Corona Treating Basics

 

Corona treating increases the surface energy of plastic films, foils, paper and polymer objects to improve wettability and adhesion of inks, coatings and adhesives.

Treating works best when a substrate is treated at the time of extrusion and in-line prior to converting.

Corona treating increases quality and productivity through improved print quality, faster press speeds and less scrap.

Why do I need it?

Polymer films and objects have chemically inert and non-porous surfaces with low surface tensions causing them to be non-receptive to bonding with substrates, printing inks, coatings, and adhesives.

Pretreated films, that is films that have been surface treated at the time they were produced, exhibit a higher surface energy that is crucial to producing quality printed, coated or laminated products.

Film that is not treated at the time of production will not accept printing, coating or lamination well. The opposite is not always true. Even if film is treated at the time of production, it will not always guarantee that printing, coating or laminating will be easily accomplished at any future time.

Each film type has an inherent surface energy (dyne level) that can be increased through corona treatment at the time of production. This level of treatment diminishes over time. So, film that can be easily printed and coated immediately after production can, within a few days or weeks, lose sufficient surface energy to become unprintable and uncoatable.

Since it’s nearly impossible to guarantee that the film you receive will be converted within the required time limit, retreating in-line is often a necessity. It’s important to note that treating in-line cannot replace primary treatment at the time of production. In fact many films, especially polyolefins (Polyethylene and Polypropylene) are almost untreatable when they set after production.

To ensure consistent quality, use films that have been treated at the time of production and retreat in-line. In order to make a product that is of acceptable quality to the converter and to the end customer, the substrate must be corona treated twice:

1. At the time of production.
2. Prior to converting.

How does surface treating work?

A corona treating system is designed to increase the surface energy of plastic films, foils and paper in order to allow improved wettability and adhesion of inks, coatings and adhesives. As a result, the materials treated will demonstrate improved printing and coating quality, and stronger lamination strength.

A corona treating system consists of two major components: the power supply and the treater station.

The power supply accepts standard 50/60 Hz utility electrical power and converts it into single phase, higher frequency (nominally 10 to 30 kHz) power that is supplied to the treater station.

The treater station applies this power to the surface of the material, through an air gap, via a pair of electrodes at high potential and roll at ground potential which supports the material. Only the side of the material facing the high potential electrode should show an increase in surface tension. (If treatment is applied to the other side of the material it is referred to as backside treatment.)

A corona treating system in its simplest form can be portrayed as a capacitor. Voltage is applied to the top plate which, in the case of a corona treating system, would be the electrode. The dielectric portion of the capacitor would be made up of some type of roll covering, air, and substrate in the corona treater.

The final component, or bottom plate, takes the form of an electrically grounded roll. In the corona treating system, the voltage buildup ionizes the air in the air gap, creating a corona, which will increase the surface tension of the substrate passing over the electrically grounded roll.

TREATMENT TERMS

• Specific energy : This measurement identifies the corona dose to which a film is subjected. Included in it are the working speed, width and applied power.
• Power density : This specifies the power per cm of discharge line. This information is important in order to assess the saturation effect of the discharge and, in my opinion, as an assessment criterion concerning the degree of thermal shock applied to the foil by the corona.
• Corona Dose : The is the specific quantity of electrical energy applied to the web material. Using this parameter, it is possible to calculate the increase in the surface energy or to dimension a corona treatment system for a particular application.
• Power Density : This is the specific power applied to a 10mm length of an individual corona section. Using this parameter, the effectiveness of a discharge can be assessed or a treatment station can be correctly dimensioned for a specific treatment application
• Treatability :
  1. Not all film materials can be equally well treated. PP is very hard to treat and requires more treatment power. PE is less difficult to treat compared to PP. PVC and PET does not require treatment for printing as their initial surface tension is high.
  2. Unit of Surface Tension : Dynes/Cm.
• Generator Power :
  1. The required generator power is determined by the following
  2. Treat width x line speed x # of sides x the watt density required to increase the surface energy of the substrate to the desired level. The watt density is determined by the type of substrate and the amount of additives in the substrate. Having calculated the correct kilowatt rating, it is important to determine the correct discharge area (size and # of electrodes) in order to match the impedance of the system properly and not over stress the electrodes. Having a good match will optimize the efficiency of the discharge area or electrode
  3. It must be kept in mind that if you have a segmented electrode, it is most likely that not all segments will be engaged at the same time. This is done by moving certain sections of the electrode out of the treat position. This increases the amount of power available to the remaining electrodes which are still engaged for treatment.
• Segmented Electrode : There are a number of factors determining the kind and style of electrode that would best suit an application. In the Blown/Cast Film industry, a typical application requires the material be strip or lane treated. In this case a segmented electrode would be best on a covered roll treater, which means the base roll would have an insulated covering such as silicone sleeve, vulcanized or ceramic dielectric. Segmented electrodes constructed of stainless steel or aluminum are widely used

How To Check Corona Treatment

1. For Plastic Film

Qualitative Method

This is the simple and widely used method to check the treatment on the film. This method only gives indication of good or weak treatment but it doesn’t give any figure of dynes level achieved. In this method, small piece of treated film is taken and thin layer of printing ink is applied across the film. Allow time to dry the ink. Stick cellotape over the ink area firmly and then peel it off in one stroke. Observe for any ink sticking on the cellotape and treatment can be classified as follows

•  Good Treatment : If peeled out ink is not more than 5 % of ink area over which cellotape is applied.
 
•  Weak Treatment : If peeled out ink is between 5% and 30% of ink over which cellotape is applied.
 
•  Poor Treatment : If peeled out ink is more than 30% of ink over which cellotape is applied.

Quantative Method

This method gives figure of treatment level in terms of dynes/cms. In this method solution is prepared by mixing Ethyl Cellosolve and Formamide by volume percentage as shown in table 1. The victoria blue dye (in powder form) is added to make the solution visible on the surface of the plastic film. To start with take a solution of 45 Dynes/cm. Take a cotton bud and dip it in the solution . Apply it over the plastic film whose treatment level is to be checked. If solution film breaks down into droplets within two seconds , then go for lower dynes solution. The correct dyne will be the solution for which solution film does breakdown into droplets in two seconds.

Formamide volume (%)	Ethyl cellosolve (%)	Wetting tension : (Dynes/cm)
54.0	46.0	38
59.0	41.0	39
63.5	36.5	40
67.5	32.5	41
71.5	28.5	42
74.7	25.3	43
78.0	22.0	44
80.3	19.7	45
83.0	17.0	46
87.0	13.0	48
90.7	9.3	50
93.7	6.3	52
96.5	3.5	54
99.0	1.0	56

2. Paper

There is no method to check treatment on paper. Corona Treatment of paper burns fine tissues on paper surface which may hinder to LD Extrusion/Coating on paper.

3. Conductive Film. Al. Foil

Basically corona treatment cleans and activates the surface of conductive film by nothing but Atmospheric Plasma Cleaning.

Corona Treatment v/s. Time

Surface Tension (i.e Level of Treatment ) gradually reduces with time. This is due to intermigration of polymers at Surface of the film. As time passes treated polymer molecules goes inside the film and untreated polymer molecules comes to the surface. The rate of decay of treatment level depends upon initial treatment level. i.e 48 dynes/cm comes to 44 dynes/cm within 2 days while it takes 15 days from 44 dynes/cm to 40 dynes/cm.

Static Electricity & Problems

Static Electricity

Denoting or pertaining to electricity at rest. How simple and inadequate this definition is of a phenomenon that creates problems which cost industry millions of dollars per year

Static electricity is generated by unbalancing the molecular construction of relatively non-conductive insulators such as plastics and paper. All matter is composed of atoms. A balanced atom contains positive charges that are present in the nucleus of the atom. An equal amount of negative charges orbits this nucleus in the form of electrons. Both charges are equal and, therefore, the overall charge of a balanced atom is zero. However, should this configuration be disturbed and several electrons removed from this atom, we end up with a greater positive charge in the nucleus and a deficiency of electrons, which gives you an overall charge in the positive direction. Conversely, should we add a few extra electrons, we have an overall charge of negative, due to the fact that we now have an excess of electrons and the net charge is now in the negative direction.

Static Electricity Problems?

Static Electricity can be defined as an electrical charge on a material due to a surplus or deficit of electrons. The problems caused by static electricity break down into five main areas

Production problems and slow downs

A static charge generates an electrical field which acts like a magnet-repelling similar charges and attracting opposite or neutral charges. This accounts for the attraction between charged materials and machine frames or rollers causing machine jams. In many cases the machine operator will be forced to run at slow speeds to avoid the problems caused by static

Dust Attraction

Many products, such as plastic molding materials and film, develop high static charges on their surfaces. This high charged surface will attract airborne dust, sometimes from over 3 feet away, and hold it tightly to the surface. Subsequent operations (such as molding, printing or laminating) and their end products can be seriously affected by such difficult-to-remove contamination.

Shocks To Personnel

When handling highly-charged materials, people receive unpleasant shocks either directly from material, or indirectly. An example of indirect shock is when a person slides out of an automobile seat and then touches the door handle.

FIRES AND EXPLOSIONS

Static is usually found on non-conductive materials where high resistivity prevents the movement of the charge. There are at least two situations where the static charge can move quickly and be dangerous in a combustible atmosphere. The first is where a grounded object intensifies the static field until it overcomes the dielectric strength of the air and allows current to flow in the form of a spark. The second case is where the charge is on a floating conductor such as an isolated metal plate. Here the charge is very mobile and will flash to a proximity ground at the first opportunity.

Damage And Interference In Electronic Components

Strong static fields and static discharges can cause interference with electronic equipment. However, even greater problems are caused by static electricity in the electronics manufacturing industry. E.O.S . (Electrical Overstress) and E.S.D. (Electro-static Discharge) have become areas of vital concern in controlling the quality of static sensitive MOS and FET devices where it is believed that 50 volts or less can cause component failure

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