NMERI 98/8/33380 

MAIN GROUP COMPOUNDS AS EXTINGUISHANTS 

G. Dana Brabson, Maya M. Garcia, Everett W. Heinonen, Gabriel Juarez, 

J. Douglas Mather, and Robert E. Tapscott 

Center for Global Environmental Technologies 

NEW MEXICO ENGINEERING RESEARCH INSTITUTE 

The University of New Mexico 

901 University Boulevard SE 

Albuquerque, New Mexico 87106-4339 

Final Report, February 1998 – March 1999 

May 2001 

Sponsored by: 

U. S. Department of Defense 

Strategic Environmental Research and Development Program 

The views and conclusions contained in this document are those of the authors and should not be 

interpreted as representing the official policies, either expressed or implied of the Strategic 

Environmental Research and Development Program or any other part of the U. S. Government. 
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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 

31 Mar 1999 

3. REPORT TYPE AND DATES COVERED 

Final Report, 19 Feb 98 - 31 Mar 99

4. TITLE AND SUBTITLE 

Main Group Compounds as Extinguishants 

5. FUNDING NUMBERS 

 

 HQ0038-7350-0009

6. AUTHOR(S) 

Robert E. Tapscott, J. Douglas Mather, Everett W.

Heinonen, Gabriel Juarez, and Maya M. Garcia 

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 

New Mexico Engineering Research Institute 

The University of New Mexico 

Albuquerque, New Mexico 87106-4339

8. PERFORMING ORGANIZATION 

REPORT NUMBER 

 NMERI 98/8/33380

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 

The Department Of Defense 

Strategic Environmental Research and Development 

Program

10. SPONSORING/MONITORING 

AGENCY REPORT NUMBER 

11 SUPPLEMENTARY NOTES 

12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE 

13. ABSTRACT (Maximum 200 words)

Under the Montreal Protocol, an international treaty, the production of the fire and explosion protection 

agent Halon 1301 (bromotrifluoromethane, BFC-13B1, CBrF3) was phased out in the United States at the end 

of 1993. No environmentally acceptable halon substitute that is equivalent to the existing halons in toxicity, 

effectiveness, and dimensionality has been identified. The objective of the overall effort is to assess 

compounds containing members of the main group elements as fire extinguishants and has potential halon 

replacements.

14. SUBJECT TERMS 

phosphorus, sulfur, fire, extinguishment, protection, halon, replacement, 

15. NUMBER OF PAGES 

discharge, evaporation, environmental

16. PRICE CODE 

17. SECURITY CLASSIFICATION OF 

REPORT

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THIS PAGE

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ABSTRACT

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Prescribed by ANSI Std 239-18 

i 
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ii 
PREFACE 

This report was prepared by the Center for Global Environmental Technologies (CGET) 

Division, New Mexico Engineering Research Institute (NMERI), The University of New 

Mexico, Albuquerque, New Mexico, for the Department of Defense, Strategic Environmental 

Research and Development Program (SERDP), under Defense Supply Service-Washington 

(DSS-W) Contract HQ0038-7350-0009, NMERI Number 8-33380. This is Project 4D/5/8 under 

the Next-Generation Fire Suppression Technology Program (NGP). 

The Start Date was 19 February 1998, and the End Date is 31 March 1999. The 

Technical Program Manager is Dr. Richard G. Gann, National Institute of Standards and 

Technology; the Contracting Officer’s Technical Representative (COTR) is Dr. Ronald S. 

Sheinson, Naval Research Laboratory, and the NMERI Principal Investigator (PI) is Dr. J. 

Douglas Mather. Dr. Robert E. Tapscott served as the Deputy PI. 

Gabriel Juarez and Maya M. Garcia were apprentices under the National Aeronautics and 

Space Administration (NASA) SHARP Plus Program. 

NMERI 98/8/33380 

iii 

(The reverse of this page is blank.) 
SUMMARY 

A. TASK OBJECTIVE 

The objective was to evaluate the potential of compounds other than simple halocarbons 

and transition metal compounds as replacements for halon fire extinguishing agents. 

B. GENERAL METHODOLOGY 

This project requires the assessment of liquid or gaseous compounds of main group 

elements (compounds containing elements from the s and p blocks of the periodic table—Groups 

IA, IIA, IIIB, IVB, VB, VIB, VIIB, and VIIIB) as halon replacements. Excluded are 

halocarbons, amines, ethers, and silicon compounds, which have been examined extensively 

elsewhere. This report, which includes the objectives, methodology, results, important findings 

and conclusions, and implications for further research, is the final project deliverable. Of 

particular importance in this effort is an assessment of the potential of various chemical families 

and their derivatives to provide compounds as Halon 1301 substitutes and the development of a 

list of potential compounds. 

C. TECHNICAL RESULTS 

With few exceptions, compounds of the s block elements (the alkali metals and alkaline 

earths—Groups IA and IIA) are solids and do not meet a primary requirement for materials 

considered here (that the compounds be gases or liquids). Most of the compounds which have 

halogens (Group VIIB) as a primary constituents and which could be considered as halon 

replacements are either halocarbons, amines, or ethers (which are excluded) or contain other 

non-carbon main group elements and are considered with those elements. The only non-halogen, 

non-transition metal elements that appear to provide any significant catalytic fire suppression 

capabilities are the alkali metals, tin, lead, and phosphorus. 

iv 
D. IMPORTANT FINDINGS AND CONCLUSIONS 

With one major exception, the results of the search have been disappointing. Few 

materials based on main group elements with potentially acceptable physical (primarily, 

volatility) and toxicological properties have been identified. The lowest molecular weight 

materials (i.e., the materials with the highest volatilities) often have hydrogen or halogen atoms 

directly attached to non-carbon atoms. The former (containing hydrogen) are often flammable, 

and the latter (containing halogens) are usually toxic (owing to hydrolysis). Both often have low 

stabilities. Moreover, few elements provide catalytic fire suppression capabilities, and 

compounds of most of those do not meet the requirements of this study. A major exception is 

fluoroalkyl phosphorus compounds, which hold significant promise despite their relatively low 

volatility. 

E. SIGNIFICANT HARDWARE DEVELOPMENTS 

The only hardware development made in this project was the addition of a mass flow 

control

F. SPECIAL COMMENTS 

None. 

G. IMPLICATIONS FOR FURTHER RESEARCH 

Fluoroalkylphosphorus compounds offer the most promise as main group element halon 

alternat

ler for the air flow in existing cup burners.

ives (as defined in this project).

v 
TABLE OF CONTENTS 

Page

Section I. Introduction .....................................................................................................................1 

A. Objective...............................................................................................................1 

B. Scope.....................................................................................................................1 

C. Approach...............................................................................................................1 

D. Background...........................................................................................................1 

Section II. Evaporation Equilibrium and Rates...............................................................................3 

A. Volatility ...............................................................................................................3 

B. Evaporation Equilibrium.......................................................................................3 

C. Evaporation Rate...................................................................................................9 

1. Fick’s Law ...................................................................................................10 

2. Diffusion coefficient ....................................................................................12 

3. Moving Drops..............................................................................................12 

Section III. Technology Review and Assessment .........................................................................14 

A. Elements..............................................................................................................14 

B. Compounds of Group I Elements .......................................................................15 

C. Compounds of Group II Elements......................................................................15 

D. Compounds of Group III Elements.....................................................................16 

1. Boron............................................................................................................16 

2. Aluminum ....................................................................................................16 

3. Gallium, Indium, Thallium ..........................................................................17 

E. Compounds of Group IV Elements.....................................................................17 

1. Carbon..........................................................................................................17 

2. Silicon ..........................................................................................................17 

3. Germanium, Tin, Lead.................................................................................17 

F. Compounds of Group V Elements......................................................................18 

1. Nitrogen .......................................................................................................18 

vi 
2. Phosphorus...................................................................................................18 

3. Arsenic, Bismuth, Antimony .......................................................................22 

G. Compounds of Group VI Elements.....................................................................23 

1. Oxygen.........................................................................................................23 

2. Sulfur............................................................................................................23 

3. Selenium, Tellurium ....................................................................................24 

H. Group VIII Elements...........................................................................................24 

Section IV. Cup Burner .................................................................................................................25 

A. Overview.............................................................................................................25 

B. Air Flow..............................................................................................................25 

1. Mass Flow Controller ..................................................................................25 

2. Digital Bubble Meter ...................................................................................26 

C. Agent flow ..........................................................................................................27 

1. Oxygen Monitoring......................................................................................27 

2. Direct Measurement of Extinguishment Concentration ..............................28 

3. Mass Flow Rate ...........................................................................................28 

4. Digital Bubble Meter ...................................................................................29 

D. Cup Burner Testing of Blends Containing Phosphorus Compounds .................30 

Section V. Sapphire Experiments..................................................................................................31 

A. Sapphire 0-7 with dimethylmethylphsophonate (DMMP). ................................31 

1. Apparatus.....................................................................................................31 

2. Objective......................................................................................................33 

3. Experiment DMMP_A.................................................................................33 

4. Experiment DMMP_B.................................................................................34 

Section VI. Results and Conclusions.............................................................................................35 

A. Results.................................................................................................................35 

References......................................................................................................................................36 

vii 
LIST OF FIGURES 

Figure Page

1 Estimated Maximum Boiling Points That Can Achieve Given 

Concentrations as Function of Ambient Temperature.............................................8 

2 Estimated Maximum Boiling Points That Can Achieve Given 

Concentrations as Function of Ambient Temperature (Simplified), 

Calculated for tc = 88 J/K-mol.................................................................................9 

3 Elements (in Boldface) Considered Under the Present Project.............................14 

4 Actual versus Mass Flow Meter Reading for Air Flow Rates...............................26 

5 The Sapphire 0-7 Atmospheric-Pressure Burner...................................................32 

viii 
LIST OF TABLES 

Table Page

1 Estimated Vapor Pressure (Torr) Versus Boiling Point and Ambient 

Temperature for Trouton’s Constant 88 J/K-mol. ...................................................5 

2 Estimated Maximum Concentration (Vol%) Versus Boiling Point and 

Ambient Temperature for Trouton’s Constant 88 J/K-mol and 1 

Atmosphere Pressure. ..............................................................................................6 

3 Estimated Maximum Boiling Point That Can Achieve a Given 

Concentration...........................................................................................................7 

4 Enthalpy of Reaction for PF3 + 3 O=CF2 → P(OCF3)3.........................................22 

5 Perfluoroalkyl Sulfur Compounds.........................................................................23 

6 Actual versus Mass Flow Meter Reading for Air Flow Rates...............................26 

7 Cup Burner Extinguishment Concentrations, n-Heptane Fuel. .............................30 

ix 
LIST OF ABBREVIATIONS 

AAWG Advanced Agent Working Group 

CCOD ID CGET Chemical Options Database Identification Number 

CGET Center for Global Environmental Technologies 

COTR Contracting Officer’s Technical Representative 

DEMP diethyl methylphosphonate 

DIMP diisopropyl methylphosphonate 

DMMP dimethyl methylphosphonate 

IMP isopropyl methylphosphonate 

ISO International Standards Organization 

MPA methylphosphonic acid 

NASA National Aeronautics and Space Administration 

NGP Next-Generation Fire Suppression Technology Program

NIST National Institute of Standards and Technology 

NMERI New Mexico Engineering Research Institute 

PI Principal Investigator 

SERDP Strategic Environmental Research and Development Program

SLM Standard Liters/Minute 

TFEP tris(2,2,2-trifluoroethyl)phosphite 

TMP trimethylphosphate 

USAF United States Air Force 

x 
LIST OF UNITS AND SYMBOLS 

C concentration 

J Joule 

K Kelvin 

log10 logarithm to the base 10 

mol mole

Pb external pressure at boiling point 

p vapor pressure 

R ideal gas constant 

T temperature 

Tb boiling point temperature 

Tc Trouton’s constant 

vol% percent by gas volume 

°C degrees Celcius 

∆Hr enthalpy of reaction 

∆Hvap enthalpy of vaporization 

∆Svap molar entropy of vaporization 

xi 
SECTION I. 

INTRODUCTION 

A. OBJECTIVE 

The objective was to evaluate the potential of compounds other than simple halocarbons 

and transition metal compounds as replacements for halon fire extinguishing agents. Though not 

specifically stated in the requirements, only compounds that are not solids at room temperature 

are targeted. 

B. SCOPE 

This project requires the assessment of liquid or gaseous compounds of main group 

elements (compounds containing elements from the s and p blocks of the periodic table—Groups 

IA, IIA, IIIB, IVB, VB, VIB, VIIB, and VIIIB) as halon replacements. Excluded are 

halocarbons, amines, ethers, and silicon compounds, which have been examined extensively 

elsewhere. Of particular importance is an assessment of the potential of various chemical 

families and their derivatives to provide compounds as Halon 1301 substitutes and the 

development of a list of potential compounds. 

C. APPROACH 

Much of the information acquired on candidate compounds and related chemicals has 

been added to the CGET Chemical Options Database© [1]. Those chemicals that appear in this 

database are assigned a CGET Chemical Options Database Identification Number (CCOD ID), 

and this number is given in this report where appropriate. 

D. BACKGROUND 

Past work under the United States Air Force (USAF) and the Advanced Agent Working 

Group (AAWG) has performed an assessment of the fire suppressant potential of compounds in 

all elemental families [2]. As a result of this initiative, it was recommended that tropodegradable 

bromocarbons [3], silicon compounds [4], phosphorus compounds [5], and metal compounds [6] 

1
receive particular attention. A large number of additional main group chemicals were, however, 

identified as having some potential promise. Due to severe funding and time limitations, no 

work was carried out to identify derivatives and evaluate these additional compounds. 

A significant amount of work has been performed that allows a comparison of flame 

suppression for compounds of main group elements with that of halocarbons, which have been 

more widely studied. McHale gives a good overview of work in this area up to 1969 [7]. 

Some results from this present project have been reported at conferences [8, 9, 10]. 

2
SECTION II. 

EVAPORATION EQUILIBRIUM AND RATES 

A. VOLATILITY 

Only compounds that are not solids at room temperature are targeted under this specific 

project. Many, if not most, of the liquids found in this study are liquids with relatively high 

boiling points. A major criterion for use of such compounds is the ability to evaporate and fill a 

space to the desired concentration within the time required for extinguishment. Thus, 

evaporation properties must be taken into account during compound review and selection. There 

are two areas that must be considered—evaporation equilibrium and evaporation rate. 

B. EVAPORATION EQUILIBRIUM 

The first question that must be answered is whether a given compound can achieve the 

required extinguishment concentration upon discharge given sufficient time for evaporation to 

occur. This question, which considers only evaporation equilibrium, is relatively easy since only 

thermodynamics are involved in the analysis. The logarithm of the ratio of the vapor pressure 

(p) at two different temperatures T2 and T1 (in absolute units) can be estimated from Equation 1 

(the Clausius-Clapeyron equation), where R is the ideal gas constant and ∆Hvap is the heat of 

vaporization.* This equation is based on the assumption that (1) ∆Hvap is temperature 

independent and (2) the vapor is an ideal gas. The ratio of ∆Hvap/Tb, where Tb is the boiling 

point temperature, is equal to the molar entropy of vaporization ∆Svap at the boiling point. For 

closely related compounds, ∆Svap at the boiling point is nearly constant with a value of tc, 

Trouton’s constant (Equation 2). Thus, by selecting T1 as the boiling point (Tb) at, for example, 

1 atmosphere pressure (i.e., p1 = 760 Torr), one can estimate the vapor pressure at some other 

temperature and develop an approximate vapor pressure/temperature curve if tc is known. 

log10(p2/p1) = (∆Hvap (T2-T1)/2.302585RT2T1) [1]

 

*Throughout this report, numbers accompanying equations are placed in brackets and numbers 

accompanying reactions are placed in parentheses. 

3
∆Hvap/Tb = ∆Svap ≈ tc [2]

The two equations can be combined to simplify the estimation. 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. 

p2 = Pb10**(0.05221065)tc[1-(Tb/T2)] [3]

For example, if tc 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 = 

50 °C at 760 Torr is 313 Torr. Table 1 gives the estimated vapor pressures as a function of 

boiling point and ambient temperature. From vapor pressure values, one can calculate the 

maximum concentration possible. For one atmosphere total pressure, the equilibrium (maximum

possible) volume fractions are p2/760 for p2 in Torr. These values, converted to percent, are 

given in Table 2. Similar tables can be prepared for other pressures. Note that this analysis does 

not say how fast the equilibrium concentration will be reached. It only gives the maximum

theoretically possible. 

4
TABLE 1. ESTIMATED VAPOR PRESSURE (TORR) VERSUS BOILING POINT AND 

AMBIENT TEMPERATURE FOR TROUTON’S CONSTANT 88 J/K-MOL. 

Boiling Ambient Temperature, °C 

Pt., °C -60 -50 -40 -30 -20 -10 0 10 20 25 30 40 50

-60 760 1221 1883 2803 4044 5673 7763 10391 13635 15512 17573 22285 27848

-50 463 760 1196 1814 2663 3795 5270 7152 9504 10879 12396 15896 20073

-40 282 473 760 1174 1753 2539 3578 4922 6625 7629 8744 11339 14469

-30 171 294 483 760 1154 1698 2429 3387 4618 5350 6168 8088 10429

-20 104 183 307 492 760 1136 1649 2331 3219 3752 4351 5769 7517

-10 64 114 195 318 500 760 1119 1605 2244 2631 3069 4115 5419

0 39 71 124 206 329 508 760 1104 1564 1845 2165 2936 3906

10 24 44 79 133 217 340 516 760 1090 1294 1527 2094 2815

20 14 28 50 86 143 228 350 523 760 908 1077 1494 2029

30 9 17 32 56 94 152 238 360 530 636 760 1065 1463

40 5 11 20 36 62 102 161 248 369 446 536 760 1054

50 3 7 13 23 41 68 110 171 257 313 378 542 760

60 2 4 8 15 27 46 74 117 179 220 267 387 548

70 1 3 5 10 18 30 51 81 125 154 188 276 395

80 1 2 3 6 12 20 34 56 87 108 133 197 285

5
TABLE 2. ESTIMATED MAXIMUM CONCENTRATION (VOL%) VERSUS BOILING 

POINT AND AMBIENT TEMPERATURE FOR TROUTON’S CONSTANT 88 J/K-MOL 

AND 1 ATMOSPHERE PRESSURE. 

Boilin

g 

Ambient Temperature, °C 

Pt., °C -60 -50 -40 -30 -20 -10 0 10 20 25 30 40 50 

-60 100

-50 61 100 

100 

100 

100 

100 

100 

10 100 

100 

84 100 

100 

50 0 1 2 3 5 9 14 22 34 41 50 71 100 

60 0 1 1 2 4 6 10 15 24 29 35 51 72 

70 0 0 1 1 2 4 7 11 16 20 25 36 52 

80 0 0 0 1 2 3 5 7 11 14 17 26 37 

-40 37 62

-30 23 39 64

-20 14 24 40 65

-10 8 15 26 42 66

0 5 9 16 27 43 67

3 6 10 18 29 45 68

20 2 4 7 11 19 30 46 69

30 1 2 4 7 12 20 31 47 70

40 1 1 3 5 8 13 21 33 49 59 71

We can also calculate the maximum allowable boiling point (Tb) that can achieve a given 

concentration (C) for various ambient temperatures, T2. Equation 4 gives the calculation for 

temperatures in K, Pb in Torr, and tc in J/K-mol. Table 3 gives the estimated maximum boiling 

point that can achieve concentrations from 1 vol% to 20 vol% for a Trouton constant of 

88 J/K-mol. The data are plotted in Figure 1. Figure 2 gives a simplified illustration (some data 

have been omitted). 

Tb = [1-(Log(760C/100Pb)/0.05221065tc)]T2 [4] 

6
TABLE 3. ESTIMATED MAXIMUM BOILING POINT THAT CAN ACHIEVE A 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 19 33 46 60 74 87 101 115 128 142 156 169

3 11 24 37 51 64 77 91 104 117 130 144 157

4 5 18 31 44 57 70 83 96 109 122 135 148

5 0 13 26 39 52 65 77 90 103 116 129 142

6 -3 9 22 35 47 60 73 85 98 111 123 136

7 -6 6 19 31 44 56 69 81 94 106 119 131

8 -9 3 16 28 40 53 65 78 90 102 115 127

9 -11 1 13 25 38 50 62 74 87 99 111 124

10 -14 -1 11 23 35 47 59 72 84 96 108 120

11 -16 -3 9 21 33 45 57 69 81 93 105 117

12 -17 -5 7 19 31 43 55 67 79 91 103 115

13 -19 -7 5 17 29 41 53 65 77 88 100 112

14 -20 -9 3 15 27 39 51 63 74 86 98 110

15 -22 -10 2 14 25 37 49 61 73 84 96 108

16 -23 -11 0 12 24 36 47 59 71 83 94 106

17 -24 -13 -1 11 22 34 46 57 69 81 92 104

18 -25 -14 -2 9 21 33 44 56 68 79 91 102

19 -27 -15 -3 8 20 31 43 54 66 78 89 101

20 -28 -16 -5 7 19 30 42 53 65 76 88 99

7
-30

-20

-10

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

160

170

180

190

200

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

Ambient Temperature, °C

Maximum Boiling Point, °

1% 20%

Figure 1. Estimated Maximum Boiling Points That Can Achieve Given Concentrations as 

Function of Ambient Temperature. 

8
-30

-20

-10

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

160

170

180

190

200

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

Ambient Temperature, °C

Maximum Boiling Point, °

1%

3%

5%

7%

10%

15%

20%

Figure 2. Estimated Maximum Boiling Points That Can Achieve Given Concentrations as 

Function of Ambient Temperature (Simplified), Calculated for tc = 88 J/K-mol. 

C. EVAPORATION RATE 

Calculation (or estimation) of evaporation rates are much more difficult than calculation 

of equilibrium agent concentrations. Here, we present an overview of some of the things that 

must be considered. In the simplest case, a spherical droplet of fire suppressant agent of radius r 

is ejected into stagnant, completely unsaturated air at velocity U0. The rate of evaporation as a 

function of droplet radius and the time for complete evaporation is of interest. It is assumed that 

the temperature of the droplet is the same as its surroundings, and that the temperature at the 

surface of the droplet does not change as the droplet evaporates. While neither of these 

9
conditions is true, considering temperature changes is too complex for a simple analysis. The 

following uses cgs units. 

1. Fick’s Law 

Fick’s first law for one-dimensional diffusion relates the flux (J in g/s-cm2

) of a 

vapor through a gaseous medium to the concentration gradient dc/dx, where c is the 

concentration (g/cm3

) and x is the direction of diffusion (Equation 5). 

dr

dc J = −D [5]

D, the diffusion coefficient of the vapor in cm2

/sec, is a function of the agent and 

the ambient conditions. Kinetic theory shows that for ideal gases of density ρ of rigid spherical 

molecules having a molecular mass m and a diameter σ, the diffusion coefficient is given by 

Equation 6, where k is the Boltzmann constant and T is the temperature [11]. 

πσ ρ

π = 2

1 / 2 ( mkT)

8

3 D [6]

Setting the density ρ = Pm/kT, one obtains Equation 7, which shows that the 

diffusion coefficient is proportional to T3/2 and inversely proportional to the pressure P for this 

idealized gas composed of hard-spheres. It is also inversely proportional to the square root of 

the molecular mass m and to the square of the molecular diameter σ. Real gases, of course, 

show significant deviations from this relationship, which may be modified to take into account 

actual molecular interactions. 

1/ 2 2 1/ 2

3 / 2

Pm

(kT)

8

3 D

π σ = [7]

Continuing to use the hard-sphere model, one can relate the diffusion coefficient 

to the coefficient of viscosity (η, g/cm-s), the heat capacity per molecule (cv, g-cm2

/s2-°C, = 

Cv/No, where No is Avogadro’s number), and the coefficient of thermal conductivity (λ, 

g-cm/s3

-°C) (Equations 8 and 9). 

10
v 25 c

12 m D

ρ

λ = [8]

ρ

η = 5

6 D [9]

Equation 5 can be rewritten to give the diffusion mass transport through a 

spherical surface of radius r (Equation 10). 

D

dr

dc 4 r

dt

dm 2 = − π [10]

Equation 10 can be used to determine the mass transport from the liquid/gas 

interface surrounding a spherical drop of radius ro with a concentration of co to a region far from

the drop (r∞) with a vapor concentration c∞. Since dm/dt, the mass transport, is independent of r, 

integration (Equation 11) gives Equation 12. If r∞ is very large and if c∞ is taken as zero, 

Equation 15 results. 

∫ ∫

∞ ∞

= − π

c

c

4 D dc

r

r r

dr

dt

dm

o o

2 [11]

(1/ r 1/ r )

c c 4 D

dt

dm

o

o

∞

∞

−

− = π [12]

If r4 is taken as sufficiently large, Equation 12 can be rewritten to give 

Equation 13. Note that as c4 approaches co (the air becomes saturated), the rate of evaporation 

approaches zero. 

4 Dr (c c ) dt

dm = π o o − ∞ [13]

Equation 13 can be rewritten to give Equation 14 

RT

4 rDM(p p )

dt

dm o π − ∞

= [14]

11
where M = molecular weight of the vapor, and po and p4 are the partial pressures of the vapor at 

the surface of the droplet and at a distance infinitely far from the droplet. In the case of a fire 

suppressant agent, it can be assumed that p4 is initially zero, since no agent has yet been released 

into the room. If c4 is zero, one obtains Equation 

o o 4 Dr c

dt

dm = π [15]

Equation 15 shows that the transport rate increases with drop size and with the 

concentration of agent at the liquid/vapor interface. 

There are two problems. First, one must determine D for a real gas (rather than 

for the hard-sphere idealized gas discussed earlier). Second, and more difficult, one must 

determine the concentration of agent at the interface. 

2. Diffusion coefficient 

Since measured values of diffusivity are often not available, the only way to 

obtain values may be to perform estimates based on available equations. Reference 12 lists 

several correlations for diffusivity; the equation estimated to have the lowest error is the FullerChapman-Giddings equation 

∑ ∑+

+

= 1/ 3 2

2

1/ 3

1

0.5

1 2

1.75

P[( v ) ( v ) ]

) M

1

M

1 0.1013T (

D [16] 

where T is the temperature of gases in K, M1 is the molecular weight of the gas diffusing into the 

air, M2 is the molecular weight of the air, P is the pressure of the gases, and v1 and v2 are atomic 

diffusion volumes which are given in tables. Unfortunately, atomic diffusion volumes are not 

available for the majority of compounds and must be estimated for many of the compounds of 

interest. 

3. Moving Drops 

The question of motionless droplets is academic at best. Droplets of fire 

suppressant chemicals will be rapidly expelled through the nozzle to ensure complete coverage 

12
of the protected area or the fire as well as providing mechanical breakup to smaller droplet sizes. 

Any realistic evaluation of evaporation must include inertia, gravity, and other forces acting on 

the droplets. The rapid acceleration of the droplets also distorts the droplets; however, one 

usually assumes spherical droplets. 

The problem quickly escalates from a simple diffusion mechanism to a heat and 

mass transfer mechanism; it reduces to the calculation of the rate of evaporation or heat transfer 

from a spherical body in a moving gas stream (in the case of interest, the droplet is in motion 

within stagnant air, but the mathematics are the same). Solutions to such problems rely on the 

use of dimensionless numbers such as Reynolds, Nusselt, Prandtl, and Schmidt numbers. The 

problem is complicated by the fact that flow may be either laminar or turbulent, depending on 

the value of the Reynolds number; heat and mass transfer are radically different in laminar and 

turbulent flow. 

A simple correction to Equation 13 to allow for the increase in evaporation rate 

due to motion through air is 

4r D(c c )(1 Sc Re ) I f

dt

dm o 1/ 3 1/ 2 = π o − ∞ + β = [17]

where dm/dt equals the evaporation rate with flow around the droplet, ∃ is equal to 0.276 

(measured for a sphere), Re is the Reynolds number characterizing the flow of the air around the 

drop, and Sc is the Schmidt number. The term f is usually called the wind or ventilation factor. 

It is reported [13] that Equation 17 accurately represents the evaporation rate of droplets in 

moving air. For very small droplets falling in air, f is small, approaching 1; for larger droplets in 

moving streams, f can be rather large, reflecting the higher heat and mass transfer rates at higher 

velocity. 

13
SECTION III. 

TECHNOLOGY REVIEW AND ASSESSMENT 

A. ELEMENTS 

The term “main group elements” is used to denote any element other than transition 

metals; however, since most work to date has involved organic compounds, particularly 

halocarbons, such materials are not covered here. Moreover, significant past work under the 

Next-Generation Fire Suppression Technology Program (NGP) has been performed on amines, 

ethers, and silicon compounds [14, 15], these compounds are also not included or, in the case of 

silicon, are only briefly discussed. Phosphorus compounds were examined in the earlier NGP 

work cited; however, due to the large promise found for such agents, phosphorus compounds are 

also covered here. 

Figure 3

Figure 3. Elements (in Boldface) Considered Under the Present Project.

shows a periodic table with the elements considered under the present project in 

boldface. Note that carbon, oxygen, and the halogens may be (in some cases, are likely to be) 

present; however, such elements do not form the primary structural features of the compounds 

considered here. As noted earlier, nitrogen, which was examined in earlier work, is ignored 

here. Though compounds of helium, neon, and argon are unknown, these are considered in their 

elemental form. Due to toxicity, the radioactive elements are excluded. Though they are 

included, the Group I and Group II elements are unlikely to have utility under the project as 

defined here since most compounds of these are solids. 

I II III IV V VI VII VIII

H He

Li Be B C N O F Ne

Na Mg Al Si P S Cl A

K Ca Ga Ge As Se Br Kr

Rb Sr Transition Metals In Sn Sb Te I Xe

Cs Ba Tl Pb Bi Po At Rn

Fr Ra

14
B. COMPOUNDS OF GROUP I ELEMENTS 

Group I elements comprise the alkali metals—lithium (Li), sodium (Na), potassium (K), 

rubidium (Rb), cesium (Cs), and francium (Fr). The last element, which is radioactive, is not 

considered here. Compounds of the alkali metals, in particular potassium and sodium salts, have 

long been used as fire extinguishing agents. A rather thorough review of the literature on fire 

extinguishment by compounds of the Group I elements has been prepared [6] and these elements 

have also been discussed in a report prepared for the USAF [2]. Although these compounds are 

highly effective extinguishants, the mechanism is still uncertain. Extinguishment may be due to 

heat absorption, free-radical recombination on particulate surfaces, or homogeneous free-radical 

removal. 

Nearly all compounds of the Group I elements are solids at room temperature, and, 

therefore, past work has been on either dry chemical powders or solutions. Exceptions are some

of the lithium alkyls, which are flammable. Some eutectic mixtures of alkali metal salts have 

melting points near room temperature; however, the vapor pressures are negligible. Thus, alkali 

metal compounds must be applied as dry powders or liquid solutions and will not be further 

considered here. It should be noted, however, that studies have shown that aqueous solutions of 

some alkali metal compounds are extremely effecting fire extinguishants [16]. 

C. COMPOUNDS OF GROUP II ELEMENTS 

No flame extinguishment data have been found for compounds of beryllium; however, 

compounds of the other Group II elements (excluding, of course, radium) have been shown to 

have lower fire extinguishing efficacies than those of the Group I compounds. The absence of 

beryllium data is likely due to the very high toxicity for beryllium compounds. Like the 

compounds of the alkali metals, most alkaline earth compounds are solids. Beryllium does, 

however, form some highly reactive liquid alkyls. Due to their generally unsuitable physical 

properties, compounds of the Group II elements will not be considered further. 

15
D. COMPOUNDS OF GROUP III ELEMENTS 

1. Boron 

A number of boron compounds are gases or liquids; however, apparently without 

exception all are either flammable, highly toxic, or act merely as halon carriers. Thus, both 

gaseous and liquid boranes (boron hydrides) are known, but all are flammable. Many of the 

simple halides such as BF3, B2Cl6, etc., are gases or liquids, but these are highly unstable in the 

presence of moisture and are all highly toxic. 

Fire extinguishment testing using standard handheld extinguishers shows that 

finely divided borax (Na2B4O7, degree of hydration not specified) is approximately equal to 

sodium bicarbonate in fire extinguishment effectiveness [17]. The fire suppression capability of 

this compound may be due to the presence of sodium. 

Common flame retardants for fabrics and plastics are sodium borate pentahydrate 

(borax pentahydrate, Na2B4O7•5H2O), boric acid [B(OH)3], boric oxide (B2O3), disodium

octaborate tetrahydrate (Na2B8O13•4H2O), ammonium pentaborate octahydrate 

[(NH4)2B10O16•8H2O], and zinc borates (e.g., Zn4B12O22•7H2O) (Reference 18). In an extensive 

study of boron compounds, sodium tetrafluoroborate (NaBF4), a solid, was the most effective 

single compound for reducing fabric flammability [19]. Note that these flame retardants are all 

solids. 

In this study, no liquid or gaseous boron compounds that have a reasonable 

toxicity and that are nonflammable have been found. There is no evidence that boron has any 

catalytic fire suppression capability. 

2. Aluminum

Aluminum oxide trihydrate (Al O •3H O) is a widely used fire retardant in 

elastomers [20], where it operates primarily by endothermic release of water to provide cooling. 

Gaseous aluminum chloride is an effective inhibitor of premixed methane/air flames [21], and 

the dispersed powder effectively inhibits spark ignition of methane/air mixtures [22]. This 

activity is probably due to the presence of chlorine rather than to any intrinsic fire extinguishing 

capability of the aluminum itself. To date, no evidence that aluminum has any catalytic fire 

2 3 2

16
suppression capabilities has been presented. Moreover, the few gaseous or liquid aluminum

compounds that exist have serious drawbacks (toxicity, flammability, and/or stability). 

3. Gallium, Indium, Thallium

No fire extinguishing studies have been reported for the relatively rare elements 

gallium, indium, or thallium. Toxicity would, moreover, be a serious problem with thallium and 

indium compounds. 

E. COMPOUNDS OF GROUP IV ELEMENTS 

1. Carbon 

Most potential or known halon replacements for flooding applications are either 

organic compounds (e.g., the alkyl halides) or have organic substituents. Thus, the presence of 

carbon is too broad a requirement to be used for a criteria in this report. Most compounds of 

interest will contain carbon. There is no evidence that carbon imparts any catalytic fire 

suppression capabilities. 

2. Silicon 

Silicon compounds have been examined in several projects [2, 4, 14, 15]; and 

there is little or no evidence that silicon offers any inherent fire suppression capabilities. The 

presence of silicon may, in some cases, offer desirable environmental and toxicological 

characteristics or physical properties. 

3. Germanium, Tin, Lead 

The only germanium compound studied as a fire extinguishant is germanium

tetrachloride,GeCl4 [23], whose extinguishing capability may be primarily due to the presence of 

chlorine. On the other hand, there have been some reports that indicate a possible chemical 

inhibition for compounds of tin and lead. Thus, lead tetraacetate (Pb(CH3COO)4) and lead 

acetylacetonate, Pb(CH3C(O)CHC(O)CH3)2), neither of which contain halogens, are effective 

extinguishants [24]. A catalytic mechanism has been proposed for fire extinguishment by tin 

17
[25]. A very recent paper has presented results indicative of a weak catalytic flame suppression 

action by tin compounds [26]. 

Despite the scattered indications of catalytic effects for compounds of the heavier 

Group IV elements, germanium is too costly to be considered, and tin and lead are 

environmentally unacceptable. 

F. COMPOUNDS OF GROUP V ELEMENTS 

1. Nitrogen 

The chemistry and flame suppression of nitrogen compounds (with an emphasis 

on fluoroalkyl amines) was discussed in earlier work performed under the Next-Generation Fire 

Suppression Technology Program (NGP) [14, 15]. One cycle that has been proposed catalyze 

recombination of hydrogen atoms and hydroxyl free radicals is given in Reactions 1 through 3 

[27]. The NO cycle, however, appears to be relatively inefficient [28]. 

H + NO + M → HNO + M* (1) 

H + HNO → H2 + NO (2) 

HNO + OH → H2O + NO (3) 

2. Phosphorus 

A review of the proposed extinguishment mechanisms and testing of phosphorus 

compounds has been presented [15]. Since the time that the earlier report was being written, a 

number of new papers and reports have appeared. 

A kinetic model has been developed for the combustion chemistry of dimethyl 

methylphosphonate (DMMP, CCOD ID 1375, O=P(CH3)(OCH3)2) [29]. The opposed-jet burner 

has been shown to be effective for studying this low-volatility compound [30], and studies with 

this apparatus have shown that DMMP is a better suppressant when introduced in the oxidizer 

stream [31] and that DMMP and trimethylphosphate (TMP, CCOD ID 1225, O=P(CH3)3) are 

approximately forty times more effective than nitrogen on a per-mole basis in suppressing flames 

and two to four times more effective than Halon 1301 [32]. Like Halon 1301, the effectiveness 

is greater when DMMP is introduced on the oxidizer side, rather than the fuel side, of non18
premixed flames even when the amounts actually reaching the flame are taken into account [33]. 

This behavior is not observed with argon, which has no chemical contribution to flame 

suppression. 

Gas-phase studies in nitrogen show that diethyl methylphosphonate (DEMP, 

O=P(CH3)(OCH2CH3)2) pyrolyzes to form ethene (CH2=CH2), ethanol (CH3CH2OH), and ethyl 

methylphosphonate (O=P(OH)(CH3)(OCH2CH3)) [34]. Formation of ethene may result from

formation of a six-membered ring transition state ( ) or by scission of a PO-CH2CH3 bond with 

subsequent loss of a hydrogen atom from the ethyl radical (•CH2CH3). Similar results have been 

found in pyrolysis studies of diisopropyl methylphosphonate (DIMP), which gives propene 

(CH2=CHCH3), 2-propanol (CH3CH(OH)CH3), isopropyl methylphosphonate (IMP, 

O=P(OH)(CH3)(OCH(CH3)2)), and methylphosphonic acid (MPA, O=P(OH)2(CH3)) [35]. A 

six-membered ring transition state is proposed for formation of 2-propene. Derivatization 

followed by gas chromatography/mass spectrometry has identified DMMP, methyl 

methylphosphonate (O=P(OH)(CH3)(OCH3)), dimethyl phosphate (O=P(OH)(OCH3)2), 

monomethyl phosphate (O=P(OH)2(OCH3)), MPA, orthophosphoric acid (O=P(OH)3), 

phosphorous acid (P(OCH3)3), and phosphonic acid (O=PH(OH)2) as combustion products in a 

CH4/O2/N2 flame doped with DMMP [36]. 

1

(1) 

C2H5

H3C O

P

O

C

H

H

H

Based on opposed-flame burner results, there has been significant interest in 

DMMP as a potential fire extinguishing material. No cup burner values have, however, been 

determined for DMMP and it would be difficult to obtain such values because of its very high 

boiling point (181 °C, [37]) and low vapor pressure (1.2 Torr at 25 °C, [38]). Attempts to 

determine a cup burner extinguishment concentration for a higher volatility nonfluorinated 

compound, trimethyl phosphite (P(OCH3)3) has also been unsuccessful, due, in this case, to its 

19
flammability. All of the phosphorus compounds discussed above in this section are, in fact, 

flammable.

There are two ways around the flammability problem. One is to blend the 

material with a nonflammable carrier; the other is to work with fluorinated alkyl derivatives of 

phosphorus. Cup burner testing using both of these approaches has been carried out and the 

results are presented in this report. 

Previous work has shown that the compound tris(2,2,2-trifluoroethyl)phosphite 

( ), TFEP, CCOD ID 1236, exhibits a particularly low cup-burner extinguishment concentration 

[15]. (More recent and careful work, performed under the present project, gave an extinguishing 

concentrations of 1.77, 1.81, 1.81, 1.79, and 1.69 vol% (average: 1.78 ± 0.04 vol%) for nheptane fuel.) This compound, however, has a relatively high boiling point of 130 to 131 °C at 

743 Torr pressure [39]. One might, therefore, consider tris(trifluoromethyl)phosphite ( ) or 

tris(trifluoromethyl)phosphate ( ), CCOD ID 1348, as promising materials. 

2

(2) 

3

(3) 

4

OCH2CF3

OCH2CF C 3 F3CH2O

P

OCF3

OCF3

CF3O

P

20
OCF3

OCF3

CF3O

P

O

(4) 

Tris(trifluoromethyl)phosphite (P(OCF3)3) has not been reported; however, 

tris(trifluoromethyl)phosphate (O=P(OCF3)3) has been prepared by oxidation of 

tris(trifluoromethyl)phosphine (P(CF3)3, CCOD 1351, [40]). The phosphate has a boiling point 

of 52 °C at 760 Torr pressure and a melting point of -86 °C. The vapor pressure p in Torr can be 

fit to the equation log10p = -1445/T + 7.33, where T is in units of K, corresponding to a 

Trouton’s Constant of 85.22 J/mol-K. This equation gives a calculated vapor pressure of 304 

Torr at 25 °C. One might consider whether tris(trifluoromethyl)phosphite could be prepared by 

reaction of trif

ond

ompounds 

tion would

PF3 + 3 O=CF2 → P(OCF3)3 (4) 

l 3 3 2

the reaction is

luorophosphine (PF3) with carbonyl fluoride (O=CF2, CCOD ID 696) 

(Reaction 4). The enthalpy of reaction (∆Hr) for Reaction 4 was estimated based on b

energies from values in References 41, 42, and 43. Table 4 gives the data used in the 

calculation. The column marked “Basis” indicates the type of compound or types of c

used to determine the bond energy. ∆Hr is estimated as 92 kJ/mol (22 kcal/mole). Thus, the 

reaction is slightly endothermic, and the reverse reaction may provide a pathway for 

decomposition. This is particularly likely since the entropy of the decomposition reac

almost certainly be positive due to the increase in the number of moles of gas. 

Were the materia to be formed as the phosphonate structure, O=P(CF )(OCF ) , 

less endothermic, with ∆Hr estimated as to be 58 kJ/mol (14 kcal/mole). Again, 

however, the reverse reaction should have a positive entropy change. Even though the reactions

21
are predicted to be endothermic, one must recognize that the use of average bond energies is 

highly suspect. 

TABLE 4. ENTHALPY OF REACTION FOR PF3 + 3 O=CF2 → P(OCF3)3. 

Bond ∆H, Total for Bond 

Formation or Dissociation

kJ/mol (kcal/mol) 

Reactants

C-F 6 490 (117) CF4 2939 (702)

C=O 3 749 (179) ketones

Product, Phosphite Structu

2248 (537) 

Sum 6682 (1596)

re

368 (88) P4O6

C-F 9 490 (117)

ics

Product, Phosphonate Struct

CF4 4410 (1053)

C-O 3 358 (85.6) organ 1075 (257) 

Sum 6590 (1574)

ure

368 (88) P4O6

P=O 1 456 (109) l3 456 

F4 4410 )

ics

6624 

O=PC (109)

P-C 1 306 (73) 306 (73) 

C-F 9 490 (117) C (1053

C-O 2 358 (85.6) organ 716 (171) 

Sum (1582)

Type Number Energy, kJ/mol (kcal/mol) Basis 

P-F 3 498 (119) PF3 1495 (357) 

P-O 3 1105 (264) 

P-O 2 736 (176) 

3. Arsenic, Bismuth, Antimony 

Antimony has been extensively used in flame retardants, usually in conjunction 

with bromine-c

uth 

ontaining compounds. This work has been reviewed [2] and the information 

presented earlier will not be repeated here. No studies on fires suppression by arsenic or bism

22
compounds has been found. Compounds of arsenic and antimony are too toxic to be considered 

as fire extinguishing compounds. 

G. COMPOUNDS OF GROUP VI ELEMENTS 

1. Oxygen 

Oxygen compounds are included under other headings and will not, therefore, be 

covered here. There is no indication that oxygen compounds have any type of catalytic fire 

suppression capability. 

2. Sulfur 

A sulfur dioxide cycle (Reactions 5 and 6) has been proposed to inhibit 

combustion by removal of hydrogen atoms [28]. 

H + SO2 + M → HSO2 + M* (5) 

H + HSO2 → H2 + SO2 (6)

Perfluoroalkyl sulfur compounds (Table 5) are of some interest due to their 

relatively high stability toward oxidation and hydrolysis. Bis(trifluoromethyl)sulfide 

(CF3-S-CF3, CCOD ID 1362) can be prepared by heating commercially available 

bis(trifluoromethyl)disulfide (CF3-SS-CF3, CCOD ID 1363) at 425 to 435 °C for 3 to 4 days 

[44]. This and other perfluoroalkylsulfides can also be prepared by heating or photolysis of 

RfOC(O)Rf, where Rf is a perfluoroalkyl group [45, 46]. The ester starting materials are 

prepared by reaction of Rf-S-Cl with silver perfluorocarboxylates (AgOOCRf). The derivatives 

Rf-SF2-Rf, can be prepared by fluorination of the corresponding sulfide with ClF or F2, and from

these, hydrolysis gives Rf-S(O)-Rf [47, 48]. A number of these materials are remarkably stable 

toward both hydrolysis and oxidation [44, 46, 47]. 

Table 5. Perfluoroalkyl Sulfur Compounds. 

23
CCOD 

ID 

Compound Formula Boiling

Point, °C 

1362 1,1,1,3,3,3-hexafluoro-2-thiopropane CF3-S-CF3 a-22. 

1282 1,1,1,3,3,4,4,4-octafluoro-2-thiobutane CF3-S-CF2CF3

b

6.3 

1364 1,1,1,3,3,4,4,5,5,5-decafluoro-2-thiopentane CF3-S-CF2CF2CF3

b

38.6 

1365 1,1,1,2,2,3,3,3-octafluoro-2-thiopropane CF3SF2CF3

b

21.0 

1366 1,1,1,2,2,3,3,4,4,4-decafluoro-2-thiobutane CF3SF2CF2CF3

b

44.1 

1367 1,1,1,2,2,3,3,4,4,5,5,5-dodecafluoro-2-thiopentane CF3SF2CF2CF2CF3

b

69.3 

1368 1,1,1,2,2,3,3,4,4,5,5,6,6,6-dodecafluoro-3-thiopentane CF3CF2SF2CF2CF3

b

69.5 

1369 1,1,1,3,3,3-hexafluoro-2-oxo-2-thiopropane CF3S(O)CF3

b

37.3 

1370 1,1,1,3,3,4,4,4-octafluoro-2-oxo-2-thiobutane CF3S(O)CF2CF3

b

58.2 

1371 1,1,1,3,3,4,4,5,5,5-decafluoro-2-oxo-2-thiopentane 3S(O)CF2CF2CF

3

b CF 64.0 

a

Reference 44. 

b

Reference 46. 

One major drawback of sulfur compounds is the formation of toxic sulfur oxides 

during fire extinguishment. 

3. Selenium, Tellurium

The only compound containing one of these elements for which fire 

extinguishment capabilities have been reported is perfluoroselenolane [49]. There is no 

indication that selenium or tellurium compounds exhibit catalytic flame inhibition or that such 

compounds would have particular utility as halon replacements. Toxicity would be of some

concern. 

H. GROUP VIII ELEMENTS 

Of the non-radioactive Group VIII elements, the noble gases (helium, neon, argon, 

krypton

rease

e

, and xenon), only xenon and to a much lesser extent, krypton are known to form

compounds. These compounds are strong oxidizing agents and would probably act to inc

flame intensity rather than decrease it. On the other hand, no studies of xenon or krypton 

compounds in flames has been carried out. Like nitrogen, all of the noble gases exhibit fir

extinguishment capabilities by dilution and cooling. 

24
SECTION IV. 

CUP BURNER 

A. OVERVIEW

In anticipation of future work on halon replacements, a number of modifications were 

made in the NMERI cup burner. These are described below. 

B. AIR FLOW

1. Mass Flow Controller 

A change was made in the cup burner to allow the use of a mass flow controller 

for the air flow for all ranges. In the past, this method has been used only for the largest air 

flows. The air flow rates are obtained from a Cole-Parmer Model 33116-60 mass flow 

controller, which covers flow rates from 0 to 50 Standard Liters/Minute (SLM) calibrated for air 

against a NIST standard. (This instrument is used to monitor the air flow rate regardless of the 

method used to determine agent concentration.) Here SLM is defined as 760 torr and 70 °F 

(21.11 °C). From 0 to 50 °C and up to 150 psia (50 psia back pressure) the outputs are 

independent of temperature and pressure. The readings, R in SLM, can be converted directly to 

air molar flow rates, nA, without concern about the actual operating conditions (Equation 18). In 

this equation, 22.41408807 is the conversion from liters to moles for a temperature of 273.15K 

and a pressure of 760 Torr. The molar flow rates can, of course, be converted to volumetric flow 

rates in any part of the air stream if the pressure and temperature are known. 

nA = R(273.15/21.11+273.15)/22.41408807 (18) 

Factory calibration gives the data shown in Table 6 and Figure 4. With a forced 

intercept of zero, the data are fit well with a line actual = 0.9928*measured. 

25
TABLE 6. ACTUAL VERSUS MASS FLOW METER READING FOR AIR FLOW RATES. 

Reading, L/min Actual Measured, L/min Difference, percent 

0.000 0.000 0

12.678 12.597 +0.6

25.359 25.052 +1.2

38.000 37.748 +0.7

50.244 49.922 +0.6

0

10

20

30

40

50

60

0 10 20 30 40 50 60

Indicated Air Flow, L/min

Actual Air Flow (L/min)

Figure 4. Actual versus Mass Flow Meter Reading for Air Flow Rates. 

2. Digital Bubble Meter 

Since the change to a mass flow controller for the NMERI Standard Cup Burner 

was made during the present project, not all air-flow data were collected using the flow 

controller. In some cases, a bubble meter with an electronic digital readout was used. The 

26
digital bubble meter used is a Humonics, Inc. digital flowmeter (a bubble meter with a digital 

electronic readout), Serial Number 006528. The digital flow meter is calibrated at 23 °C and 

1025 mb (768.8 Torr) against a National Institute of Standards and Technology (NIST) 

calibrated standard TN 244763 Teledyne HBM-1A #1265. The calibration showed percent 

errors of -1.61, +1.61, and –0.40 percent at flow rates of 1.34, 6.53, and 12.74 L/min. 

C. AGENT FLOW

A number of techniques have been used by various researchers in the past to determine 

agent flow. We discuss these below and indicate those used here. 

1. Oxygen Monitoring 

The draft ISO standard for cup burner testing [50] suggests calculating the 

extinguishment concentration, C, from the cup burner chimney oxygen concentration, O2, and 

the supply air oxygen concentration, O2(sup) (Equation 19). 

C = 100*[1-O2/O2(sup)] (19)

There are two major problems with this method: (1) Measurement of oxygen 

concentrations is usually relatively imprecise. For example, the oxygen analyzer used by one 

organization performing cup-burner determinations, is claimed to have a measurement accuracy 

of 0.15 vol% [51]. Since the concentration of oxygen in air is approximately 20 vol%, this 

corresponds to an accuracy of 0.75 percent for the oxygen concentration. Because two oxygen 

concentrations are required, the ratio of O2/O2(sup) has an accuracy of only 1.5 percent. (2) For 

halocarbon agents, the calculation of the agent concentration, C, requires subtraction of two 

numbers with nearly the same value — 1 and the O2/O2(sup) ratio, which is nearly 1 — to give a 

small number (around 0.05). For example, for FC-3-1-10 (n-perfluorobutane, CF3CF2CF2CF3) 

which has an extinguishment concentration of approximately 5.3 vol% with n-heptane fuel [52], 

the O2/O2(sup) ratio would be approximately 0.947. Thus, an error of 1.5 percent means that the 

ratio could vary from 0.933 to 0.961 with a calculated agent concentration (Equation 1) varying 

from 3.9 to 6.7 vol%, a possible error of ±24 percent for a cup burner value of 5.3 vol%. Thus, 

the oxygen concentration method, which is relatively precise for inert gas extinguishants, which 

27
have very large extinguishing concentrations, is virtually worthless for halocarbon agents. It is 

recognized, of course, that this method may have a greater utility for inert gas agents. 

Minor problems with the oxygen concentration method is that there are concerns 

about the effect of temperature and pressure on the oxygen concentration values and the 

requirement for removal of a portion of the air/agent stream below the cup during 

extinguishment if continuous measurements are required. In fact, most continuous analytical 

methods require disturbance of the agent/air flow. 

2. Direct Measurement of Extinguishment Concentration 

Gas chromatography, spectroscopy, and other methods (e.g., measurements based 

on thermal conductivity) have been used to determine the actual concentration of agent in the 

chimney following extinguishment. These methods are attractive since they measure the actual 

agent concentration; however, they are often less precise, labor intensive, and provide sources 

for error in manipulation of samples. For GC work, known concentrations of agent obtained by 

dilution of a standard are used for calibrations. Following extinguishment, the air and agent flow 

rates are maintained and aliquots are removed with a gas-tight syringe and injected into a gas 

chromatograph. Peak areas are used to determine concentrations. Past experience with GC 

analyses show that periodic recalibration is needed and that, at best, such analyses are accurate to 

within no better than 5 percent. 

3. Mass Flow Rate 

We have developed a computerized system that monitors the weight of a small 

cylinder from which agent is delivered as a function of time and calculates continuously updated 

flow rates. An electronic scale with an RS-232 output is connected to a computerized data 

acquisition. The mass flow rates, m , permit the direct calculation of the agent molar flow rates, 

n , without concern about temperature or back pressure (Equation 20). 

a

a

na = ma/MW (20)

From the molar flow rates of the agent (Equation 20) and the air, the agent 

concentration, C, can be directly calculated (Equation 21). If the mass flow controller is used to 

determine the air flow rate (Equation 18), no part of this calculation requires any knowledge of 

28
the temperature or pressure (or back pressure) within the system as long as ideality can be 

assumed (nonideality can, of course, be taken into account), the temperature at the mass flow 

controller is between 0 and 50 °C, the air pressure is less than 150 psia, and the back pressure is 

less than 50 psia. (At no time do the temperatures and pressures come close to the limits. The 

back pressures are always well under 1 psia, and the temperatures at the flow meter are around 

25 °C.) 

C = na/ nA + na (21)

The following gives an error analysis for the mass flow rate method. A typical 

mass flow rate for FC-3-1-10 (n-perfluorobutane, CF3CF2CF2CF3) to extinguish a n-heptane 

flame in the NMERI Standard Cup Burner operating with an air flow rate of 10 L/min is 

approximately 0.094 g/sec, which can be measured with the electronic balance coupled to the 

data acquisition system to ±0.001 g/sec. Since the electronic balance output is almost perfectly 

linear over small mass changes, the precision is the same as the accuracy. Note that this does not 

require the assumption that the absolute mass measured is exact, only that the mass differential is 

exact. Thus, the accuracy of the agent mass flow rates is approximately 1.0 percent independent 

of any pressures, temperatures, or back pressures. Combining the predicted accuracies for the air 

and agent mass flow rates gives an accuracy of approximately 2 percent for the extinguishment 

concentration for a n-heptane flame using FC-3-1-10 employing Equation 21). Very similar 

results are obtained for other halocarbon/fuel combinations. Note that this precision is an order 

of magnitude better than that possible for an oxygen analyzer. It is also better than the precisions 

estimated for gas chromatographic, FTIR, and rotametric methods, all of which require 

calibration for the specific agents used and, in most cases, knowledge of temperatures, pressures, 

and/or back pressures. 

4. Digital Bubble Meter 

In some cases, the flow rates of some of the less efficient highly gaseous agents 

were determined using the Humonics, Inc. digital flowmeter described earlier. The accuracy of 

this meter is approximately 2 percent for agents with extinguishing concentrations of 10 vol% in 

the NMERI Standard Cup Burner with an air flow rate of 10 L/min and the accuracy increases as 

the agent efficiency decreases (more agent required). 

29
D. CUP BURNER TESTING OF BLENDS CONTAINING PHOSPHORUS COMPOUNDS 

Trimethylphosphite, which is more volatile that DMMP, was blended with both 

HFC-227ea and HFC-125 and the extinguishment concentrations were determined using the 

NMERI Standard Cup Burner, a digital bubble meter for air flow, and mass flow rate for the 

agent. The results (Table 7) show a reduction of 1 to 2 percent (absolute) in the extinguishment 

concentration compared to the pure carriers. 

TABLE 7. CUP BURNER EXTINGUISHMENT CONCENTRATIONS, N-HEPTANE FUEL. 

Compound Formula Test

Number 

Extinguishment 

Concentration, vol% 

(Determinations) 

HFC-227ea CF3CHFCF3 981209-2 6.25 ± 0.03 (6) 

Trimethylphosphite/HFC227ea (7.2%/92.8%) P(OCH3)3/CF3CHFCF3 981209-1 5.31 ± 0.15 (6) 

HFC-125 CHF2CF3 981208-1 8.75 ± 0.21 (6) 

Trimethylphosphite/HFC125 (12.2%/87.8%) P(OCH3)3/CHF2CF3 981209-4 6.79 ± 0.15 (5) 

30
SECTION V. 

SAPPHIRE EXPERIMENTS 

A. SAPPHIRE 0-7 WITH DIMETHYLMETHYLPHSOPHONATE (DMMP). 

1. Apparatus 

The Sapphire 0-7 burner (Figure 5) was designed for the study of low volatility 

flame inhibition agents. Compounds are introduced from a heated cell, and the entire burner is 

wrapped with Nichrome resistance wire and heated to prevent condensation of the agent in the 

burner. The burner allows infrared spectroscopy of post-combustion gases. Openings are 

provided near the top of the borosilicate glass “chimney” (about 8.7 in [22 cm] above the surface 

of the burner). A Fourier Transform Infrared (FTIR) Spectrophotometer with external optics can 

be located so that the optical axis passes through the post-combustion gases. A wire mesh screen 

placed over the top of the “chimney” prevents external air currents from disturbing the test 

environment; IR-transparent windows in the openings in the borosilicate glass “chimney” are not 

required. Very satisfactory data have been obtained with this very simple arrangement. The 

Sapphire 0-7 burner also allows optical absorption photometry of the pre-combustion gases. 

31
EG IN

EG OUT

PMT

N2 IN

BLUE LED

FLOW

METERS

TC TC TC

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

AGENT

HEATER

COILS

TC

SAPPHIRE

0- 7 BURNER

OPTICAL AXIS OF

IR SPECTROPHOTOMETER

SCREEN

METHANE

AIR

IR_SAP07

Pyrex

Screen

Optical Axis of IR

Spectrophotometer

TC TC TC

EG Out

EG In

N2 In

PMT

Blue IED

Methane

Air

Flow

Meters Saphire 0-7 

Burner

Me Methane thane

Heater

Coils

Agent

TC

Figure 5. The Sapphire 0-7 Atmospheric-Pressure Burner. 

32
2. Objective 

The objective of these experiments is to characterize the extinguishment of 

atmospheric-pressure methane-air flames by dimethyl methylphosphonate (DMMP, 

O=P(CH3)(OCH3)2). These experiments should not only provide a baseline for subatmospheric 

experiments with the Sapphire 2 apparatus, but also provide insights into the mechanism of 

inhibition and extinguishment of flames by this compound. In these experiments, inhibition of 

the premixed methane-air flame is studied as a function of the mole fraction of inhibiting agent 

added to the premixed gases. Both the flow rate of methane and flow rate of air are fixed, and 

the amount of extinguishing agent is varied. Two air streams are used; one stream passes 

through a bubbler containing the extinguishing agent, while the other stream bypasses the 

bubbler. The concentration of extinguishing agent introduced into the flame is controlled by 

controlling the relative flow rates of the two air streams. 

3. Experiment DMMP_A 

A stoichiometric flame, Φ = 1.00, was used in this experiment, and the bubbler 

was thermostated at room temperature. Although information regarding the vapor pressure of 

DMMP has been sought in the available literature, no information has been found as of this 

writing. Accordingly, it was assumed that the vapor pressure is 20.3 torr, and all calculations 

were based on this value. The heat extracted by the burner was measured as a function of the 

flow rate of DMMP. Approximately 35 percent of the inhibition curve was obtained; the flame 

was not extinguished. The maximum degree of inhibition was achieved when the entire air flow 

was passed through the bubbler. To obtain a greater degree of inhibition, it will be necessary to 

use another technique for introducing the DMMP into the premixed gases. Except for the three 

data points at the highest flow rate of air through the bubbler, the data points fell on a straight 

line. This is the behavior characteristic of a thermal (physical) agent. At this point, there is no 

obvious evidence of a contribution by a catalytic extinguishment mechanism. It is suspected that 

the three data points that did not lie on the straight line (and were above the line), were the result 

of partial saturation of the air stream with DMMP at these very high flow rates. The estimated 

mole fraction of DMMP at extinguishment (obtained by extrapolation to the x axis) was 0.057 

33
(5.7 vol%). Note that, if a smaller vapor pressure had been assumed for the DMMP, the mole 

fraction at extinguishment would have been proportionately smaller. 

4. Experiment DMMP_B. 

Experiment DMMP_B was a repeat of DMMP_A. This was an atmospheric 

pressure experiment with Φ = 1.0. Midway through Experiment DMMP_A, there had been a 

jump in the data; although we believed that we knew how to treat the data for DMMP_A, a 

confirmatory experiment was desirable. Experiment DMMP_B did confirm the results of the 

prior experiment. There was a fortuitous difference between the two experiments. While 

Experiment DMMP_A was run with a DMMP reservoir temperature of 23 C, the reservoir 

temperature in Experiment DMMP_B was 19 C. The slopes of the delta-q lines were likewise 

different; the slopes were 1.71 cal/unit and 1.41 cal/unit for the 23 C and 19 C experiments, 

respectively. Making the assumptions that (1) the slope of the line is proportional to the 

extinguishment effectiveness, and (2) that the extinguishment effectiveness is proportional to the 

vapor pressure of the DMMP in the bubbler, we can estimate the heat of vaporization of DMMP 

from these data. The value obtained (using the Clausius-Clapeyron relationship) is 35 kJ/mol. 

For comparison, one obtains a value of 36 kJ/mol using Trouton's Rule, taking Trouton's Focus 

to be 88 J/mol-K and a boiling point of 181 C. The splendid agreement is far too good, given the 

small difference in temperature; however, it gives one considerable confidence in the validity of

the experimental configuration. With this heat of vaporization as a starting point, it can then be 

estimated that the vapor pressure of DMMP at 19 C is 3.83 torr. Assuming complete saturation 

of the air stream in the bubbler over the linear region of the data, it can then be estimated that the 

mole fraction at extinguishment is 0.063 (6.3% by volume). This is similar to the value expected 

for this compound. In summary, the Sapphire 0 experiments are yielding excellent data of 

considerable value. In addition to providing estimated extinguishment concentrations, these 

experiments also give insight into the extinguishment mechanism. At this point, it can be said 

that the data are consistent with a purely thermal mechanism. Experiment DMMP_C was 

performed on 29 Oct; for this experiment, Φ = 0.9. At first look, it looks like something may 

have happened part way through the experiment which invalidated the data; analysis will 

continue today. 

34
SECTION VI. 

RESULTS AND CONCLUSIONS 

A. RESULTS 

With few exceptions, compounds of the s block elements (the alkali metals and alkaline 

earths—Groups IA and IIA) are solids and do not meet a primary requirement for materials 

considered here (that the compounds be gases or liquids). Most of the compounds which have 

halogens (Group VIIB) as a primary constituents and which could be considered as halon 

replacements are either halocarbons, amines, or ethers (which are excluded) or contain other 

non-carbon main group elements and are considered with those elements. The only non-halogen, 

non-transition metal elements that appear to provide any significant catalytic fire suppression 

capabilities are the alkali metals, tin, lead, and phosphorus. 

H. IMPORTANT FINDINGS AND CONCLUSIONS 

With one major exception, the results of the search have been disappointing. Few 

materials based on main group elements with potentially acceptable physical (primarily, 

volatility) and toxicological properties have been identified. The lowest molecular weight 

materials (i.e., the materials with the highest volatilities) often have hydrogen or halogen atoms 

directly attached to non-carbon atoms. The former (containing hydrogen) are often flammable, 

and the latter (containing halogens) are usually toxic (owing to hydrolysis). Both often have low 

stabilities. Moreover, few elements provide catalytic fire suppression capabilities, and 

compounds of most of those do not meet the requirements of this study. A major exception is 

fluoroalkyl phosphorus compounds, which hold significant promise despite their relatively low 

volatility. 

35
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