Table of Contents
Introduction and basics
Evolved gas analysis in combination with Fourier transform infrared spectroscopy (EGA-FTIR) is an established method for analyzing the thermal stability and emissions of additives in thermoplastics such as polyethylene (PE), polypropylene (PP) and polyamide (PA). EGA-FTIR enables the detection of degradation products and volatile, mostly low-molecular additives, particularly in the early stages of a thermoplastic process, even before macrodefects or significant material damage occur.
Functional principle of the EGA-FTIR method
Measuring principle
In EGA-FTIR, the polymer to be analyzed is heated during a controlled temperature program. The volatile substances released (e.g. emissions of additives, cleavage products, residual monomers) are transferred directly into a gas cell of the FTIR spectrometer and analyzed there (4). The resulting infrared spectra allow qualitative and (with calibration) also quantitative identification of the released substances based on characteristic absorption bands.
Procedure
- Heating the sample: The polymer material is heated in a special thermobalance oven (e.g. in TGA mode) under controlled conditions (temperature rise, defined atmosphere).
- Release of volatile compounds: Additives, plasticizers, low-molecular components or initial degradation products evaporate even at moderate temperatures and are discharged from the oven as a gas stream (“evolved gases”).
- Transfer to the FTIR: These gases are transported continuously or step by step via a transfer line into a gas cuvette of the FTIR spectrometer.
- IR analysis: In FTIR, the molecules are identified by their characteristic infrared absorption band. Each additive or degradation product has a specific IR spectrum (fingerprint), so that even complex mixtures can be analyzed qualitatively and – with calibration – quantitatively.
Specific emission products of thermoplastics
Polyethylene (PE)
- Main products: Aliphatic hydrocarbons during pyrolysis, gaseous products such as ethane, ethene, propane, propene, pentanes and other low molecular weight alkane and alkene compounds
- Oxidation products: CO, CO₂ during oxidation, especially in the later stages or at elevated temperatures
- FTIR characteristics: Intense bands for C-H stretching vibrations of aliphatic chains
- Special features: Virtually no nitrogen-containing compounds, as PE contains no nitrogen groups
Polypropylene (PP)
- Main products: Comparable to PE, but increased alkene emissions such as propene, 2-methylpropene and various alkene and alkane derivatives
- Oxidized degradation products: Aldehydes, ketones (especially acetaldehyde, acetones) and carboxylic acids (e.g. acetic acid), especially during oxidative degradation (2)
- Other gases: CO, CO₂, H₂ and smaller quantities of hydrogen
- FTIR characteristics: Typical C-H valence vibrations at slightly different wavenumbers of PE due to the methyl group structures
Polyamide (PA)
- Specific products: Ammonia (NH₃), caprolactam (in PA6), low molecular weight amides and cyclohexanone even at moderate temperatures (150-300°C)
- Other emissions: Butadiene, alkylamides and small quantities of aliphatic and aromatic nitrogen compounds
- FTIR characteristics: Especially the carbonyl band (C=O) around 1712 cm-¹ as well as absorption bands for NH and CO groups, which clearly distinguish PA-6 from PE and PP
Comparative overview
| Polymer | Main emission products | Specific molecules | Spectral characteristics |
|---|---|---|---|
| PE | Aliphatic KW, CO, CO₂ | Ethan, ethene, propane, pentane | C–H aliphatic |
| PP | Aliphatic hydrocarbons, aldehydes, CO₂ | Propene, acetaldehyde, acetic acid | C–H + methyl groups |
| PA | Amides, nitrogen compounds | Ammoniak, Caprolactam, Cyclohexanon | NH, C=O bands, aromatic fragments |
Application examples and research results
Various studies demonstrate the efficiency of the EGA-FTIR method.Biale et al. showed that the thermal degradation profiles of polypropylene (PP) and polyethylene (PE) can be detected very sensitively using EGA recordings. For PP, for example, the method showed a reduction in the start-of-degradation temperature as a result of artificial ageing, combined with changes in gas emissions (1).
Park et al. were able to precisely determine times and temperatures for the emission of specific pyrolysis products from various thermoplastics using TG-FTIR. In particular, gases with low molecular weight – such as additives or monomers – were quantified early in the temperature program (2).
Cuthbertson et al. described the possibility of identifying additives using FTIR spectra in EGA mode and tracking their concentration via the temperature development (3).
Advantages and areas of application
Specific advantages
- High sensitivity for volatile and semi-volatile organic additives
- Early detection: All volatile and semi-volatile additives are detected in the early stages of the heating process – even before macroscopic changes to the solid are visible
- Specific identification of individual emissions via characteristic FTIR bands
- Can be integrated into existing thermobalance systems (5)
- Wide range of applications: In addition to additives, residual monomers, solvents or chemical modifications can also be monitored via their gas emissions
Areas of application
- Quality assurance of raw polymers
- Additive stability in the recycling process
- Development of low-pollutant formulations
- Error analysis in everyday laboratory work
- Fast, non-destructive quality monitoring
- Root cause analysis for laboratory, production or recycling processes
- Testing of raw materials
- Development of new additive systems
Conclusion
The EGA-FTIR method is ideal for the proactive monitoring and development of sustainable polymer formulations with controlled emission profiles. These specific emission products enable early and selective identification of thermoplastics and their additives already in the early-stage thermal process. Laboratory users and engineers will find EGA-FTIR a powerful package for routine testing, failure analysis and in-process control.
List of sources
(1) Biale, G. et al. (2021). A Systematic Study on the Degradation Products of Polypropylene and Other Common Polymers via EGA-MS and Py-GC-MS Analyses. PMC. https://pmc.ncbi.nlm.nih.gov/articles/PMC8234390/
(2) Park, K.B. et al. (2023). Pyrolysis products from various types of plastics using TG-FTIR. ScienceDirect. https://www.sciencedirect.com/science/article/abs/pii/S0165237023001274
(3) Cuthbertson, A.A. et al. (2024). Characterization of polymer properties and identification of additives: Opportunities with TGA-FTIR. RSC Publishing. https://pubs.rsc.org/en/content/articlehtml/2024/gc/d4gc00659c
(4) Measurlabs (2006). Evolved Gas Analysis (EGA) | TGA-FTIR & TGA-MS. https://measurlabs.com/methods/evolved-gas-analysis/
(5) Linseis Messgeräte GmbH (2025). Description of the Gas Analysis L40 EGA FTIR for thermobalances. https://www.linseis.com/en/instruments/additional-devices-support/l40-ega-ftir/ct*. https://www.sciencedirect.com/science/article/abs/pii/S0165237023001274
