From Melt Miscibility to Crystallization: Rheological and Thermal Analysis of Polyethylene Blends for Recycling Applications
Plastic waste management remains a major challenge, and the mechanical recycling of polyethylene (PE) is complicated by the coexistence of HDPE, LDPE, and LLDPE grades that are difficult to separate. This review critically examines the miscibility, apparent miscibility, compatibility, and crystallization behavior of PE blends, with an emphasis on linear viscoelastic rheology and thermal analysis. Rheological criteria, including additivity, Cole–Cole, time–temperature superposition, Han, and van Gurp–Palmen analyses, were evaluated, together with DSC and successive self-nucleation and annealing. Most studies indicate that HDPE/LDPE blends are melt-immiscible. HDPE/LLDPE blends more frequently exhibit apparent melt miscibility when LLDPE has low short-chain branching, typically below 15 branches per 1000 carbon atoms, and relatively low molar mass; increasing molar mass generally promotes immiscibility. LDPE/LLDPE behavior is less systematic and depends on branch content and distribution, molar mass, and catalyst-controlled molecular architecture. Crystallization does not necessarily mirror melt miscibility. HDPE/LLDPE co-crystallization is generally favored at high HDPE contents but strongly depends on branching, comonomer type, and cooling conditions, while LDPE/LLDPE may co-crystallize despite apparent melt immiscibility. These trends provide practical guidelines for designing recycled PE blends with more predictable processing and properties.