[kapital]

Why electric insulators have that litle of conductvity? If the do not have ions, or electrons, that could trnasfer charge, therir coductivity shoud be 0, not some small value?

[Enthalpy]

(1) Because we're unreasonable in our measurements... Conductivity spans 33 magnitudes from metals to good insulators. If we were more reasonable, we would just say "no conductivity".

If you've already made or seen such a measure, it's totally  . The material must be processed in a part shaped to allow the measure, because surface currents would otherwise overwhelm volume conductivity by many magnitudes. Cleanliness is  . Of course, moisture is totally excluded - good plastic insulators are essentially the hydrophobic ones. And did I mention that atmosphere is undesired?

(2) Because a few charge carriers subsist, sure. Essentially impurities: polymerization initiators, impurities in ceramic, moisture. If these could be eliminated (improbable) then thermal energy creates a few carriers, even in insulators - normally you don't see this cause.

(3) Radioactive materials (within the insulator) create charge carriers by ionization. One advantage to organic materials, but beware ¹⁴C. Is carbon from petrol better than bio-produced...?
In silicon chips, radioactivity can be a significant source for leakage that erases Prom memory over time, if not controlled.
Cosmic rays can't be avoided - just reduced by going very deep underground.

[Enthalpy]

Just to make the magnitudes more concrete...

If PTFE really has 10²⁵ ohm*m resistivity as Wiki claims
http://en.wikipedia.org/wiki/Electrical_conductivity
then a perfect capacitor using it as a dielectric would self-discharge in 7 million years - or much easier: measure 1ppm charge lost over 7 years...

I wrote "unreasonable", that's what it means.

A normally good electronician can measure the conductivity of de-ionized water, but not of glass. Still 10 magnitudes to go to PTFE  .