An investigation of defect formation and growth in epoxy resin using light scattering methods

Perkins, Elaine Ann (1989) An investigation of defect formation and growth in epoxy resin using light scattering methods. Doctoral thesis, City of London Polytechnic.


This work has used laser light scattering to investigate the possible existence of inhonogeneities and other defects in an epoxy resin system, and the role these defects play in the mechanical and electrical failure of the polymer.

Mie Theory was used in the analysis of the data throughout this thesis. Ultra clean fully degassed samples were prepared to ensure any scatterers observed were Inherent to the sample.

It was discovered that inhomogeneities and microvoids do exist in the fully cured and degassed ultra clean samples before stressing. Reproducible trends in inhomogeneity behaviour were observed on stressing, and a three phase model of inhomogeneity has been proposed to explain this behaviour. This consists of a densely crossllnked core of mainly diester crosslinks, surrounded by a less densely crosslinked network containing more ether crosslinks. These regions are then interconnected by a weaker even less densely crossllnked matrix where it is proposed that initial yield occurs followed by eventual failure.

Voids present in the network were observed to grow in a similar manner under both mechanical and electrical stress. This suggests a possible link in the deformation processes under either stress. On release of both types of stress, relaxation occurred and the voids reduced in size although some hysteresis was observed. One distribution of voids reached a plateau in size with applied electrical and mechanical stress and this suggests that not all defects have the potential to cause failure.

No wholly reproducible trends were observed for the samples that underwent electrical stress. A common observation was a step type of behaviour that occurred on the release of stress, and also during stressing around a field value of 2.0 x 10[to the power of]6 kV m[to the power of]-1. The later has been attributed to failure occurring at the pin tip epoxy interface causing relaxation in the scattering volume. The overall behaviour with applied stressed varied from sample to sample and has been attributed to complicated mechanical and electrical forces present and the remoteness of the scattering volume from the pin tip. The scattering volume, however is very much affected by what is occurring at the pin tip.

It is concluded that the defects present in the epoxy resin do play an important role in the deformation and failure of the polymer. The defomation process under both electrical and mechanical stress are thought to be the same and the deformation under electrical stress results from the mechanical force associated with the electric field.

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