Deinking principle

Current deinking technology removes most of the impurities and inks. However, with the continuous development of new printing technologies and the increasing complexity of ink formulations, the complete removal of ink has become a major challenge. The deinking operation largely depends 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 Pigment or Dyes: The pigment provides color and opacity to form a certain contrast with the sheet of paper it is intended to use. It is also important for the viscosity transfer and the required flow characteristics.

2 Carrier (Binder): The binder imparts the necessary transfer properties to the ink, and has the function of spreading and carrying ink particles. It is the carrier of the pigment particles and is advantageous for the retention of pigments or dyes on paper sheets. The binders usually have dry vegetable oils, mineral oils, synthetic or natural resins, plastics and organic solvents.

3 Others: Printing inks may also include some other components such as adhesives, solvents, drying agents, wetting agents, and paraffin waxes.

2. Printing ink must meet certain requirements

It should be able

1 provides good contrast;

2 has good printability for the printing method adopted;

3 has good drying characteristics;

4 The stability is good.

Important ink properties are viscosity, tackiness, size, and drying characteristics. 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 action of the ink against external forces, such as the tension of the printing plate, so that it does not fall off from the surface of the sheet. The ink must have a certain viscosity in order to adhere well to the paper sheet. However, if the viscosity is too high, the surface of the paper sheet may be torn or fluffed, and the powder may fall off. The size is used to measure the dispersion of the ink particles on the paper sheet. The short and coarse ink particles will cause spots and the ink layer will wrinkle: The long stick ink particles will cause ink drawing, which is a special problem in newsprint production. . 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 properties are directly related to the removal efficiency of the ink. They determine the tightness of the bond between the ink and the fiber and the ease of release.

3. Ink fixation

The fixation of the ink on the paper sheet is based on one of two main mechanisms:

1 The binder is adsorbed on the pores of the sheet;

2 The ink cures on the surface of the sheet.

Second, modern printing technology

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 particular ink is adapted to the type of printing it is intended to use. Before we discuss how to remove these new inks, let us take a look at the printing methods they apply.

1. Newsprint/magazine paper printing process

1 Toppan Printing: Toppan printing is mainly used for newsprint, magazines and cowhide bags. The embossed ink has a moderate viscosity and viscosity and is in a paste form. It is a typical formula for embossing inks by mixing pigments and desiccants into a dry persimmon. Resins and other adjuvants are sometimes added to achieve special properties such as gloss. The ink can be dried by penetration, volatilization, or gel fixation. Most sheet-fed embossed inks are anchored by oxidized conjunctiva.

2Plain printing: Printing is based on the principle of oil-water immiscibility to make the text part inking, non-graphical part of the indirect printing method does not ink. The offset ink is composed of water-insoluble or solvent-resistant water-resistant binders and pigments. Compared with the embossed inks, the pigments used in the plain inks have strong coloring power and often use flax oil as a binder. In modern ink formulations, alkyd resins and dry oils are used as the binder. Polymerization occurs when dried, resulting in a strong cross-linked film. Thermosetting inks are commonly used as a binder for hydrocarbon-based resins and are dried by evaporation. UV light-curable inks are composed of acrylic monomers and their prepolymers. They are polymerized under UV light to form a strong film, which is commonly used in newsprint color printing.

3 Gravure printing: Rotary rotogravure printing is the use of the graphic part of the concave section of the plate to form the graphic, the blank part is higher than the graphic part. Intaglio printing ink is a kind of quick-drying ink. The main mechanism of drying is: relying on the volatilization of the ink solvent, drying is usually achieved under heating conditions. These inks have low viscosity and are easily transferred from the concave portion of the printing plate to the paper under the effect of capillaries in the sheet. This type of ink is typically a polyamide or polyacrylamide dissolved in an aromatic or ester solvent as a varnish. Because this type of ink has a fire, or even the risk of explosion, some inks use water-based inks with low alcohol content, water and alcohol as the connecting material.

4 Flexographic Printing Inks: Flexographic printing is improved from letterpress printing. In relief printing, some of the ink spills from the side of the high pressure zone. Because not all inks are instantly absorbed by the sheet, it is more important that the hardness of the metal or plastic plate will cause permanent deformation of the surface structure of the sheet. These quality defects promote the development of the flexographic printing plate. The new printing plate is called aniline letterpress. Often used for low-cost packaging paper and paperback printing, this technology is now more and more widely used for newsprint printing.

Similar to gravure inks, flexible inks have two distinct properties, low viscosity, and fast drying. Commonly used alcohol and ester mixed solvents are used as connecting materials and rely on volatile drying.

2. Office paper printing process

There is growing interest in the deinking of white paper, such as office copy paper, lasers and ink jet paper. The printing of these papers does not involve the aforementioned traditional printing processes, and in fact some printing technologies do not use conventional inks with diversified binders.

The conventionally used deinking technology was developed for the removal of conventional inks and cannot meet the need to remove these new inks. This necessitates the development of new strategies for the effective removal of such inks, the development of new equipment, and the improvement of chemical deinking processes. Before discussing the ways in which such inks can be removed, it is necessary to simply test the appropriate printing technologies for these inks.

1 Electrostatic Printing: Electrostatic Printing Inks Organic pigments such as carbon black are formulated in thermoplastic resin binders. The toner particles are generally dry and will add a very small amount of zinc stearate as a drying agent (or dry lubricant). Liquid toner is a suspension formed by dispersing toner particles in an insulating liquid. Resins or vegetable oils are sometimes added as charge modifiers, in which case the ink particles are dissolved by heat on the surface of the sheet.

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 it can pass through the ink jet device and can be quickly dried after being sprayed onto the paper. However, it cannot be done too fast so as not to cause blockage due to drying in the nozzle. Inkjet printing inks consist mainly of water-based or ethanol-based water soluble pigments, glycols, polyethers, and wetting agents.

3 Laser printing: The principle is similar to electrostatic copying. The ink used is similar to the electrostatic copying ink.

Third, deinking principle

The equipment and chemicals used in a given deinking process depend on the type of waste paper ink. In the pulping process, those inks that are easily detached and broken are 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 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 with slotted screens and centrifugal separators, and those that are too large to be removed by washing. Washing is the most effective method for removing the smallest ink particles, wherein the slit screen and the centrifugal separator are suitable for removing large ink particles. Washing and flotation operations require the addition of chemicals to increase their defoaming efficiency, so an efficient deinking strategy must also include evaluation of the chemicals and equipment used.

Solvent deinking is mainly used for deinking of laser printing and photocopying paper. Paraffin wax is commonly used as a solvent in the deinking process of washing, and paraffin wax is said to emulsify the ink. The swelling of the vehicle-carrying network, which holds the pigment particles, weakens the fixing effect, and further breaks the ink into fine particles and disperses in water under the action of a surfactant and an alkali.

Solvents also dissolve waxes, stickies, other impurities and all inks. Some waste papers are washed with solvents, and little or no water is added to obtain very clean fibers. However, from the economic point of view, solvent-based deinking is difficult. First, it is not easy to recover all the solvents. To environmental pollution, the economy can not afford it. The first is the recovery of the solvent.