Purpose of Corona Treating
The purpose of corona treating is to improve the wettability and adhesion characteristics of plastic substrates to inks and adhesives. Unlike the purely mechanical bond, as in the case of an ink penetrating into a porous surface like paper, plastic films will generally need some means of surface treatment to achieve acceptable chemical bonding with the ink or adhesive.
Theory of Corona Treating
Although there are numerous theories explaining the principles behind the process of corona treating, the most commonly accepted theory appears to be the theory of high speed oxidation. Basically, this theory states that the energy of the corona breaks the molecular bonds on the surface of the non-polar substrate. The broken bonds then recombine with the free radicals in the corona environment to form additional polar groups on the film surface. These polar groups have a strong chemical affinity to the polar inks and adhesives, which results in improved adhesion. Similarly, the polar surface results in an increased surface energy which correlates with improved wettability.
Components of Corona Treating Systems
All corona treating systems have two components. The first component is the power source and the second component is the treater station.
The power source generally consists of a high frequency generator and a high voltage output transformer. In very general terms, the purpose of the power source is to raise the incoming electricity (typically 50/60 Hz, 230/460 V) to a higher frequency (10-35 kHz) and higher voltage (10 kV). The power source is commonly referred to as a power supply or a generator. Typically, power supplies are rated in kilowatts (kW) and can range from 500 W to 30 kW, depending on the application.
All treater stations have a high voltage electrode and a ground electrode. A solid dielectric (insulating) material is needed to cover one of the two electrodes in order to generate a corona atmosphere, as opposed to a "lightening bolt" charge.
In very broad terms, treater stations are classified into two categories - Covered Roll or Bare Roll.
Covered roll stations have the dielectric covering on the ground roll and the high voltage electrode is bare metal. Significant advantages and disadvantages of covered roll stations are as follows:
- Designed to treat non-conductive surfaces. It is a more efficient system than bare roll.
- Must be used for substrates that are difficult to treat (i.e., high slip polypropylene, high slip polyethylene).
- Easy to adjust treat width and to "lane" treat specific areas which leave other areas untreated.
- Can use a variety of dielectric coverings to accommodate customer preference for cost, durability, size and quality.
- Can treat any width material.
- Dielectric failures can be cumbersome to repair as the roll must be removed from the station.
- Stations are typically larger than comparable bare roll stations.
- Cannot treat metallized substrates.
Bare roll stations have the dielectric covering on the high voltage electrode and the ground electrode is bare metal. Significant advantages and disadvantages of the bare roll stations are as follows:
- Can treat metallized substrates.
- More compact than covered roll treater stations.
- Easy to repair dielectric failures on electrodes.
- Less efficient than covered roll.
- Cannot easily adjust treat width or lane treat (Pillar is introducing a new product that eliminates this disadvantage).
- Limited to maximum 120 inch treat width.
- Will not treat certain substrates.
- Requires large volumes of makeup air, as air is needed to cool the electrodes.
In addition to the broad classification of covered roll versus bare roll, treater stations are further classified by the application in which they are used. Stations are designed to be standard for these applications. Typical applications include sheet fed applications, narrow web label presses, flexo/gravure printing presses, coaters/laminators, extrusion coaters, blown film extrusion, cast film extrusion, oriented film extrusion and laboratory treaters.
How Systems are Sized
The first step in sizing corona treater systems is to determine the desired "dyne level" needed for the substrate that is being converted. The "dyne level" is a measure of surface energy and is a commonly used parameter in the converting industry.
The "watt density" that will be needed to raise the dyne to the desired level for a particular film can be determined from empirical tables. Once the watt density is known, it is multiplied by the maximum treat width of the material and the line speed of the operation to determine the maximum kW needed for the application. This kW level determines the size of the power supply. Based on the kW size of the power supply, there are a number of formulas that are used to calculate the sizes of the electrodes, treater rolls, etc.
Selecting a System
If you are new to corona treating, you will want to consult with a reputable manufacturer of corona treaters to make sure that you order the correct equipment for your application. Ultimately, you want a power supply that has proven reliability. It should be easy to service because at some time during the life of the system you will probably need minor maintenance.
Similarly, the station should be designed to be operator-friendly. The electrode gap and treat width should be easily adjusted. The station should be easy to clean and maintain. The electrode should be rigid and able to withstand high temperatures without distortion. The dielectric should be properly chosen and sized. Finally, the station construction should be rugged and designed to withstand an ozone environment.