Preliminary Results in Rheologically Obtaining Model Parameters for the Purpose of Predicting the Orientation of Concentrated Long Glass Fibers in Processing Flows

摘要

The purpose of this research is to understand the transient fiber orientation of long glass fiber (> 1mm) reinforced polypropylene, in a well-defined simple shear flow, by determining unambiguous model parameters from rheological experiments, and to ultimately predict fiber orientation in complex processing flows. A sliding plate rheometer was designed to measure stress growth in the startup and cessation of steady shear flow. Two fiber orientation models were investigated to predict the transient orientation of the long glass fiber system. One model, the Folgar-Tucker model2'3, has been particularly useful for predicting fiber orientation in short glass fiber1 systems and was used in this paper to assess its performance with long glass fibers. A second fiber orientation model~8, one that accounts for the flexibility of long fibers, was also investigated. The accuracy of both the Folgar-Tucker model and the semi-flexible orientation model, when used with the stress predicting Lipscomb~(12) tensor and a modified version of this tensor (one that tries to account for bending stresses), respectively, is evaluated by comparing orientation predictions against experimentally measured orientations. Samples consisting of 10 wt. % glass fiber in polypropylene with an average fiber length of 3 mm were prepared with near planar random initial orientation, and were sheared at different shear rates. Results show that, when parameters are determined from rheology, the combination of the Lipscomb model and Folgar-Tucker model is unable to accurately predict the transient fiber orientation. On the other hand, the combination of the semi-flexible orientation and modified Libscomb model show orientation predictions that are encouraging.