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Carbon Fibre : Sweeping technical world off its feet

In 1879, the world experienced a novel level of innovation that peacefully took over the reins of technical textile. It was in this year that Edison took out a patent for the manufacture of carbon filaments suitable for use in electric lamps. However, it was in the early 1960s when successful commercial production was started, as the requirements of the aerospace industry – especially for military aircraft – for better and lightweight materials became of paramount importance.

The use of carbon fiber fabric is now widespread and it is in everything from tennis rackets to bicycle frames. It has become a big part of Formula 1 cars and many supercars as well. Several cruisers use carbon fibre extensively. However, as carbon fibre is not part of daily textile needs of consumers, many are unaware that this fibre has become an undeniable part of everyday lives. The truth is that it is everywhere to be found in the technological world.

Carbon fibre is an exceptionally lightweight strengthening fibre derived from the element carbon. Sometimes known as graphite fibre, when this extremely strong material is combined with a polymer resin, a superior composite product is produced. Also called graphite fibre or carbon graphite, carbon fibre consists of very thin strands of the element carbon. Carbon fibres have high tensile strength and are very strong for their size. In fact, carbon fibre might be the strongest material there is. Speaking in technical terms, each fibre is 5-10 microns in diameter. One micron (um) is 0.000039 inches.

In Textile Terms and Definitions, 3k carbon fiber fabrics has been described as a fibre containing at least 90 percent carbon obtained by the controlled pyrolysis of appropriate fibres. The term graphite fibre is used to describe fibres that have carbon in excess of 99 percent. Large varieties of fibres called precursors are used to produce carbon fibres of different morphologies and different specific characteristics. The most prevalent precursors are polyacrylonitrile (PAN), cellulosic fibres (viscose rayon, cotton), petroleum or coal tar pitch and certain phenolic fibres. Based on modulus, strength, and final heat treatment temperature, carbon fibres are classified into different categories.

As far as carbon fibre cloth is concerned, spools of carbon fibre are taken to a weaving loom, where the fibres are then woven into cut resistant fabric. The two most common types of weaves are plain weave and twill. Plain weave is a balanced checker board pattern, where each strand goes over then under each strand in the opposite direction. Whereas a twill weave looks like a wicker basket. Here, each strand goes over one opposing strand, then under two. Both twill and plain weaves have an equal amount of carbon fibre going each direction, and their strengths will be almost same. The two are aesthetically different.

Carbon fibre is primarily used for producing sporting goods, which account for nearly 11 million lb of that material. Currently, the United States of America consumes nearly 60 percent of the world production of carbon fibres, while the Japanese represent for almost 50 percent of the world capacity for production. The world production capacity of pitch-based carbon cloth is almost totally based in Japan. The key to further carbon fibre market expansion is continued development of high-rate manufacturing methods and considering this, it is predicted that demand of the fibre will increase by 235 percent by 2020.

Carbon fiber is composed of carbon atoms bonded together to form a long chain. The fibers are extremely stiff, strong, and light, and are used in many processes to create excellent building materials. Carbon fiber material comes in a variety of “raw” building-blocks, including yarns, uni-directional, weaves, braids, and several others, which are in turn used to create composite parts. The properties of a carbon fiber part are close to that of steel and the weight is close to that of plastic. Thus the strength to weight ratio (as well as stiffness to weight ratio) of a carbon fiber part is much higher than either steel or plastic. Carbon fiber is extremely strong. It is typical in engineering to measure the benefit of a material in terms of strength to weight ratio and stiffness to weight ratio, particularly in structural design, where added weight may translate into increased lifecycle costs or unsatisfactory performance.

Carbon fibers or kevlar fabrics are fibers about 5–10 micrometres in diameter and composed mostly of carbon atoms. Carbon fibers have several advantages including high stiffness, high tensile strength, low weight, high chemical resistance, high temperature tolerance and low thermal expansion. These properties have made carbon fiber very popular in aerospace, civil engineering, military, and motorsports, along with other competition sports. However, they are relatively expensive when compared with similar fibers, such as glass fibers or plastic fibers.

The raw material used to make carbon fiber is called the precursor. About 90% of the carbon fibers produced are made from polyacrylonitrile. The remaining 10% are made from rayon or petroleum pitch. All of these materials are organic polymers, characterized by long strings of molecules bound together by carbon atoms. The exact composition of each precursor varies from one company to another and is generally considered a trade secret. During the manufacturing process, a variety of gases and liquids are used. Some of these materials are designed to react with the fiber to achieve a specific effect. Other materials are designed not to react or to prevent certain reactions with the fiber. As with the precursors, the exact compositions of many of these process materials are considered trade secrets.

Before the fibers are carbonized, they need to be chemically altered to convert their linear atomic bonding to a more thermally stable ladder bonding. This is accomplished by heating the fibers in air to about 390-590° F (200-300° C) for 30-120 minutes. This causes the fibers to pick up oxygen molecules from the air and rearrange their atomic bonding pattern. The stabilizing chemical reactions are complex and involve several steps, some of which occur simultaneously. They also generate their own heat, which must be controlled to avoid overheating the fibers. Commercially, the stabilization process uses a variety of equipment and techniques. In some processes, the fibers are drawn through a series of heated chambers. In others, the fibers pass over hot rollers and through beds of loose materials held in suspension by a flow of hot air. Some processes use heated air mixed with certain gases that chemically accelerate the stabilization.

Once the fibers are stabilized, they are heated to a temperature of about 1,830-5,500° F (1,000-3,000° C) for several minutes in a furnace filled with a gas mixture that does not contain oxygen. The lack of oxygen prevents the fibers from burning in the very high temperatures. The gas pressure inside the furnace is kept higher than the outside air pressure and the points where the fibers enter and exit the furnace are sealed to keep oxygen from entering. As the fibers are heated, they begin to lose their non-carbon atoms, plus a few carbon atoms, in the form of various gases including water vapor, ammonia, carbon monoxide, carbon dioxide, hydrogen, nitrogen, and others. As the non-carbon atoms are expelled, the remaining carbon atoms form tightly bonded carbon crystals that are aligned more or less parallel to the long axis of the fiber. In some processes, two furnaces operating at two different temperatures are used to better control the rate de heating during carbonization.

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