Moldable Ethylene/Vinyl Acetate Copolymer

The present invention relates to moldable explosives.

During World War II the British introduced the use of an explosive composition, containing 91.+-. .07 percent RDX (cyclotrimethylenetrinitramine) and 9.+-. .07 percent beeswax, which was highly suitable for press loading. The beeswax has been replaced with a petroleum wax designated Grade A and the explosive composition is known to the military as A-3. Grade A is a micro-crystalline parafinic wax.

The wax is used to coat the particles and act as a binding agent when the composition is pressed. When coated on the particles, the wax desensitizes them to premature explosion from friction, shock and other stimuli.

A-3 is manufactured by heating a water slurry of RDX to nearly 100.degree. C with agitation. The wax is added and the slurry is allowed to cool with continued agitation until the wax solidifies. The solids are filtered and dried.

A-3 and related explosives are moldable and therefore suitable for press loading due to the flow properties imparted to the composition by the wax. In press loading, a container, commonly a bullet-shaped projectile, is loaded from the nose by adding layers of explosive then pressing each layer with a ramrod-like device. When pressed, the material must flow outward and upward to some degree to coat the wall of the container.

Pressloading may be carried out either at room temperature (cold pressing) or at elevated temperatures (hot pressing). A-3 is loaded by cold pressing, a safer and simpler technique.

Grade A wax being a petroleum derivative, has become considerably more expensive and difficult to obtain; a search for a substitute binder with desensitizing and flow properties equivalent to Grade A wax provided the impetus for the present invention.

Polymers containing both vinyl acetate and ethylene moieties provide desensitizing and molding properties equivalent to or superior to those of Grade A wax. The adhesive properties of these polymers make them suitable for use with a variety of explosives. By modifying the ratio of vinyl acetate to ethylene or polyethylene moieties, various flow characteristics can be obtained making the compositions suitable for either hot pressing or cold pressing.

The polymers of the present invention are random or block copolymers. The two monomer units do not regularly alternate. Rather, they are present in sections of various lengths (termed herein "moieties") of ethylene monomers and vinyl acetate monomers. Such copolymers are termed polyethylene vinyl acetate or "EVA".

Both polyethylene and polyvinyl acetate are crystalline, hard materials. EVA's are pliable, adhesive materials, because polyethylene chains are rather zig-zag while vinyl acetate chains are not, so linear polymers containing both kinds of chains do not easily form the inter-chain bonds characteristic of rigid polymers.

EVA has been found to be exceptionally well suited for use in pressed explosives. The ethylene moiety provides a long hydrocarbon chain for lubricity, but not so long as to be crystalline. The vinyl acetate moiety provides polar groups which are attracted to the surface of RDX making the polymer an excellent wetting agent. Ester groups in the polymer function as Lewis bases and have an affinity for nitro groups (Lewis acids) in the explosive. Whereas most binders do not adhere to RDX at all, EVA adheres to it with remarkable strength, separating from it in strands.

Its affinity for nitro groups makes EVA adaptable for use with most, if not all, nitro explosives, and especially the cyclic nitramine class of explosives, including, in addition to RDX, HMX(1,3,5,7-tetranitrazacyclooctane).

EVA's are available commercially with various ratios of ethylene to vinyl acetate groups. For instance, EVA 638, marketed as ULTRATHENE by U.S. Industrial Chemical Co., has a 70:30 ratio. Vynathene 907, also from U.S. Industrial Chemical Co., has a 40:60 ratio. Vynathene has a lower glass transition temperature than the 638, making it softer and suitable for coldpressing.

The properties of some compositions made from the 638 are summarized in the following table:

In all cases the EVA compositions are comparable to the corresponding wax compositions. Compositions from 2 to 9 percent RDX corresponding to presently used military explosives have been prepared and found suitable. Eighteen percent EVA is the maximum amount of binder desired for the present invention, since in general, binder content is to be minimized.

The EVA 639/9% RDX composition has been successfully press loaded into 5 inch projectiles, but hot pressing was required. By substituting Vynathene 907 cold pressing was accomplished. Also, better friction sensitivity, on the order of 800 lbs., was recorded. Other operations are possible with EVA 638 (or other 40:60 EVA), such as extrusion, making possible the manufacture of explosive rods or pellets of the present composition.

Other advantages of EVA reside in its thermal properties. EVA/RDX compositions can be temperature cycled without cracking. Also, EVA, unlike other polymeric binders, endothermically breaks down at high temperatures, and acts as a heat sink for the explosive.

The compositions of the above table were prepared as follows: 10% EVA was dissolved in benzene or tetrahydrofuran and added with stirring to an RDX/water slurry, which is stirred until mixing is complete, then filtered and dried.