Semi-Aromatic Polyamides (6T, DT, 6I)

Properties and Applications

Partially aromatic polyamides are heat resistant, semi-crystalline thermoplastic resins. They are copolymers made from the condensation of aliphatic amides, such as caprolactam and hexametylene diamine, with terephthalic acid and/or isophthalic acid, i.e. these resins are partially aromatic polyamides. The resins in which the residues of terephthalic acid and/or isophthalic acid comprise at least 55 molar percent of the repeat units are known as polyphthalamide (PPA). The aromatic portion in the polymer backbone increases the melting point, rigidity, heat stability, and chemical resistance and lowers the moisture absorption. The much lower water absorption results in little dimensional changes which, in turn, results in much more stable properties because moisture absorption greatly affects the mechanical and electrical properties of polyamides.² However, these resins are much more expensive than aliphatic amides and are more difficult to process due to their high melting point. For these reasons, they are sometimes blended with aliphatic polyamide resins such as Nylon 66.

Two of the most common semi-aromaitc amides are poly(hexamethylene teraphthalamide) (PA 6T) and poly(hexamethylene isophthalamide) (6I). These resins have a very high melting point (6T: T_m ≈ 320 °C) and glass transition temperature (6T: T_g ≈ 140 °C). They are known for their excellent dimensional stability, low creep at elevated temperatures and good chemical resistance comparable to many high performance engineering plastics.

Semi-aromatic polyamides are often a cost effective alternative to the more expensive fully aromatic aramids. Common applications include automotive, aerospace, electrical, and industrial parts that must withstand prolonged exposure to harsh chemicals and/or high temperatures. This includes motor parts, fuel line connectors, coolant pumps, bushings, bearing pads in aircraft engines, charge air coolers, resonators, engine cover components and heat shields, fuel cutoff and water heater manifold valves, connectors, high voltage bushings, motor housings, and head light components.

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