Either ready-to-mold material in sheet form or readty-to-wind
material in roving form, which may be cloth, mat, unidirectional fiber,
or paper impregnated with resin and stored for use. The resin is
partially cured to a B-stage and supplied to the fabricator, who lays up
the finished shape and completes the cure with heat and pressure.
The two distinct types of PREPREGs available are:
1. Commercial PREPREGs: where the roving is coated with a hot melt
or solvent system to produce a specific product to meet specific customer
requirements
2. Wet PREPREGs: where the basic resin is installed without solvents
or preservatives but has limited room temperature shelf life.
Both thermoset and thermoplastic matrices can be used:
Thermoset
The reactants are mixed and the fibers are coated and a a partial cure
is achieved.
The material can the be cut to shape, used to fill the mold or wrap the
part.
Final cure is achieved with heat and usually pressure.
Problems: Can be difficult to achieve full cure. Also, storage
is a concern. Generally these materials must be kept at low temperatures
to prevent premature curing.
Thermoplastic
The fibers coated with resin using melted polymer or polymer in solvent.
The material can the be cut to shape, used to fill the mold or wrap the
part.
Final cure is achieved with heat and usually pressure.
Problems: It is difficult to fully recover all of the solvent used
in the process. Also, some polymers have a high melt viscosity which
makes it difficult to coat the fibers. This high viscosity can lead
to voids, slow processing and partially coated fibers.
Advantages of using PREPREG:
Very precisely controlled fiber/resin ratios
Highly controlled tack and drap (in thermoset matrices)