IT202000020590A1 - ANODIZED LAYERS RESISTANT TO ACIDS AND ALKALIS - Google Patents

Description

DESCRIPTION

Attached to a patent application for INDUSTRIAL INVENTION having the title

?Acid and alkali resistant anodized sheets?

FIELD OF THE INVENTION

The present invention relates to a surface treatment method for anodized aluminum or its alloys to make anodized aluminum resistant to corrosion by acid and/or alkali agents, in particular for use in the automotive sector.

STATE OF ART

The automotive industry has recently imposed very high standards of alkali resistance for anodized aluminum used for automotive components, which can hardly be surpassed by anodized and conventionally bonded aluminum.

The electrochemical formation of oxide on aluminum or its alloys? used for a long time to produce protective and/or decorative layers on these metals for various uses and for different sectors.

The oxide layer is formed on the metal by electrochemical way by known processes of the sector. Once formed, the oxide layer has a porosity what can? be useful in case you want to apply any colors, but has the major disadvantage of not protecting the metal from corrosion. To overcome this problem, the anodized aluminum? usually subjected to a subsequent step of fixing the pores, for example, by means of hot hydration or cold impregnation, which causes an expansion or swelling of the oxide layer and consequent closure of the pores.

The hot fixing of the pores is normally carried out with hot water in the presence of any additives, or with steam, while the cold fixing requires temperatures of 30°C and the presence of nickel fluoride.

These conventional bonding technologies allow for pore closure and resistance to corrosion and weathering.

However, this treatment is not sufficient to allow anodized aluminum to pass the tests required by the automotive industry such as, for example:

? Production standard 9.57448 (FIAT Group)

? Anorganische Schutzschicht Auf Aluminiumteilen ? Oberflaechenschutzanforderung TL182 Volkswagen AG;

? GMW 14665, ?Anodic Oxidation Coating on Aluminum? (General Motors Material Specification Finish ? Worldwide Engineering Standards, January 2009).

The tests indicated above are similar and other well-known car manufacturers also apply similar tests.

The test more critical, as written in point 2.4.5 of the FCA specifications and similarly in the other specifications listed above, provides for subjecting the anodized aluminum samples to the following test cycle:

? Immersion for 10 min. ? 30 sec. in acid solution at pH = 1 (1) ? Rinse with running water and dry with compressed air. ? Stay in stove for 1 hour? 5 mins. at 40 ?C ? 3 ?C.

? Immersion for 10 min. ? 30 sec. in alkaline solution at pH = 13.5 (2)

? Rinse with running water and dry with compressed air.

(1) 0.1 molar HCl solution

(2) Aqueous solution of 12.7 g/l of NaOH 4.64 of Na3PO4 0.33 g/l of NaCl.

At the end of the test, the standard states that no aesthetic alterations should be observed compared to an untreated sample.

? true that all car manufacturers admit the use of a sol-gel technology on the pieces that must pass this test in order to further seal the pores of the aluminum oxide, but this application tends to "opacify"? a material requested as glossy and possibly colored. A typical example of such pieces can be the windscreen wiper rods or the polished aluminum moldings of many cars.

Maintaining the typical glossy aspect of polished, anodized and possibly colored aluminum and passing the test indicated above is not required. absolutely simple, also why? the use of sol-gel technology involves the use of silanes, which usually leave a polymeric film that is not perfectly colorless or transparent on the treated surfaces.

The aspects that prove most? complexes to overcome are the following:

? Maintain a shiny metallic aspect of anodized aluminum even after a sol-gel type treatment;

? The resistance to pH = 13.5, that is? to a pH greater than the pH 12.5 to which the current finishes applied on? Anodized aluminum already? they resist;

? The resistance at pH = 1 ? certainly a critical point why? hydrochloric acid attacks aluminum with a radical and exothermic reaction.

? it is known to use a finishing treatment of anodized aluminum for window frames which consists of a transparent electrophoretic painting (possibly glossy) of the type known as ?E.D. coat? or, simply ED. This painting would allow the anodized aluminum to pass the tests of the car manufacturers, but the polymer layer of paint? visible to the naked eye and therefore pu? not be accepted in this industry.

To increase the corrosion resistance of anodized aluminum or its alloys, even when used in the automotive sector, surface treatments with silicate or silane solutions, possibly also in mixtures, are known.

For example, EP1873278 describes a treatment of anodized and fixed aluminum with a solution of alkaline silicates. The treatment serves to increase the corrosion resistance of aluminum for use in the automotive sector. According to this publication, the silane coating provides resistance up to pH 13.5.

JP2003183889 discloses a method for increasing the corrosion resistance of anodized aluminum. This method involves applying a silane solution to the aluminum surface.

US2015/0034487 relates to a method for improving the corrosion resistance up to pH 13.5 of anodized and partially fixed aluminum which consists in applying a solution containing an orthosilicate and a non-alkaline silicate to the surface of the aluminum and, subsequently, heating to 200?C for one?hour. This treatment can cause the irreversible breakdown of the oxide layer.

Also known are treatments of anodized aluminum in which a first layer of alkaline silicate is applied, followed by a second layer of silane. This process ? described in EP3245317 where it is indicated that the piece of aluminum cos? treaty resists the automotive tests listed above, the cio? at pH 1 and pH 13.5.

However, the product obtained after treatment with silicates and silane in two steps shows a weight loss of at least 10 mg/dm<2 > of surface area when subjected to the alkali resistance test required by the standards of the car manufacturers. This result indicates that the treatment does not allow the product to pass the test without altering its properties. functional and aesthetic.

In the sector, therefore, is the need felt? to provide a surface treatment method for anodized aluminum (or its alloys) which allows it to pass the acid and basic pH tests (in particular at pH 1 and 13.5) required by car manufacturers without altering the aesthetic appearance and of the properties? of aluminum. This in order to make this substrate resistant to the atmospheric and chemical agents to which motor vehicles are subjected. In particular, the anodized aluminum components used for the construction of cars must resist the products used in car washes which are solutions, foams, dispersions, etc. which have a pH of approximately 13.5 or higher.

SUMMARY OF THE INVENTION

The present invention relates to a process for coating anodized aluminum comprising the treatment of anodized aluminum with a solution containing silicates, silanes and siloxanes and subsequent heating to a temperature ranging from 55°C to 80°C.

The solution can possibly also comprising a surfactant.

Before the treatment according to the invention, the anodized aluminum substrate can be treated with conventional methods for closing the pores (fixing).

In one embodiment, the anodized aluminum substrate can be colored by methods known in the art.

The invention also relates to an anodized aluminum substrate or its alloys coated with a coating deriving from the process according to the invention.

The aluminum substrate treated with the process of the invention passes without alterations the test required by the standards of the car manufacturers, i.e. it resists unchanged at pH from 1 to 13.5.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 shows the initial phase of a resistance test at pH 13.5 of two anodized aluminum substrates: left substrate (black coloured) not treated with the coating of the invention and right substrate (silver) treated with the coating of the invention.

Figure 2 shows the end of the immersion test of figure 1.

Figure 3 shows anodized aluminum substrates colored black after immersion in a pH 13.5 solution. Left substrate not coated with the coating of the invention; right substrate coated with the coating. Figure 4 shows undyed anodized aluminum substrates subjected to immersion tests in a basic solution at pH 13.5. Left substrate not coated with the coating of the invention; right substrates coated with the coating.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a coating process of an anodized aluminum substrate or anodized aluminum alloys, comprising:

a) applying to the substrate a solution comprising at least one alkali silicate, at least one silane and at least one siloxane;

b) Dry the substrate at a temperature between 55?C and 80?C.

The anodized aluminum substrate (or its alloys) ? preferably a component used for the construction of motor vehicles, in particular automobiles, such as for example windscreen wiper rods, body parts such as doors and bonnets, wheels and other components which have an aesthetic value. These substrates can be subjected to extreme pH conditions especially when they are placed in contact with washing solutions that normally reach pH even beyond 13.5.

The substrate of anodized aluminum (or its alloys) can? also be a piece of furniture, a window frame, a door, a floor, a wall, a shower cubicle or other furnishing items.

The aluminum substrate (or its alloys) ? subjected to a conventional anodizing process. This process can include some preliminary stages of preparation of the substrate chosen from:

? Substrate cleaning;

? Mechanical polishing, if necessary for aesthetic purposes; car components are often shiny;

? Chemical brightening, if necessary for aesthetic purposes; usually the polishing is performed with chemical rather than electrochemical treatments as the substrates are small;

? Acid pickling.

Before and after acid pickling? possible to wash the substrate.

After the? application of one or more? of the preliminary treatments indicated above, the substrate is subjected to anodization by immersing the piece in an acid bath, preferably sulfuric acid, and applying a current having a density? usually between 1.0 and 3.0 A/dm<2>, preferably between 1.5 and 2.0 A/dm<2>. The acid bath? maintained at a temperature between 15?C and 23?C, preferably between 18?C and 21?C, thermostated at ?0.5 with respect to the chosen temperature.

The treatment time? between 30 and 60 minutes, preferably between 40 and 50 minutes.

After the anodization? You can apply a color, if needed, for your desired applications. In this case an organic color is used which is absorbed by the oxide or an electrolytic coloring is performed.

If the color is applied by immersing the substrate in the dye, the color must be pre-fixed by immersing the colored substrate in a solution containing nickel acetate. The nickel ion binds to the dye molecule and forms nickel hydroxide within the pore of the aluminum oxide, preventing dye bleeding resulting in discoloration of the dye substrate. Preferably, a solution of 5-10 g/L of nickel acetate is used. Preferably the solution ? heated to a temperature between 60? and 75?C.

This ? a known technique, foreseen by the Qualanod Specifications.

The thickness of aluminum oxide that is formed pu? vary from 5 to 25 ?m. Before applying the coating according to the present invention ? it is preferable to proceed with the total or partial closure of the pores of the aluminum oxide with a fixing process.

The fixing can take place by hydration with boiling water, possibly in the presence of additives, or by steam or by impregnation with a solution containing nickel ions and fluorine ions. The latter ? particularly useful when applying a color to the anodized substrate.

Preferably, the fastener used for the present invention is a fixing with nickel fluoride at a temperature between 25? C and 30?C.

Alternatively you can? carry out a treatment with nickel acetate at a temperature between 70?C and 90?C.

Alternatively, the pores can be fixed by immersion in hot water, possibly with the addition of suitable additives, at a temperature between 90°C and 100°C.

Fixing takes place for a time ranging from 1 to 3 min/?m of oxide thickness.

Once the pores have been closed, a solution comprising at least one alkaline silicate, at least one siloxane and at least one silane is applied.

The alkaline silicate ? also known as water glass because it is an amorphous material and has the appearance of clear glass. ? soluble in water and forms viscous and alkaline solutions. Chemically, it is alkali metal silicate with the general formula M2O x nSiO2 (M = Na, K, Li), where n ? between 1.5 and 4. Preferably the alkaline silicate ? a sodium silicate. The alkaline silicate ? usually a concentrated solution, obtained by dissolving glass silicates in water, or by directly dissolving silica sand in NaOH.

The alkaline silicate ? present in the solution in a quantity? between 0.5 and 10 g/l, preferably between 1 -4 g/l.

Silanes have a general formula:

R?(CH2)nSi(OR)3

where is it:

n ? between 2 and 4;

R? ? a functional group selected from: vinyl, epoxy, glycidyl-oxy, methacrylate, primary amine, diamine, mercapto.

OR ? a hydrolyzable alkoxy group, such as methoxy (OCH3), ethoxy (OC2H5) or acetoxy (OCOCH3).

In a preferred embodiment, n ? equal to 3 or 4, R? ? vinyl, epoxy or glycidyl-oxy, OR ? a methoxy group (OCH3).

In one embodiment more? favorite, silane ? chosen between:

CH2=CHSi(OCH3)3

CH2=CHSi(OC2H5)3

Preferably, the silane used in the solution of the invention is 3-glycidyloxypropyltrimethoxysilane having the formula given above. The silane ? present in the solution of the invention in a quantity? between 0.5 g/L and 4 g/L, preferably between 1 g/L and 3 g/L.

Siloxanes are a class of chemical compounds in which the functional group R2SiO repeats itself, where R ? a hydrogen or an alkyl or aryl group. The name "siloxane" comes from the combination of silicon, oxygen and alkane. They are considered part of the class of organo-siliceous compounds. The siloxanes have a main chain, linear or branched, in which silicon and oxygen atoms -Si-O-Si-O- alternate with the side chains R linked to the silicon atoms.

The polymers of siloxanes where R ? an alkyl group, are commonly known as silicones or "polysiloxanes". The best examples representative of these polymers are [SiO(CH3)2]n (polydimethylsiloxane) and [SiO(C6H5)2]n (polydiphenylsiloxane).

The synthesis of silicones generally starts from the hydrolysis of methyl chlorosilanes which form cyclosiloxanes: tetramethylchlorosilane, hexamethylchlorosilane, usually referred to as D4, D6 respectively. Cyclosiloxanes can be considered as the ?monomers? polymerization of silicones. Polymerization takes place in the presence of strongly basic catalysts in a particular synthesis process which allows the introduction of other siloxanes with alkyl, phenyl and vinyl endings into the polymeric chain. With these introductions the characteristics of the final polymer change considerably and above all they can, with appropriate dosages, be modulated according to the needs of use.

The siloxane used in the solution of the invention ? a siloxane and/or a polysiloxane.

The siloxane? preferably chosen from:

alkoxymethylsiloxane,

polyvinyl trimethoxysiloxane,

polymethylvinylsiloxane,

hexamethylcyclotrisiloxane,

hexamethyldisiloxane,

polydimethylsiloxane, e

decamethylcyclopentasiloxane;

The polysiloxane ? preferably selected from polyether siloxane and polyetherdimethylsiloxane.

Preferably the solution comprises a siloxane preferably selected from: hexamethyldisiloxane and polydimethylsiloxane and a polysiloxane preferably selected from: polyether siloxane and polyetherdimethylsiloxane. The siloxane? included in the solution in quantity? from 0.1 to 1g/L, preferably from 0.2 to 0.5 g/L.

The polysiloxane ? included in the solution in quantity? from 0.1 to 1g/L, preferably from 0.2 to 0.5 g/L.

The solution can comprising an anionic surfactant, preferably a disulfonate. The surfactant, if present, ? added in quantity? between 0.1 and 1% with respect to the volume of the solution, preferably between 0.2% and 0.5%.

The surfactant has the function of homogenizing and making the solution clear and ? particularly useful in case you want to prepare anodized substrates with a glossy aesthetic effect, while if the substrates are satin, the surfactant can? also not to be added as on satin surfaces (i.e. opaque and non-reflecting), any aesthetic alterations are not visible.

The solution of the invention is applied to the substrate by immersing it in the solution. Application time? between 5 and 20 minutes, preferably between 7 and 15 minutes.

The solution ? heated to a temperature between 40?C and 85?C, preferably between 50?C and 80?C.

After immersion of the anodized substrate in the solution, the piece is dried, preferably without rinsing, at a temperature between 55°C and 80°C, preferably between 60°C and 70°C.

Drying? preferably carried out with hot air.

The thickness of the layer obtained? between 0.5 and 3 ?m.

The process ? self-limiting, as the thickness that is formed is not ? directly proportional to the dive time; that is, once a certain value has been reached which depends on the operating conditions, the layer no longer increases.

The anodized substrate treated with the solution of the invention has demonstrated that it passes the tests required by the FCA specification which envisages subjecting the anodized aluminum samples to the following test cycle: ? Immersion for 10 min. ? 30 sec. in acid solution at pH = 1 - (1) ? Rinse with running water and dry with compressed air. ? Stay in stove for 1 hour? 5 mins. at 40 ?C ? 3 ?C.

? Immersion for 10 min. ? 30 sec. in alkaline solution at pH = 13.5 - (2)

? Rinse with running water and dry with compressed air.

(1) 0.1 molar HCl solution

(2) Aqueous solution of 12.7 g/l of NaOH 4.64 of Na3PO4 0.33 g/l of NaCl.

At the end of the test, the substrate has not and must not show any aesthetic alterations.

The invention also relates to an aluminum substrate (or its alloys) anodized and coated with a coating layer obtainable by applying the process according to the present invention.

This substrate? characterized by the fact that it passes the test set by the car manufacturers for resistance to acid pH equal to 1 and to basic pH up to 13.5 without aesthetic alterations.

The anodized aluminum substrate (or its alloys) has an aluminum oxide thickness from 0.3 µm to 30 µm.

The thickness of the coating layer according to the present invention can vary from 0.5 to 3 ?m.

The total thickness of the anodized aluminum substrate coated with the coating according to the present invention ? between 3 and 35 ?m.

EXAMPLES

Solutions according to the invention

EXAMPLE 1

4 g/L sodium silicate

0.9 g/L glycidyl trimethoxy silane

0.1 g/L alkoxymethylsiloxane

0.1 g/L polydimethylsiloxane

0.1% anionic surfactant (Dowfax 2 A 1) by volume of solution

EXAMPLE 2

5 g/l lithium silicate

1.5 g/l glycidyl trimethoxy silane

0.2 g/l alkoxymethylsiloxane

0.2 g/l anionic surfactant

EXAMPLE 3

3.5 g/l potassium silicate

2 g/l glycidyl trimethoxy silane

0.15 g/l polyvinylsiloxane

0.15 g/l anionic surfactant

EXAMPLE 4

6 g/l sodium silicate

2.5 g/l glycidyl trimethoxy silane

0.45 g/l hexamethylcyclotrisiloxane

EXAMPLE OF PREPARING SOLUTIONS

(final volume, 1 litre)

In a container of capacity? suitable introduce about 500 ml of demineralised water.

Add the silicate solution (sodium, lithium or potassium silicate) while stirring.

Mix for a few minutes, add the silane and wait until complete homogenization.

Add the siloxane/polysiloxane and leave under mechanical stirring for a few minutes.

Bring to final volume (1 litre) with demineralised water and, if necessary, add surfactant.

Warm the solution to operating temperature.

EXAMPLE OF TREATMENT ON 1050 SERIES ALUMINUM ALLOY

With the treatment described below a shiny silver finish is obtained (possibly coloured)

Mechanical cleaning of an aluminum alloy substrate with suitable polishing pastes and cloth.

Degreasing at 65?C for 5 minutes.

Chemical brightening at 96?C for 5 minutes.

Flushing.

Depatination.

Oxidation at 12.5 V for t = 40 minutes at T= 20?C, to form an oxide layer of about 10 microns.

Flushing.

Fixing by impregnation t= 1 min/?m T = 28?C

Immersion in one of the solutions according to the examples 14 t 10 min T 60°C.

Final drying t= 10 min T= 65?C.

Preliminary treatments were carried out using commercial products and performed according to Qualanod specifications.

To obtain a colored and glossy substrate it is necessary to carry out a passage of 15 min in color (for example, red; blue, orange, turquoise, green, black) at a temperature recommended by the dye manufacturer.

Rinse.

Immerse in pre-fix for 10 min at T= 65?C.

Rinse.

Fixing by impregnation t= 1 min/?m T = 28?C.

Immersion in one of the solutions according to examples 1-4 t= 10 min T= 60?C.

Final drying t= 10 min T= 65?C.

EXAMPLE OF TREATMENT ON 6060 SERIES ALUMINUM ALLOY The treatment described below gives a satin silver finish (possibly coloured).

Degreasing of the aluminum alloy substrate at 65?C for 5 minutes. Satin finishing at 65?C for 15 minutes.

Flushing.

Depatination.

Oxidation at 14.5 V for t = 45 minutes T= 20?C, to form an oxide layer of about 15 microns.

Flushing.

Fixing by impregnation t= 1 min/?m T = 28?C.

Immersion in one of the solutions of Examples 1-4, t= 10 min T= 60?C.

Final drying t= 10 min T= 65?C.

To obtain the colored satin just carry out:

a 15 min passage in color (example, orange, blue, turquoise, etc.) at the temperature recommended by the dye manufacturer.

Flushing.

Pre-fix for 10 min at T= 65?C.

Fixing by impregnation t= 1 min/?m T = 28?C.

Immersion in one of the solutions of Examples 1-4, t= 10 min T= 60?C. Final drying t= 10 min T= 65?C.

ALKALI RESISTANCE TESTS

To verify the alkali resistance of anodized aluminum substrates coated with the coating of the invention, comparison tests were carried out with uncoated substrates, performing a visual evaluation after immersion in a solution at pH 13.5.

Figure 1 shows the initial phase of a resistance test at pH 13.5 of two anodized aluminum substrates: left substrate (black coloured) not treated with the coating of the invention and right substrate (silver) treated with the coating of the invention.

In the following figures, the part of the substrate immersed in the alkaline solution? delimited by a black marker.

Figure 2 shows the end of the immersion test in Figure 1. It is clearly seen that the untreated colored substrate has completely lost its color and aluminum oxide layer ? been dissolved by the high alkalinity; instead the silver substrate treated with the coating of the invention is not ? been absolutely affected. Indeed, the basic solution ? remained clear and colourless.

If there had been basic attack, and then dissolution of aluminum, the solution would have become milky.

Figure 3 shows anodized aluminum substrates colored black after immersion in a pH 13.5 solution. The left substrate is not ? been coated with the coating of the invention: the substrate on the right? been coated with the coating. As can be seen from the figure, the left-hand substrate has a total dissolution of the color and the oxide layer, while the right-hand substrate is intact for both the color and the oxide layer.

Figure 4 shows undyed anodized aluminum substrates subjected to immersion tests in a basic solution at pH 13.5. The left substrate is not ? been coated with the coating of the invention while two of the right ones were coated. As can be seen from the figure, the aluminum oxide layer of the left substrate? been completely dissolved from the basic solution, while the substrates on the right are intact.

Claims (14)

CLAIMS 1. A process of coating a substrate of anodized aluminum or anodized aluminum alloys, including: a) applying to the substrate a solution comprising at least one alkali silicate, at least one silane and at least one siloxane and/or polysiloxane; b) Dry the substrate at a temperature between 55?C and 80?C. 2. A process according to claim 1, in which at least one alkaline silicate ? an alkali metal silicate with the general formula M2O x nSiO2, where M = Na, K, Li and n ? between 1.5 and 4. 3. Process according to claim 1 or 2, wherein the at least one silane has the general formula: R?(CH2)nSi(OR)3 where is it: n ? between 2 and 4; R? ? a functional group selected from: vinyl, epoxy, glycidyl-oxy, methacrylate, primary amine, diamine, mercapto; OR ? a hydrolyzable alkoxy group, such as methoxy (OCH3), ethoxy (OC2H5) or acetoxy (OCOCH3), preferably n ? equal to 3 or 4, R? ? vinyl, epoxy or glycidyl-oxy, OR ? a methoxy group (OCH3). 4. Process according to claim 3, wherein the at least one silane is chosen between: CH2=CHSi(OCH3)3 CH2=CHSi(OC2H5)3 5. A process according to any one of claims 1 to 4, wherein at least one siloxane ? chosen between: alkoxymethylsiloxane, polyvinyl trimethoxysiloxane, polymethylvinylsiloxane, hexamethylcyclotrisiloxane, hexamethyldisiloxane, polydimethylsiloxane, e decamethylcyclopentasiloxane. 6. Process according to any one of claims 1 to 4, wherein the at least one polysiloxane is selected from polyether siloxane and polyetherdimethylsiloxane. 7. Process according to any one of claims 1 to 6, wherein the solution of step a), comprises a siloxane preferably selected from: hexamethyldisiloxane and polydimethylsiloxane and a polysiloxane preferably selected from: polyether siloxane and polyetherdimethylsiloxane. 8. Process according to any one of claims 1 to 7, wherein the solution of step a) comprises an anionic surfactant, preferably a disulfonate. 9. Process according to any one of claims 1 to 8, wherein the solution of step a) ? applied to the substrate for a time comprised between 5 and 20 minutes, preferably between 7 and 15 minutes. The process according to any one of claims 1 to 9 comprising a step of applying a color before step a). 11. Process according to any one of claims 1 to 10, comprising a step for fixing the pores, before applying the solution of step a), preferably by hydration with boiling water, optionally in the presence of additives, or by steam or for impregnation with a solution containing nickel ions and fluorine ions. 12. Substrate of anodized aluminum or of anodized aluminum alloys coated with a coating layer obtainable by applying to the anodized substrate the process according to any one of claims 1 to 11. 13. A substrate according to claim 12, wherein the thickness of the coating layer is? between 0.5 and 3 ?m. The substrate according to claim 12 or 13, wherein the anodized aluminum substrate has an aluminum oxide thickness of from 0.3 µm to 30 µm.