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     

Structural changes in thermal stabilization of polyacrylonitrile fibre

The following conclusions were drawn from the experimental data examined: IR spectroscopy can be used to evaluate the molecular orientation of initial and oxidized PAN fibre and demonstrate its succession during processing; the deformation regimes of oxidation of PAN fibre affect the degree of preservation of its molecular orientation and the level of the properties of the carbon fibre obtained.Translated from Khimicheskie Volokna, No. 1, pp. 17–19, January–February, 1995.     

Effect of oxygen uptake and aromatization on the skin–core morphology during the oxidative stabilization of polyacrylonitrile fibers

The isothermal oxidative stabilization of polyacrylonitrile fibers has been carried out at 210, 230, and 250°C. The stabilized fibers, treated for different times, have been characterized with elemental analysis, wide‐angle X‐ray diffraction, optical microscopy, and field emission scanning electron microscopy. A parabola relationship has been established between the oxygen uptake and stabilization time, whereas the aromatization index shows a trend of moderate ascension, retention, and acceleration. With increasing temperature and time, the skin–core morphology of the stabilized fibers becomes more and more distinct, but the skin thickness is almost unchanged for 60 and 120 min at 250°C. The fracture mechanism is ductile fracture in the core but is brittle fracture in the skin. The results indicate that the initial rapid oxygen uptake at a high temperature and the subsequent intense aromatization are responsible for the formation of the skin–core morphology. On the basis of the isothermal stabilization, an onion‐like model is proposed for the structure of stabilized fibers that are treated by stepwise increasing temperatures in industrial production. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

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     

The conjugated plane formed in polyacrylonitrile during thermal stabilization

The formation process and mechanism of the pseudo‐graphite sheets of polyacrylonitrile (PAN) during thermal treatment in inert atmosphere were investigated by thermo‐gravimetry (TG) and X‐ray diffraction (XRD). According to the results, the conjugated plane originally formed during stabilization was proposed as the basic planar structure of the pseudo‐graphite sheets, and it was connected and stacked to form pseudo‐graphite microcrystal through chain scission reaction and non‐carbon atoms elimination reaction in 280–450 °C and 800–1300 °C respectively. The in situ measurements for time dependence of ultraviolet‐visible (UV‐vis) and Fourier transform infrared spectra (FTIR) were applied to study the conjugated plane of PAN during isothermal stabilization. It was found that with the higher temperature the conjugated plane forms more rapidly, and it has higher conjugated extent with the extending of heating time. The FTIR data showed that both cyclization and dehydrogenation are beneficial to the evolution of conjugated plane in the first 3 h, but only cyclization continues after 3 h at 250 °C. Further investigation indicated that stabilized PAN with large quantity and high conjugated extent of the conjugated plane would contribute to get large size of pseudo‐graphite microcrystal after carbonization, which is consistent with the former assumption. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43890.     

Preparation and characterization of carbon fibers from polyacrylonitrile precursors

The present work deals with the preparation of carbon fibers from polyacrylonitrile (PAN) fibers. The chemical composition and physical properties of the starting fibers were determined. The PAN fibers were stabilized in air at the temperatures (230, 270, and 300°C) with the heating time from 40 to 420 min. The effects of both final stabilization temperature and heating rate on the chemical and physical properties of the prepared stabilized fibers were studied. The chosen stabilized fibers samples were carbonized in argon atmosphere at the temperatures (1000, 1200, and 1400°C) with different heating rates 5, 10, 15, and 20°C min^?1. The effects of both carbonizing temperature and heating rate on the weight loss, density, elemental composition, and IR absorption spectra of carbonized fibers were also studied. The fiber sample, which was carbonized at 1400°C, contains 97.55% carbon, 1.75% nitrogen, and 1.4% hydrogen. This means that carbonizing the stabilized fibers at 1400°C in argon atmosphere is suitable to get oxygen‐free carbon fibers. Therefore, the used carbonizing temperature in the present work (1400°C) is suitable to produce moderate heat‐treated carbon fibers with the heating rate of 15°C min^?1. The modulus of the prepared carbon fibers was compared to that of industrially produced fibers using the results of X‐ray analysis. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

The role of structural evolution of polyacrylonitrile fibers during thermal oxidative stabilization on mechanical properties

The effects of chemical and physical structural evolution of polyacrylonitrile (PAN)-based carbon fibers precursor during thermal oxidative stabilization (TOS) on the mechanical properties of stabilized fibers were systematically studied. The results of Fourier transform infrared spectroscopy, wide-angle X-ray diffraction, and density gradient column showed that the PAN fibers treated with high temperature and for long time have higher extent of cyclization, oxygen content, and crosslinking content. The crystallinity and crystallite size decreased with the increase of TOS time and temperature, whereas the bulk density of the stabilized fibers increased. The mechanical property results indicated that the decrease in tensile strength was inseparable from the formation of the cyclic structure and the amorphization transition of the crystal structure. The fibers have better structural stability when the extent of cyclization was 80–83%, the crystallinity was 34–45%, and the bulk density of stabilized fibers was 1.33–1.35 g/cm³, but exceeding these ranges, a serious skin-core structure appeared.     

在氮气中处理聚丙烯腈纤维的不熔化工艺

聚丙烯腈纤维不熔化过程的前期(220℃)在氮气中热处理,后期(220℃)在空气中热处理,与常规全程空气中热处理进行对比,希望能够降低不熔化处理的成本。借助元素分析(EA)、热重分析仪(TGA)、环境扫描电子显微镜(ESEM)、力学性能分析等表征手段,研究了不同热处理方法对PAN不熔化纤维的氧化程度和皮芯结构影响以及碳化收率和碳纤维的力学性能。结果发现,与全程空气处理相比,经过氮气气氛处理后的不熔化纤维在后续处理过程中最终不熔化温度可以降低20℃,900℃碳化后的收率增加了3.8%,碳纤维的强度相当,初步估算节能8.7%。     

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.     

Structural changes in modified polyacrylonitrile copolymer

The structural changes in standard and as-spun PAN gel fibres in modification with flame retardants were investigated using dynamic isothermal heating, thermogravimetric, and x-ray structural analysis. The characteristics of thermooxidative degradation of the fibres in predominance of structure formation over degradation were established. Modified PAN gel fibres have an OI of 40 vol. % and are resistant to wet treatment with a fireproofing effect. Translated from Khimicheskie Volokna, No. 6, pp. 14–16, November–December, 2008.     

Indicators for evaluation of progress in thermal stabilization reactions of polyacrylonitrile fibers

Using different indices calculated from Fourier Transform Infrared Spectroscopy (FTIR), X‐ray Diffraction (XRD), and Differential Scanning Calorimetry (DSC) spectra, progress in stabilization reactions of three different commercial grade polyacrylonitrile (PAN) fibers is calculated. From each analysis technique quantitative indices are computed which could assess in some particular reactions. Combination of these indices gives further information about the progress of stabilization reactions which cannot be concluded from single indices. The results show that different indices are not fully consistent with each other, depending on the analysis technique and the changes they assess. The advantages and disadvantages of each index are investigated and practical indices are identified which can be used to design the optimum stabilization process. In addition, by combination of some indices it is possible to separate the temperature ranges in which reactions occur in amorphous or crystalline regions. This approach can be used to design appropriate stretching process during stabilization. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40343.     

UV assisted stabilization routes for carbon fiber precursors produced from melt-processible polyacrylonitrile terpolymer

A low-cost route for producing PAN-based carbon fibers is being developed. The approach involves forming polyacrylonitrile terpolymers that can be melt-spun into fibers. The fibers are then stabilized and carbonized to yield carbon fibers. Melt-processibility, however, precludes direct thermal stabilization of these polymeric fibers. Therefore, a precursor terpolymer containing acryloyl benzophenone (ABP) is used. The UV sensitivity of ABP moiety enhances the UV crosslinkability of the precursor fibers. After a brief exposure to UV radiation, the melt-spun terpolymer fibers can be oxidatively stabilized at 320 °C without melting and subsequently carbonized. UV-visible and ATR-IR spectroscopic analyses suggest that UV radiation induces the formation of free radicals which, in turn, cyclize the PAN. Cyclized PAN was characterized by a strong absorbance in UV-visible region (300-500 nm) due to conjugated >CC< and >CN- bonds which were also detected by infrared spectroscopy.     

Effect of nitrogen pretreatment on the skin-core structure of thermal oxidative stabilization polyacrylonitrile fibers

Polyacrylonitrile precursor was pretreated under nitrogen to study the effect of the extent of cyclization of pretreated fibers on the chemical structure, crystal structure and skin-core structure of thermal oxidative stabilization (TOS) fibers. Based on the FTIR spectra, the cyclization degree of pretreated fibers was quantitatively calculated by a combination of peak-fitting and second-derivative operations. Combining the results of wide-angle X-ray diffraction, Raman spectroscopy and scanning electron microscope analysis, pretreatment was beneficial to increase the cyclized structure of TOS fibers, but inhibited the oxidation reactions in the TOS process. As the increase of cyclization degree of pretreated fibers, the oxygen content and crystal size of TOS fibers decreased and the crystal interplanar spacing increased; in the skin layer of TOS fibers, the I_D/I_G ratio increased and brittle fracture area decreased. When the cyclization degree of pretreated fibers was in the range of 10%–35%, the skin-core structure of TOS fibers was weakened, which demonstrated that proper pretreatment before TOS process can improve the skin-core structure.     

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。     

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.     

Thermal behavior and kinetics during the stabilization of polyacrylonitrile precursor in inert gas

The thermal behaviors of polyacrylonitrile precursor during thermal stabilization in inert gas were investigated by differential scanning calorimetry, thermomechanical analysis, and thermogravimetry. Combining these methods with the tracing of chemical changes by Fourier transform infrared spectroscopy indicated that complex reactions, including cyclization and pyrolytic reactions occurred sequentially. An imine‐enamine tautomeric structure was formed at around 240°C and was converted to a conjugated structure when the temperature was increased to 400°C. A thermal stabilization mechanism was proposed and confirmed experimentally by using a two‐step heating process. The apparent activation energies and the pre‐exponential factors for these stabilization reactions were also estimated by the Kissinger, Ozawa, and “Improved Coats‐Redfern” methods. To obtain a fit to the experiment data, a new kinetic model, named the “Three Regions Kinetic Model,” was proposed using the Improved Coats‐Redfern method. The applicability of this model and the prediction of the stabilization profile at a given heating rate were verified by plotting conversion rate against conversion profiles. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.     

酯类共聚单体对PAN结构及热稳定化行为的影响

采用K2S2O8-NaHSO3氧化还原引发剂和水相沉淀聚合法制备了丙烯腈均聚物(PAN)、丙烯腈-醋酸乙烯酯共聚物(P(AN-VAc))、丙烯腈-丙烯酸甲酯共聚物(P(AN-MA));采用红外光谱、核磁共振、差示扫描量热/热重同步分析仪、X射线衍射仪等手段对聚合物的结构及热稳定化行为进行了研究。结果表明:引入VAc,MA酯类共聚单体对PAN聚合物的立构规整性几乎没有影响,3种聚合物的等规、无规、间规比例均约为1∶2∶1,平均序列长度约为2,呈现典型的无规聚合物结构特征;在空气气氛热稳定化过程中,VAc,MA酯类共聚单体不能以离子形式在低温下引发环化反应,但是都可以起到缓和放热和减少失重的作用,从而提高热稳定化过程的可控性;在相同条件下进行热稳定化处理,P(AN-MA)具有较高的相对环化率和芳构化指数,表明MA更有利于提高热稳定化程度。     

Kinetic study of the dehydrogenation reaction in polyacrylonitrile‐based carbon fiber precursors during thermal stabilization

The kinetics of dehydrogenation reaction and the structural evolution in polyacrylonitrile precursor fibers during thermal stabilization in air have been studied by Fourier transform infrared spectroscopy. The results indicate that, with the progress of dehydrogenation, the absorbance of methylene groups (? CH₂? ) gradually decreases, whereas that of methine groups (?CH? ) gradually increases. The dehydrogenation reaction in the fibers is basically completed after 20‐min stabilization above 255°C. According to the Beer–Lambert law, the values of the absorbance for both ? CH₂? groups and the resulting ?CH? groups have been calculated and converted into the concentration fractions of ? CH₂? groups via the Lorentzian multipeak fitting. According to the principles of chemical kinetics, the dehydrogenation reaction has been determined as a pseudo‐second‐order reaction with an activation energy of 107.6 kJ mol^?1. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

张力对聚丙烯腈纤维不熔化过程中氧化反应的影响

通过连续的不熔化实验,在不同温度段对聚丙烯腈纤维施加不同的张力,得到一系列不熔化纤维。借助差示扫描量热法(DSC)、元素分析(EA)、场发射扫描电镜(FESEM)等表征手段,研究张力在不熔化过程中对氧化反应的影响。结果表明,在175~218℃,随着张力的增大,纤维内氧含量减少,氧含量梯度(氧梯度)变化不大;在226~232℃,氧含量随张力增大先增多后减少,张力较大使得更多氧参与反应,纤维皮部与芯部氧含量差异增大,氧梯度较大;在238~270℃,随张力增大纤维内氧含量增多氧梯度增大。