With the growing awareness of human environmental protection, green environmental protection has become the theme of development in the 21st century. In recent years, many countries have increased hygiene and environmental protection requirements and restrictions on packaging and printing, especially for food packaging and printing. Due to the use of organic solvent inks and coating oils, hydrocarbons are emitted, air is contaminated, and As a result, health foods have been largely eliminated from the use of solvent-based printing materials in food packaging and printing in developed countries. Research and development and application of small pollution, less pollution, and even pollution-free printing materials become an inevitable requirement for the development of green packaging, thus, the green packaging printing materials that are conducive to the environment will be more and more important.
Water-based ink composition
Water-based ink is a combination of specific water-based polymer resins, pigments, water, and additives added by the physical and chemical process of ink, referred to as ink. Water-based ink uses water as a solvent. After the ink is transferred to the substrate, the water evaporates into the environment or penetrates into the substrate. The ink is dried with the evaporation of moisture. Aqueous polymer resin is the most important component of water-based ink. It plays a role as a binder in ink, allowing the pigment to be uniformly dispersed, giving the ink a certain degree of fluidity, and providing adhesion to the substrate material, making the ink Can form a uniform ink layer after printing. Alkali-soluble resins are currently used for research and development. These resins can be dissolved in water to make water-based binders that become insoluble in water after printing and drying.
The hue of water-based inks is mainly determined by the pigments. The pigments are evenly distributed in the binder in the state of particles. The pigment particles can absorb, reflect, refract, and transmit light, and thus can exhibit a certain color. General requirements Pigments have a bright color, suitable coloring power and hiding power, and a high degree of dispersion. In addition, depending on the purpose of use, it is also possible to select different resistance solvents. The role of the solvent is to dissolve the resin, so that the ink has a certain degree of fluidity, can be successfully transferred during the printing process, and adjust the viscosity and drying properties of the ink. Auxiliaries have a great influence on ink performance and are an indispensable part of ink painting. The commonly used additives in inks include pH stabilizers, slow drying agents, defoamers, dilutants, and the like.
Water-based ink's environmental safety features
Water-based inks are distinguished from solvent-based inks, the most prominent feature being the dissolving vehicle used. Solvent-based inks are dissolved in organic solvents such as toluene, ethyl acetate, and ethanol, while aqueous inks are dissolved in water and a small amount of alcohol (about 3% to 5%). Due to the use of water as a carrier for dissolving, aqueous inks have significant environmental and safety features: safe, non-toxic, non-hazardous, non-inflammable, non-explosive, and virtually free of volatile organic gases. Mainly in the following four aspects:
1. No pollution to the atmosphere;
2. To reduce residual toxic substances on the surface of printed matter to ensure food hygiene and safety;
3. Reduce resource consumption and reduce environmental costs;
4. Improve the safety of the operating environment and ensure the health of contact operators.
Application of water-based inks
From the printing point of view, water-based inks are not yet suitable for offset printing, and its main application areas are flexographic printing, gravure printing and screen printing.
Countries that have used water-based screen printing inks in large quantities are Sweden, Germany and the United States. Since the 1980s, light and matt water-based inks have been developed internationally for fabric printing, paper, PVC, PS, aluminum foil and metal on-line printing. The development of screen printing in our country is very fast, and the progress in the development and popularization of screen printing water ink is slow. What is gratifying is that China has attracted people's attention in the research and development of screen printing inks. After several years of hard work, we have achieved some results. According to reports, screen-printed water-based inks have been applied to printing materials such as paper and plastics in China.
Problems with Water-based Printing in Screen Printing
1. In screen printing, it is necessary to control the parameters of the aqueous ink, including the viscosity, fluidity, thixotropy, and plasticity of the ink.
2. Note the control index of water-based ink. The control indexes of water-based inks mainly include fineness, viscosity, pH value, and drying speed. They play a decisive role in the application of inks.
3. Because water-based inks are pure water-based systems, care should be taken not to mix water-based inks with alcoholic inks and solvent-based inks or to add organic solvents to inks in order to avoid print quality problems.
4. When using water-based ink, it should be evenly stirred, and after measuring the viscosity, pour into the ink tank. If the viscosity is not suitable, adjust with diluent or thickener, and pay attention to control the pH of the ink. Generally, it is controlled at 8.5~. 9.5. During the printing process, due to the volatilization of water, the viscosity of the ink will increase and the pH value will decrease. Therefore, the usage of the ink must be observed frequently. When the viscosity of the ink is increased or the pH exceeds the control range, diluent and pH stabilizer should be used in time. Adjustment to ensure viscosity and pH stability.
5. Under normal circumstances, the ink is very suitable for high-speed printing, if the drying speed is too low or the printing effect is not good during low-speed printing, a certain proportion of slow-drying agent can be added.
6. After the printing is finished, the remaining ink should be taken back to the corresponding original bucket or the lid of the ink tank should be tightly closed for the next use. Prevents filming, thickening or even drying out of the ink surface. Water-based ink should be stored in a cool, ventilated room with room temperature controlled at 5 to 45 °C.
7. The ink remaining on the printing device after printing can be washed with fresh water when it is not completely dry, and the ink that cannot be washed with fresh water can be cleaned with a cleaning agent.
8. The printability of ink is affected by objective conditions such as printing conditions, substrate surface characteristics, ambient temperature and humidity, and the length of storage, so when using it, some additives are needed to make fine adjustments to the ink to obtain the best printing effect. Commonly used additives in water-based ink color materials, pH stabilizers, slow drying agents, defoamers, diluting agents, etc., should be familiar with the characteristics and use of additives in printing.
1) The coloring material is used to deepen the color, and its amount should generally be controlled at 0 to 30%. More than 30% will cause the content of the binder in the ink to be too low, resulting in a decrease in adhesion and wear resistance.
2) The pH stabilizer is used to adjust and control the pH value of the aqueous ink, making it stable at 8.0-9.5, ensuring the printing state of the aqueous ink. At the same time, it can also adjust the viscosity of aqueous ink and dilute the ink.
3) The slow drying agent is used to reduce the drying degree of the ink. The slow drying agent can suppress and slow the drying speed of the water-based ink, prevent the ink from drying on the printing plate, and reduce the occurrence of printing failures such as blocking and stencil printing. The amount of slow-drying agent should generally be controlled at 0 to 10% of the total amount of ink. If the amount of slow-drying agent is too much, the ink will not dry completely and the printed matter will become dirty or have an odor.
4) Diluter is mainly used to reduce the depth of color, and the amount is not limited. However, the addition of a large amount of diluting agent may cause the ink viscosity to slightly increase. At this time, a small amount of diluent may be added at the same time to adjust.
5) Dilution is mainly used to reduce the viscosity of ink, the amount should be controlled within 3%, it will not reduce the color depth of ink, and can maintain the pH of ink. The thinner should be added slowly while stirring to prevent the viscosity from being too low. If more diluent is added or diluted with tap water with high hardness, air bubbles will be generated, and defoamer should be added at this time.
6) The main reason for foaming problems in the use of water-based inks is that water is used as a diluent. In emulsion polymerization, a certain amount of emulsifier must be used to greatly reduce the surface tension of the emulsion system. Defoamer's role is to suppress and eliminate the generation of air bubbles, to avoid the appearance of pinholes, white leaks, uneven ink and other print quality problems caused by bubbles, but the amount of general defoamers should be controlled at 0.1% to 0.2%.
7) The thickener is mainly used to increase the viscosity of the ink, while also giving the ink excellent mechanical and physicochemical stability, to control the rheological effect during printing, but the amount can not be too much, if used in water-based ink too More thickeners will increase the thickness of the bubble's membrane wall and increase its elasticity so that the foam is not easily eliminated. Generally, it is controlled at 0.5% to 1%.
8) The quick drying agent is mainly used to accelerate the drying speed of the ink, and the amount is 0 to 30%.
9) The anti-settling agent can make the pigment particles in the ink suspended and dispersed evenly and not easily aggregated and settled to avoid floating phenomenon.
The future development trend of water-based ink
Due to the excellent environmental characteristics of water-based inks, developed countries and regions are currently working hard to develop and use aqueous inks to gradually replace solvent-based inks. The packaging printing with water-based ink printing as the main development object has become a trend in the world. From the perspective of the development trend of international packaging printing, water-based ink has developed from a single carton ink to various substrates and multi-color printing.
In the United States and some European countries, the use of solvent-based inks containing benzene has been banned and green inks have been replaced. At present, 40% of U.S. plastic printing uses water-based inks. Other developed countries, such as Japan, Germany, France, etc., are also increasingly using water-based inks in plastic film printing. The United Kingdom has already banned the use of solvent-based inks to print films for food packaging. In recent years, China has made some provisions for food packaging, and the status of water-based ink printing is continuously improving in our country.