Voids equilibrium stability

Figure 6.5 Equilibrium void stability map for a typical epoxy resin system. Curves indicate stable void equilibrium states for liquid-resin pressures indicated. Growth takes place above the lines and dissolution occurs below the lines for any given resin pressure...

The model framework for describing the void problem is schematically shown in Figure 6.3. It is, of course, a part of the complete description of the entire processing sequence and, as such, depends on the same material properties and process parameters. It is therefore intimately tied to both kinetics and viscosity models, of which there are many [3]. It is convenient to consider three phases of the void model void formation and stability at equilibrium, void growth or dissolution via diffusion, and void transport. 

Let us first consider the synergistic elfect that water has on void stabilization. It is likely that a distribution of air voids occurs at ply interfaces because of pockets, wrinkles, ply ends, and particulate bridging. The pressure inside these voids is not sufficient to prevent their collapse upon subsequent pressurization and compaction. As water vapor diffuses into the voids or when water vapor voids are nucleated, however, there will be an equilibrium water vapor pressure (and therefore partial pressure in the air-water void) at any one temperature that, under constant total volume conditions, will cause the total pressure in the void to rise above that of a pure air void. When the void pressure equals or exceeds the surrounding resin hydrostatic pressure plus the surface tension forces, the void becomes stable and can even grow. Equation 6.5 expresses this relationship... 

In the preceding discussion we considered equilibrium void stability however, actual processing conditions involve changing temperature and pressure with time. Whereas equilibrium calculations provide bounds on void growth, it is the time-dependent growth process that is most important from a product quality viewpoint. 

Based strictly on equilibrium considerations, bounds can be set on the stability of voids as a function of temperature and pressure. Although this type of phase map does not depict the time dependency of an actual process, it does provide a limiting scenario toward which the actual process would be heading at any point in the curing cycle. It is surprising that high void pressures are possible if sufficient moisture is present in the resin. 

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