Model systems for thermo-oxidised epoxy composite matrices

This paper deals with the thermal oxidation of organic matrix composites, more precisely aromatic amine crosslinked epoxy resins (977-2 DGEBF-TGPAP-DDS) reinforced by carbon fibers. In this first part, it is tried to understand the effects of thermal oxidation on mechanical properties, emphasis being put on elastic and viscoelastic properties. It is shown that oxidation decreases the glass transition temperature, that results from predominant chain scission, but increases the glassy state elastic modulus. From quasi-static tensile testing, flexural and torsional dynamic testing (DMA) and ultrasonic measurement (5 MHz, 20 °C), it is shown that degradation affects essentially the amplitude of high temperature component of the sub-glass transition (internal antiplasticization). Model networks have been synthesized from amine and epoxy but in non-stoichiometric ratio. It is clearly shown that they display the same behaviour, that confirms the precedent interpretation. For the same crosslink density, the dangling chains concentration is higher in model than in degraded networks, but it remains of the same order of magnitude.