Clay-based coating could usher in new generation of green flame retardants
A new thick-forming, clay-based coating that could lead to a new generation of sustainable flame retardants has been developed by researchers at the National Institute of Standards and Technology (NIST).
The technology has a uniformly high concentration of flame-inhibiting clay particles and is said to demonstrate excellent properties for adhering to the surface of polyurethane foam, a material commonly used in furniture cushions, carpet padding and children's car seats.
"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," explained NIST 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."
To date, other researchers have built up coatings by stacking thin layers in pairs that are held together by basic electrical attraction. With no clay present, just a pure polymer, a thick coating is formed 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 be an effective fire retardant.
The NIST team's method involved 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 the researchers discovered that hydrogen bonds formed between the two negative layers and overcame this repulsive force.
The resulting trilayer yielded a unique result: a thick, fast-forming, and high concentration clay coating on polyurethane foam. According to Davis, the nanocomposite coating is 10 times thicker, contains six times more clay, and achieves this 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," he noted. "Only a few hundred nanometers thick, the final coating is transparent and the foam still has the same softness, support and feel."
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