear low‐density polyethylene (LLDPE) under high shear rates. The rheological characteristics of the three melts were compared through the systematical analyses for three significant effects, namely the end pressure loss, pressure dependence, and dissipative heating in capillary flow. Pronounced end effect begins to appear at the shear rates of 1.6 × 105, 8.0 × 105, and 2.8 × 106 s−1 for the PS, PP, and LLDPE melts, respectively. The significance of the end effect can be ordered as PS > PP > LLDPE. It seems that the polymers with more complex molecular structures exhibit a higher degree of divergence between the comprehensively corrected and uncorrected melt viscosity curves. Moreover, the dissipation effect begins to predominate over the pressure effect under the lowest shear rate of 105 s−1 for the PS melt among the three melts. POLYM. ENG. SCI., 55:506–512, 2015. © 2014 Society of Plastics Engineers

Correlations of single‐point parameters of linear rheology and molecular weight distribution of polypropylene homo‐ and copolymers

“…Even in cases where online rheometers are in use, those are often limited to high shear rates. 55,56 While on-line results can be correlated to the behavior in standard off-line capillary rheometers and injection molding, 57 they are of limited value for polydispersity calculation. Dynamic off-line rheometry has therefore been established as QC method, and the frequency range is kept narrow to accelerate the procedure.…”