Thermal Stability of Insulating Fabrics
Authors: Bartlett R. C. 
Published: August 2009

With the purpose of assembling a well-rounded picture of up-to-date ideas and data on the subject of thermal stability of insulating fabrics and elastomeric tapes, all members of Subcommittee VII on Insulating Fabrics of ASTM Committee D-9 on Electrical Insulating Materials were asked to make contributions. Scope of electrical insulation covered by this group is so broad that it was necessary to construct an organizational framework to assemble this collection logically. While most information obtained was closely related to ASTM test methods and round robin activities, other ideas and data obtained are included in this report to give a broader picture on the subject of thermal stability. Insulating materials included in this survey are varnished cloth, treated sleeving, coated glass fabrics, and elastomeric adhesive tapes. As can readily be seen, a scheme for arranging such a cumbersome mass of information becomes extremely important to obtain a logical and digestible package. Accordingly, this report is divided into two main groupings, coated fabrics and adhesive coated elastomeric tapes. Each of these two types of insulation have been subdivided into other groups based on type of test such as measurement of thermal stability by electrical tests, by mechanical tests, and by physical tests. Under the general heading of coated cloth are electrical insulating materials such as varnished cambric, silicone varnished glass cloth, “Teflon” coated glass cloth, silicone rubber coated glass cloth, and various types of varnished fabric sleeving. Stability of these materials as determined by electrical tests is based on the use of a dielectric strength or dielectric breakdown measurement of specimens aged at elevated temperatures. In most cases the specimens are flexed around a 1/8-in. diameter mandrel after the aging followed by measurement of dielectric strength as prescribed in ASTM Methods of Testing Varnished Glass Fabrics and Varnished Glass Fabric Tapes Used in Electrical Insulation (D 902-53). Percentage change in dielectric breakdown as well as actual breakdown voltage before and after aging is reported as a measure of thermal instability of the material at the temperature of accelerated aging. Another test for thermal stability is similar to Method D 902 with the exception that the specimens are not flexed prior to the dielectric strength measurement. Such test data on thermal stability of silicone varnished, silicone rubber coated, and Teflon coated glass cloth is demonstrated by composite, Fig. 1, for materials aged at 250 C, where dielectric strength of unflexed specimens is shown after various periods of time. Another set of curves. Fig. 2, shows the effect of flexing test specimens with regard to the test results for this type of test for thermal stability. Obviously, dielectric strength of some materials after aging at 225 and 250 C is greatly reduced by flexing. Close examination of these decay curves on Fig. 2 shows that a heavy glass cloth base must be considered when evaluating thermal stability data of silicone varnished glass cloth. On another set of curves, Fig. 3, can be found a distinction between silicone varnished glass cloth having good and poor thermal stability when tested according to Method D 902. Here the dielectric breakdown of specimens baked at 250 C is shown after various aging periods. Of particular interest to the general problem of measuring thermal stability of coated fabric insulation are two new methods of test for treated sleeving. Both of these are based on a dielectric breakdown test to measure embrittlement in one case and cut-through characteristic in the other. In the former method a measure of embrittlement or thermal degradation is accomplished by measuring dielectric breakdown of straight lengths of coated fibrous glass sleeving after aging various periods of time at elevated temperatures. Another variation of this new test procedure for tubular insulation is also based on a dielectric breakdown measurement on specimens aged at eleva ed temperatures, but the test lengths are bent 180 deg around a 1-in. diameter rod prior to voltage test generally similar to Method D 902. To determine resistance to cut-through, another newly developed test is based on mechanically loading a specimen with a similated wire cut-through condition at an elevated temperature. After cooling to room temperature the degree of cut-through is measured by the breakdown voltage between the loading device as outer electrode and the proper fit mandrel as the inner electrode. Since these two tests for coated sleeving have been recently developed, no test data are available.