The problem of environmental protection ink in paper recycling

With the rapid development of information technology, the traditional carrier of information, paper, is growing at an alarming rate. Paper requires a lot of wood as the main raw material, and this is running counter to the aim of environmental protection. Therefore, countries have increased the recycling of waste paper, and both the utilization and the utilization have been increasing. In 1990, the global waste paper utilization was 84.8 million tons, and the utilization rate was 35.6%. In 1995, it increased to 115 million tons and 41.4% respectively. It is expected that the waste paper utilization and utilization rate in 2005 will reach 188 million tons and 48.1 respectively. %. China is in a period of rapid economic development. The demand for paper is also growing at a level that exceeds the world average growth rate. However, China’s forest resources are extremely scarce, and the per capita forest share is only 10% of the world level. Every year, a large amount of foreign exchange is required to exchange imported pulp and paper products to meet domestic demand. However, the waste paper consumption rate in China lags far behind that of some developed countries. For example, in 1997, the waste paper consumption rate in France (the ratio of recycled paper waste to total waste paper) was 48.9%. 59%, 71% in the Netherlands, 65.2% in Switzerland, 71.5% in the UK, and only 37.8% in China. Therefore, how to recycle waste paper efficiently and with high quality has become a hot topic for researchers in China and related professional researchers.

The key issue in the recycling process of waste paper is how to completely remove the ink and impurities from the paper fibers. The deinking is the difficulty in this key issue. Nowadays, there are many types of printing inks, such as lithographic offset printing inks, flexographic printing inks, screen printing inks, gravure inks, water-based inks, and UV inks (UV-curing inks). The degree of deinking of each printing ink is not limited. To the same extent, UV inks and water-based inks are the most difficult to deink. This article discusses the problems encountered in the deinking process of UV inks and water-based inks in waste paper processing, and how the current countries around the world have solved these problems.

Waste paper recycling process

In general, the deinking of waste paper can be roughly divided into five steps: crushing pulping, filtration, centrifugal cleaning, flotation to impurities, and rinsing, which are briefly described as follows:

1. Crushing pulping: In this step, first remove the relatively large domestic wastes in waste paper, such as plastic sheets and chopped trash, then pour the pretreated waste paper into a mixing kettle, then add water and caustic alkali. Enzymes are added to the kettle. After a period of soaking and mechanical agitation and comminution, the waste paper is made into a ready-to-use paper fiber slurry. Ink and other impurities can be separated from the paper fiber and get a certain size distribution.

2. Filtration: The resulting paper fiber slurry is passed through a rotating screen so that large particles of ink and impurities are retained on the screen. The size of the ink to be sieved can be controlled by a series of screen meshes. Generally, the ink particles with a particle size of more than 360 μm can be removed by this step.

3. Centrifugal cleaning: According to the difference in density, centrifugal force is used to separate the paper fibers from the non-paper fiber impurities. The key to the complete separation of the two substances lies in the difference in density between them. The greater the difference, the more complete the separation. Therefore, only 100 μm to 360 μm of ink impurities can be removed in this step.

4. Flotation method to remove impurities: The flotation agent (ie, surfactant) is added to the pulp first. This not only stabilizes the foam and facilitates flotation, but also enhances the hydrophobicity of impurities, making it easier to break away from the bondage of water. . At this time, air is introduced into the pulp to form air bubbles, and the impurities adhere to the air bubbles due to the action of the flotation agent, and float to the liquid surface with the air bubbles, and finally the impurities are removed. The key to this process lies in the weight of the impurities: it is necessary to have enough weight to penetrate the air-liquid interface of the air bubbles and adhere to the air bubbles, and to make the air bubbles have sufficient buoyancy to bring the impurities to the liquid surface. Therefore, most of the impurities are removed during this process. The impurity particle size should be controlled within 20 μm to 150 μm. This is the most important step in the entire recycling process. The quality level of recycled paper depends on the degree of impurities such as ink removed in this step.

5. Rinse: Rinse the pulp with plenty of water to remove impurities with a particle size of less than 10 microns.

The main operational problem of deinking in waste paper recycling is how to remove the ink from the paper fibers and effectively control the particle size of the ink contaminants, resulting in an ideal particle size distribution to facilitate separation. Ink that cannot be removed remains in the pulp and greatly affects the finish of the recycled paper.

Problems and Countermeasures of UV Ink in Waste Paper Recycling

1. Problems that arise

UV ink is a new type of ink that has just entered the rapid growth in China's ink industry, and is increasing at a rapid rate every year. UV inks are emerging in various printing fields because of their advantages of solvent-free environmental protection, fast drying speed, shortened printing cycle, and various resistances. Everything has its advantages and disadvantages. The advantages of UV inks in the printing process are precisely the disadvantages in the deinking process of waste paper processing.

UV ink is a reaction drying ink, and its main components are: reactive binders, photoinitiators, pigments, auxiliaries (waxes, leveling agents, etc.), monomer diluents, and the like. Its binder is mainly made of acrylic epoxy resin and other oligomers as its main raw material, and various types of reactive monomers are used as diluents. Photoinitiators generate free radicals or cations under the excitation of ultraviolet light and initiate chain reaction. The entire system quickly undergoes a cross-linking reaction to completely convert the reaction materials such as binders and diluents into solids. The UV ink thus printed on the surface of the paper becomes an ink film similar to a plastic film. This film has the advantages of strong toughness and large mechanical strength due to the network structure of the polymer, which makes the UV ink resistant to chemical corrosion and weathering. It is particularly prominent in terms of resistance, such as resistance and friction resistance.

However, the advantage of this patience has brought difficulties to the recycling of waste paper. First, UV inks and paper fibers are more difficult to separate during crushing. For ordinary offset printing inks, the saponification and hydrolysis of the resin binder and the lye occur, and the sodium hydroxide solution causes the paper fibers to swell. Under the double action, the ink can be easily removed. However, the chemical resistance of UV inks makes them unable to undergo saponification and hydrolysis reactions in alkaline environments, and its own network structure makes its binding force with paper fibers quite strong, so it is not easy under ordinary alkaline conditions. Separated.

Secondly, UV inks are difficult to be effectively separated during filtration. Due to the above reasons, most of the inks separated from the paper fibers are large sheet-like films, and some full-printed prints even peel off the entire ink film. This membrane is similar to a plastic membrane and has excellent toughness. It is not easily crushed during the crushing process and it is difficult to achieve an ideal particle size distribution range of 20 μm to 150 μm. Although some large ink films can be separated by filtration, the UV ink film is highly flexible and easily deformable. During the filtration process, it deforms due to extrusion and passes through the filter with the paper fibers.

In addition, the UV ink film is equivalent to the density of water, and it is difficult to remove it effectively by the centrifugal cleaning method.

The subsequent flotation method is a very important part of waste paper recycling. In this process, most of the ink particles can be easily removed, but the UV ink is due to its own toughness and high Mechanical strength, it is difficult to control the size of most of the ink particles in the grinding process from 20 microns to 150 microns. Therefore, the UV ink particles that are pulverized by ordinary methods cannot be effectively removed during the flotation deinking process.

2. Solution

In order to reduce the particle size distribution of the separated UV inks to the size of the floatable particles, the manufacturer adopted a composite mechanical crushing process that uses both mechanical action such as kneading and rubbing to crush. The slurry maintains a high concentration (30% to 35%), which can reduce the damage to the paper fibers during the comminution process. With this method, the ink particle size distribution can be reduced from 150 μm to 400 μm to 30 μm to 150 μm. However, the ink film is very firmly connected to the paper fiber due to its cross-linked structure, and it is likely that it cannot be separated from the paper fiber, and part of the paper fiber is crushed at the same time with the ink and is removed together to cause loss.

With the application of ultrasonic technology, ultrasonic waves can now be used to crush the ink film to a floatable size within a controllable range. The cyclonic bubble generated by the 20KHz ultrasonic wave will produce a local temperature of 5500K and a microjet with a flow rate of 170ms-1s during the rupture process, and the pressure of 1000N/m2 will be sprayed on the surface of the ink film. Under such a huge impact force, no need for any Chemical reagents, ink particles can be crushed from large 400-micron particles into small particles suitable for flotation. And using ultrasonic comminuted particles is less than using mechanical methods.

Problems and countermeasures of water-based ink in waste paper recycling

1. Problems that arise

Due to its own characteristics, water-based inks make it difficult for the ink layer formed on the printed matter to be separated by flotation. The main causes are as follows:

(1) The resins used in aqueous inks are generally hydrophilic and difficult to separate from aqueous pulps. (2) Emulsions and polymers in aqueous ink formulations are not easily separated because the particles formed in pulping are too small. This is because the resin in water-based ink is mostly acrylic resin, which is soluble in water under alkaline conditions, and it is insoluble in water after being converted into carboxylic acid form by reverse reaction. This property is used in printing. The ink is fixed on the paper and dried. Due to this reversible conversion reaction, the aqueous ink is poor in alkali resistance. Therefore, this water-based ink can only be removed by rinsing in recycled pulp and cannot be effectively separated by flotation. Another aqueous resin is in the form of an emulsion (such as styrene-acrylic resin) into which a glycol ether is introduced. The hydroxyl group on the glycol ether and the carboxyl group on the resin condense to make the resin hydrophilic and disperse in water. The fixation of this resin is achieved by the aggregation of the latex particles in the emulsion to remove moisture from the resin and thus achieve drying. This dry form is irreversible, and this irreversible drying gives the print product excellent acid and alkali resistance. But at the same time, it can not be returned to the original colloidal state during the crushing pulping stage of paper recycling, because the thin layer formed by this ink is very small (<20 μm), and due to the hydrophilicity of glycol ethers. It makes it difficult to remove effectively by flotation.

2. Solution

The chemical balance in water-based ink formulations is complex, and this chemical balance is important for ink drying and other resistance properties, so it is difficult to solve this ink removal problem by changing the ink formulation or modifying the resin in the formulation. . At present, most of the solutions to this problem begin with the development of better flotation aids. On the one hand, the inherently hydrophilic ink particles are rendered hydrophobic, so that they can be effectively removed from the fiber pulp; on the other hand, the effective floatation of the ink particles has a wider range of particle sizes, resulting in a smaller particle size of the aqueous Ink particles can also be effectively removed.

The pressure of environmental protection has made the contradiction between waste paper recycling and ink manufacturing increasingly prominent. The quality of recycled paper and the requirements for waste paper recycling efficiency have also become higher and higher. The increase in printing quality requirements has also made ink formulations increasingly complex. This will inevitably require the improvement of deinking technology in waste paper recycling. However, unilateral efforts are still not enough. Furthermore, the environmental protection bills now introduced are increasingly restricting the ink manufacturing industry if they are used in ink manufacturing or development. The ability to consider these issues from the perspective of paper recycling and other environmental protection will greatly enhance the ink market.

Source: China Packaging Author: Shi Yiming

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