At the beginning it is transparent; it can be liquid or pasty. If it is irradiated with the appropriate light at some point, the entire special resin begins to solidify and takes on a dark color. The epoxy resin formula that makes this possible has been patented by the TU Vienna. The researchers led by Prof. Robert Liska at the Institute of Applied Synthesis Chemistry at Vienna University of Technology have now even succeeded in letting this process take place under water. The new epoxy resin can thus be used for tasks that were previously difficult to solve - such as filling cracks in bridge piers or dams under water or repairing pipes during operation. Another new feature is that the epoxy resin can also be used in combination with carbon fibers or carbon fiber mats. This opens up application possibilities in aircraft construction, wind power plants, ship and boat building, and in the automotive industry.
Epoxy resins are standard materials used in industry for many different purposes - for example to insulate electronic components or to fix mechanical parts. The research group at the TU Vienna develops additives that are added to ordinary epoxy resin to adapt its properties and enable targeted curing at the touch of a button.
"We are developing special compounds in which light triggers a chemical reaction," explains Robert Liska. "This can be a bright flash of visible light; we also have compounds that only react to UV light." At the point where the light hits the resin, a reaction is started that releases heat. This heat spreads and sets the chemical cascade in motion elsewhere - until all the resin has cured in a short time. "The decisive advantage of this method is that you do not have to illuminate the entire resin, as with other light-curing materials," Liska explains. "It is enough to hit any point with light. The rest cures even when it's deep in a dark crack you want to fill." So far, two-component formulations were mostly used for such areas of application.
Partner companies from industry asked whether this process would also be possible in the presence of "dark" fillers or fibers, because self-curing epoxy resin would be extremely useful for such difficult applications. "On the surface, this idea contradicts any theory," says Liska. "The light is very well absorbed by the black carbon fibres, so it cannot penetrate far into the material. Nevertheless, experiments at the TU Vienna have impressively shown that this works very well.
Hardening under water also contradicts any theory. "One would expect the water to react chemically with the components of the resin during curing on the one hand, and to dissipate the heat needed to maintain the reaction on the other hand. Surprisingly, however, it was also possible to run off the light-started self-curing under water. "An important reason for this is that the chemical reaction causes the water to boil," Liska explains. "A thin protective layer of water vapor forms between the hardening resin and the surrounding water."
The researchers are now looking for further industrial users to explore the possibilities of the special resin. "There are many possibilities - we hope for interesting new ideas," says Liska.