Modern armor is bullet resistant, not bulletproof. With enough ballistic energy, soft armor can be pierced. This means that body armor is a balanced compromise, with wearability and comfort balanced against effectiveness. The result is a vest that is thin, light and comfortable enough that officers will wear it, even in the warmer climates, and still efficient enough to stop the most likely bullets. One can always build a heavier vest, but if it is uncomfortable enough, officers won't wear it. Then all benefit is lost.
Modern armor does not deflect bullets. Rather, the armor catches the bullet, dispersing its energy across a wider area of fabric, thereby preventing penetration. Think of the difference between a thrown baseball and a hunting arrow launched from a bow. Upon impact, the arrow focuses all of its energy on a very small part of the target. As the arrow's point enters the target, the arrowhead's shape opens a larger wound, which allows the bulk of the projectile to pass through.
Conversely, a baseball thrown with the same energy behind it will hit a larger area of the target's surface, but the energy is spread over a larger area; the energy is dissipated without penetration.
Early soft body armor was made of nylon (a polyamide) and had fairly poor ballistic performance. It required many thick layers for increased effectiveness. Today, the high strength fibers that are used in the production of bullet resistant panels for soft body armor are of two primary types, 1) high performance polyethylene fibers, otherwise known as HPPE, and 2) aramid fibers. Honeywell's Spectra® and DSM's Dyneema® are HPPE fibers. DuPont's Kevlar® and Teijin Twaron's Twaron® are aramid fibers.
Believe it or not, HPPE fibers are made of the same basic materials that go into plastic milk cartons--but with a different structure--and they are capable of very good ballistic performance. Due to the chemical structure of these HPPE fibers, they are actually more like a plastic than a fiber, thus movement of air and release of moisture through the fabric is much more difficult. Many armor manufacturers have worked diligently to engineer solutions to these two problems.
Aramid fibers were developed in the mid-1960s by DuPont. Aramids are a subgroup of polyamides (nylon). However, due to differences in their molecular structure, they are capable of much greater ballistic performance. These are multifilament synthetic fibers characterized by very low flammability and no melting point, as well as very good fabric integrity at higher temperatures. Aramids contain aromatic rings--chemical structures that are very strong. These rings are combined into long molecules that are very linear in nature. These structures have to have some "give", but not too much. A good analogy is a glass window as opposed to a Plexiglas window: glass is brittle, a bee that flies into it will bounce off, but a baseball will shatter it. Plexiglas, on the other hand, has some give. A baseball will bounce off, but a bullet will penetrate it. The degree to which the window is resistant to a force hitting it is a function of the mass and velocity of the object combined with the flexible structure of the window.
By the way--consider this: when a bullet strikes a piece of bullet resistant material, that material has to instantly slow the projectile from a speed of perhaps 1,400 feet per second to zero feet per second. That is a tremendous amount of energy--where does it go? The law of conservation of energy tells us that it has to go somewhere--it doesn't just disappear. It travels outwards, along the strands of bullet resistant fiber, dissipating as it goes, while some is conveyed through the ballistic panel (although the projectile is stopped, some of the energy continues forward as it diminishes). The challenge for body armor fiber manufacturers, fabric weavers, and armor manufacturers, is to develop products that will dissipate the energy in the proper ratio, so that the projectile will stop and the energy will dissipate with minimal injury to the wearer.
Incidentally, another class of fibers exists, known as PBO fibers, marketed by the Japanese company Toyobo. The trade name for this fiber is Zylon, and while it is known to be very effective ballistically and lightweight, Zylon vests have given rise to concerns regarding how well they retain their ballistic integrity over time. Since these issues are broader than the scope of this article, Zylon will not be discussed here.
Early vests were woven, in a process similar to other fabrics, then multiple layers of fabric were sandwiched together to form vest panels. To raise the stopping power of these early vests, more layers were added. Of course, that made them heavier and hotter.
Current technology is different. Instead of a weave pattern, the fibers are laid out in parallel fashion. Think of a bunch of pencils laying side by side on a table, so that their long sides are touching all along their length, like the construction of a wooden raft made of logs. The second layer of fibers is laid atop the first layer, with the fibers running at right angles to those in the first layer. The next layer is also laid at right angles to the second layer, and so on. While simplistically put, this image conveys the idea of a more effective fiber arrangement.
When a projectile strikes ballistic fabric, it is important that the energy be able to flow away from the point of impact in order to dissipate the energy over a larger area. The faster the energy can flow away from the point of impact, the more the energy is spread across a larger area; thus less energy is deposited into the impact point, and less "shock" energy is transmitted through the fabric to cause backface deformation against the underlying tissue.
Backface deformation, an indentation on the wearer's side of the armor, is what causes blunt trauma, or the heavy bruising of the wearer. Weaving the fibers is less effective at energy dissipation than the previously described layering. When one weaves fibers into a sheet of ballistic fabric, the points where the fibers cross each other form something akin to energy roadblocks. Energy will flow past these junctions, but will be slowed along the way.
On the other hand, when the fibers are laid parallel to each other, the energy is allowed to flow unimpeded, and therefore more quickly and efficiently, away from the impact point. Less trapped energy near the impact site means less energy moving forward into the panels and into the body. Ergo, less injury.
This change in assembly techniques also increased the effectiveness of the fabrics at stopping rounds, since there is an energy dissipation benefit in that regard as well. One of the primary benefits of this evolution is increased capability at stopping multiple hits.
Corrections versus Patrol
In the corrections environment, protection against bullets is not the problem, instead, protection from slashing, cutting and stabbing is. Many vests intended for street law enforcement are just not suitable for work in a correctional setting. While ballistic panels offer some protection, an ice pick or similar instrument can sometimes penetrate them.
Enter armor specially designed for corrections officers. Frequently worn over the uniform as an outer garment, this armor is often thicker. Additionally, the actual construction of the panels is different. The yarns or filaments in the fiber are finer, so they are smaller and more densely packed. This helps to reduce the likelihood that a shank or an ice pick will penetrate. One option for corrections officers is a multi-threat vest, designed for both projectile resistance as well as correctional threats.
Vests Wear Out
Ballistic fibers can gradually lose their effectiveness and efficiency over time, especially if they are routinely exposed to heat and moisture, as they are when worn daily (especially in hot climates). Vests have a life expectancy of five years or so. Check manufacturer's recommendations. That doesn't mean that they suddenly "switch off" their effectiveness when they expire, but the expiration date is the date specified by the manufacturer as the last date that they expect the vest to still be within specifications taking into account normal wear and tear.
One final note; Whatever officers may have heard, or read, regarding the recent problems with Zylon, the fact is that most vests out there are not made of Zylon. And, even if your vest contains Zylon, wearing your vest until it can be replaced is absolutely the safest option.
Stay safe, and wear your vest! (and Buckle Up!)