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Molecular interaction, viscosity and rheological properties of lubricating oils containing dispersive polymethacrylate additives

Alkyl methacrylate copolymers (PAMA) of specific composition and structure are well known and widely used improvers of rheological properties of mineral lubricating oils, especially of viscosity index and pour point of the engine and transmition oils where they are added in relatively high concentrations. Recently, development of the PAMA additives is oriented towards copolymers that display detergent-dispersive properties apart from improving shear and oxidation stability. The desired effect can be reached via polymerization by incorporating comonomers containing functional groups that include oxygen or nitrogen into the macromolecules. The duty of functional group in polymeric additive is to disperse sludge in oil and/or to prevent deposition of the combustion products that form a thin surface layer on the vital parts of an engine. This improves the efficiency of the engine, reduces friction and fuel consumption. The rheological behavior of solutions of the polymeric additives of lubricating oils that display dispersive properties was investigated. Additives are synthesized by radical copolymerization of styrene, dodecyl methacrylate, octadecyl methacrylate and dimethylaminoethyl methacrylate (DMAEMA, 2 and 4 mol. % share) in base mineral oil. All solutions of copolymers in basic oil displayed high values of kinematic viscosity and viscosity index (> 150). By embedding DMAEMA into macromolecules the interactions between polymeric additives and oil changeed significantly. Therefore, much larger hydrodynamic volume of polymeric molecules compared to the conventional methacrylate additives occurs. As a result, the viscosity increased due to the hydrodynamic volume increase ; oppositely, shear stability of solutions decreased and pour points remained comparable. Relative, specific and reduced viscosities of diluted solutions of homopolymeric constituents of investigated additives: poly(alkyl methacrylate) (methyl-, PMMA ; dodecyl-, PDDMA ; octadecyl-, PODMA) and polystyrene (PS), as well as other most commonly used additives, ethylene-propylene (EPC) copolymers as well as styrene and hydrogenated butadiene (SHB) were determined, where the following ranking was obtained: EPC ≥ SHB > PDMAEMA ≥ PS >> PDDMA > PMMA ≥ PODMA. Huggins’ equation was used to describe the concentration dependence of viscosity where limiting viscosity number and Huggins’ constants were determined as well. The obtained relationships between polymer composition and structure vs. rheological properties of their solutions in oil, knowing the molecular interactions between polymers and solvents (base oil), enable formulating of lubricating oils of advanced application properties.

molecular interaction; rheological properties; lubricating oils; dispersive polymethacrylate additives

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