Thermogravimetric Testing of Chemically Active Substances

Thermogravimetric testing of chemically active substances is a core thermal analysis technique used to evaluate the decomposition behavior, thermal stability, and mass-change characteristics of reactive materials such as oxidizers, organic peroxides, and self-heating compounds. By continuously monitoring sample mass under controlled temperature programs and atmospheres, this method delivers essential data for hazard classification, process safety design, and regulatory compliance. Accredited third-party laboratories perform these tests in accordance with internationally recognized standards such as ISO 11358, ASTM E2550, and GB/T 27761.

Technical Overview

Thermogravimetric analysis (TGA, also referred to as TG testing) is an analytical technique that measures the change in mass of a specimen as a function of temperature or time while the specimen is subjected to a controlled temperature program in a specified atmosphere. For chemically active substances—materials that may readily oxidize, decompose, dehydrate, hydrate, or react with air or moisture—the mass-change curve provides direct, quantitative insight into their reactivity and thermal hazards.

Chemically active substances typically include oxidizing solids and liquids, organic peroxides, pyrophoric substances, water-reactive materials that emit flammable gases, self-heating substances, and self-reactive compounds. Because these materials can undergo exothermic decomposition under relatively mild thermal conditions, thermogravimetric testing is frequently used as a screening and characterization tool within dangerous goods classification schemes, such as those described in the UN Manual of Tests and Criteria and the Globally Harmonized System of Classification and Labelling of Chemicals (GHS).

Modern thermogravimetric analyzers can detect microgram-level mass changes, resolve overlapping decomposition stages, and be coupled with differential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy (FTIR), or mass spectrometry (MS) to identify evolved gases. Testing is generally conducted by accredited third-party laboratories in conformance with standards including ISO 11358, ASTM E2550, ASTM E1131, ASTM E1877, and GB/T 27761.

Testing Samples

Oxidizing Substances and Organic Peroxides

Solid and liquid oxidizers, peroxide formulations, and peroxide-containing polymer initiators are tested to determine decomposition onset temperatures, available oxygen release behavior, and mass-loss characteristics under inert and oxidative atmospheres. These results support classification under UN Division 5.1 and 5.2 criteria.

Pyrophoric and Water-Reactive Substances

Finely divided metal powders, metal hydrides, alkali metals, and other materials that ignite spontaneously on contact with air or release flammable gases upon contact with water can be evaluated for oxidation-induced mass gain, reaction onset, and reaction kinetics under carefully controlled humidity and atmospheric conditions.

Self-Reactive and Energetic Materials

Self-reactive substances, energetic intermediates, and thermally unstable compounds are screened for their decomposition temperature limits, exothermic mass-loss events, and accumulated thermal history, providing supporting data for self-accelerating decomposition temperature (SADT) assessment and thermal stability evaluation.

Self-Heating Substances and Bulk Reactive Powders

Coal, carbon black, metal sulfide concentrates, biomass char, dried sewage sludge, and other bulk materials prone to slow exothermic oxidation in storage are examined for oxidation mass-gain behavior and low-temperature reactivity trends relevant to transport classification.

Industrial Catalysts, Battery Materials, and Chemical Intermediates

Heterogeneous catalysts, cathode and anode active materials for lithium-ion batteries, desiccants, adsorbents, and reactive chemical intermediates are characterized for moisture and volatile content, thermal decomposition profiles, oxygen uptake, and residue composition.

Testing Items

Typical thermogravimetric test items for chemically active substances include:

• Thermal decomposition onset temperature and decomposition temperature range
• Mass-loss rate and peak mass-loss rate temperature for each decomposition stage
• Total mass loss, residual mass, and ash or inorganic residue content
• Moisture content, solvent content, and volatile matter determination
• Oxidation onset temperature and oxidation-induced mass gain behavior
• Kinetic parameters, including apparent activation energy and pre-exponential factor, derived from multi-heating-rate experiments
• Thermal stability temperature limit and maximum safe operating or storage temperature screening
• Compositional analysis of multi-component formulations based on staged mass loss
• Supporting data for self-accelerating decomposition temperature (SADT) estimation and dangerous goods classification

Testing Methods

Dynamic (temperature-scanning) tests are most common: the specimen, typically 2 to 20 mg, is heated at a constant rate—commonly 5, 10, or 20 °C/min—from ambient temperature to a target of 600 to 1000 °C while mass is recorded continuously. For chemically active substances, a conservative initial heating rate of 5 °C/min is often selected to improve resolution of overlapping decomposition events and reduce thermal lag effects.

Atmosphere control is critical for reactive materials. Tests may be conducted under high-purity nitrogen or argon to characterize intrinsic decomposition, under air or oxygen to study oxidation and combustion-related mass change, or under controlled humidity to evaluate water-reactivity trends. Crucible selection—open alumina, platinum, or sealed high-pressure crucibles with pinhole lids—is matched to the volatility, corrosivity, and reactivity of the sample.

Isothermal methods hold the specimen at a constant temperature for a defined period to quantify mass loss or mass gain over time, which is useful for storage stability studies and low-temperature oxidation screening. Kinetic analysis is typically performed using multi-heating-rate approaches such as the Kissinger method, Ozawa–Flynn–Wall method, or Friedman isoconversional analysis, following the general principles of ASTM E2890 and ASTM E1640 where applicable.

Instrument qualification includes mass calibration with certified weights, temperature calibration using Curie point standards or melting point reference materials, and temperature-modified procedures consistent with ASTM E2040 and ASTM E1582. Where evolved gas identification is required, hyphenated TGA-DSC, TGA-FTIR, or TGA-MS configurations are employed to correlate mass-loss stages with specific gaseous decomposition products.

Applications

Thermogravimetric testing of chemically active substances serves a wide range of industrial and regulatory purposes:

• Classification of dangerous goods for transport under the UN Model Regulations, including evaluation of oxidizing, self-heating, pyrophoric, and water-reactive hazard classes
• Thermal hazard assessment and thermal stability screening for process safety management in chemical manufacturing
• Supporting data generation for self-reactive substance classification and SADT determination programs
• Shelf-life evaluation, storage condition definition, and expiration dating of reactive chemical products
• Quality control of incoming raw materials, intermediates, catalysts, and battery active materials
• Research and development support for formulation optimization, filler and additive content verification, and thermal decomposition mechanism studies
• Root cause investigation of thermal incidents, unexpected mass loss, or product degradation failures
• Regulatory documentation for safety data sheets (SDS), transport compliance dossiers, and customer technical submissions

FAQ

What sample quantity is required for thermogravimetric testing?

Typically 5 to 50 mg of homogeneous material is sufficient, though testing of heterogeneous or low-density powders may require replicate runs or larger consolidated specimens to ensure representativeness.

How is the decomposition onset temperature determined?

The onset temperature is commonly defined as the temperature at which the mass-loss curve first deviates from the established baseline by a specified threshold, or as the intersection of the extrapolated baseline and the tangent to the steepest portion of the initial mass-loss step. Because onset values depend on heating rate, sample mass, and atmosphere, test conditions should always be reported with the results.

Can TGA directly determine SADT for classification?

No. TGA provides valuable supporting evidence on decomposition onset and kinetics, but the self-accelerating decomposition temperature for regulatory classification must be established through the dedicated large-scale heat accumulation and isothermal storage tests specified in the UN Manual of Tests and Criteria.

Why test under both inert and oxidative atmospheres?

Inert atmospheres reveal the intrinsic thermal decomposition behavior of the substance, while air or oxygen atmospheres expose oxidation-driven mass gain and oxidative decomposition. Comparing the two curves distinguishes pure thermal degradation from oxidation and combustion-related processes, which is essential for hazard interpretation.

Is thermogravimetric testing safe for energetic or highly reactive samples?

Yes, when performed by experienced personnel. Milligram-scale specimens, shielded furnaces, controlled atmospheres, and staged screening protocols keep the energy release within safe limits. Preliminary hazard review of the material is conducted before testing to define safe operating envelopes.

What is the typical turnaround time?

Standard thermogravimetric testing is generally completed within 5 to 10 working days after sample receipt, depending on the number of test conditions, kinetic programs, and any coupled evolved-gas analysis required.