Effects of reinforcing fibers on the crystallization of polypropylene

The effects of the incorporation of different types of fibers on the crysta llization kinetics and thermodynamics of isotactic polypropylene (IPP) are investigated. The study is mainly performed by thermal analysis, both in is othermal and constant cooling rate conditions, utilizing differential scann ing calorimetry (DSC). The crystallization kinetics of the studied composit es is successfully described by the Avrami model. Moreover, the effects of different kind of fibers are studied comparing the values of kinetics param eters such as the Avrami exponent, n, the kinetic constant Of the crystalli zation rate, K-n and the half-time of crystallization, tau (1/2). The resul ts of the investigation show that the fibers behave as effective nucleant a gents for the crystallization of polypropylene. In fact, a dramatic decreas e of the half-time of crystallization, tau (1/2), as well as a sensible inc rease of the overall crystallization rate, Kn are observed In the presence of all the fibers analyzed. These effects are more marked in the presence o f aramid fibers. The Avrami model is successfully applied to describe the c rystallization kinetics of the composites. The kinetic curves obtained unde r non-isothermal conditions confirm the results obtained under isothermal c onditions and demonstrate the nucleating action of the fibers on the PP cry stallization. Furthermore, the spherulite growth and the transcrystallinity on the surface of the fibers are investigated by Optical polarizing micros copy. It is observed that transcrystallinity takes place in all kind of fib ers studied, but also in this case, aramid fibers are the most effective in promoting transcrystallinity.