The advantages and disadvantages of corona treatment

Corona treatment is widely used for surface treatment of plastics, paper and metal foil to improve the adhesion of inks, paints, connecting agents and coatings. The reason why it is widely adopted is that it is effective, controllable and easy to master. Especially because the efficiency and role of corona treatment equipment are constantly improving, keeping pace with high-yield processing equipment.

1. Wetability and adhesion

Corona treatment is an effective method to change the surface characteristics of many non-absorbent substrates, which can make the ink have better wetting and adhesion performance, so-called wettability. The basis for paper or metal foil attachment. Wettability determines the ductility of a liquid on a solid surface. Water droplets spread into a thin layer of water on the hydrophilic surface, while small droplets form on the lipophilic surface. The contact angle (angle) formed between the tangent of the outer surface of the water drop and the solid surface indicates the strength of the surface's wetting performance. The larger the contact angle, the worse the wetting performance.

Wettability varies depending on chemical composition and surface structure. When printing or foiling plastics, the wettability of the plastic surface must be higher than that of ink or foil, otherwise it will be difficult to spread, 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 usually used as an ink solvent is: ethanol 22mN / m, ethyl acetate 24mN / m, and water 72mN / m, it is difficult to wet plastics, so water-based inks are generally not used to print plastics.

Plastic is a composite substance containing one or more polymers and various additives, such as fillers, antioxidants, lubricants, antistatic agents, pigments, etc. Although the chemical structure (group) of the main 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 plastic needs to be tested for surface energy and corona treated before printing to avoid problems such as transfer or insufficient adhesion. Generally speaking, the intermediate frequency processing of 20-40 kHz is mostly sufficient. Special treatments (such as complex products) can be treated with specific electrodes.

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

When printing polyethylene with different types of ink, the surface energy of polyethylene is required to be no less than the following:

Solvent ink 38-42mN / m

UV ink 42-46mN / m

Water soluble ink 42-48mN / m

Water-based ink 38-42mN / m

When coating polyethylene, the surface energy of polyethylene is required to be no less than the following:

Solvent adhesive 38-42mN / m

UV coating 42-46mN / m

Emulsion 42-48mN / m

Solvent-free adhesive 44-48mN / m

2. Wetability and adhesion test

In industrial practice, the determination of the surface energy of polymers is carried out by testing inks in accordance with DIN ISO 8296 or ASTM D2578-99a.

According to DINISO8296 method, use inks with different surface energies to brush ink strips about 100mm long on the film to be tested, and observe whether more than 90% of the ink strip edges shrink and form ink droplets within 2 seconds. If there is, replace the ink with a lower level of surface energy and then brush the ink strip. Observe the same until there is no shrinkage and ink droplets appear. The surface energy of the test ink corresponds to the surface energy of the film. This method is a method for measuring the intermediate value of surface energy, and small deviations are not counted.

According to the method of ASTM D2578-99a, a block of approximately 25 × 25 mm is coated with a cotton wool pad dipped in the test ink. Observation is performed by referring to the same method as above, and the measured value is the minimum surface energy value of the film. This test method is difficult to master because of the uniformity of the thickness of the ink layer, and its accuracy is not as good as the DINISO8296 method. The error of DINISO8296 is about ± 1mN / m, and the error of ASTMD2578-99a is about ± 2mN / m. Therefore, in industrial practice, the DINISO8296 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, for reference. Annotation)

However, no matter which method is used, the same type of Softal test ink can be used, with test inks of twenty-one surface energy levels ranging from 30 to 72 mN / m (each with a difference of 2 mN / m). Dyne test Pen (38mN / m) can be used as a quick 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 formed, it means that the surface energy of the material is not less than 38mN / m. If it is not connected continuously, it means that the surface energy of the material is less than 38mN / m, insufficient treatment or even untreated, does not meet the printing and processing requirements.

If the plastic film contains a large amount of additives or is covered with a coating film, when using the above test ink or test solution to test, chemical reactions often occur, affecting accuracy. In this case, it is advisable to use distilled water for contact angle test. For rapid determination of the adhesion of paints, coatings, inks, etc., tape adhesion test can be performed in accordance with ASTM D3359-97.

Adhesive Tape is one of many varieties of backing materials coated with an adhesive.

Adhesive Tape

Clear Adhesive Tape, Colored Adhesive Tape, Adhesive Tape, Double Sided Adhesive Tape

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