Structure-property relationships of injection- and compression-molded microfibrillar-reinforced PET/PA-6 composites

Abstract

Structure-property relationships of injection- and compression-molded microfibrillar-reinforced PET/PA-6 blends with different weight ratio of the components have been studied. The blends were prepared by melt mixing using a single-screw extruder. Thermal, X-ray (WAXS), morphological, and static mechanical studies of the bulk samples were carried out in order to determine an optimum processing window. The upper processing temperature was established by the onset of the melting of the fibrillized PET component in the blend. Morphological studies of the injection-molded samples indicate skin-core morphology. The skin layer is composed of fibrils that are well oriented along the flow axis. The core region is composed of a small amount of randomly oriented, fibrillar PET bundles, as well as a large amount of spherical PET domains in the isotropic PA-6 matrix. SEM observations of the fracture surface of compression-molded samples show well-extended PET fibrils embedded in an isotropic PA-6 matrix. The reason for this significant difference in the morphology of the samples is the fact that in the case of compression molding it is possible to keep very accurately the desired processing temperature in a quite narrow interval. On the other hand, an overheating of the system due to non-isothermal shear and a viscosity dissipation are the main reasons for the melting of a part of the PET fibrils and their transformation into spherical domains in the core of the injection-molded samples. The morphology strongly affects the tensile properties of the bulk samples. Compared to the values of injection-molded neat PA-6, the elastic modulus and the strength of injection-molded MFC (Microfibrillar reinforced composites) blends have been increased by a factor of 2.5 and 1.7, respectively. In the case of compression-molded samples, both modulus and strength were about four times higher than those of the neat PA-6. © 2000 John Wiley & Sons, Inc. Adv Polym Techn 19: 249–259, 2000