1. How does corona generate?
Corona is generated by the extremely nonuniform electric field created by an uneven conductor. When the voltage gradually increases to a certain threshold, the air is ionized by the electric field, triggering a discharge and forming a corona. Outside the corona, where the electric field is relatively weak, collisions and ionization do not occur. Therefore, the charged particles outside the corona are primarily composed of ions, which form the primary current of the corona discharge. Simply put, the corona phenomenon occurs when a conductor electrode with a small radius of curvature discharges air.
2. What is the purpose of corona treatment?
Corona treatment is a method that enhances the adhesion of printed substrates through electrical shock. It uses high frequency and high voltage to induce corona discharge on the plastic surface, generating a low-temperature plasma. This plasma, through electrical shock and penetration, disrupts the molecular structure of the printed substrate, further oxidizing and polarizing the molecules on the substrate surface. Ultimately, the ion shock erodes the surface, significantly improving the substrate's surface adhesion.
Corona treatment enhances substrate adhesion through a series of mechanisms: first, it removes atoms and molecules adsorbed on the surface; second, it promotes interatomic contact, thereby enhancing the material's wettability; third, it significantly increases the surface energy level and modulates the material's polarity; and finally, it creates atomic radicals and functional groups on the surface that can initiate chemical reactions.
In film production, the effectiveness of corona treatment is often tested using a liquid coating method. Key factors influencing corona treatment include 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.
The purpose of corona treatment is to induce chemical bond breakage within the material's molecules, thereby causing degradation and significantly increasing surface roughness. Furthermore, the ozone generated during the corona treatment process undergoes an oxidative reaction with molecules on the plastic's surface, which not only improves surface wettability but also enhances adhesion.
3. How Corona Treatment Works:
High-frequency, high-voltage electricity is applied to the corona treatment device, triggering a corona discharge phenomenon that produces small, dense purple-blue sparks. As the air is ionized, various plasmas are formed. Driven by the strong electric field, these plasma particles impact the plastic product at high speed. Because the energy of these plasma particles is similar to the chemical bond energy of plastic molecules, on the order of several to tens of electron volts, they effectively induce chemical bond breakage within the plastic molecules, leading to material degradation and significantly increasing surface roughness. Furthermore, the corona discharge process produces a large amount of ozone. This strong oxidant reacts with molecules on the plastic surface to form carbon-based compounds, peroxides, and other substances.
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 material's surface polarity. This treatment also increases the polarity of the film, raising surface tension and thus enhancing bonding properties. Under the influence of a high-voltage electric field, oxygen in the air is converted into ozone, which is then further decomposed into oxygen and nascent oxygen atoms. These nascent oxygen atoms possess strong oxidizing power and can react with the α-carbon atoms in polyethylene or polypropylene molecules, introducing carbonyl and hydroxyl groups. This structural change increases the polarity of the film molecules, raising their surface tension and enhancing their affinity for adhesives, thereby improving the bond strength between composite films. Furthermore, the generation of active hydrogen can chemically react with the active groups in polyester adhesives, further enhancing the bonding effect.
Corona treatment can also significantly improve ink adhesion and enhance printing compatibility. During the treatment process, a large number of plasma ozone particles interact with the surface molecules of the plastic. The high-energy particles bombard the film surface, breaking polymer bonds and generating numerous free radicals and unsaturated centers. These superficial reactive groups then cross-link with moisture adsorbed on the film surface, introducing polar groups such as hydroxyl groups on the fiber surface and activating the film surface. After corona treatment, the adhesion and fastness of inks on films are significantly improved, thereby enhancing printability.
5. Factors Affecting Corona Treatment Effectiveness:
The effectiveness of corona treatment is influenced by multiple factors. First, the treatment time directly impacts the treatment effect. Too short a treatment time may not achieve the desired cleaning effect, while too long a treatment time may damage the material surface. Second, the treatment temperature is also a key factor, affecting the physical and chemical properties of the material surface, which further affects the effectiveness of the corona treatment. Furthermore, the electric field strength is an essential factor, determining the intensity and uniformity of the corona discharge, which in turn affects the treatment quality. Furthermore, the properties of the material itself, such as surface tension and polarity, also significantly affect the effectiveness of corona treatment. Finally, impurities and moisture content in the air also affect the efficiency and quality of corona treatment to a certain extent.
The Effect of Temperature: As the temperature increases, the degree of corona treatment increases accordingly. In actual production, if the treatment level is insufficient due to power limitations of the corona treater, measures such as adding infrared radiation before the treatment device (such as cold film treatment) can be adopted to improve the treatment effect. Humidity: High humidity often affects the effectiveness of corona treatment, as the corona discharge process occurs in air.
Time: The effectiveness of corona treatment gradually fades over time.
Storage: The temperature of the storage environment significantly affects the rate of fading. Generally speaking, the higher the storage temperature, the faster and more complete the fading of the corona treatment. Other factors, such as material thickness, may also affect the fading rate. For IXPE corona treatment in particular, its optimal shelf life is typically less than two weeks. After this period, the treatment's effectiveness gradually diminishes.