38 They can also modify these parameters, and tweak related properties such as tensile strength, elongation, thermal conductivity, viscosity and density. Offering characteristics intermediate between silicones and epoxies, polyurethanes and acrylates are generally less resistant to moisture penetration, chemical attack and high temperatures, says White at Henkel. “Silicones are particularly useful in potting applications because of their extremely low Tg and high degradation temperatures, which provide consistent mechanical performance over their operating temperature range of roughly -40 C to 175 C,” they add. Silicones also have a very low surface energy and therefore wet-out surfaces extremely well, which is ideal for potting applications. The wet-out is particularly important to avoid moisture ingress and ensure excellent resistance to corrosion. White say silicones have a very low modulus (stretch a lot) relative to other polymers, which is crucial for giving stress relief during thermal cycles. “The low modulus is important for thin sections with a large CTE mismatch between components. Silicones have an immense range of molecular weights and architectures, which makes them very flexible as starting materials. Silicones are soft materials with high-temperature stability. Their high-temperature resistance compared to urethane is the main advantage, but silicones are much more expensive and adhere less well to substrates than do urethanes. “Epoxies are also widely used for high-voltage applications in EVs, but they have a much higher glass transition temperature and higher modulus, which makes the CTE mismatch much more important to manage to avoid component damage,” he adds. Over the last five years, Axalta has been developing new epoxy-based materials for the encapsulation of electric machines in industrial applications, such as servo drives, and it is looking to apply them to EV motors, Koschmieder says. The idea is to improve thermal conductivity by encapsulating the stator so that the motor can produce more power without overheating. “With regard to possible EV use, we did tests with a couple of ATF oils and got brilliant results,” he emphasises. “Some of these oils are very aggressive due to the additives included.” Essential fillers All of the polymers used in potting and encapsulation have relatively low thermal conductivity, and therefore typically have fillers such as alumina mixed into them to increase their ability to dissipate heat, improve their electrical conductivity or, in some cases, reduce cost. “A common challenge in the industry is formulating the products in a way that the filler distribution remains uniform,” says Brown of C-Therm. “Agglomeration and sedimentation are issues often faced in manufacturing, depending on the shelflife stability of the composite. “Typically, the lower the viscosity, the more quickly they tend to sediment. Several sophisticated systems are available from groups like bdtronic that provide automated dispensing of precise amounts of material. It is crucial to do so at speed and place the material precisely in scaling manufacturing.” Henkel’s Bill Brown says dispensing is vital in any thermal potting application because of the high volumes and potential abrasion of components by the hard particles of the thermal filler. “The dispensing system needs to be matched with the dosing accuracy required and must be well integrated with the overall process flow. For thermal potting, remixing is often required prior to dispensing to ensure good homogeneity of the high-density thermal filler, while thawing may be needed for one-component systems,” he says. Insight | Potting & encapsulation March/April 2024 | E-Mobility Engineering Dispensing equipment for potting must often cope with highly abrasive filler materials that are essential for thermal conductivity, and wear and tear is a major issue (Image courtesy of ViscoTec)
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