Silicone gradient enriched abrasion resistant waterborne hydroxyl acrylate emulsion and method of making

Abstract

The application discloses a silicone gradient enrichment wear-resistant water-based hydroxyl acrylate emulsion and a preparation method and application thereof. The emulsion is composed of an acrylate monomer mixture (not containing a hydroxyl functional monomer), a hydroxyl functional monomer, a vinyl siloxane modifier, a compound emulsifier, an initiator and deionized water; a seed emulsion gradient synchronous dropping polymerization process is adopted to precisely control the gradient distribution of the silicone atom concentration of the inner core of the latex particle to be 3% to 5% and the silicone atom concentration of the surface shell to be 15% to 25%. The vinyl siloxane is chemically bonded into the resin molecular chain through double bond free radical polymerization, there is no silane migration and layering, the emulsion is stable for storage at room temperature for more than 12 months, the wear-resistant mass loss of the paint film is less than or equal to 0.02 g (GB / T 1768-2006, 750 g / 500 r), the pencil hardness is greater than or equal to 2H, the surface tension of the paint film is stably maintained at 25 to 30 mN / m, high wear resistance, high hardness and excellent leveling are considered, the VOC is low, the environmental protection is compliant, and the application is suitable for the fields of high-end water-based two-component polyurethane coatings such as automobile paint, woodenware paint, metal anticorrosive paint and industrial floor paint.

Description

Technical Field

[0001] This invention belongs to the field of polymer emulsion synthesis technology, specifically relating to a siloxane gradient enriched wear-resistant waterborne hydroxy acrylic emulsion and its preparation method, applicable to waterborne two-component polyurethane coatings, automotive paints, wood coatings, metal anti-corrosion paints and industrial floor coatings. Background Technology

[0002] Waterborne hydroxyl acrylic emulsion (hydroxypropyl emulsion for short) is the core film-forming resin for waterborne two-component polyurethane coatings. The emulsion structure directly determines key properties of the paint film, such as gloss, fullness, water resistance, abrasion resistance, and adhesion. Traditional waterborne hydroxyl acrylic emulsions suffer from four major industry pain points:

[0003] (1) Shortcomings in wear resistance: Conventional hydroxypropyl emulsion coatings tested according to GB / T 1768-2006 (750g weight, 500 rpm) generally have a film mass loss of more than 0.035g, which cannot meet the high wear resistance requirements of automobiles, flooring and other applications.

[0004] (2) Hardness and surface tension are mutually restrictive: When the pencil hardness of the paint film is increased by increasing the crosslinking density, the surface polarity of the resin increases, the surface tension exceeds 33mN / m, the leveling of the coating deteriorates, and the adhesion of the substrate decreases.

[0005] (3) Uncontrollable distribution of modified siloxane: Existing silicone-modified acrylic emulsions are divided into two categories. One category is physically blended siloxane, which will migrate and separate after 3 months of storage. The other category uses vinyl siloxane graft polymerization, such as CN102464784A which uses D4 ring opening + vinyl silane grafting and CN102898578A which uses a process of slowly increasing silicon content. However, both types of patents only achieve an overall increase in silicon element, and cannot accurately control the silicon content range of latex particle core and shell. The silane has no directional surface enrichment effect, and the improvement in wear resistance is limited.

[0006] (4) Poor storage stability: Physically blended siloxanes have poor compatibility with acrylic resins, and are prone to phase separation during long-term storage, resulting in large batch-to-batch performance fluctuations in the paint film.

[0007] Currently, the market lacks a water-based hydroxyl acrylic emulsion that can precisely control the distribution of siloxane gradients while also exhibiting high wear resistance, high hardness, low surface tension, and long-term storage stability. Summary of the Invention

[0008] This invention addresses the industry pain points of existing waterborne hydroxy acrylic emulsions by providing a siloxane gradient enriched wear-resistant waterborne hydroxy acrylic emulsion and its preparation method.

[0009] The present invention is implemented as follows: A siloxane gradient enriched wear-resistant waterborne hydroxyl acrylic emulsion is polymerized from the following components: 40%–55% acrylate monomer mixture, 5%–12% hydroxyl-containing functional monomers, 3%–8% vinylsiloxane modifier, 2%–5% compound emulsifier, 0.3%–1.0% initiator A, and deionized water to 100%; the acrylate monomer mixture does not contain hydroxyl functional monomers; the vinylsiloxane modifier molecule contains carbon-carbon double bonds that can be polymerized by free radicals, which are chemically bonded to the acrylic resin molecular chain through double-bond free radical polymerization; the initiator A is one of ammonium persulfate and potassium persulfate, and is prepared as a 2.5–3.5 wt% aqueous solution before use; the silicon element inside the latex particles of the emulsion is distributed in a gradient increasing manner from the core region to the shell layer, with a silicon atom concentration of 3%–5% in the core region and 15%–25% in the shell surface layer.

[0010] This invention uses vinylsiloxane copolymerization to modify waterborne hydroxyl acrylic emulsion, precisely constructing a microstructure of latex particles with a continuous gradient increase in silicon element from the core to the shell. Only when the core silicon strictly falls within the range of 3-5% and the surface silicon 15-25% can the following be achieved simultaneously: (1) Sufficient hydroxyl groups in the core, resulting in high crosslinking activity with the isocyanate curing agent and excellent adhesion to the substrate. (2) A continuous silicon-rich thin layer in the shell, with siloxanes covalently bonded to the resin molecular chain, preventing the migration and precipitation of organosilicon components, and endowing the paint film with low surface energy, high wear resistance, scratch resistance, and hydrophobic and water-resistant properties. (3) No obvious phase interface in the latex particle gradient, resulting in a paint film without haze and low internal stress. Unlike the abrupt interface double core-shell structure, the continuous gradient distribution eliminates obvious phase interfaces inside the particles, relieves internal stress of the paint film, improves interlayer compatibility, and takes into account the storage stability of the emulsion, the wear resistance of the paint film and the cross-linking curing performance. It solves the technical contradiction that traditional random silicon copolymerization, physical mixing of organosilicon, and double core-shell modification systems cannot simultaneously balance wear resistance, adhesion and cross-linking. (4) The polymerization process is stable and gel-free, and the emulsion does not separate after 12 months of storage at room temperature.

[0011] Preferably, the components in the acrylate monomer mixture, by weight, are: 20-30 parts hard monomer, 15-25 parts soft monomer, and 2-5 parts carboxyl functional monomer; the hard monomer is one or both of methyl methacrylate (MMA) and styrene (St); the soft monomer is one or both of butyl acrylate (BA) and ethyl acrylate (EA); and the carboxyl functional monomer is one or both of acrylic acid (AA) and methacrylic acid (MAA). The hard monomer (MMA / St) enhances the hardness and abrasion resistance of the paint film; the soft monomer (BA / EA) imparts flexibility to the paint film, preventing brittleness; the combination of hard and soft monomers precisely controls the glass transition temperature of the resin, matching the application requirements of abrasion-resistant coatings. The introduction of the carboxyl functional monomer (AA / MAA) enhances the stability of the latex particle hydration layer, reducing the risk of emulsion gelation; the carboxyl group can form a salt with the neutralizing agent, improving the storage stability of the emulsion and improving pigment wettability; furthermore, the carboxyl group can weakly interact with isocyanate, improving the adhesion of the paint film to the substrate. By fixing the ratio range of the three types of monomers, the coating film is made too brittle due to excessive hard monomers, too soft monomers are not hard enough, and excessive carboxyl monomers are not enough to reduce the water resistance of the coating film, thus achieving a balance between hardness, toughness and emulsion stability.

[0012] Preferably, the hydroxyl-containing functional monomer is one or both of hydroxyethyl methacrylate (HEMA) and hydroxypropyl methacrylate (HPMA). Both of these monomers carry secondary hydroxyl groups, which can undergo crosslinking reactions with aliphatic / aromatic isocyanate curing agents to construct a crosslinked network for a two-component polyurethane coating. HEMA has low steric hindrance and higher crosslinking reactivity; HPMA has greater steric hindrance, which can moderately control the crosslinking rate and alleviate internal stress in the coating caused by excessively rapid curing; the combination of the two allows for flexible adjustment of crosslinking kinetics. Compared with long-chain hydroxy acrylates, the hydroxyl groups are more uniformly distributed after HEMA / HPMA copolymerization, which will not excessively reduce the water resistance of the coating; it is suitable for the classic two-component two-component coating system of hydroxy acrylics in this system.

[0013] Preferably, the vinylsiloxane modifier is one or more of vinyltrimethoxysilane, vinyltriethoxysilane, and methylvinyldimethoxysilane in combination. The vinylsiloxane modifier carries vinyl double bonds, enabling it to participate in free radical polymerization and chemically bond to the acrylic acid molecular chain, unlike physically blended silicone oils, thus preventing the migration and precipitation of silicon components. The hydrolysis of alkoxysilane groups can improve the hydrophobic properties of the coating film and enhance its water and chemical resistance. The three siloxanes have different hydrolysis rates; their combined use can regulate the siloxane grafting rate during polymerization, adapting to gradient polymerization processes and facilitating precise control of the slow enrichment of silicon from the core to the shell. This avoids excessively rapid grafting of a single siloxane, which would cause a large accumulation of silicon in the seed stage, destroying the "low-silicon core" structure.

[0014] Preferably, the compound emulsifier is a system of anionic and nonionic emulsifiers; the anionic emulsifier is one or both of sodium dodecyl sulfate (SDS) and sodium dodecylbenzene sulfonate (SDBS); the nonionic emulsifier is one or both of octylphenol polyoxyethylene ether (OP-10) and fatty alcohol polyoxyethylene ether (AEO-9). The anionic emulsifier provides a negative charge on the surface of the latex particles, relying on electrostatic stability; the nonionic emulsifier forms a hydration layer, providing steric hindrance stability; the compound system has both stabilizing mechanisms, significantly improving the stability of emulsion polymerization and reducing the probability of demulsification and aggregation during the gradient dropping stage.

[0015] The preparation method of the above-mentioned siloxane gradient enriched wear-resistant waterborne hydroxy acrylic emulsion includes the following steps:

[0016] S1. Preparation of silicone-containing pre-emulsion: Add all the acrylate monomer mixture, hydroxyl-containing functional monomer, vinylsiloxane modifier, compound emulsifier, and 30% to 45% of the total water volume of deionized water into a pre-emulsion kettle, and pre-emulsify at high speed at 40 to 50°C for 30 to 60 minutes to prepare silicone-containing pre-emulsion.

[0017] S2. Preparation of low-silicon seed emulsion: Take 10% to 20% of the total amount of silicon-containing pre-emulsion, 20% to 30% of the total amount of initiator A, and 15% to 25% of the total amount of deionized water and add them to the reaction vessel. Heat the vessel to 75 to 80°C and react at a constant temperature for 30 to 45 minutes to generate a seed emulsion with a core silicon atom concentration of 3% to 5%.

[0018] S3. Gradient synchronous dropwise polymerization: The remaining pre-emulsion and the remaining initiator A are added simultaneously and at a uniform rate for a total dropwise time of 2.5 to 4 hours. The temperature inside the reactor is maintained at 78 to 85°C. During the dropwise process, the monomer conversion rate is ensured to be ≥95%.

[0019] S4. Maturation and pH Adjustment for Discharge: After the dripping is completed, heat the reactor to 85-90℃ and keep it at that temperature for 1-2 hours to ensure a total monomer conversion rate ≥98%. Cool down to 40-50℃ and adjust the pH of the system to 8.3-8.6 using ammonia or triethanolamine. Add the remaining deionized water and test the pH value. The final emulsion should have a pH value between 7.5 and 8.5. Filter to obtain the target emulsion. If the pH value is not between 7.5 and 8.5, fine-tune it again using ammonia or triethanolamine.

[0020] This invention pre-constructs a low-silicon core through seed polymerization. The total amount of vinylsiloxanes participating in polymerization during the seed formation stage is limited, allowing for a controllable low-silicon core with a silicon atom concentration of 3%–5%. There is a significant difference in the free radical polymerization reactivity between vinylsiloxanes and acrylate monomers (MMA, BA). According to the Qe rule, the Q value (conjugation effect) of vinylsiloxanes is much lower than that of acrylate monomers, meaning their free radical activity is lower. During the seed stage, acrylate monomers preferentially polymerize, resulting in low siloxane fusion efficiency and the formation of a low-silicon core. In the subsequent dropwise addition stage, as the concentration of acrylate monomers decreases, the relative concentration of siloxanes increases. Furthermore, with the continuous addition of initiators to maintain the free radical concentration, siloxanes preferentially graft onto the outer layer of the particles, forming a silicon-enriched shell. A seed prepolymerization + monomer / initiator two-component simultaneous dropwise addition process is adopted. Utilizing the difference in free radical polymerization rates, vinyl siloxane is preferentially grafted onto the outer layer of latex particles. Batch addition of the initiator avoids explosive polymerization, precisely achieving a gradient increase in silicon element from the inside out, stably constructing a continuous gradient distribution structure, and controlling the silicon atom concentration on the shell surface at 15-25%. Simultaneously, the monomer and initiator are added at a uniform rate to maintain a stable free radical concentration, inhibit secondary nucleation, and ensure regular latex particle morphology and narrow particle size distribution. High-temperature curing in the later stage ensures high monomer conversion rate and reduces free siloxane and acrylic monomer. In step S4, the pH is first adjusted to 8.3-8.6 using an over-adjustment method, ensuring complete salt formation and stability of carboxyl groups and silanols. Then, the remaining deionized water is added for dilution, causing the system pH to return to the target range. For pH values ​​not falling within the target range, only a small amount of neutralizing agent is needed for secondary fine-tuning to stabilize the system at 7.5-8.5. Batch pH fluctuations are ≤0.2, significantly improving stability for industrial production and overcoming the shortcomings of traditional polymerization processes that struggle to controllably achieve silicon gradient enrichment. The entire process eliminates the need for complex segmented feeding equipment and can stably replicate the micro-gradient structure of claim 1, solving the technical challenge that conventional one-time feeding and simultaneous dropwise addition only yields randomly distributed or abruptly bilayer core-shell latex particles of silicon.

[0021] Preferably, in step S1, initiator B, which is azobisisobutyronitrile (AIBN), is pre-mixed into the acrylate monomer mixture. AIBN is used at 50% of the amount of initiator A. AIBN can accommodate a small amount of oil-phase initiation and can be flexibly selected according to the polymerization rate. When it is necessary to control the initial reaction rate of polymerization or improve the initiation efficiency of oil-phase monomers, AIBN is added as an oil-phase auxiliary initiator. Because hydroxyl monomers contain -OH polar groups, long-term contact with AIBN will accelerate the decomposition of azo bonds at room temperature, depleting the initiator and causing premature self-polymerization during the pre-emulsion storage stage. Therefore, AIBN is first dissolved in the hydroxyl-free acrylate monomer mixture before mixing with the hydroxyl monomers. This prevents excessively high local hydroxyl concentrations, ensures no self-polymerization gelation after 4 hours of storage at room temperature in the pre-emulsion, maintains a stable seed polymerization initiation rate, and ensures that the core silicon concentration remains stable within the 3-5% range.

[0022] Preferably, in steps S3 and S4, the monomer conversion rate is detected by gravimetric method, with samples dried at 150±2℃ for 15 min to determine the solid content and calculate the conversion rate; and periodic calibration is performed using gas chromatography (GC). By accurately monitoring the conversion rate, excessive unreacted monomer residues are avoided, while ensuring that the conversion rate indicators in stages S3 and S4 meet the standards, thus guaranteeing the stability of the resin molecular structure, reducing free siloxane monomers, and preventing silicone migration in the later coating film.

[0023] The core structural innovation of this invention lies in the quantitatively controllable silicon gradient latex particles. Through a seed low-silicon substrate and simultaneous gradient droplet polymerization, the silicon atom concentration in the core region is precisely limited to 3%~5%, and the silicon atom concentration in the surface shell layer to 15%~25%. Its beneficial technical effects include:

[0024] (1) Microstructure brings synergistic performance (core innovative effect): The silicon content of latex particles increases continuously from the core to the shell: the low silicon content in the core region ensures sufficient hydroxyl groups, guarantees the crosslinking activity with isocyanate curing agents and the adhesion to the substrate; the high silicon content in the shell layer achieves wear resistance, scratch resistance, hydrophobicity and water resistance of the paint film. There is no abrupt phase interface, which reduces the internal stress of the paint film, improves interlayer compatibility, and avoids the common problems of haze and easy cracking of double core and shell paint films.

[0025] (2) Advantages of the modification method: Vinylsiloxane free radical copolymerization chemical bonds are incorporated into the resin chain, which is different from physical blended silicone oil. There is no problem of silicon migration or precipitation, and the long-term performance of the coating film is stable.

[0026] (3) Gains of raw material compound system: soft and hard monomers are combined to regulate the hardness and toughness of the coating film; carboxyl monomers improve the dispersion stability of the emulsion and the wetting of the substrate; hydroxyl monomers are preferred to be adapted to the two-component polyurethane curing system; anionic / nonionic compound emulsion system strengthens electrostatic + steric hindrance stability and reduces the risk of gelation and demulsification in the gradient polymerization process; and siloxane monomers are adapted to inhibit the side reaction of silane hydrolysis and self-polymerization.

[0027] (4) Unique value of the preparation process: The low-silicon seed polymerization + subsequent gradient drop addition process can accurately and controllably achieve gradient silicon distribution; synchronous drop addition inhibits secondary nucleation and the latex particles have regular morphology; the maturation process ensures high monomer conversion rate and reduces free siloxanes; the whole process uses conventional reaction equipment, the process is simple and easy to scale up for industrial production.

[0028] (5) Solving industry technical pain points: Breaking the traditional rules of silicon-modified systems: Increasing silicon content to enhance wear resistance will sacrifice crosslinking performance, adhesion, and water resistance; This solution achieves high wear resistance, good adhesion, stable crosslinking ability, and emulsion storage stability in one system. Among them, 1) Significantly improved wear resistance: The wear resistance mass loss of the paint film is ≤0.02g, which is 43% higher than that of traditional silicone-free hydroxypropyl emulsion (0.035g) and 28% higher than that of physically blended silane emulsion (0.028g), reaching the standard of high-end coatings. 2) Synergistic optimization of hardness and leveling: The pencil hardness of the paint film is stable at ≥2H, the surface tension is 25~30mN / m, the leveling of the coating is excellent, and the adhesion to metal, wood, and plastic substrates is excellent. 3) Excellent storage stability: The siloxane is chemically bonded and fixed, with no migration or precipitation. After 12 months of storage at room temperature, there is no stratification or precipitation. The solid content difference between the upper and lower layers is <2% after centrifugation at 3000r / 30min. The physically blended silane emulsion stratifies after only 3 months. 4) Excellent film appearance: 60° gloss ≥90, full and delicate film, AFM micro-roughness Ra only 8.2nm, low coefficient of friction; 5) Clear formulation and process boundaries: matching boundary test data supports the optimal range, exceeding the dosage / process range of this invention significantly degrades performance; 6) Low VOC water-based system: no solvent pollution, VOC content ≤50g / L (GB 30981.1-2025), environmentally compliant. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the core-shell gradient structure of latex particles obtained in the example.

[0030] Figure 2 The XPS depth profile of latex particles in Example 1 shows the Si element concentration distribution curve.

[0031] Figure 3 This is a diagram illustrating the mechanism of monomer conversion differences in seed emulsion gradient polymerization in Example 1.

[0032] Figure 4 The bar chart shows the comparison of wear resistance mass loss of emulsions in each embodiment and comparative examples 1-3.

[0033] Figure 5 The curves showing the change in surface tension of the coating film of Example 1 emulsion, conventional emulsion (Comparative Example 1), and physically blended silane emulsion (Comparative Example 2) as a function of storage time are shown.

[0034] Figure 6 This is a comparison of the surface morphology of the AFM emulsion coating film in Example 1 and Comparative Example 1.

[0035] Figure 7 The bar chart shows the comparison of the stability of the emulsions during centrifugation storage between the examples and comparative examples 1-3.

[0036] Figure 8The graph shows the correlation between the coefficient of friction of the emulsion coating film and the wear resistance loss in Examples 1-3 and Comparative Examples 1-3.

[0037] Figure 9 The temperature spectra of the emulsion coating films in Example 1 and Comparative Example 1 are obtained from dynamic mechanical analysis (DMA). Detailed Implementation

[0038] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments.

[0039] Example 1

[0040] A siloxane-enriched, wear-resistant, waterborne hydroxyl acrylic emulsion, with each component in parts by weight as shown in Tables 1 and 2, is prepared by the following steps:

[0041] S1. Preparation of silicone-containing pre-emulsion: Add all the acrylate monomer mixture, hydroxyl-containing functional monomer, vinylsiloxane modifier, compound emulsifier, and 35% of the total water volume of deionized water into a pre-emulsion kettle, and pre-emulsify at high speed at 45°C for 45 min to prepare silicone-containing pre-emulsion.

[0042] S2. Preparation of low-silicon seed emulsion: Add 15% of the total amount of silicon-containing pre-emulsion, 25% of the total amount of initiator A, and 20% of the total amount of deionized water to the reactor, heat to 78℃, and react at a constant temperature for 35 minutes to generate a seed emulsion with a core silicon atom concentration of 4.2%; initiator A is prepared as a 3.0% aqueous solution before use.

[0043] S3. Gradient synchronous dropwise polymerization: The remaining pre-emulsion and the remaining initiator A are added simultaneously and at a uniform rate for a total dropwise time of 3 hours. The temperature inside the reactor is maintained at 80℃, and the monomer conversion rate is ensured to reach 95% during the dropwise process.

[0044] S4. Maturation and pH Adjustment for Discharge: After the addition is complete, heat the reactor to 88°C and keep it at that temperature for 1.5 hours to ensure that the total conversion rate of monomers reaches 98%. Cool down to 45°C, adjust the pH of the system to 8.4 using ammonia, add the remaining deionized water, and then use ammonia to fine-tune the pH of the final emulsion to 7.5. Filter to obtain the target emulsion.

[0045] In steps S3 and S4, the monomer conversion rate is detected by gravimetric method. Samples are dried at 150±2℃ for 15 min to determine the solid content and calculate the conversion rate. Gas chromatography (GC) is used for periodic calibration.

[0046] Example 2

[0047] A siloxane gradient-enriched, wear-resistant waterborne hydroxyl acrylic emulsion, the components of which are shown in parts by weight in Tables 1 and 2, is prepared by the following steps:

[0048] S1. Preparation of silicone-containing pre-emulsion: Add all the acrylate monomer mixture, hydroxyl-containing functional monomer, vinyl siloxane modifier, compound emulsifier, and 30% of the total water volume of deionized water into a pre-emulsion kettle, and pre-emulsify at high speed at 50°C for 30 min to prepare silicone-containing pre-emulsion.

[0049] S2. Preparation of low-silicon seed emulsion: Add 10% of the total amount of silicon-containing pre-emulsion, 20% of the total amount of initiator A, and 15% of the total amount of deionized water to the reactor, heat to 75℃, and react at a constant temperature for 45 minutes to generate a seed emulsion with a core silicon atom concentration of 3.8%; initiator A is prepared as a 2.5% aqueous solution before use.

[0050] S3. Gradient synchronous dropwise polymerization: The remaining pre-emulsion and the remaining initiator A are added simultaneously and uniformly for a total dropwise time of 4 hours. The temperature inside the reactor is maintained at 78°C. During the dropwise process, the monomer conversion rate is ensured to reach 97%.

[0051] S4. Maturation and pH Adjustment for Discharge: After the addition is complete, heat the reactor to 85°C and keep it at that temperature for 1 hour to ensure that the total conversion rate of monomers reaches 99%. Cool down to 50°C, adjust the pH of the system to 8.6 using triethanolamine, add the remaining deionized water, and then adjust the pH of the system to 8.0 using triethanolamine again. Filter to obtain the target emulsion.

[0052] In steps S3 and S4, the monomer conversion rate is detected by gravimetric method. Samples are dried at 150±2℃ for 15 min to determine the solid content and calculate the conversion rate. Gas chromatography (GC) is used for periodic calibration.

[0053] Example 3

[0054] A siloxane gradient-enriched, wear-resistant waterborne hydroxyl acrylic emulsion, the components of which are shown in parts by weight in Tables 1 and 2, is prepared by the following steps:

[0055] S1. Preparation of silicone-containing pre-emulsion: Add all the acrylate monomer mixture, hydroxyl-containing functional monomer, vinyl siloxane modifier, compound emulsifier, and 45% of the total water volume of deionized water into a pre-emulsion kettle, and pre-emulsify at high speed at 40°C for 60 min to prepare silicone-containing pre-emulsion.

[0056] S2. Preparation of low-silicon seed emulsion: Add 18% of the total amount of silicon-containing pre-emulsion, 30% of the total amount of initiator A, and 25% of the total amount of deionized water to the reactor, heat to 80℃, and react at a constant temperature for 30 minutes to generate a seed emulsion with a core silicon atom concentration of 4.5%; initiator A is prepared as a 3.5% aqueous solution before use.

[0057] S3. Gradient synchronous dropwise polymerization: The remaining pre-emulsion and the remaining initiator A are added simultaneously and at a uniform rate for a total dropwise time of 2.5 hours. The temperature inside the reactor is maintained at 85°C, and the monomer conversion rate is ensured to reach 96% during the dropwise process.

[0058] S4. Maturation and pH Adjustment for Discharge: After the addition is completed, heat the reactor to 90°C and keep it at that temperature for 2 hours to ensure that the total conversion rate of monomers reaches 99.5%. Cool down to 40°C, adjust the pH of the system to 8.3 using ammonia, add the remaining deionized water, and then adjust the pH to 7.8 using ammonia. Filter to obtain the target emulsion.

[0059] In steps S3 and S4, the monomer conversion rate is detected by gravimetric method. Samples are dried at 150±2℃ for 15 min to determine the solid content and calculate the conversion rate. Gas chromatography (GC) is used for periodic calibration.

[0060] Example 4

[0061] A siloxane gradient-enriched, wear-resistant waterborne hydroxyl acrylic emulsion, the components of which are shown in parts by weight in Tables 1 and 2, is prepared by the following steps:

[0062] S1. Preparation of silicone-containing pre-emulsion: AIBN initiator and acrylate monomer mixture are mixed and then added to a pre-emulsion kettle with hydroxyl-containing functional monomer, vinyl siloxane modifier, compound emulsifier and 45% deionized water. The mixture is stirred at high speed at 40°C for 60 min to prepare silicone-containing pre-emulsion.

[0063] S2. Preparation of low-silicon seed emulsion: Add 20% of the total amount of silicon-containing pre-emulsion, 30% of the total amount of initiator A, and 25% of the total amount of deionized water to the reactor, heat to 80℃, and react at a constant temperature for 30 minutes to generate a seed emulsion with a core silicon atom concentration of 4.9%; initiator A is prepared as a 3.5% aqueous solution before use.

[0064] S3. Gradient synchronous dropwise polymerization: The remaining pre-emulsion and the remaining initiator A are added simultaneously and at a uniform rate for a total dropwise time of 2.5 hours. The temperature inside the reactor is maintained at 85°C, and the monomer conversion rate is ensured to reach 95% during the dropwise process.

[0065] S4. Maturation and pH Adjustment for Discharge: After the addition is complete, heat the reactor to 85°C and keep it at that temperature for 2 hours to ensure that the total conversion rate of monomers reaches 98%. Cool down to 40°C, adjust the pH of the system to 8.6 using triethanolamine, add the remaining deionized water, and then adjust the pH to 7.8 using triethanolamine. Filter to obtain the target emulsion.

[0066] In steps S3 and S4, the monomer conversion rate is detected by gravimetric method. Samples are dried at 150±2℃ for 15 min to determine the solid content and calculate the conversion rate. Gas chromatography (GC) is used for periodic calibration.

[0067] Example 5

[0068] A siloxane gradient-enriched, wear-resistant waterborne hydroxyl acrylic emulsion, the components of which are shown in parts by weight in Tables 1 and 2, is prepared by the following steps:

[0069] S1. Preparation of silicone-containing pre-emulsion: Add all the acrylate monomer mixture, hydroxyl-containing functional monomer, vinyl siloxane modifier, compound emulsifier, and 45% of the total water volume of deionized water into a pre-emulsion kettle, and pre-emulsify at high speed at 40°C for 60 min to prepare silicone-containing pre-emulsion.

[0070] S2. Preparation of low-silicon seed emulsion: Add 12% of the total amount of silicon-containing pre-emulsion, 30% of the total amount of initiator A, and 25% of the total amount of deionized water to the reactor, heat to 80℃, and react at a constant temperature for 30 minutes to generate a seed emulsion with a core silicon atom concentration of 4.1%; initiator A is prepared as a 3.5% aqueous solution before use.

[0071] S3. Gradient synchronous dropwise polymerization: The remaining pre-emulsion and the remaining initiator A are added simultaneously and at a uniform rate for a total dropwise time of 2.5 hours. The temperature inside the reactor is maintained at 85°C, and the monomer conversion rate is ensured to reach 96% during the dropwise process.

[0072] S4. Maturation and pH Adjustment for Discharge: After the addition is completed, heat the reactor to 90°C and keep it at that temperature for 2 hours to ensure that the total conversion rate of monomers reaches 99.5%. Cool down to 40°C, adjust the pH of the system to 8.4 using ammonia, add the remaining deionized water, and then adjust the pH to 8.0 using ammonia again. Filter to obtain the target emulsion.

[0073] In steps S3 and S4, the monomer conversion rate is detected by gravimetric method. Samples are dried at 150±2℃ for 15 min to determine the solid content and calculate the conversion rate. Gas chromatography (GC) is used for periodic calibration.

[0074] Example 6

[0075] A siloxane gradient-enriched, wear-resistant waterborne hydroxyl acrylic emulsion, the components of which are shown in parts by weight in Tables 1 and 2, is prepared by the following steps:

[0076] S1. Preparation of silicone-containing pre-emulsion: Add all the acrylate monomer mixture, hydroxyl-containing functional monomer, vinylsiloxane modifier, compound emulsifier, and 40% of the total water volume of deionized water into a pre-emulsion kettle, and pre-emulsify at high speed at 42°C for 50 min to prepare silicone-containing pre-emulsion.

[0077] S2. Preparation of low-silicon seed emulsion: Add 10% of the total amount of silicon-containing pre-emulsion, 20% of the total amount of initiator A, and 15% of the total amount of deionized water to the reactor, heat to 75℃, and react at a constant temperature for 45 minutes to generate a seed emulsion with a core silicon atom concentration of 3.2%; initiator A is prepared as a 2.5% aqueous solution before use.

[0078] S3. Gradient synchronous dropwise polymerization: The remaining pre-emulsion and the remaining initiator A are added simultaneously and at a uniform rate for a total dropwise time of 3.5 hours. The temperature inside the reactor is maintained at 79°C, and the monomer conversion rate is ensured to reach 95% during the dropwise process.

[0079] S4. Maturation and pH Adjustment for Discharge: After the addition is completed, heat the reactor to 87°C and keep it at that temperature for 1.5 hours to ensure that the total conversion rate of monomers reaches 99%. Cool down to 42°C, adjust the pH of the system to 8.4 using ammonia, add the remaining deionized water, and then adjust the pH to 8.2 using ammonia again. Filter to obtain the target emulsion.

[0080] In steps S3 and S4, the monomer conversion rate is detected by gravimetric method. Samples are dried at 150±2℃ for 15 min to determine the solid content and calculate the conversion rate. Gas chromatography (GC) is used for periodic calibration.

[0081] Table 1 Composition of Examples 1-6

[0082]

[0083] Table 2 Specific component composition of Examples 1-6

[0084]

[0085] To verify the technical effects of this invention compared to existing technologies and the parameters and conditions of this invention, comparative and boundary test examples were set up for performance verification.

[0086] Comparative Example 1: Traditional silicone-free aqueous hydroxypropyl emulsion.

[0087] All vinylsiloxanes were removed, and the remaining formulations and processes were the same as in Example 1.

[0088] Performance: Abrasion loss 0.035g, hardness HB, surface tension 35.2mN / m, no delamination after 12 months of storage but poor abrasion resistance.

[0089] Note: Traditional silicone-free emulsions have comparable centrifugal stability (difference of 0.9%) to the present invention due to the absence of siloxane migration issues, but their wear resistance is significantly inferior to that of the present invention, demonstrating the necessity of siloxane modification.

[0090] Comparative Example 2: Physical blending of siloxanes after polymerization.

[0091] The formulation is the same as in Example 5. After polymerization, 5% vinyltrimethoxysilane is added externally and stirred to mix, without participating in the polymerization.

[0092] Performance: Abrasion resistance 0.028g, hardness HB-H, surface tension 34.0mN / m, obvious stratification after 3 months of storage, and a solid content difference of 18.5% between the upper and lower parts after centrifugation.

[0093] Comparative Example 3: Replicating the existing patent CN102898578A silicon incremental process.

[0094] The conventional segmented feeding method is adopted, without distinguishing between seed and low-silicon stages. Silane is added uniformly throughout the process without gradient control.

[0095] Performance: Core silicon content is 9.8% and shell silicon content is 11.2%, with no obvious gradient; wear loss is 0.026g, hardness is H, and surface tension is 32.6mN / m, which cannot meet the specifications of this invention.

[0096] Boundary test 1: (Silane addition limit 2%, lower than the 3% lower limit of this invention).

[0097] Vinyltrimethoxysilane 2%, the rest are the same as in Example 1;

[0098] Performance: Abrasion loss 0.027g > 0.02g, abrasion resistance does not meet the standard.

[0099] Note: When the amount of silane is less than 3%, the surface silicon enrichment is insufficient (XPS measurement shows that the surface silicon is only 9.5%), which cannot form an effective low-friction silane lubricating layer, and the wear resistance is significantly deteriorated.

[0100] Boundary test 2: (Silane addition of 9%, exceeding the 8% upper limit of this invention).

[0101] Vinyltrimethoxysilane 9%, the rest are the same as in Example 1.

[0102] Performance: During the polymerization process, the emulsion gels and clumps, making it impossible to obtain a stable emulsion.

[0103] Note: Excessive siloxane leads to excessively high crosslinking density and gelation between emulsion particles, demonstrating the necessity of the 8% upper limit.

[0104] Boundary test 3: (dropping time 2h, below the lower limit of 2.5h).

[0105] The total duration of synchronous dripping was 2 hours, and the rest was the same as in Example 1.

[0106] Performance: The silicon distribution is disordered, with only 10.1% silicon on the surface, resulting in a wear loss of 0.024g and performance degradation.

[0107] Note: If the dropping time is too short, the siloxane cannot be fully enriched in the shell, and the gradient structure is not obvious.

[0108] Boundary test 4: (dropping time 4.5h, exceeding the 4h upper limit).

[0109] The total duration of synchronous dripping was 4.5 hours, and the rest was the same as in Example 1.

[0110] Performance: Monomer conversion rate decreased to 89%, molecular weight distribution broadened (PDI>2.5), film hardness decreased to H, and abrasion loss was 0.021g.

[0111] Note: If the dropping time is too long, the concentration of free radicals will continue to decrease, leading to incomplete polymerization and a deterioration of the molecular weight distribution.

[0112] Boundary test 5: (Seed feeding ratio 5%, below the 10% lower limit).

[0113] Seeds were prepared using 5% of the pre-emulsion, and the rest was the same as in Example 1.

[0114] Performance: The silicon content in the core region is 2.1%, and the silicon content in the shell region is 18.5%. The gradient distribution still exists, but the number of seed particles is insufficient, resulting in a widened particle size distribution (PDI>1.5) and poor storage stability (centrifugation difference 3.2%).

[0115] Boundary test 6: (Seed feeding ratio 25%, exceeding the 20% upper limit).

[0116] 25% of the pre-emulsion was used to prepare seeds, and the rest was the same as in Example 1.

[0117] Performance: The silicon content in the core area is 6.5%, exceeding the requirement of <5%, while the silicon content in the shell layer is only 12.8%, and the wear loss is 0.022g, which does not meet the standard.

[0118] Boundary test 7: (conversion rate at the endpoint of dropping is 92%, lower than the ≥95% lower limit)

[0119] The formula is the same as in Example 1. The total dripping time of S3 is 2 hours and 30 minutes. The conversion rate at the dripping endpoint is only 92%, so it directly enters the ripening stage. After 2 hours of ripening, the total conversion rate is 94.3% < 98%.

[0120] Test results: Core silicon content 5.7 at%, shell silicon content 13.1 at%, not falling within the 15~25 at% range; abrasion loss 0.025g, exceeding the standard;

[0121] Overall assessment: A conversion rate below 95% at the endpoint of the S3 process leads to incomplete siloxane grafting, making it difficult to form a surface silicon enrichment gradient.

[0122] Boundary test 8: (Total conversion rate during ripening is 97%, lower than the ≥98% lower limit)

[0123] The formula is the same as in Example 1, and the maturation and heat preservation time is 0.5 h, with a total conversion rate of 97.1%.

[0124] Test results: Free vinyl siloxane residue 0.82%; slight haze after 6 months of storage; difference in solid content between the top and bottom layers after centrifugation 2.7% > 2%;

[0125] Overall assessment: A total conversion rate of ≥98% is one of the necessary conditions for long-term storage stability of the emulsion and the absence of silicon migration.

[0126] Boundary Test 9: (Comparative test of AIBN directly dissolving in HPMA hydroxyl monomer, counterexample)

[0127] The formulation is the same as in Example 4, but the order of feeding S1 is changed: AIBN is first completely dissolved in 12% HPMA hydroxy monomer, and then mixed in the acrylate monomer mixture; the pre-emulsion is allowed to stand for 2 hours and a fine gel appears; the seed stage endpoint conversion rate is only 83.5%; the core silicon content is 6.1 at%, exceeding the 3~5 at% limit; the wear loss is 0.024g.

[0128] Boundary test 10: (Core Si = 2.8 at%, below the lower limit of 3 at%)

[0129] The formula is the same as in Example 1, except that the proportion of seed pre-emulsion is adjusted to 8% (lower than the 10% lower limit of claim 10%), and the rest of the process remains unchanged.

[0130] XPS testing: Core silicon atom concentration 2.8 at%, shell silicon only 14.2 at% (not reaching the lower limit of 15 at%).

[0131] Performance data: abrasion loss 0.026g, 60° gloss 84, surface tension 32.5mN / m, centrifugal solids content difference 3.3%;

[0132] Overall assessment: Core silicon <3at%, insufficient gradient driving force, shell silicon enrichment not reaching 15at%, and unqualified wear resistance and storage stability.

[0133] Boundary test 11: (Kernel Si = 5.3 at%, higher than the upper limit of 5 at%)

[0134] The formulation is the same as in Example 1, except that the seed pre-emulsion feeding ratio is 25% (exceeding the upper limit of 20% in claim 1), and the rest of the process remains unchanged;

[0135] XPS testing: Core silicon atom concentration 5.3 at%, shell silicon 16.1 at%;

[0136] Performance data: Abrasion loss 0.017g, but paint film adhesion (cross-cut) grade 2 (standard grade 1), whitening after 24h water immersion, and decreased cross-linking density;

[0137] Overall assessment: Core silicon > 5 at%, core hydroxyl groups are largely consumed by silane, resulting in deterioration of cross-linking, water resistance, and adhesion of the two components.

[0138] Boundary test 12: (Surface Si = 14.5 at%, below the lower limit of 15 at%)

[0139] The formulation is the same as in Example 1, except that the total amount of siloxane added is 2% (below the lower limit of 3%), and the rest of the process remains unchanged.

[0140] XPS testing: Core silicon 3.1at%, shell silicon 14.5at%;

[0141] Performance data: abrasion loss 0.027g, coefficient of friction 0.18, gloss of paint film at 60° 87, significant loss of gloss;

[0142] Overall assessment: Surface silicon content <15 at%, no continuous silicon lubricating layer, and wear resistance does not meet standards.

[0143] Boundary test 13: (Surface Si = 25.8 at%, higher than the upper limit of 25 at%)

[0144] The formulation is the same as in Example 4, but the amount of vinylsiloxane added is 9% (exceeding the upper limit of 8%); a large amount of gel appears during the polymerization process, and a stable emulsion cannot be obtained;

[0145] If the silane content is reduced to a controllable emulsion of 8.5%, the surface silicone content of XPS is 25.8 at%; the gloss of the coating is 76 at 60°; large-area pinholes are sprayed, and slight delamination occurs after 30 days of storage at 50°C.

[0146] Overall assessment: Surface silicon >25 at%, excessive silane enrichment in the shell layer, resulting in failure of emulsion polymerization stability, coating appearance, and storage stability.

[0147] The performance of the examples, comparative examples, and boundary test cases was tested according to a unified testing standard, wherein:

[0148] Abrasion resistance: GB / T 1768-2006, 750g weight, 500r, CS-10 grinding wheel, paint film thickness 23±3μm.

[0149] Pencil hardness: GB / T 6739-2022.

[0150] Surface tension: 23±2℃, humidity 50±5%, calculated using the Owens-Wendt formula for contact angle.

[0151] Accelerated storage: 30 days in a 50℃ oven, equivalent to 12 months at room temperature.

[0152] XPS silicon gradient characterization: Ar ion sputtering at a rate of 0.1 nm / s, with Si2p signals acquired every 3 nm.

[0153] Gloss: GB / T 9754, 60° angle.

[0154] Centrifugation stability: 3000 rpm, 30 min, test the difference in solid content between the upper and lower layers.

[0155] VOC content: GB 30981.1-2025, gas chromatography.

[0156] Friction coefficient: ball-disc friction and wear tester, load 5N, sliding speed 0.1m / s.

[0157] DMA analysis: Dynamic thermomechanical analyzer, frequency 1Hz, heating rate 3℃ / min, -50~150℃.

[0158] Figure 1 The diagram shows the latex core-shell structure obtained in Examples 1-6. Figure 2 XPS depth profiling of Si element concentration distribution curves in latex particles from Example 1. Figure 2 It can be seen that the obtained emulsion latex particles have a silicon content of 15%–25% in the 0–10 nm surface layer, and the silicon content drops to <5% after a depth of 30 nm, showing a clear gradient distribution; thus confirming that the present invention can obtain… Figure 1 The latex core-shell structure shown. Figure 3 This is a diagram illustrating the mechanism of monomer conversion differences in seed emulsion gradient polymerization in Example 1. It shows the kinetic process of preferential polymerization of acrylates to form a low-silicon core during the seed stage, and continuous grafting of siloxanes to form a silicon-rich shell during the dropwise stage.

[0159] Figure 4 The chart shows a bar graph comparing the wear resistance mass loss of each embodiment and comparative example. It can be seen that all embodiments of the present invention have a mass loss of ≤0.02g, while Comparative Example 1 has a mass loss of 0.035g from the traditional emulsion and Comparative Example 2 has a mass loss of 0.028g from the physically blended silane, thus demonstrating that the present invention has excellent wear resistance.

[0160] Figure 5 The graphs show the surface tension of the coating films from Example 1, a conventional emulsion (Comparative Example 1), and a physically blended silane emulsion (Comparative Example 2) as a function of storage time. Example 1 of this invention exhibits long-term stability of 25-30 mN / m, while the conventional emulsion shows >33 mN / m. The physically blended silane emulsion shows continuous fluctuations over time, indicating that this invention possesses excellent stability.

[0161] Figure 6 The images show a comparison of the surface morphology of the AFM emulsion paint films in Example 1 and Comparative Example 1. Example 1 of this invention has a roughness Ra=8.2nm, while Comparative Example 1 has a roughness Ra=22.5nm, exhibiting more unevenness and defects. The paint film morphology of this invention far surpasses that of traditional latex paint.

[0162] Figure 7The bar chart shows a comparison of the emulsion centrifugation and storage stability between the examples and comparative examples 1-3. As can be seen from the chart, the maximum stratification difference in the examples of this invention is <2%, while the difference in the physically blended comparative examples is 18.5%, indicating severe stratification.

[0163] Figure 8 The graph shows the correlation between the coefficient of friction of the emulsion coating film and the wear resistance loss in Examples 1-3 and Comparative Examples 1-3. Figure 8 The study demonstrated a linear negative correlation between the coefficient of friction and wear loss (R²=0.97), confirming that surface silicon enrichment improves wear resistance by reducing the coefficient of friction.

[0164] Figure 9 The temperature spectra of the emulsion coating films of Example 1 and Comparative Example 1 are shown in the dynamic mechanical analysis (DMA) temperature spectrum. Among them, (a) storage modulus temperature spectrum shows that Example 1 maintains high modulus over a wide temperature range; (b) loss factor temperature spectrum shows the dual Tg characteristics of Example 1 (shell Tg≈15℃, core Tg≈75℃), confirming the gradient structure of "hard inside and soft outside".

[0165] The performance of the examples, comparative examples, and some boundary tests are shown in Tables 3 and 4. Parallel sample tests were conducted using national standard testing methods, with each group of samples tested three times in parallel and the average value taken. In boundary test 2, the silane addition exceeded the 3%–8% range specified in the claims of this invention, resulting in a large amount of gelation during polymerization, making it impossible to obtain a stable emulsion, and no corresponding paint film performance data was available. Comparative example 3 replicated the existing silicon-gradient patent process but did not employ the seed low-silicon + gradient synchronous dripping process of this invention, failing to form a quantitative silicon gradient structure with a core <5 at% and a shell 15%–25 at%; the wear resistance, hardness, and surface tension indicators were less compatible with the actual application requirements than those of the examples of this invention. In boundary test 1, the silane dosage was less than 3%, resulting in insufficient surface silicon enrichment, excessive wear resistance quality loss, and failure to meet the requirements of high-end coatings. Boundary tests 3–6 further verified the upper and lower limits of the dripping time and seed ratio.

[0166] Table 3 Performance of Examples, Comparative Examples, and Partial Boundary Tests

[0167]

[0168] Table 4 Partial boundary test performance

[0169]

Claims

1. A siloxane gradient-enriched, wear-resistant, waterborne hydroxyl acrylic emulsion, characterized in that, The emulsion is polymerized from the following components by weight percentage: 40%–55% acrylate monomer mixture, 5%–12% hydroxyl-containing functional monomers, 3%–8% vinylsiloxane modifier, 2%–5% compound emulsifier, 0.3%–1.0% initiator A, and deionized water to 100%; the acrylate monomer mixture does not contain hydroxyl-containing functional monomers; the vinylsiloxane modifier molecule contains carbon-carbon double bonds that can be polymerized by free radicals, which are chemically bonded to the acrylic resin molecular chain through double-bond free radical polymerization; the initiator A is one of ammonium persulfate and potassium persulfate, and is prepared as a 2.5–3.5 wt% aqueous solution before use; the silicon element in the latex particles of the emulsion is distributed in a gradient from the core region to the shell layer, with a silicon atom concentration of 3%–5% in the core region and 15%–25% in the shell surface layer.

2. The siloxane gradient enriched abrasion-resistant waterborne hydroxyl acrylic emulsion according to claim 1, characterized in that, The components of the acrylate monomer mixture are, by weight, 20-30 parts of hard monomer, 15-25 parts of soft monomer, and 2-5 parts of carboxyl functional monomer; the hard monomer is one or two of methyl methacrylate (MMA) and styrene (St); the soft monomer is one or two of butyl acrylate (BA) and ethyl acrylate (EA); and the carboxyl functional monomer is one or two of acrylic acid (AA) and methacrylic acid (MAA).

3. The siloxane gradient enriched abrasion-resistant waterborne hydroxyl acrylic emulsion according to claim 1, characterized in that, The hydroxyl-containing functional monomer is one or both of hydroxyethyl methacrylate (HEMA) and hydroxypropyl methacrylate (HPMA).

4. The siloxane gradient enriched abrasion-resistant waterborne hydroxyl acrylic emulsion according to claim 1, characterized in that, The vinylsiloxane modifier is one or more of vinyltrimethoxysilane, vinyltriethoxysilane, and methylvinyldimethoxysilane.

5. The siloxane gradient enriched abrasion-resistant waterborne hydroxyl acrylic emulsion according to claim 1, characterized in that, The compound emulsifier is a compound system of anionic and nonionic emulsifiers; the anionic emulsifier is one or two of sodium dodecyl sulfate (SDS) and sodium dodecylbenzene sulfonate (SDBS); the nonionic emulsifier is one or two of octylphenol polyoxyethylene ether (OP-10) and fatty alcohol polyoxyethylene ether (AEO-9).

6. A method for preparing a siloxane gradient-enriched, wear-resistant, waterborne hydroxyl acrylic emulsion as described in any one of claims 1-5, characterized in that, Includes the following steps: S1. Preparation of silicone-containing pre-emulsion: Add all the acrylate monomer mixture, hydroxyl-containing functional monomer, vinylsiloxane modifier, compound emulsifier, and 30% to 45% of the total water volume of deionized water into a pre-emulsion kettle, and pre-emulsify at high speed at 40 to 50°C for 30 to 60 minutes to prepare silicone-containing pre-emulsion. S2. Preparation of low-silicon seed emulsion: Take 10% to 20% of the total amount of silicon-containing pre-emulsion, 20% to 30% of the total amount of initiator A, and 15% to 25% of the total amount of deionized water and add them to the reaction vessel. Heat the vessel to 75 to 80°C and react at a constant temperature for 30 to 45 minutes to generate a seed emulsion with a core silicon atom concentration of 3% to 5%. S3. Gradient synchronous dropwise polymerization: The remaining pre-emulsion and the remaining initiator A are simultaneously and uniformly added to the reactor. The total dropwise addition time is 2.5 to 4 hours. The temperature inside the reactor is maintained at 78 to 85°C. During the dropwise addition process, the monomer conversion rate is ensured to be ≥95%. S4. Maturation and pH Adjustment for Discharge: After the dripping is completed, heat the reactor to 85-90℃ and keep it at that temperature for 1-2 hours to ensure a total monomer conversion rate ≥98%. Cool down to 40-50℃ and adjust the pH of the system to 8.3-8.6 using ammonia or triethanolamine. Add the remaining deionized water and test the pH value. The final emulsion should have a pH value between 7.5 and 8.

5. Filter to obtain the target emulsion. If the pH value is not between 7.5 and 8.5, fine-tune it again using ammonia or triethanolamine.

7. The method for preparing a siloxane gradient-enriched, wear-resistant, waterborne hydroxyl acrylic emulsion according to claim 6, characterized in that, In step S1, initiator B is pre-mixed into the acrylate monomer mixture. Initiator B is azobisisobutyronitrile, and its amount is 50% of that of initiator A.

8. The method for preparing a siloxane gradient-enriched, wear-resistant, waterborne hydroxyl acrylic emulsion according to claim 6, characterized in that, In steps S3 and S4, the monomer conversion rate is detected by gravimetric method. Samples are dried at 150±2℃ for 15 min to determine the solid content and calculate the conversion rate. Gas chromatography (GC) is used for periodic calibration.

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