D3377-04 – Standard Test Method Technical Guide

📐 Scope and Significance of ASTM D3377-04

ASTM D3377-04 (Reapproved 2018) defines a standard test method for determining the percent weight loss of cured solventless electrical varnishes when exposed to elevated temperatures for prescribed periods of time. The results of this test method are specifically based on 6 mm (1/4 in) thick specimens and are not applicable to specimens appreciably thinner or thicker than 6 mm (Section 1.1 and 1.2). The values stated in SI units are regarded as the standard (Section 1.3).

The significance of this test lies in the potential impact of weight loss. The amount and composition of by-products produced by exposure to elevated temperatures can possibly deteriorate other insulations in the system, cause cracking of insulation due to pressure buildup in thick cross sections, or corrode other parts in the assembly (Section 4.1). Furthermore, the amount and rate of weight loss will possibly affect the thermal rating of a varnish (Section 4.2).

⚙️ Required Apparatus and Material Definitions

Section 5 specifies the apparatus required for this test. A metal mold consisting of two flat sheets 300 by 300 by 6 mm (12 by 12 by 1/4 in) is required, separated by 6.4 ± 0.5 mm (1/4 ± 0.02 in) thick spacers along three sides. The spacers also seal the edges of the mold, and the assembly is held together with “C” clamps or bolts. Aluminum or polished steel plates are permissible. A laboratory balance with an accuracy of ± 1 mg is also required (Section 5.2). Forced-convection ovens must conform to ASTM D5423.

In terms of terminology, Section 3 defines an electrical insulating varnish as a liquid resin system applied to and cured on electrical components. Specifically, the solventless type is a liquid resin system free of volatile, non-reactable solvents, differentiating it from solvent-containing varnishes which are solutions, dispersions, or emulsions in a volatile liquid.