Aramid

  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 as an asbestos substitute. The name is a shortened form 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.

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History

Aromatic polyamides were first introduced in commercial applications in the early 1960s, with a meta-aramid fiber produced by DuPont under the tradename Nomex. This fiber, which has the hand of normal textile apparel fibers, is characterised 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 tradename Teijinconex, in China by Yantai under the tradename New Star and a variant of meta-aramid in France by Kermel under the tradename Kermel.

Based on earlier research by Monsanto 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 had a patent war 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.

Production

World capacity of para-aramid production is estimated at about 41.000 tons/yr in 2002 and increases each year by 5-10%^[1]. In 2007 this means a total production capacity of around 55.000 tons/yr.

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 (Nomex, Kevlar and Twaron) are AABB polymers. Nomex or Teijinconex 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 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 a patent war between Akzo and DuPont.

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% (water free) sulphuric acid (H₂SO₄).

Appearances

• Fiber
• Chopped fiber
• Powder
• Pulp

Other types of aramids

Beside 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). ^[2] 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 which 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^[3]

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 ^[3]

Major industrial uses

• flame-resistant clothing
• heat protective clothing and helmets
• body armor^[4], competing with PE based fiber products such as Dyneema and Spectra
• composite materials
• asbestos replacement (e.g. braking pads)
• hot air filtration fabrics
• tires, newly as Sulfron (sulphur 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
• speaker woofers
• boathull material
• fiber reinforced concrete
• reinforced thermoplastic pipes
• tennis strings (e.g. by Ashaway and Prince tennis companies)
• Sneaking Suits

See also

Para-aramid

• Kevlar
• Technora
• Twaron

Meta-aramid

• Nomex
• Teijinconex
• Kermel
• New Star

Others

• Sulfron
• Nylon
• Textile
• Ultra high molecular weight polyethylene

Notes and references

• Aramid production increases