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 fiber orientations generated during processing, such as injection molding. In an effort to optimize industrial processing, optimize mold design, and maximize desired properties of the final part, it is highly desirable to predict long fiber orientation as a function of processing conditions. The purpose of this research is to use rheology as a fundamental means of understanding the transient orientation behavior of concentrated long glass (> 1mm) fiber suspensions. Specifically, this research explores the method of using rheology as a means of obtaining stress tensor and orientation model parameters needed to accurately predict the transient fiber orientation of long glass fiber reinforced polypropylene, in a well-defined simple shear flow, with the hopes of extending the knowledge gained from these fundamental experiments for the use of simulating processing flows, such as injection molding. Two fiber orientation models were investigated to predict the transient orientation of the long glass fiber systems explored. 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 orientation model, one that accounts for the semi-flexibility of fibers, was extended to describe non-dilute suspension and coupled with an augmented stress tensor that accounts for fiber bending. Stress tensor and orientation model parameters were determined (in all cases) by best fitting these coupled equations to measured stress data obtained using a sliding plate rheometer. Results showed the semi-flexible orientation model and stress tensor combination, overall, provided improved rheological results as compared to the Folgar-Tucker model when coupled with the stress tensor of Lipscomb (1988). Furthermore, it was found that both stress tensors required empirical modification to accurately fit the measured data. Both orientation models provided encouraging results when predicting the transient fiber orientation in a sliding plate rheometer, for all initial fiber orientations explored. Additionally, both orientation models provided encouraging results when the model parameters, determined from the rheological study, were used for the purpose of predicting fiber orientation in an injection molded center-gated disk. / Ph. D.

This work is concerned with determining empirical parameters in stress and fiber orientation models required to accurately simulate the fiber orientation in injection molded composites. An independent approach aims to obtain the material parameters using a sliding plate rheometer to measure the rheology of fiber suspensions at increased fiber lengths subjected to transient shear flow. Fiber orientation was measured in conjunction with shear stress to determine the relationship between stress and fiber orientation. Using a compression molding sample preparation procedure, the transient shear stress response was measured for glass and carbon fiber suspensions up to a number average fiber aspect ratio (length/diameter) of 100. Increases in concentration or fiber aspect ratio caused the magnitude of the stress response to increase by as much as an order of magnitude when compared to the suspending matrix. The degree of shear thinning at low shear rates also increased with increases in aspect ratio and concentration. The compression molding sample preparation procedure provided poor control of the initial fiber orientation which led to the investigation of samples subjected to flow reversal and samples generated through injection molding. The samples prepared through injection molding provided improved repeatability in the measured shear stress response and fiber orientation evolution during the startup of flow compared to compression molded samples and samples subjected to flow reversal. From repeatable stress and orientation evolution data, models for stress and fiber orientation were assessed independently. Current theories for stress were unable to reflect the overshoot in the measured stress response and could at best capture the steady state. The transient behavior of the fiber orientation models were found to be highly dependent on the initial fiber orientation. The repeatable orientation data obtained from the injection molding sample preparation procedure provided material parameters in the strain reduction factor and reduced strain closure models. The injection molded samples provided evolution data from different initial fiber orientations to provide further scrutiny or validation of the material parameters. Orientation model parameters that provided reasonable agreement to multiple sets of fiber evolution data in simple shear flow should allow for a better assessment of the orientation models in complex flow simulations. / Ph. D.

Transient viscosity growth measurements at the startup of shear flow were performed on long glass fiber-filled polypropylene. Samples were prepared with fibers pre-oriented either in 1-direction, 3-direction or random in 1-3 plane, where the 1-direction is the direction of shear motion, the 2-direction is perpendicular to the shear plane and the 3-direction is the neutral direction. A sliding plate rheometer incorporating a shear stress transducer was constructed in the lab. It was shown that this device works well for the tested materials including a Newtonian oil, a low density polyethylene (LDPE) and short glass fiber-filled polypropylene. The transient viscosity growth behavior for long glass fiber suspensions was subsequently investigated. The results suggested that both, fiber length and fiber concentration have pronounced effect on the steady state suspension viscosity. It was also observed that the transient behavior of the pre-oriented samples was highly dependent on the initial orientation state of the fibers. / Master of Science