The use of UV coating on the print, sometimes yellowing phenomenon, there are two main reasons: First, UV coating itself yellow; Second, in the coating process, the coating oil is only cured on the surface of the print. The interior is not completely cured.
The yellowing of the varnish is related to the use of the oligomers and reactive diluents with a certain yellow color. Therefore, colorless oligomers and reactive diluents should be used to formulate the varnish. In addition, the influence of photoinitiators is also great, some photoinitiators will make the system yellow after curing, such as benzoin dimethyl ether, so the use of such photo initiators to avoid the preparation of coating oil.
In the coating process, when the interior of the coating oil is not completely cured, the internal coating oil thinner will gradually be absorbed by the ink and the paper, and at the same time, it will have an effect on the binder in the ink and cause the loss of the oil. The paint is insufficient With the protection, the pigment particles are free. At this time, the smoothness of the surface of the paper coating and the ink layer is reduced. The rough surface will produce diffuse reflections of light, adding some of the plant's white components to the colors, which will increase the brightness and decrease the saturation, giving the impression of overall yellowing and lightening. Therefore, the curing speed of the UV coating should be increased or an efficient UV curing light source should be selected. In addition, the use of water-based UV varnish can avoid the chemical effect of the thinner Liu ink layer and paper coating, and can avoid yellowing of the printed matter.