Interfacial Engineering of Highly Filled Recycled Polyethylene/Hump Hurd Composites through Elastomer Pre-Coating

Highly filled recycled polymer-biomass composites often suffer from poor melt processability and premature mechanical failure because of the large stiffness and polarity mismatch between the polymer matrix and lignocellulosic particles. Here, a simple elastomer pre-coating strategy was investigated to improve the mechanical compatibility of recycled high-density polyethylene (rHDPE)/hemp hurd (HH) composites without chemical surface treatment or reactive compatibilization. Three commercial elastomers, Nordel 4520, Engage 8180, and Vistamaxx 6202, were screened at coating concentrations of 0.625 and 1.25 wt % using composites containing 5 wt % HH. Engage 8180 provided the best balance of tensile strength, stiffness, and impact resistance and was selected for subsequent evaluation at HH loadings of up to 60 wt %. Field-emission scanning electron microscopy showed greater polymer retention on coated HH surfaces and less fracture localization along particle boundaries. FTIR, XRD, and differential scanning calorimetry analyses indicated that the coating did not introduce detectable chemical bonding or significantly alter the crystalline structure or crystallization behavior of rHDPE, while TGA showed no substantial stabilization of the polyethylene-rich phase. Rheological measurements instead revealed that the coating reduced particle-induced melt resistance, narrowed the distribution of relaxation responses, and accelerated stress relaxation while preserving the shear-thinning and viscously dominated behavior of the matrix. At 60 wt % HH, coating increased the tensile strength from 6.44 to 8.13 MPa and impact strength from 9.12 to 12.90 kJ m−2 relative to the uncoated composite. Water absorption measurements further demonstrated that E2 pre-coating reduced water uptake by 58.1% in composites containing 60 wt % HH after 168 h of immersion, highlighting an additional functional benefit of the proposed approach. The combined morphological, rheological, mechanical, and water absorption results support a proposed compliant-interphase mechanism in which localized elastomer-modified regions facilitate stress relaxation and contribute to improved particle−matrix interactions without requiring a continuous elastomer shell. This study demonstrates an effective elastomer pre-coating strategy for producing highly filled rHDPE/HH composites containing up to 60 wt % renewable biomass, combining improved mechanical performance with tunable melt rheology without chemical surface modification or reactive compatibilization.