Prepreg composite materials are becoming increasingly common in the composite industry due to their ease of use, consistent properties, and high quality surface finish. However, there is much to understand about prepregs prior to committing to using this material.
What are Prepregs?
The term "prepreg" is actually an abbreviation for the phrase pre-impregnated. A prepreg is an FRP reinforcement that is pre-impregnated with a resin. Most often, the resin is an epoxy resin, however other types of resins can be used, including the majority of thermoset and thermoplastic resins. Although both are technically prepregs, thermoset and thermoplastic prepregs are dramatically different.
Thermoplastic prepregs are composite reinforcements (fiberglass, carbon fiber, aramid, etc.) that are pre-impregnated with thermoplastic resin. Common resins for Thermoplastic prepregs include PP, PET, PE, PPS, and PEEK. Thermoplastic prepregs can be provided in unidirectional tape, or in fabrics that are woven or stitched.
The primary difference between thermoset and thermoplastic prepreg is that thermoplastic prepregs are stable at room temperature, and generally, do not have a shelf life. This is a direct result of the differences between thermoset and thermoplastic resins.
More commonly used in prepreg composite manufacturing is thermoset prepregs. The primary resin matrix used is epoxy. However, other thermoset resins are made into prepregs including BMI and phenolic resins.
With a thermoset prepreg, the thermosetting resin starts as a liquid and fully impregnates the fiber reinforcement. Excess resin is precisely removed from the reinforcement. Meanwhile, the epoxy resin is undergoes a partial curing, changing the state of the resin from a liquid to a solid. This is know as the "B-stage."
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.
Advantages of Prepregs
Perhaps the greatest advantage of using prepregs is their ease of use. For example, say one is interested in manufacturing a flat panel out of carbon fiber and epoxy resin. If they were to use liquid resin in a closed molding or open molding process, they would be required to obtain a fabric, the epoxy resin, and the hardener for the epoxy. Most epoxy hardeners are considered hazardous, and dealing with resins in a liquid state can be messy.
With an epoxy prepreg, only one item needs to be ordered. An epoxy prepreg comes on a roll and has the desired amount for both resin and hardener already impregnated in the fabric.
Most thermoset prepregs come with a backing film on both sides of the fabric to protect it during transit and preparations. The prepreg is then cut to the desired shape, the backing is pealed off, and the prepreg is then laid into the mold or tool. Both heat and pressure are then applied for the specified amount of time. Some of the most common types of prepregs take an hour to cure, at around 250 degrees F, but different systems are available at both lower and higher cure temperatures and times.
Disadvantages of Prepregs
Since the epoxy is in a B-stage, it is required to be stored either refrigerated or frozen prior to use. Additionally, the overall shelf life can be low.
When manufacturing composites through a process such as pultrusion or vacuum infusion, the raw fiber and resin are combined on site. However, when using prepregs, the raw material must first be prepregged. This is most often done off-site at a specialized company that focuses on prepregs. This added step in the manufacturing chain can add increased cost, and in some instances close to double the material cost.