Anchoring intercrystalline ties, improving the environmental stress crack resistance realizing effective recycling of HDPE packaging

Environmental Stress Cracking Resistance (ESCR) is a crucial property for High-Density Polyethylene (HDPE) used in rigid packaging, particularly when exposed to surface-active agents such as detergents. However, upon mechanical recycling, the so-called post-consumer recycled (PCR) HDPE grades lose the ESCR. This limits the incorporation of PCR content in new products to 12.9% today1, whereas the EU targets demand at least 35% in plastic packaging items by 2023. Under stress, tie molecules (inter-lamellar links) stretch to their limit and finally disentangle at the boundary between the crystalline and amorphous regions. This leads to chain slippage from the lamella, initiating cracks. This failure mechanism speeds-up upon exposure to “aggressive” substances as detergents. The mechanism of a failing network of tie molecules can effectively be studied by tensile testing, specifically the strain hardening process, above the -relaxation temperature. We discovered a “repair” mechanism based on the addition of a Styrene – Ethylene – Ethylene Propylene – Styrene (SEEPS) block copolymer. It is postulated that the SEEPS network, formed by Polystyrene (PS) rigid domains and polyolefin flexible mid-blocks within the crystalline structure of HDPE, functions as anchors, preventing the pullout of tie molecules. To validate this hypothesis we wish to complement our previous ESCR, SEM, and in-situ tensile FTIR measurements by following the co-crystallization of the SEEPS’s PE block with the HDPE matrix, its effect on the spatial ordering of the PS domains at different cooling and strain rates, and lastly their role in the strain hardening time-resolved SAXS/WAXD