A novel P-N structure modified halloysite nanotube for simultaneously enhancing flame retardancy and mechanical properties of unsaturated polyester resin

Unsaturated polyester resin (UPR) is highly flammable due to its long-chain polymer structure composed of hydrocarbon elements, which significantly restricts its application. In this study, an organic-inorganic hybrid flame retardant was developed by using siloxane (KH-560) as a bridging agent to graft bio-based P-N structure (the salt formation of diphenyl phosphoric acid and cytosine) onto the surface of halloysite nanotubes (HNT), resulting in the synthesis of HNT-Si-DC. Experimental results demonstrated that the UPR composite containing 15 wt% HNT-Si-DC achieved a UL-94 V-0 rating, with a limiting oxygen index (LOI) of 33.2 %. Compared to pure UPR, the peak of heat release rate (PHRR) of the composite decreased by 49.8 %. Furthermore, the mechanical properties of the UPR/HNT-Si-DC₁₅ composite were significantly enhanced, with the tensile modulus increasing by 38.7 % and the flexural strength improving by 21.3 % compared to those of pure UPR, indicating high mechanical performance. Additionally, the dense and continuous carbon layer preferentially enhanced the flame retardancy. This work presents a highly efficient approach to designing organic-inorganic hybrid modified HNT for the development of UPR composites with excellent fire resistance and high mechanical properties.