Clear plastic film on a roll can be one of several materials, and the two you will meet most often are polyester (PET) and polypropylene (BOPP). They look similar, both are made by stretching the film in two directions, and both are used in packaging, labels and industry. But they behave very differently once they are printed, heated, sealed or put under load, and choosing the wrong one is an expensive way to find out. Polyester in particular turns up in far more places than most people realise, as our guide to polyester film in everyday life shows.
PSG works with both. Our Polymex® range includes a polyester and a polypropylene grade, each with its own published data sheet, and we supply and convert both films. This guide uses that experience to set out where each one wins.
Two films, two polymers
Polyester film is made from polyethylene terephthalate, which is why it is called PET film, or BOPET when it is biaxially oriented. Polypropylene film is made from polypropylene and, when it is stretched in both directions, is called BOPP or OPP. The orientation gives both films their strength and clarity, but the polymers themselves set the limits: how hot the film can get, how stiff it is, what it keeps in or out, and how much film you get for each kilogram you buy.
Polyester and polypropylene film compared
The table summarises the differences that matter most when you specify a film. Where a value is given, it comes from PSG's published Polymex® data sheets: Polymex® A for polyester and Polymex® B for polypropylene.
| Property | Polyester (PET) | Polypropylene (BOPP) |
|---|---|---|
| Density | 1.40 (Polymex® A) | 0.91 to 0.95 (Polymex® B) |
| Heat | Melting point 265°C; service temperature minus 70°C to 130°C | Softening point 144°C |
| Stiffness and dimensional stability | Very stiff and stable, holds size through heat and print | More flexible, more stretch under heat and tension |
| Barrier | Better barrier to oxygen, aromas and grease | Better barrier to water vapour |
| Oven use | Ovenable in suitable grades | Not ovenable |
| Recycled content | Available with recycled content | No recycled-content option in our packaging range |
| Typical cost per square metre | Higher | Lower, helped by its lower density |
Where polyester film wins
Heat and dimensional stability
Polyester's melting point of 265°C, against a softening point of 144°C for polypropylene, is the single biggest difference between the two. PET keeps its strength and shape through drying ovens, hot lamination, sealing jaws and laser printers, and in suitable grades it can go into a conventional oven. That stability is why printers use it for multi-colour work that must stay in register, and why engineers use it for parts that must hold a tolerance as the temperature changes.
Strength and stiffness in thin gauges
PET is stiffer and stronger than BOPP at the same thickness, so a thinner polyester film can often do the job of a heavier polypropylene one. Windows stay flat, lids stay taut and die-cut parts keep their shape.
Electrical insulation
Polyester's combination of high dielectric strength, toughness and temperature range has made it a standard material for motor slot liners, transformer and coil insulation, cable wrap and membrane switches. Our guide to polyester film for electrical insulation covers the grades and data in detail.
Oxygen and aroma barrier, and recycled content
For packaging that must keep oxygen out or aromas in, PET is the better barrier. It is also one of the most widely recycled plastics, and our carton window films are available with 30%, 60% or up to 90% post-consumer recycled content.
Where polypropylene film wins
Yield and cost
Polypropylene is much lighter. With a specific gravity of 0.91 to 0.95 against 1.40 for polyester, a kilogram of BOPP covers roughly half as much area again as a kilogram of PET at the same thickness. Our polyester film thickness and yield guide shows how to work out the area per kilogram for any thickness, and the film calculator works it out for either film. Film is usually bought by weight and used by area, so for jobs where polyester's extra performance is not needed, polypropylene is usually the lower-cost choice.
Moisture barrier and chemical resistance
Polypropylene is a better barrier to water vapour, which suits dry foods and products that must not take up moisture. It is also highly resistant to many chemicals, and absorbs almost no water.
Thicker, lighter parts
The same properties make polypropylene a good material for thicker components where light weight and chemical resistance matter more than heat. In our Polymex® colour-coded shimstock, polyester covers the thinner gauges, from 12 to 350 microns, and polypropylene the thicker ones, from 0.375 mm to 3.000 mm.
Where either film will do
For many packaging jobs both films are suitable. In our carton window and packaging range, polyester and BOPP are both food approved, recyclable, high clarity, freezer safe and microwaveable, and both are available with anti-fog and heat-sealable grades. The choice then comes down to the machine, the pack and the price, and to whether the pack needs to go in an oven or contain recycled content, which point to polyester.
Both films also need a receptive surface for printing and lamination. Corona treatment raises the surface energy of either film, and polyester is also widely available with chemically treated surfaces for demanding inks, coatings and adhesives. Our guide to polyester film grades, finishes and treatments explains the options.
A quick way to decide
Choose polyester (PET) film when the job involves any of these:
- temperatures above about 100°C in processing or use, or an ovenable pack;
- printing or lamination that must stay in register, or parts that must hold a tolerance;
- electrical insulation;
- an oxygen or aroma barrier;
- recycled content in the film itself.
Choose polypropylene (BOPP) film when the job values:
- the lowest cost per square metre, and polyester's extra performance is not needed;
- a moisture barrier for dry products;
- light weight in thicker parts, with good chemical resistance.
If the answer is still not clear, the application usually decides it. Tell us what the film has to do, what it meets in your process and how it is packed, and we will recommend a material and grade.
From the right film to the finished format
Whichever film you choose, we can supply it ready for your line. Our slitting and sheeting service converts film to the reel width, core size, sheet size or die-cut part you need, from your material or ours. For an overview of everything we do with PET, start with our guide to polyester (PET) film supply and converting.
Need a quotation or a recommendation? Request a quote from our technical team with the application, thickness, finish and format, and we will come back to you with the right film for the job.
