Use of Reactive Diisocyanate-Terminated Polymers as Rheology Modifiers of Lubricating Greases

In this work, the use of reactive diisocyanate-terminated polymeric materials as rheology modifiers of lubricating greases has been studied. With this aim, the reaction between terminal isocyanate groups and the hydroxy group located in the hydrocarbon chain of the 12-hydroxystearate lithium soap was promoted during the processing of lubricating greases. The polymeric materials used and the final lubricating greases were characterized through 1H NMR, IR, differential scanning calorimetry (DSC), and gel permeation chromatography (GPC) techniques. The effectiveness of such additives was tested on final greases by performing small-amplitude oscillatory shear (SAOS) and viscous flow measurements as well as standardized mechanical stability tests. The rheological response was also related to the microstructure of these greases. From the experimental results obtained, it may be concluded that a reactive diisocyanate-terminated polymer with relatively low molecular weight (Mn = 1800), such as poly(1,4-butanediol)tolylene 2,4-diisocyanate (PBTDI), can be satisfactorily used as a rheology modifier in grease formulations. Their effectiveness is similar to that achieved with nonreactive polymers of much higher molecular weight, such as low-density polyethylene (LDPE), also showing better mechanical stability than polyethylene-based greases. On the other hand, a tri-block copolymer PBTDI-PEO-PBTDI was synthesized in order to increase the molecular weight of the diisocyanate-terminated polymeric additive. However, lower increments in the linear viscoelastic functions were found for greases containing PBTDI-PEO-PBTDI copolymer as additive, a fact that has been explained on the basis of its lower reactive isocyanate groups/molecular weight ratio and the presence of other macromolecules without terminal isocyanate groups.