Polymer-Driven Wax Crystal Control Using Partially Crystalline Polymeric Materials

Abstract

A long term problem confronting the transportation of crude oils and refined middle distillate fuels is the abrupt degradation of the system viscoelastic properties as temperatures fall below ∼0 °C. The prime contributor to this unfavorable event is the phase separation of paraffins (waxes) with carbon contents ranging from C₁₆ to ∼C₃₈. This problem has been addressed, with varying degrees of success, via the use of formulations containing polymeric additives. Additives that have had long use are the ethylene-rich copolymers of ethylene and vinyl acetate (EVA). Although far from being universally successful in their treatment capacity, the EVA materials serve as a prototype wax-crystal modifier in that their structure of alternating amorphous-crystalline segments serves as a model for regarding the composition of other polymeric candidates. A recent candidate is the diblock copolymer consisting of ethylene and ethylene-butene segments. This material thus consists of a semicrystalline block joined to an amorphous counterpart. After a four-year development period it became a commercial item in 2000. In hydrocarbon milieu the polyethylene block will self-assemble as the system temperature decreases to yield plate-like micelles that remain in solution due to the presence of the amorphous “hairs”. Small angle neutron scattering studies have shown that this polymer architecture is quite effective in providing a scaffold for wax nucleation, thus leading to quite effective control of wax crystal size in a variety of fuels. An architectural mimic of EVA is the random copolymer of ethylene and butene. This particular random copolymer was also shown to be highly effective in its capacity as a modifier for wax crystal size control. The mechanism by which this is done was found to be even richer than that shown by the diblock architecture.