What Is PET Film? Its Properties, Manufacturing, and Uses

PET film is a thin, flexible sheet made from polyethylene terephthalate, a semi-crystalline polyester polymer built from repeating units of ethylene glycol and terephthalic acid. It is one of the most widely used polymer films in the world, showing up in everything from food packaging and solar panels to archival document sleeves and flexible electronics. What makes it so popular is a combination of mechanical toughness, optical clarity, chemical resistance, and thermal stability that few other plastic films can match at a comparable cost.

What PET Actually Is

Polyethylene terephthalate belongs to the polyester family. Its backbone is a chain of ester linkages formed by reacting a diacid (terephthalic acid) with a diol (ethylene glycol). The chemistry of synthesizing this polymer involves a network of main reactions that drive productivity and side reactions that influence the quality of the final product.¹ When people talk about “PET,” they could mean the resin pellets used to blow bottles, the fibers spun into polyester fabric, or the flat film form. This article focuses on the film, which is produced by melting PET resin and stretching it into thin sheets, typically ranging from a few micrometers to a few hundred micrometers thick.

PET film goes by several trade names you may have encountered. Mylar (originally a DuPont brand), Melinex, and Hostaphan are all PET film products. Despite different branding, they share the same base polymer. The differences between them come down to thickness, surface coatings, and how aggressively the film was stretched and heat-treated during production.

How PET Film Is Made

PET film production starts with melting resin pellets and extruding the molten polymer through a flat die to form a thick, amorphous sheet. That sheet is then cooled rapidly on a chilled drum. At this stage, the material is relatively soft, hazy, and mechanically weak because the polymer chains are randomly tangled with little crystalline order.

The transformation into a useful film happens during biaxial stretching, where the sheet is pulled in two perpendicular directions. First it is stretched lengthwise (the “machine direction”), then widthwise (the “transverse direction”). During this process, the polymer chains align and begin to crystallize, and the film develops its characteristic strength and clarity. Research into this stretching process has identified distinct stages: an initial phase where stress and the optical properties of the film track together linearly, followed by a phase where chains become taut and develop a more ordered, liquid-crystal-like alignment, and finally a phase where the chains reach their maximum stretch.² The result is a biaxially oriented PET film (sometimes abbreviated BOPET) with balanced mechanical properties in both directions.

After stretching, the film is typically “heat set,” meaning it is held at elevated temperature under tension for a short time. This locks in the crystalline structure and improves the film’s dimensional stability, so it does not shrink dramatically when exposed to heat later in its life. The final film is wound onto large rolls, sometimes hundreds of meters long, for slitting and conversion into finished products.

Surface Treatments and Why They Matter

One quirk of PET film is that its surface is naturally quite smooth and chemically inert, which sounds like a good thing until you try to print on it, glue it to something, or coat it with a functional layer. Inks bead up, adhesives peel off, and coatings refuse to stick. To solve this, manufacturers routinely treat the surface before further processing.

The most common treatment is corona discharge, where the film passes through a high-voltage electrical field that bombards the surface with ions. This creates tiny physical bumps and cavities and introduces oxygen-containing chemical groups (like hydroxyl groups) onto the surface. Classic research on this showed that even modest increases in the density of these functional groups lead to a sharp jump in how easily the surface can be wetted by liquids.³ In practical terms, corona treatment makes the film receptive to inks, adhesives, and coatings.

The effect is not permanent, though. Surface energy decreases over time after treatment as the polar groups rearrange and migrate below the surface. One study comparing corona-treated films found that PET held up better than low-density polyethylene after treatment, but the polar component of surface energy still declined with storage time across all materials tested. In lamination trials, biaxially oriented PET showed such strong bond strength after corona treatment that samples tore before delaminating.⁴ Corona treatment has also been shown to improve how well carbon nanotube electrodes adhere to PET substrates, which matters for flexible electronics applications.⁵

Beyond corona, manufacturers use other surface modifications depending on the end use. Slip and antiblock additives are incorporated to keep sheets from sticking to processing rollers, improve stacking, and aid mold release during fabrication.⁶ Metallization, where a thin layer of aluminum or another metal is deposited on the film by vacuum evaporation, is another common treatment used to boost barrier properties or create a reflective surface.

Thermal and Mechanical Properties

PET film sits in an interesting middle ground among polymer films: tougher and more heat-resistant than commodity plastics like polyethylene, but easier and cheaper to process than high-performance engineering polymers like polyimide. A few thermal landmarks define its behavior.

The glass transition temperature is the point where the amorphous (non-crystalline) regions of the polymer shift from rigid and glassy to rubbery and mobile. For standard amorphous PET, this occurs around 78°C, though the exact number varies with processing history and any coatings present.⁷ Commercial PET film, which has been stretched and heat-set, tends to show a somewhat higher glass transition because of its increased crystallinity and chain orientation. Metallizing the film pushes it higher still: aluminum-coated and lead-coated PET films have been measured with glass transitions near 111–112°C, compared with about 90°C for the uncoated commercial film used as the starting material.⁸

Above the glass transition, if you keep heating amorphous PET, cold crystallization kicks in around 115°C. This is where polymer chains that were frozen in a disordered state during rapid cooling gain enough mobility to organize into crystals. Finally, melting occurs in the range of roughly 225–255°C.⁷ This high melting range is one reason PET film tolerates temperatures that would ruin many other plastic films.

Mechanically, biaxially stretched PET film has high tensile strength, good stiffness, and excellent resistance to creep (slow deformation under sustained load). When compared with biaxially oriented polypropylene (BOPP), PET consistently shows superior thermal resistance and slower creep under various temperatures and loads.⁹ That dimensional stability is critical for applications like printed electronics, where even small amounts of stretching or warping during processing would ruin a circuit.

Optical Properties

Unstretched PET film is largely transparent and amorphous, but biaxial stretching introduces optical anisotropy, meaning light travels through the film at slightly different speeds depending on its direction. This shows up as birefringence, which can be measured using techniques like ellipsometry and channeled spectra. Research has found that the birefringence of PET film decreases as the wavelength of light increases across the visible spectrum.¹⁰

For most everyday uses, the birefringence is not noticeable. But in optical applications like display films, sensor windows, or polarizer components, it matters a great deal. Manufacturers can control the degree of orientation during stretching to tune the film’s optical behavior for specific end uses. For packaging and general-purpose applications, PET film appears clear and glossy, with high light transmission.

Where PET Film Shows Up

The range of applications is remarkably broad, and it keeps expanding as engineers find new ways to exploit the film’s combination of properties.

Packaging is the single largest market. PET is one of the most widely used polymers in the packaging industry, valued for its clarity, mechanical strength, and reasonable gas barrier properties.¹¹ Metallized PET film, with a thin vacuum-deposited aluminum layer, serves as a barrier layer in snack food bags, coffee pouches, and other flexible packaging where keeping oxygen and moisture out is essential. The structural features of the aluminum coating, including thickness and continuity, directly influence the water and oxygen barrier performance of the finished laminate.¹²

Multilayer laminates often pair PET with polyethylene or polypropylene. The PET layer provides stiffness, printability, and barrier performance, while the polyethylene layer provides a heat-sealable surface. Aging studies have shown that PET holds up well over time, with only modest decreases in mechanical properties attributed to the polymer’s inherent UV stability.¹³ When exposed to gamma irradiation (used for sterilizing packaged foods and medical supplies), PET-based laminates proved more resistant than BOPP-based alternatives, with barrier properties actually improving slightly at moderate radiation doses.¹⁴

Solar energy is another significant application. Transparent PET films, typically around 125 micrometers thick, are used as backsheet materials in photovoltaic modules, where they protect the solar cells from moisture and mechanical damage while remaining lightweight and flexible.¹⁵ These films face decades of outdoor exposure, so understanding their long-term degradation under UV light and chemical attack is an active area of research.

In archival and museum conservation, PET film goes by names like Melinex and is prized for being transparent, strong, chemically inert, and exceptionally durable. The Canadian Conservation Institute recommends polyester film for encapsulating fragile documents and manuscripts, noting that 3-mil thickness works for most documents while 7-mil is better for heavier items like large maps.¹⁶ Unlike many other plastics, PET does not off-gas harmful chemicals that could damage paper or photographs over time.

Flexible electronics represent a growing frontier. Roll-to-roll printing of circuits, sensors, and displays onto PET substrates is attractive because the film is cheap, dimensionally stable, and can tolerate moderate processing temperatures. The PET substrate’s superior dimensional stability compared with BOPP makes it the preferred choice when registration accuracy between printed layers matters.⁹

Antimony Migration and Food Safety

PET is generally considered safe for food contact, but it is not entirely inert. The most scrutinized issue is antimony, which enters PET because antimony trioxide is the standard catalyst used during the polycondensation step of manufacturing. Trace amounts of antimony remain in the finished polymer and can migrate into food or beverages under certain conditions.

For bottled water stored at room temperature, antimony concentrations are generally low. A systematic review spanning 13 studies found that antimony levels in bottled water were typically below 5 micrograms per liter under ambient storage conditions, but reached up to about 18.5 micrograms per liter when bottles were exposed to high temperatures or acidic conditions.¹⁷ Those stress conditions include leaving bottles in a hot car or in direct sunlight for extended periods.

The concern is more pronounced for PET trays used in ready-to-eat meals that are heated in the tray. One study found that cooking in conventional ovens caused antimony migration of 8–38 micrograms per kilogram into the food, while microwave heating produced smaller increases of 0–17 micrograms per kilogram. Roughly half the products heated to 180°C exceeded the specific migration limit set by the European Commission for food contact materials.¹⁸ The practical takeaway: PET containers at room temperature pose minimal migration risk, but heating PET packaging, especially in a conventional oven, significantly increases the amount of antimony that transfers into food.

Enzymatic Recycling and Biological Degradation

PET film and PET in general are recyclable through mechanical means (grinding, washing, re-melting), but the recycled material loses quality with each pass. This has driven intense interest in enzymatic recycling, where specialized enzymes break PET all the way back down to its original monomer building blocks, which can then be repolymerized into virgin-quality material.

The enzymes that degrade PET work by cleaving the ester bonds in the polymer backbone. Microorganisms secrete extracellular PET-degrading enzymes that break the polymer into intermediate compounds, which are then further broken down into terephthalic acid and ethylene glycol, the same monomers used to make PET in the first place.¹⁹ The discovery of the bacterium Ideonella sakaiensis in 2016, which evolved the ability to use PET as a food source, was a landmark moment. Since then, researchers have been discovering PET-degrading enzymes in increasingly unexpected places.

A striking recent example came from deep-sea microorganisms. Researchers identified the first archaeal PETase, dubbed GuaPA, from an uncultured organism found in deep ocean sediments. Within 48 hours, GuaPA released roughly 3–5 millimolar concentrations of terephthalic acid and its intermediates from low-crystallinity PET. When combined with a newly discovered companion enzyme (a BHETase), the hydrolysis of untreated PET film improved by 68%.²⁰ The fact that deep-sea organisms appear to have evolved enzymes for breaking down a material that has existed for less than a century suggests that plastic pollution has already begun shaping microbial evolution in remote ecosystems.

Enzymatic recycling faces real challenges before it can work at industrial scale. The enzymes generally work faster on amorphous PET than on the highly crystalline material found in stretched films and bottles. Reaction temperatures, enzyme stability, and the cost of purifying recovered monomers all remain active research problems. Still, the field has moved remarkably fast: several companies are now operating pilot plants for enzymatic PET recycling, and the overall approach has attracted significant investment.²¹

Emerging Alternatives to PET Film

Despite PET’s dominance, researchers and manufacturers are exploring alternatives driven by sustainability concerns and performance needs. Polyethylene furanoate (PEF) is one of the most discussed. Made from biobased resources rather than petrochemicals, PEF shares PET’s polyester chemistry but uses furandicarboxylic acid instead of terephthalic acid. PEF offers improved gas barrier properties compared with PET, which could allow thinner films for the same level of protection. Both PEF and another polyester called Tritan are being positioned as alternatives to PET for food packaging applications.²²

PEF has been in development for over a decade, and commercial-scale production is only now beginning to materialize. Cost remains the biggest hurdle: producing furandicarboxylic acid at the volumes and prices needed to compete with terephthalic acid is difficult. Whether PEF displaces PET in significant volumes will depend on whether the biobased premium shrinks as production scales up, and whether regulatory or consumer pressure for non-fossil-derived plastics continues to grow. For now, PET film remains the default choice for most applications, and its established recycling infrastructure gives it an advantage that new materials lack.