Thermomechanical Properties of GFRP after Air and Cooling Fluid Exposure at Elevated Temperatures for Application in Electric Motors

Abstract This paper investigates the effect of exposure of glass fiber reinforced plastic (GFRP) laminates with epoxy resins to air and a synthetic poly-alpha-olefin (PAO) oil at 180 °C for 500 h (h) and 1000 h. The test is intended for prequalifying material systems for the intended use of the laminates as a composite seal of a stator lamination to enable a directly cooled electric motor. Three commercially available amin curing and one non-commercial anhydride curing epoxy resin systems are investigated. The results of the mechanical testing indicate post curing effects for the epoxy resins during exposure. These translate to higher moduli in the laminates and for most systems the strength rises as well. These effects dominate the change in mechanical properties for the commercial systems making these systems well suited for the intended use case. However, for one commercial and the noncommercial system, some properties drop after exposure to cooling fluid (CF) e.g. in the transverse tensile test. Here a decrease of the fiber-matrix adhesion is the identified cause. This in turn is caused by a moderate laminate quality which is evident by visible imperfections of the laminate. Thermal properties are mostly affected by the chosen temperature and time of exposure, not so much by the ambient medium. From our findings we conclude that the exposure to PAO oils of the chosen systems is not as severe as the findings from other authors suggest. Therefore, the investigated laminates are prequalified for the intended use. In addition, through the successful subsequent use of our results, we find, that the identified test is suitable for prequalifying GFRP for the use in directly cooled electric motors. Graphical Abstract