Interphase anchoring mechanism for enhanced environmental stress cracking resistance in recycled high-density polyethylene

Low environmental stress cracking resistance (ESCR) of recycled HDPE (rHDPE) is a primary barrier to increase the recycled content in rigid packaging. This study introduces a novel interphase-driven mechanism based on styrene-b-ethylene-b-ethylene/propylene-b-styrene block copolymer (SEEPS) to enhance ESCR across multiple virgin and recycled HDPE systems. Multiscale characterization techniques show that SEEPS forms an interdiffused EEP (ethylene–ethylene/propylene)/HDPE interphase in which the olefinic midblock entangles with amorphous HDPE segments, while polystyrene nanodomains act as thermoreversible physical crosslinks below the PS glass transition. This interphase suppresses chain mobility in the polymer matrix and increases resistance to tie-chain pullout and disentanglement, refining the “anchoring ties” concept into an interphase-driven mechanism. In-situ SAXS/WAXD during tensile deformation corroborates this by showing extended intermediate patterns over a wider strain window in the blend before fibrillar scattering develops. Applied to rHDPE, adding SEEPS (2.5–10.0 wt%) promoted a steady increase in ESCR by up to 130% relative to the unmodified material. ESCR improvements scaled with the matrix tie-chain concentration estimated by using the Huang–Brown approach. The interphase-anchoring mechanism enabled by adding tailor-made block copolymers offers a promising route for enhancing the performance of recycled HDPE. • SEEPS block copolymer enhances ESCR in both virgin and recycled HDPE blends. • Entanglements anchor HDPE tie molecules to SEEPS domains at their interphase. • The interphase anchoring mechanism explains the improved ESCR. • SEEPS content directly correlates with ESCR in recycled HDPE. • Incorporating SEEPS enables increased recycled HDPE content in rigid packaging.