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Aramid fibers
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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 nameNomex. 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, in Korea by Toray under the trade name Arawin, in China by Yantai Tayho 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 Bayerpara-aramid fiber with much higher tenacity and elastic modulus was also developed in the 1960s1970s by DuPont and Akzo Nobel, both profiting from their knowledge of rayonpolyester 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, (CONH) are attached directly to two aromatic rings.

 

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

n NH2ArCOCl  (NHArCO)n + n HCl

The most well-known aramids (KevlarTwaronNomex, New Star and Teijinconex) are AABB polymers. Nomex, Teijinconex and New Star contain predominantly the meta-linkage and are poly-metaphenylene isophthalamides (MPIA). Kevlar and Twaron are both p-phenylene terephthalamides (PPTA), the simplest form of the AABB para-polyaramide. 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 ionic component (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.

 

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 (H2SO4).

 

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 (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

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 electrostatic charge 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 (~3.5%)

difficult to dye  usually solution-dyed

 

Uses

flame-resistant clothing (for example, military MIL-G-181188B suits).

heat-protective clothing and helmets

body armor, 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

 



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