A newly developed clay-based coating may lead to a new generation of green flame retardants for polyurethane foam.
Researchers at the National Institute of Standards and Technology (NIST) say they have developed a thick, fast-forming coating with a high concentration of flame-inhibiting clay particles that adheres strongly to porous polyurethane foam—often used in furniture, carpet padding and other items.
NIST researchers used a cone calorimeter testing device to measure the heat-release rate and other flammability properties of materials. At top, untreated polyurethane foam catches fire from a nearby heat source. Below, foam treated with the clay-filled coating did not ignite when exposed to the same heat source. Instead, a fast-growing protective layer called char forms on the surface.
The coating is billed as a "green" alternative to many of the commercial and halogen-based fire retardants that have been linked to environmental and health concerns, according to the researchers.
New Coating Wall Performs
“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 “Innovative Approach to Rapid Growth of Highly Clay-Filled Coatings on Porous Polyurethane Foam,” in the journal ACS Macro Letters.
Unexpected Coating Advance
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,” the researchers note. “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 tried something they did not expect to work: trilayers consisting of a positively charged bottom topped by two negatively charged layers.
The team says that under most circumstances, the two negative layers would repulse each other; however, the hydrogen bonds formed between the two negative layers and overcame this repulsive force, they explain.
'Clay Brick Wall'
“This 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,” according to the team.
“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,” NIST reported.
Only a few hundred nanometers thick, the final coating is transparent and the foam still has the same softness, support and feel, the researchers note.
Moreover, when 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.
When asked about other possible applications, such as building materials and insulation, a NIST spokesperson said the team was currently concentrating on “understanding how the process and recipe impact flammability performance.”
But, Davis says he thinks markets and applications beyond polyurethane foam would be targets worth pursuing with an external partner.