High-strength and high-toughness precoated sand and preparation method thereof

Abstract

The invention belongs to the technical field of casting materials, and discloses high-strength and high-toughness precoated sand and a preparation method thereof. The precoated sand comprises the following components in parts by weight: 100 parts of crude sand, 3-8 parts of a composite modified binder, 0.7-2.9 parts of a multifunctional composite additive, 0.3-0.8 part of a curing agent and 0.1-0.3 part of a lubricant, the composite modified binder is composed of thermoplastic phenolic resin, a toughening modifier and a reinforcing modifier, and the multifunctional composite additive comprises a reinforcing filler, a toughening filler and an auxiliary functional filler. The flexible toughening component and the interface reinforcing component are introduced into a binder system, and the synergistic effect of the nanoscale reinforcing filler and the thermally-decomposable toughening filler is matched, so that the bending strength and the impact toughness of the precoated sand are remarkably improved on the premise that the gas forming amount and the collapsibility are guaranteed.

Description

Technical Field

[0001] This invention belongs to the field of casting materials technology, specifically relating to a high-strength, high-toughness coated sand and its preparation method. Background Technology

[0002] Coated sand is a type of molding sand material used in casting. It is made by using raw sand as aggregate and thermoplastic phenolic resin as the main binder, which is then heated and coated with a curing agent. It is widely used in shell casting and core-making processes. Coated sand has advantages such as high molding accuracy, good surface quality, and ease of operation, and is widely used in the production of automotive engine blocks, cylinder heads, and various complex thin-walled castings. However, with the increasing complexity of casting structures and the continuous improvement of dimensional accuracy requirements, higher demands are being placed on the comprehensive performance of coated sand, especially the synergistic improvement of strength and toughness, which has become a key focus of the industry.

[0003] In existing technologies, coated sand typically uses a single thermoplastic phenolic resin as a binder. While this type of resin has high crosslinking density and strong structural rigidity, providing high room-temperature flexural strength, it suffers from low elongation at break and poor impact resistance, making it prone to brittle fracture during handling, core casting, or filling complex cavities. Furthermore, simply increasing the resin content to improve strength leads to increased gas generation, increased porosity defects in the casting, and affects collapsibility, hindering subsequent cleaning. Some technical solutions attempt to improve performance by adding rubber-based toughening agents or inorganic fillers, but these often only improve a single property, easily resulting in insufficient interfacial compatibility, uneven system dispersion, or decreased strength, making it difficult to achieve simultaneous improvement in strength and toughness. Therefore, how to improve the strength, toughness, and overall performance of coated sand through binder structural design and the synergistic effect of multiple functional fillers without significantly increasing resin usage has become a pressing technical problem in this field. Summary of the Invention

[0004] To address the shortcomings mentioned in the background art, the present invention aims to provide a high-strength, high-toughness coated sand and its preparation method. This is achieved by constructing a composite modified binder system composed of thermoplastic phenolic resin, toughening modifier, and reinforcing modifier, and combining it with reinforcing fillers, toughening fillers, and auxiliary functional fillers to form a multifunctional composite additive, thereby achieving a synergistic effect of interface reinforcement and toughness regulation. Under the premise of controlling the resin dosage, the flexural strength and impact toughness of the coated sand are significantly improved, while also considering gas evolution, collapsibility, and flowability. The preparation process is stable and suitable for industrial applications.

[0005] The objective of this invention can be achieved through the following technical solutions: A high-strength, high-toughness coated sand comprises the following raw materials in parts by weight: 100 parts raw sand, 3-8 parts composite modified binder, 0.7-2.9 parts multifunctional composite additive, 0.3-0.8 parts curing agent, and 0.1-0.3 parts lubricant; The composite modified adhesive is composed of thermoplastic phenolic resin, toughening modifier and reinforcing modifier, wherein the weight ratio of thermoplastic phenolic resin, toughening modifier and reinforcing modifier is 100:(5~15):(2~8). The multifunctional composite additive consists of 0.3 to 1.1 parts of reinforcing filler, 0.25 to 1.0 parts of toughening filler, and 0.15 to 0.8 parts of auxiliary functional filler.

[0006] More preferably, the toughening modifier is a mixture of polyamide and liquid nitrile rubber in a weight ratio of 1:(0.8-1.2), and the reinforcing modifier is a mixture of silane coupling agent KH560 and cashew nut shell powder in a weight ratio of 1:(1.5-2.5).

[0007] More preferably, the reinforcing filler is nano-sized graphite powder with a particle size of 50-100 nm, the toughening filler is selected from one or more of α-starch, β-starch, and dextrin, and the auxiliary functional filler is selected from one of molybdenum disulfide or boron nitride.

[0008] More preferably, the raw sand is silica sand that has undergone surface coupling treatment, with a particle size of 70-140 mesh, the curing agent is hexamethylenetetramine, and the lubricant is selected from one or more of calcium stearate, zinc stearate, magnesium stearate, and paraffin wax.

[0009] More preferably, the softening point of the thermoplastic phenolic resin is 85–105°C.

[0010] More preferably, the liquid nitrile rubber has an acrylonitrile content of 25-35% and a number average molecular weight of 2000-5000.

[0011] A method for preparing high-strength, high-toughness coated sand includes the following steps: S1. Heat thermoplastic phenolic resin to a molten state, then add toughening modifier and reinforcing modifier in sequence and mix to obtain a composite modified adhesive; S2. Mix the reinforcing filler, toughening filler, and auxiliary functional filler evenly to obtain a multifunctional composite additive; S3. The raw sand is roasted, cooled, and then a silane coupling agent is added for surface modification. Sand particles of different sizes are then graded and mixed. S4. Heat the raw sand treated in step S3 and mix it with the composite modified binder obtained in step S1. Then, add the multifunctional composite additive, curing agent and lubricant obtained in step S2 in sequence and mix. S5. Cool and screen the coated material to obtain the high-strength and high-toughness coated sand.

[0012] More preferably, in step S1, the thermoplastic phenolic resin is heated to 80-90°C and maintained at this temperature, and toughening modifier and reinforcing modifier are added sequentially. The mixture is stirred at 80-90°C for 30-40 minutes at a stirring rate of 150-200 r / min, and then cooled to room temperature to obtain a composite modified adhesive.

[0013] More preferably, in step S3, the raw sand is placed in a roasting furnace and roasted for 2-3 hours, cooled to 100-120°C, and 0.1-0.2% of the weight of the raw sand is added to the silane coupling agent KH560 for 15-20 minutes. Then, sand particles with different particle size distributions are mixed at a weight ratio of 2:1.

[0014] More preferably, in step S4, the pretreated raw sand is heated to 120-140°C, a composite modified binder is added and mixed, then the material temperature is adjusted to 90-110°C, a curing agent and a lubricant are added and mixing continues.

[0015] The beneficial effects of this invention are: This invention achieves a simultaneous improvement in the strength and toughness of coated sand by constructing a synergistic structure of a composite modified binder system and a multifunctional composite additive system. Regarding the binder, by introducing polyamide and liquid nitrile rubber toughening modifiers into thermoplastic phenolic resin, the resin system forms a flexible dispersed phase with a microphase separation structure during curing. Under external force, this phase can absorb impact energy through cavitation and shear yielding mechanisms, thereby effectively improving fracture toughness. Simultaneously, the use of silane coupling agents and cashew phenol as reinforcing modifiers enhances the interfacial bonding force between resin molecular chains and between the resin and sand grain surfaces, improving overall structural density and flexural strength. In the multifunctional composite additive, nano-sized graphite powder fills the micropores in the resin film and blunts crack tips, improving the density and heat resistance of the coating layer. Starch-based toughening fillers decompose at high temperatures to form a microporous structure, helping to release internal stress and improve collapsibility, reducing the risk of sand adhesion to castings. Molybdenum disulfide or boron nitride, as auxiliary functional fillers, can reduce frictional resistance between sand grains and improve fluidity and filling capacity. Attached Figure Description

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

[0017] Figure 1 The graphs show a comparison of the flexural strength of the coated sand prepared in Examples 1-3 and Comparative Examples 1-2 after treatment at room temperature and 500°C, respectively. 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: The high-strength and high-toughness coated sand contains the following raw materials in parts by weight: 100 parts raw sand, 3 parts composite modified binder, 0.7 parts multifunctional composite additive, 0.3 parts curing agent, and 0.1 parts lubricant; The preparation steps of the high-strength and high-toughness coated sand are as follows: S1. Add 2.804 kg of thermoplastic phenolic resin to a stainless steel mixing vessel equipped with a jacketed heating system and mechanical stirring. Heat and maintain the material temperature inside the vessel at 80°C. Set the stirring speed to 150 r / min. At this temperature, add 0.078 kg of polyamide, 0.062 kg of liquid nitrile rubber, 0.022 kg of silane coupling agent KH560, and 0.034 kg of cashew phenol in sequence. Continue stirring at 80°C for 30 min to homogenize the system. Then stop heating and cool down to room temperature with cooling water to obtain 3.000 kg of composite modified binder for later use.

[0020] S2. Add 0.300 kg of nano-sized graphite powder (particle size 50 nm), 0.250 kg of α-starch and 0.150 kg of molybdenum disulfide to a high-speed mixer, set the speed to 1000 r / min and mix for 10 min until uniform, to obtain 0.700 kg of multifunctional composite additive, and seal for later use.

[0021] S3. Place 100,000 kg of silica sand (70-140 mesh) in a calcining furnace and heat it to 600℃ and keep it at that temperature for 2 h. After removing it from the furnace, cool it to 100℃. Transfer the cooled silica sand to a sand mixer, add 0.100 kg of silane coupling agent KH560 and mix for 15 min to complete the surface modification. Then mix the coarse sand and fine sand at a 2:1 ratio to obtain the pretreated raw sand.

[0022] S4. Add the pretreated raw sand obtained in step S3 to a special heated sand mixer for coated sand, heat it to 120°C, add 3.000 kg of composite modified binder and stir at 100 r / min for 3 min; then adjust the material temperature to 90°C, add 0.700 kg of multifunctional composite additive, 0.300 kg of hexamethylenetetramine and 0.100 kg of calcium stearate in sequence, adjust the stirring speed to 200 r / min and continue mixing for 5 min to obtain the coated material.

[0023] S5. After the obtained coated material is discharged and cooled to room temperature, it is screened through a 50-mesh sieve to remove agglomerated particles, thus obtaining a high-strength and high-toughness coated sand product.

[0024] Example 2: The high-strength and high-toughness coated sand contains the following raw materials in parts by weight: 100 parts raw sand, 8 parts composite modified binder, 2.9 parts multifunctional composite additive, 0.8 parts curing agent, and 0.3 parts lubricant; The preparation steps of the high-strength and high-toughness coated sand are as follows: S1. Add 6.504 kg of thermoplastic phenolic resin to a stainless steel mixing vessel equipped with a jacketed heating system and mechanical stirring. Heat and maintain the material temperature inside the vessel at 90°C. Set the stirring speed to 200 r / min. At this temperature, add 0.443 kg of polyamide, 0.532 kg of liquid nitrile rubber, 0.149 kg of silane coupling agent KH560, and 0.372 kg of cashew phenol in sequence. Continue stirring at 90°C for 40 min to homogenize the system. Then stop heating and cool down to room temperature with cooling water to obtain 8.000 kg of composite modified binder for later use.

[0025] S2. Add 1.1 kg of nano-sized graphite powder (particle size 100 nm), 1.0 kg of β starch and 0.8 kg of boron nitride to a high-speed mixer, set the speed to 1000 r / min and mix for 10 min until uniform, to obtain 2.9 kg of multifunctional composite additive, and seal for later use.

[0026] S3. Place 100.0 kg of silica sand (70-140 mesh) in a calcining furnace and heat it to 600℃ and keep it at that temperature for 3 h. After removing it from the furnace, cool it to 120℃. Transfer the cooled silica sand to a sand mixer, add 0.200 kg of silane coupling agent KH560 and mix for 20 min to complete the surface modification. Then mix the coarse sand and fine sand at a 2:1 ratio to obtain the pretreated raw sand.

[0027] S4. Add the pretreated raw sand obtained in S3 to a special heated sand mixer for coated sand, heat it to 140℃, add 8.0 kg of composite modified binder and stir at 100 r / min for 3 min; then adjust the material temperature to 110℃, add 2.9 kg of multifunctional composite additive, 0.8 kg of hexamethylenetetramine and 0.3 kg of lubricant zinc stearate in sequence, adjust the stirring speed to 200 r / min and continue mixing for 5 min to obtain the coated material.

[0028] S5. After the obtained coated material is discharged and cooled to room temperature, it is screened through a 50-mesh sieve to remove agglomerated particles, thus obtaining the high-strength and high-toughness coated sand.

[0029] Example 3: The high-strength and high-toughness coated sand contains the following raw materials in parts by weight: 100 parts raw sand, 5.5 parts composite modified binder, 1.8 parts multifunctional composite additive, 0.55 parts curing agent, and 0.2 parts lubricant; The preparation steps of the high-strength and high-toughness coated sand are as follows: S1. Add 4.783 kg of thermoplastic phenolic resin to a stainless steel mixing vessel equipped with a jacketed heating system and mechanical stirring. Heat and maintain the material temperature inside the vessel at 85°C. Set the stirring speed to 175 r / min. At this temperature, add 0.239 kg of polyamide, 0.239 kg of liquid nitrile rubber, 0.080 kg of silane coupling agent KH560, and 0.159 kg of cashew phenol in sequence. Continue stirring at 85°C for 35 min to homogenize the system. Then stop heating and cool down to room temperature with cooling water to obtain 5.500 kg of composite modified binder.

[0030] S2. Add 0.7 kg of nano-sized graphite powder (particle size 75 nm), 0.625 kg of α-starch and 0.475 kg of molybdenum disulfide to a high-speed mixer, set the speed to 1000 r / min and mix for 10 min until uniform, to obtain 1.800 kg of multifunctional composite additive, and seal for later use.

[0031] S3. Place 100.0 kg of silica sand (70-140 mesh) in a calcining furnace and heat it to 600℃ and keep it at that temperature for 2.5 h. After removing it from the furnace, cool it to 110℃. Transfer the cooled silica sand to a sand mixer, add 0.150 kg of silane coupling agent KH560 and mix for 17.5 min to complete the surface modification. Then mix the coarse sand and fine sand at a 2:1 ratio to obtain the pretreated raw sand.

[0032] S4. Add the pretreated raw sand obtained in step S3 to a special heated sand mixer for coated sand, heat it to 130°C, add 5.5 kg of composite modified binder and stir and mix at 150 r / min for 3 min; then adjust the material temperature to 100°C, add 1.8 kg of multifunctional composite additive, 0.55 kg of hexamethylenetetramine and 0.2 kg of lubricant magnesium stearate in sequence, adjust the stirring speed to 175 r / min and continue mixing for 5 min to obtain the coated material.

[0033] S5. After the obtained coated material is discharged and cooled to room temperature, it is screened through a 50-mesh sieve to remove agglomerated particles, thus obtaining the high-strength and high-toughness coated sand.

[0034] Comparative Example 1: The high-strength and high-toughness coated sand contains the following raw materials in parts by weight: 100 parts raw sand, 5.5 parts composite modified binder, 1.8 parts multifunctional composite additive, 0.55 parts curing agent, and 0.2 parts lubricant; The composite modified adhesive does not contain toughening modifiers. The preparation steps of the high-strength and high-toughness coated sand are as follows: S1. Add 5.261 kg of thermoplastic phenolic resin to a stainless steel mixing vessel equipped with a jacketed heating system and mechanical stirring. Heat and maintain the temperature of the material inside the vessel at 85°C. Set the stirring speed to 175 r / min. At this temperature, add 0.080 kg of silane coupling agent KH560 and 0.159 kg of cashew phenol in sequence. Continue stirring at 85°C for 35 min to make the system homogeneous. Then stop heating and cool down to room temperature with cooling water to obtain 5.500 kg of composite binder.

[0035] S2. Add 0.7 kg of nano-sized graphite powder (particle size 75 nm), 0.625 kg of α-starch and 0.475 kg of molybdenum disulfide to a high-speed mixer, set the speed to 1000 r / min and mix for 10 min until uniform to obtain 1.8 kg of multifunctional composite additive.

[0036] S3. Place 100.0 kg of silica sand (70-140 mesh) in a calcining furnace and heat it to 600℃ and keep it at that temperature for 2.5 h. After removing it from the furnace, cool it to 110℃. Transfer the cooled silica sand to a sand mixer, add 0.150 kg of silane coupling agent KH560 and mix for 17.5 min. Then mix the coarse sand and fine sand at a 2:1 ratio to obtain the pretreated raw sand.

[0037] S4. Add the pretreated raw sand obtained in step S3 to a special heated sand mixer for coated sand, heat it to 130°C, add 5.5 kg of composite binder and stir at 150 r / min for 3 min; then adjust the temperature to 100°C, and add 1.8 kg of multifunctional composite additive, 0.55 kg of hexamethylenetetramine and 0.2 kg of lubricant magnesium stearate in sequence, adjust the stirring speed to 175 r / min and continue mixing for 5 min to obtain the coated material.

[0038] S5. After the obtained coated material is discharged and cooled to room temperature, it is screened through a 50-mesh sieve to remove agglomerated particles, thus obtaining the high-strength and high-toughness coated sand.

[0039] Comparative Example 2: The high-strength and high-toughness coated sand contains the following raw materials in parts by weight: 100 parts raw sand, 5.5 parts composite modified binder, 0.55 parts curing agent, and 0.2 parts lubricant; The preparation steps of the high-strength and high-toughness coated sand are as follows: S1. Add 4.783 kg of thermoplastic phenolic resin to a stainless steel mixing vessel equipped with a jacketed heating system and mechanical stirring. Heat and maintain the material temperature inside the vessel at 85°C. Set the stirring speed to 175 r / min. At this temperature, add 0.239 kg of polyamide, 0.239 kg of liquid nitrile rubber, 0.080 kg of silane coupling agent KH560, and 0.159 kg of cashew phenol in sequence. Continue stirring at 85°C for 35 min to homogenize the system. Then stop heating and cool down to room temperature with cooling water to obtain 5.500 kg of composite modified binder.

[0040] S2. Place 100.0 kg of silica sand (70-140 mesh) in a calcining furnace and heat it to 600℃ and keep it at that temperature for 2.5 h. After removing it from the furnace, cool it to 110℃. Transfer the cooled silica sand to a sand mixer, add 0.150 kg of silane coupling agent KH560 and mix for 17.5 min to complete the surface modification. Then mix the coarse sand and fine sand at a 2:1 ratio to obtain the pretreated raw sand.

[0041] S3. Add the pretreated raw sand to a special heated sand mixer for coated sand, heat it to 130℃, add 5.5 kg of composite modified binder and stir at 150 r / min for 3 min; then adjust the material temperature to 100℃, add 0.550 kg of hexamethylenetetramine and 0.200 kg of lubricant magnesium stearate, adjust the stirring speed to 175 r / min and continue mixing for 5 min to obtain the coated material.

[0042] S4. After the obtained coated material is discharged and cooled to room temperature, it is screened through a 50-mesh sieve to remove agglomerated particles, thus obtaining the high-strength and high-toughness coated sand.

[0043] Performance testing 1. Tensile strength test at room temperature The room temperature tensile strength test was performed according to GB / T 2684-2025 "Test Methods for Foundry Sand and Mixtures". Each coated sand was placed into a standard figure-eight shaped tensile specimen mold, thermoset at 232±5℃ for 2 minutes, cooled to 20~30℃, and allowed to stand at this temperature for 24 hours before testing. An electronic universal testing machine was used, clamping both ends of the specimen and applying a constant loading rate of 2 mm / min until fracture. The maximum load at fracture was recorded and converted to tensile strength. At least 5 specimens were tested in each group, and the average value was taken as the room temperature tensile strength result for that group. The test results are shown in Table 1 below.

[0044] Table 1 Tensile strength at room temperature

[0045] As shown in Table 1, the room temperature tensile strength of Examples 1-3 is higher than that of Comparative Examples 1 and 2, indicating that the formulation of the present invention can effectively improve the load-bearing capacity of the coated sand at room temperature. Among them, Example 3 shows an improvement of about 25.5% compared to Comparative Example 1, indicating that after introducing polyamide and liquid nitrile rubber into the composite binder, the crack resistance and stress transfer efficiency of the resin film are improved, and the toughening component can dissipate energy and delay crack propagation during tensile testing. Example 3 shows an improvement of about 18.5% compared to Comparative Example 2, indicating that the multifunctional composite additive has a reinforcing effect on the densification of the coating layer and the interfacial bonding, thereby further improving the tensile strength.

[0046] 2. Impact toughness test The test was conducted according to GB / T 2684-2025 "Test Methods for Foundry Sand and Mixtures". Each coated sand was placed into a U-notch impact test mold, heat-cured at 232±5℃ for 2 min, demolded, cooled to 20-30℃, and allowed to stand for 24 h. A pendulum impact tester with a pendulum energy of 30 J and an impact velocity of 2.9 m / s was used to perform a single impact on the specimens. The impact energy absorbed at fracture was recorded, and the impact toughness was calculated based on the effective cross-sectional area of ​​the specimen. At least 5 specimens were tested in each group, and the average value was taken. The results are shown in Table 2 below.

[0047] Table 2 Impact toughness of coated sand for each sample

[0048] As shown in Table 2, the impact toughness of Examples 1-3 is significantly higher than that of Comparative Examples 1 and 2, indicating that the fracture resistance of the coated sand under impact load is significantly enhanced. Example 3 shows the most significant improvement of approximately 77% compared to Comparative Example 1, demonstrating that the introduction of polyamide and liquid nitrile rubber into the composite modified binder can form an energy dissipation structure in the resin film. During impact, this structure achieves toughening through plastic deformation and crack propagation inhibition, effectively improving the brittle fracture problem of traditional phenolic coated sand. Example 3 shows an improvement of approximately 46% compared to Comparative Example 2, indicating that the multifunctional composite additive also has a synergistic toughening effect on the densification and crack passivation of the coating layer.

[0049] 3. High-temperature strength retention rate test The high-temperature strength retention rate test was performed according to GB / T 2684-2025 "Test Methods for Foundry Sand and Mixtures". Standard flexural strength specimens (80mm × 10mm × 4mm) were prepared from each formulation of coated sand, heat-cured at 232±5℃ for 2 minutes, demolded, cooled to 20–30℃, and left to stand at this temperature for 24 hours. The flexural strength σ(RT) of each group of specimens was first determined at room temperature using an electronic universal testing machine (loading rate 2mm / min). Separately, specimens from the same batch were placed in a 500℃ resistance furnace and held for 5 minutes. After removal, they were cooled in air to 20–30℃, and their flexural strength σ(500℃) was determined using the same electronic universal testing machine (loading rate 2mm / min). The high-temperature strength retention rate was calculated using the formula η = σ(500℃) / σ(RT) × 100%, where η is the high-temperature strength retention rate, σ(RT) is the room temperature flexural strength, and σ(500℃) is the flexural strength after insulation at 500℃. At least 5 pieces were tested in each group, and the average value was taken. The results are shown in Table 3 below.

[0050] Table 3. High-Temperature Strength Retention Rate Test Results

[0051] As shown in Table 3, the flexural strength and strength retention rate of each embodiment after treatment at 500℃ are higher than those of Comparative Example 1 and Comparative Example 2, indicating that the coated sand of the present invention has better structural stability and load-bearing capacity under high temperature. The retention rate of Example 3 is 57%, which is about 24% higher than that of Comparative Example 1 and about 27% higher than that of Comparative Example 2. This indicates that the introduction of toughening / reinforcing components in the composite modified binder can improve the thermal stability of the resin film and the interfacial bonding at high temperature, reduce the propagation of microcracks caused by thermal stress, and thus improve the strength retention after high temperature. Comparative Example 2 lacks multifunctional composite additives and has the lowest strength and retention rate after 500℃, indicating that fillers such as nano-graphite have a significant effect on the densification of the coating layer and crack passivation. Comparative Example 1 lacks a toughening system and also has a low retention rate, indicating that the toughening structure still helps to maintain the integrity of the film layer after high temperature thermal shock.

[0052] 4. Gas output test Take 2.00 g of each formulation of coated sand sample and let it stand in a constant temperature environment of 20-30℃ for 24 h to remove surface adsorbed moisture. Place the sample in the heating furnace of the gas evolution measuring device and rapidly heat it to 1000℃ at a rate of 10℃ / min, and hold it at that temperature for 3 min to pyrolyze the organic binder in the coated sand to generate gas. The generated gas is condensed and dust removed before being measured in a gas measuring cylinder with an accuracy of 0.1 mL. The gas evolution (mL / g) is calculated according to the sample mass. Each group is tested at least 3 times, and the average value is taken as the gas evolution result of that group. The test results are shown in Table 4 below.

[0053] Table 4 Gas output test results

[0054] As shown in Table 4, the gas evolution of Examples 1 to 3 is within the range of 12.0 to 12.8 mL·g⁻¹, with little overall difference compared to Comparative Examples 1 and 2. This indicates that the present invention achieves improved strength and toughness without significantly increasing gas evolution. Specifically, Example 3 has a gas evolution of 12.6 mL·g⁻¹, which is basically equivalent to Comparative Example 1 (12.4 mL·g⁻¹). This indicates that the composite modified binder improves mechanical properties through the synergistic effect of toughening / reinforcing components, without relying on simply increasing resin dosage to achieve strength, thus avoiding an increased risk of gas evolution. Example 3 is slightly higher than Comparative Example 2 (11.9 mL·g⁻¹), mainly because Comparative Example 2 lacks multifunctional composite additives and corresponding synergistic structures, and the difference in film density and pyrolysis pathway leads to differences in gas release.

[0055] 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.

[0056] 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 high-strength, high-toughness coated sand, characterized in that, It contains the following raw materials in parts by weight: 100 parts raw sand, 3-8 parts composite modified binder, 0.7-2.9 parts multifunctional composite additive, 0.3-0.8 parts curing agent, and 0.1-0.3 parts lubricant; The composite modified adhesive is composed of thermoplastic phenolic resin, toughening modifier and reinforcing modifier, wherein the weight ratio of thermoplastic phenolic resin, toughening modifier and reinforcing modifier is 100:(5~15):(2~8). The multifunctional composite additive consists of 0.3 to 1.1 parts of reinforcing filler, 0.25 to 1.0 parts of toughening filler, and 0.15 to 0.8 parts of auxiliary functional filler.

2. The high-strength, high-toughness coated sand according to claim 1, characterized in that, The toughening modifier is a mixture of polyamide and liquid nitrile rubber in a weight ratio of 1:(0.8-1.2), and the reinforcing modifier is a mixture of silane coupling agent KH560 and cashew nut shell powder in a weight ratio of 1:(1.5-2.5).

3. The high-strength, high-toughness coated sand according to claim 1, characterized in that, The reinforcing filler is nano-sized graphite powder with a particle size of 50-100 nm. The toughening filler is selected from one or more of α-starch, β-starch, and dextrin. The auxiliary functional filler is selected from one of molybdenum disulfide or boron nitride.

4. The high-strength, high-toughness coated sand according to claim 1, characterized in that, The raw sand is silica sand that has undergone surface coupling treatment, with a particle size of 70-140 mesh. The curing agent is hexamethylenetetramine, and the lubricant is selected from one or more of calcium stearate, zinc stearate, magnesium stearate, and paraffin wax.

5. The high-strength, high-toughness coated sand according to claim 1, characterized in that, The softening point of the thermoplastic phenolic resin is 85–105°C.

6. The high-strength, high-toughness coated sand according to claim 1, characterized in that, The liquid nitrile rubber has an acrylonitrile content of 25-35% and a number average molecular weight of 2000-5000.

7. A method for preparing high-strength, high-toughness coated sand as described in claim 1, characterized in that, Includes the following steps: S1. Heat thermoplastic phenolic resin to a molten state, then add toughening modifier and reinforcing modifier in sequence and mix to obtain a composite modified adhesive; S2. Mix the reinforcing filler, toughening filler, and auxiliary functional filler evenly to obtain a multifunctional composite additive; S3. The raw sand is roasted, cooled, and then a silane coupling agent is added for surface modification. Sand particles of different sizes are then graded and mixed. S4. Heat the raw sand treated in step S3 and mix it with the composite modified binder obtained in step S1. Then, add the multifunctional composite additive, curing agent and lubricant obtained in step S2 in sequence and mix. S5. Cool and screen the coated material to obtain the high-strength and high-toughness coated sand.

8. The method for preparing high-strength, high-toughness coated sand according to claim 7, characterized in that, In step S1, the thermoplastic phenolic resin is heated to 80-90°C and maintained at this temperature. Toughening modifier and reinforcing modifier are added sequentially. The mixture is stirred at 80-90°C for 30-40 minutes at a stirring rate of 150-200 r / min. Then it is cooled to room temperature to obtain a composite modified adhesive.

9. The method for preparing high-strength, high-toughness coated sand according to claim 7, characterized in that, In step S3, the raw sand is placed in a roasting furnace and roasted for 2-3 hours, cooled to 100-120°C, and 0.1-0.2% of the weight of the raw sand is added to the silane coupling agent KH560 for 15-20 minutes. Then, sand particles with different particle size distributions are mixed at a weight ratio of 2:

1.

10. The method for preparing high-strength, high-toughness coated sand according to claim 7, characterized in that, In step S4, the pretreated raw sand is heated to 120-140°C, a composite modified binder is added and mixed, the material temperature is then adjusted to 90-110°C, a curing agent and a lubricant are added and mixing continues.

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