Research on the Application of Lignin in Flame Retardant PLA
Lignin ( Lig ) is the second most abundant substance after cellulose in nature and the largest natural aromatic polymer. According to the survey, the lignin reserve on the earth is huge, and it increases at a rate of 10 billion tons every year.
Since lignin is a complex polymer formed by the random arrangement and polymerization of phenylpropanyl compounds (see Figure 1), it also contains multiple functional groups such as hydroxyl, methoxy, and carbonyl, so compared with biomass such as cellulose, wood Elements are difficult to decompose, and most of them are incinerated in industry, so it is difficult to be effectively utilized.
The renewable nature, environmental friendliness and low price of lignin make it have great potential for development. At the same time, the development of science and technology also makes the utilization of lignin waste resources possible. At present, the separation of lignin derivatives can be achieved through extraction, membrane separation and other methods. At the same time, lignin derivatives can be used as chemical raw materials in many scenarios. The modification of lignin to produce flame retardants is an extremely important aspect. The flame retardant is composed of carbon source, acid source and gas source. Lignin contains a large number of benzene rings and can be used as the carbon source of the flame retardant. Using lignin as the raw material of the flame retardant conforms to the "double carbon economy" and is green and environmentally friendly.
Renewable resources mean that after resources are consumed, they can be regenerated within a certain period of time for self-replenishment. Lignin is mainly derived from agricultural waste such as straw and by-products of the paper industry. It is easy to obtain, and lignin has good environmental compatibility and is green and environmentally friendly.
Lignin can be widely used in common materials such as polypropylene and polyurethane, and modified lignin has a wide range of applications in flame retardants. Lignin has certain flame retardant properties, but it is still not up to the standard for industrial use. At present, lignin is often modified to improve the flame retardant properties of lignin. Lignin can be used as a raw material for flame retardants, mainly relying on its thermal stability and excellent char-forming ability.
The special positions of hydroxyl and benzene rings in lignin determine the main properties of lignin. Due to the delocalization of π electrons and the uniform distribution of the electron cloud, the benzene ring itself is very stable, so lignin has high thermal stability. Only at higher temperatures will lignin decompose. Usually, the chemical bonds of the functional groups are broken first, and then the carbon chains are completely rearranged. This process produces a large amount of residual carbon. Under the action of acid source, the residual carbon will undergo esterification reaction and then be dehydrated and cross-linked to form carbides. At the same time, under the action of gas source, it will become a honeycomb porous foam structure.
The structure is non-combustible and can be used as an isolation zone between the material and the fire source to prevent heat conduction and thus play a flame-retardant effect. Some related studies have introduced P and N elements into lignin, and tests have proved that functionalized lignin has improved flame retardancy and enhanced carbonization ability. In addition, lignin contains a large number of hydroxyl groups, and we can improve the flame retardancy of lignin through hydroxyl modification to prepare a series of flame retardants for lignin derivatives.
Lignin-based flame retardants applied to polylactic acid.
Due to the poor flame retardancy of most polylactic acid products, in order to adapt polylactic acid to more application environments, it needs to be modified. In order to maintain the biodegradability and environmental compatibility of polylactic acid, it is necessary to select environmentally friendly flame retardants, and lignin-based flame retardants are one of them.
Wei Hu et al. synthesized a lignin-based flame retardant LMD containing P. Through testing, it can be seen that the LOI value of pure polylactic acid is 18.2%. With the addition of flame retardant LMD, the LOI value of the material gradually increases. On this basis Adding 1,3,5-triglycidyl isocyanurate (TGIC) can continue to improve the flame retardancy of the system.
