Development of styrene-acrylic waterborne coating oil for printing paper (2)

2 Results and Discussion

2.1 Effect of pH Adjuster on the Properties of Emulsion and Latex Film

Using ammonium persulfate as an initiator, as the polymerization proceeds, the pH of the system will gradually decrease, which will lead to a decrease in the stability of the polymer emulsion and a gelation phenomenon. In order to reduce the gel and increase the yield of the emulsion, it is necessary to add a pH buffer or a pH adjuster to the reaction system to control the pH within a certain range. For this reason, three different pH adjusting agents were used in this work to control the pH of the reaction system. The monomer conversion, gelation rate, solids content, and latex film properties of the resulting emulsions are shown in Table 1. It can be seen that the gelling rate is too high when using a pH adjuster in which triethylamine and ammonia are mixed. The reason for this may be that although the amount of the pH adjuster is small, its strong alkalinity causes hydrolysis of the acrylate monomer and the viscosity of the system rapidly increases, resulting in the formation of a large amount of agglomerates adhering to the agitating blade. Since the gelling rate is too high, the yield and performance of the emulsion are affected; when using disodium hydrogen phosphate, the situation is similar to that of the above pH adjusting agent. Although the gel rate is reduced to 5.9%, it is still very high; When sodium is used, not only the monomer conversion rate and the solid content of the emulsion are increased, but also the gelling rate is significantly lower than that of the former two, dropping to 0.41%, and the overall performance of the coating film is also better. This shows that the use of sodium bicarbonate as a pH adjuster is effective.

2.2 Effect of Follow-up Monomer Addition Time on Performance of Emulsion and Latex Film

Table 2 shows some of the experimental results of the effect of subsequent monomer addition times on the properties of emulsions and latex films. It can be seen that by appropriately prolonging the dropping time of the monomer, the gelation rate of the system can be reduced, the solid content of the emulsion can be increased, and the gloss and water resistance of the latex film can be improved. This is because prolonging the dropping time of the monomer appropriately allows sufficient time for the monomer to be evenly distributed on the surface of the seed polymer, and a balance between the emulsifier monomer and the water is reached again, which is advantageous for controlling the latex particles. The average size improves the overall performance of the latex film. However, the long time of monomer addition will increase the probability of latex particles binding, lower the monomer conversion rate and the solid content of the emulsion, increase the gel rate, and affect the glossiness of the latex film. Therefore, it is advisable to take about 70 minutes under normal circumstances.