Reversible crosslinking powder coating

Abstract

The present invention relates to a powder coating composition comprising the cured product of a resin and a cross-linking agent, the resin being obtainable by the reaction of at least one epoxy resin having an average epoxy functionality of > 2, preferably an epoxy novolac resin, optionally one or more additional epoxy compounds, and at least one primary amine, and wherein the cross-linking agent has at least two boronic acid or boronic ester groups. In addition, the invention relates to process for preparation of a powder coating composition, to a process for the application of a powder coating on a substrate, to a coated substrate and to the use of a powder coating composition. The use of reversible bonding according to the present invention allows for the use of low Tg resins (e.g. < 40 °C, even resins that are liquid at ambient temperature) as main binders that would typically be too low for traditional powder coatings.

Description

[0001] REVERSIBLE CROSSLINKING POWDER COATING

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a powder coating composition, a process for preparation of a powder coating composition, a process for the application of a powder coating on a substrate, a coated substrate, and a use of a powder coating composition.

[0004] Background

[0005] Powder coating compositions are solid compositions that generally comprise a solid film forming (binder) polymer or mixtures of different solid film-forming polymers. The compositions can also comprise other components, for example pigments, extenders and one or more performance additives such as plasticizers, stabilizers, degassing agents, and flow aids. The film-forming polymers are usually thermosetting polymers that cure upon heating, typically in the presence of a crosslinking agent, which may itself be a polymer. Generally, the polymers have a glass transition temperature (Tg), softening point or melting point above 40 °C. An example of such a powder coating composition is disclosed in WO2024079131A1 .

[0006] Traditional powder coatings require such Tg values to ensure that they are stable (unreactive) at room temperature, and such polymers tend to have inherently high melt viscosities. The cure temperature of typical powder coatings must also be high enough to ensure the viscosity of the polymer becomes sufficiently low on heating to enable it to flow and form a film, whereas the curing rate must not be too fast as otherwise poor flow / appearance can result.

[0007] The network created during the cure of a thermoset powder coating is irreversible, preventing the film self-repairing at high temperature. Thermoplastic powders can exhibit re-flow properties when heated in-service, however these powders suffer from high melt viscosities that necessitate high curing temperatures for film formation, and the absence of crosslinks can make these coatings sensitive to solvent swelling and dissolution. Objects

[0008] It is an object of the present invention to provide an improved powder coating using reversible crosslinking. It is a further object of the present invention to provide an improved powder coating that can reversibly crosslink in order to reduce both the temperature of curing as well as the curing time.

[0009] STATEMENT OF THE INVENTION

[0010] In a first aspect, the invention relates to a powder coating composition comprising the cured product of a resin and a cross-linking agent, the resin being obtainable by the reaction of at least one epoxy resin having an average epoxy functionality of > 2, preferably an epoxy novolac resin, optionally one or more additional epoxy compounds, and at least one primary amine, and wherein the cross-linking agent has at least two boronic acid or boronic ester groups.

[0011] In a second aspect, the invention relates to a process for preparation of a powder coating composition comprising the steps of i) mixing at least one epoxy resin having an average epoxy functionality of > 2, preferably an epoxy novolac resin, optionally one or more additional epoxy compounds, at least one primary amine, and at least one cross-linking agent having at least two boronic acid or boronic ester groups at a temperature of at least 60 °C, preferably at least 70 °C, at least 80 °C or at least 90°C to react into a network polymer; ii) processing the network polymer obtained in step i) into a powder; and iii) optionally sieving the powder obtained in step ii) to obtain a powder coating composition having particles with a Dv 50 particle size distribution of less than 106 micrometer.

[0012] In a third aspect, the invention relates to a process for the application of a powder coating on a substrate comprising the steps of: a) applying a powder coating composition according to the first aspect onto at least one surface of a substrate; b) melting the powder coating composition under increased temperature so that the particles of the powder coating composition flow to obtain a continuous coating film on said at least one surface of said substrate; and c) allowing the continuous coating film to cool to obtain a coated substrate. In a fourth aspect, the invention relates to a coated substrate prepared by the process according to the invention.

[0013] In a fifth aspect, the invention relates to a use of a powder coating composition according to the invention.

[0014] Corresponding embodiments of the powder coating are also applicable for the other aspects according to the present invention.

[0015] Covalent adaptable networks (CANs), also referred to as vitrimers, are cross-linked network polymers in which covalently crosslinked networks are formed such that triggerable, reversible chemical structures persist throughout the network, (see for example Covalent adaptable networks: smart, reconfigurable and responsive network systems, Christopher J. Kloxina and Christopher N. Bowman*b Chem. Soc. Rev., 2013, 42, 7161-7173). These reversible covalent bonds can be triggered through stimuli such as temperature, light or radiation. Upon application of the stimulus the CAN polymer responds by adjusting its structure through either reversible addition / condensation or through reversible bond exchange mechanisms, either of which allow the material to re-equilibrate to its new state and condition. The stimulus can also causes these materials to alter their shape, topography, and properties including flow.

[0016] The use of reversible bonding according to the present invention allows for the use of low Tg resins (e.g. < 40 °C, even resins that are liquid at ambient temperature) as main binders that would typically be too low for traditional powder coatings.

[0017] DETAILED DESCRIPTION

[0018] The present invention is elucidated below with a detailed description.

[0019] Embodiments of the powder coating

[0020] In the first aspect, the invention relates to a powder coating composition comprising the cured product of a resin and a cross-linking agent. Each of these will be discussed below. Resin

[0021] The resin according to the invention is obtainable by the reaction of at least one epoxy resin having an average epoxy functionality of > 2, optionally one or more additional epoxy compounds, and at least one primary amine. Each of these will be discussed below.

[0022] Epoxy resin having an average epoxy functionality of > 2

[0023] A first component of the resin is a multi-epoxy resin, or an epoxy resin having an average epoxy functionality of > 2. As a preferred example thereof, epoxy novolac resin or pentaerythritol glycidyl ether may be used. An “epoxy novolac resin” as used in the present description is a polymer having a multi-functional, highly aromatic backbone with multiple glycidyl ether groups.

[0024] In an embodiment, at least one epoxy novolac resin is selected from the group consisting of epoxidized phenol-formaldehyde novolac resins, epoxidized phenolformaldehyde novolac resins diluted with difunctional cycloaliphatic glycidyl ether, and epoxidized cresol-formaldehyde novolac resins, more preferably selected from the group consisting of the following structures:

[0025] Examples of epoxidized phenol-formaldehyde novolac resins are: i) Dow Epoxy Novolac (D.E.N. or DEN® series): DEN425, DEN426, DEN428, DEN431 , DEN438, DEN439, DEN440; ii) Huntsman Araldite epoxy phenol novolac (Araldite® EPN) and EPALLOY® series: EPN1179, EPN1180, EPALLOY 8220, EPALLOY8230, EPALLOY8240, EPALLOY8250, EPALLOY8330, EPALLOY8350, EPALLOY8370; and iii) Westlake EPIKOTE™ and EPON™ Epoxy Novolac (EN) Resin series: EPON160, EPON161 , EPON162, EPIKOTE154, EPIKOTE170. Examples of epoxidized phenol-formaldehyde Novolac resins diluted with difunctional cycloaliphatic glycidyl ether are DLVNE 59 and DLVNE 61.

[0026] Examples of epoxidized cresol-formaldehyde novolac resins are EPON Resin 164, EPON Resin165, Epicion N-660, Epicion N-665, Epicion N-670, Epicion N-680.

[0027] In an embodiment, at least one epoxy resin (preferably epoxy novolac resin) has an epoxy equivalent weight (EEW) in the range of 150 to 250 grams / equivalent, such as at least 155 or at least 170, such as between 150 to 200 grams / equivalent. Epoxy Equivalent Weight (EEW) as used in the present description is a parameter that indicates the number of grams of epoxy resin / compound that is required to give 1 mole of epoxy group. This range can vary depending on the specific formulation and degree of polymerization of the novolac resin. Lower EEW values indicate a higher density of epoxy groups, while higher EEW values suggest fewer epoxy groups per unit weight of the resin.

[0028] Optionally one or more additional epoxy compounds

[0029] A possible second component of the resin is an additional epoxy compound or two or more thereof, such as a di-epoxy compound and / or a mono-epoxy compound.

[0030] Di-epoxy compound

[0031] In an embodiment, the one or more additional epoxy compounds comprises a di-epoxy compound. In an embodiment, the di-epoxy compound is selected from the group consisting of bis-glycidyl ethers of structure: (CHCH2O)-CH2-O-L-O-CH2-(CHCH2O) wherein (CHCH2O) represents an epoxide moiety, -O-L-O- is a spacer derived from a di-alcohol compound (HO-L-OH) selected from the group consisting of (hetero)aliphatic and (hetero)aromatic di-alcohol compounds, such as C2-C22 linear, branched or cyclic aliphatic di-alcohol, (poly)ether di-alcohol compounds, or C5-C22 aromatic di-alcohol compounds, such as 1 ,4-dibutanol, polyethylene glycol, bisphenol- A, and bisphenol-F.

[0032] Specific examples of di-epoxy compounds are 1 ,4-butanediol diglycidyl ether, polyethylene glycol diglycidyl ether, Bisphenol A diglycidyl ether (e.g. DER 332). The polyethylene glycol diglycidyl ether used may have an average Mn of between 200 Da and 6000 Da, such as between 300 Da and 5000 Da, for example 500 Da (as used in the Examples) and may have an EEW of between 100 and 3000 gram / equivalent such as between 250 and 300 gram / equivalent such as between 264 and 290 gram / equivalent (as used in the Examples).

[0033] Mono-epoxy compounds

[0034] In an embodiment, the one or more additional epoxy compounds comprises a monoepoxy compound. In an embodiment, the mono-epoxy compound is selected from the group consisting of glycidyl ethers of structure: R2-O-CH2-(CHCH2O) wherein (CHCH2O) represents an epoxide moiety, wherein R2 is selected from the group consisting of H, aliphatic and aromatic groups, such as C1-C22 linear, branched or cyclic aliphatic group or C5-C22 aromatic group, such as n-butyl or phenyl. For the avoidance of doubt, a cyclic aliphatic group includes aliphatic groups that comprise both cyclic and non-cyclic portions. Also for the avoidance of doubt, an aromatic or aryl group includes groups that comprise both aromatic and non-aromatic portions.

[0035] Primary amine

[0036] Another component of the resin is a primary amine, preferably at least one primary amine has the formula R1-NH2 wherein R1 is a hydrocarbyl group, preferably aliphatic or aromatic, which hydrocarbyl group is optionally substituted with one or more substituents each individually selected from the group consisting of a halide, such as F or Cl, a hydroxyl group, a C1-C4 alkoxy, a C1-C4 haloalkoxy, a -NRsaRsb group (such as dimethylamine or diethylamine), a -SiRsaRsbRsc group (such as trimethoxysilyl or triethoxysilyl), wherein Rsa, Rsb, and R3Care each independently selected from C1-C4 alkyl, preferably wherein R1 is a linear, branched or cyclic alkyl, preferably a C2-C22 alkyl, or wherein R1 is an aryl, preferably C5-C22 aryl, most preferably wherein R1 is a linear alkyl having between 6 and 12 carbon atoms, such as cyclohexyl, dodecyl or hexyl. Specific examples are cyclohexyl amine, 1-hexyl amine and 1-dodecyl amine.

[0037] Crosslinking agent The resin is crosslinked using a crosslinking agent having at least two boronic acid or boronic ester groups. In order to increase mixability with the epoxy resin it is preferred to use esters of the boronic acid compounds, such as alkyl esters, preferably linear alkyl esters, more preferably C2-C6 linear alkyl esters, for example n-butyl esters.

[0038] In an embodiment, the at least one cross-linking agent is selected from the group consisting of the structures below, being di-boronic acids, tri-boronic acid or tetra- boronic acids or esters thereof according to the formulas below or wherein the crosslinking agent is a compound having multiple boronic ester or boronic ester groups, such as a boronic function polymer: wherein R is hydrogen or a C2-C6 alkyl group, preferably a linear C2-C6 alkyl group, and wherein X2, X3, and X4 are as discussed below.

[0039] In an embodiment, the cross-linking agent is selected from the group consisting of benzene-1 ,4-diboronic acid, benzene-1 ,3,5-triyltriboronic acid; (2,6- dimethoxypyridine-3,5-diyl) diboronic acid, naphthalene-1 ,4-diyldiboronic acid, [1 ,T- biphenyl]-4,4'-diyldiboronic acid, benzofuran-2,5-diyldiboronic acid, (9,9-dioctyl-9h- fluorene-2,7-diyl) diboronic acid, or the esters thereof. In a more preferred embodiment, the cross-linking agent is a benzene-1 ,4-di-boronic acid or the C2-C6 linear alkyl ester thereof, such as tetrabutyl-1 ,4-phenylenebisboronate.

[0040] Ratios

[0041] The components of the resin and the powder coating may be mixed in specific ratios. In an embodiment, the ratio between the epoxy equivalents of the epoxy resin (preferably the novolac resin) and the epoxy equivalents of the di-epoxy compound (ratio B:C - see Examples below) is between 50 : 50 and 90 : 10, more preferably between 60:40 and 90:10, most preferably 80 : 20. In the Examples it is shown that when a ratio of 40:60 is used, the resulting powder coating composition is less preferable.

[0042] In an embodiment, the ratio between the epoxy equivalents of the epoxy (novolac) resin and optionally the di-epoxy compound on the one hand and the epoxy equivalents of the mono-epoxy compound on the other hand (ratio B+C:D - see Examples below) is between 0.8 : 1.2 and between 1.2 : 0.8, preferably between 0.9 : 1.1 and 1.1 : 0.9, more preferably 1 :1.

[0043] In an embodiment, the ratio between the epoxy equivalent of all epoxy compounds on the one hand, and the N-H equivalents of the primary amine on the other hand (ratio B+C+D:E - see Examples below) is between 0.8 : 1 .2 and between 1 .2 : 0.8, preferably between 0.9 : 1.1 and 1.1 : 0.9, more preferably 1 :1.

[0044] In an embodiment, the ratio between the boronic acid equivalent of the cross-linking agent on the one hand, and the number of nitrogen (N) atoms of the primary amine on the other hand (ratio A:E - see Examples below) is between 0.8 : 1.0 and between 1.0 : 0.8, more preferably 0.8 : 1. It is preferred that the cross-linking density is as low as possible while still making sure that the material passes the gel point; it was found that a ratio of 0.6 : 1 .0 would not result in a optimally crosslinked resin.

[0045] In an embodiment, the molar ratio between the mono-epoxy compound on the one hand and the primary amine on the other hand (ratio D:E) is between 0.8 : 1.2 and between 1.2 : 0.8, preferably between 0.9 : 1.1 and 1.1 : 0.9, more preferably 1 :1. It should be noted that a primary amine (-NH2) has two amine equivalents.

[0046] Cured product

[0047] In an embodiment, the powder coating composition is a cured powder coating composition. In an embodiment, the cured product comprises one or more of the following moieties: wherein Ri is a hydrocarbyl group, preferably aliphatic or aromatic, which hydrocarbyl group is optionally substituted with one or more substituents each individually selected from the group consisting of a halide, such as F or Cl, a C1-C4 alkoxy and a C1-C4 haloalkoxy, preferably wherein Ri is a linear, branched or cyclic alkyl, preferably a C2-C22 alkyl, or wherein Ri is a aryl, preferably C5-C22 aryl, most preferably wherein Ri is a linear alkyl having between 6 and 12 carbon atoms, such as cyclohexyl, dodecyl or hexyl; wherein X2, X3, or X4 are each independently selected from the group consisting of a C1-C22 hydrocarbyl or hetero hydrocarbyl, optionally substituted with one or more substituents each individually selected from a halide, such as F or Cl, a C1-C10 alkyl, a C1-4 alkoxy and a C1-4 haloalkoxy, preferably wherein X2, X3, or X4 is selected from the group consisting of a linear, branched or cyclic (hetero) alkyl, (hetero) aralkyl, (hetero) aryl, or (hetero) alkylaryl group, for example a phenyl group, a pyridine group, a naphthalene group, a biphenyl group, a benzofuran group, or a fluorene group.

[0048] In an embodiment, the glass-transition temperature (Tg) of the powder coating composition is above 45°C, preferably above 50°C, more preferably above 55°C, even more preferably above 60°C, such as above 62 °C, measured using a rheometer using ASTM D 7028 - 07, E 1640 - 04. This is a practical limitation to ensure non-stickiness of the powder when applying. In an embodiment, the glass-transition temperature (Tg) of the powder coating composition is below 120 °C, preferably below 110°C, such as below 100°C measured using a rheometer using ASTM D 7028 - 07, E 1640 - 04.

[0049] Additives

[0050] The powder coating composition can comprise one or more additives. In embodiments, these can be selected from stabilisers, levelling agents, anti-settling agents, matting agents, rheology modifiers, anticorrosion agents, flexibility agents, surface-active agents, UV light absorbers, light stabilisers, amine synergists, waxes, adhesion promoters, fillers, pigments, flow control agents, degassing agents, and antioxidants.

[0051] Any one or more of these additives can be incorporated into the powder coating component, for example by dry-mixing and then melt-blending (for example with an extruder, at 80-160 °C), and then grinding the solidified blend, to - for example an average particle size of 10 to 400 microns.

[0052] In addition, additives may be added by being dry-blended, and optionally melt-mixed as above, with the powder coating components. Dry-blended additives can be inorganic particulate components. For example, they can be selected from inorganic particulate materials that provide functionality to the powder coating composition, for example inorganic colour pigments, inorganic effect pigments such as metal effect pigments, biocidal pigments, anticorrosive pigments, extenders, opacifying pigments, conductive or anti-static pigments, infrared-absorbing pigments, radiation shielding pigments, glass flakes, abrasion resistance agents or any combination of two or more thereof.

[0053] The total quantities of these additional components can be in the range of from 0 to 40 wt.%, for example from 0 to 35 wt.%, or from 0 to 30 wt.% based on the total weight of the powder coating composition.

[0054] Where any of such additional components are present, their minimum concentration (individually or cumulatively) is typically at least 0.05 wt.% in the powder coating composition, for example at least 0.1 wt.%. Each additive can have an particle size distribution (Dv 50) in the range of from 5 to 100 pm, for example from 5 to 50 pm. Dv 50 as used in the present description is the particle size value at which 50% of the total volume of particles has a particle size below that value. A typical method for measuring Dv 50 is laser diffraction according to ISO 13320, which in embodiments can use the Mie model.

[0055] Specific embodiment

[0056] In a specific embodiment the powder coating composition comprises the cured product of a resin and a cross-linking agent, the resin being obtainable by the reaction of:

[0057] * an epoxy novolac resin,

[0058] * a di-epoxy compound having the structure: (CHCH2O)-CH2-O-L-O-CH2- (CHCH2O), wherein (CHCH2O) represents an epoxide moiety, -O-L-O- is a spacer derived from a di-alcohol compound (HO-L-OH) being a C2-C22 linear aliphatic dialcohol or a polyethylene glycol;

[0059] * a mono-epoxy compound, being a glycidyl ether of structure: R2-O-CH2- (CHCH2O), wherein (CHCH2O) represents an epoxide moiety and R2 is a C2-C22 linear aliphatic group; and

[0060] * at least one primary amine having the structure R1-NH2, wherein R1 is a C2- C22 alkyl; and wherein the cross-linking agent is benzene-1 ,4-diboronic acid.

[0061] Embodiments of the method of preparation of powder coating composition

[0062] The present invention also relates to a process for preparation of a powder coating composition comprising the steps of: i) mixing at least one epoxy resin having an average epoxy functionality of > 2, preferably an epoxy novolac resin, optionally one or more additional epoxy compounds, at least one primary amine, and at least one cross-linking agent having at least two boronic acid or boronic ester groups at a temperature of at least 60 °C, preferably at least 70 °C, at least 80 °C or at least 90°C to react into a network polymer; ii) processing the network polymer obtained in step i) into a powder; and iii) optionally sieving the powder obtained in step ii) to obtain a powder coating composition having a Dv 50 particle size distribution of less than 106 micrometer. In an embodiment, step i) comprises two sub-steps ia) of first mixing at least one epoxy resin and the at least one primary amine to prepare an amino-diol intermediate; and step ib) mixing the cross-linking agent with the amino-diol intermediate.

[0063] Embodiments of the method of coating

[0064] The present invention also relates to a process for the application of a powder coating on a substrate comprising the steps of: a) applying a powder coating composition according to any one of claims 1-11 onto at least one surface of a substrate; b) melting the powder coating composition under increased temperature so that the particles of the powder coating composition flow to obtain a continuous coating film on said at least one surface of said substrate; and c) allowing the continuous coating film to cool to obtain a coated substrate.

[0065] In an embodiment, the powder coating composition is applied using electrostatic spray application on a grounded substrate.

[0066] In an embodiment, the substrate is a metallic substrate or a plastic, wooden, MDF substrate.

[0067] In an embodiment, step b) is carried out at a temperature of at least 170°C for a duration of at least 10 minutes.

[0068] Embodiments of the coated substrate

[0069] The present invention also relates to a coated substrate prepared by the process according to the invention, preferably complying with the rubbing testing using methyl ethyl ketone (MEK) according to the procedure described in ASTM D4752.

[0070] Use of the powder coating composition

[0071] The present invention also relates to a use of a powder coating composition according to the invention for coating a substrate, preferably wherein the substrate is selected from the group consisting of pipes, valves, rebars, automotive or engine components, more preferably a water pipe or valve, a gas pipe or valve, an oil pipe or valve, a steam pipe or valve, an exhaust pipe or valve, HVAV pipes or valves, automotive chassis components, frames, oil pans, wheel rims, metal radiator housing, valve covers, suspension parts and interior trim.

[0072] Improved effect of the invention

[0073] The powder coating according to the present invention is applied in a covalently bonded crosslinked state. At elevated temperature the crosslinks will dynamically separate leading to non-crosslinked polymers, preferably with a low melt viscosity (due to the low Tg) enabling the powder to flow and form a film once applied on a substrate and heated. Moreover and contrary to traditional powder coatings, this reversible bonding also gives the cured film the ability to re-flow at high temperature and fill any possible defects, scratches, or marks that the coating will experience. This re-flow is well known as self-healing properties.

[0074] In an embodiment, the powder coating composition is self-healing, preferably at a temperature of above 100 °C, such as between 100 °C and 250 °C, such as between 150 °C and 225 °C or between 160 and 200 °C or between 160 °C and 180 °C, or above 170 °C. With self-healing is meant that if the cured powder coating composition applied to a substrate is damaged, e.g. by scratching, and the temperature of the coated substrate is raised, the powder coating composition will re-flow and fill the scratch to self-heal the surface.

[0075] Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. The scope of the present invention is defined by the appended claims. One or more of the objects of the invention are achieved by the appended claims. EXAMPLES

[0076] The present invention is further elucidated based on the Examples below which are illustrative only and not considered limiting to the present invention. In the Examples below several powder coating compositions have been prepared.

[0077] The coating compositions were prepared by adding to a round bottom flask all components specified in Table 6. The equivalents of boronic acid, epoxy and N-H are also specified in Table 6 and for each example 0.1 mole of A2 was used, the amounts of the rest were calculated based on that. The mixture was then stirred at 90 °C for 2 hours before casting on a Teflon sheet and allowed to cool to room temperature. The resulting material was then cured in an oven at 80 °C for 16 hours, yielding a material with a glass transition temperature (Tg) as specified in Table 7.

[0078] The coating compositions were then processed into a powder using a Retsch mill (Ultra Centrifugal Mill ZM 300, set at 16000 rpm) followed by sieving through 250 pm, 150 pm, and 106 pm sieves. The fraction with a particle size of less than 106 pm was applied via electrostatic spray application to a grounded (earthed) aluminium panel and heated in an oven at 170 °C for 10 minutes, yielding a coated panel. The results of this are provided in Table 7. The same was repeated with heating in an oven at 250 °C for 10 minutes.

[0079] The resulting cured coating film was subjected to and rubbing testing methyl ethyl ketone (MEK) according to the procedure described in ASTM D4752. The result of this test are shown in Table 7.

[0080] The cured coating film was allowed to cool down to room temperature then scored using a sharp knife before re-heating to 170 °C for 6 minutes, at which point the scoring line disappeared due to the re-flow of the coating for compositions that scored a + on this test. This is a measure of the self-healing capacities of the inventive powder coating compositions.

[0081] Compounds that have been used in these examples are shown below: Table 1: examples of cross-linking agents

[0082] Table 2: examples of multi-epoxy resins, including novolac resins Table 3: examples of di-epoxy compounds (and if used in the examples with number)

[0083] Table 4: examples of mono-epoxy compounds

[0084] Table 5: examples of primary amines Table 6 overview of the powder coating compositions.

[0085] # equivalents of boronic acid

[0086] $ epoxy equivalents

[0087] @ N-H equivalents

[0088] Table 7 overview of the results

[0089] #: +: continuous film, no continuous film; o: film formed but not fully continuous / smooth

[0090] $: +: no visible damage; visible damage n.d.: no data since test not carried out

[0091] The data above show that one or more of the objects of the present invention are obtained by the powder coating composition according to the claims.

[0092] To see the effect of the type of di-epoxy used we can compare Ex. 1 , Ex.2, and Ex. 9, each using a different di-epoxy compound, namely C1 , C2, and C3 respectively. This shows that it is preferred to have a linear alkyl or alkyl ether di-epoxy compound compared to a more sterically hindered aromatic di-epoxy compound since the latter does not allow a good film to be formed.

[0093] To see the effect of the ratio between the epoxy equivalents of the multi-epoxy and diepoxy Ex.2, Ex. 3, and Ex. 10 can be compared having ratios of 80:20, 60:40 and 40:60, respectively. The results show that for each of these films can be formed and a Tg in the range of 54 to 74 °C are observed. However, for a ratio of 40:60 the MEK rubbing test was not passed whereas this passed for the other ratios tested.

[0094] In order to review the effect of the presence of a di-epoxy compound Ex. 1 can be compared with Ex. 4. This shows that this composition does not form a continuous film at a temperature of 170 °C.

[0095] Example 2 could be compared to Example 8 to see the effect of mono epoxy component. The powder with the aliphatic mono epoxy resin was not passed the MEK test, even though it did form a film.

[0096] The case of Example 11 shows that the use of multifunctional epoxy pentaerythritol polyglycidyl ether B2 does not allow a continuous film to be produced under the cited conditions. This is despite the diepoxy component C2 and mono-epoxy component D1 , and primary amine E2 giving overall a low Tg of 52 °C, and these components A2, C2, D1 and E2 being capable in other formulations of being able to form a film, even at higher overall Tg.

Claims

CLAIMS1 . A powder coating composition comprising the cured product of a resin and a cross-linking agent, the resin being obtainable by the reaction of at least one epoxy resin having an average epoxy functionality of > 2, preferably an epoxy novolac resin, optionally one or more additional epoxy compounds, and at least one primary amine, and wherein the cross-linking agent has at least two boronic acid or boronic ester groups.

2. The powder coating composition according to any one of the preceding claims, wherein the cured product comprises (a plurality of) of one or more of the following moieties:wherein Ri is a hydrocarbyl group, preferably aliphatic or aromatic, which hydrocarbyl group is optionally substituted with one or more substituents each individually selected from the group consisting of a halide, such as F or Cl, a hydroxyl group, a C1-C4 alkoxy, a C1-C4 haloalkoxy, a -NRsaRsb group, a -SiRsaRsbRsc group, wherein Rsa, Rsb, and Rac are each independently selected from C1-C4 alkyl, preferably wherein Ri is a linear, branched or cyclic alkyl, preferably a C2-C22 alkyl, or wherein Ri is a aryl, preferably C5-C22 aryl, most preferably wherein Ri is a linear alkyl having between 6 and 12 carbon atoms, such as cyclohexyl, dodecyl or hexyl; wherein X2, X3, or X4 are each independently selected from the group consisting of a C1-C22 hydrocarbyl or hetero hydrocarbyl, optionally substituted with one or more substituents each individually selected from a halide, such as F or Cl, a C1-C10 alkyl,a C1-4 alkoxy and a C1-4 haloalkoxy, preferably wherein X2, X3, or X4 is selected from the group consisting of a linear, branched or cyclic (hetero) alkyl, (hetero) aralkyl, (hetero) aryl, or (hetero) alkylaryl group, for example a phenyl group, a pyridine group, a naphthalene group, a biphenyl group, a benzofuran group, or a fluorene group.

3. The powder coating composition according to any one of the preceding claims, wherein the at least one epoxy novolac resin is selected from the group consisting of epoxidized phenol-formaldehyde novolac resins, epoxidized phenolformaldehyde novolac resins diluted with difunctional cycloaliphatic glycidyl ether, and epoxidized cresol-formaldehyde novolac resins, more preferably selected from the group consisting of the following structures:

4. The powder coating composition according to any one of the preceding claims, wherein the at least one primary amine has the formula R1-NH2 wherein R1 is as disclosed above.

5. The powder coating composition according to any one of the preceding claims, wherein the at least one cross-linking agent is selected from the group consisting of the structures below, being di-boronic acids, tri-boronic acid or tetra-boronic acids or esters thereof according to the formulas below or wherein the cross-linking agent is a compound having multiple boronic ester or boronic ester groups, such as a boronic function polymer:wherein R is hydrogen or a C2-C6 alkyl group, preferably a linear C2- C6 alkyl group, and wherein X2, X3, and X4 are as discussed above.

6. The powder coating composition according to any one of the preceding claims, wherein the one or more additional epoxy compounds comprises an di-epoxy compound, preferably wherein the di-epoxy compound is selected from the group consisting of bis-glycidyl ethers of structure: (CHCH2O)-CH2-O-L-O-CH2- (CHCH2O), wherein (CHCH2O) represents an epoxide moiety, -O-L-O- is a spacer derived from a di-alcohol compound (HO-L-OH) selected from the group consisting of (hetero)aliphatic and (hetero)aromatic di-alcohol compounds, such as C2-C22 linear, branched or cyclic aliphatic di-alcohol, (poly)ether di-alcohol compounds, or C5-C22 aromatic di-alcohol compounds, such as 1 ,4-dibutanol, a polyethylene glycol, bisphenol-A, and bisphenol-F.

7. The powder coating composition according to any one of the preceding claims, wherein the one or more additional epoxy compounds comprises a mono-epoxy compound, preferably wherein the mono-epoxy compound is selected from the group consisting of glycidyl ethers of structure: R2-O-CH2-(CHCH2O), wherein (CHCH2O) represents an epoxide moiety, and R2 is selected from aliphatic and aromatic groups, such as C2-C22 linear, branched or cyclic aliphatic group or C5-C22 aromatic group, such as n-butyl or phenyl.

8. The powder coating composition according to any one of the preceding claims, wherein the ratio between the epoxy equivalents of the epoxy resin, preferably novolac resin, and the epoxy equivalents of the di-epoxy compound is between 50 : 50 and 90 : 10, more preferably between 60:40 and 90:10.

9. The powder coating composition according to any one of the preceding claims, wherein the ratio between the boronic acid equivalent of the cross-linking agent on the one hand, and the number of nitrogen atoms of the primary amine on the other hand is between 0.8 : 1.0 and between 1.0 : 0.8, more preferably 0.8:1.

10. The powder coating composition according to any one of the preceding claims, comprising the cured product of a resin and a cross-linking agent, the resin being obtainable by the reaction of:* an epoxy novolac resin,* a di-epoxy compound having the structure: (CHCH2O)-CH2-O-L-O-CH2- (CHCH2O), wherein (CHCH2O) represents an epoxide moiety, and -O-L-O- is a spacer derived from a di-alcohol compound (HO-L-OH) being a C2-C22 linear aliphatic dialcohol or a polyethylene glycol;* a mono-epoxy compound, being a glycidyl ether of structure: R2-O-CH2- (CHCH2O), wherein (CHCH2O) represents an epoxide moiety, and R2 is a C2-C22 linear aliphatic group; and* at least one primary amine having the structure R1 -NH2, wherein R1 is a C2- C22 alkyl; and wherein the cross-linking agent is benzene-1 ,4-diboronic acid.11 . The powder coating composition according to any one of the preceding claims, wherein the powder coating composition is a cured powder coating composition.

12. A process for preparation of a powder coating composition comprising the steps of: i) mixing at least one epoxy resin having an average epoxy functionality of > 2, preferably an epoxy novolac resin, optionally one or more additional epoxy compounds, at least one primary amine, and at least one crosslinking agent having at least two boronic acid or boronic ester groups at a temperature of at least 60 °C, preferably at least 70 °C, at least 80 °C or at least 90°C to react into a network polymer; ii) processing the network polymer obtained in step i) into a powder;iii) optionally sieving the powder obtained in step ii) to obtain a powder coating composition having particles with a Dv 50 particle size distribution of less than 106 micrometer.

13. A process for the application of a powder coating on a substrate comprising the steps of: a) applying a powder coating composition according to any one of claims 1-11 onto at least one surface of a substrate; b) melting the powder coating composition under increased temperature so that the particles of the powder coating composition flow to obtain a continuous coating film on said at least one surface of said substrate; and c) allowing the continuous coating film to cool to obtain a coated substrate.

14. A coated substrate prepared by the process according to claim 13, preferably complying with the rubbing testing using methyl ethyl ketone (MEK) according to the procedure described in ASTM D4752.

15. A use of a powder coating composition according to any one of claims 1-11 for coating a substrate, preferably wherein the substrate is selected from the group consisting of pipes, valves, rebars, automotive or engine components, more preferably a water pipe or valve, a gas pipe or valve, an oil pipe or valve, a steam pipe or valve, an exhaust pipe or valve, HVAV pipes or valves automotive chassis components, frames, oil pans, wheel rims, metal radiator housing, valve covers, suspension parts and interior trim.

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