Methods for identifying epoxy resins

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There are no simple, specific test methods for either the converted epoxy groups or the cross-linking units in cured epoxy resins. The Gibbs indophenol test for phenols using epoxy resin gives a positive result (due to the presence of bisphenol A). Unlike phenol resins, the chromium acid test for formaldehyde gives a negative result. When thermal cracking is carried out at temperatures below 260°C, all epoxy resins produce acetaldehyde. The steps are as follows: Place the sample in the thermal pyrolysis tube, cover the tube opening with filter paper moistened with a freshly prepared aqueous solution of 5% sodium nitrocyanoide and morpholine, and heat it in an oil bath to 240°C. When the gas released by thermal cracking passes through filter paper, the turning blue of the filter paper indicates that it is epoxy resin. Epoxy resins can also be identified using this method: at room temperature, take about 100 milligrams of the resin and place it in about 10 milliliters of concentrated sulfuric acid, then add about 1 milliliter of concentrated nitric acid. Five minutes later, carefully add an 5% aqueous sodium hydroxide solution to the top of the solution; if bisphenol A-type epoxy esters are present, a cherry-red color will appear at the interface between the two layers of solution.