Bio-benzoxazine structural design strategy toward highly thermally stable and intrinsically flame-retardant thermosets

The frequent fire accidents and sustainable development concerns have called for more durable and safer flame retardant polymeric materials produced from natural renewable resources. Herein, we successfully developed a molecular design strategy by incorporating oxazine ring substituents into bio-benzoxazine monomers to achieve highly thermally stable and intrinsically flame retardant thermosets. The polybenzoxazine derived from a small-molecule bio-benzoxazine resin, 3-(furan-2-ylmethyl)-3,4-dihydro-2H-benzo[e][1], [3]oxazine (PH-fa), was found to reveal a very high glass transition temperature of 317 °C. However, it was thermally unstable during the polymerization process and easy to ignite without self-extinguishing characteristic. 9,10-Dihydro-9-oxa-10-phosphaphenathrene-10-oxide (DOPO) was incorporated into PH-fa to obtain an oxazine ring substituted bio-benzoxazine (PH-fa-[4]DOPO) possessing non-ignitable capability, which reached the V-0 rating of UL-94 test. Unfortunately, a trade-off between the thermal stability and flame retardancy was discovered through the DOPO modification. Further incorporating renewable benzaldehyde into PH-fa-[4]DOPO generated a 2,4-substituted bio-benzoxazine (PH-fa-[2]ph[4]DOPO) with ideally enhanced flame retardancy as well as remained higher thermal stability. These results highlight the utility of sustainable molecular design and provide molecular-level insights into roles of oxazine ring substituents in thermal and fire-related properties of polybenzoxazines, as well as a new angle for the design of non-ignitable and high-performance bio-thermosets based on benzoxazine chemistry.