Aramid fibers, short for aromatic polyamide, are a class of heat-resistant and strong . They are used in aerospace and military applications, for ballistic-rated bulletproof vest cloth and ballistic composites, in marine cordage, marine hull reinforcement, as an asbestos substitute,
The chain molecules in the fibers are highly oriented along the fiber axis. As a result, a higher proportion of the chemical bond contributes more to fiber strength than in many other synthetic fibres in the world. Aramids have a very high melting point (>).
Common aramid include Kevlar, Nomex, and Twaron.
Terminology and chemical structure
The term
aramid is shortened from
aromatic polyamide. It was introduced in 1972,
accepted in 1974 by the Federal Trade Commission of the USA as the name of a generic category of fiber distinct from
nylon,
and adopted by the International Organization for Standardization in 1977.
Aromatic in the longer name refers to the presence of aromaticity of six carbon atoms. In aramids these rings are connected via amide linkages each comprising a CO group attached to an NH group.
In order to meet the FTC definition of an aramid,[Commercial Practices, Part 303, §303.7 Generic names and definitions for manufactured fibers.] at least 85% of these linkages must be attached to two aromatic rings.[The full definition of aramid fibre 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". (The configuration is shown as a small diagram, given in formula form here for convenience).] Below 85%, the material is instead classed as nylon.
Para-aramids and meta-aramids
Aramids are divided into two main types according to where the linkages attach to the rings. Numbering the carbon atoms sequentially around a ring,
para-aramids have the linkages attached at positions 1 and 4, while
meta-aramids have them at positions 1 and 3.
[Position 1 is simply chosen as the point where one of the chains is attached. We then count around the ring in the shortest direction until we reach the other one.] That is, the attachment points are diametrically opposite each other in para-aramids, and two atoms apart in meta-aramids. The illustration thus shows a para-aramid.
History
Aromatic polyamides were first introduced in
Commerce 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, and as a substitute for
asbestos.
Meta-aramids are also produced in the Netherlands and Japan by Teijin Aramid under the trade name Teijinconex,
Based on earlier research by Monsanto and Bayer, para-aramid fiber with much higher tensile strength and elastic modulus was also developed in the 1960s and 1970s by DuPont and AkzoNobel, both profiting from their knowledge of rayon, polyester and nylon processing. In 1973, DuPont was the first company to introduce a para-aramid fiber, calling it Kevlar; this remains one of the best-known para-aramids and/or aramids.
In 1978, Akzo introduced a similar fiber with roughly the same chemical structure calling it Twaron. Due to earlier patents on the production process, Akzo and DuPont engaged in a patent dispute in the 1980s. Twaron subsequently came under the ownership of the Teijin Aramid Company. In 2011, Yantai Tayho introduced similar fiber which is called Taparan in China (see Production).
Para-aramids are used in many high-tech applications, such as aerospace and military applications, for "bullet-proof" bulletproof vest cloth.
Both meta-aramid and para-aramid fiber can be used to make aramid paper. Aramid paper is used as electrical insulation materials and construction materials to make honeycomb core. Dupont made aramid paper during the 1960s, calling it Nomex paper. Yantai Metastar Special Paper introduced an aramid paper in 2007, which is called metastar paper. Both Dupont and Yantai Metastar make meta-aramid and para-aramid paper.
Health
During the 1990s, an
in vitro test of aramid fibers showed they exhibited "many of the same effects on
as did
asbestos, including increased
nucleotide incorporation into DNA and induction of ODC (ornithine decarboxylase) 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.
A declaration of interest correction was later provided by the author of the study stating that "This review was commissioned and funded by DuPont and Teijin Aramid, but the author alone was responsible for the content and writing of the paper."
Production
World capacity of para-aramid production was estimated at per year in 2002 and increases each year by 5–10%.
In 2007 this means a total production capacity of around 55,000 tonnes per year.
Polymer preparation
Aramids are generally prepared by the reaction between an
amine group and a
carboxylic acid halide group. Simple AB
have the connectivity −(NH−C
6H
4−CO)
n−.
Well-known aramid polymers such as Kevlar, Twaron, Nomex, New Star, and Teijinconex) are prepared from diamine and diacid (or equivalent) precursors. These polymers can be further classified according to the linkages on the aromatic subunits. Nomex, Teijinconex, and New Star contain predominantly the meta-linkage. They are called poly-metaphenylene isophthalamides (MPIAs). By contrast, Kevlar and Twaron both feature para-linkages. They are called p-phenylene terephthalamides (PPTAs). PPTA is a product of p-phenylene diamine (PPD) and terephthaloyl dichloride (TDC or TCl).
Production of PPTA relies on a co-solvent with an component (calcium chloride, CaCl2) to occupy the of the amide groups, and an organic component ( N-methyl pyrrolidone, NMP) to dissolve the aromatic polymer. This process was invented by Leo Vollbracht at Akzo. Apart from the carcinogenic HMPT, still no practical alternative of dissolving the polymer is known. The use of the NMP/CaCl2 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 (H
2SO
4).
Appearances
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 terephthaloyl chloride (TCl).
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
-
very high melting point (>)
-
low flammability
-
good fabric integrity at elevated
-
sensitive to acids and salts
-
sensitive to ultraviolet radiation
-
prone to electrostatic charge build-up unless finished
[
]
Para-aramids
-
para-aramid fibers, such as Kevlar and Twaron, provide outstanding strength-to-weight properties
-
high chord modulus
-
high tensile strength
-
low creep
-
low elongation at break (~3.5%)
-
difficult to dye – usually solution-dyed
[
]
Uses
competing with
polyethylene-based fiber products such as
Dyneema and Spectra
-
composite materials
-
asbestos replacement (e.g. )
-
hot air filtration Textile
-
, newly as Sulfron (sulfur-modified Twaron)
-
mechanical rubber goods reinforcement
-
and Wire rope
-
V-belts (automotive, machinery, equipment, and more)
-
wicks for fire dancing
-
optical fiber cable systems
-
sailcloth (not necessarily racing boat sails)
-
goods
-
-
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
-
(normally in composition with such materials as wood and carbon)
-
-
jet engine enclosures
-
fishing reel drag systems
-
Asphalt concrete reinforcement
-
for rock climbers (which slide along the main rope and can otherwise melt due to friction).
-
Snowboards core reinforcement
-
Mobile Phone case
See also
Para-aramid
Meta-aramid
Others
Notes and references
Further reading
