From Nature to Function: Green Composites Using Camphoric Acid-Based Unsaturated Polyester Resin and Bamboo/Flax Non-Woven Reinforcements

Unsaturated polyester resins (UPRs) were synthesized from camphoric acid and diluted with styrene, partially replaced (up to 30%) by trimethylolpropane triacrylate (TMPTA). Rheological tests showed increased but sustainable viscosity due to TMPTA’s higher polarity. These UPRs served as matrices for composites reinforced with non-woven bamboo and flax mats from recycled waste. Mechanical testing revealed that Cf-UPR/TMPTA30 exhibited the highest tensile strength (25.2 MPa) and modulus (0.96 GPa), compared to 18.7 MPa and 0.74 GPa for the styrene-based resin, respectively, attributed to greater cross-link density. Bamboo composites showed lower tensile properties (13.6 MPa) due to random fiber orientation and porosity, while flax-reinforced systems, especially Cf-UPR/TMPTA30–FLAX, reached 42.7 MPa tensile and 95.5 MPa flexural strength, indicating synergy between TMPTA-modified resin and flax fibers. Dynamic-mechanical analysis confirmed stable thermo-mechanical behavior, and water uptake tests showed reduced absorption (by ~10%), suggesting improved fiber/matrix adhesion. SEM images revealed brittle fracture and fiber pull-out in styrene systems, but fiber breakage and ductile textures in TMPTA-based ones, proving better stress transfer. Thermal analysis indicated slightly earlier degradation onset for TMPTA-modified resins but higher char yield in fiber composites. Overall, TMPTA substitution and flax reinforcement enhance the mechanical, interfacial, and thermal properties of bio-based UPRs, supporting sustainable high-performance composites.