Researchers at Texas A&M University, College Station, Texas, are developing eco-friendly intumescent and clay-based nanocoatings that may one day be used to provide flame resistance (FR) to cotton garments and other textiles and polyurethane foam. The coatings, made using chitosan and other renewable materials, have potential applications in such areas as childrenswear, military and protective apparel, mattresses and home furnishings, and aircraft components.
Jaime C. Grunlan, Ph.D., an associate professor in Texas A&M’s Department of Mechanical Engineering and director of the research, had been working on a layer-by-layer assembly technique using a water-based solution to create clay-based nanobrick coatings for gas barrier films and got the idea to use them in FR applications, first trying them on foam. With input from the National Institute of Standards and Technology, which is partially funding the research, it was found that
the coated foam didn’t break down, and the weight gain from the coating was less than is typical with other FR coatings. The coating penetrates the foam, significantly diminishing off gassing and reducing the heat release rate by half. “The outer surface chars, but the inside is undamaged,” Grunlan said.
Scanning electron micrographs show the intumescent coating on uncharred fabric (top) and
swollen protective foam created on the charred portion (bottom).
Meanwhile, Grunlan had been observing intumescent, foamy coatings on steel structural supports. “The coating, which contains phosphorus and nitrogen polymers, swells through the foaming process,” he explained. “The phosphorus attacks the nitrogen, which offgasses, creating bubbles that provide a thermal shield around the object.”
He decided to try a similar coating on cotton. “It totally worked,” he said. Only the surface in direct contact with flame was charred, and the swollen coating protected the fabric structure.
To make a biodegradable coating, Grunlan and his research team, including Galina Laufer, Ph.D., replaced man-made polymers with chitosan to provide the nitrogen component and phytic acid to provide the phosphorus. The water-based coating can be crosslinked to improve wash durability, Grunlan said, pointing out that the U.S. Department of Agriculture has found that the FR performance is unchanged after 10 washes conducted using an ASTM test method. “I’m very confident
we can show the same performance after 20 to 30 washes,” he added.
In current versions, the coating, though only 500 nanometers thick, does stiffen the fabric, making a very soft cotton more like canvas, such as would be suitable for a firefighter’s jacket. However, Grunlan said his team is working on recipes that would provide a softer hand and would be suitable for children’s sleepwear or other clothing.
Grunlan also has applied the coating to nylon/cotton FR and polyester fabrics. “The coating will prevent melt dripping, and a little intrinsic FR fiber in the fabric dramatically reduces the number of coating layers needed,” he said, noting that the coating will enhance a fabric’s own FR
behavior.
Left: In a vertical flame test, the coated cotton fabric chars only where the flame touches
it.
CONTACTS: For more information about the antiflammable nanocoating research at Texas A&M, contact Jaime C. Grunlan +979-845-3027; jgrunlan@tamu.edu.
November/December 2012
