What Is Mylar? Definition, Properties, Uses

Mylar is one of the brand names for stretched polyethylene terephthalate (PET) film or foil.
Mylar is one of the brand names for stretched polyethylene terephthalate (PET) film or foil. Frederick Bass / Getty Images

What is Mylar? You may be familiar with the material in shiny helium-filled balloons, solar filters, space blankets, protective plastic coatings or insulators. Here's a look at what Mylar is made of and how Mylar is made.

Mylar Definition

Mylar is the brand name for a special type of stretched polyester film. Melinex and Hostaphan are two other well-known trade names for this plastic, which is more generally known as BoPET or biaxially-oriented polyethylene terephthalate.

History

BoPet film was developed by DuPont, Hoechst, and Imperial Chemical Industries (ICI) in the 1950s. NASA's Echo II balloon was launched in 1964. The Echo balloon was 40 meters in diameter and constructed of 9 micrometer thick Mylar film sandwiched between layers of 4.5 micrometer thick aluminum foil.

Mylar Properties

Several properties of BoPET, including Mylar, make it desirable for commercial applications:

  • Electric insulator
  • Transparent
  • High tensile strength
  • Chemical stability
  • Reflective
  • Gas barrier
  • Odor barrier

How Mylar Is Made

  1. Molten polyethylene terephthalate (PET) is extruded as a thin film onto a chilled surface, such as a roller.
  2. The film is drawn biaxially. Special machinery may be used to draw the film in both directions at once. More commonly, the film is drawn first in one direction and then in the transverse (orthogonal) direction. Heated rollers are effective for achieving this.
  3. Finally, the film is heat set by holding it under tension above 200 °C (392 °F).
  4. A pure film is so smooth it sticks to itself when rolled, so inorganic particles may be embedded in the surface. Vapor deposition may be used to evaporate gold, aluminum or another metal onto the plastic.

Uses

Mylar and other BoPET films are used to make flexible packaging and lids for the food industry, such as yogurt lids, roasting bags, and coffee foil pouches. BoPET is used to package comic books and for archival storage of documents. It's used as a covering over paper and cloth to provide a shiny surface and protective coating. Mylar is used as an electrical and thermal insulator, reflective material, and decoration. It's found in musical instruments, transparency film, and kites, among other items.