Precautions for improving the adhesion of UV ink by corona method – PET, PP, PE, BOPA, BOPET, BOPP plastic film surface UV printing technology department

The surface treatment of plastic film includes flame method, chemical reagent oxidation method and corona treatment method. Corona treatment method is the most common. Corona discharge adopts high-frequency high-voltage or medium-frequency high-voltage discharge to treat the plastic surface to activate the surface and show Porous, in order to improve the adhesion of the plastic film surface to the ink and improve the printing suitability of the film. The flame method is more used in the surface treatment of high temperature plastic products, so that the plastic can remove the oil stains on the surface under the action of instantaneous high temperature. Melt a thin layer of the surface to improve inking ability. However, both corona and flame treatments are time-sensitive.

corona treatment

The principle of corona treatment is: applying high-frequency and high-voltage electricity on the processing equipment to generate high-frequency and high-voltage discharges, thereby producing small and dense purple-blue sparks. Under the action of accelerating and impacting the plastic film in the treatment device. The energy of these plasma particles is generally in the range of several to tens of electron volts, which is similar to the chemical bond energy of the plastic molecules, so it can induce the chemical bond of the plastic surface molecules to break and degrade, and increase the surface roughness. Increase its specific surface area. During discharge, a large amount of ozone is also generated. Ozone is a strong oxidant, which can oxidize plastic molecules to produce carbonyl and peroxide groups with strong polarity, thereby increasing its surface energy.

The following takes polyethylene film as an example to analyze the reasons why corona treatment improves ink fastness.

1) Oxygen in the air generates ozone due to ionization, thereby oxidizing the surface. The molecular chain of polyethylene on the surface may generate carbonyl, carboxyl, hydroxyl and other terminal p active electrodes. Nitrogen in the air is also ionized into plasma, and It interacts with polyethylene molecules to generate amine genes on the polyethylene molecular chain, and can continue to oxidize to generate a series of nitrogen-containing compounds. As a result, the polarity of the polyethylene film surface molecules is enhanced, the wettability is improved, and the ink is improved. of adhesion.

2) Eliminate some invisible oily substances (impurities, additives, oligomers, etc.) precipitated on the surface of the film, so that the ink and the film can be in close contact.

3) Due to the impact, the surface is roughened, thereby realizing the mechanical wedge head effect.

4) Due to the heating of the surface of the film, the activity of the molecular chain is intensified, that is, the degree of surface activity is increased.

5) Under the action of a high-voltage electric field, the electric charge in the molecule of the non-conductive plastic film is displaced, and an induced charge is formed on the surface. The existence of this induced charge can attract the polar ink molecules, which is beneficial to the printing ink. The improvement of layer fastness also plays a role.

The advantages of corona treatment are as follows:

(1) A wide range of processing materials can be used for polyethylene, polypropylene, polyvinyl chloride, polyamide, polyester, polycarbonate fluoroplastics, and various corresponding copolymers.

(2) The processing time is short and the speed is fast, which can be processed on the production line.

(3) Simple operation and easy control.

(4) Corona treatment only involves the extremely shallow range of the plastic surface, generally only in the nanometer order, which basically does not affect the mechanical properties of the plastic.

(5) No waste liquid discharge, basically does not pollute the environment.

Corona treatment is widely used in the surface treatment of films before printing, coating and lamination, and the surface treatment of sheets with thickness less than 0.6mm.

Precautions for improving the adhesion of uv ink by corona method

The degree of corona treatment of the film directly affects the quality of post-processing and must be strictly controlled. If the corona treatment is insufficient, the surface wettability of the film will not be significantly improved, and the adhesion of the ink will be poor, and the peeling force of the composite film will be small. On the contrary, if the corona treatment is excessive, the surface of the film will be aged and the gloss will be deteriorated; the surface molecules will be cross-linked too much, and the heat-sealability will be deteriorated; the film is easy to stick (especially in high temperature days in summer), and it is difficult to cut. Difficult to separate and other disadvantages. In short, under the premise of meeting post-processing requirements, the degree of corona treatment should be reduced as much as possible to avoid excessive treatment.

The following issues should be paid attention to when carrying out corona treatment:

1) After corona treatment, it should be used in a short period of time. Otherwise, if the polar group is combined with water molecules in the air, it will reduce the wettability and destroy the adhesion fastness of the printing ink layer. Storage environment temperature, raw material grade and film thickness and other factors are related. The higher the storage environment temperature, the faster and more complete the subsidence rate. For example, when stored in an environment below 20 degrees Celsius, the critical surface tension of pe film is 1 month or even half a year. The inner film has been kept at (3.8-4.2) 10-2n\/m; if the ambient temperature is raised to above 36 degrees Celsius, no matter how deep the original film is treated, it will retreat to below 3.810-2n\/m after 1 month. Therefore, After the film is processed, it should be stored in a dry, low-temperature warehouse, and it is best to use it up within 1 month, otherwise its printing or compounding effect will be significantly reduced. In addition, when using the film after storage for a period of time, It is recommended to re-check the test.

2) For the cold film, it is necessary to use a higher power surface electronic treatment. Before the cold film is treated, it should be preheated with iodine tungsten infrared rays.

3) Only single-sided printing, only single-sided corona treatment. Otherwise, it is easy to cause adhesion problems after printing.

4) Under high temperature conditions, if the treatment time is too long, the adhesion tends to decrease. This may be due to the formation of a "weak boundary layer" on the surface.