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Introduction to Aramid Fiber
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Aramid
Para-aramid structure
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.
[edit]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.[1] 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 forasbestos. Meta-
aramid is also produced in the Netherlands and Japan by Teijin under the trade name
Conex,[1] in China by Yantai under the trade name New Star, by SRO Group (China)
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 (see
Production).
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, (-CO-NH-) are attached directly
to two aromatic rings.
[edit]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
(ornithine decarboxylase) enzyme activity", raising the possibility
of carcinogenic implications.[2]
[edit]Production
World capacity of para-aramid production is estimated at about 41,000 tonnes/year in
2002 and increases each year by 5–10%.[3] In 2007 this means a total production
capacity of around 55,000 tonnes/year.
[edit]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 (Kevlar, Twaron, Nomex, New Star and Teijinconex)
are AABB polymers. Nomex, Teijinconex and New Star contain predominantly the
meta-linkage and are poly-metaphenylene isophtalamides (MPIA). Kevlar and
Twaron are both p-phenylene terephtalamides (PPTA), the simplest form of the
AABB para-polyaramide. PPTA is a product of p-phenylene diamine (PPD)
and terephtaloyl dichloride (TDC or TCl). Production of PPTA relies on a co-solvent
with an ioniccomponent (calcium chloride (CaCl2)) to occupy the hydrogen bonds of
the amide groups, and an organic component (N-methyl pyrrolidone (NMP)) 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.
[edit]Spinning
After production of the polymer, the aramid fiber is produced by spinning the solved
polymer to a solid fiber from a liquid chemical blend. Polymer solvent for spinning
PPTA is generally 100% anhydrous sulfuric acid (H2SO4).
[edit]Appearances
� Fiber
� Chopped fiber
� Powder
� Pulp
[edit]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 (3,4'-ODA)
with terephtaloyl chloride (TCl).[4] This relatively simple process uses only one amide
solvent and therefore spinning can be done directly after the polymer production.
[edit]Aramid fiber characteristics
Aramids share a high degree of orientation with other fibers such as ultra high
molecular weight polyethylene, a characteristic which dominates their properties.
[edit]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[5]
[edit]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 (~3.5%)
� difficult to dye – usually solution dyed [5]
[edit]Major industrial uses
� flame-resistant clothing
� heat protective clothing and helmets
� body armor,[6] competing with PE based fiber products such
as Dyneema and Spectra
� composite materials
� asbestos replacement (e.g. brake linings)
� hot air filtration fabrics
� tires, newly as Sulfron (sulfur modified Twaron)
� mechanical rubber goods reinforcement
� ropes and cables
� wicks for fire dancing
� optical fiber cable systems
� sail cloth (not necessarily racing boat sails)
� sporting goods
� drumheads
� wind instrument reeds, such as the Fibracell brand
� loudspeaker diaphragms
� boathull material
� fiber reinforced concrete
� reinforced thermoplastic pipes
� tennis strings (e.g. by Ashaway and Prince tennis companies)
� hockey sticks (normally in composition with such materials as wood and carbon)
� snowboards
� jet engine enclosures
[edit]See also
Para-aramid
� Kevlar
� Technora
� Twaron
� Heracron
Meta-aramid
� Nomex
Others
� Innegra S
� Nylon
� Textile
� Ultra high molecular weight polyethylene
� Vectran
[edit]Notes and references
1. ^ a b James A. Kent, ed. (2006). Handbook of Industrial Chemistry and Biotechnology. Springer.
p. 483. ISBN 0-387-27842-7.
2. ^ Marsh, J. P.; Mossman, B. T., Driscoll, K. E., Schins, R. F., Borm, P. J. A. (1 January
1994). "Effects of Aramid, a high Strength Synthetic Fiber, on Respiratory Cells in Vitro". Drug and
Chemical Toxicology (Informa Healthcare) 17 (2): 75–92.DOI:10.3109/01480549409014303.
Retrieved 15 August 2011.
3. ^ Committee on High-Performance Structural Fibers for Advanced Polymer Matrix Composites,
National Research Council (2005). High-Performance Structural Fibers for Advanced Polymer
Matrix Composites. The National Academies Press. p. 34. ISBN 0-309-09614-6.
4. ^ Ozawa S (1987). "A New Approach to High Modulus, High Tenacity Fibers". Polym. J. Japan 19:
199. DOI:10.1295/polymj.19.119.
5. ^ a b Kadolph, Sara J. Anna L. Langford (2002). Textiles.
6. ^ Reisch, Marc S (2005). "High-performance fiber makers respond to demand from military and
security users". Chemical and Engineering News 83 (31): 18–22. DOI:10.1021/cen-
v083n050.p018.