Science Questions

Why does line drying make clothes rough?

Mon, 12th Sep 2016

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Question

Kevin Fitch asked:

Recently our dryer died. So we had to line dry our clothes until a new dryer could be delivered. Most things, especially towels, came out rougher and stiffer. Why is this?

Answer

Laura Brooks put this to Neil Lant, research Fellow in the Fabric & Home Care division of Proctor and Gamble, who make Lenor fabric softener.

Laura - Everyone loves the feel of a soft warm towel straight from the tumble dryer but towels dried on a washing line can end up crunchy and stiff. Why does this happen?

Holly on FaceBook thinks too much soap could be to blame.

Meanwhile on our forum, Atomic S suggested that the movement from a tumble dryer would loosen up fibres making them softer.

To find out more I contacted Neil Lant, Research Fellow in the Fabric and Homecare division of Procter and Gamble who make Lenor fabric softenerÖ

Neil - Our fingertips alone have over two and a half thousand sensory receptors and our brains are constantly interpreting how things feel, sometimes at a subconscious level. When we pull clothes off the line or out of the dryer our brains are focused on the task at hand and we immediately know whether our laundry feels stiff and rough or whether it feels soft and flexible.

The feel and softness of a garment is impacted by the physical properties: smoothness, flexibility, and fullness. These physical properties are affected by the original garment construction, the type of any finish used, the fibre type and, of course, how itís laundered. A fabric thatís made from a tight weave with highly twisted yarns like a cotton terry towel has a fullness to it because the rigid fibres can support the structure, whereas a fabric made from an knit is more elastic and has a springiness to it.

Laura - Cotton fibres are made of a natural polymer called cellulose; the same stuff we extract from wood pulp to make paper. The cellulose chains stick together by a process called hydrogen bonding which makes the cotton fibres very strong. However, cellulose is also very good at absorbing water.

Neil - In the presence of large amounts of water like in a washing machine, the fibres swell, hydrogen bonds between the polymer chains are disrupted, the chains can slip past each other, and the hydrogen bonds reform upon drying.

When fabrics are line dried, water drains and evaporates causing increased cellulose-cellulose interactions between fibres in yarns driven by capillary attraction. As the fibres de-swell new hydrogen bonds are formed within and between the fibres - itís like setting the fabric in stone. Whereas when the drying happens with motion, as in a tumble dryer, there is less opportunity for these fibre-fibre adhesions to occur.

A highly structured fabric like a terry towel has twisted yarns and loops and when new hydrogen bonds are formed the towel feels very harsh since it is rough and stiff right off the line.

 

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Did you use fabric-softener sheets in your dryer? Even if you did not, the agitation of the dryer would likely loosen up the fibers some, making them more pliable. Atomic-S, Sat, 20th Aug 2016

I agree with Atomic; the movement within the mechanical dryer would have loosened up the materials by the time you came to collect the dry results. Except when it is very windy, most clothing remains relatively static on the washing line and hence is not subject to the same perturbation.

A bigger question is, therefore, why does washing stiffen in the first place, and how do fabric softeners work? chris, Sat, 20th Aug 2016

Moisture on a fabric will tend to pull fibres together. It may be that not all water molecules are lost during slow evaporation. This will tend to 'glue' fibres together. On a windy day the moisture that would have remained would then be ejected by the airflow. In a drier the hot air may have an ionising effect. Not sure on that one. jeffreyH, Sat, 20th Aug 2016

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