Acetylation wood with thioacetic acid

Figure 3. Effect of different moisture levels on vapor-phase acetylation of wood with thioacetic acid. Reaction time is 3 h.

D d During chemical modification of wood with thioacetic acid, the only by-product formed is H2S gas. This easily escapes from wood favoring a forward reaction. The liberated H2S is recycled to produce thioacetic acid, making the process a closed circuit one. Since there is no crosslinking or polymerization of the reagent, acetylation level can be directly gauged from the WPG. 

Kumar, S. and Agarwal, S.C. (1983). Biological degradation resistance of wood acetylated with thioacetic acid. International Research Group on Wood Preservation, Doc. No. IRGAVP 3223. 

Kumar, S., Dev, I. and Singh, S.P. (1991). Hygroscopicity and dimensional stabihty of wood acetylated with thioacetic acid and acetyl chloride. Journal of the Timber Development Association of India, 37(1), 25-32. 

Effect of Wood Moisture. It has been recommended that for acetylation with acetic anhydride, the moisture content of wood should be about 2% as excess moisture is likely to react with acetic anhydride and produce acetic acid (37). Goldstein et al. (28) observed that raising the moisture to 22% considerably slowed the reaction and each 1 percent of moisture in wood would lead to hydrolysis of about 5.7% acetic anhydride. Low moisture contents are not possible to attain in commercial treatment of wood. With ketene gas it has been possible to acetylate wood with as high as 20% moisture content with WPG about 25% (40). Thioacetic acid is only partially stable in cold water and dissociates at higher temperatures. The presence of moisture in wood could thus be critical in treatments with thioacetic acid also. Results of mango treated at 5 different moisture levels are depicted in Figure 3. As may be seen a moisture content up to 7.5% has no adverse effect on WPG. At 10% moisture content the WPG decreased to 4.6. With further increase in moisture, there was a gradual decrease in WPG attained. 

Limited tests on strength properties of wood acetylated with thioacetic acid showed that MOR in static bending, impact strength, maximum crushing strength in compression and density improved. It is, therefore, evident that thioacetic acid has either no adverse effect on wood components and their interlinks or the improvements affected by modification far exceed these effects, so as to be of little consequence and ending in net gains. 

Wood acetylated with thioacetic acid showed resistance to decay and termites at low WPG around 12. The available information on pattern of substitution of hydroxyls during acetylation with acetic anhydride suggests substitution of lignin hydroxyls at low acetylation levels. Resistance to micro-organisms, particularly those consuming cellulose, even at low acetylation... 

Figure 1. Vapor phase acetylation assembly. Key 1, reaction chamber 2, wood samples 3, thioacetic acid 4, thermometer 5, water condenser 6, heating mantle 7, magnetic stirrer with heater and 8, acetic anhydride and catalysts.

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