Effect of screw configuration and processing parameters on glass fiber residual length and properties of glass fiber reinforced polybutylene terephthalate composites

Glass fiber reinforced polybutylene terephthalate (GF-PBT) composite is widely applied in automobile, electronic and electrical appliances, industrial machinery due to excellent toughness, mechanical properties, heat resistance, solvent resistance and processing fluidity. However, consistent with other glass fiber reinforced composites, glass fibers are easy to fractured and hard to be dispersed during processing, limiting its improvement of material performance. A series of studies on the factors, such as screw configuration, processing parameters, were carried out through eight different screw configurations with five different screw elements in twin-screw extruder (TSE) to evaluate the effects caused by processing stress, screw speed, mixing section temperature, feeding rate and processing length in GF-PBT composite. The rotation number N_t was innovatively introduced and a novel exponential model was established to successfully predict the glass fiber residual length in the PBT system. The average glass fiber residual length at different axial positions of the screw during the processing was evaluated through the emergency stop experiment and the main reason for the glass fiber fracture was revealed. The results showed that the screw configuration affects the mechanical properties through the dispersion of glass fiber bundle, the residual length of glass fiber and the uniformity of length distribution, while the process parameters have little effect on the tensile strength and elongation at break, and have a greater impact on the notch impact strength, bending strength and bending modulus in PBT system.