Exothermic process, thermal analysis

The sample temperature is increased in a linear fashion, while the property in question is evaluated on a continuous basis. These methods are used to characterize compound purity, polymorphism, solvation, degradation, and excipient compatibility [41], Thermal analysis methods are normally used to monitor endothermic processes (melting, boiling, sublimation, vaporization, desolvation, solid-solid phase transitions, and chemical degradation) as well as exothermic processes (crystallization and oxidative decomposition). Thermal methods can be extremely useful in preformulation studies, since the carefully planned studies can be used to indicate the existence of possible drug-excipient interactions in a prototype formulation [7]. 

Measurements of thermal analysis are conducted for the purpose of evaluating the physical and chemical changes that may take place in a heated sample. This requires that the operator interpret the observed events in a thermogram in terms of plausible reaction processes. The reactions normally monitored can be endothermic (melting, boiling, sublimation, vaporization, desolvation, solid-solid phase transitions, chemical degradation, etc.) or exothermic (crystallization, oxidative decomposition, etc.) in nature. 

The sample is enclosed in a heavy walled bomb with an internal volume of approximately 1 to 1 ml. Although similar to a differential thermal analysis (DTA) test, the samples used are much larger, and the conditions of confinement allow the liquid to remain in contact with any decomposition products that form as vapors. Heat is applied so that the bath temperature increases at a constant rate and the temperature of both the heating bath and the sample are recorded continuously. When the temperature of the sample exceeds that of the bath, an exothermic reaction must be occurring in the sample, and this process is frequently accompanied by a detonation. (The bomb is equipped with a blow out disc to avoid any major damage to the equipment). In the more usual case the discrepancy between sample temperature and bath temperature increases with temperature, and the point at which this deviation is 5°F./min. is called the self-heating temperature. Typical values for some liquid materials of interest in the propellant field are listed in Table V. 

Differential thermal analysis (DTA) involves heating (or cooling) a test sample and an inert reference sample under identical conditions and recording any temperature difference which develops between them. Any physical or chemical change occurring to the test sample which involves the evolution of heat will cause its temperature to rise temporarily above that of the reference sample, thus giving rise to an exothermic peak on a DTA plot. Conversely, a process which is accompanied by the absorption of heat will cause the temperature of the test sample to lag behind that of the reference material, leading to an endothermic peak. 

Differential thermal analysis (DTA) measures the amount of heat released or absorbed by a sample as it is heated at a known rate." When the enthalpy change is determined, the method is called differential scanning calorimetry (DSC). The presence of exothermic or endothermic processes at certain temperatnres provides information about the nature of phase changes and chemical reactions occurring in the material as it is heated. DTA can often be used as a sensitive method for establishing the presence or absence of secondary phases in samples if these phases undergo phase transformations at known temperatures. ... 

A to G (Fig. 33) were obtained by integration of curves A to G (Fig. 34). Evolution of the profile of the calorimetric curves indicates that the reactivity of the oxide toward carbon monoxide increases progressively with the extent of reduction. From curve A (Fig. 34), it appears that the reaction of dose A is a relatively slow exothermic process. Curves B to F (Fig. 34) are more complex. Analysis of these curves shows that three thermal phenomena occur during the reaction of doses B to F (i) a fast exothermic process whose intensity increases with the extent of reduction, (ii) a slower exothermic process similar to that observed for dose A, whose intensity decreases from curve B to F and, (iii) a slow endothermic process which is evidently the desorption of carbon dioxide. Both exothermic processes are related to the adsorption of carbon monoxide and to the surface reduction of the solid. 

DTA Differential Thermal Analysis DSC Differential Scanning Calorimetry TGA Thermo Gravimetric Analysis ARC Accelerating Rate Calorimetry BSC Bench Scale, Heat Flow Calorimetry SEDEX Sensitive Detector of Exothermic Processes Others Oven Tests, Dewar Tests, Hot Plate Tests, etc. 

Experimental studies on the thermal decomposition and combustion processes of AP have been done and their detailed mechanisms have been reported.[1 12] Figure 5-1 shows the thermal decomposition of AP measured by differential thermal analysis (DTA) and thermal gravimetry (TG) with a heating rate of 0.33 K/s. An endothermic peak is seen at 520 K, which is the orthorhombic to cubic crystal structure lattice phase transition whose heat of reaction is -85 kj/kg without mass loss. An exothermic reaction occurs between 607 K and 720 K accompanied by mass loss. This exothermic reaction occurs through the overall reaction scheme of11,21... 

DSC is a thermal analysis technique that is used to measure the temperatures and energy flows related to transitions in materials as a function of time and temperature.These measurements provide qualitative and quantitative information about physical and chemical changes that involve endothermic or exothermic processes or changes in heat capacity. Any event, such as loss of solvent, phase transitions, crystallization temperature, melting point, and degradation temperature of the plastic sample, result in a change in the temperature of the sample. The systems available cover a wide temperature range, e g., -60°Cto>l,500°C. 

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