Glass transition temperature is an often discussed but poorly understood property of plastic materials. I want to clear up some of the mystery of glass transition temperature and help you understand how it can be used in picking the right material for an application.
I know what you’re thinking. What does ironing have to do with glass transition temperature? Read on and find out.
First, you have to understand that there are two broad categories of plastic materials: amorphous and crystalline materials. Amorphous materials are simply materials that do not have a rigid molecular structure. Crystalline materials do have a rigid structure. Here is the trick though, crystalline materials are not crystalline, they are only partially crystalline. What we call crystalline materials are partially crystalline and partially amorphous. In fact, if you could actually see the molecular structure, you would find that there are well ordered (or crystalline) regions and amorphous regions. We only call these materials crystalline for convenience. Technically they are semi-crystalline materials.
Amorphous materials are unique in that they do not go through a traditional melting point where they suddenly go from solid to liquid form. At a certain temperature, the molecules become “loose” enough where they are able to slide around a bit. This causes the material to behave like a soft pliable rubber. The molecules can be moved and rearranged.
This temperature is what is called the glass transition temperature, the temperature at which the material goes from behaving like glass to behaving like rubber. They behave similar to a glass below the glass transition temperature and more like a rubber above it. The abbreviation or symbol for glass transition temperature is Tg often pronounced T sub G.
Semi-crystalline materials undergo both a phase transition, in which the crystalline regions melt and a glass transition temperature in which the amorphous regions go from a glass like material to a rubber like material.
The thing that is the most surprising about glass transition is that it varies widely from material to material. Here are the glass transition temperatures are some common semi-crystalline materials:
Polypropylene -4 F
Polyethylene -166 F
PET 172 F
The interesting thing is that at room temperature, PP and PE are above their glass transitions but PET is below its glass transition.
The Tg can help you when picking the right material for an application. Especially for applications that are exposed to very high or very low temperatures. In the case of high temperature exposure, you need to understand that passing the Tg is going to change the properties of the material. You will see a sharp increase in the impact strength and corresponding decrease in the tensile strength and stiffness.
For parts that are going to see extreme low temperature use, it is best to pick a material that has a low Tg such as polyethylene. You should also understand that adding impact modifiers to materials will often lower the glass transition temperature which will help low temperature impact strength.
You may also be asked to provide the Tg for a material for mold flow analysis. Materials absorb heat at different rates depending on whether they are above or below their Tg. Mold flow simulation software uses the data to help determine cooling times for parts.
Another interesting fact is that the glass transition temperature is what makes ironing clothing work. When you use an iron, you are heating up the fibers in the clothing to a temperature above their glass transition which allows the molecules in the fibers to move around. The weight of the iron forces them to orient in a way that is flatter. For polyester fabric which is PET, it needs to be hotter than 172 F. By the way, cotton is a naturally occurring polymer which also has a glass transition temperature.