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Food packaging printing inks are directly related to food safety and people's health, so they are receiving more and more attention. Flexographic water-based inks are widely used in food packaging because of their non-toxic and environmentally friendly advantages. In this paper, the author discusses the advantages and printability of our company based on our experience in applying flexo-printed inks for many years.
Advantages of flexo-printed water-based inks
1. Reduce VOCs emissions, which is conducive to environmental protection
VOCs are recognized as one of the major sources of global air pollution today. Compared with the solvent-based ink, a large amount of low-concentration VOCs are emitted, and the aqueous ink is used as a dissolving carrier. The substances released during the printing process are mainly water and a small amount of alcohol, and the VOCs are hardly emitted to the atmosphere. Solvent-based inks are unmatched.
The viscosity and pH of aqueous inks are the focus of the printing technician during the printing process. After years of testing and application, the current water-based ink can maintain the pH value at a constant parameter according to requirements, reduce the pH adjustment during the printing process, and reduce the use of ammonia substances, thus enabling the use of water-based inks. Convenience is more advantageous.
2. Reduce the residual poison of the ink layer to ensure food hygiene and safety
At a time when humans are paying more and more attention to food safety, it is quite difficult to achieve the hygiene and safety requirements of foods using solvent-based inks, but it is a breeze for prints using water-based inks. Water-based ink completely solves the toxicity problem of solvent-based inks and offset inks. It does not contain organic solvents, so that the toxic substances remaining on the surface of the printed matter are greatly reduced. This feature makes food, beverages, medicines, children's toys and other packaging and printing materials with strict requirements for health and safety conditions have good health and safety.
For the water-based inks required for food packaging, the state also mandates that the main and auxiliary materials in the water-based inks need to meet the requirements in GB9685-2016 "Sanitary Standards for the Use of Additives for Food Containers and Packaging Materials". At the same time, the printed matter also needs to have a lot of resistance to resistance, such as alcohol resistance, n-hexane resistance, isooctane resistance and the like.
3. Reduce energy consumption and reduce production costs
In the process of paper packaging and printing, there are three main types of drying of water-based inks: when printing coated paper, volatile drying is the main; when printing non-coated paper, it is mainly by osmosis drying; when printing light coated paper, It takes the form of both volatilization and permeation drying. Therefore, compared with the solvent-based ink, the UVA light-curable printing ink layer which requires high energy consumption due to volatilization drying and UV ink, the drying process of the aqueous ink can adjust the energy of the drying end according to the characteristics of the surface of the material and the printing speed, even in printing. When non-coated paper is used, the ink which is mainly osmosis-drying can realize high-speed printing without using the hot air system, and reduce power consumption during printing.
In addition, when cleaning the water-based ink, the plate and the color group can be cleaned only with water or a small amount of alkaline solution, and the waste liquid generated during the cleaning process can also be filtered by using a wastewater treatment device, and the filtered water can be repeated. The use reduces the industrial water consumption of the printing enterprise and greatly saves the production materials. The use of offset inks, solvent-based inks, and UV inks, in addition to generating a large amount of waste liquid during cleaning and maintenance, inevitably generates a large amount of solid waste.
Printability of flexo-printed water-based inks
Although water-based inks have many intuitive advantages, they still have some problems in printing applicability in the practical application of flexo printing and gravure printing, which restricts the application of water-based inks and makes them stand out. At present, flexo-printed water-based inks are mainly used for printing of simple packaging such as corrugated boxes and environmental protection bags. In the industry, the application of water-based ink printing is synonymous with low-end packaging printing. Looking back at the process of applying flexographic inks by our company, with the improvement of flexographic technology and the improvement of water-based ink performance, the quality of printed matter has made a qualitative leap. At present, our company pays special attention to the following three points of printability of flexographic water-based inks: the physical and chemical properties of the ink layer, the matching with the flexographic plate, and the suitability of the spot color printing.
1. Physical and chemical properties of the ink layer
(1) Physical properties
Physical properties mainly refer to the friction resistance, because the printing materials inevitably wear and tear during printing and post-press processing, so there is a high requirement for the abrasion resistance of the flexographic aqueous ink.
The rubbing ink was subjected to a rubbing experiment using a flat friction instrument, and the rubbing resistance of the water-based ink was evaluated by examining the change in the density of the ink layer of the printed matter. Through years of test data collection, we found that the factors affecting the rubbing resistance of flexographic inkjet inks are the surface flatness of the printed material, the thickness of the ink layer (color density), and whether the surface of the ink layer is glazed.
In order to improve the rubbing resistance of the flexographic aqueous ink, it is generally required to add a certain proportion of the anti-friction aid to the ink, and to improve the proportion of the film-forming resin emulsion in the ink formulation, so that the ink layer on the surface of the printed matter has good rubbing resistance. It is particularly important to note that the ratio of the film-forming resin emulsion in the ink formulation needs to be subjected to a certain printing test, and the ratio of the film-forming resin emulsion is increased blindly, and the ink transfer failure occurs in the printing, thereby causing printing color deviation and ink stick. And other quality issues.
(2) Chemical properties
For the chemical properties of the flexographic aqueous ink layer, such as alcohol resistance, hexane resistance, and lightfastness, the ink company needs to select a suitable raw material. In particular, in terms of the chemical properties of the pigment, it should be subjected to corresponding physical and chemical tests to select the corresponding raw materials to ensure that the produced ink has good chemical properties.
2. Matching with flexo
Since the graphic carrier is a solid photosensitive resin plate during the flexographic printing process, the matching between the aqueous ink and the flexographic plate largely determines the stability of the printing quality in the flexographic printing process. The biggest problem with flexo inks is that they are prone to blockage.
(1) Reasons for blocking
In the early flexo products, for the printing of four-color mesh images, there was usually a situation where the dots were clogged. Production front-line employees often think that this is caused by factors such as too fast ink drying, unreasonable selection of anilox rolls, and too thick printing ink layer, and rarely consider the matching relationship between printing plates and inks. In 2014, our company purchased professional plate making equipment. In order to guarantee the printing quality, the printing plate was sent to a professional company to test the surface tension of the printing plate. To test the surface tension of the printing plate, the contact angle between the liquid and the solid is required, as shown in Figure 1. Through many tests, we found that the surface tension of the surface area of the flexographic surface (inking area) is 37-39 mN/m, and the surface tension of the non-image area is 39-41 mN/m, which is higher than the surface tension of the image area. 2 to 3 mN/m. A schematic diagram of the two areas is shown in Figure 2. Our analysis is that the shampoo brush affects the surface tension of the non-image area on the surface of the plate. It is precisely this difference of 2 to 3 mN/m, which makes it easier for the non-image area to adsorb ink relative to the image area during printing.