Thermoplastics are polymers whose molecular chains are not chemically crosslinked and are held together by physical forces alone. Heat releases that hold, the material softens and solidifies again on cooling, as often as required. Nine thermoplastics matter for electrical insulation, from PET through PA and PTFE to PEEK. This article puts their continuous service temperature, dielectric strength, volume resistivity and permittivity side by side in one table, draws the line to thermosets and elastomers, and shows how GOBA converts thermoplastic films and laminates from 0.023 to 3.0 mm thickness into insulation and moulded parts and slits them into widths from 5 to 1,600 mm with a width tolerance of plus minus 0.1 mm.
Weak forces between the chains make thermoplastics meltable
The chains of a thermoplastic are linear or branched and are held together by van der Waals forces, plus hydrogen bonds in polar materials such as polyamide. Those bonds are weak enough for heat to release them. The chains then slide past each other, the material turns tough, then flowable, and on cooling the bonds lock in again. The chain itself stays intact, which is why the cycle can be repeated.
Two temperatures describe this behaviour. Above the glass transition temperature (Tg) the amorphous chain regions become mobile, the material loses its brittleness and turns tough. Above the crystalline melting temperature (Tm) the ordered regions dissolve as well and the material becomes flowable. The continuous service temperature of a thermoplastic sits well below both values, because a part only stays dimensionally stable while the chains are still largely locked in place.
In practice that means thermoplastics have no hard temperature limit the way a metal does. They soften, creep and lose their shape across a range, long before they melt. The thermal classes defined in IEC 60085 map exactly this by assigning a permissible continuous temperature to each class.
Amorphous or semi-crystalline: what the structure means in practice
Thermoplastics split into two structural families, and membership decides transparency, shrinkage and warpage. Amorphous grades consist of randomly coiled chains. Semi-crystalline grades form ordered zones on cooling, known as crystallites, with amorphous regions in between.
| Property | Amorphous | Semi-crystalline |
|---|---|---|
| Chain order | randomly coiled | ordered crystallites next to amorphous zones |
| Softening | gradual above the glass transition temperature | tough above Tg, flowable only at the crystalline melting point |
| Appearance | usually clear | milky to opaque |
| Shrinkage and warpage | low, good dimensional stability | higher, parts tend to warp |
| Chemical resistance | more prone to stress cracking from solvents | better, the crystallites shield the chains |
| Typical grades | PC, PS, PMMA, PVC | PE, PP, PET, PA, POM, PTFE, PEEK |
The difference shows up immediately in converting. A biaxially oriented, semi-crystalline PET film separates cleanly at the blade and stays dimensionally stable at the edge. Amorphous films such as PVC or PC tend to crush at the edge once cutting speed runs too high. How far a material springs back also depends on this structure, covered in the article on the minimum bending radius.
Thermoplastics, thermosets and elastomers compared
The three polymer groups differ in a single property, the degree of crosslinking. Everything else follows from it.
| Property | Thermoplastic | Thermoset | Elastomer |
|---|---|---|---|
| Crosslinking | none, physical bonding forces only | densely covalently crosslinked | loosely covalently crosslinked |
| Behaviour under heat | softens, becomes flowable | stays hard until decomposition | stays rubber-elastic |
| Remeltable | yes, repeatedly | no | no |
| Weldable | yes | no, bonding only | no |
| Materials | PET, PP, PE, PA, PTFE, PEEK | epoxy resin, phenolic resin, melamine resin | silicone, EPDM, NBR |
| Role in the insulation system | films, laminates, punched parts | impregnating resins, laminated paper and fabric | seals, grommets, cable entries |
All three groups work together inside an electric motor. The slot liner is a thermoplastic film or a nonwoven-film laminate, the impregnating resin on top of it is a thermoset, and the cable entry at the terminal box is an elastomer. The counterpart to this page is the article on thermosets, while the application view of both groups is covered by electrically insulating plastics.
Which thermoplastics matter in electrical insulation?
The plastics industry sorts thermoplastics by performance level into commodity polymers (PE, PP, PS, PVC), engineering polymers (PA, PC, POM, PET, ABS) and high-performance polymers (PTFE, PEEK, PPS, PI). For material selection in insulation work that classification only gets you halfway, because it says nothing about electrical behaviour. The table below brings both sides together.
| Material | Continuous service temperature | Dielectric strength | Volume resistivity | Permittivity |
|---|---|---|---|---|
| PE, polyethylene | up to around 80 °C (PE-HD) | 20 to 45 kV/mm, thin films higher | above 10¹⁶ Ω·cm | around 2.3 |
| PP, polypropylene | up to around 100 °C | 55 to 90 kV/mm | above 10¹⁶ Ω·cm | around 2.2 |
| PET, polyester | 105 to 130 °C, class B | above 70 kV/mm, thin films considerably higher | around 4 × 10¹⁶ Ω·cm | around 3.2 |
| PA 6, polyamide | 80 to 150 °C depending on grade | 150 kV/mm dry, 80 kV/mm moist | 10¹⁴ Ω·cm dry, 10¹² Ω·cm moist | around 3.5 dry, higher when moist |
| PC, polycarbonate | up to around 130 °C | around 35 kV/mm | above 10¹⁶ Ω·cm | around 3.0 |
| POM, polyoxymethylene | up to around 100 °C | around 35 kV/mm | around 10¹⁴ Ω·cm | around 3.7 |
| PTFE, polytetrafluoroethylene | up to 260 °C | around 60 kV/mm | around 10¹⁸ Ω·cm | around 2.1 |
| PEEK, polyetheretherketone | up to around 250 °C | around 20 kV/mm on a 1 mm specimen | around 5 × 10¹⁶ Ω·cm | around 3.2 |
| PI, polyimide | around 250 °C continuous, up to 400 °C short term | above 200 kV/mm | above 10¹⁶ Ω·cm | around 3.4 |
All figures are rounded literature values for unfilled grades on standard test specimens, dielectric strength and volume resistivity to DIN 53481 and DIN 53482. Datasheet values scatter considerably with grade, additives, test frequency and specimen thickness. Design work follows the supplier datasheet, not this overview. Measurement itself and the influencing factors are covered in the article on dielectric strength.
Two numbers in the table deserve a second look. The first is the PA 6 row: dielectric strength drops from 150 to 80 kV/mm and volume resistivity from 10¹⁴ to 10¹² Ω·cm, purely through moisture picked up from ambient air. Polyamide is hygroscopic, and datasheet values apply to the conditioned, dry state. Anyone using PA in a humid environment is working with a resistivity a factor of 100 lower than published. The second is PEEK: the material withstands 250 °C and still ranks among the weakest in the dielectric strength column. Thermal endurance and electrical strength are two different properties, and no material brings both automatically.
Fibre-reinforced thermoplastics, often supplied as organosheet-type semi-finished products, formally belong here too, but in converting they behave like a layered composite. They are covered in the article on composite materials.
Use of thermoplastics: from packaging to slot insulation
The use of thermoplastics ranges from the drinks bottle to the insulating layer inside an electric motor. The reason is the same in every industry: under heat the material turns flowable and can be processed into film, pipe, profile or a moulded part, without a curing reaction fixing it in place. Which grade ends up in the part is decided by the load at the installation point, meaning temperature, media contact and mechanical stress.
| Industry | Typical thermoplastics | What they do there |
|---|---|---|
| Packaging | PE, PP, PET | films, bottles, sealing layers, moisture barrier |
| Construction | PVC, PE, PP | window profiles, pressure and waste water pipes, sealing membranes |
| Vehicle construction | PP, PA, POM | interior trim, fuel lines, gears and plain bearings |
| Medical technology | PP, PC, PEEK | sterilisable containers, viewing panels, implant components |
| Electrical engineering | PET, PI, PTFE, PP | slot insulation, layer insulation, core insulation, film capacitors |
In electrical engineering that breadth narrows to a handful of materials, because two requirements apply at once: the thermoplastic has to withstand the continuous temperature of the winding and carry the field strength at low thickness. What remains in practice is PET, polyimide and PTFE, plus PP for film capacitors. They sit in the slot insulation of stator and rotor, in layer and phase insulation, as core insulation in cable insulation and as a separating layer between the cells of a battery module. We advise against PVC in motor construction: it contains plasticisers that migrate under heat and releases hydrogen chloride as it decomposes. Which films work instead is covered in the article on the PVC film alternative.
GOBA comes in behind the material choice, at the finished part. From PET film, aramid paper and nonwoven-film laminates we produce slit rolls in the width your winding machine calls for, plus punched and creased insulating and moulded parts to your drawing. Those parts go into electric motors, transformers, household appliances and battery modules, wherever a thin layer of plastic has to withstand voltage without conducting it.
Which thermoplastic should you choose?
For sheet insulation in motors, transformers and battery modules we recommend PET as the starting point and only move away from it when a specific reason argues against it. PET combines a continuous temperature up to 130 °C per IEC 216 with more than 70 kV/mm, high tear strength from biaxial orientation, freedom from plasticisers, and a price no high-performance polymer can match. GOBA converts these polyester films as the largest European converter for Mitsubishi Hostaphan in the heavy-gauge range, and in series production that combination of data and availability is the argument.
Four questions drive the decision in practice, in this order:
- What continuous temperature actually occurs at the insulation point, not at the housing? Allow 20 to 30 °C headroom below the continuous service temperature of the material.
- How high is the field strength, and how thin may the insulation be at most? Both together define the dielectric strength you need.
- Will moisture, oil or a cleaning agent reach the part? That rules out PA in many cases and argues for PET, PP or PTFE.
- Does the part have to be creased, bent or fitted into a lamination stack? Then springback matters more than the last kelvin of temperature headroom.
Above 130 °C, PET becomes the wrong choice. At that point we move to aramid paper such as Nomex or to polyimide film, for example Kapton, which at more than 200 kV/mm also delivers the highest electrical strength in the field. We advise against POM and PC as sheet insulation in motor construction: both sit at around 35 kV/mm, less than half of PET, without offering a temperature advantage in return. Their strength lies in thick-walled machined components, not in a thin insulating layer. PTFE, in turn, is excellent electrically and thermally, but it creeps under sustained pressure and therefore relaxes in clamped assemblies.
What matters when slitting and punching thermoplastics
A datasheet says nothing about whether a material converts cleanly into a series part. That is exactly where the surprises appear. We have been processing thermoplastic films, laminates and nonwovens for more than 60 years, and four properties decide the result.
- Springback. Semi-crystalline films such as PET and PP spring back after creasing. How much more a film springs back than sheet metal, and why, is covered in the article on forming technology. We therefore emboss the fold line in the same stroke that punches the part, so the bending line sits where it should and the part stays in its final position at the customer.
- Cut edge quality. In shear slitting, two circular blades running past each other separate the material. Thin, tough films react sensitively to blade clearance and cutting speed: too fast, and the edge crushes or pulls threads. Our cutting speed reaches up to 400 m per minute depending on material, but we run the speed the edge tolerates.
- Thermal distortion. Oriented films shrink once they see heat. For moulded parts with complex geometry we therefore form under controlled heat rather than forcing them cold, staying just below the range where the orientation relaxes.
- Film thickness. We punch material from 0.023 to 3.0 mm thick, with part sizes up to 1,000 x 2,000 mm. Below roughly 0.05 mm the tool choice decides dimensional accuracy: the more economical steel rule die covers many contours, while tight tolerances and long tool life call for solid steel tooling.
In roll slitting we produce slit rolls from 5 to 1,600 mm wide with a width tolerance of plus minus 0.1 mm, wound onto cores with 25, 55, 76 or 152 mm inner diameter. Process details are described under contract slitting, tooling and forming under stamping and forming. Which thickness suits which voltage level is discussed in the article on material thickness.
The primary shaping processes that turn a thermoplastic into film or a moulded part in the first place, namely extrusion, injection moulding, blow moulding and calendering, sit upstream of our process. How the film web is produced is described in the article on film extrusion, and the systematic classification of all manufacturing processes is provided by DIN 8580.
Is a thermoplastic enough, or does the application need a thermoset?
The objection comes up regularly from design engineering: a material that softens under heat looks risky inside a motor, and a cured resin looks like the safe choice. The objection touches a real point but usually leads to the wrong conclusion. No thermoplastic stands alone in an insulation system. The punched PET or aramid layer is wound, inserted and then impregnated with resin, and that resin is a thermoset. After impregnation the composite carries the mechanical load, while the film provides dielectric strength and separation.
A pure thermoset is ruled out as sheet insulation for one simple reason: it cannot be creased, bent or slid into a slot without breaking. Laminated paper and laminated fabric are therefore used as rigid boards, support bodies and switchgear parts, not as a 0.2 mm slot liner. In practice the question is rarely thermoplastic or thermoset, but which thermoplastic, at which thickness, under which resin. Safety comes from temperature headroom rather than from the material class: 20 to 30 °C below the continuous service temperature, measured at the hottest point of the winding.
GOBA Takeaway
Thermoplastics are the working material of sheet insulation, because they can be produced as a thin web and then slit, punched and creased. Their limit is temperature, and that limit sits lower than the melting point suggests. Four figures drive the selection: continuous service temperature, dielectric strength, volume resistivity and, in high-frequency applications, permittivity. Everything beyond that is decided in production.
We convert these materials into finished parts every day, from the slit roll to the fitted moulded part made from our insulation materials. If you are unsure which thermoplastic carries your application, send us the drawing together with temperature and voltage level. For special constructions and small quantities, talk to us about custom products.
