WO2024026053A1

WO2024026053A1 - Crosslinkable fluoropolymer coating

Info

Publication number: WO2024026053A1
Application number: PCT/US2023/028914
Authority: WO
Inventors: Kelly E. LUTZ
Original assignee: Arkema Inc.
Priority date: 2022-07-28
Filing date: 2023-07-28
Publication date: 2024-02-01

Abstract

The invention relates to a coating composition and a coating comprised of fluoropolymer grafted with maleic anhydride, which is able to undergo crosslinking with polyisocyanate, to provide improved chemical resistance.

Description

Crosslinkable Fluoropolymer Coating

[0001] Field of the Invention:

[0002] The invention relates to a coating comprised of fluoropolymers, grafted with maleic anhydride, which is able to undergo crosslinking to give improved chemical resistance.

[0003] Background of the Invention

[0004] Patent US 5576106 discloses the radiation grafting of a compound that can be grafted onto the surface of particles of a fluoropolymer powder.

[0005] US7241817 describes the process to make the fluoropolymer that contains maleic anhydride. The grafted fluoropolymer can be used for coatings. There is no mention of crosslinking with a polyisocyanate to show improved chemical resistance. [0006] Typical dispersion polyvinylidene fluoride (PVDF) -based coatings (like Kynar 500® FSF® PVDF) provide excellent weatherability (meets AAMA2605-20 specification 8.9.2.2.2 Color retention) and strong chemical resistance as tested by methyl ethyl ketone (MEK) double rubs (ASTM D5402). However, these coatings must be baked at or above the melting point of PVDF (>230 °C) to form a good film. By good film, we mean the film (also referred to as coating) has a gloss value ranging from 5-50, a pencil hardness rating of F to 2H and for a white coating the b* value is less than 3. [0007] Current commercially available solvent borne PVDF-based coatings that are cured below 230 °C (25 °C to 200 °C) have poor MEK double rubs, significantly less than 150. There exists a need to develop a fluoropolymer coating with excellent chemical resistance that can be applied to more temperature sensitive substrates (e.g., non-metals), at temperatures less than the melting point of PVDF.

[0008] It has been found that a coating with a low b* value (yellowness index value) can be produced using maleic anhydride-functionalized PVDF. CIELAB color space, or L*a*b*, is a three-dimensional color space defined by the International Commission on Illumination. L* measures the lightness value, with black having an L* value of 0 and white an L* value of 100. The a* axis refers to the green-red axis and b* axis refers to the blue-yellow, b* can be used to measure relative yellowness, as a more positive value skews to the yellow spectrum as opposed to a negative value which skews to the blue. A non-yellow coating would be expected to have a b* value close to 0.

[0009] A benefit of longer lasting, chemically resistant coatings, is the need for less recoats of a substrate, ultimately lowering VOC emissions over decade time spans. [0010] Applicants have found that a crosslinked coating comprising fluoropolymer grafted with maleic anhydride crosslinked with a polyisocyanate provides superior MEK chemical resistance of greater than 100, preferably greater than 150, as compared to those cured without the polyisocyanate which are less than 100. The same affect has not been seen with fluoropolymers grafted with carboxylic acids or alcohols or with fluoropolymers grafted with maleic anhydride crosslinked with polycarbodiimides or diepoxies. The present invention has the additional benefit of being able to form a good film (as described above) at a temperature of less than 200 °C , or even less than 175 °C or preferably even less than 150 °C yet still have double rub performance of greater than 150, preferably greater than 175.

[0011] Brief Description of the Invention

[0012] The coating composition of the invention comprises a solvent, preferably an organic solvent, a fluoropolymer-miscible polymer, at least one fluoropolymer grafted with maleic anhydride, at least one polyisocyanate, and optional other additives.

[0013] The coating composition preferably is a multi-pack system that is blended just prior to use.

[0014] The invention also provides for a coating comprising a fluoropolymer-miscible polymer, fluoropolymer grafted with maleic anhydride crosslinked with a polyisocyanate.

[0015] Embodiments of the Invention

[0016] The first embodiment of the invention provides a coating composition comprising a solvent, a fluoropolymer grafted with maleic anhydride, a polyisocyanate, and a fluoropolymer-miscible polymer.

[0017] The fluoropolymer in coating composition of the invention, can comprise a polyvinylidene fluoride. [0018] The polyisocyanate in the coating composition of the invention, can be selected from aliphatic polyisocyanates, aromatic polyisocyanates, blocked isocyanates, preferably the crosslinker is an aliphatic polyisocyanates.

[0019] The coating composition of the invention can comprise polyisocyanate comprises a hexamethylene diisocyanate (HDI) or oligomers thereof.

[0020] The coating composition of any one or more of the above embodiments, wherein the amount of polyisocyanate is from 2 to 15 weight percent, preferably from 3 to 15 weight percent based on total weight of the total weight of PVDF in the coating composition

[0021] The coating composition of any one or more of the above embodiments, wherein the solvent comprises A/-methyl-2-pyrrolidone.

[0022] The coating composition of any one or more of the above embodiments, wherein the solvent comprises methyl ethyl ketone.

[0023] The coating composition of any one or more of the above embodiments, wherein the fluoropolymer-miscible polymer comprises a polymethacrylate copolymer.

[0024] The coating composition of any one or more of the above embodiments, wherein the amount of fluoropolymer-miscible polymer comprises from 15 to 70 wt percent based on total combined weight or fluoropolymer and fluoropolymer-miscible polymer in the coating composition.

[0025] The coating composition of any one or more of the above embodiments, further comprising a fluorinated copolymer. The fluorinated copolymer may be a PVDF copolymer having at least 1 weight percent of a comonomer based on total weight of the copolymer, preferably between 1 weight percent and 20 weight percent of a comonomer and preferably the comonomer is HFP.

[0026] The coating composition of any one or more of the above embodiments, wherein the amount of maleic anhydride grafted onto the fluoropolymer in the coating composition is from 0.1 to 5% by weight based on the total weight of fluoropolymer in the coating.

[0027] In another embodiment the invention provides a coating on a substrate wherein the coating comprises a fluoropolymer grafted with maleic anhydride crosslinked with a polyisocyanate and a fluoropolymer-miscible polymer. [0028] The coating on the substrate can exhibit a MEK double rub of greater than 150, preferably greater than 175.

[0029] In another embodiment, the invention provides a method of coating a substrate comprising i) applying any one or more of the above embodiments of the coating composition on a substrate, ii) baking the substrate at temperature of between 50 °C and 200 °C, wherein the coating after baking exhibits a MEK double rub of greater than 150, preferably greater than 175.

[0030] The fluoropolymer of the above method may comprise a polyvinylidene fluoride.

[0031] The polyisocyanate used in the method, may be selected from the group consisting of aliphatic polyisocyanates, aromatic polyisocyanates, and blocked isocyanates, preferably the polyisocyanate comprises an aliphatic polyisocyanate. [0032] The polyisocyanate used in the method may comprise a hexamethylene diisocyanate (HDI) or oligomers thereof.

[0033] In another embodiment, the invention provides a two pack coating product comprising a first pack and a second pack, said first pack comprising a polyisocyanate and said second pack comprising a fluoropolymer-miscible polymer and a fluoropolymer grafted with maleic anhydride.

[0034] Detailed Description of the Invention

[0035] All references cited herein are incorporated by reference. Unless otherwise stated all percentages are percentage by weight.

[0036] Polyisocyanate means a compound containing multiple isocyanate functional groups.

[0037] FP/MAH means a fluoropolymer composition comprising a fluoropolymer grafted with maleic anhydride, and optionally one or more fluorinated copolymer comprising at least two fluorinated monomers.

[0038] The term copolymer means a polymer having at least 2 different monomer units and includes terpolymers and higher degree polymers (4 or more different monomer units).

[0039] The term PVDF means a polyvinylidene fluoride polymer and includes both homopolymers and copolymers. [0040] The (meth)acrylic monomers refers to methacrylic and/or acrylic monomers. The term (meth)acrylonitrile and refers to methacrylonitrile and/or to acrylonitrile. The term (meth)acrylamide refers to methacrylamide and/or to acrylamide.

[0041] The invention provides a coating composition comprising a fluoropolymer- miscible polymer, a FP/MAH, a polyisocyanate and a solvent, preferably an organic solvent. The coating composition may further contain optional additives known in the art. The maleic anhydride in the FP/MAH chemically reacts with the polyisocyanate forming crosslinks.

[0042] To achieve lower temperature film formation, the presence of fluorinated copolymers in the coating composition is preferred. Preferably the copolymer has a comonomer amount of between 1 and 20 weight percent preferably 2 to 15 weight percent, based on the total weight of the fluorinated copolymer. One example preferred copolymer is a polyvinylidene fluoride -hexafluoropropene copolymer having between 1 and 20 weight percent HFP monomer units.

[0043] The solvent based composition of the invention is formulated to be applied directly to a substrate, preferably without any surface treatment needed.

[0044] The fluoropolymer in the coating composition can be a blend of a PVDF grafted with maleic anhydride and one or more fluorinated copolymers. The percent of maleic anhydride grafted is at least 0.01 weight percent based on total fluoropolymer present in the coating composition.

[0045] The coating composition comprises at least one polymer miscible with the FP/MAH (“fluoropolymer-miscible polymer”). The FP/MAH can be blended with a nonfunctional fluoropolymer-miscible polymer or could be blended with a fluoropolymer- miscible polymer containing hydroxyl comonomers, such as hydroxyalkyl(meth)acrylates.

[0046] Fluoropolymers

[0047] A fluoropolymer grafted with maleic anhydride is used for this invention. [0048] The term “fluoromonomer” or the expression “fluorinated monomer” means a polymerizable alkene which contains at least one fluorine atom, fluoroalkyl group, or fluoroalkoxy group attached to the double bond of the alkene that undergoes polymerization.

[0049] The term “fluoropolymer” means a polymer formed by the polymerization of at least one fluoromonomer, and it is inclusive of homopolymers, copolymers, terpolymers and higher degree polymers which are thermoplastic in their nature, meaning they are capable of being formed into useful pieces by flowing upon the application of heat, such as is done in molding and extrusion processes. The thermoplastic polymers typically exhibit a crystalline melting point.

[0050] Fluoromonomers useful in the practice of the invention include, for example, vinylidene fluoride (VDF or VF2), tetrafluoroethylene (TFE), trifluoroethylene, chlorotrifluoroethylene (CTFE), hexafluoropropene (HFP), vinyl fluoride, hexafluoroisobutylene, perfluorobutylethylene (PFBE), pentafluoropropene, 3,3,3- trifluoro-1 -propene, 2-trifluoromethyl-3,3,3-trifluoropropene, fluorinated vinyl ethers, fluorinated allyl ethers, non-fluorinated allyl ethers, fluorinated dioxoles, and combinations thereof. Fluoropolymers useful in the practice of the present invention include the products of polymerization of the fluoromonomers listed above, for example, the homopolymer made by polymerizing vinylidene fluoride (VDF) by itself. Copolymers, terpolymers and higher polymers of the fluoromonomers listed above, such as for example a copolymer of vinylidene fluoride and hexafluoropropene, may also be suitable employed in the practice if the invention.

[0051] Preferred copolymers of the invention include but are not limited to the copolymers of VDF, such as VDF with TFE, VDF with CTFE, VDF with HFP, or VDF with trifluoroethylene. Preferred copolymers may be those which comprise from about 71 to about 99 weight percent VDF, preferably 80 to about 99 weight percent VDF, and correspondingly comprise from about 1 to about 29 weight percent comonomer. Terpolymers or higher degree polymers may also be used in place or in with the copolymers. Example terpolymers or higher degree polymers useful in the invention include the polymers of VDF, HFP, and TFE, and another example is the polymer VDF, trifluoroethene, and TFE. Preferred copolymers may be those which comprise at least 71 weight percent VDF, and the other comonomers may be present in varying portions which combine to comprise up to 29 weight percent of the terpolymer.

[0052] Other useful fluoropolymers include, but are not limited to homopolymers and or copolymers of polyvinyl fluorides (PVF), chlorotetrafluoroethylenes (CTFE), polytetrafluroethylenes (PTFE), fluorinated polyethylene vinyl ethers, and fluorinated ethylene vinyl esters (FEVE).

[0053] Fluoropolymers and copolymers may be obtained using known methods of solution, emulsion, and suspension polymerization. Preferably the fluoropolymer of the invention is formed without using fluorinated surfactant in the process.

[0054] The maleic anhydride is grafted onto the fluoropolymer (post polymerization). Processes to graft the maleic anhydride onto the fluoropolymer are known in the art. For example, US5576106, US7241817, FR2476657 and EP 0214880 disclose processes for grafting a monomer onto polyvinylidene fluoride.

[0055] The amount of maleic anhydride in the FP/MAH is generally from 0.01 to 10 weight percent based on total weight of the FP/MAH, preferably 0.01 to 5 weight percent maleic anhydride.

[0056] Regarding the proportions of the fluoropolymer and the maleic anhydride in the coating composition, the proportion of fluoropolymer is advantageously, by weight, from 90 to 99.9% for 0.01 to 10% of maleic anhydride, respectively. Preferably, the proportion of fluoropolymer is from 95 to 99.9% for 0.01 to 5% of maleic anhydride, respectively.

[0057] Polyisocyanate

[0058] The crosslinker used in the present invention is a polyisocyanate.

[0059] Polyisocyanates of the invention provide two or more -N=C=O groups (“NCO”). The isocyanate groups react with the carboxyl group of the maleic anhydride to form a crosslinked, interpenetrating network that can help form a, one-phase coating. The resulting amide linkage also improves the toughness of the coating material due to presence of hydrogen bonding.

[0060] The polyisocyanate can be a diisocyanate, an oligomeric isocyanate, or can be a polymeric isocyanate. The polyisocyanate can be aliphatic, such as those based on, for example, hexamethylene diisocyanate (HDI) and oligomers thereof, bis-(4- isocyanatocyclohexyl)methane, and isophorondiisocyanate (IPDI); or the polyisocyanate can be aromatic, such as those based on, for example, toluene diisocyanate (TDI) and methylene diphenyl diisocyanate (MDI). For outside applications, the polyisocyanate is preferably aliphatic to ensure good weathering performance, which matches the excellent weathering performance of a fluoropolymer.

[0061] Some useful polyisocyanates are available under the trademarks: Desmodur® (from Covestro), Tolonate™ (from Vencorex).

[0062] The polyisocyanate should be used in an amount of from 2 to 15%, preferably 3-15 weight percent based on the weight of the FP/MAH.

[0063] Other crosslinker types were tested, but were not as effective. One type of polycarbodiimide (Carbodilite SV-02) did not increase the MEK chemical resistance, but saw a large increase in pencil hardness. Another type of polycarbodiimide (Carbodilite V-04PF) lowered the MEK chemical resistance and made the coating softer (by pencil hardness). A diepoxy crosslinker (Synocure® 899 BA) also lowered the chemical resistance, while maintaining the same pencil hardness rating as the FP/MAH coating formulation without any additional crosslinker.

[0064] Fluoropolymer-Miscible Polymer

[0065] The compositions of the present invention comprise (in addition to the FP/MAH) at least one fluoropolymer-miscible polymer. A fluoropolymer-miscible polymer is a polymer which has at least partial thermodynamic miscibility with the at least one fluoropolymer and which is a homopolymer or copolymer comprised of at least one monomer selected from the group consisting of (meth)acrylic monomers, vinyl esters, (meth)acrylonitrile and (meth)acrylamide. In certain embodiments, the fluoropolymer- miscible polymer is fully thermodynamically miscible with the FP/MAH. Suitable (meth)acrylic monomers include any of the alkyl methacrylate and/or alkyl acrylate monomers discussed below in connection with the embodiment where the fluoropolymer-miscible polymer is an acrylic polymer. Suitable vinyl esters include, for example, vinyl acetate. (Meth)acrylonitrile monomers include methacrylonitrile and acrylonitrile; (meth)acrylamide monomers include methacrylamide and acrylamide. [0066] A thermodynamically miscible polymer composition is a polymer blend that exhibits a single glass transition temperature value which typically lies intermediate between the glass transition temperatures of the individual polymeric components. Correspondingly, a partially miscible or immiscible polymer composition will exhibit two or more glass transition temperature values. Accordingly, two or more polymers are said to be thermodynamically miscible when the free energy of mixing is negative. Additionally, thermodynamic miscibility is said to exist when a mixture of two or more polymers results in a material exhibiting a single, well defined glass transition temperature.

[0067] In certain advantageous embodiments of the invention, the fluoropolymer- miscible polymer is an acrylic polymer. “Acrylic polymer”, as used herein, includes polymers and copolymers formed from alkyl methacrylate and/or alkyl acrylate monomers, and mixtures thereof. In one embodiment, the alkyl methacrylate monomer is methyl methacrylate, which may make up from 50 to 100 percent of the monomer mixture. The balance (0 to 50 percent) of the monomer mixture may be one or more other acrylate and methacrylate monomers or other ethylenically unsaturated monomers, including but not limited to, vinyl aromatic monomers such as styrene and alpha methyl styrene, acrylonitrile. Other methacrylate and acrylate monomers useful in the monomer mixture include, but are not limited to, methyl acrylate, ethyl acrylate and ethyl methacrylate, butyl acrylate and butyl methacrylate, iso-octyl methacrylate and isooctyl acrylate, lauryl acrylate and lauryl methacrylate, stearyl acrylate and stearyl methacrylate, isobornyl acrylate and isobornyl methacrylate, methoxy ethyl acrylate and methoxy ethyl methacrylate, 2-ethoxy ethyl acrylate and 2-ethoxy ethyl methacrylate, hydroxymethyl acrylate and hydroxymethyl methacrylate, hydroxyethyl acrylate and hydroxyethyl methacrylate, 2-hydroxypropylacrylate , 2-hydroxypropyl methacrylate dimethylamino ethyl acrylate and dimethylamino methacrylate monomers. (Meth)acrylic acids such as methacrylic acid and acrylic acid, (meth)acrylonitrile and (meth)acrylamide may also be used as comonomers. Where the acrylic polymer is a copolymer, the copolymer may in various embodiments of the invention be a random copolymer.

[0068] In one embodiment, the acrylic polymer is formed from 65-97 weight percent of methyl methacrylate monomer units, and 3 to 35 weight percent of one or more Ci- C 6 alkyl acrylate monomer units.

[0069] The acrylic polymer employed in the present invention may be a copolymer of two, three or more different acrylate monomers, such as a mixture of butyl acrylate, ethyl acrylate, and methyl methacrylate. Combinations or mixtures of different acrylic resins may be used.

[0070] The acrylic polymer is preferably a thermoplastic, though it can be a thermoset polymer or a mixture of the two types of polymer.

[0071] In various embodiment, the acrylic polymer is partially or fully thermodynamically miscible with the fluoropolymer.

[0072] Acrylic polymers suitable for use in the present invention can be manufactured by any means known in the art, including emulsion polymerization, solution polymerization, and suspension polymerization.

[0073] In certain embodiments, the fluoropolymer-miscible polymer (e.g., acrylic polymer) has a weight average molecular weight of from about 50,000 and 500,000 g/mol or from about 75,000 to about 200,000 g/mol, as measured by gel permeation chromatography (GPC). The molecular weight distribution of the fluoropolymer-miscible polymer may be monomodal or multimodal; the polydispersity may be low (e.g., 1 to 1 .5) or relatively high (e.g., greater than 1 .5).

[0074] Solvents

[0075] There are many acceptable solvents which can be used in the preparation of coating formulations in accordance with this invention. The solvents can be either protic or aprotic. Suitable solvents are those which are capable of dissolving the fluoropolymers used in the invention. The solvents can, for example, be amides such as dimethylformamide, /V,/V-dimethylacetamide, /V,/V-diethylacetamide, A/-ethyl pyrrolidone, /V-butyl pyrrolidone, A/-octyl pyrrolidone and A/-methyl-2-pyrrolidone; ketones such as acetone, methyl ethyl ketone and 2-butanone; carbonates such as dimethylcarbonate, diethylcarbonate, propylene carbonate and ethylene carbonate; ethers such as tetrahydrofuran; triethyl phosphate; gamma-butyrolactone; ureas such as tetramethyl urea and A/,A/'-dimethyl-trimethylene urea; esters; dimethyl sulfoxide; hexamethylphosphoramide; and mixtures thereof. Of course, other solvents capable of dissolving the fluoropolymers are also useful in this invention.

[0076] Useful solvents include, but are not limited to /V-methyl-2-pyrrolidone, /V-butyl pyrrolidone, dimethylacetamide, dimethyl sulfoxide, /V,/V-dimethylformamide, tetramethylurea, triethylphosphate, acetone, and tetrahydrofuran, or mixtures thereof. [0077] The solvent may, for example, comprise from about 40% to about 85% by weight of the coating composition comprising the FP/MAH.

[0078] Other useful additives include, but are not limited to, pigments, fillers, dyes, UV absorbers, heat stabilizers, and other additives typically found in a coating composition, and at typical effective levels.

[0079] The coating composition is prepared by dissolving the fluoropolymer-miscible polymer and FP/MAH in the solvent (the optional additives can be added to the solvent). Preferably this is done at an elevated temperature (generally between 35 °C to 10 °C below the boiling point of the solvent) normally in the range of 40 - 80 °C.

[0080] The polyisocyanate is not contacted with the FP/MAH until just prior to application (meaning 8 hours or less, preferably 6 hours or less, most preferably 4 hours or less before application) on to a substrate, as is commonly done in the art.

[0081] Substrate

[0082] The coating composition of the invention can be applied to a substrate, without any pretreatment (chemical or physical) of the substrate. The coating could also be used with a treated or paint-primed substrate.

[0083] The coating composition can be used with any substrates, including both porous and non-porous materials. The substrates included, but are not limited to metals, paper, wood, and plastics.

[0084] The coating can be applied to the substrate by known means, including but not limited to spraying, brushing, dipping and roll coating, spin coating, curtain coating, blade coating, ink jet, etc. The coatings can be cured by baking at elevated temperatures within the range of 50-200 °C, preferable in the range of from 50 to 175 °C. Generally the bake time is between 3-30 minutes. Curing and drying involves the evaporation of solvent, and the reaction between the various acid anhydride groups and NCO groups to form chemical linkages providing cross-linking of the polymers.

[0085] The coating of the invention is especially useful as a means of applying a fluoropolymer coating onto non-metal substrates. Examples of end uses include, but are not limited to coatings for glass fibers, fiberglass, external architectural coatings such as textured & decorative coatings, structural walls & curtains. The claimed coating composition/method can be applied to a variety of other substrates including metals, plastic, wood, and composites of any combination of the substrate materials mentioned above. One of skill in the art can easily imagine similar uses for this technology, based on the description and examples provided.

[0086] Coating Composition

[0087] The coating composition of the invention can be either a 1 -pack stable composition, or a multi-pack (preferably 2-pack) coating system.

[0088] In a 1 -pack stable coating, all polyisocyanates must be blocked.

[0089] Preferably the invention is used as a 2-pack system. In a 2-pack system, one pack contains polyisocyanate while the second pack contains the FP/MAH, the fluoropolymer-miscible polymer, solvent, optional polymers and optional additives. The two packs are combined just prior to using the coating composition, with agitation. The working pot life of the mixture should be at least two hours, preferably 4 hours, and more preferably 8 hours.

[0090] The levels of polyisocyanate to the fluoropolymer can be adjusted to balance coating properties such as pot life, weatherability, and chemical resistance.

[0091] Preferably, the weight ratio of fluoropolymer-miscible polymer to FP/MAH in the coating composition is from 15:85 to 70:30.

[0092] Preferably the molar ratio of isocyanate functional groups to maleic anhydride is from 20 to 1 to 1 :5, preferably 10:1 to 1 :1. Maleic anhydride functionality in the FP/MAH can be determined using FTIR. A film is made from the FP/MAH and analyzed by FTIR. The spectrum, representing maleic anhydride grafting, shows a single major peak at about 1783 cm ¹. Basic hydrolysis and back-titration, coupled with FTIR analysis, indicates the concentration of grafted maleic anhydride.

[0093] Coating and Curing Process [0094] Minimum temperature limit for film formation is above 50 °C, preferably 75 °C and above.

[0095] The invention as described is able to provide a coating without altering typical gloss or pencil hardness values from typical dispersion based fluoropolymer based coating (gloss value ranging from 5-50, a pencil hardness rating of F to 2H; and for white coatings the b* value is less than 3).

[0096] The cured coatings of the present invention are particularly desirable and advantageous in that they exhibit increased chemical resistance of PVDF-based coatings that are cured at less than 200 °C. This allows for coating of non-metal substrates that would deteriorate at the high temperature of 200 °C and above.

[0097] Examples

[0098] The fluoropolymer used in the examples is a blend of 30% by weight of a vinylidene fluoride polymer composition having between 0.01 to 5 wt percent maleic anhydride grafted on to the backbone of the fluoropolymer and 70% by weight of a copolymer having a HFP content of between 1 to 20 wt percent based on total weight of fluoropolymer in the vinylidene fluoride polymer composition.

[0099] Test Methods

[0100] L* and b* are measured by a spectrophotometer (Spectro2Guide from BYK) [0101] Gloss - ASTM D523-14 Standard Test Method for Specular Gloss

[0102] MEK (Methyl Ethyl Ketone) Double Rubs - ASTM D5402-19 Standard Practice for Assessing the Solvent Resistance of Organic Coatings Using Solvent Rubs [0103] Pencil Hardness was measured using ASTM D3363-20 for Film Hardness by Pencil Test

[0104] Example 1 : (comparative-No crosslinking)

[0105] A pigment grind was prepared by adding titanium dioxide (TiO2, 1 1.1 g) to a solution of /V-methyl-2-pyrrolidone (NMP, 44.6 g) and methyl ethyl ketone (MEK, 6.7 g). Once the TiO2 was well dispersed, fluoropolymer grafted with maleic anhydride (available from Arkema) (FP/MAH,15.8 g) and an acrylic resin (Paraloid® B44, 6.8 g, already dissolved in NMP, 32.5 g and MEK, 32.5 g) were added and stirred until well mixed. The coating was applied to an aluminum panel using a 6 mil gap drawdown square. The panel was then baked at 100 °C for 10 minutes. After allowing to cool, the panel was measured for color and gloss. MEK solvent resistance and Pencil Hardness were measured 1 day after baking.

[0106] Example 2: (Polyisocyanates crosslinker)

[0107] Same as example 1 except prior to coating the aluminum panel, an aliphatic polyisocyanate, Desmodur® N3300 was added in an amount of 9% based on weight of FP/MAH polymer in the paint.

[0108] Example 3-5: (Polycarbodiimides or diepoxy crosslinker)

[0109] Same as example 2 except polycarbodiimides or diepoxy crosslinker were used as the crosslinkers as indicated in the table.

[0110] Testing:

[0111] MEK double rubs were obtained by applying felt to the head of 1 kg ball-peen hammer. The felt was soaked in MEK and applied across the surface of the coating in a back and forth manner. One back and forth pass counts as 1 double rub.

[0112] The examples show that addition of an aliphatic polyisocyanate to the FP/MAH coating formulation gives higher MEK double rubs after baking at 100 °C. The examples also show that addition of different types of crosslinkers (polycarbodiimides or diepoxies) to the FP/MAH coating formulation do not increase the MEK double rubs after baking at 100 °C.

[0113] The crosslinking occurs after the coating containing the polyisocyanate and the FP/MAH undergoes a chemical reaction forming an amide linkage. Baking temperature above 50 °C is needed in order to form a good film.

Claims

1. A coating composition comprising a solvent, a fluoropolymer grafted with maleic anhydride, a polyisocyanate, and a fluoropolymer-miscible polymer.

2. The coating composition of claim 1 , wherein the fluoropolymer comprises a polyvinylidene fluoride.

3. The coating composition of claim 1 , wherein the polyisocyanate is selected from aliphatic polyisocyanates, aromatic polyisocyanates, blocked isocyanates, preferably the polyisocyanate is an aliphatic polyisocyanates.

4. The coating composition of claim 1 , wherein the polyisocyanate comprises a hexamethylene diisocyanate (HDI) or oligomers thereof.

5. The coating composition of claim 1 , wherein the amount of polyisocyanate is from 2 to 15 weight percent, preferably from 3 to 15 weight percent based on total weight of the PVDF in the coating composition.

6. The coating composition of claim 1 , wherein the solvent comprises /V-methyl-2- pyrrolidone.

7. The coating composition of claim 1 , wherein the solvent comprises methyl ethyl ketone.

8. The coating composition of claim 1 , wherein the fluoropolymer-miscible polymer comprises a polymethacrylate copolymer.

9. The coating composition of claim 1 , wherein the amount of fluoropolymer- miscible polymer comprises from 15 to 70 weight percent based on total combined weight or fluoropolymer and fluoropolymer-miscible polymer in the coating composition.

10. The coating composition of claim 1 , further comprising a fluorinated copolymer.

11. The coating composition of claim 10, wherein the fluorinated copolymer comprises a PVDF copolymer having at least 1 weight percent of a comonomer based on total weight of the copolymer, preferably between 1 weight percent and 20 weight percent of a comonomer and preferably the comonomer is HFP.

12. The coating composition of claim 1 , wherein the amount of maleic anhydride grafted onto the fluoropolymer in the coating composition is from 0.1 to 5% by weight based on the total weight of fluoropolymer in the coating.

13. A coating on a substrate wherein said coating comprises a fluoropolymer- miscible polymer, and a fluoropolymer grafted with maleic anhydride and crosslinked with a polyisocyanate.

14. The coating of claim 13, wherein the coating exhibits a MEK double rub of greater than 150, preferably greater than 175.

15. A method of coating a substrate comprising i) applying the composition of claim 1 on a substrate, ii) baking the substrate at temperature of between 50 °C and 200 °C, wherein the coating exhibits a MEK double rub of greater than 150, preferably greater than 175.

16. The method of claim 15, wherein the fluoropolymer comprises a polyvinylidene fluoride.

17. The method of claim 15, wherein the polyisocyanate is selected from aliphatic polyisocyanates, aromatic polyisocyanates, blocked isocyanates, preferably the crosslinker is an aliphatic polyisocyanates.

18. The method of claim 15, wherein the polyisocyanate comprises a hexamethylene diisocyanate (HDI) or oligomers thereof.

19. A two pack coating product comprising a first pack and a second pack, said first pack comprising a polyisocyanate and said second pack comprising a fluoropolymer- miscible polymer, and a fluoropolymer grafted with maleic anhydride.