Analysis of hypergolic igniters

MICROCHEMICAL

JOURNAL

9, 209-217 (1965)

Analysis

of Hypergolic

Igniters

NORMA V. SUTTON AND HANS SCHNEIDER~ Chemistry

Section, Research DepartmeTkt, Rocketdyne, A Division Aviation, Inc., Canoga Park, California

of North

American

Received March 10, 1965 INTRODUCTION

When the firing of an Atlas liquid rocket engine is initiated, the sequence of events that follows is timed in milliseconds. The most critical factor in a smooth start is called the “ignition delay.” The device placed in the fuel line to initiate smooth, steady ignition has been a cartridge which contains a mixture of 1.5 per cent triethylaluminum and 85 per cent triethylboron, called TEA/TEE, a hypergolic igniter. The quality control of the 15/85 mixture of triethylaluminum and triethylboron is fundamental to the assurance of reliability for engine ignition and the guarantee of safety for the astronaut in the capsule atop the missile. Prior to the development of the chemical analysis, quality control to obtain an overall average ignition delay was performed mechanically by smallscale destructive testing of a statistically applicable number of cartridges in a lot. The infrared spectrophotometric procedure is based upon five years of correlation studies between absorbance differences and ignition-delay data. The determination utilizes the C-C stretching mode at 10.1-10.2 ‘~1 for triethylaluminum, and the CHs rocking mode at 9.7-9.8 p for triethylboron. EXPERIMENTAL

Mass spectrometric analysis as well as gas chromatographic analysis of the hypergolic mixture (15/85) resulted in fragmentation and decomposition of the igniter fluid in such a way that the products were apparent, but the original quality of the material was not assured. Wet chemical analysis (1, 3), also involves destructive testing that does not give an accurate description of the source of the aluminum, whether tied up as triethylaluminum, diethyl aluminum hydride, aluminum ethoxide, or the most detrimental species, aluminum oxide. 1 Present address: Eau Gallie High School, Eau Gallie, Florida. 209

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V. SUTTON

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HANS

SCHNEIDER

Infrared analysis was chosen as the most informative route. Attempts to determine the concentration of triethylaluminum and triethylboron failed because a Beer’s law plot was unattainable. The ignition-delay data and spectrum assignments of over 200 samples of the pure components and their mixtures obtained from various vendors were examined in an effort to find some definitive empirical relationships. The absorbances that were found to be definitive for the purposes outlined were the C-C stretching mode for TEA at 10.1-10.2 l.~ (987 cm-‘) and the CHs rocking mode for TEB at 9.7-9.8 p (1023 cm-l). The absorbance at 4.8 u (2080 cm-l) has proven to be a satisfactory I,,. The advantages of this method are fourfold: 1. Although the relationship of TEA to TEB is not quantitatively expressed in a linear fashion by the ratio, an applicable value is determined. 2. Impurities in the original components and the mixtures are easily detected. 3. Oxidation of the sample because of handling techniques, sampling, aging, or leaking is immediately evident. 4. A permanent “fingerprint” record is available. Apparatus The apparatus consists of a well-lit dry box with gaseous nitrogen purge, drying train, and exhaust to a well-ventilated hood; a spectrophotometer, Perkin Elmer Infracord; and NaCl cells, 0.05 mm. Safety Precautions The procedure for this analysis should be carried out with utmost care because of the hazards associated with both triethylaluminum and triethylboron. In their article on pyrophoric organometallics, Mirviss, Rutkowski, Seelbach, and Oakley (6) cite that the lower-molecular-weight trialkylaluminums and trialkylboranes up to Cd are spontaneously flammable in air at ambient temperatures, regardless of the quantity. With a small exposed surface, these materials smoke badly, and their temperatures rise well above room temperature, with possible ignition. Mirviss et al. mention that no published toxicity data exist for the trialkylboranes, but that the latter are arbitrarily treated as if they were as toxic as the boron hydrides. Experience at Rocketdyne has been such that exposure to a noticeable odor (sickeningly sweet) for more than 20 minutes produces mild nausea, and until more information is known, the toxicity level is assumed similar to pentaborane. Studies (8) have shown that pentaborane affects the central nervous system, and the American Conference of Governmental

ANALYSIS

OF HYPERGOLIC

IGNITERS

211

Industrial Hygienists has established a maximum allowable concentration of 0.005 ppm for an eight-hour day of continuous exposure. The triethylboron does not react with water or alcohol under ordinary conditions in the absence of a coupling agent; hydrolysis is slow, and often droplets of TEB will float on the surface of a solution with the imminent possibility of bursting into flame. Triethylaluminum instantly ignites in air and reacts violently with water. Sampling

Sampling of the cylinder should take place in a dry box, with constant flushing, using a dry (less than 1 mg HzO/cu ft) nitrogen purge. The samples may be taken in a small glass vial for temporary storage (not more than 2 days) as long as the samples have aluminum-lined caps (TEA will react with the moisture in paper caps). In the absence of a moisture monitor or other means of measuring moisture content, the atmosphere may be considered satisfactory if only slight wisps of smoke appear when the cap is removed. Fuming, similar to that of a lighted cigar, indicates the presence of excessive moisture. Preparation inside the dry box should include all materials necessary once the analyst is confined to the enclosed area. Clean, dry, nitrogenpurged syringes (x-s ml) should be set up in a beaker. The salt cells should also be purged and fitted with Teflon plugs. An open beaker containing about 25 ml of saturated hydrocarbon diluent (heptane is satisfactory) should be available for rinsing emptied syringes as well as emergency quenching. Solutions of greater than lo-per cent aluminum-alkyl concentration should be handled as spontaneously flammable materials. Kimwipes saturated with the hydrocarbon diluent are useful for wiping up small spills, and should be kept in the glove box so that there is no possibility of moisture retention occurring. A pair of thin plastic gloves over the regular dry box gloves has made it possible for the latter to see longer service, since the hypergolic mixtures attack the rubber gloves quickly. In case of a spill, fire prevention by elimination of uncontrolled contact with reactants should be the first concern. Confinement of the reaction should follow. The CO2 extinguishers moderate the reaction and accomplish controlled burning. Also effective is dry sand, but this must cover the area to a 2- to J-inch depth. Carbon tetrachloride and chlorobromomethane should not be used because of intense smoking and the fact that one of the reaction products may be phosgene. Following the analysis, the cells should be returned to the dry box, the

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NORMA

V. SUTTON

AND

HANS

SCHNEIDER

hypergol emptied into the diluent, and the cells rinsed with diluent, purged with gN2, and allowed to dry. The diluent containing the igniter fluid can then be removed from the dry box to a suitable solvent disposal area. This solution is destroyed by reaction with isopropanol or water, or burned for final decomposition. Dry-Box Procedure Prior to opening the vials to the gN2 atmosphere, the contents should be visually examined against a good light to discover particulate matter and any incipient precipitate. The liquid should be clear and colorless, free from suspended particles and gelatinous precipitates, and contain no more than three or four salt-and-pepper-type grains in the bottom of the vial. These grains are almost unavoidable because of the reactivity of the hypergols, unless the material is prefiltered. The vial cap is then removed, set aside, and the sample removed from the container by means of the clean, dry, nitrogen-purged syringe. The NaCl cell is filled with the hypergol, and the Teflon plugs tightly fitted at each port. A Kimwipe saturated with heptane is used to cleanse any residual igniter material from the surface of the cell prior to its exit from the dry box, and the IR spectrum is then run. The cell is removed from the instrument, returned to the dry box where it is immediately Hushed into the heptane diluent, and clean heptane is flushed through the cell. Because hypergol mixtures attack NaCl cells, prompt cleaning procedures for the cells are recommended to extend the latter’s lifetime. RESULTS

AND

DISCUSSION

Examination of the IR Spectrum Since the hypergol is a mixture of triethylaluminum and triethylboron, it is distinctly advantageous to have spectra of the unmixed compounds (Figs. 1, 2, and 5). A spectrum of the material that has proved to be satisfactory for the triethylboron is shown in Fig. 1. Of special significance is the fact that there is an absence of absorbance at 6.38 p (1560 cm-l), which would indicate the presence of a tetraalkyldiborane (Fig. 2). These compounds (2) react with the hypergol, and the net result is a loss of TEA. R 28”\H,BR”

+ (W%),Al

___

Bz03 + ( ClHB)3B--

R,B + AlH (W&Al (CJ-&AlH AlH, (C$&)AlH,

B,(OR)s

ANALYSIS

OF

HYPERCOLIC

IGNITERS

213

This latter reaction is slow, but once the alkyl borate is formed, and ( CzHj) BAl added, a fast reaction occurs, and the B, (OR) X is never seen on the spectrum. B,(OR),

+ (CzHj)sAl---+

(GHE)F$ + -%4.%

The presence of diethylaluminum hydride in a fuel is not in itself an indication of the ignition quality of the fuel. The dialkylaluminum hydrides (4) exhibit a stretching mode at 5.64 u ( 1770 cm- 1). Hydride may be present initially, or it may be the product of a continuing reaction caused by other impurities (Fig. 4)) but it does not seem to adversely affect the pyrophoricity of the material. In general, the smaller the alkyl group, the more it is pyrophoric, i.e., the triethylaluminum, (&Hs)~A~, is more pyrophoric than the tributylaluminum, ( C4H,) sAl. Some indications of unusual oxidation are evident in Fig. 4 as compared with Fig. 3. The spectrum exhibited in Fig. 4 gives totally acceptable ignition-delay data, but oxidation-product build-up would commence much more quickly with this sample, where the reaction has already started, than with the sample which has not experienced an initiating oxidation. Scott et al. (9) and Ludke et al. (5) have studied spectra of some aluminum soaps which may contain bonds similar to those present in the aluminum alkoxides. Wilhoit et al. (20) have summarized these as follows: 1. Absorption bands typical of compounds containing ethoxide groups occur at 8.6 p (1160 cm-l) and 9.16 p (1090 cm-l). 2. Vibrations arising from the Al-O + Al coordinate bond occur in the vicinity of 10.1 p (990 cm-l). This absorption peak is found in aluminum oxide, aluminum soaps, and aluminum alkoxides. 3. Vibrations in the Al-O-C bonds give rise to absorptions at 9.7 u (1033 cm-l) in aluminum isopropoxide, at 9.48 p (1058 cm-l) in aluminum secbutoxide, and at 9.32 ~1(1070 cm-‘) in aluminum 2-pentoxide. References in the Russian literature (7) give reactions for the oxidation of (CJH~)~AI with air at different temperatures. Low temperatures and low concentrations favor the formation of peroxides, and have shown iodometrically that the reaction with air and (CZH;,) aAl proceeds in three steps: 1.

(CoH,).?Al + 0-b

2.

(C~H~)~AIO(C~H5) --+(C~Hs)Al(OC~H;,)s

3.

(C2Hj)Al(OC,H5)z

(C~H:)PAIO(C~H~) + (CsHj):~Al-----+

2(C2H,)zAlO(C,H,)

Texas Alkyls has reported that the exa’ct mechanism of the intermediary

214

NORMA

FIG.

V. SUTTON

1.

AND

HANS

IR spectrum of triethylboron.

WAV&ENGTH,-MICROC;S

FIG. 2.

IR spectrum of triethylboron

FIG. 3.

SCHNEIDER

with

”

‘=

tetraethyldiborane

IR spectrum of triethylaluminum.

‘*

‘-

impurity.

ANALYSIS

OF

HYPERGOLIC

215

IGNITERS

0 WVELENGTH. FIG.

FIG.

5.

4.

MlCRONS

IR spectrum of triethylaluminum

15jSS mixture

of triethylaluminum

with

reaction.

and triethylboron.

combustion reaction is not known, but it is hypothesized to begin with very rapid stepwise oxidation, as in the controlled oxidation reaction, immediately followed by : OCnH3 Al -OC&Hn ’ + 9 02+1/2 \ OC-H5

A1203 + 6 CO2 + +j Hz0

and/or OCzHB Al -OC2H;, / + 3HzO--+1/2 \ O&H>

AlaO:: . 3H20 + 3C2H50H

216

NORMA

V. SUTTON

AND

HANS

SCHNEIDER

Calculations

A.

TEA/TEB

Absorbance Difference

1. Find I, at 4.8 p (2080 cm-‘). 2. Find I for TEA, C=C stretching mode at 10.1-10.2 p (987 cm-‘). 3. Find I for TEB, CHa rocking mode at 9.7-9.8 p (1023 cm-‘), 4. Calculate log I,/1 to get the absorbances of TEA, TEB. 5. Divide the individual absorbances by the cell-path length in mm to obtain absorbance/mm. 6. Obtain difference of TEA and TEB. B.

Hydride 1. Find I, at 4.8 u (2080 cm-l). 2. Find I for diethylaluminum hydride at 5.64 lo (1770 cm-l). 3. Obtain log Io/Ihpdride an d divide by cell-path length to get As/mm.

Diflerences The conclusions drawn from these studies are: 1. TEA/TEB absorbance differences of 1 -+ 0.2 correspond to marginally acceptable hypergolic igniters. 2. TEA/TEB absorbance differences of less than 0.8 have been shown by hypergolic igniters which have had consistently unacceptable ignitiondelay values. 3. TEA/TEB absorbance differences of greater than 1.2 have been registered by those hypergolic igniters which have shown consistently acceptable ignition delays. ACKNOWLEDGMENTS The authors are indebted to G. D. Artz, J. D. Cordill, and J. E. Hilzinger of the Solid Propulsion Section of Rocketdyne Research for their entire program on ignitiondelay data. Gratitude is also expressed to Dr. K. H. Mueller, Dr. E. F. C. Cain, and R. E. Bell for their many contributions during the initial stages of this work. REFERENCES 1.

Michigan, “Triethylaluminum Analytical (1959). ASHBY, EUGENE C., New synthesis of trialkylboranes. J. Am. Chem. Sm. 81, 4791 (1959). ANDERSON

Methods”

2.

CHEMICAL

Co.,

Weston,

ANALYSIS

3.

OF HYPERGOLIC

IGNITERS

217

New York, New York, Technical Data Sheet, “Triethyl(1959). 4. HOFFMAN, E. G., AND &HOMBURG, G., Infrarotabsorption and assoziation von dialkyl-aluminumhydriden. Z. Elektrochem. 61, 1101-1109 (1957). 5. LUDKE, W. O., WIBERLEY, S. E., GOLDENSON, J., AND BAUER, W. H., Mechanism of peptization of aluminum soap-hydrocarbon gel based upon IR studies, J. Phys. Chem. 59, 222 (1955) 6. MIRVISS, S. B., RUTKOWSKI, A. J., SEELBACH, C. W., AND OAKLEY, H. T., Pyrophoric organometallics. 2nd. end Eng. Chem. 53, 58-62 (1961). 7. RAZURAEV, G. A., GRAEVSKII, A. I., DEMIN, 0. I., MINSKER, K. S., AND SUKHAREV, G., “Oxidation of Triethylaluminum and Investigation of the Catalytic Properties of the Oxidation Products,” Tr. po Khim. Khim. Tekh. 3, 373-80 (1960) ; Chem. Abstr. 55, 25430 (1961). 8. ROCKETDYNE, “Pentaborane Handling Manual,” 49 pp. Contract AF 33(616)-6939, AF/SSD-TR-61-10, Canoga Park, California, 1961. 9. SCOTT, F. A., GOLDENSON, J., WIBE~EY, S. E., AND BAUER, W. H., IR spectra of aluminum soaps and soap-hydrocarbon gels. J. Phys. Chem. 58, 61 (1954). 10. WIUIOIT, R. C., BURTON, J. R., Kuo, Fu-TIEN, HUANG, SUI-RONC, AND VIQUESNEL, A., Properties of aluminum ethoxide. /. Znorg. Nut. Chenz. 24, 851-861 (1962). ETHYL

CORPORATION,

aluminum”