Stabilization Of Polyacrylonitrile Fibers Research Articles

Effects of NaIO4 treatment on the chemical and physical properties of polyacrylonitrile fibers

Polyacrylonitrile (PAN) fiber modification can promote the stabilization efficiency and mechanical properties of the subsequent carbon fibers; however, it could cause damage to the fibers to some extent. In this article, the effects of NaIO4 treatment on the chemical and physical properties of PAN fibers were investigated. After NaIO4 modification, the cyclization reaction has partially occurred, owing to the FTIR revealed that the C≡N peak was weakened, but the C=N stretching vibration peak, the –N = C=O stretching vibration peak, the C=O stretching vibration absorption peak, and the short sequence conjugate C=N characteristic peak were enhanced simultaneously, indicating a successful cyclization and dehydrogenation reaction. Moreover, owing to more oxygen-containing functional groups successfully attached to the PAN fibers and the cyclization reaction taking place, the initial temperature, peak temperature, and heat release of cyclization decreased, which means a more efficient and safer stabilization process. X-ray diffraction (XRD) showed that the (100) crystal plane microcrystal size was the smallest at 40 °C of the modification temperature, whereas the (002) crystal plane microcrystal size was the largest at 60 °C. The modified stabilized PAN fibers presented lower crystallinity and a more aromatic structure compared with untreated fibers. Raman spectra revealed a significant increase in the intensity of the G peak after NaIO4 modification; the R-value decreased from 1.65 to 1.43, demonstrating a transformation of the sp3 hybrid carbon structure into a more graphite-like sp2 hybrid C=C structure. Meanwhile, scanning electron microscopy (SEM) images disclosed additional grooves and cracks on the fiber surface after modification.

Effect of structural evolution in high‐temperature thermal oxidative stabilized polyacrylonitrile fibers on carbonized fibers

AbstractThe effect of structure and properties of polyacrylonitrile (PAN) fibers during thermal oxidative stabilization (TOS) stage (especially high‐temperature TOS stage at 270–300°C) on the skin‐core structure of carbonized fibers was studied. The work clarified the reasons for selecting the highest TOS temperature, and also determined the optimal TOS temperature range of obtaining structurally stable carbonized fibers. The changes of chemical groups were quantitatively calculated by a combination of peak‐fitting and second‐order derivative methods. The results showed that the cyclization was dominant in the early TOS stage, oxidation reaction mainly occurred in the post‐TOS stage, and crosslinking structure significantly increased after 250°C. In the high‐temperature TOS stage, cyclization reaction was basically completed, the growth rate of oxygen content slowed down, and dehydrogenation index continued to increase. The microstructure observation found that the grooves on the fiber surface were gradually smooth after low‐temperature and high‐temperature carbonization. The radial structure of A285‐1350 and A300‐1350 fibers have obvious heterogeneity, and diameter dropped below 5 μm. Therefore, carbonized fibers have obvious structural defects when the TOS temperature is above 285°C. The TOS temperature between 200 and 285°C is more conducive to obtain structurally stable carbonized fibers.

Microstructural evolution of polyacrylonitrile fibers during industry-mimicking continuous stabilization

The industry-mimicking continuous stabilization of polyacrylonitrile (PAN) fibers was conducted in the temperature range of 180–270 °C to investigate microstructural evolutions of the fibers with comprehensive analysis at various intermediate stages. The chemical reactions during stabilization induce elongational stress along the fiber axis. In the low-temperature regime (below 240 °C), the cyclization reaction primarily occurs in the amorphous phase of the PAN structure, which possesses low thermal stability. The further stabilization including dehydrogenation and oxidation occurs actively even in the crystalline phase beyond 240 °C. The small-angle X-ray scattering (SAXS) observation confirms that the stabilized PAN fibers have two kinds of microvoids: Major large microvoids with a diameter and a length of ∼6 and ∼17 nm, respectively, located between the microfibrils are formed during the spinning of precursors by the solvent-nonsolvent exchange, and small microvoids with a diameter and a length of ∼1 and ∼2 nm, respectively, which are presumably located at the ends of molecules, are generated by gas evolution from inside the fibers during stabilization. We believe that the integrated studies on stabilization with physical and chemical structural evolution of the microvoids provide significant information for industrial manufacturing of high-performance carbon fibers.

Stabilization of polyacrylonitrile fibers with carbon nanotubes

Stabilization is an energy-intensive and time-consuming step for making PAN-based carbon fiber. Here, a new pathway is shown that uses carbon nanotubes (CNT) to accelerate the reaction rate and to reduce the PAN stabilization time. Bi-component core-sheath PAN-PAN/CNT (10 wt% CNT) fibers and single-component PAN/CNT (10 wt% CNT) fibers were made. When comparing to PAN fiber with comparable diameter, the heat of stabilization (ΔHreaction) of PAN/CNT fibers increased up to 3 times in the air, while it did not change in N2. The addition of CNTs reduced the activation energy (Ea) of PAN cyclization by up to 12% but increased it for the oxidation reaction by up to 80% compared to PAN fibers. CNTs did not increase the kinetic constant of the cyclization reaction but increased it for the oxidation reaction (koxidation) by up to 5 times, when comparing to PAN fiber at 250°C. At the same temperature, the bi-component PAN/CNT fiber bundle was fully stabilized in less than 2 hours, while the PAN fibers with comparable diameter took 6 hours to stabilize. Porosity was observed in the PAN/CNT precursor as well as in the stabilized PAN/CNT. This porosity was considered, at least partially responsible for increased oxygen diffusion, and hence for the decrease in the PAN stabilization time.

New insights into orientation differences between skin and core regions of polyacrylonitrile fibers during pre-stabilization with the stretching process

In the study, a model to systematically evaluate the differences between the orientation of polyacrylonitrile (PAN) chains both in skin and core regions during pre-stabilization with the stretching process is proposed. Consequently, there are few influences on changes in the orientation of PAN chains in core regions. By contrast, when the stretching ratios is under 1.05, the chains in skin regions mainly undergo orientation, facilitating the simultaneous progress of the crystallization and cyclization reaction. However, at the stretching ratio range from 1.05 to 1.20, there may be other types of deformations, which results in a gradual increase of the crystallinity and remarkable depression of the stabilization reactions. As the stretching ratios greater than 1.20, the crystallinity and the orientation degrees drop due to overly imposed stress. The mechanical properties of stabilized PAN fibers strongly depend on the orientation degree in quasicrystals of PAN chains in skin regions.

Structural Transformation of Polyacrylonitrile (PAN) Fibers during Rapid Thermal Pretreatment in Nitrogen Atmosphere.

The modification before the stabilization process could tune the exothermic behavior and the structural evolution of PAN fibers during stabilization. In this study, we demonstrate that a rapid thermal pretreatment in nitrogen can effectively mitigate the exothermic behavior of PAN fibers, such as decreasing the initial temperature, broadening the exothermal peak, and decreasing the nominal heat release during heating the fibers in air. The color of fibers has shown gradual changes from white to light yellow, yellow and brown during thermal pretreatment in nitrogen with the increase of pretreating temperature and time. The differential scanning calorimetry (DSC), Fourier Transform Infrared Spectrometer (FTIR), X-ray diffraction (XRD), and Thermogravimetric Analysis (TG) characterization have been applied to analyze the thermal properties, chemical and physical structural difference between PAN, and thermally pretreated PAN fibers. The thermal pretreatment of PAN fibers in nitrogen could induce cyclization, dehydrogenation, and cross-linking reactions, in which the cyclization play an important role on improving the cyclization index of stabilized PAN fibers. Meanwhile, the pretreatment can result in noticeable changes of the aggregation structure of PAN fibers, as indicated by the increase of crystallinity and crystalline size. These structural modifications can benefit the main cyclization reaction during stabilization and enhance the carbon yield in resultant carbon fibers. The rapid thermal pretreatment in nitrogen could increase efficiency of modification on PAN fibers, and that could save much time and energy. It is beneficial to manufacture low-cost carbon fibers and to spread the applications of carbon fibers.

Influence of boric acid on radial structure of oxidized polyacrylonitrile fibers

AbstractThe surface modification of polyacrylonitrile (PAN) fibers with boric acid was utilized to modulate the homogeneity of the radial structure of the PAN fibers during thermal stabilization. Exothermic peaks of the fibers were put off by boric acid, and unreacted nitrile groups of the oxidized PAN fibers increased with the boron content, indicating that boric acid on the fiber surface had an retardant effect on the thermal stabilization of PAN fibers. The relative skin thicknesses of the oxidized PAN fibers were quantitatively measured by sulfuric acid etching and SEM observation. The value increased obviously with the boron content, which could be further elevated by increasing stabilization time or decreasing stabilization temperature. Oxidized PAN fibers with more homogeneous radial structure can thus be obtained with the modification of boric acid, which might be beneficial for the preparation of high performance carbon fibers.

Carbonization of polyacrylonitrile-based fibers under defined tensile load: Influence on shrinkage behavior, microstructure, and mechanical properties

The influence of temperature and tensile mechanical load during carbonization of stabilized polyacrylonitrile fibers on the shrinkage behavior, the crystalline parameters and the mechanical properties of the resulting fibers is investigated. To this end, a newly developed experimental setup enables a defined heat treatment of fiber bundles up to a temperature of 1200 °C under control of tensile load. The attachment devices for the fiber bundles are located in the hot zone of the furnace. Additionally, carbonization treatments up to 1600 °C without application of load are performed.Three characteristic temperature regions of the carbonization process are identified.Between 250 °C and 450 °C the still ductile fibers contain rests of the polyacrylonitrile phase experiencing stabilization reactions. Stacked, heterocyclic planar structures grow due to molecular crosslinking. Between 450 °C and 1000 °C graphite-like turbostratic carbon is formed. Crystallites grow laterally, crystallinity increases, and the material becomes brittle. Above 1000 °C a pronounced increase in stacking height, lateral crystallite size and crystallinity occurs, while the interlayer distance decreases. Mechanical properties reach the values typical for commercial carbon fibers.Application of a tensile stress during carbonization influences fiber dimensions at temperatures between 250 °C and 450 °C, where the material shows ductile behavior. Shrinkage in fiber direction is hindered and radial shrinkage is favored. At temperatures above 1000 °C, crystalline parameters and mechanical properties of the resulting carbon fibers depend on the applied tensile stress. Carbonization without tensile stress results in significantly lower mechanical properties.The impact of heat treatment temperature and applied stress on carbon fiber properties demonstrate the possibility of a defined tailoring of fiber characteristics.

Stabilization of polyacrylonitrile nanofiber mats obtained by needleless electrospinning using dimethyl sulfoxide as solvent

Polyacrylonitrile can be used as a base material for thermochemical conversion into carbon. Especially nanofiber mats, produced by electrospinning, are of interest to create carbon nanofibers. Optimal stabilization and carbonization parameters, however, strongly depend on the spatial features of the original material. While differences between nano- and microfibers are well known, this paper shows that depending on the electrospinning method and the solvent used, considerable differences between various nanofiber mats have to be taken into account for the optimization of the stabilization conditions. Here, we examine for the first time polyacrylonitrile nanofiber mats, electrospun with wire electrospinning from the low-toxic dimethyl sulfoxide as a solvent, instead of the typically used needle electrospinning from the toxic dimethylformamide. Additionally, we used inexpensive polyacrylonitrile from knitting yarn instead of highly specialized material, tailored for carbonization. Our results show that by carefully controlling the maximum stabilization temperature and especially the heating rate, fully stabilized polyacrylonitrile fibers without undesired interconnections can be created as precursors for carbonization.

Melt processable polyacrylonitrile copolymer precursors for carbon fibers: Rheological, thermal, and mechanical properties

Polyacrylonitrile (PAN) copolymers containing varying amounts of methyl acrylate (MA), P(AN-co-MA), were synthesized as a melt-spinnable precursor of carbon fibers. The rheological properties of P(AN-co-MA) with MA content of 15mol% at 190°C proved to be suitable for melt-spinning and the PAN fiber was spun from an extruder. In order to prevent remelting and fusion of the fibers in the stabilization process, electron-beam irradiation of over 1500kGy was used and the melt-spun PAN fibers were successfully converted to stabilized PAN fibers by thermal treatment up to 250°C. Finally, carbon fibers (CFs) were produced by pyrolysis of the stabilized PAN fibers. The mechanical properties of the resulting-CFs were evaluated; the tensile strength, tensile modulus, and elongation at break were 1.37±0.2GPa, 110±11.1GPa, and 1.27±0.28%, respectively. These results suggest the possibility of utilizing melt-spinning as a cost-efficient method for fabrication of carbon fibers.

Significantly reduced pre-oxidation period of PAN fibers by continuous electron beam irradiation: Optimization by monitoring radical variation

A self-shielded electron beam (EB) accelerator was connected to the front of a pre-oxidation furnace to evaluate the effect of radiation on the pre-oxidation of polyacrylonitrile (PAN) fibers; with this setup, PAN fibers could be irradiated and pre-oxidized in sequence to obtain stabilized fibers. Radiation effects during different temperature stages of the pre-oxidation process were analyzed by differential scanning calorimetry, Fourier-transform infrared spectroscopy, and electron spin resonance spectroscopy. The results indicated that radiation exerted significant but different effects at various stages of the pre-oxidation process. The alkane radicals induced by radiation are rapidly transformed into polyene structures and produced polyenyl radicals after a short heat treatment period. The polyenyl radicals promoted dehydrogenation and cyclization reactions and polyene structures improved the thermal stability of the PAN fibers. Therefore, radiation can greatly shorten the time required for the stabilization of PAN fibers, owing to which the time needed for each temperature stage can be reduced by a large extent. The effects of radiation on various temperature stages of the pre-oxidation process were analyzed accurately by studying the changes in the concentration of polyenyl radicals. An optimized pre-oxidation process for irradiated PAN fibers was designed, which reduced the time period required for stabilization by 75%. The optimized pre-oxidation plus continuous EB irradiation process induced good properties in the stabilized PAN fibers, in terms of fiber density, oxygen content, and degree of cyclization. Radical concentration was found to be an appropriate indicator to express the degree of pre-oxidation.

Effects of γ-Ray Irradiation on the Radial Structure Heterogeneity in Polyacrylonitrile Fibers during Thermal Stabilization.

The radial structural heterogeneity of thermally-stabilized polyacrylonitrile (PAN) fiber is considered to be a limiting factor affecting the mechanical properties of the resulting carbon fibers. In this study, we demonstrate that a low-dose (60 kGy) γ-ray irradiation pretreatment can effectively mitigate the radial structural heterogeneity of PAN fibers after thermal stabilization. The characterization results indicate that low-dose γ-ray irradiation only affects the physical structure of PAN through decreasing its crystalline size and crystallinity, rather than inducing chemical cross-linking and/or intramolecular cyclization. It is proposed that an increased amorphous region in PAN fibers prompted by low-dose γ-ray irradiation can facilitate oxygen diffusion from skin to core during stabilization, which results in the increased structural homogeneity of stabilized PAN fibers.

Molecular modeling of the microstructure evolution during carbon fiber processing.

The rational design of carbon fibers with desired properties requires quantitative relationships between the processing conditions, microstructure, and resulting properties. We developed a molecular model that combines kinetic Monte Carlo and molecular dynamics techniques to predict the microstructure evolution during the processes of carbonization and graphitization of polyacrylonitrile (PAN)-based carbon fibers. The model accurately predicts the cross-sectional microstructure of the fibers with the molecular structure of the stabilized PAN fibers and physics-based chemical reaction rates as the only inputs. The resulting structures exhibit key features observed in electron microcopy studies such as curved graphitic sheets and hairpin structures. In addition, computed X-ray diffraction patterns are in good agreement with experiments. We predict the transverse moduli of the resulting fibers between 1 GPa and 5 GPa, in good agreement with experimental results for high modulus fibers and slightly lower than those of high-strength fibers. The transverse modulus is governed by sliding between graphitic sheets, and the relatively low value for the predicted microstructures can be attributed to their perfect longitudinal texture. Finally, the simulations provide insight into the relationships between chemical kinetics and the final microstructure; we observe that high reaction rates result in porous structures with lower moduli.

Influence of heating procedures on the surface structure of stabilized polyacrylonitrile fibers

Abstract The stabilized polyacrylonitrile (PAN) fibers were obtained after heating the precursor PAN fibers under air atmosphere by different procedures. The surface structures and compositions of as-prepared stabilized PAN fibers have been investigated by SEM, SSNMR, XPS and Raman spectroscopy. The results show that 200 °C, 220 °C, 250 °C, and 280 °C are key temperatures for the preparation of stabilized PAN fibers. The effect of heating gradient on the structure of stabilized PAN fibers has been studied. The possible chemical structural formulas for the PAN fibers is provided, which include the stable and unstable structure. The stable structure (α-type) could endure the strong chemical reactions and the unstable structure ( β - or γ -type) could mitigate the drastic oxidation reactions. The inferences of chemical formula of stabilized PAN fibers are benefit to the design of appropriate surface structure for the production for high quality carbon fibers.

Study on the Relationship Between Evolved Gases in Carbonization of Stabilized Polyacrylonitrile Fibers with 12,000 Filaments and Their Mechanical Properties

Study on the Relationship Between Evolved Gases in Carbonization of Stabilized Polyacrylonitrile Fibers with 12,000 Filaments and Their Mechanical Properties

Influence of thermal temperature on the structure and sealed micropores of stabilized polyacrylonitrile fibers

Thermal stabilization is an important process in carbon fibers’ production, during which the polyacrylonitrile fibers are heated from 180 °C to 280 °C in air. In this study, the samples were characterized by X-ray diffraction, Fourier infrared spectroscopy, differential scanning calorimetry, small angle X-ray Scattering(SAXS) and mechanical tensile tests. A new rule was suggested by the results of structural characterization for the cyclization, dehydrogenation and oxidation reactions that were observed to be drastic from 200 °C to 220 °C, from 220 °C to 250 °C, and in the later period of the thermal stabilization reactions, respectively. The sizes, shapes and distributions of the sealed micropores were obtained from the SAXS data. The breaking elongation was significantly affected by the drastic cyclization and dehydrogenation reactions. The breaking force was affected considerably by the bigger micropores, especially from 220 °C to 250 °C, owing to the drastic dehydrogenation reactions.

Optimization of the temperature program to scale up the stabilization of polyacrylonitrile fibers

The production of carbon fibers from polyacrylonitrile (PAN) includes a stabilization step before carbonization. Transferring this stabilization from the laboratory to a bigger scale cannot be performed without changes of the process parameters. Using a pure PAN polymer, fibers were spun and their stabilization as well as carbonization was studied in laboratory scale. The resulting materials were analyzed with FTIR, SEM and XRD. The stabilization step was then transferred to a pilot scale. Limitations given by the production line had to be taken into account to develop a suitable temperature treatment. The formation of a core/shell structure during stabilization was observed and was detrimental to the properties of the fiber after carbonization. Adjustments were made based on obtained results to balance the negative effects of high heating rates and strong temperature gradients. With the implemented changes the mechanical properties could be reproduced on the larger production scale.

Improving crosslinking of stabilized polyacrylonitrile fibers and mechanical properties of carbon fibers by irradiating with γ-ray

Polyacrylonitrile (PAN) fibers were stabilized continuously through a series of temperature zones, and the stabilized PAN fibers (SFs) taken out from different zones were irradiated with γ-ray, finally, some of the irradiated SFs were carbonized continuously. Structure changes of the SFs and mechanical properties of the derived carbon fibers were investigated. It is found that γ-ray not only could destroy the nitrile crosslinking bonds and form nitrile groups again in slight SFs, but could make more crosslinked structure including ether crosslinking and carbon-carbon crosslinking in slight and deep SFs, which improved thermal stability of the SFs and tensile strength of the carbon fibers. The irradiation also induced cyclization in short sequence in slight SFs, however, such a cyclization structure didn't significantly improve thermal stability.

Evolution of the crystalline structure and cyclization with changing tension during the stabilization of polyacrylonitrile fibers

ABSTRACTThe effects of tension on the crystalline structure and cyclization of polyacrylonitrile fibers during stabilization were investigated. The degree of cyclization was measured by Fourier transform infrared spectroscopy and differential scanning calorimetry. The crystalline structure was characterized by wide‐angle X‐ray diffraction. When the fibers were heat‐treated at temperatures between 175 and 218°C, the tension mainly affected the cyclization in the amorphous regions through changes in the spatial distance of the chain segments; this led to a relatively higher cyclization degree under moderate tension. When the temperatures ranged from 226 to 232°C, the reactions extended to the crystalline regions. The chemical bonds became greater in the cross section of the fibers, and this was caused by cyclization structures formed in the former stage. Therefore, the optimum tension was higher than in the prior temperature range. At higher temperatures ranging from 238 to 270°C, a crosslinked structure formed, so the optimum tension increased continually. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42182.

Nanoscale dynamic mechanical imaging of the skin–core difference: From PAN precursors to carbon fibers

In this study, nanoscale dynamic mechanical imaging was used to map the storage modulus along the cross section of fibers during the evolution from polyacrylonitrile (PAN) precursors to carbon fibers. The quantitative characterization of skin–core difference in oxidative stabilized fibers indicated that the primary skin–core structure in the fibers forms at 265°C and the skin–core difference of the final oxidative stabilized fibers in modulus was about 8%. The difference has been enhanced in modulus but reduced in spatial scale during the transition from stabilized PAN fibers to carbon fibers.