What is Mylar? A Comprehensive Technical Guide to BoPET Film for Engineers

The term "Mylar" often conjures images of shiny party balloons or protective food packaging. However, for engineers and technical professionals, particularly in the digital design, hardware, and electronics fields, Mylar® represents a high-performance material with a unique set of properties. Understanding the technical nuances of what Mylar is—beyond the common brand recognition—is essential for leveraging its capabilities in demanding applications.

Mylar® is, in fact, a well-known brand name for a specific type of polyester film chemically known as Biaxially-oriented Polyethylene Terephthalate, or BoPET. This film, derived from polyethylene terephthalate (PET) resin, possesses a remarkable combination of high tensile strength, excellent dimensional and chemical stability, transparency, and outstanding electrical insulation properties. These characteristics are largely imparted by the biaxial orientation process during manufacturing.

This article aims to provide engineers, designers, and students with a comprehensive technical understanding of Mylar/BoPET film. It moves beyond surface-level descriptions to delve into the material science, manufacturing principles, detailed property profiles, and practical implementation considerations relevant to electrical and electronic engineering.

Defining Mylar: Beyond the Brand Name

The material commonly referred to as Mylar® is chemically identified as Polyethylene Terephthalate (PET) that has undergone a specific orientation process. PET itself is a thermoplastic polymer belonging to the polyester family. It is synthesized through the polymerization of either terephthalic acid or dimethyl terephthalate with ethylene glycol. The resulting polymer chains form the basis of the film.

The term BoPET stands for Biaxially-oriented Polyethylene Terephthalate.[1] This designation highlights the critical manufacturing step that distinguishes these films. "Biaxially-oriented" signifies that the PET film has been stretched in two perpendicular directions: the machine direction (MD) and the transverse direction (TD). This process aligns the long polymer chains predominantly within the plane of the film, significantly enhancing its mechanical properties like tensile strength and stiffness compared to unoriented or uniaxially oriented PET.

Key Manufacturers and Brand History

The development of BoPET film dates back to the mid-1950s, with pioneering work done independently by DuPont, Imperial Chemical Industries (ICI) in the UK, and Hoechst in Germany.[2] DuPont introduced its BoPET film under the brand name Mylar®, which quickly gained prominence.

The Mylar® trademark was originally held by DuPont. Later, it became associated with DuPont Teijin Films (DTF), a global joint venture formed in 2000 between DuPont and Teijin Limited. In a significant rebranding initiative effective February 2024, DuPont Teijin Films changed its name to Mylar Specialty Films. Mylar Specialty Films continues to produce Mylar® PET films alongside other well-known brands like Melinex® PET and Kaladex® PEN films.

Due to Mylar's early market presence and success, the name has become somewhat genericized, often used colloquially to refer to any shiny plastic film. However, several companies manufacture BoPET films under various trade names. The table below shows some of these manufacturers and products.

The Manufacturing Process: Engineering BoPET Properties

BoPET is manufactured in large quantities for applications like packaging

The exceptional balance of properties exhibited by Mylar® and other BoPET films is a direct consequence of the sophisticated manufacturing process employed. This process transforms amorphous PET resin into a highly oriented, semi-crystalline film through carefully controlled steps, primarily extrusion, biaxial orientation, and heat setting.

From Resin to Amorphous Film

The journey begins with polyethylene terephthalate (PET) resin, typically supplied in pellet form. This resin is the product of polymerizing ethylene glycol and terephthalic acid (or its dimethyl ester). Before processing, the PET resin must be thoroughly dried to a very low moisture content because PET is susceptible to hydrolytic degradation at melt processing temperatures, which can reduce polymer chain length and compromise final film properties.

The dried PET pellets are fed into an extruder, where they are melted and homogenized under heat (e.g., 265–285°C). The molten polymer is then forced through a flat extrusion die, emerging as a thick, uniform sheet. This molten sheet is immediately cast onto a highly polished, internally cooled chill roll. The rapid cooling (quenching) freezes the polymer chains in a disordered, amorphous state before significant crystallization can occur, resulting in a clear, amorphous PET film.

Recommended reading: Extruding Plastic: Mastering Advanced Techniques and Innovations

Biaxial Orientation: The Key Step

The amorphous PET film then undergoes the critical biaxial orientation process, which aligns the polymer chains and imparts the film's characteristic strength and stability. The most common method is the sequential process:

1. Machine Direction (MD) Orientation: The film is passed through a series of rollers rotating at progressively increasing speeds. The speed differential between consecutive rollers stretches the film longitudinally, along the direction of travel. This stretching typically occurs over heated rollers to bring the film above its glass transition temperature, allowing the polymer chains to move and align. Typical MD stretch ratios are around three to four times the original length.

2. Transverse Direction (TD) Orientation: After MD stretching, the film enters a tenter frame. Here, the edges of the film are gripped by clips mounted on parallel chains. As the film moves through a heated oven within the tenter frame, the chains diverge, stretching the film sideways, perpendicular to the machine direction. Again, heating facilitates the chain alignment. Typical TD stretch ratios are also in the range of three to four times the original width.

This two-way stretching process forces the long, spaghetti-like PET molecules to align predominantly parallel to the plane of the film. This molecular orientation is the fundamental source of BoPET's high tensile strength and stiffness.

Heat Setting and Stabilization

Immediately following TD stretching, while still held under tension by the tenter frame clips, the film passes through a final oven section for heat setting (also called annealing or crystallization). The film is heated to temperatures significantly above the stretching temperature, typically exceeding 200°C.

This crucial step serves multiple purposes:

• Induces Crystallinity: The high temperature allows the oriented polymer chains to pack into ordered crystalline structures, locking the orientation in place.

• Relieves Internal Stresses: It relaxes stresses built up during the stretching process.

• Ensures Dimensional Stability: The heat setting prevents the film from shrinking back towards its original, unstretched dimensions unless exposed to temperatures near or above the heat-set temperature.

A fascinating consequence of this process is that although the film becomes semi-crystalline (typically around 50% crystallinity), it retains excellent transparency. This occurs because the orientation process induces the formation of many small crystal nuclei, which grow rapidly but are constrained by their neighbors, remaining smaller than the wavelength of visible light. Researchers have found that the addition of amide units can further improve transparency and other characteristics.[3]

Surface Properties and Additives

Unmodified BoPET film has an extremely smooth surface. If wound into a roll, these smooth surfaces can adhere strongly to each other (a phenomenon known as blocking), similar to stacked glass plates, making handling difficult. To counteract this, manufacturers often incorporate microscopic, inert inorganic particles, such as silicon dioxide (silica), into the PET resin before extrusion. These particles create a slight surface roughness, preventing blocking and improving the film's handling characteristics (slip).[4]

Beyond these intrinsic modifications, BoPET films are often subjected to further offline or inline treatments to enhance specific properties for particular applications. These include surface coatings to improve adhesion for printing or metallization, metallization itself for barrier and reflectivity, or the application of heat-sealable layers. The ability to tailor the surface and bulk properties through manufacturing and post-processing adds significantly to BoPET's versatility.

Recommended reading: PETG vs PLA: Differences and Comparison

Key Properties of Mylar/BoPET Film

The thermal and mechanical properties of Mylar make it desirable for a variety of applications

Mylar® and other BoPET films offer a distinctive and advantageous combination of physical, chemical, thermal, and optical properties, making them suitable for a vast array of demanding engineering applications. This balance of properties stems directly from the polyethylene terephthalate chemistry and the biaxial orientation process.

Mechanical Properties

• Tensile Strength: Typically 150–215 MPa, much higher than most polymer films. Some grades like Mylar® S1 reach 200 MPa.

• Dimensional Stability: Low shrinkage (~1–3% MD, 0–2% TD at 150°C) and low thermal expansion (~1.7 × 10⁻⁵ in/in/°C), important for precision applications.

• Toughness & Durability: Resistant to tearing, punctures, and aging without brittleness.

• Flexibility: Strong yet flexible, suitable for wrapping, insulation, and circuits.

Electrical Properties

• Dielectric Strength: High, typically 7,000 V/mil for thin films; decreases with thickness.

• Dielectric Constant: Stable around 3.0–3.3 at 1 kHz; rises slightly with temperature.

• Dissipation Factor: Low (0.002–0.005), supporting high AC efficiency.

• Volume Resistivity: Extremely high (~10¹⁸ ohm·cm), though reduced at high temps.

• Corona Resistance: Strong resistance to degradation from high-voltage discharges.

Thermal Properties

• Operating Range: Typically -70°C to 150°C, potentially wider in less demanding uses.

• Melting Point: 250–260°C.

• Thermal Rating: Suitable for Class B (130°C) and some Class F (155°C) insulation systems.

• Thermal Conductivity: Low (~0.15 W/m·K), useful for insulation and radiant barriers.

Chemical Properties

• General Resistance: Withstands moisture, oils, most solvents, dilute acids, and weak bases. Moisture absorption is low (~0.3–0.5%).

• Chemical Weaknesses: Degraded by strong alkalis, hot acids, and certain solvents (e.g., cresol). Susceptible to hydrolysis at high heat.

• Barrier Performance: Good gas and aroma barrier; moderate water vapor resistance. Metallization improves both significantly.

Optical Properties

• Transparency: Highly clear in thin grades (light transmittance >90%). Thicker films may appear hazy.

• Reflectivity: When metallized (e.g., aluminum), becomes highly reflective (up to 99%), useful in insulation and aesthetics.

• UV Resistance: Natural UV filtering and good weatherability. Some grades offer enhanced UV protection.

Types, Grades, and Treatments of Mylar/BoPET Film

The term Mylar or BoPET more broadly does not refer to a single, uniform material but rather a wide family of films engineered for diverse industrial and consumer needs. At its core, BoPET is made by stretching polyethylene terephthalate in two directions for enhanced strength and dimensional stability. However, the base polymer can be modified with different additives to enhance specific traits like thermal resistance, flexibility, or clarity, resulting in a range of formulations suited to different environments and stresses.

In addition to variations in base resin chemistry, BoPET films are available in a wide range of thicknesses—from as thin as 2.5 microns to over 250 microns—depending on whether the application requires lightweight wrapping or robust insulation. Surface treatments further diversify the product family. These can include chemical coatings for print adhesion, corona or plasma treatments to improve bonding with adhesives, or anti-static coatings for electronics applications. Metallization, typically with aluminum, transforms the film into a reflective, high-barrier material for packaging, insulation, or optical uses.

As a result, manufacturers categorize Mylar/BoPET films by type and grade to reflect these differences. For example, Mylar® A is a general-purpose, untreated clear film, while Mylar® ST is heat-stabilized for thermal applications, and Mylar® EM is metallized for barrier performance. Specialty grades like Mylar® D offer a matte surface ideal for printing, and others may include flame-retardant or UV-resistant variants. This wide array of configurations enables BoPET films to serve roles in electronics, aerospace, flexible packaging, imaging, and industrial insulation—all under the same material family name.

Mylar/BoPET Film Applications

Applications of Mylar/BoPET span several industries, from food packaging to aerospace

BoPET (biaxially-oriented polyethylene terephthalate) film is used across a wide range of industries due to its high strength, dimensional stability, electrical insulation, and barrier properties. Common applications include flexible food packaging, reflective insulation, printed electronics, solar and optical films, capacitor dielectrics, labels, and protective coatings. It is also used in imaging media, medical packaging, aerospace insulation, and as a durable backing in adhesive tapes and graphic overlays.

BoPET Limitations and Alternatives

Despite its wide utility, BoPET film has several limitations that can restrict its use in certain applications. One of the primary drawbacks is its sensitivity to strong alkalis and hydrolysis at elevated temperatures, which can degrade the polymer over time in harsh chemical or humid environments. While it offers good thermal resistance, it typically cannot withstand continuous exposure above 150°C, limiting its role in high-heat industrial or automotive settings. Additionally, BoPET is inherently non-biodegradable and derived from petroleum-based sources, raising environmental concerns, especially in single-use packaging where recycling infrastructure for multilayer films is limited.

To address these limitations, several alternatives are used depending on the application. For higher temperature resistance, polyimide films (like Kapton®) offer exceptional thermal stability up to 400°C and better chemical resistance. In environmentally sensitive applications, PLA-based films or cellulose-based films offer compostability and renewability, though they typically have inferior barrier and mechanical properties. For more flexible packaging needs with better sealability, BOPP (biaxially-oriented polypropylene) is a common alternative, albeit with lower tensile strength and dimensional stability than BoPET.

Conclusion

BoPET film stands out for its exceptional balance of mechanical strength, dimensional stability, thermal resistance, and electrical insulation, making it a highly versatile material across industries. Its applications range from food packaging and electrical insulation to aerospace reflectors and graphic media, driven by its durability, clarity, and barrier performance. Although not without limitations—particularly in high-alkali, extreme-heat, or sustainability-sensitive environments—BoPET remains an important material in both everyday and high-tech uses due to its adaptability and wide range of available grades and treatments.

Frequently Asked Questions

What is BoPET film?

BoPET (biaxially-oriented polyethylene terephthalate) is a strong, transparent plastic film created by stretching polyethylene terephthalate in two directions. This process gives it enhanced mechanical strength, dimensional stability, and resistance to heat, making it suitable for a wide range of applications.

What is Mylar?

Mylar is a brand name for BoPET film, originally conceived by DuPont and now manufactured by Mylar Specialty Films.

What are the common uses of BoPET film?

BoPET film is used in various industries, including packaging (especially for food products), electrical insulation, solar energy applications, and aerospace. It is valued for its durability, clarity, moisture resistance, and ability to serve as a barrier material.

Is BoPET film heat resistant?

Yes, BoPET film can withstand temperatures up to around 150°C (302°F) without significant degradation, making it suitable for applications involving moderate heat. However, it is not designed to endure extremely high temperatures for extended periods.

Is BoPET eco-friendly?

While BoPET is recyclable in certain regions, it is not biodegradable, which raises environmental concerns, particularly in packaging applications. Recycling can be challenging when the film is used in multi-layered products, leading some to explore more sustainable alternatives such as PLA-based films.

References

[1] Stepancikova R, Olejnik R, Matyas J, Masar M, Hausnerova B, Slobodian P. Pressure-Driven Piezoelectric Sensors and Energy Harvesting in Biaxially Oriented Polyethylene Terephthalate Film. Sensors. 2024 Feb 17;24(4):1275.

[2] Rappoport Z, editor. The chemistry of anilines, Part 1. John Wiley & Sons; 2007 Mar 13.

[3] Gao H, Cao W, He J, Bai Y. Highly transparent biaxially oriented poly (ester amide) film with improved gas barrier properties and good mechanical strength. European Polymer Journal. 2021 Aug 5;156:110620.

[4] DeMeuse MT. Other polymers used for biaxial films. InBiaxial Stretching of Film 2011 Jan 1 (pp. 47-58). Woodhead Publishing.