High performance opaque acrylic emulsion for water-based inks

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1 Introduction As people’s awareness of environmental protection continues to increase, more and more product packaging boxes are beginning to be printed with water-based ink. Traditional domestic water-based ink manufacturers generally use modified water-soluble rosin resin solution as a connecting material, and prepare water-based ink together with color paste. Modified water-soluble rosin resin is cheap and has strong tinting power, but has poor weather resistance and water resistance, and the prepared ink has poor overall performance. In recent years, acrylic emulsions have been gradually used as the binder for water-based inks. On the one hand, it can promote the dispersion of pigments in water and enhance the printing adaptability of ink. ； On the other hand, the ink prepared with this emulsion has good weather resistance and water resistance, and can * * Improve the grade of water-based ink. However, the effect of using acrylic emulsions commonly used in some printing materials (such as water-based varnishes) to prepare water-based inks is not very good. This is mainly because these emulsions are very transparent after drying and have poor hiding properties. The prepared inks have poor tinting power. In addition, the gloss of the printing layer is low, especially on some corrugated packaging boxes. Some well-known foreign water-based ink raw material manufacturers such as Robin & Hass and Johnson developed several covering acrylic emulsions in the early 1990s. This hard non-film-forming emulsion can provide a dry and clean hue, especially for brown kraft paper, which has excellent hiding properties. At present, few domestic manufacturers can develop and produce this type of covering emulsion. Some companies that produce high-end water-based inks import a large amount of such products from abroad every year. Therefore, the development and production of covering acrylic emulsion will have good economic and social benefits. This article uses a step-by-step polymerization method to synthesize latex particles into a swelling emulsion with a three-layer structure of "core/middle layer/shell". Through certain optical effects, the emulsion can obtain good hiding properties. The water-based ink prepared using this emulsion as a connecting material has good printing adaptability, bright colors, weather resistance, and is not easy to change color. It uses less pigment and filler and has good compatibility between the emulsion and the color paste. 2 Test part 2.1 Main raw materials The raw materials used for the test are listed in Table 1. Table 1 Main raw materials used in the test Raw material Name Specification Source Methyl methacrylate Industrial pure American imported Methacrylic acid Industrial pure Shanghai Gaoqiao Petrochemical General Plant Ethyl acrylate Industrial pure Shanghai Gaoqiao Petrochemical General Plant Polyalkenyl unsaturated monomer Chemical pure Jiangsu Kunshan Yuejin Reagent Factory Emulsifier Industrial pure Guangzhou Double Bond Company Shengsheng Industry Pure American imported potassium persulfate, analytical purity, Shantou Guanghua Chemical Factory, sodium bicarbonate, chemical purity, Shanghai Hongguang Chemical Factory, ammonia water chemical purity, Guangzhou Xingang Chemical Reagent Factory uses emulsion polymerization method to prepare covering acrylic emulsion, selects potassium persulfate as the initiator, and uses seed polymerization method to prepare emulsion particles with a three-layer structure step by step. 2.3 Analysis and testing The glass transition temperature of the emulsion resin obtained was measured using a Netzsch DSC 204 differential scanning calorimeter. ； The particle size and particle size distribution of latex particles were measured using a Malvern Mastersizer 2000 particle size distribution analyzer. ； The viscosity is measured using a 4-cup coating at 25°C. ； The covering property is to drop 5ml of emulsion on a piece of brown kraft paper lined with a plastic board. After it dries naturally, you can see if it turns into a white and shiny opaque resin. ； The compatibility with the color paste is to add 200g of the emulsion filtered through a 125 μm (120 mesh) mesh into a 1000ml enamel cup, stir with a high-speed disperser at 800r/min, slowly add 200g of the color paste filtered through a 125 μm (120 mesh) mesh, and stir for 0.5 hours. Then filter with a 125μm (120 mesh) sieve. If there is no or only a very small amount of floc or particles, it is passed. ； The tax stability after compounding with modified water-soluble rosin resin solution is to mix 200g of emulsion and rosin solution evenly, measure the viscosity with a -4 cup at 25°C, let it stand for 1 month, and then measure the viscosity under the same conditions. If the viscosity changes within ±2s, it is passed. 2.4 Performance indicators The performance indicators of the obtained emulsion are listed in Table 2. Table 2 Performance indicators of the emulsion Performance indicators of the emulsion The appearance of the emulsion is milky white, showing blue light. The appearance of the emulsion after drying is white, opaque, and shiny. Particle size before swelling: 0.17/μm. After swelling: about 0.40. Solid content: 47% pH 7.5~8.0. Viscosity before swelling: 140 (apply -4 cups)/s. After swelling: 38. Glass transition temperature: about 90°C. Good hiding properties and good compatibility with color paste. Viscosity stability when compounded with rosin solution. Free monomer content is less than 1%. Dilution stability is through freeze-thaw stability. 3 Results and Discussion 3.1 Relationship between latex viscosity structure and latex hiding properties. The synthesized latex particles have a three-layer structure of "core/middle layer/shell", in which the glass transition temperature of the core part is relatively high, exceeding 100°C, and the volume of the core is small. The glass transition temperature of the middle layer is relatively low, about 55°C, and the average diameter of the middle layer is about 1/4 to 1/3 of the outer diameter of the shell. The glass transition temperature of the shell also exceeds 100°C. The core and shell of latex particles are hard, while the middle layer is relatively soft (lower glass transition temperature), and a large number of carboxyl groups are introduced. Add a certain amount of ammonia to the emulsion. The ammonia reacts with the carboxyl groups in the middle layer, causing the molecular chains to fully stretch, and a large number of gaps will appear. The moisture in the emulsion is continuously filled in through the shell of the latex particles, and the middle layer swells significantly and increases in size. When the emulsion coating dries, the moisture in the middle layer also evaporates, resulting in a partially hollow, low-density middle layer in the latex particles. Since the density of each layer structure of latex particles is different, 3.2 Selection of monomers in each layer structure of latex particles The core requirements of latex particles are relatively hard, and hard monomers such as methyl methacrylate should be used. Core layer monomers generally account for about 10% of the total monomers. Usually when core/shell emulsion polymerization is carried out, the phenomenon of "phase inversion" is prone to occur. If some polyethylenically unsaturated monomers are selected to cause partial cross-linking between molecular chains during polymerization, the cohesion of the core can be significantly enhanced, the interpenetration between the core and the intermediate layer can be reduced, and "phase inversion" can be avoided. In order to ensure the stability of the emulsion, the amount of polyethylenically unsaturated monomer is usually no more than 5% of the total amount of core monomers, preferably 1% to 3%. The middle layer of latex particles is required to completely cover the core layer, so the amount of monomers cannot be too small, generally accounting for about 30% of the total amount of polymer monomers. In addition, the middle layer is required to be fully swollen. On the one hand, the polymer chain must be flexible enough and not too hard. ； On the other hand, there must be enough carboxyl groups on the molecular chain to react with alkaline substances such as ammonia. The selected unsaturated monomer containing carboxyl groups is methacrylic acid, accounting for approximately 30% to 35% of the total monomers in the middle layer. The reason why methacrylic acid is selected instead of the commonly used acrylic acid is mainly because acrylic acid tends to be distributed on the surface of the polymer, while methacrylic acid can be evenly distributed throughout the middle layer, which is more conducive to the swelling of the middle layer. The shell of latex particles is as hard as the core, but the monomer composition is not exactly the same. During polymerization, it is usually necessary to use a small amount of methacrylic acid monomer to introduce some carboxyl groups into the shell. During swelling, ammonia first reacts with the carboxyl groups of the shell, stretching the molecular chains, and the gaps that appear allow ammonia and water to pass through and reach the middle layer. The effect of the amount of methacrylic acid (MAA) on the degree of swelling is listed in Table 3. Table 3 Effect of the amount of methacrylic acid in the shell on the swelling degree of the emulsion Amount of MAA in the shell/% 0 0.5 1 1.5 Latex particles after swelling 0.20 0.31 0.38 0.41 Average particle size/μm Note: The average implant diameter of the latex before swelling with ammonia water was 0.17 μm. As can be seen from Table 3, if no methacrylic acid monomer is added to the shell during polymerization, the degree of swelling of the latex particles is very small. As the amount of methacrylic acid increases, the swelling degree of the emulsion gradually increases and the particle size of the latex particles increases. However, in order to ensure the water resistance of the final ink, the amount of shell methacrylic acid generally does not exceed 2% of the total amount of shell monomers. In addition, a small amount of polyethylenically unsaturated monomers must be added during shell polymerization, generally accounting for only 0.5% of the total amount of shell monomers. Adding polyethylenically unsaturated monomers can also avoid penetration between the complete layer and the middle layer. After a certain degree of cross-linking reaction with a small amount of polyethylenically unsaturated monomers, a grid structure can be formed in the shell layer. A large number of holes provide channels for ammonia and water to reach the middle layer. 3.3 The selection of emulsifier is generally based on the HLB of the polymer to screen the corresponding emulsifier. In particular, the combination of nonionic emulsifiers and anionic emulsifiers can improve the stability of the emulsification system. However, during the polymerization process of this covering emulsion, a large amount of methacrylic acid monomer needs to be added to the middle layer, so that the system contains a large amount of -COO- and H+ during the polymerization process. If an anionic emulsifier is used, the stability of the polymerization will be affected, and more flocs may appear in the system. Therefore, a nonionic emulsifier was selected in this experiment to obtain a stable emulsion system. 3.4 The influence of the swelling degree of latex particles on the hiding effect of emulsion found that if the emulsion is not swollen with ammonia after polymerization, there will be no obvious difference between the emulsion and ordinary acrylic emulsion after drying, and the ideal covering effect will not be achieved. ； If ammonia is added at a higher temperature (about 75°C) in the later stages of emulsion polymerization and kept warm for about 0.5 hours, a white, opaque, glossy resin will be obtained after the emulsion dries. This emulsion has good hiding properties. As can be seen from Table 4, the latex particle size of the emulsion treated with ammonia water at a higher temperature is larger, which means that it has a better swelling effect at high temperature. This is because high temperature can promote the reaction between ammonia and radicals and fully stretch the molecular chain. ； At the same time, the cells of the shell layer will also fully expand, which is conducive to the penetration of ammonia and water into the middle layer. Table 4 Effect of swelling conditions on swelling effect and covering effect Adding ammonia at 50°C Swelling conditions Ammonia treatment and keeping at room temperature for 30 minutes After swelling, the latex particles are 0.24 0.40 Average particle size/μm After the emulsion is dried, the resin appearance is light white, translucent white, opaque, and shiny. The viscosity of the emulsion will increase significantly after swelling treatment. Usually, adding ammonia to ordinary acrylic emulsion will also increase the viscosity of the system (i.e., alkali thickening). But there are obvious differences between the two. Ordinary alkali thickening is just that the randomly distributed carboxyl groups in the latex particles react with ammonia and other alkaline substances to stretch the molecular chains and increase the viscosity of the system, which does not affect the hiding performance of the emulsion. When the covering emulsion is treated with ammonia water, the middle layer of the latex particles is fully swollen, and the hard core and hard shell are basically not swollen, which changes the optical properties of the latex particles and gives the emulsion hiding properties. 3.5 The role of protective glue Many emulsions need to add various protective glues to improve the stability of the system. This test selected a composite protective glue, which can improve the compatibility of emulsion and color paste and prevent demulsification. In addition, some water-based ink manufacturers often use masking emulsion and water-soluble rosin resin solution together to prepare lower-cost inks. A common problem is that the viscosity stability of the compound system is not good. After the ink is left for a period of time, the viscosity changes greatly. If this protective glue is added to the covering lotion, it can * * Improve the axial stability of the compound system. Usually the protective glue is added during the polymerization process, but in this experiment, the protective glue was added after the polymerization was completed. Otherwise, it will affect the polymerization reaction (especially the polymerization of the middle layer), which will ultimately affect the hiding power of the emulsion. 4 Conclusion In summary, a step-by-step polymerization method is used to synthesize an emulsion with a three-layer structure of core/middle layer/shell, and then treated with ammonia water at a higher temperature to fully swell the middle layer of the latex particles. Through certain optical effects, the emulsion will have good hiding properties after drying. The water-based ink prepared with this emulsion can obtain brightly colored, plump, and good covering power printing effects after printing.