Anti-moisture-absorption environment-friendly precoated sand and preparation method thereof

Abstract

The invention discloses moisture-absorption-resistant environment-friendly precoated sand and a preparation method thereof. According to the invention, long-chain alkyl trialkoxysilane and an epoxy silane coupling agent are subjected to co-hydrolytic polycondensation under an acidic condition, such that the organosilicon hydrophobic agent with epoxy reaction activity is prepared. And then in a film coating process, the surface of hot sand is coated with the hydrophobic agent, then thermosetting phenolic resin and a curing agent are sequentially added, and a final product is obtained after mixing, grinding, cooling and crushing. According to the method, a hydrophobic interface layer anchored through chemical bonds is constructed between sand grains and resin, a stable physical water blocking barrier is formed through long-chain alkyl, firm interface combination is achieved through active group reaction, and therefore the moisture absorption resistance of the precoated sand is fundamentally improved. Meanwhile, the environment-friendly resin with low free formaldehyde and low free phenol is combined, so that the gas forming amount of the product is remarkably reduced.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, and in particular to an environmentally friendly, moisture-resistant coated sand and its preparation method. Background Technology

[0002] Coated sand, as a key molding material in modern precision casting, directly determines the precision and quality of castings. Traditional coated sand commonly uses thermosetting phenolic resin as a binder. While it possesses good strength and thermal stability, the residual free formaldehyde and phenol in the resin are easily volatilized during preparation and use, polluting the environment and posing a threat to the health of operators. Particularly noteworthy is the high hygroscopicity of traditional coated sand due to its hydrophilic resin groups and porous structure. Moisture in the storage environment can penetrate the resin film of the sand particles, leading to premature localized hydrolysis of the resin, a significant decrease in bonding strength, and a surge in gas generation in the mold. This, in turn, causes defects such as porosity and sand adhesion in the castings, severely impacting production stability and product yield. Although the industry has attempted to improve this by adding silane coupling agents or using hydrophobic resins, the former's effects are short-lived and prone to failure, while the latter is often costly or sacrifices other process properties. Therefore, developing a new type of coated sand that fundamentally enhances hydrophobicity while also possessing environmentally friendly characteristics has become a crucial technological bottleneck that the casting industry urgently needs to overcome. Summary of the Invention

[0003] To address the aforementioned problems, this invention proposes an environmentally friendly, moisture-resistant coated sand and its preparation method, solving the key challenges of reduced strength, numerous casting defects, and poor environmental performance caused by the high hygroscopicity of traditional coated sand.

[0004] This invention can be achieved through the following technical solutions: A method for preparing a moisture-resistant, environmentally friendly coated sand includes the following steps: Step 1: Add deionized water, anhydrous ethanol, long-chain alkyltrialkoxysilane, and epoxysilane coupling agent to a flask, and co-hydrolyze and condense at pH 3.0-5.0 to obtain a reactive organosilicon hydrophobic agent; Step 2: Heat the raw sand to 140-160℃, add reactive organosilicon hydrophobic agent, mix and grind for 60-120s, then add thermosetting phenolic resin, mix and grind until the sand particles are evenly coated, then add hexamethylenetetramine, mix well, discharge the sand, cool, crush, and sieve to obtain moisture-resistant environmentally friendly coated sand.

[0005] Preferably, in step 1, the molar ratio of long-chain alkyltrialkoxysilane to epoxysilane coupling agent is (2-6):1.

[0006] Preferably, the hydrolysis and polycondensation temperature in step 1 is 55-65℃, and the duration is 4-6 hours.

[0007] Preferably, in step 2, the mass ratio of raw sand, reactive organosilicon hydrophobic agent, and thermosetting phenolic resin is 100:(0.1-1):(2-4).

[0008] Preferably, in step 2, the mass of hexamethylenetetramine is 15%-35% of the mass of the thermosetting phenolic resin, and the concentration is 20%-40%.

[0009] Preferably, the long-chain alkyltrialkoxysilane in step 1 is one of hexadecyltrimethoxysilane, octadecyltrimethoxysilane, and octadecyltriethoxysilane.

[0010] Preferably, the epoxy silane coupling agent in step 1 is 3-glycidyl etheroxypropyltrimethoxysilane or 3-glycidyl etheroxypropyltriethoxysilane.

[0011] Preferably, the thermosetting phenolic resin is an environmentally friendly phenolic resin.

[0012] The beneficial effects of this invention are: This invention achieves a synergistic breakthrough in the hygroscopic resistance and environmental friendliness of coated sand through molecular design and process innovation. Its core lies in the co-hydrolysis and condensation of long-chain alkylsilanes and epoxysilanes under mild acidic conditions to synthesize a novel organosilicon oligomer. In the subsequent preparation process, this invention employs a step-by-step process of "hydrophobic first, then coating": first, the hydrophobic agent is coated onto the surface of the hot sand, forming a dense barrier with its long hydrophobic chains, while the active epoxy end groups face outwards; subsequently, the added phenolic resin not only binds to the sand particles but also covalently bonds with the epoxy groups of the hydrophobic agent through a chemical reaction. This dual protection mechanism of "physical barrier-chemical anchoring" constructs a stable, complete, and highly hydrophobic transition interface layer between the sand particles and the resin, fundamentally cutting off the path of moisture erosion, allowing the coated sand to maintain extremely high bonding strength even under harsh humid and hot environments. Meanwhile, this dense interface layer acts like a protective film on the resin, significantly improving the thermal stability of the resin system and making its decomposition during high-temperature casting more controllable. Combined with carefully selected environmentally friendly low-free-phenolic resin, it drastically reduces the gas evolution of the coated sand. Therefore, this invention not only revolutionizes the waterproofing technology of coated sand from a mechanistic perspective but also achieves a balance between high strength retention, low environmental emissions, and high process stability in the final product, providing an ideal material solution for high-end precision casting. Attached Figure Description

[0013] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings: Figure 1 The results show the moisture resistance and environmental friendliness of the coated sand. Detailed Implementation

[0014] The following provides a detailed description of the embodiments of the present invention: These embodiments are implemented based on the technical solution of the present invention, and provide detailed implementation methods and processes. However, the scope of protection of the present invention is not limited to the following embodiments. Experimental methods in the following embodiments that do not specify specific conditions are generally performed under conventional conditions.

[0015] The environmentally friendly phenolic resin used in the following examples was purchased from Jinan Dahui Chemical Technology Co., Ltd. The CAS number is 25085-50-1.

[0016] Example 1: A method for preparing moisture-resistant, environmentally friendly coated sand, comprising the following steps: Step 1: Add 150 mL of anhydrous ethanol, 20 mL of deionized water, 0.2 mol of hexadecyltrimethoxysilane, and 0.1 mol of 3-glycidyl etheroxypropyltrimethoxysilane to a flask, and co-hydrolyze and condense at pH 3.0 and 55 °C for 6 h to obtain a reactive organosilicon hydrophobic agent. Step 2: Heat 1 kg of raw sand to 140°C, add 1 g of reactive organosilicon hydrophobic agent, mix and grind for 60 seconds, then add 20 g of environmentally friendly phenolic resin (free formaldehyde content less than 0.2%, free phenol content less than 1.0%), mix and grind until the sand particles are evenly coated, then add 45 g of hexamethylenetetramine, mix well, discharge the sand, cool, crush, and sieve to obtain moisture-resistant environmentally friendly coated sand.

[0017] Example 2: A method for preparing moisture-resistant, environmentally friendly coated sand, comprising the following steps: Step 1: Add 150 mL of anhydrous ethanol, 20 mL of deionized water, 0.4 mol of octadecyltrimethoxysilane and 0.1 mol of 3-glycidyl etheroxypropyltriethoxysilane to a flask, and co-hydrolyze and condense at pH 4.0 and 60 °C for 5 h to obtain a reactive organosilicon hydrophobic agent. Step 2: Heat 1 kg of raw sand to 150°C, add 5.5 g of reactive organosilicon hydrophobic agent, mix and grind for 90 seconds, then add 30 g of environmentally friendly phenolic resin (free formaldehyde content less than 0.2%, free phenol content less than 1.0%), mix and grind until the sand particles are evenly coated, then add 75 g of hexamethylenetetramine, mix well, discharge the sand, cool, crush, and sieve to obtain moisture-resistant environmentally friendly coated sand.

[0018] Example 3: A method for preparing moisture-resistant, environmentally friendly coated sand, comprising the following steps: Step 1: Add 150 mL of anhydrous ethanol, 20 mL of deionized water, 0.6 mol of octadecyltriethoxysilane, and 0.1 mol of 3-glycidyl etheroxypropyltrimethoxysilane to a flask, and co-hydrolyze and polycondense at pH 5.0 and 65 °C for 4 h to obtain a reactive organosilicon hydrophobic agent. Step 2: Heat 1kg of raw sand to 160℃, add 10g of reactive organosilicon hydrophobic agent, mix and grind for 120s, then add 40g of environmentally friendly phenolic resin (free formaldehyde content less than 0.2%, free phenol content less than 1.0%), mix and grind until the sand particles are evenly coated, then add 105g of hexamethylenetetramine, mix well, discharge the sand, cool, crush, and sieve to obtain moisture-resistant environmentally friendly coated sand.

[0019] Example 4: A method for preparing moisture-resistant, environmentally friendly coated sand, comprising the following steps: Step 1: Add 150 mL of anhydrous ethanol, 20 mL of deionized water, 0.6 mol of hexadecyltrimethoxysilane, and 0.1 mol of 3-glycidyl etheroxypropyltriethoxysilane to a flask, and co-hydrolyze and condense at pH 3.0 and 55 °C for 6 h to obtain a reactive organosilicon hydrophobic agent. Step 2: Heat 1kg of raw sand to 160℃, add 10g of reactive organosilicon hydrophobic agent, mix and grind for 60s, then add 20g of environmentally friendly phenolic resin (free formaldehyde content less than 0.2%, free phenol content less than 1.0%), mix and grind until the sand particles are evenly coated, then add 105g of hexamethylenetetramine, mix well, discharge the sand, cool, crush, and sieve to obtain moisture-resistant environmentally friendly coated sand.

[0020] Comparative Example 1: The difference between this comparative example and Example 1 is that no reactive organosilicon hydrophobic agent is added.

[0021] A method for preparing moisture-resistant and environmentally friendly coated sand is as follows: 1 kg of raw sand is heated to 140°C, 20 g of environmentally friendly phenolic resin (free formaldehyde content less than 0.2%, free phenol content less than 1.0%) is added, and the mixture is ground until the sand particles are evenly coated. Then, 45 g of hexamethylenetetramine is added, the mixture is mixed, the sand is discharged, cooled, crushed, and sieved to obtain moisture-resistant and environmentally friendly coated sand.

[0022] Comparative Example 2: The difference between this comparative example and Example 1 is that the reactive organosilicon hydrophobic agent and the environmentally friendly phenolic resin are first physically mixed evenly, and then added to the original sand.

[0023] A method for preparing a moisture-resistant, environmentally friendly coated sand includes the following steps: Step 1: Add 150 mL of anhydrous ethanol, 20 mL of deionized water, 0.2 mol of hexadecyltrimethoxysilane, and 0.1 mol of 3-glycidyl etheroxypropyltrimethoxysilane to a flask, and co-hydrolyze and condense at pH 3.0 and 55 °C for 6 h to obtain a reactive organosilicon hydrophobic agent. Step 2: First, physically mix 1g of reactive silicone hydrophobic agent with 20g of environmentally friendly phenolic resin (free formaldehyde content less than 0.2%, free phenol content less than 1.0%) until uniform. Then, add the mixture to 1kg of raw sand at 140℃ and grind until the sand particles are uniformly coated. Then, add 45g of hexamethylenetetramine, mix well, discharge the sand, cool, crush, and sieve to obtain moisture-resistant environmentally friendly coated sand.

[0024] Comparative Example 3: The difference between this comparative example and Example 1 is that 3-glycidyl etheroxypropyltrimethoxysilane replaces the reactive organosilicon hydrophobic agent.

[0025] A method for preparing moisture-resistant and environmentally friendly coated sand is as follows: 1 kg of raw sand is heated to 140°C, 1 g of 3-glycidyl etheroxypropyltrimethoxysilane is added, and the mixture is ground for 60 seconds. Then, 20 g of environmentally friendly phenolic resin (free formaldehyde content less than 0.2% and free phenol content less than 1.0%) is added and ground until the sand particles are uniformly coated. Subsequently, 45 g of hexamethylenetetramine is added, and the mixture is mixed thoroughly before the sand is discharged, cooled, crushed, and sieved to obtain moisture-resistant and environmentally friendly coated sand.

[0026] Performance testing 1. Moisture resistance test The tensile strength of the coated sand samples was tested according to GB / T 2684-2025 standard. The samples were placed in a constant temperature and humidity test chamber and accelerated moisture absorption aging was carried out at 40℃ and 90% relative humidity. The instantaneous tensile strength of the samples before aging (0h) and after 24h of aging was tested respectively. The 24h strength retention rate was calculated as: (strength after 24h of aging / initial strength) × 100%. The higher the value, the better the moisture absorption resistance.

[0027] 2. Environmental protection test Using a coated sand gas emission tester, 1.0 g of sample was weighed and placed in a ceramic boat, which was then quickly pushed into a tubular furnace quartz tube preheated to 850°C. The total amount of gas (mL) generated within 90 seconds was recorded as the gas emission of the sample.

[0028] Table 1. Test results of moisture resistance and environmental friendliness of coated sand.

[0029] As shown in Table 1, the 24-hour strength retention rate of Examples 1-4 all exceeded 84%, and the gas evolution was at a low level (≤13.1 mL / g). The excellent moisture resistance of the coated sand prepared in these examples is mainly due to the successful synthesis of a novel reactive organosilicon hydrophobic agent through the controlled co-hydrolysis and polycondensation of long-chain alkylsilanes and epoxy silanes under acidic conditions. This hydrophobic agent is applied to the surface of the hot sand before resin coating. The long-chain alkyl groups in its molecular structure form a low surface energy hydrophobic barrier, while the terminal epoxy groups can chemically react with the subsequently added phenolic resin, firmly "anchoring" the hydrophobic layer to the sand-resin interface. This stable and dense interface structure constructed by chemical bonding fundamentally blocks the penetration and adsorption of moisture. Meanwhile, in terms of environmental protection, its advantages are the result of a dual effect: on the one hand, the use of environmentally friendly resin with low free formaldehyde / phenol reduces the content of harmful substances from the source; on the other hand, the above-mentioned hydrophobic interface layer also plays a role in coating and stabilizing the resin to a certain extent, reducing its abnormal decomposition at high temperatures, thereby synergistically achieving low gas emission.

[0030] Compared to Example 1, Comparative Example 1 lacked the construction of a hydrophobic interface layer, allowing moisture to directly erode the resin-sand particle interface, leading to premature hydrolysis of hexamethylenetetramine and a sharp decrease in strength. Although Comparative Example 2 added the same hydrophobic agent, it was added after mixing with the resin, disrupting the orderly process of "first interface hydrophobization, then resin coating." This resulted in the hydrophobic agent failing to effectively orient itself on the sand particle surface and fully react with the resin, significantly reducing its hydrophobic effect. Comparative Example 3 directly used a small-molecule silane coupling agent, whose simple molecular structure prevented it from forming a durable and effective physical hydrophobic barrier. Its chemical stability was also poor, and it was prone to volatilization or self-polymerization during hot sand treatment. Therefore, it could not achieve the stable and durable dual protection of hydrophobicity and chemical bonding formed by the oligomers in the examples, resulting in limited improvement in moisture resistance and a negligible contribution to reducing gas generation.

[0031] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing moisture-resistant, environmentally friendly coated sand, characterized in that, Includes the following steps: Step 1: Add deionized water, anhydrous ethanol, long-chain alkyltrialkoxysilane, and epoxysilane coupling agent to a flask, and co-hydrolyze and condense at pH 3.0-5.0 to obtain a reactive organosilicon hydrophobic agent; Step 2: Heat the raw sand to 140-160℃, add reactive organosilicon hydrophobic agent, mix and grind for 60-120s, then add thermosetting phenolic resin, mix and grind until the sand particles are evenly coated, then add hexamethylenetetramine, mix well, discharge the sand, cool, crush, and sieve to obtain moisture-resistant environmentally friendly coated sand.

2. The method for preparing moisture-resistant, environmentally friendly coated sand according to claim 1, characterized in that, In step 1, the molar ratio of long-chain alkyltrialkoxysilane to epoxysilane coupling agent is (2-6):

1.

3. The method for preparing moisture-resistant, environmentally friendly coated sand according to claim 1, characterized in that, In step 1, the hydrolysis and condensation temperature is 55-65℃, and the duration is 4-6 hours.

4. The method for preparing moisture-resistant, environmentally friendly coated sand according to claim 1, characterized in that, In step 2, the mass ratio of raw sand, reactive organosilicon hydrophobic agent, and thermosetting phenolic resin is 100:(0.1-1):(2-4).

5. The method for preparing moisture-resistant, environmentally friendly coated sand according to claim 1, characterized in that, In step 2, the mass of hexamethylenetetramine is 15%-35% of the mass of the thermosetting phenolic resin, and the concentration is 20%-40%.

6. The method for preparing moisture-resistant, environmentally friendly coated sand according to claim 2, characterized in that, The long-chain alkyltrialkoxysilane is one of hexadecyltrimethoxysilane, octadecyltrimethoxysilane, and octadecyltriethoxysilane.

7. The method for preparing moisture-resistant, environmentally friendly coated sand according to claim 2, characterized in that, The epoxy silane coupling agent is 3-glycidyl etheroxypropyltrimethoxysilane or 3-glycidyl etheroxypropyltriethoxysilane.

8. The method for preparing moisture-resistant, environmentally friendly coated sand according to claim 4, characterized in that, The thermosetting phenolic resin is an environmentally friendly phenolic resin.

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