Film-forming composition for cold isostatic pressing of green bodies, preparation method and use thereof

Abstract

A film-forming composition, which comprises 1) a water-soluble polymer, 2) a water-soluble film-forming aid, and 3) water. This composition is used to coat ceramic blanks, metal or alloy blanks, permanent magnet blanks or high-strength graphite material blanks, so as to form a protective film on the surface of the blanks. The blanks wrapped with the protective film are directly placed into the hydraulic cylinder of the oil pressure medium for cold isostatic pressing treatment, replacing the rubber sleeves, latex sleeves and plastic films currently widely used for protection. It improves production efficiency, reduces pollution, and lowers the post-treatment cost and production cost.

Description

Technical Field

[0001] The present invention relates to a film-forming composition for cold isostatic pressing of green bodies, especially for cold isostatic pressing of ceramic green bodies, powder metallurgy green bodies, permanent magnet green bodies or high-strength graphite material green bodies, and belongs to the field of material forming. Background Art

[0002] Cold Isostatic Pressing (CIP) is a process that, at room temperature, typically uses rubber or plastic as the jacket die material and a liquid (usually kerosene) as the pressure medium to form green bodies from powder materials such as ceramics, powder metallurgy, and high-strength graphite materials. It is used to increase the density of the green body to prepare high-performance, high-density ceramic materials, metals (or alloys), and high-strength graphite materials. The pressure usually used is 40 MPa - 630 MPa, and the temperature is room temperature.

[0003] In the prior art, when performing cold isostatic pressing on ceramic green bodies, in order to prevent the hydraulic medium from entering the ceramic and other objects, the ceramic green body must be coated and encapsulated with a plastic film, plastic sleeve, plastic bag, or rubber film. Only then can it be put into the hydraulic medium (such as kerosene) for pressure treatment. After isostatic pressing is completed, these coating materials are removed to obtain a dense ceramic green body. This process not only consumes a large amount of rubber or plastic film, but also the post-treatment cost of the oil-stained rubber or plastic is extremely high, causing environmental pollution.

[0004] CN111150729A discloses a film-forming composition, however, it is for a film-forming system for drugs, food, and health products.

[0005] CN115196978A discloses a technical solution of coating a binder coating before isostatic pressing, but does not disclose a technology to replace the plastic film.

[0006] CN107216155A and CN114874012A disclose that ceramic powder is mixed with low-molecular-weight polyvinyl alcohol and low-molecular-weight phenolic resin 2123 (number-average molecular weight 1500) as binders to complete the coating of powder particles in order to improve the initial strength and shape of the green body.

[0007] CN115837461A discloses a method for preparing a steel-bonded cemented carbide with low oxygen content. The method includes mixing and drying a solution M1 of a high molecular compound (such as phenolic resin, alkyd resin) with a steel-bonded cemented carbide composite powder to obtain a steel-bonded cemented carbide composite powder coated with a high molecular compound film, and then performing cold pressing or cold isostatic pressing on the composite powder to obtain a cold-pressed green body of steel-bonded cemented carbide; the cold-pressed green body of steel-bonded cemented carbide is sintered to obtain a vacuum-sintered or low-pressure-sintered ingot of steel-bonded cemented carbide.

[0008] CN111739729A discloses a method for manufacturing sintered neodymium iron boron, which comprises the following steps: 1) vacuum melting a neodymium iron boron permanent magnet alloy, followed by hydrogen decrepitation (HD) and jet milling; obtaining neodymium iron boron alloy powder with a particle size of 2-5 μm; 2) dissolving an organic coating agent in an organic solvent, stirring evenly to prepare an organic coating solution; the organic coating agent includes an antioxidant, a binder, a lubricant, a coupling agent and a surfactant; 3) under the protection of an inert gas, mixing the neodymium iron boron alloy powder obtained in step 1) into the organic coating solution to prepare coated neodymium iron boron alloy powder; 4) adding the coated neodymium iron boron alloy powder obtained in step 3) into a blanking mold for magnetic field forming to obtain a green compact; 5) subjecting the green compact to vacuum degreasing and sintering, and then aging treatment to obtain a finished neodymium iron boron permanent magnet material.

[0009] Chinese Utility Model Patent CN201020536Y discloses a plastic sheath for powder metallurgy cold isostatic pressing forming, which is made of a polyethylene film.

[0010] Currently, the isostatic pressing treatment of ceramic green bodies first requires encapsulating the ceramic green bodies with plastic and rubber encapsulating materials, performing isostatic pressing in a hydraulic cylinder, and then removing the encapsulating materials. This process not only has complex production procedures, but also the used encapsulating materials are attached with oil-based medium pollutants, resulting in high post-treatment costs and environmental pollution. The effect of the isostatic pressing treatment of ceramic green bodies by the existing process needs to be improved. Summary of the Invention

[0011] In order to overcome the problems existing in the prior art, the present application provides a film-forming composition or a double-coating system, which is used for the cold isostatic pressing treatment or process of green bodies in various shapes (such as spherical, spring-shaped, or irregular-shaped, etc.).

[0012] In the present application, the green body is or includes: a ceramic green body, a (powder metallurgy) metal green body (or alloy green body), a permanent magnet green body or a high-strength graphite material green body, etc. The metal green body or the (powder metallurgy) metal green body refers to the metal green body in the powder metallurgy pressing process. The permanent magnet green body refers to the permanent magnet green body other than the (powder metallurgy) metal green body.

[0013] The (ceramic, metal, permanent magnet or high-strength graphite material) green body or green compact is generally formed by pressing a mixture of (ceramic, metal, permanent magnet or high-strength graphite material) powder and a binder. The object of the film-forming composition or double-coating system of the present invention is the green body, not the powder material.

[0014] According to a first embodiment of the present invention, there is provided the use of a (higher molecular weight) water-soluble polymer (I) in the cold isostatic pressing process of a green body (such as a ceramic green body), or there is provided the use of a film-forming composition containing a (higher molecular weight) water-soluble polymer (I) in the cold isostatic pressing process or cold isostatic pressing treatment of a green body. A film-forming composition (containing a water-soluble polymer) is used to coat, encapsulate or impregnate a green body (such as a ceramic green body), and then the green body with a surface coated or covered with the film-forming composition (coating or film) is subjected to cold isostatic pressing. The film-forming composition comprises, consists essentially of, or consists of the following components: 1) a (higher molecular weight) water-soluble polymer (I), 2) a water-soluble film-forming aid (II), and 3) water.

[0015] According to a second embodiment of the present invention, there is provided a film-forming composition, that is, a film-forming composition (which can also be called a coating composition, a paint composition, a coating composition, or a liquid film-forming composition) for the cold isostatic pressing treatment (or cold isostatic pressing process) of a green body (such as a ceramic green body). The film-forming composition comprises, consists essentially of, or consists of the following components: 1) a (higher molecular weight) water-soluble polymer (I), 2) a water-soluble film-forming aid (II), and 3) water.

[0016] According to a third embodiment of the present invention, there is provided a dual coating system (for the cold isostatic pressing process of a green body such as a ceramic green body), which comprises: 1) a base coating (or primer coat, or base coating composition), and 2) the above-mentioned film-forming composition (as a top coating). The base coating (or base coat, or base coating composition) comprises or is: an aqueous solution of a (higher molecular weight) water-soluble polymer (I) (i.e., the first layer coating solution A). Preferably, the (higher molecular weight) water-soluble polymer (I) in the base coating is polyethylene glycol (PEG). Among them, 2) the above-mentioned film-forming composition is used as the second coating or the second layer coating (i.e., the second layer coating solution B, or also called the top coating or coating).

[0017] Preferably, in the above first, second, and third embodiments of the present application, there is provided a (liquid) film-forming composition containing a (higher molecular weight) water-soluble polymer (I) (i.e., a film-forming agent, or also called a water-soluble film-forming polymer), a water-soluble film-forming aid (II), and water, which replaces a plastic or rubber wrapping material or covering material in the cold isostatic pressing process of a green body such as a ceramic green body. That is, there is provided the use of a (liquid) film-forming composition containing a (higher molecular weight) water-soluble polymer (I), a water-soluble film-forming aid (II), and water, which is used to coat or encapsulate a green body (such as a ceramic green body) instead of a plastic or rubber wrapping material in the cold isostatic pressing process of a green body such as a ceramic green body.

[0018] (Higher molecular weight) water-soluble polymer (I)

[0019] Generally, the above-mentioned (higher molecular weight) water-soluble (film-forming) polymer (I) is in a solid state at room temperature or at normal temperature, for example, in powder form.

[0020] Preferably, as a film-forming agent, considering the film-forming performance and the strength of the film layer, the (number average) molecular weight of the (higher molecular weight) water-soluble polymer (or, water-soluble film-forming polymer) (I) is between 7,000 and 170,000 (Daltons), preferably between 8,000 and 165,000, preferably between 9,000 and 163,000, preferably between 10,000 and 160,000, preferably between 11,000 and 155,000, preferably between 12,000 and 153,000, preferably between 13,000 and 151,000, preferably between 15,000 and 150,000, preferably between 20,000 and 145,000, preferably between 30,000 and 140,000, preferably between 40,000 and 130,000, preferably between 50,000 and 120,000, preferably between 60,000 and 110,000, preferably between 70,000 and 100,000, such as 80,000 or 90,000.

[0021] Preferably, the (higher molecular weight) water-soluble polymer (or, water-soluble film-forming polymer) (I) or its type is one or two or more selected from the following components: polyvinyl alcohol (PVA) and its copolymers, polyvinylpyrrolidone (PVP) and its copolymers, polyethylene glycol (PEG) and its copolymers [such as polyethylene glycol - polypropylene glycol (random or block) copolymers (PEG-PPG)], polyethylene oxide and its copolymers, polypropylene glycol (PPG) and its copolymers, water-soluble phenolic resins, water-soluble polyurethane resins, water-soluble polyacrylates (such as polyhydroxymethyl acrylate or hydroxyethyl methacrylate) and their copolymers, polyacrylamides and their copolymers (such as copolymers of acrylamide, N,N'-methylenebisacrylamide, sodium acrylsulfonate, and acrylic acid), polyvinylimidazoles and their copolymers, polyvinylimidazolines and their copolymers, polystyrenesulfonic acid and its copolymers, poly(2-acrylamido-2-methyl-1-propanesulfonic acid) and its copolymers, polyvinylphosphonic acid and its copolymers, poly(N-hydroxyethylacrylamide) and its copolymers, polyaspartic acid and its copolymers, polyacrylic acid (salts, such as sodium salt) and its copolymers (such as acrylic acid / maleic anhydride copolymer), polymaleic anhydride and its copolymers, polymaleates and their copolymers, polyquaternary ammonium salts (such as polydiallyldialkylammonium chloride, polyacrylamidoalkylammonium chloride, polyacryloyloxyalkylammonium chloride, polydienylpropyl dimethylammonium chloride, or poly(methacryloyloxyethyltrimethylammonium chloride)) and their copolymers, polyamidoates (such as polyamic acid triethylamine salt) and their copolymers, or, polyvinylpyridine and its copolymers. The above water-soluble polymers have the property of forming a film on the surface of the green body.

[0022] Preferably, considering better film-forming properties and film layer strength (such as the property of forming a high-strength and stable film layer on a relatively large green body) as well as film layer denseness and better barrier properties against oil media, the (higher molecular weight) water-soluble polymer (or, water-soluble film-forming polymer) (I) is one or two or more selected from polyvinyl alcohol (PVA) or its copolymers, polyvinylpyrrolidone (PVP) or its copolymers, polyethylene glycol (PEG) or its copolymers, polyethylene oxide or its copolymers, polypropylene glycol (PPG) or its copolymers, water-soluble phenolic resins, water-soluble polyurethane resins, water-soluble polyacrylates (such as polyhydroxymethyl acrylate or hydroxyethyl methacrylate).

[0023] More preferably, the (higher molecular weight) water-soluble polymer (or, water-soluble film-forming polymer) (I) is one or two or more selected from polyvinyl alcohol (PVA) or its copolymers, polyvinylpyrrolidone (PVP) or its copolymers, polyethylene glycol (PEG) or its copolymers, polyethylene oxide or its copolymers, water-soluble polyurethane resins, water-soluble polyacrylates (such as polyhydroxymethyl acrylate or hydroxyethyl methacrylate).

[0024] Water-soluble film-forming aid (II)

[0025] The water-soluble film-forming aid (II) is: a water-soluble organic solvent (IIa), or, a mixture or combination comprising a water-soluble organic solvent (IIa) and a (lower molecular weight) water-soluble polymer (IIb) [such as (liquid) polyethylene glycol] or consisting of both. Generally, the (lower molecular weight) water-soluble polymer (IIb), such as a lower molecular weight (liquid) polyethylene glycol, is in a liquid state at room temperature or at normal temperature. Generally, the lower molecular weight water-soluble polymer (IIb) alone or by itself is difficult to form a film layer with sufficient strength (capable of withstanding the pressure in cold isostatic pressing treatment or process) on the surface of the green body, but it can assist the casting and film formation of the higher molecular weight water-soluble polymer (I) in the film-forming composition.

[0026] Preferably, the water-soluble film-forming aid (II) is: a water-soluble organic solvent (IIa); or, a mixture or combination comprising a water-soluble organic solvent (IIa) and a (lower molecular weight) water-soluble polymer (IIb) [such as (liquid) polyethylene glycol] or a mixture or combination consisting of both (i.e., IIa and IIb). The (number average) molecular weight of the (lower molecular weight) water-soluble polymer [such as (liquid) polyethylene glycol] (IIb) is 300 (Dalton) - 6000 (Dalton), preferably 350 - 5500, preferably 380 - 5200, preferably 400 - 5000, preferably 430 - 4700, preferably 450 (Dalton) - 4500 (Dalton), for example, 500, 600, 700, 800, 900, 1000, 1500, 2000, 2500, 3000, 3500 or 4000. Here, the (lower molecular weight) water-soluble polymer (IIb) is the same type of water-soluble polymer as those listed for the above (higher molecular weight) water-soluble polymer (I) (such as polyvinyl alcohol and its copolymers, polyvinylpyrrolidone and its copolymers, etc.), only the molecular weight is different, that is, the types of (IIb) and (I) can be the same but the molecular weights are different. The above-mentioned (higher molecular weight) water-soluble polymer (I) is a water-soluble polymer that can form a film (i.e., is easy to form a film) on the surface of the green body and the strength of the formed film can (or is sufficient to) meet the needs (or strength requirements) of cold isostatic pressing, while the (lower molecular weight) water-soluble polymer (IIb) is a water-soluble polymer that cannot form a film (i.e., is not easy to form a film) or can form a film but the strength of the formed film cannot meet the needs (or strength requirements) of cold isostatic pressing on the surface of the green body. Therefore, the (lower molecular weight) water-soluble polymer (IIb) can assist the (higher molecular weight) water-soluble polymer (I) to form a film on the surface of the green body and the strength of the formed film can (or is sufficient to) meet the needs or strength requirements of cold isostatic pressing.

[0027] When the water-soluble film-forming aid (II) is a mixture or combination comprising a water-soluble organic solvent (IIa) and a (lower molecular weight) water-soluble polymer (IIb) or consisting of both, the weight ratio of the water-soluble organic solvent (IIa) to the (lower molecular weight) water-soluble polymer (IIb) is any ratio, for example, the weight ratio of (IIa) to (IIb) is 0.01 - 100:1, preferably 0.025 - 50:1, preferably 0.033 - 30:1, preferably 0.04 - 25:1, preferably 0.05 - 20:1, preferably 0.066 - 15:1, preferably 0.08 - 12:1, preferably 0.1 - 10:1, preferably 0.2 - 5:1, preferably 0.25 - 4:1, preferably 0.33 - 3:1, preferably 0.4 - 2.5:1, preferably 0.5 - 2:1, preferably 0.75 - 1.5:1, preferably 0.8 - 1.25:1.

[0028] Preferably, in the (one-component type) film-forming composition, the content or amount of water is sufficient to dissolve the (higher molecular weight) water-soluble polymer (I) to form an aqueous solution of the (higher molecular weight) water-soluble polymer (I), that is, the content or amount of water is pre-used to dissolve the (higher molecular weight) water-soluble polymer (I) to form an aqueous solution of the (higher molecular weight) water-soluble polymer (I), and then the aqueous solution of the (higher molecular weight) water-soluble polymer (I) is mixed with the water-soluble organic solvent and optional other components (such as a binder, an antifoaming agent).

[0029] Preferably, in the primer coating of the (two-component type) two-coat system, the content or amount of water is sufficient to dissolve the (higher molecular weight) water-soluble polymer (I) to form an aqueous solution of the (higher molecular weight) water-soluble polymer (I), that is, the content or amount of water is pre-used to dissolve the (higher molecular weight) water-soluble polymer (I) to form an aqueous solution of the (higher molecular weight) water-soluble polymer (I) (i.e., the first-layer coating solution A, or, the primer coating). Additionally, in the film-forming composition of the two-coat system, the content or amount of water is sufficient to dissolve the (higher molecular weight) water-soluble polymer (I) to form an aqueous solution of the (higher molecular weight) water-soluble polymer (I), that is, the content or amount of water is pre-used to dissolve the (higher molecular weight) water-soluble polymer (I) to form an aqueous solution of the (higher molecular weight) water-soluble polymer (I). Then the aqueous solution of the (higher molecular weight) water-soluble polymer (I) is mixed with the water-soluble organic solvent and optional other components (such as a binder, an antifoaming agent) to obtain the film-forming composition (i.e., the second-layer coating solution B). When using the two-coat system, the primer coating is pre-coated on the green body (such as a ceramic green body), and then the green body with the bottom coating (such as a ceramic green body) is further coated with the film-forming composition.

[0030] "Optional" means with or without. Green body and blank have the same meaning.

[0031] Preferably, the film-forming composition comprises (or consists of, consists essentially of) the following components:

[0032] 1) 2-40 wt%, preferably 2.5-38 wt%, preferably 3-37 wt%, preferably 3.5-36 wt%, preferably 4-35 wt%, preferably 4.5-34 wt%, preferably 5-33 wt%, preferably 5.5-32 wt%, preferably 6-31 wt%, preferably 6.5-30 wt%, preferably 7-29 wt%, preferably 7.5-28 wt%, preferably 8-27 wt%, preferably 8.5-26 wt%, preferably 9-25 wt%, preferably 9.5-24 wt%, preferably 10-23 wt%, preferably 10.5-22 wt%, e.g., 11, 12, 14, 15, 17, 18 or 20 wt%, of a (higher molecular weight) water-soluble polymer (I), based on the total weight of the film-forming composition;

[0033] 2) 5-45 wt%, preferably 5.5-44.5 wt%, preferably 6-44 wt%, preferably 6.5-43.5 wt%, preferably 7-43 wt%, preferably 7.5-42.5 wt%, preferably 8-42 wt%, preferably 8.5-41.5 wt%, preferably 9-41 wt%, preferably 9.5-40.5 wt%, preferably 10-40 wt%, preferably 10.5-39.5 wt%, preferably 11-39 wt%, preferably 11.5-38.5 wt%, preferably 12-38 wt%, preferably 12.5-37.5 wt%, preferably 13-37 wt%, preferably 13.5-36.5 wt%, e.g., 14, 15, 16, 17, 18, 20, 21, 22, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34 or 35 wt%, of a water-soluble film-forming aid (II), based on the total weight of the film-forming composition;

[0034] 3) water, wherein the amount or content (wt) of water is sufficient to dissolve the (higher molecular weight) water-soluble polymer (I) (i.e., component 1 above). Obviously, since the amount or content (wt) of water in the film-forming composition is sufficient to dissolve the (higher molecular weight) water-soluble polymer (I), it is also sufficient to dissolve the lower molecular weight water-soluble polymer (IIb) in the water-soluble film-forming aid (II).

[0035] Preferably, in the above film-forming composition or in the above substrate coating (or substrate coating composition), the ratio of the content or amount (wt) of water to the content or amount (wt) of the (higher molecular weight) water-soluble polymer (I), or the weight ratio of water to the (higher molecular weight) water-soluble polymer (I), is 1.5 - 20:1, preferably 1.55 - 18:1, preferably 1.6 - 16:1, preferably 1.65 - 14:1, preferably 1.7 - 12:1, preferably 1.75 - 10:1, preferably 1.8 - 9:1, preferably 1.85 - 8:1, preferably 1.9 - 7:1, preferably 1.95 - 6:1, preferably 2 - 5:1, preferably 1.55 - 4.5:1, preferably 1.6 - 4:1, preferably 1.65 - 3.8:1, preferably 1.7 - 3.5:1, preferably 1.75 - 3.4:1, preferably 1.8 - 3.3:1, for example, 2:1, 2.5:1, 3:1 or 3.2:1.

[0036] Generally, the sum of the weights of component 1), component 2) and component 3) is 85 - 100 wt% of the weight of the film-forming composition, preferably 86 - 99.5 wt%, more preferably 87 - 99 wt%, preferably 88 - 98.5 wt%, preferably 89 - 98%, preferably 90 - 97.5 wt%, for example, 91, 92, 93, 94, 95, 96, 97 wt%.

[0037] In addition, the film-forming composition further comprises (or contains):

[0038] 4) A binder of less than 8 wt%, preferably 0.01 - 8 wt%, preferably 0.02 - 7.7 wt%, preferably 0.025 - 7.5 wt%, preferably 0.03 - 7 wt%, preferably 0.04 - 6.5 wt%, preferably 0.045 - 6 wt%, preferably 0.05 - 5.8 wt%, preferably 0.07 - 5.5 wt%, preferably 0.08 - 5.2 wt%, preferably 0.1 - 5 wt%, preferably 0.12 - 4.8 wt%, preferably 0.15 - 4.5 wt%, preferably 0.18 - 4.2 wt%, preferably 0.2 - 4 wt%, preferably 0.22 - 3.8 wt%, preferably 0.25 - 3.5 wt%, preferably 0.28 - 3.3 wt%, preferably 0.3 - 3 wt%, for example 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9 wt%. The wt% is based on the total weight of the film-forming composition.

[0039] In addition, the film-forming composition further comprises (or contains):

[0040] 5) Defoamers at less than 0.4 wt%, preferably 0.003 - 0.4 wt%, preferably 0.0035 - 0.35 wt%, preferably 0.004 - 0.3 wt%, preferably 0.005 - 0.28 wt%, preferably 0.006 - 0.25 wt%, preferably 0.007 - 0.22 wt%, preferably 0.008 - 0.2 wt%, preferably 0.009 - 0.18 wt%, preferably 0.01 - 0.17 wt%, preferably 0.011 - 0.16 wt%, preferably 0.012 - 0.15 wt%, preferably 0.013 - 0.14 wt%, preferably 0.014 - 0.13 wt%, preferably 0.015 - 0.12 wt%, preferably 0.016 - 0.11 wt%, preferably 0.017 - 0.1 wt%, such as 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09 wt%. The wt% is based on the total weight of the film-forming composition.

[0041] Preferably, the film-forming composition comprises (or consists essentially of or consists of): 1) a (higher molecular weight) water-soluble polymer (I), 2) a water-soluble film-forming aid (II), 3) water, 4) a binder.

[0042] Preferably, the film-forming composition comprises (or consists essentially of or consists of): 1) a (higher molecular weight) water-soluble polymer (I), 2) a water-soluble film-forming aid (II), 3) water, 4) a binder, 5) a defoamer.

[0043] Preferably, the binder is one or more selected from carboxymethyl cellulose, hydroxymethyl cellulose, hydroxyethyl cellulose, starch, agar, gum arabic, tragacanth gum, guar gum, carrageenan, pectin, gelatin, casein, chitosan, xanthan gum, gellan gum, or hyaluronic acid.

[0044] Preferably, the defoamer is one or more selected from low molecular weight polypropylene glycol (PPG), silicone defoamers (such as PDMS (polydimethylsiloxane)), and low molecular weight polyethylene glycol (PEG).

[0045] According to the fourth embodiment of the present invention, there is provided a method for preparing the above film-forming composition, the method comprising:

[0046] 1) Dissolving the (higher molecular weight) water-soluble polymer (I) in water to form an aqueous solution of the (higher molecular weight) water-soluble polymer (I). The concentration of the resulting solution is generally in the range of 5 - 45 wt%, preferably 7 - 43 wt%, preferably 10 - 40 wt%, preferably 12 - 37 wt%, preferably 15 - 35 wt%, preferably 17 - 33 wt%, more preferably 20 - 30 wt%.

[0047] 2) Add a water-soluble film-forming aid (II) to the aqueous solution obtained in step 1, and (stir) to obtain a mixture. Generally, the amount of the water-soluble film-forming aid (II) is in the range of 9-44 wt%, preferably in the range of 10-40 wt%, preferably 12-37 wt%, preferably 15-35 wt%, preferably 17-33 wt%, more preferably in the range of 20-30 wt%, based on the total weight of the film-forming composition.

[0048] Preferably, the method for preparing the above film-forming composition further comprises:

[0049] 3) Add a binder to the mixture formed in step 2), and (stir) to obtain a thickened mixture. The amount of the binder is as described above.

[0050] Preferably, the method for preparing the above film-forming composition further comprises:

[0051] 4) Add an antifoaming agent to the thickened mixture formed in step 3), and (stir) to obtain a defoamed mixture. The amount of the antifoaming agent is as described above.

[0052] According to the fifth embodiment of the present invention, there is provided a method for preparing the above double coating system, the method comprising:

[0053] A) Preparation of the base coating: Dissolve a (higher molecular weight) water-soluble polymer (I) in water to form an aqueous solution of the (higher molecular weight) water-soluble polymer (I). The weight ratio of the two is as described above. For example, in the above base coating, the ratio of the content or amount (wt) of water to the content or amount (wt) of the (higher molecular weight) water-soluble polymer (I), or the weight ratio of water to the (higher molecular weight) water-soluble polymer (I), is 1.5-20:1, preferably 1.55-18:1, preferably 1.6-16:1, preferably 1.65-14:1, preferably 1.7-12:1, preferably 1.75-10:1, preferably 1.8-9:1, preferably 1.85-8:1, preferably 1.9-7:1, preferably 1.95-6:1, preferably 2-5:1, preferably 1.55-4.5:1, preferably 1.6-4:1, preferably 1.65-3.8:1, preferably 1.7-3.5:1, preferably 1.75-3.4:1, preferably 1.8-3.3:1, for example, 2:1, 2.5:1, 3:1 or 3.2:1;

[0054] B) Preparation of the film-forming composition:

[0055] 1) Dissolve a (higher molecular weight) water-soluble polymer (I) in water to form an aqueous solution of the (higher molecular weight) water-soluble polymer (I). The concentration of the resulting solution is generally in the range of 5-45 wt%, preferably 7-43 wt%, preferably 10-40 wt%, preferably 12-37 wt%, preferably 15-35 wt%, preferably 17-33 wt%, more preferably 20-30 wt%.

[0056] 2) Add a water-soluble film-forming adjuvant (II) to the aqueous solution obtained in step 1 and (stir) to obtain a mixture. Generally, the amount of the water-soluble film-forming adjuvant (II) is in the range of 9-44 wt%, preferably 10-40 wt%, preferably 12-37 wt%, preferably 15-35 wt%, preferably 17-33 wt%, more preferably 20-30 wt%, based on the total weight of the film-forming composition.

[0057] In this application, "optional" means with or without.

[0058] Preferably, the method for preparing the above double coating system further includes:

[0059] 3) Add a binder to the mixture formed in step 2) and (stir) to obtain a tackified mixture. The amount of the binder is as described above.

[0060] Preferably, the method for preparing the above double coating system further includes:

[0061] 4) Add an antifoaming agent to the tackified mixture formed in step 3) and (stir) to obtain a defoamed mixture. The amount of the antifoaming agent is as described above.

[0062] The green body to be processed has various shapes, such as (quasi-) spherical, ellipsoidal (egg-shaped), cubic, cuboid, plate-shaped, sheet-shaped, olive-shaped, spring-shaped, leaf spring-shaped, rod-shaped (cylindrical, prismatic), conical, frustum-shaped, or (various) special-shaped parts. The size (diameter or length) of the green body can be from 1 mm to 5 m (meters), preferably 3 mm to 4 m, preferably 5 mm to 3 m, preferably 7 mm to 2 m, preferably 8 mm to 1.8 m, preferably 9 mm to 1.6 m, preferably 1 cm to 1.5 m, preferably 1.1 cm to 1.2 m, preferably 1.2 cm to 1 m, preferably 1.3 cm to 0.9 m, preferably 1.4 cm to 0.7 m, preferably 1.5 cm to 0.5 m, preferably 1.6 cm to 0.3 m, preferably 1.7 cm to 0.2 m, for example, 1.8, 2.0, 2.5, 3, 3.5, 4, 4.5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 50, 60, 70, 80 or 90 cm, which is particularly effective for small-sized green bodies.

[0063] In this application, ceramics include: structural ceramics such as alumina ceramics, zirconia ceramics, silicon nitride ceramics, and silicon carbide ceramics; functional ceramics such as piezoelectric ceramics, ferroelectric ceramics, pyroelectric ceramics, infrared ceramics, and transparent ceramics.

[0064] Metals include all metals and alloys formed by powder metallurgy pressing processes.

[0065] Generally, the surface of the ceramic green body is coated with a coating of a film-forming composition or a coating of a dual-coating system. The thickness of the coating is generally 0.05 - 10 mm, preferably 0.055 - 8 mm, preferably 0.06 - 6 mm, preferably 0.07 - 5 mm, preferably 0.08 - 4 mm, preferably 0.085 - 3 mm, preferably 0.09 - 2 mm, preferably 0.1 - 1 mm, preferably 0.15 - 0.9 mm, preferably 0.2 - 0.8 mm, preferably 0.25 - 0.7 mm, preferably 0.3 - 0.6 mm, preferably 0.35 - 0.55 mm, preferably 0.4 - 0.5 mm.

[0066] In this application, the water-soluble organic solvent (or water-soluble high-boiling organic solvent) in the water-soluble film-forming aid is selected from any one of the following organic solvents A, B, and C, or a mixture of any two or more of organic solvents A, B, and C:

[0067] Organic solvent A: ethylene glycol, propylene glycol, 1,2-butanediol, 2,3-butanediol, 2-methyl-1,3-propanediol, 2-methyl-2,4-pentanediol, 1,2,6-hexanetriol, 1,2,3-butanetriol, 1,2,4-butanetriol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 1,2-pentanediol, 2,4-pentanediol, 2-methyl-1,3-butanediol, 3-methyl-1,3-butanediol, 1,5-pentanediol, 2,2-dimethyl-1,3-propanediol, 1,2-hexanediol, 1,6-hexanediol, 2,5-hexanediol, 2-ethyl-1,3-hexanediol, diethylene glycol, dipropylene glycol, triethylene glycol, tripropylene glycol, tetraethylene glycol, glycerol or pentaerythritol;

[0068] Organic solvent B: ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, ethylene glycol n-butyl ether, ethylene glycol n-pentyl ether, ethylene glycol n-hexyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol n-propyl ether, propylene glycol-n-butyl ether, propylene glycol-tert-butyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol n-propyl ether, diethylene glycol n-butyl ether, diethylene glycol-n-hexyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, dipropylene glycol n-propyl ether, dipropylene glycol-n-butyl ether, dipropylene glycol-tert-butyl ether, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, triethylene glycol monopropyl ether, triethylene glycol n-butyl ether, tripropylene glycol monomethyl ether, tripropylene glycol monoethyl ether, tripropylene glycol n-propyl ether, tripropylene glycol-n-butyl ether, tripropylene glycol-tert-butyl ether, tetraethylene glycol monomethyl ether, tetraethylene glycol monoethyl ether, tetraethylene glycol monopropyl ether, tetraethylene glycol n-butyl ether, tetrapropylene glycol monomethyl ether, tetrapropylene glycol monoethyl ether, tetrapropylene glycol n-propyl ether, tetrapropylene glycol-n-butyl ether, tetrapropylene glycol-tert-butyl ether, ethylene glycol phenyl ether or propylene glycol phenyl ether;

[0069] Organic solvent C: N-methyl-2-pyrrolidone, 2-pyrrolidone, 1,3-dimethylimidazolidinone, N-methylpyrrolidone, ε-caprolactam, N-methylformamide or N,N-dimethylformamide.

[0070] In the present application, generally, cold isostatic pressing is carried out at a pressure of 40 MPa - 630 MPa, for example 150, 180, 200, 220, 250, 280, 300, 320, 350, 380, 400, 450, 500, 550 or 600 MPa. The temperature of cold isostatic pressing is usually room temperature (for example 10 - 40 °C, such as 20 - 30 °C or 25 °C).

[0071] Preferably, in the film-forming composition of the present application or in the film-forming composition of the double coating system, the mass (or weight) ratio of the higher molecular weight water-soluble polymer (I) to the sum of the mass (or weight) of the higher molecular weight water-soluble polymer (I) and the lower molecular weight water-soluble polymer (IIb) is 50-100 wt%, preferably 55-98 wt%, preferably 60-97 wt%, preferably 65-96 wt%, preferably 70-95 wt%, preferably 75-94 wt%, preferably 77-93 wt%, preferably 80-92 wt%, such as 53, 57, 63, 67, 72, 74, 76, 78, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91 wt%.

[0072] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0073] 1. For the first time, the solution of the higher molecular weight water-soluble polymer is used for the coating of the green body in cold isostatic pressing, replacing the currently widely used rubber sleeves, latex sleeves and plastic films, reducing pollution, and reducing the post-treatment cost and production cost.

[0074] 2. Rapid encapsulation of the green body is achieved, and the coating is easy to remove. The process flow of cold isostatic pressing is shortened, and the efficiency is improved.

[0075] 3. Using the high molecular coating material as the cold isostatic pressing encapsulation material is efficient, environmentally friendly and low-cost.

[0076] 4. The film-forming composition of the present invention exists in the form of a colloidal solution, and a protective coating can be formed on the surface of the green body to be cold isostatic pressed after coating at room temperature. The green body protected by the coating can be directly cold isostatically pressed in an oil medium, and the protective film layer remains intact under a (static) pressure of 200 MPa. The high molecular coating (or coating film) can effectively transfer the pressure of the liquid to densify the green body. At the same time, the protective coating can shrink together with the green body without separation, and the isolation effect is good.

[0077] 5. Particularly crucial is that the higher molecular weight water-soluble polymer (I) is combined with the lower molecular weight polyethylene glycol (IIb) as a film-forming aid and a water-soluble organic solvent (such as glycerol). Utilizing the hydrophilic (water-absorbing) characteristics of its molecular structure, the water in the solution is effectively fixed (or associated) in the molecular chain structure, and a dense film layer is formed on the surface of the green body. Not only ensures the isolation effect between the oil medium and the green body, but also the water in the solution will not be released into the green body. Ensures that the quality of the green body is not affected during the cold isostatic pressing process. Description of the Drawings

[0078] Figure 1 : Photograph of the ceramic green body after cold isostatic pressing in Example 1.

[0079] Figure 2 : Cross-sectional photograph of the ceramic green body after cold isostatic pressing in Example 1.

[0080] Figure 3 : Cross-sectional photograph of the ceramic green body after cold isostatic pressing in Example 2.

[0081] Figure 4 : A large amount of contaminated plastic film will be produced in the cold isostatic pressing process of Comparative Example 1.

[0082] Figure 5 : Cross-sectional view of the special-shaped part after cold isostatic pressing in Example 4 (no penetration of kerosene medium).

[0083] Figure 6 : Cross-sectional view of the penetration and contamination of the ceramic green body by kerosene in Comparative Example 3.

[0084] Figure 7 : Cross-sectional view of the penetration and contamination of the ceramic green body by kerosene in Comparative Example 4.

[0085] Figure 8 : Cross-sectional view of the penetration and contamination of the ceramic green body by kerosene in Comparative Example 5. Detailed implementation manners

[0086] The technical solutions of the present invention will be illustrated by examples below. The scope claimed by the present invention includes but is not limited to the following examples.

[0087] Method for checking whether the coating is complete:

[0088] Immerse the coated small balls in alcohol with a concentration of 95% for 5 minutes, and judge whether the coating of the balls is complete by comparing the weights before and after immersion.

[0089] Example 1

[0090] Preparation and application of the double coating system:

[0091] 1) First, weigh 500 grams of polyethylene glycol (number average molecular weight 10,000), add it to 1000 grams of water, heat the beaker in a water bath to 60 °C, and stir until the polyethylene glycol is completely dissolved to obtain the first layer coating solution A for standby.

[0092] 2) Prepare a mixed solution of PVA and PVP: Add 20 grams of PVA1788 (polyvinyl alcohol, alcoholysis degree 88 mol%, powdery), 160 grams of PVA0588 (polyvinyl alcohol, number average molecular weight 24,000, weight average molecular weight 30,000, powdery), and 50 grams of PVP (polyvinylpyrrolidone, number average molecular weight 45,000, powdery) into 450 grams of water in a beaker, heat the water bath temperature to 45 °C, and stir simultaneously until the solids are completely dissolved.

[0093] 3) Gradually add 30 g of polyethylene glycol (number-average molecular weight 5000) and 300 g of glycerol as film-forming aids during stirring. The reaction temperature is 80 °C, and the mixture is stirred simultaneously.

[0094] 4) Gradually add 40 g of carboxymethyl cellulose (number-average molecular weight 95000) after stirring for 1 hour, and continue stirring at 80 °C for 2 hours.

[0095] 5) Add 1 g of polyethylene glycol (molecular weight 200 Da) as an antifoaming agent, complete the reaction after stirring for 5 minutes, place the solution in a constant-temperature drying oven, and continue stirring at 85 °C until it becomes a clear and transparent colloidal liquid B.

[0096] 6) Cold isostatic pressing of the ceramic green body: First, dip-coat the silicon nitride ceramic green body in the shape of a 3-cm-diameter sphere with the first layer of solution A, take it out with tweezers, place it on a polytetrafluoroethylene plate, and air-dry (dry in the air); then, dip-coat it with a layer of solution B, take it out with tweezers, place it on a polytetrafluoroethylene plate, and air-dry (dry in the air); the coating thickness on the surface of the green body is 0.45 mm, and then put it into the container (hydraulic cylinder) of the cold isostatic pressing equipment, and perform cold isostatic pressing treatment in a kerosene medium under a static pressure of 200 MPa. The photo of the ceramic green body after cold isostatic pressing is shown in Figure 1 。

[0097] After coating by dip-coating, the thickness of the coating on the green body is measured to be 0.45 mm.

[0098] After cold isostatic pressing treatment, 100 green body samples are taken and inspected one by one, and no damage to the surface coating of the green body is found. After soaking in 95% concentration of alcohol, it is found that the coating of the ceramic spheres is complete.

[0099] Use a hammer to strike the ceramic green body after cold isostatic pressing treatment to check the fractured cross-section. The cross-sectional photo of the ceramic green body after cold isostatic pressing is as shown in Figure 2 shown in. After isostatic pressing, the surface of the green body is clean and tidy, and there is no kerosene penetration. This indicates that the kerosene medium does not penetrate into the ceramic green body during cold isostatic pressing, which shows that the coating has a good barrier effect on kerosene.

[0100] Example 2

[0101] 1) First, prepare a water-soluble polymer solution: First, weigh 50 g of polyethylene glycol 10000, 130 g of PVA1788, and 40 g of PVP polyvinylpyrrolidone (number-average molecular weight 45000), add them to 450 g of water in a beaker, heat the water bath temperature to 60 °C, and stir simultaneously until the solids are completely dissolved.

[0102] 2) Gradually add 320 g of glycerol as a film-forming aid during stirring, with the reaction temperature at 70 °C, and stir evenly at the same time.

[0103] 3) After stirring for 0.5 h, gradually add 40 g of corn starch as a binder, and continue stirring at 80 °C for 2.5 h.

[0104] 4) Add 0.1 g of polypropylene glycol (molecular weight 450 Da) as an antifoaming agent, stir for 5 minutes, and then add 0.5 g of polyethylene glycol (molecular weight 400 Da) as an antifoaming agent to complete the reaction and obtain a film-forming composition.

[0105] 5) Cold isostatic pressing of the ceramic green body: Dip a 3-cm-diameter spherical silicon nitride green body into the film-forming composition, with the coating thickness on the surface of the green body being 0.5 mm, and then place it into the container (hydraulic cylinder) of the cold isostatic pressing equipment, and carry out cold isostatic pressing treatment in a kerosene medium under a static pressure of 200 MPa.

[0106] After coating with the film-forming composition, the thickness of the coating on the green body was measured to be 0.5 mm.

[0107] After cold isostatic pressing treatment in a kerosene medium under a static pressure of 200 MPa, take 100 green body samples and check them one by one. No damage to the surface coating of the green body was found. After soaking in 95% concentration alcohol, it was found that the coating of the ceramic spheres was intact.

[0108] Use a hammer to strike the ceramic green body after cold isostatic pressing treatment and check the fractured cross-section. The cross-sectional photograph of the ceramic green body after cold isostatic pressing treatment is as Figure 3 shown. After isostatic pressing, the surface of the green body is clean and tidy, without kerosene penetration. No kerosene traces were found, indicating that the kerosene medium did not penetrate into the ceramic green body during the cold isostatic pressing process, which shows that the coating has a good barrier effect on kerosene.

[0109] Comparative Example 1

[0110] Use a polyethylene (PE) plastic film to coat a 3-cm-diameter spherical silicon nitride green body. Then place it into the container of the cold isostatic pressing equipment, and carry out cold isostatic pressing treatment in a kerosene medium under a static pressure of 200 MPa.

[0111] In addition, a large amount of contaminated plastic film was generated in the cold isostatic pressing process of the prior art in Comparative Example 1. See Figure 4 .

[0112] Example 3

[0113] Repeat Example 2, except that a wire rod of a silicon nitride ceramic green body with a diameter of 5 mm and a length of 30 cm is used instead of a spherical silicon nitride ceramic green body with a diameter of 3 cm. After cold isostatic pressing, the tensile strength of the wire rod of the silicon nitride ceramic green body is 47.5 MPa, and the elongation at break is 36.6%.

[0114] Comparative Example 2

[0115] Repeat Comparative Example 1, except that a wire rod of a silicon nitride ceramic green body with a diameter of 5 mm and a length of 30 cm is used instead of a spherical silicon nitride ceramic green body with a diameter of 3 cm. After cold isostatic pressing, the tensile strength of the wire rod of the silicon nitride ceramic green body is 44.7 MPa, and the elongation at break is 33.4%.

[0116] The inventors compared the tensile strength and elongation at break of the wire rod in Example 3 with those of the wire rod in Comparative Example 2 and found that, when comparing the cold isostatic pressing treatment of the ceramic green body using a liquid film instead of a plastic film in Example 3 of the present invention with the cold isostatic pressing treatment using a plastic film in the prior art, the strength performance of the ceramic green body treated with the technical solution of the present application is superior to that of the ceramic green body treated in Comparative Example 2.

[0117] Example 4

[0118] Repeat Example 2, except that a T-shaped special-shaped part (length 10 cm) of a silicon nitride ceramic green body is used. Photographs of the special-shaped parts after cold isostatic pressing are shown respectively in Figure 5 . From the attached Figure 5 It can be seen that when the coating of the film-forming composition is used for the cold isostatic pressing treatment of the special-shaped parts of the ceramic green body, there is no phenomenon of kerosene penetration, and the effect is very good.

[0119] Example 5

[0120] Repeat Example 1, except that a green alloy blank in the shape of a rod (40 cm in length and 4 cm in diameter) formed by pressing titanium alloy powder is used instead of the silicon nitride ceramic green body.

[0121] After cold isostatic pressing, 100 green body samples were taken and inspected one by one, and no damage to the coating on the surface of the green body was found. After soaking in 95% concentration of alcohol, it was found through inspection that the coating of the green body was intact.

[0122] The green body after cold isostatic pressing was hammered with a hammer, and the fractured cross-section was inspected. No kerosene traces were found, indicating that the kerosene medium did not penetrate into the green body during the cold isostatic pressing process, which shows that the coating has a good barrier effect on kerosene.

[0123] Example 6

[0124] Repeat Example 2, except that a green body of samarium-cobalt-iron permanent magnet is used instead of the silicon nitride ceramic green body. The green body is obtained as follows: A magnetic powder mixture consisting of magnetic powder (average particle size 4 microns) of samarium-cobalt alloy (25.5 wt% Sm, 52 wt% Co, 12.5 wt% Fe, 7 wt% Cu and 3 wt% Zr) and 3 wt% of No. 120 aviation gasoline relative to the mass of the magnetic powder is oriented and compression molded in a magnetic field of 1.6 T under a pressure of 5 MPa to obtain a green body of samarium-cobalt-iron permanent magnet with dimensions of 120 mm (front-back direction) * 65 mm (pressing direction) * 58 mm (magnetization direction), and the density is 3.8 g / cm 3 .

[0125] After cold isostatic pressing treatment, 100 green body samples are taken and inspected one by one, and no damage to the surface coating of the green body is found. After soaking in 95% concentration alcohol, it is found that the coating of the green body is complete.

[0126] The green body after cold isostatic pressing treatment is hammered with a hammer, and the fractured cross-section is inspected. No kerosene trace is found, which indicates that the kerosene medium does not penetrate into the green body during the cold isostatic pressing process, which shows that the coating has a good barrier effect on kerosene.

[0127] Example 7

[0128] Repeat Example 1, except that a green body of magnet formed by pressing neodymium-iron-boron alloy powder is used instead of the silicon nitride ceramic green body.

[0129] After cold isostatic pressing treatment, 100 green body samples are taken and inspected one by one, and no damage to the surface coating of the green body is found. After soaking in 95% concentration alcohol, it is found that the coating of the green body is complete.

[0130] The green body after cold isostatic pressing treatment is hammered with a hammer, and the fractured cross-section is inspected. No kerosene trace is found, which indicates that the kerosene medium does not penetrate into the green body during the cold isostatic pressing process, which shows that the coating has a good barrier effect on kerosene.

[0131] Comparative Example 3

[0132] 1) First, prepare a water-soluble polymer solution: First, weigh 220 grams of polyethylene glycol (number average molecular weight 4000) and add it to 450 grams of water in a beaker. The water bath temperature is heated to 60 °C and stirred simultaneously until the solid matter is completely dissolved.

[0133] 2) Gradually add 320 grams of glycerol as a film-forming aid during stirring, and the reaction temperature is 70 °C while stirring evenly.

[0134] 3) After stirring for 0.5 hours, gradually add 40 grams of corn starch as a binder and continue stirring at 80 °C for 2.5 hours.

[0135] 4) Add 0.1 g of polypropylene glycol (molecular weight 450 Daltons) as an antifoaming agent. After stirring for 5 minutes, add 0.5 g of polyethylene glycol (molecular weight 400 Daltons) as an antifoaming agent to complete the reaction and obtain the film-forming composition.

[0136] 5) Cold isostatic pressing of the ceramic green body: Dip a 3-cm-diameter spherical silicon nitride ceramic green body into the film-forming composition. The coating thickness on the surface of the green body is 0.48 mm. Then place it into the container (hydraulic cylinder) of the cold isostatic pressing equipment and perform cold isostatic pressing treatment in a kerosene medium under a static pressure of 200 MPa.

[0137] After cold isostatic pressing treatment in a kerosene medium under a static pressure of 200 MPa, take 20 green body samples and check them one by one. It is found that the surface coatings of 9 green bodies are damaged.

[0138] Use a hammer to strike the ceramic green bodies whose surface coatings are found to be damaged after cold isostatic pressing treatment, and check the fractured cross-section. As Figure 6 shown, the cross-sectional photograph reveals the penetration and contamination of kerosene into the ceramic green body.

[0139] This Comparative Example 3 shows that when the molecular weight of the water-soluble polymer is relatively low, the strength of the film layer and the barrier performance against oil media will be affected.

[0140] Comparative Example 4

[0141] 1) First, prepare a water-soluble polymer solution: Weigh 50 g of polyethylene glycol 10000, 130 g of carboxymethyl cellulose (number-average molecular weight 95000), and 40 g of PVP polyvinylpyrrolidone (number-average molecular weight 45000) and add them to 450 g of water in a beaker. Heat the water bath temperature to 60 °C and stir simultaneously until the solids are completely dissolved.

[0142] 2) Gradually add 320 g of glycerol as a film-forming aid during the stirring process. The reaction temperature is 70 °C, and stir evenly at the same time.

[0143] 3) After stirring for 0.5 hours, gradually add 40 g of corn starch as a binder and continue stirring at 80 °C for 2.5 hours.

[0144] 4) Add 0.1 g of polypropylene glycol (molecular weight 450 Daltons) as an antifoaming agent. After stirring for 5 minutes, add 0.5 g of polyethylene glycol (molecular weight 400 Daltons) as an antifoaming agent to complete the reaction and obtain the film-forming composition.

[0145] 5) Cold isostatic pressing of the ceramic green body: A silicon nitride ceramic green body in the shape of a sphere with a diameter of 3 cm was dip-coated with the film-forming composition, and the coating thickness on the surface of the green body was 0.51 mm. Then it was placed into the container (hydraulic cylinder) of the cold isostatic pressing equipment, and cold isostatic pressing was carried out in a kerosene medium under a static pressure of 200 MPa.

[0146] After cold isostatic pressing was carried out in a kerosene medium under a static pressure of 200 MPa, 20 green body samples were taken and inspected one by one, and it was found that the surface coatings of 5 green bodies were damaged.

[0147] The ceramic green bodies with damaged surface coatings after cold isostatic pressing were hammered with a hammer, and the fractured cross-sections were inspected. As Figure 7 shown, the cross-sectional photos revealed the penetration and contamination of the kerosene into the ceramic green bodies.

[0148] This Comparative Example 4 shows that when a relatively large amount of a water-soluble polymer with poor film-forming properties, such as carboxymethyl cellulose, is used as the main film-forming agent, the strength of the coating film layer and the barrier performance against oil media will decrease.

[0149] Comparative Example 5

[0150] 1) First, prepare a water-soluble polymer solution: First, weigh 50 grams of polyethylene glycol 10000, 130 grams of phenolic resin 2123 (number-average molecular weight 1500), and 40 grams of PVP polyvinylpyrrolidone (number-average molecular weight 45000), and add them to 450 grams of water in a beaker. The water bath temperature was heated to 60 °C, and stirring was carried out simultaneously until the solids were completely dissolved.

[0151] 2) During the stirring process, 320 grams of glycerol as a film-forming aid was gradually added, and the reaction temperature was 70 °C, while stirring evenly.

[0152] 3) After stirring for 0.5 hours, 40 grams of corn starch as a binder was gradually added, and stirring was continued at 80 °C for 2.5 hours.

[0153] 4) 0.1 gram of polypropylene glycol (molecular weight 450 Da) as an antifoaming agent was added, and after stirring for 5 minutes, 0.5 gram of polyethylene glycol (molecular weight 400 Da) as an antifoaming agent was added to complete the reaction, obtaining the film-forming composition.

[0154] 5) Cold isostatic pressing of the ceramic green body: A silicon nitride ceramic green body in the shape of a sphere with a diameter of 3 cm was dip-coated with the film-forming composition, and the coating thickness on the surface of the green body was 0.46 mm. Then it was placed into the container (hydraulic cylinder) of the cold isostatic pressing equipment, and cold isostatic pressing was carried out in a kerosene medium under a static pressure of 200 MPa.

[0155] After cold isostatic pressing treatment at a static pressure of 200 MPa in a kerosene medium, 20 green body samples were taken and inspected one by one, and it was found that the surface coatings of 6 green bodies were damaged.

[0156] The ceramic green bodies with damaged surface coatings after cold isostatic pressing treatment were hammered with a hammer, and the fractured cross-sections were inspected. As Figure 8 shown, the cross-sectional photographs revealed the penetration and contamination of the kerosene into the ceramic green bodies.

[0157] This Comparative Example 5 shows that when a relatively large amount of a water-soluble polymer with a lower molecular weight, such as phenolic resin 2123, is used as the main film-forming agent, the strength of the coating film layer and the barrier performance against oil media will be affected.

[0158] Comparative Example 6

[0159] Example 2 was repeated, except that the same amount of methanol was used instead of glycerol.

[0160] After cold isostatic pressing treatment at a static pressure of 200 MPa in a kerosene medium, 20 green body samples were taken and inspected one by one, and it was found that the surface coatings of 2 green bodies were damaged.

[0161] This Comparative Example 6 shows that methanol is not an ideal film-forming aid.

[0162] In summary, in all Examples 1 - 7 of the present invention, after soaking with 95% concentration of alcohol, it was found through inspection that the coating of the green body was intact. Therefore, by using a water-soluble polymer with good film-forming properties and a suitable film-forming aid, a dense and relatively high-strength wet coating film layer can be formed on the surface of the green body, so that it can be used in cold isostatic pressing treatment to withstand a relatively high pressure and there is no penetration and contamination of oil media.

[0163] Example 8

[0164] Example 1 was repeated, except that cold isostatic pressing treatment was carried out at a static pressure of 400 MPa in a kerosene medium.

[0165] After cold isostatic pressing treatment, 100 green body samples were taken and inspected one by one, and no damage to the surface coating of the green body was found. After soaking with 95% concentration of alcohol, it was found through inspection that the coating of the ceramic spheres was intact.

[0166] Example 9

[0167] Example 2 was repeated, except that cold isostatic pressing treatment was carried out at a static pressure of 400 MPa in a kerosene medium.

[0168] After cold isostatic pressing treatment, 100 green body samples were taken and inspected one by one, and no damage to the surface coating of the green body was found. After soaking in 95% concentration alcohol, it was found that the coating of the ceramic spheres was intact.

Claims

1. A green body coating for cold isostatic pressing, formed by coating a film-forming composition on the surface of the green body, the film-forming composition comprising, or consisting of, the following components: 1) 2-40 wt% of a water-soluble polymer (I), based on the total weight of the film-forming composition; 2) 5-45 wt% of a water-soluble film-forming aid (II), based on the total weight of the film-forming composition; 3) water, wherein the amount of water is sufficient to dissolve the water-soluble polymer (I); wherein the film-forming composition is used to coat the green body and is subjected to cold isostatic pressing together with the green body, and the cold isostatic pressing is carried out in an oil medium; The water-soluble polymer (I) is one or two or more selected from the following components: polyvinyl alcohol, polyvinylpyrrolidone, polyethylene glycol; the number-average molecular weight of the water-soluble polymer (I) is between 7000 and 170000; The water-soluble film-forming aid (II) is: a water-soluble organic solvent (IIa), or, a mixture comprising a water-soluble organic solvent (IIa) and a lower molecular weight water-soluble polymer (IIb), wherein the lower molecular weight water-soluble polymer (IIb) is polyethylene glycol having a number-average molecular weight of 300-6000; The water-soluble organic solvent (IIa) is any one selected from organic solvent A, organic solvent B, and organic solvent C, or a mixture of any two or more selected from organic solvent A, organic solvent B, and organic solvent C; Organic solvent A: ethylene glycol, propylene glycol, 1,2-butanediol, 2,3-butanediol, 2-methyl-1,3-propanediol, 2-methyl-2,4-pentanediol, 1,2,6-hexanetriol, 1,2,3-butanetriol, 1,2,4-butanetriol, 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 1,2-pentanediol, 2,4-pentanediol, 2-methyl-1,3-butanediol, 3-methyl-1,3-butanediol, 1,5-pentanediol, 2,2-dimethyl-1,3-propanediol, 1,2-hexanediol, 1,6-hexanediol, 2,5-hexanediol, 2-ethyl-1,3-hexanediol, diethylene glycol, dipropylene glycol, triethylene glycol, tripropylene glycol, tetraethylene glycol, glycerol or pentaerythritol; Organic solvent B: ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, ethylene glycol monon-butyl ether, ethylene glycol monon-pentyl ether, ethylene glycol monon-hexyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monopropyl ether, propylene glycol-n-butyl ether, propylene glycol-tert-butyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monopropyl ether, diethylene glycol monon-butyl ether, diethylene glycol-n-hexyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, dipropylene glycol monopropyl ether, dipropylene glycol-n-butyl ether, dipropylene glycol-tert-butyl ether, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, triethylene glycol monopropyl ether, triethylene glycol monon-butyl ether, tripropylene glycol monomethyl ether, tripropylene glycol monoethyl ether, tripropylene glycol monopropyl ether, tripropylene glycol-n-butyl ether, tripropylene glycol-tert-butyl ether, tetraethylene glycol monomethyl ether, tetraethylene glycol monoethyl ether, tetraethylene glycol monopropyl ether, tetraethylene glycol monon-butyl ether, tetrapropylene glycol monomethyl ether, tetrapropylene glycol monoethyl ether, tetrapropylene glycol monopropyl ether, tetrapropylene glycol-n-butyl ether, tetrapropylene glycol-tert-butyl ether, ethylene glycol phenyl ether or propylene glycol phenyl ether; Organic solvent C: 2-pyrrolidone, 1,3-dimethylimidazolidinone, N-methylpyrrolidone, ε-caprolactam, N-methylformamide or N,N-dimethylformamide.

2. The green body coating according to claim 1, wherein: The number average molecular weight of the water-soluble polymer (I) is between 10,000 and 160,000; and / or The number average molecular weight of the lower molecular weight water-soluble polymer (IIb) is 350 - 5500.

3. The green body coating according to claim 2, wherein: The number average molecular weight of the water-soluble polymer (I) is between 15,000 and 150,000; and / or The number average molecular weight of the lower molecular weight water-soluble polymer (IIb) is 380 - 5200.

4. The green body coating according to claim 3, wherein: The number average molecular weight of the water-soluble polymer (I) is between 20,000 and 150,000; and / or The number average molecular weight of the lower molecular weight water-soluble polymer (IIb) is 400 - 5000.

5. The green body coating according to claim 1, characterized in that: The water-soluble film-forming aid (II) is a mixture comprising a water-soluble organic solvent (IIa) and a water-soluble polymer (IIb), and the weight ratio of the water-soluble organic solvent (IIa) to the lower molecular weight water-soluble polymer (IIb) is 0.01-100:1; and / or In the film-forming composition, the mass ratio of the water-soluble polymer (I) to the sum of the masses of the water-soluble polymer (I) and the lower molecular weight water-soluble polymer (IIb) is 50-100 wt%.

6. The green body coating according to claim 5, wherein: The weight ratio of the water-soluble organic solvent (IIa) to the lower molecular weight water-soluble polymer (IIb) is 0.025-50:1; and / or The mass ratio of the water-soluble polymer (I) to the sum of the masses of the water-soluble polymer (I) and the lower molecular weight water-soluble polymer (IIb) is 55-98 wt%.

7. The green body coating according to claim 6, characterized in that: The weight ratio of the water-soluble organic solvent (IIa) to the lower molecular weight water-soluble polymer (IIb) is 0.033-30:1; and / or The mass ratio of the water-soluble polymer (I) to the sum of the masses of the water-soluble polymer (I) and the lower molecular weight water-soluble polymer (IIb) is 60-97 wt%.

8. The green body coating according to claim 1, wherein: The described film-forming composition comprises, or consists of, the following components: 1) 2.5-38 wt% of a water-soluble polymer (I), based on the total weight of the film-forming composition; 2) 5.5-44.5 wt% of a water-soluble film-forming aid (II), based on the total weight of the film-forming composition; 3) Water, wherein the amount of water is sufficient to dissolve the water-soluble polymer (I).

9. The green body coating according to claim 8, characterized in that: The described film-forming composition comprises, or consists of, the following components: 1) 3-37 wt% of a water-soluble polymer (I), based on the total weight of the film-forming composition; 2) 6-44 wt% of a water-soluble film-forming aid (II), based on the total weight of the film-forming composition; 3) Water, wherein the amount of water is sufficient to dissolve the water-soluble polymer (I).

10. The green body coating according to claim 9, characterized in that: The described film-forming composition comprises, or consists of, the following components: 1) 3.5-36 wt% of a water-soluble polymer (I), based on the total weight of the film-forming composition; 2) 6.5-43.5 wt% of a water-soluble film-forming aid (II), based on the total weight of the film-forming composition; 3) Water, wherein the amount of water is sufficient to dissolve the water-soluble polymer (I).

11. The green body coating according to claim 10, characterized in that: The described film-forming composition comprises, or consists of, the following components: 1) 4-35 wt% of a water-soluble polymer (I), based on the total weight of the film-forming composition; 2) 7-43 wt% of a water-soluble film-forming aid (II), based on the total weight of the film-forming composition; 3) Water, wherein the amount of water is sufficient to dissolve the water-soluble polymer (I).

12. The green body coating according to claim 1, characterized in that: In the above film-forming composition, the mass ratio of water to the water-soluble polymer (I) is 1.5-20:1; and / or The sum of the weights of component 1), component 2) and component 3) is 85-100 wt% of the weight of the film-forming composition.

13. The green body coating according to claim 12, characterized in that: In the above film-forming composition, the mass ratio of water to the water-soluble polymer (I) is 1.55 - 18:1; and / or The sum of the weights of component 1), component 2) and component 3) is 86 - 99.5 wt% of the weight of the film-forming composition.

14. The green body coating according to claim 13, characterized in that: In the above film-forming composition, the mass ratio of water to the water-soluble polymer (I) is 1.6 - 16:1; and / or The sum of the weights of component 1), component 2) and component 3) is 87 - 99 wt% of the weight of the film-forming composition.

15. The green body coating according to claim 14, wherein: In the above film-forming composition, the mass ratio of water to the water-soluble polymer (I) is 1.65 - 14:1; and / or The sum of the weights of component 1), component 2) and component 3) is 88 - 98.5 wt% of the weight of the film-forming composition.

16. The green body coating according to claim 1, characterized in that: The film-forming composition further comprises: 4) a binder of less than 8 wt%; this wt% is based on the total weight of the film-forming composition; and / or 5) an antifoaming agent of less than 0.4 wt%; this wt% is based on the total weight of the film-forming composition.

17. The green body coating according to claim 16, wherein: The film-forming composition further comprises: 4) a binder of 0.01 - 8 wt%; this wt% is based on the total weight of the film-forming composition; and / or 5) an antifoaming agent of 0.003 - 0.4 wt%; this wt% is based on the total weight of the film-forming composition.

18. The green body coating according to claim 17, wherein: The film-forming composition further comprises: 4) a binder of 0.02 - 7.7 wt%; this wt% is based on the total weight of the film-forming composition; and / or 5) an antifoaming agent of 0.0035 - 0.35 wt%; this wt% is based on the total weight of the film-forming composition.

19. The green body coating according to claim 18, wherein: The film-forming composition further comprises: 4) a binder of 0.025 - 7.5 wt%; this wt% is based on the total weight of the film-forming composition; and / or 5) an antifoaming agent of 0.004 - 0.3 wt%; this wt% is based on the total weight of the film-forming composition.

20. The green body coating according to any one of claims 16-19, characterized in that: The binder is one or more selected from carboxymethyl cellulose, hydroxymethyl cellulose, hydroxyethyl cellulose, gum arabic, gelatin and starch; The antifoaming agent is one or more selected from low molecular weight polypropylene glycol, silicone oil-based antifoaming agents, and low molecular weight polyethylene glycol.

21. The green body coating according to claim 1, wherein: The green body comprises: a ceramic green body, a metal green body, a permanent magnet green body or a high-strength graphite material green body.

22. A method for preparing a coating on the green body according to any one of claims 1 - 21, the method comprising: 1) dissolving the water-soluble polymer (I) in water to form an aqueous solution of the water-soluble polymer (I); 2) adding a water-soluble film-forming aid (II) to the aqueous solution obtained in step 1) to obtain a mixture.

23. The method according to claim 22, characterized in that: The method for preparing a coating on the green body further comprises: 3) adding a binder to the mixture formed in step 2) to obtain a thickened mixture.

24. The method according to claim 23, wherein: The method for preparing a coating on the green body further comprises: 4) adding an antifoaming agent to the thickened mixture formed in step 3) to obtain a defoamed mixture.

25. The method according to claim 22, wherein: The concentration of the aqueous solution obtained in step 1) is in the range of 5 - 45 wt%.

26. The method according to claim 25, wherein: The concentration of the aqueous solution obtained in step 1) is in the range of 7 - 43 wt%.

27. The method according to claim 26, wherein: The concentration of the aqueous solution obtained in step 1) is in the range of 10 - 40 wt%.

28. A double - coating system for the cold isostatic pressing process of green bodies, the double - coating system comprising: A) A base coating, where the base coating comprises or is: an aqueous solution of a water - soluble polymer (I), and the water - soluble polymer (I) is one or two or more selected from the following components: polyvinyl alcohol, polyvinylpyrrolidone, polyethylene glycol; the number - average molecular weight of the water - soluble polymer (I) is between 7000 and 170000; and B) The green - body coating as described in any one of claims 1 - 21.

29. The dual coating system according to claim 28, characterized in that: In the base coating of the double - coating system, the amount of water is sufficient to dissolve the water - soluble polymer (I) to form an aqueous solution of the water - soluble polymer (I).

30. A method for preparing the double - coating system as described in any one of claims 28 - 29, the method comprising: A) Preparation of the base coating: Dissolving the water - soluble polymer (I) in water to form an aqueous solution of the water - soluble polymer (I); B) Preparation of the green - body coating: 1) Dissolving the water - soluble polymer (I) in water to form an aqueous solution of the water - soluble polymer (I); 2) Adding a water - soluble film - forming auxiliary agent (II) to the aqueous solution obtained in step 1) to obtain a mixture.

31. The method according to claim 30, wherein: The method for preparing the green - body coating further comprises: 3) Adding a binder to the mixture formed in step 2) to obtain a thickened mixture.

32. The method according to claim 31, wherein: The method for preparing the green - body coating further comprises: 4) Adding an antifoaming agent to the thickened mixture formed in step 3) to obtain a defoamed mixture.

33. The method according to claim 30, characterized in that: In the base coating obtained in step A), the mass ratio of water to the water - soluble polymer (I) is 1.5 - 20:1; and / or The concentration of the aqueous solution obtained in step 1) is in the range of 5 - 45 wt%; and / or The amount of the water - soluble film - forming auxiliary agent (II) in step 2) is in the range of 9 - 44 wt%, based on the total weight of the green - body coating.

34. The method according to claim 33, wherein: In the base coating obtained in step A), the mass ratio of water to the water - soluble polymer (I) is 1.55 - 18:1; and / or The concentration of the aqueous solution obtained in step 1) is in the range of 7 - 43 wt%; and / or The amount of the water - soluble film - forming auxiliary agent (II) in step 2) is in the range of 10 - 40 wt%, based on the total weight of the green - body coating.

35. A green body, the surface of the green body being coated with the green - body coating as described in any one of claims 1 - 21.

36. The green body according to claim 35, characterized in that: The thickness of the green - body coating is 0.05 - 10 mm.

37. The green body according to claim 36, characterized in that: The thickness of the green - body coating is 0.055 - 8 mm.

38. The green body according to claim 37, characterized in that: The thickness of the green - body coating is 0.06 - 6 mm.

39. The green body according to claim 35, characterized in that: The green body comprises: A ceramic green body, a metal green body, a permanent - magnet green body or a high - strength graphite - material green body.

40. A green body, the surface of the green body being coated with the double - coating system as described in any one of claims 28 - 29.

41. The green body according to claim 40, characterized in that: The thickness of the double - coating system is 0.05 - 10 mm.

42. The green body according to claim 41, characterized in that: The thickness of the double - coating system is 0.055 - 8 mm.

43. The green body according to claim 42, wherein: The thickness of the double - coating system is 0.06 - 6 mm.

44. The green body according to claim 40, characterized in that: The green body is or comprises: a ceramic green body, a metal green body, a permanent - magnet green body or a high - strength graphite - material green body.

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