When you hear the word "Kevlar," you probably think of a bulletproof vest. That is one of the products created using this man-made fibre developed by DuPont in 1965. Production of belts for radial tires, cables, panels on aircraft, golf club shafts and flame-resistant clothing also involve Kevlar. Kevlar, the trademarked name for poly-paraphenylene terephthalamide, is an aramid, a fibre similar in chemical composition to nylon but with very different properties. Three grades of the fibre are available.
Several of Kevlar's properties are consistent among the three grades. The relative density is 1.44 or slightly higher. All Kevlar is highly resistant to flame, cutting and chemicals. The three grades are low in electrical conductivity and thermal shrinkage. Kevlar's high toughness means it can absorb a lot of energy before breaking. When exposed to changes in temperature and other atmospheric conditions, Kevlar holds its dimensions well, referred to as excellent dimensional stability. The properties that vary between the three grades are strength, modulus and elongation.
High Tensile Strength
Kevlar has a high tensile strength at low weight, meaning it can handle a great deal of tension without tearing apart. This measurement is expressed in gigapascals (GPa). Kevlar 29's tensile strength is 3.6 GPa. Kevlar 49's strength varies between 3.6 and 4.1 GPa while Kevlar 149 measures 3.4 GPa. The strength of the interchain bonds that make up Kevlar accounts for the fibre's might.
Kevlar has a high modulus, or structural rigidity, meaning it does not flex or bend easily under applied force. Also measured in gigapascals, the tensile modulus of the three grades are: 83 GPa for Kevlar 29, 131 GPa for Kevlar 49 and 186 GPa for Kevlar 149.
Low Elongation to Break Weight
Expressed in a percentage of the original length of the fibre, elongation at break weight represents the length of the fibre at its breaking point. Kevlar has a low percentage. Kevlar 29's tensile elongation is four per cent. Kevlar 49's elongation is 2.8 per cent and Kevlar 149's is two per cent.