Preparation method of dual-curing waterborne polyurethane-acrylate automotive coating

Abstract

This invention discloses a method for preparing a dual-curing waterborne polyurethane-acrylate automotive coating, relating to the field of coating technology. The method includes the following steps: In a mixing tank equipped with a stirring device, the following components are added sequentially in parts by weight, stirring for 5-10 minutes after each addition: 60-80 parts of waterborne polyurethane-acrylate dispersion; 3-15 parts of reactive nano-silica aqueous dispersion; 2-8 parts of reactive chlorinated polyolefin adhesion promoter; 1-3 parts of waterborne photoinitiator; 3-12 parts of blocked isocyanate crosslinking agent, selected from blocked hexamethylene diisocyanate (HDI) trimer or blocked isophorone diisocyanate (IPDI) trimer, with a deblocking temperature of 80-120°C; 0.3-2.0 parts of additives; then deionized water is added to obtain a dual-curing waterborne nanocomposite coating. The coating of this invention utilizes dual curing to solve shadow curing, achieving a synergistic improvement in hardness and flexibility, and is compatible with multiple substrate materials.

Description

Technical Field

[0001] This invention relates to the field of coating technology, specifically to a method for preparing a dual-curing waterborne polyurethane-acrylate automotive coating. Background Technology

[0002] As the automotive industry moves towards lightweighting, environmental friendliness, and high performance, the proportion of plastic components in complete vehicles continues to increase, currently reaching 20% ​​to 30% of the total vehicle weight. Automotive interior and exterior plastic parts (such as bumpers, dashboards, door panels, rearview mirror housings, and headlight bezels) are constantly exposed to sunlight, rain, temperature variations, chemicals (such as gasoline and cleaning agents), and daily mechanical friction, placing extremely high demands on the protective performance of their surface coatings. Simultaneously, increasingly stringent environmental regulations are putting traditional solvent-based coatings under pressure to be phased out due to their high VOC emissions, making water-based coatings and UV-cured coatings the mainstream direction for industry transformation and upgrading.

[0003] However, existing water-based automotive plastic coatings face the following key technical challenges: 1. The contradiction between hardness and flexibility: Waterborne polyurethane coatings have excellent flexibility and adhesion, but low crosslinking density. The pencil hardness is usually HB~2H, which is difficult to meet the scratch resistance requirements. Simply increasing the crosslinking density (such as increasing the amount of polyfunctional isocyanate) will lead to a sharp decrease in flexibility, and the plastic substrate is prone to cracking when it expands and contracts with heat or is subjected to impact.

[0004] 2. UV-cured shadow areas are difficult to cure: UV-cured coatings have fast curing speed, high hardness and low VOC emissions, but automotive plastic parts are mostly complex three-dimensional structures. Recesses, inner corners and back areas cannot be effectively irradiated by UV light, resulting in incomplete curing of the coating and a decrease in adhesion and protective performance.

[0005] 3. Poor dispersibility of nanofillers: Due to their large specific surface area and high surface energy, nano-silica, nano-alumina and other fillers are prone to agglomeration in aqueous systems. When introduced by physical blending, they lack chemical bonding with the resin matrix and are prone to migration or detachment during long-term service, becoming sources of coating defects.

[0006] 4. Insufficient adhesion of low surface energy plastics: PP and other polyolefin plastics have low surface energy (about 28~30mN / m), making it difficult for coatings to wet and adhere. Traditional processes require flame treatment, plasma treatment, or coating with chlorinated polyolefin (CPO) primer, which are complicated, costly, and introduce new VOC emissions.

[0007] 5. Insufficient water resistance and weather resistance: Residual emulsifiers and hydrophilic groups (such as carboxyl groups and sulfonic acid groups) during the film formation process of water-based coatings become water molecule penetration sites, resulting in poor water resistance; under long-term ultraviolet radiation, the ether bonds and ester bonds in the polyurethane chain are easily photo-oxidized and degraded, resulting in chalking and gloss decay.

[0008] In summary, developing an environmentally friendly coating system that combines high hardness, excellent flexibility, good adhesion to various plastic substrates, good water resistance, good weather resistance, and low VOC content is a technological direction that urgently needs to be broken through in the field of automotive plastic coatings. Summary of the Invention

[0009] To address the shortcomings of existing technologies, this invention provides a method for preparing a dual-curing waterborne polyurethane-acrylate automotive coating.

[0010] The present invention solves the above-mentioned technical problems through the following technical solutions: A method for preparing a dual-curing waterborne polyurethane-acrylate automotive coating involves adding the following components in the following order by weight to a mixing tank equipped with a stirring device, stirring for 5-10 minutes after each addition: 60-80 parts of waterborne polyurethane-acrylate dispersion; 3-15 parts of reactive nano-silica aqueous dispersion; 2-8 parts of reactive chlorinated polyolefin adhesion promoter; 1-3 parts of aqueous photoinitiator; 3 to 12 parts of blocked isocyanate crosslinking agent, selected from blocked hexamethylene diisocyanate (HDI) trimer or blocked isophorone diisocyanate (IPDI) trimer, with a deblocking temperature of 80 to 120°C; 0.3 to 2.0 parts of the additive; Then add deionized water to obtain a dual-curing waterborne nanocomposite coating; adjust the coating to the application viscosity with deionized water: 25~40 seconds with a Forte 4 cup, or 500~1500 mPa·s measured with a rotational viscometer.

[0011] Furthermore, the aqueous photoinitiator is selected from 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylphenylacetone or phenyl-2,4,6-trimethylbenzoyl lithium phosphite.

[0012] Furthermore, the additives include 0.1 to 0.5 parts of a polyether-modified organosiloxane-based substrate wetting agent, 0.1 to 0.5 parts of an acetylenic diol-based defoamer, and 0.1 to 1.0 parts of a polyurethane associative leveling agent.

[0013] The aqueous polyurethane-acrylate dispersion is prepared by the following method: Step 1, Preparation of silicone-modified polyurethane prepolymer: Under dry nitrogen protection, add 100 parts by weight of dehydrated polycarbonate diol (PCDL), 5-20 parts by weight of hydroxyl-terminated polydimethylsiloxane (PDMS-OH), and 4-8 parts by weight of dimethylolpropionic acid (DMPA); add acetone as a solvent to fully dissolve the solid components, stir and heat to 60-70°C; add 52-60 parts by weight of isophorone diisocyanate (IPDI) dropwise, controlling the reaction temperature at 70-80°C; after the dropwise addition is complete, add 0.03-0.10 parts by weight of dibutyltin dilaurate (DBTDL), and react at a constant temperature of 75-85°C for 2-4 hours to obtain: silicone-modified polyurethane prepolymer; Step 2, Preparation of acrylate-terminated silicone-modified polyurethane prepolymer: The temperature of the silicone-modified polyurethane prepolymer was lowered to 55-65℃, and under nitrogen protection, the following were added according to the mass ratio: Add 2.0~5.0 parts of end-capping agent and 0.002~0.015 parts of polymerization inhibitor dropwise, then heat to 65~75℃ and react at a constant temperature for 2~3 hours; then add 0.5~1.5 parts of methanol and continue stirring for 30 minutes; finally, an acrylate-terminated silicone-modified polyurethane prepolymer is obtained. Step 3, Neutralization and Emulsification, Preparation of Waterborne Polyurethane-Acrylate Dispersion: The acrylate-terminated silicone-modified polyurethane prepolymer was cooled to 35-45°C, and 3.6-6.4 parts by weight of the neutralizing agent triethylamine (TEA) were added. The mixture was stirred for 15-20 minutes for neutralization. Under high-speed shear dispersion conditions of 1500-3000 rpm, 120-200 parts of deionized water were slowly added. With the continuous addition of deionized water, the system underwent a phase transition from water-in-oil to oil-in-water. After the phase transition was completed, the shear rate was reduced to 500-800 rpm, and stirring was continued for 30-60 minutes to fully stabilize the system. Subsequently, the solvent acetone was evaporated at 40-50°C under reduced pressure, and deionized water was added to obtain the waterborne polyurethane-acrylate dispersion.

[0014] Furthermore, the capping agent is hydroxyethyl acrylate (HEA) or pentaerythritol triacrylate (PETA), or a mixture of both; the polymerization inhibitor is p-hydroxyanisole (MEHQ).

[0015] The preparation method of the reactive nano-silica aqueous dispersion includes: 3-15 parts of nano-silica are dispersed in a mixed solvent of 100-200 parts of ethanol and deionized water, with a volume ratio of ethanol to water of 3:1-5:1. The mixture is ultrasonically dispersed for 30-60 minutes to obtain a uniform nano-silica suspension. The pH of the suspension is adjusted to 4.0-5.0 with acetic acid. Add according to the following mass proportions under continuous stirring and ultrasonic assistance: Add 0.3 to 4.5 parts of silane coupling agent KH-570 dropwise, then heat to 55 to 65°C and react at a constant temperature for 3 to 5 hours. After the reaction was completed, the unreacted KH-570 and the hydrolysis byproduct methanol were removed by dialysis or high-speed centrifugation. The purified modified nano silica was redispersed in deionized water and ultrasonically dispersed for 30 minutes to obtain a reactive nano silica aqueous dispersion.

[0016] A reactive chlorinated polyolefin adhesion promoter, the preparation method of which includes: Under nitrogen protection, add according to the following mass proportions: Dissolve 60-100 parts of chlorinated polypropylene (CPP) in toluene or xylene. At this point, the mass concentration of chlorinated polypropylene (CPP) is 15%-25%. Heat the solution to 80-100°C and stir to dissolve. Add 1.8-8.0 parts of maleic anhydride (MAH) and 0.3-1.5 parts of initiator benzoyl peroxide (BPO), and react at 100-120°C for 2-4 hours; pour the reaction solution into acetone to precipitate, filter, dry, and vacuum dry to obtain maleic anhydride-grafted chlorinated polypropylene. Dissolve maleic anhydride-grafted chlorinated polypropylene again in toluene. At this point, the mass concentration of maleic anhydride-grafted chlorinated polypropylene is 15%~20%. Add according to the following parts by mass: 1.0-2.0 parts of hydroxyethyl acrylate (HEA) and 0.01-0.03 parts of p-toluenesulfonic acid catalyst are reacted at 110-130°C for 2-3 hours; the hydroxyl groups of HEA undergo an esterification ring-opening reaction with the anhydride groups on maleic anhydride-grafted chlorinated polypropylene. After the reaction was completed, the solvent was removed by vacuum evaporation to obtain a reactive chlorinated polyolefin adhesion promoter containing acrylate double bonds.

[0017] Compared with the prior art, the present invention has the following outstanding advantages: 1. The dual curing mechanism solves the problem of shadow curing, achieving a synergistic improvement in hardness and flexibility.

[0018] This invention employs a dual curing mode of "UV free radical photocuring + thermally induced isocyanate crosslinking" to construct an interpenetrating polymer network (IPN) of polyacrylate hard segments and silicone-modified polyurethane soft segments. During the UV curing stage, free radicals generated by the photoinitiator initiate rapid polymerization of the acrylate C=C double bonds, completing surface shaping within 30 seconds. During the thermal curing stage, the blocked isocyanate unblocks, releasing -NCO groups, which react with residual hydroxyl groups to form a polyurethane crosslinking network, compensating for insufficient curing in UV-shaded areas. Tests show that the coating achieves grade 0 adhesion (GB / T 9286-2021) in shaded areas such as recesses and inner corners of PP, PC, or ABS substrates, exhibiting complete crosslinking without defects. The IPN structure enables the coating to achieve a pencil hardness of 3H~4H (GB / T 6739-2022), while PDMS segment toughening results in no cracks in the Φ2mm shaft bending test (GB / T 6742-2007), resolving the contradiction of traditional water-based coatings being "brittle when hard and soft when flexible."

[0019] 2. Reactive nanofillers are embedded in the network through chemical bonding, eliminating interfacial defects and enhancing mechanical properties.

[0020] In-situ modification of nano-silica with KH-570 resulted in a surface grafting density of 15%–25% (mass fraction), carrying methacrylate groups capable of participating in UV polymerization. During the UV curing stage, the C=C double bonds on the surface of the nano-silica polymerized simultaneously with the resin matrix, embedding themselves into the cross-linked network via covalent bonds, rather than through physical blending. This "active cross-linking" strategy made the nanoparticles additional cross-linking nodes in the network, eliminating interfacial defects. Testing with a CS-10 grinding wheel (500g load, 100 cycles) showed a scratch resistance gloss retention rate of 88%–96% (GMW14688), more than 60% higher than the control system without nanofillers (55%–70%); Taber abrasion (CS-10 / 1000g / 1000r) was only 6.3–8.5 mg (GB / T 1768-2006), significantly better than the physically blended system (15.2–28.5 mg).

[0021] 3. The reactive adhesion promoter enables primer-free adhesion and is compatible with a variety of substrate materials.

[0022] CPP-g-MAH-HEA achieves adhesion through a dual mechanism of "physical anchoring + chemical bonding": the dispersion force and molecular chain entanglement between CPP segments and the PP substrate provide physical anchoring, while the acrylate double bonds participate in UV crosslinking, embedding the accelerator into the coating network and avoiding the migration and precipitation of traditional CPO primers. Cross-cut adhesion testing (GB / T 9286-2021) shows that the coating has an adhesion rating of 0 on PP, PC, and ABS substrates. After water resistance tests at 40℃ for 240 hours (GB / T 1733-1993) and damp heat tests at 85℃ for 85%RH for 240 hours, the adhesion retention rate is ≥95%. No flame / plasma treatment or primer step is required, simplifying the process by more than 30%.

[0023] 4. Chemically block PDMS provides long-lasting hydrophobic and weather-resistant properties, improving outdoor service life.

[0024] PDMS segments are chemically bonded into the polyurethane backbone (not a physical blend). Their low surface energy allows the coating to achieve a water contact angle of 95°~105° (GB / T 30693-2014), reducing water molecule penetration. The Si-O-Si bond energy (452kJ / mol) is significantly higher than that of the C-C bond (348kJ / mol) and CO bond (358kJ / mol), resisting UV oxidative degradation. After 1000 hours of QUV accelerated aging (GB / T 1865-2009, UVB-313 lamp, 60℃×4h illumination / 50℃×4h condensation), the coating retains 88%~94% of its 60° gloss and has a color difference ΔE≤2.2, significantly better than the unmodified system (62%~75% gloss retention, ΔE≥5.2).

[0025] 5. The water-based high-solids formula achieves ultra-low VOC emissions, which is in line with the trend of green coating.

[0026] The coating of this invention has a solid content of 35%~45% and a measured VOC content of 72~85 g / L (GB / T 23985-2009), meeting the requirements of GB24409-2020 (≤420 g / L) and GB / T 38597-2020 (≤100 g / L), which is only 1 / 4~1 / 5 of that of commercially available solvent-based coatings. Water is used as the dispersion medium during the coating application stage, resulting in low VOC in the finished product. The acetone / alcohol solvents used in the preparation process can be recycled under reduced pressure, thereby reducing overall emissions and costs. The primer-free process further eliminates primer VOC emissions (traditional primer VOC is approximately 300~500 g / L), resulting in an overall emission reduction of 60%~75%. Simultaneously, the water-based system eliminates the need for large amounts of organic solvents, reducing storage, transportation, and safety risks during use, aligning with the development direction of "environmentally friendly and low-energy consumption" in automotive coatings. Attached Figure Description

[0027] Figure 1This is a flowchart of the present invention. Detailed Implementation

[0028] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0029] Dual-curing waterborne polyurethane-acrylate automotive coating, composed of the following components (parts by weight): 1. Waterborne polyurethane-acrylate dispersion (WPUA-Si dispersion): 60~80 parts; 2. Reactive nano-silica aqueous dispersion (RNanoSiO2 dispersion): 3~15 parts; 3. Reactive chlorinated polyolefin adhesion promoter (CPP-g-MAH-HEA): 2~8 parts; 4. Aqueous photoinitiator: 1-3 parts; 5. Blocked isocyanate crosslinking agent: 3-12 parts; 6. Additives: 0.3~2 parts (including substrate wetting agent, defoamer, and leveling agent); 7. Deionized water: appropriate amount (to adjust the viscosity for application).

[0030] Figure 1 This is a flowchart of the present invention, such as... Figure 1 As shown, the dual-curing waterborne polyurethane-acrylate automotive coating includes the following steps: Step 1: Preparation of silicone-modified polyurethane prepolymer.

[0031] Under the protection of dry nitrogen, add the following by mass to a four-necked flask equipped with a stirrer, thermometer, and reflux condenser: 100 parts of dehydrated polycarbonate diol (PCDL) with a number-average molecular weight (Mn) of 1000-2000 g / mol were used as the soft segment matrix. 5-20 parts of hydroxyl-terminated polydimethylsiloxane PDMS-OH, with a number-average molecular weight Mn of 500-2000 g / mol, are used as the modified chain segment; 4-8 parts of dimethylolpropionic acid (DMPA) are used as a hydrophilic chain extender and carboxyl group source; Then, add acetone as a solvent to fully dissolve the solid components, stir, and heat to 60-70°C. The amount of acetone used should be sufficient to form a stirable homogeneous phase. The amount of acetone used, to form a stirable homogeneous phase, can be 90-110 parts.

[0032] Next, slowly add 52 to 60 parts of isophorone diisocyanate (IPDI) dropwise, controlling the dropping rate to maintain the reaction temperature between 70 and 80°C.

[0033] After the addition is complete, add 0.03~0.10 parts of the catalyst dibutyltin dilaurate (DBTDL) and react at a constant temperature of 75~85℃ for 2~4 hours to obtain: organosilicon-modified polyurethane prepolymer.

[0034] Reaction monitoring and endpoint determination: The content of -NCO groups in the reaction system is monitored by the di-n-butylamine method until the -NCO content drops to near the theoretical preset value (i.e., retaining an appropriate amount of -NCO groups required for subsequent end-capping to avoid excessive reaction leading to gelation).

[0035] Output and critical control boundaries: The reaction yielded an organosilicon-modified polyurethane prepolymer solution (containing reserved -NCO active end groups).

[0036] When the amount of hydroxyl-terminated polydimethylsiloxane PDMS-OH is less than 5% of the mass of polycarbonate diol PCDL, the content of organosilicon segments is insufficient, and it cannot effectively exert the effects of flexibility modification and hydrophobic enhancement. When the amount of hydroxyl-terminated polydimethylsiloxane PDMS-OH exceeds 20% of the mass of polycarbonate diol PCDL, the compatibility between PDMS segments and polyurethane hard segments decreases significantly, which will lead to phase separation during subsequent water dispersion, affecting the storage stability and film uniformity of the dispersion. Dimethylolpropionic acid (DMPA), a key component for water-based coatings, contains two hydroxyl groups and one carboxyl group in its molecule. The two hydroxyl groups participate in the addition reaction of isocyanate, introducing the carboxyl group into the polyurethane backbone. The carboxyl group imparts water dispersibility to the polyurethane after subsequent neutralization and salt formation. When the amount of DMPA used is less than 4% of the mass of polycarbonate diol (PCDL), the hydrophilicity is insufficient after neutralization and salt formation, and the dispersion is prone to precipitation and stratification. When the amount used is higher than 8%, there are too many residual hydrophilic carboxylate ions in the coating, leading to a significant deterioration in water resistance.

[0037] The molecular weight of hydroxyl-terminated polydimethylsiloxane PDMS-OH is preferably in the range of 1000~1500 g / mol. PDMS segments in this molecular weight range can ensure sufficient segment mobility to impart flexibility, and will not significantly reduce compatibility with polyurethane matrix due to excessively long segments.

[0038] Step 2, Preparation of acrylate-terminated silicone-modified polyurethane prepolymer.

[0039] The temperature of the silicone-modified polyurethane prepolymer was lowered to 55-65℃, and under nitrogen protection, it was added according to the following mass proportions: 2.0 to 5.0 parts of end-capping agent, wherein the end-capping agent is hydroxyethyl acrylate (HEA) or pentaerythritol triacrylate (PETA), or a mixture thereof; 0.002 to 0.015 parts of a polymerization inhibitor, wherein the polymerization inhibitor is p-hydroxyanisole MEHQ; After the addition is complete, the temperature is raised to 65-75℃ and the reaction is maintained at this temperature for 2-3 hours. The reaction is monitored at 2270 cm⁻¹ using Fourier Transmission Infrared Spectroscopy (FTIR). -1 The disappearance of the -NCO characteristic absorption peak is observed. When the -NCO peak completely disappears (absorbance drops to the baseline level), it indicates that the capping reaction has reached its endpoint.

[0040] Subsequently, 0.5–1.5 parts of methanol were added, and stirring was continued for 30 minutes to quench any remaining trace amounts of -NCO groups. The final product was an acrylate-terminated silicone-modified polyurethane prepolymer, abbreviated as UV-PU-Si prepolymer.

[0041] Output and critical control boundaries: When hydroxyethyl acrylate (HEA) is used as the end-capping agent, one acrylate double bond is introduced into each polyurethane chain end (monofunctional end-capping), resulting in better coating flexibility and suitability for interior parts. When pentaerythritol triacrylate (PETA) is used as the end-capping agent, three acrylate double bonds are introduced into each chain end (trifunctional end-capping), significantly increasing the UV curing crosslinking density and resulting in better coating hardness and scratch resistance, suitable for exterior parts. When the total amount of end-capping agent is less than 2.0 parts, excessive -NCO residue leads to poor storage stability; when it is more than 5.0 parts, excessive end-capping agent residue affects the coating crosslinking density.

[0042] Step 3, neutralization and emulsification, preparation of waterborne polyurethane-acrylate dispersion.

[0043] The acrylate-terminated silicone-modified polyurethane prepolymer was cooled to 35-45℃ and added according to the following proportions by weight: Add 3.6 to 6.4 parts of the neutralizing agent triethylamine (TEA) and stir for 15 to 20 minutes to carry out the neutralization reaction.

[0044] Under high-speed shear dispersion (1500~3000 rpm), 120~200 parts of deionized water are slowly added; as deionized water is continuously added, the system undergoes a phase transition process from water in oil (W / O) to oil in water (O / W).

[0045] After the phase transition is complete, the shear rate is reduced to 500-800 rpm, and stirring is continued for 30-60 minutes to ensure the system is fully stable. Then, the solvent acetone is evaporated at 40-50℃ under reduced pressure, and deionized water is added to adjust the solid content to 35-45% to obtain the waterborne polyurethane-acrylate dispersion, abbreviated as WPUA-Si dispersion.

[0046] The waterborne polyurethane-acrylate dispersion is a milky white to semi-transparent liquid with an average particle size of 50-150 nm, a pH of 7.0-8.5, and a room temperature storage stability of ≥6 months. The neutralizing agent, triethylamine (TEA), partially evaporates during the coating's thermal curing stage, reducing the residual hydrophilic ion content in the coating and helping to improve its final water resistance. However, excessive TEA usage leads to a higher dispersion pH, increased viscosity, and residual amines that negatively impact the coating's water resistance. When the amount of deionized water is less than 120 parts, the dispersion has an excessively high solid content and viscosity, making shear emulsification difficult and prone to aggregation. When it exceeds 200 parts, the solid content is too low, increasing the energy consumption for water evaporation and making defects such as sagging and pinholes more likely to occur during film formation.

[0047] Step 4, Preparation of reactive nano-silica aqueous dispersion.

[0048] 3-15 parts of nano-silica (native particle size 20-50 nm, specific surface area 150-380 m²) 2 The nano-silica (g) was dispersed in 100-200 parts of a mixed solvent of ethanol and deionized water (ethanol to water volume ratio of 3:1-5:1) and ultrasonically dispersed for 30-60 minutes (ultrasonic power 200-400W) to obtain a uniform nano-silica suspension. The pH of the suspension was adjusted to 4.0-5.0 with acetic acid.

[0049] Add according to the following mass proportions under continuous stirring and ultrasonic assistance: Add 0.3 to 4.5 parts of silane coupling agent KH-570 dropwise, then heat to 55 to 65°C and react at a constant temperature for 3 to 5 hours.

[0050] KH-570 is γ-(methacryloyloxy)propyltrimethoxysilane.

[0051] After the reaction was completed, the product was removed by dialysis (molecular weight cutoff 8000~14000Da) or high-speed centrifugation (10000~15000rpm, 15~20min) to remove unreacted KH-570 and hydrolysis byproduct methanol. The purified modified nano silica was redispersed in deionized water and ultrasonically dispersed for 30min to obtain reactive nano silica aqueous dispersion, abbreviated as RNanoSiO2 dispersion.

[0052] A reactive nano-silica aqueous dispersion with a solid content of 10%~20%, an average particle size of 30~80nm, and a storage stability of ≥3 months.

[0053] When the amount of KH-570 is too low, the C=C grafting density on the surface of nano-silica is insufficient. During UV curing, the nanoparticles cannot fully participate in the cross-linking reaction, and most nanoparticles are only physically embedded in the coating, resulting in insignificant enhancement of chemical bonding. When the amount of KH-570 is too high, excess KH-570 forms multiple layers of physically adsorbed or self-polymerized siloxane oligomers on the surface of nano-silica, forming a physically adsorbed layer. This reduces the effective chemical connection between the nanoparticles and the resin matrix. Furthermore, the unbonded excess C=C groups form uneven, locally highly cross-linked regions during UV curing, potentially introducing internal stress. Under acidic catalytic conditions, the KH-570 molecule undergoes the following chemical evolution process: First, the methoxy group (-OCH3) at the end of the KH-570 molecule undergoes hydrolysis in an acidic environment provided by trace amounts of water and acetic acid, removing methanol molecules and generating highly reactive silanol groups (-Si-OH). Subsequently, the newly generated silanol groups undergo dehydration condensation with the native silanol groups (Si-OH) on the surface of the nano-silica particles, forming stable Si-O-Si covalent bonds. This process is not a simple physical adsorption, but rather a process that firmly anchors the KH-570 molecule to the surface of the nanoparticles through chemical bonds.

[0054] It is worth noting that the methacryloyloxy group (CH2=C(CH3)COO-) at the other end of the KH-570 molecule remains chemically intact throughout the modification process. This group acts as a polymerizable molecular anchor, making the surface of the modified nano-silica rich in acrylate double bonds (C=C), thus enabling it to participate in free radical polymerization in the subsequent UV curing stage and embed itself into the coating network as chemical crosslinking points.

[0055] Step 5: Preparation of reactive chlorinated polyolefin adhesion promoter.

[0056] Under nitrogen protection, add according to the following mass proportions: Dissolve 60-100 parts of chlorinated polypropylene (CPP) (chlorine content 25%-35%, number average molecular weight Mn=5000-15000g / mol) in toluene or xylene. At this time, the mass concentration of chlorinated polypropylene (CPP) is 15%-25%. Heat to 80-100℃ and stir to dissolve.

[0057] Add 1.8–8.0 parts of maleic anhydride (MAH) and 0.3–1.5 parts of benzoyl peroxide (BPO) as initiator, and react at 100–120 °C for 2–4 hours. Pour the reaction solution into acetone to precipitate, filter, dry, and vacuum dry to obtain maleic anhydride-grafted chlorinated polypropylene, abbreviated as CPP-g-MAH.

[0058] Dissolve maleic anhydride-grafted chlorinated polypropylene again in toluene. At this point, the mass concentration of maleic anhydride-grafted chlorinated polypropylene is 15%~20%. Add according to the following parts by mass: 1.0-2.0 parts of hydroxyethyl acrylate (HEA) and 0.01-0.03 parts of p-toluenesulfonic acid catalyst are reacted at 110-130°C for 2-3 hours. The hydroxyl groups of HEA undergo an esterification ring-opening reaction with the anhydride groups on maleic anhydride-grafted chlorinated polypropylene.

[0059] After the reaction is complete, the solvent is removed by vacuum evaporation to obtain a reactive chlorinated polyolefin adhesion promoter containing acrylate double bonds, abbreviated as CPP-g-MAH-HEA.

[0060] When the amount of hydroxyethyl acrylate (HEA) is too low, the grafting rate is low; when the amount is too high, the CPP chain segments degrade.

[0061] The resulting reactive chlorinated polyolefin adhesion promoter is a pale yellow viscous liquid with the following characteristics: 1. The chlorinated polyolefin segments in the chlorinated polypropylene (CPP) backbone exhibit good compatibility and affinity with the surface of polyolefin substrates such as PP, achieving close contact with the substrate through molecular chain entanglement and dispersion forces. 2. The grafted acrylate double bonds (C=C) can participate in the free radical polymerization reaction during the UV curing stage of the coating system, allowing the adhesion promoter to be chemically bonded into the coating crosslinking network, rather than existing as a low-molecular-weight additive free in the coating. 3. The adhesion promoter anchored in the coating network through chemical bonding will not migrate or precipitate during use, ensuring long-term adhesion durability. Compared to traditional physically blended CPO primers, this invention directly integrates the adhesion-promoting function into the topcoat formulation, eliminating the need for a separate primer application step and achieving a primer-free process.

[0062] Step 6: Preparation of dual-curing waterborne nanocomposite coating.

[0063] In a paint mixing tank equipped with a stirring device, add the following components in the following order by weight, stirring for 5-10 minutes after each addition: 60-80 parts of aqueous polyurethane-acrylate dispersion (prepared in step 3); 3-15 parts of reactive nano-silica aqueous dispersion (prepared in step 4); 2-8 parts of reactive chlorinated polyolefin adhesion promoter (prepared in step 5); 1 to 3 parts of an aqueous photoinitiator, selected from 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylphenylacetone (Irgacure 2959) or phenyl-2,4,6-trimethylbenzoyl lithium phosphite (LAP), preferably Irgacure 2959; 3 to 12 parts of blocked isocyanate crosslinking agent, selected from blocked hexamethylene diisocyanate (HDI) trimer or blocked isophorone diisocyanate (IPDI) trimer, with a deblocking temperature of 80 to 120°C; 0.3 to 2.0 parts of additives, including 0.1 to 0.5 parts of substrate wetting agent (polyether modified organosiloxane, such as BYK-349), 0.1 to 0.5 parts of defoamer (acetylenol, such as Surfynol 104E) and 0.1 to 1.0 parts of leveling agent (polyurethane associative type, such as RM-2020).

[0064] Then add deionized water to obtain a dual-curing waterborne nanocomposite coating. Adjust the coating to the application viscosity with deionized water: 25-40 seconds with a Forco 4 cup, or 500-1500 mPa·s measured with a rotational viscometer.

[0065] Dual-curing waterborne nanocomposite coating, VOC content ≤100g / L, room temperature storage stability ≥6 months.

[0066] The compatibility of each component is explained as follows: The waterborne polyurethane-acrylate dispersion serves as the main film-forming material (base material) of the coating, providing the basic mechanical properties and adhesion foundation. The reactive nano-silica aqueous dispersion provides a nano-reinforcing effect, where the C=C groups on the surface of the nano-silica participate in cross-linking during the UV curing stage, chemically bonding the nanofillers into the coating network. The C=C groups of the reactive chlorinated polyolefin adhesion promoter also participate in UV cross-linking, achieving chemical integration between the promoter and the coating network. The waterborne photoinitiator generates free radicals under UV irradiation, initiating the polymerization reaction of the acrylate C=C double bonds to form a polyacrylate hard segment network. The blocked isocyanate does not participate in the reaction during the UV curing stage (due to protection by the blocking agent), but in the subsequent thermosetting stage, it unblocks and releases -NCO groups, reacting with residual hydroxyl groups in the system (from the ends of PCDL segments, DMPA side groups, and residual hydroxyl groups on the surface of nano-silica) to form a polyurethane soft segment cross-linked network. The UV-cured network and the thermosetting network interpenetrate each other, forming an interpenetrating polymer network (IPN) structure.

[0067] When the reactive nano-silica aqueous dispersion is used in amounts less than 3 parts, the nano-reinforcing effect is not significant, and the improvement in coating hardness and scratch resistance is limited. When the amount exceeds 15 parts, the nanoparticles tend to aggregate in the coating, which leads to a decrease in coating transparency, a deterioration in flexibility (microcracks appear when bending), and a sharp increase in coating viscosity, resulting in poor spray application. The preferred range of 3 to 12 parts in this invention achieves the best balance between the nano-reinforcing effect and the overall performance of the coating.

[0068] The unsealing temperature must be higher than the coating temperature that can be reached during the UV curing stage (typically, a small amount of heat radiation from a UV-LED light source can raise the coating temperature to 40~60℃), but lower than the heat distortion temperature of the plastic substrate (approximately 90~110℃ for PP, and approximately 100~130℃ for PC and ABS). This invention selects a blocked isocyanate with an unsealing temperature of 80~120℃, and sets the post-curing temperature to 90~120℃, ensuring complete unsealing and full cross-linking of the sealant without causing deformation of the plastic substrate.

[0069] Application methods for dual-curing waterborne polyurethane-acrylate automotive coatings include: Step 1, Substrate Pretreatment: The plastic substrate (PP, PC, ABS, or their blends) to be coated is cleaned by wiping the surface with isopropanol or ethanol to remove oil and mold release agent, and then dried at 50-60°C for 5-10 minutes. The coating of this invention already contains a reactive CPO adhesion promoter, therefore the substrate does not require traditional pretreatment processes such as flame treatment, plasma treatment, or primer coating, significantly simplifying the process.

[0070] Step 2, Coating application: Apply the coating evenly to the substrate surface using air spraying (spray gun nozzle diameter 1.0~1.5mm, atomization pressure 0.25~0.40MPa), electrostatic spraying, or high flow rate low pressure HVLP spraying, and control the dry film thickness to be 25~55μm.

[0071] Step 3, Flash-drying: Place the sprayed workpiece in a drying oven at 60~80℃ for 5~10 minutes to allow the coating to dry to the touch until it is no longer sticky. The flash-drying temperature should not exceed 80℃ to prevent the blocked isocyanate from prematurely desealing and reacting with water to form urea and CO2 before the moisture is fully removed, which could lead to defects such as pinholes and bubbles in the coating.

[0072] Step 4, UV irradiation curing: Irradiate with a 365nm wavelength LED-UV light source (or a mercury lamp UV light source), with an energy of 800~2000mJ / cm². 2 The time is 5 to 30 seconds.

[0073] Under UV irradiation, the photoinitiator (Irgacure 2959) absorbs photon energy to generate free radicals, initiating free radical polymerization of the acrylate C=C double bonds in the system, including: (a) acrylate groups at the ends of the WPUA-Si chain; (b) methacrylate groups grafted onto the surface of RNanoSiO2; and (c) acrylate groups on CPP-g-MAH-HEA. The C=C double bonds from these three sources simultaneously participate in polymerization, forming a chemically bonded network of polyacrylate hard segments, in which nano-SiO2 and the CPO adhesion promoter are covalently embedded.

[0074] Step 5, post-heat curing: Transfer the UV-cured workpiece to an oven and heat it at 90~120℃ for 20~30 minutes (90~100℃ for PP substrates, 100~120℃ for PC and ABS substrates) to deseal the blocked isocyanate and react with the residual active hydrogen groups.

[0075] During the thermosetting stage, the blocking agents in the blocked isocyanate crosslinking agents (such as methyl ethyl ketone oxime (MEKO), caprolactam, or diethyl malonate) deblock at high temperatures, releasing free -NCO groups. These -NCO groups react with residual active hydrogen groups in the system (including terminal hydroxyl groups of polycarbonate diol segments, residual hydroxyl functional groups in the system, and silanol groups -SiOH on the surface of nano-silica), forming urethane bonds and constructing a polyurethane crosslinking network. Simultaneously, the water content needs to be controlled to suppress the side reactions between -NCO and water.

[0076] Step 6, Cooling: Allow to cool naturally to room temperature to obtain the finished product with double-cured coating.

[0077] In the construction method, UV irradiation curing precedes heat curing to prevent premature desealing of the sealant, which can lead to air bubbles, and to ensure the initial strength of the coating: 1. UV curing first builds the acrylate hard segment network, providing initial strength and dimensional stability for the entire coating and preventing excessive flow or sagging of the coating due to temperature rise during the heat curing stage; 2. If heat curing is performed first, the desealed isocyanate may consume some of the active groups (such as hydroxyl groups) in the system that should participate in UV crosslinking, reducing UV curing efficiency and final crosslinking density; 3. UV curing can be completed quickly near room temperature, and the sequence of UV curing followed by heat curing is easier to implement in industrialized production lines. After the UV segment has been rapidly cured and set, the workpieces can be stacked or transferred to the heat curing oven without worrying about adhesion between coatings.

[0078] The resulting coated pencil has a hardness ≥3H, adhesion grade 0, and scratch resistance and gloss retention rate ≥88%.

[0079] This invention achieves its technical effects through the following synergistic mechanism: (1) Dual curing mechanism: Constructing an IPN structure to resolve the contradiction between hardness and flexibility. A dual curing mode of "UV free radical photocuring + thermally induced isocyanate crosslinking" is adopted: UV stage (365nm LED-UV, 800~2000mJ / cm) 2The process initiates rapid polymerization of the C=C double bonds of acrylate, forming a hard segment network of polyacrylate within 30 seconds. During the thermal phase (90~120℃, 20~30min), the blocked isocyanate is unblocked, releasing -NCO groups, which react with residual hydroxyl groups to construct a soft segment network of silicone-modified polyurethane. The two networks interpenetrate to form an IPN structure, enabling the coating to achieve a pencil hardness of 3H~4H (GB / T 6739-2022). Simultaneously, PDMS segments are used for toughening, resulting in no cracks when bending a Φ2mm shaft (GB / T 6742-2007), overcoming the shortcomings of traditional water-based coatings that are "brittle when hard and soft when soft."

[0080] (2) Reactive nanofillers: chemically bonded and embedded in the network to eliminate interfacial defects. Nano-silica (original particle size 20~50nm) is modified in situ by KH-570 (γ-methacryloyloxypropyltrimethoxysilane): under acidic conditions (pH 4~5), KH-570 hydrolyzes to generate silanol groups (-Si-OH), which condense with -OH on the silica surface to form Si-O-Si covalent bonds, with a grafting density of 15%~25% (mass fraction), and the surface carries methacrylate groups (C=C). During UV curing, C=C polymerizes synchronously with the resin matrix, and nanoparticles are embedded in the cross-linking network by covalent bonds (non-physical blending), eliminating interface defects and becoming additional cross-linking nodes, resulting in a scratch resistance gloss retention rate (CS-10 grinding wheel, 500g load, 100 cycles) of 88%~96% (GMW14688), and Taber abrasion (CS-10 / 1000g / 1000r) of only 6.3~8.5mg (GB / T 1768-2006).

[0081] (3) Reactive adhesion promoter: No primer required, compatible with multiple substrate materials. CPP (chlorine content 25%~35%) is grafted with MAH (100~120℃, 2~4h) to introduce anhydride groups, and then esterified with HEA to open the ring (110~130℃, 2~3h) to graft acrylate double bonds, thus obtaining CPP-g-MAH-HEA. Its mechanism of action is: CPP segments and PP substrate achieve physical anchoring through dispersion force and molecular chain entanglement; acrylate double bonds participate in UV crosslinking, so that the promoter is embedded in the coating network by chemical bonds, avoiding the migration and precipitation of traditional CPO primer. Cross-cut adhesion test (GB / T 9286-2021) shows that the coating has an adhesion grade of 0 on PP, PC and ABS substrates. After water resistance (GB / T 1733-1993) at 40℃ for 240h and damp heat resistance at 85℃ for 85%RH for 240h, the adhesion retention rate is ≥95%, and no flame / plasma treatment or primer process is required.

[0082] (4) Organosilicon block modification: long-lasting hydrophobicity and weather resistance, improving service life. PDMS-OH (Mn=500~2000g / mol) is introduced into the polyurethane backbone in a chemical block manner (non-physical blending): the low surface energy of PDMS enables the water contact angle of the coating to reach 95°~105° (GB / T 30693-2014), reducing water molecule penetration; the Si-O-Si bond energy (452kJ / mol) is much higher than that of CC bond (348kJ / mol) and CO bond (358kJ / mol), resisting UV oxidation degradation. After 1000 hours of QUV accelerated aging (GB / T 1865-2009, UVB-313 lamp), the 60° gloss retention rate is 88%~94%, and the color difference ΔE≤2.2, which is significantly better than the unmodified system (gloss retention rate 62%~75%, ΔE≥5.2).

[0083] The following specific embodiments further illustrate the technical solution and beneficial effects of the present invention. The raw materials used in the embodiments are all industrial grade, and can be used directly or after dehydration treatment. Solid content is the percentage by mass of the dehydrated substance relative to the wet sample. The dehydrated substance does not contain volatile components such as water, organic solvents, or unreacted low molecular weight molecules.

[0084] Example 1: Coating with hydroxyethyl acrylate (HEA) end capping and nano-silica reinforcement.

[0085] Step 1: Preparation of silicone-modified polyurethane prepolymer. Under a dry nitrogen atmosphere, the following were added to a 1000 mL four-necked flask: 100 g of dehydrated polycarbonate diol (PCDL) (number average molecular weight 1500 g / mol); 12.0 g of hydroxyl-terminated polydimethylsiloxane (PDMS-OH) (number average molecular weight 1200 g / mol); and 6 g of dimethylolpropionic acid (DMPA). 150 mL of acetone was added as solvent (to form a stirable homogeneous phase), and the mixture was stirred and heated to 65 °C to fully dissolve the solids. 60 g of isophorone diisocyanate (IPDI) was slowly added dropwise (controlling the dropping rate to maintain the reaction temperature at 70–80 °C), followed by 0.10 g of dibutyltin dilaurate (DBTDL) as catalyst. The reaction was maintained at 80 °C for 3 hours. The -NCO content was determined to be 3.82 wt% by the di-n-butylamine method, indicating complete prepolymerization.

[0086] Step 2, Preparation of acrylate-terminated silicone-modified polyurethane prepolymer.

[0087] The prepolymer was cooled to 60°C, and 0.008 g of the polymerization inhibitor p-hydroxyanisole (MEHQ) was added. Then, 3 g of hydroxyethyl acrylate (HEA) was slowly added dropwise. The reaction was carried out at 70°C for 2.5 hours, and the reaction was monitored at 2270 cm⁻¹ using Fourier transform infrared spectroscopy (FTIR). -1The complete disappearance of the -NCO characteristic peak indicates that the end-capping reaction has reached its endpoint. 1.0 mL of methanol was added to quench the remaining -NCO groups, and the mixture was stirred for 30 minutes to obtain an acrylate-terminated silicone-modified polyurethane prepolymer, abbreviated as UV-PU-Si-HEA.

[0088] Step 3, neutralization and emulsification, preparation of waterborne polyurethane-acrylate dispersion.

[0089] The acrylate-terminated silicone-modified polyurethane prepolymer was cooled to 40°C, and 5.3 g of triethylamine (TEA) was added as a neutralizing agent. The mixture was stirred for 15 minutes to carry out a neutralization reaction. 380 g of deionized water at 35°C was slowly added under high-speed shear (2000 rpm). After the system underwent a phase transition from water-in-oil (W / O) to oil-in-water (O / W), the shear rate was reduced to 600 rpm, and stirring continued for 45 minutes. Acetone was removed by vacuum distillation at 40°C, and deionized water was added to adjust the solid content to 40%, yielding an aqueous polyurethane-acrylate dispersion, abbreviated as WPUA-Si-HEA dispersion (average particle size 95 nm, pH=7.8).

[0090] Step 4, Preparation of reactive nano-silica aqueous dispersion.

[0091] 10.0 g of nano-silica (AEROSIL 200, native particle size 20 nm) was added to a mixed solvent of 150 mL ethanol and 40 mL deionized water and dispersed by sonication (300 W) in an ice bath for 45 minutes. The pH of the suspension was adjusted to 4.5 with acetic acid, and 2.0 g of silane coupling agent KH-570 (γ-methacryloyloxypropyltrimethoxysilane) was slowly added dropwise under stirring and sonication. The reaction was carried out at 60 °C with sonication for 4 hours. The reaction solution was dialyzed through a dialysis bag (molecular weight cutoff 14000 Da) for 48 hours to remove impurities, and then sonicated and dispersed in deionized water to obtain a reactive nano-silica aqueous dispersion with a solid content of 15%, referred to as RNanoSiO2 dispersion (average particle size 45 nm).

[0092] Step 5: Preparation of reactive chlorinated polyolefin adhesion promoter.

[0093] 80.0 g of chlorinated polypropylene (CPP) (chlorine content 30%, number average molecular weight approximately 10,000) was dissolved in 250 mL of toluene and stirred at 90 °C. 2.5 g of maleic anhydride (MAH) and 0.5 g of benzoyl peroxide (BPO) initiator were added, and the mixture was reacted at 110 °C for 3 hours. The reaction solution was poured into acetone to precipitate, filtered, washed, and dried to obtain maleic anhydride-grafted chlorinated polypropylene CPP-g-MAH. The maleic anhydride-grafted chlorinated polypropylene was redissolved in toluene, at a mass concentration of 20%, and 1.5 g of hydroxyethyl acrylate (HEA) and 0.02 g of p-toluenesulfonic acid were added. The mixture was reacted at 120 °C for 2.5 hours. The solvent was removed under reduced pressure to obtain a reactive chlorinated polyolefin adhesion promoter, abbreviated as CPP-g-MAH-HEA.

[0094] Step 6: Preparation of dual-curing waterborne nanocomposite coating.

[0095] Add the following components to the paint mixing tank in sequence (stir for 5-10 minutes after each component is added): 70g of waterborne polyurethane-acrylate dispersion; 9g of reactive nano-silica aqueous dispersion; 5g of reactive chlorinated polyolefin adhesion promoter; 2g of aqueous photoinitiator Irgacure 2959; 7g of blocked isocyanate crosslinking agent (blocked hexamethylene diisocyanate trimer, unblocking temperature approximately 100°C); 1.1g of additives (0.3g of substrate wetting agent BYK-349, 0.3g of defoamer Surfynol 104E, and 0.5g of leveling agent RM-2020).

[0096] After stirring evenly, add deionized water to adjust the viscosity to 30 seconds using a Forte 4 cup to obtain a double-curing waterborne nanocomposite coating.

[0097] Application methods for dual-curing waterborne polyurethane-acrylate automotive coatings include: Polypropylene (PP) sheets (100mm × 50mm × 3mm) were wiped with isopropyl alcohol and dried at 50°C for 5 minutes. Air spraying (1.3mm nozzle diameter, 0.3MPa atomization pressure) was then used to coat the sheets, resulting in a wet film thickness of approximately 120μm and a final dry film thickness of 38μm. The coating was then flash-dried at 70°C for 7 minutes and irradiated with a 365nm wavelength LED-UV light source (energy 1200mJ / cm², 15 seconds). Subsequently, it was heat-cured at 95°C for 25 minutes and allowed to cool naturally to room temperature to obtain a double-cured coating.

[0098] Example 2: Coating with pentaerythritol triacrylate PETA end capping and nano-silica reinforcement. Basically the same as Example 1, except that: Step 1: Preparation of silicone-modified polyurethane prepolymer.

[0099] 100.0g of dehydrated polycarbonate diol PCDL (number average molecular weight 1000 g / mol); 5.0g of hydroxyl-terminated polydimethylsiloxane PDMS-OH (number average molecular weight 1500 g / mol); 4.0g of dimethylolpropionic acid DMPA; 52g of isophorone diisocyanate IPDI; 0.03g of catalyst dibutyltin dilaurate DBTDL.

[0100] Step 2, Preparation of acrylate-terminated silicone-modified polyurethane prepolymer.

[0101] The end-capping agent was changed to 2g of pentaerythritol triacrylate PETA and 0.002g of polymerization inhibitor MEHQ, and the reaction was carried out at 65℃ for 2 hours. The trifunctionality of pentaerythritol triacrylate PETA introduces a higher density of acrylate double bonds C=C at the polyurethane chain ends, which significantly improves the crosslinking potential for subsequent UV curing.

[0102] Subsequently, 0.5 g of methanol was added, and stirring was continued for 30 minutes to quench any remaining trace amounts of -NCO groups. The final product was an acrylate-terminated silicone-modified polyurethane prepolymer, abbreviated as UV-PU-Si prepolymer.

[0103] Step 3, neutralization and emulsification, preparation of waterborne polyurethane-acrylate dispersion.

[0104] The acrylate-terminated silicone-modified polyurethane prepolymer was cooled to 35°C and added according to the following proportions by weight: Add 3.6g of the neutralizing agent triethylamine (TEA) and stir for 15 minutes to carry out the neutralization reaction.

[0105] Add approximately 120g of deionized water to obtain an aqueous polyurethane-acrylate dispersion (solid content 40%, average particle size 110nm, pH=7.6).

[0106] Step 4, Preparation of reactive nano-silica aqueous dispersion.

[0107] 3.0g of nano-silica, 0.3g of silane coupling agent KH-570, and the rest are the same as in Example 1, to obtain a reactive nano-silica aqueous dispersion.

[0108] Step 5: Preparation of reactive chlorinated polyolefin adhesion promoter.

[0109] Under nitrogen protection, add according to the following mass proportions: 60g of chlorinated polypropylene (CPP) (chlorine content 25%, number average molecular weight Mn=5000g / mol) is dissolved in toluene or xylene. At this point, the mass concentration of chlorinated polypropylene (CPP) is 15%. The solution is heated to 80℃ and stirred to dissolve.

[0110] Add 1.8 g of maleic anhydride (MAH) and 0.3 g of benzoyl peroxide (BPO) as initiator, and react at 100 °C for 2 hours. Pour the reaction solution into acetone to precipitate, filter, dry, and vacuum dry to obtain maleic anhydride-grafted chlorinated polypropylene, abbreviated as CPP-g-MAH.

[0111] Dissolve maleic anhydride-grafted chlorinated polypropylene again in toluene. At this point, the mass concentration of maleic anhydride-grafted chlorinated polypropylene is 15%. Add the following according to the mass percentages: 1.0 g of hydroxyethyl acrylate (HEA) and 0.01 g of p-toluenesulfonic acid catalyst were reacted at 110 °C for 2 hours. The hydroxyl groups of HEA underwent an esterification ring-opening reaction with the anhydride groups on maleic anhydride-grafted chlorinated polypropylene.

[0112] After the reaction is complete, the solvent is removed by vacuum evaporation to obtain a reactive chlorinated polyolefin adhesion promoter containing acrylate double bonds, abbreviated as CPP-g-MAH-HEA.

[0113] Step 6: Preparation of dual-curing waterborne nanocomposite coating.

[0114] Add the following ingredients to the paint mixing can in sequence: 60g of waterborne polyurethane-acrylate dispersion; 3g of reactive nano-silica aqueous dispersion; 2g of reactive chlorinated polyolefin adhesion promoter; 1g of waterborne photoinitiator Irgacure 2959; 3g of blocked isocyanate crosslinking agent (blocked isophorone diisocyanate (blocked IPDI) trimer, unblocking temperature approximately 110℃); 2.0g of additives, including 0.5g of substrate wetting agent (polyether modified organosiloxane BYK-349), 0.5g of defoamer (acetylenol 104E), and 1.0g of leveling agent (polyurethane associative type RM-2020).

[0115] Add deionized water to adjust the viscosity to 30 seconds using a Forte 4 cup to obtain a double-cured waterborne nanocomposite coating.

[0116] Application methods for dual-curing waterborne polyurethane-acrylate automotive coatings include: Polycarbonate (PC) sheets, acrylonitrile-butadiene-styrene (ABS) sheets, or ABS alloy sheets (100mm×50mm×3mm) with coating, 365 nm LED-UV irradiation energy 1500 mJ / cm² 2Heat-cured at 110℃ for 25 minutes, resulting in a dry film thickness of 42μm.

[0117] Example 3: A general-purpose coating with end-capping of hydroxyethyl acrylate (HEA) and pentaerythritol triacrylate (PETA) and reinforced with nano-silica.

[0118] Combining the advantages of Examples 1 and 2, the end capping method uses a mixture of hydroxyethyl acrylate (HEA) and pentaerythritol triacrylate (PETA) in a 2:1 molar ratio, which balances flexibility and hardness.

[0119] Step 1: Preparation of silicone-modified polyurethane prepolymer.

[0120] 100.0g of dehydrated polycarbonate diol PCDL (number average molecular weight 2000g / mol); 20g of hydroxyl-terminated polydimethylsiloxane PDMS-OH (number average molecular weight 2000g / mol); 8g of dimethylolpropionic acid DMPA; 60g of isophorone diisocyanate IPDI; 0.10g of catalyst dibutyltin dilaurate DBTDL.

[0121] Step 2, Preparation of acrylate-terminated silicone-modified polyurethane prepolymer.

[0122] The capping agent is a mixture of 2g of hydroxyethyl acrylate (HEA) and 3g of pentaerythritol triacrylate (PETA).

[0123] 0.015g of the polymerization inhibitor p-hydroxyanisole MEHQ was reacted at a constant temperature of 75℃ for 3 hours.

[0124] Subsequently, 1.5g of methanol was added, and stirring was continued for 30 minutes to quench any remaining trace amounts of -NCO groups. The final product was an acrylate-terminated silicone-modified polyurethane prepolymer, abbreviated as UV-PU-Si prepolymer.

[0125] Step 3, neutralization and emulsification, preparation of waterborne polyurethane-acrylate dispersion.

[0126] The acrylate-terminated silicone-modified polyurethane prepolymer was cooled to 45°C and added according to the following proportions by weight: 6.4g of the neutralizing agent triethylamine (TEA) was stirred for 20 minutes to carry out the neutralization reaction.

[0127] Add approximately 200g of deionized water to obtain an aqueous polyurethane-acrylate dispersion.

[0128] Step 4, Preparation of reactive nano-silica aqueous dispersion.

[0129] 15g of nano-silica, 4.5g of silane coupling agent KH-570, and the rest are the same as in Example 1, to obtain a reactive nano-silica aqueous dispersion.

[0130] Step 5: Preparation of reactive chlorinated polyolefin adhesion promoter.

[0131] Under nitrogen protection, add according to the following mass proportions: 100g of chlorinated polypropylene (CPP) is dissolved in toluene or xylene, at which point the mass concentration of chlorinated polypropylene (CPP) is 25%. The solution is heated to 100℃ and stirred to dissolve.

[0132] Add 8g of maleic anhydride (MAH) and 1.5g of benzoyl peroxide (BPO) as initiator, and react at 120℃ for 2 hours. Pour the reaction solution into acetone to precipitate, filter, dry, and vacuum dry to obtain maleic anhydride-grafted chlorinated polypropylene, abbreviated as CPP-g-MAH.

[0133] Dissolve maleic anhydride-grafted chlorinated polypropylene again in toluene. At this point, the mass concentration of maleic anhydride-grafted chlorinated polypropylene is 20%. Add the following by mass: 2.0 g of hydroxyethyl acrylate (HEA) and 0.03 g of p-toluenesulfonic acid catalyst were reacted at 130 °C for 3 hours. The hydroxyl groups of HEA underwent an esterification ring-opening reaction with the anhydride groups on maleic anhydride-grafted chlorinated polypropylene.

[0134] After the reaction is complete, the solvent is removed by vacuum evaporation to obtain a reactive chlorinated polyolefin adhesion promoter containing acrylate double bonds, abbreviated as CPP-g-MAH-HEA.

[0135] Step 6: Preparation of dual-curing waterborne nanocomposite coating.

[0136] Add the following ingredients to the paint mixing can in sequence: 80g of waterborne polyurethane-acrylate dispersion; 15g of reactive nano-silica aqueous dispersion; 8g of reactive chlorinated polyolefin adhesion promoter; 3g of waterborne photoinitiator Irgacure 2959; 12g of blocked isocyanate crosslinking agent (blocked hexamethylene diisocyanate HDI trimer, unblocking temperature approximately 120°C). 2.0g of additives, including 0.5g of substrate wetting agent (polyether modified organosiloxane BYK-349), 0.5g of defoamer (acetylenol 104E), and 1.0g of leveling agent (polyurethane associative type RM-2020).

[0137] Adjust the viscosity with deionized water to 40 seconds using a Forco 4 cup to obtain a dual-curing waterborne nanocomposite coating. The application method for dual-curing waterborne polyurethane-acrylate automotive coatings includes: Coating of polypropylene (PP) sheets, acrylonitrile-butadiene-styrene (Acrylonitrile-Butadiene-Styrene) sheets, or ABS general-purpose substrates (100mm×50mm×3mm), 365 nm LED-UV irradiation energy 1000 mJ / cm². 2 Heat-cured at 105℃ for 25 minutes, resulting in a dry film thickness of 40μm.

[0138] Comparative examples are as follows: Comparative Example 1 (Commercially available waterborne polyurethane coating): A commercially available two-component waterborne polyurethane automotive plastic coating (main agent: waterborne hydroxy acrylic dispersion; curing agent: hydrophilic modified HDI trimer) was selected and applied and cured according to the supplier's recommended ratio and process (80℃×30 min heat curing).

[0139] Comparative Example 2 (commercially available water-based UV-curable coating): A commercially available water-based UV-curable plastic coating (main component is water-based polyurethane acrylate) was selected, and photoinitiator Irgacure 2959 (2%) was added. It was cured by UV irradiation (365nm, 1500mJ / cm²) without a post-heat curing step.

[0140] Comparative Example 3 (Basic Formulation of the Invention without Nano-SiO2): The formulation is the same as in Example 1, but without the addition of reactive nano-silica aqueous dispersion (RNanoSiO2 dispersion). Water is used to replace the volume of the reactive nano-silica aqueous dispersion (RNanoSiO2 dispersion), and the remaining components and processes are completely identical. This is used to separately evaluate the reinforcing effect of nano-silica.

[0141] Performance testing and effect analysis: The coating samples of Examples 1-3 and Comparative Examples 1-3 were subjected to systematic performance tests. The test items, methods and results are as follows.

[0142] Test items and methods: 1. Pencil hardness: According to GB / T 6739-2022, Mitsubishi pencil, load 750g; 2. Adhesion: According to GB / T 9286-2021, cross-cut test (1mm spacing), 3M 600 tape; 3. Scratch resistance: According to GMW 14688 (General Motors standard), CS-10 grinding wheel, 500g load, 100 cycles, 20° gloss retention rate was measured; 4. Taber wear: According to GB / T 1768-2006, CS-10 grinding wheel, 1000g load, 1000 revolutions, the mass loss (mg) was determined. 5. Flexibility: According to GB / T 6742-2007, the shaft bending test (Φ2mm) is performed, and cracks are visually inspected. 6. Water resistance: According to GB / T 1733-1993, the adhesion retention rate was measured after immersion in deionized water at 40℃ for 240 hours and then allowed to recover for 2 hours. 7. Weather resistance: According to GB / T 1865-2009, QUV accelerated aging (UVB-313 lamp, 60℃×4h light exposure / 50℃×4h condensation alternation), after 1000 hours, the 60° gloss retention rate and color difference ΔE were measured. 8. VOC content: According to GB / T 23985-2009, gas chromatography method; 9. Chemical resistance: According to GB / T 9274-1988, soak in 0.1mol / L H2SO4, 0.1mol / L NaOH, and gasoline (92#) for 24 hours respectively, and visually evaluate the bubbling, discoloration, and softening grades (1~5, with grade 5 being the best). 10. Water contact angle: Measured according to GB / T 30693-2014, static contact angle.

[0143] The performance test results are shown in Tables 1 and 2.

[0144] Table 1. Comparison of main performance characteristics of coatings in each embodiment and comparative example.

[0145] Table 2, Detailed test results of chemical resistance in Example 1

[0146] As can be seen from the performance test data in Table 1, the coatings of Examples 1-3 of the present invention are significantly superior to Comparative Example 1 (commercially available water-based PU coating) and Comparative Example 2 (commercially available water-based UV coating) in many key performance indicators: 1. Balance between hardness and flexibility: The pencils in the examples achieved a hardness of 3H to 4H, which is 3 to 4 levels higher than that of Comparative Example 1 (HB). Simultaneously, no cracks were observed when bending at Φ2mm, verifying the effectiveness of PDMS organosilicon block modification in enhancing flexibility. Example 2, using PETA trifunctional end-capping and a higher KH-570 grafting amount, achieved a hardness of 4H, the highest among all samples, while maintaining good bending flexibility, indicating that hardness and flexibility can be flexibly controlled through adjusting formulation parameters.

[0147] 2. Adhesion: Examples 1-3 all showed adhesion of grade 0 on both PP and PC / ABS substrates, without requiring primer pretreatment. Comparative Examples 1 and 2 showed adhesion grades 3 and 2 on PP substrates, respectively, which were significantly insufficient. This is attributed to the chemical bonding and anchoring mechanism of the CPP-g-MAH-HEA adhesion promoter: the affinity of the CPO segments to the PP substrate provides a physical anchoring basis, while the acrylate functionalization fixes it covalently within the coating network, achieving stable and durable adhesion.

[0148] 3. Scratch and abrasion resistance: The scratch gloss retention rate of the examples reached 92%~96%, and the Taber abrasion was only 6.3~8.5 mg, which is a significant improvement compared to Comparative Example 1 (55% / 28.5mg) and Comparative Example 2 (70% / 18.7mg). The performance of Comparative Example 3 (without nano silica) (78% / 15.2mg) was significantly lower than that of Example 1 (92% / 8.5mg), which strongly demonstrates the reinforcing effect of nano silica: the surface chemically bonded nano silica acts as an additional crosslinking node and hard filler, and the dual mechanism jointly improves the scratch resistance and abrasion resistance of the coating.

[0149] 4. Water Resistance and Weather Resistance: The water adhesion retention rate (96%~98%) and QUV 1000h gloss retention rate (91%~94%) of the examples were significantly better than those of Comparative Example 1 (72% / 62%) and Comparative Example 2 (82% / 75%). The hydrophobic effect of the PDMS segments reduced the penetration rate of water molecules in the coating; the high bond energy and high resistance to UV degradation of the Si-O-Si bonds provided excellent weather protection. The water contact angle increased from 65° in Comparative Example 1 to 100° in Example 3, which clearly reflects the improvement in surface hydrophobicity brought about by PDMS block modification.

[0150] 5. VOC Content: The VOC content of the examples is 72~85 g / L, which is only about 1 / 4 to 1 / 5 of that of Comparative Example 1 (350 g / L) and about 1 / 2 of that of Comparative Example 2 (180 g / L), meeting the environmental protection requirements for low-VOC coatings. VOCs mainly come from trace amounts of additive solvents introduced during the coating formulation process and small molecules of blocking agents released during the deblocking of blocked isocyanates. By further optimizing the type of blocking agent (such as using low-volatility diethyl malonate blocking agents to replace MEKO blocking agents), VOCs can be further reduced to below 50 g / L.

[0151] 6. Curing Uniformity: Examples 1-3 and Comparative Example 1 (full thermosetting) all achieved complete curing in the shaded areas of the workpiece, while the shaded areas of Comparative Example 2 (pure UV curing) remained completely uncured. This result fully verifies the dual curing strategy combining UV curing and thermosetting adopted in this invention, which has a decisive advantage in solving the common problem of curing in shaded areas in the coating of three-dimensional complex-shaped workpieces such as automotive plastic parts. In actual production, the UV curing stage completes the rapid curing and shaping of most surface areas, while the thermosetting stage ensures complete cross-linking in the shaded and deep areas; the two complement each other synergistically.

[0152] 7. Overall Cost-Effectiveness Analysis: Although the examples add two preparation steps—nano-silica modification and CPO functionalization—these steps can be implemented on a large scale, and the increased raw material cost is approximately 1.2 to 1.5 times that of commercially available water-based PU coatings. However, omitting the primer step can reduce the total coating usage by about 30% and painting time by 40%, and the overall coating cost may actually be lower than the traditional primer + topcoat two-layer system. Taking PP bumper coating as an example, it is estimated that after using the coating of this invention, the overall cost of coating a single piece can be reduced by 15% to 25%, while VOC emissions are reduced by about 60% to 75%.

[0153] The dual-curing waterborne polyurethane-acrylate nanocomposite coating, its preparation method, and its application provided by this invention can be widely applied to automotive interior and exterior plastic parts (including but not limited to: bumpers, dashboards, door trim panels, rearview mirror housings, lamp cover frames, center console panels, air vent blades, pillar trim panels, seat back panels, etc.), as well as industrial fields with high requirements for coating protection and decoration, such as motorcycle and electric vehicle plastic body panels and household appliance plastic shells. The waterborne approach, dual-curing process, and primer-free technology adopted in this invention are highly consistent with the current development trend of "environmental protection, high efficiency, and short process" in the coating industry, and have clear industrial applicability and good industrialization prospects.

[0154] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for preparing a dual-curing waterborne polyurethane-acrylate automotive coating, characterized in that, In a paint mixing tank equipped with a stirring device, add the following components in the following order by weight, stirring for 5-10 minutes after each addition: 60-80 parts of waterborne polyurethane-acrylate dispersion; 3-15 parts of reactive nano-silica aqueous dispersion; 2-8 parts of reactive chlorinated polyolefin adhesion promoter; 1-3 parts of aqueous photoinitiator; 3-12 parts of blocked isocyanate crosslinking agent; 0.3 to 2.0 parts of the additive; Then deionized water is added to obtain a dual-curing waterborne nanocomposite coating.

2. The preparation method of the dual-curing waterborne polyurethane-acrylate automotive coating according to claim 1, characterized in that, The aqueous photoinitiator is selected from 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylphenylacetone or phenyl-2,4,6-trimethylbenzoyl lithium phosphite; the blocked isocyanate crosslinking agent is selected from blocked hexamethylene diisocyanate (HDI) trimer or blocked isophorone diisocyanate (IPDI) trimer, and the unblocking temperature is 80~120℃; the coating is adjusted to the application viscosity with deionized water: 25~40 seconds with a Forco 4 cup, or 500~1500 mPa·s as measured by a rotational viscometer.

3. The preparation method of the dual-curing waterborne polyurethane-acrylate automotive coating according to claim 1, characterized in that, The additives include 0.1 to 0.5 parts of a polyether-modified organosiloxane substrate wetting agent, 0.1 to 0.5 parts of an acetylenic diol defoamer, and 0.1 to 1.0 parts of a polyurethane associative leveling agent.

4. The preparation method of the dual-curing waterborne polyurethane-acrylate automotive coating according to claim 1, characterized in that, The aqueous polyurethane-acrylate dispersion is prepared by the following method: Step 1, Preparation of silicone-modified polyurethane prepolymer: Under dry nitrogen protection, add 100 parts by weight of dehydrated polycarbonate diol (PCDL), 5-20 parts by weight of hydroxyl-terminated polydimethylsiloxane (PDMS-OH), and 4-8 parts by weight of dimethylolpropionic acid (DMPA); add acetone as a solvent to fully dissolve the solid components, stir and heat to 60-70°C; add 52-60 parts by weight of isophorone diisocyanate (IPDI) dropwise, controlling the reaction temperature at 70-80°C; after the dropwise addition is complete, add 0.03-0.10 parts by weight of dibutyltin dilaurate (DBTDL), and react at a constant temperature of 75-85°C for 2-4 hours to obtain: silicone-modified polyurethane prepolymer; Step 2, Preparation of acrylate-terminated silicone-modified polyurethane prepolymer: The temperature of the silicone-modified polyurethane prepolymer was lowered to 55-65℃, and under nitrogen protection, the following were added according to the mass ratio: Add 2.0~5.0 parts of end-capping agent and 0.002~0.015 parts of polymerization inhibitor dropwise, then heat to 65~75℃ and react at a constant temperature for 2~3 hours; then add 0.5~1.5 parts of methanol and continue stirring for 30 minutes; finally, an acrylate-terminated silicone-modified polyurethane prepolymer is obtained. Step 3, neutralization and emulsification, preparation of waterborne polyurethane-acrylate dispersion: The acrylate-terminated silicone-modified polyurethane prepolymer is cooled to 35~45℃, and 3.6~6.4 parts by weight of the neutralizing agent triethylamine (TEA) are added. The mixture is stirred for 15~20 minutes to carry out the neutralization reaction. Under high-speed shear dispersion conditions of 1500~3000 rpm, 120~200 parts of deionized water are slowly added. With the continuous addition of deionized water, the system undergoes a phase transition process from water-in-oil to oil-in-water. After the phase transition is complete, reduce the shear rate to 500-800 rpm and continue stirring for 30-60 minutes to fully stabilize the system. Then, evaporate the solvent acetone at 40-50℃ under reduced pressure and add deionized water to obtain the waterborne polyurethane-acrylate dispersion.

5. The preparation method of the dual-curing waterborne polyurethane-acrylate automotive coating according to claim 4, characterized in that, The capping agent is hydroxyethyl acrylate (HEA) or pentaerythritol triacrylate (PETA), or a mixture of both; the polymerization inhibitor is p-hydroxyanisole (MEHQ).

6. The preparation method of the dual-curing waterborne polyurethane-acrylate automotive coating according to claim 4, characterized in that, The preparation method of the reactive nano-silica aqueous dispersion includes: 3-15 parts of nano-silica are dispersed in a mixed solvent of 100-200 parts of ethanol and deionized water, with a volume ratio of ethanol to water of 3:1-5:

1. The mixture is ultrasonically dispersed for 30-60 minutes to obtain a uniform nano-silica suspension. The pH of the suspension is adjusted to 4.0-5.0 with acetic acid. Add according to the following mass proportions under continuous stirring and ultrasonic assistance: Add 0.3 to 4.5 parts of silane coupling agent KH-570 dropwise, then heat to 55 to 65°C and react at a constant temperature for 3 to 5 hours. After the reaction was completed, the unreacted KH-570 and the hydrolysis byproduct methanol were removed by dialysis or high-speed centrifugation. The purified modified nano silica was redispersed in deionized water and ultrasonically dispersed for 30 minutes to obtain a reactive nano silica aqueous dispersion.

7. The preparation method of the dual-curing waterborne polyurethane-acrylate automotive coating according to claim 4, characterized in that, A reactive chlorinated polyolefin adhesion promoter, the preparation method of which includes: Under nitrogen protection, add according to the following mass proportions: Dissolve 60-100 parts of chlorinated polypropylene (CPP) in toluene or xylene. At this point, the mass concentration of chlorinated polypropylene (CPP) is 15%-25%. Heat the solution to 80-100°C and stir to dissolve. Add 1.8-8.0 parts of maleic anhydride (MAH) and 0.3-1.5 parts of initiator benzoyl peroxide (BPO), and react at 100-120°C for 2-4 hours; pour the reaction solution into acetone to precipitate, filter, dry, and vacuum dry to obtain maleic anhydride-grafted chlorinated polypropylene. Dissolve maleic anhydride-grafted chlorinated polypropylene again in toluene. At this point, the mass concentration of maleic anhydride-grafted chlorinated polypropylene is 15%~20%. Add according to the following parts by mass: 1.0-2.0 parts of hydroxyethyl acrylate (HEA) and 0.01-0.03 parts of p-toluenesulfonic acid catalyst are reacted at 110-130°C for 2-3 hours; the hydroxyl groups of HEA undergo an esterification ring-opening reaction with the anhydride groups on maleic anhydride-grafted chlorinated polypropylene. After the reaction was completed, the solvent was removed by vacuum evaporation to obtain a reactive chlorinated polyolefin adhesion promoter containing acrylate double bonds.

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