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Introduction

Para-aramid

Aromatic Polyamides were first introduced in a commercial application in the early 1960s, with the meta-aramid fiber Nomex, by DuPont. This fiber is a very heat resistant material still used as for thermal and electrical insulation and also produced by Teijin under the tradename Teijinconex.

Based on earlier research by Monsanto and Bayer, a fiber with much higher tenacity and modulus was developed also in the 1960s by Dupont and Akzo Nobel, both profiting from their knowledge from rayon, polyester and nylon processing.

Much work was done by Stephanie Kwolek in 1961 while working at DuPont and they were 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 Nobel in 1987. Due to earlier patents on the production process, Akzo Nobel and Dupont had a patent-war in the 1980s. Twaron is currently owned by the Teijin company (see Production).
Aramids are used in many high-tech applications, such as aerospace and military applications, for "bullet-proof" body armor fabric, and as an asbestos substitute.

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

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 terephtalamide (PPTA), the simplest form of the AABB para polyaramide. PPTA is a product of p-phenylene diamine (PPD) and terephtaloyl dichloride (TDC or TCl) or PPD and terephtalic acid (TPA). Production of PPTA relies on a co-solvent with an ionic component (Calcium Cloride (CaCl2) to occupy the hydrogen bonds of the amide groups, and an organic solvent N-methyl pyrrolidone (NMP) to dissolve the aromatic polymer. Prior to the invention of this process by Leo Vollbracht, working at the Dutch chemical firm Akzo Nobel, no practical means of dissolving the polymer was known. The use of this system led to a patent war between Akzo Nobel 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

  • Fibre
  • Chopped fiber
  • Powder
  • Pulp

Other types of Aramids

Beside meta aramids like Nomex, other variations which belong to the aramid fiber range. These are mainly of the copolyamide type, most wellknown under the brandname Technora, as developed by Teijin and introduced in 1976. The manufacturing process of Technora reacts PPD and 3,4'-diaminodipenylether (3,4'-ODA) with TPA. [1] This relatively simple process uses only one amide solvent and therefore spinning can be done directly after the polymer production.

Aramid fiber characteristics

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[2]

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 [2]

Major industrial uses

See also

Notes and References

  1. ^ Ozawa S (1987). Polym. J. Japan. 19: 199. {{cite journal}}: Missing or empty |title= (help)
  2. ^ a b Kadolph, Sara J. Anna L. Langford. (2002). "Textiles". Pearson Education, Inc. Upper Sadddle River, NJ.

  • JWS Hearle (2000). "High-performance fibres". Woodhead Publishing Ltd., Abington, UK - The Textile Institute (ISBN: 1855735393).


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