How does ironing work?

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Offline thedoc

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How does ironing work?
« on: 15/08/2012 13:57:23 »
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?

Thank you,

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.
Asked by Alix Martin

                                        Visit the webpage for the podcast in which this question is answered.

[chapter podcast=4055 track=12.08.12/Naked_Scientists_Show_12.08.12_10592.mp3]  ...or Listen to the Answer[/chapter] or [download as MP3]

« Last Edit: 15/08/2012 13:57:23 by _system »


Offline chris

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Re: How does ironing work?
« Reply #1 on: 13/08/2012 08:49:22 »
Hi Alix

Thanks for the kind words about the podcast. I'm glad that we provide some distraction capable of turning ironing into a more pleasurable experience. Maybe I should try that!

To answer your question, I think that the reason ironing "works" is because the heat from the iron gives energy to the fibres in the clothing; this makes it easier for them to slide past one another to adopt a flat configuration. In the cold state the fibres are all rough, so moving past each other is not easy. The fibres also have electrostatic attractions between them, locking them together and causing unsightly wrinkles.

But if you heat the fibres with the iron, and add some steam, the combination of the heat and water, which acts as a lubricant as well as re-arraning the electrostatic interactions and temporarily dissolving some of the salts binding the strands together, makes the fibres slip past each other more easily. Under the weight of the iron they then line up in neat, wrinkle-free rows. And, upon cooling they then lock into their new, pressed, condition (unless you accidentally drop the clothes on the floor or lose the shirt down the back of the wardrobe...)

I never forget a face, but in your case I'll make an exception - Groucho Marx


Offline daveshorts

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Re: How does ironing work?
« Reply #2 on: 14/08/2012 14:03:34 »
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.

The OH groups can also bond to water, so if you make the cotton hot and wet instead of bonding between molecules you bond to water, lubricating the molecules and allowing them to reconfigure. If this is done while being squashed, and then dried out the molecules will then lock into a flat configuration.