BR102014018213B1 - METHOD FOR COATING A CHROME-FREE SURFACE OF A TINNED STEEL SHEET, METHOD FOR PRODUCING A tinplate and a device for implementing the method - Google Patents

Abstract

tinplate with polymer coating and method of its production. The present invention relates to a method for coating a chromium-free surface of a tinned steel sheet with a polymer coating, wherein the chromium-free tin surface of the tinned steel sheet will initially be oxidized in an electrochemical process. In a first step, and in a second step, a polymer coating will be applied to the oxidized tin surface, as well as a device for carrying out this method. The invention furthermore relates to a tinplate coated with a polymeric coating of polyethylene terephthalate (PET), being chromium-free and passivated, wherein between the tin surface of the tinplate and the polymeric coating is only a thin layer of tin oxide and possibly an adhesion promoting layer are foreseen, as well as covering the use of tinplate of this type to produce packaging.

Description

[0001] The present invention relates to a polymer coated tinplate, as well as a method for its production and a device for carrying out the method.

[0002] Tinplate is a thin, cold-rolled steel sheet whose surface is coated with tin. The application of the tin coating on the tinplate normally takes place in an electrolytic process. Tinplate is mainly used for packaging production, especially cans for food products and animal feed. Packaging for chemical-technical filling material, aerosol cans, beverage cans and for the production of parts for such packaging, such as, for example, closures, can straps, valve discs, can lids and can rings.

[0003] The tinplate stands out for its high resistance to corrosion and stability against acids, as well as for its good deformability. For certain uses, for example for the production of food packaging and beverage cans, the surface of the tinplate is additionally given a lacquer or polymer coating, so that, in addition to corrosion protection, it ensures a Additional corrosion protection by tin coating. For this purpose, a plastic laminate of polyethylene terephthalate (PET) or polypropylene (PP) is applied to the tinplate. Laminate-coated tinplate is especially suited for the production of valve discs, aerosol can bottoms, break-away lids for cans, as well as deep-drawn vacuum containers and closures.

[0004] However, it became evident that in the deformation of tinplate sheets provided with PET coating, tension cracks appear in the PET coating, which, in contact with aggressive fillers and, especially, containing acids, make the coating underlying tin plate from the tinplate is attacked. Also, the lubricant emulsions that are applied to the surface of the tinplate for better machining, are co-responsible for the initial formation of cracks in the PET and, in addition, they can penetrate the cracks formed in the PET coating, attacking the tin surface of the sheet. tinplate, which can result in loss of adhesion and a separation of the PET coating from the tinplate surface.

[0005] From publications DE 40 09 839 A1, DE 34 36 412 C2 and EP 664 209 A1 different tinned steel sheets are known which are coated with a film of polyester resin, especially polyethylene terephthalate (PET) . Coating the surface of the tinplate with the polyester resin film takes place in a laminating process applied a polyester sheet, especially a PET sheet, to the surface of the tinplate. To ensure sufficient adhesion of the polyester resin film to the surface of the tinplate, before the applied lamination process of the polyester resin film, an adhesion layer containing chromium is applied to the surface of the tinplate, which, for example, It is formed by a single layer of hydrated chromium oxide or a double layer of metallic chromium with a superimposed layer of hydrated chromium oxide. Without this adhesive layer between the surface of the tinplate and the polyester resin film, the polyester resin film, especially a PET film, would separate from the tinplate, especially in deformations in packaging production methods or sterilization. or filling packages with hot fillers. The chromium compounds used to produce the chromium-containing adherent layer, however, are poisonous and dangerous to the environment.

[0006] As an alternative material for tinplate, the chrome electrolytic steel plate (Electrolytic Chromium Coated Steel; ECCS) became known from the state of the art. In this material, which is also known in English as "Tin-Free Steel (TFS)" - tin-free steel, it is a cold-rolled steel sheet, which, in an electrolytic process, received a chrome coating and of chromium oxide. The surface of this material has good adhesion to polymer materials, such as polyethylene terephthalate or polypropylene, and can therefore be coated with these polymers, for example, by means of a superimposed lamination of a polymer sheet, in order to provide additional corrosion protection. The adhesion of the polymer coating to the chromium surface of the ECCS, that is, the TFS, also resists intense deformations, as occurs, for example, in the production of packaging containers, as well as in sterilization processes. ECCS sheets provided with polymer coatings are therefore used especially in production processes for containers where strong sheet deformations are required, for example in the production of valve discs for aerosol cans, the organic coating in the ECCS takes place before deformation because, otherwise, there is strong tool wear.

[0007] From the European patent EP 848 664 B1, it became known, for example, a steel belt protected by a chromium coating against corrosion (ECCS, i.e. TFS), which belt was applied by rolling a film made of polyethylene terephthalate. Such steel sheets, protected with a chromium coating against corrosion, are also disadvantageous because of the toxicity of the chromium compounds used in the production process, especially liquid chromic acid (chromium VI) from the beneficiation bath.

[0008] From WO 97/03823-A, a corrosion-resistant steel sheet became known that has a metallic layer that protects against corrosion, which can be, for example, layers of tin or chromium/ chromium oxide applied in an electrolytic process, in which, on one side or on both sides, a transparent polymer film is applied by a process of superimposed lamination of a polymer film. The polymer film consists of polyethylene terephthalate (PET), polyvinyl chloride (PVC) or polypropylene (PP).

[0009] Between the metallic anti-corrosion layer of the steel sheet and the polymer film applied by lamination, an adhesion promoter is provided, especially an adhesive layer. For the production of the corrosion resistant steel sheet, a protective steel sheet coated with a metallic anti-corrosion layer and passivated with a thickness between 0.05 mm and 0.5 mm is used, being heated to temperatures of about 160 °C. On the heated steel plate, through rotating rollers, the polymer sheet will be applied by lamination. The thickness of the polymer film applied by lamination is between 5 and 100 μm. The polymer sheet preferably has, on one side, an adherent layer, which has a lower melting point than the polymer material of the polymer sheet. The polymer film will be applied by lamination with the adherent layer towards the surface of the metallic anti-corrosion layer of the steel sheet.

[00010] In this method for lamination application of a polymer sheet to the metallic anti-corrosion protective layer of a steel sheet, a special polymer sheet with an adherent layer is used in order to apply by lamination the polymer sheet to the surface of the sheet. anti-corrosion layer of steel sheet. The production of these polymer sheets with an adherent layer is very expensive. Furthermore, handling these polymer sheets with an adherent layer is more difficult and the method parameters, especially the temperature, in the lamination application process need to be kept within predetermined threshold values which are determined by the melting temperatures of the sheet. of polymer and the adherent layer. In the case of tinned steel sheets in particular, however, it has been shown that an adhesive layer cannot be dispensed with when a sufficiently good adhesion of the polymer film to the tinned surface of the steel sheet must be guaranteed. On the chromium surfaces of ECCS, i.e. TFS, in turn, the polymer sheets adhere better, however, in the production of ECCS, due to the chromium-containing substances that are used in the coating of the steel sheet, poisonous and environmentally harmful residues arise.

[00011] Starting from this situation, the invention is based on the task of presenting a totally chromium-free method for the production of steel plate with intensive resistance against corrosion formation. The highly corrosion-resistant steel sheet produced with the method must be especially suitable for the production of packaging and must also not suffer any damage in terms of corrosion resistance, even in the case of severe deformation, the production process and sterilization of the packaging produced.

[00012] In the process according to the invention for coating a chrome-free surface of a tinned steel sheet (tinplate) with a polymer coating, the chromium-free tin surface of the tinned steel sheet will initially be in a first step, oxidized in an electrochemical process and, in a second step, a polymer coating will be applied to the oxidized tin surface. By electrochemical oxidation of the tin surface, a chromium-free passivation of the tin surface is ensured, which prevents an uncontrolled growth of tin oxide on the tinplate surface. Unlike conventional processes for the passivation of tinplate against the growth of tin oxide on the surface of the tinplate, passivation of the surface of the tinplate takes place in the method according to the invention of using substances containing chromium, especially without the use of poisonous and environmentally hazardous chromium oxides. It was surprisingly found that this chromium-free passivation of the tinplate surface through the electrochemical oxidation carried out, not only an uncontrolled growth of tin oxide on the tinplate surface, but, at the same time, also constitutes a good base of adhesion for polymers. In this way, in a second step of the method according to the invention, a polymer coating can be applied without problems to the oxidized tin surface of the tinplate, the oxidized tin surface allowing a very good adhesion of the polymer coating. . It was evident that the adhesion between the oxidized tin surface and the polymer coating also resists to intense deformations as occurs, for example, in methods for producing intense deformations, as, for example, in the method for producing cans of repeated deep drawing or in the production of valve discs. The adhesion between the oxidized tin surface and the polymer coating is also perfectly resistant to sterilization, without the polymer coating being separated from the tinplate surface during sterilization.

[00013] In the method according to the invention for producing a tinplate coated with a polymer, in a first step of the process, initially on one or both sides of a tinplate, a tin coating will be precipitated in an electrolytic process . In a second step, the surface of the tin coating is electrochemically oxidized and, finally, a polymer coating is applied to the oxidized surface of the tin coating. The electrochemical oxidation of the tin surface, in this case, preferably takes place directly and especially within a few seconds after the tin coating on the steel sheet is separated. The electrochemical oxidation of the tin surface preferably also takes place without further intermediate steps, in particular without an intermediate cleaning or temperature treatment of the tinplate surface.

[00014] The electrochemical oxidation of the tin surface can be verified especially by an anodic polarization of the tinned tinplate in an aqueous electrolyte and free of chromium. For example, the electrochemical oxidation of the tin surface can be carried out by immersing the tinplate in a sodium solution (sodium carbonate solution). In this case, on the tin (chromium-free) surface of the tinplate, a thin layer of tin oxide is produced, essentially consisting of tin oxide (SnO2), quadrivalent. This quadrivalent tin oxide, compared to divalent tin oxide (SnO), which is formed on the surface of tinplate on exposure of tinplate to an oxygen-containing atmosphere, is essentially inert and prevents uncontrolled bivalent growth of a layer of tinplate. tin oxide on the surface of tinplate in contact with oxygen. The thickness of the oxide layer which is produced in the electrochemical oxidation of the tinplate surface, consisting essentially of quadrivalent tin oxide, will conveniently be in the range of nm, preferably being thinner than 100 nm. In the confirmation of this passivating action tin oxide layer on the surface of the tinplate, in the electrochemical oxidation step, preferably, a charge density will be produced on the tin surface which is at most 40 C/m2.

[00015] After the electrochemical oxidation of the surface of the tinplate, it will receive a polymer coating, and this polymer coating conveniently has a thickness in the range of 10 to 100 μm, and is preferably applied by a lamination process of a polymer sheet on the oxidized tin surface. Preferably, it is especially convenient in the case to use a plastic laminate coextruded with a polymer layer and an adhesion promoting layer which is applied by lamination to the oxidized tin surface of the tinplate, the adhesion promoting layer of the sheet being plastic is applied to the oxidized surface of tin and by applying heat, through rolling cylinders, that is, rolling rolls, it will be applied in the lamination process. By the adhesion promoting layer, the basically already good adhesion of the polymer coating on the tin oxidized surface of the tinplate is further increased.

[00016] It has been shown to be especially convenient for the tinned tinplate, during the laminating application of the polymeric sheet, to be heated to temperatures above the melting temperature of the tin coating (232°C). In this way, the tin coating of the tinplate will be melted so that at least the areas near the surface of the tin coating of the tinplate during the process of rolling the polymer sheet are present in a liquefied state. In this way, the adhesion between the tin surface (liquid, in the form of a melt) of the tinplate and the polymer coating applied by lamination will be further improved. Especially in this way, also with the use of polymeric materials in the polymeric coating with a melting point above 232 °C, or in the use of co-extruded plastic sheets, it can be ensured with an adhesion promoting layer, with a melting point above 232 °C, a sufficient adhesion between the polymeric coating and the tin surface of the tinplate. However, it is especially preferred that the tinplate, in the application by lamination of the polymeric sheet to the tinned surface, is heated to temperatures which are both higher than the melting temperature of the tin, but also higher than the temperature of the polymeric material, or of the adhesion promoter, if any, used to form the polymer coating.

[00017] It is also possible to apply a polymer coating without an additional adhesion promoter on the oxidized tin surface of the tinplate. Only when, in the subsequent work processes, when machining the tinplate produced according to the invention, very intense deformations are carried out, it will be necessary to use an adhesion promoter between the tin oxidized surface of the tinplate and the polymeric coating. in order to avoid a separation of the polymer coating in the deformation steps.

[00018] For special uses of tinplate produced according to the invention, where high strain rates with deep drawing rates are required D/d = β = 1.7 (D = edge diameter; d = diameter of bowl) and higher, it has proved convenient to use an adhesion promoter between the polymer coating and the tin surface of the tinplate. Adhesion promoters in the form of interlayers which contain glycol modified polyethylene terephthalate (PETG), polycyclohexylene methylene terephthalate (PCTG) and/or isophthalic acid (IPA) or mixtures thereof have proved to be suitable.

[00019] The polymeric material of the polymeric coating is conveniently a thermoplastic polyester, especially polyethylene terephthalate (PET). The melting point of polyethylene terephthalate is in the range of 260 to 270 °C. In order for the application of the polymer coating on the oxidized tin surface to guarantee the best possible adhesion, it will be convenient that in the application of the polymer coating, the tinplate is heated to temperatures above the melting point of polyethylene terephthalate so that in the application of the polymer coating, both the tin surface of the tinplate, as well as at least the areas of the polymer coating close to the surface, facing at least towards the tinplate, are present in a liquefied state, and can thus be carried out an intimate union of material. It has been shown to be especially convenient for the tinned tinplate, in the application of the polymer, to be kept in the temperature range between 270°C and 290°C and, preferably, at 280°C.

[00020] In order, for example, in the application by lamination of a polymer sheet to the oxidized tin surface of the tinplate through rolling rolls, to avoid a sticking of the polymer sheet in the heated rolling rolls, it will be conveniently used a multi-layer plastic sheet for forming the polymer coating which has an anti-blocking layer on its upper face. An anti-blocking layer of this form can, for example, be configured by a silicon oxide layer on the upper side of the polymer sheet.

[00021] The methods according to the invention can be carried out in belt tinning plants, whereby a steel belt is moved through a conveyor assembly at a belt speed preferably greater than 200 m/min, especially preferably higher at 500 m/min, passing through a tinning assembly, so that this steel belt is coated on one or both sides in an electrolytic process with a tin coating. The subsequent electrochemical oxidation of the tin surface takes place in an oxidation set, preferably by passing the tinned steel belt at belt speed, passing through an electrolysis bath with a liquefied electrolyte, in which case the steel belt is conveniently switched as an anode in order to oxidize the tin surface in an electrochemical (anodic) process. Then, on the oxidized tin surface of the moving steel belt, in a plastic material coating assembly, the polymer coating will be applied, for which, preferably, by means of rolling rolls on one or both sides, is laminated a polymer film to the oxidized tin surface. The tinning assembly and the oxidation assembly, viewed in the direction of travel of the belt, are arranged in sequence and preferably so close to each other that at typical belt speeds of over 200 m/min, within the shortest time and preferably within Within a few seconds after tin plating, the tinned surface of the steel belt can be oxidized in an electrochemical process.

[00022] These and other advantages of the methods according to the invention and of the tinplate according to the invention will result from the examples of embodiment described below, which will be explained with reference to the accompanying drawings. The drawings show:

[00023] Fig. 1: schematic presentation of a device for carrying out the process of the invention for producing tinplate coated with a polymer coating;

[00024] Fig. 2: schematic presentation of a tinplate according to the invention, with a polymer coating without adhesion promoting layer;

[00025] Fig. 3: Schematic presentation of a tinplate according to the invention, with a polymer coating, with an adhesion promoting layer.

[00026] The starting material for the method according to the invention for producing a tinplate coated with a polymer layer will preferably be a belt-shaped steel sheet (steel strap) preferably hot rolled and /or belt-shaped low-alloy steel sheet (steel belt) with reduced carbon content of, for example, 20 to 900 ppm. The alloy steel components conveniently meet the norms of the international standard ASTM A 623-11 (standard specification for rolling products), thus ensuring the use of tinplate produced in accordance with the invention for the production of packaging of food products. Basically, for the method according to the invention, all types of steel can be used which have a composition suitable for the production of thin sheets or very thin sheets. The hot rolled steel belt will initially be etched in an etching facility (not shown), then washed and dried, and then cold rolled in a cold rolling set. In this case, the steel belt will be laminated to a thickness of less than 1.0 mm (thin sheet) and preferably to thicknesses of 0.1 to 0.5 mm (very thin sheet). After cold rolling, the steel belt will initially be driven through a continuous annealing furnace in which the steel belt will be heated to temperatures from 550°C to 700°C in order to anneal the steel in the form of recrystallization. By the recrystallization annealing process, the deformability of the cold-rolled steel belt will be reconstituted. After recrystallization annealing, the steel belt can be submitted in a sequential rolling mill, to a final rolling or finishing process, if necessary to achieve the intended machining purposes, the necessary deformation properties. In sequential rolling, an additional and necessary thickness reduction of the steel belt may eventually also be achieved. After the final lamination or sequential lamination, the steel belt is cleaned with an alkaline electrolytic treatment and by etching with subsequent washing.

[00027] Then, as schematically shown in Fig. 1, the steel belt 10 is guided through a tinning assembly 7. In this case, the steel belt 10 will be unrolled from a roll 12 in the form of a continuous belt, being transported by a conveyor assembly 6 with a belt speed preferably greater than 200 m/min and up to 750 m/min, through a tank 7a with an electrolyte containing tin and as a cathode it is conducted between tin anodes.

[00028] In this way, the tin will be released from the anodes and separated on the steel belt in the form of a tin coating. The tin can be of random thickness and if necessary it can be precipitated on both sides of the steel belt 10. The thickness of the applied tin layer is regularly between 0.5 g/m2 and 12 g/m2. However, a coating of the steel belt with thinner or thicker tin layers is also possible.

[00029] Immediately after coating the steel belts with the tin coating and especially without other intermediate steps, the tin surface of the tinned steel belt 10 will be oxidized in an electrochemical process in an oxidation set 8. freshly tinned steel belt 10 will be led, for example, into an electrolysis bath with an aqueous, chromium-free acidic electrolyte being switched as an anode. In this way, the fresh tin surface of the tinned steel belt 10 will be anodic polarized. In this case, a tin oxide layer with a layer thickness in the range of mm is formed on the tin surface of the tinned steel belt, which essentially consists of quadrivalent tin oxide (SnO2). This quadrivalent tin oxide is essentially inert compared to the bivalent tin oxide (SnO) produced by storing tinned steel sheet in an oxygen atmosphere. With this tin oxide layer (essentially quadrivalent and inert), which is formed in the electrochemical oxidation of the freshly tinned surface, a high resistance of the tinned steel belt surface against corrosion and reaction with sulfur is ensured. The thin tin oxide layer, essentially quadrivalent tin oxide (SnO2) especially prevents uncontrolled growth of additional (bivalent) tin oxide on contact of the tinned surface with the oxygen in the air.

[00030] The electrochemical oxidation of the tin surface takes place, for example, as anodic oxidation of the tinned steel belt 10 in a sodium solution, that is, in an aqueous sodium carbonate solution. For this purpose, the tinned steel belt continues to be moved at belt speed and driven through an electrolysis bath 8a with a sodium solution. The concentration of sodium carbonate in the sodium solution will preferably be 1% by weight to 10% by weight, preferably 2% by weight to 8% by weight, preferably 3% by weight to 7% by weight, especially 4 % by weight to 6% by weight, especially about 5% by weight.

[00031] The oxidation assembly 8 for electrochemical oxidation of the tin coating surface conveniently comprises an electrolysis bath 8a with a vertical tank that is filled with electrolyte. Near the bottom, inside the vertical tank, a deflector roller is arranged on which the tinned steel belt 10 will be deflected. Between the tinned steel belt 10 and the counter electrode (eg a steel cathode) in the vertical tank, a potential is applied. The amount of charge Q transferred in the electrochemical oxidation will preferably be below 40 C/m 2 . The current density in the electrolysis bath will preferably be in the range of 1.0 A/dm2 to 3 A/dm2. The thickness of the tin oxide layer which is then formed is preferably less than 100 nm and will be especially preferably situated in the order of magnitude of 10 nm.

[00032] The anodizing time corresponds to the time the tinned steel belt remains in the electrochemical oxidation bath (electrolysis bath). This is predetermined by the duration of the electrolysis bath, i.e. its fill level, as well as anode length and belt speed and at typical belt speeds it will conveniently be in a range of 0.1 s to 1 s , especially between 0.1 s and 0.7 s, preferably in the range of 0.15 s to 0.5 s, ideally around 0.2 s. Depending on the belt speed, through the filling level, the anodizing time can be adjusted to suitable values in order to conform to the preferred layer thickness of the tin oxide layer produced in an electrochemical process.

[00033] The distance between the steel belt 10 and the counter electrode in the electrolysis bath 8a will be adjusted according to the installation. It will be situated, for example, in the range of 3 to 15 cm, preferably in the range of 5 to 10 cm and especially around 10 cm. The temperature of the electrolyte will preferably be in the range of 30 to 60 °C, especially in the range of 35 °C to 50 °C.

[00034] The current density in the electrolysis bath is regulated, for example, in the range of 1.0 to 3 A/dm2, preferably 1.3 to 2.8 A/dm2, and especially around 2.4 A /dm2. The amount of the total charge moves in a range between 0.2°C and 0.4°C being preferably, for example, 0.3°C. The corresponding charge densities (relative to the surface of the oxidized tinplate belt) will be situated in a range of 0.2 C/dm2 to 0.4 C/dm2.

[00035] The tinned steel belt 10, after the electrochemical oxidation of the tin surface, will be driven with a maximum belt speed of 200 m/min to a plastic coating set 9. As the steel belt cannot be traversed by the plastic coating set with the high belt speeds of about 750 m/min, with which the tinning of the steel belt in the tinning set is operated, conveniently the step of the polymer coating method will be separated, i.e. that is, it must be carried out with the previous winding of the tinned steel belt for storage in an intermediate coil. This can be done without any problems because, by virtue of electrochemical oxidation, the surface of the tin is resistant against an uncontrolled (additional) growth of a layer of (bivalent) zinc oxide. However, it is also possible to apply the polymer coating without intermediate storage and with the steel belt in passing, directly after tinning and oxidation of the tin surface inside the plastic coating assembly 9. There, on one or both sides of the tinned steel strap, a polymer coating will be applied. For this purpose, the steel belt will initially be increased in a heating assembly 11, which can be configured, for example, as induction heating or also as infrared or microwave heating, it will be heated to temperatures that are situated at least above of the tin melting temperature (232 °C). Conveniently, the temperature of the steel belt 10, in the application of the polymer coating, will also be above the melting temperature of the polymer material. Preferably, the polymer material is polyethylene terephthalate (PET with a melting temperature of approximately between 235 and 260 °C, depending on the degree of crystallization and degree of polymerization) or polypropylene (PP with a melting temperature of approximately 160 °C) or also polyethylene (PE with a melting temperature of approximately 130 to 145 °C).

[00036] On heating the tinned steel belt to temperatures above the melting point of tin, a very thin and very dense alloy layer consisting of iron atoms forms between the surface of the steel belt and the tin layer. of the steel and tin atoms of the tin coating. This alloy layer results in a very good adhesion of the tin coating to the surface of the tin belt and also constitutes a very effective anti-corrosion barrier. In the case of a complete applied melt of the tin coating, a well-polished surface of the tin layer will also be produced.

[00037] For the heated steel belt 10, in the plastic coating assembly 9, a laminate 16 of polymer material will be introduced on one or both sides and through rolling rolls 9a (conveniently heated) will be printed on the surface of the coating of tin. The polymer sheet 16 can be a sheet of a polyester, such as polyethylene terephthalate, and in particular a biaxially oriented or amorphous polyester sheet, or a sheet of polypropylene, or also a sheet of a laminate. polymer, consisting of polyethylene terephthalate and polypropylene and polyethylene. If necessary, a polymer sheet with an adhesion promoter will be used, which will be further described below. Based on the temperature of the heated steel belt 10, in this case at least the area close to the surface of the tin coating will be melted and (according to the temperature selected for the steel belt) eventually also at least the area facing the tinned steel belt 10, which, after printing by the shaping cylinders 9a, is adhered to the oxidized surface of the tin coating.

[00038] In order to avoid, in the applied lamination of the polymer sheet 16, on the oxidized tin surface of the steel sheet 10, by means of the rolling rolls 9a, a gluing of the polymer sheet in the rolling rolls eventually heated, will be conveniently used a multi-layer polymer sheet 16 for the configuration of the polymer coating which has an anti-blocking layer on its upper face. A blocking layer of this type can, for example, be formed by a layer of silicon oxide on the upper side of the polymer sheet.

[00039] After the applied lamination of the polymer sheet, the tin and polymer coated steel belt 10 has a cooling to about 20°C. In this way, the polymer coating may optionally still be fully melted and then quenched in a cooling assembly 15 (eg, a water bath) to a temperature below the glass transition point. In this way, for example, when using PET or PP as a polymeric material, an amorphous structure is formed in polyethylene terephthalate, that is, a minimal crystalline structure in polypropylene. Melting of the polymer coating is, in this case, especially conveniently, by repeatedly heating the steel belt 10 to temperatures above the melting point of the polymer material used, in a melting assembly 14. Melting of the polymer coating takes place in the melt assembly 14 conveniently by inductively heating the steel belt 10 in an induction coil 14a. With this post-heating, specific stresses in the polymer coating are neutralized through relaxation, which results in an increase in adhesion between the tin coating and the polymer coating and therefore results in a stabilization of the bonding of these layers. When using PET as a polymer material, for example, the relaxation time will be set at less than 0.5 seconds, so that rapid heating of the polymer coating to temperatures above the melting temperature of PET (about 260 °C) will be sufficient to produce the desired relaxation. Typical belt speeds of over 200 m/min will therefore suffice, for example an induction coil 14a which extends in the melt assembly 14 by less than one meter along the direction of travel of the belt in order to of thereby heating the steel belt 10 in this segment, thereby melting the polymer coating.

[00040] The subsequent quenching of the molten polymer coating in the quenching assembly 15 may take place, for example, by air quenching or by immersing the steel belt in a refrigerant tank. Finally, the coated steel belt 10 will be wound from the conveyor assembly 6 onto a roll 13.

[00041] Fig. 2 shows, in section, a correspondingly produced tinplate. This tinplate covers layers such as steel sheet 1, tin coating 2, tin oxide layer 3 and polymer coating 4 (eg PET).

[00042] The tinplate produced according to the invention is distinguished by a high resistance to corrosion, which is achieved by the metallic anti-corrosion protective layer of tin and the polymer coating. Also contributing to corrosion resistance is the thin layer of iron-tin alloy that forms on heating the tinned steel belt to temperatures above the melting point of tin between the surface of the steel belt and the tin layer. In this case, the combination of these anti-corrosion protective layers is especially advantageous because the polymer coating will prevent the release of tin ions from the tin coating in the application of air. The tinplates produced according to the invention, based on the polymer coating, are also inert against aggressive and especially acid-containing fillers and are therefore very well suited for producing packaging for such fillers. In comparison with the weak gray ECCS (TFS), the tinplate according to the invention - by virtue of the shiny surface of the tin coating, which is formed in the complete melting of the tin coating - has a very intense gloss. This is especially advantageous in the use of transparent or translucent polymer coatings because the tinplate in this way has a very pleasant optical gloss surface. Compared with the known methods for producing steel sheets, which have a metallic anti-corrosion layer and a polymer coating, the methods according to the invention are also distinguished by the fact that they are completely chromium-free, i.e. no substances are used. containing chromium.

[00043] The steel belts produced according to the invention also stand out for the very good adhesion of the polymer coating to the tin coating which, by virtue of the oxidized tin surface, is already achieved without adhesion promoter or adherent layers additional. The additional use of adhesion-promoting layers between the tin coating and the polymer coating is only necessary for specific applications, where very high deformation rates are present.

[00044] In the case of reduced deformation rates, which are present, for example, in the production of lids and round bottoms for cans, and which can be defined by a stretching behavior β = D/d (in the case of D = diameters of edges and d = can diameters) can be defined by β < 1.2, the use of an adherent promoter layer is not necessary. In the case of greater deformations, as for example, in the case of greater deep drawing (for example, in the production of valve discs) with β > 1.7, in turn, it will be convenient to use an adhesion promoter and, in the case of even higher strain rates of β > 2 (which occur, for example, in single or multiple deep-drawn DWI cans and cans) an adhesion promoter appears to be necessary in order to effectively prevent a coating separation from tin surface polymer.

[00045] As suitable adhesion promoters, glycol-modified polyethylene terephthalate (PETG, with less than 50% of the diol component consisting of a cyclohexadimethanol), polycyclohexylene methylene terephthalate (PCTG) being used, more than 50% of the diol component consists of cyclohexadimethanol) and/or isophthalic acid (IPA). Adhesion promoters which have a portion of PETG and 5 to 25% by volume of IPA or PCTG have proved to be especially preferred. To form an adhesion-promoting layer between the oxidized tin surface of the tinplate and the polymer coating, a multi-layer polymer laminate containing a polymer layer (e.g. PET) will conveniently be used. and an adhesion promoting layer of one of the aforementioned materials. These polymer sheets are available as co-extruded sheets, the adhesion promoters layer thickness being in the range of 3 to 6 μm with an overall polymer sheet thickness of 10 to 40 μm. The multi-layer polymer sheet will be oriented for application of the polymer coating with the adhesion promoting layer towards the tin surface, thus being applied to the oxidized tin surface. Fig. 3 shows a correspondingly produced tinplate shown in section. This comprises the layers of steel sheet 1, tin coating 2, tin oxide layer 3 and the polymer coating applied by lamination with adhesion promoting layer 5 and polymer layer 4 (eg of PET).

[00046] The tinplate sheets produced according to the invention are suitable for the production of packaging containers, especially for food products and for technical filling materials such as, for example, two-section cans (deep-drawn and stretched, cans DWI) and aerosol cans. Intersection can bottoms are also considered when the polymer coating in the solder area is removed before bottom soldering. Parts of such packaging containers can also be produced from the steel straps produced according to the invention, such as, for example, splint straps, valve discs, can lids and lid rings. Furthermore, the method according to the invention can also be used for the production of steel sheets to be used in other areas, for example for the production of sheets for the construction area or for the production of household appliances.

[00047] The invention is not restricted to the described embodiment examples. Thus, it is possible, for example, in the context that the steel belt 10 is wound after the electrochemical oxidation of the tin surface, on a roll (coil) and to drive this material in this form to the next step of the method (application of the tin coating). polymer). This was not taken into account in the schematic presentation of the device according to the invention in Fig. 1.

[00048] The polymer layer can also be applied by coating methods other than lamination, over the tin coating. Therefore, after electrochemical oxidation of the tin surface, for example, also by direct extrusion, a molten polymer material can be applied to the oxidized tin coating, as, for example, described in DE 197 30 893 C1 .

[00049] In the application of polymer coating, combinations of different polymer materials are also possible. Thus, for example, on the upper side of the tinned steel belt a PET polymer coating could be applied and on the lower side of the belt a PP polymer coating could be applied. In this case, a polymer coating (PP or PET) can also be replaced by a lacquer.

Claims (7)

1. Method for coating a chromium-free surface of a tinned steel sheet (1) with a polymer coating (4), in which the chromium-free tin surface of the tinned steel sheet (1) will initially be oxidized in an electrochemical process, in a first step, by means of anodic polarization of the tinned steel sheet (1) in an aqueous and chromium-free electrolyte and, in a second step, a polymer coating will be applied to the oxidized tin surface (4 ), characterized in that the polymer coating (4) occurs by applying a polymer sheet made of a polymer material or by direct extrusion of a molten polymer material, the polymer material being polyethylene terephthalate (PET) or polypropylene (PP) or polyethylene (PE). 2. Method for producing a tinplate coated with a polymer coating (4), characterized by the following steps, - electrolytic separation of a tin coating (2) on one or both sides of a steel sheet (1) ; - applying a polymer coating (4) to the oxidized surface of the tin coating (2) with the method defined in claim 1. 3. Method according to claim 2, characterized in that the electrochemical oxidation of the tin surface takes place directly, preferably within a few seconds after the separation of the tin coating (2) on the steel sheet (1) . 4. Method according to any one of the preceding claims, characterized in that the anodic polarization of the steel sheet (1) tinned in an aqueous and chromium-free electrolyte is carried out only until the charge density on the tin surface reaches the maximum at 40 C/m2. Method according to any one of the preceding claims, characterized in that the polymer coating (4) is applied by laminating a polymer sheet onto the chromium-free, oxidized tin surface of the steel sheet (1) , whereby the steel sheet (1) during the applied lamination of the polymer coating (4) will be maintained at temperatures above the melting temperature (TSn) of the tin coating (2). Method according to claim 1 or 2, characterized in that the polymer coating (4) is applied by lamination of a plastic sheet coextruded with a polymer layer and an adhesion promoting layer or biaxially oriented sheet or amorphous fused polyester made of polyethylene terephthalate (PET), on the oxidized tin surface of the steel sheet (1). Device for carrying out the method as defined in any one of claims 1 to 6, including, - a conveyor assembly (6) for continuously conveying a continuous steel belt (10) on a conveyor assembly with a speed of belt, - a tinning assembly (7) for galvanic coating of the steel belt (10) with the application of a tin coating (2), transported with transport speed by the coating assembly, - an oxidation assembly (8 ) with an electrolysis bath (8a), which contains an aqueous, chromium-free electrolyte, through which a tinned steel belt (10) is conducted at belt speed, in order to oxidize the tin surface in process electrochemical, - characterized in that the device also includes a plastic coating assembly (9) for application on one or both sides of a polymer coating (4) on the tin surface of the steel belt (10), which includes the application by lamination of a polymer sheet made of a polymer material or by direct extrusion of a molten polymer material, the polymer material being selected from the group consisting of polyethylene terephthalate (PET), polypropylene (PP) and polyethylene (PE).

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