Investigation of Toughening Micro-Mechanisms in Polypropylene/Ethylene-Propylene-Diene Rubber Blends at Crack and Notch Tips

Abstract

Polypropylene (PP) has the highest growth rate among commodity thermoplastics and is widely used in many applications including packaging, auto, and pipe industries. The relatively low impact strength of this polymer has led to the production of rubber-modified polypropylene with higher impact strength. To promote the applications of these blends, it is necessary to establish the relationship between the mechanical performance and fracture micro-mechanism(s). Fracture behavior is different depending on the application where either crack or notch might be present. In this study, a systematic approach is taken with the aim of understanding the toughening micro-mechanisms of polypropylene/ethylene-propylene-diene monomer (PP/EPDM) blends at both crack and notch tip using different microscopy techniques. Rubber-modified blends were prepared using a twin screw extruder. The samples were exposed to different mechanical, physical, and microscopic examinations. X-ray diffractometer (XRD) and differential scanning calorimetry (DSC) techniques were used to study the crystalline structure. Impact and fracture toughness (JIC) tests were conducted to evaluate toughness of blends. Morphology and fracture behavior of the blends were investigated via transmission optical microscopy (TOM) and scanning electron microscopy (SEM). Results indicate that both impact strength and fracture toughness (JIC) increase with increasing EPDM content. The two parameters, however, do not follow the same trend. The microscopic evaluations reveal that massive crazing coupled with particle cavitation is the dominant toughening mechanism in PP/EPDM blends under quasi-static and impact loading. Morphological features of the damage zone are different in these two loading conditions.