Void stability at equilibrium

Whether or not stable nuclei and mechanically trapped voids grow or redissolve depends on several factors. Growth may occur via diffusion of air or water vapor or by agglomeration with neighboring voids. Dissolution may occur if the changes in temperature and pressure cause an increase in solubility in the resin, as we shall see. 

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 

It is instructive to quantitatively examine how Pg might vary as the temperature is increased under equilibrium conditions. Let us assume first that the air-water vapor mixture is an ideal gas and that a Raoult s Law relationship holds for the partial pressure of the water over the water-resin solution at equilibrium. By Raoult s Law 

The assumption of an ideal solution is obviously not correct, but in the low water concentration range of interest, the resulting error will not be excessively large for a first estimate. 

It should be pointed out that the approach outlined above is perfectly general and, while water has been used as the volatile species, any solvent could in principle be used and the analysis can be applied to devolatilization problems. Furthermore, more than one solvent could be considered. Equations 6.7 and 6.8 would then contain partial pressure terms for each solvent. 

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