Towards a general kinetic model for the thermal oxidation of epoxy-diamine networks. Effect of the molecular mobility around the glass transition temperature

The system of differential equations derived from the so-called "closed-loop" mechanistic scheme was solved analytically by applying realistic proportionality assumptions between the different concentrations of reactive species during the entire course of the thermal oxidation. This new method of analytical resolution allowed obtaining a sounder kinetic model accurately describing the three first stages of the thermal oxidation kinetics: the induction period, the auto-acceleration of the oxidation kinetics at the end of the induction period, and the steady-state regime. This kinetic model was used to identify the thermal oxidation behavior at 120 and 150 °C in a large range of oxygen partial pressures (typically between 0.21 and 10 bars) of two epoxy-diamine (EPO-DA) matrices. In addition, the kinetic model was used to determine the accelerated aging conditions representative of the cruising flight of a commercial airliner. It was found that the oxygen partial pressure must be increased much more than the temperature to avoid any deformation of the structural degradation state in the two EPO-DA matrices, thus leading to the definition of extreme environmental conditions very difficult to access in practice.

No abstract

The thermal oxidation of DGEBA-DDS (bisphenol A diglycidyl ether þ 4,4 0-diaminodiphenyl sulfone) and TGMDA-DDS (4,4 0-methylenebis(N,N-diglycidylaniline) þ 4,4 0-diaminodiphenyl sulfone) was performed at 80, 120, and 200 C and was monitored by FTIR. Oxidation was shown to generate amides and carbonyls. Comparisons were done with model systems displaying some common reactive groups, which highlighted the predominating role of methylene in a position of ether in DGEBA-DDS and methylene in a position of nitrogen hold by TGMDA in TGMDA-DDS. The participation of CH 2 in a position of DDS hardener group seems to depend on the temperature and decrease when lowering it. The oxidation of such complex systems must hence be described by a co-oxidation model where each kind of reactive sites is described by its own set of kinetic constants.