Mylar balloons are a popular choice for celebrations because they retain buoyancy for a long period compared to standard latex balloons. Although often called foil balloons, “Mylar” is actually a registered trade name for a specific type of polyester film developed by DuPont in the 1950s. The balloon’s distinctive metallic appearance and unique performance characteristics come from a composite structure. This construction involves a core plastic film that is treated and then coated with a metal layer and decorative inks.
The Base Polymer Film
Biaxially-Oriented Polyethylene Terephthalate, commonly abbreviated as BoPET, is the core material of a Mylar balloon. This polyester film provides high tensile strength and durability. The material starts as a molten polyethylene terephthalate (PET) resin that is extruded into a film.
The “Biaxially-Oriented” name refers to a manufacturing process where the film is stretched in two directions—lengthwise and widthwise—using heated rollers and ovens. This stretching aligns the polymer chains, which significantly increases the film’s mechanical strength and stiffness. This process creates a thin, flexible, and highly resilient substrate that forms the main body of the balloon.
The Metallic and Decorative Layers
The characteristic metallic shine is achieved by applying a thin layer of metal, typically aluminum, onto the BoPET film. This is done using physical vapor deposition (PVD) in a vacuum chamber. In this method, aluminum is heated until it vaporizes and then condenses as a coating onto the cooler polyester film.
This aluminum layer is thin, often only a few hundred atoms thick, but it is continuous enough to create an effective barrier and a highly reflective surface. On top of this metalized surface, various inks and pigments are applied to create the colorful designs, patterns, and graphics seen on the finished product. These decorative elements are printed directly onto the film.
Material Properties and Gas Retention
The combination of the strong BoPET film and the metal coating results in a material with low gas permeability. The aluminum layer acts as a barrier against the escape of gases like helium, whose small atoms slowly diffuse through the molecular structure of plain latex. This structure allows Mylar balloons to hold helium for weeks, a significantly longer duration than porous latex.
The non-stretchable nature of BoPET means the balloon is inflated only slightly above atmospheric pressure, which reduces the internal force pushing the gas outward. Since the material is not elastic, the final balloon shape is created by heat-sealing two or more pieces of the film together to form airtight seams. This heat-sealing process is necessary to contain the helium, as the material cannot be knotted like a traditional rubber balloon.