Carbon Composite Materials: Understanding Fiber-Reinforced Polymers
By Rebecca Phan (May 2021)
Carbon composites, typically referred to as carbon fiber-reinforced polymers (CFRPs), are a class of incredibly strong, stiff, and lightweight materials. Following their invention in the 1960s, CFRPs have been adopted into a range of engineering disciplines. As well as becoming a staple material for the aerospace industry, these composite materials have also proved to be successful in civil engineering and automotive engineering applications. CFRPs are used to make an increasing number of consumer and technical products, ranging from tennis rackets to fishing poles. This technical article takes a look at how CFRPs are made and what makes them so beneficial.
CFRPs are a type of composite material comprised of a "filler" material, which is dispersed throughout a bulk ‘matrix’. In the case of CFRPs, the filler consists of carbon fibers (typically defined as fibers that contain at least 92% carbon by weight) and the matrix is a polymer.
Fabrication and Properties of Carbon Fibers
The fibers in carbon composites typically consist of small crystallites of graphite, in which carbon atoms are bonded together in sheets. Within these sheets, atomic bonding is very strong; however, bonding is much weaker between the sheets. This means that graphite is highly anisotropic: it exhibits high stiffness within its plane, but lower stiffness perpendicular to the plane. The manner in which these planes are arranged within a fiber influences the properties of the fiber, which in turn influences the properties of the carbon composite material. A ‘turbostratic’ structure, in which graphite layers are ‘crumpled’ together, tends to exhibit particularly high tensile strength; while an ordered ‘graphitic’ structure offers higher stiffness.
Carbon fibers are generally produced from polymers such as polyacrylonitrile (PAN) or petroleum pitch. These polymers can be formed into thin filaments, which are then wound onto a spool. Heat treatment of these precursor fibers at around 200 to 400°C (392 to 752°F) stabilizes them, and further treatment at around 1000°C (1832°F) removes hydrogen, oxygen, nitrogen, and other non-carbon elements to leave carbon fibers. Different types of precursor fibers and variations in production methods can produce carbon fibers with very different properties. The resulting fibers can be categorized according to their Young modulus, which typically ranges from around 4 GPa to over 500 GPa. Before incorporation into carbon composites, carbon fibers are usually treated to improve their adhesion to the matrix.
Effect of the Polymer Matrix
Because of the anisotropy of the fibers, fiber alignment has an important effect on the mechanical properties of the carbon composite. In fiber-reinforced carbon composites, carbon fibers with a diameter of around 5 to 10 μm contribute stiffness and strength to the composite material, while the surrounding polymer matrix encloses the fibers, protecting them and transferring mechanical load between them. The result is a highly strong, stiff, and low-density material that can be easily molded into complex shapes.
The polymer matrix used in carbon composites also has an important effect on its material properties. For example, carbon fiber composites from Omniseal Solutions™ boast excellent retention of mechanical properties across a wide temperature range (from cryogenic temperatures up to 316°C / 600°F) along with self-lubricating properties. Their composite solutions also offer excellent deformation resistance and thermal and electrical dissipation.
Omniseal Solutions™ brings over 65 years of manufacturing expertise to provide high-performance carbon composites to key industries such as aviation and industrial (canning, forging, rolling mills and extrusion). Their researchers and applications experts can work with your organization to develop unique carbon composite solutions for any material challenges. To find out more about carbon composite solutions, get in touch with the engineering experts of Omniseal Solutions™ today.