Kevlar is the registered trademark of DuPont for a strong, light, flexible, and flame resistant polymer fiber. The structural name for the polymer is poly-paraphenylene terephtalamide:

       _                     _
      |     __         __     |
      | O  /  \  O    /  \    |
     -|-C-- () --C-N-- () --N-|-
      |    \__/    H  \__/  H |
      |_                     _|n

Kevlar belongs to the group of polyamides; other compounds that belong in this group are the natural proteins (think of silk with its similar strength/weight properties), and the synthetic Nylon. Another descriptive name for Kevlar is "para-aramid", where para- refers to the linear linking of the two benzene groups in the structure.

The most impressive property of Kevlar is its tensile strength (3.6-4.1 GPa), combined with a low weight. Compared to an equal weight of steel, Kevlar is 5 times stronger. But there are other important features: the material has a high structural rigidity, which means that it does not elongate much by stretching the fiber up to its breaking point. Furthermore, Kevlar has a low electrical conductivity, high chemical resistance. high cut resistance, and it is flame resistant.

The physical properties of Kevlar make it a suitable material for many applications, such as:

The reason for Kevlar's remarkable properties lies in its molecular structure. As mentioned previously, Kevlar has a highly linear structure due to the para-linking on the two benzene groups in the chain. Furthermore, the large benzene groups hinder the twisting of the polymer chain: it is forced into a trans-conformation:

      C-R-C       C-R-C       C-R-C       ~
     /     \     /     \     /     \     /
    ~       N-R-N       N-R-N       N-R-N

trans-conformation


      C-R-C       C-R-C       C-R-C
     /     \     /     \     /     \
    ~       N-R-N       N-R-N       N
                                   / 
                                  R
                                 /
                                N
                               /
                              ~
cis-conformation

The large benzene (R) groups force the C-N bonds in the trans-conformation, resulting in a linear polymer structure. The cis-conformation, shown at the end of the second chain is sterically hindered; the benzene groups (not to scale) are close to each other, resulting in an energetically unfavorable conformation.

The linear structure of Kevlar results in a highly ordered, or crystalline structure. This ordering makes the Kevlar fibers exceptionally strong; randomly cluttered strands would be easy to pull apart, but lined up they can withstand enormous tensile forces.

Another reason for the tremendous strength of Kevlar is the formation of hydrogen bridge bonds between the O and H groups on the chain. These hydrogen bridges (symbolized by ":") help the structural rigidity of the polymer:


      O   O       O   O       O   O
      C-R-C       C-R-C       C-R-C       ~
     /     \     /     \     /     \     /
    ~       N-R-N       N-R-N       N-R-N
            H   H       H   H       H   H
            :   :       :   :       :   :
            O   O       O   O       O   O
            C-R-C       C-R-C       C-R-C
           /     \     /     \     /     \
          ~       N-R-N       N-R-N       N-R-N-~
                  H           H   H       H   H

The synthesis of Kevlar itself is a relatively simple polymerization. However, because of its tendency to crystallize into rigid structures, it was difficult to process into a useful product. Kevlar does not dissolve easily, and it does not melt (it decomposes at 500 C). Spinning the fibers requires the product to be in a liquid state.

The principal inventors of a synthesis and processing technique for Kevlar fibers are Stephanie Kwolek and Herbert Blades. In 1965, these DuPont researchers discovered that the addition of lithium chloride or calcium chloride to the reactant solution prevents the formation of hydrogen bridges. The polymer remains in solution until the product is synthesized by removal of the solvent, and spinning the fibers.

Nowadays, there are three grades of Kevlar: Kevlar 29, Kevlar 49, and Kevlar 149. These types have a different average molecular chain length, and vary slightly in physical properties. The most common type of Kevlar cloth is Kevlar 49.


Sources:

http://www.dupont.com/kevlar/
U.S. Patent No. 3,819,587
http://www.psrc.usm.edu/macrog/aramid.htm http://www.invent.org/hall_of_fame/90.html
http://composite.about.com/library/weekly/aa050597.htm
http://www.lbl.gov/MicroWorlds/Kevlar/index.html

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