Ultra-thin metallic coating for fire protection

Infrared radiation during a fire is known to be the main contributor to heat transfer. Therefore, coatings that can reflect infrared rays are promising for passive fire protection of materials, as they would theoretically be able to completely hinder radiative heat transfers to the substrate. This approach could be effectively used to delay the degradation and burning of polymers by limiting the heat transfers with the environment. Metals are relevant candidates since they are near perfect reflectors in the infrared range of the electromagnetic spectrum. However, they corrode and lose their properties once put under high temperatures. Moreover, coating a metallic layer on a thermally sensitive and isolating substrate is not straight-forward. Pulsed DC magnetron sputtering of pure metallic targets allows to deposit metals on substrates while maintaining their temperature relatively low. It also enables the deposition of a dielectric layer, which could effectively protect the metal without impacting its infrared reflection property. This technique was used to coat polypropylene and polyamide 6 (PA6) plates with a thin aluminum/alumina bilayer. The performance of these systems was evaluated using cone calorimetry (external heat flux of 50kW/m²) with a thermocouple to monitor the temperature at the back of the sample. With thicknesses less than 1 µm, impressive results were obtained. Time to ignition has been considerably improved, and the increase in temperature at the backside of the sample was significantly slowed down. For example, in the case of PA6, ignition was delayed to more than an hour (80s for the uncoated polymer) and the temperature at the back of the sample was kept under 320°C for the same amount of time (whereas it was reached under 5 min for the bare material). Additionally, a synergistic effect was targeted by combining the infrared coating designed with bulk fire retardant systems.