Thermogravimetry simultaneous measurements

Very often thermogravimetry alone cannot give enough information about the reactions being studied. Other measurements often add to the knowledge gained. These ancillary techniques may be applied at the same time as the TG measurement is being applied and are then referred to as simultaneous and are discussed in Chapter 6. Alternatively other measurements may take place in separate experiments in separate apparatus. This is referred to as combined measurements. Simultaneous measurements include differential thermal analysis (DTA) and evolved gas analysis (EGA). These are explained in later chapters. 

To ensure optimum conditions for a direct comparison of DSA data with results obtained by other methods, devices were constructed to provide simultaneous measurement of additional parameters [11]. The equipment for simultaneous measurements by DSA, differential thermal analysis (DTA), thermogravimetry/differential thermogravimetry (TG/DTG) or dilatometry, available from Netzsch Ltd, Germany, ensures the optimal conditions for direct comparison of the results [12]. 

In gravimetry the sample represents, according to the definition of Sect. 2.1, an open system. The mass-flow across the boundaries of the sample holder is continuously monitored by the balance. One can suggest immediately two logical extensions of thermogravimetry. In order to identify the mass flux, an analysis technique, such as mass spectrometty or exclusion chromatography can be coupled to the furnace. The other extension involves the simultaneous measurement of the heat flux by calorimetry. Instruments that couple all three techniques have been built and can fully characterize an open system. Since one should, however, always be able to precisely repeat scientific experiments, it should be possible to separately measure mass change. 

Thermal analysis is a group of techniques in which a physical property of a substance is measured as a function of temperature when the sample is subjected to a controlled temperature program. Single techniques, such as thermogravimetry (TG), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), dielectric thermal analysis, etc., provide important information on the thermal behaviour of materials. However, for polymer characterisation, for instance in case of degradation, further analysis is required, particularly because all of the techniques listed above mainly describe materials only from a physical point of view. A hyphenated thermal analyser is a powerful tool to yield the much-needed additional chemical information. In this paper we will concentrate on simultaneous thermogravimetric techniques. 

Thermogravimetry coupled to infrared spectrometry has been carried out at the anal3dical centre of the CNRS at Lyon-Vemaison. Weight loss of about 100 mg of skeleton powder during heating from ambient to 500 °C was continuously recorded. Simultaneously, emitted water and CO2 were measured by infrared absorption at 1508 cm and 2363 cm respectively (Cuif et al. 2004). 

Through the use of simultaneous thermogravimetry modulated beam mass spectrometry, optical microscopy, hot-stage time-lapsed microscopy, and scanning electron microscopy measurements, the physical and chemical processes that control the thermal decomposition of RDX 9 below its melting point (160-189 °C) have been identified (Scheme 17) <2005PCA11236>. 

The diagram on the right middle displays the principle of the Netzsch DSC 404. This instrument is based on the classical design of a heat flux DTA and can be used for heat capacity measurements up to about 1700 K. With a related design (STA 409, see also Chapter 7), differential calorimetry and thermogravimetry can be carried out simultaneously. [The temperature range is 110 K to 2700 K with different furnaces the heating rate is 0.1 to 100 K/min noise is 100 /liK, and sample volume is 100-900 mm ]. 

The next examples illustrate the greater detail that can be obtained when thermogravimetry and DTA are combined. The experiments can be carried out either simultaneously or successively. Figures 7.10 and 7.11 show copies of two pages from the "Atlas of Thermoanalytical Curves." The measurements were made with the Derivatograph as described in Fig. 7.6. In addition to differential thermal analysis and thermogravimetry, the derivative of the... 

---