Hydroxymethylation of lignin

The primarily applications with few or no modification (other than sulfonation or hydroxymethylation) of lignin are as dispersants, emulsifiers/surfactants, binders and sequestrants. These uses represent relatively low value and limited applications, but by the development of appropriate technologies to modify, control and amplify lignin s polyelectrolyte properties, chemical reactivity and compatibility properties with other monomers and polymers, it could be turn into higher-value polymer products. 

In the above paragraph it was shown that the lignin Cg units can be polymerized at the 2- and 6-positions with formaldehyde in acidic aqueous dioxane. The polymerization reaction proceeds presumably via a hydrox-ymethylated intermediate which was isolated in low yields only when large excesses of formaldehyde were employed. If the hydroxymethylation of the meta position can be achieved in high yield it would clearly afford a very... 

Van der Klashorst, G. H. The Modification of Lignin at the 2- and 6-Positions of the Phenylpropanoid Nuclei. Part III. Hydroxymethylation of Industrial Alkali Lignin. J. Wood Chem. Technol., in press. 

Table III. Evidence for Hydroxymethylation of Kraft Pine Lignin...

Table III. Beech Strip Strengths of Hydroxymethylated Bagasse Lignin Adhesives Crosslinked with Various Fortifiers...

Figure 5. Strength of three-layer particleboard (20% surface chips) glued with hydroxymethylated bagasse lignin/commercial PF resin (67 33) and pressed at 170 to 180 °C for different times, 12% chip moisture content, final density 680 kg/m3.

Figure 8. DSC thermograms of lignin hydroxymethylated under different sodium to lignin ratios a) 1.0, b) 0.75, c) 0.5, and d) 0.25.

Acidic condition. Acidic hydroxymethylation occurs mainly at the C2 or C6 position of aromatic nuclei which may be phenolic or etherified units. It is used in the preparation of wood adhesives. The influence of lignin structure on reactivity and performance of the resulting resin product is still not entirely understood, as discussed by Nimz [3951. Van der Klashorst [396-398] reported the acidic condensation of alkali lignin and lignin model compounds with formaldehyde. Initial hydroxymethylation was faster with syringyl units... 

Surface Activation. Acid Activation. Acid treatment of cellulose and hemicelluloses generally leads to hydrolysis to monosaccharides, which can subsequently dehydrate and condense to form furan-type compounds such as furfural and its 5-hydroxymethyl adduct. Further reactions lead to polymeric materials of dark color as well as to monomers such as levulinic acid, formic acid, and angelica lactones. Various condensation and solvolysis reactions also accompany the acid treatment of lignin 123). The hydrolysis, dehydration, and condensation reactions have been used to explain formation of covalent bonds between surfaces (85), increase in water resistance (85, 124), and weakening of wood (75) in nonconventional plywood or particle board production. However, very little factual information is available on how far, in terms of the consecutive reactions mentioned, and in what direction, in terms of the parallel reactions mentioned, does the surface of lignocellulosic materials actually change... 

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