【作者】 季保华；

【导师】 王成国；

【作者基本信息】 山东大学 ， 材料学， 2007， 博士

【摘要】 聚丙烯腈(PAN)原丝质量不过关是制约我国炭纤维发展的瓶颈。由于PAN纺丝溶液在凝固浴中的凝固成纤过程是纤维制备的至关重要的环节，也是各种内外部缺陷形成的源头，这些缺陷会进一步“遗传”给后续的原丝乃至碳纤维从而极大的降低其性能，因此获得结构均匀致密、缺陷较少的初生纤维对制备优质原丝及炭纤维具有十分重要的意义。为此本文采用电子探针(EPMA)、扫描电镜(SEM)、透射电镜(TEM)、热分析(DSC)、X射线衍射(XRD)等分析表征手段，研究了PAN纺丝溶液挤出胀大效应的形成机理及其对初生纤维结构与性能的影响；运用扩散系数、凝固速率和传质速率差这三个参数深入研究了初生纤维的内在成形机理，讨论了它们的表征测试方法，以及随凝固条件的变化规律；研究了凝固条件对初生纤维结构缺陷和性能的影响，提出结构缺陷的形成机理与控制措施；研究了湿法纺丝干燥致密化过程加水浸润对丝条结构与性能的影响。由于PAN纺丝溶液为粘弹性流体，从喷丝口挤出时会产生挤出胀大效应，它对纺丝细流的连续稳定乃至初生纤维结构和性能都会产生一定影响，为此对挤出胀大效应的形成机理以及变化规律进行了研究，发现随着挤出胀大比的减小，初生纤维的孔隙率降低，凝固取向度增加，皮芯结构差异变小，初生纤维及原丝的断裂强度升高。为此可采用增大喷丝孔长径比或原液温度、降低原液固含量、剪切速率或凝固负牵伸率(本论文均指绝对值，以下同)的方法来减弱纺丝原液的挤出胀大效应，以获得性能优异的初生纤维。凝固过程中溶剂与凝固剂双扩散的核心问题是扩散系数计算。本文采用一维线性扩散模型，以Fick第二定律为基础，利用返滴定法测定不同凝固时间初生丝条中溶剂与凝固剂的含量进而计算各自的扩散系数，并讨论了各种凝固条件对溶剂和凝固剂扩散系数的影响，初步揭示了纤维凝固成纤过程的动力学变化规律，为选取适当的凝固条件以调控初生纤维结构进而控制其性能提供了理论依据，得出如下结论：随着原液固含量的降低、凝固负牵伸率的减小、或者凝固浴温度的升高，溶剂和凝固剂的扩散系数升高；随着凝固浴浓度的增加，溶剂和凝固剂的扩散系数逐渐下降并且在55wt％时出现最低值，之后又逐渐增大。运用边界移动模型讨论了凝固条件对凝固速率的影响规律，发现凝固剂自身特性决定凝固速率的大小，体积较小的凝固剂具有较大的凝固速率；凝固剂与溶剂之间的分子间亲和力决定凝固激活能的大小，而凝固剂分子体积不是主要因素；凝固速率的大小不仅取决于凝固剂的扩散系数，也取决于原液的固含量等。通过对聚合物原液圆柱状试样在凝固过程中重量变化规律的研究，引入溶剂与凝固剂之间传质速率差ΔK这一极其重要的参数。凝固条件对初生纤维结构与性能的影响通过影响ΔK进而影响表面凝固层的组成及厚度而实现。ΔK随凝固剂分子体积、凝固浴温度的增大而增大，随原液固含量和凝固浴浓度的增加而减小。系统研究了凝固条件对初生纤维结构与性能的影响，并阐述了其内外部缺陷的形成机理和控制措施，得出以下结论：随着凝固浴温度、浓度的增大，以及凝固负牵伸率的降低，初生纤维截面逐渐趋于规整，其断裂强度逐渐增大。随着原液固含量、凝固浴浓度的增大，或者凝固浴温度的降低，初生纤维皮芯结构减弱，孔隙率逐渐下降，内部结构变得致密。采用凝固浴温度60℃，凝固浴浓度65wt％，凝固负牵伸率—30％的最佳凝固工艺参数制得了截面规整且结构均匀致密的优质原丝，以及拉伸强度达3.76GPa的炭纤维。对初生纤维皮芯结构形成机理提出新的解释：初生纤维的皮层厚度与原液细流刚开始凝固时所形成的表面凝固层的硬度、组成和溶胀度等性质有关，而它们又受溶剂与凝固剂之间传质速率差的控制；纺丝原液挤出胀大效应所导致的原液细流外表层的“拉伸效应”也对皮芯结构的形成起着重要作用；皮层的取向度比芯层高。干燥致密化是湿法纺丝工艺过程的一个重要阶段，丝条的致密化程度主要由温度、时间、张力等因素所控制。在一定范围内提高致密化温度、延长致密化时间可以提高纤维的晶粒尺寸和结晶度，从而使纤维的密度和强度提高。在130℃下致密化64～96S可以得到综合性能较好的纤维。致密化过程对纤维进行润湿处理可实现纤维晶粒细化，同时在其X射线衍射图谱的20／10°附近出现了新的结晶衍射峰，表明形成新的晶面；使丝条内部空洞进一步融合，孔隙率持续下降；使最终原丝的密度和强度比未浸润时增大；使浸润后的原丝在预氧化过程中的张力下降，有利于预氧化反应均匀缓慢进行。更多还原

【Abstract】 It is well-known that the poor quality of polyacrylonitrile (PAN) precursors is the "bottleneck" limiting the development of the carbon fibers in our country. As the coagulation process of the PAN spinning solution in the coagulation bath is a very critical step in fiber preparation and also the source of various defects, which will evolve and be inherited to the resultant precursors and carbon fibers and greatly lower the properties of them, thus it is very important to obtain the protofibers with compact and homogeneous microstructure and less defects.In this paper, various testing instruments such as the electron probe microanalysis (EPMA), scanning electron microscopy (SEM), transmit electron microscopy (TEM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD) were applied. Firstly, the formation mechanism of the die-swell effect of the PAN spinning dope and its effect on the structure and properties of the protofibers was discussed; Secondly, the calculation methods of the diffusion coefficients, coagulation rate and mass transfer rate difference between solvent and coagulant, and effect of coagulation variables on them were studied; Thirdly, the effect of coagulation variables on the structure and properties of the protofibers and the formation mechanism and control methods of the interior and exterior defects was dealed with; At last, the effect of wetting during collapse process on the structure and properties of the precursors was discussed.Being a viscoelastic fluid, the spinning dope will be accompanied with the die-swell effect as it is extruded from the orifice, which imposes great influence on the stability and continuity of the dope and the structure and properties of the protofibers. The die-swell effect of the spinning dope was discussed and it was found that with the decrease of the die-swell ratio the porosity of the proto fibers became lower, the spin orientation enhanced and the skin-core difference became small, and subsequently their tensile strength increased. Thus die-swell effect can be reduced by increasing the L/D of the capillary and the dope temperature, or lowering the jet stretch ratio, the shear rate or the solid content.The key aspect on the counter-diffusion during the coagulation process was the calculation of diffusion coefficients. Here the calculation formula of diffusion coefficients of solvent and coagulant was conducted by an one-dimensional linear diffusion model based on the Fick Second Law. The effect of coagulation variables on diffusion coefficients was discussed and the dynamic principle during the coagulation process was revealed preliminarily. It was found that the diffusion coefficients of solvent and coagulant both increased with the fall of the solid content or the jet stretch ratio, or the hoist of the bath temperature. It is interesting that the diffusion coefficients of solvent and coagulant first declined with the hoist of the bath concentration and achieved a minimum value as it was 55wt%, then the curve showed a trend upwards.The coagulation rate that reflects the progress of the coagulation process was introduced according to the moving-boundary model and effect of coagulation variables on it was discussed. It was found that the nature of coagulants determines the coagulation rate and the diffusion species possessing smaller molecular volume has a higher coagulation rate. It is the intermolecular affinity between coagulant and solvent that determines the coagulation activation energy, and the molecular volume of coagulants is not a dominant factor. Not only is the coagulation rate determined by the diffusion coefficient of coagulants, but also the composition of the dope such as the solid content.By observing the change of the weight loss of the cylinder samples during the coagulation process, the mass transfer rate difference between solvent and coagulant was introduced. The effect of coagulation variables on the structure and properties of the protofibers was achieved by influencing the mass transfer rate difference and further the composition and thickness of the coagulated surface layer. The mass transfer rate difference increased with the molecular volume of the coagulant or the bath temperature, but decreased with the solid content of the dope or the bath concentration.The effect of the coagulation condition on the structure and properties of the protofibers and the formation mechanism and corresponding control methods of the interior and exterior defects were systematically studied. It was found that with the increase of the coagulation bath temperature or the bath concentration, or the decrease of the jet stretch ratio, the cross-sectional shapes of the protofibers became circular and their mechanical properties were improved. With the increase of the solid content or the bath concentration, or with the decrease of the bath temperature, the skin-core difference was weakened and the porosity was lowered and the microstructure of the protofibers became compact and uniform. By adopting the optimal coagulation variables, the coagulation bath temperature 60℃, solvent content 65wt% and the jet stretch ratio—30%, the protofibers possessing circular cross-section and compact structure were gained and the carbon fibers with tensile strength of 3.76GPa were obtained.The new interpretation about the formation mechanism of the skin-core structure was introduced as follows. The thickness of the skin layer of protofibers was decided by the hardness, composition and swollen degree of the coagulated surface layer formed at the very beginning of the coagulation process, and which were controlled by the mass transfer rate difference between solvent and coagulant; The "stretching effect" of the surface layer of the dope stream caused by the die-swell effect has also great influence on the formation of the skin-core structure; The skin layer has a higher orientation degree than that of the core.Collapse process is a very critical step during the wet spinning. The denseness degree of the collapsed fibers is mainly controlled by the collapse temperature, time and tension. To increase the collapse temperature and time moderately can enhance the crystal size and crystallinity, improve the density and tensile strength of precursors. Drying the fibers 64～96S under 130℃can get precursors with excellent mechanical properties.Wetting the fibers during the collapse process has a lot of merits: It reduced the crystal size of the precursors, and a new crystal diffraction peak appeared around 2 θ/10° and new crystal surface was formed because of the soakage action of water molecules; It made the porosity decrease continually and thus improved the density and tensile strength of the resultant precursors; It lowered the tension of the precursors during stabilization process, which is prone to an even and slow stabilization reaction.更多还原