Rheology and microstructure of lithium lubricating greases modified with a reactive diisocyanate-terminated polymer: Influence of polymer addition protocol

Abstract

This work deals with the influence that the addition of poly(1,4-butanediol) tolylene 2,4-diisocyanate terminated prepolymer (PBTDI) at different stages during the manufacturing process of a traditional lithium lubricating grease exerts on its rheological and morphological characteristics. 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 intended to be promoted during lubricating grease processing. Several PBTDI-based greases just differing in the process stage at which this polymer was added were prepared. The polymer used and the final lubricating greases were characterized by FTIR spectroscopy and DSC techniques. The effect of PBTDI addition at different processing stages was tested on final greases by performing small-amplitude oscillatory shear (SAOS) measurements as well as atomic force microscopy (AFM) observations. Addition of PBTDI significantly increased the values of the linear viscoelasticity functions when it was added during the final cooling stage of the manufacturing process. It was demonstrated that this increment in relation to the values found for the additive-free lubricating grease was only observed when PBTDI chemically interacted with the soap network. However, the effectiveness of PBTDI as rheology modifier was much lower when the addition took place at earlier steps of the manufacturing process. Some competitive reactions due to either the presence of water or the high temperatures applied have been proposed to explain this loss of effectiveness depending on the lubricating grease processing stage at which PBTDI was added.

Introduction

Lubricating greases are usually made from petroleum oils thickened with metal soaps. Particularly, lithium 12-hydroxystearate soap is the most widely used thickening agent. In addition, lubricating greases usually contain some performance additives [1], [2]. Thus, the use of polymers is a common practice to modify the rheological properties of greases by reinforcing the role of the thickening agent [3], [4]. A new generation of polymeric additives consisting of polyolefins containing grafted anhydride functional groups are being progressively introduced in grease formulations [5], [6]. These additives are usually added during or prior to the saponification step in grease manufacturing process. Similarly, in a previous work [7], we have shown that reactive diisocyanate-terminated polymeric materials can be satisfactorily used as rheology modifiers of lubricating greases by promoting the reaction between terminal isocyanate groups and the hydroxy groups located in the hydrocarbon chain of the 12-hydroxystearate lithium soap. It was demonstrated that the poly(1,4-butanediol) tolylene 2,4-diisocyanate (PBTDI), with a relatively low molecular weight, shows an effectiveness similar to that achieved with non-reactive polymers of much higher molecular weights, being the mechanical stability of PBTDI-based greases under operating conditions even better.

However, taking into account both the very high potential reactivity of the isocyanate groups [8], [9] and the complex mixture of components and thermo-mechanical conditions involved in the processing of lubricating greases, the process stage at which this reactive polymer must be added is not trivial. It must be noted that the manufacture of lubricating greases, with generation in situ of the soap thickener, is a rather complex process, consisting of the saponification reaction of fatty acids, followed by dehydration, heating up to near the phase transition temperature of soap crystallites, cooling down to crystallize the soap, and final milling treatment [10], [11]. Mixing efficiency is important during the whole manufacturing process [12]. The high temperatures achieved during processing may significantly increase the reactivity of isocyanate groups [13], or even cause some thermally induced degradation [13], [14]. This research deals with the optimization of PBTDI addition during the manufacture of a traditional lithium lubricating grease. Therefore, the main objective of this work was to study the effectiveness of PBTDI as grease thickener by analysing the rheological response of PBTDI-based greases differing in the stage at which this polymer was added during processing.