To add ultraviolet (UV) coating to the common ink, it is common practice to use diffusion coating as the primer. Ordinary offset inks and ultraviolet (UV) coatings are incompatible chemicals, and this "intermediate coating" needs to be applied on the printed sheet in a short time and in a wet stacked wet form. The water-based diffusion synthesis solution contains about 40 to 45% of solids, and its flow resistance (viscosity) that changes with temperature can be adjusted by adding water.
Once the diffusion coating is applied to the printed sheet, a considerable amount of moisture must be absorbed into the printing material, and some will disappear due to volatilization. It is only at this time that the polymer starts its bonding and forms a tight coating film. During the linkage process, the primer material must be able to dry as soon as possible.
The drying speed, flexibility, infiltration capacity, traction capacity, flow resistance (viscosity), gloss and adhesiveness that can be achieved all depend on the base material and the additives used.
Primer coating function as a bonding and balancing element
From the increase in the type and number of primers used, it clearly shows its important functions.
Especially on the surface with high ink coverage, there is a change in the weight and mixing, which affects the primer coating, which is more significant. Figures 4 and 5 illustrate these relationships.
Turning 13 cubic centimeters of reticulated reels per square meter to 18 cubic centimeters of reticulated reels, the ultraviolet (UV) coating layer is smoother and has higher gloss (photos 3 and 4). This situation is more obvious in places with high coverage.
Different types of primers. The effect is also easily recognizable: Comparing Photo 5 (left) and Photo 6 (right), the primer on the left is relatively hard and has good gloss; the primer on the right contains a soft vehicle. Applying a hard, glossy primer on a 400% ink coverage surface can increase 35 gloss points; even on a 100% ink coverage surface, a difference of more than 15 gloss points can be measured (Figure six). When using soft body primer, severe paper deformation can only be seen on 200% of the surface. Hard primer coatings have a smooth surface before they are coated with ultraviolet (UV) paint. However, the ability to resist rubbing is reduced. When relatively small amounts are used, the type of primer is particularly influential.
These tests clearly show that drying between coating units is extremely important. If the drying time and drying conditions are not sufficient before applying the ultraviolet (UV) paint, although the best primer and the most effective amount can not be used to achieve the best gloss. It is for this reason that it is recommended to use two coating units and an extension drying section for high-quality coating printing.
Ultraviolet (UV) coating
Today, most offset printing uses chemically-rooted solid paint. These ultraviolet (UV) coatings contain color development agents, chemical reaction diluents, and photoinitiators. Under the irradiation of UV light, it plays a role of reticulation, making the coating firm (curing). UV light causes the photoinitiator to release chemical roots, which leads to the polymerization of acrylates; that is, the formation of long-chain networks. Oxygen hinders this reaction.
For cationic solid coatings, UV light triggers ions; it creates a solid situation in a chain. Because of the continuity of curing, although it takes a longer time, it will be completely solid. Although the reaction process is inhibited by moisture, the cationic ultraviolet (UV) coating method is mostly used in food packaging because it has no odor and is completely solid. Our testing focused on chemical-rooted solid ultraviolet (UV) coatings.
Ultraviolet (UV) coating layer and gloss that can be achieved
Increasing the amount of ultraviolet (UV) paint can improve gloss (photos 9 and 10), but the effect is not as good as increasing the amount of primer. During the test, the amount of ultraviolet (UV) paint was increased from 20 to 30 cubic centimeters per square meter, regardless of the ink coverage, the gloss increased by 10 points. When deciding the amount of ultraviolet (UV) paint, do n’t forget that depending on its flow resistance (viscosity), too much will make it difficult to apply smoothly (Photos 7 and 8).
Heating ultraviolet (UV) paint to 40 degrees Celsius can enhance fluidity and increase gloss at the same time.
The geometry of the reticulated reel also affects the smoothness of the paint. In addition to the capacity, its network cable condition also plays a role.
Only a foam-free coating then creates a good gloss effect. When the small pits of the reel are filled with paint, tiny bubbles are formed in the paint, causing micro-foam. This situation can be improved by adding a defoamer, and a properly combined paint circulation system to ensure that the paint will not be stirred too much.
Drying is necessary
Within seconds, the material that imparts color and gloss is applied to the substrate. Although manufacturers try their best to make these films quickly change from liquid to solid, high-speed printing poses limitations. Therefore, the process must choose the appropriate drying support.
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