It takes a lot of science to stop a fire. To prevent homes and workplaces from going up in smoke, manufacturers have added flame retardants to plastic, wood, and steel building materials for decades. But such additives can be toxic, expensive, and sometimes ineffective. Now, researchers in Australia and China have come up with a new flame retardant that, when exposed to extreme heat, forms a ceramic layer akin to hardened lava, squelching the flames before they spread.
“This is very good work,” says David Schiraldi, a chemist at Case Western Reserve University, who has developed other flame retardants. He notes that the ceramic’s starting materials aren’t particularly expensive or toxic, making it more likely to see widespread use. “[This] could impact public safety in the long run.”
To make a better flame retardant, researchers led by Pingan Song, a chemist at the University of Southern Queensland, Springfield, turned to lava for inspiration. Before cooling and forming igneous rocks, molten lava is made of metal and oxygen-containing glasses that are not only tolerant of heat, but also flow when heated. When exposed to intense heat, they form a nonflammable shell called “char” that prevents flames from reaching the material underneath and resists the conduction of heat.
To make their own version, Song and his colleagues used three components. First, they created a mixture of several metal oxide powders—including oxides of aluminum, silicon, calcium, and sodium. That mix begins to melt at about 350°C (below the temperature of most flames), forming a glasslike sheet. Next, the researchers added tiny flakes of boron nitride, which flow easily and help fill any spaces between the metal oxides as the glass forms. Finally, they added a fire-retardant polymer, which they described in ACS Nano in 2021. The polymer acts as a binder to glue the rest of the mixture to whatever it’s coating.
That mix dissolved in water into a milky-white solution, which they then sprayed on a variety of surfaces, including rigid foam insulation, wood, and steel. After it dried, they blasted each coated material for 30 seconds with an 1100°C butane torch. In each case, the coating melted into a viscous liquid, covering the material in a continuous glassy sheet (see video, below).
When heated by the torch, coating spewed out nonflammable gases, such as carbon dioxide. As it did, it became more dense and formed a uniform, noncombustible char layer, which blocked flames from spreading to the materials underneath. The novel flame retardant protected rigid polymer foam—the kind used to insulate homes—better than more than a dozen commonly used retardants, the researchers report today in Matter. The new coating also excelled at protecting wood and steel.
If sprayed on building materials during construction, the new coating could prevent disasters like the 2017 Grenfell Tower fire in London, where 72 people died, the researchers say. Given the new coating’s performance, lack of toxicity, and ease of application, Song says it could be a “universal” fire-protection strategy, applicable for most building materials. He hopes to commercialize the retardant soon. But at the moment, he adds, “It is just a paper.”