Polyurea coating systems as sealants for exterior of fuel tanks

Abstract

To provide a method of sealing a pre-coated surface of a substrate serving as a fuel tank.SOLUTION: There is provided a 2K coating system consisting of two components (A) and (B) being separate from each other, wherein the component (A) comprises at least one constituent (a1) containing at least one aromatic moiety and having an average of at least two primary and / or secondary amino groups, wherein the component (B) comprises at least one constituent (b1) containing at least one aromatic moiety and having an average of at least two isocyanate groups, and wherein the constituent (b1) has carbodiimide and / or uretonimine units as well as at least one structural unit (I). The method comprises a step of applying a coating composition onto a pre-coated surface of a substrate, wherein a coating composition is obtainable by mixing the components (A) and (B) of the 2K coating system.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a 2K coating system consisting of two components (A) and (B) separated from one another, wherein (A) comprises at least one component (a1) which comprises at least one aromatic moiety and has on average at least two primary and / or secondary amino groups, and (B) comprises at least one component (b1) which comprises at least one aromatic moiety and has on average at least two isocyanate groups, each of (A) and (B) having a solids content of at least 95% by weight, based on the total weight of the respective component, and wherein component (b1) present in component (B) comprises carbodiimide units and / or uretonimine units and at least one structural unit (I); to a coating composition obtained by mixing components (A) and (B) of the coating system of the present invention; to the use of said coating composition as a sealant, in particular for sealing the exterior of a fuel tank; to a method for applying the coating composition to a substrate, in particular for sealing the exterior of a fuel tank; and to a sealed substrate, such as a sealed fuel tank, obtained by this method. [Background technology]

[0002] Due to the increasing demand for aircraft and the aircraft production rate, aircraft manufacturers are interested in reducing process times in aircraft manufacturing. One process that needs to be accelerated is the application of so-called secondary fuel barrier coatings (SFB coatings) to aircraft fuel tanks. The process time varies depending on the product used. To obtain such SFB-coated fuel tanks, a sealant is sprayed onto the exterior of the fuel tank inside the aircraft fuselage. Depending on the tank's shape, other application methods, such as brushing, are also possible. Furthermore, other application methods, such as brushing, can be used for maintenance work. Fuel tanks are typically sealed from the inside with a suitable sealant, such as a polysulfide sealant. For additional protection, a further coating is applied to the external pre-coated surface of the fuel tank. In the event of a defect in the internal seal, the externally applied sealant prevents fuel or fuel vapors from leaking from the tank and entering the cargo and / or passenger areas. This externally applied sealant is referred to as the SFB coating. The sealant must be resistant to the fuel. Additionally, the sealant must be transparent (ie, clear) so that cracks and / or general damage to the tank surface can be easily and immediately detected from the outside in order to be repaired.

[0003] Polyurea and / or polyurethane coatings for various purposes are known from the prior art, e.g., US 2012 / 0183692 A1, US 6,605,684 B2, US 9,328,274 B2, JP 2012-92266 A, and JPH-11130834, and WO 2016 / 059083 A1. Transparent polyurea and / or polyurethane coatings prepared from solventless coating compositions are known, for example, from US 2012 / 0183692 A1. In particular, coatings prepared from polyaspartic amines and polyisocyanates are disclosed in this document. These coatings are used as barrier coatings to protect the substrate to which they are applied. A high-volume, low-pressure (HVLP) gun is used to apply the coating composition to the substrate. The composition is cured at an elevated temperature, such as about 54-65°C. The preparation of polyurea elastomers is disclosed in US 6,605,684 B2. Aspartic acid esters are formed, which are blended with polyoxyalkyleneamines and then reacted with isocyanates to produce the desired polyurea elastomers. US 9,328,274 B2 relates to urethane- and sulfur-containing prepolymers prepared from isocyanate-terminated urethane-containing adducts and bis(vinyl-sulfonyl)alkanols, as well as compositions containing such prepolymers and amine catalysts. Cured sealants, which are polyurethanes due to crosslinking reactions between NCO groups and OH groups of the alkanols, are obtained from these adducts. Polyurea coatings prepared from aliphatic isocyanate compounds are disclosed in JP 2012-92266 A. Polyureas prepared from mixtures of polyamines, including necessarily non-aromatic polyamines, are disclosed in JPH-11130834. WO2016 / 059083A1 discloses a polyurea coating for repairing defects in elastomeric substrates and / or for adhesively bonding these substrates together.

[0004] Polyurea sealants for aircraft antennas or other aircraft components are disclosed in US Pat. Nos. 10,052,817 B2 and 9,765,888 B2. These sealants are flame-retardant and can consist of two components that are applied using an assembly that includes a special cartridge system. A mixing rod is installed on top of this cartridge system. Both components are manually pushed into the mixing rod and mixed. The sealants are linear polyurea components and may contain carbon black to improve UV resistance. However, the sealants described in US Pat. Nos. 10,052,817 B2 and 9,765,888 B2 are not sprayable.

[0005] WO 2017 / 172906 A2 discloses a specialized cartridge system for use with a Sulzer mixing spray gun, used to apply a sprayable two-component sealant, which is a polyurea, polyurethane, and / or polyurea-polyurethane hybrid sealant, to aircraft components. Both components are separated into two chambers, the outlets of which connect to a mixing rod, where they are mixed (manually or pneumatically) by passing them through the mixing rod. This sealant is fuel-resistant and can therefore be used as a secondary fuel vapor barrier to prevent fuel leakage in the event of a leak in the tank. The sealant is transparent, i.e., clear. However, a color change is observed after application, which is a disadvantage when the sealant is applied as a secondary fuel vapor barrier, as it may interfere with the detection of leaks in fuel tanks. Furthermore, the disclosed sealant is not necessarily VOC-free, which is problematic from an environmental standpoint. A further disadvantage of the cartridge system disclosed in WO2017 / 172906A2 is that use of the Sulzer mixing system described therein typically generates a large amount of plastic waste, as the cartridges and mixing rods are made of plastic, which must be discarded after use.

[0006] Furthermore, while commercially available coating systems for providing SFB coatings, such as the commercially available polyurethane-based product "PR-1199," often require relatively long curing times and typically contain organic solvents, which is undesirable at least for environmental reasons and / or VOC regulations. For example, in many cases, a flash-off period of approximately two hours is required after application so that any solvent present begins to slowly evaporate, followed by 24 hours of curing at elevated temperatures. This process therefore requires a waiting time of at least 26 hours, which is both time-consuming and energy-consuming and therefore uneconomical. Furthermore, commercially available coating systems for providing SFB coatings typically contain not only organic solvents but also flame retardants, such as phosphorus-containing flame retardants, which is undesirable at least for environmental reasons and also economically, for example, because safety regulations (which must be considered and implemented) may apply. Polyurea compositions that necessarily contain organic solvents as stabilizers and therefore have an undesirably high VOC content are further known from WO 00 / 69943 A1.

[0007] The prior art also knows coating compositions for use as sealants that cure upon irradiation with UV light. Such sealants and respective coating methods are disclosed, for example, in WO 2016 / 128548 A1, WO 2016 / 128547 A1, EP 2836562 B1, EP 1478703 B1, and EP 1385904 B1. Furthermore, WO 2018 / 031532 A1 discloses a composition containing a polythiol and a urea- and / or urethane-containing polyalkenyl. This document further discloses a two-part sealant system, in which the polythiol is contained in a first part and the polyalkenyl is contained in a second part. The compositions and systems can be used to prepare actinic radiation-curable coatings and sealants for the aerospace industry, particularly as SFB coatings. The compositions of WO 2018 / 031532 A1 are sulfur-containing compositions, which are disadvantageous at least for environmental reasons. Furthermore, the curing reaction necessarily requires UV light, which requires expensive UV equipment and additional work steps, resulting in additional costs and time. Furthermore, additional safety regulations may apply, such as requiring employees to take special protective measures against UV light. Furthermore, the use of UV light sources does not always allow for sufficient curing in areas that are difficult to access with a UV lamp due to the shape of some parts being coated, resulting in the risk of the sealant not fully curing due to insufficient UV light exposure, resulting in so-called "shadow areas." Furthermore, such UV-curable compositions often still contain organic solvents, which is undesirable at least for environmental reasons and / or VOC regulations.

[0008] Therefore, there is a need for a coating system, a coating composition, and a method for coating a substrate that does not exhibit the aforementioned drawbacks, particularly when the substrate is a fuel tank and the coating composition is used to provide an SFB coating. In particular, there is a need for a respective coating system and composition that is essentially or completely VOC-free, yet is sprayable and / or manually processable, particularly sprayable, and allows for autocatalytic rapid curing at low temperatures, particularly curing that occurs at lower temperatures and in shorter times than conventional coating systems and compositions. At the same time, cracks and / or damage that may be present on the fuel tank surface should be easily detectable from the outside to allow for immediate repair. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] US2012 / 0183692A1 [Patent Document 2] US6,605,684B2 [Patent Document 3] US9,328,274B2 [Patent Document 4] JP2012-92266A [Patent Document 5] JPH-11130834 [Patent Document 6] WO2016 / 059083A1 [Patent Document 7] US10,052,817B2 [Patent Document 8] US9,765,888B2 [Patent Document 9] WO2017 / 172906A2 [Patent Document 10] WO00 / 69943A1 [Patent Document 11] WO2016 / 128548A1 [Patent Document 12] WO2016 / 128547A1 [Patent Document 13] EP2836562B1 [Patent Document 14] EP1478703B1 [Patent Document 15] EP1385904B1 [Patent Document 16] WO2018 / 031532A1 Summary of the Invention [Problem to be solved by the invention]

[0010]

[0006] Accordingly, an object underlying the present invention was to provide a coating system and a coating composition suitable for coating a substrate that does not exhibit the drawbacks of conventional coating systems and compositions of the prior art, particularly when the substrate is a fuel tank and the coating system and composition are used to provide an SFB coating. A specific object underlying the present invention was to provide a respective coating system and composition that is essentially or completely free of VOCs, yet still allows for sprayability and autocatalytic rapid curing at low temperatures, particularly at temperatures and in shorter times than conventional coating systems and compositions known in the prior art. At the same time, these coatings and compositions should allow for the immediate detection of cracks and / or damage that may be present on the fuel tank surface in order to carry out repairs as quickly as possible. [Means for solving the problem]

[0011] This object is solved by the subject matter of the present claims as well as by the preferred embodiments thereof disclosed herein, i.e. by the subject matter described herein.

[0012] The first subject of the present invention is a two-component (2K) coating system consisting of two components (A) and (B) separate from one another, Component (A) comprises at least one constituent (a1) containing at least one aromatic moiety and having at least two primary and / or secondary amino groups; Component (B) comprises at least one constituent (b1) containing at least one aromatic moiety and having at least two isocyanate groups; each of components (A) and (B) having a solids content of at least 95% by weight, based on the total weight of the respective component; In a two-component (2K) coating system, the amount of any filler present in component (A) does not exceed 5% by weight, based on the total weight of component (A), The constituent (b1) present in component (B) has at least one carbodiimide unit and / or uretonimine unit and further has at least one structural unit (I): [ka] (where R 1 is a C2-C8 alkylene residue, and the parameter m is an integer ranging from 1 to 200. A two-component (2K) coating system comprising:

[0013] A further subject of the present invention is a coating composition obtainable by mixing together components (A) and (B) of the coating system according to the invention.

[0014] A further subject of the present invention is the use of the coating composition of the present invention as a sealant, particularly preferably for providing a barrier coating on a precoated substrate, preferably on the exterior of a precoated fuel tank, in particular preferably on the exterior of a precoated aircraft fuel tank.

[0015] A further subject of the present invention is a method for sealing a precoated surface, preferably of a substrate, preferably of the exterior of a fuel tank, in particular of an aircraft fuel tank, comprising at least step (1), namely (1) applying a coating composition of the present invention to a preferably precoated surface of a substrate, preferably to a preferably precoated surface of the exterior of a fuel tank, particularly to a preferably precoated surface of the exterior of an aircraft fuel tank; The method includes:

[0016] A further subject of the invention is a sealed substrate obtainable by the process of the invention, for example a sealed fuel tank, preferably a fuel tank with an external seal, in particular an aircraft fuel tank with an external seal.

[0017] It has surprisingly been found that the inventive coating system and the inventive coating composition are suitable for coating substrates, in particular fuel tanks, such as fuel tanks used in the aerospace and / or aircraft industries. In particular in this regard, it has been found that the inventive coating system and the inventive coating composition are suitable for providing an SFB coating to the exterior surface of such fuel tanks, i.e., for the exterior seal of such fuel tanks.

[0018] Even more surprisingly, it has been discovered that the coating system and coating composition of the present invention allow for rapid autocatalytic curing at low temperatures, such as room temperature (18-29°C), particularly at temperatures lower and in shorter times than conventional coating systems and compositions known in the prior art. This reduction in processing time translates into reduced waiting times for customers while their products are cured. It has been discovered that the curing of the coating composition of the present invention takes less than 2 hours, particularly from 2 to 1.5 hours. Furthermore, curing does not require the application of high temperatures, such as 40°C or higher, nor does it require the application of external impulses, such as UV light, for curing to occur. Therefore, no additional energy is required.

[0019] It has further been found that the coating systems and compositions of the present invention can be provided in an essentially or completely VOC-free form, particularly for the above-mentioned purposes. Nevertheless, the coating systems of the present invention can still be sprayed using a high-pressure system for mixing and application, particularly using countercurrent injection technology, which allows mixing and spraying to occur simultaneously. It has also been found that the use of such a high-pressure system does not result in the undesirable generation of plastic waste, unlike, for example, the use of a Sulzer mixing spray gun, as disclosed in WO 2017 / 172906 A2. Furthermore, because the coating compositions of the present invention are solvent-free or essentially solvent-free (because they are essentially or completely VOC-free when applied to substrates such as fuel tanks), they do not need to be "set" after application. Therefore, no flush-off period is required.

[0020] It has further been found that the coating composition of the present invention is transparent (clear) when applied as a sealant. This allows for the immediate detection of potential cracks and / or damage to the surface of a fuel tank when used in the field of SFB coatings, allowing for the quickest possible repairs. Surprisingly, it has been found that the coating obtained from the coating composition of the present invention exhibits significantly superior properties in terms of tensile strength, Shore A hardness, and / or elongation at break than conventional opaque polyurea coatings obtained from coating compositions containing a large amount of filler, for example.

[0021] Furthermore, the coating composition of the present invention, when applied as a sealant, has been found to be color stable, i.e., does not change color after mixing, unlike applied sealants such as those disclosed in WO 2017 / 172906 A2.

[0022] Detailed Description of the Invention Coating system of the present invention and coating composition of the present invention The coating system of the present invention is a two-component (2K) coating system consisting of two components (A) and (B) that are separate from each other. For example, components (A) and (B) of the coating system of the present invention can be stored separately until they are mixed together to prepare a coating composition that can be used, in particular, as a sealant.

[0023] The coating system of the present invention comprises component (A) containing at least one component having a primary and / or secondary amino group and component (B) containing at least one component having an isocyanate (NCO) group. Thus, when these two components are mixed, a polyurea or polyurea-based coating composition is formed due to the reaction between the amino and isocyanate groups. A "polyurea coating composition" within the meaning of the present invention is formed when component (A) does not contain an additional component having OH groups. A "polyurea-based coating composition" within the meaning of the present invention is formed when component (A) contains an additional component having OH groups (however, the OH groups are present in a significantly lower amount than the amount of amino groups), which, in addition to forming a polyurea, results in the additional formation of urethane groups upon crosslinking with the isocyanate groups.

[0024] Preferably, the coating system of the present invention is a sealant system consisting of components (A) and (B).

[0025] Preferably, neither component (A) nor (B) of the coating system of the present invention contains a curing catalyst. Of course, the coating composition of the present invention also preferably does not contain a curing catalyst. The coating system of the present invention is preferably autocatalytically crosslinkable, i.e., the relevant components present therein are capable of crosslinking via an autocatalytic reaction, and therefore no curing catalyst is required.

[0026] Preferably, neither component (A) nor (B) of the coating system of the present invention contains any component containing one or more sulfur atoms. Preferably, of course, the coating composition of the present invention also does not contain any component containing one or more sulfur atoms. If any component having one or more sulfur atoms is present in one of components (A) and (B) of the coating system of the present invention and / or in the coating composition of the present invention, it is preferably selected from the group consisting of only mercaptosilanes.

[0027] Preferably, neither component (A) nor (B) of the coating system of the present invention contains a flame retardant. In particular, neither component (A) nor (B) of the coating system of the present invention contains a phosphorus-containing flame retardant. Of course, the coating composition of the present invention also preferably does not contain any flame retardants, in particular phosphorus-containing flame retardants.

[0028] Preferably, both components (A) and (B) of the coating system of the present invention are free or essentially free of organic solvents. Preferably, the same applies to the coating composition of the present invention. In the sense of the present invention, the term "free of organic solvents" preferably means that no organic solvents are present at all. In the sense of the present invention, the term "essentially free of organic solvents" preferably means that no organic solvents are present at all. This means that at least no organic solvents are intentionally added to either components (A) and (B) used in the present invention, as well as to the coating composition of the present invention. However, it cannot be excluded that residual fractions of organic solvents formed during the preparation of any of the components used to prepare components (A) and (B) used in the present invention may be present therein. Preferably, the amount of any organic solvent present in each of components (A) and (B) is less than 10% by weight, more preferably less than 8% by weight, even more preferably less than 6% by weight, even more preferably less than 5% by weight, still more preferably less than 2.5% by weight, in particular less than 1.0% by weight, or less than 0.5% by weight, and most preferably less than 0.1% by weight, or less than 0.05% by weight, or less than 0.01% by weight, in each case based on the total weight of component (A) or (B).

[0029] The solids content of component (A) of the coating system according to the invention is at least 95% by weight, even more preferably greater than 95% by weight, even more preferably greater than 97.5% by weight, in particular greater than 98% by weight or greater than 99% by weight or greater than 99.5% by weight, and most preferably 100% by weight, in each case based on the total weight of component (A). The solids content of component (B) of the coating system according to the invention is at least 95% by weight, preferably greater than 95% by weight, more preferably greater than 97.5% by weight, in particular greater than 98% by weight or greater than 99% by weight or greater than 99.5% by weight, and most preferably 100% by weight, in each case based on the total weight of component (B). The solids content of the coating composition according to the invention is preferably greater than 95% by weight, more preferably greater than 97.5% by weight, in particular greater than 98% by weight or greater than 99% by weight or greater than 99.5% by weight, and most preferably 100% by weight, in each case based on the total weight of the coating composition. The measurement of the solids content, i.e., the non-volatile content, is carried out according to the method described below.

[0030] Preferably, both components (A) and (B) of the coating system of the present invention are free or essentially free of water. The same applies to the coating composition of the present invention. In the sense of the present invention, the term "free of water" preferably means that water is completely absent. In the sense of the present invention, the term "essentially free of water" preferably means that water is essentially absent. This means that at least water is not intentionally added to either components (A) or (B) used in the present invention, or to the coating composition of the present invention. However, it cannot be excluded that residual fractions of water formed during the preparation of any of the components used to prepare components (A) and (B) used in the present invention are present therein. Preferably, the amount of any water present in each of components (A) and (B) is less than 1% by weight, more preferably less than 0.5% by weight, even more preferably less than 0.1% by weight, even more preferably less than 0.05% by weight, even more preferably less than 0.01% by weight, in particular less than 0.005% by weight or less than 0.001% by weight, in each case based on the total weight of component (A) or (B).

[0031] Preferably, neither component (A) nor (B) of the coating system of the present invention contains any component that can be crosslinked via actinic radiation, such as ultraviolet radiation. Of course, the coating composition of the present invention also preferably does not contain any component that can be crosslinked via actinic radiation, such as ultraviolet radiation. In particular, neither component (A) nor (B) of the coating system of the present invention contains any component that contains a (meth)acrylic group and / or an unsaturated C-C double bond. Of course, the same preferably applies to the coating composition of the present invention. Therefore, preferably, the coating composition of the present invention itself is not crosslinkable via actinic radiation, such as ultraviolet radiation.

[0032] In the context of the present invention, "comprising" preferably means "consisting of" in relation to components (A) and (B) of the coating system of the present invention and the coating composition of the present invention. In this case, in addition to components (a1) and (b1) present in components (A) and (B), one or more of the other components described below, such as components (a1) to (a7), which are optionally contained in each component of the coating system of the present invention or in the coating composition of the present invention, may be contained in the coating system, its components (A) and (B), or in the coating composition of the present invention. All components may in each case be present in their preferred embodiments as described below.

[0033] The proportions and amounts in % by weight of components (a1) and (b1) and any further components (a1) to (a7) present in the coating system according to the invention, its components (A) and (B) or in the composition (composition) add up to 100% by weight, based on the total weight of component (A) or (B), respectively, or based on the total weight of the coating composition.

[0034] Components (A) and (B) Component (A) comprises at least one component (a1) containing at least one aromatic moiety and having at least two primary and / or secondary amino groups. Preferably, component (a1) comprises an average of at least two primary and / or secondary amino groups. Preferably, component (a1) comprises at least two, preferably exactly two, aromatic moieties, more preferably at least two, preferably exactly two, phenyl moieties.

[0035] Preferably, component (a1) contains at least two primary amino groups.

[0036] Preferably, at least two primary and / or secondary amino groups are directly bonded to at least one aromatic moiety of component (a1). More preferably, when component (a1) comprises at least two, preferably exactly two, aromatic moieties (more preferably at least two, preferably exactly two phenyl moieties), at least one, preferably exactly one, of the at least two, preferably exactly two primary and / or secondary amino groups is directly bonded to each of the at least two, preferably exactly two aromatic moieties.

[0037] The presence of exactly two amino groups is preferred, since this allows crosslinking of component (B) with constituent (b1) to give linear polyureas.

[0038] Preferably, component (a1) further comprises at least one polyether moiety, preferably structurally arranged between at least two aromatic moieties of component (a1), if at least two aromatic moieties are present. However, in addition to or as an alternative to the at least one polyether moiety, component (a1) may comprise at least one of polyester, polybutadiene and / or poly(meth)acrylate units.

[0039] Preferably, at least one polyether moiety of component (a1) is structural unit (II): [ka]

[0040] where R 2 is a C2-C8 alkylene residue, more preferably a C2-C6 alkylene residue, and particularly a C2-C4 alkylene residue, The parameter n is an integer in the range of 1-100, more preferably in the range of 1-40, especially in the range of 1-20 or 1-10, and most preferably in the range of 1-5.

[0041] Preferably, component (a1) is a component of general formula (III): [ka]

[0042] where R 2 and n have the above-mentioned meanings, and each amino group is located independently of the others in the m-, o- or p-position on the phenyl ring, in particular in the p-position in each case.

[0043] Particularly preferred are components (a1) of general formula (IV) and / or (V):

[0044] [ka]

[0045] where n has the above-mentioned meaning in each case.

[0046] Preferably, at least one component (a1) has a number average molecular weight (M) in the range of 200 to 7500 g / mol, more preferably in the range of 200 to 5000 g / mol, in particular in the range of 250 to 3000 g / mol or 250 to 2000 g / mol, most preferably in the range of 250 to 1500 g / mol. n ) M n The determination is carried out according to the method described below.

[0047] Component (a1) (in addition to the at least two amino groups) may in principle have additional functional groups reactive towards NCO groups, such as OH groups, but preferably component (a1) does not contain such additional functional groups, i.e. the amino groups present therein are the only groups reactive towards NCO groups.

[0048] Preferably, constituent (a1) is present in component (A) in an amount in the range of 50 to 95% by weight, more preferably in the range of 55 to 90% by weight, even more preferably in the range of 60 to 85% by weight, still more preferably in the range of 65 to 85% by weight, and especially in the range of 70 to 80% by weight, based on the total weight of component (A).

[0049] Preferably, the amine value of component (a1) is in the range of 50 to 400 mg KOH / g, more preferably 75 to 300 mg KOH / g, where the amine value is determined in accordance with DIN 16945:1989-03.

[0050] The amount of any filler present in component (A) of the coating system of the present invention does not exceed 5% by weight (i.e., 5.0% by weight), based on the total weight of component (A). The term "filler" is known to those skilled in the art, for example, from DIN 55943 (dated October 2001). For the purposes of the present invention, "fillers" are preferably components that are substantially, preferably completely, insoluble in the medium surrounding them, such as each of components (A) and (B) and the composition of the present invention, and are used, in particular, to increase volume. "Fillers" in the sense of the present invention preferably differ from "pigments" in their refractive index, with fillers having a refractive index of less than 1.7 and pigments having a refractive index of 1.7 or greater. Preferably, "fillers" for the purposes of the present invention are inorganic fillers. Examples include barium sulfate or talcum.

[0051] Preferably, the amount of any filler present in component (A) of the coating system of the present invention does not exceed 4.5% by weight, more preferably does not exceed 4.0% by weight, even more preferably does not exceed 3.5% by weight, even more preferably does not exceed 3.0% by weight, still more preferably does not exceed 2.5% by weight, in particular does not exceed 2.0% by weight, or 1.5% by weight, or 1.0% by weight, or 0.5% by weight, in each case based on the total weight of component (A). Component (A) may also be free of any filler. In the sense of the present invention, this preferably means that component (A) does not contain, or at least essentially does not contain, fillers. This means that at least fillers are not intentionally added to component (A) used in the present invention and to the coating composition of the present invention. Preferably, the amount of filler present in component (A) of the coating system of the present invention is in the range of 0 to 5.0% by weight, more preferably in the range of 0 to 4.0% by weight, even more preferably in the range of 0 to 3.0% by weight, still more preferably in the range of 0 to 2.0% by weight, still more preferably in the range of 0 to 1.0% by weight or 0 to 0.5% by weight, in particular in the range of 0 to 0.1% by weight or 0 to 0.01% by weight, in each case based on the total weight of component (A). In particular, however, component (A) of the coating system of the present invention does not comprise any filler.

[0052] Preferably, the amount of any pigment present in component (A) of the coating system of the present invention is in the range of 0 to 5.0% by weight, more preferably 0 to 4.0% by weight, even more preferably 0 to 3.0% by weight, still more preferably 0 to 2.0% by weight, still more preferably 0 to 1.0% by weight or 0 to 0.5% by weight, and especially 0 to 0.1% by weight or 0 to 0.01% by weight, in each case based on the total weight of component (A). However, in particular, neither component (A) nor (B) of the coating system of the present invention contains any pigment. In the sense of the present invention, this preferably means that both components (A) and (B) are free of, or at least essentially free of, pigment. This means that at least no pigment is intentionally added to either component (A) or (B) used in the present invention, nor to the coating composition of the present invention. Preferably, the amount of pigment present in each of components (A) and (B) is less than 3% by weight, more preferably less than 2% by weight, even more preferably less than 1% by weight, still more preferably less than 0.5% by weight, even more preferably less than 0.1% by weight, and in particular less than 0.01% by weight, in each case based on the total weight of component (A) or (B), respectively. The term "pigment" is known to those skilled in the art, for example from DIN 55943 (dated October 2001). "Pigment" in the sense of the present invention preferably refers to components in the form of powders or flakes, which are colorants and / or substances that are substantially, preferably completely, insoluble in the medium surrounding them (e.g., each of components (A) and (B) and the composition of the present invention) and which can be used as pigments due to their magnetic, electric, and / or electromagnetic properties.

[0053] Preferably, both components (A) and (B) of the coating system of the present invention are transparent, i.e., clear. Preferably, of course, the coating composition of the present invention is also transparent, i.e., clear. In particular, neither components (A) nor (B) of the coating system of the present invention contain pigments and / or fillers, in particular any color and / or effect-imparting pigments and / or fillers. Of course, the same also preferably applies to the coating composition of the present invention.

[0054] Component (A) of the coating system of the present invention may comprise, in addition to component (a1), one or more further optional components, all of which are different from each other and from component (a1).

[0055] Component (A) of the coating system of the present invention may further comprise one or more components (a2) having one or more OH groups. Such components may be monomers, oligomers, and / or polymers. Examples include OH-functional polyesters, OH-functional poly(meth)acrylates, OH-functional (meth)acrylic copolymers, and / or OH-functional polyethers. However, preferably, component (A) does not comprise such components (a2).

[0056] As mentioned above, component (A) preferably does not contain a flame retardant, such as a phosphorus-containing flame retardant. However, component (A) of the coating system of the present invention may optionally contain at least one flame retardant, such as at least one phosphorus-containing flame retardant, as component (a3), particularly at least one phosphorus ester. If a flame retardant is used, it is preferably liquid (at 1 bar and 23°C). Liquid flame retardants are preferred because they do not potentially impair the desired transparency of the final coating. If a flame retardant is present in component (A), it is preferably present therein in an amount of 0.1 to 30% by weight, more preferably 1 to 25% by weight, and especially 5 to 20% by weight, based on the total weight of component (A). Amounts higher than 30% by weight are undesirable because they may degrade the mechanical properties of the final coating. An example of a phosphate suitable as a flame retardant is diphenyl cresyl phosphate. Other phosphates, such as triethyl phosphate, isopropylated triaryl phosphate, and 2-ethylhexyl diphenyl phosphate, can also be used. In principle, additionally or alternatively, other flame retardants may be used, such as halogen-based flame retardants, for example brominated or chlorinated flame retardants, such as tris-(2-chloroisopropyl)phosphate.It is also possible to use inorganic liquid compounds, such as liquid sodium water glass.

[0057] Component (A) of the coating system of the present invention may further comprise, as component (a4), one or more light stabilizers, particularly one or more UV stabilizers. Examples include sterically hindered amines (HALS: hindered amine light stabilizers). In principle, all commercially available light stabilizers from the Tinuvin® series or other manufacturers can be used. Liquid light stabilizers are preferred because they do not potentially impair the desired transparency of the final coating. If a light stabilizer is present in component (A), it is preferably present therein in an amount of 0.05 to 5% by weight, more preferably 0.1 to 3.5% by weight, and especially 0.1 to 2% by weight, based on the total weight of component (A).

[0058] Component (A) of the coating system of the present invention may further comprise one or more adhesion promoters as component (a5). In particular, organosilanes can be used as component (a5). Examples include (3-aminopropyl)trimethoxysilane, (3-aminopropyl)triethoxysilane, N-2-aminoethyl-3-aminopropyltrimethoxysilane, (3-mercaptopropyl)trimethoxysilane, (3-mercaptopropyl)triethoxysilane, (3-glycidyloxypropyl)trimethoxysilane and / or (3-glycidyloxypropyl)triethoxysilane, and vinyltrimethoxysilane. Additionally or alternatively, other adhesion promoters, such as titanates such as titanium acetylacetonate (TAA) and / or titanium n-butanolate (TnBt), may be used. If component (a5) is present in component (A), component (a5) is preferably present therein in an amount of 0.05 to 5% by weight, more preferably 0.1 to 3.5% by weight, especially 0.1 to 2% by weight, based on the total weight of component (A).

[0059] Component (A) of the coating system of the present invention may further comprise, as component (a6), one or more additives selected from the group consisting of antifoaming agents, reactive diluents such as bis-oxazolidines and / or aldimines, rheological additives, plasticizers such as phthalate esters, and viscosity reducers, in particular hydrocarbon mixtures based on naphthalene derivatives and / or indene-coumarone resins, non-reactive viscosity reducers such as tall oil and rapeseed methyl ester (biodiesel) and rapeseed oil and / or other ester-based diluents, and mixtures of these additives. These different additives used as component (a6) are different from each other. It is particularly preferred to use at least one bis-oxazolidine containing an amino group, such as a secondary amino group, as component (a6). The bis-oxazolidine does not necessarily contain free amino groups. Any free amino groups, such as secondary amino groups, contained therein can and preferably can be formed in situ later within component (A) or within the coating composition of the present invention, for example by reaction with (residual) amounts of water. In other words, the bisoxazolidine may contain an amino group in a protected form. The same applies to any OH groups present in the bisoxazolidine. An example is the commercially available product Incozol® LV. When at least one component (a6) is present in component (A), it is preferably present in an amount of 0.05 to 40% by weight, more preferably 0.1 to 30% by weight, and especially 0.1 to 20% by weight, based on the total weight of component (A). Specifically, when at least one antifoaming agent is present, its amount is preferably in the range of 0.1 to 2.5% by weight, based on the total weight of component (A). Specifically, when at least one reactive diluent is present, its amount is preferably in the range of 0.1 to 20% by weight, based on the total weight of component (A). Specifically, when at least one rheological additive is present, its amount is preferably in the range of 0.1 to 5% by weight, based on the total weight of component (A).Specifically, if present, the amount of at least one plasticizer is preferably in the range of 0.1 to 2.5% by weight, based on the total weight of component (A). Specifically, if present, the amount of at least one viscosity reducing agent is preferably in the range of 0.1 to 20% by weight, based on the total weight of component (A).

[0060] Component (A) of the coating system of the present invention can further include one or more chain extenders as component (a7). Suitable chain extenders are diamines and / or triamines different from component (a1) and preferably do not contain aromatic moieties. Rather, these chain extenders are diamines and / or triamines with aliphatic residues, including alicyclic residues. Such chain extenders are used to tailor the final properties of specific processes and final coatings. As diamines and / or triamines, dimer (fatty) amines and / or trimer (fatty) amines can be used, with dimer amines being most preferred. In particular, the use of such dimer amines can further improve the sprayability of the coating composition of the present invention by introducing chemical thixotropy. Furthermore, faster drying / curing can be achieved. Furthermore, the use of dimer diamines can significantly improve the strength and chemical resistance of the coating. An example of a suitable diamine is Priamine® 1073 from Croda. In addition to component (a1) and optionally present component (a2), the chain extender reacts with the NCO-groups of component (b1) of component (B). If component (a7), such as a diamine, especially a dimer amine, is present in component (A), component (a7) is preferably present therein in an amount of 0.1 to 20% by weight, more preferably 0.5 to 7.5% by weight, in particular 1.0 to 5% by weight, based on the total weight of component (A).

[0061] Component (A) of the coating system of the present invention may contain one or more additional components having at least two primary and / or secondary amino groups different from component (a1), such as, for example, a non-aromatic amino group-containing component and / or components (a6) and / or (a7). In that case, the amount of component (a1) is preferably greater than the amount of one or more other amino group-containing component(s) in component (A). Preferably, the relative weight ratio of component (a1) to the one or more other amino group-containing component(s) in component (A) is in the range of at least 2:1, or at least 3:1, or at least 5:1, or at least 10:1.

[0062] In particular, component (a1) is the only amino group-containing component present in component (A). Alternatively, component (a1) can be used in combination with component (a6), in particular a bis-oxazolidine containing an amino group, in particular a bis-oxazolidine containing two amino groups.

[0063] Component (B) of the coating system of the present invention comprises at least one component (b1) containing at least one aromatic moiety and having at least two isocyanate groups. Preferably, component (b1) contains an average of at least two isocyanate groups. Preferably, the NCO functionality is 2.0 to 3.0, more preferably 2.0 to 2.8, especially 2.0 to 2.5, and most preferably greater than 2.0 to 2.2.

[0064] Preferably, the NCO content, i.e. the amount of NCO groups present in component (b1), is in the range of 10 to 35% by weight, or in the range of 20 to 35% by weight, more preferably in the range of 15 to 30% by weight, and in particular in the range of 20 to 27.5% by weight, based on the total weight of component (b1).

[0065] Preferably, component (b1) is based on or prepared by utilizing an aromatic diisocyanate. The term aromatic diisocyanate in this specification refers to an isocyanate compound in which two isocyanate groups present in one molecule are directly bonded to an aromatic ring. Preferred aromatic diisocyanates are 4,4'-diphenylmethane diisocyanate and 2,4'-diphenylmethane diisocyanate (MDI), 4,4'-diphenylether diisocyanate, p-phenylene diisocyanate, m-phenylene diisocyanate, 3,3'-dimethoxy-4,4'-biphenyl diisocyanate, o-toluidine diisocyanate, naphthylene diisocyanate, 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, 3,3'-dimethyl-4,4'-diphenylether diisocyanate, and 3,3'-dimethyl-4,4'-diphenylether diisocyanate. These may be used alone or in combination of two or more thereof. Particularly preferred aromatic diisocyanates are at least one selected from the group consisting of 4,4'-diphenylmethane diisocyanate, 2,4'-diphenylmethane diisocyanate, 2,4-tolylene diisocyanate, and 2,6-tolylene diisocyanate. 4,4'-diphenylmethane diisocyanate (MDI) is the most preferred.

[0066] At least one carbodiimide and / or uretonimine structural unit in component (b1) can be formed by various methods, including subjecting an aromatic diisocyanate to a decarboxylation-induced condensation reaction accompanied by decarboxylation to produce an isocyanate-terminated component (b1) containing carbodiimide and / or uretonimine units. The decarboxylation condensation reaction of the aromatic diisocyanate compound typically proceeds in the presence of a carbodiimidation catalyst and / or a uretonimination catalyst. Examples of such catalysts include phospholene oxides such as 1-phenyl-2-phospholene-1-oxide, 3-methyl-1-phenyl-2-phospholene-1-oxide, 1-ethyl-2-phospholene-1-oxide, 3-methyl-2-phospholene-1-oxide, and their 3-phospholene isomers. The amount of catalyst is typically 0.1 to 1.0% by mass based on the aromatic diisocyanate compound used for carbodiimidation and / or uretoniminization.

[0067] The constituent (b1) present in component (B) has at least one carbodiimide unit and / or uretonimine unit. The carbodiimide unit has the following structural formula and can be formed by the condensation of two isocyanate groups:

[0068] [ka]

[0069] A uretonimine unit has the following structural formula and can be formed by the condensation of three isocyanate groups:

[0070] [ka]

[0071] Preferably, the amount of carbodiimide units and / or uretonimine units, in particular uretonimine units, in component (b1) is in the range of 4 to 18% by weight, more preferably in the range of 5 to 16% by weight, in particular in the range of 6 to 14% by weight, most preferably in the range of 8 to 12% by weight, in each case based on the total weight of component (b1).

[0072] For example, when component (b1) is based on / prepared by utilizing 4,4'-diphenylmethane diisocyanate (MDI) as the aromatic diisocyanate and contains at least one carbodiimide unit, it can have / comprise the following structure, where the parameter o is an integer from 1 to 50:

[0073] [ka]

[0074] For example, if component (b1) is based on / prepared by utilizing 4,4'-diphenylmethane diisocyanate (MDI) as the aromatic diisocyanate and contains exactly one uretonimine unit, it may have / comprise the following structure:

[0075] [ka]

[0076] As mentioned above, aromatic diisocyanates such as MDI can be used to prepare component (b1), forming at least one carbodiimide and / or uretonimine structural unit. Additionally or alternatively, it is possible to use polymeric aromatic diisocyanates such as PMDI (instead of or in addition to monomeric MDI (MMDI)). The PMDI used preferably has a number average molecular weight (M) in the range of 400 to 5000 g / mol. n ) M nThe determination of is carried out according to the method described below. The PMDI preferably has an NCO functionality in the range of 2.6 to 3.0.

[0077] However, the constituent (b1) of the component (B) used in the present invention necessarily has an ether segment since at least one structural unit (I) is present in the constituent (b1).

[0078] Therefore, aromatic diisocyanates such as MMI and PMDI used to prepare (b1) can be first reacted with an OH-functional extender, particularly an OH-functional polyether such as a polyether diol, to prepare an MMI- and / or PMDD-based prepolymer (i.e., before undergoing a condensation reaction to form at least one carbodiimide and / or uretonimine unit). As a result, an MMI- and / or PMDD-based isocyanate-functional prepolymer can be prepared, which has at least two NCO groups and contains, for example, a polyether segment. For example, MMDI can be "extended" using a polyether polyol, and the resulting prepolymer can then be subjected to a condensation reaction to form at least one carbodiimide and / or uretonimine unit.

[0079] Preferably, component (b1) is obtainable by subjecting at least one prepolymer of an aromatic diisocyanate, preferably of MDI, having at least two isocyanate groups (this prepolymer being prepared by reacting a polyether polyol with at least one aromatic diisocyanate and / or a polymer thereof) to a decarboxylation condensation reaction with the formation of at least one carbodiimide and / or uretonimine unit.

[0080] The prepolymers, such as MDI prepolymers, preferably have a number average molecular weight (M) in the range of 200 to 75,000 g / mol, more preferably in the range of 220 to 50,000 g / mol, and especially in the range of 300 to 35,000 g / mol or 350 to 20,000 g / mol. n ) M n The determination of is carried out according to the method described below. The prepolymer, such as the MDI prepolymer, preferably has an NCO functionality in the range of 1.9 to 3.0.

[0081] To prepare the isocyanate-terminated component (b1) containing at least one carbodiimide and / or uretonimine unit, it is particularly preferred to use a prepolymer, such as the MDI prepolymer described above, in the decarboxylation-condensation reaction following the decarboxylation, which allows the preparation of a component (b1) containing not only at least one carbodiimide and / or uretonimine unit but also polyether units.

[0082] Component (b1) comprises at least one polyether moiety. At least one polyether moiety of component (b1) is structural unit (I): [ka]

[0083] In the formula, R 1 is a C2-C8 alkylene residue, more preferably a C2-C6 alkylene residue, particularly a C2-C4 alkylene residue, most preferably a C2 and / or C3 alkylene residue, and the parameter m is an integer in the range of 1 to 200, more preferably in the range of 1 to 100, particularly in the range of 1 to 40, most preferably in the range of 1 to 10.

[0084] Component (B) of the coating system of the present invention may contain further components in addition to (b1). Preferably, however, component (B) of the coating system of the present invention does not contain any further components other than component (b1). When further components such as component (b2) are present, they are preferably selected from monomeric aromatic diisocyanates, such as MDI itself, and polymeric aromatic diisocyanates such as PMDI.

[0085] Component (B) of the coating system of the present invention may contain one or more additional components having at least two isocyanate groups different from component (b1), such as a component containing a non-aromatic NCO group. In that case, the amount of component (b1) is preferably greater than the amount of the one or more other NCO group-containing component(s) in component (B). Preferably, the relative weight ratio of component (b1) to the one or more other NCO group-containing component(s) in component (B) is in the range of at least 2:1, or at least 3:1, or at least 5:1, or at least 10:1.

[0086] In particular, component (b1) is the only NCO group-containing component present in component (B).

[0087] Preferably, the amount of structural unit (I) in component (b1) is at most 30% by weight, in particular at most 25% by weight, and most preferably at most 20% by weight, in each case based on the total weight of component (b1). Preferably, structural unit (I) is present in component (b1) in an amount of 1 to 30% by weight, more preferably 1.5 to 25% by weight, in particular 2 to 20% by weight, and most preferably 2.5 to 15% by weight or 3.0 to 10% by weight, in each case based on the total weight of component (b1).

[0088] Preferably, at least one component (b1) has a number average molecular weight (M) in the range of 300 to 60000 g / mol, more preferably in the range of 400 to 45000 g / mol, in particular in the range of 500 to 35000 g / mol or 600 to 25000 g / mol, most preferably in the range of 1000 to 15000 g / mol or 1500 to 10000 g / mol. n ) M n The determination is made according to the method described below.

[0089] Preferably, constituent (b1) is present in component (B) in an amount in the range of 80 to 100% by weight, more preferably in the range of 85 to 100% by weight, even more preferably in the range of 90 to 100% by weight, still more preferably in the range of 95 to 100% by weight, and especially in the range of 97.5 to 100% by weight, based on the total weight of component (B).

[0090] Preferably, component (B) of the coating system of the present invention does not contain flame retardants, such as phosphorus-containing flame retardants. In particular, component (B) does not contain organic phosphates, such as triethyl phosphate.

[0091] Preferably, the amount of any filler present in component (B) of the coating system of the present invention is in each case based on the total weight of component (B), not more than 4.0% by weight, more preferably not more than 3.5% by weight, even more preferably not more than 3.0% by weight, still more preferably not more than 2.5% by weight, even more preferably not more than 2.0% by weight, in particular not more than 1.5% by weight or 1.0% by weight. It is also possible and preferred that component (B) does not contain any filler. In the sense of the present invention, this preferably means that component (B) does not contain or at least essentially does not contain fillers. This means that at least fillers are not intentionally added to component (B) used in the present invention and the coating composition of the present invention.

[0092] A further subject of the invention is a coating composition obtainable by mixing together components (A) and (B) of the coating system.

[0093] All preferred embodiments described herein above in relation to the coating system of the present invention and its preferred embodiments are also preferred embodiments of the coating composition of the present invention.

[0094] Preferably, the coating composition of the present invention is a sealant.

[0095] Preferably, the amount of any filler present in the coating composition of the present invention is in the range of 0 to 4.0% by weight, more preferably in the range of 0 to 3.5% by weight, even more preferably in the range of 0 to 3.0% by weight, still more preferably in the range of 0 to 2.5% by weight, still more preferably in the range of 0 to 2.0% by weight or 0 to 1.5% by weight, and especially in the range of 0 to 1.0% by weight or 0 to 0.5% by weight, in each case based on the total weight of the coating composition.

[0096] Preferably, the resulting coating composition is sprayable.

[0097] Preferably, the coating composition is obtainable by mixing components (A) and (B) in a weight ratio (component (A) / component (B)) ranging from 5:1 to 1:2. More preferably, the mixing is carried out in a weight ratio ranging from 4.5:1 to 1:1.5, even more preferably in a weight ratio ranging from 4:1 to 1:1.1, in particular in a weight ratio ranging from 3.5:1 to 1:1, and most preferably in a weight ratio ranging from 3.25:1 to 1.1:1.

[0098] The mixing is preferably carried out in a high pressure apparatus using countercurrent injection techniques by utilizing the impingement mixing principle.

[0099] The mixing of components (A) and (B) is carried out in particular as follows: Components (A) and (B), located in separate containers, are separately transported to a heat-generating element, such as a block heater, via a proportioning pump. The components are then transported via preheated pipes to an impingement mixing chamber within the spray gun. This mixing chamber, with its corresponding bore, is specially shaped to allow the volumetric flows of components (A) and (B) to be homogeneously mixed with maximum turbulence within a fraction of a second. For this purpose, gaps offset by several hundred nanometers from each other are used, for example. This can be achieved with extremely strict manufacturing tolerances. The reaction mixture thus generated is discharged forward through the outlet gap and sprayed onto a substrate surface, such as the surface of a fuel tank. This means that a static mixer / mixing rod is preferably not used in this mixing system. When the spray gun's trigger is pulled, components (A) and (B) in the mixing chamber are mixed at high temperature and pressure with maximum turbulence within subseconds. Preferably, component (A) is mixed at a pressure ranging from 95 to 170 bar, particularly from above 100 to 165 bar. This mixing is preferably carried out at a temperature in the range of 70 to 80°C. Component (B) is preferably mixed at a pressure in the range of 80 to 140 bar, especially above 85 to 130 bar. This mixing is preferably carried out at a temperature in the range of 50 to 65°C. The reactive mixture is then explosively expelled from the gun orifice. When the gun trigger is released, the opening of the mixing chamber is mechanically blocked and the mixing process is interrupted. The mixing chamber can be closed with a needle-shaped bolt, which provides a mechanical self-cleaning effect. The fundamental advantage of this mixing technique over the static mixer principle is that short interruptions in operation are practically possible at any time without the need to replace the mixing module or the mixing chamber.

[0100] This mixing principle / system is particularly advantageous compared to the Sulzer mixing system (with a static mixer / mixing rod) operated by compressed air, as described in WO 2017 / 172906 A2. A disadvantage of the Sulzer mixing system is that spraying is performed by compressed air, which introduces many very small air bubbles into the sealant during the spraying process. In contrast, when mixed in a high-pressure system using countercurrent injection technology, the sealant is essentially air-bubble-free, meaning no additional air bubbles are introduced / injected into the sealant. Furthermore, using a Sulzer mixing system generates a large amount of plastic waste, as the cartridge and mixing rod are made of plastic and must be disposed of after use. In contrast, the mixing system described above does not generate plastic waste.

[0101] Suitable mixing techniques and systems for mixing components (A) and (B) and spraying the resulting coating composition of the present invention are disclosed, for example, in WO 2018 / 050482 A1 and EP 1 264 640 B1.

[0102] Use of the invention, method of the invention and coated substrate of the invention A further subject of the present invention is the use of the coating composition of the present invention as a sealant, in particular for providing a barrier coating, preferably on a precoated substrate, preferably on the exterior of a precoated fuel tank, in particular preferably on the exterior of a precoated aircraft fuel tank.

[0103] All preferred embodiments described herein above in relation to the inventive coating system and the inventive coating composition and preferred embodiments thereof are also preferred embodiments of the inventive method of use.

[0104] A further subject of the present invention is a method for sealing a preferably precoated surface of a substrate, preferably the exterior of a fuel tank, in particular the exterior of an aircraft fuel tank, comprising at least step (1), i.e. (1) applying, preferably by spraying and / or brushing, especially by spraying, a coating composition of the present invention to a preferably precoated surface of a substrate, preferably the exterior of a fuel tank, especially the exterior of an aircraft fuel tank; Includes.

[0105] All preferred embodiments described herein above in relation to the inventive coating system and the inventive coating composition and the inventive use and preferred embodiments thereof are also preferred embodiments of the inventive use.

[0106] This can be done with a 2K-compressed air support cartridge device equipped with a 2K-low pressure or 2K-high pressure dosing system. It can also be done manually. Preferably, the same device used to mix components (A) and (B) is used for step (1), i.e., the same device used to mix components (A) and (B) utilizing a high pressure device (>150 bar) with countercurrent injection technology and further utilizing the impingement mixing principle.

[0107] The coating composition of the present invention is preferably sprayed onto the exterior of a fuel tank installed inside an aircraft fuselage. Fuel tanks are typically sealed internally with an internal sealant, such as a polysulfide sealant. For added protection, if the internal seal fails, the externally applied sealant of the present invention prevents fuel or fuel vapors from leaking from the tank and entering the cargo and / or passenger areas, if the fuel tank is an aircraft fuel tank. Furthermore, the sealant of the present invention is resistant to fuel. Furthermore, the sealant of the present invention is transparent, allowing cracks and / or damage to the tank surface to be detected and repaired from the outside. The sealant of the present invention is relatively easy to remove for repair. The sealant of the present invention adheres well to a variety of substrates, including epoxy paints, aluminum and aluminum alloys, other sealants such as polysulfide sealants, polyurethane materials, and itself, allowing for easy repair.

[0108] Preferably, the method of the present invention further comprises a curing step (2), i.e. (2) curing the applied, particularly sprayed, coating composition of the present invention at ambient temperature (18-23°C) for 0.5-5 hours, preferably 1-4 hours; Includes.

[0109] Preferably, the coating composition of the present invention is applied to the surface of the substrate in a dry layer thickness in the range of 15 to 50 μm, in particular in the range of 20 to 45 μm.

[0110] A further subject of the invention is a sealed substrate obtainable by the method of the invention, for example a sealed fuel tank, preferably a fuel tank with an external seal, in particular an aircraft fuel tank with an external seal.

[0111] All preferred embodiments described herein above in relation to the inventive coating system and the inventive coating composition and the inventive method of use and the inventive method and preferred embodiments thereof are also preferred embodiments of the inventive coated substrate.

[0112] The substrates used are preferably metal substrates, particularly aluminum and / or aluminum alloy substrates. However, fiber-based composites, such as carbon fiber composites, can also be used as substrates. Preferably, these substrates are fuel tanks, particularly fuel tanks for the aviation industry. They may be pretreated and / or precoated, for example with an epoxy-based coating, before applying the sealant of the present invention thereto. Preferably, the substrates used have at least one preferably cured coating layer, such as an epoxy-based coating layer, before the coating composition of the present invention is applied thereto. If at least one such preferably cured coating layer, such as an epoxy-based coating layer, is present on the substrate before the coating composition of the present invention is applied thereto, the substrate is a precoated substrate.

[0113] method 1. Solids The solids content (non-volatile content) is determined according to DIN EN ISO 3251:2008-06 at 105° C. for 60 minutes. 2.VOC content Volatile organic compound (VOC) content is determined by ASTM D3960 (05-2018). 3.Drying time (curing time) Drying times are determined as disclosed in Federal Test Method Standard 141D, Method 4061.3 (March 22, 2001). 4.Hardness Hardness, particularly Shore A hardness, is measured according to ASTM D 2240:2015 using a Type A durometer. 5. Peel strength Peel strength is measured according to ASTM B571-18. 6. Tensile strength and elongation Tensile strength and elongation are measured according to ASTM D 412:2016. 7.Viscosity Viscosity is measured according to SAE International AS 5127 / 1 Rev. C using a RVF Brookfield viscometer with spindle 5 at 20 rpm. 8.Low temperature flexibility test The low temperature flexibility test is performed as described in SAE International's AS 5127 / 1 Rev. C. 9. Pressure rupture test The pressure rupture test is performed in accordance with SAE International AS 5127 / 1 Rev. C. 10. Bending test The flexural test is performed according to ASTM D522-17, Method A. 11.Number average molecular weight Number average molecular weight (M n ) is determined by gel permeation chromatography (GPC) using tetrahydrofuran as the eluent and polystyrene standards according to DIN 55672-1 (date: August 2007). Styrene-divinylbenzene copolymer is used as the column material. [Example]

[0114] The following examples further illustrate the invention but should not be construed as limiting its scope. Hereinafter, unless otherwise specified, all amounts shown in the tables are in parts by weight.

[0115] 1. Preparation of Components (A) and (B) Used in the Present Invention 1.1 Table 1 lists all the components used in the preparation of component (A). All components used were solvent-free. Diisopropylnaphthalene (mixture of isomers) has a boiling point of 290-300°C and therefore does not contribute to the VOC content. Versalink® P-1000 is a product of Evonik and is an oligomer diamine, i.e., poly(1,4-butanediol)bis(4-aminobenzoate), which is liquid at room temperature (23°C) (amine equivalent weight: 575-625). The reactive diluents, HALS, UV absorbers I and II, defoamers I and II, air release agents I and II, and rheological additives I and II used are in each case commercially available products.

[0116] 1.2 Table 1 also shows the ingredients used to prepare component (B). A commercially available solvent-free product was used, namely SUPRASEC® 2029, a product of Huntsman, which is a uretonimine-modified MDI containing a polyether moiety and having an average of 2.1 NCO groups (NCO content: 24.5% by weight).

[0117] 2. Preparation of the coating composition of the present invention, and investigation of its properties and the properties of the coatings obtained therefrom Exemplary coating compositions 1 to 5 (Examples 1 to 5) were prepared from components (A) and (B) as described in Table 1. 100 parts by weight of component (A) were mixed in each case with 33 parts by weight of component (B). The coating compositions obtained from components (A) and (B) were then sprayed onto substrates at 23°C.

[0118] Mixing and spraying were performed as follows: Components (A) and (B) were transferred separately to a block heater via a proportioning pump. Transfer of components (A) and (B) was achieved via heated pipes to an impingement mixing chamber and then to the spray gun. When the spray gun trigger was pulled, components (A) and (B) were mixed in the mixing chamber at high pressure (>150 bar) at 70-80°C with maximum turbulence within subseconds. The reactive mixture was then explosively expelled from the gun orifice. When the gun trigger was released, the mixing chamber orifice was mechanically blocked, halting the mixing process.

[0119] Component (A) used in the present invention, when reacted with component (B) used in the present invention, exhibits very good and unique gelation and curing properties. Surprisingly, a long pot life and rapid curing were observed. In contrast, it has been experimentally found that when component (A) is reacted with MDI containing only component (b1) (i.e., unmodified MDI), curing is less effective (slower) and high tack is observed. Furthermore, the mechanical properties of such cured materials are inferior to those of the coating system of the present invention utilizing component (B) used in the present invention.

[0120] The coating composition applied to the substrate represents a sealant. The substrates used were aircraft fuel tanks. The tanks were made of aluminum and pretreated according to MIL-DTL-5541, Class 1A, then coated with a commercially available epoxy-based coating. Additionally, a carbon fiber-based composite substrate was used and also coated with a commercially available epoxy-based coating.

[0121] The curing time (drying time) after application was 2 hours in all cases. After this 2-hour period, curing was complete. Curing was carried out at 23°C.

[0122] All of the sealants of the present invention passed the flex test according to ASTM D522 Method A.

[0123] The following properties were determined for the sealants of Examples 1, 2 and 5: The viscosity of the sealant measured 10 min after mixing was 50–150 Pa·s.

[0124] A hardness of 60-90 Shore A at 23°C was achieved in 16-24 hours. No tackiness was observed after this time. The final hardness was 81 Shore A (Example 1).

[0125] Even after seven days of storage in fuel and water at 23°C and 60°C, peel strengths ranged from 5 to 25 pounds per inch of width.

[0126] Specific gravity is 1.14g / cm 3 (Range: 1.0-1.30g / cm 3 ) (Example 1).

[0127] The sealant must exhibit a certain flexibility even at the low temperatures that can occur at flight altitude. This is verified by a low-temperature flexibility test at -54°C in accordance with AS 5127 / 1 Rev. C. The sealants shown in the examples pass this test. The sealants are applied in layers of 25 to 38 µm in all cases.

[0128] In both cases the sealant is transparent, allowing imperfections in the tank surface to be seen through the sealant.

[0129] The sealant has a pressure resistance of 30 psi at RT (pressure rupture according to AS5127 / 1 Rev. C) (Example 1).

[0130] [Table 1]

[0131] 3. Preparation of Coating Compositions Using Components (A) and (B) Used for Comparison and Comparative Examples 1 and 2 and Investigation of the Properties of the Coatings Obtained 3.1 Table 2 lists all the ingredients used to prepare the component (A) used for comparison. In each case, component (A) contained significant amounts of fillers such as barium sulfate, talc, fumed silica, and aluminum trihydroxide. The defoamers, rheological additives, dispersants, and plasticizers used were in each case commercially available products.

[0132] Table 2 also shows the ingredients used to prepare the component (B) used for comparison: Mixture M1 is a mixture of SUPRASEC® 2029, an MDI prepolymer containing NCO groups and a stabilizer. 3.2 Comparative Coating Compositions 1 and 2 were prepared from components (A) and (B) shown in Table 2. To prepare Comparative Example 1, 100 parts by weight of component (A) was mixed with 20 parts by weight of component (B). To prepare Comparative Example 2, 100 parts by weight of component (A) was mixed with 10 parts by weight of component (B). Next, the coating composition obtained from components (A) and (B) was sprayed onto a substrate.

[0133] The application method was carried out as detailed in item 2. By this method, each cured coating can be obtained. 3.3 Comparative Coatings The properties of the coatings obtained from Coating Compositions 1 and 2 were investigated and compared with those obtained from Coating Compositions 1, 2 and 5 of the present invention.

[0134] From Table 3 it can be seen that Comparative Examples 1 and 2, which contain significant amounts of filler, achieve poor tensile strength compared to Inventive Examples 1, 2 and 5. For Comparative Example 1, even poorer elongation is achieved.

[0135] [Table 2]

[0136] [Table 3]

[0137] 4. Further comparative and inventive preparation of components (A) and (B) and coating compositions obtained therefrom, and investigation of the properties of the coatings obtained 4.1 Table 4 lists all the ingredients used to prepare Components (A) and (B) used for further comparison and invention. Exemplary Coating Composition 6 (Example 6) and Comparative Coating Compositions 3, 4, 5, and 6 (Comparative Examples 3-6) were prepared from these Components (A) and (B) shown in Table 4.

[0138] The reactive diluent, UV absorbers I and II, defoamer I, and rheology additive I are all commercially available products. Masterseal® M800 PTB is the curing agent component of the commercial product Masterseal® M800 and is an MDI-based prepolymer with an NCO-group content of 12% by weight. Masterseal® M689 PTB is the curing agent component of the commercial product Masterseal® M689 and is an MDI-based prepolymer with an NCO-group content of 15% by weight. Lupranat® M20R is a commercially available curing agent (polymeric MDI) with an NCO-group content of 31.5% by weight. Lupranat® M70R is also a commercially available curing agent (polymeric MDI) with an NCO-group content of 31.5% by weight.

[0139] In Example 6, 100 parts by weight of component (A) was mixed with 33 parts by weight of component (B). In Comparative Example 3, the mixing ratio of (A) to (B) was 100:66. In Comparative Example 4, the mixing ratio of (A) to (B) was 100:53. In each of Comparative Examples 5 and 6, the mixing ratio of (A) to (B) was 100:25.

[0140] Each coating composition obtained from components (A) and (B) was then manually applied onto a substrate at 23° C. The substrates outlined in item 2 were used.

[0141] [Table 4]

[0142] 4.2 The properties of the coatings obtained from Example 6 and Comparative Examples 3-6, as well as the properties of the compositions themselves, were investigated. From Table 5, it can be seen that Comparative Examples 3-6 provide insufficient tensile strength, especially compared to Inventive Example 6. Furthermore, Inventive Example 6 advantageously provides a shorter gel time. The gel time is defined as the time until the viscosity of the composition obtained after mixing components (A) and (B) reaches a value of 15,000 mPas at room temperature (23°C), where the viscosity is measured according to the method disclosed in the "Methods" section.

[0143] [Table 5]

Claims

1. A method for sealing a preferably precoated surface of a substrate, preferably a fuel tank, in particular an aircraft fuel tank, comprising at least step (1): (1) applying a coating composition to a preferably precoated surface of said substrate, preferably to the exterior of a fuel tank, in particular to the exterior of an aircraft fuel tank, Including, The coating composition can be obtained by mixing together components (A) and (B) of a coating system, the coating system being a two-component (2K) coating system consisting of two components (A) and (B) that are separate from each other, Component (A) comprises at least one constituent (a1) containing at least one aromatic moiety and having at least two primary and / or secondary amino groups; the amount of any filler present in component (A) does not exceed 5% by weight, based on the total weight of component (A); Component (B) comprises at least one constituent (b1) containing at least one aromatic moiety and having at least two isocyanate groups; each of components (A) and (B) having a solids content of at least 95% by weight, based on the total weight of the respective component; The constituent (b1) present in component (B) has at least one carbodiimide unit and / or uretonimine unit and further has at least one structural unit (I): 【Chemical 1】 (In the formula, R １ is C ２ ~C ８ an alkylene residue, and the parameter m is an integer ranging from 1 to 200; A method comprising:

2. 2. The method of claim 1, wherein component (a1) comprises at least two aromatic moieties, and at least one of the primary and / or secondary amino groups is directly bonded to each of the at least two aromatic moieties.

3. Component (a1) further comprises at least one structural unit (II): 【Chemistry 2】 (In the formula, R ２ is C ２ ~C ８ Alkylene residue, more preferably C ２ ~C ６ Alkylene residues, especially C ２ ~C ４ is an alkylene residue, The parameter n is an integer in the range of 1 to 100, more preferably in the range of 1 to 40, especially in the range of 1 to 10, most preferably in the range of 1 to 5); 3. The method of claim 1 or 2, comprising:

4. 4. The method according to any one of claims 1 to 3, wherein component (a1) is present in component (A) in an amount in the range of 50 to 95% by weight, more preferably in the range of 55 to 90% by weight, based on the total weight of component (A).

5. The method of any one of claims 1 to 4, wherein component (b1) is the only isocyanate group-containing component present in component (B).

6. 6. The method according to claim 1, wherein component (b1) is obtainable by subjecting at least one prepolymer of an aromatic diisocyanate having at least two isocyanate groups, preferably MDI, to a decarboxylative condensation reaction under the formation of at least one carbodiimide unit and / or uretonimine unit, said prepolymer being prepared by reaction of a polyether polyol with at least one aromatic diisocyanate and / or a polymer thereof.

7. 7. The process according to any one of claims 1 to 6, wherein the NCO content present in component (b1) is in the range of 20 to 35% by weight, based on the total weight of component (b1).

8. 8. The method of any one of claims 1 to 7, wherein the solids content of component (A) is greater than 98% by weight, based on the total weight of component (A), and the solids content of component (B) is greater than 98% by weight, based on the total weight of component (B).

9. The method of any one of claims 1 to 8, wherein the coating composition is sprayable.

10. 10. The method according to any one of claims 1 to 9, wherein the coating composition is obtainable by mixing components (A) and (B) in a mass ratio (component (A) / component (B)) ranging from 5:1 to 1:

2.

11. 11. The method according to any one of claims 1 to 10, wherein mixing of components (A) and (B) of the coating system to prepare the coating composition is carried out in a high-pressure apparatus using countercurrent injection technology by utilizing the impingement mixing principle.

12. A sealed substrate, preferably a fuel tank with an external seal, in particular an aircraft fuel tank with an external seal, obtainable by a method according to any one of claims 1 to 11.

13. Use of a coating composition according to any one of claims 1 to 11 as a sealant, obtainable by mixing components (A) and (B) of the coating system, in particular for providing a barrier coating, preferably on a precoated substrate, preferably on the exterior of a precoated fuel tank, in particular on the exterior of a precoated aircraft fuel tank.

14. A coating composition according to any one of claims 1 to 11, obtainable by mixing together components (A) and (B) of the coating system according to any one of claims 1 to 8.

15. A two-component (2K) coating system consisting of two components (A) and (B) according to any one of claims 1 to 8, which are separate from each other.

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