Fiber composite armor II: Aramid
A strong fiber derived from highly aligned aromatic polyamide polymer chains, aramid is, without doubt, the most ubiquitous ballistic fiber in the world at the time of this writing. It is extensively used in standalone body armor systems, spall liners and backings, ballistic helmets and shields, and virtually every other conceivable application — including “armoring” fiber optic lines for extended stays in harsh environments, and armoring satellites from micrometeorite impact. It has excellent resistance to fire and heat, but is sensitive to UV light (photooxidative degradation) and humidity (hydrolysis), so it is typically sealed or coated prior to use, and its characteristic bright yellow color is infrequently seen.
Aramid is often supplied and utilized as woven sheets of fabric. It often isn’t a “composite,” for it very frequently isn’t reinforced with any resin at all. Aramid helmets, however, are composites typically made with 12-18% by weight of polyvinyl butyral (PVB)-phenolic resin, for rigidity and structural stiffness.
Aramid – Kevlar 29
Ultimate Tensile Strength: 3620 MPa
Modulus of elasticity: 70.3 GPa
Elongation at break: 3.6%
Aramid – Kevlar KM2
Ultimate Tensile Strength: 3880 MPa
Modulus of elasticity: 84 GPa
Elongation at break: 4.52%
Uses in armor systems:
Soft body armor and military flak jackets.
Helmets. (Growing obsolete but still very common.)
Plate backer in ceramic body armor systems. (Obsolete.)
Spall liner in combat vehicles.
Many other non-structural applications where resistance to heat is a requirement.
In 1965, just as the Vietnam War was ramping up, DuPont scientist Stephanie Kwolek was working on a research project tasked to find high-tensile-strength fibers for use in reinforcing automobile tires. To this end she was experimenting with the stiff polymer precursors poly-p-phenylene terephthalate and polybenzamide, when she noticed that the reaction product of the two precursors displayed some unusual properties. As she later explained:
“The solution was unusually (low viscosity), turbid, stir-opalescent and buttermilk in appearance. Conventional polymer solutions are usually clear or translucent and have the viscosity of molasses, more or less. The solution that I prepared looked like a dispersion but was totally filterable through a fine pore filter. This was a liquid crystalline solution, but I did not know it at the time.”
Unusual-looking chemical solutions of that sort were not often pursued; they were usually considered failed or abortive experiments. Kwolek went on to mention that “I think someone who wasn’t thinking very much or just wasn’t aware or took less interest in it, would have thrown it out.” But she persisted, so the solution was spun on a spinneret, and the mechanical properties of the resulting fibers were tested. It quickly became apparent that this new “aramid” fiber was three times as strong as nylon and possessed a far higher modulus and heat resistance. And, despite its high modulus, even these first fibers were so fine that woven fabrics derived from them were flexible, and were able to drape like nylon.
The military quickly realized the potential of this new material, though they didn’t rush to implement it in armor systems and issue it to troops in Vietnam. Military-grade aramid fabrics were first designated as PRD-29, and PRD-49 — and later as Kevlar® 29, which was previously PRD 49-IV, and Kevlar® 49, which was PRD 49-III. Kevlar 29 was judged the superior grade for ballistic protection, and prototype flak vests were made which offered significantly greater protective ability than those made of nylon and Doron, at the same weight.
In June 1978, vests comprised of 13 layers of Kevlar 29 ballistic fabric within an envelope of water-resistant ballistic nylon were issued as the “Body Armor Fragmentation Protective Vest, Ground Troops” for United States soldiers. In the early 80s, this type of vest was standardized as part of the PASGT armor system, which also included a helmet made of resin-bonded laminate Kevlar 29. The PASGT system was quite long-lived — it remained in use by the US military until the mid-2000s, and still remains extremely common worldwide, as it was extensively exported. The PASGT’s successor, the Interceptor Body Armor system, was also made of aramid — in this case, the improved KM2 fiber. The most recently fielded body armor system, the Improved Outer Tactical Vests, which are a partial replacement for the Interceptor, still incorporate aramid fibers in their soft armor component.
Yet this is not to say that aramid is the only option, nor should it even imply that aramid is the best option; as of this writing, it has largely been superseded by polyethylene-based materials.