METHOD FOR MANUFACTURING LAMINATED TINPLATE, A LAMINATED TINPLATE PRODUCED THEREBY AND USE THEREOF

Abstract

A method for manufacturing a laminated tinplate for packaging applications, the laminated tinplate including a tinplate sheet and a thermoplastic laminate layer that covers at least one side of the tinplate steel sheet, to a laminated tinplate produced thereby and use thereof in a process to produce containers for packaging purposes.

Claims

1. A method for manufacturing a laminated tinplate for packaging applications, the laminated tinplate comprising a tinplate sheet and a thermoplastic laminate layer that covers at least one side of the tinplate sheet, the laminate layer consisting of a single layer or a plurality of layers wherein each layer contains a thermoplastic aromatic (co)polyester or blend thereof or a polyolefine comprising at least 90% by mole of a propylene unit, the method comprising the subsequent steps of: producing a tinplate sheet by providing a cold-rolled steel sheet with a tin layer on one or both sides by means of electroplating; optionally heat treating the tinplate sheet by annealing above the melting temperature of tin; optionally providing the heat-treated tinplate sheet with a further tin layer on one or both sides by means of electroplating; subjecting the tinplate sheet to a surface treatment by dipping the tinplate sheet in an aqueous solution having a pH of from 8 to 12 comprising phosphate, borate, sulphate or carbonate ions or a combination thereof, and optionally simultaneously applying a cathodic current onto the tinplate; rinsing and drying the tinplate sheet; optionally applying a chromium-free, no-rinse, dry-in-place passivation treatment solution to the tinplate sheet; drying of the passivated tinplate sheet; optionally coiling the tinplate sheet for storage or transport and uncoiling for further processing; providing the thermoplastic laminate layer for coating onto at least one side of the tinplate sheet; pre-heating the tinplate sheet and laminating the thermoplastic laminate layer onto the pre-heated tinplate sheet to produce the laminated tinplate; post-heating the laminated tinplate to a temperature sufficiently high to melt the laminate layer; cooling the post-heated laminated tinplate.

2. The method according to claim 1 wherein the thermoplastic laminate layer is provided by: providing a pre-produced mono-axially or bi-axially oriented thermoplastic laminate layer, or melting thermoplastic polymer granules in one or more extruders to form the one or more layers and forming the thermoplastic laminate layer consisting of the one or more layers by passing the molten polymer or polymers through a flat (co-)extrusion die and/or two or more calendering rolls; followed by: A. cooling the thermoplastic laminate layer to form a solid thermoplastic laminate layer; optionally trimming the edges of the thermoplastic laminate layer; reducing the thickness of the solid thermoplastic laminate layer by stretching the solid thermoplastic laminate layer in a stretching unit by exerting a stretching force only in the longitudinal direction; optionally trimming the edges of the stretched thermoplastic laminate layer; laminating of the laminate layer onto the pre-heated tinplate sheet; or followed by B. drawing the extruded thermoplastic laminate layer between the flat (co-)extrusion die and a cast roll, and cast at its final desired thickness on the cast roll to rapidly cool the drawn thermoplastic layer wherein the cast & cooled thermoplastic laminate layer is essentially non-oriented; optionally trimming the edges of the cast & cooled thermoplastic laminate layer; in-line laminating the cast & cooled thermoplastic laminate layer onto the pre-heated tinplate sheet.

3. The method according to claim 1, wherein the laminate layer is formed at least on the side that becomes the inside of a packaging, such as a container or can, and the polyester in one or more of the layers in the laminate layer contains at least 80 mol. %, and preferably 85 mol. % of an ethylene terephthalate unit or wherein the polyester is a co-polyester comprising at most 35 mol. % of CHDM or 20 mol. % of IPA.

4. The method according to claim 1, wherein the laminate layer is formed at least on the side that becomes the inside of a packaging, such as a container or can, and the polyester in one or more of the layers in the laminate layer contains at least 80 mol. %, and preferably 85 mol. % of a butylene terephthalate unit.

5. The method according to claim 1, wherein the laminate layer is formed at least on the side that becomes the inside of a packaging, such as a container or can, and the polyester in one or more of the laminate layer contains a blend of a polyester containing 85% by mole of an ethylene terephthalate unit and a polyester containing at least 85% by mole of a butylene terephthalate unit.

6. The method according to claim 1, wherein the laminate layer or layers on at least one of the sides of a packaging such as a container or can, comprises one or more polypropylene layers consisting essentially of polypropylene.

7. The laminated tinplate method according to claim 1, wherein the laminate layer or layers comprises an adhesion layer consisting of a maleic anhydride grafted polypropylene.

8. The method according to claim 1, wherein the laminated tinplate is subjected to a stretching operation wherein the stretching operation is achieved by: passing the material through a temper mill and applying a thickness reduction between 0-3%, preferably of at least 0.2%; or by passing the material through a stretcher-leveller.

9. The method according to claim 1, wherein the laminate layer on the one or both sides of the passivated tinplate sheet is a multi-layer coating system, said coating system comprising at least an adhesion layer for adhering to the passivated tinplate sheet, a surface layer and a bulk layer between the adhesion layer and the surface layer.

10. The method according to claim 1, wherein the cast & cooled thermoplastic laminate layer is slit in the longitudinal direction into at least N wide laminate layers, where N is at least 2 and (N−1) narrow strips using slitting means, followed by leading the narrow strips away from the wide laminate layers by discharging means and subsequently coating the wide laminate layers onto the pre-heated tinplate by means of a nip-roll assembly to obtain a laminated tinplate with a plurality of wide laminate layers separated spatially in the longitudinal direction by narrow strips free from said wide laminate layer and wherein the edges of the tinplate remain free from said wide laminate layer, followed by the post-heating of the laminated tinplate and the cooling of the post-heated laminated tinplate.

11. The method according to claim 1, wherein the laminate layer has a thickness of between 5 and 35 μm.

12. The laminated tinplate obtained by the method process according to claim 1.

13. The laminated tinplate for producing blanks for 3-piece cans.

14. A method of use of the laminated tinplate of claim 12, comprising producing from the laminated tinplate cans or can parts for packaging purposes.

15. A method of use of the laminated tinplate of claim 13, comprising producing from the laminated tinplate cans or can parts for packaging purposes.

16. The laminated tinplate obtained by the method according to claim 2.

Description

[0058] The invention will now be explained by means of the following, non-limiting figures.

[0059] FIG. 1 shows the definition of some terms.

[0060] FIG. 2 shows a schematic depiction of the solid film lamination process.

[0061] FIG. 3 shows a schematic depiction of the cast film lamination process.

[0062] FIGS. 4 to 9 show XPS profiles of laminated tinplate after separation of the laminate layer from the tinplate to study the nature of the bond between the laminate layer and the tinplate.

[0063] FIGS. 10 to 12 show the production stages of laminated tinplate for 3-piece cans.

[0064] In FIG. 2 the tinplate sheet or strip (1) is passed through first heating device (2) where temperature of the tinplate is raised to pre-heat temperature suitable for lamination, T1. Two coils of laminate layer (3a, 3b) are simultaneously unwound and passed, together with the pre-heated tinplate, through a roll-nip assembly comprising a pair of laminating rollers (4a, 4b). The laminated tinplate (5) is passed through a second heating device (6) where the temperature of the laminated tinplate is raised to a post-heat set-point, T2. After the second heating device, the laminated tinplate is immediately cooled by passing through a quenching device (7) to reach room temperature. The method of pre-heating the tinplate in the first heating device is not particularly limited and may include passing the strip over heated rolls, conductive heating, inductive heating, radiative heating, etc. The method of post-heating the laminated tinplate in the second heating device is preferably a contactless method, such as heating in a hot gas environment or inductive heating. The method of immediate cooling in the quenching device is not particularly limited and may include applying cold air or passing through a cold water bath etc. In FIG. 2 the laminate layers are provided on a coil. However, the laminate layers may also be provided directly from the extrusion die after having been stretched and cooled to a solid and stretched thermoplastic laminate layer.

[0065] In FIG. 3 the laminate layer is extruded from a flat die (14), drawn down in a narrow gap formed between the extrusion die and the cast roll, and cast at its final desired thickness on the cast roll (13) where it is rapidly cooled. Since the draw down to the final thickness takes place in the liquid condition, the cast laminate layer is essentially non-oriented. The laminate layer can then be laminated onto the tinplate 1 in a similar way to the process depicted in FIG. 2.

[0066] To produce material for 3-piece cans the extruded laminated layer is slit (11) and the narrow polymer strips (10a-10d) in between the wide laminate layers (9a-9d) are led away and removed. The number of wide laminate layers (9a-9d) produced from the extruded polymer film can be 2 or more. In the explanatory figures a number of four wide laminate layers (9a-9d) is used by means of example, but the invention works just as well with two, three or more wide polymer films. The number of narrow polymer strips (10) cut out from between the wide laminate layers (9a-9d) to be discharged is in principle always 1 lower than the number of wide laminate layers to be laminated onto the tinplate. The width of the extruded laminate layer (3) should be smaller than that of the tinplate to allow the edges of the tinplate to remain uncoated. If the polymer film becomes too wide for the edges of the tinplate to remain uncoated (i.e. bare) then an in-line trimming of the edges of the polymer film may be needed. These cut-off edges are led away from the laminate layer and the laminate layer is coated onto the tinplate in the lamination process leaving the outermost edges of the tinplate bare from polymer. This is preferable over the alternative namely to coat the edges and grind off or otherwise remove the edges of the polymer coating after the lamination process. The leading away of the cut-off edges of the polymer film can be done by a cutting waste extraction device means such as by a sucking device (12).

[0067] FIG. 10 shows the tinplate 1 in a top view (not to scale) as well as the extruded and cooled laminate layer 3. The slit situation is depicted in the bottom drawing of FIG. 10 where the small strips to be removed are hatched (10a-10c) and the wide laminate layers to be laminated onto the tinplate with 9a-9d. FIG. 11 shows a top view of the laminated tinplate, where the bare strips and bare edges are shown. These bare edges and bare strips are needed for forming three-piece can bodies which are welded together. FIG. 11 also shows schematically (top drawing left hand side, dashed lines) how the laminated tinplate could be slit lengthwise into four narrow laminated tinplate strips, and also how individual blanks for 3-piece cans could be produced. Each of these blanks have edges which are free from polymer, and are thus weldable to produce a 3-piece can body (see FIG. 13).

[0068] The bottom picture in FIG. 11 shows a cross section along A-A. FIG. 12 shows the same where laminate layers are provided on both sides of the tinplate. FIG. 13 shows a cross-section of a welded 3-piece can body and the left hand side of the figure shows an enlarged portion of the welded portion. The bare tinplate edges are clearly shown as well as the edges of the wide polymer film strip 3a and the portion where the two bare edges are bonded together by welding. The welded and bare metal is subsequently covered with a lacquer 17 to protect the metal against corrosion. The lacquer is preferably BPA-free.