Thermal Decomposition Testing of Chemically Active Substances

Thermal decomposition testing of chemically active substances is a critical safety assessment used to identify the temperatures at which reactive materials begin to decompose, release heat, and generate pressure or hazardous gases. Accredited third-party laboratories perform these evaluations using calorimetric and thermanalytical methods in accordance with UN, ISO, IEC, ASTM, EN, and GB standards to support transport classification, safe storage, and process safety. This article outlines the sample types, testing items, methods, and typical applications of thermal decomposition testing for chemically active materials.

Technical Overview

Chemically active substances—including self-reactive chemicals, organic peroxides, oxidizers, energetic materials, and reactive intermediates—can undergo exothermic decomposition when exposed to heat, friction, contamination, or prolonged storage. Thermal decomposition testing is designed to characterize this behavior quantitatively: it determines the decomposition onset temperature, heat release rate, total heat of decomposition, gas and pressure evolution, and the kinetics of runaway reactions.

The consequences of uncontrolled decomposition include self-heating, rapid pressure buildup, vessel rupture, deflagration, and toxic gas release. For this reason, thermal stability data form the scientific basis for classifying self-reactive substances (UN Division 4.1) and organic peroxides (UN Division 5.2), for assigning temperature control and emergency temperature requirements during transport, and for defining safe operating limits in industrial processes.

Testing is typically performed in a tiered approach. Small-scale screening methods such as differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) identify thermal events using milligram-scale samples. Where significant exotherms are found, adiabatic calorimetry and storage tests are used to simulate real-world heat accumulation and determine parameters such as the self-accelerating decomposition temperature (SADT), adiabatic temperature rise, and time to maximum rate. All results are interpreted against internationally recognized criteria from the UN Recommendations on the Transport of Dangerous Goods, the Manual of Tests and Criteria, and relevant ISO, IEC, ASTM, EN, and GB standards.

Testing Samples

Self-Reactive Substances and Organic Peroxides

Solid and liquid self-reactive formulations, polymerization initiators, curing agents, peroxide-based catalysts, and pastes intended for commercial packaging are tested to determine their decomposition characteristics and SADT for transport classification.

Oxidizing Agents and Energetic Materials

Oxidizers, pyrotechnic compositions, propellant components, and other energetic substances are evaluated for thermal sensitivity, confinement effects, and rapid energy release behavior under controlled heating.

Reactive Intermediates and Process Streams

Unstable intermediates, diazo and azo compounds, nitrated aromatics, isocyanates, and concentrated reaction mixtures are screened during process development to support scale-up decisions and hazard studies.

Battery and Electrolyte Materials

Lithium-ion cell materials, electrolyte formulations, cathode and anode active materials, and separator-related chemicals are assessed for thermal runaway onset and heat release characteristics.

Pharmaceutical and Fine Chemical Products

Active pharmaceutical ingredients, intermediates, and fine chemicals with functional groups of concern are tested to establish safe drying, milling, and storage temperature limits.

Testing Items

• Decomposition onset temperature (heating-rate dependent screening values)
• Peak decomposition temperature and heat flow profile
• Total heat (enthalpy) of decomposition, expressed in J/g or kJ/mol
• Self-accelerating decomposition temperature (SADT)
• Control temperature and emergency temperature derivation for transport
• Adiabatic temperature rise (ΔTad) and phi-factor corrected adiabatic data
• Time to maximum rate (TMR) and temperature corresponding to TMR of 24 hours (TMR24 or TD24)
• Pressure and gas evolution data: maximum pressure, rate of pressure rise, gas volume
• Apparent kinetic parameters: activation energy, pre-exponential factor, reaction order, Arrhenius modeling
• Thermal stability under isothermal and long-term storage conditions
• Deflagration and confinement sensitivity indicators from time/pressure testing

Testing Methods

Differential Scanning Calorimetry (DSC)

DSC measures heat flow as a function of temperature using milligram-scale samples sealed in high-pressure crucibles. It provides decomposition onset and peak temperatures and the heat of reaction, following ASTM E537, ISO 11357, and GB/T 22232 principles. Multiple heating-rate programs allow estimation of Arrhenius kinetic parameters in line with ASTM E698.

Thermogravimetric Analysis (TGA)

TGA records mass loss during controlled heating, revealing decomposition stages, residual mass, and the temperature range of gas-releasing reactions. It is conducted in accordance with ISO 11358 and is particularly useful for distinguishing evaporation from true decomposition.

Accelerating Rate Calorimetry (ARC)

ARC provides adiabatic conditions in which the sample self-heats after an exotherm is detected. It yields adiabatic temperature rise, self-heat rate curves, time to maximum rate, pressure data, and corrected values after phi-factor adjustment, supporting process safety assessments consistent with ASTM E1981 guidance.

SADT Determination Tests

SADT is measured using the heat accumulation and storage test methods described in the UN Manual of Tests and Criteria (Tests H.1 through H.4), including Dewar vessel tests, isothermal storage tests, and package-scale heat accumulation tests performed at the actual commercial packaging size. The SADT is the lowest temperature at which self-accelerating decomposition occurs within the transport package over a defined observation period.

Pressure and Confinement Testing

Time/pressure testing in a closed vessel (UN Test C.1) evaluates whether a substance deflagrates by measuring the time for pressure to rise between defined thresholds. The Koenen test (UN Test E.1) assesses the effect of intense heat under confinement using a steel tube with a reduced orifice. These results feed directly into classification decisions.

Heat Flow and Isothermal Calorimetry

High-sensitivity Calvet-type and heat flow calorimeters quantify small heat releases at low temperatures, supporting long-term storage stability studies and shelf-life evaluation for temperature-sensitive products.

Applications

• Transport classification and temperature control requirements for self-reactive substances and organic peroxides under UN dangerous goods regulations
• Process hazard analysis, HAZOP support, and definition of safe process temperatures with defined safety margins below measured onset values
• Warehousing and storage design, including ventilation, segregation, and maximum ambient temperature limits
• Scale-up assessment for custom synthesis and fine chemical manufacturing
• Thermal runaway risk evaluation for lithium-ion battery materials and cells
• Data generation for Safety Data Sheet sections covering stability, reactivity, and transport information
• Shelf-life confirmation and stability studies for reactive commercial products
• Emergency response planning, including vent sizing input and decomposition gas characterization

FAQ

What is the difference between a DSC onset temperature and SADT?

The DSC onset temperature is a small-scale screening value obtained at a fixed heating rate with minimal heat loss, so it is always higher than the true thermal instability threshold. SADT reflects real packaging geometry and heat losses and is therefore the governing parameter for transport temperature control. A DSC onset does not by itself justify storage without additional adiabatic or storage test data.

How much sample is required for thermal decomposition testing?

DSC and TGA typically require only a few milligrams per run. ARC tests generally use grams of material, while SADT storage tests and time/pressure or Koenen tests may require tens of grams to a full commercial package, depending on the selected method and classification objective.

How are test results used to set safe operating temperatures?

Engineers combine onset temperatures, adiabatic data, and kinetics to calculate the temperature at which the time to maximum rate equals 24 hours, then apply appropriate safety margins depending on the criticality class of the process. Lower margins may be justified when reliable kinetic modeling is available, while conservative margins are applied where data are limited.

Why is pressure data collected alongside heat data?

Many chemically active substances generate permanent gases when they decompose. Even a modest exotherm can cause catastrophic vessel rupture if gas production is rapid. Pressure-versus-time records allow laboratories to distinguish condensable vapor generation from non-condensable gas formation and to support emergency relief system design.

Which standards govern this type of testing?

Commonly applied references include the UN Manual of Tests and Criteria, ASTM E537, ASTM E698, ASTM E1981, ISO 11357, ISO 11358, and corresponding EN and GB series standards. Accredited third-party laboratories select the specific method based on the physical state of the sample, the packaging configuration, and the regulatory purpose of the assessment.