THIS WAS SCANNED FROM A PDF SO THERE ARE SOME ERRORS AND IS MISSING DATA.
1962
TOXIC HAZARDS OF NEW PROPELLANTS
Anthony A. Thomas, MD
Chief, Toxic Hazards Section
6570th Aerospace Medical Research Laboratories
Wright-Patterson Air Force Base1 Ohio
The problems associated With the handling of cryogenic propellants and the resulting slow-down of missile launchings, coupled with the need for higher specific impulses, long storability and instant readiness of missiles has resulted in a concentrated effort to develop suitable new propellants. This second generation of propellants, called the storables, is finding increased use in our new weapon systems. The first typical examples are the TITAN II and Minuteman ICBM the first being a liquid, the second a solid fuel missile.
Development of course, has not stopped with these prototypes, and a. real structural. roulette is in full swing throughout the propellant industry for better, high energy propellants. The result of this search is a new brand of chemistry, and hence the terminology of “exotic” compounds Is well justified.
In the oxidizer field, basic theoretical trends are illustrated in Table 1. The synthesis of oxygenated, fluorinated, interhalogenated, oxyfluorinated, and other N-F, and N-O-F compounds, coupled with the drive to increase stability has resulted in thousands of new compounds, never encountered before by the toxicologist.
TABLE 1
Theoretical Trends in Storable High Energy Oxidizers
o - 0 Chemistry:
03, H202
N - 0 Chemistry:
HNO3, N204
0-F Chemistry:
°~ 2’ 03F2
Interhalogens:
C1F3 BrF5
N - F Chemistry:
NF3, N2F4
The fuels are equally represented by new variations on the recurring themes of hydrazine, boron, aluminum, beryllium, and hydrogen chemistry. In addition, stabilizers, plasticizers, curing agents, and. other necessary additives are being sought with relatively high energy content to improve overall performance.
Many of these newly synthesized compounds are of theoretical interest only, and are not stable enough to be useful as propellants. Therefore, various radicals are added to prevent decomposition, shock sensitivity, and explosive properties. This structural manipulation is not a new phenomenon, and is well known from the other branches of chemical industry, especially the polymer and pharmaceutical endeavors. It is needless to say that minor changes in the basic structures can result in entirely different biological activity, and therefore a systematic toxicological screening program is becoming increasingly difficult, unless it is pursued with the same intensity as the development of new drugs and therapeutic agents.
Let us therefore turn our attention to those newer propellants, which have proven their value in small scale tests at least, and to those which are approaching operational use. Table 2 is a summary of such fuels and oxidizers.
TABLE • 2
New High Energy Propellants in Use
Liquid Fuels ft H Solid Fuels
Hydrazine Aluminum
1, 1 - dimethylhydrazine JH3 Beryllium
(UDMH) K CH3
Monomethyihydrazine H> -
(MMH) H ‘tH3
Aerozine - 50 (a 50-50 mixture of hydrazine and UDMH)
Pentaborane B5H9
Decaborane
Liquid Oxidizers Solid Oxidizers
Nitrogentetroxide N204 Chlorine derivatives
Chlorinetrifluorlde C1F~ Fluorine derivatives
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Toxicity of these propellants is considerably higher than any of the cryogenics used before except liquid fluorine. For general guidance, the MMH is approximately twice as toxic as hydrazine, and UDMH is approximately half as toxic., on a mg/kg basis. The fallacy of this comparison becomes evident however, if one compares vapor pressures. To illustrate that toxicity and toxic hazards are two entirely different indexes, let us consider Aerozine-50. UDMH has a vapor pressure ten times higher than hydrazine, and although hydrazine is twice as toxic as UDMH it contributes no practical inhalation hazard to the TITAN II fuel. Hydrazine could become Important as an inhalation hazard if the UDMH was already evaporated as could occur in an open spill situation. Naturally, the skin absorption hazard from hydrazine becomes negligible if protective clothing is worn during handling.
The boranes are much more potent than the hydrazines. Since it takes so little to cause serious intoxication,, the marked differences in vapor pressures of’ pentaborane and decaborane affords no practical protection considering the 0. 001 and 0. 05 ppm suggested Threshold Limit Values (TLV). While it is true that pentaborane will evaporate much faster than decaborane, both can reach hazardous concentrations in air within a few minutes.
Another salient point in judging the toxic hazards of propellants is the amount of warning one can expect during inhalation exposure to high concentrations. Table 3 points out these differences.
TABLE 3
Warning Symptons for High Concentrations
Hydrazines: Strong Amine Odor - respiratory irritation.
Fluorines: Corrosive Fumes - bronchospasm, respiratory and eye irritation.
N2 04(2N02): Acid Odor -‘ No immediate respiratory irritation.
Boranes: Questionable, Faint Odor no immediate symptoms.
Practical human exposure experience has substantiated the observations in this table. While it is almost impossible to voluntarily tolerate acutely toxic concentrations of the hydrazines and fluorines, the unnoticed severe exposures of propellant handling personnel to NO2 and boranes is a frequent occurrence.
While the above is true in acute exposures, none of these compounds possess adequate warning character in chronic exposures, that is, during production and pilot-plant operations. There, the compliance with TLVs and good industrial hygiene practices is of utmost importance.
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Fortunately, the dose-response relation is very predictable with these propellants. Figure 1 illustrates the dose-time relations with UDMH, hydrogen fluoride, NO2, and pentaborane. The steeper these slopes, the more predictable is the ‘toxic effect (in this case, death) at any point on this graph. Also, because the toxic effects are more predictable,one can set tolerance limits with much more accuracy and validity. Consequently, one does not feel compelled to incorporate tremendous safety margins in these limits. While this is a marked advantage and can expedite significantly the handling of these propellants, there is a distinct danger in the misuse of these limits by laymen who do not realize the absence of the usual 10-fold safety margin.
To elaborate this point, Table 4 lists the criteria on which the well publicized Emergency Tolerance Limits to the TITAN II propellants were based:
TABLE 4
Acute Tolerance for Single Exposure
(in ppm)
Animal (Measured) Human (Suggested)
“No Death” “No Pathology” “No Effect”
(Rats) (dogs)
5 mm 60 mm 5 mm 60 mm 5 mm 60mm
UDMH 19,800 813 600 50 50 10
NO2 1.90 72 104 28 35 10
Pentaborane 62 7. 5 - - -
Clearly, a ten-fold excess of the suggested 35 ppm tolerance for the specified 5 minute period to NO2 could lead to severe pulmonary edema in any exposed person, and undoubtedly would result in death in some
cases. By the same token, a 5 minute exposure to 500 ppm UDMH could become a most unpleasant experience, far from the level of no effect.
The probable source of such misconceptions is illustrated in Table 5. The day-by-day, 8-hour TLV’s are set for the avoidance of chronic, repeated exposures, and entails a wide margin of safety.
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TABLE 5
Human Tolerance Limits for Inhalation (in ppm)
Propellant TLV Emergency Single
Tolerance
(8-hr. repeated) 5 mm 15 mm 30 mm 60 mm
Nitrogen Tetroxide 5.0 35 25 20 10
Hydrazine 1. 0
UDMH 0.5 50 35 20 10
Pentaborane 0. 005
Needless to say, one can occasionally be exposed to 10 times the TLV of any of the propellants for a few minutes without serious consequences. A much greater danger of the misinterpretation of Emergency Tolerance Limits is the use of these criteria for the calculation of toxic exclusion distances as they affect the communities adjacent to missile bases. Beyond 10 miles or more, the theoretical prediction of toxic vapor concentrations can easily be in error by a, factor of ten. An added consideration, is that these limits were set for Air Force use and assume a healthy, relatively young adult population. On the other hand, civilian communities consist of both healthy and sick, young and old, some with severe asthmatic and cardiac conditions. Such people could not necessarily tolerate even the emergency levels.
To complete this discussions, let it suffice to say that the toxicological problems associated with solid propellants are less numerous. The more serious problems are usually related to either exhaust products or processing accidents. Basically, the toxicology of chlorine and fluorine derivatives is not new. There are, however, some intriguing problems on the toxicology of beryllium oxide and the epidemiology of chronic beryllium disease.
There are a number of important toxic hazard considerations with large scale propellant operations, but they :‘fall beyond the scope of this presentation. The area of detection, diagnosis and therapy, environmental pollution, site selection, and toxic exclusion radii are discussed by the author In a recent issue of Industrial Medicine. The reader is referred to that publication for more detailed discussion of the problem.
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