Mylar bags are made of stretched polyethylene terephthalate (PET) film, also known as BoPET or biaxially-oriented polyethylene terephthalate. Mylar is one of the brand names for this type of stretched plastic.
PET plastics have a higher level of durability than many other plastics and do not leak into fluids or foods, making them ideal for food and water storage. Mylar has many other traits that makes it ideal for commercial uses, including its tensile strength, electrical insulation capability, reflective properties and chemical stability. The thin polyester film known as Mylar was first patented in the 1950s by the DuPont company.