Polyurethane-modified epoxy resin cathodic electrophoretic paint and method for preparing the same
By incorporating polyurethane segments with urethane bonds into the epoxy resin backbone, the problem of polyurethane soft segments and cationic groups enrichment is solved, thereby improving the stability and anti-corrosion performance of electrophoretic coatings.
Patent Information
- Application Number
- CN202611068129.5
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2026-07-17
- Publication Date
- 2026-08-25
AI Technical Summary
In existing electrophoretic coatings, polyurethane soft segments and cationic hydrophilic groups tend to accumulate locally in the deposited film, leading to the formation of low cross-linking density permeation channels in the cured film, which affects the stability and anti-corrosion performance of the film.
By attaching polyurethane segments containing tertiary amine groups to a bisphenol A type epoxy resin backbone via urethane bonds, and then subjecting the process to supplementary amination, neutralization emulsification, and paint conditioning, a stable cationic waterborne electrophoretic coating is formed, reducing the local enrichment of polyurethane soft segments and hydrophilic cationic groups.
It improves the stability of the paint film, reduces the formation of penetration channels, and enhances the anti-corrosion performance under humid, hot, and salt spray conditions.
Smart Images
Figure CN122628627A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of electrophoretic coating technology, and relates to a polyurethane modified epoxy resin cathodic electrophoretic coating and its preparation method. Background Technology
[0002] Epoxy resin is often used as the main film-forming resin in cathodic electrophoretic coatings due to its good adhesion and corrosion resistance. However, the epoxy resin skeleton is relatively rigid, and the cured film is prone to stress concentration under bending, impact, assembly deformation, or thermal cycling conditions, affecting the deformation adaptability and corrosion resistance of the film. Existing technologies typically improve the flexibility and impact resistance of epoxy cathodic electrophoretic coatings by introducing polyurethane segments. However, after the introduction of polyurethane soft segments, during resin amination, neutralization emulsification, and electrodeposition, the polyurethane soft segments, epoxy skeleton, and cationic hydrophilic groups tend to be unevenly distributed, leading to the formation of soft segment-rich regions and hydrophilic ion-rich regions in the deposited film. Soft segment-rich regions tend to form low crosslinking density areas, while hydrophilic ion-rich regions tend to increase local water absorption and ion migration, both of which can become channels for water and chloride ion permeation.
[0003] Therefore, the existing problems with electrophoretic coatings are that polyurethane soft segments and cationic hydrophilic groups tend to accumulate locally in the deposited film, resulting in the formation of low cross-linking density permeation channels in the cured film. This makes the film prone to water absorption and blistering, decreased interfacial adhesion, and corrosion propagation under humid, hot, salt spray, or deformation conditions. Summary of the Invention
[0004] To address the shortcomings of existing technologies, the present invention aims to provide a polyurethane-modified epoxy resin cathodic electrophoretic coating and its preparation method. By incorporating polyurethane segments containing tertiary amine groups into a bisphenol A type epoxy resin backbone via urethane bonds, and then subjecting the coating to supplementary amination, neutralization emulsification, and paint conditioning treatments, a stable cationic aqueous electrophoretic coating is obtained. This coating can reduce the local enrichment of polyurethane soft segments and hydrophilic cationic groups in the deposited film, reduce the formation of permeation channels, and improve the stability of the coating film.
[0005] To achieve this objective, the present invention adopts the following technical solution: In a first aspect, the present invention provides a polyurethane-modified epoxy resin cathodic electrophoretic coating, the cathodic electrophoretic coating comprising a cationic aqueous resin emulsion, a blocked isocyanate crosslinking agent, a pigment paste, an additive, and deionized water; The solid resin in the cationic aqueous resin emulsion includes polyurethane-modified epoxy resin; the polyurethane-modified epoxy resin includes a bisphenol A type epoxy resin backbone and polyurethane segments incorporated into the bisphenol A type epoxy resin backbone. The polyurethane segment contains a tertiary amine group introduced by N,N-dimethylethanolamine, and the polyurethane segment is attached to the bisphenol A epoxy resin backbone through an urethane bond formed by the reaction of the isocyanate group with the hydroxyl group in the bisphenol A epoxy resin backbone.
[0006] Preferably, the mass ratio of the polyurethane modified epoxy resin, the blocked isocyanate crosslinking agent, the pigment paste, the additives and the deionized water is 100:(20-35):(10-35):(1-5):(393-800).
[0007] Preferably, the polyurethane segment is formed by reacting a polymeric diol, a diisocyanate, and N,N-dimethylethanolamine; the polymeric diol is one of polycarbonate diol, polyester diol, and polyether diol; and the diisocyanate is one of isophorone diisocyanate, hexamethylene diisocyanate, and dicyclohexylmethane diisocyanate.
[0008] Preferably, the polyurethane-modified epoxy resin further contains tertiary amine groups introduced by the ring-opening reaction of diethanolamine with epoxy groups; The tertiary amine group introduced by N,N-dimethylethanolamine and the tertiary amine group introduced by diethanolamine together constitute the cationic group of the polyurethane-modified epoxy resin.
[0009] Preferably, the total amine value of the polyurethane-modified epoxy resin is 40-60 mgKOH / g.
[0010] Preferably, the cationic aqueous resin emulsion has a solid content of 30-36%, a pH value of 5.5-6.5, and an electrical conductivity of 1000-1600 μS / cm.
[0011] Preferably, the isocyanate crosslinking agent, based on the number of isocyanate groups that can be released after unsealing, has a molar ratio of (0.8-1.2):1 for the released isocyanate groups to the hydroxyl groups in the polyurethane-modified epoxy resin.
[0012] Preferably, the pigment paste includes titanium dioxide, carbon black, precipitated barium sulfate, and a wetting and dispersing agent; The additives include leveling agents, defoamers, and antisettling agents.
[0013] In a second aspect, the present invention provides a method for preparing the polyurethane-modified epoxy resin cathodic electrophoretic coating described in the first aspect, the preparation method comprising: S1, after dehydrating the polymeric diol, react it with diisocyanate to obtain a polyurethane reactant containing isocyanate groups; S2, N,N-dimethylethanolamine is added to the polyurethane reactant to react and obtain a polyurethane containing tertiary amine groups; S3, the polyurethane is added to bisphenol A type epoxy resin and reacted to obtain polyurethane grafted epoxy resin. S4, add diethanolamine to the polyurethane-grafted epoxy resin and react to obtain polyurethane-modified epoxy resin. S5, add an acid neutralizer to the polyurethane modified epoxy resin, then add deionized water to emulsify and disperse, to obtain a cationic aqueous resin emulsion. S6, the cationic aqueous resin emulsion, blocked isocyanate crosslinking agent, pigment paste, additives and deionized water are mixed, cured and filtered to obtain polyurethane modified epoxy resin cathodic electrophoretic coating.
[0014] Preferably, in S1, the dehydration temperature of the polymer diol is 100-120°C, and the moisture content after dehydration is not higher than 0.05%. The molar ratio of hydroxyl groups in the polymer diol to isocyanate groups in the diisocyanate is 1:(2.0-2.6). The reaction temperature is 70-90℃, and the reaction time is 1.5-4 h.
[0015] Preferably, in S2, the molar ratio of the hydroxyl groups in the N,N-dimethylethanolamine to the isocyanate groups in the polyurethane reactants is (0.25-0.45):1, the reaction temperature is 45-65℃, and the reaction time is 0.5-2h.
[0016] Preferably, in S3, the mass ratio of the polyurethane containing tertiary amine groups to the bisphenol A type epoxy resin is (8-22):(78-92), the reaction temperature is 90-115℃, and the reaction time is 2-5h.
[0017] Preferably, in S4, the mass ratio of diethanolamine to polyurethane-grafted epoxy resin is (3-6):100, the reaction temperature is 60-85℃, and the reaction time is 1-3h.
[0018] Preferably, in S5, the acid neutralizer is one of lactic acid and glacial acetic acid, and the mass ratio of the acid neutralizer to the polyurethane modified epoxy resin is (3-9):100.
[0019] Preferably, in S6, the ripening time is 12-48 hours.
[0020] When a polymeric diol reacts with a diisocyanate, the hydroxyl groups at the ends of the polymeric diol chain undergo an addition reaction with the isocyanate groups to form urethane bonds, allowing the polymeric diol segment to enter the polyurethane reactant chain. The resulting polyurethane segment retains the flexible segment from the polymeric diol and contains isocyanate groups that can continue to participate in the reaction. The hydroxyl groups in N,N-dimethylethanolamine further undergo an addition reaction with the isocyanate groups in the polyurethane reactant to form urethane bonds, linking the tertiary amine group to the polyurethane segment via the urethane structure. This linkage ensures that the tertiary amine group required for cationization is located in the same grafting unit as the polyurethane segment, preventing the tertiary amine group from being distributed only at the open-ring sites of the epoxy resin backbone and remaining independent of the polyurethane soft segment.
[0021] When polyurethane containing tertiary amine groups reacts with bisphenol A type epoxy resin, the isocyanate groups in the polyurethane molecule undergo an addition reaction with the hydroxyl groups in the bisphenol A type epoxy resin backbone, forming urethane bonds. This allows polyurethane segments to chemically integrate into the epoxy resin backbone. This grafting structure differs from the physical mixing of polyurethane and epoxy resins. In this case, the polyurethane segments are restricted by the epoxy backbone linkages within the resin molecule, making it difficult for them to migrate and aggregate as independent phases during emulsification, dispersion, and electrodeposition. The bisphenol A type epoxy resin backbone provides the aromatic and hydroxyl structures, while the polyurethane segments provide mobile segments. After being linked by urethane bonds, they form a continuous resin phase containing the epoxy backbone, polyurethane segments, and urethane linkages during subsequent film formation and curing.
[0022] Diethanolamine undergoes a ring-opening reaction with the epoxy groups in polyurethane-grafted epoxy resin, forming hydroxyl and tertiary amine structures after the epoxy ring opens. The tertiary amine groups introduced by diethanolamine, together with those introduced by N,N-dimethylethanolamine, provide cationization sites. The hydroxyl groups generated from the ring-opening of diethanolamine can also participate in the subsequent curing reaction with the blocked isocyanate crosslinking agent. The acid neutralizer reacts with the tertiary amine groups to form amine salts, enabling the polyurethane-modified epoxy resin to form a cationic aqueous dispersion system in deionized water. Since some tertiary amine groups are already linked to the polyurethane segments via N,N-dimethylethanolamine, there is an intramolecular correlation between the cationic hydrophilic sites and the polyurethane segments after resin emulsification, reducing the tendency for separation between the polyurethane soft segments and the cationic hydrophilic groups in the emulsion particles.
[0023] During electrophoresis, positively charged resin particles in the cationic aqueous resin emulsion migrate towards the cathode metal workpiece surface under the influence of an electric field. Upon reaching the cathode surface, changes in the local acid-base environment and ionic state cause the resin to destabilize and deposit into a film. After polyurethane segments are attached to the epoxy resin backbone via urethane bonds, they are deposited together with the epoxy resin backbone during deposition, making it difficult to form isolated, enriched soft segment regions. The tertiary amine groups attached to the polyurethane segments also enter the deposited film along with the same resin structure, reducing the concentrated distribution of hydrophilic cationic groups in localized areas. In the resulting wet film, the polyurethane segments, epoxy backbone, and cationization sites maintain a relatively continuous molecular distribution.
[0024] During the baking and curing process, the blocked isocyanate crosslinking agent is unblocked, releasing isocyanate groups. These released isocyanate groups undergo an addition reaction with the hydroxyl groups in the polyurethane-modified epoxy resin, generating a new urethane crosslinked structure. This reaction allows the polyurethane segments grafted onto the epoxy backbone, the hydroxyl sites formed by the ring-opening of diethanolamine, and the epoxy resin backbone to all enter the crosslinking network. The polyurethane segments do not exist as a free phase but are incorporated into the resin network through grafting points and curing crosslinking points. The cationic hydrophilic groups participate in water dispersion and electrodeposition after salt formation; during baking, they are fixed by the resin network, limiting their migration and local aggregation.
[0025] In the cured paint film, polyurethane segments are connected to the epoxy resin backbone. The flexible segments can move under external force and disperse local stress. The epoxy resin backbone and the cross-linked urethane structure maintain the continuous network of the coating film. With the reduction of local enrichment of polyurethane soft segments and cationic hydrophilic groups, the low cross-linking density regions and hydrophilic ion concentration regions within the paint film decrease, weakening the continuous penetration pathways of moisture and chloride ions. The resin-deposited film at the metal substrate interface maintains continuous coverage, reducing the channels for moisture and chloride ion migration to the interface under humid or salt spray conditions, thus inhibiting the decline in interfacial adhesion and the propagation of under-film corrosion.
[0026] Compared with the prior art, the beneficial effects of the present invention are as follows: The present invention integrates polyurethane segments containing tertiary amine groups into the bisphenol A type epoxy resin backbone via urethane bonds, forming an interconnected resin structure of polyurethane soft segments, epoxy resin backbone, and cationic groups. Compared with simple mixing of polyurethane resin and epoxy resin, this structure can reduce the independent migration of polyurethane soft segments during emulsification dispersion and electrodeposition film formation, reducing the possibility of local enrichment of soft segments in the deposited film. The tertiary amine groups introduced by N,N-dimethylethanolamine are in the same grafting structure as the polyurethane segments, which can reduce the separation and distribution of hydrophilic cationic groups from the polyurethane segments, making the resin structure in the emulsion particles and the deposited wet film more continuous. After further ring-opening amination of diethanolamine, it can supplement cationic sites and introduce hydroxyl groups that can participate in curing, which is beneficial to resin emulsification, electrodeposition, and subsequent crosslinking and curing. The blocked isocyanate crosslinking agent reacts with the resin hydroxyl groups during baking, allowing the polyurethane segments and epoxy backbone to jointly enter the crosslinking network. As a result, the low cross-linking density regions and hydrophilic ion concentration regions in the cured paint film are reduced, and the continuous penetration pathways of water and chloride ions are weakened, thereby improving the stability of the paint film. Attached Figure Description
[0027] Figure 1 The flowchart illustrates the preparation method provided by this invention. Detailed Implementation
[0028] The technical solutions of the present invention will be described in detail below with reference to specific embodiments and accompanying drawings. The embodiments described herein are specific implementations of the present invention, used to illustrate the concept of the present invention; these descriptions are explanatory and exemplary, and should not be construed as limiting the implementation methods or the scope of protection of the present invention. In addition to the embodiments described herein, those skilled in the art can employ other obvious technical solutions based on the content disclosed in the claims and specification of this application. These technical solutions include those that make any obvious substitutions and modifications to the embodiments described herein.
[0029] The chemical reagents used in the embodiments and comparative examples of this invention are all commercially available products and have not undergone any further purification treatment.
[0030] In this embodiment of the invention, the blocked isocyanate crosslinking agent is a commercially available blocked polyisocyanate crosslinking agent. Specifically, Desmodur BL 3175A (Covestro, a blocked aliphatic polyisocyanate based on HDI), Bayhydur BL 2867 (Covestro, a blocked aliphatic polyisocyanate), VESTANAT B 1358 / 100 (Evonik, a blocked alicyclic polyisocyanate based on IPDI), or VESTANAT B 1358 A (Evonik, a blocked alicyclic polyisocyanate based on IPDI) can be used as the blocked isocyanate crosslinking agent.
[0031] Example 1 This embodiment provides a polyurethane-modified epoxy resin cathodic electrophoretic coating and its preparation method. The preparation method is as follows: Figure 1 As shown, the preparation method specifically includes the following steps: S1, polycarbonate diol is dehydrated at 100°C until the moisture content is no more than 0.05%, and then reacted with isophorone diisocyanate. The molar ratio of hydroxyl groups in polycarbonate diol to isocyanate groups in isophorone diisocyanate is 1:2.0. The reaction temperature is 70°C and the reaction time is 1.5h to obtain polyurethane reactants containing isocyanate groups. S2, N,N-dimethylethanolamine is added to the polyurethane reactant for reaction, the molar ratio of hydroxyl groups in N,N-dimethylethanolamine to isocyanate groups in the polyurethane reactant is 0.25:1, the reaction temperature is 45℃, and the reaction time is 0.5h to obtain polyurethane containing tertiary amine groups; S3, the polyurethane containing tertiary amine groups is added to bisphenol A type epoxy resin for reaction, the mass ratio of the polyurethane containing tertiary amine groups to bisphenol A type epoxy resin is 8:92, the reaction temperature is 90℃, and the reaction time is 2h, to obtain polyurethane grafted epoxy resin. S4, add diethanolamine to the polyurethane-grafted epoxy resin and react. The mass ratio of diethanolamine to polyurethane-grafted epoxy resin is 3:100. The reaction temperature is 60℃ and the reaction time is 1h to obtain polyurethane-modified epoxy resin. The total amine value of the polyurethane-modified epoxy resin is 40mgKOH / g. S5, lactic acid is added to the polyurethane modified epoxy resin, the mass ratio of lactic acid to polyurethane modified epoxy resin is 3:100, and then some deionized water is added for emulsification and dispersion to obtain a cationic aqueous resin emulsion. The cationic aqueous resin emulsion has a solid content of 30%, a pH value of 5.5, and a conductivity of 1000 μS / cm. S6, the cationic aqueous resin emulsion, blocked isocyanate crosslinking agent, pigment paste, additives, and deionized water are mixed, aged for 12 hours, and then filtered to obtain a polyurethane-modified epoxy resin cathodic electrophoretic coating; the mass ratio of polyurethane-modified epoxy resin, blocked isocyanate crosslinking agent, pigment paste, additives, and deionized water is 100:20:10:1:393; the blocked isocyanate crosslinking agent is Desmodur BL 3175A, and the molar ratio of the isocyanate groups that can be released after unblocking to the hydroxyl groups in the polyurethane-modified epoxy resin is 0.8:1; the pigment paste is obtained by mixing and dispersing titanium dioxide, carbon black, precipitated barium sulfate, and a wetting and dispersing agent; the additives are obtained by mixing leveling agent, defoamer, and anti-settling agent.
[0032] Example 2 This embodiment provides a polyurethane-modified epoxy resin cathodic electrophoretic coating and its preparation method, the preparation method specifically including the following steps: S1, polyester diol is dehydrated at 105℃ until the moisture content is no more than 0.05%, and then reacted with hexamethylene diisocyanate. The molar ratio of hydroxyl groups in polyester diol to isocyanate groups in hexamethylene diisocyanate is 1:2.2. The reaction temperature is 75℃ and the reaction time is 2.3h to obtain polyurethane reactants containing isocyanate groups. S2, N,N-dimethylethanolamine is added to the polyurethane reactant and reacted. The molar ratio of the hydroxyl groups in N,N-dimethylethanolamine to the isocyanate groups in the polyurethane reactant is 0.32:1. The reaction temperature is 52°C and the reaction time is 1.0 h to obtain a polyurethane containing tertiary amine groups. S3, the polyurethane containing tertiary amine groups is added to bisphenol A type epoxy resin for reaction, the mass ratio of the polyurethane containing tertiary amine groups to bisphenol A type epoxy resin is 12:88, the reaction temperature is 100℃, and the reaction time is 3h, to obtain polyurethane grafted epoxy resin. S4, add diethanolamine to the polyurethane-grafted epoxy resin and react. The mass ratio of diethanolamine to polyurethane-grafted epoxy resin is 4:100. The reaction temperature is 68°C and the reaction time is 1.5h to obtain polyurethane-modified epoxy resin. The total amine value of the polyurethane-modified epoxy resin is 47mgKOH / g. S5, glacial acetic acid is added to the polyurethane modified epoxy resin, the mass ratio of glacial acetic acid to polyurethane modified epoxy resin is 5:100, and then some deionized water is added for emulsification and dispersion to obtain a cationic aqueous resin emulsion. The cationic aqueous resin emulsion has a solid content of 32%, a pH value of 5.8, and a conductivity of 1200 μS / cm. S6, the cationic aqueous resin emulsion, blocked isocyanate crosslinking agent, pigment paste, additives, and deionized water are mixed, aged for 24 hours, and then filtered to obtain a polyurethane-modified epoxy resin cathodic electrophoretic coating; the mass ratio of polyurethane-modified epoxy resin, blocked isocyanate crosslinking agent, pigment paste, additives, and deionized water is 100:25:18:2:520; the blocked isocyanate crosslinking agent is Bayhydur BL 2867, and the molar ratio of the isocyanate groups that can be released after unblocking to the hydroxyl groups in the polyurethane-modified epoxy resin is 0.95:1; the pigment paste is obtained by mixing and dispersing titanium dioxide, carbon black, precipitated barium sulfate, and a wetting and dispersing agent; the additives are obtained by mixing leveling agent, defoamer, and anti-settling agent.
[0033] Example 3 This embodiment provides a polyurethane-modified epoxy resin cathodic electrophoretic coating and its preparation method, the preparation method specifically including the following steps: S1, polyether diol is dehydrated at 115°C until the moisture content is no more than 0.05%, and then reacted with dicyclohexylmethane diisocyanate. The molar ratio of hydroxyl groups in polyether diol to isocyanate groups in dicyclohexylmethane diisocyanate is 1:2.4. The reaction temperature is 85°C and the reaction time is 3.2 h to obtain polyurethane reactant containing isocyanate groups. S2, N,N-dimethylethanolamine is added to the polyurethane reactant for reaction, the molar ratio of hydroxyl groups in N,N-dimethylethanolamine to isocyanate groups in the polyurethane reactant is 0.38:1, the reaction temperature is 58℃, and the reaction time is 1.5h to obtain polyurethane containing tertiary amine groups; S3, the polyurethane containing tertiary amine groups is added to bisphenol A type epoxy resin for reaction, the mass ratio of the polyurethane containing tertiary amine groups to bisphenol A type epoxy resin is 18:82, the reaction temperature is 108℃, and the reaction time is 4h, to obtain polyurethane grafted epoxy resin. S4, add diethanolamine to the polyurethane-grafted epoxy resin and react. The mass ratio of diethanolamine to polyurethane-grafted epoxy resin is 5:100. The reaction temperature is 78°C and the reaction time is 2.5 h to obtain polyurethane-modified epoxy resin. The total amine value of the polyurethane-modified epoxy resin is 53 mgKOH / g. S5, lactic acid is added to the polyurethane modified epoxy resin, the mass ratio of lactic acid to polyurethane modified epoxy resin is 7:100, and then some deionized water is added for emulsification and dispersion to obtain a cationic aqueous resin emulsion. The cationic aqueous resin emulsion has a solid content of 34%, a pH value of 6.2, and a conductivity of 1400 μS / cm. S6, the cationic aqueous resin emulsion, blocked isocyanate crosslinking agent, pigment paste, additives, and deionized water are mixed, aged for 36 hours, and then filtered to obtain a polyurethane-modified epoxy resin cathodic electrophoretic coating; the mass ratio of polyurethane-modified epoxy resin, blocked isocyanate crosslinking agent, pigment paste, additives, and deionized water is 100:30:27:4:650; the blocked isocyanate crosslinking agent is VESTANAT B 1358 / 100, and the molar ratio of the isocyanate groups that can be released after unblocking to the hydroxyl groups in the polyurethane-modified epoxy resin is 1.05:1; the pigment paste is obtained by mixing and dispersing titanium dioxide, carbon black, precipitated barium sulfate, and wetting and dispersing agent; the additives are obtained by mixing leveling agent, defoamer, and anti-settling agent.
[0034] Example 4 This embodiment provides a polyurethane-modified epoxy resin cathodic electrophoretic coating and its preparation method, the preparation method specifically including the following steps: S1, polycarbonate diol is dehydrated at 120°C until the moisture content is no more than 0.05%, and then reacted with isophorone diisocyanate. The molar ratio of hydroxyl groups in polycarbonate diol to isocyanate groups in isophorone diisocyanate is 1:2.6. The reaction temperature is 90°C and the reaction time is 4 hours to obtain polyurethane reactants containing isocyanate groups. S2, N,N-dimethylethanolamine is added to the polyurethane reactant for reaction, the molar ratio of hydroxyl groups in N,N-dimethylethanolamine to isocyanate groups in the polyurethane reactant is 0.45:1, the reaction temperature is 65℃, and the reaction time is 2h to obtain polyurethane containing tertiary amine groups; S3, the polyurethane containing tertiary amine groups is added to bisphenol A type epoxy resin for reaction, the mass ratio of the polyurethane containing tertiary amine groups to bisphenol A type epoxy resin is 22:78, the reaction temperature is 115℃, and the reaction time is 5h, to obtain polyurethane grafted epoxy resin. S4, add diethanolamine to the polyurethane-grafted epoxy resin and react. The mass ratio of diethanolamine to polyurethane-grafted epoxy resin is 6:100. The reaction temperature is 85℃ and the reaction time is 3h to obtain polyurethane-modified epoxy resin. The total amine value of the polyurethane-modified epoxy resin is 60mgKOH / g. S5, glacial acetic acid is added to the polyurethane modified epoxy resin, the mass ratio of glacial acetic acid to polyurethane modified epoxy resin is 9:100, and then some deionized water is added for emulsification and dispersion to obtain a cationic aqueous resin emulsion. The cationic aqueous resin emulsion has a solid content of 36%, a pH value of 6.5, and a conductivity of 1600 μS / cm. S6, the cationic aqueous resin emulsion, blocked isocyanate crosslinking agent, pigment paste, additives, and deionized water are mixed, aged for 48 hours, and then filtered to obtain a polyurethane-modified epoxy resin cathodic electrophoretic coating; the mass ratio of polyurethane-modified epoxy resin, blocked isocyanate crosslinking agent, pigment paste, additives, and deionized water is 100:35:35:5:800; the blocked isocyanate crosslinking agent is VESTANAT B 1358 A, and the molar ratio of the isocyanate groups that can be released after unblocking to the hydroxyl groups in the polyurethane-modified epoxy resin is 1.2:1; the pigment paste is obtained by mixing and dispersing titanium dioxide, carbon black, precipitated barium sulfate, and a wetting and dispersing agent; the additives are obtained by mixing leveling agent, defoamer, and anti-settling agent.
[0035] Comparative Example 1 This comparative example provides a polyurethane-modified epoxy resin cathodic electrophoretic coating and its preparation method. The difference from Example 1 is that N,N-dimethylethanolamine is not added in S2, but n-butanol is added instead for end-capping reaction. The molar ratio of the hydroxyl groups in n-butanol to the isocyanate groups in the polyurethane reactant obtained in S1 is 0.25:1. In S4, the amount of diethanolamine added is adjusted so that the total amine value of the polyurethane-modified epoxy resin is 40 mgKOH / g. Other process parameters and operating conditions are exactly the same as in Example 1.
[0036] Comparative Example 2 This comparative example provides a polyurethane-modified epoxy resin cathodic electrophoretic coating and its preparation method. The difference from Example 1 is that in S3, the polyurethane containing tertiary amine groups is not reacted with bisphenol A type epoxy resin. Instead, the polyurethane containing tertiary amine groups obtained in S2 is physically mixed with bisphenol A type epoxy resin that has been pre-aminated with diethanolamine to an amine value of 40 mg KOH / g. S4 is not performed. Other process parameters and operating conditions are exactly the same as in Example 1.
[0037] Comparative Example 3 This comparative example provides an epoxy resin cathodic electrophoretic coating and its preparation method. The difference from Example 1 is that the preparation of polyurethane segments, the introduction of tertiary amine groups and the grafting reaction in S1-S3 are not carried out. Instead, diethanolamine ring-opening amination is directly performed using bisphenol A type epoxy resin. Other process parameters and operating conditions are exactly the same as in Example 1.
[0038] Comparative Example 4 This comparative example provides a polyurethane-modified epoxy resin cathodic electrophoretic coating and its preparation method. The difference from Example 1 is that the mass ratio of diethanolamine to polyurethane-grafted epoxy resin in S4 is adjusted to 8:100, while other process parameters and operating conditions are exactly the same as in Example 1.
[0039] Performance testing: Polyurethane-modified epoxy resin cathodic electrophoretic coatings were prepared as electrophoretic working solutions. Cold-rolled steel plates that had undergone degreasing, phosphating, and deionized water washing were used as cathodes, and stainless steel plates were used as anodes. Electrophoretic deposition was carried out under the conditions of working solution temperature of 28℃, electrophoretic voltage of 220V, and electrophoretic time of 2min. After electrophoresis, the test plates were rinsed with deionized water and baked in an oven at 180℃ for 30min to obtain cured coating film test plates.
[0040] Adhesion test after damp heat: The cured paint film test panel was placed in a constant temperature and humidity test chamber at a temperature of 60℃ and a relative humidity of 95% for 240 hours. After the damp heat treatment, the test panel was removed and placed in an environment of 23℃ and 50% relative humidity for 2 hours. Then, the adhesion was tested according to the cross-cut adhesion test. A grid was cut on the paint film surface using a cross-cut adhesion tester, with the cuts penetrating the paint film to the metal substrate at a spacing of 1 mm. After removing the cutting debris, pressure-sensitive adhesive tape was applied to the grid area and pressed firmly. After holding for 90 seconds, it was quickly peeled off at approximately 180°. The paint film peeling was observed, and the adhesion after damp heat was evaluated on a scale of 0 to 5, with lower grades indicating better adhesion.
[0041] Impact resistance test: The cured paint film test panel was placed in an environment of 23℃ and 50% relative humidity for 24 hours before testing. A paint film impact tester was used, with a 1kg hammer and an 8mm diameter punch. The hammer was dropped freely from a set height onto the coated surface or back of the test panel. At least three impact points were tested at each height, maintaining spacing between impact points and between the impact points and the edge of the test panel. After impact, the paint film was observed for cracks, peeling, or exposure of the substrate. The maximum impact height from which no cracks, peeling, or exposure of the substrate occurred indicated the impact resistance.
[0042] Impact-induced salt spray bubbling level test: First, the cured paint film test panels were pre-treated with impact according to the impact resistance test method. The impact height was set at 50cm, the weight of the hammer was 1kg, and the diameter of the impact head was 8mm. One impact zone was set for each test panel. After impact, the test panels were exposed to corrosion using a neutral salt spray test. The salt solution was a 5% sodium chloride aqueous solution, the pH was adjusted to 6.5-7.2, the salt spray chamber temperature was 35℃, and the salt spray deposition was 1-2mL / 80cm. 2• h, continuous spraying time is 240h. After the salt spray, gently rinse the surface of the test panel with deionized water to remove salt, place it at room temperature for 2h, and observe the blistering of the paint film in and around the impact area. Evaluate according to the blistering level of the paint film: no visible blistering is recorded as level 0; only a few small-sized blisters appear in and around the impact area as level 1; many small-sized blisters or a few medium-sized blisters appear as level 2; dense small-sized blisters or many medium-sized blisters appear as level 3; dense medium-sized blisters or a few large-sized blisters appear as level 4; large-area blistering, bulging, or blistering accompanied by paint film peeling is recorded as level 5.
[0043] The test results are shown in Table 1.
[0044] Table 1. Test results of cured paint film test panels of Examples 1-4 and Comparative Examples 1-4
[0045] As shown in Table 1, compared with Example 1, the adhesion level of Comparative Example 1 increased after humid heat, the impact resistance decreased, and the salt spray bubbling level increased after impact; the adhesion level of Comparative Example 2 increased after humid heat, the impact resistance decreased, and the salt spray bubbling level increased after impact; the adhesion level of Comparative Example 3 increased after humid heat, the impact resistance decreased, and the salt spray bubbling level increased after impact; and the adhesion level of Comparative Example 4 increased after humid heat, the impact resistance remained the same, and the salt spray bubbling level increased after impact.
[0046] This is because, in Comparative Example 1, N,N-dimethylethanolamine was not used to introduce tertiary amine groups onto the polyurethane segments. Instead, n-butanol was used for end-capping, and diethanolamine was used for supplementary amination. Although the total amine value of the polyurethane-modified epoxy resin was close to that of Example 1, the cationic groups mainly came from the epoxy ring-opening sites and did not form the same grafting structure with the polyurethane segments. The polyurethane soft segments and hydrophilic cationic groups were more easily separated and distributed during emulsification and electrodeposition. The low crosslinking density region and hydrophilic penetration region in the cured film increased, resulting in decreased adhesion, reduced impact resistance, and increased salt spray foaming level after impact after humid heat. In Comparative Example 2, the polyurethane containing tertiary amine groups was only physically mixed with the amination bisphenol A type epoxy resin. The polyurethane segments did not connect to the epoxy resin backbone through urethane bonds. During emulsification, dispersion, electrodeposition, and baking curing, the polyurethane segments lacked epoxy backbone connection points to limit them, leading to increased local enrichment of soft segments and decreased resin network continuity. Therefore, the adhesion decreased after humid heat, the impact resistance decreased, and the salt spray foaming level after impact increased.
[0047] In Comparative Example 3, no polyurethane segments were introduced, and the paint film was mainly composed of a cured epoxy resin network. This reduced segment mobility made it difficult to release stress during impact deformation, leading to more cracks and localized defects. Under salt spray conditions, moisture and chloride ions entered the paint film along these impact defects, thus reducing impact resistance and increasing the salt spray blistering level after impact. In Comparative Example 4, the increased diethanolamine content led to more hydrophilic cationic groups introduced by epoxy ring-opening, increasing the localized water absorption and ion migration of the cured paint film. This resulted in decreased interfacial adhesion and blistering under humid heat and salt spray conditions.
[0048] The above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.
Claims
1. A polyurethane-modified epoxy resin cathodic electrophoretic coating, characterized in that, The cathodic electrophoretic coating comprises a cationic aqueous resin emulsion, a blocked isocyanate crosslinking agent, pigment paste, additives, and deionized water; The solid resin in the cationic aqueous resin emulsion includes polyurethane-modified epoxy resin; the polyurethane-modified epoxy resin includes a bisphenol A type epoxy resin backbone and polyurethane segments incorporated into the bisphenol A type epoxy resin backbone. The polyurethane segment contains a tertiary amine group introduced by N,N-dimethylethanolamine, and the polyurethane segment is attached to the bisphenol A epoxy resin backbone through an urethane bond formed by the reaction of the isocyanate group with the hydroxyl group in the bisphenol A epoxy resin backbone.
2. The polyurethane-modified epoxy resin cathodic electrophoretic coating according to claim 1, characterized in that, The mass ratio of the polyurethane modified epoxy resin, the blocked isocyanate crosslinking agent, the pigment paste, the additives and the deionized water is 100:(20-35):(10-35):(1-5):(393-800).
3. The polyurethane-modified epoxy resin cathodic electrophoretic coating according to claim 1, characterized in that, The polyurethane segments are formed by the reaction of polymeric diol, diisocyanate, and N,N-dimethylethanolamine; the polymeric diol is one of polycarbonate diol, polyester diol, and polyether diol; the diisocyanate is one of isophorone diisocyanate, hexamethylene diisocyanate, and dicyclohexylmethane diisocyanate.
4. The polyurethane-modified epoxy resin cathodic electrophoretic coating according to claim 1, characterized in that, The polyurethane-modified epoxy resin also contains tertiary amine groups introduced by the ring-opening reaction of diethanolamine with epoxy groups. The tertiary amine group introduced by N,N-dimethylethanolamine and the tertiary amine group introduced by diethanolamine together constitute the cationic group of the polyurethane-modified epoxy resin.
5. The polyurethane-modified epoxy resin cathodic electrophoretic coating according to claim 4, characterized in that, The total amine value of the polyurethane-modified epoxy resin is 40-60 mgKOH / g.
6. The polyurethane-modified epoxy resin cathodic electrophoretic coating according to claim 1, characterized in that, The cationic aqueous resin emulsion has a solid content of 30-36%, a pH value of 5.5-6.5, and an electrical conductivity of 1000-1600 μS / cm.
7. The polyurethane-modified epoxy resin cathodic electrophoretic coating according to claim 1, characterized in that, The isocyanate crosslinking agent is based on the number of isocyanate groups that can be released after unsealing, and the molar ratio of the releaseable isocyanate groups to the hydroxyl groups in the polyurethane-modified epoxy resin is (0.8-1.2):
1.
8. The polyurethane-modified epoxy resin cathodic electrophoretic coating according to claim 1, characterized in that, The pigment paste includes titanium dioxide, carbon black, precipitated barium sulfate, and a wetting and dispersing agent; The additives include leveling agents, defoamers, and anti-settling agents.
9. A method for preparing a polyurethane-modified epoxy resin cathodic electrophoretic coating as described in any one of claims 1-8, characterized in that, The preparation method includes: S1, after dehydrating the polymeric diol, react it with diisocyanate to obtain a polyurethane reactant containing isocyanate groups; S2, N,N-dimethylethanolamine is added to the polyurethane reactant to react and obtain a polyurethane containing tertiary amine groups; S3, the polyurethane is added to bisphenol A type epoxy resin and reacted to obtain polyurethane grafted epoxy resin. S4, add diethanolamine to the polyurethane-grafted epoxy resin and react to obtain polyurethane-modified epoxy resin. S5, add an acid neutralizer to the polyurethane modified epoxy resin, then add deionized water to emulsify and disperse, to obtain a cationic aqueous resin emulsion. S6, the cationic aqueous resin emulsion, blocked isocyanate crosslinking agent, pigment paste, additives and deionized water are mixed, cured and filtered to obtain polyurethane modified epoxy resin cathodic electrophoretic coating.
10. The method for preparing the polyurethane-modified epoxy resin cathodic electrophoretic coating according to claim 9, characterized in that, In S4, the mass ratio of diethanolamine to polyurethane-grafted epoxy resin is (3-6):100, the reaction temperature is 60-85℃, and the reaction time is 1-3h. In S5, the acid neutralizer is one of lactic acid and glacial acetic acid, and the mass ratio of the acid neutralizer to the polyurethane modified epoxy resin is (3-9):
100. In S6, the ripening time is 12-48 hours.