## 5 Tannin-Derived NIPUs Adhesive  
Tannins are natural phenolic compounds (polyhydroxy polyphenolics) available in many plants, including fruit, wood, leaves, and bark [38; 201; 202; 203], but only a few species have adequate concentration to be isolated. Tannins can be easily extracted from a wide range of plants. The largest concentration of these chemical compounds is contained in the bark of tree species [204]. In modern polymer industries, they become increasingly useful because tannins have unique features such as anti-cancer, antimicrobial, antioxidant, or anti-inflammatory properties [205]. Extensive studies on the application of tannins in the development of tannin-based wood adhesives were carried out. Tannins were used because they are more reactive than phenol, but they are also costlier than phenol. Bark tannins were industrially used around 1850 in Lyon for the black coloring of silk for women's blouses and further used in leather production since the 1970s, but they have become more important for several other industrial uses in the last decades. Besides leather, tannins were utilized in the development of wood adhesives at an industrial scale [5; 206; 207], but extensive study into their use has led to a range of other potential uses, such as metal primers, pharmaceuticals, and many other applications. Extraction of plant material and subsequent purification of isolates, accompanied by spray drying, yield powdered tannins [59]. Tannins are usually graded as hydrolyzed and as condensed compounds in two major classes (polyflavonoid tannins). Hydrolyzable tannins are derivatives of gallic acid and are categorized according to products derived from their hydrolysis. They are also classified into gallotannins (comprised of gallic acid and glucose) and ellagitannins (comprised mainly of digallic and ellagic acids and penta-galloyl-glucose). They consist of various combinations, the main components of which are n-galloyl-glucose oligomers and their re-arrangement derivatives, such as vescalin and vescalagin. Pentagaloyl glucose and polyesters made of gallic acid and hexahydroyliphene acid are the central repeating unit \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Cross-Linker** & **Type of Wood Composite** & **Bonding Strength (MPa)** & **References** \\ \hline Na2 SO3 & Plywood & 1.10 & [199] \\ KH560 (Silane coupling agent) & Particleboard & 1.06 & [200] \\ NaIO4 & Plywood & 1.72 & [70] \\ Hexamine & Particleboard & 0.77 & [71] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Lignin-based PU adhesives and their application for wood bonding. in the hydrolyzed tannin structure. Simple phenol structures of tannin are shown in Figure 7. The flavonoid units that have undergone different condensation degrees are composed of condensed tannin extracts (polyflavonoid). Their precursors (flavan-3-ols, flavan-3,4-diols), other flavonoid analogs, carbohydrates, and traces of amino- and imino-acids were always correlated with them [204; 208]. Monoflavonoids and acids containing nitrogen are present in amounts that are much too small to affect the whole extract's chemical and physical characteristics. Nevertheless, simple carbohydrates (hexoses, pentoses, disaccharides) and complex glucuronates (hydrocolloid gums), also oligomers obtained from hydrolyzed hemicelluloses, are commonly present in sufficient amounts. Likewise, carbohydrate chains of varying lengths are also often connected to flavonoid units in tannins. It is also well known that tannins are vulnerable to modifications by hydrolysis, polymerization, copolymerization, or acetylation to produce new materials [209]. [PERSON] et al. reported the preparation of carbonated glycidyl ethers of gallic acid and tannic acid by catalytic fixation of CO\({}_{2}\). Cyclic carbonates obtained were cured with amines in the next step to obtain new bio-resourced NIPUs using natural polyphenic waste [205]. There are two main approaches for the development of PU adhesives from hydrolysisable or condensed tannins.