UV-curable coatings use high-energy ultraviolet light as the energy source for coating curing. They absorb ultraviolet light through photoinitiators to produce reactive radicals, which in turn triggers a series of polymerization reactions on the photocurable resin to cure the coating. Its fast curing speed, high energy utilization rate, energy saving, 100% film reaction, and solvent-free volatilization are the new generation of environmentally friendly coatings. It has high strength, high hardness, high wear resistance, high gloss, high solvent resistance and other properties, and has been applied in many surface coating applications. In this paper, UV curable coatings with fast curing and excellent comprehensive performance for hardware substrates were developed using acrylated polyester, diacrylate oligomers, and CAB551-0.01 resin as main film-forming materials, and prepolymer structures were studied. The effects of photoinitiators and thinners on the properties of the coatings are widely used in the surface coating of hardware substrates.
1 Test part 1.1 Raw material resin: acrylated polyester, diacrylate oligomer, CAB551-0.01, domestic;
Photoinitiator: Irgacure184, domestic;
Monomer: trimethacrylate monomer, trifunctional functional monomer, homemade;
Auxiliaries: TegoRad2200, domestic;
Solvent: **** triacrylate, ethyl acrylate, homemade.
1.2 Equipment VFS-230 high-speed dispersion machine, Tu-4# cup, QXPISO scraper fineness meter, QCJ film impact tester, WGG60 gloss meter, high temperature and humidity machine, UV machine, domestic; QUV weathering tester, 7-IBB-C-CRCA paper tape wear resistance instrument, PIG-10 film thickness gauge, JM-IV wear instrument, imported.
1.3 Coating Formulations UV curing coating formulations for hardware substrates are shown in Table 1.
1.4 Curing conditions The varnish was cured on a UV machine and irradiated with a medium pressure mercury lamp (120 W/cm). No nitrogen protection during curing.
1.5 Test Methods Viscosity: Measured using a rotational viscometer at 25°C.
Curing rate: expressed as the critical curing rate. The critical cure rate is defined as the maximum permissible speed of the conveyor belt when the surface of the paint film is fully cured.
Adhesion: The coating was cut into 1 mm x 1 mm squares and tested with 600# tape.
Gloss: The gloss of the coated film at 60° was measured with a gloss meter and averaged.
Abrasion resistance: After a specified number of grinding cycles under a certain load, the weight loss (mg) of the paint film is indicated.
Hardness: Test the film hardness with a standard pencil.
Flexibility: The minimum bending diameter at which the coating film does not break after bending is tested.
Impact resistance: The impact resistance of the paint film was tested with a falling ball impact tester.
Anti-aging: The paint is sprayed on a metal aluminum plate, irradiated with strong light under a UV lamp until it is completely cured, and placed in a QUV weathering tester. After a period of time, the light loss of the coating film is detected.
1.6 Coatings and Film Performance Test results of coatings and their coating films are shown in Table 2.
2 Results and Discussion 2.1 Selection of Resins and Monomers In the design of hardware UV curing coating formulations, in addition to considering the overall curing and conversion rates, the choice of host resins and monomers should also be considered. Therefore, most of the formulations in Table 1 are selected. Acid oligomers. The main resin: with the most basic formula skeleton, to ensure the basic adhesion, stretch rate. Acrylated polyester, low viscosity, fast curing, mainly provides good flexibility and certain adhesion of the cured film. Diacrylate oligomers provide hardness, fast cure properties, and good adhesion, but their excessively high brittleness limits their use in formulations to high levels, and incomplete conversions are not possible in the system. The amount of monofunctional reactive diluent should not be too high, which will lead to excessive viscosity of the coating, so a large number of monomers capable of reducing the viscosity are selected in the formulation. On the one hand to reduce the viscosity, on the one hand to promote the leveling of the coating, get a more smooth surface of the cured film. Acrylate functional monomers and oligomers improve the wettability of the substrate and provide the formulation's adjustability (high solids content), reducing the viscosity of the formulation without affecting its performance. Trimethacrylate monomers are acidic monomers and ethyl acrylate is aggressive to the substrate and can increase adhesion. In the overall formulation, the trifunctional functional monomer has good compatibility with the acrylic prepolymer resin, has low viscosity, high curing speed, can increase the flexibility of the cured film, enhance adhesion, and give the film a good The abrasion resistance, hardness and gloss. **** Triacrylate is used to increase cross-link density, fast cure, good flexibility, and also acts as a reactive diluent. The hydroxyl-bearing multi-component acrylate itself has a higher viscosity, and the hydroxyl group can impart a certain adhesion promoting effect to the cured film. The multi-functional properties can make up for the lack of PUA polymerization activity, and increase the cross-link density of the cured film, and increase the sealing performance of the coating.
2.2 Application of CAB in Photocuring System There are different opinions on the application of CAB551-0.01 (cellulose acetate butyrate). In fact, CAB promotes the leveling process that takes place during solvent evaporation. After film formation, it is distributed on the surface of the film. It is mainly in the first half of the drying process. During the UV curing process after solvent evaporation, it must also be There must be a dry gradient, otherwise the problem of poor leveling due to shrinkage can easily occur. The use of CAB mainly focuses on its miscibility in UV coatings, followed by its own cross-linking double bonds, which can weakly participate in UV polymerization and form a whole with UV coatings.
2.3 The choice of photoinitiator Photoinitiator can absorb ultraviolet energy, produce free radicals, and then trigger oligomers and monomers to achieve polymerization, cross-linking, the system curing film. The choice of photoinitiator should be considered from the light-induced efficiency, price, pollution, toxicity, stability and other aspects. Hydrogen abstraction photoinitiator yellowing coefficient is relatively large, generally should be selected cracking photoinitiator. Using Irgacure 184 in the experiment can effectively improve the yellowing resistance of the coating. The amount of photoinitiator has a great influence on the curing performance and storage period of the coating. When the mass fraction is less than 2%, the curing rate is proportional to its mass fraction; when the mass fraction is greater than 2%, the curing rate has little relation with its mass fraction. However, when the mass fraction exceeds 4%, the shelf life of the coating decreases. Considering comprehensively, the photoinitiator mass fraction is suitable for 3%.
2.4 Selection of additives Leveling agents can effectively reduce the tension between the coating surface and components, improve the coating miscibility, increase the fluidity, ease of construction, make the coating surface smooth, flat, no pinholes, shrinkage holes, brush marks Surface defects such as orange peel. The addition of TegoRad 2200 leveling agent containing hydroxy polydimethylsiloxane to the formulation can effectively improve the smoothness, scratch resistance, flowability, leveling and non-stickiness of the paint film. Tests show that: 0.5% TegoRad2200, you can achieve good results.
2.5 Selection of diluent The inert solvent used in the photocurable coating should be a volatile solvent with a relatively low boiling point, which can be volatilized during the flash drying process of the wet film and cannot be easily taken into the cured film. Ethyl acrylate acts as a diluent for viscosity adjustment in this system. Use **** triacrylate as an oligomer cross-linker to increase cross-link density and speed up cure. The amount of diluent is adjusted according to the required viscosity within the appropriate range.
2.6 The most common problems of hardware UV-curable coatings The most common problem of hardware UV-curing coatings is that the paint film tends to cause pitting after drying. There are two main causes of pitting: First, the surface drive tension determined by the gel structure of the resin system ( Crystallization) and the other is the volatilization gradient and dissolution force control of the diluent. The matching of these two factors with the curing rate determines the appearance of pits. If the film thickness is higher, the better the leveling level is, the more the leveling times are. A relatively thick coating will have sufficient wetting and drying time, and the mutual solubility between the resins and the factors affecting the diluent. Reduction can significantly improve pitting. In general, the solid content is high and there are fewer pits. However, if thin coating is required, an issue must be made during the leveling time to improve the pitting problem, including the viscosity of the main resin (** low viscosity), the amount of initiator (as little as possible), the volatilization rate of the diluent The dosage and surface additives should be adjusted accordingly.
3 Conclusion Through experiments, selected acrylic esterified polyester, diacrylate oligomer, CAB551-0.01 resin as the main film to develop a UV curing coating for hardware substrates, under the irradiation of ultraviolet light can be quickly cured, with excellent Adhesion, high gloss, abrasion resistance and aging resistance, as well as good alcohol resistance, moist heat resistance and long-term storage stability, are widely used in the surface coating of automotive parts, equipment, metal plating and other substrates.
1 Test part 1.1 Raw material resin: acrylated polyester, diacrylate oligomer, CAB551-0.01, domestic;
Photoinitiator: Irgacure184, domestic;
Monomer: trimethacrylate monomer, trifunctional functional monomer, homemade;
Auxiliaries: TegoRad2200, domestic;
Solvent: **** triacrylate, ethyl acrylate, homemade.
1.2 Equipment VFS-230 high-speed dispersion machine, Tu-4# cup, QXPISO scraper fineness meter, QCJ film impact tester, WGG60 gloss meter, high temperature and humidity machine, UV machine, domestic; QUV weathering tester, 7-IBB-C-CRCA paper tape wear resistance instrument, PIG-10 film thickness gauge, JM-IV wear instrument, imported.
1.3 Coating Formulations UV curing coating formulations for hardware substrates are shown in Table 1.
1.4 Curing conditions The varnish was cured on a UV machine and irradiated with a medium pressure mercury lamp (120 W/cm). No nitrogen protection during curing.
1.5 Test Methods Viscosity: Measured using a rotational viscometer at 25°C.
Curing rate: expressed as the critical curing rate. The critical cure rate is defined as the maximum permissible speed of the conveyor belt when the surface of the paint film is fully cured.
Adhesion: The coating was cut into 1 mm x 1 mm squares and tested with 600# tape.
Gloss: The gloss of the coated film at 60° was measured with a gloss meter and averaged.
Abrasion resistance: After a specified number of grinding cycles under a certain load, the weight loss (mg) of the paint film is indicated.
Hardness: Test the film hardness with a standard pencil.
Flexibility: The minimum bending diameter at which the coating film does not break after bending is tested.
Impact resistance: The impact resistance of the paint film was tested with a falling ball impact tester.
Anti-aging: The paint is sprayed on a metal aluminum plate, irradiated with strong light under a UV lamp until it is completely cured, and placed in a QUV weathering tester. After a period of time, the light loss of the coating film is detected.
1.6 Coatings and Film Performance Test results of coatings and their coating films are shown in Table 2.
2 Results and Discussion 2.1 Selection of Resins and Monomers In the design of hardware UV curing coating formulations, in addition to considering the overall curing and conversion rates, the choice of host resins and monomers should also be considered. Therefore, most of the formulations in Table 1 are selected. Acid oligomers. The main resin: with the most basic formula skeleton, to ensure the basic adhesion, stretch rate. Acrylated polyester, low viscosity, fast curing, mainly provides good flexibility and certain adhesion of the cured film. Diacrylate oligomers provide hardness, fast cure properties, and good adhesion, but their excessively high brittleness limits their use in formulations to high levels, and incomplete conversions are not possible in the system. The amount of monofunctional reactive diluent should not be too high, which will lead to excessive viscosity of the coating, so a large number of monomers capable of reducing the viscosity are selected in the formulation. On the one hand to reduce the viscosity, on the one hand to promote the leveling of the coating, get a more smooth surface of the cured film. Acrylate functional monomers and oligomers improve the wettability of the substrate and provide the formulation's adjustability (high solids content), reducing the viscosity of the formulation without affecting its performance. Trimethacrylate monomers are acidic monomers and ethyl acrylate is aggressive to the substrate and can increase adhesion. In the overall formulation, the trifunctional functional monomer has good compatibility with the acrylic prepolymer resin, has low viscosity, high curing speed, can increase the flexibility of the cured film, enhance adhesion, and give the film a good The abrasion resistance, hardness and gloss. **** Triacrylate is used to increase cross-link density, fast cure, good flexibility, and also acts as a reactive diluent. The hydroxyl-bearing multi-component acrylate itself has a higher viscosity, and the hydroxyl group can impart a certain adhesion promoting effect to the cured film. The multi-functional properties can make up for the lack of PUA polymerization activity, and increase the cross-link density of the cured film, and increase the sealing performance of the coating.
2.2 Application of CAB in Photocuring System There are different opinions on the application of CAB551-0.01 (cellulose acetate butyrate). In fact, CAB promotes the leveling process that takes place during solvent evaporation. After film formation, it is distributed on the surface of the film. It is mainly in the first half of the drying process. During the UV curing process after solvent evaporation, it must also be There must be a dry gradient, otherwise the problem of poor leveling due to shrinkage can easily occur. The use of CAB mainly focuses on its miscibility in UV coatings, followed by its own cross-linking double bonds, which can weakly participate in UV polymerization and form a whole with UV coatings.
2.3 The choice of photoinitiator Photoinitiator can absorb ultraviolet energy, produce free radicals, and then trigger oligomers and monomers to achieve polymerization, cross-linking, the system curing film. The choice of photoinitiator should be considered from the light-induced efficiency, price, pollution, toxicity, stability and other aspects. Hydrogen abstraction photoinitiator yellowing coefficient is relatively large, generally should be selected cracking photoinitiator. Using Irgacure 184 in the experiment can effectively improve the yellowing resistance of the coating. The amount of photoinitiator has a great influence on the curing performance and storage period of the coating. When the mass fraction is less than 2%, the curing rate is proportional to its mass fraction; when the mass fraction is greater than 2%, the curing rate has little relation with its mass fraction. However, when the mass fraction exceeds 4%, the shelf life of the coating decreases. Considering comprehensively, the photoinitiator mass fraction is suitable for 3%.
2.4 Selection of additives Leveling agents can effectively reduce the tension between the coating surface and components, improve the coating miscibility, increase the fluidity, ease of construction, make the coating surface smooth, flat, no pinholes, shrinkage holes, brush marks Surface defects such as orange peel. The addition of TegoRad 2200 leveling agent containing hydroxy polydimethylsiloxane to the formulation can effectively improve the smoothness, scratch resistance, flowability, leveling and non-stickiness of the paint film. Tests show that: 0.5% TegoRad2200, you can achieve good results.
2.5 Selection of diluent The inert solvent used in the photocurable coating should be a volatile solvent with a relatively low boiling point, which can be volatilized during the flash drying process of the wet film and cannot be easily taken into the cured film. Ethyl acrylate acts as a diluent for viscosity adjustment in this system. Use **** triacrylate as an oligomer cross-linker to increase cross-link density and speed up cure. The amount of diluent is adjusted according to the required viscosity within the appropriate range.
2.6 The most common problems of hardware UV-curable coatings The most common problem of hardware UV-curing coatings is that the paint film tends to cause pitting after drying. There are two main causes of pitting: First, the surface drive tension determined by the gel structure of the resin system ( Crystallization) and the other is the volatilization gradient and dissolution force control of the diluent. The matching of these two factors with the curing rate determines the appearance of pits. If the film thickness is higher, the better the leveling level is, the more the leveling times are. A relatively thick coating will have sufficient wetting and drying time, and the mutual solubility between the resins and the factors affecting the diluent. Reduction can significantly improve pitting. In general, the solid content is high and there are fewer pits. However, if thin coating is required, an issue must be made during the leveling time to improve the pitting problem, including the viscosity of the main resin (** low viscosity), the amount of initiator (as little as possible), the volatilization rate of the diluent The dosage and surface additives should be adjusted accordingly.
3 Conclusion Through experiments, selected acrylic esterified polyester, diacrylate oligomer, CAB551-0.01 resin as the main film to develop a UV curing coating for hardware substrates, under the irradiation of ultraviolet light can be quickly cured, with excellent Adhesion, high gloss, abrasion resistance and aging resistance, as well as good alcohol resistance, moist heat resistance and long-term storage stability, are widely used in the surface coating of automotive parts, equipment, metal plating and other substrates.
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