Open up a small transformer, a motor or a capacitor and there is a good chance you will find polyester film inside. It is thin, strong, clean and a very good insulator for its thickness, which makes it one of the most widely used films in electrical work.
Buyers, though, often meet a confusing list of names: PET, BOPET, Mylar, milky white, transparent, metallized. This guide explains what BOPET polyester film is, the grades you will come across, and how to choose one for insulation.
What Is BOPET Polyester Film?
BOPET stands for biaxially oriented polyethylene terephthalate. The polyester is melted, cast into a thick sheet, and then stretched in two directions, along the length and across the width, before it is heat-set. That stretching lines up the polymer chains and gives the film its strength, flatness and stability.
In the trade, PET film, polyester film and BOPET film usually mean the same thing. Mylar is a well-known brand name for this type of film.
Properties That Matter for Insulation
- High dielectric strength for its thickness. A film thinner than a human hair can hold back several kilovolts, so it saves space in windings.
- High tensile strength. It does not tear easily during winding and holds its shape under tension.
- Thermal class. Electrical polyester film is generally used in Class E and Class B systems, around 120 to 130°C. Hotter designs need a higher-class film.
- Low moisture absorption. Its electrical properties stay stable in humid conditions.
- Chemical resistance. It handles oils, most solvents and impregnating varnishes well.
Common Grades of BOPET Film
Transparent Polyester Film
Clear, general-purpose film used for layer insulation, interleaving, wrapping and as the base film in laminates. See our transparent polyester / BOPET film.
Milky White (Opaque) Polyester Film
White film is easy to see on the winding line and in finished assemblies, which helps inspection. It is widely used for layer insulation in transformers and for insulating components. Our JBF milky white polyester films are E class (120°C) rated.
Electrical Grade PET Film
Some films are made specifically for electrical use, with tighter control of thickness, dielectric properties and cleanliness than packaging film. See our PET polyester films.
Metallized Polyester Film
Film coated with a very thin layer of aluminium or zinc, used mainly in capacitors. It is a specialised product, covered in our guide to metallized polyester film for capacitors.
Solar Backsheet (PV) Film
Weather-resistant polyester films form part of the backsheet of solar panels, where they insulate and protect the cells. See our solar back films.
Where BOPET Film Is Used in Electrical Work
- layer and interlayer insulation in small and dry-type transformers
- phase separators and wedges in small motors
- the middle layer of flexible laminates such as DMD insulation paper and other laminated insulation paper composites
- wrapping cables and insulating busbars
- insulating washers, barriers and die-cut parts
- capacitor dielectric, as plain or metallized film
For the wider list of uses beyond electrical work, see our post on polyester film uses. For how film fits alongside papers and pressboard, read our guide to transformer insulation materials.
BOPET Film vs Other Insulating Materials
| Material | Typical thermal class | Strengths | Typical uses |
| BOPET polyester film | E and B (about 120 to 130°C) | Strong, high dielectric strength, economical | Layer insulation, laminates, wrapping |
| Polyimide (Kapton) film | H and above | Very high temperature, flame resistant | Class H windings, high-temperature tapes |
| Polypropylene film | Lower than polyester | Very low dielectric loss | AC and high-frequency capacitors |
| Aramid paper | H | High temperature, conformable, absorbs resin | Slot liners, Class H layer insulation |
If your design runs hotter than Class B, move to polyimide film or aramid paper. Our page on DuPont Kapton polyimide film explains the step up.
How to Choose the Right BOPET Film
- Thermal class. Confirm the film rating suits the hot-spot temperature of the winding, not just its average temperature.
- Thickness. Choose it for both voltage and handling. Thinner film saves space, while thicker film is stiffer and works better for barriers. Electrical films commonly run from a few tens of microns to a few hundred microns.
- Colour. Milky white helps visual inspection. Transparent suits general use and laminating.
- Surface. If the film will be laminated, printed or bonded, ask for a treated surface that accepts adhesives.
- Width and roll size. Film slit to your exact width cuts waste on the winding line. Harnawa offers slitting services for this.
- Datasheet and consistency. Ask for dielectric strength, thickness tolerance and thermal rating, and check that they hold from batch to batch.
Choosing With Confidence
BOPET polyester film is the workhorse insulating film for Class E and B equipment: strong, thin and electrically reliable. Choose the grade by thermal class, thickness, colour and surface, and move to polyimide or aramid when temperatures climb.
Harnawa supplies a full range of polyester films for insulation. Contact us with your thickness, width and application.
Frequently Asked Questions
What is BOPET polyester film?
BOPET is biaxially oriented polyethylene terephthalate film. Polyester is stretched in two directions to give it strength, flatness and stability. It is widely used for electrical insulation, capacitors and packaging.
Is BOPET the same as PET film?
In the trade, PET film and polyester film usually refer to BOPET film. Mylar is a brand name for the same type of material.
What is the temperature rating of polyester film used for insulation?
Electrical polyester film is generally used in Class E and Class B systems, around 120 to 130°C. For higher temperatures, polyimide film or aramid paper is used.
What is the difference between transparent and milky white polyester film?
Both are polyester film. Milky white film is opaque, which makes it easier to see and inspect on the winding line. Transparent film suits general use and laminating.
Where is BOPET film used in transformers?
It is used for layer and interlayer insulation, wrapping and barriers, and as the film layer inside laminates such as DMD insulation paper.
Which is better for high temperature, polyester or polyimide film?
Polyimide film. Polyester film suits Class E and B, while polyimide film handles Class H and higher temperatures.
