TOUGHNESS PROFILE IN INJECTION-MOLDED POLYPROPYLENE - THE EFFECT OF THE BETA-MODIFICATION

Fracture toughness characterized by the J-integral method has been eva luated along injection-molded bars of isotactic polypropylene. A signi ficant increase in toughness with decreasing distance from the specime n gate has been observed with maximum steepness of the dependence in t he central part of the specimen length. Parallel analysis of wide-angl e X-ray diffraction patterns has shown a similar dependence of the cry stalline beta-phase concentration. It is suggested that polypropylene with beta-crystallites has better structural continuity in the amorpho us phase and, consequently, higher inherent ductility and superior mac roscopic toughness than the related material containing only a-crystal lites. Consequences of beta-phase gradients for practical toughness te sting are also discussed. (C) 1998 John Wiley & Sons, Inc.