Lignin-based flame retardants have the following characteristics:
The raw materials are widely available and environmentally friendly: Lignin mainly comes from agricultural waste such as straw and by-products of the papermaking industry. It is easy to obtain and has good environmental compatibility, which is in line with the “dual carbon economy” and green environmental protection concepts.
Excellent carbonization performance: Lignin contains a large number of benzene rings and can be used as a carbon source for intumescent flame retardants. Its aromatic structure has a high carbonization capacity. During the combustion process, it can form a stable carbon layer, which plays the roles of heat insulation, oxygen isolation, smoke suppression and anti-dripping, thereby effectively slowing down the combustion rate of the polymer.
Great potential for chemical modification: The structure of lignin contains various functional groups, such as methoxy, alcohol hydroxyl, phenolic hydroxyl, etc., providing abundant active sites for further chemical modification. The flame retardant performance of lignin can be significantly improved by introducing elements such as phosphorus and nitrogen. For example, after introducing P and N elements into lignin, both its flame retardant performance and carbonization ability have been enhanced.
High thermal stability: Due to the delocalization effect of π electrons and the uniform distribution of the electron cloud, the benzene ring structure in lignin is very stable, which makes lignin have high thermal stability. At higher temperatures, lignin will decompose, and a large amount of residual carbon will be produced during the decomposition process. Under the action of acid sources, these residual carbons will undergo esterification reactions and then dehydrate and cross-link to form carbides, further enhancing the flame-retardant effect.
Wide application: Lignin-based flame retardants can be widely applied in various commonly used materials such as polypropylene, polyurethane, polylactic acid, and epoxy resin, enhancing their flame retardant performance. For example, adding lignosulfonate to polyurethane can increase its LOI value and enhance its flame retardant performance. Adding P-N modified lignin to epoxy resin can make the UL-94 grade reach the V-0 grade.
Versatility: In addition to flame retardancy, lignin-based flame retardants may also possess properties such as anti-decay and anti-insect damage, and have a relatively small impact on the physical and mechanical properties as well as the processing performance of the material, thus maintaining the original characteristics of the material.
