Mechanical and Electrical Performance, at elevated service temperatures
In many electrical devices, such as generators and high performance motors, the operating temperature is a significant design consideration. Norplex-Micarta’s thermoset composite materials are not only good insulators, they are mechanically strong and have high stiffness.
Traditionally, insulation systems have a temperature index rating. Norplex-Micarta supplies materials to class B, F, and H requirements.
But, there are other measures of polymer behavior at high temperature. Heat deflection temperature, glass transition temperature, decomposition temperature, and thermal index are of a concern to designers.
How are thermosets different?
Thermosets, unlike thermoplastics, undergo a chemical reaction, called curing. After this reaction, it cannot be undone — meaning, thermosets will not melt like thermoplastics. For this reason, many standard thermoplastic measures of performance at elevated temperatures are not helpful when discussing thermosets. One of those measures is Heat Deflection Temperature (HDT). Originally a devised as a test to help injection molders determine when they could safely open a thermoplastic mold and eject parts, it sometimes is used as a proxy for the elevated temperature performance of a polymer.
Thermosets, unlike thermoplastics, do not melt.
What are good measures of a polymers elevated temperature performance?
There are two areas of general concern to designers when discussing high temperature performance. The first is the point at which the material changes from an elastic response to a viscoelastic response. This temperature is called the Glass Transition Temperature (Tg): the point at which the material begins to behave with a rubbery response.
The point at which the material begins to exhibit viscoelastic response is called the Glass Transition Temperature (Tg).
Tg can be determined through a few different analytical methods. However, on a composite, the preferred method is Dynamic Mechanical Analysis (DMA).
The second area of concern to a designer is the thermal endurance of the material. These are long term tests established by various standards organizations or OEMs to generate a Temperature Index (TI). Requirements vary from organization to organization; however, the objective of these tests is to establish a maximum deterioration in material properties for a certain number of hours (such as 20,000). Materials that can withstand a higher temperature are said to have a higher TI.
Temperature Index measures how a material performs for long periods of time at elevated temperatures.
Finally, while rarely a concern for electrical designs, the last significant temperature point for a polymer matrix composite is the Degradation Temperature (Tg). This is the point where the resin matrix begins to degrade and can no longer properly perform. For most thermosets, the Tg is far above the standard design limit of Tg.