Simultaneous Thermal Analysis Techniques

This chapter is concerned with the large family of Simultaneous Thermal Analysis (STA) techniques, in which two or more types of measurement are made at the same time on a single sample. This methodology, entailing a more complex instrument, often specially built, has been found to be essential in a variety of thermal studies, and instruments for simultaneous measurements have been constructed for more than fifty years - often as soon as it was technically possible, as the benefits of this approach were rapidly appreciated. 

In terms of an overall understanding of the system under study, the availability of two or more independent measurements that correlate precisely leads to a synergistic effect, so that the total value of the information is greater than merely the sum of the parts. F. and J. Paulik, in their review of their own ST A studies, speak of this as a multiplying effect. 

At first sight, a simultaneous instrument may seem attractive as an alternative to buying separate units. A case has been made for this on economic grounds (initial purchase, running costs etc.) but the main advantages are technical, not economic. Modern simultaneous devices are capable of excellent performance, but it is still the case that they will not perform the individual measurement tasks as well as dedicated separate units. Their advantage lies in the unambiguous correlation of the two or more sets of information. 

This chapter will describe the most common combinations of techniques in some detail, then look briefly at some less-widely used ones, which have nonetheless proved essential for some investigations. 

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Whereas Redfern [57] has pointed out the advantages of simultaneous thermal analysis techniques (particularly TG-DSC and TG-DTA) over techniques conducted singly, an even more complete thermal profile is provided when a thermal analyser is coupled to some form of gas analyser (MS or FTIR). Mohler and co-workers [51] have reported TG-DSC-MS of the thermal decomposition of the vulcanisation accelerator tetramethyl thiuram disulphide (TMTD) in rubber degradation of TMTD starts at about 155 °C, as evidenced by m/z 76 (CS2) and 44 (radical of the secondary dimethylamine). 

Warrington, S.B. Simultaneous thermal analysis techniques. In Principles of Thermal Analysis and Calorimetry Haines, P.J., Ed. Royal Society of Chemistry Cambridge, U.K., 2002 Chapter 6, 172-174. 

Coupled or simultaneous thermal analysis techniques are gaining more and more importance for several reasons ... 

Principles and Characteristics Simultaneous thermal analysis techniques, such as TG-DSC/DTA offer vital information on polymer structure based on heat flow behaviour and mass change [290], but little direct information on the composition of evolved gas products. A more complete thermal profile is provided when a thermal analyser is coupled to an identification tool. Henderson et al. [433] have recently described TG-DSC/DTA with evolved gas analysers (MS and FTIR). The skimmer coupling is the most advanced commercial way of combining a thermobalance or simultaneous TG-DSC/DTA instrument with a quadrupole mass spectrometer [338]. For descriptions of interface techniques in this coupled instrumentation, cfr. ref. [411]. Simultaneous TG-DSC-MS is capable of operation up to 2000°C [434]. 

Continuous monitoring of the carbon monoxide and carbon dioxide evolved during thermal decomposition of brominated polyester resin samples, has been carried out using a simultaneous thermal analysis-mass spectrometry technique. In order to allow measurement of the carbon monoxide evolved, the atmosphere chosen for these runs was 21% oxygen in argon, since the peak at 28 atomic mass units (amu)... 

To obtain the cure kinetic parameters K, m, and n, cure rate and cure state must be measured simultaneously. This is most commonly accomplished by thermal analysis techniques such as DSC. In isothermal DSC testing several different isothermal cures are analyzed to develop the temperature dependence of the kinetic parameters. With the temperature dependence of the kinetic parameters known, the degree of cure can be predicted for any temperature history by integration of Equation 8.5. 

Simultaneous Thermal Analysis (STA) A technique which combines DTA, TGA, DSC (cal.) and derivatives of DTA/TGA in a single instrument, offers advantages over the performance of each technique as a separate experiment on stand-alone instruments. By examining a single sample, one ensures no variation in sample homogeneity, atmosphere difference and other instrumental parameters. At the same time, there is a provision for detection and analysis of evolved gases (EGA) during thermal decomposition. Also, derivatives of TGA and DTA can be precisely measured (electronically). 

The thermal characterisation of elastomers has recently been reviewed by Sircar [28] from which it appears that DSC followed by TG/DTG are the most popular thermal analysis techniques for elastomer applications. The TG/differential thermal gravimetry (DTG) method remains the method of choice for compositional analysis of uncured and cured elastomer compounds. Sircar s comprehensive review [28] was based on single thermal methods (TG, DSC, differential thermal analysis (DTA), thermomechanical analysis (TMA), DMA) and excluded combined (TG-DSC, TG-DTA) and simultaneous (TG-fourier transform infrared (TG-FTIR), TG-mass spectroscopy (TG-MS)) techniques. In this chapter the emphasis is on those multiple and hyphenated thermogravimetric analysis techniques which have had an impact on the characterisation of elastomers. The review is based mainly on Chemical Abstracts records corresponding to the keywords elastomers, thermogravimetry, differential scanning calorimetry, differential thermal analysis, infrared and mass spectrometry over the period 1979-1999. Table 1.1 contains the references to the various combined techniques. 

The TG-DSC technique has recently been reviewed [56]. Redfern [57] has reviewed single sample simultaneous thermal analysis, i.e., TG-DSC and TG-DTA studies of polymers. 

During investigation of a new material it is unlikely that any single thermal analysis technique will provide all the information required to understand its behavior. Complementary information is usually needed, which may be from another simultaneous thermal technique such as thermogravimetric-differential scanning calorimetric-mass spectrometry (TG-DSC-MS), gas chromatography (TG-GC, or DSG-GG), or spectroscopic methods such as IR spectroscopy or X-ray photoelectron spectroscopy (XPS). 

The techniques of EGD and EGA are almost always used in conjunction with other thermal analysis techniques or multiple techniques. In multiple techniques, two options are possible (1) One sample may be employed for all the measurements or (2) two or more samples, one for each techniques, may be employed. To distinguish between the two modes, the terms simultaneous will be used for the application of two or more techniques to the same sample at the same time. The term combined will indicate the use of separate samples for each technique. Multiple techniques are indicated by the acceptable abbreviation for each technique such as TG-EGD, TG-DTA-EGD-MS, EGA-MS, and so on. Other terms that may be employed by the EGD-EGA techniques are ... 

The principal role of EGD and EGA is mainly as complementary techniques for other thermal analysis data. Samples are studied by TG, DTA, DSC and other thermal analysis techniques first and if the decomposition reactions are unknown, EGA is usually called on to determine the composition of the reaction products. With these known, as well as the other physicochemical data, the chemical pathway of the reaction can usually be elucidated. As mentioned earlier, the EGD-EGA data can often be obtained simultaneously with the other thermal data using multiple techniques with a substantial saving of time and effort. 

The techniques of DTA, and to some extent TG, have long been used by geochemists and others to study the thermal behavior of clays and minerals. Since many simultaneous reactions occur in clays, as well as solid-state reactions that alter the decomposition reaction, the use of other methods such as EGA, X-ray, IR. and so on, are used to supplement conventional thermal analysis techniques. Consequently, there has recently been an unusual amount of interest in the use of EGA (MS) in the study of these compounds. 

Although the principal thermal analysis techniques are thermogravimetry, differential thermal analysis, and differential scanning calorimetry (see Chapter 1), there are a number of other thermal techniques, besides those discussed elsewhere in this book, that are useful for solving chemical and technological problems. Some of these methods are of recent development and hence little used at the present time, but they possess the potential for wider use in the future. Many of these techniques are employed to supplement or complement the three principal techniques of TG. DTA. and DSC, either in the simultaneous (single sample) or concurrent multiple samples) modes. 

Thermoelectrometry has been reviewed in book chapters by Wendlandt (52,53) and Warfield (54), and reviews by Chiu (78), Paulik and Paulik (55), and Wendlandt (56, 98, 99). Since many of the therrooelecirometry studies involve simultaneous methods with other thermal analysis techniques, reference (55) is especially useful. 

It should be noted that, in many cases, the use of only a single thermal analysis technique may not provide sufficient information about a given system. As with many other analytical methods, complementary or supplementary information, as can be furnished by other thermal analysis techniques, may be required. For example, it is fairly common to complement all DTA or DSC data with thermogravimetry. If one or more gaseous products result, evolved gas analysis may prove useful in solving the problem at hand. Simultaneous thermal techniques are helpful in this respect in that several types of data are obtained from the same sample under identical pyrolysis conditions. 

Simultaneous thermomicroscopy/thermal analysis techniques have been developed. A TG-thermomi-croscopy system has been developed for a study of the pyrolysis and combustion of coals. The microscope is combined with a video camera to continuously monitor the thermal events involved and the sample holder for microscopy is attached to the microbalance by an aluminum oxide capillary. Events such as partial fusion, swelling, and internal structural changes associated with coal can be observed microscopically and correlated directly with noted associated mass losses. The system is particularly suitable for coal analysis involving rapid heating rates and atmosphere switching. 

While TGA provides useful data when a mass change is involved in a reaction, we have seen that many reactions do not have a change in mass associated with them. The use of both TGA and DTA or TGA and DSC provides much more information about a sample than either technique alone provides. There are several commercial thermal analysis instrument manufacturers who offer simultaneous combination systems, often called simultaneous thermal analysis (STA) systems. Simultaneous TGA-DTA and simultaneous TGA-DSC instruments are available. Instrument combinations cover a wide temperature range and come in both analytical sample size (1-20 mg)... 

Simultaneous thermal analysis (STA) refers to the simultaneous application of two or more thermoan-alytical methods on one sample at the same time, such as DTA and thermoconductivity. In practice, however, this term is mostly used for simultaneous measurement of the mass changes and caloric effects on a sample under thermal treatment. The benefits are (i) information on transformation energetics and mass change in one run, under identical conditions (ii) time saving and (Hi) no differences in sample composition for the various thermal measurements - important for non-homogeneous sample materials. Although TG-DSC and TG-DTA are the most widely used of the simultaneous techniques due to... 

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