Effect of thermal annealing on crystallinity and mechanical strength of textile glass and carbon fiber reinforced in situ polymerized ϵ-caprolactam parts

Fiber reinforced thermoplastic (FRTP) is a promising material for automobiles, airplanes, construction, home appliances and so on. In situ polymerized polyamide (PA6) is a representative polymer material for FRTP. When ϵ-caprolactam (ϵ-CL) is polymerized in situ, the polymerization of ϵ-CL and crystallization of PA6 occur simultaneously. The polymerization temperature is far below the melting temperature of PA6, and the obtained crystallinity and mechanical strength are far below those of a part prepared by injection molding. Thermal annealing is a practical approach to improve the crystallinity and mechanical properties. However, the effect of thermal annealing on the crystallinity and mechanical properties of FRTPs is still unclear. This study investigated the effect of thermal annealing on the crystallinity and mechanical properties of PA6, textile glass fiber reinforced PA6 (GF-PA6), and carbon fiber reinforced PA6 (CF-PA6) (40 vol%). The specimens were prepared by resin transfer molding with in situ polymerization of ϵ-CL. PA6 was polymerized in situ in a mold with and without fibers. The results indicated that the crystallinity, tensile modulus, and strength of PA6 were improved by thermal annealing. The crystallinity of GF-PA6 was not improved, but the tensile modulus and strength were slightly improved. The crystallinity of CF-PA6 decreased, but the tensile modulus and strength improved. The crystallinity showed a negative correlation to the tensile modulus and strength. Because the tensile modulus and strength of glass and carbon fibers are much higher than those of the crystalline phase of PA6, increasing the crystallinity had a limited effect.