Treatment of wood fibers with laccase generates phenoxy radicals in the fibers. Cross-linking reactions involving phenoxy radicals are likely to account for the improved mechanical properties observed when fibers are treated with laccase before being fabricated into fiberboards. Knowledge of the effects of raw material type and defibration conditions on the chemical structure and extent of radical formation in the fibers is therefore important. The type of wood raw material used and defibration temperature have a large impact on the number of radicals that can be formed in the fibers. Hardwood fibers are more reactive than softwood fibers produced at equal defibration temperature. The differences in reactivity between the various hardwood and softwood species, respectively, are not significant. An increase in defibration temperature brings about an increase in laccase reactivity. The magnitude of this increase is larger for hardwoods than for softwoods. The laccase reactivity of fibers depends on the chemical structure of the lignin coming into contact with the enzyme. Depolymerized lignin rich in phenolic hydroxyl groups can be extracted from the fibers with water. The water extractable fiber fraction is far more reactive toward laccase than the remaining water-extracted fibers. An increase in defibration temperature results in an increase in the degree of depolymerization of the fiber lignin. The depolymerization is more extensive for hardwood than for softwood lignin. The surface lignin coverage as determined by ESCA spectroscopy is rather similar for all the fibers studied. The results therefore indicate that the differences in radical formation in different fibers can be traced back to their content of low-molecular weight, water soluble lignin.
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