This application claims priorities filed on 9 October 2020 in INDIA with Nr 202021044019 and on 26 November 2020 in EUROPE with Nr 20306448.0, the whole content of these applications being incorporated herein by reference for all purposes.

The instant invention relates to the field of the treatment of surfaces based on metal, and more specifically metal surfaces intended to be coated with film-forming compositions such as paints, varnishes or adhesive compositions. The invention is especially directed to a treatment of said metal surfaces aiming at providing an enhancement of the adherence of the film-forming composition on the metal surface, which is especially efficient with adhesive compositions.

In order to provide an enhancement of the adherence of film-forming organic compositions such as paints, varnishes or adhesives on metal surfaces, especially on aluminum or steel, several methods have been proposed, including i.a. the deposit of inorganic coatings on the surface of the metal, especially the so-called "conversion coating".

The term "conversion coating" is well known in the art and refers to a layer formed on the surface of a metal, that is an advantageous replacement of native oxide on said surface (especially on aluminum), and which is obtained by the controlled chemical formation of a film on the metallic surface by reaction with chemical elements of the metallic surface, so that at least some of the cations dissolved from the metallic material are deposited in the conversion coating.

Typically, coating such conversion coatings are obtained by reacting the metal surface with solutions containing metal cations and fluorides. In the past, chromium- containing coatings have been proposed (typically obtained by reaction of the surface with a solution including H ₂ CrFe), and, more recently, less toxic coatings based e.g. on zirconium, titanium or other metals (for example obtained by reaction of the surface with a solution including H ₂ TiF ₆ , H ₂ ZrF ₆ , H ₂ HfF ₆ , H ₂ AIF ₆ , H ₂ SiF ₆ , H ₂ GeF ₆ , H ₂ SNF ₄ , or HBF ₄ ). A conversion coating may include other compounds such as silane precursors for example. For enhancing the adhesion on a coating such as conversion coatings it is known to add some additives, especially organic polymers. In this connection, it has been for example described the use of polyacrylic acids. A typical additive, especially suitable for paint compositions, is ACUMER™ 1510 available from DOW (and previously from Rohm & Haas) that has been widely described for this kind of application. For more details in this connection, it may be especially be referred to WO20109411 , W020109413, WO97/13588, US 4,191 ,596, or US4921552.

One aim of the present invention is to provide a new method for treating a metal surface, which imparts a good adherence of organic compositions, and especially of adhesive compositions applied to the metal surface (the adhesive may be applied by the coating of an adhesive compositions, typically an organic film forming compositions, that is generally available as a paste, more or less fluid ; or by the way of a preformed adhesive film, such as the L-F610 Epoxy Adhesive Film (commercialized by L&L).

To this end, the instant invention proposes to make use of a specific polymer, optionally (but not necessarily) together with (namely before, during, or after) the formation of a conversion coating, which leads to a treated metal surface that reveals very interesting: when coated by a film-forming composition such as a paint, varnish or adhesive composition, a good adherence is obtained between the surface and the coated composition. Besides, a good protection of the surface is obtained, especially against corrosion. When the metal surface is coated with an adhesive layer, the coated surface may typically be used for ensuring a so-called “adhesive bonding” between said coated metal surface and another surface (typically a similar metal surface treated with the same polymer) that is placed in contact with all or part of the adhesive coating. In this application, the specific polymer used according to the invention reduces the occurrence of adhesive failure (in other word it imparts a kind of “resistance to the adhesive failure”). In the scope of the invention, the inventors have now observed that the strength of the adherence between the adhesive and the metal surface is especially high, to such an extent that cohesive failure appears instead of (or at least more preferably than) adhesive failure when a sufficiently high mechanical stress is applied for separating the adhesive-bonded surfaces especially after exposure to aggressive conditions.

Cohesive failure is understood to mean that failure between two surfaces bonded by an adhesive occurs within the adhesive, which is thus retained on both surfaces. Adhesive failure is understood to mean that failure between two surfaces bonded by an adhesive occurs at the surface, the adhesive being retained on one surface.

The improvement of the bonding between two surfaces treated by the polymer of the invention and then assembled by an adhesive is thus reflected by a resistance to the adhesive failure, which means that a cohesive failure will occur instead, in particular after ageing, compared to other existing treatments.

More precisely, the instant invention make use of at least one polymer P, which is a polymer obtained by radical copolymerization of a mixture of:

(i) acrylic acid ; and

(ii) optionally methacrylic acid ; and

(iii) and at least one monomer M which is an ethylenically unsaturated monomer carrying an unsaturated heterocycle having at least two nitrogen atoms, said monomer M being preferably an ethylenically unsaturated imidazole having the Formula (I) below:

Formula (I) wherein:

R ¹ is H or a methyl group -CH3 , and preferably H ; and

A is a linkage selected from the group consisting of : a single covalent bond ; and a spacer group such as a group -CO-NH-(CH2)n- or -CO-O-(CH2)n- wherein n is an integer from 1 to 5, typically equal to 3 or 4.

All or part of the imidazole functions present in the polymer P may optionally be quaternized. In that case each of the quaternized imidazole function is associated with a proper counter anion (for example Br-, TFSI or any other suitable mono or polyanion) and exhibits typically the following formula: wherein R ² is typically H or an alkyl group, carrying typically from 1 to 12 (preferably 1 to 6) carbon atoms.

The quaternization of all or part of the imidazole functions may result from a quaternization of all or part of the monomers and/or from a post-quaternization of the imidazole functions of the polymer.

The monomer M may for example be:

- vinylimidazole of formula (la): quaternized vinylimidazole having the formula (lb): wherein R ² is as defined above ; a monomer of formula (Ic) wherein n and R ¹ are as defined above, n being typically equal to 3 ; a quaternized monomer of formula (Id) wherein n, R ¹ and R ² are as defined above, n being typically equal to 3; a mixture thereof (for example a mixture of monomers (la) and (lb) ; or a mixture of monomers (Ic) and (Id); or a mixture of the 4 monomers).

Instead of the imidazole group, the monomer M may bear another unsaturated heterocycle having at least two nitrogen atoms, for example M may bear a pyrazole group, an indazole group, or a triazole group.

The divalent spacer group A in formula (I) may typically be group -CO-NH-(CH2)n- or -CO- O-(CH2)n, but any other covalent linker group may be contemplated, for example resulting from the reaction of a compound of formula (l-X):

- with a compound of formula (l-Y):

- wherein X and Y are two groups reacting together for forming a covalent bond.

For example, Y may be a -(CH2)m-NH2 group wherein m is from 1 to 4, preferably 2 or 3. In that case, X may be for example a carboxylic acid, an acid chloride, an anhydride or an epoxy. - According to another variant, Y may be a -(CH2)m-OH group wherein m is from 1 to 4, preferably 2 or 3. In that case, X may be for example a carboxylic acid, an acid chloride, an anhydride or an ester.

The polymer P is a polymer as obtained by copolymerizing monomers (i),(ii) and (iii) namely having the structure that is obtained via such a polymerization, but the polymer P is not necessarily obtained by this process. As an alternative, the polymer P may for example be obtained by a first step (E1) of copolymerizing acrylic acid, methacrylic acid and a compound of formula (l-X) leading to a polymer P0 and then a second step (E2) of post-grafting of the polymer P0 by a reaction with compound (l-Y).

When Y is a -(CH2)m-NH2 group in the compound (l-Y) used in step (E2), the compound (l-X) used in the step (E1) may advantageously be selected from: additional acrylic or methacrylic acid, or ester thereof; maleic anhydride; vinylbenzyyl chloride; glycidylmethacrylate; and (blocked) isocyanatoethyl methacrylate.

When Y is a -(CH2)m-OH group in the compound (l-Y) used in step (E2), the compound (l-X) used in the step (E1) may advantageously be selected from additional acrylic acid, methacrylic acid, maleic anhydride or their esters.

Besides, a quaternization of all or part of the imidazole functions of polymer P may occur, resulting from a quaternization of all or part of the monomers and/or form a post-quaternization of all or part of the imidazole functions of the polymer.

The at least one polymer P, which is a polymer obtained by radical copolymerization of a mixture of:

(i) acrylic acid (AA) ; and

(ii) optionally methacrylic acid (MAA) ; and

(iii) at least one monomer M which is an ethylenically unsaturated monomer carrying an unsaturated heterocycle having at least two nitrogen atoms, said monomer M being preferably an ethylenically unsaturated imidazole having the Formula (I) may further comprise

(iv) below 10% mol of one or more further monomers M’ selected from the group consisting of hydrophobic monomers and amphiphilic monomers provided the amount of AA and optionally MAA is at least 60% mol. In this embodiment where additional monomers M’ are present in polymer P, said hydrophobic and/or amphiphilic monomers are selected from the group consisting of monoethylenically unsaturated monomers: i) alkyl esters of maleic anhydride and (meth)acrylic acid, such as monomethyl maleic anhydride ester, dimethyl maleic anhydride ester, monoethyl maleic anhydride ester, diethyl maleic anhydride ester, methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, n-hexyl (meth)acrylate, n-octyl (meth)acrylate, ii) hydroxyalkyl esters of maleic anhydride and (meth)acrylic acid, such as monohydroxyethyl maleic anhydride ester, dihydroxyethyl maleic anhydride ester, hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate iii) ethoxylates and propoxylates that derive from maleic anhydride, such as such as polypropylene oxide)-b-poly(ethylene oxide) maleic acid half ester or diester alkyl-poly(ethylene oxide) maleic acid half ester or diester, iv) ethoxylates and/or propoxylates that derive from the ethoxylation and/or propoxylation of hydroxyalkyl (meth)acrylic acid, such as polypropylene oxide)-b-poly(ethylene oxide)-ethyl (meth)acrylate v) ethoxylates and/or propoxylates that derive from the (trans)esterification of (meth)acrylic acid and esters such as polypropylene oxide)-b-polypthylene oxide) (meth)acrylate and alkyl-polypthylene oxide) (meth)acrylate vi) Vinyl ethers such as methyl vinyl ether, ethyl vinyl ether, propyl vinyl ether, 2-ethylhexyl vinyl ether, vinyl cyclohexyl ether, dodecyl vinyl ether, 2- iethylamino)ethyl vinyl ether, 2- i-n-butylamino)ethyl vinyl ether vii) allyl ethers such as methyl vinyl ether, ethyl vinyl ether, propyl vinyl ether, 2- ethylhexyl vinyl ether, viii) Vinyl esters, such as vinyl acetate or vinyl propionate ix) alkyl-substituted acrylamides such as N-tert-butyl acrylamide or N-methyl (meth)acrylamide.

Preferably, additional monomers M’ that are present in polymer P are selected from the group consisting of: i) monoethyl maleic anhydride ester, diethyl maleic anhydride ester, methyl (meth)acrylate, ethyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate ii) monohydroxyethyl maleic anhydride ester, dihydroxyethyl maleic anhydride ester, hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate iii) polypropylene oxide)-b-poly(ethylene oxide) maleic acid half ester iv) polypropylene oxide)-b-poly(ethylene oxide)-ethyl (meth)acrylate v) polypropylene oxide)-b-poly(ethylene oxide) (meth)acrylate, alkylpolyethylene oxide) (meth)acrylate vi) vinyl acetate, vinyl propionate.

Preferably there is no vinylaromatic monomer in the list, in particular no styrene.

The proportion in mol of monomers M’ cannot exceed 10% mol of the total mol of monomers (acrylic acid + methacrylic acid + M + M’) present in polymer P, otherwise the polymerization in water will occur in a dispersed medium because the polymer won’t be soluble in water anymore, requiring the use of a surfactant which is not desirable in the final application. Advantageously the proportion in mol of monomers M’ is below 5% mol.

The minimum proportion in mol of AA and optionally MAA is 60%, otherwise the performances in the application are not satisfactory.

Indeed, the presence of hydrophobic and/or amphiphilic monomers M’ due to the requirement of carrying the polymerization of the polymer P of the invention in aqueous solution without the use of surfactants and not in emulsion like it is the case for latexes. The monomers used have to be hydrosoluble or highly dispersible and should not affect the solubility of the polymer P obtained therefrom.

That’s why, in a preferred embodiment of the invention, there is no further monomer M’ in the polymer P, which means that polymer P is_obtained by radical copolymerization of a mixture consisting essentially of, notably consisting of:

(i) acrylic acid ; and

(ii) optionally methacrylic acid ; and

(iii) at least one monomer M which is an ethylenically unsaturated monomer carrying an unsaturated heterocycle having at least two nitrogen atoms, said monomer M being preferably an ethylenically unsaturated imidazole having the Formula (I).

According to this embodiment where no further monomer is present in polymer P apart from (i), (ii) and (iii), the solubility in water of polymer P is high. This solubility in water is evaluated by measuring transparency at 1% of active in water: transmittance measured in a 1cm optical path length glass cell has to be higher than 95%.

This requirement of hydrosolubility of polymer P is allowing the polymer P to be easy to use in the application, because it can be sprayed or deposited in any manner without foaming, or without using surfactants (antifoam) to control the foam. This is not the case for polymers obtained by polymerization in emulsion, for latexes for instance, where foam is a limiting factor for their sprayability as well as the latex destabilization under high shear.

Typically, the polymer P is obtained by radical copolymerization of a mixture of acrylic acid, preferably methacrylic acid, and at least one monomer M. Preferably, the polymer P is obtained by radical copolymerization of a mixture having the following molar ratio, based on the total quantity of acrylic acid, methacrylic acid and monomer M of formula (I): acrylic acid (AA): from 5 to 95%, notably from 5 to 50%, preferably from 20 to 40% (e.g., about 25 to 30%), methacrylic acid (MAA): from 0 to 90%, preferably from 25 to 90%, more preferably from 60 to 80% (e.g., about 65 to 75%) monomer M : from 1 to 50%, for example from 1 to 30%, notably from 1 to 20% and even 2 to 15% (e.g., about 3 to 10%).

The above molar ratios of each monomer in the polymer P are showing particularly good results in terms of resistance to the adhesive failure to the bonding when compared to polymers that are out of the above ranges. For example, when the quantity of AA + MAA in the polymer P is too low (below 60% mol) then the performances are not satisfactory. The same happens when no monomer M is present. When the amount of M is too high (above 50%), then there is a risk of coloration of the product and the resulting polymer is not economically viable.

Besides, the polymer P used according to the invention preferably has a number average molecular weight (/W _n ) of at least 7,500 Da, e.g. 10 kDa to 1500 kDa, for example 10 kDa to 150 kDa, notably between 10 and 100 kDa. Typically, the polymer P used according to the invention has a number average molecular weight (/W _n ) of from 20 to 100 kDa, e.g., 20 to 50 kDa. A polymer P especially suitable for the invention is a statistical (random) copolymer having a weight average molecular weight of about 20 to 50 kDa, that is the copolymerized product of a mixture of acrylic acid, optionally methacrylic acid, and a monomer M, preferably in a molar ratio of about 26/70/04 to 20/70/10.

The number and weight average molecular weights are measured by Size Exclusion Chromatography (SEC). Notably the SEC is equipped with a MultiAngle Laser Light Scattering (MALLS) Mini Dawn TREOS detector and an Agilent concentration detector (Rl detector). The SEC-MALLS system is running on three columns Varian Aquagel OH mixed H, 8 pm, 3*30 cm at a flow rate of 1 mL / min and with the following mobile phase: 85 % water, 100mM NaCI, 25mM NaH2PO4, 25Mm Na2HPO4 - 15% methanol. Polymer samples were diluted down to 0.5 active wt% in the mobile phase for at least 4 hours then filtrated in a Millipore filter 0.45 pm and 100 microliters were injected in the mobile phase flow. Absolute molar masses were obtained with the dn/dC of the poly(acrylic acid) equal to 0.1875 mL/g.

The polymer P can be prepared by conventional radical polymerization and by reversible-deactivation (controlled) radical polymerization. The reversible-deactivation (controlled) radical polymerization technology will be selected according to the composition of the targeted polymer, for example by MADIX with xanthates such as Rhodixan A1 from Solvay for polymers containing up to 30mol% of methacryl- based monomers (example AA/MAA/vinyl imidazole = 50/30/20 mol/mol/mol), or by RAFT with trithiocarbonates such as 4-((((2-carboxyethyl)thio)carbonothioyl)thio)-4-cyanopentano ic acid (BM1433, from Boron Molecular) for polymers containing more than 30mol% of methacryl-based monomers (example AA/MAA/vinyl imidazole = 20/70/10 mol/mol/mol).

According to a specific aspect, one specific object of the instant invention is the use of at least one polymer P as defined above for treating a metallic surface intended to be coated by a paint, a varnish or an adhesive, preferably an adhesive. The metal surface to be treated is preferably a surface comprising a metal selected from aluminum, steel, zinc, magnesium and their alloys. The invention is especially interesting for metal surface of aluminum or aluminum alloy.

According to a possible (but not compulsory) embodiment, a conversion coating is applied on the metallic surface to be treated, by reaction of said surface with a conversion composition (in other words, a conversion composition is applied on the metallic surface for forming a conversion coating thereon). In that case, typically:

- the conversion composition include all or part of the polymer P as an additive ; and/or

- the conversion coating is applied on the metallic surface and then all or part of the polymer P is applied on the conversion coating.

According to another possible embodiment, compatible with the previous one, all or part of the polymer P is present in a paint, a varnish or an adhesive coating applied on the surface, optionally after application of a conversion coating on the metal surface.

According to another aspect, one specific object of the invention is a process for coating a metallic surface with a paint, a varnish or an adhesive, including a step of treating said surface with at least one composition including at least one polymer P as defined above. In that scope, the composition comprising the polymer P may typically be: a conversion composition including a polymer P ; and/or a solution or a dispersion of the polymer P, preferably applied on the surface after having applied a conversion coating on the surface to be treated ; and/or the paint, varnish or adhesive, that may comprise all or part of the polymer P.

The polymer P useful according to the invention and the compositions comprising the polymer P (especially the conversion compositions including a polymer P; the paint, varnish or adhesive compositions containing the same; and the solutions or dispersions including the polymer P useful for treating the surface) also constitute specific objects of the instant invention.

Typically, the polymer P is present in the conversion composition and/or in a solution or dispersion applied on the surface to be treated. In that case, the paint, varnish or adhesive is generally applied on a surface previously treated by the polymer. According to some specific embodiments, an additional layer may applied between the treated surface and the paint, varnish or adhesive.

One more specific object of the instant invention is the use of at least one polymer P as defined above for treating a first metallic surface (S1) intended to be bonded to a second surface (S2) by adhesive bonding and for imparting a resistance to the adhesive failure to the bonding (in other words for providing the joint between surfaces S1 and S2 with a resistance to adhesive failure) An additional advantage of the adhesive bonding obtained according to the invention is that it is highly resistant to corrosive atmospheres and to wet atmospheres, which lead to long lasting adhesive bonding. In most cases, the polymer is also used for obtaining this additional effect (namely for further imparting to the bonding a resistance to corrosive atmospheres and to wet atmospheres, in other words for obtaining both a very effective, but also long lasting adhesion).

In other words, the use of at least one polymer P as defined above for treating a first metallic surface (S1) intended to be bonded to a second surface (S2) by adhesive bonding and for imparting a resistance to the adhesive failure to the bonding is also providing a very good resistance to ageing of the adhesive bonding. Such a property can be measured according to tensile tests on so-called “Single Lap Shear” (SLS) assemblies, such as defined in ASTM D-1002 10, performed on freshly bonded SLS assemblies and performed on SLS assemblies after ageing in corrosive atmospheres, wet atmospheres, or repeated cycles of corrosive atmospheres followed by wet atmospheres, such as ASTM G85 - Annex 3. Other tests simultaneously combine a corrosion stress and a mechanical stress (e.g. compression load), such as the BV 101-07, known as Ford Durability Stress Test For Adhesive Lap-shear Bonds or Arizona Proven Ground Exposure (APGE). Notably an adhesive bonding with the polymer P according to the invention between two surfaces S1 and S2 has been demonstrated to provide failure facies, after ageing, that remain more cohesive.

Typically (but not necessarily), the second surface (S2) is also a metallic surface, having or not the same nature as the first surface (S1). According to an advantageous embodiment, the second surface (S2) is a metallic surface also treated with a polymer P of formula (a), generally but not necessarily identical to the polymer P of the first surface (S1).

More generally, the polymer P used according to the invention is preferably used for treating both surfaces (S1) and (S2) before the adhesive bonding of the two surfaces, especially when (S2) is a metallic surface.

The first metal surface (S1) is preferably a surface comprising a metal selected from aluminum, steel, zinc, magnesium, titanium, copper and their alloys, or cobalt-nickel alloys. The invention is especially interesting for metal surface of aluminum or aluminum alloys. The invention is especially interesting when the surface (S1) is a metal surface of aluminum or aluminum alloy.

The second surface (S2) may be metallic or non-metallic surface.

According to an interesting embodiment, the second surface (S2) is a surface comprising a metal, advantageously selected from aluminum, steel, zinc, magnesium titanium, copper and their alloys, or cobalt-nickel alloys. According to one embodiment, the nature of the surfaces (S1) and (S2) is the same, but they can also be distinct according to other possible embodiments of the invention. According to an interesting variant, both surfaces (S1) and (S2) are metal surface of aluminum or aluminum alloys.

According to another possible embodiment, the second surface (S2) is a non- metallic surface, for example a plastic surface e.g. based on polyamide, PEEK or ABS ; or a composite surface based e.g. on CFRP or Glass Fiber Reinforced Plastics.

Whatever the exact nature of surfaces (S1) and (S2), according to a possible embodiment, a conversion coating may be applied on the metallic surface (S1), by reaction of said surface with a conversion composition (in other words, a conversion composition is applied on the metallic surface for forming a conversion coating thereon). The use of a conversion coating is however not compulsory according to the invention, and, according to a specific embodiment, no conversion coating is applied on the surface (S1). When a conversion composition is used, typically:

- the conversion composition includes all or part of the polymer P as an additive ; and/or

- the conversion coating is applied on the surface (S1) and then all or part of the polymer P is applied on the conversion coating.

The second surface (S2) may also receive a similar conversion coating, in the same conditions, especially when this second surface (S2) is a metallic surface. But again, the use of a conversion coating is not compulsory according to the invention, and, according to a specific embodiment, no conversion coating may be applied on the surface (S2). According to another possible embodiment, compatible with the previous ones, all or part of the polymer P is contained in the adhesive composition applied onto the surfaces (S1) and (S2). According to this embodiment, the polymer may typically be introduced in the adhesive composition as a solid powder, said powder comprising the polymer alone or the polymer at the surface of a mineral filler (said powder may typically be obtained by spray drying a solution or suspension of the polymer, typically in presence of mineral filler). According to another aspect, one other specific object of the invention is a process for bonding a first metallic surface (S1) with a second surface (S2) (said surfaces being preferably as defined above), including: treating said first surface (S1) with at least one composition including at least one polymer P as defined above (said surface (S1) being preferably cleaned and/or activated before the treatment with the polymer P) ; and optionally treating the second surface (S2) with at least one composition including at least one polymer P as defined above (said surface (S2) being then preferably cleaned and/or activated before the treatment with the polymer P) ; and bonding the surfaces (S1) and (S2) via an adhesive composition applied between the two surfaces.

In that scope, the composition comprising the polymer P may typically be: a conversion composition including a polymer P ; and/or a solution or a dispersion of the polymer P, preferably applied on the surface after having applied a conversion coating on the surface to be treated ; and/or the adhesive composition, that may comprise all or part of the polymer P.

Typically, the polymer P is present in the conversion composition and/or in a solution or dispersion applied on a conversion coating. In that case, the adhesive is applied on a surface previously treated by the polymer.

According to some specific embodiments, an additional layer is applied between the treated surface (S1) and the adhesive (this is for example the case for the treatment of metal coil or part on a first site, that has then to be bonded on a second site: in that case, a lubricant may be applied on the treated coil or part, in order to protect it during transportation and storage and to facilitate downstream operations (coil cutting into sheets, blanking, stamping, forming, ...).

According to yet another aspect, a specific object of the instant invention are the materials comprising two adhesive-bonded surfaces including a first metal surface comprising a metal surface (S1) which is in all or part (i) treated with a polymer P as defined above and (ii) bonded to a second surface (S2) preferably as defined above via an adhesive.

These materials include i.a. materials that have a metal surface (S1) in all or part covered by: at least one coating (typically a conversion coating and/or a paint, a varnish or an adhesive layer) comprising at least one polymer P ; and/or a layer (typically a conversion coating) comprising a reaction product of the polymer P as defined above with a metal of the treated surface or another compound present in said layer, or a polymer P strongly linked with said other compound (via a complexation, a ionic bonding or hydrogen bonding for example).

Specific features and possible embodiments will now be described in more details.

The metal surface (S1)

Any metal surface may be treated with a polymer P of the invention, but the invention is especially suitable for treating metal surfaces of: aluminum or an aluminum-based alloy ; or steel, for example galvanized steel (hot dip galvanized HDG or electrogalvanized EG) ; or cold rolled steel (CRS) ; or magnesium or magnesium-based alloys ; or

- zinc or zinc-based alloys ; or

- titanium or titanium-based alloys. The invention is especially interesting for metal surface of aluminum and aluminum alloys, such as Aluminum Alloy AA 5754, tested in the appended examples, or other alloys such as those of Series 1xxx, 2xxx, 3xxx, 4xxx, 5xxxx, 6xxx, 7xxx, such as AA 1050, 2024, 3003, 5182, 5005, 6111 , 6016, 6060, 6063, 6182, 7075.

The optional conversion coating

When a conversion coating is applied on one or both of the surfaces (S1) and/or (S2), it may be obtained by contacting the surface with any conversion composition known from the prior art.

Contacting the metal surface with the conversion composition may be made by any means known per se, such as dip coating in a conversion bath or spray coating, as illustrative examples.

The conversion composition used according to the invention may typically contain fluorides anions and cationic metals, e.g. compounds such as H2CrFe, or more preferably chromium free compounds such as H2TiFe , F^ZrFe , F^HfFe , H2AIF6 , F^SiFe , F^GeFe , H2SNF4 , or HBF ₄ .

The conversion composition may also include other compounds, such as silane precursors for example, and/or cerium salts, and/or terbium molybdate.

In addition, according to a specific embodiment, the conversion composition may contain all or part of the polymer P used according to the invention for treating the surface. In that case, the application of the conversion layer leads per se to a surface treatment according to the invention.

Otherwise, the treatment is typically obtained after the formation of the conversion layer, by contacting the metal surface carrying the conversion layer with the polymers P (they may typically be applied on the conversion layer in the form of a solution or a suspension of polymers P, or within a paint, a varnish or an adhesive composition applied on the conversion layer).

According to a specific embodiment, it may be contemplated to make use of the polymer P both in the conversion composition and within the adhesive composition applied on the conversion layer. Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

The following examples illustrate the invention.

EXAMPLES

Example 1.

Polymers according to the invention, obtained by a copolymerization of a mixture of acrylic acid, methacrylic acid, and /V-Vinylimidazole (Vim), were tested in these examples.

Example 1.1

The polymer P1 (AA/MAA/VIm 26/70/04 mol/mol/mol) has been prepared as follows: Solution 1 : 7.8g of 2,2'-Azobis(2-methylpropionamidine)dihydrochloride (V50) at 97% are dissolved in 68.3g of water.

Solution 2a (AA/Vlm=26/04 mol/mol): 35.4g of AA and 7g of Vim are diluted in 17.6g of water.

Solution 2b: caustic solution at 35% in water (113mL).

Solution 3: 114g of MAA are diluted into 74g of water.

Solution 1 , along with 5% of Solution 2a and 114g of deionized water are charged at room temperature into a 700ml reactor fitted with adequate stirring, inlets, feeding and temperature control devices.

The reactor temperature is then heated to 60°C within 1 hour, with nitrogen degassing. When temperature has reached 60°C, two feeds are started, under nitrogen blanket:

- the remaining 95% of Solution 2a over 4h, along with Solution 2b

- Solution 3, over 5 hours

Once the longest feed is over, the reaction mixture is maintained for two additional hours at 60°C, before it is cooled down to room temperature. The smooth incorporation of monomers was monitored by ¹ H NMR spectroscopy over the polymerization and the final product was analyzed by both ¹ H NMR spectroscopy and size exclusion chromatography.

A Brucker 300MHz spectrometer was used to record proton nuclear magnetic resonance ( ¹ H NMR) spectra. To measure AA, MAA and Vim conversions, four drops of the reaction mixture were diluted in around 1g of deuterated water (D2O). AA conversion > 99.6%; MAA conversion > 99.9%; Vim conversion > 99.9%

Average molecular weights were measured by Size Exclusion Chromatography (SEC) equipped with a MultiAngle Laser Light Scattering (MALLS) Mini Dawn TREOS detector and an Agilent concentration detector (Rl detector). The SEC system is running on three columns Varian Aquagel OH mixed H, 8 pm, 3*30 cm at a flow rate of 1 mL / min and with the following mobile phase: 100 % water, 100mM NaCI, 25mM NaH2PO4, 25Mm Na2H PO4, buffer pH=7. Polymer samples were diluted down to 0.5 active wt% in the mobile phase for at least 4 hours then filtrated in a Millipore filter 0.45 pm and 100 microliters were injected in the mobile phase flow. Absolute molar masses were obtained with the dn/dC of the poly(acrylic acid) equal to 0.1875 mL/g. M _n = 29.5 kg/mol; /W _w = 76 kg/mol; polydispersity index e = 2.6.

Example 1.2

The same process was used to prepare the polymer P2 (AA/MAA/VIm 20/70/10 mol/mol/mol):

Solution 1 : 7.0g of 2,2'-Azobis(2-methylpropionamidine)dihydrochloride (V50) at 97% are dissolved in 60.7g of water.

Solution 2a (AA/Vlm=20/10 mol/mol): 24.2g of AA and 15.8g of Vim are diluted in 16.6g of water.

Solution 2b: caustic solution at 35% in water (129mL). Solution 3: 101g of MAA are diluted into 66g of water.

AA conversion = 99%; MAA conversion > 99.9%; Vim conversion > 99.9% M _n = 41 kg/mol; M _w = 107 kg/mol; polydispersity index e = 2.6.

Performances of polymers P1 and P2 were assessed with AA 5754 type H111 aluminum alloys coupons from FBCG (100mm long, 25mm wide, 3mm thick) through Single Lap Shear (SLS) tests, before and after ageing in corrosive conditions. Coupons were prepared according to the protocol below and assembled to form Single Lap assemblies as described in D1002-10.

Step 1- 20 coupons are cleaned and etched all together in one single step, combining cleaning and etching, in a 4L bath at 50°C contained in a stainless steel tank, typically made by diluting a commercially available formulation, Chemtec DBT ALU 200, available from Chemtec Aertec (5g of DBT ALU 200 into 995g of water) for 3 min. under light stirring. The coupons were then rinsed twice during 1 min. with deionized water.

Step 2- the coupons are then pre-treated by dipping for 2 min. in the treatment bath, containing the polymer at 50°C and at several concentration indicated in the Table 1 below. They are then rinsed altogether with a flow of deionized water for 1 min. and dried for 30 min. at 60°C.

Step 3- the coupons are then assembled in pairs, each pair forming a so called single lap shear “assembly”: two coupons are placed horizontally, parallel, one above the other forming an overlap of 12.5mm long and 25mm wide (“overlap zone”, including one of terminal zone of each of the two coupons of 25mm wide, namely the last 12.5mm of the 100mm length of the coupon). A structural high temperature curing epoxy adhesive bead (Betamate 1496, from Dow) is applied with a gun under 7bars on the overlap zone of the lower coupon. The upper coupon is then pressed, thus forming a bonding zone of 12.5mm long, and 25mm wide. Paper clips are used to maintain the assembly integrity before and during curing. The adhesive is then cured according to adhesive producer guidelines, typically for 40min. at 180°C. Finally, paper clips are removed.

Step 4- tensile strength test I on assemblies as obtained in step 3

Used material: Zwick/Roell - Z50, with jaws grasping assembly tips over 50mm and a pulling speed of 10mm/min. (each jaw holds one of the bonded coupon of the pair, on a grasping zone of 50mm of said coupon located at the end zone of each coupon opposite to the overlap zone. The jaws are then moved for pulling each of the coupon horizontally in the direction starting from the bonding zone towards the grasping zone)

Step 5- tensile strength test II performed on assemblies as obtained in step 3 after ageing 5.1. Ageing cyclic test

A cyclic ageing test is performed according to ASTM G85 - Annex 3 (SWAAT, 2011) in a corrosion chamber Q-FOG CRH 600L, from Q-FOG in the following conditions :

• a 30-min. acidified salt fog spray followed by

• a 90-min. soak at >98% relative humidity under the following conditions:

• Chamber temperature - constant 49°C

• Air saturator temperature - constant 57°C

• Relative humidity - >98%

• pH of fall out solution - 2.8 - 3.0

• Volume of fall out solution - 1.0-2.0 ml/80cm ² /hour

• Exposure period - 1000 hours

After the exposure period is completed, the assemblies are washed down with lukewarm water to remove and neutralize excess acid and any remaining salt residues. All assemblies were then air dried using forced ambient temperature before being for submitted to lap-shear tensile testing.

5.2. tensile strength test

In the conditions of the tensile strength test I of step 4

The obtained results are reported in Tables 2 to 5 below (the values are average values: the tests were performed on 3 assemblies before ageing and on five assemblies after ageing), with the following variations in step 2. Polymers were diluted with de-ionized water and the resulting treatment bath was tested as such (no pH adjustment, “native pH”) or after acidification (pH adjusted with sulfuric acid):

Table 1 : conditions of steps 1 and 2

Below are reported performances before ageing, after ageing, and the ratio between values after ageing and values before ageing, called “retention”:

Table 2: Maximum LOAD

Table 3: STRAIN measured at the maximum load

Table 4: ENERGY measured at the maximum load

Table 5: FACIES after bond failure *** .

(c): cohesive fracture

(a): adhesive fracture

(a/c): adhesive and cohesive fracture

(~c): rather cohesive fracture * actual performance

Polymers according to the invention, with an amidopropyl spacer between the polymer backbone and the imidazole group are described in Example 2: Example 2: The polymer P3 (AA/methacrylamido propyl imidazole 92/08 mol/mol) was prepared as follows:

Poly(acrylic acid) synthesis via reversible-deactivation (controlled) radical polymerization:

219ml of DI water and 69ml of methanol were taken in a round bottom flask. 30g of acrylic acid was added to the round bottom flask and the reaction mixture was stirred till the solution became homogeneous. 0.32g of Rhodixan A1 as a controlling agent was then added to the solution. The solution was then purged with N2 for 1 hour. 0.4048g of VA-044 initiator (2,2'- Azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, TCI) was added to reaction mixture and continued purging for 15 min. The polymerization was carried out at 60°C for 5hours under N2 atmosphere. The reaction mixture was then exposed to air and and an aliquote was taken in D2O for recording proton NMR spectrum ( ¹ H NMR) using NMR (Bruker 400MHz). It confirmed complete conversion of the acrylic acid monomer. The polymer was precipitated in acetone, dissolved in water and precipitated in acetone again. The precipitated polymer was dried at 60°C for 12 hours under vacuum.

Average molecular weights were measured by Gel Permeation Chromatography (GPC) using Waters 515 instrument equipped with column oven, empower software (GPC module), Shodex RI-71 Detector. The SEC system is running on polymer Lab Aquagel-OH- 50 (Linear MW operating range: 500 to 600,000g/mol, PS equivalent) columns with Guard column, a flow rate of 1 mL/min. and with the following mobile phase: 0.25M NaNOs + 0.03M Na2HPO4 + 0.003M NaNs-pH-9. Dilute solution of polymer samples at 10mg/ml were prepared in the mobile phase and then filtrated via a Millipore filter 0.2pm before injecting to the system and 100 pLwere injected in the mobile phase flow with a run time of 30min.. The measurement was carried out at 40°C. Poly(acrylic acid-Na salt) (Polymer Lab- PL2140-0100) was used as the calibration standard. Results are: /W _n =26,000 g/mol, /W _w =57,000 g/mol, polydispersity index = 2.17.

Synthesis of an amino propyl imidazole functionalized Poly(acrylic acid) (Polymer P3) by conventional DCC (dicyclohexylcarbodiimide) coupling reaction in organic NMP solvent:

In a 500ml round bottom flask equipped with a water condenser and a mechanical agitation, were introduced at room temperature (22°C), 25g of Poly(acrylic acid) (PAA; synthesized as described above) and 100ml of NMP. The mixture was degassed by bubbling nitrogen in the bulk for 30 min. at room temperature. Then the reaction mass was heated to 60°C with constant stirring for overnight under nitrogen atmosphere. After complete dissolution of PAA, 7g of DCC (Sigma Aldrich) was added slowly under stirring at 60°C for 30 min., followed by addition of 4.34ml of aminopropyl imidazole (Sigma Aldrich) within 20 min. ; the reaction medium was then at 70°C for 48 hours.

The polymer was precipitated in ethyl acetate:THF (1 :1) mixture, dissolved in MeOH and re-precipitated in ethyl acetate. The polymer was dried in vacuum-oven at 70-75°C for 24 hours. The yield of the polymer was around 90%.

The percentage of propyl imidazole groups introduced in the polymer was calculated by recording proton NMR spectrum ( ¹ H NMR) using NMR (Bruker 400 MHz). Around 20mg of the polymer was dissolved in deuterated water (D2O) to record the spectrum. The result is: acrylic acid units/imidazolepropyl acrylamide units = 92/8 mol/mol.

Polymers according to the invention, with imidazolinium groups are described in Example 3 and 4:

Example 3: The polymer P4 was prepared by full quaternization of polymer P3 with bromo-hexane)

In a 250ml round bottom flask equipped with a water condenser and a mechanical agitation, were introduced at room temperature (22°C), 7g of aminolmidazole (8mol%) functionalized PAA (Polymer P3, from example 2) and 70ml of NMP. The mixture was degassed by bubbling nitrogen in the bulk for 30 min. at room temperature. After complete dissolution of the polymer, 11 ,88g of bromohexane (Sigma Aldrich) was added and allowed to stir at 90°C under nitrogen atmosphere for 24 hours.

The polymer was precipitated in ethyl acetate:THF (1 :1) mixture, re-dissolved in methanol and re-precipitated in ethyl acetate. The polymer was dried in vacuum-oven at 70-75°C for 24 hours. The yield of the functional polymer was about 50%.

The percentage of quaternization of the imidazole group on the polymer backbone was calculated by recording proton NMR spectrum ( ¹ H NMR) using NMR (Bruker 400 MHz). Around 20mg of the polymer was dissolved in deuterated methanol to record the spectrum. Complete quaternization of the polymer was observed (acrylic acid units I imidazolium hexyl bromide units = 92/08 mol/mol).

The bromide content of the polymer was also calculated by using argentometric titration. Details of the instrument utilized for the measurement are as follows: Metrohm Autotitrator 905, equipped with Ag/AgCI electrode (Metrohm part number: 6.0450.100) and Tiamo software (Version 2.5) as well as analytical balance with capability to weigh up to 0.0001 mg. Following reagents were utilized (obtained from Sigma Aldrich)

1. Methanol

2. 0.01 N Silver Nitrate solution (AgNOs)-Standardize before performing experiment using Anhydrous Potassium Chloride (KCI) in water

3. Potassium Chloride (KCI)

4. 50% v/v Aqueous Nitric acid(HNOs)

5. Milli Q water (Deionized water)

Sample preparation and Titration: Quarternized polymer (0.0050 to 0.0060g) was weighed and dissolved in 30 ml of methanol and stirred till sample is completely dissolved. 50% v/v aqueous HNO3 (3-4 drops) were added to acidify the sample solution. The solution was titrated against 0.01 N AgNCh solution using Ag/AgCI electrode. And the end point was noted down as EP1. peq/g of Ionic Bromide= (EP1 * 1000* Actual Normality of AgNCh)/ wt. of the sample in (g) ppm of Ionic Bromide= peq/g of Ionic Bromide *79.9(Molar mass of Bromide)

% of Ionic Bromide = ppm of Ionic Bromide /10000

The polymer contained 5% of Bromide by weight.

Example 4: The polymer P5 was prepared by copolymerizing acrylic acid, methacrylic acid and the monomer resulting from the quaternization of vinyl imidazole by hexylbromide:

Synthesis of copolymer poly(methacrylic acid-co-acrylic acid-co-(3-hexyl1-vinyl-1 H- imidazolium-3-bromide, VlmBr, synthesized as per the method described in Separation and Purification Technology 224 (2019) 388-396 in the present invention by conventional radical polymerization Initiator: a,a'-Azodiisobutyramidine dihydrochloride, (AAPH or V50, Sigma Aldrich) ;

Monomers: MAA (Sigma Aldrich) = 75mol%, AA (Sigma Aldrich) = 20mol%, VlmBr = 5mol%. Stock solution:

AA at 70wt% in H2O, MAA at 60wt% in H2O, VlmBr at 70wt% in H2O, V50 at 10wt% in H2O molar ratio: l/(VlmBr+AA+MAA) = 1.5 mol% Final solid content 35wt%

In a 100mL three necked round bottom flask equipped with magnetic stirrer, were introduced, at room temperature (22°C), 0.22g (5wt% of stock solution) of acrylic acid, 0.36g (5wt% of stock solution) of 3-hexyl1-vinyl-1 H-imidazolium-3-bromide (VlmBr), 42g of water and 0.87g of V50 (solution in 10wt% of water). The mixture was degassed by bubbling nitrogen in the bulk for 60 min. while the temperature in the solution increased up to 60°C. The remaining 95% of stock solution of AA and VlmBr was mixed together and total 11 ,08g mixture of both monomers was introduced for 3 hours into the reaction mixture. Simultaneously, MAA solution was started and performed over 4 hours (flow rate=0.0723 ml/min.). During the monomer addition, the solution became a gel and the reaction was stopped after complete addition of MAA.

Reaction mixture was precipitated in diethylether. Polymer was re-dissolved in methanol and re-precipitated in ethyl acetate. Unreacted monomer was removed by soxhlet in THF. Polymer was filtered and dried under at 60 °C vaccum for 6 hours and characterized by 1 H NMR in D2O. The polymer was free from any residual unreacted monomer. Yield of the polymer was about 50%.

Average molecular weights were measured by Gel Permeation Chromatography (GPC) using Waters 515 equipped with column oven, Clarity software (GPC module), Shodex Rl- 71 Detector. The SEC system is running on two columns-Polymer Lab Aquagel-OH-40 & Polymer Lab Aquagel-OH-30 in series with Guard column, a flow rate of 1 mL/min/ and with the following mobile phase 0.4M NaCI +0.05M Na2HPO4+ 0.003M NaNs-pH-9 (pH adjusted with few drops of 0.05M NaH2PO4). Dilute solution of polymer samples at 10mg/ml were prepared in the mobile phase and then filtrated via a Millipore filter 0.2pm before injecting to the system and 100pL were injected in the mobile phase flow with a run time of 30 min. The measurement was carried out at 40°C. Ready Cai-Kit PEO/PEG (PSS-PEOkitr1) was used as the calibration standard. Results are: /W _n =81 kDa, /W _w =162 kDa, Polydispersity index = 2.

Performances of Polymers P3, P4 and P5 were assessed with AA5754 H22 aluminum alloys coupons from FBCG (100mm long, 25mm wide, 3mm thick) through Single Lap Shear (SLS) tests, before and after ageing in corrosive conditions. Coupons were prepared and assembled to form Single Lap assemblies as described above for Polymers P1 and P2. Performances of polymer P2 were also assessed with the same batch of AA5754 H22 aluminum alloys coupons.

Process variations are reported Table 6, performance results Table 7-10.

* coupons were wiped with acetone

Table 6: conditions of steps 1 and 2

Table 7: Maximum LOAD

Table 8: STRAIN at maximum load

Table 10: FACIES after bond failure (c) : cohesive fracture

(a) : adhesive fracture

(a/c) : adhesive and cohesive fracture