## 4 Lignin-Based NIPUs Adhesive  
L Lignin is a polyaromatic macromolecule, and the second most abundant polymer in nature, preceded only by cellulose. The structure of lignin is composed of different units and repeated structures, depending on the plant species, and growing conditions, constituting approximately 24-33% of the wood substance in softwood and 16-24% in hardwood species, respectively. At present, lignin is regarded as a waste or by-product of the pulp and paper industry with an approximate annual 80-100 million tons worldwide [162]. Currently, less than 2% of the total lignin is used for value-added applications, such as dispersants, surfactants, adhesives, polymer reinforcement materials, dispersants, etc., while the rest is burnt for heat and energy purposes [162; 163]. The different technological processes, used in the pulp and paper industry to obtain lignin include mechanical, chemical, and enzymatic processes, which consequently produce different types of technical lignins, e.g., organosolv lignin by an organosolv treatment; hydrolytic lignin obtained by an enzymatic hydrolysis process; alkali lignin, derived by the Kraft process; and lignocellulosic, obtained by the sulfite pulping method. Native lignin is a complex polymer and must be partially degraded to allow its separation from the cellulose and hemicelluloses. Reactivity of different lignins depends on their chemical structure; therefore, different lignin samples contain different types and numbers of these functional groups, which can be used to manufacture industrial biomaterials and biocomposites [164; 165; 166]. The high solids content and availability of black liquor as the main by-product of the pulp and paper industry make great use of black liquor as a strategic and promising raw material for lignin. This isolated lignin from black liquor is referred to as technical lignin. Technical lignin can be converted to the industrial one by fractionation. Technical lignin has a broad molecular weight distribution, between 1000 g/mole and 100,000 g/mole, making it difficult to utilize lignin commercially. Fractionation of lignin produces lignin fractions that have useful properties for various applications. The fractions of lignin are more suitable to be used as building blocks for bio-based polymers such as bio-based PU resins [167]. Urea, melamine, and phenol-formaldehyde resins, commonly used in wood adhesives, are highly susceptible to hydrolysis and cause environmental and health problems as a result of formaldehyde volatilization over the lifetime the finished products [168]. Thus, it requires an adhesive that is environmentally friendly and with no indicated formaldehyde emission, with the characteristics of short heat curing temperatures, with short pressing time, and with high power, instead of widely used adhesives [169]. As binders in the production of engineered wood products, e.g., cross-laminated timber, NIPU adhesives can have advantageous adhesion characteristics of all adhesives [170]. They can be colorless and flexible with wood, metal, glass, plastic, rubber, ceramic, sand, and textile fiber bonds (Table 2). Some of the disadvantages of PU adhesives include high adhesive penetration in wood, low tear resistance, and unsatisfactory gap filling properties. Though rigid fillers such as calcium carbonate can be added to overcome these problems by raising viscosity, the issue is not resolved. To overcome these problems, lignin is an alternative solution. Lignin was reported to increase delamination resistance, cohesion failures, and the absence of filling properties in the NIPU adhesive formulations [171]. Lightin incorporation as a polyol substitution into the NIPU matrix is expected to improve the final polymer's aromatic content. Lignin is moderately stable at high temperatures and is an aromatic and has a cross-linked substance [172; 173]. The addition of lignin is therefore required to increase the temperature of glass transition and thermal stability of adhesives [174; 175].