Polymer-bonded explosive

Polymer-bonded explosives, also called PBX or plastic-bonded explosives, are explosive materials in which explosive powder is bound together in a matrix using small quantities (typically 5–10% by weight) of a synthetic polymer. PBXs are normally used for explosive materials that are not easily melted into a casting, or are otherwise difficult to form.

PBX was first developed in 1952 at Los Alamos National Laboratory, as RDX embedded in polystyrene with diisooctyl phthalate (DEHP) plasticizer. HMX compositions with teflon-based binders were developed in 1960s and 1970s for gun shells and for Apollo Lunar Surface Experiments Package (ALSEP) seismic experiments, although the latter experiments are usually cited as using hexanitrostilbene (HNS).

Polymer-bonded explosives have several potential advantages:

Fluoropolymers are advantageous as binders due to their high density (yielding high detonation velocity) and inert chemical behavior (yielding long shelf stability and low aging). They are somewhat brittle, as their glass transition temperature is at room temperature or above. This limits their use to insensitive explosives (e.g. TATB) where the brittleness does not have detrimental effects on safety. They are also difficult to process.

Elastomers have to be used with more mechanically sensitive explosives like HMX. The elasticity of the matrix lowers sensitivity of the bulk material to shock and friction; their glass transition temperature is chosen to be below the lower boundary of the temperature working range (typically below -55 °C). Crosslinked rubber polymers are however sensitive to aging, mostly by action of free radicals and by hydrolysis of the bonds by traces of water vapor. Rubbers like Estane or hydroxyl-terminated polybutadiene (HTPB) are used for these applications extensively. Silicone rubbers and thermoplastic polyurethanes are also in use.

Fluoroelastomers, e.g. Viton, combine the advantages of both.

Energetic polymers (e.g. nitro or azido derivates of polymers) can be used as a binder to increase the explosive power in comparison with inert binders. Energetic plasticizers can be also used. The addition of a plasticizer lowers the sensitivity of the explosive and improves its processibility.

Explosive yields can be affected by the introduction of mechanical loads or the application of temperature; such damages are called insults. The mechanism of a thermal insult at low temperatures on an explosive is primarily thermomechanical, at higher temperatures it is primarily thermochemical.