Aqueous polyurethane dispersion composition, aqueous polyurethane dispersion, method of preparation and use

Abstract

This invention relates to an aqueous polyurethane dispersion composition, an aqueous polyurethane dispersion, a preparation method, and applications. The composition comprises: 15-25 parts of a polyether soft segment polyol, 10-20 parts of a polycarbonate diol, 18-26 parts of an aliphatic or alicyclic isocyanate, 3-5 parts of a carboxyl-containing hydrophilic chain extender, 2-5 parts of ethoxylated trimethylolpropane, 0.3-1.0 parts of a phosphate-containing diol, 0.3-1.0 parts of a urea derivative chain extender, 1.5-3 parts of an organic amine neutralizer, 0.01-3 parts of a catalyst, and 25-40 parts of deionized water. The ethoxylated trimethylolpropane forms a branched structure encapsulating ester bonds, with the phosphate ester groups located at the chain ends, improving aluminum powder orientation. The urea derivative forms a reversible hydrogen bond network, balancing thixotropy. Compared with existing technologies, this invention solves the problems of poor hydrolysis resistance, insufficient aluminum powder orientation, and difficulty in balancing thixotropy in aqueous polyurethane.

Description

Technical Field

[0001] This invention relates to the field of waterborne polyurethane resin technology, and in particular to a waterborne polyurethane dispersion composition, a waterborne polyurethane dispersion, a preparation method, and applications. Background Technology

[0002] As the automotive industry rapidly moves towards low VOCs, environmental friendliness, and high aesthetic quality, waterborne metallic effect coatings are gradually replacing traditional solvent-based systems. Waterborne polyurethane, due to its excellent flexibility, adhesion, and controllable mechanical properties, has become an important film-forming resin for high-performance waterborne metallic paints.

[0003] Currently, waterborne polyurethanes in metal coatings typically employ polyester-based or polyether-based soft-segment structures. Polyester-based waterborne polyurethanes are prepared by reacting polyols (such as polybutylene adipate) with isocyanates, exhibiting good mechanical properties and adhesion, but are prone to hydrolysis in humid and hot environments. Polyether-based waterborne polyurethanes are prepared by reacting polyether polyols (such as polytetrahydrofuran diol) with isocyanates, demonstrating better hydrolysis resistance than polyester-based types, but with relatively weaker mechanical properties.

[0004] Chinese patent CN111333802A discloses a waterborne polyurethane dispersion, its preparation method, and its application. By controlling the ratio of polyisocyanate compound, macromolecular diol compound, small molecule diol compound containing acidic groups, chain extender, and polyol end-capping agent, the thixotropic index of the waterborne coating during room temperature flash-drying or low-temperature heating flash-drying stages is increased, thereby improving the directional alignment ability of aluminum powder in the waterborne coating. However, this method does not systematically solve the balance problem between hydrolysis resistance and thixotropy at the molecular structure level, and its long-term stability under humid and hot environments still needs further improvement.

[0005] Another approach, disclosed in Chinese patent CN111320928A, is a high-gloss, high-DOI waterborne polyurethane coating that improves gloss and reflectivity by optimizing the formulation and process parameters. While this approach can improve the appearance quality of the coating, it rarely addresses the synergistic improvement of hydrolysis resistance, aluminum powder orientation, and thixotropy at the molecular design level, thus limiting its application in high-end waterborne automotive metallic paints.

[0006] Furthermore, metallic pigments (especially aluminum flakes) are prone to surface oxidation, insufficient orientation, or disordered arrangement in water-based systems, making it difficult to achieve the high brilliance, clarity, and stable flip-flop effect required for high-end automotive paints. Conventional waterborne polyurethanes lack a controllable reversible hydrogen bond network in their chain structure, making it difficult to simultaneously achieve a balance between stability and thixotropy. This makes it difficult to achieve shear thinning and rapid recovery of thixotropic behavior during spraying, and to lock the arrangement of the metallic flakes.

[0007] Therefore, a new technical solution is needed that can simultaneously improve the hydrolysis resistance, aluminum powder orientation, and thixotropic properties of waterborne polyurethane through molecular structure design, so as to meet the comprehensive requirements of high-end waterborne automotive metallic paints for appearance, high durability, and application compatibility. Summary of the Invention

[0008] The purpose of this invention is to provide an aqueous polyurethane dispersion composition, an aqueous polyurethane dispersion, a preparation method, and an application, in order to solve the technical problems existing in the application of waterborne polyurethane in metal coatings, such as poor hydrolysis resistance, insufficient aluminum powder orientation, and difficulty in balancing thixotropy.

[0009] The objective of this invention can be achieved through the following technical solutions: The first aspect of this invention provides an aqueous polyurethane dispersion composition, comprising: 15-25 parts by weight of polyether-type soft segment polyol; 10-20 parts by weight of polycarbonate diol; 18-26 parts by weight of aliphatic or alicyclic isocyanates; 3-5 parts by weight of a carboxyl-containing hydrophilic chain extender; 2-5 parts by weight of ethoxylated trimethylolpropane; 0.3-1.0 parts by weight of diols containing phosphate ester groups; 0.3-1.0 parts by weight of urea derivative chain extender; Organic amine neutralizer, 1.5-3 parts by weight; Catalyst: 0.01-3 parts by weight; 25-40 parts by weight of deionized water; The ethoxylated trimethylolpropane in the composition constitutes a branched structure that encapsulates the ester bonds introduced by the polycarbonate diol, the phosphate ester group of the phosphate-containing diol is located at the chain end, and the urea derivative chain extender forms a reversible hydrogen bond network between the waterborne polyurethane molecular chains.

[0010] The ethoxylated trimethylolpropane provides a branched structure for the waterborne polyurethane. This branched structure works synergistically with the polycarbonate diol segments to increase steric hindrance, thereby improving hydrolysis resistance.

[0011] Furthermore, the polyether-type soft segment polyol is polytetrahydrofuran diol with a molecular weight of 800-1200; The polycarbonate diol has a molecular weight of 1000-2000.

[0012] Furthermore, the aliphatic or alicyclic isocyanate is isophorone diisocyanate; The catalyst is dibutyltin dilaurate; The organic amine neutralizer is dimethylethanolamine; The urea derivative chain extender is a dimethylurea derivative.

[0013] Furthermore, the carboxyl-containing hydrophilic chain extender is dimethylolpropionic acid, the phosphate-containing diol is dihydroxyphosphate, and the urea derivative chain extender is a dimethylurea derivative.

[0014] Furthermore, the hydroxyl value of the ethoxylated trimethylolpropane is 150-200 mg KOH / g.

[0015] A second aspect of the present invention provides a method for preparing an aqueous polyurethane dispersion, wherein the aqueous polyurethane dispersion is prepared using the above-described aqueous polyurethane dispersion composition as a raw material, comprising the following steps: (1) Under the protection of an inert gas, polyether-type soft segment polyol, polycarbonate diol, hydrophilic chain extender containing carboxyl groups and catalyst are mixed and heated to 60-90℃ to melt uniformly; (2) Under stirring, add aliphatic or alicyclic isocyanate to the mixture in step (1) and react at 70-100℃ until the NCO content of the system reaches 2.5-5.0wt% to obtain NCO-terminated polyurethane prepolymer. (3) Ethoxylated trimethylolpropane is added in segments to the prepolymer of step (2) and the reaction is continued at 50-80°C to form a branched structure that covers ester bonds; (4) Cool the system of step (3) to 50-80°C, add an organic solvent to reduce viscosity, then cool to 50-70°C, add a diol containing phosphate ester group to carry out a capping reaction, so as to introduce the diol containing phosphate ester group as a capping agent, so that its phosphate ester group is located at the chain end or branching point end of the branched structure. (5) Cool the system from step (4) to 40-60°C and add an organic amine neutralizer to carry out a neutralization reaction; (6) Under high-speed shearing, the neutralization product of step (5) is added to deionized water at 15-35°C to form a polyurethane emulsion. (7) The emulsion from step (6) was subjected to desolvation under reduced pressure at 35-60°C and -0.05 to -0.10 MPa until the residual organic solvent was less than 1.0 wt%. (8) Cool the desolventized emulsion from step (7) to 20-40°C, add a urea derivative chain extender to carry out a chain growth reaction, so as to form a reversible hydrogen bond network between the waterborne polyurethane molecular chains, and obtain the waterborne polyurethane dispersion.

[0016] Furthermore, an organotin catalyst was added in step (1) at a dosage of 0.01-1 parts by weight.

[0017] Further, in step (2), the NCO / OH molar ratio of the aliphatic or cycloaliphatic isocyanate to the polyether-type soft segment polyol, polycarbonate diol, and carboxyl-containing hydrophilic chain extender is 1.05-1.15; The diol containing the phosphate ester group is a dihydroxyphosphate ester, and the urea derivative chain extender is a dimethylurea derivative. The urea derivative chain extender mentioned in step (8) is added in the form of a dilute solution with a concentration of 10-20 wt%.

[0018] Optionally, in step (8), since the emulsification in step (6) and the descaling process in step (7) consume some NCO groups, by controlling the NCO content in step (2) within a specified range, a small amount of active NCO groups can be ensured to remain in the emulsion after descaling. The active hydroxyl groups on the added dimethyl urea derivative (e.g., N,N'-dihydroxymethyl-1,3-dimethylurea) react with these residual NCO groups and chemically bond to the polyurethane molecular chain. The urea groups in its structure form a dense, reversible hydrogen bond network between the molecular chains.

[0019] A third aspect of the present invention provides an aqueous polyurethane dispersion prepared by the method described above.

[0020] A fourth aspect of the present invention provides an application of the aqueous polyurethane dispersion as described above in the preparation of aqueous metallic paint.

[0021] Compared with the prior art, the present invention has the following beneficial effects: 1) By constructing a multi-segment soft segment system (polyether-type soft segment polyol, polycarbonate diol, and ethoxylated trimethylolpropane) and adopting a two-stage polymerization process (first synthesizing NCO-terminated prepolymer, then grafting ethoxylated trimethylolpropane to form a branched structure), the branched structure of ethoxylated trimethylolpropane is used to coat the ester bonds, thereby improving the hydrolysis resistance of waterborne polyurethane. After 40℃×240h of humid heat, the adhesion remains at grade 0-1, which is 2-3 grades higher than that of conventional waterborne polyurethane (adhesion grade 3-4). 2) By introducing a diol containing phosphate ester groups as a functional monomer and confining it to the end or side chain position, the phosphate ester groups generate strong chemical adsorption or complexation with the oxide layer on the surface of aluminum powder, promoting the alignment of aluminum powder parallel to the substrate during the coating drying process, improving the orientation of aluminum powder, increasing the L value from 65 to 78 (an increase of 20%), and increasing the 60° gloss by 28%, achieving high scintillation and stable flip-flop effect; 3) By introducing urea derivative chain extenders, a urea bond hydrogen bond network is formed. In the static state, the hydrogen bond network increases the viscosity of the system. Under the action of shear force, the hydrogen bonds break and the viscosity decreases, giving the coating thixotropic properties. The 3ITT thixotropic test of the rheometer shows that the viscosity recovers to 95% in 30 seconds (15-25 percentage points higher than the 70-80% of conventional waterborne polyurethane). Shear thinning is achieved during the spraying process and rapid recovery after stopping, locking the metal sheet arrangement and preventing aluminum powder from settling. 4) Excellent overall performance, with adhesion reaching grade 0, flexibility of 1mm (50-67% higher than conventional waterborne polyurethane 2-3mm), water resistance without foaming for 240h (33-100% higher than conventional waterborne polyurethane 120-180h), 60° gloss reaching 96 (20-37% higher than conventional waterborne polyurethane 70-80), and storage stability with viscosity increase ≤15% (50°C×2 weeks, 20-30% improvement in storage stability compared to conventional waterborne polyurethane), meeting the comprehensive requirements of high-end waterborne automotive metallic paints for appearance, high durability, and application compatibility. Detailed Implementation

[0022] The following detailed description is provided in conjunction with specific embodiments. It should be noted that, in the description of the embodiments of the present invention, the term polyether-type soft segment polyol refers to polytetrahydrofuran diol or other polyether polyols with a molecular weight of 800-1200; the term polycarbonate diol refers to polycarbonate diol with a molecular weight of 1000-2000; the term ethoxylated trimethylolpropane refers to trimethylolpropane polyoxyethylene ether with a hydroxyl value of 150-200 mgKOH / g; the term phosphate-containing diol refers to dihydroxyphosphate ester or other phosphate-containing diols; and the term urea derivative chain extender refers to dimethyl urea derivative or other urea derivative chain extenders.

[0023] Example 1 The aqueous polyurethane dispersion composition adopts a multi-segment soft segment system (polyether-type soft segment polyol, polycarbonate diol, ethoxylated trimethylolpropane) and introduces a phosphate ester-containing diol as an aluminum powder orientation functional monomer and a urea derivative chain extender as a thixotropic functional monomer. Through a two-stage polymerization process (first synthesizing an NCO-terminated prepolymer, then grafting ethoxylated trimethylolpropane to form a branched structure), the ethoxylated trimethylolpropane is used to coat the ester bonds, thereby improving hydrolysis resistance, aluminum powder orientation and thixotropy.

[0024] The aqueous polyurethane dispersion composition comprises the following components (by weight): 20 parts of polytetrahydrofuran diol (PTMG-1000), molecular weight 1000, analytical grade, purity 99.0%, purchased from Shandong Yinuowei Polyurethane Co., Ltd.; 15 parts of polycarbonate diol (PCDL), molecular weight 1000-2000, analytical grade, purity 99.0%, purchased from Wanhua Chemical Group Co., Ltd.; 22 parts of isophorone diisocyanate (IPDI), analytical grade, purity 99.5%, purchased from Covestro Polymers (China) Co., Ltd.; 4 parts of dimethylolpropionic acid (DMPA), analytical grade, purity 98.0%, purchased from Aladdin Reagent Company; 3 parts of ethoxylated trimethylolpropane (TMP-EO), hydroxyl value 150-200 mgKOH / g, purchased from BASF (China) Co., Ltd.; and di(2-hydroxyethyl) phosphate (Bis(2-hydroxyethyl) ester). 0.6 parts of phosphate (analytical grade, purity 97.0%) were purchased from Shanghai Maclean Biochemical Technology Co., Ltd.; (HO-CH2-CH2-O-)2P(=O)-OH; 0.6 parts of dimethyl urea derivative (DMU), analytical grade, purity 98.0%, were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.; 2 parts of dimethylethanolamine (DMEA), analytical grade, purity 99.0%, were purchased from Sinopharm Chemical Reagent Co., Ltd.; 0.2 parts of dibutyltin dilaurate (DBTDL), analytical grade, purity 95.0%, were purchased from Shanghai Maclean Biochemical Technology Co., Ltd.; 30.8 parts of deionized water, conductivity <2μS / cm, were prepared in the laboratory.

[0025] Component Description: Polytetrahydrofuran diol (PTMG-1000) is a polyether-type soft-segment polyol with a molecular weight of 1000. It provides flexibility and hydrolysis resistance. The ether bonds (COC) in the polyether segments have higher hydrolytic stability compared to the ester bonds (COC=O) in the polyester segments, making it less prone to hydrolytic chain breakage under humid and hot environments. Polycarbonate diol (PCDL) is a polycarbonate-type soft-segment polyol with a molecular weight of 1000-2000. It provides hydrolysis resistance and mechanical properties. The carbonate bonds (OC=OO) in the polycarbonate segments have higher hydrolytic stability and mechanical strength compared to ordinary ester bonds. Isophorone diisocyanate (IPDI) is an alicyclic isocyanate that constructs a hard-segment backbone, providing mechanical strength and weather resistance. The alicyclic structure has better weather resistance and yellowing resistance compared to aromatic isocyanates (such as TDI and MDI). Dimethylolpropionic acid (DMPA) is a hydrophilic chain extender containing carboxyl groups. After neutralization, the carboxyl groups form carboxylate ions, providing electrostatic stability and enabling water-based polymerization. Ethoxylated trimethylolpropane (TMP-EO) is a branched functional monomer with a hydroxyl value of 150-200 mgKOH / g. Its three hydroxyl groups react with the NCO groups of the prepolymer to form star-shaped or branched structures. The long ethoxylated segments provide steric hindrance, encapsulating the internal ester bonds, reducing the attack pathways of water molecules on the ester bonds, and improving hydrolysis resistance. Bis(2-hydroxyethyl) phosphate is a diol containing phosphate ester groups. Each molecule has an active hydroxyl group at both ends, which can react with the NCO groups of the prepolymer, thereby introducing phosphate ester groups (-PO(OH)2) into the polyurethane molecular chain or connecting them between branched segments. Its phosphate groups can be exposed on the side ends of the molecular chain, generating strong chemical adsorption or complexation with the oxide layer (Al2O3) on the surface of aluminum powder, promoting the alignment of aluminum powder parallel to the substrate during coating drying and improving metallic luster. Dimethyl urea derivative (DMU) is a urea derivative chain extender containing a large number of urea groups (-NH-CO-NH-) and hydroxyl groups. A strong intermolecular hydrogen bond network (NH···O=C) is formed between the urea groups. In the static state, the hydrogen bond network increases the viscosity of the system, and under shear force, the hydrogen bonds break and the viscosity decreases, giving the coating thixotropic properties. Dimethylethanolamine (DMEA) is an organic amine neutralizer that neutralizes the carboxyl group of dimethylolpropionic acid to form a carboxylate ion, achieving waterborne properties. Dibutyltin dilaurate (DBTDL) is an organotin catalyst that catalyzes the reaction between isocyanate groups (-NCO) and hydroxyl groups (-OH), accelerating the reaction rate. Deionized water is used as the dispersion medium, serving as the continuous phase of the waterborne polyurethane.

[0026] The solid content is 39%. The choice of solid content is based on a balance between the coating's application performance and storage stability: if the solid content is too low, the coating viscosity is low, making it prone to sagging during spraying and resulting in insufficient film thickness; if the solid content is too high, the coating viscosity is high, making spraying difficult and prone to sedimentation during storage. At a solid content of 39%, the coating viscosity is moderate (approximately 1000-1500 mPa·s, 25℃), with good spraying performance, uniform film thickness, and excellent storage stability.

[0027] The aqueous polyurethane dispersion composition formulated with the above components achieves a balance of hydrolysis resistance, flexibility, and mechanical properties through the synergistic effect of a multi-segment soft-segment system (PTMG, PCDL, TMP-EO). PTMG provides flexibility and basic hydrolysis resistance, PCDL provides mechanical strength and enhanced hydrolysis resistance, and TMP-EO further improves hydrolysis resistance by encapsulating ester bonds through a branched structure. The phosphate-containing diol (HEP) is confined to the end or side chain position, and the chemisorption of the phosphate groups with the oxide layer on the aluminum powder surface promotes the directional alignment of the aluminum powder. The urea derivative chain extender (DMU) forms a urea bond hydrogen bond network that provides thixotropy, enabling shear thinning during spraying and rapid recovery after stopping, locking the metal sheet alignment and preventing aluminum powder sedimentation.

[0028] Example 2 This embodiment provides a method for preparing an aqueous polyurethane dispersion. Using the formulation of Example 1, an aqueous polyurethane dispersion is prepared through a two-stage polymerization process (first synthesizing an NCO-terminated prepolymer, then grafting ethoxylated trimethylolpropane to form a branched structure), HEP end-capping reaction, and DMU chain extension reaction.

[0029] Under an inert gas atmosphere, 20 parts of polytetrahydrofuran glycol (PTMG-1000), 15 parts of polycarbonate glycol (PCDL), and 4 parts of dimethylolpropionic acid (DMPA) were added to a three-necked flask equipped with a mechanical stirrer, thermometer, and reflux condenser. Simultaneously, 0.2 parts of dibutyltin dilaurate (DBTDL) were added as a catalyst. Nitrogen gas was applied (nitrogen flow rate 100 mL / min), and the mixture was heated to 70-80°C at a heating rate of 5°C / min until PTMG, PCDL, and DMPA were completely melted and homogeneous, yielding a transparent, pale yellow liquid. The melting temperatures of PTMG, PCDL, and DMPA are approximately 40°C, 60°C, and 80°C, respectively. Heating to 70-80°C resulted in complete melting and homogeneous mixing, forming a homogeneous liquid. The DBTDL catalyst dissolved at this temperature, preparing for subsequent reactions.

[0030] Under stirring (300 rpm), 22 parts of isophorone diisocyanate (IPDI) were slowly added dropwise to the reactor at a rate of 2 parts / hour, maintaining the temperature at 70°C during the addition. After the addition was complete, the temperature was raised to 80°C, and the reaction continued for 2 hours until the NCO content of the prepolymer reached 3.0 wt% (determined by di-n-butylamine titration), yielding an NCO-terminated polyurethane prepolymer. The NCO / OH molar ratio of IPDI to PTMG, PCDL, and DMPA was 1.10, i.e., 10% excess isocyanate groups, ensuring complete reaction of all hydroxyl groups while reserving a certain amount of NCO groups for subsequent grafting reactions. During the reaction, the isocyanate groups (-NCO) of IPDI underwent nucleophilic addition reactions with the hydroxyl groups (-OH) of PTMG, PCDL, and DMPA to form urethane bonds (-NH-COO-), with the reaction equation: R-NCO + R'-OH → R-NH-COO-R'. The reaction is exothermic, with an enthalpy change of approximately -100 kJ / mol. Therefore, it is necessary to control the dropping rate and reaction temperature to avoid excessively high temperatures that could lead to side reactions (such as biuret reaction and isocyanate trimerization). The reaction endpoint is determined by measuring the NCO content of the prepolymer. When the NCO content reaches 3.0 wt%, it indicates that the reaction is essentially complete and the molecular weight of the prepolymer has reached the predetermined range.

[0031] Three parts of ethoxylated trimethylolpropane (TMP-EO) were added to the prepolymer in segments, taking care to avoid gelation (addition rate: 0.5 parts / hour). The reaction was continued at 60-70℃ for 1-2 hours to form a branched structure. The three hydroxyl groups of TMP-EO reacted with the NCO groups of the prepolymer to form star-shaped or branched structures. During the reaction, the hydroxyl groups of TMP-EO underwent nucleophilic addition reactions with the NCO groups of the prepolymer to form urethane bonds, grafting TMP-EO to the end or side chain positions of the prepolymer. The ethoxylated long-chain segments (polyoxyethylene segments) of TMP-EO provided steric hindrance, encapsulating the internal ester bonds (from PCDL), reducing the attack path of water molecules on the ester bonds, and improving hydrolysis resistance. The addition rate of TMP-EO needs to be strictly controlled to avoid gelation: the three hydroxyl groups of TMP-EO are highly reactive; if the addition rate is too fast, multiple prepolymer molecules will crosslink through TMP-EO, forming a three-dimensional network structure (gel) and losing fluidity. Therefore, the feeding rate is controlled at 0.5 parts / hour to allow TMP-EO to be gradually grafted onto the prepolymer, avoiding excessive crosslinking.

[0032] The prepolymer was cooled to 60°C, and a predetermined amount of acetone (30% of the prepolymer mass) was added. The mixture was stirred to reduce the viscosity of the system. Acetone, being an organic solvent, lowers the viscosity of the prepolymer, facilitating the uniform reaction of the subsequent addition of the polyfunctional alcohol (HEP). The viscosity of the prepolymer at 60°C was approximately 6000 mPa·s; after adding acetone, the viscosity decreased to 1500 mPa·s, making stirring easier and allowing for uniform dispersion of the HEP.

[0033] The system was cooled to 55°C, and 0.6 parts of di(2-hydroxyethyl) phosphate (Bis(2-hydroxyethyl) phosphate) were added for end-capping reaction, with a reaction time of 0.5 hours. The two hydroxyl groups of HEP react with the NCO groups of the prepolymer, confining HEP to the end of the chain, while the phosphate groups are exposed on the molecular chain surface for subsequent chemisorption with the aluminum powder oxide layer. The HEP end-capping reaction temperature was controlled at 55°C; excessively high temperatures may cause HEP decomposition (the thermal stability of phosphate bonds is lower than that of urethane bonds), while excessively low temperatures result in a slow reaction rate. During the reaction, the hydroxyl groups of HEP undergo nucleophilic addition with the NCO groups of the prepolymer to form urethane bonds, fixing HEP at the end of the prepolymer chain. The phosphate groups of HEP (-PO(OR)2) do not participate in the reaction, remaining on the molecular chain surface for subsequent interaction with the aluminum powder.

[0034] The system was cooled to 45°C, and 2 parts of dimethylethanolamine (DMEA) were added to neutralize the carboxyl group of dimethylolpropionic acid (DMPA). The neutralization reaction took 10-15 minutes. DMEA is an organic amine, which undergoes an acid-base neutralization reaction with the carboxyl group of DMPA to form an ammonium carboxylate salt. The reaction equation is: R-COOH + R'-N(CH3)2 → R-COO - + R'-NH + (CH3)2. Ammonium carboxylate salts ionize in water to form carboxylate ions (R-COO-). - ) and ammonium ions (R'-NH + (CH3)2), the carboxylate ions provide electrostatic stabilization, enabling the prepolymer to form a stable dispersion in water. The degree of neutralization is controlled at 90%, that is, the amount of DMEA is 0.9 molar equivalents of the DMPA carboxyl groups. If the neutralization is too low, the prepolymer will not be hydrophilic enough and will be difficult to disperse; if the neutralization is too high, excessive DMEA will remain in the system, affecting the coating performance.

[0035] Under high-speed shear conditions (shear rate 3000-5000 rpm), the neutralized prepolymer was slowly added to 30.8 parts of deionized water (at 25°C) at a feeding rate of 6 parts / hour, forming a stable polyurethane emulsion after shearing. High-speed shearing disperses the prepolymer into tiny droplets (100-300 nm in diameter). The carboxylate ions on the droplet surface provide electrostatic repulsion, preventing droplet aggregation and forming a stable aqueous dispersion. During emulsification, the NCO groups of the prepolymer react with water to form urea bonds (-NH-CO-NH-), with the reaction equations: R-NCO + H2O → R-NH2 + CO2 ↑, R-NH2 + R'-NCO → R-NH-CO-NH-R'. The formation of urea bonds further increases the molecular weight of the prepolymer while releasing carbon dioxide gas. The emulsification temperature should be controlled between 20-30℃. If the temperature is too high, the reaction rate between the NCO groups and water will be too fast, producing a large amount of carbon dioxide gas, which will cause the emulsion to foam. If the temperature is too low, the viscosity of the prepolymer will be high, making shearing difficult.

[0036] The emulsion was subjected to vacuum solvent removal at 45℃ and -0.08 MPa for 2-3 hours until the acetone residue decreased to below 0.5 wt% (determined by gas chromatography). Vacuum solvent removal removes the organic solvent (acetone) from the emulsion, improving environmental friendliness and reducing VOC emissions. The solvent removal temperature was controlled at 45℃; excessively high temperatures may lead to thermal degradation of the polyurethane molecular chains, while excessively low temperatures result in a slow solvent removal rate. The vacuum level was controlled at -0.08 MPa; excessively high vacuum levels will cause water to evaporate, leading to an increase in solid content; excessively low vacuum levels result in low solvent removal efficiency. The solvent removal endpoint was determined by measuring the acetone residue; when the acetone residue decreased to below 0.5 wt%, the solvent removal was considered essentially complete.

[0037] The solvent-free emulsion was cooled to 30°C, and 0.6 parts of a dilute solution of dimethyl urea derivative (DMU) (DMU dissolved in deionized water, concentration 15 wt%) were added to induce slow chain growth at a feeding rate of 0.15 parts / hour. Stirring continued for 1-2 hours until the chain extension reaction was complete, yielding the final aqueous polyurethane dispersion. The hydroxyl groups of DMU react with the residual NCO groups in the prepolymer to form urethane bonds, while the urea groups (-NH-CO-NH-) of DMU form an intermolecular hydrogen bond network, providing thixotropy. The chain extension reaction temperature was controlled at 30°C; too high a temperature resulted in an excessively fast reaction rate, making it difficult to control the molecular weight; too low a temperature resulted in a slow reaction rate. The feeding rate of DMU needed to be strictly controlled to avoid an excessively wide molecular weight distribution: the hydroxyl groups of DMU are highly reactive; if the feeding rate is too fast, some molecular chains will grow rapidly while others grow slowly, resulting in a widened molecular weight distribution that affects coating performance. Therefore, the feeding rate was controlled at 0.15 parts / hour to allow DMU to react gradually with the prepolymer, resulting in a uniform molecular weight distribution. After the chain extension reaction is completed, the final aqueous polyurethane dispersion is obtained with a solid content of 39%, a viscosity of 1200 mPa·s (25℃), a pH value of 8.0, and a particle size of 150-250 nm (determined by dynamic light scattering method).

[0038] The aqueous polyurethane dispersion obtained by the above preparation method achieved TMP-EO coating of ester bonds and improved hydrolysis resistance through a two-stage polymerization process (first synthesizing an NCO-terminated prepolymer, then grafting TMP-EO to form a branched structure). The ethoxylated long chain segment of TMP-EO provides steric hindrance, coating the internal ester bonds (from PCDL) and reducing the attack path of water molecules on the ester bonds. According to the hydrolysis reaction mechanism, the ester bond undergoes a nucleophilic substitution reaction in water, where water molecules attack the carbonyl carbon of the ester bond to form a tetrahedral intermediate, followed by ester bond cleavage to generate carboxylic acids and alcohols. The steric hindrance effect of TMP-EO makes it difficult for water molecules to approach the ester bonds, increasing the activation energy of the hydrolysis reaction from approximately 80 kJ / mol in conventional polyurethanes to approximately 120 kJ / mol (based on literature data and DFT calculations), and reducing the hydrolysis rate constant by approximately 100 times (calculated according to the Arrhenius equation k=Ae^(-Ea / RT)), thus improving hydrolysis resistance. HEP is confined to the end of the chain through the HEP end-capping reaction. The phosphate ester groups of HEP undergo strong chemical adsorption or complexation with the aluminum powder surface oxide layer (Al2O3). The phosphorus atoms in the phosphate ester groups form coordinate bonds (PO-Al) with aluminum atoms. The bond energy of the coordinate bonds is about 300-400 kJ / mol, which is much stronger than van der Waals forces (bond energy about 5-20 kJ / mol). Therefore, the aluminum powder is oriented parallel to the substrate by HEP during the coating drying process, improving the metallic luster. A urea bond hydrogen bond network is formed through the DMU chain extension reaction. The hydrogen bond energy between urea groups (NH···O=C) is about 20-40 kJ / mol. In the static state, the hydrogen bond network increases the viscosity of the system. Under shear force, the hydrogen bonds break and the viscosity decreases, giving the coating thixotropic properties.

[0039] The above-mentioned waterborne polyurethane dispersion was used to prepare a waterborne metallic topcoat. The formulation and performance testing were carried out according to the formula in section 4.2 and the board-making process in section 4.3 of the disclosure document. The formulation of the waterborne metallic topcoat was as follows: 40 parts waterborne polyurethane dispersion (39% solid content), 12 parts waterborne acrylic emulsion, 5 parts methylated amino resin, 5 parts waterborne directional wax paste, 3 parts co-solvent, 3 parts DMEA aqueous solution (10%), 5 parts functional additives, 5 parts aluminum powder, 5 parts co-solvent, 14 parts deionized water, and 3 parts colorant. The board-making process was as follows: the raw materials were added in sequence and stirred evenly. An appropriate amount of deionized water was added to adjust the viscosity. The mixture was filtered through a 400-mesh silk cloth and set aside. First, B1 color paint was sprayed, flash-drying at 80% for 3-5 minutes, then B2 color paint was sprayed, flash-drying at 80% for 3-5 minutes, and finally 2K clear varnish was sprayed. After leveling, the mixture was baked at 140℃ for 25-35 minutes to complete the board-making process.

[0040] The coating performance test results are as follows: Appearance is smooth and even (visual inspection); adhesion grade 0 (GB / T 9286-98); flexibility 1mm (GB / T 1731-93); water resistance without bubbling for 240h (GB / T 1733-93); 60° gloss 96 (BYK gloss meter); storage stability with viscosity increase ≤15% and no deviation in film performance (50°C × 2 weeks). Aluminum powder orientation test results are: L value 78 (20% higher than the conventional waterborne polyurethane L value of 65), 60° gloss increased by 28% (compared to the conventional waterborne polyurethane benchmark). Thixotropic test results are: 3ITT rheometer thixotropic test shows that at shear rates from 1000s... -1 Reduced to 0.1s -1 During the process, the viscosity increased from 100 mPa·s to 10000 mPa·s, with a shear-thinning ratio of 100 times. After shearing stopped, the viscosity recovered to 9500 mPa·s within 30 seconds, with a recovery rate of 95% (15-25 percentage points higher than the 70-80% recovery rate of conventional waterborne polyurethane). The hydrolysis resistance test results were as follows: after 40℃ for 240 hours of damp heat, the adhesion remained at grade 0-1 (2-3 grades higher than the 3-4 grades of conventional waterborne polyurethane adhesion).

[0041] The improved hydrolysis resistance of the waterborne polyurethane dispersion obtained by the above preparation method is due to the branched structure of TMP-EO coating the ester bonds, reducing the attack path of water molecules on the ester bonds, increasing the activation energy of the hydrolysis reaction, and decreasing the hydrolysis rate. According to hydrolysis kinetic experimental data, conventional waterborne polyurethanes under humid heat conditions of 40℃×240h exhibit an ester bond hydrolysis rate of approximately 30-40%, a molecular weight reduction of approximately 20-30%, and an adhesion decrease from grade 0 to grade 3-4; while the waterborne polyurethane of the present invention, under the same conditions, exhibits an ester bond hydrolysis rate of only 5-10%, a molecular weight reduction of approximately 5-10%, and an adhesion maintained at grade 0-1. The improved aluminum powder orientation is due to the formation of coordination bonds between the phosphate ester groups of HEP and the oxide layer on the surface of the aluminum powder. The bond energy of the coordination bonds is approximately 300-400 kJ / mol, which is much higher than the bond energy of van der Waals forces (5-20 kJ / mol). Therefore, the aluminum powder is oriented parallel to the substrate during the coating drying process due to the orientation effect of HEP. Based on aluminum powder orientation experiment data, the L value of conventional waterborne polyurethane is 65, with a 60° gloss level as the baseline; while the L value of the waterborne polyurethane of this invention is 78 (a 20% increase compared to conventional waterborne polyurethane), and the 60° gloss level is improved by 28%. The mechanism of improved thixotropy lies in the fact that the urea groups of DMU form an intermolecular hydrogen bond network, with a bond energy of approximately 20-40 kJ / mol. In the static state, the hydrogen bond network increases the system viscosity, and under shear force, the hydrogen bonds break, reducing the viscosity. According to rheological experiment data, the shear-thinning ratio of conventional waterborne polyurethane is approximately 10-20 times, and the viscosity recovery rate after shearing stops is 70-80%; while the shear-thinning ratio of the waterborne polyurethane of this invention is 100 times, and the viscosity recovery rate after shearing stops is 95%.

[0042] Comparative Example 1 This comparative example provides a formulation and preparation method for a conventional waterborne polyurethane to compare with the technical effects of the present invention.

[0043] The difference from Examples 1 and 2 is that ethoxylated trimethylolpropane (TMP-EO), di(2-hydroxyethyl) phosphate, and dimethyl urea derivative (DMU) are not used, while the other components and preparation methods remain unchanged.

[0044] The formulation of a conventional waterborne polyurethane is as follows (by weight): 25 parts polytetrahydrofuran diol (PTMG-1000), 10 parts polycarbonate diol (PCDL), 22 parts isophorone diisocyanate (IPDI), 4 parts dimethylolpropionic acid (DMPA), 2 parts dimethylethanolamine (DMEA), 2 parts dibutyltin dilaurate (DBTDL), and 35 parts deionized water. Compared with Example 1, the conventional waterborne polyurethane does not contain TMP-EO, HEP, or DMU. The amount of PTMG is increased from 20 parts to 25 parts, the amount of PCDL is decreased from 15 parts to 10 parts, and the amount of deionized water is increased from 30.8 parts to 35 parts to maintain a basically consistent solid content (approximately 38-40%).

[0045] The preparation method is as follows: Under inert gas protection, 25 parts of polytetrahydrofuran diol (PTMG-1000), 10 parts of polycarbonate diol (PCDL), and 4 parts of dimethylolpropionic acid (DMPA) are added to a reactor, along with 2 parts of dibutyltin dilaurate (DBTDL). The mixture is heated to 70°C to ensure uniform melting. While stirring, 22 parts of isophorone diisocyanate (IPDI) are slowly added dropwise to the reactor. The reaction is carried out at 80°C for 2-3 hours until the NCO content of the prepolymer reaches 3.0 wt%, yielding an NCO-terminated polyurethane prepolymer. The system is cooled to 45°C, and 2 parts of dimethylethanolamine (DMEA) are added to neutralize the dimethylolpropionic acid (DMPA). Under high-speed shear conditions, the neutralized prepolymer is slowly added to 35 parts of deionized water (at 20-30°C), forming a stable polyurethane emulsion after shearing. The emulsion was subjected to reduced pressure solvent removal at 40°C and -0.08 MPa (if an organic solvent was used) to obtain a conventional aqueous polyurethane dispersion.

[0046] A conventional waterborne polyurethane dispersion was used to prepare a waterborne metallic topcoat. The same formulation and slab preparation process as in Example 2 were used for slab preparation and performance testing. The coating performance test results are as follows: smooth and even appearance (visual inspection); adhesion grade 1-2 (GB / T 9286-98); flexibility 2-3 mm (GB / T 1731-93); water resistance (no bubbling) 120-180 h (GB / T 1733-93); 60° gloss 70-80 (BYK gloss meter); storage stability (viscosity increase 20-30%); film performance deviation (50°C × 2 weeks). Aluminum powder orientation test results: L value 65, based on 60° gloss. Thixotropic test results: 3ITT thixotropic testing with a rheometer showed that at shear rates from 1000 s⁻¹... -1 Reduced to 0.1s -1During the process, the viscosity increased from 100 mPa·s to 2000 mPa·s, with a shear-thinning ratio of 20 times; after shearing stopped, the viscosity recovered to 1400-1600 mPa·s within 30 seconds, with a recovery rate of 70-80%. The hydrolysis resistance test results were as follows: after 40℃ for 240 hours of damp heat, the adhesion decreased to level 3-4.

[0047] Compared to Example 2, Comparative Example 1 showed a decrease in adhesion by 1-2 grades (from grade 0 to grade 1-2), a decrease in flexibility by 50-67% (from 1 mm to 2-3 mm), a decrease in water resistance by 33-100% (from no bubbling for 240 hours to no bubbling for 120-180 hours), a decrease in 60° gloss by 20-37% (from 96 to 70-80), and a decrease in storage stability (viscosity increase from ≤15% to 20-30%). Aluminum powder orientation decreased by 20% (L value from 78 to 65), and 60° gloss decreased by 28%. Thixotropy decreased (shear-thinning ratio from 100 times to 20 times, viscosity recovery rate from 95% to 70-80%). Hydrolysis resistance decreased by 2-3 grades (adhesion decreased from grade 0-1 to grade 3-4 after 240 hours of humid heat).

[0048] The reduced adhesion is due to the fact that conventional waterborne polyurethane does not use TMP-EO to coat the ester bonds. These ester bonds hydrolyze under humid and hot conditions, leading to a decrease in molecular weight and a decline in the cohesive strength and adhesion of the coating. According to hydrolysis kinetics data, under humid and hot conditions of 40℃ for 240 hours, the hydrolysis rate of ester bonds in conventional waterborne polyurethane is approximately 30-40%, resulting in a molecular weight reduction of approximately 20-30%, and an adhesion grade decrease from 0 to 3-4. The reduced flexibility is due to the fact that the amount of PTMG in conventional waterborne polyurethane is increased to 25 parts, while the amount of PCDL is reduced to 10 parts. Although the total amount of soft segments is the same (35 parts), the mechanical strength of PCDL is higher than that of PTMG. The reduction in PCDL content leads to a decrease in the mechanical strength and flexibility of the coating. The reduced water resistance is due to the fact that ester bond hydrolysis reduces the density of the coating, allowing moisture to penetrate more easily and shortening the bubbling time. The reduced 60° gloss is due to the fact that conventional waterborne polyurethane does not use HEP to promote aluminum powder orientation. The aluminum powder becomes disordered during coating drying, resulting in a reduced metallic gloss. The reduced storage stability is due to the fact that conventional waterborne polyurethane does not use DMU to form a hydrogen bond network, resulting in a faster viscosity increase during storage and deviations in film performance. The reduced thixotropy is also due to the fact that conventional waterborne polyurethane does not use DMU to form a urea bond hydrogen bond network, leading to a lower shear thinning ratio and viscosity recovery rate. This results in easier aluminum powder settling during spraying and uneven coating appearance.

[0049] The experimental results of Comparative Example 1 show that ethoxylated trimethylolpropane (TMP-EO), di(2-hydroxyethyl) phosphate (Bis(2-hydroxyethyl) phosphate), and dimethyl urea derivative (DMU) play a key role in improving the hydrolysis resistance, aluminum powder orientation, and thixotropy of waterborne polyurethane. This invention constructs a multi-segment soft-segment system (PTMG, PCDL, TMP-EO) and employs a two-stage polymerization process (first synthesizing an NCO-terminated prepolymer, then grafting TMP-EO to form a branched structure). It introduces HEP phosphate functional monomers for aluminum powder orientation, introduces DMU chain extenders to form a urea-hydrogen bond network to provide thixotropy, and utilizes the TMP-EO branched structure to coat the ester bonds, achieving a synergistic improvement in hydrolysis resistance, aluminum powder orientation, and thixotropy, thus meeting the comprehensive requirements of high-end waterborne automotive metallic paints for appearance, high durability, and application compatibility.

[0050] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. An aqueous polyurethane dispersion composition, characterized in that, include: 15-25 parts by weight of polyether-type soft segment polyol; 10-20 parts by weight of polycarbonate diol; 18-26 parts by weight of aliphatic or alicyclic isocyanates; 3-5 parts by weight of a carboxyl-containing hydrophilic chain extender; 2-5 parts by weight of ethoxylated trimethylolpropane; 0.3-1.0 parts by weight of diols containing phosphate ester groups; 0.3-1.0 parts by weight of urea derivative chain extender; Organic amine neutralizer, 1.5-3 parts by weight; Catalyst: 0.01-3 parts by weight; 25-40 parts by weight of deionized water; The ethoxylated trimethylolpropane in the composition constitutes a branched structure that encapsulates the ester bonds introduced by the polycarbonate diol, the phosphate ester group of the phosphate-containing diol is located at the chain end, and the urea derivative chain extender forms a reversible hydrogen bond network between the waterborne polyurethane molecular chains.

2. The aqueous polyurethane dispersion composition according to claim 1, characterized in that, The polyether-type soft segment polyol is polytetrahydrofuran diol with a molecular weight of 800-1200. The polycarbonate diol has a molecular weight of 1000-2000.

3. The aqueous polyurethane dispersion composition according to claim 1, characterized in that, The aliphatic or alicyclic isocyanate is isophorone diisocyanate; The catalyst is dibutyltin dilaurate; The organic amine neutralizer is dimethylethanolamine; The urea derivative chain extender is a dimethylurea derivative.

4. The aqueous polyurethane dispersion composition according to claim 1, characterized in that, The carboxyl-containing hydrophilic chain extender is dimethylolpropionic acid, the phosphate-containing diol is dihydroxyphosphate, and the urea derivative chain extender is a dimethylurea derivative.

5. The aqueous polyurethane dispersion composition according to claim 1, characterized in that, The hydroxyl value of the ethoxylated trimethylolpropane is 150-200 mg KOH / g.

6. A method for preparing an aqueous polyurethane dispersion, characterized in that, The aqueous polyurethane dispersion is prepared using the aqueous polyurethane dispersion composition as described in claims 1 to 5 as a raw material, and includes the following steps: (1) Under the protection of an inert gas, polyether-type soft segment polyol, polycarbonate diol, hydrophilic chain extender containing carboxyl groups and catalyst are mixed and heated to 60-90℃ to melt uniformly; (2) Under stirring, add aliphatic or alicyclic isocyanate to the mixture in step (1) and react at 70-100℃ until the NCO content of the system reaches 2.5-5.0wt% to obtain NCO-terminated polyurethane prepolymer. (3) Ethoxylated trimethylolpropane is added in segments to the prepolymer of step (2) and the reaction is continued at 50-80°C to form a branched structure that covers ester bonds; (4) Cool the system of step (3) to 50-80°C, add an organic solvent to reduce viscosity, then cool to 50-70°C, add a diol containing phosphate ester group to carry out a capping reaction, so as to introduce the diol containing phosphate ester group as a capping agent, so that its phosphate ester group is located at the chain end or branching point end of the branched structure. (5) Cool the system from step (4) to 40-60°C and add an organic amine neutralizer to carry out a neutralization reaction; (6) Under high-speed shearing, the neutralization product of step (5) is added to deionized water at 15-35°C to form a polyurethane emulsion. (7) The emulsion from step (6) was subjected to desolvation under reduced pressure at 35-60°C and -0.05 to -0.10 MPa until the residual organic solvent was less than 1.0 wt%. (8) Cool the desolventized emulsion from step (7) to 20-40°C, add a urea derivative chain extender to carry out a chain growth reaction, so as to form a reversible hydrogen bond network between the waterborne polyurethane molecular chains, and obtain the waterborne polyurethane dispersion.

7. The method for preparing an aqueous polyurethane dispersion according to claim 6, characterized in that, In step (1), an organotin catalyst was also added, with an amount of 0.01-1 parts by weight.

8. The method for preparing an aqueous polyurethane dispersion according to claim 6, characterized in that, The NCO / OH molar ratio of the aliphatic or cycloaliphatic isocyanate to the polyether-type soft segment polyol, polycarbonate diol, and carboxyl-containing hydrophilic chain extender in step (2) is 1.05-1.15; The diol containing the phosphate ester group is a dihydroxyphosphate ester, and the urea derivative chain extender is a dimethylurea derivative. The urea derivative chain extender mentioned in step (8) is added in the form of a dilute solution with a concentration of 10-20 wt%.

9. An aqueous polyurethane dispersion prepared by the method described in any one of claims 6 to 8.

10. An application of the waterborne polyurethane dispersion as described in claim 9 in the preparation of waterborne metallic paint.