# Automatic Transmission Fluids in Electrified Transmissions: Compatibility with Elastomers
## 1 Introduction  
The electric vehicle (EV) has been adopted by governments as one of the main solutions in the struggle to minimise transportation-related emissions which, according to the Intergovernmental Panel on Climate Change [1], represent 14% of emitted global greenhouse gasses. The average efficiency of the EV is 77%, which is a great improvement when compared to the 21% of the traditional internal combustion engine (ICE) vehicle [2]. The energetic efficiency results in big reductions in CO\({}_{2}\) emissions [3], an effect which is even greater when the electricity used is generated from renewable sources [4]. There are many types of EVs, from full electric vehicles such as extended range electric vehicles (REV) or battery electric vehicles (BEV), to hybrid electric vehicles (HEV) or plug-in hybrid electric vehicles (PHEV), and mild hybrid electric vehicles (MHEV) or fuel cell electric vehicles (FCEV). Moreover, independently of the EV type, the position of the electric motor (EM) can be before, after or inside the transmission of the vehicle [5], or even directly next to the wheels [6]. When the EM is located inside the transmission, it is in contact with the transmission fluid, a circumstance that may occur both in hybrid vehicles and in 100% electric vehicles, which require a transmission to adapt the speed of the motor to the speed required at the wheels of the vehicle. For that reason, the requirements of automatic transmission fluids (ATFs) are more demanding than in ICE vehicles. Among these requirements are magnetic, thermal, electrical and materials compatibilities [6; 7; 8].  
The growing importance of EVs in the context of reducing global emissions makes it of especial interest to analyse the interaction between EV transmission lubricants and the materials used in them. The compatibility of the lubricants with seal materials that are commonly found in transmissions is tested by evaluating the variation in volume, hardness, tensile strength, and elongation at break after aging in the ATF (for 168 h) [9; 10]. This is the case of several elastomers that are frequently used in seals, gaskets, and the insulation of electrical wires [11], such as ethylene-propylene-diene monomer (EPDM), fluoroelastomer (FKM), or vinyl-methyl silicone rubber (silicone). The swelling and degradation of elastomers caused by some oils have been previously reported by other authors [12; 13], as the physical properties of these materials need to remain stable in order to prevent oil leakages.  
This research has the objective of studying the compatibility of four commercial ATFs with certain elastomers commonly used in electrified transmissions as seals, gaskets, and electrical insulators. Measurements of volume, hardness, tensile strength, and elongation at break were made for three elastomers (EPDM, silicone, and FKM), following the international standards recommendations [14]. The tested materials were aged in the four ATFs for the time recommended by different specifications or standards (168 h) and, in addition, for longer time periods (336, 504, and 672 h). To explain the changes that occurred in the properties of the elastomers, additional tests such as Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and thermogravimetric and derivative thermogravimetric analysis (TGA and DTGA) were performed on the elastomers.  
The novelty of this research consists of studying the materials compatibility of four ATFs with different compositions at longer aging times than that recommended by the corresponding standards. This compatibility study is a part of a broader research project, whose goal is the formulation of appropriate lubricants for electric vehicles and where other compatibility tests (electrical, thermal, copper corrosion by wire resistance, etc.) will be performed.