Lignin is an essential organic binding element in the cell structures and fibers of wood and plants such as grasses. There are two classes of lignin — namely sulfur-bearing and sulfur-free types — which collectively represent the second most abundant source of renewable carbon in the world. As of 2011, only sulfur-bearing lignin is commercially utilized, however, with the vast majority being discarded as waste. Common industrial uses of lignin include emulsion and dispersant agents, polymer binders, and food additives. It is also used for agricultural soil rehabilitation, as a anti-corrosion agent, and as a tanning agent.
The cells, vessels, and fibers of wood and grasses are bound together by an organic substance known as lignin. This essential substance is unique in that its chemical composition is never exactly the same from one plant to the other, with the only predictable characteristic being a phenyl-propene-based dendritic network polymer. The substance is also noteworthy for the fact that it, after cellulose, represents the second richest renewable carbon source on Earth. Two basic classes exist — sulfur-bearing and sulfur-free lignin — with the sulfur-bearing variant being the only one of any commercial interest as of 2011. In fact, very little of the substance is used, with the majority of the 40 to 50 million tons (36.3 – 45.4 million tonnes) produced annually being destined for the non-commercial waste heap.
The two sulfur-bearing lignin varieties commonly utilized are lignosulphonates and Kraft lignins, with an approximate collective global production of 600,000 tons (544,310 tonnes) per annum. One of the desirable characteristics of these substances are their hydrophilic and hydrophobic qualities, which see the substances used as multi-polarity dispersant and emulsifying agents. Being a naturally-branched and cross-linked network polymer, it is also regularly used as a binding agent in a range of materials, such as polyurethane, polyester, and several grades of particle and resin boards. Other industrial uses of the substance as a material binder include the production of composites, activated carbons, and several epoxies.
The agricultural sector also utilizes lignin as a soil rehabilitation aid as well as a component in slow release fertilizers. Other agricultural uses include components in insecticides, artificial humus, and as a granulation and pelletizing aid. The food industry is also a consumer of lignin as a component in a variety of anti-oxidant and anti-bacterial food additives. In other industries, the substance is frequently used as a tanning agent, foam stabilizer, and a component in several pharmaceutical anti-bacterial and anti-inflammatory preparations.