PET thermogravimetric analysis (TGA) testing is an essential thermal characterization technique for evaluating the thermal stability, decomposition behavior, and compositional makeup of polyethylene terephthalate materials. By continuously monitoring mass loss as a specimen is heated under controlled atmospheres, TGA provides quantitative data on degradation onset temperatures, moisture and volatile content, filler loading, and residual ash. This article presents a comprehensive technical overview of PET TGA testing, covering sample types, measurable parameters, test methods, industry applications, and frequently asked questions.
Technical Overview
Thermogravimetric analysis (TGA) is a thermal analysis technique that measures the mass of a sample as a function of temperature or time under a precisely controlled atmosphere and heating program. For polyethylene terephthalate (PET), TGA is one of the most informative methods for assessing thermal robustness, because the mass-loss profile directly reflects how the polymer degrades, oxidizes, and decomposes. A typical PET TGA curve under an inert nitrogen atmosphere exhibits a small initial mass loss below 150 °C due to moisture and residual volatiles, followed by a single, sharp decomposition step between approximately 350 °C and 500 °C, with a maximum mass-loss rate (DTG peak) near 440–460 °C. Neat, unfilled PET leaves virtually no residue at the end of the test, whereas filled, reinforced, or flame-retardant grades leave a characteristic inorganic residue that quantifies the filler content.
Because PET is processed at melt temperatures of roughly 260–290 °C, confirming that degradation does not begin until well above this window is critical for extrusion, injection molding, and fiber spinning operations. In oxidative (air) atmospheres, PET degrades in multiple overlapping steps: thermo-oxidative degradation begins at lower temperatures than in nitrogen, followed by char formation and subsequent burnout of the carbonaceous residue above 550 °C. The differences between nitrogen and air runs are themselves diagnostic, revealing the susceptibility of the material to oxidative attack. Modern TGA instruments offer temperature accuracy within ±1 °C, mass resolution of 0.1 μg, and automated gas switching, enabling highly reproducible compositional and kinetic analysis of PET materials.
Testing Samples
PET Resins and Pellets
Virgin PET resin in pellet form is the most commonly submitted sample type. Pellets are typically cut or cryo-ground into small pieces (1–3 mg fragments) to ensure uniform heat transfer and representative sampling. Testing of virgin resin establishes baseline thermal stability data for incoming material inspection and supplier qualification.
PET Films, Sheets, and Bottles
Biaxially oriented PET film, thermoforming sheet, and bottle-grade PET (including sections cut from preforms, bottles, or trays) are tested to verify that forming and orientation processes have not compromised thermal performance. Thin sections are ideal for TGA because they heat rapidly and uniformly, minimizing thermal lag artifacts.
PET Fibers and Textiles
Polyester staple fiber, filament yarn, and nonwoven fabrics are analyzed to confirm spinning stability and to detect finishes, spin finishes, or delustrants such as TiO2. Because fibers often contain optical brighteners and sizing agents, the low-temperature mass-loss region provides useful information about these organic additives.
Recycled PET (rPET) and Compounds
Recycled flake, granulate, and compounded rPET are tested to assess contamination levels, residual moisture, and the presence of foreign polymers or fillers. TGA is particularly valuable for quality control of recycled feedstock, where multi-step decomposition curves can reveal polyolefin or PVC contamination that shifts or broadens the degradation profile.
Filled, Reinforced, and Flame-Retardant PET
Glass-fiber reinforced PET (PET-GF), PET containing mineral fillers, talc, calcium carbonate, or montmorillonite nanoclays, and flame-retardant formulations are analyzed to quantify inorganic content through the residual mass at the end of the test. This is a fast, reliable alternative to ash testing in a muffle furnace.
Testing Items
A standard PET thermogravimetric analysis report can include the following quantitative results:
- Thermal stability assessment: confirmation that the decomposition profile is consistent with well-defined PET and free of anomalous low-temperature degradation.
- Moisture and volatile content: mass loss below 150 °C, quantifying absorbed water and residual solvents or monomers, typically reported as a weight percentage.
- Onset temperature of decomposition (Tonset): the intersection temperature of the baseline tangent and the leading-edge tangent of the degradation step, indicating where rapid mass loss begins.
- T5% and T10% temperatures: the temperatures at which 5% and 10% of the original mass have been lost, widely used comparative stability metrics.
- Peak decomposition temperature (Tmax or Tp): the temperature at the maximum rate of mass loss, determined from the derivative thermogravimetric (DTG) curve.
- Residue and ash content: the residual mass at 600–800 °C under nitrogen (or after burnout in air), directly quantifying glass fiber, mineral filler, pigment, or catalyst residues.
- Compositional analysis: staged determination of volatiles, polymer content, carbonaceous char, and inorganic filler by switching from nitrogen to oxygen or air at a defined temperature, following ASTM E1131 methodology.
- Oxidative stability and char yield: comparison of nitrogen and air runs to evaluate thermo-oxidative degradation onset and char-forming behavior relevant to flame retardancy.
- Kinetic parameters: activation energy, pre-exponential factor, and reaction order derived from multiple-heating-rate experiments using Kissinger, Flynn–Wall–Ozawa, or Friedman methods, supporting service-life and processing-stability predictions.
Testing Methods
Sample Preparation
Specimens of 5–10 mg are accurately weighed into open platinum or alumina pans. Pellets and thick sections are cut into small fragments to improve thermal contact; films and fibers may be loosely ball-formed. Specimens are conditioned in a desiccator when moisture quantification is a test objective, or tested as-received when measuring total absorbed moisture.
Instrumentation and Calibration
Testing is performed on a research-grade thermobalance calibrated for temperature (using the Curie point standards or indium/tin/zinc melting standards) and for mass using certified weights. A purge gas flow of 40–60 mL/min is maintained throughout the run. Baseline correction with an empty pan is performed prior to analysis.
Standard Test Programs
The most common protocols applied to PET include:
- Dynamic scan in nitrogen: equilibration at 30–40 °C, ramp at 10 °C/min to 800 °C under high-purity nitrogen; used for Tonset, T5%, Tmax, and residue determination (ISO 11358-1).
- Compositional analysis (ASTM E1131): ramp at 20 °C/min in nitrogen to 600 °C, switch to air or oxygen, hold to 900 °C; separates volatiles, polymer, carbon char, and inorganic filler.
- Rapid degradation screening (ASTM D3850): fast heating (e.g., 20 °C/min or faster) to compare the relative thermal endurance of PET formulations.
- Oxidative run in air: identical ramp in air to evaluate thermo-oxidative stability, char yield, and soot/carbon burnout behavior.
- Multi-rate kinetic study: repeat scans at 5, 10, 15, and 20 °C/min for isoconversional kinetic analysis and lifetime estimation.
Data Analysis and Reporting
The TGA curve (mass versus temperature) and its first derivative (DTG curve) are evaluated to extract step boundaries, tangent-based onset temperatures, and stepwise mass losses. Reports include the full curve, tabulated characteristic temperatures, each mass-loss step as a percentage, the atmosphere program, heating rate, and instrument parameters, ensuring traceability and comparability between laboratories.
Applications
PET thermogravimetric analysis supports a wide range of industrial, regulatory, and research objectives:
- Processing window validation: verifying that PET decomposition begins well above melt-processing temperatures of 260–290 °C, safeguarding extrusion, injection molding, stretch-blow molding, and fiber spinning operations.
- Incoming resin inspection and supplier qualification: comparing T5% and Tmax values against specifications to detect under-polymerized, contaminated, or degraded lots.
- Recycled PET quality control: identifying polymer contamination, fillers, and degradation shifts in rPET streams intended for food-contact or fiber applications.
- Filler and reinforcement quantification: rapid determination of glass fiber, mineral filler, and pigment content in engineering PET compounds without lengthy muffle-furnace ashing.
- Flame retardancy development: measuring char yield, onset of oxidative degradation, and residue stability to screen intumescent and mineral-based flame-retardant systems.
- Additive and stabilizer evaluation: assessing the effect of phosphite, hindered phenol, and chain-extension additives on thermal and thermo-oxidative stability.
- Service-life prediction: applying TGA kinetics to estimate long-term thermal aging behavior of PET films, electrical insulation, solar backsheets, and geotextiles.
- Failure analysis: comparing degraded or discolored field returns against reference material to confirm thermal or oxidative damage as the root cause.
FAQ
How much sample is required for PET TGA testing?
A typical test requires only 5–10 mg of material. For heterogeneous samples such as filled compounds or recycled flakes, several fragments can be combined or the specimen can be cryo-ground to ensure the small specimen represents the bulk material.
What temperature range is used?
Most PET runs span from ambient temperature (or 30–40 °C) to 800 °C in nitrogen, or to 900 °C when a switch to air is included for full compositional analysis and carbon burnout. This range captures moisture loss, the main decomposition step, and complete residue characterization.
What is the difference between running PET in nitrogen and in air?
In nitrogen, PET undergoes a single thermal decomposition step near 350–500 °C with almost no residue for neat resin, reflecting intrinsic thermal stability. In air, degradation begins at a lower temperature due to oxidation and proceeds in multiple steps, with carbonaceous residue burning off above approximately 550 °C. Comparing both runs distinguishes purely thermal from thermo-oxidative behavior.
Can TGA quantify moisture in PET before processing?
Yes. The mass loss below 150 °C corresponds to absorbed water and low-boiling volatiles. This is particularly useful for PET, which must be dried to very low moisture levels (typically below 50 ppm) before melt processing to prevent hydrolytic molecular-weight loss, and TGA provides a rapid verification method.
Can TGA measure the crystallinity or molecular weight of PET?
No. Crystallinity and melting behavior are measured by differential scanning calorimetry (DSC), and molecular weight requires solution viscometry or gel permeation chromatography (GPC). TGA, DSC, and GPC are complementary techniques, and a full PET characterization program often combines all three.
Which standards govern PET thermogravimetric testing?
The principal standards are ISO 11358-1 (plastics — thermogravimetry of polymers, general principles), ASTM E1131 (compositional analysis by thermogravimetry), and ASTM D3850 (rapid thermal degradation of plastics by TGA). Test programs can be tailored to client-specific or end-customer specifications as required.
How long does a PET TGA test take?
A single dynamic scan to 800 °C at 10 °C/min takes approximately 80–90 minutes including purge equilibration. Multi-atmosphere compositional runs and multi-heating-rate kinetic studies require proportionally longer total time, and standard reporting turnaround is typically three to five working days.