Method for manufacturing a laminated tinplate, laminated tinplate produced thereby and use thereof

Abstract

The present invention relates to a process for manufacturing a laminated tinplate for packaging applications, said laminated tinplate comprising a tinplate sheet and a thermoplastic lamination layer covering at least one side of said tinplate sheet, to the laminated tinplate produced thereby and to its use in a process for producing containers for packaging purposes.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method of manufacturing a laminated tinplate for packaging applications, said laminated tinplate comprising a tinplate sheet and a thermoplastic lamination layer covering at least one side of said tinplate sheet, to the laminated tinplate produced thereby and to its use in a method of producing containers for packaging purposes. BACKGROUND

[0003] Tin rolled products traditionally include electrolytic tin plated steel, electrolytic chromium plated steel (also known as tin free steel or TFS) and blackplate. Although not being limited thereto, most applications of tin rolled products are used by the container industry in the manufacture of cans, ends and closures for the food and beverage industry.

[0004] The use of polymer coated substrates in the packaging industry is becoming more and more common in the production of cans and can components. Polymer coated substrates can be produced by either extruding a molten polymer film directly onto a metallic substrate or by producing a thermoplastic polymer film which is subsequently laminated as a solid film onto a metallic substrate in an integrated or separate lamination process step. The thermoplastic polymers which are most suitable for this process are polyesters such as PET and polyolefins such as PE or PP.

[0005] The metallic substrate material used in polymer coated packaging steels is mostly electrolytic chromium coated steel (ECCS), sometimes also referred to as tin free steel (TFS), which is a cold rolled steel electrochemically coated with very thin layers of chromium and chromium oxide. The chromium / chromium oxide layer formed on the steel surface in this electrolytic process provides an excellent surface layer for the adhesion of organic coatings such as thermoplastic polymer coatings. Polymer coated packaging steels based on ECCS substrates therefore exhibit excellent adhesion and can making properties. Similarly, in a recent development described as trivalent chromium coated steel an electrochemical application of very thin layers of chromium and chromium oxide on top of a cold rolled steel provides a polymer coated steel with excellent properties.

[0006] However, the use of chromium coated steels in packaging applications has some drawbacks such as lack of weldability and poor corrosion resistance in some media, especially acidic (food) media. Tin plated steel, which is a steel substrate provided with a tin layer on either side, is both corrosion resistant and weldable and therefore represents a suitable alternative substrate material when these properties are important. Although the adhesion between the polymer coating and the tin plated steel surface is not as good as the adhesion between the polymer coating and the chromium / chromium oxide surface, a high level of adhesion can still be achieved and a product with good can making properties can be obtained.

[0007] Tin-plated steel is a light gauge steel strip coated with tin on both surfaces. The tin is usually applied by electrodeposition. Tin-plated steel can be provided with the same thickness of tin on both sides or with different thicknesses (different coatings). Tin-plated steel can be used in the as-produced state or it can undergo a heat treatment above the melting temperature of tin, for example by induction or resistance heating, so-called reflow, to enhance the corrosion resistance of the product by forming an inert FeSn2-alloy layer at the interface between the steel substrate and the tin layer. A particular type of heat treated tin-plated steel is provided with a FeSn (50 atomic % iron and 50 atomic % tin) alloy layer as disclosed in WO2012045791-A1. This is achieved by diffusion annealing the tin-plated steel containing up to 1000 mg / m 2 and preferably between at least 100 and / or at most 600 mg / m 2 of deposited tin at a temperature between 513 °C and 625 °C in a reducing atmosphere. The tin layer is converted at said temperature into an iron-tin alloy consisting of FeSn in a 1 : 1 ratio Fe:Sn. The FeSn layer can be coated with an additional tin layer. This reduces the total amount of tin although two tin layers are deposited.

[0008] An important aspect of the surface of tin-plated steel is that it is very unstable to tin oxide growth and therefore needs to be passivated. If the surface of tin-plated steel is not passivated, an oxide layer can form on its surface and this layer will continue to grow in thickness during storage depending on the storage conditions. The tin oxide layer imparts a yellowish appearance to the product and will result in adhesion loss once an organic coating such as a lacquer or a polymer coating is applied to the surface. For many years, the most common passivation treatment of tin-plated steel was based on the use of a chromate solution, i.e. a solution containing hexavalent chromium, to which the freshly tin-plated steel strip was exposed using an immersion or electrolysis assisted process (so-called "300" and "311" passivation treatments, respectively). However, hexavalent chromium is currently considered to be a toxic substance that can be harmful to the environment and poses a safety risk to workers.

[0009] Object of the invention

[0010] It is an object of the present invention to provide a method of manufacturing a laminated tin-plated steel.

[0011] It is a further object to provide a method of manufacturing a laminated tin-plated steel that does not involve hexavalent chromium technology.

[0012] It is still a further object to provide a method of manufacturing a laminated tin-plated steel that does not use hexavalent chromium, is weldable and has excellent adhesion between the laminated layer and the tin-plated steel.

[0013] It is also an object to provide a laminated tin-plated steel produced without the use of hexavalent chromium technology that is weldable and has excellent adhesion between the laminated layer and the tin-plated steel and that is suitable for the manufacture of metal cans and metal can components.

[0014] Description of the invention

[0015] One or more objectives are achieved using a method for manufacturing a laminated tin-plated steel for packaging applications, the laminated tin-plated steel comprising a tin-plated steel sheet and a thermoplastic laminate covering at least one side of the tin-plated steel sheet, the laminate consisting of a single layer or multiple layers, wherein each layer contains a thermoplastic aromatic (co)polyester or a blend thereof or a polyolefin containing at least 90 mol% propylene units, the method comprising the following steps:

[0016] Tin-plated steel sheets are produced by electroplating a tin layer onto one or both sides of cold-rolled steel sheets.

[0017] • Optionally, the tin-plated steel sheet is heat-treated by annealing at a temperature greater than that of tin;

[0018] •Optionally, an additional tin layer may be applied to one or both sides of the heat-treated tin-plated steel sheet by electroplating;

[0019] • The tin-plated steel sheet is surface-treated by immersing it in an aqueous solution containing phosphate ions, borate ions, sulfate ions, or carbonate ions, or combinations thereof, with a pH of 8-12.

[0020] • Rinse and dry the tin-plated steel sheet;

[0021] • Optionally apply a chromium-free, non-rinsing, in-situ drying passivation solution to the tin-plated steel sheet;

[0022] • Dry and passivated tin-plated steel sheets;

[0023] • Optionally, the tin-plated steel sheet can be wound for storage or transport and unwound for further processing;

[0024] • The thermoplastic laminate is provided for coating at least one side of the tin-plated steel sheet;

[0025] • Preheat the tin-plated steel sheet and stack the thermoplastic laminate onto the preheated tin-plated steel sheet to produce laminated tin-plated steel;

[0026] • The laminated tin-plated steel is then heated to a temperature high enough to melt the laminated layers;

[0027] • Cooling and then reheating the laminated tin-plated steel.

[0028] In the context of this invention, tin-plated steel is defined as a steel substrate comprising a tin layer, the laminate being a polymer coating to be laminated onto the tin-plated steel, and tin-plated steel having a laminated layer on the tin-plated steel is referred to as laminated tin-plated steel (see [link to documentation]). Figure 9 ac).

[0029] The method according to the present application provides a method which provides a tin plated steel sheet which can be provided in the form of a tin plated steel strip and provides a laminating layer to the tin plated steel sheet. The surface condition of the tin plated steel is critical in the adhesion of the laminating layer and the method according to the present application ensures that the surface of the tin plated steel is suitable for thermal bonding with the laminating layer. The way the surface of the tin plated steel is made suitable is by immersing the tin plated steel in an aqueous solution which can be a buffer solution, or by applying a cathodic current to the tin plated steel while immersed, and optionally by additionally applying a chromium-free, non-rinsing, in-situ drying passivation treatment solution to the tin plated steel sheet. This results in a clean and receptive surface from which any contaminants and tin oxides have been removed to a certain extent, so that the adhesion after thermal bonding is of the same quality as the adhesion of prior art tin plated steels passivated with hexavalent chromium. If the storage conditions of the tin plated steel are such that essentially no tin oxide growth occurs, it can be sufficient to treat the surface of the tin plated steel sheet by immersing it in an aqueous solution comprising phosphate ions, borate ions, sulfate ions or carbonate ions or a combination thereof having a pH of 8-12, wherein optionally at the same time a cathodic current is applied to the tin plated steel, and it can not be necessary to additionally apply a chromium-free, non-rinsing, in-situ drying passivation treatment solution.

[0030] Preferably, the pH of the aqueous solution is not lower than 8.5 and / or not higher than 11.5. A suitable maximum pH value is 11 or even 10.5. The aqueous solution preferably contains cations from group 1 (e.g. Na + , K + ) or group 2 (e.g. Mg 2+ , Ca 2+ ) of the periodic table or polyatomic cations (e.g. NH4 + ) and polyatomic anions (phosphate, borate, sulfate, carbonate etc.). Additionally, the anion can be the conjugate base of an organic acid (e.g. acetate, citrate). Furthermore, the electrolyte can contain other chemical additives such as surfactants, wetting agents, antifoams etc. to support the electrochemical treatment.

[0031] Preferably the aqueous solution contains only carbonate as anion, preferably added to the aqueous solution as sodium carbonate, and preferably no borate, phosphate, sulfate etc. By way of non-limiting example, a solution of sodium carbonate decahydrate having a pH between 9.5 and 10 containing not more than 1 g / l Na2CO3.10H2O in deionized water is suggested.

[0032] However, if these storage conditions of the tin plated steel are less favorable due to humidity, storage temperature, duration of storage etc., the surface of the tin plated steel sheet can be passivated without electrolytic deposition by additionally applying a chromium-free, non-rinsing, in-situ drying passivation treatment solution to the tin plated steel sheet.

[0033] The passivation treatment solution can be based on zirconium, titanium, a combination of zirconium and titanium, phosphates, for example the acidic aqueous solution containing water-soluble inorganic compounds of the elements Zr, Ti, Hf as disclosed in US10011915. Examples are M-NT1455, Bonderite M-NT1456 and Bonderite M-NT10456 (Henkel) or Z801 (AD Chemicals). In the context of the present invention, the passivation treatment solution is not a silane or siloxane based solution, as these do not improve the adhesion between the tin plated steel and the thermoplastic layer. So according to the present invention, the passivation treatment solution is silane free, siloxane free and not Si based.

[0034] The advantage of the no-rinse, in-situ drying system over the electrolytic system is that the solution is easy to apply, simple equipment is used in a compact application unit, allowing easy fit on existing production lines, and more versatile chemicals are available. The passivation treatment solution can be applied to the surface treated tin plated steel surface by application techniques that are usual for such passivation systems. Suitable application techniques include: dipping, dipping with a squeeze roll, rotor jet application, rotor jet application supported by the use of a finishing roll, spray application, spray-squeeze application, application by a roll coater system, application by slot coating, slot curtain coating and the like.

[0035] The surface treated and optionally passivated tin plated steel can be wound for storage and transport and later unwound, or it can be immediately transferred to a lamination device where the laminate is laminated in-line to the pre-heated tin plated steel.

[0036] The present invention is also embodied in a method, wherein a thermoplastic laminate is provided by:

[0037] • providing a pre-produced mono- or biaxially oriented thermoplastic laminate, or

[0038] • melting thermoplastic polymer pellets in one or more extruders to form one or more layers and forming a thermoplastic laminate consisting of said one or more layers by passing the molten polymer(s) through a flat (co-)extrusion die and / or two or more calender rolls; after which:

[0039] A.

[0040] • cooling the thermoplastic laminate to form a solid thermoplastic laminate;

[0041] • optionally finishing the edges of the thermoplastic laminate;

[0042] • reducing the thickness of the solid thermoplastic laminate by stretching the solid thermoplastic laminate in a stretching device by applying a stretching force only in the longitudinal direction;

[0043] • optionally trimming the edges of the extruded thermoplastic laminate;

[0044] • laminating the laminate to a preheated tinplate sheet;

[0045] or thereafter

[0046] B.

[0047] • drawing the extruded thermoplastic laminate between a flat (co)extrusion die and a casting roll and casting to its final desired thickness on the casting roll to rapidly cool the drawn thermoplastic laminate, wherein the cast and cooled thermoplastic laminate is substantially free of orientation;

[0048] • optionally trimming the edges of the cast and cooled thermoplastic laminate;

[0049] • laminating the cast and cooled thermoplastic laminate in-line to a preheated tinplate sheet.

[0050] The method of application of the laminate to the tinplate is preferably by extrusion coating and lamination, wherein the polymer is melted and shaped into a thin hot film in a flat (co)extrusion die, wherein the extruded polymer film is subsequently directed onto a casting or cooling roll and then laminated to a preheated tinplate substrate to form a laminated tinplate. The laminated tinplate is then typically passed through a roll-nip assembly which firmly presses the laminate against the substrate to ensure complete contact and adhesion. The preheating temperature of the tinplate must be high enough to promote adhesion of the laminate to the tinplate, but not so high as to cause the laminate to stick to the equipment or cause degradation of the laminate. The optimum preheating temperature is therefore dependent on the combination of the laminate and the lamination equipment.

[0051] An alternative to extrusion coating and lamination is film lamination, wherein a solid laminate is supplied and coated onto a heated tinplate and pressed against the tinplate by a roll-nip assembly to ensure complete contact and adhesion of the laminate to the preheated tinplate. This solid laminate can be pre-produced and even taken from an external supplier, or it can be produced on-site and then laminated to the tinplate sheet.

[0052] In both cases, after the laminate is laminated to the tinplate in the roll-nip assembly, the laminated tinplate is post-heated in a post-heating device to a temperature greater than the melting point of the one or more laminates, or if the laminates are composed of different polymers, to a temperature greater than the melting point of the laminate with the highest melting temperature in the multi-layer system. The purpose of the post-heating is to reduce or eliminate any residual orientation in the laminate.

[0053] After this post-heating, the laminated tin-plated steel is immediately cooled to a temperature low enough to suppress crystallization as much as possible, preferably to suppress crystallization completely, with a cooling rate high enough. Water quenching is appropriate and often used. A quenching temperature of less than 50°C is a good rule for most polyesters. Below the glass transition temperature (Tg), the polymer chains are no longer mobile. The value of 50°C is less than the glass transition temperature of most aromatic (co)polyesters. Polyolefins have a much lower Tg, even less than 0°C, so here the problem is to suppress crystallization as much as possible and in particular to avoid the growth of large (spherical) crystals. The cooling rate achieved in such a quenching is not particularly critical, as long as it is fast enough, and a suitable value lies between about 50 and 300°C / s, for example about 100°C / s. The pre- and post-heating temperatures and the cooling rate and cooling temperature required depend on the type of polymer used and can be easily determined on the basis of the above. The post-heating temperature is preferably at least 235°C.

[0054] As laminating layer, the invention can use a pre-produced biaxially or uniaxially oriented polymer film as claimed in claim 2, an in-line cast and stretched uniaxially oriented polymer film, or an in-line cast and drawn un-stretched polymer film.

[0055] The polyester in the laminating layer is a thermoplastic aromatic (co)polyester or a blend thereof. In particular, reference is made here to polyethylene terephthalate (PET), IPA-modified polyethylene terephthalate (IP A-PET), CHDM-modified polyethylene terephthalate (PETg), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), or a copolymer or blend thereof.

[0056] The polypropylene in the laminating layer is selected from the group consisting of polypropylene, polypropylene copolymers, chemically modified polyolefins such as maleic anhydride grafted polypropylene. The latter are mainly used as adhesion layer. Polypropylene is mainly used as bulk layer.

[0057] "In-line" in the context of the present invention is to be understood as forming an integrated part of a continuous sequence of operations. Thus, in the in-line production of the laminating layer and the laminated tin-plated steel, the production of the laminating layer and the coating of the laminating layer onto the pre-heated tin-plated steel is carried out in a continuous and uninterrupted sequence of operations.

[0058] The invention is also embodied in a laminated tin-plated steel, wherein the laminating layer is formed on at least one side which becomes the inside of a package, such as a container or a can, and the polyester in one or more of the layers of the laminating layer contains at least 80 mol% and preferably 85 mol% of ethylene terephthalate units or wherein the polyester is a copolyester containing at most 35 mol% of CHDM or 20 mol% of IPA.

[0059] The invention is also embodied in a laminated tinplate where the laminate is formed on at least one side which becomes the inside of a package such as a container or can and the polyester in one or more of the layers of the laminate contains at least 80 mole % and preferably 85 mole % of butylene terephthalate units.

[0060] The invention is also embodied in a laminated tinplate where the laminate is formed on at least one side which becomes the inside of a package such as a container or can and the polyester in one or more of the layers of the laminate contains a blend of a polyester containing 85 mole % of ethylene terephthalate units and a polyester containing at least 85 mole % of butylene terephthalate units.

[0061] The invention is also embodied in a laminated tinplate where one or more of the laminate layers on at least one side of a package such as a container or can comprises one or more polypropylene layers consisting essentially of polypropylene.

[0062] The invention is also embodied in a laminated tinplate where one or more of the laminate layers comprises an adhesion layer consisting of maleic anhydride grafted polypropylene.

[0063] In another embodiment of the invention the laminated tinplate is subjected to a stretching operation where the stretching operation is achieved by:

[0064] • passing the material through a temper mill and applying a thickness reduction of between 0 and 3 %, preferably at least 0.2 %; or

[0065] • passing the material through a stretch leveler.

[0066] In order to achieve a tight bond between the laminate layer and the tinplate it is necessary to use elevated temperatures and / or heat treatments such as pre-heating of the substrate and post heating of the laminated tinplate. These heat treatments can negatively affect the overall mechanical properties of the steel substrate due to ageing effects. By tempering the coated polymeric steel substrate by a small degree (i.e. between 0 and 3 %, preferably at least 0.2 %, more preferably at least 0.5 %) or passing the material through a stretch leveler. Such treatment improves the overall mechanical, can improve the strip shape and such material conditioning method can also be used to modify the surface texture. The inventors have found that the development of stress cracking in coated polymeric steel substrates for packaging applications is directly related to the mechanical behaviour of the substrate. There is a strong correlation between the areas where stress cracking occurs and the substrate shows inhomogeneous local deformation (Lueders lines) due to discontinuous yielding phenomena. By tempering or stretch levelling the coated polymeric substrate these discontinuous yielding phenomena are suppressed.

[0067] The invention is also embodied in a method where the laminate on one or both sides of a passivated tinplate sheet is a multi-layer 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.

[0068] The method according to the present application for producing an online cast and drawn unstretched polymer film is particularly suitable for producing a laminated tinplate for 3-piece cans as described in WO2019110616-A1.

[0069] The present application is also embodied in a preferred embodiment, wherein the cast and cooled thermoplastic laminate layer is cut into at least N wide laminate layers (9a-9d) and (N-1) narrow bands (10a-10c) in the longitudinal direction using a cutting device (11) (where N is at least 2), after which the narrow bands are guided away from the wide laminate layers by an expelling device (12) and subsequently the wide laminate layers are applied to the preheated tinplate by a pressure roller assembly (4a, 4b) to obtain a laminated tinplate having a plurality of wide laminate layers (9a-9d) spaced apart in the longitudinal direction by narrow bands (10a-10c) without said wide laminate layers, and wherein the edges of the tinplate remain without said wide laminate layers, after which the laminated tinplate is post-heated and the post-heated laminated tinplate is cooled.

[0070] This embodiment results in a laminated tinplate coated with narrow bands of thermoplastic laminate produced from a single wider thermoplastic laminate layer produced by in-line extrusion. Between the narrow bands there are very narrow bands of un-laminated tinplate and the laminated tinplate so produced is particularly suitable for producing blanks for 3-piece cans.

[0071] In an embodiment, the laminate layer has a thickness between 5 and 35 pm.

[0072] In the method according to the present application the laminate layer is defined by its main polymeric component. However, in addition to the polymeric component, additives such as antioxidants, heat stabilizers, UV absorbers, plasticizers, pigments, nucleating agents, antistatic agents, mold release agents, antiblocking agents, etc. can be present in the polymer.

[0073] Copolyesters are produced when more than one diacid or diol is used in the polymerization process. When ethylene glycol (EG) and up to 35% cyclohexanedimethanol (CHDM) are used together, a copolyester known as glycol-modified polyethylene terephthalate (PETg) is produced. When up to 20 mol% of terephthalic acid is replaced by isophthalic acid (IPA), the result is an IPA-PET copolyester.

[0074] Preferably, the steel used for the tin-plated steel steel substrate is a carbon steel, preferably a low carbon steel, an extra low carbon steel, an ultra low carbon steel or a HSLA steel. The thickness of the steel substrate is typically between 0.10 and 0.49 mm. These unalloyed (ULC, LC and ELC) or micro-alloyed (HSLA) steels are relatively inexpensive substrates and offer good strength and formability. The steel is produced by means of well-known methods such as casting, hot rolling and cold rolling. Low carbon steels typically contain 0.05 to 0.15 wt% C and ultra low carbon steels typically contain 0.02 to 0.05 wt% C. Extra low carbon steels contain typically less than 0.01 wt% C. Other elements can be present in addition to carbon according to EN 10020-2000 which defines how much of a certain element can be present still be considered a non-alloyed steel.

[0075] The present application is also embodied in a laminated tin-plated steel obtained by the method according to the present application, a laminated tin-plated steel for use as a blank for the production of a 3-piece can, and the use of a laminated tin-plated steel according to the present application in a method for producing a can or can part for packaging purposes. Examples

[0076] The present application will now be explained by means of the following non-limiting examples.

[0077] A cold rolled low carbon steel with a thickness of 0.17 mm and a TH550 flatness was tin electroplated on a commercial tin-plating line to produce tin-plated steel with a tin coating weight of 1.0, 2.0, 2.8 or 5.6 g / m 2 of product on the test side of the product. In most cases the tin layer was reflowed but in two cases (Examples 7 and 8) the reflow was switched off to produce non-reflowed tin-plated steel. The as-produced tin-plated steel was subsequently passivated in-line using a tin-plating process according to the different methods outlined above.

[0078] In Examples 1-6, which reflect the prior art, the tin-plated steel was passivated by passing the strip through a sodium dichromate solution under cathodic current, i.e. a conventional "CDC" (cathodic dichromate) or "311" passivation treatment.

[0079] In Examples 7-10, a chromium-free passivation was applied by first passing the strip through a sodium carbonate solution (characterized by a pH of 9.5-10) without the application of an electric current. After rinsing and drying, a solution of Bonderite M-NT1456 was applied by means of a spray disc together with a smoothing roller. The concentration of the Bonderite solution corresponds to 0.25 g / l Ti and was applied with a wet film thickness of about 4 ml / m 2 , aiming at a dry passivation film thickness of 0.8-1.2 mg / m 2 Ti. The wet film was dried in place and the strip was subsequently coiled.

[0080] In Examples 11 and 12, the strip was passed through the sodium carbonate solutions of Examples 7-10 without applying an electric current. After rinsing and drying, the strip was coiled without applying the Bonderite solution. These examples show that good adhesion can be obtained without applying a chromium-free passivation solution to the tinplate if the time between passing the tinplate through the sodium carbonate solution and laminating the tinplate with the thermoplastic laminate layer is short.

[0081] In Examples 13 and 14, the strip was passed through the sodium carbonate solutions of Examples 7-12 while applying an anodic current corresponding to a charge density of 40 C / m 2 After rinsing and drying, the solution of Bonderite M-NT 1456 was applied by means of a spray disc together with a smoothing roller. The concentration of the Bonderite solution corresponded to 0.25 g / 1 Ti and was applied with a wet film thickness of about 4 ml / m 2 , aiming at a dry passivation film thickness of 0.8-1.2 mg / m 2 Ti. The wet film was dried in situ and the strip was subsequently coiled.

[0082] The various tinplate materials described above were applied with a polyester coating by means of an extrusion coating and lamination process. Two types of polymeric coatings were applied:

[0083] • Coating type A consisted of a single layer of 15 μιη thickness consisting of poly(ethylene terephthalate) type N180 commercially available from Indorama (PET).

[0084] • Coating type B consisted of an adhesion layer of 3 μιη thickness in contact with the metal substrate and a top layer of 12 μιη thickness. The adhesion layer was a mixture consisting of 70 wt% of a glycol-modified poly(ethylene terephthalate) (PETg) commercially available as Eastar Copolyester 6763 from Eastman Chemical Company and 30 wt% of poly(ethylene terephthalate) type N180. The top layer consisted of poly(ethylene terephthalate) type N180 (PET).

[0085] In the polymeric coating process, the tinplate was pre-heated to a temperature such that the tinplate temperature was at least 170°C when the laminate layer was brought into contact with the tinplate. After applying the laminate layer, the laminated tinplate was briefly reheated to a temperature of 275°C, followed by quenching in a cold water tank, drying and coiling.

[0086] An overview of the different laminated tinplate steels in this example is given in Table 1 below. Examples 1-6 reflect the state of the art, where the tinplate steel substrate is passivated with a hexavalent chromium passivation solution (311). Examples 7-12 are inventive examples, where the tinplate steel substrate is a chromium-free tinplate steel. Examples 13-14 are control examples, where the tinplate steel substrate is a chromium-free tinplate steel.

[0087] Evaluation of the materials

[0088] Dry adhesion by 180° T-peel test

[0089] The quantitative evaluation of the coating adhesion was done by means of the 180° T-peel test performed on flat sheet material. For this test, a 15 mm wide strip was cut from the polymer coated material. The narrow end of the strip was put into a small volume of 18% hydrochloric acid to etch the steel substrate and obtain a short length (a few millimeters) of free polymer coating. A tape was applied on the free coating and subsequently the coating was peeled from the substrate at an angle of 180° using a tensile tester running at 25 mm / min. The T-peel force was measured as the maximum load value (in N / 15 mm) needed to start the peeling process.

[0090] Dry adhesion by cross-cut test

[0091] The coating adhesion on flat material was evaluated using cross-cut testing according to ISO 2409. A special cutting tool consisting of 4 cutting blades spaced 5 mm apart was used and a lab scale motor driven cutting device was used to apply the cross-cuts to a 15 x 7.5 cm flat panel. After applying the cross-cuts, the coating adhesion was evaluated by peeling the coating using a 25 mm wide piece of Scotch 610 tape and expressing the results by means of the well-known Gitterschnitt (GT) scale ranging from 0 (no delamination) to 5 (full delamination). All tests were performed in triplicate.

[0092] Adhesion after sterilization in various media

[0093] Flat panels of size 15 x 7.5 mm were placed in the test media in a CertoClav "pressure cooker" sterilization device and then the appropriate time and temperature conditions for sterilization were applied. The various media and test conditions are described in Table 2. After the sterilization procedure, the panels were allowed to cool and dry and the adhesion was evaluated by means of the cross-cut test as described above (within less than 4 hours).

[0094] XPS (X-ray photoelectron spectroscopy) analysis of the delamination interface

[0095] Surface and near-surface chemical analysis of freshly delaminated samples was performed by a Kratos Axis Ultra instrument using an Al monochromated source (1486.7 eV) at 15 kV acceleration voltage and 15 mA emission current. Depth profiling by XPS was performed using Ar + Sputtering was used to investigate the subsurface composition of the freshly exposed surface. After each XPS measurement, a sputter cycle was performed at an acceleration voltage of 2 kV providing a sputter rate of 1 nm / min and an extraction current of 60 μA for a sputter area of 3 x 3 mm. Analysis was performed on both the substrate and the coating side of the freshly delaminated surface. Delamination was achieved by applying an epoxy resin (Betamate 1496) to the polymer coated metal sample, followed by curing at approximately 175 °C for 20 minutes, and subsequently immersing the sample in liquid nitrogen, where the relatively thick epoxy layer peels the polymer coating from the substrate due to the epoxy shrinkage at low temperature. XPS analysis was performed at room temperature in ultra-high vacuum (1 x 10 -9 The XPS spectra obtained for each detected element were then processed using CasaXPS to produce the concentration (atomic %) of the different species at different depths from the surface.

[0096] Evaluation of adhesion on the cross-cut test on the polymer coated tinplate was always performed using at least three panels, and due to the nature of the test and the interpretation of the results, there can be small variations in the GT adhesion values between different panels. Based on our experience, we have determined that the adhesion of the polymer coating on the tinplate substrate will be sufficient for the final end-use application as long as the highest GT adhesion value (i.e. the worst adhesion result) of the three tested panels is not higher than 2. Furthermore, in order to perform the forming step without delamination of the coating, the dry T-peel adhesion value should be at least 5 N / 15 mm.

[0097] The adhesion results generated from this example are given in Table 4. Based on the above criteria, it can be seen that the materials using chromium passivation according to the prior art all show good performance. For coating type A, the dry T-peel adhesion values are in the range of 8-11 N / 15 mm, while for coating type B, the values are in the range of 6.5-8.5 N / 15 mm. Furthermore, the cross-cut adhesion values are all in the required range. The tin coating weight of the tinplate substrate in the range investigated seems to have no significant influence on the test results.

[0098] The examples 7-12 of the present application also demonstrate excellent adhesion properties, with dry T-peel adhesion values (coating type B only) in the range 7-9 N / 15 mm, and in all cases cross-cut adhesion values rating 2 or lower. Interestingly, excellent adhesion was also achieved when no Bonderite passivation solution was applied after the cleaning step (examples 11 and 12). It should be noted that in this case, which essentially corresponds to "unpassivated" tinplate, during the time elapsed between the production of the tinplate material and its coating with the polymer layer, a tin oxide layer can form on the outermost surface of the tinplate, and this tin oxide layer can impair the adhesion of the polymer coating. Therefore, when this method is used, the time elapsed between the production of the tinplate material and its coating with the polymer layer, and the storage conditions of the tinplate material (e.g. temperature, relative humidity) should be well controlled.

[0099] In the comparative examples 13-14, in which the cleaning step was performed using an anodic current, very poor coating adhesion was observed. The dry T-peel adhesion values were very low (2.0 N / 15 mm in example 13) or difficult to measure (<0.5 N / 15 mm in example 14, in which the coating almost spontaneously delaminated from the substrate), and this was similarly striking in the cross-cut adhesion values: in many cases the rating was 3, and even up to 4 or 5 in the most aggressive medium, i.e. the medium containing acetic acid.

[0100] Analysis of the delamination interface by XPS indicated that the poor adhesion in comparative example 14 could be attributed to the presence of tin oxide species, mainly Sn02, at the interface between the polymer and the tin layer. Figure 4 and 5 XPS depth profiles of the tinplate substrate and of the delaminated polymer coating, respectively, from the material of example 4 for a chromium-passivated tinplate representative of the prior art. On the substrate side and on the delaminated polymer side, the depth profiles both mainly show organic carbon. This means that delamination of the polymer coating occurs by cohesive failure within the polymer itself, at a distance of several tens of nanometres from the tin surface. This means that the adhesion between the polymer and the tin surface is very strong. Figure 6 and 7XPS depth profiles for tinplate steel substrate and the peeled polymer coating, respectively, for the material from inventive example 8 representing a chromium-free passivated tinplate steel, including the steps of cleaning in sodium carbonate solution without applying an electric current and subsequent application of Bonderite M-NT 1456 solution. On the substrate side, the depth profile shows mainly tin with a very thin layer of tin oxide (mainly SnO) of about 1 nm. This thin SnO layer can have formed during sample preparation after the polymer coating was detached. On the peeled coating side, mainly organic carbon is observed. This result indicates that the delamination occurred at a clear interface between the organic coating (i.e. the Bonderite layer and / or the organic part of the polymer coating) and the tin, thus providing a strong bond between the coating and the substrate, as reflected by the high T-peel adhesion force and the good adhesion performance in the cross-cut test. Finally, Figure 8 and 9 XPS depth profiles for tinplate steel substrate and the peeled polymer coating, respectively, for the material from comparative example 14 representing a chromium-free passivated tinplate steel, including the steps of cleaning in sodium carbonate solution with applying an electric current and subsequent application of Bonderite M-NT 1456 solution. Both depth profiles on the substrate side and on the polymer side show a significant amount of tin oxide, mainly Sn02, of a few nanometers thickness. The fact that tin oxide is present on both sides of the delamination interface means that the delamination occurred within the oxide layer. This oxide layer is clearly weak as can be seen from the T-peel adhesion values and leads to an easy delamination of the polymer coating from the tinplate steel surface. Therefore, the presence of such an oxide layer has to be avoided in order to achieve a good adhesion between the polymer coating and the tinplate steel substrate.

[0101] Table 1 Properties of tinplate steel substrates, type of passivation and polymer coating (soda = sodium carbonate) in the examples of the invention

[0102]

[0103] Table 2 Product evaluation tests

[0104] Test Description Medium Conditions Test 1 Dry adhesion Not applicable Not applicable Test 2 Bouillon Plasmal 12 g / l Maggi + 2 g / l Plasmal 121 °C / 90 min Test 3 Acetic acid 1% acetic acid 121 °C / 60 min Test 4 Salt-containing test 3.6% NaCl 121 °C / 90 min Test 5 Vitamin C 1 g / l Vitamin C + 3.6% NaCl 121 °C / 90 min Test 6 Water Demineralized water 121 °C / 60 min Test 7 Salt-acid 18.7 g / l NaCl + 30 g / l acetic acid 121 °C / 60 min

[0105] As part of the extensive research of the inventors on surface treatment of tinplate steel regarding tin oxide formation and stability and regarding adhesion of organic and polymer coatings, the inventors investigated the performance of two broad types of chromium-free passivation treatment solutions: those based on siloxanes and those based on zirconium and titanium compounds. A well-known example of a siloxane-based treatment system is Bonderite® MM0705 from Chemetall. A well-known example of a zirconium / titanium-based treatment system is Bonderite® M-NT 1456 from Henkel. TM ​M-NT1456. Both systems were applied to tin coated steel with a tin coating weight of 2.8 g / m 2 Sn on both sides using spray application installed on a commercial production line. The coating conditions and chromium-free passivation treatment solution composition were selected to give a tin coating weight of 2.8 g / m 2 Si or Ti thickness, as thereafter confirmed by surface characterisation (XPS). The tin oxide and tin oxide growth rates were then determined and compared to chromium passivated ("311") tin plated steel and unpassivated tin plated steel.

[0106] The amount of tin oxide present on the surface of the material can be determined using the Coulometric method. The tin oxide layer is reduced by a controlled small cathodic current in a 0.01 M hydrobromic acid (HBr) solution, which is free of oxygen by washing with nitrogen. The reduction process of the oxide is monitored by measuring the reduction potential and the charge passed (A*t) for complete reduction is taken as a measure of the tin oxide layer thickness. For the test, a cylindrical cell with a circular aperture of approximately 4 cm diameter at one end and an Ag / AgCl reference electrode is used. The other end of the cell contains a platinum counter electrode. The test sample covers the aperture, which is sealed using an O-ring to form a water tight connection of a well defined area and a gas pressure cylinder is used to secure the test sample in place. The cell is connected to the electrolyte solution by a flexible tube so that it can be filled and emptied under a nitrogen atmosphere. A constant potential-galvanostat is used to apply a cathodic current density of -0.50 A / m 2 to the sample and the potential is measured until reduction is complete. The test results are expressed as the total charge density (in C / m 2 The stability of the oxide layer is examined by placing the test panel in a climate chamber at 40°C and 80% relative humidity for two weeks and then measuring the amount of tin oxide present on the surface and comparing it to the amount of tin oxide present on the surface of the tin plated steel material in the as-received condition.

[0107] The results of this study are summarised in Table 3. The Zr / Ti based Bonderite system provides a thinner and more stable tin oxide layer on the tin plated steel surface compared to the Si based Oxsilan system. Based on this result, it is clear that the Zr / Ti based system is ideally suited for use in the present application, whereas the Si based system is not.

[0108] Table 3 Tin oxide layer stability comparison after passivation

[0109]

[0110] ** Exposure in a climate chamber at 40°C and 80% RH for two weeks

[0111]

[0112]

[0113] Brief description of the drawings

[0114] The application will now be explained by way of the following non-limiting drawings.

[0115] Figure 1 Definitions of some terms are shown.

[0116] Figure 2 A schematic illustration of the solid film lamination process is shown.

[0117] Figure 3 A schematic illustration of the cast film lamination process is shown.

[0118] Figures 4 to 9 The XPS profile of the laminated tinplate after separation of the laminated layer from the tinplate is shown to study the properties of the bond between the laminated layer and the tinplate.

[0119] Figures 10 to 12 The production stages of a laminated tinplate for a 3-piece can are shown.

[0120] In Figure 2 the tinplate sheet or strip (1) is passed through a first heating device (2) in which the temperature of the tinplate is raised to a preheating temperature T1 suitable for lamination. Two rolls of laminated layer (3a, 3b) are simultaneously unwound and passed together with the preheated tinplate through a nip roller assembly comprising a pair of lamination rollers (4a, 4b). The laminated tinplate (5) is passed through a second heating device (6) in which the temperature of the laminated tinplate is raised to a post heating set point T2. After the second heating device, the laminated tinplate is immediately cooled to room temperature by passing through a quenching device (7). The method of preheating the tinplate in the first heating device is not particularly limited and can include passing the strip over heated rollers, conduction heating, induction heating, radiation heating, etc. The method of post heating the laminated tinplate in the second heating device is preferably a non-contact method such as heating in a hot gas environment or induction heating. The direct cooling method of the quenching device is not particularly limited and can include applying cold air or by passing through a cold water bath, etc. In Figure 2 In

[0121] In Figure 3 the laminated layer is extruded from a flat die (14), drawn down in a narrow gap formed between the extrusion die and a casting roll, and cast to its final desired thickness on a rapidly cooled casting roll (13). Because the draw down to final thickness is performed under liquid conditions, the cast laminated layer is essentially non-oriented. It can then be oriented in a manner similar to Figure 2The laminated layers are laminated to the tinplate 1 in a similar way as described in the method.

[0122] To produce material for a 3-piece can, the extruded laminate is slit (11) and the narrow polymer bands (10a-10d) between the wide laminates (9a-9d) are pulled off and removed. The number of wide laminates (9a-9d) produced from the extruded polymer film can be 2 or more. In the illustrative drawings, the number of four wide laminates (9a-9d) is used by way of example, but the invention is also applicable to two, three or more wide polymer films. The number of narrow polymer bands (10) cut from between the wide laminates (9a-9d) to be removed is in principle always one less than the number of wide laminates to be laminated to the tinplate. The width of the extruded laminate (3) should be less than the width of the tinplate to allow the edges of the tinplate to remain uncoated. If the polymer film becomes too wide to allow the edges of the tinplate to remain uncoated (i.e. bare), it can be necessary to trim the edges of the polymer film on-line. These cut-off edges are pulled off from the laminate and the outermost edges of the tinplate are left bare of polymer during the lamination process of the laminate to the tinplate. This is preferable to the alternative of coating the edges of the polymer coating after the lamination process and grinding or otherwise removing the edges. The pulling off of the cut-off edges of the polymer film can be carried out by means of a cutting waste extraction device, for example by means of a suction device (12).

[0123] Figure 10 The tinplate 1 and the extruded and cooled laminate 3 are shown in a top view (not to scale). In the bottom view of Figure 10 the cut-out situation is depicted, where the small bands to be removed are hatched (10a-10c) and the wide laminates to be laminated to the tinplate use 9a-9d. Figure 11 A top view of the laminated tinplate is shown, where the bare bands and bare edges are shown. These bare edges and bare bands are needed to form a 3-piece can body that is welded together. Figure 11 It is also shown schematically (top view left-hand side, dotted lines) how the laminated tinplate can be cut lengthwise into four narrow laminated tinplate bands and how individual blanks for 3-piece cans can be produced. Each of these blanks has edges that are free of polymer and can therefore be welded to produce a 3-piece can body (see Figure 13 ).

[0124] Figure 11 The bottom view in shows a cross-section along A-A. Figure 12 The same situation is shown where laminated layers are provided on both sides of the tinplate. Figure 13A cross section of a welded 3-piece can body is shown and the left hand side of the figure shows an enlarged portion of the welded portion. The bare tinned steel edge as well as the edge of the wide polymer film ribbon 3a and the portion where the two bare edges are joined together by welding are clearly shown. The welded and bare metal is subsequently covered with a lacquer 17 to protect the metal from corrosion. The lacquer is preferably BPA-free.

Claims

1. A method of manufacturing a laminated tin-plated steel for packaging applications, the laminated tin-plated steel comprising a tin-plated steel sheet and a thermoplastic laminate covering at least one side of the tin-plated steel sheet, the laminate comprising a plurality of layers, wherein each layer contains a thermoplastic aromatic polyester or a blend thereof or a polyolefin containing at least 90 mol% propylene units, the method comprising the following steps: • Tin-plated steel sheets are produced by electroplating a tin layer onto one or both sides of cold-rolled steel sheets. • The tin-plated steel sheet is subjected to surface treatment by immersing it in an aqueous solution containing phosphate ions, borate ions, sulfate ions, or carbonate ions or combinations thereof with a pH of 8-12, without applying an electric current to the tin-plated steel sheet during immersion. • Rinse and dry the tin-plated steel sheet; • Optionally, a chromium-free, rinse-free, in-situ dried passivation solution is applied to the tin-plated steel sheet, wherein the passivation solution is free of silanes, free of siloxanes, and is not Si-based; • Drying and passivating tin-plated steel sheets; • The thermoplastic laminate is provided for coating at least one side of the tin-plated steel sheet; • Preheat the tin-plated steel sheet and stack the thermoplastic laminate onto the preheated tin-plated steel sheet to produce laminated tin-plated steel; • The laminated tin-plated steel is then heated to a temperature high enough to melt the laminate; • Cool the reheated laminated tin-plated steel.

2. The method according to claim 1, wherein the thermoplastic aromatic polyester is a thermoplastic aromatic copolyester.

3. The method of claim 1, wherein the thermoplastic laminate for lamination onto the preheated tin-plated steel sheet is provided by: • Provide pre-generated uniaxial or biaxially oriented thermoplastic laminates, or • Melting thermoplastic polymer pellets in one or more extruders to form one or more layers and forming the thermoplastic laminate consisting of the one or more layers by passing the molten one or more polymers through a flat extrusion die and / or two or more calendering rolls; after: A. • Cool the thermoplastic laminate to form a solid thermoplastic laminate; • Optional trimming of the edges of the thermoplastic laminate; • The thickness of the solid thermoplastic laminate is reduced by stretching the solid thermoplastic laminate in a stretching device by applying a tensile force only in the longitudinal direction; • Optional trimming of the edges of stretched thermoplastic laminates; • The laminates are stacked onto a preheated tin-plated steel sheet; • The laminated tin-plated steel is then heated to a temperature high enough to melt the laminate; • Cooling the reheated laminated tin-plated steel. or after B. • An extruded thermoplastic laminate is drawn between a flat extrusion die and a casting roll, and cast onto the casting roll to its final desired thickness for rapid cooling of the drawn thermoplastic laminate, wherein the cast and cooled thermoplastic laminate is substantially unoriented. • Optional trimming of the edges of the cast and cooled thermoplastic laminate; • The cast and cooled thermoplastic laminate is online onto a preheated tin-plated steel sheet; • The laminated tin-plated steel is then heated to a temperature high enough to melt the laminate; • Cool the reheated laminated tin-plated steel.

4. The method of claim 3, wherein the flat extrusion die is a flat co-extrusion die.

5. The method according to any one of claims 1 to 4, wherein the laminate is formed at least on the side that becomes the inside of the package, and the polyester in one or more layers of the laminate contains at least 80 mol% of polyethylene terephthalate units or wherein the polyester is a copolyester containing at most 35 mol% of CHDM or at most 20 mol% of IPA.

6. The method according to any one of claims 1 to 4, wherein the laminate is formed at least on the side that becomes the inside of the package, and the polyester in one or more layers of the laminate contains at least 85 mol% of polyethylene terephthalate units or wherein the polyester is a copolyester containing at most 35 mol% of CHDM or at most 20 mol% of IPA.

7. The method according to any one of claims 1 to 4, wherein the laminate is formed at least on the side that becomes the inside of the package, and the polyester in one or more layers of the laminate contains at least 80 mol% butylene terephthalate units.

8. The method according to any one of claims 1 to 4, wherein the laminate is formed at least on the side that becomes the inside of the package, and the polyester in one or more layers of the laminate contains at least 85 mol% butylene terephthalate units.

9. The method according to any one of claims 1 to 4, wherein the laminate is formed on at least one side that becomes the inside of the package, and the polyester in one or more of the laminate comprises a blend of a polyester containing at least 85 mol% of polyethylene terephthalate units and a polyester containing at least 85 mol% of butylene terephthalate units.

10. The method according to any one of claims 1 to 4, wherein the laminate on at least one side of the package comprises one or more polypropylene layers substantially composed of polypropylene.

11. The method according to any one of claims 1 to 4, wherein the laminate comprises an adhesive layer composed of maleic anhydride-grafted polypropylene.

12. The method according to any one of claims 1 to 4, wherein the laminated tin-plated steel undergoes a stretching operation, wherein the stretching operation is achieved by: • Pass the material through a leveling machine and apply a thickness reduction of 0-3%; or • Pass the material through the stretching straightener.

13. The method according to any one of claims 1 to 4, wherein the laminated tin-plated steel undergoes a stretching operation, wherein the stretching operation is achieved by: • Pass the material through a leveling machine and apply a thickness reduction of 0.2-3%; or • Pass the material through the stretching straightener.

14. The method according to any one of claims 1 to 4, wherein the laminate on one or both sides of the passivated tin-plated steel sheet is a multilayer coating system, the coating system comprising at least an adhesion layer for adhesion to the passivated tin-plated steel sheet, a surface layer, and a body layer between the adhesion layer and the surface layer.

15. The method according to claim 3 or 4, wherein the cast and cooled thermoplastic laminate is cut in the longitudinal direction into at least N wide laminates (9a-9d) and (N-1) narrow strips (10a-10c), wherein N is at least 2, and then the narrow strips are guided away from the wide laminates by a discharge device (12), and subsequently the wide laminates are coated onto preheated tin-plated steel by a pressure roller assembly (4a, 4b) to obtain a laminated tin-plated steel having a plurality of wide laminates (9a-9d) spaced in the longitudinal direction by narrow strips (10a-10c) without the wide laminates, wherein the edges of the tin-plated steel are kept free of the wide laminates, and then the laminated tin-plated steel is postheated and the postheated laminated tin-plated steel is cooled.

16. The method according to any one of claims 1 to 4, wherein the laminate has a thickness between 5 and 35 μm.

17. Laminated tin-plated steel obtained by the method according to any one of claims 1 to 16.

18. A laminated tin-plated steel for producing blanks for three-piece cans, wherein the laminated tin-plated steel is obtained by the method according to any one of claims 1 to 16.

19. Use of the laminated tin-plated steel of claim 17 or 18 in a method of producing cans and can parts for packaging purposes.

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