Lignin

Lignin is a complex and heterogeneous cross-linked polymer, containing aromatic rings, CC bonds, phenolic hydroxyls, and ether groups, with molar mass higher than 104 g mol-1. It is formed in chemical association with cellulose, giving lignocellulose, in the cell walls of plants. Thus lignin is not a polysaccharide, but a complex substance consisting of aromatic structures with alkoxy and hydrocarbon substituents that link the basic aromatic unit into a macromolecular structure through carbon-carbon and carbon-oxygen bonds. It is not heterogeneous both in chemical composition and molar mass. Lignocellulose is strong and tough, and provides physical, chemical and biological protection to the plant. Lignin is resistant to peroxidation as a result of the presence of many antioxidant-active phenolic groups, which act as protective agents against abiotic peroxidation and biological attack by peroxidase enzymes. Lignin is obtained mainly from wood: 25-30% of wood is lignin, filling the spaces among cellulose, hemicellulose and pectin in the cell wall of plants. It is also present in some algae. After the cellulose, it is the second most abundant organic polymer on earth, with about 50 million tons being industrially produced annually. It is the main humus-forming component, which provides nutrients and electric charges to the soils. Humus is slowly biodegradable by oxidase and peroxidase enzymes, produced especially by fungi. Although basidiomycetous white-rot fungi and related litter-decomposing fungi are the most efficient degraders of lignin, mixed cultures of fungi, actinomycetes, and bacteria in soil and compost can also mineralize lignin.

The main industrial use of lignin is still the power generation, as biofuel. A biodegradable material based on lignin, obtained as a byproduct from the manufacture of paper, mixed with vegetable fibers, is manufactured by Tecnaro under the trade name of Arboform. This is a hygroscopic thermoplastic material, that can be processed at 140 °C. Its mechanical properties show high rigidity and low deformability.