摘要
纳米纤维素具有来源广泛、生物相容、绿色环保、可再生等特点,具有的特殊纳米层级形貌赋予了其特殊的性质。本文简述了纳米纤维素的制备工艺,其中包括化学法、物理法、生物法和其他方法等,并总结了纳米纤维素作为纸浆添加剂、涂料和特种材料在制浆造纸领域的应用研究。
纤维素在地球上分布广泛,是世界上储量最大的一种自然资源,在人类社会中有着广泛的用途,而纳米纤维素来源于天然纤维素,是一种新型的高分子材料。纳米纤维素是指某一维度上具有纳米尺寸的生物质基材
通过加酸水解纤维素含量较高的原料如棉短绒或者微晶纤维素,酸液能够破坏非结晶区的氢键从而将纤维中的非结晶区溶解,结晶区因为排列整齐致密,不易被水解得以保留,剩下的这一部分高结晶度物质就形成了CNC。通常使用的酸有硫
制备纳米纤维素的原料和酸解方法很大程度的影响产物的尺寸与形态。微晶纤维素(MCC)由于自身粒径尺寸小,所以水解难度低,可以有效地制备纳米纤维素。盐酸水解制备的纳米纤维素得率低,表面电荷低且分散性差;而硫酸水解时,纳米纤维素表面会残存一些磺酸根,使得纤维素表面负电荷高,分散性
TEMPO是2,2,6,6-四甲基哌啶-1-氧化物,具有弱氧化性,在TEMPO/ NaBr / NaClO体系中,NaClO先与NaBr形成NaBrO,随后NaBrO将TEMPO氧化成亚硝鎓离子,亚硝鎓离子将羟基氧化成醛基最后生成羧基,不过这个体系存在缺陷,碱性环境下,纤维素分子解聚现象比较严重,聚合度下降,且这个体系氧化后的纤维素有醛基残留,使得纳米纤维素热稳定性变差,在温度超过80℃时会变色,还会在纤维间形成半缩醛键而影响其在溶液中的分散

图1 碱性环境下以水为介质TEMPO/ NaBr / NaClO体系中的氧化机

图2 弱酸性环境下以水为介质TEMPO/ NaClO / NaClO2体系中的氧化机
高压均质法就是在高压均质机中用高压泵将纤维素悬浮液增压后通过Z型或者Y型的均质室,当液体高速通过均质腔时会急剧改变流动的方向,由此产生的撞击力、剪切力和空穴效应可以有效地分散液体中的纤维素。经过几十次循环后,可以得到直径小于100 nm的纳米纤维
魏莉等
精细研磨类似于使用磨盘机,研磨室中纤维原料在悬浮液中受到了挤压、剪切、撕裂、摩擦等作用,纤维被切断、分丝帚化、压溃。研磨时间和速度决定了产物尺寸,这个方法不需要预处理且设备清洁方便,但是对纤维的降解程度较难把控,一般会与其他方法协同处理,何玉
由于酶解的过程是多种酶协同合作的一个过程,反应进行到一定过程会发生深度降解,影响纳米纤维素的聚合度,需要对实验条件进行很好的把控从而避免这种情况的发生,酶解法相比酸解条件要温和且更环
1886年,Brow
静电纺丝法主要是用于制备纳米尺寸的纤维交织膜。静电纺丝法是将溶液调节至合适黏度,通过泰勒锥形成射流,利用高电压使纤维素溶液带电,液体克服表面张力后拉成细丝,再由收集装置接收,方法如

图3 静电纺丝法制备CN
纳米纤维素由于其比表面积大且表面富含羟基,相互之间容易形成氢键而具有较强的结合力,可以直接作为纸浆的添加剂添加到纸浆中来提升纸张的强度。文献[
研究人员为了克服添加纳米纤维素会降低纸浆的滤水性能这个缺点,开辟出一条新的思路,将纳米纤维素和填料相结合,形成二元助留助滤体系,在提高填料保留率和纸张强度的同时还可以加快浆料的脱水速度。王俊芬等
通常使用纯CNF或者与常规涂料混合来改善纸张的强度、印刷适应性、透气性、抗水性、均匀性和光学性能等。传统的涂布剂多是一些合成的高分子化合物,在回收降解方面存在困难,而CNF是环境友好型材料,可天然降解且不会对环境造成污染。Song等
纳米纤维素不仅可以作为添加剂改善纸张性能,其自身就可以结合并形成具有特殊性能的纸张。Hentze等
纳米纤维素因为具有抵抗形变能力高、比表面积大、光学性能独特和流变性良好的特点,在制浆造纸方面有着广泛的应用前景和巨大潜力。可以直接添加到纸浆中配合轻度打浆通过增加氢键结合来提高纸张的强度,但同时会降低纸浆的滤水性能,导致生产率降低,也能与填料相结合形成二元助留助滤体系在提高纸张强度的同时提高填料留着率,进而提升纸张的不透明度。纳米纤维素的悬浮液有剪切变稀的特点使其成为涂料的良好候选者或减少混涂料中其他添加剂的使用制备可降解无污染的环保型涂料,从而改善透气性。但纳米纤维素由于自身表面上的氢键较多会破坏防潮性能,可通过化学改性解决这个问题,改性后涂布在纸张表面形成疏水层。虽然目前纳米纤维素的制备方法很多,但是大规模的工业化生产还有许多挑战,如制备纳米纤维素的成本较高,在实际生产中还需要降低制备成本和提高产率,不同批次的纳米纤维素性能会有一定的差异,在造纸过程中会影响生产系统的稳定性。总而言之,纳米纤维素在制浆造纸领域虽然存在不少挑战,但其潜在价值仍然值得研究。
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