New Options In Personal Ballistic Protection
Emerging materials and processing techniques expand choices for manufacturers of vests, helmets and shields.
By Alan S. Brown | March 2003
Today, personal ballistic protective gear is a sophisticated combination of advanced woven fibers, flexible laminates, and composite and ceramic hardplates in body armor augmented by composite helmets and shields — each component thinner, lighter, more effective, and less restrictive than ever before. In body armor, for instance, some models are scarcely visible under ordinary clothing. "The most protective soft armor vest you see today may weigh half as much as the equivalent unit three years ago," says John Dottore, who headed the DuPont Co.'s (Wilmington, Del., U.S.A.) Law Enforcement & Kevlar Body Armor segment for six years. Much of the progress has been recent, fueled by new fiber developments in the traditional aramids and some relative newcomers. And research continues unabated, as threats to military and security forces grow. "Now the goal is to stop multiple threats, from knives to high-energy bullets, using a solutions approach that takes advantage of a variety of materials and laminates," says Dottore.
Source: Honeywell
A helmet made with Spectra Shield® Plus material stops a 9 mm round and defeats a fragmentation simulator.
Lightweight aramid breakthrough
In the early 1970s, DuPont commercialized aramid fiber, under the trade name Kevlar. Long aramid molecules were dissolved and then spun into fibers that were stretched as they solidified. This process oriented the long molecules along the length of the fiber, greatly increasing the finished fiber's tensile strength (Kevlar 29: 2.9 to 3.0 Gpa, weight of 1.44 g/cc). Originally developed to replace steel in the reinforcement belts of car and truck tires, aramid proved useful as well for bulletproof vests.
The result was a revolution in personal protection. Previous forms of ballistic armor were heavy or impractical. Steel-plated World War II flak jackets were fine for aircraft gunners, but far too weighty for soldiers on foot. Vietnam-era vests with ballistic nylon and polycarbonate inserts could stop shrapnel but not rifle shot. But soldiers usually wore them open, when they wore them at all, because they were bulky and hot. Both were impractical for daily use by domestic police forces. Aramid vests, however, used layers of flexible, high-strength, densely woven fibers. When a bullet hits an aramid weave, the fibers absorb energy because they are able to stretch and resist breaking under severe impact. The vests were much thinner, lighter and more flexible — and they worked. Police could wear them under their uniforms, and they would stop bullets from most handguns wielded on city streets.
Armor makers adopted aramid composites to add helmets and riot shields to the mix, and to make lightweight hardface inserts for flexible vests, augmenting projectile stopping power. Aramid-reinforced composite helmets, for instance, are a basic part of the U.S. Army's Personal Armor System Ground Troop (PASGT) system.
Composites offer two-fold ballistic protection benefit: Fibers resist projectile penetration due to their inherent tensile strength as well as their initial rigidity because they are embedded in and stiffened by the resin matrix. Secondarily, dry fibers absorb energy as they elongate. This gives composites excellent resistance not only to shrapnel but handguns and some high-caliber rifle rounds, as well.
Kevlar competitors, of course, jumped into the market. Akzo Inc., now Teijin Twaron BV (Arnhem, The Netherlands), opened its first Twaron aramid fiber plant in 1978. Twaron has a chemical structure similar to Kevlar. Teijin also claims a patent position that covers certain deniers, contending that its fine denier yarns, which are made from 1,000 or more ultrafine microfilaments, spread ballistic impacts over more fibers and enhance the fiber's ability to withstand a hit.
Source: Honeywell
Spectra Shield Plus material engages a 124g, 9 mm FMJ/RN round, fired with a muzzle velocity over 1400 fps. Each layer, shown as vertical lines, extracts energy, thus decelerating the projectile, causing the round to mushroom just 25 percent of the way into the 36-layer, Level III-A ballistic package.
Armor makers improved ballistic properties through new weaves that increased the number of fibers involved in any ballistic event. Kevlar Correctional fiber, for instance, debuted in 1995, relying on a very tight, dense weave of thin fibers that enables more fibers to engage and absorb knife thrusts. They found that many separate woven layers of textiles actually did a better job of absorbing an impact than the same number of layers that had been stitched together. And they discovered that laminating textiles with a thin, flexible elastomer is often enough to stop a low-energy knife attack.
Polyethylene and PBO
In the 1980s, Allied-Signal, now Honeywell Performance Fibers (Colonial Heights, Va., U.S.A.), and DSM High Performance Fibers (Heerlen, The Netherlands) introduced ultra-high-molecular-weight polyethylene (UHMWPE) fibers. Honeywell's Spectra and DSM's Dyneema were large molecules spun and oriented much like aramid. Their unusual viscoelastic properties and greater stiffness enabled them to resist bullet penetration better than aramids (Spectra 900: tensile strength of 2.2 to 2.6 Gpa, weight of 0.97 g/cc; Spectra 2000, 3.2 to 3.5 Gpa, weight unchanged). These properties allow vest makers to equal aramid protection at 15 percent less weight (though not necessarily less bulk). Unlike aramid, polyethylene retains its properties when exposed to water, though its stiffness makes it more difficult to weave and its cost is much higher.
In 1998, Second Chance Body Armor Inc. (Central Lake, Mich., U.S.A.) and others began manufacturing vests that used a new fiber, poly (p- phenylene-2,6-benzobisoxazole), or PBO. Manufactured by Toyobo Co. Ltd. (Osaka, Japan) under the trade name Zylon, PBO has twice the tensile strength of aramids (5.8 Gpa; weight of 1.55 g/cc), but costs several times as much as aramid or polyethylene, adding hundreds of dollars to the final cost of a vest. Though PBOs enable vest makers to provide protection equivalent to aramid vests at half the thickness, armor manufacturer BSST Sicherheitstechnik GmbH (Nellingen, Germany) pulled its PBO vests off the market in 2001. Toyobo's own accelerated aging tests suggest that they will experience some performance decline during ordinary use, regardless of climate (based on Toyobo test figures posted on the BSST Web site, PBO showed a 15 percent decline in performance after 150 days at 40°C/104°F and 80 percent relative humidity). Despite questions, thinner vests remains a big draw. Many companies continue to sell PBO-based woven products, while engineers are looking for ways to use the fibers in hard armor.
Armor makers also have combined products to achieve synergistic effects. An all-PBO vest might be very expensive, but a vest containing a PBO/polyethylene hybrid to stop projectile penetration and aramid to prevent knife penetration may make a thinner, more wearable and more affordable vest.
