All-water-based melamine resin for metal baking varnish as well as preparation method and application of all-water-based melamine resin

Abstract

The invention provides a preparation method of all-water-based melamine resin for metal baking varnish, which comprises the following steps: mixing melamine, a formaldehyde donor, glyoxylic acid and cyclic urea, and carrying out heating reaction under alkaline conditions to obtain a carboxyl-containing polyhydroxymethyl intermediate; adding low-carbon alcohol, and carrying out etherification reaction under an acidic condition to obtain etherified resin liquid; cooling the etherified resin liquid to 45-55 DEG C, adjusting the pH value of the system to 6.0-7.0, and then adding a silane coupling agent containing an epoxy group to carry out a thermal insulation grafting reaction to obtain grafted modified resin; and carrying out reduced pressure on the grafted modified resin to remove small molecular byproducts, then adding a neutralizer to adjust the pH value to alkalescence, and adding deionized water for dispersion to obtain the all-water-based melamine resin for the metal baking varnish. The adhesive has ultralow free formaldehyde content and excellent storage stability, and has chemical bonding-level adhesive force and excellent flexibility to a metal substrate.

Description

Technical Field

[0001] This invention relates to a melamine resin, its preparation method, and its application, specifically to a water-based melamine resin for metal baking paint, its preparation method, and its application, belonging to the field of polymer resin materials technology. Background Technology

[0002] Melamine resin is widely used in metal baking paint coatings due to its excellent hardness, abrasion resistance, and chemical resistance. However, the persistently high free formaldehyde content is the primary challenge faced by traditional melamine resins and their waterborne modification technologies. The synthesis of traditional melamine resins heavily relies on the hydroxymethylation reaction of formaldehyde, resulting in a large amount of residual free formaldehyde in the final product. This not only fails to meet increasingly stringent environmental regulations but also continues to release formaldehyde during paint storage and application, endangering the health of operators and polluting the environment.

[0003] Existing technologies achieve water dispersibility by adding emulsifiers or modifying with strong hydrophilic groups. However, these methods often lead to resin hydrolysis or self-polymerization during storage, resulting in a sharp increase in viscosity or even gelation. This typically results in a short shelf life, putting pressure on paint manufacturers' inventory and limiting the industrial application of waterborne melamine resins. Furthermore, traditional resins lack active groups that form chemical bonds with metal surfaces, relying mainly on physical adsorption for adhesion. During secondary processing such as deep drawing and bending of metal sheets, the paint film is prone to peeling, failing to meet the stringent processability requirements of coatings in industries such as home appliances and automotive parts.

[0004] Furthermore, due to the rigidity of the six-membered ring structure of melamine, the cured paint film is brittle and hard, and is prone to cracking when the metal substrate is subjected to thermal expansion and contraction or mechanical impact, which will seriously affect the service life and reliability of the product.

[0005] Therefore, how to develop an all-waterborne melamine resin that combines ultra-low free formaldehyde, excellent storage stability, chemical bonding-level adhesion to metal substrates, and excellent flexibility has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] Based on the above background, the purpose of this invention is to provide a water-based melamine resin for metal baking paint, its preparation method and its application. Through molecular structure design and process parameter control, a breakthrough in the performance of water-based melamine resin is achieved, solving the problems of high free formaldehyde content, poor storage stability, weak adhesion to metal substrates and high brittleness of paint film in existing water-based melamine resins.

[0007] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0008] A method for preparing an all-water-based melamine resin for metal baking paint, the method comprising the following steps:

[0009] S1. Melamine, formaldehyde donor, glyoxylic acid and cyclic urea are mixed and heated under alkaline conditions to obtain a carboxyl-containing polyhydroxymethyl intermediate.

[0010] S2. Add a low-carbon alcohol to the carboxyl-containing polyhydroxymethyl intermediate and carry out an etherification reaction under acidic conditions to obtain an etherified resin solution.

[0011] S3. Cool the etherified resin solution to 45℃~55℃, adjust the pH of the system to 6.0~7.0, and then add an epoxy-containing silane coupling agent to carry out a heat preservation grafting reaction to obtain the grafted modified resin.

[0012] S4. The grafted modified resin is subjected to depressurization to remove small molecule byproducts, then a neutralizing agent is added to adjust the pH value to weak alkalinity, and deionized water is added for dispersion to obtain the all-water-based melamine resin for metal baking paint.

[0013] Preferably, based on 1 mole of melamine, the amounts of the other raw material components are as follows: 1.2 to 2.0 moles of formaldehyde donor (based on formaldehyde), 0.5 to 1.2 moles of glyoxylic acid, 0.2 to 0.5 moles of cyclic urea, 3.0 to 5.0 moles of low-carbon alcohol, and 0.05 to 0.15 moles of epoxy-containing silane coupling agent.

[0014] Preferably, in step S1, the cyclic urea is ethylidene urea or allyl urea; the pH value of the alkaline condition is 8.0~9.0; the temperature of the heating reaction is 65℃~75℃, and the reaction time is 1~2 hours.

[0015] Preferably, in step S2, the lower alcohol is one or more of methanol, ethanol, or butanol; the pH value of the acidic condition is 4.0~5.0; the temperature of the etherification reaction is 70℃~80℃, and the reaction time is 2~3 hours.

[0016] Preferably, in step S2, polycarbonate diol is added simultaneously with the low-carbon alcohol for co-etherification reaction; the molar ratio of polycarbonate diol to melamine is 0.1~0.2:1, and the number average molecular weight of polycarbonate diol is 1000~2000.

[0017] Preferably, in step S3, the epoxy-containing silane coupling agent is γ-glycidyl etheroxypropyltrimethoxysilane or γ-glycidyl etheroxypropylmethyldiethoxysilane; the time for the heat preservation grafting reaction is 1 to 1.5 hours.

[0018] Preferably, in step S4, the neutralizing agent is one or more of N,N-dimethylethanolamine, triethylamine, or ammonia water; and the weakly alkaline pH value is 7.5~8.5.

[0019] A water-based melamine resin for metal baking paint is prepared by the above-mentioned method for preparing a water-based melamine resin for metal baking paint.

[0020] The application of the above-mentioned water-based melamine resin for metal baking paint in the field of metal surface coating.

[0021] The all-water-based melamine resin is formulated as a water-based baking paint and applied to the surface of a metal substrate. It is then cured into a film at 130℃~150℃. During the curing process, the siloxane groups at the resin end groups hydrolyze and condense with the hydroxyl groups on the surface of the metal substrate to form covalent bonds.

[0022] Compared with the prior art, the present invention has the following advantages:

[0023] This invention discloses a method for preparing a water-based melamine resin for metal baking paint. The method utilizes the synergistic effect of glyoxylic acid and cyclic urea to reduce and capture free formaldehyde. The glyoxylic acid molecule contains bifunctional groups of aldehyde and carboxyl groups. The aldehyde group can participate in hydroxyalkylation reactions, while the carboxyl group imparts emulsifying ability to the resin, achieving water dispersibility without the need for external emulsifiers. Simultaneously, the secondary amine group of the cyclic urea can rapidly react with free formaldehyde released from the formaldehyde donor to form a stable five- or six-membered ring acetal structure, thereby locking the free formaldehyde within the resin skeleton. Tests show that the free formaldehyde content of the resin of this invention can be controlled below 0.1%, lower than the 1.5%~3.0% of traditional water-based melamine resins.

[0024] This invention introduces a carboxyl group from glyoxylic acid to form a carboxylate anion under alkaline conditions, which generates an electrostatic repulsion stabilizing effect. At the same time, the silane coupling agent introduced in step S3 is grafted to the end of the molecular chain through the ring-opening reaction of epoxy groups and residual amino groups in the resin. This not only consumes easily hydrolyzed active hydrogen, but also the introduced siloxane group forms a hydrophobic protective layer at the end of the molecular chain, effectively blocking water molecules from attacking the ether bonds of the resin. The two work together to significantly improve the storage stability of the all-water-based melamine resin.

[0025] This invention strictly controls the silane grafting reaction temperature at 45℃~55℃ and the pH value at a slightly acidic to neutral range of 6.0~7.0. Under these conditions, the epoxy groups of the epoxy silane coupling agent can undergo selective ring-opening grafting reactions with the residual amino groups on the resin molecular chain, while the methoxysilane groups at the ends are in a kinetically stable state at this temperature and pH value, and will not undergo significant hydrolysis and condensation reactions. This allows the siloxane groups to be completely preserved until the final baking and curing stage. When the coating is baked at 130℃~150℃, the siloxane groups hydrolyze to generate silanol, which then undergoes a condensation reaction with the hydroxyl groups on the surface of the metal substrate to form stable Me-O-Si covalent bonds, achieving chemical bonding and adhesion to the metal substrate.

[0026] This invention introduces polycarbonate diol with a specific molecular weight during the etherification stage. Its long carbonate chains are inserted between the six-membered rings of melamine to form a molecular network structure. When the metal sheet is subjected to secondary processing such as stamping and bending, the polycarbonate diol segments can effectively absorb and disperse stress, preventing the paint film from cracking.

[0027] In step S3 of this invention, the temperature control of 45℃~55℃ and the pH control of 6.0~7.0 have a weak synergistic relationship. If the temperature is too high or the pH deviates from this range, the methoxy group of the silane coupling agent will be hydrolyzed prematurely, causing the resin to crosslink, thicken, or even gel during storage. If the temperature is too low, the reaction rate between the epoxy group and the amino group is too slow, resulting in low grafting efficiency. Only within this process range can the goal of active grafting while maintaining stable storage be achieved. In addition, the ratio of glyoxylic acid to cyclic urea has also been optimized. If the amount of glyoxylic acid is too low, the emulsifying ability will be insufficient, while if it is too high, the curing speed will be affected. If the amount of cyclic urea is too low, the formaldehyde capture will be insufficient, while if it is too high, the crosslinking density may decrease. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0029] Figure 1 This is a schematic flowchart of a method for preparing an all-water-based melamine resin for metal baking paint according to the present invention.

[0030] Figure 2 This is a comparison chart of the free formaldehyde content of Example 1 and Comparative Examples 1-3 of the present invention. Detailed Implementation

[0031] The technical solution of the present invention will be further described in detail below through specific embodiments and in conjunction with the accompanying drawings. It should be understood that the implementation of the present invention is not limited to the following embodiments, and any modifications and / or alterations made to the present invention will fall within the protection scope of the present invention.

[0032] In this invention, unless otherwise specified, all parts and percentages are by weight, and the equipment and raw materials used are commercially available or commonly used in the art. Unless otherwise specified, the methods in the following embodiments are conventional methods in the art. Unless otherwise specified, the components or equipment in the following embodiments are general standard parts or components known to those skilled in the art, and their structures and principles can be learned by those skilled in the art through technical manuals or conventional experimental methods.

[0033] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings. In this detailed description, numerous specific details are set forth to facilitate explanation and provide a thorough understanding of the embodiments of the present invention. However, one or more embodiments may be practiced by those skilled in the art without these specific details.

[0034] The low-carbon alcohols mentioned in this invention refer to monohydric alcohols with 1-4 carbon atoms, including methanol, ethanol, n-propanol, isopropanol, n-butanol, and isobutanol. The formaldehyde donors mentioned in this invention refer to compounds capable of releasing formaldehyde under reaction conditions, including paraformaldehyde, trioxyformaldehyde, and aqueous formaldehyde solutions. Paraformaldehyde is preferred in this invention because it is easy to measure and safer to transport and store. Cyclic ureas mentioned in this invention refer to urea compounds with a cyclic structure, including ethylene urea and allyl urea. Ethylene urea is preferred in this invention because it has high reactivity with formaldehyde and the product has good stability.

[0035] Example 1

[0036] refer to Figure 1 This embodiment discloses a method for preparing an all-water-based melamine resin for metal baking paint, using the following raw materials (in molar parts):

[0037] Melamine: 1.0 part;

[0038] Paraformaldehyde (based on formaldehyde monomer): 1.6 parts;

[0039] Glyoxylic acid (50% aqueous solution): 0.8 parts;

[0040] Ethylene urea (analytical grade, purity ≥99%): 0.35 parts;

[0041] Methanol (industrial grade, purity ≥99.5%): 4.0 parts;

[0042] γ-glycidyl etheroxypropyltrimethoxysilane (KH-560, purity ≥98%): 0.1 parts;

[0043] N,N-Dimethylethanolamine (DMEA, purity ≥99%): appropriate amount;

[0044] Deionized water: appropriate amount.

[0045] The preparation method specifically includes the following steps.

[0046] S1, Synergistic hydroxyalkylation:

[0047] In a stainless steel reactor equipped with a stirrer, thermometer, reflux condenser and pH online monitoring device, add 1.0 mol of melamine, 1.6 mol of paraformaldehyde, 0.8 mol of glyoxylic acid (50% aqueous solution, equivalent to 0.4 mol of pure glyoxylic acid) and 0.35 mol of ethylurea.

[0048] Start the stirrer at 200-300 rpm to ensure thorough and uniform dispersion of the solid raw materials. Then, add a 30% (w / w) triethanolamine aqueous solution dropwise to the reaction system while simultaneously monitoring the pH online and adjusting the pH to 8.5 ± 0.1. Once the pH has stabilized, turn on the heating system and heat the system to 70 ± 1°C at a rate of 2°C / min, maintaining this temperature for 1.5 hours.

[0049] In this process, the aldehyde group (-CHO) of glyoxylic acid undergoes a hydroxyalkylation reaction with the amino group (-NH2) of melamine, while the secondary amine group of ethylurea reacts with formaldehyde released from paraformaldehyde to form a hydroxymethylated product. Ethylurea can capture free formaldehyde in the system, forming a stable cyclic acetal structure, thereby reducing the free formaldehyde content. The carboxyl group (-COOH) introduced by glyoxylic acid is partially converted into a carboxylate under these alkaline conditions, laying the foundation for subsequent water dispersibility.

[0050] S2, Etherification reaction:

[0051] After the reaction in step S1 is complete, the system is cooled to 75℃±1℃, and a 20% formic acid aqueous solution is added dropwise to the reactor to adjust the pH of the system to 4.5±0.1. Then, 4.0 mol of methanol is added in batches, with the first batch being 60% of the total amount, and the remaining 40% added after 1 hour of reaction to maintain a stable reaction.

[0052] The reaction temperature was controlled at 75℃±1℃, and the stirring speed was increased to 300~400 rpm for etherification reaction for 2.5 hours. During this process, the hydroxymethyl group (-CH2OH) in the polyhydroxymethyl intermediate undergoes etherification with methanol to generate ether structures such as methoxymethyl (-CH2OCH3) or butoxymethyl, which reduces the polarity and hydrophilicity of the resin and improves its compatibility with the coating matrix resin. At the same time, some carboxyl groups undergo esterification with methanol, further adjusting the polarity balance of the resin.

[0053] S3, Temperature-controlled silane grafting:

[0054] After the etherification reaction is complete, the cooling system is activated to reduce the system temperature to 50℃±1℃ at a cooling rate of 1℃ / min. This cooling process must be strictly controlled to avoid excessive temperature fluctuations.

[0055] When the temperature stabilizes at 50℃, begin adding a 10% (w / w) sodium hydroxide aqueous solution dropwise, while monitoring the pH change and adjusting the system pH to 6.5 ± 0.1. This pH range must be maintained; otherwise, if the pH is below 6.0, the selectivity of the reaction between the epoxy groups and amino groups of the silane coupling agent decreases, and methoxysilanes may undergo premature acid-catalyzed hydrolysis; if the pH is above 7.0, the ring-opening reaction rate of the epoxy groups is too slow, reducing grafting efficiency.

[0056] After the pH stabilizes, slowly add 0.1 mol of KH-560 dropwise over 30 minutes. During the addition, maintain the temperature at 50℃±1℃, the pH at 6.5±0.1, and the stirring speed at 250~300 rpm. After the addition is complete, maintain the reaction under these conditions for 1.2 hours.

[0057] Within this specific temperature and pH range, the epoxy groups in the KH-560 molecule can undergo nucleophilic ring-opening addition reactions with the residual primary or secondary amino groups on the resin molecular chain to form a stable β-hydroxyamine structure, thereby firmly grafting the silane coupling agent to the end of the resin molecular chain. Simultaneously, the terminal trimethoxysilyl group (-Si(OCH3)3) remains kinetically stable under these mild conditions and does not undergo significant hydrolysis-condensation reactions, as hydrolysis reactions are typically accelerated under strongly acidic or alkaline conditions, and the higher the temperature, the faster the hydrolysis. This means that the siloxane group is completely preserved until the final paint curing stage, at which point it reacts with the metal substrate.

[0058] S4, Neutralization and Dispersion:

[0059] After the grafting reaction is completed, unreacted methanol, a small amount of water and other small molecule byproducts are removed by vacuum distillation under vacuum conditions of -0.09MPa and 60~65℃. The removal time is about 30~45 minutes until the resin solid content reaches 80%±2%.

[0060] The system was then cooled to 35℃±2℃, and N,N-dimethylethanolamine (DMEA) was added as a neutralizing agent to adjust the pH to 8.0±0.2. At this point, the carboxyl groups in the resin were fully ionized, forming an anionic self-emulsifying system. Finally, deionized water was slowly added using a high-speed disperser at 1500~2000 rpm, making the resin-to-water mass ratio approximately 1:1.5, for reverse emulsification dispersion. Dispersion was continued for 30 minutes until the system transformed into a uniform, transparent, pale yellow viscous liquid with a solid content controlled at 50%±2%, thus obtaining the all-water-based melamine resin for metal baking paint.

[0061] Example 2

[0062] This embodiment discloses a method for preparing an all-water-based melamine resin for metal baking paint. The raw materials and steps are the same as in Embodiment 1, except that a polycarbonate diol long-chain extender is introduced in step S2, as detailed below:

[0063] In step S2, after adjusting the pH to 4.5, 0.15 moles of polycarbonate diol (PCDL, number average molecular weight 1500, hydroxyl value 74~78 mgKOH / g) are added simultaneously with methanol and coetherified with the polyhydroxymethyl intermediate for 2.5 hours at 75°C and pH 4.5.

[0064] Polycarbonate diol has hydroxyl groups at both ends, which can undergo etherification or condensation reactions with the hydroxymethyl groups of melamine resin, thereby introducing flexible polycarbonate long chains into the rigid six-membered ring molecular backbone of melamine. This significantly improves the flexibility and impact resistance of the coating film while maintaining the high hardness and abrasion resistance of melamine resin.

[0065] Example 3

[0066] This embodiment discloses a method for preparing an all-water-based melamine resin for metal baking paint. The raw materials and steps are the same as in Embodiment 1, except that:

[0067] In step S3, the system temperature is lowered to 45℃, the pH is adjusted to 6.0, KH-560 is added dropwise, and the reaction is kept at this temperature for 1.5 hours.

[0068] Example 4

[0069] This embodiment discloses a method for preparing an all-water-based melamine resin for metal baking paint. The raw materials and steps are the same as in Embodiment 1, except that:

[0070] In step S3, the system temperature is lowered to 50℃, the pH is adjusted to 6.5, KH-560 is added dropwise, and the reaction is kept at this temperature for 1.2 hours.

[0071] Example 5

[0072] This embodiment discloses a method for preparing an all-water-based melamine resin for metal baking paint. The raw materials and steps are the same as in Embodiment 1, except that:

[0073] In step S3, the system temperature is lowered to 55℃, the pH is adjusted to 7.0, KH-560 is added dropwise, and the reaction is kept at this temperature for 1.0 hour.

[0074] Comparative Example 1

[0075] This comparative example discloses a method for preparing melamine resin, in which 1.0 mole of melamine and 3.0 mole of paraformaldehyde are mixed, 5.0 mole of methanol is added, and the mixture is hydroxymethylated at pH 8.5 and 70°C for 2 hours, followed by etherification at pH 4.5 and 75°C for 3 hours, and finally neutralized and dispersed. No glyoxylic acid or ethyl urea is added, and no silane grafting is performed.

[0076] Comparative Example 2

[0077] This comparative example discloses a method for preparing melamine resin, with the same raw materials and steps as in Example 1, the difference being:

[0078] In step S1, no ethylurea is added; only glyoxylic acid is used.

[0079] Comparative Example 3

[0080] This comparative example discloses a method for preparing melamine resin, with the same raw materials and steps as in Example 1, the difference being:

[0081] In step S3, after etherification, the temperature is lowered to 80°C, the pH is adjusted to 6.5, and KH-560 is added directly for reaction.

[0082] Because the temperature is much higher than 55℃, the methoxy group of the silane coupling agent undergoes a large amount of hydrolysis and condensation during the reaction, causing the resin to cross-link and thicken in the reactor.

[0083] The melamine resins prepared in Examples 1-5 and Comparative Examples 1-3 were subjected to performance tests according to the following methods.

[0084] Free formaldehyde content test: The test was conducted in accordance with GB / T 23993-2009 "Determination of formaldehyde content in water-based coatings - acetylacetone spectrophotometric method".

[0085] Storage stability test: The resin sample was placed in a 50℃ constant temperature oven, and samples were taken every 7 days to test the viscosity at 25℃ using a rotational viscometer, and the viscosity increase factor was recorded.

[0086] Adhesion test: The resin and 50% solids water-based acrylic resin were mixed at a mass ratio of 3:7. Deionized water was added to adjust the application viscosity. The mixture was sprayed onto a pre-phosphated tinplate and cured at 140℃ for 30 minutes, resulting in a film thickness of 20±2μm. Adhesion was tested according to GB / T 9286-2021 "Paints and Varnishes - Cross-cut Test", with grade 0 being the best and grade 5 the worst.

[0087] Flexibility test (T-bend): The T-bend test is conducted in accordance with GB / T 6742-2007 "Paints and Varnishes Bending Test (Cylindrical Shaft)". The smaller the T value, the better the flexibility (i.e., 1T is better than 3T).

[0088] Water resistance test: Immerse the cured sample in boiling water at 100℃ and record the time when the paint film begins to bubble and peel off.

[0089] The test results of the melamine resins prepared in Examples 1-5 and Comparative Examples 1-3 are compared in Table 1.

[0090] Table 1. Comparison of test results of melamine resins prepared in Examples 1-5 and Comparative Examples 1-3

[0091]

[0092] refer to Figure 2 Compared to Comparative Example 1, the free formaldehyde content in Example 1 was only 0.08%, a reduction of over 97%. Clearly, this effect stems from the synergistic effect of glyoxylic acid and cyclic urea. The free formaldehyde content in Comparative Example 2 was 1.42%, a 50% reduction compared to the traditional process, but still significantly higher than in Example 1. This indicates that although glyoxylic acid introduces a carboxyl group, its ability to reduce free formaldehyde is limited. In Example 1, the secondary amine group in the cyclic structure of ethylene urea exhibits extremely high reactivity with formaldehyde, forming a stable five-membered acetal structure that does not decompose and release formaldehyde during subsequent etherification and storage. Furthermore, the carboxyl group of glyoxylic acid reduces the pH sensitivity of the system, allowing the formaldehyde capture reaction of ethylene urea to proceed under milder conditions. Simultaneously, the carboxyl group introduced by glyoxylic acid may form hydrogen bonds or salt bonds with the amino group of ethylene urea, promoting the uniform distribution of ethylene urea within the resin molecules, enabling it to more effectively capture dispersed free formaldehyde.

[0093] Example 1 showed excellent storage stability with a viscosity increase of only 1.2 times after 30 days of accelerated storage at 50°C. In contrast, Comparative Example 3, due to silane grafting at a high temperature of 80°C, caused the methoxy group of the silane coupling agent to hydrolyze into silanol during the reaction, which then condensed and crosslinked with the hydroxyl groups on the resin molecules or with other silanol molecules, resulting in the resin gelling during the reaction stage and making it impossible to discharge.

[0094] Comparing Examples 3, 4, and 5 reveals that while the grafting reaction rate increases with increasing grafting temperature, storage stability decreases. Even under mild conditions of pH 6.0-7.0, increased temperature still accelerates the hydrolysis kinetics of siloxane groups. When the temperature exceeds 55°C, the hydrolysis rate of methoxysilanes increases significantly, leading to mild cross-linking between resin molecules, resulting in increased viscosity during storage. Therefore, the temperature range of 45-55°C in step S3 represents the optimal process window for balancing efficient grafting with stable storage.

[0095] The adhesion of Example 1 was grade 0, significantly better than grade 2 of Comparative Example 1 and grade 1 of Comparative Example 2. This indicates that the introduction of the silane coupling agent significantly improved the adhesion. However, the key point is that only the siloxane groups retained under the temperature-controlled conditions of Example 1 can play a role in the subsequent paint curing process.

[0096] When the coating is baked at 140°C, the siloxane groups undergo thermally induced hydrolysis, generating highly reactive silanol groups. These silanol groups undergo condensation and dehydration reactions with the metal substrate (aluminum or steel plate) to form stable Al-O-Si or Fe-O-Si covalent bonds. The strength of this chemical bond is much higher than that of traditional van der Waals forces or hydrogen bonds, thus giving the coating film excellent adhesion and water resistance.

[0097] Although glyoxylic acid was introduced in Comparative Example 2, silane grafting was not performed, so this chemical bonding mechanism was lacking. The adhesion was only grade 1, and the water resistance was also poor.

[0098] Comparing Example 2 and Example 1, although both are comparable in adhesion and formaldehyde content, their flexibility differs. This difference stems from the role of the long PCDL chains in the resin network. During the T-bend test on the metal sheet, the block structure formed by PCDL connected to the rigid melamine six-membered rings at both ends through ether or condensation bonds can absorb and disperse bending stress through the movement of the flexible PCDL chain segments, preventing brittle fracture of the rigid melamine network. Furthermore, the carbonate groups in the PCDL structure are polar and interact with the metal substrate, which is one reason why the boiling water resistance of Example 2 is slightly better than that of Example 1.

[0099] Application examples

[0100] The all-waterborne melamine resin prepared in Example 1 was mixed with an aqueous hydroxyl acrylic dispersion with a solid content of 45% and a hydroxyl value of 90 mg KOH / g at a ratio of NCO / OH = 1.05 (NCO value is calculated based on the effective crosslinking groups of the melamine resin). Appropriate amounts of deionized water, wetting agent, leveling agent, and defoamer were added to prepare a waterborne metallic baking paint coating. The coating was sprayed onto the surface of a phosphated cold-rolled steel sheet, with a wet film thickness of 60 μm. After leveling for 10 minutes, it was cured by baking at 140°C for 30 minutes. The properties of the resulting paint film are as follows:

[0101] Appearance: Smooth and flat, gloss (60°) ≥95;

[0102] Thickness: 25±2μm;

[0103] Pencil hardness: ≥2H;

[0104] Adhesion: Grade 0 (cross-cut test), Grade 0 (circle test);

[0105] T-bend: 1T (no cracking);

[0106] Impact resistance: Passes through 50 kg·cm;

[0107] Water resistance: After boiling in 100℃ water for 2 hours, the paint film shows no blistering, peeling, or discoloration, and the adhesion remains at level 0.

[0108] Solvent resistance (butanone wiping): ≥100 times without exposing the substrate.

[0109] This application example demonstrates the excellent performance of the all-water-based melamine resin for metal baking paint of the present invention in the field of metal baking paint, which fully meets the requirements of high-end metal coating for home appliances, automotive parts and other products.

[0110] This article uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. It should be noted that those skilled in the art can make several improvements and modifications to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

Claims

1. A method for preparing an all-water-based melamine resin for metal baking paint, characterized in that: The method includes the following steps: S1. Melamine, formaldehyde donor, glyoxylic acid and cyclic urea are mixed and heated under alkaline conditions to obtain a carboxyl-containing polyhydroxymethyl intermediate. S2. Add a low-carbon alcohol to the carboxyl-containing polyhydroxymethyl intermediate and carry out an etherification reaction under acidic conditions to obtain an etherified resin solution. S3. Cool the etherified resin solution to 45℃~55℃, adjust the pH of the system to 6.0~7.0, and then add an epoxy-containing silane coupling agent to carry out a heat preservation grafting reaction to obtain the grafted modified resin. S4. The grafted modified resin is subjected to depressurization to remove small molecule byproducts, then a neutralizing agent is added to adjust the pH value to weak alkalinity, and deionized water is added for dispersion to obtain the all-water-based melamine resin for metal baking paint.

2. The method for preparing a water-based melamine resin for metal baking paint according to claim 1, characterized in that: Based on 1 mole of melamine, the amounts of the other raw material components are as follows: 1.2 to 2.0 moles of formaldehyde donor (based on formaldehyde), 0.5 to 1.2 moles of glyoxylic acid, 0.2 to 0.5 moles of cyclic urea, 3.0 to 5.0 moles of low alcohol, and 0.05 to 0.15 moles of epoxy-containing silane coupling agent.

3. The method for preparing a water-based melamine resin for metal baking paint according to claim 1, characterized in that: In step S1, the cyclic urea is ethylidene urea or propylene urea; the pH value of the alkaline condition is 8.0~9.0; the temperature of the heating reaction is 65℃~75℃, and the reaction time is 1~2 hours.

4. The method for preparing a water-based melamine resin for metal baking paint according to claim 1, characterized in that: In step S2, the lower alcohol is one or more of methanol, ethanol, or butanol; the pH value of the acidic condition is 4.0~5.0; the temperature of the etherification reaction is 70℃~80℃, and the reaction time is 2~3 hours.

5. The method for preparing a water-based melamine resin for metal baking paint according to claim 1, characterized in that: In step S2, polycarbonate diol is added simultaneously with the low-carbon alcohol to carry out a co-etherification reaction; the molar ratio of polycarbonate diol to melamine is 0.1~0.2:1, and the number average molecular weight of polycarbonate diol is 1000~2000.

6. The method for preparing a water-based melamine resin for metal baking paint according to claim 1, characterized in that: In step S3, the epoxy-containing silane coupling agent is γ-glycidyl etheroxypropyltrimethoxysilane or γ-glycidyl etheroxypropylmethyldiethoxysilane; the time for the heat preservation grafting reaction is 1 to 1.5 hours.

7. The method for preparing a water-based melamine resin for metal baking paint according to claim 1, characterized in that: In step S4, the neutralizing agent is one or more of N,N-dimethylethanolamine, triethylamine, or ammonia water; the weakly alkaline pH value is 7.5~8.

5.

8. A water-based melamine resin for metal baking paint, characterized in that: The water-based melamine resin for metal baking paint is prepared by any one of the preparation methods of water-based melamine resin for metal baking paint according to any one of claims 1-7.

9. The application of the all-water-based melamine resin for metal baking paint as described in claim 8 in the field of metal surface coating.

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