Lignin Bonding

The lignin-removal and fiber-separation mechanisms involved in chemical pulping are alkaline hydrolysis (lignin bond cleavage) and the formation of soluble sulfur-containing... 

The production of enzyme-lignin-bonded particleboard is much less hazardous than the production of isocyanate-bonded particleboard, for example. 

The phenoloxidase-lignin-bonded particleboards are free of any emission. 

Under acidic conditions, however, the liberation of free acetic acid during hydrolysis of the esters can increase the acidity and enhance further hydrolysis of not only additional ester groups, but acetal linkages and lignin bonds as well. A prime example is the so-called autohydrolysis reaction (2) in which acetic acid liberated by steam can cause complete hydrolysis of the hemicelluloses and convert the lignin into soluble fragments. 

Table I. Kraft and milled wood lignin bond types.

The bleaching is done in three to five stages in which the pH of the pulp is alternated between acid and alkaline conditions. During the acid cycle, the chemical reactions between the bleach and lignin bonds turn the pulp lighter in color. During the alkaline cycle, the reaction products from the acid stage are removed (38). 

During enzymatic hydrolysis, nonproductive cellulose-lignin bonding limits the conversion via the enzyme. Should an enzyme adsorb onto lignin, it is irreversibly... 

The pulping of wood is at present based on either mechanical or chemical methods or combinations thereof, as previously described in this chapter. The interfiber lignin bond is broken down by the mechanical and chemical treatments to free the cellulose fibers for papermaking. In the forest, white rot fungi perform a similar task on wood left behind. The enzymes of the fungi do the work of lignin degradation. 

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