Experimental investigation of the mechanical, thermal, and acoustic properties of polyester composites reinforced with <i>Smyrnium cordifolium</i> fiber

This study presents a comprehensive experimental investigation into the physical, mechanical, thermal, and acoustic properties of sustainable biocomposites reinforced with natural plant fibers. The composites were fabricated using two different polymer matrices—unsaturated polyester resin (UPR) and flame-retardant polyester (FRP) with varying fiber-to-resin ratios (70:30 to 40:60 wt. %). The results demonstrated that increasing the fiber content led to a decrease in composite density and compressive strength, alongside an increase in water absorption. UPR-based composites exhibited lower density and superior thermal insulation performance, with thermal conductivity values between 0.0503 and 0.0567 Wm −1 K −1 . FRP-based composites, while denser and exhibiting higher thermal conductivity (up to 0.0720 Wm −1 K −1 ), are advantageous for fire-resistance applications. Acoustic analysis revealed that samples with lower resin content and higher porosity achieved better sound absorption characteristics. Moreover, ultrasonic pulse velocity (UPV) tests and SEM analyses indicated a correlation between matrix continuity and mechanical integrity. Statistical analyses confirmed strong relationships between density, compressive strength, thermal conductivity, and water absorption. Overall, the findings suggest that natural fiber-reinforced biocomposites, especially those based on UPR, have strong potential for use as lightweight structural materials with added thermal and acoustic insulation benefits, while FRP-based alternatives offer enhanced performance in fire-sensitive environments. These results indicate that such composites are promising candidates for lightweight interior wall and ceiling panels, insulation boards, and other building components requiring combined structural, thermal, and acoustic performance.