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Introduction to Polyamides

Polyamides are derived from the poly-condensation of Amino-acid. Amino acids are connected by amide bonds. Polyamides are found as both natural and synthetic. Proteins are naturally occurring polymers. The first synthetic Polyamide was introduced commercially in 1938. It was Nylon. Nylon was developed in order to replace silk.

One of the earliest uses of nylon 6,6 fiber was for ladies' stockings and toothbrush bristles. During World War II, nylon was used as a war material such as parachutes, tire cords for bombers, ropes, flak jackets, and jungle clothing.

Structure of Nylon 6,6 - a Polyamides
Figure 01: Structure of Nylon 6,6

Nylon is synthesized by polycondensation of di-acid and di-amine.

Nylon synthesis
Figure 02: Nylon synthesis

A variety of nylon grades are available commercially. Those are Nylon 6, Nylon 66, Nylon 6/6-6, Nylon 6/9, Nylon 6/10, Nylon 6/12, Nylon 11, and Nylon 12. The nomenclature of nylon is depending on the number of carbon atoms in di-acid and di-amine.

The general structure of nylon
Figure 03: General structure of nylon

Carbon atoms in di-amineCarbon atoms in di-acidName
46Nylon 4,8
47Nylon 4,9
49Nylon 4,11
64Nylon 6,6
66Nylon 6,8
69Nylon 6,11
Table 01: . The nomenclature of nylon

Properties of polyamides

Polyamides are thermoplastic materials. Most of the polyamides tend to be semi-crystalline and it shows lamella structure. Polyamides are linear chains with a low molecular weight of around 15000 g/mol.

The tensile strength of polyamides is upon to polar groups and crystalline structure of the polymer. Polyamides have a high density of hydrogen bonds. These hydrogen bonds are formed between the oxygen atoms of the carbonyl group and hydrogen atoms in polyamide. The high amount of hydrogen bonding influences the stiffness, crystallinity, flexibility, glass transition point, and gas permeability of the polyamide. The tensile strength of nylon is 600 kg/cm2.

Polyamides are the most common and important commercial examples, of thermoplastic materials that are capable to compete with metals. When considering polyamides, polyamide is stronger than steel.

The melting temperature of polyamide is 255℃ -265 ℃. The high melting temperature has advantages and disadvantages. The advantage is Polyamides can be used in high-temperature environments. Heat-stabilized systems allow sustained performance at temperatures up to 185 ℃ (for reinforced systems). A disadvantage of a high melting point is, it should be supplied with high energy to be melted at the processing stage.

Polyamides tend to provide good resistance to Hydrocarbons. Polyamides swell in alcohols and dissolve in phenols. Polyamides can be attacked by strong acids, alcohols, and alkalis. Polyamides are good dielectric in an oily medium.

Polyamides have good resistance to abrasion.

A disadvantage of polyamide is it tends to absorb moisture from its surroundings. This absorption process will happen until equilibrium is attained and that can have a negative effect on "dimensional stability". And it also affects the tensile strength.

Dimensional change vs Moisture
Figure 04: Dimensional change vs Moisture

Moisture absorption versus tensile strength
Figure 05: Moisture absorption versus tensile strength


Aramids are a class of synthetic polyamides that are formed from aromatic monomers, and yield fibers of exceptional strength and thermal stability. The tensile strength of aramids is about 2.5 times of nylon fibers.

One of the well-known examples of aramids is Kevlar. Kevlar is a polyaromatic amide (an aramid).  Kevlar is synthesized by polymerization of Terephthalic acid and p-Phenylenediamine.

Figure 06: Kevlar

Cables of Kevlar are as strong as cables of steel, but only about 20% of the weight. Kevlar fabric is used in the production of bulletproof vests, jackets, and raincoats.

Additives used with Polyamide

Raw polyamide is blended with different types of additives such as Heat and light stabilizers, Plasticizers, Lubricants, Fillers, and Pigments in order to increase its properties.

Heat and light stabilizers are used to protect PA against oxidation and UV for outdoor applications.

o, p, toluene ethyl sulphone amide UV and heat stabilizer
Figure 07: o, p, toluene ethyl sulphone amide UV and heat stabilizer

Fillers can be blended with raw polymer up to 60%. Fillers are added to the material for two main reasons. The first one is to reduce the cost. Low-cost fillers are blended with raw materials. in order to increase the volume. This type of filler is called “Non-reinforcing filler”. Graphite, Chine clay, and Talc are some non-reinforcing fillers. They decrease the cost of compounds and improve water resistance.

The other reason is to increase the tensile strength of the material. These filters are called “Reinforcing fillers”.  Asbestos, Glass fibers are reinforcing fillers. They increase the strength of the material. Glass fibers increase the tensile strength of the material by about -3000kg/cm2

Glass fiber-reinforced nylon is a better alternative to steel. Short glass fibers (2mm-5mm length) or Long glass fibers (10-12mm length) are used as reinforcing material.

Processing of Nylon

Nylon can be processed into final products by conventional plastic processing methods. Such as injection molding, extrusion, blow molding, rotational molding, and thermoforming.

Extrusion of nylon

Nylon is extruded at 220-300℃ temperature. Nylon films and fibers are manufactured by extrusion. The main drawback of nylon extrusion is its moisture content. Due to the polar groups of nylon, it tends to absorb moisture. A too-high content of moisture results in bubbles/foaming of the melt and its results in holes in the film. The maximum allowable moisture content is 0.1 % for the extrusion of nylon.

Nylon fibers are used in textiles, fishing lines,s and carpets. Nylon film does not transmit smell and is resistant to hydrocarbons. Therefore, Nylon films are used for food packaging, offering toughness and low gas permeability. Nylon show high resistance to temperature. Therefore, nylon films are used for boil-in-the-bag food packaging. As well Nylon films are used for the sterilization of hospital equipment.

Electrical insulation, corrosion resistance, and toughness make nylon a good choice for high-load parts in electrical applications such as insulators, switch housings, and ubiquitous cable ties. Another major application is for power tool housings.

Nylon fibers
Figure 08: Nylon fibers

Nylon packaging
Figure 09: Nylon packaging

Injection molding of nylon

The water content of about 0.15% to 0.2% is preferred in nylon pellets used in injection molding.

To ensure proper water content levels in the injection molding of nylon it is necessary that the granules be dry. Therefore, the polymer should be dried in an oven at about 70-90°C. The drying time should be 10 to 48 hours.

Nylon melt at 255℃ -265 ℃. High temperature accelerates oxidation.

Molding and extrusion compounds find many applications as replacements for metal parts, for instance in car engine components. Nylon has self-lubricating properties that make it useful for gears and bearings.

nylon as replacements for metal parts
Figure 10: nylon as replacements for metal parts

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