Solvent-free drinking water tank environment-friendly paint and preparation method thereof

Abstract

This invention relates to the field of coating composition technology, specifically to a solvent-free environmentally friendly coating for drinking water tanks and its preparation method. The coating consists of component A and component B. Component A includes a bio-based hyperbranched resin, polyoxypropylene diamine, rutile titanium dioxide, talc, and additives; component B is an aliphatic isocyanate. The bio-based hyperbranched resin of this invention is obtained by co-condensation of hyperbranched polyol and citric acid-modified Span-80, wherein the hyperbranched polyol is prepared by reacting succinic anhydride with diethanolamine, and the citric acid-modified Span-80 is prepared by reacting Span-80 with citric acid. This coating can be applied without organic solvents and reactive diluents, exhibits low mixing viscosity, good adhesion between the cured coating and the substrate, meets the salt spray and water resistance requirements for drinking water tank protection, and no aromatic primary amines are detected, complying with food safety standards.

Description

Technical Field

[0001] This invention relates to the field of coating composition technology, specifically to a solvent-free environmentally friendly coating for drinking water tanks and its preparation method. Background Technology

[0002] As a core facility for the storage and transportation of drinking water, the protective coating on the inner wall of the drinking water tank is directly related to the safety of drinking water and the service life of the facility. The coating is required to be solvent-free and environmentally friendly, safe for contact with drinking water, excellent water resistance and corrosion resistance, and must also meet the requirements of not leaching or peeling off under long-term immersion conditions.

[0003] Solvent-free coatings have gained widespread attention in the field of drinking water tank protection due to their low volatile organic compound content and excellent environmental performance. Existing solvent-free drinking water tank coatings mostly utilize epoxy resin systems. Chinese patent CN102010646B discloses a low-temperature, fast-drying solvent-free epoxy drinking water tank coating and its preparation method. It consists of two components, A and B, composed of epoxy resin, reactive diluent, dispersant, silane coupling agent, thixotropic agent, titanium dioxide, talc, defoamer, modified aromatic amine curing agent, and catalyst. During use, the components are mixed in a weight ratio of A:B = 7.5 / 1 to 8.5 / 1. At a low temperature of 10-15℃, the mixed viscosity is 3000-5000 mPa·s, the workable time is 1-2 hours, the semi-curing time is 6-8 hours, and the curing time is 20-24 hours, exhibiting excellent coating workability and film drying properties. However, the resin raw materials in the above scheme are mainly derived from petroleum-based chemical products, which are non-renewable and do not meet the current requirements of green environmental protection and sustainable development. Chinese patent application CN119264803A discloses a bio-based coating, its preparation method, and its application. Using a specially formulated bio-based hyperbranched resin as a base material, combined with a first pigment, a second pigment, and fillers, the bio-based coating has a high content of non-volatile substances, thereby improving its corrosion resistance, weather resistance, acid and alkali resistance, and salt spray resistance. However, it requires a solvent system for stable dispersion and is not a solvent-free coating suitable for drinking water tank applications, thus failing to balance solvent-free environmental friendliness with food contact safety. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the present invention aims to provide a solvent-free environmentally friendly coating for drinking water tanks and its preparation method. This coating achieves low-viscosity application without the addition of any organic solvents or reactive diluents, while also possessing excellent water resistance, corrosion resistance, and food safety. Hyperbranched polyols are prepared through a nucleophilic addition reaction of succinic anhydride and diethanolamine. Citric acid-modified Span-80 is prepared through an esterification reaction of Span-80 and citric acid. A bio-based hyperbranched resin is then prepared by a co-condensation reaction of the hyperbranched polyol and the citric acid-modified Span-80, which can be used to replace petroleum-based epoxy resins. This bio-based hyperbranched resin is mixed with polypropylene diamine and other components to form component A, which is then mixed with an aliphatic isocyanate component B and cured to obtain the solvent-free environmentally friendly coating for drinking water tanks.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a solvent-free environmentally friendly coating for drinking water tanks, the environmentally friendly coating comprising component A and component B.

[0006] Preferably, component A includes bio-based hyperbranched resin, polyoxypropylene diamine, rutile titanium dioxide, talc, and additives.

[0007] Preferably, the bio-based hyperbranched resin is a product obtained by co-condensation reaction of hyperbranched polyol and citric acid modified Span-80, wherein the hyperbranched polyol is prepared by nucleophilic addition reaction of succinic anhydride and diethanolamine, and the citric acid modified Span-80 is obtained by esterification reaction of Span-80 and citric acid.

[0008] Preferably, the additives include dispersants, defoamers, and anti-settling agents; and component B is hexamethylene diisocyanate-based polyisocyanate.

[0009] Preferably, the mass ratio of component A to component B is 100:(28~35).

[0010] Preferably, in component A, the mass ratio of bio-based hyperbranched resin, polyoxypropylene diamine, rutile titanium dioxide, talc and additives is (40~50):(20~30):(10~15):(13~18.5):(1.5~2).

[0011] Preferably, in component A, the talc powder has a D50 of 8~15μm, a whiteness of ≥85%, and a moisture content of ≤0.5%.

[0012] Preferably, the mass ratio of the dispersant, defoamer and antisettling agent in the additives is (5~10):(2~5):(5~8).

[0013] Preferably, the dispersant in the additives is BYK-111.

[0014] Preferably, the defoamer in the additives is BYK-051.

[0015] Preferably, the anti-settling agent in the additives is fumed silica.

[0016] Secondly, the present invention provides a method for preparing a solvent-free environmentally friendly coating for drinking water tanks, comprising the following steps: S1. Mix the dispersant and defoamer evenly to form a primary auxiliary agent; S2. Mix bio-based hyperbranched resin and polyoxypropylene diamine, stir, add rutile titanium dioxide, talc, anti-settling agent and primary additives in sequence, homogenize, degas under vacuum, filter, and obtain component A. S3. Mix components A and B, stir, and mature to obtain a solvent-free environmentally friendly coating for drinking water tanks.

[0017] Preferably, in step S2, the stirring speed is 200-300 rpm, the stirring temperature is 20-30°C, and the stirring time is 10-20 min.

[0018] Preferably, in step S2, the rotation speed of the homogenizer is 800~1200 rpm, and the homogenization time is 45~60 min.

[0019] Preferably, in step S2, the vacuum degree of vacuum degassing is -0.08 to -0.09 MPa, the vacuum degassing temperature is 20 to 30°C, the vacuum degassing speed is 40 to 60 rpm, and the vacuum degassing time is 20 to 30 minutes.

[0020] Preferably, in step S2, the mesh size of the filter screen is 80-120 mesh.

[0021] Preferably, in step S3, the stirring speed is 300-500 rpm and the stirring time is 5-10 min.

[0022] Preferably, in step S3, the ripening time is 15-20 minutes.

[0023] Thirdly, the present invention provides a method for preparing the bio-based hyperbranched resin, comprising the following steps: A1. Dissolve citric acid in anhydrous acetic acid to obtain an acid solution; A2. Add Span-80 to the acid solution, stir, reflux acetic acid, distill off acetic acid, and react to obtain citric acid modified Span-80; A3, succinic anhydride and diethanolamine are mixed and reacted to obtain hyperbranched polyols; A4. Citric acid-modified Span-80 was added to hyperbranched polyol and reacted to obtain bio-based hyperbranched resin.

[0024] Preferably, in A1, the mass ratio of citric acid to anhydrous acetic acid is 1:(1.5~2.0).

[0025] Preferably, in A2, the mass ratio of Span-80 to acid solution is 1:(0.45~0.54).

[0026] Span-80, also known as dehydrated sorbitan monooleate, is a bio-derived renewable polyol and a very common nonionic surfactant. It possesses excellent emulsifying and dispersing properties, and is non-toxic, non-irritating, low-volatility, and odorless, making it widely used in the pharmaceutical, food, and cosmetic industries. This application involves esterifying and grafting citric acid with Span-80 in an acetic acid system to obtain citric acid-modified Span-80. All reaction materials are non-toxic biomass-derived, involving no toxic or harmful reagents, and posing no food safety risks. The citric acid-modified Span-80 can be further covalently grafted into the hyperbranched polyester polyol backbone, ultimately curing to form a cross-linked coating network. It will not migrate or dissolve into drinking water and can be safely used as a bio-based component in solvent-free drinking water tank coatings.

[0027] Preferably, in A2, the stirring speed is 200~300 rpm and the stirring time is 5~10 min.

[0028] Preferably, in A2, the reflux temperature of acetic acid is 80~90℃, the vacuum degree of acetic acid reflux is -0.03~-0.04MPa, and the reflux time of acetic acid is 80~100min.

[0029] Preferably, in A2, the rate of acetic acid distillation is 0.5~0.7 g / min.

[0030] Preferably, in A2, the reaction temperature is 115~125℃, the reaction speed is 100~200rpm, the reaction time is 80~100min, and the reaction vacuum degree is -0.05~-0.07MPa.

[0031] Citric acid is insoluble at lower temperatures; at higher temperatures, it readily dehydrates to form aconitic acid. Therefore, this application limits the reaction temperature to 115~125℃. Preferably, in A3, the molar ratio of succinic anhydride to diethanolamine is 1:(0.9~1.2).

[0032] Preferably, in A3, the reaction temperature is 135~145℃, the reaction time is 1.5~2.5h, and the stirring speed is 200~300rpm.

[0033] Preferably, in A4, the mass ratio of the citric acid-modified Span-80 to the hyperbranched polyol is (0.2~0.25):1.

[0034] Preferably, in A4, the reaction temperature is 130~150℃, the reaction time is 4~6h, and the reaction speed is 200~300rpm.

[0035] Compared with the prior art, the beneficial effects achieved by the present invention are as follows: This invention relates to a solvent-free environmentally friendly coating for drinking water tanks and its preparation method. The coating can be applied at low viscosity without the addition of organic solvents and reactive diluents, possesses water resistance and corrosion resistance, and meets food contact safety requirements. This invention uses succinic anhydride and diethanolamine to prepare a hyperbranched polyol, which is then polymerized with citric acid-modified Span-80 to prepare a bio-based hyperbranched resin. The hyperbranched molecular structure and graft modification reduce the system viscosity, enabling solvent-free application. This bio-based hyperbranched resin contains a large number of hydroxyl groups at its ends, which can react with aliphatic isocyanates to form a three-dimensional cross-linked polyurethane network. Combined with a room-temperature curing system of polypropylene diamine and aliphatic isocyanates, the cross-linking density and water resistance of the coating are improved, making the coating less prone to blistering and peeling under long-term water immersion conditions, meeting the protection requirements for the inner walls of drinking water tanks. This invention uses aliphatic isocyanates as curing agents, which do not contain aromatic structures, eliminating the risk of aromatic primary amine migration in the coating. The formula does not use small molecule plasticizers or reactive diluents, reducing the risk of small molecule substances migrating and precipitating under long-term water immersion, ensuring drinking water safety. Attached Figure Description

[0036] Figure 1 This is a schematic diagram of the preparation process of a bio-based hyperbranched resin provided by the present invention. Detailed Implementation

[0037] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the embodiments of the present invention and the specific features in the embodiments are detailed descriptions of the technical solution of the present invention, rather than limitations thereof.

[0038] The chemical reagents used in the embodiments and comparative examples of this invention are all commercially available products and have not undergone further purification or processing.

[0039] Example 1 like Figure 1 As shown, this embodiment provides a method for preparing a bio-based hyperbranched resin, comprising the following steps: A1. Dissolve 100g of citric acid in 180g of anhydrous acetic acid to obtain an acid solution; A2. Add 100g of Span-80 to 50g of acid solution, stir at 250rpm for 7min, adjust the vacuum to -0.035MPa and the temperature to 85℃, and reflux the acetic acid for 90min. After reflux, distill off the acetic acid at a rate of 0.6g / min, then raise the temperature to 120℃, adjust the vacuum to -0.06MPa, and stir at 150rpm for 90min to obtain citric acid modified Span-80. A3. Mix 1 mol of succinic anhydride and 1 mol of diethanolamine, and stir at 250 rpm for 2 h at 140 °C to obtain hyperbranched polyol. A4. Add 22g of citric acid modified Span-80 to 100g of hyperbranched polyol, and stir at 140℃ and 250rpm for 5h to obtain bio-based hyperbranched resin.

[0040] Example 2 like Figure 1 As shown, this embodiment provides a method for preparing a bio-based hyperbranched resin, comprising the following steps: A1. Dissolve 100g of citric acid in 150g of anhydrous acetic acid to obtain an acid solution; A2. Add 100g of Span-80 to 45g of acid solution, stir at 200rpm for 10min, adjust the vacuum to -0.03MPa, adjust the temperature to 90℃, and reflux the acetic acid for 80min. After reflux, distill off the acetic acid at a rate of 0.7g / min, then raise the temperature to 125℃, adjust the vacuum to -0.05MPa, and stir at 100rpm for 100min to obtain citric acid modified Span-80. A3. Mix 1 mol of succinic anhydride and 1.2 mol of diethanolamine, and stir at 200 rpm for 1.5 h at 145 °C to obtain hyperbranched polyol. A4. Add 25g of citric acid modified Span-80 to 100g of hyperbranched polyol, and stir at 150℃ and 200rpm for 4h to obtain bio-based hyperbranched resin.

[0041] Example 3 like Figure 1 As shown, this embodiment provides a method for preparing a bio-based hyperbranched resin, comprising the following steps: A1. Dissolve 100g of citric acid in 200g of anhydrous acetic acid to obtain an acid solution; A2. Add 100g of Span-80 to 54g of acid solution, stir at 300rpm for 5min, adjust the vacuum to -0.04MPa and the temperature to 80℃, and reflux the acetic acid for 100min. After reflux, distill off the acetic acid at a rate of 0.5g / min, then raise the temperature to 115℃, adjust the vacuum to -0.07MPa, and stir at 200rpm for 80min to obtain citric acid modified Span-80. A3. Mix 1 mol of succinic anhydride and 0.9 mol of diethanolamine, and stir at 300 rpm at 135 °C for 2.5 h to obtain hyperbranched polyol. A4. Add 20g of citric acid modified Span-80 to 100g of hyperbranched polyol, and stir at 300rpm for 6h at 130℃ to obtain bio-based hyperbranched resin.

[0042] Example 4 This embodiment provides a method for preparing a solvent-free environmentally friendly coating for drinking water tanks, including the following steps: S1. Mix 0.8g of dispersant BYK-111 and 0.4g of defoamer BYK-051 evenly to form a primary additive; S2. Mix 45g of the bio-based hyperbranched resin prepared in Example 1 with 25g of polyoxypropylene diamine, and stir at 250 rpm at 25°C for 15 min. Then, add 12.2g of rutile titanium dioxide, 16g of talc powder with D50=8μm, whiteness ≥85%, and moisture content ≤0.5%, 0.6g of anti-settling agent fumed silica, and 1.2g of primary additives in sequence. Stir for 3 min after each addition of a material before adding the next. Homogenize at 1000 rpm for 50 min, and degas under a vacuum of -0.085MPa at a temperature of 25°C, a speed of 50 rpm, and a time of 25 min. After vacuum degassing, filter through a 100-mesh filter to obtain component A. S3. Mix 100g of component A and 30g of hexamethylene diisocyanate-based polyisocyanate, stir at 400rpm for 7min, and cure for 17min to obtain a solvent-free environmentally friendly coating for drinking water tanks.

[0043] Example 5 This embodiment provides a method for preparing a solvent-free environmentally friendly coating for drinking water tanks, including the following steps: S1. Mix 0.5g of dispersant BYK-111 and 0.5g of defoamer BYK-051 evenly to form a primary additive; S2. Mix 40g of the bio-based hyperbranched resin prepared in Example 2 with 30g of polyoxypropylene diamine, and stir at 200rpm at 20°C for 20min. Then, add 10g of rutile titanium dioxide, 18.5g of talc powder with D50=15μm, whiteness ≥85%, and moisture content ≤0.5%, 0.5g of anti-settling agent fumed silica, and 1g of primary additives in sequence. Stir for 3min after each addition of a material before adding the next. Homogenize at 800rpm for 60min, and degas under a vacuum of -0.08MPa at a temperature of 30°C, a speed of 60rpm, and a time of 20min. After vacuum degassing, filter through a 120-mesh filter to obtain component A. S3. Mix 100g of component A and 28g of hexamethylene diisocyanate-based polyisocyanate, stir at 300rpm for 10min, and cure for 20min to obtain a solvent-free environmentally friendly coating for drinking water tanks.

[0044] Example 6 This embodiment provides a method for preparing a solvent-free environmentally friendly coating for drinking water tanks, including the following steps: S1. Mix 1g of dispersant BYK-111 and 0.2g of defoamer BYK-051 evenly to form a primary additive; S2. Mix 50g of the bio-based hyperbranched resin prepared in Example 3 with 20g of polyoxypropylene diamine, and stir at 300rpm at 30°C for 10min. Then, add 15g of rutile titanium dioxide, 13g of talc powder with D50=10μm, whiteness ≥85%, and moisture content ≤0.5%, 0.8g of anti-settling agent fumed silica, and 1.2g of primary additives in sequence. Stir for 3min after each addition of a material before adding the next. Homogenize at 1200rpm for 45min, and degas under a vacuum of -0.09MPa at a temperature of 20°C, a speed of 40rpm, and a time of 30min. After vacuum degassing, filter through an 80-mesh filter to obtain component A. S3. Mix 100g of component A and 35g of hexamethylene diisocyanate-based polyisocyanate, stir at 500rpm for 5min, and cure for 15min to obtain a solvent-free environmentally friendly coating for drinking water tanks.

[0045] Comparative Example 1 A solvent-free environmentally friendly coating for drinking water tanks and its preparation method are disclosed. The difference between this method and Example 4 is that the bio-based hyperbranched resin used does not contain Span-80. Instead, citric acid is directly polymerized with hyperbranched polyol to generate the bio-based hyperbranched resin. Other operating steps and process parameters are exactly the same as in Example 4.

[0046] Comparative Example 2 A solvent-free environmentally friendly coating for drinking water tanks and its preparation method are disclosed. The difference between this method and Example 4 is that the bio-based hyperbranched resin used does not use hyperbranched polyols during resin synthesis, and polybutylene adipate polyol is used instead. Other operating steps and process parameters are exactly the same as in Example 4.

[0047] Comparative Example 3 A solvent-free environmentally friendly coating for drinking water tanks and its preparation method differ from Example 4 in that an equal amount of bisphenol A type liquid epoxy resin is used to replace the bio-based hyperbranched resin, and 1.35g of cashew phenol glycidyl ether is added as an active diluent; and component B is replaced with an equal amount of phenolic amine type epoxy curing agent. Other operating steps and process parameters are exactly the same as in Example 4.

[0048] Performance testing: The coatings prepared in Examples 4-6 and Comparative Examples 1-3 were applied to carbon steel plates of the same size to form a dry coating with a thickness of (100±3) μm, which served as the test samples. The performance of the test samples was then tested, and the specific process is as follows: Salt spray resistance test: Spray continuously with 50g / L sodium chloride solution at 25℃ for 720h, then rinse with water and dry, and observe the damage to the surface of the sample.

[0049] Pull-off adhesion: The test column is vertically bonded to the coating surface of the sample to be tested using a two-component fast-drying epoxy adhesive. After the adhesive is fully cured, a tensile tester is used to apply a tensile force perpendicular to the coating surface at an acceleration of 5 mm / min. The force that pulls the coating off is taken as the pull-off adhesion of the coating.

[0050] Water resistance test: At 25°C, the coatings prepared in Examples 4-6 and Comparative Examples 1-3 were applied to glass plates of the same size and immersed in deionized water for 720 hours. After removal and drying, the coating condition of the test samples was observed. The immersion water was then used for hygiene testing.

[0051] Hygiene testing content and standards for soaking water: No visible matter; total bacterial count ≤100 CFU / mL; no migration of aromatic primary amines detected; total hardness (calculated as CaCO3) ≤450mg / L; iron ≤0.3mg / L; manganese ≤0.1mg / L; copper ≤1.0mg / L; zinc ≤1.0mg / L; volatile phenols (calculated as phenol) ≤0.002mg / L.

[0052] Table 1. Performance test results of the coatings prepared in Examples 4-6 and Comparative Examples 1-3

[0053] As shown in Table 1, the solvent-free environmentally friendly coatings for drinking water tanks prepared in Examples 4 to 6 have good adhesion to the substrate. After long-term salt spray testing and water immersion, the coatings remain intact without any abnormalities such as blistering, peeling, or rust. The hygiene indicators of the immersion water meet the relevant standards for marine drinking water tank coatings.

[0054] The bio-based hyperbranched resin used in Comparative Example 1 did not incorporate Span-80; it was polymerized with citric acid and hyperbranched polyols. This resulted in the absence of hydrophobic modification and substrate wetting effects brought by citric acid-modified Span-80, leading to lower coating adhesion than in the embodiments of the present invention, increased coating water absorption, and decreased resistance to salt spray and long-term water immersion. Only the hygiene indicators of the immersion water still met the standard requirements.

[0055] Comparative Example 2 uses a linear polyester polyol to replace the hyperbranched polyol synthesized from succinic anhydride and diethanolamine. It lacks the high crosslinking density and low viscosity characteristics brought by the hyperbranched structure. The network formed by the linear polyester and isocyanate has few crosslinking points and poor density. In addition, the polyester chain segment has a high ester bond content, which makes it easy to hydrolyze under water immersion conditions. Therefore, its salt spray resistance, water resistance and adhesion are the worst among the three comparative examples. The coating is prone to large-area blistering, softening and corrosion.

[0056] Comparative Example 3 replaced the bio-based hyperbranched resin with an equal amount of bisphenol A-type liquid epoxy resin, added an additional reactive diluent, and replaced the curing system with a phenolic amine-type epoxy curing agent. Unlike the polyurethane curing system of this invention, the initial adhesion, salt spray resistance, and water resistance of the coating were similar to those of the example. However, the epoxy resin contained a bisphenol A structure, and the reactive diluent and amine curing agent posed a risk of bisphenol A and aromatic primary amine migration during long-term water immersion, resulting in the hygienic indicators of the immersion water failing to meet the relevant standards for drinking water tank coatings.

[0057] The above description is merely a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A solvent-free environmentally friendly coating for drinking water tanks, characterized in that, The product comprises component A and component B. Component A includes a bio-based hyperbranched resin, polypropylene diamine, rutile titanium dioxide, talc, and additives. The bio-based hyperbranched resin is a product obtained by co-condensation reaction of hyperbranched polyol and citric acid-modified Span-80. The hyperbranched polyol is prepared by nucleophilic addition reaction of succinic anhydride and diethanolamine. The citric acid-modified Span-80 is obtained by esterification reaction of Span-80 and citric acid. The additives include dispersants, defoamers, and anti-settling agents. Component B is hexamethylene diisocyanate-based polyisocyanate.

2. The solvent-free environmentally friendly coating for drinking water tanks according to claim 1, characterized in that, The mass ratio of component A to component B is 100:(28~35); in component A, the mass ratio of bio-based hyperbranched resin, polyoxypropylene diamine, rutile titanium dioxide, talc and additives is (40~50):(20~30):(10~15):(13~18.5):(1.5~2); in the additives, the mass ratio of dispersant, defoamer and anti-settling agent is (5~10):(2~5):(5~8).

3. A method for preparing a solvent-free environmentally friendly coating for drinking water tanks as described in any one of claims 1-2, characterized in that, Includes the following steps: S1. Mix the dispersant and defoamer evenly to form a primary auxiliary agent; S2. Mix bio-based hyperbranched resin and polyoxypropylene diamine, stir, add rutile titanium dioxide, talc, anti-settling agent and primary additives in sequence, homogenize, degas under vacuum, filter, and obtain component A. S3. Mix components A and B, stir, and mature to obtain a solvent-free environmentally friendly coating for drinking water tanks.

4. The method for preparing a solvent-free environmentally friendly coating for drinking water tanks according to claim 3, characterized in that, In step S2, the stirring speed is 200-300 rpm, the stirring temperature is 20-30℃, and the stirring time is 10-20 min; the homogenization speed is 800-1200 rpm, and the homogenization time is 45-60 min; the vacuum degree of the vacuum degassing is -0.08 to -0.09 MPa, the vacuum degassing temperature is 20-30℃, the vacuum degassing speed is 40-60 rpm, and the vacuum degassing time is 20-30 min; and the filter screen mesh size is 80-120 mesh.

5. The method for preparing a solvent-free environmentally friendly coating for drinking water tanks according to claim 3, characterized in that, In step S3, the stirring speed is 300-500 rpm, the stirring time is 5-10 min, and the maturation time is 15-20 min.

6. The method for preparing a solvent-free environmentally friendly coating for drinking water tanks according to claim 3, characterized in that, The preparation method of the bio-based hyperbranched resin is as follows: A1. Dissolve citric acid in anhydrous acetic acid to obtain an acid solution; A2. Add Span-80 to the acid solution, stir, reflux acetic acid, distill off acetic acid, and react to obtain citric acid modified Span-80; A3, succinic anhydride and diethanolamine are mixed and reacted to obtain hyperbranched polyols; A4. Citric acid-modified Span-80 was added to hyperbranched polyol and reacted to obtain bio-based hyperbranched resin.

7. The method for preparing a solvent-free environmentally friendly coating for drinking water tanks according to claim 6, characterized in that, In A1, the mass ratio of citric acid to anhydrous acetic acid is 1:(1.5~2.0).

8. The method for preparing a solvent-free environmentally friendly coating for drinking water tanks according to claim 6, characterized in that, In A2, the mass ratio of Span-80 to acid solution is 1:(0.45~0.54); the stirring speed is 200~300 rpm, and the stirring time is 5~10 min; the reflux temperature of acetic acid is 80~90℃, the reflux vacuum degree of acetic acid is -0.03~-0.04 MPa, and the reflux time of acetic acid is 80~100 min; the evaporation rate of acetic acid is 0.5~0.7 g / min; the reaction temperature is 115~125℃, the reaction speed is 100~200 rpm, the reaction time is 80~100 min, and the reaction vacuum degree is -0.05~-0.07 MPa.

9. The method for preparing a solvent-free environmentally friendly coating for drinking water tanks according to claim 6, characterized in that, In A3, the molar ratio of succinic anhydride to diethanolamine is 1:(0.9~1.2); the reaction temperature is 135~145℃, the reaction time is 1.5~2.5h, and the stirring speed is 200~300rpm.

10. The method for preparing a solvent-free environmentally friendly coating for drinking water tanks according to claim 6, characterized in that, In A4, the mass ratio of citric acid-modified Span-80 to hyperbranched polyol is (0.2~0.25):1; the reaction temperature is 130~150℃, the reaction time is 4~6h, and the reaction speed is 200~300rpm.

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