Abstract
Recent experimental results showed that the Tg of cured resin scarcely decreased and the impact strength of resins increased by over 50% when a tetrafunctional epoxy named N,N,N',N'-tetraglycidyl-4,4'-diaminodiphenyl ether (TGDDE) was introduced to an appropriate flexible chain from a dimer fatty acid (DFA). In order to understand the reason for this phenomenon, the modification and the chemical structure of the prepolymer together with the curing reaction and the viscoelasticity of the cured resins were studied in detail in the present work. The results indicated that the modification would help the prepolymer improve its molecular mobility. As a result, the resins could be further cured, resulting in the cross-linking density increasing. This is because the curing efficiency was increased, but the tetrafunctional epoxy was not cured completely due to its large steric hindrance. Moreover, the flexibility of some parts of the networks was improved, which was beneficial for the toughness of the cured resins. Therefore, the toughness of the tetrafunctional resin was improved with little influence on the thermal properties when the epoxies were modified with an appropriate content of DFA.
Highlights
As important thermosetting epoxy polymers, the amine based tetrafunctional epoxies were widely used as the matrices for fiber-composites materials [1,2,3,4]
From the four calculated curing reaction parameters, it can be seen that the curing efficiency and the initial curing reaction rate would be increased on account of the decreasing activation energy and the more flexible molecular chain when TGDDE was modified with dimer fatty acid (DFA)
A tetrafunctional epoxy named TGDDE was modified with DFA, and the prepolymer was cured with methyl nadic anhydride (MNA)
Summary
As important thermosetting epoxy polymers, the amine based tetrafunctional epoxies were widely used as the matrices for fiber-composites materials [1,2,3,4]. The molecular chain modification with a flexible chain was an efficient way for improving the toughness of epoxy resins. It was nearly selected as a priority toughening method for the tetrafunctional epoxy resins, because this method was regarded to have a dispositive effect on the thermal property of the cured resin. The recent experiment results indicated that Tg of the cured resin scarcely decreased and the impact strength of the resins increased by over 50% when TGDDE was modified with an appropriate dimer fatty acid (DFA) content [10], which verified that the tetrafunctional epoxy resin could be toughened by introducing a befitting flexible chain with the premise of little influence on the thermal properties. The curing reaction between epoxy and anhydride (absence of catalyst)
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