Thermal event, exothermic

The purpose of this section is to define the various parameters that are measured by DSC. The types of thermal events, exothermic or endothermic, that can be measured by DSC are reported in Table 1. The following sections will describe some of the more fundamental thermal events. Examples from the pharmaceutical field will be given to illustrate the techniques. The examples will be based on either single components such as drug substance and bulk excipients or on a mixture of components such as physical blends of drugs and excipients, solid dispersions, formulated drugs after granulation, and/or compression. 

A 100 Degree Rule was often used in the past throughout the chemical industry to assess whether an accident would occur. According to this rule, if the operating temperature of a process is 100 "C away from the nearest detectable exotherm observed in DSC (Differential Scanning Calorimetry) experiment the operation will not experience this thermal event. In such a case no more detailed information on hazards need be searched for. The 100°C degree rule is, however, often far from the safety margin The use of this rule was the reason of many accidents. 

Differential thermal analysis (DTA) is the monitoring of the difference in temperature between a sample and a reference as a function of temperature [42], Differences in temperature between the sample and reference are observed when changes occur that require a finite heat of reaction. If AH is positive (endothermic reaction), the temperature of the sample will lag behind that of the reference. If the AH is negative (exothermic reaction), the temperature of the sample will exceed that of the reference. Differential thermal analysis is not normally used for quantitative work instead it is used to deduce temperatures associated with thermal events. 

In the DTA measurement, an exothermic reaction is plotted as a positive thermal event, while an endothermic reaction is usually displayed as a negative event. Unfortunately, the use of power-compensation DSC results in endothermic reactions being displayed as positive events, a situation which is counter to IUPAC recommendations [38]. When the heat-flux method is used to detect the thermal phenomena, the signs of the DSC events concur with those obtained using DTA, and also agree with the IUPAC recommendations. 

FIG. 5.5 Thermogram of pure potassium chlorate, KCIO thermal events are observed prior to the melting point (356°Q. Exothermic decomposition occurs above the melting point as oxygen gas is liberated. 

If an endothermic event occurs in the sample, the temperature of the sample lags that of the reference, which follows the heating program. A typical DTA curve for such a thermal event is shown in Fig. 2C. If an exothermic process occurs in the sample, the response will be in the opposite direction. The negative peak shown in Fig. 2C is termed an endotherm and is characterized by its onset temperature. The area under a DTA peak is proportional to the enthalpy associated with the thermal event. Because such proportionality is not linear with temperature, only approximate enthalpy data can be obtained from DTA. The nature of the reference material is important. It must undergo no thermal events over the DTA operational temperature range and it must not chemically react with its container. Alumina has been... 

Thermal events in the sample thus appear as deviations from the baseline, in either an endothermic or exothermic direction, depending upon whether more or less energy is supplied to the sample compared with the reference material. The operational temperature range of DSC is less than that of DTA—being typically subambient to 750°C. Temperature and energy calibration of DSC instruments is achieved using the ICTAC certified reference materials. ... 

Solution calorimetry can also be used to evaluate amorphous/crystalline content in a binary mixture. The enthalpy of solution for the amorphous compound is an exothermic event, whereas that of the crystalline hydrate is endothermic. Enthalpy of solution is a sum of several thermal events, that is, heat of wetting (incorporating sorption process, such as surface sorption and complexation), disruption of the crystal lattice, and solvation. The order of magnitude of solution enthalpy for the crystalline compound suggests that the disruption of the crystal lattice predominates over the heat of solvation. In addition, the ready solubility of the compotmd in aqueous media is probably governed by entropy considerations. 

The above example of the determination of the change in Cp as the epoxy resin underwent crosslinking is an example of a reversible thermal event Cp decreases on vitrification) convoluted by a non-reversible chemical change due to the formation of the network by the ring-opening reaction of the epoxy resin (which produced a large exotherm). Experimentally this separation of the events is achieved by applying a small temperature oscillation on top of the isothermal temperature. The conditions are thus quasi-isothermal. The condition may be described (Jones et al, 1997) by... 

It is clear from Figure 1.2 that the technique may easily and rapidly detect the temperature at which a particular thermal event is occurring. Indeed, current uses of the technique are based largely on the ability of the method to detect the initial temperatures of thermal processes and to qualitatively characterize them as endothermic or exothermic, reversible or irreversible, first order or higher order, etc. (2). The ability to run experiments in a range of atmospheres has also rendered the approach particularly useful for the construction of phase diagrams. 

Differential thermal analysis (DTA) measures the temperature difference between a sample and a reference as the temperature is increased. A plot of the temperature difference (thermogram) reveals exothermic and endothermic reactions that may occur in the sample. The temperature for thermal events such as phase transitions, melting points, crystallization temperatures, and others can be determined... 

The data is recorded and can be plotted at the computer. From these DSC plots, thermal events such as melting points, phase change temperatures, chemical reaction temperatures and glass transition temperature of polymers can be determined. An endothermic peak is plotted in the upward direction and an exothermic peak in the downward direction... 

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