Application of Thermogravimetry-Mass Spectrometry (TG-MS) Technique in Pyrolysis Analysis of Fire Retardant Materials

Fire retardant materials are widely used in modern industry and daily life, making the study of their pyrolysis processes and flame retardant mechanisms crucial. The TG-MS technique combines the advantages of thermogravimetric analysis (TG) and mass spectrometry (MS), enabling real-time monitoring of mass changes and gas product composition of fire retardant materials during heating. This makes it a powerful tool for in-depth exploration of the pyrolysis behavior and flame retardant reaction pathways of fire retardant materials.

Principles of the TG-MS Technique

2.1 Principle of Thermogravimetric Analysis (TG)

Thermogravimetric analysis is a technique for measuring the mass change of a substance as a function of temperature under programmed temperature control. In the study of pyrolysis of fire retardant materials, the sample is placed in the sample cell of a thermogravimetric analyzer, which is heated at a constant heating rate. As the temperature rises, the fire retardant material undergoes decomposition, volatilization, and other reactions, leading to continuous mass changes. The thermogravimetric analyzer records the mass change curve of the sample with temperature or time (i.e., the TG curve) through a high-precision balance, intuitively reflecting the thermal stability and thermal decomposition process of the material.

2.2 Principle of Mass Spectrometry (MS)

Mass spectrometry is a technique for separating and detecting ions based on their mass-to-charge ratio (m/z) using electromagnetic principles after ionizing sample molecules. In MS, sample molecules are first ionized into various ions in an ion source. These ions are separated by their m/z in electric and magnetic fields and then detected by a detector. The detector converts the ion signals into electrical signals, which are amplified and processed to form a mass spectrum. Analyzing the mass spectrum allows determination of the composition and relative content of gas products.

2.3 TG-MS Hyphenation Technique

The TG-MS hyphenation technique connects a thermogravimetric analyzer and a mass spectrometer through a special interface, allowing the gas products from sample pyrolysis in the thermogravimetric analyzer to directly enter the mass spectrometer for analysis. The design of the interface is critical, as it must ensure rapid and efficient transfer of gas products to the mass spectrometer while preventing interference from external air to ensure the accuracy of the analysis results.

3. Application of TG-MS in Analyzing Gas Product Composition During Pyrolysis of Fire Retardant Materials

3.1 Sample Preparation and Experimental Setup

Before TG-MS experiments, fire retardant material samples need to be ground into an appropriate particle size (typically tens to hundreds of microns) to ensure uniform heating during pyrolysis. A suitable amount of sample is placed in the sample cell of the thermogravimetric analyzer, and experimental parameters are set, including the heating rate (usually 5–20°C/min) and temperature range (determined based on material properties, typically from room temperature to 800°C or higher). For the mass spectrometer, appropriate ion sources (e.g., electron impact ion source EI), scanning ranges (determined based on the m/z of possible gas products), and detection modes (e.g., full scan mode, selected ion monitoring mode SIM) are selected.

3.2 Qualitative and Quantitative Analysis of Gas Products

During the experiment, as the temperature rises, gas products from the pyrolysis of fire retardant materials continuously enter the mass spectrometer. Qualitative analysis of gas products is performed by analyzing mass spectra. For example, an ion peak at m/z 18 typically corresponds to water (H₂O), and a peak at m/z 44 may correspond to carbon dioxide (CO₂) or CO₂ generated by the reaction of carbon monoxide (CO) with oxygen (O₂). Accurate identification of gas product composition is achieved by comparing with standard mass spectral databases.

For quantitative analysis, internal standard or external standard methods are commonly used. The internal standard method involves adding a known amount of an internal standard substance to the sample and calculating the content of the target gas product by comparing the peak areas or intensities of the internal standard and the target product. The external standard method involves preparing a series of standard gases with known concentrations to plot a standard curve, from which the content of the target gas product in the sample is determined based on its peak area or intensity.

4. Revealing Flame Retardant Reaction Pathways of Fire Retardant Materials Based on TG-MS

4.1 Division of Pyrolysis Stages and Analysis of Gas Products

Based on TG curves and MS data, the pyrolysis process of fire retardant materials can be divided into different stages. At the initial stage, at lower temperatures, the material may undergo volatilization of small-molecule additives or cleavage of partially unstable chemical bonds, producing small-molecule gases such as methanol and ethanol. As the temperature rises, the main pyrolysis stage begins, during which macromolecular chains break, generating a large amount of volatile organic compounds (VOCs), such as benzene, toluene, and ethylene. At high temperatures, the material further decomposes, potentially producing gases such as carbon monoxide and carbon dioxide.

By analyzing the composition and content of gas products at different stages, the pyrolysis reaction process of fire retardant materials can be inferred. For example, if a large amount of phosphorus-containing gas products are detected at the initial pyrolysis stage, it may indicate that phosphorus-based flame retardants start to act at lower temperatures, inhibiting combustion through a gas-phase flame retardant mechanism.

4.2 Study on the Action Mechanism of Flame Retardants

The TG-MS technique clearly demonstrates the influence of flame retardants on the pyrolysis process of materials. For halogen-based flame retardants, hydrogen halide gases are released during pyrolysis, which capture free radicals in the combustion process to interrupt the combustion chain reaction. Through TG-MS analysis, the release temperature and amount of hydrogen halide gases can be detected to study the flame retardant reaction pathways of halogen-based flame retardants.

For intumescent flame retardants, which mainly consist of acid sources, carbon sources, and gas sources, the acid source first decomposes to produce acidic substances during pyrolysis, promoting the dehydration and carbonization of the carbon source to form a char layer, while the gas source decomposes to generate gases that expand the char layer. The TG-MS technique can monitor the release of acidic gases, water vapor, nitrogen, and other gas products, thereby revealing the flame retardant reaction pathways and synergistic mechanisms of intumescent flame retardants.

5. Challenges and Solutions in TG-MS Applications

5.1 Secondary Reactions of Gas Products

During pyrolysis, gas products may undergo secondary reactions in the connecting pipeline between the thermogravimetric analyzer and the mass spectrometer or in the ion source of the mass spectrometer, leading to discrepancies between the detected gas product composition and content and the actual situation. To address this, the design of the connecting pipeline can be optimized using high-temperature-resistant, inert materials to reduce the residence time of gas products in the pipeline. Additionally, optimizing the ion source parameters of the mass spectrometer (e.g., lowering the ion source temperature) can reduce the occurrence of secondary reactions.

5.2 Analysis of Complex Mixtures

Fire retardant materials are often complex mixtures of multiple components, and their pyrolysis produces a wide variety of gas products that interfere with each other, complicating analysis. Multidimensional chromatography-mass spectrometry hyphenation techniques (such as GC-MS-MS) combined with chemometric methods can be used to process and analyze complex mass spectral data, improving the ability to identify and quantitatively analyze gas product components in complex mixtures.

Conclusion

The thermogravimetry-mass spectrometry (TG-MS) technique provides a comprehensive and accurate analytical method for studying the pyrolysis process of fire retardant materials. Through this technique, the composition of gas products during the pyrolysis of fire retardant materials can be deeply analyzed, and their flame retardant reaction pathways can be revealed, providing important technical support for the research, development, performance optimization, and flame retardant mechanism studies of fire retardant materials. Although challenges exist in its application, with the continuous development and improvement of the technique, TG-MS will play an increasingly important role in the field of fire retardant materials, driving the progress and development of the fire retardant materials industry.

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