Aramid, oraromatic polyamide fibers are a class of strong, heat-resistant,synthetic fibers, commonly used in aerospace and military applications - e.g., ballistic-ratedbody armorfabric and ballistic composites,marine cordage andhull reinforcement - as a substitute forasbestos,[1] and in lightweight consumer items, such asphone cases andtennis rackets.
Individual amide molecules forming the aramid chainpolymerise in the direction of the fiber axis, lending greater structural integrity to the resulting fiber. This is due to the higher proportion ofchemical bonds which contribute to the physical strength and thermal resistance (melting point >500 °C (932 °F)) versus other synthetic fibres, such asnylon.
Notablebrands of aramid fiber includeKevlar,Nomex, andTwaron.
The termaramid is shortened fromaromaticpolyamide. It was introduced in 1972,[3] accepted in 1974 by theFederal Trade Commission of the USA as the name of a generic category of fiber distinct fromnylon,[4][5] and adopted by theInternational Organization for Standardization in 1977.[citation needed]
Aromatic in the longer name refers to the presence ofaromatic rings of six carbon atoms. In aramids these rings are connected viaamide linkages each comprising a CO group attached to an NH group.
In order to meet the FTC definition of an aramid,[5] at least 85% of these linkages must be attached to two aromatic rings.[6] Below 85%, the material is instead classed as nylon.[5]
Aramids are divided into two main types according towhere 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, whilemeta-aramids have them at positions 1 and 3.[7] 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.
Aromatic polyamides were first introduced incommercial applications in the early 1960s, with ameta-aramid fiber produced byDuPont as HT-1 and then under the trade nameNomex.[8] 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 ofoxygen. It is used extensively in the production of protective apparel, air filtration, thermal and electrical insulation, and as a substitute forasbestos.
Meta-aramids are also produced in the Netherlands and Japan byTeijin Aramid under the trade name Teijinconex,[8] and byToray under the trade name Arawin, inChina by Yantai Tayho under the trade name New Star and by SRO Group under the trade name X-Fiper, and a variant of meta-aramid inFrance by Kermel under the trade name Kermel.
Based on earlier research byMonsanto Company andBayer,para-aramid fiber with much highertenacity andelastic modulus was also developed in the 1960s and 1970s by DuPont andAkzoNobel, both profiting from their knowledge ofrayon,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[citation needed] para-aramids or aramids.
In 1978, Akzo introduced a similar fiber with roughly the same chemical structure calling itTwaron. 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 inChina (seeProduction).
Para-aramids are used in many high-tech applications, such as aerospace and military applications, for "bullet-proof"body armorfabric.
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.[citation needed]
During the 1990s, anin vitro test of aramid fibers showed they exhibited "many of the same effects onepithelial cells as didasbestos, including increasedradiolabelednucleotide incorporation into DNA and induction of ODC (ornithine decarboxylase) enzyme activity", raising the possibility ofcarcinogenic implications.[9] However, in 2009, it was shown that inhaled aramid fibrils are shortened and quickly cleared from the body and pose little risk.[10] 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."[11]
World capacity of para-aramid production was estimated at 41,000 t (40,000 long tons; 45,000 short tons) per year in 2002 and increases each year by 5–10%.[12] In 2007 this means a total production capacity of around 55,000 tonnes per year.[citation needed]
Aramids are generally prepared by the reaction between anamine group and acarboxylic acidhalide group. Simple ABhomopolymers have the connectivity −(NH−C6H4−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 calledp-phenylene terephthalamides (PPTAs). PPTA is a product ofp-phenylene diamine (PPD) andterephthaloyl dichloride (TDC or TCl).
Production of PPTA relies on acosolvent with anionic component (calcium chloride, CaCl2) to occupy thehydrogen bonds of the amide groups, and an organic component (N-methyl pyrrolidone, NMP) to dissolve thearomaticpolymer. This process was invented by Leo Vollbracht at Akzo. Apart from thecarcinogenicHMPT, 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.
After production of the polymer, the aramid fiber is produced byspinning the dissolved polymer to a solid fiber from a liquid chemical blend. Polymer solvent for spinning PPTA is generally 100%anhydroussulfuric acid (H2SO4).
Besides meta-aramids like Nomex, other variations belong to the aramid fiber range. These are mainly of thecopolyamide type, best known under the brand nameTechnora, as developed by Teijin and introduced in 1976. The manufacturing process of Technora reacts PPD and 3,4'-diaminodiphenylether (3,4'-ODA) withterephthaloyl chloride (TCl).[13]This relatively simple process uses only one amide solvent, and therefore spinning can be done directly after the polymer production.
Aramids share a high degree of orientation with other fibers such asultra-high-molecular-weight polyethylene, a characteristic that dominates their properties.
Para-aramid
Meta-aramid
Others
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