With the development of printing technology, as well as various types of consumer demand for printed products to improve the decorative effect, UV ink printing and UV coating have been increasingly popular in PET, PVC, BOPP, PP, PE and other plastic substrates. However, it is a new issures for the printing and ink manufacturers on UV ink printing, the biggest problem is how to make the UV ink/varnish firmly adhere to the plastic substrate surface, that is, adhesion problems. In the following, we focus on the problems of UV printing on plastic substrates such as PET, PVC, BOPP, PP, PE, the key points are substrate treatment and UV ink formulation, which would has a good reference value.

1.PET, PVC, BOPP, PP, PE plastic substrate surface tension

The plastic substrate surface tension is the first factor to consider when printing. The surface tension on many plastic films is very low before treatemnt, generally need to surface treatment inpurpose of improving the surface tension of plastic substrates, plastic film suppliers would have the surface treatment but the processing effect will soon be to lose.

Corona discharge is the most common treatment for printers,  it adopts high frequency high pressure or normal frequency high pressure to discharge and treat the plastic surface, improving the the adhesion of the plastic film surface to the ink, and also improving the printability of the film. Flame method is more used for high temperature resistant plastic surface, the plastic surface is in an instant high temperature and remove the surface of the oil and melt the surface layer, improving ink capacity; chemical treatment, is also a common solution in increasing the adhesion for kinds of substrates like PET.

After surface treatment, the surface tension of ink is lower than that of plastic film. Similarly, the surface tension of the varnish is lower than the surface tension of the ink in order for the varnish to be well wetted on the ink surface and adhere firmly to the surface of the ink. The study of a variety of ink formulations found that solvent-based ink surface tension is generally lower than other types of ink, it can be a variety of plastic film surface wetting spread out.

UV plastic printing commonly used plastic surface tension value (temperature: 20 degrees Celsius):

Surface tension of plastic substrates	(Dyne / cm)
Polyethylene (PE)	31
Polypropylene (PP)	34
Polyvinyl chloride (PVC)	33~39
Polystyrene (PS)	33
Polyester (PET)	43
Polyvinylidene chloride (PVDC)	40
nylon	46

2.PET, PVC, BOPP, PP, PE plastic substrate expansion and permeability properties

Unlike many commonly used printing paper and paperboards, the surface of the plastic substrate does not allow the ink/varnish to penetrate into its internal micropores.However, there will have swelling phenomenon on some plastic substrates when contacting with some UV ink/varnish raw materials. Therefore, we can makes the plastic substrate with certain UV ink used in conjunction, so that ink / varnish can penetrate into the plastic substrate inside. At the time of curing, there is an ink/varnish internal penetration, the ink / varnish and the plastic substrate surface can be formed between the strong adhesion effect. In addition, the plant temperature can be increased to enhance the penetration of plastic substrates and ink / varnish, which is one of the ways to improve the adhesion between UV inks / varnishes and plastic films.

3. The glass transition temperature (Tg) of the ink

In physics, glass is formed when certain substances are below or equal to the glass transition temperature. UV inks/varnishes generally contain smaller molecular weight materials that are thicker and more crosslinked when cured by reaction than conventional inks/varnishes. At the same time, the glass transition temperature (Tg) after the UV ink/varnish curing is higher and the film is harder, it has good abrasion resistance and chemical resistance.

If the Tg of PET, PVC, BOPP, PP, PE film is higher than the working temperature in the hot stamping or laminating process, the UV film in the finished product will not adhere firmly to the gold foil or composite plastic film. UV inks/varnishes made with low Tg materials are good for gold foil and composite films in the finishing process. Sometimes in order to be able to make gold foil or film smoothly, we had to reduce the process temperature.

4. Degree of cure

For any UV ink formulation, the appropriate photoinitiator must be used to optimize the performance of the ink layer after curing. Each UV ink/varnish formulation works in conjunction with a specific, sufficient amount of UV energy. If the UV energy spectral band, power changes, it will affect the performance of the ink after curing.

Many may feel the UV ink/varnish has been cured in the plastic substrates surface of PET, PVC, BOPP,PP, PE, etc, but in fact, it is likely that the entire film interior is not fully cured. The degree of cure close to the bottom layer of the film is critical to achieving good adhesion. There is no point in the penetration of the UV ink/varnish inside the plastic substrate, since the inside and the bottom of the film are not completely cured. Once the curing energy changes, the surface tension of the film will also change, and ultimately affect the subsequent finishing process successfully completed.

The plastic packaging market is an exciting market, while UV inks/varnishes are commonly used for plastic packaging and printing. Rapid curing means more efficient production, no heat drying ink means that the impact on the environment and work shop is smaller.

The adhesion of UV ink/varnish is one of the few problems for in the plastic packaging and printing , mainly due to printing customers continue to need new plastic substrate materials and new applications. As long as the full consideration of the above four problems,  issues in the PET, PVC, BOPP, PP, PE plastic printing problems can be solved,