Introduced by the US Army in the 1950s, during the Korean and Vietnam wars, they were true armor, made of steel plates sewn into the cloth – which made them heavy and uncomfortable. The great revolution that allowed the emergence of modern vests was the invention, in 1965, of kevlar: an aramid fiber, a synthetic material similar to nylon, light and flexible but five times more resistant than steel. Made of several layers of fiber intertwined, these vests receive the bullet, flattening its point and distributing its impact throughout the fabric, until it paralyzes it completely. The number of layers determines the protection level of the vest – which can go from one to four, according to an American standard accepted worldwide. For certain occasions, when the heaviest weapons are expected, it is possible to achieve an even higher degree of protection, reinforcing the vest with rigid plates of especially resistant ceramic.
Kevlar reigned alone until, in the 1990s, the American company AlliedSignal patented spectra, a polyethylene fiber ten times stronger than steel. Its molecules are evenly stretched in one direction, so they don’t warp. These state-of-the-art vests, called Spectra Shield, accumulate layers of this fiber glued perpendicularly, forming a very strong network. “Unlike previous models, it does not deteriorate over time, does not lose strength in contact with water and offers more protection with fewer layers, therefore with less weight”, says Captain Régis Rocha da Rosa, from the Rio Grande Military Brigade do Sul, scholar of the subject. Since 1997, vests purchased for Brazilian police forces have been Spectra Shield.