Mechanical, Electrical, Thermal and Chemical Properties of Polyamide (PA) Resins

Polyamide (PA) is a high molecular compound synthesized from lactam, fatty carboxylic acid, fatty amine, or aromatic dibasic acid, aromatic dibasic amine, containing amide groups (-NHCO-) in the main chain. Polyamide is tough angular semi-transparent or milky white crystalline resin, as an engineering plastic polyamide's relative molecular mass is generally (1.5-3) × 104. Polyamide has high mechanical strength, high softening point, heat resistance, low friction factor, abrasion resistance, self-lubricating, vibration absorption and anechoic, oil resistance, weak acid resistance, alkali resistance and solvents in general, good electrical insulation, self-extinguishing, non-toxic, odorless, good weathering, poor dyeing, good resistance, good coloring, good colorability. Good weatherability, poor coloring. The disadvantage is that the water absorption is large, affecting the dimensional stability and electrical properties. Reinforcement with fiber can reduce the water absorption of the resin, so that it can work in high temperature and high humidity conditions.

1. Properties of commonly used polyamides

PA varieties are Pa6, Pa66, PA11, PA12, PA46, PA610, PA612, PA1010, as well as semi-aromatic polyacid amine PA6T and specialty polyamides, etc., of which PA6, PA66 production is the largest, accounting for more than 90% of the production of polyamide. The chemical structure of various polyamides is different, and their properties are also different. Table 1-1 lists the properties of some commonly used polyamides.

Properties of common polyamides

2.Mechanical properties

Polyamide molecular chain contains polar amide groups, can form intermolecular hydrogen bonding, crystallinity, the force between the molecular chain is larger, so it has high mechanical strength and modulus. With the increase of amide group density, molecular chain symmetry and crystallinity, its strength increases; with the increase of methyl group in polyamide molecular chain, the mechanical strength decreases, while the impact strength increases gradually. The introduction of aryl groups into the polyamide molecular chain structure also increases the strength due to the increase in bonding energy and the increase in forces (e.g. van der Waals forces) between the molecular chains. Among polyamides, PA66 has the highest hardness and rigidity, but the lowest toughness. All kinds of polyamide according to the toughness size ranking: PA66 < PA11 < PA12 < PA1010 < PA6 < PA610. polyamide crystallinity, it has a great impact on the mechanical properties, tensile strength, flexural strength, flexural modulus are with the increase in crystallinity and improve. The amide group in the main chain of polyamide molecule is a hydrophilic group, which makes the polyamide have water absorption. Water absorption has a great influence on the mechanical properties of polyamide, after absorbing water, tensile strength, bending strength and its bending modulus of elasticity and so on greatly decreased, the size of the product changes greatly, while the impact strength is greatly increased, see Table 2-1.

Effect of Polyamide Water Absorption on Strength

3. Electrical Properties

Polyamide has good electrical insulation properties, Table 3-1 lists some of the electrical properties of polyamide. Although polyamide has good electrical properties, it contains polar amide groups in the main chain of the molecule and is a water-absorbent polymer. As the water absorption rate increases, the volume resistivity and dielectric strength of the polyamide decreases, making polyamide engineering plastics unsuitable for use as electrical insulators for high-frequency and wet environment work.

Electrical properties of some polyamides

4. Thermal properties

The melting point of different varieties of polyamide varies greatly, the highest melting point is PA46, up to 295 ℃. Polyamide has a wide range of operating temperatures, generally from -40 to 100°C. The heat deflection temperature of polyamide is strongly dependent on the load. The heat deflection temperature of polyamides is strongly related to the load they are subjected to, and decreases rapidly with increasing load. The thermal conductivities of the various polyamides do not vary much, as shown in Table 1-1. The heat of combustion of the various polyamides is shown in Table 4-1. The heat of combustion of PA6 and PA66 is the same, while that of PA610 is higher and that of PA11 is the highest.

Heat of combustion of polyamide

(Unit: kJ/g)

5. Chemical properties

Polyamide is stable to most chemical reagents, especially to gasoline, lubricants and other oils, has a strong resistance, good oil resistance, PA11, PA12 oil resistance is excellent, is the first choice of materials for automotive fuel lines. However, it is soluble in phenol, concentrated inorganic acid and formic acid at room temperature, and in ethylene glycol, glacial acetic acid, propylene glycol, methanol solution of zinc chloride or calcium chloride, as well as fluoroacetic acid and fluoroethanol at high temperature. In general, most polyamide plastics are stable in alkaline solutions, but hydrolysis or degradation occurs at high temperatures, especially when the polyamide is melted; under these conditions, inorganic acids and amines, especially monovalent acids, can rapidly acidify and aminolyze the polyamide, causing the breakage of the phthalide-amine bond, and ultimately generating the monomer of the polyamide. Polyamides undergo thermal degradation at high temperatures. In addition to thermal degradation, oxidative degradation occurs when polyamide is heated in air. For example, PA66 becomes brittle after treatment at 250°C for 2h or 70°C for 2 years. In practice, often in the processing of polyamide products, add some antioxidants to inhibit thermal oxidative degradation. Polyamide in the light, due to ultraviolet radiation, will also occur photodegradation or aging. Polyamide contains carbonyl group, can absorb ultraviolet light in sunlight, so that the polyamide chain segment breakage and crosslinking, in the absence of oxygen light, PA6 and PA66 decomposition into H2, CO and hydrocarbons. UV absorbers and hindered amine light stabilizers can be added to improve the light stability of polyamides.