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What exactly is Kevlar?

KEVLAR

Kevlar is one of those magic modern materials people talk about all the time without ever really explaining any further. "It's made of Kevlar", they say, with a knowing nod, as though that were all the explanation you needed.

These are some of Kevlar's properties:

  • It's strong but relatively light. The specific tensile strength (pulling strength) of both Kevlar 29 and Kevlar 49 is over eight times greater than that of steel wire.
  • Unlike most plastics it does not melt: it's reasonably good at withstanding temperatures and decomposes only at about 450°C (850°F).
  • Unlike its sister material, Nomex, Kevlar can be ignited but burning usually stops when the heat source is removed.
  • Very low temperatures have no effect on Kevlar: DuPont found "no embrittlement or degradation" down to −196°C (−320°F).
  • Like other plastics, long exposure to ultraviolet light (in sunlight, for example) causes discoloration and some degradation of the fibers in Kevlar.
  • Kevlar can resist attacks from many different chemicals, though long exposure to strong acids or bases will degrade it over time.
  • In DuPont's tests, Kevlar remained "virtually unchanged" after exposure to hot water for more than 200 days and its super-strong properties are "virtually unaffected" by moisture.

What exactly is Kevlar?

Kevlar was introduced in 1971, having been discovered in the early 1960s by chemist Stephanie Kwolek and is an example of chemicals called synthetic aromatic polyamides or aramidsfor short.

  • Aromatic means Kevlar's molecules have a strong, ring-like structure like that of benzene.
  • Polyamide means the ring-like aromatic molecules connect together to form long chains. These run inside (and parallel to) the fibers of Kevlar a bit like the steel bars ("rebar") in reinforced concrete.
  • Polymer means that Kevlar is made from many identical molecules bonded together (each one of which is called a monomer). The monomers in Kevlar are based on a modified, benzene-like ring structure.

You probably know that natural materials such as wool and cotton have to be spun into fibers before they can turned into useful textile products—and the same is true of artificial fibers such as nylon, Kevlar, and Nomex. The basic polymer is turned into fibers by a process called wet spinning, which involves forcing a hot, concentrated, and very viscous solution of poly-para-phenylene terephthalamide through a spinneret (a metal former a bit like a sieve) to make long, thin, strong, and stiff fibers that are wound onto drums. The fibers are then cut to length and woven into a tough mat to make the super-strong, super-stiff finished material we know as Kevlar.

Artwork: How Kevlar is made. 1) The rodlike Kevlar molecules start off in dilute solution. 2) Increasing the concentration increases the number of molecules but doesn't make them align. At this stage, the molecules are still tangled up and not extended into straight, parallel chains. 3) The wet-spinning process causes the rods to straighten out fully and align so they're all oriented in the same direction—forming what's called a nematic structure—and this is what gives Kevlar its exceptionally high strength.




Date: 2016-01-14; view: 143


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