The Effect of Swelling Agents and Catalysts on Acetylation of Wood

Acetylation and other esterifying treatments of wood act to replace some of the hydrophilic hydroxyl groups of cellulose and lignin with less hydrophilic groups, thereby providing greatly increased dimensional stability. Experiments were carried out to establish optimum conditions for acetylation of wood. Factors investigated included temperature, amount of impregnation, moisture content, catalyst effects, and acetylating medium concentration effects. Suitable reaction conditions were first established using sawdust, and later applied to treatment of veneers. Acetylation of sawdust impregnated with urea and a catalyst proceeded at temperatures down to 85?C., the bottom of the range studied, although more time is required at lower temperatures to obtain the same degree of acetylation. A 15 percent retention of catalyst-impregnant was found sufficient to give maximum acetylation. Initial moisture contents varying from 10 to 150 percent had no effect on degree of acetylation. Of all impregnants tried, the urea-ammonium sulphate combination was found to be most effective. It was also established that anhydride to wood ratios greater than 8:1 did not affect acetylation appreciably: below 8:1, acetyl contents dropped rapidly. Dimethylformamide (DMF) offers some advantages as a swelling agent in acetylation, but requires somewhat higher temperatures than urea impregnation. Of the salts tried as catalysts, only ammonium sulphate and potassium acetate were found to be effective. Veneers were acetylated by a vapor phase technique using three modifications: 1) impregnated with a urea-ammonium sulphate solution, dried, and exposed to acetic anhydride vapor; 2) impregnated with potassium acetate solution, dried, and exposed to vapors of a mixture of DMF and acetic anhydride; and 3) impregnated with DMF and exposed to vapors of phenylisocyanate. Veneers with anti-shrink efficiencies over 70 percent were produced by all three methods. In addition, veneers treated by the third method exhibited 3-6 percent increase in MOR.

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