Approximate Heats of Explosion Using Diffe rentia I Thermal Analysis ROBERT L. BOHON Central Research Laboratories, 3 M Co., 2301 Hudson Rd., St. Paul 79, Minn.
b Techniques are described for obtaining approximate heats of explosion on milligram amounts of propellants and explosives via difFerentiaI thermal analysis. A small pressure-tight metal cup serves as a constant volume sample container. The calibration constant is calculated from the total heat capacity of the cup, which its essentially a constant, and from the relaxation curve for each run, thereby compensating for variations in the equipment. This micro method is particularly effective for homogeneous materials such as single-compound explosives or doublebase propellants. A series of nine runson JPN propellant gave an average heat of explosion of 1 1 10 + 140 cal. per gram a t about 250' C. The method should be useful for preliminary screening of new compounds and mixtures available cnly in research quantities.
T
SEARCH for highly energetic rocket fuels has led to the synthesis of many new compounds with unknown properties. Most of these materials are explosive, shock-seidtive, and toxic. The heat of explosion of some new, uncharacterized compound or mixture gives the investigator one index of its destructiveness, as well aa an indication of its potential power its a propellant or explosive (13). The heat of explosion is also a necessary parameter for the calculation of the critical size for spontaneous thermal ignition (6), and accurate values are useful for estimation of theoretical specific impulse (13). In many cases the hazardous nature of experimental rocket fuels or oxidizers restricts laboratory synthesis to tenths of a gram. Conventioiial equipment for measuring heats of combustion or explosion require such large amounts of sample (about 0.5 gram per run) that information is not obtained during the early stages of research when it would be of great assistance. In this laboratory, advantage has been taken of the highly sensitive nature of the differential ,;herma1 analysis (DTA) technique to neasure heats of explosion on 5 to 25 mg. of several typical rocket propelhnts and explosives. It was hoped that the method cvould prove sufficiently precise and HE
versatile that it could be extended to new compounds and formulations as an aid in estimating potential handling hazards, and as a guide in evaluating the thermal output of energetic materials. DTA has been widely used for measuring the heat absorbed in a variety of endothermic processes such as dehydration, decarbonation, melting, crystal inversions, etc. (1-3, 6,9, 11, 16, 17, 91), but very few efforts have been made to measure the heat evolved in highly exothermic reactions (7). For an exploding sample some method must be available for establishing equilibrium between the extremely hot product gases and the DTA sample container and thermocouple. The conventional problems of explosion calorimetry are present also: assurance of complete reaction, nonreaction with the container, appropriate calibration of the apparatus, etc. The present study shows that the DTA method is very useful for determining approximate heats of explosion of single-compound explosives which ignite and for homogeneous double-base propellants. Composite, heterogeneous propellants are less satisfactory because of the difficulty in obtaining truly representative samples weighing less than 25 mg. Explosives which did not ignite under DTA conditions gave low heats of explosion, whereas the propellants did not suffer from this limitation. The technique also appears readily applicable to isochoric thermal studies of conventional compounds such as hydrates, carbonates, sulfates, azides, and perchlorates. EXPERIMENTAL
The DTA apparatus was built specifically for work with propellants and could be operated under vacuum or up to several hundred p.s.i.g. of any gas, usually argon. Details of construction and operation are presented elsewhere (4). Temperature was measured from the reference thermocouple. The heating rate of 4.2' C. per minute was essentially linear above about 50' C. Monel metal was used for all sample containers reported here, although choice of materials of construction is optional. For the isochoric f cups described
below, the dome of the DTA cell was omitted to permit, better heat transfer from the furnace coils. The heat transfer between sample and reference thermocouples could be controlled within limits by changing the location, size, or material of construction of the uppermost baffle plnte(s) which serves as a thermal bridge as well as a spacing device and radiant energy reflector (4). The compounds used for calibration via crystal transitions and fusion were reagent grade with the exception of potassium bifluoride, KHF2, which was a sample supplied by Harshaw Chemical Co., Cleveland 6, Ohio. The compounds were used as received. The explosives and propellants were obtained from various sources. NITROGLYCERIN: (Explosives Dept., Hercules Powder Co., Rilmington, Del,) Recovered from an alcoholic solution by dilution with water according to directions by the manufacturer. Dried in a desiccator over silica gel. TETRANITROhlETHANE : (Hummel Chemical Co.. Inc.. 90 West St., New York 16, N. Y . ) ' 2.4.6-TRINITROTOLUENE (TNT) : (Easthan Chemical Go. KO: 268, Rochester 3, X. Y.). CPCLOTRIMETHYLENE TRINITRAMINE (RDX): (U. S. Naval Ordnance Laboratory, White Oak, Silver Spring, Md.) The water-alcohol solvent was removed by evaporating for 24 hours in a shallow tray in a vacuum oven a t 80" C. 2,4,6 TRINITROPHENYL METHYL NITRAJ~INE (TETRYL): (U. S. Naval Ordance Laboratory.)
-
PENTAERYTHRITOL TETRASITRATE (PETY): (U. S. Naval Ordnance Laboratory.) Dried like RDX. DOUBLE-BASE PROPELLANTS N-5 AND JPN: (Yaval Ordnance Test Station, China Lake, Calif.) ALUMINIZED DOUBLE-BASE PROPELLANT ABL-2056 : (rlllegany Ballistics Laboratory, Hercules, Powder Co., Cumberland, hId.) COMPOSITES:Prepared a t this laboratory. Isobaric Samlpe Cups. The container (9 cup) shown in Figure 1 was cwentinlly a miniature rocket chamber with a porous metal nozzle which served the dual purpose of gaseous heat exchanger and retaining filter for the reacting propellant. For decomposition reactions occurring over a period of several minutes-e.g., dehydrations, decarbonations, etc.-this arrangement gave reliable values of the heat evolved or absorbed by the sample. However, when ignition or explosion
VOL 35, NO. 12, NOVEMBER 1963
1845
CUP g
CUP f
I
I
Figure 1.
Isobaric sample container: cup g Figure 2.
occurred, -cri.iblc Iwat invariably a‘ lost in the effluent ga-e< and the 1.esulting DTA peak area !\a\ too .mall. The error \ V B ~ rpduced to about 25% 011 some con1 entional double-base propellants by u-ing ver- fine-porosity inch thick, but the metal caps up to uncertainty of the measurement seemed to preclude its use in a general method. Reducing the ambient pressure usually resulted in less violent reactions, in which case this g cup was usable. Unfortunately, however, low pressure generally re+ulted in the incomplete conibustiori of compo+te propellants, negating the measurement of heat evolved. Isochoric Sample Cups. A much more successful design for determining heats of explosion was the constant volume container (f cup) illustrated in Figure 2 . I n the present study the internal volume of the sealed cup was approximately 0.055 cc. This small bomb was tared, loaded with about 25 mg. of sample, weighed again, and the socket-head top tightly screwed against the dead-soft copper gasket making a pressure-tight seal. At the conclusion of a run, the bomb was 1846
ANALYTICAL
CHEMISTRY
lsochoric sample container: cup
\\ cighcd again to assure that no gayeous products had escaped. If leakage occurred, ai1 audible “crack” was usually heard at ignition. A Pregl micropress (A. H. Thomas 8472-C) is useful for preparing 2-mm. diameter pellets of compressible solid samples. A mechanical arbor press or large rubber stopper held in the palm of the hand should be used to apply pressure during the compaction operation t o avoid personal injury from accidental explosion recoil. 9 loading chamber was constructed (Figure 3) to permit control of the pressure and type of gas in contact with the sample at the start of the run (pa!.. Both the pressure and gas composition within the sealed cup change as the temperature is raised and as the sample slowly decomposes, but the variation for explosives is generally trivial until ignition occurs. Overloading the Monel cups resulted either in leakage around the copper gasket, bulging of the cup around the sample chamber, or in extreme cases, rupture of the threaded area and violent expulsion of the cap screw. Less ductile metals would behave differently and
f
caution must be exercised in handling the loaded cups before and after heating. Severe explosions sometimes caused sufficient distortion of the thermocouple sheath to necessitate its replacement. This was readily accomplished with the Conax glands through which the thermocouples are inserted in the base of the apparatus (4). CALIBRATION
The total area, A , swept out during nn endothermic or exothermic reaction is related to the heat of the reaction, q, by the relation
where q is expressed in calories per gram of sample, wd is the sample weight in grams, and K is the calibration constant for the particular experimental apparatus in question (cal. per O C. second). The area is given by the integral A
=
J:
ATdt
where AT is the differential temperature in O C., t the time in seconds, C the apparent total heat capacity of the sample cup in cal. per O C., OL the fraction reacted a t time t, and m the apparent heat transfer coefficient in set.-' When the reaction is complete, the rate of reaction (balbt) becomes zero, and Equation 3 reduces to
I
O-RING
COLtAR
/RETAINING
Upon integration this becomes
/-
Aiplot of log AT' vs. t produces a straight Figure 3.
Apparatus
line (relaxation or decay curve) from n hich m can be calculated
for precharging cup f with oxygen or inert gas Used up to 1000 p.5.i.g.
which can be readily evaluated by graphical methods from the DTA curve of differential temperature, AT, us. time, t.
Boersma (3) has suggested that considerable improvement in reproducibility should be obtained in measuring heats of reaction from DTA peak areas if the differential thermocouple is located outside the sample volume as in the present study, rather than imbedded within the reacting material. The sample is then free to furiction solely as a producer or absorber of heat. Changes in heat capacity or thermal conductivity as the reaction progresses have little or no effect upon the flow of heat between the differential thermocouples-i.e., upon the calibration factor, K . The constant K now bec3mes almost exclusively a function cf apparatus heat capacity and of the overall heat-transfer coefficient m between sample and reference thermocouples. This coefficient in turn is fixed by the design and materials of construction, and i o a lesser extent by the familiar operating parameters such as heating rate, temperature, etc. The usual method of calibration consists of running a number of materials with known heats of reaction, measure A , then calculate K from Equation l. This method was employed for a number of standar Is, but, unfortunately, it is not alwals possible to find materials with reliably-known heats of transition or reaction i i the temperature or energy range of interest. This is particularly true a-hen studying explosions or combustion reactions where the heats evolved are nany times those associated with the usual calibration phenomena of crystal inversions, melting, dehydration, and decarbonation. A more desirable approach is to calculate the calibration factor for any
7n
2.303 X slope
(6)
d typical example for a double-base plopellant is shown in Figures 4 and 5. The calibration constant K is then
particular run from some parameter of the DTA curve itself, thereby correcting for variations which may occur in K as a function of temperature, equipment age, cup used, heating rate, thermal contact between cup and thermocouple sheath, etc. A convenient method consists of evaluating the apparent heat transfer coefficient, m, for each run from the relaxation curve following This completion of reaction (23). method is particularly easy to apply to propellants or explosives because the end of reaction is generally so clear-cut and m becomes essentially independent of the external heating rate whenever the sample actually ignites. The differential heat-balance equatioii for the DTA sample container a t any instant may be written N0.447-J
=
readily calculated for any particular DTA run by multiplication of m by the total apparent heat capacity of the sample cup and contents, C (11, 23) : K
=
mC
(7)
A good estimate of C can be made for the apparatus used in this study by aswming that the heat capacity contribution of the small samples used is negligible compared with that of the sample cup, whereupon
c r cujc
(8)
where c is the specific heat of thc sample cup (temperature dependent) and w,,it= mass. The appropriate value of w, i, uncertain to the extent that home unde-
PN
14.6 m g .
4.2"C/min
f
I IO0
I
I
150 TEMPERATURE
200
I 2 50
"C Figure 4. Thermogram of JPN propellant showing clear-cut completion of reaction following ignition at 168" C. VOL. 35, NO. 12, NOVEMBER 1963
1847
I cu RLLAXATlOh
NO. 4 4 7
arrangement from calibration compounds having known transition heats. R is calculated from the known heat effect and the measured peak areas (Equation 1). The constant m is then determined from the relaxation curve for each of these calibration runs and Equation 7 applied to calculate an apparent average C. It is important when using this method to choose standards with well-defined relaxation curves in the temperature region of interestwhich are not always readily available, The two calibration methods are compared in Table I for a series of DTA runs on standard materials. The subscripts A or rn have been used to designate calibration constants derived from peak area or relaxation measurements, respectively. I t is apparent that for a relatively massive sample container such as the f cup, the calibration constant can be reliably determined from the relaxation constant and an estimated total heat capacity (Equations 7 and 8). As the mass of the cup becomes smaller relative to the thermocouples (1/8-inch 0.d. sheath) and standard sample (-50 mg.), the estimated heat capacity becomes increasingly less well-defined and one must use the conventional peak area method for deriving calibration constants. Since molten nitrates and perchlorates generally react with metal cups giving erroneous values for q, it is preferable to restrict calibration with these materials to crystalline inversions rather than fusion.
CURbE
- J PN
R AT
-
10-
Figure 5. Relaxation curve from thermogram of Figure 4 Relaxation constant m i s computed from slope of log AT VI. time
fined portion of the swaged thermocouple inserted into the base of the cup contributes to the total effective mass (and heat capacity) of the cup assembly. Good results were obtained by assuming that just the thermocouple well of the cup was packed with MgO, the insulation medium of the swaged thermocouple (cf. footnote b, Table I). Alternatively, C may be determined empirically for a given experimental
The data reported below on explosives and propellants have been calculated from the relation q. = K d / w , = mCA/w.
(9)
Areas were determined by cutting out Mylar replicas and weighing on an analytical balance. Mylar was found to undergo negligible variation in density with changes in laboratory humidity, etc. RESULTS
Typical DTA curves are illustrated in Figure 6; they are similar t o those obtained by Mason and Davis (14) on 1.O gram samples of military explosives in an open system. Whether a given sample ignites or smoothly decomposes a t a certain temperature depends upon the heat generation and loss situation as expressed by Equation 3. This in turn is contingent upon the reaction rate and its temperature coefficient, the heat of reaction, the loading density, and the physical parameters of the sample and cup (thermal conductivity, density, etc.) (6). The externally supplied heating rate assumes a minor role in determining whether or not ignition occurs a t the relatively slow heating rates encountered in DTA. Some materials-e.g., JPX and A B L 2056 propellants-almost invariably ignited, assuring good combustion and giving excellent relaxation curves; others-e.g., N-5 propellant, nitroglycerin, and PETN-generally gave smooth reaction exotherms. Tetryl
Table 1. Calibration of Cup f (Monel) i = crystal inversion all run under 1 atm. air m = melting q is ( - ) for endothermic proceas
0bserved
Run No.
a
W.
temp. ( " C.)
194 i 232 m 172 m 170 m 126 i 121 m 121 m 120 m 114 m 115 m 252 m
(w.1
Sample
513
KHF2
57.0
512 617 511 613 618 691 616 694 693
KCKS KCNS KNOI Benzoic acid Benzoic acid Benzoic acid o-Dinitrobenzene o-Dinitrobenzene
55.8 35.26 56.31 15.63 27.66 19.26 15.53 28.17 36.0
LiNOi
lOJK.4
-P
( cal./g. 1
54.28 (18)
(sum)
25.72 (18) 25.72 12.86 (18) 33.9 (10) 33.9 33.9 32.3 (10) 32.3 88.5 (10)
(tal./" C.
loaml
sec.)
(sec.)
5.68
6.48
4.85 3.97 5.50 5.02 6.40 6.00 4.90 7.70 7.90
6.44 5.64 6.83 6.63 8.82O 8.20 7.99 10.9 9.60
Poorly defined relaxation curve.
* Estimation of total heat capacity, C, for cup f: Components Monel body Cop er gasket Stee!' cap Thermocou le well, assumed yul1 of MgO, 3.58 g./cc.
w (g.)
c a t 200' C. (cal./g. C.) (90)
4.13 0.13 1.28
0.127 0.103 0.137
0.25
0.27
0.525 0.013 0.175
Sum -
1848
ANALYTICAL CHEMISTRY
c = WE
(cal./O C.)
=
0.067 0.780
C (cal./O C.)
KA/m 0.876
two
O.7Sb 0.753 0.704 0.803 0.757 0.725 0.729 0.613 0.706 0.822 0.749 f 0.058 av.
The theoretical heat of reaction at constant volume was corrected for the effect of temperature by means of the Kirchhoff equation (8):
Using the convention that exothermic reactions supply a positive quantity of heat to the surroundings (Q. = -LIE), and allowing for phase transitions in the reactants and products,
I
-0
where R and P refer to reactants and products, respectively, n the number of moles, C, the molar heat capacity a t constant volume, TO the reference temperature, and L the heat of transition. The equation is not strictly applicable since the DTA exotherm is occurring over a range of temperatures rather than a t T , but the error is slight. The literature values for heats of reaction mere corrected to a temperature near the end of the DTA exotherm peak since all products mere heated to this temperature during the course of the DTA exotherm reaction and the area A was evaluated up to this point. Literature values for heats of reaction at constant pressure (AH, = - Q p ) mere currected to constant volume conditions by the approximate relation (8)
E
cz W
I
c 0
x
w
&.
L 50'
I
I
looo
150'
I
zooo
I 250'
TEMPERATURE, C Figure 6 . Typical DTA curves in cup f, 8-25 mg. samples Note ignition effect of precharging RDX with oxygen) all other runs at 1 atm. air initially. C/min. rate -4.2'
gave a double exotherra of unique shape suggesting a two-step decomposition process. The DTA heat release was calculated from Equation 9 using C = 0.75 cal. per O C. (from Table I) and the appropriate values for m, ws, and A . Data from nine runs on a typical, slightly underoxidized double-base propellant, JPTU', are summarized in Table 11. Temperature and Isochoric Corrections. Literature values for heats of explosion (usually g;iven a t 0' or 2.5' C.) were adjusted to the elevated temperature of the DT-4 exotherm with appropriate corrxtions for water in the gaseous stalJe (WG). This correction is minor for the highly
Heating
exothermic reactions involved here, but can be appreciable for reactions with much lower heat effects (see below).
Reaction
E QP
+ RT An
(12)
where A n refers to the change in number of moles of gas caused by reaction. In some cases-e.g., JPN-product distribution data were unavailable and appropriate adjustment of literature values to DTA conditions could not be made. [The papers by Isaac Barshad (2) and de Bruyn and Van der Mare1 (6) neglect the heat capacity correction term for solids. A recalculation of Barshad's chosen examples, but including this term, follows: - q (cal./g.) C. corrected neglected for C#
VOL. 35, NO. 12, NOVEMBER 1963
t
( C.)
1849
Table II. Exothermic DTA Reaction Heats for JPN Double-Base Propellant q:(WG)refers t o heat evolved in cal./g. sample at constant volume with water in the gaseous state a t the temperature t q m C A / W s (cal./g.) initial pressure in cup = 1 atm. air C = 0.75 cal./'C. =2
ig = ignition d = smooth decomposition
Run NO.
Wa
(mg.)
(sec.-l)
Initial
Final
4.56 3.98 3.88 2.69 3.56 3.94 5.21 6.24 6.27
171 ig. 168 ig. 170 ig. 172 ig. 174 ig. 172 d 173 ig. 170 ig. 169 ig.
250 250 250 260 260 240 240 240 240
Product distribution unavailable for Kirclihoff correction
Off-scale; extrapolated area. Cup leaked.
Several typographical errors occur in Barshad's paper as well as in the review thereof by W. J. Smothers and Y. Chiang (21, p. 61): the m.p. of AgCl should read 455' instead of 307' C. and the inversion temperature of K L ~ ~ M oshould O ~ be 440' instead of 642' C. (I@.] Effect of Reaction Rate. Since the total heat evolved is a function of the products formed and their concentration, it is important t h a t comparisons between methods take into account the possibility of differences in this parameter. The heats of explosion used for comparison in the present study were obtained by others via the usual calorimetric method of igniting the sample with a high intensity a hot wire-in a constimulus-e.g.,
:lo).
species and their distribution, and therefore the observed reaction heat. A comparison of results from systems which did and did not ignite is given in Table 111. The next-to-last column presents the ratio of experimental DTA heats to the conventional heat of explosion (corrected wherever possible), and serves as a correlation index. As would be expected, virtually all the explosives studied gave a much closer agreement with conventional heats of explosion when the sample actually ignited. Smooth exothermic reactions for these materials generally produced lower total heat evolution than suggested from conventional heat of explosion measurements. The solid propellants, on thc other
Comparison of Ignition and Nonignition Systems
Oxidation state
DTA runs
DTA av.
Lit. ("C.)
mmt:
Under Under Over Under Under Under Under
2 1 1 1 3 8 1
780 1030 2210 1160 1680 1080 1220
485 (227') 816 (250") 2307 ( 0 ' ) 1300 (0" j 1890 (0". WL) 1215 (00; w L j 1412 (0')
1.61 1.26 0.96 0.89 0.89 0.89 0.86
Over Over
2 1
380 720
784 (0') 3510 (327")
0.48 0.20
Under Under
1 4
1340 870
1215 (00, Y L ) 816 (250 )
1.10 1.06
975" 1670 660 870 900 770 310 600 410
954 (0") 2307 ( 0 ' ) 1020 (00) 1412 (0') 1520 (227') 1300 (OD) 557 (227') 3510 (327') 2850 (227')
1.02" 0.72 0.65 0.61 0.59 0.59 0.56 0.17 0.14
(15/69/16 bw)
+
TNT 0 2 System which did not ignite: JKP Propellant N-5 Propellant TNT/NHdNOa/Paraffin (18/82/0.16 bw) RDX 0 2 Tetryl PETN Nitroglycerin RDX Tetranitromethane TNT 0 2 Tetryl 0 2 a May be too optimistic because
+
++
1850
215 ioo; wLja 215 (O", WLp 215 ( 0 " .\VI,P 215 (0"; W L j a 215 (0", WL).
I
No.
Sample Systems which ignited: TNT N-5 ProDellant RDX +' 02 RDX ABL-2056 Propellant JPN Propellant PETN TNT/NH4NOI/NaC1
YO0 1340 1480 990 1020 Mean = 1110 f 140
stant volume bomb. Actual deflagration of the sample occurs and high flame temperatures are reached, with appropriate adjustment of products as the gases are cooled back to approximately the initial temperature. I n the DTA method, on the other hand, ignition occurs only when the rate of self-generated heat production within the sample exceeds the rate at which it can be dissipated to the surroundings (Equation 3). This is a function of the sample loading density, the kinetics of the heat-producing reaction(s), the thermal conduction of the system, etc. Failure to ignite does not preclude the evolution of large quantities of heat, but it can have a profound effect upon the maximum temperature, the product
Table 111.
__ q: ( W G )(cal./g.) DTA Lit. ( t "C.) ll50b 215 (00,WL)a 103oc 215 (O", W L p 990c 215 ( O O , W L p 1080 a15 (0". WLP
DTA Temp. ("C.)
103m
Wee.) 0.15 0.18 0.13 0.06 0.08 0.06 0.08 0.11 0.13
12.6 14.6 11.5 5.3 6.7 5.1 6.8 9.58 11.53
446 447 448 449 45 1 466 467 487 488 a
Loading density
ANALYTICAL CHEMISTRY
Over 1 Over 1 3 Under 2 Under 3 Over 2 Under 1 Over 2 Over 2 Over of extraordinarily large m.
pf (WG) (calJg.1
hand, were not subject to this restriction and produced virtually the same amount of heat regardless of whether or not actual deflagration oc :urred. This observation is not surprirjing in view of the normal tendency of propellants to “burnJ’ and of explosives to “explode,” regardless of the method of initiation. Efforts to increase reaction rates to the point of ignition by initially charging the cup with seveial atmospheres of argon-Le., burning rates are normally pressure dependent-were generally unsuccessf~l,c.f. nitroglycerin and tetryl Perhaps this was due to the relatively moderate pressures used. Underoxidized materials. Systems which do not contain sufficient oxygen for complete combus ,ion of all carbon and hydrogen to caibon dioxide and water will produce ,Jarying amounts of CO, H2, CHI, aiid C, the exact proportions depending primarily upon the equilibrium ccnstants of the appropriate reaction: occurring in the systems a t the temp(3ratui-e and pressure in question. These considerations are treated in detai, by Morris and Thomas (25) and by Taylor (26), and depend principally upon the loading density (grams of explosive per cc. container volume). In the present study the loading density was always less than the literatL re conditions because of the strength limitations of the particular f cup design employed. h notation has been made in Table I11 ab to the oxidation state of each system studied. Once again using the ratio of experimental to literature heats as an index of accuracy, there appears to be no particular merit associated with an overtern. Th: total number of experimenth is probably too few to jubtify a general conclusion. The data on J P S propellant (Table 11) represent the mol4 extensive series accumulated under identical conditions of initial preisure and ignition characteristics and probably illustrate the degree of reproducibilitv one can expect for a slightly underoiidized propellant in the present appara ,us. The average deviation from the mean value of 1110 cal. per gram was 140 cal. per gram, or =t12.6%. Product distribution was unavailable for this propellant, therefore no correction m s applied to the nianufacturcr’s value of 1215 cal. per gram for the heat of explosion, which refers to 0’ C. with vater in the liquid state (WL). ,lppropriate water phase corrections would be expected to lower the heat of explosion so as to approach more clowly the observed DTA value. The individual DTA values for J P N showed no correlation with loading density. The uncertainties associated with underoxidation are generally avoided in calorimetry by addition of sufficient oxidizer (usually ga: eous oxygen) to
*
produce complete combustion. Several runs were made in which the f cup was initially charged with several atmospheres of oxygen prior to the heating cycle, but the results were inconclusive. For example, addition of oxygen to TWT and tetryl had virtually no effect upon the heat released, although a significant increase was observed in the case of
RDX. An alternate technique to achieve complete combustion involves premixing a known mass of sample with a solid oxidizer, such as sodium peroxide or ammonium nitrate or perchlorate, but this complicates the quantitative, reproducible sampling of milligram amounts of such a composite. The Tn’T-NHJS03 series (Table 111) representc a case in point; the erratic results are most probably a result of nonrepresentative sampling. Experience with other systems not reported here confirms this inability to sample properly mixed solids in very small lots unless component particle sizes are extremely fine and great care has been taken to mix thoroughly all ingredients. Excessive DTA Heats. I n a few runs the DTA heat release appreciably exceeded the conventional heat of explosion-e.g., see T N T , N-5, J P K . I n some instances this was simply experimental variation, but in others the discrepancy was so consistent as to suggest a real increase in the heat release during reaction. This may have been due to different product distribution as discussed above, or more probably to reaction n5th the cup materials (Monel, steel, or copper). il platinum-lined vezsel might obviate this latter problem in most instsnceb. DISCUSSION
The techiiiyues outlined certainly do not provide a replacement for bomb calorimetry when the investigator has access to, and handling procedures for, beveral grams of high energy sample. However, when a limited supply of material is available for evaluation, as during the initial phases of a research project, a carefully applied DTA method can yield valuable information on the explosive potential of just a few milligrams of a compound or mixture. As in all quantitative procedures, multiple determinations are essential for reliable information. As a by-product, of course, one obtains the usual DTA information regarding transition and ignition temperatures-e.g., tetryl, Figure 6. Furthermore, the product gases can be readily recovered for analysis by means of a flexible tube surrounding an Allen wrench and attached to an evacuated vessel, thereby providing valuable clues as to the course and degree of reaction achieved. This information provides a
good starting point for subsequent precision calorimetric determinations, even though bomb (intense ignition source) and DTA (spontaneous deflagration under controlled heating conditions) reactions may differ considerably. For severely underoxidized materials one is faced with the option of running explosions under identical, but probably unknown, conditions of pressure as determined by precisely equal loading densities, or attempting to obtain complete combustion by charging with sufficient oxygen (generally over 500 p s i ) or premixing with a solid oxidizer. The results with oxygen-charging were not encouraging, although larger and stronger sample vessels might eliminate the principal objections to this method. The same remedy might well apply to a solid oxidizer-sample mixture where the quantity of specimen could be increased to 50-100 mg. Increasing the sample size very far beyond this level would be pointless since modified calorimetric techniques then could be employed. Aside from the obvious errors introduced by sample impurities, incomplete combustion, reaction with the cup, and calibration errors, the most serious limitations introduced into these measurements were related t o the very small samples necessitated by the strength limitations of the particular sample container employed. For pure compounds the problem is minor, but for any kind of heterogeneous propellant or explosive mixture it is difficult, if not impossible, to obtain 10 to 25 mg. of a tiuly representative sample. Larger samples frequently overloaded the cup resulting in leakage or rupture. The upper limit for sample size can be increased by constructing a sturdier container, but the optimum size will depend upon the UTA response desired, the value of q, etc. The calculation of calibration coilstants from the relaxation curve of each experimental run seems to have great merit for reaction. which go quickly and cleanly to completion, as in the present study. Coiieiderabl(> ambiguity can occur in the determination of m nhen the reactions are slow, as in conventional phase transitions, thereby causing coniparable uncertainty in the calibration constant K and the absolute ialue of the heat evolved. If a given sample suffers from this deficiency, calibration is required just before and/or just after each run, using precisely the same esperimental conditions but a standard ban]ple. I t would seem irnniaterial whether this calibration is performed in the conventional way by measuring A and calculating K from a known q, or whether a readily ignitable explosive or propellant-e.g., J P S or ABL-2056-is used and m determined from the relaxation curve (and K from C and Equation 7). In evaluating the explo-ive hazard of a VOL. 35, NO. 12, NOVEMBER 1963
1851
given material it is important, of course, to measure the maximum amount of energy which reasonably can be obtained, regardless of the means of initiation. As seen in the present study, DTA results alone can be misleading if the sample does not ignite, thereby producing a lesser quantity of heat than might be obtained under true deflagrating conditions. On the other hand, adjustment of experimental conditions so as to promote ignition (increased loading density, higher initial pressures, etc.) can be expected to yield reliable approximations of the conventional heat of explosion. As was pointed out previously, however, the solid propellants studied produced good heats of explosion regardless of the rate of reaction. Results with the slightly underoxidized double-base propellant JPN were particularly encouraging, the success depending largely upon the consistent ignition and the homogeneous character of the material. It seems probable that equally reliable data can be obtained by DTA methods for most systems which react under controlled heating conditions with the evolution of large amounts of heat. The method should be particularly helpful for screening propellants and explosives during early stages of research and development. It should be borne in mind that the potential destructive power of a given material is related not only to the heat of explosion, but also to the rate with
which this energy is evolved. Substances which are capable of supporting true detonations-i.e., supersonic deflagrations with microsecond release of explosion energy-can inflict damage many orders of magnitude worse than that resulting from simple explosions. Detonations during DTA runs presumably would shatter the sample cup, although such destruction was never observed during the course of this work. ACKNOWLEDGMENT
The author is grateful to R. V. Wagner for experimental assistance, and to the U. S. Department of Defense for permission to publish this work. LITERATURE CITED
(1) Allison, E. B., Silicates Ind. 19, 363 (1954). (2) Barshad, I., Am. Mineralogist 37, 667 (1952). (3) Boersma, S. L., J . Am. Ceram. SOC. 38,281 (1955). (4) Bohon, R. L., ANAL.CHEM.33, 1451 (1961). (5) Bowden, F. P., Yofize, A. D., “Fast
Reactions In Solids, Butterworths, London, 1958. (6) de Bruyn, C. M. A., Van der Marel, H. W.,I Gwl. Mijnbouw 16, 69, 407
(1954). (7) Gamel, C. M., Jr., Smothers, W. J., Anal. Chim. Acta 6 , 442 (1952). (8) Glasstone, S., “Thermodynamics for Chemists,” van Nostrand, New York, 1947. (9) Hagel, W. G., Pound, G. M., Mehl, R. F., Acta Met. 4, 37 (1956).
END OF
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SYMPOSIUM
(10) ‘,‘,Handbook of Chemistry and Phys-
ics, 40th ed., Chemical Rubber Publishing Go., Cleveland, Ohio, 1958. (11) de Jong, G. de Josselin, J. Am. Ceram. SOC.40,42 (1957). (12) Kirk, R. E., Othmer, D. F., (‘ET; c clopedia of Chemical Technolo 61.6, Interscience Encyclopedia New York, 1951. (l?) Lewis, B., Pease, R. N., Taylor, H. S., Combustion Processes,” Princeton University Press, Princeton, N. J.,
E;.,
1956. (14) Mason, E. E., Davis, H. A., U. S.
Naval Weapons Station, Yorktown, Va., NAVORD Rept. 5802, (Jan. 22,
1960). (15) Morris, G., Thomas, H., Research 1, 132 (1947). (16) Murphy, C. B., ANAL. CHEM.30, 867 (1958). (17) Ibid,, 32, 168R (1960). (18) National Bureau of Standards, Circular 500 (1952). (19) “Propellant Manual SPIA/M2,” Chemical Pro ulsion Information Agency, Johns Ropkins Univ.,. Silver Springs, Md (1962) [Confidential]. (20) %nithso&m Physical Tables,” 9th rev. ed., (1954). (21) Smothers, W. J., Chiang, Y., “Dif-
ferential Thermal Analysis,” Chemical Publishing Co., Inc., New York, 1958. (22) Taylor, J., ‘‘D$mation in Condensed Explosives, The Clarendon Press, Oxford, 1952. (23) Vold, M. J., ANAL. CHEM.21, 683 (1949).
RECEIVEDfor review June 28, 1963. Accepted Septembe; 16, 1963. Division of Analytical Chemist 144th Meeting, ACS, Lo8 Angeles, a i f . , April 1963. The Advanced Research Projects Agency provided support under Contract NOrd 18688 which was monitored by the Bureau of Naval Weapons.
