DTA applications

The applications of these techniques to organic compounds have been extensively reviewed (2-8). Specific DTA applications are reviewed by Mitchell and Birnie (2), while DSC techniques are discussed by Gray (4). 

Applications of DTA for Polymers. Table 2 (Ref 5, Chapt. l) describes some of the many applications of DTA and DSC. Both DTA and DSC can be used to determine the temperature of the transitions, cited in Table 2. However, the DSC peak area, in addition, gives quantitative calorimetric information (heat of reaction, transition, or heat capacity). DTA can only do so when calibration with a standard material allows the quantitative conversion of AT to heat flow and, ultimately, heat of transition (AH) or heat capacity (Cp). Also, the response of DTA with increasing temperature may be affected by poor heat transfer in the system, detector sensitivity, etc (4). For these reasons, when there is a choice between DSC and DTA, DSC is the preferred method. The illustrations shown below for applications of DSC in characterization of polymers also generally apply for DTA, with the limitations mentioned above. Therefore, DTA applications will not be considered here. Illustrations of polymer applications for DTA can be found in the Thermal Analysis section by Bacon Ke (6) in the previous edition of this encyclopedia. 

Typical TG-DTA applications are thermal and oxidative stability, determination of relative components, decomposition temperatures and thermal decay reactions, action of heat stabilisers, thermal ageing. TG-DTA has been used to screen candidate automotive engineering plastics, elastomeric seals and lubricant additives to establish quality and understand field failures [300], The organometallic chemicals used as lubricant additives were employed to increase the thermal and/or oxidative stability of passenger car and heavy-duty diesel oils. 

The discussion of the applications of DTA is divided into three parts. The first covers the more qualitative DTA, which was historically the first major DTA application. The term qualitative is chosen for these applications because no quantitative heat measurements are made. The temperature is, however, quantitatively fixed. Often transition temperatures can be fixed more precisely by such qualitative DTA than by the more quantitative DSC because of less stringent experimental restriction on sample mass and thermometer placement. Many of the thermometric measurements given in Chapter 3 can also be carried out by DTAand often with better results, so that Sect. 4.5 is an extension of the introduction to the thermodynamics of two-component systems given in Sect. 3.5. The measurement of heat is the... 

The traditional method of DTA has deeper roots and it has been described elsewhere [3,15,602,640]. DTA applications were first based on experience and semi-quantitative evaluations so that the early articles, e.g., [641], should be noticed for using a computer in the verification of the effects influencing the shape of DTA peak. The DTA peak area was verified to be proportional to the heat of reaction (in the AT vs. t coordinates) and found dependent on the heating rate and distorted by increasing crucible diameter. It is, however, almost independent of the positioning of the theiTnocouple junction within the sample (it affects the apex temperature). 

The applications of the DTA technique are quite wide for the characterization of the catalysts and related products, but they are mainly oriented on the determination of their stmctural properties. Here below is a non exhaustive list of DTA applications ... 

The DTA applications applied to catalysts and more generally to inorganic materials have been extensively reviewed in various books [23, 33, 44, 45, 48, 61]. 

The techniques referred to above (Sects. 1—3) may be operated for a sample heated in a constant temperature environment or under conditions of programmed temperature change. Very similar equipment can often be used differences normally reside in the temperature control of the reactant cell. Non-isothermal measurements of mass loss are termed thermogravimetry (TG), absorption or evolution of heat is differential scanning calorimetry (DSC), and measurement of the temperature difference between the sample and an inert reference substance is termed differential thermal analysis (DTA). These techniques can be used singly [33,76,174] or in combination and may include provision for EGA. Applications of non-isothermal measurements have ranged from the rapid qualitative estimation of reaction temperature to the quantitative determination of kinetic parameters [175—177]. The evaluation of kinetic parameters from non-isothermal data is dealt with in detail in Chap. 3.6. 

Although it is possible to use DTA as a quantitative tool, such applications are not trivial. For this reason, DTA has historically been mostly used in a quantitative sense as a means to determine the temperatures at which thermal events takes place. Owing to the experimental conditions used for its measure-... 

The calibration of DTA systems is dependent on the use of appropriate reference materials, rather than on the application of electrical heating methods. The temperature calibration is normally accomplished with the thermogram being obtained at the heating rate normally used for analysis [16], and the temperatures... 

Calibration is by heating known reference materials of accurately known external characteristics. Applications of DTA... 

DSC essentially studies the same thermal phenomena as DTA, albeit using a different principle. Thus DTA and DSC provide very much the same information and their applications are similar. Reference back to the section on the applications of DTA will suffice to indicate the scope of DSC. Some differences in the quality of the information obtained sometimes exist however, leading to a preference for one technique over the other for particular purposes. 

Classical DTA has been developed into heat-flux DSC by the application of multiple sensors (e.g., a Calvet-type arrangement) or with a controlled heat... 

Seyler, R. J., "Applications of Pressure DTA (DSC) to Thermal Hazard Evaluation," Thermochimica Acta, 39 (1980). 

The use of DTA analysis as a means to deduce the compatibility between a drug substance and its excipients in a formulation proved to be a natural application of the technique [22], For instance, Jacobson and Reier used DTA analysis to study the interaction between various penicillins and stearic acid [23]. For instance, the addition of 5% stearic acid to sodium oxacillin monohydrate completely obliterated the thermal events associated with the antibiotic. It seems that the effect of lubricants on formulation performance was as problematic then as it is now, and DTA served as a useful method in the evaluation of possible incompatibilities. Since that time, many workers employed DTA analysis in the study of drug-excipient interactions, although the DTA method has been largely replaced by DSC technology. 

These parameters need to be considered for reactions that go towards the intended completion as well as for possible upsets (see section C). Measuring methodologies for determining characteristic material property values (Stoffkenngrofcen), e.g., differential thermal analysis ("DTA"), calorimetry, and adiabatic experiments, and their possible use and applications are given in the literature /1, 2, 3, 41. 

Some scientists describe DSC techniques as a subset of DTA. DTA can be considered a more global term, covering all differential thermal techniques, while DSC is a DTA technique that gives calorimetric (heat transfer) information. This is the reason that DSC has calorimetry as part of its name. Most thermal analysis work is DSC, and Sections 15.3.3 and 15.3.4 provide information about the instrumentation and applications of this technique. 

Most of the thermobalances have built-in, simple heating programs which allow e.g. linear heating and cooling with different rates, change to isothermal conditions, or to cycle the temperature between two preselected values (Fig. 14 E). The application of the latter is to check the reversibility of certain decompositions, or the reproducibility of DTA-peaks. 

This is another example of the application of thermogravimetry for determination of equilibrium temperatures in dissociation studies. This also enables one to calculate the heat of dissociation from the linear relation between log of dissociation pressure and 1/T. Determination of the specific heat by means of DTA was used afterwards for conversion of the heat of dissociation into the standard values of formation at 298 °K. Ba02 was chosen for these investigation56 because it has been investigated in the past by calorimetric methods and therefore gives a possibility for comparing those values obtained from static methods with those obtained from values from dynamic methods. 

As an application of the simultaneous TG- and DTA-method, the characteristic thermograms of o-acetophenetidide which were recorded at various pressures are shown in Fig. 65. The experimental details were as follows sample weight 25 mg, heating rate 4 °C/min, pressure range 10—500 torr. 

The difference of temperature between the sample under estimation and a thermally-inert reference material is continuously recorded as a function of furnace temperature in differential thermal analysis (DTA). In actual practice both TGA and DTA are regarded as complementary techniques whereby information gathered by the usage of one approach is invariably supplemented and enhanced by the application of the other method. The range of phenomena measurable during a DTA-run is found to be much larger than in a TGA-run. 

---