METHODS FOR FORMING ENERGETIC MOLDING POWDERS AND THE ENERGETIC MOLDING POWDERS FORMED BY THESE METHODS

Abstract

Methods for manufacturing explosives and the explosives manufactured by the methods are disclosed. Embodiments include mechanical mixing energetic particles with a thermoset binder material. Some embodiments include mixing the particles and binder using a bladeless mixer until homogeneous. Embodiments may also include heating a mixture of high energetic molding powder and a binder material at a low temperature (e.g., 70 C.) until the binder material is partially cured. Some embodiments include breaking up the mixture into pieces about 1 mm in size to form an energetic molding powder. And some embodiments include pressing a mixture of energetic molding powder into a die.

Claims

1. A method of manufacturing an energetic molding powder, comprising: combining energetic particles with a binder material into a mixture; partially curing the mixture of energetic particles and binder material by applying heat to the mixture of energetic particles and binder material, and removing the applied heat from the mixture of energetic particles and binder material before the mixture of energetic particle and binder mixture is fully cured; and breaking up the partially cured energetic particle and binder mixture into smaller pieces.

2. The method of claim 1, further comprising: after said partially curing, mixing the partially cured mixture of energetic particles and binder material.

3. The method of claim 1, further comprising: stopping said breaking up the partially cured energetic particle and binder mixture into smaller pieces when no piece has a largest dimension larger than 1 centimeter.

4. The method of claim 1, further comprising: placing the partially cured energetic particle and binder mixture into a die form; pressing the partially cured energetic particle and binder mixture in the die form; and removing the pressed energetic particle and binder mixture from the die form.

5. The method of claim 4, further comprising: after said pressing, slowly releasing the pressure on the energetic molding powder over a predetermined time.

6. The method of claim 1, wherein the binder is a photopolymer binder.

7. The method of claim 1, wherein the binder is a thermoset binder.

8. The method of claim 1, wherein: the energetic particles are high melting explosive (HMX) or Royal demolition explosive (RDX); and the binder material is a hydroxyl-terminated polybutadiene (HTPB) binder.

9. The method of claim 8, wherein the energetic particles and a thermoset binder material being mixed are 90 wt. % solids loading of energetic particles and a coarse (150-212 m) to fine (90-150 m) ratio of 4:1, 10 wt. % of binder including 6.56 wt. % HTPB, 1.95 wt. % dimeryl diisocyanate (DDI), and 1.5 wt. % isodecyl pelargonate plasticizer (IDP).

10. The method of claim 9, wherein said mixing includes: scraping the mixture of energetic particles and thermoset binder material that has adhered to the side of the bladeless mixer from the side of the bladeless mixer; stirring the mixture of the energetic particles and the thermoset binder material; and repeating said rotating, scraping and stirring until the mixture of energetic particles and thermoset binder material is homogeneous.

11. The method of claim 10, wherein: said heating includes placing the mixture of energetic particles and thermoset binder material in an oven heated to 70 C. for 8 hours.

12. The method of claim 11, wherein: said pressing includes pressing the partially cured energetic molding powder at 250 MPa for 10 minutes.

13. The method of claim 1, further comprising: after said partially curing, mixing the partially cured mixture of energetic particles and binder material, said mixing including scraping the mixture of energetic particles and thermoset binder material that has adhered to the side of the bladeless mixer from the side of the bladeless mixer, stirring the mixture of the energetic particles and the thermoset binder material, and repeating said rotating, scraping and stirring until the mixture of energetic particles and thermoset binder material is homogeneous; stopping said breaking up the partially cured energetic particle and binder mixture into smaller pieces when no piece has a largest dimension larger than 1 centimeter; placing the partially cured energetic particle and binder mixture into a die form; pressing the partially cured energetic particle and binder mixture in the die form; slowly releasing the pressure on the energetic molding powder over a predetermined time; and removing the pressed energetic particle and binder mixture from the die form.

14. An energetic molding powder, comprising: a mixture of energetic particles and a binder material, wherein said mixture of energetic particles and a binder material is formed by combining energetic particles with a binder material into a mixture; partially curing the mixture of energetic particles and binder material by applying heat to the mixture of energetic particles and binder material, and removing the applied heat from the mixture of energetic particles and binder material before the mixture of energetic particle and binder mixture is fully cured; and breaking up the partially cured energetic particle and binder mixture into smaller pieces.

15. The energetic molding powder of claim 14, wherein said mixture of energetic particles and a binder material is further formed by after said partially curing, mixing the partially cured mixture of energetic particles and binder material, said mixing including scraping the mixture of energetic particles and thermoset binder material that has adhered to the side of the bladeless mixer from the side of the bladeless mixer, stirring the mixture of the energetic particles and the thermoset binder material, and repeating said rotating, scraping and stirring until the mixture of energetic particles and thermoset binder material is homogeneous; stopping said breaking up the partially cured energetic particle and binder mixture into smaller pieces when no piece has a largest dimension larger than 1 centimeter; placing the partially cured energetic particle and binder mixture into a die form; pressing the partially cured energetic particle and binder mixture in the die form; slowly releasing the pressure on the energetic molding powder over a predetermined time; and removing the pressed energetic particle and binder mixture from the die form.

16. The energetic molding powder of claim 14, wherein: the energetic particles are high melting explosive (HMX) or Royal demolition explosive (RDX); and the binder material is a hydroxyl-terminated polybutadiene (HTPB) binder.

17. The energetic molding powder of claim 16, wherein the energetic particles and a thermoset binder material are 90 wt. % solids loading of energetic particles and a coarse (150-212 m) to fine (90-150 m) ratio of 4:1, 10 wt. % of binder including 6.56 wt. % HTPB, 1.95 wt. % dimeryl diisocyanate (DDI), and 1.5 wt. % isodecyl pelargonate plasticizer (IDP).

18. The energetic molding powder of claim 14, wherein the heating includes placing the mixture of energetic particles and thermoset binder material in an oven heated to 70 C. for 8 hours.

19. The energetic molding powder of claim 14, wherein said the pressing includes pressing the partially cured energetic molding powder at 250 MPa for 10 minutes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Some of the figures shown herein may include dimensions or may have been created from scaled drawings. However, such dimensions, or the relative scaling within a figure, are by way of example, and not to be construed as limiting.

[0018] FIG. 1 depicts one or more methods for forming a molding powder according to embodiments of the present disclosure.

[0019] FIG. 2 is a depiction of ranges of volume loading (and equivalent solids loading) of solid particle material that can be processed using embodiments of the present disclosure illustrated in at least FIG. 1.

[0020] FIGS. 3, 4, 5 and 6 are photographs of compacted pellets as a function of HTPB cure time using a 10 mm cylindrical die form wherein the molding powder was cured for 2, 4, 8 and 24 hours, respectively, according to embodiments of the present disclosure.

[0021] FIG. 7 depicts stress-strain curves of the compacted molding powder pellets depicted in FIGS. 3, 4 and 5, which were cured for 2, 4 and 24 hours, respectively.

[0022] FIG. 8 depicts 24 hour and 2 hour molding powders pressed together into one pellet according to at least one embodiment of the present disclosure.

[0023] FIG. 9 depicts a region at the edge of a pellet that was cured for 4 hours according to at least one embodiment of the present disclosure.

[0024] FIG. 10 depicts a central region of the same pellet depicted in FIG. 9.

[0025] FIG. 11 is an illustration of precuring times for molding powders according to embodiments of the present disclosure.

[0026] FIG. 12 depicts a percent theoretical maximum density (% TMD) vs. cure time plot, with the cure times according to embodiment of the present disclosure.

[0027] FIG. 13 depicts stress-strain curves of a compacted molding powder according to embodiments of the present disclosure.

[0028] FIG. 14 depicts the average maximum stress and modulus of elasticity as a function of cure time according to embodiments of the present disclosure.

[0029] FIG. 15 depicts X-ray tomography images of the center of the pellets according to embodiments of the present disclosure.

[0030] FIG. 16 depicts an image of a pellet made using ammonium perchlorate in the molding powder for the solids loading and an image depicting the combustion flame propagating through the same sample according to at least one embodiment of the present disclosure.

[0031] FIG. 17 depicts pellets that were cured for different preset time periods according to embodiments of the present disclosure.

[0032] FIG. 18 depicts a stress-strain curve of compacted molding powders that were cured for 4 hours in the oven and then stored for different periods at cold temperatures according to embodiments of the present disclosure.

[0033] FIG. 19 depicts a percent TMD plot for molding powders that were cured for 4 hours then stored for different periods at cold temperatures according to embodiments of the present disclosure.

[0034] FIG. 20 depicts a percent TMD plot for photocurable molding powders that were either dried over 72 hours or not at all, and then either precured with a UV light or not at all according to embodiments of the present disclosure.

[0035] FIG. 21 depicts a percent TMD plot for photocurable molding powder that is precured for different time periods then compacted according to embodiments of the present disclosure.

[0036] FIG. 22 depicts stress-strain curves of compacted molding powder that was first precured for different time periods then compacted according to embodiments of the present disclosure.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

[0037] For the purposes of promoting an understanding of the principles of the disclosure, reference will now be made to one or more embodiments, which may or may not be illustrated in the drawings, and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the disclosure is thereby intended; any alterations and further modifications of the described or illustrated embodiments, and any further applications of the principles of the disclosure as illustrated herein are contemplated as would normally occur to one skilled in the art to which the disclosure relates. At least one embodiment of the disclosure is shown in great detail, although it will be apparent to those skilled in the relevant art that some features or some combinations of features may not be shown for the sake of clarity.

[0038] Any reference to invention that may occur within this document is a reference to an embodiment of a family of inventions, with no single embodiment including features that are necessarily included in all embodiments, unless otherwise stated. Furthermore, although there may be references to benefits or advantages provided by some embodiments, other embodiments may not include those same benefits or advantages, or may include different benefits or advantages. Any benefits or advantages described herein are not to be construed as limiting to any of the claims.

[0039] Likewise, there may be discussion with regards to objects associated with some embodiments of the present invention, it is understood that yet other embodiments may not be associated with those same objects, or may include yet different objects. Any advantages, objects, or similar words used herein are not to be construed as limiting to any of the claims. The usage of words indicating preference, such as preferably, refers to features and aspects that are present in at least one embodiment, but which are optional for some embodiments.

[0040] Specific quantities (spatial dimensions, temperatures, pressures, times, force, resistance, current, voltage, concentrations, wavelengths, frequencies, heat transfer coefficients, dimensionless parameters, etc.) may be used explicitly or implicitly herein, such specific quantities are presented as examples only and are approximate values unless otherwise indicated. Discussions pertaining to specific compositions of matter, if present, are presented as examples only and do not limit the applicability of other compositions of matter, especially other compositions of matter with similar properties, unless otherwise indicated.

[0041] Embodiments of the present disclosure provide methods of manufacturing an explosive material by directly mixing the energetic particles with a binder instead of suspending energetic particles and a binder in a liquid to mix them. Example embodiments include mechanically mixing the energetic particles of a polymer-bonded explosive (PBX) (e.g., high melting explosive (HMX) or Royal demolition explosive (RDX)) with a hydroxyl terminated polybutadiene (HTPB) binder.

[0042] Referring to FIG. 1, one or more methods for producing a molding powder 100 (e.g., an explosive molding powder) according to various embodiments is depicted. In method 100, energetic particles and binder are mechanically mixed until the mixture is homogeneous. See, e.g., step 110. The mixture is then partially cured (e.g., precured). For thermoset binders, the mixture can be placed in a low temperature oven (e.g., an oven heated to 70 C.) and left in the low temperature oven for a predetermined period to partially cure the thermoset binder (e.g., HTPB). See, e.g., step 120. For photoset (e.g., photopolymer) binders, the mixture can be exposed to the appropriate type of light, such as UV light, for a predetermined period to partially cure the binder. The mixture may be then broken up into small clumped particles. See, e.g., step 130. The broken up clumped particles may then be loaded into a die form and pressed into pellets. See, e.g., step 140. The pellets may then be removed from the die and allowed to fully cure. See, e.g., step 150. It is noted that these methods are different than methods using solvents and different than methods that cast HTPB mixtures, which do not include compaction.

[0043] Embodiments include molding powders (e.g., explosive molding powders) with approximately 90 wt. % solids loading of energetic particles (and in some embodiments sugar is used as a surrogate for testing) and a coarse (approximately 150-212 m) to fine (approximately 90-150 m) ratio of 4:1, and the remaining 10 wt. % being binder with approximately 6.56 wt. % HTPB, 1.95 wt. % dimeryl diisocyanate (DDI), and 1.5 wt. % isodecyl pelargonate plasticizer (IDP).

[0044] The inventors determined that the length of the cure time impacts several physical characteristics of the PBX, such as the distribution of the binder in the molding powder, the stiffness of the pellets, the texture of the pellets, the microstructure of the pellets, and/or the macrostructure of the pellets. Changes to the microstructure are expected to impact the sensitivity to detonation, vibration, thermal cycling, etc. and the potential hotspot formation of live PBX pellets.

[0045] Embodiments of the mixture with short cure times (e.g., less than approximately 4 hours) tend to have excessive binder migration, e.g., to the top and/or bottom faces of the pellet. However, embodiments of the mixture with long cure times (e.g., approximately 24 hours or more), tend to result in expansion of the pellet, potentially resulting in pellets that have a more porous consistency. Embodiments with cure times of approximately 8 hours tended to produce homogeneous pellets.

[0046] Some embodiments utilize other partially cured thermosets such as photopolymers and/or dual cure polymers (e.g., UV and thermal cure) as binders during the manufacturing process.

[0047] Some embodiments of the present disclosure include: molding powder with 90 wt. % solids loading of energetic particles (and in some embodiments sugar is used as a surrogate for testing) and a coarse (150-212 m) to fine (90-150 m) ratio of 4:1; and the remaining 10 wt. % being binder with approximately 6.56 wt. % HTPB, 1.95 wt. % dimeryl diisocyanate (DDI), and 1.5 wt. % isodecyl pelargonate plasticizer (IDP). In at least some embodiments, the binder includes three parts: HTPB being the primary part, DDI to cross link the HTPB (e.g., make it solidify), and IDP as a plasticizer.

[0048] Ranges of volume loading (and equivalent solids loading) of solid particle material that may be processed using embodiments of the present disclosure are listed in Table 1.

TABLE-US-00001 TABLE 1 Parameter Loading Volume Loading 75-93 vol. % Equivalent sugar solids loading 81-95 wt. % Equivalent ammonium perchlorate (AP) solids loading 84-97 wt. % Equivalent RDX solids loading 83-96 wt. % Equivalent HMX solids loading 84-97 wt. %

[0049] FIG. 2 includes a depiction of ranges of volume loading (and equivalent solids loading) of solid particle material that can be processed using embodiments of the present disclosure according to embodiments of the present disclosure.

[0050] Some embodiments utilize a bladeless mixer to mix the ingredients, although bladed mixers can also be utilized. In one example, a bladeless mixer (e.g., a FlackTek DAC 512-200 Pro) is rotated at 2500 RPM for a predetermined interval (e.g., 30 seconds). At the end of each interval the mixture is stirred and/or the container sides are scraped (e.g., with a spatula). The intervals are repeated until homogeneity is achieved, which typically occurred after 3 to 4 intervals.

[0051] After mixing, the mixture is partially cured in an oven (which in at least one embodiment is heated to 70 C.) to make the molding powder before it is compacted. For example, the mixture is cured in the oven for 2, 4, 8 or 24 hours before compaction.

[0052] After partial curing, the molding powder may be broken up using a spatula, or other methods, making clumps. In at least one embodiment the breaking up of the powder is continued until the resulting clumps are no bigger than 1 cm in diameter, e.g., no larger than 1 cm as their largest dimension.

[0053] The partially cured molding powder may then be placed in a die, e.g., a 10 mm cylindrical or 10 mm square die form. The partially cured powder may then be pressed (e.g., at a pressure of approximately 250 MPa) and held for a dwell time (e.g., approximately 10 minutes) before being slowly released. In at least one embodiment, during the slow release the pressure is slowly decreased from the maximum pressure to zero over a preset time period. In at least one example embodiment, a die pressure of 2300 psi (pounds per square inch) was released over the course of 10 minutes, which was a pressure decrease rate of approximately 0.026 MPa per second.

[0054] The density of 4 hour (i.e., a partial cure time of 4 hours) HTPB molding powder pellets with different compaction parameters were measured using mineral oil as outlined in the ASTM B962-13.3. The stress-strain curve of the pellets were measured by compressing the pellets at a rate of 1 mm/min until failure (ASTM D695) on a Mark-10 universal testing machine (UTM) with a 1500-pound force compression load cell. A Keyence VR-6000 optical profilometer and a Hirox digital microscope were used to examine the binder distribution.

[0055] It was determined by the inventors that the cure time impacts the stress-strain curve of the compacted pellets. The distribution of the binder in the compacted pellets and the stiffness of the compacted pellets themselves are also affected by the HTPB partial cure time. As seen in FIGS. 3-6, short cure times (e.g., 4 hours or less) resulted in excessive binder migration to the bottom of the pellet. As seen in FIG. 6, long cure times (e.g., on the order of 24 hours) resulted in an undesirable expansion of the pellet (e.g., pellet diameters expanded to 10.4 mm) and produced a spongy consistency. As seen in FIG. 5, cure times around 8 hours produced more favorable and homogeneous pellets.

[0056] Turning to FIG. 7, the maximum compressive stress for a 2 hour molding powder was almost twice that of the 24 hour sample. It is believed that this effect is attributed mostly to the degree of crosslinking available in the partially cured HTPB binder to keep the pellets together after compaction.

[0057] The density of the pellets cured for 4 hours and compacted as a function of die pressure and dwell time were also measured. The pellet densities were compared to the theoretical max density (1.52 g/cm.sup.3) to find the percent theoretical max density (% TMD). The % TMD ranged from 85%-93% for the 4 hour cure pellets.

[0058] FIG. 8 depicts a pressing of 24 hour and 2 hour molding powders together into one pellet, which demonstrates the ability to control the mechanical properties locally.

[0059] FIG. 9 depicts a binder-rich region at the edge of a pellet that was cured for 4 hours according to at least one embodiment of the present disclosure.

[0060] FIG. 10 depicts a central region of the pellet depicted in FIG. 9 revealing a more even distribution of binder and particles.

[0061] Embodiments of the present disclosure utilize at least two different types of binders to make molding powdersthermoset and photopolymer.

[0062] Embodiments using thermoset-based molding powders can include hydroxyl terminated polybutadiene (HTPB) mixed with an isocyanate and a plasticizer. In some embodiments dimeryl diisocyanate (DDI) is the isocyanate that is used, although other embodiments use other isocyanates. The amount needed will typically vary based on the Index Cure Ratio. Turning to the plasticizer, in some embodiments isodecyl pelargonate (IDP) is the plasticizer that is used, although other embodiments use other plasticizers. However, the amount of plasticizer, or the type of plasticizer, can vary. In at least one embodiment, the amount of plasticizer is chosen to reduce the viscosity, which can change depending on the specific viscosity that is desired. Once the amount of plasticizer is set, the user can use the cure index to determine the ratio of HTPB to isocyanate.

[0063] Embodiments using photopolymer-based molding powders can include an oligomer, a photoinitiator and a reactive diluent. In some embodiments the oligomer is a Difunctional Aliphatic Polyester Urethane Acrylate. However, other oligomers are used in different ratios. IN some embodiments the photoinitiator is Phenylbis(2,4,6-trimethylbenzoyl) phosphine oxide, although other ratios and other photoinitiators can be used. Some embodiments use 1,6-hexanediol diacrylate as the reactive diluent, although the ratio and the type can vary.

[0064] In some embodiments the solid particles used to make the molding powder are sugar and ammonium perchlorate (AP), though other embodiments utilize other solid energetic material particles, such as HMX and/or RDX.

[0065] In some embodiments, the volume loading of solid particles ranges from 75-93 vol. %. This volume loading is equivalent to 81-95 wt. % for sugar, 84-97 wt. % for AP, 83-96 wt. % for RDX, and 84-97 wt. % for HMX. The upper limit for volume loading, and equivalent solids loading, has not been determined, although the range is likely higher that those that have been tested since none of the pellets processed with optimal precure times have so far lost structural integrity.

[0066] To create the molding powder according to one or more embodiments of the present disclosure, the ingredients may be first measured into separate containers before being mixed.

[0067] For thermoset-based molding powder embodiments, the molding powder is frequently bimodal and includes both fine (small) and coarse (large) particles. The coarse particles are typically about 200 microns, but can be larger (up to 400 microns), and the fine particles are to be smaller than the large particles. The mixing can begin by dividing the fine particles into two equal portions. One half of the fine particles may be thoroughly blended in a container with a plasticizer until every fine particle is evenly coated with the plasticizer. In a separate container, the other half of the fine particles may be mixed with a curing agent (e.g., isocyanate) using the same method. Once both mixtures are prepared, the two mixtures may be combined in a third container with a binder (e.g., HTPB) and manually stirred (e.g., using a spatula) until all the fine particles are fully incorporated into the binder. The coarse particles may then be added and gently mixed in, with the mixing continuing until the coarse particles are completely wetted by the binder.

[0068] For photopolymer-based molding powder embodiments, the process can begin with mixing the fine particles with a reactive diluent until each particle is evenly coated. This prepared mixture may then be combined with an oligomer in a container and stirred until fully blended. At this stage, a photoinitiator (also a powder) may be added directly to the oligomer-reactive diluent mixture, along with the coarse particles. The mixture may then be manually stirred (e.g., with a spatula) until all of the fine particles are thoroughly incorporated into the binder.

[0069] Other embodiments can use other mixing methods and/or sequences, such as combining all of the binder at once and then combining the solids, or changing the order of addition, since these should still produce viable products.

[0070] For both types of molding powders (thermoset or photopolymer based), once all the ingredients are combined (e.g., manually combined such as by manual stirring), the ingredients may then be mixed in one container using a powered mixer (e.g., a FlackTek Speed Mixer DAC 515-200 Pro). The mixture can be mixed at different speeds and/or intervals to ensure the material is homogeneous. For example, in at least one embodiment the mixture is mixed at 2500 rpm for three 30-second intervals to ensure that the material is homogenous. Varying the mixer settings, number of mixing cycles, or the type of mixer is likely to produce a viable molding powder, although to date only certain combinations of mixer settings, cycles, and types of mixers have been used.

[0071] Embodiments then utilize precuring to prepare the mixtures. Completely curing the mixture during the precuring process is typically avoided. For example, complete curing can result in molding powder mixtures that cannot be effectively pressed into pellets and/or forming pellets that lose their integrity when removed from the die molds. Similarly, not curing the mixture a sufficient amount during the precuring process is also typically avoided. For example, under-curing can result in molding powder mixtures that cannot be effectively pressed into pellets and lose their shape after being removed from the die molds.

[0072] For thermoset-based molding powders, the homogeneous mixture from the mixer can be placed onto a surface (e.g., placed into one or more weigh boats) and gently leveled for uniform distribution. While some embodiments utilize a thin layer (e.g., 5 mm), other embodiments may use thicker layers, although to date these have not been explicitly tested. The powders may then be placed in an oven and cured for various amounts of time, which is referred to as precuring. Tested precure times in the oven have been 2, 4, 8, 12 and 24 hours, and all of these tested precure times have produce a range of functional products. Typically, the oven is set to 70 C. However, other embodiments can utilize lower temperatures (e.g., temperatures as low as 60 C., or 50 C.) to produce a qualitatively similar product. The precise temperature appears to be isocyanate dependent and will frequently depend on the molding powder material being used. Lower temperatures will typically require longer times. Although not yet tested, higher oven temperatures could also be used to yield qualitatively similar products.

[0073] After removing the molding powders from the oven, the molding powders are mixed (e.g., with a spatula) to reincorporate the material and break up any large clumps. The resulting powder forms in prills (e.g., 1 cm sized prills) that are then used in a pressing process. Other embodiments utilize differently sized prills provided that the prills fit within the dies that are used for pressing.

[0074] For photopolymer-based molding powders, the homogeneous mixture from the mixer can be placed onto a surface (e.g., placed into one or more weigh boats) and gently leveled for uniform distribution to achieve uniform curing by a UV light. While some embodiments utilize a thin layer (e.g., <3 mm), other embodiments may use thicker layers, although to date these have not been explicitly tested. The molding powders may then be precured under a UV light for a given amount of time, typically between 0.5-10 seconds. Various UV light sources can be used, although doing so will likely change the optimal precure time. Afterwards, the molding powders are mixed with a spatula to reincorporate the material and break up any large clumps. The powder then forms in 1 cm sized prills so that it can be used in the pressing process. However, smaller or larger sized prills would work for this product as long as it can fit within the die.

[0075] Depicted in FIG. 11 are precuring times for molding powders according to various embodiments of the present disclosure.

[0076] In some embodiments utilizing thermoset-based molding powders, the molding powders are cured in an oven for 2 to 24 hours. In additional embodiments utilizing thermoset-based molding powders, the molding powders are cured in an oven for 4 to 16 hours. In further embodiments utilizing thermoset-based molding powders, the molding powders are cured in an oven for 8 to 12 hours. In still additional embodiments, the molding powders are cured in an oven for 2 to 4 hours. In still further embodiments utilizing thermoset-based molding powders, the molding powders are cured in an oven for 8 to 12 hours. In yet further embodiments utilizing thermoset-based molding powders, the molding powders are cured in an oven for 24 hours.

[0077] In some embodiments utilizing photopolymer-based molding powders, the molding powders are cured using UV light for 0.5 to 15 seconds. In additional embodiments utilizing thermoset-based molding powders, the molding powders are cured using UV light for 1 to 12 seconds. In further embodiments utilizing thermoset-based molding powders, the molding powders are cured using UV light for 5 to 10 seconds. In additional embodiments utilizing photopolymer-based molding powders, the molding powders are cured using UV light for 0.5 to 1 seconds. In yet further embodiments utilizing thermoset-based molding powders, the molding powders are cured using UV light for 5 to 10 seconds.

[0078] The precured molding powder may then be loaded into a die and pressed (e.g., with a hydraulic press) until the precured molding powder consolidates into a solid pellet. Example embodiments utilize a swell time of 10 minutes under a die pressure of 250 MPa, although different times and pressures may be used. After compaction, the pressure is gradually released (e.g., over a 10 minute period) to minimize failure of the pressed mixture, such as failure due to capping. Alternate embodiments utilize different dwell times and/or release times to obtain a usable product. In general, the longer the dwell time and the slower the pressure is released (longer release time), the better the product. Longer dwell and/or release times may be particularly helpful for molding powders processed with lower precure times, since those were more susceptible to failure.

[0079] After pressing, the thermoset-based pellets may be allowed to fully cure at room temperature, while the photopolymer-based pellets may be post cured using a UV light source until fully cured. In at least one example embodiment, the pellets are exposed to the UV light source for 10 minutes on each side, although the amount of post cure (e.g., the UV light source) can be varied to produce the final pellet.

[0080] For thermoset-based molding powders (e.g., those made using HTPB), a 24-hour precure time produced pellets that were porous and had fairly weak compressive properties while still being whole, which appeared to define an upper precure time limit to achieve an acceptable pellet. For the HTPB molding powders, 2-hour and 4-hour precure times produced pellets with significant binder migration towards the edges and submaximal compressive strength, which appeared to define a lower precure time limit to achieve an acceptable pellet. The HTPB molding powders using 8-hour and 12-hour precure times produced solid homogeneous pellets with the high compressive strengths, which appeared to be at least one preferred precure processing range. Although there were attempts to press uncured mixtures, the uncured mixtures broke during extraction from the die. Based on testing results, it appears as if precure times less than 2 hours do not produce viable pellets. Similarly, pressing a molding powder for too long (e.g., 168 hours, which was tested) produces a crumbly pellet that breaks during extracting from the die, indicating that there is an upper limit to how long the molding powder can be pressed.

[0081] For the photopolymer-based molding powders, precure times of 0.5-1 seconds appeared to be near the lower limit of precure times since there was binder migration in the pellets that were produced. Precuring photopolymer-based molding powders for 5-10 seconds appeared to be ideal since the pellets were more homogenous and had better density values than other precuring times. Although precure times from 1-5 seconds were not tested, it is expected that some precure times in this range will produce workable molding powders and viable pellets. Photopolymer-based molding powders precured for 25 seconds resulted in crumbly pellets that were not able to maintain their shape during die extraction, indicating that 25 seconds is at or beyond an upper limit for precuring. Mixtures that we not precured had too much binder migration and broke during die extraction, so precure times less than 0.5 seconds are thought to be too short to produce viable pellets.

[0082] Depicted in FIG. 12 is a plot of percent theoretical maximum density (% TMD) of PBXs based on HTPB precure times (percent theoretical maximum density vs. cure time), with cure times of 2, 4, 8, 12 and 24 hours according to embodiments of the present disclosure. The theoretical maximum density TMD of the pellets is 1.52 g/cm.sup.3 (equivalent to 100% TMD). In embodiments of the present disclosure, the % TMD can be tailored based on precure time of the HTPB molding powder.

[0083] FIG. 13 depicts stress-strain curves of compacted PBX molding powder using 2, 4, 8, 12 and 24 hour HTPB precure times according to embodiments of the present disclosure. The compressive stress strain curves can be tailored based on precure time of the HTPB molding powder.

[0084] Shown in FIG. 14 is a graph depicting the average maximum stress and modulus of elasticity of PBX both as a function of cure time according to embodiments of the present disclosure. The error bars are based on the standard deviation from taking measurements. The maximum compressive strength and modulus of elasticity can be tailored based on precure time of the HTPB molding powder.

[0085] FIG. 15 depicts micro x-ray computed tomography images of the center of pellets made from PBX molding powder with 4, 8 and 24 hour precure times according to embodiments of the present disclosure. The distribution of the particles and the pores were analyzed for HTPB molding powder pellets and it was determined that the pore size and densities were controllable based on HTPB precure time.

[0086] FIG. 16 depicts side-by-side images of a PBX pellet made using ammonium perchlorate in PBX HTPB molding powder for the solids loading, and a screenshot from a video depicting the combustion flame propagating through the same sample according to embodiments of the present disclosure. The burning rates were measured and it was determined the burning rate was controllable by varying the precure time.

[0087] FIG. 17 depicts four different pellets that were made by layering different molding powders that were cured for different amounts of time according to embodiments of the present disclosure. The leftmost pellet is comprised of molding powder that was cured for 4 and 24 hours (the bottom layer is 4 hour precured HTPB, and the top layer is 24 hour precured HTPB). The second from the left pellet is comprised of molding powder that was cured for 12 and 2 hours (the bottom layer is 12 hour precured HTPB, the middle layer is 2 hour precured HTPB, and the top layer is 12 hour precured HTPB). The second from the right pellet is comprised of molding powder that was cured for 8 and 12 hours (the bottom layer is 8 hour precured HTPB, and the top layer is 12 hour precured HTPB). And, the rightmost pellet is comprised of molding powder that was cured for 8- and 24-hours (the bottom layer is 8 hour precured HTPB, and the top layer is 24 hour precured HTPB).

[0088] FIG. 18 depicts a stress-strain curve of a compacted molding powder (e.g., PBX and HTPB) that was cured for 4 hours in an oven and then stored for 24, 48 and 168 hours in a low temperature environment, e.g., in a refrigerator (at approximately 10 C.) or a freezer (at approximately 0 C.) before being compressed according to embodiments of the present disclosure. It was determined that low temperature storage (in the fridge or freezer) can affect the stress strain curves of the compacted molding powder and that shelf life can be extended when the storage conditions are optimized.

[0089] FIG. 19 is a plot of percent TMD for PBX/HTPB molding powders that were precured for 4 hours in an oven then stored for 24, 48 and 168 hours in a fridge (10 C.) or a freezer (0 C.) before being compressed according to embodiments of the present disclosure. Optimal storage conditions and extended shelf life (relative to 0 hour storage) by maintaining % TMD were realized.

[0090] FIG. 20 depicts a % TMD plot for photocurable PBX molding powder that was either dried in the oven over 72 hours or not at all, and then either precured (also called preflashed) with a UV light for 0.7 seconds or not at all according to embodiments of the present disclosure. The theoretical maximum density TMD of the pellets is 1.47 g/cm.sup.3 (equivalent to 100% TMD) and the % TMD can be tailored based on preflash curing time for the UV curable binder after being dried (placed in oven) or undried (used as is).

[0091] FIG. 21 depicts a % TMD plot for photocurable molding powder that was first precured for 0.5, 1, 5 or 10 seconds and then compacted according to embodiments of the present disclosure. An optimal preflash cure could be determined to maximize % TMD and to maintain cohesion without binder migration.

[0092] FIG. 22 depicts compressive stress-strain curves of a compacted UV curable PBX molding powder based on preflash time. The molding powder was first precured for 0.5, 1, 5 or 10-seconds and then compacted according to embodiments of the present disclosure. Optimal preflash cure times to maximize for reasonable compressive strengths were found.

[0093] Reference systems that may be used herein can refer generally to various directions (e.g., upper, lower, forward and rearward), which are merely offered to assist the reader in understanding the various embodiments of the disclosure and are not to be interpreted as limiting.

[0094] To clarify the use of and to hereby provide notice to the public, the phrases at least one of A, B, . . . and N or at least one of A, B, . . . . N, or combinations thereof or A, B, . . . and/or N are defined by the Applicant in the broadest sense, superseding any other implied definitions hereinbefore or hereinafter unless expressly asserted by the Applicant to the contrary, to mean one or more elements selected from the group comprising A, B, . . . and N. In other words, the phrases mean any combination of one or more of the elements A, B, . . . or N including any one element alone or the one element in combination with one or more of the other elements which may also include, in combination, additional elements not listed. As one example, A, B and/or C indicates that all of the following are contemplated: A alone, B alone, C alone, A and B together, A and C together, B and C together, and A, B and C together. If the order of the items matters, then the term and/or combines items that can be taken separately or together in any order. For example, A, B and/or C indicates that all of the following are contemplated: A alone, B alone, C alone, A and B together, B and A together, A and C together, C and A together, B and C together, C and B together, A, B and C together, A, C and B together, B, A and C together, B, C and A together, C, A and B together, and C, B and A together.

[0095] While examples, one or more representative embodiments and specific forms of the disclosure have been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive or limiting. The description of particular features in one embodiment does not imply that those particular features are necessarily limited to that one embodiment. Some or all of the features of one embodiment can be used or applied in combination with some or all of the features of other embodiments unless otherwise indicated. One or more exemplary embodiments have been shown and described, and all changes and modifications that come within the spirit of the disclosure are desired to be protected.

Element Numbering

[0096] Table 2 includes element numbers and at least one word used to describe the element and/or feature represented by the element number. However, none of the embodiments disclosed herein are limited to these descriptions. Other words may be used in the description or claims to describe a similar member and/or feature, and these element numbers can be described by other words that would be understood by a person of ordinary skill reading and reviewing this disclosure in its entirety.

TABLE-US-00002 TABLE 2 100 Method for forming a molding powder 110 Mixing 120 Pre-curing mixture 130 Breaking up mixture 140 Pressing mixture 150 Fully curing mixture