Tests on the Epoxy Resin

The most commonly used tests for properties of the materials used in epoxy adhesive formulation and uncured mixed adhesives are viscosity, shelf life, percent of solids, and moisture content. These are generally applied to the resinous components. 

ASTM has developed a Standard Guide for Testing Epoxy Resins, which covers several procedures that can be conducted on the same starting sample (e.g., viscosity, color, and density). Other contents of the standard include test methods for nonvolatile content, acid value, epoxy content, hydrolyzable chlorine, and total chlorine. The most important of these test methods are described below. 

Viscosity is defined as the resistance of a liquid material to flow. Viscosity can be used as an indirect determination of the epoxy resin s molecular weight and chemical structure. It is usually measured in fundamental units of poise or centipoise. The unit of centipoise (cP) is sometimes confusing unless one is familiar with these particular units. Practical comparisons of the viscosities of common fluids may be of assistance in understanding the viscosity of epoxy resins measured in centipoise. 

There are also a number of specialty viscosity tests that employ their own relative units. These tests have been developed for specific industries such as paints and coatings. Generally, these viscosity units can be directly converted to poise. 

The most popular viscosity test for products ranging in viscosity from 50 to 200,000 cP uses a rotating spindle instrument such as the Brookfield viscometer. The equipment used for this measurement is shown in Fig. 20.1. The instrument measures the resistance of the fluid to a spindle of certain size that is rotating at a predetermined rate. The method is relatively simple and quick. It can be adapted to either the laboratory or production floor. 

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