Internal stress control in epoxy resins and their composites by material and process tailoring

Abstract The development of internal stress during cure of epoxy and hyperbranched polymer‐modified epoxy resins was characterized, taking into account the evolving viscoelastic properties, the volumetric shrinkage due to the chemical reaction, and the thermal expansion. A criterion for void formation during cure in a constrained mold was proposed, providing guidelines for the construction of a process window for manufacturing of void‐free composites. It was shown that the internal stress development in epoxy resins during cure is strongly influenced by the presence of hyperbranched polymer modifiers. The role of these modifiers was illustrated for the case of autoclave processing of glass fiber/epoxy composites. This study showed that higher fiber volume fractions could be used with hyperbranched polymer‐modified resins than with unmodified resins, for producing void‐free laminates. It also appeared that by suitable tailoring of the process cycle, a fully stress‐free laminate could be obtained after cure, using the modified resin.