A cathodic electrophoretic coating emulsion against punching and its preparation method

Abstract

This invention relates to a cathodic electrophoretic coating emulsion for preventing paint cracking during perforation and its preparation method, belonging to the field of electrophoretic coating technology. The preparation method includes the following steps: S1. Blocking some isocyanate groups of toluene diisocyanate, and then reacting and bonding the unblocked isocyanate groups with polyamide resin to obtain a self-crosslinking polyamide resin; S2. Amination reaction of epoxy-terminated polybutadiene rubber with a first amine compound to obtain amino-modified polybutadiene liquid rubber; S3. Ring-opening amination reaction of epoxy resin with a second amine compound to obtain amine-modified epoxy resin; S4. Grafting the self-crosslinking polyamide resin onto the amine-modified epoxy resin to obtain grafted resin; S5. Physically mixing the amino-modified polybutadiene liquid rubber and the grafted resin, neutralizing with organic acid, and dispersing with water to obtain the cathodic electrophoretic coating emulsion. This method solves the problem of edge cracking and peeling of existing electrophoretic coatings after secondary mechanical processing such as perforation.

Description

Technical Field

[0001] This invention relates to the field of electrophoretic coating technology, specifically to a cathodic electrophoretic coating emulsion for resisting pore-bursting and paint cracking, and its preparation method. Background Technology

[0002] Cathodic electrophoretic coatings have been widely used in the surface protective coating of metal products such as automotive parts and hardware accessories due to their excellent corrosion resistance, uniform film thickness distribution, and efficient automated construction characteristics. 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 coating film, which is then baked and cured to form a paint film with good adhesion and corrosion resistance.

[0003] However, in actual production, many metal workpieces require secondary machining after electrophoretic coating, such as punching, shearing, and bending. During the punching process, the punch applies extremely high local shear and impact forces to the metal substrate, and the electrophoretic coating film covering the surface of the metal substrate is subjected to local strains far exceeding its elongation at break. Traditional cathodic electrophoretic coating films are single-phase rigid cross-linked network structures. When the local strain at the edge of the punch exceeds the film's ultimate deformation capacity, cracks form in the stress concentration area and propagate rapidly along the punched corners, causing the film to peel off over a large area from the metal substrate, a phenomenon known as "punch-out paint."

[0004] Paint peeling during perforation not only affects the appearance quality but also leaves the exposed metal substrate without corrosion protection. In existing technologies, the common approach to address paint film brittleness is to reduce the overall cross-linking density of the paint film or introduce a large number of flexible segments to improve overall flexibility. However, while increasing flexibility, this approach inevitably leads to a comprehensive decrease in paint film hardness and corrosion resistance. Summary of the Invention

[0005] This invention provides a method for preparing a cathodic electrophoretic coating emulsion that resists paint cracking during perforation. The emulsion prepared by this method, after electrophoretic coating and secondary perforation processing, does not crack at the edges and corners of the paint film, thus solving the problem of cracking and peeling at the edges and corners of existing electrophoretic coatings after secondary mechanical processing such as perforation.

[0006] In a first aspect, the present invention provides a method for preparing a cathodic electrophoretic coating emulsion resistant to pore-bursting and paint cracking, comprising the following steps: S1. Part of the isocyanate groups of toluene diisocyanate are blocked by a mixture of n-butanol and polyether polyol, and then the unblocked isocyanate groups are reacted and bonded with polyamide resin to obtain self-crosslinking polyamide resin. S2. Amination reaction of terminal epoxy-terminated polybutadiene rubber with a first amine compound is carried out to obtain amino-modified polybutadiene liquid rubber; S3. The epoxy resin is subjected to a ring-opening amination reaction with a second amine compound to obtain an amine-modified epoxy resin; S4. The self-crosslinking polyamide resin is grafted onto the amine-modified epoxy resin to obtain a grafted resin; S5. After physically mixing the amino-modified polybutadiene liquid rubber with the grafted resin, the mixture is neutralized with organic acid and dispersed with water to obtain the cathodic electrophoretic coating emulsion.

[0007] Furthermore, in step S2, the amount of the terminal epoxy polybutadiene rubber is 20-30 parts by weight.

[0008] Further, in step S2, the first amine compound includes diethanolamine and diethylaminopropylamine, wherein the amount of diethanolamine is 5-10 parts by weight and the amount of diethylaminopropylamine is 10-15 parts by weight.

[0009] Furthermore, in step S2, the amination reaction is carried out in two stages: in the first stage, the temperature is maintained at 100-110℃ for 1.5-2.5h; in the second stage, the temperature is lowered to 55-65℃ and diethylaminopropylamine is added, and then the temperature is raised to 115-125℃ and maintained for 1.5-2.5h.

[0010] Furthermore, in step S2, polypropylene glycol diglycidyl ether is added as a flexible chain extender during the amination reaction, and the amount of polypropylene glycol diglycidyl ether is 10-20 parts by weight.

[0011] Further, in step S1, the mass ratio of n-butanol to polyether polyol is 1:2; And / or, the amount of toluene diisocyanate is 150-200 parts by weight, the amount of the mixture of n-butanol and polyether polyol is 100-200 parts by weight, and the amount of polyamide resin is 180-250 parts by weight. And / or, the sealing reaction temperature is 50-60℃ and the holding time is 1.5-2.5h, and the bonding reaction temperature is 75-85℃ and the holding time is 1.5-2.5h.

[0012] Further, in step S3, the epoxy resin is a medium molecular weight solid epoxy resin, and the amount used is 20-50 parts by weight; the second amine compound is diethanolamine, and the amount used is 5-10 parts by weight. And / or, the ring-opening amination reaction is carried out at a temperature of 80-90°C and a holding time of 1.5-2.5 h.

[0013] Further, in step S4, the amount of the self-crosslinking polyamide resin added is 50-80 parts by weight; And / or, in step S5, the amount of amino-modified polybutadiene liquid rubber added is 10-20 parts by weight.

[0014] Furthermore, the amount of the organic acid used is 5-8 parts by weight; And / or, the water dispersion uses deionized water, and the amount of deionized water is 120-150 parts by weight.

[0015] Secondly, the present invention provides a cathodic electrophoretic coating emulsion for resisting puncture and paint cracking, which is prepared by the aforementioned preparation method.

[0016] Through one or more embodiments of the above embodiments of the present invention, at least the following technical effects can be achieved: This invention introduces amino-modified polybutadiene liquid rubber into the cathodic electrophoretic coating system. By utilizing the high elongation at break of the polybutadiene backbone, flexible microdomains are constructed in the cured coating film. During the punching process, the coating film can absorb and disperse the local stress concentration at the punching corners through the elastic deformation of the rubber microdomains, thereby avoiding cracking and peeling of the coating film.

[0017] Meanwhile, after amino modification, the amino groups of polybutadiene liquid rubber are positively charged after acid neutralization, which can be deposited synchronously with the resin matrix during electrophoretic deposition, ensuring the uniform distribution of the rubber phase in the coating film.

[0018] In addition, the self-crosslinking polyamide resin is chemically bonded to the amine-modified epoxy resin to construct a resin network skeleton with high crosslinking density, which ensures the hardness, adhesion and corrosion resistance of the coating film. Attached Figure Description

[0019] 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.

[0020] Figure 1 This is an enlarged view of the corner of the coating after punching in Example 1.

[0021] Figure 2 The image shows a comparison of the corner shape after punching the coated part in Example 2 with the image without coating (left: no coating, right: with the above coating).

[0022] Figure 3 This is a diagram showing the overall shape of the coating after punching in Example 3.

[0023] Figure 4 This is a diagram showing the shape of the corners after punching holes in a commercially available electrophoretic coating emulsion. Detailed Implementation

[0024] 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.

[0025] 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.

[0026] As described in the background section, traditional cathodic electrophoretic coatings use ammoniated epoxy resin as the core film-forming material. Although they possess good electrodeposition and corrosion resistance, the epoxy-cured film is brittle with low elongation at break. When the workpiece undergoes secondary punching after electrophoretic coating, the edges and corners of the film cannot withstand the intense plastic deformation of the metal substrate, resulting in cracking and peeling, a problem known as "paint peeling." Existing flexibility improvement solutions often sacrifice hardness or corrosion resistance, and their effectiveness is limited under extreme localized stress.

[0027] To address the aforementioned issues, this application provides a novel resin system design approach: In a high crosslinking density network constructed by grafting amine-modified epoxy resin onto self-crosslinking polyamide resin, amino-modified polybutadiene liquid rubber is introduced as a flexible toughening component, utilizing the elastic deformation capability of the rubber microdomains to absorb local stress during punching.

[0028] Correspondingly, this application discloses a method for preparing a cathodic electrophoretic coating emulsion that resists pore-bursting and paint cracking, comprising the following steps: S1. A mixture of n-butanol and polyether polyol is used to block some of the isocyanate groups of toluene diisocyanate, and then the unblocked isocyanate groups are reacted and bonded with polyamide resin to obtain a self-crosslinking polyamide resin.

[0029] First, toluene diisocyanate is dissolved in a first solvent.

[0030] In some embodiments, the first solvent is propylene glycol methyl ether acetate (PMA), and the amount used is 90-100 parts by weight. The stirring temperature is controlled at 45-55°C, and the stirring time is about 1 hour, until the system is clear and transparent, indicating that toluene diisocyanate has been fully dissolved.

[0031] Toluene diisocyanate was selected as the isocyanate component. This molecule contains two -NCO groups, and the two -NCO groups exhibit significant differences in reactivity due to their different spatial positions. Taking 2,4-toluene diisocyanate as an example, the -NCO at the 4-position is far from the methyl group, has less steric hindrance, and is highly reactive, while the -NCO at the 2-position is adjacent to the methyl group, has greater steric hindrance, and is less reactive. First, the more reactive -NCO reacts with the blocking agent, and then the less reactive -NCO reacts with the polyamide resin.

[0032] For example, the CAS number of toluene diisocyanate used in this application is 26471-62-5.

[0033] After toluene diisocyanate is fully dissolved, a mixture of n-butanol and polyether polyol is slowly added dropwise to the system at 50-60°C. Both n-butanol and polyether polyol contain hydroxyl groups, which can react with the highly reactive -NCO groups on toluene diisocyanate 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.

[0034] n-Butanol has a small molecular weight and high reactivity, and can react with the most active -NCO in the early stage of the sealing reaction, thus playing a role in regulating the sealing rate. Polyether polyol has a larger molecular weight, and although the sealing rate is slightly lower, the polyether polyol released during subsequent baking has a very low vapor pressure, which reduces the amount of evaporation from the paint film. The polyether polyol remains inside the paint film as a flexible segment, and may even further participate in cross-linking and curing.

[0035] Therefore, the higher the proportion of polyether polyols used, the lower the proportion of volatile components during baking, and the lower the heat loss.

[0036] In some embodiments, the polyether polyol is preferably polypropylene glycol PPG400. The mass ratio of n-butanol to PPG400 is 1:2. Exemplarily, the polyether polyol may also be PPG600, PPG1000, or PTMEG650, etc. The total amount of the n-butanol and polyether polyol mixture is 100-200 parts by weight, for example, any value or range between any two of 100, 120, 150, or 200 parts.

[0037] After the blocking reaction is complete, polyamide resin is added to the system. After heating to 75-85℃, the unblocked -NCO on toluene diisocyanate reacts with the -NH- on the polyamide molecular chain via urea bond reaction, anchoring toluene diisocyanate to the polyamide backbone via covalent bonds, forming a permanent bond.

[0038] In some embodiments, a second solvent, propylene glycol methyl ether, is added as a diluent to adjust the viscosity while heating, and the temperature is maintained for 1.5-2.5 hours to allow the bonding reaction to proceed fully. In some embodiments, the amount of propylene glycol methyl ether used is 110-130 parts by weight, for example, 110 parts, 115 parts, 120 parts, or 130 parts.

[0039] In some embodiments, the amount of toluene diisocyanate used is 150-200 parts by weight, for example 150, 155, 160, 180, or 200 parts. The amount of polyamide resin used is 180-250 parts by weight, for example 180, 200, 220, or 250 parts.

[0040] The amount of TDI used determines the total amount of -NCO in the system, which directly affects the crosslinking density of the final coating film. If the amount is too low, the crosslinking density will be insufficient; if the amount is too high, the coating film will become brittle. The crosslinking density is at a suitable level within the range of 150-200 parts.

[0041] The structure of the self-crosslinking polyamide resin is as follows: S2. Amination reaction of terminal epoxy group polybutadiene rubber with a first amine compound to obtain amino-modified polybutadiene liquid rubber.

[0042] Using epoxy-terminated polybutadiene rubber as the starting material, protonable amino groups are introduced onto the polybutadiene molecule through an amination reaction with a first amine compound. The introduced amino groups are protonated into ammonium salt structures in the acid neutralization step S5, giving the rubber molecules a positive charge and hydrophilicity, enabling them to migrate and deposit synchronously with the resin matrix during electrophoresis.

[0043] In some embodiments, the amount of the epoxy-terminated polybutadiene rubber is 20-30 parts by weight, for example, any value or a range between any two of 20 parts, 25 parts, or 30 parts.

[0044] In some embodiments, the first amine compound includes diethanolamine and diethylaminopropylamine.

[0045] In some embodiments, the amount of diethanolamine used is 5-10 parts by weight, for example, any value or a range between any two of 5, 7, or 10 parts. The amount of diethylaminopropylamine used is 10-15 parts by weight, for example, any value or a range between any two of 10, 12, or 15 parts.

[0046] In some embodiments, the amination reaction in step S2 is carried out in two stages: First stage: After mixing epoxy-terminated polybutadiene rubber and diethanolamine evenly, add ethylene glycol butyl ether as a solvent and keep warm at 100-110℃ for 1.5-2.5h.

[0047] During this stage, the secondary amine group of diethanolamine undergoes a ring-opening addition reaction with the epoxy groups on the terminal epoxy groups of polybutadiene rubber and the epoxy groups on polypropylene glycol diglycidyl ether, thus completing the chain extension of the rubber and the introduction of tertiary amine / secondary hydroxyl groups.

[0048] In some embodiments, the amount of ethylene glycol butyl ether used is 30-50 parts by weight, for example, 30 parts. Ethylene glycol butyl ether serves as a reaction solvent in step S2; its boiling point is higher than the amination reaction temperature, enabling it to maintain a liquid phase state during the reaction and providing a good mass transfer environment. In subsequent steps, ethylene glycol butyl ether remains in the final emulsion system, participating as a co-solvent in the dispersion and stabilization of the emulsion.

[0049] Second stage: Cool the system to 55-65℃, add diethylaminopropylamine, mix thoroughly, and then heat to 115-125℃ and keep warm for 1.5-2.5h.

[0050] At this stage, the primary amine group of diethylaminopropylamine reacts with the residual epoxy groups in the system, introducing a primary amine group onto the rubber molecule.

[0051] Understandably, in the first stage, the ring-opening reaction between the secondary amine group and the epoxy group of diethanolamine has a low activation energy, proceeding efficiently at 100-110℃. If diethylaminopropylamine is added simultaneously, its primary amine group has higher reactivity than the secondary amine group, competing with diethanolamine for the reaction site of the epoxy group, resulting in an uncontrollable distribution of functional groups at the molecular chain ends. Therefore, the ring-opening reaction and chain extension of diethanolamine are completed first in the first stage, followed by the grafting reaction of diethylaminopropylamine in the second stage to achieve the orderly introduction of functional groups.

[0052] In some embodiments, in the first stage of step S2, polypropylene glycol diglycidyl ether is added as a flexible chain extender, and the amount is 10-20 parts by weight, for example, any value or range between any two of 10 parts, 15 parts, or 20 parts.

[0053] Polypropylene glycol diglycidyl ether (PPDD) molecules contain an epoxy group at each end, with a flexible polyether segment of PPD in the middle. In the first stage of the amination reaction, the two epoxy groups react with diethanolamine intermediates on the end groups of different rubber molecules, acting as molecular bridges to connect two short-chain rubber molecules through the flexible polyether segment, thus achieving chain extension and growth of the rubber molecules.

[0054] The structure of amino-modified polybutadiene liquid rubber is as follows: S3. The epoxy resin is subjected to a ring-opening amination reaction with a second amine compound to obtain an amine-modified epoxy resin.

[0055] Specifically, epoxy resin is dissolved in a mixed solvent and stirred at 80-90℃ until transparent. Then, a second amine compound is added to initiate a ring-opening amination reaction. The mixture is kept at this temperature for 1.5-2.5 hours to allow the active amine groups of the amine compound to fully undergo a ring-opening addition reaction with the epoxy groups on the epoxy resin molecular chain, introducing tertiary amine groups and secondary hydroxyl groups onto the epoxy resin molecular chain.

[0056] 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 20-50 parts by weight, such as 20, 30, 40, or 50 parts. Exemplarily, the medium molecular weight solid epoxy resin can be type E-20 (epoxy resin 601), type CYD-014, or type EPON1004, etc.

[0057] In some embodiments, the mixed solvent comprises 10-15 parts of propylene glycol methyl ether and 1-6 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.

[0058] In some embodiments, the second amine compound is diethanolamine, and the amount used is 5-10 parts by weight, for example, any value or range between any two of 5, 6, 7, or 10 parts.

[0059] The secondary amine group of diethanolamine undergoes a ring-opening addition reaction with the epoxy group on the epoxy resin, simultaneously introducing a tertiary amine group and a secondary hydroxyl group onto the molecular chain. The ring-opening amination reaction is carried out at 80-90℃ for 1.5-2.5h to ensure complete completion.

[0060] The structure of amine-modified epoxy resin is as follows: S4. The self-crosslinking polyamide resin is grafted onto the amine-modified epoxy resin to obtain the grafted resin.

[0061] Specifically, the self-crosslinking polyamide resin prepared in step S1 is added to the amine-modified epoxy resin product in step S3, and the mixture is kept at 75-85℃ for 1.5-2.5h to form a grafted structure through chemical bonding.

[0062] In some embodiments, the amount of the self-crosslinking polyamide resin added is 50-80 parts by weight, for example, any value or a range between any two of 50, 55, 60, 70, and 80 parts.

[0063] S5. After physically mixing the amino-modified polybutadiene liquid rubber with the grafted resin, the mixture is neutralized with organic acid and dispersed with water to obtain the cathodic electrophoretic coating emulsion.

[0064] Specifically, the amino-modified polybutadiene liquid rubber prepared in step S2 is added to the grafted resin product in step S4, and stirred at 55-65℃ for 0.5-1.5h to ensure that the rubber and grafted resin are fully and uniformly mixed.

[0065] In some embodiments, the amount of amino-modified polybutadiene liquid rubber added is 10-20 parts by weight, for example, any value or a range between any two of 10 parts, 15 parts, or 20 parts.

[0066] Furthermore, an organic acid is added to the physical mixture for neutralization. The protons of the organic acid simultaneously react with the tertiary amine groups on the amine-modified epoxy resin and the amino groups on the polybutadiene liquid rubber to form an ammonium salt structure, imparting positive charge and hydrophilicity to both components.

[0067] In some embodiments, the organic acid is glacial acetic acid, used in an amount of 5-8 parts by weight, for example, any value or range between any two of 5, 6, 7, or 8 parts. The neutralization temperature is 55-65°C, and the neutralization time is 0.5-1.5 hours.

[0068] It should be noted that the amount of glacial acetic acid used needs to be properly controlled. Insufficient dosage leads to incomplete neutralization, poor water dispersibility, and decreased electrodeposition efficiency; excessive dosage results in an excessively low pH, and the free acid may corrode the metal substrate during electrophoresis. For example, other weak organic acids such as formic acid, lactic acid, or citric acid can be used instead of glacial acetic acid.

[0069] Further, deionized water is slowly added to the neutralization product to induce a phase transition 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.

[0070] In some embodiments, the amount of deionized water used is 120-150 parts by weight, for example, any value or a range between any two of 120, 130, 140, and 150 parts. The resulting emulsion has a milky white liquid appearance and a solids content of approximately 34%-36%.

[0071] 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.

[0072] Example 1: S1: Take 160 parts of toluene diisocyanate and 90 parts of propylene glycol methyl ether acetate 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 PPG400 (mass ratio of n-butanol to PPG400 1:2) dropwise at 50-60°C, and keep at this temperature for 2 hours after the addition is complete. Add 200 parts of ketimide-modified polyamide resin, raise the temperature to 80°C, add 120 parts of propylene glycol methyl ether, and keep at this temperature for 2 hours to obtain a self-crosslinking polyamide resin.

[0073] S2: Take 20 parts of epoxy-terminated polybutadiene rubber, 10 parts of polypropylene glycol diglycidyl ether, and 30 parts of ethylene glycol butyl ether, and mix thoroughly. Add 5 parts of diethanolamine and keep warm at 105℃ for 2 hours. Cool down to 60℃, add 10 parts of diethylaminopropylamine, and heat to 120℃ for 2 hours to obtain amino-modified polybutadiene liquid rubber.

[0074] S3: Take 40 parts of medium molecular weight solid epoxy resin E-20 (epoxy resin 601), 15 parts of propylene glycol methyl ether, and 5 parts of isopropanol, and stir in a reactor at 85°C for 2 hours until clear and transparent. Add 5 parts of diethanolamine, and keep at 85°C for 2 hours to obtain amine-modified epoxy resin.

[0075] S4: Add 50 parts of the self-crosslinking polyamide resin prepared in S1 to the product of S3, and keep it at 80℃ for 2 hours to obtain grafted resin.

[0076] S5: Add 10 parts of the amino-modified polybutadiene liquid rubber prepared in S2 to the product of S4, and physically mix at 60°C for 1 hour. Add 5 parts of glacial acetic acid and neutralize at 60°C for 1 hour. Slowly add 120 parts of deionized water to obtain a cathodic electrophoretic coating emulsion with anti-piercing and anti-cracking properties.

[0077] Example 2: S1: Take 155 parts of toluene diisocyanate and 90 parts of propylene glycol methyl ether acetate 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 PPG400 (mass ratio 1:2) dropwise at 50-60°C, and keep at this temperature for 2 hours after the addition is complete. Add 180 parts of polyamide resin, raise the temperature to 80°C, add 130 parts of propylene glycol methyl ether, and keep at this temperature for 2 hours to obtain a self-crosslinking polyamide resin.

[0078] S2: Take 25 parts of epoxy-terminated polybutadiene rubber, 15 parts of polypropylene glycol diglycidyl ether, and 30 parts of ethylene glycol butyl ether, and mix thoroughly. Add 7 parts of diethanolamine and keep warm at 105℃ for 2 hours. Cool down to 60℃, add 12 parts of diethylaminopropylamine, and heat to 120℃ for 2 hours to obtain amino-modified polybutadiene liquid rubber.

[0079] S3: Take 50 parts of medium molecular weight solid epoxy resin E-20 (epoxy resin 601), 14 parts of propylene glycol methyl ether, and 6 parts of isopropanol, and stir in a reactor at 85°C for 2 hours until clear and transparent. Add 6 parts of diethanolamine, and keep at 85°C for 2 hours to obtain amine-modified epoxy resin.

[0080] S4: Add 70 parts of the self-crosslinking polyamide resin prepared in S1 to the product of S3, and keep it at 80℃ for 2 hours to obtain grafted resin.

[0081] S5: Add 20 parts of the amino-modified polybutadiene liquid rubber prepared in S2 to the product of S4, and physically mix at 60°C for 1 hour. Add 7 parts of glacial acetic acid and neutralize at 60°C for 1 hour. Slowly add 120 parts of deionized water to obtain a cathodic electrophoretic coating emulsion with anti-piercing and anti-cracking properties.

[0082] Example 3: S1: Take 150 parts of toluene diisocyanate and 90 parts of propylene glycol methyl ether acetate 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 PPG400 (mass ratio 1:2) dropwise at 50-60°C, and keep at this temperature for 2 hours after the addition is complete. Add 200 parts of polyamide resin, raise the temperature to 80°C, add 130 parts of propylene glycol methyl ether, and keep at this temperature for 2 hours to obtain a self-crosslinking polyamide resin.

[0083] S2: Take 30 parts of epoxy-terminated polybutadiene rubber, 15 parts of polypropylene glycol diglycidyl ether, and 30 parts of ethylene glycol butyl ether, and mix thoroughly. Add 7 parts of diethanolamine and keep warm at 105℃ for 2 hours. Cool down to 60℃, add 15 parts of diethylaminopropylamine, and heat to 120℃ for 2 hours to obtain amino-modified polybutadiene liquid rubber.

[0084] S3: Take 50 parts of medium molecular weight solid epoxy resin E-20 (epoxy resin 601), 14 parts of propylene glycol methyl ether, and 6 parts of isopropanol, and stir in a reactor at 85°C for 2 hours until clear and transparent. Add 7 parts of diethanolamine, and keep at 85°C for 2 hours to obtain amine-modified epoxy resin.

[0085] S4: Add 80 parts of the self-crosslinking polyamide resin prepared in S1 to the product of S3, and keep it at 80℃ for 2 hours to obtain grafted resin.

[0086] S5: Add 20 parts of the amino-modified polybutadiene liquid rubber prepared in S2 to the product of S4, and physically mix at 60°C for 1 hour. Add 8 parts of glacial acetic acid and neutralize at 60°C for 1 hour. Slowly add 130 parts of deionized water to obtain a cathodic electrophoretic coating emulsion with anti-piercing and anti-cracking properties.

[0087] Comparative Example 1: The difference between Comparative Example 1 and Example 1 is that the preparation of the amino-modified polybutadiene liquid rubber in step S2 and the rubber incorporation step in step S5 are omitted. That is, the emulsion system does not contain the polybutadiene liquid rubber component and is composed only of self-crosslinking polyamide resin grafted with amine-modified epoxy resin. The remaining steps are exactly the same as in Example 1.

[0088] Comparative Example 2: The difference between Comparative Example 2 and Example 1 is that polypropylene glycol diglycidyl ether was not added in step S2; instead, 20 parts of terminal epoxy-terminated polybutadiene rubber were directly amination reaction with 5 parts of diethanolamine and 10 parts of diethylaminopropylamine. That is, the rubber was not subjected to polyether chain extension. The remaining steps were exactly the same as in Example 1.

[0089] Comparative Example 3: The difference between Comparative Example 3 and Example 1 is that the amount of amino-modified polybutadiene liquid rubber in step S5 was adjusted from 10 parts to 25 parts. The remaining steps are exactly the same as in Example 1.

[0090] Performance testing: Examples 1-3, Comparative Examples 1-3, and commercially available cathodic electrophoretic coating emulsion HY-600 were respectively mixed with pigment paste in appropriate proportions to form electrophoretic coatings, and stirred and matured for more than 24 hours.

[0091] Electrophoretic coating preparation: After sanding the surface of the phosphated standard steel plate, it was degreased with ethanol in an ultrasonic cleaner and rinsed clean with deionized water. The steel plate was then immersed in a surface passivating agent for 5 minutes, rinsed with deionized water, and dried. Using the treated steel plate as the cathode, electrophoretic coating was applied at 60V / 25℃ / 4 minutes and cured at 180℃ / 30 minutes to obtain the electrophoretic paint film.

[0092] The test methods for paint film performance are as follows: Film thickness: Tested according to GB / T 13452.2-2008; Pencil hardness: Tested according to GB / T 6739-2006; Gloss (60° angle): Tested according to GB / T 9754-2007; Flexibility: Tested according to GB / T 1731-1993; Impact resistance: Tested according to GB / T 1732-1993; Adhesion: Tested according to GB / T 9286-1988; Salt spray resistance: Tested according to GB / T 1771-2007 (salt spray test on cold-rolled steel sheet before degreasing and rust removal).

[0093] Punching test method: The prepared emulsion is diluted with water at a volume ratio of 1:1 and then electrophoresed on a standard phosphate plate. The baking temperature is 180℃ / 20min, and the electrophoretic film thickness is controlled at 25μm. Then, a punching test is carried out in a machining plant to observe whether paint peeling occurs at the punching edge.

[0094] The test results are shown in Table 1.

[0095] Table 1 Test Result Analysis The following conclusions can be drawn from the data in Table 1: 1. The comparison between Comparative Example 1 and Examples 1-3 shows that the introduction of polybutadiene liquid rubber enables the paint film to absorb and disperse the local stress concentration at the corners of the punching through the elastic deformation of the rubber microdomains during the punching process. In contrast, the paint film of Comparative Example 1 is a rigid cross-linked network of a single phase. The extreme local strain at the corners of the punching directly exceeds the elongation at break of the paint film, resulting in paint cracking.

[0096] 2. A comparison between Comparative Example 2 and Example 1 shows that the short-chain rubber without the chain-extending effect of polypropylene glycol diglycidyl ether has a lower molecular weight and shorter chain segments, resulting in limited elastic deformation and stress absorption capacity, which is insufficient to completely buffer the extreme strain at the punched corners. The high-molecular-weight rubber after chain extension has longer flexible chain segments and a larger elastic deformation space, thus enabling it to absorb local stress more effectively.

[0097] 3. A comparison between Comparative Example 3 and Example 1 shows that excessive rubber content disrupts the integrity and density of the continuous phase matrix. On the one hand, it creates weak areas in the crosslinking network of the matrix, reducing the overall crack resistance of the coating film; on the other hand, it decreases the corrosion resistance of the coating film. Therefore, more rubber is not necessarily better. Within the range of 10-20 parts by weight, the toughening effect of the rubber microdomains achieves the best balance with the structural integrity of the continuous phase matrix.

[0098] 4. A comparison of Examples 1-3 with the commercially available HY-600 cathodic electrophoretic coating emulsion shows that the emulsion of the present invention is significantly superior to the commercially available product in all comprehensive performance aspects. The commercially available HY-600 emulsion exhibited significant paint peeling during the punching test, and all its indicators were lower than those of the examples of the present invention.

[0099] Among them, Example 3 has the best overall performance, with a pencil hardness of 3H, a gloss of 98 (60°), and a salt spray resistance of 500h without bubbling, cracking, or peeling. There is no paint cracking at the corners after punching.

[0100] 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 cathodic electrophoretic coating emulsion with anti-piercing and paint-cracking properties, 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 polyether polyol, and then the unblocked isocyanate groups are reacted and bonded with polyamide resin to obtain self-crosslinking polyamide resin. S2. Amination reaction of terminal epoxy-terminated polybutadiene rubber with a first amine compound is carried out to obtain amino-modified polybutadiene liquid rubber; S3. The epoxy resin is subjected to a ring-opening amination reaction with a second amine compound to obtain an amine-modified epoxy resin; S4. The self-crosslinking polyamide resin is grafted onto the amine-modified epoxy resin to obtain a grafted resin; S5. After physically mixing the amino-modified polybutadiene liquid rubber with the grafted resin, the mixture is neutralized with organic acid and dispersed with water to obtain the cathodic electrophoretic coating emulsion.

2. The preparation method according to claim 1, characterized in that, In step S2, the amount of the terminal epoxy polybutadiene rubber is 20-30 parts by weight.

3. The preparation method according to claim 1, characterized in that, In step S2, the first amine compound includes diethanolamine and diethylaminopropylamine, wherein the amount of diethanolamine is 5-10 parts by weight and the amount of diethylaminopropylamine is 10-15 parts by weight.

4. The preparation method according to claim 3, characterized in that, In step S2, the amination reaction is carried out in two stages: the first stage is held at 100-110℃ for 1.5-2.5h, and the second stage is cooled to 55-65℃ and then diethylaminopropylamine is added, followed by heating to 115-125℃ and holding for 1.5-2.5h.

5. The preparation method according to claim 4, characterized in that, In step S2, polypropylene glycol diglycidyl ether is added as a flexible chain extender during the amination reaction, and the amount of polypropylene glycol diglycidyl ether is 10-20 parts by weight.

6. The preparation method according to claim 1, characterized in that, In step S1, the mass ratio of n-butanol to the polyether polyol is 1:2; and / or, The amount of toluene diisocyanate used is 150-200 parts by weight, the amount of the mixture of n-butanol and polyether polyol used is 100-200 parts by weight, and the amount of polyamide resin used is 180-250 parts by weight; and / or, The sealing reaction temperature is 50-60℃ and the holding time is 1.5-2.5h, and the bonding reaction temperature is 75-85℃ and the holding time is 1.5-2.5h.

7. The preparation method according to claim 1, characterized in that, In step S3, the epoxy resin is a medium molecular weight solid epoxy resin, used in an amount of 20-50 parts by weight; the second amine compound is diethanolamine, used in an amount of 5-10 parts by weight; and / or, The ring-opening amination reaction is carried out at a temperature of 80-90℃ for 1.5-2.5 hours.

8. The preparation method according to claim 1, characterized in that, In step S4, the amount of the self-crosslinking polyamide resin added is 50-80 parts by weight; and / or, In step S5, the amount of amino-modified polybutadiene liquid rubber added is 10-20 parts by weight.

9. The preparation method according to claim 1, characterized in that, The organic acid is used in an amount of 5-8 parts by weight; and / or, The water dispersion is carried out using deionized water, and the amount of deionized water is 120-150 parts by weight.

10. A cathodic electrophoretic coating emulsion for resisting pore-bursting and paint peeling, characterized in that, It is prepared by the preparation method according to any one of claims 1-9.

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