Dynamic fracture toughness of cellulose-fiber-reinforced polypropylene: Preliminary investigation of microstructural effects

In this study, the microstructure of injection-molded polypropylene reinforced with cellulose fiber was investigated. Scanning electron microscopy of the fracture surfaces and X-ray diffraction were used to investigate fiber orientation. The polypropylene matrix was removed by solvent extraction, and the lengths of the residual fibers were optically determined. Fiber lengths were reduced by one-half when compounded in a high-intensity thermokinetic mixer and then injection molded. At low fiber contents, there is little fiber orientation; at high fiber contents, a layered structure arises. To better understand mechanisms of fracture under impact loading, dynamic fracture analysis was performed based on linear elastic fracture mechanics. Dynamic critical energy release rates and dynamic critical stress intensity factors were deduced from instrumented Charpy impact test measurements. Dynamic fracture toughness increased with cellulose content and with orientation of fibers perpendicular to the crack direction. A preliminary evaluation of a simple model relating the microstructure to the dynamic fracture toughness shows promise, but further work is needed to assess its validity.