WO2023094570A1

WO2023094570A1 - Thermosetting coating powder suitable for outdoor application

Info

Publication number: WO2023094570A1
Application number: PCT/EP2022/083219
Authority: WO
Inventors: Gerhard BUCHINGER; Ingrid HINTERSTEINER; Rosa MATA PAVIA; Lukas ROISER
Original assignee: Tiger Coatings GmbH and Co KG
Current assignee: Tiger Coatings GmbH and Co KG
Priority date: 2021-11-26
Filing date: 2022-11-25
Publication date: 2023-06-01
Anticipated expiration: 2024-05-26

Abstract

The present invention relates to a thermosetting coating powder comprising at least one fluoropolymer, at least two polyester resins and a curing agent, characterized in that the polyester resins comprise at least one amorphous polyester resin and at least one semi-crystalline polyester resin. The invention also focusses on a substrate having a powder coating thereon on at least one surface with the powder coating being obtained from the thermosetting coating powder, and the use of the thermosetting coating powder for the coating of outdoor structures, for example façade elements and bridges.

Description

Thermosetting coating powder suitable for outdoor application

The present invention relates to the field of coatings and in particular to thermosetting coating powders and powder coatings obtained therefrom suitable for architectural and outdoor applications.

Coating powders are dry coating compositions that are applied to a substrate (e.g.: an aluminum plate), typically by an electrostatic process, in the form of a free-flowing powder. The powder coating is then obtained by treating the applied coating powder with energy, such as heat, under formation of a coating film on the substrate. Both thermosetting and thermoplastic coating powders are known, whereby thermosetting coating powders and thermoset powder coatings obtained therefrom are most widely used in the industry. Thermosetting coating powders typically comprise one or more resin(s), a curing agent for crosslinking said resin(s), and optionally further compounds, such as pigments, fillers and various additives. Amongst others, powder coatings are the coating of choice for various industrial applications that require high film thicknesses (e.g.: 50 to 120 pm) and high durability, excellent substrate protection and good weatherability resulting in long-term preservation of both functional and aesthetical properties of the coating. Further, as coating powders typically are essentially free of volatile organic compounds (VOCs), such as solvents, coating powders are an environmentally friendly coating solution, as for example compared to solvent-based liquid paints. Coating powders are used in a large variety of industries, such as in the automotive industry (e.g.: for rim coatings), architectural industry (e.g.: for fagade coatings), general industry (e.g.: for coating of machine parts) and household appliances industry on many different substrates such as metal (e.g.: aluminum or steel), wood (e.g. solid wood or medium density fiberboard (MDF)), glass, ceramic, plastics and composites, to name a few.

For certain applications, in particular for outdoor and architectural applications, coating powders based on a mixture of polyester resins and fluoropolymers are considered as particularly suitable by the industry because of the good durability, weatherability and optical appearance of the obtained powder coatings. Such obtainable powder coatings may even fulfill well-known industry standards for powder coatings in the architectural industry as available from AAMA (American Architectural Manufacturers Association), Qualicoat and GSB (GSB international e.V.), in particular AAMA 2604 and/or Qualicoat class 2 and/or GSB Florida 3 or 5. Consequently, continuous improvement of the respective coating powders and obtainable powder coatings therefrom is highly desirable, in particular to achieve AAMA 2605 and/or Qualicoat class 3 and/or GSB Florida 10 quality standards.

In the following, a brief overview about the state-of-the art in the field of coating powders based on polyester resins and fluoropolymers is provided: EP 1233044 A1 relates to a thermosetting powder coating composition which comprises a fluorine-containing polymer (A) comprising a fluoroolefin unit and a vinyl ester unit and having a crosslinkable reactive group, a polyester polymer (B) having a crosslinkable reactive group, and a curing agent.

EP 2627719 A1 relates to a hybrid polyester-fluorocarbon powder coating composition, comprising 30 to 70 wt.-% based on the total weight of the powder coating composition of discrete particles comprising a polyester resin and a curing agent for said polyester resin; and 70 to 30 wt.-% based on the total weight of the powder coating composition of discrete particles comprising a fluorocarbon resin and a curing agent for said fluorocarbon resin. The document further relates to a production process of such powder coating composition.

WO 2016037807 A1 relates to a formulation for coating substrates, comprising 5 to 70 wt.-% functional fluoropolymers, 5 to 70 wt.-% polyester (based on di- or polycarboxylic acids or their derivatives and aliphatic or cycloaliphatic di- or polyols), wherein the polyester contains at least one aliphatic or cycloaliphatic di- or polycarboxylic acid or its derivatives, 2 to 25 wt.-% crosslinker, 0.01 to 2 wt.-% crosslinking catalysts, up to 20 wt.-% UV absorber and up to 10 wt.-% UV stabilizers. The document inter alia further relates to a use of such a formulation for facade and roof surface design.

WO 1999060066 A1 relates to a powder coating composition comprising a hydroxyl functional fluoropolymer (comprising a linear, branched or mixture thereof, hydroxyl functional fluoropolymer having terminal hydroxyl groups), a solid or crystalline hydroxylated aliphatic polyester (comprising the polymerization product of a cycloaliphatic diacid and a diol) and a crosslinking agent. The document further relates to a surface protection coating comprising such a powder coating composition.

EP 3670570 A1 relates to a resin composition comprising a blend of 10 to 90 wt.-% of at least one fluoropolymer resin and 90 to 10 wt.-% of at least one semi-crystalline polyester resin (based on the total weight of the fluoropolymer resin and semi-crystalline polymer resin), wherein the semi-crystalline polyester resin preferably has a linear aliphatic and/or cycloaliphatic structure. The document discloses that amorphous polyester resins and fluoropolymer resins are not compatible and thus, a heterogeneous blend is obtained upon mixing of these two kinds of resins. Thus, the amount of fluoropolymer resin that can be added to the resin blend is limited, leading to a coating having reduced quality and weathering properties. By using said composition of a fluoropolymer and a semi-crystalline polyester resin, a stable and homogeneous resin composition is obtained which is easily ground to powder and which can lead to a coating having good weathering properties, good adhesion properties, high performance durability and an excellent overall appearance. The document further discloses the use of such a resin composition for architectural powder coating.

WO 2014002964 A1 relates to a powder coating composition comprising a fluororesin (A), a polyester polymer (B), a curing agent (C) and an ultraviolet absorber (D), wherein the polyester polymer (B) is a polyester polymer having units derived from an aromatic polycarboxylic acid compound having 8 to 15 carbon atoms and units derived from a polyhydric alcohol compound having 2 to 10 carbon atoms. The document discloses that even if such a powder coating composition is processed by a one-coat coating process to a coating film, the fluororesin layer and the polyester layer are separated into two layers in the melting and curing process of the powder coating composition. As a result, a cured film having excellent water resistance, chemical resistance, and weather resistance can be formed. This layer separation is considered as crucial for obtaining the above-mentioned beneficial properties of the powder coating. The document further discloses outdoor use of such obtained powder coatings, e.g. for outdoor units of air conditioners installed along the coast, poles of traffic lights, and signs.

In the course of extensive research activities, the applicant of the present application found that the above-described self-stratifying layer separation of fluoropolymer and polyester resin is indeed of great importance in order to obtain highly durable powder coatings that are suitable for outdoor use, in particular for architectural applications. It proved difficult, however, to control and sometimes even to achieve the desired layer separation to a sufficient extent, in particular in cases where the coatings powders were manufactured as so-called one component (1C) compositions. Also, the layer separation was found to be more difficult to control and even to achieve upon increase of the film thickness of the powder coating. Apart from deteriorated functional properties being caused by poor or insufficient layer separation, also the optical appearance of such powder coatings with poor layer separation was found to be negatively impacted, in particular due to an uneven, inhomogeneous color impression and/or a mottled surface appearance.

It is thus an object of the present invention to provide an improved, polyester-fluoropolymer based thermosetting coating powder that can be processed to a powder coating with an excellent outdoor durability and improved optical appearance, in particular by improving the self-stratifying layer separation of the polyester and the fluoropolymer during formation of the powder coating from the coating powder upon melting and curing. The coating powder according to the present invention is particularly suitable for the coating of metallic substrates in the architectural industry, such as for coating of fagade elements, bridges and other parts for outdoor use.

It was surprisingly found that with a thermosetting coating powder comprising a fluoropolymer, an amorphous polyester, a semi-crystalline polyester and a curing agent, a homogeneous powder coating with distinct fluoropolymer and polyester layers that have good interlayer interactions, in particular interlayer adhesion and phase separation, and a good adhesion to the substrate is obtained. Said powder coating has excellent physical properties, such as weathering resistance, optical appearance and mechanical properties.

The present invention is directed to a thermosetting coating powder comprising at least one fluoropolymer, at least two polyester resins and a curing agent, characterized in that the polyester resins comprise at least one amorphous polyester resin and at least one semi-crystalline polyester resin. The present invention also relates to a substrate having a powder coating thereon on at least one surface, the powder coating being obtained from said thermosetting coating powder, and to the use of said thermosetting coating powder for the coating of outdoor structures, for example fagade elements and bridges.

The thermosetting coating powder according to the present invention can be crosslinked (i.e. cured), for example by means of heating. Preferably, thermal crosslinking is applied, as it allows to benefit from the viscosity drop of the powder upon heating and thus to yield a powder coating exhibiting distinct layers of polyester and fluoropolymer with good interlayer interactions, a smooth, non-corrugated surface and a uniform thickness. Further, by using thermal crosslinking, a comparably thick coating layer (e.g. with a thickness of 50 to 120 pm) can be homogeneously crosslinked.

The crystalline domains of the semi-crystalline polyester can melt upon heating. Thus, preferably, the crystalline domains of the semi-crystalline polyester are present in molten state upon heating of the powder to a specific temperature (e.g. the activation temperature of the curing agent), and at least at the beginning of the crosslinking reaction. This allows the semi-crystalline polyester to act as diluent by improving the mobility of the compounds, particularly polymers, of the powder. The enhanced mobility of polymer chains can consequently improve the arrangement of fluoropolymer and polyesters in distinct layers. Some of the polymer chains in interlayer-near regions remain securely locked (e.g. via covalent bonds) in both layers as the powder is crosslinked, leading to good interlayer interactions and thus a coating with a good mechanical performance, such as a high shear resistance.

If only the fluoropolymer and the amorphous polyester would be mixed, a resulting powder coating would exhibit a corrugated interfacial surface, especially in case of a higher film thickness (e.g. 50 to 120 pm), which means that no two distinct polymer layers are formed, but a quasisuspension of one polymer layer in the other polymer layer, a so-called bi-continuous phase or sponge phase as shown in Fig. 5. As a consequence, also the surface of the coating is often corrugated. This is not only likely to cause an inferior (e.g. inhomogeneous) optical appearance but also to decrease the weatherability. If, in contrary, only the semi-crystalline polyester would be used in a blend with the fluoropolymer, the viscosity reduction upon heating caused by melting of the crystalline domains of the semi-crystalline polyester would result in a too strong flow of the molten powder. However, such a good flowability would be likely to cause an unequal thickness (in particular at the edges) of the coating and even dripping of the molten powder from the substrate, in particular in cases where the substrate has vertically oriented surfaces. Also, such formulations would be hard to handle during the production process, especially during extrusion, crushing and grinding. Furthermore, the obtained powders would not exhibit the usually required storage stability for powder coatings due to a too low Tg of the powder. Thus, the combined use of an amorphous and a semi-crystalline polyester is crucial in the present invention.

In a preferred embodiment of the present invention, the thermosetting coating powder has a melting enthalpy in the range of 500 to 15,000 mJ/g, more preferably 800 to 13,000 mJ/g, particularly preferably 1 ,100 to 10,000 mJ/g, with a melting peak temperature of 150 °C or below, more preferably 140 °C or below, particularly preferably 130 °C or below, as determined with differential scanning calorimetry (DSC) according to the method given in the present application. The melting enthalpy is determined from a second heating cycle, so that the thermal history of the powder is fully, or at least to the widest possible extent, eliminated in a preceding first heating cycle. Care has to be taken to not heat the powder to the reaction or activation temperature of the curing agent in the first heating cycle, as this might cause partial crosslinking already in the first heating cycle and consequently alter the crystallization behavior of the semi-crystalline polyester, leading to erroneous results when determining the melting enthalpy from the second heating cycle. To avoid a pre-reaction in the first heating cycle and, more importantly, to ensure that all or at least most of the crystalline domains of the semi-crystalline polyester are present in molten state when the powder is crosslinked, said crystalline domains preferably melt well below the activation temperature of the curing agent. This is preferably ensured by selecting a semicrystalline polyester with a melting peak temperature in the given range, as the curing temperature is typically chosen to be above 150 °C. Further preferably, the offset of the melting peak of the crystalline domains of the semi-crystalline polyester is 150 °C or below, more preferably 140 °C or below, particularly preferably 130 °C or below. This allows a clear distinction between the melting range of the crystalline domains of the semi-crystalline polyester and the temperature range in which the curing agent reacts or is activated (e.g. by means of deblocking of end groups if a blocked curing agent is used).

The appearance of a pronounced melting peak in the preferred mJ/g-range having a peak temperature within the preferred temperature range is a good indication that a sufficient amount of crystalline domains of the semi-crystalline polyester is present in the powder to achieve the above-described effect of layer formation. It is noted that apart from the semi-crystalline polyester, another compound that shows a melting peak in the same or in a similar temperature range as the semi-crystalline polyester could be present in the powder. In this case, the melting peaks of the semi-crystalline polyester and the other compound might overlap. However, a melting enthalpy in said mJ/g-range having a peak temperature within said temperature range was found to be well suitable for the assessment of whether a powder comprising a certain amount of semicrystalline polyester will yield a powder coating with the desired performance. Further, it is noted that a person skilled in the art can readily make use of additional methods to determine the composition of a given coating powder in case of uncertainties, such as by means of infrared spectroscopy, NMR spectroscopy or chromatographic methods.

[Polyester resins] At least one of the polyester resins present in the inventive powder comprises a functional group that allows a reaction with the curing agent. In a preferred embodiment of the present invention, the at least one amorphous polyester resin and/or the at least one semicrystalline polyester resin comprise an OH-functional polyester resin. In this context, an OH group is understood to be a hydroxyl group. Preferably, an OH-functional polyester resin has a hydroxyl value (HV) of at least 15 mg KOH/g, more preferably of at least 20 mg KOH/g. In an alternative embodiment of the present invention, the at least one amorphous polyester resin and/or the at least one semi-crystalline polyester resin comprise a COOH-functional polyester resin. In this context, a COOH group is understood to be a carboxyl group. Preferably, a COOH-functional polyester resin has an acid value (AV) of at least 15 mg KOH/g, more preferably of at least 20 mg KOH/g. In yet an alternative embodiment of the present invention, the at least one amorphous polyester resin and/or the at least one semi-crystalline polyester resin comprise a COOH- functional polyester resin and an OH-functional polyester resin. The OH and/or COOH groups are preferably present as terminal groups of the polyester chains. Depending on the monomers used in the synthesis of the polyester, the presence of OH and/or COOH groups in side chains is also possible. A person skilled in the art will readily know which monomers in which proportions to select in order to obtain a polyester with a specific functionality. Proper selection of monomers allows to tailor the flexibility, weatherability and crosslink density of the amorphous and/or semicrystalline polyester in the obtained powder coating, and thus to adjust the balance between mechanical strength and flexibility. Preferably, polyester resins employed in the inventive powder have an AV (in case of COOH-functionality) or HV (in case of OH-functionality) from 15 to 300 mg KOH/g, more preferably 15 to 200 mg KOH/g, yet more preferably from 15 to 100 mg KOH/g and most preferably from 15 to 95 mg KOH/g. In a particularly preferred embodiment, polyester resins employed in the inventive powder have an AV (in case of COOH-functionality) or HV (in case of OH-functionality) from 20 to 300 mg KOH/g, more preferably from 20 to 200 mg KOH/g, yet more preferably from 20 to 100 mg KOH/g and most preferably from 25 to 95 mg KOH/g. Further, suitable polyester resins preferably have a weight average molecular weight (Mw) of below 30,000 g/mol, more preferably below 20,000 g/mol, even more preferably below 16,000 g/mol, yet preferably from 5,000 to 15,000 g/mol, even more preferably from 7,000 to 15,000 g/mol and most preferably from 10,000 to 13,000 g/mol.

A polyester comprising OH and/or COOH groups can be crosslinked by means of a suitable curing agent, for example a curing agent bearing amine, amide, anhydride, epoxy and/or isocyanate groups. The skilled person is capable of choosing a suitable curing agent, or mixture of curing agents depending on the nature of the employed polyester resin(s). Preferably, both the semicrystalline and the amorphous polyester present in the inventive powder are crosslinkable, as this remarkably increases the properties, such as the durability and weatherability, of the produced powder coating. Even more preferably, both the semi-crystalline and the amorphous polyester are crosslinkable with the same curing agent, or at least with curing agents bearing the same functional group(s) and/or allowing for the same reaction mechanism. This does not only facilitate processing but also allows the semi-crystalline and the amorphous polyester to be present in the same network, which increases the strength, durability and uniformity of the formed polyester layer in the powder coating.

Preferably, the polyesters used in the inventive powder are saturated with a saturation degree of at least 90%, preferably at least 95%, particularly preferably at least 99% and most preferably 100%, to provide for an excellent durability and resistance to aging, in particular towards UV- radiation. It is noted that in this context, aromatic moieties (e.g. a benzene ring resulting from the incorporation of isophthalic or terephthalic acid) are not considered as unsaturated but as saturated. In this context, only isolated or TT-conjugated carbon-carbon, carbon-heteroatom (e.g. nitrogen) or heteroatom-heteroatom double and/or triple bonds, such as in vinyl, allyl, acrylate, methacrylate units and the like are considered as unsaturated.

[Semi-crystalline polyester] The semi-crystalline polyester used in the inventive powder is preferably based on the polycondensation reaction of (cyclo)aliphatic and/or aromatic mono-, di- , tri- and/or polyfunctional alcohols with (cyclo)aliphatic and/or aromatic mono-, di-, tri- and/or polyfunctional acids or anhydrides, esters or acid chlorides based on these acids. Preferably, the semi-crystalline polyester is mainly (at least 80 wt.-%, preferably at least 90 wt.-%, with respect to the total weight of the employed monomers) based on diols and diacids, whereby these diols and diacids are particularly preferably unbranched monomers. In this context, an unbranched monomer is understood as a monomer without side chains, such as terephthalic acid, adipic acid, 1 ,4-cyclohexandiol, or 1 ,5 pentanediol. To increase the functionality of the semi-crystalline polyester (and consequently its crosslinking density), a small amount of tri- and/or tetrafunctional alcohols, and/or of tri- and/or tetrafunctional acids can be added.

The functionality of an OH-functionalized polymer correlates with its hydroxyl value (HV). In certain embodiments, the semi-crystalline polyester has a hydroxyl value (HV) from 20 to 300 mg KOH/g, preferably from 20 to 200, more preferably from 20 to 150 and yet more preferably from 20 to 100 mg KOH/g and most preferably from 25 to 60 mg KOH/g. Preferably, the at least one semi-crystalline polyester resin used in the inventive powder has a hydroxyl value from 25 to 40 mg KOH/g, more preferably from 25 to 35 mg KOH/g and even more preferably from 28 to 35 mg KOH/g (as determined by the method given in the present application). In an alternative embodiment, the semi-crystalline polyester has an acid value (AV) from 15 to 150 mg KOH/g, preferably from 15 to 100 mg KOH/g, more preferably from 20 to 90 mg KOH/g, more preferably from 20 to 80 mg KOH/g, yet more preferably from 25 to 70 mg KOH/g, and most preferably from 25 to 60 mg KOH/g.

Preferably, most of the monomers (or more preferably, all monomers) employed to synthesize the semi-crystalline polyester are aliphatic and/or cycloaliphatic, as these monomeric units are less rigid than aromatic units and thus allow for a comparatively high mobility and good alignment of the chains of the semi-crystalline polyester. In certain embodiments, the amount of aliphatic and/or cycloaliphatic monomers employed to synthesize the semi-crystalline polyester is between 51 and 100 wt.-%, preferably between 70 and 100 wt.-%, more preferably between 90 and 100 wt.-% and most preferably between 95 and 100 wt.-%, with respect to the total weight of the employed monomers. It is particularly preferred if 100 wt.-% of the monomers employed to synthesize the semi-crystalline polyester are aliphatic and/or cycloaliphatic monomers. The higher the amount of aliphatic and/or cycloaliphatic units, the more crystalline domains may be obtained in the polyester. . This allows a quick reduction of the viscosity of the powder prior to the initiation of the crosslinking reaction due to melting of the crystalline domains, which prolongs the time available for the fluoropolymer and the polyesters to form distinct layers with good interlayer interactions before the crosslink density and thus the viscosity increase.

The monomers used in the synthesis of the semi-crystalline polyester are preferably derived from a C2-C14 mono-, di-, tri- and/or polyfunctional (cyclo)aliphatic acid (in particular including the respective acid anhydrides, esters and halo acids such as acid chlorides and acid bromides) and/or a C2-C14 mono-, di-, tri- and/or polyfunctional (cyclo)aliphatic alcohol. This yields a semicrystalline polyester with a good processability, particularly good melting properties of the crystalline domains, and an optimum crosslink density after crosslinking to reach the desired coating properties, such as flexibility and strength.

The mono-, di-, tri- and/or poly-ol used in the synthesis of the semi-crystalline polyester is preferably selected from the group comprising 1 ,2-ethanediol (= ethylene glycol), 1 ,3-propanediol, 1 ,4-butanediol, 1 ,5-pentanediol, 1 ,6-hexanediol, trimethylolethane, trimethylolpropane and glycerol or a combination thereof. The mono-, di-, tri- and/or poly-acid used in the synthesis of the semi-crystalline polyester is preferably selected from the group comprising succinic acid, glutaric acid, adipic acid, azelaic acid, sebacic acid, 1 ,12-dodecanedioic acid, 1 ,4- cyclohexanedicarboxylic acid (CHDA), terephthalic acid, isophthalic acid or a combination thereof. Particularly preferably, the semi-crystalline polyester comprises a combination of 1 ,4-butanediol, ethylene glycol, succinic acid and/or CHDA, which renders a uniform powder coating with excellent outdoor durability. In certain embodiments, the semi-crystalline polyester even consists of a combination of 1 ,4-butanediol, ethylene glycol, succinic acid and/or CHDA.

Preferably, the maximum of the melting peak of the semi-crystalline polyester is in the range of 80 to 135 °C, particularly preferably in the range of 90 to 125 °C (determined by means of DSC according to the method given in the present application). As the inventive powder is preferably thermally crosslinked at a temperature above the melting range of the crystalline domains of the semi-crystalline polyester (or at least above the peak temperature of said melting range), it is hereby ensured that the crystalline domains are present in molten state upon heating to the curing temperature (e.g. the activation temperature of the curing agent) and at least in the beginning of the crosslinking reaction. Thus, a rapid decrease of the viscosity can be obtained upon heating of the powder upon melting of the crystalline domains, and a powder coating according to the present invention with the desired layer formation can be produced.

In a preferred embodiment of the present invention, the thermosetting coating powder comprises the at least one semi-crystalline polyester in an amount from 1 to 20 wt.-%, with respect to the total weight of the powder. More preferably, the content of semi-crystalline polyester in the powder ranges from 1 to 15 wt.-%, particularly preferably from 1 to 10 wt.-%, yet more preferably from 1 to 7.5 wt.-%, with respect to the total weight of the powder. In a particularly preferred embodiment, the content of semi-crystalline polyester in the powder ranges from 2 to 8.5 wt.-%, more preferably 2.5 to 7.5 wt.-%, with respect to the total weight of the powder. In an alternative embodiment, the content of semi-crystalline polyester in the powder ranges from 0.5 to 15 wt.-%, particularly preferably from 0.5 to 10 wt.-% and yet more preferably from 0.5 to 7.5 wt.-%, with respect to the total weight of the powder. It was found that the use of an amount of semi-crystalline polyester in the given ranges in the powder yields powder coatings with excellent optical properties, uniformity, weatherability and mechanical properties, as a good balance between (i) phase separation of fluoropolymer and polyester layers, (ii) presence of sufficient interlayer interactions, and (iii) desired surface appearance (for example color homogeneity and/or uniform levelling for an acceptable visual appearance for a commercial application) of the obtained powder coating are achieved. Further, using the semi-crystalline polyester within the preferred ranges as specified above allowed for excellent processability upon production, in particular upon extrusion, crushing and grinding, of the coating powder.

The use of a content of semi-crystalline polyester below said preferred weight range could be too low to evoke the desired effect of layer arrangement. The resulting coating would rather be likely to exhibit a corrugated surface, thus an inferior optical appearance and a decreased weatherability. Contrary, an increase of the content of semi-crystalline polyester to values above the upper limit of the preferred range of 1 to 20 wt.-% (with respect to the total weight of the powder) may result in a significant viscosity drop during thermal cure of the powder due to melting of the crystalline domains. This will likely result in a powder coating with only little interlayer interactions, which consequently decreases the physical properties. Further, the viscosity of the molten powder may become so low that a coating with non-uniform thickness, in particular at the edges, is obtained and in extreme cases, drop formation at the edges or drops dripping off the molten powder from the substrate, in particular from a vertically oriented surface of the substrate, may occur after and in the course of curing. Finally, it was found that coating powders comprising more than 20 wt.-% of semicrystalline polyester are more difficult to process in production, in particular upon extrusion, crushing and grinding, as such powders turned out to be rather sticky when heated (e.g. in the course of extrusion) and difficult to grind.

According to another preferred embodiment of the present invention, the at least one semi-crystalline polyester resin has a melt viscosity of below 0.5 Pas at 130 °C (as determined according to the method given in the present application), preferably below 0.4 Pas and more preferably below 0.3 Pas at 130 °C. At this temperature, at least some of the crystalline domains of the semi-crystalline polyester are generally present in molten state, which results in said low viscosity of the semi-crystalline polyester. This allows the semi-crystalline polyester to act as diluent and aid in the arrangement of fluoropolymer and polyesters in layers with excellent interface interactions, so that a powder coating according to the present invention is formed. Further, such a low melt viscosity of the polyester at 130 °C turned out to particularly favor the phase separation and results in smooth surfaces of the powder coating, thereby also improving durability and weatherability

Preferably used semi-crystalline polyesters are Additol E 04707 (Allnex, Austria), which has a hydroxyl value of 35 mg KOH/g, a melting point in the range of 110 to 115 °C and a viscosity of 0.2 Pas at 130 °C; and R 9006, which is produced according to example 1 of the present application and has a hydroxyl value of 31 mg KOH/g, a melting point of 98 °C and a viscosity of below 0.2 Pas at 130 °C. These semi-crystalline polyesters show a good processability and assist well in the formation of layers with excellent interfacial interactions, allowing to prepare a powder coating with a homogeneous optical appearance and layer thickness. Further suitable, commercially available, crystalline polyester resins that may be employed for the inventive powder, alone or in combination, without limitation, are Additol E 04654, Additol E 0469 and Additol E 04763 (Allnex, China).

[Amorphous polyester] Similarly to the semi-crystalline polyester used in the inventive powder, the amorphous polyester is preferably based on the polycondensation reaction of (cyclo)aliphatic and/or aromatic mono-, di-, tri- and/or polyols with (cyclo)aliphatic and/or aromatic mono-, di-, tri- and/or polyacids or anhydrides, esters or halo acids such as acid chlorides based on these acids. Similar monomers as described for the semi-crystalline polyester can be used. In addition to or in place of these monomers, at least one monomer (or a combination of monomers) that hinders crystallization of the polyester, such as a branched monomer and/or non-linear monomer, needs to be used in the synthesis to prepare an amorphous polyester, which can readily be selected by a person skilled in the art. Also, by using a higher number of different kinds of monomers, (e.g. 4 or more different kinds of monomers), wherein preferably at least one of these monomers hinders crystallization when incorporated into a polymer chain and/or wherein preferably at least one of these monomers is a branched and/or a non-linear monomer and wherein particularly preferably said at least one branched and/or non-linear monomer accounts for more than 15 wt.-% with respect to all monomers, crystallization can be minimized or even prevented, since an ordered arrangement of polymer chains that promotes crystallization can be avoided or at least reduced. Preferably, the amorphous polyester comprises one or more of the following monomers: isophthalic acid, terephthalic acid, neopentyl glycol and trimethylolpropane. Preferably, a weather-resistant amorphous polyester is employed. Use of such an amorphous polyester resin may significantly improve the weatherability of the obtained powder coatings. Preferably, weatherresistant amorphous polyester resins comprise isophthalic acid as an acid-functional monomer, preferably in amount of at least 50 wt.-%, more preferably at least 80 wt.-%, yet more preferably at least 90 wt.-% and most preferably at least 95 wt.-%, with respect to total weight of the acidfunctional monomers employed for the synthesis of the amorphous polyester resin. Alternatively, weather-resistant amorphous polyester resins comprise terephthalic acid as an acid-functional monomer, preferably in amount of at least 50 wt.-%, more preferably at least 80 wt.-%, yet more preferably at least 90 wt.-% and most preferably at least 95 wt.-%, with respect to total weight of the acid-functional monomers employed for the synthesis of the amorphous polyester resin. Further preferably, weather-resistant polyester resins comprise neopentyl glycol and/or trimethylolpropane.

Preferably, the content of amorphous polyester in the inventive powder is in the range of 5 to 50 wt.-%, preferably 10 to 45 wt.-%, more preferably 15 to 35 wt.-%, particularly preferably 20 to 30 wt.-%, with respect to the total weight of the powder. The content of amorphous polyester in the inventive powder is preferably chosen such that it is about 1 to 30 times, more preferably about 1 .5 to 20 times, particularly preferably about 2 to 12 times, yet more preferably about 3 to 10 times and most preferably about 4 to 8 times, higher than the content of the semi-crystalline polyester. The selection of a ratio in said range was found to yield an optimum balance between (i) formation of distinct fluoropolymer and polyester layers, and (ii) presence of sufficient interlayer interactions in the produced powder coating, hereby providing an excellent outdoor durability and mechanical strength, as well as a uniform, non-corrugated coating. In certain embodiments, the amorphous polyester has a hydroxyl value (HV) from 15 to 300 mg KOH/g, preferably from 15 to 200, more preferably from 15 to 150, more preferably from 15 to 95 mg KOH/g, yet more preferably from 25 to 100 mg KOH/g, and most preferably from 25 to 90 mg KOH/g. In a preferred embodiment, the at least one amorphous polyester resin has a hydroxyl value (HV) from 25 to 70 mg KOH/g, preferably from 25 to 40 mg KOH/g, more preferably from 25 to 35 mg KOH/g (e.g. 27 or 28 mg KOH/g) and most preferably from 30 to 35 mg KOH/g. In a further embodiment of the present invention, the at least one amorphous polyester resin has a hydroxyl value (HV) from 70 to 100 mg KOH/g, more preferably from 75 to 95 mg KOH/g (as determined by the method given in the present application). In an alternative embodiment, the amorphous polyester has an acid value (AV) from 15 to 150 mg KOH/g, preferably from 15 to 100 mg KOH/g, more preferably from 20 to 90 mg KOH/g, more preferably from 20 to 80 mg KOH/g, yet more preferably from 25 to 70 mg KOH/g, and most preferably from 25 to 60 mg KOH/g.

Preferably, the amorphous polyester used in the present invention has a Tg from 35 to 80 °C, more preferably from 40 to 70 °C, particularly preferably from 40 to 60 °C. The given upper limit allows for a sufficient mobility of the polymer chains and facilitates the formation of a powder coating with the desired properties. The given lower limit in turn improves the storage and transport stability of the inventive coating powder.

Due to its good processability and excellent performance after crosslinking as well as good weatherability, the amorphous polyester Crylcoat 4890-0 (Allnex, Austria) is preferably used.

Further suitable, commercially available, amorphous polyester resins that may be employed in the inventive powder, alone or in combination, without limitation, are llralac P 1550, llralac P 1680, Uralac P 1580, Uralac 1675, Uralac 1475, Uralac 1420, Uralac 5504, Uralac 1425, Uralac 1625 (all Covestro, Germany), Crylcoat 2814-0, Crylcoat 2818-0, Crylcoat 4823-0, Crylcoat 2920- 0, Crylcoat 2890-0, Crylcoat 2857-5, Crylcaot 2860-0 (all Allnex, Austria).

[Fluoropolymer] The fluoropolymer used in the inventive powder comprises partially or fully fluorinated monomers, such as vinylidene fluoride or tetrafluoroethylene. Fluorinated monomers can be prepared for example by substituting a hydrogen atom in a hydrocarbon olefin by a fluorine atom. The fluoropolymer can be a homopolymer or a copolymer of at least two different monomers, of which at least one monomer is fluorinated. Non-fluorinated monomers can for example be selected from ethylene or propylene. When the fluoropolymer is a copolymer, it preferably contains at least 50 wt.-%, more preferably at least 70 wt.-%, fluorinated monomers to ensure a good weatherability and resistance to other environmental influences (e.g. to UV light). Further, preferably, the fluoropolymer is saturated with a saturation degree of at least 95%, preferably at least 99% and more preferably 100%, which provides for a further enhancement of the durability and resistance to aging. Yet again, in this context, as for the polyester resins, aromatic moieties are not considered as being unsaturated.

Preferably, an amorphous fluoropolymer is employed in the present invention, with the Tg of the fluoropolymer preferably being below the curing temperature of the powder (which can for example be the activation temperature of the curing agent). Preferably, the Tg of the fluoropolymer is from 30 to 70 °C, more preferably from 35 to 60 °C, and most preferably from 40 to 55 °C. This allows for an improved mobility of the fluoropolymer chains upon heating to the curing temperature and at least in the beginning of the crosslinking reaction. Thus, a coating with good layer separation and suitable interactions between the fluoropolymer and the polyester layers can be obtained. In contrary, it is preferred to not use a semi-crystalline fluoropolymer in the present invention, or at least only in a small amount. As semi-crystalline fluoropolymers usually melt in a temperature range that is significantly higher than the curing temperature (e.g. above 300 °C), the use of a fluoropolymer with crystalline domains in the inventive powder can result in an increase of the stiffness at the curing temperature of the powder, at which the crystalline domains of the semi-crystalline fluoropolymer would be present in crystalline state. This could impede phase separation and the arrangement of fluoropolymer and polyesters in separate layers in the powder coating. In case a semi-crystalline fluoropolymer is employed in the inventive powder, it is noted that its selection is preferably chosen in accordance with the semicrystalline polyester in order to avoid an overlap in melting peaks of semi-crystalline polyester and semi-crystalline fluoropolymer. Otherwise, the ability to conclude from the melting enthalpy in a certain temperature range on the content of crystalline domains present in the semi-crystalline polyester could be impeded.

Preferably, the content of fluoropolymer in the inventive powder is in the range of 15 to 50 wt.-%, more preferably 15 to 40 wt.-%, even more preferably 20 to 40 wt.-%, particularly preferably 25 to 35 wt.-%, with respect to the total weight of the powder. This allows for the formation of a homogeneous layer with a thickness sufficient to provide a good weatherability and a high mechanical strength.

In a preferred embodiment of the present invention, the at least one fluoropolymer is crosslinkable. This allows to further improve the physical properties of the inventive powder coating, particularly its mechanical strength and outdoor durability. Particularly preferably, the fluoropolymer present in the inventive powder comprises a functional group that allows a reaction with the same curing agent used to crosslink the amorphous and/or semi-crystalline polyester, or at least with curing agents bearing the same functional group(s) and/or allowing for the same reaction mechanism. Crosslinking of both the fluoropolymer and the polyester with the same curing agent can increase the interlayer interactions between the polyester and fluoropolymer layers in the powder coating, which improves the physical properties of the powder coating, such as its shear strength.

Preferably, the at least one fluoropolymer comprises an OH-functional fluoropolymer. In this context, an OH group is understood to be a hydroxyl group. Functionalization with OH groups allows for the crosslinking of the fluoropolymer by means of a suitable curing agent, e.g. a curing agent bearing amine, amide, anhydride, epoxy and/or isocyanate groups. In yet another embodiment, the at least one fluoropolymer comprises a COOH-functional fluoropolymer which again allows crosslinking with a suitable curing agent, e.g. hydroxylalkylamids (so called PRIMIDs) and/or epoxides, such as epoxy resins, Triglycidylisocyanurat (TGIC) or Araldite PT- 910/912 (Huntsman, USA) crosslinkers. Of course, at least one OH-functional fluoropolymer and in addition at least one COOH-functional fluoropolymer may also be employed in certain coating powders according to the present invention, which allows the crosslinking with various curing agents suitable to crosslink hydroxyl and carboxyl groups as mentioned in the previous paragraph. In another embodiment, the at least one fluoropolymer comprises both an OH (hydroxyl) and a COOH (carboxyl) functional group within a single polymer chain, resulting in a so called “OH/COOH bi-functional” fluoropolymer.

So, in summary, and according to a preferred embodiment, the at least one fluoropolymer comprises an OH-functional fluoropolymer, a COOH-functional fluoropolymer and/or a OH/COOH bi-functional fluoropolymer.

According to particularly preferred embodiment, the at least one fluoropolymer comprises an OH- functional fluoropolymer and/or a COOH-functional fluoropolymer.

Further preferably, the at least one fluoropolymer has a hydroxyl value (HV) from 30 to 60 mg KOH/g, preferably from 40 to 50 mg KOH/g as determined according to ISO 14900:2017. This allows to yield a crosslink density in the fluoropolymer layer in the powder coating high enough to provide good mechanical properties and an excellent resistance to weatherability but low enough to maintain a good flexibility of the powder coating and to avoid embrittlement during application.

In a preferred embodiment of the present invention, the at least one fluoropolymer comprises a fluoroethylene vinyl ether (FEVE) polymer, i.e. a copolymer of a fluoroethylene monomer and a vinyl ether monomer (e.g. from AGC Chemicals Europe, Ltd.). FEVE does not only provide an excellent weathering resistance and durability but also can be equipped with various functional groups, preferably OH groups. Even further, it is particularly preferred if the at least one fluoropolymer consists of FEVE polymers.

According to a further embodiment, the at least one fluoropolymer is free of epoxy and/or glycidyl- functional groups. According to a further embodiment, the at least one fluoropolymer is free of polytetrafluoroethylene (PTFE).

According to a further embodiment, the at least one fluoropolymer is free of structural units that are derived from (meth)acrylate monomers; or in other words, the at least one fluoropolymer is synthesized from monomers which are free of (meth)acrylate functional groups.

According to a further embodiment, the at least one fluoropolymer is free of structural units that are derived from tetrafluoroethylene; or in other words, the at least one fluoropolymer is synthesized from monomers which are free of tetrafluoroethylene.

According to a further embodiment, the at least one fluoropolymer is free of structural units that are derived from vinylidene fluoride; or in other words, the at least one fluoropolymer is synthesized from monomers which are free of vinylidene fluoride.

Lumiflon LF-710F (AGC Chemicals Europe) is preferably used as fluoropolymer because of its excellent weatherability and processability. Further suitable, commercially available fluoropolymers that may be employed in the inventive powder, alone or in combination, without limitation, are DF-MP08 (Huatong Ruichi Materials Technology Co. Ltd., China), Kynar® PVDF 201 , Kynar® PVDF 711 , Kynar® PVDF 721 , Kynar® PVDF 741 , Kynar® PVDF 761 (Arkema, France).

[Curing agent] The curing agent used in the powder according to the present invention needs to be matched with the functional groups of the amorphous and/or semi-crystalline polyester to allow for crosslinking. In case an OH-functionalized polyester is used in the powder, the curing agent may comprise amine, amide, anhydride or isocyanate groups or a mixture thereof, which groups allow for a reaction with OH groups. It is noted that any blocked or latent form of a respective functional group that is unblocked or turns reactive under suitable curing conditions (e.g.: in between 150 and 250 °C) is also included, for example blocked isocyanates (e.g.: blocked by a suitable blocking agent, such as caprolactam) or reversibly dimerized isocyanates, so-called uretdiones. The same inclusion also holds true for any other functional group of a compound as mentioned within the present applications, such as of the employed resins.

The curing agent needs to be selected in accordance with the curing conditions, such as time and temperature. Preferably, thermal cure is used to crosslink the inventive powder, in which case the curing agent preferably initiates the curing reaction at the curing temperature (e.g. from 155 to 240 °C) and subsequently preferably allows for a quick reaction to be able to cure the inventive powder within a reasonable period of time (e.g.: within 5 to 40 minutes, preferably 10 to 35 minutes or e.g.: within 5 to 30 minutes, preferably 10 to 20 minutes). At the same time, the curing agent preferably provides for a good shelf life at room temperature to allow the storage of the inventive powder at ambient conditions (e.g. at 25 °C for at least about 6 to 12 months) without lumping or the occurrence of an undesired pre-reaction prior to processing. Finally, it is preferred that the curing agent causes no or only very little pre-reactions in the course of production of the coating powder, in particular upon extrusion. For that reason, curing agents having an activation temperature of above 150 °C are preferably used.

In case a crosslinkable fluoropolymer is employed, the curing agent also needs to be able to crosslink the fluoropolymer via its functional groups. In this case, preferably, the amorphous and/or semi-crystalline polyester and the crosslinkable fluoropolymer bear the same functional groups which enables a reaction with the same curing agent. This does not only facilitate processing but, more importantly, allows for the establishment of excellent interlayer interactions formed by means of covalent crosslinks between the polyester layer and the fluoropolymer layer. Alternatively, for example in case the polyester and fluoropolymer bear different functional groups that need to be crosslinked with different curing agents, a mixture of two or more curing agents can be employed in the powder.

Preferably, the content of curing agent in the inventive powder is in the range of 5 to 25 wt.-%, preferably 9 to 24 wt.-%, and more preferably 14 to 23 wt.-%, with respect to the total weight of the powder, which was found to yield powder coatings with an optimum crosslink density, so that an optimum balance between flexibility and mechanical strength is achieved.

In a preferred embodiment of the inventive powder, the curing agent comprises a blocked isocyanate. This is particularly suitable in case thermal cure is applied. Blocking of the isocyanate group prevents any reaction of the blocked isocyanate to take place at ambient conditions or upon production of the powder, in particular in the course of extrusion. Instead, the blocked isocyanate needs to be exposed to a certain activation temperature at which it is deblocked. Only after deblocking can the curing reaction proceed, so that the storage, transport and production stability of the inventive powder below the activation temperature of the blocked isocyanate is significantly improved. Preferably, the isocyanate used in the present invention is blocked with caprolactam that deblocks at a temperature in the range of 160 to 180 °C. This enables not only a good shelf life and a fast cure of the inventive powder but also results in a powder coating with excellent properties, such as an excellent optical appearance, weatherability and physical performance. Exemplary preferred caprolactam-blocked isocyanates are Vestagon B 1530 (Evonik, Germany) and Crelan NW 5 (Covestro, Germany). Further suitable, commercially available curing agents that may be employed, alone or in combination, without limitation, are: Vestagon B 1530, Vestagon BF 1540, Vestagon BF 1320, Vestagon B1400 (all Evonik, Germany), Crelan EF 403, Crelan NW5 (all Covestro, Germany), TGIC (available from various suppliers), PT910/912 (Huntsman, USA) and hydroxylalkylamids, so-called PRIMIDs (EMS-Griltech, Switzerland). The curing agent used in the inventive powder can comprise more than one curing compound, for example if more than one polymer is crosslinkable but if the crosslinkable polymers cannot be all crosslinked with the same curing agent (e.g. because they bear different functional groups). However, it is preferred to be able to crosslink all crosslinkable polymers in the powder with the same curing agent (or at least with curing agents bearing the same functional group(s) and/or allowing for the same reaction mechanism). Processing is hereby facilitated, and side reactions (e.g. by the reaction of different curing agents with each other) can be avoided or at least minimized. Preferably, all crosslinkable polymers present in the inventive powder bear OH groups, which allows for a reaction with the same curing agent (or at least with curing agents bearing the same functional group(s) and/or allowing for the same reaction mechanism), preferably with an isocyanate, particularly preferably with a blocked isocyanate.

In a preferred embodiment of the present invention, the curing agent comprises a first curing compound and a second curing compound, wherein the first curing compound is a first blocked isocyanate and the second curing compound is a second blocked isocyanate different from the first blocked isocyanate. This allows to tailor the crosslinking process, such as the crosslinking rate or the crosslinking density, for example by using two isocyanates which deblocking rates, deblocking temperatures and/or functionalities (i.e., number of isocyanate groups) differ from each other. Further, it can be useful to use a first blocked isocyanate with a better affinity to the fluoropolymer and a second blocked isocyanate with a better affinity to the polyester, which allows for example to tailor the network properties in accordance with the desired application and substrate. Also, in case of stepwise crosslinking (e.g. heating to a first temperature; deblocking of the first isocyanate and partial crosslinking with the first isocyanate; heating to a higher, second temperature; deblocking of the second isocyanate and continuation of crosslinking reaction with the second isocyanate), the use of two different isocyanates can be useful. This allows for example to perform the crosslinking reaction more moderately in order to avoid the formation of stress cracks in the powder coating. Of course, these principles may be exploited by the skilled person for other cases where two or more curing compounds being different from each other are employed in similar powders. Such curing compounds may comprise the same and/or different functional groups provided that these functional groups are capable of crosslinking the respective resin for which a respective curing compound is provided under the chosen curing conditions.

The curing agent can also be used in combination with an initiator, accelerator and/or catalyst, for example to speed up the curing reaction at a definite temperature. Preferably, a latent initiator, accelerator, catalyst or a mixture thereof is used, as the shelf life of the inventive powder at ambient temperature (about 25 °C) is not negatively affected but the curing reaction at processing temperature and/or the resulting crosslink density can be significantly enhanced. [Further compounds] The contents of the at least one fluoropolymer, the at least two polyester resins and the curing agent in the inventive thermosetting coating powder can be chosen such that their amount is 100 wt.-%, with regard to the total weight of the powder. However, preferably, said compounds make up to less than 100 wt.-% so that the inventive powder can comprise further compounds. These further compounds are preferably selected from the group comprising a stabilizer, a process aid, a filler, a color pigment, an effect pigment, a wax, a matting agent, a thermoplastic polymer, impact modifiers, or a combination thereof, and preferably make up to at least 10 wt.-%, more preferably at least 20 wt.-%, of the total weight of the powder, to provide the powder with further advantageous properties, e.g. improved flame resistance, color, effect, mechanical properties, weatherability, flexibility, and/or improved processability. Of course, further compounds known in the art of powder coatings may be added to the inventive powders, provided that such compounds (in the added amount) do not significantly disturb the phase separation of the polyester and fluoropolymer.

The inventive powder can comprise a stabilizer, such as a UV absorber, an antioxidant, a hindered amine light stabilizer (HALS), a heat stabilizer or a mixture thereof, to improve its resistance to aging and degradation and thus its weatherability. Preferably, a mixture of a HALS compound (preferably Tinuvin 144, BASF, Germany) and a UV absorber (preferably triazine based, such as 2-hydroxyphenyl-s-triazine, e.g. Tinuvin 405 BASF, Germany) is used in the inventive powder due to its good processing performance and excellent compatibility with the other compounds present in the powder. By adding stabilizers to the powder, the durability and weatherability of the powder coating may be further improved. The inventive coating powders preferably comprise stabilizers in an amount from 0.5 to 5 wt.-%, more preferably from 0.5 to 2.5 wt.-%, most preferably from 0.5 to 1.5 wt.-%, with respect to the total weight of the powder. Suitable, commercially available stabilizers that may be employed, alone or in combination, without limitation, are: Tinuvin 144, Tinuvin 326, Tinuvin 292, Tinuvin 328, Tinuvin P, Tinuvin 622, Tinuvin 405, Tinuvin 292, Tinuvin 152, Tinuvin 622, Tinuvin 770, Tinuvin 123, Tinuvin 477, Tinuvin 479 (all BASF, Germany).

Further, a process aid can be used in the inventive powder, such as a degassing agent, a flow agent, an anti-caking agent, a lubricant, a levelling agent, or a mixture thereof. Preferably, a degassing agent, such as Benzoin (Harke Chemicals, Germany) and/or a flow agent (such as BYK 3900 P, BYK, Germany) is used in the inventive powder. The degassing agent allows to prevent the formation of air bubbles in the powder during the curing reaction and improves the homogeneity of the resulting powder coating. The flow agent further improves the processing performance of the inventive powder. The inventive coating powders preferably comprise process aids in an amount from 1.0 to 7.0 wt.-%, more preferably from 1.0 to 5.0 wt.-%, even more preferably from 1.2 wt.-% to 5.0 wt.-% and most preferably from 1.2 wt.-% to 3.0 wt.-%, with respect to the total weight of the powder. In a preferred embodiment, the inventive coating powders preferably comprise process aids in an amount from 1.3 wt.-% to 5.0 wt.-% and most preferably 1.5 wt.-% to 5.0 wt.-%, with respect to the total weight of the powder. Suitable, commercially available process aids that may be employed, alone or in combination, without limitation, are: Powdermate 486 CFL, Powdermate 507 PFL, Powdermate 542 DG or Powdermate 579 FL (all Troy Chemical Company, The Netherlands), Resiflow P 67 or Resiflow PL 200, Resiflow PL 220 or Resiflow CP 77 (all Worlee-Chemie GmbH, Germany), BYK-364 P, BYK-3931, BYK-3932, BYK-3933 (all BYK, Germany), Ceretan ME 0825, Ceretan MF 5108, Ceretan MTZ 9335 (all Munzing Chemie, Germany), Cerafluor 920, Cerafluor 929, Cerafluor 961 , Cerafluor 965 (all BYK, Germany), Ceridust 3910, Ceridust 6050M, Ceridust 9610 F, Licowax C Micropowder PM (all Clariant, Switzerland).

Preferably, the inventive powder further comprises one or more type(s) of pigments, in particular color and/or effect pigments which can be comprised in the polyester and/or the fluoropolymer layer of the produced powder coating. The pigments can be selected from organic and/or inorganics pigments. In case the pigments are only or at least largely present in the polyester layer, the fluoropolymer layer is preferably transparent (i.e., absent of any compounds that absorb light, such as pigments or fillers), which allows for the pigmented polyester layer to be seen beneath the fluoropolymer layer. In case the pigments are present in the fluoropolymer layer, they preferably provide increased resistance to weathering, light and heat. Particularly preferably, inorganic pigments are used in the inventive powder, as these pigments do not only provide for good weathering of the powder coating, but also offer a good stability to heat and do not show any migration. The inventive coating powder preferably comprise pigments in an amount from 0.5 to 30 wt.-%, preferably from 1 to 30 wt.-%, more preferably from 5 to 25 wt.-% and most preferably from 5 to 20 wt.-%, with respect to the total weight of the powder. Preferably, the inventive coating powder comprises a black color pigment. Alternatively, the inventive coating powder preferably comprises a white color pigment. Particularly preferably, the powder according to the present invention comprises titanium dioxide (TiO2; such as Ti-Select TS6200 from Chemours (former DuPont), US), which is not only the most commonly used white pigment within the coatings industry but may also exhibit an excellent UV resistance in case a suitable quality for exterior use is employed. Further, TiO2 is chemically inert so that the crosslinking reaction of the powder is not affected. It was surprisingly found that, probably due to the low compatibility of the fluoropolymer with TiO2, migration of TiO2 into the polyester layer of the powder coating occurs upon curing. Thus, TiO2 is present only, or at least to a large extent, in the substrate-near polyester layer of the powder coating. This brings the further advantage that a photocatalytic effect (which would be likely to cause degradation of the coating) can be avoided or at least reduced. Thus, with a powder coating comprising a substrate-near, TiO2-containing polyester layer and a surface-near fluoropolymer layer, an excellent weatherability and outdoor durability is achieved. TiC>2 that is surface treated (e.g. at least partially coated) with silica, zirconia and/or alumina is particularly preferred, as such treated TiC>2 shows a reduced photocatalytic activity. The same effect has also been observed for other inorganic pigments. Suitable commercially available TiC>2 pigments that may be employed, alone or in combination, without limitation, are: TiPure R-960 TiPure R-706 (Chemours (former DuPont), USA), KRONOS 2360, KRONOS 2160, KRONOS 2310 (all Kronos, The Netherlands), D-970, D-918 (Sakai Chemical Industry, Japan), TRONOX CR880 (Tronox, Switzerland), Lomon R-996 (Lomon Billions, China).

Suitable, commercially available color pigments and/or effect pigments that may be employed, alone or in combination, without limitation, are: Bayferrox 160M or Bayferrox 316 (all Lanxess, Germany), Black 1G (The Shepherd Color company Belgium), Colortherm Green GN (Lanxess, Germany), Sicotan Yellow L 1010 or Sicotan Yellow 2110 (all BASF, Germany), Sicopal Blue L 6210 or Sicopal Blue L 6310 (all Lanxess, Germany), AL-82431 NEGRO (Al-Farben, Spain), STANDART® PCR 211 Aluminum Powder, STANDART® PCU 5000 Aluminum Powder, STANDART® PCU 3500 Aluminum Powder (all Eckart, Germany). It is noted that black and white are considered as colors within the context of the present application.

The inventive powder can also comprise a filler, preferably an active filler to improve certain physical properties of the powder coating, such as its mechanical strength or other physical properties. Fillers may be of organic (e.g.: polymeric) or inorganic nature, whereby inorganic fillers, such as inorganic salts, are preferably used. Preferably, a flame retardant filler is employed, particularly preferably aluminum hydroxide (ATH; e.g. Portafill A 40, Sibelco Europe, The Netherlands) which acts as flame retardant and smoke suppressant. ATH considerably improves the fire resistance and thermal insulation of the powder coating prepared according to the present invention without negatively affecting its mechanical properties. Also, reactive fillers comprising functional groups that are reactive with any further compound of the coating powder under curing conditions may be employed. Such reactive fillers are incorporated into the polymeric network of the coating by means of covalent bonds and thus may improve the mechanical properties of the coating. The inventive coating powder preferably comprises fillers in an amount of up to 35 wt.-%, preferably up to 25 wt.-%, more preferably from 5 to 20 wt.-%, even more preferably from 5 to 16 wt.-% and most preferably from 5 to 12 wt.-%, with respect to the total weight of the powder. Suitable, commercially available fillers that may be employed, alone or in combination, without limitation, are: Martinal OL 104, Martinal ON 310, Martinal ON 4608 (all Martinswerk GmbH, Germany), Portaryte 40/9, Portaryte B10, Portaryte B15 (all Sibelco Specialty Minerals, The Netherlands), Sachtofine LG (DEUTSCHE BARYT INDUSTRIE, Germany), Blanc fixe M 0.9 (Harke Chemicals, Germany). [Further aspects] T o produce a thermosetting coating powder according to the present invention, all compounds, i.e. fluoropolymer, amorphous and semi-crystalline polyester, curing agent, and further compounds are weighed according to the respective formulation and mixed. The mixing can be performed for example with a high-speed mixer, followed by extrusion, e.g. by means of a double screw extruder. To avoid any pre-reaction during production of the powder, care has to be taken to perform the processing steps below the activation temperature of the curing agent (e.g. below the deblocking temperature of the isocyanate). Subsequently, the mixture is broken, granulated and/or ground and optionally classified, e.g. with a mill, to obtain a powder. Optionally, the powder can additionally be sieved to tailor the particle size distribution. Preferably, the resulting powder has a d particle size ranging from 5 to 16 pm, more preferably 8 to 12 pm and/or a dso median particle size ranging from 20 to 40 pm, more preferably 26 to 31 pm and/or a dgo particle size ranging from 50 to 70 pm, more preferably 55 to 65 pm, as it provides a good processability and yields a uniform, homogeneous powder coating. Preferably, the powder is sieved (top-cut) using a sieve having a mesh size of at most 150 pm, preferably of at most 120 pm and most preferably of at most 100 pm in order to avoid particles with a size above the given mesh size in the final powder, as such large particles may negatively impact the processing of the coating powder and/or the surface appearance of the obtained powder coatings.

The Tg of all amorphous and semi-crystalline polyesters and fluoropolymers of the thermosetting coating powder according to the present invention is preferably 80 °C or below, more preferably 70 °C or below, particularly preferably 60 °C or below. This allows for a high mobility of the polymer chains during thermal crosslinking, as all polymers are present in their entropy-elastic state. To further increase the shelf life of the powder, it is preferred if the polymers present in the powder are in their energy-elastic state and thus have a limited chain mobility at ambient temperature (about 25°C). Thus, preferably, the Tg of the amorphous polyesters and fluoropolymers comprised in the powder is 30 °C or above, more preferably 40 °C or above, particularly preferably 45 °C or above.

In a preferred embodiment, the thermosetting coating powder according to the present invention has a minimum viscosity of below 60 Pas, more preferably below 50 Pas, yet more preferably below 40 Pas and most preferably below 30 Pas, as determined according to method 2a of the present application. The powder preferably reaches its minimum viscosity when at least a significant portion of the crystalline domains of the semi-crystalline polyester is present in molten state, preferably all crystalline domains. Accordingly, the temperature at which the minimum viscosity of the powder is reached is preferably higher than the temperature of the melting peak of the semi-crystalline polyester. This considerably improves the mobility of the compounds, particularly polymers, present in the powder, and allows the arrangement of fluoropolymer and polyesters in distinct, homogeneous layers with good interlayer interactions and an excellent optical appearance.

The inventive thermosetting powder has a very good shelf life (e.g.: at least 12 months) at ambient temperature (about 25 °C), which allows its production and storage as a one component (1C) system, i.e. with crosslinkable polymers and the curing agent (and any further compounds) being present in one formulation without any pre-reaction to occur. This further significantly facilitates the handling during the production of powder coatings. Thus, according to a particularly preferred embodiment the inventive coating powder is a one component (1C) composition. Of course, however, the coating powder according to the present invention may also be formulated as a two component (2C) or even a multi component composition. For example, in a 2C composition, the first component may comprise the polyester resins, a curing agent for the polyester resins and optionally further compounds and the second component may comprise the at least one fluoropolymer, a curing agent for the same and optionally further compounds. The first and second component of a 2C composition may be produced individually (in full analogy to 1C systems) and then be mixed, e.g.: by means of a dry blending process, in order to obtain the 2C coating powder.

The inventive thermosetting coating powder can be coated onto various substrates, for example onto a polymer, metal (in particular steel or aluminum), concrete, ceramic, wood including engineered wood (such as fiber board, e.g.: MDF), or a composite thereof, provided that the temperature-stability of the substrate is good enough to be not damaged, e.g. degraded, at the curing temperature of the powder. The substrate may be pre-treated and/or comprise a primer coating thereon. Further, a top-coating may be applied on top of the inventive powder coating. Preferably, a metal substrate is pre-treated in accordance with the requirements and quality standards for architectural applications as regularly provided for and updated by GSB international e.V. The substrate according to the present invention exhibits the powder coating obtained from the inventive thermosetting coating powder on at least one surface. To produce the powder coating, the coating powder can be applied electrostatically, e.g. by means of Corona or Tribo charging. A layer of coating powder is formed onto the surface of the substrate, preferably resulting in a coating after curing with a thickness of 50 to 120 pm, more preferably 55 to 100 pm, particularly preferably 60 to 90 pm, which was found to yield the best optical and mechanical properties of the resulting coating. The coating powder can then be crosslinked in an oven, such as a convection oven or an infrared oven, to obtain the powder coating. Preferably, thermal crosslinking is performed at a substrate temperature in the range from 155 to 240 °C, preferably 160 to 240 °C, more preferably 160 to 220 °C, even more preferably 180 to 220 °C, particularly preferably 190 to 205 °C, for a time period of preferably 5 to 30 min, more preferably 10 to 25 min, which yields a powder coating with good layer separation and interactions between the fluoropolymer and polyester layers, a good mechanical performance and a uniform thickness without any corrugation. In a particularly preferred embodiment, thermal crosslinking is performed at a substrate temperature in the range from 170 to 205 °C, or from 170 to 200 °C, for a time period of preferably 5 to 35 minutes, more preferably 5 to 30 minutes, more preferably 5 to 20 minutes, yet more preferably 10 to 25 minutes. In an embodiment, the inventive powder is cured at 170 °C for 35 min. In an alternative embodiment, the inventive powder is cured at 200 °C for 20 min. Further preferably, the Tg of the powder coating ranges from 40 to 105 °C, more preferably from 65 to 90 °C, which yields an optimum balance between strength and flexibility.

The present invention also relates to the use of the inventive thermosetting coating powder for the coating of outdoor structures, for example fagade elements and bridges. The powder coating obtained from the inventive powder exhibits excellent physical properties, such as a good durability and weatherability (along with an excellent resistance to aging and embrittlement), a good mechanical performance (such as a high strength, sufficient flexibility and impact resistance) and an excellent optical appearance, including superior color and gloss retention over time. Thus, the powder coating obtained from the inventive coating powder is particularly suitable for outdoor structures which need to withstand a harsh environment with an application temperature that can range from -30 to 80 °C (depending on location and season), without significant deterioration of the functional and decorative properties over time. Even coatings fulfilling AAMA 2604 or AAMA 2605 and/or Qualicoat class 2 or 3 quality standards may be obtained. It is noted, however, that the before-mentioned quality standards represent the highest possible standards in the architectural industry for powder coatings and can only be obtained for certain limited colors and/or formulations of the inventive powder. Nevertheless, it was found that a good layer separation and smooth individual layers of the polyester and fluoropolymers are of upmost importance to reach these standards for a variety of formulations.

All embodiments and characteristic features within embodiments of this invention are interrelated, and each embodiment and/or disclosed characteristic feature (within the general part of the description or within a certain embodiment of the present invention) may be combined with each other and also as any combination of two or more embodiments/characteristic features.

The use of the word “a” or “an” in the present application may mean “one” but is also consistent with the meaning of “one or more”, “at least one” and “one or more than one”.

Definitions

[Acid value (AV)] The term acid value or acid number (AV) is defined as the mass of potassium hydroxide (KOH) in milligrams that is required to neutralize one gram of chemical substance that contains acid groups. The acid value is a measure of the amount of acid groups in a chemical compound, such as an acid functional polyester, or in a mixture of compounds. Typically, a known amount of sample is dissolved in an organic solvent and is then titrated with a solution of potassium hydroxide with a known concentration and with thymolphthalein as a pH indicator. The acid value (AV) is determined according to DIN EN ISO 2114:2002-06, with the difference that a mixture of 28 parts of acetone and 1 part of pyridine (% w/w) is used as solvent.

[Curing / Crosslinking] These terms are used synonymously in the present application and are to be understood as the treatment of a thermosetting coating powder with energy, in particular heat, thereby obtaining a powder coating wherein the crosslinkable (i.e., curable) compounds, such as thermosetting resins or thermoplastics bearing functional, crosslinkable groups, are at least partially crosslinked (i.e, cured) to form a three-dimensional network.

[Curing agent / Crosslinking agent] This term is to be understood as a compound of the inventive powder that is capable of reacting in any form with one or more other reactive compounds of the powder, such as with a monomer, oligomer or polymer, to form a three- dimensional network, particularly a thermoset, by at least partially crosslinking said reactive compounds (for example a polyester resin). In addition to a curing agent, a catalyst and/or initiator can be present to assist in the crosslinking reaction.

[Coating powder] This term is to be understood as a coating composition in powder form (= a plurality of particles) that is suitable for coating onto a substrate.

[Fluoropolymer] This term is to be understood as an at least partially organic (co)polymer comprising fluorine atom(s), e.g. an organic polymer having fluorine-containing monomers incorporated in its polymeric backbone. The terms fluoropolymer and fluororesin are used synonymously herein.

[Glass transition temperature (Tg)] The glass transition is the gradual transition of an amorphous or (semi-)crystalline polymer from its energy-elastic (glassy) state into its entropyelastic (rubbery) state. In the present application, the point of inflection of the endothermic step indicating the glass transition is referred to as the glass transition temperature (Tg) of the inventive powder.

[Homogeneity / homogeneous] Within the scope of the present application, the terms homogeneity and homogeneous are understood to refer to all compounds of the powder or the powder coating obtained thereof being present in well-dispersed state, with a random distribution of the compounds, either in the powder or in in the respective layers (i.e., fluoropolymer and/or polyester layers) of the powder coating. A homogeneous powder coating further describes a powder coating having a smooth, non-corrugated surface and non-corrugated, stratified interface between the fluoropolymer and polyester phases and a uniform optical appearance. [Hydroxyl value (HV)]

Polyester resin: The term hydroxyl value or hydroxyl number (HV) is the value which is defined as the number of milligrams (mg) of potassium hydroxide (KOH) required to neutralize the acetic acid taken up on acetylation of one gram of a chemical substance that contains free hydroxyl groups. The hydroxyl value is a measure of the content of free hydroxyl groups in a chemical substance, usually expressed in units of the mass of potassium hydroxide in milligrams equivalent to the hydroxyl content of one gram of the chemical substance. The analytical method used to determine the hydroxyl value preferably involves acetylation of the free hydroxyl groups of the substance in organic solvent. After completion of the reaction, water is added, and the remaining unreacted acetylation reagent, which is preferably acetic anhydride, is hydrolyzed and measured by titration with potassium hydroxide. The hydroxyl value of a polyester resin is determined according to DIN EN ISO 4629:1998-07.

Fluoropolymer: In contrast, the hydroxyl value of a fluoropolymer is determined according to ISO 14900:2017.

[d_x particle size] The d_x particle size, also known as the d_x value of the particles size distribution (PSD) is to be understood as the particle size at X% of a cumulative distribution (e.g. dso referring to the particle size at 50% of said cumulative distribution). For example, a dso value of 60 pm means that 50% of the particles are larger than 60 pm and 50% of the particles are smaller than 60 pm. The values d and dgo refer to the 10% and the 90% quantiles of the cumulative distribution, respectively. The d_x particle size is determined by laser diffraction; e.g. by using a Malvern Mastersizer Scirocco 2000 manufactured by Malvern Panalytical GmbH (Germany). The laser diffraction is a volume-based measurement method, which means that the particle size distribution (PSD) is a volume distribution and the % are to be understood as vol.-%. The d_x particle size was determined according to ISO 8130-13:2019.

[Melting enthalpy] The melting enthalpy, also referred to as enthalpy of fusion, of a compound/composition (e.g.: a coating powder or a (semi-)crystalline polyester resin) is the change in its enthalpy resulting from providing energy, typically heat, to a specific quantity of the compound/composition to change its state from a solid to a liquid at constant pressure.

[Melting point] The melting point of a substance (Tm) is the temperature at which it changes its state from solid to liquid. As polymers have a melting range rather than a melting point, the melting point of a (semi-)crystalline polymer is signified in the present application to be the peak temperature of the endothermic melting peak of crystalline domains of said polymer and is determined from DSC analysis (according to the method given in the present application). [Weight average molecular weight (Mw)] The weight average molecular weight (Mw) is a statistic number relating to the molecular weight distribution of a chemical sample, e.g. a polyester resin or a fluoropolymer. Mw of compounds such as polyester resins is determined by gel permeation chromatography (e.g. by using a SECurity2 GPC System, as available from PSS- Polymer (Germany)) against polystyrene standards. For amorphous compounds such as amorphous polyester resins, tetrahydrofurane (THF) is employed as eluent; for (semi)-crystalline compounds such as (semi)-crystalline polyester resins, chloroform is employed as eluent.

[Polyester (resin)] This term is to be understood as a polyester (resin) comprising at least two ester functionalities in its polymeric backbone, e.g.: derivable by a polycondensation reaction of one or more mono, di-, tri- and/or polyfunctional carboxyl- and one or more mono, di-, tri- and/or polyfunctional hydroxyl monomers. The terms polyester, polyester resin and polyester polymer are used synonymously herein. It is further noted that the term polyester (resin) in its singular can comprise one or more polyesters (e.g. an amorphous and a semi-crystalline polyester) if not explicitly stated otherwise.

[Amorphous polyester (resin)] This term is to be understood as a polyester (resin) consisting essentially of amorphous, non-crystallizable domains in its solid state. A polyester is considered to be amorphous if it does not show a discernible crystallization or melting peak after elimination of the thermal history (e.g. obtained from DSC analysis).

[(Semi-)crystalline polyester (resin)] This term is to be understood as non-amorphous, in particular a polyester resin comprising crystalline domains and optionally amorphous domains in its solid state. A polyester is considered to be (semi-)crystalline if it shows at least one crystallization peak or melting peak after elimination of the thermal history (e.g. obtained from DSC analysis).

[Powder coating] This term is to be understood as an at least partially crosslinked coating powder, typically in the form of a coating layer on a substrate.

Measurement methods

Method 1 - DSC analysis

The DSC measurements of the thermosetting coating powders or individual compounds are performed by means of a commercially available DSC measurement device, such as a DSC device (NETZSCH (Germany), DSC 204 F1 Phoenix), using a heating/cooling rate of 20 K/min (10-15 mg sample weight). The temperature program given in Table 1 is applied; samples are measured under nitrogen atmosphere. The Tg is determined by evaluating the point of inflection of the endothermal step (only endothermal steps above 0 °C are considered except for semicrystalline polyesters, which may have a Tg well below 0°C). The Tg of the coating powder (or an individual compound) prior to crosslinking is determined from heating cycle 2, step #5 of the measurement program. The Tg of the crosslinked coating powder (which corresponds to the Tg of the powder coating obtained from the inventive powder) is determined from heating cycle 3, step #9 (note that the coating powder is crosslinked in the course of heating cycle 2, steps #5 and #6). The melting enthalpy of the thermosetting coating powders or individual compounds is determined from the endothermic area of the heating cycle 2, step #5. The melting point is determined as the local extremum (i.e. , the peak) of said endothermic area from heating cycle 2, step #5.

Table 1 - Temperature program for DSC measurements

Method 2 - Viscosity

Method 2a - Determination of the viscosity of the thermosetting coating powders: The viscosity, in particular the minimum viscosity, of the coating powder is determined by using a commercially available parallel plate rheometer, such as AR2000ex by TA Instruments (US). For the sample preparation, 1.1 g of the coating powder is pressed into a tablet (diameter = 25 mm, height = approximately 1.8 mm) at a pressure of 10 bar by using a manual hydraulic press (such as from Mauthe Maschinenbau, Germany). The tablet is clamped between the two plates of the parallel plate rheometer, the chamber of the rheometer is closed, and the measurement is started with the following heating program:

20 to 40 °C at a heating rate of 5 K/min,

40 to 60 °C at a heating rate of 10 K/min, 60 to 200 °C at a heating rate of 5 K/min.

The sample is kept at 200 °C for the rest of the measurement (typically 35 to 40 min in total). A frequency of 1 Hz and an amplitude of 0.05% are applied. The storage modulus (G’), the loss modulus (G”), the complex shear modulus (G*) and the complex shear viscosity (q*) are determined. The absolute value of the complex shear viscosity ( | q* | ) is denoted as the viscosity of the coating powder in Pas and is provided by the analysis software of the rheometer. Supplementary, the standard ISO 6721-10:2015 is employed. Also, the minimum viscosity (= minimum of the absolute value of the complex shear viscosity = min( | q* | )) is determined by the analysis software of the rheometer.

Method 2b - Determination of the viscosity of the polyester resins: The dynamic melt viscosity (q) of the polyester resins is determined by using a commercially available cone-plate viscometer device (such as Brookfield CAP 2000+ by Brookfield Ametek, US) equipped with a suitable spindle; e.g. spindle 06 (CAP-S-06); depending on the expected dynamic viscosity of the sample, also other spindles might be appropriate (e.g. spindle 02 for dynamic viscosities of below 0.5 Pas at 130 °C).

For the amorphous polyester resin, the plate is pre-heated to 200 °C and an appropriate amount of sample (typically about 0.1g for solid compounds and spindle 06) is applied onto the plate. The sample is heated to 200 °C and the measurement is started by applying a rotational speed of 700 rounds per minute (rpm) for a time period of 115 sec. The dynamic melt viscosity at a temperature of 200 °C is then obtained from the display of the device.

For the (semi-)crystalline polyester resin, the plate is pre-heated to 130 °C and an appropriate amount of sample (typically about 0.1 g for solid compounds and spindle 06 or spindle 02) is applied onto the plate. The sample is heated to 130 °C and the measurement is started by applying a rotational speed of 700 rounds per minute (rpm) for a time period of 115 sec. The dynamic melt viscosity at a temperature of 130 °C is then obtained from the display of the device.

Supplementary information (for example regarding appropriate choice of spindle, guideline for the appropriate amount of sample) can be found in the user manual of the device (Manual No. M02- 313-1091699 as available from: https://www.brookfieldengineering.com/-

/media/ametekbrookfield/manuals/lab%20viscometers/cap2000%20instructions.pdf?la=eri) [retrieved on 06 May 2021],

Method 3 - SEM images

Scanning electron microscopy (SEM) images are taken by using a scanning electron microscope (JSM-IT100, JEOL, Japan) equipped with a backscatter electron detector (MP-04040BED, JEOL, Japan) and an analysis software InTouchScope (Version 1.060, JEOL, Japan) with following measurement parameters: Acceleration 10 kV, width=10 mm, vacuum 10 Pa, magnification=200.

For the sample preparation, a powder-coated aluminum substrate is cut into 10x10 mm pieces. The layer thickness of the powder coating is typically between 60 and 80 pm. The sample piece is degreased with isopropanol and then embedded in a cold embedding medium (LevoCit-2 Kit, Struers, diameter of embedding mould of 40 mm) so that the cross-section of the sample piece is visible from above. The embedded cross-section is then ground and polished with a polishing machine (Labopol-5, Struers, Germany) with the following program:

1 . Rough surface grinding with SiC foil on MD Gekko disk, SiC foil #220, Struers followed by SiC foil #320, Struers and SiC foil #500, Struers (pressure while grinding is applied to the sample by hand, 300 rpm for 2 min)

2. First pre-polishing with 9 pm suspension (DiaPro Allegro/Largo 9 pm, Struers) on MD Largo honeycomb disk (spring pressure 30 N, 150 rpm for 5 min)

3. Second pre-polishing with 3 pm suspension (DiaPro Dac 3 pm, Struers) on a polishing cloth on DAC 3 pm disk (spring pressure 20 N, 150 rpm for 4 min)

4. Polishing with 0.04 pm suspension (OP-ll NonDry 0,04 pm, Struers) on a polishing cloth on MDChem disk (spring pressure 15 N, 150 rpm for 1 min)

It is noted, however, that also alternative electron microscopy measurement methods and sample preparation procedures may be employed as known by the skilled person, which provide sufficient image quality to observe the layer separation of the polyester and fluoropolymer phase.

Method 4 - Light microscopy images

The light microscopy images are taken by using a Science MTL-201 microscope (Bresser, Germany) equipped with an HD USB-camera. The images are taken at 5-fold magnification using the MikroCamLab7 measurement software.

Brief description of the figures

Fig. 1-4 display the relevant part of DSC curves showing step #5 of the second heating cycle of thermosetting coating powders. The DSC analysis was performed by means of method 1 of the present application. In Fig. 5-13, SEM images of powder coatings obtained from thermosetting coating powders are shown. Samples were prepared and measured in accordance with method 3 of the present application.

In Fig. 14-17, light microscopy images of powder coatings obtained from thermosetting coating powders are shown. The recording of the images was performed according to method 4 of the present application.

Examples

Example 1 - Production of the semi-crystalline polyester resin R 9006

The semi-crystalline polyester resin R 9006 is produced via a two-step process: Monomers for the first reaction step (550 g succinic acid, 167.4 g 1 ,4-cyclohexanedicarboxylic acid (CHDA) and 650 g 1 ,4-butandiol) are weighed in a reaction flask. Further, 0.6 g phosphite-based processing stabilizer and 1.2 g monobutyltin oxide (MBTO) are added. The flask is purged with nitrogen gas. The formulation is heated to 40 °C, then 45 g methylisobutylketone (MIBK) is added using a dropping funnel. The formulation is subsequently heated to 210 °C. The reaction water is collected and compared with the theoretical amount as calculated from the resin formulation. When the reaction is completed and the resin formulation has become clear (after about 1 .5 h), the reaction mass is cooled to 180 °C and the monomers for the second step of the process (120 g succinic acid and 0.6 g phosphite-based processing stabilizer) are added. The formulation is heated again to 210 °C and the reaction water is collected. When the reaction is completed and the reaction mixture has become clear (about 1 h) a vacuum is applied (500 +/- 50 mbar). After 10 min, the pressure is lowered to 300 +/- 50 mbar. The total time for the vacuum step is 5 h. Finally, the resin is discharged onto aluminum plates and cooled overnight. The parameters of the final resin are: Acid value (AV) of 11.8 mg KOH/g, hydroxyl value (HV) of 31.4 mg KOH/g, viscosity of below 0.2 Pas (at 130 °C, as determined with method 2b of the present application), melting peak temperature of about 98 °C (as determined with method 1 of the present application).

Example 2 - Thermosetting coating powder formulations

The chemicals used to prepare thermosetting coating powders according to the present invention are listed in Table 2.

Table 2: Chemicals used to produce thermosetting coating powders according to the present invention

Tables 3 and 4 disclose thermosetting coating powder formulations comprising Additol E 04707 or R 9006 as semi-crystalline polyester. Table 3 further lists one reference formulation (Ref-1) that does not comprise a semi-crystalline polyester. Table 5 shows thermosetting coating powder formulations comprising a combination of two blocked isocyanates (Vestagon B 1530 and Crelan NW 5). The thermosetting coating powders are produced according to example 3 of the present application. Table 3: Formulations of thermosetting coating powders according to the present invention (comprising Additol E 04707 as semi-crystalline polyester) and reference (Ref-1); contents in wt.-%

Table 4: Formulations of thermosetting coating powders according to the present invention (comprising R 9006 as semi-crystalline polyester); contents in wt.-%

Table 5: Formulations of thermosetting coating powders according to the present invention comprising a combination of two blocked isocyanates; contents in wt.-%

Example 3 - Production of a thermosetting coating powder according to the present invention

All compounds, such as polymers (i.e.: polyesters and fluoropolymers), curing agents, pigments, fillers, additives and further compounds of a respective formulation are weighed according to the formulations of example 2 (0.5 kg in total), put in a plastic bag and roughly premixed by hand in said plastic bag. The so-formed pre-mixture is further mixed in a high-speed mixer (Thermo PRISM Pilot 3, Thermo Fisher Scientific, US) for 10 s with a rotor speed of 1 ,000 rpm and then extruded by means of a double screw extruder (TSK-PCE-20/24 D, Theysohn Extrusionstechnik GmbH, Germany) at a screw speed of 400 rpm with a temperature of at most 150 °C in order to avoid pre-reactions. In case the composition significantly pre-reacts upon extrusion, a lower extrusion temperature, e.g. at most 130 or 140 °C, is advisable. Further, a cooling device for the feeding area is used in order to avoid overheating. The extrudate is then platted and cooled down to ambient temperature (about 25 °C), granulated manually (e.g. by use of a plastic hammer) and finely ground with an impact mill (ICM 2.4, Neumann & Esser, Germany) and optionally sieved, e.g.: top cut at 120 pm thereafter to obtain the coating powder having a suitable particle size distribution (PSD) (e.g.: d = 5-20 pm, dso = 20-50 pm, dgo = 50-110 pm; such as dw = 10 pm, dso = 30 pm, dgo = 70 pm). The Tg of the obtained powders is typically about from 45 to 60 °C. Depending on the formulation or the desired application of the composition, the above-described procedure might have to be adjusted (e.g.: the extrusion parameters and/or PSD). The process steps as required for the preparation of coating powders are well known and the skilled person is capable of adjusting the mixing, extrusion, milling and sieving parameters as may be required for a specific formulation and application. Further, it is noted that the above set-up describes the production on laboratory scale but of course, the coating powders according to the present invention may also be produced on suitable production lines on industrial scale. Formulations A-9 and B-9 could not be processed into a coating powder. The high amount of semi-crystalline polyester (25 wt.-%) lead to a rapid drop in viscosity during the extruding process which caused extrusion to be very difficult. Nevertheless, such formulations may be processed to a coating powder using an alternative mixing process, such as melt mixing in an appropriately heated and stirred vessel, or spray drying.

Example 4 - Application and curing of the thermosetting coating powder to prepare a powder coating

Each of the coating powder formulations produced according to example 3 is applied electrostatically (Corona charging) by using a spray gun (60 kV, 40 pA; GEMA OptiTronic, ITW GEMA Easy Select, Austria). The charged particles are applied onto an aluminum substrate (aluminum panel 147 x 75 x 0.7 mm, Wurm & Awender Kunststofftechnik GmbH, Austria). The formed coating powder layer is then cured at 200 °C (substrate temperature) for 20 min in a convection oven (Heraeus, Germany). The coating powder is applied such that the layer thickness of the powder coating is about 80 pm as determined by byko-test 4500 Fe/NFe (BYK, Germany). The Tg of the thus obtained powder coating is typically from about 65 to 90 °C. Depending on the formulation of the composition and the desired application, the above-described procedure might have to be adjusted and another application technique (e.g.: tribo charging) and/or curing method (e.g.: infrared oven) may be more suitable in order to obtain a powder coating from the thermosetting coating powder according to the present invention. These methods are well-known and the skilled person is capable of choosing suitable applications and curing conditions for a coating powder with a specific formulation. The described set-up relates to the production of a coated substrate on laboratory scale but of course, the coating powder according to the present invention may also be used on industrial scale production lines using suitable equipment.

Example 5 - Thermal properties of thermosetting coating powders and powder coatings

Thermal properties of the thermosetting coating powders with the formulations given in example 2 were determined by means of DSC analysis according to method 1 of the present application. The melting enthalpy and Tg of the reference powder and inventive powders are listed in Table 6. Step #5 of the second heating cycle of DSC measurements is shown in Fig. 1-4 for selected powders.

Fig. 1 displays the relevant part of step #5 of the second DSC heating cycle of Ref-1. A comparatively small melting peak with a melting enthalpy of 300 mJ/g and a peak temperature of 117 °C is shown, indicating that the reference powder comprises a small amount of crystalline domains that do not originate from a semi-crystalline polyester (but rather from e.g. a process aid or stabilizer). A further peak with an enthalpy of 8,500 mJ/g and a peak temperature of approx. 178 °C is shown which signifies the deblocking of the blocked isocyanate.

Fig. 2-4 show the relevant parts of step #5 of the second DSC heating cycles of powders according to the present invention, namely A-6 (Fig. 2), B-6 (Fig. 3), and C-1 (Fig. 4). A pronounced melting peak with a peak temperature in the range of 109 to 111 °C for powders comprising Additol E 04707 and 94 to 120 °C for powders comprising R 9006 is observed. Depending on the content of semi-crystalline polyester, the melting enthalpy of the analyzed powders according to the present invention ranges from 500 to 12,400 mJ/g, as can be seen from Table 6.

Table 6: Thermal properties of thermosetting coating powders according to the present invention and references

Example 6 - Optical appearance of powder coatings obtained from thermosetting coating powders

SEM images of cross-sections of powder coatings obtained from thermosetting coating powders selected from the formulations given in example 2 are displayed in Fig. 5-13. The layers of the powder coatings are displayed in-between the aluminum substrate (seen in the lower part of the figures) and the embedding medium (seen in the upper part of the figures). As can be seen from Fig. 5 which displays Ref-1 , a certain separation of the fluoropolymer and polyester layers is observed. However, as can be seen, both the interlayer interface and the surface of the coating are corrugated, which results in inhomogeneous coating properties, particularly in an inhomogeneous appearance. Also, in case the bottom polyester layer is corrugated, the durability and weatherability of the coating is deteriorated as at those regions where the polyester layer is comparatively thick, the protective fluoropolymer layer on top is typically comparatively thin and thus, degradation of the coating is favored in these regions. This degradation is believed to be caused, at least to a certain extent, by UV-radiation, which penetrates into the polyester layer and which negative effect is further enhanced by inorganic compounds that tend to enrich in the polyester layer, in particular TiC>2 which is known for its photocatalytic activity. Further, the coating exhibits a pronounced edge formation in the surface-near region, which is likely to result in a poor adhesion. In contrary, the powder coatings obtained from inventive powders exhibit a much better homogeneity than the reference, as is exemplary shown for formulations A-1 (Fig. 6), A-4 (Fig. 7), A-6 (Fig. 8), A-7 (Fig. 9), A-8 (Fig. 10), C-1 (Fig. 11), B-1 (Fig.12) and B-7 (Fig.13). These coatings show both a smooth surface and a smooth interlayer interface, which allows to achieve homogeneous physical, particularly optical, properties as well as excellent outdoor durability and weathering resistance.

Light microscope images of the surface of powder coatings obtained from thermosetting coating powders selected from the formulations given in example 2 are displayed in Fig. 14-17. As visible in Fig. 14 (Ref-1), the corrugated phase separation of the two resin layers leads to the formation of pinhole-like structures. In contrary to classic pinholes which are mostly caused by degassing processes, the structures in Ref-1 are pockets in the substrate-near, pigmented polyester phase filled with fluoropolymer phase. While the coating can still exhibit a closed and non-corrugated surface in some cases, the optical appearance of the surface is negatively impacted by these structures. This leads to an uneven, inhomogeneous color impression and/or a mottled surface appearance. Also, the durability and weatherability of such a surface will be deteriorated. The powder coatings obtained from inventive powders (Fig. 15-17) show a highly reduced number of these defects (A-6 in Fig. 15) or no defects at all (B-5 in Fig. 16 and C-1 in Fig. 17), resulting in both an excellent optical appearance as well as excellent durability and weatherability of such powder coatings.

Claims

- 37 - Claims

1 . A thermosetting coating powder comprising at least one fluoropolymer, at least two polyester resins and a curing agent, characterized in that the polyester resins comprise at least one amorphous polyester resin and at least one semi-crystalline polyester resin.

2. A thermosetting coating powder according to the previous claim, characterized in that the powder comprises the at least one semi-crystalline polyester resin in an amount from 1 to 20 wt.-%, with respect to the total weight of the powder.

3. A thermosetting coating powder according to any of the previous claims, characterized in that the powder has a melting enthalpy in the range of 500 to 15,000 mJ/g with a melting peak temperature of 150 °C or below, as determined with differential scanning calorimetry (DSC) by the method given in the description.

4. A thermosetting coating powder according to any of the previous claims, characterized in that the at least one fluoropolymer comprises an OH-functional fluoropolymer and/or a COOH- functional fluoropolymer.

5. A thermosetting coating powder according to any of the previous claims, characterized in that the at least one fluoropolymer comprises an OH-functional fluoropolymer.

6. A thermosetting coating powder according to the previous claim, characterized in that the at least one fluoropolymer has a hydroxyl value (HV) from 30 to 60 mg KOH/g, as determined according to ISO 14900:2017.

7. A thermosetting coating powder according to any of the previous claims, characterized in that the at least one fluoropolymer comprises a fluoroethylene vinyl ether (FEVE) polymer.

8. A thermosetting coating powder according to any of the previous claims, characterized in that the at least one amorphous polyester resin and/or the at least one semi-crystalline polyester resin comprise an OH-functional polyester resin.

9. A thermosetting coating powder according to any of the previous claims, characterized in that the at least one semi-crystalline polyester resin has a hydroxyl value (HV) from 25 to 40 mg KOH/g, as determined according to DIN EN ISO 4629:1998-07.

10. A thermosetting coating powder according to any of the previous claims, characterized in that the at least one semi-crystalline polyester resin has a melt viscosity of below 0.5 Pas at 130 °C, as determined by the method given in the description.

11 . A thermosetting coating powder according to any of the previous claims, characterized in that the curing agent comprises a blocked isocyanate. - 38 -

12. A thermosetting coating powder according to any of the previous claims, characterized in that the curing agent comprises a first curing compound and a second curing compound, wherein the first curing compound is a first blocked isocyanate and the second curing compound is a second blocked isocyanate different from the first blocked isocyanate.

13. A thermosetting coating powder according to any of the previous claims, characterized in that the powder is a one component (1C) composition.

14. A substrate having a powder coating thereon on at least one surface, the powder coating being obtained from a thermosetting coating powder according to any of the previous claims.

15. Use of a thermosetting coating powder according to any of claims 1 to 13 for the coating of outdoor structures, for example fagade elements and bridges.