China Coatings Online News Information Abstract: Ultraviolet light (UV) curing powder coating combines the many advantages of traditional powder coating and radiation curing technology, is a forward-looking product of the coating industry. This paper introduces the preparation, curing process, coating process and potential application of UV-cured powder coating, and summarizes its latest research progress.
key words: powder coatings, radiation curing coatings, UV curing
in the last 20 years of the last century, the production of powder coatings increased rapidly. The world powder coatings market had a total output of 479,000t in 1995, with an average annual growth rate of 11%. The greatest advantages of powder coatings are environmental friendliness (no VOC emissions), good economy, high outdoor durability and other properties. However, there are also shortcomings in powder coatings: on the one hand, the curing temperature of the thermoclytic resin used is high (180 to 220 degrees C), the curing time is long (10 to 30min), which limits its use only for metals and other heat-resistant base, and relatively large energy consumption, time-saving. On the other hand, the melting leveling and curing start stage of the thermo-cured powder coating have some overlap, so that if the coating is not well mastered, the coating will appear on the flatness defects, such as shrink holes, orange peel and so on.
UV-cured liquid coating is an environmentally friendly and energy-saving coating developed in the 1960s. Because it has no or low VOC emissions, energy saving (energy consumption is only 1/5 to 1/10 of thermoclysting coating), fast curing speed (0.1 to 10s), high production efficiency, suitable for pipeline production, low curing temperature, suitable for applying thermal substrates and other advantages, the development is very rapid. However, the deficiency of UV-cured liquid coating is that the coating preparation process must be added to the active monomorphic thinner to regulate the viscosity of the coating, the active monomorphic thinner will partially penetrate the substrate, the environment may still have a certain degree of pollution.
UV-cured powder coating (UV-cured powder coating) is a new technology that combines traditional powder coatings with UV-curing technology. The biggest feature of UV cured powder coating is that the process is divided into two obvious stages, the coating will not occur in the melting level phase of the early curing of resin, thus providing ample time for the coating to fully level and remove bubble operation, which fundamentally overcomes the persistent disease of thermoclytic powder coating, but also eliminates the deficiency of UV-cured liquid coating. The use of UV curing technology can significantly reduce the temperature of the heating and curing process (120 to 140 degrees C), improve production efficiency, not only save more resources, energy, manpower, time and space, but also avoid excessive heating of substrates, open up a broader range of applications of powder coatings, such as wood, plastics, paper, thermal alloys and metal components containing thermal parts.
1, UV curing powder coating raw material selection
UV curing powder coating formula consists of photo-curing resin, light trigger, curing agent, pigment, filler, additive (including leveling agent, desmoulizer, anti-lighting agent, lightener, modifier, promoter, etc.). Among them, the selection of main resin and light trigger is particularly important.
resin is the main film-forming substance of UV-cured powder coating, and it is the main component that determines the properties of coating and coating properties. The preparation of powder coatings that can achieve low temperature UV curing requires, on the one hand, that the resin can give the powder good storage stability, that is, the powder must be stored for 3 to 6 months without cladding under conditions of up to 40 degrees C, and on the other hand, the raw materials used must have a lower melting viscosity at lower temperatures (e.g. 100 degrees C or lower) to ensure that the coating has good flow and leveling properties before and during the photo-curing process, followed by a photo-curing reaction at less than 120 degrees C. This requires that the Tg of the selected resin should be 50 to 70 degrees C (at least above 40 degrees C), the average molecular weight is 1000 to 4000, and the molecular weight distribution is narrow. It is not easy to obtain such a resin, Tg above 50 degrees C resin melting is difficult to control, because CC double-bonds can start polymerization at 80 degrees C, and below 80 degrees C its viscosity is too high to handle. If the solvent is used during the synthesis process, it is difficult to remove the solvent so that its content in the powder is less than 0.3% to prevent powder lumps. Therefore, conventional thermooenstic resins do not meet the requirements of UV powder coatings. A common way to reduce the melting temperature of resins is to synthesize semicrystalline resins, add crystalline compounds, or amorphous lymers. The preparation of low-temperature cured UV powder resin is also a feasible method through polymer structure design, synthetic branch-like and overspending semi-crystalline polymers, which has recently attracted much attention.
in UV-cured powder coating formulations, try to avoid the use of curing agents, thus avoiding early curing during the coating melt leveling phase. However, in order to improve the performance of the coating film, a small amount of curing agent is sometimes added, but the amount added is much smaller than the thermoclytic powder coating.
is an essential component in UV-cured powder coating formulations. According to the different light-causing factors, the light-triggering agents used in UV-cured powder coatings are mainly free-based light-triggers and cation light-triggering agents, and now a variety of light-triggering agents are available.
2, powder coating UV curing process
UV curing powder coating photo-curing process has free-form-triggered polymerization and cation-triggered polymerization, both of which have their advantages and disadvantages. The advantages of free-based polymerization reaction are that water has no blocking effect on the system and the curing speed is fast, the disadvantage is that wrinkle is obvious and oxygen has a blocking effect on the reaction;
of solid BPA epoxy and vinyl ether resins can be achieved by cation polymerization. However, in most cases at present, the photo-polymerization of UV powder coatings is still free-based polymerization, such as methyl acrylate polyester system, unsaturated polyester/polyurethane acrylates system, unsaturated polyester/vinyl ether system.
3, UV curing powder coating manufacturing and coating process
UV curing powder coating manufacturing and spraying process and thermocly curing powder coating is basically similar. Since UV powder coatings require leveling at relatively low temperatures, their melting viscosity is much lower than conventional thermocly cured powder coatings, so extrusion operations are appropriate at 70 degrees C. UV-cured powder coating is generally sprayed with electrostation powder. Static powder spraying method can be divided into high-voltage electrostation powder spraying method and friction static powder spraying method. The electrostational spraying equipment used has a corona discharge electrostation powder spray gun and a friction-charged electrostation powder spray gun. The heating leveling process for powder coatings is usually done in the oven. The oven is heated by hot air, infrared and infrared heating. After the coating melts flat, the coated object shines directly under the UV lamp and cures into a film.
reports of UV-cured powder coatings began in the mid-1970s, but did not attract much attention. It was not until the mid-1990s that reports emerged in large numbers, but mostly in patents. According to recent reports, UV powder coatings in foreign countries have been in the early stages of market development, but the domestic is still in the initial stage, the following classification is introduced.
4.1 acrylic system
Johansson and others report on a UV powder coating system that can be cured both at low temperatures (80 to 110 degrees C) and light-cured at lower temperatures. This low temperature curing system consists of amorphous methyl acrylates A1 and crystalline compounds C1 and C2. Analytic resin A1 is synthesized by a two-step reaction, starting with the preparation of hydroxymethyl hydromethyl formal grouping polymethyl acrylates by free radical polymerization, hydroxyl hydroxyl formal grouping of polymethyl acrylates and then reacting with methyl acrylamide chlorine. C1 is bipropylate (mp-106-C) and C2 is biphenol-A bimethyl acrylic (mp-72-74 degrees C). The system uses benzoyl peroxide (BPO) as a thermo-curing trigger, and lucirin8728 as a photo-curing trigger.
study found that the viscosity-temperature relationship of the resulting hybrid system is suitable for low temperature cured powder coating system by mixing a crystalline monosome with an amorphous resin. Compared with UV curing and thermocuring, UV curing reaction speed is fast, significantly reducing the effect of oxygen blocking on the reaction, because UV curing occurs after full leveling, the system is easier to control accurately and the coating performance is better.
4.2 Unsaturated Polyester System
4.2.1 Pink of Hirst, Germany, et al. have developed a UV-cured powder coating system that can be used on thermal substrates such as medium density fiberboard (MDF) and thermoplastics. This system is composed of solid unsaturated polyester resin as the main resin, solid polyurethane acrylates as a curing agent, the performance of the two is listed in Table 1. The photo-curing agent is free-based polymerization, and the light trigger used is "Irgacure651". The results found that the performance of the coating is influenced by the composition of the formula, the particle size of the powder, the intensity of the infrared light source, the distance between the light source and the substrate, the time of exposure, the intensity of the ultraviolet light source and the time of exposure. If well-provisioned and the paint performs well, this system is suitable for thermally sensitive substrates such as MDF and thermoplastics.
4.2.2UCB successfully developed the "UvecoatTM" series of UV-cured powder coating resins, Buysens and Zune proposed UV powder coating formulations. The chemical composition of the "UvecoatTM" resin is polyester containing (methyl) acrylic double bonds, which are cured into film by a free-form polymerization process. The "UvecoatTM1000" resin is suitable for woody foundations such as MDF, while the "UvecoatTM2000" resin is suitable for coated metal substrates. The light triggers used in the formulation are "Irgacure651" or "Irgacure2959" and "Irgacure819". Coatings based on the "UvecoatTM2000" resin have excellent storage stability at 35 degrees C, and the formulation of the "UvecoatTM2100 and 2200" resins can be stored at 38 degrees C. The preparation of the coating is made by melting and mixing, and the melt extrusion temperature is 65 to 85 degrees C. The powder coating is sprayed on the surface of the object with an electrostectant spray gun and the thickness of the coating is 50 to 90 m. The powder coating is heated by using medium-wave infrared light source combined with the oven, and the melting level temperature is 90 to 110 degrees C. The melted flat film layer uses two 1600W/cm mercury lamps to cure them at a speed of 3m/min.
coating performance tests show that MDF and metal substrates are coated with UV-cured powder coatings made of "UvecoatTM" resins, which are very adhesive, flexible, corrosion-resistant and weather resistant.
4.2.3Daly has developed a fast cryogenic heat, UV dual-cured powder coating for thermal substrates. The biggest feature of this technique is the addition of both heat and light triggers to the formulation, which cure the surface of the coating film, while the thermal trigger promotes rapid and complete curing inside the coating, which is particularly effective for pigment-containing coatings and thicker coatings. It is said that light, heat double curing not only will not bring bubbles, erudicoes and other ills to the coating film, coating episodural performance and mechanical performance are very good. Unsaturated polyester resin as the main resin of the formula, vinyl ether polyurethane as the crosslinking agent of the main resin. The light triggers used in the formulation are Lucerin TPO and Irgacure184, and the thermal trigger is the peroxide Lupersol231WL.
4.3 Malayate/Vinyl Ether
Witte reports on a UV-curable powder coating system. The system is based on a Malay ester (MA)/vinyl ether (VE) non-acrylic UV curing system developed by DSM Resin. The system is actually made up of two polymers, unsaturated polyester containing Malay acid or fumaric acid and polyurethane containing unsaturated groups of vinyl ethers. The system is UV-cured, i.e. the electron-rich vinyl ether group and the electron-deficulated Malayate or fumarate group occur 1/1 copolymer. Understandably, there is a chemical measurement balance of Malay esters and vinyl ether groups in the system.
because the MA/VE system is solid at room temperature, light triggers, colors and additives are added to the formulation, powder coatings can be prepared using traditional powder processing processes. The powder is electrostatized by a friction spray gun applied to the substrate surface, and the coating is heated by IR 30s to fully melt the level, which is then cured into a film within 5s under UV exposure. The research found that the advantages of the system is that the system does not occur ionization under control conditions, the flat performance of the coating is very good, the viscosity at 100 degrees C is only 26Pa.s, even if the temperature is as high as 200 degrees C viscosity will not increase;
a UV-cured powder coating for thermal substrates. Powder coating formulations are based on cation catalytic resins (mainly epoxy resins), including bisphenol resins, line-type phenolic modified bisphenol resins, lipid epoxy compounds, including hydrogenated bisphenol-A, shrinking glycelin ether-based methyl acrylic lipids, shrunk glycerol ether acrylic or related compounds, vinyl ethers and mixed systems of the above compounds. Light triggers are sulfur salt, aromatic iron salt, iodized salt, triphenol-based light trigger and related compounds. Light shields, leveling agents, plasticizers, charging additives and degassing agents are also added to the formulation. Light triggers need to be solidified before use, with hydroxyl polyester added to promote curing and improve film performance.
4.5 overspending polyester acrylic system
Johansson, etc., synthesizes semi-crystalline cryogenic low-temperature cured powder coating resins based on overspending polyester. The resin is synthesized with ε-tetryl-gylitol branching hydroxyl-phenolic overspending polyester to obtain a semi-crystalline polymer. The crystallization and fluidization properties of the polymer can be adjusted by the selection of the overspending resin and the polymerization of the joining chain, and then the hydroxyl polymer at the end of methyl acrylic seal is used, and finally the UV-cured resin is obtained. The analysis shows that the fluidization performance of the synthetic resin meets the requirements of low temperature cured powder coating, and only the unsaturated group of traces remains after the resin cures. The performance of curing coating is mainly determined by the structure of overspending polyester resin, when the branch chain is long, the branch chain can crystallize in the network, generating a semi-crystalline network structure, and when the branch chain is short, there is no crystallization.