PFC capacitors are always made from aluminium, either as a thin foil with an insulating spacer ( and part of the dielectric) or a sputtered film on a substrate. They are filled with either an oil, or a proprietary formula as a dielectric and arc suppressor. This often can break down with time, as it is often not designed to be stable in the presence of oxygen from the air. Thus a leaky seal lets in oxygen, and may not let the reaction products out again, causing the cap to swell.