We’ve been covering a mini-series on plastic temperature characteristics. This is the final part, and today’s topic is the “Glass Transition Temperature.”
That might sound like a strange term. You may be thinking:
“Glass? Aren’t we talking about plastic?”
Let’s break it down in a simple and relatable way.
When you heat plastic, it gradually softens. At first it’s hard and rigid, but with more heat, it becomes rubbery, and eventually turns into a molten liquid.
The reverse also happens: a molten plastic cools into a rubbery phase, and further cooling makes it rigid again.
This point—where the plastic changes from a flexible to a rigid, glass-like state—is known as the “glass transition.”
More precisely, the Glass Transition Temperature (Tg) is the temperature at which a plastic transitions from a soft, flexible state to a hard, brittle state (or vice versa).
Note: this is different from the melting point. The melting point is when a material becomes fully liquid, while the glass transition is a change in material properties without a visible phase change.
Why does plastic become hard at lower temperatures?
Because the molecular chains in the material stop moving—they “freeze” in place, restricting flexibility.
Now here’s a fun little quiz!
There’s something in your daily life that demonstrates the glass transition effect. Can you guess what it is?
Hint: It’s edible. You’ve definitely had it before—maybe even today! Let’s give you three options:
- (1) Pickled plum (umeboshi)
- (2) Chewing gum
- (3) Hard candy (“ame-chan,” as we say in Osaka!)
Got your answer? Here’s the correct one:
It’s (2) Chewing gum.
Think about gum’s texture. It’s soft and stretchy, like rubber. But if you recall, when you first pop a stick of gum into your mouth, it’s a little stiff. It breaks when bent. But after a few moments, it softens and becomes flexible.
That softening is due to your body heat raising the gum’s temperature past its glass transition point. In other words, gum is engineered to take advantage of glass transition!
That’s also why gum left in a hot car in summer becomes sticky and unpleasant—it’s partially transitioned already due to heat.
Here are some typical glass transition temperatures for common plastics:
- PEEK: approx. 140 °C
- PPS: approx. 90 °C
- PVC (Polyvinyl chloride): approx. 80 °C
- PP (Polypropylene): approx. –20 °C
- PE (Polyethylene): approx. –80 °C
These values vary depending on the specific formulation and reference source.
Now, you might wonder:
“Isn’t this kind of like the melting point?”
Good question! Let’s clarify:
- Melting point is the temperature at which a material becomes liquid.
- Glass transition point is the temperature at which a material’s mechanical behavior changes—without becoming liquid.
It’s a subtle shift in flexibility, not a dramatic visual change like melting.
This is also different from the heat resistance or “continuous use temperature” we talked about earlier.
For example, at Kashima Bearings, we tell customers that PEEK bearings have a heat resistance of around 200 °C. But the glass transition temperature of PEEK is only about 140 °C.
That doesn’t mean you can’t use PEEK above 140 °C—it doesn’t suddenly collapse or lose all performance. It just means its internal structure becomes more flexible, and the mechanical characteristics begin to shift.
So please don’t confuse glass transition with melting. It’s not the point where plastic turns useless—it’s a threshold of behavioral change in the material.
Understanding the glass transition point can help you make smarter choices when selecting materials. It’s especially useful for components that need to maintain shape and stability under fluctuating temperatures.
And who knows—maybe you’ll invent the next “gum” by leveraging this hidden but powerful property of plastics!
