Flexo inks used in packaging and printing

In recent years, the flexographic printing process has achieved great success in the field of packaging and printing. Flexo printing is called the “best and most promising” printing method. Flexo printing inks have played a role in the promotion and development of the flexographic printing process. How to make a suitable packaging flexo ink is placed on people The foremost task, Siegwerk, a renowned high-end ink manufacturer, has made important contributions to the research and development in this area.

First, the composition of flexographic ink

The ink is made by mixing, grinding and other processes in a certain proportion of pigments, binders and auxiliary materials. Among them, the pigment is a color forming substance in the ink, which determines the hue of the ink; and the binder is a dispersion medium and a film former of the pigment, which is a key factor for determining the performance of the ink. Different types of inks often use different types of linking materials. At present, the linking materials mainly include oils, resins, and solvents. Only the pigments and binders in the ink can not satisfy various printing requirements. A certain amount of auxiliary materials must also be added in order to achieve specific printability and improve ink performance. According to the different solvents, flexo printing inks can be divided into three categories: water-based inks, solvent-based inks and UV-curable inks.

Water-based inks consist of water-soluble (or alcohol-soluble) resins, pigments, water, and excipients. The composition ratios are as follows: water (40%-60%), pigment (10-25%), resin (15-25%) and auxiliary material 5%. Because the ink does not contain organic solvents, it has two major advantages of no pollution and economy, and is highly valued by the printing industry. The main purpose of adding the water-soluble resin to the ink is to adjust the viscosity and fluidity of the ink, impart a dispersion effect to the pigment, give the ink ink film shape and ink fixing performance, and stabilize the dispersion effect. Most of the water-based ink resins currently used are alkali-soluble resins. The resin itself is insoluble in water, but it is soluble in ammonia or amines to make salty water-based binders. Therefore, water-based inks require pigments with alkali resistance. Water-based ink cosolvents are mostly alcohols or glycol esters, and the content is generally about 5%. Its function is to reduce the surface tension of water-based inks, improve the wetting effect on substrates, improve the solubility of resins, maintain proper viscosity, and adjust Drying speed and so on.

Solvent ink is mainly composed of organic solvents, resin binders, pigments and auxiliary materials. The composition ratios are as follows: organic solvent (45% to 65%), resin binder (10 to 20%), pigment (15 to 25%), and auxiliary material 5%. The role of adding a solvent to a solvent-based ink is to dissolve the resin so that the ink has fluidity and a certain viscosity so that the ink can be transferred during printing and the drying speed can be controlled. The following organic solvents are commonly used in the manufacture of solvent inks:

● Alcohols (ethanol, isopropanol, etc.)
Ethylene glycol derivatives (methyl cellosolve, methyl ethyl benzoate, etc.)
● 酉 same class (equivalent to B and C)
● Hydrocarbons (gasoline, mineral turpentine, etc.)
● Ester (butyl acetate, ethylene glycol ester, etc.)

Solvent inks can be divided into synthetic resins and natural resins. Natural resins in the true sense are not suitable as ink binders, such as rosin, shellac, etc. In the ink manufacturing industry, chemical reactions are generally used to modify natural resins to make them suitable for making binders, such as rosin-modified phenolic resins. Resin, dry oil-modified carboxylic aldehyde resin, etc.; and synthetic resin is currently the most used resin in inks, such as: light-curing resin, plastic resin, alkyd resin and so on.

UV curable inks are inks that can produce photochemical reactions and cure in just a few seconds. The main components of inks are photopolymerizable resins, photopolymerization initiators, pigments and auxiliary materials. In general, UV curable inks do not contain organic solvents. The four types of component ratios are: binders (55% to 80%), pigments (10% to 20%), photopolymerization initiators (5% to 15%), and 5% to 10% of excipients. At present, photopolymerizable resins are mainly oligomers or low-viscosity, high-boiling monomers and the like that are polymerized by several basic molecular phases. Photopolymerization initiators are catalysts that induce photopolymerization reactions. Its reaction mechanism is that the photopolymerization initiator absorbs ultraviolet energy to generate free radicals, and its atomic group reacts with monomers or oligomers, and begins to polymerize, forming a hardened ink film in an instant through a series of chain reactions.

The above three types of inks can all use the same pigments. Pigments are divided into inorganic pigments and organic pigments. Inorganic pigments are mostly simple metal oxides or inorganic salts. Usually, they are not very strong in terms of hue and tinting strength, but they have hiding power. There are advantages in terms of heat resistance, solvent resistance, and price; organic pigments have bright color, strong coloring power, and high dispersion, but hiding power, heat resistance, and solvent resistance are not very good. At present, the ink manufacturing industry uses more synthetic organic pigments, and the structure is usually more complicated. In addition, in order to achieve a specific color effect in the field of package printing, people also use metals, pearlescent substances, and luminescent materials, etc., as pigments for the ink.

Second, application areas

Solvent-based inks have a wider range of applications than water-based and UV-curable inks. It is currently the most widely used type of ink. It is not limited by the chemical composition and physicochemical properties of the substrate. It can be printed on absorbent materials such as paper, cardboard, etc., or it can be printed on aluminum foil. , or other synthetic materials represented by non-absorbing materials; can be printed on the front of the printing material, but also printed on the back of the printing material.

Water-based ink uses water and resin as its own binder. Since the water is less volatile, the water-based ink has a good solvent balance. This is also the reason why the drying speed of water-based ink is slower than that of solvent-based ink. In many cases, the printing materials suitable for water-based inks are relatively narrow, and the production efficiency of water-based ink printing jobs is not high. Water-based inks can only select absorbent materials for printing under normal drying conditions because a portion of the water in the ink is absorbed by the absorbent material during printing, thereby accelerating the ink drying process and reducing the consumption of ink for drying. Thermal energy. At present, water-based inks are widely used in corrugated paper printing and gift packaging, wallpaper printing, napkin printing, fruit juice soft packaging, soft laundry liquid packaging, cigarette packaging, and the like. If the printing equipment is equipped with an ideal ink drying system, we can also use water-based inks for printing on non-absorbent substrates, such as printing PE bags or carrying bags on polyamide or PVC materials. The ink is required to have good adhesion, the ink still has good abrasion resistance after being subjected to hot stamping and thermal disinfection treatment, and has properties such as no fading, cold resistance, water resistance and solvent resistance.

UV curable inks can be printed on absorbent materials or on non-absorbent materials. Since the UV-curable ink does not contain organic solvents or water, but is a mixture of oligomers and pigments, the ink has a high concentration, good stability, good printability and printing effects, and its prints are high enough. luster. At present, the most typical application area is the use of paper or polyester film for label printing, cup printing, cardboard printing, as well as paper bags and plastic bag printing for food packaging.

UV-curing inks are advanced printing materials like water-based inks. The market share is not very high, and there is still a long way to go. However, as far as the current packaging printing is concerned, people should consider how to choose the right printing materials to match with different formulations of water-based inks and UV-curable inks, so as to obtain more ideal prints to further promote water-based inks and UV curing. The purpose of type ink. Here we take the graphic food packaging bag as an example:

This packaging material is a synthetic material made of a mixture of polyester, aluminum foil, and other polymers. Because it uses less raw materials, is lighter in quality, and has better adhesion to ink, it is more economical and sustainable in terms of resources than food cans made of metal or aluminum. It can be imagined that with the advancement of material synthesis technology, the promotion and use of ink will inevitably be promoted. There is still an unresolved issue in the application of flexo-curable UV curable inks: UV rays tend to emit odors, which will limit their use in odor-sensitive printing applications.

III. Environmental Protection and Safe Production

Solvent inks contain lightly-volatile organic linkers, and the consumption of ink during printing is relatively large, thus contaminating the surrounding environment, causing acute or chronic poisoning in print workers, and easily causing fires. The European Union’s regulations for safe production stipulate that the content of organic solvents in the air cannot exceed its limit value; explosion protection devices should be equipped on the electrical equipment of the printing units, and inspections and maintenance should be carried out frequently to avoid fires; and workplace ventilation should be exhausted as much as possible. The concentration of organic solvents in the air is minimized; solvent collectors should be installed wherever possible.

Since the water-based ink contains only a small amount of alcohol, it is not necessary to install an exhaust gas treatment device. However, in view of safety in production, it is necessary to install an exhaust fan in the workshop in order to reduce the concentration of ammonia in the air. According to tests, the MAK value of ammonia in the air during the printing process can be as high as 20 DDm, so we can use as many alcohols as possible to reduce the amount of ammonia used during the use of water-based inks, but this has resulted in A new problem, because alcohol is also a volatile substance, in the printing process is easily heated to the ambient air, if the content of alcohol in the air is too high, there is the risk of causing a fire or even an explosion.

The use of UV-curable ink printing does not involve the problem of solvent evaporation, so people can save huge investment in solvent treatment devices and explosion-proof devices on electrical appliances, and at the same time, because they do not need huge and expensive drying devices, they also make flexo printers become More compact.

Three types of inks involve three different types of chemical compounds. We should consider the potential hazards in the treatment of such chemical substances, and give them an indication according to their respective hazard levels, such as solvent-based inks. On the "flammable (F)" mark, UV curable ink should be marked with "irritant (Xi)" mark.

Fourth, the remaining ink processing

How to deal with the remaining ink after printing will involve environmental protection, safe production, and reusability of resources. For solvent-based inks, if there is enough solvent in the remaining ink, we can distill the solvent in it, the obtained liquid can be reused, and the remaining residue can be treated by simply burning. For UV curable inks, we can use a special combustion heat treatment device to process the remaining ink. For water-based inks, we obviously cannot use the combustion process to treat the remaining ink. However, by using physical and chemical methods, certain metal salts are added to the ink at a specific pH to make the pigment and metal in the ink. The salt reacts to form a precipitate. The mixture is then filtered. The residue is filtered and disposed of as special waste. The filtered water is discharged into the sewer after it is detected without any quality problems. Therefore, the cost of processing water-based ink remains. Relatively high.

V. Summary

Economical cost, ease of printing, and good safety performance will be the reasons for the continuous expansion of the scope of water-based ink applications. Currently, the proportion of water-based inks in the field of flexographic printing accounts for 35%, and this ratio meets environmental protection, safety requirements, and solvents. Changes in costs, solvent profitability and other factors have further increased. The UV-curable inks are relatively low in equipment investment and operating costs due to their drying, there is no evaporation of organic solvents, there is no air pollution, the prints are almost odorless, tasteless, bright color and a series of advantages will also compete in the ink market. In an advantageous position.

Reprinted from: China Packaging Machinery Business Network