Nylon (PA)

What is Nylon (Polyamide)

Nylon is the trade name for a family of semi-crystalline thermoplastic polymers whose repeating chain is joined by amide linkages (CO-NH). The generic chemical term is polyamide, abbreviated PA, and that is the designation used on every engineering datasheet and in the ISO and ASTM material standards. The amide bond is the same chemistry that holds together natural proteins, which is why nylon shares some of silk's strength and elasticity, the property that first drove DuPont to commercialize it as a textile fiber.

The polymer was invented at DuPont by chemist Wallace Carothers, whose team produced the first sample of nylon 66 from adipic acid and hexamethylenediamine on 28 February 1935. The name 66 records that both monomers carry six carbon atoms. DuPont announced the material publicly in 1938, first selling it as toothbrush bristles, then famously as women's stockings displayed at the 1939 New York World's Fair and put on sale in 1940. The Second World War redirected nylon to parachutes, ropes, and tire cord, and after the war it became the foundational synthetic engineering material it remains today.

A polyamide qualifies as an engineering plastic because it holds usable mechanical properties under sustained mechanical and thermal load, not merely at room temperature for a moment. Unreinforced nylon delivers tensile strength in the 50 to 85 MPa range, far higher than commodity polyolefins, combined with low friction, good abrasion resistance, fatigue endurance under cyclic load, and a self-lubricating surface that makes it the default material for gears and small bearings that must run without grease.

The scale of the polyamide industry is large and still growing. Worldwide production of the two commodity polyamides reached roughly 9 million tonnes in 2024, with PA6 the clear volume leader at around 6.5 million tonnes per year (close to 70 percent of PA6 plus PA66) and PA66 about 2.5 million tonnes per year (the remaining 30 percent). The Asia-Pacific region alone accounts for well over half of global consumption. A large share of PA6 flows into textile fibers for sportswear, carpet, and industrial fabric, while the balance serves injection molding, extrusion, and film.

Three engineering characteristics define how nylon is selected: its grade (which carbon-chain chemistry, PA6, PA66, PA12 and so on), its reinforcement (unfilled, glass fiber, carbon fiber, or lubricant filled), and its moisture state. The third is unique among common plastics. Because nylon is hygroscopic, the same part can read one set of numbers dry as molded and a different set fully conditioned, so a competent specification always states the moisture condition alongside the mechanical values.

The Nylon Grade Family

The polyamide family is named by the number of carbon atoms in its monomers. A single number (PA6) means one monomer of that carbon count; two numbers (PA66, PA612) mean a diamine and a diacid of those two counts. Higher carbon numbers mean longer aliphatic segments between amide groups, which lowers the density of polar amide bonds, and that single fact explains most of the property differences across the family: longer chains absorb less water, flex more easily at low temperature, and melt lower. The table below compares the production grades an engineer is most likely to encounter.

PA6 (polycaprolactam) is polymerized from a single monomer, caprolactam, by ring-opening polymerization. It melts around 220 degrees Celsius, processes easily because of its lower and broader melt window, takes color and surface finish well, and is slightly tougher in impact than PA66. Its drawbacks are the highest moisture uptake of the commodity grades and lower stiffness when wet. PA6 is the volume leader of the family and the basis of most machinable cast and extruded stock shapes.

PA66 (polyhexamethylene adipamide) is condensed from two monomers, hexamethylenediamine and adipic acid. Its more regular chain crystallizes to a higher melting point near 260 degrees Celsius and gives greater stiffness, hardness, and abrasion resistance than PA6, at the cost of being slightly more brittle and harder to process. PA66 is the default for hot, load-bearing duty: automotive under-hood components, electrical connectors, and structural brackets, usually in a glass-reinforced grade.

Long-chain grades PA11, PA12, PA610, and PA612 trade peak strength and heat for low moisture absorption, flexibility, and excellent low-temperature toughness. PA12 absorbs only about 1.5 percent at saturation against roughly 9 percent for PA6, so it holds dimensions and properties far more stably in humid or wet service, which is why it owns automotive fuel and brake lines, pneumatic tubing, and powder-bed 3D printing. PA11 is partly bio-based from castor oil. PA46 (Stanyl) sits at the high-heat end with a melting point near 295 degrees Celsius for the hottest gear and engine duty, while aromatic high-temperature polyamides (PPA, PA6T, PA9T) extend the family above what the aliphatic grades can reach.

Reinforcement and Additives

The base polymer is only the starting point. Most structural nylon parts use a compounded grade in which fillers, fibers, lubricants, flame retardants, and stabilizers are blended into the resin to tune properties for the application. The most important of these is glass fiber, which transforms nylon from a tough but flexible material into a stiff, dimensionally stable structural plastic. The table below shows how property values move as glass loading rises in a PA66 base.

Glass fiber is the dominant reinforcement. Chopped E-glass strands at 15 to 50 percent by weight carry tensile load that the polymer matrix transfers into the fibers, so 30 percent loading roughly doubles tensile strength from about 80 MPa to about 180 MPa, raises tensile modulus three to four times to around 10 GPa, and lifts the heat deflection temperature at 1.8 MPa from below 90 degrees Celsius to roughly 250 degrees Celsius, close to the melt point. The price is brittleness: elongation at break collapses to a few percent, the part becomes notch sensitive, and shrinkage turns anisotropic, which can warp flat or thin parts. PA66-GF30 is the industry workhorse for structural and under-hood components.

Mineral and bead fillers such as talc, wollastonite, and glass beads raise stiffness and dimensional stability with far less anisotropic shrinkage than fiber, giving flatter parts and a better surface, at a smaller strength gain. Mineral-glass hybrids are common where both warp control and strength matter. Impact modifiers, typically an elastomer phase, are blended into supertough grades that restore ductility for snap fits, living hinges, and parts that must survive drops, trading some stiffness and heat resistance.

Internal lubricants matter wherever nylon slides against metal or itself. Molybdenum disulfide (MoS2) acts as a solid lubricant and nucleating agent, increasing crystallinity, rigidity, hardness, and wear life; the grey MoS2-filled cast grades sold as the Nylatron GSM type are the standard for heavy-duty gears, sprockets, and wear pads. Encapsulated oil grades release lubricant at the bearing surface for maintenance-free running with about 25 percent lower coefficient of friction. PTFE and graphite are added for low-friction bearing grades. Flame retardants bring grades to the UL 94 V-0 rating required for electrical and electronic housings, since unmodified PA6 only reaches V-2 because of burning drip behavior; UL-recognized grades carry a Yellow Card stating the V rating at a given wall thickness.

Water Absorption and Chemical Resistance

The single most important difference between nylon and competing engineering plastics is moisture absorption, and ignoring it is the most common selection mistake. The polar amide groups in the chain hydrogen bond with water molecules, so every aliphatic polyamide is hygroscopic to some degree. Unreinforced PA66 absorbs roughly 1.1 to 1.5 percent of its weight in water in 24 hours and 8 to 9 percent at full saturation; PA6 absorbs even more. The long-chain grades absorb far less, with PA12 near 1.5 percent at saturation because it has fewer amide groups per unit length.

Absorbed water behaves as a plasticizer. As nylon takes up moisture, its impact strength and elongation rise, which is often desirable, but its tensile strength, stiffness, hardness, and creep resistance fall, and the part physically swells: a saturated unfilled part can grow 2 to 3 percent in linear dimension. This is why dry-as-molded datasheet numbers can mislead. For a part that will live in humid air or water, design and verify against conditioned (equilibrium-moisture) properties, and for tight-tolerance machined parts, condition the stock to equilibrium before final cuts so the dimensions stop moving. Where moisture stability is critical, switch to a glass-reinforced grade, which suppresses dimensional change, or to a low-absorption long-chain grade such as PA12.

Chemical resistance is one of nylon's strengths, with a clear pattern. Nylon resists hydrocarbons, mineral oils, greases, fuels, esters, ketones, and most organic solvents well, which is exactly why it owns fuel lines, gears, and oil-wetted mechanical parts. Its vulnerabilities are strong mineral acids, oxidizing acids, phenols, and formic acid, all of which hydrolyze the amide bond, plus prolonged hot water and steam that cause slow hydrolytic chain scission. Some metal-halide salt solutions and certain alcohols can stress-crack stressed parts. The table below is a first-pass screening guide.

Because resistance depends sharply on concentration, temperature, exposure time, and applied stress, treat this table as a screening filter only. Before committing, obtain the resin maker's full chemical-resistance chart and confirm behavior at the actual concentration and temperature, and where the part carries load, account for environmental stress cracking, which can fail a material that survives the same chemical unstressed.

Key Specification Parameters

A nylon datasheet lists many lines, but a handful drive the selection. Always read the moisture condition first: serious datasheets list both dry-as-molded (DAM) and conditioned (50 percent relative humidity) values, and confusing the two is a frequent error. The table below gives typical values for unfilled PA6 and PA66 so the family ranges are anchored; remember these are dry-as-molded figures that shift with moisture.

Tensile strength, modulus, and elongation from the ISO 527 test define the structural envelope. Strength and modulus tell you how much load and how little deflection the part can take; elongation at break tells you how ductile it is before fracture. Reinforcement trades elongation for strength and stiffness, so a GF30 grade is strong and rigid but brittle, while an unfilled or impact-modified grade is weaker but forgiving. Read all three together, never strength alone.

Melting point and heat deflection temperature describe two different limits. The melting point (around 220 degrees Celsius for PA6, 260 for PA66) is a processing and absolute-ceiling figure. The heat deflection temperature under load, measured by ISO 75 at 1.8 MPa, is the practical short-term limit for a loaded part: roughly 75 to 90 degrees Celsius unfilled, but near 250 degrees Celsius for a GF30 grade because the glass network carries load up to near the melt. Neither is the same as the continuous service temperature.

Continuous service temperature is set by long-term thermo-oxidative aging, not melting, and is far lower than the melt point: about 80 to 95 degrees Celsius continuous in air for standard PA66, with short intermittent excursions to roughly 170 degrees Celsius. Heat-stabilized and reinforced grades extend continuous use toward 120 to 150 degrees Celsius. A grade can survive a brief 200 degree spike yet degrade if held at 110 degrees for years, so match the rating to the duty cycle, not the peak.

Water absorption belongs on the same priority level as strength for nylon because, as Chapter 4 covered, it moves dimensions and every mechanical property. Electrical and flammability ratings close the list: dielectric strength around 25 to 30 kV/mm makes nylon a capable insulator, but note that absorbed moisture lowers volume resistivity, and electrical housings usually require a UL 94 V-0 flame-retardant grade backed by a UL Yellow Card stating the rating at the design wall thickness.

Selection Decision Factors

To turn the preceding five chapters into a specific grade callout, follow the decision sequence below. The frequent failure is fixing on a single number (usually tensile strength) before settling the temperature, moisture, and reinforcement context that determines whether that number is even meaningful. These steps double as a fixed RFQ template.

1. Service temperature and duty cycle: Fix the continuous and peak temperatures first. Up to about 80 to 95 degrees Celsius continuous, unfilled PA6 or PA66 serves; for sustained heat choose a heat-stabilized or glass-reinforced PA66, and for the hottest gear or engine duty consider PA46 or a high-temperature polyamide. Rate against continuous service temperature, not melting point.
2. Mechanical load and stiffness: Decide whether the part needs ductility or rigidity. Tough impact and snap-fit parts want unfilled or impact-modified grades; stiff, dimensionally stable structural parts want glass reinforcement (GF15 for balance, GF30 to GF35 for structural duty, GF50 for maximum rigidity), accepting reduced ductility and possible warp.
3. Moisture environment: For humid air or wet service, design to conditioned (not dry) properties and account for swell. Where dimensional stability is critical, choose a glass-reinforced grade to suppress movement, or a low-absorption long-chain grade such as PA12 or PA612.
4. Chemical and media compatibility: Confirm against the resin maker chemical-resistance chart at the real concentration and temperature. Nylon is excellent with oils and fuels but poor with strong acids and degraded by long-term hot water and steam; switch to PTFE, PVDF, or a hydrolysis-stabilized PA66 where needed.
5. Wear and friction: For gears, bushings, and wear pads specify a lubricated grade: MoS2-filled cast nylon for load-bearing wear, oil-filled cast grades for maintenance-free sliding (about 25 percent lower friction), or PTFE-filled grades for the lowest friction.
6. Regulatory and flammability requirements: Electrical and electronic parts usually need a UL 94 V-0 flame-retardant grade with a UL Yellow Card at the design wall thickness; food and potable-water contact needs FDA, EU 10/2011, or NSF compliant grades. Confirm the certificate, not just a verbal claim.
7. Form and process: Choose injection-molded compound for high-volume net-shape parts; choose extruded PA6 or PA66 rod, sheet, and tube for small machined parts; choose cast nylon (PA6 G, MC nylon) for large diameters, thick slabs, and heavy wear duty that extrusion cannot supply.
8. Total cost of ownership: Weigh resin price against tooling, drying and processing yield, scrap from warp, and field life. A cheaper unstabilized grade that embrittles from heat aging or distorts from moisture can cost more over the part lifetime than the right compounded grade specified upfront.

One last dimension is supplier consistency and documentation: lot-to-lot viscosity and moisture control, a current UL Yellow Card where required, regulatory certificates, and reliable regional supply. For molding resin the reference brands are BASF Ultramid (A series PA66, B series PA6), DuPont Zytel, Ascend Vydyne, Lanxess Durethan, Envalior (formerly DSM) Akulon and Stanyl, and EMS Grivory; for machinable stock shapes the references are Mitsubishi Chemical Nylatron and Ertalon, Ensinger Tecamid and Tecast, and Quadrant. Specify by the ISO 1874 designation (for example PA66-GF30) plus reinforcement, stabilization, and flammability rating so the quote is unambiguous.