水分吸着过程中杉木黏弹行为的经时变化规律及其频率依存性

摘要

【目的】研究木材黏弹行为在水分吸着过程中的经时变化，明确水分对木材黏弹行为频率依存性的影响，补充和完善“水分－机械力”耦合作用下木材黏弹行为的变化规律，并为模拟和预测木材在切削、热压、磨浆等实际复杂过程中黏弹行为的变化提供科学依据。【方法】以含水率0.6%的杉木木材为研究对象，采用动态机械分析仪(DMA Q800)在30℃、不同相对湿度条件(30%，60%和90%)下测定木材贮存模量 E'和损耗因子 tanδ的变化情况，比较不同频率(1～50 Hz)之间木材黏弹行为的异同。水分吸着过程分为升湿和恒湿2个阶段：在升湿阶段，相对湿度由0以2%·min －1的速率分别升高至30%，60%或90%；在之后的恒湿阶段，相对湿度在30%，60%或90%下分别恒定240 min。【结果】在任一频率下，随着吸着时间的延长，木材贮存模量E'减小，损耗因子tanδ增大，并且贮存模量的变化率|ΔE'|明显小于损耗因子的变化率|Δtanδ|；单位含水率的贮存模量和损耗因子变化率(|ΔE'/ΔMC|和|Δtanδ/ΔMC|)随着吸着时间的延长均减小。此外，在水分吸着过程中的任一时间节点处，贮存模量随频率的增加而增大，损耗因子随频率的增加先减小后增大，损耗因子极小值对应的特征频率出现在10～30 Hz 范围内，并随着吸着时间的延长向高频方向移动；在1 Hz和20 Hz频率下贮存模量的比值约为0.98，该比值基本不随吸着时间的延长而变化，但损耗因子的比值在升湿和恒湿过程中先增大后减小，并在升湿阶段结束时达到最大值。【结论】在水分吸着过程中，水分子的“塑化效应”是引起木材贮存模量减小和损耗因子增大的主要原因，并且单分子层吸着水的“塑化效应”最为明显；机械吸湿蠕变效应的存在使得升湿阶段木材黏弹性的变化较恒湿阶段明显；在水分吸着过程中，随着含水率增加，木材细胞壁聚合物分子的运动速度加快，松弛时间减少，并且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的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)的转变向高频方向移动；在水分吸着过程中，含水率的变化可引发木材细胞壁的不稳定化现象，并且相对湿度的变化加剧了这种不稳定化。%[Objective]This study was designed to investigate the changes of viscoelastic properties of Chinese fir during moisture adsorption process,to clarify the effects of moisture on frequency-dependency viscoelastic properties,and to supplement the coupling effects of moisture and mechanical force on wood viscoelasticity.[Method]Dynamic mechanical analysis (DMA Q800) was used to determine the changes of storage modulus and loss factor of Chinese fir samples with a moisture content of 0. 6%. The temperature was 30 ℃,and the relative humidity ( RH) was selected as 30%,60% and 90%. The differences of viscoelastic properties among frequencies (1 -50 Hz) were compared. The adsorption process in this study was divided into RHramp period and RHisohume period. During the RHramp period,the RH increased from 0% to 30%,60% or 90%,respectively with a ramping rate of 2%·min -1 . During the RHisohume period,RH kept constant at 30%,60% or 90% for 240 min.[Result]Regardless of frequency,storage modulus decreased and loss factor increased with the increasing moisture adsorption time,respectively. Changes of storage modulus were significantly less than those of loss factor. Both the changes of storage modulus and loss factor per unit change of moisture content decreased with the increasing moisture adsorption time. Furthermore,in any time point during the moisture adsorption process,higher value of storage modulus was found at higher frequency. As frequency increased,loss factor decreased at the beginning and then increased. The character frequency according to the minimum value of loss factor occurred from 10 to 30 Hz,and shifted to higher frequency with the prolonged adsorption time. The ratio of storage modulus tested at 1 and 20 Hz was about 0. 98,and no remarkable changes were found during the adsorption process. The differences of loss factor at 1 and 20 Hz increased at the beginning and then decreased,achieving its maximum value at the end of RHramp period.[Conclusion]During the adsorption process,the plasticize effect of water molecular was the major reason for decreasing storage modulus and increasing loss factor. The plasticization effect of the monomolecular water layer was greater than that of each additional polymolecular water layers. The mechanosorptive effect led more significant changes of dynamic viscoelastic properties during RHramp period than RHisohume period. The character frequency according to the minimum value of loss factor was shifted to higher frequency with the increase of moisture content,which meant the faster movements of polymer molecular and less relaxation time with more moisture content. The transition of the α-relaxation process ( attributed to the glass transition of hemicellulose ) and the β-relaxation process ( assigned to the reorientation of the methylol groups in amorphous wood cell walls and the reorientation of adsorbed water molecules ) moved to a higher frequency with the increasing moisture content. The changes of moisture content caused the unstable state of wood cell wall,and aggravated by the change of RH.