How to measure
What dose is to be applied in the corona treatment?
So that the surface of a material provides wettability, the surface tension must be
the same than that of the liquid which is intended to adhere. Otherwise, adhesion problems will appear (for example with printing inks and adhesives), making it necessary to raise,
by corona treatment, the surface tension of the substrate to ensure the optimum required level of adhesion.
To determine the level of corona treatment which is appropriate for each application is necessary to take into account different factors as: the type of material, the quantity and types of additives (sliding, etc..) that the material contains, the line’s speed, the temperature of the material, its width and thickness, wether the application of the treatment is performed in an extruder or in a printer or a laminator (untreated material or refresh treatment).
The power density of the treatment level required is measured in watts / m². Min.,
And is calculated using the following formula:
Dw = W / Ve x N x L ( ) expressed in W / m². minute.
Dw = power density (watts / m². Min.)
W = Power in (W)
Ve = Line’s speed (m. / Min)
N = number of faces to treat
L = length of the electrode (Mts)
Whenever possible, it is recommended to do a laboratory test to determine the dynas
and power density required for each application.
The surface tension of the material is measured in mN / m (millinewtons per meter,
equivalent in numerical value to Dynas / cm), according to DIN 53 364 rules.
The most common method for checking the surface tension of a material is the use of test inks formulated for different test values. These inks must be applied by performing a stroke on the material’s surface where we want to determine the treatment.
We will know the surface tension of the substrate, when the ink is spread on a continuous and homogeneous stroke (without retraction) during the first 2 seconds [4A]. By contrast,
if the ink is retracted and forms drops [4B], it would indicate that the substrate tension is lower than the one of the test ink applied.
When we have obtained a consistent stroke, we can apply a new test ink with a Dynas’
level higher than the previous, so we get to know the precise surface tension level that the analyzed substrate has.
The surfaces of the treated materials, lose the treatment over time if have not been printed,
laminated or bonded after being treated. In contrast, once processed, the improvement
in the linkage of the substrates is permanent.
The presence of additive fillers in the materials (besides adding difficulty to their treatment), reduces its ability to maintain the effects of corona treatment, since the additives tend to migrate to the surface of the material, weakening the treatment’s effect in cases where after treating it is not done then the printing, lamination or bonding of the film.
Apart from the time elapsed since the application of the treatment to the material, in the loss of its effect also influence the conditions under which storage is carried out and the temperatures to which it is exposed.
The unwanted treatment of the back side of a film, to which corona treatment is being applied, occurs when air bubbles are trapped between the back of the film and the treater roll. That air, when ionized, results in the generating of the corona treatment and, therefore, the consequent unwanted treatment of the backside of the film.
A well-designed treater station, designed it so that the roller treater embraces the maximum possible area of the film, helps to avoid this problem. It also helps that the coating silicone treater roll is vulcanized and rectified; it’s always clean and not damaged.
In some applications it will be convenient to incorporate a “pressure roller”, in order to assure optimum contact with the surface material of the roller.