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Material hierarchy: Polymer - Engineering Thermoplastic - Polyamide
Nylon was originally developed to be used as a fiber. Its natural color is white or ivory. Today there are many uses for nylon. Nylon is the material of choice for applications involving high service temperatures, low coefficient of friction, chemical resistance, good abrasion or wear, and toughness. Nylon comes in numerous variations depending on the properties required. There is nylon 6, 6/6, 11, 12, filled (glass or moly), alloys and more. Some of the largest applications include gears, cams, bearings, guides, and housings. Nylon does tend to absorb water over time from the environment, some grades more than others. This can affect dimensional stability. Spiratex has a lot of experience with nylon tubing, spiral wrap, and profiles, with many of the profiles being used in the conveyor industry. Some grades can be used in food contact applications when used in accordance with FDA standards.
An important polymer family that forms characteristic amide linkages (CO-NH) during polymerization is the polyamides (PA). The most important members of the PA family are nylons, of which the two principal grades are nylon-6 and nylon 6,6 (the numbers are codes that indicate the number of carbon atoms in the monomer). Nylon-6,6 was developed at DuPont in the 1930s. Nylon-6 was developed in Germany. Nylon is strong, highly elastic, tough, abrasion resistant, and self-lubricating. It retains good mechanical properties at temperatures up to about 125°C (250°F). One shortcoming is that it absorbs water with an accompanying degredation in properties. The majority of applications of nylon (about 90%) are in fibers for carpets, apparel, and tire cord. The remainder (10%) are in engineering components; nylon is commonly a good substitute for metals in bearings, gears, and similar parts where strength and low friction are needed.
Nylons are polyamides. The nylon family members are identified by the number of carbon atoms in the monomers. Where two monomers are involved, the polymer will carry two numbers (e.g., nylon 6/6). Crystalline nylons have high tensile strength, flex modulus, impact strength, and abrasion resistance. Nylons resist nonpolar solvents, including aromatic hydrocarbons, esters, and essential oils. They are softened by and absorb polar materials such as alcohols, glycols, and water. Moisture pickup is a major limitation for nylons because it results in dimensional changes and reduced mechanical properties. However, overly dry material can cause processing problems, and particular attention needs to be paid to moisture control.
Several different types of nylon are on the market, the two most widely used being nylon 6/6 (hexamethylene diamine adipic acid) and nylon 6 (polycaprolactam).
Nylon 6/6 is the most common polyamide molding material. Is special grades include: (1) heat-stabilized grades, for molding electrical parts; (2) hydrolysis-stabilized grades, for parts to be used in contact with water; (3) light-stabilized grades, for weather-resistant moldings; and (4) higher-melt-viscosity grades, for modling of heavy sections and for better extrudability.
Special grades of nylon 6 include: (1) grades with higher flexbility and impact strength; (2) heat- and light-stabilized grades, for resistance to outdoor weathering; (3) grades for incorporating nucleating agents to promote consistent crystallinity throughout sections, thereby providing better load-bearing characteristics; and (4) higher viscosity grades, for extrusion of rod, film, pipe, large shapes, and blow-molded products.
Other types of nylon include: 6/9, 6/10, 6/12, 11, 12, 4/6, and 12/12. A transparent, amorphous nylon has been commercialized. Although type 6 absorbs moisture more rapidly than type 6/6, both eventually reach equilibrium at about 2.7% moisture content in 50% relative humidity air, and about 9 to 10% in water.
Creep rates for nylons at various stress levels under both tension and compression show only a small deformation within the initial 24-hour period and increase from this point in a linear manner. This means that long-term deformation usually can be predicted accurately on the basis of short-term tests.
General-purpose nylon molding materials are available for extrusion, injection molding, blow molding, rotational molding, and (for the nylon 6 materials) casting or anionic polymerization. Nylon sheet and film also are marketed.
The properties of nylon resins can be improved by filling and reinforcing. Mineral-filled and glass-fiber-reinforced compounds are widely available. Several manufacturers offer specially toughened grades where extra impact strength is required.
Nylon is a semi-crystalline thermoplastic, with a continuous use temperature range of up to 210°F (99°C). It has a low difficulty to machine. It has high dimensional stability, high impact resistance, high stiffness, and good chemical resistance. It has good wear resistance and is light-weight. It has high moisture absorption. It has a low coefficient of friction.
For specific engineering applications, a number of specialty nylons have been developed, including; molybdenum disulfide-filled nylons (to improve wear and abrasion resistance, frictional characteristics, flexural strength, stiffness, and heat resistance); glass-fiber-filled nylons (to improve tensile strength, heat distortion temperatures, and, in some cases, impact strength); and sintered nylons. Sintered nylons are fabricated by processes similar to powder metallurgy (the same as those used for TFE fluorocarbons). The resulting materials have improved frictional and wear characteristics, as well as higher compressive strength.
Nylons have also found increasing use in alloys, notably with ABS and with PPE. The materials' aroma barrier properties have led to applications in multilayer packaging film constructions. In many electrical applications, the mechanical strength and resistance to oils and greases also are important properties.