波谱学杂志, 2023, 40(4): 448-461 doi: 10.11938/cjmr20233054

研究论文

基于TD-NMR的PMMA木塑复合材吸湿和吸水研究

赵万磊, 赵芝弘, 张明辉,*, 刘文静

内蒙古农业大学，内蒙古 呼和浩特 010020

Study on Moisture Absorption and Water Absorption of PMMA Wood-plastic Composites Based on TD-NMR

ZHAO Wanlei, ZHAO Zhihong, ZHANG Minghui,*, LIU Wenjing

Inner Mongolia Agricultural University, Hohhot 010020, China

通讯作者: * Tel: 15849376426; E-mail:zhangminghui@imau.edu.cn.

收稿日期: 2023-02-28  

基金资助: 国家自然科学基金资助项目(31860185)

Corresponding authors: * Tel: 15849376426; E-mail:zhangminghui@imau.edu.cn.

Received: 2023-02-28  

摘要

本研究采用时域核磁共振（TD-NMR）技术探究聚甲基丙烯酸甲酯（PMMA）木塑复合材在吸湿与吸水过程中水分的状态和自旋-自旋弛豫（T2）特征．并比较PMMA木塑复合材的阻湿率和拒水率，分析PMMA木塑复合材的阻湿和防水性能与原理．研究对象为浸渍甲基丙烯酸甲酯（MMA）单体聚合生成的PMMA木塑复合材．首先对绝干的PMMA木塑复合材进行核磁共振T2检测，然后分别进行吸湿和吸水实验，并每隔一段时间测定试件质量及其T2信号量．结果表明：PMMA木塑复合材的吸湿率和吸水率低于未处理材，具有良好的阻湿和防水性能，且MMA浸渍浓度为100%，浸渍时间为24 h时，在温度为 (40±0.2) ℃相对湿度为 (96.4±0.4)%恒温恒湿环境下木塑复合材吸湿率低于15%，常温下吸水试件的吸水率低于30%．PMMA木塑复合材的吸湿率和吸水率明显降低，T2峰面积也逐渐减小．这说明木塑复合材内部的PMMA起到了防止水分进入的作用．

关键词： 木塑复合材; 聚甲基丙烯酸甲酯; 时域核磁共振; 弛豫特征; 木材吸湿性

Abstract

In this study, time-domain nuclear magnetic resonance (TD-NMR) technology was employed to explore the moisture state and spin-spin relaxation (T2) characteristics of polymethyl methacrylate (PMMA) wood-plastic composites during the moisture absorption and water absorption process. The moisture resistance and water repellency of PMMA wood-plastic composites were compared and analyzed. The experimental samples were PMMA wood-plastic composite material produced by impregnated methyl methacrylate (MMA) monomer polymerization. Firstly, the NMR T2 test was carried out on the dry PMMA wood-plastic composite, and then the moisture absorption and water absorption experiments were carried out respectively, and the mass of the specimen and its T2 signal were measured at intervals. The results showed that the moisture absorption rate and water absorption rate of PMMA wood-plastic composite were lower than those of untreated wood, and PMMA had good moisture resistance and water resistance. When the impregnation concentration of MMA was 100 % and the impregnation time reached 24 h, the moisture absorption rate of wood-plastic composites was lower than 15% under the constant temperature of (40±0.2)℃ and relative humidity of (96.4±0.4)%, and the water absorption rate of water-absorbing specimens at room temperature was lower than 30%. At the same impregnation time, with the increase of impregnation concentration of MMA, the moisture absorption rate and water absorption rate of PMMA wood-plastic composites decreased significantly, and the T2 peak area decreased gradually. This indicates that the PMMA inside the wood-plastic composite material contributes to preventing water from entering.

Keywords： wood-plastic composites; PMMA; TD-NMR; relaxation characteristics; moisture absorption of wood

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本文引用格式

赵万磊, 赵芝弘, 张明辉, 刘文静. 基于TD-NMR的PMMA木塑复合材吸湿和吸水研究[J]. 波谱学杂志, 2023, 40(4): 448-461 doi:10.11938/cjmr20233054

ZHAO Wanlei, ZHAO Zhihong, ZHANG Minghui, LIU Wenjing. Study on Moisture Absorption and Water Absorption of PMMA Wood-plastic Composites Based on TD-NMR[J]. Chinese Journal of Magnetic Resonance, 2023, 40(4): 448-461 doi:10.11938/cjmr20233054

引言

人工速生材是重要建筑结构材料之一，其良好的特性能够满足大部分的使用需求[1].然而大多数人工林速生材本身存在很多缺陷，如纤维结构疏松、材质较差、易吸水变形、腐朽发霉等问题，这些问题严重影响了木制品的质量需求，因此有必要对速生材进行物理、化学或生物改性，进一步提高速生材的物理力学性能[2⇓-4].

木塑复合工艺是一种常用来赋予木材防水、防潮和提高可塑性的木材改性技术.它使用不饱和单体甲基丙烯酸甲酯（MMA）、乙烯醋酸乙烯酯等对木材进行改性[5⇓⇓-8]，单体聚合生成的聚合物填充木材细胞腔，能有效提高木材的物理力学性能[9].其中，MMA是一种低粘度、廉价易得的有机单体，聚合物力学性能好，呈透明状，因此常用作制备木塑复合材的原材料[10,11].Yue等[12]使用MMA浸渍制备透明木材，发现以聚甲基丙烯酸甲酯（PMMA）橡胶木为基材制备的透明木材有望成为高强度木塑复合材.有研究[13]发现，硅烷偶联剂能够进一步增强MMA与木材之间的界面相容性，从而显著改善透明木材的机械性能和光透性.木塑复合材相较于普通木材具有更好的耐水性.但木塑复合材吸水后，仍然存在体积变形、力学性能下降以及发霉等问题[14].因此，研究木塑复合材的阻湿防水性能，对于解决其在吸水后产生的问题具有重要意义.

时域核磁共振（TD-NMR）技术是基于弛豫时间检测的一种快捷、无损的磁共振技术，已在各个领域得到广泛应用，包括农业食品[15]、自然科学[16]、地质探索[17,18]、化学化工[19]、医疗制药[20]以及材料科学[21]等多个领域.国内外研究者利用TD-NMR对木材阻湿和防水特性做了大量研究[22⇓-24]，发现木材中细胞腔水和细胞壁水的自旋-自旋弛豫时间（T2）信号存在明显的差异，且由于T2信号和木材含水率（MC）呈正相关，因此可以利用T2信号准确地对木材中细胞壁水和细胞腔水进行定性和定量分析[25⇓-27].同时，TD-NMR也能够对单体、低聚物和聚合物改性后木材中的水分状态进行分析.Xu等[28]利用TD-NMR研究水溶性三聚氰胺-脲醛（MUF）树脂浸渍白杨干燥过程中T2分布以及含水率分布，发现树脂处理没有明显增加T2分布峰的数目，而且树脂浸渍样品能够观察到更显著的含水率梯度.Jin等[29]使用TD-NMR技术对木塑复合材吸水过程中水分的运动和孔隙分布进行研究探讨，结果表明核磁共振技术可以准确反映不同时间段木塑复合材的透水性和孔隙分布的变化，且木塑复合材含水率和膨胀率的测量结果变化与低场核磁共振弛豫法得到的孔径变化趋势一致.

研究者们主要研究了PMMA木塑复合材的力学性能、热学性能以及半透明的特性[30]，而对PMMA木塑复合材的阻湿和防水性能研究较少，因此在对PMMA木塑复合材的阻湿和防水性量化方面有必要进行深入研究.为此，本文采用浸渍固化的方法制备PMMA木塑复合材，并通过TD-NMR技术对PMMA木塑复合材进行阻湿性和防水性测定.通过TD-NMR T2信号研究PMMA木塑复合材在不同吸湿和吸水时间的水分状态和含量，从而确定出本实验条件下PMMA木塑复合材阻湿性和防水性最佳工艺.

1 实验部分

1.1 材料与仪器

采购自内蒙古呼和浩特市森和木业有限责任公司的30块北京杨（Populus × beijingensis W. Y. Hsu）边材，大小为10 mm（径向）×10 mm（弦向）×20 mm（纵向）（R×T×L），称重法（烘干法）测定含水率8%~12%，分为4组.

MMA与偶氮二异丁腈（AIBN）均购自上海易恩化学技术有限公司；无水乙醇购自天津汇杭化工科技有限公司.

德国Bruker公司生产的时域核磁共振仪minispec mq20，磁场频率为19.96 MHz，90°脉冲宽度为15.1 μs，探头死时间为4.5 μs，磁体温度为40 ℃，配备该公司研发的Bruker the minispec 软件和CONTIN 反演软件；北京科伟永兴仪器有限公司生产的ZF6210型真空干燥箱，压强范围为0~0.08 kPa；北京赛多利斯科学仪器有限公司生产的Sartori BSA223S电子天平，精度为0.001 g；上海新苗医疗器械制造有限公司生产的DHG91435III电热恒温鼓风干燥箱，精度为0.1 ℃；常州国华电器有限公司生产的HH4数显恒温水浴锅.

1.2 PMMA木塑复合材制备

将30块杨木边材绝干后，一组（3块）不做任何处理，命名为PM#0，其余分别在不同浸渍浓度25%、50%、100%的MMA溶液真空浸渍6 h、12 h、24 h后，水浴80 ℃下添加0.02 g AIBN，木材内部发生热聚合反应制得PMMA木塑复合材.制备完成后，于 (103±0.2) ℃的干燥箱中绝干24 h后备用.将制备的PMMA木塑复合材试件按照浸渍时间各自分为3组，每组按照浸渍浓度分为3种，每种各有3个重复样品.样品组分别命名为PM#6，PM#12，PM#24.3组PMMA木塑复合材的制备条件如表1所示.

表1   PMMA木塑复合材的制备条件

Table 1  Preparation conditions of PMMA wood-plastic composites

样品编号浸渍时间/hMMA浓度/%反应温度/℃偶氮二异丁腈/g
PM#00000
PM#6-25625800.02
PM#6-50650800.02
PM#6-1006100800.02
PM#12-251225800.02
PM#12-501250800.02
PM#12-10012100800.02
PM#24-252425800.02
PM#24-502450800.02
PM#24-10024100800.02

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1.3 TD-NMR测试

首先，将绝干后的未处理材和PMMA木塑复合材称重，放入样品管中，使用CPMG脉冲序列在Minispec mq20核磁共振谱仪上测量T2.具体参数如下：扫描次数为64次，增益值为59 dB，扫描循环间隔时间为2 s，采样点数设定为4 000，回波时间设置为0.12 ms.然后将试件放入温度为40 ℃、相对湿度为 (96.4±0.4)%的恒温恒湿环境中进行吸湿实验，每隔一定时间（2 h, 4 h, 6 h, 8 h, 12 h, 24 h, 36 h, 48 h, 60 h, 72 h, 84 h, 96 h, 108 h, 120 h）称重并做记录.然后，采用CPMG脉冲序列测量T2.吸湿实验结束后，将试件浸泡至去离子水中，进行吸水试验，每隔一定时间（3 h, 6 h, 12 h, 24 h, 36 h, 48 h, 60 h, 72 h, 84 h, 96 h, 108 h, 120 h）取出试件擦拭表面水分并称重并作记录后，采用CPMG脉冲序列测量T2.每个试件在扫描结束后使用CONTIN[31]算法进行数据反演.

1.4 吸湿率与阻湿率和吸水率与拒水率计算公式

未处理材与PMMA木塑复合材吸湿率（MA）与阻湿率（MEE）的计算公式[32]如(1)、(2)式所示：

$\text{MA}=\frac{{{m}_{\text{M}}}-{{m}_{\text{D}}}}{{{m}_{\text{D}}}}\times 100\%$
$\text{MEE}=\frac{\text{M}{{\text{A}}_{\text{C}}}-\text{M}{{\text{A}}_{\text{T}}}}{\text{M}{{\text{A}}_{\text{C}}}}\times 100\%$

(1)式中，${{m}_{\text{M}}}$为不同时间段吸湿后的试件质量（g）；${{m}_{\text{D}}}$为绝干后的试件质量（g）；(2)式中$\text{M}{{\text{A}}_{\text{C}}}$为未处理材的吸湿率；$\text{M}{{\text{A}}_{\text{T}}}$为PMMA木塑复合材的吸湿率.

未处理材与PMMA木塑复合材吸水率（WA）与拒水率（RWA）的计算公式如(3)、(4)式所示：

$\text{WA}=\frac{{{m}_{\text{W}}}-{{{{m}'}}_{\text{D}}}}{{{{{m}'}}_{\text{D}}}}\times 100\%$
$\text{RWA}=\frac{\text{W}{{\text{A}}_{\text{C}}}-\text{W}{{\text{A}}_{\text{T}}}}{\text{W}{{\text{A}}_{\text{C}}}}\times 100\%$

(3)式中${{m}_{\text{W}}}$为不同时间段吸水后的试件质量（g）；${{{m}'}_{\text{D}}}$为进入去离子水之前的试件质量（g）；(4)式中$\text{W}{{\text{A}}_{\text{C}}}$为未处理材的吸水率；$\text{W}{{\text{A}}_{\text{T}}}$为PMMA木塑复合材的吸水率.

2 结果与讨论

2.1 未处理材与PMMA木塑复合材的阻湿性和防水性对比分析

图1和图2分别为PMMA木塑复合材阻湿率和拒水率随时间的变化曲线.木塑复合材的阻湿率和拒水率通过上述公式计算后取平均值.由图1可以看出PMMA木塑复合材的阻湿率随着吸湿时间的增加先降低后趋于稳定.木塑复合材在吸湿前期，阻湿率达到60%以上，随着吸湿的不断进行，阻湿率逐渐降低，吸湿24 h之后，木塑复合材的阻湿率下降的趋势逐渐平缓.此时木塑复合材的阻湿率的范围在27.31%~72.11%.在吸湿120 h时，木塑复合材的阻湿率由最初的63.19%~91.09%下降至27.78%~68.55%. 通过对相同浸渍时间下不同浸渍浓度的木塑复合材的阻湿率进行对比，PM#6-25、PM#6-50、PM#6-100的阻湿率先降低后逐渐趋于平衡，平衡时阻湿率分别是29.78%，33.71%，59.69%.可知，PM#6-100的阻湿率最大，该材料表现出良好的防潮性能.另外两组木塑复合材的阻湿率与PM#6表现出相似的特征，PM#12-100和PM#24-100在同组中阻湿率最高，达到60%以上，分别为61.55%，68.55%，防潮效果最好.从图1中还可以观察到PM#6-100、PM#12-100与PM#24-100三组中PM#24-100的阻湿率最高.由此可发现，木塑复合材阻湿率随浸渍时间和浸渍浓度的增加而逐渐增大.与此同时，为保证试验的完整性，在本试验中另对一组浸渍浓度为100%的木塑复合材试件进行了超过24 h的检测.结果发现，真空浸渍24 h后木材内MMA已基本饱和.因此通过对比得出在本实验条件下浸渍浓度100%，浸渍时间24 h制备的木塑复合材的阻湿性能最佳.

图1

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图1   PMMA木塑复合材吸湿过程阻湿率变化曲线

Fig. 1   Curve of moisture resistance rate of PMMA wood plastic composite during moisture absorption process


图2

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图2   PMMA木塑复合材吸水过程拒水率变化曲线

Fig. 2   Water repellency curve of PMMA wood-plastic composites during water absorption process


由图2可以看出PMMA木塑复合材的拒水率随着吸水时间的延长而逐渐减小.在吸水36 h之前，拒水率降低较快，在吸水48 h之后，拒水率逐渐趋于平缓，在吸水120 h后，木塑复合材的拒水率从最初的51.68%~84.64%下降至31.65%~78.45%.通过对比相同浸渍时间，不同浸渍浓度的木塑复合材拒水率，发现PM#6-100、PM#12-100、PM#24-100的平衡拒水率为相同浸渍时间组中的最高值，分别为73.02%、76.75%、78.45%.由此可知浸渍浓度越高，木塑复合材的拒水率越好.在图2中还可观察到在相同浸渍浓度下，PM#24-100的平衡拒水率高于其他两组，表明在相同浸渍浓度下，浸渍时间较长的木塑复合材的拒水率更好.

通过对三组平行试件数据求均值和标准差，得到PM#6-25、PM#6-50与PM#6-100三组木塑复合材阻湿率和拒水率的误差图，如图3所示.由图中可以看出，木塑复合材的阻湿率偏差相对较大；拒水率的偏差相对较小，比较稳定.随着吸湿和吸水过程的进行，试件阻湿率和拒水率的误差逐渐渐小.通过观察误差棒的长短发现，三组木塑复合材的误差棒的前期较长，后期逐渐变短，则表明误差随着吸湿和吸水时间的延长逐渐变小.同时表明实验数据离散程度较低，数据较好.

图3

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图3   PMMA木塑复合材阻湿率和拒水率误差图

Fig. 3   Error diagram of moisture resistance and water repellency of PMMA wood-plastic composites


PMMA木塑复合材的阻湿性和防水性主要源于其制备过程中，单体MMA与木材细胞壁上的羟基发生共聚反应，形成较强的氢键，从而降低了水与木材羟基的亲合力[33].此外，生成的聚合物填充了细胞腔内的大部分空间，使得进入木材的流体渗透性降低.同时，木材的纹孔被聚合物单体填充，聚合反应与细胞壁上的羟基相互作用生成较强的化学键[34]，增强了聚合物基质与亲水性木材的相容性[35,36].在它们共同作用下，PMMA木塑复合材的阻湿性和防水性得到显著的提高.

2.2 PMMA木塑复合材吸湿过程中的水分状态

图4为未处理材和PMMA木塑复合材吸湿过程的T2弛豫分布.表2为未处理材和PMMA木塑复合材在吸湿120 h时的吸湿率和平均T2及T2峰面积.根据图4可知未处理材在吸湿2 h之前，只有一个T2弛豫峰，弛豫时间为0.9 ms左右，随着未处理材吸湿时间延长，弛豫峰右移，弛豫时间逐渐变长，由最初的0.9 ms增加到1.48 ms.当吸湿6 h之后，吸湿率为12.89%，未处理材的T2反演图谱出现两个弛豫峰，第一个弛豫峰的T2值小于0.1 ms，约在0.05 ms左右；第二个弛豫峰的T2值为1.95 ms.随着吸湿过程不断进行，木材吸湿的吸湿率也越来越高.当吸湿率达到19.30%时，达到吸湿平衡，图中可清晰看到一个T2为2.68 ms的弛豫峰.该弛豫峰位于10 ms以内且吸湿率低于纤维饱和点（FSP）30%[37].因此认为该木塑复合材内部吸湿的水分状态主要是结合水或是细胞壁水[38,39].

图4

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图4   未处理材与PMMA木塑复合材吸湿水分T2分布

Fig. 4   Moisture T2 distribution of untreated wood and PMMA wood-plastic composite during moisture absorption process


表2   吸湿120 h的未处理材和PMMA木塑复合材吸湿率和平均T2及T2峰面积

Table 2  Moisture absorption rate, average T2 and peak area of untreated wood and PMMA wood plastic composite material after 120 h

试件编号吸湿率/%T2(1)/msT2(2)/ms峰面积A(1)峰面积A(2)总面积A(总)
PM#019.300.362.680.1229.5229.64
PM#6-2513.230.302.490.2324.2124.44
PM#6-5012.580.602.470.3722.9423.31
PM#6-1008.740.201.960.3721.3721.74
PM#12-2513.030.302.600.2223.4623.68
PM#12-509.90-2.21-22.4422.44
PM#12-1007.420.201.950.3118.6018.91
PM#24-2513.53-2.60-24.5824.58
PM#24-509.51-1.85-17.8217.82
PM#24-1006.07-1.82-17.2617.26

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如图4所示，在相同的浸渍浓度下，PM#6、PM#12、PM#24样品与未处理材具有很多的相似的特征.在吸湿的初始阶段，均没有观测到具体的峰.随着吸湿的进行，当吸湿率大于3%时，逐渐出现第一个稍明显的峰.随着吸湿的进行，峰值逐渐向右移动，弛豫时间逐渐变长，个别T2分布图中开始出现第二峰.该弛豫峰的T2弛豫时间处于0.01~1 ms之间.但木塑复合材在达到吸湿平衡时的吸湿率仍处于FSP之下.这表明木塑复合材内的水分主要是结合水，这与未处理材的结果一致.

通过表2和图4可知，在相同浸渍时间下，PMMA木塑复合材的吸湿率随着浸渍浓度增加逐渐降低. 浸渍浓度为25%时，PM#24-25的吸湿率和峰面积稍大于PM#12-25，这可能是因为MMA的浓度较低时，聚合生成的PMMA的含量较少，试件中的羟基没有完全消耗，导致PMMA木塑复合材的阻湿效果一般.而由于木材试件本身也存在各向异性，低浓度的浸渍未能展现较好的阻湿性能. 对比其它组，PM#6-100、PM#12-100、PM#24-100三组木塑复合材T2 (2)较短，吸湿率在相同浸渍时间组内最低.这是由于木塑复合材在高湿环境下水蒸气的扩散受到木塑复合材本身性质和聚合物填充多孔结构阻力的影响.木塑复合材中一部分的孔隙与吸附位点被PMMA固化体系或化学取代[40]；且PMMA在木材中固化后，会形成一层密集的聚合物表面，这层表面会覆盖木材的表面，从而防止水分渗透进入木材内部，使得木材变得疏水.蒸气难以进入木塑复合材内部，因而吸湿率降低，低吸湿率又使得水分自由度降低，导致更小的T2弛豫时间[28].

2.3 PMMA木塑复合材吸水过程中的水分状态

图5为未处理材和PMMA木塑复合材吸水过程中不同吸水率的T2弛豫分布.图6为未处理材和PMMA木塑复合材吸水过程中吸水率与T2峰面积的关系.表3为未处理材和PMMA木塑复合材吸水120 h时的测定吸水率和平均T2及T2峰面积.

图5

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图5   PMMA木塑复合材吸水过程水分T2分布

Fig. 5   Moisture T2 distribution of PMMA wood-plastic composites during water absorption process


图6

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图6   不同浸渍时间和浓度的PMMA木塑复合材吸水率与T2峰总面积关系（吸水时间分别为0 h, 3 h, 6 h, 12 h, 24 h, 36 h, 48 h, 60 h, 72 h, 84 h, 96 h, 108 h和120 h）

Fig. 6   The relationship between water absorption and T2 peak area of PMMA wood-plastic composites with different impregnation time and concentrations (The water absorption times are 0 h, 3 h, 6 h, 12 h, 24 h, 36 h, 48 h, 60 h, 72 h, 84 h, 96 h, 108 h and 120 h, respectively)


表3   吸水120 h的未处理材和PMMA木塑复合材吸水率和平均T2及T2峰面积

Table 3  Water absorption, T2 average relaxation time and peak area of untreated wood and PMMA wood plastic composite material after 120 h

试件编号吸水率/%T2(2)/msT2(3)/ms峰面积A(2)峰面积A(3)总面积A(总)
PM#0130.734.50102.3469.9013389.1013459.60
PM#6-2596.683.6589.0956.7510254.0210310.77
PM#6-5081.653.4089.0758.448295.258353.69
PM#6-10034.482.7633.7047.734237.274280.99
PM#12-2589.353.4189.0745.927672.147718.06
PM#12-5059.673.40155.5250.785154.885205.66
PM#12-10031.702.9677.5351.602316.822368.42
PM#24-2590.033.65135.1050.517688.597739.10
PM#24-5047.812.5864.4732.745035.805068.54
PM#24-10020.792.4031.1438.362098.092136.45

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如图5所示，木塑复合材吸水过程中，随着时间的增加，吸水率逐渐升高，图中逐渐出现3~4个峰.不同吸水时间下的PM#6、PM#12、PM#24样品的T2都发生较大变化，信号峰值和峰面积逐渐增大，表明木塑复合材的吸水率逐渐增加.在相同浸渍时间（PM#6、PM#12、PM#24）组内分别进行T2弛豫分布对比，可知浸渍浓度为100%的弛豫时间分布较窄，且峰总面积较小.这说明在木塑复合材吸水过程中，随着MMA浓度的增加，木塑复合材的孔径和水分分布发生了变化.通过对木材吸水过程中水分状态的研究，可知不同组分的T2时间的范围是不同的，用于区分自由水和结合水[41,42].随着吸水过程的不断进行，吸水率不断升高，未处理材T2弛豫峰也随着吸水时间逐渐向右移动.在吸水120 h后，吸水率逐渐趋于稳定，T2弛豫峰几乎不再移动，且吸水时间逐渐延长后，没有明显的变化.因此，将120 h作为平衡时间点.在平衡时可以从图中观察到两个明显的峰，其中一个峰弛豫时间[对应T2(2)]由吸湿结束后的2.68 ms增加到4.50 ms，代表水分为结合水，此时结合水已饱和；另一个峰弛豫时间为102.34 ms，代表水分为自由水.通过对木塑复合材的水分弛豫时间的测定，发现木塑复合材在吸水稳定时，个别图谱出现三个不同的T2范围，T2(1)约在0.01~0.1 ms的范围内，T2(2)约在1~10 ms的范围内，这两部分都代表着结合水，T2(3)在10~100 ms以及大于100 ms的范围内代表自由水.发生这种变化是因为细胞壁由于修饰而变得疏水[43].

如表3和图6所示，PM#0、PM#6、PM#12、PM#24不同浸渍浓度下的峰面积随着吸水率的增大而增大；对比未处理材和PMMA木塑复合材的峰面积和吸水率的大小可以看出，木塑复合材的平衡吸水率和峰面积远低于未处理材.表明聚合物PMMA对木材的整体防水性能有重要影响.PMMA木塑复合材的吸水率在相同浸渍时间下，吸水率和峰总面积随着浸渍浓度的增加而逐渐降低；相同浸渍浓度下，吸水率和峰总面积基本上随浸渍时间增加呈降低趋势. 但在相同浸渍浓度下，PM#24-25的吸湿率和峰总面积略高于PM#12-25，这可能是由于浸渍浓度为25%时，试件内的羟基没有被完全覆盖，聚合生成的PMMA含量较少，致使PMMA木塑复合材的防水性能不佳，木材存在各向异性，导致吸水率变化较小.PM#6-100、PM#12-100和PM#24-100三组木塑复合材吸水率与峰总面积始终保持最低，同时，在100%浸渍浓度下，PM#24-100的峰总面积要低于其它两组.这表明在浸渍浓度为100%状态下的木塑复合材内部几乎被聚合物PMMA网络结构填满，木材细胞腔内三维聚合物网络起到了阻碍水分进入的作用；同时PMMA聚合物填料的存在可能改变了木材纤维的分布和连接方式，减少了水分侵入的连接链[44,45].

2.4 未处理材与PMMA木塑复合材吸湿率和吸水率与峰面积的关系

图7为未处理材和PMMA木塑复合材（PM#6-25）吸湿率和吸水率与T2峰总面积的变化关系.从图中可以看出，未处理材和PMMA木塑复合材吸湿和吸水过程的峰总面积与称重法测得的吸湿率和吸水率呈正相关，相关系数R2都在0.93以上.说明方程的拟合程度较好.T2峰面积与吸湿率和吸水率呈线性相关，因此可利用T2弛豫峰面积来预测木塑复合材和未处理材的水分含量的变化.同时有利于在进一步的研究中量化木材中不同水分状态（即结合水和自由水）的吸水率变化.

图7

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图7   未处理材和PMMA木塑复合材吸湿率和吸水率与峰总面积的线性拟合

Fig. 7   The linear fitting between moisture absorption rate, water absorption rate and total peak area of untreated wood and PMMA wood-plastic composites


3 结论

（1）通过对比PMMA木塑复合材的吸湿和吸水过程中的阻湿率和拒水率变化可知，PMMA木塑复合材具有较强的阻湿性和防水性.通过对比可知，木塑复合材的阻湿率和拒水率在随着浸渍浓度和浸渍时间的增加逐渐增大，在本实验条件下浸渍浓度100%，浸渍时间为24 h的木塑复合材表现出良好的阻湿防水性.

（2）PMMA木塑复合材的吸湿过程中主要出现一个峰，个别图中出现两峰.弛豫峰的T2值处于0.1~10 ms之间，因此木塑复合材中增加的主要是结合水.T2值随着浸渍浓度增大逐渐减小.PMMA木塑复合材在吸水过程中，主要出现两个峰，其中一个弛豫峰的T2值处于1~10 ms之间，代表结合水；另一个峰处于10~100 ms以及大于100 ms范围，代表自由水.个别图谱会在0.01~0.1 ms范围出峰，也代表结合水.但在相同浸渍时间下，随着浸渍浓度增加，木塑复合材的弛豫分布变窄，吸水率随着降低，峰总面积也随之减少.

（3）PMMA木塑复合材吸水过程的峰面积随着吸水过程中吸水率的增加而增大.对比未处理材与PMMA木塑复合材的峰总面积可知，木塑复合材的平衡吸水率越小，峰总面积越低.在相同浸渍时间下，木塑复合材吸水过程中平衡吸水率随着浸渍浓度增大逐渐降低，峰总面积也随之减小.在相同较高浸渍浓度下，木塑复合材的平衡吸水率和峰总面积在随着浸渍时间的增大而减小.

（4）通过线性回归发现，PMMA木塑复合材和未处理材的吸湿率和吸水率与核磁共振T2图谱峰总面积呈线性相关，相关系数达到0.93以上.有利于未来研究不同水分状态的木塑复合材吸水率的变化.

利益冲突

无

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This review summarizes the development of the experimental technique and analytical method for using TD-NMR to study wood-water interactions in recent years. We briefly introduce the general concept of TD-NMR and magnetic resonance imaging (MRI), and demonstrate their applications for characterizing the following aspects of wood-water interactions: water state, fiber saturation state, water distribution at the cellular scale, and water migration in wood. The aim of this review is to provide an overview of the utilizations and future research opportunities of TD-NMR in wood-water relations. It should be noted that this review does not cover the NMR methods that provide chemical resolution of wood macromolecules, such as solid-state NMR.

ROSTOM L, COUYTIER-MURIAS D, RODTS S, et al.

Investigation of the effect of aging on wood hygroscopicity by 2D 1H NMR relaxometry

[J]. Holzforschung, 2020, 74(4): 400-411.

DOI:10.1515/hf-2019-0052      URL     [本文引用: 1]

Two-dimensional proton nuclear magnetic resonance (2D 1H NMR) relaxometry is increasingly used in the field of wood sciences due to its great potential in detecting and quantifying water states at the level of wood constituents. More precisely, in this study, this technique is used to investigate the changes induced by “natural” and “artificial” aging methods on modern and historical oak woods. Two bound water components are detected and present differences in terms of association to the different wood polymers in cell walls: one is more strongly associated with wood polymers than the other. The evolution of the two bound water types is discussed in regard to aging methods and is related to the structure of the cell wall, especially with the S2 layer and the evolution of wood chemical composition (cellulose, hemicelluloses and lignin). The evolution of hydric strains is also discussed taking into account the effect of aging methods on the two bound water components. The obtained results confirm the ability of 2D 1H NMR relaxometry to evaluate the effect of aging at the molecular level and on hydric deformation. Furthermore, this method shows that it is possible to determine the moisture content of wood without the necessity to oven-dry the wood material.

LI J Y, MA ER N.

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[J]. Journal of Forestry Engineering, 2021, 6(03): 61-68.

[本文引用: 1]

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Low-field nuclear magnetic resonance (LFNMR) relaxometry was applied to determine the spin-spin relaxation time (T\n 2) of water-saturated Norway spruce (Picea abies (L.) Karst.) specimens cut from mature sapwood (sW) and mature and juvenile heartwood (hW), where earlywood (EW) and latewood (LW) were separated. In combination with quantitative wood anatomy data focusing on the void volumes in various morphological regions, the NMR data served for a more reliable assignment of free-water populations found in water-saturated solid wood. Two free-water populations were identified within most sample types. One was assigned to water in the tracheid lumen and the other to water inside bordered pits. Whether water in the ray cell lumina was included in one or the other of these two populations depends on the curve-fit method applied (continuous or discrete). In addition, T\n 2 differences between the different tissue types were studied and, for comparison, sorption isotherms were measured by means of a sorption balance. There was a significant difference between EW and LW as well as between juvenile wood and mature wood in terms of T\n 2 related to the cell wall water. However, no differences were seen between the sorption isotherms, which indicates that the observed T\n 2 differences were not due to differences in cell wall moisture content (MC).

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In wood–polymer composites (WPCs), regardless of the origin of the filler and its dimensions, their significant role in changing the properties of the WPCs’ material was found. Given the above, it is of particular importance to determine the size of the wood filler particles after their production. In addition, it is also important to determine the degree of distribution of the filler in the polymer matrix. The methodology for determining particle size and distribution is complex, even when using image analysis computer systems. This article presents the application and implementation of the multi-stage procedure for determining the size of wood particles and the degree of their distribution in the WPCs by means of image analysis using a numerical calculation program. The procedure, co-authored by the researchers at the Koszalin University of Technology and School of Mechanical and Materials Engineering, is published in the Industrial Crops and Products 2016 Comparing the results obtained for the PP/Lignocel 3-4 and PP/Lignocel C120 composites produced under highly different conditions in the target zone, it was found that the degree of the component distribution in the polymer matrix was significantly influenced by the width of the target gap. In both cases, the best homogeneity of the material and a good distribution of the filler in the polymer matrix was achieved within the parameters that have a mild effect on the material and allow it to stay longer in the plasticizing system, i.e., Ws = 1.0–3.0 mm with simultaneous impact medium to high speed in the range n = 26–40 rpm.

ČREŠNAR K P, BEK M, LUXBACHER T, BRUNCKO M, et al.

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