Polyester and nylon are plastics and they fail like plastics

Extruded, not grown

Polyester and nylon start as polymer chips, are melted, and are forced through fine holes to form continuous filaments that are then cooled and drawn out to align the molecules. Nothing about that process resembles the growth of a cotton boll or a wool fibre, and the differences that follow are structural rather than a matter of quality.

A grown fibre is short, irregular, and full of internal structure. An extruded filament is smooth, uniform, and can be made any length the machine runs for. It has no scales, no natural crimp and no internal channels unless somebody engineers them in.

That uniformity is the source of most of the advantages. Strength is consistent, dye batches are consistent, and the fibre can be made in whatever thickness and cross-section the application wants.

Strength, and what it does to the garment around it

Both fibres are strong, and nylon is notably strong and abrasion-resistant, which is why it dominates in hosiery, linings for hard-wearing outerwear, straps, and the face fabric of anything expected to be dragged over rock.

Strength has a less obvious cost. A very strong yarn stitched into a weaker fabric concentrates stress at the seam and cuts the surrounding material instead of giving way itself, and a strong fibre blended into a weak one can hold a garment together long after the rest of it has thinned to nothing.

The same property is why synthetic fabrics pill so persistently. Pills form on every fabric, but on cotton or wool the little balls break off because the fibres anchoring them are weak enough to snap. On polyester those anchoring fibres do not snap, so the pills stay put and accumulate.

Both are hydrophobic, and that decides comfort

Neither fibre absorbs much water into its structure. Polyester in particular takes up almost nothing, which is why it dries fast and why it does not lose strength when wet, and it is also why it feels clammy in heat.

Sweat that cannot be absorbed sits on the skin or on the fabric surface as liquid. Technical sportswear deals with this by engineering the yarn shape and the knit structure to move liquid along the surface by capillary action rather than by absorbing it, which works, but it is a fabric-engineering trick and not a property of the fibre.

The same water-hating chemistry attracts oils. Polyester is oleophilic, meaning body oils and greasy soils bond to it readily and are hard to remove at low wash temperatures. This is the real reason synthetic sportswear retains odour after washing while a cotton shirt does not: the residue that feeds the smell is still in there.

Heat is the failure mode people forget

These are thermoplastics. Below a certain temperature they hold their shape; above it they soften, and above that they melt. That is not a defect, it is how the fibre is made and how permanent pleats are set into synthetic fabric in the first place.

In practice it means an iron that is fine on cotton will glaze or melt polyester on contact, leaving a shiny mark that cannot be reversed. It means a tumble dryer on high heat gradually stiffens and shrinks synthetic garments. And it means a stray spark or an oven shelf does far more damage to a synthetic than to wool, which chars rather than melting.

Heat also degrades elastic recovery in blends. A garment with elastane in it is often ruined by dryer heat long before the fabric wears out, and the label's low-temperature instruction is protecting the stretch fibre more than the polyester.

Where they genuinely earn their place

It is easy to be sniffy about synthetics and the honest position is more mixed. Nylon and polyester hold pleats, resist creasing, dry quickly, do not shrink much, resist moths entirely, and cost a fraction of the alternatives. In outerwear, rainwear, swimwear and workwear they do things no natural fibre does as well.

They also last, which cuts both ways. A polyester garment often outlives its owner's interest in it, so the fibre's durability is spent on a garment nobody wants. The environmental argument about synthetics is more about volume and disposal than about the fibre being inherently poor.

Shedding is the other genuine issue. Synthetic fabrics release microscopic fibre fragments during washing, and unlike cellulosic ones these do not readily break down. Washing less often, on shorter and cooler cycles, and with full loads reduces it. It does not eliminate it, and it would be dishonest to imply otherwise.