以動態水分吸附法探討熱處理材吸脫濕之遲滯性質

Abstract

本研究以 170、190、210 ºC 熱處理1 h 與未處理之大葉桃花心木(Swietenia macrophylla)為材料，用色差計量測，探討L*(明暗值)、a*(紅綠值)、b*(黃藍值)，以及ΔE (色差值)受熱處理影響之變化；以FTIR(Fourier transform infrared spectroscope)量測其吸收光譜，觀察試材受熱處理影響其吸收光譜特性之改變；另以DVS(Dynamic vapor sorption，動態水分吸附儀)精測試材於吸濕及脫濕程序下之遲滯曲線差異及吸脫濕特性(Hygroscopic Properties)；末以GAB(Guggenheim -Anderson-de Boer)模型分析遲滯曲線特性，以及PEK(parallel exponential kinetics)模型擬合試驗吸脫濕動能曲線，並以數值方法分析試材內部吸脫濕特性。 顏色量測結果為 L*、a*、b*皆隨熱處理溫度上升及熱處理溫度而下降，而ΔE 隨熱處理溫度上升而增加，表示試材受熱處理後，顏色偏暗、偏綠、偏藍，以及色差值增加。 FT-IR 光譜結果為隨熱處理溫度上升，特徵頻帶1743 cm-1 吸收特性降低，1513 cm-1 吸收特性增加表示半纖維素的降解以及木質素相對含量增加。1503cm-1 與1106 cm-1 峰面積值隨處理溫度升高而增加，新的木質素交聯可導致水吸收減少，同時也表示木材收縮與膨潤的降低。且1335 cm-1 與1316 cm-1 的雙峰比值隨熱處理溫度升高而上升，表示纖維素非結晶區經熱處理後降解。 動態水分吸附法之結果顯示，在相同相對濕度(Relative moisture, RH)下之平衡含水率(Equilibrium moisture content, EMC)隨熱處理溫度升高而較低，EMC 明顯於熱處理後降低，表示吸濕性隨熱處理溫度上升而明顯下降。比較樹種或是處理方法之差異無反映在逐步吸脫濕程序下之EMC、帄衡時間與吸脫濕速率上，而是反映在數值尺度上。吸脫濕程序之遲滯曲線結果可知，其最高EMC 於未處理及熱處理170、190、210 ºC 分別為18.85、18.81、17.44 及14.04 %，表示最大EMC 與熱處理溫度有負相關性。 以 GAB 分析遲滯曲線數值結果顯示，經過熱處理後單層水之總容積皆隨熱處理溫度升高而下降，分別為0.04697、0.04864、0.04314 及0.03372 (cm3/g)，且單層水吸收特定表面積也具類似趨勢，分別為179、185、164 及129 (m2/g)，隨處理溫度升高而降低。 以 PEK 模型擬合分析吸濕動能曲線之結果為，吸濕程序中，多數情況為代表木材中單層結合水之快速曲線所達帄衡值，高於代表木材多層結合水之緩慢曲線所達帄衡值；脫濕程序中，快速曲線則與緩慢曲線相差不多。吸濕程序中含水率上升總和(快速/緩慢)在未處理、以170、190、210 ºC 熱處理樣本分別為21.14( 4.98 / 16.16)、20.69( 6.03 / 14.66)、18.22 ( 4.57 / 13.65) 、14.27 ( 2.89 / 11.38) (%)， 與DVS 於吸濕程序中95 %RH 時之樣本含水率18.85、18.10、17.44、14.04 (%)有相近結果，因此擬合總值具參考價值。

The wood dealt with in this work is Swietenia macrophylla, with four groups:untreated (control), heat treated by 170 ºC, 190 ºC and 210 ºC for 1 h. The color changes were determined by the color meter and showed that L*, a* and b* were all decrease when heat-treated at higher temperatures. ΔE is also shown to increase when heat-treated at higher temperatures. Results show that after heat treatment the samples all darken and change to greenish and bluish tints. Analysis using the FTIR (Fourier transform infrared spectroscope) spectra showsthe chemical changes after heat treatment. When treated, there is found a decrease at the 1743 cm-1 mark peak, and an increase at the1513 cm-1 mark peak, meaning there is a degradation of hemicellulose causing the relative content of lignin to increase. The peak areas of 1503 cm-1 and 1106 cm-1 both increase when treated, showing the new formation of cross-link of lignin and causing a decrement of shrinkage and swelling in the treated sample. On the other hand, the ratios of the 1335 cm-1 and 1316 cm-1 peaks represent a degradation in the amorphous region of the treated sample. The DVS (Dynamic vapor sorption) can be used to analyze the sorption properties of water of materials. The results of DVS (Dynamic vapor sorption) show that the EMC (Equilibrium moisture content) and equilibrium time decrease when treated. There is no significant difference in the equilibrium rate when treated. The highest EMC in untreated, treated at 170 ºC, 190 ºC and 210 ºC are 18.85, 18.81, 17.44 and 14.04 %, respectively. Which gives a negative correlation between the heat-treated temperature and the highest EMC. Numerical analysis of the hysteresis loop via the GAB model(Guggenheim-Anderson-de Boer) shows that the volume of adsorbed water of dry wood (untreated – 0.04697, 170 ºC – 0.04864, 190 ºC – 0.04314, 210 ºC – 0.03372 cm3 /g) decreases when treated, as does the monolayer capacity. (untreated –179, 170 ºC – 185, 190 ºC – 164, 210 ºC – 129 m2 /g) In Curve fitting all the experimental data of adsorption and desorption kinetics curves, and plotting the parameters for division into the fast and slow curves by using the PEK (parallel exponential kinetics) model. The results shows that the fast curve, which represents the monolayer of water in the woods internal surface, is higher than the slow curve, which represents the multilayer of water in the woods external surface, in adsorption. It is also shown that the fast curve is much closer to the slow curve for the desorption process.