Development of NaOH-borax crosslinked PVA-xanthan gum-lignin hydrogel as green fire retardant coating

In this paper, we report synthesis of green fire-retardant hydrogels comprising polyvinyl alcohol (PVA), xanthan gum (XG), and lignin nanoparticles (LNP, synthesized using nanoprecipitation). The hydrogel network was achieved through crosslinking by borax and NaOH. XPS analysis of LNPs revealed no change in chemical state of lignin after formation of nanoparticles. The hydrogels were characterized using FTIR, FESEM, and thermogravimetric analysis. The rheological properties of the hydrogels were measured. Among different hydrogels, the best combination of properties was achieved for hydrogel PX3B0.4N2L20, viz. elastic modulus (G′) = 23,428.57 Pa (indicating strong elastic network), crosslinking density = 870,350.12 kmol m−3 and mesh size of 5.60 nm. High char formation (34.61 wt%) by PX3B0.4N2L20 hydrogel helped in enhancing fire retardancy. With coating of PX3B0.4N2L20 hydrogel, limiting oxygen index (LOI) for cotton cloth improved from 20 % to 39 %. Kinetic analysis of TGA data yielded the highest activation energy of 493.8 kJ mol−1 for hydrogel, which corroborated the LOI test results. The cone calorimeter tests of PX3B0.4N2L20 hydrogel also revealed the least peak heat release rate of 90.3 ± 10 kW/m2 and the least total heat release rate of 1.56 ± 4 MJ/m2. In summary, PVA-based hydrogels synthesized in this study have high potential as effective and biodegradable fire retardant coatings.