Kevlar

Kevlar
Ben Mills and Jynto · Public domain · source
Name	Kevlar
Caption	Chemical structure of Kevlar
OtherNames	Poly-paraphenylene terephthalamide
Density	1.44 g/cm³
MeltingPt	Decomposes at approximately 500°C
CASNo	24938-64-5

Kevlar is a high-strength synthetic aramid fiber developed by DuPont scientist Stephanie Kwolek in 1965. Its exceptional strength-to-weight ratio, derived from its rigid molecular structure, makes it five times stronger than steel on an equal weight basis. The material is renowned for its use in ballistic vests and body armor, but its applications extend to numerous fields requiring high tensile strength and thermal stability, including aerospace engineering, marine cordage, and composite materials.

History and development

The discovery of Kevlar occurred in 1965 at DuPont's Pioneering Research Laboratory in Wilmington, Delaware, under the direction of polymer chemist Stephanie Kwolek. While searching for a next-generation lightweight fiber for reinforced tires, Kwolek synthesized a unique liquid crystalline solution of poly-paraphenylene terephthalamide. Her team, including Herbert Blades, developed the critical spinning process to produce the first high-modulus aramid fibers. Commercial production began in the early 1970s, with initial applications in racing tires before its adoption by the National Institute of Justice for ballistic protection. The material's development was a landmark in polymer science, earning Kwolek the National Medal of Technology.

Chemical structure and properties

Kevlar is a polyamide where the polymer chains are connected by strong aromatic rings and amide linkages in a para-orientation, forming poly-paraphenylene terephthalamide. This structure creates rigid, rod-like molecules that align in parallel during fiber formation, resulting in extensive hydrogen bonding between the carbonyl and amine groups of adjacent chains. This molecular arrangement grants Kevlar its extraordinary properties: high tensile strength, significant cut resistance, and stability across a wide temperature range. It exhibits high crystallinity and maintains its strength from cryogenic temperatures up to about 450°C, though it is susceptible to degradation by strong acids and prolonged ultraviolet light exposure.

Manufacturing process

The production of Kevlar begins with the polymerization of 1,4-phenylene-diamine and terephthaloyl chloride in a solvent like N-methyl-pyrrolidone and calcium chloride, a reaction pioneered at DuPont. The resulting polymer is dissolved in concentrated sulfuric acid to create an anisotropic spinning solution. This solution is extruded through a spinneret in a process called dry-jet wet spinning, where it passes through an air gap before entering a coagulation bath of water or dilute acid. This step precipitates the solid fibers while maintaining molecular alignment. The fibers are then washed, dried under tension, and wound onto spools. The final product can be woven into fabric or incorporated into composite materials for various end uses.

Applications

Kevlar's primary and most famous application is in personal armor, including bulletproof vests, combat helmets used by the United States Army, and vehicle armor for Humvees. In the aerospace industry, it is used in pressure vessels, rocket motor casings, and components for aircraft like the Boeing 787 Dreamliner. Its high strength and low weight make it ideal for marine uses such as sailcloth in the America's Cup and mooring lines. Other significant applications include safety gloves for industrial work, reinforcement in optical fiber cables, asbestos replacement in brake pads, and protective gear for motorsport drivers in Formula One.

Variants and related materials

DuPont has developed several grades of Kevlar optimized for specific properties. Kevlar 29 is a high-tensile strength fiber used in ballistic vests and ropes, while Kevlar 49 possesses a higher modulus and is favored in aerospace and marine composites. Other variants include Kevlar 100 for colored fabrics and Kevlar KM2 for enhanced ballistic performance. Related high-performance aramids include Twaron manufactured by Teijin, and Technora developed by Teijin and DuPont. Similar high-strength fibers in different material families include Spectra and Dyneema, as well as the carbon fiber used in advanced composite materials.

Category:Synthetic fibers Category:DuPont Category:Polyamides Category:Armor