Cathode electrophoretic coating having high corrosion resistance, and preparation method therefor

Abstract

A cathode electrophoretic coating having high corrosion resistance, and a preparation method therefor. The cathode electrophoretic coating comprises a cationic epoxy emulsion and a compound functional pigment filler color paste. The cationic epoxy emulsion is obtained from polyethyleneimine modified epoxy resin, and the cationic epoxy emulsion comprises a heat-releasable fully-blocked isocyanate, polyvinyl alkyl ether resin and a neutralizer. The cathode electrophoretic coating having high corrosion resistance is developed for matching a film pretreatment process, such that the throwing power, the salt spray resistance and the edge protection performance of a film treated component are improved.

Description

A cathodic electrophoretic coating with high corrosion resistance and preparation method thereof Technical Field

[0001] The present invention relates to the technical field of coatings, and in particular to a cathode electrophoretic coating with high corrosion resistance and a preparation method thereof. Background Art

[0002] Cathodic electrophoretic coatings are widely used in the coating and protection of automobiles and their components due to their excellent corrosion protection, efficient application, and excellent coating performance on a variety of complex, irregularly shaped workpieces. When cathodic electrodeposition coatings are applied to metal substrates, the workpiece surface is typically pretreated to improve various coating properties, such as corrosion resistance and adhesion. Zinc phosphate-based surface treatment compositions, which offer excellent adhesion and corrosion resistance, are widely used in the surface treatment field. However, zinc phosphate surface treatment compositions have high metal ion and acid content and exhibit very high reactivity. This is undesirable due to economic and processability considerations, such as expensive wastewater treatment. Furthermore, the phosphating process produces water-insoluble salts that separate as sediment in the working bath. This sediment, commonly known as "phosphating slag," is costly to remove and dispose of. Furthermore, phosphate ions can cause environmental pollution, such as nutrient enrichment in rivers and oceans.

[0003] The thin-film, phosphorus-free pretreatment process represents an innovation in the traditional phosphating process. This process applies a very thin coating (20-200nm) to the vehicle body, replacing the traditional phosphating layer (3μm). It also features a short production process, requires no heating, reduces product consumption, and reduces waste disposal costs. It is free of harmful heavy metal ions such as nickel, zinc, and manganese, and contains no phosphorus, resulting in excellent environmental performance. However, the electrophoretic penetration of the workpiece after thin-film treatment is somewhat lower than that of the phosphating process, and its corrosion resistance is also reduced. Summary of the Invention

[0004] The object of the present invention is to provide a high-corrosion-resistant cathodic electrophoretic coating and a preparation method thereof, so as to solve one or more of the above-mentioned problems in the prior art.

[0005] On the one hand, the present invention provides a high-corrosion-resistant cathodic electrophoretic coating, which includes a cationic epoxy emulsion and a compounded functional pigment and filler paste. The cationic epoxy emulsion is obtained by polyethyleneimine-modified epoxy resin, and the cationic epoxy emulsion includes a fully enclosed isocyanate that can be deblocked at high temperature, a polyvinyl alkyl ether resin, and a neutralizer.

[0006] The cationic epoxy emulsion and the compounded functional pigment and filler paste are used to prepare an electrophoretic paint with a pH value of 5.2-6.0 and an electrical conductivity of 1000-2000 μs / cm.

[0007] Electrodeposition coating is performed by applying a voltage of 50 to 450 V between the workpiece and the anode in an electrophoretic paint working solution. The temperature of the electrodeposition bath is generally 20 to 40°C. The thickness of the electrodeposition coating film is preferably 10 to 25 μm. If it is less than 10 μm, the corrosion resistance is insufficient, and if it exceeds 25 μm, it will lead to waste of coating. The wet film resistance of the electrodeposition coating is preferably 800 to 1300 kΩ·cm 2 If the wet film resistance is less than 800kΩ·cm 2 If the wet film resistance exceeds 1300kΩ·cm, the resistance will be insufficient, which will lead to poor throwing power. 2 , the appearance of the coating will be significantly deteriorated.

[0008] In certain embodiments, the weight percentage of polyethyleneimine in the cationic epoxy emulsion to the total resin is 0.02%-2.0%, preferably 0.2%-1.0%; the polyethyleneimine contains 35% by mole of primary amine, 35% by mole of secondary amine and 30% by mole of tertiary amine.

[0009] Polyethyleneimine is highly reactive and possesses the highest cationic density among existing materials. It exhibits strong adsorption to negatively charged surfaces, resulting in excellent wet film adhesion during electrodeposition. Polyethyleneimine also acts as a potential anchoring point within the electrodeposited coating. During the baking process, it can further cross-link with the cross-linking agent to form a higher molecular weight resin, modifying the coating's rheological properties and reducing shrinkage around the workpiece edge, providing excellent edge protection.

[0010] In certain embodiments, the epoxy resin is a mixture of one or more of bisphenol A, bisphenol F, bisphenol S, phenol linear novolac resin, aliphatic monofunctional epoxy resin, aromatic monofunctional epoxy resin or aliphatic difunctional epoxy resin, preferably bisphenol A glycidyl ether resin.

[0011] In certain embodiments, the cationic epoxy emulsion is obtained by introducing a primary ammonium group, a secondary ammonium group, a tertiary ammonium group, a quaternary ammonium group, a tertiary sulfonium group or a quaternary phosphonium group to open the epoxy ring, and then cationizing the epoxy emulsion with a neutralizing acid.

[0012] Prior to the ring-opening reaction with amines, the epoxy resin can be chain-extended with difunctional polyester polyols, polyether polyols, bisphenol A, dicarboxylic acids, or the like to further increase its molecular weight and introduce functional groups. This modified epoxy resin has a weight-average molecular weight of approximately 3,000 to 6,000, and an epoxy equivalent weight (EPW) of preferably 500 to 1,500. Adjusting the amount of bisphenol A can adjust the wet film resistance of the electrodeposited coating.

[0013] In certain embodiments, the cationic epoxy emulsion forms a protonated ammonium salt by introducing an amine group. The amine compound selected is one or more mixtures of diethylamine, dibutylamine, diethanolamine, N-methylethanolamine, diethylenetriamine, triethylenetetramine, aminoethylethanolamine ketimine, diethylenetriamine methylisobutylketimine, or polyamide. Polyethyleneimine also provides a large number of amino groups, and after acid neutralization, a cationic resin with a high charge density is formed.

[0014] In certain embodiments, the fully blocked isocyanate that can be deblocked at high temperature is blocked by a blocking agent to obtain a cross-linking agent with complementary reactive functional groups, wherein the polyisocyanate is a mixture of one or more of diphenylmethane-4,4'-diisocyanate, toluene-2,4-diisocyanate, hexamethylene diisocyanate, and isophorone diisocyanate, and the blocking agent is a mixture of one or more of alcohol ether solvents, methyl ethyl ketone oxime, alcohols, or caprolactam.

[0015] In certain embodiments, the weight percentage of the polyvinyl alkyl ether resin to the total weight of the resin is 0.05%-3.0%, preferably 0.2%-2.0%; its structure is as follows:

[0016] Wherein: R1, R2 and R3 represent a hydrogen atom or a hydrocarbon group having 1 to 8 carbon atoms, and they may be the same or different from each other;

[0017] R a represents a divalent hydrocarbon group having 2 to 4 carbon atoms;

[0018] R b It represents hydrogen atoms and aliphatic hydrocarbons having 1 to 10 carbon atoms.

[0019] Among them: Polyvinyl alkyl ether resin has a low Tg and can be used as a plasticizer. It also contains a large number of wetting groups -COC-, which can significantly improve the wetting ability of the electrophoretic paint, thereby enhancing the adhesion between the electrophoretic paint and the substrate.

[0020] In certain embodiments, the neutralizing agent is a mixture of one or more of lactic acid, glacial acetic acid, formic acid, and sulfamic acid.

[0021] In certain embodiments, the compounded functional pigment and filler paste is a pigment and filler paste modified with nano-SiO2.

[0022] Titanium dioxide, red iron oxide, and carbon black serve as conventional pigments and fillers; kaolin, aluminum silicate, precipitated barium sulfate, mica, and clay serve as filler pigments. Cationic or nonionic low-molecular-weight surfactants containing quaternary ammonium groups and / or tertiary sulfonium groups and modified epoxy resins serve as pigment dispersing resins. The nano-SiO2-modified functional pigment resin is modified and dispersed primarily through mechanical dispersion using a silane coupling agent and a hyperdispersant. The volume effect of the nanomaterial fills the "structural pores" that are inevitable in organic coatings, preventing the penetration of various corrosive media. The nanomaterial's enormous specific surface area and surface energy have the most direct effect on the coating: significantly enhancing the degree of unsaturated bonds between the protected metal and the coating. The silane coupling agent is at least one of an amino-containing silane coupling agent, its hydrolyzate, and its polymer. Amino-containing silane coupling agents are compounds with at least one amino group and a siloxane bond in their molecules. Their hydrolyzate and polymer can act on both the chemical conversion coating and the coating film, improving the adhesion between the two coatings.

[0023] On the other hand, the present invention provides a method for preparing a high-corrosion-resistant cathodic electrophoretic coating, comprising the following steps:

[0024] Preparation of fully blocked isocyanates that can be deblocked at high temperatures: blocking polyisocyanates with blocking agents to obtain crosslinkers with complementary reactive functional groups;

[0025] Preparation of cationic epoxy emulsion: by introducing primary ammonium group, secondary ammonium group, tertiary ammonium group, quaternary ammonium group, tertiary sulfonium group or quaternary phosphonium group, opening the epoxy ring, and then cationizing it with neutralizing acid;

[0026] Preparation of compound functional pigment and filler paste;

[0027] The cationic epoxy emulsion, the compounded functional pigment and filler paste and deionized water are mixed to prepare the high anti-corrosion performance cathode electrophoretic coating.

[0028] In certain embodiments, the preparation method of the fully blocked isocyanate that can be deblocked at high temperature is:

[0029] To a round-bottom four-necked flask equipped with a stirrer, condenser, temperature probe, and nitrogen feed line, diphenylmethane diisocyanate (MDI) (Wanhua PM200, NCO equivalent of 135 g / eq) and dibutyltin dilaurate were added sequentially. Under nitrogen, stirring and heating to 60°C were performed. Diethylene glycol butyl ether was added dropwise at a constant rate over 2 hours, maintaining the addition temperature between 60-80°C. After completion of the addition, the temperature was raised to 80°C and held for 1 hour. After the holding period, the temperature was lowered to 70°C, and a mixed solution of methyl isobutyl ketone and trimethylolpropane was added dropwise, with the addition temperature not exceeding 100°C. After completion of the addition, the temperature was held at 100°C for 1 hour. Subsequently, n-butanol and methyl isobutyl ketone were added to dilute the mixture, and the temperature was lowered to 65°C for discharging. The solids content was 70%.

[0030] In certain embodiments, diethylenetriamine ketimine is prepared by:

[0031] A 70% by mass solution of diethylenetriamine in methyl isobutyl ketone is subjected to azeotropic removal of reaction water at 110-140° C., and then diluted with methyl isobutyl ketone until the solution has an amine equivalent weight of 120.

[0032] In certain embodiments, the preparation method of the cationic epoxy emulsion is:

[0033] To a round-bottom four-necked flask equipped with a stirrer, condenser, temperature probe, and nitrogen feed line, NPEL128 epoxy (Nan Ya Epoxy, EEW = 188 g / eq), bisphenol A, dodecylphenol, polyvinyl alkyl ether resin (Luta-50, Hubei Jinghong Biotechnology), and methyl isobutyl ketone solution were added in sequence and slowly heated to 130°C for melting. After complete melting, N,N-dimethylaminobenzylamine was added, allowing a spontaneous exotherm to be maintained at a maximum temperature of no more than 175°C. After the exotherm ended, the mixture was cooled to 145°C, N,N-dimethylaminobenzylamine was added, and the temperature was maintained at 145°C. The epoxy equivalent weight (EEW) was measured every half hour during this holding period. When the EEW reached 1250 g / eq, methyl isobutyl ketone and polypropylene glycol (molecular weight, 1000) were rapidly added. Stir and cool to 70°C, add methylethanolamine and diethylenetriamine ketimine, raise the temperature to 110-120°C and keep stirring for 2 hours. After the insulation is completed, add ethylene glycol butyl ether and ethylene glycol hexyl ether and cool to below 100°C, then add polyethyleneimine and cross-linking agent, and keep stirring at 90-100°C for 1 hour. After the insulation is completed, add glacial acetic acid and keep stirring at 80-90°C for 1 hour. After the insulation is completed, slowly add pure water and stir to emulsify. At 40-50°C, vacuum extract the solvent to remove the residual methyl isobutyl ketone solution in the emulsion to obtain an emulsion with a solid content of 35% and a pH of 5.8.

[0034] In certain embodiments, the preparation method of the compounded functional color filler paste is:

[0035] (1) Premixing: Add WC-00-03 grinding resin, 20% mass concentration acetic acid aqueous solution and deionized water to a container and disperse at 600 rpm for 10 minutes to obtain a uniform solution. While stirring, add silane coupling agent KH-560 and ethylene glycol butyl ether, add AEROSIL R972 stepwise and stir for 30 minutes to prepare a nano-SiO2 modified premixed slurry. Then, add dibutyltin oxide, carbon black, kaolin, barium sulfate and titanium dioxide at 900 rpm and stir for 60 minutes to obtain a premixed slurry. During the premixing process, the temperature is controlled at 5-40°C and the viscosity of the slurry is controlled at 60-80s after four cups of coating.

[0036] (2) Sand milling: Place the premixed slurry into a sand mill, select zirconium beads of 0.8-1.25 mm, grind at a speed of 4000-5000 rpm for 1.5 h, control the grinding temperature to be less than 40 ° C, and the output fineness is ≤13 μm.

[0037] Compared with the prior art, the beneficial effects of the present invention are: the present invention develops a high-corrosion-resistant cathodic electrophoretic coating for a matching thin film pre-treatment process, aiming to improve the throwing power, salt spray performance and edge protection performance of the paint film on the thin film treated workpiece. DETAILED DESCRIPTION

[0038] The technical solution of the present invention is described in detail below in conjunction with various embodiments.

[0039] Example 1

[0040] Preparation of fully blocked polyisocyanate crosslinker:

[0041] To a 2-liter, four-necked, round-bottom flask equipped with a stirrer, condenser, temperature probe, and nitrogen feed line, 523g of diphenylmethane diisocyanate (MDI) (Wanhua PM200, NCO equivalent of 135g / eq) and 1g of dibutyltin dilaurate were added sequentially. Under nitrogen, the mixture was stirred and heated to 60°C. 481g of diethylene glycol butyl ether was added dropwise at a constant rate over 2 hours, maintaining the addition temperature between 60-80°C. After completion, the temperature was raised to 80°C and held for 1 hour. After the temperature was lowered to 70°C, a mixed solution of 387g of methyl isobutyl ketone and 43g of trimethylolpropane was added dropwise, maintaining the addition temperature at no more than 100°C. After completion, the mixture was held at 100°C for 1 hour. 48g of n-butanol and 15g of methyl isobutyl ketone were then added to dilute the mixture, and the temperature was lowered to 65°C. The solids content was 70%.

[0042] Example 2

[0043] Preparation of diethylenetriamine ketimine:

[0044] A 70% by mass solution of diethylenetriamine in methyl isobutyl ketone is subjected to azeotropic removal of reaction water at 110-140° C., and then diluted with methyl isobutyl ketone until the solution has an amine equivalent weight of 120.

[0045] Example 3

[0046] Preparation of cationic epoxy emulsion:

[0047] In a 2-liter round-bottom four-necked flask equipped with a stirrer, condenser, temperature probe, and nitrogen feed line, 256.5 g of NPEL128 epoxy (Nan Ya Epoxy, EEW = 188 g / eq), 86.4 g of bisphenol A, 40.5 g of dodecylphenol, 2.3 g of polyvinyl alkyl ether resin (Luta-50, Hubei Jinghong Biotechnology), and 18 g of methyl isobutyl ketone solution were added in sequence and slowly heated to 130°C for melting. After complete melting, 0.4 g of N, N-dimethylaminobenzylamine was added, and heat was released on its own, with the maximum temperature controlled not to exceed 175°C. After the heat release ended, the material was cooled to 145°C, and 0.2 g of N, N-dimethylaminobenzylamine was added, and the temperature was maintained at 145°C. During the heat preservation process, the epoxy equivalent EEW is detected every half an hour. When the epoxy equivalent reaches 1250g / eq, 40g of methyl isobutyl ketone and 70g of polypropylene glycol (molecular weight is 1000) are quickly added. Stir and cool to 70°C, add 40g of methylethanolamine and 39g of ketimine of diethylenetriamine (prepared in Example 2), heat to 110-120°C and keep stirring for 2 hours. After the heat preservation is completed, 60g of ethylene glycol butyl ether and 30g of ethylene glycol hexyl ether are added, the temperature is cooled to below 100°C, and then 1.8g of polyethyleneimine ( MI 6735, BASF) and 283 g of the crosslinker (prepared in Example 1) were stirred at 90-100°C for 1 hour. After the incubation period, 9.5 g of glacial acetic acid was added, and the mixture was stirred at 80-90°C for 1 hour. After the incubation period, 1080 g of pure water was slowly added, stirred, and emulsified. The solvent was vacuum extracted at 40-50°C to remove the residual methyl isobutyl ketone solution in the emulsion, yielding an emulsion with a solids content of 35% and a pH of 5.8.

[0048] Example 4

[0049] Preparation of compound functional pigment and filler paste:

[0050] The components are as follows: Note: WC-00-03 is Haolisen's production-type grinding resin with a solid content of 70%;

[0051] AEROSIL R972 is fumed silica from Evonik Degussa.

[0052] Premixing: Add WC-00-03 grinding resin, 20% acetic acid aqueous solution, and deionized water to a container and disperse at 600 rpm for 10 minutes to form a homogeneous solution. While stirring, add silane coupling agent KH-560 and ethylene glycol butyl ether. Add AEROSIL R972 in stages and stir for 30 minutes to prepare a nano-SiO2-modified premix slurry. Then, add dibutyltin oxide, carbon black, kaolin, barium sulfate, and titanium dioxide at 900 rpm and stir for 60 minutes to obtain a premix slurry. During the premixing process, maintain a temperature of 5-40°C and a four-cup viscosity of 60-80 seconds.

[0053] Sand milling: put the premixed slurry into the sand mill, select zirconium beads of 0.8-1.25mm, grind at a speed of 4000-5000rpm for 1.5h, control the grinding temperature to be less than 40℃, and the output fineness is ≤13μm.

[0054] Example 5

[0055] Preparation of electrophoresis bath:

[0056] An electrophoresis bath was prepared by mixing 2000 parts by weight of the emulsion from Example 3, 400 parts by weight of the color paste from Example 4, and 2600 parts by weight of deionized water. The bath had a pH of 5.75 and a conductivity of 1540 μs / cm. After aging at 30°C for 48 hours, the plate was formed.

[0057] Electrophoresis sample preparation:

[0058] Pretreatment agent: Chemetall 9810 silane treatment liquid;

[0059] Workpiece sample: 100mm*200mm*0.8mm cold-rolled steel plate;

[0060] Electrophoresis conditions: bath temperature 28-30°C, stirring speed 500 rpm, electrophoresis soft start time 15 s, electrodeposition time 120 s, voltage adjustable between 150 V and 320 V.

[0061] Baking conditions: 180℃ / 15 minutes.

[0062] Electrophoretic paint performance indicators:

[0063] In summary: the present invention uses an electrophoretic coating process to deposit on the surface of the workpiece, and obtains a paint film with excellent anti-corrosion performance and good appearance after curing, which is particularly suitable for anti-corrosion of the surface of the workpiece without phosphorus pretreatment.

[0064] It will be apparent to those skilled in the art that various modifications to the above embodiments may be made without departing from the overall spirit and concept of the present invention. Such modifications fall within the scope of protection of the present invention. The protection scheme of the present invention shall be subject to the claims appended hereto.

Claims

1. A cathodic electrophoretic coating with high corrosion resistance, characterized in that: The cathode electrophoretic coating comprises a cationic epoxy emulsion and a composite functional pigment and filler paste. The cationic epoxy emulsion is obtained by polyethyleneimine-modified epoxy resin and comprises a fully enclosed isocyanate that can be deblocked at high temperature, a polyvinyl alkyl ether resin and a neutralizer.

2. The high corrosion resistance cathodic electrophoretic coating according to claim 1, characterized in that: The weight percentage of polyethyleneimine in the cationic epoxy emulsion to the total resin is 0.02%-2.0%, preferably 0.2%-1.0%; the polyethyleneimine contains 35% by mole of primary amine, 35% by mole of secondary amine and 30% by mole of tertiary amine.

3. The high corrosion resistance cathodic electrophoretic coating according to claim 1, characterized in that: The epoxy resin is one or more mixtures of bisphenol A, bisphenol F, bisphenol S, phenol linear phenolic resin, aliphatic monofunctional epoxy resin, aromatic monofunctional epoxy resin or aliphatic difunctional epoxy resin, preferably bisphenol A glycidyl ether resin.

4. The high corrosion resistance cathodic electrophoretic coating according to claim 1, characterized in that: The cationic epoxy emulsion is obtained by introducing primary ammonium groups, secondary ammonium groups, tertiary ammonium groups, quaternary ammonium groups, tertiary sulfonium groups or quaternary phosphonium groups to open the epoxy ring, and then cationizing the epoxy emulsion with a neutralizing acid.

5. The high corrosion resistance cathodic electrophoretic coating according to claim 4, characterized in that: The cationic epoxy emulsion forms protonable ammonium salt by introducing amine groups, and the selected amine compounds are one or more mixtures of diethylamine, dibutylamine, diethanolamine, N-methylethanolamine, diethylenetriamine, triethylenetetramine, ketimine of aminoethylethanolamine, diethylenetriamine methylisobutylketimine or polyamide.

6. The high anti-corrosion performance cathodic electrophoretic coating according to claim 1, characterized in that: The fully blocked isocyanate that can be unblocked at high temperature blocks polyisocyanate through a blocking agent to obtain a cross-linking agent with complementary reactive functional groups. The polyisocyanate is a mixture of one or more of diphenylmethane-4,4'-diisocyanate, toluene-2,4-diisocyanate, hexamethylene diisocyanate, and isophorone diisocyanate. The blocking agent is a mixture of one or more of alcohol ether solvents, methyl ethyl ketone oxime, alcohols, or caprolactam.

7. The high anti-corrosion performance cathodic electrophoretic coating according to claim 1, characterized in that: The weight percentage of the polyvinyl alkyl ether resin to the total amount of the resin is 0.05%-3.0%, preferably 0.2%-2.0%; its structure is as follows: Wherein: R1, R2 and R3 represent a hydrogen atom or a hydrocarbon group having 1 to 8 carbon atoms, and they may be the same or different from each other; R a It represents a divalent hydrocarbon group having 2 to 4 carbon atoms; R b It represents hydrogen atoms and aliphatic hydrocarbons having 1 to 10 carbon atoms.

8. The high anti-corrosion performance cathodic electrophoretic coating according to claim 1, characterized in that: The neutralizing agent is a mixture of one or more of lactic acid, glacial acetic acid, formic acid and aminosulfonic acid.

9. The high anti-corrosion performance cathodic electrophoretic coating according to claim 1, characterized in that: The compound functional pigment and filler paste is a pigment and filler paste modified by nano-SiO2.

10. A method for preparing the cathodic electrophoretic coating with high corrosion resistance according to any one of claims 1 to 9, characterized in that: The following steps are involved: Preparation of fully blocked isocyanate that can be unblocked at high temperature: blocking polyisocyanate with a blocking agent to obtain a cross-linking agent with complementary reactive functional groups; Preparation of cationic epoxy emulsion: by introducing primary ammonium group, secondary ammonium group, tertiary ammonium group, quaternary ammonium group, tertiary sulfonium group or quaternary phosphonium group, opening the epoxy ring, and then cationizing it with neutralizing acid; Preparation of compound functional pigment and filler paste; The cationic epoxy emulsion, the compound functional pigment and filler paste and deionized water are mixed to prepare the high anti-corrosion performance cathode electrophoretic coating.

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