Professor Grunlan and his team are developing a new flame-retardant coating using renewable, nontoxic materials readily found in nature that could provide effective fire protection for several widely used materials.
Tech Briefs: What motivated you to start this project?
Professor Jaime Grunlan: I had an existing relationship with a group of researchers at KTH Royal Institute of Technology in Stockholm, Sweden. They do work on what we call nanocellulose, or cellulose nanofibrils obtained from processing wood-based materials, which are Sweden’s biggest products. Since I work in flame retardancy and gas barrier and clay, I wanted to see how that material would function with my clay nanocoatings.
Tech Briefs: What made you think that this combination might be fire retardant?
Grunlan: I knew that clay with various polymers will give a good heat shielding/flame retardant coating. Cellulose, because of its structure, looks like it should even be better than a typical polymer because it looks like something that will form char. And if you form char, you can stop fires. So, I thought it would promote char formation in addition to acting as a spacer for the clay. It was an educated guess.
Tech Briefs: Could you tell me something more about the two materials?
Grunlan: Clay is a component in what people would commonly call dirt. There are large clay deposits all over the United States and Europe, and I’m sure all over the world. You can mine it and purify it and what you’ll get is a powder. When that powder is dispersed by putting it into water, it breaks apart into nanosilicate platelets that are sometimes hundreds of nanometers in diameter and one nanometer thick. (Silicate means silicon and oxygen make up their structure.) They’re little nanocrystalline plates, which are impermeable to gas. They also tend to have very nice mechanical properties so they can even be used to make a composite stronger, for example.
For the cellulose, when you break wood down to its very smallest components that you can actually physically separate, you get several hundred nanometer-long fibers that are maybe ten nanometers in diameter, which are also very strong. They’re not flame retardant because cellulose burns very well — as you know, wood burns nicely. But wood also forms char. For example, when you burn your wood in your fireplace, it doesn’t turn into nothing, it turns into charred ashes.
Tech Briefs: I can’t visualize the clay that my kids play with being used as a coating.
Grunlan: It’s the same clay. The difference is the ratio of clay to water. When I make a coating, I use one weight percent of clay in water. Clay that’s used for sculpting is probably 90% clay and 10% water, which makes it like a dough that can be shaped, but ultimately, it’s the same material.
Tech Briefs: How did you apply the coating?
Grunlan: We exposed a piece of polyurethane foam, which we chose because it’s one of the most common types of foam that’s used, for example, in upholstered furniture or in automobiles — any type of cushioning that you sit on. We soaked it into the water-based solutions containing either clay or cellulose fibers. The cellulose fibers had been chemically treated to be cationic, meaning they had a positive charge, while the clay naturally has a negative charge. When I dipped it back and forth between these two solutions, I was building up nanolayers of cellulose, and then clay, and then cellulose, and then clay. We built kind of a nanobrick wall structure. The synergy between these two materials gives the structure its fire-retardant property.
Tech Briefs: Does uncoated polyurethane burst into flame or does it just melt?
Grunlan: It ignites and then it melt-drips drops of fire. In a home, when your couch catches on fire, it’s that melt-dripping effect that tends to spread the fire more quickly because it will hit the ground, which means your carpet or whatever your flooring is will catch on fire, the drapes will catch on fire, etc.
Tech Briefs: What other materials could this be used for besides polyurethane?
Grunlan: It could be used on any flammable polymeric material. It could be used, not just to stop fire, but also to add to the heat shielding. The coating enables the polyurethane foam to withstand a higher temperature than it could without the coating. There are all kinds of places where polymers are used, where if they could survive even 10 or 20 degrees more temperature, and not melt or drip, they could be used in new applications with reduced cost.
Tech Briefs: Do you plan to continue research along these lines?
Grunlan: Our flame-retardant research will go on indefinitely. We’re always developing new recipes and working with companies to commercialize the technologies. We’re working on a number of different types of water-based flame-retardant coatings that are combinations of clay with polymers.
Tech Briefs: Why polymers rather than cellulose?
Grunlan: So, to be super technical, cellulose is a polymer. It’s a naturally occurring polymer. But for now, it’s expensive to produce the fibrils. The cellulose nanofibers take a lot of processing to produce, and then have to be chemically modified in another step to get them to be cationic. I can buy polymers that are already positively charged, and, in some cases, those polymers have chemistry that’s even more beneficial for flame retardancy. However, these polymers are not naturally occurring, except perhaps for chitosan, which is similar to cellulose, but also contains nitrogen, which increases its fire-retardancy. A chemical called chitin is found in crustacean shells, such as lobster, crab, and shrimp. If you dissolve the shells, one of the major components would be chitin, which is a garbage material for the most part. But you can turn it from chitin into chitosan by the simple chemical process called deacetylation. It then becomes a water-soluble cationic cellulose-like material that has this very natural origin. So, you could call it an environmentally benign kind of polymer.
Tech Briefs: Why have you chosen cellulose and chitin instead of commercial polymers?
Grunlan: If we can make functional coatings using naturally occurring materials, then when you’re talking about waste and throwing packaging or a flame-retardant object away, you don’t need to worry about the coating having a negative environmental impact.
Tech Briefs: Finally, what excites you about this project?
Grunlan: I enjoy making materials with stuff the earth just happens to have sitting around naturally, often waste material — it’ a win-win. I jokingly say I use dirt and shrimp shells to make functional materials. I feel blessed every day. I have a job I love, even though it can sometimes be daunting because when you see one nice piece of work, there’s typically 20 failures behind it.
An excerpted version of this interview appeared in the April 2019 issue of Tech Briefs.