Detection of multiple flow regimes in capillary flow at low shear stress

Abstract

The examination of capillary flow of polymers has resulted in reports of two distinct flow regimes at stresses well below that required for gross melt fracture. The break (kink) between the two regimes has been associated with the onset of sharkskin melt fracture (SSMF), which does not appear to be consistent with the popular hypothesis that SSMF is purely an exit phenomenon. We scrutinized 37 sets of data from 15 laboratories to detect the reported regimes in a systematic and objective fashion using several statistical tests including the Akaike information content method of model discrimination. Many data sets failed to pass one or more of the tests, and several failed all the tests, suggesting the absence of two distinct regimes as opposed to the expected increase in shear-thinning behavior with increasing stress. On the other hand, four out of the 37 sets passed all the tests and 11 out of 37 passed at least 3 of the 4 tests, leaving open the hypothesis of an abrupt change in flow behavior in the capillary itself or in the entrance region. No global correlation of the results with polymer type, capillary geometry, or temperature was found, although with certain resins the abrupt change appeared most prominently with low-aspect-ratio capillaries, suggesting an entrance phenomenon. A method of quantitatively judging the sharpness of the transition from one regime to another was developed. This method, comprising a single nonlinear but explicit generalized Newtonian fluid function with a sharpness parameter, can be easily applied to any data set and is recommended as a method of checking for the presence of a kink.