Modification of mechanical properties of Kevlar fiber by polymer infiltration

While high performance organic fibers such as PBZT and Kevlar possess excellent mechanical properties under axial tension, their strength under compression is generally poor. Poor adhesion between the fibrils as well as the weak secondary forces in the lateral fiber direction have been considered to be the main factors limiting the compressive strength of Kevlar fiber. This study focuses on a polymer infiltration approach to modify the mechanical properties of the Kevlar 49 fiber in tension as well as compression. Various polymeric resins, such as epoxies, novolacs and bismaleimide, were infiltrated in an opened fibrillar network of Kevlar single filaments to increase the adhesion between the fibrils. Fiber opening was achieved using sulfuric acid solutions of various concentrations. While an increasing extent of opening was achieved with increasing acid concentrations, it was also accompanied by a strength loss at high acid concentrations. However, compared to the acid treated fiber, both the tensile strength and strain to failure of the fibers were found to increase after infiltration with epoxy resins and bismaleimide polymers. A noteworthy result of this approach was the improvement in the compressive strength observed for the Kevlar fiber after infiltrating with the epoxy and bismaleimide polymers. Plasma modification of the fibril surfaces using ammonia was also used to further enhance interfibrillar adhesion by incorporating reactive amine groups on the fibril surface.; A similar study was also conducted on wet-never-dried PPTA fibers, which was presumed to offer more accessibility for polymer infiltration. Although no further improvement was observed in properties of these fibers when infiltrated as-received, infiltration after opening with acid resulted in over 30% increase in the compressive strength of the fiber. Further, the infiltrated fiber has been modeled as a fiber micro-composite with the fibrils constituting the reinforcing phase and the infiltrated polymer as the matrix phase. The effects of various load sharing behavior amongst the fibrils on the predicted strength of the fiber micro-composite has been studied.