What is water-based UV resin?

In the late 1960s, ultraviolet (UV) curing technology was developed and applied to the coating resin industry as a new type of green technology. 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 main 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 is quickly cross-linked and cured. After the UV coating is cured, the basic properties of the coating film depend to a large extent on its main film-forming material—UV resin, and the properties of the UV resin are determined by the macromolecular polymer that constitutes the resin, the molecular structure, molecular weight, and molecular weight of the polymer. Double bond density and glass transition temperature will affect the properties of the resin. Traditional oil-based UV resins have large molecular weight and high viscosity, and are insufficient in coating process and paint film performance control. Acrylate reactive diluents contain unsaturated double bonds and have low viscosity. When added to UV curing systems, they can reduce the viscosity of the resin, increase the crosslinking density of the resin, and improve the film properties of the resin, so they are widely used. However, most active diluents are toxic and irritating to human skin, mucous membranes and eyes. In addition, it is difficult for diluents to react completely during UV irradiation, and residual monomers will directly affect the long-term performance of the cured film. performance, which limits its application in packaging materials for food hygiene products.

Water-based UV resin inherits and develops the characteristics of both traditional UV coatings and water-based coatings, and has the advantages of safety and environmental protection, energy saving and high efficiency, adjustable viscosity, thin coating coating, and low cost. In particular, water-based UV resin is a high-molecular-weight water-based dispersion whose 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 developed rapidly and has become a major direction of coating development.

1. Types of water-based UV

Water-based UV resins refer to UV resins that are soluble or dispersible in water, and contain a certain amount of hydrophilic groups such as carboxyl, hydroxyl, amino, ether or amide groups in the molecule, as well as acryl, methacryloyl or alkene groups. Unsaturated groups such as propyl. At present, waterborne UV resins mainly include waterborne polyacrylate, waterborne polyester acrylate, waterborne epoxy acrylate and waterborne polyurethane acrylate.

①Water-based polyacrylate

Water-based polyacrylate is cheap, 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 combination to improve some properties of photocurable coatings and inks. Water-based polyacrylates are generally first polymerized by acrylic acid and various acrylates, and part of the carboxyl groups introduced by acrylic acid react with the hydroxyl groups of hydroxyethyl acrylate or the epoxy groups of glycidyl methacrylate, thereby introducing photoactive carbon-carbon bicarbonate. bond, and then salify the carboxyl group with an organic amine.

②Water-based polyester acrylate

Water-based polyester acrylate is easy to prepare, cheap, and has a full paint film, good gloss, good softness, but poor yellowing resistance. Generally, diol and trimellitic anhydride (or pyromellitic dianhydride) are used. Reaction, esterification reaction with acrylic acid, introduction of carboxyl group, and neutralization with amine to form a salt.
③Water-based epoxy acrylate

Waterborne epoxy acrylate has the advantages of low price, high hardness of coating film, good adhesion, high gloss and good chemical resistance, but it also has the shortcomings of traditional bisphenol A epoxy resin such as brittleness and poor yellowing resistance. . Many scholars choose aliphatic epoxy resins with excellent physical and mechanical properties and excellent anti-yellowing properties to replace traditional bisphenol A epoxy resins as the matrix of waterborne UV epoxy acrylates, which greatly improves the overall performance of the resins. Generally, acrylic acid is used to esterify epoxy resin to obtain epoxy acrylate (EA), and the hydroxyl group in epoxy acrylate is reacted with acid anhydride (such as maleic anhydride, trimellitic anhydride, etc.) to introduce hydrophilic group, and then neutralized with organic amine Obtain water-based epoxy acrylate resin (EB),

④Water-based urethane acrylate

Waterborne polyurethane acrylate light-curing system has attracted much attention due to its good wear resistance, chemical resistance, low temperature resistance and flexibility. It is currently the most researched and commercialized waterborne UV resin. See Table 1. In recent years, some foreign companies, such as Bayer, AKZONOBEL, BASF, etc., have made great breakthroughs in the performance improvement of water-based UV urethane acrylates. Such as automotive primers, topcoats and finish varnishes.

Using diisocyanate as raw material, polyester or polyether diol as soft segment chain extender, carboxyl-containing diol (such as dimethylol propionic acid) as hydrophilic chain extender, hydroxy acrylate as end-capping agent, through Multi-step polycondensation can produce curable urethane acrylate, and then neutralize it with ammonia or organic amine to form a salt to obtain water-based UV urethane acrylate (WPUA).

2. New progress of water-based 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 the molecular chains are not entangled. Hyperbranched polymers have the advantages of easy solubility, low melting point, low viscosity, and high reactivity. Therefore, acryl groups and hydrophilic groups can be introduced to synthesize water-based photocurable oligomers, which opens up a new way for the preparation of water-based UV resins. .
Asif et al. used the hyperbranched polyester BoltornTMHn rich in terminal hydroxyl groups to react with succinic anhydride and IPDI-HEA prepolymer, and then neutralized with organic amine to form a salt to obtain a UV-curable water-based hyperbranched polyester (WHPUA), As shown in Figure 3. The research shows that the resin has a rapid photocuring rate and good physical properties. With the increase of the hard segment (IPDIHEA) content, the glass transition temperature of the resin increases, and 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 first prepare hyperbranched polyesters, which were further reacted with the terminal hydroxyl and carboxyl groups of hyperbranched polymers by introducing glycidyl methacrylate (GMA), and then adding triglycerides. Ethylamine (TEA) is neutralized into a salt to obtain a UV-curable water-based hyperbranched polyester. The results show that the more terminal carboxyl groups in 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-curable organic/inorganic hybrid system is an effective composite of water-based UV resin and inorganic materials. The advantages of high wear resistance and high weather resistance of inorganic materials are introduced into the resin to improve the comprehensive performance of the cured film. By introducing inorganic particles such as nano-SiO2 or montmorillonite into the UV curing system by direct dispersion method, sol-gel method or intercalation method, the photocurable organic/inorganic hybrid system can be prepared. Monomers are incorporated into the molecular chains of aqueous UV oligomers.
Zhan Chuyin et al. used di-hydroxybutyl polydimethylsiloxane (PDMS) to introduce polysiloxane groups into the soft segment of polyurethane, and diluted them appropriately with acrylic monomers to obtain an organic/inorganic hybrid emulsion (Si- PUA). After the resin-made coating is cured, the paint film has good physical properties, high contact angle and water resistance.

Liang Hongbo et al. 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 was then hydrolyzed with ethyl orthosilicate and n-butyl titanate in different proportions to prepare a SiO2/TiO2 organic-inorganic hybrid sol of photocurable hyperbranched polyurethane. The results show that with the increase of inorganic content, the pendulum hardness of the hybrid coating increases, the surface roughness increases, and 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 and 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 and other curing systems. Light curing/thermal curing, photocuring/redox curing, free radical photocuring/cationic photocuring and photocuring/moisture curing are common dual-curing systems, and some systems have been applied, such as UV electronic protective adhesive is a A light-curing/redox or light-curing/moisture-curing dual cure system.

Zeng Fanchu et al. introduced functional monomer acetoacetoxyethyl methacrylate (AMME) into polyacrylic acid emulsion, and introduced photocurable group through Michael addition reaction at low temperature to synthesize thermal curing/UV curing waterborne polyacrylate. Dry at a constant temperature of 60 °C, 2 × 5. Under the irradiation of a 6 kW high-pressure mercury lamp, the hardness of the resin after film formation reaches 3H, the resistance to alcohol wiping is up to 158 times, and the alkali resistance is up to 24 hours.

④Epoxy acrylate/urethane acrylate composite system

Epoxy acrylate coating 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 poor weather resistance. The effective compounding of the two resins by chemical modification, physical blending or hybridization can improve the performance of a single resin and give full play to the advantages of both, thereby developing a high-performance photocuring system that combines the advantages of both.

Wang Cundong et al. first used acrylic acid to esterify the epoxy group in epoxy resin E44 to obtain EA; then used TDI, polytetrahydrofuran diol (PTMG), DMPA and HEMA to synthesize water-based UV urethane acrylate; Mixing in different proportions, water/ethanol as initiator, water-based polyurethane anionic water-based polyurethane acrylate as emulsifier, UV-curable epoxy acrylate/polyurethane acrylate composite emulsion is obtained by emulsification. The results show that the modification greatly improves the flexibility of the coating film, but has little effect on other properties.

⑤ Macromolecular or polymerizable photoinitiator

Most photoinitiators are small molecules of aryl alkyl ketones, which cannot be completely decomposed after light curing, and the residual small molecules or photolysis products will migrate to the surface of the coating, causing yellowing or odor, affecting the performance of the cured film and its application. . The researchers synthesized water-based macromolecular polymerizable photoinitiators by introducing photoinitiating groups, acryl groups and hydrophilic groups into hyperbranched polymers to overcome the disadvantages of small molecular photoinitiators. Wang Zhansi of Anhui University of Science and Technology first used methyl acrylate and diethanolamine as raw materials to react to synthesize an AB2-type monomer MB, and then reacted with trimethylolpropane (TMP) as the core to synthesize hydroxyl-terminated hyperbranched polyurethane, and then used Maleic anhydride was modified into hyperbranched polyurethane containing terminal carboxyl groups, and after Z, the photoinitiator 1173 was used to modify the terminal carboxyl hyperbranched polyurethane to prepare two polymerizable hyperbranched macromolecular photoinitiators HPAE-1- MA-1173 and HPAE-2-MA-1173. The research results show that the UV absorption of the product has a red shift of the maximum absorption compared with 1173, but the photoinitiator rate is lower than that of the molecular photoinitiator 1173.

3. Application of water-based UV resin

With the improvement of people’s awareness of environmental protection, water-based photocurable systems have received more and more attention in recent years, but there are few researches on their application. 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 metal paint, water-based UV flexo printing ink, water-based UV gravure ink, water-based screen printing ink, etc. Water-based UV paper varnish, including water-based UV varnish and water-based UV primer, is the earliest water-based UV coating applied, with a glossiness of over 90. The application value of water-based UV coatings in the wood finishing industry is very high, especially in the coating of formed wood and plywood. Therefore, water-based UV wood coatings are also the most commonly used water-based UV coatings at present. 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 the in-depth study of water-based photocurable 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 resin is still in the research and development stage. Although there are many relevant literature reports, few products have actually been put into 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. Waterborne 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 substrates, poor water resistance, poor washing resistance, and poor storage stability, as well as residual small-molecule photoinitiators and photolysis products during the photocuring process, which need to be further Improve. Therefore, it is imperative to develop water-based UV resin technology to overcome the current shortcomings of water-based UV resin and develop a water-based photocuring system with better performance and wider application.