Polystyrene film, PS film for short, is a film made from the amorphous thermoplastic polystyrene, produced crystal clear as general-purpose polystyrene (GPPS), opaque and impact modified as HIPS, or stretched lengthwise and crosswise as oriented polystyrene (OPS). It has a density of 1.05 g/cm³, withstands only 70 °C continuously and has a dissipation factor of 0.9 × 10⁻⁴ at 50 Hz, less than PP at 2.5 × 10⁻⁴ and PET at 20 × 10⁻⁴ (Kern, PS 2101 data sheet). This article sorts out the grades, puts the values into context and explains why we almost always recommend PP or PET film for insulating parts. We convert the PET film for this in our polyester film range into rolls, cut sheets and die-cut parts.
GPPS, HIPS, OPS, EPS and XPS: five materials under one name
Behind “PS film” there are three solid films and two foams. The foams share nothing with a film except the raw material. All of them belong to the amorphous thermoplastics. They differ in toughness and clarity, and in density by more than an order of magnitude.
| Grade | Structure | Density | Properties | Typical use |
|---|---|---|---|---|
| GPPS (general-purpose PS) | pure, atactic polystyrene | 1.04 to 1.09 g/cm³ | crystal clear, stiff (modulus 3,200 MPa), brittle, elongation at break 3 % | windows, lids, covers |
| HIPS (high-impact PS) | PS with 5 to 15 % rubber | 1.03 to 1.06 g/cm³ | opaque white to black, tougher, modulus 1,800 to 2,200 MPa | thermoforming sheet 0.18 to 1.8 mm, model making, inserts |
| OPS (oriented PS) | GPPS film, biaxially oriented | around 1.05 g/cm³ | crystal clear, tougher than unoriented GPPS film | cups, blisters, clear packaging |
| EPS (Styropor) | bead foam from pre-expanded beads | 15 to 90 kg/m³ | insulation board, not a film | impact sound and thermal insulation, packaging |
| XPS | extruded closed-cell foam | 25 to 50 kg/m³ | compression resistant, low water uptake | perimeter insulation |
Table sources: Kern (GPPS), Stocker Kunststoff and S-Polytec (HIPS), ECTA (HIPS thermoforming sheet), a Brückner Maschinenbau patent on oriented PS film, verpackungswirtschaft.de (EPS and XPS densities). Anyone looking for “PS film” for model making usually means white HIPS or PS sheet from 0.5 mm, sold in hobby shops. Anyone looking for “EPS film” is looking for insulation: under a floating screed the right product is EPS-T impact sound board with a dynamic stiffness of no more than 30 MN/m³, and standard EPS without the T is not designed for it. Styropor is BASF’s registered trademark for EPS. From here on this article stays with solid polystyrene as a technical film.
One detail from the chemistry helps when reading data sheets. Commercial PS is atactic: the side groups sit randomly along the chain, so it does not crystallise, stays crystal clear and has no melting point. Syndiotactic PS (SPS) is semi-crystalline, melts at 270 °C and withstands hot water and many chemicals (Wikipedia, Polystyrol). SPS is mainly injection moulded and rarely met as a film.
Polystyrene withstands 70 °C continuously, 100 °C is the glass transition
Published temperature limits for polystyrene range from 55 to 101 °C. All of them are correct, they simply measure different things. Anyone who reads the glass transition as a service temperature designs a part 30 K too hot.
| Temperature | What it means | Source |
|---|---|---|
| 55 °C | Onset of accelerated ageing | Wikipedia |
| 70 °C | Maximum continuous service temperature of GPPS and HIPS | Kern, S-Polytec, Wikipedia |
| 80 °C | Permitted for short periods | Kern |
| 85 °C | Upper limit of Styroflex capacitors | Reichelt, Wikipedia |
| 86 °C | Heat deflection temperature HDT A: the test bar reaches the standard deflection under 1.8 MPa bending stress | Kern |
| around 100 °C | Glass transition, PS softens and can be thermoformed | Wikipedia |
| 101 °C | Vicat softening temperature B50 | Kern |
Measured against the thermal classes of IEC 60085, polystyrene at 70 °C sits below the lowest class Y at 90 °C. Polyester film runs in class B at 130 °C. That rules PS out for slot, layer and phase insulation in motors and transformers from the start. The full ladder is shown under thermal classes of insulation.
Electrically, PS is one of the lowest-loss film plastics
Polystyrene is non-polar like PE and PP. The chain carries no dipoles that rotate with an alternating field, so PS turns almost no field energy into heat. The permittivity is 2.5, at 50 Hz just as at 1 MHz. The dissipation factor falls from 0.9 × 10⁻⁴ at 50 Hz to 0.5 × 10⁻⁴ at 1 MHz, which puts it below polypropylene. What the value means physically is explained in the article on dielectric losses.
That is why PS film served as a capacitor dielectric for decades. Styroflex capacitors were widely used in resonant circuits and IF filters until around 1990. A distributor data sheet gives Styroflex types as 56 pF to 10 nF, 50 to 500 V, a dissipation factor of no more than 0.1 %, a temperature coefficient of minus 150 ±50 ppm/K and an operating range of minus 40 to plus 85 °C (Reichelt, Styroflex capacitors). Two limits pushed them out: the film could not be made thinner than about 10 µm, and 85 °C was the ceiling. PP film is available from 1.9 µm and runs up to 105 °C (Wikipedia, Kunststoff-Folienkondensator). How the dielectrics are used today is covered under capacitor insulation.
For dielectric strength the published figures for PS scatter more than for any other property:
- 20 to 55 kV/mm at 1 mm reference thickness (Wikipedia, dielectric strength table)
- 43 kV/mm at 1.6 mm to IEC 60243-1 (Kern, GPPS)
- 50 kV/mm for a PS-SB sheet material, test thickness not stated (Reely)
- 155 kV/mm to VDE 0303 for HIPS, test thickness not stated (S-Polytec)
We have not found a reliable published value for thin PS film. As with PET and PP, strength rises as thickness falls, so a value without a test thickness is no basis for design. The rule is explained under dielectric strength. PS is weaker than PP and PVC on tracking resistance: CTI 425 against 600, and only PET is lower at 350. And under UL 94 it only reaches class HB.
PS film compared with PP, PET and PVC
The table sets the four film plastics side by side with values from one data sheet series, measured to the same standards on semi-finished stock. Oriented films reach higher breakdown values, and PET gains 30 K of continuous temperature as BOPET film.
| Property | PS | PP | PET | Rigid PVC |
|---|---|---|---|---|
| Density | 1.05 g/cm³ | 0.90 g/cm³ | 1.40 g/cm³ | 1.38 g/cm³ |
| Continuous temperature | 70 °C | 100 °C | 100 °C as stock, 130 °C as BOPET film | 60 °C |
| Permittivity εr at 50 Hz | 2.5 | 2.3 | 3.4 | 3.5 |
| Dissipation factor tan δ at 50 Hz | 0.9 × 10⁻⁴ | 2.5 × 10⁻⁴ | 20 × 10⁻⁴ | 120 × 10⁻⁴ |
| Dielectric strength (test thickness) | 43 kV/mm (1.6 mm) | 50 kV/mm (1 mm) | 25 kV/mm | 40 kV/mm (0.6 mm) |
| CTI | 425 | 600 | 350 | 600 |
| UL 94 | HB | not stated | HB | V-0 |
| Water absorption in water storage | below 0.1 % | below 0.1 % | 0.5 % | 1.6 % |
Values: Kern data sheets PS 2101, PP-H 1502, PET 1301 and PVC-U 2690, the 130 °C for BOPET film from Mitsubishi’s general HOSTAPHAN data sheet (grades RN and WN). The films that replace PS in insulation are covered in detail under PP film and polyester film. If you want to move away from PVC, the options are under PVC film alternatives, and values for further plastics under electrically insulating plastics.
When PS works as an insulating film and when it does not
PS film has a narrow but real place in electrical engineering. It fits when the part stays cool, sees no solvents and above all has to be transparent or low-loss: windows in front of displays and instruments, covers in high-frequency assemblies, thermoformed HIPS carriers and inserts in equipment without a heat source. A technical film distributor stocks colourless PS film from 50 to 750 µm for exactly these windows and as a die-cuttable material (Müller-Ahlhorn, PS polystyrene films).
We advise against PS as soon as one of these points applies:
- The insulation point runs continuously above 70 °C, as in almost every motor and transformer.
- Petrol, dichloromethane, acetone, toluene or xylene reach the part, including as a cleaning agent or adhesive vapour. Non-polar solvents and longer-chain ketones attack PS.
- The part is under bending or clamping stress. Solid PS is prone to stress cracking, and media accelerate it.
- The barrier is to be scored or folded. GPPS breaks at 3 % elongation and easily cracks along a sharp fold line.
- The part is exposed to sunlight. UV radiation embrittles PS.
- The standard requires flame retardancy better than HB.
- The part sits in an ESD-sensitive assembly. PS film builds up static charge easily and attracts dust.
In these cases PP film is the right choice when losses and chemicals matter and the temperature stays below 100 °C, and PET film when it gets warmer or the part has to be bent and bonded.
Is the low dissipation factor not reason enough for PS?
The objection often comes from RF engineering, and it holds as long as the part is a component with a quality factor, such as a filter capacitor. For a die-cut insulating part the calculation changes. Dielectric heating grows linearly with tan δ. At 50 Hz and a film a few tenths of a millimetre thick it is negligible with PS and with PP alike, whether the factor is 0.9 or 2.5 × 10⁻⁴. What remains are the differences that matter in service: PP withstands 30 K more and has a CTI of 600, PET as a film withstands 60 K more. Our recommendation is therefore: PS only where transparency or a resonant circuit forces the choice, otherwise PP or PET.
What we convert instead of polystyrene film
For insulating parts we convert the two films that replace PS in almost every application. We convert PET film from 12 to 350 µm, including brands such as HOSTAPHAN and Mylar, and PP film for low-loss and chemically stressed parts. The production data:
- Slit rolls from 5 to 1,600 mm width with ±0.1 mm tolerance
- Die-cut parts from 0.023 to 3.0 mm material thickness, up to 1,000 x 2,000 mm part size
- Quantities from 1 piece, lead time no more than two weeks for material in stock
We slit rolls through contract slitting, cut parts and formed parts are made through stamping and forming. Send us your drawing, quantity, thickness, operating temperature and voltage. If your drawing specifies PS, ask us anyway: we will check with you whether PP or PET film meets the requirement.
GOBA Takeaway
Polystyrene film is crystal clear, stiff and electrically almost loss-free, and it is the weakest film plastic as soon as heat, solvents or bending come into play. At 70 °C continuous temperature it reaches no thermal class. For windows and cool RF parts it remains an option, for insulating parts we recommend PP or PET. For an overview of all materials we convert, see our page on insulation materials.
