Silicone greases are commonly prepared by introducing various thickeners into silicone fluids. Silicone fluids are generally represented by polyalkylsiloxanes with alkyl (e.g., methyl, ethyl, octyl, or phenyl) substituents of various viscosities. Halogenated silicone oils such as polychlorosiloxanes have also been used. Thickeners have been represented by amorphous pyrogenic silica, higher fatty acid salts (soaps), polytetrafluoroethylene, pigments, and various carbon modifications. Table 1 presents silicone greases that were developed in the USSR and are still relevant today.

Table 1. Greases prepared by thickening of silicone fluids [1]
Full size table

Silicone greases have a wide range of applications due to a variety of properties that other lubricants lack: water-washing resistance (in cold or hot water), excellent chemical stability, low volatility, and good compatibility with metals, plastics, and elastomers [2].

Research periodicals offer almost no research data on silicone greases. Prior relevant studies were basically focused on potential modification of silicone lubricants to affect their physicochemical properties and operating characteristics. Yu et al. [3] prepared a number of greases by thickening silicone oils with graphene nanosheets and reduced graphene oxide. Graphene was shown to be an effective heat-conducting filler: it provides high thermal conductivity of silicone grease with low filler content. To enhance the thermal conductivity of silicone greases, Chen et al. [4] introduced metallic oxide particles (Al2O3 and ZnO) stabilized by carbon nanotubes (CNTs). These CNTs formed a three-dimensional network structure that decreased the thermal impedance of the lubricant by more than 35% compared to the pristine sample. There has also been research into the functional performance of silicone greases on coating surfaces. In particular, Wang et al. [5] assessed the applicability of silicone greases in various electrical devices, including the electrical and mechanical properties, moisture resistance, corona resistance, and other characteristics. Jiang et al. [6] propose a prediction method for silicone thermal grease degradation in spacecraft friction units over a long service period.

To prepare lubricating formulations, some researchers have used silicone fluids mixed with other synthetic base oils from Groups IV and V as a dispersion medium. A Russian patent [7] proposes a universal grease formulation prepared by thickening a mixture of an oligomethylethylsiloxane fluid and an ester, specifically di-2-ethylhexyl sebacate, with a diurea based on aniline, dodecylamine, and 2,4-toluene diisocyanate. Furthermore, hydrophobic modified aerosil and ceresin 75 were also proposed as thickeners. This grease exhibits improved wear performance and a wide operating temperature range.

The prototype for the patent above was an earlier-patented invention [8], in which a mixture of pentaerythrite ethers and C5–C9 fatty acids with a silicone fluid was used as a synthetic base for the dispersion medium, and the thickener consisted of polyurea and hydrophobic silica gel. A number of studies have demonstrated that the physicochemical properties and operating characteristics of silicone-based lubricants can be improved by introducing synthetic oils. For example, adding about 30 wt % of a hydrocarbon or ester component into a silicone-based lubricating formulation significantly enhanced the wear performance of the lubricant: the wear scar diameter of this formulation was lower than that of the hydrocarbon component [9, 10].

Rapid advances in methodologies and equipment, as well as the appearance of high-performance machines and equipment packages have dramatically increased the requirements for lubricants. In the mid-20th century, a series of greases (referred to as polyurea greases) was prepared by thickening base oils with organic thickeners based on diureas and tetraureas. These greases are superior, for example, to soap greases in physicochemical properties, tribological performance, and operating characteristics. Of late, synthesis of novel greases by thickening a polyorganosiloxane-containing dispersion medium with polyurea thickeners, as well as practical implementation of these greases, has become increasingly relevant for lubricating materials science.

The purpose of the present study was to synthesize a number of polyurea greases based on silicone fluids and poly-α-olefin oils, and to characterize their physicochemical properties and wear performance.

EXPERIMENTAL

Polyethylsiloxane fluid PES-5 manufactured by Silan GNIICHTEOS, Russia according to GOST 13004-77 and poly-α-olefin oil PAOM-12 manufactured by Tatneft-Nizhnekamskneftekhim-Oil, Russia according to TU 0253-014-54409843-2007 were used as dispersion medium components to prepare lubricating formulations. Polymethylsiloxane fluid PMS-5, fluorosiloxane fluid 161-44, and spindle oil AU produced by selective purification and dewaxing were also used.

The dispersion medium was thickened by synthesizing diurea directly in base oil in accordance with the technique described in [11]. For this purpose, synthetic base oils (namely the polyethylsiloxane fluid and poly-α-olefin oil) were mixed in a stirred reactor. The mixture was then separated into two unequal parts in a ratio of 1 : 10, followed by adding nominal amounts of aniline and the second amine to the larger portion and adding diisocyanate to the smaller portion. The amine-enriched base oil solution was heated to 90°C, after which the diisocyanate suspension was introduced into the dispersion medium under vigorous stirring, with the generation of the grease structure being closely observed. The grease was then heated to 140°C under stirring until a uniform structure formed, followed by bulk cooling to room temperature. After cooling, the resultant mixture was homogenized.

Table 2 presents the compositions of all the greases prepared.

Table 2. Composition of studied greases
Full size table

For all the greases, major physicochemical properties were determined: drop point (as per GOST 6793-74, Petroleum products. Method for drop point determination); ultimate strength (GOST 7143-73, Greases. Method for determination of ultimate strength and thermal strengthening); and colloidal stability (GOST 7142-74, Greases. Method for determination of oil separation). The tribological performance of the greases were characterized by a four-ball friction testing machine (GOST 9490-75, Liquid and plastic lubricating materials. Test method for lubricating properties on four-ball machine).

RESULTS AND DISCUSSION

Introducing the organic thickeners based on various diureas into pure PES-5 was unable to generate the desired grease structure. We tried to thicken the silicone oil either by introducing pre-synthesized diureas or by synthesizing the thickener in situ directly in the silicone oil. In both cases the thickener flocculated. Attempts to thicken other silicone fluids such as PMS-5 and 161-44 with diureas also failed. This suggests that pure silicone fluids cannot be thickened with polyureas.

Further experiments showed that adding small amounts of hydrocarbon oils to the silicone fluids during the introduction of the organic thickeners allowed the desired greases to be generated. To evaluate the effects of the amount of the hydrocarbon component in the grease prepared from the diurea-thickened silicone fluid, we synthesized a series of greases from PES-5 with the addition of PAOM-12 in varying concentrations.

Figure 1 illustrates the physicochemical properties of the greases as functions of the PES-5 to PAOM-12 ratios, with the concentration and composition of the thickener being constant. No significant changes were observed until the weight of the hydrocarbon component added to the grease reached 20%. From this point, however, increasing the PAOM-12 content promoted a significant, almost linear variation in the physicochemical properties of the lubricant. For example, as the PAOM-12 content increased from 20 to 50 wt %, the drop point dramatically rose from 240 to 310°C (Fig. 1a). A similar enhancing effect on colloidal stability was observed (the increase in colloidal stability was expressed as a decrease in the amount of oil separated during the test, Fig. 1b). Therefore, a higher content of the hydrocarbon component favors a better retention of the base oil by the newly-forming structural framework of the thickener. On the other hand, we see a slight decrease in the ultimate strength (Fig. 1c), and this may impair the operating characteristics of this lubricant type. This behavior may be due to an overall decrease in the oil viscosity when the hydrocarbon component was added to PES-5. As expected, increasing the hydrocarbon oil content in the silicone greases improved the tribological performance, indicated by a significant reduction in the wear scar diameter. This is because the oil evolved during tribocontact had a larger content of the hydrocarbon component, which is known to improve wear performance [9].

Fig. 1.
figure 1

Physicochemical properties of greases as functions of PES-5/PAOM-12 ratio: (a) drop point; (b) colloidal stability; (c) ultimate strength; and (d) wear scar diameter. Thickener: TDI + n-hexadecylamine + aniline; thickener content 20 wt %.

Full size image

In addition, we synthesized a series of grease samples differing in the thickener content, with the PES-5/POAM-12 ratio equal to 2.33 in all the samples (Table 3). Raising the thickener content from 15 to 25 wt % increased the framework elements of the grease, an effect manifested in an improvement both of the ultimate strength and colloidal stability. At the same time, the increase in the wear scar diameter may indicate that the hydrocarbon component was better retained by the framework, whereas the tribological contact primarily evolved silicone oil with lower wear performance.

Table 3. Physicochemical properties of studied greases
Full size table

Previously, in a study on the effects of polyurea thickener compositions on the properties of greases based on poly-α-olefin oils, we demonstrated how diurea thickeners differing in hydrocarbon radical length influence the properties of the lubricants synthesized [11]. In the present work, to identify the effects of polyurea thickener type on the properties of silicone greases containing hydrocarbon components, the thickeners introduced to prepare greases had different compositions (Table 3). Both the amine type and diisocyanate type were varied.

Predictably, increasing the number of carbon atoms in the hydrocarbon radical of diureas enhanced the drop point and ultimate strength and improved the colloidal stability, with the wear performance being negligibly affected (Table 3).

It is worth noting that replacing TDI with MDI (Table 3) in the thickener formulation increased several-fold the ultimate strength of the grease. The PES-5/AU-based grease (with MDI + n-hexadecylamine + aniline as a thickener) exhibited the highest ultimate strength (485 Pa) and the lowest wear scar diameter (0.7 mm). This is good performance for silicone-based greases free of antiwear additives. Thus, these greases have the best wear performance among all the studied samples, and this, in combination with the other advantageous physicochemical properties, extends their service life.

CONCLUSIONS

A number of synthetic lubricants were synthesized and investigated; they contained a silicone oil and a poly-α-olefin oil as well as an polyurea thickener. The effects of the compositions both of the dispersion medium and polyurea thickener on the major physicochemical properties of greases were revealed. The study finally identified the optimum grease formulation, one that provided adequate levels of ultimate strength and colloidal stability and the best wear performance.