Help in identifing these 2 materials
They're structural, and not electrically or thermally conductive.

The first one is for sticking the capacitor onto the board (likely for assembly or structural reasons).

The second one is just an insulative layer, probably to prevent the 2 boards from shorting out (they have the solder sides facing each other) if they should be bent or ve in a high-vibration environment (or potentially to allow cheaper and smaller mounting to the chassis without compromising electrical properties).