Effect of interfacial shear strength on the mechanical response of polycarbonate and PP reinforced with basalt fibers

The effect of shear strength of the interphase on the mechanical properties of polycarbonate (PC), isotactic polypropylene (PP) and ithaconic acid grafted polypropylene (ITAPP) reinforced with bare unidirectional basalt fibers (BF) was investigated in a dry state and after immersion in hot water. A single embedded fiber test was utilized to determine the average shear strength of the interphase, tau(a). Four millimeters thick unidirectional fiber reinforced laminates were prepared by laying up eight 0.5 mm thick UD fiber reinforced PC, PP and ITAPP prepregs. Bare and PC oligomer grafted basalt fibers were utilized in PC based composites while only bare basalt fibers were used in PP and ITAPP based composites. Both model single fiber composites and UD laminates were exposed to 85 degrees C water for 100 h followed by 100 h drying at 100 degrees C in a vacuum oven. In the dry state, the shear strength of the interphase varied from 10 MPa for bare BF/PP to 40 MPa for bare BF/PC annealed at 245 degrees C for 10 min. The tau(a) decreased after water exposure approximately 20% for all the model composites investigated, from 8 MPa for bare BF/PP to 33 MPa for bare BF/PC annealed at 245 degrees C for 10 min. Longitudinal elastic modulus and dry transverse modulus were almost independent of the tau(a), whereas the transverse modulus increased with the tau(a) by 75% after water exposure. On the other hand, both longitudinal and transverse strength of all the composites investigated increased with tau(a) by 50% and 100% respectively. In PC/bare BF and PP/bare BF, greatest reduction of mechanical properties upon water immersion was observed. PC/bare fiber composite annealed at 245 C for 10 min and PC with PC oligomer grafted fibers exhibited excellent retention of properties after long-term immersion in hot water. Similar results were found for ITAPP/bare GF. This finding was attributed to the formation of high modulus PC interphase relatively impermeable by water and by formation of stable chemical bond between surface hydroxyls on basalt fibers and carboxylic groups of the ITAPP.