KEVLAR

Kevlar is the DuPont Company's brand name for material made out of synthetic fibre of poly-paraphenylene terephthalamide which is constructed of para-aramid fibres that the company claims is five times stronger than the same weight of steel, while being lightweight, flexible and comfortable. It is also very heat resistant and decomposes above 400 °C without melting. It was invented by Stephanie Kwolek of DuPont from research into high performance polymers, and patented by her in 1966 and first marketed in 1971. Kevlar is a registered trademark of E.I. du Pont de Nemours and Company.

Originally intended to replace the steel belts in tyres, it is probably the most well known name in soft armour as bullet-proof vests. It is also used in extreme sports equipment, high-tension drumhead applications, animal handling protection, composite aircraft construction, fire suits, yacht sails, and as an asbestos replacement. When this polymer is spun in the same way that a spider spins a web, the resulting commercial para-aramid fibre has tremendous strength, and is heat and cut resistant. Para-aramid fibres do not rust or corrode, and their strength is unaffected by immersion in water. When woven together, they form a good material for mooring lines and other underwater objects. However, unless specially waterproofed, para-aramid fibre’s ability to stop bullets and other projectiles is degraded when wet.

Properties

Kevlar is a type of aramid that consists of long polymeric chains with a parallel orientation. Kevlar derives its strength from inter-molecular hydrogen bonds and aromatic stacking interactions between aromatic groups in neighbouring strands. These interactions are much stronger than the van der Waals interaction found in other synthetic polymers and fibres 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 silk protein.

 

Polyparaphenylene Terephthalamide Intermolecular Hydrogen Bonding


These properties result in its high mechanical strength and its remarkable heat resistance. Because it is highly unsaturated, as 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 fibre 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 fibres to other types of polymer, forming a fibre reinforced plastic. This same property also makes the fibres feel more natural and "sticky" compared to nonpolar polymers like polyethylene.

 

In structural applications, Kevlar fibres 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 fibres it tends to buckle in compression.




Production

Kevlar is synthesized from the monomers 1,4-phenyl-diamine (para-phenylenediamine) and terephthaloyl chloride. The result is a polymeric aromatic amide (aramid) with alternating benzene rings and amide groups. When they are produced, these polymer strands are aligned randomly. To make Kevlar, they are dissolved and spun, causing the polymer chains to orientate in the direction of the fibre.

Kevlar has a high price, in part, due to the difficulties arising from the use of concentrated sulphuric acid in its manufacture. These harsh conditions are needed to keep the highly insoluble polymer in solution during synthesis and spinning. Kevlar Synthesis

 

Kevlar® The Quick and Simple.

Kevlar® is 5 times stronger than steel on an equal weight basis, yet, at the same time, is lightweight and flexible.

Lots of products are made with it— from protective apparel and sports equipment to automotive parts.

Kevlar® is a manmade fibre developed in 1965 by two research scientists, Stephanie Kwolek and Herbert Blades. The two scientists worked for the DuPont company. Their product offered a number of benefits that led to its commercial introduction in the early 1970's. Because of the protection it provides, Kevlar® fibres quickly became the technology of choice for bullet-resistant vests! This does not mean that hiding behind your drum in a shoot out is a good idea.

  

Fibres of Kevlar® consist of long molecular chains produced from poly-paraphenylene terephthalamide - say that ten times fast. The chains are highly oriented with strong interchain bonding which result in a combination of properties.

 
High Tensile Strength at Low Weight
Low Elongation to Break High Modulus (Structural Rigidity)
Low Electrical Conductivity
High Chemical Resistance
Low Thermal Shrinkage
High Toughness (Work-To-Break)
Excellent Dimensional Stability
High Cut Resistance
Flame Resistant, Self-Extinguishing		  

It's also found in:

Ropes that secure the airbags in the crucial landing apparatus of the Mars Pathfinder Small-diameter, lightweight ropes that hold 22,000 pounds and help moor the largest U.S. Navy vessels
Shrapnel-resistant shielding in jet aircraft engines that will protect passengers in case an explosion occurs
Run-flat tires that allow for greater safety because they won't ruin the rim when driving to the nearest assistance
Kayaks that provide better impact resistance with no extra weight
Strong, lightweight skis, helmets and racquets that help lessen fatigue and boost exhilaration
High Tension Drumheads.