Jaakko Laine

The influence of high defibration temperature on the properties of medium-density fiberboard (MDF) made from laccase-treated softwood fibers

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.

Effect of high-temperature defibration on the chemical structure of hardwood

Summary The present paper aims at elucidating the effect of high-temperature defibration at different temperatures on the bulk and surface chemical properties of defibrated birch, aspen and eucalypt. The results indicate that defibration of these hardwoods results in partial depolymerization of fiber lignin via (homolytic) cleavage of interunit alkyl-aryl (β-O-4) ether bonds. This increases the phenolic hydroxyl content and produces relatively stable (phenoxy) radicals. Syringyl-type lignin is more extensively depolymerized than guaiacyl-type lignin. Defibration generates water-extractable material, which is enriched in hemicellulose-derived carbohydrates and has a substantial content of aromatic compounds rich in phenolic hydroxyl groups. The amount of water-extract and the extent of lignin interunit ether bond cleavage increase with an increase in defibration temperature. The differences between various hardwood species in this respect are small. The surface chemical composition of the fibers differs considerably from their bulk composition, but is not significantly influenced by variations in defibration temperature. Lipophilic extractives cover a large portion of the fiber surface, while the lignin content of lipophilic extractives-free fiber surfaces is 2–3 times as high as the bulk lignin content of the fibers.

EFFECT OF HIGH-TEMPERATURE FIBERIZATION ON THE CHEMICAL STRUCTURE OF SOFTWOOD

Softwoods fiberized at high temperatures (above 170°C) were subjected to bulk and surface chemical analyses. It was found that the frequency of lignin β-O-4 linkages declined while that of phenolic hydroxyl groups increased with an increase in fiberization temperature. The amount of water extractable aromatic compounds increased with increasing temperature of fiberization, which was associated with cleavage of lignin ether linkages. The water extractable material generated was enriched in hemicelluloses and contained aromatic compounds rich in phenolic hydroxyl groups and low in β-O-4 linkages. The amount and hemicellulose content of the water extracts increased with increasing fiberization temperature. Lipophilic extractives covered most of the fiber surfaces while the surface lignin content of extractives-free fibers roughly doubled their bulk lignin content.

Manufacture of Fiberboard from Wood Fibers Activated with Fentons Reagent (H2O2/FeSO4)

Summary Fiberboard was manufactured without synthetic resins from spruce and beech fibers activated by treatment with Fentons reagent (H2O2/FeSO4) and the formation of radicals in the fibers by the action of Fentons reagent was studied. The treatment resulted in a strong improvement of board strength and a large increase in fiber free radical content. The improved adhesion is probably largely due to interfiber bonds formed by reactions of radicals or other reactive groups generated in the fibers by Fentons reagent. The reactivity of spruce fibers in terms of radical formation increased as defibration temperature increased. The thickness swell of the boards depended more on the amount of sizing agent or other additive than on the defibration temperature or fiber reactivity toward Fentons reagent.