Why Clothing Shrinkage Happens in the Wash

When a favourite dress or shirt comes out of the wash smaller than before, it can be deeply frustrating – particularly when you have carefully followed the care instructions. Some fabrics are, unfortunately, simply more likely to shrink than others. But what causes this?

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

It all comes down to fibres

To understand why clothing shrinks, it helps to first look at how textiles are produced.

Widely used textile fibres including cotton and linen come from plants. In their natural state, these fibres are uneven and crimped. Looking more closely reveals millions of tiny cellulose molecules with long chains, which naturally form coiled or tangled shapes.

As textiles are manufactured, the fibres are mechanically drawn out, stretched and twisted. This straightens the cellulose chains and lines them up, creating long, smooth threads.

At a chemical level, the chains are joined by links known as hydrogen bonds. These bonds reinforce the fibre and thread, helping to hold them together.

The threads are then woven or knitted into fabric, trapping the tension that keeps the fibres arranged alongside one another.

Yet these fibres have a strong "memory". When they encounter heat, moisture or mechanical movement – including agitation in a washing machine – they are inclined to relax and revert to their original crimped form.

This fibre memory explains why certain fabrics crease so readily and why they can also shrink in the wash.

How washing causes fabric shrinkage

To examine shrinkage, we need to return to the molecular scale. When laundry is washed in hot water, the fibres gain energy. They vibrate more quickly, disturbing the hydrogen bonds that keep them positioned as they are.

Whether a fabric is woven or knitted also matters. Loosely knitted materials contain more gaps and loops, so they are more vulnerable to shrinkage. Closely woven fabrics resist it better, as their threads are held in place with less freedom to move.

Cellulose is also hydrophilic, meaning that it attracts water. Water molecules enter the fibres, making them swell and increasing their flexibility and movement. The tumbling and twisting action of the washing machine adds further to the effect.

Together, these factors allow the fibres to loosen and spring back towards their naturally less-stretched, crinkled condition. The garment therefore becomes smaller.

It is not only hot water – here is why

Hot water is not the sole cause of this effect, 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 mechanical force. Cold water produces a less severe effect, but shrinkage can still occur.

To reduce the risk, use cold water, the slowest spin speed and the gentlest available programme, particularly for cotton and rayon. Care labels do not always fully communicate how spin speed and agitation affect fabric. If you are unsure, select a "delicate" programme.

What happens with wool?

Fibres do not all shrink in the same manner; no single process applies to every material.

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

When washed, these cuticles open and catch on nearby fibres, leading to fibre entanglement or "felting". The result is clothing that feels thicker and looks smaller – or, put simply, has shrunk.

Why synthetics shrink less

Synthetic materials such as polyester and nylon are produced from petroleum-based polymers that are designed to be stable and hard-wearing.

Their polymers have more crystalline areas, which are highly ordered and form an internal "skeleton" that stops the fibres from becoming crimped.

Some scientists are developing shape-memory polymers that can alter their form – or revert to an earlier form – in response to factors such as heat or water.

This differs from stretch fabrics, including those found in activewear. These consist of very elastic fibres that "bounce back" to their original shape after being stretched.

How to unshrink clothing

If a much-loved item has become smaller in the wash, this straightforward technique may help rescue it.

Carefully soak the garment in lukewarm water with hair conditioner or baby shampoo, using approximately one tablespoon per litre. Next, gently pull the material back into shape and leave it to dry flat or under light tension – for instance, peg the item to a drying rack.

This can work because conditioners contain chemicals called cationic surfactants. They temporarily lubricate the fibres, increasing their flexibility so that you can gently draw the material back into position.

The method will not fully undo severe shrinkage, but it may restore some of the lost size and make the garment 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.