Kevlar was introduced by DuPont in the 1970s. It was the first organic fiber with sufficient tensile strength and modulus to be used in advanced composites. Originally developed as a replacement for steel in radial tires, Kevlar is now used in a wide range of applications.
Kevlar is an aramid, a term invented as an abbreviation for aromatic polyamide. The chemical composition of Kevlar is poly para-phenyleneterephthalamide, and it is more properly known as a para-aramid. Aramids belong to the family of nylons. Common nylons, such as nylon 6,6, do not have very good structural properties, so the para-aramid distinction is important. The aramid ring gives Kevlar thermal stability, while the para structure gives it high strength and modulus. The University of Southern Mississippi Department of Polymer Science has a good description of aramid chemistry, including drawings of the chemical structure.
Like nylons, Kevlar filaments are made by extruding the precursor through a spinneret. The rod form of the para-aramid molecules and the extrusion process make Kevlar fibers anisotropic--they are stronger and stiffer in the axial direction than in the transverse direction. In comparison, graphite fibers are also anisotropic, but glass fibers are isotropic.
[The Kevlar precursor is very stable both chemically and thermally. This presented a big processing challenge, which DuPont scientist Stephanie Kwolec ultimately solved as poetically described on the UMiss site mentioned above.]
Today, there are three grades of Kevlar available: Kevlar 29, Kevlar 49, and Kevlar 149. The table below shows the differences in material properties among the different grades. If you purchase Kevlar cloth, it is most likely Kevlar 49.
The tensile modulus and strength of Kevlar 29 is roughly comparable to that of glass (S or E), yet its density is almost half that of glass. Thus, to a first approximation, Kevlar can be substituted for glass where lighter weight is desired. Kevlar 49 or 149 can cut the weight even further if the higher strength is accounted for. What gives Kevlar its high strength? Berkeley Lab's MicroWorlds provides some clues. Of course, Kevlar's weight savings does come at a price: Kevlar is significantly more expensive than glass (see Buying Materials: Small Quantities, Dry Fabrics and Tapes for a rough price comparison).
Kevlar has other advantages besides weight and strength. Like graphite, it has a slightly negative axial coefficient of thermal expansion, which means Kevlar laminates can be made thermally stable. Unlike graphite, Kevlar is very resistant to impact and abrasion damage. It can be used as a protective layer on graphite laminates. Kevlar can also be mixed with graphite in hybrid fabrics to provide damage resistance, increased ultimate strains, and to prevent catastrophic failure modes.
Like all good things, Kevlar also has a few disadvantages. The fibers themselves absorb moisture, so Kevlar composites are more sensitive to the environment than glass or graphite composites. Although tensile strength and modulus are high, compressive properties are relatively poor. Finally, Kevlar is also very difficult to cut. You will need special scissors for cutting dry fabric or prepreg, and special drill bits for drilling cured laminates. Cutting of cured laminates without fraying is very difficult.
Kevlar is used both as a raw fiber and in composites. For a sample of these applications, see the DuPont Kevlar site. For a good example of working with Kevlar composites, see Ray Jardine's Kayak Construction page.