When drying-oil-modified asphaltic resins are heated in air, they will gradually harden by slow oxidation, becoming stronger and with a higher softening temperature. However, the introduction of hydrogen-cooled generators in the 1930s removed the oxygen from contact with the insulation and nearly eliminated the in-service oxidation or drying process that had helped avoid the tape separation phenomenon. Insulation engineers recognized that a new binding and filling resin that cured during coil manufacture to a thermoset, infusible state was needed. The wartime advances in polyester resin chemistry provided the materials to begin the development of a new generation of improved generator insulation.