23rd International Colloquium Tribology - January 2022 101 An experimental study of the effect of thermal aging on the lubrication performance of Environmental Acceptable Lubricants (EALs) Mar Combarros IQL, Castellgalí, Spain Corresponding author: m.combarros@iql-nog.com Ariadna Emeric IQL, Castellgalí, Spain Gerard Cañellas IQL, Castellgalí, Spain Ángel Navarro IQL, Castellgalí, Spain Marc Alumà IQL, Castellgalí, Spain Taro Ehara IQL, Castellgalí, Spain 1. Introduction Synthetic esters are currently used in many applications, including automotive and marine engine oils, gear oils, compressor oils, hydraulic fluids or greases since they can offer outstanding properties. These properties such as biodegradability, low temperature flow or thermal oxidative stability can be tailor-made by molecular design [1]. In this paper thermal-oxidative behaviour was investigated and its influence on tribological performance assessed. An ester degrades by two main mechanisms at high temperatures: physically, evaporation of the volatiles, and chemically, due to oxidation / polymerization but also to an evolution of gases CO, CO 2 , especially at higher temperatures [2]. These phenomena cause an increase in viscosity and formation of deposits which may limit the lubricant applications specially nowadays were an extended service life is usually required. 2. Methods 2.1 Materials The aging behavior of four different products was investigated. The influence of sterical hindrance in the propagation of the polymerization was studied using two different structures: Product A and B. The influence of antioxidant additives on polymerization was investigated adding a mixture of aminic/ phenolic antioxidant additives. Product C, product with no additives was compared to product D which has a 0.5 % of additive. Table 1: Main properties for the products used A B C D KV 40 °C (cSt) 67.1 68.7 100 100 KV 100 °C (cSt) 10.8 11.7 14.0 14.0 Viscosity index 152 165 141 137 AN (mgKOH/ g) 0.09 0.06 0.15 0.36 RSSOT (min) 144 159 247 1082 Volatility-TGA (%) 61 45 24 17 Thermo-oxidative properties were studied by means of RSSOT for the oxidation and TGA for its volatility. TGA measurements were measured in a large furnace after a 1h isothermal step at 250 o C. The main properties are shown in Table 1. 102 23rd International Colloquium Tribology - January 2022 An experimental study of the effect of thermal aging on the lubrication performance of Environmental Acceptable Lubricants (EALs) 2.2 Thermal aging: Thermal degradation was evaluated using 600 g of product in a round bottom flask, heated with an electrical heater at 180 o C for 168 h. As a catalyst 0.6 g of powder Fe was used. The product was maintained at a constant speed of 60 rpm with a magnetic stirrer and at open air. After the test, the product was vacuum filtered. 2.3 Analytical evaluation of aging The evolution of the aging process was monitored evaluating the change on total acid number, viscosity, weight, FT-IR, and GPC/ SEC. The presence of oxidized compounds was assessed using FT-IR [3]. The polymerization degree was evaluated using Permeation chromatography / size exclusion chromatography (GPC/ SEC) with a refractive index (RI) detector. GPC has been used by several researchers to study degradation of oils [4,5,6]. 2.4 Tribological performance analysis Stribeck curves were generated using a PCS Instruments MiniTraction Machine at 150 °C with an applied load of 36 N. An ANSI 52100 steel disk and a ¾” ball were used. A slide-to-roll ratio of 150 % was maintained during the measurements. 3. Results 3.1 Chemistry The mechanism by which the products are degraded is a radicalary mechanism. Initial steps of oxidation originate hydroperoxides, oxoand hydroxy-compounds and it is initiated by high temperatures. In later stages high molecular weight products are formed [2,7]. The main properties of the thermal aged products are depicted in Table 2. As we can see, viscosity increases in all four cases specially if the sterically hindrance degree is lower (Product A). During the aging process, volatiles have formed due to two main effects: physical effect of evaporation of the initial products and formation of low molecular compounds. Apart from that, the acid number also increases significantly in all cases. Thanks to the addition of an antioxidant, the initiation of polymerization can be delayed as we can see if we compare viscosity of product D with that of product C. From the FT-IR analysis (Figure 1) we can conclude that the polymerization is in a later stage since the presence of aldehydes is limited. This phenomenon can also be extracted from the GPC/ SEC analysis. Higher molecular weight products are present which relates to the viscosity increase. Table 2: Products properties after aging A B C D KV 40 °C (cSt) 1194 422 586 146 KV 100 °C (cSt) 83.1 41.8 45.6 17.6 Viscosity index 146 151 128 133 AN (mgKOH/ g) 26.9 16.5 10.5 5.5 RSSOT (min) 135 103 135 317 Weight loss (%) 22 17 8.6 2.9 Figure 1: FT-IR of product B 3.2 Tribology Figure 2: Stribeck curve for product A, before and after aging Lubricity studies of the compounds before and after aging show that the friction coefficient in the hydrodynamic regime increases because of the viscosity increase as we can see in the Stribeck curve, Figure 2. The higher viscosity of the products also diminishes the coefficient in boundary and mixed regime as a thicker film is formed. 23rd International Colloquium Tribology - January 2022 103 An experimental study of the effect of thermal aging on the lubrication performance of Environmental Acceptable Lubricants (EALs) 4. Conclusion As a result of the aging process, the main and most obvious change of the product is a viscosity increase due to polymerization and to a lesser extent, evaporation, and formation of gases. There are no relevant functionality changes after the aging process as we have seen per FT- IR analysis. Sterical hindrance of the product delays the propagation of the polymerization. Moreover, the presence of an antioxidant delays the oxidative initiation, extending the service life. The increase of viscosity due to polymerization increases coefficient of friction in hydrodynamic and it leads to a higher film formation in boundary. Using the presented methodology, degradation can be predicted; thus, we can design new organic structures extending service life while maintaining a superior performance. References [1] Boyde, S. (2020). Esters. In L. Rudnick, Synthetics, Mineral Oils, and Bio-Based Lubricants (S. Chapter 3). Boca Raton: : CRC Press. [2] Bakunin, V., & Parenago, O. (1992). A Mechanism of Thermo-oxidative Degradation of Polyol Ester Lubricants . Journal of Synthetic Lubrication. [3] Duong, S., Lamharess-Chlaft, N., Sicard, M., Raepsaet, B., Galvez, M., & Da Costa, P. (2018). New Approach for Understanding the Oxidation Stability of Neopolyol Ester Lubricants Using a Small-Scale Oxidation Test Method. ACS Omega, 3, 10449-10459. [4] Ali, A., Lockwood, F., Klaus, E. E., & Duda, J. L. (1979). The Chemical Degradation of Ester Lubricants. ASLE Trans., 267-276. [5] Mousavi, P., Wang, D., Grant, C., Oxenham, W., & Hauser, P. (2005). Measuring Thermal Degradation of a Polyol Ester Lubricant in Liquid Phase. Ind. Eng. Chem. Res., 44, 5455-5464. [6] Wu, Y., Li, W., Zhang, M., & Wang , X. (2013). Oxidative degradation of synthetic ester and its influence on tribological behaviour. Tribology International, 16-23. [7] Yu, Z., & Yang, Z. (2011). Fatigue failure analysis of a grease-lubricated roller bearing from an electric motor. J. Fail. Anal. Prev., 11, 158-166.