Thermal studies on polymers and propellants

Abstract

This limited pyrolysis study of three commercial carboxy-terminated polybutadienes used in present-day propellant industry was conducted to gain some information on their kinetics and mechanism of decomposition in inert atmosphere. From an analysis of the decomposition products by a mass spectrometer, the mechanism seems to be random scission of the backbone structure producing low molecular weight hydrocarbons. The combustion chemistry of the binder can thus be considered to be that of these hydrocarbons by the decomposition products of HClO?, which are shown to be oxygen, chlorine and oxides of chlorine. However, the present investigation of pyrolysis in an inert atmosphere does not contribute anything to understand the role of solid-phase reactions. The next section is therefore devoted to a study of solid-phase reactions and to compare these results with the study on polymeric amine perchlorates. The kinetics of thermal decomposition of the binder is shown to be dependent on the heating rate as well as the atmosphere in which the experiment is conducted. This casts serious doubts as to the advisability of extending these data to combustion conditions. The present study is undertaken with a view to: (i) compare the thermal and explosive (impact) sensitivities of certain polymeric amine perchlorates with those of the corresponding model compounds, (ii) study the effect of molecular weight on the thermal and impact sensitivities of polymers, and (iii) understand the role of condensed-phase reactions in solid propellant ignition and combustion. In order to get a deep insight into the ignition and combustion mechanism of solid propellants, the pyrolysis study of three currently used carboxy-terminated polybutadiene binders is undertaken. Extensive studies on the thermal decomposition of mixtures of ammonium perchlorate with different polymers have emphasized the role of solid-state/heterogeneous reactions in propellant decomposition, ignition and combustion. The different techniques used in this investigation are differential thermal analysis, thermogravimetric analysis, isothermal weight-loss measurements, mass spectrometric investigations of the decomposition products, explosion/ignition delay measurement, impact sensitivity measurements, etc. The materials-polymers and model compounds—are characterized by chemical analyses and by different spectroscopic methods.