Buying Materials: Prepregs

Dateline: 05/03/98

Prepregs are probably the most difficult material to purchase. Because of their high price, long lead times, and limited shelf life, you want to make sure you get the order right the first time. And with hefty minimum order requirements, you don't want to find yourself short a half-roll at the end of the project.

Why Prepregs?

If you already know what a prepreg is, and why you want to buy one, then you can safely skip to the next section.

Prepreg is short for preimpregnated, and refers to a tow, unidirectional tape, or fabric which comes from the manufacturer already impregnated with resin. The resin is B-staged, or partially cured, to give it some tack, but this also limits its sheld life and usually requires freezer storage.

In addition to cold storage requirements, prepreg costs are higher than buying dry fibers and mixing your own resins. Why, then, would you want to use prepregs?

The answer can be boiled down to two words: quality control. The prepregger can tightly control both the resin mix ratio (resin:hardener) and the fiber volume fraction. Tolerances are commonly within a percent or two, and can even be held to less than one perccent.

Wet layups, and even filament winding, simply cannot maintain the uniformity of a prepreg. Prepregs are also less messy, so handling and cleanup costs can be lower.

Prepregs are shipped in dry ice and should be expedited to cold storage as quickly as possible. It doesn't happen too often, but I have seen shipments sit on the loading dock until they cure into a solid mass.

Large Suppliers

Any prepregger will supply you with a large quantity of material, but the largest will only supply large quantities. You may be able to order a single roll, but the hefty setup fees discourage such "small" orders.

This is not to say such suppliers are bad: their materials are quite good. You just shouldn't expect to approach them with a small order.

In the U.S., the largest prepreg suppliers are Hexcel, Cytecfiberite, and Toray. The former two now include portions of Hercules' former product line (Hercules spun off its advanced material division several years ago). Toray also has a plant in Japan. Thiokol has recently gotten into the business with its line of prepregs which can be stored at room temperature for a year.

All prepreggers formulate their own resins: this, in part, is what distinguishes the materials. Most purchase their fibers from outside sources. A few weave their own fabrics. Hexcel and Toray are somewhat unique in that they also manufacture their own fibers (Aldila (619-592-0404) does, too, but they haven't begun selling prepregs to external customers yet).

Small Suppliers

Small suppliers will fill large orders, but they usually don't charge a setup fee, and minimum orders are often a single roll. That's still a lot of material (50--100 yards), but better than you can do at the larger houses.

Most of the small suppliers specialize in a certain type of material, or they have a reputation in a specific field. BRYTE and YLA (707-747-2750) are probably best known for their cyanate ester prepregs. The Advanced Composites Group specializes in low temperature cure prepregs. FiberCote (203-755-1344) prepregs mainly glass and quartz fabrics with resins optimized for RF transparency.

Sometimes the large suppliers will sell you a single roll if they can fit it on the end of a larger run for another customer. They are most likely to offer this option if you are a regular customer. You shouldn't count on placing such an order, but the odds are improved if you purchase a common material.

Very Small Suppliers

When you want less than a roll, you have very few options. The only company I am aware of that regularly sells small quanities of prepreg is HeatCon. HeatCon isn't a prepregger, but the materials are certified. You will pay a premium for these small quantities: often double the original price.

FiberCote has a hand prepregger which can produce a few yards, but this is mainly intended for research purposes.

If you are cautious, you can purchase surplus materials. Usually these are materials ordered for a project which was cancelled, or simply leftovers from a large project.

Some companies, such as Composite Tooling Corporation and Advanced Composite Materials, specialize in the resale of surplus materials. You can also check the ads in the back of High Performance Composites.

A final option is to contact a composite manufacturer. Often, they will have excess material from a project which has been completed.

Other Sources

The prepreg suppliers with Web sites are listed on my Material Suppliers: Prepregs page. That page includes suppliers not listed on this page, and contains the most up-to-date links.

A good general source for materials and other manufacturing supplies is the Thomas Register. After registering, which is free, you can search the entire catalog online. A search for "prepreg" turns up 25 suppliers, most of which don't have Web sites.

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