Ethylene-Vinyl Acetate Copolymer (EVA) and Resin-Stabilized Asphaltenes Synergistically Improve the Flow Behavior of Model Waxy Oils. 3. Effect of Vinyl Acetate Content

In the last two published papers, the influences of wax and asphaltene content on the synergistic performance of ethylene-vinyl acetate (EVA) copolymer together with resin-stabilized asphaltenes on the flow behavior improving of model waxy oil were systematically investigated, and a relevant mechanism has been proposed. Here, the effects of vinyl acetate (VA) content (12–40 wt %) on the synergistic performance between EVA and asphaltenes is continuously studied to develop and complete the synergistic theories. Results show that different VA contents slightly influence the rheological properties of model waxy oils doped with neat EVA but play a significant role in the flow behavior improvement of the oil doped with EVA and asphaltenes. EVA with moderate VA content (28 wt %) possesses the best flow-improving efficiency among the neat EVA PPDs, but associated with asphaltenes, EVA with a higher VA content (33 wt %) does the best. According to the DSC tests, when the VA content is low or moderate (12–33 wt %), the wax precipitation temperature (WPT) of the waxy oil is found to be decreased after adding neat EVA, while the phenomenon for the EVA with too high VA content (40 wt %) is the opposite. The WPT of oil would not be further suppressed by EVA/asphaltenes, but EVA/asphaltenes can facilitate the crystallization of paraffin waxes and accelerate the precipitation process of wax crystals below WPT. Increasing the polarity of EVA (12–33 wt %) can strengthen the polar interaction between EVA and asphaltenes in the oil phase, promoting EVA molecules to adsorb onto the asphaltene aggregates to form the EVA/asphaltenes composite particles. The composite particles favor the formation of large, compact, and spherical wax flocs to release more of the liquid oil phase, reduce the solid–liquid interfacial areas, and weaken the interactions between wax crystals, therefore facilitating the outstanding rheological improvement of waxy oils. As VA content increases to a much higher level (40 wt %), however, the rigidity of EVA molecules is high, which is adverse for the good oil-dispersing ability of EVA and the corresponding interactions with wax molecules. Meanwhile, the high polar EVA disperses the wax crystals into smaller sizes. Both of these two sides enlarge the solid–liquid interfacial area and strengthen the interactions between wax crystals, making them more able to build up a continuous wax crystal’s network structure and leading to the performance deterioration of the EVA together with asphaltenes. This conclusion that the modest increase of PPD’s polarity facilitates the improving efficiency between PPDs and asphaltenes gives another powerful proof to the correctness of the EVA/asphaltenes composite particles mechanism.