摘要
本研究利用氢键阻滞剂氯化钠对制备的纤维素纳米纤丝(CNF)进行了干燥前预处理,探讨了真空干燥和冷冻干燥两种干燥方式对CNF再分散性能的影响,借助扫描电子显微镜、X射线衍射仪、多重光散射分析仪、流变仪对CNF的性能进行了分析表征。结果表明,真空干燥(55℃)可以促进氯化钠的氢键阻滞作用,采用氯化钠预处理后,真空干燥得到CNF的结晶度为64.7%,相比冷冻干燥(-84℃)处理的CNF的结晶度(66.7%)降低了2个百分点。真空干燥所得CNF水溶液再分散性能较高,真空干燥后水溶液再分散的CNF稳定性指数在0~0.45之间,相比冷冻干燥处理的CNF稳定性指数范围(0~4.5)要小。采用氯化钠预处理有利于提高CNF再分散悬浮液的稳定性,经过氯化钠预处理后真空干燥CNF的表观黏度明显低于经冷冻干燥CNF的表观黏度。
植物纤维素是自然界中一类资源丰富、可再生、可生物降解的优质天然高分子材料,其特殊晶体结构赋予的良好力学性能使其在植物细胞中扮演着重要角
因此,为了克服CNF在干燥过程中出现不可逆的团聚现象,得到干燥后纳米尺寸稳定、性能良好的CNF,本研究采用NaCl在一定条件下对CNF进行预处理,分别采用真空干燥与冷冻干燥两种干燥方式对其进行干燥,借助X射线衍射仪、多重光散射分析仪、稳态流变性测量仪,分析检测CNF的结晶度、物理稳定性、表观黏度等性能,探究NaCl作为“氢键阻滞剂”时,真空干燥和冷冻干燥对CNF再分散性能的影响。
氯化钠(NaCl,分析纯,天津市大茂化学试剂厂);氢氧化钠(NaOH,分析纯,天津市大茂化学试剂厂);盐酸(HCl,分析纯,莱阳经济技术开发区精细化工厂);蒸馏水(电导率1.0 μS/cm,25℃,实验室自制);漂白桉木浆板(巴西)。
疏解打浆处理:经过纤维解离器疏解后的桉木浆料以10%浆浓置入Vally打浆机(日本熊谷)打浆至加拿大标准游离度(CSF)为100 mL。
纳米均质处理:将经过疏解打浆处理的桉木浆料放入高压微射流纳米均质机中进行纳米均质处理;M-110EH-30高压微射流纳米均质机(美国)处理条件为:在20 MPa 下通过200 μm腔室10次,然后在100 MPa下依次通过200 μm和87 μm 腔室15
调节CNF悬浮液pH值为8,然后加入一定量NaCl进行预处理以控制CNF悬浮液的离子强度为10 mmol/
将CNF悬浮液放入冰箱冷冻处理24 h,冷冻温度为-40
将CNF悬浮液放入真空干燥箱(蓝豹,上海一恒科学仪器有限公司)中在55℃和-50 kPa下干燥48
通过多重光散射分析仪(IKA T25,IKA-沃克有限公司)将干燥后的CNF粉末再分散在蒸馏水中,CNF再分散质量分数分别为1%、0.5%、0.25%和0.125 %,搅拌时间为30
采用D8-ADVANCE型X射线衍射仪(德国布鲁克AXS公司)进行样品结晶度测试,测试衍射角为10°~40°,工作电压40 kV,工作电流20 mA。结晶度(CrI)按照
(1) |
式中,代表结晶区峰值强度的I002是在2θ角接近23°处的最大峰值强度,而代表无定形区峰值强度的Iam是在2θ角接近18.25°处的最小衍射强
使用多重光散射分析仪分析CNF的稳定性。将不同CNF浓度的样品放置在圆柱形玻璃池中,然后在25℃下放置在检测器中1 h。根据透射率的变化(ΔBS)评估每个样品的稳定性。稳定性指数(TSI,Turbiscan Stability Index)根据
(2) |
式中,xi表示每3 min测量的背散射光平均值;xbs表示xi的平均值;n表示扫描次
扫描电子显微镜(日立8220,日本)在低加速电压(5 kV)和短工作距离(7.5 mm)下工作。将质量分数为0.05%的CNF再分散水悬浮液滴在覆盖有碳导电带的样品架上进行真空干燥或冷冻干燥处理。干燥后样品涂有2 nm的金/钯层,以确保导电
为了研究两种不同干燥方式(冷冻干燥和真空干燥)对CNF再分散性的影响,制备了两种类型的CNF样品。根据制备工艺不同分为CNF1和CNF2,CNF1为收集的经“干燥-再分散”所得样品;CNF2为收集的经“NaCl预处理-干燥-再分散”所得样品,其中真空干燥制备CNF2的过程如

图1 CNF2真空的制备过程

图2 CNF1和CNF2的XRD谱图
由于CNF悬浮液是热力学不稳定体系,其稳定性受到粒子碰撞及聚集作用的影响,随着时间长短而发生变化。本研究利用Turbiscan技术评价了CNF水溶液再分散后体系的物理稳定性,探究了冷冻干燥、真空干燥及CNF再分散时质量分数对CNF悬浮液稳定性的影响。




图3 经过NaCl预处理的CNF2真空在不同质量分数下再分散后的稳定性谱图
由
比较
TSI是衡量纳米悬浮液稳定性的指标,TSI越小,说明稳定性越

(a) CNF1真空

(b) CNF1冷冻

(c) CNF2真空

(d) CNF2冷冻
图4 CNF1和CNF2再分散后的TSI图
由
可见,CNF再分散时质量分数越低,CNF悬浮液越不稳定;真空干燥CNF的稳定性高于冷冻干燥CNF;在CNF干燥前经过NaCl预处理可以提高其干燥后的稳定性。

(a) CNF1真空

(b) CNF1冷冻

(c) CNF2真空

(d) CNF2冷冻
图5 CNF1和CNF2再分散后的SEM图

图6 CNF1和CNF2的表观黏度
本研究以漂白桉木浆为原料,通过高压均质制备了纤维素纳米纤丝(CNF),并用NaCl对CNF悬浮液进行预处理,从CNF的物理稳定性、稳态流变性等方面分析评价了真空干燥和冷冻干燥对CNF再分散性能的影响。
3.1 在改善CNF再分散方面,NaCl起到了“氢键阻滞剂”的作用,且真空干燥方式对NaCl阻滞作用的增强效果好于冷冻干燥方式。NaCl预处理后,真空干燥(55℃)得到的CNF结晶度为64.7%,比冷冻干燥(-84℃)处理的CNF的结晶度(66.7%)降低了2个百分点,而且其CNF再分散稳定性好,其稳定性指数在0~0.45之间,小于冷冻干燥的CNF再分散稳定指数范围(0~4.5)。
3.2 在一定条件下,提高CNF在水溶液中的质量分数即CNF再分散浓度,有利于提高CNF悬浮液的稳定性。对于不经过NaCl预处理的CNF,干燥方式对其水溶液再分散后的表观黏度影响较小。
3.3 对于经NaCl预处理的CNF样品,经真空干燥CNF的稳态流变性明显低于经冷冻干燥CNF的稳态流变性。NaCl预处理与真空干燥组合是提高CNF水溶液再分散性的一种绿色高效途径。
参考文献
Yang G H, Ma G R, He M, et al. Application of Cellulose Nanofibril as a Wet-end Additive in Papermaking: A Brief Review [J]. Paper and Biomaterials, 2020, 5(2): 76. [百度学术]
Pei A, Butchosa N, Berglund L A, et al. Surface quaternized cellulose nanofibrils with high water absorbency and adsorption capacity for anionic dyes [J]. Soft Matter, 2013, 9(6): 2047. [百度学术]
Liu C, Du H S, Yu G, et al. Simultaneous Extraction of Carboxylated Celulose Nanocrystals and Nanofibrils via Citric Acid Hydrolysis-A Sustainable Route [J]. Paper and Biomaterials, 2017, 2(4): 19 [百度学术]
牟楷文, 刘卓燕, 周景蓬, 等. 纳米纤维素的研究进展: 2001~2015年收录文献检索分析 [J]. 中国造纸学报, 2016, 31(4): 55. [百度学术]
MOU Jiewen , LIU Zhuoyan, ZHOU Jingpeng, et al. Research Progress of Nano-cellulose: Literature Retrieval Analysis from 2001 to 2015 [J]. Transactions of China Pulp and Paper, 2016, 31(4): 55. [百度学术]
陈丽卿, 刘振华. 我国纳米纤维素的文献计量分析 [J].中国造纸学报, 2020, 35(1): 81. [百度学术]
CHEN Liqing, LIU Zhenhua.Bibliometric Analysis of Nano Cellulose in China [J].Transactions of China Pulp and Paper, 2020, 35(1): 81. [百度学术]
杜海顺, 刘 超, 张苗苗,等. 纳米纤维素的制备及产业化 [J]. 化学进展, 2018, 30(4): 448. [百度学术]
Du Hai-shun, Liu Chao, Zhang Miao-miao, et al. Preparation and Industrialization Status of Nanocellulose [J]. Chemical Progress, 2018, 30(4): 448. [百度学术]
Paãakko M, Ankerfors M, Kosonen H, et al. Enzymatic Hydrolysis Combined with Mechanical Shearing and High-Pressure Homogenization for Nanoscale Cellulose Fibrils and Strong Gels [J]. Biomacromolecules, 2007, 8(6): 1934. [百度学术]
He M, Yang G H, Chen J C, et al. Production and Characterization of Cellulose Nanofibrils from Different Chemical and Mechanical Pulps [J]. Journal of Wood Chemistry & Technology, 2018, 38(2): 149. [百度学术]
He M , Yang G H , Chen J C, et al. Nanofibrillation of a Bleached Acacia Pulp by Grinding with Carboxymethylation Pretreatment [J]. Paper and Biomaterials, 2018, 3(3): 32. [百度学术]
Saito T, Nishiyama Y, Putaux J L, et al.Homogeneous Suspensions of Individualized Microfibrils from TEMPO-Catalyzed Oxidation of Native Cellulose [J].Biomacromolecules, 2006, 7(6): 1687. [百度学术]
He M, Cho B U, Won J M. Effect of precipitated calcium carbonate--Cellulose nanofibrils composite filler on paper properties [J]. Carbohydrate Polymers, 2016, 136: 820. [百度学术]
Habibi Y, Lucia L A, Rojas O J. Cellulose Nanocrystals: Chemistry, Self-Assembly, and Applications [J]. Chemical Reviews, 2010, 110(6): 3479. [百度学术]
Moon R J, Martini A, Nairn J, et al. Cellulose nanomaterials review: structure, properties and nanocomposites [J]. Chemical Society Reviews, 2011, 40(7): 3941. [百度学术]
Kalousek V, Wang P, Minegishi T, et al. Conversion of Toluene and Water to Methylcyclohexane and Oxygen using Niobium-Doped Strontium Titan ate Photo electrodes [J]. Chemsuschem, 2014, 7(9): 2690. [百度学术]
Hietala M, Mathew A P, Oksman K.Bio nanocomposites of thermoplastic starch and cellulose nanofibers manufactured using twin-screw extrusion [J].European Polymer Journal, 2013, 49(4): 950. [百度学术]
Nogi M, Yano H. Transparent Nanocomposites Based on Cellulose Produced by Bacteria Offer Potential Innovation in the Electronics Device Industry [J]. Advanced Materials, 2008, 20(10): 1849. [百度学术]
Fras-Zemlji L, Stenius P, Laine J, et al. The effect of adsorbed carboxymethyl cellulose on the cotton fibre adsorption capacity for surfactant [J]. Cellulose, 2006, 13(6): 655. [百度学术]
Butchosa, Núria, Zhou Q.Water redispersible cellulose nanofibrils adsorbed with carboxymethyl cellulose [J].Cellulose, 2014, 21(6): 4349. [百度学术]
Eyholzer C, Bordeanu N, Lopez-Suevos F, et al. Preparation and characterization of water-redispersible nanofibrillated cellulose in powder form [J]. Cellulose, 2010, 17(1): 19. [百度学术]
Ho T T T, Zimmermann T, Hauert R, et al.Preparation and characterization of cationic nanofibrillated cellulose from etherification and high-shear disintegration processes[J].Cellulose, 2011, 18, 1391. [百度学术]
Vuoti S, Talja R, Johansson L S, et al. Solvent impact on esterification and film formation ability of nanofibrillated cellulose [J]. Cellulose, 2013, 20(5): 2359. [百度学术]
Missoum K, Bras J, Belgacem M N.Water Redispersible Dried Nanofibrillated Cellulose by Adding Sodium Chloride[J]. Bioma⁃cromolecules, 2012, 13(12): 4118. [百度学术]
Fall A B, LindströmS B, Sundman O, et al. Colloidal Stability of Aqueous Nanofibrillated Cellulose Dispersions[J].Langmuir, 2011, 27(18): 11332. [百度学术]
Peng Y, Gardner D J, Han Y. Drying cellulose nanofibrils: in search of a suitable method [J]. Cellulose, 2012, 19(1): 91. [百度学术]
Peng Y, Gardner D J, Han Y, et al. Influence of drying method on the material properties of nanocellulose I: thermostability and crystallinity [J]. Cellulose, 2013, 20(5): 2379. [百度学术]
刘 欢, 尹 婵, 魏晓奕, 等. 干燥方式对再生纳米纤维素结构及性能的影响 [J]. 化工新型材料, 2015(2): 184. [百度学术]
Liu Huan, Yin Chan, Wei Xiao-yi, et al. Effects of Drying Methods on Structure and Properties of Regenerated Nanocellulose[J]. New Chemical Materials, 2015(2): 184. [百度学术]
Samyn P, Barhoum A, Hlund T, et al. Review: nanoparticles and nanostructured materials in papermaking [J]. Journal of Materials ence, 2018, 53(1): 146. [百度学术]
Im W, Lee S, Rajabi Abhari A, et al. Optimization of carboxymethylation reaction as a pretreatment for production of cellulose nanofibrils [J]. Cellulose, 2018, 25(7): 3873. [百度学术]
Zhu Y, Ding J, Fu Y, et al. Fabrication and characterization of vapor grown carbon nanofiber/epoxy magnetic nanocomposites[J]. Polymer Composites, 2014, 37(6): 1728. [百度学术]
Caddeo C, Pucci L, Gabriele M, et al. Stability, biocompatibility and antioxidant activity of PEG-modified liposomes containing resveratrol [J]. International Journal of Pharmaceutics, 2017, 538(1/2): 40. [百度学术]
李秀雯,姜学泓,王静芳,等. TEMPO氧化法制备五节芒纤维素纳米纤丝及其悬浮液稳定性和流变行为表征 [J]. 浙江农林大学学报,2016,33(4): 667. [百度学术]
Li Xiu-wen, Jiang Xue-hong, Wang Jing-fang, et al. Preparation of Five-section Mangrove Cellulose Nanofibers by TEMPO Oxidation Method, Its stability of suspension and Rheological Behavior Characterization [J]. Journal of Zhejiang Agriculture and Forestry University, 2016, 33(4): 667. [百度学术]