Underwear elastic is under tension all day and goes through many wash and dry cycles. The elastic yarn, usually elastane or rubber, slowly loses its ability to spring back. How fast depends on the type and quality of the yarn, the construction of the band and what the garment is exposed to. A new sample usually looks fine, so testing has to look at the aged garment.

Common causes

Heat is a major one: hot washing and high-heat tumble drying age elastane faster. Chlorine bleach is known to damage elastane in many fabrics, and body oils, lotions and some detergents can also weaken it over time. Another cause is a band made with too little elastic or one stretched too far in sewing, which leaves it permanently extended.

Construction

A covered, stitched band with elastic of the right width and tension keeps its shape better than a thin band. Check the elastic’s width, the stretch ratio applied in sewing and the number of stitching lines that anchor it. Check that leg and waist openings are the same stretch at every size.

Test recovery after ageing

ASTM D4964 describes tension and elongation testing for elastic fabrics, which can give stretch and recovery figures for elastic bands and fabrics. Ask your lab to test the finished band before and after repeated washing with a named procedure such as ISO 6330, and, if useful, after repeated stretching cycles. Compare the recovery with an approved sample and set a limit.

Link to the care label

The FTC Care Labeling Rule, 16 CFR Part 423, requires warnings where a usual care procedure would harm the garment. If hot drying or chlorine bleach damages the elastic, the label must say so. Test the elastic under those conditions to find what warning is needed.

What to specify

Write the elastic type, width, fibre content, supplier and stretch ratio in sewing, plus recovery after washing as a pass limit. Keep an aged sample from a previous order as the reference, and check first bulk pieces against it. If you change elastic supplier to save cost, treat it as a new product: repeat the aged recovery test before you approve bulk, since two elastics that feel the same new can age very differently, and the difference only shows after months of wear.