Determination of the decomposition reaction heat of polyethylene terephthalate (PET) delivers essential thermodynamic data for thermal stability assessment, process safety evaluation, and the design of mechanical and chemical recycling routes. Using techniques such as differential scanning calorimetry (DSC), simultaneous TGA-DSC analysis, and reaction calorimetry, laboratories quantify the enthalpy released or absorbed when PET degrades under precisely controlled temperature and atmosphere conditions. This article explains the underlying principles, sample requirements, measurement procedures, and typical applications of PET decomposition heat testing.
Technical Overview
Polyethylene terephthalate (PET) is a semicrystalline engineering polyester widely used in beverage bottles, packaging films, synthetic fibers, and injection-molded components. The polymer is thermally stable through its glass transition (approximately 70–80 °C) and melting range (approximately 250–260 °C), but when it is heated above roughly 350–400 °C under an inert atmosphere, the polymer backbone undergoes random chain scission. This degradation produces volatile fragments such as acetaldehyde, benzoic acid, terephthalic acid, vinyl benzoate, carbon monoxide, carbon dioxide, and light hydrocarbons.
The decomposition reaction heat, also expressed as the enthalpy of degradation, is the net thermal energy absorbed or released per unit mass (J/g) or per mole of repeating unit (kJ/mol) as the polymer breaks down. Under nitrogen, PET pyrolysis is predominantly an endothermic bond-scission process, whereas in air or oxygen the degradation includes strongly exothermic oxidation and combustion reactions. Quantifying this enthalpy is critical for four principal reasons: defining safe processing windows in extrusion and molding, evaluating fire hazard and flammability behavior, supplying input data for pyrolysis reactor design, and characterizing depolymerization reactions used in chemical recycling.
Modern thermal analysis instrumentation integrates heat flow signals with mass loss data, allowing laboratories to correlate each decomposition stage with its corresponding enthalpy change. Because the measured value depends strongly on heating rate, atmosphere, sample geometry, and residual moisture or catalyst content, standardized test protocols are essential for obtaining reproducible and comparable results.
Testing Samples
Virgin PET Resin
Pelletized virgin PET resin is the reference material for baseline decomposition studies. Pellets are typically cryo-milled or cut into small fragments (2–5 mg pieces) to improve thermal contact and ensure representative sampling. Virgin resin measurements establish the intrinsic thermal degradation profile of the polymer and serve as the control against which modified or recycled materials are compared.
Post-Consumer Recycled (rPET) Flakes
Washed bottle flakes from post-consumer recycling streams frequently carry trace contaminants, residual moisture, and additives that shift the decomposition onset and alter the measured heat. Testing rPET flakes after controlled drying reveals how thermo-oxidative history and impurities affect degradation enthalpy, which is a key input for approving recycled feedstock for food-contact and high-temperature applications.
PET Fibers, Films, and Technical Parts
Oriented fibers, biaxially stretched films, and glass-filled or mineral-filled molded parts can be tested directly after cutting. Filled composites require careful interpretation because the measured mass-normalized heat is diluted by the non-degrading filler fraction, and the residual char yield at 600 °C is used to back-calculate the filler content.
PET Blends and Copolyesters
Blends with polycarbonate, PLA, or impact modifiers, as well as copolyesters such as PETG and modified PET with isophthalate or CHDM units, exhibit distinct multi-step decomposition profiles. Separate integration of each exothermic or endothermic event allows the enthalpy contribution of each component to be resolved.
Chemical Depolymerization Reaction Mixtures
For chemical recycling development, samples of PET reacted with ethylene glycol, methanol, water, or amines under catalyst loading are analyzed to measure the heat flow of glycolysis, methanolysis, hydrolysis, or aminolysis reactions. These reaction heats directly inform reactor design, energy balance calculations, and thermal runaway risk assessment.
Testing Items
A comprehensive PET decomposition heat determination typically reports the following parameters:
- Extrapolated onset temperature of decomposition (°C), determined by the tangent method
- Peak decomposition temperature and temperature at maximum mass-loss rate (°C)
- Heat of decomposition in inert atmosphere (J/g or kJ/mol of repeating unit), obtained by peak-area integration
- Oxidative decomposition heat in air or oxygen (J/g), including the exothermic oxidation and char burn-off steps
- Total heat evolved or absorbed across the full degradation range, with baseline correction
- Residual mass (char yield) at a defined final temperature such as 600 °C (%)
- Apparent activation energy and pre-exponential factor derived from multi-rate experiments (ASTM E698)
- Adiabatic temperature rise and self-heating rate for safety-focused studies (ARC)
- Reaction enthalpy for depolymerization reactions such as glycolysis or methanolysis (kJ/mol PET)
Testing Methods
Differential Scanning Calorimetry (DSC)
Heat-flux or power-compensation DSC is the primary tool for measuring the decomposition enthalpy. A 3–10 mg sample is placed in an open or pierced aluminum pan, purged with high-purity nitrogen (typically 50–60 mL/min), and heated from ambient to approximately 550–600 °C at 10 °C/min. Temperature and enthalpy calibration are performed with certified reference materials, most commonly indium (melting point 156.6 °C, enthalpy 28.45 J/g) and zinc. The decomposition peak observed above the melting transition is integrated between properly constructed baselines to yield the heat of decomposition. Tests conducted in air or oxygen separately quantify the exothermic oxidative decomposition heat, following the general approach of ASTM E537.
Simultaneous Thermogravimetry–DSC (TGA-DSC / STA)
Simultaneous thermal analysis records mass loss and heat flow on the same specimen in a single experiment, typically using an open alumina crucible. This coupling allows each mass-loss step (for example, moisture evolution, main-chain pyrolysis, and carbonaceous residue oxidation) to be matched with its endothermic or exothermic enthalpy. The method complies with the framework of ISO 11358 for thermogravimetry of polymers and ASTM E1131 for compositional analysis, and it is the preferred technique when overlapping events make standalone DSC interpretation ambiguous.
Kinetic Analysis by Multiple Heating Rates
Running the decomposition at several heating rates (for example 5, 10, 15, and 20 °C/min) enables determination of apparent activation energy using the Kissinger or Ozawa–Flynn–Wall approaches, consistent with ASTM E698. These kinetic parameters, combined with the measured enthalpy, allow extrapolation of decomposition heat release to real process time scales and support thermal lifetime predictions.
Reaction and Adiabatic Calorimetry for Depolymerization
For chemical recycling applications, a jacketed reaction calorimeter measures the heat flow of PET depolymerization in real time while reagent addition, stirring, and temperature ramps are controlled. Accelerating Rate Calorimetry (ARC) or adiabatic calorimeters are used to evaluate worst-case runaway scenarios by tracking self-heating rates and adiabatic temperature rise. Together, these methods provide the reaction enthalpy and safety margin data required for scale-up of glycolysis, methanolysis, and hydrolysis processes.
Quality Control of Test Conditions
- Pre-dry hygroscopic samples (for example, 2 h at 120 °C under vacuum) to remove absorbed moisture that distorts the baseline
- Use identical purge gas, flow rate, and crucible type between calibration and measurement runs
- Verify baseline repeatability with an empty-pan run and apply subtraction where required
- Keep sample mass within 3–10 mg to limit thermal gradients and volatile back-reaction
- Clean or replace crucibles between runs to prevent residue carryover
Applications
Determining the decomposition reaction heat of PET supports a broad range of industrial and research objectives:
- Processing window definition: confirming that extrusion, injection molding, and melt-spun fiber lines operate safely below the degradation onset while accounting for exothermic oxidation during prolonged residence
- Chemical recycling process design: supplying reaction enthalpies for glycolysis, methanolysis, hydrolysis, and aminolysis reactors, including energy balances and heat-removal requirements
- Pyrolysis and energy recovery: providing input data for reactor modeling, product yield prediction, and process integration in PET upcycling or fuel production
- Fire safety engineering: characterizing the exothermic heat release associated with thermal and thermo-oxidative degradation as input for flame-retardant formulation and hazard analysis
- Quality control of recycled PET: detecting shifts in degradation onset and enthalpy caused by contaminants, prior thermal abuse, or chain modifiers in rPET streams
- Material development: comparing the thermal stability of novel copolyesters, nucleated grades, and bio-based PET alternatives
- Regulatory and standards compliance: demonstrating thermal behavior consistent with ASTM E537, ASTM E698, ASTM E1131, ISO 11357, and ISO 11358 frameworks
FAQ
At what temperature does PET begin to decompose?
Under nitrogen at a standard heating rate of 10 °C/min, PET typically shows an extrapolated decomposition onset between 350 °C and 400 °C, with the main degradation peak near 430–450 °C. In air, thermo-oxidative degradation can begin substantially earlier, often between 250 °C and 350 °C, depending on residence time and sample history.
Is PET decomposition endothermic or exothermic?
In an inert atmosphere, the primary chain-scission pyrolysis of PET is largely endothermic. In air or oxygen, oxidation of the degradation products and char burn-off introduce strong exothermic contributions. A complete determination therefore reports the enthalpy separately for inert and oxidative atmospheres, as the sign and magnitude of the heat change with the test environment.
How is the decomposition heat distinguished from the melting peak?
The melting endotherm of PET occurs near 250–260 °C, well separated from the decomposition region above 350 °C. The analyst integrates only the high-temperature decomposition event using a tangent baseline established after the melt, and any cold crystallization or annealing peaks at intermediate temperatures are excluded from the degradation enthalpy calculation.
How much sample is required for a test?
Standard DSC and TGA-DSC measurements require only 3–10 mg of material, typically a few pellet fragments or cut pieces. Additional material is requested for repeat runs, multi-rate kinetic studies, and verification testing in a second atmosphere.
Which standards govern PET decomposition heat determination?
Although no single standard is dedicated exclusively to PET decomposition enthalpy, testing is performed under the general frameworks of ASTM E537 (thermal stability by DSC), ASTM E698 (Arrhenius kinetic constants by DSC), ASTM E1131 (compositional analysis by TGA), and the ISO 11357 and ISO 11358 series covering DSC and thermogravimetry of plastics.
Can the test evaluate the quality of recycled PET?
Yes. Comparing the decomposition onset, peak temperature, enthalpy, and residual mass of rPET against a virgin reference reveals the extent of prior thermal degradation, contamination, and additive loading. A depressed onset or altered enthalpy indicates reduced thermal stability, guiding the suitability of the recycled feedstock for demanding applications.