Long Glass Fiber Orientation In Extensional Flow

Long Glass Fiber Orientation in Extensional Flow

Kevin J. Meyer, Kevin C. Ortman and Donald G. Baird

Department of Chemical Engineering, Virginia Tech, Blacksburg VA 24061

With the need for alternative lightweight materials growing daily, significantly more research is being conducted in the area of reinforced composites. A cheap and effective way of increasing the properties of a polymer is to embed both short (L < 1mm) and long glass fibers (L > 1mm) in the matrix. The bulk properties of the composite material in question are highly governed by the orientation of the embedded fibers. While significant effort and emphasis has been placed on studying the orientation of fibers in injection molding, which are shear-dominated flow fields, the flows are usually very complicated with final fiber orientation being governed primarily by the processing technique. In extensional flow (shear-free flow) we present a way to govern the fiber orientation kinetics completely independent of processing technique. Under biaxial deformations, due to isotropic stresses occurring during squeezing, the fibers retain their initial orientation. With this knowledge, initial fiber orientation can be chosen a priori thus governing final material properties. Currently, modeling of fiber orientation in thermoplastic composites is done by way of the widely used Folgar-Tucker model modified for slow orientation kinetics. Due to the over prediction of fiber orientation in shear and extensional flow fields, a different modeling approach first provided by Strautins and Latz, is presented as an alternative to modeling thermoplastic composites. More specifically, we present a way to obtain model parameters used in the orientation equations from rheological data only. The orientation parameters are obtained by using the Dinh-Armstrong variation of the Lipscomb stress model for concentrated suspension. This technique provides a way to model fiber orientation without any knowledge of transient orientation data to predict final orientation. It is the purpose of this paper to apply current theoretical models and compare these models to experimentally obtained orientation data to gain insight into the orientation kinetics of long glass fibers in shear-free flow fields for predictive ability of composite properties of compression molded samples.