Coated date palm wood with biopolyester films reinforced thermal insulation composites based on poly(β-hydroxybutyrate)

Fully bio-based thermal insulation composites were engineered by reinforcing poly(β-hydroxybutyrate) (PHB) with date palm fibers (DPFs) surface-coated with poly(lactic acid) (PLA) or PHB. The fibers were coated via solution casting and subsequently incorporated into PHB through melt blending, thermocompression molding, and annealing at 20 and 40 wt.% loadings. Scanning electron microscopy revealed increased fiber surface roughness and improved interfacial adhesion, while FT-IR results were consistent with enhanced interfacial interactions between the polyester coating and the fibers. Both PLA- and PHB-coated DPF composites exhibited reduced density (1292–1313 kg/m 3 ) compared to uncoated PHB–DPF systems (1314–1334 kg/m 3 ) and significantly improved moisture resistance, with cold water absorption decreasing from 1.93% to 1.38% after 48 h at 40 wt.% loading. In terms of thermal performance, the composites achieved a minimum thermal conductivity of 0.0906 W /( m . K ) at 20 wt.% loading, remaining below 0.10 W /( m . K ) for all coated formulations, along with specific heat capacity values of 1286–1506 J /( kg.K ) and low thermal diffusivity (0.046–0.058 mm 2 /s). Mechanically, tensile strength increased to 25.8 MPa at 20 wt.% loading, whereas higher fiber content (40 wt.%) resulted in reduced strength and a slight increase in thermal conductivity, highlighting a performance trade-off. Overall, 20 - 40 wt.% coated DPF composites provide the most balanced combination of thermal insulation efficiency and mechanical integrity, making them promising sustainable alternatives for rigid interior building insulation applications.