18" round Mylar
18" Mylar Helium HB Rainbow
18" Mylar balloons are a foil type of balloon, It is not used for release because they could get caught up in electrical lines.
They are expand in the heat and deflate in a cooler enviornment so may be delivered a little deflated until the balloon adjusts to the new enviornment, then they will puff up.
Mylar balloons should last 2 days but depending on the size and make they can last up to 7 days.