3.3 Crystallization behavior and crystal morphology observation
Polarizing optical microscope (POM) has been proven to be an effective
technique for observing the crystallization behavior and crystal
morphology of sample fuels at low temperature [32-35]. In the POM
images, the crystals exert polarized reflective light and present bright
white images, as well as the amorphous substances absorbed light to form
black images [34-36]. The POM images of neat BWCO (100 vol.% BWCO),
BWCO+ DDCL (20 vol.% BWCO + 80 vol.% DDCL), BWCO+ET+DDCL (20 vol.%
BWCO + 10 vol.% ET + 70 vol.% DDCL) and BWCO+ET+DDCL (20 vol.% BWCO +
10 vol.% BT + 70 vol.% DDCL) blends at −15 °C and −20 °C are shown in
Figure 4.
As shown in Figure 4, the amounts, sizes, and shapes of wax crystals
were varied in neat BWCO, and its blends. In the neat BWCO, the wax
crystals have its unique characteristics of large particle sizes, large
number and concentrated distribution at the low temperature of −15 °C
and −20 °C. The strip-shaped wax crystal were formed in a 3D net
structure through cross-linking, and both of them lost their flowability
due to the poor low temperature performance (A1and A2 ). In the binary blends of BWCO+DDCL, some
smaller sized particle-shaped crystals were formed, and the quantity and
size of crystals grew as the temperature decreased from −15 °C to −20 °C
(B1 and B2 ). After blending ET
or BT together DDCL with BWCO, distinct differences in the amounts,
sizes, and shapes of crystals were noticed (Figs.4C and 4D), and there
was an obvious growth in Figure 4(C2 ) and
Figure 4(D2 ). Also, the crystals amounts and
sizes in BWCO+ET+DDCL and BWCO+BT+DDCL significantly decreased, as well
as the distributions of the crystals were more orderly compared to the
neat BWCO and BWCO+DDCL blends. As shown in Figure 4D, the morphology of
crystals in BWCO+BT+DDC blends were observed in smaller number and sizes
than those of BWCO+ET+DDC blends. Such result can be attributed to the
large dissolving capacity of BT for alkenes, hydrocarbons, alkanes,
cycloalkanes, and aromatic hydrocarbon in DDCL, and different FAMEs are
presented in BWCO in contrast to those of ET. In addition, DDCL together
with BT acted as an excellent diluent for the high melting compositions
of saturated FAMEs that retarded the aggregation and growth of large wax
crystals. Thus, the size and number of wax crystal in the BWCO+BT+DDC
ternary system were smaller, and could filter through filters easily,
providing such ternary blends with better low-temperature flow
properties (Figure 4D).