Endothermic 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. 

Experimental studies on the thermal decomposition and combushon processes of AP have been carried out and their detailed mechanisms have been reported.P-iil Fig. 5.1 shows the thermal decomposition of AP as measured by differential thermal analysis (DTA) and thermal gravimetry (TG) at a heating rate of 0.33 K s"f An endothermic peak is seen at 520 K, corresponding to an orthorhombic to cubic lattice crystal structure phase transition, the heat of reaction for which amounts to... 

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. 

The Thermal Decomposition of /3-HMX , Proc7thSympExpl Pyrots, FIRL, Phila (1971) [DTA analysis of /3-HMX in air at 1 atm using a heating rate of 2°/min revealed the endothermic process of the irreversible cryst phase change from /3 to 6 at 192°, and the violent decompn of <5 -HMX at 276°. Integration of the dx/dt or dT/dt curves for the activation energy exponent yields the data shown in Table 4... 

Morphological changes, which are classified into four groups, have been correlated to the degree of topotactic control. Thermal analysis has been studied on DSP poly-DSP in some detail. Two main endothermic peaks of as-polymerized poly-DSP crystals are characterized as thermal depolymerization in the crystalline state and crystal melting point followed by thermal depolymerization in the molten state. From the results of the studies on the heat treatment of as-polymerized polymer crystals, a reversible topochemical processe has been established. 

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. 

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. 

Differential thermal analysis (DTA) is based upon the measurement of the temperature difference (AT) between the sample and an inert reference such as glass or AIjO as they are both subjected to the same heating programme. The temperature of the reference will thus rise at a steady rate determined by its specific heat, and the programmed rate of heating. Similarly with the sample, except that when an exothermic or endothermic process. occurs a peak or trough will be observed. Typical behaviour is shown schematically in Figure 11.7. 

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