The difference between semi aromatic and aromatic polyamides

Polyamide (PA) is usually called nylon. It is a general term for heterochain polymers containing repeated structural unit amide groups (NHCO -) in the polymer macromolecule chain. It is mainly obtained by condensation or self polymerization of binary acids and binary amines or amino acid Lactam. It is a thermoplastic engineering plastic with early development and large use.
There are many varieties of PA, which can be divided into aliphatic polyamides, semi aromatic polyamides, fully aromatic polyamides, heterocyclic aromatic polyamides, and alicyclic polyamides based on their main chain structure. When one of the amines or acids in nylon raw materials contains a benzene ring, it is called "semi aromatic nylon", and when both materials contain a benzene ring, it is called "fully aromatic nylon". Introducing a benzene ring into the main chain of polyamide to form semi aromatic or fully aromatic polyamides can further enhance heat resistance and rigidity. Similar to aliphatic polyamides, aromatic polyamides can be formed by condensation of binary acids and diamines, or by self condensation of amino acids.
Semi aromatic polyamides can be formed by condensation of aromatic dicarboxylic acids (such as terephthalic acid) and aliphatic dicarboxylic acids (such as hexanediamine), such as nylon 6. A simple all aromatic polyamide is poly (p - Benzamide), which can be synthesized by self polycondensation of amino benzoic acid. However, the benzene ring and amide groups in the main bond are dense and structurally symmetrical, resulting in high melting point, difficulty in processing, high cost, and difficulty in industrialization. At present, the successful all aromatic polyamide is poly (p-phthaloyl P-Phenylenediamine) (PPTA), which is a lyotropic liquid crystal polymer and can be processed into fibers, called Kevlar. However, in practical applications, due to the high processing temperature of fully aromatic nylon, which is not suitable for operation, semi aromatic nylon is generally promoted as the main type in the market. As a new variety of nylon, semi aromatic nylon retains the excellent characteristics of nylon and in some aspects overcomes the shortcomings of the original PA6 and PA66. The semi aromatic nylon that has been commercialized mainly includes MXD6, PA6T and PA9T, and the fully aromatic nylon mainly includes PPTA, MPIA and PBA.
Aromatic nylon can be extruded, molded, laminated, and impregnated, and can be used to manufacture fibers, films, impregnated films, decorative laminates, glass fiber laminates, firewalls, and more. In addition, PA6 and PA66 are also lacking in dimensional stability. Because the Flexural modulus is not high enough, the shrinkage rate during injection molding is large, so production cannot be carried out. However, semi aromatic nylon has a low water absorption rate due to the presence of benzene rings in its molecular chains, which has changed the previous impression of "nylon=water absorption".