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The influence of fibre chemical constitutents in oil-tempering of hardboard. Paszner, Laszlo

Abstract

This study was designed to examine effects of alcohol-benzene solubles, hemicellulose, modified cellulose, lignin and modified lignin residuals in refined Douglas fir Asplund fibre on the heat-activated polymerization of tempering oil applied to wet-batch process hardboards. Thin (0.05 cm.thickness) experimental boards were prepared with good formation properties and reproducibility for the study. Results of oil-tempering, heat-treatment and humidification treatments were compared with and without modifying the raw stock. Oil-tempering was most effective on boards made from unmodified or alcohol-benzene extracted fibre. An ultimate tensile strength increase of 105% approximates gains had in commercial practice. Heat-treatment alone was ineffective in developing extra strength, possibly because of extended hot-pressing of the boards. Among the wood constituents investigated lignin was involved.in approximately 80% of the extra strength development on oil-tempering. It was found that oil-tempering effects could be severely depressed by mild oxidation of the fibre with acidified sodium chlorite (NaClO₂) solution at 70° C. Accompanying weight loss was below 5%. Alternatively, partial deactivation was obtained by inhibition of the fibre surface or precondensing the lignin in the raw stock with hot-water-soluble hemlock bark tannins. The tannins were introduced into the fibre structure by a new method including hot-soaking and cold-precipitation at 3% slurry consistency. Strength development on oil-tempering was thus reduced by the oxidation treatment to approximately 20% which was unaffected by further chemical treatment. This residual strength increase may be due to some other effect than lignin. Evidence for a chemical mechanism is suggested by observations that only a limited portion of the oil takes part in strength development, pointing toward limited sites available for polycondensation. Possibly, these sites are inactivated.by even mild oxidizing treatments of the fibre. This suggests that lignin quality is more important than quantity of lignin, alcohol-benzene solubles, hemicellulose or cellulose in the oil-tempering mechanism. These findings are contradictory to the literature. To date, lignin has not been considered an important wood constituent in strength development of different wood products by impregnation, and condensation-polymerization systems with unsaturated compounds. Further removal of lignin from the fibre (10 to 25% weight loss) improved formation and bonding of fibres and the subsequent strength properties of boards. This conforms to well described mechanisms accompanying fibre delignification. Additional lignin removal (25 to 35% weight loss with 5% or less residual lignin) lowered fibre viscosity and board strength. Experimental strength and elasticity data across the 0 to 5% and 10 to 15% weight loss range were fitted according to a mathematical model. The site and bonding mechanism between lignin and tempering oil have not been described. Some suggestion is made as to how this might occur.

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