PAPER

Paper from
the International CFC and Halon Alternatives Conference,
Washington, DC, Oct. 20-22, 1993

FLUOROIODOCARBONS AS HALON REPLACEMENTS

Jon Nimitz, Ph.D.
Environmental Technology & Education Center
3300 Mountain Road NE
Albuquerque, NM 87106-1920 USA
Phone:(505) 256-1463
And
Lance Lankford, P. E.
J.J. Research Labs, P. 0. Box 381
Sacramento, CA 95658 USA

Introduction
The authors have identified a small set of chemicals with outstanding physical properties, extinguishment abilities comparable to halons, low toxicity, clean evaporation, and low environmental impact. These chemicals appear to be prime candidates to replace Halon 1211 as streaming agents and Halon 1301 as total flooding agents for firefighting. These chemicals are in the class of fluoroiodocarbons (FICs). A FIC is a molecule that contains fluorine, iodine, and carbon; in some cases FICs also contain hydrogen. FICs have been shown to be highly effective fire extinguishing agents (in some cases, more effective than existing halons). Both neat FICs and blends of FICs with hydrofluorocarbons (HFCs) show great promise as high-efficiency halon replacements. This paper examines the physical properties, fire extinguishing abilities, toxicity, thermal stability, environmental effects, materials compatibility, manufacturability, and costs of FICs. Attractive blends with HFCs are also described. Tables 1 and 2 show some data on Halons 1211, 1301, and 2402 (reference compounds) plus candidate FICs and relevant inorganic compounds (HI and I₂).

Table 1. Identifying Data on Reference Compounds, Fluoroiodocarbon Candidates, and Relevant Inorganic Compounds

IUPAC NAME	OTHER NAME(S)	STRUCTURAL FORMULA	CONDENSED FORMULA	CFC NO.	CAS NO.
REFERENCE COMPOUNDS
bromochlorodifluoromethane	Halon 1211	CBrClF2	CBrClF2	12B1	353-59-3
bromotrifluoromethane	Halon 1301	CF3Br	CF3Br	13I1	75-63-8
1,2-dibromo-1,1,2,2-tetrafluoromethane	Halon 2402 Freon 114B2	CBrF2CBrF2	CBr2F4	114B2	124-73-2
FLUOROIODOCARBON CANDIDATES
trifluoroiodomethane	trifluoromethyl iodide iodotrifluoromethane	CF3I	CF3I	13I1	2314-97-8
pentafluoroiodoethane	iodopentafluoroethane iodoperfluoroethane perfluoroethyl iodide	CF3CF2I	C2F5I	115I1	354-64-3
difluoroiodomethane	iododifluoromethane	CHF2I	CHF2I	22I1	1493-03-4
1,1,2,2-tetrafluoro-1-iodoethane	iodo-1,1,2,2-tetrafluoroethane	CF2ICHF2	C2HF4I	124aI1	3831-49-0
1,1,1,2,3,3,3-heptafluoro-2-iodopropane	2-iodoperfluoropropane perfluoroisopropyl iodide	CF3CFICF3	C3F7I	217I1	677-69-0
1,1,2,2,3,3,3-heptafluoro-1-iodopropane	1-iodoperfluoropropane perfluoropropyl iodide heptafluoro-1-iodopropane perfluoro-n-propyl iodide	CF3CF2CF2I	C3F7I	22I1	1493-03-4
fluoroiodomethane	iodofluoromethane	CH2FI	CH2FI	31I1	373-53-5
1,1,2,2,3,3,4,4,4-nonafluoro-1-iodobutane	perfluoroiodobutane nonafluorobutyl iodide iodoperfluorobutane perfluorobutyl iodide perfluoro-n-butyl iodide	CF3CF2CF2CF2I	C4F9I	319I1	423-39-2
difluorodiiodomethane	diiododifluoromethane	CF2I2	CF2I2	12I2	1184-76-5
1-iodoundecafluoropentane	perfluoro-npentyl iodide	CF3(CF2)4I	C5F11I	4-1-11-I2	638-79-9
tridecafluoro-1-iodohexane	perfluoro-nhexyl iodide	CF3(CF2)5I	C6F13I	5-1-13-I2	355-43-1
RELEVANT INORGANIC COMPOUNDS
hydriodic acid	hydrogen iodide	HI	HI		10034-85-2
iodine		I2	I2		7553-56-2

Table 2. Physical Properties and Firefighting Effectiveness of Reference Compounds, Fluoroiodocarbons, and Relevant Inorganic Compounds

FORMULA	MW	BP °Ca	LIQUID DENSITY g/mLa	CUP BURNER EXTING. %b	REL. WT.	REL. VOL.
REFERENCE COMPOUNDS
CBrClF2 (Halon 1211)	165.37	-4	1.85	3.2	1.00c	1.00c
CF3Br (Halon 1301)	148.91	-58	1.50	2.9	1.00d	1.00d
CBrF2CBrF2 (Halon 2402)	259.82	47.3	2.16	2.1	0.82c	1.11c
FLUOROIODOCARBON CANDIDATES
CF3I	195.91	-22.5	2.36	3.0	1.36d	0.87d
CF3CF2I	245.92	12	2.085	2.1	0.98c	0.96c
CHF2I	177.92	21.6	3.24
CF2ICHF2	227.93	39.4 41-42
CF3CFICF3	295.93	38, 40	2.099	3.2	1.79c	1.74c
CF3CF2CF2I	295.93	40, 41	2.06	3.0	1.68c	1.66c
CH2FI	159.93	53.4	2.37
CF3CF2CF2CF2I	345.94	67, 68	2.01
CF2I2	303.81	80
CF3(CF2)4I	395.95	94	2.05
CF3(CF2)5I	445.95	117	2.05	2.5	2.11c	2.10c
INORGANIC COMPOUNDS
HI	127.91	127	1.701
I2	253.81	184	4.930 (solid)
aPhysical property data taken from Reference 2. Where two values are reported, both are listed. bFor heptane fuel fires; data averaged from References 3 through 8. cRelative to Halon 1211. dRelative to Halon 1301.

The ideal halon replacement has the following properties: effective three-dimensional firefighting ability, cleanliness, low toxicity, electrical non-conductivity, stability on storage, compatibility with engineering materials, low cost, zero ozone-depletion potential (ODP), short atmospheric and terrestrial lifetimes, and negligible global warming potential (GWP) (Ref 1). In addition, the combustion products of the agents should be no more toxic than those of Halons 1211 and 1301.

Physical Properties
Properties of interest include (as a minimum) boiling point, vapor pressure, vapor heat capacity, heat of vaporization, density, and viscosity. As a rough guideline, candidates with boiling points between -80°C and -10°C are deliverable as gases and can be considered as possible total-flooding agents, while those with boiling points between -10°C and 90°C are deliverable as liquids and can be considered as possible streaming agents. The optimal boiling point for a total flooding agent is estimated to be in the range of -70°C to -30°C to provide rapid distribution without use of extremely high-pressure equipment. The optimal boiling point for a streaming agent is estimated to be in the range of 10°C to 50°C to provide effective delivery as a liquid plus rapid knockdown.

Extinguishing Ability
Extinguishing agents can operate physically (through heat removal) and/or chemically (by terminating radical chain reactions that propagate flames). Halons owe their extinguishment effectiveness to a combination of physical and chemical mechanisms (Refs. 3 and 9). The physical mechanism is heat removal by molecular vibration, primarily by the vaporized agent. For effective physical extinguishment, a high heat capacity is desirable. In general, the larger the molecule the higher the vapor heat capacity and the greater the physical contribution to extinguishment.

The chemical mechanism involves termination of radical reactions. The presence of a bromine atom contributes substantially to chemical extinguishment (Refs. 3 and 9). One of us (JN) has found in unpublished work that an iodine atom supplies approximately 70% of the extinguishing effectiveness of a bromine atom; the effect of chlorine is negligible. The presence of either a bromine or iodine atom greatly enhances extinguishing effectiveness, and appears necessary for a halocarbon agent to have extinguishment capabilities equal or superior to those of existing halons.

Recent Army tests on simulated tank engine compartment fires showed that, when the normal 3-lb bottle of Halon 1301 was replaced with CF₃I, the fire was successfully extinguished using 2.8 lbs of CF₃I at 550 psi (Ref 10). The test results depended greatly on delivery pressure. Our analysis is that these results tell us three things:

1. With proper delivery, CF₃I is a highly effective extinguishant, even more than expected on the basis of cup burner tests (which predict that 40% more by weight is needed than Halon 1301),
2. CF₃I is more difficult to deliver than Halon 1301, because of its lower volatility, and
3. the lower volatility makes near-azeotropic blends with HFC propellants attractive to avoid the need for high-pressure systems.

Optimization of nozzles and pressure will also be needed.

Attractive Blends with Hydrofluorocarbons
Blends of agents must be considered carefully, because by appropriate choice of components many properties such as boiling point, vapor pressure, toxicity, and effectiveness can be modified to more desirable values. In addition, it is often found that two agents exhibit synergistic extinguishment effects (better extinguishment than expected from a linear interpolation of effectiveness of the two components). Such synergism may occur because one agent is acting primarily chemically while one is acting primarily physically. Azeotropic blends would be particularly attractive because they do not change composition on evaporation or handling, and have more predictable properties than other blends.

As blending agents for FICs in firefighting applications, HFCs are judged the most attractive group because of their reasonable firefighting effectiveness, high availability, low toxicity, and low cost. Perfluorocarbons, while very effective firefighting agents and extremely nontoxic, are judged less attractive because of their high global warming potentials. Fluoroethers show similar extinguishment abilities to HFCs but are unavailable, are expensive to produce, have little toxicity data available, and sometimes show serious thermal instabilities.

The limited information available regarding the acute toxicity of the candidate FICs is highly encouraging. In one study no lethality was observed during a 2-hour exposure of mice to 250,000 ppm by weight of CF₃CF₂CF₂I (Ref 11). In another report, a dog was exposed to a 910,000 ppm concentration of CF₃I by weight (91% by weight or 50% by volume) for 30 seconds. It survived and exhibited no signs of anesthesia (Ref 12). These results indicate extremely low acute toxicity. Blending FICs with low-toxicity agents such as BFCs would further decrease any chronic or subchronic toxic effects that might eventually be found.

The combustion products of the FICs are similar to those from current halons, with the exception that HI is formed instead HBr. Although virtually no quantitative data on HI toxicity has been reported in the literature, its properties are very similar to those of HBr and toxic effects are expected to be similar. Both HI and HBr are strong acids and irritate tissues. Long-term health effects of HI may well be less than those of HBr because the human body uses and excretes iodine efficiently. Iodine in the form of iodide (as it occurs in HI) is in fact a necessary nutrient for humans. In contrast, the human body does not metabolize bromine.

Thermal decomposition occurs when molecular vibration (caused by heat) stretches the weakest bond in the molecule until it breaks. Carbon-to-iodine bonds are weaker than carbon-to-bromine bonds, and therefore fluoroiodocarbons are less stable thermally than the corresponding bromofluorocarbons. Typical C-Br bond strengths are in the range of 60-70 kcal/mole, while typical C-I bond strengths are in the range of 50-60 kcal/mole. The C-I bond strength in CF₃I is 54 kcal/mole, compared to the C-Br bond strength in CF₃Br of 63 kcal/mole (Ref 13). Lower bond strength means greater reactivity.

Although iodinated compounds are in general more reactive, more toxic, and less stable chemically than brominated compounds, the presence of fluorine atoms bonded to the carbon atom attached to the iodine atom provides substantial additional stability and greatly decreased toxicity. The presence of strongly-bonded, highly electron-withdrawing fluorine atoms on the carbon atom bonded to iodine prevents both common mechanisms of reaction for iodocarbons from occurring. Steric hindrance(physical blocking by the fluorine atoms) prevents back-side attack by nucleophiles (S_N2 or substitution, nucleophilic, bimolecular mechanism) and the increased C-I bond strength prevents unimolecular bond-breaking (S_N1 or substitution, nucleophilic, unimolecular mechanism). Published studies on thermal decomposition of CF₃I indicates that the compound is very stable (Refs. 14 and 15). In another study it is reported that CF₃I is stable in contact with metals up to 170°C (340°F) (Ref 16). An additional indication of stability is that many fluoroiodocarbons can be purchased from vendors and stored without unusual precautions such as refrigeration.

Environmental Properties
In order to achieve an ODP of zero or nearly zero, three approaches can be taken. An alternative chemical must either

1. contain no ozone-depleting elements;
2. contain at least one weakly-bonded hydrogen atom in the molecule; or
3. absorb ultraviolet radiation in the wavelengths found in the troposphere and undergo rapid photolytic destruction.

The photolytic lifetime of CF₃I can be estimated based on the known photolytic lifetime of methyl iodide, CH₃I. The C-I bond strengths of CH₃I and CF₃I are virtually identical, and CF₃I would have a larger photolytic cross-section because of the replacement of fluorine for hydrogen atoms, so the lifetime of CF₃I would be shorter than that of CH₃I. Methyl iodide has a photolytic lifetime of 5 days at sea level (Refs. 17 and 18). The fraction of CF₃I released at ground level that would reach the stratosphere can be estimated, assuming an upper limit of the photolytic lifetime of 5 days and a conservatively low average transit time of 3 months (90 days) to reach the stratosphere. Since the transit time is more than eighteen photolytic lifetimes, the fraction surviving is less than 1/e^l8, which is 0.00000001 or 0.000001%. This is a conservative(high) estimate for several reasons. First, the higher electron density of CF₃I should give it a greater photolytic cross-section than CH₃I, leading to more rapid photolysis. Second, because the intensity of sunlight increases with height the photolytic lifetime will decrease as a molecule rises. And third, the transit time may well be six months to two years, rather than three months. If the average photolytic half-life is 3 days and the transit time is one year, the fraction reaching the stratosphere would be 1/e^(365/3) = 1 x 10^-53. Thus any reaction of iodine from FICs released at ground level with stratospheric ozone will be negligible. Although there is evidence that iodine that contacts ozone reacts with it, iodine from FICs released at ground level will never reach the stratosphere in any measurable quantities.

Tropospheric (ground-level) ozone is one of the most dangerous components of smog. Addition of iodine to smog at levels near 0.1 ppm has been shown to decrease levels of undesirable tropospheric ozone (Refs. 18-20). A series of eye tests showed that the iodized atmospheres were less irritating to eyes than the control atmospheres. Thus it appears likely that FICs are "anti-VOCs" and that their release will improve air quality in urban areas. In fact, the possibility arises that FICs could be used to remediate high tropospheric ozone levels. FICs therefore provide the best of all possible worlds: they will destroy "bad" tropospheric ozone while leaving the "good" stratospheric ozone intact.

Materials Compatibility
Materials compatibility is a major concern with any new chemical. Agents should ideally be compatible with materials used on existing firefighting systems. These materials include, for example, the sealing materials (e.g., neoprene, polychloroprene, Buna-N-acrylonitrile, butadiene) and the metals used in distribution piping (copper, steel, stainless steel). Because none of the agents under consideration in this paper are corrosive or possess high solvency, it is anticipated that selection of compatible materials will not pose an obstacle.

It has been reported that CF₃I does not react with aluminum, alloys of aluminum with magnesium or zinc, or mixtures of aluminum with lead and thallium up to about 170°C (Ref. 16).

Manufacturability and Costs
At this time, FICs are only available in research quantities (up to a few pounds) and cost about the same as HFC-134a in research quantities (on the order of $1/g). However, as has been the case for HFC-134a, chemicals drop in price dramatically (by factors of 30 or more) as larger quantities are produced. HFC-134a is now available in bulk at about $6/lb and it is anticipated that the ultimate prices of FICs will be in the range of $5 to $20/lb. FICs are relatively simple molecules, and several synthetic routes are available. Blending with BFCs would also lower costs of FIC-containing agents.

Top Candidates for Total Flooding
The only gaseous FIC (when delivered from a pressurized container with the attendant cooling) is CF₃I. Therefore the top candidates for total flooding all contain CF₃I. It can be considered as a neat agent, in blends with HFC-134a and/or HFC-227ea, and may be propelled with HFCs 23, 32 and/or 125.

From the data in Table 3, it is apparent that any blend of these agents will require a weight from about 1.2 to 2.5 times the weight of Halon 1301. The minimum weight agent would be a blend of about 50% CF₃I and 50% CH₂F₂, with a relative weight of 1.19 and relative volume of 0.84. This agent is predicted to have the best per-weight and per-volume performance, assuming that published data on the firefighting effectiveness of CH₂F₂ are correct. Because of the reported flammability of CH₂F₂ at some concentrations in air, it may not be safe or effective to have more than about 50% in a firefighting blend. However, it is possible to design blends that are isovolumic with Halon 1301. An isovolumic agent is one with the same firefighting effectiveness per unit volume as Halon 1301. The mole percentages of components in a blend of CF₃I and HFC-134a that is isovolumic with Halon 1301 can be determined by equation (1), where X represents the mole fraction of CF₃I.

               0.86 X + 1.92 (1.00-X) = 1.00                                        (1)

Solving equation (1) for X yields 87%. Thus an agent composed of 87% (by moles) CF₃I and 13% HFC-134a would have a relative volume of 1.00. This agent would have a relative weight given by equation (2).

               1.36 * 0.87 + 2.40 * 0.13 = 1.50                                     (2)

Thus there would be no change in volume and a 50% increase in agent weight. In other words, the quantity of a blend of 87% CF₃I and 13% HFC-134a needed to extinguish a particular fire would have the same volume but weigh 50% more than the amount of Halon 1301 required. This blend may be an attractive candidate for a drop-in replacement in existing Halon 1301 systems. Similar calculations on blends with HFC-23, HFC-125, HFC-227ea, and perfluoropropane yield the results shown in Table 4. It should be kept in mind that these are not necessarily the top candidate Halon 1301 replacements in terms of effectiveness, physical properties, or cost; they are just possible "drop-in" candidates.

Table 3. Extinguishment Data for Top Candidate Components of Total-Flooding Firefighting Agents.

CHEMICAL	BOILING POINT, °C	CUP BURNER GAS VOL %A	REL. WT.b	REL. VOL.B
CFIBrc	-58	2.9	1.00	1.00
CF3I	-22.5	3.0	1.36	0.86
HFC-23	-82	12.4	1.94	1.97
HFC-32d	-52	8.8	1.02	0.82
HFC-125	-48.5	9.4	2.53	2.07
HFC-134a	-26.5	10.5	2.40	1.92
HFC-227ea	-17	5.9	2.25	2.13
perfluoropropane	-36	6.1	2.57	2.31
aPercentages by gas volume to extinguish n-heptane fires, also equal to mole percent. Data are averages of published values taken from References 3 through 8. bCalculated as described in Reference 1. cReference compound. dFlammable at some concentrations in air; could only be minor component of blends.

Table 4. Blends Isovolumic with Halon 1301

BLENDING AGENT	MOLE FRACTION CF3I	MOLE FRACTION BLENDING AGENT	REL. WT.	AZEOTROPE OR NEAR-AZEOTROPE?
HFC-23	0.93	0.07	1.40	no
HFC-125	0.93	0.07	1.44	no
HFC-134a	0.87	0.13	1.50	yes
HFC-227ea	0.93	0.07	1.42	yes
perfluoropropane	0.94	0.06	1.43	yes

Top Candidates for Streaming
The top candidates for streaming applications at this time appear to be CF₃CF₂I (bp 12°C) and CF₃CF₂CF₂I (bp 41&3176;C), and blends of these agents with low-boiling liquid HFCs. Both of these FICs have demonstrated outstanding extinguishment and have physical properties intermediate between those of Halon 1211 and Halon 2402.

The firefighting effectiveness of top-ranking components of total-flooding blends is shown in Table 3.

The authors have developed a proprietary computer program, called AZEO, that calculates properties of mixtures and predicts azeotrope formation and composition (Ref. 1). The AZEO program reproduces properties of all known azeotropes tested (such as R-500 and R-502) within 1% accuracy. AZEO runs on a PC, works for up to five-component mixtures, and allows a choice of units. It identifies probable azeotropes, near-azeotropes, and non-azeotropes. For azeotropes and near-azeotropes, it gives the approximate azeotropic composition. It calculates vapor pressure curves and gives enthalpies of vaporization and specific heats of liquid and vapor as functions of temperature. AZEO provides pressure-volume-temperature data with an accuracy within 1% and enthalpies of vaporization within 2%. AZEO is only a tool for initial screening to identify attractive blends and possible azeotropes; all results obtained from AZEO must be validated by laboratory measurements.

Toxicity and Thermal Stability
For toxicity and stability considerations, FICs containing a terminal iododifluoro group (-CF₂I) are preferred over other molecular arrangements. If the iodine atom is on a primary carbon atom (a carbon atom attached to only one other carbon atom) the C-I bond is stronger than if the iodine atom is on a secondary or tertiary carbon. By having two (or, in the case of CF₃I, three) fluorine atoms on the same carbon atom as the iodine the C-I bond is further strengthened and reactions are inhibited.

Other Uses of Fluoroiodocarbons
The potential usefulness of FICs is not limited to firefighting agents. FICs can impart nonflammability to refrigerants, solvents, foam blowing agents, hydraulic fluids, and aerosol propellants. The present authors have already demonstrated a blend of CF₃I and HIC-152a as a nonflammable drop-in replacement for R- 12 in a refrigerator, without changing the oil. This instrumented refrigerator has been in constant operation for over 1500 hours with no operational problems, and is showing equal or slightly better energy efficiency and refrigeration capacity compared to R-12.

A companion paper to this one on uses of FIC-containing blends as refrigerants appears in this conference proceedings under the title "A New Class of High-Performance, Environmentally Sound Refrigerants," by Lance Lankford and Jon Nimitz.

Further Research Needed
The remaining validation work consists of additional testing in four areas: effectiveness (laboratory and large-scale), toxicity, long-term stability, and materials compatibility. Additional laboratory testing will be needed to select the optimal agents, optimize blends, and determine azeotropic compositions. Large-scale testing will be needed to validate the agents and determine optimal delivery pressures and nozzles. Additional acute, chronic, and subchronic toxicity testing will be needed. Acute toxicity testing can be conducted under an accelerated schedule in about 90 days. Thermal stability and materials compatibility will also need to be tested in the laboratory.

Conclusions and Recommendations
Several of the FICs described in this paper are highly effective, clean firefighting agents with negligible environmental effects and high probabilities of acceptable toxicity, thermal stability, and materials compatibility. At this time FICs appear to be the only candidate halon replacements possessing all these desirable properties. For much less than the cost of one F-16 jet or Abrams tank, these agents can be screened, optimized, and validated for general use.

References

1. Nimitz, J.S. "The Ultimate Halon Replacements are in Sight," Proceedings of the 1993 Halon Alternatives Working Conference, University of New Mexico, Albuquerque, NM, 11-13 May, 1993.
2. Bellstein Online Database produced by the Beilstein Institute for Organic Chemistry, Frankfurt, Germany, 1993, accessed through STN International, Columbus, OH.
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7. Moore, T.A., Moore, J.P., Nimitz, J.S., Lee, M.E., Beeson, H.D., and Tapscott, R.E., "Alternative Training Agents, Phase II -- Laboratory-Scale Experimental Work," ESL-TR-90-39, Vol. 2 of 4, Air Force Engineering and Services Laboratory, Tyndall Air Force Base, Florida, August 1990. NMERI OC 91/1.
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18. Chameides, W.L., and Davis, D.D., "Iodine: Its Possible Role in Tropospheric Photochemistry," Journal of Geophysical Research, Vol. 85, 1980, pp. 7383-7398.
19. "Iodine May Be Smog Weapon," Chemical and Engineering News, Vol. 40, No. 39, 1962, p. 68.
20. Hamilton, W.F., Levine, M., and Simon, E., "Atmospheric Iodine Abates Smog Ozone," Science, Vol. 140, 1963, pp. 190-191.

For more information regarding this subject, please contact Dr. Nimitz at mailto:jnimitz@etec-nm.com