In 1986, the U.S. Air Force recognized the potential for regulation of halons and initiated a program to identify halon replacements. This program demonstrated that halon replacements with decreased or zero ODP could be obtained by (1) eliminating bromine and (2) substituting hydrogen for fluorine to decrease atmospheric lifetime. 55 The Air Force program identified HCFCs, HFCs, and PFCs as the most likely near-term replacements.
The National Institute of Standards and Technology (NIST) issued a report in 1990 whose objective was to initiate a systematic search for halon replacements.56 This report is firmly grounded in the science and technology of combustion and current understanding of the role of halogenated compounds in ozone depletion in the stratosphere. The centerpiece of the publication is a list of 103 chemicals ''covering a range of chemical and physical principles thought to affect flame suppression capability.'' The list includes saturated halocarbons, halogenated ketones, esters and anhydrides, unsaturated halocarbons, halogenated ethers, sulfur halides, compounds containing phosphorus, silicon, germanium and metals, and inert gases. The list contains both candidate halon replacement agents and compounds chosen to test principles of fire suppression. While the NIST list is not claimed to be exhaustively inclusive, it is sufficiently extensive to suggest that the most obvious classes of alternative agents are included. The list includes most of the commercially available agents listed in Tables 2.2 and 2.3, omitting HFC-227ea and HFC-236fa. These compounds were later considered by NIST for aircraft nacelle and dry bay applications. 57,58
Flame suppressants that do not endanger the ozone layer either do not contain the heavy halogens (chlorine, bromine, or iodine) or have atmospheric lives so short that they do not reach the stratosphere. Perfluorocarbons satisfy the first condition but do not offer chemical flame suppression activity and are somewhat less effective in extinguishing fires. Compounds that contain the heavy halogens but are destroyed before they reach the stratosphere are of special interest as halon replacement agents. Substitution of H for a halogen, increasing the dipole moment, and shifting the absorption toward the red are strategies pursued in the NIST list, including compounds containing hydride, ketone, ester, anhydride, and double or triple bonds.
In connection with halon 1301 replacement agents, only a limited number of compounds have matching physical properties, i.e., are storable as a liquid at normal ambient temperatures, but have a high vapor pressure and so evaporate rapidly on discharge. Of the 103 compounds listed by NIST, only 31 have boiling points below 0°C (the b.p. for halon 1301 is -68°C), and some of these are toxic or have interest only for determining mechanisms. A low boiling point and high vapor pressure strongly indicate compounds of low molecular weight and/or high fluorine content. The point is that the number of candidate halocarbon halon replacement agents is not very large.
Tapscott and coworkers59 are testing various classes of compounds as candidates for total-flooding fire suppression, including brominated fluoro ethers, alkenes and aromatics, amines, and carbonyls, as well as phosphorus and silicon compounds. Some of these materials have good fire suppression characteristics, but satisfactory performance in terms of toxicity, materials compatibility, and other properties remains to be proved.
FINDING: After reviewing research, development, toxicology, and engineering activities directed toward finding alternative and replacement agents for halon 1301 and halon 1211, the committee finds that in this context, the relevant aspects of the problem are being studied effectively and a comprehensive body of scientific and engineering knowledge is being developed, and the committee has identified no obvious gaps in these important efforts.
FINDING: It is unlikely that a drop-in replacement agent will be discovered that will exhibit all of the beneficial properties of halon 1301 and not also exhibit a significant environmental impact .