Most people are aware that plastic waste is a problem. Almost all types of plastics that we use in our everyday lives are derived from fossil sources. When they end up in the environment, they cause pollution for generations. When incinerated in a waste incineration plant, they release climate-warming CO₂ into the atmosphere. Recycling is therefore the better option: Used plastics provide the raw materials for new ones, closing the loop.

However, not all plastics can be recycled. What is already standard practice for PET, for example, is all but impossible for epoxy resin. This is because epoxy belongs to the group of so-called thermosets. In these polymers, the long molecular chains are cross-linked in such a way that they cannot be melted down again after initial curing. “Today, we only have two options for disposing of epoxy resin: incineration or landfills,” says Empa researcher Arvindh Sekar from the Advanced Fibers laboratory in St. Gallen.

Nevertheless, this durable plastic is widely used, both in its pure form, for example in coatings or adhesives, and as part of fiber-reinforced materials, where epoxy resin is combined with carbon or glass fibers for everything from aircraft and car parts to sports equipment and wind turbines. Now, the Empa team has succeeded in developing a recyclable epoxy resin. Their polymer can not only be reclaimed using various methods, it is also flame-retardant and easy to manufacture, paving the way for industrial applications.

It's all in the chemistry

The element that makes all these properties possible is phosphorus. “Phosphorus-based additives are commonly used as flame retardants,” says Sekar. “Normally, they are simply mixed into the epoxy resin as a powder.” The Empa researchers go one step further and add a phosphorus-containing polymer to the resin before curing, which reacts with the epoxy. The flame-retardant effect of the phosphorus is retained, as are the advantageous mechanical properties of the resin.

However, the phosphorus polymer allows the cross-links between the polymer chains in the cured epoxy to rearrange themselves when heated. After use, the material can simply be ground into powder and pressed into a new shape while heated, causing the bonds to rearrange themselves. This is known as thermomechanical recycling. “We have carried out ten such recycling cycles, and the epoxy has not lost any significant mechanical strength in the process,” states Sekar.

But what is to be done if the epoxy is part of a composite material mixed with fibers and cannot simply be ground down? Even her, the new material is at an advantage, because in addition to thermomechanical recycling, it can also be chemically dissolved, enabling fiber recovery without significant damage – a step that was previously almost impossible. “In addition to the fibers, we can also recover over 90 percent of the epoxy and phosphorus,” adds Sekar. Unlike thermomechanical recycling, however, chemical recycling requires a lot of energy and larger quantities of solvents, the researcher warns – as does the chemical recycling of other polymers. “Chemical recycling should always be the last resort. Thermomechanical recycling is preferable wherever possible,” he says. However, for fiber-reinforced epoxy resins, there is currently no alternative.

Industry-ready

The Empa researchers have been working on their epoxy resin for several years. They have now improved the manufacturing process so that it can be scaled up for industrial production. “We are looking for industrial partners who would be interested in commercializing the flame-retardant recyclable epoxy,” says Sekar. The first areas of application could include indoor and outdoor coatings. Here, the material scores additional points because, thanks to the addition of phosphorus, it has enhanced color stability and reduced yellowing than conventional epoxy resin.

Another area of application would be as an adhesive in the construction of wind turbines. “Wind turbines are vulnerable to fire incidents because short circuits or lightning strikes can cause fires,” says Sekar. “In addition to improving fire safety, our material would facilitate maintenance and component replacement because it can be reshaped under the right conditions even after curing.” In the meantime, the researchers are working on combining the phosphorus additive with other polymers to make them fire-resistant and recyclable as well.