Controlled rate thermogravimetry

In order to increase the resolution of TG curves, it is necessary to change the heating rate in coordination with the decrease in mass. This technique is called controlled rate thermogravimetry (CRTG). Several kinds of technique for controlling the temperature, such as step-wise isothermal control, dynamic rate control and constant decomposition rate control, are employed. The above technique is mainly achieved using software with commercial TG apparatus (2). 

CRTG Controlled rate thermogravimetry (3) Direct liquid interface... 

A technique, developed by Rouquerol [1] and known as constant rate thermogravimetry requires more sophisticated controllers to adjust the furnace temperature in such a way as to maintain a constant preselected value of do/dt over the major portion of the reaction. This increased complexity of the apparatus results in a simplification in the data treatment. 

Thermogravimetry (TG). A technique whereby the weight of a substance, in an environment heated or cooled at a controlled rate, is recorded as a function of time or temperature. 

However, it is these very procedures which exploit the unique capability of DSC. There has been something of a drive towards obtaining kinetic constants from a single dynamic experiment. Although the results obtained in this way may fulfil a useful function further tests are invariably needed to explore the possibility of limitations to their applicability. Advantages have been claimed for sample controlled kinetic experiments in which the experimental conditions are varied in order to maintain the rate of reaction constant. This has proved a popular method of temperature control in thermogravimetry although in principle it can be applied to DSC. 

For further information the reader is referred to some recent reviews on thermogravimetry [80,214, 215], in particular related to polymers [216], and on controlled rate thermal analysis and related techniques [195,196] many textbooks are available (cfr. Bibliography). 

Thermogravimetry (TG) data were obtained on the NH4+- and H30+-exchanged ZSM-5 samples, as well as on a separately prepared NH -exchanged sample of similar composition, for confirmation purposes. A duPont Model 951 thermogravimetric analyzer was used in conjunction with a Model 990 Recorder-Controller. The TG curves were obtained on approximately 25 mg samples, at a heating rate of 10°C per minute, and under flowing air at a rate of 38 cnP per minute. 

Thermogravimetry (TG) or thermogravimetric analysis (TGA). In this technique, the mass of a sample is followed as a function of temperature or time. The amount and rate of mass change with temperature or time in a controlled atmosphere are obtained. Such information can tell us about thermal stability as well as the compositional profile of a variety of elastomers and polymers. It is an excellent quantitative technique but qualitatively there may be some doubt as to what material is lost during heating. 

In thermogravimetry (TG or TGA) the change in sample mass is determined as a function of temperature and/or time. The instrument is a thermobalance that permits the continuous weighing of a sample as a function of time. The sample holder and a reference holder are bounded to each side of a microbalance. The sample holder is in a furnace, without direct contact with the sample, the temperature of which is controlled by a temperature programer. The balance part is maintained at a constant temperature. The instrument is able to record the mass loss or gain of the sample as a function of temperature and time [m = /( )]. Most instruments also record the DTG curve, which is the rate of the mass change dm/dt = f(T). 

Thermogravimetry is an attractive experimental technique for investigations of the thermal reactions of a wide range of initially solid or liquid substances, under controlled conditions of temperature and atmosphere. TG measurements probably provide more accurate kinetic (m, t, T) values than most other alternative laboratory methods available for the wide range of rate processes that involve a mass loss. The popularity of the method is due to the versatility and reliability of the apparatus, which provides results rapidly and is capable of automation. However, there have been relatively few critical studies of the accuracy, reproducibility, reliability, etc. of TG data based on quantitative comparisons with measurements made for the same reaction by alternative techniques, such as DTA, DSC, and EGA. One such comparison is by Brown et al. (69,70). This study of kinetic results obtained by different experimental methods contrasts with the often-reported use of multiple mathematical methods to calculate, from the same data, the kinetic model, rate equation g(a) = kt (29), the Arrhenius parameters, etc. In practice, the use of complementary kinetic observations, based on different measurable parameters of the chemical change occurring, provides a more secure foundation for kinetic data interpretation and formulation of a mechanism than multiple kinetic analyses based on a single set of experimental data. 

A more complex system of temperature and atmosphere control used in the study of decomposition rates is quasi-isothermal and quasi-isobaric thermogravimetry [6]. In this method a heating programme linearly increases the sample temperature until a mass change is detected. On detection of a mass... 

It has already been stated that SCTA envelopes the original CRTA techniques. Essentially, there are two forms of CRTA - the quasi-isothermal/quasi-isobaric methods (Paulik) and the constant rate method (Rouquerol). The Paulik method is based on thermogravimetry (TG) and is known as Q-TG . It depends on a control system that maintains a constant rate of mass loss and the measured parameter is temperature as a function of time. CRTA relies on using a transducer to monitor the pressure of evolved gas in a continuously evacuated chamber. The sample is heated in such a way as to maintain the monitored gas pressure constant. Since the pressure is maintained constant, the rate of gas pump-off is maintained constant and thus the rate of mass loss (when a single gas is evolved) is also maintained constant. Hence, CRTA is effectively vacuum thermogravimetry. The similarity between the Paulik and Rouquerol techniques is immediately apparent. Both maintain the reaction rate constant and both control the pressure of the evolved species in the reaction environ ment. Hence, the differences in the techniques are purely semantic. These techniques allow a precise... 

Thermogravimetry is the most commonly used method for studying the kinetics of corrosion reactions at high temperatures. The sample is placed in an oven with a controlled atmosphere, where one monitors the variation of its mass by means of a microbalance (Figure 9.3). The precision of the technique is on the order of micrograms. If all corrosion products remain on the surface of the sample, the measured mass increase as a function of time permits to identify the rate law that governs the reaction kinetics. 

Thermogravimetry involves the continuous recording of mass versus temperature or time as a sample is heated in a furnace with a controlled environment. The sample may be heated at a constant rate or held at an isothermal temperature. Madorsky (35) and Jellinek (36) authored major books dealing with thermogravimetry of polymers. The era of modem automated thermogravimetiy started with the introduction of the electrobalance by Cahn and Schultz (37). Other competitors, such as DuPont, Mettler, and Perkin-Elmer, introduced their products in rapid succession. 

The solid resins in powder paints are produced in two different ways. In the conventional method, the resin is produced in isobutyl methyl ketone. In the fusion method, bisphenol-A is condensed catalytically without solvents to give a liquid epoxy resin. In this study, the powders were cured in a device permitting precise control of temperature, type of atmosphere, rate of gas flow, and the collection of the volatile compounds. The compounds were studied with a high-resolution GC and a GC-MS. The mass loss of the powders during curing was studied by thermogravimetry. 

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