In the late 1960s, ultraviolet (UV) curing technology, as a new type of green technology, was developed and applied to the coating resin industry. The earliest UV-curable coatings were born in Bayer, Germany. my country began to enter the field of photo-curable coatings in the 1970s and has developed and applied rapidly in recent years. UV resin is the most important component of the light curing system. It is an oligomer that can undergo physical and chemical changes in a short period of time after being irradiated by ultraviolet light, and quickly crosslink and cure. After the UV coating is cured, the basic performance of the coating depends to a large extent on its main film-forming material—UV resin, and what determines the performance of the UV resin is the high molecular polymer that constitutes the resin, the molecular structure, molecular weight, The double bond density and glass transition temperature will affect the performance of the resin. Traditional oil-based UV resins have large molecular weights and high viscosity, which are insufficient in coating process and paint film performance control. Acrylic reactive diluents contain unsaturated double bonds and have low viscosity. Adding to the UV curing system can reduce the viscosity of the resin and increase the crosslinking density of the resin, and improve the coating performance of the resin, so it is widely used. However, most active diluents are toxic and have irritating effects on human skin, mucous membranes and eyes of water-based UV resin manufacturers. In addition, diluents are difficult to react completely during UV irradiation, and residual monomers will directly affect the long-term curing film. Performance, which limits its application in food hygiene product packaging materials.
Water-based UV resin inherits and develops the characteristics of both traditional UV coatings and water-based coatings, and has the advantages of safety, environmental protection, energy saving, high efficiency, adjustable viscosity, thin coating coating, and low cost. In particular, the water-based UV resin is a high-molecular-weight water-based dispersion, and its viscosity can be adjusted by water, thereby avoiding the harm of reactive diluents and solving the contradiction between the hardness and flexibility of traditional UV coatings. In the past ten years, this kind of coating has been developed rapidly and has become a main direction of coating development.
1. Types of water-based UV
Water-based UV resin refers to a water-soluble or water-dispersible UV resin. The molecule contains a certain amount of hydrophilic groups such as carboxyl, hydroxyl, amino, ether or amide, as well as acryl, methacryl or alkene. Unsaturated groups such as propyl group. Currently, water-based UV resins mainly include water-based polyacrylate, water-based polyester acrylate, water-based epoxy acrylate, and water-based polyurethane acrylate.
①Waterborne polyacrylate
Water-based polyacrylate is inexpensive, has good yellowing resistance, and has good adhesion to various substrates, but has low mechanical strength and hardness, and poor acid and alkali resistance. Therefore, water-based polyacrylate is generally not used as the main resin in practical applications, and is only used in conjunction to improve certain properties of light-cured coatings and inks. Water-based polyacrylates are generally polymerized by acrylic acid and various acrylates. Part of the carboxyl groups introduced by acrylic acid react with the hydroxyl groups of hydroxyethyl acrylate or the epoxy groups of glycidyl methacrylate to introduce photoactive carbon-carbon doubles. Bond, and then use organic amine to salt the carboxyl group.
②Waterborne polyester acrylate
Water-based polyester acrylate is easy to prepare, low in price, and has full paint film, good gloss, good flexibility, but poor yellowing resistance. Generally, diol and trimellitic anhydride (or pyromellitic dianhydride) are used. Reaction, then esterification reaction with acrylic acid, introducing carboxyl group, and finally neutralizing with amine to form a salt. ③Waterborne epoxy acrylate
Waterborne epoxy acrylate has the advantages of low price, high film hardness, good adhesion, high gloss and good chemical resistance, but it also has the shortcomings of traditional bisphenol A epoxy resins such as brittleness and poor yellowing resistance. . Many scholars choose aliphatic epoxy resins with excellent physical and mechanical properties and yellowing resistance to replace traditional bisphenol A epoxy resins as the matrix of waterborne UV epoxy acrylates, which greatly improves the overall performance of the resin. Generally, acrylic acid is used to esterify epoxy resin to obtain epoxy acrylate (EA). The hydroxyl group in epoxy acrylate is reacted with acid anhydride (such as maleic anhydride, trimellitic anhydride, etc.) to introduce hydrophilic groups, and then neutralized with organic amine Obtain water-based epoxy acrylate resin (EB),
④Waterborne polyurethane acrylate
The water-based polyurethane acrylate light-curing system has attracted much attention due to its good abrasion resistance, chemical resistance, low temperature resistance and flexibility. It is currently the most researched and commercialized water-based UV resin, see table 1. In recent years, some foreign companies, such as Bayer, AKZONOBEL, BASF, etc., have made great breakthroughs in the improvement of the performance of water-based UV polyurethane acrylates. The performance of some products meets the requirements of automotive coatings and is used in various automotive coatings. Such as car primers, topcoats and varnishes.
Using diisocyanate as raw material, polyester or polyether diol as soft segment chain extender, carboxyl-containing diol (such as dimethylolpropionic acid) as hydrophilic chain extender, and hydroxy acrylate as blocking agent. Multi-step polycondensation reaction can produce curable urethane acrylate, which is then neutralized with ammonia or organic amine to form a salt to obtain water-based UV urethane acrylate (WPUA).
2.New Development of Waterborne UV Resin
①Hyperbranched system
As a new type of polymer, hyperbranched polymer has a spherical structure with a large number of active end groups and no entanglement between molecular chains. Hyperbranched polymers have the advantages of easy solubility, low melting point, low viscosity, high reactivity, etc. Therefore, acryl groups and hydrophilic groups can be introduced to synthesize water-based light-curing oligomers, which opens up a new way for the preparation of water-based UV resins . Asif et al. used a hyperbranched polyester with rich terminal hydroxyl groups, BoltornTMHn, to react with succinic anhydride and IPDI-HEA prepolymer, and finally neutralized with an organic amine to form a salt to obtain a UV-curable water-based hyperbranched polyester (WHPUA), such as As shown in Figure 3. Studies have shown that the light curing rate of the resin is rapid and the physical properties are better. With the increase of the hard segment (IPDIHEA) content, the glass transition temperature of the resin increases, the hardness and tensile strength also increase, but the elongation at break decreases.
Su Lin et al. used polybasic acid anhydrides and monofunctional epoxides as raw materials to prepare hyperbranched polyesters, and further reacted with the terminal hydroxyl and carboxyl groups of hyperbranched polymers by introducing glycidyl methacrylate (GMA), and finally added triethyl The amine (TEA) is neutralized to form a salt to obtain a UV-curable water-based hyperbranched polyester. The results show that the more terminal carboxyl groups of the water-based hyperbranched resin, the better the water solubility; the curing rate of the resin increases with the increase of terminal double bonds.
②Organic/inorganic hybrid system
The water-based UV light-curing organic/inorganic hybrid system is an effective combination of water-based UV resins and inorganic materials. It introduces the high wear resistance and high weather resistance of inorganic materials into the resin to improve the overall performance of the cured film. By direct dispersion method, sol-gel method or intercalation method, etc., inorganic particles such as nano SiO2 or montmorillonite are introduced into the UV curing system to obtain a light-curable organic/inorganic hybrid system. In addition, organic silicon The monomer is introduced into the molecular chain of the waterborne UV oligomer. Zhan Chuyin et al. used di-terminated hydroxybutyl polydimethylsiloxane (PDMS) to introduce polysiloxane groups into the soft polyurethane segment, and diluted it with acrylic monomers to obtain an organic/inorganic hybrid emulsion (Si- PUA). After the resin is formulated into the coating and cured, the paint film has good physical properties, and has high contact angle and water resistance.
Liang Hongbo and others used self-made polyhydroxy hyperbranched polyurethane, succinic anhydride, silane coupling agent KH560, glycidyl methacrylate (GMA) and hydroxyethyl methacrylate as raw materials to prepare hyperbranched hybrid polyurethane and light curing The hyperbranched polyurethane is then blended with ethyl orthosilicate and n-butyl titanate in different proportions to hydrolyze to prepare a light-cured hyperbranched polyurethane SiO2/TiO2 organic-inorganic hybrid sol. The results show that with the increase of inorganic content, the pendulum hardness of the hybrid coating increases, and the surface roughness becomes larger. The surface quality of the SiO2 hybrid coating is better than that of the TiO2 hybrid coating.
③Dual curing system
In order to solve the shortcomings of difficult three-dimensional curing of water-based UV resins, difficult curing of thick coatings and colored systems, and to improve the overall performance of the coating film, researchers have developed a dual curing system that combines light curing with other curing systems. Light curing/thermal curing, light curing/redox curing, free radical light curing/cationic light curing and light curing/moisture curing are common dual curing systems at present, and some systems have been applied, such as UV electronic protective glue. A dual curing system of light curing/redox or light curing/moisture curing.
Zeng Fanchu introduced the functional monomer acetoacetoxyethyl methacrylate (AMME) into the polyacrylic emulsion, and introduced the photo-curing group through Michael addition reaction at low temperature to synthesize the heat-curing/UV-curing water-based polyacrylate. Drying at a constant temperature of 60 ℃, 2 × 5. Under the condition of a 6 kW high-pressure mercury lamp, the hardness of the resin after film formation reaches 3H, the resistance to alcohol wipes reaches 158 times, and the resistance to alkali reaches 24 hours.
④Epoxy acrylate/urethane acrylate composite system
Epoxy acrylate coating film has the advantages of high hardness, good adhesion, high gloss and good chemical resistance, but it has poor flexibility and high brittleness. Waterborne polyurethane acrylate has the characteristics of good abrasion resistance and flexibility, but it has poor weather resistance. Using chemical modification, physical blending or hybridization to effectively compound the two resins can improve the performance of a single resin and give play to the advantages of both, thereby developing a high-performance light-curing system with both advantages.
Wang Cundong and others first esterified the epoxy group in the epoxy resin E44 with acrylic acid to obtain EA; then used TDI, polytetrahydrofurandiol (PTMG), DMPA, and HEMA to synthesize water-based UV polyurethane acrylate; finally, the two were different Proportion mixing, water/ethanol as initiator, waterborne polyurethane anionic waterborne urethane acrylate as emulsifier, and emulsification to obtain a UV curable epoxy acrylate/urethane acrylate composite emulsion. The results show that the modification greatly improves the flexibility of the coating film, but has little effect on other properties.
⑤Macromolecular or polymerizable photoinitiator
Most photoinitiators are small aryl alkyl ketone molecules, which cannot be completely decomposed after light curing. Residual small molecules or photolysis products will migrate to the surface of the coating, causing yellowing or odor, affecting the performance and application of the cured film . Researchers have synthesized water-based macromolecular polymerizable photoinitiators by introducing photoinitiator groups, acryl groups and hydrophilic groups into hyperbranched polymers, which overcomes the disadvantages of small molecule photoinitiators. Wang Zhansi of Anhui University of Science and Technology and others first used methyl acrylate and diethanolamine as raw materials to synthesize an AB2 monomer MB, and then react with trimethylolpropane (TMP) as the core to synthesize hydroxyl-terminated hyperbranched polyurethane, and then use Maleic anhydride was modified into a hyperbranched polyurethane containing terminal carboxyl groups, and finally the carboxyl-terminated hyperbranched polyurethane ester was modified by photoinitiator 1173 to prepare two polymerizable hyperbranched macromolecular photoinitiators HPAE-1-MA -1173 and HPAE-2-MA-1173. The results of the study show that the ultraviolet absorption of the product produces a maximum absorption red shift than 1173, but the photoinitiation rate is lower. The molecular photoinitiator 1173 is slower.
3.Application of water-based UV resin
With the improvement of people’s awareness of environmental protection, water-based light curing systems have received more and more attention in recent years, but there are few researches on their applications. At present, water-based UV resins are mainly used in UV coatings and UV inks, including water-based UV paper varnish, water-based UV wood paint, water-based UV metallic paint, water-based UV flexo printing ink, water-based UV gravure ink, water-based screen printing ink, etc. Water-based UV paper varnishes, including water-based UV varnishes and water-based UV primers, are the first water-based UV coatings to be applied, and the gloss can reach more than 90. Water-based UV coatings have a high application value in the wood finishing industry, especially in the coating of shaped wood and plywood. Therefore, water-based UV wood paints are also the most commonly used water-based UV coatings. At present, some water-based UV resin products developed by a few developed countries meet the requirements of automotive coatings, and are also used in various automotive coatings, such as automotive primers, topcoats, and varnishes. With people’s in-depth research on water-based light curing systems, there will be more types of water-based UV resins, and the application fields will continue to expand.
4.Conclusion and outlook
Water-based UV resins are still in the research and development stage. Although there are many related literature reports, few products are actually put on the market. They are mainly produced by developed countries such as Europe and the United States, such as UCB, ICI, CYTEC, BASF and other companies. Water-based UV resin has the advantages of environmental protection, energy saving, high efficiency, controllable viscosity, and excellent film performance. It can take into account the hardness and flexibility of the cured film, and has extremely high application value and broad market prospects. However, water-based UV resins have defects such as poor wettability to the substrate, poor water resistance, poor washing resistance, and poor storage stability. There are also small molecule photoinitiators and photolysis products that will remain during the photocuring process. These all need to be further Improve. Therefore, to overcome the current shortcomings of water-based UV resins and develop a water-based photocuring system with better performance and wider application is the top priority for the development of water-based UV resin technology.