Cotton fabrics were modified to take up 1-4% phosphorus (P) plus 0-0.8% nitrogen by phosphorylation (application of phosphoric acid, urea, potassium hydroxide, sodium xylenesulphonate and heat treatment) and 0-1% silicon by a sol-gel process. These processes showed good durability (resistance to textile washing)
Increased P content generally resulted in better fire performance. Si and N addition improved fire performance (LOI) for the same P content. Addition of only N did not do so at low P content. The authors suggest that urea (N source) acts by improving accessibility of hydroxyl groups in the cotton fibre and silicon by forming stable silicates in char. These PIN flame retardant treatments reduced smoke density very considerably (75% lower specific smoke density with 2% P).
“Synergistic effects in the pyrolysis of phosphorus-based flame retardants: the role of Si- and N-based compounds”, S. Deh et al., Polymer Degradation and Stability 130 (2016) 155e164 http://dx.doi.org/10.1016/j.polymdegradstab.2016.06.009