23rd International Colloquium Tribology - January 2022 121 Polyglycols as High Performant Base Oil Component in Modern Greases Cristina Schitco Clariant Produkte GmbH, Frankfurt am Main, Germany Corresponding author: cristina.schitco@clariant.com 1. Introduction The industrial growth, rise in automation, and the electromobility trend increase the need of high-performance greases [1]. Simultaneously, there is an increase focus on environmentally friendly greases to meet the worldwide sustainability efforts [2]. The base oil is a major component in greases and influences several important properties of the final product [3]. This work describes the relevant properties of polyglycols and how these synthetic base oils can meet the modern requirements of the grease formulators. 2. Chemical Structure and Properties Polyalkylene glycols (PAGs) are synthetic polymers with highly adjustable properties. By varying such factors like starting alcohol, amounts of ethylene and propylene oxide as shown in Figure 1, PAGs can be tailored to a manyfold of properties and application requirements. Some of the adjustable properties include viscosity, viscosity index, polarity, miscibility in various oils, pour points, thermal properties, lubrication, etc. PAGs can be theoretically available in any ISO VG class, e.g., starting with ISO VG 15 up to ISO VG 1000 and beyond. Figure 1: Chemical structure of a PAG. R stands for the starting alcohol 2.1 High Temperature Behavior PAGs posses typical flash points well above 200 °C. The flash points depend on the molecular weight and polarity, i.e., the amount of ethylene oxide contained in the molecule. Higher the molecular weight and the polarity of the molecule, higher flash points up to 270 °C can be observed. The thermal-oxidative resistance rises with the ethylene oxide and molecular weight as well. PAGs have a clear burn-off which facilitates the cleaning processes. 2.2 Material Compatibility and Oil/ Oil Miscibility Material compatibility is an important parameter when choosing the base oil for a certain grease application. Incompatibilities may promote premature failure of the material or the entire system. PAGs are notoriously not compatible with simple paints. Our tests show that PAGs exhibit compatibility with EPDM RM 69, elastomer NBR 28/ SX, and two component epoxy-based primers Primer M 20 and P 22. Table 1 shows the results of EPDM compatibility test. Table 1: EPDM compatibility test in analogy to ISO 4925, average of two measurements, EPDM RM 69, 100 °C, 7 days ISO VG 46, hydrophilic * requirements ISO 4925 Relative change in volume +0,9% min. 0% max. 10% Change in hardness IRHD -3 min. -15 max. 0% * contains antioxidant package The miscibility of PAGs depends strongly on its molecular weight and hydrophobicity. Low molecular weight hydrophobic PAGs are typically miscible in low viscous naphtenic, parrafinic, vegetable, or some type of esters, e.g., polyol esters, trimelliate esters. In limited concentrations, the low molecular weight hydrophobic PAGs are miscible in low viscous polyalphaolephines (PAOs). 2.3 Low Temperature Behavior The pour points of the base oils are an important characteristic for the low temperature applications. The pour points of the PAGs are typically low, decreasing with the increase of propylene oxide content and the decrease of molecular weight. Hydrophobic PAGs reach pour points well below -50 °C as shown in Table 2. Tailored version can reach values even below -70 °C. 122 23rd International Colloquium Tribology - January 2022 Polyglycols as High Performant Base Oil Component in Modern Greases Table 2: Pour points of selected PAGs according to DIN ISO 3016 type of PAGs pour point (°C) ISO VG 15, hydrophobic -66 ISO VG 100 - 1000, hydropho-bic -40-36 ISO VG 46 - 1000, hydrophilic -50-35 ISO VG 15, hydrophilic, modified -72 * * according to ASTM D 7346-14 2.4 Lubrication Properties High Viscosity Index (VI) allows applications over a wide range of temperatures. Polyglycols have typically very high VI, e.g., around 200 - 290, allowing high load carrying capacities. One of the strongest benefits of PAGs lies in its lubrication properties. Figure 2 shows a SRV curve of an ISO VG 1000, hydrophilic PAG, non-additivated. Figure 2: SRV curve of an ISO VG 1000, hydrophilic PAG according to DIN 51834-2-2010 at 50 °C The friction coefficient remains stable over the entire run of the test, with a value of around 0,1. The wear scar shows a low average value for a non-additivated oil of only 0,560 mm. 2.5 Heat Transfer Properties Figure 3: Thermal conductivity of several PAGs measured by TPS technique: 0°C (first column), 50 °C (second column), 100 °C (third column) Hydrophilic PAGs possess high thermal conductivity in the range of 0,19 W/ m/ K suitable for applications which require high heat transfer properties. On the other hand, hydrophobic PAGs display lower thermal conductivities in the range of 0,16 W/ m/ K. 2.6 Sustainability and Regulatoric Considerations PAGs have an excellent sustainability profile. Nearly all hydrophilic PAGs up to ISO VG 1000 and the hydrophobic PAGs up to VG 100 are readily biodegradable. With very few exceptions, nearly all PAGs are label free. 2.7 Grease Formulation To show case the use of a PAG in grease application, a Li/ Ca mixed grease of NLGI 2 has been manufactured based on an ISO VG 15, hydrophilic PAG with a very low pour point of -72 °C. The properties of the grease are compared with a similar grease based on PAO in Table 3. To highlight is the high VI of the PAG oil and superior mechanical stability of the PAG based grease. Table 3: Comparative properties of a PAG and PAO from ISO VG 15 properties PAG grease PAO grease Oil Viscosity index 1 210 127 Pour point 2 (°C) -73 -75 Grease Consistency 3 275 281 Work stability 3 291 322 Drop point 4 (°C) 186 190 Oil separation 5 -2,3 -6,7 Wear scar 6 (mm) 0,54 0,73 1 ASTM D 2270, 2 ASTM D 7346-14, 3 ASTM D 217, 4 ASTM D 566, 5 ASTM D 1742, 6 ASTM 2266 3. Conclusion The polyglycol class of synthetic fluids offer a lot of development opportunities for the grease formulators. The PAGs are especially suitable for those grease applications requiring excellent lubricity properties, low pour points, and superior sustainability profile. References [1] Andrew, J. M., “The future of lubricating greases in the electric vehicle era”, Tribology and Lubrication Technology, 75, 5, 2019, 38-44. [2] Commission Decision 05/ 360/ EC of 26 April 2005 establishing ecological criteria and the related as- 23rd International Colloquium Tribology - January 2022 123 Polyglycols as High Performant Base Oil Component in Modern Greases sessment and verification requirements for the award of the community eco-label to lubricants, Off. J. E.C., 2005, p. L118. [3] Fischer, D., Jacobs, G., Stratmann, A. and Burghardt, G., “Effect of base oil type in grease composition on the lubricating film formation in EHD contacts”, Lubricants, 32, 6, 2018.