A polyurethane-modified acrylic water-based resin and a method for preparing the same

Abstract

This invention relates to the field of waterborne polymer material synthesis technology, and discloses a polyurethane-modified acrylic waterborne resin and its preparation method. The preparation method includes: mixing a polyurethane prepolymer and an acrylate monomer in a dispersion medium, adding a pH-responsive block copolymer as a dispersant stabilizer, and performing ultrasonic-assisted pre-emulsification to form a fine emulsion; adding an initiator solution dropwise to the fine emulsion, and carrying out a free radical graft copolymerization reaction under gradient heating conditions, wherein the gradient heating is from a first temperature at a first heating rate to a second temperature, and then to a third temperature at a second heating rate; after the copolymerization reaction is completed, cooling is performed, a neutralizing agent is added to adjust the pH, and the dispersion medium is removed by vacuum distillation to obtain the product. This invention achieves emulsifier-free fine emulsion polymerization through pH-responsive block copolymer synergistic ultrasonic pre-emulsification, and precisely controls the graft copolymerization process using a gradient heating process. The resulting waterborne resin is suitable for use in waterborne coatings, adhesives, and finishing agents.

Description

Technical Field

[0001] This invention relates to the field of waterborne polymer material synthesis technology, specifically to a polyurethane-modified acrylic waterborne resin and its preparation method. Background Technology

[0002] Polyurethane materials, due to the presence of polar groups such as urethane and urea bonds in their molecular structure, exhibit excellent wear resistance, adhesion, flexibility, and low-temperature resistance in their cured coatings, making them widely used in coatings, adhesives, and leather finishing agents. Acrylic polymers, on the other hand, hold a significant position in the coatings market due to their superior weather resistance, gloss and color retention, chemical resistance, and cost advantages. Chemically modifying polyurethane and acrylate polymers can achieve complementary advantages between the two materials, resulting in composite resin materials that combine the high wear resistance and elasticity of polyurethane with the excellent weather resistance and high hardness of acrylates. This has always been an important research direction in the field of high-performance waterborne resins.

[0003] With increasingly stringent global restrictions on volatile organic compound (VOC) emissions and stricter environmental regulations, the application space for traditional solvent-based polyurethane coatings has been significantly reduced, making waterborne polyurethane and its modified systems the mainstream development direction. Waterborne polyurethane-acrylate composite emulsions are typically prepared using physical blending or chemical copolymerization. Physical blending is simple to operate, but polyurethane and acrylate polymers are thermodynamically incompatible, easily leading to macroscopic phase separation after blending. This results in a cloudy coating appearance and decreased mechanical properties, making it difficult to meet the demands of high-performance applications. Chemical copolymerization, through grafting, block polymerization, or interpenetrating networks, achieves bonding between the two polymers at the molecular scale, effectively suppressing phase separation and significantly improving compatibility. It is currently the preferred method for preparing high-performance polyurethane-acrylate composite materials.

[0004] Among existing chemical modification technologies, relatively mature methods include using polyurethane as a macromolecular initiator to initiate the polymerization of acrylate monomers, the condensation reaction of polyurethane prepolymers with hydroxyl-containing acrylate resins, and seed emulsion polymerization by dropwise addition of acrylate monomers to polyurethane emulsions. However, these methods still have many shortcomings in practical applications. First, traditional emulsion polymerization systems use a large amount of small-molecule emulsifiers. These emulsifiers remain in the coating after film formation and migrate to the coating surface, reducing the coating's water resistance and adhesion. The presence of small-molecule emulsifiers also adversely affects the resin's transparency and gloss. Second, the significant polarity difference between polyurethane prepolymers and acrylate monomers makes it difficult to form a uniform and stable dispersion system during pre-emulsification. The wide distribution of monomer droplet size leads to low grafting efficiency and poor reaction reproducibility in subsequent copolymerization reactions. Furthermore, conventional isothermal polymerization processes can lead to excessively rapid initiation rates in the early stages of the reaction, which can result in localized burst polymerization and gelation. In contrast, a single heating strategy cannot effectively control the extent of graft copolymerization at different stages, resulting in uneven polymer molecular structures and significant batch-to-batch fluctuations in the final product performance.

[0005] In recent years, researchers have attempted to improve the compatibility of polyurethane-acrylate systems using fine emulsion polymerization technology. In fine emulsion polymerization, the monomer droplet size can be controlled within the range of 50-500 nm. Each droplet can be considered an independent nanoreactor, effectively suppressing the diffusion and mass transfer process of monomers in the aqueous phase, thereby improving the uniformity of the polymerization reaction. However, relying solely on fine emulsion technology is still insufficient to solve the problems of high interfacial tension and strong droplet co-aggregation tendency between polyurethane prepolymers and acrylate monomers. Highly efficient stabilizers are needed to maintain the long-term stability of droplet size. Furthermore, in existing fine emulsion polymerization processes, the hydrophilic carboxyl groups of polyurethane need to be neutralized to achieve water dispersion. The increase in pH after adding a neutralizing agent alters the ionization state of the carboxyl functional monomers in the acrylate monomers, thus affecting their distribution behavior at the oil-water interface. This contradiction has not yet been effectively resolved in existing processes. Summary of the Invention

[0006] The purpose of this invention is to provide a polyurethane-modified acrylic waterborne resin and its preparation method, so as to solve the problems mentioned in the background art.

[0007] To achieve the above objectives, the present invention provides a polyurethane-modified acrylic waterborne resin and a method for preparing the same, the method comprising: Step (1): After mixing the polyurethane prepolymer and acrylate monomer in a dispersion medium, a pH-responsive block copolymer is added as a dispersion stabilizer, and a fine emulsion is formed by ultrasonic-assisted pre-emulsification. Step (2): Add an initiator solution to the fine emulsion and carry out a free radical graft copolymerization reaction under gradient heating conditions. The gradient heating conditions are: heating from a first temperature to a second temperature at a first heating rate, and then heating to a third temperature at a second heating rate. Step (3): After the copolymerization reaction is completed, the system is cooled to the fourth temperature, a neutralizing agent is added to adjust the pH of the system to the target value, and the dispersion medium is removed by vacuum distillation to obtain the polyurethane modified acrylic waterborne resin.

[0008] Preferably, in step (1), the method for preparing the polyurethane prepolymer is as follows: Polyether diol and polyester diol were added to a reaction vessel at a mass ratio of 1:(0.5-2) and dehydrated for 1.5-3 hours under vacuum conditions ≤-0.095MPa and temperature 100-120℃. After dehydration, the temperature is lowered to 65-85℃, and diisocyanate monomer, hydrophilic chain extender and catalyst are added. The mixture is stirred and reacted for 2-4 hours under nitrogen protection. The NCO content in the reaction system is monitored until it reaches the theoretical value to obtain the polyurethane prepolymer.

[0009] Preferably, the diisocyanate monomer is one or more combinations of isophorone diisocyanate, hexamethylene diisocyanate, dicyclohexylmethane diisocyanate, toluene diisocyanate or diphenylmethane diisocyanate; The hydrophilic chain extender is dimethylolpropionic acid or dimethylolbutyric acid; The catalyst is dibutyltin dilaurate or stannous octoate, and the amount of catalyst added is 0.05%-0.5% of the mass of the diisocyanate monomer.

[0010] Preferably, the polyether diol is polyethylene glycol, polypropylene glycol, or polytetrahydrofuran ether diol, with a number average molecular weight of 800-4000; The polyester diol is polyethylene adipate diol, polybutylene adipate diol or polycaprolactone diol, with a number average molecular weight of 1000-4000. The total mass ratio of the polyether diol and polyester diol to the diisocyanate monomer is 1:(0.8-2.5).

[0011] Preferably, in step (1), the acrylate monomer is a combination of hard monomers, soft monomers and functional monomers; The hard monomer is one or more of methyl methacrylate, styrene, acrylonitrile, or isobornyl methacrylate. The soft monomer is one or more of butyl acrylate, isooctyl acrylate, ethyl acrylate or lauryl acrylate; The functional monomer is one or more of acrylic acid, methacrylic acid, hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate, or glycidyl methacrylate; The mass ratio of the hard monomer, soft monomer and functional monomer is (20-50):(30-60):(2-15).

[0012] Preferably, in step (1), the pH-responsive block copolymer is polyacrylic acid-b-polystyrene, polymethacrylic acid-b-polybutyl acrylate or polyacrylamide-b-polystyrene, with a number average molecular weight of 2000-15000; The dispersion medium is deionized water or a mixture of deionized water and ethanol. The mass ratio of the polyurethane prepolymer to the acrylate monomer is 1:(0.5-4). The amount of pH-responsive block copolymer added is 1%-10% of the total mass of polyurethane prepolymer and acrylate monomer; The mass ratio of the dispersion medium to the total mass of the polyurethane prepolymer and acrylate monomer is (1-3):1; The ultrasonic power for the ultrasonic-assisted pre-emulsification is 200-800W, and the ultrasonic time is 5-30min.

[0013] Preferably, in step (2), the initiator solution is an aqueous solution of ammonium persulfate, an aqueous solution of potassium persulfate, or an aqueous solution of azobisisobutylamidine hydrochloride, and the concentration of the initiator solution is 1%-10%. The initiator solution is added over a period of 1-4 hours, and the total amount of initiator added is 0.3%-3% of the total mass of the acrylate monomers. The first temperature is 40-60℃, the second temperature is 65-75℃, and the third temperature is 80-90℃; The first heating rate is 0.5-2℃ / min, and the second heating rate is 0.2-1℃ / min; After holding at the second temperature for 0.5-1.5 hours, the temperature is increased to the third temperature at the second heating rate, and then held at the third temperature for 1-3 hours to mature.

[0014] Preferably, in step (3), the fourth temperature is 30-50℃; The neutralizing agent is triethylamine, ammonia, dimethylethanolamine, or triethanolamine; The target pH value is 6.5-8.5; the amount of neutralizing agent added is 80%-120% of the theoretical amount required to bring the system pH value to the target value; The vacuum distillation temperature is 35-50℃, the vacuum degree is ≤-0.08MPa, and the distillation time is 0.5-2h.

[0015] Preferably, the preparation method further includes the following step after step (3): The resin emulsion after vacuum distillation is cooled to room temperature, filtered through a 200-800 mesh filter, and then discharged. The solid content of the resin emulsion is 25%-50%.

[0016] Preferably, the present invention further includes a polyurethane-modified acrylic waterborne resin, wherein the polyurethane-modified acrylic waterborne resin is prepared by the above-described method for preparing polyurethane-modified acrylic waterborne resin.

[0017] Compared with the prior art, the beneficial effects of the present invention are: The present invention provides a method for preparing polyurethane-modified acrylic waterborne resin. By constructing a pH-responsive block copolymer synergistic with ultrasonic-assisted pre-emulsification, it achieves fine emulsification of polyurethane prepolymer and acrylate monomers without small-molecule emulsifiers, effectively overcoming the negative impact of emulsifier residue on coating performance in traditional emulsion polymerization. The pH-responsive block copolymer acts as a dispersant and stabilizer in the fine emulsion system, playing a dual role of spatial and electrostatic stability. The hydrophilic segments in its molecular chain fully extend in the medium to form a hydration layer, while the hydrophobic segments are anchored to the droplet surface or embedded within the droplets, forming a robust interfacial adsorption film. This significantly reduces the aggregation rate between droplets, ensuring a highly uniform size distribution in the fine emulsion during polymerization. The pH-responsive characteristics of this block copolymer endow it with the ability to undergo conformational changes by adjusting the system pH after polymerization. This allows it to maintain stability during polymerization and promote uniform dispersion of the stabilizer into the polymer matrix through conformational changes during post-treatment, avoiding the problem of traditional non-responsive stabilizers forming independent micro-regions in the coating.

[0018] This invention employs an ultrasonic-assisted pre-emulsification process. The localized high temperature and pressure generated by ultrasonic cavitation, along with the strong shock waves, effectively break up the aggregated structure at the oil-water interface, promoting the formation of homogeneous nanoscale droplets from the polyurethane prepolymer and acrylate monomers in the dispersion medium. Ultrasonic treatment simultaneously facilitates the rapid adsorption and rearrangement of pH-responsive block copolymers on the droplet surface, enabling stabilizer molecules to reach thermodynamic equilibrium at the interface. The resulting interfacial film possesses high dilatational modulus and elastic recovery capability, effectively resisting Laplace pressure fluctuations caused by changes in osmotic pressure within the droplets during polymerization, thereby maintaining the long-term stability of the reaction system. This pre-emulsification method allows for uniform molecular-level mixing of polyurethane and acrylate components of different polarities at the droplet scale, providing an ideal microenvironment for subsequent graft copolymerization.

[0019] This invention employs a free radical graft copolymerization process under gradient heating conditions. Through a multi-stage temperature control strategy—raising the temperature from a first temperature at a first heating rate to a second temperature, and then to a third temperature at a second heating rate—precise control of the polymerization process is achieved. In the first temperature stage, the initiator decomposes at a low rate, and the free radical concentration in the system remains relatively low. At this stage, the polymerization reaction mainly occurs in the acrylate monomer-rich region inside the droplets, where the active sites on the polyurethane prepolymer molecular chains preferentially graft with the acrylate monomers. As the temperature rises at the first heating rate to the second temperature range, the initiator decomposition rate gradually increases, the free radical concentration in the system increases, and the graft copolymerization reaction enters the chain growth-dominated stage, with acrylate graft segments successively forming on the polyurethane molecular chains. When the system temperature continues to rise at the second heating rate to the third temperature range, the diffusion of residual monomers intensifies, the polymerization reaction tends to be complete, and the degree of crosslinking and entanglement between molecular chains is moderately developed. Compared to isothermal polymerization or a simple one-time heating process, this gradient heating strategy can provide suitable initiation efficiency and chain growth rate at different reaction stages, allowing the graft copolymerization reaction to occur uniformly along the polyurethane molecular chain. This effectively suppresses gelation caused by excessively fast local reaction rates and avoids the problem of low grafting rate caused by insufficient reaction at active sites.

[0020] In this invention, the polyurethane prepolymer adopts a compound system of polyether diol and polyester diol. The polyether segment gives the resin good flexibility and hydrolysis resistance, while the polyester segment provides high cohesive energy and adhesion. The synergistic effect of the two enables the final cured film to maintain sufficient flexibility while having good mechanical strength.

[0021] In the polyurethane molecular chains formed by the reaction of compound diol systems with diisocyanates, hard and soft segments undergo microphase separation under thermodynamic drive, forming a physically cross-linked network structure. This microphase separation morphology is further enriched and regulated after grafting acrylate segments, which helps to improve the tensile strength and elongation at break of the coating film. The introduction of hydrophilic chain extenders introduces carboxyl side groups into the polyurethane molecular chains. After polymerization, the carboxyl groups are ionized by neutralizing agents, giving the resin self-emulsifying properties. This allows the final product to form a stable aqueous dispersion in deionized water, avoiding the use of external emulsifiers. Attached Figure Description

[0022] Figure 1 This diagram illustrates the working steps of the preparation method of a polyurethane-modified acrylic waterborne resin according to the present invention. Detailed Implementation

[0023] This invention provides a method for preparing a polyurethane-modified acrylic waterborne resin. The core process involves preparing a composite polyurethane prepolymer by compounding polyethers and polyester diols, combining it with a soft-hard functional composite acrylate monomer, using a pH-responsive block copolymer as a dedicated dispersant and stabilizer, and employing ultrasonic pre-emulsification and gradient temperature copolymerization processes to obtain a polyurethane-modified acrylic waterborne resin with excellent stability, water resistance, and mechanical properties. The raw materials used in the following examples and comparative examples are all industrial-grade conventional raw materials, and the testing standards all adopt industry-standard waterborne resin testing standards, specifically as follows: emulsion appearance is observed visually; solid content is tested by drying and weighing; emulsion stability is tested by standing at room temperature for 30 days and at 50°C for 7 days to check for stratification, precipitation, and gelation; coating adhesion is tested according to GB / T 9286-1998 standard; coating water resistance is tested by immersion in water at room temperature for 72 hours to observe whitening, blistering, and peeling; tensile strength and elongation at break are tested according to GB / T 1040-2018 standard.

[0024] Example 1

[0025] See appendix Figure 1 This embodiment describes a method for preparing a polyurethane-modified acrylic waterborne resin, comprising the following specific steps: Step (1) Preparation of polyurethane prepolymer: Polypropylene glycol with a number average molecular weight of 2000 and polybutylene adipate diol with a number average molecular weight of 2000 are added to a dry and clean reaction vessel at a mass ratio of 1:1. The vacuum degree is controlled at -0.098MPa and the temperature is 110℃. The free water in the raw materials is completely removed by vacuum dehydration for 2 hours. After dehydration, the vacuum system was shut off, and nitrogen gas was continuously introduced for protection. The system was cooled to 75°C. Hexamethylene diisocyanate monomer was added according to the mass ratio of polyether diol to polyester diol and hexamethylene diisocyanate 1:1.2. At the same time, dimethylolbutyric acid (DMHA), a hydrophilic chain extender, was added at a rate of 5.0% of the mass of hexamethylene diisocyanate monomer. Dibutyltin dilaurate catalyst was added at a rate of 0.2% of the mass of hexamethylene diisocyanate monomer. The reaction was stirred at a constant temperature for 3 hours, and the NCO content of the system was monitored in real time. When the NCO content dropped to the theoretical design value of 3.5%, the reaction was stopped, and the mixture was cooled for later use to obtain the composite polyurethane prepolymer.

[0026] Step (2) Ultrasonic-assisted pre-emulsification to prepare a fine emulsion: The polyurethane prepolymer and acrylate monomers prepared above are mixed at a mass ratio of 1:2, wherein the acrylate monomers are composed of hard monomer methyl methacrylate, soft monomer butyl acrylate, and functional monomer acrylic acid in a mass ratio of 35:50:15. Deionized water is used as the dispersion medium, and the mass ratio of the dispersion medium to the total mass of polyurethane prepolymer and acrylate monomers is 2:1. 5% of the total mass of the polyurethane prepolymer and acrylate monomers is added to the mixture, which contains a pH-responsive block copolymer polyacrylic acid-b-polystyrene (number average molecular weight 8000). After mixing evenly, the mixture is ultrasonically treated with an ultrasonic power of 500W for 15 minutes to perform ultrasonic-assisted pre-emulsification and form a uniform and stable fine emulsion.

[0027] Step (3) Gradient heating free radical graft copolymerization: Prepare a 5% (w / w) ammonium persulfate aqueous solution as an initiator solution. The total amount of initiator added is 1.5% of the total mass of acrylate monomers. Initially heat the fine emulsion system to 50℃ (first temperature), then heat it to 70℃ (second temperature) at a first heating rate of 1℃ / min, and keep it at this temperature for 1h; then heat it to 85℃ (third temperature) at a second heating rate of 0.5℃ / min, and slowly add the initiator solution dropwise over a period of 2.5h. After the addition is complete, keep it at 85℃ for 2h to complete the free radical graft copolymerization reaction.

[0028] Step (4) Neutralization, solvent removal, and filtration: After the copolymerization reaction, the system is cooled to 40℃ (fourth temperature) at a uniform rate. Triethylamine is slowly added dropwise as a neutralizing agent, with the amount of neutralizing agent added being 100% of the theoretical amount required to achieve the system pH target. The pH of the system is adjusted to 7.5. After adjustment, the system is distilled under reduced pressure for 1 hour at 40℃ and a vacuum of -0.09MPa to remove the residual dispersion medium. After distillation, the resin emulsion is allowed to cool naturally to room temperature. Impurities are filtered using a 400-mesh standard filter, and a polyurethane-modified acrylic waterborne resin with a solid content of 38% is obtained.

[0029] Example 2

[0030] This embodiment describes a method for preparing a polyurethane-modified acrylic waterborne resin, comprising the following specific steps: Step (1) Preparation of polyurethane prepolymer: Polytetrahydrofuran ether diol with a number average molecular weight of 1000 and polycaprolactone diol with a number average molecular weight of 1000 were added to a reaction vessel at a mass ratio of 1:0.8. The vacuum degree was controlled at -0.096 MPa and the temperature at 105℃, and vacuum dehydration was carried out for 2.5 h. After dehydration, nitrogen gas was introduced for protection and the temperature was lowered to 70℃. The mass ratio of the total mass of polyether diol and polyester diol to that of isophorone diisocyanate was 1:1.5. Isophorone diisocyanate monomer was added, along with the hydrophilic chain extender dimethylolpropionic acid. The amount of hydrophilic chain extender dimethylolpropionic acid added was 6.0% of the mass of isophorone diisocyanate monomer. Stannous octoate catalyst was added at 0.3% of the mass of diisocyanate monomer. The reaction was stirred at a constant temperature for 3.5 h. The NCO content of the system was monitored until it reached the theoretical value. The theoretical NCO content was 4.0%, and the polyurethane prepolymer was obtained.

[0031] Step (2) Ultrasonic-assisted pre-emulsification to prepare a fine emulsion: The mass ratio of polyurethane prepolymer to acrylate monomer is 1:1. The acrylate monomer is a compound of hard monomer styrene, soft monomer isooctyl acrylate, and functional monomer hydroxyethyl methacrylate in a mass ratio of 40:55:5. The dispersion medium is a mixture of deionized water and ethanol (volume ratio 9:1), and the mass ratio of dispersion medium to total materials is 1.5:1. 3% of the total mass of pH-responsive block copolymer poly(methacrylate-b-polybutyl acrylate) (number average molecular weight 5000) is added. After uniform mixing, the mixture is ultrasonicated at 400W for 20 minutes to prepare a uniform fine emulsion.

[0032] Step (3) Gradient heating free radical graft copolymerization: Prepare an 8% potassium persulfate aqueous solution as the initiator solution. The total amount of initiator added is 2.0% of the total mass of acrylate monomers. The initial temperature of the system is 45℃, and the temperature is increased to 68℃ at a rate of 0.8℃ / min and kept at a constant temperature for 1.2h. Then the temperature is increased to 82℃ at a rate of 0.3℃ / min, and the initiator solution is added dropwise at a uniform rate for 3h. After the addition is completed, the mixture is kept at a constant temperature for 2.5h to complete the copolymerization reaction.

[0033] Step (4) Neutralization, solvent removal, and filtration: After the reaction, the temperature was lowered to 35°C. Ammonia was used as a neutralizing agent, with an addition amount of 90% of the theoretical amount, and the pH of the system was adjusted to 7.0. The residual solvent was removed by vacuum distillation for 1.5 hours at 38°C and a vacuum degree of -0.085 MPa. After cooling to room temperature, the product was filtered through a 600-mesh filter to obtain a polyurethane-modified acrylic waterborne resin with a solid content of 42%.

[0034] Example 3

[0035] This embodiment describes a method for preparing a polyurethane-modified acrylic waterborne resin, comprising the following specific steps: Step (1) Preparation of polyurethane prepolymer: Polyethylene glycol with a number average molecular weight of 4000 and polyethylene adipate diol with a number average molecular weight of 4000 were added to a reaction vessel at a mass ratio of 1:1.5. The mixture was dehydrated for 1.8 h under a vacuum of -0.099 MPa and a temperature of 115 °C. After dehydration, the temperature was lowered to 80 °C under nitrogen protection. The mass ratio of the total material mass to dicyclohexylmethane diisocyanate was 1:2.0. Dicyclohexylmethane diisocyanate was added, along with dimethylolbutyric acid (DMHA) as a hydrophilic chain extender. The amount of DMHA added was 5.0% of the mass of the dicyclohexylmethane diisocyanate monomer. Dibutyltin dilaurate (DICDI) was added at a mass ratio of 0.1% of the monomer mass. The reaction was carried out at a constant temperature for 2.5 h. The NCO content was monitored and found to be 3.0%, thus obtaining the polyurethane prepolymer.

[0036] Step (2) Ultrasonic-assisted pre-emulsification to prepare a fine emulsion: The mass ratio of polyurethane prepolymer to acrylate monomer is 1:3. The acrylate monomer is a compound of hard monomer isobornyl methacrylate, soft monomer ethyl acrylate, and functional monomer glycidyl methacrylate in a mass ratio of 25:60:15. The dispersion medium is deionized water, with a media-to-material mass ratio of 2.5:1. 8% of the total mass of the material is added to a pH-responsive block copolymer polyacrylamide-b-polystyrene (number average molecular weight 12000). After mixing, the mixture is ultrasonicated at 700W for 10 minutes to obtain a stable fine emulsion.

[0037] Step (3) Gradient heating free radical graft copolymerization: Prepare a 3% (w / w) aqueous solution of azobisisobutylamidine hydrochloride as the initiator solution. The total amount of initiator added is 0.8% of the total mass of acrylate monomers. The initial temperature of the system is 55℃, and the temperature is increased to 72℃ at a rate of 1.5℃ / min and held for 0.8h. Then the temperature is increased to 88℃ at a rate of 0.8℃ / min, and the initiator solution is added dropwise for 1.5h. After the addition is completed, the mixture is kept at a constant temperature for 1.5h to complete the copolymerization reaction.

[0038] Step (4) Neutralization, solvent removal, and filtration: After the reaction, the temperature was lowered to 45℃, and dimethylethanolamine was used as a neutralizing agent at 110% of the theoretical amount. The pH was adjusted to 8.0. The dispersion medium was removed by vacuum distillation for 0.8 h at 45℃ and a vacuum degree of -0.092 MPa. The product was then filtered through a 300-mesh filter at room temperature to obtain a polyurethane-modified acrylic waterborne resin with a solid content of 32%.

[0039] Example 4

[0040] This embodiment describes a method for preparing a polyurethane-modified acrylic waterborne resin, comprising the following specific steps: Step (1) Preparation of polyurethane prepolymer: Polypropylene glycol with a number average molecular weight of 3000 and polybutylene adipate diol with a number average molecular weight of 3000 were added to a reactor at a mass ratio of 1:2. The reactor was dehydrated for 1.5 h under vacuum of -0.097 MPa and temperature of 120 °C. After dehydration, the temperature was lowered to 85 °C under nitrogen protection. The mass ratio of the total material to toluene diisocyanate was 1:0.9. Toluene diisocyanate monomer was added, along with dimethylolpropionic acid chain extender. The amount of hydrophilic chain extender dimethylolpropionic acid added was 7.0% of the mass of toluene diisocyanate monomer. Stannous octoate catalyst was added at 0.4% of the monomer mass. The reaction was carried out at a constant temperature for 4 h. The NCO content was monitored and found to be 5.5%, thus obtaining the polyurethane prepolymer.

[0041] Step (2) Ultrasonic-assisted pre-emulsification to prepare a fine emulsion: The mass ratio of polyurethane prepolymer to acrylate monomer is 1:4. The acrylate monomer is a compound of hard monomer methyl methacrylate + styrene (mass ratio 1:1), soft monomer lauryl acrylate, and functional monomer hydroxyethyl methacrylate in a mass ratio of 50:30:20. The dispersion medium is deionized water, and the mass ratio of medium to total material is 3:1. 10% of polyacrylic acid-b-polystyrene (number average molecular weight 15000) is added, and the mixture is ultrasonicated at 200W for 30 minutes to prepare a fine emulsion.

[0042] Step (3) Gradient heating free radical graft copolymerization: Prepare a 10% (w / w) ammonium persulfate aqueous solution, and add the initiator at a total amount of 3.0% of the total mass of the acrylate monomers. The initial temperature of the system is 60℃, and the temperature is increased to 75℃ at a rate of 2℃ / min and held for 0.5h; then the temperature is increased to 90℃ at a rate of 1℃ / min, and the initiator solution is added dropwise for 4h. After the addition is completed, the system is kept at a constant temperature for 3h to complete the copolymerization reaction.

[0043] Step (4) Neutralization, solvent removal, and filtration: After the reaction, the temperature was lowered to 50℃, and triethanolamine was used as a neutralizing agent at 120% of the theoretical amount. The pH was adjusted to 8.5. The solvent was removed by vacuum distillation for 2 hours at 50℃ and a vacuum degree of -0.08MPa. The product was then filtered through an 800-mesh filter at room temperature to obtain a polyurethane-modified acrylic waterborne resin with a solid content of 48%.

[0044] Comparative Example 1: The preparation method of this comparative example is basically the same as that of Example 1, except that: In step (2), pH-responsive block copolymers are not added as dispersants and stabilizers. Instead, conventional sodium dodecylbenzenesulfonate emulsifier is used. The ultrasonic-assisted pre-emulsification process is cancelled, and mechanical stirring for 30 minutes is used for emulsification. The remaining raw material ratios, reaction parameters, and operation steps are exactly the same as in Example 1. Finally, the comparative sample waterborne resin is obtained.

[0045] Comparative Example 2: The preparation method of this comparative example is basically the same as that of Example 1, except that: Step (3) The copolymerization reaction adopts a constant temperature reaction process, eliminating the gradient heating process, and directly keeping the system constant at 85℃ for initiation copolymerization and maturation. The remaining raw material ratios, reaction parameters, and operation steps are exactly the same as in Example 1, and finally the comparative sample waterborne resin is obtained.

[0046] Comparative Example 3: The preparation method of this comparative example is basically the same as that of Example 1, except that: When preparing the polyurethane prepolymer, only polypropylene oxide diol with a number average molecular weight of 2000 and a hydroxyl value of 56 mgKOH / g, namely PPG-2000, was used. Its functionality was 2 and it did not contain ethylene oxide links. No polyester diol was compounded. The other raw material ratios, reaction parameters, and operation steps were exactly the same as in Example 1. Finally, the comparative sample waterborne resin was obtained.

[0047] The polyurethane-modified acrylic waterborne resins prepared in the above four examples and three comparative examples were uniformly tested for emulsion storage stability, coating basic properties, and mechanical properties. The test data are shown in the table below. All test samples were cured and tested under the same environmental conditions (temperature 25℃, humidity 50%) to ensure the accuracy and comparability of the data.

[0048] Table 1: Comparison of storage stability and basic physical properties of resin emulsions from each example and comparative example

[0049] As shown in Table 1, the polyurethane-modified acrylic waterborne resin emulsions prepared in the four examples of this invention have excellent overall performance, uniform and delicate appearance with no obvious impurity particles, and the solid content can be adjusted in the range of 32.0%-48.0% according to the process parameters, which can meet the needs of different construction scenarios.

[0050] It remained stable for 30 days at room temperature without any layering, precipitation, or gelation. Under accelerated aging test at 50°C, only the high-solids-content Example 4 showed slight thickening without any performance failure, demonstrating excellent heat resistance and storage stability.

[0051] In Comparative Example 1, after replacing the dedicated pH-responsive block copolymer and eliminating ultrasonic pre-emulsification, the appearance of the emulsion deteriorated, with the appearance of turbid particles. Layering and precipitation occurred during storage at room temperature, and severe gelation occurred under high temperature conditions. This demonstrates that the pH-responsive block copolymer has a better dispersion and stability effect than traditional small molecule emulsifiers. Combined with ultrasonic pre-emulsification, it can significantly improve the uniformity of emulsion dispersion and storage stability, effectively preventing latex particle agglomeration and sedimentation.

[0052] Comparative Example 2 used a constant temperature copolymerization process without a gradient heating program. The emulsion had uneven color and local agglomeration, and its storage stability was significantly reduced. This was because the initiation rate was too fast in the early stage of the constant temperature reaction, and the monomer instantaneous polymerization was intense, which easily formed large-diameter latex particles. The uneven particle distribution made it easy for stratification and precipitation to occur during long-term storage.

[0053] This fully demonstrates that the gradient heating process can precisely control the polymerization reaction rate, achieve uniform particle growth, and improve the overall stability of the emulsion.

[0054] Comparative Example 3, which uses only a single polyether diol to prepare the prepolymer, showed a decrease in emulsion gloss and fineness, and a deterioration in high-temperature stability. This indicates that the polyether diol and polyester diol compound system can synergistically optimize the molecular structure and compatibility of the polyurethane prepolymer, and can effectively improve the storage stability of the modified resin emulsion compared to the single polyether system.

[0055] Table 2: Comparison of water resistance and adhesion properties of resin coatings in each example and comparative example

[0056] In the industry standard for adhesion rating, grade 0 is the best and grade 4 is the worst. Excellent water resistance is indicated by the absence of whitening, blistering, or peeling of the coating.

[0057] From the data in Table 2, we can obtain: The coatings in Examples 1 and 2 of this invention exhibit the best performance, with adhesion reaching level 0. The coatings are smooth, dense, and have good gloss. They show no damage after being immersed in water at room temperature for 72 hours, demonstrating excellent water resistance. Due to adjustments in monomer ratio and solid content, Examples 3 and 4 show adhesion at level 1 with only slight edge whitening, yet still possess excellent practical water resistance, meeting the needs of most water-based coatings and coating application scenarios.

[0058] Due to defects in the emulsification system, the latex particles in Comparative Example 1 were unevenly dispersed, which easily led to pinholes and defects during the film formation process. The density of the coating film decreased significantly, and moisture easily penetrated into the interior of the coating film, resulting in large-area whitening, blistering and peeling. At the same time, the adhesion between the coating film and the substrate was weakened, and the adhesion was only level 2. The overall performance of the coating film was significantly worse than that of the sample in the example.

[0059] This confirms that the stabilizing and dispersing effect of pH-responsive block copolymers can effectively improve the packing density of latex particles, optimize film formation, and enhance coating adhesion and water resistance.

[0060] The resin prepared by the isothermal polymerization process in Comparative Example 2 has uneven distribution of molecular chain segments, incomplete graft copolymerization reaction, poor regularity of resin molecular structure, uneven film thickness and structural defects after film formation, and easy penetration of water into the film interface, resulting in a significant decrease in water resistance and adhesion. This fully demonstrates that the gradient heating process can precisely control the graft copolymerization reaction process, ensuring that polyurethane and acrylate molecules are fully grafted to form a well-structured and dense resin system, thereby fundamentally optimizing the coating performance.

[0061] Comparative Example 3: The single polyether type polyurethane prepolymer modified resin has a soft coating with insufficient rigidity and slightly worse water resistance than the compound system examples. This is because the introduction of polyester diol can improve the rigidity and density of polyurethane molecules. To overcome the shortcomings of excessive flexibility and insufficient water barrier properties in single polyether systems, the combination of polyether and polyester diol can achieve a balance between resin flexibility and rigidity, significantly optimizing the water resistance and adhesion performance of the coating film.

[0062] Table 3: Comparison of mechanical properties of resin coatings in each example and comparative example

[0063] Tensile strength, elongation at break, and pencil hardness are the core indicators for measuring the mechanical properties of resin coatings, corresponding to the coating's tensile strength, flexibility and toughness, and surface rigidity and hardness, respectively.

[0064] As shown in Table 3, the mechanical properties of the four sets of sample examples of this invention are distributed in a gradient and are adapted to different application requirements: Example 4 is high in hardness and strength, with a tensile strength of 20.3 MPa and a hardness of 3H, making it suitable for high wear resistance and high rigidity coating scenarios; Example 3 is highly flexible, with an elongation at break of 512%, and excellent resistance to bending and cracking, making it suitable for flexible substrate coatings; Examples 1 and 2 achieve a balanced match between rigidity and flexibility, with the best comprehensive mechanical properties and extremely strong versatility.

[0065] Compared to the embodiments, all three comparative examples showed significant shortcomings in mechanical properties. Comparative Example 1, due to poor emulsification and dispersion, insufficient copolymerization uniformity of the resin system, and weak intermolecular forces, exhibited a coating tensile strength of only 10.5 MPa, an elongation at break of 295%, and a hardness of only HB. The overall mechanical properties were significantly reduced, demonstrating that the dedicated pH-responsive block copolymer and ultrasonic pre-emulsification process can optimize resin polymerization uniformity, strengthen intermolecular cross-linking, and simultaneously improve the rigidity and toughness of the coating.

[0066] Comparative Example 2 shows that the isothermal polymerization process resulted in a low resin grafting rate, numerous molecular chain segment defects, and insufficient crosslinking density. The tensile strength and elongation at break were significantly lower than those of the sample from the gradient heating process. This demonstrates that gradient heating can control the initiation, polymerization, and curing processes in stages, improve the graft copolymerization efficiency of polyurethane and acrylic resin, increase the crosslinking density of the system, and significantly optimize mechanical properties.

[0067] Comparative Example 3 shows that the single polyether-type prepolymer modified resin has a high elongation at break but low tensile strength and insufficient rigidity. However, the polyether and polyester diol compound system can combine the advantages of high toughness of polyether and high rigidity of polyester to achieve a synergistic improvement in the mechanical properties of the resin, effectively solving the technical shortcomings of single system with single performance and poor comprehensive performance.

[0068] Based on the preparation process and performance test results of the above embodiments and comparative examples, it is clear that this invention significantly improves the overall performance of polyurethane-modified acrylic waterborne resin through three core technological innovations. First, a polyurethane prepolymer is prepared by compounding polyether diol and polyester diol, balancing the resin's flexibility and rigidity, thus solving the performance imbalance problem of traditional single-system formulations. Second, a pH-responsive block copolymer is used to replace traditional small-molecule emulsifiers, coupled with an ultrasonic-assisted pre-emulsification process, significantly improving the uniformity of emulsion dispersion and storage stability, and reducing film-forming defects. Third, a gradient-temperature free radical graft copolymerization process is employed to precisely control the polymerization reaction rate, increasing the molecular grafting rate and system crosslinking density, fundamentally optimizing the adhesion, water resistance, and mechanical properties of the resin coating.

[0069] The polyurethane-modified acrylic waterborne resin prepared by this invention has a wide range of adjustable solid content and excellent emulsion stability. The coating film exhibits high adhesion, excellent water resistance, and balanced mechanical properties. Compared with traditional modified resins, its overall performance is significantly improved, making it widely applicable in various fields such as waterborne wood coatings, metal anti-corrosion coatings, plastic coatings, and textile coatings. It possesses strong industrial application value and market prospects. Furthermore, the process parameters of this invention are controllable, the operation is stable, and the repeatability is good, making it suitable for large-scale continuous industrial production.

[0070] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a polyurethane-modified acrylic waterborne resin, characterized in that, The preparation method includes the following steps: Step (1): After mixing the polyurethane prepolymer and acrylate monomer in a dispersion medium, a pH-responsive block copolymer is added as a dispersion stabilizer, and a fine emulsion is formed by ultrasonic-assisted pre-emulsification. Step (2): Add an initiator solution to the fine emulsion and carry out a free radical graft copolymerization reaction under gradient heating conditions. The gradient heating conditions are: heating from a first temperature to a second temperature at a first heating rate, and then heating to a third temperature at a second heating rate. Step (3): After the copolymerization reaction is completed, the system is cooled to the fourth temperature, a neutralizing agent is added to adjust the pH of the system to the target value, and the dispersion medium is removed by vacuum distillation to obtain the polyurethane modified acrylic waterborne resin.

2. The method for preparing polyurethane-modified acrylic waterborne resin according to claim 1, characterized in that, In step (1), the preparation method of the polyurethane prepolymer is as follows: Polyether diol and polyester diol were added to a reaction vessel at a mass ratio of 1:(0.5-2) and dehydrated for 1.5-3 hours under vacuum conditions ≤-0.095MPa and temperature 100-120℃. After dehydration, the temperature is lowered to 65-85℃, and diisocyanate monomer, hydrophilic chain extender and catalyst are added. The mixture is stirred and reacted for 2-4 hours under nitrogen protection. The NCO content in the reaction system is monitored until it reaches the theoretical value to obtain the polyurethane prepolymer.

3. The method for preparing polyurethane-modified acrylic waterborne resin according to claim 2, characterized in that, The diisocyanate monomer is one or more combinations of isophorone diisocyanate, hexamethylene diisocyanate, dicyclohexylmethane diisocyanate, toluene diisocyanate or diphenylmethane diisocyanate; The hydrophilic chain extender is dimethylolpropionic acid or dimethylolbutyric acid; The catalyst is dibutyltin dilaurate or stannous octoate, and the amount of catalyst added is 0.05%-0.5% of the mass of the diisocyanate monomer.

4. The method for preparing polyurethane-modified acrylic waterborne resin according to claim 2, characterized in that, The polyether diol is polyethylene glycol, polypropylene glycol or polytetrahydrofuran ether diol, with a number average molecular weight of 800-4000. The polyester diol is polyethylene adipate diol, polybutylene adipate diol or polycaprolactone diol, with a number average molecular weight of 1000-4000. The total mass ratio of the polyether diol and polyester diol to the diisocyanate monomer is 1:(0.8-2.5).

5. The method for preparing polyurethane-modified acrylic waterborne resin according to claim 1, characterized in that, In step (1), the acrylate monomer is a combination of hard monomers, soft monomers and functional monomers; The hard monomer is one or more of methyl methacrylate, styrene, acrylonitrile, or isobornyl methacrylate. The soft monomer is one or more of butyl acrylate, isooctyl acrylate, ethyl acrylate or lauryl acrylate; The functional monomer is one or more of acrylic acid, methacrylic acid, hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate, or glycidyl methacrylate; The mass ratio of the hard monomer, soft monomer and functional monomer is (20-50):(30-60):(2-15).

6. The method for preparing polyurethane-modified acrylic waterborne resin according to claim 1, characterized in that, In step (1), the pH-responsive block copolymer is polyacrylic acid-b-polystyrene, polymethacrylic acid-b-polybutyl acrylate or polyacrylamide-b-polystyrene, with a number average molecular weight of 2000-15000. The dispersion medium is deionized water or a mixture of deionized water and ethanol. The mass ratio of the polyurethane prepolymer to the acrylate monomer is 1:(0.5-4). The amount of pH-responsive block copolymer added is 1%-10% of the total mass of polyurethane prepolymer and acrylate monomer; The mass ratio of the dispersion medium to the total mass of the polyurethane prepolymer and acrylate monomer is (1-3):1; The ultrasonic power for the ultrasonic-assisted pre-emulsification is 200-800W, and the ultrasonic time is 5-30min.

7. The method for preparing polyurethane-modified acrylic waterborne resin according to claim 1, characterized in that, In step (2), the initiator solution is an aqueous solution of ammonium persulfate, an aqueous solution of potassium persulfate, or an aqueous solution of azobisisobutylamidine hydrochloride, and the concentration of the initiator solution is 1%-10%. The initiator solution is added over a period of 1-4 hours, and the total amount of initiator added is 0.3%-3% of the total mass of the acrylate monomers. The first temperature is 40-60℃, the second temperature is 65-75℃, and the third temperature is 80-90℃; The first heating rate is 0.5-2℃ / min, and the second heating rate is 0.2-1℃ / min; After holding at the second temperature for 0.5-1.5 hours, the temperature is increased to the third temperature at the second heating rate, and then held at the third temperature for 1-3 hours to mature.

8. The method for preparing polyurethane-modified acrylic waterborne resin according to claim 1, characterized in that, In step (3), the fourth temperature is 30-50℃; The neutralizing agent is triethylamine, ammonia, dimethylethanolamine, or triethanolamine; The target pH value is 6.5-8.5; The amount of neutralizing agent added is 80%-120% of the theoretical amount required to bring the pH value of the system to the target value; The vacuum distillation temperature is 35-50℃, the vacuum degree is ≤-0.08MPa, and the distillation time is 0.5-2h.

9. The method for preparing polyurethane-modified acrylic waterborne resin according to claim 1, characterized in that, The preparation method further includes the following steps after step (3): The resin emulsion after vacuum distillation is cooled to room temperature, filtered through a 200-800 mesh filter, and then discharged. The solid content of the resin emulsion is 25%-50%.

10. A polyurethane-modified acrylic waterborne resin, characterized in that, The polyurethane-modified acrylic waterborne resin is prepared by the preparation method according to any one of claims 1-9.

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