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Low VOC drying of lumber and wood panel products. Progress report No. 7

Description: Green pine blocks (2x1x 1) were dried to different moisture levels at 120 degrees C. They were immersed in D{sub 2}O (greater than 99% isotopic Content) for different periods at room temperature, and were then cut in halves. One piece from each set was then wrapped in plastic, and microwaved at 110 W, for 30 minutes, with the field being cycled to keep the wood surface at 90-100 degrees C. Fibers taken from just inside the wet surface from five regions along the length of the piece were then analysed by mass spectrometry with a direct insertion probe. The m/e profiles of the three isotopic forms of water, namely H{sub 2}O, HOD, and D{sub 2}O, remained unchanged as the wood was heated inside the spectrometer, indicating that they were bound equally strongly to the wood. The water released from the green wood had the same isotopic composition regardless of whether or not the wood was microwaved (Table 1), indicating that the exchangeable protons in wood were not affected by microwaving. However, as the wood progressively dried, the water released from the microwaved wood was of lower isotopic content, which means that microwaving increases access of the exchangeable protons in wood tissue to water. The only exchangeable protons in dried wood are those sited on hydroxyl groups, and the difference in isotopic exchange is the greatest for dried wood. This must mean that as wood dries, internal hydrogen bonding restricts access of D{sub 2}O to the hydroxyl protons. Presumably the energy transferred to water upon microwaving is sufficient to at least partially overcome this barrier. The effect is akin to the hysteresis that occurs for moisture sorption to green and dried wood. Similar isotope exchange work with D{sub 2}O has been previously conducted to determine the accessibility of cellulose to water.
Date: March 1, 1998
Creator: Yan, Hui; Hooda, Usha & Banerjee, Sujit
Partner: UNT Libraries Government Documents Department