The first composites were a mixture of straw and mud prepared by our ancestors nearly 4,000 years ago, said Dr. Andrew George of Brigham Young University at yesterday’s CAMX pre-conference tutorial, “Introduction to Composite Materials Technology.” Today’s materials are obviously far more complex, offering a dizzying array of choices that can overwhelm industry newcomers. However, with Dr. George as a guide, industry novices left yesterday’s tutorial ready to make savvy materials decisions.
The “mud” in modern composites is the resin or matrix. Ninety percent of resins are polymer based, while the remaining matrices are primarily metal or ceramic. Most polymer-based resins are unsaturated polyester (UPE) because they are inexpensive and can be cured at room or elevated temperatures. Building the perfect UPE, said George, is simply a matter of working with your resin supplier to choose the right combination of properties like strength, flame resistance, thermal properties, durability, weatherability and resistance to solvents and water. Additives such as pigments and flame retardants also can be included. Epoxies are the second most widely used resins, followed by vinyl esters and phenolics. These produce increased stiffness and thermal properties, but are more expensive and require higher temperatures than UPE.
Today’s “straw” – the fiber or reinforcements that provide strength – come in three basic types: fiberglass, carbon or aramid fibers. Ninety-six percent of composite materials are reinforced with fiberglass, the most cost-effective fiber. Carbon fiber, which George said is “one of the stiffest (materials) for its density on the face of the planet” are the second, but far more expensive. Most carbon fiber (90 percent) is made from polyacrylonitrile (PAN) precursor, with the rest derived from petroleum pitch and rayon. Aramid fibers, used in bulletproof vests, are the most expensive.
Modern fibers are very thin. With a diameter of just seven microns, carbon fibers are barely visible to the naked eye. (By comparison, a human hair is 100 microns.) Consequently, fibers are most often purchased in the form of roving or tow (yarn); mats made from chopped strand fibers held together with glue; woven fabrics; or prepreg, non-crimp fabrics with unidirectional fibers held together by glue or stitches.
Fiber and resin matrices have to be combined. The strongest materials have the highest amount of fibers and least amount of resin possible while still allowing the resin to transfer well (or surround every fiber). Fiberglass composites, for example, are generally 65 percent fiber. The most common method of manufacturing composites remains hand lay up of materials into an open mold. Thermoplastic processes use very short fibers, sometimes called “whiskers” of fiberglass, nylon, polycarbonate or polypropene with injection molding technology. Thermosets use compression to produce higher fiber content, but require very large, expensive presses to do so.