DOPO was reacted (in solvents) with piperazine (an organic compound containing nitrogen) and then with a chloro-silane compound, to produce a DOPO-N-Si molecule (not containing the chlorine): 1-(9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide)-4-(trimethoxysilylmethyl) piperazine (DOPO-PiP-Si). This was applied to cotton fabric (122 g/m2) by dissolving the FR in solvent, adjusting pH to hydrolyse and then soaking the cotton (two dips), drying and then curing at 110°C, with 25 – 40% weight gain FR loading. LOI (limiting oxygen index) was increased from 18% to 25 – 28%, peak heat release rate was reduced by 30% and the treated cotton passed the vertical flammability test (GB/T 5455-2014). Analysis showed that this P-N-Si FR acted by both char formation and in the gas phase: in particular, ammonia release in “bubbles” both directly acts as a fire quenching gas and also carries phosphorus – silicon particles into the gas phase, where they act as radical inhibitors. The silicon also generates SiO2 reinforcing char.
“Preparation of a synergistic reactive flame retardant based on silicon, phosphorus and nitrogen and its application to cotton fabrics”, Z. Zhang et al., Cellulose, 2019 https://doi.org/10.1007/s10570-019-02900-4