Choosing Between Formex, Nomex, and Mylar – Information and Comparison

Selecting the right electrical insulation material is one of those decisions that looks straightforward on the surface but carries real consequences when equipment runs hot, fails early, or falls short of certification requirements. Formex, Nomex, and Mylar are three of the most widely specified insulation materials in industrial manufacturing, yet each one serves a distinct set of conditions. This comparison covers what each material is, where it performs best, and where its limitations begin, so you can match the right solution to your application.

How Formex, Nomex, and Mylar Are Used in Industry

All three materials are established electrical insulation solutions found across motors, transformers, power electronics, EV battery systems, industrial controls, and consumer electronics. They are not interchangeable, but they often appear together within the same insulation system. The NMN laminate, which combines Nomex outer layers with a Mylar core, is a standard construction in motor slot liners and dry-type transformer windings, where each material contributes a different property to the composite.

Formex is the youngest of the three in commercial terms and has carved out a strong position in electronics enclosures, PCB barriers, LED lighting assemblies, and EV battery packs. Nomex, developed by DuPont in the early 1960s, remains the benchmark for high-temperature motor and transformer insulation. Mylar, a biaxially oriented polyethylene terephthalate film, has been a primary dielectric barrier in electrical insulation systems for over six decades. Understanding where each material sits in this landscape is the starting point for any material comparison.

Side-by-Side Comparison: Formex vs. Nomex vs. Mylar

Formex: Cost-Effective, Flame-Retardant, and Highly Formable

Formex is a proprietary flame-retardant polypropylene thermoplastic manufactured by ITW Formex. Its defining commercial advantage is the combination of UL 94 V-0 flame resistance, strong dielectric strength, and a non-hygroscopic structure, all at a cost point well below aramid-based alternatives. Moisture absorption is below 0.06%, which makes it genuinely suitable for humid or outdoor environments where other insulation materials would require additional protection.

Key Properties

The standard Formex GK grade carries a continuous-use temperature rating of 115°C and a dielectric strength of 2,200 V/mil at 0.005 inch thickness. Formex GL extends that to 125°C, while Formex N3, a halogen-free multi-layer polycarbonate variant, reaches 130°C. A particularly useful characteristic is that Formex can be scored and folded into three-dimensional parts without cracking, a fabrication capability that distinguishes it from traditional insulation papers and films. It is also compatible with die-cutting, heat forming, laser cutting, and waterjet cutting, and parts can be shipped flat to reduce freight costs.

Best Suited For

Formex is the practical choice for PCB barriers, EV battery pack insulation, onboard chargers, power supplies, industrial control panels, LED drivers, and panelboard barriers. For applications requiring electrostatic discharge protection alongside flame resistance, Formex GS adds static-dissipative properties. The Formex TCI grade, introduced in early 2025, adds thermal conductivity of 1 W/(m·K), making it a dual-purpose option as both an electrical insulator and a heat-dissipation layer in power electronics.

Limitations

The ceiling is temperature. Formex is a thermoplastic and will soften and melt above its service rating. At 115 to 130°C depending on grade, it is outperformed by both Mylar and Nomex in high-temperature motor and transformer applications. It is not suitable for Class H insulation systems, and it should not be specified in applications where short-term peak temperatures approach or exceed its rated threshold.

Nomex: The High-Temperature Standard for Motors and Transformers

Nomex is a meta-aramid paper made from aromatic polyamide fibrids and short fibers using a wet papermaking process. It is the material of choice when operating temperatures are high, failure consequences are severe, and long service life is non-negotiable. Its thermal class of 220°C continuous, recognized by multiple international standards bodies including UL, IEC, and NEMA, places it in a category that Formex and standard Mylar cannot reach.

Key Properties

Nomex 410, the most widely used grade in electrical insulation, achieves a dielectric strength above 40 kV/mm and maintains mechanical and electrical integrity up to 220°C continuously, with short-term tolerance to 300°C. Critically, Nomex does not melt. It does not support combustion at its rated operating temperature, and its Limiting Oxygen Index remains above the threshold for combustion in air at 220°C. UL has recognized over 100 insulation systems based on Nomex paper, covering temperatures from 130 to 220°C and voltages up to 34.5 kV.

Best Suited For

Nomex is the specified material for motor slot liners, wedges, and phase insulation in Class H motors and generators, dry-type transformer windings, and EV traction motors where high power density and sustained high temperatures are design requirements. Nomex 910, a cellulose-aramid hybrid, is designed for oil-immersed transformer applications. For aerospace and industrial drives operating at extreme thermal loads, Nomex is effectively the baseline specification. The material is also compatible with automotive transmission fluids, which broadens its applicability in hybrid electric vehicle drivetrains.

Limitations

Cost is the primary constraint. Nomex paper commands a significantly higher price per kilogram than polyester films or polypropylene-based alternatives. Its rigidity in thicker grades can complicate fabrication in transformer geometries with complex shapes, and cutting aramid materials requires specialized equipment. Nomex also absorbs moisture from the environment, which can cause dimensional changes in the cross-machine direction. While largely reversible upon re-drying, this behavior requires controlled storage and handling, particularly in laminating and precision converting operations.

Mylar: Thin, Precise, and Dielectrically Powerful

Mylar is a biaxially oriented polyethylene terephthalate film, originally developed by DuPont in the early 1950s and now produced under the Celanese brand. It is not a structural insulation material in the way Nomex is, but as a dielectric barrier it is exceptionally capable. Its dielectric strength of 7,000 V/mil (AC) is among the highest of any plastic films, and its chemical inertness, resistance to oils and solvents, and dimensional consistency across production runs make it a reliable component in composite insulation systems.

Key Properties

Standard Mylar A is classified at IEC/UL insulation Class B, with continuous service up to 130°C depending on grade and applicable standard. Mylar E extends this to 180°C, and the Kaladex 2000 grade (polyethylene naphthalate) reaches rated temperature indices of 160°C mechanical and 180°C electrical under UL 746B. Moisture absorption is below 0.5%, and the material does not become brittle with age under normal conditions because it contains no plasticizers. Mylar is available in thicknesses from around 6 to 480 micrometers, supporting a wide range of converting operations including die cutting, laminating, slitting, coating, and roll-to-roll lamination.

Best Suited For

Mylar functions as the primary dielectric core in NMN (Nomex-Mylar-Nomex) and DMD (Dacron-Mylar-Dacron) composite laminates, which are standard constructions for motor slot liners and transformer winding insulation. It is also used as intermediate and top-layer insulation in transformers, chokes, and relay coils, as well as for cable and wire wrapping. For applications where thin-gauge precision converting and cost-effectiveness are the primary procurement criteria, Mylar is the natural starting point.

Limitations

Standard Mylar PET film carries a UL 94 HB flammability rating, the lowest classification in the UL 94 system. This makes it unsuitable as a standalone material in applications requiring V-0 flame performance. Flame-retardant grades such as Mylar FR exist but are grade-specific and should be verified against the relevant application standard. Mylar is also susceptible to thermal aging over time, and its volume resistivity decreases as temperature rises, which limits its use in sustained high-temperature environments without composite support.

What Sets These Three Materials Apart?

The clearest differentiator across Formex, Nomex, and Mylar is thermal class. Under IEC 60085, the governing standard for thermal classification of electrical insulating materials, Formex sits at Class E/B (115 to 130°C depending on grade), standard Mylar A at Class B (130°C), and Nomex 410 at Class R/N (220°C). An insulation system operated continuously at its rated class temperature is designed for a service life of 20,000 hours. Specifying a material below the required thermal class shortens that life considerably.

Flame retardancy is the second major differentiator. Formex achieves UL 94 V-0, self-extinguishing within 30 seconds on a vertical specimen. Nomex does not melt and does not support combustion at its rated temperature, also meeting UL 94 V-0. Standard Mylar carries only UL 94 HB, the lowest rating. Flame-retardant Mylar grades are available, but they are not the default.

Fabricability separates the three in practical terms as well. Formex excels at scoring, folding, and forming into three-dimensional enclosures and barriers. Mylar excels in thin-gauge precision converting: die cutting, laminating, slitting, and coating at high consistency. Nomex is the most difficult to cut and process, requiring specialized tooling, and its rigidity in thicker grades can limit design flexibility. On moisture resistance, Formex is non-hygroscopic, Mylar is moisture-resistant, and Nomex absorbs moisture and requires controlled handling. On cost, Nomex is the most expensive, while Mylar and Formex are the most cost-effective, with composite laminates sitting in between.

Which Material Fits Your Application?

The selection framework is temperature-first. For applications operating up to 130°C, Formex and Mylar are practical and cost-effective. For sustained operation between 130°C and 220°C, Nomex is the appropriate material. For applications above 220°C, none of these three materials is sufficient, and fiberglass, mica, or ceramic-based systems are required.

Within the up-to-130°C range, the choice between Formex and Mylar depends on the physical role the material must perform. If the application requires a three-dimensional barrier, an enclosure wall, or a folded channel, Formex is the better fit. If the application requires a thin, flat dielectric layer, a wrap, or a component within a laminated composite, Mylar is the appropriate starting point. For applications where flame performance to UL 94 V-0 is mandatory, Formex holds the advantage over standard Mylar.

For motor and transformer insulation at Class F (155°C) or Class H (180°C), NMN laminates combining Nomex outer layers with a Mylar dielectric core are the standard construction. NMN is available in both Class F and Class H variants depending on the specific construction and adhesive system. For oil-immersed transformers, kraft paper or Nomex 910 are the established options. For EV traction motors and high-power-density drive systems where thermal performance is the design constraint, Nomex 414 or full aramid constructions are the specified solution.

Engineers should define voltage requirements, operating temperature, peak temperature exposure, geometry constraints, moisture environment, and flame certification requirements before selecting a material. Comparing only dielectric strength and price without defining the physical role of the insulation is a common cause of incorrect material selection.

Finding the Right Insulation Material for Your Needs

Insulation failure is a leading cause of electrical equipment breakdown in service, which makes material selection a decision worth getting right the first time. The right material is not always the most thermally capable one; it is the one that meets all relevant requirements, including thermal class, flame rating, fabricability, moisture behavior, and certification compliance, at the appropriate lifecycle cost for the application.

For halogen-free compliance under IEC 61249-2-21, Formex N3 and Formex CND are specifically marketed as halogen-free options. Mylar and Nomex grades are available with RoHS and REACH compliance documentation, but specific compliance declarations should be requested from the manufacturer for current product versions, as SVHC candidate lists are updated periodically.

Supply chain considerations also matter in B2B procurement. Formex parts can be die-cut and shipped flat, reducing warehousing and freight costs. Mylar films are available from multi-site stock in Europe and North America with short lead times through established converting networks. Nomex requires specialized cutting equipment, which affects both lead time and processing cost.

Tekniseri is here to help you find the right insulation solution. Whether your application calls for Formex, Nomex, Mylar, or a composite laminate, Tekniseri’s team brings decades of experience in industrial insulation manufacturing across Finland, Sweden, and China. Contact Tekniseri to discuss your specific requirements and get expert guidance on material selection, converting, and supply.