Fatigue crack propagation evaluation of different commercial grade propylene polymers

JESSICA WAINSTEIN; MIRCO CHIAPETTI; PABLO E. MONTEMARTINI; PATRICIA FRONTINI

INTERNATIONAL JOURNAL OF POLYMERIC MATERIALS

Taylor&Francis

Lugar: Philadelphia; Año: 2005 vol. 54 p. 575 - 589

0091-4037

Commercial Polypropylene Polymer - PP- are available in various grades having different characteristics due to modifications done during polymerization (e.g., production of syndiotactic homopolymers, copolymers, with different comonomer content or polymers with narrow molecular mass distribution) [1]. Their characteristics not only depend on molecualar weight and comonomer content of the matrix but also on tacticity, crystallinity, different crystallinity forms, supermolecular architecture and the processing history [2-7]. Although mechanical behaviour of PP polymers has been extensively studied [7-15], many features are still unknown. Moreover, the use of PP is growing in new fields where an improvement of performance is required. For instance, components, and structures where cyclic loads are experienced [16-17], composite matrixes prosthetics devices machine components and piping [18-19], among others. In all this applications an understanding of the fatigue behaviour is needed. Besides this, low frequency cyclic fatigue tests are more suitable for studying morphological effects than static and dynamic fracture tests [20]. To summarize, the evaluation of PP fatigue response appears interesting not only from a technological point of view but also from a scientific one. The present paper reports the mechnical behaviour under cyclic loading of commercially available PP polymers. Two different polymers were assayed: one extrusion grade homopolymer, and one impact block copolymer. In order to enhance PP toughness below its glass transition temperature (Tg @ 0ºC), propylene is copolymerized with ethylene [22]. However, copolymerization also induces a considerable loss in stiffness and strength characteristics. Another way of improving isotactic PP toughness is promoting further crystallization [2, 11] by thermal annealing over 140 ºC. This article aims to investigate how the differences in the mechanical behaviour of these polymers are reflected in fatigue loading condition performance.