A polymer-bonded explosive, also called PBX or plastic-bonded explosive, is an explosive material 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 in 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).
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, 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.
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 however 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 effect to safety. They are also difficult to process.
Elastomers have to be used with more mechanically sensitive explosives, e.g. 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.
Insults (potential explosive inhibitors)
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.
Thermomechanical mechanisms involve stresses by thermal expansion (namely differential thermal expansions, as thermal gradients tend to be involved), melting/freezing or sublimation/condensation of components, and phase transitions of crystals (e.g. transition of HMX from beta phase to delta phase at 175 °C involves a large change in volume and causes extensive cracking of its crystals).
Thermochemical changes involve decomposition of the explosives and binders, loss of strength of binder as it softens or melts, or stiffening of the binder if the increased temperature causes crosslinking of the polymer chains. The changes can also significantly alter the porosity of the material, whether by increasing it (fracturing of crystals, vaporization of components) or decreasing it (melting of components). The size distribution of the crystals can be also altered, e.g. by Ostwald ripening. Thermochemical decomposition starts to occur at the crystal nonhomogeneities, e.g. intragranular interfaces between crystal growth zones, on damaged parts of the crystals, or on interfaces of different materials (e.g. crystal/binder). Presence of defects in crystals (cracks, voids, solvent inclusions...) may increase the explosive's sensitivity to mechanical shocks.
Some example PBXs
|Name||Explosive ingredients||Binder ingredients||Usage|
|EDC-29||β-HMX 95%||5% HTPB||UK composition|
|EDC-37||HMX/NC 91%||9% polyurethane rubber|
|LX-04-1||HMX 85%||Viton-A 15%||High-velocity; nuclear weapons (W62, W70)|
|LX-07-2||HMX 90%||Viton-A 10%||High-velocity; nuclear weapons (W71)|
|LX-09-0||HMX 93%||BDNPA 4.6%; FEFO 2.4%||High-velocity; nuclear weapons (W68). Prone to deterioration and separation of the plasticizer and binder. Caused serious safety problems.|
|LX-09-1||HMX 93.3%||BDNPA 4.4%; FEFO 2.3%|
|LX-10-0||HMX 95%||Viton-A 5%||High-velocity; nuclear weapons (W68 (replaced LX-09), W70, W79, W82)|
|LX-10-1||HMX 94.5%||Viton-A 5.5%|
|LX-11-0||HMX 80%||Viton-A 20%||High-velocity; nuclear weapons (W71)|
|LX-14-0||HMX 95.5%||Estane & 5702-Fl 4.5%|
|LX-15||HNS 95%||Kel-F 800 5%|
|LX-16||PETN 96%||FPC461 4%||FPC461 is a vinyl chloride:chlorotrifluoroethylene copolymer and its response to gamma rays has been studied.|
|LX-17-0||TATB 92.5%||Kel-F 800 7.5%||High-velocity, insensitive; nuclear weapons (B83, W84, W87, W89)|
|PBX 9007||RDX 90%||Polystyrene 9.1%; DOP 0.5%; rosin 0.4%|
|PBX 9010||RDX 90%||Kel-F 3700 10%||High-velocity; nuclear weapons (W50, B43)|
|PBX 9011||HMX 90%||Estane and 5703-Fl 10%||High-velocity; nuclear weapons (B57 mods 1 and 2)|
|PBX 9205||RDX 92%||Polystyrene 6%; DOP 2%||Created in 1947 at Los Alamos, later given the PBX 9205 designation.|
|PBX 9404||HMX 94%||NC 3%; CEF 3%||High-velocity; nuclear weapons, widely used (B43, W48, W50, W55, W56, B57 mod 2, B61 mods 0, 1, 2, 5, W69). Serious safety problems related to aging and decomposition of the nitrocellulose binder.|
|PBX 9407||RDX 94%||FPC461 6%|
|PBX 9501||HMX 95%||Estane 2.5%; BDNPA-F 2.5%||High-velocity; nuclear weapons (W76, W78, W88). One of the most extensively studied high explosive formulations.|
|PBS 9501||-||Estane 2.5%; BDNPA-F 2.5%; sieved white sugar 95%||inert simulant of mechanical properties of PBX 9501|
|PBX 9502||TATB 95%||Kel-F 800 5%||High-velocity, insensitive; principal in recent US nuclear weapons (B61 mods 3, 4, 6–10, W80, W85, B90, W91), backfitted to earlier warheads to replace less safe explosives.|
|PBX 9503||TATB 80%; HMX 15%||Kel-F 800 5%|
|PBX 9604||RDX 96%||Kel-F 800 4%|
|PBXN-106||RDX||polyurethane rubber||Naval shells|
|PBXN-3||RDX 85%||Nylon||AIM-9X Sidewinder Missile|
|PBXN-5||HMX 95%||fluoroelastomer 5%||Naval shells|
|PBXN-9||HMX 92%||HYTEMP 4454 2%, Diisooctyl adipate (DOA) 6%||Various|
|X-0242||HMX 92%||polymer 8%|
|XTX 8003||PETN 80%||Sylgard 182 (silicone rubber) 20%||High-velocity, extrudable; nuclear weapons (W68, W76)|
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