Chemistry » Organic Chemistry » Amines and Amides

Kevlar

Kevlar

Kevlar (see the figure below) is a synthetic polymer made from two monomers 1,4-phenylene-diamine and terephthaloyl chloride (Kevlar is a registered trademark of DuPont). Kevlar’s first commercial use was as a replacement for steel in racing tires. Kevlar is typically spun into ropes or fibers. The material has a high tensile strength-to-weight ratio (it is about 5 times stronger than an equal weight of steel), making it useful for many applications from bicycle tires to sails to body armor.

A structural formula is shown for the polymer Kevlar. The structure appears inside brackets which have single dashes extending from them at the left and right ends. Outside the lower right corner of the brackets, an italicized n appears. The structure inside the brackets includes a C atom forming a double bond with an O atom and a bond with a benzene ring. The benzene ring forms a bond with another C atom which has a double bond with an O atom. The C atom is bonded to an N atom. The N atom is bonded to an H atom and a benzene ring. The benzene ring bonds with another N atom which is also bonded to an H atom.

This illustration shows the formula for polymeric Kevlar.

The material owes much of its strength to hydrogen bonds between polymer chains (refer back to the tutorial on intermolecular interactions). These bonds form between the carbonyl group oxygen atom (which has a partial negative charge due to oxygen’s electronegativity) on one monomer and the partially positively charged hydrogen atom in the N–H bond of an adjacent monomer in the polymer structure (see dashed line in the figure below). There is additional strength derived from the interaction between the unhybridized p orbitals in the six-membered rings, called aromatic stacking.

This diagram shows the repeating, interlinked units that exist in Kevlar, taking on a sheet-like appearance. Dashed line segments are indicated between units. Individual units are composed of nitrogen, hydrogen, oxygen and carbon atoms. The repeating structural units include benzene rings and double bonds.

The diagram shows the polymer structure of Kevlar, with hydrogen bonds between polymer chains represented by dotted lines.

Kevlar may be best known as a component of body armor, combat helmets, and face masks. Since the 1980s, the US military has used Kevlar as a component of the PASGT (personal armor system for ground troops) helmet and vest. Kevlar is also used to protect armored fighting vehicles and aircraft carriers. Civilian applications include protective gear for emergency service personnel such as body armor for police officers and heat-resistant clothing for fire fighters. Kevlar based clothing is considerably lighter and thinner than equivalent gear made from other materials (see the figure below).

Three photos are shown. In the first, two male soldiers are shown sorting through green brown material on a table. In the second, two people are shown paddling a canoe. In the third, heavy white rope is being manipulated with a hand tool.

(a) These soldiers are sorting through pieces of a Kevlar helmet that helped absorb a grenade blast. Kevlar is also used to make (b) canoes and (c) marine mooring lines. (credit a: modification of work by “Cla68”/Wikimedia Commons; credit b: modification of work by “OakleyOriginals”/Flickr; credit c: modification of work by Casey H. Kyhl)

In addition to its better-known uses, Kevlar is also often used in cryogenics for its very low thermal conductivity (along with its high strength). Kevlar maintains its high strength when cooled to the temperature of liquid nitrogen (–196 °C).

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