Phenolated kraft lignin

The effectiveness of betaine can be explained better by its blocking proton uptake from solution. Betaine lessened the amount of protonated phenolic hydroxyl groups at any pH and consequently the intermolecular association between kraft lignin molecules. The chemical reactions can be shown as follows ... 

The variation of catechol and total monomers identified with NaOH concentration is shown in Figure 7B. A maximum of approximately 11% total phenols and 6% catechol is reached at 2% NaOH with increasing alkaline concentration for the same treatment severity. It should be noted that Clark and Green (5) also obtained a maximum of 11% phenolic compounds when they cooked kraft lignin in 4% NaOH at 300°C for 30 minutes. The quantity of catechol was found to be 5.3% of the initial lignin under these conditions. 

It is becoming apparent that wood components, especially lignin, are chemically modified by solvents during wood dissolution, and that the resulting wood tars or pastes become highly reactive. Attempts have therefore been made to prepare effective adhesives, moldable resins and other products from wood after dissolution in phenols or polyhydric alcohols. This review presents recent progress on wood dissolution, and on the preparation of epoxy and phenol resin adhesives from kraft lignin. 

In this study, kraft lignin was chemically modified with phenol and with bisphenol-A in order to enhance its potential as adhesive. 

Epoxy resin adhesives from lignin were reported by Tai et al. (19,20) in 1967. Satisfactory gluability was found. The solubility in organic solvents, however, was found to be poor. In essence, the phenolic hydroxyl groups of kraft lignin were glycidylated directly. 

We have also made an attempt to prepare lignin-epoxy resin adhesives (21). However, in order to improve its reactivity, kraft lignin was first phenolated with bisphenol-A. For phenolation, a small amount of aqueous hydrochloric acid or BF3-ethyl etherate was used as catalyst, and thus two kinds of glycidylation methods were adopted (21). The phenolation with bisphenol-A was found to enhance the solubility of the lignin derivative. In fact, the lignin-epoxy resins obtained were found to be completely soluble in certain organic solvents, including acetone. 

Resol-Type Phenol Resin Adhesives from Kraft Lignin... 

There have been many attempts to prepare resol resin adhesives from kraft lignin (23). Phenolation of lignin can, in these cases, also be considered a key reaction. That is, effective modification of lignin with phenol can enhance its reactivity, resulting in good gluability. 

M.W.L. contains free phenolic (guaiacyl) hydroxy groups in about every third unit kraft lignin contains about twice as much. In addition, probably about every 15th unit of kraft lignin has a catechol ring structure 2,13). 

By means of Fremy salt oxidation, Adler and Lundquist (5) estimated that about half of the phenolic units contain free S-positions in M.W.L. Mikawa (J/), by Mannich reaction, obtained evidence that about 25-40% of the phenolic units in his sulfate lignin preparation contained no substituent in 5-position. In this investigation we also conclude that at least one-third of the phenolic units in our kraft lignin preparation has free 5-positions. 

Figure 2 shows a difference curve obtained from methylol kraft lignin when untreated lignin was used as reference. The difference curve has a distinct maximum at 590 m/z, characteristic of the phenol alcohol structures formed. By comparing the absorbance at 590 m/z with the molar absorbance (e) of models investigated, the amount of phenol alcohols in lignin can be estimated. The numerical values depend somewhat on the e value of the model chosen. In this study a value of e = 840 was used, the increase found at 590 m/z between the molar absorbances of the dimeric phenol alcohol and its parent phenol of model II. 

Regarding these uncertainties the agreement of phenol alcohols found in kraft lignin with FeCl3 test and other methods is very reassuring. This colorimetric method is less applicable for M.W.L. preparations in which only a small amount of phenol alcoholic groups can be formed. 

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. 

In the latter preparation, about 0.05 a-CO groups per C6-C3 were found in phenolic units (26). It is known that the phenolic hydroxyl in acetovanillone or in similar phenolic ketones is quite acidic (21) (pK = 8) reducing these ketones would decrease the acidity to the usual level of phenols (pK = 10, or still higher in sterically hindered phenols). This observation is a further indication that our present concept concerning the presence of a-CO groups in kraft lignin (24) is right. 

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