WO2003014417A1

WO2003014417A1 - Treatment of tinned steel before applying a coating

Info

Publication number: WO2003014417A1
Application number: PCT/EP2002/008228
Authority: WO
Inventors: Jörg Riesop; Reinhard Seidel
Original assignee: Henkel Kommanditgesellschaft Auf Aktien
Priority date: 2001-08-03
Filing date: 2002-07-24
Publication date: 2003-02-20
Also published as: DE10138096A1; EP1412553A1

Abstract

The invention relates to a method for treating the surface of tinned steel with an aqueous treatment solution containing phosphate or silicate. Said method is characterised in that a surface which is newly produced or cleaned before being treated is brought into contact with an aqueous treatment solution having a pH value between 8 and 13.5 and containing between 0.1 and 15 g/l of phosphate, calculated as phosphor, or between 0.1 and 3 g/l of silicate, calculated as silicon, for between 1 and 30 seconds. The treatment solution is then dried in without rinsing.

Description

"Treatment of tinned steel before painting"

The invention is in the field of treating tinned steel before painting. Tinned steel, also called tinplate, is mainly used for the production of cans, especially for the food sector. The usual tin coatings for tinplate are about 1 to about 10 g / m ² . After forming, for example drawing cans, the amount of free tin can be 0.2 to 7 g / m ² . Tinplate objects are heated to temperatures in the range of 70 to 250 ° C for baking the lacquer and / or for sterilizing the contents of the container. Here, the tin oxidizes on the surface to tin oxide. Since tin oxide has a lower reflectivity for light than elemental tin, the brightness of the surface decreases considerably due to the formation of tin oxide. This leads to an optically unsightly appearance and gives the impression that the surfaces are corroded. The tin layer can oxidize almost completely to tin oxide, particularly in the case of tin layers thinned by drawing. This reveals the blue-gray color of the iron underneath the tin layer. This makes the surface even darker. Darkened surfaces, which may still be visible under a light layer of lacquer, can give the impression that the containers are corroded and the contents may be spoiled. Such containers are then heavier or no longer salable. Therefore, a pretreatment process for tinned surfaces is desirable, which reduces the tin oxide formation when heated, so that the surfaces largely retain their bright metallic appearance.

Pretreatment processes for tinplate surfaces are known in the prior art. For example, European Patent EP-B-533 823 describes a solution for treating the surface of tin-plated steel which contains 1 to 30 g / l phosphate ions, 0.1 to 5 g / l condensed phosphate ions and 0.1 to 20 g / l one contains water-soluble polyvinylphenol derivative. The pH of the treatment solution must be in the range between 2.0 and 6.5, since the polyvinylphenol polymer fails at higher pH values. The tinned surface is brought into contact with the treatment solution for a period of between 2 and 60 seconds. Then the surfaces are rinsed with tap water, then with deionized water and dried. US 5 603 754 describes a treatment solution for tinned steel which has a pH in the range between 2.0 and 4.0. It contains at least 10 ppm of dissolved phosphate ions, but mainly at least one dissolved zirconium or titanium compound, an oxidizing agent and fluoride ions.

WO 93/18205 describes a cleaning solution for tinned steel, with which the surface is prepared for a subsequent corrosion-protective conversion treatment. This cleaning solution has a pH in the range from 8 to 11 and contains 0.5 to 10 g / l of a builder component, for example phosphates or silicates, 0.05 to 1 g / l of a chelating complexing agent such as, for example, condensed phosphates, and 0.05 up to 2 g / l surfactant. After contact with this cleaning solution and before the subsequent conversion treatment, the tinned surfaces are rinsed with water.

WO 94/13855 describes a treatment solution for tinned surfaces which has a pH in the range from 2.0 to 4.5 and which has 1 to 30 g / l phosphate ions, 0.1 to 5 g / l of a chelating complexing agent and 0 , 01 to 2.0 g / l of tin ions. The chelating complexing agent can be, for example, condensed phosphate. After treatment with this solution, the surfaces are rinsed with water.

The problem mentioned at the beginning of the formation of dark tin oxide when the tinned surfaces are heated is only unsatisfactorily solved by the prior art. There is therefore a need for an improved method of treating tinned surfaces prior to painting that reduces the formation of tin oxide when heated.

In a first aspect, the invention relates to a method for treating the surface of tin-plated steel with an aqueous treatment solution containing phosphate or silicate, characterized in that a surface freshly produced or cleaned before the treatment is treated with an aqueous treatment solution with a pH value in the Range from 8 to 13.5, which contains 0.1 to 15 g / l phosphate, calculated as phosphorus, or 0.1 to 3 g / l silicate, calculated as silicon, for a period in the range from 1 to 30 seconds Brings contact and then dries the treatment solution without rinsing. The treatment solution can be brought into contact with the metal surface by dipping, spraying or roller application. The treatment solution preferably has a temperature in the range from 15 to 60 ° C.

The process can be used in a tinning plant immediately after the tinning. In this way, the treatment solution comes into contact with a freshly produced tinned surface. In this case, no cleaning is required between the tinning and the treatment method according to the invention. However, if the tinned surfaces are contaminated, for example with anti-corrosive oils or with drawing agents, it is necessary to clean and preferably rinse the surfaces with a conventional cleaner suitable for this purpose before the treatment according to the invention.

The process according to the invention can be used on the one hand for tinned strip, preferably directly after its production. On the other hand, it is suitable for the treatment of objects made from tinned steel by forming processes such as deep drawing. An example of such objects are cans, in particular those for holding food.

According to the invention, the surface is not rinsed with water after contact with the treatment solution. Rather, the treatment solution is dried on the surface. As a result, a layer of phosphate or silicate remains on the surface, which reduces the formation of tin oxide during a subsequent heat treatment. After the treatment according to the invention, the surfaces can be painted immediately. No further corrosion-protective conversion treatment is required.

If phosphates are used in the process according to the invention, they are preferably selected from the group consisting of orthophosphate, diphosphate, triphosphate and / or higher condensed phosphates. Of course, one must choose those phosphates which are water-soluble in the concentration range given at the pH values mentioned. Alkali metal phosphates and in particular the sodium phosphates are preferred for this.

When using phosphates, the aqueous treatment solution preferably has a pH in the range of at least 8 and in particular at least 8.5 to a maximum of 11, in particular a maximum of 10.5. Depending on the phosphate used, this pH value can be adjusted with NaOH or phosphoric acid.

If such a phosphate-containing solution is applied to the metal surface in the process according to the invention, after drying on the metal surface a layer coating in the range of 2 to 100 mg / m ² , preferably 15 to 30 mg / m ² phosphorus is obtained.

In a further embodiment of the treatment method according to the invention, an aqueous solution of silicate is used. For this one selects silicates which are water-soluble in the concentration and pH range mentioned. Alkali metal silicates with a molar ratio of SiO ₂ : M ₂ O, where M is an alkali metal, are preferably chosen in the range from 0.5 to 4, in particular in the range from 1 to 3. Such alkali metal silicates are commercially available in solid form or in the form of aqueous solutions available. For example, a sodium water glass solution from Cognis can be used for this purpose, which is known commercially as type HK 37/40.

A treatment solution containing silicate preferably has a pH in the range from at least 9.5, in particular from at least 10 to a maximum of 13.5, in particular up to 13. If a pH in this range is not already available when the water glass solution is diluted or when the alkali metal silicate is dissolved, it can be adjusted upwards with sodium hydroxide solution, downwards with a less alkaline silicate solution or with sulfuric or nitric acid.

If such a silicate-containing treatment solution is brought into contact with the metal surface in the process according to the invention, a layer coating in the range of 3 to 100 mg / m ² , preferably 5 to 30 mg / m ^{2 of} silicon is obtained after drying on the metal surface.

The layer of silicon or phosphor on the treated surface can be determined by conventional analytical techniques. In particular, an X-ray fluorescence analysis is recommended after an appropriate calibration, since it works non-destructively and quickly delivers a result.

For the treatment method according to the invention, it is not necessary that the treatment solution contain other components, such as complexing agents other than Contains phosphates, organic polymers and / or complex fluorides. The additional use of organic polymers in particular could lead to stability problems in the alkaline treatment solutions. Furthermore, organic polymers can have a negative impact on paint adhesion and corrosion behavior after the cans have been sterilized. Accordingly, it is preferred in the process according to the invention that the aqueous treatment solution does not contain more than 1 mg / l of complexing agents other than phosphates, organic polymers and / or complex fluorides.

However, it can be advantageous to add so-called drainage aids to the treatment solution. These are agents for reducing the surface tension of H ₂ O, for example surfactants. Such drainage aids reduce the formation of water stains after the treatment solution has dried out. Furthermore, the treatment solution can contain agents to facilitate sliding. These reduce the friction between the tinplate objects, for example the cans, during transport processes.

The treatment method according to the invention replaces a conventional conversion treatment such as phosphating or chromating. Accordingly, a particular aspect of the method is that the metal surface is painted after the aqueous treatment solution has dried without further corrosion protection treatment, such as conversion treatment, being carried out between the treatment according to the invention and the painting.

In a further aspect, the present invention relates to an article made of tinned steel which has a coating on the tin layer with phosphate or silicate with a layer coating of 2 to 100 mg / m ² phosphorus or 3 to 100 mg / m ² silicon. The object can consist of a tinned steel strip or a section thereof. Alternatively, the article can be a shaped part such as a can, especially a can for holding food. The object can carry a lacquer layer above the phosphate or silicate layer. If the object coated according to the invention is a steel strip or a section thereof, an additional advantage of the coating according to the invention is that forming oils spread better on the coated surface than without such a coating. This makes forming easier and reduces the need for forming oil. However, as mentioned at the outset, the most important advantage of the treatment according to the invention is that it reduces the formation of tin oxide when the tinplate objects are heated. As a result, the surface retains a more metallic appearance than without the treatment according to the invention. In particular, food cans have a "fresher" effect on a buyer than cans that have not been treated according to the invention.

embodiments

For the subsequent experiments, a solution containing 3 g / l silicon was used as solution A. This was prepared by diluting a commercial sodium water glass solution (type HK 37/40, company Cognis). It had a pH of 10.8. A sodium pyrophosphate solution with 3 g / l phosphorus was used as solution B. It had a pH of 8.5.

The treatment solutions were applied to flat tinplate with rollers with a wet film thickness of 6 μm and dried at a temperature of 75 ° C. after an exposure time of 1 second. The sheets were then annealed at a temperature of 220 ° C. for a period of 15 minutes. The results are shown in the list below.

example 1

Tinplate flat, application on rollers, 6 μm wet film, concentration solution A 3 g / l Si, solution B 3 g / l P (as pyrophosphate); Silicate = water glass (37/40)

Measurement of the tin oxide coating (the lower, the better): The coating was measured using coulombmetry.

Blank sample untempered 26 mg / m ² blank sample annealed 36 mg / m ² sample A annealed 21 mg / m ² sample B annealed 30 mg / m ²

Example 2

Cleaned tin cans were immersed in treatment solutions at a temperature of 40 ° C for 6 seconds and then dried at 50 ° C. The treatment solutions with different concentrations of silicon or phosphorus were prepared analogously to embodiment 1. The tempering conditions also corresponded to Example 1. The table below contains the results. Tin cans, applied for 6 seconds at 40 ° C

Claims

claims

1. A process for treating the surface of tin-plated steel with an aqueous treatment solution containing phosphate or silicate, characterized in that a surface freshly produced or cleaned before the treatment is treated with an aqueous treatment solution with a pH in the range from 8 to 13, 5, which contains 0.1 to 15 g / l of phosphate, calculated as phosphorus, or 0.1 to 3 g / l of silicate, calculated as silicon, for a period of time in the range from 1 to 30 seconds and then the treatment solution dries without rinsing.

2. The method according to claim 1, characterized in that the aqueous treatment solution contains phosphate in the form of orthophosphate, diphosphate, triphosphate and / or higher condensed phosphates.

3. The method according to claim 2, characterized in that the aqueous treatment solution has a pH in the range of at least 8 to a maximum of 11.

4. The method according to one or both of claims 2 and 3, characterized in that a layer of 2 to 100 mg / m ² phosphorus is produced after drying on the metal surface.

5. The method according to claim 1, characterized in that the aqueous treatment solution contains silicate in the form of alkali metal silicates with a molar ratio SiO ₂ : M ₂ O (M = alkali metal) in the range from 0.5 to 4.

6. The method according to claim 5, characterized in that the aqueous treatment solution has a pH in the range from at least 9.5 to a maximum of 13.5.

7. The method according to one or both of claims 5 and 6, characterized in that a layer of 3 to 100 mg / m ² silicon is produced after drying on the metal surface.

8. The method according to one or more of claims 1 to 7, characterized in that the aqueous treatment solution contains no more than 1 mg / l of complexing agents other than phosphates, organic polymers and / or complex fluorides.

9. The method according to one or more of claims 1 to 8, characterized in that the metal surface is coated after the aqueous treatment solution has dried, without a corrosion protection treatment being carried out before the coating.

10. Object made of tinned steel which has a coating on the tin layer with phosphate or silicate with a layer coating of 2 to 100 mg / m ² phosphorus or 3 to 100 mg / m ² silicon.