The current deinking technology can remove most of the impurities and ink. However, with the continuous development of new printing technologies and increasingly complex ink formulations, completely removing the ink has become a major challenge. The deinking operation depends largely on the type of ink and impurities in the waste paper.
1. Composition and characteristics of printing ink
1. The composition of the ink depends on the type of paper to be printed, the printing method, the drying process and the application of the paper.
There are two main components:
â‘ Pigment or dye: Pigment provides color and opacity to form a certain contrast with the paper sheet it is to be used for. It is also important for the transmission of viscosity and the required flow characteristics.
â‘¡Vehicle (linking material): The connecting material gives the ink the necessary transfer properties, and has the function of spreading and carrying ink particles. It is a carrier of pigment particles, which is conducive to the retention of pigments or dyes on paper sheets. The binders are usually dry vegetable oils, mineral oils, synthetic or natural resins, plastics and organic solvents.
â‘¢Others: Printing ink can also include some other components such as adhesives, solvents, desiccants, wetting agents and paraffin.
2. Printing ink must meet certain requirements
It should be able to
â‘ Provide good contrast;
â‘¡It has good printability for the printing method used;
â‘¢ Has good drying characteristics;
â‘£Good stability.
Important ink characteristics are viscosity, adhesion, size, and drying characteristics. The viscosity of the ink determines the flow properties of the ink and how the ink yields under the pressure of the printing plate. The fluidity of the ink will also affect the penetration of the ink. Viscosity refers to the property that the ink resists external forces, such as the pulling force of the printing plate, so that it does not fall off the surface of the paper sheet. The ink must have a certain viscosity in order to be well fixed on the paper sheet. However, if it is too sticky, it will cause the surface of the paper to tear or fluff or powder. The size is used to measure the degree of dispersion of ink particles on the paper. The thick and short ink particles will cause spots and the ink layer is wrinkled: long rod-shaped ink particles will cause ink drawing, which is a special problem in newsprint production. . In addition, the fixing characteristics of the ink are particularly important, because further processing after printing must be carried out after the ink is fixed.
All these ink characteristics are directly related to the ink removal efficiency, and they determine how tightly the ink is bound to the fiber and how easy it is to remove.
3. Ink fixation method
Ink fixation on paper is based on one of the following two main mechanisms:
â‘ The connection material is absorbed in the pores of the paper;
â‘¡ The ink solidifies on the surface of the paper sheet.
2. Modern printing process
With the continuous development of printing technology in recent years, a variety of new printing processes have emerged, such as flexographic printing, laser printing, xerography and inkjet printing, and corresponding special ink formulations. An obvious requirement for printing ink is to be able to adapt to the required printing method. Naturally, the formulation of a special ink should be adapted to the printing method it is to be used for. Before discussing how to remove these new inks, let's take a look at the applicable printing methods.
1. Newsprint / magazine paper printing process
â‘ Letterpress printing: Letterpress printing is mainly used for the printing of newsprint, magazines and kraft paper. The viscosity and viscosity of embossing ink are moderate, in the form of paste, which is made of pigment and desiccant mixed with dry oil binder. Carbon black dispersed in oil (hydrocarbon) is a typical formula for relief printing inks. Sometimes resins and other auxiliary agents are added to obtain special properties such as gloss. The ink can be dried by penetration, volatilization or gelation. Most flat sheet embossed inks are fixed by oxidized conjunctiva.
â‘¡ Offset printing: Offset printing is an indirect printing method based on the principle that oil and water are immiscible, so that the graphic part is inked, and the non-graphic part is not inked. Lithographic printing inks are composed of water-resistant binders and pigments that are insoluble in water or solvents. Compared with letterpress inks, the pigments used in lithographic printing inks have strong coloring power and often use linseed oil as binder. In modern ink formulations, alkyd resins and dry oils are used as connecting materials. When dried, they will polymerize to form a strong cross-linked film. Thermosetting inks often use hydrocarbon-based resins as binders, which are dried by volatilization. The bonding material of ultraviolet light solid ink is composed of acrylic monomer and its prepolymer, which is polymerized into a strong film under the irradiation of ultraviolet light, which is often used for newsprint color printing.
â‘¢ Gravure printing: Rotating gravure printing is to use the concave part of the graphic on the printing plate to form the graphic, the blank part is higher than the graphic part. Gravure printing ink is a kind of quick-drying ink, and its main mechanism of drying is: relying on the volatilization of the ink solvent, it is usually dried under heating. The viscosity of these inks is low, and it is easy to transfer from the concave part of the printing plate to the paper under the action of the capillary in the paper sheet. This type of ink usually uses polyamide or polyacrylamide dissolved in aromatic hydrocarbon or ester solvent as the ink. Because this type of ink has the risk of fire or even explosion, some inks use water-based inks with low alcohol content and water and alcohol as the connecting material.
â‘£ Flexographic printing ink: Flexographic printing is improved from relief printing. During letterpress printing, some ink will overflow from the side of the high pressure area. Because not all the ink will be absorbed by the paper in an instant, it is more important that the hardness of the metal or plastic plate will permanently deform the surface structure of the paper. These quality defects have promoted the development of flexible printing plates. The new printing plate is called flexographic relief. Commonly used for low-cost packaging paper and paperback printing, this technology is now more and more widely used for printing newsprint.
Similar to gravure inks, flexible inks have two notable characteristics of low viscosity and fast drying. Commonly used alcohol. Ester mixed solvent as a linking material, rely on volatile drying.
2. Office paper printing process
There is growing interest in the deinking of white paper, such as office copy paper, laser and inkjet printing paper. The printing of these papers does not involve the aforementioned traditional printing process. In fact, some printing technologies do not use the traditional inks of various connecting materials.
The commonly used deinking technology is developed to remove traditional inks, and cannot meet the needs of removing these new inks. It is necessary to formulate new strategies for the effective removal of such inks, develop new equipment, and improve the chemical deinking process. Before discussing the ways in which such inks can be removed, it is necessary to simply test the appropriate printing techniques for these inks.
â‘ Electrostatic printing: Organic pigments such as carbon black in electrostatic printing ink are dissolved in thermoplastic resin binder and formulated. Generally, the toner particles are dry, and a very small amount of zinc stearate will be added as a dry agent (or dry lubricant). Liquid toner is a suspension formed by dispersing toner particles in an insulating liquid. Sometimes resin or vegetable oil is added as a charge regulator, in these cases the ink particles are dissolved on the surface of the paper by heating.
â‘¡Inkjet printing: Inkjet printing is to convert the digital image of the original document stored in the magnetic tape or magnetic disk into a printed image by controlling the projection of ink droplets on the paper. The viscosity of the ink must be very low to ensure that it can pass through the inkjet device and can be dried quickly after being sprayed onto the paper. However, it should not be dried too fast to avoid clogging due to drying in the nozzle. Inkjet printing ink is mainly composed of water-based or ethanol-based water-soluble pigments, glycol, polyether and wetting agent.
â‘¢Laser printing: The principle is similar to electrostatic copying, and the ink used is similar to electrostatic copying ink.
3. Principle of deinking
The equipment and chemicals used in the established deinking process depend on the ink type of the waste paper. Ink that is easy to fall off and break during pulping is easier to remove from the pulp than those that are resistant to treatment.
The size of the ink particles to be removed is the main basis for choosing a suitable deinking device. The commonly used deinking processes now include flotation deinking, washing deinking, washing-flotation deinking, and solvent deinking. The mechanism of flotation deinking is to remove ink particles that are too small to be removed by slit screens and centrifugal separators, as well as particles that are too large to be removed by washing. Washing is the most effective method for removing the smallest ink particles. The slit screen and centrifugal separator are suitable for removing large ink particles. Washing and flotation operations need to add chemicals to improve their defoaming efficiency, so high-efficiency deinking strategies must also include an evaluation of the chemicals and equipment used.
Solvent deinking is mainly used to deink laser printing and photocopying paper. Paraffin wax is usually used as a solvent in the deinking of the washing method. Paraffin wax is said to emulsify the ink. The vehicle network that fixes the pigment particles is swelled to weaken its fixing effect, and the ink is further broken into fine particles and dispersed in water under the action of surfactant and alkali.
The solvent can also dissolve wax, stickies, other impurities and all inks. Some waste paper can be cleaned with solvent and a little or no water can be used to obtain very clean fibers. However, from an economic point of view, it is difficult to remove ink with the solvent method. The environmental pollution that comes here cannot be afforded economically. One is the treatment of the solvent after recovery.
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