Understand thermal properties like a pro

For almost every material, you can find information about its "heat resistance". However, this term is very broad and does not have a clear definition across websites. If the heat resistance of the material is key for your application, a deeper understanding of the individual terms on the technical sheets is necessary. Let's start with what we encounter most often:

Glass Transition Temperature (Tg)

Tg is the temperature at which the crystalline or semi-crystalline part of the polymer begins to melt and changes from an ordered structure to an amorphous structure.

Simply put, it is the temperature at which the internal structure of the material begins to change and the material "begins to lose" its mechanical properties. At first glance, it may seem that if we exceed Tg, we can no longer rely on the print from the given material. However, Tg is treacherous in that it doesn't say anything specific about the mechanical properties, only that they change, but by how much, Tg won't tell us. Tg is not a mechanical stress test but a chemical property.

So if Tg doesn't tell me much about mechanical properties, what should I be looking at?

The second term we will introduce is:

Heat Deflection Temperature (HDT)

"HDT is a measure of the polymer's resistance to deformation under a given load at elevated temperature." In practice, it is the temperature at which a given polymer sample will be bent by 0.25 mm under a given load. 2 the most commonly used loads are: - 1.8 MPa (264 psi) ISO 75 -A– 0.46 MPa (67 psi) ISO 75-B

HDT represents the degree of stiffness of the material when the temperature increases, for mechanical applications it is much more relevant than Tg!

The difference between Tg and HDT can best be illustrated in the example of two materials directly from our e-shop: PLA Filament-PM:

• Tg: 55°C, HDT (method unknown, we will assume 0.46MPA): 55°C
• PolyMide™ PA6-CF PolyMaker: Tg: 56.6°C , HDT 0.46MPa: 215°C!
  

Fun fact: We deliberately use some materials above their Tg (TPE, TPU) because then they have more useful properties for us (flexibility).

Sources:
https://www.toppr.com/guides/chemistry/the-solid-state/crystalline-and-amorphous-solids/
https://omnexus.specialchem.com/polymer-properties/properties/glass-transition- temperature
https://omnexus.specialchem.com/polymer-properties/properties/heat-deflection-temperature-of-plastics
https://www.iso.org/standard/55653.html