Ion – complexed chitosan formulations as effective fire-retardant coatings for wood substrates

In the present work, two chitosan based formulations – intended to be used as flame retardant coatings - were prepared by ionic crosslinking of hypophosphorous acid-modified chitosan with either nitrilotris(methylenephosphonic acid) (NTMP) or phytic acid (PA). Thermal properties and flammability of the two ion – complexed chitosan formulations were subsequently investigated through thermogravimetric analysis (TGA), microcombustion calorimetry (MCC), and cone calorimeter (CC). As a case study, the two formulations were used to coat wood specimens. Coated samples with both NTMP and PA crosslinked chitosan showed an increase in char residue compared to pristine wood and self-extinguishing behavior in horizontal configuration. The most significant reduction in flammability was found in NTMP crosslinked chitosan coating, being capable of (i) extinguishing the flame in vertical configuration; (ii) preventing ignition under a 50 kW/m2 heat flux and (iii) reducing both total heat release rate (THR) and peak heat release rate (pHRR) by over 96% compared to the uncoated control during CC experiments. Furthermore, we demonstrated good compatibility with an epoxy-based, waterproofing top coat in order to preserve crosslinked chitosan coatings. Durability and propensity for fungal growth of the coatings were also assessed. As follows, this newly developed ionic crosslinked chitosan coating has considerable potential for applications in renewable intumescent flame retardant systems.