Polyacrylonitrile Precursor Fibers Research Articles

Microstructural Evolution during Dry-Jet Wet Spinning Postprocessing from Coagulation Bath Fiber to High-Performance Polyacrylonitrile Precursor Fiber

Microstructural Evolution during Dry-Jet Wet Spinning Postprocessing from Coagulation Bath Fiber to High-Performance Polyacrylonitrile Precursor Fiber

Confined pyrolysis-driven one-dimensional carbon structure evolution from polyacrylonitrile fiber and its microwave absorption performance

The evolution of one-dimensional carbon structures from polyacrylonitrile (PAN) fiber precursors is investigated based on confined pyrolysis strategy, which is employed to convert electrospun PAN fibers into carbon in a closed system. Carbon tube with ultra-large diameter derived from silica-coated organic PAN precursor fiber is observed and its low-performance of microwave absorption is revealed. To enhance microwave absorption, cobalt acetylacetonate is introduced as a magnetic resource into PAN, facilitating one-dimensional carbon structure evolution and yielding magnetic/carbon nanofibers. The mechanism resulting in such one-dimensional carbon structure evolution is discussed. The microwave absorption performances of the prepared magnetic carbon nanofibers are analyzed based on their complex permittivity and permeability. Remarkably, the magnetic carbon nanofibers exhibit strong microwave absorption intensity of −42 dB at only 20 wt% filler content. Further simulations are conducted based on radar-absorbing structure with prepared magnetic carbon nanofibers-coated laminate and wing models by using CST software. The laminated composite structure appears the maximum RCS reduction value of 18.70 dBsm at 6 GHz with 3.0 mm thickness in the frequency range of 2–18 GHz. In the corresponding wing structure, highly efficient electromagnetic wave absorption is observed and the RCS reduction can achieve −14.12 dBsm at 10 GHz with 3.5 mm thickness.

Investigation of the Microstructure of PAN Precursor Fibers Obtained at Various Processing Stages

The production of high-performance carbon fibers relies heavily on the structural transformations of polyacrylonitrile (PAN) fiber during processing. In this article differential scanning calorimetry (DSC), synchrotron small-angle X-ray scattering (SAXS), and synchrotron wide-angle X-ray diffraction (WAXD) were used to study the changes of the thermal properties, voids and crystal structure of the PAN precursor fibers on industrial lines during the continuous processing stages, i.e., after the treatment in the coagulation bath, after stretching in boiling water and after stretching in high-temperature steam. The findings of this study suggest that after boiling water stretching the molecular chain motion of the PAN precursor fiber weaken, resulting in the expansion of the voids. After high-temperature steam stretching the voids became elliptical and oriented in the stretching direction. Although the boiling water stretching process partially damaged the (100) crystal planes of the PAN precursor fibers, it does not hinder their overall crystalline structure. The high stretching ratio primarily affected the orientation of the molecular chains. This orientation, even in the context of partial crystal plane disruption, further promoted the crystallization of the (100) crystal planes in the fibers. These experiments explored the correlation between the processing technology and fiber structure transformation of PAN-based carbon fibers, resulting in producing a guide for the preparation of high-performance carbon fibers with fewer defects.

Evolution of microstructure within carbon fiber during pre-carbonization revealed by mean field theory

Microstructure dominating the conductivity in carbon fiber (CF) was investigated to construct an initial model for the interpretation of the sudden shift in conductivity of pre-oxidized polyacrylonitrile precursor fibers (POF) during pre-carbonization. The electrical conductivity of POF treated at different pre-carbonization temperatures was measured by four-point probe meter. According to the result of high-resolution transmission electron microscopy and X-ray diffraction, the microstructure of CF was approximately regarded as nanocomposite consisting of pseudo-graphite sheet (PGS) and an amorphous carbon matrix. Based on the analysing of this nanocomposite microstructure model with mean-field theory and applying effective-medium approximation, the evolution of electrical conductivity and microstructure of POF was investigated. The microstructure of CF during pre-carbonization process, including the volume fraction and geometric factors of PGS, was determined by X-ray diffraction and Raman spectroscopy. The existence of percolation effect was determined by applying a combination of simulation calculating and experiment. The results indicated that a carbon skeleton consisting of connected PGSs was formed within CF during the pre-carbonization process. In addition, the carbon skeleton provides efficient pathways for the lattice vibrations and phonon propagation required for heat conduction and has a significant impact on the subsequent crystallization of the final CF and its properties.

Microstructure Analysis of Drawing Effect and Mechanical Properties of Polyacrylonitrile Precursor Fiber According to Molecular Weight.

Polyacrylonitrile (PAN) fiber is the most widely used carbon fiber precursor, and methyl acrylate (MA) copolymer is widely used for research and commercial purposes. The properties of P (AN-MA) fibers improve increasingly as the molecular weight increases, but high-molecular-weight materials have some limitations with respect to the manufacturing process. In this study, P (AN-MA) precursor fibers of different molecular weights were prepared and analyzed to identify an efficient carbon fiber precursor manufacturing process. The effects of the molecular weight of P (AN-MA) on its crystallinity and void structure were examined, and precursor fiber content and process optimizations with respect to molecular weight were conducted. The mechanical properties of high-molecular-weight P (AN-MA) were good, but the internal structure of the high-molecular-weight material was not the best because of differences in molecular entanglement and mobility. The structural advantages of a relatively low molecular weight were confirmed. The findings of this study can help in the manufacturing of precursor fibers and carbon fibers with improved properties.

New insight into structure-property correlation of polyacrylonitrile precursor fibers and resultant carbon fibers

New insight into structure-property correlation of polyacrylonitrile precursor fibers and resultant carbon fibers

The impact of guanidine carbonate incorporation on the molecular structure of polyacrylonitrile precursor fiber stabilized by a multistep heat treatment strategy

AbstractThermal‐oxidative stabilization of the polyacrylonitrile (PAN) precursor was performed employing a multi‐step heat treatment strategy in an air circulating furnace. In this approach, the applied temperature was gradually increased from 200°C to 250°C employing several stages for different stabilization durations. Fifteen percent guanidine carbonate (GC) was found as optimum to incorporate with the PAN precursor fibers to accelerate the thermal‐oxidative stabilization process. Characterization techniques, including X‐ray diffraction (XRD), Fourier transform infrared spectroscopy (FT‐IR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), tensile strength, volume density, linear density, fiber thickness, and burning test have been performed to monitor the changes in PAN structure. Test results of the stabilized samples were compared with the reference sample results to demonstrate the accelerating effect of GC integration. Findings showed that GC pretreatment enhanced and accelerated the cyclization of nitrile groups in the PAN polymer structure and allowed the quicker formation of a thermally stable structure. The analysis of the experimental results revealed that GC integration and employing the multi‐step heat treatment strategy helps greatly to cut the overall PAN‐based carbon fiber production cost.

Interior morphological feature of PAN nascent fibers and precursor fibers revealed by ultrathin section and solution etching

Polyacrylonitrile (PAN) fiber is a vital precursor for the high-performance carbon fibers. The interior morphologies of PAN nascent fibers and precursor fibers were exposed by the ultrathin section and solution etching, and elucidated by scanning electron microscope (SEM) and high-resolution transmission electron microscope (HRTEM). The results indicated the unequivocal presence of fibril structure features in the PAN fibers. Electron microscope observation revealed different morphological types: microfibrillar network, microfibrils, fibrils, fibril bundles, ribbon fibrils, crystal layers, pore/voids between fibrils, and different transverse connections between microfibrils/fibrils. The microfibrillar network formed in nascent fibers, and the aligned fibrils arranged in precursor fibers. The microfibrils tended to aggregate and amalgamate into the fibrils/fibril bundles, which were interlinked by the transverse connections. The investigation of the detailed interior morphologies and the modeling of the compressive structural types contribute to optimization of the spinning process, thereby regulating the fibril morphology and improving the fiber mechanical properties.

Designing Materials and Processes for Strong Polyacrylonitrile Precursor Fibers.

Although polyacrylonitrile (PAN)-based carbon fibers have been successfully commercialized owing to their excellent material properties, their actual mechanical performance is still much lower than the theoretical values. Meanwhile, there is a growing demand for the use of superior carbon fibers. As such, many studies have been conducted to improve the mechanical performance of carbon fibers. Among the various approaches, designing a strong precursor fiber with a well-developed microstructure and morphology can constitute the most effective strategy to achieve superior performance. In this review, the efforts used to modulate materials, processing, and additives to deliver strong precursor fibers were thoroughly investigated. Our work demonstrates that the design of materials and processes is a fruitful pathway for the enhancement of the mechanical performance of carbon fibers.

Effect of stretching on the orientation structure and reaction behavior of PAN fiber during the thermal stabilization

The evolution of structures and properties of Polyacrylonitrile (PAN) precursor fiber during the thermal stabilization was directly related to the mechanical properties of carbon fiber. In the present study, the effects of stretching on the orientation structure and reaction behavior of PAN fibers at different thermal stabilization stages were experimentally investigated. The multi-dimensional structural characterization and mechanical properties tests were used, such as XRD, FT-IR, 13C-NMR, Raman and material testing machine, etc, which would help to reveal the evolution mechanism of the microstructure and reaction behavior of PAN fiber during the thermal stabilization. The results showed that in the process of thermal stabilization, stretching caused the rearrangement of crystalline orientation structure, promoted the cyclization reaction process at higher temperature (>200 °C), and furthermore promoted the formation of oriented aromatic like lamellar structure. It can be found that when the drawing ratio of 10% and the exposed temperature at 200 °C were applied to the PAN fiber, the increasing of crystalline orientation was obvious and more aromatic lamellar structures generated. Based on the above process technology, the carbon fiber with the higher graphitization degree and better mechanical properties was finally obtained.

Effect of Polyacrylonitrile Precursor Orientation on the Structures and Properties of Thermally Stabilized Carbon Fiber.

The thermal stabilization process of polyacrylonitrile (PAN) precursor fiber was the key step to prepare high-performance carbon fiber. During the thermal stabilization process, the aggregation structure and the reactivity of molecular chains have significant effects on the microstructures and mechanical properties of carbon fiber. In the present paper, the effects of the orientation structure of PAN precursor fiber on the thermal stabilization reaction and the mechanical properties of carbon fiber were experimentally studied. Using multi-dimensional structural and mechanical properties tests, such as XRD, DSC, 13C NMR and Instron machine testing, the crystalline and skeleton structure, exothermic behavior, and tensile properties of PAN precursor fiber with different orientations in the process of thermal stabilization were characterized to reveal the relationship between microstructure evolution and tensile properties. The results showed that the orientation structure of PAN precursor fiber had an obvious effect on the thermal stabilization process and the tensile stress–strain characteristic. When the heat treatment temperature was lower than 200 °C, the crystallinity and crystallite size of PAN fibers with higher orientation degrees increased significantly. After sufficient thermal stabilization, the original PAN precursor fiber with a higher orientation degree could form more aromatic lamellar structures and showed better regularity. Furthermore, the yield strength and initial modulus of the fibers with a higher orientation degree increased due to the formation of more aromatic rings. The maximum increase in the percentages of yield strength and tensile modulus of the PAN fibers were achieved when the heat-treated temperature was 200 °C, and the percentage values were 138.4% and 158.7% compared to the precursor without heat-treatment. In addition, the elongation at break of the fibers with a higher orientation degree was also relatively larger.

Microstructure and Mechanical Properties of Polyacrylonitrile Precursor Fiber with Dry and Wet Drawing Process.

Polyacrylonitrile (PAN) fibers are typically used as precursor fibers for carbon fiber production, produced through wet-spinning processes. The drawing process of the spun fiber can be classified into dry and wet drawing processes. It is known that the drawing stability and stretching ratio differ depending on the drawing process; however, the elementary characteristics are approximately similar. In this study, the mechanical properties of PAN fibers have been examined based on these two drawing processes with the differences analyzed through the analysis of microstructures. Further, to examine the composition of the fiber, element analysis has been conducted, and thereafter, the microstructure of the fiber is examined through X-ray diffraction analysis. Finally, the characteristics of PAN fibers and its mechanical properties has been examined according to each drawing condition. There are differences in moisture content and microstructure according to the drawing process, and it affects the tensile behavior. The results obtained could have potential implications if the processes are combined, as it could result in a design for a stable and highly efficient drawing process.

Evolution of polyacrylonitrile precursor fibers and the effect of stretch profile in wet spinning

AbstractWet spinning of polyacrylonitrile‐based polymers is a common technique to manufacture carbon fiber precursors. Understanding the role of stretch profile on structural evolution will support efforts to reduce cost and improve process robustness. Fiber stretch generally occurs in three sequential stages: jet stretch, wet stretch (first draw), and hot draw (second draw). In this study, total fiber stretch was kept constant, but distributed differently across the stretch stages yielding three different fiber variants. Samples were collected and analyzed after each stretch stage in order to correlate process parameters to structural information. For all variants, orientation of the ordered phase increases gradually for each stage of stretch while activation energy for the structural relaxation decreases. Alternatively, crystallite size increases substantially during hot draw, which is shown to have the most pronounced effect on cyclization behavior. Given the process conditions, the variant with the lowest jet stretch and highest hot draw demonstrates the highest tenacity and modulus along with the greatest orientation, crystallite size, and highest peak exotherm temperature.

Continuous, Pilot-Scale Production of Carbon Fiber from a Textile Grade PAN Polymer

Presented here is an investigation into the successful wet spinning and carbonization of a low-cost textile grade (TG) polyacrylonitrile (PAN) polymer blended with a more traditional carbon fiber (CF) grade PAN precursor. Both wet spinning and carbonization were performed using continuous pilot scale facilities directly analogous to larger scale industrial processes which provide validation for further scale-up. The blended PAN precursor fibers (TG-CF) showed consistently uniform cross-sections with the benchmark CF grade PAN precursor exhibiting mechanical properties similar to a commercially available precursor fiber. Despite a TG-PAN only precursor failing to adequately stabilize, the benchmark CF-PAN and the blended (TG-CF) precursor fibers and carbon fibers produced are high quality and exhibit uniform properties. The TG-CF carbon fiber containing 70wt% of TG-PAN has a tensile strength of 2.2 GPa, a tensile modulus of 210 GPa and a 1% elongation to failure. Although less than the CF-PAN carbon fiber, the nonetheless very good mechanical properties of TG-CF affirm that this approach has the potential to develop scalable low-cost carbon fiber suitable for a wide range of important applications.

Melt Spinning of Polyacrylonitrile (PAN) Fibers Based on Ionic Liquids

The production of high strength carbon fibers is an energy intensive process where a significant cost involves the wet or dry spinning of polyacrylonitrile (PAN) fiber precursors. Melt spinning PAN fibers would allow for significant reduction in production cost and production hazards. Ionic liquids (ILs) are an attractive fiber processing medium due to their negligible vapor pressure and low toxicity. In addition, they are carbon forming precursors; upon carbonization residual ILs can enhance carbon yield although primarily useful for plasticized melt spinning of PAN precursor fibers. In this presentation, the influence of the molecular structure of the ILs and the control of the plasticizing interactions with PAN during melt spinning will be discussed. The structure, thermal and mechanical properties of the melt spun PAN fibers are investigated by a combination of various characterization methods, such as differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X-Ray Diffraction (XRD) and mechanical testing. Our results demonstrate that ionic liquid structure and counter-anions influence the PAN fiber formation. More specifically, ILs containing bromide counter-anions produced PAN precursor fibers with increased mechanical properties compared to ILs containing chloride anions. We believe that our research can provide foundation to understand the influence of ILs on melt spinning of PAN fibers and gives the guidelines for the more cost/energy efficient production of PAN-based carbon fibers.

Large‐scale synthesis of hollow carbon fibers with ultra‐large diameter by thermally controlled pyrolysis

AbstractHollow carbon fibers (HCF) with ultra‐large diameter have been synthesized and the versatility to convert them into the corresponding carbon‐based composites has been demonstrated. The hollow carbon fibers were fabricated by thermal controlled carbonization of electrospun polyacrylonitrile fibers. For the existence of inorganic silica shell during pyrolysis, heat release will be blocked at the boundary, driving the polyacrylonitrile precursor fiber to form hollow structure. The diameter of the as‐prepared hollow carbon fibers can exceed 150 nm. Sol‐gel‐derived Fe3O4 nanoparticles can grow on the outer‐surface and the inner‐surface of hollow carbon fibers. The microwave absorption performance of ternary HCF@Fe3O4@PPy composite is testified and the values of reflection loss exceeding −10 dB can be obtained in the frequency of 3.3‐11.3 GHz. The large diameter of hollow carbon fibers can have inner and outer interfaces in the corresponding composites, which make them great potential for a variety of applications in future.

Углеродное волокно. Получение, модификация, свойства, области применения.

The main methods for producing a polyacrylonitrile precursor, methods for producing carbon fiber, its properties, and applications are presented. Patent research in the field of polyacrylonitrile precursor and carbon fiber. Technological problems in the subject area are identified, namely the development of technologies and equipment for producing high-strength carbon fiber, the development of technologies and equipment to reduce the cost of carbon fiber production, the development of technologies for improving the quality of carbon fiber-based composites, and the main ways to solve them are given. Ways to solve them are developing a technology for producing a polyacrylonitrile precursor for producing high-strength carbon fibers by the wet spinning method, developing a “dry-wet” method for producing polyacrylonitrile, developing high-performance equipment for producing technical polyacrylonitrile precursor in the form of bundles, developing technologies and equipment for efficient regeneration and utilization waste, heat and emissions from the production of carbon fibers, the development of new compositions of precursors and the transition to materials with a higher linear density, optimization of the structure of carbon fiber reinforced plastic to increase strength, the development of technologies and the creation of production of modern types of binders, including the addition of nanoparticles. The main methods for modifying the surface of a carbon fiber that are currently existing are considered.

Force field in coagulation bath at low temperature induced microfibril evolution within PAN nascent fiber and precursor fiber

AbstractThe coagulation process was vital for the microfibril evolution and mechanical properties of polyacrylonitrile (PAN) fibers. The PAN nascent fibers and precursor fibers were prepared by controlling drawing ratio of coagulation bath at low temperature during the dry‐jet wet spinning process. The microfibril morphological changes induced by force fields in coagulation bath were investigated by scanning electron microscopy and high‐resolution transmission electron microscopy. During the coagulation process, the spinning solution evolved into an interconnected network composed of random microfibrils and tie joints as a building block of the network. At low drawing ratio, the random interconnected network existed in nascent fibers. Increasing drawing ratio, the fiber filaments underwent shrinkage and the network was transformed into the transverse lamellae. Furthermore, the lamellar thickness also decreased. After the treatment of post‐spinning, similar transverse lamellae were formed in all of precursor fibers, and random microfibrillar network was stretched and oriented to develop into the aligned microfibrils in precursor fibers. The transverse fold‐chain crystal layers were densely stacked in the microfibrils. Increasing drawing ratio, the lamellae and microfibrils in precursor fibers were packed more densely and orderly. Consequently, the order and homogeneity of microfibrils in nascent fibers and precursor fibers were improved by increasing drawing ratio, which were benefit to increase fiber tensile strength and modulus.

Effect of the Ionic Liquid Structure on the Melt Processability of Polyacrylonitrile Fibers.

The production of high-strength carbon fibers is an energy-intensive process, where a significant cost involves the wet or dry-spinning of polyacrylonitrile (PAN) fiber precursors. Melt-spinning PAN fibers would allow for significant reduction in the production cost and production hazards. Ionic liquids (ILs) are an attractive fiber-processing medium because of their negligible vapor pressure and low toxicity. In addition, they are carbon-forming precursors; upon carbonization, residual ILs can enhance the carbon yield, although primarily useful for plasticized melt-spinning of PAN precursor fibers. In this research, we investigated the influence of the molecular structure of ILs and the control of plasticizing interactions with PAN during melt-spinning. The structural, thermal, and mechanical properties of the melt-spun PAN fibers were obtained by a combination of various characterization methods, such as differential scanning calorimetry, thermogravimetric analysis, Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray diffraction, and mechanical testing. These results demonstrated that the IL structure and counteranions influence the PAN fiber formation. More specifically, ILs containing bromide counteranions produced PAN precursor fibers with increased mechanical properties compared to ILs containing chloride anions. Our research can provide a foundation to understand the influence of ILs on melt-spinning of PAN fibers and provides us the guidelines for a higher cost-/energy-efficient production of PAN-based carbon fibers.

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

ABSTRACTThe 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.