Interfacial strength of PTFE fiber/i-PP composites with transcrystalline layers of different thickness

Transcrystallization of isotactic polypropylene (i-PP) on the surface of poltetrafluoroethylene (PTFE) fibers was investigated. Using the single-fiber pull-out test, the effect of the transcrystalline layers (TCLs) on the adhesive fracture energy was studied as well. To induce TCL with different thickness, two-step crystallization of i-PP was carried out; the PTFE/i-PP composites were first crystallized at 140°C for a specified time and were subsequently quenched in 25°C water to conclude the crystallization. Specimens with TCL thickness of 45 ± 6 and 126 ± 15 μm were thus prepared for crystallization time of 1 and 3 hrs, respectively. In addition, the bulk portion surrounding the TCL showed a transparent appearance that indicated crystallization of i-PP spherulites was much limited. In spite of the difference in the crystallization time, it is suggested that the mechanical properties of the individual constituents in the matrix (TCL and spherulites) prepared in this way remain unchanged, leaving the TCL thickness alone as the major controlling factor. Pull-out results show that adhesive fracture energy is independent of the TCL thickness and the measured value is 3.0 ± 1.5 J/m² in spite of the large variation in the TCL thickness. Compared to specimens without TCL whose adhesive fracture energy is 2.1 ± 1.1 J/m², promotion of the adhesion level seems much limited, if any, through the presence of TCL and its thickness. However, the friction stress at the debonded PTFE fiber/i-PP TCL is increased when TCL are produced at high crystallization temperatures. Toughness of composites with TCL can be enhanced in this manner although the neat adhesive fracture energy remains intact.