Oct., 1957
REACTIONS OF
THE
SYSTEM BARIUM PEROXIDE-MAGNESIUM-CALCIUM RESINATE
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PRE-IGNITION AND IGNITION REACTIONS OF THE SYSTEM BARIUM PEROXIDE-MAGNESIUM-CALCIUM RESINATE1 BY VIRGINIA D. HOGAN AND SAULGORDON Pyrotechnics Chemical Research Laboratory, Picatinny Arsenal, Dover, New Jersey Received April 4, 1967
The chemical reactions involved in the thermal ignition of the propagatively burning system 81% barium peroxide~l776~ magnesium-’2% calcium resinate were studied by the techniques of thermogravimetry, differential thermal analysis and determinations of the time to ignition as a function of temperature. From these data, including the energy of.activation for ignition and an intercept value proportional to the Arrhenius constant A , a mechanism is proposed for the ignition reactions of the system and a correlation is obtained between the calculated “activity” and that obtained using the absolute rate theory.
Introduction The system barium peroxide-magnesium-calcium resinate (81/17/2) is a standard pyrotechnic igniter composition that was empirically developed without a knowledge of the reaction mechanisms involved in the propagative ignition.2 I n order t o investigate the reactions of this system with respect t o the thermal parameters of ignition and propagative burning, and to elucidate the reaction mechanisms responsible for these phenomena, the ingredients, various binary mixtures, and the above ternary composition were studied by means of thermogravimetry, differential thermal analysis, and determinations of the time to ignition as a function of temperature. The thermogravimetric curves yield data concerning any physical or chemical reactions accompanied by weight changes which are characteristic of the sample under study, while differential thermal analyses furnish data relative to the absorption or evolution of heat as a result of these physico-chemical phenomena. The experimental values of the time to ignition were used to calculate the energy of activation for ignition and an intercept value proportional t o the Arrhenius constant A . On a basis of the experimental data, a mechanism is proposed for the ignition reactions of the system and a correlation is obtained between the value of the “activity” calculated from these data and that obtained using the absolute rate theory as previously applied t o propagatively reacting ~ y s t e m s . ~ Experimental
The inaccuracy of this assumption has been reported. Approximately 50 g. of each composition investigated was prepared from the weighed ingredients and samples for individual determinations were taken from the bulk supply. These compositions are listed in Table I.
TABLE I COMPOSITIONS INVESTIGATED Barium peroxide
Binaries
87.5 97.6
..
Ternary
81
Composition, % Magnesium
Calcium resinate
12.5
..
89.5 17
2.4 10.5 2
..
Apparatus and Procedures.-Thermogravimetry consists in continuously weighing a sample as it is heated a t constant temperature or to elevated temperatures at a constant rate so that the weight change curves are obtained as a function of time or temperature. The apparatus used in this investigation was a Chevenard photographically recording thermobalance converted to electronic recording as previously described.8 A Gardsman stepless rogrammiiig and controlling pyrometer (West Instrument was used to control the heating rate of the furnace a t a nominal 15’ per minute, which was monitored by recording the temperature in the furnace above the sample as function of time on a Brown Electronik potentiometric strip chart recorder.B Thermogravimetric curves were obtained with. a Mosely Autograf X-Y recorder which plots the change 111 weight as a function of furnace temperature. This furnace temperature was found to be five to thirty degrees higher than the actual sample temperature. Due to the possibility of damage to the balance resulting from a sudden ignition, only the individual ingredients were studied by this technique. The samples, of sizes chosen so that the expected weight change would be no more than 200 milligrams, were contained in No. 000 high form Coors crucibles. Reagents.-Barium peroxide, analytical reagent grade, Differential thermal analysis (DTA) involves heating the. assay 88.0% barium peroxide (Mallinckrodt Chemical sample under study together with a thermally inert referWorks); atomized magnesium 98.0% free metallic mag- ence material to elevated temperatures a t a constant rate nesium4 (Golwynne Chemical Co.); and calcium resinate, while continuously recording the temperature difference 77.7% calcium resinates (Barium and Chemicals Inc.). between them. The apparatus employed has been described The nominal purity of calcium resinate, a natural product, by Gordon C a m ~ b e l l . ~A nominal 15’ per. minute is determined by analyzing for the amount of calcium as- heating rateand was employed. The temperature difference suming that the formula for the resinate ion is (CZOH~BO~)-.~’~ between the sample and reference materials was recorded as a function of time on a General Electric photoelectric record( 1 ) Presented before the Division of Physical and Inorganic Cheming microammeter while the Sam le temperature was reistry at the 130th Meeting of the American Chemical Society, Atlantic corded as a function of time on a %rown Electronik potenCity, N. J., Sept. 21, 1956. tiometric strip chart recorder. Synchronous reference (2) (a) T. Stevenson and E, R. Rechel, U. 9. Patent 2,115,047, marks were placed on both charts a t one minute intervals (April 26, 1938); (b) D . Hart, “Investigation of Stability of Igniter by a cam-operated synchronous-motor-driven timer. Sample Composition “K” and Red Tracer Composition,” Picatinny Arsenal sizes of ingredients and mixtures varied from about one to Technical Report No. 1645, 1947. 6ve grams and were chosen to produce approximately full (3) E. 8. Freeman and S. Gordon, THISJOURNAL, 60, 867 (1956). scale differential temperature deflections. (4) Purchase Description, PA-PD-265, March 21, 1953, Powders, Measurements of the time to ignition as a function of temMetals, Atomized (for use in Ammunition). perature were used to determine the thermal parameters (5) S. M. Kaye, “A Method for the Determination of the Purity of of the propagative ignition for the two binary fuel-oxidant Barium, Calcium and Magnesium Stearates and Calcium Resinate by Titration in Non-aqueous Medium,” Picatinny Araensl Technical Report No. 1926. 1953. (6) Military Specification, MIL-C-20470. Nov. 27, 1951, Calcium Reainate.
80.)
(7) W. F. Fowler, Ore. C h e n . Bull. Easlman Kodak Co., 26, 3 (1954). (8) S. Gordon and C. Campbell, Anal. Chem., 28, 124 (1956). (9) 8. Gordon and C. Campbell, ibid., 27, 1102 (1955).
VIRGINIAD. HOGAN AND SAULGORDON
1402
Vol. 61
minations were made for the magnesium and barium peroxide powders using the Fisher Sub-sieve Sizer. This is an air permeability method which gives a value for the average particle diameter, assuming a spherical shape." 1,09269
Results and Discussion
150180I
IO0
200
I
300
400
500
600
700
800
900
TEMPERATURE PC).
Fig. 1.-Thermogravimetric analyses of the ingredients barium peroxide, magnesium and calcium resinate a t a heating rate of 15" per minute.
i,. '
Ba O1
Ih
Temperature ("C.). Fig. 2.-Differential thermal analyses of the ingredients barium peroxide, magnesium and calcium resinate a t a heating rate of 15' per minute. and the ternary compositions. The method and apparatus employed have been described previously.lo To calculate the activity of the reactants using the absolute rate theory, i t is necessary to know the particle size of the materials considered. Particle size deter(10) s. Gordon and C. Campbell. "Fifth Symposium on Combustion (Proceedings)," Reinhold Publ. Corp., New York, N. Y . , 1955, PP. 277-284.
Since thermogravimetric curves are quantitative representations of weight changes, they can be related to the chemical and physical changes taking place in a sample as it is heated and can often be used to determine the nature of the products. These studies showed that all three ingredients undergo thermal reactions involving weight changes. The curves obtained by plotting weight change in milligrams as a function of temperature in degrees centigrade are illustrated in Fig. 1. Barium peroxide exhibits a weight loss beginning at about 600" corresponding t o the loss of one atom of oxygen. Magnesium shows a continuous gain in weight which begins at about 600' and continues on pmt the maximum temperature. The curve varies in slope becoming perceptibly steeper at 850" and markedly less steep at 675". This weight gain is attributed to the successive formation of magnesium nitride and magnesium oxide. Calcium resinate first shows a continuous weight loss extending from about 110 to 560°, which markedly changes in slope in the region 350 to 450". This loss is followed by a more gradual one which levels off at about 800°, leaving a 12% residue. Differential thermal analyses were run on the ingredients and all of the mixtures, with differential temperature plotted on the ordinate and time on the abscissa. The zero of differential temperature is placed at the center of the record so that exothermal reactions are plotted as upward deflections while endothermal reactions are plotted as downward deflections. The vertical lines crossing the curves are the reference timing marks. Deflections are curvilinear because of the construction of the photoelectric recorder. The DTA curves for the ingredients are illustrated in Fig. 2. Barium peroxide exhibits no thermal effects other than a small endothermic reaction in the region of 100" which is due to adsorbed moisture. The differential thermogram of Mg s h o w a step-like exothermic peak beginning at about 406" which may be due t o either the formation of magnesium nitride on the surface of the sample or reaction with the glass tube. This peak is successively followed by the endothermic fusion of magnesium and another exothermic peak probably caused by oxidation of molten magnesium. Differential thermal analysis of calcium resinate reveals varied thermal effects. First, the curve exhibits an endothermic peak where water is given off. A small endothermic pip at 214" is followed by a broad region of altered baseline during which white fumes are given off and a yellow liquid refluxes in the top of the sample tube. At about 420" the baseline shifts again, this time in an endothermic direction, and in this region boiling and spattering sounds are heard accompanying the refluxing, By 600", almost the end of the run, charring is observed in the tube. As shown in Fig. 3 the binary fuel-oxidant mixture magnesium-barium peroxide ignites under (11) Fisher Scientific Co.,'717 Forbes St., Pittsburgh 19,
Pa
.
,
Oct., 1957
REACTIONS OF THE SYSTEM BARIUM PEROXIDE-MAGNESIUM-CALCIUM RESINATE
DTA conditions at 540". A small endotherm at 78" and an exotherm a t 465" precede the exothermal pre-ignition reaction that culminates in a highly exothermal ignition and combustion. The binary fuel mixture magnesium-calcium resinate gives a DTA curve which exhibits, first, an exothermic baseline shift during which the formation and charring of a yellow liquid condensate are observed. An exotherm at 578" and the endothermic fusion of magnesium complete the thermal phenomena. This suggests that the calcium resinate decomposes without reacting with the bulk of the magnesium while protecting the magnesium from reaction with air. The binary fuel-oxidant mixture calcium resinate-barium peroxide ignites at 278". The curve, Fig. 4, shows a shallow endotherm around 90" presumably due to moisture in the sample, prior t o the exothermic pre-ignition reaction. A small endotherm at 368" follows the large exothermic ignition peak. As shown in Fig. 4, the ternary composition barium peroxide-magnesium-calcium resinate, K-29 igniter, ignites at 299" without showing any thermal effects prior to the exothermic pre-ignition reaction. The magnesium-barium peroxide ignition was the only system for which it was feasible to write combustion reactions based upon the composition of the solid residues, since the complexity of calcium resinate and the lack of knowledge concerning its combustion products with solid oxidants preclude writing plausible reactions or calculating reasonable heats of reaction from known thermodynamic properties. For the simpler case, where magnesium oxide and barium oxide were the only solid products found, it seems clear that the reaction is Mg BaOn --c MgO BaO. From these TGA and DTA data it is concluded that the ignitions involving calcium resinate occur after it has begun to decompose but before any of the other ingredients undergo any thermal reactions, Thermal parameters for ignition can be discussed quantitatively in terms of the time t o ignition for the various ignitible conipositions as a function of the temperature at which the ignition takes place. AE,, the energy of activation for ignition, is the amount of energy required to raise a mole of the system to a point where it will initiate a propagative reaction and can be calculated from these data, assuming that the time to ignition is reciprocally related to the specific reaction rate of the limiting ignition reaction in the Arrhenius rate equation.lo A plot of the logarithm of the time to ignition versus the reciprocal of the absolute temperature should give a straight line of slope AEa/2.3R and intercept B / A , where B / A is a constant, and R is the universal gas constant. Figure 5 shows these curves for the three ignitible compositions. A straight line may be drawn over the entire ignition temperature range of 290 to 440°, for the ternary composition. The activation energy calculated from the slope of this line is 13 kcal. per mole. However, the plot for the barium peroxide-calcium resinate binary can be resolved into two lines, one for the ignition temperature range 280 to 350", whose slope yields a value of 23 kcal./
+
1403
Mg- BO 0 2
g
I l l
-
U
$ M g - C o R2 578
I
636
Temperature ("(3.). Fig. 3.-Differential thermal analyses of the binary mixtures magnesium-barium peroxide and magnesium-calcium resinate a t a heating rate of 15' per minute.
+
Temperat,ure ("C.). Fig. $.-Differential thermal analyses of .the binary mixture barium peroxide-calcium resinate and the ternary mixture barium peroxide-magnesium-calcium resinate at a heating rate of 15" per minute.
mole for the activation energy, and another for the temperature range 350 t o 440' with an activation energy of 12 kcal./mole. The slope of the line obtained from the data for the magnesium-barium peroxide composition over its ignition temperature range of 530 to 650" gives an activation energy value of 37 kcal./mole. On a basis of these experimental data, it was concluded that the thermal degradation products of calcium resinate determine the ignition parameters for compositions containing this organic material. Furthermore, it is proposed that the thermal decomposition mechanism of calcium resinate
VIRGINIAD. HOGAN AND SAULGORDON
1404
CORRESPONDING TEMPERATURE,
Vol. 61
OC.
300
400
600
0 Mo-BoOr
0
BoO,-CoR,
A Boot-Mg-CoRt
x io3, Pig. 5.-Ignition
OK;‘,
time-temperature plots for the ignitible compositions magnesium-barium peroxide, barium peroxide-calcium resinate and barium peroxide-magnesium-calcium resinate.
varies with temperature. Below 350” a degradation product, X, is formed that reacts propagatively with barium peroxide, the binary system having an activation energy for ignition of 23 kcal./mole. At temperatures above 350” a different degradation product, Y, with a lower energy of activation for ignition, 12 kcal./mole, is responsible for the propagative reaction. This product may be formed from calcium resinate directly or via X as intermediate. When magnesium is present, either the high temperature product, Y, is formed below 350°, or some other ignitible product, Z, with about the same energy of activation for ignition, 13 kcal./mole, is formed. The following pre-ignition and ignition reaction mechanisms are proposed for barium peroxide-calcium resinate compositions. At temperatures below 350°, X is responsible for the ignition of the composition because the rate of formation of Y is such that its concentration is essentially zero. However, at temperatures above 350°, the concentration of Y becomes appreciable. Since Y is more easily ignitible than X, then at these temperatures the rate of ignition of Y will be much greater than the rate of ignition of X. If Y forms directly from calcium resinate, this implies that the rate of formation of Y increases more rapidly with temperature than the rate of formation of X. Alternatively, if Y is formed from calcium resinate via X as intermediate then above 350°, X decomposes as soon as it is formed. I n both temperature regions the barium peroxide ignition of the degradation product, X, Y or Z, is the rate-determining step in the over-all combustion mechanism. Ignition temperature data for the ternary composition indicate that, over the complete ignition temperature range of 290 to 440°, the system has the same energy of activation for ignition as the degradation product Y, 12 and 13 kcal./mole, re-
spectively. Therefore, for compositions containing magnesium it is proposed that the metal powder affects the system as previously described. The activation energy, 37 kcal./mole, and ignition temperature range, 530 t o 650°, show that the propagative reaction between magnesium and barium peroxide is not responsible for the ignition of the ternary composition. The intercept values obtained from the log time vs. reciprocal temperature plots correspond to the constant B / A in the modified Arrhenius equation mentioned previously. This constant can be expressed in terms of thermodynamic properties and a concentration factor3
where B / A is the modified Arrhenius constant equal to the experimental intercept value; [H q ] is the amount of heat per mole that the system requires for ignition, approximated by the experimentally determined AEa; AH, is the thermochemical heat of the pre-ignition reaction calculated from the heats of formation12J3; h, Planck’s constant; k, Boltzmann’s constant; T, the absolute temperature at which the reaction takes place; and a, the concentration factor called the activity. The activity can thus be calculated from the experimental data for a particular composition by means of this equation providing that a specific temperature is chosen and a pre-ignition reaction is assumed. The activity calculated from equation 1 for the magnesium-barium peroxide composition
+
(12) J. P. Coughlin, “Contribution t o the Data on Theoretical Metallurgy. XII. Heats and Free Energies of Formation of Inorganic Oxides,” U. 8. Bur. of Mines Bulletin 542, 1954. (13) I
