Synthetic Fibers
Polyester, nylon, acrylic, spandex and the semi-synthetics (rayon, acetate, lyocell) explained - how they behave, how to press and sew them, how to identify an unknown fabric with a burn test, and their environmental trade-offs.
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Fibres fall into three broad groups. Natural fibres are grown: cotton, linen, silk and wool. Synthetic fibres are made from chemical polymers, most of them derived from petroleum, melted or dissolved and extruded through tiny holes (spinnerets) into continuous filaments. In between are the semi-synthetic or regenerated cellulose fibres, such as rayon, made by chemically processing wood pulp or other plant material into a new fibre.
Synthetics changed what clothes could do. Nylon stockings went on sale in the US in 1940. Polyester made the drip-dry, easy-care clothes of the 1960s and 70s possible. Spandex, patented by DuPont in 1958 and sold as Lycra from 1962, gave us stretch fabrics that recover their shape. Today polyester is the most widely produced textile fibre in the world.
The main synthetic fibres
- Polyester: strong, inexpensive, wrinkle-resistant, quick-drying, and holds its shape. It doesn't absorb moisture, so it can feel clammy, and it holds onto oils and odours. Found in everything from crepe and satin to fleece, linings, thread, and blends with cotton.
- Nylon (polyamide): very strong, elastic and abrasion-resistant. Used for hosiery, swimwear, activewear, outerwear and tricot linings.
- Acrylic: soft, warm and wool-like, used for knitwear and fleece. It pills easily and can stretch out of shape.
- Spandex / elastane (Lycra): stretches to several times its length and snaps back. Almost always blended in small amounts (2-20%) with other fibres. See Knit Fabrics and Elastic.
The semi-synthetics
These are made from cellulose, so they behave more like natural fibres. They breathe, absorb moisture, and press well.
- Rayon / viscose: fluid drape and a silky feel, cool to wear. Weak when wet and prone to shrinking, so always pre-wash it.
- Modal and lyocell (often sold as Tencel): more refined versions of rayon, stronger when wet and more stable. Lyocell is made in a closed-loop process that recovers most of its solvent.
- Acetate: a lustrous fibre used mainly for linings and satins. It's sensitive to heat and dissolves in acetone (nail polish remover).
- Cupro (Bemberg): made from cotton linter, used for high-quality linings. Breathable, smooth and anti-static.
Sewing synthetics
Pressing is the biggest difference. Many synthetics melt, glaze or pucker under a hot iron instead of scorching slowly the way cotton does. Use the synthetic setting, test on a scrap, and use a press cloth. Polyester also resists holding a crease: it bounces back from pressing, which is useful for wrinkle-free garments but frustrating when you want a sharp pleat or crisp seam. A clapper, a damp press cloth, and patience help. Heat-set pleats in polyester, on the other hand, are permanent.
Other differences:
- Needles: smooth, tightly woven polyester (crepe, microfibre, satin) sews best with a sharp Microtex needle. Synthetic knits and spandex blends need a stretch needle to avoid skipped stitches.
- Puckered seams are common on fine polyesters, because the fibres don't compress. A shorter stitch, slightly lower tension, and holding the fabric gently taut while sewing all help.
- Fraying and slipping: slippery synthetics such as satin and chiffon slide while cutting. Cut single-layer, over paper, with a rotary cutter. Some synthetic satins and woven polyesters fray readily.
- Easing: synthetics resist being shrunk with steam, so set-in sleeves with a lot of cap ease are harder to sew smoothly than in wool or cotton.
- Static and pilling: polyesters and acrylics build up static. Anti-static linings such as cupro help.
Identifying an unknown fabric: the burn test
When a fabric has no label, a careful burn test narrows it down. Work outdoors or over a sink, hold a small snippet with tweezers over a heatproof dish, and bring it to a flame:
| Fibre | In the flame | Smell | Residue |
|---|---|---|---|
| Cotton, linen, rayon | Burns quickly, glows after | Burning paper or leaves | Soft grey ash |
| Wool, silk | Burns slowly, self-extinguishes | Burning hair | Crushable black ash |
| Polyester | Melts and shrinks from flame, black smoke | Sweet or chemical | Hard, dark bead |
| Nylon | Melts, then burns | Celery-like | Hard, pale bead |
| Acrylic | Burns and melts, flares | Acrid | Hard, irregular bead |
| Acetate | Melts and burns | Vinegar | Hard, dark bead |
Blends give mixed results. If a snippet burns but also leaves a hard bead, it contains some synthetic fibre.
Environmental trade-offs
Synthetics are a mixed picture. They're durable, need less water to produce than cotton, and recycled polyester reduces the use of new petroleum. On the other hand, they're made from fossil fuels, don't biodegrade, and shed microfibres into waterways when washed. Washing synthetic garments less often, in cooler water, in a full machine, or in a microfibre-catching wash bag reduces shedding. From a sewing point of view, the most sustainable garment is the one that's well made and worn for years, whatever it's made of.