Polyester film, often referred to as PET (Polyethylene Terephthalate), has outstanding physical properties for a plastic film. These include high tensile strength, chemical resistance, thermal stability, flatness, clarity, high temperature resistance and thermal and electrical insulation properties.
With hundreds of types of film in a wide variety of thicknesses and coating combinations, polyester film gives outstanding performance in a huge range of applications and is often far superior to other substrates such as PVC, acetate and polypropylene.
Polyester, or Polyethylene Terephthalate (PET) is a plastic material produced by reacting terephthalic acid with ethylene glycol – two substances produced in the crude oil / naptha cracking chain. During manufacture, polyester “chips” are fed into a heated extruder and the molten polyethylene terephthalate (PET) is then extruded onto a chill roll, which quenches it into the amorphous state. It is then biaxially oriented by a process known as drawing. The most common way of doing this is the sequential process, in which the film is first drawn in the machine direction using heated rollers and subsequently drawn in the transverse direction, i.e. at 90° to the direction of travel, in a heated oven. Draw ratios are typically around 3 to 4 in each direction.
Once the drawing process is completed, the film is “heat set” or crystallized under tension in the oven at temperatures typically above 200 °C. The heat setting step prevents the film from shrinking back to its original unstretched shape and locks in the molecular orientation in the film plane. The orientation of the polymer chains is responsible for the high strength and stiffness of biaxially oriented PET film. Another important consequence of the molecular orientation is that it induces the formation of many crystal nuclei. The crystallites that grow rapidly reach the boundary of the neighboring crystallite and remain smaller than the wavelength of visible light. As a result, unfilled biaxially oriented PET film has excellent clarity, despite its semicrystalline structure
The many advantages of polyester film can be exploited in different ways according to the wide variety of possible applications, some of which are listed below…
- Strength – hanging displays, tags and labels
- Clarity – overlamination, archieval storage, safety window film
- Thermal Stabilty – ovenable packaging film, electrical insulation at elevated temperatures
- Printability – by a variety of techniques including offset, flexo, gravure, digital
- Whiteness – as a synthetic paper mostly for print applications
- Rigidity & Flatness – in tickets, tags and card applications
- Sealability – can be heat sealed over a wide range of temperatures
- Chemical Stability – medical test strips, hazardous substance labelling