Effect of glass fiber length on the creep and impact resistance of reinforced thermoplastics

Impact and flexural creep testing were conducted at temperatures between −22°F (−30°C) and 250°F (121°C) to evaluate and compare the end-use performance of continuous long glass fiber-reinforced thermoplastic sheet composites to that of short glass fiber-reinforced thermoplastics. The matrices studied consisted of amorphous (polycarbonate and acrylonitrile-butadiene-styrene) and semicrystalline (polypropylene) polymers. Data were obtained from both injection-molded specimens (short fibers), and from specimens machine-cut from compression-molded test panels (continuous long fibers). The creep results of this study demonstrated that continuous long fibers are more efficient than short fibers in reinforcing the thermoplastic matrices, resulting in enhanced load-bearing ability at elevated temperatures. The addition of continuous long glass fibers to the thermoplastic matrices led to a significant increase in the notched Izod impact strengths between the temperatures of −22°F (−30°C) and 77°F (25°C), and only slight improvement in the drop-weight impact strengths. The lack of correlation between notched Izod impact and drop-weight strengths is largely due to the difference in crack propagation and fracture initiation energies. Results of the Rheometrics instrumented impact test indicated a higher total fracture energy for the long glass-reinforced thermoplastic sheet composites than for the short glass-reinforced injection-molded thermoplastics. The decreased ease of crack propagation in thermoplastic sheet composites is associated with the high energy-absorbing mechanisms of fiber debonding and interply delamination. The results of this study point to the significant property improvement of continuous long fibers vs. short fibers. The creep strength of short fiber-reinforced thermoplastics are greatly affected by the nature of the stress transfer which in turn is influenced by the critical fiber length and temperature, which is not the case for the long fiber-reinforced thermoplastic sheet composites. Long fibers dramatically increase the impact resistance of thermoplastics. The retention of toughness at low temperatures coupled with elevated temperature performance greater than similar short glass fiber-reinforced thermoplastics effectively extends the capabilities of thermoplastic sheet composites at both temperature extremes.