The anomalous properties of viscosity of water-dilution adhesive

Usually, the acid resin cannot be diluted with water only after neutralization with the amine, and it can be dissolved in water only by adding an auxiliary solvent for solubilization. In general, the viscosity of the organic vehicle solution is always reduced when diluted with organic solvents, but it is different when diluted with water.

When the aqueous solution of the binder is diluted, the viscosity rises rapidly after a period of decline, and then drops rapidly after passing the highest point of viscosity. Since the printing quality has a high requirement for the viscosity of ink, it is necessary to understand in detail the variation of the viscosity of the ink.

The viscosity drops first and then rises because of the presence of amine salts and cosolvents in the system. When the amine salt is diluted with water, two changes occur simultaneously: the resin concentration decreases, and the ratio of cosolvent to water also decreases. In the initial stage of dilution, the viscosity of the water-diluted resin decreases faster than the solvent-diluted system. Dilution of the water will cause the viscosity of the system to decrease. This is easily understood; in the aqueous solution of the co-solvent, the cosolvent is started. The ratio of water to water is high, but the proportion of cosolvent and water gradually decreases after dilution with water. After the ratio is reduced to a certain point, a part of the resin molecules will no longer dissolve in the mixed solvent. The non-polar part of these resin molecules may be Associating with each other to form aggregates, the non-polar intramolecular aggregates are formed inside the aggregates, and the highly polar carboxylate groups exist in the periphery of the aggregates. Once aggregates are formed, the system changes from a solution state to an aggregate dispersion with a dispersed phase.

As dilution continues, more and more molecules are added to the ranks of aggregates. Some of the solvent dissolves in the aggregate and swells the volume of the aggregate. When the number of aggregates increases and the volume increases, the volume fraction in the internal phase increases, and the aggregates are packed more and more tightly, resulting in an increase in viscosity. In simple terms, the solubility of the mixed solvent of co-solvent and water decreases as the water is continuously added, resulting in the increase of the interaction energy between the molecules of the binder and the increase of the viscosity. This is the reason why the viscosity of the system first decreases and then rises.

Continuously adding water, the above effects will continue to be maintained, and the viscosity of the system will continue to rise; but at the same time, the binder will be continuously diluted, and the interaction between the binder molecules will gradually weaken. When diluted to a certain degree, the viscosity will reach the highest point. When you continue to add water, it is simply diluted and the viscosity drops rapidly, just as high solids emulsions do. Therefore, this seemingly abnormal viscosity change is entirely the result of two competing effects. Therefore, when designing an ink formulation, it is necessary to select the system and the amount that satisfy the printing suitability, and in particular the choice of the cosolvent, so that the interaction between the amine salt and the cosolvent can exhibit a viscosity value that satisfies the printability and the viscosity. Appeared in the ink solid content meet the formula design and printing requirements of the interval. This work should be one of the basic and key tasks in ink formulation design.

Source: China Tobacco Packaging Ink Information Network