Glass transition temperature vs Melting Temperature

In summary, there are two temperatures commonly mentioned in the context of polymers: T_g (glass transition temperature) and T_m (melting temperature). For plastic extrusion applications, the melting temperature T_m should be considered. T_g is used to provide information about the softening of plastics and can be important in producing objects with specific shapes through softening, though extrusion and molding typically involve melting the polymer. Extrusion temperatures for metals and non-metals are determined by optimizing flowability and extrusion forces, and generally lie between the recrystallization temperature and melting temperature. Glass transition temperature is similar to the recrystallization temperature of metals.
  • #1
fog37
1,568
108
Hello Forum,

In the context of polymers, I have see two different temperatures being mentioned:

T_g and T_m (T_g is the glass transition temperature and T_m the melting temperature).

Which temperature should be considered for plastic extrusion applications? The melting temperature T_m?

What is T_g used for? what information does it provide? Can T_g be important when plastics are not melted but softened to produce objects with particular shapes? Extrusion, molding seem to always have to melt the polymer...

thanks,
Fog37
 
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  • #2
fog37 said:
Hello Forum,

In the context of polymers, I have see two different temperatures being mentioned:

T_g and T_m (T_g is the glass transition temperature and T_m the melting temperature).

Which temperature should be considered for plastic extrusion applications? The melting temperature T_m?

What is T_g used for? what information does it provide? Can T_g be important when plastics are not melted but softened to produce objects with particular shapes? Extrusion, molding seem to always have to melt the polymer...

thanks,
Fog37

Extrusion doesn't really happen at liquid state as it can induce some practical limitations, for metals or non metals an extrusion temperature is determined by optimizing flowability and extrusion forces.

Extrusion temperature lie between recrystallization temperature and melting temperature.

Glass transition temperature is very similar to re-crystallization temperature of metals.
 

1. What is the difference between glass transition temperature and melting temperature?

The glass transition temperature (Tg) is the temperature at which an amorphous material changes from a rigid, glassy state to a more flexible, rubbery state. This transition is not accompanied by a change in molecular structure, but rather a change in molecular mobility. On the other hand, melting temperature (Tm) is the temperature at which a crystalline material changes from a solid to a liquid state. This transition is accompanied by a change in molecular structure and a loss of long-range order.

2. Can a material have both a glass transition temperature and a melting temperature?

Yes, a material can have both a glass transition temperature and a melting temperature. This is because some materials can exist in both amorphous and crystalline forms, depending on how they are processed. For example, glass can have a Tg when it is in its amorphous state, but can also have a Tm when it is in its crystalline state.

3. Which temperature is more important for understanding the properties of a material?

It depends on the application and the properties of interest. For materials that are used in their glassy state, the glass transition temperature may be more important as it affects properties such as stiffness and brittleness. For materials that are used in their liquid state, the melting temperature may be more important as it affects properties such as viscosity and flow behavior. Both temperatures can provide valuable information about the properties and behavior of a material.

4. Do all materials have a glass transition temperature?

No, not all materials have a distinct glass transition temperature. Only amorphous materials, which lack a regular crystalline structure, exhibit a glass transition. Crystalline materials, on the other hand, do not undergo a glass transition and instead have a melting temperature when they change from a solid to a liquid state.

5. Can the glass transition temperature and melting temperature of a material be changed?

Yes, the glass transition temperature and melting temperature of a material can be changed by altering its composition, processing conditions, and external factors such as pressure and additives. For example, the addition of plasticizers can lower the Tg of a polymer, making it more flexible and less brittle. Similarly, the addition of impurities or changing the crystallinity of a material can affect its melting temperature.

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