Thermal studies on substituted ammonium and phosphonium perchlorates

Abstract

The investigations described in this thesis have been aimed at contributing to the understanding of the thermal decomposition characteristics of some of the substituted ammonium and phosphonium perchlorates. The importance of such a study arises from the extensive applicabilities of the high?energy compounds of nitrogen and phosphorus in propellants, pyrotechnics and/or explosives for various civil and military purposes. Ammonium perchlorate is the oxidizer of choice in solid rocket propellants and mainly because of this reason, its thermal decomposition studies have been carried out in detail. The study of the thermal properties of the substituted ammonium perchlorates, some of which have been tried as auxiliary oxidizers, has, however, not been paid due attention and hence methyl?substituted ammonium perchlorates were chosen for thermal studies carried out in the present investigation. Ammonium perchlorate, NH?ClO?, is relatively stable at ordinary conditions whereas its immediate analogue phosphonium perchlorate, PH?ClO? (nitrogen and phosphorus being in the same group of the periodic table of elements) is extremely unstable and explodes violently in the dry state. Substituted phosphonium perchlorates are, however, stable at ordinary conditions. The thermal decomposition studies of none of these phosphonium perchlorates have been reported so far. It was, therefore, interesting to synthesise and study the thermal behaviour of some of these compounds and compare their thermal properties with those of the corresponding substituted ammonium perchlorates. The work presented in this thesis is divided into seven chapters. Chapter 1 is an introduction to the present work. This incorporates an account of the importance and properties of perchlorates in general, the various aspects of thermal decomposition and explosion of solids and liquids/ melts and the decomposition characteristics of a few typical non?metallic oxy?salts. In addition, the scope of the present investigation is outlined. Chapter 2 deals with the various experimental techniques and analytical methods employed in the present study. The chief techniques are differential thermal analysis (DTA), thermogravimetric analysis (TGA), mass spectrometry and explosion delay measurements. The methods of chemical analysis and spectroscopic techniques such as nuclear magnetic resonance (NMR) and infrared (IR) spectroscopy which have been used for characterizing the compounds prepared during the present investigation are briefly outlined. Chapter 3 describes the studies on the thermal decomposition and explosion sensitiveness of mono?, di? and tri?methylammonium perchlorates. A new feature in the DTA trace of monomethylammonium perchlorate, viz., another exotherm beyond the one due to the decomposition of the compound, has been observed. This has been attributed to the decomposition of ammonium perchlorate formed as a decomposition product of monomethylammonium perchlorate. The possible reactions that can lead to the formation of ammonium perchlorate as a result of the decomposition of monomethylammonium perchlorate have been discussed. In addition, the relative thermal stability of mono?, di? and tri?methylammonium perchlorates has been studied by various techniques. Decomposition temperatures as observed from DTA and TGA, the values of activation energy for decomposition and explosion and the results of the mass?spectrometric analysis of the gaseous decomposition products have been discussed in terms of the relative thermal stability of these compounds. The observations have been explained essentially in terms of decomposition via a dissociation step. An attempt has been made to correlate thermal decomposition with explosion. The study of the thermal behaviour of tetramethylammonium and phosphonium perchlorates is presented in Chapter 4. A decomposition process via a primary dissociation step, involving a methyl?group transfer from the cation to the anion, has been proposed. Evidence for such a proposition comes from the results of some decomposition experiments at low pressures and by mass?spectrometric analysis of the gaseous decomposition products. Results of the investigations on the kinetics of thermal decomposition of these compounds are also included in this chapter. Chapter 5 is devoted to a comparative study of the thermal decomposition characteristics of tetramethylammonium and ?phosphonium perchlorates, nitrates and picrates. This study sheds light on the decomposition mechanism of these quaternary onium salts. The explosion sensitiveness of these compounds is also presented in this chapter. The relative thermal stability of the tetramethylammonium and phosphonium compounds is in the order, perchlorate > nitrate > picrate. The ammonium compounds are found to be more stable than the corresponding phosphonium compounds. The observations have been explained in terms of a dissociation step involving a methyl?group transfer in the decomposition of these compounds. Chapter 6 is concerned with the thermal characterization of some triphenyl?substituted phosphonium perchlorates where the fourth substituent is –H, –CH?, –n?C?H? or –NH? group. The relative thermal stabilities of the alkyl?substituted triphenylphosphonium perchlorates have been determined by DTA, TGA and explosion delay measurements. The results of the residual analysis of the decomposition products and other data presented in this chapter support the primary step of decomposition proposed in Chapter 4. From the point of view of the use of ammonium perchlorate (AP) as an oxidizer in solid rocket propellants, it may be of interest to see if some of these substituted onium compounds studied during the present investigation, when doped into or mixed with AP crystals, modify its thermal decomposition characteristics. The effects on the thermal behaviour of AP using tetramethylammonium and ?phosphonium perchlorates as additives are dealt with in Chapter 7. It is seen that the thermal characteristics of AP when co?crystallised or mixed with these compounds are considerably modified.