[mikethedruid]

"The permittivity is the characteristics of dielectric materials and for Mica it is 3 - 6 while for Plexiglas (clear plastic) is 3.4." Notice that for mica the figure is indefinite, "3 - 6", so it could be less than plastic, or more, and, depending on humidity which can affect mica by getting between the layers, it can even vary on an individual sheet from day to day, whereas plastic is fixed and predictable. Except in applications where heat that would affect the plastic are involved, I see plastic as not only an acceptable replacement for mica, but actually a superior replacement. Of course, in 50 years or so, one is likely to have to replace the plastic pieces since they will age and degrade, where the mica would not, I will grant that, but I won't be around to worry about it, and it will be an east fix for some technician of a new generation.