How Kevlar is made

Stephanie Kwolek's cloudy, runny liquid was nearly thrown away, and it turned into a fiber that did not break.

stations: 6
things upstream: 12
raw sources: 6
steps from the ground: 4

The short version

Kevlar is made by joining two petrochemicals, para-phenylenediamine and terephthaloyl chloride, into a rigid polymer. The polymer is dissolved in concentrated sulfuric acid, which keeps it liquid. The solution is spun through fine holes into cold water, and the fibers are washed, dried and stretched.

Make the two building blocks

One monomer is a diamine built from refinery aromatics and ammonia. Its partner is a chlorinated form of terephthalic acid, which is made from p-xylene and chlorine.

Goes in
Para-xylene Ammonia Chlorine
Join them in solution

In a solvent, the two react in a condensation reaction that yields hydrochloric acid as a byproduct. Long, stiff polymer chains settle out as a yellow powder.
Dissolve it in strong acid

The water-insoluble polymer is dissolved in concentrated sulfuric acid. This is difficult to handle, which is a reason Kevlar costs so much to make.

Goes in
Sulfuric acid
Spin it into a cold bath

The thick liquid is pushed through a spinneret and drops into cold water, where the polymer solidifies. The stiff chains line up along the thread as they come out.
Wash out the acid

Water rinses the sulfuric acid from the new fiber, and the acid is neutralized or reclaimed. The fiber is then dried on heated rolls.
Wind it onto spools

The yarn is wound onto bobbins for weavers. It is cut into short lengths or woven into cloth for armor, tires and ropes.

End of the lineKevlar Next it goes into brake pad, canoe.

Worth repeating

Stephanie Kwolek developed Kevlar at DuPont in 1965, and its first commercial use in the early 1970s was in racing tires as a replacement for steel.
Her polymer solution was cloudy and runny and was usually thrown away, until she persuaded technician Charles Smullen to try spinning it.
By tensile strength for its weight, Kevlar is five times stronger than steel.