: Using a testing device called a cone calorimeter as seen in the top image taken at NIST, untreated polyurethane foam catches fire from a nearby heat source. Below, foam treated with the clay-filled coating did not ignite when exposed to same heat source.
Clay is one of the oldest materials known to man. It is used to make products ranging from walls and bricks, to edible supplements. In that regard, the National Institute of Standards and Technology (NIST) researchers are doing something more appetizing with clay than eating it. They are trying to set it on fire.
Home furnishings require a coating of flame retardants so that your couch doesn’t ignite like an explosion scene from a Chuck Norris movie. However, these retardants are generally deemed toxic. Similar toxic fire retardants have become headline news with the recent High Park wildfire near Fort Collins, Colo. NIST researchers are trying to address the problem of toxic flame retardants by developing a new generation of sustainable, non-halogenated flame retardant technology. The answer may be found in clay.
The sustainable flame retardant technology creates a thick, fast-forming coating which has a uniformly high concentration of flame-inhibiting clay particles. Polyurethane foam was a well-suited testing material because of its broad use in home furnishings and automobile seating. Luckily, the sustainable coating adheres strongly to polyurethane foam’s open-cell surface.
"In effect, we can build the equivalent of a flame-retarding clay wall on the foam in a way that has no adverse impact on the foam manufacturing process," explains NIST fire researcher Rick Davis. "Our clay-based coatings perform at least as well as commercial retardant approaches, and we think there's room for improvement. We hope this new approach provides industry with practical alternative flame retardants."
Davis and his NIST colleagues describe the new coating and the process they used to make it in the journal ACS Macro Letters.
To date, researchers have built up coatings by stacking thin layers in pairs held together by basic electrical attraction. With just a pure polymer present, a thick coating forms rapidly, but it isn't a fire retardant. With clay in every other layer, either the coating is too thin or the clay content is too low to discourage flames.
The NIST team tried something you would expect not to work: trilayers consisting of a positively charged bottom topped by two negatively charged layers. Under most circumstances, the two negative layers would repulse each other, but it turns out that hydrogen bonds formed between the two negative layers and overcame this repulsive force.
The resulting trilayer yields a unique result: a thick, fast-forming, and high-concentration clay coating on polyurethane foam. This nanocomposite coating is 10 times thicker, contains six times more clay, and does so using at least five times fewer total layers than the traditional bilayer coatings.
"The eight trilayer system thoroughly coated all internal and external surfaces of the porous polyurethane foam, creating a clay brick wall barrier that reduced foam flammability by as much as 17% of the peak heat release rate," the team reported. Only a few hundred nanometers thick, the final coating is transparent and the foam still has the same softness, support and feel.
More info: (NIST) http://www.nist.gov/el/fire_research/clay-062712.cfm
