Low-Emissivity Metal/Dielectric Coatings as Radiative Barriers for the Fire Protection of Raw and Formulated Polymers

Radiative heat transfer dominates in medium- to large-scale fires, with infrared ray absorption causing the rapid ignition of combustible materials. In an attempt to hinder this phenomenon, low-emissivity coatings consisting of an aluminum layer topped by alumina have been deposited on polypropylene (PP) and polyamide 6 (PA6) substrates by pulsed DC magnetron sputtering. Emissivity measurements have confirmed the decrease in the emissivity of both surfaces. The samples were tested with a mass loss cone (MLC) calorimeter at 50 kW/m(2) to mimic the radiative thermal constraint of the last stages of a developing fire. The evolution of the temperature was monitored by a thermocouple to confirm the radiative barrier effect. With this setup, it was evidenced that the presence of low-emissivity coatings reduced the heat absorption, effectively increasing the time to ignition of both substrates. Indeed, coated PP ignited after 7 min, whereas a neat PP ignited in 1 min, and PA6 had a time to ignition of as long as 1 h, as opposed to the neat polymer igniting in only 75 s. However, the effect of the coating on flammability parameters such as the peak heat release rate (PkHRR) and the total heat release (THR) was found to be very low. To counterbalance this downside and to take advantage of the reduction of heat absorption, Al/Al2O3 coatings were deposited on PP and PA6 filled with thermally triggered fire-retardant additives, respectively, expandable graphite and a commercial mixture of melamine polyphosphate and aluminum diethylphosphate. Without surface treatment, the presence of the additives caused a shorter time to ignition, but a reduction in the PkHRR of similar to 50%. The combination of the Al/Al2O3 coating with flame retardants in the bulk led to both a long time to ignition and a reduction in the PkHRR and the THR under MLC testing.