Self Healing cast cured energetic materials – MECA

Modern ammunitions such as bombs, penetrators, torpedo warheads, missile warheads of all sizes and some large calibre shells now contain cast cured PBX (Plastic Bonded eXplosive) formulations. Cast cured PBX formulations consist of explosive solid fillers coated with a polymeric binder. Depending on the desired effects and their applications, these explosive solid fillers may sometimes be associated with oxidizers and metal fuels. The solid charge rates are high, about 85 to 90% by weight.
The same family of formulations, in which oxidizing crystalline charges or reducing metal fuel particles are coated with a polymeric binder, are also used as solid propellants. These solid propellants are for example used for space launchers (Ariane 5 ‘P240’ current booster motor as well as the future Ariane 6 new generation booster ‘P120C’). The rate of solid charges is currently of the order of 85%. Same kinds of composite propellants are also used in many tactical or strategic missiles engines.
The mechanical integrity of the loadings containing these formulations is fundamental and critical, as it guarantees the safety of use during the firing phase, either the rocket motor or the explosive ordnance airborne or launched by a gun. Indeed, in the case of a solid propellant motor, the presence of a crack in the energetic composition will offer more surface to the combustion than expected and then cause an unwanted pressure rise that can lead to the destruction of the motor. In the case of the firing of a shell, the internal displacements generated by the acceleration forces (10 to 30000g) can lead to the mutual friction between the lips of the crack and cause ignition by localized heating of the formulation, which can lead to a reaction of the shell in the gun barrel.
The objectives are therefore to demonstrate experimentally the feasibility of obtaining self-repairing polymers based on HTPB (hydroxyl terminated polybutadiene) and optionally on GAP (Glycidyl Azide Polymer), compatible with high enough solid charge rates (at least 80%). ) while maintaining its ability to repair and offering mechanical properties equivalent to existing formulations.
To do this, ICPEES and EURENCO are joining forces to develop a research strategy based on three chemical repairing concepts, the first self-repairing concept is based on Diels-Alder's cycloaddition, the second is based on click chemistry. All of these self-repairing ligation reactions will be studied on innovative multivalent molecular platforms.
These concepts will be developed to integrate cast PBX formulations as defined and produced by EURENCO. These are mainly composed of solid explosive charges and a polymeric binder, plasticizers and additives. The matrix of the binder is composed of HTPB. This matrix is subsequently cross-linked with an isocyanate so as to form a polyurethane. HTPB is a di-functional polymer which has hydroxyl functional groups at the chain end and pendant alkene chains. These alkene chains will allow self-healing reactions.
The two ways (and variants) of repair will be tested and validated first in the form of films without solid charges. Then, after checking the chemical compatibilities of the ingredients, especially with the live explosive charges, formulations with inert fillers will be studied on a small scale using the resonant acoustic mixing (RAM) technology. The best conditions will be applied to an energetic formulation equivalent to those commonly employed in airborne bomb loadings and also close to a generic composite propellant formulation. The demonstrations of self-repair will be carried out firstly after manual cracking and then with an equipment allowing the appearance of cracks in the thermal shrinkage.