Carbon fibers
Carbon fibers are thin, strong filaments primarily composed of carbon atoms, with diameters typically ranging from 5 to 10 micrometers and containing more than 90 wt% carbon, making them a high-performance material renowned for their exceptional strength-to-weight ratio and use as reinforcement in advanced composites.[1] These fibers exhibit tensile strengths often exceeding 3.5 GPa and moduli up to 500 GPa or more, depending on the grade, while maintaining a low density of approximately 1.7 to 2.0 g/cm³, which is about one-fourth that of steel, enabling significant weight savings in structural applications.[2] Produced mainly through the pyrolysis of polyacrylonitrile (PAN) precursors, which dominate over 90% of global production, carbon fibers undergo stabilization, carbonization at around 1,000°C, and optional graphitization up to 3,000°C to achieve their graphitic structure and tailored properties.[3] The material's key attributes include high chemical and thermal stability in non-oxidizing environments, excellent electrical and thermal conductivity (up to 500 W/mK for high-modulus variants), and a negative coefficient of linear thermal expansion, rendering it ideal for demanding conditions.[2] Carbon fibers are categorized by modulus—standard, intermediate (high-strength), and high-modulus—and available in forms like continuous tows (1K to 320K filaments) or chopped strands, with global production capacity reaching 290,000 tons in 2023 and demand projected to grow 144% by 2030 due to expanding uses.[3] In composites like carbon fiber-reinforced polymers (CFRPs), they provide specific strengths around 1300 MPa/(g/cm³)—over 15 times that of steel—and specific moduli of 131 GPa/(g/cm³), about five times steel's, while offering superior fatigue resistance (70–80% of tensile strength) and energy absorption.[4] Applications of carbon fibers span aerospace, where they constitute over 50 wt% of structures in aircraft like the Boeing 787 and Airbus A350 for fuselages and wings; automotive lightweighting, as in BMW i3 body panels and NIO ES6 chassis to reduce vehicle mass and emissions; wind turbine blades for renewable energy; and sporting goods, pressure vessels, and construction for enhanced durability and efficiency.[5] Their integration via processes like filament winding or resin transfer molding has driven a compound annual growth rate of 12.5% over the past two decades, underscoring their role in sustainable, high-performance engineering.[1]History
Early Discovery
The early discovery of carbon fibers dates back to 1879, when Thomas Edison experimented with carbonized cellulose threads derived from cotton or bamboo as filaments for incandescent light bulbs, achieving a viable electrically heated lighting solution that lasted over 13 hours.[6] These fibers, formed by baking cellulose materials at high temperatures to carbonize them, represented an initial recognition of carbon's potential for high-temperature resistance, though Edison's focus remained on lighting rather than broader industrial applications, and the technology was not scaled for fiber production.[6] Post-World War II, renewed interest in advanced materials for aerospace prompted systematic research in the United Kingdom at the Royal Aircraft Establishment (RAE) during the 1950s, where scientists sought high-temperature resistant fibers suitable for aircraft components.[6] By the early 1960s, the team at the RAE, led by William Watt and colleagues, developed processes using polyacrylonitrile (PAN) as a precursor that aligned carbon atoms into oriented structures for improved strength and stability under extreme heat.[7] In parallel, early U.S. efforts in the late 1950s at Union Carbide's Parma Technical Center, led by chemist Roger Bacon, produced high-performance graphite whiskers through the pyrolysis of hydrocarbon gases, such as natural gas heated to approximately 2,000°C in a vacuum, yielding structures with tensile strengths up to 20 GPa.[6] These whiskers demonstrated the potential for exceptionally strong carbon-based materials, though initial outputs were limited to small quantities and extremely high costs.[8] Throughout these pioneering experiments, researchers encountered significant challenges, including the inherent brittleness of the fibers due to uneven carbonization and low yields—often below 20%—resulting from material degradation during processing.[8] These issues were mitigated through refined heat treatment techniques, such as staged pyrolysis up to 3,000°C in vacuum or inert gas environments, which reduced defects and improved molecular orientation without fracturing the structure.[6] Such innovations paved the way for the commercial scalability of carbon fibers in the 1960s.Commercial Development
The commercial development of carbon fibers accelerated in the 1960s, building on early experimental discoveries such as Roger Bacon's 1958 production of high-performance graphite whiskers at Union Carbide.[6] In late 1965, Union Carbide achieved a key breakthrough with the development of high-modulus fibers through optimized hot-stretching and carbonization processes applied to rayon precursors, resulting in the commercial launch of Thornel 25 yarn with a Young's modulus of 172 GPa.[8] This advancement marked the shift from laboratory-scale production to viable industrial methods, initially targeting high-value applications.[6] The first widespread commercial production emerged in the late 1960s, driven by aerospace sector demands during the Space Race and military programs requiring lightweight, high-strength materials.[9] Companies like Celanese initiated commercial-scale manufacturing of rayon-based carbon fibers around 1965, supplying early applications in military aircraft and missiles.[6] Concurrently, Toray Industries in Japan began test production of polyacrylonitrile (PAN)-based carbon fibers in 1971 under the Torayca brand, scaling to full commercial output by 1973 at a rate of 5 tons per month.[10] These efforts were propelled by the need for materials in space exploration and defense, where carbon fibers offered superior performance over metals.[9] Significant patent developments facilitated this industrialization, notably Akio Shindo's 1959 Japanese patent for a PAN-based carbonization process that achieved 50-60% carbon yield by oxidizing precursors in air rather than enclosed furnaces.[11] Licensed to Japanese firms like Nippon Carbon in 1961 and Toray in 1970, Shindo's innovation enabled efficient production of high-strength PAN fibers, with the U.S. equivalent patent issued in 1970.[12] Although vapor-grown carbon fibers emerged later in the 1970s through separate catalytic processes, Shindo's work laid the groundwork for PAN dominance in commercial markets.[13] By the 1970s, production capacity expanded from lab-scale grams to industrial tons per year, supported by continuous processing techniques that boosted output for aerospace and emerging civilian uses.[9] Initial costs, exceeding $400 per pound in the late 1960s due to batch methods, dropped below $100 per pound by the early 1970s through process optimizations and economies of scale, eventually approaching $25 per pound for standard grades.[14] This cost reduction, combined with rising demand from programs like the U.S. Department of Defense, propelled annual production growth at double-digit rates, establishing carbon fibers as a cornerstone of advanced composites.[9]Types
Precursor-Based Classification
Carbon fibers are primarily classified based on their precursor materials, which are the starting polymers or compounds converted into the final fibrous structure through processes like spinning, stabilization, and carbonization. The choice of precursor significantly influences the fiber's microstructure, mechanical properties, and suitability for applications, with polyacrylonitrile (PAN), pitch, and rayon being the main types. Emerging bio-based precursors are also gaining attention for sustainability.[15] PAN-based carbon fibers dominate the market, accounting for approximately 90% of production as of 2024 due to their versatility and cost-effectiveness. Derived from copolymers of acrylonitrile with comonomers like itaconic acid or methyl acrylate, PAN precursors are solution-spun into fibers that undergo oxidative stabilization to prevent melting during subsequent heating. This results in high-strength fibers with tensile strengths often exceeding 3 GPa, making them ideal for aerospace and automotive composites.[16][15] Pitch-based carbon fibers, comprising about 9% of the market, are produced from petroleum pitch, coal tar pitch, or synthetic pitches rich in aromatic hydrocarbons. These isotropic or mesophase pitches are melt-spun and yield fibers with a highly graphitic structure, providing exceptional stiffness with Young's moduli up to 900 GPa. Their ordered molecular alignment suits applications requiring high rigidity, such as structural reinforcements in high-temperature environments.[16][15] Rayon-based carbon fibers, derived from regenerated cellulose (viscose rayon), were the first commercially produced type in the 1950s but now hold less than 1% market share. The cellulose precursor is spun into fibers and dehydrated during carbonization, leading to a turbostratic structure suitable for flame-resistant textiles and high-temperature insulation. Their production is energy-intensive, limiting widespread use to specialty applications.[17][16] Emerging precursors like lignin, a renewable byproduct from wood pulping, offer sustainable alternatives to petroleum-based materials, potentially reducing costs by up to 50% and lowering carbon footprints. Lignin is processed via melt or solution spinning, though it typically yields around 40-50% carbon, comparable to PAN's approximately 50% yield, but with ongoing optimizations to improve efficiency. As of 2025, advancements in lignin-based fibers have achieved higher stiffness, with some blends reaching tensile moduli approaching 200 GPa, and pilot productions demonstrating feasibility for automotive applications. Bio-based options, including alginate-lignin blends, are under development to enhance environmental viability while maintaining fiber integrity. These precursors result in modulus and strength variations that align with specific performance categories.[18][19][20]Performance-Based Classification
Carbon fibers are classified based on their performance characteristics, primarily tensile strength and modulus of elasticity, which determine their suitability for specific applications. This performance-based categorization emphasizes end-product mechanical and functional properties rather than precursor materials, though precursors like polyacrylonitrile (PAN) and pitch influence these traits. High-strength fibers prioritize tensile strength for load-bearing roles, while high-modulus variants focus on stiffness; intermediate types offer a balance, and specialty forms like vapor-grown carbon nanofibers (VGCNF) provide unique nanoscale properties for advanced functionalities.[15] High-strength carbon fibers exhibit tensile strengths exceeding 3.5 GPa, often reaching 4-7 GPa, making them ideal for applications requiring high load capacity and impact resistance, such as sporting goods like tennis rackets and bicycle frames. These fibers, typically derived from PAN precursors, achieve their performance through optimized carbonization processes that enhance fiber integrity without excessive brittleness. For instance, commercial grades like Toray's T700S offer a tensile strength of 4.9 GPa alongside a standard modulus of around 230 GPa, enabling lightweight composites in recreational and industrial uses.[21][15] High-modulus carbon fibers, with Young's modulus values greater than 300 GPa and up to 500 GPa or more, are designed for applications demanding superior stiffness and dimensional stability, such as aerospace structural stiffeners and pressure vessels. Predominantly produced from mesophase pitch precursors, these fibers undergo high-temperature graphitization to align graphite layers, resulting in exceptional rigidity but moderate tensile strengths around 2-4 GPa. Examples include pitch-based fibers with moduli of 588 GPa, which provide thermal stability and are used in satellite components where vibration damping is critical.[15][21] Intermediate modulus/strength carbon fibers bridge the gap between high-strength and high-modulus types, featuring moduli of 240-300 GPa and tensile strengths of 3-5 GPa, which balance performance and cost for broader industrial adoption. These hybrids, often PAN-based, are particularly suited for automotive applications like chassis components and body panels, where weight reduction and moderate stiffness improve fuel efficiency without the premium expense of ultra-high-modulus variants. Commercial offerings, such as those with 290 GPa modulus and strengths over 4.8 GPa, support high-volume manufacturing in electric vehicles and recreational vehicles.[21][22] Specialty carbon fibers, such as vapor-grown carbon nanofibers (VGCNF), deviate from traditional microscale fibers with diameters under 100 nm and high aspect ratios exceeding 100, emphasizing electrical and thermal conductivity over bulk mechanical strength. These nanofibers, synthesized via catalytic vapor deposition, exhibit resistivities as low as 5 × 10^{-7} ohm·cm and thermal conductivities up to 910 W/m·K in composites, making them valuable for electronics, electromagnetic interference shielding, and battery anodes in energy storage systems. Unlike conventional fibers, VGCNF's nanoscale structure enables unique functionalities like enhanced gas absorption and percolation in polymer matrices at low loadings (e.g., 1.5 vol% for conductivity).[23][23]| Category | Key Metrics (Tensile Strength / Modulus) | Primary Applications | Typical Precursor Influence |
|---|---|---|---|
| High-Strength | >3.5 GPa / 200-250 GPa | Sporting goods, industrial composites | PAN for strength optimization |
| High-Modulus | 2-4 GPa / >300 GPa | Aerospace stiffeners, pressure vessels | Pitch for graphite alignment |
| Intermediate Hybrids | 3-5 GPa / 240-300 GPa | Automotive components, vehicles | PAN for cost-performance balance |
| Specialty (VGCNF) | 2.5-3.5 GPa / 100-1200 GPa | Electronics, energy storage | Vapor deposition for nanoscale conductivity |