Phosphorylated lignin as a halogen-free flame retardant additive for epoxy composites

Sustainable, non-halogenated flame retardants are desired for a variety of industry applications. Lignin, an industrially processed wood derivative, has been examined as a potential sustainable flame retardant additive to polymer systems. Here, the lignin is phosphorylated using a pyridine-catalysed esterification reaction with diphenyl phosphoryl chloride to improve its char-forming abilities. The chemical modification of the lignin was characterised by nuclear magnetic resonance spectroscopy and showed the formation of phosphorylated structures on the lignin. The thermal decomposition profile and char-forming characteristics of the modified lignin and modified lignin–epoxy composites were investigated using thermogravimetry. The flammability performance of modified lignin–epoxy composites was tested using mass loss calorimetry. With the addition of 10% modified lignin, the peak heat release rate decreased by 40% and the total heat of combustion decreased by 20%. Scanning electron microscopy was used to investigate the char morphology of the post-flame test samples and showed closed cell foam structures.