The reduction in fiber length during extrusion and injection molding of two commercial glass fiber-reinforced polypropylene products containing 30 percent by weight of glass fibers was studied. The first product had very small fibers of average length around 0.5 mm and also contained a coupling agent. The second product contained relatively longer glass fibers of 9 mm length and no coupling agent. In both cases, fiber attrition occurs predominantly at the solid-melt interface in the melting zone of the extruder. However, in the short fiber granules, the maximum of the length distribution, which for the initial sample is around 0.5 mm, moved to shorter fiber lengths along the screw channels further from the hopper. In the long fiber granules, a bimodal length distribution was obtained in the intermediate channels; the first maximum was around the original length of 9 mm and the second centered around 0.5 mm. Thus, the forces at the solid-melt interface result in fiber breakage to lengths which are predominantly around 0.5 mm. The fiber attrition was observed to be more severe in injection molding apparently because of higher shear rates and also because the fibers had to pass through narrow channels. The measured distributions of fiber length along the screw channels for the two products are presented, and the possible mechanisms of fiber breakage are discussed. The mechanical properties of samples containing different fiber length distributions and the effects of fiber length and interfacial adhesion on properties are presented and discussed in Part 11. , FEBRUARY 1989, Yo/. 10, No. 1 POLYMER COMPOSITES

An improved high-order theory is presented to investigate the dynamic behavior of thin and thick fiber-reinforced plastic (FRP) plates with a soft viscoelastic flexible core. Shear deformation theory is used for the face sheets while three-dimensional elasticity theory is used for the soft core. The analysis permits nonlinear distortions of the cross-sectional plane of the core as well as changes in its height. The analysis determines the damped natural frequencies, loss factors, and local and global mode shapes of plates. Some of the results are hitherto not reported in the literature based on a higher-order plate theory (HSAPT). Transverse shear and rotary inertia effects of face sheets are taken into consideration. For simply supported boundary condition, closed-form solutions are obtained by Navier’s technique. Numerical results are presented and compared with the experimental and theoretical results found in literature.

Two commercial grades of glass fiber-reinforced polypropylene granules, one containing short fibers of average length around 0.5 mm with a coupling agent and the other containing relatively much longer fibers mostly around 9 mm, but no coupling agent, were injection-molded into dumbbells and tested in tension between -43 and 90°C. There is considerable fiber attrition during injection molding; the fiber lengths are reduced to average values of 0.4 to 0.8 mm for these two samples. Also during injection molding of the test sample, partial molecular alignment of the matrix (polypropylene) occurs which supplements the reinforcement of the matrix due to the aligned glass fibers (30 percent by weight) present in the composite sample. The stiffness and strength of these samples do not reflect the effects of fiber lengths since most of the fibers are of very small length in the molded specimen and also since the sample with longer fibers has a non-uniform distribution of fibers. While the interfacial shear strength does not appear to play a significant role in determining stiffness, it turns out to be extremely important in controlling strength, particularly at the higher test temperatures. The room temperature impact strength is high for the sample containing relatively longer fibers of average length around 0.8 mm in which fiber dispersion is non-uniform and fiber agglomerates are present. Acoustic emission data shows that debonding and fiber pull-out are the main contributors to sample toughness: this observation is supported by scanning electron micrographs of the fracture surfaces.

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