Application status and development of the hottest

Application status and development of UV curable ink

Abstract: the development process of UV curable ink at home and abroad is summarized, the film-forming principle and composition of UV curable ink are analyzed, and the characteristics and application of UV curable ink are described. The factors affecting the performance of UV curable inks were reviewed, and the existing problems and future development direction of UV curable inks were pointed out

key words: UV curable ink; Printing; Photoinitiator

0 preface

ultraviolet curing (UV curing) refers to the chemical reaction of photosensitive substances in the system under the action of ultraviolet light to produce active fragments, which leads to the polymerization and cross-linking of active monomers or oligomers in the system, so that the system changes from a liquid coating into a solid coating instantly. Since the first generation of UV curing coating was developed by Bayer company in 1967, UV curing technology has developed rapidly all over the world, and has been rapidly promoted and applied in electronics, printing, architecture, decoration, medicine, machinery, chemical industry, automobile and other industries. UV curable materials have maintained a high growth rate of 8% - 10% since they were commercialized abroad in the 1970s. The main reasons are: it solidifies quickly and meets the needs of modern automated production; No pollution, conforming to the development direction of modern coatings and inks; The advantages of high coating quality, high hardness, scratch resistance, corrosion resistance and so on have attracted much attention [1]. However, with the progress of society and the improvement of human living standards, people pay more and more attention to efficiency and environmental protection. There are two development trends of printing inks: using environmentally friendly and pollution-free solvent-based inks; Use UV curable ink. It is predicted that the world UV ink market will increase from 1.1 billion US dollars in 2001 to 1.5 billion US dollars in 2005, with an annual increase of 6% - 8%. At present, UV curable printing inks, as a large category of UV curable materials, have been rapidly developed and widely used in the packaging and printing industry and the printed circuit industry, and have been widely used in offset printing, embossing (including flexographic printing), gravure printing, printing and inkjet printing

1 UV curing mechanism and composition of UV curing ink

1 1 curing mechanism of UV curing ink

the UV curing mechanism of UV curing materials can be roughly divided into four types: free radical polymerization, free radical addition, cationic polymerization and acid curing. Among them, the research work of UV curing reaction by free radical polymerization is more mature than other types, and it is also the most widely used type of UV ink at present. Under UV irradiation, the photopolymerization initiator in UV ink absorbs photons of a certain wavelength and turns into free molecules, which become free radicals. Then, through the transfer of energy between molecules, the polymeric prepolymer and photosensitive monomer become excited, generating charge transfer complexes. The complexes are continuously crosslinked and polymerized, and solidified into films. The curing process of UV ink is a photochemical reaction process, that is, under the action of UV energy, the prepolymer solidifies into a film in a very short time; In addition to causing the surface curing of the ink, ultraviolet light can penetrate into the liquid UV curing ink and stimulate the further curing of the deep ink film. In traditional inks, oil-based inks solidify under oxidation, while solvent or water-based inks solidify mainly by evaporation of water or solvent, and some inks can penetrate into paper. Therefore, compared with traditional inks, UV curing inks are more thoroughly polymerized and dried without any evaporation or solvent pollutants, and the ink film is 100% cured

1. 2 composition of UV curing ink

the main components of UV curing ink include photopolymerization prepolymer, photosensitive monomer, photopolymerization initiator, organic pigment and additives, etc. the photoinitiator in the ink is a compound that is easily excited by light. After absorbing light, it is excited into free radicals, and the energy is transferred to photosensitive molecules or photo crosslinking agents, so that UV ink has a UV curing reaction

1. 2.1 prepolymer

prepolymer, also known as oligomer, is a low molecular polymer containing unsaturated functional groups. It is the most important component in UV ink. Its performance plays a decisive role in the curing process and the properties of the cured film. Structurally, oligomers are low molecular resins containing C = C unsaturated double bonds, with an average molecular weight of about hundreds to thousands, mostly acrylic resins. The purpose of preparing polymers is to obtain molecules with unsaturated bonds. When such molecules are in certain conditions, such as UV irradiation, they can be crosslinked with other unsaturated molecules to become macromolecules, which change from liquid to solid coating, and these unsaturated molecules need to remain stable and non reactive before their crosslinking. The most commonly used oligomers in UV curing inks are epoxy acrylate, polyurethane acrylate, polyester acrylate and alkali soluble acrylic resin

1. 2.2 active diluent

active diluent, also known as crosslinking monomer, is a small molecular compound with unsaturated groups (such as double bonds), which can be crosslinked with oligomers under light, and its molecular weight is about hundreds to thousands. The function of adding active diluent to viscous prepolymer is to adjust the viscosity of UV curing ink, control the curing crosslinking density of UV curing ink, and improve the physical and mechanical properties of curing film, such as flexibility and hardness. Active diluents are divided into single functional diluents (i.e. each molecule has a double bond) and multi-functional diluents (i.e. each molecule has several double bonds). Monofunctional active diluents include monomers commonly used in addition polymerization, most of which are flammable and volatile; Relatively speaking, multi-functional active diluent has the characteristics of low volatility and high flash point, so multi-functional active diluent is better than single functional active diluent in terms of dilution effect and light curing speed. As active diluent plays an extremely important role in the curing process of UV curing ink and the properties of the cured film, it is very important to select a suitable active diluent. Generally, the following important properties should be considered: viscosity, functionality, volatility, odor, toxicological properties, solubility, etc. Commonly used active diluents are the following: dipropylene glycol diacrylate (TPGDA), tetraethylene glycol diacrylate (tegda), trimethylol triacrylate (TMPTA), etc

1. 2.3 photoinitiator

photoinitiator is the main component required by any UV curing system. It is generally a substance that can decompose into free radicals and initiate polymerization and crosslinking under UV irradiation with a wavelength of 200 ~ 400 nm. As a photochemical reaction, the initiator becomes very active after being excited by light and absorbing photons, producing free radicals, transferring energy to high molecular polymers, producing a chain reaction, changing the linear resin into a shape structure, and curing the ink into an ink film with tough functional characteristics. Because it is difficult for ultraviolet light source to directly open the double bond of photopolymerizable prepolymer, even photosensitive materials will not cure quickly without photopolymerization initiator. A small amount of photopolymerization initiator is added to the ink and exposed to ultraviolet light at the same time; After the photoinitiator absorbs light quanta, it decomposes into free radicals, which starts the chain initiation and initiates the polymerization of unsaturated double bonds, so that the polymer molecules are continuously crosslinked to form a like structure; When the free radical loses its activity and the growth of the chain terminates, the ink is completely cured. Photosensitive initiators for UV curing inks are generally divided into three types: homogeneous initiators, hydrogen extraction initiators and cationic initiators

1. 2.4 polymerization inhibitor

polymerization inhibitor refers to the substance that can quickly interact with free radicals, slow down or inhibit unwanted chemical reactions, and is used to prolong the storage life of some monomers and resins, also known as polymerization terminator, which includes polymerization inhibitor and polymerization retarder. According to its polymerization inhibition principle, it can be divided into two types: in the absence of oxygen, benzoquinone reacts directly with free radicals to form a semiquinone intermediate, and then reacts with another free radical to form a stable compound; In the presence of oxygen, hydroquinone and its derivatives react with oxygen to form peroxy radical, which reacts with diphenol formed by the reduction of atoms in p-benzene particles to form a free radical complex, which reacts with another peroxy radical to form a stable compound. Common polymerization inhibitors are hydroquinone, p-methoxyphenol, etc

1. 2.5 organic pigments and additives

pigments also play a color rendering role in UV inks, which has a direct impact on some characteristics of inks. Pigments are colored, black or white powders with high dispersion that are insoluble in water and organic solvents. According to their source and chemical composition, they are divided into organic pigments and inorganic pigments. The requirements for pigments used in printing inks are quite high, especially color, dispersion, transparency, etc. the hue of color pigments is required to be close to the spectral color, and the saturation should be as large as possible. The transparency of magenta, cyan and yellow pigments used in three primary color inks must be high. All pigments must not only have water resistance, but also be able to quickly and evenly combine with connecting materials. The oil absorption capacity of pigments should not be too large, and pigments should preferably have alkali resistance, acid resistance and other properties

ink additives are auxiliary materials used to adjust ink to different printing conditions. The performance of additives directly affects various indicators of ink. There are many kinds of ink additives, commonly used are viscosity remover, diluent, desiccant, anti desiccant, diluent, friction resistant agent, gloss oil, Anti Dirt agent, inking oil, etc

2 characteristics and application of UV curable ink

2 1 Characteristics of UV curing ink

there are still many problems unsolved in traditional printing ink so far [6], which hinders the development of printing technology, and the use of UV curing ink just makes up for the defects of traditional printing ink. Its advantages are as follows: the shape structure produced by the high-density cross-linking of acrylate polymers gives the ink film a high degree of toughness, stain resistance, wear resistance, solvent resistance and other high printing quality; UV ink has good thixotropy, and under certain conditions, it can be treated with ice, refraction, sanding, wrinkles and other effects, so that it can be used for high-precision printing and special printing of high-line number; The high curing speed of tens to hundreds of meters per minute meets the production requirements of modern large industry; 100% high solid content, energy conservation, safety, no organic volatiles during production and curing, conforms to the requirements of environmental protection and energy conservation in today's society, and is an ideal green ink [7]. However, there are also some defects such as skin irritation before curing and poor recoating performance. In a word, UV inks comply with the "5E principle" (energysaving, ecology, economics, ease of application, excellenceof finish), and have good application and development prospects

2. 2 Application of UV curing ink

2 2.1 application of UV curable ink in packaging and printing industry

UV ink has attracted the attention of the packaging and printing industry because of its excellent performance and characteristics, and has been widely used in offset printing, embossing (including flexo printing), gravure printing, printing and inkjet printing. For example, in embossing, UV ink is widely used in the printing of gold 120 ° diamond cone silver paperboard, synthetic paper and processed PP, PE and other substrates. Its printing quality is good (compared with ordinary ink), and it is convenient to immediately arrange the production of post process, forming assembly line operation, and high production efficiency; In gravure printing, UV ink is generally used for the printing and anti-counterfeiting of bank bills, securities, cheques and stamps, as well as the printing of plastic packaging for some cooking resistant food and beverages. It has a large number of prints, rich ink levels and good product quality; In printing, UV inks are more widely used. According to their different substrates and uses, a variety of UV inks suitable for printing have been developed