Acetylation mango

Two species, chir (Pinus roxburghii Sargent.) size 7.5 x 2.75 x 1.25 cm3and mango (Mangifera indica Linn.) size 7.5 x 1.25 x 1.25 cm3 were studied to establish different reaction variables. Four replicates were taken for each treatment and experiments were carried out with oven-dried samples except in case of those where effect of moisture content was studied. Samples were placed in a reaction chamber and reaction was carried out under reflux conditions at 93+1 C, the boiling point of thioacetic acid. Experimental set-up for vapor phase acetylation is shown in Figure 1. 

To study the rate of reaction and obtain adequate weight percent gain (WPG), reaction periods varying from 1 to 8 hours were used. Effect of sample thickness on overall acetylation levels was examined on 7.5 cm long mango samples of different cross-sections (6x6 mm2, 12.5 x 12.5 mm2, 20 x 20 mm2, 25 x 25 mm2, and 30 x 30 mm2). These samples were acetylated in vapor phase for 3 and 6 hours. 

Five sets (4 samples each set) of mango samples were conditioned to five different moisture levels and conditioned samples were acetylated in vapor phase for 3 hours to study the effect of wood moisture content on WPG. 

Effect of Time and Sample Thickness. Table I depicts the acetylation level obtained in mango at various time intervals. 

Figure 2. Relationship between weight percent gain (WPG) and acetyl content. Key O, mango A, chir.

Table I. Effect of Time on Vapor Phase Acetylation of Mango...

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. 

A detailed study with respect to acetylation time was made in case of monochloroacetic acid and it was observed that an acetylation time of 2 hours was sufficient to obtain a WPG of 16.78%. In the case of mango, only pyridine and monochloroacetic acid were tried. While pyridine had very little effect, monochloroacetic acid improved WPG to over 17% in six hours treatment (Table IV). 

Table IV. Vapor Phase Acetylation of Mango Samples Pretreated with Pyridine/Monochloroacetlc Acid...

Figure 4. Relationship between acetyl content and antishrink efficiency. Key O, spruce (31) > chir (41) and X> mango.

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