Transferring of powdery particles The present invention relates to a method for transferring powdery particles onto a web to form a coating layer or a film layer in a dry surface treatment process. The present invention also relates to a device for forming a coating layer or a film layer in a dry surface treatment process comprising means for electrically charging powdery particles, and means for attaching the charged particles onto a web.

The dry surface treatment process comprises an application step in which the powdery particles are applied on the web. The application step is followed by a finishing step, for example thermomechanical fixing. The application of the powdery particles utilises an electric field to transfer the powdery particles to a surface to be treated and to enable an electrostatic adhesion prior to the finishing. Both the final adhesion and the surface smoothening of the applied layer are executed simultaneously through a thermomechanical treatment or another suitable treatment.

In the dry surface treatment process, the powdery particles are normally applied to the web by blowing them against the web through at least one nozzle, which is integrated with charging means. A problem related to this kind of application may be a difficulty to form an even layer of powdery particles on the web.

The method of the invention is characterized in that the powdery particles are attached to an intermediate substrate from which they are released to the web. The device of the invention is characterized in that the device comprises an intermediate substrate onto which surface the charged powdery particles are arranged to be attached before transferring them to the web.

An even charge over the whole surface of the intermediate substrate may be achieved by corona charging, which means an electric discharge from an electric field under normal pressure. As a consequence of the electric discharge, the electric conductivity of a medium, such as air, becomes remarkably higher. Charged ions

deposit themselves on the surface of the intermediate substrate. The intermediate substrate is substantially dielectric, i. e. the resistivity of the intermediate substrate is on a certain level, and thus it is capable of storing electric charges for a long enough time. When charges are transferred to an ideal dielectric surface the potential Vs accumulated on the surface decreases the ionic current according to an equation Is = AS (V-VS) (V-VS-Vo), wherein Vo is a level of voltage when the electric discharge takes place, and the electric conductivity of the medium suddenly increases. For the constant As has been given an estimate according which it depends on the dielectric constant of the receiving surface (the surface of the intermediate substrate) (es), the ionic mobility in air (F), the diameter of the corona wire (a), and the distance between the corona charging electrode and the receiving surface (R) according to the equation As--28gEsu<BR> S R2 ln (R/a) To enhance the effect caused by the electric discharge the intermediate substrate can be coupled with an opposite potential, which is approximately on the same level of magnitude as the corona potential, and thus the ionic mobility further increases, and the potential of the intermediate substrate becomes higher. The electric field created between the corona electrode and the intermediate substrate exerts a force to the powdery particles, which are brought above the intermediate substrate. The force F = QE, wherein Q is the charge and E is the electric field. The normal force of the intermediate substrate is F = qEz (y, z), wherein q is a charge of the powdery particle. The created electric field at the intermediate substrate is described by the equation Q = density of the surface charge of the intermediate substrate,

relative permittivity between the surfaces of the intermediate substrate, Relative permittivity between the surface of the intermediate substrate and the electrode, s = thickness of the intermediate substrate, and d = distance between the surface of the intermediate substrate and the electrode.

To transfer the powdery particles to the web, the web and the powdery particles must be in close proximity, and a force, which exceeds the adhesion or cohesion between the powdery particles and the intermediate substrate must be exerted in the layer of the powdery particles. The transfer is executed by an electric field. The tension, which is created by the transfer can be described by the equation q (z') = electric charge density of the powdery particles at the location z', E2 (z') = electric field in the layer of the powdery particles at the location z', Pm = mechanical stress, E3 = electric field between electrodes, £3 = dielectric constant of the intermediate substrate, and E3£32= electric stress caused by the electrodes exerted on the layer of the powdery particles.

The transfer happens when the tension exceeds the adhesion or cohesion forces. The powdery particles to be transferred can be particles from which a coating layer can be formed on a paper web, or they can be particles from which a film layer can be formed on a suitable web, such as a paper web or a plastic web.

The intermediate substrate can be an endless belt, which rotates around guiding rolls close to the web to be treated. The surface of the belt has a certain potential, and the powdery particles having an

opposite charge are transferred to the belt. The belt is substantially dielectric because the belt shall maintain its charge until the powdery particles are released from it in a controlled manner. The belt conveys the particles to a location where they are released from the belt and attached to the moving web. No contact of the intermediate belt and the treated web is necessary. The speed of the belt is adjusted so that the accumulated particle layer can be transferred to the web evenly.

The invention is explained by referring to a figure, which shows a schematic view about the method and the device of the invention.

An endless belt 1 rotates around guiding rolls 2. The surface of the belt 1 is charged by a corona charging electrode 3. Powdery particles having a different charge are attached in the powder application area 5 to the charged belt 1. When the belt 1 having a layer of powdery particles on it rotates, the layer of the powdery particles is transferred to the side where the belt 1 is in close proximity of a web W. The web W, for example a paper or plastic web, travels in a production line.

Underneath the web W there is an electrode 6 (additional corona charging electrode), which is an opposite potential compared to that of the particle layer on the belt 1, and having a potential difference compared to the belt. As a consequence, the powdery particles are separated from the belt 1, and attached to the surface of the moving web W.

The invention is not restricted to the above-mentioned embodiment of the invention but may vary in the scope of the claims.