Probing the interactions between pour point depressants (PPDs), viscosity index improvers (VIIs), and wax in octane using fluorescently labeled PPDs

Abstract

A poly(octadecyl methacrylate) sample fluorescently labeled with 6.7 mol% of pyrene (Py(6.7)-PC18MA) was used as a mimic of a pour point depressant (PPD) to investigate how Py(6.7)-PC18MA interacts with wax found in engine oils and ethylene-propylene (EP) copolymers used as mimics of viscosity index improvers (VIIs). The fluorescence spectra of Py(6.7)-PC18MA solutions in octane were acquired in octane at low and high concentrations of Py(6.7)-PC18MA and analyzed to obtain the molar fraction (finter) of pyrene labels, that formed excimer intermolecularly, a measure of the level of intermolecular interactions between Py(6.7)-PC18MA molecules in the solution. The finter-versus-T profile obtained for Py(6.7)-PC18MA alone in octane confirmed that Py(6.7)-PC18MA formed microcrystals at solution temperatures below 0 oC. The effect induced by the addition of wax and an amorphous (EP(AM)) and semicrystalline (EP(SM)) EP copolymer on the interactions experienced by Py(6.7)-PC18MA were characterized by monitoring finter as a function of temperature and comparing the different finter-versus-T plots obtained after the addition of the different components with the finter-versus-T plot obtained for Py(6.7)-PC18MA alone. These studies demonstrated that wax and EP(AM) increased the level of intermolecular interactions between the Py(6.7)-PC18MA molecules at all temperatures in octane. EP(SM) increased the interactions between Py(6.7)-PC18MA molecules at high temperature, where it was soluble in octane, but finter reverted to its value in the absence of EP(SM) at low temperatures, where EP(SM) had crystallized. These experiments illustrate how pyrene excimer fluorescence can be applied to probe the complex interactions taking place between the different components found in engine oils.