The purpose of thermal sensitivity testing is to determine the effects of time and temperature on explosives. A thermal sensitivity test is also conducted as a part of the DOT Classification (See DOT Classification).
Differential Scanning Calorimeter (DSC) Tests
A DSC can be used to determine the exotherms and endotherms of an explosive. This test identifies critical temperatures where a phase change or run-away reaction may occur.
A DSC compatibility test can be used to determine the ability of a material to coexist in intimate contact with an explosive without adverse reaction, for an acceptable period of time. This test measures heat increase or heat loss. It is used when looking for an exothermic or endothermic reaction. This test may be used for composite explosives.
Fisher-Johns Auto-Ignition Test
The Fisher-Johns Auto-ignition Test is performed to determine the thermal/time conditions under which an explosive of a specified sample will ignite. Auto ignition is investigated in two ways: (a) Heat the sample at a constant rate (normally 15°C/min.). (b) Time the sample maintained at a constant, elevated temperature until ignition occurs (normally the test material is held at 120°C for up to one hour).
During heating, any physical changes of the sample such as melting, fuming, or smoking, etc., is recorded noting the temperature at which these changes occur. During a normal auto-ignition study, three tests are conducted and an average auto-ignition temperature is reported. During a normal auto-ignition study, three tests are conducted using method (a). For a screening study, one test is sufficient.
Reference: Standard thermal analysis methodology and theory.
The Henkin bath test is used to determine the thermal stability of explosives by isothermally testing samples in a heated metal bath. A sample of explosive is weighed (50 mg) and placed in a No. 8 blasting cap shell. The cap is then sealed (aluminum shell). Testing is done on the sealed test samples at various temperatures. The test measures the time to explosion. A graph is made from final data showing time vs. temperature to determine the thermal stability of the explosive sample being tested (cook off time, and reaction rates). This differential thermal analysis combined with other tests (impact) may be used to determine proper explosive handling techniques.
This equipment can also be used to determine long term compatibility between a test sample (e.g. paints, adhesives, ingredients etc.) and an explosive.
The modified Taliani test is used to determine the ability of a material to coexist in intimate contact with an explosive without adverse reaction, for an acceptable period of time. The usual conditions for the test are 93 °C (200 °F) for 23 hours in a closed inert atmosphere, as opposed to 5 hours in the standard Taliani test. This test uses a measuring device to measure pressure and is valid for about 70% of compatibility tests required. However it is not valid for the following:
1) Volatile solvents
3) Materials producing heat only (no gaseous products)
4) Some composite explosives
Simulated-Bulk Auto-Ignition Temperature (SBAT)
In the past, companies have commonly used the Henkin Time-to-Explosion test to calculate a sample’s Critical Temperature and its dependency on the sample’s size. However, the Henkin Time-to-Explosion test is an older test whose techniques are more archaic in comparison to safer, more modern techniques.
A modern technique for calculating the critical temperature consists of using a Simulated Bulk Auto-Ignition Temperature (SBAT) apparatus. This apparatus utilizes a 3-5 gram sample and a heavily-insulated sample holder to accurately simulate the ignition behavior of the bulk material in response to temperature.
The SBAT critical temperature is advantageous because it is sample-size independent; the critical temperature for the bulk material is the lowest, or worst-case critical temperature determined by the testing. Hence, despite sample configuration, provided the temperature of the sample is below the SBAT’s critical temperature and that the material is non-autocatalytic, thermal ignition of the material will not occur.
Another advantage of the SBAT test is that it can monitor the temperature of the sample, relative to the surrounding heater temperature. This ensures that the sample’s heat production can be detected and measured well before a run-away reaction proceeds to ignition.
Taliani Stability Test
The Taliani stability test is used as a standard method for the evaluation of the relative stability of explosive of known composition by subjection to a specified temperature and atmosphere. The test determines stability by measuring the pressure of gasses evolved from a sample under conditions of constant temperature and volume. The test is limited to compositions of known or predictable stability, which normally will not undergo rapid reaction or decomposition when exposed to temperatures of 110 +/- 0.1 °C for short periods of time. This test cannot be run on samples that contain large quantities of volatiles. A plot of time versus pressure makes up the stability curve.
A dry sample is placed in a tube at 100°C. The tube is evacuated. The pressure above the sample is measured. The increase in pressure is a measure of the rate of decomposition of the sample.
This test is used to determine the reaction of samples when subjected to elevated temperatures for a determined period of time. The test uses an oven to verify handling, transportation, and storage requirements. The 50 g sample is placed in a constant temperature (explosion-proof) oven at 75 °C for a period of forty-eight hours. The temperature is continuously monitored and recorded. At the completion of the test, the sample is examined for discoloration, weight loss, and dimensional change for evidence of decomposition. This test is part of a series of tests used for establishing hazard classification.
Although usually performed at 100 °C, other temperatures can be used in the vacuum stability test. The sample is placed under 5 mm Hg at constant temperature for 40 – 48 hours. The volume of gas liberated is calculated. The test is discontinued if 11+milliliters of gas is evolved. Rate data may be obtained if gas released is measured at intervals. In the vacuum stability test, increased reactivity is indicated by the mixture evolving more than five milliliters of gas over the sum of the amount of gas produced by the ingredients tested separately.