## 4 Lignin-Based NIPUs Adhesive
The structure, chain length, and several hard and soft segments are parameters that significantly affect the temperature of the glass transition, thermal decomposition, and the mechanical strength of the NIPU lignin-based compound [175]. [PERSON] et al. indicated that the shear strength decreases as polyethylene glycol (PEG) chain length is increased, and the NIPUs that contain 50% W/V of polyol have optimum characteristics (PEG with a molecular weight of 200). Additionally, the initial decomposition temperature increased to 50% W/V of polyol from 270 to 285 \({}^{\circ}\)C as lignin content increased [174]. The mechanical characteristics of polyurethanes are largely determined by the interconnecting density, which depends on their stoichiometry and functionality. Even though lignin substitutes polyol in the soft segment NIPU (NIPU structure), its aromatic structure normally makes it the tough segment, thus increasing the mechanical strength of lignin-based NIPU [175]. Lignin, because of its aromatic composition, typically enhances the mechanical resistance and thermal properties of blends, copolymers, and composites. Lignin affects the quantity and chemical composition of the final adhesive. It contains different functional groups, i.e., methoxyl, hydroxyl, and carbonyl groups, which allow its chemical modification, intended to increase its reactivity. The main reason for its application in the formulation of wood adhesives is due to its polyphenic structure, mostly as a partial substitution of phenol (C\({}_{6}\)H\({}_{6}\)O) in PF resins. Lignin was reported to increase the thermal stability and cause phase separation, resulting in a poor stress transfer and eventually weakening the mechanical properties of end products. One of the key barriers to broader usage of condensing adhesives is the poor reactivity of lignin and lignin-derived compounds towards condensation reactions [174], delamination [171], and abrasion resis \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Lignin- and Tannin-Based NIPUs** & **Other Wood Adhesives** & **References** \\ \hline \multirow{4}{*}{Bonding properties} & High delamination resistance, adhesion, and cohesion strength. & UF, MF, MUF give over penetration in & \\  & Tensile strength and deformation are & wood, lower tear resistance, and & \\  & comparable to traditional & lower adhesion compared to NIPUs & \\  & isocyanate PUs. & & \\ \hline \multirow{2}{*}{Physical properties} & Comparable water resistance and & Lower water resistance of & \\  & dimensional stability to & formaldehyde-based adhesives & \\  & isocyanate PUs & compared to NIPUs & \\ \hline \multirow{2}{*}{Mechanical properties} & High value of hard to soft ratio & Mechanical properties are lower than & \\  & resulting in satisfactory & NIPUs adhesives & \\  & mechanical properties & & \\ \hline \multirow{2}{*}{Chemical properties} & Having greater chemical resistance & Lower chemical resistance and & \\  & for about 30–50\% than & permeability than NIPUs adhesives, & \\  & other adhesives & except for isocyanates & \\ \hline \multirow{2}{*}{Thermal properties} & Thermal stability enhancement owing & Lower thermal stability than NIPUs, & \\  & to the presence of aromatic lignin & except for isocyanates & \\  & and tannin & & \\ \hline \multirow{2}{*}{Renewability} & Derived from renewable biomass of & Derived from petroleum that is not & \\  & lignin and tannin & renewable & \\ \hline \multirow{2}{*}{Toxicity} & Less toxicity due to being & Release formaldehyde and contain & \\  & isocyanate free & isocyanate that are carcinogenic & \\ \hline \multirow{2}{*}{Price} & Cheaper than isocyanate PUs, but still & Isocyanates are expensive, but & \\  & more expensive that those of & formaldehyde-based resins & \\  & formaldehyde-based resins & are cheaper & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Advantages of NIPUs compared to other adhesives. tance [176] by significantly adding it to the polyurethane matrix.