Differential-thermal analysis

In differential thermal analysis (DTA), the temperature difference that develops between a sample and an inert reference material is measured, when both materials are subjected to an identical heat treatment [87], The related technique of DSA relies on differences in the energy required to maintain the sample and reference at an identical temperature. 

Sample holder comprising thermocouples, sample containers, and a ceramic or metallic block 

FIGURE 4.35 (a) DTA profiles of A1P03 3H20, (b) a mixture of A1P03 3H20 (20wt %) and Na-LTA 

FIGURE 7.30 Ideal differential thermal analysis (DTA) response the temperature difference between the sample and the reference is plotted as a function of the reference temperature, showing endothermic and exothermic reaction peaks. 

FIGURE 7.31 DTA curves for some important soil minerals (a) kaolinite, (b) halloysite, (c) montmorillonite, (d) gibbsite, and (e) allophane. (Reprinted from Tan et al., Methods of Soil Analysis. Part 1. Physical and Mineralogical Methods, American Society of Agronomy-Soil Science Society of America, Madison, Wisconsin, 2010. With permission from the Soil Science Society of America.) 

The fundamental equations for differential thermal analysis have been derived in section 2 namely, 

These equations are based on the assumptions that the heat evolved in a small temperature interval is directly proportional to the weight of polymer reacting during that temperature interval (eqn. (5), p. 9) and that the heat capacity terms are negligible compared with the other terms (eqns. (7) and (8), p. 9). 

If eqns. (80) and (81) are substituted into eqn. (72), it is possible to convert the expressions used in thermogravimetric analysis into those for differential thermal analysis [36, 37]. The activation energy and order of reaction can then be obtained by similar methods. 

Irgashi and Kambe [3] also studied the thermal degradation of polyethylene and used dynamic thermal analysis (DTA) as well as TGA techniques. The experiments were carried out in both air and nitrogen. The PE studied were, two low-density samples. By means of DTA, the crystallinities of the high-pressure samples were found to be 33% and 36% while those for the low-pressure samples were 64% and 77%, and the melting points of the latter samples were higher than those of the former. 

Schwartz and co-workers [97] used isothermal differential thermal analysis to study the diffusion of Irganox 1330 (1,3,5 tris (3,5 di-tor -butyl-4-hydroxyl benzyl) mesitylene) in extruded sheets of isotactic polypropylene (iPP). Studies were conducted over the temperature range 80-120 °C. The measurements showed a clear relation between oxidation induction time and oxidation maximum time [both determined by isothermal dynamic thermal analysis (DTA)] and the concentration of stabiliser. It was possible to calculate the diffusion coefficients and the activation energy of diffusion of Irganox 1330 in iPP by measuring the oxidation maximum times across stacks of iPP sheets. 

For quantitative determination of the concentrations of antioxidants in PP that are required for the analysis of diffusion data, an isothermal DTA technique was developed that directly uses the effect of antioxidants on the thermooxidative stability of the polymers. Especially at elevated temperatures and in the presence of oxygen, polyolefines undergo thermooxidative degradation which follows a radical mechanism [98]. 

The time from the start of an isothermal DTA experiment to the beginning of exothermal decomposition is the so-called oxidation induction time (OIT). After this period, which depends on the antioxidant concentration, effectiveness, and temperature used, autocatalytic oxidation produces an exothermal peak [99-102]. The time from the start of the test to the maximum of this peak is the so-called oxidation maximum time (OMT) [103], which means the complete consumption of antioxidants and the loss of thermal stability of polymer. At elevated test temperatures, corresponding to short reaction times, it was difficult, or even impossible, to determine the OIT in the usual manner. For 

Films of iPP having the dimensions 15 mm x 15 mm x 100 pm with an antioxidant level of 0.03 or 0.10% were chosen for the diffusion measurements. Fifteen films having 0.03% antioxidant concentration were stacked and placed together and then placed over 15 films 

Reproduced from Schwarz and co-workers. Journal of Applied Polymer Science [97] 

reference material S, sample HC, furnace heating current Th, furnace thermocouple. 

Two thermocouples pass through the sample block and into cavities which contain the sample and the inert reference material respectively. These thermocouples are joined in opposition, so that the potential difference seen by the amplifier is a measure of the temperature difference (AT between the sample and the reference material. One of the thermocouples is used to measure the sample temperature (T) directly. The output of both signals from the amplifier can be measured on a two pen recorder. It is usually necessary to control the furnace temperature automatically to ensure a uniform rate of heating or cooling. Many different types of commercial apparatus are available with various refinements and convenience features .  

The major problem with dta stems from the difficulty of obtaining a uniform temperature throughout the sample. For this reason, there is rarely quantitative agreement between the results of different workers. The peak shape and area on the thermogram are affected by non-uniformity of furnace temperature, rate of heating, sample geometry, particle size, and dilution with inert material. In many instances, the atmosphere above the sample also influences the reaction. 

All the factors mentioned above become more important in any attempt to make kinetic measurements for a solid state reaction. Kissinger has attempted to use the temperature of maximum reaction rate to derive reaction order by using the following expression 

A basic assumption in Kissinger s analysis is that reaction occurs equally in all parts of the specimen. Disagreement between the results of various authors shows that this assumption is not generally true and the results are particularly influenced by particle size and sample geometry. Borchardt and Daniels have derived an expression for the rate coefBcient k) derived from a dta curve 

A linear high-pressure PE blend upon heating, undergoes three phase changes from its high-pressure form the 115 °C peak is associated with the high-pressure PE, whereas the 134 °C is shown to be proportional to the linear content of the system. 

Clampit [80] also applied DTA to a study of the 124 °C peak which he describes as the co-crystal peak. His results appear to indicate that there are two classes of co-crystals in linear high-pressure PE blends with the linear component being responsible for the division of the blends into two groups. The property of the linear component that is responsible for the division is related to the crystallite size of the pure linear crystal. 

This technique measures heat-related phenomena that are associated with transitions in materials. In this technique the polymer sample is temperature programmed at a controlled rate and, instead of determining weight changes as in TGA, the temperature of the sample is continually monitored. Just as a phase change from ice to water or vice versa is accompanied by a latent heat effect, so when a polymer undergoes a phase change from, for example, crystalline to an amorphous form, heat is either evolved or absorbed. 

This technique has been used to characterise polymers at temperatures up to 150 °C. Under a reactive gas (oxygen) or an inert gas (nitrogen) plots of the applied temperature versus the temperature of specimens (or calories per second) detect positive (i.e., exothermic) or negative (i.e., endothermic) temperature changes (AH) in the reactions or phase changes occurring upon heating the polymer. The theory of this technique has been discussed by Earnest [3]. 

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Phospholipid molecules form bilayer films or membranes about 5 nm in thickness as illustrated in Fig. XV-10. Vesicles or liposomes are closed bilayer shells in the 100-1000-nm size range formed on sonication of bilayer forming amphiphiles. Vesicles find use as controlled release and delivery vehicles in cosmetic lotions, agrochemicals, and, potentially, drugs. The advances in cryoelec-tron microscopy (see Section VIII-2A) in recent years have aided their characterization [70-72]. Additional light and x-ray scattering measurements reveal bilayer thickness and phase transitions [70, 71]. Differential thermal analysis... 

Kissinger H E 1957 Reaction kinetics in differential thermal analysis Ana/. Chem. 29 1702... 

Assessing the Thermal Stability of Chemicals by Methods of Differential Thermal Analysis, American Society for Testing and Materials, Philadelphia. 

Thermal analysis iavolves techniques ia which a physical property of a material is measured agaiast temperature at the same time the material is exposed to a coatroUed temperature program. A wide range of thermal analysis techniques have been developed siace the commercial development of automated thermal equipment as Hsted ia Table 1. Of these the best known and most often used for polymers are thermogravimetry (tg), differential thermal analysis (dta), differential scanning calorimetry (dsc), and dynamic mechanical analysis (dma). 

The glass-tiansition tempeiatuiesfoi solution-polymeiized SBR as well as ESBR aie loutinely determined by nuclear magnetic resonance (nmr), differential thermal analysis (dta), or differential scanning calorimetry (dsc). 

Melting temperatures of as-polymerized powders are high, ie, 198—205°C as measured by differential thermal analysis (dta) or hot-stage microscopy (76). Two peaks are usually observed in dta curves a small lower temperature peak and the main melting peak. The small peak seems to be related to polymer crystallized by precipitation rather than during polymerization. 

Fig. 13. Differential thermal analysis of wood and its components at a heating rate of 12°C pet minute and a gas flow rate of 30 cm pet minute. Sample...

Thermal Properties. The thermal stabiUty of cellulose esters is deterrnined by heating a known amount of ester in a test tube at a specific temperature a specified length of time, after which the sample is dissolved in a given amount of solvent and its intrinsic viscosity and solution color are deterrnined. Solution color is deterrnined spectroscopically and is compared to platinum—cobalt standards. Differential thermal analysis (dta) has also been reported as a method for determining the relative heat stabiUty of cellulose esters (127). 

Differential thermal analysis curves of iUite show three endothermic peaks in the ranges 100—150, 500—650, and at about 900°C, and an exothermic peak at about 940°C, or immediately following the highest endothermic peak. Minerals formed from iUite at high temperature vary somewhat with the... 

The hydrated alumina minerals usually occur in ooUtic stmctures (small spherical to eUipsoidal bodies the size of BB shot, about 2 mm in diameter) and also in larger and smaller stmctures. They impart harshness and resist fusion or fuse with difficulty in sodium carbonate, and may be suspected if the raw clay analyzes at more than 40% AI2O2. Optical properties are radically different from those of common clay minerals, and x-ray diffraction patterns and differential thermal analysis curves are distinctive. 

A variety of instmmental techniques may be used to determine mineral content. Typically the coal sample is prepared by low temperature ashing to remove the organic material. Then one or more of the techniques of x-ray diffraction, infrared spectroscopy, differential thermal analysis, electron microscopy, and petrographic analysis may be employed (7). 

Order of thermal stabiUty as determined by differential thermal analysis is sebacic (330°C) > a2elaic = pimelic (320°C) > suberic = adipic = glutaric (290°C) > succinic (255°C) > oxahc (200°C) > malonic (185°C) (19). This order is somewhat different than that in Table 2, and is the result of differences in test conditions. The energy of activation for decarboxylation has been estimated to be 251 kj/mol (60 kcal/mol) for higher members of the series and 126 kJ/mol (30 kcal/mol) for malonic acid (1). 

Crystallization kinetics have been studied by differential thermal analysis (92,94,95). The heat of fusion of the crystalline phase is approximately 96 kj/kg (23 kcal/mol), and the activation energy for crystallization is 104 kj/mol (25 kcal/mol). The extent of crystallinity may be calculated from the density of amorphous polymer (d = 1.23), and the crystalline density (d = 1.35). Using this method, polymer prepared at —40° C melts at 73°C and is 38% crystalline. Polymer made at +40° C melts at 45°C and is about 12% crystalline. 

Phase equilibria of the isothiazole-water system have been investigated by differential thermal analysis (76BSF1043), and it has been established that a stable crystalline clathrate (isothiazole-34H20) forms below 0 °C. 

In given work the possibilities enumerated above of varieties of thermal analysis used to reseai ch of solid solutions of hydrated diphosphates with diverse composition. So, for example, the results of differential-thermal analysis Zn Co j P O -SH O showed, that it steady in the time of heating on air to 333 K. A further rise of temperature in interval 333 - 725 K is accompanied with the masses loss, which takes place in two basic stages, registered on crooked TG by two clear degrees, attendant to removal 4,0 and 1,0 mole H O. On crooked DTA these stages dehydration registers by two endothermic effects. In interval 603 - 725 K on crooked DTA is observed an exothermal effect. 

Differential thermal analysis (DTA) Onset temperature of exotherms, heat of reaction, Cp, approximate kinetics... 

What are the consequences What is the maximum pressure Vapor pressure of solvent as a function of temperature Gas evolution Differential Thermal Analysis (DTA) / Differential Scanning Calorimetry (DSC) Dewar flask experiments... 

The procedures of measuring changes in some physical or mechanical property as a sample is heated, or alternatively as it is held at constant temperature, constitute the family of thermoanalytical methods of characterisation. A partial list of these procedures is differential thermal analysis, differential scanning calorimetry, dilatometry, thermogravimetry. A detailed overview of these and several related techniques is by Gallagher (1992). 

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