Aerospace composite parts made by lay-up of prepreg onto a mold surface followed by vacuum bagging and autoclave provide a good example. Typically, once the resin is infused, the mold would be placed in a large oven at approximately 180 degrees Celsius for several hours to cure and achieve the necessary properties such as stiffness, elastic modulus and glass transition temperature.
“We replace this curing process by using an energy source such as a small heater to trigger an exothermic reaction in the infused part,” says Sottos. “The heat input needs to last just a few seconds to launch the reaction locally. After that, the heat generated internally by the reaction moves quickly, continuing the polymerization throughout the rest of the part in a wave-like pattern. With a large part, the reaction can be initiated at multiple points such as corners or on various layers of thickness.” She adds that the result is an FRP part with similar mechanical properties to those cured conventionally.
The challenge lies in precise control of the polymerization kinetics. While the front of the wave can reach 180 C and plenty of energy is stored in the resin’s chemical bonds to fuel the process, the width of the front is only millimeters. “You need to keep the energy front moving so that the polymerization wave isn’t quenched,” notes Sottos. “Identifying the best balance of resin and inhibitor in the thermoset matrix to prevent premature curing without quenching the frontal polymerization wave is key.”
The team has identified several next steps for its research. “We want to optimize the fiber matrix interface to enhance part properties, especially where multiple polymerization fronts meet in the part,” says Sottos.
While their current research has used dicyclopentadiene (DCPD) as the monomer, the team is actively exploring the potential for other monomers, such as epoxy. “Down the road, we are interested in developing a prepreg-like material usable in a tape-laying process,” Sottos says.