Nylon: History, Properties, Types, Process, Dyeing, and Safety Tips

By Eman Abdallah Kamel

Eman is a writer and an engineer. She obtained a bachelor’s degree in textile science from the Faculty of Applied Arts in Egypt.

This article covers the history of nylon, its chemistry, manufacturing process, physical and chemical properties, types, dyeing, uses, and safety tips.

Nylon

Nylon is a trade name for a type of polyamide thermoplastic. Nylon is a class of synthetic polymers with amide backbones, usually linking aliphatic or semi-aromatic groups. Nylon is a thermoplastic material, so it can be processed by melting. It is white or colorless and soft. The properties of nylon are often modified by blending it with other materials.

Du Pont Company developed it in the United States in 1935. Nylon has been used primarily as a material for women’s stockings since its industrial production.

Nylon is the second most common synthetic fiber. It has many uses and can be easily dyed.

Nylon is used in many products, such as textiles, electrical conductors, fishing nets, pulleys, surgical sutures, plastics, and parachutes.

Despite the many applications of nylon, like all synthetic fibers and plastics, it has many risks to humans and the environment. Visit Synthetic Fibers: Process and Risks to Humans and the Environment to find out more about the hazards associated with synthetic fibers.

In 2022, the United States, Germany, China, Italy, and the Netherlands were the largest exporters of nylon, while the largest importers were. Germany, China, Italy, Mexico, and India.

DuPont nylon advertisement 1939. Image source: digital.sciencehistory.org

History

In 1926, the head of research at DuPont proposed conducting research in several areas, including polymerization. To lead this work, he hired Dr. Wallace Carothers from Harvard University and staffed him with a new Ph.D. cohort. Carothers decided to focus on polymers, giant molecules that are the building blocks of familiar materials such as rubber and cotton. Carothers believed that polymers were molecules linked end to end in long chains by regular bonds. He proposed creating polymers using known chemical reactions to link many smaller molecules to test his theory. He then succeeded in producing polyesters with molecular weights ranging from 2300 to 5000 using basic diacids and glycol. He then introduced the molecular distillation apparatus, a laboratory instrument that enabled the production of polyesters with molecular weights up to 25,000. Carothers called these materials “superpolymers.” They were opaque solids that turned into transparent, viscous liquids when heated.

By the early 1930s, new technology was needed to process raw materials and shape them into fibers. Choosing a material that could compete with cotton, silk, wool, and rayon was critical. The decision to focus on socks was critical. The market was limited and high-quality.

In 1934, renewed efforts were made to make a polymer suitable for fibres. A nine-carbon amino acid ester was chosen as the starting material, and a high-melting-point polyamide, the first nylon, was produced. Carothers’ group then looked at 81 polyamide structures, of which polymer 66 was one.

The patent for nylon was issued in September 1938. In December 1939, the Seaford plant began production. The name “nylon,” which was intended to be a generic name for a class of polymers, became another word for stockings.

Currently, 20% of the manufactured fibre produced worldwide—that is, almost half of all fibre production—comes from nylon.

Properties

1. Physical Properties

Texture: Soft and silky.
Resiliency: Excellent.
Persistence: 4-9 g/dn (dry), at 90% dry humidity.
Hardness: 20-40g/dn.
Flexibility: Breaking elongation is 20-40%.
Specific gravity: 1.14.
Dimensional stability: It is good.
Corrosion resistance: It is excellent.
Melting point: Nylon 6.6–2520°C, Nylon 6–2150°C.
Softening point: Nylon 6.6–2290°C, Nylon 6–1490°C.
Moisture recovery: 3.5-5%, non-absorbent due to crystallization.

2. Chemical properties

Flammability: Burns slowly.
Alkali affected: Nylon is largely inert to alkalis.
Acidic action: Mineral acids can damage nylon 6; however, it is resistant to dilute organic acids when boiled. Mineral acids strongly affect nylon 6.6, causing it to dissolve or disintegrate. But it is inert to dilute acetate and formic acid even when boiled. It dissolves in concentrated formic acid.
Bleach: Nylon is not affected by oxidizing and reducing bleaches, but chlorine and strong oxidizing bleaches can damage it.
Electrical effect: High insulating properties cause static charges on the fibers.
Organic solvents: formic acid and phenol metacressol dissolve fibers, but solvents used for stain removal and dry cleaning do not damage them.
Biological organisms: Neither microorganisms nor moths and larvae attack the nylon.

The condensation polymerization reaction of Nylons. Image source: researchgate.net

Chemistry

Nylons belong primarily to the polyamide class, which also includes Kevlar. Condensation polymerization between dicarboxylic acids and diamines forms all nylons except nylon 6.

A newer method of producing nylon uses a carbonyl chloride group instead of a carboxylic acid group to form an amide bond with an alcohol. Sometimes, acid anhydrides are also used instead of carboxylic acid groups.

In the interfacial condensation technique used to produce nylon, polymerization can proceed at the interface between an aqueous and an organic medium. Since polymer formation at the interface is a diffusion-controlled process, very high molecular-weight polymers can be achieved with this method.

Structure of nylon 6 and nylon 66. Image source: Polymer Science Learning Center.

Types

Many types of nylon are used, including:

Nylon-6,
Nylon-6,6,
Nylon-6,10,
Nylon-6,12,
Nylon-11,
Nylon-12,
Nylon MXD6.

1. Nylon-6 is a type of semicrystalline polyamide polymer. Synthesizing nylon 6 involves the ring-opening polymerization of caprolactam, a molecule with six carbon atoms. When caprolactam is heated at approximately 533 K in an inert nitrogen atmosphere for approximately 4-5 hours, the ring breaks and undergoes polymerization. After that, the resulting molten mass is passed through spinnerets to form nylon 6 fibers. Nylon 6 can be modified during the polymerization process by incorporating co-monomers or stabilizers, which results in new functional groups or chain ends. This modification alters its reactivity and chemical properties, often to improve dyeability or flame resistance.

2. Nylon 66, also known as nylon 6.6, is a type of nylon polyamide. It is one of the most widely used materials in the textile and plastics industries, along with nylon 6. Nylon 6:6 gets its name from the two monomers it is made of, hexamethylene diamine and adipic acid, each of which has 6 carbon atoms. Nylon 66 is preferred because of the low cost of its raw materials. The synthesis of nylon 66 involves the condensation of hexamethylene diamine and adipic acid. Equal amounts of these two monomers are combined in water. The resulting salt, composed of ammonium and carboxylates, is isolated. It is then heated, in batches or continuously, to initiate polycondensation. The removal of water promotes the polymerization process by facilitating the formation of amide bonds between the acid and amine groups. This polymerization can occur in a concentrated aqueous mixture containing hexamethylene diamine and adipic acid.

Did You Know?

Nylon 6 is not ideal for applications exposed to water at high temperatures because of its low thermal conductivity and high water absorption, while the good tensile strength, stiffness, and bending modulus of nylon 66 make it ideal for applications that require long-term, repetitive performance. Applications include friction bearings, radiator caps, and tire cords.

3. Nylon 6,10 consists of two monomers, one with six carbon atoms and the other with ten carbon atoms. The 10-carbon monomer is sebacoyl chloride (ClOC-(CH₂)-COCl), an acidic chlorine with a -COCl group at each end, while the other monomer is a 6-carbon chain with an -NH₂ amine group at each end. This is 1,6-diaminohexane, H₂N-(CH₂)₆NH₂.. When these monomers polymerize, the amine group and the acid group combine, causing hydrochloride (HCl) molecules to be lost during the polymerization process and forming the nylon 6-10 polymers with the formula (CO-CH₂ )₈-CONH-(CH₂)₆-NH) as in the structure: n ClOC-(CH₂)₈-COCl + n H₂N-(CH₂)₆-NH₂ -> (CO-(CH₂)₈-CONH-(CH₂)₆-NH) n.

4. Nylon 6/12 (C18H36N2O3) is a copolymer with a composition that sets it apart from other nylon materials. It is an engineering resin that combines the desirable properties of nylon 6 and nylon 12. It offers less moisture absorption than nylon 6 and is more economical than nylon 12.

Dodecanedioic acid and hexamethylene diamine are combined to form a single precursor compound, which is then used to make nylon 6/12. This compound is then polymerized to produce the final material.

5. Nylon 11 properties are similar to those of nylon 6 but with a lower melting point of about 182-185°C, low water absorption of 1.2-1.4% at 21°C, and 65% relative humidity.

Ricinoleic acid, which constitutes 85-90% of castor oil, is used in the traditional method of producing nylon 11, which ends with the polymerization of 11-aminoundecanoic acid at 215°C to nylon.

Did You Know?

Another method of producing nylon-11 is from a compound derived from the seed oil of Vernonia galammensis, an annual herb that grows in tropical and subtropical regions of Africa. This process is relatively simple and can be performed under relatively mild conditions. It offers advantages over the conventional castor oil method, including fewer steps and energy-intensive thermal decomposition reaction elimination. To learn more about this invitation, visit patents.google.com.

Nylon 12 polymer chemical structure. Image source: europlas.com

6. Nylon 12 comprises amide units (-CO-NH-) and carbon atoms. This structure gives it strength and wear resistance. It is manufactured through a polymerization process, where nylon 12 monomers are chemically bonded together to form long chains. This process can be controlled to achieve specific molecular weights and desired material properties.

What is the difference between nylon 6 and nylon 12?

They differ in their monomer structure and the number of carbon atoms in their repeating units.
Nylon 12 is derived from the monomer laurolactam, while nylon 6 is made from the caprolactam.
Nylon 12 has more carbon atoms in its repeating unit than nylon 6. This difference in molecular structure results in distinct properties.
Nylon 12 exhibits higher tensile strength and impact resistance than nylon 6.

Nylon MXD6 polymer chemical structure. Image source: sciencedirect.com

7. Nylon-MXD6 is a crystalline polyamide resin produced by polycondensation of meta-xylene diamine (MXDA) with adipic acid under MGC’s proprietary technology. It is an aliphatic polyamide resin containing meta-xylene groups in the molecule.

The most notable property of MXD6 nylon is its gas barrier against oxygen and carbon dioxide. Nylon-MXD6 is easily applied for co-extrusion or co-injection with other resins to produce multi-layer containers, sheets, or films for the packaging industry because of its excellent heat stability and wide processing window.

Manufacturing Process

The nylon production stages include the following steps:

Polymerization to Manufacture Nylon Chips
Drying
Extrusion
Spinning
Winding

1. Polymerization

For nylon-6 polymer, caprolactam with 3-5% water and additives—like titanium dioxide for deglazing—is heated to 250°C and 270°C under specific pressure conditions. There are two types of polymerization processes: batch and continuous. The batch process is used in low-capacity plants to produce a variety of polymers with different molecular weights. The constant process has many advantages, such as consistency of quality, ease of operation and control, simplicity of design and process, and high capacity.

2. Drying

It is a process that helps to remove moisture from nylon because it can cause degradation in the linear chain of the molecules, which results in decreasing viscosity.

The chips enter the fluid/crystallization bed, which removes the moisture and increases the crystallization orientation of the molecules by fluidization. The chips are then forwarded to the column dryer to remove the remaining moisture in the chips. The air temperature of the fluid bed and column dryer ranges from 170 °C to 185 °C.

The dryer consists of a cyclone attached to a dust bag to remove microparticles such as dust and other impurities. The heaters in the dryer filter and heat the air that circulates inside to the proper temperature. The air pressure regulator allows dehumidified atmospheric air to enter the dryer system. It consists of a blower to suck the air and pass it through chilled tubes maintained at a temperature of 5-7°C, resulting in the condensation of water vapor present in the air. Then, it passes through two towers containing silica cylinders or cubes and acts simultaneously. The working hours of each tower are 6–8 hours, and its function is to remove the extra moisture from the atmospheric air.

3. Extrusion

Extrusion involves melting and shaping polymer chips into a constant shape. The process begins by feeding dry chips from a hopper into an extrusion barrel. Screws and heaters arranged along the barrel generate mechanical power that melts the flakes.

The rotating screw pushes the nylon polymer chips forward into the hot barrel. In most processes, the barrel heating profile is set so that three or more independent PID-controlled heating zones gradually increase the barrel temperature from the back to the front. This allows the chips to melt gradually as they are pushed through the barrel and reduces the risk of overheating, which can cause chip deterioration.

4. Melt Spinning

The nylon 6 flakes are melted and then extruded through the spinnerets. A metering pump controls the flow of the molten liquid to the spinning head, where it is filtered before extrusion to ensure that any unmelted material is removed so that it does not form burrs, which can cause weak points. The air cools the fibers as they emerge. The cooling process works on the principle of allowing the extruded yarns from the yarn bundle to pass through an air-cooling chamber. It has moderately cooled air at about 20°C with moderate relative humidity. When the cooling air comes into contact with the spun yarns, it removes heat and facilitates their solidification.

5. Winding

The yarn is passed through the take-up rollers to break the vertical path of the yarn, and allows the winder to be easily adjusted within the available space. The speed of the first rotating surface the yarn contacts after the yarn beam determines the spinning speed. In high-speed spinning industries today, friction rollers and godets are not used because they can abrade yarn, leading to subpar quality. Consequently, new winders with motor-driven rollers are employed. An automatic feedback mechanism is installed where the winder speed is regulated to maintain constant tension in the yarn line.

Nylon Dyeing

Nylon is dyed using acid dyes, metallic pre-dyes, and reactive dyes.

Acid dyes are anionic dyes that dissolve in water and are typically applied from an acid bath. Acidic groups like SO₃H and COOH are present in acid dyes. They are used to dye nylon, wool, and silk when an ionic bond is created between the oxidizing NH₂ group of the fiber and the acidic group of the dye.
Complex metal dyes can be generally divided into two categories: 1:1 complex metal dyes and 1:2 complex metal dyes. The dye molecule usually has a monoazo structure with additional groups such as carboxyl, amine, or hydroxyl. Commonly used metals include copper, nickel, and cobalt. Complex metal dyes have a good affinity with protein fibers such as wool. Complex metal dyes with a 1:2 ratio are also suitable for polyamide fibers such as nylon.
Many reactive dyes have been synthesized and dissolved by combining one or two cationic groups. Each possesses a mono- or dichlorotriazine reactive group or a bifunctional heterogeneous reactive system. All dyes are efficiently fixed on nylon under alkaline dyeing conditions. The fixation and accumulation of cationic reactive dyes on nylon are excellent. Because the covalent bond formation between the dye and nylon is effective, the resulting dye exhibits excellent moisture fastness. Monocationic dyes appear to accumulate better than bis-cationic types. This is probably due to the accumulation of positive charge on the fibers with increased dye fixation, resulting in reduced electrostatic attraction between the dye and fibers at dense shade depths.

Did You Know?

According to a study, cotton-nylon blends in one dye bath after treating the fabric with anionic chloroacetic acid and then converting the fabric to cation using magnesium chloride. The results showed that the 75:25% cotton-nylon blend had the highest colorimetric values and the highest color fastness. The 25:75% cotton-nylon blend had the highest air permeability and abrasion resistance. The process was characterized by shortening time and saving costs.

Safety Tips

During nylon production,

Wear protective gloves, a long-sleeved shirt, and long pants when handling molten polymer.
Safety glasses are recommended to prevent particles from entering the eyes during grinding or operation.
If you inhale vapors, move to fresh air, and if you develop fever or flu-like symptoms, consult a physician.
Use a self-contained breathing apparatus to protect yourself from the fumes released by large fires.
Thermal burns can result from hot-molded nylon; if skin or eyes come into contact with it, immediately cool with cold water. Never try to remove nylon by peeling it off the skin. For thermal burns, get medical help. When contact with the eyes occurs, mechanical irritation will occur; rinse with water.
In the event of a leak or spill, vacuum or wet sweep to minimize exposure to dust.
Use dry storage containers and keep them tightly closed.
According to local and federal laws, dispose of waste by burning or landfilling.

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