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).