Porous sintered membrane and method for preparing the same

Abstract

A porous sintered body and a method for manufacturing the porous sintered body by steps including an injection molding step are described.

Description

[Technical field]

[0001] The invention described relates to a porous sintered body and includes a method for producing the same by steps including an injection molding step. [Background technology]

[0002] Porous sintered bodies are used in a variety of industrial applications, including filtration of materials used in the electronics and semiconductor manufacturing industries, as well as other industries that require high purity materials for processing. For example, in the semiconductor and microelectronics industries, in-line filters are often used to remove particulate matter from fluids to prevent their introduction into the manufacturing process. The fluids may be in the form of gases or liquids.

[0003] Currently, the common method for commercially preparing porous sintered bodies includes forming and sintering steps that involve manually moving and handling the intermediate (in-process) form of the porous body. These steps are labor intensive. Furthermore, the sintered bodies are fragile and the forming steps can be poorly defined. These characteristics make the process prone to significant waste, undesirably low efficiency, and undesirably high cost. Summary of the Invention

[0004] Described herein is an alternative technique for forming porous sintered bodies. The disclosed method does not suffer from the inefficiencies and cost drawbacks comparable to current techniques, and replaces labor-intensive, less accurate, and potentially variable manual steps with a more accurate, less labor-intensive injection molding step. The injection molding step may be more automated, more accurate, and may generate less waste compared to older and currently used methods of forming porous sintered bodies. The described method can be implemented to achieve high-volume production of highly repeatable (accurate) porous sintered bodies. Current (non-injection molding) commercial processes are hampered by manual and labor-intensive steps, with production yields in the range of 60-80%. In comparison, the well-controlled, automated injection molding process described herein has the potential to achieve improved uniformity and precision (e.g., reduced variation within parts, between parts, and between batches) with higher production volumes (increased throughput), lower waste, and higher yields.

[0005] Metal injection molding techniques have been used to prepare bodies with relatively low porosity, but the present invention can produce bodies with higher porosity. Current commercial examples of metal injection molding methods involve injection compositions ("feedstock") and resulting sintered bodies that contain a large amount of particles and relatively low porosity, often or typically targeting less than 1% porosity (voids by volume) for the injected metal parts. Although not usually striving to produce metal parts with higher porosity, the metal injection molding methods and materials used are capable of producing injection molded metal parts with somewhat higher porosity levels, for example, up to 15, 20, 25, or 30% volume porosity, and a corresponding solids (metal) % of the resulting sintered body of at least 70%.

[0006] In contrast, the presently described method can use an injection composition containing a relatively low percentage of particles, e.g., 20-50% (by volume) particles, and 50-80% binder. The process can correspondingly produce a final porous sintered body having a porosity in the range of 50-80%. To successfully process in this manner, it has surprisingly been found that the particles used to form the sintered porous body can be selected to exhibit a relatively low "relative apparent density," which can be a function of the morphology (e.g., shape) of the particles.

[0007] In one aspect, a method is disclosed for producing a porous sintered body by particle injection molding a liquid injection composition into a molded mold cavity. The liquid injection composition includes a polymeric binder and 20-50 volume percent solid inorganic particles, based on the total volume of the liquid injection composition. The method includes flowing the liquid injection composition into the molded mold cavity and causing solidification of the liquid binder within the molded mold cavity to form a solidified injection composition including the solid binder surrounding the solid inorganic particles.

[0008] In another aspect, a liquid ejection composition is disclosed, comprising 50-80 volume % of a polymer binder and 20-50 volume % of solid inorganic particles having a relative apparent density in the range of 5-35% of the theoretical density of the particles, based on a total volume of the liquid ejection composition.

[0009] In another aspect, disclosed herein is an injection molded porous sintered body comprising sintered particles and having a porosity in the range of 50-80%.

[0010] The present disclosure may be more fully understood in consideration of the following description of various exemplary embodiments in conjunction with the accompanying drawings. [Brief description of the drawings]

[0011] [Figure 1] 1A-1D illustrate exemplary steps of the above-described method for forming a porous sintered body. [Figure 2A]FIG. 2 illustrates an aggregate of inorganic (metal) particles as described herein. [Figure 2B] FIG. 2 illustrates an aggregate of inorganic (metal) particles as described herein. [Figure 2C] FIG. 2 illustrates an aggregate of inorganic (metal) particles as described herein. [Figure 2D] FIG. 2 illustrates an aggregate of inorganic (metal) particles as described herein. [Figure 3A] FIG. 3A illustrates various shapes of exemplary injection-molded and sintered porous bodies described herein. [Figure 3B] FIG. 3B illustrates various shapes of exemplary injection-molded and sintered porous bodies described herein. [Figure 3C] FIG. 3C illustrates various shapes of exemplary injection-molded sintered porous bodies described herein. [Figure 3D] FIG. 3D illustrates various shapes of exemplary injection-molded and sintered porous bodies described herein. [Figure 4A] FIG. 4A illustrates an exemplary injection molded and sintered porous body described herein. [Figure 4B] FIG. 4B illustrates an exemplary injection molded and sintered porous body described herein. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] According to the following description, a porous sintered membrane is prepared by an injection molding method, such as what is sometimes called the "particle injection molding" technique, more specifically the "metal injection molding" technique, or "MIM", which involves the use of metal particles. The described method involves the use of an injection composition containing inorganic (e.g., metal) particles and a polymeric binder. The injection composition is flowed (injected) as a liquid into a mold cavity and solidified in the mold cavity to form a solidified injection composition in the form of a solidified compact. The solidified compact can be removed from the cavity and is a self-supporting form. The compact, although potentially fragile, can be handled and further processed by steps including removing the polymeric binder and sintering the inorganic (e.g., metal) particles to form a porous sintered body.

[0013] The resulting porous sintered body comprises (or consists of, or consists essentially of) a solid (e.g., rigid or semi-rigid) matrix of fused and thereby interconnected particles. The matrix is ​​porous (e.g., highly porous) and the particles of the matrix are connected to each other at adjacent surfaces (e.g., "interconnected") during the sintering step.

[0014] The porous body may be in the form of a flat sheet, e.g., a membrane, which may have the shape of a substantially planar, essentially two-dimensional (having a very thin thickness) flat sheet or membrane. However, injection molding techniques for forming these objects may allow for greater flexibility in choosing the shape or form of the molded body. Other examples of shapes of porous bodies may be non-planar, e.g., three-dimensional. For example, the porous body may be in the form of a curved or round plate or "cup". Alternatively, the porous body may be three-dimensional, e.g., an annular membrane, such as in the form of a tube, i.e., a tube with a round or circular cross section when viewed along the axis of the cylinder. Other tubes may have shapes that are non-circular in cross section, such as shapes that include angles, corners, or pleated patterns (multi-pointed stars, or circular "zigzag" patterns). A membrane (any shape) includes two opposing major surfaces and a thickness between the two opposing major surfaces.

[0015] The thickness of the membrane (e.g., the thickness of the wall of a tube or cylinder) can be in a range that is effective for using the porous body as a filter. Examples of useful thicknesses can range from 0.5 to 5 millimeters, for example, from 1 to 4 millimeters.

[0016] The porous body may have a relatively high porosity, for example 50-80%, or 55-75%, and other useful properties related to the performance of the porous body as a filtration membrane, such as flow and retention properties known to be required for a useful filtration membrane. As used herein and in the art of porous sintered bodies, the "porosity" of a porous sintered body (sometimes called porosity) is a measure of the void (i.e., "empty") space within the porous sintered body as a percentage of the total volume of the porous sintered body, calculated as the ratio of the volume of the voids of the porous sintered body to the total volume of the porous sintered body. An object with 0% porosity is completely solid.

[0017] Methods for preparing porous bodies include injection molding the injection composition by injecting a liquid injection composition (i.e., a "liquid injection composition") into a mold cavity to form a molded body. The molded body can be solidified in the mold cavity to form a solidified molded body. In a subsequent step, the molded body can be removed from the mold cavity, the polymer binder can be separated from the particles of the molded body, and the particles can be fused together by a sintering step to form a free-standing porous sintered body.

[0018] The injection composition contains a polymer binder, and inorganic particles are distributed throughout the polymer binder. To allow the particles to be processed to form a porous membrane by a step including injection molding, the particles are selected to exhibit physical properties including morphological (including shape) and density properties that allow the particles to be included in the injection composition in relatively small amounts, but still be interconnected by the sintering step to form a free-standing porous sintered body. More specifically, the particles have a low "relative apparent density". At a low "relative apparent density", the particles can be present in the injection composition at a low volume percentage, for example, at an amount of particles less than 50 volume % based on the total volume injection composition, but still be processed to form a free-standing porous sintered body. At a low "relative apparent density", even when present at a low percentage of the volume of the injection composition, the particles can be effectively fused together by sintering to form a useful porous sintered body, for example, a porous body consisting of "free-standing" and fused interconnected particles, which is useful as a filtration membrane as described herein, for example.

[0019] The particles, as an aggregate, have physical properties including size, shape, and density that allow the particles to be distributed in a liquid polymer binder at a relatively low volumetric amount, but still be processed by injection molding and sintering to form a useful (e.g., interconnected and self-supporting) porous sintered body. The volumetric amount of particles in the injection composition is desirably low so that the resulting sintered body exhibits a relatively high porosity, so that the sintered body can be effectively used as a filtration membrane. However, even at a low volumetric amount in the injection composition (to produce a highly porous sintered body), the particles contained in the injection composition must have sufficient proximity between adjacent surfaces in a sufficient amount to effectively fuse and interconnect during sintering, so that the particles forming the sintered body are highly interconnected, and therefore the sintered body is self-supporting.

[0020] As used herein, a "free-standing" object is one that can support its own weight in a given form or shape for a useful period of time (e.g., between processing steps) without collapsing and preferably without sagging more than a small extent. The sintered bodies described herein that are free-standing can be handled, moved (some free-standing sintered bodies are fragile and require great care to move), and further processed without requiring support from another structure, such as a polymer binder.

[0021] Specifically with respect to free-standing sintered bodies, a collection of particles (e.g., to be molded) can be formed into a free-standing sintered body if the collection of particles includes a high enough percentage of particles that are close enough to each other (e.g., have surfaces that are in contact or near contact) to fuse together (i.e., be "connected" or "interconnected") during sintering. Preferably, a high percentage of the particles of the injected composition are located close enough together, e.g., have at least one surface that is in contact or near contact with at least one other particle surface, such that most or essentially all of the particles (e.g., 95, 99, or 99.9% of the total amount of particles) of the injected composition become fused particles of the sintered porous body. A high degree of contact or proximity (near contact) between particle surfaces can be present in the molded body contained in the mold cavity in the form of the solidified injected composition. A high degree of contact or proximity between particle surfaces also remains during the solvent debinding step, as well as during and after the thermal debinding step. After the thermal debinding step and before the sintering step, the object consists only of particles with a high degree of contact or proximity between adjacent particle surfaces and with voids between the surfaces. The object can be free-standing even if it is not held together by a polymer, even if the particles have not yet been fused together by sintering, due solely to the high degree of proximity and contact between the particles of the object.

[0022] To be effective in forming a self-supporting porous sintered body by injection molding and sintering, the size, shape, and density characteristics of the particles allow the particles to be formed into a molded body (e.g., a solidified injection composition contained in a mold cavity) that includes the particles distributed in a polymer binder, where there is a high degree of proximity and contact between the particles, but also an amount of voids that results in a useful (relatively high) porosity of the resulting porous sintered body. As part of the injection composition, especially as part of the solidified injection composition, the particles are spaced apart from each other to a sufficient degree that the resulting sintered body has voids (porosity) that allow the sintered body to be effective as a filtration membrane. At the same time, the particles have some proximity or contact between their surfaces so that when the molded body is later processed to remove the polymer, only the remaining particles (no polymer present) form a self-supporting body. The same high contact or proximity between the particles also allows the particles (after all of the polymer has been removed) to be subsequently processed by sintering, where the particles are fused and interconnected to a degree that the resulting sintered body is self-supporting.

[0023] Referring to FIG. 1, the method described involves the use of a feedstock (10) that includes inorganic particles, a polymeric binder, and optional ingredients effective to form a useful ejection composition.

[0024] Useful inorganic particles include inorganic particles that can be processed as described herein because they can be effectively formed into a liquid injection composition for dispensing into a mold cavity to form a molded body, followed by forming a free-standing object comprised of the particles alone, which can then be processed by a sintering step that effectively fuses the particles together to form an interconnected, free-standing, porous, sintered body.

[0025] The particles may be in the form of an aggregate of small particles, for example as a powder, and the particles may be any of a variety of known particle morphologies, such as individual particles referred to as "agglomerates," "dendritic particles," or "fibrous particles," among others. The particles may be of any effective size or range of sizes, including small or relatively small particles on the micron scale (e.g., having an average size of less than 500 microns, less than 100 microns, less than 50 microns, 10 microns, or less than 5 microns).

[0026] The particles may comprise one or more different types of inorganic materials, including metallic particles, ceramic particles, or a combination of both ceramic and metallic particles. As used herein, the term "metal" refers to any metallic or semi-metallic chemical element, or an alloy of two or more of these elements.

[0027] The particles can be selected to achieve the efficiency of processing as described above, and can be injection molded, formed into a molded, self-supporting body of particles, and then sintered to form a self-supporting porous sintered body that will effectively function as a filtration membrane. The size, shape, and chemical composition of the particles can be any that is effective for these purposes. In some embodiments, the particles identified as useful as described herein can be selected based on size, shape (including morphological properties), and density properties, for example, that can be processed by injection molding to form a porous sintered body as described (composed of interconnected particles, self-supporting, and exhibiting relatively high porosity and effective flow and filtration properties).

[0028] The density properties of selected particles can be described as apparent density (also known as bulk density) and relative apparent density (apparent density divided by theoretical (or "particle" density). Exemplary particles made of nickel, nickel alloys, or stainless steel, measured in powder form, may have an apparent ("bulk") density of less than 2 grams per cubic centimeter (g / cc), such as less than 1.8 g / cc, or less than 1.5 g / cc. Other materials may have higher density values ​​(e.g., refractory metals) or lower apparent density values ​​(e.g., certain ceramic materials). As is known, the apparent (bulk) density of a powder (collection of particles) refers to the mass of the powder for a given volume of powder, where the volume includes the volume of the particles as well as the volume of the spaces between the particles in powder form. Methods for measuring apparent (bulk) density are well known and include ASTM B703-17 "Standard Test Method for Apparent Density of Metal Powders and Related Compounds Using the Arnold Meter".

[0029] Exemplary particles in the form of powders can also be selected to have a "relative apparent density" that allows for processing as described to produce porous sintered bodies by steps including injection molding steps. As specified herein, particles can be selected based on the relative apparent density to allow the particles to be successfully processed by injection molding and subsequent sintering to produce porous sintered bodies with desirably high porosity, with the particles also interconnected to form a free-standing body. As generally understood herein, the term "relative apparent density" is calculated as the ratio of the apparent density of the powder divided by the theoretical density of the powder. The theoretical density of a collection of particles (e.g., powder), also referred to as the "particle density" of the particles, refers to the density of a collection of particles calculated based on the density of the material (e.g., metal, ceramic) that constitutes the particle, e.g., the density (mass per volume) of a single particle, or weight per volume, where the volume is calculated to include only the volume of the particles and not the volume of voids between the particles. Exemplary particles useful according to the described method can be in the form of powders with a relative apparent density in the range of 5-35% of the theoretical density.

[0030] It has been found herein that particles exhibiting low "relative apparent density" can be processed by injection molding to form porous sintered bodies with high porosity and correspondingly low solid content (less than 50%) (i.e., high porosity). Low relative apparent density particles have physical shape and size characteristics that cause a high degree of contact or proximity between the surfaces of the particles when included in the injection composition (even if present in small amounts (low volume %) in the injection composition) with high voids between the particles. A high degree of contact or proximity between the surfaces of the particles also exists in objects formed from the injection composition, for example, by molding. With high contact or proximity between the particle surfaces, even with high voids, the polymer binder can be removed and the particles remain in the form of free-standing, self-supporting objects that can be further processed by sintering to fuse the particles together sufficiently at their surfaces to be interconnected and self-supporting to form useful porous sintered membranes.

[0031] A relatively low "relative apparent density" is a property of an aggregate of particles that can be directly influenced by the physical size and shape characteristics of the particles. The size and shape characteristics of inorganic powders (metallic or ceramic) can vary widely, and known particles have many different shapes. Some examples of common particle shapes include those called spherical, round, angular, flaky, cylindrical, acicular, cubic, columnar, dendritic, fibrous, elongated, and branched. Other particle shapes and other terms used to describe specific shapes are also known. Different types of particles may also be agglomerated or non-agglomerated, or "fibrous". Certain types of particles or their branches or fibrils that have a predominant length dimension relative to a small thickness and width dimension can be characterized by having a high aspect ratio.

[0032] Particles useful in the injection molding method as described have shape and size characteristics that cause the particles to have a low relative apparent density, for example to form an aggregate of particles with a high level of voids between particles, e.g., low packing density, as a powder. Size and shape characteristics of particles with low relative apparent density include characteristics that cause low packing density ("packing efficiency"). Shape characteristics of particles that can produce low packing density (and high voids) include irregular (non-geometric) shape characteristics including multiple fibrils or branches in a random (non-repeating) arrangement between particles, elongated shapes (e.g., high aspect ratios) of particles or parts of particles, high surface areas, branched, twisted, bent, or curved filaments or branches, etc., that prevent close packing of particles and result in the presence of substantial voids between particles when the particles are part of a powder.

[0033] Examples of particle shapes that may result in low relative apparent density include branched shapes, shapes referred to as "dendritic", and shapes referred to as "fibrous".

[0034] Dendritic particles include particles with dendritic morphology as described in US Pat. No. 5,814,272. As indicated therein, the term "dendritic" refers to a highly anisotropic irregular morphology that includes one or more filaments that individually have one dimension substantially greater than the other two dimensions of the filament. The filaments may be straight or curved, and may be branched or unbranched with irregular surfaces. Dendritic particles are characterized by a lower packing efficiency compared to particles with more regular morphology, and therefore form powders with lower apparent (bulk) density than those formed by particles with more regular morphology. Examples of dendritic particles include Nickel 255 particles shown in FIG. 2A, and treated stainless steel particles shown in FIG. 2C.

[0035] Dendritic particles can be prepared and processed so that the particles achieve the desired dendritic morphology and useful relative apparent density. An example of a process useful for producing dendritic particles with the described density characteristics is provided in U.S. Patent No. 5,814,272, which is incorporated herein by reference in its entirety. As described herein, the particles can be processed to have a relatively low "relative apparent density" by processing the particles to make them dendritic. In general, an effective processing method includes (1) heating a powder containing non-dendritic particles under conditions suitable to form a lightly sintered material, and (2) breaking the lightly sintered material to form a powder containing dendritic particles.

[0036] The term "lightly sintered material" refers to a material that has been processed to induce fusion of metal powder particles through the early stages of sintering, as defined by Randall (Randall in "Powder Metallurgy Science," second edition, German, ed., Metal Powder Federation Industry (1994)), the contents of which are incorporated herein by reference. In the early stages of sintering or short-range diffusion sintering, bonds are formed between particles at contacting particle surfaces, resulting in fusion of particles only with their immediate neighbors. Thus, the early stages of sintering result in a brittle structure with low mechanical strength. For a given material, sintering proceeds slowly beyond this early stage at temperatures at the lower end of the sintering range of the material. For the purposes of this description, the term "early stage sintering" refers to the sintering of a powder under conditions in which sintering does not proceed substantially beyond the early stage.

[0037] Figure 2A is a micrograph showing dendritic particles made from Nickel 255 (an example of a commercially available pure nickel metal powder), Figure 2B is a micrograph of a stainless steel particle before the particle has been processed to have a dendritic morphology, and Figure 2C is a micrograph of the stainless steel particle of Figure 2B after the particle has been processed to become dendritic.

[0038] Another example of particles characterized by low packing efficiency and relatively low "relative apparent density" are particles referred to as "fibrous" particles. Fibrous particles have a high aspect ratio, such as an aspect ratio (length to diameter ratio) of at least 10:1 (length:diameter), at least 30:1, at least 50:1, or at least 75:1, or at least 100:1, and are elongated (e.g., "noodle-like") and optionally curved or bent. Examples of fibrous particles include fibrous stainless steel particles, such as those shown in FIG. 2D.

[0039] Other types of inorganic particles in powder form that are identified as non-dendritic and non-fibrous are known and are also useful for preparing metal bodies by sintering. These particles exhibit relatively high packing efficiency compared to dendritic or fibrous particles and do not usually have low relative apparent density (without being combined with dendritic or fibrous particles). Examples of these types of particles include particles that are generally (substantially) unbranched and have relatively low aspect ratios (e.g., less than 5:1 or less than 3:1 or less than 2:1), including particle types that are referred to as spherical, round, angular, flaky, cylindrical, acicular, and cubic.

[0040] A population of particles in the form of a powder and having a low relative apparent density useful in the described methods may contain particles all having substantially the same or similar size, shape, and morphological characteristics, e.g., a population of all dendritic particles, or a population of all fibrous particles. Alternatively, if desired, the population of particles may contain a combination of two or more different types of particles having different size, shape, or morphological characteristic characteristics. The particles of the powder may include, for example, a combination of both dendritic and non-dendritic particles, or a combination of both fibrous and non-fibrous particles, etc., where the combination has a relative apparent density sufficient to be processed to form sintered porous bodies and precursors thereof as described.

[0041] The particle assembly may include one or more different types of inorganic particles, including metallic particles, ceramic particles, or a combination of both ceramic and metallic particles. Examples of useful particles include particle assembly made substantially or entirely of metal particles, for example, particle assembly made of at least 90, 95, 99, or 99.9% by weight of a metal (or metal alloy), such as steel particles (e.g., stainless steel), nickel particles, nickel alloy particles, or particles made of another metal or metal alloy. Commercially available examples include those sold under the following names: Nickel 255, "Alloy 22" (Hastelloy® C-22), and 316L Stainless Steel.

[0042] The nickel particles typically contain at least 99% by weight nickel, with little or no impurities such as carbon.

[0043] An example of a nickel alloy may be an alloy containing a combination of nickel (e.g., 45-56 wt%), chromium (e.g., 15-30 wt%), and molybdenum (e.g., 8-18 wt%), along with minor amounts of metals such as iron, cobalt, tungsten, manganese, silicon, carbon, vanadium, and copper. A specific example of a nickel alloy commonly referred to as nickel "alloy 22" (e.g., HASTELLOY® C-22®) contains (by weight) nickel (56 balance), chromium (22), molybdenum (13), iron (3), cobalt (up to 2.5), tungsten (3), manganese (up to 0.5), silicon (up to 0.08), carbon (up to 0.01), vanadium (up to 0.35), and copper (up to 0.5).

[0044] An example of a stainless steel alloy is Stainless Steel Alloy 316L, which may contain (by weight %) Chromium (16-18), Nickel (10-14), Molybdenum (2-3), Manganese (max 2.0), Silicon (max .75), Carbon (max 0.08), Phosphorus (max 0.045), Sulfur (max 0.30), Nitrogen (max 0.10), and Iron (balance).

[0045] The useful and preferred particles described can have apparent densities and relative apparent densities, as described, with particular metal alloys having characteristic density properties and characteristic combinations of density properties.

[0046] Useful or preferred stainless steel particles may have an apparent density in the range of 0.5 to 2 grams per cubic centimeter, such as 0.8 to 1.2 grams per cubic centimeter, and a relative apparent density in the range of 5 to 25, such as 7 to 20 percent of the theoretical density.

[0047] Useful or preferred nickel particles may have an apparent density in the range of 0.3 to 1.5 grams per cubic centimeter, e.g., 0.4 to 0.8 grams per cubic centimeter, and a relative apparent density in the range of 4 to 17% of the theoretical density, e.g., 5 to 9% of the theoretical density.

[0048] Useful or preferred particles made of nickel alloys having major (wt%) amounts of nickel (e.g., 45-56 wt%), chromium (e.g., 15-30 wt%), and molybdenum (e.g., 8-18 wt%), such as Hastelloy® C-22, may have an apparent density in the range of 0.5 to 2 grams per cubic centimeter, e.g., 1.2 to 1.8 grams per cubic centimeter, and a relative apparent density in the range of 5 to 13% of the theoretical density, e.g., 7 to 11% of the theoretical density.

[0049] The amount of particles in the injection composition can be any amount useful for producing the porous sintered body described herein having the porosity described herein. An example can be in the range of 20-50% by volume, e.g., 25-45%, based on the total volume injection composition, on a total volume basis. According to a useful process, the particles are combined with a polymeric binder and optionally one or more additional ingredients (e.g., to facilitate processing) to form an injection composition that can be in the form of a liquid (e.g., at a "liquid binder temperature"), which can be injected into a mold cavity and can also solidify within the mold cavity (e.g., at a "solid binder temperature" at an effective pressure), i.e., thereafter in the form of a solidified injection composition having the shape of the mold cavity. The polymeric binder can be a thermoplastic or a thermosetting material, with thermoplastic materials potentially being preferred. A thermoplastic polymer composition is a polymeric composition that can be reversibly heated and cooled to change between a heated liquid state and a cooled solid state.

[0050] Useful polymeric binders can be any polymeric binder (including combinations of binder materials) capable of dispersing the particles, which can take the form of a liquid injection composition (e.g., at elevated temperature) followed by a solidified injection composition (e.g., at a cooling temperature, which can be ambient temperature (25 degrees Celsius), or at another temperature below the elevated temperature). The polymeric binder is also a polymeric material that is strong and cohesive enough to support the solidified injection composition after it is removed from the mold cavity as a molded body. The polymeric binder should also be capable of being completely or essentially completely washed from the particles with little or no residue remaining on the particles to produce a substantially residue-free porous sintered body.

[0051] According to a useful or preferred injection composition, the polymeric binder may contain at least two different types of polymeric materials: a first polymeric binder (aka "primary binder") that can be removed using a liquid solvent (e.g., organic or aqueous), and a second polymeric binder (aka "secondary binder") that is not removed by exposure to a liquid solvent and remains with the object after the first polymeric binder is removed using a liquid solvent. The second polymeric binder, sometimes referred to as the "backbone polymer" of the binder system, may be effective in supporting the particles as part of the molded body (e.g., brown body) during subsequent processing after the first polymeric binder is removed.

[0052] The first binder, sometimes referred to as a "filler," is soluble in water or organic solvents at room temperature (25 degrees Celsius) or at elevated temperatures, typically between 40 degrees Celsius and 80 degrees Celsius. Examples of organic solvents that may be effective in removing the first binder include heptane, hexane, HFE (hydrofluoroether), dichloroethylene, and trichloroethylene. As another example, the polymer can be removed from the object by exposing the object to supercritical carbon dioxide.

[0053] Some non-limiting examples of types of first binders include waxes such as paraffin wax, carnauba wax, polyethylene glycol, agar, and mineral oil.

[0054] The amount of the first binder in the injection composition can be any amount that enables processing as described herein, with examples ranging from 10 to 25 weight percent based on the total weight of the injection composition.

[0055] The exemplary injection composition also contains a second binder, sometimes called a "backbone polymer," that is not soluble in water or the organic solvent used to remove the first binder. The second binder will remain as part of the object (sometimes called the "brown object") upon removal of the first binder. The second binder stays with the grains of the brown object, thereby supporting the brown object for further processing.

[0056] Examples of useful second binders include polymers that are stable (insoluble) in water and in the organic solvent used to remove the first binder, but can be removed from the brown body to a very high degree (low residue) by heat treatment, for example in an oven. Examples of useful secondary binders include certain polyolefins, polyacetals, polyoxymethylenes, polymethyl methacrylates, ethyl vinyl acetates (including homopolymers and copolymers thereof), and combinations of two or more of these types of polymers. Specific examples of commercially available polymers useful as second binders include polypropylene and Celanese Celon® M450 acetal copolymer.

[0057] The amount of the second binder in the injection composition can be any amount that enables processing as described herein, with examples ranging from 5 to 20 weight percent based on the total weight of the injection composition.

[0058] If desired and deemed effective, other ingredients may also be included in the injection composition, including ingredients known and believed to be useful in injection molding processes. Examples include antioxidants in small amounts, surfactants (including emulsifiers), lubricants, etc.

[0059] Referring again to FIG. 1, one step of the exemplary process 100 is mixing the ingredients (20). During the mixing step, the ingredients can be compounded together and mixed until a homogenous mixture is obtained. The ingredients can be completely "dry", meaning that they are in substantially solid form and do not contain any liquid ingredients, in which case the mixing step is a "dry mixing step". The dry ingredients can include polymeric binders (e.g., first and second binders), inorganic (e.g., metal) particles, and any other dry ingredients such as antioxidants, surfactants, lubricants, etc.

[0060] Alternatively, the components of the mixing step need not be in "dry" or solid form. The mixing step can be performed to mix components that include one or more components in liquid form and can be a "wet mixing" step. For example, the mixing step can include compounding components that include liquid polymer components, e.g., polymers dissolved in a solvent. As a single example of a liquid polymer component, polyethylene glycol (among many other polymers) can be in solid (dry) form or alternatively dissolved in water or an organic solvent during the mixing step.

[0061] The mixture of the mixing step can include, consist of, or consist essentially of binders (e.g., first and second binders), inorganic (e.g., metal) particles, and other optional dry ingredients. As used herein, a component, composition, or combination of components that "consists essentially of" the specified component, composition, or combination of components is considered to contain the recited component, composition, or combination of components and no more than insignificant amounts of other materials, e.g., less than 5, 2, 1, or 0.5 weight percent of any other material based on the total weight of the component, composition, or combination of components.

[0062] After the dry blending step, the blended dry components can be further compounded by a compounding step (30) to form a feedstock that can be used as a liquid injection composition in a molding step. The combining step is typically performed at a temperature where the components are in liquid form. Exemplary temperatures for the combining step can be above room temperature (25 degrees Celsius), for example, in the range of 125 degrees Celsius to 200 degrees Celsius (depending on the type of polymer). After combining, the mixture can be cooled to form a pelletized feedstock in solid form (e.g., at room temperature), which can then be placed in an injection molding system and heated to remelt the polymer and form a liquid injection composition that can be dispensed (e.g., injected) into a mold cavity to form a molded body. The particles in the liquid injection composition can be present in a useful volume, in total volume percent, such that the surfaces of the particles are in sufficient proximity or contact with each other to allow the particles to produce a resultant porous sintered body that is self-supporting but still has a porosity in the range of, for example, 20-50% as described herein.

[0063] The next step may be that of injection molding (40), which may include remelting the solid feedstock produced during the combining step to form a liquid injection composition that is injected into the mold cavity, solidifying the liquid injection composition within the mold cavity (e.g., by decreasing the temperature and increasing the packing pressure) to form a solidified injection composition, and removing the solidified injection composition from the mold in the form of a molded body containing particles dispersed in a solid (solidified) polymeric binder.

[0064] The compact is then processed to remove the polymer (binder) from the particles and to fuse the particles together. For example, as shown in FIG. 1, the compact can be processed by a water / solvent debinding step (50) in which the compact is contacted with a solvent, which may be organic, water, or a combination of water and organic solvent. The solvent removes any amount of polymer binder or other solid (non-particle) material that is soluble in the solvent (i.e., the primary binder) from the compact. The remaining matter (which may be referred to herein as the "brown matter") includes particles suspended in the remaining polymer binder (secondary binder) that is not soluble in the solvent, optional antioxidants and surfactants, etc. The water / solvent debinding step is performed by exposing the compact to the solvent under conditions, e.g., temperature, sufficient to remove substantially all of the water or solvent soluble material, including the first (primary) binder. The temperature of the water / solvent debinding step may be any effective temperature, such as a temperature in the range of 40 degrees Celsius to 80 degrees Celsius.

[0065] According to an exemplary step of the process 100 of FIG. 1, the brown body is then exposed to heat in a thermal debinding step (60) to remove the polymer (second or "secondary" binder) and any other remaining non-particulate material, such as antioxidants, surfactants, etc., from the brown body. In this step, the brown body is exposed to a high temperature sufficient to remove the remaining solid (non-particulate) material and provide a substantially residue-free porous body that substantially contains only particles that form a free-standing structure but are not yet interconnected by fusion. For example, after the thermal debinding step (or alternately after the sintering step), the porous body can contain 1, 0.5, 0.1, 0.05, or 0.01 wt. % or less of any component, such as binders, antioxidants, surfactants, etc., i.e., at least 99, 99.5, 99.9, 99.95, or 99.99 wt. % of the particles of the injected composition.

[0066] The temperature of the thermal debinding step can be any temperature useful for substantially removing the binder and other solid non-particulate materials from the brown body (without causing sintering or melting of the particles). The particular temperature can depend on features such as the type of polymer and the morphological properties and composition of the particles (as well as the sintering and melting temperatures). In certain embodiments, the thermal debinding step can be performed by slowly increasing the temperature with a "hold" or plateau (period of constant temperature) based on the thermogravimetric analysis (TGA) profile of the polymer of the brown body. Any rate of increase in temperature ("ramp rate") during the thermal debinding step can be similarly useful, with an exemplary rate being 2 degrees Celsius per minute. The maximum temperature reached during the thermal debinding step can be similarly useful depending on the type of polymer, with an example being up to 500 degrees Celsius or at least 600 degrees Celsius. Typically, the thermal debinding temperature does not exceed 600 degrees Celsius.

[0067] The porous body is also subjected to a sintering step (70) to fuse and connect the particles together. As used herein, the term "sintering" has a meaning consistent with that given to the term when used in the art of porous sintered metal structures, such as porous sintered metal membranes of the type that may be used as metal filtration membranes. Consistent with this, the term "sintering" can be used to refer to a process of bonding together (e.g., "solid-state welding" or "fusion") a collection of small sinterable particles of one or more different types (sizes, compositions, shapes, etc.) by applying heat to the particles (i.e., to the porous body) in a non-oxidizing environment to a temperature that fuses the particle surfaces together by physical (mechanical) bonding between the particle surfaces, but does not melt the particles (i.e., neither metal material reaches its melting temperature).

[0068] The sintering step is carried out at a temperature higher than the sintering point of the particles of the object, but lower than the melting temperature of the particles. As used herein, the "sintering point" of a particle is the temperature at which the material of the particle can be sintered, i.e., the temperature at which the particle starts to adhere to other particles of the object being sintered and can fuse to another particle at a certain pressure, e.g., atmospheric pressure. The sintering point of a material (e.g., a metal) is usually lower than the melting temperature of the material, i.e., the temperature at which the metal becomes liquid.

[0069] Thus, useful temperatures for carrying out the sintering step may depend on the composition of the particles, the sintering point of the particles, and the size of the particles being sintered, e.g., whether the particles are "coarse" (larger) or fine (smaller). For nickel, the sintering point may be in the range of 550-750 degrees Celsius, and the sintering step may be carried out at a temperature in the range of 550-800 degrees Celsius. For nickel and stainless steel alloys, the sintering point may be in the range of 950-1250 degrees Celsius, and the sintering step may be carried out at a temperature in the range of 950-1300 degrees Celsius. The sintering step may be carried out in a furnace or oven, and in a non-oxidizing atmosphere that does not react with or adversely affect the metal particles of the body being sintered, e.g., in a vacuum or an atmosphere of concentrated or pure hydrogen, concentrated or pure inert gas, or a combination of concentrated or pure hydrogen and inert gas.

[0070] According to one exemplary method with added details, an injection composition can be prepared using known polymer combination equipment such as a twin screw extruder or prep mixer, using a binder system including a first solvent soluble binder and a second solvent insoluble binder, along with optional processing aids such as surfactants, antioxidants, lubricants, etc. The combined feedstock (injection composition) can be pelletized for use in an injection molding machine. The injection composition can be injection molded by methods similar to conventional plastic or metal injection molding processes. The compact formed by the injection molding step can be debonded in two separate steps. The first (solvent) debonding step exposes the compact to a heated water or solvent bath to remove the primary binder component and create a brown morphology. The second (thermal) debonding step can be performed to thermally remove the secondary binder and additives in a furnace, leaving a free-standing particle structure substantially free of residue. This structure is then sintered to its final density in a furnace containing a non-oxidizing, e.g., hydrogen, inert gas, or vacuum atmosphere, to achieve the desired porosity of the metal film.

[0071] The porous sintered bodies prepared according to the described methods may be useful as filtration membranes for filtering gases, such as gases used in semiconductor processing. Various characteristics of the porous sintered bodies are believed to affect the usefulness of the porous bodies as filtration membranes. When filtering gaseous materials for use in semiconductor processing, the gaseous fluid may be supplied at a pressure that is about atmospheric pressure (e.g., under 2 atm), above atmospheric pressure, or below atmospheric pressure (e.g., under vacuum conditions). Processes using gaseous fluids may require very high removal rates of nanoscale and micron-scale particles, e.g., at least 3, 4, 5, 7, or 9, as measured by the "log reduction value" (LRV) of the filtering step. Processes for filtering these gaseous materials may also be performed at relatively low flow rates, e.g., less than 50, 25, 10, 5, 2, 1, or 0.5 standard liters per minute per square centimeter of frontal filter area (slpm). The methods described herein may be useful for preparing filtration membranes that meet such requirements, allowing the filtration membranes to be effectively used as filtration membranes, e.g., for filtering gaseous materials for use in semiconductor processing.

[0072] Advantageously, the sintered porous bodies formed by the injection molding step can be prepared to have any of a wide variety of three-dimensional shapes, including certain types of shapes that may not be possible to produce by previous non-injection molding techniques for forming porous bodies of the type useful as filtration membranes.

[0073] Exemplary shapes of injection molded films can be generally three-dimensional, including tubular forms, including non-tubular (e.g., somewhat or substantially flat or planar) forms, and substantially circular or cylindrical forms or variations thereof.

[0074] An example of a non-tubular shape may be in the form of a flat, curved, or rounded plate or "cup" having two opposing major surfaces and a thickness between the two opposing surfaces. The opposing major surfaces may generally be flat or curved, and may further be flat or have surface structures including raised ridges or walls, depressions or channels, or non-flat patterned or non-patterned three-dimensional structures such as "waffling". Referring to Figures 3A, 3B, 3C, and 3D, Figures 3A and 3C are top views, and Figures 3B and 3D are perspective views of filtration membranes 200 and 204 that may include two opposing major surfaces having a width and a length, and a thickness between the two surfaces that is substantially less than the width and length. At least one surface (e.g., surface 202 in Figures 3A and 3B) can include three-dimensional structures, including repeating or non-repeating patterns of depressions (e.g., sunken channels) or ridges or elevated ridges, walls, etc., an example being a waffling pattern as shown in Figures 3A, 3B, 3C, and 3D. Other shapes of surface structures and patterns are possible using the presently described injection molding techniques.

[0075] Alternatively, the porous sintered body may be three-dimensional, such as a tubular membrane, such as in the form of a tube (e.g., tube, cylinder) with a round or circular cross section when viewed along the axis of the tube, i.e., cylinder. Other tubes may have non-circular shapes in cross section, such as shapes that include repeating patterns of angles, corners, curves (e.g., fluting), or pleated patterns (multi-pointed star, or circular "zigzag" pattern) that run around the inner or outer surface of the tube. The membrane (any shape) includes two opposing major surfaces and a thickness between the two opposing major surfaces. At least one end of the tubular membrane may be open, and the second end may be open or closed. Figures 4A and 4B show perspective views of an annular filtration membrane 210 with a non-circular cross section that includes multiple repeating curved surfaces, such as "pedals" or "grooves," with one open end and one closed end.

[0076] As used herein, an object referred to as an "injection molded porous sintered body" (or "injection molded body", etc.) is an object that is structurally or physically identifiable as an object produced by an injection molding step, i.e., an object that includes physical features that indicate that the object is formed by an injection molding technique. During injection molding, a mold cavity having a mold surface is used to form the shape of the molded body. The mold cavity includes one or more structural features that generate different physical markings on the surface of the object prepared by the mold. Exemplary mold cavities can include, among others, a mold number on the mold surface, a parting line (the boundary or edge where two parts of the mold come together), an injector pin, a gate, or a gate opening. These or other structures of the cavity mold can form a physical mark ("injection mold mark") that remains on the surface of the object formed using the mold. Thus, an "injection molded porous sintered body" can be identified by the presence of one or more of these injection molding marks, such as a parting line mark (formed by a parting line), an injector pin mark (formed by an injection pin), a gate mark (formed by a gate or gate opening), or a cavity number (formed by a cavity number mark on the mold surface).

[0077] Exemplary binder components are shown in Tables 1 and 2 below. TIFF0007650859000001.tif79170TIFF0007650859000002.tif81170

[0078] Having thus described some exemplary embodiments of the present disclosure, those skilled in the art will readily appreciate that still other embodiments may be made and used within the scope of the appended claims. Many advantages of the present disclosure covered by this document have been set forth in the foregoing description. It will be understood, however, that the present disclosure is in many respects merely illustrative. Changes may be made in details, particularly in matters of shape, size, and arrangement of parts, without exceeding the scope of the present disclosure. The scope of the present disclosure is, of course, defined in the language in which the appended claims are expressed.

[0079] In a first aspect, a method of making a porous sintered body by particle injection molding a liquid injection composition into a molded mold cavity includes flowing a liquid injection composition into the molded mold cavity, the liquid injection composition comprising at least one polymeric binder and 20-50 volume percent solid inorganic particles, based on a total volume of the liquid injection composition, and causing solidification of the polymeric binder in the molded mold cavity to form a solidified injection composition comprising the solid binder surrounding the solid inorganic particles.

[0080] A second aspect according to the first aspect further includes removing the solidified injected composition from the mold cavity, removing the solid binder from the solidified injected composition to form a porous non-sintered body, and sintering the porous non-sintered body to form a porous sintered membrane.

[0081] A third embodiment according to the first or second embodiment, wherein the solid inorganic particles are dendritic or fibrous and have an apparent density of less than 2.0 grams per cubic centimeter.

[0082] A fourth embodiment according to any preceding embodiment, wherein the solid inorganic particles have a relative apparent density in the range of 5 to 35% of the theoretical density of the particles.

[0083] A fifth aspect according to any preceding aspect, wherein the porous sintered membrane has a porosity in the range of 50-80%.

[0084] A sixth embodiment according to any preceding embodiment is wherein the polymeric binder comprises a thermoplastic polymer selected from wax, polypropylene, polyethylene glycol, polyoxymethylene, polymethyl methacrylate, ethyl vinyl acetate, and combinations thereof.

[0085] A seventh embodiment according to any preceding embodiment, wherein the polymeric binder comprises a primary binder that can be removed from the solidified ejected composition by contacting the solidified ejected composition with a liquid solvent selected from water, an organic solvent, and combinations thereof at a temperature in the range of 40 degrees Celsius to 80 degrees Celsius.

[0086] An eighth embodiment according to the seventh embodiment is wherein the primary binder is selected from wax and polyethylene glycol.

[0087] A ninth aspect according to the seventh or eighth aspect further comprises removing the primary binder from the solidified ejectable composition by contacting the solidified ejectable composition with a liquid solvent selected from water and an organic solvent at a temperature in the range of 40 degrees Celsius to 100 degrees Celsius.

[0088] A tenth aspect according to the ninth aspect is where the organic solvent is selected from heptane, hexane, hydrofluoroether, dichloroethylene, trichloroethylene, and mixtures thereof.

[0089] An eleventh aspect according to the seventh to tenth aspects is where the polymeric binder comprises a secondary binder that can be removed from the solidified injected composition by heating the solidified injected composition to a temperature of up to 600 degrees Celsius.

[0090] A twelfth aspect according to the eleventh aspect is wherein the secondary binder is selected from polypropylene, polyoxymethylene, polymethyl methacrylate, ethyl vinyl acetate.

[0091] The thirteenth aspect of the ninth through eleventh aspects further includes, after removing at least a portion of the primary binder, heating the solidified injected composition to a temperature of no greater than 600 degrees Celsius to remove the solid binder.

[0092] A fourteenth embodiment according to any preceding embodiment is wherein the porous sintered body is an annular filtration membrane having a shape that includes a three-dimensional tube.

[0093] A fifteenth aspect according to the fourteenth aspect, wherein the tube has a circular cross section when viewed in the direction of the tube's axis.

[0094] A sixteenth aspect according to the fourteenth aspect, wherein the tube has a non-circular cross section when viewed in the direction of the tube's axis.

[0095] A seventeenth aspect according to the sixteenth aspect, in which the non-circular cross-section includes a pleated pattern.

[0096] An eighteenth embodiment according to any preceding embodiment is wherein the porous sintered body is a three-dimensional non-tubular filtration membrane.

[0097] In a nineteenth embodiment, the liquid ejection composition comprises 50-80 volume percent of a polymer binder and 20-50 volume percent of solid inorganic particles having a relative apparent density in the range of 5-35% of the theoretical density of the solid inorganic particles, based on the total volume of the liquid ejection composition.

[0098] A twentieth aspect according to the nineteenth aspect, in which the solid inorganic particles are dendritic or fibrous and have an apparent density of less than 2.0 grams per cubic centimeter.

[0099] In a twenty-first aspect, the injection-molded porous sintered body includes sintered particles and has a porosity in the range of 50 to 80%.

[0100] A twenty-second embodiment according to the twenty-first embodiment is such that in that embodiment the particles are dendritic particles.

[0101] A twenty-third embodiment according to the twenty-first embodiment is such that in that embodiment the particles are fibrous particles.

[0102] A twenty-fourth embodiment according to the twenty-first to twenty-third embodiments is in the form of a ring-shaped filtration membrane having a shape including a three-dimensional tube.

[0103] A twenty-fifth embodiment according to the twenty-fourth embodiment, wherein the tube has a circular cross section when viewed in the direction of the tube's axis.

[0104] A twenty-sixth aspect according to the twenty-fourth aspect, wherein the tube has a non-circular cross section when viewed in the direction of the tube's axis.

[0105] A twenty-seventh aspect according to the twenty-sixth aspect, in which the non-circular cross-section includes a pleated pattern.

[0106] A twenty-eighth aspect according to the twenty-first to twenty-third aspects includes two opposing main surfaces and a thickness between the two opposing surfaces, and has a non-cylindrical shape.

[0107] A twenty-ninth embodiment according to the twenty-eighth embodiment is one in which at least one major surface has a structure including a pattern such as raised ridges, channels, or waffles.

[0108] A 30th aspect according to the 21st to 29th aspects further includes an injection mold marking.

[0109] A thirty-first aspect according to the twenty-first to twenty-ninth aspects further includes one or more of a parting line mark (formed by a parting line of the mold), an injector pin mark (formed by an injection pin of the mold), a gate mark (formed by a gate or gate opening of the mold), or a cavity number (formed by a cavity number mark on the mold surface).

Claims

1. 1. A method for producing a porous sintered body by particle injection molding of a liquid injection composition into a molded mold cavity, comprising: In the molded mold cavity, Based on the total volume of the liquid ejection composition, 50-80% by volume of at least one polymeric binder; and 20-50% by volume of solid inorganic particles having a relative apparent density in the range of 5-35% of the theoretical density of the solid inorganic particles. flowing a liquid ejection composition comprising: causing solidification of the polymeric binder within the formed mold cavity to form a solidified injected composition including a solid binder surrounding the solid inorganic particles; removing the solidified injected composition from the mold cavity; removing the solid binder from the solidified injected composition to form a porous non-sintered body; sintering the porous non-sintered body to form a porous sintered body; Including, The porous sintered body has a porosity in the range of 50 to 80%. method.

2. 10. The method of claim 1, wherein the solid inorganic particles are dendritic or fibrous and have an apparent density of less than 2.0 grams per cubic centimeter.

3. 3. The method of claim 1 or 2, wherein the polymeric binder comprises a primary binder that can be removed from the solidified jetted composition by contacting the solidified jetted composition with a liquid solvent selected from water, organic solvents, and combinations thereof at a temperature in the range of 40 degrees Celsius to 80 degrees Celsius.

4. 4. The method of any one of claims 1 to 3, wherein the polymeric binder comprises a secondary binder that can be removed from the solidified injected composition by heating the solidified injected composition to a temperature of up to 600 degrees Celsius.

5. 1. A liquid ejection composition comprising: Based on the total volume of the liquid ejection composition, 50-80% by volume of a polymer binder; 20 to 50 volume percent of solid inorganic particles having a relative apparent density in the range of 5 to 35% of the theoretical density of the solid inorganic particles; 1. A liquid ejection composition comprising:

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