Strong Adhesion of High-Density Polyethylene (HDPE) and Isotactic Polypropylene (iPP) with Ethylene-Styrene-Butadiene (ESB) Copolymers

Polyethylene (PE) and isotactic polypropylene (iPP) are the most widely used semi-crystalline polyolefins, but their inherent immiscibility limits the bonding of dissimilar polyolefins and the upcycling of mixed waste streams. Conventional compatibilizers, such as olefin block copolymers (OBCs), bridge the PE/iPP interface through co-crystallization between their segments and the matrices, and therefore require processing close to the melting points ( T m ) of the bulk polymers. In this study, we show that an ethylene-styrene-butadiene (ESB) random terpolymer adheres strongly to both high-density polyethylene (HDPE) and iPP through a mechanism that does not rely on co-crystallization. T-peel testing reveals that ESB establishes measurable adhesion from temperatures close to its own broad melting (approximately 60°C) to both matrices — that is, far below the matrix T m — and that the peel strength rises further once the temperature exceeds the crystalline relaxation temperature of the matrix. Crucially, this low-temperature adhesion is observed even for the iPP/ESB system, in which co-crystallization between ESB and iPP is unexpected, demonstrating that co-crystallization is not a prerequisite for strong polyolefin compatibilization. Raman microscopy reveals that ESB and matrix chains interdiffuse over 5–10 μm across the interface. Cryo-TEM further confirms broad interdiffusion in both systems; an additional, system-dependent contribution from needle-like HDPE crystals growing into the ESB phase is observed only in the HDPE/ESB system. Together, these results establish amorphous-phase interdiffusion, likely driven by thermodynamic affinity rather than by segmental matching, as a sufficient mechanism for strong adhesion between dissimilar polyolefins, and provide a route to bond HDPE and iPP under thermal conditions that preserve the structural integrity of both matrices.