Exothermal peaks

The cure of novolaks with hexa has been studied with differential scanning calorimetry (dsc) and torsional braid analysis (tba) (46) both a high ortho novolak and a conventional acid-cataly2ed system were included. The dsc showed an exothermic peak indicating a novolak—hexa reaction ca 20°C higher than the gelation peak observed in tba. Activation energies were also calculated. 

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... 

Quantitative analysis of the peroxy group of macroinitiators is performed by iodometry [38] and that of the azo group is done by ultraviolet (UV) spectrometry. Recently, type II MAI composed of PU was determined of its azo concentration by UV [20]. When the UV absorption spectral peak of the azo group overlaps other peaks, DSC is available by determining the azo group from the exothermal peak area [1IJ. 

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. 

Benzoquinone moistened by water gives rise to an important thermal load. A DTA showed that the exothermic peak is reached at 40-50 C and the compound decomposes at a temperature starting at 60-70°C. When a bottle of benzoquinone is exposed to sunlight, its temperature reaches 50-60°C rapidly. 

Dimethyiacetamide (DMA) behaves the same way the interaction exothermicity is higher than with DMF. A differential thermal analysis of the 1 1 mixture of DMA/CCI4 shows two exothermic peaks at 91-97 and 97-172°C (maximum at 147°C), respectively. An experiment made in an autoclave shows that the final temperature reaches 450°C and a pressure of 12.8 bar that is bonded to a substantial gas release. If 1 % of iron is present, the first exotherm occurs at 71 C. 

The dry salt was more or less explosive, depending on the method of preparation [1], DTA examination of a 15% solution of the diazonium salt in hydrochloric acid showed an exothermic decomposition reaction with the exotherm peak at 65°C. Adiabatic decompositon of the solution at 20° C took 80 min to attain the maximum... 

Mostly, the convention with exothermic peaks (exotherms) shown as positive and endothermic peaks (iendotherms) as negative is accepted. However, there are exceptions and care should always be exercised in the interpretation of recorded data. 

An example of an idealized DSC curve with an exotherm peak is represented in Figure 2.13. The shape of the DSC curve depends on the reaction order, the occurrence of autocatalytic decomposition, and on parameters such... 

A sketch of the thermogram obtained for the thermal decomposition of Fe(CO)(l,3-C4H6)2 at 418 K is shown in figure 9.5 [163]. The endothermic part reflects the heating (from 298 K to 418 K) and the melting of the sample and probably also some thermal decomposition. The exothermic peak of the thermogram was attributed to the polymerization of butadiene. Because area B is larger than A, the overall process (equation 9.10) is exothermic. 

Figure 3.2. Differential calorimetric curves for the molecular glasses (a) Spiro-sexiphenyl (second heating curve) and (b) Spiro-PBD (first and second heating curve). The glass transition is indicated by a characteristic step, the melting point by an endothermic peak. In (a) recrystallization occurs above Tg, which can be seen by an exothermic peak. The material in (b) forms a stable amorphous glass without recrystallization. The melting point from the first heating curve of a crystalline sample (dotted line) disappears in the second heating cycle (solid line). Only the glass transition is visible.

An easy method for investigating the thermal-oxidative degradation of PET is differential thermal analysis (DTA), which indicates thermal degradation by the appearance of an exothermic peak in the range of the melting temperature. This approach also can be used to assess the efficiency of stabilizers [40],... 

The DSC exothermic peaks can be used to obtain Xc(tmax) from /max. Figures 20.24 and 20.25 illustrate the linear regression analysis of the... 

Endothermic features are visible at 116°C and 278°C before three weight loss stages each with accompanying endotherms. The first weight loss of 5.8% occurs with an endothermic feature at 313°C, the second 1.9% weight loss occurs with an endothermic feature at 353°C, the last weight loss of 0.56% occurs with an exothermic peak at 39I°C. A steady increase in sample weight occurs from 450°C to 600°C. A final endotherm is visible at 586°C... 

The two most popular methods of calculation of energy of activation will be presented in this chapter. First, the Kissinger method [165] is based on differential scanning calorimetry (DSC) analysis of decomposition or formation processes and related to these reactions endo- or exothermic peak positions are connected with heating rate. The second method is based on Arrhenius equation and determination of formation or decomposition rate from kinetic curves obtained at various temperatures. The critical point in this method is a selection of correct model to estimate the rate of reaction. 

Figure 3.9a shows a typical DSC trace at the heating rate of 10°C/min obtained in our laboratory from as-received LiAlH (purity 97%). It is clearly seen that (Rla) is proceeded by an exothermic peak centered at 152.5°C. In the literature, this first... 

Kim et al. [119] synthesized a mixture of Mg(AlH )2 and NaCl after 1 h of milling in a SPEX 8000 ball mill. DSC test was carried out only up to slightly over 300°C and showed the first exothermic peak at the range of 115-150°C and the second endothermic peak at the range of 240-290°C. They concluded that the first exothermic reaction corresponds to the decomposition of Mg(AlH )2 into MgH, Al and and the second endothermic peak to the decomposition of MgH. Apparently,... 

Figure 3.30 shows the DSC traces for the (MgH + 20, 30, 50 and 70 wt%LiAlH ) composites. Only single endothermic peak centered at 350°C is visible in DSC traces for the (MgH + 20 wt%LiAlH ) composite (Fig. 3.30a). This peak corresponds to the decomposition of MgH. The first low temperature exothermic effect observed in Fig. 3.9 for a pure LiAlH (both unmilled and milled), which is usually assigned to the interaction of LiAlH with hydroxyl impurities [67], is not observed in Fig. 3.30a-c but it appears in Fig. 3.30d for (MgH + 70 wt%LiAlH ). Four endothermic events occur for (MgH + 30, 50 and 70 wt%LiAlH ) (Fig. 3.30b-d). The first endothermic peak at 174-182°C has almost exactly the same temperature range as (Rla) in Fig. 3.9. No exothermic peak (Rib) of melting from Fig. 3.9 is seen in Fig. 3.30a-d. It seems that the addition of just 30 wt%MgH suppresses melting of LiAlH and its first decomposition into LijAlH and Al ((Rib) in Fig. 3.9) occurs from a solid phase and is endothermic. This is supported by the observation of partial decomposition of LiAlH into (LijAlH + Al) during milling as discussed before. The second endo peak in Fig. 3.30b-d at 198,193 and 223°C, respectively, corresponds to the decomposition...

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