Lignin reaction with formaldehyde

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... 

Compared to previous gluten based adhesives the present resins are applicable in liquid form, thus without any need for modifications of the application systems in particleboard factories. Relative to other protein adhesives such as soy-based adhesives based on reaction with formaldehyde some of the resins presented here have several advantages (i) they cannot and do not produce any aldehyde emission as neither formaldehyde nor any other volatile aldehyde was used in some of the formulations (ii) the percentage of natural materials was increased up to 70% for one type of formulation and up to 95% for others. Furthermore, in relation to resin formulations based on different cross-linking reactions other than those with formaldehyde the resins presented here have other advantages they are competitive with alternate natural resin systems such as those based exclusively on tannins and/or lignins. 

The aromatic ring of a phenoxy anion is the site of electrophilic addition, eg, in methylolation with formaldehyde (qv). The phenoxy anion is highly reactive to many oxidants such as oxygen, hydrogen peroxide, ozone, and peroxyacetic acid. Many of the chemical modification reactions of lignin utilizing its aromatic and phenoHc nature have been reviewed elsewhere (53). 

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). 

The reaction of formaldehyde with the meta positions of lignin clearly have considerable potential for the use of lignin, particularly heavily condensed alkali lignin, in polymeric applications. 

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... 

Figure 7. Reaction of soda bagasse lignin with formaldehyde in 1.0 hydrochloric acid in 23% aqueous dioxane at 80°C (—,-------) and in 0.4N...

Figure 8. Generalized reaction of formaldehyde with lignin at the meta positions in acidic aqueous dioxane.

Alkali-Catalyzed Reactions of Formaldehyde with Lignins... 

The alkali-catalyzed reaction of lignin as a polyphenolic material with formaldehyde is the subject of this paper. 

This paper deals mainly with the lignin structural aspects of the alkali-catalyzed reactions of lignin with formaldehyde thus, the technique used will be illustrated briefly with two experiments (Figures 3 and 4). 

The main phase of reaction of pine kraft lignin with formaldehyde proceeds at almost constant pH (Figure 3). The extent of Cannizzaro reaction is insignificant since it can be estimated from the formic acid content. The formation of phenol alcohols (as determined by FeCl3-color reaction) reached its peak after 2 hours, and the stability is rather high even after a day. 

The reaction of lignin with formaldehyde is of practical interest not only for the potential use of lignin as reactive extender in phenol-formaldehyde resins but also for wood pulping. 

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. 

Lignin was obtained from an industrial soda bagasse spent liquor as before (5). The lignin was initially evaluated as a thermosetting adhesive by the beech strip test. Prior to its application as adhesive, the lignin was reacted with formaldehyde in alkali at temperatures below 60 °C to afford a hydroxymethylated lignin (fi). The hydroxymethylation reaction was done at pH 12 and 13, and samples of the reaction mixtures were evaluated on beech strips with overlaps of 25 x 25 mm, cured for 4 hours at 90 °C and 12% equilibrium moisture content. 

Free formaldehyde-1.8% with formaldehyde/lignin ratio of 1/1 after reaction for 1 hour at 50 °C,... 

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