METHOD FOR PRODUCING POLYMER COATED STEEL SHEET FOR 3-PIECE CANS AND USE THEREFOF

Abstract

A method for producing polymer coated steel sheet for 3-piece cans and 3-piece cans produced thereof.

Claims

1. A method for manufacturing polymer coated tinplate for 3-piece can bodies comprising the steps of: providing tinplate in the form of a strip; producing one or two polymer films by means of in-line extrusion; slitting the in-line extruded polymer film or films in the longitudinal direction into at least N wide polymer film strips where N is at least 2 and (N−1) narrow polymer film strips cut out from between the wide polymer film strips using slitting means; in-line trimming of the edges of the polymer film to ensure that the width of the polymer film is smaller than the width of the tinplate to enable the edges of the polymer coated tinplate to be free from polymer film; leading the narrow polymer film strips, and the cut-off edges, away from the wide polymer film strips by discharging means; conveying the separate two or more wide polymer film strips to a nip-roll assembly; preheating the tinplate and subsequently coating the two or more wide polymer film strips onto the preheated tinplate by means of the nip-roll assembly to obtain a polymer coated tinplate with the two or more wide polymer film strips separated spatially in the longitudinal direction by narrow strips free from said polymer film and wherein the edges of the polymer coated tinplate remain free from said polymer film; post-heating the polymer coated tinplate in a post-heat device to a temperature above the melting point of the polymer film or films, or if the polymer film is a multilayer system, to a temperature above the melting point of that polymer film layer in the multilayer system that has the highest melting temperature; quenching the post-heated polymer coated tinplate.

2. The method according to claim 1, wherein the tinplate is preheated to at least 190° C. prior to entering the nip-roll assembly.

3. The method according to claim 1, wherein the quenched polymer coated tinplate is slit into a plurality of strips wherein the slitting is performed in the longitudinal direction and in the narrow strips free from polymer film.

4. The method according to claim 1, wherein blanks for producing 3-piece can bodies are produced from the quenched polymer coated tinplate.

5. The method according to claim 1, wherein the polymer film comprises polyamides, polyolefins, polyesters, co-polyesters, or blends of polyesters.

6. The method according to claim 1, wherein the polymer film has a thickness of between 5 and 35 μm.

7. The method according to claim 1, wherein the polymer film or films produced by the extrusion process is essentially non-oriented.

8. The method according to claim 1, wherein the direction of travel of the extruded polymer film, the resulting wide polymer film strips and the tinplate is the machine direction of the extruder and nip-roll combination and wherein the direction of travel remains unchanged and parallel to the machine direction so that the width of the gap, that was made by removing the narrow polymer film strips from between the wide polymer film strips, is the same as the width of the polymer free narrow strips of the polymer coated tinplate.

9. The method according to claim 1, wherein the post-heat temperature is above the melting point of tin to achieve that the resulting tin layer will become alloyed with iron from the steel substrate to improve the adhesion of the tinplate layer to the steel substrate and the adhesion of the polymer to the tin or tin-alloy layer.

10. The method according to claim 9, wherein the post-heat temperature is at least 235° C.

11. The method according to claim 1, wherein the polymer film is not uniaxially or biaxially oriented and therefore will not shrink when heated by the preheated tinplate or during the lamination in the roll-nip assembly.

12. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The invention will now be further illustrated by reference to the following drawings.

[0038] FIG. 1 (not to scale) shows the tinplate 1, the polymer film 2, and the polymer film slit into four wide strips (3a-3d) and three narrow strips (4a-4c) which are removed so that only the wide strips (3a-3d) remain for further processing in the coating line.

[0039] FIG. 2 (not to scale) shows the tinplate coated with the four wide strips (8a-8d) which have been laminated onto the tinplate, post-heated and quenched. The polymer film strips (8a-8d) are in an amorphous state. Below a cross section is depicted over the line A-A showing the tinplate and the four wide strips (8a-8d) as well as the polymer free edges (10a-10b) and the polymer free narrow strips (9a-9c).

[0040] FIG. 3 (not to scale) shows schematically how the polymer coated tinplate could be slit lengthwise into four narrow polymer coated tinplate strips, and also (on the left hand side) how individual blanks could be produced from the polymer coated tinplate or the narrow polymer coated tinplate strips. These blanks have edges which are free from polymer, and are thus weldable to produce a 3-piece can body.

[0041] FIGS. 4A-4C (not to scale) show a cross section over the line A-A showing the polymer coated tinplate when coated on both sides with four wide strips. Note that in this drawing the wide polymer film strips are positioned symmetrically in relation to the centre line of the tinplate. In principle it is possible to have a degree of asymmetry in this positioning depending on the blank size, can body and body maker. The wide polymer strips need not have the same width on both sides. FIG. 4B shows that 8a is wider than 11a, etc. so that the polymer free parts depicted in FIG. 2 and indicated with 9a-9c are different in width on either side of the strip. Also 8a and 11a may have the same width. But they may be shifted in respect to the other (FIG. 4C).

[0042] FIG. 5 shows an extruder 12 which extrudes a molten polymer onto a cooled guide roll 13 to form an extruded polymer film 2. This film is subsequently transported to cutting means 5 for slitting the extruded polymer film 2 into wide polymer film strips 3a-3d and narrow polymer film strips 4a-4c and optionally the edges 4d, 4e of the extruded polymer film. These narrow polymer film strips and optionally the edges are led away and removed by means of the cutting waste extraction device 6. The wide polymer film strips 3a-3d travel further to the two rolls 7 who are positioned such that a nip exists between the two rolls through which a preheated tinplate 1 is led along with the narrow polymer film strips. The tinplate is preheated in heating device 14 and by a pressing action of the two rolls the wide polymer film strips 3a-3d are laminated onto the tinplate and fixedly bonded thereto by this film lamination process. The polymer coated tinplate is led to a post-heat device 15 where the polymer coated tinplate is heated to above at least the melting point of the polymer film or films, or if the polymer film is a multilayer system, to a temperature above the melting point of that polymer film layer in the multilayer system that has the highest melting temperature, and subsequently quenched in quenching device 16. After quenching the polymer coated tinplate is coiled and ready for further processing into 3-piece can bodies. The heating device 14 works on the basis of induction, heated guide rolls, hot air or otherwise.

[0043] The device depicted in FIG. 5 is a one-sided lamination. However, the device can be easily made symmetrical so that polymer coating on both sides can occur simultaneously. It is noted that the device in FIG. 5 is a schematic drawing, and that the dimensions are not intended to be accurate. For instance, the cutting means 5 are positioned close to the lamination roll, and the distance the wide polymer strips travel after removing the narrow polymer strips is as short as possible to prevent dimensional inaccuracies. As soon as the narrow strips and optional edge cuts 4a-4e are cut they are led away to the cutting waste extraction device 6 to prevent any disturbance of the lamination process.

[0044] FIG. 6 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.

EXAMPLES

[0045] Example 1: A tinplate 1 with a thickness of 0.20 mm is heated to a temperature of 230° C. by means of heating device 14. A polymer film (PET) is extruded via nozzle 12, on a guide roll 13. The cooled extruded polymer film 2 is then conveyed to the preferably rubber coated contact roll 7. As it travels it is possible to monitor the thickness, colour and strip tension and to trim to the correct width. Cutting devices 5 slit the film into wide polymer film strips 3a-3d and narrow polymer film strips 4a-4c as cut of the edges 4d, 4e, which are removed by suction through 6. The thickness of the extruded polymer film is around 30 μm. The polymer film strips are coated onto the preheated tinplate by pressing the polymer onto the tinplate in the nip between the two rolls 7. The rubber of these rolls is cooled externally, for example by metal cooling roll, or by an air-blade on the rubber surface. The coated strip is then subjected to a brief extra heat treatment to 260° C. in order to optimise adhesion. A good product results, particularly suitable for example for the covers of three-piece cans.

[0046] Example 2: An ETP strip was coated with 2.0 g/m.sup.2 tin on both sides and subjected to 311 passivation treatment. The strip was coated with a polymer coating which will become the internal coating of the can body and another coating on the other side (external coating). The internal coating consisted of a 4 μm adhesion layer, a 12 μm middle layer and a 4 μm top layer, all three layers consisting of blends of PET or co-polymers of PET. The external coating consisted of a 4 μm adhesion layer, a 12 μm middle layer and a 4 μm top layer, all three layers consisting of blends of PET and PBT. During the coating process 2 narrow polymer film strips with a width of 5 mm were removed and the edges were removed so that 10 mm edges of the tinplate were bare. Three polymer coated strips of ETP with a width of 310 mm were obtained. From these strips can bodies can be produced.

[0047] Typical recipes for polymer films consisting of three sublayers for use in the process according to the invention are given below. In the Examples below, five different types of polyester resin were used to produce different types of polyester sublayers: [0048] IPA-PET: poly(ethylene terephthalate) copolymer in which about 3 mole % of terephthalic acid monomer units has been replaced with isophthalic acid monomer units [0049] PETg: poly(ethylene terephthalate) copolymer in which about 30 mole % of ethylene glycol monomer units has been replaced with cyclohexane-dimethanol monomer units [0050] PBT: poly(butylene terephthalate) homopolymer [0051] TiO.sub.2 MB: a 50/50 weight % mixture of TiO.sub.2 and CHDM-PET

TABLE-US-00001 TABLE 1 Polyester film recipes (typical dimensions are 4:12:4 μm, total 20 μm) Main layer or Code Adhesion layer barrier layer Top layer B 70% PETg 100% IPA-PET 100% IPA-PET 30% IPA-PET C 70% PETg 67% IPA-PET 100% IPA-PET 30% IPA-PET 33% TiO2 MB D 100% IPA-PET 67% IPA-PET 70% PETg 33% TiO2 MB 30% IPA-PET E 75% IPA-PET 75% IPA-PET 75% IPA-PET 25% PBT 25% PBT 25% PBT

[0052] Another example of a polymer film is comprises, or consists only of an inner layer comprising a combination of PET and modified PET (IPA-PET), as adhesion layer, a layer consisting of a blend and/or copolymer of PET and PBT as a main layer or barrier layer and an outer (top) layer comprising PET, modified PET or a PET: PBT blend.

[0053] Known blends of PET and PBT can be used. Ratios of 25:70 PBT and 30:75% of PET are common nowadays.

[0054] To the expert it will be clear that the invention can be applied for single-side or two-side coating of a metallic substrate with on each side the same polymer layer, or a different polymer layer for example coating system A on one side and coating system E on the other side.