Epoxy Prepreg

Advantages of Epoxy Prepreg

Basic knowledge of Epoxy Prepreg

“Prepreg” is broad terminology in composite fabrication used for describing fabrics that are pre-impregnated with a fully curable, mixed resin system during manufacture. Prepregs are often regarded as the most advanced system of composite fabrication. By simply adding a mold, heat, and pressure, use of prepreg fabrics virtually eliminate process variation found in traditional layups or vacuum infusion processes. As a result, prepregs have revolutionized composite fabrication as we know it and are most often chosen for achieving the highest performance characteristics in fiber reinforced structures.

Carbon Fiber, Kevlar, and Fiberglass are the most used fabrics in Prepreg reinforced composites. Historically, Prepregs most popular use was aerospace and aviation applications. As technology has advanced, prepreg fabrics are now more affordable and have found additional market value in high performance applications of automobile manufacture and racing, boating, sporting goods, and even construction.

Prepregs are a calculated combination of reinforcing fabric paired with a semi-cured resin system that is most often Epoxy based. This matrix, a calculated weight ratio for prepregs is responsible for achieving the light-weight performance characteristics in a composite laminate. Prepregs can achieve a ratio as low as 35% resin, which is unheard of with any other layup methods.

Basic prepreg selection is based on characteristics such as fiber, tow size, and weave selection, all of which are similar features to that of traditional fabrics. Prepregs will often come with a defined “cured ply thickness” and density in addition to providing the precise fiber to resin ratio. The resin added to the fabric also doubles to make layup processes much more effective as they carry a tack in making layup more effective.

Because resins in prepregs flow and cure in a different manner than infusion or traditional layup methods, they require pressure and heat for cure. Once a layup of prepreg fabrics has been completed, it must then be cured by a combination of heat and vacuum. This is often referred to as a cure cycle performed inside an oven or an autoclave. For complex laminates, an autoclave, like a large pressure cooker, provides pressure far beyond vacuum alone. General epoxy prepregs are commonly cured between 250F and 350F.

How to use Epoxy Prepreg

First, prepare your mold as you usually would. To obtain a glossy finished part, it is necessary to have a flawless mold which has been waxed and coated with PVA. The prepreg is supplied between two backing sheets which need to be peeled away before laying the fabric into the mold. Because the prepreg surface is very tacky, it is necessary to place the material carefully into the mold. Unlike a dry fabric in a hand lamination, you cannot reposition the prepreg without fixing your mold release. Repositioning will pull up the PVA. You can continue to place layers on top of each other up to a total thickness of one-quarter inch at a single time.

The prepreg is partially cured which means that it is in B-Stage. It can be trimmed, pleated, and formed using a knife or a pair of scissors. The prepreg handles very easily and because of its tackiness, it will lay tightly into your mold. You might still have relief cuts or pleats to contend with just like you would with a dry fabric.

After placing the layers in the mold, the prepreg must be compressed to allow bonding, to remove any trapped air between the layers and to squeeze out any excess resin. Although there will be very little excess, the resin will naturally become thinner as the temperature is raised and there will be resin flow prior to a full cure of the resin.

The needed compression can be accomplished using a press but unless the part is perfectly flat, a press is not an option. Consequently, the part is generally vacuum bagged.

With prepreg fabric, there is a benefit in having plenty of time to build your bag and get the part ready for the oven. You are not fighting the natural pot life of the resin. However, the more layers you have laid, the more likely that there will be air trapped between the layers. You should really have perfected your vacuum bagging technique before laying multiple layers of prepreg for a single curing.

A big advantage of prepregs is that after curing and cooling, additional layers can be laid onto the part without scuffing or additional surface preparation. Again, up to one-quarter inch can be laid at a single time with the same consideration of not trapping air between layers.

After the layup is complete, the vacuum bag is built and sealed, and you are ready to pull vacuum and begin the heat cycle. There are three recommended cure cycles for Fibre Glast prepregs and all will produce similar properties.

All curing cycles begin with a temperature ramp up and end with a ramp down. The difference is the target temperature and the amount of time required for a complete cure. Always ramp up at a rate of no more than 5°F per minute until the target temperature is attained. Maintain the target temperature throughout the cure cycle and then ramp down at a rate of less than 5°F per minute to at least 150°F before removing from the oven.

Let the part cool to room temperature, remove the vacuum bagging materials, and release the part from the mold. Your part should be fully cured and can be put into immediate service.

There are several considerations when determining the amount of time and heat to use. First, if you are not in a hurry, plan to go low on temperature and long on time. You can leave the part in the heat up to 24 hours without any risk of damage to the part. You cannot "overcook" your prepreg. You can, however, "undercook" it.

Second, when determining your temperature and cure cycle, consider the mass and conductivity of your mold. If it is thin metal (low mass and conductive), it will heat up quickly and not interfere with the curing cycle. If it is thick fiberglass (high mass and not conductive), it will take a while to heat up and you will want to be on the conservative side of time and temperature.