We have all heard the term, and most of us know how important Corona treatment is for printing on the surface of synthetic materials, but how many of us actually understand the science behind the process? Nick Coombes met up with Jan Eisby, of Danish manufacturer Vetaphone, to learn more.
“The difficulty of writing or printing on plastic film is well known. You have a plastic bag and you would like to write on it with a pen, but unfortunately the ink doesn’t stick to the surface because the untreated plastic has a structure and this makes for poor adhesion. Corona treatment is the solution to this well known problem,” said Eisby, whose Father, Verner, was the inventor of the process, and who today runs the international family business with his brother Frank.
Avoiding adhesion problems
Corona treatment is a high frequency discharge that increases the adhesion of a plastic surface. Whether a liquid wets a material well or poorly primarily depends on the chemical nature of both the liquid and the substrate. Wetting is defined as the ratio between the surface energies of the liquid and substrate.
“In general, a material will be wetted if its surface energy, expressed in dyne/cm, is greater than the surface energy of the liquid. If it is not, there will be an adhesion problem,” explained Eisby.
The pre-treatment offered by the Corona process is necessary to obtain sufficient wetting and adhesion on plastic film or metallic foil before printing, laminating or coating takes place. A Corona discharge unit is used to optimise the wetting and adhesion, and the technique has proved to be both highly efficient and cost-effective, especially as it can take place inline.
What is corona treatment?
Plastic is a man-made synthetic material that contains long homogeneous molecular chains that form a strong and uniform product. The chains of molecules are normally joined end-to-end forming even longer chains, which leave only a few open chain ends, and therefore only a small number of surface bonding points.
“It is these few bonding points that cause the low adhesion and wettability, which becomes a problem in the converting process,” said Eisby, whose father, Verner, was originally asked to solve the problem back in the 1950s. “He came up with a theory that a high frequency charge would provide a more efficient end controllable method of increasing the adhesion and wettability of a plastic surface. During Corona discharge treatment, electrons are accelerated into the surface of the plastic causing the long chains to rupture, and this creates a multiplicity of open ends and free valences” Jan Eisby explained.
The ozone in the electrical discharge is accelerated onto the surface, which in turn forms new carbonyl groups with a higher surface energy. The result is an improvement of the chemical connection (dyne/cm) between the molecules in the plastic and the applied media/liquid.
“Importantly, this surface treatment does not reduce or change the strength of the substrate, and nor does it change the appearance of the material,” he said. Corona treatment changes only the top molecule chains, which are 0.00001 micron thick.
Determination of treatment level
The effectiveness of Corona treatment depends on the specific material being used.
“Materials have different characteristics and different amounts of slip and additives, which will determine the effect of the Corona treatment. There are no limits with regard to the materials that can be Corona treated, but the required intensity of the treatment, measured in watt/min/m2, may vary significantly” he said.
The treatment level can be calculated by using the following formula:
Power (watt) = T x S x W x M
P = Total Power (Watt) required
T = Number of sides to Treat (single/double sided)
S = Line Speed (in metres per minute)
W = Film Width (in metres)
M = Material factor (required Watt per m² per minute)
The exact value is best determined by testing a sample of the actual film that is being used for a specific application.
Durability
Over time, the obtained dyne level can decrease and it may be necessary to Corona treat the material again just before use. When a material contains slip, the dyne level falls faster, and thicker and older film stocks are more difficult to treat as slip agents may have migrated to the surface.
“Storage conditions and temperature can also affect the loss of Corona treatment. Generally speaking, the higher the amount of slip, the quicker it is likely to decay, it has also been proven that some films with very high slip additives, say over 1200 ppm, can be totally resistant to printing just 24 hours after treatment, and it may be necessary to process this film immediately after treatment, or boost the treatment inline with the printer,” he added.
Material that has not been treated under extrusion can be difficult to treat afterwards.
“We recommend treating film just after extrusion, and then use a refreshment treatment, if necessary, just before the liquid or media is supplied to the surface in the converting process,” he concluded.
If the science of Corona treatment is lost on many, the realities of its effects and the benefits it brings are all too obvious. Vetaphone claims a market share in narrow web of around 80%, and is looking to extend its business into the wider markets of flexible packaging.
