Syntactic foam and method for producing the same

Abstract

To provide a buoyant material having high hydrostatic pressure breaking strength and relatively low density.SOLUTION: The syntactic foam (102) includes a matrix (150), a quantity of cellular polymeric beads (200) disposed within the matrix (150), each of the cellular polymeric beads (200) having a breadth of less than 1.1 5mm, and a quantity of hollow microspheres dispersed within the matrix (150) and between the quantity of cellular polymeric beads (212). The largest of the hollow microspheres (300) is no more than 1 / 7 of the smallest of the porous polymeric beads (200), and the hollow microspheres (300) have a size distribution within ± 10 μm.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] FIELD OF THE DISCLOSURE The present disclosure relates to buoyancy materials, and in particular to syntactic foams containing inclusions with controlled size distribution. [Background technology]

[0002] Buoyancy materials are used in undersea vessels, such as unmanned undersea vehicles, to maintain stability at various depths. Buoyancy materials are also used in subsea packages, such as cabled observation nodes and unmanned sensors, to improve deployment efficiency and keep the subsea packages submerged at a specific depth. Syntactic foam has a relatively high hydrostatic crush strength, making it widely used as a buoyancy material in various undersea platforms, such as subsea vessels and subsea packages.

[0003] Many subsea platforms are subject to volumetric constraints due to transportation requirements. These volumetric constraints reduce the space available for buoyancy materials. As a result, buoyancy materials must be designed to fit into irregularly shaped voids within the subsea platform, potentially requiring many uniquely shaped buoyancy members to meet the buoyancy requirements. The need for uniquely shaped buoyancy members increases design and manufacturing costs and assembly time. Furthermore, the presence of many uniquely shaped buoyancy members complicates maintenance and servicing, as many buoyancy members must be removed to access the various systems of the subsea platform.

[0004] In view of the above, there is a need in the art for buoyancy materials that have relatively high hydrostatic crush strength and relatively low density to meet the buoyancy requirements of subsea applications. Summary of the Invention

[0005] The aforementioned needs related to buoyancy materials are met by the present disclosure, which provides a syntactic foam. The syntactic foam, in its green state, includes a matrix, a quantity of porous polymer beads disposed within the matrix, and a quantity of hollow microspheres dispersed within the matrix among the quantity of porous polymer beads. Each of the porous polymer beads has a width of less than 1.5 mm. The largest of the hollow microspheres is no more than 1 / 7 of the smallest of the porous polymer beads, and the hollow microspheres have a size distribution within ±10 μm.

[0006] Also disclosed herein is a post-cured syntactic foam comprising a matrix, a predetermined amount of matrix cavities in the matrix, and a predetermined amount of hollow microspheres dispersed within the matrix among the predetermined amount of matrix cavities. Each of the matrix cavities encloses a space partially occupied by polymer particles and has a maximum width of less than 1.5 mm. The largest of the hollow microspheres is no more than 1 / 7 of the smallest of the matrix cavities, and the hollow microspheres have a size distribution within ±10 μm.

[0007] Also disclosed herein is a method for producing syntactic foam. The method includes placing a quantity of porous polymer beads, each less than 1.5 mm wide, in a mold. The method also includes dispersing a select amount of hollow microspheres throughout the quantity of porous polymer beads in the mold to obtain a powder bed. The hollow microspheres have a particle size distribution within ±10 μm, with the largest hollow microspheres being no more than 1 / 7 the size of the smallest porous polymer beads. The method further includes impregnating the powder bed in the mold with a resin mixture comprising a resin and a curing agent. The method also includes curing the resin mixture in the powder bed to obtain the syntactic foam.

[0008] The above-described features, functions, and advantages may be achieved individually in various embodiments of the present disclosure and may be combined in other embodiments, details of which will become apparent from the following description and drawings. [Brief explanation of the drawings]

[0009] The present disclosure can be better understood by reference to the following detailed description in conjunction with the accompanying drawings illustrating preferred and exemplary embodiments, which are not necessarily to scale, and which are provided by way of example only and are not intended to be limiting on the scope of the description or claims herein.

[0010] [Figure 1] 1A-1C illustrate examples of unmanned undersea vehicles that include syntactic foam as a buoyancy material. [Figure 2] FIG. 2 is a cross-sectional view of a syntactic foam of the present disclosure taken along line 2-2 of FIG. 1. [Figure 3] FIG. 1 is an enlarged view of a portion of a syntactic foam, the syntactic foam being composed of a matrix containing a plurality of porous polymer beads and a plurality of hollow microspheres. [Figure 4]FIG. 1 is an enlarged view showing an example of porous polymer beads in the form of expanded polystyrene beads. [Figure 5] FIG. 5 is a cross-sectional view of a portion of the porous polymer bead of FIG. 4, showing the internal structure of the synactic foam when it is in the green state. [Figure 6] FIG. 1 is a magnified view showing an example of a hollow microsphere. [Figure 7] FIG. 7 is a cross-sectional view of the hollow microsphere of FIG. 6. [Figure 8] FIG. 1 is an enlarged view showing an example of a syntactic foam in which the porous polymer beads have an ellipsoidal shape. [Figure 9] FIG. 1 is a close-up view showing an example of a syntactic foam in which the porous polymer beads have irregular rounded shapes. [Figure 10] 1 is a chart showing the isostatic fracture strength of two types of hollow glass microspheres, each having a number of different size distributions, and also showing the isostatic fracture strength for bulk quantities of the same two types of hollow glass microspheres. [Figure 11] 1 is a graph showing the size distribution of a bulk amount of hollow microspheres and the size distribution of a sorted amount of hollow microspheres. [Figure 12] 1 is a chart showing hydrostatic pressure for several different syntactic foam configurations, each containing hollow microspheres of several different size distributions. [Figure 13] 1 is a graph showing water displacement as a function of hydrostatic pressure in hydrostatic crush strength testing of syntactic foam specimens. [Figure 14] 1 is a flowchart of the steps involved in the production of syntactic foam. [Figure 15] FIG. 1 is a schematic diagram showing a quantity of porous polymer beads being poured into a cylindrical mold as part of the process for manufacturing syntactic foam. [Figure 16] FIG. 1 is a schematic diagram showing a mold being placed on a mechanical vibration device to apply high frequency vibrations to the mold. [Figure 17] FIG. 1 is a schematic diagram illustrating sieving a bulk quantity of hollow microspheres using one of at least two mesh sieves (e.g., a 50 μm sieve and a 70 μm sieve) required to obtain a selected quantity of hollow microspheres having a desired particle size distribution (e.g., within ±10 μm). [Figure 18] FIG. 1 is a schematic diagram showing pouring a selected amount of hollow microspheres into a mold while the mold is vibrated using a mechanical vibrator. [Figure 19] FIG. 1 is a schematic diagram showing a mold containing a powder bed. [Figure 20] FIG. 20 is an enlarged view of a portion of the powder indicated by the reference numeral 20 in FIG. 19. [Figure 21] FIG. 1 is an exploded schematic view showing a mold assembly with porous frit disposed on the top and bottom mold surfaces. [Figure 22] FIG. 22 is a schematic diagram showing the mold assembly of FIG. 21 in an assembled state. [Figure 23] FIG. 1 is a schematic diagram illustrating a manufacturing system including a vacuum pump fluidly connected to a top surface of a mold and a resin reservoir fluidly connected to a bottom surface of the mold. [Figure 24] FIG. 1 is a schematic diagram illustrating a manufacturing system during operation of a vacuum pump. [Figure 25] FIG. 1 is a schematic diagram showing the manufacturing system after room temperature curing of syntactic foam to the green state. [Figure 26] FIG. 26 is an enlarged view of a portion of the syntactic foam designated 26 in FIG. 25, showing the internal structure of the porous polymer beads when the syntactic foam is in the green state. [Figure 27] FIG. 1 is a schematic diagram showing a mold placed in an oven for final curing of a green syntactic foam to fully crosslink the matrix (i.e., thermoset resin). [Figure 28] FIG. 1 is a schematic diagram showing the removal of the syntactic foam from the mold after final curing. [Figure 29]FIG. 29 is an enlarged view of a portion of the post-cured syntactic foam designated by numeral 29 in FIG. 28. DETAILED DESCRIPTION OF THE INVENTION

[0011] The figures shown in this disclosure illustrate various aspects of the disclosed embodiments, and only the differences will be discussed in detail.

[0012] Embodiments of the present disclosure are described more fully below with reference to the accompanying drawings, in which some, but not all, of the disclosed embodiments are shown. Indeed, several different embodiments are presented, and the present disclosure should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.

[0013] Although the present specification includes references to a "configuration" and "configurations," these "configurations" and "configurations" do not necessarily refer to the same configuration. Similarly, the present specification includes references to an "example" and an "example," these "example" and "example" do not necessarily refer to the same example. Particular features, structures, or characteristics may be combined as appropriate consistent with the present disclosure.

[0014] As used herein, the term "comprising" is open-ended and, when used in the claims, does not exclude additional elements, structures, or steps.

[0015] As used herein, "configured" means that various parts or components may be described or defined as being "configured" to perform a task or tasks. In this context, "configured" is used to describe a structure and includes structures in which parts or components perform those tasks when in operation. Thus, the above-described parts or components may be said to be configured to perform a task even if the particular part or component is not currently operating (e.g., not powered on).

[0016] As used herein, the singular reference to an element or step does not necessarily exclude a plurality of elements or steps. As used herein, the term "and / or" includes all combinations of one or more of the associated listed elements. Also, as used herein, the phrase "combinations thereof" includes combinations with at least one of the associated listed elements, which combinations may additionally include other elements, such as unlisted elements.

[0017] As used herein, when the phrase "at least one" is used in conjunction with a list of elements, it means that one or more of the listed elements can be used in various combinations, and that only one of each listed element may be required. That is, "at least one" means that any number of the listed elements can be used in any combination, and it is not required that all of the listed elements be used. An element in this context may be, for example, a particular object, thing, or category.

[0018] Reference is now made to the drawings, which illustrate various examples of the present disclosure. FIG. 1 illustrates an unmanned undersea vehicle 100 including a syntactic foam 102. The syntactic foam 102 has a relatively high hydrostatic crush strength and a relatively low density, which allows the syntactic foam 102 to efficiently meet the buoyancy requirements of undersea applications such as the unmanned undersea vehicle 100 illustrated in FIG. 1 and other undersea applications such as cabled observation nodes and unmanned sensors. Advantageously, the syntactic foam 102 can be easily molded into a foam product 108 shaped to complement the interior voids and exterior contours of the undersea vehicle or other applications. The syntactic foam 102 is not limited to undersea applications and can also be implemented in terrestrial, airborne, and space applications.

[0019] 2-7, Fig. 2 shows a cross-sectional view of an example of a syntactic foam 102 of the present disclosure. This syntactic foam includes a matrix 150 that forms a rigid or flexible space, boundary, enclosure, container, or region, a quantity of porous polymer beads 200 dispersed within the matrix 150, and a quantity of hollow microspheres 300 dispersed within the matrix 150 in the spaces between and among the porous polymer beads 200. As will be described below, the hollow microspheres 300 are provided in a relatively narrow size distribution, which allows the syntactic foam 102 to have high hydrostatic crush strength and low density.

[0020] The porous polymer beads 200 and hollow microspheres 300 are encapsulated in a matrix 150. The matrix 150 bonds the porous polymer beads 200 to each other, the hollow microspheres 300 to each other, and the porous polymer beads 200 to the hollow microspheres 300. The matrix 150 does not substantially penetrate the porous polymer beads 200 or the hollow microspheres 300. For example, the matrix 150 does not penetrate each porous polymer bead 200 to a depth greater than 10 percent of the bead width 202 (e.g., FIG. 3). Similarly, the matrix 150 does not penetrate each hollow microsphere 300 to a depth greater than 10 percent of the microsphere diameter 302 (FIG. 7). In other examples, the porous polymer beads 200 and / or hollow microspheres 300 are configured such that the matrix 150 does not penetrate more than 5 percent of each of the bead width 202 and the microsphere diameter 302.

[0021] The matrix 150 is formed from a thermosetting resin mixture 424 (FIG. 23), which is composed of a liquid resin and a hardener or catalyst. The matrix 150 (i.e., resin) can be a thermosetting material such as epoxy, silicone, polyurethane, polyurea, polyester, cyclic olefin, phenolic resin, polyimide, polybenzimidazole, and / or other thermosetting materials. The epoxy resin can be, for example, bisphenol A diglycidyl ether epoxy resin. The hardener can be any suitable polymerization initiator, such as an amine-based or alcohol-based hardener, mixed with the resin in the appropriate ratio to produce a resin mixture 424 that hardens or hardens over time. Examples of hardeners include aliphatic polyamine hardeners and cyclic polyamine hardeners. An example of a commercially available two-part resin system (i.e., resin and hardener) is DENEEF®, available from GCP Applied Technologies, Inc., Alpharetta, Georgia. TM Denepox I-40, Ultra Thin2 Epoxy available from Pace Technologies, Tucson, Arizona TMand CI-SLV available from Simpson Strong-Tie Co., Pleasanton, California TM Ultra-low viscosity structural injection epoxy.

[0022] In the manufacture of syntactic foam 102, as described below, porous polymer beads 200 (FIG. 15) are mixed with hollow microspheres 300 (FIG. 18) in a mold 402 (FIG. 15) to form a powder bed 310 (FIGS. 19-20). This powder bed is then impregnated with a resin mixture 424 (FIG. 23). After the powder bed 310 is impregnated, the resin mixture 424 is cured (e.g., at room temperature or by heat curing) to a green state. In this state, the syntactic foam 102 can maintain its shape in a solid state, but may be pliable and / or flexible. The green state syntactic foam 104 (FIG. 26) is finally cured by heat treatment at an elevated temperature for a predetermined period of time, which fully crosslinks the matrix 150 (i.e., the resin mixture 424) and results in the post-cured syntactic foam 106 (FIGS. 28-29).

[0023] 3 is an enlarged view of a portion of a green state syntactic foam 104, which is comprised of a matrix 150 containing a quantity of porous polymer beads 200 and a quantity of hollow microspheres 300. Generally, the hollow microspheres 300 maintain a desired spacing between the porous polymer beads 200. In the illustrated example, some of the hollow microspheres 300 are in contact with each other and some of the hollow microspheres 300 are in contact with the porous polymer beads 200. In any of the configurations of syntactic foam 102 of the present disclosure, whether in the green state or in the post-cured state, the syntactic foam 102 has 55 to 75 volume percent porous polymer beads 200 relative to the combined volume of the hollow microspheres 300, the porous polymer beads 200, and the space filled by the matrix 150. The spaces filled with matrix 150 are a combination of the spaces between adjacent hollow microspheres 300, the spaces between adjacent porous polymer beads 200, and the spaces between a hollow microsphere 300 and its adjacent porous polymer bead 200.

[0024] The porous polymer beads 200 and hollow microspheres 300 are preferably dispersed substantially uniformly within the matrix 150. For example, the porous polymer beads 200 are dispersed within the matrix 150 such that the average distance between each pair of adjacent porous polymer beads 200 in all regions of the syntactic foam 102 is shorter than the bead width 202 of the smallest porous polymer bead 200 within the syntactic foam 102. Alternatively, or in addition, the average distance between each pair of adjacent porous polymer beads 200 may be equal to or greater than the microsphere diameter 302 of the largest hollow microsphere 300. Similarly, the hollow microspheres 300 are dispersed within the matrix 150 such that the average distance between each pair of adjacent hollow microspheres 300 in the syntactic foam 102 is less than twice (2x) the microsphere diameter 302 of the largest hollow microsphere 300.

[0025] 3-5, the porous polymer beads 200 are formed of a low-density thermoplastic foam material. For example, the porous polymer beads 200 are expanded polystyrene beads and / or expanded polypropylene beads. FIG. 4 shows an example of an expanded polystyrene bead 203. FIG. 5 is an enlarged view showing an example of the cellular internal structure 204 of the expanded polystyrene bead 203 on an exaggerated scale. In each structure of the syntactic foam 102 of the present disclosure, the porous polymer beads 200 have a bulk density of 0.1 g / cm. 3 More preferably, it is less than 0.05 g / cm 3 Additionally, in each configuration of syntactic foam 102 of the present disclosure, the bead width 202 or diameter is less than 1.5 mm. By way of example, the bead width 202 or diameter is between 0.5 and 1.5 mm. In the present disclosure, the bead width 202 of a porous polymer bead 200 is a measure of its maximum width, as shown in the examples of Figures 4, 8, and 9.

[0026] The porous polymer beads 200 are generally circular. In the example of Figures 3-4, the porous polymer beads 200 are generally spherical 210. However, the shape of the porous polymer beads 200 may have a variety of alternative shapes, such as, for example, an elliptical shape 212 (i.e., a spheroidal shape) shown in Figure 8 and / or an irregularly rounded shape 214 shown in Figure 9. While each configuration of syntactic foam 102 shown in Figures 3, 8, and 9 includes porous polymer beads 200 of a single shape, syntactic foam 102 may be manufactured to include porous polymer beads 200 of two or more different shapes.

[0027] The hollow microspheres 300 (FIGS. 6-7) are composed of glass, ceramic, polymer, or other suitable materials. Furthermore, the density of the hollow microspheres 300 is 0.6 g / cm. 3 less than 0.4 g / cm 3In one example, hollow microspheres 300 are hollow glass microspheres (HGMS), such as S38 HGMS and S38hs HGMS available from 3M Company, St. Paul, Minnesota. S38 HGMS and S38hs HGMS are made of soda-lime borosilicate glass and have a bulk density of 0.38 g / cm. 3 K25 HGMS is also available from 3M and is also made from soda-lime borosilicate glass with a bulk density of 0.25 g / cm. 3 is.

[0028] In the syntactic foam 102 of the present disclosure, the largest hollow microsphere 300 is no more than 1 / 7 the size of the smallest porous polymer bead 200. As shown in FIG. 7, the size of the hollow microsphere 300 refers to the microsphere diameter 302. By making the size of the hollow microspheres 300 relatively small, the ability to fill the gaps 312 (FIG. 3) between the porous polymer beads 200 is increased, thereby reducing the effective density of the syntactic foam 102. As an example, the largest hollow microsphere 300 is no more than 1 / 10 the size of the smallest porous polymer bead 200. In some examples, the microsphere diameter 302 of the hollow microspheres 300 in the syntactic foam 102 is 40 μm or greater.

[0029] The hollow microspheres 300 in the syntactic foam 102 have a size distribution within ±10 μm. In this disclosure, size distribution refers to the percentage of hollow microspheres 300 whose microsphere diameters 302 fall within a particular size range. For example, the syntactic foam 102 may be configured such that at least 70 percent of the hollow microspheres 300 have microsphere diameters 302 within the ±10 μm size range (e.g., microsphere diameters between 50 and 70 μm), with the remaining 30 percent or more of the hollow microspheres 300 having microsphere diameters 302 outside that size range (e.g., smaller than 50 μm or larger than 70 μm). In another example, the syntactic foam 102 is configured such that at least 90 percent of the hollow microspheres 300 fall within the ±10 μm size range. In yet another example, at least 95 percent of the hollow microspheres 300 fall within the ±10 μm size range. In yet another example, at least 99 percent of the hollow microspheres 300 fall within a size range of ±10 μm.

[0030] In some examples of syntactic foam 102, the size distribution of hollow microspheres 300 is within ±5 μm. For example, syntactic foam 102 is configured such that at least 70 percent of the hollow microspheres 300 have microsphere diameters 302 within a ±5 μm size range (e.g., microsphere diameters of 53-63 μm). In other examples, syntactic foam 102 is configured such that at least 90 percent of the hollow microspheres 300 have microsphere diameters within a ±5 μm size range. In yet other examples, at least 95 percent of the hollow microspheres 300 have microsphere diameters within a ±5 μm size range. In yet other examples, at least 99 percent of the hollow microspheres 300 have microsphere diameters within a ±5 μm size range.

[0031] As discussed above, the hollow microspheres 300 have a relatively narrow particle size distribution, which allows the syntactic foam 102 to have a relatively high hydrostatic crush strength at a relatively low density. The relatively high hydrostatic crush strength of the syntactic foam 102 is related to the isostatic crush strength of the hollow microspheres 300, which is a function of the bulk density and size (i.e., microsphere diameter 302) of the hollow microspheres. Generally, the density and isostatic crush strength of hollow microspheres 300 increase as size decreases.

[0032] For example, Figure 10 is a chart showing the properties of S38hs and K25 hollow glass microspheres in several different size distributions. The size distributions were obtained by sieving bulk amounts of S38hs and K25 hollow glass microspheres in 10-63 μm increments using a stainless steel mesh sieve 412. Each increment was characterized by isostatic fracture strength and density. As shown in Figure 10, K25 hollow glass microspheres in the 75-63 μm size range had a density of 0.19 g / cm. 3 and an isostatic fracture strength of approximately 300 psi. In comparison, smaller K25 hollow glass microspheres in the size range of 53-45 μm have a density of 0.25 g / cm 3 and an isostatic fracture strength of approximately 600 psi, K25 hollow glass microspheres less than 45 μm in diameter have a density of 0.34 g / cm 3 and an isostatic fracture strength of approximately 800 psi. Note that the isostatic fracture strength of K25 hollow glass microspheres smaller than 53 μm is higher than the isostatic fracture strength (tested at 400 psi) of bulk K25 hollow glass microspheres (which have a relatively broad size range of 20-100 μm and an average diameter of 40 μm).

[0033] Furthermore, Figure 10 shows the properties of S38hs hollow glass microspheres in the size range of 53–45 μm, which exhibit a density of 0.30 g / cm 3 and an isostatic fracture strength of approximately 2,100 psi. In comparison, smaller S38hs hollow glass microspheres in the 38-20 μm size range have a density of 0.41 g / cm 3 and an isostatic fracture strength of approximately 4,400 psi. As shown in Figure 10, S38hs hollow glass microspheres sized above 38 μm have a density comparable to that of K25 hollow glass microspheres, but more than twice the isostatic fracture strength. Additionally, the isostatic fracture strength of S38hs hollow glass microspheres in the 38-20 μm size range is higher than the isostatic fracture strength (tested at 3,000 psi) of bulk S38hs hollow glass microspheres in the 20-100 μm size range.

[0034] FIG. 11 is a graph showing the size distribution of S38hs hollow glass microspheres in a bulk quantity 304 and a sort quantity 306 (i.e., the quantity being sieved). The size range of the sort quantity 306 is 53-63 μm, which is an example of a ±5 μm size distribution. The 53-63 μm size range was achieved using mesh sieves 412 with mesh sizes of 53 μm and 63 μm, respectively. In the bulk quantity of S38hs, the volume percentage of hollow glass microspheres between 50-73 μm (i.e., a ±11.5 μm size distribution) is 43 percent of the total volume. When sieved, the volume percentage of 50-73 μm hollow glass microspheres increases to 75% of the total volume. As shown in FIG. 11, most of the volume of the hollow glass microspheres falls within the mesh size range of sieve 412, indicating that sieving selectively narrows the size distribution within the sieving range.

[0035] Advantageously, a relatively narrow size distribution of hollow microspheres 300 (e.g., ±10 μm or ±5 μm) maximizes packing efficiency within powder bed 310, thereby allowing the density of syntactic foam 102 to be reduced while maintaining high hydrostatic crush strength. In contrast, a low-density syntactic foam 102 containing a bulk amount of unsorted hollow microspheres in a wide size range (e.g., 20-100 μm) has a relatively low hydrostatic crush strength.

[0036] For example, Figure 12 shows the increased hydrostatic crush strength (i.e., also referred to herein as hydrostatic pressure) and corresponding underwater service depth for several test specimens (not shown) formed with several different syntactic foam 102 configurations, each containing a different size distribution of hollow microspheres 300. Shown in the figure is the hydrostatic crush strength of a syntactic foam 102 containing a bulk amount of hollow microspheres, as well as the hydrostatic crush strength of a prior art low density foam 110.

[0037] The hydrostatic crush strength data shown in FIG. 12 was generated by testing multiple test specimens (not shown), each with a different syntactic foam 102 configuration. For example, test specimens were fabricated from various syntactic foams 102, each consisting of 0.8 mm expanded polystyrene (EPS) beads combined with 53-63 μm S38hs hollow glass microspheres, 38-20 μm S38hs hollow glass microspheres, and a bulk amount of S38hs hollow glass microspheres. Each test specimen was cored from a larger syntactic foam 102, and the exterior of the specimen was sealed using epoxy resin. Each specimen was then subjected to hydrostatic compression, during which the water volume displacement was recorded.

[0038] FIG. 13 is a graph showing water displacement as a function of hydrostatic pressure (i.e., hydrostatic compression) applied to the test specimen. The hydrostatic fracture strength of each test specimen is characterized based on the relationship of water displacement to pressure. For example, a change in water displacement without a change in hydrostatic pressure indicates that the test specimen has undergone a fracture event 112. In FIG. 13, the initial fracture event 112 is represented by the first breakdown in the linearity of the graph, occurring at approximately 2900 psi. A subsequent slight increase in hydrostatic pressure results in another breakdown, indicating further fracture of the test specimen.

[0039] Referring again to Figure 12, the test results showed that the specimen containing 0.8 mm EPS beads and 53-63 μm S38hs hollow glass microspheres had a density of 0.33 g / cm 3 The hydrostatic fracture strength (i.e., pressure resistance) was found to be approximately 3,000 psi. The specimen containing 0.8 mm EPS beads and 20–38 μm S38hs had a density of 0.32 g / cm. 3 and a hydrostatic crush strength of approximately 1,800 psi. In comparison, a test specimen of syntactic foam 102 containing 0.8 mm EPS beads and a bulk amount of S38hs had a density of 0.29 g / cm. 3 and a hydrostatic crush strength of about 1,600 psi. The prior art low density foam 110 shown in Figure 12 exhibited even lower hydrostatic crush strength than various syntactic foams 102 containing S38hs hollow glass microspheres. As shown, the syntactic foam 102 of the present disclosure having controlled size distribution of hollow microspheres 300 exhibits significantly improved performance compared to the prior art low density foam 110.

[0040] The enhanced performance of the syntactic foam 102 of the present disclosure is due to the narrow size distribution of the hollow microspheres 300, which controls the interstices between the porous polymer beads 200. To this end, the spherical shape 210 of the hollow microspheres 300 results in uniform size of the hollow microspheres 300, which, combined with the narrow size distribution, allows for uniform wall thickness 152 (FIGS. 3, 8, and 9) of the matrix 150. The uniformity of wall thickness 152 results in a syntactic foam 102 with higher hydrostatic crush strength and lower density compared to syntactic foam 102 including hollow microspheres 300 with the relatively broad size distribution typical of bulk or unsorted amounts of microspheres.

[0041] The syntactic foam 102 of the present disclosure can be manufactured to have different densities and corresponding hydrostatic crush strengths, for example, densities between 0.20 and 0.25 g / cm 3 The syntactic foam 102 has a hydrostatic crush strength of more than 500 psi. In another example, the density is 0.28 to 0.32 g / cm. 3 The syntactic foam 102 has a hydrostatic crush strength of more than 1500 psi. In yet another example, the density is 0.32 to 0.35 g / cm. 3 The syntactic foam 102 has a hydrostatic crush strength of greater than 2000 psi. The above-described combination of density and hydrostatic crush strength is applicable to the syntactic foam 102 in a post-cured state after heat curing at an elevated temperature (e.g., 100-150°C) for a predetermined period of time (e.g., 2 hours). Advantageously, heat curing the syntactic foam 102 increases the hydrostatic crush strength by 25-50% compared to the hydrostatic crush strength in the green state.

[0042] When the syntactic foam 102 is in its green state, the porous polymer beads 200 have an internal structure 204 similar to the example shown in FIG. 26. When the green state syntactic foam 104 is heated in its final cure, each porous polymer bead 200 shrinks, forming a matrix cavity 206. In this respect, each matrix cavity 206 is defined by the cured matrix 150, not the original porous polymer bead 200. Each matrix cavity 206 surrounds a space partially occupied by a polymer particle 208, as shown in the example of FIG. 29. As explained below, the polymer particle 208 is the result of the porous polymer bead 200 shrinking when the green state syntactic foam 104 is heated in its final cure.

[0043] While FIGS. 3 and 8 depict syntactic foam 102 having a single size of porous polymer beads 200, syntactic foam 102 may be configured with porous polymer beads of different sizes. For example, syntactic foam 102 may be provided in a trimodal configuration (not shown) that includes only two sizes of porous polymer beads 200 in addition to hollow microspheres 300. As described above, the bead width 202 of each size of porous polymer beads 200 in a trimodal syntactic foam is less than 1.5 mm. For example, the bead width 202 of each of the two sizes of porous polymer beads 200 in a trimodal syntactic foam is within the range of 0.5 to 1.5 mm. Furthermore, the largest of the hollow microspheres 300 is no more than 1 / 7 of the smallest of the porous polymer beads 200. Furthermore, the hollow microspheres 300 have a size distribution within ±10 μm.

[0044] In other examples of trimodal syntactic foam 102, the porous polymer beads 200 may be provided in only a single size, and the hollow microspheres 300 may be provided in only two sizes. As noted above, in any configuration of syntactic foam 102 of the present disclosure, including trimodal configurations, the bead width 202 of the porous polymer beads 200 is less than 1.5 mm (e.g., 0.5-1.5 mm), and the largest of the hollow microspheres 300 is no more than 1 / 7 of the smallest of the porous polymer beads 200. Furthermore, the two different sizes of hollow microspheres 300 are within the same size distribution, within ±10 μm.

[0045] In a further example, the syntactic foam 102 may optionally include one or more additives to improve the manufacturability and / or performance of the syntactic foam 102. For example, expanded aluminum or carbon nanotubes (not shown) may be added to transfer heat away from the interior of the syntactic foam 102 during final curing. Alternatively, or in addition, a wetting agent (not shown) may be added to improve the wettability of the surfaces of the porous polymer beads 200 and hollow microspheres 300 and facilitate resin impregnation.

[0046] Referring to Figure 14, and with further reference to Figures 15-29, Figure 14 shows a flowchart of steps involved in a method 500 for manufacturing a syntactic foam 102 of the present disclosure. Step 502 of method 500 involves placing a quantity of porous polymer beads 200 of one or more sizes into a mold 402, as shown in Figure 15. In the illustrated example, mold 402 is cylindrical in shape. However, mold 402 can have any shape, size, and / or configuration. Mold 402 has a top mold surface 406 and a bottom mold surface 408.

[0047] In step 502, the width of each porous polymer bead 200 is less than 1.5 mm. In some examples, the width of the porous polymer bead 200 is greater than 0.5 mm (i.e., 0.5-1.5 mm). As described above, the porous polymer beads 200 are formed from a thermoplastic foam material, such as expanded polystyrene beads or expanded polypropylene beads. The bulk density of the porous polymer beads 200 is 0.1 g / cm. 3 More preferably, it is less than 0.05 g / cm 3 is less than.

[0048] Optionally, the method 500 may include agitating the porous polymer beads 200 during and / or after placing the porous polymer beads 200 into the mold 402, thereby optimizing the packing of the porous polymer beads 200 within the mold 402. Agitation of the porous polymer beads 200 may be accomplished by directly agitating the porous polymer beads 200, by tapping the mold 402, and / or by mechanically vibrating the mold 402 using a mechanical vibrator 410 as shown in Figure 16. The mold 402 may be vibrated at a relatively high frequency (e.g., 1500-2000 rpm) for a short period of time.

[0049] Step 504 of method 500 involves dispersing a select amount 306 of hollow microspheres 300 throughout a quantity of porous polymer beads 200 (FIG. 18) in mold 402 to obtain powder bed 310 (FIG. 19). As noted above, the size distribution of the hollow microspheres 300 in the select amount 306 is within ±10 microns. In some instances, the size distribution of the hollow microspheres 300 is within ±5 microns. The largest of the hollow microspheres 300 is no more than 1 / 7 the size of the smallest of the porous polymer beads 200. The hollow microspheres 300 are formed from glass, ceramic, polymer, and / or other materials and have a density of 0.6 g / cm or less. 3 less than 0.4 g / cm 3 is less than.

[0050] Before or during step 504, the method 500, in some instances, includes sieving the bulk quantity 304 (FIG. 17) of hollow microspheres 300 to obtain a sort quantity 306 of hollow microspheres 300 having a desired size distribution (e.g., ±10 μm, ±5 μm, etc.). For example, FIG. 17 illustrates a process for sieving the bulk quantity 304 of hollow microspheres 300 using a mesh sieve 412. Depending on the initial size range of the hollow microspheres 300 in the bulk quantity 304 (i.e., 20-100 μm), multiple mesh sieves 412 of different sizes (e.g., at least two mesh sieves 412) may be required in the sieving step.

[0051] As described above, the hollow microspheres 300 allow the plurality of porous polymer beads 200 to remain spaced apart. The hollow microspheres 300 are sieved to relatively large sizes (e.g., 53-63 μm), which can remove at least a portion of the hollow microspheres 300 with low isostatic crushing strength, as shown in FIG. 10 . These are the largest sizes of hollow microspheres 300 that can reduce the hydrostatic crushing strength of the syntactic foam 102. For example, sieving is performed so that the microsphere diameters 302 of some of the hollow microspheres 300 fall within a range (e.g., 58-63 μm) greater than the average in the size distribution (e.g., 53-63 μm). Sieving can also remove a portion of the high-density, relatively small-sized hollow microspheres 300. These contribute to the relatively thin wall thickness 152 (FIGS. 3, 8, 9) of the cured matrix 150, reducing the fracture strength of the syntactic foam 102.

[0052] To improve filling efficiency within the mold 402, step 504 may optionally include pouring the hollow microspheres 300 into the mold 402 while the mold 402 is being rocked. For example, FIG. 18 shows a mechanical vibrator 410 vibrating the mold 402 as the hollow microspheres 300 are being poured into the mold 402, as a means of filling the interstices 312 (FIG. 3) between the porous polymer beads 200 with the hollow microspheres 300. Rocking of the mold 402 can also occur when the hollow microspheres 300 are sieved through one or more mesh sieves 412 and poured into the mold 402. The mold 402 can be rocked or vibrated before, during, and / or after pouring the hollow microspheres 300 into the mold 402. In some instances, rocking of the mold 402 can continue until the free volume of the interstices 312 between the porous polymer beads 200 is filled with hollow microspheres 300 .

[0053] In some instances, a select amount 306 of hollow microspheres 300 can be added to the porous polymer beads 200 to achieve a volume percentage of 55-75 volume percent of the porous polymer beads 200 relative to the combined volume of the hollow microspheres 300, porous polymer beads 200, and the space filled by the matrix. Figure 19 shows a mold 402 containing a powder bed 310 of porous polymer beads 200 and hollow microspheres 300. Figure 20 shows hollow microspheres 300 dispersed throughout the porous polymer beads 200.

[0054] 21-22, an example mold assembly 404 is shown in which a porous frit 416 or other porous material is disposed on the mold top surface 406 and a porous frit 416 is also disposed on the mold bottom surface 408. These porous frits 416 allow for liquid impregnation of the powder bed 310. A manifold 418 is attached to the side of the mold 402 where the resin mixture 424 (FIG. 23) is introduced to ensure that the resin mixture 424 is distributed across the entire cross section of the powder bed 310.

[0055] Prior to impregnating the powder bed 310, the method 500 includes mixing a resin with an appropriate amount of a hardener to obtain a homogenous resin mixture 424 (FIG. 23), which will solidify or harden over time to form a hardened resin (i.e., matrix 150). As discussed above, the resin mixture 424 is a thermosetting material such as an epoxy, silicone, polyurethane, polyurea, polyester, cyclic olefin, phenolic resin, polyimide, polybenzimidazole, or other thermosetting material.

[0056] 23 illustrates an example of a manufacturing system 400 for impregnating a powder bed 310 contained in a mold 402. The manufacturing system 400 includes a resin container 422 containing a resin mixture 424. The resin container 422 is fluidly connected to a bottom mold surface 408 of the mold 402. The manufacturing system 400 also includes a vacuum source 420, such as a vacuum pump, fluidly connected to a top mold surface 406 of the mold 402. The powder bed 310 is sealed at both ends of the mold 402 by porous frits 416, which allow liquid impregnation of the powder bed 310 with pore sizes smaller than the pore size of the smallest hollow microspheres 300 (e.g., less than 10 μm) but large enough to allow the resin mixture 424 to pass through.

[0057] Step 506 of method 500 includes impregnating powder bed 310 with resin mixture 424. In the example of FIGS. 23-24 , impregnation of powder bed 310 with resin mixture 424 involves introducing resin mixture 424 into powder bed 310 by applying vacuum pressure to the side of mold 402 opposite the side where resin mixture 424 was introduced. Impregnation is initiated by activating a vacuum pump to create vacuum pressure, which draws resin mixture 424 upward from resin reservoir 422, into manifold 418 at mold bottom 408, through porous frit 416, through powder bed 310, and out porous frit 416 at mold top 406. Pressure can also be applied to the resin source (e.g., resin reservoir 422) to increase the rate of impregnation. Drawing the resin mixture 424 upward improves debulking of the impregnated powder bed 310 compared to the reduced debulking capability of drawing the resin mixture 424 downward through the powder bed 310. In step 506, vacuum pressure is applied for a predetermined time (e.g., 1-3 hours) and / or until a suitable amount of the resin mixture 424 has been expelled from the mold top surface 406 and / or until the entire powder bed 310 is saturated with the resin mixture 424 and preferably free of air bubbles.

[0058] 25-26, step 508 of method 500 includes initially curing the resin mixture 424 in the powder bed 310 to bring the syntactic foam 102 to a green state. In some examples, the resin mixture 424 can solidify or harden at room temperature within a predetermined time (e.g., 12 hours). FIG. 26 shows the cellular interior structure 204 of the porous polymer beads 200 when the syntactic foam 102 is in a green state.

[0059] After room temperature curing is complete, the green state syntactic foam 104 is heated to an elevated temperature (e.g., 135°C) and held at that temperature for a predetermined period of time (e.g., 2 hours) to fully crosslink the matrix 150 (i.e., resin mixture 424) and produce the post-cured syntactic foam 106. For example, as shown in Figure 27, the green state syntactic foam 104 is placed in an oven 426 for heat treatment during curing. After cooling from the final cure is complete, the post-cured syntactic foam 106 is removed from the mold 402, as shown in Figure 28.

[0060] Figure 29 shows a cross section of a post-cured syntactic foam 106 containing matrix cavities 206, which were porous polymer beads 200 with internal structures 204 (e.g., Figure 26) when the syntactic foam 102 was in its green state. In the post-cured state shown in Figure 29, each matrix cavity 206 encloses a space partially occupied by polymer particles 208. The polymer particles 208 in each matrix cavity 206 are the result of shrinkage of the internal structures 204 of the porous polymer beads 200 during heating of the green state syntactic foam 104 in the final cure.

[0061] In its post-cured state, the syntactic foam 102 can be molded as needed into a foam product 108. For example, the syntactic foam 102 can be machined into a foam product 108 having a shape complementary to the shape and / or outer contour of an interior void of an undersea application, such as an unmanned undersea vehicle 100 (e.g., FIG. 1).

[0062] Many modifications and other embodiments of the disclosure described herein will be apparent to one skilled in the art to which this disclosure pertains having the benefit of the foregoing descriptions and the associated drawings. The embodiments described herein are illustrative and not intended to be limiting or comprehensive. Although specific terms are employed herein, these terms are used in a generic and descriptive sense only and not to limit the disclosure. Functionally equivalent methods and apparatuses within the scope of the disclosure, in addition to those enumerated herein, may be implemented from the above description. All such modifications and variations are intended to be encompassed within the scope of the appended claims. The disclosure is limited only by the terms of the appended claims, along with the full scope of equivalents to which such claims are entitled.

Claims

1. The matrix and a quantity of porous polymer beads disposed within the matrix, each having a width of less than 1.5 mm; A syntactic foam comprising: a predetermined amount of hollow microspheres dispersed within the matrix among the predetermined amount of porous polymer beads, wherein the largest of the hollow microspheres is no more than 1 / 7 of the smallest of the porous polymer beads, and the hollow microspheres have a size distribution within ±10 μm.

2. 10. The syntactic foam of claim 1, wherein the matrix is ​​comprised of at least one thermosetting material selected from the group consisting of epoxy, silicone, polyurethane, polyurea, polyester, cyclic olefin, phenolic resin, polyimide, and polybenzimidazole.

3. 10. The syntactic foam of claim 1, wherein the porous polymer beads comprise at least one thermoplastic material selected from the group consisting of expanded polystyrene beads and expanded polypropylene beads.

4. The density of the porous polymer beads is 0.1 g / cm ３ The syntactic foam of claim 1 , wherein the tensile strength is less than 1 / 2.

5. The syntactic foam of claim 1 , wherein the hollow microspheres comprise at least one of the following materials: glass, ceramic, and polymer.

6. The density of the hollow microspheres is 0.6 g / cm ３ The syntactic foam of claim 1 , wherein the tensile strength is less than 1 / 2.

7. As a combination of density and hydrostatic crushing strength, 0.20~0.25g / cm ３ and a hydrostatic crush strength of greater than 500 psi; 0.28-0.32g / cm ３ and a hydrostatic crush strength of greater than 1500 psi; 0.32-0.35g / cm ３ and a hydrostatic crush strength greater than 2000 psi.

8. The matrix and a quantity of matrix cavities present within the matrix, each of which encloses a space partially occupied by polymer particles and each of which has a maximum width of less than 1.5 mm; A syntactic foam comprising: a predetermined amount of hollow microspheres dispersed among the predetermined amount of matrix cavities within the matrix, wherein the largest of the hollow microspheres is no more than 1 / 7 of the smallest of the matrix cavities, and the hollow microspheres have a size distribution within ±10 μm.

9. 9. The syntactic foam of claim 8, wherein the matrix is ​​comprised of at least one thermosetting material selected from the group consisting of epoxy, silicone, polyurethane, polyurea, polyester, cyclic olefin, phenolic resin, polyimide, and polybenzimidazole.

10. As a combination of density and hydrostatic crushing strength, 0.20~0.25g / cm ３ and a hydrostatic crush strength of greater than 500 psi; 0.28-0.32g / cm ３ and a hydrostatic crush strength of greater than 1500 psi; 0.32-0.35g / cm ３ and a hydrostatic crush strength greater than 2000 psi.

11. 1. A method for producing a syntactic foam, comprising: placing a quantity of porous polymer beads, each having a width of less than 1.5 mm, into a mold; dispersing a selected amount of hollow microspheres throughout the quantity of porous polymer beads in the mold to obtain a powder bed, wherein the hollow microspheres have a particle size distribution within ±10 μm, and the largest of the hollow microspheres is no more than 1 / 7 the size of the smallest of the porous polymer beads; impregnating the powder bed in the mold with a resin mixture comprising a resin and a hardener; curing the resin mixture in the powder bed to obtain the syntactic foam.

12. 12. The method of claim 11, further comprising rocking the mold during and / or after placing the quantity of porous polymer beads in the mold, thereby optimizing the packing of the porous polymer beads in the mold.

13. 12. The method of claim 11, further comprising sieving the bulk quantity of hollow microspheres to obtain a select quantity of hollow microspheres having a particle size distribution within ±5 μm.

14. Dispersing the selected amount of hollow microspheres throughout the predetermined amount of porous polymer beads comprises:

12. The method of claim 11, comprising pouring the hollow microspheres into the mold while rocking the mold until the free volume between the porous polymer beads is filled with the hollow microspheres.

15. 12. The method of claim 11, wherein dispersing the selected amount of hollow microspheres throughout the predetermined amount of porous polymer beads is performed such that the volume percentage of the porous polymer beads is 55 to 75 volume percent relative to the combined volume of the hollow microspheres, the porous polymer beads, and the space filled with the resin mixture.

16. 12. The method of claim 11, wherein the step of impregnating the powder bed with the resin mixture comprises applying a vacuum pressure to a side of the mold opposite to the side where the resin mixture is introduced.

17. The method of claim 11 , wherein the porous polymer beads comprise at least one of the following thermoplastic materials: expanded polystyrene beads and expanded polypropylene beads.

18. The density of the porous polymer beads is 0.1 g / cm ３ The method of claim 11 , wherein the

19. The method of claim 11 , wherein the hollow microspheres comprise at least one of the following materials: glass, ceramic, and polymer.

20. The density of the hollow microspheres is 0.6 g / cm ３ The method of claim 11 , wherein the

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