Kaolin-embedded cellulose hydrogel with tunable properties as a green fire retardant

This study reports the synthesis of methylene bisacrylamide (MBA) crosslinked cellulose-kaolin (CMK) composite hydrogels. The internal structures of hydrogels were deduced using network parameters, viz. elastic modulus, average molecular weight, mesh size, and effective crosslink. Meanwhile, combustion behavior was investigated using the cone calorimeter test (CCT), limiting oxygen index (LOI) test, vertical flammability test (VFT), and open fire test (OFT). Our results revealed that kaolin addition improves the fire retardancy of hydrogels but reduces their swelling ability. Hydrogel having cellulose to MBA ratio of 1:2 and 2 % w/v kaolin (CM2K2) produced 63 % wt. char residue and the hydrogel-coated cotton fabric exhibited the lowest heat release rate (HRR) of 26.60 kJ/m² and total heat release (THR) of 0.9 MJ/m². The LOI of the cotton fabric surged from 20 % to 34.37 % after hydrogel coating. Kinetic analysis using the isoconversional model yielded the highest activation energy (216 kJ/mol) for the CM2K2 hydrogel, corroborating the increased LOI after kaolin addition. VFT and OFT validated the delay in the burning process and the formation of a char layer, which protected the underlying layer of cotton from burning. Overall, cellulose-kaolin hydrogels developed in this study are effective green fire retardant coatings for flammable materials.