# Lignin to Materials: A Focused Review on Recent Novel Lignin Applications
## 2 Technical Lignins  
In nature, lignin is an amorphous phenolic polymer that is randomly branched and crosslinked with cellulose and hemicellulose [7]. To utilize lignin, it needs to be isolated from biomass through various processes, and the extracted lignin is called technical lignin. Since most technical lignins are available in the pulp and paper industries as byproducts, the production of lignin will continue to increase as lignocellulosic ethanol emerges. Considering that the lignin extraction method modifies the native structure of lignin, it is important to understand how the process affects lignin structures to develop effective lignin valorization technologies. There have been many review articles that thoroughly discuss lignin separation methods and their influence on the structures and properties of the extracted lignin [8; 9; 10; 11]. The following overview briefly introduces common industrial processes in which lignin is extracted from biomass.  
Figure 1 illustrates several processes to extract technical lignins from lignocellulosic biomass. Among the technical processes, Kraft pulping is the major chemical pulping process, accounting for 85% of the total lignin production in the world [12]. The process is performed at a high pH, and about 90-95% of the lignin is dissolved into the black liquor. Kraft lignin is typically precipitated and recovered from black liquor by the addition of acidifying agents. Predominantly, the acidification is carried out by adding either mineral acid (e.g., sulfuric acid) or carbon dioxide, followed by filtering, washing, and drying for the recovery of Kraft lignin. Around 630,000 tons of Kraft lignin are annually produced, and most Kraft lignin is combusted for heat generation, resulting in low-value utilization. The sulfite pulping process is conducted between a pH of 2-12, depending on the cationic composition of the pulping liquor [13]. Lignosulfonates, isolated lignins from the sulfite process, contain significant amounts of sulfur in the form of sulfonate groups. Since lignosulfonates are widely available, lignosulfonates were used in a wide range of applications, such as dispersants, flocculants, concrete additives, and composites [14].  
The organosolv process constitutes the fractionation of biomass components through treatment using an organic solvent, such as ethanol, ethylene glycol, acetone, tetrahydrofuran, and \(\gamma\)-valerolactone [15; 16]. Since the organosolv process is conducted in the absence of sulfur, it has recently been utilized more so than Kraft and sulfite pulping. Furthermore, the large-scale production of organosolv lignin is expected from the emerging cellulosic ethanol sectors, which offers significant opportunities for lignin valorization.  
Figure 1: Processes for the extraction of technical lignins.  
In addition to the conventional pulping and biomass pretreatment processes such as steam explosion, dilute acid, and ammonia fiber explosion, there have been significant advances in biomass fractionation to extract high-quality lignin using novel solvents. For example, ionic liquids (ILs) and deep eutectic solvents (DESs) have attracted considerable attention as promising agents for biomass fractionation due to their high solvation capacity in the dissolution of biomass components [17; 18]. The technical lignins obtained from ILs and DESs have been found to retain their original structures; the eco-friendly properties of such solvents offer new options for lignin extraction.