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 dioctyl phthalate 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). Potential advantages Polymer-bonded explosives have several potential advantages: If the polymer matrix is an elastomer (rubbery material), it tends to absorb shocks, making the PBX very insensitive to accidental detonation, and thus ideal for insensitive munitions. Hard polymers can produce PBX that is very rigid and maintains a precise engineering shape even under severe stress. PBX powders can be pressed into a particular shape at room temperature, which when casting normally requires hazardous melting of the explosive. High pressure pressing can achieve density for the material very close to the theoretical crystal density of the base explosive material. Many PBXes are safe to machine—to turn solid blocks into complex three-dimensional shapes. For example, a billet of PBX can, if necessary, be precisely shaped on a lathe or CNC machine. This technique is used to machine explosive lenses necessary for modern nuclear weapons. Binders Fluoropolymers 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 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