Explosive and thermic compositions or charges – Structure or arrangement of component or product – Solid particles dispersed in solid solution or matrix
Reexamination Certificate
2002-01-22
2004-05-04
Miller, Edward A. (Department: 3641)
Explosive and thermic compositions or charges
Structure or arrangement of component or product
Solid particles dispersed in solid solution or matrix
C528S028000, C149S019400, C149S019600
Reexamination Certificate
active
06730181
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method for making cured poly(glycidyl nitrate) having high stability against de-cure. This invention also relates to the production of solid energetic compositions, such as propellants, explosives, pyrotechnics, and gas generants, comprising poly(glycidyl nitrate) binders.
2. State of the Art
Solid high energy compositions, such as propellants, explosives, pyrotechnics, and gasifiers, or the like, generally comprise solid particulates, such as fuel particles and/or oxidizer particles, dispersed and immobilized in a cured binder matrix.
In recent years, energetic polymers have been developed and evaluated as replacements for inert polymeric binders in cast propellant systems, explosive compositions, and pyrotechnics. The substitution of an energetic polymer for an inert polymer in a typical pressable or extrudable explosive composition generally increases the detonation pressure and detonation velocity of the explosive.
Poly(glycidyl nitrate) (also known as “PGN” and “polyGLYN”) has been known and recognized for years as a possible energetic polymer suitable for use in propellants, explosives, gas generants, pyrotechnics, and the like. PGN is commonly synthesized in the industry by preparing a difunctional glycidyl nitrate polymer and curing the PGN with a polyfunctional isocyanate having a functionality of greater than about 2.3 to give urethane cross-linked polymers. Aliphatic polyisocyanates have been selected as the curing agents.
Although glycidyl nitrate prepolymers have a satisfactory shelf life, it is known that conventionally cured PGN inherently de-cures when subjected to elevated temperatures for prolonged periods. If precautions are not taken, over time, cured PGN can de-cure to the point of reverting to a pourable liquid. Accordingly, special care must be taken in the handling and storing of energetic compositions containing cross-linked PGN. The special care required to avoid a de-curing problem has impeded the widespread use of PGN as a binder, despite its attractive energetic properties.
One solution to this de-curing problem has been suggested by N. C. Paul et al.,
An Improved polyGLYN Binder Through End Group Modification
, ICI Explosives (1998). The article indicates the de-curing problem as being caused by the proximity of the terminal hydroxyl groups of the polymer to nitrate ester groups. The authors conclude that the de-curing problem is an inevitable consequence of the end group structure. To overcome this problem, the article describes a two-step process that modifies the end groups by removing the adjacent nitrate esters and replacing the nitrate ester groups with hydroxyl groups by base-catalyzed hydrolysis. Aging tests have shown that this technique is successful in preventing de-cure of the polymer. However, one of the major drawbacks to this solution of PGN de-curing is the decrease of energetic performance. It is estimated that approximately 10 percent of the nitrate esters may be removed from the polymer chain in accordance with this technique. Another drawback is that the extra chemical process steps cause additional expense and chemical waste in production.
BRIEF SUMMARY OF THE INVENTION
Therefore, in one embodiment, the present invention provides a poly(glycidyl nitrate) (PGN) production process that produces cured PGN having desired stability against de-cure without sacrificing nitrate ester moieties in the production process.
The present invention further provides a process for the production of a cured energetic composition wherein the process can yield a cured energetic composition having desirable stability against de-cure without sacrificing nitrate ester moieties in the production process.
In accordance with the purposes of the invention as embodied and broadly described in this document, a process is provided in accordance with a first aspect of the invention in which at least one polyol having a hydroxyl functionality of at least three, preferably four, is provided as a polymerization initiator. The polyol initiator is optionally, although preferably, reacted with a catalyst to form a catalyst-initiator complex, which is then used in the polymerization of glycidyl nitrate. The resulting polyfunctional poly(glycidyl nitrate) has a functionality substantially equivalent in number to the hydroxyl functionality of the polyol. The poly(glycidyl nitrate) is then either cross-linked or combined into an energetic formulation and subjected to cross-linking. Cross-linking is performed with at least one aromatic diisocyanate having at least one aromatic ring and two isocyanate moieties bonded directly to the at least one aromatic ring. Examples of the aromatic diisocyanate include toluene diisocyanate, phenylene diisocyanate, and/or methylene di-p-phenylene diisocyanate.
The selection of an aromatic diisocyanate curative for curing of the polyfunctional PGN imparts a high stability against de-cure, especially at elevated temperatures, while not sacrificing the energetic performance contributed by the nitrate ester moieties of the glycidyl nitrate. Further, by selecting a polyol initiator having a hydroxyl functionality of three or more, cross-linking of the PGN (instead of exclusive chain extension, as in the case of using a difunctional initiator and difunctional curing agent) is attained with an aromatic diisocyanate for enhancing physical properties of the cured binder.
In accordance with another aspect of the invention, a process is provided for the production of a cured energetic composition. The process comprises providing at least one polyol initiator having a hydroxyl functionality of at least three, optionally reacting the polyol initiator with a catalyst to form a catalyst-initiator complex, reacting glycidyl nitrate monomers with the polyol initiator and/or the catalyst-initiator complex to form poly(glycidyl nitrate) having a functionality substantially equal in number to the hydroxyl functionality, preparing an energetic formulation comprising the poly(glycidyl nitrate), and cross-linking the poly(glycidyl nitrate) with at least one aromatic diisocyanate. The diisocyanate has at least one aromatic ring and two exocyclic isocyanate moieties bonded directly to the aromatic ring.
Additional aspects and advantages of the invention will be set forth in the description that follows and in part will be apparent from the description, or may be learned by practice of the invention. The aspects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations pointed out in the appended claims.
DETAILED DESCRIPTION OF THE INVENTION
Reference will now be made in detail to the presently preferred embodiments and methods of the invention as described below. It should be noted, however, that the invention in its broader aspects is not limited to the specific details, representative devices and methods, and examples described in this section in connection with the preferred embodiments and methods. The invention according to its various aspects is particularly pointed out and distinctly claimed in the attached claims read in view of this specification and appropriate equivalents.
In accordance with one aspect of the invention, a process is provided for the production of poly(glycidyl nitrate). The process comprises providing at least one polyol initiator having a hydroxyl functionality of at least three.
The preferred polyol initiator employed generally has at least three sterically unhindered hydroxyl groups and preferably is a liquid at room temperature. An exemplary liquid triol is glycerin. Examples of solid triols are trimethylol propane ((2-ethyl-2-hydroxymethyl)-1,3-propanediol) and butane triol, which may be dissolved in a suitable solvent, such as methylene chloride. Preferably, the polyol initiator has a hydroxyl functionality of four, i.e., a tetraol. Tetraols useful in forming tetrafunctional polymers according to this invention have the formula:
wherein R
1
is nothing or a nonpolar extender, such as an al
Martins Laura J.
Sanderson Andrew J.
Alliant Techsystems Inc.
Miller Edward A.
TraskBritt
LandOfFree
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