NEMA Standard LI-1 (Industrial Laminated Thermosetting Products) gives specifications for many types of composite materials, including the NEMA G-11 mentioned in the other thread (G-11 and many more are defined in LI-1).
The specifications of the NEMA standard include mechanical properties and electrical properties, including dielectric constant (a material property) and dielectric strength (a little trickier to define, depends on geometry... there are standards).
Also, the Gund Company offers a catalogue of composite products for use in electrical machinery. Every item in their catalogue has dielectric constant and strength identified. I think any company that serves the specialty market of electric machinery would provide that info in its catalogue.
So, I guess my suggestions to ask your supplier for the information, or else ask if this material meets specifications for one of the materials identified in NEMA LI-1.
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(2B)+(2B)' ?