Why Clothes Shrink in the Wash and How to Unshrink Them

When a much-loved dress or shirt comes out of the wash smaller than before, it can feel disastrous - particularly when you have followed the care instructions carefully. Yet certain fabrics are simply more likely to shrink than others. What causes this?

Knowing the science behind textile fibres can help you avoid clothing shrinkage and may even allow you to "rescue" a garment after the occasional laundry mishap.

It all comes down to the fibres

To understand why clothes shrink, it helps to first look at how textiles are produced.

Many everyday textile fibres, including cotton and linen, come from plants. In their natural state, these fibres are uneven and crimped. At a deeper level, they contain millions of tiny cellulose molecules with long chains that naturally form coiled or convoluted structures.

In textile production, the fibres are mechanically drawn out, stretched and twisted, straightening and lining up the cellulose chains. This process produces long, smooth threads.

At the chemical level, the chains are also joined by links known as hydrogen bonds. These bonds reinforce both the fibre and thread, giving them greater cohesion.

The threads are then knitted or woven into fabric, preserving the tension that keeps the fibres positioned beside one another.

But these fibres have a strong "memory". When subjected to heat, moisture or mechanical action - such as movement in a washing machine - they tend to loosen and revert to their original crimped form.

This fibre memory explains why certain fabrics crease so readily, and why they can shrink during washing.

How washing causes fabric shrinkage

To examine shrinkage, we need to return to the molecular scale. When laundry is washed in hot water, the energy within the fibres rises. They therefore vibrate more quickly, disrupting the hydrogen bonds that keep them in position.

A fabric's knit or weave also matters. Loosely knitted materials contain more gaps and loops, leaving them more vulnerable to shrinkage. Closely woven fabrics are more resistant, as their threads are held in place with less space for movement.

Cellulose is also hydrophilic, meaning it draws in water. Water molecules enter the fibres, making them swell and become more mobile and flexible. The tumbling and twisting action of the washing machine adds further force.

Together, these effects allow the fibres to relax and spring back towards their naturally crimped, less-stretched state. The garment consequently becomes smaller.

Why clothes can shrink without hot water

Hot water is not the only cause of shrinkage, as anyone who has washed rayon clothing may have discovered.

Even cold water can enter fibres and make them swell, while the washing machine's tumbling action contributes further. The result is usually less pronounced in cold water, but shrinkage can still occur.

To reduce the risk, use cold water, the lowest available spin speed or the gentlest programme, particularly for cotton and rayon. Garment labels do not always make the effects of agitation and spin speed clear. If you are unsure, select a "delicate" programme.

What happens to wool?

Fibres do not all shrink in the same manner; there is no universal process.

Cellulose-based fabrics shrink in the way described above, whereas wool is an animal fibre composed of keratin proteins. Its exterior is coated in tiny overlapping scales, known as cuticle cells.

While wool is washed, these cuticles open and catch on nearby fibres, causing entanglement or "felting". This leaves the garment denser and smaller - or, put simply, shrunken.

Why synthetic fabrics shrink less

Synthetic fibres, such as polyester and nylon, are made using petroleum-based polymers designed to be stable and durable.

These polymers have more highly ordered crystalline regions, which function like an internal "skeleton" and stop the fibres from becoming crimped.

Some researchers are developing shape-memory polymers, which can alter their shape - or revert to an earlier one - in response to factors such as heat or water.

This differs from stretch fabrics, including those used in activewear. Such materials contain highly elastic fibres that "bounce back" to their original form once stretched.

How to unshrink clothing

If a cherished item has shrunk in the wash, this straightforward approach may help restore it.

Soak the garment gently in lukewarm water containing hair conditioner or baby shampoo, using approximately one tablespoon per litre. Next, cautiously stretch the material back to its shape, then dry it flat or with light tension - for instance, by pegging it onto a drying rack.

This works because conditioners contain chemicals called cationic surfactants. They temporarily lubricate the fibres, making them more pliable and enabling you to gently pull the garment back into position.

The method cannot fully undo severe shrinkage, but it may restore some of the lost size and make the clothing wearable once more.

Nisa Salim, Director, Swinburne-CSIRO National Testlab for Composite Additive Manufacturing, Swinburne University of Technology

This article is republished from The Conversation under a Creative Commons licence. Read the original article.