Figure 1: Polyamides and nylon are used in consumer devices, machines, high-impact designs, and many other places. Source: Dr. Reiner Düren aka RedPiranha/CC BY-SA 4.0

Polyamides are a class of engineering plastics widely regarded for their strength and wide application of uses. Nylon is a well-known type of polyamide that became the first widely commercialized polymer. Polyamides and nylon are used in consumer devices, machines, high-impact designs, and many other places where strong, lightweight materials are needed.

Theory of Operation

Polyamides, including nylons, are a type of synthetic polymer that can be made into a variety of products, including fibers, films, and engineering plastics. These materials are notable for their strength, durability, resistance to heat and chemicals, and other desirable properties.

Polyamides and nylon resins achieve their chemical structure through the process of polymerization. Here are the critical steps in achieving these engineered plastics:

Monomer Synthesis

The first step in creating polyamides is to produce the necessary monomers. For nylon, these are typically diamines and dicarboxylic acids or lactams. These compounds form the basic building blocks of polyamide polymers.

Polymerization

The monomers are then reacted together in a process called polymerization. In the case of nylons, this is a type of condensation polymerization, where a small molecule (usually water) is eliminated as the polymer is formed. The monomers link together via amide bonds, forming a long chain molecule with repeating units.

Figure 2: Polyamide-6 from caprolactam. Source: Public domain

Processing

The resulting polymer can then be processed into the desired form. For fibers, this involves extrusion through a spinneret and then drawing to align the polymer chains and improve properties like strength and toughness. For resins, the polymer might be pelletized for later use in injection molding or other processes.

Use

In their final form, polyamides like nylon are used in a wide variety of applications, from clothing and carpets to car parts and packaging. Their properties can be tuned by adjusting the types of monomers used, the polymerization conditions, and the subsequent processing.

This process allows for the production of a material that combines strength, flexibility, and resistance to wear and tear, making it an excellent choice for many different applications.

Specifications

The specifications for polyamide and nylon resins can vary greatly depending on the exact type of resin and its intended use. The shape and bonding type of the polymer can greatly impact the resulting characteristics of the polymer. Some common specifications include:

Strength

The strength of polyamides can vary wildly between different types. Different polymer chains and additions of materials like glass cause these wide fluctuations. Tensile strength of polyamides can range from as low as 40 MPa up to 3,600 MPa depending on these parameters. Flexural strength, impact strength, and elongation at break are all parameters that can vary greatly between different types of polyamides and nylons.

Physical Properties

These might include things like density, melting point, glass transition temperature, and moisture absorption. These parameters are specific to the type of polyamide but most polyamide and nylon resins have a sensitivity to moisture. Exposure to moisture can alter these parameters and reduce the strength of the material.

Thermal Resistance

Nylons and polyamides can be exceptionally resistant to heat. Certain types have excellent heat deflection temperatures and thermal expansion coefficients.

Electrical Properties

As with thermal properties, electrical properties can vary between polyamide and nylon types. These properties include dielectric strength, volume resistivity, and dielectric constant.

Processing Properties

These are important for manufacturers and might include things like the recommended processing temperature, shrinkage, and flow rate.

Types

Polyamides and nylons come in many different types, each with its own unique properties that make it suitable for specific applications. The types of polyamides and nylons are often differentiated by the number of carbon atoms in the diamine and dicarboxylic acid monomers used to produce them. Here are a few of the most common types:

Nylon 6,6

This is one of the most common types of nylon. It's named for the fact that it's made from a diamine and a dicarboxylic acid, each containing six carbon atoms. Nylon 6,6 has good mechanical properties and heat resistance, making it suitable for use in things like textiles, carpets, and plastic parts.

Nylon 6

This is another common type of nylon, but it's made in a slightly different way from nylon 6,6. Instead of a diamine and a dicarboxylic acid, it's made from a single monomer called caprolactam, which has six carbon atoms. Nylon 6 has properties similar to nylon 6,6, but it tends to absorb more moisture.

Nylon 4,6

This nylon is made from a four-carbon diamine and a six-carbon dicarboxylic acid. It has better heat resistance than nylon 6 or 6,6, but it's also more difficult to process.

Nylon 11 and Nylon 12

These nylons are made from 11 or 12 carbon monomers, respectively. They have lower moisture absorption and better impact resistance than the shorter-chain nylons, but they also have lower strength and stiffness.

Aromatic Polyamides (Aramids)

These are a special type of polyamide where at least a portion of the carbon atoms in the chain are part of an aromatic ring. They include materials like Kevlar and Nomex, which are known for their exceptional strength and heat resistance respectively.

Figure 3: Source: 2007 Toronto Fire Service

Bio-based and Transparent Polyamides

With advancements in technology, new types of polyamides have been developed, including those made from renewable resources (bio-based) and those that are transparent.

Each type of nylon has its own set of properties, which can be further modified by adjusting the polymerization process, adding fillers or additives, or blending with other polymers. This makes the family of polyamides and nylons extremely versatile, able to be tailored to a wide range of applications.

Features

Polyamide and nylon resins have a number of distinguishing features that make them valuable in various industries. Some of the most notable features include:

Durability

Polyamides are known for their durability. They have high tensile strength, meaning they can withstand a lot of pulling force without breaking. They're also highly resistant to wear and tear, which makes them ideal for components that need to withstand friction.

Heat Resistance

Many polyamides, including several types of nylon, have good resistance to heat. This makes them suitable for applications such as engine components or kitchen utensils, where they may be exposed to high temperatures.

Chemical Resistance

Polyamides are generally resistant to many chemicals, including oils and many solvents. This makes them useful in industrial applications where they may come into contact with various chemicals.

Flexibility

Despite their strength, many polyamides are also quite flexible. This allows them to be formed into a variety of shapes and sizes without breaking or cracking.

Low Friction

Polyamides, especially when used as a fiber, exhibit low surface friction, making them ideal for applications such as bearings or gears.

Moisture Absorption

While this can be a drawback in some applications, the ability of polyamides to absorb moisture can also be beneficial. For example, in clothing, this property can help to wick moisture away from the body.

Electrical Properties

Polyamides are generally good insulators, with low electrical conductivity. This makes them suitable for use in electrical and electronic components.

Recyclability

Many types of polyamides are recyclable, which can help to reduce their environmental impact.

Figure 4: Polyamide Resin Code, PA. Source: B LaMont/CC BY-SA 4.0

Versatility

Perhaps one of the most remarkable features of polyamides and nylon resins is their versatility. They can be produced with a wide range of properties and can be processed in a variety of ways, including injection molding, extrusion, and spinning into fibers. This allows them to be used in a vast array of applications, from clothing and carpets to car parts and electronics.

Manufacture

Polyamide and nylon resins are manufactured through a process called polymerization, where small molecules (monomers) are chemically bonded together to form a large, chain-like molecule (polymer). The specific type of polymerization used to make polyamides is called condensation polymerization.

Here's a simplified explanation of the process for manufacturing nylon, one of the most common types of polyamide:

Monomer Production

The first step is to produce the necessary monomers. For nylon 6,6, the monomers are adipic acid and hexamethylenediamine. For nylon 6, the monomer is caprolactam.

Polymerization

The monomers are then combined in a high-temperature, high-pressure reactor. In the case of nylon 6,6, the adipic acid and hexamethylenediamine react to form nylon 6,6 and water. The reaction is a type of condensation polymerization, so named because a small molecule (in this case, water) is produced as a byproduct. In the case of nylon 6, the caprolactam is polymerized in a ring-opening polymerization to form the nylon 6 polymer and water.

Extrusion

The molten polymer is then extruded through a die to form a long, thin strand. This is then cooled (usually in water) to solidify it.

Figure 5: Extrusion process. Source: NikhilJNimbalkar/CC BY-SA 4.0

Drawing

The solid strand is then drawn, or stretched, which aligns the polymer chains and improves properties like strength and toughness. The amount of drawing affects the final properties of the nylon.

Cutting

Finally, the drawn nylon is cut into small pellets. These pellets can be sold as is for use in processes like injection molding, or they can be further processed into fibers, films, or other forms.

This is a simplified explanation, and the actual manufacturing process can be quite complex, requiring precise control of conditions like temperature and pressure to produce a high-quality product. Additionally, different types of polyamides may require different manufacturing processes.

Applications

Polyamide and nylon resins have a wide range of applications thanks to their combination of strength, durability, heat resistance, and other properties. Here are some examples of how these materials are used:

Textiles and Apparel

Nylon is commonly used in the production of textiles due to its strength, elasticity, and resistance to abrasion. It is used in a wide variety of clothing items, including sportswear, swimwear, and hosiery. Nylon’s ability to wick moisture makes it very useful in sportswear and performance clothing. It's also used in fabrics for furniture and in carpets.

Automotive Industry

Polyamides are used in many automotive applications due to their heat resistance, durability, and light weight. They can be found in components such as fuel lines, air intake manifolds, radiator end tanks, and various other under-the-hood components. Polyamides also find applications in tires, where they are used as reinforcing material.

Figure 6: Large treaded tire. Source: Angie/CC BY 2.0

Packaging

Polyamide resins are often used in food packaging because of their excellent barrier properties, especially against oxygen and aromas, and their heat resistance. They're used in films for packaging meats, cheeses, and other food products.

Electronics

Polyamides are used in a variety of electronic components due to their electrical insulating properties and heat resistance. They're used in connectors, switches, and other components. Certain types of polyamides are used as base materials for printed circuit boards.

Engineering Plastics and Machine Parts

Due to their strength and durability, polyamides are often used in mechanical parts, such as gears, bearings, and bushings. They can withstand significant wear and tear, making them suitable for applications with moving parts.

Figure 7: Drive wheel made of Lauramid with steel hub. Source: Handtmann Elteka GmbH & Co. KG/CC BY-SA 4.0

Sporting Goods

Polyamides, particularly nylon, are used in a variety of sporting goods. These include things like ropes and climbing gear, racquet strings, and the fabric of inflatable boats.

Bulletproof Vests

Kevlar, a type of polyamide is known for its incredible impact strength. Kevlar is often used in bulletproof vests for military and law enforcement personnel.

Figure 8: Bullet and stab proof vest. Source: SimmeD/CC BY-SA 4.0

Medical Devices

Some polyamides have been approved for use in certain medical devices, where they are valued for their strength, flexibility, and biocompatibility.

Filaments for 3D Printing

Nylon is commonly used as a filament material in fused deposition modeling (FDM) 3D printers. It offers strength, flexibility, and heat resistance, making it suitable for printing a wide range of items.

These are just a few examples of the many applications for polyamide and nylon resins. The versatility of these materials, along with the ability to modify their properties through changes in the polymerization process and the addition of fillers or additives, allows them to be tailored to a wide range of uses.

Standards

Due to the widespread use of polyamides, many industry standards exist concerning the production and quality standards of various polyamide types. Some of the most common standards include:

• ISO 1874-1:2010
• ASTM D6779-21
• ISO 16396-1:2015
• ASTM D4066-13(2019
• Many others

Types of polyamide plastics are differentiated from each other by a classification system based on appropriate levels of designatory properties like viscosity number and tensile modulus of elasticity. Standards use information about the composition, intended application, method of processing, important properties, additives, colorants, fillers, and reinforcing materials to further distinguish the materials. This designation system is applicable to all polyamide homopolymers, copolymers, and blends. It applies to both unmodified materials ready for normal use and materials that have been modified, for example, by colorants, additives, fillers, reinforcing materials, and polymer modifiers.

References

Xometry—Polyamide vs. Nylon: What Are the Differences and Uses?

Nexeo—Polyamide (PA, Nylon) Materials

Thomas Net—All About Polyamide Resins—Properties and Uses

AMCO Polymers—Polyamide (PA or Nylon)

Omnexus—Polyamide (PA) or Nylon: Complete Guide (PA6, PA66, PA11, PA12…)

Sewport—What is Polyamide Fabric: Properties, How its Made and Where

Natural Clothing—Polyamide Fabric and Nylon Fabric

ISO 1874-1:2010 Plastics—Polyamide (PA) moulding and extrusion materials—Part 1: Designation system and basis for specification

ASTM D6779-21  Standard Classification System for and Basis of Specification for Polyamide Molding and Extrusion Materials (PA)

ISO 16396-1:2015 Plastics—Polyamide (PA) moulding and extrusion materials—Part 1: Designation system, marking of products and basis for specifications

ASTM D4066-13(2019) Standard Classification System for Nylon Injection and Extrusion Materials (PA)

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