Thermal Stability.Pdf

Thermochimica Acta 566 (2013) 137–148 Contents lists available at SciVerse ScienceDirect Thermochimica Acta jo urnal homepage: www.elsevier.com/locate/tca The effect of molecular structure on thermal stability, decomposition kinetics and reaction models of nitric esters a a a,∗ Qi-Long Yan , Martin Künzel , Svatopluk Zeman , b c Roman Svoboda , Monika Bartoskováˇ a Institute of Energetic Materials, Faculty of Chemical Technology, University of Pardubice, 53210 Pardubice, Czech Republic b Department of Physical Chemistry, Faculty of Chemical Technology, University of Pardubice, 53210 Pardubice, Czech Republic c Department of Environment, Faculty of Chemistry, Brno University of Technology, CZ-612 00, Brno, Czech Republic a r t i c l e i n f o a b s t r a c t Article history: In this paper, the thermal stability and decomposition mechanism functions of 10 nitric esters including Received 8 March 2013 nitroglycerine (NG), pentaerythritol tetranitrate (PETN), trimethylolethane trinitrate (TMETN), dipen- Received in revised form 21 May 2013 taerythritol hexanitrate (DiPEHN), trimethylolpropane trinitrate (TMPTN), erythritol tetranitrate (ETN), Accepted 22 May 2013 xylitol pentanitrate (XPN), sorbitol hexanitrate (SHN), mannitol hexanitrate (MHN) and nitroisobutyl- Available online 31 May 2013 glycerol trinitrate (NIBGT) are determined by means of non-isothermal TG and DSC techniques. It has been found that the mean activation energies for most nitric esters are comparable at constant heating Keywords: −1 rate (around 145 kJ mol ), indicating that their main decomposition pathways might be the same. The Nitric esters −1 mass loss activation energies of NG, TMETN and TMPTN are less than 100 kJ mol due to partial evap- Thermal stability oration. Based on the critical temperature of thermal decomposition, the order of molecular stability Critical temperature Reaction models for involved nitric esters is found to be MHN < XPN < TMPTN < SHN < NIBGT < ETN < PETN < DiPEHN. The Kinetic compensation effect introduction of function groups to the tertiary carbon is in favor of increasing thermal stability due to increase of symmetry and rigidity of the molecule. The decomposition kinetics was described in terms of the Johnson-Mehl-Avrami and Sesták-Berggrenˇ models. Two types of kinetic behavior were observed and most nitrate esters followed typical decomposition kinetics close to the first order reaction. However, cer- tain materials showed complex behavior caused by overlapping of more mechanisms/processes, which were represented either by simultaneous evaporation and decomposition or by different decomposition mechanisms originating from varying morphology and structure of the samples. © 2013 Elsevier B.V. All rights reserved. 1. Introduction powerful explosives used mainly for military purposes due to greater compatibility and higher performance than other nitric Nitric esters have been used as plasticizers or energetic fillers esters [4–7]. In particular, with regard to spark detonators, PETN in detonators, propellants and explosives for mining, artillery, and can be used to avoid the need for primary explosives due to its lower engineering since hundreds years ago [1,2]. In the past decades, electric spark initiation energy (10–60 mJ). On the other hand, some considerable interest in nitric esters has been expressed by not only nitric esters could be used as drugs in medical treatment. In fact, the specialists but also the amateurs and terrorists due to require- nitroester drugs have been shown to relax the smooth muscle of ments of little synthetic expertise and availability of cheap raw blood vessels, and hence were widely accepted for the treatment materials from the shops [3]. There has been growth in use of those of angina pectoris [8]. nitric esters such as erythritol tetranitrate (ETN), most of which are Because of growing practical demands on nitric esters, more and so-called “homemade” explosives (HME). On the one hand, a num- more investigations are carried out with regard to their synthe- ber of polynitroesters, including nitrocellulose (NC), nitroglycerin sis and physiochemical properties. On the purpose of utilization (NG), the nitroester of pentaerythritol (PETN), trimethanolethane as energetic ingredients, recent studies have been concentrated trinitrate (TMETN), and bis(2-nitroxyethyl)nitramine (DINA) are mainly on their detailed thermal decomposition mechanisms, com- bustion and detonation performances [9–11]. For instance, density function theory (DFT) has been employed to study the geometric ∗ and electronic structures of trinitrate esters including NG, TMETN, Corresponding author. Tel.: +420 466038503; fax: +420 466038024. butanetriol trinitrate (BTTN), and trimethylolpropane trinitrate E-mail addresses: [email protected] (Q.-L. Yan), [email protected], [email protected] (S. Zeman). (TMPTN) at the B3LYP/6-31G* level [12]. It has been found that 0040-6031/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2013.05.032 138 Q.-L. Yan et al. / Thermochimica Acta 566 (2013) 137–148 the oxygen balance, volume, density, detonation velocity and pres- 3. Results and discussions sure of trinitrate esters linearly decrease with the increase of methylene group’s number. In order to clarify the initial decompo- 3.1. Thermal responses during heating sition mechanism for nitric esters, T-Jump/FTIR and T-jump/Raman spectroscopies were used to analyze the gaseous products of The thermal responses (heat flow) for involved 10 nitric esters several aliphatic nitric esters [13]. Kimura [14] also used chemi- were recorded by DSC under pressure of 0.1 MPa as shown in Fig. 1. luminescence (CL) method to determine the light-emitting species The DSC curves for most of the studied nitric esters are not avail- ◦ ◦ during low temperature decomposition (between 40 C and 90 C) able in the literature. It has been shown that the melting points of of PETN and NC. It has been shown that the thermal decomposi- nitric esters are much lower than those of secondary explosives, and tion of nitric esters is accompanied by some oxidation reactions, some of them are liquid at room temperature. There is a big differ- which could be generated in the course of recombination of per- ence with regard to melting point due to discrepancy in molecular oxy radicals. On this basis, Chen and Brill [15] further studied structure. For instance, MHN and PETN have melting point higher ◦ the fast thermal decomposition kinetics and mechanism of some than 110 C, while those of TMETN, NG, and NIBGT are less than polymeric nitric esters by SMATCH/FTIR technique (heating rate: zero. With regard to heat release process, it has been observed that, ◦ −1 100–150 C s ), and their activation energies were obtained as the exothermic peaks for NG and TMETN are not well formed due −1 −1 around 129.8–142.4 kJ mol with log(A) values of 14.7–16.9 s . to strong evaporation. In order to make a quantitative view of the Agrawal [16] synthesized some aromatic nitric esters, among thermal responses, the corresponding parameters are summarized which the compound 1,3,5-tris(2-nitroxyethylnitramino)-2,4,6- in Table 1. The density and detonation velocity parameters are also trinitrobenzene was found to be the potential alternative of PETN. In included in this table. addition, quantum chemical calculations are used to compute the As shown in Table 1, the density of these nitric esters is around −3 heats of formation for 24 nitric esters among which only 5 com- 1.4–1.8 g cm with theoretical detonation velocity of between −1 pounds have the available experimental values [17]. As a result, 7000 and 9000 m s . As it could be seen, all of the nitric esters considerable progress toward an understanding of the decompo- decompose in liquid state, and for the solid nitric esters, there sition mechanism for nitric esters has been achieved. However, are obvious endothermic heat change before decomposition due undoubtedly, the effect of molecular structure on the performances to fusion. It is interesting that the peak temperature of SHN and and physiochemical properties of nitric esters could not be clearly XPN is the same while their onset temperature is very different, identified due to discrepancy in principles and physical condi- and this might be caused by their very similar carbon chain struc- tions of corresponding measurements carried out by different ture. NG, ETN and TMETN are comparable. PETN has the highest researchers. In this paper, the non-isothermal behavior, isocon- fusion energy (H1), and melting point due to its highly symmet- versional decomposition kinetics, reaction models and thermal ric molecule and great molecular rigidity. For NG, there is also one stability for 10 typical nitric esters will be systematically inves- endothermic peak before decomposition due to evaporation, and tigated, based on which the effect of molecular structure on these under dynamic nitrogen atmosphere the NG could partially decom- parameters will be expounded. pose. With regard to heat of decomposition H2, DiPEHN has the highest heat release due to higher hydrogen and carbon content with sufficient oxygen balance. The heat release from decomposi- 2. Experimental tion depends not only on the energy content of the compound, but also on the experimental condition. Here the nitric esters decom- 2.1. Materials pose under 0.1 MPa dynamic nitrogen atmospheres with relatively slow heating rate. The heat change is also slow and could be well There are 10 nitric esters

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