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PEEK polymer has a glass transition temperature of 289°F (143°C) and, because it is a semi-crystalline thermoplastic, retains a high degree of mechanical properties close to its melting temperature of 649°F (343°C).
 Short Term Effects
The short term thermal performance of a material may be characterized by determining the Heat Distortion Temperature (HDT, ASTM D648). This involves measuring the temperature at which a defined deformation is observed in a sample under constant applied stress. A comparative chart of high performance materials using ASTM D648 HDT values (Figure 19) for a defined applied stress of 2639 psi shows that 450G compounds are superior to the other materials tested.
Figure 19 - Heat Distortion Temperature for a Range of High Performance Materials
Long Term Effects
Polymeric materials are subject to chemical modification (often oxidation) at elevated temperatures. These effects may be evaluated by measuring the Continuous Use Temperature (CUT) otherwise known as the Relative Thermal Index (RTI) as defined by Underwriters Laboratories (UL 746B). This test determines the temperature at which 50% of material properties are retained after a conditioning period of 100,000 hours. The UL RTI rating for natural PEEK polymer is charted against other engineering materials in Figure 20.
The bar chart in Figure 20 shows that PEEK polymer materials can only be matched in thermal performance by certain polyimides. However, these polyimides are only available as rod-stock or finished product, which often precludes their use on a production cost or component design basis.
Figure 20 - Relative Thermal Index (RTI) for a Range of High Performance Materials
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