A METHOD FOR ACTIVATING A METAL SURFACE

Abstract: The method for activating a metal surface, for example for improving the flow of lacquer over and/or the adhesion of lacquer to a tin surface (3), comprising the steps of: 1) providing a lightly greased or oiled metal surface (3), for example the surface of a tin plate of strip (3); 2) subjecting that metal surface to a corona discharge through application of an alternating voltage between that metal surface and a conductor (22) situated thereabove.

Description: The British patent specification GB-A-1 108 346 relates to a method of improving the adhesion of substances to metal surfaces. This prior art specification discloses a method for activating a metal surface, for example for improving the flow of lacquer over and/or adhesion of lacquer to a tin surface, comprising the steps of: 1) providing a metal surface, for example the surface of a tin plate or strip; and 2) subjecting that metal surface to a discharge by applying an alternating voltage between that surface and a conductor situated thereabove. As stated in the mentioned prior art specification, preferably the metal surfaces have been cleaned or degreased by known chemical or physical processes before being subjected to the corona treatment. It is, relative to this prior art, a purpose of the invention to provide a method, in which the degreasing step can be omitted. Furthermore it is a purpose of the invention to provide a method that gives the treated metal surface superior qualities in connection with the flow over and the adhesion to that surface of the lacquer or ink. To this end the invention provides a method for activating a metal surface, for example improving the flow of lacquer over and/or adhesion of lacquer to a tin surface, comprising the steps of: 1) providing a greased or oiled metal surface, for example the surface of a tin plate or strip, the grease or oil concentration being in the order of 0,8-15 mg/m²; and 2) subjecting that metal surface to a corona discharge by applying an alternating voltage between that surface and a conductor situated thereabove. Surprisingly, it has been found that, contrary to what would be expected, the grease or oil after the corona treatment does not adversely affect the flowing and adhering properties of the metal surface, but rather contributes to a surface which is superior in this respect. It will be apparent that in the case of an electrically conducting product or article, for example a tin plate or strip, this product can be carried over a conducting base, for example while simultaneously being pressed thereon by pressure rollers, whereby the conductor extends in a transverse direction relative to the transporting direction of the product. Hereby achieved in a continuous through-feed is that one of both surfaces of the product can be treated. If required both sides of the product can be consecutively treated in one line. It is noted that many products are already lightly greased or oiled during manufacture. The purpose of such a treatment can on the one hand be to provide a certain lubrication whereby the surfaces of the plates for stacking can slide over each other more easily, while on the other hand the greasy covering layer can form an oxygen barrier in order to counter corrosion. A frequency in the order of 15-40 kHz is preferably used. The corona voltage can be in the order of 10 kV. With a method wherein the metal surface is carried past a transverse conductor which is provided with a dielectric jacket, a well functioning corona treatment can be effectively performed and nevertheless avoid the danger of sparkover. The dielectric covering layer or jacket ensures a high strength of field and a good homogeneity of the corona discharge. Very good results are achieved with a corona discharge wherein the energy density lies between 45 and 90 Watts per running centimeter of transverse conductor. Below this region the effectiveness of the treatment is irregular, while above this value a certain "saturation" occurs. In other words, it is well possible to increase the energy density but this is not accompanied by increasing effectiveness of the treatment and is therefore undesirable from an economic point of view. Use can particularly be made of ester-based lubricants such as DOS (dioctyl sebacate) ATBC (acetyl tributyl citrate) and/or BSO (butyl stearate/palmitate). The dielectric can comprise for example sinter material, plastics such a hypalon (trademark) and/or silicon rubbers, alone or in combination. The corona discharge can advantageously take place in the surrounding air. As a consequence of the corona discharge a part of the oxygen present in the air is converted into ozone. This ozone can bring about a chemical activation of the metal surface. Use can also be made of additional reactive components, e.g. a reactive gas such as chlorine, one or more amines, aldehydes, and/or unsaturated organic compounds. These compounds generally have to be of the type capable of starting radical-reactions. The types mentioned as examples are capable thereof. A good effectiveness is acquired with a method wherein the distance chosen between the metal surface and the conductor is a maximum of approximately 1.5 mm. It has been found that the distance chosen between the metal surface and the electrode is preferably smaller the higher the frequency chosen. In order to cause the homogeneity of the treatment over the surface to be as good as possible the method is recommended in which steps are taken to ensure the flatness of the metal surface at the location of the conductor such that the distance between the metal surface and the conductor is substantially constant. This method can for example be performed such that the article of which the metal surface forms a part can be held by making use of underpressure on a supporting surface, for example a perforated or divided endless conveyor belt with an underpressure source connected thereunder. The article or product of which the metal surface is a part can also be held on a supporting surface by using magnetic forces. It is also possible to roll the plate beforehand. The surface in question can form part of an article or product (semi-manufactures such as metal plates or strips), tin (iron with a covering layer of tin), aluminium, chrome-passivated steel or ECCS (electro-chromium coated steel) or pure steel or "blackplate". The treatment according to the invention gives great durability in the lacquering and printing qualities. Tests have demonstrated that up to approximately four months after the treatment no decrease whatever can be discerned in the lacquering and/or printing qualities. It is thus apparent that the method according to the invention leads to a very stable result. The invention will now be explained with reference to the accompanying drawings. In the drawings: Figure 1 is a schematic longitudinal section of an apparatus according to the invention; Figure 2 partially in side elevation and partially in longitudinal section according to figure 1 a total transport and treating apparatus having an apparatus according to figure 1; Figure 3 is a rear elevation, i.e. an elevation view at the output side of the apparatus according to figure 2; and Figure 4 is a perspective view of a corona-electrode. A supply conveyor 1 serves for supplying in a supply pinch 2 flat metal plates 3 between the pinch of two supply rollers 4, 5, said roller 4 being driven by an electro-motor 6 in a manner to be herein described below, and said roller being spring-loaded in the direction of supply roller 4 and being freely rotatable. The convey- or transport-direction of metal plates 3 is indicated with an arrow 7. Downstream relative to rollers 4, 5 the roller pairs 8, 9, 10, 11, 12, 13 and 14, 15 are positioned. Rollers 8, 10, 12, 14 are simultaneously with supply roller 4 driven by means of a chain 1 by means of electro-motor 6. Between the respective rollers 4, 8, 10, 12 and 14 reversing rollers indicated with 17 are arranged. Said rollers 9, 11, 13, 15 are spring-loaded in the direction of the driven rollers 8, 10, 12, 14. Thus a positive transport of the conveyed plates 3 to be treated is ensured. The apparatus 18 shown in figures 1, 2 comprises two corona-stations 19, 20. Station 19 treats plates in the region between the roller pairs 8, 9 and 10, 11, whilst station 20 is arranged downstream relative thereof and subjects the plates treated by station 19 to another corona-treatment in the region between roller pairs 12, 13 and 14, 15. The corona-stations 19, 20 are identical. For the sake of ease therefore only station 19 will be discussed. During the transport of metal plates 3 by the apparatus 18 the plates are carried by support bars 21 (see figure 3). In this connection it also should be noted that rollers 8-15 are actually designed as shown in figure 3, namely each as rows of nine rollers arranged on one common shaft. Figure 3 shows the rows of rollers 14 and 15. The corona-station 19 (and 20) comprises six corona-electrodes 22. Such an electrode is shown on an enlarged scale in figure 4 and comprises a rest-proof core 24 and a ceramic jacket 24. The corona-electrodes 22 are arranged in a common horizontal plain on a certain distance of support bars 21, in such a way that they have also a desired distance relative to the conveyed metal plates 3. By means of a cable 25 the corona-electrodes 22 are connected with an adjustable high-tension generator 26, the earth side of which is connected with earth, of the total frame, of the apparatus 18. Also support bars 21 are conductively connected with said frame, such that the high-tension generator 26 generates a high tension between the cores 23 of the corona-electrodes 22, on the one hand, and the upper surface of the transported metal plates 3, on the other hand, which, as mentioned, shift during transport by rollers 8-15 electrically conductive lay over support bars 21. As a result of the presence of the dielectric ceramic jacket 24 between this jacket 24 and the upper surface of a passing metal plate 3 a strong electric field is generated. As a result thereof this mentioned upper surface of the related metal plate is subjected to a corona-discharge. In corona-station 19 the full surface is subjected by six successive electrodes 22 to six corona-treatments, followed by the same series of treatments in corona-station 20. A hood 27 serves as cover for the corona-stations 19, 20. The hood can be hingedly removed by means of a hinge 28 into the position shown with interrupted lines and indicated with 27′. In this position the interior of the apparatus is excessible, e.g. for service purposes. Over electrodes 22 fan-means 29 are arranged for sucking off ozone-gas generated by the corona-discharge. The poisonous ozone-gas can, by means of means not-shown be removed to a place, where is it harmless. It will be obvious that the lengths of the corona-electrodes 22 has to be at least equal to the width of the conveyed metal plates 3.

Claims: A method for activating a metal surface, for example improving the flow of lacquer over and/or adhesion of lacquer to a tin surface, comprising the steps of: 1) providing a greased or oiled metal surface, for example the surface of a tin plate or strip, the grease or oil concentration being in the order of 0,8-15 mg/m²; and 2) subjecting that metal surface to a corona discharge by applying an alternating voltage between that surface and a conductor situated thereabove. A method as claimed in claim 1, characterized in that the frequency of the alternating voltage is located approximately in the region of 15-40 kHz. A method as claimed in any of the foregoing claims, characterized in that the voltage between the metal surface and the conductor amounts to in the order of 10kV. A method as claimed in any of the foregoing claims, characterized in that the metal surface is carried past at least one transverse conductor with a dielectric jacket. A method as claimed in claim 4, characterized in that the energy density is in the order of 45-90 watts per running centimetre of transverse conductor. A method as claimed in any of the foregoing claims, characterized in that ester-based lubricants are used as oil or grease, such as DOS (dioctyl sebacate), ATBC (acetyl tributyl citrate) and/or BSO (butyl stearate/palmitate). A method as claimed in claim 4, characterized in that a sinter material is used as dielectric. A method as claimed in claim 4, characterized in that hypalon is used as dielectric. A method as claimed in claim 4, characterized in that a silicon rubber is used as dielectric. A method as claimed in any of the foregoing claims, characterized in that the corona discharge is allowed to take place in the surrounding air, for example at a temperature of approximately 20°C and a relative humidity of 50%. A method as claimed in any of the claims 1-9, characterized in that the corona discharge is allowed to take place in the presence of additional reactive components, e.g. a reactive gas such as chlorine, one or more amines, aldehydes and/or unsaturated organic compounds. A method as claimed in any of the foregoing claims, characterized in that the distance chosen between the metal surface and the conductor is a maximum of approximately 1.5 mm. A method as claimed in any of the foregoing claims, characterized in that the distance chosen between the metal surface and the electrode is smaller the higher the frequency chosen. A method as claimed in any of the foregoing claims, characterized in that steps are taken to ensure the flatness of the metal surface at the location of the conductor such that the distance between said metal surface and said conductor is substantially constant. A method as claimed in claim 14, characterized in that by making use of underpressure the article or product of which the metal surface is a part is held on a supporting surface, for example a perforated or divided endless conveyor belt with an underpressure source connected thereunder. A method as claimed in claim 14, characterized in that the article or product of which the metal surface is a part is held on a supporting surface by making use of magnetic forces. A method as claimed in claim 14, characterized in that the plate is rolled flat beforehand.

Inventors: TER HORST, GERRIT HARMEN FREERK, VAN DONGEN, CORNELIS JACOBUS
Applicants: THOMASSEN & DRIJVER-VERBLIFA N.V.
Application year: 1989
Main classification: B05D3
IPC codes: B05D3, B05C9, H01T19