Structure-Property Relationships in Anhydride-Cured Epoxy Resin

Anhydride-cured epoxy after amines are among the most widely used curing agents.
Anhydride-cured epoxy resins exhibit excellent thermomechanical properties as matrices
in polymeric composite materials and are widely used in the aerospace and automotive industries. This study investigates the influence of anhydride structures and initiator choice on the cure kinetics, thermal, and mechanical properties of the cured diglycidyl
ether of bisphenol A (DGEBA).

N, N-dimethylbenzylamine (BDMA) and 1-methylimidazole (1MI) were employed as the
initiator, and the hardeners used were cis-1,2,3,6-tetrahydrophthalic anhydride (THPA),
a combination of cis and trans-hexahydro-4-methylphthalic anhydride (MHHPA) and cis-hexahydrophthalic anhydride (HHPA).

Isothermal cure kinetics data revealed that the anhydride structure and the choice of
initiator influenced the conversion rate and rate constant of the epoxy-anhydride
network. Kinetic triplet (Ea, A, and f(α)) parameters were obtained by fitting the
experimental data to the Kamal-Sourour, Sestak-Berggren, and isoconversional models.

Data on the glass transition temperature (Tg) of the different epoxy-anhydride networks
indicate that the molar mass of the alicyclic component in the diester crosslink
predominates in determining the stiffness of the chains.

ATR-FTIR analysis confirmed the formation of ester linkages in the epoxy-anhydride
networks. PALS data also revealed that the anhydride's structure influenced the average
free volume in the network.

The size of the free volume has been shown to affect the compressive modulus. An inverse relationship was observed between the flexural modulus and compressive modulus. Additionally, a positive correlation emerged between the density of the different networks and the fracture toughness results.

Characterising these epoxy-anhydride-cured networks based on curing kinetics and their
responses to both static and dynamic stresses provide insights into their potential for
demanding applications in polymer composites and in the aerospace and automotive
sectors.