Two-layer can strip coating

Abstract

The present invention relates to a strip-shaped semi-finished product (11) for manufacturing can lids or can tabs for cans, preferably beverage cans, wherein the semi-finished product (11) comprises an aluminum alloy strip (4) having at least one first at least partial surface coating (11a), the at least one first at least partial surface coating (11a) provided on the side of the aluminum alloy strip (4) that will be used on the outside of the can. The present invention further relates to a coated aluminum alloy strip (16) manufactured from the strip-shaped semi-finished product (11) according to the present invention for manufacturing can lids or can tabs for cans, preferably beverage cans, wherein the aluminum alloy strip (16) comprises at least one second coating (16a) positioned on top of the at least one first coating (11a) of the semi-finished product (11). The objective of defining a simple possibility for providing an aluminum alloy strip having a scratch-resistant lacquer layer, preferably a colored lacquer layer, for the manufacture of can lids or can tabs, particularly beverage can lids and beverage can tabs, is achieved by the strip-shaped semi-finished product described in claim 1, the aluminum alloy strip described in claim 9, and the method for manufacturing the same described in claim 14.

Description

Technical Field

[0001] The present invention relates to a strip-shaped semi-finished product for manufacturing a can lid or a can tab of a can, preferably a beverage can, which semi-finished product has an aluminum alloy strip having at least one first at least partial surface coating with a crosslinkable coating substance, and this surface coating is provided on the side of the aluminum alloy strip used for the outside of the can. The present invention further relates to an aluminum alloy strip for manufacturing a can lid or a can tab, preferably a beverage can, manufactured from the strip-shaped semi-finished product according to the present invention, which aluminum alloy strip has at least one second coating disposed on at least one first coating of the semi-finished product. Finally, the present invention relates to a method for the production of an aluminum alloy strip according to the present invention.

Background Art

[0002] Preferably, can lids and can tabs of beverage cans are often manufactured using coated aluminum alloy strips. The coated aluminum alloy strips are adapted for their respective applications, i.e., for a specific can application, from both the perspective of the aluminum alloy strip used and the perspective of the coating. For this reason, different requirements are imposed on the inside and outside of a can, preferably a beverage can. The coated inside of the can generally needs to protect the aluminum alloy from the influence of the contents of the can in addition to improving the forming properties, and vice versa protect the contents of the can from the aluminum alloy, while the outside mainly has the role of determining the appearance, i.e., the look, of the can, especially a beverage can. Therefore, it is also increasingly common to use colored lacquers and clear lacquers for the coating of aluminum alloy strips for the manufacture of can lids or can tabs.

[0003] Most lacquers or other coatings typically undergo a heat-curing process, also known as baking. This plays a crucial role in the properties of the subsequent coated aluminum alloy strip, as the baking process affects, or defines, the strength of the coated aluminum alloy strip.

[0004] The curing process is performed at the so-called curing temperature. The curing temperature indicates the temperature required for complete curing of the coating material for a specified curing time. In this document, the curing temperature is always specified in the form of the “peak metal temperature” (PMT) of the aluminum alloy strip. For this purpose, the holding time at the maximum temperature (PMT) is also specified to define the curing process. The PMT is reached only after a specific preheating time. Therefore, the PMT is usually lower than, for example, the circulating air temperature in a convection furnace. The PMT can be easily measured by a thermocouple on a test specimen in the furnace pass (German: Ofendurchlauf, English: furnace pass).

[0005] The use of colored lacquers is increasing, particularly on the exterior of cans, such as can lids, where there is a high contrast to the natural aluminum color. This presents problems when processing aluminum alloy strips. Can lids are stacked and stored for processing and further processing after storage. During storage or further processing in the can manufacturing process, the lids rotate relative to each other. The protruding crown, which is the ridge around the can lid, is particularly susceptible to lacquer defects caused by lacquer peeling or chipping. The lacquer layer in these areas is especially vulnerable after forming at the top of the crown because the lacquer layer needs to withstand the high tensile stresses caused by forming. In addition, due to the small contact surface with other can lids, the crown is subjected to relatively high contact pressure, further increasing the risk of lacquer damage. Due to the strong contrast between the uncoated silver aluminum and the colored lacquer layer, even very small chips within the colored lacquer layer can be visible to the naked eye. Damage to the lacquer layer results in an undesirable appearance for cans, such as beverage cans. The same is generally true for can tabs.

[0006] Therefore, to reduce the risk of lacquer damage, a two-layer lacquer system is created by coating the colored lacquer layer with a clear lacquer. Thus, the corresponding aluminum alloy strip experiences two separate baking steps, which are usually performed at the same baking temperature. Compared to a single baking step, this results in a decrease in the strength of the aluminum alloy strip because the baking temperatures of both coatings initiate a softening process in the aluminum alloy strip. The strength of the aluminum alloy strip is significantly reduced compared to an aluminum alloy strip exposed to only a single baking process. This is especially true for typical baking temperatures of 245°C to 270°C, which partially exceeds the temperature threshold for recrystallization of the aluminum alloy in the aluminum alloy strip.

[0007] In addition, because the clear lacquer layer does not adhere sufficiently to the colored lacquer layer in certain areas, this two-coating system continues to cause delamination of the clear lacquer layer. These areas are also visible in the finished can. The temperature of the baking step can also affect the color of the lacquer layer, and it has been shown that discoloration of the colored lacquer layer may occur. Finally, the energy required to provide a two-coating system by the same baking step is significantly higher compared to an aluminum alloy strip with a single layer of lacquer.

[0008] A two-layer lacquer-coated aluminum strip in which both lacquer layers are cured independently is known from Patent Document 1. Partial crosslinking of the first coating is not known.

[0009] Patent Document 2, an international patent application, shows an aluminum strip coated with a water-based coating for a beverage can formed in only a single layer.

[0010] The use of polyester as a substitute for PVC in a single-layer coating on the inside of a can lid is known from Patent Document 3, an international patent application.

[0011] A single-layer coating for beverage cans that does not release potentially harmful substances is provided by Patent Document 4, an international patent application relating to beverage cans.

[0012] Patent Document 5, an international patent application, discloses a single-layer coating containing latex for beverage containers. [Prior art documents] [Patent Documents]

[0013] [Patent Document 1] U.S. Patent No. 4,253,584 [Patent Document 2] International Publication No. 2008 / 036628(A1) [Patent Document 3] International Publication No. 98 / 23198(A1) [Patent Document 4] International Publication No. 2004 / 013240(A1) [Patent Document 5] International Publication No. 2015 / 002958(A1) [Overview of the project] [Means for solving the problem]

[0014] Therefore, an object of the present invention is to provide a simple way to provide an aluminum alloy strip having a scratch-resistant lacquer layer, preferably a colored lacquer layer, for the manufacture of can lids or can tabs, particularly beverage can lids and beverage can tabs. In addition, this aluminum alloy strip should preferably require less energy to produce.

[0015] According to the first teaching, the objective shown above is achieved by a strip-shaped semi-finished product, where at least one first coating of the aluminum alloy strip has a degree of crosslinking of at least 20% to 80%, preferably 30% to 70%, and more preferably 45% to 60%, and the aluminum alloy strip achieves the result of an unchanging surface on the coating side in a so-called block test, for this block test, two blanks having dimensions such as 10 cm × 10 cm from a semi-finished product having a partially crosslinked first coating pointing in the same direction are placed overlapping between two flat pressure bodies, the overlapping blanks are pressed against each other from above the pressure bodies with a pressure of at least 2.942 kPa, and in this state are heated in PMT at 50°C and held for 24 hours, then the blanks are separated again, and after the blanks are separated, the changes in the surface of the coatings of the two blanks are investigated.

[0016] An unchanging surface result is achieved when the coated side of the blank shows no visible optical change after separation, i.e., it remains unchanged. As a result, the semi-finished product can be wound onto a coil without any problems and processed later.

[0017] On the other hand, negative results from block testing are characterized by surface changes due to the formation of dull spots in the areas of the blank that were pressured after separation. These dull areas on the surface are due to excessive adhesion of the blank during block testing. This results in damage to the coating surface when the blank is separated.

[0018] In addition, the stronger adhesion of the blanks in block testing can even lead to partial or complete peeling of the coating on the blanks. Both dull spots and partial or complete peeling of the coating result in surface changes to the coating. As a result, the semi-finished product cannot be wound into a coil for further processing.

[0019] The degree of crosslinking of at least one first coating can be determined, for example, by a so-called "sol fraction test." A semi-finished sample in a fixed shape is first weighed (unloaded). The sample is then placed in a methyl ethyl ketone (MEK) bath at room temperature for 30 minutes and dried in a laboratory oven at 180°C for 2 minutes (loaded). In this step, the uncrosslinked portion of the coating is essentially removed. The sample is weighed again, and then the lacquer layer is removed, leaving the sample consisting only of metal (metal). Finally, the sample is weighed again. The difference in weight between the unloaded and loaded samples, and the quotient between the weight of the unloaded sample and the weight of the metal, then yields the uncrosslinked portion of the coating.

[0020] Surprisingly, it has been shown that the strip-shaped semi-finished product according to the invention can provide a significantly improved crosslinking between the first coating and the second coating due to the insufficiently crosslinked state of at least one first coating. After coating with at least one second coating and baking both layers, it has been found that crosslinking occurs strongly between those layers. As a result, at least one second coating adheres significantly better to the first coating. For example, chipping of the transparent lacquer layer at important points of the can lid can be significantly reduced. The unchanged surface after the block test ensures that the semi-finished product according to the invention with an insufficiently crosslinked coating is difficult to adhere, for example when wound around a coil. When the block test is successful, the semi-finished product can then be wound around the coil and stored without any problems during subsequent processing. Thus, the semi-finished product according to the invention can, for example, be easily stretched again after storage and processed into a finished coated aluminum alloy strip for can lids or can tabs. This enables an economical production of aluminum alloy strips for the manufacture of can lids and can tabs of beverage cans, especially those having at least two layers of coating.

[0021] The aluminum alloy strip optionally has a passivation layer before coating with at least one first coating, and this passivation layer is provided on one or both sides, on which at least one first coating is arranged. The passivation layer is preferably formed without chromium. This brings the advantage of processing with respect to safety measures involving chromium-containing chemicals during operation. The passivation layer can be provided, for example, by treating the aluminum alloy strip with zirconium phosphate. The passivation layer increases the adhesion of at least one first coating in the aluminum alloy strip.

[0022] According to a first embodiment of the semi-finished product according to the invention, at least one first coating of the aluminum alloy strip has a baking temperature (PMT) of 180°C to 240°C, preferably 200°C to 230°C, particularly preferably 210°C to 220°C, and a holding time of 1 to 20 seconds, preferably 2 to 12 seconds. By reducing the baking temperature, the loss of strength of the aluminum alloy strip during baking of at least one first coating can be minimized, and partial curing of at least one first coating can still be achieved. At the same time, the lower baking temperature also reduces the energy required for the first baking process.

[0023] According to a further embodiment, at least one first coating of the aluminum alloy strip is formed by an epoxyamine lacquer system or a polyesteramine lacquer system, which can make it possible to finely adjust the baking temperature via the lacquer composition. For example, by changing the chemical composition, such as by changing the binder, the lacquer system can be well adjusted by the lacquer manufacturer for a specified baking temperature.

[0024] Since the baking time also depends particularly on the thickness of the layer of at least one first coating, it is advantageous for at least one first coating of the aluminum alloy strip to have a layer thickness of 2 g / m 2 ~5 g / m 2 This layer thickness ensures a short baking time and at the same time provides good crosslinking or adhesion to at least one second coating. At the same time, the thickness of these coatings is sufficient to ensure sufficient coloring of the colored lacquer coating.

[0025] Due to the good properties of the strip-shaped semi-finished product according to the invention in the block test, this strip-shaped semi-finished product can be further processed or stored in a simple manner by winding it into a coil. For example, the semi-finished product can be easily fed to a further coating step, and an economical production method can be ensured.

[0026] In further embodiments, when the semi-finished aluminum alloy strip has an aluminum alloy of type AA3004, AA3104, AA3105, AA5052, AA5042, or AA5182, and the metal thickness of the aluminum alloy strip is 0.12 mm to 0.30 mm, preferably 0.16 mm to 0.25 mm, and particularly preferably 0.16 mm to 0.23 mm, an aluminum alloy strip with the required strength can be economically provided for the manufacture of can lids or can tabs.

[0027] According to a preferred embodiment, at least one first coating is a colored lacquer layer. As already shown above, the semi-finished product according to the present invention can be advantageously used for the production of aluminum alloy strips for can lids and can tabs that are particularly resistant to lacquer peeling, i.e., resistant to scratches and abrasion of the lacquer layer.

[0028] Finally, according to a further embodiment, the semi-finished product has a passivation layer on the side of the aluminum alloy strip that is used for adhesion to the outside and / or inside of the can before coating each side. The passivation layer improves the adhesion between the coating and the aluminum material of the aluminum alloy strip. This applies to both the side of the aluminum alloy strip intended for the outside of the can and the side of the aluminum alloy strip intended for the inside. Passivation of the side of the aluminum alloy strip provided for the inside may also be optionally performed after the application of at least one first coating on the side of the aluminum alloy strip provided for the outside.

[0029] Further teachings indicate that the stated objective is achieved by an aluminum alloy strip for manufacturing can lids or can tabs, preferably for beverage cans, where the aluminum alloy strip is manufactured from a strip-shaped semi-finished product according to the present invention, which is provided with at least one second coating, the second coating being placed on top of at least one first coating of the semi-finished product. As already stated above, the aluminum alloy strip according to the present invention has improved crosslinking between at least one first coating and at least one second coating. Thus, the aluminum alloy strip having two-layer coatings differs from conventionally manufactured aluminum alloy strips in that the two coatings are more strongly crosslinked with each other, and thus the adhesion between the two coatings is stronger. At the same time, the use of a semi-finished product according to the present invention ensures particularly economical manufacturing of the aluminum alloy strip.

[0030] According to the first embodiment, at least one first coating and at least one second coating have a degree of crosslinking greater than 90%, preferably greater than 95%, and particularly preferably greater than 98%, and at least one first coating has a lower baking temperature than the second coating, preferably the baking temperature of at least one second coating is 245°C to 270°C PMT, preferably 245°C to 260°C PMT, and particularly preferably 248°C to 255°C PMT, and the holding time is 1 to 20 seconds, preferably 2 to 12 seconds.

[0031] An aluminum alloy strip may be provided having a two-layer coating that avoids the aforementioned disadvantages of greater loosening and significantly higher energy consumption by having a lower baking temperature for at least one first coating. Improved crosslinking between at least one first coating and at least one second coating makes this two-layer coating simultaneously more scratch-resistant than conventionally manufactured two-layer coatings.

[0032] Preferably, the aluminum alloy strip according to the present invention has exactly two coatings having a degree of crosslinking greater than 90%, preferably greater than 95%, and particularly preferably greater than 98%. The corresponding two-layer coating can meet the requirements for can lids and can tabs in a particularly economical way, as each additional coating requires an additional baking process.

[0033] As already mentioned above, the baking temperature can be set very precisely by providing at least one second coating via an epoxyamine lacquer system or a polyesteramine lacquer system. The aforementioned lacquer systems can be well tuned by selecting a composition that is modified with respect to the baking temperature, for example by changing the binder.

[0034] At least one second coating has a layer thickness substantially identical to that of at least one first coating. For this purpose, at least one second coating is preferably 2-5 g / m². 2 It has a surface weight of . Furthermore, at least one second coating can be optionally formed as a transparent lacquer layer to protect the colored lacquer layer without interfering with the color impression of the colored lacquer layer, for example.

[0035] The following embodiment of the aluminum alloy strip has an aluminum alloy of type AA5182, and after baking of at least one first coating and a second coating, the tensile strength Rm of the aluminum alloy strip is 380 MPa to 425 MPa, and the yield strength R p0.2 When the pressure is 330 MPa to 380 MPa, a high-strength aluminum alloy strip for can lids can be provided, having at least one two-layer coating, preferably with the two-layer coating on the outside.

[0036] According to further teachings of the present invention, the objective of a method for producing an aluminum alloy strip having at least one first coating and at least one second coating placed on the first coating is achieved by first producing a semi-finished product by coating the aluminum alloy strip with at least one first coating, where the at least one first coating of the semi-finished product is cured in a first baking step to a degree of crosslinking of 20% to 80%, preferably 30% to 70%, more preferably 45% to 60%, and the semi-finished product achieves an unchanging surface result on the coated side after block testing, where the at least one first coating of the semi-finished product is coated with at least one second coating, and in a subsequent second baking step, the at least one first coating and the at least one second coating are cured together. The semi-finished product may be preferably wound on a coil and stored before the application of the second coating.

[0037] For example, the problem of delamination of lacquered areas in the case of can lids or can tabs can be significantly reduced by providing improved adhesion between at least one first coating that was not previously fully crosslinked and at least one second coating placed on top of the first coating, by baking both coatings in a second baking step. This results in significantly stronger crosslinking of the two coatings to each other. At the same time, in the manufacture of coated aluminum alloy strips for the manufacture of can lids or can tabs, high economic efficiency can be achieved by producing storable semi-finished products with a subsequent second coating process of the finished aluminum alloy strip, while simultaneously providing a favorable lacquer system of at least two layers on the aluminum alloy strip.

[0038] In a first embodiment of this method, at least one first coating is cured at a curing temperature lower than the curing temperature of a second coating placed on top of the first coating, where preferably at least one first coating is cured at a curing temperature (PMT) of 180°C to 240°C, preferably 200°C to 230°C, and particularly preferably 210°C to 220°C for a holding time of at least 1 to 20 seconds, preferably 2 to 12 seconds. This not only reduces the energy required for the curing process despite providing a two-layer coating system, but also limits loosening of the aluminum alloy strip. As a result, a fully cured aluminum alloy strip having at least two layers of coating can be provided with lower energy requirements.

[0039] When a semi-finished product is wound onto a coil after coating with at least one first coating and partial curing of at least one first coating, this semi-finished product can be stored in a simple manner and provided for subsequent coating with at least one second coating.

[0040] Following coating with at least one second coating, at least one first coating and at least one second coating are preferably cured together in a second baking step, where the baking temperature in the second baking step is higher than the baking temperature of at least one first coating, and preferably in the second baking step, curing is performed by holding for 1 to 20 seconds, preferably 2 to 12 seconds, at a baking temperature (PMT) of 245°C to 270°C, preferably 245°C to 260°C, and particularly preferably 248°C to 255°C. This ensures safe curing of the two-layer coating system and simultaneously achieves a coated aluminum alloy strip with lower looseness than conventional aluminum alloy strips having a two-layer lacquer system.

[0041] The present invention will be described in more detail below based on exemplary embodiments. [Brief explanation of the drawing]

[0042] [Figure 1] This is a schematic cross-sectional view showing the manufacturing of aluminum alloy strips for beverage cans in particular. [Figure 2] This is a schematic cross-sectional view illustrating a method for manufacturing semi-finished products by coating aluminum alloy strips. [Figure 3] This is a schematic cross-sectional view illustrating a method for manufacturing an aluminum alloy strip having two layers of lacquer, with one layer placed on top of the other, using a semi-finished product according to the present invention. [Figure 4] This is a schematic diagram showing the progress of the baking process over time at different baking temperatures, while maintaining the same holding time. [Figure 5] This is a schematic cross-sectional view showing embodiments of a semi-finished product and an aluminum alloy strip according to the present invention. [Figure 6] This is a schematic top view showing a sample of a lacquered strip with marks for evaluating impact bending tests. [Modes for carrying out the invention]

[0043] Figure 1 shows, in a schematic diagram, the individual method steps for manufacturing an aluminum alloy strip, from the production of a rolled ingot to the cold rolling of the aluminum strip to its final thickness.

[0044] First, a rolled ingot 1 is produced, for example, in a DC casting process. Similarly, strip casting, which is not shown here, may also be used to produce a cast strip. Next, in step 2a, the rolled ingot 1 is homogenized, and then in step 3a, it is hot-rolled to form a hot strip 3. Hot rolling may be carried out in an inverting stand and / or a tandem stand having multiple passes. Next, in step 4a, the hot strip is cold-rolled to a final thickness to form a cold strip 4. Cold rolling may be carried out in a rolling rack having a single rolling pass or in multiple racks having two or more rolling passes. During cold rolling, one or more intermediate annealing processes 4b may be carried out in a chamber furnace 5 or a continuous furnace, which is not shown. However, a final heat treatment is also not excluded, and the cold-rolled aluminum alloy strip, preferably in a fully hardened H18 or H19 rolled state, is sent to the next coating method step. At the end of cold rolling, the cold-rolled aluminum alloy strip 4 is preferably wound onto a coil 6.

[0045] Figure 2 shows an embodiment of a method for producing a semi-finished product by coating an aluminum alloy strip. The cold-rolled aluminum alloy strip 4 is unrolled from the coil 6 and sent to an optional passivation step 7, which is designed here as a roll coating process. Alternatively, passivation can be performed by spraying a passivation chemical, such as by electrostatic spraying of a passivation chemical. Alternatively, passivation can also be performed by passing the strip through a bath containing a passivation liquid. However, the roll coating process has proven its value due to the high processing speed and the good precision of the application of the passivation chemical. A rinse-free process is preferred for surface passivation, in which the passivating agent, preferably zirconium phosphate, remains on the aluminum strip and does not need to be rinsed off. For this purpose, after the application of the passivation chemical, the aluminum alloy strip 4 coated with the passivation chemical is dried in an oven 8.

[0046] Figure 2 does not show any optional pretreatment of the underside of the aluminum alloy strip 4, and a passivation layer, for example, may also be provided on the underside of the aluminum alloy strip 4. The passivation agent can then be dried. Here again, the use of zirconium phosphate is preferred. Thus, optionally, both sides of the aluminum alloy strip have a passivation layer.

[0047] In the next step, the aluminum alloy strip 4 for providing the semi-finished product according to the present invention is coated with at least one first coating 11a, for example, in a roll coating method 9. Alternatively, other application processes not shown herein may be used.

[0048] Preferably, at least one first coating 11a is provided via a colored lacquer. The coating is preferably lacquer-based, particularly preferably epoxyamine lacquer-based or polyesteramine lacquer-based, and has a baking temperature (PMT) of 180°C to 240°C, preferably 200°C to 230°C, particularly preferably 210°C to 220°C, and a holding time of 1 to 20 seconds, preferably 2 to 12 seconds.

[0049] Next, in the furnace 10, the first baking step cures at least one first coating 11a of the aluminum alloy strip, where a degree of crosslinking of at least 20% to 80%, preferably 30% to 70%, and more preferably 45% to 60%, is achieved, and the aluminum alloy strip achieves the result of an unchanged surface on the coating side after the block test described above. Lower baking temperatures reduce the energy required for the first baking step in the furnace 10.

[0050] Subsequently, the semi-finished product 11 according to the present invention, i.e., an aluminum alloy strip provided with at least one first coating, is optionally wound onto a coil 12. As an alternative to winding the semi-finished product 11 onto a coil 12, the semi-finished product 11 may be sent directly to a subsequent coating process for coating at least one second coating to be placed on top of the at least one first coating.

[0051] However, since the semi-finished product according to the present invention has consistent surface results in block testing, the semi-finished product 11 wound on the coil 12 can be easily stored and subsequently further processed to further coat at least one second coating on top of at least one first coating.

[0052] Figure 2 shows the coating in the roll coating method 9 with at least one first coating related to the application of a passivation layer 7 and drying 8. Alternatively, for different process speeds, the application of the passivation layer on the strip surface and the application of at least one first coating can be performed separately in a device that performs only one coating step and one drying step at a time, as shown in Figure 3. This makes it possible to appropriately adapt the strip speed to different application processes and material properties, particularly drying properties.

[0053] In the embodiment shown in Figure 3, the semi-finished product 11 is stretched from the coil 12 and sent to a further coating step 14 in the form of a roll coating process. Again, other alternative coating processes may be used, but they are not shown. The aluminum alloy strip 16, which has been given a two-layer coating, is then cured in a second baking step in a furnace 15 and wound onto the coil 13.

[0054] In the second baking step, at least one first coating and at least one second coating are cured in the furnace 15 to a degree of crosslinking greater than 90%, preferably greater than 95%, and particularly preferably greater than 98%, where the at least one second coating has a higher baking temperature than the at least one first coating. In the furnace 15, both layers are baked at the higher baking temperature.

[0055] In the second baking step in furnace 15, a baking temperature (PMT) of 245°C to 270°C is preferably used, more preferably 245°C to 260°C, and particularly preferably 248°C to 255°C, and the holding time is 1 to 20 seconds, preferably 2 to 12 seconds.

[0056] As shown in Figure 3, the underside of an aluminum alloy strip, for example, to be supplied to the inside of a beverage can, can also be passivated and / or coated in a roll coating process, and then dried or baked in an oven. The use of other coating processes may also be considered.

[0057] Figure 4 shows schematic diagrams of two time-course PMTs of an idealized bake process, resulting from different bake temperatures of 220°C for the first bake step in furnace 10 and 255°C for the second bake step in furnace 15, and the same holding time of 8 seconds. The curves are shown in a highly idealized manner.

[0058] Figure 5 shows both a semi-finished product 11 and a finished aluminum alloy strip 16 based on an aluminum alloy strip, preferably of type AA3004, AA3104, AA3105, AA5052, AA5042, or AA5182, having a metal thickness of 0.12 mm to 0.30 mm, preferably 0.16 mm to 0.25 mm, and particularly preferably 0.16 mm to 0.23 mm. The semi-finished product 11 and the finished aluminum alloy strip 16 are shown in Figure 5 without an optional passivation layer.

[0059] The thickness of the layer of at least one first coating 11a on the semi-finished product 11 and at least one second coating 16a on the finished aluminum alloy strip 16 is preferably 2 g / m² in order to provide the necessary properties for the coating system, such as an aluminum alloy strip for manufacturing can lids or can tabs. 2 ~5g / m 2 That is the case.

[0060] To demonstrate the advantages of the semi-finished product and the finished aluminum alloy strip according to the present invention, various studies were conducted on coated aluminum alloy strips.

[0061] Table 1 shows the results of the investigation. The aluminum alloy strips have an aluminum alloy of type AA5182 and are used for the manufacture of can lids for beverage cans by the manufacturing process schematically shown in Figure 1.

[0062] Two comparative semi-finished products A and C, as well as semi-finished product B according to the present invention, were manufactured from aluminum alloy strips produced in this manner. These semi-finished products differed only in the degree of crosslinking and the baking temperature of each coating with a fixed holding time of 2 to 12 seconds. Conventional coatings with a baking temperature of 250°C to 340°C with a holding time of 1 to 12 seconds were used for comparative semi-finished product A, while the baking temperature for the semi-finished product according to the present invention was 200°C to 290°C, and 150°C to 240°C for the comparative semi-finished products. Each test strip was then baked under the conditions specified in Table 1.

[0063] At least one first coating on comparison strips A and C was baked by PMT at 249°C (comparison band A) and 170°C (comparison band C), respectively. The holding time for each was 2 seconds.

[0064] Differences in the degree of crosslinking were observed after the baking of the first coating in tests A, B, and C. Test A had nearly complete crosslinking at 91% in the "sol fraction test," while semi-finished product B according to the present invention had a degree of crosslinking of 55%, and comparative semi-finished product C had only a degree of crosslinking of 17%. The area weight of at least one first coating of the inspected strip-shaped semi-finished products was all the same at 4 g / m². 2 That was the case.

[0065] Block testing was performed on coated aluminum alloy strips manufactured using this method. In the block testing, two blanks from the semi-finished product, each measuring 10 × 10 cm, were placed on a first flat pressure body with an edge length of, for example, 15 cm × 15 cm, with the first coating facing upwards, and pressed against each other with a pressure of 2.942 kPa via a further pressure body. The blanks were heated to 50°C PMT while pressed together and maintained in this state for 24 hours. The blanks were then separated again, and after separation, the surface changes of the first coating on at least one of the two blanks were examined.

[0066] Since tests A and B showed no change in results, the corresponding test strips A and B could be easily wound onto the coil. A negative result was obtained in the block test against comparison strip C. Here, the excessively low crosslinking rate of 17% due to slight bonding on the surface of the cut-out strip was significant. As a result, the second coating could not be applied to test semi-finished product C because the layers bonded together when wound onto the coil. In this regard, the pressure of at least 2.942 kPa used in the block test determines the behavior of the semi-finished product in the coil state.

[0067] Processability and susceptibility to lacquer peeling were also tested. For this purpose, impact bending tests were performed using Erichsen's impact bending test device Model 471. The impact tests simulate common sheet processing steps such as punching, bending, and flange forming on samples with a size of 50 × 140 mm and a thickness ranging from 0.1 mm to 0.35 mm.

[0068] For the impact bending test, 50 × 140 mm specimens were cut from test strips A and B, each provided with at least a second coating. Each specimen had its longitudinal side perpendicular to the rolling direction and was bent along its longitudinal centerline around a cylindrical bending mandrel with a diameter of 5 mm. The impact stress caused deformation from the previously cylindrical bent edge with a diameter of 5 mm to a conical shape. An evaluation was performed to determine the bending radius at which the coating began to show damage. The smaller the radius, the better the coating could withstand the mechanical load without problems.

[0069] The impact bending test device consists of a hammer with parallel guides, having a weight of 2300 ± 100 g and a drop height of 650 ± 5 mm. A specially molded conical anvil acts as a support for the pre-bent sample sheet. The hammer is placed between two upper retaining pins. The pre-bent sample sheet is placed on the anvil so that one of its two side edges touches the stop. The bending impact is then initiated.

[0070] In 1 liter of distilled water 100g of copper sulfate (CuSO4·5H2O), 50g of citric acid, 200g of 37% hydrochloric acid = 168ml The sample was immersed in the mixed test solution for 5 minutes, and then thoroughly rinsed under running water. Damage to the coating is visualized as corrosion lines or corrosion spots. As a result of the measurement, the length of the outer corrosion line in the area of ​​conical deformation outside the maximum bending is measured in mm.

[0071] Here, Figure 6 shows a schematic plan view of a coated aluminum strip sample after an impact bending test. First, the area of ​​the sample in which the conical anvil did not cause bending is defined as t0. Then, as shown in Figure 6, point t1 is determined where the maximum bending of the sample ends, with both sides overlapping, and the bending transitions to a bending of the sample with a bending radius that increases to the left due to the shape of the anvil.

[0072] Furthermore, the point along the bent edge of the sample where the corrosion lines become invisible due to the attack of the acidic test solution is also indicated as t2 in Figure 6. This is done, for example, with a 10x magnification magnifying glass. The shorter the measurement distance between t1 and t2, the smaller the bending radius that can be enabled by the coating without corrosion problems, and the better the deformability of the coating (see operating instructions for Erichsen's impact bending test device Model 471). Figure 6 also shows schematic diagrams of the corresponding sample cross-sections at points t1 and t2.

[0073] Comparative strip A achieved an average measurement of 25 mm, which corresponds to the tolerance limit. Aluminum alloy strip B according to the present invention achieved an average result of 15 mm in the impact bending test, which shows significantly better processing characteristics. Therefore, they have higher resistance to chipping.

[0074] The semi-finished product according to the present invention has very good processing characteristics, and this test has shown that aluminum alloy strips coated according to the present invention are significantly more resistant to damage than conventionally manufactured aluminum alloy strips with a two-layer coating.

[0075] [Table 1]

Claims

1. A strip-shaped semi-finished product (11) for manufacturing a can, preferably a can lid or can tab for a beverage can, comprising an aluminum alloy strip (4) having at least one first at least partial surface coating (11a) having a crosslinkable coating material, wherein the at least one first at least partial surface coating (11a) is provided on the side of the aluminum alloy strip (4) that will be used on the outside of the can. The aluminum alloy strip (4) has at least one first coating (11a) with a degree of crosslinking of at least 20% to 80%, preferably 30% to 70%, more preferably 45% to 60%, and the degree of crosslinking is measured according to the “sol fraction test” referred to in the specification, and the aluminum alloy strip (4) achieves the result of an unchanging surface on the coating side in a block test according to the specification, and for the block test, two blanks from the semi-finished product (11) having partially crosslinked first coatings pointing in the same direction are placed overlapping between two flat pressure bodies, and the overlapping blanks are pressed against each other from above the pressure bodies with a pressure of at least 2.942 kPa, and are heated in this state at 50°C PMT and held for 24 hours, and then the blanks are separated again, and after the separation of the blanks, the surface changes of the coating (11a) of the two blanks are investigated. A semi-finished product characterized by the following features.

2. The semi-finished product according to claim 1, characterized in that the at least one first coating (11a) of the aluminum alloy strip (4) has a PMT (peak metal temperature) of 180°C to 240°C, preferably 200°C to 230°C, and particularly preferably 210°C to 220°C, and has a holding time of 1 to 20 seconds, preferably 2 to 12 seconds.

3. The semi-finished product according to claim 1 or 2, characterized in that the at least one first coating (11a) of the aluminum alloy strip (4) is formed of an epoxyamine lacquer system or a polyesteramine lacquer system.

4. The at least one first coating (11a) of the aluminum alloy strip (4) is 2 g / m ２ ~5g / m ２ A semi-finished product according to any one of claims 1 to 3, characterized by having a layer thickness of the specified amount.

5. The semi-finished product according to any one of claims 1 to 4, characterized in that the strip-shaped semi-finished product (11) is wound around a coil (12).

6. The semi-finished product according to any one of claims 1 to 5, characterized in that the aluminum alloy strip is an aluminum alloy of type AA3004, AA3104, AA3105, AA5042, AA5052, or AA5182, and the metal thickness of the aluminum alloy strip is 0.12 mm to 0.30 mm, preferably 0.16 mm to 0.25 mm, and particularly preferably 0.16 mm to 0.23 mm.

7. The semi-finished product according to any one of claims 1 to 6, characterized in that the at least one first coating (11) is a colored lacquer layer.

8. The semi-finished product according to any one of claims 1 to 7, characterized in that the semi-finished product has a passivation layer on the side of the aluminum alloy strip which is used for adhesion to the outside and / or inside of the can before coating each side.

9. A coated aluminum alloy strip (16) for manufacturing a can lid or can tab, preferably a beverage can, manufactured from a semi-finished product (11) according to any one of claims 1 to 8, the aluminum alloy strip having at least one second coating (16a) disposed on the at least one first coating (11) of the semi-finished product (11).

10. The aluminum alloy strip according to claim 9, characterized in that the at least one first coating (11a) and the at least one second coating (16a) have a degree of crosslinking greater than 90%, preferably greater than 95%, and particularly preferably greater than 98%, the at least one first coating (11a) has a lower baking temperature than the second coating (16a), preferably the baking temperature of the at least one second coating is PMT at 245°C to 270°C, more preferably 245°C to 260°C, and particularly preferably 248°C to 255°C, and the holding time is 1 to 20 seconds, preferably 2 to 12 seconds.

11. The aluminum alloy strip according to claim 9 or 10, characterized in that the at least one second coating (16a) is an epoxyamine lacquer or polyesteramine lacquer.

12. The above-mentioned at least the second coating (16a) is 2 to 5 g / m² ２ An aluminum alloy strip according to any one of claims 9 to 11, characterized in that it has an area weight and the at least one second coating (16a) is optionally a transparent lacquer layer.

13. The aluminum alloy strip (16) has an aluminum alloy of type AA5182, and after the baking of the at least one first coating and one second coating, the aluminum alloy strip (16) has a tensile strength R of 380 MPa to 425 MPa. ｍ and yield strength R of 330 MPa to 380 MPa ｐ０．２ An aluminum alloy strip according to any one of claims 9 to 12, characterized by having the following:

14. A method for manufacturing an aluminum alloy strip according to any one of claims 9 to 13, using a semi-finished product according to any one of claims 1 to 8, A method characterized in that a semi-finished product (11) is produced by coating an aluminum alloy strip (4) with at least one first coating (11a), the at least one first coating (11a) of the semi-finished product (11) is cured in a first baking step to a degree of crosslinking of 20% to 80%, preferably 30% to 70%, more preferably 45% to 60%, the semi-finished product (11) achieves the result of an unchanging surface on the coated side in a block test according to the specification, the at least one first coating (11a) of the semi-finished product (11) is coated with at least one second coating (16a), and then in a second baking step the at least one first coating and the at least one second coating (11a, 16a) are cured.

15. The method according to claim 14, characterized in that the at least one first coating (11a) is cured at a baking temperature lower than the baking temperature of the second coating (16a) placed on the first coating, and the at least one first coating (11a) is preferably partially cured at a baking temperature (PMT) of 180°C to 240°C, preferably 200°C to 230°C, particularly preferably 210°C to 220°C, by a holding time of at least 1 to 20 seconds, preferably 2 to 12 seconds.

16. The method according to claim 14 or 15, characterized in that the semi-finished product (11) is wound onto a coil after the coating by the at least one first coating (11a) and the partial curing of the at least one first coating (11a).

17. The method according to any one of claims 14 to 16, characterized in that, after the coating with the at least one second coating (16a), the at least one first coating (11a) and the at least one second coating (16a) are cured together in a second baking step having a baking temperature, wherein the baking temperature in the second baking step is higher than the baking temperature of the at least one first coating, and in the second baking step, the curing is preferably carried out in a PMT of 245°C to 270°C, preferably 245°C to 260°C, particularly preferably 248°C to 255°C, for a holding time of 1 to 20 seconds, preferably 2 to 12 seconds, and the at least one first coating (11a) and the at least one second coating (16a) are cured to a degree of crosslinking greater than 90%, preferably greater than 95%, and particularly preferably greater than 98%.

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