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In order to analyze the electrical properties, the loss factor (tan 5) and the permittivity (er) were determined before and after water storage in accordance with IEC 60250. The specimens were stored 100 days at 50 °C. Before water storage, all specimens had excellent values of tan 5 lower than 0.05 and a permittivity er lower than 6.
After 100 days of water storage, all specimens showed a loss factor higher than 0.1. The permittivity increased to more than 15. Due to the dielectrical properties,
Fig. 4 Heat conductivity of alumosilicate filled EP depending on volume/mass-concentration of additive (HPF The Mineral Engineers 2014b)
alumosilicate is not recommended for outdoor (subjected to direct weathering) but for indoor applications only.
The mechanical properties were characterized in accordance with ISO 178 (three- point bending test) and ISO 179 (impact strength). Due to the high filling load, all specimens showed a high stiffness and a loss of flexibility:
By use of the alumosilicate additive in casting resins, thermal conductivities up to 2.0 W/(m K) could be achieved. For the purpose of comparison, unfilled epoxy resin has a thermal conductivity of only 0.2 W/(m K).
The filling degree can be increased up to 58.85% vol. providing good flow ability and casting process ability at the same time when using the alumosilicate with a special surface coating (SST) (Table 5).
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