Safety Management Services, Inc. (SMS) produces the Simulated Bulk Auto-Ignition Temperature (SBAT) test apparatus, a laboratory tool designed by Alliant Techsystems (ATK) to quickly and accurately characterize the thermal response of a large mass of energetic material (bulk) using only gram quantities of the sample.

The SBAT apparatus contains six insulated ports or test chambers (including 1 reference test chamber) arranged in a circular configuration in an aluminum block. Each test chamber can be filled with 3 - 5 grams of a test sample. A thermocouple is inserted into the center of each sample, and the chamber is closed with a vented cap. The aluminum block is heated by internal cartridge heaters.

SBAT Test Apparatus

Insulated Test Chambers

The SBAT apparatus was developed by ATK to produce results similar to the Accelerating Rate Calorimeter (ARC). The ARC is highly insulated and a 'heat-wait-search' mode results in a very accurate estimate of how the material will behave in a bulk, well-insulated configuration. The SBAT can test multiple samples in a shorter time frame than the ARC with a smaller operating cost. The experimental auto-ignition temperature (AIT) determined by the SBAT is nearly equivalent to that determined by the ARC (0 - 1°C temperature difference typical).

The SBAT apparatus can either ramp the heater temperature (for determining the auto-ignition temperature of a bulk sample) or hold it at a constant isothermal temperature (for determining whether a bulk sample exhibits autocatalytic behavior or has a critical auto-ignition temperature). These two test methods can characterize a substance's thermal properties to support safe handling, processing, storage, or transportation in bulk quantities.

The reaction hazard of a bulk material may be qualitatively predicted based on the type of reaction observed in the SBAT. For example, if it were observed that a sample burns, detonates, or exothermically decomposes from SBAT heating, a similar type of reaction would likely be observed from heating the bulk material (unconfined).

Ramped SBAT Application

A ramped temperature test using the SBAT apparatus can yield the temperature at which exothermic or endothermic events occur under conditions of low heat loss from the sample. The faster heat leaves the material, the higher the observed temperature required for ignition. The rate of heat removal depends on the amount of thermal isolation and the ambient conditions. The experimental temperatures determined with the SBAT apparatus represent a worse-case scenario where the rate of heat transfer is very low. Similar tests such as Differential Scanning Calorimetry (DSC) and Differential Thermal Analysis (DTA) also yield exothermic or endothermic temperatures with a higher rate of heat loss.

Performing Differential Thermal Analysis (DTA) and Differential Scanning Calorimeter (DSC) tests on Explosive Sample A indicates the onset of an endotherm around 75°C and the onset of an exotherm around 260°C. A sample DTA curve for this explosive is shown below.

DTA Curve for Explosive Sample A

Performing a Ramped SBAT test on the same Explosive Sample A indicates the onset of an endotherm around 70°C (5°C lower than DTA / DSC) and the onset of an exotherm around 170°C (90°C lower than DTA / DSC).

Ramped SBAT on Explosive Sample A

This large discrepancy between exothermal onset temperatures found with the DTA/DSC and SBAT can be explained by considering the application of each test method. Tests like the DTA and DSC are designed around small sample sizes (milligram quantities), high heating rates (e.g. 20°C/min), and a low degree of thermal isolation (i.e. high heat losses from the sample). These test methods accurately characterize the thermal response of small quantities of unconfined energetic materials whose configuration allows for heat dissipation from the sample.

The SBAT is designed around a larger sample size (gram quantities), slow heating rates (e.g. 0.2°C/min), and a high degree of thermal isolation (low heat losses from the sample). This enables the SBAT to simulate the poor dissipation of heat that can occur in a large, bulk mass of energetic material.

Isothermal SBAT Application

Performing an Isothermal SBAT test at 182°C on the same Explosive Sample A indicates an immediate exotherm onset as the sample reaches the isothermal heater temperature, with the exotherm peaking at 14 hours.

Isothermal SBAT (182°C) on Explosive Sample A

Additional Isothermal SBAT tests at 193°C, 171°C, and 160°C reveal that the sample is autocatalytic, undergoing an exothermic decomposition at any given temperature above it's melting point. Analysis of the data from the Isothermal SBAT tests yields the following model.

Predicted Stability for Explosive Sample A

As can be seen from the chart, given an isothermal temperature on the x-axis, the time to exothermal onset (blue line) and time to exothermal peak (red line) for Explosive Sample A can be estimated, provided the rate of heat transfer from the sample is low. For example, a bulk quantity of Explosive Sample A held at 160°C would begin to exothermally decompose around 20 hours, with peak decomposition occurring around 100 hours.