Plant-derived acid modification to enhance interfacial properties of bamboo-plastic composites

Objective Bamboo-plastic composites have been widely used in construction and outdoor products, and their performance largely depends on the interfacial bonding state between the fibers and the matrix. Current studies often rely on traditional chemical coupling agents to improve interfacial compatibility, which suffer from environmental unfriendliness, high cost, and limited mechanistic understanding. There remains a notable lack of analysis regarding the multi-phase interface involving “bamboo fiber-filler-polymer”. This study aims to use two environmentally friendly plant-derived acids for the surface modification of calcium carbonate, transforming it from a conventional filler into an active coupling agent. The research further seeks to elucidate its mechanism in enhancing the interfacial bonding and comprehensive mechanical properties of bamboo fiber/polypropylene/calcium carbonate (BFs/PP/CaCO₃) composites, thereby providing a reference for developing green, low-cost bamboo-plastic composites.

Method Sodium citrate and DL-malic acid were used to modify the surface of calcium carbonate, respectively, and their effects on the surface properties of calcium carbonate were compared with those of the traditional silane coupling agent KH570 under varying additive amounts. Using calcium carbonate, bamboo fibers, and polypropylene as raw materials, BFs/PP/CaCO₃ composites were prepared under controlled calcium carbonate addition levels. Their physical-mechanical properties were investigated, and their microstructure, dimensional stability, and processability were characterized. Finally, the interfacial enhancement mechanism was clarified, and an optimized production formulation for the composites was proposed by balancing cost and performance.

Result Through oil absorption value and activation degree analyses, the optimal additive amounts of sodium citrate, DL-malic acid, and KH570 were determined to be 15%, 5%, and 10% of the calcium carbonate mass, respectively. When the modified calcium carbonate addition was 10%, the composite exhibited optimal performance. The traditional KH570-modified group achieved a tensile strength of 41.30 MPa, with flexural strength and elastic modulus reaching 63.5 MPa and 4.48 GPa, respectively. The sodium citrate and DL-malic acid-modified groups demonstrated interfacial bonding capabilities comparable to KH570, with tensile strengths of 40.31 and 39.62 MPa, rivaling engineering plastics like ABS (38−45 MPa). Their flexural strengths and elastic moduli were 63.1 MPa, 63.5 MPa, and 4.41 GPa, 4.37 GPa, respectively, exceeding twice of the values specified in LY/T 2565-2015 Bamboo-Plastic Composites. The sodium citrate-modified group met mechanical performance requirements while offering lightweight advantages, with density reduced to 1.08 g/cm³ (a decrease of 4.42%). The DL-malic acid-modified group exhibited superior dimensional stability, with a water absorption thickness swelling rate of only 0.211%, a 46.31% reduction compared with the unmodified group. Additionally, after DL-malic acid modification, the composite’s melt flow index reached 22.77 g/(10 min), comparable with pure PP of 21.00 g/(10 min), indicating significantly improved processability.

Conclusion Plant-derived acid modifiers significantly enhance the surface activity of calcium carbonate. Through carboxyl bonding with Ca²⁺ and hydrogen bonding with bamboo fibers, they improve the interfacial compatibility among CaCO₃, bamboo fibers, and polypropylene, thereby enhancing the physico-mechanical properties of composites. These plant-based modifiers achieve mechanical properties comparable with those of KH570 in certain aspects, while also offering additional advantages: DL-malic acid improves processability, and sodium citrate reduces material costs by approximately 7%.