木质纤维素及其组分转化木材胶粘剂的发展趋势

Wood Adhesives Derived from Lignocellulosics: History and Future

木质素是造纸工业废液中的主要成分。其具有和酚醛树脂相似的结构，因此可以部分替代苯酚用于酚醛树脂的生产。但是由于木质素化学结构复杂，具有化学反应活性的住点少、反应官能团所收到的空间位阻大，一般添加量不超过10％～20％。有机溶剂法制浆分离的木质紊分子量大，纯度高，酚醛树脂允许的添加量可达20％～30％，但是要求提高固化温度、延长固化时间来克服木质素反应活性低的限制。时木质素进行化学改性如羟甲基化、酚解、醇解、还原等，可以降低分子量，增加活性基团如酚羟基的含量、并使苯环上更多的活性位点暴露出来，从而提高木质素的化学反应活性。酚醛树脂经过改性木质素的允许的添加重可达30％～40％。部分木质素含量高的木质纤维素类原料如核桃壳粉等，经过研磨和化学活化处理后可以直接添加到酚醛树脂中，部分替代苯酚而不对胶接性能产生影响。环碳酸酯类和多元醇混合物被用来作为全新的液化剂来把木质纤维素快速液化为有用的化学原料。所得到的液化产物具有较高的反应活性，是今后利用木质纤维素开发绿色木材胶粘剂的一条可资利用的途径。

Lignocellulosics is the most abundant terrestrial biomass which consists of cellulose, hemicellulose and lignin. The proportion of this three component fluctuates dramatically among different genetic origins and different organs of individual plant. Lignin, the main component of the black liquor of pulping industry, with big quantity and low industrial value, was firstly used as the additives of wood adhesive. The concentrated black liquor was once used directly but failure out due to the disturb of the impurities. Lignin extract from black liquor possessing the network chemical structure that similar to PF resin can partially substitute the phenol for the PF resin production. Limited by the inactive chemical property, the proportion of lignin of conventional pulping process, like kraft lignin, added can hardly excess 10%～20% （w/w） of PF resin. Organic solvent pulping lignin owns a relatively high molecular weight and purity, the upper limit is between 20% ～ 30%, but higher curing temperature and longer curing time is needed to compensate the inactiveness of this kind lignin. Lignin was chemically modified to increase the activity by means of hydroxymethylation, phenolization, alcoholysis, chemical reduction using sodium dithionite, that results in lower the molecular weight and increasing fuctional group like phenolic hydroxyl group, lets more reactive site in the aromatic ring exposure. Up to 30% ～40% of the modified lignin can be added to the PF resin. Special originate lignocellulosics like wood barks and nut shells which are rich in lignin can be directly added to the PF resin after well milling and chemical activated. The liquefied product of lignocellulosics phenolysised or phenol-polyhydric alcoholysised under acid catalyst can be used to prepare the low formaldehyde emission PF resin. The liquefied product of polyhydric alcoholysised lignocellulosics under acid condition can react with the isocynanate, which can be used as wood adhesive. The intermixture of cyclic carbonate and polyhydric alcohol was used to liquefying lignocellulosics rapidly under atmosphere pressure, acid catalyst and elevate temperature. The liquefied product was far more reactive than polyhydric alcoholysised liquefied products. It provides with a possible new route to prepare lignocellulosic based wood adhesive.