Phytic acid and CaCl2 co-treated rice straw fibers/unsaturated polyester composites with excellent flame retardant, water resistant and dimensional stability properties

The application range of rice straw (RS) is limited by its poor flame and water resistance. It’s still a great challenge to endow RS fibers with excellent flame and water resistance properties without compromising the intrinsic mechanical properties. Here, a simple surface treatment strategy of RS fibers via co-treatment with phytic acid (PA) and calcium chloride (CaCl₂) was developed. First, PA was grafted onto RS fibers by esterification reaction. Then RS fibers were ionically crosslinked by CaCl₂ treatment. Modified RS fibers (PCRS) were then mixed with unsaturated polyester resin (UPR) to produce PCRS/UPR composites (PCRU). Benefiting from Ca²⁺ complexes via the ionic crosslinking between Ca²⁺ and –P (=O)–OH of the PA, –OH of the RS fibers, PCRU demonstrates enhanced tensile strength (12.04 MPa), good waterproofing capability, and flame-retardant performance. PCRU exhibites low water absorption rate (5.90%) and dimensional stability (thickness deformation less than 1.76%) after 168 h of water immersion. In addition, the limiting oxygen index and the total heat release of the prepared PCRU composites are 29.7 and 11.3 kJ/g, which reach the standard for flame retardant materials. Therefore, this study provides a simple and green pretreatment strategy of rice straw fiber for the applications in flame retardant, low water absorption, and dimensionally stable composites.

Graphical Abstract