Effect of high defibration temperature on the properties of medium-density fiberboard (MDF) made from laccase-treated hardwood fibers

High-temperature defibration of hardwood improves the reactivity of hardwood fibers during laccase-catalyzed oxidation. This is due to the progressive breakdown of the lignin polymer with increasing refining temperature, which makes it a more amenable substrate for laccase. Dryprocess 12-mm thick medium-density fiberboards (MDF) of high mechanical strength can be made from fibers treated with laccase in the refiner blowline and without synthetic resins or long laccase incubation times. The internal bond strength and thickness swell properties of the boards improve with an increase in defibration temperature. The improvement of board properties correlates with the number of radicals formed in the fibers on laccase treatment in water suspension. The extent of radical formation depends on the amount of low-molecular weight lignin and other phenolic substances extractable with water or other polar solvents and present in the fibers after defibration. As approximately equal amounts of lignin and other phenolic substances not extractable with acetone are present on the fiber surfaces regardless of refining temperature, differences in fiber surface chemical composition do not account for the different fiber reactivities during oxidation with laccase in water suspension and properties of MDF boards bonded with laccase.