Highly water mark resistant cathodic electrophoretic emulsion and method for preparing the same

Abstract

This application relates to a highly water-mark-resistant cathodic electrophoretic emulsion and its preparation method, belonging to the field of electrophoretic coating technology. The preparation method of the highly water-mark-resistant cathodic electrophoretic emulsion includes the following steps: S1. Blocking some isocyanate groups of toluene diisocyanate with a mixture of n-butanol and PEG400, and then reacting and bonding the unblocked isocyanate groups with oleamide resin and polyamine to obtain a polyamine-modified isocyanate curing agent; S2. Performing a ring-opening amination reaction between epoxy resin and a first amine compound, and then adding a second amine compound to continue the reaction to obtain an amine-modified epoxy resin; S3. Grafting the polyamine-modified isocyanate curing agent onto the amine-modified epoxy resin to obtain a cationic epoxy resin; S4. Neutralizing the cationic epoxy resin with an organic acid, adding deionized water for emulsification and dispersion to obtain the highly water-mark-resistant cathodic electrophoretic emulsion. After electrophoretic film formation, the emulsion exhibits good hydrophilicity on the wet film surface, thereby significantly reducing the formation of watermarks during rapid baking.

Description

Technical Field

[0001] This invention relates to the field of electrophoretic coating technology, and in particular to a highly water-resistant cathodic electrophoretic emulsion and its preparation method. Background Technology

[0002] Cathodic electrophoretic coatings, due to their excellent corrosion resistance, uniform film thickness distribution, and efficient automated application, have been widely used in the surface protective coating of metal products such as automotive parts, household appliance housings, and hardware accessories. In the cathodic electrophoretic coating process, the coating emulsion is deposited on the surface of the metal workpiece under the action of a DC electric field to form a dense wet film. After washing away excess coating with water, the workpiece is heated and cured in an oven tunnel, ultimately forming a paint film with good adhesion and corrosion resistance.

[0003] However, in industrial production, a large number of water droplets inevitably remain on the surface of workpieces after electrophoresis and water washing. If these residual water droplets fail to evaporate in time and evenly before the paint film gels and cures during the curing process, they will form irreversible pits or rings on the surface of the cured paint film, i.e., "watermarks" defects, which seriously affect the uniformity of the coating appearance.

[0004] This problem is particularly prominent in the high-efficiency hardware processing and automotive painting industries: the increased speed of the production line has greatly reduced the time that the workpiece stays in the drying tunnel, and the window period for water droplet evaporation has been shortened sharply, which puts forward extremely high requirements for the watermark resistance of electrophoretic coatings.

[0005] In existing technologies, the wet film of traditional electrophoretic emulsion systems has high surface tension and insufficient hydrophilicity. Residual water droplets gather in beads on the surface of the wet film and are difficult to spread into a thin film. In addition, rapid heating causes the outer layer of the paint film to gel first, locking the water droplets inside the film. This results in watermarks occurring frequently on high-speed production lines, becoming a key technical bottleneck that restricts product appearance quality and production cost control.

[0006] In view of this, it is necessary to provide a highly watermark-resistant cathodic electrophoretic emulsion and its preparation method to overcome the shortcomings of the prior art. Summary of the Invention

[0007] This invention provides a method for preparing a highly water-mark resistant cathodic electrophoretic emulsion. The emulsion prepared by this method has good hydrophilicity on the wet film surface after electrophoretic film formation, and water droplets can easily spread into a thin water layer, thereby significantly reducing the formation of watermarks during rapid baking. This solves the problem that existing electrophoretic coatings are prone to watermark defects under rapid baking process conditions.

[0008] In a first aspect, the present invention provides a method for preparing a highly water-mark-resistant cathodic electrophoretic emulsion, comprising the following steps: S1. Part of the isocyanate groups of toluene diisocyanate are blocked by a mixture of n-butanol and PEG400, and then the unblocked isocyanate groups are reacted and bonded with oleamide resin and polyamine to obtain a polyamine-modified isocyanate curing agent. S2. After the epoxy resin is subjected to a ring-opening amination reaction with the first amine compound, a second amine compound is added to continue the reaction to obtain an amine-modified epoxy resin; S3. Graft the polyamine-modified isocyanate curing agent onto the amine-modified epoxy resin to obtain a cationic epoxy resin; S4. The cationic epoxy resin is neutralized with an organic acid and then emulsified and dispersed with deionized water to obtain the highly watermark resistant cathodic electrophoretic emulsion.

[0009] Further, in step S1, the mass ratio of n-butanol to PEG400 is 1:2.

[0010] Further, in step S1, the amount of toluene diisocyanate used is 160-190 parts by weight, and the total amount of the mixture of n-butanol and PEG400 used is 99-150 parts by weight.

[0011] Further, in step S1, the amount of oleamide resin used is 200-230 parts by weight, and the amount of polyamine used is 10-15 parts by weight.

[0012] Further, in step S2, the epoxy resin is a medium molecular weight solid epoxy resin, and the amount used is 20-40 parts by weight.

[0013] Further, in step S2, the first amine compound is diethanolamine, and the amount used is 5-6 parts by weight.

[0014] Further, in step S2, the second amine compound is triethanolamine, and the amount used is 2-7 parts by weight; the second amine compound is added after the ring-opening amination reaction is completed and the temperature is lowered.

[0015] Further, in step S3, the amount of the polyamine-modified isocyanate curing agent added is 50-60 parts by weight.

[0016] Further, in step S4, the organic acid is glacial acetic acid, and the amount used is 5-6 parts by weight.

[0017] Furthermore, in step S4, the amount of deionized water used is 110-130 parts by weight.

[0018] Secondly, the present invention provides a highly water-mark resistant cathodic electrophoretic emulsion, which is prepared by any of the aforementioned methods; the solid content of the emulsion is 34%-36%.

[0019] Thirdly, the present invention provides the application of the high watermark-resistant cathodic electrophoretic emulsion described above in electrophoretic coating of metal parts.

[0020] Through one or more embodiments of the above embodiments of the present invention, at least the following technical effects can be achieved: This invention constructs a multi-dimensional hydrophilicity regulation network in the curing film system by simultaneously introducing PEG400 polyether segments and oleamide resin amphiphilic groups into a polyamine-modified isocyanate curing agent, and introducing triethanolamine polyhydroxy segments into an amine-modified epoxy resin. The polyether segments of PEG400 impart good hydrophilicity to the wet film surface, making it easy for residual water droplets to spread into a thin water layer. The amphiphilic structure of oleamide resin effectively reduces and homogenizes the surface tension of the wet film, further promoting the uniform spreading of water droplets. The polyhydroxy structure of triethanolamine adds hydrophilic functional groups to the system from the epoxy resin side. This allows water droplets to spread rapidly into a thin film on the wet film surface, significantly increasing the evaporation area and enabling uniform and rapid evaporation under rapid heating and baking conditions, greatly reducing the formation of watermark defects. Attached Figure Description

[0021] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 It is a colloid prepared from the electrophoretic emulsion of Example 1 of this application.

[0023] Figure 2 It is a colloid prepared from the electrophoretic emulsion of Example 2 of this application.

[0024] Figure 3 It is a colloid prepared from the electrophoretic emulsion of Example 3 of this application.

[0025] Figure 4 It is a swimming plate made from commercially available KNT831LF. Detailed Implementation

[0026] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0027] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. Furthermore, the character " / " in this document, unless otherwise specified, generally indicates that the preceding and following related objects have an "or" relationship. Unless otherwise specified, "parts by weight" as used herein refers to relative parts by mass.

[0028] As described in the background section, traditional cathodic electrophoretic coatings use ammoniated epoxy resin as the core film-forming material. While they possess good electrodeposition and corrosion resistance, their wet film surface lacks sufficient hydrophilicity and has high surface tension. Residual water droplets accumulate in beads on the wet film surface, making spread difficult. During rapid baking, the outer layer of the coating gels and solidifies first, trapping the water droplets within the film layer. Ultimately, these droplets leave irreversible indentations on the coating surface, known as "watermarks." Existing improvements are mostly limited to adjusting the baking curve or adding a drying process. While these methods have some effect, they significantly increase production costs and equipment investment, and cannot solve the problem of water droplet spread on the wet film surface from the coating itself.

[0029] To address the above problems, this application provides a method for preparing a highly watermark-resistant cathodic electrophoretic emulsion, comprising the following steps: S1. Part of the isocyanate groups of toluene diisocyanate are blocked by a mixture of n-butanol and PEG400, and then the unblocked isocyanate groups are reacted and bonded with oleamide resin and polyamine to obtain a polyamine-modified isocyanate curing agent.

[0030] It should be noted that the polyamine-modified isocyanate curing agent is a mixture, with two main active ingredients, whose structural characteristics are as follows: When preparing the above-mentioned polyamine-modified isocyanate curing agent, toluene diisocyanate is first dissolved in an organic solvent.

[0031] In some embodiments, the organic solvent is preferably methyl isobutyl ketone, and the amount used is 90-95 parts by weight, for example, any value or a range between any two of 90, 92, 94, and 95 parts. TDI is dissolved in MIBK and stirred at 45-55°C for about 1 hour until the system becomes clear and transparent, indicating that the TDI has been fully dissolved.

[0032] After the TDI is fully dissolved, a mixture of n-butanol and PEG400 is slowly added dropwise to the system at 50-60℃. Both n-butanol and PEG400 contain hydroxyl groups, which can react with the highly reactive -NCO groups on the TDI to form urethane bonds, thus achieving the blocking effect. After the addition is complete, the mixture is kept at this temperature for about 2 hours to allow the blocking reaction to proceed fully.

[0033] TDI molecules contain two -NCO groups, and their reactivity differs significantly due to their different spatial positions. Taking 2,4-toluene diisocyanate as an example, the 4-position -NCO is far from the methyl group, has less steric hindrance, and is highly reactive. It reacts first with the blocking agents (the hydroxyl groups of n-butanol and PEG400) to form a blocked urethane bond. The 2-position -NCO is adjacent to the methyl group, has greater steric hindrance, and is less reactive. It subsequently undergoes a urea bond reaction with the active hydrogen of oleamide resin and polyamine under elevated temperature conditions, thus achieving differentiated stepwise reaction control.

[0034] n-Butanol, with its small molecular weight and high reactivity, reacts first with the most reactive -NCO group in the initial stage of the sealing reaction, thus regulating the sealing rate. PEG400, due to its large molecular weight and low vapor pressure, retains its polyether segments in the coating system after baking and desealing. These retained PEG400 polyether segments impart hydrophilic properties to the coating: during the wet film stage after electrophoretic film formation, the polyether segments tend to migrate to the membrane-water interface, forming hydrogen bonds with water molecules through ether oxygen atoms. This significantly enhances the hydrophilicity of the wet film surface, allowing water droplets to quickly spread into a thin water layer upon contact with the wet film, rather than aggregating into beads.

[0035] In some embodiments, the mass ratio of n-butanol to PEG400 is 1:2. The higher the proportion of PEG400, the more hydrophilic polyether segments are retained in the coating film, and the more significant the watermark resistance effect. However, if the proportion of PEG400 is too high, it may lead to a slow blocking reaction rate, increased system viscosity, and decreased process controllability.

[0036] At a mass ratio of 1:2, a good balance can be achieved between the blocking reaction rate and the hydrophilic function. For example, PEG400 can also be replaced with polyols containing polyether segments such as PEG200, PEG600, polypropylene glycol PPG400, or PTMEG650.

[0037] In some embodiments, the total amount of the n-butanol and PEG400 mixture is 99-150 parts by weight, for example, any value or a range between any two of 99, 120, 135, and 150 parts.

[0038] After the blocking reaction is complete, oleamide resin and polyamine are added to the system. The oleamide resin molecule contains amide bonds and long-chain fatty acid segments, while the polyamine molecule contains multiple primary and secondary amine groups. The active hydrogens on both can react with the unblocked -NCO groups on TDI to form urea bonds, covalently anchoring TDI to the oleamide and polyamine backbones. After heating to 75-85℃, propylene glycol methyl ether is added as a diluent to adjust the viscosity, and the temperature is maintained for approximately 2 hours to allow the bonding reaction to proceed fully.

[0039] In some embodiments, the amount of oleamide resin used is 200-230 parts by weight, for example, any value or a range between any two of 200 parts, 210 parts, 220 parts, and 230 parts.

[0040] In some embodiments, the polyamine is preferably triethylenetetramine (hereinafter referred to as TETA), used in an amount of 10-15 parts by weight, for example, any value or range between any two of 10, 11, 13, and 15 parts. TETA contains two primary amine groups and two secondary amine groups, providing multiple reaction sites to form urea bonds with the -NCO group of TDI, ensuring that the curing agent has sufficient amine functionality. Of course, other polyamines such as diethylenetriamine or tetraethylenepentamine can also be used instead of TETA.

[0041] In some embodiments, the amount of toluene diisocyanate used is 160-190 parts by weight, for example, any value or a range between any two of 160, 170, 180, and 190 parts.

[0042] The amount of TDI used determines the total amount of -NCO in the system, directly affecting the crosslinking density of the final paint film. If the amount is too low, curing will be incomplete, and the hardness and corrosion resistance of the paint film will decrease; if the amount is too high, the paint film will become over-crosslinked and brittle. The crosslinking density is at a suitable level within the range of 160-190 parts.

[0043] In some embodiments, the amount of propylene glycol methyl ether used is 110-120 parts by weight, for example, any value or a range between any two of 110 parts, 113 parts, 116 parts, and 120 parts.

[0044] S2. After the epoxy resin is subjected to a ring-opening amination reaction with the first amine compound, a second amine compound is added to continue the reaction to obtain an amine-modified epoxy resin.

[0045] The structural characteristics of amine-modified epoxy resin are as follows: Specifically, the epoxy resin is first dissolved in a mixed solvent and stirred at 80-90°C for about 2 hours until it becomes clear and transparent, indicating that the resin has been fully dissolved and uniformly dispersed. Then, the first amine compound, diethanolamine, is added, and the mixture is kept at 80-90°C for about 2 hours.

[0046] In some embodiments, the epoxy resin is preferably a medium molecular weight solid epoxy resin (e.g., bisphenol A type solid epoxy resin), and the amount used is 20-40 parts by weight, for example, any value or a range between any two of 20 parts, 25 parts, 30 parts, 35 parts, and 40 parts.

[0047] In some embodiments, the mixed solvent comprises 10-15 parts of propylene glycol methyl ether and 1-3 parts of isopropanol. Propylene glycol methyl ether is the main solvent, used to dissolve the solid epoxy resin; isopropanol is used in smaller quantities, mainly serving to aid dissolution and adjust viscosity.

[0048] The secondary amine group of diethanolamine undergoes a ring-opening addition reaction with the epoxy group on the epoxy resin molecular chain, simultaneously introducing a tertiary amine group and a secondary hydroxyl group onto the molecular chain.

[0049] In some embodiments, the amount of diethanolamine used is 5-6 parts by weight, for example, any value or a range between any two of 5 parts, 5.5 parts, 6 parts.

[0050] It should be noted that after the ring-opening amination reaction of diethanolamine is completed and before adding triethanolamine, the reaction solution needs to be cooled appropriately. Cooling is necessary to prevent the triethanolamine from reacting too violently with the residual epoxy groups at high temperatures, which could lead to gelation.

[0051] In some embodiments, the amount of triethanolamine used is 2-7 parts by weight, for example, any value or a range between any two of 2, 3, 5, or 7 parts.

[0052] S3. The polyamine-modified isocyanate curing agent is grafted onto the amine-modified epoxy resin to obtain a cationic epoxy resin.

[0053] In some embodiments, the amount of polyamine-modified isocyanate curing agent added is 50-60 parts by weight, for example, any value or a range between any two of 50 parts, 52 parts, 55 parts, 58 parts, and 60 parts.

[0054] S4. The cationic epoxy resin is neutralized with an organic acid and then emulsified and dispersed with deionized water to obtain the highly watermark resistant cathodic electrophoretic emulsion.

[0055] When an organic acid is added to a cationic epoxy resin product, the protons of the organic acid react with the tertiary amine groups and residual primary / secondary amine groups on the resin molecular chain to form an ammonium salt structure, which imparts a positive charge and hydrophilicity to the resin.

[0056] In some embodiments, the organic acid is glacial acetic acid, used in an amount of 5-6 parts by weight, for example, any value or range between any two of 5, 5.3, 5.6, or 6 parts. The neutralization temperature is approximately 60°C, and the neutralization time is approximately 1 hour.

[0057] It should be noted that the amount of glacial acetic acid used needs to be properly controlled. Too little acetic acid will result in insufficient neutralization, poor water dispersibility, and decreased electrodeposition efficiency; too much acetic acid will cause the system pH to be too low, and the free acid may corrode the metal substrate during electrophoresis.

[0058] For example, other weak organic acids such as formic acid, lactic acid, or citric acid can be used instead of glacial acetic acid.

[0059] Deionized water is slowly added to the neutralized product to induce a phase transition and dispersion. The water addition process should be slow and uniform to avoid a sudden increase in local water concentration that could lead to resin precipitation or coagulation.

[0060] In some embodiments, the amount of deionized water used is 110-130 parts by weight, for example, any value or a range between any two of 110 parts, 115 parts, 120 parts, 125 parts, and 130 parts. The resulting emulsion has a milky white liquid appearance and a solid content of approximately 34%-36%.

[0061] Furthermore, this application provides a highly watermark-resistant cathodic electrophoretic emulsion, prepared by the aforementioned method. In some embodiments, the resulting emulsion has a solid content of 34%-36% and a particle size of approximately 400-450 nm.

[0062] Furthermore, this application provides the application of the aforementioned high watermark-resistant cathodic electrophoretic emulsion in electrophoretic coating of metal parts. This emulsion is particularly suitable for fields with stringent requirements for appearance quality, such as automotive bodies, appliance housings, and hardware accessories. It can also be applied in scenarios with special requirements for process tolerance, such as factories in humid geographical environments or with limited water resources.

[0063] The present application will be further described below with reference to embodiments and comparative examples. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the present application.

[0064] Example 1: S1: Take 160 parts of toluene diisocyanate and 90 parts of methyl isobutyl ketone in a reactor and stir at 50°C for 1 hour until clear and transparent. Slowly add 100 parts of a mixture of n-butanol and PEG400 (mass ratio of n-butanol to PEG400 1:2) dropwise at 50-60°C, and keep at this temperature for 2 hours after the addition is complete. Add 200 parts of oleamide resin and 10 parts of triethylenetetramine, raise the temperature to 80°C, add 110 parts of propylene glycol methyl ether, and keep at this temperature for 2 hours to obtain a polyamine-modified isocyanate curing agent.

[0065] S2: Take 20 parts of medium molecular weight solid epoxy resin, 10 parts of propylene glycol methyl ether, and 1 part of isopropanol, and stir in a reactor at 80-90℃ for 2 hours until clear and transparent. Add 5 parts of diethanolamine and keep at 80-90℃ for 2 hours. After cooling to 60-70℃, add 2 parts of triethanolamine and keep at this temperature for 30 minutes to obtain amine-modified epoxy resin.

[0066] S3: Add 50 parts of the polyamine-modified isocyanate curing agent prepared in S1 to the product of S2, heat to 80℃ and keep warm for 2 hours to obtain cationic epoxy resin.

[0067] S4: Add 5 parts of glacial acetic acid to the product of S3 and neutralize at 60℃ for 1 hour. Then slowly add 120 parts of deionized water and stir to disperse evenly to obtain a highly watermark-resistant cathodic electrophoretic emulsion.

[0068] The resulting emulsion had a particle size of approximately 450 nm and a solid content of 35.5%.

[0069] Example 2: S1: Take 180 parts of toluene diisocyanate and 95 parts of methyl isobutyl ketone in a reactor and stir at 50°C for 1 hour until clear and transparent. Slowly add 120 parts of a mixture of n-butanol and PEG400 (mass ratio 1:2) dropwise at 50-60°C, and keep at this temperature for 2 hours after the addition is complete. Add 230 parts of oleamide resin and 12 parts of triethylenetetramine, raise the temperature to 80°C, add 110 parts of propylene glycol methyl ether, and keep at this temperature for 2 hours to obtain a polyamine-modified isocyanate curing agent.

[0070] S2: Take 35 parts of medium molecular weight solid epoxy resin, 10 parts of propylene glycol methyl ether, and 2 parts of isopropanol, and stir in a reactor at 80-90℃ for 2 hours until clear and transparent. Add 6 parts of diethanolamine and keep at 80-90℃ for 2 hours. After cooling to 60-70℃, add 7 parts of triethanolamine and keep at this temperature for 30 minutes to obtain amine-modified epoxy resin.

[0071] S3: Add 60 parts of the polyamine-modified isocyanate curing agent prepared in S1 to the product of S2, heat to 80℃ and keep warm for 2 hours to obtain cationic epoxy resin.

[0072] S4: Add 6 parts of glacial acetic acid to the product of S3 and neutralize at 60℃ for 1 hour. Then slowly add 110 parts of deionized water and stir to disperse evenly to obtain a highly watermark-resistant cathodic electrophoretic emulsion.

[0073] The resulting emulsion had a particle size of approximately 440 nm and a solid content of 35.2%.

[0074] Example 3: S1: Take 190 parts of toluene diisocyanate and 90 parts of methyl isobutyl ketone in a reactor and stir at 50°C for 1 hour until clear and transparent. Slowly add 150 parts of a mixture of n-butanol and PEG400 (mass ratio 1:2) dropwise at 50-60°C, and keep warm for 2 hours after the addition is complete. Add 215 parts of oleamide resin and 15 parts of triethylenetetramine, raise the temperature to 80°C, add 120 parts of propylene glycol methyl ether, and keep warm for 2 hours to obtain a polyamine-modified isocyanate curing agent.

[0075] S2: Take 40 parts of medium molecular weight solid epoxy resin, 15 parts of propylene glycol methyl ether, and 3 parts of isopropanol, and stir in a reactor at 80-90℃ for 2 hours until clear and transparent. Add 6 parts of diethanolamine and keep at 80-90℃ for 2 hours. After cooling to 60-70℃, add 7 parts of triethanolamine and keep at this temperature for 30 minutes to obtain amine-modified epoxy resin.

[0076] S3: Add 52 parts of the polyamine-modified isocyanate curing agent prepared in S1 to the product of S2, heat to 80℃ and keep warm for 2 hours to obtain cationic epoxy resin.

[0077] S4: Add 5.6 parts of glacial acetic acid to the product of S3 and neutralize at 60℃ for 1 hour. Then slowly add 130 parts of deionized water and stir to disperse evenly to obtain a highly watermark-resistant cathodic electrophoretic emulsion.

[0078] The resulting emulsion had a particle size of approximately 400 nm and a solid content of 34.8%.

[0079] Comparative Example 1: The difference between Comparative Example 1 and Example 1 is that in step S1, an equal amount of n-butanol (100 parts) was used instead of the mixture of n-butanol and PEG400 for blocking. The remaining steps are exactly the same as in Example 1.

[0080] Comparative Example 2: The difference between Comparative Example 2 and Example 1 is that oleamide resin was not added in step S1. The remaining steps are exactly the same as in Example 1.

[0081] Comparative Example 3: The difference between Comparative Example 3 and Example 1 is that in step S2, triethanolamine was not added; that is, only diethanolamine was used to amination and modify the epoxy resin. The remaining steps are exactly the same as in Example 1.

[0082] Performance testing Examples 1-3, Comparative Examples 1-3, and commercially available cathodic electrophoretic emulsion KNT831LF were prepared into electrophoretic coatings by mixing deionized water and pigment paste in a mass ratio of 6:7:1, and then stirred and matured for at least 24 hours before use.

[0083] Electrophoretic coating preparation: The stainless steel plate is sanded with sandpaper to remove surface deposits and obtain a clean and smooth surface. Then, it is washed with ethanol in an ultrasonic cleaner to remove surface grease, and rinsed thoroughly with deionized water. The sanded sample is immersed in a prepared surface passivating agent for 5 minutes, then rinsed again with deionized water and dried. Using the stainless steel plate as the cathode and the aluminum alloy plate as the anode, electrophoretic coating is applied at 70V / 30℃ / 120s, and cured at 110℃ / 30min or 200℃ / 10min to obtain the electrophoretic paint film.

[0084] The test methods for paint film performance are as follows: film thickness is tested according to GB / T 13452.2-2008; pencil hardness is tested according to GB / T6739-2022; impact resistance is tested according to GB / T 1732-2020; flexibility is tested according to GB / T 1731-2020; neutral salt spray resistance is tested according to GB / T 1771-2007; damp heat resistance is tested according to GB / T 1740-2007; solvent wiping resistance (MEK) is tested according to GB / T 23989-2009; and liquid media resistance is tested according to the immersion method in GB / T 9274-1988.

[0085] Watermark test method: Dilute the emulsion with water at a volume ratio of 1:1 and then perform electrophoretic coating. After washing with water, do not dry it. Put it directly into an oven and set the oven to rapidly heat up to 200℃. Bake for 10 minutes and then take it out. Visually observe and count the watermarks on the board surface.

[0086] The test results are shown in Table 1.

[0087] Table 1 Performance test data of the examples and comparative examples Test Result Analysis The following conclusions can be drawn from the data in Table 1: A comparison of Comparative Example 1 and Examples 1-3 shows that Comparative Example 1 exhibited 7 watermarks in the rapid baking test, while Examples 1-3 showed only 1-2 watermarks. The difference lies in the fact that in Comparative Example 1, pure n-butanol was used instead of the mixture of n-butanol and PEG400 as a sealing agent. Since the n-butanol completely evaporates during baking, no hydrophilic polyether segments remain in the paint film. The wet film surface lacks sufficient hydrophilicity, causing water droplets to aggregate in a bead-like shape and fail to spread. Under rapid heating, these droplets are blocked by the paint film, forming numerous watermarks. This indicates that the introduction of PEG400 polyether segments is the key factor in achieving the watermark-resistant performance of this invention.

[0088] A comparison of Comparative Example 2 and Example 1 shows that Comparative Example 2 has 6 watermarks, significantly more than the 1 watermark in Example 1. Without oleamide resin, the surface tension of the wet film was not effectively reduced and homogenized. Although the water droplets showed some spreading tendency due to the hydrophilic effect of PEG400, the surface tension remained high, resulting in insufficient and uneven spreading. Many water droplets accumulated in large local areas, ultimately forming watermarks. Furthermore, the salt spray resistance of Comparative Example 2 was 2800 hours, slightly lower than the 3000 hours of Example 1. This may be related to the decreased density of the paint film network after the absence of oleamide resin.

[0089] A comparison of Comparative Example 3 and Example 1 shows that Comparative Example 3 has 4 watermarks, falling between Example 1 and the commercially available product. Without triethanolamine, the polyhydroxy hydrophilic groups on the epoxy resin side are absent, and the hydrophilicity of the system relies solely on the single source of PEG400 on the curing agent side. Although PEG400 still provides basic hydrophilic spreading function, the uneven distribution of hydrophilic groups in the resin network affects the uniformity of water droplet spreading, resulting in some areas where a small number of water droplets still accumulate and form watermarks.

[0090] A comparison of Examples 1-3 with the commercially available KNT831LF cathodic electrophoretic emulsion shows that the number of watermarks in the three examples of the present invention is significantly better than that of the commercially available products. Among them, Examples 1 and 3 have the best overall performance, with only 1 watermark, which effectively solves the problem of watermark defects in electrophoretic coatings under rapid baking conditions.

[0091] Meanwhile, the film thickness, pencil hardness, impact resistance, MEK wiping resistance, salt spray resistance, and other comprehensive properties of Examples 1-3 are basically the same as those of commercially available products, proving that while the present invention significantly improves the watermark resistance, the hardness, adhesion, and corrosion resistance of the paint film are still at a good level.

[0092] In summary, although the present invention has been disclosed above with reference to preferred embodiments, these preferred embodiments are not intended to limit the present invention. Those skilled in the art can make various modifications and refinements without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention is defined by the scope of the claims.

Claims

1. A method for preparing a highly watermark-resistant cathodic electrophoretic emulsion, characterized in that, Includes the following steps: S1. Part of the isocyanate groups of toluene diisocyanate are blocked by a mixture of n-butanol and PEG400, and then the unblocked isocyanate groups are reacted and bonded with oleamide resin and polyamine to obtain a polyamine-modified isocyanate curing agent. S2. After the epoxy resin is subjected to a ring-opening amination reaction with the first amine compound, a second amine compound is added to continue the reaction to obtain an amine-modified epoxy resin; S3. Graft the polyamine-modified isocyanate curing agent onto the amine-modified epoxy resin to obtain a cationic epoxy resin; S4. The cationic epoxy resin is neutralized with an organic acid and then emulsified and dispersed with deionized water to obtain the highly watermark resistant cathodic electrophoretic emulsion.

2. The preparation method according to claim 1, characterized in that, In step S1, the mass ratio of n-butanol to PEG400 is 1:

2.

3. The preparation method according to claim 2, characterized in that, In step S1, the amount of toluene diisocyanate used is 160-190 parts by weight, and the total amount of the mixture of n-butanol and PEG400 used is 99-150 parts by weight.

4. The preparation method according to claim 1, characterized in that, In step S1, the amount of oleamide resin used is 200-230 parts by weight, and the amount of polyamine used is 10-15 parts by weight.

5. The preparation method according to claim 1, characterized in that, In step S2, the epoxy resin is a medium molecular weight solid epoxy resin, and the amount used is 20-40 parts by weight.

6. The preparation method according to claim 5, characterized in that, In step S2, the first amine compound is diethanolamine, and the amount used is 5-6 parts by weight.

7. The preparation method according to claim 6, characterized in that, In step S2, the second amine compound is triethanolamine, and the amount used is 2-7 parts by weight. The second amine compound is added after the ring-opening amination reaction is completed and the temperature is cooled to 60-70°C.

8. The preparation method according to claim 1, characterized in that, In step S3, the amount of the polyamine-modified isocyanate curing agent added is 50-60 parts by weight.

9. The preparation method according to claim 1, characterized in that, In step S4, the organic acid is glacial acetic acid, and the amount used is 5-6 parts by weight; and / or, The amount of deionized water used is 110-130 parts by weight.

10. A highly watermark-resistant cathodic electrophoretic emulsion, characterized in that, It is prepared by the preparation method according to any one of claims 1-9.

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