EP0230903B1 - Process for cleaning aluminium containers - Google Patents

Abstract

Aluminium containers are cleaned using an alkaline cleaning soln., followed by rinsing at least once with an aq. soln. whose pH is not more than 2.5. The use of the process as a pretreatment for aluminium containers before the application of coatings is also claimed. Pref. the aluminium containers are rinsed with an acid aq. soln. of pH about 2, whose temp. is less than 65.6 deg.C, esp. 32.2-54.4 deg.C, before and/or after treatment with the alkaline cleaning soln. After the alkaline treatment, the containers are rinsed in several steps, in which the second step involves rinsing with an aq. soln. whose pH is not more than 2.5. Pref. the rinsing with the aq. soln. is with a spray, the aq. acid soln. contains fluoride ions, and rinsing with water takes place after the rinsing with aq. acid. Pref. after treatment with the alkaline cleaning soln. and rinsing, the aluminium containers are dried.

Description

The invention relates to a process for cleaning aluminum containers with the aid of alkaline cleaning solutions and subsequent rinsing with an aqueous solution, and to the use thereof as a pretreatment of aluminum containers before the application of conversion coatings.

Aluminum containers have found widespread use and are used in particular for packaging food and beverages, such as soft drinks and beer. It is common practice in the manufacture of such containers to first punch round disks from aluminum sheet, to bring them into a preliminary cup shape with suitable devices and finally to deform them in one or more stages by pulling and smoothing them into the final can. The cans are then trimmed and subjected to various cleaning and treatment stages. The conclusion is usually the application of an inner varnish and a decorative outer coating.

To facilitate the aforementioned process with rolling, punching, drawing and smoothing, various lubricants and cooling lubricants are used which - together with fine metal particles that are formed during the shaping process - remain in certain quantities. It is essential that such lubricant residues and metal particles are removed before the containers are subjected to further treatment.

There are numerous acidic or alkaline cleaners for this purpose. Although these cleaners are quite effective in terms of residue removal, problems often arise with regard to the taste of the contents of the container. This problem is particularly annoying because it occurs only occasionally and in an unpredictable manner and then leads to rejection of entire batches of cans due to impairment of the contents of the can. The actual reasons for the sudden appearance of such taste impairments are not known. A somewhat speculative assumption is that residues from cleaning and any chemical treatment lead to the problem in question. In this respect, adjustments or changes in the concentration of the active components of the cleaning solution have no effect. In addition, there is a limit to the amount of change in the concentrations of the active ingredients since they lead to adverse effects on the aluminum surfaces, e.g. an undesirable matt appearance.

The type and thoroughness of the rinsing after the cleaning treatment is also of crucial importance for a possible taste impairment, but also the appearance of the surface. In particular, the detergent solution adhering to the aluminum container and added to the rinsing stage must be practically completely removed.

The prior art includes a process for cleaning aluminum containers using alkaline ones Detergent solutions and subsequent water rinsing, in which one works in the rinsing stage with circulating rinsing water, which is kept approximately neutral to acidic by pH control (EP - A2 - 181 673). Although this method has considerable advantages, particularly with regard to the consumption of rinsing water, the desired advantages with regard to the cleanliness of the containers and their mobility in the transport device of the treatment system and on the way from there to the further production stages, such as printing etc., cannot be achieved on a regular basis.

EP-A-0 157 382 describes a process for cleaning aluminum surfaces with acidic, aqueous cleaning solutions or cleaning solutions suitable therefor, which contain a soluble boron compound and a soluble fluorine compound, the boron content compared to the free fluoride content being greater than the stoichiometric requirement corresponds to the formation of BF₄- and which has a pH of <3.5. The cleaning solution contains at least 10 ppm free fluoride, the stoichiometric excess of boron compared to the free fluoride is at least 4 ppm.

FR-A-2 147 336 describes a process for removing oxide and dirt layers by bringing the layer to be cleaned into contact with an acidic, aqueous solution which contains between 5 and 21 g ions / l iron ions, 0.5 to 2, Contains 0 g ions / l fluoride ions and 0.05 to 3.0 g / l thiourea, the pH of the solution being between 0.1 and 1.8. After applying the solution, the treated surface is rinsed with water.

WO 85/01302 describes a method for producing clean surfaces of aluminum and aluminum alloys before gluing or applying paint to these surfaces.

For this purpose, the surface is first cleaned with alkali, washed with hot water and then etched with an acid (sulfuric acid containing fluoride ions). It is then washed again with water and then coated with a chromate-phosphate layer, after which the surface is washed again.

US-A-3 634 262 describes a composition for the treatment of aluminum and aluminum alloy surfaces which are suitable for subsequent further treatment processes. The composition consists essentially of a dry mixture of 5 to 20 wt .-% of a compound selected from alkali metal peroxodiphosphate, alkaline earth metal peroxodiphosphate and ammonium peroxodiphosphate. The composition further consists of 77 to 95% by weight of an acidic salt of sulfuric acid and between 0 and 3.0% by weight of a water-soluble fluoride salt. In the process for cleaning the aluminum and aluminum alloy surfaces, also described in this document, the aluminum or aluminum alloy object is soaked in an aqueous solution of the composition for 1 to 10 minutes at a temperature of from 15.5 ° C (60 ° F) to 76.7 ° C (170 ° F) treated. The pH of this solution is not more than 3.

The object of the invention is to provide a method for cleaning aluminum containers, which regularly leads to aluminum containers which have a high level of purity and which, owing to their great mobility, permit high throughputs, in particular also in a printing device which may be connected downstream.

The object is achieved by designing the method of the type mentioned at the outset in accordance with the invention in such a way that the aluminum containers treated with alkaline cleaning solution are rinsed in at least one stage with a solution whose pH is set at ≦ 2.5.

Due to the pH value of ≦ 2.5 in the aqueous solution, it is possible to avoid the brown or yellow discoloration that frequently occurs on the aluminum container after alkaline cleaning and to considerably improve the mobility of the containers. Such discolouration occurs in particular when, for unforeseeable reasons, the machine comes to a standstill Treatment plants for the aluminum containers is coming. According to the current state of knowledge, the reason for the improved mobility of the aluminum containers on transport devices and against one another is the removal of the aluminum oxide film formed during the alkaline cleaning.

A preferred development of the method according to the invention provides for rinsing with an acidic aqueous solution before treatment with alkaline cleaning solution. This measure already allows a considerable part of the contamination on the aluminum container consisting of fine aluminum particles and organic residues to be removed, so that the subsequent step of the alkaline cleaning is less stressed.

It is particularly effective to rinse before and / or after treatment with an alkaline cleaning solution with an aqueous solution whose pH is adjusted to about 2.

It is advisable to set the temperature of the solution to <65.6 ° C, in particular 32.2 to 54.4 ° C.

The method according to the invention can be used with particular advantage if the aluminum containers are cleaned with a cleaning solution which, in addition to alkaline components, contains a complexing agent and at least one surfactant with an HLB number of at least 12 and a pH of at least 10.

The alkaline reaction can occur in this cleaning solution Part of alkali or alkaline earth borate, carbonate, hydroxide or phosphate and mixtures thereof. Alkali hydroxide and carbonate are the preferred ingredients. On the one hand, their concentration must be sufficient to remove the aluminum particles on the container surface, but on the other hand it must not cause excessive etching of the aluminum surface. The result must be a clean, shiny appearance of the surface. The pH value, which is limited to 10, can go up to around 13 depending on the nature of the container surface. It is preferably in the range from 11.5 to 12.5. The concentration is usually between 0.05 and 10 g / l, preferably between 0.4 and 3.5 g / l.

The complexing agent must be soluble in the cleaning solution and compatible with the other components. He must also be able to complexly bind the ions in the solution to the extent that the formation of troublesome precipitates is avoided. Sugar acids such as gluconic acid or glucoheptanoic acid, citric acid, ethylenediaminetetraacetic acid, tartaric acid or salts of these acids and sodium tripolyphosphate are particularly suitable as complexing agents. The concentration of the complexing agents is usually in the range from 0.01 to 5 g / l, preferably in the range from 0.05 to 1 g / l.

The HLB number of the surfactant is a measure of the size and strength of the hydrophilic surfactant component in relation to the lipophilic surfactant component (cf. Römpps Chemie-Lexikon, Franckh'sche Verlagsbuchhandlung, Stuttgart, 8th edition, page 1715). By choosing surfactants with an HLB number An effective removal of lubricants and organic contaminants, which result in particular from the deformation and smoothing of the aluminum containers, is achieved by at least 12 and preferably at most 15. The formation of white rust is also inhibited. When using surfactants with an HLB number of more than 15, higher concentrations are usually required if complete cleaning is to be achieved and redeposition of contaminants on the container surface is to be avoided.

R

ein Kohlenwasserstoffrest mit 6 bis 30 C-Atomen,

R'

eine Ethylen- und gegebenenfalls Propylen-Gruppierung, wobei der Ethylenanteil überwiegt, und

n

eine ganze Zahl von 5 bis 100

darstellt.Suitable surfactants are, for example, those whose hydrophobic group is formed by alkylphenol, straight-chain or branched-chain alcohols and propylene glycol / propylene oxide polymers. The hydrophilic constituent can originate from ethylene oxide groups, the end groups of which may be etherified or otherwise occupied. Alkoxylated hydrocarbon-based surfactants, which are particularly suitable, have the general formula



R (OR ') _n OH



on, where

R

a hydrocarbon residue with 6 to 30 carbon atoms,

R '

an ethylene and optionally propylene grouping, the ethylene content predominating, and

n

an integer from 5 to 100

represents.

Tergitol 15-5-9

der Firma Union Carbide Corporation (ethoxylierter sekundärer Alkohol; HLB-Zahl 13,5)

Neodol 91-8

der Firma Shell Chemical Company (ethoxylierter linearer Alkohol; HLB-Zahl 14,1) und

Igepal CA 630

der Firma GAF Corporation (ethoxyliertes Octylphenol; HLB-Zahl 13,0).

Examples of particularly suitable surfactants are

Tergitol 15-5-9

from Union Carbide Corporation (ethoxylated secondary alcohol; HLB number 13.5)

Neodol 91-8

from Shell Chemical Company (ethoxylated linear alcohol; HLB number 14.1) and

Igepal CA 630

from GAF Corporation (ethoxylated octylphenol; HLB number 13.0).

All of the aforementioned surfactants can be used individually or in combination. Their concentration should usually be 0.003 to 5 g / l, preferably 0.02 to 1.0 g / l.

This cleaning solution is applied as usual by e.g. Flooding, dipping or spraying at temperatures of about 15 to 65 ° C, preferably 32 to 54 ° C, for a period of time that ensures the purity of the aluminum surface.

Another advantageous embodiment of the invention provides for rinsing the aluminum containers treated with alkaline cleaning solution in several stages. The second stage should then be operated with an aqueous solution adjusted to a pH ≦ 2.5. In this embodiment of the invention as well as in the case immediately downstream of the alkaline cleaning acidic rinse is usually followed by a water rinse, which can be multi-stage and equipped with countercurrent flow of the rinse water. The final rinse is carried out regularly with deionized water.

The conditions with which the water rinse is carried out should correspond to those of the acid rinse with regard to the type and temperature of treatment. Flushing in the syringe is particularly advantageous.

Common organic or inorganic acids are suitable for adjusting the pH of the acidic rinsing solution. It is particularly expedient to use sulfuric acid and / or hydrofluoric acid. The presence of fluoride ions in sufficient quantities has the advantage that the mobility of the aluminum containers is further increased and the suppression or removal of any discoloration is promoted. Accordingly, a further advantageous embodiment of the invention consists in rinsing with an aqueous acidic solution which contains fluoride ions. The fluoride levels can be up to 1000 ppm. Under special circumstances, if the concentrations are too high, caustic phenomena can occur, so that 200 ppm should not be exceeded. It is particularly expedient to keep the fluoride concentration below 100 ppm. In general, 40 ppm or less is sufficient. The aforementioned fluoride contents or concentrations refer to free fluoride.

In addition to hydrofluoric acid, the fluoride can also be introduced via soluble simple or complex Fluoride compounds are made. The required pH value may then be adjusted with other acids, for example with sulfuric acid.

If no further wet chemical treatment is intended, the aluminum containers are dried after the rinsing treatment, usually also painted, printed or otherwise provided with decorative coatings.

The method according to the invention is particularly advantageously suitable for the treatment of aluminum containers before the formation of conversion coatings. Conversion coatings greatly improve the mobility of the containers (rotation, sliding against each other) as they pass through the treatment plant and through the subsequent production stages and also increase the corrosion resistance. Conversion coatings of this type can be produced, for example, from solutions based on chromium phosphate or titanium, zirconium and hafnium fluoride with or without additional tannin content. Examples of processes for forming such conversion coatings are given in US Pat. Nos. 4,017,334, 4,054,466 and 4,338,140.

Also in the case of the aforementioned production of conversion coatings, the aforementioned drying, painting, etc. usually follows, depending on the type of process, after a previous rinsing treatment.

The invention is illustrated by the following examples, for example and in more detail.

Examples

Various tests were carried out to illustrate the effectiveness of the method according to the invention with regard to the achievable mobility of aluminum cans in a high-speed treatment plant. The first three-stage part of the plant was used for cleaning, the first stage of pre-cleaning with an aqueous acidic solution, the second stage of alkaline cleaning with a cleaner of the type specified above having a pH of 11.9 and the third stage of Rinsing with an acidic solution adjusted to pH 2.5 was used. After a water rinse, the aluminum cans were fed to the part of the plant in which the conversion coating was produced with a solution with a pH of 2.8 to 3.2. After rinsing again with water and deionized water, the mixture was dried. The aluminum cans were then fed via a transport device and chutes to a printing device which was designed for a capacity of 980 to 1000 cans per minute.

If the above-mentioned process conditions were maintained, the printing device could be operated with full performance and a perfect result.

If, on the other hand, the pH of the aqueous washing-up liquid was increased from 2.5 to 5 in the third stage, the mobility of the aluminum cans was reduced, so that the performance of the printing device had to be reduced to 710 cans per minute if its perfect operation was ensured should.

As soon as the pH of the washing-up liquid was adjusted to 2.5 again, the mobility of the aluminum cans increased again and thus the feed to the printing device, so that their output of 980 to 1000 aluminum cans per minute could be achieved again.

Another series of experiments showed the positive influence of the production of a conversion coating according to the preferred embodiment of the invention on the mobility of the aluminum cans. This coating formation was suspended for a period of 24 hours. As a result, the mobility of the aluminum cans was significantly reduced and the performance of the printing device had to be reduced to 820 cans per minute. When the conversion coating was started again, the original performance of the printing device was achieved again.

Claims (10)

1. A process for cleaning aluminum containers by means of alkaline cleaner solutions and subsequently rinsing with an agueous solution, characterized in that the aluminum containers treated with an alkaline cleaner solution are rinsed in at least one stage with a solution, the pH value of which has been adjusted to ≦2.5.
2. The process according to claim 1, characterized in that a rinsing step using an acidic aqueous solution is carried out prior to the treatment with the alkaline cleaner solution.
3. The process according to claims 1 or 2, characterized in that a rinsing step using an agueous solution, the pH value of which has been adjusted to about 2, is carried out prior to and/or after the treatment with the alkaline cleaner solution.
4. The process according to claims 1, 2 or 3, characterized in that a rinsing step using an aqueous solution, the temperature of which has been adjusted to <65.6 °C, and especially to from 32.2 °C to 54.4 °C, is carried out prior to and/or after the treatment with the alkaline cleaner solution.
5. The process according to any one or several of claims 1 to 4, characterized in that the aluminum containers treated with alkaline cleaner solution are rinsed in several steps, wherein the second step is carried out using an aqueous solution, the pH value of which has been adjusted to ≦2.5.
6. The process according to any one or several of claims 1 to 5, characterized in that the rinsing step is carried out by spraying with an aqueous solution.
7. The process according to any one or several of claims 1 to 6, characterized in that the rinsing step is carried out using an aqueous solution containing fluoride ions.
8. The process according to any one or several of claims 1 to 7, characterized in that a step of rinsing with water is carried out after the step of rinsing with the aqueous acidic solution.
9. The process according to any one or several of claims 1 to 8, characterized in that the aluminum containers treated with the alkaline cleaner solution and then rinsed are dried.
10. Use of the process according to any one or several of claims 1 to 8 as a pre-treatment of aluminum containers prior to an application of conversion coatings.