Investigation of polyurethane toughened epoxy resins for composite cryotank applications. Part I: Phase separation phenomenon and cryogenic mechanical behaviors

For composite cryotanks used in aerospace engineering, polyurethane toughened epoxy (PU/EP) resins are promising as matrices by introducing soft segments. There is a special phase separation phenomenon during preparation process, and the microscale morphology is of great significance for mechanical behaviors. In this work, the relationship between phase separation phenomenon and room temperature (RT)/cryogenic mechanical behaviors is explored in-depth for PU/EP systems. As the second phase particle nucleation and growth are demonstrated by the numerical method, scanning electron microscopy, transmission electron microscope, energy dispersive spectroscopy, and viscosity test are executed to reveal particle size distributions for different PU contents. Notably, it is shown that the phase separation starts at a relatively high rate in the initial curing stage, and particle mergences and absorptions play significant roles in enlarging particle diameter subsequently. Afterwards, the RT and cryogenic tensile and fracture behaviors of PU/EP systems are systematically investigated. The addition of PU would reduce tensile strength and modulus at RT while the reducing effect vanishes at 90 K. Fracture toughness results reach the maximum values at 40 phr which are 137.3% and 60.3% higher than those of the untoughened epoxy at RT and 90 K, respectively. This work provides an effective guidance for composite cryotank designs and also offers the precise acknowledge of PU/EP phase separation structures and mechanical properties for theoretical toughening mechanism analysis in our following Part-II investigation.