Modern printing ink and removal strategy
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Current deinking technology removes most of the impurities and inks. But as new printing technologies continue to be developed and ink formulations become more complex, the complete removal of ink has become a challenge. The deinking operation depends to a large extent on the type of ink and impurities in the waste paper.
First, the 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 purpose of the paper.
There are two main components:
1.1 Pigment or Dye: The pigment provides color and opacity to form a certain contrast with the sheet of paper to which it is to be used. It is also important for the transfer of viscosity and the required flow characteristics.
1.2 Vehicle (linking material): The binder gives the ink the necessary transfer properties, has the function of spreading and carrying the ink particles, and it is the carrier of the pigment particles, which is favorable for the retention of the pigment or dye on the paper. The binders are usually dry vegetable oils, mineral oils, synthetic or natural resins, plastics and organic solvents.
1.3 Others: The printing ink may also include some other components such as a binder, a solvent, a desiccant, a wetting agent, and a paraffin wax.
2. Printing ink must meet certain requirements
It should be able
2.1 provides good contrast;
2.2 has a good printability for the printing method used;
2.3 has good drying characteristics;
2.4 good stability.
Important ink characteristics are viscosity, tack, size and dryness. The viscosity of the ink determines the flow properties of the ink and how the ink yields under plate pressure. The fluidity of the ink also affects the penetration of the ink. Viscosity refers to the effect of the ink against external forces, such as the tensile force of the printing plate, so that it does not fall off the surface of the paper. The ink must have a certain amount of tack to be well adhered to the paper. However, too much viscosity can cause the surface of the sheet to tear or lint and fall off the powder. The size is used to measure the degree of dispersion of the ink particles on the paper. The short and short ink particles will cause spots and the ink layer will wrinkle. The long rod-shaped ink particles will cause the ink to be drawn, which is a special problem in the production of newsprint. . In addition, the fixing properties of the ink are particularly important because further processing after printing must be performed after the ink is fixed.
All of these ink characteristics are directly related to the efficiency of ink removal, which determines the tightness of ink-to-fiber bonding and the ease of extraction.
3. How the ink is fixed
The fixation of the ink on the paper sheet is based on one of two main mechanisms:
3.1 the binder is adsorbed in the pores of the paper sheet;
3.2 The ink is cured on the surface of the paper.
Second, the 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, xerographic and inkjet printing, and corresponding special ink formulations. An obvious requirement for printing inks is to be compatible with the printing method required. Naturally, the formulation of a particular ink is 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 printing methods that apply to them.
1. Newsprint / magazine paper printing process
1.1 Letterpress: Letterpress printing is mainly used for printing newsprint, magazines and kraft paper. The embossed ink has a moderate viscosity and a viscous viscosity, and is formed by mixing a pigment and a desiccant in a dry oil binder. Dispersion of carbon black in oil (hydrocarbons) is a typical formulation for embossed inks. Resins and other adjuvants are sometimes added to achieve special properties such as gloss. The ink can be dried by penetration, volatilization or gelation. Most flat sheet embossed inks are fixed by oxidative conjunctiva.
1.2 lithography: lithography is based on the principle that oil-water is immiscible, so that the graphic part is inked, and the non-image part is not indirect printing method. The lithographic ink is composed of a water-resistant binder and a pigment which are insoluble in water or solvent. Compared with the embossed ink, the pigment used for the lithographic ink has a strong coloring power, and linseed oil is often used as a binder. In modern ink formulations, alkyd resins and drying oils are used as binders, which polymerize upon drying to form a strong crosslinked film. Thermosetting inks are commonly used as a binder for hydrocarbon-based resins and are dried by evaporation. The UV-curable ink is composed of acrylic monomer and its prepolymer. It is polymerized into a strong film under ultraviolet light and is often used for newsprint color printing.
1.3 Gravure printing: Rotary gravure printing uses the concave part of the printing plate to form the graphic, and the blank part is higher than the graphic part. Gravure printing ink is a fast drying ink. The main mechanism of drying is: relying on the evaporation of ink solvent, usually under heating conditions to achieve drying. These inks have a low viscosity and are easily transferred from the concave portion of the printing plate to the paper by the capillary in the sheet. Such inks are typically used as a varnish in a polyamide or polyacrylamide dissolved in an aromatic or ester solvent. Because this type of ink has the risk of fire or even explosion, some inks use a water-based ink with a low alcohol content and water and alcohol as a binder.
1.4 Flexographic printing inks: Flexographic printing is an improvement from letterpress printing. In letterpress printing, some of the ink will overflow from the side of the high pressure zone. Because not all inks are absorbed by the paper in an instant, more importantly, the hardness of the metal or plastic printing plate will permanently deform the surface structure of the paper. These quality defects promote the development of the flexographic printing plate. The new printing version is called aniline relief. Often used for low-cost wrapping paper and book printing, this technology is now more and more widely used in the printing of newsprint.
Similar to gravure inks, flexible inks have two distinct properties, low viscosity, and are fast drying. A common alcohol-ester mixed solvent is used as a binder, and is dried by evaporation.
2. Office paper printing process
There is a growing interest in the deinking of white paper, such as office copy paper, laser and inkjet paper. The printing of these papers does not involve the aforementioned conventional printing process, and in fact some printing techniques do not use conventional inks of diverse binders.
Conventional deinking techniques have been developed to remove conventional inks and do not meet the need to remove these new inks. This necessitates a new strategy for effectively removing such inks, developing new equipment, and improving the chemical deinking process. Before discussing the ways in which such inks can be removed, it is necessary to simply test the printing techniques that are appropriate for these inks.
2.1 Electrostatic printing: Electrostatic printing ink Organic pigments such as carbon black are dissolved in a thermoplastic resin binder. In general, the toner particles are dry, and a very small amount of zinc stearate is added as a dry agent (or a dry lubricant). The liquid toner is a suspension in which toner particles are dispersed in an insulating liquid. Resin or vegetable oil is sometimes added as a charge regulator, in which case the ink particles are dissolved on the surface of the paper by heating.
2.2 Inkjet Printing: Inkjet printing converts a digital image of an original document stored on a magnetic tape or disk into a printed image by controlling the projection of ink droplets onto the paper. The viscosity of the ink must be low to ensure that it passes through the inkjet device and can be dried quickly after spraying onto the paper. But you can't do it too quickly to avoid clogging in the nozzle. Inkjet printing inks are primarily composed of water-based or ethanol-based water-soluble pigments, glycols, polyethers, and wetting agents.
2.3 laser printing: The principle is similar to electrostatic copying, and the ink used is similar to electrostatic copying ink.
Third, the principle of deinking
The equipment and chemicals used in a given deinking process are determined by the type of ink used in the waste paper. Those which are easily peeled off and broken during pulping are more easily removed from the pulp than those which are resistant to treatment. The size of the ink particles to be removed is the primary basis for selecting a suitable deinking device. The commonly used deinking processes now include flotation deinking, washing deinking, washing-flotation deinking, and solvent deinking processes. The mechanism of flotation deinking is to remove too small ink particles that cannot be removed with a slit screen and a centrifugal separator, as well as those that are too large to be removed by washing. Washing is the most efficient way to remove the smallest ink particles, with slit screens and centrifugal separators being suitable for removing large ink particles. Washing and flotation operations require the addition of chemicals to increase their defoaming efficiency, so a highly efficient deinking strategy must also include an evaluation of the chemicals and equipment used.
Solvent deinking is mainly used for deinking laser printing and photographic copy paper. Paraffin wax is generally used as a solvent in deinking by washing, and it is said that paraffin wax can emulsify an ink. The vehicle network of the fixed pigment particles is swollen to weaken its fixation, and the ink is further broken into fine particles and dispersed in water by the action of a surfactant and a base.
The solvent also dissolves waxes, stickies, other impurities and all inks. Some waste papers are washed with solvent, with little or no water, to obtain very clean fibers. However, from the economic point of view, solvent deinking is difficult. First, it is not easy to recover all the solvents. Environmental pollution is not affordable. One is the treatment of the solvent after recovery.