Toward better Halon substitutes: Effects of H content on pyrolytic and fire-suppressing mechanisms of ozone-friendly fluorinated alkanes

Abstract

Halon extinguishants have been prohibited due to the caused damages to the stratospheric ozone layer. For Halon replacement, ozone-friendly fluorinated alkanes are used as transitional Halon substitutes. In order to unravel the relationship between chemical structures and fire-extinguishing performance of fluoroalkane agents, we investigated the effects of hydrogen content on pyrolytic and fire-extinguishing mechanisms of fluoroalkanes including HFC-236fa (CF3CH2CF3), HFC-227ea (CF3CHFCF3) and PFC-218 (CF3CF2CF3), which may provide some guidance on efficient exploration of new substitutes for Halon extinguishant. Both the experimental and theoretical methods were employed in this study. It was found that the energy barrier of CF3· fire extinguishing free radical generated by the three fluorinated alkanes increases slightly with the increase of H content. The reduction of hydrogen content notably promotes the thermal stability of fluorinated alkanes and the productivity of incombustible pyrolytic products. The HFC-236fa agent exhibits remarkable cooling effect upon the combustion cycle via endothermic pyrolysis, while the PFC-218 agent extinguishes flame predominantly through the dilution effect on the flammable gas. Remarkably, a lower hydrogen content effectively inhibits the production of flammable alkenes, corrosive and toxic pyrolytic products. Although PFC-218 agent exhibits the highest fire-extinguishing concentration (FEC) relative to the other two counterparts, the PFC-218 manifests the subtlest combustion promotion and releases the least corrosive products, which inhibits the suddenly risen temperature in the combustion region and reduces corrosive and poisonous products. These findings provide new understanding and guidance on the molecular designing and screening of environmental-friendly and efficient Halon substitutes.