UV Inkjet Inks – Photoinitiator
Inkjet printing is a non-contact, no pressure, no printing plate printing method, by the system controller, inkjet controller, printheads and substrate drive mechanism, etc., through the computer editing graphics and text, and control the inkjet printer nozzle jet ink droplets to the substrate to obtain accurate images, is completely digital printing process. As a combination of digital printing technology and computer technology products, inkjet printing has become the most rapid development in the field of digital printing, the most widely used a printing method.
Inkjet printing as a fully digital printing method has many characteristics.
① completely separated from the traditional printing process of cumbersome procedures, fast response to the market, job preparation time and short production cycle.
② can be very high time efficiency and low-cost efficiency to complete the production of digital samples and short-run jobs.
③ Variable data printing and on-demand printing are possible to realize personalized printing.
③ Variable data printing and on-demand printing are possible, realizing personalized printing. ④ Off-site printing can be realized through Internet transmission.
⑤ Can be printed on different materials such as paper, cardboard, film, plastic, metal, wood, ceramics, glass, fabrics, leather, it is no exaggeration to say that at present, inkjet imaging technology has been able to print on all planar and non-planar materials in addition to water and air.
Inkjet printing is divided into continuous printing and on-demand printing in different ways according to the printing method, the current main form of inkjet printing is on-demand printing, on-demand printing has thermal jet and piezoelectric.
Thermal spray inkjet device consists of an ink cavity, nozzle and heater, the heater is a thin film resistor, in the ink ejection zone will be heated, the formation of a bubble, the bubble instantly expand and rupture, the ink from the nozzle ejection. The entire process of bubble formation and rupture is less than 10μs, and then the ink chamber is filled and ready for the next process.
Piezoelectric inkjet device consists of piezoelectric ceramics, diaphragm, pressure chamber and nozzle. Applying voltage to the piezoelectric ceramics causes it to deform, resulting in a change in the volume of ink in the pressure chamber, which generates a high pressure and ejects the ink from the nozzle (see Figure 3-23). Currently piezoelectric inkjet due to faster inkjet speed, easy to control the shape and size of the ink droplets, the nozzle life is also long, applicable to all types of inkjet ink, although the price of the equipment is slightly more expensive, but the cost of use is low, so large-format inkjet machine are used piezoelectric inkjet.
UV inkjet ink preparation
The most important consumable in inkjet printing is inkjet ink, which is a kind of liquid ink that can be jetted onto the substrate to produce image text as required after being subjected to the electric field between the nozzle of the inkjet printer and the substrate. It is a very demanding special ink, it must be stable, non-toxic, do not block the nozzle, moisture retention and jet to be good, no corrosive effect on the nozzle and other metal objects, but also not swallowed by bacteria, not easy to burn and fade, etc.. Ink surface tension, viscosity, drying and color density is the key to inkjet printing, ink should be able to dry on absorbent and non-absorbent materials, and not dry on the nozzle to avoid clogging the nozzle. The chemical composition and properties of inkjet inks determine the quality of the printed image, the characteristics of the ink droplet jet and the reliability of the printing system.
Inkjet inks can be divided into dye-based and pigment-based inks according to the colorant. Dye-based inkjet inks are mainly water-based inks. Since the dye is dissolved in the carrier, each dye molecule is surrounded by carrier molecules, and no particulate matter can be observed under the microscope, so it is a completely dissolved homogeneous solution. Dye-based water-based inkjet ink is not easy to block the nozzle, easy to be absorbed by the substrate material after spraying, and because the dye can express the color gamut is generally larger than that expressed by the pigment, the embodiment of the color range is also large, so that the prints are more vibrant and bright, and its cost cost is also lower, but its waterproof performance is poorer, not friction resistance, optical density is low, and due to the relatively poor chemical stability, light resistance is also poor, and Easy to blot. Dye-based inkjet inks are colorful, layered and less expensive than pigment-based inks, which are the preferred products for printing pictures and making color inkjet business cards. Currently, most inkjet photos use dye-based water-based inkjet inks.
Pigment-based inkjet inks are solid pigments ground into very fine particles, dispersed in a special solvent, a suspension solution. The emergence of this ink to solve the shortcomings of the dye-based inkjet ink, it is water-resistant, light-resistant, not easy to fade, fast-drying, due to the weak penetration of pigment inkjet ink to the medium, will not be like the dye inkjet ink as diffusion, so it is not easy to blot. Pigment inkjet ink has become the inevitable trend of inkjet ink development with its many advantages.
Pigment-based inkjet inks currently have water-based inkjet ink, solvent-based inkjet ink, weak solvent-based inkjet ink, UV inkjet ink and UV-LED inkjet ink developed in recent years.
UV ink as early as the 1970s in the letterpress, screen printing, offset printing, flexographic printing, gravure and pad printing and other printing processes, until the end of the 20th century, due to the progress of inkjet technology, improve the accuracy of the UV light source production technology, the development of UV light source to the inkjet with a suitable UV light source, design and manufacture of UV inkjet. At the same time, new raw materials for the preparation of UV ink are constantly being developed, and the UV ink manufacturing technology is constantly being improved, finally making UV ink enter the inkjet printing field.
UV ink for inkjet printing, in addition to general UV printing ink characteristics, especially to solve the technical difficulties in three areas: viscosity, stability and oxygen barrier.
① inkjet ink must have a low viscosity. General thermal inkjet inkjet at 25 ℃, viscosity in 3 ~ 5mPa – s; piezoelectric inkjet inkjet at 25 ℃, viscosity in 3 ~ 30mPa – s. Therefore, the UV inkjet inkjet inkjet to achieve the viscosity of <30mPa – s is a certain degree of difficulty. Must choose low viscosity oligomer and active diluent, and low viscosity oligomer and active diluent is often lower functionality, which is not conducive to the improvement of the light curing speed, but also not conducive to the dispersion of pigments and stability.
② The colorant used for UV inkjet ink is mainly pigment, the fineness of the pigment is required to be 1μm or lower, and then dispersed in the resin system with low viscosity, without aggregation, and without settling when placed. Therefore, it is necessary to choose oligomer with good wettability for pigment, pigment with good dispersibility and dispersant with good stability, and obtain the ink with fineness <1μm and placing without settling and aggregation after grinding process.
③ UV inkjet ink from the nozzle spray tiny ink droplets, which has a great surface area, the air contact surface increases, there are more oxygen dissolved into the ink, oxygen barrier polymerization seriously impede the ink droplets UV curing. At present, large-scale airbrush nozzle moving speed of 30m/min or so, which requires the ink has a faster light curing speed and anti-oxygen barrier polymerization ability. So UV inkjet ink in the formulation must be considered to overcome the oxygen barrier to polymerization, the use of highly active photoinitiators and light curing speed of oligomers and active diluents, to take the necessary measures to prevent oxygen barrier to polymerization.
UV inkjet printing machine manufacturers in order to solve the problem of UV inkjet ink low viscosity in the ink storage device equipped with heating devices, the ink can be warmed up to about 50 ℃, which provides favorable conditions for the UV inkjet ink manufacturers to choose oligomers and active diluents. At the same time, UV inkjet inks are mainly used for advertising, there is no requirement for the physical and mechanical properties of the ink layer, so the oligomer content used in the ink formulation is very small, and can be used more active diluents.
UV inkjet ink with oligomer hope to have a lower viscosity, faster light curing rate, good wettability and dispersion of the pigment stability, the substrate has a good adhesion. Now many domestic and foreign manufacturers of oligomer developed hyperbranched oligomer has the above characteristics, especially suitable for UV inkjet ink manufacturing.
Reactive diluent should also choose low viscosity, high activity, good wettability of the pigment dispersion of acrylic functional monomer, the actual use of mono-, bifunctional, multifunctional acrylic ester mixing, in order to achieve a better overall performance.
Photoinitiator selection should also consider the high activity, conducive to surface drying varieties, the amount added can be slightly higher than the general UV ink, but also to use a variety of photoinitiators with the use.
In order to reduce the impact of oxygen barrier polymerization, can choose to have anti-oxygen barrier polymerization function of raw materials, such as amine-modified oligomer, oligomer containing polyether structure, alkoxylated active diluent, tertiary amine initiator and so on.
Water-based UV inkjet ink is also under development, its viscosity is lower than the UV ink, can provide a lower ink film weight. However, it has higher requirements for the substrate, during printing, water should be absorbed by the substrate, if the substrate is not permeable to water, then the water should be dried during UV curing, for the substrate that is not permeable to water, it should be more difficult to control its wettability, so it is able to adhere to special surfaces, and it is suitable for application on porous-permeable and semi-porous-permeable substrates, and it can achieve a better curing effect.
So far, the sales of inkjet inks are still dominated by solvent-based inks that have been used for a long time, but the market share of UV inkjet inks is increasing year by year, showing an upward trend. From the perspective of environmental protection to consider the rapid development of UV inkjet ink is an inevitable trend.
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