Ink type and effect of deinking method (2)

Second, the printing method on ink characteristics and flotation deinking effect

The basic requirement for a corresponding brush ink is to meet the needs of the printing method. For different printing methods, the contact route of the ink and the fiber is different, and the physical and chemical properties of the ink are changed differently. The special changes of these inks directly affect the flotation deinking efficiency.

According to the ink solidification mechanism, printing methods can be divided into two categories.

The first type is the printing method where ink is attached by absorption and evaporation. In this case, the ink is adapted to different printing needs with different viscosities.

The lithography method is based on the principle that ink and water cannot be mixed. The image area on the printing plate is made into an acceptable oil-based ink instead of water. The lithographic printing ink is often called offset oil-based ink and is based on a water-resistant vehicle ( The linker) and the pigment in water or ethanol cannot flow out. This method is used for brushing newsprint, glossy magazines, books and photographic reproductions. In this method, the offset printing chilled ink is simply absorbed by the capillary pores of the paper at room temperature, and the pigment is then left on the surface of the paper without further drying. This is the liquid composition of the ink penetrating into the paper sheet. Whilst leaving solid components on paper. Even if the adhesion is carried out under dry or chemical reaction, there is still absorption. Therefore, the properties of offset cold-oil based inks do not change significantly during the printing process.

Letterpress printing and its improved printing, aniline relief printing is also this printing method, the ink is used for the convex surface of the printing plate, when the printing plate is pressed on the paper, the ink on the convex surface of the printing plate is Transfer to paper. Letterpress inks have medium or low viscosity and fast drying properties, commonly used oil-based and water-based inks. This method is a typical method of printing newspapers and magazines.

Rotary gravure printing is the use of the area of ​​the printing plate recessed to form the image, the ink is filled in the recessed area, and the paper is pressed against the ink by the elastic rubber roller pressed against the recessed area. Rotary gravure printing is typically used to print colored newspapers and magazines.

In letterpress and gravure printing, water-based inks and oil-based offset thermosetting inks are dried by evaporation of volatile solvents (such as water or low-boiling organic solvents such as aromatic hydrocarbons or esters) at 60°C. Other boring and pigments stay on paper. Because no chemical reactions occur during printing, oil-based offset plastisol inks and dried water-based inks retain their original hydrophobic properties after printing.

The second important type of printing method is indirect printing. A relatively high temperature is required during printing to allow the ink to adhere to the fibers. When a finely divided solid ink is used, these printing methods are photocopies and laser printing.
In the photocopying method, a latent image is formed on a charged photoconductive surface and then transferred to paper, that is, a charged surface reflects light to reflect the recorded latent image for copying. The light reflected from the non-printed area forms surface charges and dissipates. The areas that do not see light retain their original charge. These charged areas are exposed to the oppositely charged pigment particles, and the pigment particles adhere to the charged area to form a visible image. Finally, this image is transferred to paper. When the pigment particles are heated to 200° C., the styrene oxyester is melted and the color material is melted, and is combined with the paper fiber to become a polymer particle having a diameter of the order of mm to form an image. These particle size changes can be clearly seen from scanning electron microscope (SEM) photographs and optical microscopy (OM) photographs. At the same time, it is also possible to clearly see from the atomic force microscope (AFM) photographs the variation range of the small particles of the original photoprinting material after melting.
Laser printing is an image formed by a laser beam, and its printing principle is similar to photocopying. The laser beam forms a latent image by neutralizing the electric charge on the charged drum without the image area. After the toner particles adhere to the drum, they adhere to the charged image area and form a visible image. Transferred to the paper lie female crown, the pigment particles melted on the surface of the paper. Therefore, as mentioned above, the composition of the laser printing pigment is similar to the composition of the photocopying color material. During the printing process, the laser printing particles also undergo similar physical and chemical changes. The laser printing pigment particles are printed from 20 μm after printing. Increase to a few mm (see Figure 4).

Since the above two kinds of printed colorants are subjected to heat (melting), light and oxygen (air) in the short-time during the printing process, the styryl group, especially the styrene group on the surface of the pigment particles, is oxidized and polymerized. Oxide, printing is pigment polymerization and the formation of larger particles, which makes cellulose chemically combined with it, also makes cellulose fibers surrounded by pigment ink particles; In addition, oxidation generated on the ink surface The greater polarity (magnetic), which makes the flotation more difficult, even if the ink can be separated from the fiber to the flotation slurry, there will still be some cellulose fibers and ink particles bonded together. This effect will make the pigment ink particles more hydrophilic, which is why the efficiency of office waste paper flotation deinking is low. For this reason, it is necessary to design an improved deinking method such as a magnetic separation technique; it is also possible to design specific pigment particles.

In summary, oil-based (offset cold and offset thermoset) inks for newsprint printing contain w=45% to 60% oil and w=5% to 35% resin, water-based inks contain w=40% water and w. = 30% resin (polystyrene). Laser printing and photocopying pigments contain styrene acrylate copolymers with w=60% to 90%. In addition to water-based inks that dissolve in water (but their dried residue is hydrophobic), other inks, including dried water-based inks, are originally hydrophobic. Therefore, the ink can be separated from hydrophilic cellulose fibers by froth flotation.

Because of different printing methods, the ink undergoes different physical and chemical surface property changes during the printing process. Oil-based offset chilling inks for newsprint are simply absorbed by paper pores to complete printing, while water-based inks and oil-based offset plastisol are physically dried (evaporated) at temperatures below 60°C to complete printing. of. Therefore, these inks have no major change in their chemical properties, especially their hydrophobic properties, which make them easily separated from the cellulose in flotation. Photocopies and laser-printing toner particles are thermally fused to the cellulose by the action of heat/light during the printing process. The toner particles undergo polymerization due to exposure to heat (up to 200°C), light, and oxygen (air). And oxidation, followed by the formation of peroxides attached to the fibers. This polymerization creates a strong physical combination of tiger holes and produces larger particles. Oxidation makes the surface of the pigment particles produce greater polarity (magnetic), reducing the efficiency of flotation deinking, which also proves that the printing method changes the surface chemistry and physical properties of the printing ink, and deinking the flotation The effect has a decisive influence.

In order to improve the flotation deinking efficiency of laser printing photocopying waste paper, the design of the new toner particles and the surface properties of the printed toner particles must be considered.

Source: Starfire Design