This study was carried out to elucidate the effect of defibration temperature in the range 171–202°C on the properties of 12-mm thick MDF boards made without synthetic resins from softwood fibers activated by laccase treatment for the generation of phenoxy radicals on the fiber surfaces. Laccase treatment generated radicals in the fibers. An increase in defibration temperature improved the reactivity of fibers during laccase-catalyzed oxidation. The number of radicals detected in the fibers after laccase treatment in water suspension and the fiber oxygen consumption during the treatments increased with an increase in defibration temperature, while a concurrent improvement was observed in the mechanical strength and thickness swell of dry-process MDF boards made from fibers refined at different temperatures and treated with laccase in the refiner blowline. The different fiber reactivities or board properties were not due to a presence of different amounts of lignin remaining on the fiber surfaces after acetone extraction. The probable reason for them was the fact that the amount of low-molecular weight lignin, a reactive substrate for laccase, increases with increasing defibration temperature. The adhesion occurring during pressing is thus likely to involve coupling or other reactions of radicals located on adjacent fibers, whereby interfiber covalent bonds are formed.
- heat treatment