Water-based coating drying

(HOW WATERbaseD COATINGS DRY)

After the printing process is completed, it needs to be coated on the printed matter to protect the printed matter.

One of the things that printers often do is calculate how long it takes for the coating to lose its ability to stick. This is what is commonly called the drying time of aqueous coatings. Whether it can resist the ink soaking and the coating layer can prevent the paper from fluffing a great deal.

First understand the main process of coating formation. The formation of the coating is divided into two phases: the first is the ready (solidification) phase and the second is the final drying phase. The first stage will determine whether the coating will prevent the paper from fluffing. The second stage will determine whether the ink can penetrate.

Coating ready (solidification) refers to the process of evaporation of the solvent contained in the coating from the beginning of the coating. The main component of most solvents is water, and there are many complex solvents. The role of the solvent is to cause the coating to form a continuous film, which takes several seconds to several minutes to evaporate, sometimes even after infrared or hot air drying to leave the coating surface.

As the water evaporates from the coating film, the polymer composition gradually increases and the hardness of the coating film gradually increases until the coating layer “feels” “feeling good”. This process is entirely dependent on the person's subjective feelings, so mistakes are often made.

Adding paraffin or silicon to the solvent, they will accumulate on the surface of the coating film, and the coated layer is ready (solidified) quickly, preventing fuzzing. This seems to be a wonderful thing, but these additives usually have certain Side effects, such as effects on stamping, COF, and gluing, and the coating can undergo certain changes due to the easy migration of silicon atoms.

To ensure that the water-coated material does not fluff in 15 to 60 seconds, the best way is to form a layer of material that stops the material from fluffing. This can be achieved by adding a resist or by selecting the correct polymeric material. If the coating material is designed properly, even if the coating ready time is prolonged due to the thick ink or the paper is too tight, it will not fluff.

In fact, if the coating material is ready (solidified) too fast, the coating layer on the upper part of the ink will also be embrittled. The shorter the ready time of the coating material, the thicker the ink and the more severe the cracking phenomenon. This is extremely common on poorly coated, non-voided printed materials.

The second step in the coating process is the actual drying of the coating. This process may take a few minutes or even several hours. As the solvent evaporates upwards through the pores in the coating, the drying process proceeds down the surface of the coating.

The speed of this process determines whether the coating resists ink immersion and the final hardness of the coated layer. Unlike the first phase, the second phase has little to do with the infrared and hot air dryers of the offset press. It is related to the composition of the aqueous coating itself.

Sometimes a coated paper will feel dry even after 10-15 seconds, and it will not guarantee that the ink will not soak or the paper will not have burrs. A coating with insufficient coagulation must incorporate a bulge blocker to prevent the paper from fluffing within a few seconds after coating begins.

Even if some coating materials require more than 20 seconds of ready (solidification) time, the results are still good. Coatings with short (ready-to-cure) times do not necessarily guarantee that the ink is not saturated, and materials that are ready (solidified) for a long time do not necessarily indicate poor resistance to ink penetration.

Under normal conditions, the coating should harden between 30 and 45 seconds. Taking into account the solidification rate of the ink, long time is also possible. Of course, the long time does not mean that the material is not suitable, and the coated layer should be randomly loaded to test whether it has enough ink penetration resistance. The coating ready (solidification) time may also vary from 10 seconds to 20 seconds, when the coating is ready (solidified) and the drying process is almost completed.

The above is the key to coating ink resistance, which is more important than paper fluffing. Today's printers must have a clear understanding of how the coating is dried in order to meet customer requirements and avoid the huge waste caused by rework.