PPS insulation film thermal aging tests determine the long-term thermal endurance of polyphenylene sulfide films used in motors, transformers, capacitors, and other high-temperature electrical systems. This article explains the applicable standards, sample preparation, testing items, and Arrhenius-based data analysis methods used to establish the temperature index of PPS insulation film. Understanding these procedures helps engineers select and qualify reliable insulation materials for thermally demanding applications.
Technical Overview
Polyphenylene sulfide (PPS) is a semi-crystalline aromatic engineering thermoplastic whose backbone of benzene rings linked by sulfur atoms gives it outstanding thermal stability, inherent flame retardancy, chemical resistance, and stable dielectric properties. PPS film typically exhibits a melting point near 280 to 285 °C, a glass transition temperature of approximately 85 to 92 °C, and a continuous-use temperature commonly rated between 200 and 240 °C. Because PPS film is widely used as a high-temperature electrical insulation material, its service life under sustained heat must be quantified rather than assumed.
A thermal aging test accelerates the natural degradation of the film by exposing specimens to elevated temperatures in forced-air ovens for extended periods, then measuring the decay of key mechanical and electrical properties. The resulting data are used to determine the temperature index (TI) and halving interval (HIC) in accordance with the IEC 60216 series, or the relative temperature index (RTI) under UL 746B. Principal degradation mechanisms in PPS film during thermal aging include oxidative crosslinking, chain scission at the surface, gradual embrittlement, changes in crystallinity, and eventually loss of dielectric strength.
- Applicable standards: IEC 60216 series, UL 746B, IEC 60085 for thermal class assignment
- Typical aging temperatures: 180 °C to 260 °C, selected to bracket the expected endurance range
- Primary endpoint criterion: 50% retention of elongation at break, or a defined drop in dielectric strength
- Typical outcome: temperature index supporting thermal class F (155), H (180), or higher ratings
Testing Samples
Base Film Specimens
Specimens are die-cut directly from production-grade PPS film rolls of representative thickness, commonly 25 μm, 50 μm, 100 μm, 125 μm, or 250 μm. The film lot, thickness, manufacturer, crystallinity condition (as-extruded versus heat-treated), and production date are documented so that results are traceable. Dumbbell-shaped or strip specimens are prepared for tensile and elongation testing in the machine direction and transverse direction, since PPS film can exhibit pronounced anisotropy.
Electrical Test Specimens
Disc or rectangular specimens of sufficient size are cut for dielectric strength testing, typically with electrodes applied per IEC 60243-2 or ASTM D149. Additional specimens are prepared for volume and surface resistivity, dielectric constant, and dissipation factor measurements. If the film is used in metallized form, such as in film capacitors, representative metallized samples may be included to evaluate the film-metal interface under thermal stress.
Specimen Conditioning and Quantity
All specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 24 hours before baseline property measurement. A statistically meaningful number of specimens is allocated for each aging temperature and each withdrawal interval, typically at least five specimens per interval for tensile testing and ten or more for breakdown measurements, so that median property values can be established with confidence.
Testing Items
The thermal aging program tracks properties that reflect both the mechanical integrity and the electrical function of the insulation film. Baseline values are measured before aging, and the same properties are remeasured after each aging interval to construct degradation curves.
- Tensile strength and elongation at break per ISO 527-3 or ASTM D882, with elongation at break serving as the most sensitive indicator of embrittlement
- Dielectric strength (breakdown voltage) per IEC 60243-2 or ASTM D149, expressed in kV/mm
- Volume resistivity and surface resistivity per IEC 62631-3 series
- Dielectric constant (relative permittivity) and dissipation factor (tan δ) per IEC 60250
- Mass loss determined gravimetrically after each aging period
- Degree of crystallinity and melting behavior by differential scanning calorimetry (DSC)
- Oxidative degradation analysis by Fourier-transform infrared spectroscopy (FTIR), tracking carbonyl and sulfone absorption bands
- Visual examination for discoloration, shrinkage, warping, cracking, and embrittlement
Testing Methods
Aging Procedure
Specimens are aged in forced-air convection ovens meeting the uniformity and air-exchange requirements of IEC 60216-4-1, with temperature controlled within ±2 °C of the setpoint. At least three, and preferably four or more, aging temperatures are selected so that the median time to endpoint ranges from roughly 100 hours at the highest temperature to several thousand hours at the lowest. Common temperature sets for PPS film include 260, 240, 220, and 200 °C, adjusted according to the expected thermal class. Specimens are hung or supported to allow free air circulation, kept from contacting each other or metal fixtures, and oriented consistently to avoid mechanical stress during aging.
Withdrawal Intervals and Measurement
Sampling intervals follow a geometric progression, for example at 24, 72, 168, 336, 672, and 1344 hours, refined as the degradation curve develops. At each withdrawal, specimens are removed from the oven, reconditioned at 23 °C and 50% relative humidity for at least 24 hours, and tested promptly using the same instruments and procedures as the baseline measurements. The measured property is plotted against aging time at each temperature, and the time at which the median property crosses the endpoint criterion is recorded as the time to failure.
Endpoint Criteria and Arrhenius Analysis
For flexible films, the most widely used endpoint is 50% retention of the initial elongation at break, because embrittlement precedes catastrophic dielectric failure. Alternative or supplementary criteria include a fixed minimum dielectric strength, a specified percentage increase in dissipation factor, or a defined mass loss. The logarithm of the median time to endpoint is then plotted against the reciprocal of absolute temperature (1/T) on an Arrhenius diagram. If the relationship is linear over the tested range, the line is extrapolated to a reference time of 20,000 hours, and the corresponding temperature is reported as the temperature index (TI). The halving interval (HIC), expressed in kelvin or degrees Celsius, describes the temperature change that halves the lifetime and characterizes the slope of the endurance line. Statistical techniques from IEC 60216-3 are applied to compute confidence limits and verify that extrapolation does not exceed justified bounds.
Result Interpretation
Depending on film thickness, crystallinity, and the chosen endpoint criterion, PPS films typically achieve temperature indices in the range of 180 to 220 °C, supporting thermal class H or class C ratings in many cases. Deviations from Arrhenius linearity, a sharp acceleration of degradation at the highest temperatures, or large scatter between specimen sets should trigger a review of oven uniformity, specimen preparation, and possible non-Arrhenius mechanisms such as rapid surface oxidation or crystallinity change. Well-designed reports include the regression equation, confidence intervals, all raw degradation curves, and a clear statement of the endpoint criterion used.
Applications
Thermal aging data for PPS insulation film directly support material qualification and design decisions across industries where sustained high temperature and reliable electrical insulation must coexist. Verified temperature indices allow engineers to specify PPS film with confidence and to justify extended service life or higher operating temperatures in thermal class H and class C equipment.
- Slot, phase, and layer insulation in motors and generators, including traction motors for electric vehicles
- Interlayer and wrap insulation in dry-type transformers and magnetic components
- Metallized film capacitors for automotive power electronics and industrial drives
- Flexible printed circuit substrates and high-temperature adhesive tapes
- Battery pack insulation, barrier films, and busbar wrapping in energy storage systems
- Aerospace avionics, downhole instrumentation, and other harsh-environment electronics
FAQ
What standards govern PPS insulation film thermal aging tests?
The IEC 60216 series defines the general procedures, test methods, statistical analysis, and oven requirements for determining the thermal endurance of electrical insulating materials. UL 746B covers long-term thermal aging for polymeric materials and is used to assign relative temperature index (RTI) values. IEC 60085 provides the framework for mapping the measured temperature index to a thermal class such as 155 (F) or 180 (H).
How long does a complete thermal aging test take?
A full determination of the temperature index commonly requires four to twelve months, sometimes longer, because the lowest aging temperature must run long enough to establish the endurance line with statistical confidence. Shorter comparative aging studies at one or two temperatures can be completed in weeks when the goal is screening or quality control rather than full certification.
Why is elongation at break used as the primary endpoint criterion?
Thermally aged PPS film tends to embrittle through oxidation and crosslinking well before it suffers a dramatic loss of dielectric strength. Elongation at break is highly sensitive to this embrittlement, so a 50% retention criterion marks the onset of mechanical fragility that could cause cracking during winding, vibration, or thermal cycling in service, providing a conservative and reproducible endpoint.
What is the difference between TI and RTI?
The temperature index (TI) comes from IEC 60216 and represents the temperature at which the extrapolated time to endpoint equals a specified reference time, usually 20,000 hours. The relative temperature index (RTI) is assigned under UL 746B using comparable long-term aging methods and may be differentiated by application, such as electrical only, mechanical with impact, or mechanical without impact. Both express long-term thermal capability but arise from different standardization systems.
Do humidity and atmosphere affect the test results?
PPS absorbs very little moisture, so ambient humidity has only a minor influence compared with polyamide films. However, oven air-exchange rate and the presence of circulating fresh air matter significantly, because oxygen drives the surface oxidation reactions responsible for embrittlement. Tests intended for certification must therefore control and document oven air throughput in accordance with IEC 60216-4-1.
