Corona Treatment : Overview & Working For BOPP /BOPET Film Industry

 

Corona Treatment: Overview & Working F BOPP/BOPET Film Industry 

Corona Discharge is a COLORED ELECTRICAL DISCHARGE GENERATING DUE TO IONISATION OF AIR SURROUNDING A CONDUCTING BAR. In many industrial processes such as extrusion, molding etc. the output product surface has low surface energy which makes them chemically inactive. For example, in manufacturing of Polypropylene (BOPP) or PET (Polyethylene Terephthalate, BOPET) Films, the surface of final product has low surface tension, which makes it non-receptive to bonding with other chemicals. Low surface tension reduces printability, wettability and affects coating, sealing, and labelling.

To measure surface energy of the film, “Dyne level” is used as unit of measurement. Higher the Dyne value, better the printability. 

Methods to increase Surface Tension: 

If the surface energy of the film is less than the surface energy of the liquid/paint/coating, then, it will oppose each other and will result in poor printability. If the surface energy of the film is greater than the surface energy of liquid, it shows increased molecular attraction and improves printability. By breaking the molecular bonds of the primary surface, and excite it the free radicals, surface energy can be increased. There are many methods to do so: CORONA TREATMENT, FLAME TREATMENT, PLASMA TREATMENT, CHEMICAL TREATMENT. In this article, we will focus on “corona Treatment”. 

What is Corona Treatment: 

Corona is a violet-colored electric discharge caused due to ionization of air surrounding two electrodes with high voltage. When electrodes are supplied with high volage, ions present in the surrounding air are accelerated by the electric field and strike to nearby ions, transferring their kinetic energy to other ions. As the power increases, it causes die electric breakdown of air and a violet-colored discharge can be seen. These accelerated ions, when come in contact with the plastic surface, strike and try to penetrate its surface molecular structure. Continuous penetration of ions causes the surface oxidation and creates polar units, which increases its surface tension. 

Corona Discharge

Components of Corona Treater: 

Corona treater system consists of 3 parts: 

• Electronic generator 

• High voltage transformer

• Discharge station (Electrode & Roller) 

 1. Electronic Generator: Electronic generator converts 3-phase 50Hz power supply into 1-phase high frequency (usually 10kHz – 30kHz), and feed it to high voltage transformer. It consists of: 

• Rectifier: Which converts AC supply into DC supply. 

• Chopper: Adjusts output power of rectifier before it fed to the inverter section. 

• Inverter: Consists of IGBT (insulated-gate bipolar transistor), which converts it into high frequency AC supply. 

2. High Voltage Transformer (HV Transformer): It is a step-up type transformer with working frequency from 15kHz-30kHz. 

3. Discharge Station: Discharge station consists of discharge Electrodes and roller. Discharge electrode can be a fixed bar or choke type (for single treatment). Electrode is made of Stainless Steel or Aluminum or Ceramic and connected to HV transformer through HV cable. Roller provides supports for surface to be treated and connected to ground potential. Discharge rollers have dielectric cladding. The cladding must be rigid, and have low dielectric losses, and high ozone resistance. A uniform air gap (2.5 -2.8 MM), (depending on the material and length of electrode, and type of dielectric coating on roller) must be maintained. Earthing plays an important roll in Corona treatment. Rollers should be well grounded.