MECHANICAL STRENGTH OF INTERFACES IN THERMOPLASTIC POLYMERS

In this introductory lecture a brief review is given first on the mechanical problems, which derive from the existence and the large variety of interfaces in thermoplastic polymers, taking into account their specific organization and the particularities of the geometry of stress and strain. Then (fracture mechanics) methods are discussed to determine interfacial "strength". Subsequently the molecular conditions required for stress transfer at the interface of crystal lamellae are investigated as well as the mechanisms leading on the one hand to the establishment of interfacial strength during crack healing and welding and on the other hand to the weakening and final rupture of a joint under load. These studies, performed with the industrially important semi-crystalline polyethylene, polypropylene and polyamide have permitted to define optimum welding conditions. The influence of a heterogeneous phase, glass fibers, on the quality of a weld-line in polyamide has been assessed using laser extensometry to follow damage development in space and time. From these considerations conclusions could be drawn with respect to the design and execution of joints, their resistance under thermo-mechanical and environmental solicitation and the testing procedure. 1