Ironing, something we all hate to do. I know that I do not specifically enjoy the process. I always seem to burn myself somehow. So, we all know that ironing removes wrinkles from clothes but the question is why this process of heat and force straightens fabrics. Now, some may not care and I am aware of this but if you have ever wondered why, then here is your answer.

Before we can talk about how the process of ironing straightens clothes, we need to talk about a few chemistry concepts. First, clothing is a polymer. A polymer is a compound that is made up of many repeat units that are called monomers. The number of times something can repeat can be hundreds or thousands! These can become rather large molecules called macromolecules.

How these macromolecules are processed and how big they determines the properties that they will have. So, an example would be one of those styrofoam cups and a petri dish (those clear plastic dishes that scientist use to grow bacteria in). It turns out that the styrofoam cup and the petri dish are comprised of the same material, polystyrene. The reason they are completely different is due to the difference in their size and how they were processed at the factory.

Ok, so an important property that polymers have is how well they hold their shape. Let's take a candle as an example. Imagine that you have a candle that you left in a hot car on a summer day. When you return to your car you notice that you no longer have a candle but just this blob that is gooey. Now, think about that same candle and instead of leaving it in the hot car, you take it home and light it. What happens to the wax? Well, you can watch it run down the candle like a liquid. This liquid is of course melted candle wax. However, the candle itself wasn't running like a liquid when left in the car so this must be different than melting. It turns out that there is another property of polymers that describe their ability to hold a shape, this is called their glass transition temperature. This temperature is not melting but when the material becomes gooey and can't hold a shape (technically isn't hard like a piece of glass).

Now, we can apply this to clothes. Let's take a look at 2 types of fabric, polyester (70's disco music starts playing in my head) and cotton.

Polyester is inherently more wrinkle resistant than cotton. Why? And why should a hot iron be used? Well, it turns out that this glass transition temperature is the reason. Polyester clothing has a higher glass transition temperature than cotton and makes it more wrinkle resistant. A a hot iron is used to heat the fabric above its glass transition temperature and the pressure of the iron forces the polymers to straighten. Then the fabric cools to below its glass transition temperature and the polymer chains are locked in the straightened state. So there you go, that's it!

Hope you fellow science nerds enjoyed it!