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Scientia Silvae Sinicae ›› 2016, Vol. 52 ›› Issue (8): 96-103.doi: 10.11707/j.1001-7488.20160812

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Changes of Time Dependent Viscoelasticity of Chinese fir and its Frequency-Dependency During Moisture Adsorption Processes

Zhan Tianyi1,2,3, Jiang Jiali1,2, Peng Hui2, Chang Jianmin3, Lü Jianxiong2   

  1. 1. Research Institute of Forestry New Technology, CAF Beijing 100091;
    2. Key Laboratory of Wood Science and Technology of State Forestry Administration Research Institute of Wood Industry, CAFBeijing 100091;
    3. College of Materials Science and Technology, Beijing Forestry University Beijing 100083
  • Received:2015-07-07 Revised:2016-03-01 Online:2016-08-25 Published:2016-09-19

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Abstract: [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.

Key words: Chinese fir, moisture adsorption, viscoelastic behavior, frequency, mechanosorptive effect, unstable state of wood cell wall

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