When were foil balloons invented and how are they made?

Silver metalized balloons were first developed for the New York City ballet in the late 1970s. These balloons are sometimes (mistakenly) called "Mylar" balloons, but they are almost always made from sandwiched sheets of plastic (polyethylene) and nylon that are then coated with aluminum. Because of this, the balloon industry prefers to call them "foil" balloons. They are much more expensive to produce than ordinary Latex balloons. While the molecular structure of foil balloons is much "tighter" than stretched Latex, even with these balloons, Helium will still eventually seep out through the inflation seal of a foil balloon and leave them loosely sagging and flat.