Thermoplastics - Thermal Properties

The service temperature is the range between the maximum and minimum temperatures that the polymer is subjected to in service. The maximum temperature is limited by softening and burning, while the minimum temperature is limited by embrittlement. Insight into the ability of a polymer to withstand short term exposure to elevated temperatures under load can be gained by referring to the heat deflection temperature (HDT). However, the HDT does not give an absolute measure of the thermal stability of a material.

One of the most common measures of long term temperature resistance is the Underwriters’ Laboratory (UL) thermal index. This index is derived by exposing test specimens to different temperatures and testing them after various time intervals. When property levels drop to 50% of their initial values, this is deemed a failure. Oxidation of the polymer is the most common cause of degradation at high temperatures.

The molecular weight of the polymer decreases as the long chain molecules ‘un-zip’, and the physical strength and impact strength drop. As degradation continues, other properties are affected, including electrical properties. Hence, long-term temperature resistance is often rated according to the application, e.g. mechanical with impact, mechanical without impact and electrical. An indication of HDT and UL index values are given in Table 1.

Polymers tend to become brittle at low temperatures, and the variation of impact strength or notched impact strength with temperature can be used to determine minimum service temperature.

Table 1. Thermal properties of unfilled polymers (data for guidance only-absolute values are a function of grade)(ABS – Acrylonitrile Butadiene Styrene, POM – Polyoxymethylene, PMMA – polymethylmethacrylate, PAryl – Polyarylates, PA6 – Polyamide/Nylon, PC – Polycarbonate, PET – Polyethylene Terapthalate, PPO- Polyphenylene Oxide, PEI- Polyetherimide, PES – Polyethersulphone, PPS – Polyphenylene Sulphide, PSul – Polysulphone, PEEK – Polyaryletherketone).

Source : Materials Information Service, edited by Justin Furness

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