Aromatic polyamides are linear macromolecules composed of aromatic rings linked by amide bonds, in which at least 85% of the amide bonds are directly connected to two aromatic rings, and less than 50% of the amide bonds may be replaced by imide bonds. Fibers made from these aromatic polyamide long-chain polymers are known as aramid fibers.
Isophthaloyl chloride is dissolved in tetrahydrofuran and then added at room temperature into a vigorously stirred aqueous solution of m-phenylenediamine and sodium carbonate. The polycondensation reaction occurs rapidly at the interface within a few minutes, during which the generated acid is neutralized by sodium carbonate. After cooling, separation, washing, and drying, poly(m-phenylene isophthalamide) polymer is obtained.
Using dimethylformamide (DMF) or dimethylacetamide (DMAc) as solvent, m-phenylenediamine is dissolved with a small amount of acid acceptor and cooled to 0–1°C. Isophthaloyl chloride is then added slowly under continuous stirring. After completion of the reaction, water is added to precipitate the polymer. The solid is filtered, washed, and dried to obtain poly(m-phenylene isophthalamide).
The polymer is dissolved in DMF or DMAc containing certain chlorides to prepare the spinning dope. The solution is then spun by dry spinning. The as-spun fibers contain inorganic salts on the surface and are washed repeatedly with water. Subsequently, the fibers are stretched 4–5 times at 300°C. Both filament yarn and staple fiber can be produced by this method.
The spinning dope is controlled at 22°C and extruded through a spinneret with 0.07 mm holes and 34,000 capillaries into a coagulation bath at 60°C containing DMAc and calcium chloride (density: 1.366 g/cm³). The nascent fibers are washed, stretched 2–3 times in hot water, dried on heated rollers, and further stretched 1.5–1.8 times on a 320°C hot plate to obtain final products, mainly staple fibers.
The tensile strength and toughness of Aramid 1313 are comparable to nylon and polyester fibers.
Shrinkage in dry air at 260°C is about 1.7%, and around 2% in boiling water. Heat-set fabrics exhibit virtually no shrinkage in boiling water.
Aramid 1313 is inherently flame-retardant. It only burns when exposed to direct flame and self-extinguishes once the flame is removed.
Aramid 1313 is resistant to most acids, though prolonged exposure to hydrochloric, nitric, or sulfuric acid can reduce strength. It is also stable against alkalis, except for strong bases like sodium hydroxide under long-term exposure. It shows good resistance to bleach, reducing agents, phenol, formic acid, and many organic solvents such as acetone.
Hollow fibers made from Aramid 1313 can also be used in desalination based on reverse osmosis principles.
Poly(p-phenylene terephthalamide) Fiber (PPTA) – Aramid 1414
Chemical structural formula of PPTA
Aramid 1414 (commercially known as Kevlar®) was first introduced in 1972 by DuPont as a high-strength aromatic polyamide fiber primarily developed for tire reinforcement and other rubber products.
The monomers used are terephthaloyl chloride and p-phenylenediamine. Polymerization is carried out by low-temperature solution polycondensation using weakly basic solvents such as hexamethylphosphoramide (HMPA), DMAc, or N-methyl-2-pyrrolidone (NMP). A mixed solvent system of HMPA and NMP (1:2 by weight) is commonly used to achieve higher molecular weight.
Polymerization is conducted at 0–20°C under strictly anhydrous conditions. After reaction, the polymer is precipitated in water, filtered, washed, pulverized, and dried to obtain fiber-forming polymer.
The spinning dope is heated to 70–90°C and extruded through a spinneret, passing through a 0.52 cm air gap before entering a coagulation bath at about 10°C containing 20–27% sulfuric acid.
Due to the high degree of molecular orientation, the nascent fibers do not require additional drawing and already exhibit excellent properties. After thorough washing and drying at 150°C, they can be used as tire cord fibers.
Aromatic polyamide fibers (aramid fibers) represent a leading class of high-performance materials. With outstanding mechanical strength, heat resistance, chemical stability, and unique functional properties, they play an irreplaceable role across many industries. From Aramid 1313 in high-temperature filtration and protective apparel to Aramid 1414 in tire reinforcement and composites, these materials continue to drive technological advancement in high-performance applications.
