Recommended Test Setup: SBAT
Test Purpose
The purpose of completing the in-process thermal test is determine the effects of time and temperature on energetic materials in the process configuration. Thermal tests generally consist of measuring a sample’s response to either: (1) gradually increasing the temperature and recording the temperature at which the substance gives off heat (autoignition temperature), or (2) holding the substance at a given temperature for an extended period of time to evaluate stability at a given temperature (thermal stability). Both auto-ignition and thermal stability tests are used for in-process risk evaluation.
Variations
Many different pieces of equipment can be used to obtain an autoignition temperature. Some of them are
- Accelerating Rate Calorimetry (ARC),
- Simulated Bulk Auto-Ignition Temperature (SBAT) (UN Test Method 3 (c) (ii)),
- Differential Thermal Analysis (DTA), and
- Differential Scanning Calorimetry (DSC).
The above pieces of equipment can also be used for thermal stability tests. Other pieces of equipment can also be used:
- Taliani,
- Time to explosion (Henkin), and
- Thermal stability in an oven (UN Test Method 3 (c) (i)).
The most conservative auto-ignition or thermal stability result is obtained when there are no heat losses from the sample, the sample is confined, and the heating rate is very slow. The Accelerated Rate Calorimeter (ARC) uses gram sized quantities with high insulation, high confinement, with a heat-wait search routine. Auto-ignition temperatures from the ARC apparatus are typically the lowest. The Simulated Bulk Auto-Ignition test (SBAT) also has a high insulation value, gram sized quantities, a slow heating rate, and the sample can be confined. The DSC on the other hand has a high rate of heat loss and very small samples, although the sample can be confined. With these variations between tests, there can be significant differences in the auto-ignition and thermal stability results. Shown below is a table giving the auto-ignition results for multiple substances for the ARC, SBAT, and DSC.
| Substance | ARC °C | SBAT °C | DSC °C |
|---|---|---|---|
| Nitroglycerine sample | 115 | 124 | 178 |
| Liquid nitrate ester sample | 115 | 117 | 171 |
| Double-base propellant sample | 120 | 131 | 184 |
| Sample containing both nitramine and nitrate ester groups | 125 | 135 | 185 |
| Nitrate ester propellant sample | 125 | 136 | 171 |
| Nitrate ester polyether propellant sample | 125 | 135 | 169 |
| Nitrate ester polyether propellant sample | 126 | 135 | 168 |
| Organic azide sample | 150 | 164 | 224 |
| Aliphatic nitrocompound sample | 165 | 172 | 225 |
| AP composite propellant sample | 180 | 195 | 248 |
| Pyrotechnic sample | 185 | 184 | 231 |
| RDX sample | 189 | 195 | 218 |
| BKNO3 sample | 277 | – | 368 |
On-average, the SBAT gives auto-ignition temperatures just 9°C greater than the ARC. The DSC on the other hand with smaller quantities and much greater heat losses gives auto-ignition temperatures 50°C greater than the ARC. The SBAT has been used by Thiocol, ATK, SMS, and others to obtain auto-ignition temperatures and thermal stability results for more than 20 years.
Key Parameters
| Key Parameter | Objectives | Origin | Specs |
|---|---|---|---|
| Temperature profile | Slow heating rates (e.g. < 0.5°C/min) allow the sample to respond to the temperature increase; the response of the sample can lag behind when fast heating rates (e.g. 10°C/min) are employed, potentially yielding an artificially elevated auto-ignition temperature. Testing at different isothermal temperatures enables detection of time-dependent behaviors, such as auto-catalytic reactions. | SBAT standardized to 12°C/hr | |
| Heat loss | Low heat loss is representative of in-process configurations where heat generated by the sample cannot readily dissipate (insulated); high heat loss is representative of in-process configurations where heat generated by the sample can be readily dissipated or removed. | SBAT standardized to utilize sample tubes that are insulated to inhibit heat loss; insulated to at least a temperature decay time constant (i.e. time to decay 36.8% of the initial temperature difference) of 10 minutes. | |
| Confinement | Open configurations allow evaporation of volatiles and pressure relief; sealed configurations resist loss of volatiles and/or permit pressurization that can increase the reaction rate. | SBAT test tubes may be open or sealed; standardized materials of construction is glass, but other materials may be utilized as well. | |
| Sample size | The larger the sample size the greater the resistance in transferring heat from the center of the sample and thus the more sensitive is the test. | SBAT standardized to 3 – 5 grams | |
| Sample response to temperature profile | Observation, detection, and documentation of a material’s response to the test stimuli | SBAT standardized to utilize 30-gauge thermocouples (sensitive to small temperature changes); thermocouples can be placed in the sample or on the outside of the sample tube |
Indicators
| Indicators | Detection Method | Assessment |
|---|---|---|
| Temperature Increase | Thermocouple | For a ramped test, the temperature at which the substance begins to exotherm is the auto-ignition temperature. For an isothermal test, if a temperature rise of 1.5°C over the baseline is observed then the substance is not thermally stable at that temperature. |
Example SBAT Results: Auto-Ignition
