Foil vs. mylar
Aluminium foil and Mylar are two very different materials. Although when most people think of Mylar, they think of the shiny, silver balloons, that is not actually what Mylar naturally looks like. Real Mylar is completely transparent plastic and contains no metal.
Both Mylar and aluminium foil have their own places, benefits and uses.
Aluminium Foil Properties
Aluminium is refined from bauxite ore. The ore is melted and mixed to remove aluminum oxide. The aluminum oxide is electrolytically reduced in a mixture of molten cryolite to remove the oxygen from the metal. Aluminium foil is made from heated aluminium that is rolled between large rollers. The rollers press the metal into thin sheets. Horizontal tension and pressure keeps the metal flat and still as the rollers press the metal.
Mylar is actually not a form of metal at all. Mylar is a brand name for polyester resin, which is a type of clear, thin plastic. The foil-covered Mylar used to make balloons and other shiny products is an extremely thin layer of aluminium metal (less that 1/100th of the width of a human hair in some cases). Mylar is made by melting the polyester resin and stretching it into thin, flat sheets.
Aluminium foil is used in many different applications including: household food storage, food packaging, foil containers, military packaging, gift wrap, decorations, candy wrappers, Christmas trees, name plates and many other uses. Mylar is used for many applications as well, including: clothing stays (such as for lingerie), book jackets, liners, protective surfaces, duct liners, plastic ribbons, tape, labels and of course, balloons.
There are many benefits to both Mylar and aluminium. Aluminium foil is used for many applications where Mylar would be inappropriate. Areas where heat is necessary, such as in industrial and household settings, work better with aluminium foil. Aluminium is also formable, and can wrap around objects. Mylar is perfect for areas where aluminium is unsuitable. Mylar is more flexible than foil and will not tear as easily. Mylar can also fuse with objects, which is nearly impossible for aluminium to do.
In some cases, such as with balloons, the properties of aluminium and Mylar can be fused together. This gives the material some of the properties of both materials. Usually when aluminium is added to Mylar, the aluminium must be applied in small amounts to retain the flexible nature of the plastic.