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Morphological, mechanical, and rheological properties of poly(butylene terephthalate) (PBT)/glass fiber (GF) composites and their fiber lengths after extrusion were studied, along with the molecular weight of PBT and the sizing effect of the fiber. From the molecular weight effect of PBT on mechanical properties, the lower molecular weight PBT composites prepared showed higher flexural and tensile strengths compared to the PBT/GF composites prepared with a higher molecular weight PBT when the fiber content was over 20 wt %. This behavior was mainly due to the increased fiber length of the PBT/GF composite prepared with lower molecular weight PBT. For the PBT composites with GF coated with organic materials having polarity similar to that of the PBT matrix, better mechanical properties were observed due to higher bonding forces between the matrix and fibers. Due to the effect of fiber length on the complex viscosity of the PBT/GF composite and its resulting high elasticity, the shear thinning behavior was more significant for the composites containing long fibers than in composites containing short fibers. From the above results, it is suggested that the optimum mechanical properties of the fiber-reinforced polymer composite can be achieved by varying the processing conditions such as molecular weight of the matrix polymer, surface treatment of the fiber, and feeder location of the fiber.

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The electrical, morphological and rheological properties of melt-and dry-mixed composites of polypropylene (PP)/nickel-coated carbon fiber (NCCF) with multi-walled carbon nanotubes (MWCNTs) was investigated. Melt mixed PP/NCCF/MWCNT composites were prepared by mixing the materials with a twin screw extruder. From the results of the electrical conductivity of the PP/NCCF/MWCNT composites, the MWCNTs increased the electric conductivity of the PP/NCCF composites appreciably. A higher aspect ratio of MWCNTs can increase the conductivity of PP/NCCF composites in a polymer matrix compared to the composites without the MWCNTs. The tensile and flexural strengths of the PP/NCCF/MWCNT composites were higher than the composites without MWCNTs. This is might be due to the reinforcing effect of the well dispersed MWCNT in the PP matrix. The results of electrical, morphological and mechanical studies indicate that the MWCNTs increased the electrical conductivity and mechanical strength of the PP/NCCF composites.