Thermochemical analysis of improvised energetic materials by laser-heating calorimetry

Abstract

Thermochemical analysis of six improvised energetic materials was carried out using laser-heating calorimetry to demonstrate the feasibility of this methodology to provide distinctive thermal signatures and information on the material shelf life. The chemicals evaluated were erythritol tetranitrate, hexamethylene triperoxide diamine (HMTD), poor-man's C-4 (blend of potassium chlorate and petroleum jelly), R-salt (represented by 1,3,5-trinitroso-1,3,5-triazinane), triacetone triperoxide (TATP), and urea nitrate. The measurement technique records the temperature rise with time, from which one can estimate the material endothermic/exothermic behavior, energy release rate, and total specific energy release (heating value, enthalpy of explosion), as well as the sample mass rate of change. Measurements were carried out in an inert nitrogen environment at laser heating rates up to 60 K/s with steady-state temperatures reaching about 933 K. Sample initial mass was between 1.0 mg and 4.0 mg. Experiments were carried out with freshly prepared samples, as well as refrigerated samples and those stored at room (laboratory) temperature for three years. Results indicated that the samples reacted rapidly between 0.50 s and 0.75 s, being initiated near the material decomposition temperature. The total specific energy release, using two different thermal-analysis models, was calculated and compared to values available in the literature. One model represents sample reaction and decomposition within the spherical reactor volume, while the second represents reactions emanating from sample in a pan centrally positioned within the sphere; the former model was found to be the more appropriate approach for these faster-reacting energetic materials. The thermal signatures (temperature-time derivatives with temperature) were different for each chemical, a feature that may be important for energetic material identification. The initiation and peak reaction temperatures were found to decrease with increasing initial sample mass. Also, the shelf life for TATP and HMTD was found not to degrade under nonideal conditions after three years.