Prepregs

Pre-preg is a term for "pre-impregnated" composite fibers where a matrix material, such as epoxy, is already present. The fibers often take the form of a weave and the matrix is used to bond them together and to other components during manufacture. The matrix is only partially cured to allow easy handling; this is called B-Stage material and requires cold storage, normally at -18 ºC (-0.4 ºF) to prevent complete curing. B-Stage pre-preg is always stored in cooled areas since heat accelerates complete polymerization. Hence, composite structures built of pre-pregs will mostly require an oven or autoclave to cure.

In the B-stage, the resin is partially cured, and usually tacky. When the resin is brought up to an elevated temperature, it often returns briefly to a liquid state prior to hardening completely. Once cured, the thermoset resin which was in the b-stage, is now fully cross-linked.

Pre-preg allows one to impregnate the fibers on a flat workable surface, or rather in an industrial process, and then later form the impregnated fibers to a shape which could prove to be problematic for the hot injection process. Pre-preg also allows one to impregnate a bulk amount of fiber and then store it in a cooled area for an extended period of time to cure later. Unfortunately the process can also be time consuming in comparison to the hot injection process and the added value for pre-preg preparation is at the stage of the material supplier.

The primary resin matrix used is epoxy. However, other thermoset resins are made into prepregs including BMI (mainly for tooling) and phenolic resins (fire retardant applications).