The use of UV inks in the label printing industry seems to be a consensus in the industry. Whether it is flexo labels or embossed labels, UV ink is used in the ink selection. And when the printing material expands from coated paper to plastic film, it seems that there is no other choice but to use UV ink due to the adhesion problem of the ink film on non-absorbent materials. Even rotary offset printing, which has just emerged in the label printing industry, also chooses UV ink.
UV ink is indeed a good thing, it uses the radiation energy of the spectrum to achieve the drying of ink, low VOC emissions, and low environmental pollution; The ink solid content is high, and the color concentration is also high; As long as it is not irradiated by UV lamp, the ink film is basically not dry, so there is no need to clean the ink path when the production task is finished, and the reasonable loss of ink is small. However, the price of UV ink is high, which is several times that of solvent-based ink and water-based ink, and because the photoinitiator sensitivity frequency of UV ink and the luminous frequency of UV lamps often cannot fully match and achieve resonance, the energy utilization rate of UV ink is low and the energy consumption is huge. Theoretically, UV drying should use the spectrum of UV lamps, but in reality most of the heat energy emitted by UV lamps is used. In recent years, the LEDEV technology launched in the European and American markets is to solve this problem, using LED cold light source as the irradiation source, adjusting the wavelength of ultraviolet light to 365nm, improving the utilization rate of UV energy and reducing energy consumption. Unfortunately, this technology is still in its infancy, and the price is very high, making it difficult for the domestic label printing market to accept.
Water-based ink is an environmentally friendly ink that the printing industry can and is willing to accept. Water-based inks rarely contain organic solvents except for a very small amount of additives, so water-based inks also have no VOC emission problems. However, if you want to replace flexo UV ink with flexo water-based ink on high-end labels, especially film self-adhesive labels, there are many doubts among industry colleagues. Therefore, as an environmentally friendly ink with incomparable advantages in cost and environmental friendliness, what market demand must be met in terms of printing performance if flexo water-based ink can replace flexo UV ink?
As we all know, the water-soluble resin in water-based ink must be amiminated before it can be dissolved in water, so the water in water-based ink is very different from the tap water we use in our lives. Water-based inks are alkaline, and their pH value is generally 8.5~9.5. Some water-based inks are highly alkaline, with a pH level of 10.0 or even above. The pH value of water-based inks changes with the ambient temperature, so in water-based ink applications, just as solvent-based inks measure viscosity with a viscosity cup every 15~20 minutes, water-based inks must also monitor the pH value regularly. When the pH value of water-based inks drops below the lower limit of the specified value, their viscosity will increase, the drying speed will increase, and the transfer performance will deteriorate. Therefore, it is necessary to add stabilizers provided by ink suppliers (actually alkaline additives such as ammonia) in time to restore the pH value of water-based inks to the original level. At present, the application of water-based inks is like a set of sawtooth wave curves in controlling pH value, with time t as the axis, the pH value gradually decreases with the extension of the t axis, and once a stabilizer is added, the pH value will rise immediately, and so on, and it has been fluctuating. The pH of water-based inks is actually as unstable as a sawtooth curve, and to stabilize, the time to determine pH and add stabilizers must be shortened, that is, the frequency of pH measurement and stabilizer application must be increased. Therefore, foreign label printing companies often use real-time pH control devices in water-based ink applications, measure pH value at any time, and add stabilizers at any time to maintain the stability of water-based ink performance.
Water-based inks should be measured in time and adjusted in time, which is related to the resin amination technology in the manufacturing process of water-based inks. According to the current technology, acrylic resin amination generally adopts three processes: one is organic amine process, the second is ammonia process, and the third is NaOH or NaHCO3 process. These three processes are very different in terms of performance, cost, and stability after resin amination. Organic amines volatilize slowly, so they have good stability, but they are expensive; Ammonia is cheap, but it volatilizes quickly and must be replenished in time; Inorganic alkali solutions are cheap and non-volatile, but have poor performance. At present, the water-based inks used in the industry basically use one of the first two processes, or combine these two processes, using both organic amines and ammonia, in order to take into account performance and cost. At present, water-based inks with high quality requirements basically do not need to monitor their pH value regularly, and they do not need to add stabilizers frequently.
Water-based inks with stable pH value have a pH value between 8.25~8.5, with very little fluctuation. Such water-based inks have stable transferability and stable viscosity (due to the large latent heat of water evaporation, the probability of ink viscosity increasing due to water evaporation in water-based inks is extremely low, and the increase in viscosity of water-based inks is often caused by the volatilization of ammonia to reduce the pH value), so the color concentration is stable.