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 66
Polyamide 66 is produced through condensation polymerization of adipic acid and hexamethylenediamine. The repeating unit of this plastic is as follow:
The molecular mass of polyamide 66 fibers is about (1500 to 13000) g/mol, while for the production of parts, the molecular mass of polyamide is usually higher than 24000.
Advantages of polyamide
• High impact resistance
• High hardness
• Abrasion resistance
• Flexibility
• Stability against many solvents and chemicals
Polyamides show very good resistance against petroleum substances and aliphatic solvents, but they are strongly affected by concentrated mineral acids at room temperature and alkalis at high temperature.
Polyamides are hard to catch fire and if they get out of the flame, they are self-extinguishing. Polyamide 66 fibers are resistant to the attack of insects such as willows.
If they are exposed to UV light, environmental factors and temperature of 130 oC during long-term operation, their color will fade and the mechanical properties will become weak and fragile. At a higher temperature, they are destroyed due to hydrolysis. The addition of light and thermal stabilizers slows down the effect of these factors on polyamides.
Research shows that polyamide parts that are used outside the room temperature should be stable and protected from sunlight, so an ultraviolet light absorber suitable for polyamides should be used in the polymeric compound. This can also be done by adding carbon black, so no significant change in their properties and performance in the long term at normal temperature.
Thermal properties of polyamide 66
In DSC thermogram, the melting phenomenon of polyamide 66 occurs around 264 oC, and at a temperature of 275 oCand above, this polymer is on the beginning of thermal degradation.
Another prominent feature of this plastic is the flexibility and good toughness of its fibers, which has made it widely used in the preparation of textiles, especially for women’s use. The permanent set of these fibers is occurred at 100 oC in which if they cooled slowly, the permanent crease will be created. Polyamide 66 maintains its mechanical properties up to 150°C, although conservatively, the temperature applied to this plastic (such as ironing) should not exceed 125°C.
Polyamides benefit from very good electrical insulation capabilities at low temperatures and low humidity, but these properties quickly disappear when the temperature rises or the humidity increases.
Increasing the methylene (-CH2) functional group in the acidic part of the polyamide chain makes this plastic have a lower melting range and less moisture absorption ability, at the same time it reduces its stiffness and weakens its mechanical properties.
Mechanical properties of polyamide 66
Polyamide 66 contains a set of excellent properties such as mechanical properties (high strengths, high toughness and excellent resistance to wear) and very good thermal and chemical properties.
Two widely used industrial polyamides 6 and 66, despite the differences in molecular structure, are relatively similar to each other in terms of most mechanical and chemical properties.
The most important mechanical properties of polyamides are expressed through the information obtained from measuring of tensile, bending, compressive, shear and hardness tests. The set of results obtained from these 5 tests provides a major part of the technical information needed for the designer.
Polyamide 66 has high tensile strength, high impact resistance, good dimensional stability at relatively high temperatures and good resistance to wear. It also can be lubricated, therefore it is used in the manufacture of bearings.
As a result of stretching, the degree of crystallinity of polyamide fibers increases and many of its mechanical properties are improved. The tensile strength of polyamide 66 fibers is high and reaches more than 8 gr/denier.
At first impression, samples made of polyamide 6 are softer than polyamide 66.
It is worth mentioning that after absorbing moisture, due to the increase in flexibility of polyamide, many of its mechanical properties decrease. Only the impact resistance increases.
The moisture absorption of polyamides, especially polyamide 6 and 66, causes a sharp decrease in their modulus, because the presence of water between the chains (especially in polyamides that have the ability to absorb high moisture) acts as a softening agent for them and causes a drop in the mechanical properties of polyamides. The moisture absorption has a very favorable effect on the electrical properties of these types of plastics.
Types of polyamide 66
Polyamide 66 contains a set of excellent properties and has the ability to be used in many applications and various processes. In the first and most important classification, the types of polyamides 66 are divided into six groups: homopolymer, copolymer, blend, improved, filled, and reinforced. Several varieties of glass fiber reinforced polyamide 66 are made in weight ratios of 10%, 15%, 20%, 30%, 33%, 40%, 50% and 60%.
With the aid of additives and with the aim of improving the properties and increasing the performance of polyamide 66, various types of this plastic have been offered to the consumer market such as the fire-resistant type, which is placed in V-0 site in accordance with UL 94 guidelines.
There is a type of polyamide 66, in which due to mixing with fluoroplastics the decrease in friction is appeared.
Another type of this plastic is offered in black color, which is reinforced against harmful environmental factors such as ultraviolet light. It is worth mentioning that in this type of polyamide the presence of carbon black acts as the ultraviolet light absorber.
There are varieties of polyamide 66 homopolymer that are made for the injection process with low viscosity and high melt flow rate.
Another type of polyamide 66 homopolymer is available in the market, which its high viscosity enables the preparation of thick parts in the injection process.
Another type of polyamide 66 made with molybdenum disulfide additive can show excellent resistance to wear.
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.
• Application in automobile industry
Polyamide 6 and polyamide 66 are used in the automotive industry. This industry accounts for 25% of the global consumption of polyamides.
Polyamide 66, in addition to manufacturing engineering parts, is used to make textile fibers in a very large volume. Due to its linear structure and good physical and chemical properties, suitable fibers are obtained from this polymer. First, this fiber replaced silk and was used in textiles, then it was noticed in carpet weaving. In military purposes, it is used to prepare parachutes and life jackets.
The speedometer gear of odometers, and especially the timing gear (chain) of some cars, have been proven to be able to work for years if they are made of polyamide 66.
Woodworking hammer handles are made of polyamide 66 reinforced with glass fibers, which is a good substitute for wood.
Polyamide 6 and 66 are compatible with wool and cotton, so if they are added to natural fibers at a ratio of 30%, they increase their resistance to wear and tear, while also improving their ironability.
Polyamide 66 granules and powder must be dehumidified before the processing, otherwise the fabricated parts will be damaged in terms of mechanical properties and appearance.
The moisture absorbed by the granules of polyamides turns into steam during the molding process. The steam causes the hydrolysis of the polyamide, the reduction of the molecular mass, the loss of mechanical properties. It makes bubbles in the parts and causes some parts of the molded piece to be incomplete and creates adverse effects on the surface.
Moisture absorbed by polyamides changes and loses their good electrical properties. The electrical properties of polyamides are limited to their use in low frequencies because this plastic has polar groups.
The most important uses of polyamide 66, which is an engineering plastic, can be seen in these applications:
Replacement for metals in bearings, gears, rollers (cylindrical rollers) and cams
Power wires are coated with polyamide 66 because it provides a toughness, wear resistance, good insulation properties, and heat stability.
If multi-functional reagents such as triamines, tetraamines and trifunctional acids are used, it leads to the production of network polymers with major differences in their properties.