From Interface to Impact: Strengthening Polyethylene Adhesion for Sustainable Recycling

Abstract

Multilayer polymer packaging relies on polyethylene (PE) layers bonded by ethylenemethacrylic acid (EMAA) copolymer tie layers, but the adhesion mechanisms governing interfacial strength are not fully understood. This thesis presents a systematic peel strength study across EMAA compositions varying from 5 to 22 wt% methacrylic acid (MAA) content and sodium neutralization levels from unneutralized to 83%, paired with both low-density polyethylene (LDPE) and high-density polyethylene (HDPE) substrates under two thermal histories (aircooled and quenched). Peel strength decreases monotonically with increasing MAA content for both PE substrates, consistent with increasing polarity mismatch reducing interfacial cocrystallization. A partial recovery in peel strength at 22 wt% MAA is attributed to the glass transition temperature of the EMAA amorphous phase rising through room temperature, producing a mechanically asymmetric bilayer where energy dissipation during peeling increases. Sodium neutralization uniformly reduces adhesion relative to unneutralized EMAA, with an abrupt drop upon introduction of any ionic crosslinks attributed to conformational entropy penalties imposed by ionic aggregates on interfacial chain segments. Aircooled samples consistently exhibit higher peel strength than quenched samples, reflecting more extensive interfacial co-crystallization during slow cooling, while quenched samples provide greater reproducibility for isolating compositional effects. These results establish quantitative, reproducible relationships between EMAA composition, processing conditions, and bilayer adhesion, offering practical guidance for designing multilayer packaging systems with controlled adhesion for both mechanical performance and end-of-life recyclability.