Aramid Fibers/Polymers: History, Synthesis, Properties, Applications/Uses

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."

History of Aramids:

Aramids were first introduced in commercial applications in the early 1960s, with a meta-aramid fiber produced by DuPont as HT-1 and then under the trade name "Nomex". 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 for asbestos.

Meta-aramid is also produced in the Netherlands and Japan by Teijin under the trade name "Conex", In Korea by Toray under the trade name "Arawin", in China by Yantai Tayho under the trade name "New Star", by SRO Group (China) under the trade name "X-Fiper", and a variant of meta-aramid in France by Kermel under the trade name "Kermel".

Based on earlier research by Monsanto Company and Bayer, a fiber "para-aramid" with much higher tenacity and elastic modulus was also developed in the 1960s–1970s by DuPont and Akzo Nobel, both profiting from their knowledge of rayon, polyester and nylon processing.

Much work was done by Stephanie Kwolek in 1961 while working at DuPont and that company was 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 in 1978. Due to earlier patents on the production process, Akzo and DuPont engaged in a patent dispute in the 1980s. Twaron is currently owned by the Teijin Company.

Para-aramids are used in many high-tech applications, such as aerospace and military applications, for "bullet-proof" body armor fabric.

The world capacity of para-aramid production is estimated at about 41,000 tons/year in 2002 and increases each year by 5–10 %. In 2007 this means a total production capacity of around 55,000 tons/year.

POLYMER SYNTHESIS

Aromatic polyamides are synthesized most frequently from an aromatic di-amine and an aromatic di-acid or di-acid chloride by the following reaction:

Low Temperature Poly-Condensation

A classic example of low-temperature polymerization is the preparation of poly (p-phenylene terephthalamide) (PPD-T) from p-phenylenediamine (PPD) and terephthaloyl chloride (TCl) in an amide solvent as shown below:

This reaction can also be carried out by several polymerization methods like interfacial polymerization and solution polymerization. The most convenient method is low temperature polycondensation in a solvent. In general, the initial monomer concentration is in the range of 5 ± 20 wt. %, depending on the polymer solubility and viscosity. Anhydrous diamine and diacid chloride are used in equimolar quantities to ensure maximum polymer molecular weight. One of the monomers, usually the diamine, is dissolved in an amide solvent. The second monomer is then added to the monomer solution to initiate the polycondensation. The reaction is then allowed to proceed in a tinder dry nitrogen atmosphere at -10 to 60 oC for a period of several minutes to several hours. As polycondensation proceeds, the reaction mixture will become increasingly viscous and the polymer may precipitate at a certain point. After allowing the reaction to continue for some time, the polymer is finally isolated from the reaction mixture by a nonsolvent such as water. It is thoroughly washed, neutralized, and then dried. In some cases, the polymer may remain soluble in the polymerization solvent throughout the course of reaction. Such a reaction mixture can be processed directly to form fibers or other shaped products.

Two major types of solvents for the low-temperature polycondensation of aromatic polyamides: halogenated nonaromatic hydrocarbons and organic amide solvents. Examples of the nonaromatic hydrocarbons are chloroform, methylene chloride, methyl ethyl ketone, acetonitrile, tetra-methylene sulfone, dimethylcyanamide, and propionitrile. The amide solvents which are most often used in the synthesis of aromatic polyamides include dimethyl acetamide (DMAc), N-methyl-2-pyrrolidone (NMP), hexamethyl phosphoramide (HMPA), and tetramethyl urea (TMU). In many instances, a small amount of alkaline and alkali earth metal salts such as lithium chloride, lithium hydroxide, calcium chloride, calcium hydroxide, is used in combination with the solvent to increase the polymer solubility or to neutralize the acid chlorides produced in the reaction.

Other Synthesis Methods

Several alternate methods have been reported for the polymerization of aromatic polyamides. Preston and Hofferbert (1979) attempted to synthesize aromatic polyamides by a phosphorylation reaction in DMAc/5% LiCl in the presence of triphenylphosphite and pyridine. This method gave only modest polymer molecular weight in the synthesis of PPD-T. Higashi and Taguchi in 1981 refined the phosphorylation reaction by reacting an aromatic diamine and an aromatic dicarboxylic acid in NMP/LiCl in the presence of triphenylphosphite and poly (4-vinylpyridine). The refinement also included the addition of various pyridine derivatives. They obtained PPD-T with a high inherent viscosity of 4.5 dL g-1 by the polycondensation of p-phenylenediamine and terephthalic acid in NMP/LiCl/CaCl2/pyridine/triphenylphosphite at 1.2% polymer concentration.

Properties of Aramid Fibers

Aromatic polyamides are characterized by high melt temperatures, excellent thermal stability and flame resistance, and poor solubility in many inorganic and organic solvents. An important aspect here is the effect of polymer composition on physical properties. This can be detected by comparing homopolymers vs. copolymers, unsubstituted vs. substituted polymers, and para- vs. meta-oriented polymers.

DENSITY

Unsubstituted polymers, such as PPD-T, PPD-I, MPD-T, and MPD-I, were reported to exhibit densities of 1.43-1.46 g cm(-3). Substituted polymers as a group give considerably lower density of 1.2-1.4 g cm(-3).

GLASS TRANSITION TEMPERATURE

PPD-T was reported to have a Tg of >375 OC, while polymers containing pendant substitutions and meta-oriented phenylene segments showed Tgs in the range of 255-260 OC.

MELT TEMPERATURE

PPD-T, PPD-I, MePPD-T, and MeMPD-T failed to show any melting point. However, melt temperatures of 530 and 518 OC for PPD-T and MePPD-T, respectively. The discrepancy may be attributed to differences in polymer molecular weight and molecular weight distribution.

DECOMPOSITION TEMPERATURE

Aromatic polyamides are generally very stable at high temperatures. The decomposition temperature (Td) is often determined by the thermo gravimetric analysis (TGA). PPD-T is thermally stable to about 550 OC. Most unsubstituted polyamides in the literature were stable at 400-500 OC. Chloro, methyl, and other substituted polyamides were reported to be stable at 300-400 OC.

SOLUBILITY

Wholly aromatic polyamides with rigid chains are soluble in only a few solvents such as concentrated sulfuric acid, hydrofluoric acid, and methane sulfonic acid. They are insoluble in formic acid and m-cresol, which are good solvents for aliphatic polyamides. Wholly aromatic polyamides dissolve very slightly in amide solvents like hexamethyl phosphoramide, N-methyl-2-pyrrolidone and N, N'-dimethyl acetamide, often in the presence of lithium chloride, calcium chloride, and other metal halides. The solubility in organic solvents is improved only by polymer modification such as copolymerization and substitution.

Applications/Uses of Aramid (Kevlar) Fibers

• Kevlar is five times stronger than steel, yet it is extremely lightweight. Kevlar does not rust or corrode, and it absorbs vibrations readily. Kevlar is expensive because special precautions are necessary to handle the concentrated sulfuric acid used in its production.
• Kevlar breaks down when exposed to the ultraviolet rays in sunlight. Dry-cleaning agents' bleach, and repeated washing can affect Kevlar negatively also. To protect against these problems, the layers of Kevlar in bullet-resistant vests have fabric coverings to prevent exposure to sunlight and moisture.
• Kevlar is made in three common grades: Kevlar, Kevlar 29, and Kevlar 49. Kevlar is typically used in tires. Kevlar 29 is used in body armor, industrial cables, asbestos replacements, and brake linings. Kevlar 49 is used in applications such as plastic reinforcement for boat hulls, airplanes, and bicycles.
• Most North American police officers engaged in frontline law enforcement now wear bullet-resistant vests. However, as the name implies, bullet-resistant vests do not prevent injury from edged weapons that police officers may encounter, such as knives, arrows, or ice picks. Because the force of a blow from such weapons is focused on a very small area, knives and other pointed objects can penetrate many layers of Kevlar causing injury or death. However, specially designed vests that protect against edged weapons are often worn by correctional officers.
• Kevlar is used in many other industries to make such items as badminton and tennis rackets, helmets and bulletproof vests. It can also be used to make gloves that are extremely strong and resistant to being torn or cut. Blankets made out of Kevlar are used to protect individuals from blasts from explosions or fire. As Kevlar continues to gain in popularity, it will likely be used in many other industrial applications.
• Many boats have parts made out of Kevlar. It is used in boat hulls of many types of watercrafts, as well as in some canoes made entirely out of the material. These canoes are resistant to being punctured by rocks are other elements in nature. Windsurfing vessels and sailboats often have sails made out of Kevlar due to the fact that it is difficult to tear, even in high wind.
• Kevlar was initially created as a material to be used in automobile tires to prevent damage along with wear and tear. The material is still used in this application, as well as in many engine parts that need to have a high heat resistance and tensile strength. Kevlar is also used in engines and fuel lines in high performance race cars that require strength and heat resistance.
• Kevlar is currently used in a variety of applications in both airlines and spacecraft. It is used as a protective material in airplane cockpits in case of debris or an emergency landing. It is used in fuel systems to create stronger lines that will not break as easily. Kevlar has many uses in commercial airlines in a variety of parts as well as in high-speed aircraft used by the U.S. government.