- Aramid
Aramid fibers are a class of heat-resistant and strong
synthetic fiber s. They are used in aerospace and military applications, for ballistic rated body armor fabric, and as anasbestos 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 thechemical bond can be exploited.History
Aromatic polyamides were first introduced in commercial applications in the early
1960 s, with a meta-aramid fiber produced byDuPont under the tradenameNomex . 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 byTeijin under the tradenameTeijinconex , in China by Yantai under the tradenameNew Star and a variant of meta-aramid in France by Kermel under the tradename Kermel.Based on earlier research by
Monsanto andBayer , a fiber - para-aramid - with much higher tenacity and elasticmodulus was also developed in the 1960s-1970s by DuPont andAkzo Nobel , both profiting from their knowledge of rayon, polyester and nylon processing.Much work was done by
Stephanie Kwolek in 1961 while working atDuPont , and that company was the first to introduce a "para-aramid" calledKevlar in 1973. A similar fiber calledTwaron 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% [http://books.nap.edu/openbook.php?record_id=11268&page=34 High-Performance Structural Fibers for Advanced Polymer Matrix Composites (2005)] ] . 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::"n"NH2-Ar-COCl → -(NH-Ar-CO)n- + "n"HCl
The most well-known aramids (Nomex, Kevlar, Twaron and New Star) are AABB polymers. Nomex, New Star and Teijinconex contain predominantly the meta-linkage and are poly-"metaphenylene isophtalamide"s (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
ion ic component (calcium chloride (CaCl2)) to occupy thehydrogen bond s of the amide groups, and an organic component (N-methyl pyrrolidone (NMP)) to dissolve thearomatic 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.pinning
After production of the polymer, the aramid fiber is produced by spinning the solved polymer to a
solid fiber from aliquid chemical blend. Polymer solvent for spinning PPTA is generally 100% (water free)sulfuric acid (H2SO4).Appearances
*
Fiber
* Chopped fiber
* Powder
* PulpOther 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 byTeijin and introduced in 1976. The manufacturing process of Technora reacts PPD and 3,4'-diaminodiphenylether (3,4'-ODA) with terephtaloyl chloride (TCl). [cite journal |author= Ozawa S| title= | journal= Polym. J. Japan | year= 1987| volume= 19 | issue= | pages= 199 | url= ] 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
* nomelting point , degradation starts from 500°C
* low flammability
* good fabric integrity at elevatedtemperature s
* sensitive to acids and salts
* sensitive toultraviolet radiation
* prone to static build-up unless finishedPara-aramids
* para-aramid fibers such as Kevlar and Twaron, provide outstanding strength-to-weight properties
* highYoung's modulus
* high tenacity
* low creep
* low elongation at break (~3.5%)
* difficult to dye - usually solution dyed cite journal |author= Kadolph, Sara J. Anna L. Langford.| title= Textiles| journal= Pearson Education, Inc. Upper Saddle River, NJ| year= 2002| volume= | issue= | pages= | url= ]Major industrial uses
* flame-resistant
clothing
* heat protective clothing andhelmet s
* body armor cite journal |author= | title= High-performance fiber makers respond to demand from military and security users| journal= Battle Tested| year= | volume= | issue= | pages= | url= http://pubs.acs.org/email/cen/html080205142022.html ] , competing with PE based fiber products such asDyneema andSpectra
*composite material s
*asbestos replacement (e.g. braking pads)
* hot airfiltration fabrics
*tire s, newly as Sulfron (sulfur modified Twaron)
* mechanicalrubber goods reinforcement
*rope s andcable s
* wicks forfire dancing
* optical fiber cable systems
*sail cloth (not necessarily racingboat sails)
*sport ing goods
*drum heads
* wind instrument reeds, such as the Fibracell brand
* speaker woofers
*boat hull material
*fiber reinforced concrete
*reinforced thermoplastic pipe s
* tennis strings (e.g. by Ashaway and Prince tennis companies)
* hockey sticks (normally in composition with such materials as wood and carbon)ee 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
*
*
* [http://www.compositesworld.com/hpc/issues/2004/September/548 Aramid production increases]
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