University of Massachusetts Amherst researchers have demonstrated a possible new avenue for developing flame-retardant and generally low-conductivity (low heat transfer) plastics that retain the benefits of being strong and flexible by limiting the accessibility of heat-carrying vibrational channels of the material. This new design framework has promising applications, including lightweight thermal insulation materials for spacesuits, thermal protection components for spacecraft and advanced building materials that reduce heating and cooling losses.
An illustration of THDBT (tetrahydroxy deoxybenzoin triazole) filler aggregates at the molecular level. In this “slow chaos” state, there are fewer vibrational pathways available for heat transport, resulting in lower thermal conductivity. Reproduced from Materials Horizons with permission from the Royal Society of Chemistry.
Thermal conductivity is a measure of how efficiently heat can move across a material. When heat moves quickly, the material is conductive. If heat moves slowly, the material is a good insulator. Conventionally, materials are made more insulative by the introduction of pockets of air, which are poor conductors. While effective for inorganic materials, this method does not work for plastics because it can weaken the material and complicate manufacturing.
Yanfei Xu, corresponding author of the study and assistant professor in the Riccio College of Engineering at UMass Amherst, and her team investigated a new way to reduce conductivity without introducing porosity. Instead, they looked at the material’s vibration on an atomic level. Heat moves when vibrational energy is passed from one atom to another, much like a bucket brigade passes water down a line. Firefighters (here representing the atoms) move the bucket (representing heat) in coordinated movement, efficiently from point A to point B.
To reduce conductivity, Xu and her team used vibrational engineering so that, instead of strong firefighters efficiently passing big buckets from one person to the next, the polymer behaves like a group of disorganized toddlers – no two children are moving in the same direction and the small hands can only carry small cups instead of big buckets.
As a result, the heat moves along the material very slowly. In their initial trial of this new method (tested using a polymer hybrid of polyurethane and tetrahydroxy deoxybenzoin triazole), the researchers found that this “slow chaos,” as Xu describes the polymer’s behavior, reduced conductivity by 17%. The material also demonstrated flame-retardant behavior.
Xu points out that their reduction in thermal conductivity is small in this initial testing, but she is excited about their discovery of a new mechanism for governing thermal conductivity.
“There is a lot of potential,” she says. “By reducing the density of thermally accessible vibrational channels available for heat transport, thermal conductivity is suppressed. The materials remain dense, mechanically compliant and flame-retardant.”
This research, published in Materials Horizons, was featured on the journal’s front cover. The work was conducted in collaboration with scientists from North Carolina State University, Massachusetts Institute of Technology, Texas A&M University, and Brookhaven, Oak Ridge and Argonne national laboratories.
The research was supported by the U.S. National Science Foundation, the Federal Aviation Administration and UMass Amherst.