A key element in low temperature engine oil pumpability is wax, and in particular its composition and behavior. The wax composition is determined by the basestocks used in the formulation and its impediment to oil flow is reduced by additives [wax crystal modifiers].
Basestocks are desirable mixtures of hydrocarbons in which the undesirable components are weeded out by refinery separation and conversion sciences. The refinery unit operations take into account the chemical and physical properties of the hydrocarbons that are present in the crude oil. This hydrocarbon molecular selectivity is predicated upon [A] vapor pressure or molecular weight/structure, i.e., vacuum pipestill, [B] chemical bond strengths, i.e., hydrocracking, [C] solubility, i.e., solvent dewaxing/extraction, and [D] molecular shape/chemical bond strengths, i.e., catalytic dewaxing. The refinery unit operations can be combined to manufacture basestocks by four pathways. The basestocks produced by these divergent pathways may have the same physical properties--i.e., viscosity at 40°C, viscosity index, and pour point -- and yet be different mixtures of desirable hydrocarbons. The latter, however, can make themselves known in having unequal low temperature engine oil pumpability properties. In other words, the basestocks with the same physical properties can have different wax compositions; hence their pumpability characteristics may not be equal.
Wax gelation obstructs the flow of oil and the hindrance is dependent upon the size, shape, number of wax crystals -- paraffin composition parameters -- as well as the total wax concentration. Additives are designed to participate in the wax crystallization process so that the crystals produced impede the flow of oil less than the unmodified crystals. The wax-additive interaction is a physical process and it is temperature dependent. The two physical properties which control this interaction are solubility and affinity. Additive solubility is adjusted by manipulating t' chemistry in order to have the wax crystal modifier precipitate just prior to or with the wax -- for maximum effectiveness. Affinity is the weak Van der Walls attractive forces that exist between non-polar molecules and, accordingly, wax-like segments must be incorporated into the additive. The chemistry of the wax crystal modifier is designed, therefore, to match the solubility of a particular wax composition and tailored to attract specific paraffins.