A water-based printing paint for aluminum coil and a method for preparing the same

Abstract

This invention provides a water-based printing coating for aluminum coils and its preparation method, belonging to the field of coating technology. The invention introduces siloxane active groups into the chain ends of a water-based polyurethane prepolymer with isocyanate-terminated groups via an aminosilane coupling agent. This is combined with the chelation control of titanium alkoxides by acetylacetone and the formation of a titanium-silicon composite sol through acidic hydrolysis and condensation of tetraalkoxysilane and epoxysilane coupling agents. Finally, this sol is cured and mixed with a silane-terminated water-based polyurethane emulsion. This allows the coating to construct a uniform organic-inorganic hybrid network in situ during curing and film formation, and to form a more stable chemical anchoring and dense shielding structure at the aluminum substrate interface. Thus, while maintaining the workability of the water-based system, it significantly improves the adhesion, hardness, and corrosion resistance of the aluminum coil coating.

Description

Technical Field

[0001] This invention belongs to the field of coating technology, specifically relating to a water-based printing coating for aluminum coils and its preparation method. Background Technology

[0002] Aluminum coil pre-coating technology, as a paradigm of modern industrial coating, is renowned for its high efficiency, continuous operation, uniform coating quality, and high material utilization. It is widely used in high-end building curtain walls, aerospace skins, honeycomb aluminum panels, and white goods panels. For a long time, this industry has primarily relied on solvent-based coating systems, such as solvent-based polyvinylidene fluoride (PVDF) and solvent-based polyester. With increasingly stringent restrictions on volatile organic compound (VOC) emissions (such as GB 30981-2020), water-based coatings for aluminum coils have become an important development direction.

[0003] However, the application conditions for coating aluminum coils are extremely demanding: the coating needs to be cured under extremely high-temperature baking conditions in a very short time, while also withstanding extreme deformation during subsequent processing and meeting requirements such as scratch resistance and pencil hardness. While existing waterborne acrylics and traditional waterborne polyurethanes can be used in some light corrosion protection applications, they generally exhibit the following shortcomings in aluminum coil scenarios: First, the alumina layer on the aluminum surface easily absorbs moisture, forming a weak boundary layer. Traditional waterborne resins mostly rely on physical adsorption such as hydrogen bonds formed between carboxyl / hydroxyl groups and the substrate, which easily fails after being exposed to moisture or humid heat, resulting in problems such as blistering and peeling. Second, the hardness-flexibility contradiction is prominent: to improve hardness, the crosslinking density usually needs to be increased, but increased crosslinking density easily leads to brittleness of the coating during bending and deep drawing, making it difficult to balance forming processing and surface wear resistance. Third, waterborne systems often introduce hydrophilic groups and emulsifier residues for stable dispersion, which may form ion penetration channels, resulting in significantly lower corrosion resistance and electrochemical impedance compared to solvent-based systems, making it difficult to meet the harsh environments such as marine or aerospace applications.

[0004] Therefore, there is a need for a water-based printing coating system and its preparation method that is suitable for the harsh curing and post-processing conditions of aluminum coil coating, while taking into account adhesion, hardness-flexibility balance, corrosion resistance and shielding performance, and construction stability, so as to meet the actual needs of water-based and high-performance aluminum coil coatings. Summary of the Invention

[0005] In view of the above situation and to overcome the defects of the prior art, the purpose of the present invention is to provide a water-based printing coating for aluminum coils and a method for preparing the same, so as to at least partially solve the problems mentioned in the background art.

[0006] The technical solution adopted in this invention is as follows: The first aspect of this invention provides a method for preparing a water-based printing coating for aluminum coils, comprising the following steps: An aminosilane coupling agent is subjected to a capping reaction with an aqueous polyurethane prepolymer with terminal isocyanate groups to graft siloxane groups onto the ends of the polyurethane molecular chains. After water dispersion treatment, a silane-capped aqueous polyurethane emulsion is obtained. In an anhydrous solvent, titanium alkoxides are mixed with acetylacetone for a chelation reaction, followed by the addition of tetraalkoxysilane and epoxysilane coupling agents. Hydrolysis and polycondensation reactions are carried out under acidic conditions, and after aging, titanium-silicon composite sol is obtained. The titanium-silicon composite sol and the silane-terminated aqueous polyurethane emulsion are stirred and mixed, and then allowed to stand for aging to obtain an aqueous printing coating for aluminum coils.

[0007] In some embodiments of the present invention, the method for preparing the aqueous polyurethane prepolymer with terminal isocyanate groups includes the following steps: After dehydrating polycarbonate diol, it undergoes an addition reaction with isophorone diisocyanate, followed by the addition of hydrophilic chain extender dimethylolpropionic acid and small molecule chain extender 1,4-butanediol for chain extension reaction. The amount of isophorone diisocyanate added is excessive, so that the prepolymer ends with active isocyanate groups.

[0008] In some embodiments of the present invention, the aminosilane coupling agent comprises 3-aminopropyltriethoxysilane, and the amount added is 30%-50% of the theoretical molar amount of residual isocyanate groups in the aqueous polyurethane prepolymer; the end-capping reaction temperature is 50°C, and the reaction time is 30-60 minutes.

[0009] In some embodiments of the present invention, the anhydrous solvent includes ethanol, the titanium alkoxide includes tetrabutyl titanate, the tetraalkoxysilane includes tetraethyl orthosilicate, and the epoxysilane coupling agent includes 3-glycidyl etheroxypropyltrimethoxysilane.

[0010] In some embodiments of the present invention, the molar ratio of acetylacetone to titanium alkoxide is (2-4):1; the molar ratio of tetraalkoxysilane to epoxysilane coupling agent is 4:1.

[0011] In some embodiments of the present invention, the total molar ratio of silicon to titanium in the titanium-silicon composite sol is (4-8):1.

[0012] In some embodiments of the present invention, the chelation reaction is carried out under sealed conditions at room temperature for 1 hour; the co-hydrolysis and polycondensation reaction is initiated by adding an acidic aqueous solution with a pH of 3.0-4.0 for 4-6 hours; and the aging time is 24 hours.

[0013] In some embodiments of the present invention, before mixing the titanium-silicon composite sol with the silane-terminated aqueous polyurethane emulsion, the pH value of the titanium-silicon composite sol is adjusted to 5.0-6.0; the mass ratio of the titanium-silicon composite sol to the silane-terminated aqueous polyurethane emulsion is 1:(10-20).

[0014] A second aspect of the present invention provides an aqueous printing coating for aluminum coils prepared by the above method.

[0015] The third aspect of this invention discloses the application of the above-mentioned water-based printing coating for aluminum coils in the coating of aluminum alloy coils, including the following coating curing process: The coating is applied to the surface of a pretreated aluminum alloy substrate and then baked and cured at high temperature. The high-temperature baking and curing temperature is 232℃±5℃, and the time is 40-60 seconds.

[0016] The beneficial effects achieved by this invention are as follows: This invention introduces siloxane active groups into the chain ends of an isocyanate-terminated aqueous polyurethane prepolymer by end-capping with an aminosilane coupling agent. It combines this with the chelation control of titanium alkoxides by acetylacetone and the formation of a titanium-silicon composite sol through acidic hydrolysis and condensation of tetraalkoxysilane and epoxysilane coupling agents. This sol is then cured and mixed with a silane-terminated aqueous polyurethane emulsion. This allows the coating to construct a uniform organic-inorganic hybrid network in situ during curing and film formation, creating a more stable chemical anchoring and dense shielding structure at the aluminum substrate interface. This significantly improves the adhesion, hardness, and corrosion resistance of the aluminum coil coating while maintaining the workability of the aqueous system. Furthermore, the chelation helps reduce the risk of precipitation and system instability caused by rapid hydrolysis of the titanium source, improving formulation stability and industrial compatibility. Detailed Implementation

[0017] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those familiar to those skilled in the art. Furthermore, any methods and materials similar to or equivalent to those described herein may be applied to this invention. The preferred embodiments and materials described herein are for illustrative purposes only and do not limit the scope of this invention.

[0019] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0020] To address the problems raised in the background art, the first aspect of this invention provides a method for preparing a water-based printing coating for aluminum coils, comprising the following steps: An aminosilane coupling agent is subjected to a capping reaction with an aqueous polyurethane prepolymer with terminal isocyanate groups to graft siloxane groups onto the ends of the polyurethane molecular chains. After water dispersion treatment, a silane-capped aqueous polyurethane emulsion is obtained. An aminosilane coupling agent is subjected to an end-capping reaction with an aqueous polyurethane prepolymer with terminal isocyanate groups, thereby introducing siloxane groups into the ends of the polyurethane molecular chains and dispersing them to obtain a silane-capped aqueous polyurethane emulsion. This silane-capped structure can serve as a chemical bridge connecting the organic and inorganic phases, providing reactive sites for subsequent sol-gel polycondensation. This allows the inorganic network to no longer be a simple physical filler but to structurally couple with the resin skeleton, thereby significantly reducing the risk of phase separation due to insufficient compatibility and improving the continuity and stability of the hybrid network.

[0021] In an anhydrous solvent, titanium alkoxides are mixed with acetylacetone for a chelation reaction, followed by the addition of tetraalkoxysilane and epoxysilane coupling agents. Hydrolysis and polycondensation reactions are carried out under acidic conditions, and after aging, titanium-silicon composite sol is obtained. Titanium alkoxides, due to their extremely rapid hydrolysis rate, readily form titanium dioxide precipitates upon direct contact with the aqueous phase, leading to system instability. However, chelation with acetylacetone can increase the coordination number of the titanium center and significantly reduce the hydrolytic activity of the Ti-OR bond through steric hindrance, enabling kinetically matched co-hydrolysis and co-condensation of titanium and silicon. This inhibits precipitation and aggregation at the source, improving the transparency, fineness, and reproducibility of the sol. Simultaneously, tetraalkoxysilane, as the main inorganic network framework, can provide higher crosslinking density and hardness. Furthermore, the epoxy groups of the epoxy silane coupling agent can react with the amino or carboxyl groups on the polyurethane chain to further enhance the organic-inorganic interface bonding. Its longer organic segments can also act as a toughening buffer while improving crosslinking and hardness, thus establishing a better balance between hardness and flexibility.

[0022] Titanium-silicon composite sol was stirred and mixed with silane-terminated waterborne polyurethane emulsion, and allowed to stand for aging to obtain a waterborne printing coating for aluminum coils.

[0023] After mixing the aforementioned titanium-silicon composite sol with a silane-terminated aqueous polyurethane emulsion and allowing it to stand for curing, the inorganic precursor can undergo an in-situ sol-gel reaction between the organic polymer chains during film formation and curing. This generates nanoscale Si-O-Ti and Si-O-Si inorganic networks, which then interpenetrate and entangle with the polyurethane network at the molecular level, forming semi-interpenetrating or fully interpenetrating structures. The rigid Si-Ti-O network restricts the macroscopic movement of organic chain segments, thereby significantly improving the coating's storage modulus, surface hardness, and scratch resistance. Meanwhile, the interpenetrating structure allows for microscopic slippage during deformation to dissipate energy, exhibiting strong yet non-brittle characteristics. This better adapts to the post-processing deformation requirements such as T-bending in aluminum coil coating. Simultaneously, the densification and tortuous path of the hybrid network reduce water vapor and ion penetration channels, enhancing the coating's corrosion resistance and shielding ability. Furthermore, this sol-gel reaction can be efficiently completed within the high-temperature, short-time curing window of coil coating, meeting the requirements for industrial process adaptability.

[0024] According to the method for preparing waterborne printing coatings for aluminum coils according to embodiments of the present invention, siloxane active groups are introduced into the chain ends of waterborne polyurethane prepolymers with isocyanate-terminated groups by end-capping with an aminosilane coupling agent. This is combined with the chelation control of titanium alkoxides by acetylacetone and the formation of a titanium-silicon composite sol through acidic hydrolysis and condensation of tetraalkoxysilane and epoxysilane coupling agents. Finally, this sol is cured and mixed with a silane-terminated waterborne polyurethane emulsion. This allows the coating to construct a uniform organic-inorganic hybrid network in situ during curing and film formation, and to form a more stable chemical anchoring and dense shielding structure at the aluminum substrate interface. This significantly improves the adhesion, hardness, and corrosion resistance of the aluminum coil coating while maintaining the workability of the waterborne system. Furthermore, chelation helps reduce the risk of precipitation and system instability caused by rapid hydrolysis of the titanium source, improving formulation stability and industrial compatibility.

[0025] In some embodiments, the preparation method of the aqueous polyurethane prepolymer with isocyanate-terminated groups includes the following steps: After dehydrating polycarbonate diol, it undergoes an addition reaction with isophorone diisocyanate, followed by the addition of hydrophilic chain extender dimethylolpropionic acid and small molecule chain extender 1,4-butanediol for chain extension reaction. The addition of an excessive amount of isophorone diisocyanate ensures that the prepolymer ends with active isocyanate groups. The use of polycarbonate diol and isophorone diisocyanate as raw materials imparts good weather resistance and hydrolysis resistance to the coating, effectively solving the problem of easy powdering and failure of outdoor aluminum coils.

[0026] In some embodiments, the aminosilane coupling agent includes 3-aminopropyltriethoxysilane, and the amount added is 30%-50% of the theoretical molar amount of residual isocyanate groups in the aqueous polyurethane prepolymer.

[0027] The amino group of 3-aminopropyltriethoxysilane can efficiently add to the terminal isocyanate group to form a urea bond, achieving chemical bonding of the silane group. During subsequent aging and curing, the triethoxysilane terminal readily hydrolyzes to generate silanols, which then participate in polycondensation. This constructs a more continuous Si-O-Si and Si-O-Ti hybrid network in the coating, improving its hardness, scratch resistance, and corrosion barrier properties. Limiting the addition amount to 30%-50% avoids excessive silane end-capping, which can lead to rapid polycondensation, increased viscosity, or demulsification and agglomeration. It also reduces the risk of embrittlement and bending cracking caused by an overly dense inorganic network, allowing the coating to maintain good flexibility and process compatibility while improving adhesion and corrosion resistance.

[0028] In some embodiments, the end-capping reaction temperature is 50°C and the reaction time is 30-60 minutes. Controlling the silane end-capping reaction temperature to 50°C and the reaction time to 30-60 minutes helps to ensure the full consumption of the terminal isocyanate groups while suppressing side reactions, thereby improving end-capping efficiency and system stability.

[0029] In some embodiments, the anhydrous solvent includes ethanol, the titanium alkoxide includes tetrabutyl titanate, the tetraalkoxysilane includes tetraethyl orthosilicate, and the epoxysilane coupling agent includes 3-glycidyl etheroxypropyltrimethoxysilane. Using ethanol as the anhydrous solvent, in conjunction with tetrabutyl titanate, tetraethyl orthosilicate, and 3-glycidyl etheroxypropyltrimethoxysilane, to construct a composite sol allows for better reaction controllability and organic-inorganic compatibility. Ethanol exhibits good miscibility with the aforementioned titanium and silicon precursors, facilitating the formation of a homogeneous reaction system and mitigating the sudden hydrolysis of the precursors by reducing the system's water activity, thereby reducing gel clumps and precipitation, and improving sol transparency and storage stability. Tetrabutyl titanate provides a highly active titanium source, which facilitates the formation of titanium oxide structures that interact with the aluminum substrate surface during subsequent film formation, enhancing interface anchoring and corrosion-resistant shielding. Tetrabutyl orthosilicate, as the main precursor of the silicon oxide network, can form a dense Si-O-Si framework, improving hardness, scratch resistance, and barrier properties. 3-Glycidyl etheroxypropyltrimethoxysilane participates in the formation of a silicon-oxygen network in a sol-gel process at one end, while the epoxy group at the other end can undergo a ring-opening reaction with the active groups in the resin system, achieving chemical coupling between the inorganic phase and the polyurethane phase. This reduces the risk of phase separation and embrittlement, while improving hardness and corrosion resistance, and also taking into account the toughness and processing adaptability of the coating film.

[0030] In some embodiments, the molar ratio of acetylacetone to titanium alkoxide is (2-4):1; the molar ratio of tetraalkoxysilane to epoxysilane coupling agent is 4:1. Limiting the molar ratio of acetylacetone to titanium alkoxide to (2-4):1 allows acetylacetone to fully chelate the titanium center, significantly reducing the hydrolytic activity of the Ti-OR bond kinetically. This prevents the titanium alkoxide from undergoing sudden hydrolysis in the aqueous system, resulting in precipitation or coarse agglomeration, thereby improving the transparency, particle size refinement, and long-term stability of the titanium-silicon composite sol. At the same time, moderate chelation still retains reactive sites, ensuring that subsequent co-hydrolysis and condensation with the silicon component forms a continuous Si-O-Ti structure, enhancing the coating's densification and shielding effect. By controlling the molar ratio of tetraalkoxysilane to epoxysilane coupling agent to 4:1, the Si-O-Si framework constructed with tetraalkoxysilane becomes the main inorganic network, preferentially providing hardness, scratch resistance, and barrier properties. A small amount of epoxysilane acts as an interfacial coupling unit, achieving chemical connection and toughening buffering with the resin without significantly increasing the amount of organic phase flexible segments introduced. This improves crosslinking density while inhibiting embrittlement and phase separation, resulting in a more stable hardness-flexibility balance and corrosion resistance reliability.

[0031] In some embodiments, the total molar ratio of silicon to titanium in the titanium-silicon composite sol is (4-8):1. Controlling the total molar ratio of silicon to titanium in the titanium-silicon composite sol to (4-8):1 is beneficial for achieving synergistic optimization between inorganic network structure, interface anchoring ability, and system stability. When the Si / Ti ratio is in the range of 4-8, the network dominated by the silicon-oxygen framework (Si-O-Si) can be fully formed and maintain high continuity and density, thereby significantly improving the coating's hardness, scratch resistance, and barrier ability against water vapor and ions. At the same time, an appropriate amount of titanium component can effectively introduce the Si-O-Ti bridging structure, improve the crosslinking strength and thermal stability of the inorganic phase, and more easily condense with the hydroxyl sites on the aluminum substrate surface during film formation, forming a more stable interfacial chemical anchoring, improving adhesion retention and corrosion resistance reliability.

[0032] In some embodiments, the chelation reaction is carried out under sealed conditions at room temperature for 1 hour; the co-hydrolysis and polycondensation reactions are initiated by adding an acidic aqueous solution with a pH of 3.0-4.0 for 4-6 hours; and the aging time is 24 hours. Conducting the chelation reaction under sealed conditions at room temperature and controlling the reaction time to 1 hour facilitates the full coordination of acetylacetone with the titanium center, forming a stable chelate complex. This significantly reduces the hydrolytic activity of the titanium alkoxide without introducing additional moisture, providing a controllable starting point for subsequent synergistic reactions with the silicon precursor. The co-hydrolysis and polycondensation reactions are initiated by adding an acidic aqueous solution with a pH of 3.0-4.0 and continue for 4-6 hours. This allows for the equilibrium of hydrolysis and polycondensation rates under acid catalysis, avoiding localized supersaturation and gel formation caused by instantaneous water addition. This makes it easier for the Ti and Si components to form uniform, fine titanium-silica sol particles and Si-O-Ti bridging structures, thereby improving the stability of the compound and the uniformity of the coating. Further controlling the aging time to 24 hours can stabilize the condensation and structural rearrangement within the sol, reduce the risk of subsequent rapid thickening or demulsification caused by unreacted small molecules and active silanols, and facilitate the formation of a denser inorganic network precursor. This can more effectively improve the hardness, scratch resistance, and barrier corrosion resistance of the coating during the film formation and curing stage, and improve the consistency between formulation batches and industrial repeatability.

[0033] In some embodiments, before mixing the titanium-silicon composite sol with the silane-terminated aqueous polyurethane emulsion, the pH of the titanium-silicon composite sol is adjusted to 5.0-6.0; the mass ratio of the titanium-silicon composite sol to the silane-terminated aqueous polyurethane emulsion is 1:(10-20). Adjusting the pH of the titanium-silicon composite sol to 5.0-6.0 before compounding with the silane-terminated aqueous polyurethane emulsion can significantly alleviate the pH shock between the acidic sol and the emulsion system (usually weakly alkaline or near-neutral), reduce the risk of demulsification, flocculation, or sudden increase in viscosity caused by charge shielding and abrupt changes in ionic strength, thereby improving the stability and pot life of the compounding process, and avoiding gel particles and appearance defects caused by localized rapid condensation. Further controlling the mass ratio of titanium-silicon composite sol to silane-terminated aqueous polyurethane emulsion to 1:(10-20) ensures that the inorganic phase provides sufficient Si-O-Si and Si-O-Ti reinforcement and barrier contribution, improving hardness, scratch resistance and corrosion resistance shielding performance. It also avoids problems such as thickening, embrittlement or T-bend cracking caused by excessive inorganic phase, so that the coating achieves a better balance between high performance and flexible processing adaptability, and improves batch consistency and industrial feasibility.

[0034] A second aspect of the present invention provides an aqueous printing coating for aluminum coils prepared by the above method.

[0035] The third aspect of this invention discloses the application of the above-mentioned water-based printing coating for aluminum coils in the coating of aluminum alloy coils, including the following coating curing process: The coating is applied to the surface of a pretreated aluminum alloy substrate and then baked and cured at high temperature. The high-temperature baking curing temperature is 232℃±5℃, and the time is 40-60 seconds.

[0036] The present invention will be described below through specific embodiments. It should be noted that these embodiments are for illustrative purposes only and should not be considered as limiting the scope of the invention. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0037] Example 1: (1) 100g of polycarbonate diol (PCDL, Mn=2000) was added to a reactor and dehydrated under vacuum at 110℃ and -0.095MPa for 1.5 hours. The temperature was lowered to 85℃, and 45.6g of isophorone diisocyanate (IPDI) and 0.05g of dibutyltin dilaurate (DBTDL) were added. The reaction was carried out for 2.5 hours to obtain a polyurethane prepolymer with isocyanate-terminated groups.

[0038] The temperature was lowered to 70°C, and 6.8 g of dimethylolpropionic acid (DMPA, pre-dissolved in a small amount of N-methylpyrrolidone) and 2.5 g of 1,4-butanediol (BDO) were added. The reaction was allowed to proceed for 1.5 hours for chain extension. Subsequently, the reaction temperature was lowered to 50°C, and 13.6 g of 3-aminopropyltriethoxysilane (APTES) was added. The reaction was maintained at this temperature for 45 minutes.

[0039] The temperature was lowered to 40°C, and 4.8g of triethylamine (TEA) was added for neutralization for 30 minutes. Then, 320g of deionized water was added under high-speed shearing at 2500rpm for emulsification. After desolventizing under reduced pressure, a silane-terminated waterborne polyurethane emulsion with a solid content of about 35% was obtained.

[0040] (2) In a dry container, add 10 g of anhydrous ethanol and 6.8 g of tetrabutyl titanate (TBT, 0.02 mol). Under magnetic stirring, slowly add 6.0 g of acetylacetone (AcAc, 0.06 mol). Stir and chelate for 1 hour at room temperature under sealed conditions to obtain a chelated titanium solution. Add 16.7 g of tetraethyl orthosilicate (TEOS, 0.08 mol) and 4.7 g of 3-glycidyl etheroxypropyltrimethoxysilane (GPTMS, 0.02 mol) sequentially to the above solution. Prepare an ethanol-water solution containing a trace amount of hydrochloric acid (pH adjusted to 3.5), and slowly add it dropwise to the above mixture. Stir and carry out hydrolysis and polycondensation reactions at room temperature for 5 hours. After the reaction is complete, allow to stand and age for 24 hours to obtain a titanium-silicon composite sol.

[0041] (3) Adjust the pH of the aged titanium-silicon composite sol to 5.5 using ammonia. Under low-speed stirring, slowly add the pH-adjusted titanium-silicon composite sol dropwise to the silane-terminated aqueous polyurethane emulsion at a mass ratio of 1:15 (sol:emulsion = 1:15). After mixing thoroughly, allow to stand for 12 hours to mature, thus obtaining the aqueous printing coating for aluminum coils.

[0042] Example 2: The only difference from Example 1 is: In step (1), the amount of APTES added was adjusted to 10.2 g. The capping reaction time was 30 minutes.

[0043] In step (2), the amount of acetylacetone added was adjusted to 4.0 g. The amount of tetraethyl orthosilicate (TEOS) added was adjusted to 13.3 g, and the amount of 3-glycidyl etheroxypropyltrimethoxysilane (GPTMS) added was adjusted to 3.8 g. The pH of the hydrolysis reaction was controlled at 3.0, and the reaction time was 4 hours.

[0044] In step (3), the mass ratio of the titanium-silicon composite sol to the silane-terminated aqueous polyurethane emulsion is adjusted to 1:10. The pH of the sol is then adjusted to 5.0.

[0045] Example 3: The only difference from Example 1 is: In step (1), the amount of 3-aminopropyltriethoxysilane (APTES) added was adjusted to 17.0 g, and the end-capping reaction time was extended to 60 minutes.

[0046] In step (2), the amount of acetylacetone (AcAc) added was adjusted to 8.0 g; the amount of tetraethyl orthosilicate (TEOS) added was adjusted to 26.7 g; and the amount of 3-glycidyl etheroxypropyltrimethoxysilane (GPTMS) added was adjusted to 7.6 g. The pH value of the hydrolysis reaction was controlled at 4.0, and the reaction time was 6 hours.

[0047] In step (3), the mass ratio of the titanium-silicon composite sol to the silane-terminated aqueous polyurethane emulsion is adjusted to 1:20. The pH of the sol is then adjusted to 6.0.

[0048] Example 4: The only difference from Example 1 is: In step (2), the amount of tetraethyl orthosilicate (TEOS) added was adjusted to 20.0 g, and the amount of 3-glycidyl etheroxypropyltrimethoxysilane (GPTMS) added was adjusted to 5.7 g.

[0049] Example 5: The only difference from Example 1 is: In step (3), the mass ratio of the titanium-silicon composite sol to the silane-terminated aqueous polyurethane emulsion is adjusted to 1:12.

[0050] Example 6: The only difference from Example 1 is: In step (1), the temperature of the end-capping reaction is controlled at 50°C.

[0051] In step (2), the pH value of the hydrolysis reaction is controlled at 3.2.

[0052] Comparative Example 1: This comparative example provides a simple waterborne polyurethane coating. The difference from Example 1 is: Steps (2) and (3) are omitted. The silane-terminated aqueous polyurethane emulsion prepared in step (1) is used directly as a coating.

[0053] Comparative Example 2: This comparative example provides a physically blended coating. The difference from Example 1 is: In step (1), APTES is not added for the end-capping reaction. Ethylenediamine (EDA) is used directly to perform post-chain extension and end-capping on the chain-extended prepolymer (to consume the residual NCO groups), followed by neutralization and water dispersion to prepare a common waterborne polyurethane emulsion.

[0054] Steps (2) and (3) are the same as in Example 1, where the titanium-silicon composite sol is mixed with a common waterborne polyurethane emulsion.

[0055] Comparative Example 3: This comparative example provides a pure silica sol hybrid coating. The difference from Example 1 is: In step (2), tetrabutyl titanate and acetylacetone are not added; only tetraethyl orthosilicate (TEOS) and glyoxysilane (GPTMS) are used to prepare pure silica sol.

[0056] Test method: The coatings from Examples 1-6 and Comparative Examples 1-3 were applied to the surface of pretreated 3003 aluminum alloy coils and then placed in a high-temperature oven. The plate temperature was controlled to reach 232°C, and the high-temperature baking and curing time was 50 seconds, followed by immediate water quenching.

[0057] (1) Pencil hardness: The test was conducted according to GB / T 6739-2022 "Determination of paint film hardness by pencil method". Using a pencil, push the pencil at a 45-degree angle under a load of 750g. Record the hardest pencil grade that did not pry the paint film.

[0058] (2) T-bend flexibility: The T-bend test method was performed according to GB / T 13448-2019 "Test Methods for Color-Coated Steel Sheets and Strips". The coated sample was folded in half (180 degrees). 0T means the flat plate was folded directly in half, 1T means the plate with a layer of the same thickness in the middle was folded in half, and so on. After bending, the bending point was observed with a magnifying glass to see if there were any cracks, and a peel test was performed with tape. The minimum T value without cracks and coating peeling was recorded.

[0059] (3) Adhesion (cross-cut test): Dry adhesion: Tested according to GB / T 9286-2021 "Cross-cut test for paints and varnishes". Draw 100 squares using a cross-cutting tool with a 1mm spacing, apply the tape and then quickly peel it off to observe the peeling situation. Grade 0 indicates that the cut edges are completely smooth and no squares are peeled off; Grade 5 indicates that the peeling is severe.

[0060] Water-resistant adhesion: Immerse the sample in boiling deionized water for 2 hours, remove, dry, and allow to recover at room temperature for 1 hour. Then, perform a cross-cut adhesion test according to the GB / T 9286 method described above. This is a key indicator for verifying the hydrolysis resistance of aluminum coil coating interfaces.

[0061] (4) Solvent resistance to wiping: The test shall be conducted in accordance with GB / T 23989-2009 "Determination of solvent resistance to wiping of coatings". A 1kg weight wrapped in gauze shall be dipped in methyl ethyl ketone (MEK) and rubbed back and forth on the coating surface until the substrate is exposed or the specified number of wiping cycles is reached. The highest number of wiping cycles without damage shall be recorded.

[0062] (5) Resistance to neutral salt spray: The test was conducted in accordance with GB / T 1771-2007 "Determination of resistance to neutral salt spray of paints and varnishes". An "X" cut was made on the surface of the sample and placed in a 35℃ salt spray chamber (5% NaCl solution). After 500 hours of testing, the corrosion spread width at the cut and the blistering on the surface of the plate were observed.

[0063] Table 1 presents the statistical results of the above tests on pencil hardness, T-bend flexibility, and adhesion.

[0064] Table 1

[0065] Table 2 presents the statistical results of the above solvent resistance and neutral salt spray resistance tests.

[0066] Table 2

[0067] Analysis of the test results in Tables 1 and 2 shows that, compared with Comparative Example 1, the pencil hardness of the coating significantly increased from HB to 2H-3H after the introduction of the titanium-silicon composite sol. This is because, through the sol-gel reaction, rigid inorganic Si-O-Si and Si-O-Ti skeletons are generated in situ between the polyurethane polymer chains, restricting the macroscopic slippage of organic segments, thereby greatly improving the storage modulus and scratch resistance of the coating.

[0068] More importantly, comparing Example 1 and Comparative Example 2, although both introduced inorganic components, their performance differed significantly. Example 1 employed APTES end-capping technology to achieve chemical bonding between the organic and inorganic phases. This nanoscale interpenetrating network (IPN) structure allows for micro-stress dissipation of molecular chains during deformation, thus maintaining high hardness while withstanding extreme deformation at 0T (flat plate folding) without cracking. Comparative Example 2, lacking the chemical bridging of a silane coupling agent, experienced inorganic particle aggregation in the resin matrix, forming micro-defects and stress concentration points. This resulted in limited hardness improvement and a reduction in flexibility to 3T.

[0069] The adhesion failure of aluminum coil coatings in humid environments is a major pain point in the industry. Test results show that the introduction of titanium components is key to solving this problem. In the 2-hour boiling test, Comparative Example 1 showed complete peeling at level 5, while Comparative Example 3 also reached severe peeling at level 4. This indicates that relying solely on hydrogen bonds (polyurethane) or Si-O-Al bonds (pure silica sol) cannot resist the erosion of high-temperature water molecules, resulting in poor interfacial hydrolysis resistance. In Examples 1-6, all titanium-containing examples maintained an adhesion level of 0 (no peeling) after 2 hours of boiling. This is because the active Ti-OH groups generated by the hydrolysis of titanium alkoxide can undergo an irreversible condensation reaction with the oxide layer on the aluminum alloy surface, forming highly energetic and hydrolysis-resistant Ti-O-Al chemical bonds. This chemical anchoring effect is significantly better than physical adsorption, giving the coating excellent wet adhesion.

[0070] In the neutral salt spray (500h) and MEK wiping tests, the example group showed significant advantages. Example 1 exhibited over 100 MEK wiping cycles with no further corrosion at the salt spray cut. This is attributed to two factors: firstly, the highly cross-linked IPN structure effectively blocked the swelling and penetration of solvent molecules; secondly, the titanium oxide readily accumulates at the interface, forming a dense gradient layer that effectively cuts off the lateral diffusion channels of water vapor and chloride ions, thereby preventing electrochemical corrosion of the aluminum substrate. In contrast, Comparative Example 1, due to the presence of hydrophilic groups forming ion channels, resulted in dense bubbling and severe corrosion during the salt spray test.

[0071] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0072] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the protection scope of the present invention.

Claims

1. A method for preparing a water-based printing coating for aluminum coils, characterized in that, Includes the following steps: An aminosilane coupling agent is subjected to a capping reaction with an aqueous polyurethane prepolymer with terminal isocyanate groups to graft siloxane groups onto the ends of the polyurethane molecular chains. After water dispersion treatment, a silane-capped aqueous polyurethane emulsion is obtained. In an anhydrous solvent, titanium alkoxides are mixed with acetylacetone for a chelation reaction. Tetraalkoxysilane and epoxysilane coupling agents are added, and hydrolysis and polycondensation reactions are carried out under acidic conditions. After aging, titanium-silicon composite sol is obtained. The titanium-silicon composite sol and the silane-terminated aqueous polyurethane emulsion are stirred and mixed, and then allowed to stand for aging to obtain an aqueous printing coating for aluminum coils.

2. The preparation method according to claim 1, characterized in that, The preparation method of the aqueous polyurethane prepolymer with terminal isocyanate groups includes the following steps: After dehydrating polycarbonate diol, it undergoes an addition reaction with isophorone diisocyanate, followed by the addition of hydrophilic chain extender dimethylolpropionic acid and small molecule chain extender 1,4-butanediol for chain extension reaction. The amount of isophorone diisocyanate added is excessive, so that the prepolymer ends with active isocyanate groups.

3. The preparation method according to claim 1, characterized in that, The aminosilane coupling agent includes 3-aminopropyltriethoxysilane, and the amount added is 30%-50% of the theoretical molar amount of residual isocyanate groups in the waterborne polyurethane prepolymer; the end-capping reaction temperature is 50°C, and the reaction time is 30-60 minutes.

4. The preparation method according to claim 1, characterized in that, The anhydrous solvent includes ethanol; the titanium alkoxide includes tetrabutyl titanate; the tetraalkoxysilane includes tetraethyl orthosilicate; and the epoxysilane coupling agent includes 3-glycidyl etheroxypropyltrimethoxysilane.

5. The preparation method according to claim 1, characterized in that, The molar ratio of acetylacetone to titanium alkoxide is (2-4):1; the molar ratio of tetraalkoxysilane to epoxysilane coupling agent is 4:

1.

6. The preparation method according to claim 1, characterized in that, In the titanium-silicon composite sol, the total molar ratio of silicon to titanium is (4-8):

1.

7. The preparation method according to claim 1, characterized in that, The chelation reaction is carried out under sealed conditions at room temperature for 1 hour; the co-hydrolysis and polycondensation reaction is initiated by adding an acidic aqueous solution with a pH of 3.0-4.0 for 4-6 hours; and the aging time is 24 hours.

8. The preparation method according to claim 1, characterized in that, Before mixing the titanium-silicon composite sol with the silane-terminated aqueous polyurethane emulsion, the pH value of the titanium-silicon composite sol is adjusted to 5.0-6.0; the mass ratio of the titanium-silicon composite sol to the silane-terminated aqueous polyurethane emulsion is 1:(10-20).

9. A water-based printing coating for aluminum coils prepared by the method according to any one of claims 1-8.

10. The application of a water-based printing coating for aluminum coils according to claim 9 in the coating of aluminum alloy coils, characterized in that, Including the following coating curing processes: The coating is applied to the surface of a pretreated aluminum alloy substrate and then baked and cured at high temperature. The high-temperature baking and curing temperature is 232℃±5℃, and the time is 40-60 seconds.

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