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Corona Treatment

‘The difficulty of writing or printing on plastic film is well known. The ink doesn’t stick to the surface because the untreated plastic’s structure makes for poor adhesion. Corona treatment is the solution to this well-known problem.’

 

So explains Jan Eisby, whose father, Verner Eisby, was the inventor of the process, and who today runs this international family business with his brother Frank.

Corona treatment is a high frequency discharge that improves adhesion to 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’s not, there will be an adhesion problem,’ Jan explains. The graphic alongside shows the basic dyne level of a material, and the required dyne level in the different applications.

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,’ Jan continues.

It was his father, Verner, who originally solved 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,’ Jan explains. ‘During corona discharge treatment, electrons are accelerated into the surface of the plastic causing the long chains to rupture, creating a multiplicity of open ends and free valences.’

The ozone in the electrical discharge is accelerated on to the surface, which in turn forms new carbonyl groups with 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 doesn’t reduce or change the strength of the substrate, nor does it change the appearance of the material. Corona treatment changes only the top molecule chains, which are 0.00001µm 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 determine the effect of the corona treatment. According to Jan, there are no limits with regard to the materials that can be treated, but the required intensity of the treatment, measured in Watt/min/m2, may vary significantly.

The treatment level can be calculated 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 (m/min)

W = Film width (metres)

M = Material factor (required Watt per m² per minute)

The exact value is best determined by testing a sample of the actual film 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,’ Jan comments.

‘Generally speaking, the higher the amount of slip, the quicker it is likely to decay. It has been proven that some films with very high slip additives, say over 1 200 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 treatment inline with the printer,’ he add.

Material not 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 applied to the surface in the converting process,’ Jan concludes.

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.

Family Eisby certainly has the background and pedigree to achieve that aim.

The company is represented in South Africa by Ipex Machinery.

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