Who says insulating paper is resistant to pressure?

The mica insulation sheets that permeates the electrolyte is only the negative plate, and the vulcanized fiber sheet is the oxide layer of the outer layer of the positive plate.
As for the shiny aluminum foil is the negative lead.
When the electrolytic capacitor plus reverse voltage, it will layer that layer of decomposition, so leakage,
If long-term plus a small amount of reverse (to ensure that the capacitor will not explode), and even in the negative aluminum foil to establish oxide,
So that the polarity of the electrolytic capacitor is reversed.