UV curing coating composition and performanceread count [649] release time：2019-12-27 17:06:00
UV coatings refer to coatings that are cured using UV radiation. UV curing Coatings can be applied with ink printing and exposed to UV radiation. Its solid content can be as high as 100%, so it has no volatile components and does not pollute the environment. The high solids content also enables application in very thin films. With the in-depth research on oligomers, reactive diluents, photoinitiators, additives, pigments and fillers, etc. enter,New materials are constantly emerging,Liquid UV curable coatings will contribute to various aspects of traditional coatings answer Rapid expansion of application fields,An overview of the research on its components is described below.
1. Oligomers for UV curing coatings
Oligomers, also called oligomers or prepolymers, are the matrix resin of UV-cured coatings. As the skeleton, they occupy a large proportion in the UV-cured coating system and play a decisive role in the basic properties of the system (including adhesion, hardness, flexibility, wear resistance, heat resistance, chemical resistance, durability, optical properties and aging resistance, etc.). The types of oligomers that are widely researched and used in UV curing coatings mainly include unsaturated polyester, epoxy acrylate and polyurethane acrylate.
1.1 Unsaturated polyester
Unsaturated polyester (UPE) was first used in UV curing coatings
Oligomers refer to linear or branched polyester macromolecules containing reactive =C double bonds in the molecule. Under the trigger of active free radicals, active vinyl and other monomers copolymerize with UPE, which can cross-link and solidify the network structure. However, the polymerization reaction time is long, the temperature is high, and the oxygen inhibition phenomenon during the polymerization process is serious, which increases the yellowing of the coating film, so its application is subject to certain limitations. Using hyperbranching technology to prepare multifunctional unsaturated polyesters is one direction to solve the problem. Hyperbranched oligomers have a unique three-dimensional molecular structure, which makes them have good compatibility, low viscosity and high reactivity. The shrinkage rate of the cured film becomes smaller, it has good substrate adhesion properties, and it can avoid the use of volatile reactive diluents, so it is more environmentally friendly.
1.2 Epoxy acrylate
Epoxy acrylate (EA) is composed of epoxy resin and acrylic or methacrylic
Acrylic acid is obtained by ring-opening esterification under the action of a catalyst. According to the structure type, it can be divided into bisphenol A epoxy acrylate, phenolic epoxy acrylate, modified epoxy acrylate and epoxidized oil acrylate. EA light-curing reaction rate is fast, the cured film has good adhesion, hardness, strength, gloss and chemical resistance, and the price is low. It is the most consumed light-curing oligomer in the light-curing industry. EA has high viscosity, which affects construction and leveling. The cured film is brittle, has poor flexibility, and is not resistant to aging. Therefore, research on improving the properties of epoxy acrylate has been ongoing, such as increasing the relative molecular weight of oligomers to reduce the shrinkage during curing, introducing silicon-containing compounds to synthesize a prepolymer to improve the heat resistance of the coating film, and introducing flexible long chains to overcome the brittleness of epoxy resin.
1.3 Polyurethane Acrylate (PUA)
Polyurethane acrylate (PUA) is obtained by reacting the NCO group of polyisocyanate with the hydroxyl group of polyol, and using hydroxyl-containing acrylate to introduce photoactive groups. As the molecular weight increases and the number of photoreactive groups contained in the molecule increases, the curing speed accelerates. It is an important type of light-curing oligomer. Star-shaped hyperbranched polymers that give PUA a three-dimensional spherical structure are becoming a hot research topic. This structure is different from traditional linear polymers.
Different, the polymer has the characteristics of high functionality, no inter- and intra-molecular entanglement, etc., so the
Polymer-like materials have high activity, low viscosity, good solubility, and easy modification of functional groups, so they can obtain suitable construction performance and excellent coating performance. Although the price of PUA is relatively high, the polyurethane acrylate cured film has excellent flexibility and wear resistance, good chemical resistance and impact resistance, and good adhesion. Therefore, PUA is an oligomer whose dosage is second only to EA.
2. Reactive diluents, photoinitiators and additives and pigments and fillers for UV curable coatings
2.1 Active diluent
Reactive diluents are small organic molecules containing polymerizable functional groups that can dissolve and dilute oligomers, adjust the viscosity of the system, improve construction performance, and can participate in polymerization and curing film formation, adjust the light curing speed and various properties of the cured film, such as wear resistance, hardness, flexibility, etc.
1) Monofunctional reactive diluent. (Meth)acrylate, etc., each molecule contains only one group that can participate in the curing reaction, and generally has the characteristics of low viscosity, high conversion rate, low curing rate, small volume shrinkage, and low cross-linking density.
2) Bifunctional reactive diluent. Containing two (meth)acrylate functional groups, compared with single-functional reactive diluents, it generally has the characteristics of good dilution, accelerated curing rate, and increased cross-linking density.
3) Multifunctional reactive diluent. Contains 3 or more (meth)acrylate functional groups,
Generally, it has the characteristics of large viscosity, fast light curing speed, and high film hardness.
4) Reactive diluent for cationic UV curing systems. Such as alicyclic epoxy resin, polyol and vinyl ether, etc. When selecting reactive diluents, the following issues should be considered: compatibility with oligomers, dilution capacity, curing speed, curing shrinkage and impact on the properties of the cured coating film.
2.2 Photoinitiator
Photoinitiator is one of the key components of the photocuring system. Its performance determines the curing rate and degree of curing of UV-cured coatings. According to the difference in reaction mechanism, it is a free radical polymerization photoinitiator and a cationic polymerization photoinitiator. Free radical photoinitiators are divided into two types: cracking type and hydrogen abstraction type. In the field of UV curing coatings, small molecule free radical photopolymerization initiators are mostly used. Among them, unreacted photoinitiators and photolysis
Debris can easily cause the coating film to age and turn yellow; you can also use a polymeric hydrogen-abstracting photoinitiator, in which the incomplete reaction of the photoinitiator will also cause the coating film to age and turn yellow. If the photoinitiator is macromolecular and multifunctional (containing two or more photochemically active groups), the yellowing of the coating film caused by the photoinitiator can be reduced. For cationic light curing systems, the only suitable oligomers are resins with vinyl ether functional groups, epoxy resins, and epoxy functional polysiloxane resins, which limits the use of cationic light curing agents.
Free radical photoinitiators have the advantage of being low-priced. Most UV-cured coatings use free radical curing, and some use dual UV curing by mixing cations and free radicals, which can form an interpenetrating network structure to improve coating film properties. By rationally utilizing the type and dosage of photoinitiators and compatibility with photosensitizers, the curing rate and degree of curing can be adjusted to meet different needs.
When UV curing coatings are used on components with complex shapes, shadows will appear and are difficult to cure. It is also difficult to cure thick coatings, opaque media and colored systems. These can be overcome with a dual curing system, that is, through a light curing reaction stage and a dark reaction (including heat curing, moisture curing, oxidative curing or anaerobic curing reaction, etc.) stage. The light curing reaction allows the system to quickly shape or reach surface dryness, while the dark reaction completely cures the bottom part or the shadow part.
2.3 Auxiliaries
In actual UV curing coating applications, due to the fast photocuring rate, in addition to the basic ingredients, various additives (including leveling agents, defoaming agents, wetting and dispersing agents, coupling agents and matting agents, etc.) must be added to meet the usage requirements. The addition of leveling agents can solve coating film surface defects caused by poor leveling in UV cured coatings; the addition of wetting and dispersing agents and defoaming agents can increase product stability and construction performance; the addition of coupling agents can improve construction performance and adhesion. As the application fields of UV curing coatings continue to expand, in order to meet the use requirements of the objects being coated, available additives include matting agents, thermal polymerization inhibitors, sensitizers, stabilizers, etc.
2.4 Pigments and fillers
In order to prepare UV-curable colored coatings, pigments and fillers must also be added.
1) Pigment. Attention should be paid to the influence of pigments on coating performance. Many pigments (such as carbon black, iron yellow, etc.) will scatter or absorb UV radiation and hinder UV curing. There is a big difference in the UV absorption of pigments on the surface of the coating film and deep inside the coating film, which may cause the surface and bottom to be cured out of synchronization and cause the coating film to shrink and wrinkle. Therefore, it is necessary to choose pigments that match the system.
2) Filling. Tests have shown that talc and calcium carbonate can be used as fillers in UV curable coatings. The addition of some nanofillers can significantly improve the wear resistance, antibacterial properties, anti-aging properties, flexibility and gloss of the coating film. Recommended reading: A high-quality electroplating company made a wedding dress for a small electroplating factory. Whose fault is it?
3. Conclusion
UV curable coating is a high-efficiency, energy-saving and environmentally friendly coating. The coating film has excellent performance and is further expanding into various application fields of traditional coatings. With the deepening of research, it has huge development potential in industrial applications. There are still some gaps between my country's UV curing coating industry and developed countries in terms of research and development, market size, etc., but the development speed is amazing. It is facing an opportunity to cooperate with the implementation of sustainable development strategies. I believe that with the efforts of the majority of scientific researchers and others, it will be further improved and developed.
