EP2845929A1 - Tinplate with a polymer coating and method for producing the same - Google Patents

Abstract

The invention relates to a process 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 is first electrochemically oxidized in a first step and a polymer coating is applied to the oxidized tin surface in a second step, and a device for carrying out of the procedure. The invention further relates to a coated with a polymer coating of polyethylene terephthalate (PET) and chromium passivated tinplate, between the tin surface of the tinplate and the polymer coating only a thin tin oxide layer and optionally a primer layer is present, and the use of such a tinplate for the production of packaging ,

Description

The invention relates to a coated with a polymer coating tinplate and method for its preparation and an apparatus for performing the method.

Tinplate is a thin, cold-rolled steel sheet, the surface of which is coated with tin. The application of the tin coating on the steel sheet is usually carried out electrolytically. Tinplate is mainly used for the production of packaging, in particular cans for food and pet food, packaging for chemical and technical products, aerosol cans, beverage cans and for the production of parts for such packaging as e.g. Closures, flap bands, valve plates, can ends and lid rings used.

Tinplate is characterized by high corrosion resistance and stability against acids as well as good formability. For certain applications, for example for the production of food packaging and beverage cans, the tinplate surface is additionally provided with a paint or a polymer coating, in addition to the corrosion protection by the tin coating to ensure additional protection against corrosion. For this purpose, for example, a plastic film of polyethylene terephthalate (PET) or polypropylene (PP) is applied to the tinplate. Foil-coated tinplate is particularly suitable for the production of valve plates, aerosol can bottoms, can openers, deep-drawn containers and vacuum closures.

However, it has been shown that during the forming of tinplates provided with PET coatings, stress cracks develop in the PET coating, which, when in contact with aggressive and in particular acidic contents, causes the underlying tin coating of the tinplate to be attacked. The emulsions of lubricants, which are applied to the tinplate surface for better processability, are also responsible for the crack initiation in the PET and, moreover, can penetrate into the formed cracks in the PET coating and the tin surface of the PET Whiteboard attack, which can lead to a loss of adhesion and a replacement of the PET coating from the tinplate surface.

From the publications DE 40 09 839 A1 . DE 34 36 412 C2 and EP 664 209 A1 Tinned steel sheets are known, which are coated with a polyester resin film, in particular of polyethylene terephthalate (PET). The coating of the tinplate surface with the polyester resin film is carried out by laminating a polyester film, in particular a PET film, on the surface of the tinplate. In order to ensure sufficient adhesion of the polyester resin film to the tinplate surface, a chromium-containing adhesive layer is applied to the tinplate surface prior to laminating the polyester resin film which is formed, for example, by a monolayer of hydrated chromium oxide or by a double layer of metallic chromium with an overlying layer of hydrated chromium oxide , Without this adhesive layer between the tinplate surface and the polyester resin film, the polyester resin film, especially a PET film, would peel off the tinplate, particularly in forming processes in the packaging manufacturing processes or in sterilizing or filling the packages with hot fillers. However, the chromium compounds used for the preparation of the chromium-containing adhesive layer are toxic and environmentally hazardous.

As an alternative material for tinplate, electrolytic chromium coated steel (ECCS) is known from the prior art. This material, also known as "Tin-Free Steel (TFS)", is a cold-rolled steel sheet which has been electrolytically coated with a coating of chromium and chromium oxide. The surface of this material has good adhesion to polymeric materials, such as polyethylene terephthalate or polypropylene, and therefore can be coated with these polymers, for example, by laminating a polymeric film to allow additional corrosion protection. The adhesion of the polymer coating on the chromium surface of the ECCS or TFS also holds strong transformations, such as occur in the production of packaging containers, as well as sterilization processes stood. Therefore, ECCS sheets provided with polymer coatings are used particularly in manufacturing processes for containers in which large deformations of the sheets are required, such as in the production of Valve plates for aerosol cans, wherein the organic coating in the ECCS before forming takes place, otherwise severe tool wear occurs.

From the EP 848 664 B1 For example, a steel strip protected by a chromium coating against corrosion (ECCS or TFS) on which a film of polyethylene terephthalate has been laminated is known. However, such steel sheets, which are protected with a chromium coating against corrosion, also prove to be disadvantageous because of the toxicity of the chromium compounds used in the manufacturing process, in particular the liquid chromic acid (chromium VI) of the finishing bath.

From the WO 97/03823-A a corrosion-resistant steel sheet is known, which has a metallic corrosion protection layer, which may be, for example, electrolytically applied tin or chromium / chromium oxide layers on which one or both sides a transparent polymer film is applied by laminating a polymer film. The polymer film consists of polyethylene terephthalate (PET), polyvinyl chloride (PVC) or polypropylene (PP).

In this case, an adhesion promoter, in particular an adhesive layer, is provided between the metallic corrosion protection layer of the steel sheet and the laminated polymer film. To produce the corrosion-resistant steel sheet, a galvanically coated with a metallic corrosion protection layer and passivated steel sheet with a thickness between 0.05 mm and 0.5 mm is used and heated to temperatures of about 160 ° C. On the heated steel sheet, the polymer film is laminated by means of rotating rollers. The thickness of the laminated polymer film is between 5 and 100 μm. The polymer film preferably has on one side an adhesive layer which has a lower melting point than the polymer material of the polymer film. The polymer film is laminated with the adhesive layer on the surface of the metallic corrosion protection layer of the steel sheet oriented.

In this method of laminating a polymer film on the metallic anticorrosion layer of a steel sheet, a special polymer film having an adhesive layer is used to laminate the polymer film on the surface of the anticorrosion layer of the steel sheet. Such polymer films with an adhesive layer are very expensive to manufacture. Furthermore, the handling of such polymer films with an adhesive layer more complex and the process parameters, in particular the temperatures must be kept during the lamination within the specified limits, which are determined by the melting temperatures of the polymer film and the adhesive layer. In particular, with tin-plated steel sheets, however, it has been found that an adhesive layer can not be dispensed with if sufficiently good adhesion of the polymer film to the tinned surface of the steel sheet is to be ensured. On the chrome surfaces of ECCS and TFS, however, the polymer films adhere better, but in the production of ECCS due to the chromium-containing substances used in the coating of the steel sheet toxic and environmentally harmful waste.

Proceeding from this, the object of the invention is to provide a method which is as completely as possible free of chromium for the production of a highly corrosion-resistant steel sheet. The highly corrosion-resistant steel sheet produced by the method should be suitable in particular for the production of packaging and suffer no deterioration in terms of corrosion resistance even with severe deformations in the manufacturing process and sterilization of the package produced.

These objects are achieved by the method according to claims 1 and 2 and by a tinplate having the features of claim 11. Preferred embodiments of the methods are shown in the dependent claims 3-10 and the dependent claims 12-17 represent preferred embodiments of the tinplate. To achieve the object further contributes to an apparatus for performing the method having the features of claim 18.

In the method according to the invention for coating a chromium-free surface of a tinned steel sheet (tinplate) with a polymer coating, the chromium-free tin surface of the tinned steel sheet is first electrochemically oxidized in a first step and in a second step a polymer coating is applied to the oxidized tin surface. The electrochemical oxidation of the tin surface ensures chromium-free passivation of the tin surface, which prevents unhindered growth of tin oxide on the tinplate surface. Unlike the known methods for passivating tinplate against the growth of tin oxide on the tinplate surface, the passivation of the tinplate surface takes place in the process according to the invention without the use of chromium-containing substances. especially without the use of toxic and environmentally hazardous chromium oxides. It has surprisingly been found that this chromium-free passivation of the tinplate surface by electrochemical oxidation not only prevents unhindered growth of tin oxide on the tinplate surface, but at the same time also forms a good adhesion base for polymers. As a result, in the second step of the process 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 has been found that the adhesion between the oxidized tin surface and the polymer coating also withstands severe deformation, as occurs, for example, in processes for the production of multi-deep-drawn cans or in the manufacture of valve plates. The adhesion between the oxidized tin surface and the polymer coating also readily withstands sterilization, without any detachment of the polymer coating from the tinplate surface during sterilization.

In the process according to the invention for producing a tinplate coated with a polymer coating, in a first process step, first of all, a tin coating is deposited electrolytically on one or both sides of a steel sheet. In a second step, an electrochemical oxidation of the surface of the tin coating takes place and finally a polymer coating is applied to the oxidized surface of the tin coating. The electrochemical oxidation of the tin surface is preferably carried out directly and in particular within a few seconds after the deposition of the tin coating on the steel sheet. The electrochemical oxidation of the tin surface preferably takes place here without further intermediate steps, in particular without intermediate cleaning or temperature treatment of the tinplate surface.

The electrochemical oxidation of the tin surface can be carried out in particular by anodic polarization of the tinned steel sheet in an aqueous and chromium-free electrolyte. For example, the electrochemical oxidation of the tin surface may be carried out by immersing the tinplate in a soda solution (sodium carbonate solution). In this case, a thin tin oxide layer, which consists essentially of tetravalent tin oxide (SnO ₂ ), forms on the (chromium-free) tin surface of the tinplate. This tetravalent tin oxide is resistant to bivalent tin oxide (SnO), which occurs when tinplate is stored in an oxygen-containing atmosphere on the tinplate surface is formed, substantially inert and prevents unhindered growth of a (divalent) tin oxide layer on the tinplate surface on contact with oxygen. The thickness of the oxide layer of substantially tetravalent tin oxide formed in the electrochemical oxidation of the tinplate surface is expediently in the nm range and is preferably thinner than 100 nm. In the formation of this passivating tin oxide layer on the tinplate surface, a charge density is preferred in the step of electrochemical oxidation the tin surface produced, which is at most 40 C / m ² .

After the electrochemical oxidation of the tinplate surface, this is provided with a polymer coating, wherein the polymer coating expediently has a thickness in the range of 10 to 100 microns and is preferably applied by laminating a polymer film on the oxidized tin surface. The use of a coextruded plastic film with a polymer layer and an adhesion promoter layer, which is laminated to the oxidized tin surface of the steel sheet, by applying the adhesion promoter layer of the plastic film to the oxidized tin surface and laminating it under the action of heat by means of laminating rolls or lamination rolls is particularly expedient. The adhesion promoter layer further increases the inherently good adhesion of the polymer coating to the oxidized tin surface of the tinplate.

It has proven particularly expedient if the tinned steel sheet is heated to temperatures above the melting temperature of the tin coating (232 ° C.) during the lamination of the polymer film. As a result, the tin coating of the tinplate is melted, so that at least the near-surface areas of the tin coating of the tinplate are in the molten state during the lamination of the polymer film. This further improves the adhesion between the (molten) tin surface of the steel sheet and the laminated polymer coating. In particular, even when using polymer materials of polymer coating having a melting point of more than 232 ° C or when using coextruded plastic films with a primer layer whose melting point is greater than 232 ° C, sufficient adhesion between the polymer coating and the tin surface of the tinplate ensured become. However, it is particularly preferred if the steel sheet is heated during the lamination of the polymer film on the tinned surface to temperatures which are both higher than the melting temperature of the tin and higher than the Melting temperature of the polymer material used for the formation of the polymer coating or the adhesion promoter may be present.

It is also possible to apply a polymer coating without an additional adhesion promoter to the oxidized tin surface of the tinplate. Only if very strong transformations are carried out in the subsequent processing during the processing of the tinplate produced according to the invention, the use of an adhesion promoter between the oxidized tin surface of the tinplate and the polymer coating is required in order to prevent detachment of the polymer coating in the forming steps.

For special applications of the tinplate according to the invention, in which high forming rates with deep-drawing rates of D / d = β = 1.7 (D = diameter of the tube, d = bowl diameter) and higher are required, the use of a coupling agent between the polymer coating and the tin surface has become necessary of the tinplate proved to be useful. Intermediate layers which comprise glycol-modified polyethylene terephthalate (PETG), glycol-modified polycyclohexylenedimethylene terephthalate (PCTG) and / or isophthalic acid (IPA) or mixtures thereof have proven suitable adhesion promoters.

The polymer material of the polymer coating is expediently a thermoplastic polyester, in particular polyethylene terephthalate (PET). The melting point of polyethylene terephthalate is in the range of 260-270 ° C. In order to ensure the best possible adhesion when applying the polymer coating to the oxidized tin surface, it is expedient to heat the tinned steel sheet to temperatures above the melting point of polyethylene terephthalate when the polymer coating is applied, so that both the tin surface of the tinplate and at least when the polymer coating is applied the near-surface areas of the polymer coating facing the tinplate are in the molten state and can therefore form an intimate material connection. It has proven to be particularly useful if the tinned steel sheet during application of the polymer coating in the temperature range between 270 ° C and 290 ° C and preferably maintained at about 280 ° C.

For example, in order to prevent sticking of the polymer film to the heated laminating rollers when laminating a polymer film onto the oxidized tin surface of the steel sheet by means of laminating rollers, it is expedient to use a multilayer plastic film for forming the polymer coating, which has an antiblocking layer on its upper side. Such an antiblocking layer may be formed, for example, by a silicon oxide layer on top of the polymer film.

The processes according to the invention can be carried out in strip-tinning plants in which a steel strip is moved by means of a transport device at a strip speed of preferably more than 200 m / min and more preferably of more than 500 m / min through a tinning device, around one or both sides of the steel strip electrolytically coated with a tin coating. The subsequent electrochemical oxidation of the tin surface is carried out in an oxidation device preferably by passing the tinned steel strip at the belt speed through an electrolytic bath with an aqueous electrolyte, wherein the steel strip is suitably connected as an anode to electrochemically (anodically) oxidize the tin surface. Thereafter, the polymer coating is applied to the oxidized tin surface of the running steel strip in a plastic coating device, for which purpose preferably by means of laminating rollers on one or both sides a polymer film is laminated onto the oxidized tin surface. The tinning device and the oxidation device are arranged one behind the other in the belt running direction and preferably so close to each other that at the typical belt speeds of more than 200 m / min within a very short time and preferably within a few seconds after the tin coating, the tinned surface of the steel belt can be electrochemically oxidized ,

Fig.1:

Schematische Darstellung einer Vorrichtung zur Durchführung des erfindungsgemäßen Verfahrens zur Herstellung von mit einer Polymerbeschichtung beschichtetem Weißblech;

Fig. 2:

Schematische Darstellung eines erfindungsgemäßen Weißblechs mit einer Polymerbeschichtung ohne Haftvermittlerschicht;

Fig. 3:

Schematische Darstellung eines erfindungsgemäßen Weißblechs mit einer Polymerbeschichtung mit Haftvermittlerschicht.

These and other advantages of the inventive method and the tinplate according to the invention will become apparent from the embodiments described below, which are explained with reference to the accompanying drawings. The drawings show:

Fig.1:

Schematic representation of an apparatus for carrying out the method according to the invention for the production of coated with a polymer coating tinplate;

Fig. 2:

Schematic representation of a tinplate according to the invention with a polymer coating without adhesive layer;

3:

Schematic representation of a tinplate according to the invention with a polymer coating with a bonding agent layer.

Starting material for the process according to the invention for the production of a polymer coating coated tinplate is preferably a hot rolled and unalloyed or low-alloyed sheet steel (steel strip) with a low carbon content of, for example, 20 to 900 ppm. The alloy components of the steel suitably meet the requirements of the international standard ASTM A 623-11 (Standard Specification for Tin Mill Products), whereby a use of the tinplates according to the invention produced for the production of food packaging is ensured. In principle, all steel grades which have a composition suitable for the production of fine or superfine sheets can be used for the method according to the invention. The hot-rolled steel strip is first pickled in a (not shown) pickling plant, then rinsed and dried and then cold-rolled in a cold rolling mill. The steel strip is rolled to a thickness of less than 1.0 mm (thin sheet) and preferably to thicknesses of 0.1 to 0.5 mm (fine sheet). After cold rolling, the steel strip is first passed through a continuous annealing furnace in which the steel strip is heated to temperatures of 550 ° C to 700 ° C to recrystallize the steel to anneal. The recrystallizing annealing reestablishes the formability of the cold rolled steel strip. After recrystallization annealing, the steel strip may be dressed or regrooved in a post-rolling mill if necessary to achieve the forming properties necessary for the intended processing purposes. When re-rolling, if necessary, a required further reduction in the thickness of the steel strip can be achieved. After the temper rolling or rolling, the steel strip is cleaned by an alkaline electrolytic treatment and by pickling with subsequent rinsing.

Subsequently, the steel strip 10, as in FIG. 1 shown schematically, guided by a tinning 7. The steel strip 10 is unwound as a continuous belt from a roll 12 and moved by a conveyor 6 at a belt speed of preferably more than 200 m / min and up to 750 m / min through a tank 7a with a tin-containing electrolyte and carried out as a cathode between tin anodes , As a result, the tin of the anodes is dissolved and deposited on the steel strip as a tin coating. The tin can be deposited in any thickness and, if necessary, on both sides of the steel strip 10. The thickness of the applied tin layer is regularly between 0.5 g / m ² and 12 g / m ² . However, it is also possible to coat the steel strip with thinner or thicker tin layers.

Immediately after the coating of the steel strip with the tin coating and in particular without further intermediate steps, the tin surface of the tin-plated steel strip 10 is electrochemically oxidized in an oxidizer 8. For this purpose, the freshly tinned steel strip 10 is, for example, passed into an electrolysis bath with an acidic, chromium-free and aqueous electrolyte and connected as an anode. Thereby, the fresh tin surface of the tinned steel strip 10 is anodically polarized. In this case, a tin oxide layer having a layer thickness in the nm range is formed on the tin surface of the tin-plated steel strip, which consists essentially of tetravalent tin oxide (SnO ₂ ). This tetravalent tin oxide is much more inert to divalent tin oxide (SnO) that forms when tinned steel sheet is stored in an oxygen atmosphere. This (essentially tetravalent and inert) tin oxide layer, which forms during the electrochemical oxidation of the freshly tinned surface, ensures high resistance of the tinned steel strip surface against corrosion and reaction with sulfur. The thin tin oxide layer of essentially tetravalent tin oxide (SnO ₂ ) prevents, in particular, unimpeded growth of further (bivalent) tin oxide upon contact of the tinned surface with atmospheric oxygen.

The electrochemical oxidation of the tin surface takes place, for example, as anodic oxidation of the tinned steel strip 10 in soda solution, i. in an aqueous sodium carbonate solution. For this purpose, the tin-plated steel strip is moved further at the strip speed and passed through an electrolytic bath 8a with a soda solution. The concentration of sodium carbonate in the soda solution is preferably 1 wt .-% to 10 wt .-%, preferably 2 wt .-% to 8 wt .-%, preferably 3 wt .-% to 7 wt .-%, especially 4 Wt .-% to 6 wt .-%, in particular about 5 wt .-%.

The oxidation device 8 for the electrochemical oxidation of the surface of the tin coating expediently comprises an electrolytic bath 8a with a vertical tank which is filled with the electrolyte. Near the bottom is inside the vertical tank one Deflection arranged over which the tinned steel strip 10 is deflected. Between the tinned steel strip 10 and the counter electrode (eg, a steel cathode) in the vertical tank, a potential is applied. The amount of charge Q transferred during the electrochemical oxidation is preferably below 40 C / m ² . The current density prevailing in the electrolysis bath is preferably in the range from 1.0 A / dm ² to 3 A / dm ² . The thickness of the thereby forming tin oxide layer is preferably less than 100 nm and is particularly preferably of the order of 10 nm.

The anodization time corresponds to the residence time of the tinned steel strip in the electrochemical oxidation bath (electrolyte bath). This is predetermined by the length of the electrolyte bath or its fill level as well as the anode length and the belt speed and is advantageously in the range of 0.1 s to 1 s, in particular between 0.1 s and 0.7 s, preferably in the typical belt speeds Range from 0.15s to 0.5s, and ideally 0.2s. The fill level can be used to set the anodization time to suitable values, depending on the belt speed, in order to form the preferred layer thickness of the electrochemically produced tin oxide layer.

The distance between the steel strip 10 and the counter electrode in the electrolytic 8a is set due to the system. It is, for example, in the range of 3 to 15 cm, preferably in the range of 5 to 10 cm and in particular by 10 cm. The temperature of the electrolyte is preferably in the range of 30 to 60 ° C, in particular in the range of 35 to 50 ° C.

The current density in the electrolysis bath is set, for example, in the range from 1.0 to 3 A / dm ² , preferably 1.3 to 2.8 A / dm ² , and in particular by 2.4 A / dm ² . The total amount of charge moves in the range between 0.2 C and 0.4 C and is preferably, for example, 0.3 C. The corresponding charge densities (with respect to the surface of the oxidized tinplate strip) are in the range of 0.2 C / dm ² to 0.4 C / dm ² .

The tin-plated steel strip 10 is guided into a plastic coating device 9 after the electrochemical oxidation of the tin surface at a maximum strip speed of 200 m / min. Since the steel strip can not be performed by the plastic coating device with the high belt speeds of about 750 m / min, which are driven when tinning the steel strip in the tinning, it is expedient the process step of Polymer coating separately, ie to carry out with previous rolling of the tinned steel strip to a coil and intermediate storage of the coil. This is easily possible since the tin surface is resistant to a (further) unhindered growth of a (divalent) tin oxide layer due to the electrochemical oxidation. However, it is also possible to apply the polymer coating without intermediate storage and in continuous steel strip directly after tinning and oxidation of the tin surface in the plastic coating device 9. There, a polymer coating is applied to one or both sides of the tinned steel strip. For this purpose, the steel strip is first heated in a heater 11, which may be formed, for example. As induction heating or as infrared or microwave heating, to temperatures which are at least above the melting temperature of the tin (232 ° C). Suitably, the temperature of the steel strip 10 during application of the polymer coating is also above the melting temperature of the polymer material. The polymer material is preferably polyethylene terephthalate (PET having a melting point of approx. Between 235 and 260 ° C., depending on the degree of crystallization and the degree of polymerization) or polypropylene (PP having a melting temperature of approx. 160 ° C.) or PE (with a melting temperature of about 130-145 ° C).

When the tin-plated steel strip is heated to temperatures above the tin melting point, a thin and very dense alloy layer consisting of iron atoms of the steel and tin atoms of the tin coating is formed between the steel strip surface and the tin layer. This alloy layer leads to a very good adhesion of the tin coating on the steel strip surface and also represents a very effective corrosion barrier. In a complete melting of the tin coating also a glossy surface of the tin layer is produced.

The heated steel strip 10 is supplied in the plastic coating device 9 on one or both sides of a film 16 made of a polymer material and pressed by (suitably heated) laminating rollers 9a to the surface of the tin coating. The polymer film 16 may be a film of a polyester such as polyethylene terephthalate and in particular a biaxially oriented or amorphous polyester film or a film of polypropylene or else a film of a polymer laminate consisting of polyethylene terephthalate and polypropylene and polyethylene. If necessary, a polymer film with a primer layer is used, which will follow is described. Due to the temperature of the heated steel strip 10, at least the near-surface region of the tin coating and (depending on the selected temperature of the steel strip) possibly also at least the region of the polymer film 16 facing the tin-plated steel strip 10, which then oxidizes on pressing by the laminating rolls 9a Surface of the tin coating adheres.

In order to prevent sticking of the polymer film to the optionally heated laminating rollers during lamination of the polymer film 16 to the oxidized tin surface of the steel sheet 10 by means of the laminating rollers 9a, it is expedient to use a multilayer polymer film 16 for forming the polymer coating, which has an antiblocking layer on its upper side. Such an antiblocking layer may be formed, for example, by a silicon oxide layer on top of the polymer film.

After lamination of the polymer film, the tin and polymer coated steel strip 10 undergoes cooling to about 20 ° C. Thereafter, the polymer coating can optionally be completely melted and then quenched in a cooling device 15 (for example a water bath) to a temperature below the glass transition point. As a result, for example, when using PET or PP as the polymer material, an amorphous structure is formed in the polyethylene terephthalate or a minimal crystalline structure in the polypropylene. The melting of the polymer coating is carried out particularly expediently by re-heating the steel strip 10 to temperatures above the melting point of the polymer material used in a melting device 14. The plastic coating of the polymer coating takes place expediently in the melting device 14 by induction heating of the steel strip 10 in an induction coil 14a. As a result of this reheating, residual stresses in the polymer coating are broken down by relaxation, which leads to an increase in the adhesion between the tin coating and the polymer coating and, as a result, to a stabilization of the composite of these layers. For example, when PET is used as the polymeric material, the relaxation time is less than 0.5 seconds, so that a brief heating of the polymer coating to temperatures above the PET melt temperature (about 260 ° C) is sufficient to provide the desired relaxation. For example, at the typical belt speeds of more than 200 m / min., An induction coil 14a extending in the melting device 14 over less than 1 meter along the strip running direction is sufficient thus inductively heat the steel strip 10 in this section and thereby melt the polymer coating.

The subsequent quenching of the molten polymer coating in the cooling device 15 can be done, for example, by air cooling or by immersing the steel strip in a tank with cooling liquid. Finally, the coated steel strip 10 is wound on a roller 13 by the transport device 6.

In FIG. 2 a correspondingly manufactured tinplate is shown in section. This comprises the layers of steel sheet 1, tin coating 2, tin oxide layer 3 and the polymer coating 4 (for example of PET).

The tinplates produced according to the invention are distinguished by high corrosion resistance, which is achieved by the metallic corrosion protection layer made of tin and the polymer coating. The thin iron-tin alloy layer, which is formed when the tin-plated steel strip is heated to temperatures above the tin melting point between the steel strip surface and the tin layer, also contributes to the corrosion resistance. The combination of these anticorrosive layers is particularly advantageous because the release of tin ions from the tin coating is avoided when exposed to air by the polymer coating. Because of the polymer coating, the tinplates produced according to the invention are also inert towards aggressive and, in particular, acidic filling products and are therefore very well suited for the production of packaging for such filling products. In comparison with matt-gray ECCS (TFS), the tinplates according to the invention have a high brilliance due to the glossy surface of the tin coating, which results from a complete melting of the tin coating. This is particularly advantageous when using transparent or translucent polymer coatings, because the tinplate thereby has an optically very attractive glossy surface. Compared to the known processes for the production of steel sheets, which are provided with a metallic corrosion protection layer and a polymer coating, the inventive method is further characterized in that they are completely free of chromium, ie, no chromium-containing substances are used.

The steel strips produced according to the invention are further distinguished by a very good adhesion of the polymer coating to the tin coating, which is already achieved without adhesion promoters or additional adhesive layers due to the oxidized tin surface. The additional use of primer layers between the tin coating and the polymer coating is only required for special applications where very high forming rates occur.

At low forming rates, which occur, for example, in the production of round lids or bottoms for cans and can be defined by a draw ratio β = D / d (at D = ring diameter and d = can diameter) of β <1.2, the use is a bonding agent layer not required. For larger transformations, such as e.g. with larger deep draws (eg in the production of valve plates) with β> 1.7, it is expedient to use an adhesion promoter and at even larger forming rates of β> 2 (the example. In single and multiple deep-drawn cans and DWI Doses), a coupling agent appears to be required to reliably prevent peeling of the polymer coating from the tin surface.

Suitable adhesion promoters are glycol modified polyethylene terephthalate (PETG wherein less than 50% of the diol component is cyclohexadimethanol), glycol modified polycyclohexylenedimethylene terephthalate (PCTG wherein more than 50% of the diol component is cyclohexadimethanol) and / or isophthalic acid (IPA). Coupling agents which have a proportion of PETG and 5 to 25% by volume of IPA or PCTG have proven to be particularly preferred. In order to form a bonding agent layer between the oxidized tin surface of the tinplate and the polymer coating, it is expedient to use a multilayered polymer film which contains a polymer layer (for example of PET) and an adhesion promoter layer of one of the abovementioned materials. Such polymer films are available as coextruded films, wherein the thickness of the adhesion promoter layer in the range of 3 to 6 microns with a total thickness of the polymer film from 10 to 40 microns. This multilayered polymer film is oriented toward the tin surface for application of the polymer coating with the adhesion promoter layer and is thus laminated onto the oxidized tin surface. In FIG. 3 a correspondingly manufactured tinplate is shown in section. This includes the layers steel sheet 1, tin coating 2, tin oxide layer 3 and the laminated Polymer coating with the adhesion promoter layer 5 and the polymer layer 4 (for example made of PET).

The tinplates produced according to the invention are suitable for the production of packaging containers, in particular for foodstuffs and for technical filling goods, such as e.g. Two-piece cans (deep-drawn and stretched, DWI cans) and aerosol cans. Canned hulls of three-piece cans are also possible if the polymer coating in the welding area is removed before the core is welded. Parts of such packaging containers may also be made from the steel strips made in accordance with the invention, e.g. Flap tapes, valve plates, can ends and cover rings. In addition, the process according to the invention can also be used for the production of steel sheets for use in other fields, such as e.g. for the production of sheet metal for the construction sector or for the production of household appliances.

The invention is not limited to the described embodiments. For example, within the scope of the invention, it is possible to wind the steel strip 10 onto a roll (coil) after the electrochemical oxidation of the tin surface and to feed it in this form to the next process step (application of the polymer coating). This is in the schematic representation of the device according to the invention FIG. 1 not considered.

The polymer coating may also be applied to the tin coating by other than laminating methods. Thus, after the electrochemical oxidation of the tin surface, for example, by means of direct extrusion, a molten polymer material can be applied to the oxidized tin coating, as described, for example, in the patent DE 197 30 893 C1 is described.

When applying the polymer coating, combinations of different polymer materials are possible. So z. B. on the upper side of the tin-plated steel strip, a polymer coating of PET and on the underside of the belt, a polymer coating of PP are applied. In this case, a polymer coating (PP or PET) can also be replaced by a coating.

Claims (15)

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 is first electrochemically oxidized in a first step and a polymer coating is applied to the oxidized tin surface in a second step. Process for the production of a polymer coating coated tinplate with the following steps: electrolytically depositing a tin coating on one or both sides of a steel sheet, electrochemical oxidation of the surface of the tin coating, - Applying a polymer coating on the oxidized surface of the tin coating. A 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 deposition of the tin coating on the steel sheet. Method according to one of the preceding claims, wherein the charge density on the tin surface after the electrochemical oxidation is at most 40 C / m ² . Method according to one of the preceding claims, wherein the electrochemical oxidation of the tin surface is carried out by anodic polarization of the tinned steel sheet in an aqueous and chromium-free electrolyte. Method according to one of the preceding claims, characterized in that the polymer coating by lamination of a polymer film, in particular a coextruded plastic film with a polymer layer and a primer layer, on the chromium-free and oxidized tin surface of the Steel sheet is applied, wherein the steel sheet during the lamination of the polymer coating is maintained at temperatures above the melting temperature (T _Sn ) of the tin coating. Method according to one of the preceding claims, characterized in that it is the polymer material of the polymer coating is a polyester, in particular polyethylene terephthalate (PET) or polypropylene (PP) or polyethylene (PE). A method according to claim 7, characterized in that the polymer coating is applied by lamination of a film, in particular a biaxially oriented or amorphous cast polyester film of polyethylene terephthalate (PET), on the oxidized tin surface of the steel sheet. With a polymer coating of polyethylene terephthalate (PET) coated and chromium-free passivated tinplate, between the tin surface of the tinplate and the polymer coating only a thin tin oxide layer and optionally a primer layer is present, the tin oxide layer consists essentially of tetravalent tin oxide (SnO ₂ ) and preferably has a thickness of at most 0.1 .mu.m and in particular less than 0.01 microns. Tinplate according to claim 9, characterized in that an adhesion promoter layer is present between the tin oxide layer and the polymer coating. Tinplate according to claim 10, characterized in that the primer layer contains glycol-modified polyethylene terephthalate (PETG), glycol-modified polycyclohexylenedimethylene terephthalate (PCTG) and / or isophthalic acid (IPA). Tinplate according to one of claims 9 to 11, characterized in that on the upper side of the polymer coating, which faces away from the tinned steel sheet, an antiblocking layer is present, which consists in particular of silicon oxide. Tinplate according to one of Claims 9 to 12, characterized in that it consists of a cold-rolled steel strip 0.05 to 0.50 mm thick, made of a low-carbon and low-alloy or steel, by coating with a tin coating in an overlay of 0, 5 to 12 g / m ^{2 is} made. Use of a tinplate according to one of claims 9 to 13 for the production of packaging, in particular cans for food and pet food, packaging for chemical and technical products, aerosol cans, beverage cans, or parts for such packaging, in particular closures, flap bands, valve plate, can lid or lid rings. Apparatus for carrying out the method according to one of claims 1 to 8, comprising a transport device (6) for the continuous transport of an endless steel strip (10) in a transport direction at a transport speed which is preferably greater than 200 m / min, a tin-plating device (7) for galvanically coating the steel strip (10) moved at the transport speed by the coating device with a tin coating, an oxidizer (8) having an electrolytic bath (8a) containing an aqueous, chromium-free electrolyte through which the tinned steel strip (10) is passed at belt speed to electrochemically oxidize the tin surface, a plastic coating device (9) for applying one or both sides of a polymer coating to the tin surface of the steel strip (10).

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