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The most notable property of Kevlar is its strength. Kevlar has a tensile strength of 3.6 GPa.
This is considerably stronger than mild strength steel which has a breaking stress of 414 MPa and aluminium, 310 MPa.
However the compressive strength is low so if you tried to stretch Kevlar, it would be very hard but if you put
a large mass on top of it, it may buckle or, in some cases, be pierced.
Another property which makes Kevlar very useful in bulletproof vests is its density; Kevlar 29, the one used in
such protection, has a relative density of 1.44 gcm-3.
Steel, however, has a density of about 7.8gcm-3 although
because of the many different recipes for mixing iron, carbon and other elements to make steel, this can vary. Another material
which is widely regarded as lightweight, particularly in motorsport is carbon fibre, the density of this can also vary but
most samples are of equal weight or heavier than Kevlar 29. To sum up, Kevlar combines extraordinary strength with a very
low density to make a material which can be worn easily on the torso and still stop a speeding bullet.
Kevlar is also quite heat resistant for a polymer. It maintains around 80% of its strength at temperatures where a lot
of polymers would have melted or disintegrated considerably. It even gains strength at slightly lower temperatures and continues
to work effectively down to cyrogenic temperatures (below –150 °C.)
However, Kevlar has some undesirable properties which can cause problems during manufacture and use. Alongside the expense
of production and low compressive strength discussed earlier, Kevlar is very difficult to cut into the desired shape, adding
to the cost even more. However this cut resistance can also be seen as a desirably feature in protection against attacks from
sharp objects.
Another obvious problem that can arise from using Kevlar is its stiffness. Kevlar has a very high Young's Modulus, 130
GPa, so, because E = stress/strain a large stress only creates a small strain and the material is very rigid. This means,
in the case of bulletproof clothing, garments can only protect one part of the body. A police officer wearing
a bulletproof vest must have his arms, legs and neck exposed if he does not want to restrict his movement.
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