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 to change the surface characteristics of many non-absorbent substrates, can make the ink have better wetting and adhesion properties, the so-called wettability, which is such as ink, bottom The basis for the attachment of plastic, paper, or foil to plastics, coatings, etc. 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
Solventless adhesive 44-48 mN/m
(To be continued)

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