Toughness mechanism in semi-crystalline polymer blends: I. High-density polyethylene toughened with rubbers

The mechanical response of rubber-modified high density polyethylene (HDPE) was investigated. The rubbers were either ethylene-propylene copolymers (E PDM) or ethylene-octene copolymers (EOR), blended into HDPE at volume fract ions of up to 0.22. These rubbers were in the form of finely dispersed sphe rical inclusions with sizes well below 1 mu m The incorporation of rubber i nto HDPE does not substantially change its crystallinity, but produces spec ial forms of preferential crystallization around the rubber particles. The notch toughness of the rubber-modified HDPE increases by more than 16-fold as a result. The single parameter, controlling the notch toughness of these blends was found to be the matrix ligament thickness between rubber inclus ions. When this thickness is above a certain critical value, the notch toug hness of the material remains as low as that of the unmodified HDPE. When t he average ligament thickness is less than the critical value a dramatic to ughness jump results. The critical ligament thickness for the HDPE-rubber s ystems was found to be around 0.6 mu m, independent of the type of the rubb er used. The sharp toughness threshold in the rubber-modified HDPEs results from a specific micro-morphology of the crystalline component of HDPE surr ounding the rubber particles. The PE crystallites of approximately 0.3 mu m length perpendicular to the interface are primarily oriented with their (1 00) planes parallel to the particle interfaces. Material of this constituti on has an anisotropic plastic resistance of only about half that of randoml y oriented crystallites. Thus, when the interparticle ligaments of PE are l ess than 0.6 pm in thickness the specially oriented crystalline layers over lap, and percolate through the blend, resulting in overall plastic resistan ce levels well under that which results in notch brittle behaviour, once ru bbery particles cavitate in response to the deformation-induced internal ne gative pressure. This renders ineffective the usual strength-limiting micro structural flaws and results in superior toughness at impact strain rates. (C) 1999 Elsevier Science Ltd. All rights reserved.