Determination of fiber orientation of composites using wide angle X-ray diffraction

Studies of both rheological properties and fiber orientation development due to flow of short fiber filled thermoplastic systems are described. These composite systems are mechanical mixtures of an amorphous polymethyl-methacrylate (PMMA) matrix with short crystalline aramid (Kevlar 49) reinforcing fibers in a proportion 80/20 in volume. Rheological measurements only cover the shear viscosity (n) in the high shear rate range due to flow instabilities and experimental difficulties which did not permit carrying out any other rheological measurements. Fiber orientation development due to flow was investigated in systems having different fiber aspect ration. Qualitative studies involved the use of x-ray flat-film techniques and scanning electron microscopy (SEM). Quantitative evaluations of fiber orientation as represented by Herman-Stein orientation functions were carried out in extrudate and frozen in situ specimens by means of a wide angle x-ray technique proposed in this research.

Addition of aramid fibers increases the viscosity but does not affect the basic character of the viscosity shear rate behavior of the polymer melt. A little change in viscosity is observed with fiber length at high rates of deformation, but a significant increase can be expected at low shear rates with longer fiber length distributions. Pressure effects observed in the system filled with fibers having a modal length larger than the diameter of the capillaries used in its characterization are associated with this fact.

Studies of fiber orientation carried out in composite specimens prepared by capillary extrusion using the pro-posed x-ray technique showed that the orientation function values obtained are dependent in both shear rate and die swell at constant processing geometry. Analysis of composite specimens from material frozen inside the rheometer barrel and the capillary die showed that the fiber orientation development took place along the capillary die and was greatly affected by the die swell effect in the extrudate. Reduced die swell effects were observed in all composite samples as compared to that observed in the unfilled melt.