Kevlar was first synthesized by Stephanie Kwolek in a DuPont research laboratory in 1964. It was only in 1971 that Kevlar was marketed because of the problems they encountered in processing this rigid and very insoluble fiber. Kevlar’s chemical composition is poly para-phenyleneterephthalamide or para-aramid.
A single chain of its polymer could have anywhere from five to a million monomers bonded together. Each Kevlar monomer, H[-NH-C6H4(para)-NHCO-C6H4(para)CO]n, contains 14 carbon atoms, 2 nitrogen atoms, 2 oxygen atoms and 10 hydrogen atoms.
The amide groups of Kevlar are separated by para-phenylene groups. The attachment of the amide groups to the phenyl rings is at carbons 1 and 4. Aside from containing aromatic and amide molecular groups, many factors contribute to the strength of Kevlar. The polymers have a crystalline arrangement when the liquid Kevlar is spun into fibers.
The orientation of the polymer chains is parallel to the axis of the fiber. The aromatic components of Kevlar polymers also have a spoke-like orientation which allows a high level of regularity and symmetry to the internal make up of the fibers.
What hold the polymer chains together are the hydrogen bonds between these polymer chains. These hydrogen bonds are formed by the amide groups. The following are the general characteristics of Kevlar: High Tensile Strength at Low Weight Low Elongation to Break High Modulus (Structural Rigidity).
Low Electrical Conductivity High Chemical Resistance Low Thermal Shrinkage High Toughness (Work-To-Break) Excellent Dimensional Stability High Cut Resistance Flame Resistant, Self-Extinguishing Kevlar is produced from the reaction of para-phenylenediamine (PPD) and molten terephthaloyl chloride.
It is difficult to produce p-phenylenediamine due to the coupling and diazotization of aniline. The resulting polymer is filtered, washed and dissolved in concentrated sulfuric acid and is extruded through spinnerets.
It then passes through a narrow duct and goes through the wet spin process where it is coagulated in sulfuric acid. The filament can take two different paths at this point. It can be formed into a yarn, washed and dried which is wound into spools that produces a modulus of 400-500 g/denier. Conversely, the filament can go under further heat treatment with tension and produce a fiber with a modulus of 900-1000 g/denier. The end product can take several forms. It can form filament yarns, pulp, or spun-laced sheets and papers. (www. chem. uwec. edu) Nomex Nomex is the meta-aramid fiber as opposed to para-aramid Kevlar.
It was first developed in the 1961 also by the DuPont Company, particularly by Dr. Wilfred Sweeny. The chemical name of Nomex is poly (m-phenyleneisophthalamide). It is generated from the reaction of m-phenylenediamine and isophthaloyl chloride. The solution is dry spun through spinnerets. The remaining solvent is evaporated, the filament is washed and wound into tow, heated, and finally stretching into rolls at a temperature of 150 degree’s Celsius. Nomex can be produced as a continuous filament yarn, staple, spun yarn, floc, pressboard, paper, needle felt, or as a fabric.
Next we will take a look at the economics of producing Nomex (www. chem. uwec. edu). As compared to Kevlar, Nomex has meta-phenylene groups. The amide groups are bonded to the phenyl ring at the 1 and 3 positions. Nomex does not melt or flow when heated and it also doesn’t char in extreme temperatures until over 370° Celsius. Nomex III, which is a combination of 95% Nomex and 5% Kevlar, is the usual material found in turnouts and airline seat covers. The following are the general characteristics of Nomex: • Heat and Flame Resistant High Ultraviolet Resistance High Chemical Resistance.
Low Thermal Shrinkage Formable for Molded Parts Low Elongation to Break Low Electrical Conductivity What sets Nomex apart from Kevlar is the location of the amide linkages on the aromatic ring. Compared to Kevlar, Nomex has a lower tensile strength and modulus and a higher elongation and solubility in organic solvents. (www. chem. uwec. edu) Polybenzimidazoles Polybenzimidazoles or PBI fiber is a “manufactured fiber in which the fiber-forming substance is a long-chain aromatic polymer having recurring imidazole groups as an integral part of the polymer chain”.
Polybenzimidazoles or PBI fiber was developed in 1961 by Carl Shipp Marvel and H. Vogel and developed by the Celanese Corporation. It was used for defense and aerospace applications and was introduced to the fire service during the 1980s. It became commercially available in 1983 and began to be functional in various industries like automotive and aircraft. These are produced mainly from the condensation of 3, 3′, 4, 4′-tetraaminobiphenyl (diaminobenzidine) and diphenyl isophthalate, which are dry-spun using dimethyl acetamide as solvent. The result of this reaction is a long-chain aromatic polymer with recurring imidazole groups.
They have a high level of chemical and thermal stability which made them the choice for the make up of turnouts and other protective clothing such as race driver and astronaut space suits. PBI fiber is non-flammable. It doesn’t melt, ignite or drip as it was tested in the Federal Vertical Flame Tests. Even after contact with a large number of organic solvents and chemicals, it maintains its tensile strength, aside from being chemically resistant to inorganic acids and bases. Cloths that are made up of PBI fiber do not embrittle or shrink when exposed to high temperature or flame.
It integrity and suppleness is retained even up to 1000°F. PBI fibers are also blended with other synthetic fibers, like aramids, to produce garments with excellent tear strength and penetration resistance. These combinations are applied to demanding applications such as bullet-proof vests. The electrical conductance of materials that contain PBI fibers is similar to that of unprocessed pure cotton. It has a low static build up quality which is intrinsic in the fabric and will not wash out. The following are the characteristics of PBI: Chemical Resistance Acids – concentrated Poor.
Acids – dilute Fair-Poor Alcohols Good Alkalis Good-Poor Aromatic hydrocarbons Good Greases and Oils Good Halogenated Hydrocarbons Good Ketones Good Electrical Properties Dielectric constant @1MHz 3. 2 Dielectric strength ( kV mm-1 ) 21 Volume resistivity ( Ohmcm ) 8×1014 Mechanical Properties Coefficient of friction 0. 19-0. 27 Compressive modulus ( GPa ) 6. 2 Compressive strength ( MPa ) 400 Elongation at break ( % ) 3 Hardness – Rockwell K115 Izod impact strength ( J m-1 ) 590 unnotched Poisson’s ratio 0. 34 Tensile modulus ( GPa ) 5.
9 Tensile strength ( MPa ) 160 Physical Properties Density ( g cm-3 ) 1. 3 Flammability Does not burn Limiting oxygen index ( % ) 58 Radiation resistance Good Water absorption – over 24 hours ( % ) 0. 4 Thermal Properties Coefficient of thermal expansion ( x10-6 K-1 ) 23 Heat-deflection temperature – 0. 45MPa ( C ) 435 Thermal conductivity @23C ( W m-1 K-1 )
0. 41 Upper working temperature ( C ) 260-400 (www. goodfellow. com) Works Cited “Aramid Fibers. ” 30 April 2001. 17 May 2008 <http://www. chem. uwec. edu/Chem405_S01/malenirf/project. html> “Aramids. ” P. Leo. 1996-1998.