Many epoxy adhesives require high temperature curing to bond composite materials. Of course, this limits their use to materials that are not damaged by this heating process. US researchers, however, have taken inspiration from the way plasmonic nanoparticles heat when illuminated to cure such adhesives at much lower temperatures. Writing in this journal [AT Roberts, et al., Mater Today (2018); DOI: 10.1016/j.mattod.2018.09.005], they explain how they have embedded gold nanoparticles in a high-temperature epoxy material, then used laser light to concentrate heat and initiate the curing process in the resin without the need for oven baking of the entire composite structure.
Composite materials are increasingly replacing traditional metals for many applications, such as the growing need to make vehicles and aircraft lighter and more fuel efficient. Of course, heat-cured thermoset composites are formed, then oven cured, while thermoplastic composites are melted then pressed into their final form. In terms of repairs, high-temperature epoxy resins are frequently used to bind composite materials, and this too requires oven heating.
The team's new approach converts a conventional thermal curing process into a photothermally driven one. The team has worked up a theoretical understanding of what happens, showing that plasmonic nanoparticle-based epoxy photocuring proceeds through a four-stage process. When the laser illuminates the semi-transparent epoxy film sprinkled with gold nanoparticles, the nanoparticles grow hot enough to cure the adjacent epoxy. Because the cured epoxy is dark, it can absorb the laser light directly, eventually growing hot enough to trigger a cure front that rapidly sweeps through the remainder of the epoxy.
Their theoretical insights are then supported by their experimental observations. Not only do they validate the four-stage model, they also show that highly local photocuring can actually create a far stronger bond between composite materials than is possible with conventional thermal curing without nanoparticles. Indeed, their curing process can make a material that is stronger than the composite material itself. "Altogether this 'spot welding' technique for curing epoxy films concentrates heating only where it is needed, while significantly reducing the time required for the curing process," says Henry Everitt, the senior scientist at the U.S. Army Aviation and Missile Research, Development, and Engineering Center who led this project with researchers at Redstone Arsenal, Alabama and colleagues at Rice University. "Not only do our findings advance our understanding of rapid, highly efficient bonding and repair of composite materials, they may be extended to any adhesive or material that becomes darker when heated."
The approach could find utility in fabricating or repairing composite structures that contain embedded sensors and other electronics that would not survive oven baking to cure the adhesive. "Plasmonic nanoparticle-based photocuring is a highly attractive solution for the bonding of composite materials, providing several significant advantages over current oven-curing methods," Everitt says. They conclude that commercialization would be more tenable once the process can be adapted to work with more abundant metal nanoparticles like aluminum and with a wide range of epoxies.
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase. His popular science book Deceived Wisdom is now available.