Crosslinkable base for producing an Anti-corrosion coating for a metal surface, in particular for aeronautics

Abstract

Disclosed is a base that is crosslinkable for producing an anti-corrosion coating, in particular for aeronautics, the main component of the base being a trans (1,4) polyisoprene, the base comprising a crosslinking agent and an initiator, wherein the crosslinking of the base produces an insoluble crosslinked polyisoprene which is the polymer matrix of the coating and which is capable of adhering to the metal surface, in particular of an aluminum alloy.

Description

[0001] Base capable of being crosslinked to form an anti-corrosion coating for metal surfaces, particularly for aeronautics

[0002] Technical field

[0003] The invention relates to anti-corrosion coatings, particularly in the aeronautical field.

[0004] The invention relates in particular, but not exclusively, to anti-corrosion paints.

[0005] The invention relates in particular, but not exclusively, to anti-corrosion coatings for aluminum alloys.

[0006] The invention aims, among other things, to provide paints using in particular Eucommia gum (EUG).

[0007] State of the art of anti-corrosion paints and treatments in aeronautics

[0008] State-of-the-art anti-corrosion paints are composed of three elements:

[0009] - a base, containing a polymer resin in which various additives are added, introduced for their property of interest, in particular corrosion inhibitors (chromates), pigments;

[0010] - a primer and a hardener, the function of the primer being to trigger the polymerization / crosslinking of the base polymer, and thus transform the liquid paint into a solid film;

[0011] - a solvent, water being increasingly favored today as a replacement for organic solvents, for example alcohol, whose VOCs (Volatile Organic Compounds), dangerous for health, can reach up to 400 g / L.

[0012] A state of the art of anti-corrosion coatings of aluminum alloys was presented in 2019 by Vargel Corrosion of aluminum, ISBN 978-0080999258).

[0013] Aluminum alloys, particularly the 2000 and 7000 series, are widely used in aeronautics. For example, it is estimated that 75% of the mass of an A380 aircraft is related to aluminum alloy structural parts. The 2024-T3 alloy is the most widely used in the 2000 series, and forms a large part of the aircraft fuselage.

[0014] The 2000 series alloys, particularly the 2024-T3 alloy, have poor corrosion resistance (especially pitting corrosion, intergranular corrosion).

[0015] Three approaches are implemented for the protection of metal parts in aeronautics, particularly aluminum alloy parts: passive protection, active protection and sacrificial protection, these three approaches can be coupled.

[0016] Passive protection results from a barrier effect of a coating, against water, oxygen or ions such as chlorides. Passive protection is often reinforced by the presence of anti-corrosion pigments, such as titanium dioxide.

[0017] Active protection results from the use of corrosion inhibitors.

[0018] In the aerospace industry, multi-layer coating systems are used to protect metal substrates, particularly for 2024-T3 aluminum alloy.

[0019] These multi-layer coating systems include, after surface preparation (by cleaning, degreasing, stripping, deoxidation), a surface treatment by electrochemical anodization with or without sealing or chemical conversion, the application of an adhesion-promoting primer and the application of an organic finish (top-coat). In paint systems used in aeronautics, the thickness of the primer layer is usually 15 to 25 microns, the thickness of the top coat being usually between 50 and 120 microns.

[0020] Not all structural parts of an aircraft are painted, as surface treatment by anodization or chemical conversion must then provide sufficient protection against corrosion.

[0021] The primer is often an epoxy containing pigments such as strontium chromate SrCrC or zinc chromate ZnCrCh.

[0022] The top coat is most often epoxy or polyurethane based.

[0023] Surface treatment by anodizing or conversion usually involves corrosion-inhibiting compounds. Conversion coatings based on hexavalent chromium have been used for over 50 years, for example with the product marketed under the brand name Alodine 1 200®. Such compounds include, for example, chromium trioxide (CrCh), potassium dichromate (K2Cr2O7), sodium dichromate (Na2Cr2O?), or strontium chromate (SrCrCh). Hexavalent chromium is classified as carcinogenic to humans by the CI RC. Several derivatives of hexavalent chromium, such as chromium trioxide, are also classified as carcinogenic by the European Union.

[0024] Due to the potential carcinogenic effects of hexavalent chromium and the negative environmental effects associated with the use of compositions containing this compound, which require the use of a large quantity of sometimes toxic organic solvents and water, its use is now strictly regulated in Europe by REACH Regulation No. 1907 / 2006 (Registration, Evaluation, Authorization and restriction of Chemicals) and in the United States by the Environmental Protection Agency (EPA).

[0025] Current anti-corrosion paints contain metals in their base that selectively oxidize instead of the substrate, with Chromium (VI)-based compounds being the most common. In addition to inhibiting the metal's oxidation-reduction reactions, their transformation into metal oxide acts as a barrier compound to the molecules that cause corrosion.

[0026] These paints have many disadvantages.

[0027] Firstly, these paints contain chromates in very large quantities, these Cr(VI)-based compounds having a high environmental impact. For example, potassium hydroxyoctaoxodizincatedichromate (CAS 11103-86-9) and strontium chromate (CAS 7789-06-2) are subject to Annex XIV of the REACH Regulation, their placing on the market and use being prohibited after 22 January 2019, unless an authorisation is granted for a limited period for specifically defined uses.

[0028] For a period of 7 years from 22 January 2019, the European Commission has permitted the use of these substances, subject to supply from a company covered by an authorisation dossier. These authorisations represent a significant additional cost, both for the supplier and for the client.

[0029] Secondly, these paints in which the solvent is water clog the filters of the paint booths four times faster than solvent-based paints (alcohol). Thus, the benefit of the reduction of volatile organic compounds (VOCs) is counterbalanced by a working atmosphere highly concentrated in CM R compounds (carcinogenic, mutagenic, and toxic for reproduction), which requires operators to use specific protection against inhalation or skin contact with these substances.

[0030] To take into account the ban on hexavalent chromium, various alternative approaches have been proposed to form anti-corrosion protection in the aeronautics field, in particular for aluminium alloys, including: the replacement in the chemical conversion of hexavalent chromium by trivalent chromium (for example, the product marketed under the reference "SurTec650" from the company SurTec GmbH, or the product marketed under the reference "Lanthane 613.3" from the company Element Solutions, or the product "Socosurf TCS PACS" from the company Socomore);

[0031] - the replacement in anodizing processes of chromic anodic oxidations based on chromium VI by sulfuric anodic oxidations (OAS) or sulfuric and tartaric anodic oxidations (TSA);

[0032] - the use of phosphate compounds, titanium and zirconium compounds in a fluorinated medium, or rare earths such as cerium or lanthanum;

[0033] - the use of inhibitors existing in different degrees of oxidation, such as vanadium, manganese, molybdenum; the formation of a sol-gel film.

[0034] A state of the art review of these alternative coating techniques for aluminum alloys was presented in 201 9 by Becker (Chromate-free chemical conversion coatings for aluminum alloys doi.org / 10. 1515 / corrrev-2019-0032). Reference can also be made to the review published in 2022 by Peltier et al. Review of Cr-Free coatings for the corrosion protection of aluminum aerospace alloys, doi.org / 10.3390 / coatings 12040518). Reference can also be made to document FR3053352 (Airbus, 201 8).

[0035] Replacing chromium VI with chromium III is not without its drawbacks. For example, surface treatments using sulfuric anodic oxidation based on sulfide and trivalent chromium (Cri 11) take a long time to implement, since they require several successive treatment stages and intermediate rinsing.

[0036] State of the art regarding the use of Eucommia gum Eucommia ulmoides, or gum tree, sometimes referred to as guttapercha tree (although unrelated to Palaquium gutta), is cultivated in China and Vietnam, and is used in herbalism and traditional Chinese medicine.

[0037] From the Eucommia ulmoides tree, it is possible to extract poly(1,4)trans isoprene (CAS 104389-32-4):

[0038] It is also possible to extract poly(1,4)trans isoprene from natural latex obtained from leaves of Palaquium gutta, or from latex of Manilkara bidentata (balata gum).

[0039] A state of the art review of poly(1,4)trans isoprene extraction methods is presented by Liu et al. Review on extraction, characteristic, and engineering of the Eucommia ulmodies rubber for industrial application, Industrial Crops & Products 180, 2022, 1 14733).

[0040] While the cis stereoisomer of polyisoprene is produced by more than 2000 plants and is a polymer of great industrial importance, particularly for the manufacture of tires, the trans stereoisomer is produced naturally only very rarely, by a few tree species.

[0041] The properties of trans 1,4 polyisoprene are very different from those of cis 1,4 polyisoprene (CAS 104389-31-3). The chains of cis 1,4 polyisoprene are tortuous while the chains of trans 1,4 polyisoprene are regular. Trans 1,4 polyisoprene is a semi-crystalline polymer material, with a glass transition temperature between -60°C and -70°C, while cis 1,4 polyisoprene is an amorphous material at room temperature. The elastic modulus of trans 1,4 polyisoprene is 120 M Pa at room temperature, higher than the elastic modulus of cis 1,4 polyisoprene.

[0042] To suppress the crystallization of trans 1,4 polyisoprene (TPI), and enable the production of elastomers from TPI, a wide variety of chemical modifications of TPI have been proposed (chlorination, hydrogenation, epoxidation, grafting, Alder-ene reaction).

[0043] For example, Yan et al. (Industrial Crops & Products 191, 2023, doi.org / 10.1016 / j.indcrop.2022.115943) describes a copolymerization of a Eucommia extract with Tung oil via a Diels-Alder reaction. Eucommia gum, dissolved in xylene, is mixed with Tung oil and the mixture is heated to 75° for 8 hours. The resulting material is in the form of a flexible, hydrophobic film. The film is claimed to be corrosion-resistant, based on test results conducted by immersing coated tin plates in sulfuric acid (1 mol / L) for 7 days.

[0044] In an attempt to find a natural substitute for cis-1,4 polyisoprene rubber produced from Hevea brasiliensis latex, it has been proposed to modify Eucommia gum to produce elastomers, by reaction with mercaptans, by a click chemistry-type thiol-ene reaction. For example, CN 109265580 describes the click chemistry-type thiol-ene reaction of a mercaptan (dodecanthiol) with Eucommia gum, in the presence of a photoinitiator (2,2-bis(hydroxymethyl)propionic acid, DM PA), under 365 nm ultraviolet radiation.

[0045] The thiol-ene reaction has been proposed for the photo-vulcanization of cis-polyisoprene, for the manufacture of surgical gloves. For example, the paper Schlôgl et al. (Photo-vulcanization using thiol-ene chemistry: film formation, morphology and network characteristics of UV crosslinked rubber, Polymer 55, 2014, 5584-5595) describes a pre-vulcanization by thiol-ene reaction of an industrial latex based on cis-polyisoprene ("Kraton I R0401 BU", from the Kraton company), the thiol being a trithiol (trimethylolpropane tris(3-mercaptopropionate)), the reaction being carried out in the presence of a photo-initiator (ethyl phenyl(2,4,6-trimethylbenzoylphosphinate), under ultraviolet light, at room temperature. The film produced has a thickness of between 0.5 mm and 1 mm and is then vulcanized, the films obtained after vulcanization having a thickness of between 200 and 300 microns.

[0046] It has also been proposed to crosslink TPI by gamma irradiation. For example, Zhao et al. Achieving high elasticity of trans-1,4-polyisoprene with a combination of radiation crosslinking and thiol-ene grafting. Polymer chemistry, 14(1), 81-91, 2022) describe crosslinking of TPI by gamma rays. The resulting product is then diluted in toluene and mixed with n-dodecanethiol, in the presence of a photoinitiator (phenyl bis(2,4,6-trimethylbenzoyl)-phosphine oxide). After keeping the resulting mixture in the dark for 24 h, the mixture was subjected to ultraviolet radiation at a wavelength of 365 nm, the tensile mechanical properties of the resulting amorphous material being those of an elastomer.

[0047] Eucommia gum or trans 1,4 polyisoprene (TPI) are not considered in the state of the art as effective biopolymers as corrosion inhibitors. For example, Eucommia gum or TPI are not cited in the meta-review published in 2021 by Shahini et al. Recent advances in biopolymers / carbohydrate polymers as effective corrosion inhibitory macro-molecules: A review study from experimental and theoretical views Journal of Molecular Liquids 325, 2021, 1 151 10).

[0048] It has been proposed in the prior art to form anti-corrosion coatings comprising epoxidized EUG. Reference may be made, for example, to Wang et al. {J Mater Sci, 2021, 56: 1855-1878), Chen et al. {A novel and green method to synthesize an epoxidized eucommia gum as the nanofiller in the epoxy composite coating with excellent anticorrosive performance, Chemical Engineering Journal 379, 2020, 122323), Feng et al. {Preparation and properties of Eucommia ulmoides gum / epoxy resin anticorrosive coating. Mater Rep 33(22): 3847-3852, doi.org / 10.11896 / cldb.18090258). Reference may also be made to documents CN 1 1 2457503, CN 1 1 081 881 8, CN 1 093851 81 . The document Liu { Trans- 1 ,4-polyisoprene extracted from Eucommia bark as natural corrosion inhibitor for carbon steel in the simulated concrete pore solution, I nt. J. Electrochem. Sci., 17, 2022, article number: 220615, doi: 10.20964 / 2022.06.34) describes the dissolution of trans 1,4 polyisoprene extracted from Eucommia gum, at concentrations of 0.3 to 2.0 g / L, in a saline solution containing calcium hydroxide, potassium hydroxide, sodium hydroxide and sodium chloride, in contact with a Q235 carbon steel bar. The saline solution simulates the pore solution contained in the pores of concrete, in a marine environment, Q235 steel being an ordinary structural steel, used in construction for the manufacture of reinforced concrete.

[0049] Document CN 10651 9342A (Univ Shenyang Chemical Tech) describes the mixture of nitrile rubber and Eucommia gum, for the manufacture of vulcanized rubbers intended for offshore or port equipment, the Eucommia gum being in the minority and forming only 10 to 40 parts by weight of the mixture of materials to be vulcanized, the nitrile rubber representing between 60 and 90 parts by weight of the mixture of materials to be vulcanized.

[0050] Document CN 1 1 6253920A (Univ South China Tech) describes a porous material prepared from Eucommia gum, the material having low density and high porosity, the crosslinking being carried out in emulsion. The intended applications are sound absorption, thermal insulation, filtration. The porous material obtained is not designed for anti-corrosion applications and its porosity is incompatible with anti-corrosion applications.

[0051] CN 1093851 81 A (Univ Shenyang Chemical Tech) discloses an anti-corrosion coating comprising partially epoxidized Eucommia gum, and further comprising an epoxy resin and a crosslinking agent in an organic solvent.

[0052] Document CN 1 1 081 881 8A (Univ Hunan) describes an anti-corrosion coating comprising partially epoxidized Eucommia gum, the coating further comprising an epoxy resin and a crosslinking agent in an organic solvent. The epoxy resin is largely in the majority in the mixture (40 to 85 parts by weight of epoxy, and 0.1 to 10 parts by weight of epoxidized Eucommia gum, the examples showing percentages of epoxy resin between 61.3 and 64.3, and percentages of epoxidized Eucommia gum between 0.5 and 5).

[0053] Subject of the invention

[0054] The invention aims to propose new coating bases, and in particular paint bases using advantageously biosourced compounds, in particular Eucommia gum (Eucommia Ulmoides Gum EUG) or poly(1,4)trans isoprene from Eucommia ulmoides, or from other plants such as Palaquium gutta, or Manilkara bidentata. Poly(1,4)trans isoprene is thus for example extracted from gutta-percha or balata.

[0055] By bio-sourced, we mean a material entirely or partially made from materials of renewable biological origin.

[0056] The invention aims to develop new coatings, and in particular new paints from natural compounds, respecting the constraints of the REACH regulation and having anti-corrosive properties comparable to those of surface treatments and in particular to those of petroleum-sourced paints.

[0057] The invention aims to provide an alternative to the use of compounds comprising hexavalent chromium, which are doomed to disappear.

[0058] For these purposes, the invention relates, according to a first aspect, to a base capable of being crosslinked to form an anti-corrosion coating for a metal surface, in particular for aeronautics, the main component of the base being a trans (1,4) polyisoprene, giving the base its anti-corrosion properties, the base further comprising an initiator and a crosslinker which is advantageously a thiol, the crosslinking of the base forming a crosslinked polyisoprene which is the polymer matrix of the coating and which is capable of adhering to the metal surface, in particular of an aluminum alloy.

[0059] Advantageously, the main component trans(1,4)polyisoprene represents between 40% and 80% of the total mass of the base, the initiator a quantity less than or equal to 2% of the total mass of the base, and the crosslinker the remaining quantity of the total mass of the base.

[0060] Preferably, the main component trans(1,4)polyisoprene represents between 45% and 60% of the total mass of the base, the initiator an amount less than or equal to 2% of the total mass of the base, and the crosslinker the remaining amount of the total mass of the base. Advantageously, the main component trans(1,4)polyisoprene is the only organic polymer of the base, which is without surfactants.

[0061] Advantageously, the coating is insoluble. Insoluble here means that the soluble fraction of the coating in dichloromethane is low, preferably less than 20%. Advantageously, the main component is a trans (1,4) polyisoprene from Eucommia ulmoides, or natural latex obtained from leaves of Palaquium gutta, or latex from Manilkara bidentata.

[0062] In some implementations, the main component comprises synthetic trans(1,4)polyisoprene (CAS 104389-32-4).

[0063] Thus, trans(1,4)polyisoprene is the main component of the base capable of being crosslinked to form a coating intended for anti-corrosion, the coating being adherent to the metal surface.

[0064] Advantageously, the trans(1,4)polyisoprene is contained in natural Eucommia gum resin (EllG).

[0065] Advantageously, the trans(1,4)polyisoprene is non-epoxidized or partially epoxidized, for example with an epoxidation rate of less than 60%.

[0066] Depending on the various implementations, the crosslinker is a thiol chosen from the list:

[0067] - a multifunctional aliphatic thioalcohol whose hydrocarbon chain length varies from 2 to 12 methylene groups, for example ethanedithiol or hexanedithiol;

[0068] - or a multifunctional aliphatic thioalcohol whose aliphatic chain length varies from 4 to 20 methylene groups, and contains organic functions of the acetoxy type, for example 1,4-butanediol bis(thioglycolate) or trimethylolpropane tris(3-mecraptopriopionate);

[0069] - or a mixture of these thiols.

[0070] Advantageously, the main component is the only component of the base giving the base its anti-corrosion properties and which allows the base, once crosslinked, to adhere to the metal surface, advantageously without surface treatment of the latter. Advantageously, the rate of thiol functions or the molar fraction of the thiol functions is between 10 and 100 molar percent (mol%) relative to the number of moles of C=C functions of the TPI in the base (denoted mol% [C=C]).

[0071] In some implementations, the crosslinking reaction is initiated photochemically.

[0072] The initiator comprises, for example, 2-hydroxy-2-methylpropiophenone (“Darocur1 1 73”), 1-hydroxy-cyclohexyl-phenyl-ketone (“Irgacure 1 84”). The rate of photochemical initiator is advantageously between 2 and 10 molar percent (mol%) relative to the number of moles of SH functions in the base (denoted %mol / [SH]).

[0073] In certain particular implementations, the crosslinker is hexanedithiol, the initiator rate being between 2 and 10 mol percent (mol%) relative to the number of moles of thiol functions in the base, the rate of thiol functions being between 40 and 80 mol% relative to the number of moles of alkene functions.

[0074] In some implementations, the crosslinking reaction is initiated thermally.

[0075] The thermal initiator comprises, for example, azobisisobutyronitrile (AI BN), benzoyl peroxide (POB), dicyclohexyl peroxydicarbonate (PCDH) or 1,1'-azobis(cyclohexanecarbonitrile) (marketed under the name "Vazo 88"), or a mixture of these initiators.

[0076] Advantageously, the initiator rate is between 2 and 20 mol% relative to the number of moles of thiol functions.

[0077] In certain particular implementations, the crosslinker is hexanedithiol, the initiator level being advantageously between 2 and 10 mol% relative to the number of moles of thiol functions, the level of thiol functions being between 20 and 60 mol% relative to the number of moles of alkene functions.

[0078] Advantageously, the base is chrome-free and / or epoxy synthetic resin-free.

[0079] According to a second aspect, there is provided a base as presented above, the base being formed before application, the trans(1,4)polyisoprene, the crosslinker and the initiator being contained in a mixture containing a solvent, the solvent advantageously being an aromatic solvent, advantageously xylene.

[0080] In other implementations, the base is formed upon application, by mixing the trans(1,4) polyisoprene from a first container, with a crosslinker and an initiator from a second container.

[0081] According to a third aspect, there is provided a method of applying a base as presented above, to a metal part, the base being crosslinked, the main component of the base being a trans (1,4) polyisoprene, the base comprising a crosslinker and an initiator, the crosslinker being a thiol, the crosslinking of the base forming a polyisoprene which is the polymer matrix of the coating and which is capable of adhering to the metal surface, the crosslinking of the base being photochemical, under ultraviolet and / or visible light, the crosslinking time being between 60 and 300 seconds.

[0082] In certain particular implementations, the crosslinker is hexanedithiol, the initiator level being advantageously from 2 to 10 mol% relative to the number of moles of thiol functions, the level of thiol functions being from 10 to 100 mol% relative to the number of moles of alkene functions.

[0083] The thickness of the base applied to the support is preferably between 30 and 1000 microns, advantageously between 30 microns and 60 microns.

[0084] In some implementations, the application of the base on the metal part is carried out by dip coating, dip-removal, or by spraying.

[0085] In particular implementations, the coating is carried out by several immersions, for example 10 to 20 immersions of the metal part in the base, photochemical crosslinking being carried out between each immersion, making it possible to produce different layers of the crosslinked base on the metal part.

[0086] According to a fourth aspect, there is provided a method of applying a base as presented above, on a metal part, this base being crosslinked, the main component of the base being a trans (1,4) polyisoprene, the base comprising a crosslinker and an initiator, the crosslinker being a thiol, the crosslinking of the base forming a crosslinked polyisoprene which is the polymer matrix of the coating and which is capable of adhering to the metal surface, the crosslinking of the base being thermally initiated, the crosslinking temperature being between 40°C and 120°C.

[0087] In particular implementations, the crosslinking agent is hexanedithiol, the level of thiol functions being advantageously between 20 and 60 mol% relative to the number of moles of alkene function, the level of initiator being between 2 and 20 mol% relative to the number of moles of thiol functions, the crosslinking time being between 2 and 24 h, the crosslinking temperature being between approximately 60°C and 100°C. The application of a base to the metal part is for example carried out by evaporation casting.

[0088] According to another aspect, there is provided a paint system, in particular for aeronautics, comprising a base as presented above, the base being applied directly to a metal part, advantageously an aluminum alloy part, without surface treatment of the latter or addition of a primer adhesion layer. In various embodiments, the paint comprises, in addition to the base, pigments and fillers mixed with the base.

[0089] Advantageously, the paint includes a quantity of pigments and fillers mixed with the base which is between 10% and 70% of the total volume of the paint.

[0090] List of figures

[0091] Other objects and advantages of the invention will appear in the light of the description of embodiments, given below with reference to the appended drawings in which:

[0092] - Figure 1 is a graph representing the variations in the soluble fraction (in %) in dichloromethane CH2CI2, and the variations in the mass setting (in %) at 24 h in dichloromethane of the polyisoprene formed by photochemical crosslinking, as a function of the rate of thiol functions (%SH), the crosslinker being hexanedithiol, the rate of initiator and the duration of irradiation being fixed and constant;

[0093] - Figure 2 is a graph representing the variations in the soluble fraction (in %) in dichloromethane CH2CI2 and the variations in the mass setting (in %) at 24 h in dichloromethane of the polyisoprene formed by photochemical crosslinking, as a function of the duration of ultraviolet irradiation;

[0094] - Figure 3 is a graph representing the variations in the soluble fraction (in %) in dichloromethane CH2CI2, and the variations in the mass setting (in %) at 24 h in dichloromethane of the polyisoprene formed by photochemical crosslinking, as a function of the photoinitiator rate, the crosslinker being hexanedithiol, the rate of thiol functions and the duration of ultraviolet irradiation being fixed and constant parameters;

[0095] - Figure 4 is a graph representing the variations in the soluble fraction (in %) in dichloromethane CH2CI2, and the variations in the mass setting (in %) at 24 h in dichloromethane of the polyisoprene formed by photochemical crosslinking, as a function of the thickness of the film, the crosslinker being hexanedithiol, at fixed initiator level, irradiation time and level of thiol functions, the initiator level being 2 mol%, the ultraviolet irradiation time being 180 seconds, the level of thiol functions being 70 mol%;

[0096] - Figure 5 is a graph representing the variations in the soluble fraction (in %) in dichloromethane CH2CI2, and the variations in the mass setting (in %) at 24 h in dichloromethane of the polyisoprene formed by thermal crosslinking, as a function of the rate of thiol functions (% SH), the crosslinker being hexanedithiol, at constant initiator rate, temperature and reaction time, the initiator rate being 10 mol%, the temperature being 80°C, the reaction time being 16 h;

[0097] - Figure 6 is a graph representing the variations in the soluble fraction (in %) in dichloromethane CH2CI2, and the variations in the mass setting (in %) at 24 h in dichloromethane of the polyisoprene formed by thermal crosslinking, as a function of the reaction temperature, the crosslinker being hexanedithiol, with the initiator rate, the reaction time, and the rate of fixed and constant thiol functions;

[0098] - Figure 7 is a graph representing the variations in the soluble fraction (in %) in dichloromethane CH2CI2, and the variations in the mass setting (in %) at 24 h in dichloromethane of the polyisoprene formed by thermal crosslinking, as a function of the reaction time, the crosslinker being hexanedithiol, for constant reaction parameters such as the initiator level, the level of thiol functions and the temperature;

[0099] - Figure 8 is a graph representing the variations in the soluble fraction (in %) in dichloromethane CH2CI2, and the variations in the mass setting (in %) at 24 h in dichloromethane of the polyisoprene formed by thermal crosslinking, as a function of the initiator level, the crosslinker being hexanedithiol, with the level of thiol functions, the temperature, the reaction time fixed;

[0100] - Figure 9 is a histogram showing the values ​​of soluble fraction (in %) in dichloromethane CH2CI2, of the polyisoprene formed by crosslinking, using as crosslinker hexanedithiol (R-SH) or 1,4 butanediol bis (thi 0 g lyco I ate) (R-OOC-SH), the crosslinking being carried out thermally (EUG-SH Th), or being carried out photochemically, with ultraviolet radiation (EUG-SH-UV);

[0101] - Figure 10 is a histogram showing the mass gain values ​​(in %) in dichloromethane CH2CI2, of the polyisoprene formed by crosslinking, using as crosslinker hexanedithiol (R-SH) or 1,4 butanediol bis (thi 0 g lyco I ate) (R-OOC-SH), the crosslinking being carried out thermally (EUG-SH Th), or being carried out photochemically, with ultraviolet or visible radiation, (EUG-SH UV);

[0102] - Figure 1 1 is a graph showing the film thickness values ​​obtained, as a function of the number of immersions during a dip coating, on the one hand with photochemical crosslinking, the base being a poly(1,4 trans-isoprene) (EUG-SH UV), and on the other hand with photochemical crosslinking, the base being an epoxidized poly(1,4 trans-isoprene) (EEUG-SH UV);

[0103] - figure 1 2 is a sectional diagram of an aluminum alloy support onto which is cast a mixture comprising a reaction mixture, comprising poly(1,4 trans-isoprene), a thiol crosslinker (SH), an initiator and a solvent, the mixture being placed in a housing delimited by a polytetrafluoroethylene border, to carry out a deposition by evaporation casting;

[0104] - Figure 13 is a table showing the inhibition times, in hours, of different coatings, during potentiodynamic measurements, two coatings obtained by thermo-crosslinking of poly(1,4 trans-isoprene) (EUG-SH Th, EEUG-SH Th) being compared to two commercial coatings (PAC33, P60);

[0105] - Figure 14 is a graph showing the variations of the intensity Icorr, in micro amperes, as a function of time, in hours, of different coatings, during potentiodynamic measurements, two coatings obtained by thermo crosslinking of poly(1,4 trans-isoprene) (EUG-SH Th, EEUG-SH Th) being compared to a surface treatment (SAO), and two commercial coatings (PAC33, P60);

[0106] - Figure 15 is a table similar to Figure 13, showing the inhibition times, in hours, of different coatings, during potentiodynamic measurements, two coatings obtained by photocrosslinking of poly(1,4 trans-isoprene) (EUG-SH UV, EEUG-SH UV) being compared to two commercial coatings (PAC33, P60); - Figure 16 is a graph similar to Figure 14, showing the variations of the intensity Icorr, in micro amperes, as a function of time, in hours, of different coatings, during potentiodynamic measurements, two coatings obtained by photocrosslinking of poly(1,4 trans-isoprene) (EUG-SH UV, EEUG-SH UV) being compared to a surface treatment (SAO), and two commercial coatings (PAC33, P60);

[0107] - Figure 17 shows the state of the samples tested by potentiodynamic polarization, the samples being 2024 T3 aluminum alloy plates, provided with conventional treatments on the one hand and coated according to the invention on the other hand;

[0108] - Figure 18 shows the appearance of the specimens during a salt spray test according to ASTM B1 17, the specimens being coated with a layer of thermo-crosslinked poly(1,4 trans-isoprene) (EUG-SH Th), this layer having a thickness of 56 microns;

[0109] - figure 1 9 is similar to figure 1 8, the test pieces being made of 2024 T3 aluminum alloy, and being coated with a layer of photo-crosslinked poly(1,4 trans-isoprene) (EUG-SH UV), this layer having a thickness of 34 microns;

[0110] - figure 20 is similar to figure 18, the test pieces being made of 2024 T3 aluminum alloy, and being coated with a layer of partially epoxidized, photo-crosslinked poly(1,4 trans-isoprene) (EE UG-SH UV), this layer having a thickness of 35 microns;

[0111] - Figure 21 shows the visual appearance of test pieces after adhesion testing according to ISO 2409.

[0112] Detailed description

[0113] In the rest of this description:

[0114] P60 designates a product marketed by the company Akzo Nobel, this product being a water-dilutable base for paint systems used in aeronautics, based on epoxy;

[0115] PAC33 designates a product marketed by the company PPG, this product being a solvent-based polyurethane-based paint system used in aeronautics;

[0116] - % mol denotes the molar percentage. The soluble fraction and the solidification in dichloromethane CH2CI2 of the obtained thiol crosslinked polyisoprene are shown in Figures 1 to 4 (photochemical crosslinking) and Figures 5 to 8 (thermal crosslinking). The lower the soluble fractions and / or the lower the swelling in dichloromethane, the more the polymer is crosslinked.

[0117] Soluble fraction values ​​appear as squares in Figures 1 to 8, and bulking values ​​as circles.

[0118] The proposed anti-corrosion coatings are obtained by crosslinking poly(1,4 trans-isoprene) (TPI), for example contained in Eucommia gum (EUG Eucommia Ulmoides Gum).

[0119] The thiol-ene crosslinking reaction can be presented as follows:

[0120] This crosslinking reaction allows the formation of a thiol crosslinked polyisoprene, advantageously insoluble, from TPI, a thiol crosslinker, in the presence of an initiator.

[0121] The reaction is carried out in the presence of a solvent, for example an aromatic solvent such as xylene.

[0122] Poly(1,4 trans-isoprene) is advantageously derived from Eucommia.

[0123] In some implementations, poly(1,4 trans-isoprene) is partially epoxidized.

[0124] In various implementations, the crosslinker is a multifunctional aliphatic thioalcohol whose aliphatic chain length can vary from 2 to 12 methylene groups, for example ethanedithiol or hexanedithiol.

[0125] The crosslinker may also be a multifunctional aliphatic thioalcohol whose aliphatic chain length may vary from 4 to 20 methylene groups, for example 1,4-butanediol bis(thioglycolate) or trimethylolpropane tris(3-mecraptopriopionate). The crosslinker may also be a mixture of these thiols.

[0126] The thiol crosslinker is for example chosen from the group comprising hexanedithiol (CAS 1191-43-1), 1,4-butanediol bis(thioglycolate) (CAS 10193-95-0), trimethylolpropane tris(3-mercaptopropionate) (CAS 33007-83-9), or a mixture of these thiols.

[0127] In other implementations, the thiol crosslinker is n-octyl mercaptan (CAS 111-88-6), isooctyl mercaptoacetate (CAS 251-03-09-7).

[0128] In some implementations, the crosslinking reaction is initiated thermally, the thermal initiator comprising, for example, azobisisobutyronitrile (AI BN), benzoyl peroxide (POB), dicyclohexyl percarbonate (PCDH) or 1,1'-azobis(cyclohexane carbonitrile (for example marketed under the name "Vazo 88"), or a mixture of these initiators.

[0129] Alternatively, the crosslinking reaction is initiated photochemically, in particular under ultraviolet and / or visible light, the initiator being, for example, 2-hydroxy-2-methylpropiophenone (CAS 7473-98-5), marketed under the reference “Darocur1 1 73” or 1-Hydroxy-cyclohexyl-phenyl-ketone (CAS 947-19-3), marketed under the reference “Irgacure 184”.

[0130] Examples of implementation will now be described, as well as the properties of the products obtained.

[0131] 1 - Photochemical crosslinking

[0132] 1 - 1 Effect of the rate of thiol functions

[0133] Figure 1 is a graph representing the variations of the soluble fraction (in %) in dichloromethane CH2CI2, and the variations of the mass setting (in %) at 24 h in dichloromethane of the polyisoprene formed by photochemical crosslinking, as a function of the rate of thiol functions (%SH), the crosslinker being hexanedithiol, with a fixed rate of initiator and a fixed duration of irradiation. The initiator is 2-hydroxy-2-methylpropiophenone, marketed under the reference “Darocur1 1 73”.

[0134] An insoluble polyisoprene is obtained for thiol function levels between 10 and 100 mol% / [C=C]. The soluble fraction is substantially constant for thiol function level values ​​between 10 and 100 mol%. The mass setting values ​​decrease when the thiol function level increases.

[0135] 1-2 Effect of ultraviolet irradiation duration

[0136] Figure 2 is a graph representing the variations of the soluble fraction (in %) in dichloromethane CH2CI2 and the variations of the mass setting (in %) at 24 h in dichloromethane of the polyisoprene formed by photochemical crosslinking, as a function of the duration of ultraviolet irradiation.

[0137] The crosslinker is hexanedithiol with a fixed thiol function rate between 10 and 100 mol%.

[0138] The initiator is 2-hydroxy-2-methylpropiophenone, marketed under the reference “Darocurl 173”, with an initiator level of between 10 and 100% mol.

[0139] An insoluble polyisoprene is obtained for ultraviolet irradiation times between 60 and 300 seconds.

[0140] The soluble fraction is approximately constant for irradiation times between 60 and 300 seconds. The mass gain values ​​decrease as the irradiation time increases.

[0141] 1-3 Effect of the photoinitiator rate

[0142] Figure 3 is a graph representing the variations of the soluble fraction (in %) in dichloromethane CH2CI2, and the variations of the mass setting (in %) at 24 h in dichloromethane of the polyisoprene formed by photochemical crosslinking, as a function of the rate of photoinitiator, the crosslinker being hexanedithiol, the rate of thiol functions and the duration of ultraviolet irradiation being fixed parameters.

[0143] The initiator is 2-hydroxy-2-methylpropiophenone, marketed under the reference “Darocurl 173”.

[0144] An insoluble polyisoprene is obtained for photoinitiator rates between 2 and 10% mol / [SH],

[0145] The soluble fraction is substantially constant for photoinitiator levels between 2 and 10 mol%.

[0146] 1-4 Effect of the of the film before crosslinking Figure 4 is a graph representing the variations in the soluble fraction (in %) in dichloromethane CH2CI2, and the variations in the mass setting (in %) at 24 h in dichloromethane of the polyisoprene formed by photochemical crosslinking, as a function of the thickness of the film, the crosslinker being hexanedithiol, at fixed initiator level, irradiation time and level of thiol functions.

[0147] The initiator is 2-hydroxy-2-methylpropiophenone, marketed under the reference “Darocur1 173”.

[0148] For a film thickness of 1000 microns, the solidification rate is around 750%, compared to around 250% for a film thickness of 50 microns. The efficiency of photopolymerization is increased when the film thickness is reduced.

[0149] 2- Thermal crosslinking

[0150] 2- 1 Effect of the rate of thiol functions

[0151] Figure 5 is a graph representing the variations in the soluble fraction (in %) in dichloromethane CH2CI2, and the variations in the mass setting (in %) at 24 h in dichloromethane of the polyisoprene formed by thermal crosslinking, as a function of the rate of thiol functions (% SH), the crosslinker being hexanedithiol, at constant initiator rate, temperature and reaction time.

[0152] The initiator is azobisisobutyronitrile (AI BN).

[0153] An insoluble polyisoprene is obtained for thiol function rates between 25 and 100% mol / [C=C].

[0154] The soluble fraction remains approximately constant, around 20%. Mass setting decreases sharply when the thiol function level increases. Increasing the thiol crosslinker level leads to a decrease in mass setting.

[0155] 2-2 Effect of temperature

[0156] Figure 6 is a graph representing the variations of the soluble fraction (in %) in dichloromethane CH2CI2, and the variations of the mass setting (in %) at 24 h in dichloromethane of the polyisoprene formed by thermal crosslinking, as a function of the reaction temperature, the crosslinker being hexanedithiol with the initiator rate, the reaction time, and the rate of thiol functions constant. The initiator is azobisisobutyronitrile (AI BN).

[0157] As the reaction temperature increases, especially between 40°C and 60°C, the soluble fraction decreases sharply.

[0158] 2-3 Effect of reaction time

[0159] Figure 7 is a graph representing the variations of the soluble fraction (in %) in dichloromethane CH2CI2, and the variations of the mass setting (in %) at 24 h in dichloromethane of the polyisoprene formed by thermal crosslinking, as a function of the reaction time, the crosslinker being hexanedithionitrile, for constant reaction parameters such as the initiator level, the level of thiol functions and the temperature. The initiator is azobisisobutyronitrile (AIBN).

[0160] As the reaction time increases, the soluble fraction and the solidification decrease, this decrease being strong when the reaction time increases from 2 to 4 hours.

[0161] 2-4 Effect of primer rate

[0162] Figure 8 is a graph representing the variations of the soluble fraction (in %) in dichloromethane CH2CI2, and the variations of the mass setting (in %) at 24 h in dichloromethane of the polyisoprene formed by thermal crosslinking, as a function of the initiator rate, the crosslinker being hexanedithiol with the rate of thiol functions, the temperature, the reaction time fixed.

[0163] The initiator is azobisisobutyronitrile (AI BN).

[0164] The mass setting and soluble fraction decrease as the initiator level increases, particularly between 2 and 10% mol.

[0165] 3- Influence of the type of thiol crosslinker

[0166] Figures 9 and 10 are respectively histograms showing the values ​​of soluble fraction (in %) and mass gain (in %), in dichloromethane CH2CI2, of the polyisoprene formed by crosslinking, using as crosslinker hexanedithiol (R-SH) or 1,4 butanediol bis(thioglycolate) (R-OOC-SH), the crosslinking being carried out thermally (EUG-SH Th), or being carried out photochemically, with ultraviolet radiation (EUG-SH UV).

[0167] The tests were carried out for films with a thickness of 1000 microns. The change of thiol crosslinker does not affect the values ​​of soluble fraction and solids obtained, for thermal crosslinking. For photochemical crosslinking, the change from hexanedithiol to 1,4 butanediol bis(thioglycolate) leads to an increase in the soluble fraction and a decrease in solids.

[0168] 4- Influence of the number of immersions during dip coating In order to obtain a film of controlled thickness, the part supporting the film, for example a metal plate in 2024T3 aluminum alloy, is immersed in a bath containing the reaction mixture (isoprene, crosslinker, initiator).

[0169] The part is immersed in the bath using a dip-coating process.

[0170] Dip-and-remove involves immersing the substrate to be coated in a solution, and removing the substrate, for example vertically, at a constant speed. Dip-and-remove allows a thin layer of solution to be deposited on both sides of a plate.

[0171] The thickness of the film deposited by dip-shrinking depends on the shrinkage speed. In some applications, shrinkage is carried out at a constant speed.

[0172] In particular implementations, the immersion time is 5 seconds and the part is illuminated with ultraviolet light for a period of 30 seconds between each immersion.

[0173] Figure 1 1 is a graph showing the film thickness values ​​obtained, as a function of the number of immersions during dip coating, the film being formed from poly(1,4 trans-isoprene) (EUG-SH UV), or the film being formed from an epoxidized poly(1,4 trans-isoprene) (EEUG-SH UV).

[0174] Similar film thickness values ​​are obtained from either epoxidized or non-epoxidized TPI.

[0175] The thickness of the final film obtained increases with the number of immersions, a thickness of 50 microns being obtained after 15 immersions.

[0176] In a particular example of photocuring with dip immersion, the parameters are:

[0177] Rate of thiol functions [SH] / [C=C]: 93%

[0178] Primer rate [Darocur] / [SH]: 10% Number of immersions: 15

[0179] Irradiation time: 135 seconds (15*5s + 2*30s)

[0180] Drying: 24 hours

[0181] The resulting material has the following properties:

[0182] Thickness: 50 microns

[0183] Fraction soluble in dichloromethane: 14%

[0184] Mass setting in dichloromethane, after 24 hours: 360%

[0185] Water recovery after 24 hours: 15%

[0186] In other words, it is advantageous that the quantity of thiol functions is equivalent to 93% of the molar quantity of TPI introduced and that the quantity of initiator is 10% of the molar quantity of thiol crosslinker introduced.

[0187] 5- Evaporation casting

[0188] Figure 12 is a cross-sectional diagram of an aluminum alloy support onto which a reaction mixture is cast, comprising poly(1,4 trans-isoprene), a thiol crosslinker (SH), an initiator and a solvent, the mixture being placed in a housing delimited by a polytetrafluoroethylene (Teflon®) border, to form an evaporative casting deposit.

[0189] The bracket is made of 2024 T3 aluminum alloy.

[0190] The Teflon® border extends around the perimeter of the aluminum alloy part, to a height of 2 mm.

[0191] After pouring the mixture, the crosslinking reaction is carried out for 16 hours, at a temperature of 80°C.

[0192] A film with a thickness of 56 microns is obtained, with a mixture comprising poly(1,4 trans-isoprene) (EUG-SH Th).

[0193] A film with a thickness of 45 microns is obtained, with a mixture comprising partially epoxidized poly(1,4 trans-isoprene) (EEUG-SH Th).

[0194] In a particular example of evaporative casting deposition, the parameters are:

[0195] Rate of thiol functions [SH] / [C=C]: 53%

[0196] [AI BN] / [SH] primer rate: 10%

[0197] Crosslinking temperature: 80°C Crosslinking time: 16 hours

[0198] The resulting material has the following properties:

[0199] Thickness: 45 microns

[0200] Fraction soluble in dichloromethane: 1.8%

[0201] Mass setting in dichloromethane, after 24 hours: 520%

[0202] Water recovery after 24 hours: 13%

[0203] 6- Anti-corrosion performance of coatings obtained by thermo-crosslinking of polv(1,4 trans-isoprene)

[0204] Corrosion monitoring was carried out by linear polarization, using an electrochemical cell in hydrochloric acid (2 mol / L).

[0205] The reference electrode is saturated calomel. The measuring device also includes a graphite electrode.

[0206] Linear polarization consists of performing a potential sweep of a few millivolts, in the vicinity of the corrosion potential, and determining the current density.

[0207] The potentiodynamic polarization method is based on the Butler-Volmer relationship, and the Tafel method or the Stern and Geary polarization resistance method.

[0208] Initially, no corrosion is observed. After an inhibition period, the intensity increases and then stabilizes.

[0209] Figure 13 is a table showing the inhibition times, in hours, of different coatings, during potentiodynamic measurements, two coatings obtained by thermo-crosslinking of poly(1,4 trans-isoprene) (EUG-SH Th, EEUG-SH Th) being compared to two commercial coatings (PAC33, P60).

[0210] Figure 14 is a graph showing the variations of the intensity Icorr, in microamperes, as a function of time, in hours, of different coatings, during potentiodynamic measurements, two coatings obtained by thermo-crosslinking of poly(1,4 trans-isoprene) (EUG-SH Th, EEUG-SH Th) being compared to a surface treatment (SAO), and two commercial coatings (PAC33, P60).

[0211] The coatings were deposited on a 2024 T3 aluminum alloy support.

[0212] The EUG-SH Th coating has a thickness of 56 microns, and is prepared as described with reference to Figure 12. The EEUG-SH Th coating has a thickness of 56 microns, and is prepared as described with reference to Figure 12.

[0213] The SAO surface treatment is sulfuric anodizing, or sulfuric anodic oxidation. With this type of surface treatment, immediate corrosion is observed during potentiodynamic polarization tests.

[0214] The PAC33 product marketed by PPG is a solvent-based polyurethane paint system used in aeronautics. This PAC33 product is used here for comparison purposes, the test sample being a 2024T3 aluminum alloy plate coated with a 56-micron-thick layer of PAC33.

[0215] Akzo Nobel's P60 product is a water-based epoxy-based paint system for aeronautical applications. This P60 product is used here for comparison purposes, with the sample tested being a 2024T3 aluminum alloy plate coated with a 50-micron-thick layer of P60.

[0216] Coatings obtained by thermo-crosslinking of poly(1,4 trans-isoprene), non-epoxidized (EUG-SH Th), or partially epoxidized (EEUG-SH Th) have corrosion resistance properties similar to commercial P60 and PAC33 products.

[0217] 7- Anti-corrosion performance of coatings obtained by photo-crosslinking of poly(1,4 trans-isoprene)

[0218] Figure 15 is a table similar to Figure 13, showing the inhibition times, in hours, of different coatings, during potentiodynamic measurements, two coatings obtained by photocrosslinking of poly(1,4 trans-isoprene) (EUG-SH UV, EEUG-SH UV) being compared to two commercial coatings (PAC33, P60).

[0219] Figure 16 is a graph similar to Figure 14, showing the variations of the intensity Icorr, in micro amperes, as a function of time, in hours, of different coatings, during potentiodynamic measurements, two coatings obtained by thermo crosslinking of poly(1,4 trans-isoprene) (EUG-SH UV, EEUG-SH UV) being compared to a surface treatment (SAO), and two commercial coatings (PAC33, P60).

[0220] The SAO treatment and the PAC33, P60 products are those presented when describing the results in Figures 13 and 14. The reference electrode is saturated calomel. The measuring device also includes a graphite electrode. The working solution contains hydrochloric acid (2 mol / L).

[0221] The coatings were deposited on a 2024 T3 aluminum alloy support.

[0222] The EUG-SH UV coating has a thickness of 56 microns, and is made by photopolymerization of poly(1,4 trans-isoprene).

[0223] The EEUG-SH UV coating has a thickness of 45 microns, and is produced by photopolymerization of poly(1,4 trans-isoprene), partially epoxidized.

[0224] Figure 17 shows the condition of the samples tested by potentiodynamic polarization, the samples being 2024 T3 aluminum alloy plates, provided with conventional treatments on the one hand and coated according to the invention on the other hand.

[0225] The sulfuric acid anodized (SAO) plate was pierced after 7 hours.

[0226] The plate coated with a layer of P60, this layer being 50 microns thick, was pierced in 580 hours.

[0227] The plate coated with a layer of PAC33, this layer being 56 microns thick, was pierced in 504 hours.

[0228] In contrast, the same 2024 T3 aluminum alloy plate, coated with a layer of photo-crosslinked trans 1,4 polyisoprene, this layer having a thickness of 40 microns (EUG-SH UV) is only slightly attacked after 672 hours of testing.

[0229] The same plate in 2024 T3 aluminum alloy, coated with a layer of partially epoxidized, photo-crosslinked trans 1,4 polyisoprene, this layer having a thickness of 50 microns (EEUG-SH UV) is only slightly attacked after 504 hours of testing.

[0230] Figure 1 8 shows the appearance of the specimens during a salt spray test according to ASTM B1 1 7 or ISO9227.

[0231] The test is of the neutral salt spray type (NaCI, concentration 45 g / L), the pH being between 6.5 and 7.2, the chamber being at a pressure of 1 bar and the temperature being 35°C.

[0232] The specimens seen in Figure 18 are made of 2024 T3 aluminum alloy, and are coated with a layer of thermo-crosslinked poly(1,4 trans-isoprene) (EUG-SH Th), this layer having a thickness of 56 microns. The visual appearance shows the appearance of pitting on the edges of the specimen, after 336 hours.

[0233] Figure 19 is analogous to Figure 18, the test pieces being made of 2024 T3 aluminum alloy, and being coated with a layer of photo-crosslinked poly(1,4 trans-isoprene) (EUG-SH UV), this layer having a thickness of 34 microns.

[0234] The visual aspect shows the appearance of pitting on the edges of the test piece, after 336 hours.

[0235] Figure 20 is analogous to Figure 18, the test pieces being made of 2024 T3 aluminum alloy, and being coated with a layer of partially epoxidized, photo-crosslinked poly(1,4 trans-isoprene) (EE UG-SH UV), this layer having a thickness of 35 microns.

[0236] The visual appearance shows the absence of corrosion pitting for test durations of 700 hours.

[0237] The table below summarizes the properties of photo-crosslinked, epoxidized or non-epoxidized poly(1,4 trans-isoprene) coatings providing corrosion inhibition protection; these properties are compared to those of the conventional PAC33 product:

[0238] The table below summarizes the properties of thermo-crosslinked poly(1,4 trans-isoprene) coatings, providing protection against substrate corrosion, compared to those of bare 2024 T3 alloy, or coated with conventional P60 product:

[0239] The coatings obtained by photocrosslinking of poly(1,4 trans-isoprene), non-epoxidized (EUG-SH UV), or partially epoxidized (EEUG-SH UV) have corrosion resistance properties superior to those of commercial P60 and PAC33 products: significantly higher inhibition times (figure 1 5), significantly lower corrosion intensities (figure 1 6), significantly higher resistance over time (figure 1 7).

[0240] 8- Adhesion

[0241] Figure 21 shows the visual appearance of specimens after adhesion testing according to ISO 2409.

[0242] The adhesion test is a grid test carried out using a device with claws spaced 1 mm apart, a standardized adhesive being applied and then removed. An adhesion level, from 0 to 5, is obtained depending on the surface of the gridded part affected by the flaking or detachment of the coating.

[0243] An example of a test device is marketed under the reference “Erichsen 295”, by the Erichsen company.

[0244] No delamination is observed for a P60 coating, with a thickness of 50 microns.

[0245] No delamination is observed for a partially epoxidized, photo-crosslinked (EEUG-SH UV) poly(1,4 trans-ispoprene) coating, this coating having a thickness of 35 microns.

[0246] For a PAC33 coating, with a thickness of 56 microns, delamination is observed.

[0247] 9- ACET tests

[0248] Accelerated cyclic electrochemical tests (ACET) were performed.

[0249] The principle of these tests is presented for example by Usman et al. Exploring the use of an AC-DC-AC technique for the accelerated evaluation of anticorrosion performance of anodic films on aluminum alloys, Progress in organic coatings, 144, 2020, 105648).

[0250] Reference may also be made to document ES2317803 (Mediciones y Corrosion, 2009). Reference may also be made to document Pérez (Doctoral studies, Universitat Jaume, 2016) and to the ISO 1 7463:2022 standard. ACET consists of applying measurement cycles of electrochemical impedance spectroscopy (EIS), cathodic polarizations and potential relaxation. The degradation of the coating system is accelerated by cathodic polarization. EIS and potential relaxation allow monitoring the modification of the coating system caused by cathodic polarization. This technique allows evaluating the permeability of the coating and the properties that may be associated with adhesion to the substrate.

[0251] Depending on the evolution of potential relaxation over time, corrosion inhibition (Type A) or substrate corrosion (Type C) can be identified.

[0252] In ACET tests, the evolution of potential relaxation is stable over time, for 2024 T3 aluminum alloy plates coated with a 40 micron thick layer of photo-crosslinked trans(1,4) polyisoprene (EUG-SH UV), showing that the coating inhibits corrosion. No precipitate was observed in the electrolyte.

[0253] An ACET test performed with a 2024 T3 aluminum alloy plate coated with a 50 micron thick P60 layer resulted in decreasing potential relaxation values ​​over time, related to substrate corrosion. A precipitate appeared in the electrolyte during the ACET test, identified as sodium tetrachloroaluminate NaAlCU.

[0254] Advantages of the invention

[0255] The invention offers several advantages.

[0256] Firstly, the coatings obtained by crosslinking poly(1,4 trans-isoprene) are advantageously bio-sourced, poly(1,4 trans-isoprene) being the main component of Eucommia gum. It is also possible to extract poly(1,4) trans isoprene from natural latex obtained from Palaquium gutta leaves, or from Manilkara bidentata latex (balata gum). This provides an alternative to petro-sourced anti-corrosion coatings or paints, for example P60 or PAC33, with anti-corrosion properties similar to these petro-sourced products or surface treatments. Secondly, the coatings obtained by crosslinking poly(1,4 trans-isoprene) are chromium-free, in particular hexavalent or trivalent chromium-free.

[0257] Thirdly, partially epoxidized and advantageously non-epoxidized Eucommia gum is used as the basis for new anti-corrosion coatings. Crosslinking can be achieved by thiol-ene addition. This irreversible reaction can be carried out on the metal support to be treated.

[0258] Crosslinking can be initiated thermally or photochemically. The base can be deposited in a thin layer on the support to be protected, by dip-coating or by evaporation casting, or even by spraying, with control of the thickness of the solution deposited. The thickness of the deposited layer is, for example, of the order of 30 to 60 microns.

[0259] The base-coated part is then placed in an oven or photopolymerization chamber.

[0260] The single-step coating application offers economic benefits, as well as decarbonation benefits (lightening), compared to state-of-the-art paint systems.

[0261] The invention finds advantageous application in aeronautics, in particular for the protection against corrosion of aluminum alloy parts, in particular for parts made of 2024 T3 alloy.

[0262] The invention also finds application in the railway sector, in the construction of motor vehicles and in marine construction.

Claims

AMENDED CLAIMS received by the International Bureau on December 16, 2024 (16.12.2024) 1. Paint system, in particular for aeronautics, comprising: a base capable of being crosslinked to form an anti-corrosion coating of a metal surface, pigments and fillers mixed with the base, a solvent, the main component of the base intended to form the anti-corrosion coating being a trans (1,4) polyisoprene giving the base its anti-corrosion properties, the base further comprising an initiator and a crosslinker which is a thiol, the crosslinking of the base forming a crosslinked polyisoprene which is the polymer matrix of the coating and which is capable of adhering to the metal surface, in particular of an aluminum alloy.

2. Paint system for forming an anti-corrosion coating of a metal surface according to claim 1, characterized in that the main component trans (1,4) polyisoprene represents between 40 and 80% of the total mass of the base, the initiator an amount less than or equal to 2% of the total mass of the base, and the crosslinker the remaining amount of the total mass of the base.

3. Paint system for forming an anti-corrosion coating of a metal surface according to claim 1 or 2, characterized in that the main component trans (1,4) polyisoprene represents between 45 and 60% of the total mass of the base, the initiator an amount less than or equal to 2% of the total mass of the base, and the crosslinker the remaining amount of the total mass of the base.

4. Paint system for forming an anti-corrosion coating of a metal surface according to any one of claims 1 to 3, characterized in that the main component trans (1,4) polyisoprene is the only organic polymer of the base, which is without surfactants.

5. Paint system for forming an anti-corrosion coating of a metal surface according to any one of claims 1 to 4, AMENDED SHEET (ARTICLE 19) characterized in that the main component is a trans (1,4) polyisoprene from Eucommia ulmoides, or natural latex obtained from leaves of Palaquium gutta, or latex from Manilkara bidentata.

6. Paint system for forming an anti-corrosion coating of a metal surface according to any one of claims 1 to 5, characterized in that the main component comprises synthetic trans (1,4) polyisoprene.

7. Paint system for forming an anti-corrosion coating of a metal surface according to any one of claims 1 to 6, characterized in that the trans (1,4) polyisoprene is non-epoxidized.

8. Paint system for forming an anti-corrosion coating of a metal surface according to any one of claims 1 to 6, characterized in that the trans (1,4) polyisoprene is partially epoxidized.

9. Paint system for forming an anti-corrosion coating of a metal surface according to any one of claims 1 to 8, characterized in that the crosslinker is a thiol chosen from the list: a multifunctional aliphatic thioalcohol whose hydrocarbon chain length varies from 2 to 12 methylene groups, for example ethanedithiol or hexanedithiol, or a multifunctional aliphatic thioalcohol whose aliphatic chain length varies from 4 to 20 methylene groups, and contains organic functions of the acetoxy type, for example 1,4-butanediol bis(thioglycolate), or trimeethylolpropane tris(3-mecraptopriopionate), or a mixture of these thiols.

10. Paint system for forming an anti-corrosion coating of a metal surface according to any one of claims 1 to 9, characterized in that the main component is the only component of the base giving the base its anti-corrosion properties and which allows the base, once crosslinked, to adhere to the metal surface, advantageously without surface treatment of the latter. AMENDED SHEET (ARTICLE 19) 1 1. Paint system for forming an anti-corrosion coating of a metal surface according to any one of claims 1 to 10, characterized in that the base is chrome-free.

12. Paint system for forming an anti-corrosion coating of a metal surface according to any one of claims 1 to 11, characterized in that the base is without epoxy synthetic resin.

13. Paint system for forming an anti-corrosion coating of a metal surface according to any one of claims 1 to 12, characterized in that the crosslinker is hexanedithiol 14. Paint system for forming an anti-corrosion coating of a metal surface according to any one of claims 1 to 13, characterized in that the level of thiol functions is between 10 and 100 mol% relative to the number of moles of C = C functions in the base.

15. Paint system for forming an anti-corrosion coating of a metal surface according to any one of claims 1 to 14, characterized in that the crosslinking reaction is initiated photochemically, and in that the initiator level is between 2 and 10 mol% relative to the number of moles of SH functions in the base, the initiator being for example chosen from the group comprising 2-hydroxy-2-methylpropiophenone or 1-hydroxy-cyclohexyl-phenyl-ketone, the level of thiol functions being for example between 40 and 80 mol% relative to the number of moles of alkene functions.

16. Paint system for forming an anti-corrosion coating of a metal surface according to any one of claims 1 to 14, characterized in that the crosslinking reaction is initiated thermally and in that the initiator level is between 2 and 20 mol% relative to the number of moles of thiol functions, AMENDED SHEET (ARTICLE 19) and in that the initiator is chosen for example from the group comprising azobisisobutyronitrile (AI BN), benzoyl peroxide (POB), peroxide dicyclohexyl peroxydicarbonate (PCDH), or 1,1'-azobis(cyclohexane carbonitrile, or a mixture of these initiators, and in that the initiator level is between 2 and 10 mol% relative to the number of moles of thiol functions, the level of thiol functions being for example between 20 and 60 mol% relative to the number of moles of alkene functions.

17. A paint system according to any one of claims 1 to 16, characterized in that the paint comprises an amount of pigments and fillers mixed with the base which is between 10% and 70% of the total volume of the paint.

18. Method for producing an anti-corrosive coating by applying a paint system comprising a base intended to form the anti-corrosion coating as presented in any one of claims 1 to 17, on a metal part, this base being crosslinked, the main component of the base being a trans (1,4) polyisoprene, the base comprising a crosslinker and an initiator, the crosslinker being a thiol, the crosslinking of the base forming a crosslinked polyisoprene which is the polymer matrix of the coating and which is capable of adhering to the metal surface, characterized in that the crosslinking of the base is photochemical, under ultraviolet and / or visible light, the crosslinking time being between 60 and 300 seconds, the base being for example applied directly to a metal part, without surface treatment of the latter or addition of a primer adhesion layer, advantageously for example an aluminum alloy part.

19. Method according to claim 1 8, characterized in that the thickness of the base applied to the metal part is between 30 and 1000 microns, advantageously between 30 microns and 60 microns, and in that the application of the base to the metal part is carried out for example by coating by dipping, or by spraying or by casting evaporation. AMENDED SHEET (ARTICLE 19) 20. Method according to claim 19, characterized in that the coating is carried out by several immersions, for example 10 to 20 immersions of the metal part in the base, photochemical crosslinking being carried out between each immersion making it possible to produce different layers of the crosslinked base on the metal part.

21. Method for applying a base as presented in any one of claims 1 to 17, on a metal part, this base being crosslinked, the main component of the base being a trans (1,4) polyisoprene, the base comprising a crosslinker and an initiator, the crosslinker being a thiol, the crosslinking of the base forming a crosslinked polyisoprene which is the polymer matrix of the coating and which is capable of adhering to the metal surface, characterized in that the crosslinking of the base is thermally initiated, the crosslinking temperature being between 40°C and 120°C, the base being for example applied directly to a metal part, without surface treatment of the latter or addition of a primer adhesion layer, advantageously for example an aluminum alloy part. AMENDED SHEET (ARTICLE 19)

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