Better Bonding of Carbon Fibers

A new process increases the adhesion of carbon fibers to the surrounding matrix

Composites based on carbon fibers exhibit exceptional strength and toughness. Adhesion of carbon fibers to the matrix is an important criterion in this regard. If the surface of the fibers loosens its attachment to the matrix resin in response to application of a force, the absorption of force is concentrated in the fibers alone and material failure can result.

Until now, the fibers were chemically treated to improve the fiber-matrix bonding – oxidation processes or a plasma treatment create functional groups on the fiber's surface. While this improves the fiber-matrix bonding, it also reduces the strength of the fibers. For this reason, ITCF Denkendorf developed a new process to increase adhesion of the carbon fibers to the epoxy matrix. In the new process, polymer chains are grafted onto the surface of the fibers. The polymers bond to functional groups on the carbon fibers, grow into polymer chains on the fiber's surface and form three-dimensional structures. These then protrude into the epoxy resin like anchors. This yields exceptional fiber-matrix bonding without compromising the original stability of the fibers.

According to the information provided, the chemical surface modification can be easily incorporated into industrial production. The fibers are prepared with a monomer solution and then undergo a thermal treatment. A washing and drying process follow.

Investigations of test specimens produced by means of the new method show a definite improvement in mechanical properties. Tensile strength increased by 15% compared to that of composites with conventionally treated fibers. The modulus of elasticity increased by 6% as well. The treatment had the greatest effect on the interlaminar shear strength, which rose 20%. Microscopic images of fracture surfaces of the modified material show that in tensile tests the fibers separate much less from the matrix material.