Preparation of Microcellular Epoxy Foams through a Limited-Foaming Process: A Contradiction with the Time–Temperature–Transformation Cure Diagram

Abstract

3D cross-linking networks are generated through chemical reactions between thermosetting epoxy resin and hardener during curing. The curing degree of epoxy material can be increased by increasing curing temperature and/or time. The epoxy material must then be fully cured through a postcuring process to optimize its material characteristics. Here, a limited-foaming method is introduced for the preparation of microcellular epoxy foams (Lim-foams) with improved cell morphology, high thermal expansion coefficient, and good compressive properties. Lim-foams exhibit a lower glass transition temperature (T_g) and curing degree than epoxy foams fabricated through free-foaming process (Fre-foams). Surprisingly, however, the T_g of Lim-foams is unaffected by postcuring temperature and time. This phenomenon, which is related to high gas pressure in the bubbles, contradicts that indicated by the time–temperature–transformation cure diagram. High bubble pressure promotes the movement of molecular chains under heating at low temperature and simultaneously suppresses the etherification cross-linking reaction during post-curing.

A new molecular mechanism of the curing of epoxy foams fabricated through a limited-foaming process is proposed. These foams exhibit two distinctive T_g, which are unaffected by postcuring temperature and time. High gas pressure in the bubbles promotes the movement of molecular chains under heating at low temperature and simultaneously suppresses the later cross-linking reaction during postcuring.

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