Study On Formation Mechanism Of Polyacrylonitrile As-Spun Fibers

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.