(361b) 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.