Selection of a PET capacitor film reduces to three variables: grade, thickness, and standard. Biaxially oriented polyethylene terephthalate film intended for capacitor dielectrics is produced from a constrained resin formulation rather than a generic bottle- or packaging-grade feedstock. The grade-defining parameters are intrinsic viscosity of 0.62–0.68 dL/g when determined by ASTM D4603, diethylene glycol content below 1.5 mol%, carboxylic end group concentration below 30 mmol/kg, and total inorganic residue below 100 mg/kg. This formulation window exists because tenter-frame sequential stretching at machine-direction draw ratios of 3.0–3.5 and transverse-direction draw ratios of 3.3–4.0 applies tensile stress to a melt curtain generated by a single-screw extruder. Insufficient chain length at the die lip manifests as melt resonance, transverse thickness bands, and gel streaks that reduce dielectric strength after vacuum metallization. Melt filtration through sintered stainless-steel media with 15 µm absolute cutoff is standard for thin-gauge production. The casting drum is controlled between 30 °C and 50 °C to limit spherulitic crystallization before sequential orientation.

Capacitor-grade formulations contain no antiblock, slip, or nucleating additives. Even sub-0.1 wt% inorganic particulates behave as field-concentrating defects in a 3–5 µm dielectric. Film for high-voltage pulse duty is therefore produced from essentially particle-free resin, and the absence of additive roughness increases film-to-film and film-to-metal friction. Winding equipment is specified with load-cell tension control and lay-on roller profiles to prevent air entrapment and blocking. Hydrolytic stability defines another operational boundary. PET hydrolyzes at rates that become significant above 70 % relative humidity and 40 °C; resin pre-drying below 50 ppm moisture and sealed film storage are therefore required.

At production scale, batch-to-batch variability in metal catalyst residues is controlled by antimony or titanium catalyst dosing. Residual antimony levels in film-grade PET frequently fall between 50 ppm and 90 ppm, while germanium catalyst systems are reserved for selected low-loss grades. Elevated catalyst residue increases direct-current leakage under high temperature and high humidity. The extruder barrel for capacitor-grade resin is operated with a decreasing temperature profile from 285 °C in the feed zone to 270 °C at the metering zone to limit thermal degradation. Process data from biaxial lines indicate that a 2 °C casting drum deviation produces a measurable shift in amorphous orientation and film density.

What Separates Capacitor-Grade PET from Packaging-Grade Resin in a Tenter-Frame Line?

Dielectric loss in PET is governed by molecular mobility, residual oligomer content, and crystalline orientation state. Capacitor-grade resins are polymerized to lower diethylene glycol and cyclic oligomer concentrations than packaging grades. Cyclic oligomers migrate to the film surface during heat-setting and interfere with vacuum metallization. The relevant production control is the isothermal crystallization half-time measured by differential scanning calorimetry. A capacitor-grade film typically exhibits a melting peak temperature of 256–260 °C at a heating rate of 10 K/min according to ISO 11357-3, with crystallinity above 40 % after heat-setting. The dielectric constant at 1 kHz is 3.2 ± 0.1, and the dissipation factor is not more than 0.005 when measured with guarded electrodes under ASTM D150. Volume resistivity at 23 °C is typically greater than 10¹⁷ Ω·cm when measured according to ASTM D257.

Heat-setting temperature and transverse relaxation determine shrinkage and dimensional stability. Capacitor-grade film is normally heat-set at 210–230 °C with transverse relaxation of 2–5 %. This leaves residual shrinkage below 2.0 % in the machine direction and below 1.0 % in the transverse direction at 150 °C when tested to IEC 60674-2. Relaxation is a process conflict: excessive relaxation improves shrinkage but lowers tensile modulus and creep resistance during capacitor winding.

Thickness specification begins with winding stress and margin control, not capacitance density alone. For stacked SMD capacitor film, nominal thicknesses between 1.5 µm and 3.5 µm are used. Wound power electronic capacitors more commonly use 5 µm to 12 µm film because the dielectric must survive winding tension, edge contact, and partial discharge. The relationship between thickness and dielectric strength is nonlinear. As thickness decreases, the probability that a single gel, pinhole, or contamination particle spans the dielectric increases. Electrical clearing of metallized defects is used in some pulse applications, but operational field strength must be derated when film is processed below 3 µm. Published data for the precise defect density of commercial sub-2 µm capacitor PET is limited; converter qualification therefore includes spark testing at 200–400 V/µm and segmented electrode mapping.

Thickness tolerance affects capacitance match in series pairs. A ±5 % thickness variation on a 3.0 µm film produces a capacitance variation of approximately ±5 %, so stacked capacitor producers routinely request ±3 % or tighter in machine and transverse directions. In tenter-frame production, transverse thickness profile is controlled by die-bolt actuators and measured with a scanning beta gauge. Machine-direction thickness is controlled by casting drum speed and draw-ratio stability. The relevant specification method is IEC 60674-2 or supplier optical profilometry; handheld caliper measurement is not acceptable for film below 6 µm.

Metal adhesion and oil impregnation place opposing constraints on surface roughness. Metallized film requires roughness adequate to prevent blocking but fine enough to avoid metal shadowing. A surface roughness of 0.2–0.6 µm Rz measured with stylus or optical profilometry is commonly specified. The roughness must be uniform across the transverse direction; roughness bands produce variation in metal adhesion and capacitance loss after aging.

When the Purchase Specification Lacks a Surface Treatment Clause

Surface treatment is specified for metallized PET capacitors even when the base film is dielectric grade. Corona discharge in air or nitrogen introduces carbonyl and hydroxyl groups on the PET surface, raising surface energy from an untreated 38–42 mN/m to 52–58 mN/m. Aluminum vacuum metallization requires a minimum surface energy of 48 mN/m for adequate wetting and peel strength; zinc metallization is somewhat more tolerant. Overtreatment above 60 mN/m can leave low-molecular-weight oxidized species at the surface, which increase dielectric loss and reduce breakdown strength under high humidity. The specification must therefore include a dyne-level window, a water-contact-angle maximum, and an aging limit after corona treatment, because treatment decays with log time. If a converter receives untreated film labeled only as capacitor grade, metal adhesion failure may not appear until thermal cycling at 85 °C / 85 % RH for 1000 h. The relevant surface tension is measured by ISO 8296 dyne pens or contact-angle goniometry.

For oil-impregnated capacitors, corona treatment is usually unnecessary and may be forbidden because polar surface groups can increase moisture uptake. Solid-resin encapsulated cells, by contrast, require controlled treatment to prevent delamination between the dielectric and the encapsulant. For high-frequency SMPS capacitors, overtreatment can raise the dissipation factor measured at 100 kHz by 0.001–0.003. Converter qualification therefore includes loss measurement before and after metallization, not merely a wetting check.

Standards Crosswalk Without Equivalency Claims

Electrical and mechanical qualification of PET capacitor film is distributed across multiple standards. IEC 60674 provides the general classification and test framework for plastic films used for electrical insulation. The material-specific specification for biaxially oriented PET is IEC 60674-3-4. Electric strength is determined by IEC 60243-1 using 25 µm specimen thickness and 25 mm / 75 mm electrodes, with a short-time acceptance value not less than 350 V/µm at 5 µm. AC permittivity and dissipation factor are tested under ASTM D150; tensile strength and elongation at break are obtained under ASTM D882 for films below 250 µm. Dimensional stability is assessed using IEC 60674-2 shrinkage without constraint at 150 °C for 15 min. Compliance with RoHS Directive 2011/65/EU and REACH Regulation (EC) No 1907/2006 is required for capacitor films entering the European Union, although these regulations do not define electrical performance.

Incoming film is also tested for haze and gel content. Haze below 3 % at 5 µm is typical for clear capacitor grade; gels larger than 0.1 mm are counted per square meter using transmitted light methods adapted from ASTM D7310. Conductive particles are controlled by melt filtration and cleanroom slitting. No single standard can be substituted for another without experimental correlation because electrode geometry, voltage ramp rate, and conditioning differ.