Plastic substrates are popular due to good stability, water resistance, softness and strength. They have various types. However, due to the features of plastic itself, there are some difficulties to printing.

For example, in silkscreen printing, plastics are highly hygroscopic. They are easily deformed by the relative humidity of the surrounding air. During the processing of printed materials, within the strength allowable range, the elongation increases with the increase of tension.

The surface is smooth, and has no pores, which makes it difficult for the ink layer to fix. The ink of the next color overprinting after the first color printed is easy to be stuck off, causing the image and text to be damaged.

The additives in the film made by adding additives are very easy to seep out during the printing process, forming an oily layer on the film surface. It is not easy for coatings or other adhesives to firmly adhere to the surface. The ink layer is not easy to dry.

These features have caused difficulties in printing. Therefore, in order to make plastic printing go smoothly, the surface must be treated physically and chemically. These treatments mainly polarize and roughen the plastic surface to facilitate the smooth adhesion of ink on it.

The substrates for silkscreen printing are polyethylene, methacrylate resin, polystyrene, polypropylene, polycarbonate, polyvinyl chloride plastics, etc.. During printing, to improve ink adhesion, there are many ways for surface treatment.

There are chemical treatment and physical treatment. Let's talk about one popular method: plasma surface treatment, and how does it improve the adhesion of silkscreen printing ink.

Plasma surface treatment

Plasma surface treatment can make ink have better wettability and adhesion. Wettability determines the ductility of a liquid on a solid surface. When printing on plastic, the wettability of the plastic surface must be higher than the wettability of the ink or foil. Otherwise, it is hard to spread, transfer and adhere.

Water droplets spread into a thin water layer on a hydrophilic surface, but form small water droplets on an oleophilic surface. The contact angle formed between the outer layer of the water droplet tangent and the solid surface indicates the strength of surface wettability. The larger the contact angle, the worse the wettability.

Under the influence of strong electric field, high temperature and laser, neutral atoms or molecules will lose electrons and ionize into ions. These ions contain equal positive and negative charges, so they are called plasma.

What's the principle of plasma surface treatment? Radio frequency generator emits laser energy at a higher voltage, producing a characteristic glow discharge phenomenon, causing the passing gas substance to produce many excited electrons, ions and atoms, which then bombard the plastic surface, causing the active groups on the plastic surface to change or form new groups or free radicals, resulting in deposition, thereby finishing chemical and physical modification of the polymer surface. It generates polarity, which will dock with the polar groups in ink binder, thereby improving ink adhesion.

Plasma surface treatment can increase the adhesion of plastics, because it can remove atoms and molecules absorbed on the surface. The treatment promotes atomic contact and increases wetting. It also increases surface energy and adjusts polarity. It creates atomic groups or functional groups that can react chemically.