Study On The Relationship Between Spinning Process And Performance Of Pan Precursor Fibers

The high quality of polyacrylonitrile (PAN) precursor fibers is a significant prerequisite for obtaining carbon fibers with excellent performance. It is important to study the spinning technics of PAN precursor fibers and relationship between structures and properties of PAN fibers, which is advantageous to improve quality of carbon fibers. In this paper, the effects of every technics on the structures and properties of fiber were systematically studied by means of electron probe microscopy analysis (EPMA), transmission electron microscopy (TEM), scanning electron microscopy (SEM), wide angle X-ray diffraction (WAXD) and element anasysis (EA). The effects of spinning process on the evolvement of structures and defects were also included.The coagulation process play a great important on the structure and properties of fiber. Uniform structure of PAN precursor fibers is favorable to make high quality carbon fiber, and moreover uniform structure (including the shape of cross-section and core-skin structure) of nascent fiber is an important factor to affect uniformity of precursor fibers' structure. The effect of coagulation concentration, coagulation temperature and jet stretch ratio on the shape of cross-section were studied. It was found that increasing coagulation concentration or reducing coagulation temperature could reduce the diffuse speed, and it was adapt to form round cross-section. Increasing jet stretch ratio within limits could form round cross-section, reduce the titer of fiber, and increase the orientation degree and strength of fiber. If the jet stretch ratio was too big, properties of fiber will reduce.Molecular structure of nascent fiber is short of directional arrangement, and strength of fiber is low. In order to make high performance precursor fiber, fiber must be drawn to increase orientation degree. In order to increase the total stretch ratio, stretch is separated into four segments, so it is necessary to optimize the distribution of stretch ratio. The stretch ratio of coagulation bath 2 was in the range of 1.1～1.5, and the pre-stretch ratio was in the range of 1.3～1.6, besides the stretch temperature must be above 80°C. Boiling stretch ratio could reach to 1.7, and the stream stretch ratio was in the range of 2.0～3.0. The strain-stress curve of every fiber were tested, and it was found that fibers of coagulation bath 1, coagulation bath 2, coagulation bath 3 existed obvious elastic deformation, yield point and neck-shrinkage. Begin with pre-stretch, the yield point and neck-shirnkage became unconspicuous and till vanish. During the whole stretch process, strength of fiber increased, elongation reduced from 150% to 10%, and crystallinity and crystal size of fiber increased.After being drawn, pores in nascent fibers with net structure were elongated along tropism direction. When collapsed, water in fibers was removed, pores in fibers shrinked and fibers became compact. When collapse time increased, crystallnity, crystal size and strength of fibers increased, but elongation and stress reduced. If collapse time was too short, density and strength of fibers would be low. If collapse time was too long, chemical reaction would occur, and defect probability would increase. When collapse temperature increased, mechanical properties of fibers improved, inner stress reduced. If collapse temperature was too low, fiber's properties would not be improved evidently. If collapse temperature was too high, fibers would be oxidated and scalded. Based on experiment, when collapse time was in the range of 64～80s and temperature was 140°C, high performance fiber could be obtained. If fibers were wetted during collapse, crystallinity and crystal size of fibers reduced, and strength of precursors fibers increased.In this paper, boiling shrinkage of fibers was studied, and it was found that inner stress of fibers was related to stretch ratio and temperature. Crystal property of fibers during spinning process was studied. It was found that during spinning process crystallinity and crystal size of fiber increased, and collapse and steam stretch played an important part on the increasing. There is netlike structure and samdwich, and the crunode of netlike structure was crystal, which can prevent propagate of crack and increase fiber's strength. Samdwich was made up of crystal areas, the direction of crystal areas was vertical to axes of fibers. During the process of spinning, there are many grooves on the surface of fibers. These grooves may be caused by many reasons such as irregular shape of spinneret, low machining precision, roughness of rollers. Most of the grooves are reflection of high polymer's supermolecular structure, and these grooves became deeper and thinner during stretching. Moreover, there were some concavo-convex spots on the surface of fibers, and a little impurity and dust was adhered to the surface of fibers. Therefore, clean condition is necessary during spinning process.