The present invention relates to a self-adhesive film as defined in the preamble of claim 1 and to a procedure for its manufacture.

It is known that charges are formed on the surface of die¬ lectric films when the films are rubbed or reeled or when the films are manufactured by previously known continuous manufacturing methods. The electric charges thus produced may generate electrostatic forces and the films may e.g. adhere to different surfaces. However, such electrostatic forces of attraction are very weak and the charges can easi¬ ly disappear e.g. due to humidity, so the action of these forces may be of a very short duration. When this is the case, the films will not set straight on a surface and they are easily detached e.g. by air current.

Previously known are also electrets produced from solid material in which charges are generated in the film surface structure by subjecting it to a DC corona treatment. Such electret films can be utilized in the first place in various detector applications, such as e.g. the electret microphone.

Producing a powerful charge penetrating deep into a solid film material is very difficult because it is not possible to incease the surface potential of such film material to a sufficiently high level during the corona treatment (char¬ ging) because of the leakage current flowing through the film. A leakage current flows because in solid material the resistance of the material is reduced as a function of vol¬ tage.

A further known method is to manufacture e.g. from polypro- pylene a bubble or cavitated film which contains small, preferably flat gas bubbles placed in an overlapping arran-

gement inside the film. Due. to the whiteness of the film resulting from the bubble structure, such film is an excel¬ lent material for printing. This kind of white film is gene¬ rally used e.g. as packaging material.

In a film containing flat gas bubbles, resistivity is not reduced in the gas bubbles and the leakage current is not increased with voltage. Therefore, it is possible during charging to achieve a high electric field, e.g. in the range 100 - 200 MV/ , which in turn produces partial discharges in the bubbles contained in the film. The object of the present invention is to achieve a completely new type of film made self-adhesive by an electrostatic force, a film whose strong adhesion is based on a large unipolar charge created inside the film as explained above. The features characteristic of the film are presented in greater detail in the attached claims.

The most significant advantage provided by this type of self-adhesive film is that no separate adhesive substances, such as glue, or fixing tools are needed to make it adhere to a surface.

With the film of the invention, a very good adhesion is achieved. Moreover, the film of the invention retains its charge for a very long time, typically several years. In addition, because of the charge inside the film, an electro¬ static force is active on both sides of the film, which means that films will stick together very firmly. This makes it possible to place several films, typically 15 - 30 pcs and even as many as 50 pcs, on top of each other like the leaves of a so-called flip chart. It is also possible to attach other materials, such as paper, onto the film.

The film of the invention s.ets straight and neat onto a surface. The film can resist e.g. air currents, humidity and heat without being detached from the surface. In addition, it adheres well even on a rough surface.

To produce a self-adhesive bubble film as provided by the invention, a bubble film made of a plastic material, e.g. polypropylene, of a thickness of a few tens of μm, e.g. 50 μm, is charged by means of a very intensive electric field (e.g. in the range of 100 - 200 MV/m). During charging, partial discharges occur in the air bubbles in the film, and the charges are injected into the plastic material of the film. Due to the large resistivity of the bubble structure and the film material, the charges are well retained within the film. As a result, a film is obtained which has a high net charge and whose strong adhesion is based on a large unipolar charge inside the film.

Because of the powerful net charge, such a film is especial- ly applicable as a self-adhesive film which can be used e.g. as a medium of visual information e.g. by printing the in¬ formation directly on the film. As the charge inside the film generates an adhesion force on both sides of the film, information printed on paper or other light material can be attached to the film. It is also possible to pile films on top of each other e.g. on a wall to form a so-called flip chart.

One can also cut holes or patterns of arbitrary shape in the film, so it can be used e.g. as an anti-glare barrier in automobiles or for various decorative purposes.

Furthermore, it is possible to create charges of opposite sign inside the film so that certain areas of the film con-

tain a positive charge and certain other areas a negative charge, e.g. so that positively and negatively charged strips or areas of any shape placed side by side are formed in the film. When the positive and negative charges are equal, the net charge of the film is zero and the film can also be used between metal sheets without generating in the metal sheets voltage differences according to the capacitan¬ ce law

where ΔU is the change in voltage, Q is the charge of the film and ΔC is the change in capacitance occurring when the sheets are being detached or piled up. The change in capaci¬ tance may be of the order of 1 μF/m ² and, without the use of strips, the voltage differences thus produced may be several kV and therefore even dangerous to life. One of the applica- tions made possible by such a film is its use a self-ad¬ hesive protective film on steel and sheet metal plates.

To improve its adhesion and printability, the bubble film may also be advantageously subjected to an AC corona treat- ent. Possible uses of the film may be further increased by providing one side of the film with a coat of slightly sticker-like material, which allows the adhesion of the film to be adjusted both via AC corona treatment and by adjusting the intensity of the charging process described above.

Further, the amount of net charge formed inside the film can be adjusted by doping the film with charge binding addi¬ tives, such as ferrochloride, which generally contain impu¬ rities which act as charge centres.

It is obvious to a person skilled in the art that different

embodiments of the invention are not restricted to the exam¬ ple described above but that they may instead be varied within the scope of the claims presented below.