Fluctuation of Operating Viscosity and Effect of Solvent Enrichment on Solvent Residue (Part 2)

The isopropanol curve in both plots is the volumetric evaporation rate plot for yellow ink, and the other plot is the evaporation rate trend plot for blue or red ink. In our opinion, the volatilization rate of isopropanol is relatively slow, but the volatilization rate is faster in a hydrocarbon-based system. If mixed in systems without benzene or less hydrocarbons, isopropanol volatilizes at a relatively slow rate. The volumetric evaporation rate of methyl ethyl ketone is fastest, and therefore the content ratio sharply decreases, and also because the volume fraction thereof decreases, the evaporation rate also decreases, resulting in a decrease in the total evaporation rate of the mixed solvent represented by the black curve. It should be noted that the example of this process uses an automatic viscosity controller, but the problem of the slow-drying solvent concentration in the ink is still very prominent, and the overall effect is that the volatilization rate of the solvent is reduced. If you do not adjust the drying parameters, it is prone to accidents where residual solvents are too large.

Another aspect of solvent enrichment is the absence of a certain solvent. Let's analyze the above process example and see how the composition of the ink in the ink tray changes. See graphs 3 and 4.

In the compositional changes of yellow ink, blue ink, and red ink, the common trend is the decrease in the percentage of methyl ethyl ketone. Methyl ethyl ketone is a strong solvent for certain ingredients in BOPP ink binders. If methyl ethyl ketone is missing, the solubility of the mixed solvent in the binder resin will be reduced. As a result, the viscosity of the ink will increase and the fluidity will deteriorate. In a certain range, the increase in viscosity will increase the amount of partial coating in the field, and the residual problem will be more serious. The poor flowability will cause the loss of high-profile parts of small dots. In this case, the ink factory will suggest that we add some methyl ethyl ketone. Conditional manufacturers can sample at different times to detect changes in solid ink content, solvent composition, and viscosity. If the solid content does not change and the viscosity changes, the solubility of the solvent to the binder changes. The effect of changes in the solvent composition of each type of ink on the fluidity of the ink is not generally provided by the ink manufacturer. It is necessary for us to explore, accumulate, and make evaluations.

The solvent composition of the ink will change during the production process, and the concentration of the high-boiling solvent will cause the total evaporation rate to decrease. Due to the poor solubility of the solvent to the binder, the viscosity of the ink changes, and these factors can affect the amount of residual solvent in the product and have caused our attention.

Author/(Jiang Zhongqiu)