Development of Color Paste Dispersion Resin and Color Paste for Cathodic Electrophoretic Coatings
Paint Industry
Coating Industry:
introduction
Cathodic electrophoretic coating is a water-based anti-corrosion coating with excellent anti-corrosion performance, high degree of coating automation, high coating utilization rate, and good environmental performance. Widely used in the field of electrophoretic coating for automobiles and their accessories, agricultural machinery, hardware and household appliances, etc. Cathodic electrophoretic coatings are divided into single component and dual component systems, both of which have coloring properties and include pigment and filler components. The dispersion stability and construction performance of pigments and fillers are non important performance indicators. The key to developing pigment pastes with excellent storage and construction performance is to prepare pigment dispersion resins with excellent dispersion performance.
At present, in cathodic electrophoretic coatings, pigment dispersion resins are usually quaternary ammonium salt dispersion resins, which are made of small molecule organic amine ring opening epoxy resins and neutralized by organic acids. This type of dispersion resin has good dispersibility for pigments and fillers, but storage stability and workability still need to be improved. This study used nonylphenol to modify epoxy resin to prepare dispersed resin. Nonylphenol serves as a flexible side chain in the dispersed resin structure, increasing its anchoring effect on pigments and fillers, and improving the storage stability and construction performance of the developed color paste.
Experimental part
1.1 Experimental Materials and Instruments
Epoxy resin (epoxy equivalent: 580-610 g/mol): industrial grade, South Asia Epoxy Resin Co., Ltd; Ethylene glycol butyl ether and diethanolamine: industrial grade, Jiangyin Dexian Chemical Co., Ltd; Nonylphenol: purity ≥ 98.0%, Western Asia reagent; Grinding additives, pigments and fillers, special lotion for electrophoresis: industrial grade, CNOOC Changzhou Environmental Protection Coating Co., Ltd; Acetic acid, triphenylphosphine: chemically pure, China National Pharmaceutical Chemical Reagent Co., Ltd; Deionized water: industrial grade, self-made.
Fourier transform infrared spectrometer: VECTOR-22 type, Brooke Company, Germany; Salt spray test chamber: SF260 type, United States Onckel Co., Ltd; Electrophoresis instrument: Type 300, Shanghai Precision Instrument Co., Ltd; Paint film thickness gauge: QNIX4500 type, Nix, Germany; Conductivity and pH measuring instrument: PC510 type, Shanghai Guyu Environmental Protection Technology Co., Ltd; Blast drying oven: DHG-9030 type, Shanghai Yiheng Scientific Instrument Co., Ltd.
1.2 Synthesis and characterization of dispersed resins
Add epoxy resin, nonylphenol, and ethylene glycol butyl ether to a four necked flask equipped with a thermometer and stirring device in sequence, heat up to 120~130 ℃, and stir for dissolution; Add catalyst triphenylphosphine, stir at 130-140 ℃ for 3 hours, and cool to 100-110 ℃; Add diethanolamine for insulation reaction for 2 hours, and cool to 70-80 ℃; Add glacial acetic acid for insulation reaction for 1 hour, adjust the solid fraction of the dispersed resin to 60% with deionized water, stir for 1 hour, cool down and discharge the material. The dispersed resin structure was tested and characterized using Fourier transform infrared spectroscopy. The reference formula for dispersed resin is shown in Table 1.
Table 1 Reference Formula for Disperse Resin
1.3 Preparation and characterization of electrophoretic color paste
Add the dispersed resin to a clean grinding tank, add a certain amount of grinding aids, stir evenly, and then add a small amount of deionized water multiple times. Stir evenly and gradually add pigments and fillers. After fully dispersing and wetting, add the same volume of grinding zirconium beads. Grind with a grinder to a fineness of ≤ 15 μ m. Add an appropriate amount of deionized water to adjust the solid content of the electrophoretic color paste, and filter out the material for later use. Use a fineness scraper to measure the fineness of the color paste according to GB/T 1724-1979, and the fineness of the color paste is ≤ 15 μ M; The solid content of the color paste shall be determined according to GB/T 1725-2007; The pH value and conductivity of the color paste were tested using a PC510 pH meter conductivity meter.
Table 2 Color Paste Reference Formula Table
1.4 Performance testing of color paste matching use
In a clean electrophoresis tank, add special lotion for electrophoresis, deionized water and color paste (mass ratio: 5:6:1) to prepare a working solution, and stir and mature at room temperature for 24 hours. Perform electrophoretic coating on the surface of tinplate and standard phosphating board at a temperature of (30 ± 1) ℃. After coating, dry the board at room temperature and place it in a 150-160 ℃ oven for 30 minutes. Test the performance of the working solution and paint film according to the HG/T 3952-2007 cathodic electrophoretic coating standard.
Results and Discussion
2.1 Synthesis and characterization of dispersed resins
(1) Synthesis conditions of dispersed resin
The synthesis conditions of the dispersed resin are shown in Figure 1. From Figure 1, it can be seen that after 4 hours of reaction at a reaction temperature of 130 ℃, the epoxy equivalent of the dispersed resin is difficult to reach the theoretical value of 800 g/mol. The reaction temperature is 140 ℃. After 3 hours of reaction, the epoxy equivalent of the dispersed resin reaches the theoretical value, and the reaction is complete. When the reaction temperature is raised to 150 ℃ and after 2.5 hours of reaction time, the epoxy equivalent of the dispersed resin meets the standard. At different reaction temperatures, the initial reaction rate is faster, mainly due to the high concentration of reactants and the high effective collision frequency between molecules. In the later stage of the reaction, the viscosity of the system increases, and the concentration of reactants is low, which limits the collision between relative molecular masses and significantly reduces the reaction rate. However, from a practical application perspective, the higher the reaction temperature or the longer the reaction time, the more side reaction products there are, which may lead to problems such as cross-linking between dispersed resin molecular chains and increased reaction viscosity. Therefore, the synthesis condition of dispersed resin is selected to be 140 ℃ for a 3-hour insulation reaction.
Figure 1 Synthesis temperature and time of dispersed resin
(2) Characterization of dispersed resin structure
The structure of the dispersed resin was characterized using Fourier transform infrared spectroscopy, and the results are shown in Figure 2. On the infrared spectrum of epoxy resin, 3460 cm ⁻ ¹ The characteristic peak of hydroxyl groups in epoxy resin, 2970 cm ⁻ ¹ Is the characteristic peak of methyl group, 1510-1610 cm-1 is the characteristic peak of benzene ring in epoxy resin, 1240 cm ⁻ ¹ The characteristic peak of phenyl ether on the molecular chain, 1040 cm ⁻ ¹ It is a characteristic peak of epoxy group, 829 cm ⁻ ¹ The characteristic peak of para substitution of benzene ring. On the infrared spectrum of nonylphenol, 3340 cm ⁻ ¹ The characteristic peak of the hydroxyl group on the nonylphenol molecule, 2960 cm ⁻ ¹ Nearby is the stretching vibration peak of methyl group, 1510-1610 cm ⁻ ¹ Nearby is the characteristic peak of the benzene ring in the nonylphenol molecule, 1240 cm ⁻ ¹ The stretching vibration peak of diphenyl ether, 831 cm ⁻ ¹ The para substitution peak of the benzene ring. On the main chain of dispersed resin, 3460 cm ⁻ ¹ And 1040 cm ⁻ ¹ Epoxy resin characteristic peak appears at 2960 cm ⁻ ¹ And 831 cm ⁻ ¹ The characteristic peak of nonylphenol appears at, indicating a successful reaction between epoxy resin and nonylphenol.
Figure 2 Infrared spectra of epoxy resin, nonylphenol, and dispersed resin
2.2 Development of Color Paste
(1) The Effect of Grinding Time on the Fineness of Color Paste
The grinding time affects the final dispersion effect of the color paste. If the grinding time is not enough, the resin cannot fully wet the dispersed pigments and fillers; Long grinding time can increase production costs. As shown in Figure 3, as the grinding time increases, the fineness of the color paste gradually decreases, and the color paste becomes more stable. However, after the grinding time reaches 2.5 hours, the fineness of the color paste reaches 15 μ Below m, it meets the usage requirements and continues to extend the grinding time, resulting in a small change in the fineness of the color paste. Therefore, the suitable grinding time for the color paste is about 2.5 hours.
Figure 3 Effect of grinding time on the fineness of color paste
(2) The Effect of Solid Content and Color Base Ratio on the Stability of Color Paste
Study the effect of solid content and color base ratio on the state and storage stability of color paste. Solid content and color base ratio are important factors affecting the state and stability of color paste. The results are shown in Table 3, as can be seen from Table 3. When the solid content of the color paste is 42%, the color paste state is slightly thin; When the solid content of the color paste is 45%, the viscosity of the color paste is appropriate and the storage stability is good. But when the solid content of the color paste increases to 48%, the color paste becomes slightly thicker, and false thickening occurs during storage. In addition, under the same solid content conditions, increasing the color to base ratio of the color paste from 1.5 to 2.0 can improve the stability of the color paste, but attention should be paid to the false consistency during the storage process of the color paste. When the solid content of the color paste is 45% and the color to base ratio is 1.5, the viscosity of the color paste is moderate and the storage stability is good after 30 days.
Table 3: Effect of Solid Content and Color Base Ratio on the State and Stability of Color Paste
(3) Performance testing of color paste
The performance test results of the color paste are shown in Table 4. From Table 4, it can be seen that the color paste appears as a black paste like liquid without stratification. The solid content of the color paste is 45.6%, the color to base ratio is 1.51, and the conductivity is 1268 μ S/cm, pH 6.9, fineness 12.5 μ m. Stable storage state after 30 days. The comprehensive performance of the color paste meets the technical requirements for industry use.
Table 4 Performance Test Results of Color Paste
2.3 Matching performance of color paste
2.3.1 Performance testing of electrophoresis tank solution
In the clean electrophoresis tank, add special lotion for electrophoresis, deionized water and color paste (mass ratio: 5:6:1) to prepare the working solution. The performance test results of the working solution are shown in Table 5. From Table 5, it can be seen that the solid fraction of the working fluid is 18.5%, and the conductivity is 1716 μ S/cm, pH value of 6.2, color to base ratio of 0.15, precipitability of 0.5 mm, L-effect and resolubility qualified, breakdown voltage of 380V, meeting the technical requirements for using electrophoretic coating working fluid.
Table 5 Performance Test Results of Working Fluid
2.3.2 Paint film performance testing
Use the above working fluid to electrophoretically coat the surface of the tinplate and standard phosphating board. After coating, dry the board at room temperature and let it dry for 30 minutes in a 150-160 ℃ oven. The performance test results of the paint film are shown in Table 6. The appearance of the paint film is flat and smooth, the pencil hardness is H, the impact resistance is 50 cm, the flexibility is 1 mm, the cup protrusion is 8 mm, the acid alkali and gasoline resistance meet the standards, and the neutral salt spray resistance is 1200 hours, The comprehensive performance of the paint film is excellent.
Table 6 Paint Film Performance Test Results
epilogue
This study used nonylphenol to modify epoxy resin to prepare a dispersed resin for cathodic electrophoretic coatings, and further developed a color paste. The results showed that the optimal synthesis condition for dispersed resin was 140 ℃ for 3 hours of insulation reaction. The chemical structure of the dispersed resin was characterized by Fourier transform infrared spectroscopy. The dispersed resin was ground with pigments and fillers to prepare a color paste. The grinding time was 2.5 hours, the solid content of the color paste was 45%, the color to base ratio (P/B) was 1.5, the viscosity of the color paste was moderate, and the storage stability was good. All technical indicators met the standards. The color paste is used together with the special lotion for electrophoresis. The electrophoresis working solution meets the requirements of the industry's technical indicators, and the indicators of the paint film meet the use requirements. Compared with conventional quaternary ammonium salt dispersion resins, color paste dispersion resins prepared using nonylphenol modified epoxy resins can effectively improve the storage stability and construction performance of electrophoretic color pastes, reduce the use of organic solvents in electrophoretic coatings, and have good practical value and application prospects.
