Low-shrinkage anti-cracking water-soluble all-inorganic salt core bonding glue and preparation method thereof

Abstract

The application provides a low-shrinkage anti-cracking water-soluble all-inorganic salt core bonding glue and a preparation method thereof, and belongs to the technical field of inorganic adhesives.The bonding glue is prepared by compounding a paste-like inorganic mortar with silica sol as a liquid-phase film-forming matrix, aluminum oxide powder as a skeleton reinforcing filler and boric acid as a flexible anti-cracking water-soluble modifier.The Si-O-B flexible crosslinking network is constructed in situ by the boric acid, and the industry problems of the traditional silica sol glue layer, such as great brittleness, easy cracking due to shrinkage during cooling, and poor cold and hot cycle stability of the pure inorganic glue, are solved;meanwhile, the all-inorganic reversible water-soluble characteristics of the system are retained, the cured glue layer can be completely powdered and removed by hot water immersion, and there is no stubborn glue layer residue.The formula of the application is simple, the construction can be carried out at room temperature, the curing shrinkage rate is extremely low, the room-temperature bonding strength is high, the cold and hot impact resistance is excellent, and the application is suitable for the room-temperature splicing of die-casting salt cores and cast water-soluble salt core components, salt core repair and repair, salt core forming positioning and fixing and the like.

Description

Technical Field

[0001] This invention relates to the field of inorganic adhesives, specifically to a low-shrinkage, crack-resistant, water-soluble, all-inorganic salt core adhesive and its preparation method. Background Technology

[0002] Salt cores are core components formed by pressing and sintering water-soluble inorganic salts. Due to their rapid dissolution and disintegration upon contact with water, they are widely used in the internal cavity forming processes of aluminum alloy die casting, automotive oil passage housings, and engine cavity castings. During the salt core pressing, assembly, repair, and module assembly processes, adhesives are required for temporary fixation and positioning of the salt core blank.

[0003] Currently, salt core adhesives on the market are mainly divided into two categories: organic adhesives and traditional inorganic adhesives, both of which have obvious technical defects. 1. Organic adhesives (epoxy AB glue, instant adhesive, polyurethane glue, etc.): They have high bonding strength and good toughness at room temperature, but after curing, they form a dense polymer cross-linked structure that is completely insoluble in water and non-degradable. When castings are water-cooled and cored, the organic adhesives cannot dissolve synchronously with the salt core, leaving a stubborn carbonized adhesive layer that easily clogs the casting's flow channels and oil passages, causing batches of castings to be scrapped. They are completely unsuitable for the production of precision die-casting salt cores.

[0004] 2. Traditional silicate (water glass) inorganic adhesives: They are inexpensive and water-soluble, but the adhesive layer has a very high curing shrinkage rate, and the thermal stress is concentrated during the cooling process, making the adhesive seams very easy to crack and fall off; at the same time, the system contains sodium ions, which are prone to alkali return and whitening, which changes the overall dissolution rate of the salt core and affects the molding quality of the inner cavity of the casting.

[0005] 3. Phosphate inorganic adhesive: It has high bonding strength at room temperature and can be pulverized in hot water, but it has poor adaptability to hot and cold temperatures. After high and low temperature cycles, the adhesive layer is prone to pulverization and failure. The bonding joint cracks prematurely during the die casting preheating stage, resulting in poor production stability.

[0006] 4. Pure silica sol adhesive: It has low curing shrinkage and high temperature resistance, but the pure silica layer has a rigid and brittle structure, which is prone to cracking during the cooling process, has poor impact resistance, insufficient initial tack at room temperature, and extremely low bonding reliability when used alone.

[0007] In summary, among the currently available publicly available technologies, there is no dedicated salt core adhesive that can simultaneously meet the requirements of high-strength bonding at room temperature, low curing shrinkage, no cracking under thermal cycling, complete powdering with no residue in hot water, and being entirely inorganic and not interfering with the salt core dissolution rate. Summary of the Invention

[0008] This invention aims to overcome the shortcomings of existing salt core adhesives, such as easy cracking during thermal cycling, residual adhesive clogging of casting channels, poor bonding stability, and interference with the salt core dissolution rate. It provides a low-shrinkage, crack-resistant, water-soluble, all-inorganic salt core adhesive and its preparation method. The adhesive of this invention uses silica sol as the liquid-phase film-forming matrix, alumina powder as the skeleton reinforcing filler, and boric acid as a flexible, crack-resistant, water-soluble modifier, compounded to form a paste-like inorganic adhesive. This invention solves the industry problems of high brittleness, easy cracking due to cooling shrinkage, and poor thermal cycling stability of traditional silica sol adhesives by constructing a Si-OB flexible cross-linking network in situ with boric acid. Simultaneously, it retains the system's all-inorganic reversible water-soluble characteristics; the cured adhesive layer can be completely powdered and removed by hot water immersion, leaving no stubborn adhesive residue. The formula of this invention is simple, can be applied at room temperature, has extremely low curing shrinkage, high bonding strength at room temperature, and excellent resistance to thermal shock. It is suitable for applications such as room temperature splicing of die-cast salt cores and cast water-soluble salt core components, salt core blank repair, and salt core forming and positioning. It has extremely high industrial practical value and promising prospects for promotion.

[0009] To achieve the above objectives, the present invention adopts the following technical solution: a low-shrinkage, crack-resistant, water-soluble, all-inorganic salt core adhesive, which is prepared by mixing liquid ammonium silica sol and composite solid powder at a mass ratio of 1:0.99 to 1.01; the composite solid powder is composed of calcined α-alumina powder and boric acid powder.

[0010] Preferably, the composite solid powder is formulated in the following proportions by mass: 92-95 parts of calcined α-alumina powder and 5-8 parts of boric acid.

[0011] Preferably, the ammonium silica sol has a solid content of 28% to 32%, a nano-silica particle size of 10 to 20 nm, a pH value of 8.0 to 9.0, and is free of sodium ion impurities, thus avoiding alkali return of the sol layer and changes in the salt core dissolution rate.

[0012] Preferably, the calcined α-alumina powder is 300-400 mesh, with a purity of ≥99%, and its thermal expansion coefficient matches that of the salt core matrix, reducing the curing shrinkage of the adhesive layer. It also has strong chemical inertness and does not contaminate the molten aluminum at high temperatures during die casting.

[0013] This invention also discloses a method for preparing a low-shrinkage, crack-resistant, water-soluble, all-inorganic salt core adhesive, characterized by comprising the following steps: (1) Solid phase premixing: Take calcined α-alumina powder and boric acid powder according to the ratio, put them into a high-speed powder mixer, stir at a uniform speed for 15-20 minutes, and after mixing evenly, seal and store to obtain composite solid phase powder; (2) Mixing and molding: At room temperature, slowly pour ammonium silica sol into the composite solid powder and stir at low speed for 3 to 5 minutes until a uniform paste-like slurry without dry powder or particles is formed. (3) Letting stand to defoam: Let the prepared putty stand at room temperature for 2-3 minutes to eliminate the internal stirring bubbles, and the preparation is complete. It can be directly used for salt core bonding construction.

[0014] The stirring speed in step (1) is 200-300 r / min, and the stirring is carried out in a closed environment at room temperature to avoid rapid evaporation of moisture.

[0015] This invention also discloses the application of a low-shrinkage, crack-resistant, water-soluble, all-inorganic salt core adhesive, used for room temperature splicing of die-cast salt cores and cast water-soluble salt core components, salt core blank repair, and salt core forming, positioning, and fixing processes.

[0016] The core mechanism of action of this invention: 1. Silica sol film formation and bonding mechanism The surface of the nano-silica particles inside the ammonium-type silica sol carries a large number of active silanol groups (-Si-OH). After the sol is applied to the surface of the salt core, the moisture in the system evaporates slowly and uniformly. The silanol groups undergo intermolecular dehydration condensation to construct a continuous and dense three-dimensional inorganic network structure of Si-O-Si, which can form chemical bonds with the hydroxyl groups on the surface of the inorganic salt matrix of the salt core, achieving high-strength room-temperature adhesion with no gaps and good sealing performance at the bonding interface.

[0017] 2. Mechanism of alumina powder skeleton reinforcement and shrinkage reduction Calcined α-alumina powder exhibits high mechanical strength and strong thermal stability, with its coefficient of thermal expansion closely matching that of the inorganic salt core. The powder fills the pores of the silica sol gel network, forming a rigid skeletal support structure that significantly reduces curing shrinkage and thermal shrinkage of the adhesive layer, physically preventing cracking due to cooling shrinkage. Simultaneously, it enhances the hardness and load-bearing capacity of the adhesive layer. As an inert inorganic powder, alumina does not release harmful gases at high die-casting temperatures. During water-cooling core removal, it disperses into powder along with the salt core and is discharged from the mold cavity.

[0018] 3. Boric acid flexible crosslinking + reversible water-soluble modification mechanism Boric acid is the core modifying agent of this invention, possessing dual key functions: 1) Anti-cracking buffer function: During the dehydration and curing stage, boric acid can undergo a cross-linking reaction with silanol to generate Si-OB flexible boron-oxygen cross-linking bonds, breaking the single rigid and brittle structure of pure silica sol. A rigid-flexible composite network is formed inside the adhesive layer, which effectively buffers the thermal expansion and contraction stress generated by temperature rise and fall, and completely solves the problem of self-cracking and thermal cycling cracking of traditional inorganic adhesives, ensuring the integrity of the bonding throughout the entire process of salt core transfer, preheating, and die casting.

[0019] 2) Reversible water-soluble degumming function: The boron-oxygen crosslinking bond has the characteristic of reversible hydrolysis. When the casting is water-cooled and cored after molding, the boron-oxygen bond breaks first, the dense glue layer is loosened and disintegrated as a whole, and completely decomposed into inorganic powders of silicon dioxide and aluminum oxide, which are discharged with the salt core dissolving water flow. There is no carbonized glue residue or hard glue film, which prevents the casting from being scrapped due to blockage of oil passages and small holes.

[0020] 4. Liquid-to-solid ratio of approximately 1:1, suitable for compatibility mechanisms. Strictly controlling the liquid-to-solid mass ratio to approximately 1:1 is crucial for the optimal performance of this invention. When the liquid-to-solid ratio is >0.99–1.01, there is an excess of liquid silica sol, leading to a prolonged curing period, a softer adhesive layer, and insufficient bond strength. Excess water will also dilute the surface of the salt core, causing premature local dissolution. When the liquid-to-solid ratio is <0.99–1.01, there is too much solid powder, resulting in a dry and non-flowable adhesive paste, making application difficult, causing poor bonding at the interface, and resulting in gaps in the adhesive. An approximately 1:1 ratio forms a uniform paste-like adhesive that balances application fluidity, curing density, bond strength, and crack resistance without damaging the salt core substrate.

[0021] The beneficial effects of this invention are as follows: Compared with existing salt core adhesives, this invention has five outstanding innovative advantages: 1) High bonding strength at room temperature and excellent bonding stability Relying on the dual reinforcement of Si-O-Si chemical bonding and a rigid alumina skeleton, the adhesive layer is dense and hard after room temperature curing. It has strong wettability and adhesion to various water-soluble inorganic salt cores, meeting the needs of salt core module splicing, damage repair, and temporary bonding for transportation and load-bearing.

[0022] 2) Ultra-low shrinkage, completely eliminating cooling cracking defects. Alumina powder reduces curing shrinkage, boric acid introduces flexible boron-oxygen crosslinking to buffer thermal stress, and the thermal expansion coefficient of the adhesive layer is highly matched with that of the salt core. During repeated hot and cold cycles from room temperature to 200℃ and natural long-term cooling, the adhesive layer does not crack, peel, or fall off, making it suitable for the entire process of salt core drying and preheating die casting.

[0023] 3) Fully water-soluble powdering for core removal, with no residual adhesive clogging the casting channels. The completely inorganic reversible water-soluble system requires no organic solvents, grinding, or high-temperature burning. During the water-cooling and core-removal stage after die casting, the adhesive layer loosens and pulverizes simultaneously with the salt core. The powder is discharged from the cavity with the water flow, leaving no hard adhesive film residue, which significantly reduces the rate of casting blockage and scrap.

[0024] 4) Completely inorganic, environmentally friendly, and high-temperature resistant; does not contaminate molten aluminum castings. The system contains no organic resins or organic solvents, and there are no volatile harmful substances during the curing process; there is no black smoke or carbonization products under the high temperature of die casting, and it will not cause porosity or inclusion defects in the castings, making it suitable for high pressure die casting of aluminum alloys.

[0025] 5) The preparation process is simple, the raw materials are inexpensive and easy to mass-produce industrially. All processes are completed at room temperature, without the need for heating or sintering equipment; silica sol, alumina powder, and boric acid are all common bulk chemical raw materials on the market, with simple proportions and easy mixing for application, making it suitable for batch continuous production in salt core processing plants. Detailed Implementation

[0026] The effects are illustrated below with reference to specific examples. The ammonium silica sol of this invention has a solid content of 30%, nano-silica particle size of 10-20 nm, pH value of 8.0-9.0, and is free of sodium ion impurities, thus avoiding alkali return in the sol layer and altering the salt core dissolution rate. The calcined α-alumina powder has a particle size of 300-400 mesh, purity ≥99%, and a thermal expansion coefficient matching that of the salt core matrix, reducing sol layer curing shrinkage. Furthermore, it exhibits strong chemical inertness and does not contaminate the molten aluminum at high die-casting temperatures.

[0027] General preparation process 1) Solid-phase premixing process: Weigh calcined α-alumina powder and boric acid powder according to the ratio, put them into a high-speed powder mixer, stir at a uniform speed of 300r / min for 15min, and after the powder is mixed evenly, seal and store it in a moisture-proof container to obtain composite solid-phase powder. 2) On-site glue preparation process: Under normal temperature conditions, according to the liquid-to-solid mass ratio of 1:1, slowly pour the ammonium silica sol with a solid content of 30% into the composite solid phase powder, stir at low speed for 4 minutes until there is no dry powder or particles and a uniform paste-like mortar. 3) Defoaming and standing process: Let the prepared putty stand at room temperature for 2 minutes to eliminate the internal air bubbles generated by stirring. Then it can be applied to the die-cast salt core test block and left at room temperature for 24 hours to fully cure before performance testing.

[0028] Example 1 (Optimal Example): Solid phase mass fractions: 94 parts calcined α-alumina powder, 6 parts boric acid; Liquid raw material: Ammonium silica sol with a solid content of 30%; The liquid-solid mixture has a mass ratio of 1:1.

[0029] Example 2: Solid phase mass fractions: 95 parts calcined α-alumina powder, 5 parts boric acid; the remaining raw materials and process parameters are consistent with those in Example 1.

[0030] Example 3: Solid phase mass fractions: 92 parts calcined α-alumina powder, 8 parts boric acid; the remaining raw materials and process parameters are consistent with those in Example 1.

[0031] Comparative Example 1: Pure silica sol (without alumina or boric acid modification): 30% ammonium silica sol was used to directly cure into a film.

[0032] Comparative Example 2: Silica sol + alumina compound (without boric acid, otherwise the same as Example 1).

[0033] Comparative Example 3: Traditional water glass (silicate) inorganic adhesive.

[0034] Comparative Example 4: Conventional phosphate inorganic adhesive.

[0035] The performance test results of Examples 1-3 and Comparative Examples 1-4 are shown in Table 1 below.

[0036] Table 1 Performance test results of Examples 1-3 and Comparative Examples 1-4

[0037] Performance Summary: Comparison of multiple examples and comparative examples demonstrates that the silica sol-alumina-boric acid ternary compound, liquid-solid 1:1 system of this invention achieves six core performance breakthroughs compared to existing inorganic and organic salt core adhesives: high strength, zero cooling cracking, complete water-soluble powdering with no residue, stability under thermal cycling, and no interference with salt core dissolution. This solves a long-standing technical pain point in the field of water-soluble salt core bonding for aluminum alloy die casting.

[0038] Application scenarios: Used for room temperature splicing of die-cast salt cores and cast water-soluble salt core components, salt core blank repair, salt core forming and positioning processes, etc., and is more suitable for the following scenarios: 1) Aluminum alloy die-cast water-soluble salt core separate splicing and module assembly; 2) Repairing defects in salt-core billets, filling gaps, and shaping the billets; 3) Assembly and positioning fixation before salt core drying and die casting preheating; 4) Matching processes for room temperature bonding and non-destructive water-cooled core removal of various casting water-soluble inorganic salt cores.

[0039] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, process improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A low-shrinkage, crack-resistant, water-soluble, all-inorganic salt core adhesive, characterized in that it is composed of... The liquid ammonium silica sol and the composite solid powder are mixed at a mass ratio of 1:0.99 to 1.01; the composite solid powder is composed of calcined α-alumina powder and boric acid powder.

2. The low-shrinkage, crack-resistant, water-soluble, all-inorganic salt core adhesive as described in claim 1, characterized in that, The composite solid powder is formulated in the following proportions by mass: 92-95 parts of calcined α-alumina powder and 5-8 parts of boric acid.

3. The low-shrinkage, crack-resistant, water-soluble, all-inorganic salt core adhesive as described in claim 1, characterized in that... The ammonium silica sol has a solid content of 28%–32%, a nano-silica particle size of 10–20 nm, a pH value of 8.0–9.0, and is free of sodium ion impurities.

4. The low-shrinkage, crack-resistant, water-soluble, all-inorganic salt core adhesive as described in claim 1, characterized in that... The calcined α-alumina powder has a mesh size of 300-400 and a purity of ≥99%.

5. The low-shrinkage, crack-resistant, water-soluble, all-inorganic salt core adhesive as described in claim 1, characterized in that... The mass ratio of the liquid ammonium silica sol to the composite solid powder is 1:

1.

6. The method for preparing the low-shrinkage, crack-resistant, water-soluble, all-inorganic salt core adhesive according to any one of claims 1-5, characterized in that, Includes the following steps: (1) Solid-phase premixing: Take calcined α-alumina powder and boric acid powder, put them into a high-speed powder mixer, stir at a uniform speed for 15-20 minutes, mix evenly and then seal and store to obtain composite solid-phase powder; (2) Mixing and molding: At room temperature, slowly pour ammonium silica sol into the composite solid powder and stir at low speed for 3 to 5 minutes until a uniform paste-like slurry without dry powder or particles is formed. (3) Let stand to defoam: Let the prepared clay stand at room temperature for 2-3 minutes to eliminate internal stirring bubbles.

7. The preparation method according to claim 6, wherein the stirring speed in step (1) is 200-300 r / min, and the stirring is carried out in a closed environment at room temperature to avoid rapid evaporation of moisture.

8. The application of the low-shrinkage, crack-resistant, water-soluble, all-inorganic salt core adhesive as described in any one of claims 1-5 in the room temperature splicing, salt core blank repair, and salt core molding, positioning, and fixing processes of die-cast salt cores and cast water-soluble salt core components.

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