Kevlar: Why is it so good at stopping bullets?
50 years have passed since kevlar was originally developed by DuPont in 1965, which was developed by a Polish-American chemist called Stephanie Kwolek. It also goes by its chemical name, poly paraphenylene terephthalamide, and was created to be used in tyres, to keep them strong whilst also making them lighter. At this point in time little was known about kevlar; it wasn’t until chemists started experimenting with the compound that it’s full range of applications were imagined.
When Kevlar is spun an incredibly strong fibre is created, with a tensile strength in the region of 3,620 MPa. This strength, which is five times that of steel of the same weight, is due to the huge number of inter-molecular forces that are present within the molecule, particularly the hydrogen bonds that form between the carbonyl group and the Nitrogen-Hydrogen centres.
The repeated molecule forms a sheet-like structure that’s very rigid, comparable to silk protein. Another interesting property of Kevlar is that it’s actually stronger at lower temperatures, making it extremely useful in cryogenics. If you heat it however its tensile strength is immediately reduced by up to 20%; at 260 degrees Centigrade its strength is reduced by 50% after 70 hours.
Today, Kevlar is found in hundreds of different products, far too many to list. Its most famous use is without doubt in personal armour, such as combat helmets and bulletproof vests. Some less well known uses include in tennis string, textiles, musical instruments, frying pans, particle physics and smart phones.
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