Carboxyl group Lignin

The method of choice for determining carboxyl groups in lignin is based on potentiometric titration in the presence of an internal standard, /)-hydroxybenzoic acid, using tetra- -butylammonium hydroxide as a titrant (42). The carboxyl contents of different lignins are shown in Table 6. In general, the carboxyl content of lignin increases upon oxidation. 

Figure 3.8(a) Possible ester linkage between an a-OH of lignin and a carboxyl group of a 4-0-methyl-/3-d-glucuronic acid residue on the xylan backbone. 

Thus, bleaching of non-phenolic lignin derivatives is possible at both pH extremes, and conditions may be chosen according to process convenience. The formation of new functionality during color removal is inevitable, and this can be assessed by FTIR spectroscopy. Acidic conditions appear to result in the formation of carbonyl and carboxyl groups, and alkaline H2O2 is suggested to alter aromatic character. Hydroxylation and hydrolysis are encountered also. 

Ozonization of lignin forms derivatives of muconic acid that have the unique chemical structure of conjugated double bonds with two carboxyl groups. These derivatives have great potential for chemical modification. The ozonized lignin of white birch was soluble in epoxy resin at 120°C, and the free carboxyl groups were found to react with epoxide. This paper discusses developmental work on the preparation of pre-reacted ozonized lignin/epoxy resins the dynamic mechanical properties of cured resins and preliminary results of the application of these resins as wood adhesives. 

The lignin isolated from white birch after steaming was ozonized. The ozonized lignin was soluble in epoxy resins at 120°C. It was also found that free carboxyl groups (introduced by ozonization) react with epoxide only by heating. Previous developmental work on the blending of lignin with epoxy resin has been reported by Ball and his co-workers (3). They synthesized... 

Other experiments show that phenolcarboxylic acids labelled in the carboxyl group form radioactive polymers by simultaneously forming Cl4(>2. This means that not all C1402 is split off, and the polymerization does not occur only through quinones as intermediate steps. Demethylation of the lignin decomposition products is a further important reaction because o-benzoquinones can be formed under certain conditions. 

The main causes for the formation of carbonyl and carboxyl groups in cellulose are isolation and purification procedures besides natural aging. This applies in particular to cellulosic pulps from wood, which has undergone a number of processing steps to be freed from lignin, hemicelluloses, and extractives. 

In conclusion, even though there is the definite correlation between carboxylic group index and minimum in respect to graftability in our system, a detailed work with model compounds would be required to futher elucidate the mechanism of lignin participation in the copolymerization reaction. 

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