Gaithersburg, Maryland - A team at the US National Institute of Standards and Technology (NIST) researching flame retardants for polyurethane foam has discovered a potential clay-based nanocomposite coating.
UTI reported on the team's development a layered carbon nanofibre-based coating with flame retardant potential in December 2011. NIST continued its experiments before discovering a "surprising result," the institute said in a 27 June news release.
The thick, fast-forming coating that the NIST team created has a uniformly high concentration of flame-inhibiting clay particles, and it adheres strongly to surface of polyurethane foam.
"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 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."
To date, 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, NIST said. 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 then tried 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, according to NIST.
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 6 times more clay, and achieves this using at least 5 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 percent of the peak heat release rate," the team reported.
The final coating, which is only a few hundred nanometers thick, is transparent and the foam still has the same softness, support and feel.
NIST concluded by saying that compared with amounts of current flame retardant applied to polyurethane foam, only half as much of the new clay-based coating was required to achieve comparable levels of performance.