Introduction

”Halon” 1301 (CF3Br) and ”Halon” 1211 (CF2ClBr) are widely used high-performance fire-extinguishers. However, Halons are ozone-depleting substances (ODSs) and synthetic greenhouse gases (GHGs), which caused serious damage to the ozone layer and had a significant impact on the ecological environment. Therefore, the Montreal Protocol (1987) and subsequent amendments have prohibited the production and usage of Halons and Chlorofluorocarbons (CFCs). Besides, the European Union is requested to accomplish the replacement of Halon 1301 and 1211 in newly constructed aircraft by 2018 and in existing ones by 2040. Therefore, the exploration for low ozone depletion potential (ODP) and pronounced fire-extinguishing performance is of crucial significance1-3.
Among the product candidates for Halon substitutes, Hydrofluorocarbons (HFCs), 2-bromo-3, 3, 3-trifluoropropane (2-BTP) and perfluoro2-methyl-3-pentone (PFMP) products have been well studied and were considered as potential alternative to clean fire-extinguishing agents. For example, HFC-227ea is a colorless, odorless, low-toxic and non-conductive clean gas. It has good thermal and chemical stability4 and can reliably extinguish fires with a low concentration (6.3vol%). PFMP5 was found to be a Halon substitute owing to its negligible global warming potential (GWP), zero ODP, and promising fire-extinguishing performance. And 2-BTP was considered as the potential replacement6, because it has an atmospheric lifetime of days to weeks and can be removed from the atmosphere by OH· radicals in the photolysis reaction7. Despite the obvious advantages of the above alternatives, HFCs, 2-BTP, and PFMP also have drawbacks to a certain extent that limit their use in some critical applications. For example, HFCs have long atmospheric life and high GWP, which do not meet the requirements of environmental protection. 2-BTP contains Br, which is too toxic and can cause harm to the human body. And the boiling point of PEMP is too high, hard to vaporize, which has impact on fire efficiency. Therefore, the above fire-extinguishing agents will be phased out in the future8-10.
Considering the drawbacks of these Halon substitutes, we have attempted to assess the possibility of using the environmentally friendly Iodinated alkanes as the Halon replacements and found that CF3I has a FEC of 3.0vol% on n-heptane−air flame in a cup burner, which is similar to that of Halon 1301. Besides, CF3I is a new type of environmentally friendly (GWP<5, ODP=0.008) alternative refrigerant and insulating gases with low toxicity, and CF3I also has good compatibility with the material. At present, the reaction kinetics of CF3I and OH· have been investigated by using Ab initio calculation in a published paper11 which analyzed the formation enthalpy of the main reaction product HOI. Therefore, a further study on the initial pyrolysis reaction mechanism of CF3I is essential to evaluate the fire-extinguishing performance of this potential Halon substitute.
In this paper, the thermal decomposition mechanism of CF3I has been simulated by DFT with high precision quantum mechanics, and the subsequent reaction channels between CF3I and hydroxyl and hydrogen radicals have been calculated, which reveals the fire-extinguishing mechanism of this halogenated hydrocarbon containing iodine. Furthermore, the FEC of methane-air flame was measured to compare the fire-extinguishing performance of CF3I with HFCs. The results show that the fire-extinguishing performance of CF3I is better, even equal to that of Halon 1301. Therefore, we believe that CF3I has the potential to be used as a substitute of Halon for clean fire-extinguishing agents.