Elastane and stretch

Heat

The dominant cause. Sustained temperature loosens the physical bonds in the hard segments and permits permanent deformation. A hot wash, a tumble dryer and a radiator all do the same damage by the same mechanism. This is the entire reason intimates are washed cool and dried flat.

Chlorine

Attacks the polymer directly. Swimming pool water is the obvious source, but chlorine bleach in a wash is worse, and so is a bleach based cleaner splashed near a drying rack.

Sustained tension

Elastic held stretched for long periods recovers less. A band worn on the tightest hook from day one, or a garment stored stretched over a hook, ages faster than one that is not.

Body oils and antiperspirant

Accumulated oils and aluminium salts sit in the fibre and accelerate breakdown. This is why washing after every two or three wears extends life rather than shortening it.

Ultraviolet light

Yellows and embrittles elastane over long exposure, which is why drying in direct sun is a false economy.

A bra almost never fails because the fabric wore through. It fails because the elastic stopped coming back, and a band that does not come back cannot support anything.

Content varies enormously by component, and a single bra will contain several different fabrics. A cup fabric may have very little. A band or a wing is typically much higher, often in the region of a fifth of the fibre content, because it has to recover under load thousands of times. Power mesh used in shapewear is at the top of that range.

The practical implication is that a garment is only as durable as its most loaded elastic component. The band goes first, then the straps, then the leg elastics on briefs, then everything else. Judging a bra by whether the cup fabric looks fine is a category error.

1. Fasten the bra on the loosest hook and put it on. If it is only comfortable on the tightest hook and still rides up, the band has lost its recovery.
2. Hold the band flat and stretch a section by hand, then release. Live elastic snaps back immediately. Tired elastic returns slowly or not fully.
3. Look at the elastic edges for a rippled, wavy line where the fabric no longer lies flat. That is permanent deformation.
4. Check the strap. If it is on the tightest setting and still slipping, the strap elastic is gone.

When to retire a bra sets out the full set of tests and what can be salvaged.

• Wash at 30 degrees or by hand. Every ten degrees of avoided heat is worth months.
• Never tumble dry anything containing elastane. This is the single highest impact rule in the whole care section.
• Never dry on a radiator or in front of a heater, which is a tumble dryer with extra steps.
• Rotate. Elastane recovers better if given a day out of use between wears, because recovery is time dependent as well as tension dependent.
• Store flat or lightly folded, never stretched over a hook or hanger by one strap.
• Skip fabric softener, which coats the fibre and interferes with recovery.
• Rinse swimwear immediately in cool fresh water after pool use to remove chlorine before it has time to work.

Natural rubber elastic still appears in some waistbands and is more durable against heat than elastane, though it perishes with age, ozone and oils. Elastomultiester and similar newer fibres offer better chlorine resistance and appear mostly in swimwear. Neither displaces elastane in mainstream intimates, so the care rules above remain the operative ones for almost everything in an ordinary drawer.

The one genuinely elastane free approach is cut: a bias cut silk slip, a woven cotton brief, a structured cut and sewn bra with a firm woven band. These rely on geometry rather than recovery, last far longer, and demand a much more precise fit because they cannot adapt to the body at all.