The use of UV ink in the label printing industry seems to be a consensus in the industry. Whether it's flexo labels or convex labels, UV ink is used in ink selection. And when the material is expanded from the coated paper to the plastic film, the adhesion of the ink film on the non absorbent material seems to have no choice except for the use of UV ink. Even the rotary offset printing, which has just emerged in the label printing industry, chooses UV ink.
UV ink is really a good thing. It uses the radiation energy of the spectrum to dry the ink, the VOC emission is low, the pollution of the environment is small; the ink solid content is high and the color concentration is high. As long as the ink film is not dry after the UV lamp is irradiated, the ink road is not cleaned when the production task is finished, and the reasonable loss of the ink is less. But the price of UV ink is high, it is a few times the price of solvent type ink and water-based ink, and because the photosensitive frequency of the photoinitiator of the UV ink is often not fully matched with the luminescence frequency of the UV lamp, the resonance can not be reached, so the energy utilization ratio of the UV ink is low and the energy consumption is huge. In theory, UV drying should use the spectrum of UV lamps and lanterns, but in fact, most of the use of the thermal energy emitted by the UV lamps and lanterns, the commonly used printing machine UV drying device metal suction pipe is often ironed directly, so it is necessary to pay extra attention to the discharge of heat. In recent years, the LEDUV technology introduced in the European and American markets is to solve this problem. Using the LED cold light source as the irradiation source, the ultraviolet light wavelength is adjusted to 365nm, and the utilization rate of UV energy is improved and the energy consumption is reduced. Unfortunately, this technology is still in its initial stage and the price is very high, and the domestic label printing market is more difficult to accept.
Water-based ink is acceptable and acceptable to the printing industry. Water-based inks contain little organic solvents, so there is no VOC emission problem for water-based ink. However, if we want to replace flexo UV ink with high-grade flexographic labels, especially film adhesive labels, the industry colleagues will be puzzled. Therefore, as a kind of environmental ink with unparalleled advantages in cost and environmental friendliness, it is necessary to replace the soft printing UV ink. What market demand must it be achieved in printing performance?
As we all know, water soluble ink in water soluble resin must be amidiated to dissolve in water. Therefore, water in water-based ink is very different from the tap water used in our life. Water-based ink is alkaline, and its pH value is generally 8.5 to 9.5. Some water-based inks are highly alkaline and the pH value even reaches or exceeds 10. The pH value of water-based ink will change with the change of ambient temperature, so in the application of water-based ink, like the solvent type ink to measure the viscosity every 15~20 minutes with the viscosity cup, the water-based ink must also monitor the pH value regularly. When the pH value of water-based ink drops to the lower limit of the specified value, the viscosity will increase, the drying speed will increase and the transfer performance will become worse. Therefore, it is necessary to add the stabilizer provided by the ink supplier in time (in fact, the alkaline additive such as ammonia water), so that the pH value of the water-based ink can be restored to the original level. At present, in the application of water based ink control, the control of pH value is like a set of sawtooth curve, with time t as the axis, and the pH value gradually decreases with the extension of the T axis, and once the stabilizer is added, the pH value will recover immediately, so it is in the circle and has been in the fluctuation. In fact, the pH value of water-based ink is as unstable as the sawtooth curve. If it is to be stable, it is necessary to shorten the time to determine the value of pH and add the stabilizer, that is, to improve the frequency of measuring pH value and adding stabilizer. Therefore, foreign label printing enterprises often use pH value real-time control device in the water-based ink application, determine pH value at any time, and add stabilizer at any time, so as to maintain the stability of water-based ink performance.
The pH value of water-based ink should be measured in time and the pH value should be adjusted in time. This is related to the amination of resin in the manufacture of water-based ink. According to the current technology, acrylic resin amination generally uses 3 processes: one is the organic amine process, the other is the ammonia process, and the three is the NaOH or NaHCO3 process. The 3 processes differ greatly in their performance and differ greatly in their cost. Moreover, the difference in stability after amination is also great. Organic amine volatilization is slow, so the stability is good, but the price is expensive; the price of ammonia water is cheap, but the volatilization is fast, it must be replenished in time; the price of the inorganic alkali solution is cheap and does not volatilize, but the performance is poor. At present, the water-based ink used in the industry is basically one of the first two processes, or the combination of the two processes, both with organic amines and ammonia, for both performance and cost. At present, water-based inks with high quality requirements basically do not regularly monitor their pH values, nor do they often add stabilizers.
The pH value of pH value stable water-based ink is between 8.25 and 8.5, and the fluctuation is very small. This kind of water-based ink is stable, and the viscosity is stable (because the latent heat of water evaporation is big, the probability of the ink viscosity of the ink in water-based ink is very low because of the evaporation of water, the increase of the viscosity of the water-based ink is often caused by the decrease of the pH value of ammonia), so the color concentration is stable.