Development of styrene-acrylic waterborne coating oil for printing paper (3)

2.4 Effect of Acid Monomer on the Viscosity of Emulsion

When coating paper, the viscosity of the emulsion is a crucial factor. It will directly affect the ease of construction, the leveling of the coating, and the glossiness.

There are three main ways to increase the viscosity of the emulsion, namely thickening of the thickening base and thickening of the solvent, the most common being alkali thickening. Emulsions that can be used for alkali thickening are mostly carboxylate latexes, and the molecular chains with carboxyl groups can be dissolved in the aqueous phase as a whole, or partially in the latex particles. Most of the carboxyl groups of these molecular chains extend into the aqueous phase. Under acidic or neutral conditions, these carboxyl groups exist in the form of non-ionic -COOH, the molecular chain of which collapses into random coils; after addition of the base, the carboxyl groups become carboxylates and ionize to the negative ion -COO-, in the same molecule With the same number of ions on the chain, due to the effect of electrostatic repulsion, the twisted state of the molecular chain is relatively stretched, and when it is affected by external forces, the interaction between macromolecular chains and latex particles increases, and macroscopically, The viscosity of the emulsion appears to increase. Figure 1 shows the change in morphology of latex particles before and after alkali addition.

Alkali thickening mechanism is used in this work: the aqueous emulsion adjusted from acidic to alkaline with ammonia will increase the viscosity of the emulsion. Experiments have found that the properties of carboxyl-containing monomers directly affect the thickening effect of the emulsion. Table 4 shows the effect of acid-2/acid-1 ratio on the viscosity of the emulsion.

It can be seen from Table 4 that the ratio of Acid-2/Acid-1 greatly affects the viscosity of the emulsion. When acid-2 is used in combination with acid-1, although the total amount of carboxyl groups does not change, the viscosity of thickened emulsion increases as the ratio of acid-2/acid-1 increases, when acid-2/acid When the ratio of -1 is 1/1, the viscosity of the emulsion reaches a maximum of 4350 mPa·s. As the ratio of acid-2/acid-1 continues to increase, the viscosity of the emulsion decreases.

With the introduction of acid-2, the thickening range of the emulsion becomes larger, mainly due to -CH3 on the acid-2, and a certain degree of hydrophobicity with the longer segments formed by other comonomers, when reaching a certain concentration in water Hydrophobic groups in the molecular chains are associated with each other to form a polymer network with micro micelles as crosslinking points, which enhances the thickening ability. However, as the amount of acid-2 continues to increase, the number of hydrophobic groups in the polymer backbone increases, the probability of internal association of the polymer chains increases, and the polymer chain segments do not fully expand and become distorted. On the other hand, due to the increase of hydrophobic groups, the hydrophilicity of the polymer is weakened, and the elongation of the molecular chain caused by electrostatic repulsion weakens, causing the emulsion to have a reduced viscosity. Therefore, the amount of acid or the ratio of acid-2 to acid-1 can be appropriately adjusted depending on the requirements for the consistency of the construction and the coating quality of the emulsion.

3 Conclusion

a. Emulsions prepared using the emulsion polymerization process and technology of the present work, the amount of emulsifier is only 2.95% of the total monomer amount, the solid content of the emulsion and the monomer conversion rate are 42.66% and 95.31%, respectively, and the gel rate can be controlled. About 0.4%;

b. The effect of using NaHCO3 as a pH adjuster is good; the subsequent monomer addition time and incubation reaction time are respectively 70min and 2h;

c. Emulsion viscosity can be adjusted in the range of 472 to 4350 mPa.s (or more), and it is easy to meet the requirements of different construction requirements and paper finishing requirements;

d. The use of non-pre-emulsified semi-continuous feeding method, and absorbs the characteristics of microemulsion polymerization and soap-free emulsion polymerization of this printing paper with water-based glazing, its latex film gloss, hardness, water resistance, etc. Excellent overall performance.

Source: China Printing Chemicals Network

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