Four problems with solvent-based inks
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Question 1: What is the cause of the dirty plate (a foggy appearance in the non-image portion of the gravure roll) in gravure solvent-based ink printing?
In gravure printing, the cause of the dirty plate is the ink transfer by the doctor blade. Specifically, the ink in the non-image portion is not scraped off during the ink scraping process. The reason why the blank portion of the ink is not cleaned is mainly mechanical and ink components. problem.
Mechanical factors: For the first reason, the squeegee is very worn. During the normal use of the squeegee, it will eventually wear out. To solve this problem, the direction of the doctor blade along the gravure cylinder can be checked to ensure that the doctor blade is evenly pressed against the gravure cylinder. At the same time, it is necessary to ensure that the squeegee itself has no wear points or dirty spots, which will affect the squeegee effect of the squeegee. If these problems occur, it is best to change the squeegee again. The second reason is that if the surface of the copper plate and the chrome layer of the gravure roll is rough, it will also cause the squeegee to be dirty, sometimes Changing the angle of the squeegee can solve this problem. False or adjustment of the squeegee can not overcome the roughness on the gravure roll, then the plate should be polished to light.
Ink component factors: If there is a need to frequently change the doctor blade during the production process, it is likely that the colorant in the ink has a strong abrasive property, considering whether to change another ink. In addition, improper use of the solvent in the ink or excessive use of the solvent may cause flocculation of the colorant in the ink, and the flocculation causes the toner to remain under the squeegee, causing a dirty plate or causing the squeegee to wear.
The lubrication performance of the ink is also a cause of the dirty version. The lubricity of the ink is related to the resin in the ink composition. Some gravure ink manufacturers add a small amount of dark asphalt to the ink composition to improve the ink squeegee performance.
Question 2: Please explain the reasons for the increased viscosity and gelation of solvent-based liquid inks. What is the relationship between pigments and this problem?
In general, the solvent-based liquid ink exhibits an increase in viscosity because the resin is insufficiently adsorbed on the surface of the toner particles. If the surface of the crystal of the toner particles does not completely adsorb the polymer resin, the surface of the toner particles which do not adsorb the resin will aggregate with each other to form flocculation.
From a technical point of view, the surface of the colorant is not surrounded by the resin and is not isolated, but the solvent is adsorbed on the surface of the toner particles. Although the solvent sometimes wets the colorant, the solvent is colored. Dispersion cannot be in a stable position. Therefore, it appears that a solvent, not a polymer resin, surrounds the surface adsorbed on the toner particles. The surface of the pigment particles over time. As time passes, the toner particles slowly flocculate, and the end result is a change in the fluidity of the ink.
Theoretically, the problem of solving solvent-adsorbing toner particles is to use a resin having a high affinity with the colorant particles, so that the affinity of the solvent to the surface of the colorant particles is lower than that of the resin and the colorant particles. . Also, if the solubility of the polymer resin in the solvent mixture system is good, the flocculation probability of the toner particles can be minimized because the solvent/resin is better than the colorant/solvent.
Question 3: How to improve the adhesion of nitrocellulose (NC)/polyurethane (PU) solvent-based inks?
There are several different ways to solve this problem. First, you can add titanate or zirconium aluminate auxiliaries to the ink. These agents can synergize with the oxygen-containing groups on the surface of the substrate to improve the adhesion of the ink to the substrate. The second solution is to modify the nitrocellulose/polyurethane resin with polychloroolefins. The problem to be considered in the formulation of nitrocellulose/polyurethane solvent-type gondola is nitrocellulose/polyurethane. ratio.
Obviously, the nitrocellulose resin is hard and brittle and does not have sufficient plasticity, so the solvent-based ink using the nitrocellulose resin as a binder lacks toughness and elasticity, and has poor adhesion. The addition of polyurethane to the continuous material can effectively improve the plasticity of the nitrocellulose and improve the adhesion of the ink, and the cost of the ink is also improved. The addition of titanate (DOP, BBP, DBP) or benzoate can soften the nitrocellulose and is also less expensive. However, some titanates are limited to some food packaging printing applications.
Question 4: How to measure the surface tension value of gravure solvent-based ink? How does the surface tension of the ink affect printability and composite strength?
If you need to measure the surface tension of the ink during work, we can use a tensiometer to make the measurement. However, this measurement method can only measure the static surface tension of the ink (the surface tension of the ink when it is at rest). However, the ink will definitely receive dynamic forces on the press. Therefore, the truly effective surface tension should be the surface tension measured in the presence of dynamic forces, and sometimes dynamic surface tension is not easy to measure.
There is a bubble pressure tensiometer that measures surface tension under dynamic forces, but these devices are expensive and are generally not used in ink labs.
Surface tension is an important issue when printing with water-based inks. This is because the surface tension of the aqueous ink is higher than the surface energy of the substrate. In these aqueous inks, it is often necessary to add some additives to lower the surface tension of the ink so that the aqueous ink can be well wetted on the surface of the substrate.
For any particular aqueous ink formulation, the primary effect of surface tension is to affect the leveling of the ink on the surface of the substrate. Solvent-based inks generally have a low surface tension so they can wet well on the surface of a variety of substrates. However, good wettability does not mean that the adhesion is good, but only indicates that the printing quality is good. The bonding property of the resin component in the ink to the surface of the substrate is the primary factor affecting the adhesion and composite strength of the ink. The composite strength is also related to the adhesion of the ink itself. The physical interaction between the polymer resin in the ink and the various raw materials in the ink is the basis for the ink to resist the peeling action of the composite film. Therefore, the structure of the resin molecule (linear molecules or molecules containing branches) and molecular weight also increase the ink. Adhesion, so the composite bond strength increases.