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Epiform F-6975 Grey Coil Insulation Epoxy Coating Powder was designed specifically to be used to coat the copper hairpin on 400V and 800V Interior Permanent Magnet of Electric Vehicle motors. It is a gray fusion-bonded functional epoxy coating powder formulated to have adhesion to both copper and PEEK and designed to be used in a continuous automatic transmission fluid (ATF) environment.
Epiform F-6975 has a proven track record and is being used by most automotive manufacturers across the globe in Japan, China, India, Europe and the USA at car manufacturers from all of these regions. It achieves a heat resistance classification of 180°C and therefore can be used in applications that require a Class H rating. F6975 is perfect for fludized bed application methods.
What happens if we cure in a lower temperature?
There is always a contour plot of Tg that depends on time and temperature. Please see this article that discusses this and the following image:
The recommended cure schedule at 190°C is 20mins and the general rule of thumb is that for every 10°C (up or down) generally halves or doubles the rate of reaction. Dropping the temperature by 15°C or 25°C would then mean a cure time of approximately 60-80 minutes. However, as you see in the image, the rings get further apart as the cure temperature and the Tg get higher. Given that the Tg of the powder is only 130C and the cure temperature is still 165C or 175C, we would feel comfortable saying that a cure time of 30-40 mins would be sufficient still. There are cure kinetic curves that can be generated but this should last least give you a starting point to work with.
PEEK, also known as magnet wire, can add a smaller heat capacity compared with non-PEEK bare copper wires, which can result in thin epoxy coating results and cause adhesion problems. This is not the norm but it can happen.
It is probable that due to this, oxidation proceeds easily, and peeling can occur. The epoxy is being oxidized and oxidation manages to occur at the interface between the epoxy and PEEK. Oxidation occurs in all epoxy layers.The action at the interface means that the thin part of the epoxy oxidizes easily, dampening the plasma effects that normally aid in the adhesion between the PEEK and the epoxy.
This phenomenon can be resolved by coating the wire in a way that reduces the coating area. Reducing the coating area means painting only near the non-PEEK bare copper wire. Non-PEEK bare copper wire has higher heat conduction compared to PEEK. During coating the non-PEEK bare copper wire can adhere a large amount of powder to increase the film thickness. This is thought as an effective countermeasure because dripping occurs after coating and the epoxy layer on PEEK becomes thin.
If delamination has not occurred in all samples, or you never faced this issue before, it is also probable that the pretreatment plasma irradiation was non-uniform and has to be tried again.
Another cause of delamination of the coating to PEEK has been by having too large of a temperature differential between subsequent dips of epoxy coating powder. Because the recommended pre-heat temperature causes an expansion rate difference between the PEEK and the copper, it is important to limit the pre-heat temperature to 160°C (and not higher) to avoid large temperature differentials between the pre-heat and dipping process. Paying attention to this temperature differential will ensure a coating without delamination.
Please see the following image for a standard coating process: