Alkaline cleaner for aluminum surfaces

Abstract

The present invention relates to a method for cleaning and, if necessary, degreasing aluminum surfaces, particularly in the serial treatment of aluminum can cylinders. A high degree of gloss is achieved in the inventive method by using an alkaline cleaner containing, in addition to a builder and surfactant, an amount of at least one aluminate dissolved in water. The invention also includes an alkaline cleaner as an application solution, which is particularly suitable for use in an inventive method.

Description

[0001] The present invention relates to a method for cleaning and, if necessary, degreasing aluminum surfaces, particularly in the serial treatment of aluminum can cylinders. A high degree of gloss is achieved in the inventive method by using an alkaline cleaner containing, in addition to a builder and surfactant, an amount of at least one aluminate dissolved in water. The invention also includes an alkaline cleaner as an application solution, which is particularly suitable for use in an inventive method.

[0002] While good cleaning and degreasing of surfaces can generally be achieved when cleaning semi-finished products and components made of aluminum, and the cleaning agents used for this purpose are generally known, a significant challenge lies in reliably achieving high-gloss surfaces, especially during the serial processing of large numbers of semi-finished products / components. Particularly with high component throughput, changes in the composition of the cleaning bath due to consumption and the introduction of contaminants, as well as inconsistent pickling or temporary over-pickling, can lead to a high rate of material rejects or require components to undergo multiple cleaning steps. This is especially problematic in can manufacturing, a highly standardized and automated production process with extremely high throughput rates of several hundred to several thousand cans per minute.Furthermore, in can manufacturing, where transparent can exterior lacquers are used for decoration, cans with low or inconsistent gloss levels are perceived as unattractive by the end consumer and must be rejected during the production process and recycled. Established cleaning systems for aluminum materials, and aluminum cans in particular, often rely on aqueous acidic, fluoride-containing solutions. For these systems, the fluoride content in the cleaner must be precisely controlled during the throughput of the batch of components to ensure a uniform gloss, possibly with the addition of inhibitors. US Patent 5,565,076 serves as an example of the process engineering effort involved. It describes a special design of a fluoride-sensitive glass electrode for the reliable determination of the fluoride content in acidic cleaning baths during the treatment of aluminum cans.

[0003] Other alkaline cleaning systems, which are ideally suited for removing oil and grease residues from prior manufacturing steps such as deep drawing of aluminum surfaces, are often based on builder substances such as borates and / or phosphates that pose health or environmental hygiene risks. Furthermore, these alkaline builders, selected from phosphates, borates, and / or silicates, can cause surface defects during painting, necessitating sophisticated process control of the cleaning bath composition. Finally, even with alkaline cleaners, the pickling effect must be precisely adjusted to achieve a consistent gloss level with short treatment times and high throughput rates. This ensures, for example, low reject rates in can manufacturing, resulting in high economic efficiency and environmental compatibility.

[0004] Based on the aforementioned requirements for a cleaning and degreasing step in the production of semi-finished products and components made of aluminum, and considering the current state of the art, it is clear that existing processes must be improved to enable the wet-chemical cleaning and degreasing of a large number of semi-finished products or components within short treatment times. This should reliably produce high-gloss material surfaces that, when coated with transparent lacquers, also meet high aesthetic standards and exhibit no dulling on the outer surfaces. In particular, a cleaner should be provided that, within a wide application window in terms of application temperature and duration, delivers consistently good cleaning results and gloss levels without the need for complex process control of bath parameters, even in high-throughput processes such as can manufacturing.Furthermore, wet chemical cleaning and degreasing should be possible with a composition that is largely environmentally safe and, from an occupational hygiene point of view, does not cause any health hazard for the workers tasked with carrying out and monitoring the cleaning.

[0005] The present invention solves this requirement profile by establishing a method for cleaning surfaces of the metal aluminium, in which the aluminium surface to be cleaned is brought into contact with an alkaline cleaner having a pH value greater than 8.0 and containing water. a) contains at least one alkaline builder, b) contains a total of at least 0.20 g / kg of aluminates dissolved in water, calculated as AlO2, and c) contains at least one surfactant.

[0006] Unless explicitly defined otherwise in individual cases, the concentration or quantity specifications characterizing the alkaline cleaner in the context of the present invention always refer to the overall composition of the alkaline cleaner.

[0007] Within the scope of the present invention, an alkaline builder is defined as alkaline-reacting compounds or a mixture of such compounds, wherein at least one alkaline-reacting compound must be a component of the builder at a concentration of at least 1 mmol / kg, and the alkalinity-inducing equilibrium reaction has a pKa B value in the range of 3.0–6.0. ​​Suitable builders in the context of the present invention are therefore carbonates, borates, silicates, and / or phosphates, as well as mixtures of these compounds with hydroxides.

[0008] In the context of the present invention, the alkaline builder is preferably selected from alkali metal carbonates and / or alkali metal carbonates, particularly preferably from alkali metal carbonates and especially preferably from sodium carbonate.

[0009] To effectively remove the drawing oils and greases commonly used in metalworking from the component surfaces to be cleaned, it is preferred that the cleaner contains a total of at least 0.20 g / kg, particularly preferably at least 0.30 g / kg, and especially preferably at least 0.40 g / kg of alkali metal carbonates, calculated as COs. Cleaning and degreasing are also supported by pickling the aluminum substrate; however, high pickling rates, even with short treatment times, are detrimental to surface gloss due to the significantly increased surface roughness. To prevent the aluminum surfaces from matting, cleaners with a total alkali metal carbonate content of no more than 3.00 g / kg, and particularly preferably no more than 2.00 g / kg, calculated as COs, are preferably used according to the invention.

[0010] For similar reasons, sufficient cleaning and, if necessary, removal of drawing oils and greases requires that the pH value of the alkaline cleaner be greater than 8.0, preferably greater than 8.5. However, in order to prevent over-pickling of the aluminum surfaces treated in the process according to the invention, preferably without the use of inhibitors, it is preferred that the pH value of the alkaline cleaner is not greater than 11.0, particularly preferably not greater than 10.5, and especially preferably not greater than 10.0.

[0011] For sufficient pH stability and a consistent, but not excessively high, pickling effect, it is preferred that the alkaline cleaner has a total alkalinity of at least 1.5 points, particularly preferably at least 1.8, but preferably not exceeding 4.0 points, particularly preferably 3.0 points, and most preferably 2.4 points. The total alkalinity corresponds to the consumption of 0.1 N hydrochloric acid in milliliters after titration of a 10 ml sample volume of the cleaner diluted with 50 ml of deionized water (κ < 1 µScm⁻¹) to a pH of 3.6 at a temperature of 25 °C.

[0012] The free alkalinity of the alkaline cleaner, expressed in points, is preferably not above 1.5, particularly preferably not above 1.0, and especially preferably not above 0.8, but preferably at least 0.2 points, and particularly preferably 0.3 points, for a moderate pickling effect during the process according to the invention. The free alkalinity corresponds to the consumption of 0.1 N hydrochloric acid in milliliters after titration of a 10 ml sample volume of the cleaner diluted with 50 ml of deionized water (κ < 1 µScm-1) to a pH of 3.6 at a temperature of 25 °C.

[0013] Essential for providing a glossy surface and maintaining the gloss during the cleaning process according to the inventive method is the presence of aluminates dissolved in water, which exist as complex hydroxoaluminate ions. All water-soluble aluminates are suitable as a source of the aluminates dissolved in water, preferably alkali metal aluminates, for example sodium aluminate and / or potassium aluminate, but sodium aluminate is particularly preferred.

[0014] Due to the aluminates, cleaning and degreasing in the alkaline medium is highly efficient and does not impair the gloss properties, as over-etching of the aluminum substrate is prevented in the presence of the aluminates. For this to occur, a total of at least 0.20 g / kg of aluminates dissolved in water is required, preferably at least 0.40 g / kg, and particularly preferably at least 0.60 g / kg, each calculated as AlO₂. Significantly higher concentrations of aluminates can be achieved in the cleaner without any loss of performance, especially without loss of gloss. However, for reasons of economy, the concentration of aluminates dissolved in water should be limited, and therefore preferably less than 4.00 g / kg, particularly preferably less than 3.00 g / kg, and most preferably less than 2.00 g / kg of aluminates dissolved in water, calculated as AlO₂, are contained in the alkaline cleaner.Overall, this results in a wide application window with regard to the content of aluminates dissolved in water, which opens up a high degree of process engineering flexibility.

[0015] For good cleaning and degreasing performance, the presence of at least one surfactant is necessary. In principle, all surfactants known in the prior art that are capable of emulsifying nonpolar components in a continuous aqueous phase are suitable. These include anionic surfactants, such as the aforementioned fatty acids, but especially also nonionic surfactants (nioteric surfactants). In a preferred embodiment of the process according to the invention, the alkaline cleaner therefore contains at least one nonionic surfactant with an HLB value above 10.0, preferably above 11.0, and particularly preferably above 12.0. Within the scope of the present invention, the HLB value is an empirical value and is calculated as follows: HLB = 20 ⋅ 1 − M L / M with ML: molar mass of the lypophilic group of the niotenoid; M: molar mass of the niotenoid.

[0016] Preferred niotensides with an HLB value greater than 10.0 are selected from alkylbenzyl alkoxylates having no more than 12 carbon atoms in the alkyl group, which in turn is preferably saturated, wherein the alkylbenzyl alkoxylates have at least 6 but no more than 14 alkoxy units.

[0017] Equally suitable, and therefore equally preferred, are niotenes with an HLB value greater than 10.0, which are selected from fatty alcohol alkoxylates having at least 8, but not more than 16 carbon atoms in the alkyl group, which is preferably saturated and particularly preferably saturated and unbranched, which have at least 10, but not more than 30 alkoxy units, wherein the alkoxy units are preferably selected from ethoxy and / or propoxy units.

[0018] At high throughput rates of semi-finished products and components made of aluminum, or during prolonged operation of a cleaning bath according to the inventive method, the precipitation of aluminum salts can become problematic due to the increased input of aluminum ions. The microcrystalline deposits that occur on the semi-finished products and components with these precipitates impair the gloss properties of the aluminum surfaces and also represent surface defects with potentially insufficient adhesion to subsequently applied coating systems.To prevent this, the alkaline cleaner preferably contains additional organic complexing agents, which in turn are preferably selected from α-hydroxycarboxylic acids, particularly preferably from α-hydroxycarboxylic acids with no more than three carboxyl groups, most preferably from sugar acids, particularly preferably aldonic and aldaric acids, and most preferably from citric acid and / or tartaric acid. For sufficient dosage, it is generally advantageous if the molar ratio of organic complexing agents to aluminates contained in the cleaner is in the range of 1:1 to 1:10, and particularly preferably in the range of 1:2 to 1:8.

[0019] An advantage of the alkaline cleaner used in the process according to the invention is that it can be formulated without fluoride and without the addition of inhibitors. Furthermore, potentially harmful and environmentally damaging building blocks such as phosphates and / or borates can be avoided.

[0020] According to the invention, processes are therefore preferred in which the alkaline cleaner contains less than 100 mg / kg, particularly preferably less than 10 mg / kg, and especially preferably less than 1 mg / kg of phosphates dissolved in water calculated as PO4, or of borates dissolved in water calculated as BOs, or of silicates dissolved in water calculated as SiO4.

[0021] According to the invention, processes in which the alkaline cleaner contains less than 100 mg / kg, particularly preferably less than 10 mg / kg, and especially preferably less than 1 mg / kg of fluorides dissolved in water, calculated as the total fluoride content, are also preferred. The total fluoride content is determined in a TISAB-buffered aliquot of the alkaline cleaner using a fluoride-sensitive electrode at 20 °C (TISAB: "Total Ionic Strength Adjustment Buffer"), wherein the volumetric mixing ratio of buffer to aliquot of cleaner is 1:1. The TISAB buffer is prepared by dissolving 58 g NaCl, 1 g sodium citrate and 50 ml glacial acetic acid in 500 ml deionized water (κ < 1µScm -1< ) ​​and adjusting the pH to 5.3 using 5 N NaOH, then filling up to a total volume of 1000 ml again with deionized water (κ < 1µScm -1< ).

[0022] Equally preferred is the absence of otherwise usual corrosion inhibitors for aluminium and therefore that the alkaline cleaner contains a total of less than 100 mg / kg, preferably less than 10 mg / kg, particularly preferably less than 1 mg / kg of benzoic acid and benzoates dissolved in water, calculated as C 7 H 6 O 2, or of organic compounds dissolved in water with at least one nitrogen-containing heterocycle, calculated as N 3.

[0023] In the inventive method, the contacting of the alkaline aqueous cleaner can be carried out using any application method established in the prior art, including immersion, flooding, spraying, and misting of the aluminum surfaces, with immersion and spraying being particularly preferred and established in high-throughput processes such as can manufacturing. The contact times are preferably in the range of 5 to 180 seconds, typically and therefore particularly preferably less than 60 seconds, and more preferably 10 to 30 seconds.

[0024] The temperature range at which contact with the alkaline aqueous cleaner takes place and cleaning is particularly effective is 30 to 65 °C, particularly preferably 40 to 60 °C, and especially preferably 45 to 55 °C, in each case as the temperature of the cleaner at the time of contact.

[0025] The surfaces of the metal aluminum treated in the cleaning process according to the invention are preferably the surfaces of semi-finished products and components made of aluminum or clad with aluminum. Both aluminum cladding and aluminum materials forming surfaces of the metal aluminum within the meaning of the present invention consist of more than 50 at.%, preferably more than 80 at.%, and particularly preferably more than 90 at.% of the metal aluminum, and thus aluminum alloys can also be cleaned in the process according to the invention. Further alloying elements can be selected, for example, from manganese, magnesium, iron, copper, zinc, chromium, zirconium, and / or silicon.A particularly preferred aluminium material that can be cleaned very reliably using the inventive method is the 3xxx alloy series, in particular the 3004 alloy, which is specifically used for the manufacture of beverage cans.

[0026] In principle, all semi-finished products and components made from the aforementioned materials can be cleaned and degreased using the inventive method. Particularly noteworthy are flat products such as sheets and semi-finished products formed from these, for example by deep drawing, such as cans open at one end. However, as mentioned at the outset, the method is especially suitable for cleaning and degreasing in the context of high-throughput mass production of a large number of semi-finished products and components. Therefore, the present invention focuses on the cleaning and degreasing of can cylinders for the production of beverage or aerosol cans, which thus represents the preferred application example of the inventive method.

[0027] In the inventive method, surfaces of the metal aluminium are therefore particularly preferred, which represent the surfaces of a large number of can cylinders made of aluminium and its alloys.

[0028] In a second aspect, the present invention relates to a composition that is particularly suitable for use as an aqueous alkaline cleaner in a process according to the invention. This composition according to the invention is an alkaline cleaner for aluminum surfaces, having a pH value greater than 8.0 and preferably not greater than 11.0, and containing, in addition to water, a) at least 0.20 g / kg, but preferably not more than 3.00 g / kg of alkali metal carbonates calculated as COs, b) at least 0.20 g / kg, but preferably less than 4.00 g / kg of aluminates dissolved in water, preferably sodium aluminate calculated as AlO₂, c) at least one niotenoid with an HLB value greater than 10.0, and d) at least one organic complexing agent, preferably selected from α-hydroxy carboxylic acids, particularly preferably selected from α-hydroxy carboxylic acids with no more than three carboxyl groups, and most preferably selected from gluconic acid and / or citric acid.

[0029] Further preferred embodiments of the alkaline cleaner can be carried out analogously to those described in the context of the inventive method with regard to the alkaline cleaner used therein. Examples:

[0030] To illustrate the cleaning and degreasing performance of alkaline cleaners according to the invention, open-ended beverage cans made of AI3004 with different formulations were cleaned by spraying. The cleaners were applied by spraying at 1.0 bar. After spraying, the semi-finished products were freed from cleaner residues by brief immersion in deionized water (κ < 1 µScm). After re-immersion and blowing off the surfaces, the respective gloss level was determined. To determine the gloss level, the reflectance of the respective substrate surface was measured and compared to the reflectance of a surface of the respective semi-finished product degreased with a 1:1 volumetric mixture of n-hexane / ethanol. The corresponding results are shown in Table 1. The application solution according to the invention contains as 1 wt.-% solution of the cleaner concentrate (see B1, Table 1) contains the following components: . Sodium aluminate 0.90 g / kg Na₂CO₃ 0.47 g / kg Niotenes (HLB value 10-12) 0.26 g / kg Sodium gluconate 0.26 g / kg citric acid 0.30 g / kg Tab. 1 Quantity / wt.% PH value Duration / s T / °C Gloss level #< B1 1,0 9,6 40 60 95 B1 1,0 9,6 180 60 94 B2 1,5 9,6 40 60 94 B2 1,5 9,8 180 55 95 VB1 1,5 9,8 40 60 70 VB1 1,5 9,8 180 60 20 VB2 1,5 2,1 40 60 98 VB2 1,5 2,1 180 60 50 # Determined at an angle of 60° with Picogloss Model 503, Erichsen B1 / B2 alkaline cleaners according to the invention based on the same concentrate VB1 commercial alkaline cleaner based on NaOH / borate (Bonderite® < C-AK 60036 of Henkel AG) VB2 commercial fluoride-containing cleaner based on H₂SO₄ / HF (Bonderite® < C-IC 740 E of Henkel AG)

Claims

1. A process for cleaning surfaces of the metal aluminum, in which the aluminum surface to be cleaned is brought into contact with an alkaline cleaner which has a pH value of greater than 8.0 and contains, in addition to water, a) at least one alkaline builder, b) a total of at least 0.20 g / kg of aluminates dissolved in water, calculated as AlO2, and c) at least one surfactant.

2. Method according to claim 1, characterized in that the alkaline builder of the cleaner is selected from alkali metal hydroxides and / or alkali metal carbonates, preferably from alkali metal carbonates and particularly preferably from sodium carbonate.

3. Method according to claim 2, characterized in thatthe alkaline cleaner contains a total of at least 0.20 g / kg, preferably a total of at least 0.30 g / kg, very particularly preferably a total of at least 0.40 g / kg, but preferably a total of not more than 3.00 g / kg, particularly preferably a total of not more than 2.00 g / kg of alkali metal carbonates calculated as COs.

4. Method according to one or more of the preceding claims, characterized in that the pH of the cleaner is greater than 8.5, but preferably not greater than 11.0, more preferably not greater than 10.5 and most preferably not greater than 10.

0.

5. Method according to one or more of the preceding claims, characterized in that alkali metal aluminates, preferably sodium aluminate, are contained in the alkaline cleaner as a source of the aluminates dissolved in water.

6. Method according to one or more of the preceding claims, characterized in thatthe alkaline cleaner contains a total of at least 0.40 g / kg, preferably a total of at least 0.60 g / kg, but preferably a total of less than 4.00 g / kg, particularly preferably a total of less than 3.00 g / kg, especially preferably less than 2.00 g / kg of aluminates dissolved in water, calculated as AlO2.

7. Method according to one or more of the preceding claims, characterized in thatthe at least one surfactant contained in the alkaline cleaner is selected from nonionic surfactants, preferably from nonionic surfactants with an HLB value greater than 10.0, particularly preferably from alkylbenzyl alkoxylates having not more than 12 carbon atoms in the alkyl radical, which is preferably saturated, which have at least 6 but not more than 14 alkoxy units, and / or from fatty alcohol alkoxylates having at least 8 but not more than 16 carbon atoms in the alkyl radical, which is preferably saturated and particularly preferably saturated and unbranched, which have at least 10 but not more than 30 alkoxy units, wherein the alkoxy units are preferably each selected from ethoxy and / or propoxy units.

8. Method according to one or more of the preceding claims, characterized in thatthe alkaline cleaner additionally contains organic complexing agents which are preferably selected from α-hydroxycarboxylic acids, particularly preferably selected from α-hydroxycarboxylic acids having not more than three carboxyl groups, very particularly preferably from sugar acids, in particular aldonic acids and aldaric acids, as well as from citric acid and / or tartaric acid.

9. Method according to claim 8, characterized in that the molar ratio of organic complexing agents to aluminates contained in the alkaline cleaner is in the range of 1:1 to 1:10, preferably in the range of 1:2 to 1:

8.

10. Method according to one or more of the preceding claims, characterized in that the alkaline cleaner contains a total of less than 100 mg / kg, preferably less than 10 mg / kg, particularly preferably less than 1 mg / kg of phosphates dissolved in water, calculated as PO4.

11. Method according to one or more of the preceding claims, characterized in thatthe cleaner contains a total of less than 100 mg / kg, preferably less than 10 mg / kg, particularly preferably less than 1 mg / kg of borates dissolved in water, calculated as BOs.

12. Method according to one or more of the preceding claims, characterized in that the alkaline cleaner contains a total of less than 100 mg / kg, preferably less than 10 mg / kg, particularly preferably less than 1 mg / kg of benzoic acid and benzoates dissolved in water, calculated as C7H6O2.

13. Method according to one or more of the preceding claims, characterized in that the alkaline cleaner contains a total of less than 100 mg / kg, preferably less than 10 mg / kg, particularly preferably less than 1 mg / kg of organic compounds dissolved in water with at least one nitrogen-containing heterocycle calculated as N3.

14. A method according to one or more of the preceding claims for cleaning and degreasing surfaces of the metal aluminum, which are the surfaces of a plurality of can cylinders made of aluminum.

15. Alkaline cleaner for aluminum surfaces, which has a pH value of greater than 8.0 and preferably not greater than 11.0, containing in addition to water a) at least 0.20 g / kg, but preferably not more than 3.00 g / kg of alkali metal carbonates calculated as CO2, b) at least 0.20 g / kg, but preferably less than 4.00 g / kg of aluminates dissolved in water, preferably sodium aluminate, calculated as AlO2, c) at least one nonionic surfactant with an HLB value greater than 10.0 and d) at least one organic complexing agent, which is preferably selected from α-hydroxycarboxylic acids, more preferably selected from α-hydroxycarboxylic acids with not more than three carboxyl groups, most preferably selected from gluconic acid and / or citric acid.

Images