Under the direction of Steven Nutt, M.C. Gill Professor and director of the M.C. Gill Composites Center, the team at USC has been working on a solution: promote extraction of gases in the through-thickness direction of laminates, which is much shorter than the planar direction. This paradigm shift for prepreg formats relies on discontinuities or openings in the resin films coated onto the fibers, through which entrapped gases can be evacuated. “The relatively simple idea is to get the air flow out of the plane in the Z direction,” says Anders. “It really improves the robustness of out-of-autoclave prepregs if you have that feature – that out-of-plane breathability or through-thickness gas permeability.”
These prepregs – known as semi-pregs – have several advantages, according to Anders. They reduce debulk times from hours to minutes without sacrificing part quality. In addition, semi-pregs exhibit a much lower sensitivity to adverse process conditions, such as humidity and embedded ply drops, than conventional VBO prepregs.
To further development of this new class of prepregs, Nutt’s team has set out to answer three primary questions: Does extraction of gases in the through-thickness direction work? How can you make these semi-preg materials? What is the best design? The first question has been answered. “Our lab has fabricated semi-pregs with through-thickness permeability using various reinforcement architectures and resin types, demonstrating the feasibility and versatility of the concept,” says Anders. To date, the team has done lab-scale work, making small quantities of laminates measuring 12 square inches or less and conducting in-situ diagnostics to better understand how the gas transport happens.
Recently, Anders has examined how to make prepreg with through-thickness permeability, testing a variety of methods in the lab. One system his team built is a printer that programs any desired pattern and could potentially make prepreg on demand. However, it’s slower than traditional prepreg manufacturing machines. Another method the team has pursued is retrofitting existing prepreg equipment to make semi-pregs. “We are looking at how to translate these ideas into something that can be done at scale and won’t cost an arm and a leg,” he says.
Finally, the researchers are looking at design options. “Suddenly, you have this whole new design space,” says Anders. “Before, you were only considering the through-thickness degree of impregnation, but now you are considering in-plane patterns in the resin.” The team has tried numerous patterns, resins and fabrics, such as stripes of resin on plain weave fabrics and “islands” or separate dots of resin on spread tows.