Page 155 - Carrahers_Polymer_Chemistry,_Eighth_Edition

118 Carraher’s Polymer Chemistry

by DuPont by Stephanie Kwolek and Roberto Berendt in 1965 (Equation 4.65). Like Nomex, Kevlar
o
exhibits good thermal stability decomposing above about 500 C. By weight it has higher strength
and modulus than steel and is used in the manufacture of the so-called “bullet-proof” clothing.
Because of its outstanding strength, it was used as the skin covering of the Gossamer Albatross,

which was flown using only human-power across the English Channel. Aramids are also used as

fiber reenforcement in composites and as tire cord.
O
NH
Cl O O
+ H 2 N NH 2
O O Cl
(4.65)
R NH O
R

Terephthaloyl chloride p -Phenylenediamine Poly(p -phenylene terephthalamide)
(PPT)
Kevlar fiber has a high tensile strength (about 3,000 MPa) and a relatively low density (about

1.4 g/mL) leading to the often used statement that Kevlar is five times stronger than steel (on an equal
weight basis). It gains part of its strength from the interchain hydrogen bonds formed between the
carbonyl groups and hydrogen atoms on amines on neighboring chains and on the partial stacking
of the phenylene rings allowing pi–pi interactions between the members of the stacking units. The
molecularly rigid chains tend to form sheet-like structures similar to those of silk.
Kevlar is used in the manufacture of so-called “bullet-proof” clothing used by the military,
policemen, and SWAT teams. In truth, most bullet-proof clothing is bullet-resistant and unless quite
bulky not able to “stop” most rifle bullets or high caliber hand guns. It is also used in the construc-

tion of bullet-resistant facemasks by the military and motorcycle riders and to protect against abra-
sion by motorcycle riders.
There are many other commercial uses for Kevlar. It is used as the inner liner for some bicycle tires
to protect against puncture. It is also being used for bow strings in archery. Drumheads have been
made from Kevlar. Kevlar is widely employed as the protective outer sheath for fiber optic cable and as

the reinforcing layer in rubber bellow expansion joints and hoses for high-temperature applications.
Aramid materials are also employed in the U.S. space program along with other “space-age”
materials. The thermal Micrometeroid Garment on the Extravehicular Mobility Unit, Advanced
Crew Escape Suit, thermal blankets, and the fuselage, bay doors, upper wind surfaces of the Space
Shuttle Orbiter employs Nomex, Kevlar, and Gore-Tex materials. The airbags for the Mars Pathfi nder
and MER rovers, the Galileo atmospheric probe and the new Crew Exploration Vehicle all have
aramid materials. Aramid materials are also used in the form of thin pads to protect sintered silica-

fiber mats from stress and vibrations during the flight of the space shuttle.

Aramid fibers are widely employed in the construction of composites where high strength is required.
The continuous phase is often an epoxy resin. Applications include cricket bats, helicopter rotor blades,
bodies for formula one race cars, kayaks, tennis rackets, lacrosse sticks, and ice hockey sticks.
Several so-called semiaromatic nylons have been produced. Nylon-6,T is produced from conden-
sation of terephthalic acid and 1,6-hexanediamine (4.66). Both reactants are readily available and
inexpensive and the resulting materials offer greater strength than simply wholly aliphatic nylons
such as nylon-6,6. Nylon-6,T has a very high T of 370 C and a T of 180°C. The high T results
o
m
g
m
in the need for a high temperature to be employed in processing so that a third reactant is often
introduced to lower the T and the processing temperature. “Third reactants” often used are adipic
m
acid, caprolactam, isophthalic acid, and 1,5-hexyldiamine. These materials are sold under the trade
names of Zytel HTN, Ultramid T, and Amodel R.

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