4. Pigments are essential components of UV printing inks. Pigment particles are dispersed in the binder to express the color of the printing ink. The addition of pigments will change the rheological properties of the ink and also have a great impact on the gloss of the printed film.
For UV printing inks, the addition of pigments will cause the absorption, reflection or scattering of UV light, and reflection and scattering will weaken the absorption of UV light by the ink system, reduce the photoinitiation efficiency of the photosensitizer, and cause the ink to cure and dry more slowly.
Commonly used color pigments for UV inks include benzidine yellow, Hansa yellow, phthalocyanine blue, phthalocyanine green, permanent red, red powder, etc., and color-reducing pigments include titanium dioxide, high-pigment carbon black, etc.
For overprinting inks, the back color sequence ink layer is required to have sufficient UV transmittance to ensure that the lower ink layer has sufficient UV exposure to avoid printing failures such as the surface ink drying but the bottom ink not drying.
Different types of pigments have different transmittances to UV light. The transmittance of many pigments to UV light increases with the increase of wavelength. The transmittance of inorganic pigments is higher, and the thickness of the pigment absorption layer of UV light is large. This is related to the structural stability of inorganic pigments. The active group (azo group) in the molecular structure of organic pigments has a strong absorption effect on UV light. The transmittance of carbon black to UV light is extremely low, which is the result of black body absorption.
Further analysis of typical four-color UV inks (magenta, yellow cyan, and black) shows that magenta ink has the maximum transmittance to 313.1nm light, which is 50%; yellow ink has the maximum absorption rate to 320nm light, which is 32%; cyan ink has the maximum transmittance to 285nm light, which is 25%; black ink has a transmittance to light close to zero.
Therefore, when using four-color overprinting, the printing color sequence of the four-color ink is: black ink - cyan ink - yellow ink - magenta ink. If a UV light source that matches the maximum transmittance of each ink can be set after each color, better printing results will be achieved.
However, in actual applications, the printing color sequence of the four-color ink is: black ink - cyan ink - magenta - yellow ink. The reason is that on the one hand, the transmittance of yellow ink to ultraviolet light is higher, which can make the bottom ink solidify in time, and on the other hand, the transparency of yellow ink is greater, which is more conducive to the expression of printing effect.
The role of fillers in ink is mainly to increase the solid content, improve the rheological properties of ink, and reduce the price of ink.
The addition of fillers will also hinder the ink from absorbing UV light and reduce the drying rate of ink, which should be fully considered when selecting. Commonly used fillers in UV printing inks include calcium carbonate, aluminum hydroxide, silicon dioxide, talcum powder, kaolin, etc.
5. Auxiliary agents In order to improve the printing suitability and quality suitability of UV printing inks, in addition to the four components mentioned above, auxiliary agents are also required in the UV ink formula.
The auxiliary agents used in UV inks are characterized by small dosage, wide variety and high degree of specialization. Here we select the main varieties for overview.
● Inhibitors An auxiliary agent added to prevent UV printing inks from crosslinking during storage and improve the storage stability of inks. Commonly used inhibitors include hydroquinone, p-methoxyphenol, p-benzoquinone, etc. These inhibitors do not inhibit photopolymerization reactions, but are added only to prevent thermal polymerization reactions.
● Defoamer This auxiliary agent is used to eliminate bubbles generated during the manufacture and use of inks. It is mainly for thin ink varieties with low viscosity, such as UV inks used for gravure printing and flexible relief printing. Commonly used defoamers include silicone oils (F810, FN), silicone acrylates (R2500), etc.
● Leveling agent is used to improve the leveling of the ink layer, reduce the shrinkage of the printed film, and help improve the gloss of the printed film. Commonly used leveling agents include silicone acrylate (R2100, R2200), silicone (Deqian 457), etc.
● Anti-impact agent UV printing ink used for metal coil printing, after printing, the printed product needs to be sheared, bent, stamped and other mechanical processing, and the printed film often breaks and falls off during the mechanical processing. In order to improve the performance of the ink against pressure processing, it is usually added with anti-impact agent.
Commonly used anti-impact agents include (methyl) acrylate phosphate (DAY-18, PM-2, DAY-0211, SR9008). In addition to the above-mentioned auxiliary agents, there are many varieties of auxiliary agents used in UV printing inks. For example, wax is usually added to improve the rheology of the ink, improve the water resistance of the ink, and enhance the wear resistance of the ink. With the advancement of technology, auxiliary agents for UV printing inks will continue to be introduced.
6. Application of UV printing inks UV printing inks are currently used in the fields of sign printing, container printing, and packaging cardboard printing. UV printing inks have been applied to many different printing types, usually including wet offset printing, letterpress printing, screen printing, and flexographic printing. Gravure printing is less used. The printing substrates are also constantly increasing. UV printing inks can be used for printing on substrates such as paper, wood, plastic, and metal. The performance of UV printing inks used for different printings varies.
7. Development Trends of UV Ink In order to further improve the performance of UV printing ink, the current focus is on the development and application of new prepolymers, active diluents and photosensitizers related to ink performance, such as improving the wettability of pigments and fillers, curing speed, enhancing adhesion and reducing the toxicity of active diluents.
● Improve pigment wettability The wettability of the binder to the pigment directly affects the manufacturing process, printing operation adaptability and printing quality adaptability of the ink. If the wettability of the pigment is poor, the pigment will not be easily dispersed during the preparation process of the ink. Even after strong dispersion, the pigment will flocculate in a short period of time during storage; the ink will not be transferred well on the printing press, the printing dots will be shrunken, and the printed products will lose tone and color; especially in offset printing, the balance of water and ink cannot be guaranteed, resulting in problems such as difficulty in controlling or even failure of printing. In fact, many printing failures are caused by poor wettability of the pigment. Therefore, improving the wettability of the pigment is the key to obtaining excellent performance UV printing ink.
● Improve ink curing speed The advantage of UV printing ink is that it can dry instantly, which is one of the reasons why UV ink has received much attention and has developed rapidly. With the further modernization of printing equipment, the printing speed is getting faster and faster. Faced with such a high printing speed, the initial UV printing ink curing speed has been difficult to adapt, and it is inevitable to increase the ink curing speed to meet the requirements of high-speed printing.
● Enhance ink adhesion Adhesion reflects the bonding strength between the printing film and the printing substrate.
● Reduce the toxicity of active diluents. Many acrylic compounds used as active diluents are restricted due to their high toxicity and strong skin irritation. For this reason, many countries in the world have developed many low-toxic active diluents. The more successful new products mainly include alkoxylated allyl ether compounds.
● Composite light curing The composite light curing method mainly adopts a free radical/cationic composite light curing system. Since the curing of this system is not affected by oxygen, it can be deeply cured and the curing volume shrinks little. Therefore, the use of a free radical/cationic composite light curing system can increase the light curing speed. It is particularly beneficial for the curing of dark and thick layers of screen printing ink. The prepolymers and active diluents used in the free radical/cationic light curing system are mainly epoxy resins, epoxy phenolic resins, polycyclic compounds, vinyl ether compounds, etc. Cationic photosensitizers are mainly iodine salts, thionium salts, etc. However, free radical/cationic composite photocuring is not perfect, and there are many defects. For example, cations are highly sensitive to active impurities in the ink system, which can easily cause cation inactivation and make the ink dry slowly or not dry.
●Water-based structure of ink The water-based structure of UV printing ink is a new research direction in the field of UV ink. In addition to the environmentally friendly characteristics of UV ink, the ink is safer, the system has low viscosity, good fluidity, and has a certain mitigation effect on oxygen inhibition. However, there are also some disadvantages. Water reduces the solid content of the system, and residual water in the cured film will reduce the performance of the printed film. The preparation of water-based UV printing ink prepolymer is relatively complicated. At present, it is mainly prepared by mechanically dispersing or emulsifying the prepolymer, or by introducing water-soluble groups (--COOH, -O-, -COO-, -OH, -NH-). Photosensitizers should also be selected with high water solubility. Water-based UV printing inks are mainly limited to volatile places or water-absorbing substrate surfaces.