Water-based acrylic acid-polyurethane composite emulsion, paint and preparation method and application of water-based acrylic acid-polyurethane composite emulsion

Abstract

The invention discloses a waterborne acrylic acid-polyurethane composite emulsion, a waterborne acrylic acid-polyurethane composite paint, and a preparation method and application of the waterborne acrylic acid-polyurethane composite emulsion. According to the preparation method of the water-based acrylic acid-polyurethane composite emulsion, a core-shell structure polyacrylic acid-polyurethane emulsion is synthesized in situ by utilizing a core-layer monomer pre-emulsion and a shell-layer monomer pre-emulsion through a semi-continuous seed emulsion polymerization method. The shell layer of the core-shell structure has hydrophilic functional groups including carboxyl, sulfo group and hydroxyl, so that the emulsion has a self-thickening effect, and rheological properties and good constructability required by the coating can be provided. A nonionic hydrophilic group composite sodium sulfonate type post chain extender is selected as waterborne polyurethane, the emulsifying capacity is high, the emulsion stability is good, strong-acid monomers such as acrylic acid and methacrylic acid can be selected, high-thixotropy resin can be obtained through adjustment, meanwhile, the PA content can be effectively increased, the water resistance and weather resistance of a paint film can be improved, and the water resistance and weather resistance of the paint film can be improved. Therefore, the requirements of the automobile paint on moisture resistance, especially high-pressure flushing resistance, are met.

Description

Technical Field

[0001] The invention relates to the field of automobile paint, in particular to a water-based acrylic acid-polyurethane composite emulsion, paint and a preparation method and application thereof. Background Art

[0002] Automotive coatings include primers, midcoats, basecoats and topcoats. The VOC emissions of traditional oil-based automotive coatings are one of the main sources of pollution in the automotive manufacturing industry, especially basecoats. Due to performance requirements, basecoats can only be applied at a lower solid content, so they have the highest solvent content among all coatings used, accounting for more than 50% of the total VOC emissions. In recent years, with the widespread improvement of environmental awareness and increasingly stringent environmental regulations, the development of water-based automotive coatings has flourished. The water-based systems of basecoats and midcoats include the traditional 3C2B (three coatings and two bakings, i.e., primer (C) → baking (B) → basecoat (C) → varnish (C) → baking (B)), 3C1B (three coatings and one baking, i.e., primer (C) → basecoat (C) → varnish (C) → baking (B)) and the most mainstream B1B2 (primer B1 → topcoat B2) midcoat-free process. The film-forming resins used are mainly acrylic emulsions and waterborne polyurethane dispersions. Acrylic emulsion has certain rheological properties, which helps the directional arrangement of metal powder, but the leveling property may be affected, and it is easy to cause color difference between dry and wet films, which is not conducive to color matching. Waterborne polyurethane dispersion can help adjust the leveling and color matching properties of the coating, while providing excellent adhesion, flexibility, light resistance, weather resistance and chemical resistance of the paint film. At present, most waterborne automotive paints are obtained by physically compounding acrylic self-thickening emulsion and waterborne polyurethane dispersion, combined with color paste, thickener, wetting dispersant, etc., which are very prone to problems such as poor compatibility and poor paint film appearance.

[0003] The water-based acrylic acid modified polyurethane composite emulsion combines the two organically at the resin synthesis stage, and the obtained polyacrylic acid segment (PA) and polyurethane segment (PU) can achieve molecular compatibility, so the transparency, mechanical properties, thermal properties, etc. of the corresponding paint film are more uniform. There are a lot of literature reports on the preparation of water-based acrylic acid modified polyurethane composite emulsion. The US patent with publication number US6063861 discloses that water-based polyurethane emulsion is used as a seed, and then an emulsion polymerization of acrylic acid is carried out to obtain an acrylic acid-polyurethane composite emulsion. The hydrophilic group used in the water-based polyurethane in this scheme is a carboxylic acid type, which is unstable when the pH is low. It is easy to demulsify when adding a strong acid monomer such as acrylic acid, thereby limiting the selection of acrylic acid monomers, and the performance adjustment of the composite emulsion is inconvenient. The Chinese patent applications with publication numbers CN107428888B, CN117384325A, and CN102093516A all use carboxylate-type polyurethane dispersions, which have insufficient emulsification properties, so the selection of acrylic acid monomers is limited, and the PA content is difficult to increase. The Chinese patent application with publication number CN102675553A discloses a method for preparing a polyurethane / polyacrylate core-shell emulsion, using a sulfonate type polyurethane emulsion as a seed, and the obtained product mainly shows the performance of polyurethane after film formation, has excellent bonding performance, low temperature resistance, etc., and can be used as a fabric finishing agent, composite adhesive, etc. For automotive paint, the high thixotropy of the film-forming resin is crucial. However, there are few reports on thixotropy in the existing literature of water-based acrylic modified polyurethane. The Chinese patent application with publication number CN113831814A designs the structure of polyurethane segments and polyacrylate segments, also uses hydroxy acid hydrophilic monomers, and uses epoxy-modified amine compounds for chemical bonding and physical blending to develop a new type of high thixotropic waterborne polyurethane acrylate aqueous dispersion, which is applied to water-based metallic paint, and the coating has high thixotropy, but its process is complicated and cumbersome, and operability is poor. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a water-based acrylic-polyurethane composite emulsion, paint and its preparation method and application in view of the deficiencies in the prior art, which has a self-thickening effect, high thixotropy, and can provide the rheological properties and good construction properties required by the coating.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0006] A method for preparing an aqueous acrylic acid-polyurethane composite emulsion, in situ synthesizing a core-shell structured polyacrylic acid-polyurethane emulsion using a core layer monomer pre-emulsion and a shell layer monomer pre-emulsion;

[0007] The core layer monomer pre-emulsion includes at least one acrylic ester monomer containing two double bonds;

[0008] The shell monomer pre-emulsion includes at least an aqueous polyurethane dispersion, at least one acrylic ester monomer containing two double bonds, at least one acrylic ester monomer containing a carboxyl group, and at least one acrylic ester monomer containing a hydroxyl group;

[0009] The weight ratio of the acrylic acid ester monomers in the core layer monomer pre-emulsion and the shell layer monomer pre-emulsion is 30 / 70-85 / 15;

[0010] The number average molecular weight Mn of the aqueous polyurethane dispersion is 10,000 to 150,000, and a non-ionic hydrophilic group composite sulfonic acid sodium salt type post-chain extender is selected.

[0011] The core-shell structure of the present invention includes hydrophilic functional groups such as carboxyl, sulfonic acid and hydroxyl groups, which make the emulsion have a self-thickening effect and can provide the rheological properties and good construction properties required by the coating.

[0012] The acrylic acid ester monomer is an acrylic acid (ester) monomer containing one or two double bonds. The acrylic acid ester monomer can be butyl (meth)acrylate, methyl methacrylate, isooctyl (meth)acrylate, isobornyl (meth)acrylate, hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, allyl methacrylate, 1,6-hexanediol di(meth)acrylate, glycidyl methacrylate, (meth)acrylic acid, etc.

[0013] More preferably, the weight ratio of the acrylic acid ester monomers in the core layer monomer pre-emulsion and the shell layer monomer pre-emulsion is 40 / 60-70 / 30, more preferably 50 / 50-60 / 40.

[0014] The semi-continuous seed emulsion polymerization method adopts an existing method, such as the semi-continuous seed emulsion polymerization method in the patent with publication number CN109810218B.

[0015] More preferably, the number average molecular weight of the aqueous polyurethane dispersion is 20,000 to 100,000, more preferably 30,000 to 60,000.

[0016] In a preferred embodiment of the present invention, the core-shell structure polyacrylic acid-polyurethane emulsion is synthesized by semi-continuous seed emulsion polymerization. Preferably, the weight ratio of the polyurethane segment structure to the acrylic acid segment structure in the core-shell structure polyacrylic acid-polyurethane emulsion is 25 / 75-75 / 25, more preferably 25 / 75-50 / 25.

[0017] The polyurethane structure (PU) is derived from an aqueous polyurethane dispersion and contains acrylate double bonds. The acrylic structure (PA) is mainly derived from acrylate monomers, which are copolymerized with double-bond aqueous polyurethane dispersions to achieve covalent bonding between PU and PA. The sulfonic acid groups and long polyethylene glycol chains in the PU structure work synergistically with the carboxyl and hydroxyl groups in PA to give the system special rheological properties, namely, self-thickening and high thixotropy. PU can provide excellent adhesion and flexibility to the paint film, while helping to adjust the leveling and colorability of the coating. PA has excellent film-forming properties, water and weather resistance, and contributes significantly to rheology.

[0018] In a preferred embodiment of the present invention, the method for synthesizing the aqueous polyurethane dispersion comprises the following steps:

[0019] S1. Add isocyanate to polymer polyol, raise the temperature, and react;

[0020] S2, adding hydrophilic monomer and catalyst, and continuing the reaction;

[0021] S3, adding functional monomer, inhibitor, and diluent, and continuing the reaction;

[0022] S4. Add deionized water to disperse evenly, add sodium sulfonate salt and chain extender aqueous solution, and continue stirring to disperse evenly.

[0023] In a preferred embodiment of the present invention, the sodium sulfonate salt post-chain extender in S4 includes one or more of sodium ethylenediamine sulfonate and α,ω-polypropylene glycol-diamine-sulfonyl sodium salt.

[0024] The post-chain extender can significantly increase the molecular weight of polyurethane, which helps to obtain a high molecular weight waterborne polyurethane dispersion, while improving the emulsification stability of the waterborne polyurethane dispersion and providing support for the thixotropy of the final waterborne acrylic-polyurethane composite emulsion.

[0025] The polymer polyol in S1 is dehydrated at 100-120° C. in vacuum until the water content is less than 0.05%, and nitrogen is filled for protection.

[0026] The temperature at which the polymer polyol in S1 is added to the isocyanate is lower than or equal to 50°C.

[0027] The temperature after heating in S1 is 80-90°C, and the reaction time is 1-1.5h.

[0028] The NCO content of S2 after reaction is 1.5-1.2%.

[0029] The NCO content of S3 after reaction is 0.8-0.2%.

[0030] The solution obtained by the reaction in S3 is cooled to 30-50° C. and deionized water is added thereto. The dispersion time is 10 min to 1 h.

[0031] After adding the sodium sulfonate salt type chain extender aqueous solution into S4, the stirring time is 1 h to 2 h.

[0032] The isocyanate is at least one of isophorone diisocyanate IPDI, hexamethylene diisocyanate HDI, dicyclohexylmethane diisocyanate HMDI, toluene diisocyanate TDI, diphenylmethane diisocyanate MDI and the like.

[0033] The polymer polyol and isocyanate are the main raw materials for forming the high molecular weight polymer, and provide the main macromolecular chain structure of the waterborne polyurethane resin.

[0034] The hydrophilic monomer is a hydrophilic chain extender, which provides a hydrophilic group and provides emulsification and dispersion stability for the aqueous polyurethane dispersion. The hydrophilic chain extender is a non-ionic chain extender, such as trimethylolpropane polyethylene glycol monomethyl ether.

[0035] The catalyst is an organic tin catalyst, such as at least one of dibutyltin dilaurate DBTDL, stannous octoate, and the like.

[0036] The diluent is a low-viscosity acrylic ester monomer, such as butyl acrylate.

[0037] The polymerization inhibitor is at least one of 2,6-di-tert-butyl-p-cresol, p-hydroxyanisole and the like.

[0038] The functional monomer is a hydroxyl-containing acrylic ester monomer, which can react with the isocyanate group to form an end cap, and can also participate in the reaction in the subsequent PA emulsion polymerization stage, so as to form an organic covalent bond between the PU segment and the PA segment, and realize organic hybridization on a molecular scale. The functional monomer can be at least one of hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl acrylate, etc.

[0039] The isocyanate index R of the aqueous polyurethane dispersion is 1.05 to 1.6, preferably 1.2 to 1.4. The isocyanate index is helpful to obtain high molecular weight polyurethane.

[0040] The mass ratio of polymer polyol, isocyanate, hydrophilic monomer and catalyst is 30-40:5-10:3-5:0.01-0.05.

[0041] The mass ratio of the diluent, the inhibitor and the functional monomer is 0.5-1.0:0.1-0.3:10-15.

[0042] The mass ratio of the polymer polyol to the aqueous solution of the sodium sulfonate post-chain extender is 30-40:0.5-1.0.

[0043] In a preferred embodiment of the present invention, the polymer polyol includes a diol with a short-chain side group; preferably, the polymer polyol includes at least one of 3-methyl-1,5-pentanediol, neopentyl glycol, 2-methyl-1,3-propanediol, 1,3-butanediol and 2,2,4-trimethyl-1,3-pentanediol; further preferably, the polymer diol has a number average molecular weight Mn of 500 to 10,000.

[0044] The short-chain side groups help to reduce the viscosity of the polymer, facilitate the use of solvent-free polymerization, and help improve the water resistance of the polymer. Other raw materials include conventional dibasic acids (anhydrides) such as adipic acid, phthalic anhydride, and terephthalic acid.

[0045] Preferably, the number average molecular weight Mn of the polymer diol is 1000-5000, more preferably 1500-3500.

[0046] In a preferred embodiment of the present invention, the synthesis of core-shell structured polyacrylic acid-polyurethane emulsion in situ using core layer monomer pre-emulsion and shell layer monomer pre-emulsion by semi-continuous seed emulsion polymerization method comprises the following steps:

[0047] Step 1, add water, emulsifier, acrylate seed monomer, mix evenly, increase the temperature, and add initiator to react;

[0048] Step 2, heating, adding core layer monomer pre-emulsion and initiator dropwise, and keeping warm;

[0049] Step 3, continue to dropwise add the shell monomer pre-emulsion and initiator, and keep warm;

[0050] Step 4: Cool down, add neutralizer to neutralize the reaction, and filter.

[0051] After heating in step 1, the temperature is greater than or equal to 70° C., and the reaction time is 30 min to 60 min.

[0052] In step 2, the temperature is raised to 80°C.

[0053] The dropping time in step 2 is 2 to 3 hours.

[0054] In step 2, the temperature is kept at 78-82° C. and the reaction time is 1-2 h.

[0055] In step 3, the pre-emulsion of the shell monomer is added dropwise over 2 to 3 hours, and the initiator is added dropwise 0.5 to 1 hour later than the monomer.

[0056] In step 3, the insulation temperature is 78-82° C. and the insulation time is 1-2 h.

[0057] Step 4: Cool down to 40-50°C.

[0058] Step 4: Add a neutralizing agent and react for 30 to 60 minutes.

[0059] In step 4, 1 μm filter cloth is used for filtration.

[0060] The emulsifier is at least one of sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, and the like.

[0061] The acrylic ester seed monomers include emulsifier, water, butyl acrylate and acrylic acid.

[0062] The initiator is at least one of ammonium persulfate, sodium persulfate, potassium persulfate, azobisisobutyronitrile and the like.

[0063] The neutralizing agent is a compound that can ionize the hydrophilic group, such as at least one of the organic bases triethylamine, triethanolamine, N,N-dimethylethanolamine DMEA, etc.

[0064] In step 1, the mass ratio of butyl acrylate, acrylic acid and initiator is 1-5:0.15-0.5:0.1-0.5.

[0065] In step 2, the mass ratio of the core layer monomer pre-emulsion to the initiator is 110-130:0.1-0.5.

[0066] In step 3, the mass ratio of the shell monomer pre-emulsion to the initiator is 500-600:0.1-0.5.

[0067] In step 4, the mass ratio of the shell monomer pre-emulsion to the neutralizer is 500-600:0.5-1.

[0068] The invention also discloses a water-based acrylic acid-polyurethane composite emulsion prepared by the preparation method.

[0069] The invention also discloses an aqueous acrylic acid-polyurethane composite emulsion, wherein the acid value of the aqueous acrylic acid-polyurethane composite emulsion is 5-12 mgKOH / g; the viscosity of the aqueous acrylic acid-polyurethane composite emulsion is 10-2000 mPa.s; the solid content of the aqueous acrylic acid-polyurethane composite emulsion is 20-60%; and the particle size of the aqueous acrylic acid-polyurethane composite emulsion is 100-200 nm.

[0070] More preferably, the acid value of the aqueous acrylic-polyurethane composite emulsion is 6 to 9 mgKOH / g.

[0071] More preferably, the viscosity of the aqueous acrylic-polyurethane composite emulsion is 10 to 1000 mPa.s, more preferably 20 to 300 mPa.s.

[0072] More preferably, the solid content of the aqueous acrylic-polyurethane composite emulsion is 20 to 50%, more preferably 35 to 45%.

[0073] More preferably, the particle size of the aqueous acrylic-polyurethane composite emulsion is 120 to 180 nm.

[0074] The invention also discloses a paint, which is made from the water-based acrylic acid-polyurethane composite emulsion.

[0075] The invention also discloses the application of the water-based acrylic acid-polyurethane composite emulsion and the paint in automobile color paint and mid-coat.

[0076] Compared with the prior art, the present invention has the following beneficial effects:

[0077] (1) The synthesis process is to first synthesize an aqueous polyurethane dispersion, and then synthesize a core-shell structure polyacrylic acid-polyurethane emulsion in situ through a pre-emulsification semi-continuous method. It is solvent-free and environmentally friendly.

[0078] (2) Waterborne polyurethane uses a non-ionic hydrophilic group complex sulfonic acid sodium salt type post-chain extender, which has high emulsification ability and good emulsion stability. In the PA polymerization stage, strong acid monomers such as acrylic acid and methacrylic acid can be selected to facilitate the adjustment of highly thixotropic resins. At the same time, it helps to effectively increase the PA content, which is beneficial to the improvement of the water resistance and weather resistance of the paint film, thereby meeting the requirements of automotive paint for moisture resistance, especially high-pressure water resistance.

[0079] (3) The shell layer of the core-shell structure has hydrophilic functional groups such as carboxyl, sulfonic acid, hydroxyl, and long-chain polyethylene glycol. Under alkaline conditions, the carboxyl and sulfonic acid groups are ionized, and the macromolecular long-chain structure stretches out. The hydrogen bonding effect between the hydroxyl and long-chain polyethylene glycol structures and the steric hindrance increase the viscosity of the emulsion, which has an obvious self-thickening effect and can provide the rheological properties and good construction properties required by the coating. BRIEF DESCRIPTION OF THE DRAWINGS

[0080] Figure 1 1K is the rheological curve of the water-based 1K metallic base paint in one embodiment of the present invention. DETAILED DESCRIPTION

[0081] 1. Synthesis of homemade polyester diol PEPL:

[0082] Polyester diol 1: Add 2,2,4-trimethyl-1,3-pentanediol, hexanediol and adipic acid into the reactor in the ratio of 0.6:0.6:1.1, heat to 165°C, react for 1.5h, heat to 185°C, react for 2h, heat to 200°C, and dehydrate by vacuum distillation until the acid value is 0.45mgKOH / g and the hydroxyl value is ~56mgKOH / g.

[0083] Polyester diol 2: Add 3-methyl-1,5-pentanediol, isophthalic acid and adipic acid into the reactor in a ratio of 1:0.25:0.65, heat to 165°C, react for 1.5h, heat to 185°C, react for 2h, heat to 200°C, and dehydrate by vacuum distillation until the acid value is 0.37mgKOH / g and the hydroxyl value is ~55mgKOH / g.

[0084] 2. Synthesis of waterborne polyurethane dispersion PUD

[0085] Table 1. Synthesis formula of waterborne polyurethane dispersion

[0086]

[0087] PUD reaction example 1

[0088] 36 parts of homemade polyester diol 1 were placed in a reactor, and water was removed under vacuum at 100-120°C until the water content was <0.05%, and nitrogen was filled for protection. Cool down to 50°C, add 6.6 parts of IPDI, heat up to 80-90°C, and react for 1-1.5h. Add 3 parts of YmerN120 and 0.01 parts of DBTDL, and continue to react until the NCO content is 1.5-1.2%. Add 13 parts of BA, 0.1 parts of BHT, and 0.8 parts of HMPA, and continue to react until the NCO content is 0.8-0.2%. Cool down to 30-50°C, slowly add 90 parts of deionized water to disperse evenly, which takes 10min-1h. Under strong shear conditions, add 0.8 parts of AAS aqueous solution, and continue to stir and disperse for 10min-2h. Filter to obtain PUD reaction example 1. The solid content of Example 1 is 31.4%, and the viscosity is 35cps (24°C, 100s -1 ), particle size is 48nm.

[0089] PUD reaction example 2-4

[0090] According to the formula composition in Table 1 and the synthesis method of Reaction Example 1, Examples 2-4 were completed in sequence.

[0091] PUD reaction example 5

[0092] The same process was carried out according to the formula composition in Table 1 and the synthesis method of Reaction Example 1, except that the last step of AAS salt post-chain extension was not performed. Therefore, the molecular weight of the product was low, and the lack of hydrophilic functional group sodium sulfonate salt also affected its hydration reaction. The product viscosity and particle size were high, and the emulsification stability was poor.

[0093] PUD reaction example 6

[0094] Put 25 parts of homemade polyester diol PEPL1 into the reactor, remove water in vacuum at 100-120℃ until the water content is <0.05%, and fill with nitrogen for protection. Cool down to 50℃, add 7 parts of IPDI, raise the temperature to 80-90℃, and react for 1-1.5h. Add 1.8 parts of DMPA and 0.01 parts of DBTDL, and continue to react until the NCO content is 1.5-1.2%. Add 12 parts of BA, 0.1 parts of BHT, and 1 part of HMPA, and continue to react until the NCO content is 0.8-0.2%. Cool down to 30-50℃, add 1.2 parts of DMEA and react for 10min-1h. Under strong shear conditions, slowly add 47 parts of deionized water to disperse evenly, which takes 10min-1h. Filter to obtain PUD reaction example 6. The solid content of reaction example 6 is 36.2%, and the viscosity is 45cps (24℃, 100s -1 ), particle size is 65nm.

[0095] 3. Synthesis of waterborne acrylic-polyurethane composite emulsion PUA

[0096] Table 2. Synthesis formula of water-based acrylic-polyurethane composite emulsion PUA

[0097]

[0098]

[0099] PUA Example 1

[0100] 20 parts of water, 0.45 parts of SDS, 45 parts of BA, 50 parts of MMA and 1.8 parts of HDDMA were placed in an emulsifying homogenizer and sheared and emulsified at 3Krps for 5 minutes to obtain a core layer monomer pre-emulsion.

[0101] 500 parts of PUD reaction example 1, 45 parts of MMA, 2 parts of MAA, 1.8 parts of HDDMA and 2.7 parts of GMA were placed in an emulsifying homogenizer and sheared and emulsified at 3Krps for 5 minutes to obtain a shell monomer pre-emulsion.

[0102] Put 120 parts of water, 0.01 parts of ammonia water, 0.3 parts of SDS, 3.7 parts of BA, and 0.15 parts of MAA into the reactor, mix well, heat to 70°C, add a mixed solution of 0.1 parts of APS and 2 parts of water, and react for 30 minutes. Heat to 80°C, and at the same time, drip the core layer monomer pre-emulsion and the core layer initiator solution (a mixed solution of 0.15 parts of APS and 25 parts of water) at a uniform rate, add the dripping for 2 to 3 hours, and keep the reaction at 80°C for 1 hour. Continue to drip the shell layer monomer pre-emulsion and the shell layer initiator solution (a mixed solution of 0.25 parts of APS and 25 parts of water), the monomer is added in 2 to 3 hours, and the initiator is added 0.5 hours slower than the monomer. Keep warm at 80°C for 1 to 2 hours. Cool to 40 to 50°C, add 0.75 parts of neutralizer and react for 30 minutes. Filter the material with 1um filter cloth to obtain PUA Example 1. The solid content of Example 1 is 39.0%, and the viscosity is 23 cps (24°C, 100 s -1 ), particle size is 140.5nm, acid value is 8.04mgKOH / g, number average molecular weight is 78342.

[0103] PUA Examples 2-4

[0104] According to the formula composition in Table 2, the corresponding PUD reaction examples 2-4 and the synthesis method of PUA example 1 are used to complete PUA examples 2-4 in sequence.

[0105] PUA Comparative Example 1

[0106] Also according to the formula composition of Table 2, using PUD reaction example 5 and the synthesis method of PUA embodiment 1, PUA comparative example 1 is completed. Since the molecular weight of the PUD used is low, the molecular weight of the final PUA is also low. At the same time, the viscosity and particle size of the PUD used are high, and the corresponding PUA viscosity and particle size are also high, especially the particle size, which reaches 221.3nm, and the stability of the emulsion is poor.

[0107] PUA Comparative Example 2

[0108] When the shell monomer pre-emulsion was prepared according to the formula in Table 2, MAA was added to PUD reaction example 6, and the system immediately demulsified and precipitated, and the preparation could not be performed. This is because the PUD was prepared using a carboxyl hydrophilic monomer, and the carboxyl salt was unstable under strong acidic conditions and easily demulsified.

[0109] 4. Preparation of water-based midcoat and 1K metallic basecoat for automobiles

[0110] Using the synthesized aqueous acrylic-polyurethane composite emulsions of Examples 1, 2, 3, and 4 of the present invention, Comparative Example 1, and commercially available DAOTAN TW 6466 as Comparative Example 2, automotive aqueous midcoats and automotive aqueous 1K metallic basecoats were prepared according to the formulations in Tables 3 and 4, respectively.

[0111] Table 3. Automotive water-based midcoat formulation

[0112] raw material m / g Water-based acrylic-polyurethane composite emulsion 27 Amino resin 6 Water-based polyester 12 Color paste 22 Wetting Agents 0.3 Defoaming agent 0.1 Deionized water 27

[0113] Table 4. Automotive water-based 1K metallic basecoat formulation

[0114]

[0115]

[0116] The rheological behavior of water-based 1K metallic basecoat was tested, and the rheological curve is as follows: Figure 1 The rheological test was conducted using a HAAKEMARS 40 rheometer, with the following test conditions: #100 rotor; T 25.00°C, and four consecutive sections: 30s; 180.00s; 30.00s; 180.00s.

[0117] The electrophoresis plate was used as the substrate and the plate was sprayed according to the 3C1B process. The performance test results are shown in Table 5.

[0118] Table 5. Test results of automotive coating performance

[0119]

[0120] FI value is tested using BYK-mac i Pro. Adhesion (cross-hatch) test is tested according to GB / T 9286-2021. Impact strength is tested according to GB / T 1732-2020. Stone impact resistance is tested according to ISO 20567. Adhesion (high pressure water flushing) is tested according to Q / JLJ174007. Moisture resistance is tested according to GB / T 1766 and Q / JL J174007. Water resistance is tested according to GB / T 1766.

[0121] from Figure 1 It can be seen that the water-based 1K metallic basecoat prepared based on the water-based acrylic-polyurethane composite emulsion Examples 1, 2, 3, and 4 respectively has obvious pseudoplasticity. When subjected to high-speed shearing, the viscosity is less than 35 cps, which is conducive to the atomization spraying of the coating. When sheared at a low speed, the viscosity increases rapidly, which is conducive to the directional arrangement of the metal powder, and the FI value is ≥18.5. This fully demonstrates that the water-based acrylic acid-polyurethane composite emulsion prepared in the present invention has rheological properties, making it very suitable for use in water-based automotive paint.

[0122] In the performance tests of coating examples 1-4, it can be seen that the paint films have a flat and smooth appearance, the adhesion can reach level 0, and they are resistant to high-pressure water flushing without falling off, and the moisture resistance and water resistance can meet the standard requirements.

[0123] Compared with coating example 1, coating comparative example 1 does not undergo post-chain extension, and the resulting water-based acrylic-polyurethane composite emulsion has a lower acid value, a larger particle size, and poor emulsification stability. From the rheological curve of the water-based 1K metallic base paint, its rheological viscosity is low and its FI value is low. The molecular weight is low and the adhesion is poor, especially when the high-pressure water impact occurs, the shedding area is >5%, which does not meet the standard. Its impact strength is also low, and the stone impact resistance only reaches level 2. During the water resistance test, a small amount of blistering occurs.

[0124] Compared with coating Example 1, coating Comparative Example 2 has a higher rheological final viscosity, but a slower shear recovery rate. After cutting, The time required for the viscosity to recover to 75% is 7.5s, which is much higher than 2s in the embodiment, which means that after the coating is sprayed onto the substrate, the viscosity cannot be increased more quickly, so the directional arrangement efficiency of the metal powder will be reduced, the paint film has a dark appearance, and the FI value is only 16.5. At the same time, DAOTAN TW 6466 is not very compatible with other resins in the formula, and the formula has little room for adjustability. The adhesion, flexibility, water resistance, etc. of the obtained paint film are not ideal and cannot meet the use requirements.

[0125] The abbreviations represent the following meanings:

[0126] BA: Butyl acrylate

[0127] IPDI: Isophorone diisocyanate

[0128] HPMA: Hydroxypropyl Methacrylate

[0129] Ymer N120: trimethylolpropane polyethylene glycol monomethyl ether

[0130] DBTDL: Dibutyltin dilaurate

[0131] BA: Butyl acrylate

[0132] BHT: 2,6-di-tert-butyl-p-cresol

[0133] AAS: 50% ethylenediamine sulfonic acid sodium salt aqueous solution

[0134] DMPA: dimethylolpropionic acid

[0135] DMEA: N,N-dimethylethanolamine

[0136] Mn: number average molecular weight

[0137] SDS: Sodium dodecyl sulfate

[0138] MAA: Methacrylic acid

[0139] APS: Ammonium Persulfate

[0140] EHA: Isooctyl acrylate

[0141] MMA: Methyl Methacrylate

[0142] HDDMA: 1,6-Hexanediol dimethacrylate

[0143] GMA: Glycidyl methacrylate.

Claims

1. A method for preparing an aqueous acrylic acid-polyurethane composite emulsion, characterized in that: In situ, core-shell structured polyacrylic acid-polyurethane emulsion is synthesized by using core-layer monomer pre-emulsion and shell-layer monomer pre-emulsion; The core layer monomer pre-emulsion includes at least one acrylic ester monomer containing two double bonds; The shell monomer pre-emulsion includes at least an aqueous polyurethane dispersion, at least one acrylic ester monomer containing two double bonds, at least one acrylic ester monomer containing a carboxyl group, and at least one acrylic ester monomer containing a hydroxyl group; The weight ratio of the acrylic acid ester monomers in the core layer monomer pre-emulsion and the shell layer monomer pre-emulsion is 30 / 70-85 / 15; The number average molecular weight Mn of the aqueous polyurethane dispersion is 10,000 to 150,000, and a non-ionic hydrophilic group composite sulfonic acid sodium salt type post-chain extender is selected.

2. The method for preparing the aqueous acrylic acid-polyurethane composite emulsion according to claim 1, characterized in that: The core-shell structure polyacrylic acid-polyurethane emulsion is synthesized by semi-continuous seed emulsion polymerization. Preferably, the weight ratio of the polyurethane segment structure to the acrylic segment structure in the core-shell structure polyacrylic acid-polyurethane emulsion is 25 / 75-75 / 25.

3. The method for preparing the aqueous acrylic acid-polyurethane composite emulsion according to claim 1, characterized in that The synthesis method of the aqueous polyurethane dispersion comprises the following steps: S1. Add isocyanate to polymer polyol, raise the temperature, and react; S2, adding hydrophilic monomer and catalyst, and continuing the reaction; S3, adding functional monomer, inhibitor, and diluent, and continuing the reaction; S4. Add deionized water to disperse evenly, add sodium sulfonate salt and chain extender aqueous solution, and continue stirring to disperse evenly.

4. The method for preparing the aqueous acrylic acid-polyurethane composite emulsion according to claim 3, characterized in that: The sodium sulfonate salt post-chain extender in S4 includes one or more of sodium ethylenediamine sulfonate and α,ω-polypropylene glycol-diamine-sulfonyl sodium salt.

5. The method for preparing the aqueous acrylic acid-polyurethane composite emulsion according to claim 3, characterized in that: The polymer polyol includes a diol with a short-chain side group; preferably, the polymer polyol includes at least one of 3-methyl-1,5-pentanediol, neopentyl glycol, 2-methyl-1,3-propanediol, 1,3-butanediol and 2,2,4-trimethyl-1,3-pentanediol; further preferably, the polymer diol has a number average molecular weight Mn of 500 to 10,000.

6. The method for preparing the aqueous acrylic acid-polyurethane composite emulsion according to claim 1, characterized in that: The method of synthesizing a core-shell structured polyacrylic acid-polyurethane emulsion in situ by using a core layer monomer pre-emulsion and a shell layer monomer pre-emulsion by a semi-continuous seed emulsion polymerization method comprises the following steps: Step 1, add water, emulsifier, acrylate seed monomer, mix evenly, increase the temperature, and add initiator to react; Step 2, heating, adding core layer monomer pre-emulsion and initiator dropwise, and keeping warm; Step 3, continue to dropwise add the shell monomer pre-emulsion and initiator, and keep warm; Step 4: Cool down, add neutralizer to neutralize the reaction, and filter.

7. An aqueous acrylic acid-polyurethane composite emulsion prepared by the preparation method according to any one of claims 1 to 6.

8. A water-based acrylic-polyurethane composite emulsion, characterized in that: The acid value of the aqueous acrylic-polyurethane composite emulsion is 5-12 mgKOH / g; the viscosity of the aqueous acrylic-polyurethane composite emulsion is 10-2000 mPa.s; the solid content of the aqueous acrylic-polyurethane composite emulsion is 20-60%; and the particle size of the aqueous acrylic-polyurethane composite emulsion is 100-200 nm.

9. A paint, characterized in that: It is made from the aqueous acrylic acid-polyurethane composite emulsion described in claim 7 or claim 8.

10. Use of the aqueous acrylic-polyurethane composite emulsion as claimed in claim 7 or claim 8 and the paint as claimed in claim 9 in automotive color paint and mid-coat.

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