How do Polypropylene and Polyester compare as capacitor dielectric?

Polypropylene (PP) and Polyester (polyester terephthalate, or PET) are established as the most widely used plastic films for capacitors for decades. Each of these has its own characteristic properties, making them a preferred choice for specific applications. They may be used as plain film or metallized film for capacitors. It is necessary to go through their properties, mechanical, thermal and electrical to understand, which will help in proper selection of film for manufacture and usage in various applications.

Polyester Film

PET is most commonly used for DC capacitors. Its main advantages are

• Higher dielectric constant of 3.2 (PP dielectric constant – 2.2), which enables smaller capacitor dimensions.
• Good mechanical strength, which allows high speed winding on machines. There is no stretching of film even at high winding speed.
• PET can be made into very thin films of up to 0.7 μ, which enables very compact capacitance density, and maximum capacitance in small volume.
• High service temperature of up to 130℃, as against maximum of 100℃ for PP.
• Polyester absorbs very little moisture, and this makes ‘naked’ designs possible, without the need for encapsulation or coating.
• PET is most inert among films to corrosive atmosphere and chemical pollutants in atmosphere.

In short, small sizes, higher temperature range, and high processing speeds go in favour of selection of PET film for DC applications. Polyester is a polar medium, and is limited in use for mostly in DC, and a few low frequency AC applications at low voltage levels.

Since it packs high capacitance in small volumes (volumetric efficiency), and it is suitable for high service temperatures up to 125℃, PET is ideally suitable for SMD capacitors. Its low cost and compact dimensions have made PET a preferred choice for general low cost DC applications, mostly in not-so-critical circuits. PET also comes in handy for stacked film capacitor construction.

Limitations of Polyester film

• Higher change of value over working temperature range, as much as 5%
• Capacitance change of -3% from 100HZ to 100 kHz.
• It absorbs some amount of water. (PP does not absorb water at all)
• High dissipation factor makes it unsuitable for most AC applications, except for low voltages and those for short time duty.
• PET can generate good amount of static electric charge during winding, which attracts dust.

Further, PET is not advised for AC applications mainly because of its high loss factor (tan delta). In addition, when it comes to temperature stability, PP capacitors undergo a change of about -3% between 25℃ to 100℃, whereas PET capacitors may change by as much as +6%. If high stability is needed over working temperature range, PP is the preferred choice.

Applications of PET capacitors

• Most DC applications
• Coupling, decoupling and blocking applications in DC circuits
• General Dc and low frequency circuits
• DC Power supplies
• Noise suppression
• Audio circuits

Polypropylene Film

Polypropylene Film (PP) is the most widely used film for AC capacitors, including power factor capacitors. It is also used for stack capacitors and ‘naked’ capacitors due to low moisture absorption.  Its main advantage is its low loss factor and better capacitance stability compared with PET film. Advantages of PP film are:

• PP is clear, high clarity and chemically stable plastic.
• Low density of 0.9 makes for high yield of capacitors per unit weight.
• Does not absorb water.
• Good resistance against chemicals and moisture.
• Low capacitance changes over temperature range
• Low and negative temp. coeff. (TCC) Of capacitance of ±2.5% within working range
• Lowest dielectric absorption among plastic films
• Suitable for precision capacitors
• Low dielectric loss and high stability even at high frequencies
• PP film can be made in several forms- plain, matt finish, hazy (roughened) etc.
• Low electrostatic charge during processing on machines.

Its lower dielectric constant of 2.2 makes capacitor size larger. Further, its low mechanical strength limits lowest available thickness to 1.6 – 2 μ. Capacitors of 1% tolerance are available. PP capacitors have good stability over time and over a large voltage range. PP capacitors are available only as capacitors with leads, and are not suitable for SMD versions, or capacitors without leads.

PP capacitors are able to stand high voltages, and capacitors of up 600-1000 V (both AC and DC) are available. Their low tan delta (loss angle) makes them ideal for most AC capacitors even for high frequency applications. Excellent electrical properties, low loss factor, coupled with their low cost makes them preferred material for plastic film capacitors, particularly for Ac applications, including KVAR capacitors as also several Dc applications.

Limitations of PP dielectric film

• Lower service temperature (max. 105℃) compared to PET
• More stretchable and can get elongated easily under excess winding tension

Applications of PP capacitors

• AC capacitors- both power frequencies and high frequencies
• High pulse discharge
• Induction heating
• Sample and hold circuits
• Precision timing circuits
• High frequency resonant circuits.

Properties of polypropylene and polyester films are summarized in following table.

Property	BOPP	PET
Density	0.91	1.395
Melting point	165℃	>255℃
Long Term Temp. range	-60 to 100℃	-60 to 130℃
Shrinkage in air m/c dir.	<1% @125℃, 15 Min	<0.25% @150℃, 15 Min
Shrinkage in air transverse dir.	<1% @125℃, 15 Min	<1.5% @150℃, 15 Min
Tensile strength md @23℃	165 N/ mm2
Tensile strength td @23℃	206 N/ mm2
Elongation at  break md @23℃	160%
Elongation at  break td @23℃	50%
Modulus of elasticity md @23℃	2600 MPa
Modulus of elasticity td @23℃	4600 MPa
Field strength in air DC @23℃	≥600 V/μ	≥400 V/μ
Field Strength in air AC (50HZ) 23℃	≥350 V/μ	≥350 V/μ
Loss factor (Tan δ)  @ 1 kHZ, 23℃	0.0002	0.005
Dielectric  Constant 23℃	2.2-2.25	3.2-3.3
Specific resistance @23℃	≥1018 Ω.cm	≥1017 Ω. cm
Thinnest film available	1.6 μ	0.6- 0.7 μ
Thickest film available	20 μ	15 μ