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.
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.
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 40øC/104øF 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.
Composites facilitate light hard armor
"None of the soft vests alone offer protection beyond handguns," says Alan Hannibal, president of composites manufacturer Composiflex Inc. (Erie, Pa., U.S.A.). His company uses Honeywell's Spectra Shield, a roll product that consists of 0ø/90ø unidirectional Spectra tapes bonded with a layer of elastomeric thermoplastic resin, usually a Kraton styrene block copolymer from Kraton Polymers LLC (Houston, Texas, U.S.A.) that absorbs low-energy impacts. Spectra Shield-based composites combined low weight and better bullet penetration resistance compared to aramids of equivalent weight. They soon found application as vest inserts, both as multi-ply hardface panels (e.g., breastplates) and as wraps for ceramics and other hard materials, and in helmets and riot shields.
"When it comes to protecting against high-powered rifles, you can use Spectra by itself to make a very lightweight breastplate," Hannibal says. "Composite plates help reduce the trauma associated with high-velocity impacts. If you're wearing a vest and you're hit over the heart, the trauma will actually bruise or even stop your heart. A chest plate reduces that trauma."
Plates generally involve simple layups, since they require only minor curvatures to accommodate the shoulders. "We calibrate our processing equipment regularly and it's all computer-controlled," Hannibal explains. "We want material that is consistent and that has a wide processing window. Spectra Shield has always done that for us, and it makes a lighter breastplate."
Composiflex usually compression molds the plates. At one time, they were processed 200 psi, but now run at 2,500 to 3,000 psi and even higher because Honeywell's research demonstrated that greater forming pressure reduces backface deformation caused when bullets that partially penetrate the shield push out part of the plate's backside. Too much deformation can cause trauma and damage internal organs. Consolidation temperatures run =127øC/= 260øF. Above this temperature=, Spectra fibers lose their uniform orientation, and therefore some of their tensile strength. Composiflex currently consolidates four 10-inch by 12-inch breastplates at a time. "We're planning to switch to a larger press and increase that to 12 plates at a time", says Hannibal. "When we're fully operational, we'll be able to make 100,000 units per year."
The company plans to make an all-composite, polyethylene-based plate and, for military use, a thin ceramic plate wrapped with and/or bonded to Spectra in an autoclave. In the latter, the hard alumina, silicon carbide, or boron carbide ceramic face breaks up the bullet into small fragments and the composite wrap prevents them from penetrating through the plate.
Gentex Corp. (Carbondale, Pa., U.S.A.), one of the largest manufacturers of military helmets, uses a variety of materials and processes to make them, says rapid response engineering manager Brad Sutter. While some government agencies specify starting materials, others provide performance requirements such as weight, impact, and allowable deformation on the inside of the helmet.
"We use aramid, polyethylene and carbon fiber, to meet those performance goals," says Sutter. "We might use separate layers of each material, or hybrid fabrics."
Many of the attributes that differentiate fiber performance in soft armor do not translate to hard composites. Kevlar and Twaron tend to behave similarly, while Spectra offers better penetration resistance than Kevlar, though it tends to pose more drapability problems, a significant issue in helmet production, due to the deep draw. To circumvent this problem, Gentex cuts aramid or polyethylene fibers or weaves into pinwheel patterns and hand lays them up into preforms, overlapping each layer to ensure complete coverage prior to compression molding. Gentex uses both Spectra Shield and unidirectional Spectra tapes, the latter for helmets that require lighter weight and improved performance. The pinwheel strategy has some disadvantages, says Shawn Walsh, team leader of the U.S. Army Research Laboratory's Intelligent Materials & Processing Group (Nattick, Mass., U.S.A.), because overlapped pinwheel patterns sometimes produce variations in fiber volume and areal density — and protection — in parts of the helmet.
For that reason, Walsh recently signed an agreement with the Diaphorm Div. of Solectria Corp. (Woburn, Mass., U.S.A.) to fund development of an automated preform process that could produce a more consistent, easier-to-handle preform while lowering costs at least 20 percent by reducing touch labor and materials waste. The company's Diaphorm performing process is designed to glue fabrics together using minimal amounts of off-the-shelf binders, says division vice president/general manager Bob Miller. Too much binder prevents resin from infiltrating the preform, a serious problem in helmets that require high fiber volume. Binders also can cause bridging in corners. The Diaphorm process uses a proprietary method to apply binder that reportedly achieves tight tolerances of ñ5 percent fabric weight. To make the preform, a vacuum is drawn on a male mold and fibers are layed up. The vacuum keeps the fibers in place while a reusable, transparent silicone diaphragm comes down over the preform. A bank of infrared lamps then rolls to the preform and heats the binder. It takes only 5 to 7 minutes to lay up and sinter a four-layer preform, says Miller. The process makes thicker preforms by laying up to eight layers at a time, sintering them in 7 to 8 minutes. The process yields a stable "drop-in" preform that require no special handling when used in resin transfer molding (RTM), structural reaction injection molding (SRIM), or vacuum-assisted RTM (VARTM) station, he says.
Diaphorm also has developed a thermoplastic composite forming process — in which a preform is infused with resin at low pressure — that could be used for helmets and protective inserts. Miller says it is cost-competitive with compression molding in product runs of 2,000 to 50,000 pieces. The equipment also costs much less, since it operates at 40 psi instead of 300 to 1,500 psi for compression molding units. Diaphorm starts with Hexcel Corp.'s (Stamford, Conn., U.S.A.) thermoplastic-coated Twoflex fabric or Saint-Gobain Vetrotex America Inc.'s (Maumee, Ohio., U.S.A.) thermoplastic commingled Twintex fabric. In a riot helmet, Diaphorm might sandwich Twintex with Kevlar for additional reinforcement, says Miller.
Using the Diaphorm process, the armor maker lays up the thermoplastic prepregs on a single-sided mold and covers it with a diaphragm. A bank of infrared lamps applies heat to wetout the fabric. A proprietary method is used to apply pressure and consolidate. It takes only three to six minutes to consolidate a four-layer system, says Miller.
While many companies sell police riot shields, Composiflex is one of a handful that actually manufactures them, favoring polyethylene fibers because shields tend to be large, heavy objects, though he uses aramid in some models.
In either case, he typically starts with woven fabrics, though he also uses 0ø/90ø tapes to enhance certain properties. The most common matrix is polyvinyl butyral (PVB), a very tough resin used to manufacture shatterproof glass for automobiles. PVB has been written into military specifications that many law enforcement organizations have adopted for their own use. He also has worked with vinyl esters, urethanes, Kraton, and other elastomeric matrices that deform easily.
Most shields have only modest curvatures, which makes for easy layups and consolidation. Once molded, though, they require significant post-processing. Composiflex, for example, uses a water jet to cut them to shape and carve out the viewing window. A 5-axis router cuts holes to attach them to aluminum or ABS skins and handles. Depending on the threat level, the window may require several thick, clear polycarbonate inserts and a machined bezel to hold them in place. Finally, an elastomeric edging is attached to the part to keep out moisture.
Stopping future threats
Despite advances, present technologies do not provide complete protection, especially for the military. Fiber vests and composite helmets do a good job against shrapnel from artillery or grenades, which account for 70 to 85 percent of the battle casualties since World War II. The remaining casualties come from high-powered rifle fire.
Second Chance's research director Aaron Westrick, for example, is looking for better hardface inserts to meet this threat. He has experimented with cermets — ceramic particles in a metal matrix. A bullet that strikes a cermet tends to fragment when it hits the hard ceramic particles in the relatively soft metal matrix. Steve Monette, who works with Westrick, notes that by controlling how cracks propagate through metal matrix composites, the company hopes to improve on ceramic insert performance without incurring a price penalty.
Howard L. Thomas, a professor of textile engineering at Auburn University (Auburn, Ala., U.S.A.), points out that the most dangerous weapons discharge long, spin-stabilized projectiles that use their aspect ratio to punch through armor. Thomas proposes a series of metal pyramids and balls suspended in composites to destabilize the round. "The more surfaces a projectile encounters, the more strain or shock waves are set up to knock it off course and blunt its impact," he explains. Although this concept was originally developed for vehicular armor, Thomas believes it will work with personal protective gear, as well.
There is much to be done in the area of weight management, as well. U.S. soldiers carry up to 48 kg/105 lb of equipment into battle — far too much for a mobile force to maintain under combat conditions. In the U.S., the Army Research Laboratory (ARL) is rethinking how future armor should function in its Objective Force Warrior (OFW) program. Its goal is to reduce weight by integrating some of the soldier's gear into his lightweight armor.
"OFW is developing the soldier-as-system concept," ARL's Walsh explains. "Soldiers now look like Christmas trees with all the equipment they carry, and this limits their ability to accomplish their mission." OFW seeks to integrate range finders, electronics, antennas, and optical and acoustic sensors into the hard composites used, for example, in helmets. Making the systems modular would enable soldiers to reconfigure them depending on their mission. In the same way, says OFW project leader Matt Correa, body armor would act as a chassis and load carriage for ammunition, medical supplies, water, and radio.
To get there, the Army will require new materials. OFW has penciled in PBO for its probable approach to composite helmets and composite armor, says Correa, adding that there is an outside chance OFW will use a new type of reinforcement called M5. Developed by Magellan Systems International LLC (Bethesda, Md., U.S.A.), it is a rigid rod molecule whose hydrogen bonds reinforce it in three dimensions rather than just two. According to Correa, M5's tensile strength is three times that of Kevlar 129 (8.5 Gpa vs. 3.2 Gpa) and nearly two-thirds greater than PBO (5.8 Gpa).
PBO's excellent ballistic properties do not fully translate to composites, says Westrick, contending that it tends to hug the matrix too tightly, and so it does not achieve its energy absorption potential through fiber pullout. Also, the Berry Amendment prohibits the use of imported fibers for military applications. (This amendment to the Buy American Act of 1993, under DFAR Subpart 225.7002, permanently codified restrictions present in the U.S. Dept. of Defense (DoD) Appropriations Act since 1941, specifically requiring, with certain exceptions, that all equipment sold to the U.S. military be manufactured in the U.S. and be made of U.S. materials.) For the technology to move forward, a PBO plant would have to be built in the United States or a U.S. producer would have to be licensed to use the PBO production process.
While the Objective Force Warrior program's goals lie many years in the future, it seems certain to help set the research agenda for the personal protection industry, push engineers to rethink what personal armor can do — something armor makers have become accustomed to in their rapidly changing industry.