Alkaline cleaning agent for aluminum surfaces

Abstract

The present invention relates to a method for cleaning and optionally degreasing aluminum surfaces, in particular during serial treatment of can cylinders made of aluminum. In the method according to the invention, a high degree of gloss is achieved when using an alkaline cleaning agent containing, in addition to a builder and a surfactant, a quantity of at least one aluminate dissolved in water. The invention also relates to an alkaline cleaning agent as an application solution, which agent is especially suitable for being used in a method according to the invention.

Description

[0001] Alkaline cleaner for aluminum surfaces

[0002] 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 using an alkaline cleaner containing, in addition to builder and surfactant, a quantity of at least one aluminate dissolved in water. The invention also encompasses an alkaline cleaner as a working solution, which is particularly suitable for use in a method according to the invention.

[0003] When cleaning semi-finished products and components made of aluminum, good cleaning and degreasing of the surfaces can usually be achieved, and the cleaning agents used for this purpose are generally known. However, a greater challenge is the reliable provision of high-gloss surfaces, especially in the series treatment of a large number of semi-finished products / components. Particularly with high component throughputs, 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 mean that high material waste must be accepted or that components must undergo a cleaning step multiple times. This is particularly problematic in can production, a highly standardized and automated manufacturing process with extreme throughput rates of several hundred to several thousand cans per minute.In addition, in can production, where transparent exterior can coatings are used for decoration, cans with low or inconsistent gloss levels are perceived as unsightly by the end consumer and must be sorted out during the production process and recycled. Established cleaning systems for aluminum materials, and aluminum cans in particular, are often based on aqueous, acidic, fluoride-containing solutions, for which the fluoride content in the cleaner must be precisely controlled during the throughput of the series of components to ensure a uniform gloss appearance, if necessary with the addition of inhibitors. As an example of the process engineering effort involved, reference can be made to US Pat. No. 5,565,076, which deals with 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.

[0004] Other alkaline cleaning systems, which are excellently suited for removing oil and grease residues from previous production steps such as deep drawing from aluminum surfaces, are often based on builder substances such as borates and / or phosphates, which are problematic from a health or environmental perspective. Furthermore, alkaline builders selected from phosphates, borates, and / or silicates can cause surface defects during painting, thus requiring sophisticated process-engineered control of the cleaning bath composition. Last but not least, 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, thus ensuring low waste and thus high cost-effectiveness and environmental compatibility, for example in can production.

[0005] Based on the aforementioned requirements for a cleaning and degreasing step in the production of semi-finished products and components made of aluminum, and the state of the art, it follows that existing processes must be improved to enable the wet-chemical cleaning and degreasing of a wide variety of semi-finished products or components in short treatment times. This reliably produces high-gloss material surfaces that, when coated with transparent coatings, also meet high aesthetic standards and exhibit no matting on the outer surfaces. In particular, a cleaner is to be provided that delivers consistently good cleaning results and gloss levels over a broad application window in terms of application temperature and duration without the need for complex process control of bath parameters, even in high-throughput processes such as can production.Furthermore, wet-chemical cleaning and degreasing should be carried out using a composition that is largely ecologically safe and, when used from an occupational hygiene point of view, does not pose a health risk to the workers responsible for carrying out and monitoring the cleaning.

[0006] The present invention solves this requirement profile by establishing a method 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, in addition to water, contains 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.

[0007] 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, unless explicitly defined otherwise in the individual case.

[0008] For the purposes of the present invention, an alkaline builder refers to alkaline-reacting compounds or a mixture of such compounds. At least one alkaline-reacting compound must be a component of the builder in a concentration of at least 1 mmol / kg, at which the alkalinity-inducing equilibrium reaction has a pKß value in the range of 3.0-6.0. Therefore, highly suitable builders in the context of the present invention are carbonates, borates, silicates, and / or phosphates, as well as mixtures of these compounds with hydroxides.

[0009] In the context of the present invention, the alkaline builder is preferably selected from alkali metal carbonates and / or alkali metal carbonates, more preferably from alkali metal carbonates and especially preferably from sodium carbonate.

[0010] In order to be able to effectively remove the drawing oils and greases commonly used in metalworking from the component surfaces to be cleaned, it is preferred if the cleaner contains a total of at least 0.20 g / kg, more preferably a total of at least 0.30 g / kg, especially preferably a total of at least 0.40 g / kg of alkali metal carbonates, calculated as CO3. The cleaning and degreasing is also supported by pickling the aluminum base substrate, but high pickling rates, even with short treatment times, are detrimental to the surface gloss due to the significantly increased surface roughness. In order to prevent the aluminum surfaces from becoming dull, cleaners are preferably used according to the invention whose proportion of alkali metal carbonates totals no more than 3.00 g / kg, more preferably no more than 2.00 g / kg, each calculated as CO3.

[0011] For similar reasons, for adequate cleaning and, if necessary, removal of drawing oils and greases, the pH of the alkaline cleaner must 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, if possible without the use of inhibitors, it is preferred that the pH of the alkaline cleaner be no greater than 11.0, more preferably no greater than 10.5, and especially preferably no greater than 10.0.

[0012] For sufficient pH stability and consistent, but not too high, pickling effect, it is preferred if the alkaline cleaner has a total alkalinity in points of at least 1.5, particularly preferably of at least 1.8, but preferably not exceeding 4.0 points, particularly preferably 3.0 points, most preferably 2.4 points. The total alkalinity corresponds to the consumption of 0.1 N hydrochloric acid in milliliters after titration of a solution of 50 ml of deionized water (K ​​< 1 pScm -1 ) diluted sample volume of 10 ml of the cleaner to a pH of 3.6 at a temperature of 25 °C.

[0013] For a moderate pickling effect during the process according to the invention, the free alkalinity of the alkaline cleaner in points is preferably not above 1.5, more preferably not above 1.0 and especially preferably not above 0.8, but preferably at least 0.2 points, particularly preferably 0.3 points. The free alkalinity corresponds to the consumption of 0.1 N hydrochloric acid in milliliters after titration of a sample volume of 10 ml of the cleaner diluted with 50 ml of deionized water (K ​​< 1 pScm-1) to a pH of 3.6 at a temperature of 25 °C. The presence of the aluminates dissolved in water, which are present as complex hydroxoaluminate ions, is essential for providing a shiny surface and maintaining the shine during the cleaning process according to the process according to the invention.All water-soluble aluminates are suitable as a source for the aluminates dissolved in water, preferably alkali metal aluminates, for example sodium aluminate and / or potassium aluminate, but particularly preferably sodium aluminate.

[0014] Due to the aluminates, cleaning and degreasing in the alkaline medium is very efficient and without any loss of gloss properties, since over-pickling of the aluminum substrate is prevented in the presence of the aluminates. For this to happen, a total of at least 0.20 g / kg of aluminates dissolved in water is required, preferably a total of at least 0.40 g / kg, particularly preferably a total of at least 0.60 g / kg, each calculated as AlO2. Significantly higher aluminate contents can be achieved in the cleaner without any loss of performance, in particular without any loss of gloss. However, for reasons of cost-effectiveness, the content of aluminates dissolved in water should be limited. Therefore, the alkaline cleaner preferably contains a total of less than 4.00 g / kg, particularly preferably a total of less than 3.00 g / kg, and especially preferably less than 2.00 g / kg of aluminates dissolved in water, calculated as AlO2.Overall, there is a broad application window with regard to the content of aluminates dissolved in water, which opens up a high degree of process engineering flexibility.

[0015] The presence of at least one surfactant is necessary for good cleaning and degreasing performance. Suitable surfactants include, in principle, all surface-active substances known in the art that are capable of emulsifying non-polar components in a continuous aqueous phase. These include anionic surfactants, such as the fatty acids mentioned above, but especially also non-ionic surfactants (nonionic surfactants). In a preferred embodiment of the process according to the invention, the alkaline cleaner therefore contains at least one non-ionic surfactant with an HLB value above 10.0, preferably above 11.0, 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 -ML / M) where ML: molecular weight of the lypophilic group of the nonionic surfactant; M: molecular weight of the nonionic surfactant.

[0016] Preferred nonionic surfactants with an HLB value greater than 10.0 are selected from alkylbenzyl alkoxylates having no more than 12 carbon atoms in the alkyl radical, which in turn is preferably saturated, wherein the alkylbenzyl alkoxylates have at least 6 but no more than 14 alkoxy units. Equally suitable, and therefore equally preferred, are nonionic surfactants with an HLB value greater than 10.0 that are selected from fatty alcohol alkoxylates having at least 8 but no more than 16 carbon atoms in the alkyl radical, which is preferably saturated and particularly preferably saturated and unbranched, and that have at least 10 but no more than 30 alkoxy units, wherein the alkoxy units are preferably each selected from ethoxy and / or propoxy units.

[0017] At high throughput rates of aluminum semi-finished products and components, or during prolonged operation of a cleaning bath according to the process according to the invention, the precipitation of aluminum salts can become problematic due to the increased input of aluminum ions. The microcrystalline deposits that occur with these precipitations on the semi-finished products and components impair the gloss properties of the aluminum surfaces and also represent surface defects with potentially insufficient adhesion to subsequently applied paint systems.To prevent this, the alkaline cleaner preferably additionally contains organic complexing agents, which in turn are preferably selected from α-hydroxycarboxylic acids, particularly preferably selected from α-hydroxycarboxylic acids with no more than three carboxyl groups, very particularly preferably from sugar acids, especially preferably aldonic acids and aldaric acids, and particularly 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, particularly preferably in the range of 1:2 to 1:8.

[0018] One 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 harmful builder substances such as phosphates and / or borates can be omitted.

[0019] According to the invention, therefore, processes are preferred in which the alkaline cleaner contains a total of less than 100 mg / kg, particularly preferably less than 10 mg / kg, especially preferably less than 1 mg / kg of phosphates dissolved in water, calculated as PU4, or of borates dissolved in water, calculated as BO3, or of silicates dissolved in water, calculated as SiO4.

[0020] Also preferred according to the invention are processes in which the alkaline cleaner contains a total of less than 100 mg / kg, particularly preferably less than 10 mg / kg, especially preferably less than 1 mg / kg of fluorides dissolved in water, calculated as the total fluoride content. 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"), with the volume-related mixing ratio of buffer to the aliquot of the cleaner being 1:1. The TISAB buffer is prepared by dissolving 58 g of NaCl, 1 g of sodium citrate, and 50 ml of glacial acetic acid in 500 ml of deionized water (K ​​< 1 pScm -1 ) and adjusting the pH to 5.3 using 5 N NaOH and filling to a total volume of 1000 ml with deionized water (K ​​< 1 pScm -1 ).

[0021] Likewise preferred is the absence of otherwise conventional corrosion inhibitors for aluminum 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 C7H6O2, or of organic compounds dissolved in water with at least one nitrogen-containing heterocycle, calculated as N3.

[0022] In the process according to the invention, the alkaline aqueous cleaner can be brought into contact with the aluminum surfaces using any of the application methods established in the prior art, including dipping, flooding, spraying, and misting, particularly dipping and spraying, with the latter application method being 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 especially preferably 10 to 30 seconds.

[0023] The temperature interval at which the contacting with the alkaline aqueous cleaner takes place and cleaning is carried out particularly effectively is 30 to 65 °C, particularly preferably 40 to 60 °C, especially preferably 45 to 55 °C, in each case as the temperature of the cleaner when bringing into contact.

[0024] 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 the material aluminum or having an aluminum plating. Both aluminum platings and aluminum materials that form surfaces of the metal aluminum within the meaning of the present invention consist of more than 50 at.%, preferably more than 80 at.%, 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. Other alloy constituents can be selected, for example, from manganese, magnesium, iron, copper, zinc, chromium, zirconium, and / or silicon.A particularly preferred aluminum material that can be cleaned very reliably in the process according to the invention is the 3xxx alloy series, in particular the 3004 alloy, which is used specifically for the production of beverage cans.

[0025] In principle, all semi-finished products and components made from the aforementioned materials can be cleaned and degreased using the process according to the invention. Particular mention should be made here of flat products such as sheet metal and semi-finished products formed from these, for example, by deep drawing, such as single-sided open-end cans. However, as mentioned at the beginning, the process is particularly suitable for cleaning and degreasing in the context of series production of a large number of semi-finished products and components at high throughput rates, so that the focus of the present invention is the cleaning and degreasing of can cylinders for the production of beverage or aerosol cans, which thus represents the correspondingly preferred application example of the process according to the invention.

[0026] In the process according to the invention, therefore, those surfaces of the metal aluminum which represent the surfaces of a large number of can cylinders made of aluminum and its alloys are particularly preferably cleaned and degreased.

[0027] In a second aspect, the present invention relates to a composition which 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 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 CO3, 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 a-hydroxycarboxylic acids, more preferably selected from a-hydroxycarboxylic acids having not more than three carboxyl groups, most preferably selected from gluconic acid and / or citric acid.

[0028] Further preferred embodiments of the alkaline cleaner can be carried out analogously to those which were described in the context of the process according to the invention with regard to the alkaline cleaner used there.

[0029] Examples:

[0030] To illustrate the cleaning and degreasing performance of the alkaline cleaners according to the invention, single-sided open beverage cans made of AI3004 were cleaned using different formulations using a spray process. The cleaners were applied by spraying at 1.0 bar. After spraying, the semi-finished products were briefly immersed in deionized water (K ​​< 1 pScm -1) were freed from cleaner residues, and after 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 mixture of n-hexane / ethanol (volumetric 1:1). The corresponding results are shown in Table 1. The application solution according to the invention contains the following components as a 1 wt.% solution of the cleaner concentrate (see B1, Table 1):

[0031] Sodium aluminate 0.90 g / kg

[0032] Na2CO30.47 g / kg

[0033] Niotensides (HLB value 10-12) 0.26 g / kg

[0034] Sodium gluconate 0.26 g / kg

[0035] Citric acid 0.30 g / kg

[0036] Table 1

[0037] # determined at an angle of 60° with Picogloss Model 503, Erichsen

[0038] B1 / B2 alkaline cleaners according to the invention based on the same concentrate

[0039] VB1 commercial alkaline cleaner based on NaOH / borate (Bonderite® C-AK 60036 from Henkel AG)

[0040] VB2 commercial fluoride-containing cleaner based on H2SO4 / HF (Bonderite® C-IC 740 E from Henkel AG)

Claims

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. Process 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. Process according to claim 2, characterised in that the 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 CO3.

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

0.

5. Process 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 for the aluminates dissolved in water.

6. Process according to one or more of the preceding claims, characterized in that the 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, particularly preferably less than 2.00 g / kg of aluminates dissolved in water, calculated as AlO2.

7. The method according to one or more of the preceding claims, characterized in that the 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 alkyl benzyl alkoxylates having not more than 12 carbon atoms in the alkyl radical, which is preferably saturated, which contain 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. Process according to one or more of the preceding claims, characterized in that the alkaline cleaner additionally contains organic complexing agents which are preferably selected from a-hydroxycarboxylic acids, particularly preferably selected from a-hydroxycarboxylic acids having not more than three carboxyl groups, very particularly preferably from sugar acids, in particular aldonic acids and aldaric acids, and from citric acid and / or tartaric acid.

9. Process 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. Process 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 PCM.

11. Process according to one or more of the preceding claims, characterized in that the 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 BO3.

12. Process 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. Process 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 represent 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 CO3, 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.

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