Kevlar is a type of aramid that consists
of long polymeric chains with a parallel
orientation. Kevlar derives its strength
from intra-molecular hydrogen bonds and
phenyl stacking interactions between aromatic
groups in neighboring strands. These interactions
are much stronger than the van der Waals
interaction found in other synthetic polymers
and fibers like dyneema. The presence of
salts and certain other impurities, especially
calcium, would interfere with the strand
interactions and has to be avoided in the
production process. Kevlar consists of relatively
rigid molecules, which form a planar sheet-like
structure similar to
These properties result in its high mechanical
strength and its remarkable heat resistance.
Because it is highly unsaturated, i.e. the
ratio of carbon to hydrogen atoms is quite
high, it has a low flammability.
Kevlar molecules have polar groups accessible
for hydrogen bonding. Water that enters
the interior of the fiber can take the place
of bonding between molecules and reduce
the material's strength, while the available
groups at the surface lead to good wetting
properties. This is important for bonding
the fibers to other types of polymer, forming
plastic. This same property also makes the
fibers feel more natural and "sticky"
compared to non-polar polymers like
solution during synthesis and spinning.