Matériaux composites à matrice organique

Propriétés thermomécaniques des matrices et mise en œuvre

Organic matrices for composites: thermomechanicaland processing properties

Ecole Nationale Supérieure d’Arts et Métiers (ENSAM), Paris

Abstract

Some important properties of organic matrices are reviewed. The thermal stability ceiling depends essentially on the dissociation energy of the weakest bond of the monomer unit and the eventual existence of a mechanism of chain propagation of the degradation. At long term, the lifetime of materials is generally limited by this ceiling. At short term glass transition or melting are the limiting properties. The structure-property relationships in this latter field, especially the importance of chain stiffness and crosslinking, are relatively well known.

Concerning elastic properties, the influence of cohesion, cristallinity and -partially- temperature, are relatively well known. In contrast, viscoelastic effects linked to the local motions (β relaxation), are more difficult to predict. The ductility/toughness seems to be sharply linked to the local mobility. However certain aspects, especially the eventual influence of crosslinking, are not yet well understood.

Concerning processing properties, (time-temperature) or (molecular weight-temperature) maps allow to show that all the linear polymers are not thermoplastics. For these latter, processing can be made only in a sharp molecular weight interval. In the case of thermosets, the (temperature-conversion) maps allow to illustrate the main characteristics and to put in evidence the problems linked to the fact that, in the final steps, the thermal degradation interferes with vitrification. Some important aspects of processing, especially damagement linked to differential shrinkages and build-up of morphological heterogeneities are briefly evoked.