Carbon fibers are a wonder material and a problem child at the same time. Lighter than aluminum. Stiffer than steel. But also sensitive and difficult to process. Bare fibers are brittle. They break easily. They bond poorly to resins. The solution: a functional coating, known as a finish or sizing. Coating carbon fibers turns the difficult raw material into a processable high-performance material.

Without coating, carbon fibers would hardly be usable. They would break during processing. They would not bond with the resin. They would not enable composite materials. The coating is the key to the exceptional properties of modern CFRP components.

What makes carbon fibers so special

Carbon fibers consist almost entirely of carbon. Finest filaments, about ten times thinner than a human hair. Thousands of these filaments are bundled into a roving. The structure of the fibers is highly ordered. Graphite planes aligned in the fiber longitudinal direction. This gives the fibers their unique properties.

Tensile strength is enormous. Up to 7,000 MPa for high-performance fibers. That is more than five times that of high-strength steel. At the same time, density is low. About 1.8 grams per cubic centimeter. Steel weighs more than four times as much. This combination of strength and lightness makes carbon fibers indispensable.

But the same structure that provides strength also makes the fibers challenging. The surface is chemically inert. Smooth. Non-polar. It offers hardly any anchoring points for resins or other bonding partners.

Why carbon fibers need to be coated

Carbon fiber coating fulfills several important functions. Without it, carbon fibers would be difficult to use in industrial practice.

• Protection against mechanical damage. The individual filaments are extremely sensitive. During processing, weaving, winding, uncoated fibers break easily. The coating protects them.
• Improved handling. The coating holds the filaments together. It prevents the roving from splitting. It makes the fibers less unruly and easier to process.
• Improved fiber-to-matrix adhesion. The most important function in composites. The coating acts as an interface between the non-polar fiber surface and the polar resin matrix.
• Reduction of friction. In processing on textile machinery, friction is an issue. The coating acts as a lubricant and protects both fibers and machines.
• Antistatic effect. Carbon fibers are electrically conductive. Uncoated fibers can cause electrostatic issues. The right coating prevents this.

Which coatings exist

Not every coating fits every application. The sizing must be matched to the later resin system. There are different basic types.

Epoxy-compatible sizings

The standard for most structural applications. Epoxy resins are the most common matrix for CFRP. The sizing contains components that react chemically with the epoxy and form a strong bond.

Polyamide-compatible sizings

For thermoplastic composites. The chemistry differs from epoxy. The sizing must be adapted accordingly to ensure good adhesion.

Polyurethane-compatible sizings

For special applications that require flexibility or particular resistances. PU-based composites have their own requirements for the fiber-to-matrix bond.

Universal sizings

Compromise solutions for applications where different resin systems might be used. They provide acceptable adhesion across different matrices, but not the optimal performance of specialized sizings.

How carbon fibers are coated

Carbon sizing is applied in a continuous process. The fibers run as an endless strand through the coating line. Several methods are possible.

Dip coating. The classic method. The fibers are drawn through a bath containing the sizing solution. Pickup is controlled via speed, concentration, and doctoring. Drying follows.

Spray coating. The sizing solution is sprayed onto the running fibers. Enables more precise control of application amount. Particularly suitable for sensitive fibers.

Roll application. The fibers are guided over rollers wetted with sizing. Good control at high speeds.

After application, drying follows. Typically via heated sections or infrared emitters. Water or solvent evaporates. What remains is a thin, uniform film on the fibers. The layer thickness is typically in the nanometer range. A few percent of the fiber weight.

Where coated carbon fibers are used

Wherever low weight and high strength are required. Applications have expanded significantly in recent years.

• Aerospace. The original domain. Fuselages, wings, tail units. Every gram counts.
• From motorsport to series production. Body parts, structural components, drive shafts.
• Rotor blades are getting longer and longer. Without CFRP, today’s dimensions would not be possible.
• Sports and leisure. Bicycle frames, tennis rackets, golf clubs, fishing rods. Wherever performance matters.
• Robot grippers, rollers, shafts. Stiffness at low weight enables higher speeds.
• Prosthetics, orthotics, medical devices. Low weight increases wearing comfort.

In all these applications, the quality of the fiber coating is decisive for the performance of the end product. An overview of further applications of functional coatings can be found on our website.

What matters for quality

The coating may account for only a few percent of fiber weight. But its quality significantly influences the properties of the finished composite.

Sizing content. Too little sizing means poor protection and insufficient adhesion. Too much sizing can hinder resin impregnation. The optimal range is narrow.

Uniformity. The coating must be uniform over the entire length and across all filaments. Non-uniformities lead to variations in the composite.

Fiber-to-matrix adhesion. The most important parameter for composites. Measured by microbond tests or other adhesion tests on composite specimens.

Fiber damage. The coating must not damage the fibers. Tensile strength and modulus must be maintained.

Coating short fibers in the drum process

In addition to continuous fibers, there are also short fibers and chopped fibers. They are used for injection molding compounds, molding compounds, or as filler. They also need a coating. This is where drum coating can leverage its advantages. Coating carbon fibers in the SC-Coater® offers specific advantages for short fibers.

• Gentle processing. The fibers are tumbled, not stirred. This minimizes fiber breakage and preserves fiber length.
• Uniform coating. Each fiber is reached from all sides. No uncoated areas.
• High material efficiency. 95 percent of the coating material ends up on the fibers. Minimal loss.
• Flexible formulations. Different sizings and additives can be applied. Multi-layer systems are also possible.

More about the process can be found on our contract coating page.

The path to a tailored fiber coating

Every application is different. The sizing must be matched to fiber type, resin system, and processing method. We develop the appropriate solution together with you.

It starts with requirements analysis. Which fibers? Which resin? Which processing? Which properties should the composite have? On this basis, we select the coating system or develop a new formulation.

Then coating trials follow on a small scale. The coated fibers are analyzed. Sizing content, uniformity, visual appearance. If necessary, we optimize parameters.

The decisive test is the application. The coated fibers are processed into composites and tested. Only when results meet requirements do we move to production.

Special Coatings supports you throughout the entire process. From project planning through samplingto series delivery.

High-performance material optimally prepared

Coating carbon fibers is more than a finishing step. It is the prerequisite for carbon fibers to deliver their exceptional properties in composites. The right coating determines the performance of the end product.

With our experience in functional coatings and our know-how in processing sensitive materials, we are the right partner for demanding fiber coatings. We understand the requirements. We deliver solutions.

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Frequently asked questions

Why do carbon fibers need to be coated?

The coating (sizing) protects sensitive fibers from mechanical damage, improves handling, mediates adhesion to the resin matrix, and reduces friction during processing. Without coating, carbon fibers would hardly be industrially usable.

Which types of sizing exist?

The main types are epoxy-compatible, polyamide-compatible, and polyurethane-compatible sizings. Each is optimized for specific resin systems. In addition, there are universal sizings for broader use with slightly lower specialization.

How much sizing is applied?

Sizing content is typically 0.5 to 2 percent of fiber weight. The exact amount depends on fiber type, application, and resin system. Layer thickness is in the nanometer range.

Can short fibers also be coated?

Yes. Short fibers and chopped fibers can be coated using the drum process. The process is particularly gentle and preserves fiber length. It is suitable for fibers that are later used in injection molding compounds or molding compounds.

How is coating quality tested?

Key parameters are sizing content, coating uniformity, and fiber-to-matrix adhesion in the finished composite. Adhesion is determined by microbond tests or mechanical testing on composite specimens.

Can the coating be adapted to a specific resin system?

Yes. The sizing can be tailored to the customer’s specific resin system. If needed, we develop custom formulations that ensure optimal adhesion and processability.