High Performance Carbon Fiber Research Articles

Correlation between fibril structures and mechanical properties of polyacrylonitrile fibers during the dry‐jet wet spinning process

ABSTRACTThe high quality polyacrylonitrile (PAN) fibers, which were used to manufacture the high performance carbon fibers, were prepared by the dry‐jet wet spinning process. The fiber samples were treated by using the ultrathin sectioning and solution etching methods. Subsequently, the changes of the fibril structures and mechanical properties were investigated by utilizing scanning electron microscopy, high‐resolution transmission electron microscopy and mechanical tester. The fiber titer and diameters, the amount of the pores decreased during the spinning process. The microfibrils were oriented along the fiber axis to form the fiber backbone in PAN fibers. Furthermore, the microfibrils with small size were incorporated into the large fibrils and some fold chains formed the interfibril transverse joint bridges to reinforce the connection of the fibril elements. The fibrils served as self‐reinforcing element to efficiently resist the cracks propagation and dissipate much energy to increase the tensile strength and initial modulus. Consequently, the PAN precursor fibers with the homogeneous, compact, well‐oriented, and closely interlinked fibrils, presented high tensile strength and initial modulus. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47336.

Characterization and mechanical properties of Cf/ZrB2-SiC composites fabricated by a hybrid technique based on slurry impregnation, polymer infiltration and pyrolysis and low-temperature hot pressing

The continuous carbon fiber reinforced ZrB2-SiC composite was fabricated successfully via a hybrid technique based on nano ceramic slurry impregnation, polymer infiltration and pyrolysis and low-temperature hot pressing. The Cf/ZrB2-SiC composites exhibited non-brittle fracture modes and the chemical interaction at the fiber/matrix interfaces was effectively inhibited owing to the low sintering temperature. The S2-Cf/ZrB2-SiC composite presented the highest mechanical properties with fracture toughness of 4.47 ± 0.15 MPa m1/2 and the work of fracture of 877 J/m2, which was attributed to the multiple length-scale toughening mechanisms including the macroscopic toughening mechanisms of crack deflection and crack branching, the micro toughening mechanisms of fiber bridging and fiber pull-out. This work presented a novel and effective method to fabricate high-performance continuous carbon fiber reinforced ceramic matrix composites.

Thermal Analysis on the Stabilization Behavior of Ternary Copolymers Based on Acrylonitrile, Methyl Acrylate and Itaconic Acid

Polyacrylonitrile (PAN)-based copolymers are widely used as a precursor for manufacturing high performance carbon fibers via a series of processes of thermal stabilization, carbonization, and graphitization. We have recently synthesized a series of copolymers with various compositions of acrylonitrile (AN), methyl acrylate (MA) and itaconic acid (IA) by using an efficient aqueous suspension polymerization. In this study, the influences of MA and IA units on thermal stabilization behavior of AN/MA/IA-based terpolymers has been investigated by thermal analyses using DSC and TGA. It was found that the glass transition temperatures (Tg) of AN/MA/IA-based terpolymers with a constant AN content increased with the IA content due to a specific interaction between carboxylic acid and nitrile groups, while the MA unit played a role of lowering Tg of the copolymers owing to the interruption of AN sequence with a strong dipole-dipole interaction. The exothermic peaks of DSC curves as well as the weight loss of TGA/DTG curves under air condition revealed that the IA unit in AN/MA/IA-based terpolymers contributed to accelerate the oxidation reaction especially under air condition and also to slow down the following cyclization and dehydrogenation reactions including isomerization, unlike PAN homopolymer and AN/MA-based bipolymers. On the other hand, the MA unit in AN/MA-based bipolymers and AN/MA/IA-based terpolymers served as a delaying agent on the overall thermal stabilization reactions of oxidation, cyclization and dehydrogenation.

Recovery and characterisation of carbon fibres from waste CFRP using supercritical n-butanol

ABSTRACTThe high-performance carbon fibres can be recycled from waste carbon fibre/epoxy resin composites by supercritical n-butanol. Recycling experiment designed by response surface method was used to investigate quantitative relation between degradation rate of epoxy resin and process parameters. Thus, the optimum process parameters could be obtained, and effects of process parameters and layers on degradation rate and mechanical performance of the recycled carbon fibres were analysed. The tensile strength of the recycled carbon fibre under the optimum process parameters was 94.53% of that of the original carbon fibre, tensile modulus was 93.57% of that of the original carbon fibre and interfacial shear strength was 90% of that of the original carbon fibre.

Highly isotactic (>60%) polyacrylonitrile-based carbon fiber: Precursor synthesis, fiber spinning, stabilization and carbonization

High performance carbon fibers are mostly produced from polyacrylonitrile (PAN). Its chemical structures strongly affect the further steps of carbon fiber manufacturing. In this work, highly isotactic (>60%) PAN was synthesized by template-assisted solid phase polymerization in the presence of magnesium chloride. We systematically studied the effect of isotacticity of PAN on its solution rheology, fiber spinning, thermal stabilization and carbonization. Highly isotactic PAN-based carbon fibers were first produced in this work. The higher isotacticity of PAN was observed to affect the storage stability of PAN solutions, coagulation processes, the structures of PAN fibers, stabilization reactions and carbonization yields. The improvement of PAN isotacticity benefits its thermal stabilization, but also introduces many problems during carbon fiber manufacturing which need to be carefully considered.

Numerical simulation of stress distribution for CF/EP composites in high temperatures

The high-performance carbon fiber materials can be obtained by decomposing carbon fiber reinforced resin matrix composites using thermally activated oxide semiconductors. This paper established the representative volume element (RVE) of carbon fiber/epoxy resin composites, and investigated the structure destruction of composites in the recycling process based on analysis of the stress and strain distribution by the thermomechanical coupling module of Digimat. The results indicated that the maximum thermal stress of the epoxy resin appeared in the poor resin region, while the minimum appeared in the resin-rich region; the stress of the carbon fibers in poor resin region was greater than that in the resin-rich region; the maximum stress of composites appeared in the interface layers when the temperature ranged from 350 to 500 °C, and the maximum thermal stress was 196.9–281.3 MPa, as well as the maximum shear stress was 98.2–140.3 MPa; the maximum peeling stress perpendicular to the fiber directions was 53.7–157.3 MPa; the strain of the interface layers and carbon fibers were the smaller than that of the resin matrix, whose maximum strain ranged from 0.0622 to 0.0889. The structure destruction of the composites was caused by both the peeling stress and the interfacial shear stress in recycling.

Comparison of different surface treatments of carbon fibers used as reinforcements in epoxy composites: Interfacial strength measurements by in-situ scanning electron microscope tensile tests

In-situ characterization of the fiber/matrix interfacial failure behavior at microscopic scale is important to optimize the fiber surface treatment and to design high performance composites. In this study, in-situ tensile tests in scanning electron microscope (SEM) were used to investigate the interfacial adhesion strength of epoxy composites reinforced by four kinds of carbon fibers (CF)—raw CF, desized CF, carbon nanotube-grafted CF (CNT-CF) and oxidized CNT-CF. The crack initiation position and fracture failure mode were well recorded. The strains and the interfacial adhesion strength were obtained for these four kinds of composites. It was found that the interfacial strength decreased from 53 MPa to 48 MPa after removing the sizing on carbon fiber surface. However, by grafting CNTs on the CF surface, the interfacial strength reached 55 MPa and was further increased to 58 MPa after a simple thermal oxidation treatment. Moreover, energy dispersion X-ray analysis (EDX) was carried out using scanning transmission electron microscopy (STEM). The EDX mapping demonstrated that oxygen aggregated at the interfaces of raw CF/epoxy and oxidized CNT-CF/epoxy. Thus, a combination of CNT grafting with chemical functionalization should be necessary to achieve high performance carbon fiber reinforced polymer composites.

Layer-by-layer assembled flame-retardant architecture toward high-performance carbon fiber composite

Fire safety is a permanent challenge in carbon fiber (CF) epoxy resin (EP) composites, which largely restrict their application especially in high-performance fields with flame-retardant demands. Herein, an effective flame-retardant architecture consisting of branched polyethyleneimine (PEI) and ammonium polyphosphate (APP) was successfully fabricated on CFs surface through the facile layer-by-layer (LbL) assembly to achieve a bilayer polyelectrolytes coated CF (BL@CF), which did not only endow excellent flame retardance to the relevant composites (EP/(BL@CF)) with no additional flame retardants, but also maintained desirable mechanical properties. The as-prepared BL@CF were characterized by SEM-EDX, XPS and XRD, indicating that PEI/APP coating was successfully deposited on CFs surface with no damage to the structure of CF. The phosphorus-containing PEI/APP coating imparted high fire safety to the composite, exhibiting a high limiting oxygen index (LOI) value of 41.0%, V-0 rating in UL-94 V test with only 6 deposited bilayers on CFs, and the decrease of peak-heat release rate (PHRR) and total heat release (THR) by 33.5% and 22.5%, respectively, obtained from cone calorimetry. Moreover, thanks to the increased surface roughness and chemical bonds between BL@CF and EP matrix, satisfactory mechanical properties and glass transition temperature of the composite were obtained. Taking these advantages, this facile manufacturing method from layer-by-layer assembly toward superior flame retardance and favorable mechanical properties can bring more chances for advanced composites.

Study on the interfacial properties of the dual-activity silicone resin/carbon fibers composites

Dual-activity silicone resins (DASR) with double bonds and epoxy groups were prepared via the hydrolysis and condensation of γ-glycidoxypropyltrimethoxysilane (γ-GPS) and γ-methacryloxypropyltrimethoxysilane (γ-MPS). The hydrolysis and condensation degree was monitored during the preparation process of DASR. The epoxy groups of DASR were directly reacted with amine groups on the fiber surface attempting to improve the interfacial properties of carbon fiber (CF) composites. The CF/DASR composites represented 36.91% and 20.85% enhancement in interfacial shear strength (IFSS), compared to those of MASR composites reinforced with untreated CF and amine modified CF. In addition, the tensile strength of CF/DASR remained stable after the UV irradiation. Thus, these attractive results demonstrate that the designed dual-activity silicone resin/carbon fiber composites provide a promising approach for preparing high-performance carbon fibers composites.

Research on the multi-scale microstructure of polyacrylonitrile precursors prepared by a dry-jet wet spinning process

An understanding of the properties of polyacrylonitrile (PAN) precursors is an essential precondition for manufacturing high-performance carbon fibres, and the structure of the precursors has a direct and profound effect on the performance of carbon fibres. In this study, PAN precursors, formed in a multistage coagulation bath, were spun by a dry-jet wet spinning process, and the multi-scale microstructure and morphology of the precursors were investigated by separating the fibrils from the precursors. Scanning electron microscopy and high-resolution transmission electron microscopy were employed to examine the surface morphology, cross-sectional morphology and microstructure of the precursors. X-ray diffraction was used to characterize the crystal structure. The micropore sizes of the precursors were determined with nitrogen adsorption experiments; the adsorption increased after ultrasonic etching and decreased with an increase in the treated concentration. All the results demonstrated that the PAN precursors had a multi-scale microstructure, the precursors consisted of fibrils with diameters of 80–200 nm and the fibrils consisted of some microfibrils with diameters of 20–40 nm, including the periodic tissues with thicknesses of 16–30 nm perpendicular to the fibre axis.

Investigation into the Gelation of Polyacrylonitrile Solution Induced by Dry-jet in Spinning Process and Its Effects on Diffusional Process in Coagulation and Structural Properties of Carbon Fibers

The jet effect in dry-jet wet spinning of a polyacrylonitrile (PAN) solution was investigated. The two parameters, jet-stretch ratio and air gap length, of the jet were controlled to elucidate each effect on PAN precursors and resulting carbon fibers. Excessively high jet-stretch ratio (>4) or air-gap (>1 cm) resulted in the development of the internal pore structure in PAN precursors. The pores remained even after the densification by thermal treatment acting as defects for poor tensile properties of carbon fibers (CFs). It was revealed that two parameters critically controlled the bidirectional diffusion of both solvent and non-solvent by determining the degree of the surface gelation at the jet. Excessively high jet-stretch ratio or high air-gap length created a thick solid skin on extruded dope limiting solvent/non-solvent diffusion. As a method to limit the development of the pores under the condition of high jet stretch ratio (>4), raising the dope temperature for limiting the degree of gelation at the jet was attempted and successfully manufactured mechanically improved fiber with a dense structure without pores under high jet-stretch condition. The study suggests that the high quality PAN precursors for high performance CFs can be manufactured under high jet-stretch ratio condition with proper management on gelation at the jet.

Effect of KH550 on the Preparation and Compatibility of Carbon Fibers Reinforced Silicone Rubber Composites

Silicone rubber has good heat resistance, but its mechanical properties are poor. To improve the mechanical properties of the silicone rubber, the carbon fiber / silicone rubber composites were prepared in this paper, in which the silicone rubber was used as the matrix, the high-performance carbon fiber (CF) treated with coupling agent is used as the reinforcement. The best composite formulation, the types and content of coupling agent are determined by testing mechanical properties of the composites. The morphology structures of the composites were observed by scanning electron microscope (SEM), the compatibility between carbon fiber and silicone rubber was studied by infrared spectrum analysis (IR), dynamic thermal mechanical analysis (DMA) and X-ray photoelectron spectroscopy (XPS). The results show that the best composite formulation is silicone rubber of 100 phr, carbon fiber of 12 phr, coupling agent KH550 of 2.5 phr. The best first curing conditions are at 175 ∘C under 10 MPa for 30 min, the postcuring conditions are at 200 ∘C for 2h. The compatibility between carbon fiber treated with KH550 and silicon rubber is the best by SEM, IR, DMA and XPS analysis, and confirm that the coupling agent KH550 plays a compatilizer role in the preparation process of the carbon fiber/silicone rubber composites.

Single-step process to improve the mechanical properties of carbon nanotube yarn.

Carbon nanotube (CNT) yarns exhibit low tensile strength compared to conventional high-performance carbon fibers due to the facile sliding of CNTs past one another. Electron beam (e-beam) irradiation was employed for in a single-step surface modification of CNTs to improve the mechanical properties of this material. To this end, CNT yarns were simultaneously functionalized and crosslinked using acrylic acid (AA) and acrylonitrile (AN) in an e-beam irradiation process. The chemical modification of CNT yarns was confirmed by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and scanning electron microscopy (SEM). The best improvement in mechanical properties was achieved on a sample treated with an aqueous solution of AA and subsequent irradiation. CNT yarn treatment with AA enhanced the strength (444.5 ± 68.4 MPa) by more than 75% and the modulus (21.5 ± 0.6 GPa) by more than 144% as compared to untreated CNT yarn (strength 251 ± 26.5 MPa and modulus 8.8 ± 1.2 GPa).

Double resonance calibration of g factor standards: Carbon fibers as a high precision standard

The g factor of paramagnetic defects in commercial high performance carbon fibers was determined by a double resonance experiment based on the Overhauser shift due to hyperfine coupled protons. Our carbon fibers exhibit a single, narrow and perfectly Lorentzian shaped ESR line and a g factor slightly higher than gfree with g=2.002644=gfree·(1+162ppm) with a relative uncertainty of 15ppm. This precisely known g factor and their inertness qualify them as a high precision g factor standard for general purposes. The double resonance experiment for calibration is applicable to other potential standards with a hyperfine interaction averaged by a process with very short correlation time.

A revolution in low-cost carbon fiber production

LeMond Composites’ patent pending manufacturing process will produce low-cost, high performance carbon fiber for a range of industries, from transportation to renewable energy.

Effect of acrylonitrile sequence distribution on the thermal stabilization reactions and carbon yields of poly(acrylonitrile-co-methyl acrylate)

Polyacrylonitrile (PAN) copolymers are important precursors for making high performance carbon fibers. In this study, a type of inert comonomer, methyl acrylate (MA), was copolymerized with AN, and the effects of AN sequence distribution on the cyclization, oxidation and carbon yield were systematically studied. The sequence distribution of P(AN-co-MA) could be adjusted by altering MA/AN feeding ratio during radical polymerization. The number-average sequence length of AN in the copolymers decreases quickly along with the increase of MA content in PAN. The stabilization and carbonization weight losses were determined by thermogravimetric analysis, and the corresponding chemical changes were detected by Fourier transform infrared spectrums and X-ray photoelectron spectroscopy. The thermal cyclization and oxidation reactions were separated by altering gas environment from nitrogen to air, and were monitored by differential scanning calorimetry. It is concluded that the sequence distribution of AN in P(AN-co-MA) strongly affects reaction activation energies, cyclized structures and carbon yield.

On the successful chemical recycling of carbon fiber/epoxy resin composites under the mild condition

The research of green chemical recycling of thermosetting carbon fiber composites has been widely concerned by scholars. In this paper, the recycling of high-performance carbon fiber epoxy/resin composites(CF/EP) were conducted in a shorter time and at much lower temperature than those reported recycling methods. The CF/EP were pretreated in nitric acid to be initially decomposed and layered, then the layered CF/EP were subjected to macrogol 400 in presence of potassium hydroxide at 160 °C for 200min. As a result, the carbon fiber was separated and recovered from epoxy resin. Specifically, resin removal rate was more than 95 wt%, and the tests of X-ray photoelectron spectra and scanning electron microscope revealed that surface of recovered fiber was close to that of virgin fiber. In addition, dynamic contact angle meter shown that surface wettability of recovered fiber was improved, which might contribute to the infiltration of resin. On the other hand, the mechanical properties of recovered carbon fiber decreased slightly compared with those of virgin fiber. As mentioned above, the recycling of CF/EP was realized under mild conditions and the performance of recovered carbon fiber was well preserved. This means that the efficient method to recover CF/EP showed great commercial application potential.

Processing of Carbon Nanotubes and Carbon Nanofibers towards High Performance Carbon Fiber Reinforced Polymers

Carbon fiber reinforced polymers (CFRPs) are promising composite materials for high-performance and lightweight applications, gaining increasing interest in aerospace and automotive industries. Epoxy thermosets are frequently used as polymer matrices of CFRPs, which are usually responsible for failure of the composite. In this work different types of carbon nanotubes (CNTs) and carbon nanofibers (CNF) are added to the epoxy resin to improve mechanical properties of the whole CFRP composite. The dispersion of the fillers on a three-roll mill (TRM) is shown comparing their dispersion behavior in the resin. Results of increased modulus and strength of the hierarchical composite in four-point bending tests are presented.

Quantitative analysis of the effects of comonomers and heating conditions on the stabilization reactions of polyacrylonitrile fibers as carbon fiber precursors

Thermal stabilization of polyacrylonitrile fibers is an important step in the production of carbon fibers. A detailed map of stabilization reactions is necessary to adjust the processing parameters - such as temperature and tension - that lead to high performance carbon fibers with minimum structural imperfections. We have investigated the effect of comonomer type and heating conditions on the thermal behavior of polyacrylonitrile fibers during stabilization process. In doing so, three types of PAN fibers having different chemical composition and characteristics were studied using Differential Scanning Calorimetry, Thermogravimetry Analysis and Thermomechanical Analysis. It was observed that chemical composition and characteristics of PAN fibers affect the initiation mechanism and temperature of the stabilization reactions, i.e. the sequence and temperature range of the stabilization reactions change in presence of different types of comonomers. However, heating rate as an important processing parameter does not affect the sequence of these reactions. An increase in the heating rate can only shift the reaction temperature to higher values. These results help the design of the processing unit and adjusting the tension.

Enhancing the interfacial strength of carbon fiber reinforced epoxy composites by green grafting of poly(oxypropylene) diamines

We report on a green method of using poly(oxypropylene) diamines (D400) as coupling and curing agent to functionalize carbon fiber in water. We propose to enhance the interfacial properties of carbon fiber composites, together with the tensile strength of carbon fibers. The microstructure and mechanical properties of carbon fibers before and after modification are investigated. The results show that D400 do not change the surface morphology, but significantly increase the polarity, wettability and roughness of the carbon fiber surface. The interfacial shear strength (IFSS) of modified carbon fiber/epoxy composite and the tensile strength of carbon fibers increase by 79.1% and 8.2%, respectively. It is believed that D400 can effectively improve the interfacial adhesion of the composites by improving resin wettability, increasing chemical bonding and mechanical interlocking. This green and simple method can have applications in continuous production of high-performance carbon fiber composites.