POM Analysis
A polarizing optical microscope can be used to observe the wax crystallization behavior and crystal morphology changes. Fig 5 shows the POM of B20 treated or untreated with 2000 ppm PPDs, and dispersants were examined at different temperatures.
As shown in Fig 5 (a), the wax crystals of untreated B20 exhibit feathery or flaky shapes. These larger wax crystals have a strong tendency to aggregate, and they can flow to form a three-dimensional network structure. Thus, B20 loses its fluidity at higher temperatures. Fig 5 (b) shows that the volume of wax crystals is significantly reduced, and the shapes were oval and rod after adding single PPD. However, the wax crystals are unevenly distributed and still tend to aggregate. The reason for this phenomenon is that biodiesel has a different composition from diesel (Rawat et al., 2014; Moser, 2014). B20 is a multicomponent system, which consist of 20 vol.% biodiesel and 80 vol.% diesel. The CFPP of FAME is usually higher than that of n-alkanes. Therefore, when components with different CFPP interact, the CFPP temperature of the B20 system will decreases (Magalhães et al., 2019). However, because the structure of pure PPD is more similar to n-alkanes and has better compatibility with diesel, PPD preferentially co-crystallizes with diesel components to form small crystals, and the remaining high CFPP FAME is likely to self-polymerize into larger crystals. Therefore, although PPD delays the precipitation time of wax crystals in B20, the wax crystals still tend to polymerize.
The wax crystals generated when PPDC-DMP (4:1) or PPDC-FAPE7 (4:1) was added to B20, as shown in Fig 5 (c-d), and the shape of the wax crystal is sphere-like or long needle-like. However, at the same temperature, the wax crystals produced by PPDC-FAPE7 (4:1) are less than PPDC-DMP (4:1) and are more dispersed, thereby proving that the former has a lower CFPP than the latter. PAEs were selected as wax dispersants because of their highly polar functional groups brought by the two carbonyl groups; accordingly, they achieved surface-active properties and good oil solubility (Magalhães et al., 2008). In addition, FAPEs have larger volume and polarity due to methoxy bonds. This finding can increase the interfacial adsorption capacity of PPDs and shield the nonpolar component of oil, thereby preventing the wax crystals from growing further and exhibiting the best synergy with PPDs. Thus, B20 treated with PPDC-FAPE 7 (4:1) produces uniformly dispersed and smaller wax crystals at low temperatures and exhibits the best cold flow ability.