The first use of carbon fibers coincides with Edison's patent in 1879 for using carbon filaments in lamps. But the first real use is in the late 1950s. The needs of the aircraft and space industry have been the most important factor in this. The first successful commercial application was carried out by William Watt and his team at the Royal Aircraft Establishment in Farnborough, England.
The real history of carbon fibers began in the early 1960s and the use of carbon fiber and its composites has gradually increased due to their high performance.
Both carbon and graphite structures are composed of the element carbon as the main building block. According to the definition of textile, carbon fiber is a fiber containing at least 90% carbon in its structure. Carbon fibers with different morphologies and properties are produced by processing a wide variety of raw materials called precursors in different ways.
What is expected from a precursor is that the amount of carbon element it contains should be as high as possible in order to ensure easy conversion to carbon fiber structure. Leading materials are a very important factor in the production forms, structure, properties and end uses of carbon fibers.
Classification of Carbon Fibers
- Ultra-high modulus type (UHM): They are carbon fibers with a modulus above 500 Gpa. An example is the P120 type (820 Gpa) from Union Carbide. This fiber is mezphase-pitch based.
- High modulus type (HM): Carbon fibers with a modulus of 300 – 500 Gpa and a strength / modulus ratio of 5 – 7 10-3 are included in this group. Toray's PAN based M50 model (500 Gpa) is a good example of this group.
- Intermediate Module (IM): Carbon fibers with a modulus of up to 300 Gpa and a strength / modulus ratio of around 10-2 are included in this group. An example is Toray's PAN-based M30 (294 Gpa).
- Low Modulus (LM): Carbon fibers with a modulus less than 100 Gpa fall into this group. These fibers, which have an isotropic structure, generally have low strength properties.
According to Strength:
- Ultra High Strength (UHS): Carbon fibers with a strength of more than 5 Gpa and a strength/hardness ratio of 2 – 3.10-2 are included in this group. An example is Toray's PAN based T1000 model (7.06 Gpa).
- High Strength (HS): Carbon fibers with a strength of more than 3 Gpa and a strength/hardness ratio of 1.5 – 2.10-2 are included in this group. Hercules' PAN-based AS-6 model (4.14 Gpa) is an example of this group.
According to Final Heat Treatments:
- Carbon fibers with a finishing temperature above 2000 OC: High modulus types are included in this group.
- Carbon fibers with a finishing temperature of around 1500 OC: High-strength types are included in this group.
- Carbon fibers with a finishing temperature up to 1000 OC: Low modulus and strength types are included in this group.
The most important precursor materials in the production of carbon fibers are Polyacrylonitrile (PAN), cellulosic fibers (viscose - rayon, cotton) and structures such as pitch. From 1960 to 1980, a wide variety of patents were obtained in the USA on different production possibilities of carbon fibers depending on the precursors. It is most appropriate to separate the production forms according to the precursor fiber type, as explained below.
Production of PAN Based Carbon Fibers
Today's high-tech carbon fibers are aromatic polymers with desirable molecular orientation and crystallinity, often also nitrogen-containing. PAN-based carbon fibers have received much more commercial attention than other precursors. There are three main steps in the production of carbon fiber from PAN.
- Oxidative stabilization at 200 – 300 OC.
- Carbonization at 1000 OC (It can go up to 1500 OC.
- Graphitization between 1500 – 3000 OC depending on the fiber type.
In the first phase, the PAN precursor is held in tension and undergoes oxidation at 200 – 300 OC. This process transforms the PAN into a non-plastic annular compound. Watt and Johnson recommended the 150 – 400 OC range for this process. The formation of this structure takes place in two steps. These steps are cyclicization and dehydrogenation.
During these two steps, the temperature is also gradually increased. It is recommended to wait a few hours for stabilization to complete. The reason for keeping the fiber taut is to prevent the fiber from loosening and losing its orientation during oxidation. The amount of elongation during stretching may vary according to the production method.
A recent patent advocates rapid stabilization of PAN precursors. In this patent, the first stage takes place at the temperature at which maximum plasticity is obtained from the material (10 – 50% shrinkage). The second stage takes place at 0.01 – 0.2 g/denier tension and at 200 – 300 °C. Total treatment time is 15 – 60 minutes (time in an oxygen atmosphere).
With the oxidative process, the fibers gain resistance to processes at high temperatures. After oxidation, the fibers are carbonized without tension at temperatures above 1000 °C. During the carbonization process, non-carbon structures (CHN, NH3, H2) are removed and a structure is obtained that is about half the weight of the initial PAN.
Carbonization consists of two stages. Denitrogenation is performed between 400 – 600 OC and nitrogen elimination continues at 700 OC and reaches its maximum level at 900 OC. At 1300 OC, the nitrogen in the fiber is at a minimum level.
The fibers obtained after carbonization were almost free of non-carbon structures and a graphite-like structure was formed. With heat treatments above 2500 OC (graphitization), orientation and crystallinity are increased in the direction of the fiber axis.
Production of Rayon Based Carbon Fibers
There are three steps in producing carbon fiber from rayon.
- Stabilization (25 – 400 OC)
- Carbonization (400 – 700 OC)
- Graphitization (700 – 2700 OC)
Stabilization is basically an oxidation process and again consists of three steps.
- Physical discharge of water (25 – 150 OC)
- Dehydration of the cellulosic structure (150 – 240 OC)
- Basic breaking of circular bonds, formation of C – C bonds instead of ether C – O bonds and aromatization (240 – 400 OC)
Production of Mesophase Pitch Based Carbon Fibers
If the thermodynamic nature of a hydrocarbon mixture is known, there may be possibilities to produce a variety of carbon fibers. The production of carbon fiber from some components of the pitch is also carried out within the framework of this logic. It is possible to prepare the pitch for carbon fiber production with a suitable solvent system. High molecular weight aromatic pitches are generally anisotropic in nature. To these mesophase is called. After the attraction, the mesophase molecules are oriented and made parallel to the fiber axis and a thermodynamically robust structure is obtained. Before actual transformation, the pitch becomes the fiber to be drawn. The general processes of this production are as follows, respectively.
- Commercial pitch => Mesophase polymerization
- Melt shooting
- Stabilization in air
The pitch precursor turns into mesophase pitch by heat treatment at 350 °C. This structure includes both isotropic and anisotropic structures. After extraction, the isotropic portion becomes injectable at a temperature lower than the softening point. After that, the fiber undergoes carbonization at 1000 OC. The advantage of this method is that no stretching is required during the stabilization and graphitization phases.
The structure of the carbon fiber was revealed by X-ray and electron microscopy methods. Unlike graphite, carbon fiber does not have a regular three-dimensional structure. In general, the high strength of the PAN fiber means that the carbon fiber to be produced should also be durable. The strength of the PAN precursor drops drastically during the first stage of the oxidation process, and the elongation rate first increases and then decreases. The orientation increases significantly with the increase of the heat treatment temperature during carbonization. After carbonization, there is a serious increase in the Young's Modulus of the fiber. The shell and core structures of the fiber also have a great influence on the strength properties. If a moderate stabilization is applied, the modulus and strength increase significantly with carbonization under tension. In a high modulus fiber, the crystals should be arranged in layers in the direction of the fiber.
The general usage areas of carbon fiber are as follows.
- Aircraft and space industry
- Sports equipment
- General engineering applications
The main reasons for the use of carbon fibers in the aerospace industry are as follows.
- Considering the weight, the specific strength of carbon fibers is about seven times higher than that of metals, and their breaking strength is about 5 times higher.
- Their tendency to expand with temperature is very low.
- It has a better fatigue strength than steel and aluminum.
- They are very advantageous in terms of performance / cost ratio.
Given a suitable strength and stiffness, carbon fibers become an indispensable material for the aerospace and aircraft industries. Parts made with carbon fibers are approximately 30% lighter than parts made with substitute metals.
The biggest advantages of carbon fibers are their stiffness and non-expansion tendency. In addition, carbon fiber composites can be used as very good thermal insulation elements. An example of such applications is the isolation of the ignition sections of airplanes and space shuttles.
In the sports industry, carbon fibers have a wide range of applications such as tennis rackets, hockey sticks, skis, fishing rods, racing cars, bicycles, racing engines. The biggest gain in these applications is strength and lightness.
The chemical resistance of carbon fibers is also at a good level. This gives the fiber good corrosion resistance. Therefore, carbon fibers are also used in the construction of chemical and fuel tanks.
The biological compatibility of carbon fibers is better than any other material. Carbon fibers are highly compatible with soft tissues, blood and bone. Therefore, carbon composites are used in prosthetics and bone transplants.