Photoinitiator  Improved adhesion to substrates

Because UV offset inks have instantaneous curing and low-temperature curing characteristics, they can be used not only on ordinary paper, but also on a variety of non-absorbent papers and plastics such as synthetic paper, metal-coated paper, PVC, PC, and so on. However, during free radical polymerization, the distance between monomers or oligomers changes from the distance of van der Waals force action before curing to the distance of covalent bonding action after curing, and the distance between the two shrinks, so the volume shrinkage is obvious. Coupled with the curing time is very short, the ink layer of the internal residual stress is very large, so compared with the general solvent-based ink, UV offset ink has a tendency to deteriorate adhesion.

At present, for some special plastic substrates (such as PE, PP and part of the PET) UV offset ink can not provide good adhesion, mainly due to such materials have active diluent can not be dissolved, low surface polarity characteristics, has been cured between the ink film and the substrate can not form an effective force, resulting in poor adhesion. Solving the adhesion problem of UV offset ink to these plastics is an insurmountable technical problem in the industry. According to practical experience, to solve the adhesion problem of UV offset ink on difficult substrates, usually from the following aspects: ① Increase the active diluent with low surface tension to improve the wettability of the ink on the substrate. ② Select low-functionality active diluent and oligomer to reduce the ink stress and volume shrinkage. ③ Select oligomer with high molecular weight and low Tg. ④ Use special functional group modified oligomer such as chlorinated polyester acrylate. ⑤ Add adhesion promoter.

However, such treatment often leads to poor curing performance of the ink and deterioration of printability. From the application situation, it is very difficult to make UV offset ink have good adhesion performance to all the substrates, so while improving the ink performance, it is also necessary to carry out surface pretreatment of the substrate (such as corona treatment, flame treatment) or do viscosity enhancement of the primer coating, etc., in order to improve the surface performance of the substrate. In short, must be combined with the printing process to solve the problem of UV offset ink for the substrate adhesion difficulties.

The use of UV Offset Ink also requires attention to the following issues.

(1) Printing Plate
Offset printing can use both positive and negative PS plates. In the case of a positive-type PS plate, a positive-type PS plate specializing in UV ink must be used because the active diluents and photoinitiators can decompose the dots of the ordinary PS plate, resulting in a decrease in the print resistance of the printing plate. If you use a general positive-type PS plate, you need to bake the plate, that is, according to the conventional method of sun-made ordinary PS plate, you need to bake the plate for 10 to 15min (temperature 250 ℃).CTP plate also need to bake the plate for 10 to 15min (temperature 250 ℃).

(2) Rubber rollers and blankets
To use UV special rollers, the use of oil-based ink and UV ink type rollers can also be used for its effect. Blanket selection of general oil-based ink, UV ink with the blanket can be, but due to the UV ink in the active diluent will be in the printing process on the ordinary rollers and blankets to penetrate, on the one hand, resulting in a reduction of active diluent in the ink, destroying the fluidity of the ink; on the other hand, resulting in the corrosion of the rollers and blankets, short-term will lead to the surface of the rubber rollers and blankets and the expansion of the surface deformation of the surface of the glass, the long-term printing of the rollers and blankets. Long-term printing rollers and blanket surface will be localized peeling or cracks. So usually choose EPDM rubber (EPDM rubber) or silicone rubber made of rubber rollers, the best hardness in Shaw’s 40SH or more. Select UV special rubber blanket, the rubber blanket mixture to add anti-UV resin components, UV radiation can be resistant to rubber blanket is not easy to powder. Or use UV self-adhesive blanket, the blanket also contains anti-UV resin components, for UV printing, life can be extended.

(3) Disperse and grind
UV offset printing ink pigment used in a variety of inks in the largest, generally in 15% to 21%, so dispersion, grinding is very important, usually the first dispersing resin, active diluent, wetting and dispersant and pigment dispersion in the sand mill on the dispersion, grinding, made of color paste, and then mix the rest of the components, with three-roll machine grinding made of ink.

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