24th International Colloquium Tribology - January 2024 71 Antioxidative Action and Tribological Performance of CuDTP as a Potential Additive for Hydraulic Fluids N. Ayame 1* , K. Yagishita 1 , T. Oshio 1 1 ENEOS Corporation, Yokohama, Japan * Corresponding author: ayame.noriko@eneos.com 1. Introduction In order to contribute to the achievement of the United Nations’ Sustainable Development Goals (SDGs), the reduction of environmental impact and the prevention of environmental pollution are being considered in various industrial sectors. Lubricants are also expected to provide solutions to these challenges, and environmentally friendly lubricants have been developed. For example, efforts in hydraulic fluids are shown in Figure 1. Hydraulic fluids are lubricants used in hydraulic systems such as hydraulic excavators, which are responsible for preventing wear in hydraulic equipment and for transferring the hydraulic energy generated in a hydraulic pump to the individual components such as a drive motor and cylinder. Figure 1: Efforts to reduce the environmental impact with hydraulic fluids For CO 2 reduction, very high viscosity index fluids, which present small change of viscosity with temperature, are effective for improving fuel efficiency. Using such fluids allow to keep sufficiently low viscosity at low temperature and sufficiently high viscosity at high temperature in order to obtain the best compromise in terms of lubrication capability and fuel economy. For preventing environmental pollution, lubricant base oils with excellent biodegradability are used for hydraulic fluids. For waste oil reduction, antioxidant technologies are important for improving oxidation stabilities of hydraulic fluids. From these considerations, the most extensive studies have been antioxidants, which lead to reduce waste oil. Zinc dialkyl dithiophosphate (ZnDTP) has been used as a useful additive in various lubricants because of its multifunctional characteristics providing antioxidative, antiwear, and extreme pressure properties, together with its availability at low cost. For this reason, ZnDTP has long been utilized as an important additive of hydraulic fluids (HFs), and thus HFs with ZnDTP (Zn HFs) have been the mainstream formulation in practical hydraulic applications. On the other hand, recent hydraulic systems have required the operation under severer conditions with increasing pressure and also with decreasing oil volumes, resulting in higher oxidation and heat loads on HFs. It is known that ZnDTP decomposes by oxidation and heat, this eventually causes insoluble sludge in fluids, and it makes trouble such as filter blockage and hydraulic pressure failure. Under the circumstances, HFs without ZnDTP tend to be used in hydraulic systems. But they are less cost competitive than Zn HFs because they need several ashless additives to meet demanded performance. So, Zn HFs still account for the majority of the market. 2. Results As an alternative to ZnDTP, this study investigated the potential of copper dialkyl dithiophosphate (CuDTP) shown in Figure 2. CuDTP was reported not only to have antiwear characteristics equivalent to those of ZnDTP but also to deactivate ROO • and ROOH, the active species in oxidative degradation. The tribological properties of CuDTP were evaluated using the FZG test, four-ball test, and V104C pump test, and the evaluation results demonstrated HFs with CuDTP (Cu HFs) had the performance equivalent or superior to that of Zn HFs. Further, the A2F pump test showed that the oxidation stability of Cu HFs was more than twice as much as that of Zn HFs. This fact suggests that CuDTP, compared with ZnDTP, can achieve the satisfactory antioxidative action with a smaller addition amount, resulting in lower cost and less sludge. Figure 2: Structure of CuDTP