Keywords

1 Introduction

Electric vehicles (EV) are one of the main solutions developed to reduce greenhouse gas emissions and thus mitigate the effects of climate change. Certain EV configurations have the EM within the transmission case. In these circumstances, the EM is contact with the ATF, which makes it necessary for the ATF to present a series of additional properties to the usual ones for this type of fluid: electrical, magnetic and materials compatibility, protection against corrosion and resistance to the formation of foams, a part from reducing friction and wear and improving the EM cooling.

The aim of this work is to analyze the compatibility of four conventional ATFs with three structural polymers (Polyether Ether Ketone – PEEK, Polytetrafluoroethylene – PTFE and Polyamide 66 – PA66), used in supports and moorings, and three elastomers (Fluorocarbon – FKM, Monomer of Ethyl – Propyl – Diene – EPDM and Vinylmethylsilicone Rubber – silicone), used in seals and joints, as well as in the insulation of electrical cables [1]. For this, the variations in volume and hardness after a period of aging of the materials in the ATFs will be analyzed.

2 Materials y Methods

2.1 Materials

Four conventional automatic transmission fluids (ATFs), commonly used by many automobile manufacturers in their current models, were selected for materials compatibility testing. The composition of the ATFs is shown in Table 1. The specimens used in the compatibility tests were extracted from sheets of each material with the dimensions specified (type V) in the ASTM D638 standard.

Table 1. ATFs composition.
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2.2 Aging of Materials

According to the recommendations of the ASTM D7216-15 standard [2], the variations in the volume and hardness of the materials after aging were studied. The test samples of the materials were immersed in the ATFs at 100 ℃ in a MEMMERT Universal Oven (Model U, precision: ±0.5  ℃) for 168 h. In addition, other material samples were aged for longer periods (336, 504 and 672 h). The specimens were cleaned after the machining process with a damp cloth, immersed in ethanol and dried with paper and hot air. After removing the specimens from the oven, they were cleaned with blotting paper to drain the oil. Subsequently, ethanol was used to ensure complete cleaning and excess ethanol was removed with blotting paper to prevent its evaporation on the surface of the samples.

2.3 Materials Characterization

Volume and hardness variations were also measured according to ASTM D7216-15 [2]. Volume changes were determined using an analytical balance (readability: 0.1 mg). The hardness of three specimens of each material was measured with a SAUTER HBD 100 durometer (structural polymers) and with a SAUTER HBA 100 (elastomers), before and after the aging process, in four different positions. The mean value of these four measurements was considered as the result to be compared.

2.4 Complementary Tests

X-Ray Diffraction (XRD) experiments were performed to calculate the degree of crystallinity of the polymers before and after aging in the different oils using a PHILIPS X’PERT PRO diffractometer in a range of 2θ between 5° and 80°, with a step angle of 0.0167°. Fourier transform infrared spectroscopy (FTIR) measurements were performed on the structural polymers and elastomers on a Varian 670-IR FTIR spectrometer with a Golden Gate ATR device to verify possible variations in their composition after the aging process. Aditionally, the elastomers underwent a thermal characterization using thermo-gravimetric analysis (TGA) to analyze how the aging process could affect the structure of the elastomers and their thermal stability. The same SDT Q600 thermal analyzer (TA Instruments) was used. A nitrogen atmosphere was used. The samples were heated from room temperature to 800 ℃ at a heating rate of 10 ℃/min.

3 Results

3.1 Volume Variation

The compatibility of the structural polymers and the elastomers with the ATF was first verified in terms of the variation in volume, according to the ageing time specified in the ASTM D7216-15 standard (168 h) [2]. In the case of structural polymers, the volume changes depended more on the type of material than on the oil used (Table 2). The three structural materials (PEEK, PTFE and PA66) experienced a slight reduction in volume. Although there are no specifications on the compatibility of these materials with ATFs, the measured volume variations, which are around -0.5%, indicate that these ATFs and materials are compatible. For longer times (more than 168 h), only PTFE showed some variability, although it could be concluded that all structural materials underwent negligible volume changes.

Table 2. Volume variation of the materials after ageing.
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Regarding the elastomers, there are compatibility specifications set by transmission manufacturers [3]. Table 2 shows the volume variations of them after the aging process. The silicone perfectly complied with the maximum recommended limit values (∆V + 20%). On the other hand, the FKM constitutes a special case, since the volume of the samples was reduced between 2.5 and 3.5%, with which they would be out of specification (∆V between −2% and +5%). Therefore, the tested oils are not compatible with FKM in terms of volume change. The EPDM experienced a considerable increase in volume, obtaining the maximum values with oil A. Despite the lack of reference, the results obtained allow us to conclude that these oils are not compatible with EPDM. On the other hand, the long-term results (more than 168 h) showed that both FKM and EPDM worsened their results, while silicone remained stable.

3.2 Hardness Variation

There are also no specifications for hardness changes of ATF-aged structural polymers. The hardness variation after the first 168 h in the three materials was small (Table 3). On the one hand, in PEEK and PTFE it decreased around 3 points (Shore D). On the other hand, the hardness increased slightly in PA66 after the aging period. The changes in the hardness of each material did not depend on the oil used. In the long-term tests, the hardness values remained stable in all cases, which allows us to conclude that these materials are compatible with the ATF in this study.

Table 3. Hardness variation of the materials.
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Regarding the evolution of the hardness of the different elastomers during the aging process. The FKM showed small changes in hardness in the first 168 h, confirming what was reported by Drobny [4]. On the other hand, EPDM and silicone showed a great reduction in hardness, although in the case of silicone it complies with the values recommended by manufacturers (−15 ShA), which confirms what has been found in the literature [5]. There is no bibliographical reference on the behavior of EPDM in oils, but its loss of hardness is the greatest of the three elastomers. The studies by Nakamura et al. [6] and de Souza [7] confirm this trend and that it is due to immersion in fluids, although EPDM would not be affected by high temperatures. These results did not vary according to the oil used (Table 3).

3.3 Complementary Tests

Hardness increases with the degree of crystallinity, so XRD results can be correlated with experimentally measured hardness and volume variation results. The spectra obtained in the case of structural polymers did not show notable changes in the proportion of crystalline and amorphous areas, which confirms the small variations measured in volume and hardness for these materials. The analysis of these values showed that the measured changes in hardness are explained by the significant changes in the percentage of crystallinity showed in Table 4.

Table 4. Percentage of crystallinity of elastomers FKM and Silicone by ATF and ageing time.
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In FTIR spectra, qualitative differences (peak positions) in the same material were almost negligible for structural polymers. For this reason, a principal component analysis (PCA) [8] was performed and the results suggested that, in the case of PEEK, time affects to a greater extent than the type of oil. This would explain, in general, the similarity of results in the hardness and volume tests of PEEK after aging in the different ATFs. The other two structural polymers were analyzed in a similar way and the results showed that on PTFE, contrary to PEEK and PA66, the type of oil had more influence than the aging time.

The TGA showed that the mass loss of the fresh FKM below the TOnset was higher than that of the aged samples probably due to the volatile components presence in the material [9], which is not present in the aged samples. Meanwhile, the EPDM [10] and the Silicone [11] showed the higher mass loss due to loss of the oil previously absorbed, which explains the huge volume increase shown (Table 2) along with the huge reduction in hardness (Table 3).

4 Conclusions

The compatibility between four conventional ATFs, three structural polymers (PEEK, PTFE and PA66) and three elastomers (FKM, EPDM and silicone) which can be used in electrified transmissions was researched by determining volume and hardness variations after ageing the materials separately immersed in the ATFs. The main conclusions obtained were: (a) PEEK and PTFE showed short and long term compatibility according to volume and hardness variations; (b) PA66 experienced an increase in hardness due to the increase in crystallinity during aging, (c) ATFs A and C increased excessively EPDM volume witch made the no compatible. The remaining elastomers endured well all ATFs, (d) XRD test denote a significate reduction in the crystalline structure of silicone aged in ATF C. The variation in FKM was not remarkable and no test where run on EPDM due to its semi-crystalline structure; and (e) due to oil absorption, verified by TGA test on EPDM and silicone, a volume increase occurred which led to a hardness reduction. FKN did not absorbed oil, but volatile components evaporation in the fresh rubber provoked a volume decrease and a hardness increase.