Melting endotherms

The heat flow into (endothermic) or out (exothermic) of a sample as a function of temperature and time is measured using the technique of DSC. In particular, it is used to study and determine the temperature of thermal transitions. For polymers, these include Tg, the glass transition temperature, Tc, the (exothermic) temperature of crystallisation for polymers that can crystallise, and Tm, the (endothermic) melting temperature. A DSC measurement requires only a small amount of sample 2-20 mg of a film, powder, fibre or liquid samples can be analysed in a DSC pan. 

Measurements of thermal analysis are conducted for the purpose of evaluating the physical and chemical changes that may take place in a heated sample. This requires that the operator interpret the observed events in a thermogram in terms of plausible reaction processes. The reactions normally monitored can be endothermic (melting, boiling, sublimation, vaporization, desolvation, solid-solid phase transitions, chemical degradation, etc.) or exothermic (crystallization, oxidative decomposition, etc.) in nature. 

Lot number Initial freezing maximum (°C) Second freezing maximum (°C) Third freezing maximum (°C) Fourth freezing maximum (°C) First endotherm maximum (°C) Second endotherm (melt) maximum (°C)... 

A particularly critical case is the exothermal decomposition immediately following the endothermal melting peak. In such cases, the decomposition is faster in the liquid than in the solid state. In an industrial environment, this could mean that a hot spot may melt a small part of the solid, which begins to decompose and the decomposition may propagate through the entire volume of the product. In such cases, the definition of a safe operation temperature becomes critical. 

The endothermic melting transitions were integrated, enabling a determination of the enthalpies of fusion for the two forms. For Form-I it was found that AHf = 70.6 J/g, while for Form-II it was found that... 

Partially crystalline behavior at low temperature (endothermic melting peaks in DSC at approximate -50 °C) is observed for weakly crosslinked siloxane particles and the corresponding graft copolymers. 

The decomposition of LiAlH4 involves structural phase transformations and several decomposition steps and has been intensively investigated [69-73]. First, the endothermic melting of LiAlH4 between 165 and 175 °C is observed, followed by the exothermic decomposition (AH= —lOkJ mofi H2) and recrystallization of Li3AlHfi between 175 and 220 °C (Eq. (5.18)). During this first step the hexahydride structure. 

Appelqvist, I.A.M. Cooke, D. Thermal properties of polysaccharides at low moisture an endothermic melting process and water-carbohydrate interactions. Carbohydr. Polym. 1993, 20. 

When confined in the closed cell and subjected to the isothermal storage test performed at a T 2 cm of a powdery chemical of the quasi-AC type warms slowly up to the T, but the temperature remains near the T, over a long time, because a phase transition, the endothermic melting, of the chemical occurs almost simultaneously with the exothermic decomposition reaction all through this time. Once, however, the chemical finishes melting in the course of time, an apparently sudden quasi-autocatalytic reaction of the resultant liquefied chemical starts. 

The property of each individual powdery chemical of the quasi-AC type varies in a continuous fashion with the interval between the cndothcnnic peak and the exothermic peak in the DTA curve which the chemical affords. Both peaks arc in particular close together in the case of ABCN or AMVN, as shown in Fig. 9. The pattern of the DTA curve of a typical powdery chemical of the quasi-AC type, such as ABCN or AMVN, is thought to reflect the fact that, when heated, a phase transition, the endothermic melting, takes place in parallel with a chemical reaction, the exothermic decomposition reaction, in it, as shown diagrammatically in Fig. 10. 

Figure 10, Simultaneous occurrence of a phase transition, the endothermic melting, and a chemical reaction, the exothermic decomposition reaction, in ABCN as a representative of powdery chemicals of the quasi-AC type.

The above phenomenon that the endothermic melting takes place in parallel with the exothermic decomposition reaction is thought to occur in every powdery chemical of the qiiasi-AC type, although this fact is not necessarily revealed explicitly in the DTA curve. 

A differential scanning calorimetry curve of salicylic acid was obtained (Fig. 9) on a Perkin Elmer DSC 20 differential calorimeter. Nitrogen was used as purge gas. Scan was performed at the rate of 10°C/min from 100-200°C. The DSC curve revealed an endothermic melting peak (Max. 158.2°C). 

Differential scanning calorimetry Samples of gels (8 to 11 mg) were placed in hermetically sealed stainless steal pans and analyzed with a DSC QlOO (TA Instruments, New Castle, DE, USA). An empty pan was used as reference. Samples were heated at 10°C/min from -40 to 120°C. At least triplicate samples were analyzed for each gel. The endothermic melting peak around 0°C was taken as an evidence of ice melting. Endothermic peaks in the 50 to 70°C range were taken as evidence of starch retrogradation (stored samples). 

Another useful tool that the automotive chemist will use is the differential scanning calorimeter. The component will be measured against a reference as heat is applied to both samples. A constant temperature will be maintained and the difference in temperature (AT) required to raise the temperature of the sample in relation to the reference sample is measured. Because the reference sample is not undergoing phase transitions, the AT of the measured sample will show an endothermic (melting point) or exothermic (crystallization) peak as heat is applied [7-9],... 

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