Cathode electrophoretic coating additive as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of coatings, and discloses a cathode electrophoretic coating additive as well as a preparation method and application thereof. The cathode electrophoretic coating additive disclosed by the invention is a grafted copolymer formed by semi-closed polyisocyanate and polyvinyl alcohol. The cathode electrophoretic coating additive is good in compatibility with a traditional cathode electrophoretic coating, only simple mixing is needed, production line equipment does not need to be transformed, the cathode electrophoretic coating additive is applied to the cathode electrophoretic coating, on the basis that low-temperature (130 DEG C) curing is guaranteed, the salt spray resistance (longer than 900 h) and the impact strength (larger than or equal to 80 kg.cm) of a paint film formed through curing can be obviously improved, and the service life of the cathode electrophoretic coating additive is prolonged. And the leveling effect of the paint film is not negatively influenced.

Description

Technical Field

[0001] This invention belongs to the field of coating technology, and specifically relates to a cathodic electrophoretic coating additive, its preparation method, and its application. Background Technology

[0002] Cathodic electrophoretic coatings are widely used. In some special applications, such as the automotive industry, the neutral salt spray resistance of cathodic electrophoretic coatings is required to be ≥1008h (42 days) or 30 cycles (42 days) of cyclic salt spray corrosion test, such as the corrosion resistance standard of Volkswagen PV1210. However, the salt spray resistance of traditional low-temperature (130-160℃) curing cathodic electrophoretic coatings is insufficient, usually 500-800h, which can no longer meet the requirements.

[0003] Traditional low-temperature curing cathodic electrophoretic coatings mainly suffer from the following three problems:

[0004] (1) Increasing the crosslinking density can reduce the permeation rate of moisture and oxygen, effectively slowing down the corrosion of the substrate. However, increasing the crosslinking density will lead to embrittlement of the coating and a decrease in impact resistance (impact strength < 50 kg·cm).

[0005] (2) Adhesion promoters are used to increase the adhesion between the coating and the substrate, improve the coating’s anti-foaming ability, and inhibit the paint film from peeling off and corroding the substrate. However, adhesion promoters generally have applicability requirements for the type of substrate. Different substrates often require different types of adhesion promoters, and there may be compatibility issues that lead to rough paint film or pinhole defects, so their practicality is not high.

[0006] (3) Using organic zinc to passivate the substrate produces strong sealing and anchoring properties, improves water resistance, and inhibits corrosion expansion. However, in the actual use of electrophoretic coatings, organic zinc will ionize under working conditions to produce zinc ions. Zinc ions accumulate in the bath solution, causing the conductivity of the bath solution to continuously increase, which easily forms pinholes that break through the paint film.

[0007] Therefore, how to improve the salt spray resistance of cathodic electrophoretic coatings without increasing the curing temperature is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0008] The present invention aims to at least solve one of the technical problems existing in the prior art. To this end, the present invention provides a cathodic electrophoretic coating additive, its preparation method, and its application. This cathodic electrophoretic coating additive, when used in cathodic electrophoretic coatings, can improve the salt spray resistance and impact strength of the coating film formed after curing.

[0009] In a first aspect, the present invention provides a cathodic electrophoretic coating additive, wherein the cathodic electrophoretic coating additive is a graft copolymer formed from a semi-blocked polyisocyanate and polyvinyl alcohol.

[0010] In some embodiments of the present invention, the weight ratio of the semi-blocked polyisocyanate to polyvinyl alcohol is (5-10):1, preferably (6-10):1, and more preferably (8-10):1.

[0011] In some embodiments of the present invention, the raw materials of the cathodic electrophoretic coating additive include: polyisocyanate, polyvinyl alcohol, sealing agent and organic solvent.

[0012] In some embodiments of the present invention, the raw materials of the cathodic electrophoretic coating additive, by weight, include: 160-380 parts of polyisocyanate, 40-60 parts of polyvinyl alcohol, 50-150 parts of sealant, and 600-1300 parts of organic solvent.

[0013] In some embodiments of the present invention, the raw materials of the cathodic electrophoretic coating additive, by weight, include: 180-360 parts of polyisocyanate, 45-55 parts of polyvinyl alcohol, 70-145 parts of sealant, and 700-1300 parts of organic solvent.

[0014] In some embodiments of the present invention, the polyisocyanate includes at least one of isophorone diisocyanate (IPDI), hexamethylene diisocyanate and phenyl diisocyanate, preferably isophorone diisocyanate.

[0015] In some embodiments of the present invention, the blocking agent includes at least one of alcohol ether solvents, methyl ethyl ketone oxime (MEKO), alcohols and caprolactam, preferably methyl ethyl ketone oxime.

[0016] In some embodiments of the present invention, the organic solvent includes at least one selected from methyl isobutyl ketone (MIBK), butanone, methanol, ethanol, benzene, and toluene, preferably methyl isobutyl ketone.

[0017] In some embodiments of the present invention, the degree of polymerization of the polyvinyl alcohol is 800-1000.

[0018] In some embodiments of the present invention, the hydroxyl value of the polyvinyl alcohol is 100-150 mg KOH / g.

[0019] In some embodiments of the present invention, the viscosity of the cathodic electrophoretic coating additive at room temperature (25±2℃) is 2000-5000 cP; its appearance is a pale yellow liquid.

[0020] In a second aspect, the present invention provides a method for preparing the cathodic electrophoretic coating additive described in the first aspect, comprising the following steps:

[0021] S1. Mix polyisocyanate and organic solvent, heat, then add blocking agent dropwise, and keep the reaction at the temperature to obtain semi-blocked isophorone diisocyanate;

[0022] S2. The semi-closed isophorone diisocyanate is heated and then mixed with polyvinyl alcohol, and the reaction is carried out under heat to obtain the cathodic electrophoretic coating additive.

[0023] In some embodiments of the present invention, in step S1, the heating temperature is 40-45°C; and / or, the dropping rate is 0.5-2.5 g / min; and / or, the heat preservation reaction is carried out until the NCO value in the system is 11.3 ± 0.2%; and / or, the heat preservation reaction time is 2-2.5 h.

[0024] In some embodiments of the present invention, in step S2, the heating temperature is 80-85°C; and / or, the heat preservation reaction is carried out until the NCO value in the system is below 0.2%; and / or, the heat preservation reaction time is 5-5.5 h.

[0025] In some embodiments of the present invention, the synthesis route of the cathodic electrophoretic coating additive is as follows:

[0026] ①

[0027] ②

[0028] It is understood that in the synthetic route of the present invention, R1 corresponds to a group in different existing polyisocyanates.

[0029] In a third aspect, the present invention provides a cathodic electrophoretic coating, the cathodic electrophoretic coating comprising a cathodic electrophoretic emulsion and the cathodic electrophoretic coating additives described in the first aspect of the present invention.

[0030] In some embodiments of the present invention, the mass of the cathodic electrophoretic coating additive accounts for 1%-10% of the mass of the cathodic electrophoretic emulsion, preferably 2%-8%, and more preferably 3%-5%.

[0031] In some embodiments of the present invention, the cathodic electrophoretic coating further includes a color paste.

[0032] In some embodiments of the present invention, the curing temperature of the cathodic electrophoretic coating is 130-160°C, preferably 130°C.

[0033] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0034] The cathodic electrophoretic coating additive provided by this invention achieves controllable adjustment of salt spray resistance by grafting semi-closed polyisocyanate and polyvinyl alcohol, and retaining sufficient free -NCO groups for low-temperature crosslinking, thus solving the contradiction between traditional low-temperature curing and high salt spray resistance. At the same time, this cathodic electrophoretic coating additive has good compatibility with traditional cathodic electrophoretic coatings, requiring only simple mixing and no modification to production line equipment. When used in cathodic electrophoretic coatings, it can significantly improve the salt spray resistance (>900h) and impact strength (≥80kg·cm) of the cured coating film while ensuring low-temperature (130℃) curing, and has no negative impact on the leveling effect of the coating film. Detailed Implementation

[0035] The present invention will be further described in detail below through specific embodiments. Unless otherwise specified, the raw materials, reagents, or apparatus used in the embodiments can be obtained from conventional commercial sources or by existing technical methods. Unless otherwise specified, the experimental or testing methods are conventional methods in the art.

[0036] Example 1

[0037] A cathodic electrophoretic coating additive, the raw materials for which are prepared include 50g of polyvinyl alcohol, 180g of isophorone diisocyanate, 70g of methyl ethyl ketone oxime and 700g of methyl isobutyl ketone; wherein, the degree of polymerization of polyvinyl alcohol is 800 and the hydroxyl value is 120mgKOH / g.

[0038] The preparation method of the above-mentioned cathodic electrophoretic coating additive includes the following steps:

[0039] S1. Pre-dry polyvinyl alcohol in an oven at 80℃ for 12 hours to obtain dried polyvinyl alcohol;

[0040] S2. Add isophorone diisocyanate and methyl isobutyl ketone to a four-necked flask, control the temperature at 42℃, and add methyl ethyl ketone oxime at a dropping rate of 1g / min. After the addition is complete, keep the reaction at the temperature for 2 hours and detect the NCO value. The NCO value is controlled at 11.3±0.2%, and a semi-closed isophorone diisocyanate product, namely semi-closed IPDI, is obtained for later use.

[0041] S3. Heat the semi-closed IPDI to 82°C, add dry polyvinyl alcohol (PVA), and keep the reaction at 82°C for 5 hours. Detect the NCO value. If the NCO value is controlled below 0.2%, the cathodic electrophoretic coating additive is obtained. The weight ratio of semi-closed IPDI to dry PVA is 5:1.

[0042] Example 2

[0043] A cathodic electrophoretic coating additive is prepared from raw materials including 50g of polyvinyl alcohol, 215g of isophorone diisocyanate, 85g of methyl ethyl ketone oxime, and 817g of methyl isobutyl ketone; wherein the degree of polymerization of polyvinyl alcohol is 800 and the hydroxyl value is 120mgKOH / g.

[0044] The preparation method of the above-mentioned cathodic electrophoretic coating additive includes the following steps:

[0045] S1. Pre-dry polyvinyl alcohol in an oven at 80℃ for 12 hours to obtain dried polyvinyl alcohol;

[0046] S2. Add isophorone diisocyanate and methyl isobutyl ketone to a four-necked flask, control the temperature at 40℃, and add methyl ethyl ketone oxime at a dropping rate of 1.5 g / min. After the addition is complete, keep the reaction at the temperature for 2 hours and detect the NCO value. The NCO value is controlled at 11.3±0.2%, and a semi-closed isophorone diisocyanate compound, namely semi-closed IPDI, is obtained for later use.

[0047] S3. Heat the semi-closed IPDI to 82°C, add dry polyvinyl alcohol (PVA), and keep the reaction at 82°C for 5 hours. Detect the NCO value. If the NCO value is controlled below 0.2%, the cathodic electrophoretic coating additive is obtained. The weight ratio of semi-closed IPDI to dry PVA is 6:1.

[0048] Example 3

[0049] A cathodic electrophoretic coating additive is prepared from raw materials including 50g of polyvinyl alcohol, 287g of isophorone diisocyanate, 113g of methyl ethyl ketone oxime and 1050g of methyl isobutyl ketone; wherein the degree of polymerization of polyvinyl alcohol is 800 and the hydroxyl value is 120mgKOH / g.

[0050] The preparation method of the above-mentioned cathodic electrophoretic coating additive includes the following steps:

[0051] S1. Pre-dry polyvinyl alcohol in an oven at 80℃ for 12 hours to obtain dried polyvinyl alcohol;

[0052] S2. Add isophorone diisocyanate and methyl isobutyl ketone to a four-necked flask, control the temperature at 45℃, and add methyl ethyl ketone oxime at a dropping rate of 2 g / min. After the addition is complete, keep the reaction at the temperature for 2.5 h, and detect the NCO value. The NCO value is controlled at 11.3±0.2%, and a semi-closed isophorone diisocyanate compound, namely semi-closed IPDI, is obtained for later use.

[0053] S3. Heat the semi-closed IPDI to 82°C, add dry polyvinyl alcohol (PVA), and keep the reaction at 82°C for 5 hours. Detect the NCO value. If the NCO value is controlled below 0.2%, the cathodic electrophoretic coating additive is obtained. The weight ratio of semi-closed IPDI to dry PVA is 8:1.

[0054] Example 4

[0055] A cathodic electrophoretic coating additive is prepared from raw materials including 50g of polyvinyl alcohol, 359g of isophorone diisocyanate, 141g of methyl ethyl ketone oxime and 1283g of methyl isobutyl ketone; wherein the degree of polymerization of polyvinyl alcohol is 800 and the hydroxyl value is 120mgKOH / g.

[0056] The preparation method of the above-mentioned cathodic electrophoretic coating additive includes the following steps:

[0057] S1. Pre-dry polyvinyl alcohol in an oven at 80℃ for 12 hours to obtain dried polyvinyl alcohol;

[0058] S2. Isophorone diisocyanate and methyl isobutyl ketone are added to a four-necked flask. The temperature is controlled at 45℃. Methyl ethyl ketone oxime is added dropwise at a rate of 2.5 g / min. After the addition is completed, the reaction is maintained at this temperature for 2.5 h. The NCO value is measured. The NCO value is controlled at 11.3 ± 0.2%. The semi-closed isophorone diisocyanate product, namely semi-closed IPDI, is obtained and set aside for later use.

[0059] S3. Heat the semi-closed IPDI to 82°C, add dry polyvinyl alcohol (PVA), and keep the reaction at 82°C for 5 hours. Detect the NCO value. If the NCO value is controlled below 0.2%, the cathodic electrophoretic coating additive is obtained. The weight ratio of semi-closed IPDI to dry PVA is 10:1.

[0060] Comparative Example 1

[0061] Salt spray resistant additive M3 from Huangshan Aoshengyuan New Material Technology Co., Ltd. was used.

[0062] Comparative Example 2

[0063] A cathodic electrophoretic coating additive is prepared from raw materials including 50g of polyvinyl alcohol, 280g of isophorone diisocyanate, 220g of methyl ethyl ketone oxime and 1283g of methyl isobutyl ketone; wherein the degree of polymerization of polyvinyl alcohol is 800 and the hydroxyl value is 120mgKOH / g.

[0064] The preparation method of the above-mentioned cathodic electrophoretic coating additive includes the following steps:

[0065] S1. Pre-dry polyvinyl alcohol in an oven at 80℃ for 12 hours to obtain dried polyvinyl alcohol;

[0066] S2. Add isophorone diisocyanate and methyl isobutyl ketone to a four-necked flask, control the temperature at 45℃, and add methyl ethyl ketone oxime at a dropping rate of 4 g / min. After the addition is complete, keep the reaction at the temperature for 2.5 h, and detect the NCO value. If the NCO value is controlled below 0.2%, the fully enclosed isophorone diisocyanate compound, i.e., fully enclosed IPDI, is obtained and set aside for later use.

[0067] S3. Heat the fully enclosed IPDI to 82°C, add dry polyvinyl alcohol (PVA), and keep the reaction at 82°C for 5 hours. Detect the NCO value. If the NCO value is controlled below 0.2%, the cathodic electrophoretic coating additive is obtained. The weight ratio of semi-enclosed IPDI to dry PVA is 10:1.

[0068] Performance testing

[0069] The additives provided in Examples 1-4 and Comparative Examples 1-2 were added to a commercial cathodic electrophoresis emulsion (KD-8841F2) (the additives accounted for 4% of the mass of the commercial cathodic electrophoresis emulsion). These were then mixed with black paste (KD-8841F1) and pure water at a weight ratio of 4:1:5 to prepare the bath solution, which was then aged at room temperature for 24 hours. The test plate was a standard phosphated test plate (cold-rolled steel plate). The electrophoresis parameters were: bath temperature 30℃, voltage 220V, and electrophoresis time 3 minutes. After electrophoresis, the plate was cured at 130℃ for 20 minutes, forming a film thickness controlled at 20-22 micrometers, thus obtaining the performance test plate (hereinafter referred to as the sample plate). A blank control group was also set up, i.e., the commercial cathodic electrophoresis emulsion did not contain any additives.

[0070] The formulation of the commercial cathodic electrophoresis emulsion (KD-8841F2) is shown in Table 1; the formulation of the black paste (KD-8841F1) is shown in Table 2.

[0071] Table 1. Commercial cathodic electrophoresis emulsion formulations

[0072] Components Mass content (%) Blocked isocyanates 15 Modified epoxy resin 25 Methyl isobutyl ketone 5 Ketoimine 2 glacial acetic acid 0.5 pure water margin bactericide 0.2

[0073] A method for preparing commercial cathodic electrophoretic emulsion includes the following steps:

[0074] (1) Add the formulated amount of modified epoxy resin and methyl isobutyl ketone to a four-necked flask, stir (250 r / min), heat to 80℃, and dissolve until transparent;

[0075] (2) Add ketimine, stir (500 r / min), and keep the reaction at 80℃ for 120 min;

[0076] (3) Add the blocked isocyanate, stir (500 r / min), and keep warm at 60℃ for 30 min;

[0077] (4) Add glacial acetic acid, stir (800 r / min), and keep warm at 60℃ for 90 min;

[0078] (5) After the heat preservation is completed, stir (800r / min), turn off the heating, divide the pure water into 3 equal portions, and add each portion at 30min intervals;

[0079] (6) After adding pure water, add bactericide and stir (250r / min) for 30 minutes.

[0080] Table 2 Commercial Black Paste Formulation

[0081] Components Mass content (%) Modified epoxy-polyurethane resin 30 Ethylene glycol butyl ether 5 glacial acetic acid 0.5 pure water margin Organic bismuth 0.5 carbon black 5 Kaolin 25

[0082] A method for preparing commercial black paste includes the following steps:

[0083] (1) Add modified epoxy-polyurethane resin, ethylene glycol butyl ether and glacial acetic acid to the sand mill, stir (500 r / min), and keep warm at 50℃ for 60 min;

[0084] (2) Add pure water and stir (500 r / min) at 25℃ for 60 min;

[0085] (3) Add organic bismuth, stir (500 r / min), and stir at 25℃ for 30 min;

[0086] (4) Add carbon black and kaolin, stir (1600 r / min), and stir at 25℃ for 210 min.

[0087] Salt spray resistance performance was tested according to GB / T 10125 standard. The sample was subjected to a cross-cut test until the maximum erosion width on one side along the cut line was 2.00 mm. The salt spray resistance performance was then measured. The results are shown in Table 3.

[0088] The roughness of the paint film was measured using a roughness tester. The roughness was tested at three points: the top, middle, and bottom of the sample plate, and the arithmetic mean was taken as the test result. The results are shown in Table 4.

[0089] The impact strength test was conducted according to GB / T 1732 standard. The impact strength was tested at three points: top, middle and bottom of the sample plate. The minimum value of paint film impact cracking was taken as the test result. The results are shown in Table 5.

[0090] Table 3. Results of Salt Spray Resistance Test of Coating Film

[0091] Sample group Salt spray resistance time of paint film (h) Example 1 942 Example 2 1012 Example 3 1208 Example 4 1488 Blank control group 846 Comparative Example 1 870 Comparative Example 2 820

[0092] Table 4 Results of paint film roughness test

[0093] Sample group Roughness (μm) Example 1 0.118 Example 2 0.124 Example 3 0.120 Example 4 0.125 Blank control group 0.128 Comparative Example 1 0.332 Comparative Example 2 0.282

[0094] Table 5 Results of paint film impact strength test

[0095]

[0096]

[0097] As shown in Table 3, compared with the blank control group without additives, the coatings with additives from Examples 1-4 exhibited significantly improved salt spray resistance time. Furthermore, the salt spray resistance increased with increasing grafting degree, with the coating based on Example 4 achieving a salt spray resistance time as high as 1488 hours. In contrast, the salt spray resistance time of the coatings based on additives from Comparative Examples 1 and 2 showed no significant change.

[0098] As shown in Table 4, compared with the blank control group without additives, the roughness of the paint film formed by adding the additives of Examples 1-4 did not change significantly, and the roughness of the paint film did not increase with the increase of grafting degree, that is, the additives of the present invention have no negative effect on the leveling effect of the paint film. However, the additives provided in Comparative Examples 1 and 2 have a greater impact on the roughness of the paint film.

[0099] As shown in Table 5, compared with the blank control group without additives, the coatings with additives from Examples 1-4 showed significantly improved impact strength, and the impact strength did not increase with the increase of grafting degree. However, no significant change was observed in the impact strength of the coatings based on the additives of Comparative Examples 1 and 2.

[0100] The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited to the embodiments described. Those skilled in the art can make various equivalent modifications or substitutions without departing from the spirit of the present invention, and these equivalent modifications or substitutions are all included within the scope defined by the claims of this application.

Claims

1. A cathodic electrophoretic coating additive, characterized in that, The cathodic electrophoretic coating additive is a graft copolymer formed from semi-blocked polyisocyanate and polyvinyl alcohol.

2. The cathodic electrophoretic coating additive according to claim 1, characterized in that, The weight ratio of the semi-closed polyisocyanate to polyvinyl alcohol is (5-10):

1.

3. The cathodic electrophoretic coating additive according to claim 1, characterized in that, The raw materials for the cathodic electrophoretic coating additive include: polyisocyanate, polyvinyl alcohol, sealant and organic solvent.

4. The cathodic electrophoretic coating additive according to claim 3, characterized in that, The raw materials of the cathodic electrophoretic coating additive, by weight, include: 160-380 parts of polyisocyanate, 40-60 parts of polyvinyl alcohol, 50-150 parts of sealant, and 600-1300 parts of organic solvent.

5. The cathodic electrophoretic coating additive according to claim 3 or 4, characterized in that, The polyisocyanate includes at least one of isophorone diisocyanate, hexamethylene diisocyanate, and phenyl diisocyanate; the blocking agent includes at least one of alcohol ether solvents, methyl ethyl ketone oxime, alcohols, and caprolactam; and the organic solvent includes at least one of methyl isobutyl ketone, butanone, methanol, ethanol, benzene, and toluene.

6. A method for preparing a cathodic electrophoretic coating additive as described in any one of claims 1-5, characterized in that, Includes the following steps: S1. Mix polyisocyanate and organic solvent, heat, then add blocking agent dropwise, and keep the reaction at the temperature to obtain semi-blocked isophorone diisocyanate; S2. The semi-closed isophorone diisocyanate is heated and then mixed with polyvinyl alcohol, and the reaction is carried out under heat to obtain the cathodic electrophoretic coating additive.

7. The preparation method according to claim 6, characterized in that, In step S1, the heating temperature is 40-45℃; and / or the dropping rate is 0.5-2.5 g / min; and / or the heat preservation reaction is carried out until the NCO value in the system is 11.3±0.2%; and / or the heat preservation reaction time is 2-2.5 h.

8. The preparation method according to claim 6, characterized in that, In step S2, the heating temperature is 80-85℃; and / or, the heat preservation reaction is carried out until the NCO value in the system is below 0.2%; and / or, the heat preservation reaction time is 5-5.5h.

9. A cathodic electrophoretic coating, characterized in that, The cathodic electrophoretic coating comprises a cathodic electrophoretic emulsion and the cathodic electrophoretic coating additives according to any one of claims 1-5.

10. The cathodic electrophoretic coating according to claim 9, characterized in that, The mass of the cathodic electrophoretic coating additive accounts for 1%-10% of the mass of the cathodic electrophoretic emulsion.

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