(17g) Predicting the Orientation of Concentrated Long Glass Fiber Suspensions In Simple Shear Flow: Application to Processing Flows

Great interest exists in adding long fibers into polymeric fluids due to the increase in properties associated with the composite, as compared to the neat resin.  These properties, however, are dependent on the orientation of the fibers within the composite.  In an effort to optimize industrial processing, optimize mold design, and maximize desired properties of the final part, it is highly desirable to be able to predict long fiber orientation as a function of processing conditions.  For this reason, it is desirable to understand the link between the deformation response (rheology) of a long fiber reinforced thermoplastic and the effect it has on its underlying microstructure.  

The purpose of this research is to understand the transient fiber orientation and the associated rheology of long glass fiber (> 1 mm) reinforced polypropylene, in a well-defined simple shear flow, to extend the knowledge gained from these fundamental experiments for the use of simulating (more complex) molding processes.  Specifically, we are interested in associating the rheological behavior of long glass fiber reinforced polypropylene with the transient evolution of fiber orientation, in simple shear flow, to ultimately model 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 model, has been particularly useful for predicting fiber orientation in short glass fiber systems and was used in this paper to assess its performance with long glass fibers.  A second fiber orientation model, 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 Lipscomb model for the stress 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.