Modification of lignins

Once lignin is separated from other plant products, it is not now widely used in resource recovery. Extensive studies on the modification of lignin have been made(24) because of the enormous mass of kraft lignin produced each year by the pulp and paper... 

A considerable amount of data has accumulated regarding the modification of lignins to engineering plastics. Unfortunately, the incorporation of various monomers and polymers, such as di- and polyvalent epoxyphenols, esters and isocyanates, in the lignin structure in most cases resulted in brittle or tarry materials whose properties designated them as potential adhesives, lacquers, dispersants and films, but not as structural materials (36-40). 

New trends in the modification of lignins related to the formation of polymeric materials with such special properties as thermal stability, fire resistance and use as carriers for controlled release, bioactive compounds are discussed. Several properties of new polymeric materials, especially their thermal behavior, were studied. 

The modification of lignins with chlorophosphazenes allows the manufacture of products characterized by flame resistance and thermal stability. This can be attributed to the aromatic structure of the lignin-phosphazene polymer as well as to the presence of such flame inhibiting elements as phosphorous, nitrogen and sulfur. Other useful properties may also result from this combination. It has previously been reported (8-13) that the modification provides crosslinked products with suitably low chlorine content. This is a consequence of incomplete substitution of the phosphazenes cycles. Additional modification of the reaction products by chemical compounds with reactive hydroxyl or amine groups reduces the unreacted chlorine content and improves product properties (8-13). Some properties of the derivatives obtained are presented in Table I. 

An additional modification of lignins with hydroxyl or amine group-containing compounds was found to further improve the product properties (Table III). 

The modification of lignins by chlorophosphazenes allows the formulation of polymeric materials characterized by ... 

The modification of lignins with chlorophosphazenes results in changes of E that correspond to the degree of substitution and the phosphorous content (Table V). 

Some results of the modification of lignin sulfonate Ultra B002 by reaction with terephthaloyl chloride are summarized in Table VI. The total hydroxyl content of the lignosulfonates as well as their derivatives are presented in Table VII. The hydrolytic resistance of selected products is evaluated in Table VIII. The results presented in Tables VI-VIII stress several advantages of the derivatives with terephthaloyl chloride. The modified lignin sulfonates were insoluble, or only very slightly soluble, in organic solvents. They were, however, soluble in dimethyl sulfoxide. Ordered structures were identified by X-ray studies (16,17). 

The modification of lignin sulfonates with terephthaloyl chloride produces new polymeric materials containing ester groups. This modification can be used to utilize lignins also for improvement of chemical fiber properties. This is presently under investigation. 

These examples thus clearly show 4-hydroxy-3-methoxyphenyl and 4-hydroxy-3,5-dimethoxyphenylalkanes (typical of the lignin structure) to be reactive at positions 2 and 6 towards electrophilic substitution reactions under acidic conditions. The subject matter of this paper deals with the development of procedures whereby the 2- and 6- (i.e., meta) positions are utilized for the polymerization and modification of lignin. 

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. 

It is therefore very likely that the lignin itself is highly modified by solvents, especially by phenols. In consequence, effective modification of lignin can be considered to enhance its reactivity. 

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. 

In a somewhat unusual example of electrochemical fluorination, a group of Russian workers have reported [50] the modification of lignins by fluorination of the organic substrate in aqueous alkaline solutions of sodium and potassium fluorides, at platinum or platinised titanium anodes. The fluorine content in the product was said to reach 20%. 

Subsequent efforts to inhibit photochemical yellowing of lignin-containing paper were based on what was believed to be the dominant reaction pathways to yellow chromophores the formation of phenoxy free-radicals by direct excitation of phenolic groups and the abstraction of phenolic hydrogen by the aromatic carbonyl triplet excited state, as shown by reactions 1 and 2 in Scheme 1. The approaches used by researchers were modification of lignin by ... 

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