Two-layer can strip coating

Abstract

The invention relates to a strip-shaped pre-product (11) for producing can lids or can tabs of a can, preferably of a beverage can, comprising an aluminum alloy strip (4) having at least one first coating (11a) over at least part of the surface, which coating is provided on the side of the aluminum alloy strip (4) that is used for the outside of the can. The invention further relates to a coated aluminum alloy strip (16) for producing a can lid or a can tab, preferably of a beverage can, which coated aluminum alloy strip is produced from a strip-shaped pre-product (11) according to the invention comprising at least one second coating (16a) which is arranged on the at least one first coating (11a) of the pre-product (11). The problem of providing a simple way of providing aluminum alloy strips with more resistant lacquer layers, preferably colored lacquer layers, for the production of can lids or can tabs, in particular beverage can lids and beverage can tabs, is solved by a strip-shaped pre-product according to claim (1) and with an aluminum alloy strip according to claim 9 as well as by a method for producing same according to claim (14).

Description

[0001] Two-layer can strip coating

[0002] The invention relates to a strip-shaped precursor for producing can ends or can tabs of a can, preferably a beverage can, comprising an aluminum alloy strip with at least one first, at least partial, coating comprising a crosslinkable coating substance provided on the side of the aluminum alloy strip used for the outer side of the can. Furthermore, the invention relates to an aluminum alloy strip for producing a can end or a can tab, preferably a beverage can, made from a strip-shaped precursor according to the invention, comprising at least one second coating arranged on the at least one first coating of the precursor. Finally, the invention relates to a method for producing an aluminum alloy strip according to the invention.

[0003] Can ends and can tabs, preferably for beverage cans, are often manufactured using coated aluminum alloy strip. The coated aluminum alloy strips are adapted to the respective application, i.e., to the specific can application, both in terms of the aluminum alloy strip used and the coating. This results in different requirements for the inside and outside of a can, preferably a beverage can. While the coated inside of a can generally has to protect the aluminum alloy from the influences of the can's contents and, conversely, the can's contents from the aluminum alloy, the outside, in addition to improving the forming properties, primarily has the task of determining the appearance, i.e., the visual appearance, of the can, especially the beverage can.Colored paints and clear coats are therefore increasingly being used in the coating of aluminum alloy strip for the production of can ends or can tabs. Most paints or other coatings are typically subjected to a curing process involving heating, also known as a baking process. This process plays an important role in the final properties of the coated aluminum alloy strip, as the baking process influences or determines its strength.

[0004] The baking process is carried out at the so-called baking temperature. The baking temperature is the temperature required for the complete curing of coating materials within a specified baking time. In this document, it is always specified as the "peak metal temperature" (PMT) of the aluminum alloy strip. The holding time at the maximum temperature (PMT) is also specified to determine the baking process. The PMT is only reached after a certain preheating time; it is therefore usually lower than, for example, the circulating air temperature of a convective baking oven. The PMT can be easily measured using thermocouples on test specimens passing through the oven.

[0005] Colored lacquers are increasingly being used, particularly those that contrast sharply with the natural aluminum color on the outside of cans, for example on can ends. This creates problems when processing aluminum alloy strips. Can ends are stored in stacks for processing and then further processed after storage. During storage or further processing in the can manufacturing process, the ends rotate against each other. Lacquer defects due to scraping or flaking of lacquer develop particularly quickly on the protruding crown, a raised portion of the can end. The lacquer layer in these areas is particularly sensitive on the top side of the crown after forming, as the lacquer layer here has to withstand the high tensile stresses caused by the forming process.In addition, due to the small contact surface compared to other can ends, the Crown is subjected to comparatively high contact pressures, further increasing the risk of paint damage. Due to the strong contrast between the uncoated, silver-colored aluminum and the colored paint layer, even extremely small chips in the colored paint layer can be visible to the naked eye. The damage to the paint layer results in an undesirable appearance of the can, such as a beverage can. The same applies to the can tabs.

[0006] To reduce the risk of paint damage, the colored paint layer is coated with a clear varnish, creating a two-layer paint system. The corresponding aluminum alloy strip therefore undergoes two separate baking steps, which are usually carried out at a uniform baking temperature. Compared to a single baking step, this leads to a reduction in the strength of the aluminum alloy strip because the baking temperatures of both coatings initiate softening processes in the aluminum alloy strip. The strength of the aluminum alloy strip is significantly reduced compared to aluminum alloy strips exposed to only a single baking process. This applies in particular to the usual baking temperatures of 245 °C to 270 °C, at which the temperature threshold for recrystallization of the aluminum alloy in the aluminum alloy strip is sometimes exceeded.

[0007] Furthermore, the two-layer system continues to result in flaking of the clear coat because the clear coat lacks sufficient adhesion to the colored coat in some areas. This is also visible on the finished can. It has also been shown that the temperatures of the baking steps can also influence the color of the coat, and discoloration of the colored coat can occur. Finally, the energy required to produce a two-layer coating system with identical baking steps is significantly higher than for an aluminum alloy strip with a single-layer coating. US Patent 4,253,584 discloses a two-layer coated aluminum strip in which both coats are cured separately. Partial crosslinking of the first coating is unknown.

[0008] The international patent application WO 2008 / 036628 Al shows an aluminum strip for a beverage can coated with a water-based coating, which is formed in only one layer.

[0009] The use of polyester as a replacement for PVC in a single-layer coating on the inside of a can end is known from the international patent application WO 98 / 23198 A1.

[0010] Single-layer coatings for beverage cans that do not release potentially hazardous substances are provided by the international patent application WO 2004 / 013240 A1 for beverage cans.

[0011] The international patent application W02015 / 002958 Al discloses single-layer, latex-containing coatings for beverage containers.

[0012] The invention therefore aims to provide a simple method for producing aluminum alloy strips with less sensitive coatings, preferably colored coatings, for the production of can ends or can tabs, in particular beverage can ends and beverage can tabs. Preferably, the aluminum alloy strip should also require less energy for production.

[0013] According to a first teaching, the above-mentioned object is achieved by a strip-shaped precursor product in that the 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%, more preferably 45% to 60%, and the aluminum alloy strip achieves an unchanged surface result of the coated side in a so-called block test, wherein for the block test, two blanks, for example with the dimensions 10 cm x 10 cm, from the precursor product with their partially crosslinked, first coatings pointing in the same direction are placed on top of each other between two flat pressure bodies, the superimposed blanks are pressed against each other via the pressure bodies with a pressure of at least 2,942 kPa and are heated and maintained in this state at 50 °C PMT for 24 hours,The blanks are then separated again and, after separating the blanks, the surface of the coating of the two blanks is examined for changes.

[0014] An unaltered surface finish is achieved when the coated sides of the blanks exhibit no visible optical changes after separation, i.e., remain unchanged. With this result, the pre-product can be wound into a coil and later processed without any problems.

[0015] A negative result in the block test, on the other hand, is characterized by a change in the surface due to the formation of dull spots in the area where the blanks were subjected to pressure after separation. These dull spots on the surface are due to excessive adhesion of the blanks in the block test. This leads to damage to the coating surface when the blanks are separated.

[0016] Furthermore, even stronger adhesion of the blanks during the block test can even lead to partial or complete tearing of the coating on a blank. Both dull spots and partial or complete tearing of the coating lead to a change in the surface of the coating. As a result, the precursor cannot be wound onto a coil for further processing.

[0017] The degree of crosslinking of at least one initial coating can be determined, for example, using the so-called "sol fraction test." In this test, a sample of fixed geometry of the precursor 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 two minutes (loaded). This step essentially removes any uncrosslinked coating components. The sample is reweighed, and the paint layer is then removed so that the sample consists only of metal (metal). Finally, the sample is weighed once more. The quotient of the difference between the weight of the unloaded sample and the loaded sample and the difference between the weight of the unloaded sample and the metal weight then yields the uncrosslinked coating component.

[0018] It has surprisingly been found that the ribbon-shaped precursor according to the invention can provide significantly improved crosslinking between the first and second coatings due to the undercrosslinked state of the at least one first coating. It was found that after coating with the at least one second coating and baking of both layers, crosslinking also takes place significantly between the layers. This means that the at least one second coating adheres significantly better to the first coating. The flaking of a clear coat, for example, at critical points on a can end was significantly reduced. The unchanged surface after the block test ensures that the precursor according to the invention with the undercrosslinked coating does not tend to stick, for example when wound onto a coil.If the block test is successful, the precursor product can be wound onto a coil and stored without causing problems during subsequent processing. The precursor product according to the invention can therefore, for example, be easily unwound after storage and processed into the finished coated aluminum alloy strip for can ends or can tabs. This enables economical production of an aluminum alloy strip for the manufacture of can ends and can tabs, in particular for beverage cans with at least two-layer coating. Optionally, before being coated with the at least one first coating, the aluminum alloy strip has a passivation layer, which is provided on one or both sides and on which the at least one first coating is arranged. The passivation layer is preferably chromium-free.This results in processing advantages with regard to safety precautions regarding chromium-containing chemicals in operation. The passivation layer can be provided, for example, by zirconium phosphating the aluminum alloy strip. The passivation layer increases the adhesion of the at least one first coating to the aluminum alloy strip.

[0019] If, according to a first embodiment of the preliminary product according to the invention, the at least one first coating of the aluminum alloy strip has a baking temperature (PMT) between 180°C and 240°C, preferably 200°C to 230°C, particularly preferably 210°C to 220°C, with a holding time of 1 to 20 seconds, preferably 2 to 12 seconds, the reduced baking temperature can minimize the strength loss of the aluminum alloy strip during baking of the at least one first coating, while still achieving partial curing of the at least one first coating. At the same time, less energy is required for the first baking process due to the lower baking temperature.

[0020] A good adjustability of the baking temperature via the coating composition can be achieved by a subsequent embodiment in which the at least one first coating of the aluminum alloy strip is formed by an epoxy-amino coating system or a polyester-amino coating system. The coating systems can be easily adjusted by the coating manufacturer to predetermined baking temperatures by changing the chemical composition, for example, by changing the binder. Since the baking time depends in particular on the layer thickness of the at least one first coating, it is advantageous if the at least one first coating of the aluminum alloy strip has a layer thickness of 2 g / m 2 up to 5 g / m 2This layer thickness ensures short curing times while simultaneously providing good crosslinking and adhesion to at least a second coating. At the same time, these layer thicknesses are sufficient to ensure adequate coloration in a colored coating.

[0021] Due to the excellent properties of the inventive strip-shaped precursor in the block test, it can be easily further processed or stored by winding the strip-shaped precursor onto a coil. This allows the precursor to be easily fed into further coating steps, for example, ensuring cost-effective production.

[0022] If, according to a further embodiment, the aluminum alloy strip of the precursor product comprises an aluminum alloy of the 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, particularly preferably 0.16 mm to 0.23 mm, aluminum alloy strips for the production of can ends or can tabs with the necessary strengths can be provided economically.

[0023] According to a preferred embodiment, the at least one first coating is a colored lacquer layer. As already explained above, the precursor according to the invention can be advantageously used for the production of aluminum alloy strips for can ends and can tabs, which are particularly resistant to paint flaking, i.e., resistant to scratches and abrasions of lacquer layers. Finally, according to a further embodiment, the precursor has a passivation layer on the side of the aluminum alloy strip used for the outside and / or the inside of the can to promote adhesion before coating the respective sides. A passivation layer achieves improved 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. Optionally, the passivation of the side of the aluminum alloy strip intended for the inside can also be carried out after the application of the at least one first coating to the side of the aluminum alloy strip intended for the outside.

[0024] According to a further teaching, the indicated object is achieved by an aluminum alloy strip for producing a can end or a can flap, preferably a beverage can, in that it is produced from a strip-shaped precursor according to the invention and in that at least one second coating is provided, which is arranged on the at least one first coating of the precursor. As already explained above, the aluminum alloy strip according to the invention has improved crosslinking between the at least one first and the at least one second coating. The aluminum alloy strip having a two-layer coating therefore differs from conventionally produced aluminum alloy strips in that the two coatings are more strongly crosslinked with one another and thus in that the two coatings have stronger adhesion.At the same time, the use of the precursor product according to the invention ensures particularly economical production of the aluminum alloy strips.

[0025] According to a first embodiment, the at least one first coating and the at least one second coating have a degree of crosslinking of more than 90%, preferably more than 95%, particularly preferably more than 98%, wherein the at least one first coating has a lower baking temperature than the second coating, and preferably the baking temperature of the at least one second coating is 245 °C to 270 °C PMT, preferably 245 °C to 260 °C PMT, particularly preferably 248 °C to 255 °C PMT with a holding time of 1 to 20 seconds, preferably 2 to 12 seconds.

[0026] Due to the lower curing temperatures of the at least one first coating, an aluminum alloy strip with a two-layer coating can be produced, which avoids the aforementioned disadvantages of increased softening and significantly higher energy consumption. Due to the improved crosslinking between the at least one first and at least one second coating, the two-layer coating is also more scratch-resistant than conventionally produced two-layer coatings.

[0027] The aluminum alloy strip according to the invention preferably has exactly two coatings with a degree of crosslinking of more than 90%, preferably more than 95%, particularly preferably more than 98%. Such two-layer coatings can meet the requirements for can ends and can tabs in a particularly economical manner, since each additional coating requires additional baking processes.

[0028] As already explained above, the curing temperatures can be adjusted very precisely by applying at least one second coating using an epoxy-amino coating system or a polyester-amino coating system. The aforementioned coating systems can be easily adjusted by selecting a modified composition with regard to the curing temperatures, for example, by changing the binder.

[0029] The at least one second coating has a substantially identical layer thickness to the at least one first coating. For this purpose, the at least one second coating has a basis weight of preferably 2 to 5 g / m 2 Furthermore, the at least one second coating is optionally formed as a clear coat, so that it can protect the colored coat, for example, without disturbing the color impression of the colored coat.

[0030] If, according to a next embodiment, the aluminum alloy strip comprises an aluminum alloy of type AA5182 and, after firing the at least one first and one second coating, the tensile strength Rm of the aluminum alloy strip is 380 MPa to 425 MPa and the yield strength R P o,2 of the aluminum alloy strip 330 MPa to 380 MPa, high-strength aluminum alloy strips for can ends can be provided which have at least a two-layer coating, preferably a two-layer coating on the outside.

[0031] According to a further teaching of the invention, the object for a method for producing an aluminum alloy strip with at least one first coating and at least one second coating arranged on the first coating is achieved in that firstly a precursor is produced by coating an aluminum alloy strip with at least one first coating, wherein the at least one first coating of the precursor 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%, wherein the precursor achieves an unchanged surface result of the coated side after a block test,and the at least one first coating of the precursor is coated with at least one second coating, and then the at least one first and the at least one second coating are cured together in a second baking step. Preferably, the precursor can be wound onto a coil and stored before the second coating is applied. It has been shown that the problems with flaking coating areas, for example on can ends or can tabs, can be significantly reduced by improving the adhesion between the previously not fully crosslinked,at least one first coating and the at least one second coating arranged on the at least one first coating can be provided by baking both coatings in the second baking step. This results in significantly stronger cross-linking of the two coatings. At the same time, by producing a storable precursor product with a subsequent second coating process to produce the finished aluminum alloy strip, high cost-effectiveness can be achieved in the production of the coated aluminum alloy strip for the manufacture of can ends or can tabs, while simultaneously providing an advantageous at least two-layer coating system on the aluminum alloy strip.

[0032] If, according to a first embodiment of the method, the at least one first coating is cured at a baking temperature which is lower than the baking temperature of the second coating arranged on the first coating, wherein preferably the at least one first coating is 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 with a holding time of at least 1 to 20 seconds, preferably 2 to 12 seconds, not only is less energy required for the baking process, but the softening of the aluminum alloy strip is also limited despite providing a two-layer coating system. As a result, fully baked aluminum alloy strips having at least two-layer coatings can be provided with lower energy requirements.

[0033] If the pre-product is wound into a coil after coating with the at least one first coating and partial curing of the at least one first coating, it can be easily stored and made available for subsequent coating with the at least one second coating.

[0034] Preferably, after coating with the at least one second coating, the at least one first coating and the at least one second coating are cured together in a second baking step, wherein the baking temperature in the second baking step is higher than the baking temperature of the at least one first coating, wherein preferably in the second baking step the curing takes place at a baking temperature (PMT) of 245 °C to 270 °C, preferably 245 °C to 260 °C, particularly preferably 248 °C to 255 °C with a holding time of 1 to 20 seconds, preferably 2 to 12 seconds. This achieves reliable curing of the two-layer coating system and, at the same time, a coated aluminum alloy strip with lower softening than with conventional aluminum alloy strips with two-layer coating systems is achieved.

[0035] The invention will be explained in more detail below using exemplary embodiments. The drawing shows

[0036] Fig. 1 shows a schematic sectional view of the production of an aluminum alloy strip for cans, in particular beverage cans,

[0037] Fig. 2 shows a schematic sectional view of a process for producing a precursor product by coating an aluminum alloy strip,

[0038] Fig. 3 shows a schematic sectional view of the process for producing an aluminum alloy strip with two layers of lacquer arranged on top of one another using the precursor product according to the invention, Fig. 4 shows a schematic diagram of the course of the baking process over time at different baking temperatures but with the same holding time,

[0039] Fig. 5 embodiments of the preliminary product according to the invention and the aluminum alloy strip according to the invention in a schematic sectional view and

[0040] Fig. 6 shows a schematic plan view of a sample of a painted strip with markings for the evaluation of the impact folding test.

[0041] Fig. 1 shows a schematic view of the individual process steps for producing an aluminum alloy strip from the production of the rolling ingot to the cold rolling of the aluminum alloy strip to the final thickness.

[0042] First, a rolling ingot 1 is produced, for example using the DC casting process. Analogously, a strip casting process (not shown) can also be used to produce a cast strip. The rolling ingot 1 is then subjected to homogenization in step 2a and then hot-rolled into a hot strip 3 in step 3a. Hot rolling can take place in reversing stands and / or in tandem stands with multiple passes. The hot strip is then cold-rolled to the final thickness in step 4a into a cold strip 4. Cold rolling can take place in rolling stands with a single rolling pass or in multiple stands with two or more rolling passes. During cold rolling, one or more intermediate annealing steps 4b can take place in the batch furnace 5 or in a continuous furnace (not shown).A final heat treatment is also not excluded, although the cold-rolled aluminum alloy strips are preferably fed to the next coating process step in the as-rolled state (hardened to H18 or H19). At the end of cold rolling, the cold-rolled aluminum alloy strip 4 is preferably wound onto a coil 6. An exemplary embodiment of a method for producing a precursor by coating an aluminum alloy strip is shown in Fig. 2. The cold-rolled aluminum alloy strip 4 is unwound from a coil 6 and fed to an optional passivation step 7, which is designed here as a roll-coating process. Alternatively, the passivation chemical can also be sprayed on, for example, electrostatically sprayed. Further alternatively, the passivation can also be carried out by passing the strip through a bath containing the passivation liquid.However, the roll-coating process has proven successful due to the high throughput speeds possible and the high precision of the passivation chemical application. For surface passivation, no-rinse processes are preferred, in which the passivating agent, preferably zirconium phosphate, remains on the aluminum strip and does not need to be rinsed off. For this purpose, the aluminum alloy strip 4 coated with a passivation chemical is dried in an oven 8 after the passivation chemical has been applied.

[0043] Not shown in Fig. 2 is an optional pretreatment of the underside of the aluminum alloy strip 4, which can also be provided with a passivation layer, for example. The passivation agent can then be dried. Zirconium phosphate is also preferred here. Optionally, both sides of the aluminum alloy strip have passivation layers.

[0044] In the next step, the aluminum alloy strip 4 is coated with at least one first coating 11a, for example, using the roll-coating method 9 to provide the preliminary product according to the invention. Alternatively, it is also conceivable to use other application methods that are not shown here. Preferably, the at least one first coating 11a is provided using a colored lacquer. The coating preferably comprises a lacquer system, particularly preferably an epoxy-amino lacquer system or a polyester-amino lacquer system, which has a baking temperature (PMT) between 180°C and 240°C, preferably 200°C to 230°C, particularly preferably 210°C to 220°C, with a holding time of 1 to 20 seconds, preferably 2 to 12.

[0045] In the furnace 10, the at least one first coating 11a of the aluminum alloy strip is then cured in a first baking step, achieving a degree of crosslinking of at least 20% to 80%, preferably 30% to 70%, more preferably 45% to 60%, and the aluminum alloy strip achieves an unchanged surface finish on the coated side after the block test described above. Due to the lower baking temperature, less energy is required for the first baking step in the furnace 10.

[0046] Subsequently, the preliminary product 11 according to the invention, i.e., the aluminum alloy strip provided with at least one first coating, is optionally wound into a coil 12. Alternatively to winding the preliminary product 11 into a coil 12, the preliminary product 11 could be fed directly to the next coating process for coating the at least one second coating arranged on the at least one first coating.

[0047] Since the preliminary product according to the invention has an unchanged surface result in the block test, the preliminary product 11 wound on the coil 12 can, however, be readily stored and later processed for further coating with at least one second coating on the at least one first coating.

[0048] In Fig. 2, the coating in the roll coating process 9 with at least a first

[0049] Coating is shown in conjunction with the application 7 and drying 8 of the passivation layer. Alternatively, due to different process speeds, the application of the passivation layer to the strip surfaces and the application of the at least one first coating can also be carried out separately, namely in devices which, as shown in Fig. 3, each perform only one coating and one drying step. This allows the strip speeds to be optimally adapted to the different application processes and material properties, in particular drying properties.

[0050] Fig. 3 now shows an embodiment in which the precursor 11 is unwound from coil 12 and fed to a further coating step 14 in the form of a roll-coating process. Other alternative coating processes could also be used here, but these are not shown. Subsequently, the aluminum alloy strip 16, provided with a two-layer coating, is cured in a second baking step in furnace 15 and wound onto a coil 13.

[0051] In the second baking step, the at least one first coating and the at least one second coating are cured in oven 15 to a degree of crosslinking of more than 90%, preferably more than 95%, particularly preferably more than 98%, with the at least one second coating having a higher baking temperature than the at least one first coating. In oven 15, both layers are baked at the higher baking temperature.

[0052] Preferably, in the second baking step in the oven 15, a baking temperature (PMTJ) of 245°C to 270°C, preferably 245°C to 260°C, particularly preferably 248°C to 255°C, is used with a holding time of 1 to 20 seconds, preferably 2 to 12 seconds.

[0053] Analogous to the coating step shown in Fig. 3, the lower side of the aluminum alloy strip, which is intended, for example, for the inside of the beverage can, can also be provided with a passivation and / or coating using the roll-coating process, which is then dried or baked in an oven. The use of other coating processes is also conceivable here.

[0054] Fig. 4 schematically shows two time profiles of the PMT of idealized curing processes, which result from different curing temperatures: 220 °C for the first curing step in furnace 10 and 255 °C for the second curing step in furnace 15, with an identical holding time of 8 seconds. The curves are shown in a highly idealized manner.

[0055] Starting from the aluminum alloy strip, which preferably comprises an aluminum alloy of type AA3004, AA3104, AA3105, AA5052, AA5042, or AA5182 and has a metal thickness of 0.12 mm to 0.30 mm, preferably 0.16 mm to 0.25 mm, particularly preferably 0.16 mm to 0.23 mm, both the precursor 11 and the finished aluminum alloy strip 16 are shown in Fig. 5 without the optional passivation layer.

[0056] The layer thicknesses of the at least one first coating 11a of the precursor 11 and the at least one second coating 16a of the finished aluminum alloy strip 16 are preferably 2 g / m 2 up to 5 g / m 2 to provide the necessary properties for the coating system of, for example, an aluminum alloy strip for the production of can ends or can tabs.

[0057] Various tests were conducted on coated aluminum alloy strips to demonstrate the advantages of the precursor product according to the invention and the finished aluminum alloy strip according to the invention. The results of the tests are presented in Table 1. Aluminum alloy strips comprising an aluminum alloy of type AA5182 were investigated, which is typically used for the production of can ends for beverage cans and was manufactured according to the manufacturing process schematically shown in Fig. 1.

[0058] Starting from the aluminum alloy strips thus produced, two comparative precursors A and C and one inventive precursor B were manufactured. The precursors differ only in the degree of crosslinking and the baking temperature with a fixed holding time of 2 to 12 seconds for the respective coating. While comparative precursor A used a conventional coating with a baking temperature of 250 °C to 340 °C and a holding time of 1 to 12 seconds, the baking temperature with an identical holding time was 200 °C to 290 °C for the inventive precursor and 150 °C to 240 °C for the comparative precursor. The respective test strips were then baked under the conditions specified in Table 1.

[0059] At least one first coating of the comparison tapes A and C was baked with a PMT of 249 °C (comparison tape A) and 170 °C (comparison tape C), respectively. The holding time was 2 s in each case.

[0060] Differences arose after baking the first coating in Experiments A, B, and C due to their degree of crosslinking. While Experiment A exhibited almost complete crosslinking of 91% in the sol fraction test, the inventive precursor B exhibited a degree of crosslinking of 55%, and the comparative precursor C only a degree of crosslinking of 17%. The basis weights of at least one first coating of the tested strip-shaped precursors were all identical and amounted to 4 g / m². 2 .

[0061] A block test was carried out on the coated aluminum alloy strips produced in this way. In the block test, two blanks made from the precursor product, measuring 10 x 10 cm and made from the coated aluminum alloy strip, were placed on a first flat pressure body, for example with an edge length of 15 cm x 15 cm, with the first coating facing upwards, and pressed against each other using another pressure body at a pressure of 2,942 kPa. While pressed together, the blanks were heated to 50 °C PMT and held in this state for 24 hours. The blanks were then separated again, and after separation, the surface of at least one of the first coatings on both blanks was examined for changes.

[0062] Tests A and B showed unchanged results, so the corresponding test strips A and B could be wound into a coil without any problems. The comparison strip C produced a negative result in the block test. The insufficient degree of cross-linking (17%) was noticeable in the slight sticking of the surfaces of the blanks. As a result, a second coating could not be applied to the test precursor C because the layers would stick together during winding onto a coil. The pressure used in the block test, at least 2,942 kPa, therefore determines the behavior of the precursor in the coiled state.

[0063] Furthermore, the processability and susceptibility to paint flaking were investigated. For this purpose, an impact folding test was conducted using an Erichsen Model 471 impact folding tester. The impact folding test simulates common sheet metal processing steps such as punching, folding, and flanging on specimens measuring 50 x 140 mm and with thicknesses ranging from 0.1 mm to 0.35 mm.

[0064] For the impact bending test, a 50 x 140 mm sample was cut from each of the test strips A and B, each coated with at least the second coating, with the long side perpendicular to the rolling direction. The sample was bent along the long centerline around a 5 mm diameter cylindrical bending mandrel. The deformation occurs from a previously cylindrical bending edge with a diameter of 5 mm into a conical one due to impact stress. The bending radius at which the coating begins to show damage is assessed. The smaller the radius, the better the coating can withstand mechanical stress without problems.

[0065] The impact-folding tester consists of a parallel-guided impact hammer weighing 2300 ± 100 g and a drop height of 650 ± 5 mm. A specially shaped, conical anvil serves as a support for the pre-bent sample sheet. The impact hammer is suspended between the two upper retaining pins. The pre-bent sample sheet is placed on the anvil so that one of its two side edges abuts the stop. The folding impact is then triggered.

[0066] In a test liquid containing 11 distilled water

[0067] 100 g copper sulfate (Cu S04 • 5 H2O),

[0068] 50 g citric acid,

[0069] The samples were immersed in a solution containing 200g of 37% hydrochloric acid (168ml), for 5 minutes and then thoroughly rinsed under running water. Damage to the coating becomes visible either as corrosion lines or as corrosion spots. The measurement result is the length of an outer corrosion line in the area of ​​conical deformation outside the maximum fold, measured in mm.

[0070] Fig. 6 shows a schematic plan view of a sample of coated aluminum strip after the impact bending test. First, the area of ​​the sample is defined as t0 where the conical anvil did not bend the sample. Then, the point t1 is determined at which the maximum folding of the sample, where both sample sides lie on top of each other, ends and transitions into a bend of the sample with a bending radius increasing to the left in Fig. 6 due to the anvil geometry. Furthermore, t2 in Fig. 6 marks the point along the bending edge of the sample at which a corrosion line is no longer visible due to the attack of the acidic test liquid. This is carried out, for example, using a magnifying glass with 10x magnification.The shorter the measured distance between ti and t2, the smaller the bending radii the coating allows without corrosion problems and the better the coating's formability (see the operating instructions for the Erichsen Model 471 impact bending tester). Fig. 6 also shows the corresponding specimen cross-sections at points ti and t2 schematically.

[0071] The comparison strips A achieved measured values ​​of 25 mm on average, which corresponded exactly to the permissible limit. The aluminum alloy strips B according to the invention achieved results of 15 mm in the impact bend test, indicating significantly better processing properties. They are therefore more resistant to spalling.

[0072] The tests showed that the precursors according to the invention have very good processing properties and that the aluminum alloy strips coated according to the invention are significantly more resistant to damage than previously manufactured aluminum alloy strips with a conventionally produced two-layer coating.

[0073] Table 1

Claims

Patent claims 1. Strip-shaped precursor product (11) for the production of can ends or can tabs of a can, preferably a beverage can, comprising an aluminum alloy strip (4) with at least one first, at least partial, coating (11a) comprising a crosslinkable coating substance, which is provided on the side of the aluminum alloy strip (4) used for the outside of the can, characterized in that the at least one first coating (11a) of the aluminum alloy strip (4) has a degree of crosslinking of at least 20% to 80%, preferably 30% to 70%, more preferably 45% to 60%, wherein the degree of crosslinking is measured according to the "sol fraction test" mentioned in the description and the aluminum alloy strip (4) achieves an unchanged surface result of the coated side in a block test according to the description, wherein for the block test two blanks from the precursor product (11) with their partially crosslinked,first coatings are placed on top of each other facing in the same direction between two flat pressure bodies, the superimposed blanks are pressed against each other via the pressure bodies with a pressure of at least 2.942 kPa and are heated and maintained in this state at 50 °C PMT for 24 hours, then the blanks are separated again and after separating the blanks, the surface of the coating (11a) of the two blanks is examined for changes.

2. Precursor product according to claim 1, characterized in that the at least one first coating (11a) of the aluminum alloy strip (4) has a baking temperature between 180°C and 240°C PMT (peak metal temperature), preferably 200°C to 230°C PMT, particularly r,r '7,,rrt 71 n° r up to 220 °C PMT with a holding time of 1 to 20 seconds, preferably 2 to 12 seconds.

3. Precursor 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 by an epoxy-amino lacquer system or a polyester-amino lacquer system.

4. Pre-product according to one of claims 1 to 3, characterized in that the at least one first coating (11a) of the aluminum alloy strip (4) has a layer thickness of 2 g / m 2 up to 5 g / m 2 has.

5. Pre-product according to one of claims 1 to 4, characterized in that the strip-shaped pre-product (11) is wound on a coil (12).

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

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

8. Precursor according to one of claims 1 to 7, characterized in that the precursor product has a passivation layer on the side of the aluminum alloy strip used for the outside and / or inside of the can to promote adhesion before coating the respective side.

9. Coated aluminum alloy strip (16) for producing a can end or a can tab, preferably a beverage can, made from a precursor (11) according to claims 1 to 8, comprising at least one second coating (16a) which is arranged on the at least one first coating (11) of the precursor (11).

10. 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 of more than 90%, preferably more than 95%, particularly preferably more than 98%, wherein the at least one first coating (11a) has a lower baking temperature than the second coating (16a), and preferably the baking temperature of the at least one second coating is 245 °C to 270 °C PMT, preferably 245 °C to 260 °C PMT, particularly preferably 248 °C to 255 °C PMT with a holding time of 1 to 20 seconds, preferably 2 to 12 seconds 11. Aluminum alloy strip according to claim 9 or 10, characterized in that the at least one second coating (16a) comprises an epoxy-amino lacquer system or a polyester-amino lacquer system.

12. Aluminium alloy strip according to one of claims 9 to 11, characterized in that the at least second coating (16a) has a basis weight of 2 to 5 g / m 2 wherein the at least one second coating (16a) is optionally a clear coat layer.

13. Aluminium alloy strip according to one of claims 9 to 12, characterized in that the aluminium alloy strip (16) comprises an aluminium alloy of the type AA5182 and the aluminium alloy strip (16) after baking the at least one first and one second coating has a tensile strength R m from 380 MPa to 425 MPa and a yield strength of R P o,2 from 330 MPa to 380 MPa.

14. A method for producing an aluminum alloy strip according to one of claims 9 to 13 using a precursor according to claims 1 to 8, characterized in that a precursor (11) is produced by coating an aluminum alloy strip (4) with at least one first coating (11a), wherein the at least one first coating (11a) of the precursor (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%, wherein the precursor (11) achieves an unchanged surface result of the coated side in a block test according to the description, and the at least one first coating (11a) of the precursor (11) is coated with at least one second coating (16a) and then the at least one first and the at least one second coating (11a, 16a) are cured in a second baking step.

15. The method according to claim 14, characterized in that the at least one first coating (11a) is cured at a baking temperature which is lower than the baking temperature of the first coating arranged second coating (16a), wherein preferably the at least one first coating (11a) is 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 with 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 precursor (11) is wound into a coil after coating with the at least one first coating (11a) and 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 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 at 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, wherein preferably in the second baking step the curing takes place at a PMT of 245 °C to 270 °C, preferably 245 °C to 260 °C, particularly preferably 248 °C to 255 °C with 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 of more than 90%, preferably more than 95%, particularly preferably more than 98%.

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