基于生长轮的杉木早材黏弹性

The Viscoelasticity of Chinese Fir Earlywood in Individual Growth Rings

【目的】 比较不同生长轮木材早材黏弹性,探讨全干密度、管胞胞壁率、微纤丝角等因子与木材黏弹性之间的关系,从细胞水平理解木材黏弹行为的作用机制。【方法】 以人工林杉木为研究对象,采用X射线剖面密度仪、ZEISS Imager A1显微镜、X射线衍射仪、动态力学分析仪(DMA 2980)分别测定第3、6生长轮(心材)以及第14、18生长轮(边材)早材的全干密度、管胞胞壁率、微纤丝角、弹性模量、贮存模量和损耗模量。【结果】 1) 边材(第14、18生长轮)的全干密度比心材(第3、6生长轮)略高,4个生长轮管胞胞壁率无明显差异,微纤丝角随树龄增加呈减小趋势。2) 早材的弹性模量、贮存模量和损耗模量均随树龄增加而增大,微纤丝角与弹性模量、贮存模量、损耗模量呈显著负相关关系。3) 在本研究测量温度范围内(-120~120 ℃),4个生长轮早材均出现2个力学松弛过程: 一是在10 ℃附近的α力学松弛过程,关于其分子运动归属目前尚无统一定论;二是在-40 ℃附近的β力学松弛过程,是基于木材细胞壁无定型区中伯醇羟基的回转取向运动引起的。不同生长轮之间的力学损耗峰温度几乎无差异。4) 随测量频率(1、2、5和10 Hz)增加,β力学松弛过程的损耗峰温度移向高温方向,α力学松弛过程的损耗峰温度并不随测量频率增加而改变,即无频率依存性。5) 与位于边材区域(第14、18生长轮)的早材相比,位于心材区域(第3、6生长轮)的早材发生力学松弛过程所需的表观活化能均较大,可能是由于杉木心材相较于边材有更多的抽提物,抽提物的沉积限制细胞壁中分子链段运动所致。【结论】 微纤丝角是影响不同生长轮内早材刚度和阻尼的关键因子,心材与边材表观活化能的差异可能是抽提物沉积所致。

【Objective】 This paper was proposed to compare the viscoelasticity of earlywood within different growth rings, and to investigate the relationships between absolutely dried density, rate of tracheid cell walls, microfibril angel and wood viscoelastic behavior, respectively,with an aim to reveal the deep mechanisms at cellular level. 【Method】 The absolutely dried density, rate of tracheid cell walls, microfibril angel, modulus of elasticity, storage modulus, and loss modulus of earlywood within the third and sixth growth rings(heartwood)and the fourteenth and eighteenth growth rings(sapwood)were respectively measured by X-ray profile densimeter, ZEISS Imager A1 microscope, X-ray diffractometer, and dynamic mechanical analysis(DMA 2980). 【Result】 1) The absolutely dried density of sapwood(the fourteenth and eighteenth growth rings)was slightly higher than that of the heartwood(the third and sixth growth rings), and the rate of tracheid cell walls among the four growth rings exhibited no significant differences. With the increase of tree age, the microfibril angle decreased. 2) The elastic modulus, storage modulus and loss modulus increased with the increasing of tree age. The microfibril angle showed a significant negative correlation with elastic modulus, and the microfibril angle was a key factor affecting the stiffness and damping of earlywood. 3) In the temperature range(-120-120 ℃)of measurement, two mechanical relaxation processes were observed in all four growth rings earlywood. One was the α mechanical relaxation process at the temperature around 10 ℃, and there was no unified conclusion about its molecular movement;While the other β mechanical relaxation process at temperature around -40 ℃ was based on the reorientation of methylol groups in amorphous of wood cell wall. There was almost no difference in mechanical loss peak temperature among different growth rings. 4) With the testing frequency(1,2,5 and 10 Hz)increasing, the loss peak temperature of β mechanical relaxation process moved to a higher temperature range. But for the α mechanical relaxation process, the loss peak temperature almost did not change with the increase of testing frequency, that is to say, it had no frequency dependence. 5) Compared to the earlywood of sapwood(the fourteenth and eighteenth growth rings), the apparent activation energy of mechanical relaxation processes of the earlywood located in heartwood(the thirdth and sixth growth rings)were slightly higher, this could be because the heartwood had more extractives than sapwood for Chinese fir, and then the deposition of extractives limited the movement of molecular segments in cell walls. 【Conclusion】 The MFA might be the key factor affecting the stiffness and damping of early wood in eralywood of different growth rings and the difference of apparent activation energy between heartwood and sapwood may be caused by the deposition of extracts.