Carbon fiber is a composite material celebrated for its exceptional strength-to-weight ratio, allowing for robust structures with minimal mass. It is up to 75% lighter than steel, yet possesses tensile strength several times greater than most metals. This unique combination of lightness and performance has made it a transformative material in demanding applications. The manufacturing process is a complex thermal and chemical journey that converts an organic polymer into a highly ordered, stiff, and strong carbon filament. This controlled transformation enables the high-performance characteristics relied upon in aerospace, automotive, and wind energy.
Selecting the Precursor Material
The initial properties of the final carbon fiber are determined by the choice of the starting material, known as the precursor. Polyacrylonitrile (PAN) is the dominant precursor globally, accounting for over 90% of all carbon fiber production. PAN is a synthetic polymer spun into fine filaments, sometimes containing co-monomers like methyl acrylate to improve spinning. The molecular structure of the PAN fiber, consisting of long, aligned polymer chains, must be maintained throughout subsequent high-temperature processes. Alternative precursors, such as petroleum pitch or rayon, are sometimes used for specialized grades. Pitch-based fibers, for instance, are chosen when high stiffness (modulus) is prioritized over tensile strength.
Thermal Conversion Stages
The manufacturing process involves a series of heat treatments that chemically transform the precursor fiber into pure carbon. The first step is stabilization (oxidation), where PAN fibers are continuously heated in air between 200°C and 400°C for 30 to 120 minutes. This process chemically alters the polymer chains.
During stabilization, linear PAN molecules undergo cyclization, forming a thermally stable, ladder-like structure. This prevents the fibers from melting or fusing during the subsequent high-temperature stage. Oxygen atoms are incorporated into the structure, which is necessary before the final conversion to carbon.
The stabilized fibers are then transferred to carbonization furnaces, where they are heated in an inert atmosphere (usually nitrogen or argon) to prevent combustion. Carbonization occurs at high temperatures, ranging from 1000°C to 3000°C, depending on the desired final properties. This intense heat drives off nearly all non-carbon atoms as gaseous byproducts. The remaining carbon atoms reorganize into a tightly packed, turbostratic structure characterized by stacked, graphite-like crystalline layers oriented parallel to the fiber axis. The final temperature directly influences performance; lower temperatures yield high-strength fibers, while higher temperatures result in higher stiffness.
Preparing the Fiber for Composite Use
Once carbonization is complete, the carbon filaments must undergo specific treatments to bond effectively with a polymer matrix. The first step is surface treatment, typically electrochemical oxidation in an acidic or basic electrolyte bath. This treatment microscopically etches the fiber surface, creating roughness to enhance mechanical interlocking with the resin.
The surface treatment also introduces chemically active functional groups, such as carboxyl and hydroxyl groups, onto the carbon surface. These groups are essential because they form strong chemical bonds with the encapsulating polymer resin. Controlling this process is delicate, as over-etching can create defects that compromise the fiber’s strength.
Following this, a protective coating called sizing is applied to the fiber strands. The sizing agent is a thin, polymeric film that serves two primary purposes before the composite is formed. It protects the brittle filaments from damage during handling and weaving. The sizing is also formulated to be chemically compatible with the final matrix resin (e.g., epoxy) to enhance adhesion and load transfer.
From Fiber to Finished Composite
The final stage of carbon fiber manufacturing involves converting the treated filaments into a usable composite material for structural applications. Individual carbon filaments are bundled together into strands called tows, which can contain anywhere from 1,000 (1K) to 50,000 (50K) or more filaments. These tows are the fundamental building blocks and are often woven into fabrics or unidirectional sheets.
The carbon fibers are then combined with a matrix material, most commonly an epoxy resin, which binds the fibers together and facilitates the transfer of mechanical load throughout the finished part. The resin’s role is to stabilize the fibers and protect them from environmental damage, while the fibers provide the majority of the part’s strength and stiffness. The combination of fiber and resin creates a carbon fiber reinforced polymer (CFRP).
Several manufacturing techniques are employed to combine the fiber and resin into a rigid shape, with the choice depending on the part’s complexity, volume, and required performance. One common method uses “prepreg,” which is carbon fiber fabric pre-impregnated with a partially cured epoxy resin. Prepreg layers are meticulously laid up in a mold and then cured under high heat and pressure in an autoclave, a pressurized oven, to achieve the highest possible strength and lowest void content, making it the preferred method for aerospace components.
For high-volume production of complex parts, Resin Transfer Molding (RTM) is often utilized, where dry fiber preforms are placed into a closed mold, and a low-viscosity resin is then injected under pressure. Alternatively, filament winding is used for hollow, rotationally symmetric parts like pressure vessels, where resin-impregnated tows are continuously wrapped around a mandrel. In all these processes, the final step involves curing the resin with a specific temperature cycle to fully cross-link the polymer, permanently locking the carbon fibers into a rigid, load-bearing composite structure.