glass transition, plasticizer, thermoplastic

I'm just guessing that everyone out there knows what plastic is. We call plastics plastic because they are pliable, that is, they can be shaped and molded easily. As plastics become easier to mold and shape when they're hot, and melt when they get hot enough, we call them thermoplastics. This name can help you tell them apart from crosslinked materials that don't melt, called thermosets.

But like I said, everyone knows what plastic is, so why do we have a page here? Well, there are a few nuances and details of what makes a plastic a plastic that it might be useful to go over. For example, why do we call a material a plastic and not a rubber, or elastomer? The answer is in the bouncing. You can stretch an elastomer, and it bounces back. Plastics tend to either deform permanently, or just plain break, when you stretch them too hard.

But that's not bad. You see, although plastics don't behave as well as rubber when they're stretched, it takes a lot more energy to stretch them in the first place. The fancy way to say that is "plastics resist deformation better than elastomers do". This is good when we don't want our material to stretch.

But wait a minute! At the top of this page we said that plastics were called "plastic" because we could deform them, and mold them. That's just the point. It takes more energy to stretch the plastic, making it resistant to deformation. But at the same time, if you pull hard enough, you can not only stretch a plastic, but it will stay in the shape you stretched it into once you stop stretching it. Elastomers bounce back when you let go.

And plastics are also much more pliable than some other materials, like fibers. Fibers stretch very little when you pull on them. This makes them good for things like rope.

Hard Plastic and Soft Plastic

Of course, we've all seen plastics that are hard, and some that are soft. The plastic keys on your keyboard are hard, while the plastic around the cables of the same computer is soft. This is because all plastics have a certain temperature above which they are soft and pliable, and below which they are hard and brittle. This is called the glass transition temperature, or Tg. The Tg is different for each plastic. At room temperature, some plastics are below their Tg, and so they are hard. Other plastics are above their Tg at room temperature, and these plastics are soft.

Sometimes additives are added to a plastic to make it softer and more pliable. These additives are called plasticizers.

Some polymers used as plastics are:

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