Determination of Volatile Matter in PET Insulation Film: Testing Standards, Methods, and Application

PET insulation film volatile content determination is a critical quality control test that quantifies the residual moisture, solvents, oligomers, and other low-molecular-weight substances released when polyethylene terephthalate film is heated under controlled conditions. Excessive volatile matter in PET film can cause outgassing, voids, delamination, and premature electrical failure in capacitors, motors, transformers, and other insulated equipment. This article explains the testing principle, sample types, standard methods, calculation procedures, and typical applications of volatile matter testing for PET insulation film.

Technical Overview

PET insulation film is a biaxially oriented polyethylene terephthalate (BOPET) material widely used as dielectric and electrical insulation in capacitors, motors, transformers, cables, and flexible printed circuits. During film manufacturing, traces of moisture, residual processing solvents, cyclic oligomers, plasticizers, and other low-molecular-weight additives may remain trapped within the polymer matrix. These substances are collectively referred to as volatile matter.

Determination of volatile content is performed by heating a conditioned specimen at a specified temperature for a defined period and measuring the resulting mass loss gravimetrically. The test result, expressed as a mass percentage, provides a direct indication of the drying completeness of the film and its suitability for high-reliability electrical applications. Elevated volatile levels are associated with gas evolution during winding and impregnation, reduced insulation resistance, degraded adhesion of metallized or coated layers, and shortened service life of the finished insulation system.

Volatile matter testing is routinely specified in incoming material inspection, manufacturing quality control, supplier qualification, and failure analysis programs for electrical-grade PET film.

Testing Samples

Biaxially Oriented PET Film (BOPET)

Standard electrical insulation grade PET film in thicknesses typically ranging from 3 micrometers to 350 micrometers, in plain or corona-treated forms, supplied as sheets or roll samples.

Capacitor-Grade PET Film

Thin films intended for metallized film capacitors and foil capacitors, where extremely low volatile content is required to prevent gas formation and capacitance drift during thermal aging.

Motor and Transformer Insulation Film

PET film used in slot insulation, layer insulation, flexible laminates, and adhesive-bonded composite structures such as PET film laminated with aramid paper or presspaper.

Metallized and Coated PET Film

Films carrying aluminum or zinc metallization, heat-sealable coatings, or surface treatment layers, where volatiles can impair coating adhesion and vacuum deposition quality.

PET Composite Insulation Materials

Multi-layer insulation products and prepreg-style materials containing PET film combined with adhesives, resins, or fibrous substrates, tested to evaluate total releasable volatiles.

Testing Items

Core and related test parameters available for PET insulation film volatile characterization include:

• Total volatile matter content (mass loss on heating)
• Moisture and water vapor content
• Residual solvent content
• Cyclic oligomer and extractable low-molecular-weight species
• Volatile content as a function of temperature and time (thermal release profile)
• Supporting characterization: thickness, tensile strength, elongation, dielectric strength, volume resistivity, and thermal shrinkage

Testing Methods

Gravimetric Oven Method (Primary Method)

The specimen is conditioned in a standard laboratory atmosphere, cut to the required size, and weighed to an accuracy of 0.1 mg using an analytical balance to obtain the initial mass. The specimen is then placed in a forced-convection oven maintained at the specified temperature, commonly 105 ± 2 degrees Celsius or 160 ± 2 degrees Celsius depending on the applicable standard and end-use requirement, and held for a defined period such as 1 hour or longer until consistent results are achieved. After heating, the specimen is cooled to room temperature in a desiccator and reweighed. The volatile matter content is calculated as:

Volatile content (%) = [(m1 − m2) / m1] × 100, where m1 is the initial specimen mass and m2 is the mass after heating.

Testing is typically performed in replicate, and preconditioning, heating temperature, and dwell time are strictly controlled to ensure reproducibility.

Thermogravimetric Analysis (TGA)

TGA measures continuous mass loss of a small specimen while the temperature is ramped at a controlled rate under nitrogen or air. This method distinguishes moisture release, solvent evaporation, and thermal degradation onsets, and is useful for developing drying schedules and investigating volatiles released at service temperatures.

Karl Fischer Titration

Coulometric or volumetric Karl Fischer titration selectively quantifies moisture content in the film, separating water from organic volatiles and providing a sensitive measure of hygroscopic uptake.

Headspace Gas Chromatography (GC and GC-MS)

Headspace GC techniques identify and quantify individual residual solvents and volatile organic compounds released from the film at elevated temperature, supporting root-cause analysis of contamination originating from coating, printing, or adhesive processes.

Reference Standards

Test procedures are performed with reference to recognized standards including IEC 60674-2 (methods of test for plastic films for electrical purposes), IEC 60674-3-3 (requirements for balanced biaxially oriented PET films for electrical insulation), the GB/T 13542 series of Chinese national standards for electrical insulation films, and client-specified or supplier-agreed internal specifications.

Applications

Volatile matter determination for PET insulation film supports quality assurance and engineering decisions across the electrical and electronics industry:

• Quality control of PET film production lines, including verification of drying and annealing processes
• Incoming material inspection by capacitor, motor, transformer, and cable manufacturers
• Supplier qualification and lot acceptance testing against purchase specifications
• Failure analysis of insulation breakdown, delamination, blistering, and capacitor gas generation
• Research and development of film formulations, coatings, surface treatments, and composite laminates
• Compliance verification for customer technical specifications and product safety certification support
• Shelf-life and storage condition studies evaluating moisture reabsorption over time

FAQ

What sample quantity is required for volatile matter testing?

Generally, several grams of film or sheets of approximately 100 mm × 100 mm are sufficient for replicate gravimetric testing. Smaller quantities are acceptable for TGA, Karl Fischer, and headspace GC analysis.

Which test temperature should be selected, 105°C or 160°C?

The temperature is defined by the applicable product standard or customer specification. Lower temperatures primarily measure moisture, while elevated temperatures near 160°C release additional oligomers and processing residues, giving a more complete picture of total volatiles for high-temperature insulation applications.

What is a typical acceptance limit for volatile content?

Requirements vary by application, but capacitor-grade and high-reliability insulation films are commonly specified with volatile matter limits of 0.5 percent or lower. The governing limit should always be confirmed against the relevant product standard or purchase agreement.

How long does testing take?

Standard gravimetric volatile content determination is typically completed within 3 to 7 working days, including conditioning. Multi-temperature studies, Karl Fischer moisture testing, and GC-MS identification may require additional time.

Can moisture be distinguished from other volatile substances?

Yes. Combining gravimetric testing with Karl Fischer titration quantifies the moisture fraction, while TGA and headspace GC-MS characterize organic volatiles and residual solvents, enabling precise source identification.

Why is volatile content control important for metallized film capacitors?

Volatiles released during capacitor operation generate internal gas pressure, degrade the metallization-layer interface, reduce insulation resistance, and accelerate capacitance loss, so low volatile matter is essential for long-term reliability.