The study of bitumen by differential scanning calorimetry: The interpretation of thermal effects

Interpretation of Double-Peak Endotherm on DSC Heating Curves of Bitumen

“…Thus, we can confirm the hypothesis that the double-peak endotherm on the DSC heating curve of bitumen is formed by the overlap of the dissolution endotherm (background) of the rearranged primary wax crystals of different sizes and the perfection and sharp exothermic peak of the recrystallization of the secondary crystals (secondary crystallization cannot be observed as an isothermal process). The onset of the recrystallization peak must be at the annealing temperature.…”

“…If one takes into consideration the relative amount of macro- and microcrystalline waxes in the analyzed bitumen (7.5% and 12.2%, respectively), it can be concluded that the shown heating curve profiles should have another shape: the low temperature endotherm peak (representing the dissolution of the microcrystalline wax) must be broader and more smoothed at the lower temperature range than is seen in Figure b, and the high-temperature peak (representing the dissolution of the macrocrystalline wax) must be narrower and sharper. ,, The above arguments prove, as we believe, that there are not two distinct populations of crystallites in bitumen and the hypothesis that polydispersity is responsible for the double-peak endotherm can be taken out of consideration. However, it is interesting to note that the DSC heating curve of as-received macrocrystalline paraffins separated from analyzed bitumen (Figure a) is very similar to the DSC curves of some pure n -alkanes which have a polymorphic nature (Figure 6). This fact gives rise to the hypothesis that polymorphism is responsible for the double-peak endotherm.…”

“…There is no endothermic signal on the curve of the model bitumen 1 sample which contains no wax. On the curve of model bitumen 2 sample which differs from model bitumen 1 by the added microcrystalline wax, the dissolution endotherm of crystals formed in the process of cold crystallization appears . Only when introducing into the composition of model bitumen 3 macrocrystalline wax, its nonreversing curve acquires a profile similar to the profile of the curve of the base bitumen.…”

“…This pattern is confirmed by a gradual increase in the size and perfection of the crystalline structures, the dissolution of which causes this endotherm (the dissolution of larger and more perfect crystals no longer fits into the temperature modulation cycle). Since the temperature ranges are the same (from 0 to 80 °C) for the endotherms on all of the nonreversing curves, it is obvious that the structure of the crystalline fraction changes during the reorganization process and is formed by the same molecules of wax . This process occurs most actively during the 16 h annealing.…”

“…In this work, TMDSC analysis was used to study the precipitation/dissolution processes in bitumen in terms of the thermal structuring of the fractions of bitumens. 15 All of the earlier proposed interpretations of double-peak endotherm appearance were critically considered, and a probable descriptive model for this phenomenon was suggested.…”