【作者】 戴路；

【导师】 戴红旗；

【作者基本信息】 南京林业大学 ， 制浆造纸工程， 2011， 博士

【摘要】 棉纤维细胞壁的纤维素含量高,结晶度高,不含木素等其它成分,表面吸附的类脂、色素等杂质也很容易除去,因此常常将棉纤维作为天然纤维素材料研究纤维素的化学结构和各方面化学性能。但是,棉纤维的高结晶度限制了其利用价值,因此在进行反应之前,通常要进行溶胀或活化预处理。2,2,6,6-四甲基哌啶-1-氧化物自由基(TEMPO),具有弱氧化性,在含TEMPO的共氧化剂体系的存在下对伯羟基具有良好的选择性,反应条件比较缓和,反应过程也较简单。研究工作以棉纤维为原料,TEMPO/NaClO/NaBr氧化体系为处理方法,首先采用预处理体系(NaOH/水体系；NaOH/硫脲/尿素；PFI磨浆)对棉纤维进行预处理,以提高后续TEMPO氧化体系对棉纤维氧化的促进作用。其次以Olympus BH-2型多功能显微镜、扫描电镜(SEM)、红外光谱分析(FTIR)、X-射线衍射分析(XRD)、差热扫描量热分析(DSC)和热重分析(TG)为主要分析手段,对TEMPO氧化棉纤维的工艺条件进行了机理分析,重点分析了氧化产物受氧化工艺条件如氧化剂用量,反应时间,反应温度以及pH值等因素的影响。并依据氧化反应在棉纤维表面导入的羧基量,研究了表面羧基含量对纸浆干,湿抗张强度的影响。最后对研究的结果进行动力学分析,求出影响氧化反应速率的关键因素,深入了解该氧化体系的作用机制,为后续的产品应用(如膜改性、造纸增强剂、纳米纤维素)提供参考依据。研究结果如下：采用NaOH/水体系、NaOH/硫脲/尿素体系、PFI磨以及先PFI磨后分别采用NaOH/水体系、NaOH/硫脲/尿素体系这四种方法来对棉纤维进行预处理,然后再进行TEMPO氧化体系进行氧化,均能够加快TEMPO氧化反应速率,增加羧基生成量。NaOH/水体系中NaOH浓度为10%时、NaOH/硫脲/尿素体系中纤维质量分数为6%时,对氧化反应速率促进最为明显；PFI磨预处理对氧化反应速率促进作用较小；先PFI磨后分别采用NaOH/水体系、NaOH/硫脲/尿素体系对氧化反应速率的促进也很明显,棉纤维的反应性得到提高。通过几种预处理方法发现当磨浆转速为3万转时,随后采用纤维质量分数为6%的NaOH/硫脲/尿素进行处理,此条件是纤维较为理想的预处理体系。通过研究氧化剂NaCIO用量,反应时间,反应温度,pH值等因素对获得特定纤维羧基含量和纸浆纤维的制备工艺条件发现,较适宜的TEMPO氧化工艺条件是：NaCIO用量小于3mmol/g,反应时间控制在1h以内,pH为10.5,温度为室温25℃。在纤维表面导入的羧基对浆料手抄片的干、湿抗张强度的影响得出,提高干抗张强度的工艺条件为:NaCIO用量小于3mmol/g,温度为25℃,此时获取的羧基量为0.562mmol/g,干抗张强度为136.07N·m/g,比原纤维的90.35N·m/g提高了50.6%；提高湿抗张强度的工艺条件为：NaCIO用量小于lmmol/g,温度为低于室温25℃,原因是低温反应活性较低,将醛基转换为羧基的速率降低,更有利于羟醛形成半缩醛结构,此时湿抗张强度为12.55N·m/g,比原纤维的6.03N·m/g提高了2倍多。反应时间和pH同上。光学显微镜测定经过TEMPO氧化体系处理后纤维的形态特征图并没有发生明显的变化,随着氧化剂NaClO用量的增大,羧基的引入增加了纤维的水溶性,致使部分纤维被转换成水溶性组分。扫描电镜(SEM)图可以在纤维表面附近可观察到大量的细小纤维,并可见部分纤维壁上发生明显的分丝帚化现象。深度氧化后可以清楚地看到纤维细胞壁表面异常光滑。X-射线衍射曲线图(XRD)可知棉纤维经TEMPO选择性氧化处理后其晶体类型无变化仍为纤维素Ⅰ晶体,保持晶区与非晶区两相共存的状态,且曲线中各X衍射峰相对应的衍射角2e变化不大,均为纤维素Ⅰ的特征衍射峰。红外光谱分析(FTIR)表明,经TEMPO氧化后的棉纤维红外图谱在1740cm-1左右处出现了新的特征吸收峰,此处即为非共轭羰基或羧基C=O伸缩振动峰,由此说明棉纤维大分子链中已经生成了新的活性基团。聚合度的变化表明,TEMPO氧化体系选择性氧化过程中,由于纤维素的氧化降解反应使纤维素分子链断裂,纤维被剥离、溶失,导致纤维产生重量损失,是由于在pH 10-11时TEMPO氧化体系在反应过程中所形成的·OH引起β消除反应,其反应非常迅速,造成聚合度下降,而聚合度下降是引起纤维重量损失的直接原因。差热扫描量热分析(DSC)和热重分析(TG)的研究结果表明,棉纤维经TEMPO选择性氧化处理后,纤维吸热分解温度和热失重温度都往低温方向偏移,表明随着反应条件的加深,棉纤维的热稳定性逐渐在降低,但是幅度并不是非常剧烈,同时最大热失重速率温度也下降,但是几乎都在350℃左右,并没有太大的变化趋势,这两点和X射线衍射结果相类似,氧化反应对纤维结晶度的影响并不是很大。由一级反应建立的动力学方程求得了不同条件下TEMPO氧化棉纤维的反应动力学常数,并通过温度的曲线求得了该氧化反应的活化能和指前因子。棉纤维的特殊构造使得这一反应动力学过程可用关于氧化基质的两个一级反应动力学来描述,即纤维素初生壁区和次生壁区的动力学过程。在一定浓度范围内,初生壁区的氧化反应速率与TEMPO的浓度和NaBr的浓度成正比,但是当NaBr的浓度大于1.0mmol/gi[绝干浆)后,氧化反应速率随着NaBr用量的增加而稍有下降。次生壁区的氧化反应速率几乎不随催化剂的用量变化而变化。pH值对氧化反应速率也有较大影响,就氧化速率而言,pH最佳控制在10-11之间较好。温度对氧化反应速率常数的影响可很好地用Arrhenius方程来描述,由此求得的表观活化能为56.66kJ/mol,指前因子A为1.84×107min-1。以深度氧化后的棉纤维为原料,根据氧化后纤维形态特殊将其作为一种绿色的化学品加入到纸浆纤维中,随着纤维添加量(5%-25%)的增加,纸张的干,湿抗张强度逐渐提高；将改性后的纤维材料涂覆在微滤膜的表面,随着纤维添加量的增加(0.1g-0.5g),微滤膜的截留率提高到25.10%,但是纯水通过量下降幅度明显；将改性后的纤维作为添加物跟高打浆度的纤维复合抄造成滤纸,随着纤维添加量的增加(0.1g-0.5g),抄造成的滤纸截留率从25.36%提高到33.28%,纯水通过量不仅下降明显,而且比涂覆在微滤膜表面的水通量更低。更多还原

【Abstract】 The cotton cell wall contents of high cellulose and crystallinity index. It contains no lignin or surface-adsorbed lipids, pigments and other impurities can be easily removed. For these reasons cotton fiber is often used as a natural cellulosic fiber material to study the structure of cellulose and various chemical properties. However, pretreatment should be used for its high crystalline.2,2,6,6-tetramethyl piperidine-1-oxyl radical (TEMPO), having a weak oxidizing ability, has a good selectivity to primary hydroxyl oxidation for polysaccharides. The reaction conditions are moderate and the reaction process is relatively simple. In this study, the cotton fiber is used as experimental material and TEMPO/NaBr/NaClO system is adopted as process method. In order to improve the performance, cotton fiber is treated by NaOH/water, NaOH/thiourea/urea and PFI refining to improve TEMPO oxidation first. Sequently, Light Microscope, SEM, FTIR, XRD, DSC and TG technology are employed to analyze the mechanism of conditions in the TEMPO oxidation cotton fiber. The influences of NaClO dosage, reaction time, reaction temperature and pH value are discussed in detail. The effect of surface carboxyl groups on paper’s dry or wet tensile strength have been researched according to the carboxyl groups introduced on cotton fiber surfaces by TEMPO-mediated oxidation. The key factor of oxidation reaction rate are calculated and the mechanism of oxidation reaction could be understood finally by performing the results of kinetic analysis which offers a reference to subsequent applications such as membrane modification, paper reinforcing agent, nano-cellulose.The research results were shown as the following:The pretreatment with NaOH/water, NaOH/thiourea/urea and PFI refining were performed before TEMPO selective oxidation could increase the oxidation rate and improve the carboxyl content. The pretreatment facilitates the oxidation rate obviously when the NaOH concentration was 10% in NaOH/water and the fiber consistency was 6% in NaOH/thiourea/urea, while PFI refining only had a weak improvement. With 30000 rpm grinding speed and 6 fiber consistency of in NaOH/thiourea/urea, this pretreatment facilitates the oxidation rate most and improves the reactivity of the cotton fibers.By studying the effect of NaClO dosage reaction time, reaction temperature and pH value on the specific carboxyl group content of cotton faiber and Pulp fiber production process conditions, moderate TEMPO oxidiation condition is:NaCIO dosage less than 3mmol/g, reaction time within Ih, pH value at 10.5 and reaction temperature at 25℃.The impact of introduced carboxyl groups on surface of cotton fiber on paper’s dry or wet tensile strength was proposed. Dry tensile strength was 136.07 N·m/g increased 50.6% compared to original fiber while fiber was treated at 25℃and NaClO dosage was less than 3mmol/g obtained 0.562 mmol/g carboxyl. However, NaClO dosage should be less than lmmol/g and reaction temperature was lower than 25℃in order to increase wet tensile strength for the hemiacetal structure was formed easily at low temperature when converte rate of aldehyde group to carboxyl group was reduced. Under this condition, the wet tensile strength was 12.55N·m/g twice more than the original fiber wet tensile strength.The morphology of cotton fiber was obviously unchanged during the TEMPO oxidation according to Light Microscope, The fiber was partly water-soluble with the increasing dosage of NaClO for the carboxyl group increased the water-solublity of fibei. lots of tiny fiber near the fiber surface and occurred points silk in fiber wall can be observed obviously from the SEM figure, the removal phenomenon in fiber cell layer can be clearly seen by depth oxidation left the abnormal smooth fiber on surface. The X-ray diffraction patterns of the cotton fibers before and after the TEMPO-mediated oxidation was barely changed, the fiber always have its cellulose I crystal morphology and keep the state of two-phase coexistence. The 20 diffraction angle of cotton fiber was nearly unchanged through the oxidation. FTIR indicated that the new absorption peaks appearanced in 1740cm-1 after TEMPO oxidiation was the conjugate carbonyl or C=O streching vibration peaks, it can be illustrated that a new active groups generated on macromolecular fiber chain. The change polymerization degree showed that rupture of cellulose molecules chain, degradation, dismantling, dissolve loss weight loss was caused during TEMPO selective oxidation process. It was possible to form a hydroxyl radical from the hydroxylamine and NaBrO followed byβelimination participated in depolymerization under alkaline conditions, the reaction was rapid and minimised the DP, then led to the obvious decrease of the fiber molecular weight. DSC and TG research results showed that the endothermic decomposition temperature and thermogravimetric temperature offset to low temperature direction after TEMPO selective oxidation process cotton fiber. The thermal stability of cotton fiber was gradually reducing with the depth reaction conditions, but the range was not severe and maximum thermal conversion rate also declined reached the temperature at 350℃almost all around. This two points are similar to X-ray diffraction results, the oxidiation reaction was not influence crystallinity of fibers greatly.It was revealed that the oxidation reaction of the cotton fibers by TEMPO/NaBr/NaClO system can be approximately described as two pseudo-first-order reaction kinetics which based on the cellulose structures, namely the kinetic process of the primary wall area and the secondary wall area. In the concentration range used in this study, the rate constant k was directly proportional to the concentration of TEMPO. As to NaBr, the rate constant was proportional to the concentration in a relatively lower range while it tended to level off at higher concentration, but the oxidation reaction rate increased with concentration when the concentration was above 1.0mmol/g. The pH value had a great impact on oxidation rate; the optimum pH was controlled from 10 to 11. The effect of temperature on the rate constant could be well described by the Arrhenius equation, the apparent activation energy and pre-exponential factor measured was about 56.66kJ/mol and 1.84×107min-1.With the increase of fiber(5%-25%), dry, wet paper tensile strength improved gradually; then coat modified fiber materials on the microfiltration membrane surface, the retention ration of microfiltration membrane will reach up to 25.10% with the increasing of addition amount of fiber(0.1g～0.5g), but pure water flow declines significantly. When the modified fiber was added to high beating degree fiber to manufacture filter parer, the retention ration of filter paper increase from 25.36% to 33.28%, not only the pure water flux decreased significantly, but aslo has more lower water flow than microfiltration membrane which was coated by Microcrystalline fiber with the increase of fiber(0.1g-0.5g).更多还原