Low-thermal-conductivity high-stability composite insulating coated sand and preparation method thereof

Abstract

The application belongs to the technical field of coated sand for casting, and discloses a composite heat-insulating coated sand with low heat conduction and high stability and a preparation method thereof.The coated sand comprises raw sand, phenolic resin, boron-silicon hybrid porous ceramic microshell modifier, heat-triggered boron-silicon-benzoxazine interface bridging microcapsule modifier, curing agent and lubricant.During preparation, the boron-silicon hybrid porous ceramic microshell modifier is mixed with preheated raw sand, and then the first part of phenolic resin, the heat-triggered interface bridging microcapsule modifier, the second part of phenolic resin, the curing agent and the lubricant are sequentially added to obtain the composite heat-insulating coated sand.The coated sand forms a layered structure of a raw sand core, a boron-silicon microshell heat-insulating inner layer, a benzoxazine bridging interface layer and a phenolic resin outer sealing layer, forms an interface bridging network in the core-making heating stage, and thus reduces heat conduction and improves thermal strength and storage stability.

Description

Technical Field

[0001] This invention belongs to the technical field of coated sand for casting, specifically relating to a composite heat-insulating coated sand with low thermal conductivity and high stability and its preparation method. Background Technology

[0002] Coated sand is an important material for preparing sand cores and sand molds in casting production. It is usually produced by thermal coating of raw sand, phenolic resin, curing agent, and lubricant. With the development of complex thin-walled castings, precision castings, and high-temperature alloy castings, higher requirements are placed on the thermal insulation performance, hot strength, dimensional stability, and storage stability of coated sand. Traditional coated sand is prone to rapid heat transfer during the pouring of molten metal, resulting in excessively rapid local temperature rise in the sand core. This leads to problems such as thermal decomposition of the resin film, weakening of the sand core strength, casting veins, sand adhesion, sintering, and dimensional deviations.

[0003] In existing technologies, to reduce the thermal conductivity of coated sand, low thermal conductivity fillers such as hollow glass microspheres, cenospheres, expanded perlite, diatomaceous earth, ceramic microspheres, or aerogel powder are often used for modification. Although these fillers can block heat transfer to a certain extent, their compatibility with phenolic resin systems is poor. Direct addition can easily lead to agglomeration, detachment, or uneven distribution, disrupting the continuity of the resin coating and causing a decrease in the room temperature and hot strength of the coated sand. Furthermore, low thermal conductivity fillers are usually randomly dispersed between sand particles, making it difficult to form a stable and continuous insulating layer on the surface of individual sand particles, thus making it difficult to achieve both insulation and reinforcement effects. In addition, existing coated sand preparation processes mostly use one-time feeding or conventional sequential coating methods, making it difficult to precisely control the spatial distribution of the insulating filler, resin layer, and interface modification components on the sand particle surface. Even when using silane coupling agents to improve the bonding between inorganic fillers and resin, it mainly remains at the level of filler surface treatment and cannot solve the problems of weakening of the resin film interface after filler introduction and interface instability during high-temperature service. Summary of the Invention

[0004] To address the shortcomings mentioned in the background art, the present invention aims to provide a composite thermal insulation coated sand with low thermal conductivity and high stability, and its preparation method. By introducing a borosilicate hybrid porous ceramic microshell modifier and a thermally triggered borosilicate-benzoxazine interface bridging microcapsule modifier, and employing a layered positioning coating process, the coated sand particles form an original sand core, a borosilicate microshell thermal insulation inner layer, a benzoxazine bridging interface layer, and a phenolic resin outer sealing layer, thereby reducing thermal conductivity and improving thermal strength and storage stability.

[0005] The objective of this invention can be achieved through the following technical solutions: A composite thermal insulation coated sand with low thermal conductivity and high stability comprises the following components by weight: 100 parts raw sand; 1.0-4.0 parts phenolic resin; 0.5-8.0 parts borosilicate hybrid porous ceramic microshell modifier; 0.1-2.0 parts thermally triggered borosilicate-benzoxazine interface bridging microcapsule modifier; 0.1-0.8 parts curing agent; and 0.03-0.5 parts lubricant. The borosilicate hybrid porous ceramic microshell modifier includes a hollow ceramic microsphere core, a borosilicate-oxygen hybrid layer covering the surface of the core, and an organosilicon compatible layer bonded to the outer surface of the borosilicate-oxygen hybrid layer; the thermally triggered borosilicate-benzoxazine interface bridging microcapsule modifier includes a capsule core containing benzoxazine and boron-containing structures, a siloxane-phenolic composite capsule wall covering the outer side of the capsule core, and an organosilicon interface layer bonded to the outer surface of the siloxane-phenolic composite capsule wall.

[0006] More preferably, the raw sand is one or more of silica sand, alumina sand, ceramsite sand, chromite sand, and zircon sand; the phenolic resin is selected from one of thermoplastic phenolic resin, a combination of thermoplastic phenolic resin and heat-resistant modified phenolic resin, and the heat-resistant modified phenolic resin is selected from one or two of boron-modified phenolic resin and organosilicon-modified phenolic resin; the curing agent is selected from one or two of hexamethylenetetramine and microencapsulated hexamethylenetetramine; and the lubricant is selected from one or more of calcium stearate, zinc stearate, polyethylene wax, paraffin wax, and ethylene bis-stearamide.

[0007] More preferably, the composite heat-insulating coated sand particles comprise, from the inside out, a raw sand core, a borosilicate microshell heat-insulating inner layer, a benzoxazine bridging interface layer, and a phenolic resin outer sealing layer; the borosilicate microshell heat-insulating inner layer contains the borosilicate hybrid porous ceramic microshell modifier, and the benzoxazine bridging interface layer contains the thermally triggered borosilicate-benzoxazine interface bridging microcapsule modifier.

[0008] More preferably, the volume median particle size (D50) of the borosilicate hybrid porous ceramic microshell modifier is 5-40 μm; the volume median particle size (D50) of the thermally triggered borosilicate-benzoxazine interface-bridged microcapsule modifier is 10-80 μm; and the mass content of the capsule core is 40-75% based on the total mass of the thermally triggered borosilicate-benzoxazine interface-bridged microcapsule modifier.

[0009] More preferably, the borosilicate hybrid porous ceramic microshell modifier is prepared from the following raw materials in parts by weight: 100 parts hollow ceramic microspheres; 5-25 parts tetraethyl orthosilicate; 1-10 parts boric acid; 1-8 parts phenyltriethoxysilane; 1-4 parts organosilane compatibilizer; 100-300 parts ethanol; 10-80 parts water; 0.1-2 parts acid catalyst; wherein the organosilane compatibilizer is selected from one or two of γ-aminopropyltriethoxysilane and γ-glycidoxypropyltrimethoxysilane; and the acid catalyst is selected from one or more of glacial acetic acid, citric acid, and oxalic acid.

[0010] More preferably, the borosilicate hybrid porous ceramic microshell modifier is prepared by the following method: tetraethyl orthosilicate, boric acid, phenyltriethoxysilane, water, ethanol, and an acid catalyst are mixed to form a borosilicate hybrid sol; hollow ceramic microspheres are added to the borosilicate hybrid sol to allow the borosilicate-oxygen hybrid layer to be deposited in situ on the surface of the hollow ceramic microspheres; an organosilane compatibilizer is added for surface compatibility treatment; and the borosilicate hybrid porous ceramic microshell modifier is obtained after drying and heat treatment.

[0011] More preferably, the thermally triggered borosilicate-benzoxazine interface bridging microcapsule modifier is prepared from the following raw materials in parts by weight: 30-50 parts cashew phenol; 10-25 parts furfurylamine; 10-25 parts paraformaldehyde; 2-8 parts phenylboronic acid; 10-30 parts tetraethyl orthosilicate; 3-12 parts phenyltriethoxysilane; 2-10 parts low molecular weight phenolic resin; 0.5-8 parts organosilane interface agent; 1-8 parts emulsifying dispersant; 100-300 parts ethanol; and 200-600 parts water.

[0012] More preferably, the thermally triggered borosilicate-benzoxazine interfacial bridging microcapsule modifier is prepared according to the following method: S101, cashew nut shell powder, furfurylamine, and paraformaldehyde are reacted in ethanol to obtain a cashew nut shell powder-furfurylamine type benzoxazine prepolymer, and then phenylboronic acid is added for boron modification treatment to obtain a core liquid containing benzoxazine structure and boron structure; S102, the core liquid is added to an aqueous phase containing an emulsifying dispersant to disperse and form a core emulsion; S103, tetraethyl orthosilicate, phenyltriethoxysilane, low molecular weight phenolic resin, ethanol, water, and acid catalyst are mixed and pre-hydrolyzed to obtain a siloxane-phenolic composite sol, and then the siloxane-phenolic composite sol is added to the core emulsion so that the siloxane-phenolic composite capsule wall covers the outside of the core; S104, an organosilane interfacial agent is added for interfacial treatment, and after separation, washing, and drying, the thermally triggered borosilicate-benzoxazine interfacial bridging microcapsule modifier is obtained.

[0013] A method for preparing composite heat-insulating coated sand includes the following steps: S1. Preheat the raw sand, add borosilicate hybrid porous ceramic microshell modifier to the preheated raw sand and mix to obtain the first mixture; S2. Add the first portion of phenolic resin to the first mixture and mix to obtain the second mixture; S3. Add the thermally triggered borosilicate-benzoxazine interface bridging microcapsule modifier to the second mixture and mix, then add the second part of phenolic resin and mix to obtain the third mixture; S4. Add curing agent and lubricant to the third mixture, and after cooling, crushing and sieving, obtain the composite heat insulation coated sand.

[0014] More preferably, before step S1, a surface activation step is included, in which a γ-glycidyl etheroxypropyltrimethoxysilane diluent is sprayed onto the surface of the raw sand; in step S1, the preheating temperature of the raw sand is 135-165℃; in step S2, the first part of the phenolic resin accounts for 25-45% of the total mass of the phenolic resin; in step S3, the addition temperature of the thermally triggered borosilicate-benzoxazine interface bridging microcapsule modifier is 100-130℃, and the second part of the phenolic resin accounts for 55-75% of the total mass of the phenolic resin; in step S4, the addition temperature of the curing agent and the lubricant is 80-105℃.

[0015] The beneficial effects of this invention are: This invention, through the synergistic design of a borosilicate hybrid porous ceramic microshell modifier, a thermally triggered borosilicate-benzoxazine interfacial bridging microcapsule modifier, and a layered positioning coating process, transforms coated sand from a simple mixture of low thermal conductivity filler and resin into a composite coating structure with spatial layers on the surface of individual sand particles. The borosilicate hybrid porous ceramic microshell modifier, using hollow ceramic microspheres as the thermal insulation core, can utilize the cavity structure to block continuous heat transfer. Its surface borosilicate-oxygen hybrid layer improves the heat resistance and structural stability of the microshell, while the organosilicon compatible layer improves the interfacial bonding between it and the phenolic resin film, reducing resin film damage caused by uneven dispersion or poor interfacial bonding of ordinary hollow fillers. The thermally triggered borosilicate-benzoxazine interfacial bridging microcapsule modifier remains relatively stable during preparation and storage. During the core-making heating stage, it undergoes thermally triggered activation, enabling the benzoxazine-containing and boron-containing cores to participate in ring-opening polymerization or interfacial cross-linking. This forms a bridging network between the borosilicate microshell inner insulating layer and the phenolic resin outer sealing layer, thereby enhancing the locking effect of the insulating microshell within the resin film. By employing a layered, targeted coating method—first adding the borosilicate microshell, then the first portion of phenolic resin, followed by the bridging microcapsules, and finally the second portion of phenolic resin—the coated sand particles can form a structure consisting of the original sand core, the borosilicate microshell inner insulating layer, the benzoxazine bridging interfacial layer, and the phenolic resin outer sealing layer, avoiding performance fluctuations caused by the random distribution of functional components. Attached Figure Description

[0016] The invention will now be further described with reference to the accompanying drawings.

[0017] Figure 1The bar chart shows the comparison of room temperature tensile strength and hot tensile strength of the coated sand in Examples 1-3 and Comparative Examples 1-3; Figure 2 The bar chart shows the comparison of the tensile strength retention rate of the coated sand in Examples 1-3 and Comparative Examples 1-3 after 30 days of storage. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] Example 1 I. Preparation of Boron-Silicon Hybrid Porous Ceramic Microshell Modifier By weight, 5.5 parts of boric acid were dissolved in 45 parts of water, and 200 parts of ethanol, 15 parts of tetraethyl orthosilicate, and 4.5 parts of phenyltriethoxysilane were added. Glacial acetic acid was added to adjust the pH of the system to 4.8. The mixture was hydrolyzed and condensed at 45°C for 60 min to obtain a borosilicate hybrid sol. 100 parts of hollow ceramic microspheres were added to the borosilicate hybrid sol and reacted at 65°C for 2 h to allow the borosilicate-oxygen hybrid layer to be deposited in situ on the surface of the hollow ceramic microspheres. Subsequently, 2.5 parts of γ-glycidyl etheroxypropyltrimethoxysilane were added and reacted at 65°C for 1 h to perform surface compatibility treatment. After drying at 105°C for 2 h and heat-treated at 300°C for 1 h, a borosilicate hybrid porous ceramic microshell modifier was obtained.

[0020] II. Preparation of Thermally Triggered Borosilicate-Benzoxazine Interfacial Bridged Microcapsule Modifier S101. Add 40 parts of cashew nut shell powder, 17.5 parts of furfurylamine, 17.5 parts of paraformaldehyde and 100 parts of ethanol to a reaction vessel equipped with a stirrer and a reflux condenser. Heat to 85°C and maintain the temperature for 3.5 hours to obtain a cashew nut shell powder-furfurylamine type benzoxazine prepolymer solution. Then add 5 parts of phenylboronic acid and continue the reaction at 80°C for 1.5 hours. Remove some of the ethanol under reduced pressure to obtain a core liquid containing benzoxazine structure and boron structure. S102. Add 4.5 parts of polyvinyl alcohol to 300 parts of deionized water, stir and dissolve at 85°C, and cool to 45°C to obtain a polyvinyl alcohol aqueous phase; add the core liquid to the polyvinyl alcohol aqueous phase and disperse at 5000 r / min for 10 min to obtain a core emulsion. S103. Mix 20 parts of tetraethyl orthosilicate, 7.5 parts of phenyltriethoxysilane, 6 parts of low molecular weight phenolic resin, 100 parts of ethanol, 100 parts of deionized water, and 0.8 parts of glacial acetic acid. Adjust the pH of the system to 4.8 and pre-hydrolyze at 45°C for 60 min to obtain a siloxane-phenolic composite sol. Slowly add the obtained siloxane-phenolic composite sol to the core emulsion and react at 60°C for 3 h to coat the outer side of the core with the siloxane-phenolic composite capsule wall. S104, then 4.25 parts of γ-glycidyl etheroxypropyltrimethoxysilane were added, and the reaction was continued at 60℃ for 1 h for interfacial treatment. After the reaction was completed, the mixture was centrifuged, washed twice with an ethanol-water mixture, and vacuum dried at 55℃ for 8 h to obtain the thermally triggered borosilicate-benzoxazine interfacial bridging microcapsule modifier.

[0021] III. Preparation of Composite Thermal Insulation Coated Sand with Low Thermal Conductivity and High Stability A composite thermal insulation coated sand with low thermal conductivity and high stability comprises the following raw materials by weight: 100 parts raw sand; 1.0 part phenolic resin; 0.5 parts borosilicate hybrid porous ceramic microshell modifier; 0.1 parts thermally triggered borosilicate-benzoxazine interface bridging microcapsule modifier; 0.1 parts curing agent; and 0.03 parts lubricant. S1. Dilute γ-glycidoxypropyltrimethoxysilane with an ethanol-water mixture to prepare a 2% (w / w) γ-glycidoxypropyltrimethoxysilane diluent; the mass ratio of ethanol to water in the ethanol-water mixture is 9:1. Spray 5g of the diluent onto the surface of 1000g of silica sand, mix for 120s, remove the ethanol and water from the diluent during heating, and preheat the silica sand to 135℃; add 5g of borosilicate hybrid porous ceramic microshell modifier to the preheated silica sand, mix for 60s, and obtain the first mixture; S2. Add 2.5g of the first part of thermoplastic phenolic resin to the first mixture and mix at 135℃ for 90s to obtain the second mixture; S3. Control the temperature of the second mixture to 100℃, add 1g of thermally triggered borosilicate-benzoxazine interfacial bridging microcapsule modifier, and mix for 50s; then add 7.5g of the second part of thermoplastic phenolic resin, mix for 80s, and obtain the third mixture. S4. Reduce the temperature of the third mixture to 80℃, add 1g of hexamethylenetetramine and 0.3g of calcium stearate, mix for 60s, and then cool, crush, and sieve to obtain a composite heat-insulating coated sand with low thermal conductivity and high stability.

[0022] Example 2 The preparation steps of the borosilicate hybrid porous ceramic microshell modifier and the thermally triggered borosilicate-benzoxazine interface bridging microcapsule modifier are the same as in Example 1. The difference is that the volume median particle size D50 of the borosilicate hybrid porous ceramic microshell modifier is controlled to 40 μm, the volume median particle size D50 of the thermally triggered borosilicate-benzoxazine interface bridging microcapsule modifier is controlled to 80 μm, and the core mass content is controlled to 75%.

[0023] A composite thermal insulation coated sand with low thermal conductivity and high stability comprises the following raw materials by weight: 100 parts raw sand; 4 parts phenolic resin; 8 parts borosilicate hybrid porous ceramic microshell modifier; 2 parts thermally triggered borosilicate-benzoxazine interface bridging microcapsule modifier; 0.8 parts curing agent; and 0.5 parts lubricant. The phenolic resin is a combination of thermoplastic phenolic resin and boron-modified phenolic resin in a weight ratio of 3:1.

[0024] S1. Dilute γ-glycidoxypropyltrimethoxysilane with an ethanol-water mixture to prepare a 2% (w / w) γ-glycidoxypropyltrimethoxysilane diluent; the mass ratio of ethanol to water in the ethanol-water mixture is 9:1. Spray 5g of the diluent onto the surface of 1000g of zircon sand, mix for 120s, and preheat the zircon sand to 165℃; add 80g of borosilicate hybrid porous ceramic microshell modifier to the preheated zircon sand, mix for 60s, and obtain the first mixture; S2. Add 18g of the first part of phenolic resin to the first mixture and mix at 165℃ for 90s to obtain the second mixture; S3. Control the temperature of the second mixture to 130℃, add 20g of thermally triggered borosilicate-benzoxazine interface bridging microcapsule modifier, and mix for 50s; then add 22g of the second part of phenolic resin, mix for 80s, and obtain the third mixture; S4. Reduce the temperature of the third mixture to 105℃, add 8g of microcapsule-type hexamethylenetetramine and 5g of ethylene bis-stearamide, mix for 60s, and then cool, crush, and sieve to obtain a composite heat-insulating coated sand with low thermal conductivity and high stability.

[0025] Example 3 The preparation methods of the borosilicate hybrid porous ceramic microshell modifier and the thermally triggered borosilicate-benzoxazine interface bridging microcapsule modifier are the same as those in Example 1, except that the volume median particle size D50 of the borosilicate hybrid porous ceramic microshell modifier is controlled to 23 μm, the volume median particle size D50 of the thermally triggered borosilicate-benzoxazine interface bridging microcapsule modifier is controlled to 45 μm, and the core mass content is controlled to 57.5%.

[0026] A composite thermal insulation coated sand with low thermal conductivity and high stability comprises the following raw materials by weight: 100 parts of abrasive sand; 2.5 parts of thermoplastic phenolic resin; 4.25 parts of borosilicate hybrid porous ceramic microshell modifier; 1.05 parts of thermally triggered borosilicate-benzoxazine interface bridging microcapsule modifier; 0.45 parts of curing agent; and 0.265 parts of lubricant. The lubricant is a combination of calcium stearate and polyethylene wax, with 2.0 g of calcium stearate and 0.65 g of polyethylene wax. S1. Dilute γ-glycidyl etheroxypropyltrimethoxysilane with an ethanol-water mixture to prepare a 2% (w / w) γ-glycidyl etheroxypropyltrimethoxysilane diluent; the mass ratio of ethanol to water in the ethanol-water mixture is 9:1; spray 5g of the diluent onto the surface of 1000g of abrasive sand, mix for 120s, and preheat the abrasive sand to 150℃; add 42.5g of borosilicate hybrid porous ceramic microshell modifier to the preheated abrasive sand, mix for 60s, and obtain the first mixture; S2. Add 8.75g of the first part of thermoplastic phenolic resin to the first mixture and mix at 150°C for 90s to obtain the second mixture; S3. Control the temperature of the second mixture to 115℃, add 10.5g of thermally triggered borosilicate-benzoxazine interfacial bridging microcapsule modifier, and mix for 50s; then add 16.25g of the second part of thermoplastic phenolic resin, mix for 80s, and obtain the third mixture. S4. Reduce the temperature of the third mixture to 92.5℃, add 4.5g hexamethylenetetramine, 2.0g calcium stearate and 0.65g polyethylene wax, mix for 60s, cool, crush and screen to obtain a composite heat-insulating coated sand with low thermal conductivity and high stability.

[0027] Comparative Example 1: No borosilicate hybrid porous ceramic microshell modifier added The preparation method of the thermally triggered borosilicate-benzoxazine interface bridging microcapsule modifier is the same as that in Example 1.

[0028] This comparative example provides a composite heat-insulating coated sand, which, by weight, comprises the following raw materials: 100 parts of abrasive sand; 2.5 parts of thermoplastic phenolic resin; 1.05 parts of thermally triggered borosilicate-benzoxazine interfacial bridging microcapsule modifier; 0.45 parts of curing agent; and 0.265 parts of lubricant; wherein the lubricant is a combination of calcium stearate and polyethylene wax, with calcium stearate comprising 2.0 g and polyethylene wax comprising 0.65 g. S1. Dilute γ-glycidoxypropyltrimethoxysilane with an ethanol-water mixture to prepare a γ-glycidoxypropyltrimethoxysilane diluent with a mass concentration of 2%; the mass ratio of ethanol to water in the ethanol-water mixture is 9:1; take 5g of the diluent and spray it onto the surface of 1000g of abrasive sand, mix for 120s, preheat the abrasive sand to 150℃, and continue mixing for 60s to obtain the first mixture; S2. Add 8.75g of the first part of thermoplastic phenolic resin to the first mixture and mix at 150°C for 90s to obtain the second mixture; S3. Control the temperature of the second mixture to 115℃, add 10.5g of thermally triggered borosilicate-benzoxazine interfacial bridging microcapsule modifier, and mix for 50s; then add 16.25g of the second part of thermoplastic phenolic resin, mix for 80s, and obtain the third mixture. S4. Reduce the temperature of the third mixture to 92.5℃, add 4.5g hexamethylenetetramine, 2.0g calcium stearate and 0.65g polyethylene wax, mix for 60s, cool, crush and screen to obtain the coated sand of Comparative Example 1.

[0029] Comparative Example 2: Microencapsulation modifier without thermally triggered borosilicate-benzoxazine interface bridging agent The preparation method of the borosilicate hybrid porous ceramic microshell modifier is the same as that in Example 1.

[0030] This comparative example provides a composite heat-insulating coated sand, which, by weight, comprises the following raw materials: 100 parts of abrasive sand; 2.5 parts of thermoplastic phenolic resin; 4.25 parts of borosilicate hybrid porous ceramic microshell modifier; 0.45 parts of curing agent; and 0.265 parts of lubricant; wherein the lubricant is a combination of calcium stearate and polyethylene wax, with calcium stearate at 2.0 g and polyethylene wax at 0.65 g. S1. γ-glycidyl etheroxypropyltrimethoxysilane was added to an ethanol-water mixture to prepare a 2% (w / w) silane dilution, wherein the mass ratio of ethanol to water was 9:1. 5g of the silane dilution was evenly sprayed onto the surface of 1000g of abrasive sand. After mixing for 120s, the treated abrasive sand was heated to 150℃. Subsequently, 42.5g of borosilicate hybrid porous ceramic microshell modifier was added, and mixing continued for 60s to obtain the first mixture. S2. Add 8.75g of the first part of thermoplastic phenolic resin to the first mixture, mix at 150°C for 90s, so that the resin melts and coats the surface of the first mixture to obtain the second mixture; S3. Adjust the temperature of the second mixture to 115℃, do not add the thermally triggered borosilicate-benzoxazine interface bridging microcapsule modifier, directly add 16.25g of the second part of thermoplastic phenolic resin, mix for 80s to obtain the third mixture; S4. Cool the third mixture to 92.5℃, add 4.5g hexamethylenetetramine, 2.0g calcium stearate and 0.65g polyethylene wax, mix for 60s, then cool, crush and sieve to obtain the coated sand of Comparative Example 2.

[0031] Comparative Example 3: Without using a layered positioning and coating process The preparation methods of the borosilicate hybrid porous ceramic microshell modifier and the thermally triggered borosilicate-benzoxazine interface bridging microcapsule modifier are the same as in Example 1.

[0032] This comparative example provides a composite heat-insulating coated sand, which, by weight, comprises the following raw materials: 100 parts of abrasive sand; 2.5 parts of thermoplastic phenolic resin; 4.25 parts of borosilicate hybrid porous ceramic microshell modifier; 1.05 parts of thermally triggered borosilicate-benzoxazine interface bridging microcapsule modifier; 0.45 parts of curing agent; and 0.265 parts of lubricant; wherein the lubricant is a combination of calcium stearate and polyethylene wax, with calcium stearate at 2.0 g and polyethylene wax at 0.65 g. S1. Dilute γ-glycidoxypropyltrimethoxysilane with an ethanol-water mixture to prepare a γ-glycidoxypropyltrimethoxysilane diluent with a mass concentration of 2%; the mass ratio of ethanol to water in the ethanol-water mixture is 9:1; take 5g of the diluent and spray it onto the surface of 1000g of abrasive sand, mix for 120s, and then preheat the abrasive sand to 150℃; S2. Add 42.5g of borosilicate hybrid porous ceramic microshell modifier, 10.5g of thermally triggered borosilicate-benzoxazine interface bridging microcapsule modifier and 25g of thermoplastic phenolic resin to the preheated alumina in one go, and mix at 150℃ for 150s to obtain the mixture. S3. Reduce the temperature of the mixture to 92.5℃, add 4.5g hexamethylenetetramine, 2.0g calcium stearate and 0.65g polyethylene wax, and mix for 60 seconds; S4. After cooling, crushing and screening, the coated sand of Comparative Example 3 is obtained.

[0033] Performance testing Thermal conductivity, room temperature tensile strength, hot tensile strength and storage stability of the coated sand obtained in Examples 1-3 and Comparative Examples 1-3 were tested.

[0034] Thermal conductivity test: Each group of coated sand was pressed into circular samples with a diameter of 50 mm and a thickness of 10 mm under the same pressure, cured at 230℃ for 120 s, cooled to 25℃, and the thermal conductivity was measured using a thermal conductivity tester. Three samples were tested for each group, and the average value was taken.

[0035] Room temperature tensile strength test: Each group of coated sand was made into a standard figure-eight shaped tensile specimen, cured at 230℃ for 120s, cooled to 25℃, and the room temperature tensile strength was determined using a coated sand strength tester. Five specimens were tested in each group, and the average value was taken.

[0036] Hot tensile strength test: Each group of coated sand was made into a standard figure-eight shaped tensile specimen, cured at 230℃ for 120s, and then kept at 300℃ for 60s. The hot tensile strength was measured using a hot tensile strength tester. Five specimens were tested in each group, and the average value was taken.

[0037] Storage stability test: Each group of coated sand was placed in a sealed bag and stored in an environment of 25℃ and 50%±5% relative humidity for 30 days. After storage, the tensile strength at room temperature was measured according to the room temperature tensile strength test method, and the tensile strength retention rate after 30 days of storage was calculated according to the following formula: tensile strength retention rate after 30 days of storage / % = tensile strength at room temperature after 30 days of storage / initial tensile strength at room temperature × 100%.

[0038] The test results are shown in Table 1 below.

[0039] Table 1. Performance analysis results of coated sand in Examples 1-3 and Comparative Examples 1-3

[0040] The results in the table show that the thermal conductivity of Examples 1-3 is lower than that of Comparative Example 1, indicating that the borosilicate hybrid porous ceramic microshell modifier can reduce the thermal conductivity of the coated sand. In Example 2, the amount of borosilicate hybrid porous ceramic microshell modifier is relatively high, so the thermal conductivity is the lowest. Example 3 shows a good overall balance between thermal conductivity, room temperature tensile strength, hot tensile strength, and storage stability.

[0041] Compared with Comparative Example 2, Example 3 contains borosilicate hybrid porous ceramic microshell modifier, but Comparative Example 2 does not contain thermally triggered borosilicate-benzoxazine interface bridging microcapsule modifier. Its hot tensile strength and tensile strength retention rate after 30 days of storage are significantly reduced, indicating that the bridging microcapsule modifier can improve the interfacial stability between the heat insulation microshell and the phenolic resin film.

[0042] Comparative Example 3 used the same raw materials but did not employ the layered positioning coating process. Its performance was lower than that of Example 3, indicating that the combination of raw materials alone is insufficient to achieve the best results; the layered coating sequence plays a crucial role in forming a stable composite structure. Example 3, while exhibiting a lower thermal conductivity, also demonstrated the highest room-temperature tensile strength, hot-state tensile strength, and storage retention rate, indicating that the present invention achieves a synergistic improvement in thermal insulation and stability through microshell insulation, microcapsule bridging, and layered coating.

[0043] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0044] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A composite thermal insulation coated sand with low thermal conductivity and high stability, characterized in that, By weight, it comprises the following components: 100 parts raw sand; 1.0-4.0 parts phenolic resin; 0.5-8.0 parts borosilicate hybrid porous ceramic microshell modifier; 0.1-2.0 parts thermally triggered borosilicate-benzoxazine interface bridging microcapsule modifier; 0.1-0.8 parts curing agent; and 0.03-0.5 parts lubricant. The borosilicate hybrid porous ceramic microshell modifier includes a hollow ceramic microsphere core, a borosilicate-oxygen hybrid layer covering the surface of the core, and an organosilicon compatible layer bonded to the outer surface of the borosilicate-oxygen hybrid layer; the thermally triggered borosilicate-benzoxazine interface bridging microcapsule modifier includes a capsule core containing benzoxazine and boron-containing structures, a siloxane-phenolic composite capsule wall covering the outer side of the capsule core, and an organosilicon interface layer bonded to the outer surface of the siloxane-phenolic composite capsule wall.

2. The composite heat-insulating coated sand according to claim 1, characterized in that, The raw sand is one or more of silica sand, alumina sand, ceramsite sand, chromite sand, and zircon sand; the phenolic resin is selected from thermoplastic phenolic resin, a combination of thermoplastic phenolic resin and heat-resistant modified phenolic resin, and the heat-resistant modified phenolic resin is selected from one or two of boron-modified phenolic resin and organosilicon-modified phenolic resin; the curing agent is selected from one or two of hexamethylenetetramine and microencapsulated hexamethylenetetramine; the lubricant is selected from one or more of calcium stearate, zinc stearate, polyethylene wax, paraffin wax, and ethylene bis-stearamide.

3. The composite heat-insulating coated sand according to claim 1, characterized in that, The composite heat-insulating coated sand particles include, from the inside out, a raw sand core, a borosilicate microshell heat-insulating inner layer, a benzoxazine bridging interface layer, and a phenolic resin outer sealing layer; the borosilicate microshell heat-insulating inner layer contains the borosilicate hybrid porous ceramic microshell modifier, and the benzoxazine bridging interface layer contains the thermally triggered borosilicate-benzoxazine interface bridging microcapsule modifier.

4. The composite heat-insulating coated sand according to claim 1, characterized in that, The volumetric median particle size (D50) of the borosilicate hybrid porous ceramic microshell modifier is 5-40 μm; the volumetric median particle size (D50) of the thermally triggered borosilicate-benzoxazine interface-bridged microcapsule modifier is 10-80 μm; and the mass content of the capsule core is 40-75% based on the total mass of the thermally triggered borosilicate-benzoxazine interface-bridged microcapsule modifier.

5. The composite heat-insulating coated sand according to claim 1, characterized in that, The borosilicate hybrid porous ceramic microshell modifier is prepared from the following raw materials in parts by weight: 100 parts hollow ceramic microspheres; 5-25 parts tetraethyl orthosilicate; 1-10 parts boric acid; 1-8 parts phenyltriethoxysilane; 1-4 parts organosilane compatibilizer; 100-300 parts ethanol; 10-80 parts water; and 0.1-2 parts acid catalyst. The organosilane compatibilizer is selected from one or two of γ-aminopropyltriethoxysilane and γ-glycidoxypropyltrimethoxysilane. The acid catalyst is selected from one or more of glacial acetic acid, citric acid, and oxalic acid.

6. The composite heat-insulating coated sand according to claim 5, characterized in that, The borosilicate hybrid porous ceramic microshell modifier is prepared by the following method: tetraethyl orthosilicate, boric acid, phenyltriethoxysilane, water, ethanol, and an acid catalyst are mixed to form a borosilicate hybrid sol; hollow ceramic microspheres are added to the borosilicate hybrid sol to allow the borosilicate-oxygen hybrid layer to be deposited in situ on the surface of the hollow ceramic microspheres; an organosilane compatibilizer is added for surface compatibility treatment; and the borosilicate hybrid porous ceramic microshell modifier is obtained after drying and heat treatment.

7. The composite heat-insulating coated sand according to claim 1, characterized in that, The thermally triggered borosilicate-benzoxazine interfacial bridging microcapsule modifier is prepared from the following raw materials in parts by weight: cashew phenol 30-50 parts; furfurylamine 10-25 parts; paraformaldehyde 10-25 parts; phenylboronic acid 2-8 parts; tetraethyl orthosilicate 10-30 parts; phenyltriethoxysilane 3-12 parts; low molecular weight phenolic resin 2-10 parts; organosilane interfacial agent 0.5-8 parts; emulsifying dispersant 1-8 parts; ethanol 100-300 parts. 200-600 parts water.

8. The composite heat-insulating coated sand according to claim 7, characterized in that, The thermally triggered borosilicate-benzoxazine interfacial bridging microcapsule modifier is prepared according to the following method: S101, cashew nut shell powder, furfurylamine, and paraformaldehyde are reacted in ethanol to obtain a cashew nut shell powder-furfurylamine type benzoxazine prepolymer, and then phenylboronic acid is added for boron modification treatment to obtain a core liquid containing benzoxazine structure and boron structure; S102, the core liquid is added to an aqueous phase containing an emulsifying dispersant to disperse and form a core emulsion; S103, tetraethyl orthosilicate, phenyltriethoxysilane, low molecular weight phenolic resin, ethanol, water, and acid catalyst are mixed and pre-hydrolyzed to obtain a siloxane-phenolic composite sol, and then the siloxane-phenolic composite sol is added to the core emulsion so that the siloxane-phenolic composite capsule wall covers the outside of the core; S104, an organosilane interfacial agent is added for interfacial treatment, and after separation, washing, and drying, the thermally triggered borosilicate-benzoxazine interfacial bridging microcapsule modifier is obtained.

9. A method for preparing the composite heat-insulating coated sand according to any one of claims 1-8, characterized in that, Includes the following steps: S1. Preheat the raw sand, add borosilicate hybrid porous ceramic microshell modifier to the preheated raw sand and mix to obtain the first mixture; S2. Add the first portion of phenolic resin to the first mixture and mix to obtain the second mixture; S3. Add the thermally triggered borosilicate-benzoxazine interface bridging microcapsule modifier to the second mixture and mix, then add the second part of phenolic resin and mix to obtain the third mixture; S4. Add curing agent and lubricant to the third mixture, and after cooling, crushing and sieving, obtain the composite heat insulation coated sand.

10. The preparation method according to claim 9, characterized in that, Before step S1, there is a surface activation step of spraying a diluted solution of γ-glycidyl etheroxypropyltrimethoxysilane onto the surface of the raw sand; in step S1, the preheating temperature of the raw sand is 135-165℃; in step S2, the first part of phenolic resin accounts for 25-45% of the total mass of phenolic resin; in step S3, the addition temperature of the thermally triggered borosilicate-benzoxazine interface bridging microcapsule modifier is 100-130℃, and the second part of phenolic resin accounts for 55-75% of the total mass of phenolic resin; in step S4, the addition temperature of the curing agent and lubricant is 80-105℃.

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