Mechanism and kinetics of oxidation during the thermal stabilization of polyacrylonitrile fibers

The thermal behavior and structural evolution during the thermal stabilization of polyacrylonitrile (PAN) fibers in N₂ and air were investigated using differential scanning calorimetry and solid‐state ¹³C nuclear magnetic resonance. It was found that an oxidation reaction, that generated carbonyl (C?O) groups could occur at 160°C which has not been reported in the literature. It is proposed that the cyclized structures in the PAN macromolecule chains are a prerequisite for the oxidation. Further investigations indicate that with more cyclized structures in the PAN macromolecule chains, the oxidation proceeds more readily, which is consistent with the proposed mechanism. The kinetic parameters for the oxidation and cyclization reactions were estimated using the Kissinger method. The activation energies for the reactions of oxidation and cyclization for PAN fibers are about 96.4 kJ/mol and 190.0 kJ/mol, respectively, which implies that the cyclization is the rate determining step during the thermal stabilization of PAN fibers. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Chemically Enhanced Wet‐Spinning Process to Accelerate Thermal Stabilization of Polyacrylonitrile Fibers

In this study, a fast and inexpensive approach is introduced to assist stabilization of polyacrylonitrile (PAN) fibers by adding ammonium iron(II) sulfate in coagulation bath. Effects of chemical treatment on stabilization process and structural evolution of fibers are studied using calorimetric, infrared, and X‐ray techniques. A stepwise infrared study confirms the assisted cyclization reaction, and an X‐ray analysis reveals a significant improvement in crystallinity and orientation of polymer chains which lead to an increase in tensile strength and modulus of PAN fibers. Differential scanning calorimetry results show 13 °C reductions in peak temperature of the stabilization reaction which means a sign of chemical activation at lower temperature by adding sulfate ions. Quantification of IR spectra shows a 7% increase in extent of reaction of chemically treated fibers and higher degree of conjugation compared with untreated and post‐treated fibers. Finally, mechanical properties of chemically treated fibers are improved due to an increase in size and orientation of polymer chains after chemical treatment in the coagulation bath. Compared to control and post‐treated PAN fibers, thermochemical properties of presented fibers are improved due to chemically assisted stabilization, and as a consequence, energy consumption of the stabilization step will be reduced by a simple and facile treatment.     

Ferric Chloride Assisted Thermal Stabilization of Polyacrylonitrile Precursor Fibers Prior to Carbonization

The effect of ferric (Fe³⁺) ion incorporation on the thermal stabilization of polyacrylonitrile (PAN) fibers was investigated using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), X-ray diffraction analysis and infrared spectroscopy methods. The results obtained from the DSC and TGA measurements indicated that there was an improvement in thermal stability when Fe³⁺ ions were incorporated into the polymer structure. TGA thermograms showed a relative improvement in thermal stability as indicated by increasing carbon yield with progressing time. The degree of lateral order and apparent crystallite size estimated by X-ray diffraction decreased with increasing stabilization time. Incorporation of Fe³⁺ induced distinct changes in the infrared spectra of the thermally stabilized PAN fibers. The spectral changes appeared to be due to the formation of coordination bonds between the nitrogen atom of the nitrile group and Fe³⁺. The formation of ferric ion-nitrile coordination bonds accelerated thermal stabilization by catalyzing the occurrence of dehydrogenation, cyclization and oxidation reactions.     

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.     

Kinetics of the cyclization and isomerization reactions in polyacrylonitrile based carbon fiber precursors during thermal-oxidative stabilization

The kinetics of reactions in polyacrylonitrile (PAN) based carbon fiber (CF) production should be of significance to the guidance of process control, fiber structure formation. PAN precursor fibers were isothermally stabilized at 210, 225, 240, 255, and 270 °C, respectively, for 10 to 100 min in an air oven to study the kinetics of the cyclization and isomerization reactions. The structural evolution of PAN precursor fibers during thermal-oxidative stabilization was characterized by Fourier transform infrared (FTIR) spectroscopy and solid state ¹³C nuclear magnetic resonance (¹³C NMR). The results indicate that the FTIR absorbance of  CN (the resultant of the cyclization) in PAN shows a trend of first increasing and then decreasing. And then the NMR peak assigned to the carbon atoms linking imino groups ( NH ) proves the isomerization of  CN into  NH in pyridone structure. Based upon the FTIR absorbance method, the entire process of the cyclization and isomerization reactions is considered as a consecutive first-order reaction. A kinetic model for the consecutive reaction has been established via the evaluation of the reaction rate constants of two single reactions. According to the model, the simulated kinetic curves of the characteristic groups ( CN,  CN , and  NH ) conform to the FTIR absorbance trends of these groups based on experimental data. This study is expected to furnish in-depth information on the crucial reaction kinetics during stabilization of PAN precursors, which is of advantage to the process optimization of the CF production. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48819.     

聚丙烯腈纤维的稳定化研究进展

聚丙烯腈(PAN)纤维的稳定化是制备PAN碳纤维的关键步骤之一,综述了PAN纤维在不同介质中的稳定化过程研究以及国内外纤维改性后稳定化的研究,并简要介绍了时间、温度、张力等对稳定化过程的影响。     

Magnetic polyacrylonitrile-Fe@FeO nanocomposite fibers - Electrospinning, stabilization and carbonization

Uniform and bead-free pure polyacrylonitrile (PAN) and its magnetic polymer nanocomposite (PNC) fibers reinforced with different core-shell Fe@FeO nanoparticles (NPs) loadings are prepared using electrospinning method. The morphology of the resulting products is correlated to the corresponding rheological behaviors of the pure PAN and PAN/Fe@FeO solutions. The diameter of the PAN fibers is linearly related to the polymer solution concentration. However, with a fixed PAN concentration of 10 wt%, the Fe@FeO NP loading shows a negligible effect on the morphology of the PNC fibers. Thermogravimetric analysis (TGA) results indicate an enhanced thermal stability of the PNC fibers than that of the pure PAN fibers. Magnetic carbon nanocomposite (MCNC) fibers are prepared through the stabilization and carbonization of the electrospun PNC fibers. The effects of the heating procedures, including the stabilization and carbonization temperature and time, on the fiber morphology are systematically investigated. Both short and long MCNC fibers could be easily produced by changing the heat procedures. Room temperature magnetic properties of the nanocomposite fibers based on different heating procedures are also studied in this work.     

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.     

Evolution of tension during the thermal stabilization of polyacrylonitrile fibers under different parameters

Complicated physical and chemical reactions can occur during the thermal stabilization of polyacrylonitrile (PAN) fibers, and they can be macroscopically reflected by the evolution of tension in the fibers. In this work, PAN fibers were oxidized under different parameters in a continuous production line. The tension in the fibers was examined in detail and found to be influenced greatly by the stretching ratio, temperature, and time, as well as the porosity of the PAN precursors. As the thermal stabilization proceeded, tension with different characteristics could result from various reaction mechanisms. At the initial stage, a higher temperature was helpful for lowering the tension, but the tension increased with an increasing stretching ratio. In a later stage, the tension was dominantly dependent on the cyclization reaction and increased with increasing temperature or time. Under the same stabilization conditions, the tension in low‐porosity fibers was higher than that in high‐porosity fibers. The microstructures, characterized by high‐resolution transmission electron microscopy, provided some direct evidence for the partially stabilized fibers that the stabilization in the crystalline phase was slower than that in the amorphous phase. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5500–5506, 2006     

Influence of oxygen on the stabilization reaction of polyacrylonitrile fibers

Stabilized polyacrylonitrile (PAN) fibers pretreated under N₂ and air atmospheres were prepared and their thermal behaviors were compared by differential scanning calorimetry and thermogravimetry methods. The results indicated that the subsequent stabilization reaction of PAN pretreated in air was more obvious than that in N₂. In addition, the thermal stability of PAN pretreated in air is better than that in N₂. The structural analysis by Fourier transform infrared spectroscopy and solid state ¹³C nuclear magnetic resonance implied that oxygen promoted dehydrogenation and a compact conjugated structure was formed in PAN. In addition, the C?O structures were generated in air and increased gradually with temperature. The contents of oxygen in PAN fibers studied by elemental analysis corresponded with the structural evolution. Further investigation indicated that the C?O structures helped dehydration and also promoted formation of the cross‐linked structures. A mechanism for structural evolution in PAN during stabilization in air was proposed. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Structural changes and molecular motion of polyacrylonitrile fibers during pyrolysis

The stabilization process of polyacrylonitrile (PAN) fiber is necessary to develop high-performance carbon fiber. This work is concerned with studies of the activation energy of PAN fibers during the stabilization process. A wide-angle X-ray diffractometer combined with a fiber specimen holder for heating was used to measure the activation energy of crystal transition E_c. E_c is almost the same as the activation energy of the cyclization reaction E which is measured by a differential thermal analysis (DTA). The variation of crystal size in PAN fiber and a model of the ladder polymer in stabilized fiber transformed from acrylontrile (AN) units of PAN fiber are discussed also. The crystal size of PAN fiber increases when the thermal treatment temperature is raised. When exposed to temperatures above the crystal degradation temperature T_d, the molecular rods of PAN fiber are destroyed completely, and ladder polymer is formed in the ordered phase of the original PAN fiber. The transformation of ladder polymer is initiated in the disordered phase, and then at the boundaries of the ordered phase.     

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.     

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.     

光密度法研究聚丙烯腈纤维的热稳定化过程

采用梯度升温曲线,在170～310℃内,每间隔10 min升高10℃对聚丙烯腈(PAN)纤维进行热处理,使用光密度法研究了PAN纤维的热稳定化。结果表明:PAN纤维在热稳定化过程中光密度发生了变化,热稳定化过程中纤维表面会发生反应生成致密的氧化层,导致皮芯结构的形成。光密度法是系统研究PAN纤维热稳定化程度与氧化均匀性的理想方法,具有良好的准确性灵敏性;引入了光密度标准差有效评估PAN纤维皮芯结构程度。     

Study of electrospun polyacrylonitrile fibers with porous and ultrafine nanofibril structures: Effect of stabilization treatment on the resulting carbonized structure

Electrospun polyacrylonitrile (PAN)-based carbon nanofibers (CNFs) with high surface area have been of promising interest because of their potential for applications in various fields, especially energy devices. In this study, PAN nanofibers with porous and ultrafine nanofiber structures were prepared by electrospinning PAN/poly(vinyl pyrrolidone) (PVP) immiscible solutions and then selectively removing the PVP component from the electrospun PAN/PVP bicomponent nanofibers. The chemical reaction and microstructure of the PAN fibers with porous and ultrafine nanofibril structures in the stabilization process were investigated. The results revealed the effects of PAN fibers with porous and ultrafine nanofibril structures on the crosslinking reaction, microstructure, and morphology during the stabilization process. According to the in situ Fourier transform infrared spectroscopy results, the intermolecular and intramolecular reactions of the nitrile group for the PAN fibers with ultrafine nanofibril structures exhibited slower reaction rates than those for the neat PAN fibers during stepwise and isothermal heating. Selecting a good stabilization temperature for ultrafine PAN-crosslinked nanofibrils can enhance the surface area and carbonized structure of CNFs. The possible applications of CNFs with porous and ultrafine nanofibril structures in supercapacitors were also evaluated. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48218.     

Rheology of polyacrylonitrile‐based precursor polymers produced from controlled (RAFT) and conventional polymerization: Its role in solution spinning

Polymer solutions in dimethyl sulfoxide (DMSO) as a solvent, made from reversible addition fragmentation chain transfer (RAFT)‐mediated polyacrylonitrile (RAFT^¥ PAN) terpolymer with molecular weight (MW) of 260,000 g/mol and dispersity (Ð) of 1.29, behave differently under applied shear stress than polymer solutions made from conventional PAN (Control PAN) with similar MW (258,000 g/mol) but Ð of 2.05 in the same solvent. The unique rheology of RAFT PAN is because of the reduced amount of high MW polymer fractions. Specifically, a 25% (w/v) polymer solution of RAFT PAN had a viscosity of 198 Pas while the equivalent control PAN polymer solution had a viscosity of 968 Pas at a shear rate of 1 s^?1. Also, RAFT PAN polymer solutions had a longer Newtonian plateau than control PAN polymer solutions. This exhibits more liquid character in RAFT PAN polymer solutions than control PAN polymer solutions at same temperature and concentration. In dynamic tests, RAFT PAN polymer solutions gelled slower than their equivalent control PAN polymer solutions because of their longer polymer chain relaxation times. Slow gelling and higher liquid character in RAFT PAN polymer solutions can result in obtaining stronger and finer precursor fibers during wet spinning. Since RAFT PAN polymer solutions exhibit low viscosity and higher liquid character when compared to its equivalent control PAN at same concentration and temperature, these can allow a wider working window for wet spinning and can also allow higher solid content in the polymer solutions that remain easy to wet spin. This is expected to lead to compact and finer fibers with less voids and higher strength. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44273.     

富氮多孔碳纤维的制备及其在超级电容器中的应用

以聚丙烯腈（PAN）为前驱体,制备具有发达三维网络结构的大/中孔PAN纤维,采用水合肼和盐酸羟胺处理使PAN纤维在产生交联反应的同时进一步引入氮原子,制备了富氮多孔碳纤维,并考察了富氮多孔碳纤维的形貌结构、孔结构特征及电化学性能。结果表明：富氮多孔碳纤维中的N和O原子含量分别为13.53%和8.01%,在电流密度为0.1 A/g时,比电容为222 F/g,比表面电容达0.80 F/m²。     

生产碳纤维的关键设备-预氧化炉

预氧化的目的是使PAN原丝的线型大分子链经预氧化处理后转变为耐热梯型结构,不熔不燃,经得起高温碳化而保持纤维形态。实现这一转化的关键设备就是预氧化炉。研究表明,预氧化炉首先要满足工艺条件的需求：即加热系统的温度场均匀,温度梯度分布合理,风向最好垂直于丝束,风速最好在0.5m/s以上,牵伸系统低温区实施正牵,高温区实施负牵。同时,大型预氧化炉对纤维的处理量要大,以降低生产成本。     

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.     

Characterization of PAN-based nonburning (nonflammable) fibers

In this article, a continuous stabilization process is used to make nonburning (nonflammable) fibers from polyacrylonitrile (PAN) fibers. The effect of the shrinkage behavior and the stretching process of PAN fibers during the stabilization process on the physical properties, morphology, and flammability of the resultant nonourning fibers is studied for the first time. The higher shrinkage of PAN fibers during the continuous stabilization process is found to increase the diameter, the core proportion, and flammability and decrease the Al value, density, mechanical properties, and formation of oriented molecular chains in the resultant nonburning fibers. The effect of the shrinkage behavior of PAN fibers on the fracture surface of the nonburning fibers is also discussed. The nonburning fibers show a fracture structure radiating from the fiber center to the boundary. The structures are composed of small and fine radial strip-layer–like fibrils. Nonburning fibers developed using an optimum stretch process, not only had increased preferred orientation and density, but also had improved mechanical properties. Those fibers also have sufficient nonflammability. © 1993 John Wiley & Sons, Inc.