The most common resin used in thermoset prepreg materials is epoxy. These prepregs are generally stored in a low-temperature environment and have a limited shelf life. Room-temperature prepregs are also becoming available. Usually, the resin is partially cured to a tack-free state called B-staging. Several additives (e.g., flame retardants, catalysts, and inhibitors) are added to meet various end-use properties and processing and handling needs. Thermoset prepregs require a longer process cycle time, typically in the range of 1 to 8 hr due to their slower kinetic reactions. Due to higher production needs, rapid-curing thermoset prepregs are being developed.
Thermoset prepregs are more common and more widely used than thermoplastic prepregs. They are generally made by solvent impregnation and hot melt technology. In the solvent impregnation method, the resin is dissolved by a chemical agent, creating a low-viscosity liquid into which fibers are dipped. Due to growing environmental awareness, disposal of the solvent resulting from this process is becoming a concern. The hot melt technology eliminates the use of solvents. In this process, the matrix resin is applied in viscous form. The drawback of this process is that fiber wetting is not easily achievable due to the higher viscosity of the resin.
Prepregs are generally used for hand lay-up, roll wrapping, compression molding, and automatic lay-up processes. Once the prepregs are laid on a tool, it is cured in the presence of pressure and temperature to obtain the final product.
Thermoplastic prepregs have an unlimited shelf life at room temperature and are generally processed at the melting temperature of the resin. The most common resins are nylon, polyetheretherketone (PEEK), polyphe-nylene sulfide, polyimide, etc. The process cycle time for thermoplastic composites is much faster than thermoset composites, in the range of a few minutes. It is a relatively new technology and provides several processing and design advantages over thermoset prepregs. The benefits of thermoplastic prepregs are:
Recyclability
Good solvent and chemical resistance
Reduced process cycle time
Higher toughness and impact resistance
Indefinite shelf life with no refrigeration
Reshaping and reforming flexibility
Greater flexibility for joining and assembly by fusion bonding and in situ consolidation
Better repairability potential
The disadvantages of thermoplastic prepregs are that they require higher temperatures and pressures for processing. They provide some processing difficulties because of their poor drape capabilities.
Thermoplastic prepregs are manufactured by solvent impregnation and hot melt coating techniques similar to thermoset prepreg manufacturing. Solvent impregnation becomes difficult because thermoplastics offer more chemical resistance. The hot melt coating technique is similar to an extrusion process, wherein fibers and resins are extruded simultaneously in sheet form. There are other manufacturing methods such as film stacking and dry powder deposition methods for prepreg fabrication. In the film stacking process, the thermoplastic resin film is stacked together with the reinforcements and consolidated under heat and pressure to fully imprepregnate the fibers. This process is clean and solvent-free but requires proper care for the production of a void-free prepreg. In the dry powder deposition technique, the resin must be in powder form as a starting material. The powder is fluidized and charged to form a resin cloud. The fibers are passed through the cloud and coated with the charged resin as they get attracted to the fibers. The coated fibers are then passed through a heat source to fully melt the resin and form a continuous sheet of material.