In the label printing industry, the use of UV ink is the consensus of the industry. Whether it is letterpress printing labels or gravure printing labels, UV ink will be used when choosing ink. And when the printing material extends from coated paper to plastic film, it seems that there is no choice but to use UV ink due to the adhesion of the ink film on the non-absorbent material. Even rotary offset printing, which has just emerged in the label printing industry, has chosen to use UV ink. UV ink is indeed a good thing. It uses spectral radiation energy to dry the ink, with low emission of volatile organic compounds and low environmental pollution.
The ink has high solid content and high color concentration. As long as the UV lamp is not irradiated, the ink film is basically not dry. Therefore, there is no need to clean the ink at the end of the production task, and the reasonable loss of ink is small. However, the price of UV ink is very high, which is several times the price of solvent-based ink and water-based ink.
Moreover, since the photosensitive frequency of UV ink and the lighting frequency of UV lamp often cannot be completely matched, resonance cannot be achieved, so the energy efficiency of UV ink is low. Energy consumption is huge. In theory, UV drying should use the spectrum of the UV lamp, but in practice, most of the heat energy emitted by the UV lamp is utilized. At present, the metal exhaust pipes of commonly used printing machine UV drying devices are usually very hot to the touch and cannot even be touched directly, so special attention must be paid to heat dissipation.
In recent years, LEDUV technology has been launched in the European and American markets to solve this problem. It uses LED cold light sources as lighting sources and adjusts the UV wavelength to 365nm to improve UV energy utilization and reduce energy consumption. Unfortunately, this technology is still in its infancy, the price is very high, and it is difficult for the domestic label printing market to accept it. Water-based ink is an environmentally friendly ink that can be accepted and willing to be accepted by the printing industry. Water-based ink rarely contains organic solvents except for a small amount of additives, so water-based ink does not emit volatile organic compounds.
However, if you want to use gravure water-based ink instead of gravure UV ink on high-end labels, especially on film self-adhesive labels, industry peers have many doubts about this. Therefore, as an environmentally friendly ink with unparalleled advantages in cost and environmental friendliness, gravure water-based ink must meet market demands in terms of printing performance in order to replace gravure UV ink. As we all know, the water-soluble resin in water-based ink must undergo amination treatment before being dissolved in water. Therefore, the water in water-based ink is very different from the tap water we use in daily life. Water-based ink is slightly alkaline, and its pH value is usually 8.5-9.5. Some water-based inks are highly alkaline, with pH levels reaching or exceeding 10.0. The pH value of water-based ink will change with changes in ambient temperature, so when applying water-based ink, just like solvent-based ink using a viscosity cup to measure the viscosity every 15 to 20 minutes, the pH value of water-based ink must also be monitored regularly.
When the pH value of a water-based ink is lower than the lower limit of the specified value, its viscosity will increase, the drying speed will be accelerated, and the transfer performance will become worse. Therefore, the stabilizer (actually an alkaline additive such as ammonia) provided by the ink supplier must be added in time to restore the pH value of the water-based ink to the original level.
At present, in the application of water-based ink, the control of pH value is like a set of zigzag waveform curves, with time t as the axis. As the t axis extends, the pH value gradually decreases. Once the stabilizer is added, the pH value immediately rises again, and so on, constantly fluctuating. In fact, the pH value of water-based ink is as unstable as a sawtooth waveform curve. In order to be stable, the time for determining the pH value and adding stabilizers must be shortened, that is, the frequency of determining the pH value and adding stabilizers must be increased.
Therefore, when applying water-based ink, foreign label printing companies often use real-time pH control equipment to determine the pH value at any time, and add stabilizers at any time to maintain the stability of the performance of the water-based ink. The pH value of water-based ink should be determined and adjusted in time, which is related to the resin amination technology in the manufacturing process of water-based ink. According to current technology, acrylic resin amination generally uses three processes: one is the organic amine process, the second is the ammonia process, and the third is the NaOH or NaHCO3 process.
The three processes vary widely in terms of performance, cost and stability after amination. Organic amines volatilize slowly, so they have good stability, but are expensive; ammonia is cheap, but 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 a combination of the two processes, using both organic amines and ammonia to balance performance and cost.
At present, high-quality water-based ink basically does not require regular monitoring of its pH value, nor does it require frequent addition of stabilizers. The pH value of stable water-based ink is between 8.25 and 8.5, with very small fluctuations. Such water-based ink has stable transferability and stable viscosity (due to the large latent heat of water, the probability of an increase in ink viscosity caused by the evaporation of water during the evaporation of water-based ink is very low. The increase in viscosity of water-based ink is often due to the decrease in pH value caused by the volatilization of ammonia) , so the color density is stable.