Method for treating the surface of a metal foil with an ultraviolet-curable protective varnish

Abstract

The method comprises the following steps: applying atmospheric pressure plasma (PA) to each area portion of the surface of a metal foil (10) to be processed; increasing the surface free energy of the metal foil (10) so that the surface free energy is compatible with the surface tension of a varnish to be applied to the metal foil (10); applying an ultraviolet-curable varnish layer (CV), which may be an oligomer, a monomer or a photoinitiator and has a solids content of 99% to 100%, to the surface of the metal foil (10) that has been subjected to the atmospheric pressure plasma (PA); and curing the varnish layer (CV) by using ultraviolet irradiation to form a varnish coating adhered to the surface of the metal foil (10).

Description

Technical Field

[0001] The present invention relates to a method for treating the surface of a metal foil by applying an atmospheric pressure plasma, the surface of the metal foil being coated or uncoated with chromium and tin, in particular a metal foil for the metal packaging industry, thereby allowing a protective varnish to be applied with good adhesion and high wettability on one or both sides of the metal foil used to form the bodies and components of these packages, the protective varnish being curable by ultraviolet irradiation, and these packages are generally in the form of cans of two, three or more pieces. Background Art

[0002] The metal packaging industry uses steel foils formed with a high iron content and a low carbon content as the basic substrate for its products, and these steel foils need to be moisture-proof to avoid oxidation, which can cause losses of the package body and the products packaged therein.

[0003] The steel industry provides coils of steel foil for forming metal packages, and generally an electrolytic deposition process is applied to the metal foil in order to form an intermetallic compound surface layer of tin or chromium therein, and the metal foil is heat-treated to consolidate the crystallography / grain science and porosity of the chromium or free tin layer.

[0004] The above treatments are provided to increase corrosion resistance. However, since the above treatments are carried out by another metal (chromium or tin) and are generally applied in the range of 1.0 to 11.2 g / m 2 it produces an extremely thin layer (1 to 2 nm), which is not effective for the finished products that will withstand mechanical stress, not only in the assembly of cans and their components (lids and bottoms), but also in their transportation, packaging, handling, and exposure to the climate in the environment where the packages will be stored or sold. Therefore, it is necessary to apply a varnish on one or both sides of the metal foil to protect the steel forming the package body, thereby preventing its oxidation.

[0005] In order to obtain a uniform spreading and adhesion degree with the surface of the metal foil, the varnishes currently used to protect metal packages have approximately 50% solvent in their chemical composition. This characteristic results in at least half of the varnish content being lost during the thermal curing process, and the solvent becomes a highly toxic vapor that must be incinerated before reaching the atmosphere.

[0006] The solvent is the standard medium that allows the varnish to reach the required and correct spreading degree on the metal foil. It enters the grooves of the metal substrate and carries the solids of the varnish to the entire surface of the metal foil, and the varnish solids are easily adhered to the surface of the metal foil.

[0007] The current process has flexibility, good spreading, stretching and adhesion properties. In addition, it minimizes the impact of grease or residual dirt on the metal surface and dissolves or disperses these contaminants. However, the solvents used in these varnishes in the current process have a high percentage of volatile organic compounds, which evaporate when the varnish is cured or dried by heating and forced convection in large equipment, thus consuming high energy and processing time.

[0008] The volatile compounds in solvent-based varnishes can be defined by a single compound or by a mixture such as ethers, acetates, aromatic compounds, glycol ethers and aliphatic hydrocarbons, thus requiring the use of expensive incineration systems so that they are not released into the atmosphere because they can cause great harm to the environment.

[0009] Approximately 99% to 100% of the components of the varnish without volatile compounds (i.e., the varnish after the curing process) solidify on the metal foil, forming a single polymer, as suggested for the purposes of the method of the present invention. If the surface free energy of the metal foil is not changed to a value compatible with the surface tension of the varnish to be applied in liquid form, it cannot be used.

[0010] The compatibility of the surface tension value of the liquid varnish with the free energy of the solid surface of the metal foil is necessary to avoid the formation of spherical droplets, which results in a high concentration of the varnish layer at certain points on the metal foil surface, while the layer applied at other points is reduced or even absent. The free energy of a solid is closely related to its wettability, that is, its ability to form a common interface with the liquid in contact with it. The compatibility of the numerical values of the free energy and the surface tension allows uniform spreading and the desired and safe adhesion of the varnish to the metal foil surface. Summary of the Invention

[0011] Due to the inconveniences associated with the use of solvent-based varnishes and the difficulty of obtaining sufficient spreading and adhesion of varnishes with high solid concentrations, the present invention aims to provide a method that allows the application of a protective varnish cured by ultraviolet irradiation in a metal foil, which is uncoated or coated with chromium or tin and is used in the metal packaging industry for the internal and external protection of cans and components.

[0012] The method discussed allows the replacement of thermally cured solvent-based varnishes with varnishes cured by ultraviolet light and compatible with the environment, which solvent-based varnishes require large furnaces that use gas to generate heat and a large amount of organic vapors that are very harmful to humans and the environment.

[0013] The method discussed saves a large amount of process costs because it allows the deactivation of gas furnaces and organic vapor incinerators, which have high maintenance costs compared to the new ultraviolet curing production line used in the technical solution proposed herein.

[0014] The method includes treating the surface of a metallic substrate by cleaning and activating it with an atmospheric pressure plasma. This operation raises the free energy of the metallic substrate to a value equal to or greater than the surface tension value of the ultraviolet curable varnish to be applied to the substrate, thereby increasing the polar part of the free energy by about 8 times, which has hydrophilic properties, thus improving the adhesion properties and wettability of the metal foil and allowing proper spreading of the varnish and a high degree of adhesion to the metallic substrate.

[0015] Each substance seeks the lowest possible free energy. In the absence of the influence of weight, a liquid forms spherical droplets. Conversely, solids cannot deform their surfaces to concentrate in the smallest possible space, but they can form a common and compatible interface with a liquid, thereby reducing the free energy, that is, they can be wetted. Therefore, the surface free energy of a solid is closely related to its wettability and thus closely related to the degree of spreading of the varnish. The terms "surface free energy" and "surface tension" are physically equivalent. The term surface free energy is usually used for the surface tension of solid surfaces and liquid surfaces. However, the term surface tension of solids is sometimes also used. The expression "free" refers to the part of the energy that can be converted into mechanical work, as opposed to internal energy, which also includes entropy related to heat. The term "free" is often omitted. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The present invention will be described below with reference to the drawings, in which:

[0017] Figure 1 The steps of the treatment method of the present invention when used for surface protection of a metal foil are presented in a schematic and simplified manner. DETAILED DESCRIPTION

[0018] As mentioned and shown in the drawings, the method of the present invention aims to provide a surface protection treatment on one or both sides of a metal foil 10 by applying a protective varnish that can be cured by ultraviolet irradiation. Figure 1 The metal foil 10 on which the method of the present invention is carried out may or may not be coated with chromium or tin and is particularly used in the metallic packaging industry in the form of two-piece or three-piece cans.

[0019] According to the method, any surface of the metal foil 10 is brought into contact with an ionized gas called atmospheric pressure plasma PA, which is considered the fourth state of matter and is released from a plasma nozzle 20 onto the surface to be treated.

[0020] An atmospheric pressure plasma PA is generated inside the plasma nozzle 20 by a controlled release of electrical energy generated by a high-frequency and high-voltage plasma generator 21, and compressed air is further supplied to the plasma nozzle from a compressed air generator 22. In the drawings

[0021] Figure 1 ​Only one high-frequency and high-voltage plasma generator 21, a compressed air generator 22 and a plasma nozzle 20 are shown, and the atmospheric pressure plasma PA generated by them impacts on the surface of the metal foil 10. It should be understood that the number of plasma nozzles 20 and plasma generators 21 can vary according to the lateral dimension of the metal foil 10, the relative moving speed between the metal foil 10 and the plasma nozzle 20, and the intensity of the plasma nozzle.

[0022] In the attached drawings Figure 1 An example is shown where the metal foil 10 is moved through the atmospheric pressure plasma PA by a conveyor belt 30. The linear velocity is determined by the above-mentioned other parameters and is related to the type of the metal foil 10 and the capacity of the plasma nozzle 20, assuming that the latter is sufficient in number and position to cover the width of the metal foil 10 passing through the atmospheric pressure plasma PA.

[0023] Before the metal foil 10 is exposed to the atmospheric pressure plasma, the average surface free energy of its surface is 28 to 45 mN / m. The polar part is considered to be hydrophilic and is relatively low. The average surface free energy of the tin-coated metal foil is 1 to 3, and that of the chromium-plated foil is 4 to 5, depending on its production, machining, the layer of free tin or chromium, and passivation. The cohesive force between atoms and molecules causes the surface energy / tension of a substance, which can be explained by different types of interactions. In particular, dispersion interactions and polar interactions can be distinguished from each other. The interaction caused by the transient fluctuations of the charge distribution in atoms / molecules is called dispersion interaction (van der Waals interaction).

[0024] The said polar interactions include the Coulomb interaction between permanent dipoles and the Coulomb interaction between a permanent dipole and an induced dipole (e.g., hydrogen bond). Since van der Waals interaction occurs between all atoms and molecules, there is no substance whose surface energy / tension consists only of the polar part. The surface energy / tension σ i of component i is composed in an additive manner by the dispersion part σ i d and the polar part σ i p as follows:

[0025]

[0026] By comparing the ratios between the dispersion part and the polar part of the surface energy / tension between the substrate and the varnish, it is allowed to predict the adhesion force between the said two components. The closer the relationship between the substrate and the varnish, the more likely the interaction between the components, and thus the greater the adhesion force between them.

[0027] After exposing the metal substrate (steel foil) to the atmospheric pressure plasma PA, using a high-frequency and high-voltage plasma generator 21, the plasma nozzle 20 is placed at a position 1 to 3 mm away from the steel foil. At the jet outlet, with a nitrogen flow rate of 50 to 60 liters per minute and a pressure of 245 mbar (24.5 KPa), the substrate (in the form of metal foil 10) is moved at a speed of 40 to 80 m / min. The surface free energy of the substrate increases to a parameter that can be adjusted to be compatible with the surface tension of the varnish to be applied. For a metal foil coated with tin or chromium, the surface free energy of the treated foil can be defined as 29 to 72 mN / m, with a high polar part. If a tin-coated metal foil is used, the surface free energy before treatment is 41 mN / m, the dispersibility is 39 / polarity 2, and the result after plasma application leads to an increase in free energy to 56 mN / m, and the polar part is 16. If a chromium-coated metal foil is used, the surface free energy is 41 mN / m, the dispersibility is 35 / polarity 6, then after plasma application, it leads to an increase in free energy to 56 mN / m, and the polar part is 16.

[0028] The high-frequency and high-power plasma generator 21 must be configured to generate 300 to 400 volts, with a pulse frequency between 20 and 30 kHz, to achieve a power between 900 and 1000 W. The gas to be compressed can be oxygen or nitrogen, and the latter (N2) is the most effective. In both cases, the amount of gas released should be 50 to 60 liters per minute, and the pressure at the jet outlet is 240 to 250 mbar (24.5 to 25.0 KPa).

[0029] Among the above parameters, the stability between the metal foil surface and the varnish is found, thus allowing good wetting and adhesion of the ultraviolet-curing varnish used. Then, the foil can successfully pass the standard tests of "scratch and tape - adhesion" and reverse impact.

[0030] This pretreatment process produces the following effects: cleaning of organic substances, polarization of the metallic surface during treatment, and an increase in the surface free energy of the steel surface of the metal foil 10, so that the surface free energy is compatible with the surface tension of the ultraviolet-curing varnish to be applied to the metal foil 10, allowing good adhesion and spreading of the varnish on the foil. The speed is greater than 3500 steel foils (foils coated with tin or chromium) per hour, considering the atmospheric pressure plasma PA generated by nitrogen or compressed oxygen, using 56 liters of gas per minute, with a pressure of 245 mbar (24.5 KPa) at the jet outlet, and the speed of the metal foil is about 50 m / min.

[0031] The metal foil 10 surface-treated by atmospheric pressure plasma PA is conveyed by a belt 30 to a varnish applicator 40, which can be defined, for example, by a varnish roller 41 that supplies an application roller 42, and the application roller runs relative to a support roller 43. When passing between the application roller 42 and the support roller 43, an ultraviolet curable varnish layer CV is formed on the surface of the metal foil 10 that contacts the application roller 42. Immediately thereafter, the metal foil 10 enters a curing unit 50, which is formed, for example, by a mercury vapor or LED lamp, and which generates sufficient ultraviolet irradiation to activate the photoinitiator in the chemical composition of the varnish, thereby causing its polymerization, its adhesion to the metal foil surface, and the complete coverage and protection of the said surface of the metal foil, which is to be used for producing the body, dome, and bottom of a metal package.

[0032] Varnishes with a high solids content must be used, without dispersants or chemical solvent-based carriers, and cured by ultraviolet irradiation generated by mercury vapor lamps.

[0033] The varnish used in this process contains oligomers, monomers, and photoinitiators, is applied by a lithographic roller 42 without dilution, has a viscosity of 40 to 60 seconds CF4 / 25 °C, a specific gravity of 1.01 to 1.05 / 25 °C, and a solids content of 99 to 100%.

[0034] The method under discussion allows the use of the said protective product, i.e., the varnish on the internal and external parts of the body and components (dome and bottom) of a metallic package consisting of three or two pieces, which has 99% solids cured by ultraviolet irradiation, and the method also allows replacing a gas furnace with a curing station with ultraviolet lamps.

[0035] Curing of the varnish occurs through the polymerization of the varnish caused by the interaction of the photoinitiator in the varnish composition, using ultraviolet irradiation (exceeding 100 mJ / cm 2 ) emitted by the lamp of the curing unit 50, thereby resulting in a final dry layer of 4.65 to 7.75 g / m 2 .

[0036] The beneficial effects of the method now proposed can be listed as follows:

[0037] - Elimination of emissions of gases harmful to health and the environment;

[0038] - Elimination of the use of gases in the industrial process;

[0039] - Elimination of environmental hazards due to the use of non-flammable varnishes, which is very different from the method using solvent-based varnishes, which are eliminated through heat curing and toxic gas treatment;

[0040] - Reduction of factory space, elimination of the gas furnace for heat curing of solvent-based varnishes, and use of a significantly smaller ultraviolet curing station;

[0041] - Reduce the maintenance of the application line and the treatment line as they are smaller and simplified;

[0042] - Significant savings in maintenance, rent, and fewer operators;

[0043] - Save on freight as there is no longer a need to transport previously used varnish solvents, but only the solids to be applied to the steel foil;

[0044] - Greater stability of the metallic substrate, thus ensuring better adhesion and wettability.

Claims

1. A method for treating the surface of a metal foil with an ultraviolet-curable protective varnish, characterized in that, It includes the following steps: - Applying atmospheric pressure plasma (PA) to each area portion of the surface to be treated of the metal foil (10) to increase the surface free energy of the metal foil (10) and make the surface free energy compatible with the surface tension of the varnish to be applied to the metal foil (10). The atmospheric pressure plasma (PA) is generated by a controlled discharge of electrical energy released from at least one plasma nozzle (20) and generated within the plasma nozzle (20) by a plasma generator (21). The plasma generator (21) generates a voltage of 300 to 400 volts, a pulse frequency of 20 to 30 kHz, and a power of 900 to 1000 W. The gas to be compressed is selected from oxygen or nitrogen, with an amount of 50 to 60 liters per minute and a pressure of 240 to 250 mbar; - Applying an ultraviolet-curable varnish layer (CV) to the surface of the metal foil (10) that has been subjected to the atmospheric pressure plasma (PA). The varnish layer (CV) contains oligomers, monomers, and photoinitiators, has a viscosity of 40 to 60 seconds CF4 / 25°C, a specific gravity of 1.01 to 1.05 / 25°C, and a solid content of 99% to 100%; and - Curing the varnish layer (CV) applied to the surface of the metal foil (10) by causing the photoinitiator to interact with ultraviolet irradiation in a curing unit (50), through the polarization of the varnish compound, to form a varnish coating adhered to the surface of the metal foil (10).

2. The method according to claim 1, wherein The thickness of the varnish coating is 4.65 to 7.75 g / m 2 .

3. The method according to any one of claims 1 and 2, characterized in that, Compressed air is also supplied from a compressed air generator (22) to the plasma nozzle (20).

4. The method according to any one of claims 1 and 2, characterized in that The metal foil (10) is held at a distance of 1 to 3 mm from the plasma nozzle (20), moved at 50 m / min, and subjected to the atmospheric pressure plasma (PA) with a flow rate of 56 liters of nitrogen per minute and a pressure of 245 mbar (24.5 KPa) so as to exhibit a surface free energy of 56 mN / m.

5. The method according to any one of claims 1 and 2, characterized in that, The curing unit (50) applies ultraviolet irradiation emitted by mercury vapor or an LED lamp with an intensity greater than 100 mJ / cm 2 to the varnish layer (CV).

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