The current carbon fiber producers are less than a dozen around the world and operate in (almost) total secrecy. Notwithstanding that, in this article we will do our best to give some information relating to the production process of this incredibly useful material
Marine industry is very much increasing the use of CFRP, due to its outstanding qualities. There is a lot of talk about carbon fibers, but its production is a topic not much under the limelights because it is complex and expensive.
Nonetheless, understanding how carbon fiber is produced can help not only to improve how the material is used, but also to understand the future outlook and prospects for the carbon fiber market.
Carbon fiber: the most relevant details
Carbon fiber is a long, thin strand of material about 0.0002-0.0004 in (0.005-0.010 mm) in diameter and composed mostly of carbon atoms bonded together in microscopic crystals that are more or less aligned parallel to the long axis of the fiber. It is used to produce a large diversity of composites, i.e. materials built up of two or more components.
Its main features are:
- high mechanical resistance;
- low density;
- a very low coefficient of thermic expansion;
- resistance to the effects of several chemical agents;
- good fireproof properties.
Carbon fibres are divided into two main categories, PAN fibers and PITCH fibers:
- PAN fibers are characterized by high resistance and toughness. Tensile strength up to 7000 MPa, Young modulus = 230-600 GPa;
- PITCH fibers are characterized by very high modulus but low resistance. Tensile strength up to 3400 MPa, Young modulus up to 930 GPa.
A fundamental difference
Unlike metals, which are homogeneous and – by design – have properties that comply with established standards (for example, each manufacturer’s P20 steel is interchangeable with that of others), composite materials are heterogeneous.
Being made combining different materials (fiber and resin), their variability and therefore their customization are central to their appeal. As a result, carbon fiber manufacturers produce similar but not identical products.
Carbon fiber varies in tensile modulus (or stiffness determined as strain under stress) and resistance to traction, compression, and fatigue. PAN-based carbon fiber is now available in:
- low modulus (less than 32 million lbf/in² or <32 Msi);
- standard modulus (33 to 36 Msi);
- intermediate modulus (40 to 50 Msi);
- high (from 50 to 70 Msi);
- ultra-low modulus (from 70 to 140 Msi).
The fiber, available in bundles called “tow”, is available in different sizes from 1K to 350K (1K is equivalent to 1,000 filaments with a diameter of 5 to 10 microns). The products also vary in the degree of carbon content and the type of surface treatment/coating.
The inherent complexity of carbon fiber composites is what adds value to structures made of carbon fiber, because they offer ten times the strength of steel at half the weight.
Carbon fiber is produced by pyrolysis of an organic precursor fiber in an inert atmosphere at temperatures above 982°C / 1800°F. Manufacturing of carbon fiber, however, is a complex undertaking. Throughout the process, tight tolerances define the ultimate usefulness of the fiber, i.e. its future sector of use: to make aircraft and spacecraft parts, racing car bodies, tennis rackets, bicycle frames, fishing rods, automobiles, boats and many other components where light weight and high strength are needed.
Treating the surface and sizing
The next step is critical to fiber performance and, apart from the precursor, it makes a supplier’s product more distinct from a competitor’s one. The adhesion between matrix resin and carbon fiber is essential in a reinforced composite; during the production of carbon fiber, a surface treatment is performed to improve this adhesion.
Manufacturers use different treatments, but a common method is to pull the fiber through an electrochemical or electrolytic bath that contains solutions such as sodium hypochlorite or nitric acid. These materials etch or roughen the surface of each filament, which increases the surface area available for interfacial fiber/matrix bonding and adds reactive chemical groups, such as carboxylic acids.
Next, a coating called “sizing” is applied: it protects the carbon fiber during handling and processing (for example, weaving) in intermediate forms, such as dry fabric and prepreg. The sizing also holds the filaments together in individual trailers to reduce fuzz, improve processability and increase the interfacial shear strength between the fiber and the matrix resin.
Carbon fiber manufacturers increasingly use a sizing with the desired specifications in the appropriate composite for the customer’s end use. The crystal alignment makes the fiber incredibly strong for its size. Several thousand carbon fibers are twisted together to form a yarn, which may be used by itself or woven into a fabric. The yarn or fabric is combined with epoxy and wound or molded into shape to form various composite materials. Forty years of processing perfection have brought technological maturity and the ability to translate superior performance and application versatility through fibers to advanced composite materials.
Trends in carbon fiber production
Technological changes have made carbon fiber available and more practical for use by OEMs in a wide range of markets and applications. The surveyors and suppliers who build the kilns and furnaces with which the pyrolysis was carried out have recently outlined some of the most significant developments.
1. Sizing for many matrices
Since most carbon fiber has historically been used with epoxy matrices, the sizing is predominantly epoxy-based and low molecular weight to promote flexibility and spreadability of the fibers. However, research is underway to create chemical sizing to accommodate the variety of matrix resins currently required for end-use applications.
2. More efficient new generation ovens
In the production of carbon fiber, much depends on the design of the ovens that pyrolyze the fibers.
In the oxidation process, the furnace airflow plays a fundamental role in controlling the temperatures of the process and preventing exothermic reactions. Airflow models can be single flow (parallel or perpendicular or multipath). Three important elements are required by carbon fiber manufacturers for oxidation furnaces: productivity, scalability and energy efficiency. The newer generation of larger furnace systems is more efficient, producing a larger volume of carbon fiber with lower energy costs.