Polypropylene films, PP films for short, are films made from the semi-crystalline thermoplastic polypropylene, produced either unstretched as cast film (CPP) or stretched lengthwise and crosswise as BOPP. With a density of around 0.90 g/cm³ they are the lightest common plastic film, and they withstand 100 °C continuously. In electrical engineering PP film is bought for its dissipation factor: at 1 kHz it is 1 to 5 × 10⁻⁴, ten to a hundred times lower than PET film. This article shows when PP is the better insulating film and when you should choose polyester instead. We convert PP films into slit rolls from 5 mm width through contract slitting.
PP film from GOBA: supply forms, thicknesses and enquiry
GOBA supplies PP film from a single piece, as material or as a finished part to your drawing.
- Supply forms
- Roll and slit roll, Cut-to-size, Die-cut part to drawing
- Thicknesses
- To your specification. GOBA processes materials from 7 µm to 3 mm.
- Versions
- BOPP and CPP, as slit roll, cut-to-size or die-cut part.
- Slit rolls
- Widths from 5 to 1,600 mm, tolerance ±0.1 mm, cores of 25, 55, 76 or 152 mm
- Die-cut and formed parts
- Material thickness 0.023 to 3.0 mm, dimensions up to 1,000 × 2,000 mm
- Quantity
- From 1 piece, from sample to series
- Lead time
- Two weeks at most when the material is in stock
What we need for your quotation
- Drawing or dimensions
- Quantity
- Material and thickness
For rolls also width, length, core diameter, core material (for example plastic) and tolerances.
BOPP, OPP and CPP: two production routes, three names
Both film types come out of film extrusion and only differ afterwards. Cast film cools directly on a chill roll and stays unstretched. BOPP film is then stretched in both directions. In trade, OPP usually means the same as BOPP. Almost 90 percent of the PP film on the market is biaxially oriented.
| Type | Production | Typical thickness | Strength |
|---|---|---|---|
| BOPP (OPP) | stretched lengthwise and crosswise | 18 to 25 µm in packaging, from 1.9 µm in capacitors | stiff, clear, high dielectric strength, hardly sealable without coating |
| CPP | cast, unstretched | 20 to 150 µm | soft, easy to seal, tougher in the cold |
| Technical PP film and sheet | extruded, partly flame retardant | 25 µm to about 1.6 mm | die-cuttable, scorable, usable as an insulation barrier |
For electrical insulation two groups matter: thin BOPP film as a dielectric and thicker technical PP film as a die-cut insulating part. The 20 µm packaging film is a different product even though it carries the same name. If you request PP film for an electrical part, always state the type and thickness.
Why PP film is bought in electrical engineering
Polypropylene is non-polar. Its molecular chains have no dipoles that rotate with an alternating field, so almost no energy turns into heat. That is exactly what the dissipation factor tan δ measures. PP sits at 1 to 5 × 10⁻⁴ at 1 kHz, PET at 50 to 100 × 10⁻⁴. The permittivity of 2.2 stays stable across frequency and temperature, and the film absorbs less than 0.1 percent water.
This defines the applications where PP film is the standard:
- Film capacitors (MKP) for inverters, motor drives and line filters, metallised and self-healing
- Separators in lithium-ion cells, single-layer or as PP/PE/PP trilayer
- Cell and module insulation in battery systems, where moisture and electrolyte are a problem
- Protective and release films that have to resist solvents and acids
- Insulation barriers made of flame-retardant PP film in power supplies and battery housings
In a capacitor the dissipation factor decides how much the part heats up at high currents. That is why BOPP is the standard there, as we describe in more detail under capacitor insulation. In a battery, chemical resistance counts, and the PP layer in a trilayer separator keeps the cell mechanically intact when the PE layer shuts down at overtemperature.
PP dielectric strength depends on thickness, stretching and temperature
Values between 50 and 650 kV/mm circulate for PP. All four figures are correct, they belong to different material states.
| Material state | Dielectric strength | Test condition |
|---|---|---|
| PP sheet, unstretched, 1 mm | 50 kV/mm | IEC 60243-1, room temperature |
| BOPP film, 25 µm | 524 kV/mm | 25 °C |
| BOPP film, 25 µm | 413 kV/mm | 125 °C, around 21 percent lower |
| BOPP capacitor film, a few µm | at least 600 V/µm (DC), at least 350 V/µm (AC 50 Hz) | 23 °C |
Three effects are at work. Thin films contain fewer defects per area, so the value rises as thickness falls. Stretching aligns the molecular chains and raises it further. Heat lowers it again. The consequence for design: never use the capacitor value when you dimension a 0.5 mm cut part. The basics are covered under dielectric strength, choosing the right gauge under material thickness.
100 °C is the limit: the temperature ceiling of PP film
PP film has the lowest temperature limit of the common insulating films. Published values range from 80 to 140 °C because they mean different things:
| Temperature | What it means |
|---|---|
| minus 10 to minus 15 °C | Glass transition, below it standard PP turns brittle |
| 80 to 85 °C | Rated and derating temperature of many BOPP power capacitors |
| 100 °C | Continuous service temperature of the material |
| 105 °C | Upper limit for PP film capacitors |
| 115 °C | RTI of flame-retardant PP insulating film |
| 140 °C | Short-term peak, not a design value |
| 160 to 166 °C | Melting range of isotactic PP |
Measured against the thermal classes of IEC 60085, plain PP film at 100 °C does not even reach class A (105 °C). Polyester film runs in class B at 130 °C. For slot, layer and phase insulation in motors and transformers we therefore advise against PP. There, polyester film or a laminate is the right choice. The full ladder of classes is shown under thermal classes of insulation. Which film takes over above 100 °C is compared for nine materials in the article on electrical insulating films.
PP film or PET film: the direct comparison
Most enquiries come down to PP against PET. The table shows where each film leads.
| Property | PP film | PET film |
|---|---|---|
| Density | around 0.90 g/cm³ | 1.395 g/cm³ |
| Continuous temperature | 100 °C | 130 °C, class B |
| Permittivity εr at 1 kHz | 2.2 | 3.3 |
| Dissipation factor tan δ at 1 kHz | 1 to 5 × 10⁻⁴ | 50 to 100 × 10⁻⁴ |
| Water absorption | below 0.1 % | 0.3 % |
| Surface energy, untreated | 29 to 30 mN/m | around 41 mN/m |
| Behaviour in the cold | brittle below about 0 °C | dimensionally stable down to about minus 70 °C |
Our recommendation: PP film when losses, moisture or chemicals drive the design and the temperature stays below 100 °C. PET film when heat, dimensional stability and bondability count. For most insulating parts in motors, household appliances and switchgear that means PET. Compared with polyethylene, PP is stiffer and about 20 K more heat resistant, but more brittle in the cold. Both belong to the thermoplastics and are equally non-polar.
Bonding, printing, laminating: the low surface energy
Untreated PP film has a surface energy of 29 to 30 mN/m. Printing inks and adhesives need 38 to 46 mN/m depending on the system, otherwise they bead up or come loose later. PP film is therefore corona treated before printing or laminating, a controlled corona discharge over the web. The effect fades over days to weeks.
For adhesive cut parts and laminations, clarify these points before ordering:
- Which side is corona treated, and to what value in mN/m?
- When was it treated, and how long has the roll been stored since?
- Is the adhesive explicitly approved for polyolefins?
- Can the joint be made mechanically instead, for example by inserting or clamping?
More on composites of PP and other layers under laminating film.
Standard PP burns, flame-retardant PP film does not
Standard polypropylene only reaches class HB under UL 94 and, without UV stabiliser, is unsuitable for outdoor use because it embrittles in sunlight. For insulating parts in equipment there are flame-retardant PP films. A well-known example is Formex GK: UL 94 V-0, thicknesses from 0.127 to 1.57 mm, RTI 115 °C and a comparative tracking index above 600 V. These films can be scored and folded into three-dimensional barriers without cracking at the bend. Typical parts are inserts in power supplies, covers on busbars and barriers in high-voltage battery housings.
PP film at GOBA: slit rolls, cut parts, die-cut parts
We convert PP films into slit rolls, cut sheets and die-cut parts. The raw film is cheap. It only becomes expensive through wrong widths, edge trim waste and parts that do not hold in assembly. That is where our equipment comes in:
- Slit rolls from 5 to 1,600 mm width with ±0.1 mm tolerance
- Winding cores with 25, 55, 76 or 152 mm inner diameter
- Parent rolls up to 1,000 kg, slitting speed up to 400 m/min depending on material
- Die-cut parts from 0.023 to 3.0 mm material thickness, up to 1,000 x 2,000 mm part size
We slit PP protective films under protective film slitting, PP and trilayer separators under battery film slitting. Insulation barriers and formed parts made of PP are produced through stamping and forming. If you are torn between PP and another material, send us your drawing, operating temperature and voltage. We will recommend the film that suits the application, even if it is not PP.
GOBA Takeaway
PP film is the lightest and one of the lowest-loss films in the insulation range, and the weakest once it gets warm. Selecting it by the dissipation factor and the 100 °C limit rarely goes wrong. In capacitors, battery cells and as a chemically resistant protective film it is the first choice, in motor windings it is not. For an overview of all materials we convert, see our page on insulation materials.
