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.