Paraaramid

Para-aramids are a family of aromatic polyamides characterized by para-linked phenylene rings along the polymer backbone. The most widely known member is poly(p-phenylene terephthalamide), the basis for fibers marketed under names such as Kevlar. Para-aramids are distinguished from meta-aramids by the arrangement of the aromatic rings, which confers higher tensile strength and modulus but different thermal behavior.

Their repeating unit consists of amide linkages between para-substituted phenylene rings: -NH-Ph-CO-Ph-CO-NH-, giving poly(p-phenylene terephthalamide). They are typically formed by polycondensation of p-phenylenediamine with terephthaloyl dichloride or terephthaloyl chloride, often processed in concentrated sulfuric acid to form a fiber-ready dope. Fibers are produced by dry or wet spinning and subsequent drawing to align chains, yielding high strength and stiffness.

Properties include very high tensile strength and modulus, outstanding abrasion and impact resistance, and excellent chemical stability. They resist many solvents and fuels but are not melt-processable; they decompose at high temperatures rather than melting, with thermal stability typically up to about 400–500°C in air, depending on formulation and post-treatment. They also exhibit low creep and good dimensional stability.

Applications cover ballistic and stab protection, helmets and body armor, cut-resistant clothing, reinforcement fibers for composites, tires and hoses, and cables. The para-aramid class remains widely used where high strength-to-weight ratio and durability are critical.