Solvent balance problem in printing of plastic gravure ink

The gravure composite ink using polyurethane resin as a connecting material is prone to a series of printing failures such as stencil printing, no coloring, dirty printing, sticky printing roller, thick and sticky ink, and poor fluidity during the printing process. The reason is that whether the solvent volatilization of the ink is balanced in the printing process is a key issue. Since the true solvent such as butanone volatilizes too quickly, the polyurethane resin precipitates, the pigment flocculates, and the above series of printing failures occur. 

We know that when the temperature, humidity, speed, pressure, and tension of a printing press are constantly controlled, the only factor that changes with time is ink. The reason for the ink change is that the solvent in the ink is constantly evaporating, and the characteristics of the ink itself also change as follows:

1 The volatilization rate changes and may slow down or become faster. 
2 Dissolution performance changes. Due to the inconsistent volatilization rate of the solvent, the solvent composition changes, resulting in a change in the solubility of the mixed solvent to the resin in the ink. 
3 The viscosity changes. As the solvent volatilizes, the viscosity of the ink gradually increases. 

When the ink changes to a certain degree, the operator will add a diluent to the ink at a specified ratio in order to adjust the viscosity and the evaporation speed. Adjusting the viscosity of the ink can be controlled with a viscosity cup to achieve consistency before and after. However, the volatilization rate and the dissolution performance are different. Since the proportion of the components in the added diluent is different from the proportion of the volatilized solvent, the volatilization rate and the dissolution performance of the ink cannot be consistent before and after, and the mixed solvent in the ink dissolves the resin. Performance has also changed. 

According to the view of physical chemistry, under the constant temperature and constant pressure, the volatilization of the mixed solution will reach an “equilibrium point”. At this equilibrium point, the vapor pressure of each component in the volatilization will not change (that is, the volatilization ratio of each component will not change). The concentration of each component in the mixed solution remains unchanged. Therefore, when we analyze the ink in an ink tank as a system, for a selected ink type and concentration, the solvent in the ink tank will reach an equilibrium point after a certain time under constant temperature and constant pressure. . At this point of equilibrium, the examples of each of the volatilized solvents remain unchanged, and the concentration of each solvent in the ink remains unchanged. Even if you add a certain amount of release agent into the ink, you will still reach this balance point. 

To balance this point, we do the following analysis and discussion:

1 This equilibrium point only changes with temperature and pressure, and varies with the type of component. However, the initial concentration of each solvent component does not affect the equilibrium point. 

2  For chlorinated polypropylene type compound inks, the solvents used for preparing the diluents are usually toluene, xylene, ethyl acetate, butanone, and the like. These solvents, whether alone or in any combination, are good solvents for several resins in chlorinated polypropylene type inks. Therefore, chlorinated polypropylene inks are difficult to cause series of defects such as scumming due to resin precipitation during use, and the volatilization rate adjustment is also relatively easy. 

3 For polyurethane-based composite inks, commonly used solvents include isopropyl alcohol in addition to the above four types. However, toluene, xylene, and isopropanol alone are not good solvents for polyurethanes. Butanone and ethyl ester are good solvents for polyurethanes, but they are too volatile. Therefore, when adjusting the viscosity with the diluent ratio, attention should be paid to the two factors of volatilization rate and solubility. Adding too much toluene (xylene) will result in poor solubility; adding too much butanone (or ethyl ester) will volatilize too quickly; At present, there is no solvent with a good volatilization rate and good solubility. There are two ways to resolve this contradiction: First, to improve the toluene tolerance of the polyurethane resin (which requires the re-synthesis of the polyurethane), so that the minimum concentration of butanone (or ethyl ester) at the time of resin precipitation is lower than that at the volatile equilibrium point (or Concentration of ethyl ester, the ink will only thicken during printing, and the resin will not precipitate. According to past experience, if the toluene tolerance per gram of polyurethane resin is greater than 15 ml, the resulting ink has good printability. The second is based on the principle of solubility parameters using mixed solvent method. That is, when preparing the diluent, an appropriate amount of isopropyl alcohol is added to mix the toluene, the dispersive force of the mixed solvent is enhanced, and the solubility parameter of the mixed solvent is increased so as to be as close to the solubility parameter of the polyurethane resin as possible so as to achieve a good solubility. Since isopropanol volatilizes slowly, the volatilization speed of toluene is moderate. The combination of the two can better control the problem of excessive evaporation of ink due to excessive addition of butanone (or ethyl ester), while maintaining the ink in the printing process. Polyurethane resin does not precipitate, the ink always presents a good uniform fluid state, ensuring the smooth progress of the printing work.