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Flame retardant nanocoating helps fight furniture fires without toxic materials

Flame retardant nanocoating helps fight furniture fires without toxic materials

14-Jun-13

The U.S. National Fire Protection Association has estimated that upholstery and bedding were the first items ignited in an average of 17,300 fires annually, resulting in 871 civilian deaths and millions of dollars in property loss. Furniture cushions are made of flammable polyurethane foam that, when burned, puddles at high temperatures, a quality called the “melt-dripping” effect that further spreads fires. Furniture foam must be treated with flame-retardant chemicals that are known to be harmful to human health and the environment. Amid concerns over the potential health effects of existing flame retardants for home furniture, fabrics and other material, scientists are reporting development of an exceptionally effective new retardant that appears safer and more environmentally friendly. A thin polymer coating on foam used in furniture can prevent the spread of flames, said Dr. Jaime Grunlan, the Gulf Oil/Thomas A. Dietz Career Development Professor at Texas A&M University. The coating developed consists of a sulfur-based polymer and chitosan, a carbohydrate polymer found in crustacean shells. This new coating could be an environmentally friendly alternative to the toxic chlorine- and bromine-based flame retardants used today in furniture foam. "This new flame retardant nanocoating functions by generating a 'gas blanket to starve the surface of burning object of oxygen," Grunlan said. "No oxygen means no fire." The “nanocoating” is so thin that 1,000 layers of it would fit across the width of a human hair, and it is made with a relatively benign polymer that creates a “gas blanket,” preventing oxygen from fueling a fire. It is the first flame retardant that both reduces the heat released from fire and prevents the foam from dripping and spreading flames to the rest of the room or house. R Grunlan's technology involves covering every microscopic fiber in a fabric with a thin composite coating of two polymers that exhibit an intumescent effect, producing a protective carbon foam coating when exposed to high temperatures. The thin films are about one-tenth of a micron thick, or about one-thousandth the thickness of a human hair, and are created with the layer-by-layer assembly technique in which the coating is deposited onto the surface of the fiber being coated. This layer-by-layer process allows to control the thickness of the coating down to the nanometer level. The technology will be suitable for clothing, including children's clothing; lab coats; and medical clothing for both doctors and patients. It can even be used in military camps, where a fire in a single tent can wipe out an entire camp. But the technology's applications go far beyond just clothing and fabric. The coating could be used in foams, such as those found in sofas, mattresses, theater and auditorium seats, airplane seat cushions, and building insulation. The coating is so thin that it adds only 4 to 5 weight-percent to the foam and does not negatively alter its color, texture or flexibility.

As per Phys.org, a surprising result that may lead to a new generation of nonhalogenated, sustainable flame retardant technology for polyurethane foam. 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 the Swiss cheese-like surface of polyurethane foam, which is used in furniture cushions, carpet padding, children's car seats, and other items. "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 that are held together by basic electrical attraction. With no clay present, just a pure polymer, a thick coating is formed rapidly, but it is not 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 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 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% 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. 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.

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