Process for decorating metal substrates

Abstract

The present invention relates to a process for decorating metal substrates, particularly those used as food or beverage cans or aerosol cans, in which the decoration is printed using a digital printer.

Description

[Technical field]

[0001] The present invention relates to a process for decorating metal substrates, particularly metal substrates that can be used as containers for food cans, general line cans, or aerosol cans. [Background technology]

[0002] Metal packaging plays an important role in addressing many food safety concerns or maintaining the proper shelf life of chemicals or cosmetics. Such packaging is strong, seamless, and provides a durable solution for preserving food while keeping out contaminants and light (e.g., UV light). The most commonly used metal substrates for such packaging are aluminum, electrolytic tinplate, or tin-free steel.

[0003] To identify the contents of such packaging, and also to identify the manufacturer of such containers, decoration is most often applied to the outside of the packaging with such information. Packaging decoration also serves as a marketing tool, as it often has some attractive color or design.

[0004] The conventional process for making metal substrates that can be used as packaging involves applying a clear or white coating to the substrate and then printing the decoration onto the substrate using well-known printing techniques, such as offset lithography printing. In this process, ink is transferred in the desired pattern from the surface of an aluminum plate to a rubber blanket and then to the metal substrate. This process has several drawbacks, such as: -Only one color can be printed at a time on the substrate during each pass through an offset lithography press. For decorations with multiple colors, the substrate must be passed through the press multiple times and carefully aligned before each print pass. - A multicolour offset press consists of a number of single colour stages in register, where the substrate passes through all the stages, each colour of ink is applied separately, and there is one image plate per colour. -Images need to be aligned for each color. Color densities need to be carefully adjusted for each run. The operator needs to adjust the ink / water balance to ensure that ink flow is sufficient and consistent for accurate color reproduction. This set-up process takes a lot of time. - Small batch runs are made after each setup adjustment to check all the above parameters. A lot of time and material is wasted at this stage. Because the offset lithography printing process is time-consuming and difficult to set up, multiple passes usually need to be made before the final decoration is obtained, making the cost of the printed sheet relatively high. -Due to the inability to change the desired decoration during the printing process, only one decoration can be printed on multiple metal substrates, and it is not possible to print each custom-made decoration on different metal substrates. -Once printed, the substrate with the same image cannot be used for other images or customers. If the substrate does not go to an assembly line, all the metal becomes waste. Often, to achieve optimal production costs, companies print more sheets than the initial order requires and leave the printed sheets on the floor to be assembled later. Downtime and inventory space costs are relatively high. -Offset lithography presses for metals are very specialized machines. They are very expensive and require a lot of maintenance. They also require a lot of space to install. -An over varnish coating is always required to protect the ink layer.

[0005] US 2018 / 0164719 discloses a more flexible process for decorating metal containers. In this process, a digital printer is used to print an image onto a transfer member, which is then transferred to the metal container. This process is more flexible than lithographic or flexographic printing processes, as each decoration is printed separately, allowing a variety of different decorations to be printed and then transferred to the metal substrate. A drawback of this process is that the image on the metal container is not protected, making the decoration more susceptible to damage during handling of the container. This could be solved by applying an additional protective layer to the decorated can (not disclosed in this document), but this would involve additional process steps, which would also make the process of decorating the can more complicated and more susceptible to failure.

[0006] WO 2008 / 152137 discloses a method for decorating a substrate using a printed adhesive film. In this process, the decoration is printed onto a thermoplastic adhesive film, which is then applied to the substrate. The decoration is on the opposite side of the adhesive, meaning that when the adhesive film is applied to the substrate, the decoration is in direct contact with the environment. This process has the same drawbacks as the process of US 2018 / 0164718, i.e. the image is not protected once applied to the substrate. An alternative process is also disclosed in which the decoration is further protected with a second adhesive film. This makes the process more complicated and prone to failure. Furthermore, such films or film layers containing the decoration easily detach from the substrate. Summary of the Invention [Means for solving the problem]

[0007] The present invention relates to a process that allows the decoration of metal substrates in a simple and flexible manner, without presenting the drawbacks of known processes. According to the process of the present invention, the process for decorating metal substrates comprises: a) applying a coating composition to a metal substrate; b) curing the coating composition applied in step a) at a temperature T1 for a time sufficient to obtain a non-tacky coating; c) printing a decoration onto the polymer film using a digital printer; d) laminating the polymer film having the decoration onto the coated metal substrate, whereby the decoration is positioned between the surface of the coated metal substrate and the polymer film; e) heating to a temperature T2 and applying a pressure P1 during lamination; f) after lamination, exposing the substrate to a temperature T3 for a time sufficient to fully cure the coating.

[0008] In order to further explain the present invention, some figures are provided. These figures are for illustrative purposes only and should not be taken as a detailed description of the present invention. For the sake of clarity and conciseness, some objects depicted in the figures may not be drawn to scale. [Brief description of the drawings]

[0009] [Figure 1] One embodiment for decorating a polymer film is shown: a transparent polymer film (1) is decorated by passing it through a digital printer (2). The decorated polymer film (3) is then collected by winding it onto a bobbin. [Diagram 2] One embodiment for coating a metal sheet is shown. A sheet feeder (4) is used to feed a raw metal sheet (5) into a roll coater (6), where the metal sheet is coated with a coating composition (7). After the coating process, the coated metal sheet passes through an oven to cure the coating composition. Once the coating composition has cured to a non-tacky coating, the coated sheets (10) are stacked (9) together. [Diagram 3]One embodiment for decorating a coated metal sheet is shown. The coated sheet (10) is fed into a feeder system (11) and passes through a heated calendar roll (12) and a pressure roll (13) together with the decorated polymer film (3). The decorated film is laminated to the coated sheet by heat and pressure. The laminated coated sheet is discharged onto a conveyor belt (14) with a heating system (15) above the belt. After a thermal separator (16), the decorated metal sheet (18) is collected on a stack (17). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] definition Within the context of this application, the following terms have the following meanings. - Multicolour decoration means a decoration which not only has a combination of black and white but also has at least one further colour, for example cyan, magenta, yellow, etc. - Peak Metal Temperature (PMT) refers to the temperature of the metal substrate in question. Generally, the peak metal temperature is lower than the temperature of the oven or other means used to heat the substrate and depends on the residence time in the oven, or more generally, the time the substrate is heated. - Coating life means the period during which the coating exhibits less than a 10% increase in viscosity when stored at ambient conditions (23°C, 65% relative humidity). - Non-stick coating means that a stack of coated metal substrates can be formed and then individual coated metal substrates can be separated without visible damage to the coating and without noticeable sticking of adjacent coated substrates to each other. -coating composition means a mixture of various components that can be transferred into a coating film by exposure to elevated temperature and / or by evaporation of the solvents present in the composition.

[0011] The metal substrate used in the process of the present invention can be a thin metal plate suitable for use as a food can or a general line can or an aerosol can. For such metal substrates, aluminum, electrolytic tinplate, or tin-free steel are usually used. Metal substrates used for food or beverage packaging usually have a thickness in the range of 0.1-0.5 mm.

[0012] The metal substrate used in the process of the present invention may be flat or curved. If the substrate is flat, it can be formed into the desired shape as a food or beverage container in a further process step. However, it is also possible to use the process of the present invention to apply decoration to preformed cans or containers.

[0013] The coating composition applied to the metal substrate may be a clear coating composition or a white coating composition. Clear coating compositions are typically used when it is desired to have the surface of the container visible in combination with decoration. White coating compositions are typically used to completely cover the surface of the substrate.

[0014] The coating composition is applied to protect the substrate from corrosion and to act as an adhesive layer for the polymer film carrying the decoration. To allow for an environmentally friendly process, the initial cure temperature of the coating composition should be in the range of 130-140°C peak metal temperature (PMT), preferably in the range of 130-140°C PMT. After the initial cure, the coating should be non-tacky. If the temperature of this initial step is higher than 140°C PMT, the amount of energy required to cure the coating will be significant, and if the cure temperature is lower than 90°C, the storage stability of the coating will be affected, limiting the life of the coating. Typically, a life of at least 3 months, more preferably at least 6 months, is desired for these types of coatings. The initial cure is performed for 10-12 minutes at a PMT in the range of 130-140°C.

[0015] Coating compositions containing 75-80 wt. % saturated polyester resin (wherein the wt. % is based on the solids content of the coating without pigment) have been found to be highly suitable for use as coating compositions in the process of the present invention. The coating compositions are typically solvent-based, with preferred solvents being solvent naphtha and butyl glycol. Other components optionally present in the coating composition are epoxy acrylate resins, blocked polyisocyanate resins, and melamine resins. To enable a non-tacky coating after a relatively short initial curing stage, the coating composition also contains a small amount of lanolin, typically 0.02-0.06 wt. % lanolin (wherein the wt. % is based on the solids content of the coating without pigment).

[0016] In parallel with the process of applying the coating composition to the metal substrate, or following such a process, the decoration is printed onto the polymer film using a digital printer. Typically, the polymer film is flexible and has a thickness in the range of 5 to 50 μm. Usually, films of a type well known in the art are used, such as polyester or polycarbonate films. In principle, any suitable digital printer can be used to print the decoration onto the polymer film. The decoration printed onto the polymer film can have a single color or be a black and white image (and various shades of gray) or be a multicolor image.

[0017] The polymer film with the printed decoration is then laminated onto a non-stick coated metal substrate. In this process step, the side of the polymer film carrying the decoration is laminated onto the coated substrate, so that after the lamination process is completed, the decoration is located between the surface of the coated substrate and the polymer film. In this lamination process, care must be taken to ensure that the decoration and / or the polymer film maintain their original dimensions, so that the polymer film is not stretched or wrinkled during the lamination process. In one embodiment of the process of the invention, the lamination process is carried out in two steps. In step 1, the polymer film with the decoration is laminated onto the coated substrate with gentle pressure at a PMT in the range of 110-140°C, and then in step 2, the pressure is increased and lamination continues at a PMT in the range of 130-170°C. At this latter temperature, the coating is finally fully cured for a period of about 10 minutes. More generally, the two-step lamination process can be described as a process in which a polymeric film bearing a decoration is laminated onto a coated metal substrate, where in step 1, the laminate is heated to a temperature T 2,1 and pressure P 2,1 In step 2, the lamination is performed at a temperature T 2,2 and pressure P 2,2 where T 2,1 <T 2,2 And P 2,1 <P 2,2 It is.

[0018] It was found that once lamination was complete, the decoration adhered securely to the coated substrate, and further, that the adhesion of the decoration to the coated substrate was better than the adhesion to the polymer film, such that the decoration remained on the coated substrate even when the polymer film was removed from the substrate.

[0019] The metal substrate obtained by the process of the present invention can then be formed into any form or shape, for example into a container or packaging material for food or beverages. For example, if the metal substrate is a sheet metal, it can be formed into a container by a metal forming process such as deep drawing techniques or stamping.

Claims

1. 1. A process for decorating a metal substrate, comprising: a) applying a coating composition (7) to said metal substrate (5); b) curing the coating composition applied in step a) at a temperature T1 for a time sufficient to obtain a non-tacky coating (10); c) printing a decoration onto the polymer film (1) using a digital printer (2); d) laminating (12, 13) the polymer film having the decoration onto the coated metal substrate, whereby the decoration is located between the surface of the coated metal substrate and the polymer film; e) heating to a temperature T2 and applying a pressure P1 during lamination; f) after lamination, exposing the substrate to a temperature T3 for a time sufficient to fully cure the coating.

2. The process of claim 1 , wherein the metal substrate is a thin metal plate.

3. 2. The process of claim 1, wherein the metal substrate is formed into a container for food can or general line can or aerosol can after lamination of the polymer film with the decoration.

4. The process of claim 1 , wherein the coating composition applied in step a) is a white coating composition or a clear coating composition.

5. The process of claim 1 , wherein the decoration is a multi-color decoration.

6. the coating composition comprises 75-80% by weight of a saturated polyester resin, 0.02-0.06% by weight of lanolin, and solvent naphtha or butyl glycol; 10. The process of claim 1, wherein the weight percentage is based on solids of the coating composition without pigment.

7. step e) is a two-step lamination process; In step 1, the laminate is heated to a temperature T ２，１ and pressure P ２，１ In step 2, the lamination is performed at a temperature T ２，２ and pressure P ２，２ Continued in T ２，１ <T ２，２ And P ２，１ <P ２，２ 2. The process of claim 1 ,

8. T ２，２ 8. The process of claim 7, wherein T3.

9. A metal packaging container for food cans or general line cans or aerosol cans, at least a part of said container being decorated by the process according to claim 1.

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