## 4 Lignin-Based NIPUs Adhesive
However, with lignin volumes greater than 60% in total, the breakability is increased [175] and causes phase separation [177], resulting in a poor stress transfer and eventually deteriorated mechanical properties of end products [177, 178, 174]. Various methods of lignin modification, such as demethylation [179, 180, 181, 182], hydroxymethyl [183, 184], phenolation [185, 186, 187], oxidation [188], etc. were reported to improve lignin reactivity. Among them, demethylation seems to be an efficient and forward-looking process [181, 182], as certain aromatic methoxyl groups are converted into phenolic hydroxyl groups under mild reaction conditions. The proportion of reactive sites would therefore be increased [179, 180], providing the presence of more catechol moieties in lignin and their reactivity enhanced by the use of HI [189], HCl [190], and various other hydrolysis reactions, fungi (white or brown-red fungi) [191], and bacteria (_Pseudomonas_ or _Sphingomonas_) [192]. However, some disadvantages were determined, including environmental concerns such as acid state, high processing temperature (up to 225 \({}^{\circ}\)C) [179], longer processing time, and high processing pressure [182]. More effective approaches should therefore be preferred to replace these intense reaction conditions. In the demethylation phase, where a nucleophilic substitution reaction occurs between hydroxyl and sulphite ions, lignin is converted to sulphite pulp [179]. Then, aryl methyl ethers of aromatic compounds are broken down by nucleophilic substitution reactions with sulphite ions, resulting in a decrease in the methoxy group's content and an increase in the reactivity of modified lignin. Figure 6 shows the potential route of conversion of lignin to NIPUs. Demethylation of lignin could be performed using 1-dodecanetiol (DSH) and dimethylformamide (DMF). Because demethylated lignin has more phenolic hydroxyl groups, lignin has been regarded as building blocks for NIPUs. Carbonation with dimethyl carbonate may create carbonyl groups that can be used as precursors for NIPUs. The NIPUs are cured with the aid of hexamethylene diamine. Polyester polyols synthesized from potato starch and vegetable oils by transesterification were used in the development of PU wood adhesives by reaction with toluene diisocyanate [193], with adhesive performance being similar or superior than the commercial synthetic PU adhesives. Oxidation is a standard process used in the production of starch wood adhesives [194]. Recently, some researchers drew attention to the selective oxidation of plant meals in terms of their efficacy in the application of adhesives [195, 196]. Periodate is a particular oxidant that cleaves carbon to carbon bonds, presenting vicinal aliphatic hydroxyl groups to form aldehydes and dialddehydes [195, 196, 197]. In certain instances, the lignin structural units present on their aliphatic portion both hydroxyl groups vicinal to the \(\beta\)-O-4 bonds to another lignin unit. The \(\beta\)-O-4 linkage is easily oxidized so that aldehydes and possibly dialdehydes are formed. Moreover, the more significant proportion of aromatic hydroxyl groups also increased aromatic ring reactivity due to demethylation. Thus, aldehydes formed by periodate oxidation are likely to respond to create a more robust hardened network at hydroxyl-activated reactive phenolic areas. This adhesive can be known as a bio-based product. In their study, [PERSON] et al. aimed at improving and testing its bonding efficiency as a sustainable, lignin-derived wood adhesive that has a high biomass level without any external-added aldehydes by two steps: demethylation and oxidation. The sodium sulfite was synthesized under a moderate condition as a catalyst for directly mixed sodium periodate (NaIO\({}_{4}\)) at atmospheric pressure [70]. Because certain methoxy groups have become phenolic hydroxyl groups through a nucleophilic substitution reaction, the process of demethylation will increase lignin reactivity so that more reactive areas can be formed in the lignin.