What is the ink of an inkjet printer?
The ink of an inkjet printer is a kind of “liquid ink” that is sprayed on a predetermined area on the surface of the substrate by means of the electric field force between the nozzle of the inkjet printer and the substrate to form a print of graphic information. The inkjet ink can be referred to as the blood of the inkjet system. The performance of the inkjet ink directly affects the output quality of the inkjet image, and some even affect the smooth progress of the inkjet operation. General inkjet ink should have: stable physical and chemical properties, non-corrosive to metal objects such as nozzles, in addition, it is not swallowed by bacteria, and is not easy to be burned and fade; the ink has good playability and drying performance, and it is not corrosive during the spraying process. The nozzle is blocked and it does not dry on the nozzle wall, but the ink can dry and adhere firmly on the surface of various types of printing materials after being ejected; the color density, viscosity, surface tension, and other printing properties of the ink meet the needs of digital printing because Ink affects the color vividness of inkjet printed images, and viscosity and surface tension directly affect the normal progress of inkjet operations.
Types of inkjet inks, chemical components, and classification
The chemical components of inkjet inks are similar to those of printing inks and are generally composed of pigments, binders, and other additives. Colorants mainly play a significant role in coloring inks. Common colorants are divided into dyes and pigments. At present, dye inks are commonly used in China. The main reason is that dye inks have the advantages of low price and good printing effect, which is more important. Most of the compatible inks on the market are dye inks. Therefore, for inkjet printer users, the choice of dye ink means that there is more room for choice and the use cost is relatively low. However, the disadvantage of dye ink is that it is only suitable for indoor use. It will quickly fade under outdoor light and is not scratch-resistant and not waterproof. In addition, the use of dye ink for output requires the use of printing materials with special coatings. Pigment inks have been widely used internationally. Pigment inks have overcome the shortcomings of the photochemical activity of dyes. They have strong light and water resistance, and their fading and changes are extremely slow, and they can be kept outdoors for a long time without fading. In addition, it does not require high-output media.
The binder of the ink can be water, oily substance, solvent, resin, etc. Generally, people divide the ink into water-based ink, oil-based ink, solvent-based ink, and thermosetting ink according to different binders.
1) Water-based ink
Water-based ink refers to an “ink system” formed by dissolving water-soluble dyes or dispersing pigments in water or a mixture of water and organic solvents as a carrier.
The basic composition of water-based ink:
① Solvent (80~90%), the purpose is to uniformly dissolve/disperse color formers and other additives; “water” is generally deionized water with extremely high purity, and the general pH value is about 10.
②Additives (5-10%), mainly high-boiling liquids, to reduce the volatilization of water solvents and reduce the clogging of dye deposition nozzles caused by the volatilization of water solvents. Common additives: glycerin, water-soluble oligomers, etc.
③Coloring agents (2~6%), according to different colorants, can also be divided into dye type and pigment type. In the dye-based water-based ink, because the dye molecules are dissolved in the carrier, each dye molecule is surrounded by the carrier molecules, no particulate matter can be observed under the microscope, and its optical density is low, and the light resistance and water resistance are poor, but the output There are many types of image colors, and the vividness is better. In the pigment-based water-based ink, since the pigment is insoluble in water, it is dispersed in the carrier in a loose particle state, so the pigment particles must be ground very fine, requiring a small size of 30μm or less to have a better printing effect. , Otherwise, it is easy to cause blocking when using high resolution. In addition, pigment particles usually need to be treated with a dispersant or stabilizer to prevent the pigment from forming agglomerates and causing precipitation.
Water-based ink is an environmentally friendly ink. It does not contain volatile organic solvents. Therefore, it has no pollution to the atmosphere and will not harm the health of ink manufacturers and printing operators. As the binder uses water, the drying time of water on the printing material is relatively long, but the nozzles of the inkjet head also tend to dry quickly. Therefore, frequent maintenance of the inkjet head is required. Water-based ink is suitable for porous and non-coated printing materials, such as cardboard. Sometimes, in order to obtain the best printing effect and durability outdoors, expensive special printing materials are required, or coating treatment is required, so the application of water-based ink is subject to certain restrictions.
2) Oil-based ink
Oil-based ink is an “ink system” formed by using low-volatility oils (vegetable oil, mineral oil, etc.) as a binder and dissolving dyes or dispersing pigments in it. The colorant of oil-based ink can also be divided into two types: pigment type and dye type, but the common one is pigment type. Oil-based ink is most suitable for printing on porous absorbent materials because it is dried by absorption. However, due to the characteristics of oil, it can never be “dried” on non-permeable or semi-permeable materials. “. It is characteristic of oil-based ink that dries quickly, so it is more commonly used in industry, such as bar code printing on outer boxes. Because it does not dry up in the print head, it also greatly reduces the maintenance cost of the print head.
3) Solvent ink
Solvent ink refers to an ink system formed by dissolving dyes or dispersing pigments in the carrier using organic solvents or solvent-based polymer film-forming agents as the carrier.
The basic composition of solvent ink:
① Color developer (2~6%), generally a pigment, uniformly dispersed in the resin solution, the particle size is in the micron-submicron level.
②Additives (5-10%), mainly dispersants, surfactants, and high-boiling organic solvents.
③Solvent (80~90%), the purpose is to uniformly dissolve and disperse resin and other additives, generally organic solvent. The solvent is the main component in the solvent-based inkjet ink binder. It is an important component for adjusting the viscosity, surface tension, and dryness of the inkjet ink. When the inkjet is transferred to the substrate, the volatile solvent quickly evaporates and evaporates. The low-performance solvent penetrates into the substrate by capillary action, so that the resin binder remaining on the surface of the paper is fixed on the surface of the paper and dried.
Compared with other types of inks, solvent inks do have certain advantages in terms of image quality, mainly in terms of vivid colors and good color reproduction. Because the solvent ink is dried by the principle of solvent volatilization, it spreads less laterally on the printing material and the image drying speed is fast. Therefore, it does not have high requirements on the substrate. It can be used in such as paper, plastic film, self-adhesive film, and Direct printing on uncoated materials such as mesh fabrics. If printing on the surface of non-absorbent materials, sometimes in order to increase the drying speed, drying measures are often considered, which can speed up the drying speed and prevent the ink from forming stains in high-density areas. Since solvents are easy to volatilize, you must pay attention to the daily maintenance of the print head when using solvent-based inks, and cover the print head when not in use. The disadvantages of solvent-based inks are also obvious. The level of VOC (volatile organic compounds) of solvent-based inks is very high, which will have an adverse impact on the environment. Therefore, the use, storage, and disposal of solvent-based inks need to be controlled, and, They must be used under well-ventilated conditions.
4) Light-curable ink
UV/EB light-curable inks have been used in traditional offset printing, screen printing, flexo printing, and gravure printing units for at least ten years, and compared with other inks in the above printing methods, they have been continuously expanding their market share. Its environmental protection, stability, and quick drying, even on non-absorbent media can also dry quickly performance is the main factor in its occupation of the market. In contrast, the use of photocurable inks in inkjet printing is relatively late, but the development is extremely rapid.
UV light-curable ink is developed on the basis of light-curable coatings. As a new type of environmentally friendly ink, it has energy saving, no solvent emission, no pollution to the environment, high production efficiency, and is suitable for heat-sensitive substrates, The advantages of printed products, such as excellent performance and low equipment investment, have been valued by countries all over the world.
UV light curing ink uses ultraviolet radiation energy to excite the photoinitiator in the liquid ultraviolet material into free radicals or cations, thereby triggering the chemical reaction of the unsaturated double bonds in the ultraviolet material, forming a cured network structure. The ink is mainly composed of photo-curable resin, active diluent, photoinitiator, coloring agent, additives, and so on.
① Light-curing resin, also known as an oligomer, is a low-molecular polymer containing unsaturated functional groups, most of which are acrylate oligomers, which are the main film-forming substances of ink, with a content of about 30%-50%. Its performance determines to a large extent the physical and chemical properties of the ink, the printability, and the performance of the printed matter.
②Reactive diluent, also known as a monomer, monomer plays a key role in the system. It not only adjusts the viscosity, but also acts as a film-forming substance, which affects the final performance of the film, the curing kinetics of the reaction, the degree of polymerization, and the formation of polymerization. The nature of things has a direct impact.
③Photoinitiator, a small amount of photopolymerization initiator is added to the ink. Under the action of ultraviolet light, the photoinitiator absorbs light quanta, decomposes to generate free radicals, initiates polymerization of unsaturated double bonds, and polymer molecules interact to form a network. Structure, when the free radical loses its activity, the chain growth stops. At this point, the ink is completely cured.
④Colorant, the colorant in UV curable ink is generally pigment. Generally, if a system contains pigment, its application performance is largely affected by the performance of the pigment. These effects include stability to light, heat, climate, organic solvents, etc., and involve dispersion in the processing process, blooming, and color matching of the final product, and so on. The pigment is the key to determining the color and optical characteristics of the final product, and the color and optical characteristics are closely related to the nature of the pigment and its dispersion, that is, the particle size. Under the same composition, hue, tinting power, and hiding power are all related to the dispersion of pigment particles.
⑤Additives, in order to improve certain properties of products, when preparing UV-curable resins, an appropriate amount of leveling agents, plasticizers, polymerization inhibitors, wetting agents, defoamers, heat stabilizers, etc. are often added.
Thermosetting ink means that the ink is solid at room temperature, it is a hot melt liquid when ejected from the nozzle, and solidifies quickly when it reaches the surface of the substrate. This rapid solidification feature can prevent the ink from spreading or penetrating into the substrate. , To ensure the printing quality, can output high-quality, high-gloss images, while adapting to the demand of unlimited high-speed printing.