DSC Analysis
The energy changes in the phase change process of B20 and combination with PPDs at a low temperature were studied by DSC. Fig 3 and Table 3 show the energetic changes and data in the crystallization of neat B20, B20 with C16MC-MB-NVP, B20 with PPDC-DMP, and B20 with PPDC-AEO 7. The starting temperature of the peak (Tonset ) indicates the temperature, in which sedimentation of wax crystals in biodiesel begins, and the solid–liquid phase change energy (ΔH ) reflects the stability of dispersion.
As shown in Fig 3 and Table 3, the ∆H of pure B20 sample is 20.35 J/g, and the Tonset of B20 is -5.81 °C, which is higher than that of B20 with C16MC-MB-NVP (-6.05 °C), PPDC-DMP (-6.26 °C) and PPDC-AEO 7 (-6.81 °C). The peak temperatures of the absorbance peak (Tpeak ) was also reduced from -6.36 °C to -6.56 and -6.65 °C, then to -7.09 °C. The absolute value of the ΔH of B20 with PPDC-AEO 7 was 18.19 J/g, which is lower than that of B20 with C16MC-MB-NVP (20.27 J/g) and B20 with PPDC-DMP (19.94 J/g). These results suggest that the solid–liquid phase change energy of B20 with PPDC-AEO 7 is smaller, and the biodiesel blends with PPDC-AEO is very stable.
Rheological Measurements
Dynamic viscosity, a representation of the flow ability of biodiesel blends, at temperatures from 20 to -40 °C, has been depicted in Fig 4. The figure clearly shows that the viscosity of all samples increases as the temperature decreases.
The figure indicates that the viscosity of the samples did not change remarkably before the temperature reached CFPP. As the temperature reached the inflection point, which is lower than CFPP, the viscosity of the biodiesel blend oil with pure PPDs or PPDCs was remarkably reduced, compared with B20. By contrast, B20 with additives showed a decrease in magnitude of viscosity change compared with pure B20 sample, where PPDCs show a lower magnitude of viscosity change than PPDs. This finding is caused by the addition of PPDs and dispersants in the biodiesel blends, thereby appropriately increasing the dispersibility of wax crystals. As a result, the amount of lager wax crystals in the low-temperature crystallization stage is reduced, thereby decreasing the viscosity of the biodiesel blends.