Thermogravimetric analysis (TGA) is the definitive method for evaluating the thermal stability and weight-loss behavior of PET high-temperature insulation films used in motors, transformers, and flexible electronics. This technical guide covers the testing principles, sample types, measurement parameters, analysis methods, and industrial applications of thermal weight loss testing for PET insulation films. Understanding TGA results enables engineers to select the correct insulation grade and predict long-term reliability under elevated operating temperatures.
Technical Overview
Polyethylene terephthalate (PET) film is one of the most widely used organic insulation materials in the electrical and electronics industry, valued for its excellent dielectric strength, mechanical toughness, chemical resistance, and cost efficiency. For high-temperature insulation applications such as motor slot liners, transformer layer insulation, and flexible printed circuits, the thermal stability of the film directly determines device safety and service life.
Thermogravimetric analysis (TGA) is a thermal characterization technique that continuously measures the mass of a specimen while it is subjected to a controlled temperature program. By recording weight loss as a function of temperature or time, TGA reveals moisture content, volatile additives, onset of polymer decomposition, maximum degradation rate, and residual char or filler content. For PET insulation film, the resulting thermogravimetric (TG) curve and its derivative (DTG) curve provide a quantitative fingerprint of thermal robustness that cannot be obtained from simple heat-aging tests.
In quality control and incoming inspection workflows, thermal weight loss analysis serves three purposes: verifying that the film meets specification limits for low-temperature volatiles, confirming that the polymer backbone decomposes within the expected temperature window, and detecting formulation defects such as excessive oligomer content or inadequate heat stabilization. Typical acceptance criteria require less than 0.5 percent weight loss below 300°C and a 5 percent decomposition temperature above 370°C for premium insulation grades.
Testing Samples
Standard Biaxially Oriented PET Film
Standard biaxially oriented PET film, typically 23 to 250 micrometers thick, is the baseline sample type for TGA evaluation. The biaxial stretching process aligns the polymer chains and increases crystallinity to approximately 35 to 45 percent, which influences the decomposition profile. Specimens are cut into small pieces of 2 to 3 millimeters and dried at 105°C for 2 hours before testing to remove surface moisture that could distort the low-temperature region of the TG curve.
Heat-Stabilized PET Film
Heat-stabilized grades undergo an additional annealing treatment above the glass transition temperature, typically between 150°C and 220°C, which locks in dimensional stability for continuous operation up to 120°C to 155°C. TGA of these films verifies that the stabilization process has not introduced low-molecular-weight residues and that the main decomposition stage remains above 380°C. Comparing stabilized and unstamped film is a common comparative study in material qualification.
Coated and Filled PET Composite Films
Testing Items
The thermal weight loss analysis of PET high-temperature insulation film typically reports the following parameters:
- Moisture and volatile content, measured as weight loss between room temperature and 150°C
- Onset decomposition temperature, defined as the temperature at 1 percent or 2 percent weight loss
- Five percent and ten percent weight-loss temperatures, commonly abbreviated as T5 percent and T10 percent
- Maximum mass-loss rate temperature, identified as the DTG peak temperature
- Final residue or char yield at 600°C to 800°C, indicating inorganic filler or coating fraction
- Total weight loss within the main decomposition stage, typically between 350°C and 500°C
- Comparative oxidation induction behavior under air versus inert nitrogen atmosphere
- Apparent activation energy of decomposition derived from multi-heating-rate kinetic analysis
Testing Methods
Instrumentation and Standard Conditions
Testing is performed on a precision thermogravimetric analyzer equipped with a microbalance having 1 microgram resolution. A specimen mass of 5 to 10 milligrams is placed in an open alumina or platinum pan and heated from ambient temperature to 800°C at a heating rate of 10°C per minute under nitrogen flowing at 50 to 60 milliliters per minute. The procedure follows the general principles of ISO 11358 and ASTM E1131, adapted to thin-film specimens.
Inert Atmosphere Analysis
Under nitrogen, high-purity PET film shows an initial slight mass loss below 150°C from absorbed moisture and residual oligomers, followed by a plateau region extending to approximately 350°C. The dominant single-step decomposition occurs between 380°C and 460°C, corresponding to random chain scission of the ester backbone with evolution of volatile products. A well-defined single DTG peak near 430°C to 445°C confirms a homogeneous polymer with minimal additive interference.
Oxidative Atmosphere Analysis
A duplicate run in air or oxygen evaluates thermo-oxidative stability, which is more representative of actual service conditions for insulation exposed to air. In oxygen, the decomposition onset shifts 10°C to 30°C lower and the degradation proceeds in two stages: primary chain scission followed by oxidative burn-off of carbonaceous residue. The difference between nitrogen and air onset temperatures is a useful indicator of oxidation sensitivity.
Kinetic Analysis
For advanced reliability assessment, specimens are scanned at multiple heating rates of 5, 10, 15, and 20°C per minute. Applying the Kissinger or Ozawa-Flynn-Wall method to the DTG peak shifts yields the apparent activation energy, typically 180 to 250 kilojoules per mole for PET thermal degradation. These kinetic parameters support extrapolation of degradation behavior to long-term, lower-temperature service conditions used in establishing the thermal class of the insulation system.
Data Interpretation and Acceptance Criteria
The TG curve is normalized to the initial dry mass, and each mass-loss step is quantified with tangent or stepwise analysis software. For premium high-temperature insulation film, typical specification values include weight loss below 0.5 percent up to 300°C, T5 percent greater than 370°C, a single main DTG peak above 425°C, and residue consistent with the declared inorganic content. Deviations such as early mass loss, multiple low-temperature steps, or depressed T5 percent indicate contamination, inadequate drying, or insufficient polymerization.
Applications
Thermogravimetric analysis of PET insulation film supports material qualification and process control across multiple industries:
- Slot insulation, phase insulation, and wedges in small and medium electric motors, where thermal class verification is mandatory
- Layer and interlayer insulation in transformers, reactors, and magnetic components operating at continuous elevated temperature
- Base film and coverlay substrates for flexible printed circuits requiring strict volatile control to prevent delamination during lamination
- Insulation barriers and wrap layers in lithium-ion battery modules, where outgassing behavior is safety-critical
- Dielectric films in film capacitors, where long-term thermal endurance correlates with decomposition kinetics
- Photovoltaic backsheet films, where TGA confirms stabilization package integrity after damp-heat aging
- Cable wrapping and heat-shrinkable product substrates evaluated for thermo-oxidative lifetime prediction
- Incoming inspection and supplier audits where TGA fingerprints authenticate resin grade and detect substandard substitutions
FAQ
What temperature does PET high-temperature insulation film begin to decompose?
High-quality heat-stabilized PET film typically shows an onset of significant decomposition near 370°C to 390°C under nitrogen, with the maximum degradation rate occurring between 430°C and 445°C. Measurable weight loss from moisture and volatiles appears below 150°C but should remain under 0.5 percent for premium grades.
Why are tests run in both nitrogen and air atmospheres?
Nitrogen reveals the intrinsic thermal stability of the polymer backbone, while air reflects real-world thermo-oxidative degradation. The difference between the two curves quantifies oxidation sensitivity, which is essential for films used in open-air high-temperature environments.
How much sample is required and how is it prepared?
Only 5 to 10 milligrams is needed. The film is cut into small pieces, dried at 105°C for 2 hours, and loaded into an open pan so that evolved gases escape freely and the mass signal reflects true decomposition rather than diffusion effects.
Can TGA results predict long-term service life?
TGA alone does not provide a service-life guarantee, but kinetic parameters such as activation energy, combined with long-term oven-aging tests, support thermal endurance assessment and temperature index determination in accordance with insulation system qualification practice.
What does a high residue at 800°C indicate?
Residue at 800°C corresponds to inorganic content such as coatings, pigments, or flame-retardant fillers. For pure film it is usually below 2 percent, while coated composite films show residue matching the declared inorganic loading, which is useful for verifying coating weight.