Thermal Stability and Screening Testing

The air over layer test is conducted to establish the onset temperature of exothermic activity on thin layers of materials when exposed to elevated atmospheric temperatures. It replicates thin layer (up to 15 mm) deposits in a drying situation i.e. drier walls or roofs where hot air may rush over the surface.

Using a small tray test cell, a sample is placed inside a cell and thermocouples are placed into the material at three locations. This assembly is then placed into a digitally controlled horizontal laboratory tube oven with a controlled heated air supply.

Either of the below tests can be performed using a data acquisition system to monitor, detect and record any exothermic activity.

An environmental temperature is digitally increased from ambient conditions up to 400°C at 0.5°C.min-1. This test configuration is used for estimating exothermic activity for onset temperature.

The loaded test cell is placed into a heated oven and set to a specified temperature for a duration of 24 hours. The temperature selected can be based on working or proposed process conditions, plus factors for safety. Alternatively, full determination can be undertaken, whereby several isothermal tests can be conducted to establish a definitive ignition temperature.

Not only is the air over layer test a necessity in evaluating whether current or proposed drying and process temperatures are safe, but it can also confirm the possibility of using higher temperatures to increase productivity.

The aerated bulk powder diffusion cell test is conducted to establish the onset temperature of the exothermic activity of a material when exposed to elevated atmospheric temperatures. It replicates small bulk deposits in any drying situation where heated air is percolated through a material i.e. fluid bed driers.

Using a cylindrical glass test cell with a sintered glass bottom, the sample is placed inside the cell. Thermocouples are then inserted through its wall into the material at four locations from the bottom upwards. A glass lid is applied, and a regulated hot air supply that pushes air through the test material is connected. The assembly is placed into a digitally controlled laboratory oven and either of the below tests are performed using a data acquisition system to monitor, detect and record any exothermic activity.

An environmental temperature is digitally increased from ambient conditions up to 400°C at 0.5°C.min-1. This test configuration is used for estimating exothermic activity for onset temperature.

The loaded test cell is placed into a heated oven and set to a specified temperature for a duration of 24 hours. The temperature selected can be based on working or proposed process conditions, plus factors for safety. Alternatively, full determination can be undertaken, whereby several isothermal tests can be conducted to establish a definitive ignition temperature.

Not only is the aerated bulk powder diffusion cell test a necessity in evaluating whether current or proposed drying and process temperatures are safe, it can also confirm the possibility of using higher temperatures to increase productivity.

ASTM E 537-12

The Differential Scanning Calorimetry (DSC) test is conducted as a screening method to assess the extent and presence of enthalpic changes and to approximate both the temperature of initiation and enthalpies (heats) of solids, liquids or slurries. Very small sample sizes of 5-10 mg are used, complimenting lab-stage process developments for hazard assessment.

This test is conducted by inserting the desired material and a thermally inert reference material into isolated crucibles within the instrument. Both the testing sample and the reference material are then simultaneously heated at a controlled rate of 2 °C/min to 20 °C/min under an equilibrated atmosphere. As the furnace temperature ramps up at the desired rate, and temperature differential between the sample and reference are recorded as heat flow (Δq) and is associated with changes in enthalpy of the sample. Changes in state (solid-liquid, liquid-vapour, glass transitions) typically come with an endothermic heat flow as the sample takes on energy to achieve a higher energy state, whereas thermal decompositions typically involve chemical bond breaking and heat release as exothermic energy.

A record of heat flow as a function of furnace temperature is established, and when the sample experiences an event that involves a change of enthalpy indicated by a departure from the established baseline, the data is analysed to indicate possible hazardous events.

The DSC test can detect potentially hazardous reactions from volatile chemicals while estimating the probability of a hazard and the time at which a hazardous reaction may occur. The DSC can also be used as an early-stage test for detecting the potentially hazardous thermal components of uncharacterised materials.

The DSC test is often run as a screening test complimentary to Carius Tube test, as it provides the valuable Heat of reaction data not available from the Carius Tube.

Industry standard tool for screening reactive systems, developed to compliment DIERS

The Advanced Reactive System Screening Tool (ARSST) is a pseudo-adiabatic calorimeter batch reactor tool that is used to quickly identify potential reactive chemical hazards in processes.

It consists of a well-instrumented pressure vessel that usually holds a spherical glass test cell that can heat a 10 ml sample at a specified ramp or rate, determining onset temperatures of up to 400 °C. The reaction mixture, magnetic stirrer and thermocouple are introduced to this cell, while the pressure inside the vessel is measured by a pressure transducer. Due to the unique heating method by a wraparound heater, the sample is kept in a pseudo-adiabatic mode and no heat loss to the surroundings occurs. Typical data outputs can include the following:

• Rate of pressure rise
• Heat of reaction
• Heat of mixing
• Total adiabatic temperature rise
• Onset temperature
• Tempering temperature
• Time to maximum rate (induction time profile)

Flexible screening tool for analysis of varied Reactive Systems: basic reaction calorimetry possible with reagent mixing, thermal stability screening of mixtures and materials, and evolved pressure data to boot. Uses typically small sample sizes to compliment early-stage development.

Pseudo-adiabatic methodology for accurate determination of thermal decomposition onsets of materials, with the added benefit of pressure data collection.

ASTM E 1981-22

The Accelerating Rate Calorimetry (ARC) test can predict thermal and pressure events in the processing, storage, and shipping of potentially hazardous chemicals. The ARC can also be used to investigate and determine the effects of catalysts, inhibitors, initiators, reaction atmospheres and materials of construction.

The Carius Tube test can quickly identify thermal regions of interest (exothermic events, structural changes, permanent gas generation), which can then in turn be investigated in more detail using adiabatic or more precise techniques. The Carius Tube is often run as a screening test complimentary to DSC, as it provides the valuable pressure data not available from the DSC.

Recognised industry standard test, developed first at ICI and AstraZeneca hazard labs, and included in ABPI and IChemE publications on Chemical Reaction Hazard Assessment.

The Carius tube apparatus is used as a small-scale screening tool for thermal stability. It is primarily undertaken to investigate exothermic activity and gas generation. The Carius tube can also aid in the detection of exothermic activities such as onset temperatures. It can also identify the pressure effects of permanent gas generation.

The Carius tube apparatus works by placing approximately 10 g of a test material in a heavy-walled sealed glass tube and then placing it in an oven. The oven temperature is then increased at a controlled rate (typically 0.5 °C/min). The temperature and pressure of the test cell are then detailed and recorded to assess the thermal stability of the sample upon heating.

The Carius Tube test can quickly identify thermal regions of interest (exothermic events, structural changes, permanent gas generation), which can then in turn be investigated in more detail using adiabatic or more precise techniques. The Carius Tube is often run as a screening test complimentary to DSC, as it provides the valuable pressure data not available from the DSC.