Making Sense of The Thermal Properties

Have you ever wondered what thermal test data to use to evaluate a given application? It is critical for applications where the part will experience elevated temperatures. These temperatures differ whether the application is under a load, reaching the highest temperature point over a specified duration, or is simply operating in a high temperature environment at a constant rate. We will discuss the differences between the four tests that are commonly used to compare the thermal properties of plastic materials.

Melting Point (Tm)

Simply put, the melting point of a plastic is the temperature at which a crystalline or semi-crystalline material changes from solid to liquid. This is the point where the polymer chain shifts from a 3-dimensional orderly structure to a disordered, viscous state.

Glass Transition Temperature (Tg)

This is a phenomenon of amorphous materials, which are composed of long-chain polymers randomly packed together with no order. Glass is a non-polymeric amorphous material. Under elevated heat, glass goes from hard and brittle to soft and pliable material. This is the same for amorphous polymers; the point where the polymer loses its rigidity is the Tg.

The most common way to measure the Tg of amorphous polymers is differential scanning calorimetry, or DSC. The polymer sample is placed in the calorimeter, which measures quantities of absorbed or emitted heat in the sample as the temperature increases. Since the changes to these polymers are gradual, the Tg is reported as the midpoint of the transition.

Heat Deflection Temperature (HDT)

HDT describes the temperature at which the material deforms under a specified load. ASTM D648 and ISO 75 are the test standards used to determine the values. One major difference between the tests is that ASTM measures the edge of the sample while ISO measures deflection on the flat side of the test sample.

For HDT, the test uses a 3-point apparatus submerged in oil that is heated to determine the temperature at which the material reaches deflection of 0.25 mm (ASTM). The tests are commonly run at 66 psi and 264 psi. The test data provides insight into how the material will perform under increased thermal conditions while under load.

Continuous Operating Temperature

This is a bit of an ambiguous data point as it is not guided by ASTM or ISO test methods. Essentially, it is the maximum ambient temperature a material can operate at while maintaining 50% of its mechanical and electrical properties. One common test used to evaluate materials for operating temperatures is the UL test Relative Thermal Index (RTI).