Resulting Carbon Fibers Research Articles

Curing kinetics and mechanical properties of the epoxy vitrimer based on transesterification and its composite laminates

AbstractThe resin is a fundamental factor in determining the performance and usage conditions of its composites. The epoxy vitrimer, which combines both thermoplastic and thermosetting properties, shows significant potential in extending the service life of material. The curing process significantly affects the macroscopic properties of the vitrimer and the resulting carbon fiber reinforced plastics (CFRP). Effective control of curing parameters constitutes the critical factor for the assurance of product quality. In the study, a differential scanning calorimetry (DSC) was utilized to establish a curing kinetics model for a modified, self‐healable epoxy vitrimer based on transesterification. Then the molecular structure, thermodynamic, and mechanical properties of the cured vitrimer were analyzed. Next vitrimeric CFRP (vCFRP) laminates were fabricated, and the tensile and bending properties of the vCFRP laminates were investigated. The results showed the curing process was obtained: 95°C/1 h + 150°C/2 h + 170°C/2 h. The thermodynamic and mechanical properties of cured vitrimer could meet practical requirements. The minimal differences observed in the stress–strain curves of the specimens indicated that the curing process was appropriately configured. The prepared laminates exhibited performance comparable to that of commercial thermosetting composites.

Synthesis of spinnable isotropic pitch from low-softening-point coal tar pitch via Blanc bromomethylation-dehydrobromination towards carbon fibers with enhanced mechanical properties

Blanc bromomethylation-dehydrobromination is proposed to synthesize isotropic pitches with superior spinnability, utilizing refined coal tar pitch as the feedstock. This strategy involves introducing bromomethyl functional groups into pitch molecules through the Blanc bromomethylation reaction. In the subsequent process of dehydrobromination, methylene bridges generate between neighboring aromatic molecules. The pitch precursor prepared by this method not only displays an increased level of oligomerization and pitch yield, but also exhibits a more linear molecular structure compared with that produced through purely thermal polycondensation. The enhanced linear molecular configuration contributes to the improved spinnability of the pitch precursor and the oxidation reactivity of its derived fiber. Furthermore, the tensile strength of the resulting carbon fibers rises with an elevation in the amount of polyoxymethylene, and it exhibits the trend of an initial growth followed by a subsequent decrease as the stabilization temperature escalates. The carbon fibers obtained from the pitch prepared through thermal polycondensation present a low Young’s modulus of 24 GPa and a low tensile strength measuring of 456 MPa. In contrast, carbon fibers derived from the pitch synthesized via Blanc bromomethylation-dehydrobromination exhibit superior mechanical performance, offering Young’s modulus and tensile strength of 58 GPa and 1210 MPa, separately.

Kinetics on thermal soaking of mesophase pitch at processing temperature and its influence on the derived carbon structure

Mesophase pitch (MP) is an attractive precursor for tailoring high-performance carbon fibers due to its aromatic hydrocarbons mesogenic components that self-assemble into highly ordered microstructures. Relating processing conditions to the evolution of MP microstructure and its inheritance in carbon materials remains a challenge. This work quantitatively determined the kinetics of mesogenic aggregation in MP at spinning temperatures for the first time. Polarized optical microscopy tracked domain size changes in MP during 1–24 h thermal soaking. Analysis using an exponential growth model revealed nonlinear growth kinetics with an activation energy of just 39.3 kJ·mol−1, suggesting mesogenic aggregation is facile at processing temperatures compared to mesophase formation. Prolonged thermal soaking promoted microcrystalline development in derived carbon fibers, evident from increased crystallite stacking and reduced interlayer spacing. Relating microstructural evolution in the MP to the resulting carbon fiber structure provides new insights to tailor carbon materials for applications. This work revealed the kinetics of MP at its processing temperature, provides a valuable insight into the future optimization of processing conditions for MP based carbon materials.

Enhancing spinnability and properties of carbon fibers through modification of isotropic coal tar pitch precursor

Producing general-purpose carbon fibers (GPCFs) with excellent properties using an economical raw material such as coal tar pitch (CTP) holds significant importance. To synthesize carbon fibers, the spinnability of precursor material plays an important role. However, the CTP impedes flowability during the spinning process due to possessing high molecular weight complex aromatic structures. To address this limitation, CTP was blended with varying proportions of petroleum pitch (5, 10 and 20 %) and spinnability and flow behavior were analyzed using Rheology measurements. The distinct constitutional properties of petroleum pitch also contributed to alterations in both the stabilization and carbonization processes. Carbon fibers produced from 10PP showed the highest tensile strength and modulus of ∼784 MPa and ∼47 GPa, respectively. The impact of oxidative and carbonization treatments on the resultant microstructural and mechanical properties of carbon fibers was thoroughly examined using various techniques such as FTIR, XPS, XRD, SEM, Raman and UTM. The results showed that the composition of petroleum pitch influenced the stabilization of fibers which eventually affected the carbonization process and resultant properties of carbon fibers. This study offers an understanding of the connection between the precursor's composition, stabilization conditions, and mechanical properties of the resultant carbon fibers.

Non-Coagulant Spinning of High-Strength Fibers from Homopolymer Polyacrylonitrile Synthesized via Anionic Polymerisation.

The rheological properties, spinnability, and thermal-oxidative stabilization of high-molecular-weight linear polyacrylonitrile (PAN) homopolymers (molecular weights Mη = 90-500 kg/mol), synthesized via a novel metal-free anionic polymerization method, were investigated to reduce coagulant use, enable solvent recycling, and increase the carbon yield of the resulting carbon fibers. This approach enabled the application of the mechanotropic (non-coagulating) spinning method for homopolymer PAN solutions in a wide range of molecular weights and demonstrated the possibility of achieving a high degree of fiber orientation and reasonable mechanical properties. Rheological analysis revealed a significant increase in solution elasticity (G') with increasing molecular weight, facilitating the choice of optimal deformation rates for effective chain stretching prior to strain-induced phase separation during the eco-friendly spinning of concentrated solutions without using coagulation baths. The possibility of collecting ~80 wt% of the solvent at the first stage of spinning from the as-spun fibers was shown. Transparent, defect-free fibers with a tensile strength of up to 800 MPa and elongation at break of about 20% were spun. Thermal treatment up to 1500 °C yielded carbon fibers with a carbon residue of ~50 wt%, in contrast to ~35 wt% for industrial radically polymerized PAN carbonized under the same conditions.

A novel strategy for the design of cooperatively reinforced, shape-memory, recyclable ESO-based carbon fiber composites

To improve the poor performance of epoxidized soybean oil (ESO) thermosetting materials in preparing high-performance carbon fiber composites, we synthesized novel bio-based epoxy monomers using naringin (EN) as an epoxy monomer. EN's hydrogen bonding and benzopyrenone structure enhanced the crosslinking density, rigidity, bond shear strength, glass transition temperature (Tg), tensile and lap shear strength, and shape memory ability of ESO/MeHHPA/EN. The resulting carbon fiber composites had high tensile strength (349 MPa) and could be fully recovered using ethanolamine, with the recovered fibers maintaining almost 100% of the mechanical properties of the original sample. This study provides a new approach for fabricating multifunctional, high-performance, and recyclable ESO-based carbon fiber reinforced composites.

In-situ monitoring fiber orientation evolution during stabilization process by two-dimensional X-ray diffraction technology

The tensile performance of fibers is highly dependent on their structural orientation. In this study, a two-dimensional X-ray diffraction spectrometer equipped with a precisely stretching hot stage was used to in-situ monitor the structural changes, especially for orientation, of PAN fibers during stabilization. A correlation of structural orientation between precursor, stabilized and carbonized fibers are proposed. Our results demonstrate that applied tension is a crucial factor in achieving a well-oriented structure during the complex chemical conversions that occur during stabilization. The oriented stabilized structures are then inherited by the resulting carbon fibers. The tensile properties of the resultant carbon fibers suggest that a higher modulus could be achieved at a higher structural orientation. Furthermore, by implementing a temperature gradient during stabilization, the oxidized fibers can be subjected to increasing tension, resulting in a better orientation structure. The roles of temperature and tension during stabilization were further analyzed by a design of orthogonal experiment (DOE), and it was found that tension is critical to achieve better orientation, while temperature has little effect. Overall, this technique provides efficient tool and method to systematically reveal the effects of processing conditions, and build the structure and performance correlations between precursor fiber and the resultant carbon fiber.

Influence of Oxygen Uptake on Pitch Carbon Fiber.

Carbon fiber precursor materials, such as polyacrylonitrile, pitch, and cellulose/rayon, require thermal stabilization to maintain structural integrity during conversion into carbon fiber. Thermal stabilization mitigates undesirable decomposition and liquification of the fibers during the carbonization process. Generally, the thermal stabilization of mesophase pitch consists of the attachment of oxygen-containing functional groups onto the polymeric structure. In this study, the oxidation of mesophase pitch precursor fibers at various weight percentage increases (1, 3.5, 5, 7.5 wt%) and temperatures (260, 280, 290 °C) using in situ differential scanning calorimetry and thermogravimetric analysis isinvestigated. The results areanalyzed to determine the effect of temperature and weight percentage increase on the stabilization process of the fibers, and the fibers aresubsequently carbonized and tested for tensile mechanical performance. Thefindings provide insight into the relationship between stabilization conditions, fiber microstructure, and mechanical properties of the resulting carbon fibers.

Study of mesophase pitch based carbon fibers: Structural changes as a function of anisotropic content

This study investigates the synthesis of mesophase pitch (MP) with varying anisotropic (mesophase) content of 19–74% by thermal soaking of isotropic coal tar pitch for different time intervals at 410 °C without using any catalyst/additive and pre-treatment processing. These mesophase pitches were converted into continuous pitch fibers by melt spinning process, then stabilized in air and carbonized at 1000 °C to obtain the carbon fibers. The anisotropic (mesophase) content increases with increasing soaking time period from 11 to 15 h and as a result, alter the various microstructural properties of carbon fibers. Physio-chemical and microstructural properties of prepared MP were systematically studied with TGA, FTIR, Raman spectroscopy, XRD and 1H‐NMR. The overall result suggested enhancement in aromaticity and naphthenic hydrogen with increased anisotropy. Rheological studies were carried out to understand the spinning behavior of MPs. Furthermore, transformation of mesophase pitch into carbon fibers at 1000 °C, showed a decline in the microcrystal properties due to the evolution of gases which create disorderness in the lattice structure. It was observed that the increased anisotropic content improves the microstructure of the mesophase pitch and their resultant carbon fibers as the microstructure attained during mesophase formation is retained during its carbonization.

Evolution of the Composition and Melting Behavior of Spinnable Pitch during Incubation.

The physical and chemical properties of spinnable pitch showed a huge impact on the performance of resultant pitch carbon fiber even if its physical and chemical properties were slightly changed. Various polycyclic aromatic compounds and abundant free radicals existed in spinnable pitch, and there are many interactions among molecules and free radicals. The molecular structure and composition of spinnable pitch were investigated during incubation, and the effect of molecular evaluation on rheological properties of spinnable pitch was illustrated using various characterization methods in this work. It indicated that n-hexane soluble fraction mainly occurred condensation or cleavage, and a small number of heavy components were generated after a long period. The fraction of n-hexane insoluble/toluene soluble underwent molecular condensation and cross-linking in the presence of oxygen-containing radicals and aromatic hydrocarbon radicals, while toluene insoluble/tetrahydrofuran soluble fraction tended to change in large molecules of polycyclic aromatic hydrocarbons. Lastly, tetrahydrofuran insoluble fraction was condensed due to its high aromaticity during the incubation process, and the content of aromatic carbon increased. These changes of composition and structure of spinnable pitch led to its softening point, increase in viscosity and flow activation energy, and deterioration of the rheological property.

Highly electrically conductive biomass-derived carbon fibers for permanent carbon sequestration

Carbon fiber derived from biomass is an excellent method for permanently sequestering carbon from the atmosphere because of the chemical inertness of carbon fiber and its versatile applications. However, many biomass-derived carbon fibers fall short in producing high electrical conductivities. Here, we demonstrate bamboo cellulose derived carbon with high electrical conductivity that can permanently sequester 0.85 t of carbon/ha of bamboo/year. To fabricate carbon fibers, bamboo cellulose macrofibers were extracted from bamboo via performic acid treatment and air-drying, followed by stabilization and carbonization to create conductive carbon fibers. The carbon fibers were sequentially Joule-heated to form graphitized carbon fiber at ∼2000 °C for 20–30 s. The resulting carbon fibers demonstrated very high alignment of nanofibrils with high electrical conductivity of 25,300 ± 6270 S/m, which is one of the higher values among reported biomass-derived carbon fibers. Furthermore, a hectare of bamboo can capture 17 t of carbon per year, and the obtained carbon fiber can permanently store 0.85 t of the captured carbon. Additionally, lignin from the delignification process could be processed into valuable products such as battery anodes, wound dressings, supercapacitors, and biobased epoxies. Compared to existing biomass-derived products, the fabrication method presented here is inexpensive and produces fibers with very high electrical conductivity, suggesting fibers could be used in low-cost, high quantity applications such as concrete or bridge sensors requiring electrical conductivity and thermoplastic or resin impregnation.

Effects of ambient pressure on structural characteristics of polyacrylonitrile pre‐oxidized fibers

AbstractDuring the pre‐oxidation process, skin‐core structure is easily formed in the fibers due to the competitive mechanism of oxygen capture and oxygen diffusion, which would degrade the tensile properties of the resulting carbon fibers. To prepare high‐performance carbon fibers, it is vital to optimize the radial structure of polyacrylonitrile (PAN) pre‐oxidized fibers. Methods commonly used to solve the skin‐core structure would result in low productivity and comprehensive performance. Here, we examine the effects of pressure on the chemical structure and the evolution of radial structure in PAN pre‐oxidized fibers under 225°C in the air, by using 13C Solid‐state NMR analysis (13C‐NMR), Optical density test (OD), and Thermogravimetric Analysis (TG). It was found out that under the positive pressure of 30 kPa, the reaction area expands to the core of the fibers with the shell thickness increasing from 88.11% under normal pressure to 98.67%, and the radial structure homogeneity is effectively improved. As the pressure increases above 22 kPa, oxygen diffusion is accelerated, allowing for breaking the barrier of the fiber surface capture and promoting the oxygen catalytic cyclization reaction in the core. Consequently, PAN pre‐oxidized fibers prepared under positive pressure have a more uniform radial structures and higher thermal stability. The carbonization yield at 800°C of the fibers prepared at 30 kPa is 13.10% higher than that of the fibers prepared at atmospheric pressure.

Evaluation of Keratin-Cellulose Blend Fibers as Precursors for Carbon Fibers.

One main challenge to utilize cellulose-based fibers as the precursor for carbon fibers is their inherently low carbon yield. This study aims to evaluate the use of keratin in chicken feathers, a byproduct of the poultry industry generated in large quantities, as a natural charring agent to improve the yield of cellulose-derived carbon fibers. Keratin–cellulose composite fibers are prepared through direct dissolution of the pulp and feather keratin in the ionic liquid 1,5-diazabicyclo[4.3.0]non-5-enium acetate ([DBNH]OAc) and subsequent dry jet wet spinning (so-called Ioncell process). Thermogravimetric analysis reveals that there is an increase in the carbon yield by ∼53 wt % with 30 wt % keratin incorporation. This increase is comparable to the one observed for lignin–cellulose composite fibers, in which lignin acts as a carbon booster due to its higher carbon content. Keratin, however, reduces the mechanical properties of cellulose precursor fibers to a lesser extent than lignin. Keratin introduces nitrogen and induces the formation of pores in the precursor fibers and the resulting carbon fibers. Carbon materials derived from the keratin–cellulose composite fiber show potential for applications where nitrogen doping and pores or voids in the carbon are desirable, for example, for low-cost bio-based carbons for energy harvest or storage.

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

Morphological influence of graphitic carbon nanofibers by N–F dual-doping on Pt electrocatalytic activity and stability for oxygen reduction reaction in polymer electrolyte membrane fuel cells

Morphology of carbon nanofibers significantly effects Pt nanoparticles dispersion and specific interaction with the support, which is an important aspect in the fuel cell performance of the electrocatalysts. This study emphasizes, the defects creation and structural evolution comprised due to N–F co-doping on graphitic carbon nanofibers (GNFs) of different morphologies, viz. GNF-linearly aligned platelets (L), antlers (A), herringbone (H), and their specific interaction with Pt nanoparticle in enhancing the oxygen reduction reaction (ORR). GNFs–NF–Pt catalysts exhibit better ORR electrocatalytic activity, superior durability that is solely ascribed to the morphological evolution and the doped N–F heteroatoms, prompting the charge density variations in the resultant carbon fiber matrices. Amongst, H–NF–Pt catalyst performed outstanding ORR activity with exceptional electrochemical stability, which shows only 20 mV loss in the half-wave potential whilst 100 mV loss for Pt/C catalyst on 20,000 potential cycling. The PEMFC comprising H–NF–Pt as cathode catalyst with minimum loading of 0.10 mg cm−2, delivers power density of 0.942 W cm−2 at current density of 2.50 A cm−2 without backpressures in H2–O2 feeds. The H–NF–Pt catalyst owing to its hierarchical architectures, performs well in PEMFC at the minimized catalyst loading with outstanding stability that can significantly decrease total price for the fuel cell.

Laser surface treatment of carbon fiber reinforced polymer using near-infrared laser wavelength with variated process parameters

In this study, the surface pretreatment of carbon fiber reinforced polymer with near-infrared laser radiation is investigated. Several primary and computable secondary laser parameters were varied to identify their influences and interactions. The generated surface structures were examined qualitatively and quantitatively using optical and electronic microscopes. To quantify the ablation behavior, depth measurements of the exposed and covered carbon fibers were performed on cross sections. A correlation analysis was applied to determine process parameters, which have an influence on the matrix and carbon fiber ablation. With regard to the detachment and removal of the matrix, it was shown that with a pulse overlap of 50% and a single pulse energy of 195–344 μJ or a laser energy density of 3–6 J/cm2, 70%–85% of the carbon fibers could be exposed with minimal damage or ablation of carbon fibers at the same time. With 90% pulse coverage, more matrix residues remain on the surface due to the melting of polyamide particles embedded in the matrix. These retain the detached matrix on the surface. To ablate these, more laser energy is necessary, which leads to a higher fiber damage and ablation. However, a completely damage-free processing could not be realized. It is also shown that the matrix layer thickness, the local carbon fiber density, and the resulting carbon fiber topography have an influence on the local matrix removal. Furthermore, the damage caused by processing, such as fiber breaks, cratering, and others, was recorded and described.

Screening of spinning oils for melt‐spun lignin‐based carbon fiber precursors

AbstractFor the improvement of the processing of melt‐spun lignin‐based fibers, the use of a spinning oil is necessary. The use of spinning oils is standard for the processing of PAN based carbon fiber precursors. Since no commercial spinning preparations for lignin fibers are available on the market, a study on different oils, such as technical oils, food‐grade oils or spinning avivages, is performed. A stepwise approach with applied tests (viscosity measurement, fiber etching, melt‐spinning, stabilization, carbonization) was developed to eliminate oils which are not suitable. The physical properties of the tested oils are determined and spectroscopic studies of these avivages are performed. Mechanical properties are characterized for as‐spun lignin fibers, stabilized lignin‐based fibers and lignin‐based carbon fibers with and without a spinning preparation. Four of seven tested spinning oils improve the handling during the processing of lignin‐based fibers but only two spinning oils lead to good quality carbon fibers and improve even the tensile strength of the resulting carbon fibers.

New insights into the radial structural differences of polyacrylonitrile fibres during thermal stabilization by the synchronous processing adjustment of time and temperature.

In this study, the synchronous effects of time and temperature on the radial structural differences of polyacrylonitrile (PAN) fibres during thermal stabilization were investigated. For each sample to achieve equal densities (∼1.36 g cm−3), PAN fibres were thermally stabilized for various times between 8–32 min and at corresponding temperatures of 279–252 °C, which was considered to give a synchronous processing adjustment as a time–temperature integral (TTI). Besides, a previously developed mathematic model was utilized to quantitatively evaluate the differences in the radial heterogeneous structures of the stabilized PAN fibres as a function of TTI. It was found that several structural parameters (e.g., the stabilization degrees, the present crystallinities, and the orientation degrees) of PAN chains in the skin regions that mainly determine the fibres' overall performances were dramatically different from those in the core regions. Meanwhile, based on the TTI model, these skin–structure parameters demonstrated a strong correlation with the tensile properties of the resultant carbon fibres. However, while the stabilized PAN fibres had equal densities, their structural parameters, as well as the properties of the resultant carbon fibres, were obviously different.

Study on the structural evolution of polyacrylonitrile fibers in stepwise heat treatment process and its relationship with properties

AbstractThe structural evolution of polyacrylonitrile fibers in stepwise heat treatment process and its relationship with properties were investigated systematically. Results demonstrated that the cyclization degree and dehydrogenation index of fibers reached 84.13% and 1.361, respectively after stabilization at 265°C. The oxygen reaction was enhanced after stabilized at 245°C. Refinement of the microcrystalline structure took effect during stabilization and low temperature carbonization, while growth of microcrystalline dominated during high temperature carbonization. The results of surface topography study not only provided the evolution characteristics of surface microstructure under the influence of chemical reaction, but also further complements the internal aggregation structural rearrangement process from the side. Additionally, the temperature zone from 245 to 265°C was the key stage of the formation of the skin‐core structure, which would be transmitted to the resultant carbon fiber. Tensile strength and tensile modulus decreased during stabilization stage, reaching 4.273 GPa and 221.132 GPa, respectively after high temperature carbonization. The elongation at break which first increased and then decreased proved a toughness failure to brittleness failure. The investigation elucidated the relationship between the mechanical property with chemical and physical microstructure. And the possible rearrangement process of the aggregation structure was proposed.

Fabrication of carbon fibers from the cupric ion impregnated and thermally stabilized poly(hexamethylene adipamide) precursor

The conversion of poly (hexamethylene adipamide) or polyamide 66 precursor fiber to carbon fibers was accomplished through thermal stabilization and carbonization processes. Thermal stabilization was conducted of cupric chloride (CuCl2)–ethanol-impregnated polyamide 66 (PA66) fibers in the air. To determine the influence of heating rate on the fiber structure and properties of the resultant carbon fibers, carbonization experiments were performed at selected temperatures of 500, 700, 900, and 1100°C using 2.5 and 5 °C/min heating rates with no dwelling. The results conclusively revealed that the volume density and tensile properties of the PA66 fiber were higher at 2.5 °C/min heating rate. After fixing the heating rate as 2.5°C/min, further carbonization experiments were conducted at temperatures from 500 to 1100°C, using increments of 100°C with no dwelling time. Linear density, volume density, fiber diameter, carbon yield, elemental composition, tensile, and electrical properties exhibited a strong dependence on the carbonization temperature. After taking into account the effects of structural defects (i.e., microvoids), tensile strength, and tensile modulus of the carbon fibers increased to 794 MPa and 92.4 GPa, respectively, when carbonized at 1100°C. X-ray diffraction analysis of the carbon fibers further revealed the existence of a greatly disordered (i.e., amorphous) structure, which developed during the carbonization process. FT-IR analysis confirmed the formation of highly aromatic carbon clusters at temperatures of 500°C and higher. The outcomes of electrical conductivity in this study confirm that the PA66 precursor was converted into a semi-conducting state once it was carbonized.