Fibers of different materials can be used as reinforcement, the most common ones include carbon, glass or basalt.
Other important reinforcing fibers include aramid fibers, known under the tradename Kevlar, or natural fibers like hemp or flax.
In principle, fibers increase the stiffness and the strength of a composite material.
However, fibers make composites also anisotropic, as their beneficial effect on the strength and stiffness is much more pronounced in fiber direction than to the direction perpendicular to that.
How to produce fibers for composites?
Carbon fibers are mainly produced from Polyacrylonitrile (PAN), but sometimes also from other carbon-rich constituents like pitch or rayon.
First, threads are drawn from these raw materials. These threads are then heated to 200-300°C and oxidized which facilitates rearranging their atomic lattice.
On the other hand, this process step gives carbon fibers their typical black color. Often this procedure is referred to as "stabilizing".
This step is followed by carbonization in inert conditions at 800-3000°C. The lack of oxygen in the atmosphere prevents the fibers from burning.
In addition, foreign atoms are removed and the carbon atoms form a tight crystal structure along the fiber axis. In most cases, the fiber surface is slightly oxidized after carbonization to improve their bonding properties.
After this surface treatment, usually, a coating is applied to prevent the fibers from damage during further processing.
This last step is known as sizing.
Glass fibers are produced by heating silicon dioxide (SiO2) and specific additives above their melting point and bypassing the viscous glass melt through wear-resistant nozzles.
The melt is then drawn into filaments while being cooled with aqueous solutions containing additives which improve their further processability. As with carbon fibers, usually, a size is applied once the fibers are in their final shape.
Individual fibers are only rarely directly used for fiber-reinforcement. Normally, fibers are further processed into textiles.
This avoids the presence of just a single fiber orientation and allows for a mechanical interlocking of the individual fibers.
Advanced Fibre-Reinforced Polymer (FRP) Composites for Structural Applications edited by J Bai