Coating for casting and preparation method thereof

Abstract

The invention provides a coating for castings. The coating comprises the following components: 60-65 parts of modified quartz powder; 3-5 parts of elastic PI particles; 3-5 parts of phenolic resin; 2 to 3 parts of bentonite; 3 to 5 parts of PI chopped fiber; 0.8 to 1 part of CMC (carboxymethyl cellulose); 1 to 1.8 parts of PVA (Polyvinyl Alcohol); 0.2 to 0.4 part of a defoaming agent; 25 to 30 parts of water; the modified quartz powder is obtained by compounding quartz powder with different particle size ranges, the quartz powder with the particle size of 200-350 meshes accounts for 40-50%, the quartz powder with the particle size of 400-600 meshes accounts for 30-40%, and the quartz powder with the particle size of 700-1000 meshes accounts for 10-20%. Through synergistic optimization of the components, the core pain points of drying cracking, high-temperature failure and air hole sand burning of a traditional phenolic resin-based coating are solved, the environment-friendly phenolic resin-based coating has the advantages of being environmentally friendly, compliant and feasible in industrialization, and an efficient solution is provided for research, development and application of high-performance coatings in the casting industry.

Description

Technical Field

[0001] This invention relates to the field of coating technology, specifically to a coating for castings and its preparation method. Background Technology

[0002] Casting is a fundamental process in the machinery manufacturing industry, and the surface quality of castings is directly related to the coating protection. Coatings must simultaneously meet three core requirements: room temperature molding stability, high temperature impact resistance, and ease of cleaning. Especially in high-temperature casting conditions such as cast steel and cast iron, the coating's crack resistance, high-temperature strength, and permeability become key factors in determining the casting yield. With the development of precision casting technology, traditional coatings are no longer suitable for high-requirement scenarios: on the one hand, high-end equipment, such as automobile engine blocks and engineering machinery parts, have continuously increased requirements for the surface roughness (Ra≤3.2μm) and defect-free rate of castings; on the other hand, tightening environmental protection policies have driven the research and development of chromium-free, low-VOC, and easily degradable coatings, and traditional chromium-containing anti-adhesion sand agents and highly polluting solvent-based coatings are gradually being phased out.

[0003] Coatings using phenolic resin as a binder often suffer from cracking after drying and insufficient high-temperature strength. During coating drying, if the additives have poor crack resistance or the aggregate powder is too fine, shrinkage stress can cause the coating to crack. Under high-temperature conditions, organic binders such as phenolic resin are prone to decomposition at 300-500℃, causing the coating to lose its support and become easily damaged when exposed to molten metal. Furthermore, a mismatch in the thermal expansion coefficients of the coating and the mold can lead to cracks or even peeling, resulting in sand adhesion to the casting. Against this backdrop, the development of new functional coatings that combine crack resistance, high-temperature stability, and environmental compatibility has become an urgent industry need. Summary of the Invention

[0004] Technical problem to be solved: The purpose of this invention is to provide a coating for castings that comprehensively solves the core pain points of traditional casting coatings, such as cracking, high-temperature failure, porosity and sand adhesion, through the synergistic effect of functional components.

[0005] Technical solution: A coating for castings, comprising the following components: Modified quartz powder: 60-65 parts Elastic PI particles: 3-5 parts Phenolic resin: 3-5 parts Bentonite: 2-3 parts PI short-cut fibers: 3-5 parts CMC: 0.8~1 part PVA: 1~1.8 parts Defoamer: 0.2~0.4 parts Water: 25-30 parts; The modified quartz powder is obtained by compounding quartz powders of different particle size ranges, with 200-350 mesh quartz powder accounting for 40-50%, 400-600 mesh quartz powder accounting for 30-40%, and 700-1000 mesh accounting for 10-20%.

[0006] Preferably, the defoamer is a silicone defoamer, and the silicone defoamer is any one of BYK-066N, SILFOAM SC 120 or Dow Corning DC-1500; The PI short-cut fibers have a length of 1~5mm and a diameter of 8~10um.

[0007] Preferably, the method for preparing the elastic PI particles includes the following steps: ODA was added to DMF and stirred until dissolved. Then, amino-terminated polydimethylsiloxane and PMDA were added in sequence. The reaction was carried out under ice bath conditions and stirred until complete, resulting in a viscous solution. Toluene and isoquinoline were added to a viscous solution, and the mixture was heated to reflux to achieve partial iminoization, resulting in a siloxane-modified PI solution. PVA and ammonium bicarbonate were added to water and dissolved, and the mixture was stirred to obtain an aqueous phase. A siloxane-modified PI solution was added dropwise to the aqueous phase and emulsified by high-speed shearing to obtain an oil-shell-aqueous-core emulsion structure. The emulsion was transferred to a reaction vessel and subjected to a gradient temperature increase reaction until the droplets in the emulsion solidified into microspheres; The solidified microspheres were filtered and separated, dried and heated to obtain PI particles. The PI particles were then modified with KH-560 to obtain elastic PI particles.

[0008] Preferably, the molar ratio of ODA to PMDA is 1:1 to 1.02, the molecular weight of amino-terminated polydimethylsiloxane is 3000 to 5000, and the mass ratio of amino-terminated polydimethylsiloxane to ODA is 1 to 2:10.

[0009] Preferably, the mass ratio of toluene, isoquinoline and amino-terminated polydimethylsiloxane is 2~3:0.4~0.8:2, the reaction temperature is 150~160℃, and the reaction time is 6~8h.

[0010] Preferably, the mass ratio of PVA, ammonium bicarbonate and water in the aqueous phase is 1~2:4~6:50.

[0011] Preferably, the rotational speed of the high-speed shearing is 3000~3500 r / min.

[0012] Preferably, the gradient heating reaction involves first heating to 70-80°C and holding for 4 hours, then heating to 150°C and holding for 4 hours.

[0013] Preferably, the heating temperature is 300~350℃ and the holding time is 2h.

[0014] The above-mentioned method for preparing coatings for castings includes the following steps: S1. Quartz powder, Tris and dopamine are added to water, with a mass-to-volume ratio of quartz powder, Tris, dopamine and water of 100g:1~1.5g:3~5g:100~150mL. The mixture is stirred at room temperature at 800~1000rpm for 6~8h to obtain modified quartz powder. S2. Add bentonite to water, stir well, then add CMC and PVA, and stir to obtain a homogeneous solution; S3. Add quartz powder to the solution obtained in S2 in three portions, stir well, then add phenolic resin, elastic PI particles, and chopped PI fibers in sequence, stir well again, add defoamer, mix well, and obtain a coating for casting.

[0015] Beneficial effects: The coating prepared by this invention has the following advantages: In this invention, the elastic PI particles possess excellent elastic buffering capacity, while also buffering the impact of molten metal and thermal expansion stress at high temperatures; the short-cut PI fibers form a three-dimensional interwoven network in the coating, acting like micro-reinforcing bars to bridge micro-cracks and prevent crack propagation, forming a dual protection of buffering and crack prevention with the elastic PI particles; the compounded modified quartz powder (coarse, medium and fine three-level gradation) forms a dense packing structure, reducing shrinkage space, while dopamine modification enhances interfacial bonding force, avoiding interfacial delamination caused by stress concentration; In this invention, the elastic PI particles contain siloxane segments, which maintain their structural integrity even after the phenolic resin decomposes at 300-500℃. Together with the compounded quartz powder, they form a rigid-elastic composite structure that resists molten metal erosion and penetration. The PI chopped fibers withstand temperatures ≥500℃ and maintain a three-dimensional network morphology at high temperatures, supporting the coating skeleton, preventing coating collapse, and further enhancing high-temperature resistance to breakage. The porous structure of the elastic PI particles can store gases generated by the decomposition of phenolic resin, preventing gas accumulation and the formation of pores. Simultaneously, the closed-cell structure reduces direct contact between the molten metal and the sand mold, combined with the dense packing of modified quartz powder. The overall permeability of the coating is controllable (gas emission 25-30 mL / g), allowing gas to be orderly discharged through the interconnected pores of the particles and the sand mold, eliminating defects such as pores and inclusions, improving the surface roughness of the casting, and meeting the requirements of precision casting. Detailed Implementation

[0016] The present invention will be further described below with reference to embodiments. These embodiments are illustrative of the present invention, but the present invention is not limited to these embodiments: Example

[0017] A coating for castings comprises the following components: modified quartz powder: 60 parts, elastic PI particles: 3 parts, phenolic resin: 3 parts, bentonite: 2 parts, PI chopped fibers: 3 parts, CMC: 0.8 parts, PVA: 1 part, defoamer BYK-066N: 0.2 parts, and water: 25 parts. The modified quartz powder is obtained by compounding quartz powders with different particle size ranges, with 40% being 200-350 mesh quartz powder, 40% being 400-600 mesh quartz powder, and 20% being 700-1000 mesh quartz powder; the PI short chopped fiber has a length of 1-5 mm and a fiber diameter of 8-10 μm.

[0018] The above-mentioned method for preparing coatings for castings includes the following steps: S1. Quartz powder, Tris and dopamine were added to water in a mass-volume ratio of 100g:1g:3g:100mL. The mixture was stirred at room temperature at 800rpm for 8 hours to obtain modified quartz powder. S2. Add bentonite to water, stir well, then add CMC and PVA, and stir to obtain a homogeneous solution; S3. Add the quartz powder to the solution obtained in S2 in three portions, stir well, then add the phenolic resin, elastic PI particles, and chopped PI fibers in sequence, stir well again, add the defoamer, mix well, and obtain the casting coating. Preferably, the method for preparing the elastic PI particles includes the following steps: ODA was added to DMF and stirred until dissolved. The molar ratio of ODA to PMDA was 1:1. An amino-terminated polydimethylsiloxane with a molecular weight of 5000 and PMDA were added. The mass ratio of amino-terminated polydimethylsiloxane to ODA was 2:10. The reaction was carried out under ice bath conditions and stirred until complete to obtain a viscous solution. Toluene and isoquinoline were added to a viscous solution. The mass ratio of toluene, isoquinoline and amino-terminated polydimethylsiloxane was 2:0.4:2. The mixture was heated to reflux at 150°C for 8 hours to achieve partial iminolation and obtain a siloxane-modified PI solution. PVA and ammonium bicarbonate were added to water and dissolved. The mass ratio of PVA, ammonium bicarbonate and water was 2:6:50. The mixture was stirred and dissolved to obtain an aqueous phase. A siloxane-modified PI solution was added dropwise to the aqueous phase and emulsified by high-speed shearing at a speed of 3000 r / min to obtain an oil-shell-aqueous-core emulsion structure. The emulsion was transferred to a reaction vessel and subjected to a gradient temperature increase reaction. First, the temperature was increased to 80°C and held for 4 hours, then the temperature was increased to 150°C and held for 4 hours until the droplets in the emulsion solidified into microspheres. The solidified microspheres were filtered and separated, dried, and then heated to 300℃ for 2 hours to obtain PI particles. The PI particles were then modified with KH-560 to obtain elastic PI particles. Example

[0019] A coating for castings comprises the following components: modified quartz powder: 65 parts, elastic PI particles: 5 parts, phenolic resin: 5 parts, bentonite: 3 parts, PI chopped fibers: 5 parts, CMC: 1 part, PVA: 1.8 parts, defoamer BYK-066N: 0.4 parts, and water: 30 parts. The modified quartz powder is obtained by compounding quartz powders with different particle size ranges, with 50% being 200-350 mesh quartz powder, 40% being 400-600 mesh quartz powder, and 10% being 700-1000 mesh quartz powder; the PI short-cut fiber has a length of 1-5 mm and a fiber diameter of 8-10 μm.

[0020] The above-mentioned method for preparing coatings for castings includes the following steps: S1. Quartz powder, Tris and dopamine were added to water in a mass-volume ratio of 100g:1.5g:5g:150mL. The mixture was stirred at room temperature at 1000rpm for 6 hours to obtain modified quartz powder. S2. Add bentonite to water, stir well, then add CMC and PVA, and stir to obtain a homogeneous solution; S3. Add the quartz powder to the solution obtained in S2 in three portions, stir well, then add the phenolic resin, elastic PI particles, and chopped PI fibers in sequence, stir well again, add the defoamer, mix well, and obtain the casting coating. Preferably, the method for preparing the elastic PI particles includes the following steps: ODA was added to DMF and stirred until dissolved. The molar ratio of ODA to PMDA was 1:1.02. A 5000-molecular-weight amino-terminated polydimethylsiloxane and PMDA were added. The mass ratio of amino-terminated polydimethylsiloxane to ODA was 1:10. The reaction was carried out under ice bath conditions and stirred until complete, resulting in a viscous solution. Toluene and isoquinoline were added to a viscous solution. The mass ratio of toluene, isoquinoline and amino-terminated polydimethylsiloxane was 3:0.8:2. The mixture was heated to reflux at 160°C for 6 hours to achieve partial iminolation and obtain a siloxane-modified PI solution. PVA and ammonium bicarbonate were added to water and dissolved. The mass ratio of PVA, ammonium bicarbonate and water was 1:4:50. The mixture was stirred and dissolved to obtain an aqueous phase. A siloxane-modified PI solution was added dropwise to the aqueous phase and emulsified by high-speed shearing at a speed of 3500 r / min to obtain an emulsion structure with an oil shell and an aqueous core. The emulsion was transferred to a reaction vessel and subjected to a gradient temperature increase reaction. First, the temperature was increased to 70°C and held for 4 hours, then the temperature was increased to 150°C and held for 4 hours until the droplets in the emulsion solidified into microspheres. The solidified microspheres were filtered and separated, dried, and then heated to 350℃ for 2 hours to obtain PI particles. The PI particles were then modified with KH-560 to obtain elastic PI particles. Example

[0021] A coating for castings comprises the following components: modified quartz powder: 62 parts, elastic PI particles: 3.5 parts, phenolic resin: 3.5 parts, bentonite: 2 parts, PI chopped fibers: 3.5 parts, CMC: 0.8 parts, PVA: 1.2 parts, defoamer BYK-066N: 0.3 parts, and water: 28 parts. The modified quartz powder is obtained by compounding quartz powders with different particle size ranges, with 45% being 200-350 mesh quartz powder, 35% being 400-600 mesh quartz powder, and 20% being 700-1000 mesh quartz powder; the PI short-cut fiber has a length of 1-5 mm and a fiber diameter of 8-10 μm.

[0022] The above-mentioned method for preparing coatings for castings includes the following steps: S1. Quartz powder, Tris and dopamine were added to water in a mass-to-volume ratio of 100g:1.2g:3.5g:120mL. The mixture was stirred at room temperature at 850rpm for 8 hours to obtain modified quartz powder. S2. Add bentonite to water, stir well, then add CMC and PVA, and stir to obtain a homogeneous solution; S3. Add the quartz powder to the solution obtained in S2 in three portions, stir well, then add the phenolic resin, elastic PI particles, and chopped PI fibers in sequence, stir well again, add the defoamer, mix well, and obtain the casting coating. Preferably, the method for preparing the elastic PI particles includes the following steps: ODA was added to DMF and stirred until dissolved. The molar ratio of ODA to PMDA was 1:1. 5000-molecular-weight amino-terminated polydimethylsiloxane and PMDA were added. The mass ratio of amino-terminated polydimethylsiloxane to ODA was 1.5:10. The reaction was carried out under ice bath conditions and stirred until complete to obtain a viscous solution. Toluene and isoquinoline were added to a viscous solution. The mass ratio of toluene, isoquinoline and amino-terminated polydimethylsiloxane was 2.2:0.5:2. The mixture was heated to reflux at 150°C for 8 hours to achieve partial iminolation and obtain a siloxane-modified PI solution. PVA and ammonium bicarbonate were added to water and dissolved. The mass ratio of PVA, ammonium bicarbonate and water was 1.4:4.5:50. The mixture was stirred and dissolved to obtain an aqueous phase. A siloxane-modified PI solution was added dropwise to the aqueous phase and emulsified by high-speed shearing at a speed of 3000 r / min to obtain an oil-shell-aqueous-core emulsion structure. The emulsion was transferred to a reaction vessel and subjected to a gradient temperature increase reaction. First, the temperature was increased to 70°C and held for 4 hours, then the temperature was increased to 150°C and held for 4 hours until the droplets in the emulsion solidified into microspheres. The solidified microspheres were filtered and separated, dried, and then heated to 350℃ for 2 hours to obtain PI particles. The PI particles were then modified with KH-560 to obtain elastic PI particles. Example

[0023] A coating for castings comprises the following components: modified quartz powder: 64 parts, elastic PI particles: 4.5 parts, phenolic resin: 4.5 parts, bentonite: 3 parts, PI chopped fibers: 4.5 parts, CMC: 1 part, PVA: 1.6 parts, defoamer BYK-066N: 0.4 parts, and water: 30 parts. The modified quartz powder is obtained by compounding quartz powders with different particle size ranges, with 50% being 200-350 mesh quartz powder, 30% being 400-600 mesh quartz powder, and 20% being 700-1000 mesh quartz powder; the PI short-cut fiber has a length of 1-5 mm and a fiber diameter of 8-10 μm.

[0024] The above-mentioned method for preparing coatings for castings includes the following steps: S1. Quartz powder, Tris and dopamine were added to water in a mass-volume ratio of 100g:1.4g:4.5g:140mL. The mixture was stirred at room temperature at 950rpm for 6 hours to obtain modified quartz powder. S2. Add bentonite to water, stir well, then add CMC and PVA, and stir to obtain a homogeneous solution; S3. Add the quartz powder to the solution obtained in S2 in three portions, stir well, then add the phenolic resin, elastic PI particles, and chopped PI fibers in sequence, stir well again, add the defoamer, mix well, and obtain the casting coating. Preferably, the method for preparing the elastic PI particles includes the following steps: ODA was added to DMF and stirred until dissolved. The molar ratio of ODA to PMDA was 1:1.02. 5000-molecular-weight amino-terminated polydimethylsiloxane and PMDA were added. The mass ratio of amino-terminated polydimethylsiloxane to ODA was 1.2:10. The reaction was carried out under ice bath conditions and stirred until complete to obtain a viscous solution. Toluene and isoquinoline were added to a viscous solution. The mass ratio of toluene, isoquinoline and amino-terminated polydimethylsiloxane was 2.7:0.7:2. The solution was heated to reflux at 160°C for 6 hours to achieve partial iminolation and obtain a siloxane-modified PI solution. PVA and ammonium bicarbonate were added to water and dissolved. The mass ratio of PVA, ammonium bicarbonate and water was 1.8:5.5:50. The mixture was stirred and dissolved to obtain an aqueous phase. A siloxane-modified PI solution was added dropwise to the aqueous phase and emulsified by high-speed shearing at a speed of 3500 r / min to obtain an emulsion structure with an oil shell and an aqueous core. The emulsion was transferred to a reaction vessel and subjected to a gradient temperature increase reaction. First, the temperature was increased to 80°C and held for 4 hours, then the temperature was increased to 150°C and held for 4 hours until the droplets in the emulsion solidified into microspheres. The solidified microspheres were filtered and separated, dried, and then heated to 320℃ for 2 hours to obtain PI particles. The PI particles were then modified with KH-560 to obtain elastic PI particles. Example

[0025] A coating for castings comprises the following components: modified quartz powder: 63 parts, elastic PI particles: 4 parts, phenolic resin: 4 parts, bentonite: 2.5 parts, PI chopped fibers: 4 parts, CMC: 0.8 parts, PVA: 1.5 parts, defoamer BYK-066N: 0.35 parts, and water: 30 parts. The modified quartz powder is obtained by compounding quartz powders with different particle size ranges, with 50% being 200-350 mesh quartz powder, 35% being 400-600 mesh quartz powder, and 15% being 700-1000 mesh quartz powder; the PI short-cut fiber has a length of 1-5 mm and a fiber diameter of 8-10 μm.

[0026] The above-mentioned method for preparing coatings for castings includes the following steps: S1. Quartz powder, Tris and dopamine were added to water in a mass-volume ratio of 100g:1.3g:4g:130mL. The mixture was stirred at room temperature at 900rpm for 7h to obtain modified quartz powder. S2. Add bentonite to water, stir well, then add CMC and PVA, and stir to obtain a homogeneous solution; S3. Add the quartz powder to the solution obtained in S2 in three portions, stir well, then add the phenolic resin, elastic PI particles, and chopped PI fibers in sequence, stir well again, add the defoamer, mix well, and obtain the casting coating. Preferably, the method for preparing the elastic PI particles includes the following steps: ODA was added to DMF and stirred until dissolved. The molar ratio of ODA to PMDA was 1:1.02. 5000-molecular-weight amino-terminated polydimethylsiloxane and PMDA were added. The mass ratio of amino-terminated polydimethylsiloxane to ODA was 1.5:10. The reaction was carried out under ice bath conditions and stirred until complete to obtain a viscous solution. Toluene and isoquinoline were added to a viscous solution. The mass ratio of toluene, isoquinoline and amino-terminated polydimethylsiloxane was 2.5:0.6:2. The solution was heated to reflux at 155°C for 7 hours to achieve partial iminolation and obtain a siloxane-modified PI solution. PVA and ammonium bicarbonate were added to water and dissolved. The mass ratio of PVA, ammonium bicarbonate and water was 1.6:5:50. The mixture was stirred and dissolved to obtain an aqueous phase. A siloxane-modified PI solution was added dropwise to the aqueous phase and emulsified by high-speed shearing at a speed of 3500 r / min to obtain an emulsion structure with an oil shell and an aqueous core. The emulsion was transferred to a reaction vessel and subjected to a gradient temperature increase reaction. First, the temperature was increased to 75°C and held for 4 hours, then the temperature was increased to 150°C and held for 4 hours until the droplets in the emulsion solidified into microspheres. The solidified microspheres were filtered and separated, dried, and then heated to 330℃ for 2 hours to obtain PI particles. The PI particles were then modified with KH-560 to obtain elastic PI particles.

[0027] Comparative Example 1 The difference between Comparative Example 1 and Example 5 is that no elastic PI particles were added to the coating.

[0028] Comparative Example 2 The difference between Comparative Example 2 and Example 5 is that no PI short-cut fibers were added to the coating.

[0029] Comparative Example 3 The difference between Comparative Example 3 and Example 5 is that the modified quartz powder is not compounded using quartz powder of different particle sizes.

[0030] Comparative Example 4 The difference between Comparative Example 4 and Example 5 is that the quartz powder was not modified. A coating for castings comprises the following components: quartz powder: 63 parts, elastic PI particles: 4 parts, phenolic resin: 4 parts, bentonite: 2.5 parts, PI chopped fibers: 4 parts, CMC: 0.8 parts, PVA: 1.5 parts, defoamer BYK-066N: 0.35 parts, and water: 30 parts. The quartz powder is a blend of quartz powders with different particle sizes, of which 50% is 200-350 mesh, 35% is 400-600 mesh, and 15% is 700-1000 mesh; the PI short-cut fibers have a length of 1-5 mm and a diameter of 8-10 μm.

[0031] The above-mentioned method for preparing coatings for castings includes the following steps: S1. Add bentonite to water, stir well, then add CMC and PVA, and stir to obtain a homogeneous solution; S2. Quartz powder is added to the solution obtained in S1 in three portions. After stirring evenly, phenolic resin, elastic PI particles, and PI short-cut fibers are added in sequence. After stirring evenly again, defoamer is added and mixed evenly to obtain a casting coating. Preferably, the method for preparing the elastic PI particles includes the following steps: ODA was added to DMF and stirred until dissolved. The molar ratio of ODA to PMDA was 1:1.02. 5000-molecular-weight amino-terminated polydimethylsiloxane and PMDA were added. The mass ratio of amino-terminated polydimethylsiloxane to ODA was 1.5:10. The reaction was carried out under ice bath conditions and stirred until complete to obtain a viscous solution. Toluene and isoquinoline were added to a viscous solution. The mass ratio of toluene, isoquinoline and amino-terminated polydimethylsiloxane was 2.5:0.6:2. The solution was heated to reflux at 155°C for 7 hours to achieve partial iminolation and obtain a siloxane-modified PI solution. PVA and ammonium bicarbonate were added to water and dissolved. The mass ratio of PVA, ammonium bicarbonate and water was 1.6:5:50. The mixture was stirred and dissolved to obtain an aqueous phase. A siloxane-modified PI solution was added dropwise to the aqueous phase and emulsified by high-speed shearing at a speed of 3500 r / min to obtain an emulsion structure with an oil shell and an aqueous core. The emulsion was transferred to a reaction vessel and subjected to a gradient temperature increase reaction. First, the temperature was increased to 75°C and held for 4 hours, then the temperature was increased to 150°C and held for 4 hours until the droplets in the emulsion solidified into microspheres. The solidified microspheres were filtered and separated, dried, and then heated to 330℃ for 2 hours to obtain PI particles. The PI particles were then modified with KH-560 to obtain elastic PI particles.

[0032] Comparative Example 5 The difference between Comparative Example 5 and Example 5 is that the elastic PI particles were not partially imidized; A coating for castings comprises the following components: modified quartz powder: 63 parts, elastic PI particles: 4 parts, phenolic resin: 4 parts, bentonite: 2.5 parts, PI chopped fibers: 4 parts, CMC: 0.8 parts, PVA: 1.5 parts, defoamer BYK-066N: 0.35 parts, and water: 30 parts. The modified quartz powder is obtained by compounding quartz powders with different particle size ranges, with 50% being 200-350 mesh quartz powder, 35% being 400-600 mesh quartz powder, and 15% being 700-1000 mesh quartz powder; the PI short-cut fiber has a length of 1-5 mm and a fiber diameter of 8-10 μm.

[0033] The above-mentioned method for preparing coatings for castings includes the following steps: S1. Quartz powder, Tris and dopamine were added to water in a mass-volume ratio of 100g:1.3g:4g:130mL. The mixture was stirred at room temperature at 900rpm for 7h to obtain modified quartz powder. S2. Add bentonite to water, stir well, then add CMC and PVA, and stir to obtain a homogeneous solution; S3. Add the quartz powder to the solution obtained in S2 in three portions, stir well, then add the phenolic resin, elastic PI particles, and chopped PI fibers in sequence, stir well again, add the defoamer, mix well, and obtain the casting coating. Preferably, the method for preparing the elastic PI particles includes the following steps: ODA was added to DMF and stirred until dissolved. The molar ratio of ODA to PMDA was 1:1.02. 5000-molecular-weight amino-terminated polydimethylsiloxane and PMDA were added. The mass ratio of amino-terminated polydimethylsiloxane to ODA was 1.5:10. The reaction was carried out under ice bath conditions and stirred until complete to obtain a viscous solution. PVA and ammonium bicarbonate were added to water and dissolved. The mass ratio of PVA, ammonium bicarbonate and water was 1.6:5:50. The mixture was stirred and dissolved to obtain an aqueous phase. A viscous solution was added dropwise to the aqueous phase and emulsified by high-speed shearing at a speed of 3500 r / min to obtain an oil-shell-aqueous-core emulsion. Toluene and isoquinoline were added to the emulsion. The mass ratio of toluene, isoquinoline and amino-terminated polydimethylsiloxane was 2.5:0.6:2. The mixture was heated to reflux and reacted at 120°C for 10 hours to solidify the microspheres in the emulsion. The solidified microspheres were filtered and separated, dried, and then heated to 300℃ at a rate of 3℃ / min and held for 3 hours. The temperature was then increased to 500℃ and held for 4 hours to obtain PI particles. The PI particles were then modified with KH-560 to obtain elastic PI particles.

[0034] Comparative Example 6 The difference between Comparative Example 6 and Example 5 is that no amino-terminated polydimethylsiloxane is added to the elastic PI particles; A coating for castings comprises the following components: modified quartz powder: 63 parts, elastic PI particles: 4 parts, phenolic resin: 4 parts, bentonite: 2.5 parts, PI chopped fibers: 4 parts, CMC: 0.8 parts, PVA: 1.5 parts, defoamer BYK-066N: 0.35 parts, and water: 30 parts. The modified quartz powder is obtained by compounding quartz powders with different particle size ranges, with 50% being 200-350 mesh quartz powder, 35% being 400-600 mesh quartz powder, and 15% being 700-1000 mesh quartz powder; the PI short-cut fiber has a length of 1-5 mm and a fiber diameter of 8-10 μm.

[0035] The above-mentioned method for preparing coatings for castings includes the following steps: S1. Quartz powder, Tris and dopamine were added to water in a mass-volume ratio of 100g:1.3g:4g:130mL. The mixture was stirred at room temperature at 900rpm for 7h to obtain modified quartz powder. S2. Add bentonite to water, stir well, then add CMC and PVA, and stir to obtain a homogeneous solution; S3. Add the quartz powder to the solution obtained in S2 in three portions, stir well, then add the phenolic resin, elastic PI particles, and chopped PI fibers in sequence, stir well again, add the defoamer, mix well, and obtain the casting coating. Preferably, the method for preparing the elastic PI particles includes the following steps: ODA was added to DMF and stirred until dissolved. The molar ratio of ODA to PMDA was 1:1.02. The reaction was carried out under ice bath conditions and stirred until complete, resulting in a viscous solution. Toluene and isoquinoline were added to a viscous solution, with a mass ratio of toluene, isoquinoline and ODA of 2.5:0.6:12. The mixture was heated to reflux at 155°C for 7 hours to achieve partial iminolation and obtain a PI solution. PVA and ammonium bicarbonate were added to water and dissolved. The mass ratio of PVA, ammonium bicarbonate and water was 1.6:5:50. The mixture was stirred and dissolved to obtain an aqueous phase. The modified PI solution was added dropwise to the aqueous phase and emulsified by high-speed shearing at a speed of 3500 r / min to obtain an emulsion structure with an oil shell and an aqueous core. The emulsion was transferred to a reaction vessel and subjected to a gradient temperature increase reaction. First, the temperature was increased to 75°C and held for 4 hours, then the temperature was increased to 150°C and held for 4 hours until the droplets in the emulsion solidified into microspheres. The solidified microspheres were filtered and separated, dried, and then heated to 330℃ for 2 hours to obtain PI particles. The PI particles were then modified with KH-560 to obtain elastic PI particles.

[0036] Comparative Example 7 The difference between Comparative Example 7 and Example 5 is that PVA and ammonium bicarbonate are not added; A coating for castings comprises the following components: modified quartz powder: 63 parts, elastic PI particles: 4 parts, phenolic resin: 4 parts, bentonite: 2.5 parts, PI chopped fibers: 4 parts, CMC: 0.8 parts, PVA: 1.5 parts, defoamer BYK-066N: 0.35 parts, and water: 30 parts. The modified quartz powder is obtained by compounding quartz powders with different particle size ranges, with 50% being 200-350 mesh quartz powder, 35% being 400-600 mesh quartz powder, and 15% being 700-1000 mesh quartz powder; the PI short-cut fiber has a length of 1-5 mm and a fiber diameter of 8-10 μm.

[0037] The above-mentioned method for preparing coatings for castings includes the following steps: S1. Quartz powder, Tris and dopamine were added to water in a mass-volume ratio of 100g:1.3g:4g:130mL. The mixture was stirred at room temperature at 900rpm for 7h to obtain modified quartz powder. S2. Add bentonite to water, stir well, then add CMC and PVA, and stir to obtain a homogeneous solution; S3. Add the quartz powder to the solution obtained in S2 in three portions, stir well, then add the phenolic resin, elastic PI particles, and chopped PI fibers in sequence, stir well again, add the defoamer, mix well, and obtain the casting coating. Preferably, the method for preparing the elastic PI particles includes the following steps: ODA was added to DMF and stirred until dissolved. The molar ratio of ODA to PMDA was 1:1.02. 5000-molecular-weight amino-terminated polydimethylsiloxane and PMDA were added. The mass ratio of amino-terminated polydimethylsiloxane to ODA was 1.5:10. The reaction was carried out under ice bath conditions and stirred until complete to obtain a viscous solution. Toluene and isoquinoline were added to a viscous solution. The mass ratio of toluene, isoquinoline and amino-terminated polydimethylsiloxane was 2.5:0.6:2. The solution was heated to reflux at 155°C for 7 hours to achieve partial iminolation and obtain a siloxane-modified PI solution. A siloxane-modified PI solution was added dropwise to water and emulsified by high-speed shearing at a speed of 3500 r / min to obtain an emulsion. The emulsion was transferred to a reaction vessel and subjected to a gradient temperature increase reaction. First, the temperature was increased to 75°C and held for 4 hours, then the temperature was increased to 150°C and held for 4 hours until the droplets in the emulsion solidified into microspheres. The solidified microspheres were filtered and separated, dried, and then heated to 330℃ for 2 hours to obtain PI particles. The PI particles were then modified with KH-560 to obtain elastic PI particles.

[0038] Coating adhesion strength: The adhesion strength between the coating and the metal mold was measured using an impact test. First, a thin steel plate with dimensions of 300 mm × 300 mm was heated to (200±5)℃. Then, a pressure of 0.5~0.6 MPa was applied by spraying. During spraying, the coating direction was at a 45° angle to the surface of the thin steel plate, and the nozzle was approximately 0.4~0.5 m away from the surface. A coating layer with a thickness of 0.5~0.7 mm was uniformly sprayed onto the steel plate surface. After the coating dried, a 300 g circular weight was dropped freely from a height of 60 cm to continuously impact the steel plate. Ten tests were conducted on the back side. Based on the peeling of the coating on the steel plate surface, the coating adhesion strength was divided into four levels: I, II, III, and IV. Level I was characterized by an intact coating surface with no peeling as observed by the naked eye; Level II was characterized by slight peeling with a peeling area of ​​less than 10%; Level III was characterized by a peeling area between 10% and 30%; and Level IV was characterized by large-area peeling with a peeling area of ​​more than 30%.

[0039] High-temperature thermal crack resistance: First, the coating is sprayed onto a 300 mm × 300 mm metal plate heated to (200±5)℃. After the coating is completely dry, the metal plate sample is placed in a box furnace at 700℃ for rapid heating and holding for 2 minutes. The furnace door is then quickly opened to observe whether there are cracks in the coating, as well as the size and number of cracks. The crack condition of the coating can be evaluated according to the following 4 levels: ① Level I: The surface is smooth with no cracks, or only very fine cracks, and there is no peeling between the substrate and the coating; ② Grade II: The coating has fine network or dendritic cracks, with a crack width of <0.5 mm, and there is no peeling between the substrate and the coating; ③ Grade III: The coating has network or dendritic cracks, with a width of <1 mm, and the cracks are relatively deep. There are no through-cracks, and there is no peeling between the substrate and the coating; ④ Grade IV: The coating has network or dendritic cracks, with a width of >1 mm, and there are through-cracks. There is peeling between the substrate and the coating.

[0040] Coating adhesion strength / grade High temperature crack resistance / grade Gas generation volume (mL / g) Example 1 Ⅰ Ⅰ 22.6 Example 2 Ⅰ Ⅰ 25.7 Example 3 Ⅰ Ⅰ 26.1 Example 4 Ⅰ Ⅰ 23.9 Example 5 Ⅰ Ⅰ 24.3 Comparative Example 1 Ⅲ Ⅱ 24.9 Comparative Example 2 Ⅱ Ⅲ 23.5 Comparative Example 3 Ⅲ Ⅱ 26.3 Comparative Example 4 Ⅱ Ⅱ 27.2 Comparative Example 5 Ⅱ Ⅱ 28.4 Comparative Example 6 Ⅲ Ⅱ 26.6 Comparative Example 7 Ⅰ Ⅱ 26.8 As can be seen from the table above, in Comparative Example 1, no elastic PI particles were added. The epoxy groups on the surface of the PI microspheres formed chemical bonds with the hydroxyl groups of the modified quartz powder and the hydroxyl groups of the phenolic resin. At the same time, the elastic deformation of the microspheres could offset the shrinkage stress during the curing of the coating and avoid the formation of microcracks at the interface. Without the elastic buffer of PI microspheres, the shrinkage stress could not be released during the drying of the coating, and microcracks would be generated inside the coating.In Comparative Example 2, no PI chopped fibers were added. The PI chopped fibers form a three-dimensional interwoven network in the coating, strengthening the interfacial bonding through mechanical interlocking and simultaneously inhibiting crack initiation during curing. At high temperatures, after the phenolic resin decomposes, without the support of the fiber-supported three-dimensional skeleton, the coating is prone to localized collapse due to molten metal erosion. Furthermore, the high-temperature resistance of the fibers (maintaining their shape above 500℃) can compensate for the strength loss after the binder decomposes. In Comparative Example 3, the modified quartz powder was not particle size-blended. In the examples, coarse, medium, and fine particle sizes were blended, resulting in a densely packed structure of quartz powder. The binder can fully fill the voids, forming… The aggregate and binder are tightly bonded as a whole; without compounding, the porosity increases, the binder cannot completely fill, and there are a large number of pores inside the coating, reducing the interfacial contact area. These pores become stress concentration points, leading to a decrease in adhesion strength. Simultaneously, the uneven distribution of thermal expansion stress from single-size aggregates causes cracks. Without a dense packing structure for support, the coating density is insufficient, allowing molten metal to easily penetrate into the pores. Furthermore, the inability of gas to escape in an orderly manner leads to accumulated pressure, causing the coating to bulge and crack. Crack resistance and sand adhesion resistance decrease simultaneously. In Comparative Example 4, the quartz powder was not modified. In the examples, the dopamine-modified quartz powder surface contains hydroxyl and amino groups, and the phenolic resin and PI... Microspheres form chemical bonds. Unmodified quartz powder contains only a small amount of silanol groups on its surface, resulting in poor compatibility with the organic phase, easy agglomeration, extremely weak bonding force, and a significant decrease in adhesion strength. In Comparative Example 5, the elastic PI particles were not partially imidized. Partial imidization reduces the fluidity of polymer segments, which can limit the aggregation of siloxane segments and ensure uniform dispersion in the PI matrix. This ensures that the siloxane segments can form a continuous high-temperature resistant network during subsequent high-temperature crosslinking, increasing the decomposition temperature of the microspheres. In Comparative Example 6, the elastic PI particles do not contain amino-terminated polydimethylsiloxane. The siloxane segments of PDMS can optimize the surface polarity of PI particles, improving compatibility with the coating system. At the same time, the flexible characteristics of PDMS can alleviate the interfacial stress between the particles and surrounding components, while improving the heat resistance of PI microspheres. The high bond energy of the siloxane segments (Si-O-Si) is the core guarantee for the high-temperature stability of PI particles. Without PDMS, the thermal decomposition temperature of PI particles decreases, making it unable to support the coating structure. Without the flexible segments of PDMS, the PI... The elasticity of the particles decreased significantly, making them unable to buffer high-temperature stress and the impact of molten metal. They also had poor compatibility with sand molds and castings, and cracks were prone to initiation and propagation. In Comparative Example 7, PVA and ammonium bicarbonate were not added. Ammonium bicarbonate was the only source of the pore structure of the PI microspheres. After the simultaneous imidization was completed, the ammonium bicarbonate decomposed during the high-temperature crosslinking stage to produce gases such as CO2 and NH3, forming a "closed-pore + semi-open-pore" structure in the PI matrix. This pore structure can not only buffer the shrinkage stress during coating drying, but also store the gases produced by the high-temperature decomposition of phenolic resin, preventing gas accumulation that could lead to coating bulging. PVA can slightly participate in the interfacial interaction during the simultaneous imidization process, improving the compatibility of PI microspheres with siloxane segments.

[0041] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A coating for castings, characterized in that, Includes the following ingredients: Modified quartz powder: 60-65 parts Elastic PI particles: 3-5 parts Phenolic resin: 3-5 parts Bentonite: 2-3 parts PI short-cut fibers: 3-5 parts CMC: 0.8~1 part PVA: 1~1.8 parts Defoamer: 0.2~0.4 parts Water: 25-30 parts; The modified quartz powder is obtained by compounding quartz powders of different particle size ranges, with 200-350 mesh quartz powder accounting for 40-50%, 400-600 mesh quartz powder accounting for 30-40%, and 700-1000 mesh accounting for 10-20%.

2. The coating for castings according to claim 1, characterized in that: The defoamer is a silicone defoamer, and the silicone defoamer is any one of BYK-066N, SILFOAM SC 120 or Dow Corning DC-1500; The PI short-cut fibers have a length of 1-5 mm and a diameter of 8-10 μm.

3. The coating for castings according to claim 1, characterized in that: The preparation method of the elastic PI particles includes the following steps: ODA was added to DMF and stirred until dissolved. Then, amino-terminated polydimethylsiloxane and PMDA were added in sequence. The reaction was carried out under ice bath conditions and stirred until complete, resulting in a viscous solution. Toluene and isoquinoline were added to a viscous solution, and the mixture was heated to reflux to achieve partial iminoization, resulting in a siloxane-modified PI solution. PVA and ammonium bicarbonate were added to water and dissolved, and the mixture was stirred to obtain an aqueous phase. A siloxane-modified PI solution was added dropwise to the aqueous phase and emulsified by high-speed shearing to obtain an oil-shell-aqueous-core emulsion structure. The emulsion was transferred to a reaction vessel and subjected to a gradient temperature increase reaction until the droplets in the emulsion solidified into microspheres; The solidified microspheres were filtered and separated, dried and then heated to obtain PI particles. The PI particles were then modified with KH-560 to obtain elastic PI particles.

4. The coating for castings according to claim 1, characterized in that: The molar ratio of ODA to PMDA is 1:1 to 1.02, the molecular weight of amino-terminated polydimethylsiloxane is 3000 to 5000, and the mass ratio of amino-terminated polydimethylsiloxane to ODA is 1 to 2:

10.

5. The coating for castings according to claim 1, characterized in that: The mass ratio of toluene, isoquinoline, and amino-terminated polydimethylsiloxane is 2~3:0.4~0.8:2, the reaction temperature is 150~160℃, and the reaction time is 6~8h.

6. The coating for castings according to claim 1, characterized in that: The mass ratio of PVA, ammonium bicarbonate and water in the aqueous phase is 1~2:4~6:

50.

7. The coating for castings according to claim 1, characterized in that: The high-speed shearing rotation speed is 3000~3500 r / min.

8. The coating for castings according to claim 1, characterized in that: The gradient heating reaction involves first heating to 70-80℃ and holding for 4 hours, then heating to 150℃ and holding for 4 hours.

9. The coating for castings according to claim 1, characterized in that: The heating temperature is 300~350℃, and the holding time is 2h.

10. The method for preparing a coating for castings according to claim 1, characterized in that, Includes the following steps: S1. Quartz powder, Tris and dopamine are added to water, with a mass-to-volume ratio of quartz powder, Tris, dopamine and water of 100g:1~1.5g:3~5g:100~150mL. The mixture is stirred at room temperature at 800~1000rpm for 6~8h to obtain modified quartz powder. S2. Add bentonite to water, stir well, then add CMC and PVA, and stir to obtain a homogeneous solution; S3. Add quartz powder to the solution obtained in S2 in three portions, stir well, then add phenolic resin, elastic PI particles, and chopped PI fibers in sequence, stir well again, add defoamer, mix well, and obtain a coating for casting.