The invention relates to the corona treatment technique.

It is well known that the corona treatment basically consists in applying an high intensity electric discharge to the surface of a certain substrate, in order to increase the active tension and consequently to improve the fixing power of printing inks, stickers or coatings..

The substrate can be either a film or an extruded item, sheets or plates, wires, tubes, metal plates etc.

For example, the corona treatment is widely used to increase the wettability of plastic, metal or paper films, and to help the adhesion of inks, stickers, lacquers... ; it is often used for polyethylene, polypropylene, PVC or polyester films, but it is suitable for almost any plastic material and also for some non-plastic materials such as paper and aluminium.

A corona treater, as known from prior art, basically comprises a high frequency generator, a step-up transformer connected with one or more metal electrodes, and a roll (or a sheet) electrically connected to the ground and acting as a counter electrode. Said roll can be coated with insulating material (silicon, ceramic or others).

The substrate is inserted between the electrodes and the roll, with known dragging means. This way, the surface of the substrate is activated by the electric discharge generated between the electrodes in tension and the roll connected to the ground.

With reference to the known corona treaters, the enclosed schemes from a) to e) in Fig. 1 show the currently used techniques.

1. a) For the treatment of a non conductive substrate S, it is known a treater basically comprises a metallic electrode 1 and a metallic roll 2 acting as a counter electrode. Roll 2 is coated with a layer of a non-conductive material 3 and connected to the ground. When operating, a discharge E between electrode 1 and roll 2 is generated. As shown in the figure, substrate S is placed between the electrode and the roll, so that it is hit by the aforesaid discharge E.
2. b) As in the previous case, but using a naked roll (i.e. not coated with dielectric material). This solution is chosen when the non-conductive substrate S, thanks to its thickness, can act as an insulator.
3. c) The treater comprises many ceramic tube electrodes 4 (or another kind of insulating material) with conductive material inside them. In this case, substrate S can be conductive or non-conductive. As in the previous cases, the counter electrode comprises a metallic roll 2, placed on the opposite side of the substrate and connected to the ground.
4. d) As in the previous case, with a counter electrode comprising a metallic sheet 5 connected to the ground replacing roll 2.
5. e) The treater uses an electrode 6 comprising a ceramic tube (or another kind of insulator) filled with conductive material, and next to electrode 6 is placed a counter electrode 7 comprising a similar ceramic tube filled with conductive material and connected to the ground. The corona effect develops between the sides of electrode 6 and counter electrode 7 and is directed towards substrate S by a forced air flow F. This method is suitable only for small surfaces or section bars in general.

Following those schemes, a large number of devices for the treatment of extruded, sheets, plates, tubes and so on can be produced. Machines can be devised to work on their own or connected to another unit, for example an extruder, a flexo printer, a rotogravure, a coupling machines, a sheets production line, etc.

To trigger the corona effect in devices from a) to d) a very high voltage is required. Moreover, such voltage must be increased if the dielectric substrate is thicker.

This disadvantage is the direct consequence of the treatment method used. According to such method, the electrode and the counter electrode (roll or sheet) are placed in the opposite parts of the substrate (on the upper and on the lower side), so that the discharge has to pass through the whole substrate. Therefore, the distance between the live electrodes and the counter electrode connected to the ground must not be lower than the sum of the thickness of the substrate, the air gap that must exist between the electrodes and the surface to treat, and the thickness of the non-conductive layer covering the roll (in case there is one).

As an example, we can take into consideration a typical device, in which the substrate comprises a sheet or an extruded 10 mm thick; the air gap should be 1,5 mm, and the dielectric roll covering is usually 3 mm thick. The total distance between the electrodes and the conductive part of the roll is more or less 15 mm, and considering an average dielectric rigidity of a substrate of 2.000 V/mm, the resulting trigger tension is equal to 30.000 V. So high tensions can be a problem for safety and can damage and/or burn some materials. Moreover, such tensions require a proper design of the insulation and protection means of the device, which cause overall dimensions of the machine.

In addition, the above mentioned prior art is not suitable for certain applications, in particular for the treatment of curved surfaces such as the tubes sides. These applications are not that uncommon, for example for ink-jet printing of the name of the producer, nominal data... on polyethylene tubes.

More in detail, the surface to treat is basically a narrow band along the external surface of the tube, and so the prior art turns out to be not really suitable. As a matter of fact, it is difficult to produce the desired effect, i.e. the corona discharge, on said band, due to the lack of a counter electrode in the tube in extrusion.

Device e) in Fig. 1, in which the electrodes are one beside the other, partially solves this problem since the discharge does not have to pass through the substrate layer. Nevertheless, the discharge tends to develop between the sides of the electrodes, being said electrodes totally filled with conductive material, and it is not directed towards the underneath substrate. (see also US-A-3376208)

For this reason it is necessary to introduce the forced air flow F; anyway, the discharge still remains localized in the space between the electrodes where it does not have any useful effect. Moreover, as previously stated, this known art can be used only for small surfaces or section bars in general and does not solve the above mentioned problems related to the tubes treatment. The corona treatment device disclosed in EP-A-1047165 is of the type wherein the active electrode is arranged opposite the counter-electrode, said active electrode including high voltage conductor in the portion facing said counter electrode.

The prior art turns out to be inadequate or unsatisfactory for the treatment of three-dimensional objects plane sides.

The object of the invention is to overcome all these problems.

In greater detail, the scope of the invention is to reduce the tension necessary for the trigger of the discharge and to direct it towards the substrate, without the use of a forced air flow even if the electrode and the counter electrode are one beside the other, as in the prior art.

A further aim of the invention is to treat more easily the sides of tubes or other curved surfaces, as well as complex three-dimensional objects surfaces.

Such aims are achieved using the device of claim 1. Preferred embodiments of the invention are characterised in the remaining claims.

With "hollow inner part" we mean that the inner volume created by the tubular body is not filled with conductive material, since the only conductive part is said internal coating layer. Typically, the interior is filled with air.

Advantageously, the invention enable to avoid the use of rolls or plates connected to the ground, due to the fact that the electrodes are placed on the same side of the substrate (i.e. next to it, or over it), and to the fact that the electric discharge is produced directly between the active and passive electrode/s. The discharge does not have to pass through neither the whole layer of the substrate, nor the insulating layer of the roll. It follows that the trigging tension required is much lower than with the method in which electrode and counter electrode are on opposite parts of the substrate, with the same effect.

In addition, the described structure of the electrode with hollow interior and lined with conductive material only in the inner part near the substrate, produces an arch discharge projecting towards the substrate instead of concentrating in the space between the sides of the electrodes, as in the known systems shown in Fig. 1 e).

Another important advantage is the structural simplicity of the treater: the passive roll counter electrode is eliminated; the voltages used are lower and so imply fewer safety and insulating problems; the machine is more compact.

The features and advantages of the invention will become more apparent from the following detailed description of preferred embodiments, which are disclosed by way of non-limitative examples, and with the help of the accompanying drawings, in which:

• Fig. 2 shows a scheme of the treatment, according to a preferred embodiment of the invention, applied on the side of a tube;
• Fig. 3 shows a scheme of the treatment, according to the invention, applied on a plane sheet or an extruded;
• Fig. 4 shows a tube electrode according to the invention;
• Figs. 5 and 6 show further embodiments of the invention.

With reference to figures from 2 to 4, the invention relates to a method for the corona effect treatment on a surface S, or part of it, of a substrate, such as an extruded, a tube or a sheet of material. The treatment is the effect of the electric discharge E generated between at least an active electrode 10 and a corresponding passive electrode (or counter electrode) 11, which are placed one beside the other and on the same part, with respect to the substrate, i.e. over it or near it.

The active electrode 10, as shown in the scheme, has a connection AT to a generator and possibly to a tension raiser transformer; the passive electrode 11 has an ground connection G.

Active electrode 10 comprises a ceramic bar or tube with hallow interior and a side 12 towards substrate S, lined with a conductive layer 14 connected to high tension AT. Said conductive layer is preferably made of metal or even more preferably of silver.

According to a preferred embodiment, shown in Figs. 2-4, the counter electrode or passive electrode 11 has the same structure as electrode 10, with a face 13 likewise towards substrate S and lined with a conductive layer 14.

Internal volume 15 of the electrode 10 is filled with air; internal volume 16 of the counter electrode 11 is likewise filled with air.

Electric discharge E is generated between the metallic side 12 of the active electrode 10 and the metallic side 13, placed next to it, of the counter electrode 11, hitting surface S of the substrate. As an effect of the conductive layer 14, the electric discharge E tends to produce an arch projecting towards the underlying surface S of the substrate, instead of generating useless sparks between the sides of the electrodes as it could happen, for example, if there were any conductive material in interiors 15 and 16 of the electrodes.

So, it is clear that the method provided by the invention enables a strict control of the discharges and, thanks to the proper placing of the electrodes with respect to the plane on which the substrate slides, allows to direct electricity towards the substrate to treat instead of dispersing it in spaces useless for the corona treatment, such as the sides of the electrodes.

Fig. 2 shows the position of electrodes 10 and 11 to treat the side of a tube: electrodes 10 and 11 are placed next to the tube and immediately close to its external surface S, preferably with sides 12 and 13 perpendicular to the radial direction defined by the tube.

Furthermore, in the embodiment shown in Fig. 3, there are many electrodes one beside the other and alternated on the same horizontal disposition, in order to generate many discharges E1, E2...

Preferably, electrodes are in odd number, the external ones are connected to the ground. Thanks to this, the protective casing of the machine can be placed closer to the external electrodes, since these ones are not in tension (i.e. live). In Fig. 3 for example, discloses an active electrode 10 and two external electrodes connected to the ground 11A, 11 B.

Further lay-outs are possible, due to the fact that, basically, the invention is based on the fact that each active electrode discharges towards at least one passive electrode. E.g., with reference to a three-electrodes disposition, a reversed disposition can be used, instead of that shown in Fig. 3, with the passive electrode in the middle and two active electrodes on the sides.

The electrodes disposition in Fig. 3, or more in general with electrodes one next to the other over the substrate, in even or odd number, is suitable to treat surfaces of plane sheets, extruded or plates or the like, which are carried continuously and by known means, sliding underneath the electrodes.

The examples make it clear that when the electrodes are placed on the same part of the substrate, i.e. over it or one side of it, it is not necessary to produce the discharge throughout the whole thickness of the substrate, but on the contrary it is possible to direct the corona discharge exactly where it is required, i.e. on the surface of a tube, an extruded item or another substrate.

The treatment can be equally carried out with a steady substrate and a mobile electrode moved to treat the whole required surface. More in detail, another version (not showed) implies the connection of the active and passive electrodes to a manual or automatic moving system (mobile electrodes), to work on a steady substrate or even on plane surfaces of three-dimensional objects. The treatment of objects' surfaces, as known, is possible if said surfaces are physically reachable by the electrodes, considering the electrodes' dimensions and shapes and the limited corona discharge action field (usually 2-3 mm).

Fig. 4 shows the preferred shape of the electrode, referring indifferently to an active or a passive electrode, having face 12 or 13, towards the substrate, coated inside with an electricity conductive layer (14). In similar devices, as known to the skilled man, electrodes can have a transversal section different from the basically square section of Figs. 2-4. As an example, the electrodes can have a trapezoidal section, preferably with the smaller side facing the substrate, or a polygonal section, having one or more internal sides lined with conductive material, or even a circular section, part of which is lined with conductive material.

It is also clear that the insulating material of the electrodes can be different from the mentioned ceramic; and the conductive material of the covering layer 14 can be different from the mentioned silver. Said materials being described as preferred.

It is preferable to use the above described structure both for the active electrode and for the counter electrode, to obtain the greates effect of arch discharge. Nonetheless, a simplified and cheaper version of the invention implies the use of non-ceramic conductive counter electrodes: as an example, Fig. 5 shows a scheme with counter electrode 20 composed by a metallic T beam; Fig. 6 shows an even cheaper version in which the counter electrode is simply a metal wire 21 connected to the ground.

Thanks to such simplified version it is possible to reduce the manufacturing cost of the device and still achieve a positive result, since the discharge, which is generated between the metallic coated face of the active electrode and the metallic counter electrode, is directed mainly towards the substrate.

The invention is suitable for treating both plastic and non-plastic materials, such as paper and aluminium, and can be used with known types of devices for corona treatment.