Coating composition for enhancing protective layer

Abstract

The present invention relates to a coating composition comprising: a) a resin system comprising an organic film-forming resin and optionally a curing agent reactive with the organic film-forming resin; b) a lithium salt having a solubility in water of 0.01-120 g / L at 20° C., selected from lithium carbonate, lithium phosphate, lithium bicarbonate, lithium tetraborate, and lithium oxalate, in an amount of at least 1.3% by weight of lithium, based on the weight of the solids of the resin system; and c) a zinc salt of 2,5-dimercapto-1,3,4-thiadiazole (DMTD). The zinc salt of DMTD has been shown to have a synergistic effect on enhancing the barrier function of the protective layer formed on coating defects due to the presence of the lithium salt in the coating composition.

Description

Field of the Invention

[0001] The present invention relates to a coating composition having protective properties, particularly for use on metal substrates.

[0002] background

[0003] Protective coatings are widely used to protect substrates, particularly metal substrates, from corrosion. Hexavalent chromium compounds have long been used as corrosion inhibitors in protective and conversion coatings for metal surfaces. However, hexavalent chromium is toxic and is being phased out for environmental, worker safety, and regulatory reasons. Alternative chromium-free inhibitors have been proposed in recent years, but many formulations struggle to meet industry corrosion resistance standards.

[0004] Despite great progress in chromium replacement in anti-corrosion coatings, there remains a need to provide Cr-free coatings with improved and long-term corrosion resistance properties comparable to or better than conventional Cr-containing coatings.

[0005] In recent years, coating compositions containing lithium salts have been proposed as alternatives to Cr-containing coatings. For example, see Visser, P., Liu, Y., Terryn, H. et al., “Lithium salts as leachable corrosion inhibitors and potential replacement for hexavalent chromium in organic coatings for the protection of aluminum alloys,” J Coat Technol Res 13, 557-566 (2016). As described in the article, coating compositions containing lithium salts have been shown to have anti-corrosion activity by forming a protective layer on bare metal at locations where the coating deposited from the coating composition containing the lithium salt has defects. This layer was found to contain aluminum, oxygen, and also lithium leached from the coating. The protective layer provides a barrier between the metal surface in the coating defects and the corrosive environment. The barrier function and strength of the protective layer may be important for long-term protection. Insufficient barrier function or low strength of the protective layer formed may lead to localized metal defects and corrosion.

[0006] It would be desirable to provide coating compositions having improved barrier properties. SUMMARY OF THE INVENTION

[0008] The inventors discovered that if the coating composition also contains a zinc salt of 2,5-dimercapto-1,3,4-thiadiazole (DMTD), the protective layer formed in the coating defects due to the action of the lithium salt exhibits significantly higher barrier function, as observed by electrochemical impedance spectroscopy (EIS). The zinc salt of DMTD acts as an enhancer of the barrier function of the protective layer formed on the exposed metal surface.

[0009] Therefore, the present invention provides a coating composition in a first aspect, comprising:

[0010] a) a resin system comprising an organic film-forming resin and optionally a curing agent reactive with the organic film-forming resin,

[0011] b) a lithium salt having a solubility in water at 20° C. in the range of 0.01 to 120 g / L, selected from lithium carbonate, lithium phosphate, lithium bicarbonate, lithium tetraborate and lithium oxalate, in an amount of at least 1.3 wt % lithium based on the solids weight of the resin system, and

[0012] c) a zinc salt of 2,5-dimercapto-1,3,4-thiadiazole (DMTD), wherein the zinc salt of DMTD is 2,5-dimercapto-1,3,4-thiadiazole zinc salt (VII).

[0013] In a second aspect, the present invention provides a multi-layer coating system on a metal substrate, wherein the coating system comprises a layer obtained from the coating composition according to the first aspect of the present invention.

[0014] In another aspect, the present invention provides a metal substrate coated with a coating composition according to the first aspect of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 and 2 Shown are EIS measurement results of coating compositions according to some embodiments of the present invention. Detailed Description of the Invention

[0018] Surprisingly, it was discovered that the use of a zinc salt of DMTD in a coating composition comprising a lithium salt selected from lithium carbonate, lithium phosphate, lithium bicarbonate, lithium tetraborate, and lithium oxalate enhances the protective layer formed by lithium ions of the lithium salt in coating defects on the metal surface to which the coating composition is applied. The formation of a protective layer by lithium ions has previously been shown to be the anti-corrosion mechanism of lithium salts. When coating defects are present in the cured coating such that the metal substrate is exposed and susceptible to corrosion, lithium ions leach from the coating to the metal surface and contribute to the formation of a protective layer on the surface. The presence of the protective layer can be determined and its barrier properties can be quantified using electrochemical impedance spectroscopy (EIS). See Visser, P., Liu, Y., Terryn, H. et al. “Lithium salts asleachable corrosion inhibitors and potential replacement for hexavalent chromium in organic coatings for the protection of aluminum alloys,” J Coat Technol Res 13, 557-566 (2016). This electrochemical method allows quantifying the barrier performance of the protective layer in damaged areas (scribes) resulting from the active protection mechanism (leaching) of the coating after exposure to accelerated corrosion tests (e.g. neutral salt spray exposure).

[0019] The magnitude of the barrier function is obtained from electrochemical impedance spectroscopy. The impedance modulus graph shows the impedance modulus (Ωcm) as a function of the frequency range (Hz). 2 The increase in the impedance modulus in the mid-frequency range of 10 Hz may be associated with the formation of a protective layer in the damaged area (scribe line). The increase in the impedance modulus at low frequencies (10 mHz) may be associated with the suppression of corrosion processes at the substrate. An important advantage of using EIS is that it allows the quantification of differences between coating formulations that cannot be observed or quantified using visual methods.

[0020] The inventors of the present invention have tested many compounds and found an enhancer that allows the protective layer formed from the lithium salt to exert a significantly higher barrier function. The inventors believe that this enhanced protective layer provides better and / or longer-term protection for the metal substrate. Higher barrier function here refers to a higher impedance modulus value (Ωcm) for the same frequency range (Hz) as measured by EIS. 2 ) and, in particular, higher impedance modulus values ​​for frequencies of 10 Hz and 10 mHz. This enhancement effect is synergistic, meaning that it is much higher than would be expected based on the individual components. In particular, the effect is preferably at least 2 times, more preferably at least 3 times, higher than the effect of the individual components (in this case, the lithium salt and the zinc salt of DMTD).

[0021] The coating composition of the present invention is preferably chromium-free. "Chromium-free" means that it does not contain any Cr compounds, in particular Cr(VI) compounds such as chromates.

[0022] The coating composition of the present invention comprises a) a resin system comprising an organic film-forming resin and optionally a curing agent reactive with the organic film-forming resin. In this specification, the term "film-forming resin" includes polymers, but also monomers or oligomers that form the polymer system during the curing process of the coating. An organic film-forming resin refers to a polymer, monomer, or oligomer that is organic in nature (carbon-containing compound). The coating composition preferably does not contain polysiloxane. The coating composition is preferably not a sol-gel composition.

[0023] The film-forming resin may be selected from, for example, epoxy resins, hydroxyl-functional resins (such as polyesters and poly(meth)acrylates), resins having one or more blocked hydroxyl groups (such as acetals), Oxazolidine resins, carboxylic acid-functional resins, polyacrylates, polyurethanes, polyethers, polyaspartates, (blocked) isocyanates, thiol-functional resins, amine-functional resins, amide-functional resins, imide-functional resins (e.g. maleimides), alkyd resins, resins containing at least one ethylenically unsaturated bond, resins containing silanes, polysiloxane resins, acetoacetate resins, functional (curable) fluorinated resins and mixtures and hybrids thereof.

[0024] Preferably, the film-forming resin is selected from epoxy resins and hydroxyl-functional resins such as hydroxyl-functional poly(meth)acrylates or polyester polyols and polyurethanes. Epoxy resins are epoxy-functional polymers having an epoxy equivalent weight greater than 1 and typically about 2. The most commonly used epoxy-functional polymers are cyclic polyols such as polyglycidyl ethers of bisphenol A, resorcinol, hydroquinone and catechol, or polyglycidyl ethers of polyols such as 1,2-cyclohexanediol, 1,4-cyclohexanediol and 1,2-bis(hydroxymethyl)cyclohexane.

[0025] The hydroxy-functional resin preferably has a hydroxy functionality of 2.1 to 3.5 and an equivalent weight of at least 200 g / mol.

[0026] Epoxies and polyurethanes are preferred film-forming resins for use in the compositions of the present invention.

[0027] The film-forming resin is preferably present in the coating composition of the present invention in an amount of 1-98.7 wt %, more preferably 10-90 wt %, still more preferably 20-80 wt %, based on the total weight of the non-volatile components of the coating composition.

[0028] The coating composition may contain a curing agent that is reactive with the film-forming resin. The curing agent comprises functional groups that are reactive with the functional groups of the resin. The type of curing agent will depend on the nature of the film-forming resin. Suitable curing agents for specific film-forming resins are common knowledge to those skilled in the art. For example, acetoacetate resin-based coating compositions preferably contain a ketimine-based curing agent.

[0029] The composition containing epoxy resin preferably contains aliphatic or aromatic amine curing agents, polyamide curing agents or thiol-based curing agents. Suitable epoxy resins are bisphenol A, bisphenol F, bisphenol A / F, novolacs and aliphatic epoxy resins. Suitable amine curing agents are aliphatic amines and their adducts (e.g. 2021), phenalkamine, alicyclic amines (e.g. 2196), amidoamines (e.g. 2426), polyamides and adducts thereof, and mixtures thereof. The epoxy / NH molar ratio in epoxy-amine type coating compositions is preferably in the range of 0.6-2.0, more preferably 0.8-1.7. For solventborne epoxy-amine coating compositions, the epoxy / NH molar ratio is preferably in the range of 0.6-1.4, more preferably 0.8-1.2, and most preferably in the range of 0.85-1.1. For waterborne coating compositions, the epoxy / NH molar ratio is preferably in the range of 0.6-2.0, more preferably 0.9-1.7, and most preferably in the range of 1.3-1.7.

[0030] Preferred curing agents for hydroxy-functional resins are isocyanates and isocyanurates. Suitable isocyanate curing agents are aliphatic, cycloaliphatic and aromatic polyisocyanates, such as trimethylene diisocyanate, 1,2-propylene diisocyanate, tetramethylene diisocyanate, 2,3-butylene diisocyanate, hexamethylene diisocyanate, octamethylene diisocyanate, 2,4-trimethylhexamethylene diisocyanate, 2,4,4-trimethylhexamethylene diisocyanate, dodecamethylene diisocyanate, α,α′-dipropyl ether diisocyanate, 1,3-cyclopentylene diisocyanate, 1,2-cyclohexylene diisocyanate, 1,4-cyclohexylene diisocyanate, 4-methyl-1,3-cyclohexylene diisocyanate, 4,4′-dicyclohexylene diisocyanate, 3,3′-dimethyl-4,4′-dicyclohexylene diisocyanate, 1,3-diisocyanate, 1,2-diisocyanate, 1,4-diisocyanate, 1,3-diisocyanate, 4,4′-diisocyanate, 1,2-diisocyanate, 1,4-diisocyanate, 1,2-diisocyanate, 1,4-diisocyanate, 1,3-diisocyanate, 4,4′-diisocyanate, 1,3 ...3-diisocyanate, 1,2-diisocyanate, 1,4-diisocyanate, 1,2-diisocyanate, 1,4-diiso Hexyl diisocyanate methane, m- and p-phenylene diisocyanate, 1,3- and 1,4-bis(isocyanate methyl)benzene, 1,5-dimethyl-2,4-bis(isocyanate methyl)benzene, 1,3,5-triisocyanate benzene, 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, 2,4,6-toluene triisocyanate, α,α,α',α'-tetramethyl-o-, m- and p-xylylene diisocyanate, 4,4'-diphenylene diisocyanate methane, 4,4'-diphenylene diisocyanate, 3,3'-dichloro-4,4'-diphenylene diisocyanate, 1,5-naphthalene diisocyanate, isophorone diisocyanate, trans-vinylidene isocyanate and mixtures of the above polyisocyanates.

[0031] Suitable curing agents are also adducts of polyisocyanates, such as biurets, isocyanurates, allophanates, uretdiones and mixtures thereof. Examples of such adducts are adducts of two molecules of hexamethylene diisocyanate or isophorone diisocyanate with a diol, such as ethylene glycol, the reaction product of three molecules of hexamethylene diisocyanate and one molecule of water, the adduct of one molecule of trimethylolpropane with three molecules of isophorone diisocyanate, the adduct of one molecule of pentaerythritol with four molecules of toluene diisocyanate, the isocyanurate of hexamethylene diisocyanate ( N3390, available from Bayer), uretdione of hexamethylene diisocyanate ( N3400, available from Bayer), allophanate of hexamethylene diisocyanate ( LS2101, available from Bayer) and isocyanurate of isophorone diisocyanate ( T1890, available from Hüls). In addition, isocyanate-functional monomers such as (co)polymers of α,α′-dimethyl-m-isopropenylbenzyl isocyanate are suitable for use. Finally, the above-mentioned isocyanates and their adducts can be present in the form of blocked or latent isocyanates.

[0032] In a preferred embodiment, the film-forming resin is a hydroxyl-functional resin and the curing agent comprises isocyanate-functional groups. In another preferred embodiment, the film-forming resin comprises epoxy-functional groups and the curing agent comprises amine-functional groups.

[0033] The coating composition can be a one-component or two-component (2K) composition, or even a composition having more than two components. Preferably, the composition is a 2K composition. A 2K coating composition consists of two components that are stored separately and mixed just before application to the substrate. Typically, the first component comprises an organic film-forming resin and the second component comprises a curing agent.

[0034] The coating composition of the present invention comprises a lithium salt having limited solubility in water.

[0035] The lithium salt has a solubility in water of at least 0.01, preferably 0.05, more preferably at least 0.1 g / L, measured in water at 20°C. Lithium salts with a solubility of at least 0.5, more preferably at least 1, still more preferably at least 5 g / L show particularly good results. Lithium salts with a solubility of less than 0.01 g / L do not show sufficient activity. The solubility of the lithium salt is less than 120 g / L, preferably less than 100 g / L, more preferably less than 85 g / L. Too high a solubility may cause problems in the formulation of the coating composition, such as an inhomogeneous coating composition, which may cause problems in application. The lithium salt has a solubility in the range of 0.01-120 g / L, measured in water at 20°C. The solubility is preferably in the range of 1-100 g / L. The solubility in water is measured according to OECD Guideline No. 105, EU Method A.6 (Flask Method, Procedure §23), which describes the preparation of a saturated solution and then measuring the concentration in the solution using a suitable analytical method. Suitable analytical methods for determining the concentration of the relevant ions are known to those skilled in the art and can be selected depending on the ion in question. In particular, for lithium salts, inductively coupled plasma mass spectrometry (ICP-OES) methods are used.

[0036] The lithium salt is selected from lithium carbonate, lithium phosphate, lithium bicarbonate, lithium tetraborate and lithium oxalate. Phosphates mentioned herein refer to orthophosphates unless otherwise specified. The solubility of lithium carbonate in water is 13 g / L, lithium phosphate is 0.39 g / L, lithium oxalate is 62 g / L, and lithium tetraborate is 28 g / L, all at 20°C. Preferably, the lithium salt is selected from lithium carbonate, lithium phosphate, lithium oxalate and any mixture of two or more thereof. In some embodiments, lithium carbonate is a preferred salt. It can be used alone or in mixtures with other lithium salts, such as lithium carbonate and lithium phosphate. In other embodiments, lithium phosphate may be preferred.

[0037] Preferably, the coating composition does not contain a lithium salt having a solubility of less than 0.01 g / L at 20° C. In other embodiments, preferably, the coating composition does not contain a lithium salt having a solubility of more than 120 g / L in water at 20° C., such as lithium chloride or lithium nitrate. Preferably, the coating composition does not contain any lithium salt other than lithium carbonate, lithium phosphate, lithium bicarbonate, lithium tetraborate, and lithium oxalate.

[0038] The lithium salt selected from lithium carbonate, lithium phosphate, lithium bicarbonate, lithium tetraborate and lithium oxalate is present in the coating composition in an amount of at least 1.3% by weight, preferably at least 1.6% by weight, more preferably at least 2% by weight, even more preferably at least 2.5% by weight, and still more preferably at least 3% by weight, based on the weight of the solids of the resin system. "The weight of the solids of the resin system" refers to the total weight of the solids of the resin system, i.e., the total weight of the solids of all film-forming resins and, if present, the curing agent in the coating composition. The amount of the lithium salt selected from lithium carbonate, lithium phosphate, lithium bicarbonate, lithium tetraborate and lithium oxalate in the coating composition should be such that the weight of lithium is preferably in the range of 1.3-40% by weight, based on the weight of the solids of the resin system. The coating composition preferably does not contain greater than 40%, greater than 30%, greater than 25%, greater than 20%, greater than 15%, or greater than 10% by weight of lithium, based on the weight of the solids of the resin system. If the lithium salt is less than 1.3%, the reinforcement of the protective layer may be insufficient, especially when lithium carbonate is used as the lithium salt. As for the upper limit, including more lithium in the composition may be impractical due to increased cost and because the effect is achieved at lower concentrations. If the lithium salt is lithium phosphate, the lithium phosphate is preferably present in an amount of at least 2.0 wt. %, more preferably at least 3.0 wt. %, even more preferably at least 4.0 wt. %, and still more preferably at least 5.0 wt. % lithium based on the solids weight of the resin system.

[0039] The coating composition further comprises a zinc salt of 2,5-dimercapto-1,3,4-thiadiazole (DMTD). The zinc salt of DMTD can exist directly or exist as a part for a mixed compound. The mixed compound mentioned herein (sometimes referred to as mixed pigment) refers to an inseparable close blend of two or more solid compounds comprising organic and inorganic compounds, which is usually formed by coprecipitation. The example of mixed compound includes a host-guest compound, wherein the host matrix is ​​inorganic and has a layered structure, and the guest compound is organic. However, in some embodiments, it can be preferred that the zinc salt of DMTD does not form a part for a mixed pigment.

[0040] The zinc salt of DMTD is zinc 2,5-dimercapto-1,3,4-thiadiazole (VII) (CAS 63813-27-4), which is a 1:2 salt of Zn and DMTD. The description "1:2 salt" here means that it is formed from 1 mole of Zn and 2 moles of DMTD. The salt is further referred to herein as Zn(DMTD)2 or "zinc salt of DMTD". The zinc salt of DMTD can be obtained by known methods, for example by reacting ZnO and DMTD as described in Example 1 of WO 02 / 092880. The zinc salt of DMTD is commercially available, for example, from WPC Technologies as part of Inhibicor1000. The solubility of Zn(DMTD)2 in water has been reported to be 0.4 g / L at 24°C.

[0041] In some embodiments, the zinc salt of DMTD is present in the coating composition in the absence of any other zinc-containing compound, such as zinc oxide and / or zinc salts, for example zinc orthophosphate or zinc cyanamide (CH2N2Zn). In particular, the coating composition does not contain any of ZnO, zinc orthophosphate, zinc cyanamide, or any combination of these. In some embodiments, the coating composition does not contain ZnO. In some embodiments, the coating composition does not contain zinc orthophosphate. In other embodiments, the coating composition does not contain zinc cyanamide. In some embodiments, the composition does not contain ZnO, zinc orthophosphate, and zinc cyanamide. Without wishing to be bound by any theory, it is believed that only Zn(DMTD)2 is responsible for enhancing the barrier properties of the protective layer formed by the lithium ions in the lithium salt and therefore the presence of other Zn compounds is not necessary.

[0042] However, in other embodiments, it may be beneficial for the coating composition to include other zinc-containing compounds, such as any one of ZnO, zinc orthophosphate, zinc cyanamide, or any combination of these. Examples of combinations are Zn(DMTD)2 / ZnO, or Zn(DMTD)2 / zinc orthophosphate, or Zn(DMTD)2 / zinc cyanamide. If Zn(DMTD)2 is present in combination with other zinc compounds, the amount of Zn(DMTD)2 is 1-99 wt%, preferably 10-98 wt%, more preferably 20-95 wt%, and most preferably 30-90 wt%, based on the total weight of the zinc compounds.

[0043] In some embodiments, the content of Zn(DMTD)2 in a mixture with other zinc compounds can be greater than 50% by weight, preferably greater than 60% by weight, more preferably greater than 70, most preferably greater than 80% by weight, and more preferably greater than 90% by weight, based on the weight of the solids of the mixture. An example of a suitable mixture is 1-30% by weight of zinc orthophosphate, 1-20% by weight of ZnO, and at least 50% by weight, such as 50-98% by weight, of Zn(DMTD)2, based on the weight of the solids of the mixture. Much lower contents of Zn(DMTD)2 in the Zn compound mixture may result in insufficient protection, especially if the DMTD content in the coating composition is less than 0.05% by weight, based on the weight of the solids of the resin system.

[0044] In some embodiments, Zn(DTMD)2 is present in combination with any one of other zinc compounds, such as ZnO, zinc orthophosphate, zinc cyanamide, or any combination of these, wherein the amount of Zn(DTMD)2 in the mixture is greater than 50 wt%, preferably greater than 60 wt%, more preferably greater than 80 wt%, and most preferably greater than 90 wt%, based on the solids weight of the mixture, and wherein the DMTD content in the coating composition is at least 0.05 wt%, preferably at least 1 wt%, and more preferably at least 2 wt%, based on the solids weight of the resin system.

[0045] Preferably, the zinc salt of DMTD is present in this coating composition to provide at least 0.05 % by weight DMTD, more preferably at least 0.1 % by weight, still more preferably at least 0.4 % by weight, or at least 0.6 % by weight, or at least 1 % by weight, more preferably at least 2 % by weight or at least 4 % by weight DMTD based on the solid weight of this resin system.Preferably, this amount is lower than 50 % by weight, or lower than 40 % by weight, or lower than 30 % by weight, or 20 % by weight DMTD based on the solid weight of this resin system.When using with the amount lower than 0.05 % by weight, it is believed that the synergistic effect with lithium is too low and can not provide any benefit in practice.It is believed that it is impractical to use the amount higher than 50 % by weight, for example due to potential formulation problems or high cost.

[0046] The coating composition preferably contains a lithium salt and a zinc salt of DMTD in an amount such that the weight ratio of lithium to DMTD, calculated as the weight ratio of lithium content to DMTD content, is from 0.01 to 20, preferably from 0.1 to 10. In some embodiments, it may be preferred that the content of DMTD (present as the zinc salt of DMTD) is equal to or greater than the content of lithium (present as the lithium salt). In other embodiments, it may be preferred that the content of DMTD (present as the zinc salt of DMTD) is equal to or less than the content of lithium (present as the lithium salt).

[0047] As discovered by the inventors of the present invention and as demonstrated in the examples, the combination of a lithium salt and a zinc salt of DMTD in an organic coating exhibits unexpectedly enhanced barrier properties for a protective layer formed on an exposed metal substrate. This protective layer is formed at locations where coating defects occur, which in the examples are simulated by a line drawn through the coating to expose the bare metal to the environment. Unlike other well-known corrosion inhibitors, lithium salts are known to form an irreversible protective layer on a metal substrate (e.g., an aluminum alloy) by a mechanism of leaching from the coating matrix. This protective layer provides a barrier between the metal substrate and the environment. The barrier function of this layer cannot be observed visually, but can be measured using electrochemical impedance spectroscopy (EIS). The presence of Zn(DMTD)2 in the coating composition together with the lithium salt and the organic film-forming resin produces a much higher impedance value for the protective layer than when only the lithium salt is used or when the lithium salt is used in combination with other potentially active compounds screened. A higher impedance value means that the protective layer has a higher barrier function or is "enhanced." An enhanced protective layer may result in improved or longer-term protection. This enhancement of the protective layer is unexpected since, as shown in the examples below, Zn(DMTD)2 by itself neither exhibits barrier properties nor forms a protective layer.

[0048] The coating composition may further comprise at least one magnesium compound. Suitable magnesium compounds are, for example, magnesium-containing materials such as magnesium metal, magnesium oxide, magnesium oxyaminophosphates (e.g. 465M), magnesium carbonate, and magnesium hydroxide. Magnesium metal is suitably used in particulate form, such as powder, flakes, spheres, or spheroids. It should be noted that magnesium metal and magnesium metal alloy particles require specific stabilizers when used in aqueous coating compositions. Such stabilizers are generally known and commercially available. Magnesium oxide is preferably used as the magnesium compound.

[0049] The one or more magnesium compounds are preferably present in the coating composition in an amount of 0.1-50 wt%, more preferably 1-35 wt%, most preferably 5-20 wt%, based on the dry weight of the coating composition (the sum of the weights of the non-volatile components of the coating composition).

[0050] The magnesium oxide or magnesium salt is preferably present in an amount such that the Mg:Li weight ratio is at least 0.1: 1, more preferably at least 0.5: 1, even more preferably at least 1: 1, still more preferably at least 3: 1. This ratio is preferably less than 30: 1, more preferably less than 25: 1, even more preferably less than 15: 1, still more preferably less than 10: 1, and most preferably less than 8: 1.

[0051] If magnesium metal or alloy is present in the present composition, the Mg:Li weight ratio is preferably less than 500:1, more preferably less than 300:1, even more preferably less than 250:1, even more preferably less than 100:1, even more preferably less than 50:1, and most preferably less than 25:1.

[0052] Preferably, the composition comprises a combination of the lithium salt, magnesium oxide and zinc salt of DMTD as described above. These compounds are preferably present in a weight ratio of (1-5):(1-2):1, preferably about 3:1.5:1, to the lithium salt, magnesium oxide and zinc salt of DMTD, respectively.

[0053] In some embodiments, the coating composition further comprises one or more additional corrosion inhibitors. Additional corrosion inhibitors can be organic or inorganic. Examples of inorganic inhibitors are phosphates such as zinc orthophosphate, zinc orthophosphate hydrate, zinc aluminum orthophosphate, polyphosphates such as strontium aluminum polyphosphate hydrate, zinc aluminum polyphosphate hydrate, magnesium aluminum polyphosphate, zinc aluminum triphosphate, and magnesium aluminum triphosphate. Inhibitors further include metal oxides such as oxides of zinc, magnesium, aluminum, lithium, molybdate, strontium, cerium, and mixtures thereof; metals such as metallic Zn, metallic Mg, and Mg alloys. Examples of organic inhibitors are thiol compounds and azoles such as imidazoles, thiazoles, tetrazoles, triazoles such as (substituted) benzotriazole and 2-mercaptobenzothiazole.

[0054] In some embodiments, the additional corrosion inhibitor is a thiol compound other than 2,5-dimercapto-1,3,4-thiadiazole zinc salt (VII) or an azole. Azoles are 5-membered N-heterocyclic compounds containing a nitrogen atom and at least one other non-carbon atom (i.e., nitrogen, sulfur, or oxygen) as part of the ring. Examples of suitable compounds include 5-methylbenzotriazole and 2-mercaptobenzothiazole (2-MBT). Preferably, 2-MBT is present as an additional corrosion inhibitor.

[0055] Other compounds that may be present in the coating compositions of the present invention are color pigments (e.g. titanium dioxide or yellow iron oxide), extenders (e.g. talc, barium sulfate, mica, calcium carbonate, silica or wollastonite), rheology modifiers (e.g. Bentone SD 2 or organic rheology modifiers), flow and leveling agents (e.g. polysiloxane and polyacrylate leveling additives) and solvents (e.g. ketones such as methyl isobutyl ketone, aromatic compounds such as xylene, alcohols such as benzyl alcohol, esters such as butyl acetate and aliphatic solvents).

[0056] In a preferred embodiment, the coating composition of the present invention is a liquid coating composition. The composition may include a volatile liquid diluent, such as a volatile organic solvent or water. The composition may be aqueous, solvent-based or solvent-free. The term "solvent-free" is defined as containing less than 5% by weight of a total volatile liquid diluent content, including water and an organic solvent. The term "aqueous" is defined as containing at least 50% by weight of water based on the total weight of the volatile liquid diluent (including both water and the organic solvent). The term "solvent-based" is defined as containing an organic solvent in an amount of at least 50% by weight based on the total weight of the volatile liquid diluent (including both water and the organic solvent). Preferably, the coating composition is solvent-based.

[0057] In some embodiments, the coating composition can be substantially free of water (non-aqueous), which means that the water content is less than 1 wt %, preferably less than 0.1 wt %, based on the total weight of the coating composition. More preferably, the coating composition is completely free of water.

[0058] The coating composition of the present invention is preferably a low temperature curable composition, which means that it is curable, i.e., can form a network, at a temperature below 120° C., preferably below 100° C., more preferably below 80° C., even more preferably below 50° C., and most preferably under ambient conditions. In other embodiments, the coating composition can be a high temperature curable composition, for example, curable at a temperature of 120° C. and above, preferably 140° C. and above.

[0059] The non-volatile content of the coating composition is preferably 10-95 wt%, more preferably 25-75 wt%, even more preferably 30-70 wt%.For waterborne coating compositions, the non-volatile content is preferably in the range of 30-60 wt%.

[0060] The coating composition preferably has a volatile organic content (VOC) (determined according to ASTM D3960) of less than 700 g / L, such as less than 500 g / L, more preferably less than 300 g / L.

[0061] The coating composition can be advantageously used as an anticorrosive primer for coating non-ferrous metal substrates such as magnesium, magnesium alloys, titanium, aluminum, aluminum alloys, and aluminum / lithium alloy substrates. Preferred non-ferrous metal substrates are aluminum alloys. Examples of suitable aluminum alloys are 2024-T3 (bare or coated), 7075-T6 (bare or coated), 2098, 2099, 2198, 6061, 6111, 6022, 5052, 5083, 5251, 5454, 7475, 7017, and 7020.

[0062] The coating composition of the present invention is also suitable for coating ferrous substrates. Examples of suitable ferrous substrates are cold rolled and hot rolled steel, stainless steel 304, B952 (zinc phosphate modified), B1000 (iron phosphate modified), and zinc modified steels such as EZG 60G, zinc phosphate modified EZG 60G, G90, and Galvanneal HIA Zn / Fe A45.

[0063] The coating composition of the present invention can be used as a primer, a bonding primer, a self-priming topcoat, an intermediate coating and / or a topcoat. It can also be used in a coating system in which at least two layers have the composition described in the present disclosure.

[0064] The coating composition can be applied to substrates with and without hexavalent chromium-free pretreatment, such as sol-gel systems such as 131 (AC Tech) or (Pantheon Chemical) or chemical conversion coatings.

[0065] The coating composition can also be applied to anodized surfaces, such as chromic acid anodized (CAA) surfaces, tartaric acid-sulfuric acid anodized (TSA) surfaces, phosphoric acid anodized (SAA) surfaces, phosphoric acid anodized (PAA) surfaces, phosphoric acid-sulfuric acid anodized (PSA) surfaces, and boric acid-sulfuric acid anodized (BSAA) surfaces.

[0066] This coating composition can advantageously be used as the primer coating of non-ferrous metal substrates. It can be used as a single layer or with multilayer application. In a preferred embodiment, this coating composition is applied to substrate to form at least one primer layer in the multilayer coating system. The finish coat applied on the primer layer can be a clear coat or a colored finish. Alternatively, this coating system can also comprise a base coat that imparts color and / or effect and a clear coat that is applied on the base coat top. Said composition can also be used as a finish coat that can be transparent or colored.

[0067] The present invention also relates to a multi-layer coating system comprising at least one layer obtained from a coating composition according to the first aspect of the invention, i.e. comprising the resin system a), a lithium salt b) and a zinc salt c). After application, components a), b) and c) are present in a single layer. The layer formed from the coating composition is preferably used as a primer layer applied to a substrate, which substrate may have been pretreated. The topcoat may be a clearcoat or a pigmented topcoat, or, when the topcoat comprises a basecoat that imparts color and / or effects and is applied to the primer layer, a clearcoat is applied on top of the basecoat layer.

[0068] The present invention further provides a metal substrate coated with the coating composition of the present invention. The metal substrate can be a non-ferrous metal substrate such as aluminum or an aluminum alloy. Alternatively, the metal substrate can be a ferrous metal substrate. The substrate can be intended for exterior or interior use. Examples include structural components of aircraft, cabins, or components of vehicles.

[0069] The coating composition is particularly suitable for use in the aerospace, automotive or coil coating industries. Example

[0070] Chemicals used

[0071] Lithium carbonate—ACS reagent, ≥99.0%, from Sigma Aldrich Desmophen 650 MPA—branched polyester with hydroxyl groups from Covestro AG, 65 wt

[0072] %, in 1-methoxy-2-propyl acetate (MPA), 5.3% OH content Tioxide TR92 - titanium dioxide from Huntsman

[0073] Zn(DMTD)2—prepared according to Example 1 of WO 02 / 092880A1

[0074] Hybrid 1 - Hybrid pigment prepared according to Example 2 of WO 02 / 092880 A1, containing 45 wt% Zn(DMTD)2, 32 wt% Zn3(PO4)2.H2O, 23 wt% ZnO (wt% based on the total weight of the mixture)

[0075] Hybrid 2 - Hybrid pigment prepared according to Example 1 of WO 2008 / 140648 A9, containing a neutralized salt of Zn(DMTD)2 neutralized with the disodium salt of DMTD (Na2(DMTD)) with an estimated content of 97 wt% Zn(DMTD)2

[0076] Blend 3 - prepared as blend 1, but containing 75 wt% Zn(DMTD)2, 14 wt% Zn3(PO4)2.H2O, 11 wt% ZnO (wt% based on the total weight of the mixture)

[0077] Blend 4 - prepared as blend 1, but containing 15 wt% Zn(DMTD)2, 49 wt% Zn3(PO4)2.H2O, 36 wt% ZnO (wt% based on the total weight of the mixture)

[0078] Blend 5 - prepared as blend 1, but containing 0 wt% Zn(DMTD)2, 58 wt% Zn3(PO4)2.H2O, 42 wt% ZnO (wt% based on the total weight of the mixture)

[0079] Blend 6 - prepared as blend 1, but without phosphoric acid and containing 75 wt% Zn(DMTD)2 and 25 wt% ZnO (wt% based on the total weight of the mixture)

[0080] Blend 7—Prepared as Blend 1, but without phosphoric acid and containing 45 wt% Zn(DMTD)2 and 55 wt% ZnO (wt% based on the total weight of the mixture)

[0081] Blend 8—Prepared as Blend 1, but without phosphoric acid and containing 15 wt% Zn(DMTD)2 and 85 wt% ZnO (wt% based on the total weight of the mixture)

[0082] Blend 9—Prepared as Blend 1, but without phosphoric acid and containing 7 wt% Zn(DMTD)2 and 93 wt% ZnO (wt% based on the total weight of the mixture)

[0083] Airwhite AW 15—Barium sulfate from Sibelco Specialty Minerals

[0084] Desmodur N-75MPA / X—aliphatic polyisocyanate resin based on hexamethylene diisocyanate (HDI) from Covestro AG, dissolved in n-butyl acetate and xylene (1:1), solids content 75% by weight, NCO content 16.5%

[0085] Electrochemical Impedance Spectroscopy (EIS) of Silquest A-187—Epoxy-Functional Silane from Momentive Performance Materials

[0086] The barrier performance of the protective layer in defect areas was quantified and evaluated using electrochemical impedance spectroscopy (EIS) as described in Visser, P., Liu, Y., Terryn, H. et al. “Lithium salts as leachable corrosion inhibitors and potential replacement for hexavalent chromium in organic coatings for the protection of aluminum alloys”, J Coat Technol Res 13, 557-566 (2016).

[0087] EIS measurements were performed at open circuit potential (OCP) using a computer-controlled potentiostat over a frequency range of 0.01–30,000 Hz, measuring seven points per decade and applying a sinusoidal amplitude of 10 mV. The measurements were performed using a three-electrode setup in a Faraday cage equipped with a saturated calomel electrode (SCE) as a reference electrode, a platinum mesh as a counter electrode, and a ruled plate as a working electrode using a 0.05 M NaCl electrolyte. The area exposed to the electrolyte was 12.5 cm 2 The effective bare electrode (i.e., coating defect) area is 0.48 cm 2 And the electrolyte volume is 60cm 3 The measurements were recorded on at least 3 samples after 2 hours of exposure to 0.05 M NaCl electrolyte.

[0088] The results of these examples are shown in table form with Zmod values ​​at 10 mHz and 10 Hz. The higher the impedance value, the more the protective layer is reinforced.

[0089] Preparation of test plates

[0090] Unless otherwise specified, the test panels were typically 7.0 cm x 7 cm (3 x 6 inches) and 0.8 mm thick AA2024-T3 bare aluminum alloy, anodized in tartaric-sulfuric acid (TSA) according to aerospace requirements (AIPI 02-01-003).

[0091] The coating was applied using a high-volume, low-pressure (HVLP) spray gun under ambient conditions (23°C, 55% RH). Following application and a 1-hour air-dry period, the coated panels were cured at 80°C for 1 hour and dried under ambient conditions for 7 days. After drying, the coating had a dry film thickness of 20-25 μm. The coated panels were scored using a mechanical milling device (cross-hatch 2 cm x 2 cm), leaving U-shaped scores 1 mm wide and 100-150 μm deep (cross-hatch 2 cm x 2 cm). Following scoring, the samples were exposed to a neutral salt spray test (ASTM-B117) for 168 hours.

[0092] Example 1: Synergistic Effect between Li Salt and Zn(DTMD)2

[0093] Formulations 1-1 to 1-4 were prepared using the ingredients listed in Table 1. The amounts of the ingredients are shown in parts by weight (g). The amounts of the active ingredients Li and DMTD are also shown as wt % based on the solids of the resin system.

[0094] While stirring, add the ingredients for component A sequentially to a 370 ml glass jar. Then, add 400 grams of Zirconox beads (1.7-2.4 mm) to the mixture to grind and disperse the pigment. Shake the sample on a Skandex paint shaker for 20 minutes to achieve a fineness of grind of less than 25 μm. After shaking, separate the beads from the paint. Prepare component B separately by mixing.

[0095] Part B was added to Part A with stirring to ensure thorough mixing to obtain a homogeneous sample. The coating was induced for 30 minutes after mixing the separate components.

[0096] Table 1

[0097] 1-1* 1-2* 1-3* 1-4 Component A Methyl isobutyl ketone 60 60.0 60.0 60.0 Desmophen 650MPA 47.7 47.7 47.7 47.7 lithium carbonate - - 12.0 10.0 Tioxide TR92 19.9 19.9 19.9 19.9 <![CDATA[Zn(DMTD)2]]> - 6.0 - 6.0 Airwhite AW 15 47.3 34.0 21.2 14.0 magnesium oxide 26.6 19.0 19.0 19.0 Component B Desmodur N 75MPA / X 28.5 28.5 28.5 28.5 Silquest A-187 5.2 5.2 5.2 5.2 Methyl isobutyl ketone 50.0 50.0 50.0 50.0 <![CDATA[ Active ingredient, weight % based on resin system solids ]]> Li - - 4.4 3.6 DMTD - 9.5 - 9.5 <![CDATA[ EIS Results ]]> Zmod (kΩ) at 10mHz 27 31 180 821 Zmod at 10Hz (kΩ) 0.5 0.7 2.5 6

[0098] *Comparative Example

[0099] Formulations 1-1, 1-2, and 1-3 are comparative formulations, while formulation 1-4 is according to the present invention. The results of the EIS measurements are shown in FIG. Figure 1 and Table 1.

[0100] It can be seen that the coatings containing both Zn(DMTD)2 and Li salt (1-4) show significantly higher impedance |Z| (also called "Zmod") values ​​compared to the formulations without any active component (1-1), or with only Zn(DMTD)2 (1-2), or with only Li salt (1-3). This effect is much stronger than the sum of the individual effects of Li salt (1-3) and Zn(DMTD)2 (1-2) and is therefore a synergistic effect.

[0101] The results also show that Zn(DMTD)2 (1-2) alone has no effect on forming a protective layer in defect areas. In particular, coating 1-2 has a resistance value as low as that of the coating without (negative control) (1-1).

[0102] Example 2: Comparison with other azoles

[0103] Formulations 2-1 to 2-7 were prepared in the same manner as Example 1, but using the ingredients shown in Table 2. Ingredient amounts are shown in parts by weight (g). The active ingredient content is also shown in weight % based on the resin system solids. The molar amount of azole was the same in all formulations of this example. BTA is benzotriazole, and 2-MBT is 2-mercaptobenzothiazole.

[0104] Table 2

[0105]

[0106] *Comparative Example

[0107] Formulations 2-1, 2-2, 2-3, 2-4, 2-5, and 2-6 are comparative formulations, while formulation 2-7 is according to the present invention. The results of the EIS measurements are shown in FIG. Figure 2 and are listed in Table 2.

[0108] The results also show that the combination of Li salts with other azoles (BTA or 2-MBT) does not have the same synergistic effect as the combination with Zn(DMTD) 2. The combination of Li salts and Zn(DMTD) 2 provides much higher impedance values ​​associated with the enhancement of the protective layer in the defective areas.

[0109] Example 3: Zn(DMTD)2 in mixed pigments

[0110] Formulations 3-1 to 3-7 were prepared in the same manner as Example 1, but using the ingredients shown in Table 3. The ingredient amounts are shown in parts by weight (g). The amount of active ingredient is also given in weight % based on the solids of the resin system.

[0111] Table 3

[0112]

[0113] *Comparative Example

[0114] Formulations 3-1, 3-2, 3-3 and 3-4 are comparative formulations, while formulations 3-5, 3-6 and 3-7 are according to the present invention. The results of the EIS measurements are listed in Table 3.

[0115] The results show that Zn(DMTD)2 when incorporated into the hybrid pigments exhibits the same effect as Zn(DMTD)2 used as a pure component with the lithium salt. The total amount of Zn(DMTD)2 in the coating was the same in all formulations.

[0116] Example 4: Different ratios of Zn(DMTD)2 in mixed pigments

[0117] Formulations 4-1 to 4-7 were prepared in the same manner as Example 1, but using the ingredients shown in Table 4. The ingredient amounts are shown in parts by weight (g). The amount of active ingredient is also given in weight % based on the solids of the resin system.

[0118] Table 4

[0119]

[0120]

[0121] *Comparative Example

[0122] Formulations 4-1, 4-2 and 4-6 are comparative formulations, while formulations 4-3, 4-4, 4-5 and 4-7 are according to the present invention. The results of the EIS measurements are listed in Table 4.

[0123] In these examples, the weight ratio of Zn(DMTD)2 in the hybrid pigment varied from low to high, but the amount of Zn(DMTD)2 in the coating was the same. All examples with hybrid pigments containing Zn(DMTD)2 showed an enhancement effect of the protective layer in defect areas. There was little difference in the enhancement between the different hybrid pigments. Therefore, this effect was not dependent on the presence or amount of ZnO or Zn3(PO4)2.

[0124] Example 5: Replacement of mixed pigments; combination of ZnO and Zn(DMTD)2

[0125] Formulations 5-1 to 5-7 were prepared in the same manner as Example 1, but using the ingredients shown in Table 5. The ingredient amounts are shown in parts by weight (g). The amount of active ingredient is also given in weight % based on the solids of the resin system.

[0126] Table 5

[0127]

[0128]

[0129] *Comparative Example

[0130] Formulations 5-1 and 5-2 are comparative formulations, while the other formulations are according to the present invention. The results of the EIS measurements are listed in Table 5.

[0131] In these examples, Li salts were combined with hybrid pigments based on Zn(DMTD)2 and ZnO alone. The amount of Zn(DMTD)2 was the same in all examples, and all samples showed the same enhanced effect of the protective layer. It can be concluded that the presence of ZnO does not affect the synergistic effect of Li and Zn(DTMD)2; this synergistic effect is due to the combination of Li salt and Zn(DMTD)2.

[0132] Example 6: Replacement of mixed pigments; combination of ZnO and Zn3(PO4)2

[0133] Formulations 6-1 to 6-5 were prepared in the same manner as Example 1, but using the ingredients shown in Table 6. The ingredient amounts are shown in parts by weight (g). The amount of active ingredient is also given in weight % based on the solids of the resin system.

[0134] In these experiments, the performance of alternative hybrid pigments containing ZnO and Zn3(PO4)2 was investigated with and without Zn(DTMD)2.

[0135] Table 6

[0136]

[0137]

[0138] *Comparative Example

[0139] Formulations 6-1, 6-2, and 6-3 are comparative formulations, while the other formulations are according to the present invention. The results of the EIS measurements are listed in Table 6.

[0140] It can be concluded that the combination of the hybrid pigments of ZnO and Zn3(PO4)2 with Li salts has no synergistic effect on the barrier effect of the protective layer. The presence of only Zn(DMTD)2 in this formulation provides the effect of the invention.

[0141] Example 7: Effect of ZnO

[0142] Formulations 7-1 to 7-6 were prepared in the same manner as Example 1, but using the ingredients shown in Table 7. The ingredient amounts are shown in parts by weight (g). The amount of active ingredient is also given in weight % based on the solids of the resin system.

[0143] In these experiments the effect of ZnO was investigated. These examples comprise ZnO with and without Li salts and / or Zn(DTMD) 2. These were used in the form of a physical mixture and were not part of a hybrid pigment.

[0144] Table 7

[0145]

[0146] *Comparative Example

[0147] Formulations 7-1, 7-2, 7-3 and 7-4 are comparative formulations, while the other formulations are according to the present invention. The results of the EIS measurements are listed in Table 7.

[0148] From the EIS results, it can be seen that ZnO alone does not show any synergistic effect with the Li salt. From this example, it can be concluded that Zn(DMTD)2 is necessary for the enhancement effect on the protective layer.

[0149] Example 8: Effect of Zn3(PO4)2

[0150] Formulations 8-1 to 8-6 were prepared in the same manner as Example 1, but using the ingredients shown in Table 8. The ingredient amounts are shown in parts by weight (g). The amount of active ingredient is also given in weight % based on the solids of the resin system.

[0151] In these tests, the example formulations contained Zn3(PO4)2, with and without Li and Zn(DTMD)2. These were used in the form of a physical mixture and were not part of a hybrid pigment.

[0152] Table 8

[0153]

[0154] *Comparative Example

[0155] Formulations 8-1, 8-2, 8-3 and 8-4 are comparative formulations, while 8-5 and 8-6 are according to the present invention. The results of the EIS measurements are listed in Table 8.

[0156] From the results it can be seen that Zn3(PO4)2 alone does not show any enhancing effect with Li salts in the formation of the protective layer in defect areas. The addition of Zn(DMTD)2 to the formulation containing Li salts is necessary for a synergistic enhancing effect on the barrier effect of the protective layer.

[0157] Example 9: Combination of ZnO and Zn3(PO4)2

[0158] Formulations 9-1 to 9-6 were prepared in the same manner as Example 1, but using the ingredients shown in Table 9. The ingredient amounts are shown in parts by weight (g). The amount of active ingredient is also given in weight % based on the solids of the resin system.

[0159] In these tests, the coating compositions contained ZnO and Zn3(PO4)2, with and without Li and Zn(DTMD)2. The ratio of ZnO to Zn3(PO4)2 was the same as for Hybrid 1, however they were used in the form of a physical mixture and were not part of the hybrid pigment.

[0160] Table 9

[0161]

[0162]

[0163] *Comparative Example

[0164] Formulations 9-1, 9-2, 9-3 and 9-4 are comparative formulations, while 9-5 and 9-6 are according to the present invention. The results of the EIS measurements are listed in Table 9.

[0165] From the results, it can be seen that the combination of ZnO and Zn3(PO4)2 does not show any enhancement effect with Li salt. The addition of Zn(DMTD)2 is necessary to obtain the enhancement effect of the protective layer in the defective area.

[0166] Example 10: Li salts of different concentrations

[0167] Formulations 10-1 to 10-8 were prepared in the same manner as Example 1, but using the ingredients shown in Table 10. The ingredient amounts are shown in parts by weight (g). The amount of active ingredient is also given in weight % based on the solids of the resin system.

[0168] In these experiments, the formulations contained Li salts and Zn(DTMD)2, where the concentration of the Li salts was varied.

[0169] Table 10

[0170]

[0171]

[0172] *Comparative Example

[0173] Formulations 10-1, 10-2, 10-3, 10-4 and 10-5 are comparative formulations, while the other formulations are according to the present invention. The results of the EIS measurements are listed in Table 10.

[0174] It can be seen that significantly higher impedance values ​​are obtained for the compositions containing at least 1.3 wt. % Li based on resin system solids in combination with Zn(DMTD)2 compared to samples with only Li salt.

[0175] Example 11: Different concentrations of Zn(DMTD)2

[0176] Formulations 11-1 to 11-8 were prepared in the same manner as Example 1, but using the ingredients shown in Table 11. The ingredient amounts are shown in parts by weight (g). The amount of active ingredient is also given in weight % based on the solids of the resin system.

[0177] In these experiments, the active material contained Li salt and Zn(DTMD)2, where the concentration of Zn(DTMD)2 was varied.

[0178] Table 11

[0179]

[0180]

[0181] *Comparative Example

[0182] Formulations 11-1, 11-2, and 11-3 are comparative formulations, while the other formulations are according to the present invention. The results of the EIS measurements are listed in Table 11.

[0183] It can be concluded that Zn(DTMD)2 shows a synergistic effect with Li salts at any amount, even at very small amounts, such as 0.6 wt% DMTD. Even at these low concentrations of DMTD, there is a clear improvement (enhancing effect) in the barrier properties of the protective layer in defect areas.

[0184] Example 12: Different Li salts (4.4% Li)

[0185] Formulations 12-1 to 12-8 were prepared in the same manner as Example 1, but using the ingredients shown in Table 12. The ingredient amounts are shown in parts by weight (g). The amount of active ingredient is also given in weight % based on the solids of the resin system.

[0186] In these tests, the coating compositions contained different soluble Li salts, wherein the Li content was 4.4 wt. % based on the resin system solids. The lithium salts were selected based on their different solubilities.

[0187] Table 12

[0188]

[0189]

[0190] *Comparative Example

[0191] Formulations 12-1 to 12-5 are comparative formulations, while the other formulations are according to the present invention. The results of the EIS measurements are listed in Table 12.

[0192] It can be concluded that Zn(DTMD)2 shows an enhanced effect with Li salts. The moderate effect of lithium phosphate is believed to be due to its lower solubility in water.

[0193] Example 13: Different Li salts (1.3 wt% Li)

[0194] Formulations 13-1 to 13-6 were prepared in the same manner as Example 1, but using the ingredients shown in Table 13. The ingredient amounts are shown in parts by weight (g). The amount of active ingredient is also given in weight % based on the solids of the resin system.

[0195] In these experiments, the coating compositions contained different soluble Li salts, wherein the Li content was 1.3 wt. % based on the resin system solids.

[0196] Table 13

[0197]

[0198]

[0199] *Comparative Example

[0200] Formulations 13-1 to 13-3 are comparative formulations, while the other formulations are according to the present invention. The results of the EIS measurements are listed in Table 13.

[0201] It can be concluded that Zn(DTMD)2 shows an enhancement of the protective layer with most Li salts (and when used in lower amounts than in Example 12), with the exception of lithium phosphate. Better results were achieved with lithium phosphate at higher Li contents (Examples 12 and 15), which is believed to be due to its lower solubility in water.

[0202] Example 14: No magnesium oxide

[0203] Formulations 14-1 to 14-4 were prepared in the same manner as Example 1, but using the ingredients shown in Table 14. The ingredient amounts are shown in parts by weight (g). The amount of active ingredient is also given in weight % based on the solids of the resin system.

[0204] In these experiments it was tested whether the reinforcing effect also exists in compositions without magnesium oxide.

[0205] Table 14

[0206] 14-1* 14-2* 14-3* 14-4 Component A Methyl isobutyl ketone 60.0 60.0 60.0 60.0 Desmophen 650MPA 47.7 47.7 47.7 47.7 lithium carbonate - 12.0 - 12.0 Tioxide TR92 19.9 19.9 19.9 19.9 barium sulfate 70.6 44.4 57.8 31.5 <![CDATA[Zn(DMTD)2]]> - - 6.0 6.0 Component B Desmodur N 75MPA / X 28.5 28.5 28.5 28.5 Silquest A-187 5.2 5.2 5.2 5.2 Methyl isobutyl ketone 50.0 50.0 50.0 50.0 <![CDATA[ Active ingredient, weight % based on resin system solids ]]> lithium carbonate - 23.0 - 19.0 <![CDATA[Zn(DMTD)2]]> - - 11.4 11.5 Li - 4.4 - 3.6 DMTD - - 9.5 9.5 <![CDATA[ EIS Results ]]> Zmod (kΩ) at 10mHz 40 94 57 459 Zmod at 10Hz (kΩ) 0.6 1.9 0.8 4

[0207] *Comparative Example

[0208] Formulations 14-1 to 14-3 are comparative formulations, while 14-4 is according to the present invention. The results of the EIS measurements are listed in Table 14.

[0209] It can be concluded that the enhancement effect also exists in the absence of MgO and is purely due to the combination of lithium salt and Zn(DMTD)2.

[0210] Example 15: Lithium phosphate at different concentrations

[0211] Formulations 15-1 to 15-6 were prepared in the same manner as Example 1, but using the ingredients shown in Table 15. Ingredient amounts are shown in parts by weight (g). The amount of active ingredient is also given in weight % based on the solids of the resin system.

[0212] In these experiments, the concentration of lithium phosphate was varied so that the Li content was in the range of 2.0-7.0 wt % based on the resin system solids.

[0213] Table 15

[0214]

[0215] *Comparative Example

[0216] Formulations 15-1 to 15-3 are comparative formulations, while the other formulations are according to the present invention. Test panels were prepared and scribed as described above. After scribe, the samples were exposed to a neutral salt spray test (ASTM-B117) for 96 hours. The results of the EIS measurements are listed in Table 15.

Claims

1. A coating composition comprising: a) a resin system comprising an organic film-forming resin and optionally a curing agent reactive toward the organic film-forming resin, b) a lithium salt having a solubility in water at 20° C. in the range of 0.01 to 120 g / L, selected from lithium carbonate, lithium phosphate, lithium bicarbonate, lithium tetraborate, and lithium oxalate, in an amount of at least 1.3 wt % and no more than 10 wt % lithium based on the solids weight of the resin system, and c) a zinc salt of 2,5-dimercapto-1,3,4-thiadiazole (DMTD), wherein the zinc salt of DMTD is zinc salt of 2,5-dimercapto-1,3,4-thiadiazole (VII) in an amount to provide at least 4 wt% to less than 20 wt% DMTD based on the solid weight of the resin system.

2. The composition according to claim 1, wherein said composition does not contain any zinc-containing compound other than said zinc salt of DMTD.

3. The composition according to claim 1, further comprising a zinc-containing compound selected from the group consisting of zinc oxide, zinc orthophosphate, zinc cyanamide, and any combination thereof.

4. The composition according to claim 1, wherein the lithium salt b) is selected from the group consisting of lithium carbonate, lithium bicarbonate, lithium tetraborate and lithium oxalate.

5. The composition according to claim 4, wherein the lithium salt b) is selected from lithium carbonate and lithium oxalate.

6. The composition according to claim 5, wherein the lithium salt b) is lithium carbonate.

7. The composition according to claim 1, wherein the lithium salt b) is lithium phosphate, and the lithium phosphate is present in an amount of at least 2.0 wt. % lithium based on the solid weight of the resin system.

8. The composition of claim 7, wherein the lithium phosphate is present in an amount of at least 4.0 wt. % lithium based on the solids weight of the resin system.

9. The composition according to claim 1, wherein the film-forming resin is selected from the group consisting of epoxy resins, hydroxyl-functional poly(meth)acrylates, polyester polyols and polyurethanes.

10. The composition according to claim 1, further comprising a magnesium compound selected from the group consisting of magnesium, magnesium oxide, magnesium oxyaminophosphate, magnesium carbonate and magnesium hydroxide.

11. The composition according to claim 1, further comprising a thiol or azole other than 2,5-dimercapto-1,3,4-thiadiazole zinc salt (VII).

12. A multilayer coating system on a metal substrate comprising a layer obtained from a coating composition according to any one of claims 1 to 11.

13. The multi-layer coating system according to claim 12, wherein the layer obtained from the coating composition according to any one of claims 1 to 11 is a primer layer on the metal substrate.

14. A metal substrate coated with a coating composition according to any one of claims 1 to 11.

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