CompositesWorld
Issue link: https://cw.epubxp.com/i/517026
CompositesWorld.com 31 NEWS N E W S N S N E W S E N W S W OOA Void Reduction "One difculty in qualifying new OOA prepregs is the wide range of available choices. Te distribution of resin — with respect to the dry areas of the prepreg — can be confgured in diferent ways, depending on the supplier," says the report. In short, there is not yet a defned level of OOA prepreg impregnation that is considered optimum for efective gas evacuation. "Te goal of this work is to characterize and model the fow processes associated with partially impregnated prepregs." The experiments used three prepreg types: • Gurit ST94 RC200T, 200 g/m 2 , 3K tow, 2x2 twill, 42% resin, by weight. • Gurit ST94 RC303T, 303 g/m 2 , 12K tow, 2x2 twill, 42% resin, by weight. • Cytec (formerly ACG) VTM264 CF0100, 283 g/m 2 , 3K tow, 4x4 twill, 43% resin, by weight. Resin is distributed as a flm on each twill type and compressed with a consolidation block at 55°C onto and into the fabric to a variety of impregnation percentages: 0%, 5%, 10%, 20%, 30%, 40%, 50%, 70% and 90%. To halt resin fow at the desired impregnation state, the laminate is immediately cooled with ice water over the vacuum bag. With the resin immobilized, the prepreg's permea- bility is assessed, using what is called the unsteady fow test, which measures pressure drop in a laminate over a given time period. Fig. 2 shows the results of this test, which indicate that air pathways in the fber reinforcement begin to close at >75% impreg- nation (as measured by surface area of impregnated fbers). As pathways close, it becomes much more difcult to evacuate gases and, thus, increases the likelihood of trapped gases becoming bubbles and hardening, during cure, into voids. "Once the inter- tow regions are saturated, any further resin impregnation into the fber tows results in a linear decrease in the laminate's perme- ability to air," says the paper. Ganglof notes, "In traditional composites processes, such as resin transfer molding [RTM], a dry fber bed is impregnated with liquid resin. Te goal is to direct the incompressible resin into all the dry fber areas for complete saturation. In OOA processes, the prepreg is, typically, partially saturated and is complicated by air trapped between prepreg laminates. Tis air must be evacuated for full resin saturation. Lack of process understanding has made the edge and through-thickness breathing dominated by trial-and- error and very expensive." If vacuum is not sufently achieved inside the laminate's air pathways, gases will inevitably become trapped and manifest as voids. By coupling the technique for monitoring resin saturation with the technique for measuring air permeability, the required vacuum dwell time can be reliably predicted for a given prepreg and part geometry. Ganglof notes, "When probing the multiphase fow of bubbles and resin together through the tortuous fow pathways in OOA fabrics, more detailed fuid fow modeling is necessary to deter- mine if moving bubbles in moving resin can navigate and escape prepregs during processing before resin cure." Bubble behavior "Te most important aspect of the group's work has been just the documentation of bubble behavior under vacuum," Cender reports. FIG. 3 Inter-tow channels provide the largest escape routes for air bubbles. The team's research showed that bubbles are most efectively evacuated if they move ahead of the resin fow front in the direction of the fber. Intra-tow resin saturation increases the likelihood of trapping air bubbles, which become voids. 0 20 40 60 80 100 % Area Filled 0% 10% 30% 50% Gurit ST94 RC200T Gurit ST94 RC303T ACG VTM 264 NA NA Kgas (m 2 ) Air Pathways Close 10 -12 10 -13 10 -14 10 -15 FIG. 2 Results of the unsteady fow test revealed that air pathways in a fber reinforcement begin to close at 35-40% impregnation, limiting evacuation routes for entrapped air. Sample 1 ST94-RC200T Sample 2 ST94-RC200T Sample 1 ST94-RC303T Sample 2 ST94-RC303T Sample 1 VTM-264 Sample 2 VTM-264