1. Concept
How is corona generated?
Corona is generated by the extremely nonuniform electric field created by an uneven conductor. When the voltage near an electrode with a small radius of curvature rises to a certain threshold, air is ionized and discharges occur, forming a corona. It is noteworthy that outside the corona, due to the weaker electric field, no collisions or ionization occur. Therefore, the charged particles outside the corona are primarily composed of ions, which form the current of the corona discharge. Simply put, the generation of corona is the result of a small radius of curvature conductor discharging into the air.
Corona Treatment
Corona treatment is a method that enhances the adhesion of printed substrates through electrical shock. Its core principle is to generate a high-frequency and high-voltage corona discharge on the plastic surface (high-frequency AC voltage can reach up to 5,000-15,000 V/m²), generating a low-temperature plasma. This plasma penetrates the printed substrate surface through electrical shock and penetration, disrupting its molecular structure, and subsequently oxidizing and polarizing the treated surface molecules. This significantly improves the adhesion of the printed substrate surface. How Corona Treatment Improves Substrate Adhesion
Corona treatment enhances substrate adhesion through a series of mechanisms. First, it removes atoms and molecules adsorbed on the substrate surface, creating a clean surface for subsequent treatment. Second, corona treatment promotes interatomic contact, increasing wettability and, in turn, raising the surface energy level and polarity. Ultimately, this process creates reactive atomic groups and functional groups on the substrate surface that can initiate chemical reactions, significantly improving adhesion.
Corona treatment is commonly used in film production, and its effectiveness is influenced by a variety of factors, including electrode type, film temperature, line speed, electrode exhaust volume, and surface material. Notably, corona treatment also affects physical properties such as the film's coefficient of friction, shrinkage, and heat seal strength.
2. Purpose of Corona Treatment:
Corona treatment is intended to induce chemical bond breakage within the material's molecules, leading to material degradation and increasing the substrate's surface roughness. Furthermore, the ozone generated during the corona treatment process further oxidizes the plastic's surface molecules, significantly improving surface wettability and adhesion.
3. Principle of Corona Treatment:
A specialized corona treatment device applies high-frequency, high-voltage electricity, inducing a corona discharge phenomenon that produces numerous fine, dense purple-blue sparks. During this discharge, the air is ionized into various plasmas. These plasmas, under the influence of a strong electric field, impact the plastic product at high speed. Because the energy of these plasma particles is similar to the chemical bond energy of plastic molecules, they effectively induce chemical bond breakage and degradation, thereby increasing the surface roughness of the substrate. Corona discharge also produces a large amount of ozone. This strong oxidant further oxidizes the surface molecules of the plastic, promoting the formation of carbon-based compounds and peroxides.
4. Effects of Corona Treatment:
Corona treatment not only effectively removes oil, moisture, and dirt from the plastic surface, but also significantly improves wettability and adhesion, enhancing the polarity of the material surface. This process involves multiple physical and chemical changes, including increasing the polarity of the film, enhancing surface tension, and improving bonding properties.
Under the influence of the high-voltage electric field, oxygen in the air is converted into ozone, which then decomposes into oxygen and nascent oxygen atoms. These newly generated oxygen atoms possess strong oxidizing properties and can react with the α-carbon atoms in polyethylene or polypropylene molecules to form carbonyl and hydroxyl groups. The presence of these groups increases the polarity of the film molecules, raising their surface tension, thereby enhancing the film's affinity for adhesives and improving the bond strength between composite films.
Corona treatment also improves ink adhesion and enhances printability. During the treatment process, a large number of plasma ozone particles interact with the plastic's surface molecules, breaking polymer bonds on the film's surface. These broken polymer bonds generate abundant free radicals and unsaturated sites on the material's surface. These shallow surface active groups then cross-link with moisture adsorbed on the film's surface, introducing polar groups such as hydroxyl groups onto the fiber surface and activating the film's surface. Corona-treated films significantly enhance ink adhesion and fastness, thereby enhancing their printability.