Microstructure and properties of polyester/urethane acrylate thermosetting blends,and their use as composite matrices

Thermosetting blends of a rigid polyester (PE) and a poly(urethane acrylate) elastomer (UA) have been investigated over a wide range of compositions in the cast condition, and in use as matrices of unidirectional glass-fibre composites. Transmission electron microscopy showed that blends in the composition range 20%–60% UA have a two-phase structure, probably with a phase inversion from PE-rich to UA-rich matrix between 40% and 50% UA. It was found that the observed variation in Young's modulus with blend composition could be represented by a simple geometrical model based on series/parallel combination of phases with a regular dispersion. This analysis provided supporting evidence for the proposed phase structures of the blends. Further evidence as to the phase structures was the rapid decline in indentation hardness and in temperature of distortion under load which occurred between 40% and 50% UA. Blends with up to 15% UA were found to have higher tensile strength, and slightly higher failure strain, than the unblended polyester. For composites with PE-rich matrices, the transverse Young's modulus exceeded that of the matrix in bulk, and it was found that the relationship could be expressed approximately by a version of the series/parallel model referred to above. Transverse tensile strengths of the composites were, in all cases, lower than the bulk matrix strength. To account for the observed relationship, a modified version of the Cooper and Kelly model for transverse strength is presented.