【作者】 董兴广；

【导师】 王成国；

【作者基本信息】 山东大学 ， 材料学， 2009， 博士

【摘要】 在PAN基碳纤维的生产制备中,作为PAN原丝纺丝过程的第一步,纤维的凝固成形是决定碳纤维性能的关键。针对自制PAN原丝的纺丝成形过程展开基础性科学研究,深入了解PAN纤维在凝固成形过程中的物理化学变化,明确结构的演变规律,探讨成形工艺、结构及性能的相互关系,为高性能碳纤维的制备提供理论指导。本文针对PAN原丝的凝固成形过程,在PAN原丝的纺丝实验线上开展一系列的试验,利用RS75流变仪、差示扫描量热仪（DSC）、热重分析仪（TG）、傅立叶变换红外光谱仪（FTIR）、元素分析仪（EA）、X射线衍射仪（XRD）、电子探针（EMPA）、扫描电镜（SEM）、高分辨透射电镜（HRTEM）及动态力学分析仪（DMA）等测试技术,在确定PAN纤维凝固成形工艺并制备出高性能PAN原丝的基础上,对PAN纤维的凝固成形机理、凝固条件以及热性能与断裂行为等进行全面深入的研究,并探讨了纺丝工艺对PAN纤维性能的影响。在对PAN纺丝溶液流变学性质的分析和研究中发现,共聚物的分子量和溶液浓度大小对溶液体系的粘性、弹性及稳定性等有明显影响。分子量越高,浓度越大,PAN溶液粘度越大,结构化程度越高,非牛顿性越强,物理稳定性越差,非牛顿指数越低,弹性模量越高,拟形成的弹性网络结构更为致密,但均未形成稳定的弹性网络结构。在实际凝固成形阶段纺丝溶液流动的温度范围（60-80℃）内,温度的变化对中低分子量的溶液粘度影响较大,而高分子量（100万）的溶液粘度对温度的变化不敏感。本文利用Fick定律,采用Bessel函数等求解了湿法纺丝过程中PAN/DMSO溶液体系在凝固浴（DMSO/H₂O体系）中的扩散方程,计算了扩散系数,并讨论了凝固条件对扩散过程的影响。研究发现,在湿法纺丝中,溶剂和凝固剂的扩散系数均随凝固浴温度的升高而增大,随凝固浴浓度的升高而降低,随凝固浴牵伸比的增大而增大,随原液中聚合物浓度的提高而下降。扩散缓和度随温度升高而下降,随凝固浴浓度的升高变化较小,随凝固浴牵伸比的增大而增大,随原液中聚合物浓度的提高而增大。利用XRD、SEM、纤维细度仪以及纤维强伸力仪等,研究了PAN初生纤维成形过程中的组织结构变化以及凝固浴条件对初生纤维的影响,获得了湿法纺丝过程中的最优凝固浴条件。随着凝固浴浓度的升高,初生纤维的横截面形状逐渐趋于规则的圆形,在浓度介于60-70wt%时,横截面形状变化不大;随着凝固浴浓度的增加,初生纤维及相应原丝的结晶度先升高后降低,在浓度为65wt%时,结晶度最大,残留溶剂含量较低。随着凝固浴温度的升高,初生纤维及相应原丝的结晶度增大,初生纤维的残留溶剂含量不断降低,在温度高于55℃后,残留溶剂含量的变化较小;凝固浴温度从50℃升高到60℃的过程中,初生纤维的横截面由腰子形逐渐变为规则的圆形,温度继续升高到70℃,圆形横截面形状崩溃,且有粘丝和并丝现象发生。随着凝固浴负牵伸的增加,初生纤维及相应原丝的结晶度不断降低,初生纤维内部的残留溶剂含量不断下降,在负牵伸增加到-20%以后,残留溶剂的含量变化较小,初生纤维的横截面形状逐渐趋于规则、均匀,初生纤维表现出更好的韧性;但是过高的负牵伸不利于高性能PAN原丝的制备,尤其是趋于0%时的牵伸会造成初生纤维的断丝现象。随着凝固浴长度的增加,在从15cm升高到90cm的过程中,初生纤维的结晶度不断增大,残留溶剂含量不断降低,在凝固浴长度升高到75cm以后,结晶度和残留溶剂含量的变化不再明显,初生纤维的横截面形状趋于圆形,直径不断降低且离散系数变小。在湿法纺丝中,采用溶液聚合,粘均分子量为16万,原液固含量为21wt%,凝固浴表观负牵伸为-10%,凝固浴温度为60℃,凝固浴浓度为65wt%时,可得到均匀性高、截面为圆形、性能优良的初生纤维,得到的原丝纤度在1.04dtex左右,强度高达7.50cN/dtex,最终碳纤维强度可达3.86GPa。在干喷湿纺过程中,空气层的存在改善了原液细流在凝固浴中双扩散的情况,抑制了凝固剂水和溶剂DMSO的快速扩散,有利于初生纤维均匀结构的形成,实验得到了合理的干喷湿纺工艺。随着空气层厚度的增加,凝固浴牵伸的增大,凝固浴长度的增加,初生纤维的结晶度不断增大;随着凝固浴浓度的升高,当凝固浴浓度到达80wt%时,初生纤维表现出疏松且均匀的横截面,截面形状呈现规则的圆形:随着凝固浴牵伸的增大初生纤维的直径不断减小,横截面逐步趋向致密;随着凝固浴牵伸的不断增加,初生纤维的断裂强度不断增大、断裂延伸率逐渐降低,表现出更高的密度、更低的孔隙率。在干喷湿纺中,采用水相沉淀聚合,粘均分子量为23万,原液固含量为18wt%,空气层厚度为3mm,凝固浴牵伸为+130%,凝固浴温度为20℃,凝固浴浓度为80wt%时,可得到组织均匀、横截面形状为圆形、性能优良的初生纤维,得到的原丝纤度在1.01dtex左右,强度高达7.52cN/dtex以上。利用DSC、TG、FTIR、EPMA等表征了初生纤维不同凝固条件下的热性能、化学结构变化和断裂行为。结果表明:随着凝固浴牵伸的增加以及干喷湿纺中空气层厚度的升高,初生纤维的放热量增大,热稳定性提高;随着凝固浴长度的增加,初生纤维表现出更高的放热量,但当长度增到一定范围（＞75cm）后,初生纤维的结构趋于稳定,放热量变化微小。在湿法纺丝和干喷湿纺下,初生纤维在牵伸过程中均表现出明显的“颈缩”现象,但是湿纺纤维的组织结构较为致密并表现出难以消除的皮芯结构,干喷湿纺纤维表现出较为疏松但均匀的组织结构。在较慢的牵伸速率下,初生纤维的断口表现出明显的韧性断裂特征,在较快的牵伸速率下,初生纤维的断裂特征不明显。从化学结构上看,在波数950-1050cm^-1附近,DMSO中S=O的特征峰与PAN的指纹区的叠加出现明显的双峰,其中,950cm^-1附近的峰变化最大,是初生纤维后续处理工艺DMSO含量的变化依据。利用DMA、SEM以及XRD等分析了纺丝工艺对纤维结构和性能的影响,优化了工艺参数。研究发现:多级凝固浴牵伸的存在有利于纤维结构的致密和力学性能的提高;沸水牵伸和蒸汽牵伸倍数的提高,有利于PAN纤维内部结构获得稳定而巩固的取向,提高其机械性能;致密化温度的升高或时间的延长,有利于结晶度的增大。随着纺丝工艺的不断进行,PAN纤维的结晶度不断增大,残留溶剂的含量不断降低。通过对比正常水洗和直接水洗的初生纤维发现,前者残留溶剂的含量较低,在红外图谱中950cm^-1附近的峰不明显;后者残留溶剂含量较多且结构疏松,不利于预氧化工艺的进行。更多还原

【Abstract】 As the first step of polyacrylonitrile spinning process, the coagulation formation process is the key factor which determine the quality of polyacrylonitrile （PAN）-based carbon fibers during the preparation of carbon fibers. In order to obtain high quality carbon fibers, it is important to carry out deeply studies on the coagulation formation process of PAN fibers. The reaserches on the physical and chemical changes, structure evolution, as well as the relationship between formation processing, structure and properties during coagulation can provide academic help for the preparation of the high-quality carbon fibers. In this work, a series of coagulation and spinning experiments were conducted on a spinning line by using home coploymers and spinning solutions as the raw material fibers. Several technologies, such as the rheometer, differential scanning calorimetry （DSC）, thermal gravimetry （TG）, Fourier transform infared spectroscopy （FTIR）, elemental analyzer （EA）, X-ray diffraction （XRD）, scanning electron microscopy （SEM）, electron probe micro-analyser （EPMA）, high resolution transmission electron microscopy （HRTEM） and dynamic mechnics analyser （DMA） were used to systemically charaterize the structure and feature of the samples. At the same time, the influences of processsing conditions on the changes of the properties of nascent fibers and PAN fibers were also discussed.The investigation on the PAN spinning solution rheology indicates that the viscosity, elasticity and stability of spinning solutions depended on the molecular weight of copolymers and the spinning solution concentration at a certain extent. The molecular weight was higher, the solution concentration was bigger, the viscosity was bigger, the structure viscous index was higher, the Non-Newtonian index was smaller, the physical stability was worse, the elastic modulus was higher, the elastic network formed was more compact. During the actual spinning solution flowing temperature range from 60℃to 80℃, the effect of change of temperature on the viscosity of PAN solutions with low molecualr weight （23×10⁴ and 50×10⁴ ） was biggish, and it was not sensitive to temperature for spinning solutions with high molecular weight （100×10⁴）.The coagulation formation process of PAN nascent fibers is effected by the double diffusion of DMSO and H₂O. The diffusion equation of PAN/DMSO system was solved by use of Bessel funtion, and the diffusion coefficients under different coagultion conditions were received. The experimental results showed that the diffusion coefficients of solvent DMSO and precipitator H₂O hoisted with the rise of coagulation bath temperature, decreased with the rise of coagulation bath concentration, increased with the rise of coagulation drawing ratios, decrased with the rise of spinning solution concentrations. Diffusion relaxative degree descended with the rise of coagulation bath temperature, changed a little with the rise of coagulation bath concentration, increased with the rise of coagulation drawing ratios, and increased with the rise of spinning solution concentrations.Structure changes of PAN nascent fibers were further clarified and the excellent coagulation bath conditions for wet spinning were acquired in the forming process by use of XRD, SEM, fiber fineness and fiber strength and elongation analysers. With the increase of coagulation bath concentration, cross-sectional shapes of nascent fibers became round, and the cross-sectional shapes had little change when the concentration was 60-70wt%. With the increase of coagulation bath concentration, crystallinity of nascent fibers and corresponding PAN precursors firstly increased and then reduced, the crystallinity got the maximum when the concentration was 65wt%, and the residual solvent content got the minimusm. With the increase of the coagulation bath temperature, crystallinity of nascent fibers and corresponding PAN precursors increased, and the residual solvent content in nascent fibers became lower, and the residual solvent content hardly had any changes when the temperature was higher than 55℃. During the process of coagulation bath temperature varying from 50℃to 60℃, the cross-sectional shapes of nascent fibers gradually became from the kidney shapes to circular shapes. When the temperature was 70℃, circular cross-sectional shapes of the fibers collapsed. As the rise of minus stretch ratios, crystallinity of the nascent fibers and corresponding PAN precursors and the residual solvent contents in nascent fibers continuously reduced. When the minus stretch ratio reached -20%, the residual solvent contents had smaller changes, cross-sectional shapes of nascent fibers became regular and homogeneous, and nascent fibers showed better toughness. But higher drawing was not favorable for producing high-performance PAN precursor, especially the minus stretch ratio reached about 0%. With the increase of coagulation bath length from 15cm to 90cm, crystallinity of the nascent fiber increased, and residual solvent content reduced. When the coagulation bath length was over 75cm, changes of crystallinity and residual solvent content was no longer obvious, and cross-sectional shapes of the nascent fibers tended to become circular, whose diameters became lower and the dispersion coefficient became smaller. In wet spinning, when solution polymerization was used, viscosity-average molecular weight was 16×10⁴, spinning solution concentration was 21wt%, and the drawing ratios of coagulation bath with concentration of 65wt% in 60℃was -10%, the excellent performance PAN fibers with circular cross-section were obtained, whose fineness was about 1.04dtex with strength up to 7.50cN/dtex, and the indensity of corresponding carbon fiber was about 3.86Gpa.The existence of air gap improved double diffusion of spinning solution droplets at the coagulation bath during dry-jet wet spinning, which inhibited the rapid diffusion of water, and it was in favor of nascent fiber formation with uniform structure. With the increase of air gap distance, coagulation bath drawing ratios and the length of coagulation bath, crystallinity of nascent fibers increased continuously. With the increase of coagulation bath concentration, especially when it reached 80wt%, the nascent fibers showed loose and symmetrical cross-section, and the cross-section shape presented regular circle. With the rise of coagulation bath drawing ratio from +5% to +105%, the diameter of nascent fiber continuous decreased and the cross section progressively tended to compact. With increase of coagulation bath drawing ratios, the fracture intensity of nascent fibers continuously increased and the fracture elongation decreased, showing a better toughness, higher density, lower porosity, and the corresponding PAN precursor fiber showed the similar rules. In dry-jet wet spinning, when aqueous precipitation polymerization was used, viscosity-average molecular weight was 23×10⁴ , spinning solution concentration was 18wt%, air gap distance was 3mm, and drawing ratio of coagulation bath with concentration of 80wt% in 20℃was +130%, we could also get the excellent performance nascent fibers with circular cross-section, whose PAN fiber fineness was about 1.01 dtex with intensity up to 7.52cN/dtex.The thermal properties, chemical structure changes and fracture behavior of nascent fibers under different coagulation conditions were studied by use of the DSC, TG, FTIR, EPMA. The results showed that the nascent fiber heat release increased and thermal stability improved with the increase of coagulation bath drawing ratios and the air gap distance during dry-jet wet spinning. And the nascent fibers showed higher heat release with the rise of coagulation bath length, but when the length increased to a certain value （>75cm）, the nascent fibers’ structure stabilized and the heat release changed a little. In wet spinning and dry-jet wet spinning, the nascent fibers showed obvious necking phenomenon, but the wet-spinning fiber’s structure was more dense and showed skin-core structure which is difficult to eliminate, and dry-jet wet spinning fibers showed a more loose but uniform microstructure. Nascent fibers showed obvious ductile fracture characteristics at a slower rate, and the fracture characteristics were not obvious at a faster rate. Judging from the chemical structure, the wave number near 950cm^-1 peak was S=O characteristic peak in DMSO, which was used to indicate the changes of DMSO in the follow-up process based on nascent fibers.The effect of spinning process to the structure and properties of the PAN fiber was analyzed, and the parameters were optimized by the use of DMA, SEM and XRD etc. The study found that the improving of drawing was conducive to get the stable orientation of the PAN fibers’ internal structure, and improved its mechanical properties. The improving of densification temperature was helpful to the rise of crystallinity. With the spinning process to be carried out, the crystallinity of PAN fiber increased, residual solvent content became lower and lower. Comparing the normal water bath fiber with the nascent fiber washed directly, the former had the lower residual solvent content and is not obvious in FTIR, the latter had the higher residual solvent content and its structure was looser, which was not conducive to the following preoxidation process.更多还原