Basic knowledge of corona treatment to improve ink adhesion (1)

Corona treatment is widely used for the surface treatment of plastics, paper and metal foils to improve the adhesion of inks, paints, adhesives and coatings. The reason why it is widely adopted is that it is effective, controllable and easy to master. This is especially true because of the continuous improvement in the efficiency and effectiveness of corona treatment equipment, and keeping pace with high-production processing equipment.

First, wettability and adhesion

Corona treatment is an effective means of changing the surface properties of many non-absorbent substrates, enabling the ink to have better wetting and adhesion properties, so-called wettability, which is what plastics, such as inks, primers, coatings, etc. Paper or foil attached to the foundation. Wetability determines the ductility of a liquid on a solid surface. Water droplets spread on the hydrophilic surface into a thin layer of water, while small droplets form on the lipophilic surface. The contact angle (angle) formed between the tangent of the outer surface of the water droplet and the solid surface indicates the wettability of the surface, and the greater the contact angle, the worse the wetting property.

Wettability varies depending on chemical composition and surface structure. When the plastic is printed or hot-foiled, the wettability of the plastic surface must be higher than the wettability of the ink or foil, otherwise it will be difficult to extend, transfer and attach. The surface properties of several polymers are roughly as follows:

Polyethylene (PE) 31-33mN/m

Polypropylene (PP) 29-30mN/m

Polyester Polyester (PET) 41-42mN/m

The surface energy commonly used as an ink solvent is: ethanol 22 mN/m, ethyl acetate 24 mN/m, and water 72 mN/m, it is difficult to wet plastics, so water-based inks are generally not used to print plastics.

Plastic is a composite material containing one or more polymers and various additives such as fillers, antioxidants, lubricants, antistatic agents, pigments, and the like. Although the chemical structure (group) of the host polymer of the plastic determines the wettability and adhesion of the ink or primer, its additives will migrate from the interior of the plastic to the surface, affecting the surface energy of the plastic. Therefore, the longer the plastic is stored, or the higher the content of certain additives (such as lubricants), the greater the change in surface energy. Therefore, the surface energy of the plastic must be tested before printing, and corona treatment, in order to avoid the transfer or adhesion problems. In general, 20-40 kilohertz IF processing is mostly sufficient. Special treatments (such as complex products) can be handled with specific electrodes.

UV inks require higher surface energy for plastic films than solvent inks. Water-based ink systems also require high surface area for high alcohol content, and require a narrow range of adhesion for good adhesion.

When printing polyethylene with different types of ink, the surface energy of polyethylene is required to be not lower than the following:

Solvent ink 38-42 mN/m

UV ink 42-46 mN/m

Water-soluble ink 42-48 mN/m

Water base ink 38-42 mN/m

For polyethylene coating, the surface energy of polyethylene is required to be not lower than the following:

Solvent type adhesive 38-42 mN/m

UV coating 42-46 mN/m

Emulsions 42-48 mN/m

Solvent free adhesive 44-48 mN/m

Second, the wettability and adhesion test

In industrial practice, the determination of the surface energy of a polymer is performed by testing the ink in accordance with DIN ISO 8296 or ASTM D 2578-99a.

According to the DIN ISO 8296 method, an ink about 100 mm long is brushed on the film to be measured with an ink having a different surface energy, and whether more than 90% of the ink strips contract and form within 2 seconds is observed. The ink droplets, if any, are replaced by ink of a lower surface energy level and the ink strips are brushed again to perform the same observation until no shrinkage or ink droplets occurs. The surface energy of the test ink corresponds to the surface energy of the film. This method is a method for testing the intermediate value of the surface energy, which is small and does not count.

In accordance with ASTM D 2578-99a, a block of approximately 25 x 25 mm is applied using a cotton padding test ink, and the minimum surface energy value of the film is measured by the same method as described above. This test method is difficult to grasp due to ink layer thickness uniformity, its accuracy is less than DIN ISO 8296 method, DIN ISO 8296 error is about ± 1 mN / m range, while the ASTM D 2578-99a error is about ± 2 mN /m. Therefore, in the industrial practice, the DIN ISO 8296 method is mostly used, and it is simpler and faster. (In China, there is a national standard GB10003-88 for the corona treatment effect test method provisions, for reference.

Regardless of the method used, the same Softal test ink can be used, with test inks of 30 to 72 mN/m of twenty-one surface levels (each differing by 2 mN/m). Dyne test pen (38mN/m) can be used as a rapid test tool for surface energy after corona treatment, but it is not suitable as a system test for printed or coated surfaces. When the test pen draws a line on the corona-treated surface, if it is continuously lined, it means that the surface energy of the material is not less than 38mN/m, such as intermittently not connected, indicating that the surface energy of the material is less than 38mN/ m. Insufficient or even untreated, not meeting the printing process requirements.

If the plastic film contains a large amount of additives or is coated with a coating film, chemical reactions tend to occur when testing with the above test ink or test liquid, which affects accuracy. In this case, distilled water is recommended for contact angle testing.

Rapid determination of the adhesion of paints, coatings, inks, etc., can be used for tape adhesion testing according to ASTM D 3359-97.