smagennis

I bought a small tank of SF6 so my son and I could do buoyancy experiments and watch balloons filled with it expands over time. One thing I didn’t expect though was that even is a semi-sealed container, the gas seems to dissipate even after just a couple hours. Can someone help me understand (in simple terms) why a gas that is much heavier than air ‘escapes’ and what sort of container I would I need to use to keep a couple cu ft of the stuff around to float balloons on?