Differential Thermal Studies with Simultaneous Gas Evolution Profiles WILLIAM M. AYRES and EVERETT M. BENS Chemistry Division, Research Department, U. S. Naval Ordnance Test Station, China lake, Calif. b Differential thermal studies of propellants have been greatly implemented by the addition of a simultaneous and continuous gas detection system. An inert gas flowing through the system removes gaseous reaction products which are detected b y a thermal conductivity cell, thus minimizing further reaction. A study of a propellant and its components makes possible an understanding of the effect of additional components. A complex and its starting materials are found to behave in an entirely different manner. Other examples presented demonstrate the value of this technique as a research tool.
D
IFFERENTIALTHERMAL and thermo-
gravimetric techniques have become increasingly useful tools in chemical investigations. From their initial application to the study of complex minerals, they have been extended, in more recent investigations, to the study of inorganic hydrates and organic compounds, especially plastics. The differential thermal techniques as used in this study are essentially a measurement of the temperature difference between an inert reference material (glass beads) and a sample diluted with the same material as the temperature of both is increased a t a uniform rate. The beads, being of the same material as the cells, provide the means of uniformly transferring heat from the furnace to the sample and to the temperature sensing probes throughout the entire temperature range of interest, thereby minimizing variables in the construction of the apparatus and probes. The signal due to the heat either liberated or taken up by the sample is recorded as an exothermic (positive) or endothermic (negative) deviation from a base line. A deflection of 100 mm. corresponds to a differential temperature of 1' C. A comprehensive review of the technique, including a bibliography of the literature to 1957, has been published by Smothers and Chiang ( 7 ) . The introduction of a gas sweeping system and gas detector cell provides additional information that can be correlated with the thermogram. A recent publication by Stone (8) describes a dynamic gas flow system for differential thermal analysis with variable pressure 568
ANALYTICAL CHEMISTRY
and atmosphere. The gas flow is directed through the sample to remove gaseous reaction products and vented into a pressure chamber. Methods of detecting gases evolved during differential thermal reaction have also been described in recent publications. Murphy et al. (4)describe an apparatus in which selected gas samples were taken from a closed evacuated system during differential thermal analysis. An apparatus described by Rogers et al. (6) pyrolyzes a sample a t a constant heating rate in a flowing system. The gaseous products are swept into a combustion chamber and then to a thermal conductivity cell for detection of the combustion products. The system used in the present work gives a gas evolution profile under normal pressures, allowing continuous monitoring of the gas evolution and permitting selective sampling of any desired portion of the evolved gases. In either case the gas evolution with thermal reaction would be similar to the combinations presented in Table I. Gas evolution during an endothermic reaction, for example, would indicate a decomposition or vaporization as opposed to a crystal transition or fusion. Differential thermal investigation of a typical double-base propellant is presented and application of the system to other fields of chemical research is demonstrated. il comparison of data obtained from the study of a doublebase propellant and its individual constituents is made. In addition, these data are also compared to those ob-
Table I. Thermal Reaction and Gas Evolution during Chemical or Physical Change of a Sample
Chemical/ EndoGas Physical thermic Evolution Change Reaction Yes No Decomposition" x x x Fusion X X Crystal transition X X Desorption x x x Vaporization Desolvation x x Ebullition x Xb Sublimation x Xb 4 Also undergoes an exothermic reaction. b Possible condensation before reaching gas detector.
tained from an organic complex showing the difference between a mixture and a true compound. Investigations of an inorganic oxidizer and an inorganic coordination complex are presented to demonstrate further the usefulness of the technique. EXPERIMENTAL
Apparatus. d block diagram of the apparatus is shown in Figure 1. The furnace and heating block arrangement of the differential thermal apparatus was similar to that of Pakulak and Leonard (6),except that the reference and sample cells were connected by a borosilicate glass manifold as shown in Figure 2. The sample, reference, and gas preheater tubes, as well as the gas inlet and outlet systems, are connected to this manifold by standard ground glass joints. Standard taper joints were used to position the thermal probes and permit disassembly for cleaning. All connections were made without sealant or lubricant on the joints, The differential temperature and temperature probes were constructed of commercially available Chromel-Alumel glass-insulated thermocouple wire (B and S No. 28) lightly sealed in borosilicate glass thermowells. The gas detector system consisted of a VECO thermal conductivity cell (Yiodel 182) and the associated bridge circuit. The signal from the differential temperature probes was fed through a Leeds & Northrup Xodel 9835A d.c. amplifier to a Sargent hlodel h4R recorder. The gas detector signal was fed directly to a Sargent Model SR recorder. In both the differential temperature and gas detector circuits, a simple attenuator ( I ) was used for ease in attenuating large signals above and below a preset zero line. The signals from the temperature probe were fed directly to a Brown recorder equipped with a special circuit to short out the differential temperature and gas detector signals a t calibrated temperatures of the sample cell. In this recorder a second slidewire bar, wound with bare copper wire a t intervals of approximately 1/2 inch, and trolley have been added. As the recorder trolley moves up scale this second trolley moves with it and makes contact with bare copper wire. As contact is made a time delay relay is actuated, shorting both the DTA and G P recorders for 5 seconds. The sample cell temperature a t which each wire is crossed by the secondary trolley was determined by
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