Nylon is the trade name for a type of polyamide thermoplastic. It was first developed by DuPont engineers in the mid-1930s and has since been used in almost every industry. Polyamide nylon has a wide range of uses, such as ropes, gears and even hosiery. It is often formed into fibres for use in microfilaments and yarns, but can also be meltblown.
General properties of polyamide nylon
Polyamide nylon has several advantages that make it an ideal candidate for a wide variety of applications. Below you will find the main advantages and disadvantages of the material.
High abrasion resistance – Higher levels of wear resistance due to mechanical action
Good thermal resistance – Special grades of nylon can have a melting point of almost 300°C.
Good fatigue resistance – This makes it ideal for components in constant cyclic motion such as gears.
High machinability – Cast billets can be machined into various components that would be too costly to cast into intricate shapes
Noise dampening – Nylon is a very effective noise dampener
Water absorption – Water absorption causes a decrease in mechanical properties. Nylon 6/12 is specially formulated to resist moisture absorption.
Chemical resistance – Nylon has a low resistance to strong acids and bases.
High shrinkage – High shrinkage rates in foundry applications.
The following table indicates some of the main grades of nylon used in the industry.
Types of polyamide nylon and their applications
Nylon comes in four main grades of polyamide nylon: nylon 66, 11, 12 and 46. These names come from the length of their polymer chains. The first number is the number of carbon atoms of the diamine and the second is the number of carbon atoms of the acid. Typical applications are:
Guitar strings and picks
Tennis racket strings
Nylon 6 was developed in an attempt to reproduce the properties of nylon 66 without infringing the patent. This grade of nylon is very strong and has a high tensile strength. It should be noted that nylon 6 is produced by a unique process called ring-opening polymerisation.
Nylon 66 is similar to nylon 6, but has a higher melting point and is more resistant to acids. It is made from two monomers, whereas Nylon 6 is made from only one.
Nylon 11 has a higher resistance to dimensional changes due to moisture absorption. This is partly due to the lower concentration of amides. It should be noted that it generally has less desirable mechanical properties than other types of nylon.
This nylon compound has the lowest melting point of the major polyamides. It is often used as a flexible film or sheet for coating food and pharmaceutical products. It also has relatively good resistance to water absorption.
Nylon 46 was developed primarily to have a higher operating temperature than other nylon grades.
Production and processing
Polyamides are typically made by combining two monomers, adipic acid and 1,6-diaminohexane. Once these two monomers have reacted together, they form water as a by-product of each bond in the polymer chain. This bonding of the two monomers is known as polymerisation. A nylon salt is created and heated to evaporate the water. This heating is done in an autoclave at 280C and 18 Bar. After the polymerisation process, various additives and pigments are added. These additives can modify the physical properties of the polymer.
Once the additives have been added, the molten polyamide nylon is extruded through holes to form long nylon cords. These strands are extruded in a water bath that allows the strands to cool and solidify. They are then cut into granules with a length of 3 to 4 millimetres. These pellets are packaged and shipped to processing plants, where they are re-melted and extruded through dies to create fibres and various extruded or cast shapes.
Nylons are manufactured in cast or extruded form. There are differences in properties, but some of the main advantages of cast nylon are listed below.
Less internal stresses
Less water absorption
More crystalline structure resulting in higher mechanical strength
Higher melting temperature