Alix Martin asked:
Dear Dr. Chris,
I did my ironing last night while listening to Naked Astronomy and realized that you might be able to answer some questions I have about this humble household chore.
First, I am guessing that the heat and pressure of the iron do something to relax the fibers, but how does this work on a molecular level and why does it work with multiple types of materials? Since some fabrics burn or melt at different temperatures, requiring different heat settings on my iron, it would seem that there might be different effects on the various types of molecules involved in the various fabrics. What is the underlying process and is it the same for synthetics, cotton, wool and so on? Why does the fabric stay flat when you lift the iron and release the pressure? Why do some creases never iron out, particularly on synthetics?
Alix Martin, New Jersey, USA
P.S. For those people who don't like to iron, may I suggest listening to the Naked Scientists while doing the job. It makes it much more enjoyable.
Chris - Thinking of the pure physics of it, if youíve got a very hot iron and youíve got say, a cotton, then the cotton is a polymer, and polymers are made of lots of long strands of molecules and in order for molecules to get past or slip past one another, they've got to be vibrating. So if you look at rubber for example, itís very stretchy when itís at room temperature because if you make it warm or it's room temperature, then the molecule shaking around makes it very easy for them to slip past one another and to line up neatly. So your cotton is quite similar. If the iron puts some heat in and you steam iron, so you put some water in as well, then the water probably helps to dissolve some salts which are from the washing powder and things between the particles. It also provides lubrication and the heat increases the energy of the molecules so they're all shaking around. And this enables them all to slip past each other very beautifully and all line up, and make a crease-free shirt. So I think thatís basically the physics of why ironing works because if you use a cold iron of exactly the same weight and even some steam, or some water, it doesnít work in the same way. You need that temperature, donít you?
The other reason for ironing needing water in cotton is that cotton is made of cellulose which is long chains of sugar, these are covered with OH groups - almost a water molecule. The O is slightly negative and the H positive. This means that when it is dry they form bonds between molecules locking them together.