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a polymer containing amide groups, — CO— NH—, bound to aliphatic or aromatic radicals (aliphatic and aromatic polyamides, respectively) in the main chain of the mac-romolecule.
The main industrial methods for the production of polyamides are polymerization with ring opening (mainly of lactams) and polycondensation of ω-aminocarboxylic acids or their esters, as well as of dicarboxylic acids or their esters or acid halides, with diamines. Substances commonly used for the production of polyamides include ∊-caprolactam and ω-dodecalactam; adipic, sebacic, and phthalic acids (terephthalic and isophthalic acids); hexamethylenediamine; and phenylenediamines (para- and meto-isomers). The most widespread are aliphatic polyamides, particularly polyhexamethyleneadipamide, polycaproamide, polyhexamethylenesebacamide, and polydodecanamide. Among the aromatic polyamides produced industrially are poly-m-phenyleneisophthalamide (from m-phenylenediamine and isophthalic acid) and poly-p–enzamide (from p–aminobenzoic acid).
Most polyamides are solid, horny, crystalline white substances with crystallinity of up to 40–60 percent, although some are viscous liquids (resins). The melting point of aliphatic polyamides ranges from 150° to 260°C; that of aromatic polyamides is about 400°C and higher. Polyamides are light thermoplastic polymers. Among their characteristics are the following.
(1) Good mechanical strength—for example, tensile strength, 60–120 meganewtons per sq m (MN/m2), or 60–1,200 kilograms-force per sq cm (kgf/cm2), and bending strength, 70–100 MN/m2, or 700–1,000 kgf/cm2.
(2) Good hardness and elasticity—relative elongation of aliphatic polyamides, 100–400 percent.
(3) Good wear resistance and heat resistance—Vicat softening point, 160°-200°C for aliphatic polyamides and 270°–320°C for aromatic polyamides.
(4) Good chemical resistance—polyamides are stable in water and solutions of acids, alkalies, and amines at room temperature.
Polyamides are soluble only in highly polar solvents, such as concentrated sulfuric and formic acids, cresol, and fluorinated alcohols. They are readily processed by pressing, injection molding, and extrusion and are worked by machine tools; the fiibers are formed from melts or solutions.
Because of this combination of properties, polyamides are commonly used in industry, mainly in the production of synthetic fibers and films, and also as a structural material for manufacturing various machine parts, such as gears, bushings, and bearings.
World production of polyamides for structural purposes in 1973 was about 300,000 tons.
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V. V. KURASHEV