Lignin condensation with formaldehyde

Many of the chemical reactions used to modify lignosulfonates are also used to modify kraft lignins. These include ozonation, alkaline—air oxidation, condensation with formaldehyde and carboxylation with chloroacetic acid (100), and epoxysuccinate (101). In addition, cationic kraft lignins can be prepared by reaction with glycidjiamine (102). 

Condensation of Lignins with Formaldehyde. A sample of 10 grams of kraft lignin (45 meq. of OCH3) was dissolved in 50 ml. of IN sodium hydroxide, and commercial (35%) formalin was added, containing 120 mmoles of CH20. The solution was set aside at room temperature for... 

In alkaline media, phenolic units may react with formaldehyde, forming methylol derivatives that condense with themselves or with another phenol (J, Fig. 1.4). This formaldehyde condensation reaction forms the basis for using technical lignins in the production of adhesives. 

Lignin condensation reactions have been studied for many years, mainly because they possibly interfere with efficient delignification of wood. Numerous model compound studies established the various types of condensation reactions that may occur between lignin fragments (1-5) or between lignin and carbohydrates (1,2,6,7) during alkaline pulping. An important class of condensation reactions involves formaldehyde. 

Figure 2. Condensation of formaldehyde with lignin free-phenolic units.

Progress in the use of condensed tannins in adhesive formulations might be expected to be more rapid than is the case for lignins because of the impetus provided by the commercialization of wattle tannin-based adhesives and because of the extraordinarily high reactivity of tannins in reactions with formaldehyde. This reactivity offers an opportunity to substitute tannin for resorcinol (currently priced at about 1.80/lb) instead of phenol (about 0.40/lb). Now that wattle tannins have been successfully introduced, their application can be expected to continue to expand. The situation remains difficult, on the other hand, for use of conifer bark tannins in adhesives. Herb Hergert is certainly... 

Additionally, formaldehyde may condense with two aromatic units to form another diphenylmethane-type condensed structure (59). These condensation reactions have been well established in lignin model studies (32,300,301]. 

Lignin can be modified by condensation with phenol, formaldehyde, amino acids [3731, or diazonium salts 1387,3881. 

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... 

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