A low-temperature curing cross-linking agent for epoxy cathodic electrophoretic coating and its preparation and application

Abstract

The present invention discloses a low-temperature curing cross-linking agent for epoxy cathode electrophoretic coating, and its preparation and application. The low-temperature curing cross-linking agent is composed of the following raw materials by weight: 154 parts of diisocyanate, 100-150 parts of dispersant, 1-5 parts of catalyst, 55-140 parts of sealant A, and 150-300 parts of sealant B. The low-temperature curing cross-linking agent provided by the present invention has the characteristics of low curing temperature, good corrosion resistance, excellent appearance, and good mechanical strength. It can be stored stably for more than 6 months and can be baked into a film with materials such as rubber and plastic at low temperatures, with a curing temperature of 110-140°C. The epoxy cathode electrophoretic coating prepared with the low-temperature curing cross-linking agent of the present invention has a film thickness of 10-30 microns, a neutral salt spray resistance time of more than 500 hours, good corrosion resistance, and excellent stability.

Description

(1) Technical field

[0001] The invention relates to a low-temperature curing cross-linking agent for epoxy cathode electrophoretic coating, and the preparation and application thereof. (2) Background technology

[0002] Electrophoresis is one of the most effective methods for coating metal workpieces. Cathodic electrophoretic coatings, developed and industrialized in the mid-to-late 1970s, are widely used in various fields, particularly in the automotive industry, where the vast majority of vehicles utilize the electrophoretic coating process. Furthermore, the emulsion used is a water-diluted, low-solids electrophoretic emulsion with low organic solvent content, resulting in low VOC emissions and a more environmentally friendly finish. Conventional epoxy cathodic electrophoretic coatings utilize alcohol-blocked isocyanates as curing agents, which require curing and crosslinking at temperatures above 170°C, resulting in significant energy consumption. With market development, an increasing number of metal-composite workpieces, such as rubber and plastic, require electrophoretic coatings for drying. Excessive temperatures can cause degradation and melting of these materials. Furthermore, to maintain the electrophoretic coating's corrosion resistance, the primary resin in epoxy cathodic electrophoretic coatings is typically bisphenol A epoxy resin, which contains a large number of benzene rings and often exhibits varying degrees of yellowing during high-temperature baking. Low-temperature curing can effectively reduce this yellowing.

[0003] Currently, cathodic electrophoretic coatings that can cure at 150°C have been introduced overseas, such as PPG's ED-6. However, their lower limit for curing is 150°C, while domestically, there are no mature formulations or processes. To address this issue, patents such as CN101928513 A use methyl ethyl ketone oxime to block TDI or MDI to synthesize a low-temperature curing agent, which can cure at temperatures as low as 120-140°C. However, methyl ethyl ketone oxime evaporates during the curing process, creating an unpleasant odor in the workshop and exhibiting a certain degree of toxicity. Both TDI and MDI used are highly toxic compounds, making their use relatively dangerous. Patent CN 114231077 B uses N-methylacetamide as a blocking agent, with a curing temperature of 140°C. The high boiling point of N-methylacetamide mitigates some of the risks, but its curing temperature is too high, exceeding the temperature tolerance of some rubbers and plastics.

[0004] In summary, there is an urgent need for a product that can make up for the poor stability of the current low-temperature epoxy cathodic electrophoretic coating, the workshop odor caused by the volatilization of methyl ethyl ketone oxime during the coating baking process, poor salt spray performance, and excessively high curing temperature, so as to adapt to market development, environmental protection needs and excellent appearance performance. (3) Summary of the invention

[0005] The purpose of the present invention is to provide a low-temperature curing cross-linking agent for epoxy cathode electrophoretic coating, and its preparation method and application. The curing cross-linking agent has the properties of low curing temperature, good corrosion resistance, excellent appearance, and good mechanical strength. It can be baked together with rubber, plastic and other materials at low temperature, overcoming the defect that the curing temperature of current epoxy cathode electrophoretic coating is too high.

[0006] The technical solution adopted in the present invention is:

[0007] The present invention provides a low-temperature curing cross-linking agent for epoxy cathode electrophoretic coating, wherein the low-temperature curing cross-linking agent is composed of the following raw materials in parts by mass: 154 parts of diisocyanate, 100-150 parts of dispersant, 1-5 parts of catalyst, 55-140 parts of blocking agent A, and 150-300 parts of blocking agent B;

[0008] The blocking agent A is one or more of polypropylene glycol 400, polypropylene glycol 600, polypropylene glycol 1000, polytetrahydrofuran 650, and polytetrahydrofuran 1000;

[0009] The blocking agent B is one or more of ethyl p-hydroxybenzoate, p-hydroxyacetophenone, and p-hydroxypropiophenone;

[0010] The catalyst is dibutyltin dilaurate;

[0011] The dispersant is a ketone substance.

[0012] Furthermore, the low-temperature curing cross-linking agent is composed of the following raw materials in parts by mass: 154 parts of diisocyanate, 100-120 parts of dispersant, 1 part of catalyst, 40-140 parts of blocking agent A, and 177-228 parts of blocking agent B.

[0013] Furthermore, the diisocyanate is isophorone diisocyanate (IPDI).

[0014] Furthermore, the dispersant is methyl isobutyl ketone (MIBK), acetone or ethyl ketone.

[0015] Furthermore, the low-temperature curing cross-linking agent is composed of the following raw materials in parts by mass: 154 parts of IPDI, 110 parts of MIBK, 1 part of dibutyltin dilaurate, 83 parts of polypropylene glycol 600, and 202 parts of ethyl p-hydroxybenzoate.

[0016] The present invention also provides a preparation method of the low-temperature curing crosslinking agent, which comprises the following steps: placing a diisocyanate, a 4 / 9 dispersant and a catalyst in a reaction kettle according to the formula amount, mixing, heating to 60-80°C (preferably 70°C), adding a blocking agent A dropwise, reacting at 60-80°C for 1-3 hours (preferably 70°C for 2 hours), detecting that the NCO equivalent reaches 298, then adding a blocking agent B, heating to 80-85°C and reacting for 4-5 hours, detecting that the NCO equivalent is greater than 2000, adding the remaining dispersant to adjust the solid content to 70-80%, and obtaining the low-temperature curing crosslinking agent.

[0017] The present invention also provides an application of the low-temperature curing cross-linking agent in preparing an epoxy cathode electrophoretic coating film.

[0018] Furthermore, the application method includes: (1) main resin: epoxy resin, bisphenol A, and isooctanoic acid are placed in a reactor, heated to 145°C and reacted for 30 minutes; a catalyst is added to the reactor, heated to 155°C and reacted for 85 minutes, methylethanolamine and ketimine are added, and the reaction is carried out at 120°C for 2 hours to obtain the main resin; the catalyst includes benzylamine; (2) epoxy cathode electrophoretic emulsion: the acid is mixed with water and heated to 40°C, and then the main resin at 90°C is added, stirred for 20-40 minutes, and the temperature is controlled below 50°C. , add the low-temperature curing cross-linking agent, stir for 1-3h, add pure water in batches, stir for 50-70min each time, and obtain epoxy cathode electrophoretic emulsion; (3) epoxy cathode electrophoretic coating: mix the epoxy cathode electrophoretic emulsion and black paste 7954 in a mass ratio of 4.5:1, use mechanical stirring to obtain a wet film, and perform electrophoretic deposition under the conditions of electrophoretic temperature of 29-30℃, voltage of 100-200V, and electrophoretic time of 100-300s. After the deposition is completed, bake in an oven at 120℃ for 25min to obtain a coating.

[0019] Furthermore, the molecular weight of the epoxy resin in step (1) is 188; the mass ratio of the epoxy resin to bisphenol A is 72.5:30.7, the mass ratio of isooctanoic acid to epoxy resin is 3:72.5; the mass ratio of the catalyst to epoxy resin is 0.8:72.5; the mass ratio of the epoxy resin to methylethanolamine and ketimine is 72.5:3.8:8.6.

[0020] Furthermore, in step (2), the acid is formic acid; the mass ratio of the acid to water is 1:40-60, preferably 1:42; the mass ratio of the acid to the main resin is 1:30-70, preferably 1:37.6; the mass ratio of the acid to the low-temperature curing crosslinker is 3:50-150, preferably 1:100; the mass ratio of the acid to pure water is 1:50-80, preferably 1:60.

[0021] Compared with the prior art, the beneficial effects of the present invention are mainly reflected in:

[0022] (1) The low-temperature curing cross-linking agent provided by the present invention has the characteristics of low curing temperature, good corrosion resistance, excellent appearance, and good mechanical strength, and can be stably stored for more than 6 months.

[0023] (2) The low-temperature curing cross-linking agent of the present invention can be baked with materials such as rubber and plastic to form a film at a low temperature, and the curing temperature is 110-140°C.

[0024] (3) The epoxy cathode electrophoretic coating prepared by the low-temperature curing crosslinking agent of the present invention has a film thickness of 10-30 microns, a neutral salt spray resistance time of more than 500 hours, good corrosion resistance and excellent stability.

[0025] (4) The present invention uses a new sealing agent, ethyl parahydroxybenzoate, to avoid the volatilization of MEKO during the baking process of the coating film caused by the use of MEKO for sealing in traditional low-temperature coatings; at the same time, the relatively less toxic IPDI can be used to replace the highly toxic TDI and MDI, making the production and application environmentally friendly and safe. (IV) Description of the accompanying drawings

[0026] Figure 1 , structural formula diagram of the low-temperature curing cross-linking agent prepared in Example 1.

[0027] Figure 2 , infrared spectrum of the low-temperature curing cross-linking agent prepared in Example 1.

[0028] Figure 3 , DSC curve of wet film.

[0029] Figure 4 , Appearance after wet film curing.

[0030] Figure 5 , Salt spray image of the coating film of Example 1 after 500h.

[0031] Figure 6 , Salt spray image of the coating film of Example 2 after 200h. (V) Specific implementation methods

[0032] The present invention is further described below with reference to specific embodiments, but the protection scope of the present invention is not limited thereto:

[0033] Example 1. Preparation and application of low-temperature curing crosslinking agent

[0034] 1. Preparation of low temperature curing crosslinking agent

[0035] The mass formula of low-temperature curing crosslinker is: 154 parts IPDI, 110 parts MIBK, 1 part dibutyltin dilaurate, 83 parts polypropylene glycol 600, and 202 parts ethyl parahydroxybenzoate.

[0036] Synthesis of a low-temperature curing crosslinker: According to the formula, 154g of IPDI, 40g of MIBK, and 1g of dibutyltin dilaurate are placed in a reaction kettle and mixed, the temperature is raised to 70°C, 83g of polypropylene glycol 600 is added dropwise, and the reaction is carried out at 70°C for 2 hours to obtain an initial product; after the isocyanate group (NCO) equivalent of the initial product is detected to reach 298 (if it does not reach 298, additional polypropylene glycol 600 is added), 202g of ethyl p-hydroxybenzoate is added, the temperature is raised to 80-85°C, and the reaction is carried out for 4-5 hours to obtain a blocked product; after the NCO content of the blocked product is detected to be greater than 20,000, 70g of MIBK is added to make the solid content of the crosslinker 80%, to obtain 550g of a low-temperature curing crosslinker, wherein the molar ratio of IPDI, polypropylene glycol 600, and ethyl p-hydroxybenzoate is 1:0.2:1.6.

[0037] According to the national standard GB / T 12009.4-1989, the isocyanate content in the reaction system is determined by the di-n-butylamine method. The NCO equivalent is greater than 20,000, and the mass ratio of the NCO group is less than 0.2%.

[0038] The molecular structure of the low temperature curing crosslinker is shown in Figure 1 , infrared spectrum see Figure 2 From the spectrum, we can see that at 2260cm -1 There are no characteristic peaks of isocyanate groups on the left and right, which proves that it can fully block IPDI. The higher blocking rate makes the curing agent more stable.

[0039] Stability test of low-temperature curing crosslinker (Table 1): The low-temperature curing crosslinker was sealed and placed at room temperature to observe whether there was precipitation or stratification. The results showed that no stratification or precipitation occurred after 6 months, and the crosslinker can be stable for more than 6 months.

[0040] 2. Preparation of epoxy cathode electrophoretic emulsion

[0041] (1) Preparation of the main resin: 72.5 g of epoxy resin (molecular weight 188), 30 g of bisphenol A, and 3.7 g of isooctanoic acid were placed in a reactor, heated to 145°C and reacted for 30 min; 0.8 g of benzylamine was added to the reactor, heated to 155°C and reacted for 85 min, and the epoxy equivalent was measured to be 1130; 3.4 g of methylethanolamine and 8.6 g of ketimine were added, and the reaction was carried out at 120°C for 2 h to obtain 119 g of the main resin.

[0042] (2) Preparation of epoxy cathode electrophoretic emulsion: Place 3g of formic acid in an emulsifying kettle, add 126g of pure water, and heat to 40°C. Pour 113g of the main resin prepared in step (1) into the emulsifying kettle, with the main resin temperature at 90°C, and stir for 30 minutes; ensure that the temperature is below 50°C, add 100g of the low-temperature curing crosslinker prepared in step 1, and stir for 2 hours; add a total of 180g of pure water in two batches, stirring for 60 minutes each time, to obtain 522g of epoxy cathode electrophoretic emulsion. The stability of the epoxy cathode electrophoretic emulsion (Table 1) was tested in the same manner as in step 1, and the results showed stability >4 weeks.

[0043] 3. Epoxy cathodic electrophoretic coating

[0044] The epoxy cathode electrophoretic emulsion prepared in step 2 was mixed with black paste 7954 (Zhejiang Mingfu Metal Co., Ltd.) at a mass ratio of 4.5:1, and a wet film was obtained after mechanical stirring. The DSC curve is shown in FIG. Figure 3 , electrophoretic deposition was carried out on the surface of a standard phosphate plate under the conditions of electrophoretic temperature of 30℃, voltage of 160V and electrophoretic time of 160s. After the deposition, it was baked in an oven at 120℃ for 25min to obtain a smooth coating film ( Figure 4 ), the coating thickness is 20 microns. Figure 3 It can be seen that the wet film can undergo curing reaction above 109°C.

[0045] The pencil hardness, adhesion, impact and salt spray resistance of the coating are shown in Table 1, and the 500h salt spray diagram is shown in Table 1. Figure 5 .

[0046] Example 2. Preparation and Application of Curing Crosslinking Agent

[0047] 1. Preparation of curing crosslinking agent

[0048] The mass formula of the low-temperature curing crosslinker is: 154 parts of IPDI, 90 parts of MIBK, 1 part of dibutyltin dilaurate, 83 parts of polypropylene glycol 600, and 106 parts of methyl ethyl ketone oxime (MEKO).

[0049] Synthesis of a low-temperature curing crosslinker: According to the formula, 154g of IPDI, 40g of MIBK, and 1g of dibutyltin dilaurate were placed in a reaction kettle and mixed. The mixture was heated to 70°C, and 83g of polypropylene glycol 600 was added dropwise. The mixture was reacted at 70°C for 2 hours to obtain an initial product. After the isocyanate group (NCO) equivalent of the initial product reached 298 (if it did not reach 298, additional polypropylene glycol 600 was added), 106g of MEKO was added, and the mixture was heated to 80-85°C and reacted for 4-5 hours to obtain a blocked product. After the NCO equivalent of the blocked product was greater than 20,000, 50g of MIBK was added to bring the solid content of the crosslinker to 80%, to obtain 434g of a low-temperature curing crosslinker. The molar ratio of IPDI, polypropylene glycol 600, and MEKO was 1:0.2:1.6.

[0050] According to the national standard GB / T 12009.4-1989, the isocyanate content in the reaction system is determined by the di-n-butylamine method. The NCO equivalent is greater than 20,000, and the mass ratio of the NCO group is less than 0.2%.

[0051] 2. Preparation of epoxy cathode electrophoretic emulsion

[0052] (1) Preparation of the main resin: 72.5 g of epoxy resin (molecular weight 188), 30 g of bisphenol A, and 3.7 g of isooctanoic acid were placed in a reactor, heated to 145°C and reacted for 30 min; 0.8 g of benzylamine was added to the reactor, heated to 155°C and reacted for 85 min, and the epoxy equivalent was measured to be 1130; 3.4 g of methylethanolamine and 8.6 g of ketimine were added, and the reaction was carried out at 120°C for 2 h to obtain 119 g of the main resin.

[0053] (2) Preparation of epoxy cathode electrophoretic emulsion: 3 g of formic acid was placed in an emulsifying kettle, 126 g of pure water was added, and the mixture was heated to 40°C. 113 g of the main resin prepared in step (1) was poured into the emulsifying kettle, the temperature of the main resin being 90°C, and stirred for 30 min. While ensuring that the temperature was below 50°C, 100 g of the low-temperature curing cross-linking agent prepared in step 1 was added, and the mixture was stirred for 2 h. A total of 180 g of pure water was added in two batches, and the mixture was stirred for 60 min each time to obtain 522 g of epoxy cathode electrophoretic emulsion.

[0054] 3. Epoxy cathodic electrophoretic coating

[0055] The epoxy cathode electrophoretic emulsion prepared in step 2 was mixed with black paste 7954 in a mass ratio of 4.5:1, and the wet film was obtained after mechanical stirring. Electrophoretic deposition was performed on the surface of a standard phosphating plate at an electrophoretic temperature of 30°C, a voltage of 160V, and an electrophoretic time of 160s. After the deposition was completed, the film was baked in an oven at 120°C for 25 minutes to obtain a smooth coating with a coating thickness of 20 microns.

[0056] The pencil hardness, adhesion, impact and salt spray resistance of the coating are shown in Table 1, and the 200h salt spray diagram is shown in Table 1. Figure 6 .

[0057] Table 1. Cross-linking agent, emulsion, and coating performance test

[0058] project Example 1 Example 2 standard Curing crosslinker stability >6 months >6 months Whether there is precipitation or stratification Stability of Epoxy Cathodic Electrophoretic Emulsion >4 weeks >4 weeks There is obvious precipitation or stratification Coating appearance Smooth Smooth Observation with the naked eye Pencil hardness 2H 2H GB / T6739-2006 Adhesion Level 0 Level 0 GB / T9286-1998 Impact strength (Kg / cm) 50 50 GB / T1732-2020 Salt spray resistant >500h <200h (and bubbling) GB / T 1771—2007

[0059] in conclusion

[0060] (1) The low-temperature curing cross-linking agent of the present invention can be applied to epoxy cathode electrophoresis, has a low curing temperature of 120°C, and has excellent other properties.

[0061] (2) The curing cross-linking agent of the present invention has good storage stability and good stability when applied to cathode electrophoretic emulsion.

[0062] (3) The present invention uses a new blocking agent, ethyl p-hydroxybenzoate, so that the relatively less toxic IPDI can be used to replace the highly toxic TDI and MDI, making the production and application environmentally friendly and safe.

[0063] (4) The present invention uses a new sealing agent ethyl paraben, which avoids the volatilization of MEKO during the coating baking process caused by the use of MEKO in traditional low-temperature coatings, and is safe.

Claims

1. A low-temperature curing crosslinking agent for epoxy cathodic electrophoretic coating, characterized in that: The low-temperature curing cross-linking agent is composed of the following raw materials by mass: 154 parts of diisocyanate, 100-150 parts of dispersant, 1-5 parts of catalyst, 55-140 parts of blocking agent A, and 150-300 parts of blocking agent B; The blocking agent A is one or more of polypropylene glycol 400, polypropylene glycol 600, polypropylene glycol 1000, polytetrahydrofuran 650, and polytetrahydrofuran 1000; The blocking agent B is ethyl p-hydroxybenzoate; The catalyst is dibutyltin dilaurate; The dispersant is a ketone substance; The preparation method of the low-temperature curing crosslinking agent comprises the following steps: placing a diisocyanate, a dispersant in an amount of 4 / 9 and a catalyst in a reaction kettle according to the formula, mixing the mixture, heating the mixture to 60-80° C., adding a blocking agent A dropwise, reacting the mixture at 60-80° C. for 1-3 hours, detecting that the NCO equivalent reaches 298, then adding a blocking agent B, heating the mixture to 80-85° C. and reacting the mixture for 4-5 hours, detecting that the NCO equivalent is greater than 2000, then adding the remaining dispersant to adjust the solid content to 70-80%, thereby obtaining the low-temperature curing crosslinking agent.

2. The low-temperature curing cross-linking agent according to claim 1, characterized in that The low-temperature curing cross-linking agent is composed of the following raw materials in parts by mass: 154 parts of diisocyanate, 100-120 parts of dispersant, 1 part of catalyst, 55-140 parts of blocking agent A, and 177-228 parts of blocking agent B.

3. The low-temperature curing cross-linking agent according to claim 1, characterized in that The diisocyanate is isophorone diisocyanate.

4. The low-temperature curing cross-linking agent according to claim 1, characterized in that The dispersant is methyl isobutyl ketone or acetone.

5. The low-temperature curing cross-linking agent according to claim 3, characterized in that The low-temperature curing cross-linking agent is composed of the following raw materials in parts by mass: 154 parts of isophorone diisocyanate, 110 parts of methyl isobutyl ketone, 1 part of dibutyltin dilaurate, 83 parts of polypropylene glycol 600, and 202 parts of ethyl p-hydroxybenzoate.

6. Use of the low-temperature curing cross-linking agent according to claim 1 in preparing epoxy cathodic electrophoretic coating.

7. The use according to claim 6, characterized in that The application method includes: (1) main resin: epoxy resin, bisphenol A, and isooctanoic acid are placed in a reaction kettle, heated to 145°C and reacted for 30 minutes; catalyst is added to the reaction kettle, heated to 155°C and reacted for 85 minutes, methylethanolamine and ketimine are added, and reacted at 120°C for 2 hours to obtain main resin; the catalyst includes benzylamine; (2) epoxy cathode electrophoretic emulsion: acid and water are mixed and heated to 40°C, and then 90°C main resin is added, stirred for 20-40 minutes, the temperature is controlled below 50°C, and the reaction is continued. Add the low-temperature curing cross-linking agent, stir for 1-3h, add pure water in batches, stir for 50-70min each time, and obtain epoxy cathode electrophoretic emulsion; (3) epoxy cathode electrophoretic coating: mix the epoxy cathode electrophoretic emulsion and black paste 7954 in a mass ratio of 4.5:1, use mechanical stirring to obtain a wet film, and perform electrophoretic deposition under the conditions of electrophoretic temperature of 29-30℃, voltage of 100-200V, and electrophoretic time of 100-300s. After the deposition is completed, bake in an oven at 120℃ for 25min to obtain a coating.

8. The use according to claim 7, characterized in that Step (1) The molecular weight of the epoxy resin is 188; the mass ratio of the epoxy resin to bisphenol A is 72.5:30.7, the mass ratio of isooctanoic acid to the epoxy resin is 3:72.5; the mass ratio of the catalyst to the epoxy resin is 0.8:72.5; the mass ratio of the epoxy resin to methylethanolamine and ketimine is 72.5:3.8:8.6; In step (2), the acid is formic acid; the mass ratio of the acid to water is 1:40-60; the mass ratio of the acid to the main resin is 1:30-70; the mass ratio of the acid to the low-temperature curing crosslinker is 3:50-150; and the mass ratio of the acid to pure water is 1:50-80.

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