Aramid fibers are a class of heat-resistant and strong synthetic fibers. They are used
in aerospace and military applications, for ballistic rated body armor fabric and ballistic
composites, in bicycle tires, and as an asbestos substitute. The name is a portmanteau of "aromatic
polyamide". They are fibers in which the chain molecules are highly oriented along the fiber
axis, so the strength of the chemical bond can be exploited.
History Aromatic polyamides were first introduced
in commercial applications in the early 1960s, with a meta-aramid fiber produced by DuPont
as HT-1 and then under the trade name Nomex. This fiber, which handles similarly to normal
textile apparel fibers, is characterized by its excellent resistance to heat, as it neither
melts nor ignites in normal levels of oxygen. It is used extensively in the production of
protective apparel, air filtration, thermal and electrical insulation as well as a substitute
for asbestos. Meta-aramid is also produced in the Netherlands and Japan by Teijin under
the trade name Conex, In Korea by Toray under the trade name Arawin, in China by Yantai
Tayho under the trade name New Star, by SRO Group under the trade name X-Fiper, and a
variant of meta-aramid in France by Kermel under the trade name Kermel.
Based on earlier research by Monsanto Company and Bayer, a fiber – para-aramid – with
much higher tenacity and elastic modulus was also developed in the 1960s–1970s by DuPont
and Akzo Nobel, both profiting from their knowledge of rayon, polyester and nylon processing.
Much work was done by Stephanie Kwolek in 1961 while working at DuPont, and that company
was the first to introduce a para-aramid called Kevlar in 1973. A similar fiber called Twaron
with roughly the same chemical structure was introduced by Akzo in 1978. Due to earlier
patents on the production process, Akzo and DuPont engaged in a patent dispute in the
1980s. Twaron is currently owned by the Teijin company.
Para-aramids are used in many high-tech applications, such as aerospace and military applications,
for "bullet-proof" body armor fabric. The Federal Trade Commission definition for
aramid fiber is:
A manufactured fiber in which the fiber-forming substance is a long-chain synthetic polyamide
in which at least 85% of the amide linkages, are attached directly to two aromatic rings.
Health During the 1990s, an in vitro test of aramid
fibers showed they exhibited "many of the same effects on epithelial cells as did asbestos,
including increased radiolabeled nucleotide incorporation into DNA and induction of ODC
enzyme activity", raising the possibility of carcinogenic implications. However, in
2009, it was shown that inhaled aramid fibrils are shortened and quickly cleared from the
body and pose little risk. Production
World capacity of para-aramid production is estimated at about 41,000 tonnes/year in 2002
and increases each year by 5–10%. In 2007 this means a total production capacity of
around 55,000 tonnes/year. Polymer preparation
Aramids are generally prepared by the reaction between an amine group and a carboxylic acid
halide group. Simple AB homopolymers may look like:
nNH2-Ar-COCl → -(NH-Ar-CO)n- + nHCl The most well-known aramids are AABB polymers.
Nomex, Teijinconex and New Star contain predominantly the meta-linkage and are poly-metaphenylene
isophthalamides. Kevlar and Twaron are both p-phenylene terephthalamides, the simplest
form of the AABB para-polyaramide. PPTA is a product of p-phenylene diamine and terephthaloyl
dichloride. Production of PPTA relies on a co-solvent with an ionic component) to occupy
the hydrogen bonds of the amide groups, and an organic component) to dissolve the aromatic
polymer. Prior to the invention of this process by Leo Vollbracht, who worked at the Dutch
chemical firm Akzo, no practical means of dissolving the polymer was known. The use
of this system led to an extended patent dispute between Akzo and DuPont.
Spinning After production of the polymer, the aramid
fiber is produced by spinning the dissolved polymer to a solid fiber from a liquid chemical
blend. Polymer solvent for spinning PPTA is generally 100% anhydrous sulfuric acid.
Appearances Fiber
Chopped fiber Powder
Pulp Other types of aramids
Besides meta-aramids like Nomex, other variations belong to the aramid fiber range. These are
mainly of the copolyamide type, best known under the brand name Technora, as developed
by Teijin and introduced in 1976. The manufacturing process of Technora reacts PPD and 3,4'-diaminodiphenylether
with terephthaloyl chloride. This relatively simple process uses only one amide solvent
and therefore spinning can be done directly after the polymer production.
Aramid fiber characteristics Aramids share a high degree of orientation
with other fibers such as ultra high molecular weight polyethylene, a characteristic that
dominates their properties. General
good resistance to abrasion good resistance to organic solvents
nonconductive no melting point, degradation starts from
500°C low flammability
good fabric integrity at elevated temperatures sensitive to acids and salts
sensitive to ultraviolet radiation prone to static build-up unless finished
Para-aramids para-aramid fibers such as Kevlar and Twaron,
provide outstanding strength-to-weight properties high Young's modulus
high tenacity low creep
low elongation at break difficult to dye – usually solution dyed
Major industrial uses flame-resistant clothing.
heat protective clothing and helmets body armor, competing with PE based fiber
products such as Dyneema and Spectra composite materials
asbestos replacement hot air filtration fabrics
tires, newly as Sulfron mechanical rubber goods reinforcement
ropes and cables wicks for fire dancing
optical fiber cable systems sail cloth
sporting goods drumheads
wind instrument reeds, such as the Fibracell brand
loudspeaker diaphragms boathull material
fiber reinforced concrete reinforced thermoplastic pipes
tennis strings hockey sticks
snowboards jet engine enclosures
See also Para-aramid
Kevlar Technora
Twaron Heracron
Meta-aramid Nomex
Others Innegra S
Nylon Textile
Ultra high molecular weight polyethylene Vectran
Notes and references
Further reading J M Garcia, F C Garcia, F Serna, J L de la
Peña. "High-performance aromatic polyamides". Progress in Polymer Science 35: 623–686.
doi:10.1016/j.progpolymsci.2009.09.002. Karlheinz Hillermeier. "Prospects of Aramid
as a Substitute for Asbestos". Research Journal, Vol. 54, No. 9.
JWS Hearle. "High-performance fibers". Woodhead Publishing Ltd., Abington, UK – the Textile
Institute. Doetze J. Sikkema. "Manmade fibers one hundred
years: Polymers and polymer design". J Appl Polym Sci, John Wiley & Sons, Inc.: 484–488.
Kh. Hillermeier and H.G. Weijland. "An aramid yarn for reinforcing plastics". Plastica:
374–380. DuPont and Teijin to expand aramid production
- September 2004