Role of comonomer type in determining Environmental Stress Cracking resistance in Polyethylene

The long-term durability of polyethylene (PE) products is primarily limited by failure mechanisms such as Slow Crack Growth (SCG) and Environmental Stress Cracking (ESC). While traditional testing methods provide useful material rankings, they often lack the predictive power needed for robust engineering design. This work presents a quantitative evaluation of ESC resistance in different PE grades using a Linear Elastic Fracture Mechanics (LEFM) framework, aiming to establish clear relationships between morpho-structural parameters and fracture toughness. Four PE materials were analyzed: two linear low-desnity (LLDPE) grades with different comonomer type (1-butene vs. 1-hexene), a high molecular weight LLDPE, and a high-density (HDPE) homopolymer. The results confirm that molecular weight is a dominant factor in enhancing fracture resistance. More importantly, the study reveals the decisive role of comonomer type: the LLDPE with 1-hexene (longer side chains) exhibits significantly higher resistance to crack initiation and propagation compared to its 1-butene counterpart, despite having similar bulk structural parameters. Furthermore, the investigation identifies three distinct fracture regimes when testing in a surface-active environment, determined by the applied stress intensity factor (K): (1) a high-K regime dominated by the material's inherent toughness, (2) an intermediate-K, flow-controlled regime, and (3) a low-K regime where full plasticization of craze fibrils drastically reduces lifetime. By validating the LEFM approach, this study provides a transferable framework to guide the design of PE materials for improved long-term durability.