Abstract:
A pyrotechnic composition comprising microcrystalline nitrocellulose which is characterized as an ultra low-smoke composition. The pyrotechnic composition includes at least one flame coloring agent, and may be produced with or without an optional oxidizing agent, with or without an optional metal powder, with or without an optional chlorine donor. Upon combustion, the pyrotechnic composition produces illuminating emissions having desired colors and luminosity characteristics with significantly reduced or toxic combustion products and smoke.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application is a continuation-in-part of U.S. patent application Ser. No. 11/058,677 herein incorporated by reference, filed on Feb. 15, 2005 and published as U.S. Patent Application Publication No. 2006/0180253 on Aug. 17, 2006. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH  
       [0002]     Not Applicable.  
       BACKGROUND OF THE INVENTION  
       [0003]     The present invention is related to the manufacture and use of pyrotechnic compositions such as are used in fireworks and flares for providing an illuminating visual display, and in particular to the use of microcrystalline nitrocellulose as a fuel component in a pyrotechnic composition to provide vivid colors during combustion.  
         [0004]     Nitrocellulose, also known as cellulose nitrate, pyroxylin, colloxylin, xyloidin, celloidin, and parlodion, is a fast burning, easily ignitable, high-nitrogen energetic material which has been used in such applications as explosives and as a gun and rocket propellant. Nitrocellulose burns cleanly with non-toxic combustion byproducts, such as nitrogen, carbon dioxide, and water vapor. In the fireworks industry, nitrocellulose is typically utilized as an energetic binder in a lacquer form, added to pyrotechnic compositions.  
         [0005]     Nitrocellulose is manufactured from cellulose, which has the chemical formula C 6 H 10 O 5 , and which is the principle structural component of cell walls in higher plants. An almost pure form of cellulose, known as alpha cellulose, occurs naturally in cotton fibers. The physical properties of cellulose are a result of an association of cellulose chains which form crystalline structures called microfibrils, typically having a diameter of 2-20 nm, a length of between 100-40,000 nm, and which contain about 2000 cellulose molecules. Within a plant cell, the orientation of microfibrils changes from layer to layer. Often, especially in very strong plant cell walls, the microfibrils are arranged screw-like about an axis of the cell, with changes in the turning angle from layer to layer.  
         [0006]     Nitrocellulose is produced by the well known process of nitrating cellulose with a mixture of concentrated acids including nitric acid, which has the chemical formula HNO 3 . During the resulting chemical process, the nitric acid converts the cellulose into cellulose nitrate, having the chemical formula of C 6 H 9 O 5 (NO 2 ), C 6 H 8 O 5 (NO 2 ) 2  or C 6 H 7 O 5 (NO 2 ) 3  depending on the degree of nitration. Sulfuric acid generally is used in conjunction with the nitric acid during the nitrating process to prevent the water produced in the reaction from diluting the concentration of nitric acid.  
         [0007]     Pyrotechnic and explosive compositions are typically fairly dense mixtures of crystalline salts for flame color, crystalline oxidizers, carbonaceous fuels, and powdered metals. The flame coloring agents are commonly the salts of the metals; barium, strontium, copper, boron, calcium, potassium, sodium and lithium. The oxidizing agents may include at least one compound selected from a group consisting of ammonium perchlorate, alkali metal perchlorates, alkali metal chlorates, alkali metal nitrates and alkaline earth metal nitrates. Additional fuels may include carbon, titanium, titanium alloys, zirconium, zirconium alloys, iron, alloys of iron, magnesium, alloys of magnesium, aluminum and alloys of aluminum. These materials are easily solidified with binders, agglomerated, or compressed, to form pellets or granules.  
         [0008]     It is known that pyrotechnic compositions including nitrocellulose as the sole propellant in a color generating composition are not capable of generating sufficient temperature during combustion to produce an acceptable light output and depth of color suitable for use as a pyrotechnic star or other fireworks visual display element. Initially, it was thought that the inability of nitrocellulose to produce adequate colors was due to the presence of contaminants in the combustion flame including sodium salts and un-oxidized carbon, but extensive testing has shown that the lack of functionality is mainly due to the low combustion temperature inherent to nitrocellulose itself. Even when compressed to a great degree, nitrocellulose does not produce a sufficient combustion temperature to generate usable colors or useful luminosity as a fireworks display component by itself.  
         [0009]     Common nitrocellulose is not capable of reaching sufficient temperatures during combustion to allow the use of a variety of color generating salts because the combustion temperature of nitrocellulose does not reach sufficient levels to cause the decomposition of the colorant salts into their constituent ions, enabling them to reform into the proper color radiating species in combination with free chlorine. To solve this problem, it is known to combine the nitrocellulose with additional chemical additives to increase the temperature of combustion. Typically both oxidizers and fuels are used for this purpose, and usually in combination with each other, since adding either an oxidizer or a fuel alone would alter the oxygen balance of the pyrotechnic composition, thereby lowering the performance.  
         [0010]     It is further well known that it is advantageous to provide free and excess chlorine ions to color generating compositions to ensure the prompt and efficient production of the preferred “mono-chloride” color emitting species. The favored color generating species in pyrotechnic flames is generally considered to be a mono-chloride of an alkali metal or alkaline earth metal. In the past it has been found to be advantageous to add supplemental chlorine donating compounds to increase the chances of forming the desired mono-chlorides and thus the intended color. Recently, perchlorates such as potassium and ammonium perchlorate have been discovered to pose a heath hazard in lower levels than previously believed.  
         [0011]     Since a majority of chlorine-containing compounds normally employed in color-generating pyrotechnic compositions, are either toxic, hazardous, or pose a pollution hazard, particularly in geographic locations near which regular fireworks displays are provided, it would be advantageous to provide a pyrotechnic composition for use in fireworks and flares which produces illuminating emissions having desired color and luminosity characteristics while reducing or eliminating harmful or toxic compounds.  
         [0012]     Another problem with the use of nitrocellulose in a pyrotechnic composition is the inherent low density nature of nitrocellulose. The addition of fuels or oxidizers to a pyrotechnic composition containing nitrocellulose may serve to increase the density of the composition, but the nitrocellulose molecules remain overall a low density material. Coloiding and partial dissolution of the pyrotechnic composition has been employed in pyrotechnic components, as well as propellant applications for many years. The use of additional binders, solvents, plasticizers, and coloiding agents generally only results in slower rate or burn and an increase in the smoke output of the pyrotechnic composition upon combustion, and increases the complexity of the manufacturing process.  
         [0013]     Accordingly, it would be advantageous to provide a pyrotechnic composition which reduces the requirement for additional binders or densifying agents, which has a dense crystalline form, and which provides self binding characteristics under minor or moderate compression to a plastic-like substance, but which is harder and more durable then current pyrotechnic compositions.  
       BRIEF SUMMARY OF THE INVENTION  
       [0014]     Briefly stated, the present disclosure describes a pyrotechnic composition comprising microcrystalline nitrocellulose which is characterized as an ultra low-smoke composition. The pyrotechnic composition further includes at least one flame coloring agent, and may be produced with or without an optional oxidizing agent, with or without an optional metal powder, with or without an optional chlorine donor. The pyrotechnic composition produces illuminating emissions having desired colors and luminosity characteristics with significantly reduced toxic combustion products and smoke.  
         [0015]     The present disclosure further describes a pyrotechnic composition including microcrystalline nitrocellulose that does not require the use of additional binders or densifying agents, which has a dense crystalline form, and which provides self binding characteristics under minor or moderate compression to a plastic-like substance, but which is harder and more durable then current pyrotechnic compositions.  
         [0016]     The present disclosure provides a functional color producing fireworks system based on nitrated microcrystalline cellulose products that require no additional oxidizers or fuels to combust at an acceptable level for fireworks and pyrotechnic uses.  
         [0017]     The foregoing features, and advantages set forth in the present disclosure as well as presently preferred embodiments will become more apparent from the reading of the following description.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]     The following detailed description illustrates the invention by way of example and not by way of limitation. The description enables one skilled in the art to make and use the present disclosure, and describes several embodiments, adaptations, variations, alternatives, and uses of the present disclosure, including what is presently believed to be the best mode of carrying out the present disclosure.  
         [0019]     Previously, pyrotechnic chemistry has attempted to take advantage of traditional nitrocellulose compositions, such as described in U.S. Pat. No. 6,599,379 B2 to Hiskey et al. because of the clean burning, low smoke, low temperature and generally environmentally friendly nature of conventional nitrocellulose. However, conventional nitrocellulose has the undesirable property of not being able to attain combustion temperatures which are sufficient to generate the vivid and vibrant colors desired in fireworks displays without the use of supplemental energetic oxidizers, fuels, and/or combinations of same. The oxidizers and fuels employed with conventional nitrocellulose are the same as used in current state of the art fireworks compositions. Thus, the problems inherent in utilizing these oxidizers and fuel in combination with conventional nitrocellulose remain the same, albeit at a reduced amount.  
         [0020]     The present invention serves to remove nearly completely these smoke causing and potentially environmentally hazardous materials both from the standpoint of the combustion byproducts from the fireworks display and from the manufacture of same by providing pyrotechnic compositions which include microcrystalline nitrocellulose (MCNC) as a principal component by weight.  
         [0021]     Microcrystalline nitrocellulose, described in U.S. Patent Application Publication No. 2006/0180253 A1 to Nickel et al. combusts at a higher temperature than conventional nitrocellulose. Theoretical basis for the increased combustion temperature include: (1) the inherent density of the material which allows for a greater compression and density of loading, intensifying and magnifying the heat created; (2) since the microcrystalline nitrocellulose consists of individual crystals of nitrated alpha cellulose, there is a greater combustion surface area and for the same nitration level burn more intensely; and (3), microcrystalline nitrocellulose, like microcrystalline cellulose, is highly absorptive due to the capillary action of its surface porosity.  
         [0022]     During the nitration process for producing microcrystalline nitrocellulose, the nitrating mix, typically a mix of sulfuric and nitric acids tend to penetrate the surface of microcrystalline cellulose to a greater extent than in the nitration of fibrous cellulose during the production of conventional nitrocellulose. The residual acids are bound in the crystals tightly. During the neutralization and stabilization process of MCNC, typically promoted by treatment with solutions of alkali metal or alkaline earth metal carbonates, bicarbonates or hydroxides. The sulfates and, to a greater degree, the nitrates of the respective alkaline treatment compounds are created and retained in the porous crystal structure of microcrystalline nitrocellulose. Suitable materials for treatment during the neutralization and stabilization process include potassium carbonate, potassium bicarbonate and potassium hydroxide and to a lesser degree the carbonates or hydroxides of strontium, barium and copper. While the nitrates and sulfates formed by the neutralization of the residual material consists of less than 0.5% by weight of the MCNC, the tight incorporation of these oxidizing agents with the MCNC crystals tends to catalyze pyrotechnic combustion of the MCNC with or without any additional oxidizing additives.  
         [0023]     Similar to conventional nitrocellulose-based pyrotechnic compositions, the size of a flame envelope generated by combustion of an MCNC composition of the present invention is greatly increased over that of standard solid oxidizer/fuel pyrotechnic compositions. The larger the flame envelope and better the gas production, the better the resulting pyrotechnic star or display.  
         [0024]     Microcrystalline nitrocellulose based pyrotechnic compositions do not require additional additives to attain sufficient temperatures for adequate luminosity, and combust very well without the inclusion of supplemental oxidizers or fuels which produce solid particulate matter as byproducts of combustion. During combustion, solid particulate material present in a composition tends to radiate white light while in an excited state within a flame envelope. This white light often will wash out intended colors of a pyrotechnic display. Incorporation of microcrystalline nitrocellulose in a pyrotechnic composition enables a reduction in the quantity of solid smoke causing particulate matter, providing improved color purity and color depth during combustion. Additionally, since MCNC based pyrotechnic compositions of the present disclosure combust in a clean and orderly manner, it is not necessary to add additional chlorinated compounds to generate acceptable color purity when using metal chloride salts as a color producing agent.  
         [0025]     It is well known that various metal salts can be advantageously employed as flame coloring agents (flame colorants), for example calcium salts such as calcium carbonate for the color orange, strontium salts such as strontium nitrate for the color red, barium salts and or boron compounds for the color green, sodium salts such as sodium nitrate for the color yellow, copper salts such as copper chloride for the color blue, potassium salts such as potassium chloride for the color violet. Combinations of metal salts can yield other desirable colors. For example, a combination of copper carbonate and strontium nitrate has a purple color, and a combination of copper carbonate and barium nitrate has a blue-green color, and a combination of barium nitrate and sodium nitrate has a lime color. In spite of their toxicities, other metal salts such as cadmium, uranium, gold, mercury, arsenic and lead may be used to provide other colors if desired. Carbonate or nitrate salts are generally preferred over salts such as chloride salts as the chloride salts tend to be hydrates and contribute undesired water.  
         [0026]     It is additionally known to those of ordinary skill in the art that metal flakes or powder can be added to pyrotechnic compositions to increase the temperature and thus the light output of the flame or to produce a spark effect. Suitable metals can include aluminum, magnesium, titanium and iron or their alloys such as magnesium/aluminum alloy.  
         [0027]     In addition to exhibiting excellent combustion properties, microcrystalline nitrocellulose compacts well under minimum compression pressures, resulting in an exceptionally strong compacted form which exhibits strength similar to that of the plastic Delrin (acetal). A high binding capability enables the production of pellets or tablets of microcrystalline nitrocellulose which are extremely hard and stable, and which can be machined in much the same manner as plastic, by mechanical cutting or adhesively joining using solvents commonly utilized to dissolve nitrocellulose.  
         [0028]     Additional characteristics of microcrystalline nitrocellulose include low friability, inherent lubricity, high bulk density of 0.58 gm/cc to 0.72 gm/cc uncompressed, and a high dilution potential. It has been observed that microcrystalline nitrocellulose exhibits a lower pressure exponent than ordinary nitrocellulose when burned under elevated pressures. Fragments of microcrystalline nitrocellulose readily bond together without the use of an adhesive, and can be mixed with other substances so as to hold an additive while self-bonding.  
         [0029]     These characteristics of microcrystalline nitrocellulose enable a pyrotechnic composition including microcrystalline nitrocellulose and any optional color agents and metals to be produced with a reduction in the use of additional binders or densifying agents as compared to compositions utilizing convention nitrocellulose, which has a dense crystalline form, and which provides self binding characteristics under minor or moderate compression to form a plastic-like substance which is harder and more durable then current pyrotechnic compositions composed primarily of nitrocellulose.  
         [0030]     While the pyrotechnic compositions of the present invention which utilize microcrystalline nitrocellulose do not require supplemental fuels or oxidizers, a supplemental fuel or oxidizer may optionally be used with the pyrotechnic compositions of the present invention to increase combustion temperatures thereby increasing light output and burn speed. Since no processing other than simple mixing and compression of the components of the pyrotechnic composition is required, there is no problem with incompatibility with binders or the solvents for same. For example, the addition of magnesium, aluminum or other suitable high-temperature generating metallic compounds may simply be incorporated in the dry mix during processing of the MCNC pyrotechnic composition. Since no water, alcohol, acetone or the like is needed in the processing, no incompatibilities to the metals will be encountered. A few percent of a metallic fuel component is all that is required to increase the levels of light output approaching that of standard color generating pyrotechnic compositions.  
         [0031]     The pyrotechnic compositions of the present invention which utilize microcrystalline nitrocellulose in combination with metal chloride colorants, do not require additional chlorine doning compounds such as parlon, saran, chlorowax, hexachloroethane or ammonium perchlorate. However additional chlorine containing compounds may be optionally employed in the present invention in applications where hazardous combustion byproducts are of less concern, to provide enhanced depth and clarity to well defined colors during combustion while still greatly reducing the smoke output compared to conventional pyrotechnic compositions.  
         [0032]     In addition, the MCNC pyrotechnic composition of the present invention, which is advantageous in fireworks stars, may be used in fireworks shells and mines to fuel other pyrotechnic devices such as gerbs, lance, flares, airburst and all manner of fireworks flames where good color purity, low smoke and low environmental impact is important.  
         [0033]     The MCNC pyrotechnic compositions of the present invention generally consist of microcrystalline nitrocellulose fuel from about 20 to 97 percent by weight, and a flame coloring agent from about 1 to 8 percent by weight. The MCNC pyrotechnic compositions are more particularly described in the following examples, which are intended as illustrative only, as numerous modifications and variation will be readily apparent to those skilled in the art.  
       EXAMPLE 1  
       [0034]     A blue star MCNC pyrotechnic composition comprised of: 87% microcrystalline nitrocellulose; 5% potassium nitrate; 4% copper oxide; and 4% hexachloroethane was dry mixed by hand and sieved several times to obtain a homogenous mixture. The homogenous mixture was then fed into a stokes rotary tablet press, and tablets of 0.21 inches in diameter by 0.21 inches tall were formed for use as star components by simple compression. The star components are at this point ready to be loaded into any desired fireworks device without further processing.  
       EXAMPLE 2  
       [0035]     A red magnesium star MCNC pyrotechnic composition comprised of: 80% microcrystalline nitrocellulose; 10% strontium nitrate; 4% strontium carbonate; 3% magnesium metal powder; and 3% saran was dry mixed by hand and sieved several times to obtain a homogenous mixture. The homogenous mixture was then fed into a stokes rotary tablet press and tablets of 0.21 inches in diameter by 0.21 inches tall were formed by simple compression for use as star components. The star components are, at this point ready, to be loaded into any desired fireworks device without further processing.  
       EXAMPLE 3  
       [0036]     A red flare MCNC pyrotechnic composition comprised of: 96% microcrystalline nitrocellulose and 4% strontium chloride was dry mixed by hand and sieved several times to obtain a homogenous mixture. The homogenous mixture was then incrementally consolidated by hydraulic press into a paper tube having an inside diameter of 0.5 inches and a length of 2.0 inches.  
         [0037]     As various changes could be made in the above compositions without departing from the scope of the disclosure, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.