Pressable infrared illuminant compositions

Abstract: An infrared producing illuminant composition comprising: (a) from 40% to 90% by weight of an oxidizer which produces infrared radiation upon burning, (b) from 1% to 35% by weight binder, and (c) from 5% to 40% by weight organic fuel, distinct from the binder, the fuel comprising a compound having at least 3 to 6-membered heterocyclic ring and containing 1 to 4 oxygen atoms, the ratio of infrared radiation to visible radiation is not less than 6.0 and the burn rate of the composition is not less than 0.075cm/s, wherein the oxidizer is selected from the group consisting of potassium nitrate, caesium nitrate, rubidium nitrate and combinations thereof.

Description: BACKGROUND 1. The Field of the Invention The present invention is related to illuminant compositions which emit significant quantities of infrared radiation. More particularly, the present invention is related to pressable/tampable infrared illuminant compositions which exhibit high initial burn rates, burn cleanly, and emit relatively small quantities of visible light in proportion to the infrared radiation emitted. 2. Technical Background There is a need in various situations for an ability to see clearly at night, or during periods of substantially reduced sunlight. Such situations may, for example, include search and rescue operations, police surveillance, and military operations. In these types of situations, it is often important that key personnel have the ability to see clearly, even though there is limited sunlight. In order to solve the problem of visibility at night, or during periods of substantially reduced sunlight, devices have been developed which allow one to see based upon available infrared illumination, rather than visible light. While the infrared vision devices take on various configurations, perhaps the most common type of infrared vision devices are night vision goggles. These devices provide individual users with the ability to see much more clearly at night, while not significantly limiting the mobility of the individual user. In order to facilitate the use of infrared vision devices, it has been found advantageous to enhance the available infrared radiation in the area of interest. In that regard, infrared emitting flare mechanisms have been developed. Such mechanisms have taken on a variety of configurations; however, the most widely used mechanisms comprise flares which emit relatively large quantities of infrared radiation in addition to any visible light that may be produced. Infrared emitting flares are generally configured in much the same manner as visible light emitting flares. Such flares may provide infrared radiation at a single position on the ground, or they may provide such radiation above the ground. In the case of above-ground operation, the flare system includes an internal or external means of propulsion which allows the user to fire the flare in a desired direction. In addition, the flare itself includes a material which, when burned, produces significant quantities of infrared radiation. In general operation the flare is propelled over the area of interest and ignited. The emitted infrared radiation then greatly enhances the usefulness of infrared viewing devices, such as night vision goggles. A number of problems have been encountered in the development of suitable infrared emitting compositions for use in such flares. For example, it will be appreciated that it is often desirable to provide an infrared emitting flare which does not emit excessive quantities of visible light. In situations where it is desirable to conduct operations under cover of night with a degree of secrecy, this capability is imperative. Excessive emission of visible light from the flare may alert individuals in the area to the existence of the flare, which may in turn significantly reduce the effectiveness of the overall operation. It has been found with known infrared flare compositions that excessive visible light is in fact emitted. In that regard, the performance of infrared emitting devices can be judged by the ratio of the amount of infrared radiation emitted to the amount of visible light emitted. This ratio is found to be low for many conventional infrared emitting compositions, indicating a high proportion of visible light being emitted from the flare. Another problem encountered in the use of infrared emitting compositions relates to the burn rate achieved. Many known compositions have burn rates which are lower than would desired, resulting in less infrared radiation than would be desired. In order to provide an effective flare, relatively high burn rates are required. It is often observed that the burning (surface area) of the flare composition increases dramatically over time. This characteristic is also generally undesirable. In the case of an infrared emitting flare which is launched into the air, this means that less infrared radiation is emitted when the flare is high above the surface, while more infrared radiation is emitted while the flare is near the surface. Indeed, it is often found that the flare continues to burn after it has impacted with the ground. It will be appreciated that this burn rate curve is just the opposite of that which would be generally desirable. It is desirable to have a high intensity infrared output when the flare is at its maximum altitude in order to provide good illumination of the ground. It is less critical to have high infrared output as the flare approaches the ground simply because the distance between the ground and the flare is not as great (illumination can be expressed by the equation Illumination = (I x 4π)/(4πR 2 ) where I is the intensity in watts/steradian, R is the distance in feet from the flare to the object being illuminated, and illumination is expressed in units of watts/meter 2 ). Ultimately, it is desirable that the flare cease operation before impact with the surface in order to reduce detection and obvious problems, such as fire, which may be caused when a burning flare impacts with the ground. Another problem often encountered with known infrared emitting materials is "chunking out." This phenomenon relates to breakup or unbonding separation of the flare illuminant grain during operation. In these situations it is found that large pieces of the infrared emitting composition may break away from the flare and fall to the ground. This is problematic because the flare fails to operate as designed when large pieces of the infrared producing composition are missing, the amount of infrared output over the subject location is curtailed, and falling pieces of burning flare material create a safety hazard. It has also been found that the use of conventional flare compositions results in soot formation. Soot formation can adversely affect the operation of the flare device in several ways, including causing an increase in visible light emitted. When soot or carbon is heated it may radiate as a blackbody radiator. Soot formation is encountered primarily due to the fuels and binders employed in the infrared producing composition. Conventional infrared producing compositions have generally been unable to adequately deal with the problem of soot formation. A further problem relates to aging of the IR emitting composition. It is often observed that known compositions substantially degrade over time. This is particularly true if the storage temperature is elevated. In some situations, it may be necessary to store these materials for long periods of time at temperatures at or above 50°C. This has not been readily achievable with known compositions. In summary, known infrared emitting compositions have been found to be less than ideal. Limitations with existing materials have curtailed their effectiveness. Some of the problem areas encountered have included low overall burn rates, undesirable burn rate curves, chunking out, poor aging, and undesirable levels of visible emissions. It would, therefore, be a significant advancement in the art to provide infrared emitting compositions which overcame some of the serious limitations encountered with known compositions. It would be an advancement in the art to provide compositions which provided high levels of infrared emissions, while limiting the level of visible light output. It would be another significant advancement in the art to provide such compositions which had acceptably high burn rates. It would also be an advancement in the art to provide infrared emitting compositions which substantially eliminated soot formation and which also substantially eliminated chunking. It would also be an advancement in the art to provide compositions which did not readily degrade with age, even when stored at relatively elevated temperatures. Such compositions and methods are disclosed and claimed herein. BRIEF SUMMARY OF THE INVENTION The present invention is related to novel and inventive compositions which produce significant quantities of infrared radiation when burned. At the same time, the compositions avoid many of the limitations of the existing art. The compositions are pressable/tampable compositions, have high burn rates, produce relatively little visible light in proportion to infrared radiation produced (in that they substantially avoid soot formation). The compositions also avoid common problems such as chunking and poor high temperature aging. Thus viewed from one aspect, the present invention provides an infrared producing illuminant composition comprising: (a) from 40% to 90% by weight of an oxidiser which produces infrared radiation upon burning, (b) from 1% to 35% by weight binder, and (c) from 5% to 40% by weight organic fuel, distinct from the binder, the fuel comprising a compound having at least one 3 to 6-membered heterocyclic ring and containing 1 to 4 oxygen atoms, the ratio of infrared radiation to visible radiation is not less than 6.0 and the burn rate of the composition is not less than 0.075cm/s, wherein the oxidizer is selected from the group consisting of potassium nitrate, caesium nitrate, rubidium nitrate and combinations thereof. The basic components of the composition include a binder, an oxidiser and a fuel. The fuel may preferably include nitrogen containing compounds. Other optional ingredients may also be added in order to tailor the characteristics of the composition to a specific use. Such optional ingredients include combustion rate catalysts and heat producing materials. The fuel comprises molecules containing 3 to 6 member heterocyclic rings and 1 to 4 oxygen atoms in the ring. Alkali metal salts of such heterocyclic compounds are also excellent fuels. In addition, fuels which fall within the scope of the present invention may further include bridged polycyclic amines, urea, guanidine, azodicarbonamide, and short chain alkyls. All of these fuels result in very little soot production in the context of the present invention. As mentioned above, it is critical to reduce visible light produced. This severely limits the fuels that can be used. Boron and silicon have been used in small amounts and act well as heat sources and as combustion rate catalysts. In addition, these materials are known to have some atomic emission lines located outside the visible spectrum, while producing tolerable amounts of visible light. Hydrocarbon fuels have been evaluated and many tend to produce soot, which can lead to high visible light output. The hydrocarbon fuels/binders used, therefore, must burn cleanly and provide nonluminous fragments that can burn with ambient air in the plume in order to increase the heat output and size of the radiation surface. At the same time, the material must serve to form a composition which is processible, avoids chunking, and is compatible with the oxidizers used. The hydrocarbon binders (polymers) that have proven to reduce soot formation include polyesters, polyethers, polyamines, polyamides; particularly those with short carbon fragments in the backbone, alternating with oxygen or nitrogen atoms. It has been found that polymer binders which include relatively short carbon chains (about 1-6 continuous carbon atoms) are preferred. These molecules do not generally product significant soot. Further, the additional desirable features of the invention can be achieved using these materials. The oxidizers potassium nitrate, cesium nitrate, rubidium nitrate, and combinations of these compounds produce large quantities of infrared radiation when the flare composition is burned. These oxidizers contain a metal with characteristic radiation wavelength in the near infrared (0.700 to 0.900m microns). The primary radiation comes from this line, whose width has been greatly broadened by the thermal energy in the plume. It is believed to be important to provide free metal (potassium, cesium, or rubidium) during the burning of the flare composition in order to produce significant levels of infrared radiation. These metals appear to augment one another when used in certain combinations. Significantly, high levels of cesium nitrate in the composition are found to greatly increase performance. Cesium nitrate is found to provide several significant advantages. Cesium nitrate is found to accelerate the burn rate. In addition, cesium nitrate broadens the infrared spectral output and improves infrared efficiency. Accordingly, it is preferred that cesium nitrate form from about 10% to about 90%, by weight, of the overall composition. In particular, excellent results are achieved when cesium nitrate is added to make up from about 25% to about 90% of the composition, preferably 25 to 80%. It is found that the compositions of the present invention produce relatively high burn rate materials. Burn rates at ambient pressures in the range of from about 0.075 to about 0.4 cm/sec. (0.030 to about 0.15 inches/sec.), and even somewhat higher, are readily achievable using the present invention. The more preferred range is above about 0.15 cm/sec. (0.060 inches/sec.). Conventionally, it has been found that burn rates in this range are not readily achievable. The present invention maintains the capability of tailoring desired characteristics by selecting specific combinations of fuels, oxidizers, and binders. Thus, particular burn rates and burn rate curves can be produced, the ratio of infrared radiation to visible light can be optimized, and the general physical and chemical properties can be carefully selected. Thus, the present invention provides a flexible illuminant material. As mentioned above, the present invention is related to pressable/tampable illuminant compositions which emit significant quantities of infrared radiation. The present invention also provides infrared propellant compositions which exhibit high initial burn rates, burn cleanly, and emit relatively small quantities of visible light in relation to the infrared radiation emitted. As the title implies, pressable/tampable compositions are pressed into the desired configuration. This is a convenient form for illuminant to take and is readily usable in flares and related devices. Methods of pressing the illuminant compositions into the desire configurations are known in the art. One suitable method and apparatus for pressing infrared illuminant compositions is disclosed in United States Patent No. 5,056,435 to Jones et al., granted October 15, 1991, which is incorporated herein by this reference. Other conventional foot presses may also be used because the compositions of the present invention exhibit significantly less chunking than conventional formulations, and are even significant improvements over the formulations disclosed in United States Patent No. 5,056,435. A typical pressable/tampable composition will include the following components in the following percentages by weight: Materials Percent Oxidizing Salt(s) (such as Potassium Nitrate and Cesium Nitrate) 40-90 Boron 0-10 Silicon 0-25 Organic Fuel 5-40 Polymer Binder 1-35 It will be appreciated that equivalent materials may be substituted for those identified above. Specifically, the nitrate salts may be substituted for one another, depending on the specific characteristics desired. One such example is rubidium nitrate, which may be added to the compositions, or may be substituted for some or all of the identified oxidizers. The ultimate objective in that regard is to provide a strong oxidizer which is also capable of substantially contributing to the output of infrared radiation during burning of the composition. The identified compounds possess those characteristics. As mentioned above, the use of high levels of cesium salts (such as cesium nitrate) increases the burning rate by as much as 400% and reduces visible output by up to 50%. This occurs while at the same time maintaining high levels of infrared light in the 700 to 1100 nm region. Thus, specifically tailored formulations may include high levels of cesium nitrate in order to achieve specific performance criteria. It is presently preferred that the composition include from about 10% to about 90% cesium nitrate. In some embodiments of the invention the preferred range will be from about 25% to about 80% cesium nitrate. It will be appreciated that the cesium nitrate comprises a portion of the total oxidizing salt added to the composition. The compositions also include a polymer binder. The binder facilitates the formulation, processing, and use of the final composition. At the same time, the binder provides a source of fuel for the composition. Suitable binders in the present invention also insure a clean burning composition by substantially reducing soot formation. As mentioned above, binders which are preferred in the present invention include polymers which have relatively short carbon chains (1-6 continuous carbon atoms) connected together by ether, amine, ester, or amide linkages (polyethers, polyamines, polyesters, or polyamides). Examples of such polymers include polyethylene glycol, polypropylene glycol, polybutylene oxide, polyesters, and polyamides. Binders of this type are commercially available and are well known to those skilled in the art. A specific example of a suitable binder is Formrez 17-80 polyester of Witco Chemical Corp. and more particularly, a curable polyester resin composition comprising by weight, from about 81% to about 83% to, preferably about 82.5% Formrez 17-80 polyester resin, about 15 to about 17%, preferably about 16.5% epoxy such as ERL 510 of Ciba-Geigy Corporation and about 0 to about 2%, and preferably 1% of a catalyst such as iron linoleate. More preferably, the binder may comprise about 82.5% Formrez 17-80 polyester resin, about 16.5% ERL epoxy and about 1% iron linoleate. Such a binder composition is referred to herein as WITCO 1780. As discussed above, in the pressable/tampable compositions of the present invention, a separate fuel is provided comprising molecules with a 3 to 6 member heterocyclic ring, which also contains 1 to 4 oxygen atoms. Fuels which fall within the scope of the present invention may further include nitrogen and oxygen containing compounds. Examples of such compounds include tetrazoles, triazoles, triazines, imidazoles, oxazole, pyrazole, pyrroline, pyrrolinidene, pyridine, pyrimidine, and similar compounds. Combinations of such ring systems can be fused or joined by covalent bonds, such as in bitetrazole. Such heterocyclic rings may be substituted with nitrogen containing groups (such as nitro, nitroso, cyano, and amino) at any or all substitutable sites on the ring. Alkali metal salts of such heterocyclic compounds, or their derivatives, are also useful. Preferred alkali metal include potassium, rubidium, and cesium, alone or in combination. Fuels which fall within the scope of the present invention may further include bridged polycyclic amines. Also useful are salts arising from combinations of polycyclic amines and organic or inorganic acids. Such compounds include dicyanodiamide, cyanonitramide, hydrogencyanide, dicyanamide, and the like. Fuels which fall within the scope of the present invention may further include urea, guanidine, azodicarbonamide, and short chain alkyls that contain 1 to 8 carbons. In addition, derivatives of such compounds, substituted with nitrogen containing groups, are also useful. Substitution may be made with NO 2 , NO, CN, and / or NH 2 . It is apparent that the fuels must burn cleanly, rapidly, and at high temperatures. The fuels do not produce significant amounts of soot, with its associated increase in visible light output. The fuels identified above meet these performance criteria. As mentioned above, it is also possible to add combustion rate catalysts and heat sources to the overall composition. These materials provide for further tailoring of the performance characteristics of the resulting composition. These materials, however, must also fit the other parameters of an acceptable composition such as producing little visible light and not contributing to the other undesirable characteristics identified herein. Two examples of such preferred materials include silicon and boron, while magnesium is not preferred because of its propensity to emit large quantities of visible light. In the pressable/tampable compositions described herein, boron is preferably added to constitute from about 0% to about 10%, by weight of the total composition. Silicon preferably makes up from about 0% to about 25% of the total composition. One measure of a preferred composition is the ratio of infrared radiation to visible light produced during burning of the composition. The composition will have an IR/Vis. ratio of not less than 6.0. Indeed, ratios of from about 10 to about 20 are achievable with the present invention. These levels of infrared output per unit of visible output have not been easily achievable using conventional compositions. It is found that the compositions within the scope of the present invention also provide increased burn rates. Burn rates within the range of not less than 0.075 to about 0.4 cm/sec (0.030 to about 0.15 inches per second), and even above, are characteristic of the compositions of the present invention. As mentioned above, the preferred burn rates are in excess of 0.15 cm/sec (0.060 inches/second). Compositions within the scope of the present invention also age and store well. It has been found that a composition was still acceptable after being stored at 57°C (135°F) for one year. This is a further feature which has not generally been available in known compositions. Compositions within the scope of the present invention can be formulated and prepared using known and conventional technology. Formulation techniques such as those generally employed in mixing and preparing propellant, explosive, and pyrotechnic compositions are preferably used in the preparation of the compositions within the scope of the present invention. Summary In summary, the present invention provides new and useful illuminant formulations which produce large quantities of infrared radiation, but produce relatively small quantities of visible light. Accordingly, some of the major drawbacks with known infrared producing materials are avoided. The compositions of the present invention have high burn rates. The compositions emit infrared while producing only limited soot and, therefore, limited visible light is produced. The compositions of the present invention also substantially eliminate chunking. The compositions do not significantly degrade with age, even when stored at relatively elevated temperatures. Thus, the compositions of the present invention represent a significant advancement in the art.

Claims: An infrared producing illuminant composition comprising: (a) from 40% to 90% by weight of an oxidiser which produces infrared radiation upon burning, (b) from 1% to 35% by weight binder, and (c) from 5% to 40% by weight organic fuel, distinct from the binder, the fuel comprising a compound having at least one 3 to 6-membered heterocyclic ring and containing 1 to 4 oxygen atoms, the ratio of infrared radiation to visible radiation is not less than 6.0 and the burn rate of the composition is not less than 0.075cm/s, wherein the oxidizer is selected from the group consisting of potassium nitrate, caesium nitrate, rubidium nitrate and combinations thereof. A composition as claimed in claim 1, in which the fuel is an oxazole. A composition as claimed in claim 1, in which at least 25% by weight of the composition comprises caesium or rubidium nitrate. A composition as claimed in claim 1, which includes from 10% to 90% caesium nitrate, preferably from 25% to 80% caesium nitrate. A composition as claimed in claim 1, wherein the binder comprises materials selected from the group consisting of polyesters, polyethers, polyamines and polyamides. A composition as claimed in claim 1, in which the binder is selected from the group consisting of polyethylene glycol, polypropylene glycol, polybutylene oxide, polyesters and polyamides. A composition as claimed in claim 1, in which the illuminant has a burn rate in the range of from 0.15 to 0.4 cm/s. A composition as claimed in claim 1, which includes at least one combustion rate catalyst selected from the group consisting of boron and silicon. A composition as claimed in claim 8, further comprising up to 20% by weight silicon. A composition as claimed in claim 8, further comprising up to 10% by weight boron. A composition as claimed in claim 1, in which the binder comprises polymers selected from the group having continuous carbon chains of 1 to 6 molecules linked together by linkages selected from the group consisting of ether, amine, ester and amide linkages. A composition as claimed in claim 1, in which the ratio of infrared radiation to visible radiation is in the range 10 to 20. A composition as claimed in claim 1, in which the burn rate of the composition is greater than 0.15 cm/s at ambient pressure.

Inventors: NIELSON DANIEL B, NIELSON, DANIEL B.
Applicants: ALLIANT TECHSYSTEMS INC, ALLIANT TECHSYSTEMS INC.
Application year: 1993
Main classification: C06B45
IPC codes: C06C15, C06B33, C06B45