Phosphorylated lignin intumescent bio-flame retardants for bio-polymers

Polymeric materials are common in all areas of our lives because of their exceptional mechanical, physical, and chemical properties. The major weakness of these polymeric materials is their high flammability. This limits their applications in cases where fire resistance is needed. Flame retardants have been developed to reduce flammability, combustibility, and release of toxic fumes but, unfortunately, most flame retardants, especially the halogenated FRs, are non-biodegradable and tend to remain in the environment. This has resulted in the exploration of bio-based, biodegradable flame suppressant alternatives which include cellulose, hemicellulose, starch, cyclodextrin, isosorbide, lignin, phytic acid, tannic acid, DNA, proteins, vegetable oils, and other saccharide-based compounds. Lignin is a large source of carbon and can be used in combination with other flame retardants for creating intumescent systems for polymeric materials due to its high charring capabilities. Because of its significant char output after decomposition from its aromatic framework, lignin has recently gained a lot of attention as a bio-based flame-retardant ingredient. Nonetheless, because of its diverse structural heterogeneity, chemical reactivity, and broad molecular mass spread, its performance is strongly influenced by the botanical origin and delignification technique employed. The phosphorylation of phenolic and aliphatic hydroxyl groups in lignin can yield renewable flame retardants, providing a genuine market opportunity for its valorization. In this review, we present the suggested structures for native lignin, the five key technical lignins from the major industrial extraction processes, intumescent flame-retardant properties of lignin, several studies on lignin-based intumescent systems, innovative work on the functionalization of lignin with phosphorus-based compounds, their applications as flame retardants in bio-polymers as well as important flame retardancy results from various scholarly articles.