Кevlar — the trading name the synthetic fibre possessing high durability (is five times stronger some steel). It is developed by American company DuPont in 1965, with the beginning 1970-x years there is begun its commercial application. Mechanical properties of a material do its suitable for manufacturing bulletproof waistcoats. This one of the most known applications of kevlar.
In 1970th years to one of the most significant achievements in development of flak jacket became the invention of the company Dupont of a reinforcing fibre from kevlar. Originally the fibre was is intended for replacement of steel armature in automobile tire covers. Development flak jacket from kevlar National institute of justice (National Institute of Justice) occured within several years in four stages. At the first stage the fibre from kevlar was tested to define, whether it is capable to stop a bullet.The second stage consist in definition of quantity of layers of the material necessary for prevention of punching by bullets of various calibre and flying with different speed, and development of the prototype of the waistcoat, capable to protect employees from the most widespread threats: bullets of calibre 38 Special and 22 Long Rifle. By 1973 from seven layers of a fibre from kevlar the waistcoat has been developed for field tests. It has been established, that at drench protective properties kevlar worsene. Ability to protect from bullets also decreased after influence of a ultraviolet, including a sunlight. The dry-cleaner and bleaches also negatively affected protective properties of a fabric, also as well as numerous washings. To bypass these problems, the waterproof waistcoat having a covering from a fabric for prevention of influence of solar beams and other negative factors has been developed.
Кevlar keeps durability and elasticity at low temperatures, down to cryogenic (-196°C), moreover, at low temperatures it even hardly becomes stronger. At heating kevlar not blow, and decays at rather heats (430-480°C).The temperature of decomposition depends on speed of heating and duration of influence of temperature. At the raised temperatures (more 150°C) durability kevlar decreases eventually. For example, at temperature 160°C breaking strength decreases for 10-20 % after 500 hours. At 250°C kevlar loses 50 % of the durability for 70 hours.