Abstract
The motivation for this study was to assess how different polymer materials perform in contact with glycol-based coolants for electrical vehicles (EVs), in order to select the best materials for use in couplings, valves, pipes etc for use in thermal management (TM) systems for both battery EVs and hydrogen fuel cell EVs.
The role of the TM system is to ensure that the Li-ion battery, as well as the power electronics and powertrain, have the right temperature during charging and driving. For fuel cells, the TM system material requirements are even more challenging than for Li-ion batteries. In particular, the electrical conductivity of the coolant must be very low. Hence, migration of ions (or ion forming compounds) from the polymer material into the coolant must be very low. In addition, the polymer must of course be chemically resistant to the coolant.
About 15 polymer materials were studied, mainly glass-fibre-reinforced poly(phenylene sulphide) (PPS) grades, but also grades of polypropylene, syndiotactic polystyrene and polyphthalamide. Injection moulded polymer specimens (tensile specimens, type 1BA of ISO 527) were immersed in a coolant developed for fuel cells (consisting of ethylene glycol, water and additives), and also in de-ionised water. Borosilicate containers were used. The ratio of polymer specimen surface area to liquid volume was 90 mm2/ml. The temperature was 90 °C in most tests, and the electrical conductivity of the immersion liquid was measured during and after the test (2000 h). After the test, the liquids were analysed to identify and quantify compounds that had migrated from the polymers, as well as coolant additives and glycol degradation products. The polymer specimens were also subjected to tensile testing before and after immersion.
For tests with the coolant at 90 °C, the electrical conductivity increased as function of time for all polymer materials; typically by a factor 10-15 over the 2000 h test. The main driving factor for the conductivity increase seems to be glycol degradation. (A methodology was developed for blanketing the containers with N2 after each measurement.) Still, some material types and grades performed better than others, regarding giving a lower increase in the conductivity. Among PPS grades, the differences could be due to polymerisation method and glass fibre sizing. For two PPS grades immersed at 60 °C, the electrical conductivity increased by less than a factor 2 during the 2000 h exposure. Regarding tensile properties, several materials maintained more than 95% of the strength after 2000 h immersion. In general, materials with good retention of tensile properties also gave a low increase in conductivity.
Compared to immersion in the glycol-based coolant, immersion in de-ionised water (at 90 °C) showed a larger increase in conductivity and a larger reduction in tensile properties. Chemical analyses of the water after exposure showed ions that probably originated from the polymer materials.