Ultra-low VOC (volatile organic compound) baking type quick-drying coating and preparation method thereof

Abstract

The invention relates to an ultra-low VOC baking type quick-drying coating and a preparation method thereof, and belongs to the technical field of coatings. The coating is prepared from the following components: composite hybrid resin, methylated melamine formaldehyde resin, neopentyl glycol diglycidyl ether, trimethylolpropane triacrylate, composite filler, zinc fatty acid, fumed silica, a silane coupling agent KH-560, aminoethyl aminopropyl polydimethylsiloxane and the like. The composite hybrid resin is prepared by adopting a special method; the composite hybrid resin is matched with methylated melamine formaldehyde resin, neopentyl glycol diglycidyl ether, trimethylolpropane triacrylate, a composite filler, zinc fatty acid, fumed silica, a silane coupling agent KH-560, aminoethyl aminopropyl polydimethylsiloxane and other components according to a certain proportion for use, and has the advantages of low VOC emission, quick drying during baking, high heat resistance, high heat resistance, high heat resistance, high heat resistance, high heat resistance, high heat resistance and the like. Meanwhile, the adhesive force is good, the hardness and the flexibility are balanced, and the chemical resistance is excellent.

Description

Technical Field

[0001] The present invention belongs to the technical field of coatings, and particularly relates to an ultra-low VOC baking-type fast-drying coating and a preparation method thereof. Background Art

[0002] Coatings are a widely used class of materials, playing important roles such as protection and decoration. However, traditional coatings have problems with the emission of volatile organic compounds (VOCs) during use. Some of the VOCs are toxic, endangering people's health. With the increasing awareness of environmental protection and the growing strictness of environmental protection regulations, the restrictions on VOC emissions in coatings are becoming more and more severe. Against this background, the research and development and application of low-VOC coatings have become an important development direction in the coatings industry. Low-VOC coatings not only help improve indoor and outdoor air quality and reduce potential threats to human health, but also conform to the concept of sustainable development, can reduce the negative impact on the environment, and promote the transformation and upgrading of the coatings industry towards the direction of green environmental protection.

[0003] In the process of researching and developing low-VOC coatings, baking-type fast-drying coatings have attracted much attention due to their unique advantages. Traditional coatings have a slow drying speed, which not only prolongs the construction period and increases the time cost, but also is easily affected by external factors during the drying process, resulting in film defects such as sagging and dust adhesion, reducing the coating quality. While baking-type fast-drying coatings can be cured into a film through baking in a short time, greatly improving the construction efficiency and being suitable for large-scale industrial production scenarios. Moreover, rapid drying can effectively reduce various problems caused by too long drying time, ensuring the quality and performance stability of the coating.

[0004] However, developing an ultra-low VOC baking-type fast-drying coating is not easy and faces many technical challenges. On the one hand, while reducing the VOC content, it is necessary to ensure that the coating has good film-forming properties, adhesion, hardness, corrosion resistance and other key properties, and maintain its effective protection and decoration functions for the coated object. For example, many manufacturers use a large amount of small molecule resins and active diluents in the formula to reduce the coating viscosity to achieve low VOC, but this results in a short pot life of the coating and a decline in water resistance and salt spray resistance. On the other hand, it is also necessary to consider the applicability of the coating on different substrates and its compatibility with existing coating equipment and processes to ensure its smooth application in actual production.

[0005] Therefore, researching and developing an ultra-low VOC baking-type fast-drying coating that can not only significantly reduce VOC emissions, meet the requirements of rapid drying, but also perform well in terms of performance and applicability has become an urgent problem to be solved in the coatings industry, with important practical significance and market value. Summary of the Invention

[0006] In order to significantly reduce VOC emissions, meet the requirements of rapid drying during baking, and at the same time ensure the service performance and applicability of the paint film, the present invention provides an ultra-low VOC baking-type fast-drying paint and its preparation method. A composite hybrid resin is prepared by a special method and used in combination with components such as methylated melamine formaldehyde resin, neopentyl glycol diglycidyl ether, trimethylolpropane triacrylate, composite filler, zinc fatty acid, silane coupling agent KH-560, aminoethyl aminopropyl polydimethylsiloxane, etc. in a certain proportion. It has low VOC emissions, rapid drying during baking, and at the same time has good adhesion, balanced hardness and flexibility, and excellent chemical resistance. The specific technical solution is as follows:

[0007] An ultra-low VOC baking-type fast-drying paint, the paint comprising the following raw materials in parts by mass: 45 parts to 50 parts of composite hybrid resin, 15 parts to 20 parts of methylated melamine formaldehyde resin, 5 parts to 7 parts of neopentyl glycol diglycidyl ether, 3 parts to 5 parts of trimethylolpropane triacrylate, 8 parts to 10 parts of composite filler, 1 part to 1.5 parts of zinc fatty acid, 0.5 part to 0.8 part of fumed silica, 0.5 part to 0.8 part of silane coupling agent KH-560, 1 part to 1.5 parts of sodium polyacrylate dispersant, 0.1 part to 0.3 part of polyether-modified silicone defoamer, 2 parts to 3 parts of aminoethyl aminopropyl polydimethylsiloxane, 0.6 part to 1 part of fluorocarbon-modified polyether leveling agent, and the balance being deionized water, and the paint viscosity being 1500 mPa·s to 2000 mPa·s.

[0008] In the above paint, the preparation method of the composite hybrid resin includes: under nitrogen protection, in parts by mass, mixing 20 parts to 25 parts of isophthalic acid, 10 parts to 15 parts of adipic acid, 15 parts to 20 parts of neopentyl glycol, 5 parts to 8 parts of propylene glycol, 10 parts to 15 parts of propylene glycol monobutyl ether, 15 parts to 20 parts of dimethyl carbonate, 10 parts to 12 parts of dipropylene glycol monobutyl ether and 0.2 part to 0.3 part of tetrabutyl titanate evenly, carrying out a reflux reaction at 180°C to 200°C, collecting the water generated by the reaction through a water separator, when the acid value of the reaction system drops to 5 mgKOH / g to 7 mgKOH / g, cooling to 70°C to 80°C, successively adding 8 parts to 10 parts of methyl methacrylate, 6 parts to 8 parts of butyl acrylate, 4 parts to 6 parts of trimethylolpropane triacrylate, 3 parts to 8 parts of ethylene-vinyl acetate copolymer, 5 parts to 8 parts of polyester-type polyurethane prepolymer I, 3 parts to 5 parts of polyester-type polyurethane prepolymer II and 1 part to 2 parts of silane coupling agent KH-560 and mixing evenly, adding 0.8 part to 1.2 parts of dibenzoyl peroxide, carrying out a reflux reaction at 130°C to 140°C for 4 h to 5 h, removing dimethyl carbonate by vacuum distillation, and filtering through a sieve to remove impurities to obtain the composite hybrid resin.

[0009] In the preparation method of the above composite hybrid resin, the polyester-type polyurethane prepolymer I is the polyester-type polyurethane prepolymer T80; the polyester-type polyurethane prepolymer II is the polyester-type polyurethane prepolymer T100.

[0010] In the preparation method of the above composite hybrid resin, benzoyl peroxide is diluted with 10 to 15 times the mass of dipropylene glycol butyl ether before addition, divided into 3 to 4 volume parts on average, and 1 volume part is added every 30 to 40 minutes.

[0011] In the preparation method of the above composite hybrid resin, the nitrogen protection is a nitrogen protection with a slightly positive pressure of 0.02 MPa to 0.05 MPa.

[0012] In the preparation method of the above composite hybrid resin, the temperature of vacuum distillation is 50 °C to 60 °C.

[0013] In the preparation method of the above composite hybrid resin, the mesh number of the sieve is 150 mesh to 200 mesh.

[0014] In the above coating, the D90 particle size of the fumed silica is 15 μm or less.

[0015] In the above coating, the component mass ratio of the composite filler is mica powder: modified barium sulfate = (3 to 4):(7 to 9). The D90 particle sizes of the mica powder and the modified barium sulfate are 15 μm or less.

[0016] The preparation method of the above ultra-low VOC baking-type quick-drying coating includes the following steps:

[0017] S1: By mass parts, mix the composite hybrid resin, neopentyl glycol diglycidyl ether and 10 to 15 parts of deionized water evenly, and sequentially add sodium polyacrylate dispersant, fumed silica and composite filler, and mix and disperse evenly to form a homogeneous base material;

[0018] S2: By mass parts, sequentially add methylated melamine formaldehyde resin and zinc fatty acid to the homogeneous base material and mix evenly, and then grind with a sand mill until the fineness ≤ 15 μm to obtain a ground material;

[0019] S3: By mass parts, sequentially add trimethylolpropane triacrylate, silane coupling agent KH-560, polyether-modified silicone defoamer, aminoethylaminopropyl polydimethylsiloxane and fluorocarbon-modified polyether leveling agent to the ground material, stir at a low speed, adjust the coating viscosity to 1500 mPa·s to 2000 mPa·s with deionized water, filter with a sieve, and take the filtrate to obtain the coating.

[0020] In the above preparation method, the low-speed stirring is carried out at a low speed of 300 rpm to 350 rpm for 20 to 30 minutes; the mesh number of the sieve is 150 mesh to 200 mesh.

[0021] An ultra-low VOC baking-type fast-drying coating provided by the present invention and its preparation method have the following beneficial effects:

[0022] I. In the coating system, zinc fatty acid has the dual advantages of accelerating drying and promoting crosslinking, and regulating surface properties. The metal zinc ions of zinc fatty acid can form a complexation reaction with groups such as carboxyl groups in the composite hybrid resin, accelerating the crosslinking process between the composite hybrid resin and the methylated melamine formaldehyde resin. Zinc fatty acid plays an irreplaceable role in promoting drying and crosslinking. The improvement of the crosslinking degree helps to form a denser coating structure, thereby improving the hardness, adhesion and corrosion resistance of the coating. Zinc fatty acid can also effectively reduce the surface tension of the coating system, making the coating have better wettability and leveling property on the substrate surface. This helps the coating to form a flat and smooth paint film during construction, reducing surface defects such as shrinkage holes and orange peel caused by uneven surface tension, and ensuring the appearance quality of the coating.

[0023] II. In the coating system, the advantage of controlling the addition amount of aminoethyl aminopropyl polydimethylsiloxane is reflected in two aspects: performance optimization and avoiding negative effects. Appropriate addition of aminoethyl aminopropyl polydimethylsiloxane can fully play the role of improving the leveling property and low-temperature flexibility of the coating. Its silicone structure can reduce the surface tension of the coating, enabling the coating to spread better during the baking film-forming process and form a flat and smooth coating surface; at the same time, the flexibility of its molecular chain helps to improve the deformation ability of the coating in a low-temperature environment, avoiding the coating from becoming brittle and cracking due to low temperature. When the addition amount is too high, it will damage the compatibility of the coating system. Excessive silicone chain segments will accumulate on the coating surface, hindering the crosslinking reaction between the composite hybrid resin and the methylated melamine formaldehyde resin, resulting in a decrease in crosslinking density, and then weakening the adhesion between the coating and the substrate and reducing the properties such as hardness and corrosion resistance of the coating. Therefore, precisely controlling its addition amount can ensure the improvement of coating performance while avoiding performance deterioration caused by excessive addition.

[0024] III. In the preparation method of the composite hybrid resin, ethylene-vinyl acetate copolymer mainly plays an advantageous role in improving flexibility, enhancing the wettability of fillers and strengthening the denseness of the coating in the coating system. The ethylene chain segment in its molecular chain endows the material with good flexibility, and the vinyl acetate chain segment provides polar groups. In the composite hybrid resin, by forming intermolecular forces (including hydrogen bonds and van der Waals forces) with other components, the overall flexibility of the resin is improved. Ethylene-vinyl acetate copolymer can improve the compatibility of polyester-type polyurethane prepolymer with other components, improve the uniformity of the reaction, help to enhance the denseness of the coating after crosslinking of the composite hybrid resin, reduce the porosity, reduce the penetration of corrosive media (such as water, acids and alkalis), improve the chemical resistance and water resistance of the coating, and the dense coating can also improve other comprehensive properties.

[0025] The ethylene-vinyl acetate copolymer is used in combination with the silane coupling agent KH-560 to improve the coating performance in terms of enhancing interfacial bonding, improving crosslinking effect, and optimizing overall properties. The siloxane group at one end of the silane coupling agent KH-560 can condense with the hydroxyl groups on the substrate surface to form a Si-O-M (M is the substrate metal atom or other hydroxyl-containing materials) chemical bond, and the active groups such as epoxy groups at the other end react with the active groups in the composite hybrid resin; the ethylene-vinyl acetate copolymer binds to the resin and filler through intermolecular forces. The two act together to form a strong chemical and physical bond between the resin-filler-substrate, significantly enhancing the adhesion between the coating and the substrate. The ethylene-vinyl acetate copolymer improves the resin flexibility and filler dispersibility, creating a more uniform environment for the crosslinking reaction; the silane coupling agent KH-560 promotes the perfection of the resin crosslinking network. The two work together to make the composite hybrid resin crosslink more fully, forming a denser three-dimensional network structure, and improving the hardness, corrosion resistance, and water resistance of the coating. When the two act synergistically, they can effectively make up for the deficiencies of their individual actions, achieve performance complementarity, and improve the comprehensive performance of the coating.

[0026] IV. The polyester-type polyurethane prepolymer I contains active groups such as isocyanate groups in its molecular structure. When the composite hybrid resin is blended and baked with the methylated melamine formaldehyde resin, it can react quickly with the hydroxyl groups in the system, increase the crosslinking points, improve the crosslinking density, and accelerate the film-forming process. The rigid urethane groups of the polyester-type polyurethane prepolymer I are embedded in the crosslinking network, restricting the movement of molecular segments and increasing the intermolecular forces, thereby improving the hardness of the coating. It endows the composite hybrid resin with good mechanical properties such as tensile strength and abrasion resistance, enabling the coating to better resist deformation and wear when subjected to external forces, and is suitable for application scenarios with high requirements for mechanical properties.

[0027] The polyester-type polyurethane prepolymer II contains flexible polyester segments, which can increase the flexibility of the composite hybrid resin, enabling the coating to deform to a certain extent without cracking when subjected to external forces, and improving the impact resistance of the coating. It effectively improves the low-temperature resistance of the composite hybrid resin, maintains the movement ability of molecular chains in a low-temperature environment, prevents the coating from becoming brittle and cracking due to low temperature, and expands the use temperature range of the coating.

[0028] The polyester-based polyurethane prepolymer II and the polyester-based polyurethane prepolymer I cooperate with each other to balance the rigidity and flexibility of the crosslinked network. The combination of the two enables the crosslinked network to have both sufficient strength and appropriate flexibility, realizing the advantages of complementary performance, optimized crosslinked network, and expanded application scope. The combination of the two enables the composite hybrid resin to simultaneously possess good hardness, tensile strength, and flexibility, overcoming the limitations of a single prepolymer in terms of performance. The active groups of the two prepolymers undergo multiple crosslinking reactions with the composite hybrid resin and the methylated melamine formaldehyde resin to form a more complex and stable three-dimensional crosslinked network structure. This network structure gives the coating better integrity and improves the comprehensive properties of the coating such as hardness, adhesion, and chemical resistance.

[0029] Control the addition amounts of the polyester-based polyurethane prepolymer I and the polyester-based polyurethane prepolymer II to ensure system stability and optimize cost-effectiveness. Design the addition amounts and ratios of the two to achieve the performance of the composite hybrid resin. Ensure that the prepolymers are evenly dispersed and fully reacted in the composite hybrid resin system to avoid excessive viscosity and uneven dispersion of the system caused by too high addition amounts, which will affect the subsequent film-forming performance. At the same time, it also prevents the inability to achieve the expected crosslinking effect and performance improvement when the addition amount is too low.

[0030] V. The coating prepared by the present invention has the following comprehensive advantages:

[0031] (1) Low VOC emissions: Most of the residues of dimethyl carbonate are removed by vacuum distillation in the later stage of the synthesis of the composite hybrid resin, reducing the solvent-based VOC emissions. At the same time, the crosslinking density of the composite hybrid resin increases, and the formed three-dimensional network structure physically encapsulates the residual solvent molecules. Moreover, the reactive groups in the crosslinking process consume the volatile monomers, further reducing the VOC emissions.

[0032] (2) Fast drying: The multi-polymerization structure of the composite hybrid resin forms multiple crosslinking reaction paths when blended and baked with the methylated melamine formaldehyde resin, greatly accelerating the formation of the crosslinked network. Compared with a single resin system, the crosslinking speed is significantly improved, and the film-forming time is greatly shortened.

[0033] (3) Good adhesion: The silane coupling agent KH-560 forms a firm chemical connection between the resin and the substrate, and the dense network structure formed by the crosslinking of the composite hybrid resin and the methylated melamine formaldehyde resin further enhances the adhesion to the substrate through mechanical interlocking.

[0034] (4) High hardness: The highly crosslinked network structure restricts the movement of molecular segments, and the rigid urethane groups of the polyester-based polyurethane prepolymer are embedded in the crosslinked network, further enhancing the rigidity and hardness of the network.

[0035] (5) Excellent chemical resistance: The dense cross-linked network increases the difficulty of corrosive medium molecules penetrating into the interior of the coating. The high cross-linking density makes the pore size extremely small and tortuous, extending the diffusion path. At the same time, chemical bonds and intermolecular forces can interact with the medium molecules, consuming the penetration energy of the medium molecules. Detailed implementation method

[0036] Example 1

[0037] An ultra-low VOC baking-type fast-drying coating, the coating comprises the following raw materials in parts by mass: 45 parts of composite hybrid resin, 15 parts of methylated melamine formaldehyde resin, 5 parts of neopentyl glycol diglycidyl ether, 3 parts of trimethylolpropane triacrylate, 8 parts of composite filler, 1 part of zinc fatty acid, 0.5 part of fumed silica, 0.5 part of silane coupling agent KH-560, 1 part of sodium polyacrylate dispersant, 0.1 part of polyether-modified silicone defoamer, 2 parts of aminoethyl aminopropyl polydimethylsiloxane, 0.6 part of fluorocarbon-modified polyether leveling agent, and the balance is deionized water, and the coating viscosity is 1500 mPa·s.

[0038] Among them, the preparation method of the composite hybrid resin includes: under the protection of nitrogen with a slightly positive pressure floating between 0.02 MPa and 0.03 MPa, taking 20 parts of isophthalic acid, 10 parts of adipic acid, 15 parts of neopentyl glycol, 5 parts of propylene glycol, 10 parts of propylene glycol butyl ether, 15 parts of dimethyl carbonate, 10 parts of dipropylene glycol butyl ether and 0.2 part of tetrabutyl titanate by mass, mixing them evenly, carrying out reflux reaction at 180 °C, collecting the water generated by the reaction through a water separator. When the acid value of the reaction system drops to 5 mgKOH / g, cooling to 70 °C, and successively adding 8 parts of methyl methacrylate, 6 parts of butyl acrylate, 4 parts of trimethylolpropane triacrylate, 3 parts of ethylene-vinyl acetate copolymer, 5 parts of polyester-type polyurethane prepolymer I, 3 parts of polyester-type polyurethane prepolymer II and 1 part of silane coupling agent KH-560 and mixing them evenly. Dilute 0.8 part of dibenzoyl peroxide with 10 times its mass of dipropylene glycol butyl ether, divide it into 3 parts by volume, add 1 part by volume every 30 min, carry out reflux reaction at 130 °C for 4 h, remove dimethyl carbonate by vacuum distillation at 50 °C (residual amount less than 1%), and filter through a 150-mesh sieve to remove impurities to obtain the composite hybrid resin.

[0039] Among them, the component mass ratio of the composite filler is mica powder: modified barium sulfate = 3:7; the D90 particle size of mica powder and modified barium sulfate is 13 μm. The D90 particle size of fumed silica is 10 μm.

[0040] The preparation method of the above-mentioned ultra-low VOC baking-type fast-drying coating includes the following steps:

[0041] S1: By mass parts, mix composite hybrid resin, neopentyl glycol diglycidyl ether and 10 parts of deionized water by stirring at 300 rpm for 10 min. Then, successively add sodium polyacrylate dispersant, fumed silica and composite filler, and stir and mix for dispersion at 500 rpm for 20 min to form a homogeneous base material;

[0042] S2: By mass parts, successively add methylated melamine formaldehyde resin and zinc fatty acid to the homogeneous base material, stir and mix at 400 rpm for 15 min, and then grind with a sand mill at 1000 rpm for 30 min until the fineness is ≤ 15 μm to obtain a ground material;

[0043] S3: By mass parts, successively add trimethylolpropane triacrylate, silane coupling agent KH-560, polyether-modified silicone defoamer, aminoethyl aminopropyl polydimethylsiloxane and fluorocarbon-modified polyether leveling agent to the ground material, stir at low speed at 300 rpm for 20 min, adjust the coating viscosity to 1500 mPa·s with deionized water, filter through a 150-mesh sieve, and take the filtrate to obtain the coating.

[0044] Example 2

[0045] An ultra-low VOC baking-type fast-drying coating, the coating comprising the following raw materials in mass parts: 48 parts of composite hybrid resin, 18 parts of methylated melamine formaldehyde resin, 6 parts of neopentyl glycol diglycidyl ether, 4 parts of trimethylolpropane triacrylate, 9 parts of composite filler, 1.2 parts of zinc fatty acid, 0.7 part of fumed silica, 0.7 part of silane coupling agent KH-560, 1.2 parts of sodium polyacrylate dispersant, 0.2 part of polyether-modified silicone defoamer, 2.5 parts of aminoethyl aminopropyl polydimethylsiloxane, 0.8 part of fluorocarbon-modified polyether leveling agent, and the balance being deionized water, with the coating viscosity being 1800 mPa·s.

[0046] Among them, the preparation method of the composite hybrid resin includes: under the protection of nitrogen with a slightly positive pressure floating between 0.03 MPa and 0.04 MPa, by mass, 23 parts of isophthalic acid, 12 parts of adipic acid, 18 parts of neopentyl glycol, 6 parts of propylene glycol, 12 parts of propylene glycol monobutyl ether, 16 parts of dimethyl carbonate, 11 parts of dipropylene glycol monobutyl ether and 0.25 part of tetrabutyl titanate are mixed evenly, and reflux reaction is carried out at 190 °C. The water generated by the reaction is collected through a water separator. When the acid value of the reaction system drops to 6 mgKOH / g, the temperature is lowered to 75 °C, and 9 parts of methyl methacrylate, 7 parts of butyl acrylate, 5 parts of trimethylolpropane triacrylate, 5 parts of ethylene-vinyl acetate copolymer, 6 parts of polyester-type polyurethane prepolymer I, 4 parts of polyester-type polyurethane prepolymer II and 1.5 parts of silane coupling agent KH-560 are added in sequence and mixed evenly. 1 part of dibenzoyl peroxide is diluted with 12 times its mass of dipropylene glycol monobutyl ether, divided into 3 equal volume parts, and 1 volume part is added every 40 min, and reflux reaction is carried out at 135 °C for 4.5 h. Dimethyl carbonate is removed by vacuum distillation at 55 °C (residual amount below 1%), and impurities are removed by filtration through a 200-mesh sieve to obtain the composite hybrid resin.

[0047] Among them, the mass ratio of the components of the composite filler is mica powder: modified barium sulfate = 3.5:8; the D90 particle size of mica powder and modified barium sulfate is 11 μm. The D90 particle size of fumed silica is 12 μm.

[0048] The preparation method of the above-mentioned ultra-low VOC baking-type fast-drying coating includes the following steps:

[0049] S1: By mass, the composite hybrid resin, neopentyl glycol diglycidyl ether and 12 parts of deionized water are stirred and mixed at 400 rpm for 15 min. Sodium polyacrylate dispersant, fumed silica and composite filler are added in sequence, and stirred and mixed and dispersed at 600 rpm for 25 min to form a homogeneous base material;

[0050] S2: By mass, methylated melamine formaldehyde resin and zinc fatty acid are added to the homogeneous base material in sequence, stirred and mixed at 500 rpm for 20 min, and then ground by a sand mill at 1200 rpm for 35 min until the fineness ≤ 15 μm to obtain a ground material;

[0051] S3: By mass, trimethylolpropane triacrylate, silane coupling agent KH-560, polyether-modified silicone defoamer, aminoethylaminopropyl polydimethylsiloxane and fluorocarbon-modified polyether leveling agent are added to the ground material in sequence, stirred at low speed at 350 rpm for 20 min, the viscosity of the coating is adjusted to 1800 mPa·s with deionized water, and filtered through a 200-mesh sieve, and the filtrate is taken to obtain the coating.

[0052] Example 3

[0053] An ultra-low VOC baking-type fast-drying coating, the coating comprising the following raw materials in parts by mass: 50 parts of a composite hybrid resin, 20 parts of a methylated melamine formaldehyde resin, 7 parts of neopentyl glycol diglycidyl ether, 5 parts of trimethylolpropane triacrylate, 10 parts of a composite filler, 1.5 parts of zinc fatty acid, 0.8 parts of fumed silica, 0.8 parts of silane coupling agent KH-560, 1.5 parts of sodium polyacrylate dispersant, 0.3 parts of polyether-modified silicone defoamer, 3 parts of aminoethylaminopropyl polydimethylsiloxane, 1 part of fluorocarbon-modified polyether leveling agent, and the balance being deionized water, with the coating viscosity being 2000 mPa·s.

[0054] Among them, the preparation method of the composite hybrid resin includes: under the protection of nitrogen with a slightly positive pressure floating between 0.04 MPa and 0.05 MPa, in parts by mass, 25 parts of isophthalic acid, 15 parts of adipic acid, 20 parts of neopentyl glycol, 8 parts of propylene glycol, 15 parts of propylene glycol monobutyl ether, 20 parts of dimethyl carbonate, 12 parts of dipropylene glycol monobutyl ether and 0.3 parts of tetrabutyl titanate are mixed evenly, refluxed at 200 °C, and the water generated by the reaction is collected through a water separator. When the acid value of the reaction system drops to 7 mgKOH / g, the temperature is lowered to 80 °C, and 10 parts of methyl methacrylate, 8 parts of butyl acrylate, 6 parts of trimethylolpropane triacrylate, 8 parts of ethylene-vinyl acetate copolymer, 8 parts of polyester-type polyurethane prepolymer I, 5 parts of polyester-type polyurethane prepolymer II and 2 parts of silane coupling agent KH-560 are added in sequence and mixed evenly. 1.2 parts of dibenzoyl peroxide are diluted with 15 times the mass of dipropylene glycol monobutyl ether, divided into 4 parts by volume, and 1 part by volume is added every 40 min, and refluxed at 140 °C for 5 h. Dimethyl carbonate is removed by vacuum distillation at 60 °C (residual amount below 1%), and impurities are removed by filtration through a 200-mesh sieve to obtain the composite hybrid resin.

[0055] Among them, the mass ratio of the components of the composite filler is mica powder: modified barium sulfate = 4:9; the D90 particle size of mica powder and modified barium sulfate is 8 μm. The D90 particle size of fumed silica is 14 μm.

[0056] The preparation method of the above-mentioned ultra-low VOC baking-type fast-drying coating includes the following steps:

[0057] S1: In parts by mass, the composite hybrid resin, neopentyl glycol diglycidyl ether and 15 parts of deionized water are stirred and mixed at 500 rpm for 10 min, and the sodium polyacrylate dispersant, fumed silica and composite filler are added in sequence, and stirred and mixed and dispersed at 600 rpm for 30 min to form a homogeneous base material;

[0058] S2: By mass parts, successively add methylated melamine formaldehyde resin and zinc fatty acid to the homogeneous base material, stir and mix at 600 rpm for 15 min, then grind at 1500 rpm for 30 min using a sand mill until the fineness is ≤ 15 μm to obtain a ground material;

[0059] S3: By mass parts, successively add trimethylolpropane triacrylate, silane coupling agent KH-560, polyether-modified silicone defoamer, aminoethylaminopropyl polydimethylsiloxane, and fluorocarbon-modified polyether leveling agent to the ground material, stir slowly at 350 rpm for 30 min, adjust the coating viscosity to 2000 mPa·s with deionized water, filter through a 200-mesh sieve, and take the filtrate to obtain the coating.

[0060] Example 4

[0061] An ultra-low VOC baking-type fast-drying coating, the coating comprising the following raw materials by mass parts: 50 parts of composite hybrid resin, 15 parts of methylated melamine formaldehyde resin, 7 parts of neopentyl glycol diglycidyl ether, 3 parts of trimethylolpropane triacrylate, 10 parts of composite filler, 1 part of zinc fatty acid, 0.8 part of fumed silica, 0.5 part of silane coupling agent KH-560, 1.5 parts of sodium polyacrylate dispersant, 0.1 part of polyether-modified silicone defoamer, 3 parts of aminoethylaminopropyl polydimethylsiloxane, 0.6 part of fluorocarbon-modified polyether leveling agent, and the balance being deionized water, with the coating viscosity being 2000 mPa·s.

[0062] Among them, the preparation method of the composite hybrid resin includes: under the protection of nitrogen with a slightly positive pressure floating between 0.03 MPa and 0.04 MPa, by mass parts, mix 20 parts of isophthalic acid, 15 parts of adipic acid, 15 parts of neopentyl glycol, 8 parts of propylene glycol, 10 parts of propylene glycol monobutyl ether, 20 parts of dimethyl carbonate, 10 parts of dipropylene glycol monobutyl ether, and 0.3 part of tetrabutyl titanate evenly, reflux and react at 180 °C, collect the water generated by the reaction through a water separator, when the acid value of the reaction system drops to 6.5 mgKOH / g, cool down to 70 °C, successively add 10 parts of methyl methacrylate, 6 parts of butyl acrylate, 6 parts of trimethylolpropane triacrylate, 3 parts of ethylene-vinyl acetate copolymer, 8 parts of polyester polyurethane prepolymer I, 3 parts of polyester polyurethane prepolymer II, and 2 parts of silane coupling agent KH-560 and mix evenly. Dilute 0.8 part of dibenzoyl peroxide with 15 times its mass of dipropylene glycol monobutyl ether, divide it into 3 equal volume parts, and add 1 volume part every 40 min, reflux and react at 130 °C for 5 h, distill off dimethyl carbonate under reduced pressure at 50 °C (residual amount below 1%), and filter through a 200-mesh sieve to remove impurities to obtain the composite hybrid resin.

[0063] Among them, the mass ratio of the components of the composite filler is mica powder: modified barium sulfate = 3:9; the D90 particle size of mica powder and modified barium sulfate is 12 μm. The D90 particle size of fumed silica is 9 μm.

[0064] The preparation method of the above-mentioned ultra-low VOC baking-type quick-drying coating includes the following steps:

[0065] S1: By mass, mix the composite hybrid resin, neopentyl glycol diglycidyl ether and 10 parts of deionized water with stirring at 500 rpm for 15 min. Then, successively add sodium polyacrylate dispersant, fumed silica and the composite filler, and stir and mix and disperse at 500 rpm for 30 min to form a homogeneous base material;

[0066] S2: By mass, successively add methylated melamine formaldehyde resin and zinc fatty acid to the homogeneous base material, stir and mix at 500 rpm for 20 min, and then grind with a sand mill at 1200 rpm for 40 min until the fineness ≤ 15 μm to obtain a ground material;

[0067] S3: By mass, successively add trimethylolpropane triacrylate, silane coupling agent KH-560, polyether-modified silicone defoamer, aminoethylaminopropyl polydimethylsiloxane and fluorocarbon-modified polyether leveling agent to the ground material, stir at a low speed of 350 rpm for 20 min, adjust the coating viscosity to 2000 mPa·s with deionized water, filter through a 200-mesh sieve, and take the filtrate to obtain the coating.

[0068] The recommended baking parameters for the coatings in the above-mentioned examples: The construction is carried out by spraying or brushing. The construction environment temperature is controlled at 15°C to 30°C, and the relative humidity is controlled at 40% to 70%. After construction, put the coated workpiece into an oven, pre-bake at 60°C to 70°C for 5 min to 10 min, and then raise the temperature to 120°C to 140°C for baking. At the same time, ultraviolet light irradiation with appropriate intensity can be selected (the power is adjusted according to the actual situation) to achieve rapid curing of the coating and form a paint film with excellent performance.

[0069] Raw material sources in the above embodiments: Methylated melamine formaldehyde resin is from Oriole (Jining) Chemical Co., Ltd., model 2526. Neopentyl glycol diglycidyl ether is from Wuhan Chengtian Fine Chemical Co., Ltd., model 17557-23-2. Trimethylolpropane triacrylate is from Wuhan Chengtian Fine Chemical Co., Ltd., model TMPTA. Zinc fatty acid is from Jinan Jinyu Chemical Co., Ltd., model ZnF A. Fumed silica is from Shandong Kasong New Materials Co., Ltd. Silane coupling agent KH-560 is from Jinan Yunuo Chemical Co., Ltd. Sodium polyacrylate dispersant is from Guangdong Zhongtailianhua New Materials Co., Ltd., model AG-22. Polyether-modified silicone defoamer is from Dongguan Haoyouduo New Materials Co., Ltd., model D-001. Aminoethylaminopropyl polydimethylsiloxane is from Guangzhou Youbao Chemical Co., Ltd., model GY22. Fluorocarbon-modified polyether leveling agent is from Kunshan Luolis High Polymer Materials Co., Ltd., model LS3024. Isophthalic acid is from Shandong Wuyang Biotechnology Co., Ltd. Adipic acid is from Shandong Wuyang Biotechnology Co., Ltd. Neopentyl glycol is from Shanghai Gaoming Chemical Co., Ltd. Propylene glycol is from Tianjin Zhongheshengtai Chemical Co., Ltd., 1,2-propylene glycol. Propylene glycol monobutyl ether is from Jinan Shengda Chemical Co., Ltd., model 416, propylene glycol monobutyl ether. Dimethyl carbonate is from Shandong Chuangying Chemical Co., Ltd. Dipropylene glycol monobutyl ether is from Guangzhou Honghai Chemical Co., Ltd. Tetrabutyl titanate is from Jinan Jiayi New Materials Co., Ltd. Methyl methacrylate is from Shandong Yanshuo Chemical Co., Ltd. Butyl acrylate is from Chuangyida (Shandong) Biotechnology Co., Ltd. Ethylene-vinyl acetate copolymer is from Dongguan Yuehaoxuan Plastic Co., Ltd., model 7470M. Polyester-type polyurethane prepolymer I is polyester-type polyurethane prepolymer T80, from Jiangsu Qianmeite Polyurethane New Materials Co., Ltd., model K1235, with a MOCA dosage of 10.5 g / 100 g prepolymer. Polyester-type polyurethane prepolymer II is polyester-type polyurethane prepolymer T100, from Jiangsu Qianmeite Polyurethane New Materials Co., Ltd., model K1150, with a MOCA dosage of 14.5 g / 100 g prepolymer. Benzoyl peroxide is from Jiangyin Jianheng Chemical Co., Ltd. Mica powder is from Lingshou County Chuangkai Mineral Products Co., Ltd. Modified barium sulfate is from Guangzhou Xianwan Chemical Co., Ltd., model 1336.

[0070] Comparative Example 1

[0071] The composite hybrid resin is replaced with an acrylic resin (waterborne organosilicon-modified acrylic resin, sourced from Qingdao Wanjiahuixin Surface Materials Technology Co., Ltd., model S-611, content 40%); and an isocyanate curing agent accounting for 10% of the mass of the acrylic resin is additionally added (the isocyanate curing agent is sourced from Shanghai Kaizhi New Materials Technology Co., Ltd., model N3300), and the curing agent is added after the defoaming agent in S3; other parameters and methods are the same as in Example 1.

[0072] Comparative Example 2

[0073] The composite hybrid resin is replaced with an epoxy resin (sourced from Qingdao Jinwanli Fine Chemical Co., Ltd., content 70%, model J-990); and an N-(2-hydroxyethyl)ethylenediamine curing agent accounting for 8% of the mass of the epoxy resin is additionally added (sourced from Guangzhou Jiangshun Chemical Technology Co., Ltd., model HS0595), and the curing agent is added after the defoaming agent in S3; other parameters and methods are the same as in Example 1.

[0074] Comparative Example 3

[0075] The methylated melamine formaldehyde resin is replaced with an acrylic resin (waterborne organosilicon-modified acrylic resin, sourced from Qingdao Wanjiahuixin Surface Materials Technology Co., Ltd., model S-611, content 40%); and an isocyanate curing agent accounting for 12% of the mass of the acrylic resin is additionally added (the isocyanate curing agent is sourced from Shanghai Kaizhi New Materials Technology Co., Ltd., model N3300), and the curing agent is added after the defoaming agent in S3; other parameters and methods are the same as in Example 1.

[0076] Comparative Example 4

[0077] The methylated melamine formaldehyde resin is replaced with an epoxy resin (sourced from Qingdao Jinwanli Fine Chemical Co., Ltd., content 70%, model J-990); and an N-(2-hydroxyethyl)ethylenediamine curing agent accounting for 7% of the mass of the epoxy resin and a polyamide curing agent accounting for 3% of the mass of the epoxy resin are additionally added (the polyamide curing agent is sourced from Shanghai Canal Materials Technology Co., Ltd., model 8115), and the curing agents are added after the defoaming agent in S3; other parameters and methods are the same as in Example 1.

[0078] Comparative Example 5

[0079] The zinc fatty acid is replaced with sodium dodecylbenzenesulfonate; other parameters and methods are the same as in Example 1.

[0080] Comparative Example 6

[0081] The addition amount of aminoethylaminopropyl polydimethylsiloxane is 10 parts; other parameters and methods are the same as in Example 1.

[0082] Comparative Example 7

[0083] In the preparation method of the composite hybrid resin, ethylene-vinyl acetate copolymer is not added; other parameters and methods are the same as those in Example 1.

[0084] Comparative Example 8

[0085] In the preparation method of the composite hybrid resin, silane coupling agent KH-560 is not added; other parameters and methods are the same as those in Example 1.

[0086] Comparative Example 9

[0087] In the preparation method of the composite hybrid resin, both ethylene-vinyl acetate copolymer and silane coupling agent KH-560 are not added; other parameters and methods are the same as those in Example 1.

[0088] Comparative Example 10

[0089] In the preparation method of the composite hybrid resin, ethylene-vinyl acetate copolymer is replaced by silane coupling agent KH-560; other parameters and methods are the same as those in Example 1.

[0090] Comparative Example 11

[0091] In the preparation method of the composite hybrid resin, polyester-type polyurethane prepolymer I is not added; other parameters and methods are the same as those in Example 1.

[0092] Comparative Example 12

[0093] In the preparation method of the composite hybrid resin, polyester-type polyurethane prepolymer II is not added; other parameters and methods are the same as those in Example 1.

[0094] Comparative Example 13

[0095] In the preparation method of the composite hybrid resin, both polyester-type polyurethane prepolymer I and polyester-type polyurethane prepolymer II are not added; other parameters and methods are the same as those in Example 1.

[0096] Comparative Example 14

[0097] In the preparation method of the composite hybrid resin, polyester-type polyurethane prepolymer I is replaced by polyester-type polyurethane prepolymer II; other parameters and methods are the same as those in Example 1.

[0098] Comparative Example 15

[0099] In the preparation method of the composite hybrid resin, polyester-type polyurethane prepolymer II is replaced by polyester-type polyurethane prepolymer I; other parameters and methods are the same as those in Example 1.

[0100] Comparative Example 16

[0101] In the preparation method of the composite hybrid resin, 15 parts of polyester-type polyurethane prepolymer I are added; other parameters and methods are the same as those in Example 1.

[0102] Comparative Example 17

[0103] In the preparation method of the composite hybrid resin, 10 parts of polyester polyurethane prepolymer II are added; other parameters and methods are the same as in Example 1.

[0104] Comparative Example 18

[0105] In the preparation method of the composite hybrid resin, 12 parts of polyester polyurethane prepolymer I and 8 parts of polyester polyurethane prepolymer II are added; other parameters and methods are the same as in Example 1.

[0106] 1. Testing of baking film forming time: air spraying method was adopted, using a spray gun with a caliber of 1.5 mm, and the distance between the spray gun and the test plate (standard tinplate with a size of 150 mm × 70 mm × 1 mm) was maintained at 20 cm, so that the final coating thickness reached 80 ± 2 μm, and several parallel samples were made for each type.

[0107] Several parallel samples were set with different baking times at 120℃. The baking time was set to increase by 1min after the 10th minute. The preliminary conditions of the parallel samples were the same. The coated workpieces were quickly placed in a blast drying oven preheated to 60℃ for 8min. After the prebaking, the temperature of the drying oven was raised to 120℃ within 3min, and the timing was started. The baking time was set to increase by 1min after the 10th minute. The coating surface was tested and scratched lightly until the sample coating surface was taken out without obvious scratches and was not sticky. This time was recorded as the baking film formation time.

[0108] 2. VOC emission detection: Refer to GB / T 23986 "Paints and varnishes - Determination of volatile organic compound (VOC) content by gas chromatography" to measure the total volatile VOC content at 150°C for 30 minutes.

[0109] 3. Adhesion test: refer to GBT 9286 "Scratch test for paints and varnishes", coating thickness 80±2μm, 1mm spacing, evaluate adhesion level: 0 is the best and 5 is the worst.

[0110] 4. Hardness test: Refer to GB / T 6739 "Determination of film hardness of paints and varnishes by pencil method", coating thickness is 100±2μm, the pencil is at a 45° angle to the coating surface, a pressure of 1kg is applied, and the pencil is pushed forward at a speed of 0.5cm / s. Each hardness level is tested 3 times, and the hardest pencil hardness that does not scratch the coating is taken as the coating hardness.

[0111] 5. Low temperature flexibility test: The PET thin substrate coating thickness is 80±2μm. After being placed at -25℃ for 2h, take it out and bend the sample 180° on a shaft with a diameter of 3mm within 10s to observe whether the coating has cracking, peeling, etc.

[0112] VI. Water resistance: The coating thickness on the glass plate is 80 ± 2 μm. Immerse it in deionized water at 40°C for 240 h, and observe the conditions such as blistering, peeling, and discoloration.

[0113] Evaluation grades: Grade 1: No change on the surface of the paint film. Grade 2: Slight loss of gloss, discoloration, small bubbles, slight whitening, etc. are allowed on the surface of the paint film. Grade 3: Obvious loss of gloss, discoloration, dense small bubbles, obvious whitening, wrinkling, etc. appear on the surface of the paint film. Grade 4: Severe discoloration, large bubbles, peeling, dissolution and other damage phenomena appear on the surface of the paint film.

[0114] VII. Acid resistance: The coating thickness on the glass plate is 80 ± 2 μm. Immerse it in 5 wt% hydrochloric acid solution at room temperature, take it out and observe every 2 h, and record the time when corrosion phenomena (any one of small bubbles, wrinkling, peeling, etc.) appear on the coating.

[0115] VIII. Alkaline resistance: The coating thickness on the glass plate is 80 ± 2 μm. Immerse it in 5 wt% sodium hydroxide solution at room temperature, take it out and observe every 2 h, and record the time when corrosion phenomena (any one of small bubbles, wrinkling, peeling, etc.) appear on the coating.

[0116] Table 1 Detection data results of each example and comparative example

[0117]

[0118] As can be seen from the above results, the comprehensive performance advantages of Examples 1 to 4 are obvious, and the multi-polymerization structure of the composite hybrid resin is the core factor. During the synthesis of the composite hybrid resin, the polyester chain segments generated by the early polyesterification reaction provide abundant active groups such as hydroxyl groups. In the middle stage, monomers such as methyl methacrylate and butyl acrylate are introduced, and a large number of carbon-carbon double bonds are introduced into the molecular chain through free radical polymerization. In the later stage, the addition of polyester-type polyurethane prepolymers I and II further introduces active groups such as isocyanate groups, and a quaternary composite hybrid resin with special properties is synthesized. When the quaternary composite hybrid resin is blended and baked with methylated melamine formaldehyde resin, multiple cross-linking reaction paths are formed: the hydroxyl group of the polyester chain segment undergoes an ether exchange reaction with the methoxy group of the methylated melamine formaldehyde resin to form a methylene bridge bond; the acrylate double bond undergoes a synergistic free radical-nucleophilic substitution reaction with the active group of the methylated melamine formaldehyde resin; the isocyanate group of the polyurethane prepolymer can also react quickly with the hydroxyl group in the system. Multiple reactions proceed simultaneously, greatly accelerating the formation of the cross-linking network. Compared with the single resin system (Comparative Example 1, Comparative Example 2), the cross-linking speed is significantly improved, thus greatly shortening the film-forming time. Trimethylolpropane triacrylate, as an active diluent, plays the role of a "cross-linking node amplifier" with its three acrylate double bonds in the system. After the initiator benzoyl peroxide decomposes to generate free radicals, the double bonds of trimethylolpropane triacrylate quickly open, not only participating in its own self-polymerization reaction, but also serving as a connection point to promote the cross-linking between the composite hybrid resin and the methylated melamine formaldehyde resin, making the construction of the cross-linking network more efficient.

[0119] Root cause of the reduction in VOC emissions: During the later stage of the synthesis of the composite hybrid resin, most of the residues are effectively removed by vacuum distillation with dimethyl carbonate. Compared with traditional high-VOC solvents (such as toluene and xylene), the solvent-based VOC emissions are greatly reduced. At the resin structure level, as the cross-linking density of the composite hybrid resin increases, the formed three-dimensional network structure physically wraps the remaining small amount of solvent molecules in the network pores, restricting the volatilization of VOCs; at the same time, the reaction of active groups during the cross-linking process consumes the volatile monomers that originally existed in a free state, further reducing the VOC emissions.

[0120] The silane coupling agent KH-560 plays the role of a "molecular bridge" during the synthesis of the composite hybrid resin and the film-forming process of the coating. One end of its siloxane group can undergo a condensation reaction with the hydroxyl group on the substrate surface to form a Si-O-M (M is the substrate metal atom or other hydroxyl-containing material) chemical bond in a humid environment or during high-temperature baking; the active groups such as epoxy groups at the other end undergo a chemical reaction with the active groups in the composite hybrid resin, thus forming a strong chemical connection between the resin and the substrate. In addition, the dense network structure formed by the cross-linking of the composite hybrid resin and the methylated melamine formaldehyde resin further enhances the adhesion to the substrate through mechanical interlocking.

[0121] The highly cross-linked network structure greatly restricts the movement of molecular chain segments. When subjected to external forces, the cross-linking points hinder the relative sliding of molecular chains, and higher energy is required to deform the coating, which macroscopically manifests as increased hardness. The addition of polyester-type polyurethane prepolymers I and II, with their rigid urethane groups embedded in the cross-linking network, further enhances the rigidity and hardness of the network.

[0122] Improved chemical resistance: The dense cross-linking network is like a "molecular fence", significantly increasing the difficulty for corrosive medium molecules such as water, acids, and alkalis to penetrate into the coating interior. On the one hand, the medium molecules need to diffuse through the pores of the cross-linking network, and the high cross-linking density makes the pore size extremely small and tortuous, prolonging the diffusion path; on the other hand, the chemical bonds and intermolecular forces in the cross-linking network can interact with the medium molecules, consuming the penetration energy of the medium molecules. For example, in the water resistance test, the penetration of water molecules into the coating interior can cause resin swelling and cross-linking point destruction, while the coating with high cross-linking density in the examples can effectively resist the erosion of water molecules and extend the water resistance time.

[0123] Replacement with acrylic resin (Comparative Example 1, Comparative Example 3): The molecular chain of acrylic resin is mainly composed of carbon-carbon single bonds and ester groups, and its cross-linking method mainly relies on the free radical polymerization of acrylate double bonds, forming a relatively loose cross-linking network. When acting synergistically with methylated melamine formaldehyde resin, due to the lack of active groups (such as insufficient hydroxyl groups) in acrylic resin that can react efficiently with methylated melamine formaldehyde resin, not enough cross-linking structures such as methylene bridge bonds can be formed, resulting in low cross-linking density. This causes a decrease in the hardness, adhesion, and corrosion resistance of the coating. At the same time, the loose structure is not conducive to the encapsulation and fixation of solvents, increasing the VOC emissions.

[0124] Replacement with epoxy resin (Comparative Example 2, Comparative Example 4): The curing of epoxy resin mainly depends on the reaction of epoxy groups with curing agents such as amines and anhydrides, and its reaction with methylated melamine formaldehyde resin in this system is not well-matched. The reaction activity of the epoxy groups of epoxy resin with the methoxy groups of methylated melamine formaldehyde resin is relatively low, making it difficult to form a stable cross-linking structure. In addition, although the cross-linking network formed after the curing of epoxy resin has relatively high hardness, it has poor flexibility and is prone to cracking due to stress concentration at low temperatures (such as in the low-temperature flexibility test). At the same time, due to insufficient cross-linking, there is more solvent residue, increasing the VOC emissions.

[0125] Fatty acid zinc substitution (Comparative Example 5): Fatty acid zinc not only has a drying effect in the coating system, but also participates in the cross-linking reaction. Its metal zinc ions can react with carboxyl groups and other groups in the resin to promote cross-linking between resin molecules, while reducing the surface tension of the system, which is beneficial to the leveling and drying of the coating. As a surfactant, sodium dodecylbenzene sulfonate mainly reduces surface tension and disperses pigments, and cannot replace the functions of fatty acid zinc in cross-linking and drying, resulting in a slower drying speed of the coating and a lower degree of cross-linking, which in turn affects properties such as adhesion, hardness and corrosion resistance.

[0126] Too much aminoethylaminopropyl polydimethylsiloxane (Comparative Example 6): Although this substance can improve the leveling and low-temperature flexibility of the coating, it is essentially a silicone polymer. Excessive addition will destroy the compatibility of the coating system. Too many silicone segments will be enriched on the coating surface, hindering the cross-linking reaction between the composite hybrid resin and the methylated melamine formaldehyde resin, resulting in a decrease in cross-linking density. At the same time, the enrichment of silicone segments will also weaken the adhesion between the coating and the substrate because it reduces the polarity matching between the coating surface and the substrate.

[0127] Ethylene-vinyl acetate copolymer is missing (Comparative Example 7): Ethylene-vinyl acetate copolymer has good flexibility and polarity. The ethylene segment in its molecular chain provides flexibility, and the vinyl acetate segment provides polar groups. In the composite hybrid resin, it can form intermolecular forces (such as hydrogen bonds and van der Waals forces) with other components to improve the flexibility of the resin and the wettability of the filler. After the component is missing, the coating has weakened the mobility of the molecular segments at low temperatures (low temperature flexibility test), the internal stress cannot be effectively released, and it is easy to crack.

[0128] Silane coupling agent KH-560 is missing (Comparative Example 8): Silane coupling agent KH-560 is the key bridge between the resin and the substrate. After it is missing, the resin and the substrate mainly rely on physical adsorption, which is far less strong than chemical bonds. The coating is easy to fall off from the surface of the substrate, and various properties deteriorate.

[0129] Comparative Example 9 does not add ethylene-vinyl acetate copolymer and silane coupling agent KH-560 at the same time; and Comparative Example 10 lacks the synergistic effect of ethylene-vinyl acetate copolymer and silane coupling agent KH-560, the fusion of various components is poor, the reaction synthesis rate is reduced, and various performances are deteriorated.

[0130] Absence, substitution or increment of polyester-based polyurethane prepolymer (Comparative Examples 11 - 18): Polyester-based polyurethane prepolymers I and II introduce rigid urethane groups and flexible polyester segments into the composite hybrid resin, playing a role in regulating the balance between the hardness and flexibility of the resin. When one of them is absent or both are absent simultaneously (Comparative Examples 11 - 13), the structural balance of the resin is disrupted, and both the hardness and flexibility are affected. While substitution or increment of the prepolymer (Comparative Examples 14 - 18) will change the composition and structure of the crosslinked network. An excessive content will lead to too high a crosslinking density, uneven dispersion in the system, and insufficient components such as emulsification, dispersion and fusion to provide sufficient effective dispersion ability, poor compatibility, and difficulty in achieving a good homogeneous state after dilution, affecting the performance of subsequent film formation.

[0131] In addition, analysis of VOC volatilization: The acrylic resins in Comparative Example 1 and Comparative Example 3 contain VOC volatile solvents and volatile substances of additional curing agents. The epoxy resins in Comparative Example 2 and Comparative Example 4 contain VOC volatile solvents and volatile substances of additional curing agents, releasing benzene series and small molecule amines. Sodium dodecylbenzenesulfonate in Comparative Example 5 has no metal passivation effect, increasing the porosity of the coating and slightly rising the VOC penetration. Self-polymerization occurs in excessive aminoethylaminopropyl polydimethylsiloxane in Comparative Example 6, releasing trace amounts of small molecule siloxanes. The absence of ethylene-vinyl acetate copolymer in Comparative Example 7 leads to a decrease in the denseness of the coating, poor fusion of polar components, and accelerated volatilization of solvent residues. The absence of silane coupling agent in Comparative Example 8 results in poor binding at the filler-resin interface, increased porosity, and increased VOC release. The simultaneous absence of ethylene-vinyl acetate copolymer and silane coupling agent in Comparative Example 9 leads to a loose coating structure and a significant increase in VOC release. The substitution of silane coupling agent for ethylene-vinyl acetate copolymer in Comparative Example 10 results in poor fusion of polar components and insufficient flexibility, but the interfacial binding is improved, partially offsetting the increase in VOC. The absence of prepolymer I in Comparative Example 11 reduces the crosslinking density and slightly increases the residue of unreacted monomers. The absence of prepolymer II in Comparative Example 12 slightly increases the residue of unreacted monomers and decreases the flexibility, but the crosslinking degree is still relatively high, and the VOC release is slightly lower. The simultaneous absence of prepolymer I and II in Comparative Example 13 results in a very poor crosslinked network, and a significant increase in the residues of solvents and monomers. The interchange of prepolymers I and II in Comparative Examples 14 and 15 results in a similar crosslinking degree, and the VOC release is close to that of Example 1. Excessive prepolymer I in Comparative Examples 16 - 18 leads to too high a viscosity of the system, uneven dispersion, and a slightly increased residue.

Claims

1. An ultra-low VOC baking-type fast-drying coating, characterized in that, The coating comprises raw materials in the following parts by mass: 45 to 50 parts of composite hybrid resin, 15 to 20 parts of methylated melamine formaldehyde resin, 5 to 7 parts of neopentyl glycol diglycidyl ether, 3 to 5 parts of trimethylolpropane triacrylate, 8 to 10 parts of composite filler, 1 to 1.5 parts of zinc fatty acid, 0.5 to 0.8 part of fumed silica, 0.5 to 0.8 part of silane coupling agent KH-560, 1 to 1.5 parts of sodium polyacrylate dispersant, 0.1 to 0.3 part of polyether-modified silicone defoamer, 2 to 3 parts of aminoethyl aminopropyl polydimethylsiloxane, 0.6 to 1 part of fluorocarbon-modified polyether leveling agent, and the balance being deionized water, and the viscosity of the coating is 1500 mPa·s to 2000 mPa·s.

2. An ultra-low VOC baking-type fast-drying coating according to claim 1, characterized in that The preparation method of the composite hybrid resin includes: under nitrogen protection, by mass, mixing 20 to 25 parts of isophthalic acid, 10 to 15 parts of adipic acid, 15 to 20 parts of neopentyl glycol, 5 to 8 parts of propylene glycol, 10 to 15 parts of propylene glycol monobutyl ether, 15 to 20 parts of dimethyl carbonate, 10 to 12 parts of dipropylene glycol monobutyl ether and 0.2 to 0.3 part of tetrabutyl titanate evenly, carrying out reflux reaction at 180°C to 200°C, collecting the water generated by the reaction through a water separator, when the acid value of the reaction system drops to 5 mgKOH / g to 7 mgKOH / g, cooling to 70°C to 80°C, successively adding 8 to 10 parts of methyl methacrylate, 6 to 8 parts of butyl acrylate, 4 to 6 parts of trimethylolpropane triacrylate, 3 to 8 parts of ethylene-vinyl acetate copolymer, 5 to 8 parts of polyester-type polyurethane prepolymer I, 3 to 5 parts of polyester-type polyurethane prepolymer II and 1 to 2 parts of silane coupling agent KH-560 and mixing evenly, adding 0.8 to 1.2 parts of dibenzoyl peroxide, carrying out reflux reaction at 130°C to 140°C for 4 h to 5 h, removing dimethyl carbonate by vacuum distillation, and removing impurities by screening with a sieve to obtain the composite hybrid resin.

3. An ultra-low VOC baking-type fast-drying coating according to claim 2, characterized in that, The polyester-type polyurethane prepolymer I is polyester-type polyurethane prepolymer T80; the polyester-type polyurethane prepolymer II is polyester-type polyurethane prepolymer T100.

4. An ultra-low VOC baking-type fast-drying coating according to claim 2, wherein The dibenzoyl peroxide is diluted with 10 to 15 times the mass of dipropylene glycol monobutyl ether before addition, evenly divided into 3 to 4 volume parts, and 1 volume part is added every 30 min to 40 min.

5. An ultra-low VOC baking-type fast-drying coating according to claim 2, characterized in that The nitrogen protection is nitrogen protection with a slightly positive pressure of 0.02 MPa to 0.05 MPa; the temperature of the vacuum distillation is 50°C to 60°C; the mesh number of the sieve is 150 to 200 meshes.

6. An ultra-low VOC baking-type fast-drying coating according to claim 1, characterized in that, The D90 particle size of the fumed silica is below 15 μm.

7. An ultra-low VOC baking-type fast-drying coating according to claim 1, characterized in that, The component mass ratio of the composite filler is mica powder: modified barium sulfate = (3 to 4):(7 to 9).

8. An ultra-low VOC baking-type fast-drying coating according to claim 7, characterized in that, The D90 particle sizes of the mica powder and the modified barium sulfate are below 15 μm.

9. A method for preparing an ultra-low VOC baking-type fast-drying coating according to claim 1, characterized in that, It includes the following steps: S1: By mass, mixing the composite hybrid resin, neopentyl glycol diglycidyl ether and 10 to 15 parts of deionized water evenly, successively adding the sodium polyacrylate dispersant, fumed silica and composite filler, and mixing and dispersing evenly to form a homogeneous base material; S2: By mass parts, methylated melamine formaldehyde resin and zinc fatty acid are successively added to the homogeneous base material and mixed evenly, and then ground by a sand mill until the fineness is ≤ 15 μm to obtain a ground material; S3: By mass parts, trimethylolpropane triacrylate, silane coupling agent KH-560, polyether-modified silicone defoamer, aminoethyl aminopropyl polydimethylsiloxane, and fluorocarbon-modified polyether leveling agent are successively added to the ground material, and stirred at a low speed. The viscosity of the coating is adjusted to 1500 mPa·s to 2000 mPa·s with deionized water, filtered through a sieve, and the filtrate is taken to obtain the coating.

10. The preparation method of an ultra-low VOC baking-type quick-drying coating according to claim 9, characterized in that, The low-speed stirring is carried out at a low speed of 300 rpm to 350 rpm for 20 min to 30 min; the mesh number of the sieve is 150 mesh to 200 mesh.

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