TROPODEGRADABLEHALOCARBONSAND 

MAIN GROUP ELEMENT COMPOUNDS 

J. Dougla5 Mather and Robert E. Tapscott 

Center for Global Environmental Technologies 

The University of New Mexico 

Albuquerque, NM X7 106-4339 USA 

INTRODUCTION 

Although it is not certain that a “drop-in” chemical substitute (as opposed to a new technology) 

for halons that can be used in normally occupied areas will ever be identified. there are two 

promising paths to such an agent. The first is to target bromocarbons with very short atmospheric lifetimes. We refer to these materials as “Tropodegradable Halocarbons.” The second 

path is to develop non-halocarbons-chemicals that are unlike the classic halocarbon agents used 

in the past (halons) and those halocarbons now being commercialized (HFCs, HCFCs, and 

perfluorocarbons). We refer to these non-halocarbon materials as “Compounds of Main Group 

Elements.” Herein we discuss projects sponsored by the Department of Defense Strategic 

Environmental Research and Development Program (SERDP) under the Next-Generation Fire 

Suppression Technology Program (NGP) to explore these two paths. The initiation of this work 

was discussed at the Halon Options Technical Working Conference 1998 [I]. 

TROPODEGRADABLEHALOCARBONS 

Overview 

One approach to halon substitutes is to keep bromine in the molecule, but add features that 

decrease the tropospheric lifetime. The resultant compounds are said to be “tropodegradable,” a 

term used by Dr. Jon Nimitz. A very short tropospheric lifetime can solve two problems at once. 

First, if the lifetime is sufficiently short, ozone depletion can be negligible, even when bromine is 

present. Second, a short lifetime means a low global warming potential. Research work has 

established the potential of tropodegradable bromocarbons as fire suppressants: however, insufficient flame extinguishment experiments have been carried out to relate performance and (for 

some compounds) flammability to structure. This project was established, in part, to address the 

need for laboratory-scale fire extinguishment information for tropodegradable compounds. 

Four primary processes exist for the removal of organic molecules from the troposphere: 

(I) reaction with atmospheric hydroxyl free radicals: (2) photolysis: (3) physical removal: and 

(4) reaction with tropospheric ozone [2]. Hydroxyl free radicals, which are found in low 

concentrations throughout the atmosphere, are nature’s way of cleaning the troposphere. These 

highly reactive species react with many atmospheric contaminants converting them to chemicals 

that are rapidly eliminated. The reaction is slow for most halocarbon contaminants: however, 

chemical features can speed up the reaction with hydroxyl free radicals. Photolysis is nothing 

more than decomposition by sunlight. Only a few types of chemicals undergo significant 

photolysis. Rainout is the most common physical removal process. Finally, tropospheric ozone 

132 Halon Option* Tcchnical Working Conterence 21-29 April I9YY 
can rcact with a few molecules to remove them from the atmosphere. Though chemically 

identical, troposphcric ozonc and stratospheric ozone play entirely different roles in the 

atmosphcre. Stratospheric ozone is welcome since it protects the earth from ultraviolet radiation. 

Tropospheric ozone. on the other hand, is an environmentally harmful contaminant. Removal by 

reaction with tropospheric ozone is actually the reaction of one contaminant with another. 

Various substituents can be added to molecules to enhance each of the tropospheric removal 

processes nientioncd hcrc. 

Tropodegradable compounds that we consider the most promising ;ire bromine-containing 

tluoroethers, tluoroalkyl amines, and fluoroalkenes. This paper provides some information on 

the ethers and the amines: however, the alkenes are emphasized. 

Bromofluoroalkcnes contain carbon-carbon douhle bonds. These materials react rapidly with 

both hydroxyl frec radicals and tropospheric ozone. The atmospheric lifetimes arc about one 

day. in some cases less. This compares with atmospheric lifctimes of 65 years for Halon 1301 

and 20 years for Halon 121 I. The commercialized halon replacements havc even longer atmospheric lifetimes (except for CF21). 

Some of these materials look remarkable good. Some examples of extinguishment concentrations determined by cup burner with n-heptane fuel are shown below (Table I). These concentrations arc all lower than most of the commercialized halocarbon rcplaccments (one exception 

being CF31), and are comparable to the extinguishing concentration of approximately 3 vel.%, for 

Halon I30 I. 

TABLE I. EXAMPLES OF EXTINGUISHMENT CONCENTRATES. 

Agent n-Heptane Extinguishment Concentration, 

vel.% 

CH?=CHCBrF? 4.50 

CHz=CBrCF? 2.55 

CH2=CHCF2CBrF2 3.50 

Under sponsorship of the US Air Force and the Advanced Agent Working Group (AAWG), 

toxicity evaluations have recently been carried out on eight hromotluoroalkenes. Ten (IO) rats 

were exposed for 30 min at 5 vel.%. The results are shown in Table 2. 

Compounds 873 and 903 appeared to be the most innocuous in this test. Both compounds 

induced a degree of ancsthesia during exposure, and animals awoke following the cxposure 

without any apparent toxicity or lung impairment. Post-exposure sleeping time was slightly 

greater for Compound 903 compared with 873. 

Although not lethal under these conditions. Compound 707 had apparent nervous system effects 

as evidenced by tremors during exposure and abnormal muscular contractions postmortem. 

Exposure to Compound 707 did not result in any lung impairment. and respiration was normal 

during the exposure. Compound 1 I16 was also nonlethal, but animals did not achieve a normal 

clinical appearance after exposure, and some lung toxicity was associated with exposure. 

Hxlon Oplions Technical Worhins Conferencc 27-24 April I999 133 
TABLE 2. TOXICOLOGICAL RESULTS FOR EXPOSURE. 

CCOD Formula Number of Rat Lung Tissue Observations 

ID Deaths 

873 CH?=CBrCF3 0 All rat tissues normal 

707 CHBr=CHCF3 0 All rat tissues normal 

872 CHZ=CHCBrF? 7 At necropsy lungs were mottled 

903 CH2=CHCF?CBrF2 0 All rat tissues normal 

1 116 CHZ=CBrCF2CF3 0 At necropsy lungs were mottled 

1360 CH?=CBr(OCF3)CFCF3 10 At necropsy lungs were mottled 

1358 CH?=CBrCFZCFZCF3 I (post exposure) At necropsy lungs were mottled 

1359 CH?=CBrCFC(CFh IO At necropsy lungs were mottled 

Compound Selection and Acquisition 

Approximately 70 tropodegradable bromofluoro-alkenes, -amines and -ethers with potential as 

Halon 130 1 replacements have been identified. These organobromine compounds are estimated 

to have acceptably low Ozone Depletion Potential (ODP) and Global Warming Potential (GWP). 

Twenty-five (2.5) alkenes representing highly fluorinated with a single vinyl bromine or bromine 

substitution on saturated carbons well removed from the double bond were identified. Some of 

the alkenes initially considered have fluorine and bromine substitution on unsaturated carbons. 

While compounds with this substitution pattern may exhibit higher cup-bumer values and 

possibly increased toxicity, actual testing is required to determine their potential as halon 

replacements. 

The eight bromofluoroalkenes listed in Table 2 and the two additional bromofluoroalkenes, 

1 -bromo-3,3,3-trifluoro-2-trifluoromethyl propene (CF&C=CHBr and 1 -bromo-2,2,3,3,3- 

pentafluorobutene (CF?CF?CH=CHBr), more recently acquired all have no substitution by 

fluorine on the carbon-carbon double bond. Cup-bumer flame extinguishment values for all of 

the alkenes are expected to be less than 5% and atmospheric lifetimes may well be on the order 

of a few days. 

A list of 24 bromofluoroamines was developed based on estimates of flame extinguishment. 

physical properties, toxicity and environmental performance. From this a list of 8 promising 

candidate compounds was identified and targeted for synthesis. 

Amine synthesis is perhaps the most difficult challenge. A facile method based on the preparation of perfluoro-2-azapropene with subsequent bromination to form bromobis(trifluoromethy1)- 

amine enables preparation of a series of compounds has been identified and is being attempted, 

(Reaction I). By substituting a variety of fluoroethenes (e.g., CHF=CH*, CF?=CHz, CHF=CHF, 

CF2=CHF) for the ethylene in Reaction I, a series of new tropodegradable compounds can be 

obtained. 

(CF3)zNBr + CH2=CH2 + (CF3)2NCH2-CHZBr (1) 

I34 Halon Options Technical Working Conference 27-29 April 19YY 
A list of 20 hromotluoroethers was developed based on estimates of their tlame extinguishment, 

physical properties. toxicity and environmental performance. Work on the synthesis of bromotluoroamines is currently in progress under the direction of Dr. Jean’ne Shrceve of the University 

of Idaho (USA). Efforts to synthesize bromofluoroethers are currently underway in the NMERI 

1;rhoratory 

Summary 

This research has demonstrated the potential of tropodegradablc bromofluoroalkenes to replace 

Halon 1301 (as well as Halon 121 I). Efforts to characterize further the toxicity and fire suppression performance of selected compounds are in the planning stagc. Based on the results observed 

to date. continued research, focused on the acquisition and testing of new tropodegradahle 

hromotluoroalkenes, is warranted. 

COMPOUNDS OF MAIN GROUP ELEMENTS 

Overview 

Most studies to date on chemical substitutes foi- halons have focused on halocarbons. Some 

limited past work, however. has looked at chemicals containing silicon, phosphorus. and 

nitrogen, i.c., materials based on chemical elements other than carbon [3]. The present pro,jecl 

expands that past work to ensure that no promising non-halocarbon candidate substitutes are 

overlookcd. 

The tcnn “main group elements” denotes elements other than transition metals. This pro-ject, 

however, cxcludcs the main group of families-halocarbons, amines, ethers, and silicon compounds that have been examined extensively elsewhere. One class of non-halocarbon agents has 

long been used for fire and explosion protectiondry powder agents such as potassium 

bicarbonate and monoammonium phosphate. The present project. however, is restricted to 

liquids or gases. 

With one major exception. the rcsults of the search have been disappointing. Few materials with 

potentially acceptable physical (primarily, volatility) and toxicological properties havc been 

idcntificd. The lowest molecular weight materials (i.e., the materials with the highcst volatilities) 

oftcn have hydrogen or halogen atoms directly attached to noncarhon atoms. The fomer 

(containing hydrosen) are often tlammable, and the latter (containing halogens) are usually toxic 

(owing to hydrolysis). Both often have low stabilities. The major exception is that tluoroalkyl 

phosphorus compounds, though having a relatively low volatility, continue to hold significant 

promise. 

In the remainder of this paper, two areas of major interest in this study are examined: volatility 

(evaporation) and phosphorus compounds. 

Halon Options Technical Working Conlcrencr 21-20 April I999 I35 
Evaporation 

Compounds that are not solids at room temperature are targeted under the present project. Most 

of the compounds identified as having potential extinguishment capabilities are liquids with 

relatively high boiling points. A major criterion for such liquids is an ability to evaporate and fill 

a space to a required concentration within the time required for extinguishment. Thus, evaporation properties must be taken into account during compound review and selection. Areas to be 

considered are evaporation equilibrium and evaporation rate. Only the first are looked at herein. 

The question of whether a given compound can achieve the required extinguishment concentration upon discharge, given sufficient time for evaporation to occur, is easily answered since only 

thermodynamics are involved. The logarithm of the ratio of the vapor pressure (p) at two different temperatures T? and TI can be estimated from Equation 1 (Clausius-Clapeyron equation), 

where R is the ideal gas constant and AH,,, is the heat of vaporization. This equation is based on 

the assumption that (I) AHvap is temperature independent and (2) the vapor is an ideal gas. The 

ratio of AHvap/Tbr where Tb is the boiling point temperature, is equal to the molar entropy of 

vaporization ASvilp at the boiling point. For closely related compounds, AS,,, at the boiling point 

is nearly constant with a value of L, Trouton’s constant (Equation 2). Thus, by selecting TI as 

the boiling point (Tb) at, for example, 1 atmosphere pressure (it., pl = 760 Torr), one can 

estimate the vapor pressure at some other temperature and develop an approximate vapor 

pressure/temperature curve, if t, is known. 

The two equations can be combined to simplify the estimation. Thus, for a boiling point Tb in K 

at a pressure Pb (Equation 3), where Trouton’s constant tc in units of J/K-mol is a variable to be 

determined for a particular family of compounds, is obtained. 

pZ = Pb10”*(0.0522106S)t,[ I-(T,/Tz)] [31 

For example, if is taken to be equal to 88 J/K-mol, which holds for many compounds, one can 

estimate that the vapor pressure at 25 “C for a compound with a boiling point of Tb = SO “C at 

760 torr is 3 13 torr. From vapor pressure values, one can calculate the maximum concentration 

possible. For one atmosphere total pressure, the maximum volume fractions are p2/760 for p2 in 

torr. Of particular interest is the maximum allowable boiling point (Tb) that can achieve a given 

concentration (C) for various ambient temperatures, Tz. Equation 4 gives the calculation for 

boiling points at Pb = 760 torr, where temperatures are in K and tc is in J/K-mol. Table 3 gives 

the estimated maximum boiling point that can achieve concentrations from 1 to 20 vol.% for a 

Trouton constant of 88 J/K-mol. The data are plotted in Figure I (some data have been omitted 

to simplify the figure). 

Tb = [ 1 -(Log(760C/l 00Pb)/0.05221 06StC)]T2 = [ I-(LOg(C/I 00)/0.0S22106Stc)]T7 [41 

136 Halon Options Technical Working Conference 27-29 April 1YY9 
TABLE 3. ESTIMATED MAXIMUM BOILING POINT FOR GIVEN CONCENTRATION. 

Conc. Ambient Temperature. "C 

% -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 

1 33 47 61 76 90 105 119 133 148 162 176 191 

2 

3 

4 

5 

6 

7 

8 

9 

I0 

II 

12 

13 

14 

15 

16 

17 

18 

I9 

19 

II 

5 

0 

-3 

-6 

-9 

-1 I 

-14 

-16 

-17 

- I 9 

-20 

-22 

-23 

-24 

-25 

-27 

33 

24 

18 

13 

9 

6 

3 

I 

-I 

-3 

-5 

-7 

-9 

-10 

-I I 

-13 

-14 

-15 

46 

37 

31 

26 

22 

19 

16 

13 

11 

9 

7 

5 

3 

2 

0 

-1 

-2 

-3 

60 

51 

44 

39 

35 

31 

28 

25 

23 

21 

19 

17 

15 

14 

12 

II 

9 

8 

7 

74 

64 

57 

52 

47 

44 

40 

38 

35 

33 

31 

29 

27 

25 

24 

22 

21 

20 

87 

77 

70 

65 

60 

56 

53 

50 

47 

45 

43 

41 

39 

37 

36 

34 

33 

31 

101 

91 

83 

77 

73 

69 

65 

62 

59 

57 

55 

53 

51 

49 

47 

46 

44 

43 

1 15 

I04 

96 

90 

85 

81 

78 

74 

72 

69 

67 

65 

63 

61 

59 

57 

56 

54 

I28 

I17 

I09 

I03 

98 

94 

90 

87 

84 

81 

79 

77 

74 

73 

71 

69 

68 

66 

I42 

I30 

I22 

1 I6 

Ill 

I06 

102 

99 

96 

93 

91 

88 

86 

84 

83 

81 

79 

78 

1 56 

144 

I35 

129 

123 

I I9 

I 15 

111 

I08 

I 05 

103 

I00 

98 

0 6 

94 

92 

91 

89 

I69 

157 

148 

I42 

I36 

13 I 

I17 

124 

120 

I I7 

115 

I12 

I 10 

I 08 

I06 

I04 

I02 

101 

0 

II 

20 -28 -16 -5 19 30 42 53 65 76 88 99 

This analysis shows that one that a compound with a boiling point of over 100 "C can likely 

achieve a concentration of 5 vol.% at normal room temperature (25 "C). Whether such a 

compound can achieve such a concentration in a reasonable amount of time is, however, a quite 

separate question. 

Phosphorus Compounds 

Compounds of phosphorus have been of particular interest [4]. Phosphorus compounds appear 

to provide a chemical fire extinguishment mechanism, just like the bromine-conlaining halons, 

and some appear to be highly effective. Much work has been 

HC /'.- \ OCH, 

OCH 3 

3 

DMMP 

performed on dimethyl methyl phosphonate (DMMP) and 

related compounds by Professor Elizabeth Fisher and her coworkers at Cornell [5.6]. Though DMMP shows very promising 

flame suppression characteristics. this compound and most of 

the related compounds studied to date have some serious 

drawbacks. First, the material is tlammahle (it is actually 

possible. though not practical, to extinguish flames with a 

flammable material). Second, most compounds studied to date 

have low vapor pressures and are not very volatile. 
200 

I 90 

I80 

170 

160 

1 so 

140 

5 120 

130 

g iio 

100 

.- 90 

.- 

.- -

0 

m xo 

2 70 

.E 60 

y. 5 so 

40 

30 

20 

10 

0 

-in 

-20 

-30 

-ho -so -40 -30 -20 -in n 10 20 30 40 50 

Ambient Temperature, "C 

Figure 1. Estimated maximum boiling points that can achieve given concentrations as a 

function of ambient temperature (simplified), calculated for t, = 88 JK-mol. 

One way to address both of these problems is to work with phosphorus compounds containing a 

fluoroalkyl group-substituents with fluorine replacing part or all of the hydrogen atoms. The 

presence of fluorine decreases or eliminates flammability and increases volatility. For example, 

tris(2.2.2-trifluoroethyl)phosphite (TFEP) has a 

lower boiling point than DMMP and is nonexcellent fire suppressant. We have determined 

that the concentration required to extinguish an 

n-heptane flame is I .78 + 0.04 vol.%. This can 

be compared with the concentration of 

approximately 3.2 vol.% for Halon 1301. 

flammable. Moreover, the material is an / 'L. OCH2CF, CF,CH,O 

OCH2cF, 

TFEP 

138 Halon Options Technical Working Conference 21-29 April 1999 
Heat extraction experiments also indicate an excellent extinguishing ability for TFEP (Figure 2). 

In these experiments, the heat extracted by a burner top from a premixed methane flame is 

measured as a function of added TFEP. As the flame approaches extinguishment, less and less 

heat is extracted. A distinct (and, as yet, unexplained) break occurs in the curve. To the left of 

the break, the fitted straight line has a slope of -107 cal/cc CHI. Extrapolation 10 21 zero heat 

output gives a predicted extinguishment concentration of 0.008 mole fraction TFEP (0.8 vel.%). 

To the right of the break the fitted straight line has a slope of -50 cal/cc CHI and gives an 

extrapolated extinguishment mole fraction of0.01.5 (I .S vel.%). 

01 

m 

2 0.8 

a 

2 0.6 

a 

0 

0 0.4 

-i 0.2 

E 

. 

0 

- 

- 

a 

N .- 

L 

zoo 

0 0.001 0.002 0.003 0.004 0.005 

X. Mole Fraction TFEP 

Figure 2. Heat absorption curves for methane llanie with TFEP. 

Figure 3 plots the heat abstraction for a fixed concentration of TFEP in a premixed methane 

llame as a function of the fuel/air ratio @. For a stoichiometric flame. @ is I .OO. Higher values 

of@ indicate a fuel-1-ich flame: lower values, a fuel lean flame. The plot is symmetric. which 

indicates that the TFEP is not acting as a fuel in the flame. 

There is one major problem with TFEP. Though the volatility is 

reason, the synthesis of tluoroalkyl phosphorus compounds with 

lower molecular weights should he undertaken. For example, 

tris(tril1uorometliyl)phosphite (TFMP), which should have a 

relatively high volatility, is particularly promising. A major 

never been made! Though 

PI OCF improved. it is still not nearly what one would like. For that 

CFO/ h 3 

OCF, 3 

0 

II 

TFMP problem exists-TFMP has 

this appears to he a severe drawback. there is no reason to /'-. OCF 3 

believe that it cannot be made. The very closely related 

compound, tris(trifluoromethy1)phosphate has. in fact, been 

prepared, though, as yet, it is untested, and has a boiling 

point of52 "C [7]. This boiling point is a little high; 

however, there is good reason to believe that it could he 

acceptable. The unknown compound TFMP is expected to 

have a lower boiling point. 

CF,O OCF, 

Tris(trit1uoromethvl)uhosDhate 

H;ilon Options Technica Working Conlcrcnue 27-29 April I999 139 
1.6 

Gi 

I 1.4 ~ 

0 

0 

U 

0 

% 1.2 

z 

m 

- 

E 1. ~ 

.6 

... e .6 

m .4 

.2 

m 

a, 

u 

.. 

x 

W 

m 

I 

I 

.6 .7 .6 .9 1. 1.1 1.2 1.3 1.4 1.5 

PHI 

Figure 3. Heat extraction from methane flame with TFEP versus fuel/air ratio (@). 

The work to date indicates that, of all of the non-halocarbon and non-metal compounds, phosphorus-containing derivatives hold the most promise as halon substitutes. Little is still known 

about toxicity, and this is always of major concern. Also, the volatility and discharge properties 

may not be ideal. We have been working on ways to increase discharge rates, and it may be 

possible to use these materials as aerosols or as blends with appropriate camers. 

ACKNOWLEDGMENTS 

This work was sponsored, in part, by the Department of Defense Next-Generation Fire 

Suppression Technology Program, funded by the DoD Strategic Environmental Research and 

Development Program (SERDP) under Contracts HQ0038-7350-0009 and HQ0038-7350-0007. 

The tropodegradable bromocarbon toxicity studies were conducted under Contract SEAMAS 

SSG Task 1.4.6, F08637-98-C-6001 for the US Air Force and Applied Research Associates with 

additional support from the Advanced Agent Working Group through Contract 349. 

REFERENCES 

1. Mather, J. D., and Tapscott, R. E., “Next-Generation Fire Suppression Technology 

Program: NMERI/CGET Projects Overview,’’ Proceedings, Halon Options Technical 

Working Conference, Albuquerque, NM, pp. 339-346, 1998. 

140 Halon Options Technical Working Conference 27-29 April I999 
2. Tapscott, R. E., Heinonen, E. W., and Mather, J. D., /den/ij?ccrtio/r arid P,-oofTostitrg of 

New Totti/ F/oodin,q Agrnts: Tosic.ity und Glohctl Eniiror7nienrul A.s,sc~.s.snren/, Interiiii 

Report. NMERI 97/29/33010, US Department of Defense, Strategic Environmental 

Rescarch arid Devclopment Program and Defense Advancc Research Projects Agency, 

Arlington, VA. Feh. 1998. 

Tapscott, R. E., Moore. T. A,, Kaizerman. J. A,, Kibert, C. J.. and Tetla, R. A,, “Advanced 

Agent Halon Substitutes,” Proc.eedirrgs. Ii7tcrnurioriul CFC‘ und Hulori A/tcvxutii 

Corifi.r.enc.c, Washington, DC. pp. 644-648, 23-25 Oct. 19%. 

Kairerman, J. A., and Tapscott. R. E.. Adwtic.c,d Str.eurning Agmt Dn~c~/o[iniort. b”i/irmr 

//I: Pho.sphorrrs Com/~oii~id.s, NMERI 96/5/32540, Vol. 3 of 5. Wright Laboratory 

(WWFIVCF), Tyndall Air Force Base, Florida and Applied Rcsearch Associates, Inc., 

Tyndall Air Force Base, FL, May 1996. 

MacDonald, M. A., Jayaweera. T. M.. Fisher, E. M.. and Gouldin, F. C., “Inhihition oi 

Non-Premixed Flamcs by Dimethyl Methylphosphonatc.“ Technical Meeting, Central 

Statcs Section of The Combustion Institule. Point Clear. AL, 27-29 April 1997. 

MacDonald. M. A,, Jayaweera. T. M.. Fisher. E. M., and Gouldin, F. C.. “Inhibition of 

Non-Premixed Flames by Phosphorus-Containing Compounds.” Comhu,stio~i uiid F/ume. 

Vol. I 16, pp. 166- 176, 1999. 

Mahler. W., “Oxidation of Tris(tritluoromethy1)phosphine.” /riorCqunic. Chemistry, Vol. IS, 

No. 2, pp. 352-354, 1979. 

3. 

4. 

5. 

6. 

7. 

Halon Oplions Technical Working Conlcrcnce 27-29 April IWU 14 I 
142 Halon Option< Technicd Workin$ Conference 21-29 April IYYY