Tinplate, coated with a polymer coating, and methods for its production

Abstract

A method for the coating of a chromium-free surface of a tin-plated steel sheet with a polymer coating, with the chromium-free tin surface of the tin-plated steel sheet first electrochemically oxidized in a first step and a polymer coating applied on the oxidized tin surface in a second step, and an apparatus for the carrying out of the method. A tinplate coated with a polymer coating made of polyethylene terephathalate (PET) and passivated chromium-free, wherein only one thin tin oxide layer and perhaps an adhesion promoter layer are present between the tin surface of the tinplate and the polymer coating, and the use of such a tinplate for the production of packagings.

What is claimed is: 
     
         1 . Method for coating of a chromium-free surface of a tin-plated steel sheet with a polymer coating, wherein the chromium-free tin surface of the tin-plated steel sheet is first electrochemically oxidized in a first step and a polymer coating is applied on the oxidized tin surface in a second step. 
     
     
         2 . Method for the production of a tinplate coated with a polymer coating, with the following steps:
 electrolytic deposition of a tin coating on one or both sides of a steel sheet,   electrochemical oxidation of the surface of the tin coating,   application of a polymer coating on the oxidized surface of the tin coating.   
     
     
         3 . Method according to  claim 2 , wherein the electrochemical oxidation of the tin surface takes place immediately, preferably within a few seconds, after the deposition of the tin coating on the steel sheet. 
     
     
         4 . Method according to  claim 2 , wherein the charge density on the tin surface after the electrochemical oxidation is at most 40 C/m 2 . 
     
     
         5 . Method according to  claim 2 , wherein the electrochemical oxidation of the tin surface is carried out by the anodic polarization of the tin-plated steel sheet in an aqueous and chromium-free electrolyte. 
     
     
         6 . Method according to  claim 2 , wherein the polymer coating is applied by laminating a polymer film on the chromium-free and oxidized tin surface of the steel sheet. 
     
     
         7 . Method according to  claim 6 , wherein the polymer coating is applied by laminating a co-extruded plastic film with a polymer layer and an adhesion promoter layer on the oxidized tin surface of the steel sheet, wherein the adhesion promoter layer faces the tin surface. 
     
     
         8 . Method according to  claim 6 , wherein the steel sheet is maintained, during the laminating on of the polymer coating, at temperatures above the melting temperature (T Sn ) of the tin coating. 
     
     
         9 . Method according to  claim 2 , wherein the polymer material of the polymer coating is a polyester, in particular polyethylene terephthalate (PET) or polypropylene (PP) or polyethylene (PE). 
     
     
         10 . Method according to  claim 9 , wherein the polymer coating is applied by laminating a film, in particular a biaxially oriented or amorphous cast-polyester film made of polyethylene terephthalate (PET), on the oxidized tin surface of the steel sheet. 
     
     
         11 . Tinplate coated with a polymer coating made of polyethylene terephthalate (PET) and passivated chromium-free, wherein only one thin tin oxide layer and optionally an adhesion promoter layer are present between the tin surface of the tinplate and the polymer coating. 
     
     
         12 . Tinplate according to  claim 11 , wherein the tin oxide layer is essentially made of divalent tin oxide (SnO 2 ) and preferably has a thickness of at most 0.1 μm, and in particular less than 0.01 μm. 
     
     
         13 . Tinplate according to  claim 11 , wherein an adhesion promoter layer is present between the tin oxide layer and the polymer coating. 
     
     
         14 . Tinplate according to  claim 13 , wherein the adhesion promoter layer contains glycol-modified polyethylene terephthalate (PETG), glycol-modified polycyclohexylenedimethylene terephthalate (PCTG), and/or isophthalic acid (IPA). 
     
     
         15 . Tinplate according to  claim 11 , wherein an antiblock layer, which, in particular, is made of silicon oxide, is present on the upper side of the polymer coating that is turned away from the tin-plated steel sheet. 
     
     
         16 . Tinplate according to  claim 11 , wherein the tinplate is made from a cold-rolled steel strip with a thickness of 0.05 to 0.50 mm from a low-carbon and unalloyed or low-alloy steel by coating with a tin coating in a layer of 0.5 to 12 g/m 2 . 
     
     
         17 . Packagings, in particular cans for food products for human consumption and pet food, packagings for chemical-technical goods, aerosol cans, beverage cans, or parts for such packagings, in particular closures, lashing belts, valve plates, can lids, or lid rings comprising the tinplate of  claim 11 . 
     
     
         18 . Apparatus for application of the method of  claim 1 , comprising:
 a transporting device for continuous transport of a continuous steel strip in a transporting direction, at a transporting speed which is preferably greater than 200 m/min,   a tin-plating device for galvanic coating of the steel strip with a tin coating, moved through the coating device at the transporting speed,   an oxidation device with an electrolysis bath, in which an aqueous, chromium-free electrolyte is contained, through which the tin-plated steel strip is conducted at the strip speed so as to electrochemically oxidize the tin surface,   a plastic coating device for application of a polymer coating on the tin surface of the steel strip, on one or both sides.   
     
     
         19 . Method according to  claim 1 , wherein the electrochemical oxidation of the tin surface takes place immediately, preferably within a few seconds, after the deposition of the tin coating on the steel sheet. 
     
     
         20 . Method according to  claim 1 , wherein the charge density on the tin surface after the electrochemical oxidation is at most 40 C/m 2 . 
     
     
         21 . Method according to  claim 1 , wherein the electrochemical oxidation of the tin surface is carried out by the anodic polarization of the tin-plated steel sheet in an aqueous and chromium-free electrolyte. 
     
     
         22 . Method according to  claim 1 , wherein the polymer coating is applied by laminating a polymer film on the chromium-free and oxidized tin surface of the steel sheet. 
     
     
         23 . Method according to  claim 22 , wherein the polymer coating is applied by laminating a co-extruded plastic film with a polymer layer and an adhesion promoter layer on the oxidized tin surface of the steel sheet, wherein the adhesion promoter layer faces the tin surface. 
     
     
         24 . Method according to  claim 22 , wherein the steel sheet is maintained, during the laminating on of the polymer coating, at temperatures above the melting temperature (T Sn ) of the tin coating. 
     
     
         25 . Method according to  claim 1 , wherein the polymer material of the polymer coating is a polyester, in particular polyethylene terephthalate (PET) or polypropylene (PP) or polyethylene (PE). 
     
     
         26 . Method according to  claim 25 , wherein the polymer coating is applied by laminating a film, in particular a biaxially oriented or amorphous cast-polyester film made of polyethylene terephthalate (PET), on the oxidized tin surface of the steel sheet.