TL;DR:
- PET (polyester) film — best known by the trade name Mylar® — delivers dielectric strength of 150–300 kV/mm at roughly one-eighth the cost of aramid paper.
- Standard PET film is rated IEC 60085 Class E (120 °C), and heat-stabilized electrical grades qualify for Class B (130 °C) service.
- It is the most widely used slot liner, slot wedge and phase insulation material in Class B fractional-horsepower and general-purpose motors.
- For Class F (155 °C) and Class H (180 °C) windings, PET film is laminated with Nomex® or polyester fleece into NMN and DMD composites.
- Typical slot liner thicknesses run 0.15–0.35 mm, slit to width in rolls or supplied as formed cuffed liners.
PET film insulation is biaxially oriented polyethylene terephthalate (BOPET) film — sold under trade names such as Mylar® — used as slot liners, phase insulation and layer insulation in electric motors and transformers because it combines high dielectric strength, good mechanical toughness and very low cost. This article explores what PET polyester film is and why it dominates electrical insulation, the thermal class PET film actually achieves, and how to apply and specify it for motor slot liners, giving purchasing engineers a clear basis for choosing between PET, Nomex and NMN.
What Is PET (Polyester) Film and Why Is It Used in Electrical Insulation?
PET film is a thermoplastic polyester film produced by extruding polyethylene terephthalate resin and stretching it in both the machine and transverse directions. This biaxial orientation aligns the polymer chains, which is what gives electrical-grade PET film its unusual combination of tensile strength (typically 190–220 MPa), tear resistance and dimensional stability. Mylar® is simply DuPont Teijin Films’ registered trade name for its BOPET film; in insulation catalogs, “polyester film,” “PET film” and “Mylar” describe the same class of material.
Electrically, PET film is outstanding for its price. Thin electrical grades reach dielectric strengths of 150–300 kV/mm depending on thickness, with a dielectric constant of about 3.2 at 1 kHz, a dissipation factor near 0.005 and volume resistivity around 1018 Ω·cm. Moisture absorption stays below 0.8%, so properties remain stable in humid workshops. Because PET resin is produced at commodity scale for packaging and fibers, electrical film costs a fraction of engineered papers — the main reason it became the default insulation for mass-produced motors, transformers, capacitors and wire-and-cable wrapping.
Related product: SIDA PET Film (Milky White / Transparent, 6021/6020)
What Thermal Class Does PET Film Achieve?
Under IEC 60085 thermal classification, standard PET film is rated Class E (120 °C), and heat-stabilized electrical grades are widely certified for Class B (130 °C) insulation systems. UL 746B long-term evaluations assign typical relative thermal indices (RTI) of 105–130 °C depending on grade and thickness. The practical meaning: in a motor whose winding hot spot stays at or below 130 °C, a quality PET slot liner will deliver the expected 20,000-hour design life.
The limits matter just as much. PET melts at about 254 °C, but its mechanical properties begin to fall off well before that — the film embrittles under sustained exposure above its thermal class, and hydrolysis accelerates aging in hot, humid, sealed environments. That is why bare PET film should not be specified for Class F (155 °C) or Class H (180 °C) systems. Instead, manufacturers laminate PET with temperature-resistant outer layers: NMN insulation paper (Nomex-PET-Nomex) achieves Class F, while DMD (polyester fleece-PET-fleece) covers Class B/F economy applications.
Related product: SIDA NMN Insulation Paper (Class F)
How Is PET Film Used in Motor Slot Liners and Phase Insulation?
The slot liner is PET film’s signature application. A U-shaped strip of film, typically 0.15–0.35 mm thick, is inserted into each stator slot before winding to insulate the copper coils from the grounded laminated core. PET’s stiffness lets the liner hold its shape during automated coil insertion, its tear strength survives the pull of winding needles, and its smooth surface lets magnet wire slide into place without abrasion. Many liners are supplied with folded-back cuffs at each end to prevent the liner from slipping and to reinforce the edge where the coil exits the slot.
Beyond the slot cell, the same film serves as phase insulation — separators placed between the end-turns of different phase groups — and as slot closure wedges or top sticks that retain the winding inside the slot. In transformers and inductors, PET film appears as layer insulation between winding layers, and slit Mylar tape wraps coils, bundles leads and provides interlayer insulation in small coils. For Class F motors, the identical geometry is executed in F-grade DMD or NMN flexible laminates, so tooling and process remain unchanged when a design is upgraded.
Related product: SIDA Polyester Film Tape (PET/Mylar Tape)
PET vs Nomex vs NMN: Cost & Performance Comparison
Choosing among PET film, Nomex® aramid paper and NMN composite comes down to matching thermal class to real winding temperature and paying no more than necessary. PET film wins every cost comparison — it typically sells for one-eighth to one-tenth the price of Nomex per kilogram — but it is capped at Class B. Nomex 410 is rated for 220 °C continuous service and does not melt, while NMN sandwiches a PET core between thin Nomex layers to deliver Class F performance at a mid-range price, as detailed in our guide to choosing composite insulation paper.
| Property | PET Film | NMN Composite | Nomex® 410 |
|---|---|---|---|
| Thermal class (IEC 60085) | E / B (120–130 °C) | F (155 °C) | C (220 °C RTI) |
| Dielectric strength | 150–300 kV/mm | ~50–70 kV/mm (composite) | ~20–35 kV/mm |
| Mechanical toughness | Excellent stiffness & tear strength | Very good | Good, flexible |
| Moisture absorption | <0.8% | Low | Higher (aramid absorbs moisture) |
| Relative cost | 1× | 3–4× | 8–10× |
| Typical use | Class B slot liners, phase & layer insulation | Class F motor slot liners | Class H traction, aerospace, dry-type transformers |
The practical selection rule: if the winding hot spot is verified below 130 °C, bare PET film is the economical, technically sound choice. Between 130 °C and 155 °C, specify NMN or F-grade DMD. Above 155 °C, move to pure Nomex aramid paper or aramid alternatives — see our analysis of aramid paper at 15–30% lower cost. Over-specifying Nomex for a Class B appliance motor simply multiplies insulation spend by eight with zero reliability benefit.
Related product: SIDA Nomex® Aramid Paper Tapes
How to Specify PET Film Thickness and Width for Your Application
Thickness drives both dielectric margin and slot fill. Electrical-grade PET film is produced from 0.023 mm up to 0.35 mm; motor slot liners most commonly use 0.15, 0.19, 0.25 or 0.35 mm. Thinner film (0.05–0.125 mm) suits phase separators, layer insulation and capacitor applications, while 0.25–0.35 mm gives the stiffness needed for large slots and automated insertion. As a rule of thumb, specify the thinnest film whose breakdown voltage — roughly its thickness multiplied by 150 kV/mm, derated by a 3–4× safety factor — still exceeds your hipot test voltage, then check that the liner survives insertion mechanically.
Width and format depend on your process. Slot liner lines take slit rolls matched to the developed slot perimeter plus cuff allowance — SIDA slits from 6 mm up to full master-roll width (1,000 mm+) with ±0.2 mm tolerance. Transformer layer insulation is normally ordered as wide rolls or cut sheets. Also specify surface finish (plain, or resin-coated for varnish adhesion), color (transparent or milky white 6021), and whether you need pre-formed, cuffed liners rather than flat stock. For prototype quantities, our guide on buying insulation materials in small quantities explains minimum-order options.
Related product: SIDA PET Film — custom slitting and sheeting
FAQ
Is Mylar the same as PET film?
Functionally, yes. Mylar® is the registered trademark of DuPont Teijin Films for its biaxially oriented PET film, in the same way Nomex® is a trademark for aramid paper. Generic electrical-grade PET film from qualified producers meets the same IEC 60674-3-2 and ASTM D5213 specifications and delivers equivalent dielectric and mechanical performance, usually at significantly lower cost. When a drawing calls for “Mylar slot liner,” a certified generic PET film of the same thickness and grade is normally an acceptable substitution — confirm with the insulation system’s UL file if the motor carries UL recognition.
Can PET film alone be used in Class F (155 °C) motors?
No. Bare PET film tops out at Class B (130 °C); sustained operation at 155 °C embrittles the film and drastically shortens winding life. For Class F systems, use NMN (Nomex-PET-Nomex) or F-grade DMD flexible laminates, which keep PET’s mechanical core but protect it with thermally stable outer layers, or step up to pure aramid paper for Class H. The liner geometry, insertion tooling and winding process stay identical, so upgrading thermal class is a material swap, not a redesign.
Summary
PET film insulation remains the workhorse of electrical insulation because nothing else matches its combination of 150–300 kV/mm dielectric strength, mechanical toughness and commodity pricing. Use bare PET film for Class E/B slot liners, phase insulation and layer insulation; move to NMN or DMD composites for Class F; and reserve Nomex for genuine Class H duty. Specify thickness from your hipot requirement and slot fill, and order slit widths matched to your liner tooling.
SIDA supplies electrical-grade PET film (6020 transparent / 6021 milky white) in thicknesses from 0.023 mm to 0.35 mm, custom-slit rolls and sheets, plus NMN, DMD and the full range of flexible insulation laminates — all export-ready with datasheets and third-party test reports. Browse our PET film product page or contact us for same-day quotations.
Contact SIDA for PET film and slot liner materials:
📞 +86-15958243831
📧 jessie.feng@sidanm.com
💬 WhatsApp: https://wa.me/8615958243831
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References
- IEC 60085:2007. Electrical insulation – Thermal evaluation and designation. International Electrotechnical Commission, Geneva. https://webstore.iec.ch/publication/636
- IEC 60674-3-2. Plastic films for electrical purposes – Part 3: Specifications for individual materials – Sheet 2: Requirements for balanced biaxially oriented polyethylene terephthalate (PET) films. International Electrotechnical Commission. https://webstore.iec.ch
- ASTM D5213-12(2017). Standard Specification for Polymeric Resin Film for Electrical Insulation and Dielectric Applications. ASTM International, West Conshohocken, PA. https://www.astm.org/d5213-12r17.html
- UL 746B. Standard for Polymeric Materials – Long Term Property Evaluations. Underwriters Laboratories. https://standardscatalog.ul.com/ProductDetail.aspx?productId=UL746B
- DuPont Teijin Films. Mylar® Polyester Film – Electrical Properties, Product Information. https://www.dupontteijinfilms.com
- Boulter, E. A., & Stone, G. C. (2004). Historical development of rotor and stator winding insulation materials and systems. IEEE Electrical Insulation Magazine, 20(3), 25–39. https://ieeexplore.ieee.org/document/1307083
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