The influence of continuous stabilization on the properties of stabilized fibers and the final activated carbon fibers. Part I

This study found that, during continuous stabilization and carbonization, the shrinkage behavior of polyacrylonitrile (PAN) fibers affects the morphology and properties of the stabilized fibers, the carbon fibers, and the final activated carbon fibers. In the stabilized fibers, a higher shrinkage of the PAN fibers during the stabilization process increased the oxygen content and the core proportion and decreased the formation of ladder polymers. The effect of the shrinkage behavior of the PAN fibers on the fracture surfaces of the stabilized fibers is discussed. A microstructure model of stabilized fibers is presented, depicting fine radial structure at the fiber center. When stabilized fibers were carbonized during a continuous carbonization process, a hole structure was found in the fiber center at the temperature of 800°C, and a hollow core was found at the temperature of 1300°C. The shrinkage behaviors during the stabilization stage and the formation of the hole and the hollow core in the fiber's center during the carbonization stage are discussed. The carbon fibers developed from shrunk stabilized fibers have a lower density and lower preferred orientation than fibers developed from unshrunk stabilized fibers. But the fibers developed in this new process have greater nitrogen and oxygen content, and have a greater porosity than the traditionally-produced fibers. The mechanical properties df the new and the traditional fibers are comparable. These characteristics are very valuable in the production of activated carbon fibers, which will be described in our next paper.     

Influence of continuous stabilization on the physical properties and microstructure of PAN-based carbon fibers

A continuous stabilization and carbonization process was used to prepare polyacrylonitrile (PAN)-based carbon fibers. The stepwise stabilization of PAN fibers was tried at various temperatures. The effect of stepwise stabilization on the physical properties and microstructure of the final carbon fibers is reported in this article. The fixed temperature in stepwise stabilization is kept below the fusion temperature of PAN precursors to avoid overstabilization of the fibers. The optimum stepwise stabilization process not only increases the amount of ladder polymer in stabilized fiber but also improves the physical and mechanical properties of the resultant carbon fibers. The formation of closed pores from open pores in carbon fiber occurs at 1100°C, but the formation of closed pores occurs at 200°C lower for carbon fiber developed from overstabilized fiber. The effect of continuous stepwise stabilization on the properties of resulting stabilized fibers and the variation in physical properties, element composition, and microstructure of carbon fibers during the carbonization process are also reported in this article.     

The influence of pyrolysis on physical properties and microstructure of modified PAN fibers during carbonization

Modification of polyacrylonitrile (PAN) fibers with potassium permanganate has reduced the time required for stabilization and also improved the mechanical properties of the resulting carbon fibers. In this study, the effect of modification on the physical properties, microstructure, and elemental composition of fibers during the carbonization process was examined for the first time. The resulting carbon fibers developed from modified PAN fibers had a higher density, a greater stacking size (Lc), and a higher preferred orientation than those developed from unmodified PAN fibers. The carbon fibers developed from the modified PAN fibers also showed an improvement in tensile strength from 20 to 40%. These fibers showed a radial structure in the fracture surface and were somewhat different structurally in the cross section than were the carbon fibers developed from the original PAN fibers. A model for the structure of both carbon fibers is presented. The relationship between the formation of closed pores from open pores and the variety of cumulative pore area during the heat-treatment stage is also discussed.     

Comparative investigation on the thermal degradation and stabilization of carbon fiber precursors

Thermal degradation and stabilization of two kinds of polyacrylonitrile (PAN) fibers have been investigated by a combination of FT-IR, differential scanning calorimetry (DSC), modulated DSC, thermogravimetry (TG), thermal shrinkage behavior, in situ mass spectrometry (MS), and tensile property examinations. The two types of precursor fibers exhibit distinct properties after oxidative stabilization, but they can both make carbon fibers with equivalent mechanical properties. Compared with PAN/itaconic acid precursor fibers, the fibers containing acrylamide comonomers show a doublet appearance, broader exothermic peak, lower threshold degradation temperature, and more amount of heat evolved in DSC thermogram, which is favorable to obtain uniform microstructures in oxidative stabilization process. The two types of samples produce different ring structures in the thermal degradation and stabilization process, as evidenced by results from tensile test, TG–MS and thermal shrinkage behavior analyses. In addition, the molecular rearrangement or melting of ordered structures accompanying with nitrile polymerization was also detected from modulated DSC.     

The effect of modification on structure and dynamic mechanical behavior during the processing of acrylic fiber to stabilized fiber

Polyacrylonitrile (PAN) fibers pretreated with potassium permanganate have reduced the time required for stabilization, and also improved mechanical properties of the resultant carbon fibers. In this study, the effect of modification on the stabilization process and the dynamic mechanical properties of PAN fibers have been examined. The beta peak appeared at about 125°C on the loss tangent curves caused by molecular motion in the PAN fiber. Appearing at about 254°C, the alpha peak is attributed to chemical reactions and molecular motion in the formation of the crystalline phase of stabilized fibers. The alpha peak of the modified PAN fiber had lower absorption and had a smaller peak in the temperature range of 212–239°C. This indicated that potassium permanganate acts as a catalyst to lower the reaction temperature by about 20°C of the initial cyclization reaction. The dynamic storage modulus analysis indicated that modified PAN fibers have a lower initial transition temperature and that formation of the ladder polymer is gradual and steady.     

Effect of heating and stretching polyacrylonitrile precursor fibers in steam on the properties of stabilized fibers and carbon fibers

Polyacrylonitrile (PAN) fibers were heated and stretched in a steam bath mixed with nitrogen in the early stage of stabilization. During this process, the flexible linear chain of PAN molecule was converted to a rigid ladder structure by intramolecular cyclization. The shrinkage tension along the fiber axis decreased during this heat treatment, because water molecules here acted as a plasticizer for stretching. Consequently, the preferred orientation of ladder molecules in the stabilized fibers was enhanced. As a result, the preferred orientation, crystallite dimensions, and mechanical properties of the resultant carbon fibers (CFs) and graphite fibers (GFs) were improved, comparing with the corresponding CFs and GFs without pretreatment in the steam bath. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers.     

Gel-spun carbon nanotubes/polyacrylonitrile composite fibers. Part III: Effect of stabilization conditions on carbon fiber properties

The oxidative stabilization process of gel-spun carbon nanotube (CNT)/polyacrylonitrile (PAN) composite fibers have been studied and optimized. Optimum stabilization time depends on both the applied tension and temperature. Various characterization methods including thermal shrinkage, dynamic mechanical analysis, infrared spectroscopy, and wide angle X-ray diffraction are used to monitor the chemical and structural evolution during stabilization and carbonization. The relationship between the stabilization conditions of CNT/PAN composite fiber and the tensile properties of the resulting carbon fibers were investigated. By optimizing stabilization conditions, CNT/PAN based carbon fibers with a tensile strength of 4 GPa and a tensile modulus of 286 GPa were obtained using batch carbonization processing at 1100 °C.     

Influence of γ‐ray irradiation on structure and properties of PAN precursor fibers

To improve the oxidative efficiency of polyacrylonitrile (PAN) precursors, modification of PAN precursors was employed by gamma ray irradiation. Effects of irradiation on the structure and properties of PAN precursors were investigated. The results showed that cyclized structure in the precursors was formed after γ‐ray irradiation. Meanwhile, irradiation could reduce the heat evolved, relax the exothermic behavior, decrease the mechanical properties and enhance the density of PAN fibers. It is supposed that the modification of γ‐ray irradiation could remarkably accelerate the stabilization process of PAN precursors and reduce the production cost of resulting carbon fibers. POLYM. ENG. SCI., 56:1313–1318, 2016. © 2016 Society of Plastics Engineers     

Polyacrylonitrile/acrylamide‐based carbon fibers prepared using a solvent‐free coagulation process: Fiber properties and its structure evolution during stabilization and carbonization

Polyacrylonitrile (PAN)/acrylamide (AM) fibers were fabricated via dry‐jet wet spinning process using a solvent‐free coagulation bath. The effects of AM loading as comonomer on the mechanical and thermal properties of PAN‐based carbon fiber have been studied. The thermal stability and mechanical stability of the fibers were characterized using differential scanning calorimetry (DSC) and tensile testing. Fibers fabricated from PAN with 5 wt% AM had the highest Young Modulus at 5.54 GPa. It also showed better exothermic trend process with broader exothermic peak and lower initiation stabilization temperature compared with homopolymer PAN. The elemental composition and chemical structure evolution of the fibers during the heat treatment processes were evaluated by elemental analyzer and Fourier Transform Infrared Spectroscopy. Crystal structure evolution of the fibers during the heat treatment process was elucidated by X‐ray diffraction (XRD) analysis. The elemental analyzer, XRD and FTIR results revealed that pyrolysis process used had successfully transformed PAN/AM fibers produced from solvent free coagulation bath into carbon fibers that were comparable with the conventional coagulation bath. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

Gel-spun carbon nanotubes/polyacrylonitrile composite fibers. Part I: Effect of carbon nanotubes on stabilization

Addition of carbon nanotubes (CNTs) in polyacrylonitrile (PAN) fibers significantly improves the mechanical properties of the resulting carbon fibers. This study focuses on the effect of different types of CNTs on chemical, mechanical and structural changes during the stabilization of gel-spun CNT/PAN composite fibers. Among the different types of CNTs, it was observed that CNTs containing more walls had lower reinforcement efficiency than CNTs containing fewer walls. Similarly CNTs containing fewer walls exhibited higher orientation of the ladder polymer and greater effect on the formation of β-amino nitrile in the stabilized fibers. Wide angle X-ray diffraction, infrared spectroscopy, and scanning electron microscopy were used to determine the optimum stabilization time. Additionally, it was found that the higher tension applied during stabilization improved the properties of the stabilized fibers, and the addition of CNTs increased the maximum tension that the fiber can bear.     

Structure and properties development during the conversion of polyethylene precursors to carbon fibers

In this study, the differences of structure and properties development during stabilization and carbonization between highly oriented and partially oriented polyethylene precursors were investigated. The effect of stabilization temperatures, time, and tension on the conversion process was studied using thermal analysis such as differential scanning calorimetry and thermogravimetric analysis, scanning electron microscopy, and single filament mechanical properties. Comparison of the resultant carbon fibers from these two kinds of precursor fibers showed that using partially oriented polyethylene precursor may reduce the stabilization time and temperature, with a diffusion-controlled process of sulfonation for stabilization. The mechanical properties of the resultant carbon fibers were evaluated. © 1996 John Wiley & Sons, Inc.     

Aspects on prestretching of PAN precursor: Shrinkage and thermal behavior

Polyacrylonitrile (PAN) fibers of a special grade have been modified by a method of prestretching with various stretching ratios from negative to positive before the onset of stabilization. The effect of such pretreatments on the thermorhelogical and thermal behaviors of PAN fibers was followed by free shrinkage experiments and differential scanning calorimetry (DSC) analyses. It was found that prestretching had a significant influence on the physical shrinkage of PAN fibers. DSC results of PAN fibers showed dependence not only on atmospheric conditions but also on the extent of prestretching. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:1185–1190, 1998     

The influence of cobaltous chloride modification on physical properties and microstructure of modified PAN fiber during carbonization

Modification of polyacrylonitrile (PAN) fibers with cobaltous chloride has increased crystal size, crystallinity, and density, and also improved tensile strength and modulus of the resulting carbon fibers. In this study, the effect of cobaltous chloride modification on the physical properties, microstructure, and elemental composition of PAN fibers during the carbonization process was examined. The resultant carbon fibers developed from modified PAN fibers had a lower formation temperature of carbon basal planes than those fibers that developed from the original one. The modification process not only improved the tensile strength but also increased the tensile modulus by about 15% of the resulting carbon fibers at carbonization temperature of 1300°C. A higher stacking size (L_c), or a greater carbon basal plane in crystalline, is one of the reasons to improve the modulus and conductivity of the final carbon fibers. The modification process also increased the electrical conductivity by about 15% at 1300°C and by about 150% at 2500°C. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 2409–2415, 1998     

Optimization of stabilization and carbonization treatment of PAN fibres and structural characterization of the resulting carbon fibres

The properties of carbon fibres made from PAN are controlled by the heat treatment cycles during stabilization and carbonization. Optimum conditions can be derived by simple but time-consuming trials. The paper presents an indirect laboratory method, which is based on TMA and DSC measurements during the stabilization period. Results from this method contribute additionally to the explanation of the various simultaneously occurring chemical reactions, mainly dehydrogenation, intra- and intermolecular cyclization.Shrinkage measurements during stabilization under time linear heating show the start and end of the chemical stabilization reactions. The optimum stabilization conditions are derived from the shrinkage behavior as a function of the heating rate. As the stabilization is strongly affected by the heat of reaction, DSC measurements contribute to the accuracy but also to the explanation of the results from the TMA method.These stabilization conditions were applied for the preparation of carbon fibres which are characterized by mechanical and physical methods.     

Continuous and rapid stabilization of polyacrylonitrile fiber bundles assisted by atmospheric pressure plasma for fabricating large-tow carbon fibers

A bundle of carbon fibers (CFs) with 24,000 filaments was prepared using atmospheric pressure plasma and heat simultaneously for only 30 min, followed by the carbonization process. Conventional thermal stabilization process takes more than few hours to prevent the ignition of the polyacrylonitrile (PAN) fiber when large-tow PAN fibers are stabilized. The CFs produced using the plasma stabilization process had a tensile strength as high as 2.6 GPa. This strength level, which is slightly higher than that of CFs stabilized by the conventional process for 120 min, satisfies the demands in automobile applications. It is believed that the plasma-based stabilization process provides a potential solution not only for shortening the process time but also for providing continuous stabilization of large-tow carbon fibers.     

A comparison of coagulation and gelation on the structures and stabilization behaviors of polyacrylonitrile fibers

The fiber spinning methods determine the formation of the physical structures of polyacrylonitrile (PAN) fibers which further affect stabilization reactions and the mechanical performances of the resultant carbon fibers. In this study, PAN fibers were prepared by both dry-jet gel spinning (g-PAN) and dry-jet wet spinning (w-PAN), and their stabilization behaviors were compared. While the stabilized w-PAN fibers show sheath-core structures, the stabilized g-PAN fibers exhibit relatively uniform stabilized structures along the radial direction. Additionally, the stabilization reactions of g-PAN fibers occur faster than that of w-PAN fibers, and the cyclization, oxidation, and crosslinking reaction activation energies of g-PAN fibers are lower than that of w-PAN fibers, respectively. Moreover, the carbon yield of g-PAN is higher than that of w-PAN fibers. We believe that above changes are possibly ascribed to the formation of different PAN sheath structures and oriented chain structures during dry-jet wet spinning and dry-jet gel spinning. It is concluded that gel spinning could significantly reduce the sheath-core difference of PAN fibers and the stabilized fibers as compared with wet spinning, which leads to a faster stabilization and more uniform stabilized structures. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 137, 48671.     

Shrinkage of short PP and PAN fibers under hot-stage microscope

Marked shrinkage behavior when heated is typical of semicrystalline polymer fibers such as polypropylene (PP) and polyacrylonitrile (PAN). Shrinkage of PP and PAN fibers may give the possibility to control the spalling tendency of fiber concrete under the heat exposure of fire. Cut staple fibers are normally delivered for concrete reinforcement. Modern methods for continuous fibers cannot be used by the end-user for shrinkage determination of commerical staple fiber grades. The shrinkage of five different commercial staple fibers specially designed for concrete reinforcement was studied under a hot-stage microscope. Significant differences in cumulative shrinkages of the various PP and PAN fibers were detected, shrinkages being 3–15% with PP fibers and 6–7% with PAN fibers at a temperature of 150–170°C. At about 160–165°C, PP fibers melt, whereas PAN fibers continue shrinking. Hot-stage microscopy provides a simple and a relatively accurate method for estimating thermal shrinkage of staple PP and PAN fibers, the deviations from measured average values remaining typically at 10–15%. © 1995 John Wiley & Sons, Inc.     

Structural and thermal property changes of plasticized spinning polyacrylonitrile fibers under different spinning speeds

The effect of the spinning speed on structural and thermal properties of polyacrylonitrile (PAN) fibers prepared by plasticized spinning was investigated. The PAN fibers were characterized by scanning electron microscopy, X‐ray diffraction, differential scanning calorimetry, and thermogravimetric analysis. We found that the surface morphology of the fibers was relatively smooth. The presence of a small amount of surface defects was caused by the instability of spinning process. The final fibers may have had two tensile fracture modes, that is, cluster breaking and axial split fracture. The structure of the as‐spun fibers was destroyed when the spinning speed was up to 500 m/min; this led to chain scission in the amorphous region. The final fibers exhibited mechanical properties that were roughly comparable to those of commercial PAN fibers. The changing trend in the cyclization temperature of the final fibers was consistent with that of crystallinity, which first increased and then decreased. The decomposition temperature in the amorphous region increased with increasing spinning speed. The decomposition temperature in the crystalline region increased with increasing crystallinity. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45267.     

The effect of pre-carbonization on the properties of PAN-based carbon fibers

A continuous stabilization and two-stage carbonization process was used to prepare polyacrylonitrile (PAN)-based carbon fibers, The effect of pre-carbonization (300 to 550°C) on the final properties and microstructure of carbon fibers was measured. Experimental results using an X-ray diffractometer indicated the presence of a less ordered structure at 2Θ from 5 to 18° in the pre-carbonized fibers and the final carbon fibers. This study found that the pre-carbonization process strongly affects the microstructure of the resulting carbon fibers. The results also showed that a suitable pre-carbonization was very conducive to improvement in tensile strength or in Young's modulus of the final carbon fibers. When the final carbon fiber was pre-carbonized at 300 and 550°C, respectively, these fibers had a higher tensile strength and higher Young's modulus than carbon fibers pre-carbonized at other conditions.     

Combined Effect of Processing Parameters on Thermal Stabilization of PAN Fibers

Summary Thermal stabilization parameters are the key factors in the production of carbon fibers. The combined effect of stepwise temperature, dwell time and stretching on the properties and structures of PAN fibers was studied by elemental analysis, bulk density determination, tension measurement, FTIR and WAXD. It is indicated that the whole process of thermal stabilization can be roughly divided into four stages by three temperatures of 220 °C, 240 °C and 270 °C, of which the increment rate of oxygen content, as well as the bulk density of fibers, is different, while the dwell time has no obvious effect on them until about 240 °C. Stretching plays a great role on the tension in fibers at early stabilization stage, while temperature contributes more at later stage. The stretching is also helpful to suppress the growth of crystallite below 220 °C, which has not been reported in published documents.