Slow crack growth resistance in resin blends of chromium and metallocene catalyzed ethylene‐hexene copolymers for pipe applications

A bimodal system has been developed by blending a high density and linear medium density ethylene‐hexene copolymers synthesized with chromium and metallocene catalysts, respectively. The resistance to slow crack growth examined by the Pennsylvania Edge‐Notch Tensile test (PENT test‐ASTM F1473) and the crack opening displacement was determined at 80°C and 2.4 MPa. The effects of molecular and morphological structure on the slow crack growth (SCG) resistance were evaluated, the molecular weight and lamellar thickness being the most critical parameters for this system. The great importance of the short chain branching content and distribution was determined and discussed by SEC‐FTIR. Therefore, two main factors were found determinant in the SCG resistance. On the one hand, the increase of tie molecule density and therefore the continuity in the network formed by crystals and tie molecules and, on the other hand, the short chain branching density increase as the linear medium density ethylene‐hexene copolymer fraction does so. In addition, a morphological analysis of the fracture surface was performed in order to analyze the fracture mechanisms that took place. Differences observed in the fracture surface morphology were related to the molecular and morphological structure. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers