• 研究简报 •

### 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

1. 1. 南京林业大学材料科学与工程学院 南京 210037;
2. 中国林业科学研究院木材工业研究所 国家林业局木材科学与技术重点实验室 北京 100091;
3. 北京林业大学材料科学与技术学院 北京 100083
• 收稿日期:2016-01-26 修回日期:2016-10-19 出版日期:2017-08-25 发布日期:2017-09-27
• 基金资助:
江苏省自然科学基金项目（BK20170926，BK20150878）；江苏高校优势学科建设工程资助项目（PAPD）。

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

Zhan Tianyi1,2, Lü Jianxiong2, Zhang Haiyang1, Jiang Jiali2, Peng Hui2, Chang Jianmin3

1. 1. College of Materials Science and Engineering, Nanjing Forestry University Nanjing 210037;
2. Key Laboratory of Wood Science and Technology of State Forestry Administration Research Institute of Wood Industry, CAF Beijing 100091;
3. College of Materials Science and Technology, Beijing Forestry University Beijing 100083
• Received:2016-01-26 Revised:2016-10-19 Online:2017-08-25 Published:2017-09-27

Abstract: [Objective]The changes of viscoelastic behavior of Chinese fir (Cunninghamia lanceolata) wood during moisture 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 from 1 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℃ 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-down period and RHisohume period. During the RHramp-down period, 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 RHisohume period, 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-down period or RHisohume period 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-down period|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 RHisohume period,|ΔE'| was less than |ΔE"|, and |ΔE'/ΔMC|was less than |ΔE"/ΔMC| as well. At both the end of RHramp-down period or RHisohume period, 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%RHisohume conditions, 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 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 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.