G. Prasad

Mechanism of ageing of composite solid propellants

Composite propellants mostly consist of two major ingredients: oxidiser and binder. It has almost been established now that propellants undergo slow decomposition during the course of ageing [1-3]. To increase the longevity of the propellants, it is necessary to know the mechanism of the ageing process. Thus it has to be established whether the rate-controlling step lies in the oxidiser decomposition or in the binder decomposition. The objective of the present investigation, therefore, is to identify the constituent responsible for the ageing. It may be noted that contrary to the available literature on the mechanism of the ageing of double-base propellants, very little is known about the mechanism of composite propellant ageing [4-11]. From accelerated ageing studies in the temperature range 40-750C of five cast composite propellants, Kuletz and Pakulak [4] have shown that the primary component altered at the exposed surface is the binder, whereas in the interior it is the oxidiser. However, they found the activation energy (E) for surface, subsurface, and bulk material to be almost similar (23-27 kcal/mole). Schedlbauer [6] observed that polyurethane and carboxy terminated polybutadiene (CTPB)-based propellants harden during ageing, which he attributed to cross-linkage through the double bonds present in the main chain. He suggested that HCIO4 generated due to the interaction of AP and moisture may act as an excellent cross-linkage agent for double bonds. Myers [7] also observed the hardening of the CTPB propellant, which he explained on the basis of cross-linkage reactions due to the oxidative attack of AP on CTPB double bonds. Layton [9-11] correlated the chemical structural changes of CTPB, hydroxy terminated butadiene (HTPB), and terepolymer of butadiene, acrylic acid, and acrylonitrile (PBAN)-based propdlants with the change in their mechanical properties during the ageing process. He proposed that the cross-linkages are formed along the polymer chains at the unsaturated sites, with the maximum percentage at the pendant vinyl group. He estimated energy (E) values (5-7 kcal/mole) for the chemical and mechanical changes during ageing and attributed this to the diffusion process. It may be seen from the works of Kuletz and Pakulak [4] and Layton [9-11 ] that there is controversy over the E values for the ageing process. Another objective of the present work, therefore, is to estimate the E values for changes occurring in the mechanical properties, burning rate (~), and thermal decomposition (TD) during ageing and to see whether there is a correlation between them.

Storage stability of solid rocket fuel. 1. Effect of temperature

Ageing behaviour of polystyrene (PS)/ammonium perchlorate (AP) propellent leading to ballistic changes has been studied. It follows a zero-order kinetic law. Ageing behaviour leading to change in burning rate ( ) in the temperature range of 60–200 ° C was found to remain the same. The dependence of the change of the average thermal decomposition (TD) rate at 230 and 260°C on the change in burning rate for the propellant aged at 100 ° C in air suggests that the slow TD of the propellant is the cause of ageing. The safe-life (for a pre-assigned burning-rate change limit) at 25 ° C in air has been calculated as a function of the rate of change.

Effect of Storage Temperatures on the Mechanical Properties of the Composite Solid Propellants

Abstract Accelerated ageing studies for three composite propellant formulations, namely polystyrene (PS)/ ammonium perchlorate (AP), polymethylmethacrylate (PMMA)/AP and poly phenol formaldehyde (PPF)/AP have been carried out in the temperature range of 55-125°C. Measurements of the ultimate compression strength (U c) and isothermal decomposition rate (TD rate) were monitored as a function of storage time and temperature. The change in U c was found to be linearly dependent on the change in TD rate irrespective of the propellant systems. Analysis of the results further revealed that the cause of ageing for both U c and burning rate (r) is the thermal decomposition of the propellant. The safe-life for the change in mechanical properties was found to be higher compared to the change in r for PS and PMMA based propellants.

Storage stability of solid rocket fuels. 2. Effect of oxidizer loadings

Ageing behaviour, leading to ballistic changes, has been studied as a function of oxidizer loading in polystyrene/ammonium perchlorate solid-propellants. The ageing studies were carried out at 100 °C in air. Change in burning rate decreased as the oxidizer loading increased from 75 to 80%. The change in thermal decomposition rates both at 230 and 260 °C also decreased as the oxidizer loading in the propellants increased. The shapes of the plots of the changes in burning rate and thermal decomposition rate (230 and 260 °C) at different storage times for different oxidizer-loaded propellants seem to be exactly similar. These results lead to the conclusion that the thermal decomposition of the propellant may be responsible for bringing about the ballistic changes during the ageing process. Infrared studies of the binder portion of the aged propellant indicate that peroxide formation takes place during the course of ageing and that peroxide formation for a particular storage time and temperature increases as the loading decreases.

Effect of ageing on ballistic and mechanical properties of propellants having various oxidiser loadings

Propellants having high performance and goodstorage stability are always desirable for space applications. It is well known that the burning rate (1:) of composite solid propellants generally increases with oxidiser loading [1]. Recently, we have published a paper [2] on the effect of oxidiser loading on burning rate and thermal stability of polystyrene (PS) and ammonium perchlorate (AP) propellant systems and it was shown that higher oxidiser-loaded propellant has better storage stability as judged by burning rate changes. In general, the ageing of the propellant leads to changes in ballistic and mechanical properties which characterize the performance of the propellant. The tolerance limit of changes in these parameters depends upon the applications of the rocket motor in a specified mission. However, mechanical properties are generally used to characterise the ageing characteristics. The object of this short note is to present some results on the effect of oxidiser loading on the mechanical property of the propellant during ageing. The experimental details have been presented in our recent papers [2, 3] . The ageing studies on mechanical-property changes were carried out for PS/AP propellants having different AP loading. The results on the ultimate-compression strength (uc) for different AP-loaded propellants (aged at 100°C for 20 days) are presented in Table 1. The data for ~: changes (obtained from our earlier work

Thermo-chemical interpretation of polymer degradation

SummaryA correlation has been established between the heat of depolymerization (ΔH) of vinyl polymers for going from solid polymer state to gaseous monomer state and the activation energy (E) of degradation. On this basis it has been shown that the rate controlling step in the degradation lies in the initiation step. Attempt has been made to correlate theE and ΔH with glass transition temperature (Tg) and melting temperature (Tm) of the polymers.[/ p]