On-substrate mixing

Abstract

The present invention provides a method for coating a substrate, comprising applying to the substrate: i. a solution A comprising at least a polysilazane and a solvent, ii. a solution B comprising at least a catalyst that promotes cross-linking of the polysilazane and a solvent, wherein solution A and solution B are applied to the substrate substantially separately, and thereafter mixed.

Description

[0001] ON-SUBSTRATE MIXING

[0002] Technical Field

[0003] The present invention relates to a polysilazane coating method.

[0004] Background Art

[0005] Polysilazanes, a group of polymers characterised by their Si-N-Si backbone, have attracted increasing interest over the recent years for their use in coatings. Depending on the type and formulation of the coating, polysilazane coatings can display a range of favourable properties; the high reactivity of the polysilazanes can result in coatings with e.g., high hardness and weatherability, excellent adhesion properties and scratch and wear resistance, low surface roughness, and high gloss on painted surfaces. Excellent thermal, chemical and UV resistances have been documented. Organic polysilazane coatings are reported to have a pencil hardness of 5H when cured at room temperature, contrasting the more widely used (poly)siloxane coatings with pencil hardness of 5B using the same curing conditions. Other polysilazane coatings have a coefficient of friction between 0.03 and 0.05, similar to the 0.04 of the famous anti-stick, Teflon, but with far better scratch and wear resistance. Coating hardness as high as 3GPa have been reported for inorganic perhydropolysilazane coatings cured under UV I ight / H2O2 or H2O2 / 80 °C (yielding SiO2), and an impressive 13 GPa with curing in air at 700-1000°C, attesting to the depth of cross-linking in the functional units of polysilazane. Set apart from other common polymers such as (poly)siloxane, polyurethane, epoxy resin, PMMA, often employed because of their ease of use and / or low reactivity at ambient conditions, polysilazanes have, with their high reactivity, been referred to as the ultimate binder among its class (polymers used in wet chemistry formulation).

[0006] Coating formulations comprising polysilazane and other components are known in the art. Due to the high reactivity of polysilazane to nucleophilic groups, additives used in coating formulations are often devoid of these groups, or their concentrations are kept very low. Such nucleophilic groups, which include hydroxyl, amine, and unsaturated bonds involving hetero atoms (e.g., carbonyl, S=O), are known to cause fragmentation of polysilazane. However, these groups may be useful for covalently binding additives to polysilazane in a coating; thus, their exclusion means that polysilazane and additives are not covalently and strongly bound together, which may lead to negative effects like leaching of the additives from the coating or phase separation.

[0007] A polysilazane coating method described in WO 2022 / 002844, addresses the issue described above. In that method, polysilazanes, catalysts, and reactive nanomaterials and / or reactive molecules are introduced into a coating composition vessel and the solution is mixed for a predetermined time period. Then, within a further predetermined time after the mixing step, the mixed solution is applied to a substrate, and a coating layer is formed thereon.

[0008] However, when using the above method, it has been observed that the quality of the coatings is highly determined by both the mixing time and the time after mixing when the mixed solution is applied to the substrate.

[0009] Consequently, an issue associated with the coating method described in WO 2022 / 002844 is that industrial / commercial applications of it require expensive and complex continuous mixing systems with process control. Further, in the event of planned or unplanned interruptions, e.g., delays which occur after mixing and before application, process control is lost, resulting in wastage of mixed solutions, as they go beyond their shelf life, and time-consuming cleaning of coating application systems to remove such solutions.

[0010] These issues are also relevant to the use of any polysilazane coating compositions / solutions where cross-linking of the polysilazane is promoted through the inclusion of cross-linking catalysts. Specifically, the inclusion of such components imposes a significantly reduced shelf-life on the coating composition, making continuous application processes challenging. Furthermore, a relationship between the amount of cross-linking catalyst included in any polysilazane composition and the time before coatings applied using such compositions are sufficiently cured to facilitate handling of the coated substrates results in a trade-off between shelf life of coating compositions and process time. The consequence of this is that a compromise must be made between commercially important short processing time and ease and cost of process control must be made. The objective of the present invention is to provide coatings which are virtually particle free and cure very well with a simple coating process that minimizes wastage. The inventors have discovered a simplified approach for performing the coating method along the lines described in WO 2022 / 002844 that avoids or at least ameliorates the issues described above.

[0011] Summary of invention

[0012] The present invention provides a method for coating a substrate as stated in Claim 1 , comprising applying to the substrate: i. a solution A comprising at least a polysilazane and a solvent, ii. a solution B comprising at least a catalyst that promotes cross-linking of the polysilazane and a solvent, wherein solution A and solution B are applied to the substrate substantially separately, and thereafter mixed.

[0013] Further embodiments of the method according to the present invention are described in the dependent claims.

[0014] Specifically, the inventors have discovered that simply by independently applying solution A and solution B to a substrate without time dependent mixing and coating steps, consistently and repeatably results in coatings with similar or even enhanced curing characteristics. This is surprising, particularly given the method described in WO 2022 / 002844, includes a pre-coating mixing time and in the present invention that mixing time is essentially set to zero.

[0015] Detailed description of the invention

[0016] As used herein, the term “polysilazane” refers to a polymer in which silicon and nitrogen atoms alternate to form the basic backbone, the polymer containing chains and / or rings of the formula [R-iF^Si-NRsjn, wherein Ri, R2, and R3 can be hydrogen atoms and / or identical or different organic substituents. The term polysilazane may be used to refer to any one of an inorganic polysilazane, an organic polysilazane, a polyborosilazane, a polysiloxazane, or any combination of these, or any crosspolymer comprising any of these or a combination of any of these, or any copolymer comprising a -Si-N-Si- backbone and Si-H and N-H functional units.

[0017] The terms “perhydropolysilazane” (“PHPS”) and “inorganic polysilazane” are used interchangeably to refer to any polysilazane of the formula [H2Si-NH]n.

[0018] The term “organic polysilazane” (“OPSZ”) is used to refer to any polysilazane of the formula [RiR2Si-NRs]n, wherein at least one of Ri, R2, and R3 is an organic substituent, wherein an organic substituent is defined as any substituent comprising carbon.

[0019] The term “polysilazane coating composition” as used herein refers to any composition that comprises a polysilazane and that may be used for coating a substrate.

[0020] The term “solution” as used herein refers to a liquid mixture of two or more substances.

[0021] The term “component” is used herein to refer to any compound or material that is or is to be included in a polysilazane coating composition. The term “component” thus includes any material, compound, additive or the like that the skilled person would consider including in a polysilazane coating composition.

[0022] As used herein, the term “reactive”, used with reference to certain components, refers to the ability of said component to partake in a spontaneous chemical reaction or physical interaction, such as aggregation, that results in a chemical or physical transformation, with any other component that is or may be present in a polysilazane coating composition, on the time scale t for making or storing a polysilazane coating composition, t may be hours, such as 5 hours, such as 3 hours, such as 2 hours, t may be minutes, such as 60 minutes, such as 30 minutes, such as 10 minutes, such as 5 minutes, such as 2 minutes, t may be seconds, such as 60 seconds, such as 30 seconds.

[0023] As used herein, the terms “spontaneous” and “spontaneously”, used with reference to a reaction, refers to a reaction taking place at the temperature T and pressure p for making a polysilazane coating composition without the need for a catalyst or any other activator. T may be ambient temperature, p may be ambient pressure. The term “simultaneously” as used herein refers to any two or more processes that are occurring at the same time or roughly at the same time, and is not to be understood strictly. It is not intended that the processes need to begin nor end together.

[0024] The term “substantially separately” as used herein means that solution A and solution B are not mixing prior to implementation of an application method and any substantial mixing of solution A and solution B occurs only on the substrate.

[0025] As used herein, the term “mixing”, or any variations thereof, refers to any method for combining components of a coating composition, such as, but not limited to, admixing, contacting, blending, stirring or allowing to admix, contact, blend, stir.

[0026] As used herein, the term “fragmentation” refers to a cleavage of a backbone Si- N bond in a polysilazane polymer.

[0027] In the following, general embodiments as well as particular exemplary embodiments of the invention will be described.

[0028] It is clear to the person skilled in the art that not only the coatings disclosed herein, but also a wide range of other polysilazane based coatings, can also be formed on a substrate in accordance with the present invention, using the disclosed method.

[0029] The inventors have invented a method for forming cured polysilazane coatings with a less complicated coating application process which offers an additional advantage of suppression or reduction of particle growth, where uncontrolled or difficult to control formation of particles and / or fragmentation of polysilazane are eliminated or at least limited.

[0030] Two discrete solutions, solution A and solution B, are prepared and applied substantially separately to the substrate to form a coating.

[0031] In one embodiment, solution A comprises a polysilazane and a solvent, whilst solution B comprises a cross-linking catalyst for polysilazane and a solvent.

[0032] In a second embodiment, solution B comprises a reactive nanomaterial and / or reactive molecules that can react spontaneously with a polysilazane polymer backbone to cause fragmentation, a cross-linking catalyst for polysilazane and a solvent.

[0033] In another embodiment, solution A is applied to the substrate and subsequently, solution B is applied before the solvent in solution A has substantially evaporated.

[0034] As used herein, the term “substantially evaporated” means 90% or more of the solvent in solution A has evaporated.

[0035] In yet another embodiment, solution B is applied to the substrate and subsequently, solution A is applied.

[0036] In still yet another embodiment, solution A and solution B are applied simultaneously on the substrate.

[0037] The polysilazane component may be present in an amount of 0.5-70 wt%, based on the weight of the total composition of solution A and B, preferably 5 to 40 wt%, more preferably 10 to 35 wt%. The skilled person will appreciate that the amount of polysilazane may be selected based on the technique that will be used for application of the coating composition to a substrate.

[0038] Solutions A and B include a solvent. The term “solvent” as used herein means an inert solvent and refers to a liquid substance in which a compound is soluble or partially soluble enough at a given concentration to dissolve or partially dissolve the compound. The term refers both to solvent blends (i.e. , solvents consisting of a plurality of constituents) and to pure compounds (i.e., solvents consisting of a single constituent) unless the context indicates otherwise. As used herein, the term “inert solvent” means a solvent that is known by the skilled person not to react with other components of the coating composition. The solvent used in solution A and solution B is independently selected from the list comprising or consisting of tetrahydrofuran (THF), 2-methyl tetrahydrofuran (MTHF), dibutyl ether (DBE), methoxyperfluorobutane (MPB), cyclopentyl methyl ether, aromatic solvents such as xylene and toluene, and other polar aprotic and non-polar solvents which do not comprise functional groups that may react with polysilazane. It is typically an ether or a hydrocarbon. In some embodiments, the solvent is selected from the list comprising or consisting of THF, DBE, MTHF, MPB, cyclopentyl methyl ether and xylene. THF, MTHF, cyclopentyl methyl ether and dibutyl ether are examples of preferred solvents. The ratio of the solvent of solution A to the solvent of solution B is 70:30 to 30:70, preferably 60:40 to 40:60, more preferably 55:45 to 45:55, even more preferably approximately 50:50. The solvent in solutions A and B may be the same or different.

[0039] In some embodiments, the catalyst in solution B is or comprises a nucleophile which can activate a Si atom for nucleophilic attack.

[0040] In some embodiments, an organic nucleophile is used. Organic nucleophiles facilitate easy use with polar aprotic or non-polar solvents. In specific embodiments, tetrabutylammonium fluoride, TBAF is used as a catalyst.

[0041] The catalyst may be or comprise a quaternary ammonium salt R1R2R3R4N+X’. As used herein, the term “quaternary ammonium salt” refers to a univalently positively charged group R1R2R3R4with a tetravalent nitrogen and a negative counterion (the anion). The anion, X may be selected from fluoride, chloride, bromide, and iodide. Alternatively, X’ may be a polyatomic anion such as PFe’ or BF or OH-. In some embodiments, X is selected from F, Cl, Br, I, PFe or BF4 and OH. In some embodiments, X is selected from F, Cl, and Br. Each of R1, R2, R3, and R4is independently selected from the group comprising or consisting of C1-C10 alkyl, aryl, arylalkyl, alkoxysilyl, and alkenyl. Advantageously, the alkoxysilyl group is able to hydrolyse to a silanol, which can undergo polycondensation with other silanols from polysilazane hydrolysis to give a covalent bond post coating. Other groups that can react with polysilazane or an additive post coating in order to establish covalent bonds, such as alkenyl groups having activated C=C double bonds, are also advantageous.

[0042] The salt may promote reaction of nucleophilic groups on reactive nanomaterials and molecules, including polyhedral oligomeric silsesquioxane (POSS) included in some embodiments, with polysilazanes. This activation may be the result of an interaction of X’ with a silicon atom of the polysilazane, such as a coordination, resulting in activation of the silicon atom for nucleophilic attack.

[0043] The salt may also act as a catalyst for defragmentation. As discussed above, the presence of nucleophilic groups in nanomaterials (e.g. POSS) or molecules (e.g. isopropanol) leads to fragmentation of polysilazane chains. Fragmentation may start immediately upon mixing the polysilazane component and the POSS. Resulting fragments may be volatile, less reactive than the original polysilazane, or even unreactive. Due to their smaller size, they may be lost to evaporation. They may also leave behind in the cured coating unreacted polar groups, decreasing the anti-soiling properties of the coating. The salt may therefore catalyse the reaction of the fragments with other fragments or original polysilazane chains, thereby limiting fragmentation. Likewise, the salt may also catalyse the cross-linking of polysilazane chains (non fragmented). It therefore may be used to increase the molecular weight of polysilazane prior to curing. This may have beneficial effect in increased coating hardness.

[0044] Since cross-linking of polysilazane involves consumption of Si-H and N-H functional groups (from fragmented or non fragmented chains), the cross-linking catalyst, e.g TBAF may reduce the time needed for post coating application curing.

[0045] The salt, for example TBAF, may be present in solution B in an amount of 0.0001-2 wt%, based on the weight of the combined composition of solution A and solution B, such as 0.001-1 wt%, such as 0.1 -0.5 wt%.

[0046] In some embodiments the solution B comprises a reactive nanomaterial and / or reactive molecules that can react spontaneously with a polysilazane polymer backbone to cause fragmentation.

[0047] The reactive nanomaterial may be selected from the list comprising, but not limited to inorganic nanomaterials such as silica, titanium dioxide, graphene, boron nitride; magnetic nanomaterials; antibacterial nanomaterials such as graphene oxide, silver; hybrid nanomaterials such as POSS. The nanomaterials may be organic nanomaterials. In some embodiments, at least two types of nanomaterials are included in the coating composition. The reactive nanomaterial is chosen based on the desired properties of the coating.

[0048] The reactive nanomaterial may be replaced by or combined with a reactive molecule that can react spontaneously with a polysilazane polymer backbone to cause fragmentation. Said molecule must comprise a functional group known to the skilled person to be able to react spontaneously with a polysilazane to break Si-N bonds, for example amine and hydroxyl groups. In some embodiments, said reactive molecule is a monomer, such as 1 ,2-benzenedimethanol, such as hexane-1 ,6 diol diacrylate. In some embodiments, said reactive molecule is an oligomer, such as urethane acrylate. In some embodiments, said reactive molecule is a short, medium or long chain organic compounds, such as 2-fluoroethanol, 2,2,2-trifluoroethanol, triethoxyvinylsilane, 1 ,8-octanediol, 1 H, 1 H,2H,2H-perfluorooctyltriethoxysilane, erucamide, behenamide. In some embodiments, said reactive molecule is a polymer, such as polyethylene glycol, a polyethylene glycol derivative, a polyethylene glycol co-polymer, cellulose. For example, if the aim is a coating with very low friction coefficient, inorganic nanomaterials, which can increase surface roughness, may be substituted or combined with 2-fluoroethanol to take advantage of the low surface energy of C-F bonds.

[0049] In some embodiments, the solution B comprises more than one reactive molecule that can react spontaneously with a polysilazane polymer backbone to cause fragmentation. In some embodiments, the coating composition comprises at least one reactive molecule that can react spontaneously with a polysilazane polymer backbone to cause fragmentation and at least one reactive nanomaterial.

[0050] POSS are compounds of the chemical formula [RSiOs / 2]n (R = H, alkyl, aryl, arylalkyl, or alkoxyl, fluoroalkyl, perfluoroalkyl, siloxane / (poly)siloxane, ether / polyether), which may have a cage-like structure, and their chemical composition is a hybrid, intermediate (RSiOi.5)n between that of silica (SiO2) and silicone (R2SiO)n. Each POSS molecule may contain covalently bonded reactive functionalities suitable for polymerisation or grafting POSS monomers to polymer chains, as well as nonreactive organic functionalities for solubility and compatibility of the POSS with various polymer systems. A range of different POSS molecules are available, and more are under development.

[0051] Advantages of POSS includes the fact that the small size of 1-3 nm (Si-O-Si core) makes mixing with polysilazane at the molecular level possible, and that POSS behave like molecules rather than like the typical nanoparticles, enabling highly concentrated and stable dispersions in different solvents. Hence, formulation of a coating composition is facilitated.

[0052] The ball-like nature of the POSS molecule may contribute to reducing the coefficient of friction of the resulting coatings, relative to comparable coatings not comprising POSS, when the rigid Si-O-Si frame is combined with and surrounded by low friction (slippery) molecules, for example alkyl, fluoroalkyl from either polysilazane or POSS itself - the POSS may be seen as balls acting like a roller for a slippery component.

[0053] The R groups of the POSS may be independently selected from the group comprising or consisting of H, alkyl, aryl, arylalkyl, siloxane, ether, alkoxyl, alkoxysilyl, or alkenyl. In some embodiments, the R groups are selected from H, C1-C18 alkyl, Ce- C24 aryl, C7-C34 arylalkyl, C1-C18 alkoxyl, C1-C12 (C1-C9 alkoxysilyl)alkyl, and C2-C18 alkenyl, such as from H, C2-C12 alkyl, C6-C12 aryl, C7-C22 arylalkyl and C2-C12 alkoxyl, C2-C8 (C1-C6 alkoxysilyl)alkyl, and C4-C12 alkenyl, preferably from H, C-i-Ce alkyl (such as methyl, ethyl, propyl, butyl, isobutyl, pentyl, hexyl, cyclohexyl), phenyl, C7- C18 arylalkyl, C-i-Ce alkoxyl, C-i-Ce (C1-C4 alkoxysilyl)alkyl, and C-i-Ce alkenyl. The alkyl, arylalkyl, alkoxyl, alkoxysilyl, and alkenyl groups may be linear and / or branched. The alkyl, arylalkyl, alkoxyl, alkoxysilyl, and alkenyl groups may be cyclic. The R groups may all be the same, or one, two or more, or all the R groups may be different from the other R groups.

[0054] Advantageously, an alkenyl group or alkoxysilyl group may be able to react post-coating to bind covalently to the coating. Such bonding may prevent leaching of POSS. However, the number of such groups should not be too high; it should be limited to maximum five, such as maximum three. The presence of terminal methyl groups anywhere in the POSS, such as in highly branched alkyl groups, may result in a coating having favourable anti-soiling properties.

[0055] The POSS useful for the invention comprises at least one group that can react, with polysilazane (bind covalently to Si) to cause fragmentation. Unsaturated bonds such as C=O and S=O are not usually thought of as nucleophiles, but both can act as nucleophiles in some circumstances. The authors have found out that carbonyl in butyl acetate and S=O in dimethyl sulfoxide react with polysilazane in our coating formulation. Therefore, nucleophilic groups are used in this invention in a broad sense to include unsaturated bonds capable of reacting with polysilazane to cause fragmentation. The nucleophilic group comprises a heteroatom selected from 0, N, and S. The nucleophilic group may be selected from the group comprising C=O, OH, NH2, NH, S=O, SH, C=N, and CHN. In some embodiments, the nucleophilic group is selected from NH2, C=O, and OH. The nucleophilic group may be present as a substituent on an R group, or it may be directly bonded to a Si atom. In some embodiments, the POSS comprises 1 -8, preferably 1-5, more preferably 1-3 nucleophilic groups.

[0056] POSS molecules may be of the open cage or of closed cage type or of a random structure or of a ladder-like structure. Closed cage POSS molecules of various cage sizes, such as a T8 cage (8 Si atoms), a T10 cage, a T12 cage, and mixtures of any two or more, may be particularly useful in the coating compositions of the invention. The POSS structure may be a dumbbell shape, that is two POSS molecules joined together or a combination of more than two molecules.

[0057] Advantageously, the POSS further comprises at least one fluoro substituent. The fluoro substituent may be directly bonded to a Si atom. Advantageously, the fluoro substituent is present as a substituent on an R group, making the R group a fluorinated R group. In some embodiments, the fluorinated R group is based on a siloxane, such as a (poly)siloxane. Preferred POSS molecules comprise at least one group selected from fluoroalkyl, fluoroaryl, fluoroarylalkyl, fluoroalkoxyl, fluoroalkoxysilyl, or fluoroalkenyl, as an R group. In some embodiments, the R groups are selected from C1-C18 fluoroalkyl, C6-C24 fluoroaryl, C7-C34 fluoroarylalkyl, C1-C18 fluoroalkoxyl, C1-C12 fluoro(Ci-C9 alkoxysilyl)alkyl, and C2-C18 fluoroalkenyl, such as from C2-C12 fluoroalkyl, C6-C12 fluoroaryl, C7-C22 fluoroarylalkyl and C2-C12 fluoroalkoxyl, C2-C8 fluoro(Ci-Ce alkoxysilyl)alkyl, and C4-C12 fluoroalkenyl, such as from Ci-Ce fluoroalkyl (such as fluoromethyl, nonafluorohexyl, fluoroethyl, trifluoropropyl, pentafluorobutyl, isofluorobutyl, heptafluoropentyl, cyclofluorohexyl), pentafluorophenyl, C7-C18 fluoroarylalkyl, C-i-Ce fluoroalkoxyl, C-i-Ce fluoro(Ci-C4 alkoxysilyl)alkyl, and C-i-Ce fluoroalkenyl. The alkyl, arylalkyl, alkoxyl, alkoxysilyl, and alkenyl groups may be linear and / or branched. The alkyl, arylalkyl, alkoxyl, alkoxysilyl, and alkenyl groups may be cyclic. The R groups may all be the same, or one, two or more, or all the R groups may be different from the other R groups. For all of the mentioned fluorinated R groups, the fluorinated R group may comprise one or more fluoro substituents, such as 1 , 2, 3, 4, 5, 6, 7 or more fluorine substituents. In preferred embodiments, the POSS comprises at least one C1-C10 fluoroalkyl group bearing 1 -15 fluorine substituents, as an R group.

[0058] In some embodiments, the POSS does not comprise a fluoro substituent. In particular embodiments, the POSS comprises 5-7 R groups that are non-fluorinated and comprise aryl, such as phenyl, and / or alkyl, such as methyl, and / or (poly)siloxane, and 1 -3 R groups that are non-fluorinated and comprise one or more nucleophilic groups selected from the group comprising or consisting of C=O, OH, NH2, NH, S=O, SH, C=N, or C N.

[0059] POSS molecules comprising fluoro substituents may lead to improved antisoiling and lower friction coefficient of the resulting coatings compared to POSS without fluoro substituents. POSS molecules comprising fluoro substituents may reduce the refractive index of the resulting coatings compared to POSS without fluoro substituents.

[0060] Advantageously, the POSS structures make it possible to enrich the surface of a coating with desirable groups, e.g. phenyl, isoalkyl, CF2 and CF3 groups without introducing too many reactive groups.

[0061] The POSS may further, or alternatively, comprise other substituents not mentioned above, such as at least one chloro substituent. The POSS molecules may comprise, as or in R groups, groups or substituents selected specifically for their properties, such as anti-pathogenic agents, UV absorbers / stabilizers, IR absorbers, and / or up-converting phosphors.

[0062] All POSS molecules comprising fluoro substituents may further comprise any other type of fluorinated and / or non-fluorinated R groups, for example, but not limited to, straight-chained or branched alkyl, aryl (e.g. phenyl), (poly)siloxane, and alkoxysilyl (alkyl-O-Si). The combination of fluorinated and non-fluorinated substituents may help improve solubility of POSS, especially in non-fluorinated solvents. It may also reduce the tendency of fluorinated molecules, such as compounds comprising CF2 and / or CF3 groups, to stick together or phase separate, a phenomenon that can lead to high surface roughness, hazy coatings and even increase in friction coefficient. Alkoxysilyl, e.g. methoxysilyl and ethoxysilyl, hydrolyse very fast in the presence of moisture to form silanols, which contributes towards fast curing of polysilazane. POSS molecules with only alkoxysilyl groups or a majority of alkoxy silyl groups may also be used, with or without one R group containing a reactive functional group.

[0063] The inclusion of one or more phenyl groups in a POSS molecule used in a composition of the invention may increase toughness of the resulting coating, such as mechanical toughness, such as measured by tensile strength. Furthermore, it may improve resistance to damaging radiation, such as electron, proton and / or UV in space. Thus, in some embodiments, at least one R group comprises a phenyl group.

[0064] Furthermore, the POSS may not be fluorinated, yet with R groups which are inert or contain inert groups. Examples of such inert groups include phenyl, alkyl, alkene and siloxane. Non-fluorinated POSS may have a combination of more than one R groups. Such combination, e.g. phenyl and vinyl, alkyl and vinyl or alky and phenyl may improve dispersion and compatibility in solvent and the coating matrix. An example of non-fluorinated POSS includes aminopropyl isobutyl POSS (structure IV with trifluoropropyl replaced by isobutyl) and trisilanol isobutyl POSS (open cage, structure I).

[0065] In some embodiments, Solution A or B may further comprise a curing agent which is known to the skilled person to enhance the curing of polysilazane. A typical curing agent include a molecule comprising alkoxysily group, e.g. methoxysily or ethoxysilyl. 3-Aminopropyl triethoxy silane is common as curing agent for polysilazane but there exist several other molecules comprising alkoxysilane groups which can act as curing agent.

[0066] In some embodiments, the total / combined composition of solution A and solution B comprises: i) 0.5-70 wt%, based on the weight of the composition, of the polysilazane; the polysilazane is selected from the group comprising organic polysilazanes, inorganic polysilazanes, and mixtures of any two or more organic and / or inorganic polysilazanes;

[0067] - In solution B: ii) 0.1 -30 wt%, based on the weight of the composition, of a reactive nanomaterial and / or reactive molecules that can react spontaneously with a polysilazane polymer backbone to cause fragmentation; iii) 0.0001 -2 wt%, based on the weight of the composition, of the catalyst; the catalyst comprising a quaternary ammonium salt R1R2R3R4N+X wherein each of R1, R2, R3, and R4is independently selected from the group comprising alkyl, aryl, arylalkyl, alkoxysilyl, and alkenyl, and wherein X is selected from F, Cl, Br, I, PFe or BF4and OH; and iv) the balance being a solvent.

[0068] In some embodiments, the reactive nanomaterial and / or reactive molecules is a POSS.

[0069] In some embodiments, solution A and / or solution B may further include one or more components selected from the group comprising or consisting of flow levelling agents, photoinitiators, fibres, polymer stabilisers, fillers, and pigments.

[0070] As used herein, the terms “flow agent and levelling agent” refers to one or more compounds known by the skilled person to be able to enhance the uniformity of a coating and eliminate defects such as pinholes, “fish eyes”, “orange peel”, high roughness etc. The same additive may serve as both the flow and levelling agent. Therefore, flow levelling agent is used here to indicate an additive which performs as a flow agent or a levelling agent or both.

[0071] The flow levelling agent may not have a reactive nucleophilic group selected from the group of C=O, OH, NH2, NH, S=O, SH, C=N, or C=N. One example of a flow levelling agent is the polyether siloxane copolymer commercially available as “TEGO® Glide 410”(TG4). In some embodiments, a flow levelling agent comprising an organofluorine group is preferred, such as to achieve a coating formulation with a lower surface tension, especially when additives or polysilazane contain fluoro substituents are used. The flow levelling agent may be present in an amount of 0.01-5 wt%, such as 0.1 -0.8 wt%, such as 0.3-0.5 wt%, based on the weight of the coating composition.

[0072] The solution A and / or B may further comprise a photoinitiator, such as 1- hydroxycyclohexylphenyl ketone. A photoinitiator may further aid curing of coatings, when curing is aided by UV or visible light.

[0073] The photoinitiator may be present in an amount of 0.05-5 wt%, such as 0.1-4 wt%, such as 0.3-1 wt%, based on the weight of the coating composition.

[0074] The solution A and / or B may further comprise a fibre, such as a nanofibre, such as a cellulose nanofibre. The fibre may bear one or more alkyl and / or fluoroalkyl groups as well as group containing a nucleophile which is capable of covalently bonding to polysilazane. The inclusion of fibres may reduce the refractive index of the resulting coating, such as through formation of air pockets in the coating. Fibres may also improve mechanical properties of the coating, such as impact toughness and tensile strength.

[0075] The fibre may be present in an amount of 0.01-10 wt%, such as 0.1-7 wt%, such as 1-3 wt% based on the weight of the composition.

[0076] The solution A and / or B may further contain one or more straight, branched or cyclic (poly)siloxane. A particular advantage of (poly)siloxane-based additives is that they may not be as crystalline and rigid as POSS, and therefore may improve the flexibility of the resultant coatings. Other advantages may include improvement in light transmittance.

[0077] (Poly)siloxane-based additives may be present in the coating formulation in an amount of 0.05-15 wt%, such as 0.08-10 wt%, such as 1-3 wt%, based on the weight of the composition.

[0078] The solution A and / or B may further comprise a polymer stabiliser. As used herein, the term “polymer stabiliser” refers to any chemical compound or composition that may inhibit or retard any degradation of the polysilazane component and / or the coating. Common polymer degradation processes include oxidation, UV-damage, thermal degradation, ozonolysis, combinations thereof such as photo-oxidation, as well as reactions with catalyst residues, dyes, or impurities. The polymer stabiliser may be selected from the group comprising or consisting of antioxidants such as radical scavengers, hydroperoxide scavengers, antiozonants; light stabilisers such as UV stabilisers, quenchers, hindered amine light stabilisers; acid scavengers; metal deactivators; heat stabilisers; flame retardants; biocides; and any combination thereof. Preferably, the polymer stabiliser is a UV stabiliser, i.e. a compound or composition that prevents photodegradation of the polysilazane component or the coating, such as by absorbing UV radiation.

[0079] Solution A and / or B may further comprise one or more further fillers and / or pigments, for example, wollastonite, carbon black, micaceous iron oxide, and / or a thixotrope.

[0080] The application of the coating compositions A and B to a substrate may be performed using any technique known to the skilled person, such as by a method selected from the list comprising but not limited to, spraying; such as ultrasonic spray coating, such as spray painting, such as pneumatic spraying; spin coating; inkjet printing, and other processes known in the art for transforming solution processed chemical compositions into coatings.

[0081] In some embodiments, the application is performed by spraying, such as by ultrasonic spray coating. Ultrasonic spray coating displays unique advantages in minimising material loss, achieving uniform coating over large area in short time via atomisation of coating formulation and uniformly distributing coating components, compatibility with in-line processing (e.g. roll-2-roll) and optimizing surface roughness of the coating.

[0082] Examples

[0083] Experiments representing examples of the application of the coating method according to the subject invention are described wherein a coating is applied by ultrasonic spraying onto glass microscope slides. Solution A and solution B are prepared in advance, prior to spraying.

[0084] Coating compositions A and B are detailed in Table 1 and Table 2, respectively, wherein the following acronyms have the meanings detailed below:

[0085] IPA - isopropanol

[0086] PS - polysilazane

[0087] TG4 - TEGO® Glide 410

[0088] A85 - Trisilanol isobutyl POSS (obtained from HybridPlastics) PX6 - Dimethylpolysiloxane (viscosity 20 cSt (25 °C)(lit. ), obtained from SigmaAldrich)

[0089] PX6 stock - used in the experiment is pure, that is not dissolved in a solvent until introduced into the coating formulation.

[0090] Table 1: Solution A Compositions

[0091] Table 2: Solution B Compositions In the preparation of the compositions, because only small quantities were needed with very small quantities of the solution components, stock solutions are pre-prepared comprising levelling agent (TG4), and reactive nanomaterial and / or reactive molecules (POSS), each dissolved in solvent (THF). In the examples:

[0092] - the concentration of the TG4 in the stock solution is 10wt% - the concentration of POSS in the stock solution is 10wt%.

[0093] The spray parameters used are defined in Table 3. Table 3: Spray coating parameters and curing

[0094] In each case the solution flow rate was set at 0.4mL / min, the nozzle excitation frequency was 120kHz and the line spacing, being the distance between adjacent parallel passes of the spray nozzle was set at 10mm.

[0095] Solution A was first loaded into a syringe pump and sprayed onto the glass slide through an ultrasonic spray nozzle, repeatedly traversing first in an X-direction, then stepping over the line spacing distance (10mm) in the y-direction, before returning in the x-direction until each of 8 samples in a lot, placed side-by-side were covered.

[0096] After the A solution was applied to the samples and unused solution purged from the spray system, solution B was loaded into the syringe pump and sprayed onto the samples, over the solution A

[0097] The delay between the application of solution A and solution B was approximately 5 minutes, determined by the solution change-over time. In table 3 this is recorded as a dwell time of 0 seconds. Extending the dwell time by as much as 7 minutes had no adverse effects on the coating quality.

[0098] After the samples were coated, all were subjected to a known curing process comprising drying, exposure to H2O2 vapor, followed by baking.

[0099] The curability of the sample, attesting to the efficacy of the subject invention, was assessed by Fourier transform infrared spectroscopy (FTIR) run on transmittance mode, that is percent transmittance. Specifically, the extent of curing was determined having regard to a “curing” ratio between the intensities of the peaks representing Si-O-Si (at about 1010-1020 cm’1) and Si-CHs at about (1262 cm’1).

[0100] Curing of polysilazane proceeds via hydrolysis of the Si-H and N-H functional groups to yield silanols (Si-OH) which then polycondense to form Si-O-Si is very important because it not only confers good mechanical properties on the coating, when driven to a high degree, it also ensures that the concentration of hydrophilic / polar groups (Si-H, N-H and Si-OH) are reduced to a very low level such as not to compromise the anti-soiling properties of the coating. A cured OPSZ coating will consist mainly of Si-O-Si and Si-CHs, which will render the coating non- polar / inert, thereby ensuring robust anti-soiling character. In Table 3, the FTIR score (Si-O-Si / Si-CHs ratio is used to indicate the level of curing:

[0101] - Values less than 0.24 represents excellent curing, EC;

[0102] - Values of 0.24-0.27 represents very good curing, VGC;

[0103] - Values of 0.28 - 0.31 represents good curing, GC;

[0104] - Values of 0.32-0.36 represents acceptable curing AC;

[0105] - Values of 0.37-0.42 represents poor curing and

[0106] - values greater than 0.42 indicate very poor curing.

[0107] An experiment 13 (not included in Table 3) was prepared using the pre-mixing method reported in WO 2022 / 002844. Solution A was made up of 1182 pl of D1500 RC and 590 pl of 5.5 wt% TG4 in THF. Solution B was made up of 1182 pl of 10 wt% A85 in THF, 400 pl of 0.5 wt% TBAF in THF, and 2556 pl of THF. Solutions A and B were mixed for 5 minutes and sprayed onto glass substrates using a 120 KHz ultrasonic nozzle. The spray coatings parameters were:

[0108] - flow rate -0.8 ml / min,

[0109] - nozzle speed - 25 mm / sec,

[0110] - line spacing - 10 mm,

[0111] - height (nozzle to substrate distance) - 50 mm, and

[0112] - air shaping pressure of 0.45 bar.

[0113] The coating was dried and cured with the same procedures used in experiments 1 to 12. The FT-IR rating of the coating was VGC (very good curing).

[0114] The level of curing of the coatings of the present invention are comparable to that of the reference sample and / or are reasonably cured to be used in normal conditions of atmospheric pressure and temperature. All the coatings from experiments 1 to 12 showed a high degree of consumption of Si-H and N-H peaks located at about 2160 and 900 cm, respectively, to silanols and subsequently to Si- O-Si. Therefore, the coatings of the present invention are suitable for applications where near complete curing is desirable. For example, coatings on windshield which will be subject to snow, rain and outdoor pollution.

Claims

CLAIMS1 . A method for coating a substrate, the method comprising applying to the substrate: i. a solution A comprising at least a polysilazane and a solvent, ii. a solution B comprising at least a catalyst that promotes cross-linking of the polysilazane in solution and a solvent, wherein solution A and solution B are deposited onto the substrate substantially separately, and thereafter mixed.

2. The method of claim 1 , wherein solution B further comprises a reactive nanomaterial and / or reactive molecules that can react spontaneously with a polysilazane polymer backbone to cause fragmentation.

3. The method of claim 1 or 2, wherein solution A and solution B are applied simultaneously to the substrate.

4. The method of claim 1 or 2, wherein solution A is applied to the substrate and subsequently solution B is applied before the solvent in solution A has substantially evaporated.5 The method of claim 1 or 2, wherein solution B is applied to the substrate and subsequently, solution A is applied.

6. The method of any of the preceding claims, wherein the solution A comprises 1-70 wt%, preferably 5 to 40 wt%, more preferably 10 to 35 wt% of polysilazane based on a total composition of solution A and solution B.

7. The method of any of the preceding claims, wherein the solvent used in solution A and solution B is independently selected from the group of solvents: THF, DBE, MTHF, MPB, cyclopentyl methyl ether and xylene.

8. The method of any of the preceding claims, wherein the ratio of the solvent of solution A to the solvent of solution B is 70:30 to 30:70, preferably 60:40 to 40:60, more preferably 55:45 to 45:55, even more preferably approximately 50:50.

9. The method of any of the preceding claims, wherein the catalyst is selected from quaternary ammonium salts, preferably tetra-n-butylammonium fluoride (TBAF), tetra-n-butylammonium bromide (TBAB), tetra-n-butylammonium chloride (TBAC), and tetra-n-butylammonium hydroxide.

10. The method of any of the preceding claims 2 or 3 to 9 when depending on claim 2, wherein the reactive nanomaterial is selected from polyhedral oligomeric silsesquioxanes (POSS).11 . The method of any of the preceding claims 2 or 3 to 10 when depending on claim 2, wherein the total composition of solution A and solution B comprises: i) 0.5-70 wt%, based on the weight of the composition, of the polysilazane; the polysilazane is selected from the group comprising organic polysilazanes, inorganic polysilazanes, and mixtures of any two or more organic and / or inorganic polysilazanes; ii) 0.1 -30 wt%, based on the weight of the composition, of the reactive nanomaterial and / or reactive molecules that can react spontaneously with a polysilazane polymer backbone to cause fragmentation; iii) 0.0001 -2 wt%, based on the weight of the composition, of the catalyst; the catalyst comprising a quaternary ammonium salt R1R2R3R4N+X wherein each of R1, R2, R3, and R4is independently selected from the group comprising alkyl, aryl, arylalkyl, alkoxysilyl, and alkenyl, and wherein X is selected from F, Cl, Br, I, PFe or BF4and OH; and iv) the balance being the solvent.

12. The method of any of the preceding claims wherein the solution A and solution B are applied by spray coating.

13. The method of any of the preceding claims wherein one of or both solution A and solution B further includes a flow levelling agent.

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