Isothermal thermogravimetry

Thermal analysis helps in measuring the various physical properties of the polymers. In this technique, a polymer sample is subjected to a controlled temperature program in a specific atmosphere and properties are measured as a function of temperature. The controlled temperature program may involve either isothermal or linear rise or fall of temperature. The most common thermoanalytical techniques are (1) differential scanning analysis (DSC), (2) thermomechanical analysis (TMA), and (3) thermogravimetry (TG). 

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. 

However, if we set the furnace temperature just slightly greater than T2, we would obtain a reaction limited to that of A - B, and thus could identily the intermediate reaction product, B. This technique is called isothermal thermogravimetry. Thus, we can follow a solid state reaction by first surveying via d3mamic TGA. If there are any intermediate products, we can isolate each in turn, and after cooling (assmning each is stable at room temperature) cam identify it by x-ray analysis. Note that we can obtain an assay easily ... 

J.C. Oxley, J.L. Smith and S. Naik, Determination of Urea Nitrate and Guanidine Nitrate Vapour Pressures by Isothermal Thermogravimetry , submitted to Propellants, Explosives, Pyrotechnics. 

Temperature programmed reduction (TPR) was followed both by thermogravimetry and conventionally, i.e. by measuring the change in composition of a 5% H2 in Ar mixture. Slight reduction occurred between 570 and 670 K, due perhaps to the presence of small amounts of Ni(OH)2 or basic carbonate reduction was rapid above 670 K but H2 consumption continued until at least 1000 K. In isothermal studies, high degrees of... 

The above discussion illustrates some problems in detailed kinetic analysis of non-isothermal thermogravimetry of pitch and... 

Thermogravimetry is a technique that measures the weight change of a sample as a function of temperature or time (time is suitable only when thermal analysis is performed at specified constant temperature increments). The solid or liquid sample is heated or cooled at a selected rate or isothermally maintained at a fixed temperature. TG is used to measure degradation, oxidation, reduction, evaporation, sublimation, and other heat-related changes occurring in polymers. 

A kinetic model of the physical process of loss of plasticiser di-(2-ethylhexyl)-phthalate from plasticised PVC foil is developed. Experimental investigations are carried out using isothermal thermogravimetry in the temperature range of 120-150 deg.C. The thickness of the foil is 0.1 mm and the amount of the plasticiser in the plasticised polymer is 10-40%. The kinetic parameters... 

Celaschi and Mascarenhas (1977) studied nearly dry lysozyme by elec-tret thermal depolarization, thermal-stimulated pressure, isothermal polarization decay, and thermogravimetry. For a change in temperature of the sample from 250 K to room temperature, desorption of water dipoles was the main process responsible for electrical depolarization. 

P. Staszczuk, D. Stemik, G. W. Ch dzyhski Application of quasi-isothermal thermogravimetry and sorptometry for estimation of heterostructure and fractal dimension of high temperature superconductors. J. Therm. Anal. Cal. (2003) in print. 

Selcuk and Price [101] reported that the thermal decomposition of PbC204 in Nj between 611 and 707 K, studied by isothermal thermogravimetry, fitted the Avrami-Erofeev equation with n = 2, ascribed to random nucleation followed by two dimensional growth, , = 119 7 kJ mol. Incorporation of 1% Zn was found to increase E, to 137 5 kJ mol but did not change the kinetic fit. It is suggested that zinc oxalate within the host crystal restricts the formation of Smekal cracks along which decomposition is favoured. 

The relative thermal stabilities obtained by dynamic thermogravimetry above 200°C and by isothermal methods at 220°C are given in Figs. 51 and 52. (A) is a slightly branched polymer sample produced using benzoyl peroxide. Polymer (B) is a linear syndiotactic sample prepared at low temperature. The figures show that copolymers (C) and (D) are both much less stable than (A) and (B). Tertiary hydrogen and tertiary chlorine are... 

Figure 2.1. Three modes of thermogravimetry. (c) Isothermal thermogravimetry (6) quasi-isothermat thermogravimetry (c) dynamic thermogravimetry.

The thermogravimetry is an experimental technique based on registration of loss in weight carried out for isothermal or tempera-ture/time programmed conditions and can be displayed in cumulative Thermal Gravimetric Analysis (TGA) or derivative Differential Thermal Gravimetry (DTG) forms. 

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