水分解吸过程中杉木黏弹行为的经时变化规律及其频率依存性

Changes of Time Dependent Viscoelasticity of Chinese Fir Wood and Its Frequency-Dependency during Moisture Desorption Processes

【目的】研究木材黏弹行为在水分解吸过程中的经时变化规律及其频率依存性,明确细胞壁不稳定化现象对木材松弛转变行为的影响机制。【方法】以含水率为22.2%(对应的温度和相对湿度为30℃和85%)的杉木木材为研究对象,采用动态机械分析仪(DMA Q800)测定水分解吸过程中木材贮存模量E'和损耗模量E″的经时变化,并比较不同频率(1~50 Hz)之间黏弹行为变化的异同。在水分解吸过程中,共设置3个相对湿度水平(0%、30%和60%),任一相对湿度水平下的水分解吸过程分为降湿和恒湿2个阶段:在降湿阶段,相对湿度由85%以2%·min^(-1)分别降低至0%、30%或60%;在恒湿阶段,相对湿度在0%、30%或60%下恒定120 min。此外,选取6个平衡含水率水平(0.6%、3.2%、7.4%、13.1%、17.9%和22.2%)研究水分平衡状态下木材黏弹行为在不同频率之间的差异。【结果】在所有频率下,随着解吸时间延长,木材贮存模量增加,损耗模量减小。以1 Hz频率为例,选取降湿阶段和恒湿阶段结束为时间节点,贮存模量和损耗模量变化率(|ΔE'|和|ΔE″|)及其与含水率变化率的比值(|ΔE'/ΔMC|和|ΔE″/ΔMC|)随解吸时间的变化为:在降湿阶段结束时,|ΔE'|基本等于或大于|ΔE″|,|ΔE'/ΔMC|基本等于或大于|ΔE″/ΔMC|;但在恒湿阶段结束时,|ΔE'|小于|ΔE″|,|ΔE'/ΔMC|小于|ΔE″/ΔMC|。在降湿和恒湿阶段,|ΔE'|、|ΔE″|、|ΔE'/ΔMC|和|ΔE″/ΔMC|均随相对湿度水平的减小而增大。在解吸过程中,贮存模量随频率的增加而增大,损耗模量随频率的增加先减小后增大;损耗模量极小值对应的特征频率出现在10~30 Hz范围内。特征频率随着解吸时间的延长向低频方向移动。在含水率相同时,解吸过程对应的理论特征频率要较水分平衡状态的高。在解吸过程中,木材黏弹行为的变化在不同频率间有所差异。以损耗模量在1 Hz和20 Hz频率下的比值(E″1Hz/E″20Hz)为例,随着解吸时间的延长,E″1Hz/E″20Hz先增大后减小;在解吸过程的任一时间节点处,E″1Hz/E″20Hz在相对湿度为0%时最大,60%时最小。根据E″1Hz/E″20Hz的变化量计算细胞壁不稳定化程度的残余率,对应于相对湿度0%、30%和60%,残余率分别为2.22%、0.48%和0.37%。【结论】随着解吸时间的延长,木材刚度增加,阻尼减小;刚度的变化主要与"分子间氢键重建"效应有关,而阻尼的变化则是"分子间氢键重建"效应和"自由体积"效应的叠加作用。随着解吸时间延长,吸着水分子和"吸着水-聚合物分子链"复合基团的取向运动难度增大,表现为α力学松弛过程(由半纤维素玻璃化转变引起)和β力学松弛过程(基于木材细胞壁无定形区中伯醇羟基回转取向运动的力学松弛过程与吸着水分子回转取向运动的力学松弛过程二者叠加而成)的转变向低频方向移动,但细胞壁不稳定化的存在使得聚合物分子链的取向运动难度减小。随着解吸时间延长,细胞壁的不稳定化程度逐渐减弱;根据损耗模量在1 Hz和20 Hz频率下的比值可以预测达到新的平衡含水率时细胞壁不稳定化程度的残余率。此外,理论残余不稳定化程度随着相对湿度水平的降低而增大。

【Objective】 The changes of viscoelastic behavior of Chinese fir （Cunninghamia lanceolata） wood duringmoisture desorption processes and the influence of unstable state of cell wall on relaxation behaviors of wood polymers were investigated in this paper.【Method】 Dynamic mechanical analysis （DMA Q800） was used at the given frequencies from1 to 50 Hz to determine the changes of storage modulus E＇ and loss modulus E″ of Chinese fir samples with a moisture content （MC） of 22.2%,which was corresponded to the environmental condition as 30° C and 85% relative humidity （RH）.The moisture desorption tests were performed by changing the RH levels into the DMA chamber.In this study,three RH conditions were selected as 0% RH,30% RH and 60% RH.Under each RH condition,the moisture desorption process was divided into RHramp-downperiod and RHisohumeperiod.During the RHramp-downperiod,the RH decreased from 85%RH to 0% RH,30% RH or 60% RH,respectively with a ramping rate of 2% RH·min-1.During the RHisohumeperiod,RH kept constant at 0% RH,30% RH or 60% RH for 120 min.【Result 】 Regardless of frequency,the storage modulus increased and loss modulus decreased with the increasing desorption time.Taken 1 Hz as an example,the time points that at the end of each RHramp-downperiod or RHisohumeperiod were selected to evaluate the differences of the changing rate of storage modulus （｜ ΔE＇ ｜） and loss modulus （｜ ΔE″ ｜） and the changing rates per unit of MC （｜ ΔE＇/ΔMC ｜ and ｜ ΔE″/ΔMC ｜） with the increasing desorption time.At the end of each RHramp-downperiod ｜ ΔE＇ ｜ was basically equal to or even higher than ｜ ΔE″ ｜,and ｜ ΔE＇/ΔMC ｜ was basically equal to or even higher than ｜ ΔE″/ΔMC ｜ as well.Whereas,at the end of each RHisohumeperiod,｜ ΔE＇ ｜ was less than ｜ ΔE″ ｜,and ｜ ΔE＇/ΔMC ｜ was less than ｜ ΔE″/ΔMC ｜ as well.At both the end of RHramp-downperiod or RHisohumeperiod,the less the RH levels,the higher values of ｜ ΔE＇ ｜,｜ ΔE″ ｜,｜ ΔE＇/ΔMC ｜and ｜ ΔE″/ΔMC ｜ were obtained.Furthermore,during the moisture desorption processes,higher value of storage modulus was tested at higher frequency.As frequency increased,the loss modulus decreased at first and then increased.The character frequency according to the minimum value of loss modulus occurred basically at 10-30 Hz,and shifted toward lower frequency with the increasing desorption time.The character frequency of wood during moisture desorption process was higher than that under an equilibrium state of moisture.During the moisture desorption processes,wood viscoelastic properties varied among frequencies.Taken the changes of loss modulus at 1 and 20 Hz （E″1Hz/E″20Hz） as an example,this value increased at first with the increasing desorption time,and then decreased.At each time points,0% RH desorption condition had the most value of E″1Hz/E″20Hz,and 60% RH condition had the least one.Based on the changes of E″1Hz/E″20Hz,the residual ratio of unstable state of cell wall was 2.22%,0.48% and 0.37%,corresponding to 0%,30% and 60%RHisohumeconditions,respectively.【Conclusion】 With the increasing desorption time,the wood stiffness increased and the damping decreased.The changes of stiffness was basically only related to the effect of reformed hydrogen bonds,and the changes of damping was related to the double-effects of reformed hydrogen bonds and free volume.With the increasing desorption time,the movements of water molecular and ＂water molecular-wood polymers ＂ composed group became difficult,behaving as the transition of the α-relaxation process （attributed to the glass transition of hemicellulose） and theβ-relaxation process （assigned to the reorientation of the methylol groups in amorphous wood cell walls and the reorientation of adsorbed water molecules） moved into lower frequency.Due to the existence of unstable state of wood cell wall,the movement difficulty of polymers weakened.With the increasing desorption time,the unstable state of wood cell wall diminished.The ratio of loss modulus at 1 and 20 Hz could predict the residual ratio of unstable state when wood approaching a new equilibrium moisture content.