Effect of the stoichiometric ratio on the crosslinked network structure and cryogenic properties of epoxy resins cured at low temperature

Recently, epoxy resins with outstanding cryogenic performance have attracted much attention for low-cost space exploration. Since the structure-property relationship is the foundation of materials design, the present study investigates the effect of the stoichiometric ratio of active-hydrogen to epoxy-group ([H]/[E]) on the crosslinked network structure and cryogenic properties of a polyurethane modified epoxy resin (PUE). The average molecular weight (Mc) and integrity of the crosslinked network structure in PUE thermosets are studied by dynamic mechanical analysis and FTIR. Cryogenic properties including the coefficient of thermal expansion (CTE) and tensile properties of all PUE thermosets are determined. The results show that increasing the [H]/[E] stoichiometric ratio from 0.7 to 1.3 can decrease the Mc of PUE thermosets cured at low temperature. The crosslinked network structure of PUE thermosets with lower Mc possesses smaller CTE. The tensile strength of the thermosets at 77 K is increased to about 105 MPa when the [H]/[E] stoichiometric ratio reaches 1.3. And appropriately excessive content of curing agent is necessary to get optimum mechanical properties at RT and 77 K for these low-temperature cured PUE thermosets. Finally, the influence of the [H]/[E] stoichiometric ratio on the fracture mechanism of PUE thermosets is discussed in terms of the morphological characteristic of the propagation zones on the fracture surface of the tensile samples as well.