Nylon is a group of man-made materials made from repeating units connected by chemical bonds called amide links. These units are often linked to different types of chemical groups.

Nylons are usually brown and feel soft. Some types look like silk. Because they are thermoplastics, nylons can be melted and shaped into fibers, thin sheets, and other forms. Their properties can be changed by mixing with other materials.

There are many types of nylon. One group, called nylon-XY, is made from chemicals called diamines and dicarboxylic acids with specific carbon chain lengths. A common example is nylon-6,6, which has a repeating structure of specific chemical units. Another group, called nylon-Z, is made from chemicals called aminocarboxylic acids with a specific carbon chain length. An example is nylon-6.

Nylon is used in many products, including clothing, floor coverings, and rubber materials. It is also used to make shaped parts for cars and electrical devices, as well as thin sheets for food packaging.

History

Researchers at DuPont started working on fibers made from cellulose, which led to the creation of the synthetic fiber rayon. Their experience with rayon helped them later develop and sell nylon.

DuPont’s work on nylon took 11 years, from research on polymers in 1927 to its announcement in 1938, just before the 1939 New York World’s Fair. This effort began with a new plan at DuPont, proposed by Charles Stine in 1927, where the chemical department would have small research teams focused on "pioneering research" that could lead to useful products. Harvard teacher Wallace Hume Carothers was hired to lead the polymer research group. At first, he studied pure science, building on theories by German chemist Hermann Staudinger. His work greatly improved understanding of polymers and helped advance the science.

Nylon was the first synthetic thermoplastic polymer to be successfully used in products. DuPont started its research in 1927. The first type of nylon, called nylon 66, was created on February 28, 1935, by Wallace Hume Carothers at DuPont’s research facility. Around the same time, Paul Schlack at IG Farben made a different type of nylon, called nylon 6, based on caprolactam, on January 29, 1938.

In 1930, Carothers and his team had already made two new polymers. The first was neoprene, a synthetic rubber used during World War II. The second was an elastic paste that became strong when cooled, which later became nylon. After these discoveries, Carothers’ team shifted focus from pure research to finding a chemical that could be used in industry.

In early 1935, a polymer called "polymer 6-6" was finally produced. Carothers’ coworker, Julian W. Hill, had used a method called "cold drawing" to make a polyester in 1930. Carothers used this method in 1935 to fully develop nylon. The first example of nylon (nylon 6.6) was made on February 28, 1935, at DuPont’s research facility. It had the right properties of elasticity and strength but required a complex manufacturing process that became the basis for future production. DuPont patented the polymer in September 1938 and quickly controlled its production. Carothers died 16 months before nylon was announced, so he never saw his success. The name "Nylon" came from changing "norun" (no run) into a unique name for marketing, but it was not trademarked.

Nylon was first used in a toothbrush with nylon bristles in 1938. It became more famous in women’s stockings, which were shown at the 1939 New York World’s Fair and sold in 1940. They sold 64 million pairs in their first year. During World War II, most nylon production was used for military purposes like parachutes. This increased the market for plastics and other materials.

Making nylon required teamwork between three departments at DuPont: the Chemical Research Department, the Ammonia Department, and the Rayon Department. The Ammonia Department specialized in high-pressure chemistry, which was needed to make some nylon ingredients. Nylon helped the Ammonia Department recover financially after the Great Depression by creating jobs and revenue.

DuPont’s nylon project showed the importance of chemical engineering in industry, created jobs, and advanced chemical engineering techniques. It built a chemical plant that provided 1,800 jobs and used modern technology still used today. The ability to quickly hire chemists and engineers helped the project succeed. The first nylon plant started production in Seaford, Delaware, on December 15, 1939. In 1995, the Seaford plant was named a National Historic Chemical Landmark by the American Chemical Society.

DuPont’s marketing strategy helped nylon become popular. They promoted nylon before it was widely available. The material was announced on October 27, 1938, at a public forum near the New York World’s Fair. It was described as a "first man-made organic textile fiber" made from "coal, water, and air" and promised to be "as strong as steel, as fine as a spider’s web." The audience, mostly middle-class women, responded positively, and newspapers covered the event. Nylon was shown at the 1939 World’s Fair and at the Golden Gate International Exposition in San Francisco. Actual stockings were sold in stores in May 1940, but a limited number were sold in Delaware earlier. The first public sale was in Wilmington, Delaware, on October 24, 1939, with 4,000 pairs sold in three hours.

DuPont’s campaign also reduced reliance on silk imported from Japan, which helped convince customers. Nylon was even discussed by President Roosevelt’s cabinet, which noted its "vast and interesting economic possibilities."

However, early excitement about nylon caused problems. People expected it to be better than silk, a miracle fabric that would never tear. DuPont later reduced claims about its strength, such as "New Hosiery Held Strong as Steel" and "No More Runs."

Some consumers were uneasy about synthetic fabrics. A damaging news story suggested nylon might be made from cadaverine, a chemical found in corpses. Scientists said cadaverine could also be made from coal, but the public remained skeptical. A woman even confronted a DuPont scientist about the rumor.

DuPont changed its marketing focus, emphasizing that nylon was made from "coal, air, and water" and highlighting its beauty and convenience instead of its technical qualities. Slogans like "If it’s nylon, it’s prettier, and oh! How fast it dries!" helped make nylon more appealing.

After nylon was sold nationwide in 1940, its production increased significantly.

Chemistry

The terms "PA" (short for polyamide) and "Nylon" are often used in the same way and mean the same thing.

The system for naming nylon polymers was created when the first simple nylons were made. This system uses numbers to describe the number of carbon atoms in each monomer unit, including the carbon atoms from the carboxylic acid part. Later, when cyclic and aromatic monomers were used, letters or groups of letters were added to the names. A single number after "PA" or "Nylon" describes a homopolymer made from one amino acid (without water) as the monomer.

Two numbers or groups of letters describe a dyadic homopolymer made from two monomers: one diamine and one dicarboxylic acid. The first number shows the number of carbon atoms in the diamine. The two numbers are usually separated by a comma for clarity, but the comma is often left out.

For copolymers, the comonomers or pairs of comonomers are separated by slashes.

The term polyphthalamide, abbreviated as PPA, is used when 60% or more of the carboxylic acid part in the repeating unit of the polymer chain comes from a mix of terephthalic acid (TPA) and isophthalic acid (IPA).

Types

Nylon 66 and similar polyamides are condensation polymers made by combining equal amounts of diamine and dicarboxylic acid. In the first case, the "repeating unit" has an ABAB structure, like in many polyesters and polyurethanes. Each monomer in this copolymer has the same reactive group on both ends, so the amide bond direction alternates between monomers. This is different from natural polyamide proteins, which have a consistent direction from the C terminal to the N terminal. In the second case, called AA, the repeating unit comes from a single monomer.

Wallace Carothers at DuPont patented nylon 66. When making nylons from diamines and dicarboxylic acids, it is hard to get the exact proportions right. If the amounts are not precise, the polymer chains may stop growing before reaching a desired molecular weight of less than 10,000 daltons. To solve this, a crystalline, solid "nylon salt" can be made at room temperature using an exact 1:1 ratio of the acid and base to neutralize each other. The salt is purified by crystallization to ensure the correct chemical balance. When heated to 285°C (545°F), the salt reacts to form nylon polymer, releasing water.

Nylon 510, made from pentamethylene diamine and sebacic acid, was included in Carothers' patent for nylon 66. Nylon 610 is made similarly using hexamethylene diamine. These materials are more expensive because sebacic acid is costly. Nylon 610 has a high hydrocarbon content, making it more hydrophobic. This property makes it useful for applications like bristles.

Examples of commercially available polyamides include:
• PA46 DSM Stanyl
• PA410 DSM Ecopaxx
• PA4T DSM Four Tii
• PA66 DuPont Zytel

These polymers are made from lactams or amino acids. The method using lactams (cyclic amides) was developed by Paul Schlack at IG Farben, leading to nylon 6, or polycaprolactam. This is formed through a ring-opening polymerization process. The peptide bond in caprolactam breaks, and the active groups on each side form new bonds as the monomer joins the polymer chain.

Nylon 6 has a melting point of 220°C (428°F), which is lower than the 265°C (509°F) melting point of nylon 66. Homopolymer nylons are made from a single monomer.

Examples of commercially available homopolymer nylons include:
• PA6 Lanxess Durethan B
• PA11 Arkema Rilsan
• PA12 Evonik Vestamid L

Nylons can also be made from dinitriles using acid catalysis. For example, this method is used to make nylon 1,6 from adiponitrile, formaldehyde, and water. Nylons can also be made from diols and dinitriles using this method.

It is simple to mix monomers to create copolymers, which reduces crystallinity and lowers the melting point.

Examples of commercially available copolymers include:
• PA6/66 DuPont Zytel
• PA6/6T BASF Ultramid T (6/6T copolymer)
• PA6I/6T DuPont Selar PA
• PA66/6T DuPont Zytel HTN
• PA12/MACMI EMS Grilamid TR

Most nylon polymers can mix with each other, allowing blends to be made. The two polymers can react through transamidation to form random copolymers.

Polyamides can be classified based on their crystallinity:
• Semi-crystalline: High crystallinity (PA46 and PA66); Low crystallinity (PAMXD6 made from m-xylylenediamine and adipic acid)
• Amorphous: PA6I made from hexamethylenediamine and isophthalic acid

Based on this classification, PA66 is an aliphatic semi-crystalline homopolyamide.

Environmental impact

Nylons can react with water, especially in the presence of strong acids, in a process that is the opposite of how they are made. This reaction reduces the size of the molecules in nylon and causes cracks to form quickly in the affected areas. Nylons with smaller numbers, like nylon 6, are more likely to be damaged than those with larger numbers, such as nylon 12. Because of this, nylon parts should not be used where they might come into contact with strong acids, such as sulfuric acid found in lead–acid batteries.

During manufacturing, nylon must be dried before being molded. Water heated to high temperatures in the molding machine can also damage the material. This reaction is shown in the image above.

The average amount of greenhouse gases produced during the manufacturing of nylon used in carpets is about 5.43 kilograms of CO2 equivalent per kilogram of nylon, when made in Europe. This is similar to the carbon footprint of wool, but nylon lasts longer, which reduces its overall environmental impact.

According to data from PlasticsEurope, nylon 66 has a greenhouse gas footprint of 6.4 kilograms of CO2 equivalent per kilogram and requires 138 kilojoules of energy per kilogram. When evaluating the environmental effects of nylon, it is important to consider how the material is used over time.

When exposed to fire, many types of nylon break down and release dangerous smoke, toxic fumes, or ash that may contain hydrogen cyanide. Burning nylon to recover energy is costly, so most nylon ends up in landfills, where it decays very slowly. Discarded nylon fabric can take 30 to 40 years to break down completely. Nylon used in fishing nets and other gear that is thrown away often becomes ocean debris. However, nylon is strong and can be recycled. Some nylon is reused directly in a closed-loop system during injection molding, where old parts are ground up and mixed with new material.

Recycling nylon is expensive and complicated, so many companies prefer using new plastic instead. In the United States, clothing company Patagonia uses recycled nylon in its products and supported Bureo, a company that recycles old fishing nets into items like sunglasses and skateboards. In Italy, Aquafil has shown how to recycle ocean fishing nets into clothing. Vanden Recycling processes nylon and other polyamides in several countries, including the UK, Australia, Hong Kong, the UAE, Turkey, and Finland.

Nylon is the most commonly used fiber in home carpets. In 2018, the U.S. Environmental Protection Agency estimated that 9.2% of carpet materials were recycled, 17.8% were burned in waste-to-energy facilities, and 73% were thrown away in landfills. Some of the world’s largest carpet and rug companies are working to promote "cradle to cradle" recycling, which involves reusing non-virgin materials, even those not traditionally recycled, as a way to improve sustainability in the industry.

Properties

Above their melting temperature, thermoplastics like nylon are non-crystalline solids or thick liquids where the long chains form randomly shaped structures. Below the melting temperature, non-crystalline areas alternate with areas that form layered crystal structures. The non-crystalline parts provide flexibility, while the crystalline parts provide strength and stiffness. The flat amide (-CO-NH-) groups are highly polar, so nylon forms many hydrogen bonds between nearby strands. Because the nylon structure is regular and symmetrical, especially when all amide bonds are in the trans position, nylons often have high crystallinity and make strong fibers. The level of crystallinity depends on how the material is made and the type of nylon.

Nylon 66 can have many parallel strands lined up with their peptide bonds spaced exactly six and four carbon atoms apart for long distances. This allows the carbonyl oxygen and amide hydrogen atoms to align and form repeated hydrogen bonds between strands (see the figure opposite). Nylon 510 can have similar spacing with five and eight carbon atoms. This allows parallel (but not opposite) strands to form long, unbroken, multi-strand β-pleated sheets, a strong structure similar to that found in natural silk and feather proteins. (Proteins have only one carbon atom between sequential -CO-NH- groups.) Nylon 6 can form continuous hydrogen-bonded sheets with mixed directions, but the β-sheet structure is slightly different. The three-dimensional shape of each hydrocarbon chain depends on how the carbon atoms rotate around their 109.47° tetrahedral bonds.

When forced through holes in a spinneret to make fibers, the polymer chains tend to line up because of the flowing liquid. If stretched later in cold conditions, the fibers align more, increasing their crystallinity and making the material stronger. In practice, nylon fibers are often stretched quickly using heated rollers.

Block nylon is usually less crystalline, except near the surfaces where forces during formation cause some crystallization. Nylon is clear or milky, but it can be easily colored. Multistranded nylon cords and ropes are slippery and may untangle. The ends can be melted and joined using heat, such as a flame or electrode, to prevent this.

Nylons absorb or release moisture based on the humidity in the air. Changes in moisture affect the polymer in several ways. First, the size of the material may change, but more importantly, moisture acts as a plasticizer, lowering the glass transition temperature (Tg) and reducing the material’s stiffness at temperatures below Tg.

When dry, polyamide is a good electrical insulator. However, polyamide absorbs water, which changes some properties, such as its electrical resistance. Nylon absorbs less water than wool or cotton.

Key features of nylon 66 include:
– Pleats and creases can be set with heat at higher temperatures
– More tightly packed molecular structure
– Better resistance to weather and sunlight
– Softer feel
– High melting point (256 °C (493 °F))
– Better color retention
– Excellent resistance to wear

Nylon 6 is easier to dye but fades more quickly. It has higher impact resistance, absorbs moisture faster, is more elastic, and recovers better after stretching.

• Luster: Nylon can be shiny, semi-shiny, or dull
• Durability: Its strong fibers are used in seatbelts, tire cords, ballistic cloth, and other applications
• High stretchability
• Excellent resistance to wear
• Highly resilient (nylon fabrics can be heat-set)
• Helped create easy-care clothing
• Resists insects, fungi, animals, and many chemicals
• Used in carpets and stockings
• Melts instead of burning
• Used in military applications
• Good strength-to-weight ratio
• Transparent to infrared light (−12 dB)

Nylon clothing is less flammable than cotton or rayon, but nylon fibers may melt and stick to the skin.

Uses

Nylon was first used in toothbrushes in 1938. It became more famous when it was used in women's stockings, called "nylons," which were shown at the 1939 New York World's Fair and sold in 1940. During World War II, the need for fabrics grew, and nylon became more widely used.

Bill Pittendreigh, DuPont, and others worked hard to find a way to use nylon instead of Asian silk and hemp in parachutes. Nylon was also used to make tires, tents, ropes, ponchos, and other military supplies. It was even used to make a special type of paper for U.S. currency. At the start of the war, cotton made up over 80% of all fibers used, and wool made up nearly all the rest. By August 1945, manufactured fibers had taken 25% of the market, reducing cotton's share. After the war, nylon parachute material was sometimes used to make dresses because both silk and nylon were in short supply.

Nylon 6 and 66 fibers are used in making carpets. Nylon is one type of fiber used in tire cord. Herman E. Schroeder helped develop the use of nylon in tires.

Nylon resins are used a lot in the car industry, especially in the engine area. Molded nylon is used in hair combs and parts like machine screws, gears, and gaskets. Engineering-grade nylon is made through processes like extrusion, casting, and injection molding. Type 6,6 Nylon 101 is the most common type of commercial nylon, and Nylon 6 is the most common type of molded nylon. For tools like spudgers, nylon comes in versions with glass or molybdenum disulfide to make it stronger and more durable.

Nylon can be used as a base material in composites with glass or carbon fibers. These composites are heavier than pure nylon and are used in car parts near the engine, like intake manifolds, because they can handle heat well.

Nylon was used to make the stock of the Remington Nylon 66 rifle. The frame of the modern Glock pistol is made from a nylon composite.

Nylon resins are used in food packaging films that need to block oxygen. Some nylon-based materials are used daily in packaging, like for meat and sausages. Nylon's heat resistance makes it useful for oven bags.

Nylon filaments are mainly used in brushes, like toothbrushes and string trimmers. They are also used in fishing lines. Nylon 610 and 612 are the most common polymers for filaments.

Nylon's properties make it useful in 3D printing, especially as a filament in consumer and professional-grade 3D printers.

Nylon resins can be made into rods, tubes, and sheets through extrusion. Nylon powders are used to coat metals. Nylon 11 and 12 are the most commonly used types.

In the mid-1940s, guitarist Andrés Segovia talked about the shortage of good guitar strings, especially his favorite Pirastro catgut strings, to some diplomats. A month later, a British general gave him some nylon strings. Segovia found the sound was clear but had a slight metallic tone. In 1944, Olga Coelho used nylon strings on stage in New York. In 1946, Segovia met Albert Augustine, and they worked together to develop nylon strings. DuPont agreed to provide nylon if Augustine could make the strings. After three years, Augustine created a high-quality nylon string that impressed Segovia and DuPont. Wound strings were harder to make, but after testing different metals and smoothing techniques, Augustine made good quality wound strings.