Epoxy resin microvoid

Figure 4 TEM micrograph of microvoids and cavities of a hole band. The rectangular cavities are filled by an epoxy resin and appear light grey. Around the cavities lamellae tilting (up to 90°) is visible. Between the edges of the cavities broad PP-connections with undamaged lamellae (1) but also fibrillian structures (2) can be found. Perpendicular to the stacked lamellae black lines arise (3), which represents the first step of the formation of microvoids and cavities (4).

Lazzeri and Bucknall [131] have proposed that the pressure dependence of yield behaviour caused by the presence of microvoids can explain the observation of dilatation bands in rubber-toughened epoxy resins [132], rubber-toughened polycarbonate [133] and styrene butadiene diblock copolymers [134]. These dilatation bands combine in-plane shear with dilatation normal to the shear plane. Whereas true crazes contain interconnecting strands, as described in Section 12.5.1 above, dilatation bands contain discrete voids that, for rubber-toughened polymers, are confined to the rubber phase. 

There is evidence to suggest that exposure to humid environments, at temperatures above about 60°C can produce permanent damage in epoxy adhesives.Apicella and coworkers have explained this in terms of the formation of microcavities produced by clusters of water molecules. This mechanism of damage is expected to be evident in thermoset resins in which the rigid crosslinked structure does not allow the matrix to relax after microvoid formation. 

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