Void stabilization

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... 

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...

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. 

A plot of Equation 6.32 for two relative humidities (50 and 100%) yields a void stability map, which is shown in Figure 6.10. It is evident from this map that vacuum can be applied... 

Figure 6.10 Void stability map for pure water void formation in epoxy matrixes. Note the significant effect of initial relative humidity exposure of the resin...

Similar kinds of resin pressure calculations must also be made for other processes to examine the potential for void stability and growth. This has become a subject of considerable interest [26-30] as quality is sought in RTM parts. 

Any on-line process control model used for computer-aided manufacturing of high-performance composite laminates must include a thorough treatment of void stability and growth as well as resin transport. These two key components, along with a heat transfer model and additional chemorheological information on kinetics and material properties, should permit optimized production of void-free, controlled-thickness parts. A number of advances have been made toward this goal. 

The resin pressure is almost never equal to the autoclave pressure. If the resin pressure drops due to resin flow, then it may become less than the minimum pressure necessary to prevent void stability and growth. In order to produce quality void-free laminates consistently, accurate resin pressure predictive software is a necessity. 

Percent Air Voids Stability, lbs. Flow, 1/100" Percent VMA Percent VMA-Filled... 

A plot of Eq. (32) for two relative humidities (50 and 100%) yields a void stability map which is shown in Fig. 10. It is evident from this map that vacuum can be applied without encouraging void growth if such application is coordinated with the temperature of the system. Brown and McKague 14) have experimentally observed that the void content is reduced significantly when pressure is applied to the prepreg early in the cycle, which is in accordance with the stability map. 

Entrainment. The minimum amount of entrainment for a given separator is determined by the void stability and the geometry of the gas takeoff... 

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