Nylon — The First Fully Synthetic Fiber from Coal, Air, and Water

Nylon 6,6 is made from two monomers. Hexamethylenediamine (HMD, H₂N-(CH₂)₆-NH₂) is a six-carbon chain with an amine group (-NH₂) at each end. Adipic acid (HOOC-(CH₂)₄-COOH) is a six-carbon chain with a carboxylic acid group (-COOH) at each end. When an amine meets a carboxylic acid, they react to form an amide bond (-CO-NH-) and release one molecule of water. This is condensation polymerization — building a long chain by repeatedly linking small molecules while expelling water.

Dissolve equal molar quantities of hexamethylenediamine and adipic acid in methanol. When combined, the acid and base react instantly to form a white crystalline salt — 'nylon salt' (hexamethylenediammonium adipate). This salt ensures a perfect 1:1 ratio of the two monomers, which is critical: even a 1% imbalance limits the polymer chain length and produces weak fiber. The salt is filtered, dried, and stored as the polymerization feedstock.

Load the nylon salt into an autoclave (a sealed, pressurized reactor) with a small amount of water. Heat to 220°C under pressure (approximately 18 atmospheres). The elevated temperature drives the condensation reaction: amine groups react with acid groups to form amide bonds, linking monomers into increasingly long polymer chains. Water vapor — the byproduct — is trapped by the pressure. After 1–2 hours, the polymer has reached a molecular weight suitable for fiber formation.

Slowly release the autoclave pressure over 1–2 hours while maintaining 270–280°C. As pressure drops, the trapped water vapor escapes, driving the equilibrium reaction further toward polymerization (Le Chatelier's principle). The molten polymer thickens steadily as chain lengths grow. A final vacuum stage removes the last traces of water, pushing chain lengths to 60–100 repeat units — the range that gives nylon its strength and melt-spinnability.

The finished polymer is a clear, viscous melt at 270°C. Force it through a die plate as thick strands, which are cooled in a water bath and chopped into small chips (pellets) approximately 3 mm in diameter. These nylon chips are the intermediate product — they can be stored, transported, and re-melted for fiber spinning. DuPont's nylon chips were the first industrial polymer pellets, a format that became universal for all thermoplastics.

Re-melt the nylon chips at 260–270°C and pump the molten polymer through a spinneret — a metal plate with dozens to hundreds of precision-drilled holes, each 0.2–0.3 mm in diameter. The molten polymer emerges as thin streams that solidify almost instantly in cool air. This is melt spinning — fundamentally different from the wet spinning used for viscose rayon. Melt spinning is faster, uses no solvents, and produces no chemical waste at the spinning stage.

A cross-flow of cold air solidifies the filaments within centimeters of leaving the spinneret. The solidified filaments converge into a single yarn bundle, are lubricated with a spin finish (a light oil that reduces static and friction), and wound onto a bobbin at speeds of 1,000–4,000 meters per minute. At this stage, the filaments are undrawn — relatively weak, opaque, and extensible. The molecular chains are randomly oriented, like uncooked spaghetti.

Pass the undrawn filaments over heated rollers or through a steam chamber while stretching them to 3–5 times their original length. This 'cold drawing' aligns the polymer chains parallel to the fiber axis, transforming random coils into ordered, crystalline regions. The fiber becomes thinner, stronger, more transparent, and less extensible. This is the same principle as stretching viscose rayon — molecular orientation determines mechanical properties — but nylon's chains pack more efficiently, producing higher strength.

Pass the drawn filaments through a heated zone at 180–200°C under controlled tension. This heat-setting step relaxes internal stresses and stabilizes the crystalline structure, preventing the fiber from shrinking or distorting during later processing or garment use. Heat-set nylon retains its shape through washing, drying, and wearing — a property that made nylon stockings revolutionary: they kept their shape far longer than silk stockings.

The drawn, heat-set filaments are wound onto large packages for use as continuous filament yarn (stockings, parachute fabric, fishing line). Alternatively, the filament tow can be crimped and cut into staple lengths for blending with cotton or wool on conventional spinning machinery. Texturizing — heating and twisting the filaments to add bulk and stretch — produces the stretchy, soft yarns used in activewear, socks, and knitted fabrics.

Nylon was designed to mimic silk, and it succeeded remarkably. Both are polyamides — silk's amide bonds come from amino acids, nylon's from synthetic monomers. Nylon matches silk's tensile strength (4–5 grams per denier), exceeds its abrasion resistance by a factor of ten, and surpasses its elasticity (nylon stretches 15–20% before breaking). It resists mildew, moths, and most chemicals. Where silk costs $30–60 per kilogram, nylon costs $2–4. The one property nylon cannot match is silk's moisture absorption — nylon absorbs only 4% of its weight in water versus silk's 11%, making nylon feel clammy against skin in warm weather.

The monomers are hazardous: hexamethylenediamine is corrosive and toxic by inhalation; adipic acid is a mild irritant. Polymerization occurs at 270°C under 18 atmospheres of pressure — autoclave failure risks a molten polymer explosion. Melt spinning releases trace fumes. Nylon itself is safe in finished form but melts at 260°C and drips flaming droplets — a fire hazard that led to strict regulations on nylon use in children's sleepwear and building furnishings.

Nylon proved that humans could design materials from first principles — choosing monomers to achieve desired properties, then manufacturing them at industrial scale. This was a conceptual revolution as profound as synthetic dyes sixty years earlier. Within fifteen years of nylon, chemists created polyester (1941), acrylic (1950), and polypropylene (1954). Today, synthetic fibers account for over 60% of global textile production — roughly 80 million tonnes per year. Every one of these fibers descends from the research program Carothers directed at DuPont, and from his insight that polymer chemistry could be systematic, predictable, and industrially useful.