Surface engineering of recycled high‐density polyethylene‐ based wood–polymer composites via carbon black‐containing surface film coating

Abstract This study evaluates carbon black nanoparticle (CB NP)‐reinforced surface films for improving the durability, thermal stability, fire performance and mechanical retention of recycled high‐density polyethylene (HDPE)‐based wood‐plastic composites (WPCs) after 12 months of natural weathering. Characterisation was performed on film surfaces (colorimetry, fourier‐transform infrared spectroscopy [FTIR], field emission scanning electron microscopy [FE‐SEM]), isolated film layers (thermogravimetric analysis [TGA], differential scanning calorimetry [DSC]) and complete film–WPC systems (flexural properties, fire performance). CB NPs improved surface colour retention, which may be partly influenced by reduced spectrophotometric sensitivity of dark, broadband‐absorbing surface. Independently, stable FTIR peaks at 2915 and 2848 cm −1 , along with quantitatively stabilised carbonyl index (CI) values, demonstrated suppressed photo‐oxidative degradation at the polymer surface. Uncoated WPCs showed severe degradation, with a 47% reduction in modulus of rupture (MOR), along with surface cracking and interfacial debonding observed via FE‐SEM. Film coating slightly reduced MOR loss to 44%, whereas CB incorporation significantly limited it up to 19%. FE‐SEM confirmed reduced crack density, although some NP agglomeration was observed. TGA results indicated that CB increased T onset (~13%) and mitigated weathering‐induced reductions. DSC showed slight melting temperature increases and reduced crystallinity, while weathering promoted partial recrystallisation. Fire performance improved (~12.5%) with higher char formation. Overall, CB‐reinforced films enhance WPC durability for outdoor applications.