Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
for PU chemistry) via the so-called in-situ method. The need of specialised equipment such as a refrigeration unit and personnel safeguard measures have made it difficult for the small to medium grease manufacturers to get in and to do it safely and easily.
Even for the grease manufacturers that are fully equipped to manufacturer PUG, the in-situ method poses a challenge to modify and to introduce new PUG compositions. For decades, the concept of pre-form PUGT exists whereby the thickener is first made with or without the solvent or the diluent/ others and is then converted to grease via grease making process. The concept is attractive as it takes away the difficulties presented by the in-situ method. However, there are numerous failures and too few successes in attempting to control the morphology of the pre-form PUGT thickeners, and during the grease making process, to achieve the desired PUG grease products with consistency.
In this paper we wish to present our solutions to the pre-form thickeners that not only provide the chemistry required to make the desirable thickener composition but also address/control the challenges posed by the thickener to thickener interactions during the so-called gelling process in the making of PU grease day in and day out with reliability and consistency.
The design and the construction of PUGT is a critical part of the powder making. By varying the types of amines, from primary to secondary, aliphatic to aryl, mono amine to poly amine, and from amines to alcohols (see Table 2), a variety of powder can be produced, tuned to specific needs with the consistency required for the subsequent grease making. The process is a multi-step proprietary process. The PUGT products thus formed are in fine powder forms and can be shaped into pellets or extrudates, mixed with diluents and base oils, or even made into masterbatch forms with pre-determined oil contents prior to grease making process.
20 LUBE MAGAZINE NO.158 AUGUST 2020 Figure 3: PUGT chemistry. Table 2: PUGT component composition.
Prior to our discussion in the making of polyurea grease, it is important to recognise the unique thickening characteristics for the urea molecules to gel and form extensive intermolecular interactions via hydrogen bonding (see Figure 3) that is vastly different than traditional soap based grease. For instance, the soap grease making process is a gelling forming process after the formation/melting of the soap thickener. For polyurea grease made via the in-situ method, such gelling begins almost instantly as soon as the urea molecule is formed in the process. For PUGT however, the gelling (or the formation of hydrogen bonds) has already taken place in the powder. Any attempt to breakup or to enhance the extent of the gelling is a delicate process.
An interesting observation is made in an atomic force microscopy (AFM) study (Figure 4, Reference 2) where the entrapped thickener in the rolling EHL lubricated contact under fully flooded conditions at medium speeds, is shown to exist in fibre forms (Lithium soap and complex grease), platelets (PUG), and spheres (OBCSG). The formation of platelets, which may be viewed as the bundle of fibres, is an indication of higher film strength under rolling conditions. This observation makes sense for an enhanced thickener- thickener interaction for polyurea grease under stress.
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