Aramids

Aramid is an acronym originating from aromatic poly (amide). Wholly aromatic poly(amide)s (PA)s were described in the 1960s. Aramid fibers were originally intended to replace steel belting in vehicle tires. Aramids 

In addition to wholly aromatic PAs, PAs are termed as aromatic PAs, when at least one constituting component is of an aromatic nature. These types of partially aromatic PAs are in between nylons and aramids. The particular advantages are easier techniques of polymerization and fabrication. So, if the properties of partially aromatic PAs are sufficient for certain particular applications they can be used instead of wholly aromatic PAs. Partially aromatic PAs are dealt with in this chapter only marginally they are summarized in the poly(phthalamide) chapter. 

Wholly aromatic PAs based on 1,4-bis(4-carboxyphenoxy)naphthal-ene or 2,6-bis(4-carboxyphenoxy)naphthalene and aromatic diamines are readily soluble in a variety of organic solvents such as dimethylacetam- 

4-Phenylenediamine 1,3 -Phenylenediamine 3,4 -Diaminodiphenyl ether 4,4 -Diaminodiphenyl ether Kevlar , Twaron Nomex Technora Fihns  

Terephthaloyl chloride Isophthaloyl chloride 2-Chloroterephthaloyl chloride 1,4-B is (4-carboxyphenoxy)naphthalene 2,6-B is (4-carboxyphenoxy)naphthalene 5-Amino-2-(4-aminophenoxy)-pyridine Kevlar , Twaron Films Organically soluble Organically soluble Organically soluble  

Aramid yarns (Kevlar of DuPont, Twaron of Teijin-Twaron) are produced from poly(p-phenylene terephthalamide), PPTA (2), which is specially developed for fiber spinning and not used in any other application. DuPont had experience with poly(m-phenylene isophthalamide) in a fiber product called Nomex for high-temperature applications. The polymer is produced in dimethylacetamide and the solution is dry-spun. This cannot be done with the stiff-chain para-para analogue PPTA. The polymer does not dissolve in organic solvents. A special polymerization route had to be developed, and the discovery of lyotropic behavior of concentrated solutions in sulfuric acid then led the way to the production of a magnificent new fiber material. 

Dissolution of the polymer powder in water-free sulfuric acid can be achieved by freezing the acid and blending it with the powder. This dry blend melts at 60-70°C. An alternative is direct dissolution in a kneader. 

The key invention for aramid spinning was the discovery of the liquid rystalline behavior (lyotropy) of PPTA-sulfuric acid systems at polymer concentrations above 10%. Morgan (Monsanto) was the first to describe spinning of fully aromatic polymers from concentrated solutions (in organic solvents), but did not report their liquid-crystalline behavior [11]. Kwolek (DuPont) patented optically anisotropic solutions of PPTA in sulfuric acid [12] and Blades (DuPont) further specified the use of an air gap in the spinning process [13]. 

Solidification of the aramid filaments is called coagulation but is in fact a simple freezing-in of the solution. It is not a speed-limiting step. The subsequent removal of sulfuric acid from the solid filaments in the washing process is diffusion-controlled, however, and hence relatively slow. 

the rest of the spinning process is careful washing, neutralization, further washing, drying, and winding. The standard titer of aramid yarns is 1670 dtex f 1000 (1.7 dtex filament titer). The elongation at break is around 3%, the tenacity 2100 mN tex , and the modulus 50 N tex (75 GPa). 

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Nomex is another aramid fiber Kevlar and Nomex differ only in that the substitution pattern in the aromatic rings is para in Kevlar but meta in Nomex Nomex is best known for its fire resistant properties and is used in protective clothing for fire fighters astronauts and race car drivers... 

Aromatic nylons, [—NH—C5H4—CO—] (also called aramids), have specialty uses because of their improved clarity. 

Nylon 6/9, molding and extrusion Nylon 6/12 Nylon 11, molding and extrusion Nylon 12, molding and extrusion Aromatic nylon (aramid), molded and unfilled ... 

Fig. 17. Distribution of U.S. synthetic fiber consumption A, acryUc I, olefin +, nylon and aramid A, polyester (71,72).

Fine adjusting and optimization of each step of this process is stiU underway, and a PVA fiber having a single fiber strength as high as 2 N/tex (21 gf/dtex), which is close to that of aramid fiber, has been reported (18). 

Rigid-rod polymers are often Hquid crystalline polymers classified as lyotropic, such as the aramids Nomex and Kevlar, or thermotropic Hquid crystalline polymers, such as Vectran. 

Properties. As prepared, the polymer is not soluble in any known solvents below 200°C and has limited solubiUty in selected aromatics, halogenated aromatics, and heterocycHc Hquids above this temperature. The properties of Ryton staple fibers are in the range of most textile fibers and not in the range of the high tenacity or high modulus fibers such as the aramids. The density of the fiber is 1.37 g/cm which is about the same as polyester. However, its melting temperature of 285°C is intermediate between most common melt spun fibers (230—260°C) and Vectran thermotropic fiber (330°C). PPS fibers have a 7 of 83°C and a crystallinity of about 60%. 

Chemically Resistant Fibers. Fibers with exceUent chemical resistance to corrosive and/or chemical warfare agents or extreme pH conditions (eg, very acidic or very alkaline) were initially used for protective clothing. However, appHcations for filtration of gases and Hquids in numerous industrial faciHties are now the more important. For example, PPS is suitable for use in filter fabrics for coal-fired boilers because of its outstanding chemical and heat resistance to acidic flue gases and its exceUent durabUity under these end use conditions. Many high tenacity fibers are also chemically inert or relatively unaffected under a variety of conditions. Aramids, gel spun polyethylene, polypropylene, fluorocarbon, and carbon fibers meet these criteria and have been used or are being considered for appHcations where chemical resistance is important. 

In addition to carbon and glass fibers ia composites, aramid and polyimide fibers are also used ia conjunction with epoxy resias. Safety requirements by the U.S. Federal Aeronautics Administration (FAA) have led to the development of flame- and heat-resistant seals and stmctural components ia civiUan aircraft cabias. Wool blend fabrics containing aramids, poly(phenylene sulfide), EDF, and other inherently flame-resistant fibers and fabrics containing only these highly heat- and flame-resistant fibers are the types most frequently used ia these appHcations. 

Composites. The history of phenoHc resin composites goes back to the early development of phenoHc materials, when wood flour, minerals, and colorants were combined with phenoHc resins to produce mol ding compounds. In later appHcations, resin varnishes were developed for kraft paper and textile fabrics to make decorative and industrial laminates. Although phenoHcs have been well characterized in glass-reinforced composites, new developments continue in this area, such as new systems for Hquid-injection molding (LIM) and sheet-molding compounds (SMC). More compHcated composite systems are based on aramid and graphite fibers. 

Cydohexanedimethanol can be the comonomer. TerephthaHc acid can also be the diacid in specialty nylons (see Polyamides). Specialty fibers, including certain high modulus aramid fibers, are made from terephthaHc acid—dimethyl terephthalate derivatives. 

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