Does modal shrink less than cotton in underwear?

Short answer: Modal can be more dimensionally stable than cotton in many fabrics, but the fibre name alone does not guarantee it. Shrinkage in knitted underwear depends mostly on how the fabric was knitted, relaxed, heat-set and finished, and on how the garment is washed and dried. A well-finished cotton can shrink less than a poorly finished modal, so set a shrinkage limit for the finished fabric and test it rather than relying on fibre reputation.

What drives shrinkage in knits

Knitted fabrics are stretched lengthwise during knitting and finishing. When they are wetted and tumbled, the loops relax back towards their natural shape, so the fabric gets shorter and sometimes wider. Cellulose fibres, both cotton and modal, also swell when wet, which contributes to change. Finishing steps such as relaxation, compacting and, for elastane blends, heat-setting reduce this residual shrinkage before the fabric is cut.

How modal compares

Modal is a regenerated cellulose fibre engineered to have higher wet strength than standard viscose, and fabrics made from it are often described as stable and soft after washing. In practice the comparison depends on the specific fabrics. Lighter, more open modal jerseys can still shrink or grow if not finished properly, and modal blends combine the behaviour of both fibres.

How to specify and test

• State a maximum dimensional change in length and width for the finished fabric and for the garment.
• Test with ISO 5077 using a washing and drying procedure from ISO 6330 agreed with the lab.
• Test after more than one cycle if your complaints appear after several washes.
• Test garments as well as fabric, because sewing and elastic can change behaviour.

When moving from cotton to modal

Keep the pattern only if dimensional change and stretch are similar. Modal is often more fluid and can stretch differently, so run a fit sample after washing. Update the care label based on your own tests.

A worked example

Suppose a cotton jersey brief shows 6% length shrinkage after three washes and a modal jersey from the same mill shows 3%. That does not mean modal always shrinks less. Ask both mills for their finishing route, such as compacting or heat setting, and repeat the test on bulk lots. Set the limit in the specification, for example within 5% in length and width, and approve the pattern on washed fabric.

Limits

Tumble drying at high temperature increases shrinkage in most cellulose fabrics. If complaints mention dryers, care advice may matter as much as the fabric.

Facts, recommendations and uncertainty

Established: shrinkage in knits depends mostly on finishing and wash conditions. Recommendation: set a dimensional change limit and test washed garments for both fabrics. Uncertain: whether your specific modal fabric outperforms your cotton until tested.

Sources and further reading

• ISO 5077:2007 — Determination of dimensional change in washing and drying
• ISO 6330:2021 — Domestic washing and drying procedures for textile testing
• Lenzing — TENCEL™ Modal with Micro technology

Standards and regulations are referenced by their published scope. Test parameters, cycles and pass levels should be agreed with your laboratory for the specific product; legal requirements should be confirmed for each destination market.