DSC in an inert atmosphere

The model substances for DSC were the same as those used for thermogravimetry (see section 3.1) such as the series of w-alkanes and some of the substances listed in ASTM D 2887-84. If an appropriate gas flow rate was used, then evaporation without residue of all the tested n-alkanes resulted, as already described for thermogravimetry. Fig. 3-39 demonstrates this behaviour for n-hexacontane. At a temperature of 99 °C there is a sharp fusion (melting) peak exhibiting a heat of fusion 243.3 J g. At 439 °C the maximum of energy resorption for the evaporation process appears, which has an onset temperature of 409 °C. The low energy resorption of 136 J g indicates the occurrence of an evaporation process. 

Change of the heating rate p causes a shift of the peak maximum temperatures. In that case the curve of the peak maximum temperatures versus the carbon number will be shifted to a parallel position (Fig. 3-41). They all display a pattern similar to the graph of extrapolated boiling points versus the carbon numbers of the n-alkanes (Fig. 3-15) and likewise obey a logarithmic function  

Temperatures of the Endothermal Peaks versus C-Numbers Atmosphere Argon, 5 cm /min 1 and 10 bar Heating Rate P lOK/min 

The coefficients a, aj, and aj are primarily dependent on the heating rate j8 For the four applied heating rates (/3= 5, 10, 20, and 50 K/min) correlation coefficients of Eq. 3-5 between r = 0.993 and r = 0.998 were found. Comparison of the peak maximum temperatures to the corresponding boiling points BP results in linear functions for each heating rate /3  

Application of the correlation of the peak temperatures to the heating rates for calculation of the Arrhenius kinetic parameters and of the heat of vaporization will be considered in chapter 3.3.2. 

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