The Physics of Kevlar
- Jay Sardesai
- Oct 23, 2020
- 4 min read
Poly-paraphenylene-terephthalate, more commonly known by the brand name Kevlar, is a polymer with an extremely high tensile strength and heat resistance. It has found many uses, from cars to bulletproof vests to shoes. Despite this, Kevlar's uses are limited by its high cost and degradation during exposure to UV light.
Kevlar is produced from the monomers terephthaloyl chloride and para-phenylenediamine, with hydrochloric acid as a byproduct. After it has formed, it's spun into fibres. Most of the cost of Kevlar is because it must be in solution throughout this process, and so concentrated sulfuric acid is needed.
Kevlar has a very high tensile strength due to the number of interactions between the long polymer chains. Hydrogen bonds between oxygen and hydrogen atoms are responsible for most of the interactions. Pi stacking, a form of interactions between aromatic rings (such as benzene), accounts for most of the rest of the interactions.
Kevlar was first invented as a replacement for steel in tyres. Kevlar coils are used in the beads of tires, and in the sub-tread, which is positioned between the rubber and inner fabric of the tyre. Kevlar tyres are most commonly used in mountain biking, where it protects the type from punctures. Kevlar is used over steel as it is lighter and more flexible, allowing tyres to be folded up and carried easily.
Kevlar's most famous use is undoubtedly as the material from which bulletproof vests are made. Kevlar is widely used for this purpose because it is very effective at spreading the force delivered by a bullet, and also is very lightweight, not restricting the wearer's movement, which is very useful in combat zones. In Kevlar, the individual polymer strands are tangled together. This means that when a bullet strikes it, it is unable to push apart the individual strands, making a hole for it to get through. Instead, the energy is absorbed by the Kevlar strands, which stretch, before returning back to their usual shape.
Kevlar is also used in heat resistant clothing for personal protection, such as gloves. Kevlar is used in applications where lightweight, thin gloves are needed, but is not preferable for the mass market, as its cost isn't competitive with other cheaper, bulkier, materials. Kevlar is also used in protective padding for motorcyclists, skaters, and other people at risk of high-velocity impacts with the ground. In spite of its use in thermal protection, Kevlar itself isn't immune to being affected by heat and slowly loses its tensile strength if heated for a long period of time (after 70 hours at 260°C it loses 50% of its tensile strength). For that reason, Kevlar is unsuitable for applications where both heat resistance and high tensile strength are needed.
Kevlar fibres can be spun into cords with a very high tensile strength. As a result, it is used to connect parachutes to people during skydiving, as it is capable of supporting their weight. This property of Kevlar can also be used to great effect in the construction of suspension bridges. Suspension bridges consist of roadways supported by overhanging cables, which are in turn attached to towers interspersed throughout the bridge. Unfortunately, Kevlar's cost again prohibits it from being used much for this purpose; very few bridges are built using Kevlar.
Kevlar fibres have also been considered as a potential material to build a space elevator from, due to their very high strength to weight ratio, far in excess of steel. While Earth's gravity is too much for Kevlar to be used in building a space elevator on Earth, it would be suitable for a space elevator on the Moon, significantly reducing the cost of spaceflight.
One of the problems which Kevlar and many other synthetic polymers face is UV degradation. This is because many of the bonds found in the polymers, such as those found in the amide group, have a high absorption rate for photons with a wavelength within the bounds of UV light. These high-energy photons sometimes provide enough energy to break bonds, compromising the structural integrity of the polymer, and leading to a weakening of the material over time.
Due to Kevlar's high cost, other synthetic materials with high tensile strength have been replacing it in some niches, with varying success. Zylon, a similar polymer, has a tensile strength 1.6 times that of Kevlar, and as such was thought to be a suitable material for making bulletproof vests. However, this was soon found to not be the case. Zylon rapidly degrades at normal heat and humidity, meaning that after a reasonably short period of time, Zylon vests are no longer capable of stopping bullets. This was tragically only found out after the death of a police officer after his vest failed to stop bullets. The manufacturer, Toyobo, was fined $66 million by the US Department of Justine after it transpired that they knew that Zylon degraded quickly, but did not publicise this, choosing to keep the knowledge secret.
Some promising composite materials have a tensile strength much higher than Kevlar. For example, researchers at Northeastern University in the US have experimented with adding carbon nanotubes to cheap polymers, in order to increase their tensile strength. The carbon nanotubes help the polymer form a more regular arrangement, aligned in the same direction. Currently, composite materials with a tensile strength similar to Zylon have been achieved, however, these materials are not commercially available yet.
Overall, Kevlar is a high-performance fabric with many uses, but also significant downsides. Despite this, the lack of a credible alternative, especially with regards to bulletproof vests, means that Kevlar will continue to be dominant in many fields for decades.
Sources:
Comments