Safety Management Services, Inc. (SMS™) produces a laboratory tool that can characterize a material’s bulk sensitivity to thermal energy for a wide variety of materials. The SMS Simulated Bulk Auto-Ignition Test (SBAT) Apparatus, originally developed at Alliant Techsystems Inc. (see Chemical Engineering Progress, September 1994, Vol 90 No 9, page 67), can be used to quickly and accurately determine the critical temperature of a given material as well as its bulk auto-ignition temperature. Its strength lies in its ability to rapidly and accurately obtain critical parameters to determine temperatures at which a material can be safely handled, processed, stored, or transported in bulk quantities.
Unplanned for incidents of runaway exothermic reactions can result in injury or fatality of personnel, large restoration costs, and larger future hidden costs. Knowledge of the thermal reactivity of energetic material can prevent an unplanned energetic event. Two properties that are critical to safely handling, processing, storing, and transporting energetic material are the bulk critical reactivity temperature and the bulk auto-ignition temperature.
The critical reactivity temperature or critical temperature is the surrounding medium temperature at which the material could eventually reach ignition due to self-heating. The bulk auto-ignition temperature (bulk AIT) is the sample’s temperature at which the material will begin to give off significant amounts of energy in a short period of time and proceed to ignition. Knowledge of these two properties can assure safe operation, storage, or transportation by ensuring that when the ambient temperature is greater than the critical temperature, the material’s temperature is monitored to prevent against energetic decomposition at the bulk AIT.
There are multiple methods to determine the critical temperature and the auto-ignition temperature including Differential Scanning Calorimetry (DSC), Simulated Bulk Auto-Ignition Test (SBAT), and Accelerating Rate Calorimetry (ARC). Important parameters to consider when choosing a particular method are the sample size, minimum detectible exothermic rate (or heating rate), and insulating quality. The sample size is important as the heat retained in the sample (particularly in a powdered form) can be greater with a greater quantity due in part to changes in the heat transfer rate from the sample to the surroundings. To better approximate the behavior of bulk material, testing of a larger sample size is best. Closely related to sample size dependencies is the minimum detectible rate and magnitude of temperature change between the heating element and the sample. Larger heating rates can mask the self-heating that frequently takes place in energetic materials. Also, the amount of thermal isolation between the sample and the environment affects the accuracy and sensitivity of the test. The differences in the three parameters (sample size, heat rate (or detectible heat rate), and level of thermal isolation) for DSC, SBAT, and ARC are given in the below table.
Because of the level of thermal isolation, increased sample size, and confinement, the ARC test apparatus is widely used and regarded to give the most accurate results in determining the auto-ignition and critical reactivity temperatures, particularly for auto-catalytic materials. Additionally, unlike the DSC and SBAT, the ARC doesn’t merely increase the temperature at a prescribed rate but checks the temperature difference between the sample and heating element and attempts to match the temperature of the sample after a given time during an exothermal event. Such a ‘heat-wait-search’ mode results in a very accurate estimate of how the material will behave in a bulk, well-insulated configuration. In comparison, due to the differences described above and in the table above, the experimental DSC auto-ignition temperature (AIT) can be elevated more than 50°C over the bulk AIT found using the ARC apparatus. The below plot shows the comparison between the auto-ignition temperature found with the DSC, SBAT and ARC for a range of different energetic materials. Note that the SBAT agrees with the ARC within 9 degrees Centigrade on average.
Some of the disadvantages of the ARC include its high operating cost, long sampling time, and slow turnaround. The SBAT apparatus was developed to drastically reduce the sampling time and sample turnaround while still maintaining an accurate determination of bulk material behavior. The SBAT can test 5 samples simultaneously and the experimental AIT temperature found ~9°C above the AIT value of the ARC. In many cases, the SBAT is the ideal test apparatus to identify bulk critical reactivity and auto-ignition temperatures due to the number of ports, the rugged features, and the amount of insulation.
The SBAT apparatus contains 6 ports or test chambers (including 1 reference test chamber) arranged in a circular configuration in an aluminum block containing three 600W cartridge heaters. 3-5 grams of a sample can be loaded into each sample chamber, a thermocouple is then inserted into the sample, and the chamber is then closed with a vented cap. Depending on the test, the sample then undergoes heating at a rate of 0.2 °C/min or the sample can be tested isothermally at a given temperature.
The ramped temperature test can yield the temperature at which exothermic or endothermic events occur under conditions of low heat loss from the sample. Isothermal test can be run to determine the critical temperature, or the temperature at which given an infinite storage time the sample will not react violently.
