A D I A B A T I C C A L O R I M E T E R FOR T H E S T U D Y OF E X O T H E R M I C CHEMICAL REACTIONS EDWlN L. A L L E N Rohm and Haas Co., Redstone Research Laboratories, Huntsville, Ala. 55807 An adiabatic calorimeter developed for the study of the exothermic chemical reactions was used to determine heats of polymerization, perform curing-rate studies, and establish thermal decomposition rates as a function of temperature for high energy propellants. The calorimeter was designed for remote operation in a test cell with the control system located in an adjacent laboratory. The control system was designed to terminate a test automatically after the test sample temperature reached a preselected value. The construction and operating procedure are described in detail. Illustrative examples and data reduction techniques are discussed.
EVELOPMENT of the adiabatic calorimeter described was initiated by the need for a simple but reliable method for establishing the net quantity of energy released during the cure of high energy propellant formulations. Since the specific heats for several propellants currently under study were well known, determination of the adiabatic temperature rise during cure offered a theoretically simple route to evalute the heat of polymerization. A method was sought, therefore, for maintaining adiabatic conditions in a sample undergoing an exothermic reaction and for measuring the temperature rise. A cylindrical adiabatic oven was designed to eliminate heat loss (or gain) from a reacting sample contained in a centrally located capsule. The temperature rise produced by the reaction was measured with a thermocouple in the sample. Accurate measurement of the heat evolved in the chemical reaction required that the heat capacity of the sample container be very small compared to the heat capacity of the sample itself and that temperature control of the environment surrounding the sample be sufficient to maintain sample and surroundings at essentially the same temperature throughout the reaction to ensure that the sample remained adiabatic. These requirements were met by the use of a thin-walled aluminum capsule having a heat capacity less than one tenth that of the sample and of a current-adjusting proportional controller to regulate electrical power to the surrounding oven. The outstanding performance of the calorimeter in maintaining adiabatic conditions during heat-of-polymerization experiments suggested that rate studies of exothermic reactions might also be performed with the apparatus. Subsequent tests involving adiabatic curing and decomposition of propellant demonstrated the utility of the calorimeter for these studies. Apparatus
The adiabatic calorimeter consists of a sample container, an oven, and an automatic control system. The apparatus is shown schematically in Figure 1, with the guard heater and its control system omitted for simplicity. I n operation, the oven is preheated to a selected initial temperature with the control system in the set-point mode. The sample container with the sample is suspended in the preheated oven and allowed to approach thermal equilibrium. K h e n the temperature a t the center of the sample reaches the oven temperature (typically 30 to 40 minutes), the heater power control system is switched from the set-point mode to the automatic control mode. In the automatic mode, the control a28
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FUNDAMENTALS
system adjusts the power supplied to the heater to maintain the oven wall and the sample container at the same temperature. The temperature of the sample, its container, and the oven increases with time as the exothermic reaction progresses, and the sample temperature history is recorded using the thermocouple in the center of the sample. The sample container is a thin-walled aluminum capsule 3/4 inch in diameter and approximately 2 inches long (Figure 2 ) . An iron-constantan thermocouple junction is attached t o its inside wall by a small amount of epoxy cement. This thermocouple constitutes one half of the differential thermocouple which provides the control signal for the oven heater. The thermocouple for recording the sample temperature history passes through and is cemented to a thin cork disk which is used to seal the top of the capsule. These thermocouples pass upward through small holes in the oven top, and the openings around the wires are sealed by a strip of adhesive tape. Two Teflon (Du Pont) grommets center the capsule in the oven during the test. The polygon-shaped outer profile of the grommets ensures contact with the oven wall a t only a few points. The capsule is supported midway between the ends of the oven by the thermocouple wires. During a test, the upper end of the oven is covered by the oven top. The top supports a set of six radiation shields which reduce the end heat losses, and a cork closing the lower end of the oven supports a similar set of radiation shields. A leaf spring is attached to the oven top for use in removing the top remotely. The oven for the adiabatic calorimeter (Figure 3 ) consists of two concentric tubular heaters having independent power control systems. The oven (inner) heater and its control system provide the adiabatic environment for the sample; the guard (outer) heater and its control system isolate the inner heater from environmental temperature changes. The oven and guard heater walls are fabricated from copper tubes 14 inches long having inside diameters of 1 and 1; inches, respectively, and a 1/16-inch wall thickness. Both heater tubes are electrically insulated on the outer surfaces by a double thickness of glass-cloth thermosetting adhesive tape. A 30-gage iron-constantan thermocouple junction is peened into a hole in the inner heater wall midway between the ends t o serve as one half of the capsule differential thermocouple. Three 30-gage copper-constantan thermocouples are located between the two layers of insulating tape on the inner heater. The junctions of these thermocouples are midway between the ends of the tube and are equally spaced on the circumference. One thermocouple provides a signal to the set-point unit used for preheating the oven t o the selected initial temperature. The second provides a signal for an overheat alarm which automatically
t o minimize the difference signal, maintaining the guard heater temperature at the temperature of the inner-heater wall. Environmental temperature changes which tend to change the rate of heat loss from the oven are detected by the differential thermocouple in the outer-heater wall and corrective action is taken by the guard heater control system, Since heat transfer between the guard heater and the inner heater is primarily by radiation, a small perturbation in the guard heater temperature can be corrected without significantly changing the inner heater temperature. Thus, the guard heater tends to isolate the inner heater from environmental temperature changes. Nichrome (Driver-Harris Co.) alloy V ribbon is used for the oven heating elements. The effect of heat loss from the ends of the oven on the temperature distrihution near the center of the oven is reduced by closer snacine of the heater ribbon over a 2-inch span at both I
uinerenrial tnermocouple for the guard heater. The second half of the differential thermocouple for the guard heater is peened into a hole in the outer heater wall midway between the ends. This provides a signal to a control system which regulates power to the guard heater
spacidg of the heater windings is used over the idsection of the tubes. Each heating element is y a double thickness of thermosetting glass-cloth tape. The heaters are electrically isolated from each other by phenolic spacers between the tubes at both ends, and the outer heater is thermally insulated from the environment by a section of standard magnesia pipe insulation. The ends of the oven are protected by a 1/8-inch layer of porcelain adhesive cement. The power control systems for the oven heater and the guard heater are shown in Figure 4. The oven-heater control system consists of a set-point unit, a d.c. null detector, a current-adjusting-type proportional control unit, a magnetic amplifier, a mode-selector switch, a power switch, and an overheat alarm. The power control system for the guard heater contains a d.c. null detector, a current-adjustingtype control unit, and a magnetic amplifier. The overheat alarm is an auxiliary component which provides a means for terminating a test automatically when a preset tempers, ture is reached. The adiabatic calorimeter developed in this study has been utilized primarily for investigating curing reactions and decomposition reactions in solid propellants and solid propellant ingredients. L
~",.~~~"~
Since adiabatic thermal decomposition tests with propellants occasionally terminate in a fire or explosion, the apparatus was designed for remote operation in a reinforced concrete test cell. Components in the cell include the oven, a revolving platform, a quenching bath, and associated quick disconnects for electrical power, pneumatic lines, and thermocouples (Figure 5 ) . The electrical power control system and the control valves for the pneumatic cylinders are located in an adjacent laboratory. The overheat alarm is used to initiate the sequence of test-termination operations automatically. When a preselected oven temperature is reached, the overheat alarm
adiabatic oven. When the push rod attached t o the oven top reaches its uppermost position, a limit switch energizes another solenoid for an air cylinder which rotates the plab form supporting the oven and quenching bath so that the bath is directly beneath the capsule. Another limit switch then causes the sample to be lowered into the bath. This automatic shutdown feature permits automatic termination of a test when the system is unattended. The sample temperature during a test is recorded by a variabkspan continuous recorder. In experiments to investigate rates of reaction, data analysis requires a knowledge of time-temperature information. To circumvent the requirement for the tedious and time-consuming task of reading recorder charts following each test, a K-120 (Datex Carp.) digital recording system is used in conjunction with the recorder. This consists of an encoder for sensing the recorder slidewire shaft position, a control chassis to interpret the signal from the encoder and provide a digital signal t o the printer, and a printer equipped for three-decade operation (0 t o 999). The digital printer is triggered by a clock-driven signaling control. Printout periods of 10, 1, and 1/10 minute can he selected manually or automatically. With the printsignaling control in the automatic mode, the print period is shifted to the next shorter period when a preselected digital limit is reached. This flexibility in the printsignaling schedule is particularly useful in adiabatic decomposition tests in which the rate of temperature rise typically increases exponentially as the reaction progresses. Procedure
The procedures for the preparation and conduct of adiabatic curing and adiabatic decomposition experiments differ only slightly in the manner of sample preparation and test termination.
. .
thin-walled aluminum containers for cigars. Aluminum and other lightweight materials are used in the sample capsule fabrication to reduce the mass of inert material which must be heated by the reacting sample. Preliminary calorimeter operations involve preheating the oven and establishing a suitable temperature drift rate due to a small net loss of heat to the environment. To preheat the oven to any desired temperature, an e m f . corresponding t o the desired initial temperature for copperconstantan thermocouples is dialed on the set-point unit and the mode-selector switch is placed in the "set-point" position. I n the set-point mode, the millivolt output supplied by the set-point unit is opposed by the e.m.f. output of a copper-constantan thernioeonple fixed to the oven wall. The difference signal between the set-point unit and the thermocouple e.m.f. is supplied t o a null detector which initiates corrective action by the control system t o minimize the signal, maintaining the oven wall a t the desired initial temperature. At low adiabatic heating rates the net heat transfer rate between the test capsule and the test-cell environment a t the initial test temperature must be small in comparison with the initial sample self-heating rate. The procedure for establishing an acceptable net heat transfer rate for the calorimeter uses an inert sample in a capsule identical to the test container. With the control system in the setpoint mode, the capsule containing the inert sample is lowered into the oven and heated to the desired initial test temperature. When thermal equilibrium has been established between the inert sample and heater wall as determined with a precision potentiometer, the control system is switched to the automatic mode and the temperature drift of the inert sample is measured with the aid of the
I0F/22O
an inhibitor is included to extend the pot life and allow sufficient time for casting before the slurry gels. In a n adiabatic curing experiment, the inhibitor permits the monomer slurry to come to thermal equilibrium in the heater prior to initiation of the curing reaction, Polymerization typically begins slowly, then progresses rapidly through a major portion of the cure, and finally diminishes as the concentration of unreacted monomer diminishes. Determination of the net heat of polymerization requires an accurate knowledge of the temperature rise during polymerization and the masses and specific heats of all components which are heated by the energy generated by the reaction. For polymerization in an adiabatic system, equating the energy generated by polymerization to the enthalpy gain of the heated masses yields
MIN
280.5
280.0
250.0
DRIFT N I L
X s A h p=
249.5
I 0
I
20
40
60
80
100
120
TIME-MINUTES
Figure 6. rate
Temperature dependence of drift
potentiometer. An adjustment on the null detector is used to reduce the temperature drift rate of the inert sample to an acceptable value by permitting the addition of a small bias in the control signal. I n ordinary operations, the maximum allowable temperature drift rate is 1' F. per 100 minutes a t the initial test temperature, which corresponds to a maximum heat transfer rate of 2 cal. per hour for a typical test sample. The temperature drift rate is not significantly affected by a change in the sample temperature from the initial temperature, as shown in Figure 6. After an acceptable net heat transfer rate has been established for the inert sample, the mode selector switch for the control system is returned to the set-point position, the inert sample is removed, and the test specimen is placed in the oven. Temperature of the test sample during the preheating phase is monitored using a precision potentiometer. TThen the sample temperature reaches the oven temperature, the mode selector switch is turned immediately to the automatic mode and the output from the thermocouple in the center of the sample is switched from the potentiometer to the recorder. Choice of temperature for test termination depends solely on the material being tested. The termination procedure may be sequenced manually or automatically a t the discretion of the operator. Under manual control, the operator activates solenoids controlling air to pneumatic cylinders by toggle switches. Under autoniatic control, the overheat alarm begins the automatic shutdown procedure. Applications
The adiabatic calorimeter developed in this study was found both convenient and effective as a tool for est,ablishing heats of polymerization, determining the curing rate of monomerbased propellants, and investigat'ing the rate of thermal decomposition as a function of temperature. Polymerization
Determination of the net quantity of energy liberated during the cure of monomer-based solid propellants was the first application of the adiabatic calorimeter. During the mixing of propellants containing monomeric binder systems,
(Jf,C,)AT
(1)
in which Alfais the mass of the polymerizing sample, Ahp is the net heat of polymerization per unit mass, JI, represents the mass in individual components vihirh are heated by the energy generated by the reaction, C, represents the corresponding specific heats, and AT is the total temperature rise during polymerization. I n Equation 1, it is assumed that there is no vaporization during polymerization, that all monomer is converted to polymer, and that heats of solution are negligible. If vaporization during polymerization is appreciable, another term must he included on the right-hand side of Equation 1 to account for the heat of vaporization. I n tests to determine the heat of polymerization of methyl methacrylate, the samples were weighed before and after the test, and a correction was applied for the quantity vaporized when the loss was measurablc. Table I contains data obtained from adiabatic curing experiments involving methyl methacrylate in which an inert plasticizer was incorporated so that the maximum temperature reached would be low enough to prevent significant monomer loss from evaporation. The agreement between these experimental values and the literature values for methyl methacrylate suggests that the adiabatic calorimeter is operating satisfactorily.
Table 1.
Experimental Data for Curing Methyl Methacrylate
Test Weight data Monomer weight, g. Inert plasticizer weight," g. Rlonomer in sample, 55 Capsule weight, g. Cork weight, g. Epoxy weight, g. Grommet weight, g. Specific heat data Methyl methacrylate, cal./g.-" C. Inert plasticizer," cal./g.-' C. Capsule (aluminum), cal./g.-" C. Cork, cal./g.-" C. Epoxy cement, cal./g.-" C. Grommet (Teflon), cal./g.-" C. Temperature data Initial temperature, ' C. Final temperature,o" C. Temperatnre rise, C. Heat of polymerization Ahp, cal./g. of monomer (experimental) A H p , kcal./mole (experimental) A H p , kcal./mole (literature)b
Test 2
1
2.734 14.431 15.93 2.752 0.264 0.030 0.432
2.618 13.805 15.44 2.813 0.288 0.030 0.432
0.546 0.401 0.22 0.45 0.3 0.22
0.546 0.401 0.22 0.45 0.3 0.22
49.7 95.1 45.4
50.0 95.1
45 .O
134.64 134.24 13.48 13.44 13.0 to 1 3 . 8
Santicizer-160, benzyl butyl phthalate, Monsanto Chemical Co. and Madams (1956), Bywater (lY55), Dainton et al. (1960) Ivin (1955), Joshi (1962), Levin et nl. (1959), Tong and a
* Boxendale
Kenyon (1945).
VOL.
8
NO.
4 NOVEMBER
1969
831
The use of the calorimeter to establish the heat of polymerization and rate of polymerization for a monomer-based solid propellant formulation has been discussed by Stonecypher et al. (1966). The results were used in an analysis of the thermal problem associated with the cure of large solid propellant grains.
Since the rate of decomposition is intimately associated with the shelf life of propellants, propellant thermal stability can be conveniently and usefully compared on the basis of the adiabatic heating rate as a function of temperature. An enthalpy balance for a simple, zero-order reaction under adiabatic conditions gives
(M$i) ( d T / d t ) = M,QZ exp (- E / R T ) Decomposition
Decomposition of high energy propellant binders is typically exothermic, and the rate of decomposition generally increases with increasing temperature. To get accurate data from the first part of a decomposition test, it is important that the drift rate for the control system at the initial test temperature be very low compared to the decomposition rate. The drift rate becomes less important as the temperature (and rate of decomposition) increases. Representative temperature histories during adiabatic decomposition of some high energy propellants are shown in Figure 7.
(2)
in which -V$represents the masses of individual materials which are heated by the decomposition (propellant, container, etc. ) , grams; C, represents the corresponding specific heats, cal./g.-' C.; M, is the mass of propellant, grams; T is the temperature, K.; I represents time, minutes; Q is the heat of decomposition, cal./gram; Z is the frequency factor, min.-l; E is the activation energy, kcal. per mole; and R is the universal gas constant. (The zero-order assumption is justified when only a small fraction of the sample decomposes during the test.) Dividing Equation 2 by M, and taking the logarithm of both sides gives
R H-SA-IO3
170 ov 160 I YI
5
5 a 2
5k
-
150
-
140
-
130
-
RH-SA-IO3 RH-P-I12 RH-SE-103
30
40
50
60 70 EO 90 TIME- MINUTES
100
110
120
130
Figure 7. Temperature histories during adiabatic decomposition
I
from which it follows that a plot of In [( iU,C,/M,)(dT/dt)] against reciprocal temperature should give a straight line having slope (- E / R ) and intercept In QZ. A computer program was prepared to determine heating rates during decomposition using the recorded temperaturetime history. The total temperature rise was divided into several uniform increments, and the rate of temperature rise a t the nodal point between two increments was determined by differentiation of a second-degree polynomial fitted by the method of least squares to a t least eight of the most adjacent time-temperature data points. The heating rate and reciprocal absolute temperature for each nodal point were substituted in Equation 3, and the method of least squares was used to solve for E and QZ. A logarithm plot of heating rate us. reciprocal absolute temperature for some typical decomposition tests is shown in Figure 8. The experimentally determined rate data shown in Figure 8 are in good agreement with the simple mathematical model given by Equation 3 .
h
literature Cited
2
10-2
2 2.14
2 20
IOOO/T-
Figure 8.
832
2.40
2.30
2.50
2.58
OK-'
Heating rate during adiabatic decomposition
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FUNDAMENTALS
Boxendale, J. H., Madaras, G. W., J . Polymer Sci. 19, 171 (1956). Bywater, S., Trans. Faraday SOC.61, 1267 (1955). Dainton, F. S., Ivin, K. J., Walmsley, D. A. G., Trans. Faraday Soc. 66, 1784 (1960). Ivin, K. J., Trans. Faraday SOC.61, 1273 (1956). Joshi, R. RI., J . Polymer Sci. 6 6 , 313 (1962). Levin, P. I., Miller, I-, B., NevskiI, L. V., n'eiman, hI. B., Kargin, 5'. A., Rylov, E. E., Tr. Khim. i Khim. Tekhnol. 2, 3 (1959). Stonecvpher, T. E., Allen, E. L., Mastin, D. E., Willoughby, D. A,,Chem. Eng. Progr. Symp. Ser. 62 (Xo. 61), 7 (1966). Tong, L. X. J., Kenyon, W. O., J . Am. Chem. SOC.67, 1278 (1945). RECEIVED for review August 19, 1968 May 9, 1969 ACCEPTED Work carried out under the sponsorship of the U. S. Army Missile Command, Redstone Arsenal, Ala., under Contracts DA-01-021 ORD-l1878(Z) and DA-01-021 AlIC-l1536(Z).
