Thermal degradation is often a major factor affecting the life of insulating materials and the equipment in which they are used. The temperature index provides a means for comparing the thermal capability of different materials in respect to the degradation of a selected property (the aging criterion). This property should directly or indirectly represent functional needs in application. For example, a change in dielectric strength may be of direct, functional importance. However, more often a decrease in dielectric strength may indirectly indicate the development of undesirable cracking (embrittlement). A decrease in flexural strength may be of direct importance in some applications, but may also indirectly indicate a susceptibility to failure in vibration. Often two or more criteria of failure should be used; for example, dielectric strength and flexural strength.

Other factors, such as vibration, moisture and contaminants, may cause failure after thermal degradation takes place. In this test method, water absorption provides one means to evaluate such considerations.

For some applications, the aging criteria in this test method may not be the most suitable. Other criteria, such as elongation at tensile or flexural failure, or resistivity after exposure to high humidity or weight loss, may serve better. The procedures in this test method may be used with such aging criteria. It is important to consider both the nature of the material and its application. For example, tensile strength may be a poor choice for glass-fiber reinforced laminates, because the glass fiber may maintain the tensile strength even when the associated resin is badly deteriorated. In this case, flexural strength is a better criterion of thermal aging.

When dictated by the needs of the application, an aging atmosphere other than air may be needed and used. For example, thermal aging can be conducted in an oxygen-free, nitrogen atmosphere.

1.1 This test method provides procedures for evaluating the thermal endurance of rigid electrical insulating materials. Dielectric strength, flexural strength, or water absorption are determined at room temperature after aging for increasing periods of time in air at selected-elevated temperatures. A thermal-endurance graph is plotted using a selected end point at each aging temperature. A means is described for determining a temperature index by extrapolation of the thermal endurance graph to a selected time.

1.2 This test method is most applicable to rigid electrical insulation such as supports, spacers, voltage barriers, coil forms, terminal boards, circuit boards and enclosures for many types of application where retention of the selected property after heat aging is important.

1.3 When dielectric strength is used as the aging criterion, this test method may also be used for some thin sheet (flexible) materials, which become rigid with thermal aging, but is not intended to replace Test Method D 1830 for those materials which must retain a degree of flexibility in use.

1.4 This test method is not applicable to ceramics, glass, or similar inorganic materials.

1.5 The values stated in metric units are to be regarded as standard. Other units (in parentheses) are provided for information.

1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. A specific warning statement is given in 10.3.4.