Carbon forms substances of various forms by atoms bonding successively mainly through covalent bonds. Carbon materials such as diamond and graphite formed by the carbon are used in various industries, and their application fields are continuously expanding.

Carbon fiber is also one of such carbon materials. The specific gravity of carbon fiber is 1/4 in comparison with steel, and in view of the unit specific gravity, the strength of the carbon fiber is higher by approximately 10 times and the deformation resistance (elastic modulus) is higher by approximately 7 times. Thus, carbon fibers are "stronger than steel and lighter then aluminum," and show excellent resistance to corrosion and fatigue. The carbon fibers are not used alone but are usually mixed into plastic, rubber, metal or concrete to be used as a "carbon fiber composite material." In these composites, the incorporation of carbon fiber improves the strength, electrical conductivity, and heat resistance of the original material. For example, in addition to the products shown in the figure above, the carbon fiber is used for shafts of golf clubs and fishing rods, and reinforcement of reinforced concrete for earthquake resistance and fireproofing. Among the different types of carbon-fiber composite materials, the high-toughness CFRP used in the construction of the medium- sized passenger airplane "Boeing 787" by The Boeing Company, which significantly reduces the weight of the airplane structure, is particularly interesting.

Fibrous carbon materials have a long history. For example, bamboo-derived carbon materials were used as a filament in light bulbs which were put into practical use by Edison in the 19th century. Thereafter, light-bulb filaments were replaced by tungsten. However, in the 1950s, carbon fibers regained prominence on being used as jet-orifice materials in rockets.

At that time, products made of carbon and graphite were limited to molded products such as electrodes, or powdery products such as activated carbon and carbon black. The production of fibrous graphite was considered to be difficult. In 1956, Union Carbide Corporation in the United States succeeded in developing the world's first carbon fiber by using rayon as a precursor material.

Around that time, Dr. Akio Shindo, a researcher (joined in 1952) of the Osaka National Research Institute of the Agency of Industrial Science and Technology (present Kansai Center in National Institute of Advanced Industrial Science and Technology), was searching for new carbon materials with a desire to "be useful to society." In the process, Dr. Shindo came upon a newspaper article on carbon fibers related to graphite felt, etc. He perceived that "this will lead to innovation," and immediately started research aimed at the industrialization of carbon fibers.

In order to efficiently obtain carbon fibers, it was necessary to find a precursor material having a fibrous form which maintains the fibrous form and constitutes only carbon atoms even after being subjected to heat treatment. The utilization of carbon materials as carbon- fiber precursor materials requires extremely high energy (the melting and stretching of carbon materials involves covalent-bond breakage); therefore, this process is unrealistic. On the other hand, synthetic fibers that had already been put into practical use at that time were fibers that mainly contain carbon atoms, and therefore suitable as precursors for carbon fibers.

Therefore, Dr. Shindo attempted to convert different types of synthetic fibers into carbon fibers. However, he had difficulty in converting the materials into carbon fibers, because the heat treatment caused some materials to disperse like mist, and others to melt before carbonizing, etc. One day, he noticed that the list showing properties of synthetic fibers indicated that many fibers decompose and melt under heat treatment, whereas only polyacrylonitrile (PAN) fibers become "tenacious at 235°C." This meant that PAN fibers have heat resistance, and could be carbonized while keeping the fibrous form.

By repeating experiments using the obtained PAN fibers, Dr. Shindo finally produced PAN based carbon fibers that were flexible enough to be wrapped around the finger. He repeatedly examined the heat-treatment temperature, oxidizing atmosphere, and tension of PAN fibers to stably manufacture large amounts of carbon fiber. Blessed with serendipity (accidental discoveries), he finally published these results in a paper in 1961. At the same time, he filed a basic patent in 1959, which served as a basis for the future.

Thereafter, while the results became known to worldwide researchers, one researcher's opinion that "fibers with high specific modulus are promising as reinforcing fibers for composite materials" prompted the promotion of research and development of carbon fibers as structural materials and research and development of composite materials (CFRP: Carbon Fiber Reinforced Plastic) with resin.

On heating, PAN fibers are converted into carbon fibers with a strong graphite-crystal structure owing to intermolecular- bonding changes. PAN-based carbon fibers differ from rayon-based carbon fibers; they contain fused benzene rings that are regularly aligned in the direction of the fiber, which impart high strength and elastic modulus to the structure.

After the development of a method for synthesizing carbon fibers using PAN-based fibers, several companies began manufacturing carbon fibers. However, the long manufacturing time and high cost of the process prevented industrial production. Toray Industries, Inc. (hereinafter referred to as "TORAY") was one of many companies that focused on the carbon fibers. TORAY was originally a fiber manufacturer, and thus mainly advanced the research on PAN fibers for carbon fibers.

Hydroxyethyl acrylonitrile (HEN), a compound that is structurally similar to acrylonitrile (the starting material for PAN fibers), was discovered during nylon-production research at the Basic Research Laboratory in TORAY. This new compound HEN has an effect of promoting oxidation of PAN fibers, and it was found that mixing with PAN produced carbon fibers with excellent mechanical properties.

TORAY took this opportunity to start a project aimed at producing its own carbon fibers. In 1970, after obtaining permission to use a basic patent for the production of carbon fibers using PAN, TORAY began full-scale industrialization. However, with no technology for carbonizing ( heat treatment) large quantities of PAN fibers, TORAY approached the Union Carbide Corporation for technology exchange and acquiring knowledge on carbonizing fibers.

TORAY started the commercial production of carbon fibers in 1971. This initiated the major lead of Japanese companies in the production of PAN-based carbon fibers against Western companies in later years. However, owing to limited knowledge regarding the utility and applications of carbon fibers, the carbon-fiber market was not a growing market at the time.

TORAY aimed to immediately start using carbon fibers in airplanes owing to the high strength and elastic modulus of CFRPs. However, certification by airplane manufacturers took a significant amount of time.

TORAY pioneered the carbon-fiber market through trial and error to maximally expand the use of carbon fibers. FRP-based fishing rods gained popularity due to their lighter weight compared to conventional glass-fiber fishing rods, and the use of carbon fibers expanded to sports equipment such as golf club shafts and tennis rackets, for which high prices are more acceptable as long as they deliver high performance.

At the same time, TORAY advanced improvement in the performance of carbon fibers . Consequently, carbon fibers manufactured by TORAY became widely recognized as "lightweight and strong." In 1978, these carbon fibers were finally certified as airplane material by The Boeing Company.

In general, to guarantee the safety of an airplane structure, new materials are first used in low-risk components (nonstructural materials). They are used in components that support the load as a mainframe (structural materials) only after validation.

CFRPs underwent a similar process. First they were used as nonstructural materials (such as interior materials), and then as structural materials ( first in secondary components, such as control surfaces, and finally in primary components, such as the main wings and fuselage). In 1986, The Boeing Company proposed the required specifications for certifying CFRP as the primary structural material responsible for the strength of the airplane.

The basic structure of conventional airplanes comprises an aluminum alloy with high processability, toughness, strength, lightweight properties, corrosion resistance, and material costs. An innovative material was required for realizing further weight reduction. Thus, high expectations were held for CFRP, which made a quantum jump in lightweight property and strength and was free of corrosion, as the primary structural material.

Resin which gives shape and toughness to CFRP is referred to as the matrix, and carbon fibers which give strength is referred to as the reinforcement.

The carbon fibers used as the reinforcement were designed based on deep and elaborate analysis in property controlling factors, and high- strength, high-elastic modulus carbon fibers were developed.

To manufacture airplane structures using carbon fibers, an intermediate material, pre-preg, was used as the base material. The pre-preg is a sheet material in which carbon fibers are aligned in one direction and a thermosetting resin is impregnated therein. To fabricate airplane structures, several sheets of pre-preg were stacked to form a specific shape, followed by the application of pressure and heat in an oven (autoclave) to cure the resin.

CFRP, in which carbon fibers being the reinforcement are aligned in one direction, is strong in the fiber direction but is weak in the transverse direction of the fiber. Therefore, sheet-like prepregs are stacked in multiple and optimum directions against the applied external force to form the structure. Although this laminated structure can be strengthened in any planar direction, the toughness of the material must be improved to reduce delamination between the laminated sheets.

Airplanes are operated under extreme conditions. If a large hailstone strikes the airplane in cold climate, the aluminum alloy currently used receives a strong impact that will cause damage in the struck area. Moreover, sand and rocks strike the airplane during take- off and landing. As mentioned previously, CFRP materials may exhibit typical delamination damage on impact. It is crucial to maintain this damage below a design-allowable level.

While designing composite materials, the carbon fibers (as reinforcement) and matrix should both be meticulously designed. The epoxy resin typically used as the CFRP matrix exhibits excellent adhesion and chemical resistance, and its properties can be modified by changing the additives used with the base resin. Joining of the sheets takes place between the matrices, and thus the issue of improving toughness against delamination between the sheets became the issue of epoxy resin as the matrix.

A difficulty encountered during the research for increasing toughness of the epoxy resin itself as a matrix was the trade-off relationship between toughness and strength in CFRP. To meet the airplane-structural-material criteria proposed by Boeing, this issue required resolution.

A breakthrough came by an idea of improving only the portion where the fracture occurs, instead of uniformly improving the entire matrix. Epoxy resin was used to maintain the overall strength of the CFRP material, while thermoplastic-resin particles were arranged between the fracture-susceptible regions of the sheets for additional toughness (as shown in the figure above). By doing so, when an impact is applied to the surface of the CFRP, the thermoplastic particles become deformed, some particles break to absorb energy, and therefore the propagation of cracks can be suppressed.

To achieve this target, TORAY developed its own thermoplastic particles. Mr. Makoto Endo, Director of TORAY's Composite Materials Research Laboratories, who was involved in their development, recounts the following:

"The reason why we were able to create such an innovation was that the target of development was a composite material, which required a team of experts from various fields, such as physics, organic chemistry, material mechanics, and polymer chemistry. Once the team members understand each other's different perspectives, that stimulation leads to unexpected ideas."

TORAY's high-toughness CFRP was certified in 1990 as the primary structural material for the empennage and floor beams of the Boeing 777.

Low production cost is a prerequisite for the widespread and abundant use of materials. Consequently, to facilitate the low-cost mass production of carbon fibers for CFRP, TORAY attempted to improve the productivity of the carbon-fiber manufacturing strategy. Conventional production methods involve numerous time-consuming processes and are unsuitable for mass production. Without changing the fundamental strategy, TORAY thoroughly revamped the entire synthesis process, including the synthesis of yarn (the carbon-fiber precursor), and reduced the total number of steps.

Consequently, the revamped spinning method was developed by TORAY. This method comprises a dry-wet spinning method that combines the low-cost wet method with the high-quality dry method for high-speed carbon-fiber production. In this method, a gap is provided between the spinneret and the bath where the coagulant is retained, and the raw material is deformed in the gap while being extruded into the coagulant to form the yarn.

TORAY realized high speed operation by the new spinning method, and improved productivity by a technology which enable s uniform fiberization even if the number of fibers to be produced at one time is doubled from 12,000 filaments to 24,000 filaments. The orderly production of a large number of microfibers requires advanced production techniques. TORAY could develop this advanced method owing to the availability of technology to control fiber spinning.

Approximately 60 years have passed since basic research on carbon fibers began under Dr. Shindo of the former Osaka Industrial Research Institute. The carbon-fiber market has grown into an industry that covers approximately 80% of the world market (in terms of production by Japanese companies). Carbon fibers manufactured by TORAY occupy a global market share of approximately 50%.

Carbon fibers were first used in airplanes in 1975 (to fabricate the interior components of the Boeing 737). Carbon fibers were used in the airplane structure for the first time in 1983, and in the empennage and floor beams of the Boeing 777 in 1992. Approximately 7 tons of carbon fiber were used per airplane. Furthermore, in 2003, high-toughness CFRP was extensively used for the main wings and fuselage of the Boeing 787. In 2006, the production of the Boeing 787 was initiated, resulting in the fabrication of a "black airplane."

In Boeing 787, the amount of carbon fibers used per airplane was about 35 tons, and the weight of the airplane was considered to be 20% lighter than the conventional airplane.

Carbon fibers, owing to their production method, exhibit a higher material-production energy per unit weight than metals such as steel. Therefore, the LCA perspective is important for a proper analysis of carbon fibers. LCA is a technique for comprehensively evaluating the environmental impact of a product throughout its life cycle (from resource collection to product design, production, use, recycling, and disposal) (see Introduction to GSC No.1). According to CO₂ emission calculations of the Japan Carbon Fiber Manufacturers Association, carbon fibers are expected to significantly reduce the CO₂ emissions from airplanes.

In the Boeing 787, CFRP comprises 50% of the weight of the airplane structure. Model calculations using the same CFRP composition ratio on the Boeing 767 (an existing airplane comprising an aluminum alloy) indicate a 7% reduction in CO₂ emissions in the entire life cycle of the product during 10 years of operation. For Boeing 767-class airplanes, the percentage of CO₂ emission during operation relative to the total CO₂ emissions from standard domestic services during the period from the production of the material to the scrapping of the airplane reaches up to 99%. Therefore, an improving fuel efficiency through weight reduction is a direct effect of reducing CO₂ emissions.

The entire world focused attention on carbon fibers, and many companies took on the challenge but most of them gave up development. Even so, the reason TORAY was able to patiently continue development of carbon fibers was because TORAY was confident that no other material had excellent properties like carbon fibers, and believed in their future prospects. Another reason was that researchers involved in the development had strong hopes of "flying a black airplane" by the use of black carbon fibers

The key to successful carbon-fiber innovation is collaboration: collaboration between government (industrial technology seeds) and private sectors (industrialization of technology seeds), between material manufacturers and airplane manufacturers, and researchers across specialized fields within the research team. Material manufacturers sometimes collaborate with parts manufacturers, but usually not with airplane manufacturers.

Mr. Endo states: "We and Boeing were not sure how to use this material because there was no precedent using the new material. Much discussion took place. Of course, we collaborated with parts manufacturers to develop processing technology. Innovation in materials and innovation in processing technology occurred in parallel."

Mr. Endo says that he will continue to pursue reducing the weight of airplanes. Although the Boeing 777X (an airplane larger than the Boeing 787) is in production, carbon fibers have not yet been used in small airplanes. The cost of carbon fibers must be reduced further for application in small airplanes.

According to Mr. Endo , " in the past, development was targeted on the goal of 'stronger and stiffer,' but from now on, we want to proceed with a comprehensive development to meet the user's needs." Today, the application of carbon fibers is expanding rapidly to ships and watercraft, civil engineering and construction, as well as automobiles and airplanes. The widespread application is expected to save energy and reduce environmental impact.