DTA/DSC curve

If no exothermic peak appears up to 500 K in DTA/DSC curves for heating rates of 5-10 K min and no other signs of vigorous reaction are observed, then the material can usually be considered safe. In case of the occurrence of a temperature peak, measurements should be repeated at lower heating rate. 

The maximum operating temperature may be evaluated from DTA/DSC curves if the shift of the peak due to reduction of the heating rate to 0.5-1 K min does not exceed 40 K. This corresponds to moderate activation energies of the secondary reactions, i.e. less than 50 kJ/mol for values obtained from the Arrhenius plots . 

DTA/DSC curves reflect changes in the energy of the system under investigation—changes that may be either physical or chemical in origin. DSC measures the heat required to maintain the same temperature in the sample versus an appropriate reference material in a furnace [9]. Enthalpy changes due to a change of state of the sample are determined. DTA differs from DSC in that the temperature difference is determined, rather than enthalpy differences between the sample and the reference material [9]. 

Wendlandt (290) studied various mercury(I, II) compounds using DTA/ DSC and other TA techniques. The DTA/DSC curves of the yellow and red forms of mercury(II) oxide are shown in Figure 7.29. The initial procedural AT deviation temperature (A7J) was about 475°C for the yellow form and, as expected, 550°C for the red form. The minimum procedural AT temperatures (A7, ) were 575 and 655°C for the yellow and red forms, respectively. Using the DSC data, A7J and ATm values of450 and 550°C, respectively, were found for the yellow form. In both the TG and DTA/DSC curves of the yellow and red forms of mercury (II) oxide, the lower procedural dissociation temperatures for the yellow form are consistent with a decrease in particle size. It is well known that a reduction in particle size of a compound such as this lowers the 7J (or A7J) values. The extent of the lowering, however, cannot be predicted. 

C) When the heating rate is high, the reaction takes place with greater speed in the higher temperature region. The reaction finishes within a narrow temperature interval, and on this account the derivative curves (e.g. DTG, DTA, DSC curves) become sharper. 

Figure 2.6C shows the temperature difference between reference and sample as recorded by differential thermal analysis (DTA). Note also the similar differential scanning calorimeter (DSC) curve later in Figure 2.13. 

Many different test methods can be used to study polymers and their physical changes with temperature. These studies are called thermal analysis. Two important types of thermal analysis are called differential scanning calorimetry (DSC) and differential thermal analysis (DTA). DSC is a technique in which heat flow away from a polymer is measured as a function of temperature or time. In DTA the temperature difference between a reference and a sample is measured as a function of temperature or time. A typical DTA curve easily shows both Tg and T . 

A means of recording the energy necessary to establish zero temp difference between a substance and a reference material, as for DTA. The record is the DSC curve it represents the amount of heat applied per unit time as ordinate against either time or temp as abscissa... 

Figure 3-7 Fraction transformed via partial area integration of a DTA/DSC peak. A partial area, such as the shaded region, divided by the area under the entire endotherm contributes one datum to the fraction transformed versus temperature plot. Successive area calculations with increasing temperature will permit generation of the entire curve.

Phase diagrams can either be calculated [15] or determined experimentally. On the experimental side, cooling curves have often been used in which a molten mixture at sufficiently high temperature is slowly cooled and its temperature recorded as a function of time. At the transformation temperature, the sample temperature will remain invariant until the transformation is complete. By comparison, cooling traces using DTA/DSC provide greater phase equilibria sensitivity, since signals from only transformation events are detected and amplified. 

Another problem is the ease with which a DSC apparatus degrades. Briefly, after it is used for a long time, it begins to produce noise, such that no definite DSC curve can be obtained. This may be attributable to the contamination of cell holders by gas from samples. The authors have conducted experiments with sealed cells by passing nitrogen gas through them, but contamination and its related noise production could not be avoided. For a time, a DTA apparatus considered less prone to contamination was borrowed from Nippon Kayaku Co., Ltd. and used for investigative purposes. 

DTA and DSC curves are commonly recorded by plotting the temperature or enthalpy differences, respectively, against temperature (T) or time (t). 

As an amorphous polymer, lignin undergoes chain segment motion upon heating. This motion, a glass transition, is characteristic of all amorphous polymers, and is indicated by an endothermic shift in the DTA or DSC curves. This glass transition is accompanied by abrupt changes in free volume, heat capacity, and thermal expansion coefficient. 

The differential calorimetric curves (DSC) of the various crystalline forms of triamterene grown from organic solutions containing water and from absolute organic solutions, and the DSC curves of triamterene crystals dried under reduced pressure have been described. The differential thermal analysis-thermogravimetry analysis (DTA-TG) thermograms are also given. 

Commonly, heat flow into a sample is indicated as an upward feature of the DSC curve. This differs from a DTA curve in which an endothermic event is indicated as a downward feature. The DSC curves are commonly recorded over a temperature range by heating or cooling a sample with a constant rate, similar to DTA curves. 

Figure 10.9 Examples of sample baseline interpolation for different types of differential thermal analysis (DTA) and DSC curves. (Reproduced with kind permission of Springer Science and Business Media from M. Brown, Introduction to Thermal Analysis, Kluwer Academic Publishers, Dordrecht. 2001 Springer Science.)...

DSC curves. The only difference is that the heat flow in DSC replaces temperature change in DTA. 

DSC curves can also be used to identify individual polymers in a polymer mixture. This is a rather unique capability of DSC or DTA. Figure 10.17 shows DSC curves of plastic waste. The individual melting peaks reveal the waste s polymer content. By comparing these curves with DSC curves of pure polymers, we can assign the melting peaks as those of low density polyethylene (LDPE), high density polyethylene (HDPE), polypropylene (PP), Nylon-6, Nylon-66 and polytetrafluoroethylene (PIPE). 

People often run a sample twice in DSC or DTA. First, the sample is heated and cooled at a constant rate, then the sample is heated again to obtain the DSC curve for analysis. What are the reasons for this ... 

Clearly, it would be desirable if the area under the peak was a measure of the enthalpy associated with the transition. However, in the case of DTA, the heat path to the sample thermocouple includes the sample itself. The thermal properties of each sample will be different and uncontrolled. In order for the DTA signal to be a measure of heat flow, the thermal resistances between the furnace and both thermocouples must be carefully controlled and predictable so that it can be calibrated and then can remain the same in subsequent experiments. This is impossible in the case of DTA, so it cannot be a quantitative calorimetric technique. Note that the return to baseline of the peak takes a certain amount of time, and during this time the temperature increases thus the peak appears to have a certain width. In reality this width is a function of the calorimeter and not of the sample (the melting of a pure material occurs at a single temperature, not over a temperature interval). This distortion of peak shape is usually not a problem when interpreting DTA and DSC curves but should be borne in mind when studying sharp transitions. 

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