楸木早/晚材水分吸着与湿胀行为

Hygroscopicity and Swelling Behavior of Catalpa bungei Earlywood and Latewood

【目的】实时、同步测量楸木生长轮、早/晚材的水分吸着等温线和湿胀应变量,从生长轮尺度上揭示早/晚材的径、弦向湿胀行为规律及其相互作用机制。【方法】以楸木心材同一生长轮内的独立早材、独立晚材、生长轮内早材、生长轮内晚材及生长轮为研究对象,采用动态水分吸附分析仪联用白光显微镜成像技术,在(25±0.1)℃条件下,相对湿度以10%为梯度从0%升至90%,每个相对湿度梯度分为水分吸着阶段和平衡含水率恒定阶段:在水分吸着阶段,同步测量水分吸着等温线和径、弦向湿胀应变量;在平衡含水率恒定阶段,考察“湿胀行为”与“平衡含水率”之间是否存在滞后现象。【结果】1)在0~90%范围内任一相对湿度水平,晚材的平衡含水率均大于早材,且二者之间的差值随相对湿度升高呈先增大后减小的趋势,晚材试样的重现性优于早材试样;2)在径向湿胀方面,无论独立试样亦或生长轮试样的内部组织,晚材的湿胀应变量均大于早材,生长轮的湿胀应变量介于二者之间,且早材与晚材之间径向湿胀应变量的差值随相对湿度增加而增大;3)在弦向湿胀方面,独立早材的湿胀应变量最小,生长轮内早材表现出与生长轮内晚材、独立晚材和生长轮相似的情况;4)生长轮、生长轮内晚材和独立晚材的径、弦向差异湿胀均约为1.30;当相对湿度大于40%时,生长轮内早材和独立早材的径、弦向差异湿胀呈明显增大趋势,其中独立早材增大至1.83,生长轮内早材增大至2.28;5)在平衡含水率恒定阶段,生长轮、早材和晚材的径、弦向湿胀应变量与水分吸着阶段相比低4个数量级。【结论】在水分吸着阶段任一相对湿度水平,楸木早材的平衡含水率均小于晚材;楸木生长轮的径向湿胀行为取决于早材和晚材发生共同湿胀的净效果,晚材的湿胀行为占主导;楸木生长轮的弦向湿胀行为主要取决于晚材;早材表现出比晚材更高的径、弦向差异湿胀。生长轮、早材和晚材均可视为同步达到“含水率平衡态”和“径、弦向湿胀应变平衡态”,即“湿胀行为”与“平衡含水率”之间不存在滞后现象。

【Objective】The hygroscopicity and swelling behavior of Catalpa bungei earlywood,latewood and the growth ring was real-timely and synchronously documented.This work aimed to reveal the coupling and interaction between the swelling behavior of the earlywood and latewood in individual growth ring.【Method】Dynamic vapor sorption analysis with Dino X Lite Digital Microscope(DVS Resolution)was used to determine the separated earlywood(EW),separated latewood(LW),combined earlywood(ELW-E),combined latewood(ELW-L)and the growth ring(ELW)of C.bungei heartwood in individual growth ring.The measurements were taken at a constant temperature of(25±0.1)℃,starting at 0%relative humidity(RH)and increasing in increments of 10%RH up to 90%RH.The each RH process was divided into the water vapor sorption period and equilibrium moisture content(EMC)constant period.During the water vapor sorption period,radial and tangential swelling strain and the sorption isotherm were synchronously measured.During the EMC constant period,whether there was hysteresis between“the swelling behavior”and“EMC”or not was investigated.【Result】1)LW exhibited a slightly higher EMC than EW at the range of 0%-90%RH.As the RH increased,the difference between EW and LW increased at first and then decreased.The reproducibility of latewood samples was better than that of the earlywood samples.2)In the radial direction,the LW shown a higher swelling strain than EW,and ELW-L was also higher than ELW-E.The growth ring was between the earlywood and latewood.The difference of earlywood and latewood radial swelling strain increased with the increasing of RH.3)In the tangential direction,the EW showed obviously the lowest swelling strain.However,ELW-E exhibited a similar situation as the ELW,ELW-L and LW.4)The ratio of tangential to radial swelling for ELW,ELW-L and LW were all about 1.30.At RH above 40%,ratio of tangential to radial swelling for ELW-E and EW suddenly increased,and finally reached 2.28 and 1.83,respectively.5)Compared with those at‘the water vapor sorption period’,the radial and tangential swelling strain of ELW,EW and LW at‘the EMC constant period’were less four orders in magnitude.【Conclusion】At any RH during the moisture sorption,the EMC of C.bungei earlywood was less than that of the latewood.The radial swelling of C.bungei growth ring was determined by the net effect of earlywood and latewood,and that of the latewood was dominant.Moreover,the tangential swelling of C.bungei growth ring might be also dominated by the latewood.The ratio of tangential to radial swelling for earlywood was higher than that of the latewood.‘EMC’and‘the equilibrium of radial and tangential swelling strain’of the growth ring,earlywood and latewood were changed synchronously,indicating that there may be no hysteresis between‘the swelling behavior’and‘EMC’.