In the technical-economics of capacitance, the greatest ROI in research dollars comes from the manipulation of the available surface area of the traditional dielectric materials: Ceramic, Plastic, Aluminum, and Tantalum.
Because paper is porous it has to be impregnated in order to prevent corona effects and flash-overs. It is done by use of melted wax or different kinds of oils, among other things mineral and silicone oils. The oils increase the tensional stability but decrease to a certain extent the εr. A fibrous paper has a εr ≈ 6.6 and the mineral oil ≈ 2.3 which gives the impregnated winding a εr varying between 3.1 and 4.5. The differences depend above all on the winding pressure produced by the tensile force during winding.
The polyester film is most reliable and together with PP most used of plastic films. It can be produced in thicknesses down to 0.7 μm (0.03 mils). Its tensional stability is high and it’s εr ≈ 3.2. This has facilitated the manufacture of one for organic dielectrics very space-saving capacitor. A typical field of application is decoupling. Certain applications like switched mode power supplies (SMPS) require for filtering and decoupling purposes large capacitance and moderate losses which have made the MKT capacitor an attractive replacement for the relatively expensive ceramic X7R capacitors.
Polypropylene (PP) is from a molecular point of view a non-polar dielectric with small losses and a relatively straight and moderate TC. Since the smallest film thickness is approx. 3.5 μm (0.14 mils) and εr ≈ 2.3 the capacitor can not come down to those sizes characterizing
Polystyrene (PS) represents a non-polar dielectric material that just as polycarbonate to a great extent has been replaced by polypropylene. The availability is strongly limited due to ceased production.
Polyphenylene Sulfide or simply PPS has come to stay mainly because of its relatively high-temperature resistance which has allowed SMD manufacture.