Chitosan reacted with phosphoric acid was tested as a liquid PIN FR for natural fibres used to reinforce vinyl ester resin. Chitosan solution, prepared using acetic acid, was phosphorus-loaded by reacting with phosphoric acid. The natural fibres (wool, mulberry silk) were treated with sodium hydroxide, soaked in the liquid P-chitosan solution, then dried, resulting in c. 7% w/w PIN FR uptake.. Finally fibres were set into vinyl ester (VE) resin at c. 30% fibre loading. All three VE-fibre composites after treatment achieved UL 94 V-0 (3 mm) and showed peak heat release rate reduced by 2 – 4 x. The authors suggest that the flame retardant effect is the result of release of phosphoric acid and ammonia, generating dense, protective char, and release of non-combustible gases (water, CO2, ammonia and for silk SO2). They suggest that this could be a cost-effective, bio-based solution for fire protection of natural fibres in polymer composites.
“Preparation of a novel bio-flame-retardant liquid for flame retardancy of natural fibers and their composites”, L. Maksym et al., J. Industrial textiles, 2022 https://doi.org/10.1177/15280837221079273