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

Abstract

The method includes the steps of applying atmospheric plasma (PA) to each area portion of the surface of the metal foil (10) to be treated, increasing the surface free energy of the metal foil (10) and matching the surface free energy to the surface tension of the varnish to be applied to the metal foil (10), applying a UV-cured varnish layer (CV), which can be an oligomer, monomer, or photoinitiator with a solids content of 99-100%, to the surface of the metal foil (10) that has been subjected to the atmospheric plasma, and curing the varnish layer (CV) using UV radiation 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 providing a treatment of the surface of chromium and tin coated or uncoated metal foils, in particular those used in the metal packaging industry, by the application of atmospheric plasma, which allows a UV-curable protective varnish to be applied with good adhesion and high wettability to one or both sides of the metal foils used to form the bodies and parts of these packages, which are generally in the form of two-piece, three-piece or more piece cans. [Background technology]

[0002] The metal packaging industry uses steel foil as the basic substrate for its products, which has a high iron content, a low carbon content, and must be protected from moisture to prevent oxidation, which leads to loss of the packaging and the products contained within.

[0003] The steel industry, which supplies steel foil for the formation of metal packaging, typically applies an electrolytic deposition process to the foil to form an intermetallic surface layer of tin or chromium therein, and the foil undergoes a heat treatment to establish the crystallography / grain size distribution and porosity of the chromium or free tin layer.

[0004] The above treatment is carried out to improve corrosion resistance. However, this treatment is carried out with another metal (chromium or tin), and the amount is generally 1.0 to 11.2 g / m 2 This creates an extremely thin layer of 1-2 nm, which is applied in the range of 100-150°C. It is therefore ineffective not only in the assembly of the can and its components (lid and base), but also in the finished product, which is subjected to mechanical stresses due to transport, packaging, trade, and exposure to the elements in the environment in which the package is stored or sold. For this reason, it is necessary to apply a varnish to one or both sides of the metal foil to protect the steel that makes up the packaging and prevent its oxidation.

[0005] In order to obtain the uniform spreading and adhesion associated with the surface of the metal foil, the varnishes currently used to protect metal packaging have in their chemical composition about 50% solvents. This specific property means that the thermal curing process loses at least half of the varnish content, turning the solvent into highly toxic vapors that must be incinerated before they reach the atmosphere.

[0006] The solvent is a standard medium that makes it possible to obtain the required and precise spreading of the varnish on the metal foil, entering the grooves of the metal substrate and carrying the varnish solids over the entire surface of the metal foil, where they easily adhere.

[0007] Current methods minimize the effects of grease or residual dirt on metal surfaces, while providing flexibility, good spreading, extensibility, and adhesive properties that dissolve or disperse these contaminants. However, these varnishes of current methods use solvents with a high percentage of volatile organic compounds, which are evaporated by heat and forced convection in large equipment during curing or drying, requiring a lot of energy and processing time.

[0008] The volatile compounds of solvent-based varnishes can be characterized by a single compound or by a mixture of, for example, ethers, acetates, aromatic compounds, glycol ethers and aliphatic hydrocarbons, and require the use of expensive incineration systems, which are not released into the atmosphere, which can be very harmful to the environment.

[0009] Varnishes that do not contain volatile compounds, i.e. varnishes in which after the curing process about 99% to 100% of their components are solidified on the metal foil and form a single polymer, such as proposed in the subject matter of the method of the present invention, cannot be used without modifying the surface free energy of the metal foil to a value compatible with the surface tension of the varnish applied in the liquid state.

[0010] A match between the surface tension of the varnish in the liquid state and the free energy of the solid surface of the metal foil is necessary to avoid the formation of spherical droplets, which would result in a thick layer of varnish at some points on the metal foil surface and a thinner layer or even no layer at other points. The free energy of a solid is closely related to its wetting ability, i.e., its ability to form a common interface with the liquid that comes into contact with it. The match between the free energy and surface tension values ​​allows the varnish to spread evenly on the surface of the metal foil and achieve the desired, reliable adhesion. Summary of the Invention

[0011] Due to the inconveniences associated with the use of solvent-based varnishes and the difficulty of obtaining adequate spreading of the varnish and adhesion of the varnish at high concentrations to solids, the present invention aims to provide a method that allows the application of protective varnishes, cured by ultraviolet light, on metal foils that are uncoated or coated with chromium or tin and that are used in the metal packaging industry for the internal and external protection of can bodies and parts.

[0012] This method makes it possible to replace heat-curing solvent-based varnishes, which are cured with ultraviolet light and are environmentally compatible, requiring large ovens using heat-generating gases and generating large amounts of organic vapors that are highly harmful to humans and the environment.

[0013] The present method allows for significant process reductions as it allows for the inerting of gas furnaces and organic vapor incinerators, which have high maintenance costs compared to those of the new UV curing lines used in the technical solution proposed herein.

[0014] The method consists in treating the surface of a metal substrate, cleaning it and activating it with air plasma. This procedure increases the free energy of the metal substrate to a value equal to or greater than the surface tension value of the UV-curable varnish applied to said substrate, allowing an approximately eight-fold increase in the polar part of this free energy, which has hydrophilic properties, thereby improving the adhesive properties and wetting of the metal foil, allowing the varnish to spread correctly and adhere well to the metal substrate.

[0015] Every substance strives for the lowest possible free energy. Liquids, without the influence of gravity, form spherical droplets. Secondly, solids cannot deform their surfaces to occupy the smallest possible space, but they can form a common and compatible interface with the liquid, reducing their free energy. That is, they can wet. Therefore, the surface free energy of a solid is closely related to its wettability and, consequently, the spreading ability of a varnish. The terms "surface free energy" and "surface tension" are physically equivalent. The term "surface free energy" is commonly used for solid surfaces, while the term "surface tension" is commonly used for liquid surfaces. However, the term "surface tension" of a solid is sometimes used as well. The term "free" refers to the portion of energy that can be converted into mechanical work, as opposed to internal energy. This also includes entropy, which is related to heat. The term "free" is often omitted.

[0016] The present invention will now be described with reference to the accompanying drawings. [Brief explanation of the drawings]

[0017] [Figure 1] 1 shows, in a schematic and simplified manner, the steps of the treatment method of the invention when applied for surface protection of metal foils. DETAILED DESCRIPTION OF THE INVENTION

[0018] As already mentioned and shown in Figure 1 of the accompanying 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 application of a protective varnish that is curable by ultraviolet light.

[0019] The metal foil 10 presented in this method may be chrome or tin coated or uncoated and is particularly used in the metal packaging industry in the form of cans formed in two or three pieces.

[0020] According to this method, a metal foil 10 is brought into contact on either side with an ionized gas, called atmospheric plasma PA, which is then emitted from a plasma nozzle 20, considered a fourth state of matter, onto the surface to be treated.

[0021] The atmospheric plasma PA is generated by a controlled discharge of electrical energy produced by a high frequency, high voltage plasma generator 21 inside a plasma nozzle 20 which is further supplied with compressed air from a compressed air generator 22. Figure 1 of the accompanying drawings shows a single high frequency, high voltage plasma generator 21, a compressed air generator 22 and a plasma nozzle 20 which generates the atmospheric plasma PA acting on the surface of the metal foil 10. It should be understood that the number of plasma nozzles 20 and plasma generators 21 may vary according to the lateral dimensions of the metal foil 10, the relative displacement speed between the metal foil 10 and the plasma nozzle 20 and the strength of the plasma nozzle.

[0022] FIG. 1 of the drawings shows an example of the displacement of metal foils 10 by a conveyor belt 30 passing under the atmospheric plasma PA at a linear speed determined by the above-mentioned and other parameters related to the type of metal foil 10 and the power of the plasma nozzle 20, assuming that the latter is adequate for positioning the required range of number and width of metal foils 10 passing under the atmospheric plasma PA.

[0023] The surface free energy of the metal foil 10 before exposure to atmospheric plasma is an average of 28-45 mN / m, with the polar parts considered hydrophilic being lower: 1-3 for tin-coated foils and 4-5 for chrome-plated foils, depending on their manufacture, processing, free tin or chrome layers, and passivation. The cohesion between atoms and molecules that causes the surface energy / tension of a material can be explained by different types of interactions. In particular, dispersion interactions and polar interactions can be distinguished from each other. Interactions resulting from temporary fluctuations in the charge distribution in atoms / molecules are called dispersion interactions (van der Waals interactions).

[0024] Polar interactions include Coulomb interactions between permanent dipoles and between permanent and induced dipoles (e.g., hydrogen bonds). Van der Waals interactions occur between all atoms and molecules, so no material has a surface energy / tension that consists solely of polar moieties. The surface energy / tension σ of element i is i is the variance part σi according to d and polar part σi p It is additively composed by

[0025]

number

[0026] Comparing the ratio between the dispersive and polar parts of the surface energy / tension between the substrate and the varnish makes it possible to predict the adhesion between the two parts: the closer the relationship between the substrate and the varnish, the easier the interaction between the parts will be and the better the adhesion between them will be.

[0027] After exposing a metal substrate (steel foil) to atmospheric plasma PA using a high-frequency, high-voltage plasma generator 21 with a plasma nozzle 20 positioned 1-3 mm away from the steel foil, a nitrogen flow of 50-60 liters / min, and a jet outlet pressure of 245 mbar (24.5 kPa), the substrate in the form of a metal foil 10 under displacement at a speed of 40-80 m / min had an increased surface free energy, a parameter that could be adjusted to match the surface tension of the applied varnish. The surface free energy of the treated foil could be determined to be 29-72 mN / m in the highly polarized portion for tin- or chromium-coated metal foils. Using a tin-coated metal foil with a surface free energy of 41 mN / m (dispersion 39 / polarity 2) before treatment, the result after plasma application was an increase in the free energy to 56 mN / m with a polarity of 16. When using a metal foil coated with chromium with a surface free energy of 41 mN / m (dispersion 35 / polarity 6), the result after plasma application was a polar fraction of 16, with an increase in free energy to 56 mN / m.

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

[0029] The above parameters allowed for stability between the metal foil surface and the varnish, and the UV curing used do Good wetting and adhesion of the varnish was possible, after which the foil could be successfully subjected to the standard tests "scratch and tape-adhesion" and back shock.

[0030] This pretreatment procedure, considering an atmospheric plasma PA generated from nitrogen or compressed oxygen, using 56 liters of gas per minute at a pressure of 245 mbar (24.5 KPa) at the outlet of the jet and a foil speed of approximately 50 m / min, results in cleaning of organics, polarization of the metal surface during treatment and an increase in the surface free energy of the steel of the metal foil 10, which surface free energy is then increased by the UV curing agent applied to the metal foil 10. do It matches the surface tension of the varnish, allowing excellent adhesion and spreading of the varnish onto the foil at speeds exceeding 3500 steel foils (tin or chrome coated foils) per hour.

[0031] The metal foil 10 superficially treated with atmospheric plasma PA is transported by a belt 30 to a varnish applicator 40, which can be defined, for example, by a varnish roller 41 feeding an applicator roller 42 acting against a support roller 43. Having passed between the applicator roller 42 and the support roller 43, the metal foil 10 is subjected to UV curing on its side in contact with the applicator roller 42. do Immediately after receiving the varnish layer CV, the metal foil 10 enters a curing unit 50, which is formed, for example, by a mercury vapor lamp or an LED lamp, which generates sufficient ultraviolet radiation to activate the photoinitiator of the chemical composition of the varnish, causing its polymerization, its adhesion to the surface of the metal foil, and the total coating and protection of said surface of the metal foil used to manufacture the body, dome and bottom of the metal packaging.

[0032] High solids, no dispersants or chemical solvent-based vehicles, and cured with UV light generated by mercury vapor lamps do It is necessary to use a varnish.

[0033] The varnish used in this method contains oligomers, monomers and photoinitiators, is applied by a lithographic roller 42, does not require dilution, has a viscosity of 40-60 sec CF4 / 25°C, a specific gravity of 1.01-1.05 / 25°C and a solids content of 99-100%.

[0034] This method allows the use of protective products, namely varnishes, both inside and outside the body and parts, dome and base of metal packaging made up of pieces of a three-piece can or two-piece can, the varnishes being UV-curable. do With 99% solids, this method also allows for the replacement of gas ovens with curing stations using UV lamps.

[0035] The curing of the varnish is achieved by combining the photoinitiator in the varnish composition with 100 mJ / cm emitted by the lamp of the curing unit 50. 2 It occurs due to polymerization of the varnish caused by interaction with ultraviolet radiation exceeding 4.65-7.75 g / m 2 This results in a final dry layer of

[0036] The advantageous effects of the currently proposed method can be listed below. - Eliminates gas emissions that are harmful to health and the environment; - Abolition of the use of gas in industrial processes; - Elimination of environmental hazards through the use of non-flammable varnishes, as opposed to solvent-based varnishes, which are removed by thermal curing and treatment with toxic fumes; - Reduction of plant space with the elimination of gas ovens for thermal curing of solvent-based varnishes and the use of significantly smaller UV curing stations; - Reduced maintenance of coating and treatment lines due to compactness and simplification; - Significant savings in terms of maintenance, rental costs and fewer operators; - The transportation of the varnish solvent that was previously used is no longer necessary, and only the solid material to be applied to the steel foil needs to be transported, resulting in savings in transportation costs. - Increased stability of metal substrates, ensuring better adhesion and wetting.

Claims

1. 1. A method for treating the surface of a metal foil with a UV-cured protective varnish, comprising the steps of: - applying atmospheric plasma (PA) to each area of ​​the surface of the metal foil (10) to be treated, in order to increase the surface free energy of the metal foil (10) and adapt it to the surface tension of the varnish applied to the metal foil (10); - applying to the surface of the metal foil (10) subjected to atmospheric plasma a UV-curable varnish layer (CV) containing a compound of oligomers, monomers and photoinitiators, with a viscosity of 40-60 sec CF4 / 25°C, a specific gravity of 1.01-1.05 / 25°C and a solids content of 99-100%; and - curing the varnish layer (CV) applied to the surface of the metal foil (10) by polarization of the varnish compounds through interaction of the photoinitiator with UV radiation in a curing unit (50), forming a varnish coating adhered to the surface of the metal foil (10).

2. The varnish coating has a thickness of 4.65 to 7.75 g / m ２ 2. The method of claim 1, wherein the thickness of the substrate is 0.05 μm.

3. 3. The method according to claim 1 or 2, characterized in that atmospheric plasma (PA) is emitted from at least one plasma nozzle (20) onto the surface of the metal foil (10) to be treated, the atmospheric plasma (PA) being generated by controlled discharge of electrical energy produced by a high frequency, high voltage plasma generator (21) inside the plasma nozzle (20), the plasma nozzle (20) being further supplied with compressed air from a compressed air generator (22).

4. 4. The method of claim 3, wherein the plasma generator (21) generates 300-400 volts at a pulse frequency of 20-30 kHz and a power of 900-1000 W, and the compressed gas is selected from oxygen or nitrogen, at a volume of 50-60 liters per minute, and at a pressure of 240-250 mbar (24.5-25.0 KPa).

5. 5. The method of claim 4, wherein the metal foil (10) is maintained at a distance of 1-3 mm from the plasma nozzle (20), moves at 50 m / min, and is subjected to atmospheric plasma (PA) at a flow of 56 liters per minute of nitrogen and a pressure of 245 mbar (24.5 KPa), exhibiting a surface free energy of 56 mN / m.

6. The curing unit (50) applies 100 mJ / cm 2 emitted by a mercury vapor lamp or an LED lamp to the varnish layer (CV). ２ 6. The method according to claim 1, wherein more than 1000 W of ultraviolet radiation is applied.

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