Abstract
Lignin is one of the wood and plant cell wall components that is available in large quantities in nature. Its polyphenolic chemical structure has been of interest for valorization and industrial application studies. Lignin can be obtained from wood by various delignification chemical processes, which give it a structure and specific properties that will depend on the plant species. Due to the versatility and chemical diversity of lignin, the chemical industry has focused on its use as a viable alternative of renewable raw material for the synthesis of new and sustainable biomaterials. However, its structure is complex and difficult to characterize, presenting some obstacles to be integrated into mixtures for the development of polymers, fibers, and other materials. The objective of this review is to present a background of the structure, biosynthesis, and the main mechanisms of lignin recovery from chemical processes (sulfite and kraft) and sulfur-free processes (organosolv) and describe the different forms of integration of this biopolymer in the synthesis of sustainable materials. Among these applications are phenolic adhesive resins, formaldehyde-free resins, epoxy resins, polyurethane foams, carbon fibers, hydrogels, and 3D printed composites.
Highlights
Up to 10% of a bisphenol A (BPA) was replaced by lignin, resulting in a resin with high thermal stability, with 50% weight degradation at 390 ◦ C, similar value to that obtained with a reference BPA-based epoxy resin
In the results reported by Mazloom et al [75], it was shown that maize crop plants were taller, had a higher amount of phosphorus, and a higher biomass when they were grown in soils with lignin hydrogels, compared to synthetic hydrogels and no hydrogels
In the biocomposites based on polylactic acid (PLA)/lignin, an important antioxidant activity was observed, increased by 50% in the composite made with 40% of lignin
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Lignin is an amorphous biopolymer with a phenolic macromolecular structure, that can be extracted from wood, annual plants, and agricultural residues [1,2]. Industrial processes are designed to extract lignin from lignocellulosic materials to convert the other components, such as cellulose, into accessible and purified fibers for different purposes. The industrial sector that uses these processes with great economic profit is the pulp and paper, which employing chemical cooking of wood, solubilizes lignin and converts cellulose into a commodity of high commercial value [4]. Its use as a versatile chemical would give rise to additional economic benefits for the pulp and paper industry as it can be available in large volumes and at a relatively low cost, compared to phenolic compounds derived from oil [11,12]. Based on the background available, the objective of this review is to provide fundamental and updated information on lignin chemistry and potential use as polymer or building block for the manufacture of new and sustainable biomaterials
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