Coating composition for enhancing a protective layer

Abstract

The invention relates to a coating composition comprising a resin system comprising an organic film-forming resin and optionally a curing agent reactive to the organic film-forming resin, 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 hydrogen carbonate, lithium tetraborate and lithium oxalate, and a zinc salt of 2,5-dimercapto-1,3,4-thiadiazole (DMTD), wherein the zinc salt of DMTD is present as solid particles, the surface of which is at least partially covered by a film-forming polymer layer. The zinc salt of DMTD proves to have a synergistic effect on the enhancement of the barrier function of the protective layer formed in coating defects due to the presence of the lithium salt. Furthermore, the coating composition has an improved pot life.

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. Additionally, it would be desirable to provide coating compositions having improved pot life and minimal side reactions during open time. SUMMARY

[0008] The inventors of the present invention have 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. However, when the zinc salt of DMTD is used in a coating composition, its activity may be reduced due to its reactivity with coating components, such as epoxy resin, which in turn may adversely affect the pot life of the coating composition.

[0009] In the present invention, the reinforcing agent is provided in a surface-modified form, which allows both retaining its reinforcing activity and preventing it from reacting with coating components.

[0010] Therefore, the present invention provides a coating composition in a first aspect, comprising:

[0011] a) a resin system comprising an organic film-forming resin and optionally a curing agent reactive with the organic film-forming resin,

[0012] b) a lithium salt having a solubility in water at 20° C. in the range of 0.01-120 g / L, selected from the group consisting of lithium carbonate, lithium phosphate, lithium bicarbonate, lithium tetraborate and lithium oxalate, and

[0013] 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),

[0014] The zinc salt of DMTD exists as solid particles, the surface of which is at least partially covered by a film-forming polymer layer.

[0015] In another aspect, the present invention provides a method for coating a metal substrate, comprising the steps of: a) applying the coating composition of the present invention to the metal substrate, and

[0016] b) curing the applied coating composition.

[0017] In another aspect, the present invention provides a metal substrate coated with the coating composition of the present invention. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 and 2 The development of flow cup viscosity with time after mixing is shown for some embodiments of the present invention and comparative coating compositions. DETAILED DESCRIPTION

[0021] The inventors surprisingly discovered that the use of a zinc salt of DMTD, when used in a coating composition together with a lithium salt selected from lithium carbonate, lithium phosphate, lithium bicarbonate, lithium tetraborate, and lithium oxalate, enhances the protective layer formed by lithium ions in coating defects on metal surfaces. 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 form in a cured coating, exposing the metal substrate and making it 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 this protective layer can be observed and its barrier properties can be quantified using electrochemical impedance spectroscopy (EIS). See Visser, P., Liu, Y., Terryn, H. et al., “Lithium salts as leachable corrosion inhibitors and potential replacement for hexavalent chromiuminorganic 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).

[0022] 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.

[0023] However, the use of zinc salts of DMTD in coating compositions is not direct because of the active thiol groups of the DMTD molecule, which may be reactive towards other coating components. In the present invention, the reinforcing agent is provided in an encapsulated form (covered by a film-forming polymer layer), which allows both to retain its activity and to prevent it from reacting with coating components.

[0024] In the present invention, the zinc salt of DMTD is present as solid particles, the surface of which is at least partially covered by a film-forming polymer layer. The surface may be partially or completely covered by the film-forming polymer layer, as can be observed, for example, microscopically. Preferably, at least 80%, more preferably at least 90%, and still more preferably at least 95% of the surface is covered, as can be estimated from a representative number of microscopic images.

[0025] As used herein, a "film-forming polymer" refers to a polymer that has formed a film (often referred to as 'cured') and will no longer form a film when heated, dried, or contacted with a curing agent.

[0026] 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.

[0027] 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.

[0028] 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), The present invention also includes but is not limited to the following: 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.

[0029] The present invention is particularly beneficial for embodiments in which the film-forming resin comprises an epoxy resin. Epoxy resins are epoxy-functional polymers having an epoxy equivalent weight greater than 1 and typically about 2. The most commonly used epoxy resins are polyglycidyl ethers of cyclic polyols such as 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.

[0030] The film-forming resin is preferably present in the coating composition of the present invention in an amount of 1-99 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.

[0031] 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 functional groups of the resin. The type of curing agent depends on the properties of the film-forming resin. Curing agents suitable for specific film-forming resins are common knowledge to those skilled in the art.

[0032] The composition containing the epoxy resin preferably contains an aliphatic or aromatic amine curing agent, a polyamide curing agent or a thiol-based curing agent. More preferably, the curing agent comprises an amine-functional compound. Suitable epoxy resins are bisphenol A, bisphenol F, bisphenol A / F, novolacs and aliphatic epoxy resins, such as Epikote 828 from Hexion Europe BV. Suitable amine curing agents are aliphatic amines and their adducts (e.g. 2021, 2500), phenalkamine, alicyclic amine (e.g. 2196), amidoamines (e.g. 2426), polyamides and adducts thereof, and mixtures thereof. Particularly preferred are melamine resins, preferably methylated melamine resins, such as hexamethoxymethylmelamine (HMMM), which are available, for example, as 350 was purchased from Allnex.

[0033] 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.

[0034] 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.

[0035] The coating composition of the present invention comprises a lithium salt having limited solubility in water.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] For best results, the lithium salt selected from lithium carbonate, lithium phosphate, lithium bicarbonate, lithium tetraborate, and lithium oxalate is preferably present in the coating composition in an amount of at least 1.3% by weight, more preferably at least 1.6% by weight, still more preferably at least 2% by weight, more preferably at least 2.5% by weight, and even more preferably at least 3% by weight of lithium, based on the weight of the solids of the resin system. "The weight of the solids of the resin system" refers to the total dry weight of the resin system, i.e., the total dry weight of all film-forming resins and, if present, the curing agent in the coating composition. The amount of 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 more than 40%, or more than 30%, or more than 25%, or more than 20%, or more than 15%, or more 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 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.

[0040] The coating composition further comprises a zinc salt of 2,5-dimercapto-1,3,4-thiadiazole (DMTD).

[0041] 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.

[0042] In some embodiments, the zinc salt of DMTD is present in the coating composition in the absence of other zinc-containing compounds, 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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).

[0048] According to the present invention, the zinc salt of this DMTD exists as solid particles, and its surface is at least partially covered by a film-forming polymer layer. In this way, the zinc salt of this DMTD is by this polymer encapsulation. Although many polymers for encapsulation can be used, some of them are more suitable. For example, on the one hand, the polymer layer needs to protect this reinforcing agent (zinc salt of this DMTD) from the coating component that it may be reactive to it. Simultaneously, it is necessary to maintain the activity of this reinforcing agent in coating after encapsulation. It is not desirable to be bound by a specific theory, but it is believed that this activity is based on the zinc salt of this lithium salt and this DMTD leaching onto the metal substrate from coating in coating defects. Therefore, it is necessary to allow the zinc salt of this DMTD to leach from coating and give play to its strengthening effect by this encapsulated polymer matrix, while protecting this zinc salt from some coating component influences.

[0049] The polymer layer may contain one or more film-forming polymers. The polymers may be organic or inorganic. Best results are achieved with melamine-formaldehyde, urea-formaldehyde, and silica sol-gel-based encapsulations. The most preferred film-forming polymer for encapsulation is melamine-formaldehyde.

[0050] Encapsulation in silica microcapsules via a sol-gel reaction of a silica precursor is generally known. This method involves the reaction of a silane and an alkoxyorthosilicate in the presence of the compound to be encapsulated. Preferred silanes include epoxy-functional alkoxysilanes such as Dynasilan Glymo, a difunctional 3-glycidoxypropyltrimethoxysilane from Evonik Industries. A suitable alkoxyorthosilicate is, for example, tetraethoxyorthosilicate (TEOS).

[0051] Melamine-formaldehyde encapsulation is carried out by polycondensation of melamine-formaldehyde monomers in the presence of the encapsulated compound. The condensation is caused by lowering the pH and is preferably carried out in the presence of a surfactant. As the surfactant, any suitable surfactant is used, preferably a polymeric surfactant. Poly(ethylene-alt-maleic anhydride) (polyEMA) is preferably used. The surfactant can be used as a solution in a suitable solvent, for example a solution in water. The surfactant is preferably used in an amount of 0.01-20 wt %, more preferably 0.1-10 wt %, based on the metal salt solids of DMTD. Higher amounts of surfactant may require a washing step for the encapsulated material. As a result of the encapsulation, the particles are at least partially covered by a film-forming poly(melamine-formaldehyde) layer.

[0052] If it is desired to reduce the level of free formaldehyde in the encapsulating polymer, a formaldehyde scavenger may be used. A particularly preferred formaldehyde scavenger is ammonium chloride.

[0053] The solid particles covered by the polymer layer may also contain other Zn-containing compounds, such as zinc oxide and other Zn salts.

[0054] In some embodiments, the solid particles can comprise the zinc salt of DMTD as a part for mixed compound and in such cases it is preferably encapsulated as a whole. 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 formed by coprecipitation. Examples of mixed compounds include host-guest compounds, in which the host matrix is ​​inorganic and has a layered structure, and the guest compound is organic. However, in some embodiments, the zinc salt of DMTD preferably does not form a part for mixed pigment.

[0055] The inventors of the present invention have found that the combination of a lithium salt and a zinc salt of DMTD in an organic coating shows unexpectedly enhanced barrier properties of a protective layer formed on an exposed metal substrate. The protective layer is formed at locations where coating defects occur, which in the examples are simulated by scratches 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. The 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 much higher impedance values ​​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. Higher impedance values ​​mean that the protective layer has a higher barrier function or is "enhanced." Higher barrier function here refers to a higher impedance modulus value (Ωcm) measured by EIS for the same frequency range (Hz). 2 ) and higher impedance modulus values, particularly for frequencies of 10 Hz and 10 mHz. The enhanced protective layer may result in improved or longer-term protection. This enhancement of the protective layer is unexpected because, as shown in the examples below, Zn(DMTD)2 itself does not exhibit barrier properties or protective layer formation.

[0056] In addition, the present inventor has also found and embodiment has proved that the zinc salt of encapsulating this DMTD in film-forming polymer layer helps to prolong the pot life of coating composition, otherwise it is short and even too short in some cases for practical application.Preferred pot life is in 1-8 hour, more preferably in the range of 2-8 hour.Longer pot life is also acceptable usually.Conventionally unacceptable less than 30 minutes.Simultaneously, it is important that the zinc salt of this DMTD is still retained after encapsulation as the activity of reinforcing agent, and this is supported by higher impedance value in EIS measurement among the embodiment.

[0057] 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.

[0058] The one or more magnesium compounds are preferably present in the coating composition in an amount of 0.1 to 50 wt.-%, more preferably 1 to 35 wt.-%, most preferably 5 to 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).

[0059] 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. The 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, most preferably less than 8: 1.

[0060] If magnesium metal or alloy is present in the composition of the present application, 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, most preferably less than 25: 1.

[0061] Preferably the composition contains a combination of a lithium salt, magnesium oxide and a 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, for the lithium salt, magnesium oxide and zinc salt of DMTD, respectively.

[0062] In some embodiments, the coating composition further comprises one or more additional corrosion inhibitors. The 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 mercaptan compounds and azoles such as imidazoles, thiazoles, tetrazoles, triazoles such as (substituted) benzotriazoles and 2-mercaptobenzothiazole.

[0063] In some embodiments, the additional corrosion inhibitor is a mercaptan compound or azole other than zinc salt of 2,5-dimercapto-1,3,4-thiadiazole (VII). 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 the additional corrosion inhibitor.

[0064] Other compounds which can be present in the coating composition of the present application are colour pigments (e.g. titanium dioxide or yellow iron oxide), extenders (e.g. talc, barium sulphate, mica, calcium carbonate, silica or wollastonite), rheology modifiers (e.g. Bentone SD 2 or organic rheology modifiers), flow and levelling agents (e.g. polysiloxane and polyacrylate levelling additives) and solvents (e.g. ketones such as methyl isobutyl ketone, aromatics such as xylene, alcohols such as benzyl alcohol, esters such as butyl acetate and aliphatic solvents).

[0065] In a preferred embodiment, the coating composition of the present application is a liquid coating composition. The composition can comprise a volatile liquid diluent, such as a volatile organic solvent or water. The composition can be aqueous, solvent borne or solventless. The term "solventless" is defined as having a total volatile liquid diluent content of less than 5 wt.%, including water and organic solvents. The term "aqueous" is defined as having at least 50 wt.% water based on the total weight of the volatile liquid diluents, including both water and organic solvents. The coating composition is preferably solvent borne, which means that it contains organic solvents in an amount of at least 50 wt.%, more preferably at least 80 wt.% and still more preferably at least 95 wt.% based on the total weight of the volatile liquid diluents, including both water and organic solvents.

[0066] In some embodiments, the coating composition is 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 free of water (non-aqueous), which means that it does not contain any water.

[0067] The coating composition of the present application 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 and even more preferably below 50°C, most preferably at ambient conditions (25°C). In other embodiments, the coating composition is a high temperature curable composition, for example curable at a temperature of 120°C and above, preferably 140°C and above.

[0068] The non-volatile content of the coating composition is preferably in the range of 10 to 95 wt.%, more preferably 25 to 75 wt.% and even more preferably 30 to 70 wt.%.

[0069] The volatile organic content (VOC) of the coating composition (determined according to ASTM D3960) is preferably less than 700 g / L, such as less than 500 g / L and more preferably less than 300 g / L. The present application is particularly advantageous when applied in high solids systems, for example in non-aqueous solvent borne compositions having a low VOC, such as less than 350 g / l.

[0070] The coating composition can be advantageously used as a corrosion preventing primer to coat non-ferrous metal substrates such as magnesium, magnesium alloys, titanium, aluminum, aluminum alloys and aluminum / lithium alloy substrates. The preferred non-ferrous metal substrate is an aluminum alloy. Examples of suitable aluminum alloys are 2024-T3 (bare or clad), 7075-T6 (bare or clad), 2098, 2099, 2198, 6061, 6111, 6022, 5052, 5083, 5251, 5454, 7475, 7017 and 7020.

[0071] The coating composition of the present application is also suitable for coating ferrous containing substrates. Examples of suitable ferrous containing substrates are cold rolled steel, stainless steel 304, B952 (zinc phosphate modified), B1000 (ferric phosphate modified) and zinc modified steels such as EZG 60G, zinc phosphate modified EZG 60G, G90 and Galvanneal HIA Zn / Fe A45.

[0072] The coating composition of the present application can be used as a primer, tie primer, topcoat of a self-priming primer, midcoat and / or topcoat. It can be used in a coating system wherein at least two layers have the composition described in the present disclosure.

[0073] The coating composition can be applied to a substrate with and without the use of a hexavalent chromium free pretreatment such as a sol-gel system such as (AC Tech) or (Pantheon Chemical) or a chemical conversion coating.

[0074] The coating composition can also be applied to an anodized surface such as a chromic acid anodized (CAA) surface, a tartaric acid-sulfuric acid anodized (TSA) surface, a phosphoric acid anodized (SAA) surface, a phosphoric acid anodized (PAA) surface, a phosphoric acid-sulfuric acid anodized (PSA) surface and a boric acid-sulfuric acid anodized (BSAA) surface.

[0075] The coating composition can be advantageously used as a primer coating for non-ferrous metal substrates. It can be applied as a single layer or in multiple layers. In a preferred embodiment, the coating composition is applied to a substrate to form at least one primer layer in a multi-layer coating system. The topcoat layer applied over the primer layer can be a clear coating or a pigmented topcoat. Alternatively, the coating system can also include a basecoat imparting color and / or effect applied over the primer layer and a clear coating applied over the top of the basecoat. The composition can also be used as a topcoat which can be clear or pigmented.

[0076] 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.

[0077] The present invention further provides a method for coating a metal substrate comprising the steps of: a) applying a coating composition according to the first aspect of the present invention to the metal substrate, and b) curing the applied coating composition. Any suitable coating application method may be used, such as spraying, roller coating, etc. Curing may be carried out under ambient conditions (i.e., 25° C.) or alternatively at an elevated temperature, such as 60-120° C. The skilled person will be aware of suitable conditions for curing the coating composition.

[0078] The present invention further provides a metal substrate coated with a coating composition according to the first aspect of the present invention. The metal substrate may be a non-ferrous metal substrate such as aluminum or an aluminum alloy. Alternatively, the metal substrate may be a ferrous metal substrate. The substrate may be intended for exterior or interior use. Examples include structural components of aircraft, cabins, or components of vehicles.

[0079] The coating composition is particularly suitable for use in the aerospace, automotive or coil coating industries. Example

[0080] Chemicals used

[0081] ZnO—zinc white from James M. Brown Ltd

[0082] DMTD—2,5-dimercapto-1,3,4-thiadiazole from Chemical Point Ltd.

[0083] Poly-EMA—poly(ethylene-alt-maleic anhydride) from Sigma-Aldrich, average molecular weight approximately 400,000

[0084] Cymel 350—a highly methylated melamine resin from Allnex, consisting of the commercial form of hexamethoxymethylmelamine (HMMM)

[0085] Epikote 828—a medium viscosity liquid epoxy resin produced from bisphenol A resin and epichlorohydrin from Hexion Europe BV

[0086] Solsperse 32500—polymeric dispersant in n-butyl acetate from Lubrizol Corp.

[0087] MPA 2000X - Rheology Additive from Elementis

[0088] BYK 358N - Polyacrylate-based surface additive from BYK (member of the Altana Group)

[0089] Actiron NX3—2,4,6-Tris(dimethylaminomethyl)phenol, a catalyst for epoxy coatings from Protex

[0090] Ancamine 2500—aliphatic amine-based curing agent from Evonik Industries Dynasylan DAMO—N-2-aminoethyl-3-aminopropyltrimethoxysilane from Evonik Industries

[0091] Dynasylan Glymo—Difunctional 3-glycidyloxypropyltrimethoxysilane from Evonik Industries

[0092] DABCO—1,4-diazabicyclo[2.2.2]octane from Evonik Industries

[0093] TEOS—Tetraethoxyorthosilicate from Sigma Aldrich, 98% grade

[0094] Lithium carbonate—ACS reagent, ≥99.0%, from Sigma Aldrich

[0095] Lithium oxalate, lithium phosphate, MgO, NH4Cl—all from Sigma Aldrich Desmophen 650MPA—branched polyester with hydroxyl groups from Covestro AG, 65 wt

[0096] %, in 1-methoxy-2-propyl acetate (MPA), 5.3% OH content

[0097] Tioxide TR92 - Titanium dioxide from Huntsman

[0098] Zn(DMTD)2—prepared according to Example 1 of WO 02 / 092880A1

[0099] Airwhite AW 15—Barium sulfate from Sibelco Specialty Minerals

[0100] 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%

[0101] Silquest A-187—epoxy-functional silane from Momentive Performance Materials Hybricor 204—Zn(DMTD)2-containing pigment from WPC Technologies

[0102] Preparation of test plates

[0103] Unless otherwise specified, the test panels were 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).

[0104] 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.

[0105] Electrochemical impedance spectroscopy (EIS)

[0106] 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).

[0107] 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.

[0108] 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.

[0109] Viscosity measurement

[0110] The viscosity of the mixed coating composition was measured using an ISO-#4 flow cup according to standard method ISO 2431-19. In this method, the viscosity is measured in seconds while the cup with a defined aperture is emptied. The sample is stirred while avoiding the formation of bubbles. The orifice of the flow cup is closed with a finger and the cup is filled until the sample flows over the edge. Excess paint is scraped off by sliding a glass plate over the edge. A container is placed under the flow cup. The time measurement begins simultaneously with the removal of the finger from the orifice. As soon as the first interruption in the flow occurs near the orifice, the time measurement is stopped and the flow time is recorded to the nearest 0.5 s. This is performed within 2 hours after mixing the coating composition components.

[0111] Example 1—Preparation of Zn(DMTD)2

[0112] The procedure was adapted from Example 1 of WO 02 / 092880A1 to fit a 2-liter, 3-necked flask. 62g of ZnO (0.76mol) was mixed with 222g of demineralized water and heated at 60°C for 1 hour in a 2-liter, 3-necked glass flask. 222g of DMTD (1.48mol) was simultaneously mixed with 1480g of demineralized water. The DMTD / water mixture was added to the ZnO / water in 4 portions to prevent the temperature from dropping too much. After mixing all the ingredients, the reaction was carried out at 60°C for 2 hours. After the reaction was complete, the agitator was stopped and the material was cooled. After decanting and increasing the solid content to >20%, the aggregate particle size was reduced by using a 0.5mm ceramic bead mill (using WAB's ECM Dynomill Multi Lab). The slurry was passed through the mill until substantially no particles with a particle size >20μm were left. The particle size was determined by laser diffraction (Malvern Mastersizer 2000). The final slurry had a d of approximately 4 microns. 90 The solids content was 16% by weight.

[0113] Example 2—Preparation of MF-modified Zn(DMTD)2

[0114] MF1 : 1406 g of the slurry obtained in Example 1 was used for MF modification. 90 g of poly(ethylene-alt-maleic anhydride) (poly-EMA, average molecular weight approximately 400,000) powder was mixed with 1410 ml of demineralized water, shaken for 3 days, and then allowed to stand. 375 g of this mixture was mixed with 1.7 g of NHCl for 1 hour. This equates to 22.5 g of poly-EMA, representing 10% based on the solid Zn(DMTD)₂. The Zn(DMTD)₂ slurry and the EMA / water / NH₄Cl mixture were added to a 2000 ml three-necked flask and heated to 55° C. The pH of the emulsion measured at 55° C. was 3.0. A solution of 45 g of Cymel 350 (20% based on the solid Zn(DMTD)₂) and 55 g of demineralized water was added to the system. The pH of the slurry was adjusted to 2.3 at 55° C. using 10% by volume H₂SO₄. After 3.33 hours, the heat was turned off and the mixture was stirred overnight without heating. Agitation was stopped the next morning. The dry material was recovered after centrifugation once and drying.

[0115] The product was washed to remove the surfactant according to the following method. The material was centrifuged in a Heraeus Varifuge F at 2500 / 3000 rpm for 10 minutes, and the upper phase was removed. Water was added, the material was homogenized, and the material was centrifuged again. This process was repeated until the material had been centrifuged four times. The water was removed and the material was dried in a vacuum oven. The material was then ground using a pestle and mortar.

[0116] MF2 and MF3:The above method was repeated by using different amounts of the surfactants poly-EMA and Cymel 350. In addition, because the amount of surfactant was low, the material was not washed, which did not adversely affect the viscosity test. The differences between procedures MF mod 1, mod 2 and mod 3 are summarized in Table 1 below.

[0117] Table 1

[0118] Number <![CDATA[聚-EMA / Zn(DMTD)2]]> %Cymel 350 / Zn(DMTD)2 Start pH adjustment Washing MF 1 10％ 20％ 2.3 Yes MF 2 1％ 12.5％ 2.3 No MF 3 2.5％ 20％ 2.3 No

[0119] Example 3 - Coating Composition

[0120] Formulations 1-1 to 1-3 were prepared using the ingredients listed in Table 2. The ingredient amounts are shown in parts by weight (g). The formulations were prepared according to the above method. The formulations had a PVC of 30% and a VOC of 350 g / L.

[0121] Table 2

[0122] 1-1* 1-2* 1-3 Li only Li + Zn(DTMD)2 Li+MF modified Zn(DMTD)2 Component A 2-heptanone 19.18 19.18 19.18 Methyl ethyl ketone 11.34 11.34 11.34 Epikote 828 64.60 64.60 64.60 Solsperse 32500 4.90 4.90 4.90 M-P-A 2000X 2.01 2.01 2.01 BYK 358N 0.62 0.62 0.62 Magnesium oxide 48.60 48.60 48.60 Titanium dioxide 27.59 27.59 27.59 Barium sulfate 44.17 22.43 22.43 Lithium carbonate 18.06 18.06 18.06 Zn(DMTD)2 - 10.15 - <![CDATA[MF 1:MF改性Zn(DMTD)2]]> - - 10.15 Component B Xylene 6.72 6.72 6.72 Benzyl alcohol 5.34 5.34 5.34 Actiron NX3 2.24 2.24 2.24 Ancamine 2500 37.22 37.22 37.22 Dynasilan DAMO 5.43 5.43 5.43 Component C Methyl ethyl ketone 17.36 17.36 17.36

[0123] *Comparative Example

[0124] Formulations 1-1 and 1-2 are comparative formulations, whereas Formulation 1-3 is according to the present invention.

[0125] The pot life of the coating compositions was evaluated by measuring the development of flow cup viscosity over time. The results are shown in Figure 1 middle.

[0126] It can be seen that the sample containing unmodified Zn(DMTD)2 develops a high viscosity from the beginning, likely due to the reaction of Zn(DTMD)2 with the coating components. However, when Zn(DTMD)2 is encapsulated in a melamine-formaldehyde polymer layer, the viscosity remains low and comparable to the sample without Zn(DMTD)2.

[0127] Formulations 1-1 to 1-3 were applied to Al 2024T3 panels as described in Preparation of Panels and tested as described in Methods Used. The results of the EIS measurements are listed in Table 3.

[0128] Table 3

[0129] 1-1* 1-2* 1-3 <![CDATA[ EIS results ]]> Zmod (kQ) at 10 mHz 162.5 500 425 Zmod (kQ) at 10 Hz 1.9 3.7 4.4

[0130] The results in Table 3 indicate that the synergistic effect between the Li salt and Zn(DMTD)2 is retained when Zn(DMTD)2 is encapsulated.

[0131] Example 4—Encapsulation with alternative polymers

[0132] Encapsulation with silane

[0133] A Zn(DMTD)2-containing pigment (Hybricor 204, available from WPC Technologies) was encapsulated with silane according to the following method. 75 g of Hybricor 204 was added to 87 g of methyl amyl ketone. 0.83 g of Dynasylan Glymo and 0.015 g of DABCO were then added to the mixture. The mixture was heated to 40°C with stirring for 3 hours, after which 21 g of tetraethoxyorthosilicate (TEOS) was added along with 0.4 g of 0.1 M NaOH. The mixture was stirred at 40°C for 6 hours using a glass stirrer to yield a silane-encapsulated Zn(DMTD)2-containing pigment (55% solids in MEK).

[0134] Encapsulation with acrylic polymer

[0135] The encapsulation of Zn(DMTD)2 with acrylic polymers was carried out according to a proprietary method similar to that described in EP0477433 B1, Example 1. For the sample "Acrylic Modified 1", 11.5 wt% of polyacrylate polymer based on Zn(DMTD)2 solids was used, while for the sample "Acrylic Modified 2", 9.7 wt% of polyacrylate polymer based on Zn(DMTD)2 solids was used.

[0136] Coating compositions were prepared in the same manner as formulations 1-3 in Example 3, but using different encapsulated pigments instead of MF 1. The viscosity of the coating compositions after mixing was followed over time as described above.

[0137] The results of the viscosity measurements are shown in Figure 2 middle.

[0138] It can be concluded that encapsulation with melamine-formaldehyde (MF) and silane provides the best encapsulation.

[0139] The synergistic effect between the lithium salt and the zinc salt of DMTD is further shown in Comparative Examples 1-7, which are all comparative because the zinc salt of DMTD was used in an unencapsulated form.

[0140] Comparative Example 1—Synergistic Effect Between Li Salt and Zn(DTMD)2

[0141] Formulations C1-1 to C1-4 were prepared using the ingredients listed in Table 4. The ingredient amounts are shown in parts by weight (g). The amounts of the active ingredients Li and DMTD are also shown as weight % based on the solids of the resin system.

[0142] 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.

[0143] 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.

[0144] The results of EIS measurements are also listed in Table 4.

[0145] Table 4

[0146] C1-1 C1-2 C1-3 C1-4 Component A Methyl isobutyl ketone 60 60.0 60.0 60.0 Desmophen 650 MPA 47.7 47.7 47.7 47.7 Lithium carbonate - - 12.0 10.0 Tioxide TR92 19.9 19.9 19.9 19.9 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 75 MPA / 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 ingredients, in weight-% based on the solids of the resin system ]]> Li - - 4.4 3.6 DMTD - 9.5 - 9.5 <![CDATA[ EIS results ]]> Zmod (kQ) at 10 mHz 27 31 180 821 Zmod (kQ) at 10 Hz 0.5 0.7 2.5 6

[0147] It can be seen that the coating with both Zn(DMTD)2 and Li salt (C1-4) shows significantly higher impedance |Z| (also called "Zmod") values ​​compared to the formulations without any active component (C1-1), or with only Zn(DMTD)2 (C1-2), or with only Li salt (C1-3). This effect is much stronger than the sum of the individual effects of Li salt (C1-3) and Zn(DMTD)2 (C1-2) and is therefore a synergistic effect.

[0148] The results also show that Zn(DMTD)2 (C1-2) alone has no effect on forming a protective layer in defect areas. In particular, coating C1-2 has a resistance value as low as the coating without (negative control) (C1-1).

[0149] Comparative Example 2—Comparison with other azoles

[0150] Formulations C2-1 to C2-7 were prepared in the same manner as Comparative Example 1, but using the ingredients shown in Table 5. 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 for all formulations in this example. BTA is benzotriazole, and 2-MBT is 2-mercaptobenzothiazole. The results of EIS measurements are also listed in Table 5.

[0151] Table 5

[0152]

[0153]

[0154] The results 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 regions, indicating a better enhancement of the protective layer in the defective regions.

[0155] Comparative Example 3—Li salts of different concentrations

[0156] Formulations C3-1 to C3-8 were prepared in the same manner as Comparative Example 1, but using the ingredients listed in Table 6. Ingredient amounts are shown in parts by weight (g). The amount of active ingredient is also given as wt % based on the resin system solids. The results of the EIS measurements are also listed in Table 6. In these experiments, the formulations contained a Li salt and Zn(DTMD)2, with the Li salt concentration being varied.

[0157] Table 6

[0158]

[0159] It can be seen that significantly higher impedance values ​​are achieved 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.

[0160] Comparative Example 4—Zn(DMTD)2 at different concentrations

[0161] Formulations C4-1 to C4-8 were prepared in the same manner as Comparative Example 1, but using the ingredients shown in Table 7. Ingredient amounts are shown in parts by weight (g). The amounts of active ingredients are also given as weight percent based on the resin system solids. The results of EIS measurements are also listed in Table 7. In these experiments, the active materials included a lithium salt and Zn(DTMD)2, with the Zn(DTMD)2 concentration being varied.

[0162] Table 7

[0163]

[0164] 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.

[0165] Comparative Example 5—Different Li Salts (4.4% Li)

[0166] Formulations C5-1 to C5-8 were prepared in the same manner as Comparative Example 1, but using the ingredients shown in Table 8. The ingredient amounts are shown in parts by weight (g). The amounts of active ingredients are also given in weight percent based on the resin system solids. The results of the EIS measurements are also listed in Table 8. In these tests, the coatings contained various soluble lithium salts, with a lithium content of 4.4 weight percent based on the resin system solids. The lithium salts were selected based on their varying solubility.

[0167] Table 8

[0168]

[0169]

[0170] It can be concluded that Zn(DTMD)2 shows an enhanced effect with Li salts regardless of the Li salt used. The moderate effect of lithium phosphate is believed to be due to its lower solubility in water. Comparative Example 6 - Different Li salts (1.3 wt% Li)

[0171] Formulations C6-1 to C6-6 were prepared in the same manner as Comparative Example 1, but using the ingredients shown in Table 9. Ingredient amounts are shown in parts by weight (g). The amount of active ingredient is also given in weight percent based on the resin system solids. The results of the EIS measurements are also listed in Table 9. In these experiments, the coatings contained various soluble Li salts, with a Li content of 1.3 weight percent based on the resin system solids.

[0172] Table 9

[0173]

[0174]

[0175] 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 Comparative Example 5), with the exception of lithium phosphate. Better results are achieved with higher Li contents for lithium phosphate (Comparative Example 5), which is believed to be due to its lower solubility in water.

[0176] Comparative Example 7—No Magnesium Oxide

[0177] Formulations C7-1 to C7-4 were prepared in the same manner as Comparative Example 1, but using the ingredients shown in Table 10. Ingredient amounts are given in parts by weight (g). The amount of active ingredient is also given in weight percent based on the solids content of the resin system. The results of the EIS measurements are also listed in Table 10. These experiments tested whether the reinforcing effect also exists in compositions without magnesium oxide.

[0178] Table 10

[0179] C7-1 C7-2 C7-3 C7-4 Component A Methyl isobutyl ketone 60.0 60.0 60.0 60.0 Desmophen 650 MPA 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 Zn(DMTD)2 - - 6.0 6.0 Component B Desmodur N 75 MPA / 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 Active ingredients, in weight-% based on the solids of the resin system ]]> ​ Lithium carbonate - 23.0 - 19.0 Zn(DMTD)2 - - 11.4 11.5 Li - 4.4 - 3.6 DMTD - - 9.5 9.5 EIS results ]]> ​ Zmod (kQ) at 10 mHz 40 94 57 459 Zmod (kQ) at 10 Hz 0.6 1.9 0.8 4

[0180] 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.

[0181] Comparative Example 8—Lithium Phosphate at Different Concentrations

[0182] Formulations C8-1 to C8-6 were prepared in the same manner as Comparative Example 1, but using the ingredients shown in Table 11. 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.

[0183] 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.

[0184] Table 11

[0185]

[0186] The 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 11.

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 0.01-120 g / L, selected from the group consisting of lithium carbonate, lithium phosphate, lithium bicarbonate, lithium tetraborate and lithium oxalate, and c) a zinc salt of DMTD, wherein the zinc salt of DMTD is 2,5-dimercapto-1,3,4-thiadiazole zinc salt, The zinc salt of DMTD is present as solid particles, the surface of which is at least partially covered by a film-forming polymer layer, and the organic film-forming resin is an epoxy resin.

2. The coating composition according to claim 1, wherein the resin system comprises a curing agent, and wherein the curing agent is selected from the group consisting of aliphatic or aromatic amines, polyamides and thiols.

3. The coating composition according to claim 2, wherein the curing agent comprises a melamine resin.

4. The coating composition according to claim 1, wherein the film-forming polymer is a melamine-formaldehyde polymer.

5. The coating composition according to claim 1, wherein the lithium salt is present in an amount of at least 1.3 wt% lithium based on resin system solids.

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

7. The coating composition according to claim 6, wherein the lithium salt is selected from lithium carbonate and lithium oxalate.

8. The coating composition according to claim 6, wherein the lithium salt is lithium carbonate.

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

10. The coating composition according to claim 9, wherein the lithium phosphate is present in an amount of at least 3.0 wt. % lithium.

11. The coating composition according to claim 9, wherein the lithium phosphate is present in an amount of at least 4.0 wt. % lithium.

12. The coating composition according to claim 1, wherein the zinc salt of DMTD is present in an amount of 0.05 to 50 weight percent DMTD based on the solid weight of the resin system.

13. The coating composition according to claim 1, which is a solvent-borne composition.

14. A multilayer coating system on a metal substrate comprising a layer obtained from a coating composition according to any one of claims 1 to 13.

15. The multi-layer coating system according to claim 14, wherein the layer obtained from the coating composition is a primer layer on the metal substrate.

16. A method for coating a metal substrate, comprising the steps of: a) applying a coating composition according to any one of claims 1 to 13 to the metal substrate, and b) curing the applied coating composition.

17. A metal substrate coated with a coating composition according to any one of claims 1 to 13.

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