Introduction
Polyamides are high molecular weight polymers that have an amide group in their main chain which is considered as the main component of the polymer chain. The amide groups strongly adhere to each other and ensure high strength. These groups, which have become almost linear due to stretching, create strong hydrogen bonds with each other which are very strong.
Polyamides are classified into two groups according to their constituent monomers. 1) Polyamides that are prepared from the reaction of amino acids (AB) by the addition polymerization method. This method is used to prepare some AB type polyamides whose monomers are cyclic lactams such as ε-caprolactam or pyrrolidinone. 2) Polyamides that are formed from the condensation of bifunctional diamine and diacids that are called AABB type polyamides in which A indicates the amino group and B indicates the acidic group.
Aliphatic polyamides are named from the number that indicates the number of carbons in their constituent monomers. For AABB type polyamides two numbers are used. The first one refers to the number of diamine carbon atoms and the second one shows the number of diacid carbon atoms. Polyamide derived from ε-aminocaproic acid (6-aminohexanoic acid) or lactam is known as polyamide 6. Components with benzene rings are marked with a letter. For example, terephthalic and isophthalic acid are marked with T and I. Some authors also use TA and IA.
Linear polyamides are condensation products of bifunctional monomers. The hydrocarbon part between the amide groups may include branched or linear condensed hydrocarbons, aromatic rings or aliphatic rings, which can also include oxygen, sulfur and nitrogen. The hydrocarbon part used in the chain affects the flexibility of the chain and its structural arrangement, which is an important factor for the formation of the crystalline phase.
Aromatic polyamides are polymers in which an amide bond is placed between two aromatic rings. These polymers are prepared from the reaction of aromatic diamines with aromatic diacids in an amide solvent. Fibers with good heat resistance and high tensile strength and modulus are prepared from these polymers. Due to the unusual physical properties of aromatic polyamides, the general name aramid was chosen for them in 1974.
Aramid is a synthetic fiber made from long-chain polyamide in which at least 85% of the amide bonds (CONH) are directly attached to two aromatic rings.
In commercial aramids, 100% of amide bonds are connected to aromatic rings. With the creation of the new term aramid, synthetic fibers made from long-chain polyamide in which less than 85% of the amide bonds are directly attached to the aromatic ring were called nylon.
Properties
Polyamides comprise properties such as excellent toughness and impact resistance, excellent abrasion resistance, low friction coefficient, high tensile strength properties and optimal creep resistance, maintaining mechanical and electrical properties in a wide range of temperatures, excellent resistance to oils, greases, solvents and bases. These polymers can be processed through all special methods of thermoplastics.
Polyamide 6
It is obtained from polymerization of w-aminocaproic lactam, which is more famous under the name of caprolactam. For this reason, this plastic is also called polycaprolactam. Another chemical name of this compound is 2-oxohexamethyleneimine. By opening the caprolactam ring and the formation of a molecule with two very active ends and joining of these structural units by intercondensation and addition method, the polymerization is carried out and finally the aliphatic polyamide is obtained.
Thermal properties of polyamide 6
The properties of polyamide 6 are very similar to polyamide 66, but there are also differences between them in terms of thermal and mechanical properties. Tensile properties, hardness, thermal properties and density of polyamide 66 is superior to polyamide 6, so the toughness and impact resistance of polyamide 66 is higher than polyamide 6. Polyamide 6 shows high shrinkage after molding due to its semi-crystalline nature. The melting range of polyamide 6 is in the thermal range of 222°C, its glass transition temperature is 53°C and its long-term operating temperature is 90°C.
Polyamide 6 with the high molecular mass and high degree of crystallinity which is used in the casting method, shows improved mechanical and thermal properties compared to other molding methods. Its moisture absorption is also reduced and retains its positive characteristics against changes in humidity.
The table below shows the most important thermal characteristics of polyamide 6.
Mechanical properties of polyamide 6
Polyamide 6 possesses high strength and stiffness, high hardness, good stability against creep, good resistance to wear and thermal aging. Reducing the number of methylene functional groups in the structural unit of the polyamide 6 chain, makes this plastic benefit from a higher melting range of 220, more moisture absorption and higher mechanical properties compared to polyamide 11 and 12. Polyamide 6 fibers have high tensile strength and toughness, high elasticity and luster. These fibers are resistant to wrinkling and wear. Moisture absorption of polyamides, especially polyamide 6 and 66, causes a sharp decrease in their modulus because the presence of water between polyamide chains, which have high moisture absorption capabilities, makes them soft. Polyamide 6 fibers absorb moisture in 50% to 2.7% relative humidity condition, which this amount of water has a favorable reducing effect on its mechanical properties. The amount of moisture absorption of polyamide 6 fibers in saturated relative humidity (100%) is equal to 9.5-11%. The increase in moisture absorption causes a sharp decrease in the mechanical properties of the polyamide 6 products. As the temperature increases, the elastic modulus of the parts made of polyamide 6 decreases sharply.
In the table below, some of the most important mechanical and electrical properties of polyamide 6 are presented.
Different types of polyamide 6
Polyamide 6, like polyamide 66, contains a set of excellent properties, so it can be used in wide range of applications and diverse processes.
In the first and most important classification, polyamide 6 is divided into six groups: homopolymer, copolymer, alloy, improved, filled, and reinforced. In some of the polyamide 6 homopolymer due to the mixing with fluoroplastics at a weight ratio of 15%, 20%, 30%, a fluid-like state and reduction of friction for the final product is occurred.
Homopolymers: There are varieties of polyamide 6 homopolymer that have been developed for the injection process with low viscosity and high melt flow rate. Other types of polyamide 6 homopolymer are suitable for the extrusion process due to their high viscosity.
Reinforced: Reinforced polyamide 6 with glass fibers (long and short) are made in weight ratios of 10%, 15%, 20%, 30%, 40% and 50% to produce composite parts.
Improved: Using additives and with the aim of improving the properties and increasing the efficiency of polyamide 6, various types of this plastic have been supplied to the market, such as the types which are resistant to UV, containing softener or heat stabilizer, or with the higher degree of crystallinity and having a nucleating agent (clarifier). It is worth mentioning that the nucleating agent, in addition to making the product opaque and transparent, increases the speed of the molding and consequently improves the production efficiency. Several types of polyamide 6 have been made with molybdenum disulfide additive which can show excellent resistance to wear.
Filled: There are other types of polyamide 6 that have powdery mineral fillers such as gypsum or lime at 20% and 30% weight ratios.
Polyamide 6 copolymers: Several copolymers and terpolymers have been made from polyamide 6 that some of which have been commercially available for years. The most important polyamide 6 copolymers are 6/610 and 6/66. Copolymer 6/66 was provided by BASF, Germany, which its two comonomers has the ratio of 85:15.
Polyamide terpolymers 6/610/66: These are copolymers with high flexibility and ability to dissolve in water and alcohol (in a mixture), which have high impact absorption ability. Ultramid 1c terpolymer is made of polyamide 6. Terpolyamide include equal amounts of polyamide 6 and polyamide 66 and another amide compound called diaminodicyclohexylmethane. This terpolymer is used as a coating and finishing operation.
Typical applications of polyamide
Transportation: Transportation represents the largest market for polyamides. Applications of unreinforced materials include electrical connectors, wire coatings and lightweight gears, windshield wipers and speedometers. Stone guards and trim clips have been used to protect the windshield of the car. Glass-reinforced polyamides have been used in engine fan shrouds, radiator heads, steering and brake of fluid reservoirs and valves, sensors, and fuel injectors. Mineral reinforced resins have been used in mirror tools and tire hub covers. A combination of glass and mineral materials is used in exterior parts such as fender extensions.
Electrical and electronic applications: Flame retardant polyamides, including those that are performed with UL-94V0 requirements, play a major role in the electrical goods markets (plugs, fasteners or connectors, coils, wiring harnesses, terminal blocks, and antenna mounting tools).
Home Appliances: Polyamides are used not only for components in electrical goods, but also for mechanical parts and tools, power tools, washing machines, and various small household appliances.
Special applications in telecommunications: Power amplifiers or radio/telegraph amplifiers, amplifier stations, and connectors.
Industrial applications: Including hammer or sledgehammer handles, lawnmower parts, non-greased gears, bearings, anti-friction parts and a wide range of applications requiring spring clips or mounting the load on the spring.
Food and textile processing equipment: Including pumps, valves, measuring devices, agricultural and printing devices, office and sales machines.
Consumer Products: Applications of tough and toughened polyamide include ski boots, roller skates and ice skates bases, racket sports equipment, bicycle wheels, kitchenware, toys, and photography equipment.
Polyamide films: These films are used in a wide range for packing all kinds of meats and cheeses, as well as in non-stick bags for cooking and frying food. They are also used as an enclosing coating for making small airplane wings made of thermosetting polymers.
Wire and cable coating: Polyamides are often used as a protective layer on the primary insulation layer.
Piping and piping materials: Polyamides are used to transfer special brake fluids, special fluids for refrigerators, or as an inner lining for flexible cables.
Extrusion: sheets, bars, and handle-like shapes in machining.
- Heat-resistant materials:
This application includes filter bags for hot gases coming out of the chimney, under-press fabrics in industrial presses such as use in the permanent press of the final stage of linen fabric and linen polyester clothes, ironing board cover and sewing thread for very fast sewing, insulating paper for electric motors, tubes made for wire insulation, and dryer belts for papermaking. Another application is as pressure sensors in fuel tanks.
• Flame-resistant materials:
This application includes industrial protective clothing such as welders’ clothing and other protective clothing, firefighter clothing, flight clothing for military pilots and mail bags, carpets, curtains, sofas, fabric and cargo covers, boat covers and tents.
• Materials with dimensional stability
Fire hoses, V-belts, and power transfer belts made with high modulus aramid fibers such as Namax are examples of this application.
• Cases with very high strength and high modulus
These materials are used in V-belts of cables, parachutes, bulletproof vests, rigid reinforced plastics, antenna components, electrical circuit boards, sports equipment, ship ropes, telephone and power line cables, and fiber optic cables. Other usage is as a substitute for fireproof cotton.
• Cases with special properties
This application includes the fabrication of hollow fiber permeable separation membranes used for seawater and saltwater purification.
• Automobile industry
Polyamide 6 and polyamide 66 are used in the automotive industry. This industry accounts for 25% of the global consumption of polyamides.
In addition to making parts, polyamide 6 is also used in the production of a very large volume of textile and non-textile fibers. Due to its linear structure, fibers with particular properties are obtained from this resin.
Polyamide 6 has a wide range of applications in the manufacture of products that comprise high strength, wear resistance, high hardness and resistance to thermal wear, or the part has the ability to be machined.
This plastic is used in making gears, bearings, connections, fibers, car parts, all kinds of films, and films used for food packaging. Polyamide 6 is also used in making the collar of wheelchairs, propellers, strings of musical instruments such as violin, guitar, viola, cello, sitar, and etc.