Preparation of Acrylic Waterborne Varnish

Abstract : The development process of an acrylate copolymer emulsion was introduced. It was applied to the printing glazing process of paper. It has good glossiness, abrasion resistance and water resistance.

Foreword

With the rapid development of printing technology, various forms of printed materials have brought more and more problems to related industries, especially the adhesive industry, taking the transformation of packaging paper as an example: from the earliest single corrugated box From laminating paper, coated paper to glazing paper to multi-layer "Lehr" composite beverage packaging paper, adhesive from scorodine to modified starch glue, SBS solvent laminating adhesive, copolymer emulsion water-based laminating adhesive , styrene butadiene latex composite paper coating liquid and oily, waterborne polyurethane and acrylate printing adhesive, "Lehrer" paper special composite adhesive (one-component polyurethane adhesive). Although BOPP and PET plastic film composite papers have the advantages of high gloss and aesthetic durability, they are not conducive to the recovery and recycling of waste paper products. Therefore, some packaging papers exported to foreign countries are gradually replaced with glazing papers that are easily crushed and recycled. Because the packaging paper has special requirements for abrasion resistance and water resistance, the quality of the coating adhesive is particularly important. Currently, the most used by the manufacturer is the solvent-based PU, PET, and UV oil (UV curing) modified PSt. Varnish and water-based polish products are not as good as solvent-based products due to their glossiness, water resistance, and abrasion resistance. They are subject to greater restrictions in application. The most commonly used products are two-component waterborne polyurethane coatings (Bayer). ) Acrylate Copolymer Emulsion Coating Varnish. This article will introduce a synthesis method and characteristics of acrylate copolymer emulsion varnish, proved by the manufacturer: The varnish has the characteristics of good gloss, wear resistance, water resistance, and resistance to yellowing.

Varnish adhesives require a high glass transition temperature (Tg at around 30 °C) and good film formation. Low temperature does not cause brittle cracking, and high temperature is not sticky. Therefore, there is a certain degree of monomer selection. Requirement: Emulsion copolymerization of methyl methacrylate and methyl acrylate as the main monomers, with a small amount of functional monomers such as methacrylic acid, and thermal cross-linking monomers such as N-methylol acrylamide. Made, and finally adding a small amount of solvent as a filming agent is to become a water-based polish.

1 Experimental section

1.1 Raw materials

The main monomer is methyl methacrylate (MMA), the auxiliary monomer is methyl acrylate (MA), methacrylic acid (MAA) functional monomer A and N-methylol acrylamide, the emulsifier has anion S and non- Two kinds of ion O are used together, the protective colloid is PMA, buffer and initiator (persulfate), coalescent T, ammonia water.

1.2 Preparation method

Semi-continuous drop polymerization was used. A three-necked round-bottom flask equipped with a stirrer, a thermometer, a reflux condenser, and a dropping funnel was placed on a temperature-controlled water bath, and a portion of water, an emulsifier, a buffer, and a monomer were rapidly stirred to perform pre-emulsification for half an hour before pouring. In the dropping funnel, another portion of water is added to the flask, the protective colloid is stirred and the temperature of the water bath is raised to about 86° C., a part of the initiator is added, and a pre-emulsion is rapidly added to carry out seed polymerization. The remaining pre-emulsion was added dropwise until the end. In the course of the reaction, the initiator aqueous solution was added on time to keep the reaction speed. The emulsified droplets continued stirring and kept for a certain period of time to complete the reaction of the monomers. The temperature was lowered, the pH regulator was added, and the film formation was assisted. After the agent is filtered out, the relevant properties are determined.

2 Results and Discussion

2.1 Performance Test The viscosity, gloss, hardness and adhesion were measured with BYK-GARDNER viscosity cups, dry film tester, Brinell hardness tester and other instruments. The relationship between the polymerization stability and the amount of emulsifier was determined by the collection method, the water absorption rate of the coating film was measured by the impregnation method, and the abrasion resistance of the coating was tested by the friction method.

2.2 Effect of Emulsifiers (including Protective Colloids) on the Stability of Emulsion Polymerization

The emulsifier has a great influence on the stability of the emulsion polymerization and on the size of the latex particles generated. In general, the amount of emulsifier is increased and the amount of agglomerates is reduced. The use of an anionic/nonionic complex emulsifier can give the emulsion excellent polymerization stability and storage stability. The selection of emulsifiers should take into consideration various comonomer pairs for emulsification. The different requirements of the agent oil-water balance (HLB) are selected based on the solubility in water. The emulsifier required for the water-repellent monomer should be high in lipophilicity (the non-ionic emulsifier with smaller HLB value, such as polyoxyethylene alkylate The ethers) and hydrophilic monomers require highly hydrophilic emulsifiers (various anionic emulsifiers with large HLB values). The proportion of anionic/nonionic emulsifiers is determined according to the proportion of various monomers in the copolymerization reaction. Acrylic polymer latexes have a small particle size and therefore require more emulsifiers. The suitable amount is 4 to 5% of the monomer amount. Excessive emulsifiers will cause the polymer to have a decreased water resistance and foam during the synthesis process. serious.
If too little, gel (agglomerate or floe precipitated, and there are more coarse particles) will appear during the polymerization, so that the emulsion will not reach the required solid content and quality, and the appearance of the emulsion will be rough and blue. use.
The relationship between the amount of emulsifier (including the amount of protective colloid) and the amount of polymeric agglomerate (Wt%) is shown in the following table. Although the control condition of the reaction may also affect the formation of agglomerates, its main cause is related to the amount of emulsifier. Only the influence of the emulsifier is considered here.

From Table 1, it can be seen that with the increase of the amount of emulsifier (Wt% of emulsifier in the total weight of the emulsion), the amount of aggregates gradually decreases, the particle size of the emulsion decreases, and the viscosity increases significantly. The finer the particle size of the emulsion, the more uniform the distribution, the higher the gloss of the coating, the denser, and the better the abrasion resistance and adhesion. After comprehensive consideration, we chose the amount of emulsifier 2.4%, and the amount of protective colloid was 0.8%.

2.3 Effect of MMA and MA Monomer Ratio on Performance

The hardness and adhesion of the coating have a great relationship with the ratio of soft and hard monomers. The ratio of soft and hard monomers affects the glass transition temperature of the final polymer. The glass transition temperature of the product can vary from Gibbs--Di marzio. Formula estimate: 1/Tg=Wa/Tga+Wb/Tgb+...Wm/Tgm where Tg is the glass transition temperature of the copolymer; Wa, Wb...Wm are each component a, b...m The mass fraction in the comonomer; Tga, Tgb...Tgm are the glass transition temperatures of their homopolymers, respectively.

The hard monomer can give the coating a higher use temperature and a certain gloss while making the coating scratch-resistant. The soft monomer gives the coating flexibility and film-forming properties, and the cross-linking monomer and the functional monomer comprehensively give the network a three-dimensional structure of the polymer, so that the hardness and water-resistant abrasion resistance are improved. It has been reported that the introduction of polar groups of acrylic acid can impart polar groups to the polymer side chains, which will increase the adhesion between the coating and the substrate. The addition of methacrylic acid also has a good effect on the improvement of the emulsion polymerization process. It is a hydrophilic monomer that dissolves both in the aqueous phase and in the oil phase, which can significantly inhibit the formation of agglomerates, and at the same time make the synthetic emulsions good The mechanical stability and storage stability. Excessive amounts (3 to 4%) of the total amount of monomers are generally selected to result in a decrease in water resistance and abrasion resistance of the copolymer, and too little to achieve an effect. Practice has proved that when the glass transition temperature of the copolymer is about 28°C, it is most suitable for use as a coating binder, the glass transition temperature is too low, the surface of the coating is sticky, the hardness is extremely poor, and the abrasion resistance is extremely poor and cannot be satisfied. As a requirement for the use of coating adhesives, if the glass transition temperature is too high, the hardness is too high, the coating exhibits cracking and brittleness, it is not easy to form into films, there is no toughness, adhesion is extremely poor, and the water resistance and abrasion resistance are also Extremely poor. According to the requirements of printing varnishes, it is necessary to have high hardness and good flexibility. Wear resistance and water resistance are important quality requirements. In practice, we have increased the comprehensive index of the coating to a more ideal value by adding a certain amount of water-based adhesive-specific curing agent, DesmodurDA (Bayer).

In the above sample, no curing agent DesmodurDA was added, and the results represent the properties of the copolymer itself.
Through the above control experiments, the final comonomer ratio was determined to be MMA/MA/MAA/HAM/HEMA=50/50/4/4/1. The addition of a small amount of coalescent is to overcome the difficulty of coating due to the high glass transition temperature and has no effect on the overall performance of the coating. We have used xylene as a coalescent.
The above products have been scaled up to 1 ton, and have been tried by the manufacturers, all of which meet the requirements for use.

3. Conclusion

(1) By screening the synthesis process and various grouping ratios, a polyacrylate emulsion polish with good appearance, good storage stability, moderate viscosity, and easy polymerization can be obtained, and the coating is resistant to water and abrasion. Sex, hardness and adhesion are all better.
(2) The properties of the polish can be further improved by adding a certain amount of an isocyanate curing agent, particularly abrasion resistance, water resistance and adhesion.

Reprinted from: China Carton Network