The Effect Of Pretreatment On TEMPO Oxidation System And Kinetic Study

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).