The kinetics of alpha and beta transcrystallization in fibre-reinforced polypropylene

The kinetics of alpha (monoclinic) and beta (hexagonal) transcrystallizatio n of isotactic polypropylene on aramid Kevlar 149 fibres, glass fibres and high modulus carbon fibres was investigated under isothermal and gradient c ooling conditions. No difference was found between growth rates of bulk sph erulites and transcrystalline layers, and Hoffman's theory led to the same results in both cases. Regarding oc transcrystallization, a transition betw een regimes II and III occurred near 137 degrees C and the ratio of the slo pes of the two regimes was close to the theoretical value of 2. Regarding b eta transcrystallization, only regime II was exhibited in the temperature r ange studied. However, the induction time for transcrystallization was stro ngly influenced by the type of fibre, which in turn-based on Ishida' s appr oach-resulted in variations in free energy differences at the fibre-crystal lite interface for various fibres and bulk polypropylene. The respective va lues were 1.3, 1.5 and 2.1 X 10(-3) J m(-2) for Kevlar 149 fibres, high mod ulus carbon fibres than in polypropylene, showing that or crystallization i s more likely to occur in Kevlar 149 fibres and high modulus carbon fibres and bulk polypropylene. Gradient-thermal measurements were performed for or transcrystallinity which allowed estimation of the activation energy of tr anscrystallization for the different composites. Activation energies of tra nscrystallinity promoted on Kevlar 149 and high modulus carbon fibres were found higher than the activation energy for bulk crystallization. (C) 2000 Elsevier Science Ltd. All rights reserved.