Resin-void surface tension

The pressures inside and outside of the void are effectively equal until the resin viscosity becomes so high that viscous effects become important. As the resin proceeds toward solidification, the pressure in the void can rise significantly above the resin pressure. Surface tension effects are also negligible for voids larger than 100 pm. 

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... 

The surface tension is found from an empirical formula and is a function of temperature (determined in the thermochemical submodel). The surrounding pressure P is determined in the resin flow or compaction submodels. The pressure within the void is determined by the partial pressures of the water vapor and air within the void. The mass of water vapor within the void changes during processing and can be described by Fickian diffusion across the void-composite interface [29], Once the mass of vapor inside the void and the pressure at the location are known, the change in void size can readily be calculated from Equation 13.19. Changes in void size are halted when the resin has solidified. 

ACC-4 still has about 3-4% voids and fissures, which are detrimental to the performance of the material. In its use as a missile nose cone or rocket nozzle, the extremely hot gas environment can result in rapid degradation of the part because the chemically reactive gases can rapidly permeate the structure via the interconnecting fissures and voids and attack the carbon fibers. Furthermore, in the bow shock wave of a missile nose cone that reenters the ionosphere, the atomic oxygen that is formed can similarly permeate and attack the fibers. Further densification of ACC-4 by another PIC cycle (to reduce the voids to below the 3% level) is virtually impossible because the resin or pitch cannot be forced into the microcracks and pores of the composite even under extremely high hydrostatic pressure of 700-1,500 bar, because of viscosity and surface tension considerations. 

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