Kevlar is a processed polyparaphenylene terephthalamide that is an aramid consisting of long polymeric chains with a parallel orientation. Kevlar derives a portion of its improved strength from inter-molecular hydrogen bonds formed between the carbonyl groups and protons on neighboring polymer chains and the partial pi stacking of the benzenoid aromatic stacking interactions between stacked strands. These interactions have a greater influence on Kevlar than van der Waals interactions and chain length that typicaly influence the properties of other synthetic polymers and fibers like Dyneema. In addition the presence of salts and certain other impurities, especially calcium, could interfere with the strand interactions and caution is used to avoid inclusion in its production. Kevlar’s structure consists of relatively rigid molecules, which tend to form mostly planar sheet-like structures that have similarities to silk protein.
These properties result in its high mechanical strength and additionally kevlar’s remarkable heat resistance. The degree of unsaturated carbons (the ratio of carbon to hydrogen atoms) is quite high, and decreases kevlar’s 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 composite material. This same property also makes the fibers feel more natural and “sticky” compared to nonpolar polymers like polyethylene.
There are three common grades of Kevlar: Kevlar, Kevlar 29, and Kevlar 49. Kevlar is typically used as reinforcements in tires and other rubber mechanical goods. Kevlar 29 is used in industrial applications such as cables, asbestos replacement, brake lines, body armor (knife, bullet, etc). Kevlar 49 is considered to have the greatest tensile strength of all the aramids, it is used in applications such as plastic reinforcement for boat hulls, airplanes, and bikes.
In structural applications, Kevlar fibers can be bonded to one another or to other materials to form a composite.
Kevlar’s main weaknesses are that it decomposes under alkaline conditions or when exposed to chlorine. While it can have a great tensile strength, sometimes in excess of 4.0 GPa, like all fibers it tends to buckle in compression.