Heat-resistant powder coating composition

Abstract

The present invention refers to a powder coating composition comprising a silicone polymer and a polyethersulfone. The use of the aforementioned composition for the external coating of metal articles, preferably for domestic use, subjected to frequent thermal and / or chemical stress is also described.

Description

[0001] Heat-resistant powder coating composition

[0002] Description

[0003] The present invention refers to a powder coating composition comprising a silicone polymer and a polyethersulfone.

[0004] The use of the aforementioned composition for the external coating of metal articles, preferably for domestic use, subjected to frequent thermal and / or chemical stress is also described.

[0005] State of the art

[0006] Coating products are used industrially in many different applications. Depending on the type of substrate (metal, concrete, wood, glass, asphalt, etc.), specific qualities (performance) of the coating obtained from the application of the coating product are required.

[0007] Some industrial applications involve the coating of surfaces that will be subjected to significant thermal stress such as, for example, parts of heat engines, parts of heat generators (boilers), parts or cooking utensils, etc.

[0008] The coating products formulated for this purpose should be based on a composition containing heat-resistant polymers suitable for the need.

[0009] Furthermore, the coating products industry is continuously looking for systems for the coating of various surfaces, which contain an increasingly limited amount of volatile organic compounds (VOCs), among these, in particular, organic solvents.

[0010] Traditional solvent-based systems have therefore given way to products with a higher solid content, and consequently a lower VOCs content (High-solid coatings or solvent-free coatings). In various industrial coatings sectors, the water-based polymer technology (waterborne) has replaced the old solvent-based technology (solventborne) with excellent qualitative results.

[0011] Powder coating products represent an alternative technology to solventborne systems, with great development potential within an eco-sustainable future industrial horizon.

[0012] The polymers mostly used for this type of use are those derived from silicon chemistry, i.e. silicone polymers.

[0013] However, pure silicone polymers used as sole binders for producing powder coating products have the following issues:

[0014] 1) difficulties in the extrusion process;

[0015] 2) difficulties in incorporating / wetting the pigment load;

[0016] 3) rheological difficulties in leveling and wetting the substrate.

[0017] To overcome these problems, the silicone component is added, in variable percentages, with organic polymers having a thermostable and self-compatible structure (aromatic or aliphatic in nature).

[0018] This solution adequately solves the issues described in the above points, but introduces an additional issue induced by the variation in color (browning) and gloss, resulting from the more or less marked decomposition of the polymer which is the partner of the silicone polymer.

[0019] These hybrid (silicone / organic) powder coating products are often used in those applications where resistance to high temperatures is required for the coating without particular color requirements.

[0020] The need to make available new silicone-based coating products being not only heat- resistant, but at the same time maintaining optimal color and gloss, is therefore clear. Summary of the invention

[0021] The object of the present invention is a hybrid (silicone / organic) composition, in a powder form which does not show browning or loss of gloss, thanks to the introduction of a heat-resistant polyethersulfone (PES or PESU) organic technopolymer.

[0022] The incompatibility of the two polymers (silicone / PES) has always represented an obstacle to the formulation of a coating product in the liquid phase (solventborne coating).

[0023] One object of the present invention is to make available new powder coating products that maintain their color (preventing yellowing) and gloss over time.

[0024] A further object of the present invention is to make available new coating products which maintain or improve the performance of traditional solventborne products, while having, at the same time, a low environmental impact due to the absence of organic solvents (such as N-methyl-2-pyrrolidone (NMP), Toluene or other aromatic solvents).

[0025] The inventors have surprisingly found that by combining the two incompatible polymers (silicone / PES) in a blend through thermal extrusion, a powder coating product is obtained with all the advantages of the hybrid compositions described above, except for the problems of yellowing and loss of gloss.

[0026] The main objects described above are achieved with a powder coating composition according to claim 1 , with the use thereof for the coating of metal articles according to claim 9, with metal articles externally coated with a film according to claim 10, and with a process for producing the powder coating composition according to claim 11. Description of the Figures

[0027] Figure 1. Thermal shock test lasting 5 minutes at a temperature of 350°C. Figure 2. Thermal shock test lasting 5 minutes at a temperature of 350°C at a specific dosage.

[0028] Figure 3. Hot hardness test (150°C) at a specific dosage.

[0029] Definitions

[0030] Unless otherwise defined, all terms of the art, notations and other scientific terminology used herein are intended to have the meanings commonly understood by those skilled in the art to which this disclosure pertains. In some cases, terms with commonly understood meanings are defined herein for clarity and / or for ready reference; thus, the inclusion of such definitions in the present disclosure should not be construed to represent a substantial difference over what is generally understood in the art.

[0031] The terms “approximately” and “about” used herein refer to the range of the experimental error that is inherent in performing an experimental measurement.

[0032] The terms “comprising”, “having”, “including” and “containing” are to be intended as open-ended terms (j.e., meaning “comprising, but not limited to”), and are to be considered as a support also for terms such as “consist essentially of”, “consisting essentially of’, “consist of’, or “consisting of’.

[0033] The terms “consist essentially of, “consisting essentially of’ are to be intended as semi-closed terms, meaning that no other ingredients affecting the novel features of the invention are included (optional excipients may therefore be included).

[0034] The terms “consists of’, “consisting of’ are to be intended as closed terms.

[0035] The term “room temperature” refers to a temperature between 15°C and 25°C, preferably between 20°C and 25°C.

[0036] The term “glass transition temperature (Tg)” refers to the temperature value below which an amorphous polymer behaves like a glassy solid. Common methods used to determine the glass transition temperature are differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA).

[0037] Detailed description of the invention

[0038] An object of the present invention is represented by a powder coating composition comprising a silicone polymer and a polyethersulfone.

[0039] The powder coating products according to the invention are made of solid particles (microparticles) containing inside them all the key ingredients of a heterogeneous system.

[0040] In a powder coating product, the binder (resin) represents the main component and can consist of one or more elements

[0041] During the filming process, the thermoplastic binder undergoes an exclusively physical and reversible transformation.

[0042] The thermosetting binder consists of a hardening resin and a hardener. These two components contain functional groups in the polymer chain which, during the filming process itself, chemically react with each other producing an irreversible polymeric state.

[0043] Silicone resins (SIL) are polymers consisting of a siloxane (silicon-oxygen) network with at least one part made of silicate (SiO4 / 2) or silsesquioxane (R-SiO3 / 2), structures, where R represents various alkyl or aryl organic groups (most commonly methyl or phenyl)). Compared to organic resins (with a carbon-carbon backbone), silicone resins show greater resistance to thermal degradation and radiation.

[0044] The superior performance of silicone resins is attributed to the bond strength between silicon and oxygen (of 108 versus 82.6 kcal / mol for the carbon-carbon bond), transparency to visible and ultraviolet light, and intrinsic partially oxidized structure.

[0045] Silicon is a tetravalent atom, forming four sigma bonds in its base state, and is considered reactive through oxygen bonding.

[0046] If an oxygen is bonded to a silicon atom, it is referred to as a monofunctional or “M” unit; silanes containing two silicon-oxygen bonds are difunctional or “D” units. The “T” units are trifunctional monomers and “Q” represents a quadrifunctional silane.

[0047] Resins used in powder coatings are primarily made of “T” units, which maximize the number of Si-0 bonds.

[0048] The presence of “D” units contributes to increasing flexibility. Incorporation of a high degree of Si-0 units (siloxy) with a binding energy more than 20% higher of the organic component will increase the heat- and UV-resistance of the system.

[0049] Silanes can also be divided into a classification system based on the types of organic substituents present, which are represented by an “R”. The presence of octyl carbon chains (octyl groups), for example, makes them ideal for use as water repellents, while the amino groups make them ideal for use as adhesion promoters.

[0050] Thermal decomposition of silicone resins occurs through the oxidation of these organic substituents.

[0051] The type and concentration of certain substituents will determine the performance of the material in different environments.

[0052] Silicone resins high in phenyl (aromatic) groups are ideal for applications requiring high heat-resistance.

[0053] When exposed to 250°C, the phenyl substituent has a half-life of 100,000 hours or 11 .4 years.

[0054] This makes phenyl silicone resins ideal for applications requiring good gloss retention and flexibility at high temperatures.

[0055] The methyl substituent has a half-life of 10,000 hours or 1.14 years and the half-life of the propyl substituent is seven hours.

[0056] The methyl substituent is used in applications requiring increased hardness. The organic substituent will also affect the compatibility of the silicone polymer with organic resins.

[0057] Substituents with more than three carbon atoms will increase the compatibility of the silicone resin with organic resins.

[0058] Substituents with lower numbers will produce polymers that are generally incompatible with organic substances.

[0059] Methyl silicone resins are incompatible with organic polymers; however, they remain an excellent product when used alone.

[0060] In a preferred embodiment, the silicone polymer suitable for the present invention comprises methyl or phenyl substituents or a combination thereof.

[0061] In a preferred embodiment, the silicone resins suitable for use are those with a ratio between phenyl substituent and methyl substituent ranging from phenyl 1 / methyl 0 to phenyl 1 / methyl 1.

[0062] The modification of coating products with silicone polymers may be achieved by prereacting or post-mixing the silicone with the organic polymers used.

[0063] Silicone polymers are supplied in a solid form (flake), as a solution in solvent or as an aqueous dispersion (emulsion).

[0064] In the present invention silicone polymers are used in a solid form, preferably as flakes.

[0065] Silicone polymers with a high molecular weight, soluble in specific solvents, can hardly be emulsified in an aqueous medium, while the solid form is usually represented by polymers with a lower molecular weight, still containing reactive functionality (alkoxy-methoxy). Despite the difficult compatibility of silicone polymers with organic polymers of various nature, their solubility in organic solvents is fairly good. In a preferred embodiment, the molecular weights (Mw) of the silicone polymers suitable for use in the present invention are between 1000 and 4000.

[0066] Preferred examples of silicone polymers that can be used in the present invention, and that are commercially available, are the following products: SILRES® 601 , 603, 604 by Wacker Chemie; RSN 0217, 0220, 0233 by DOW Chemical.

[0067] Polyethersulfone (PES or PESO) is a thermoplastic polymer resistant to high temperatures.

[0068] It is produced through an aromatic polysulfonation process and consists mainly of aromatic rings (phenyl and diphenyl groups) alternatively linked by ether and sulfone groups.

[0069] It is an amorphous polymer having bonds with high thermal and oxidative stability and a glass transition temperature of 225°C.

[0070] The -SO2- (Sulfone) group confers polarity (adhesion in coatings) and resistance to high temperatures to the polymer structure.

[0071] The ether linkage, -O-, confers flexibility to the polymer chain.

[0072] PES is supplied in a solid form; normally as granules (pellets) or powder. Through an industrial manufacturing process, solutions in solvent or aqueous emulsions are obtained from the solid form. Preferred examples of polyethersulfones that can be used in the present invention, and that are commercially available, are the following products: Sumical 4100 (Sumitomo), Ultrasone E2010 (BASF), Radel R500, R5500, R5800 (Solvay).

[0073] State-of-the-art knowledge (publications) describing PES and SIL polymers never report their binary combination in a coating product. This fact is largely attributable to their extreme incompatibility in the solvent phase.

[0074] To make a PES-SIL powder coating product according to the present invention, the inventors started from the solid form (flake) of the two polymers and proceeded to combine them in a 1 :1 ratio.

[0075] The extrusion process leading to the making of the powder, which will then be applied electrostatically by spray application, led to a well-distributed homogeneous powder.

[0076] Unexpectedly and contrary to expectations, the coating resulting from the application and the baking of the PES-SIL powder coating product, appeared sufficiently glossy, well uniform and fairly transparent.

[0077] Although the inventors do not want to be limited by any interpretative theory, it is believed that, despite the high temperatures (350°C) which should have led to the non-uniformity of the coating with separation of the polymers into distinct islands originating in the molten state, as observed in applications in solvent phase, unexpectedly and contrary to expectations, the coating resulting from the application and baking of the powdered PES SIL coating product according to the invention appeared sufficiently glossy, well uniform and fairly transparent.

[0078] This data could confirm the hypothesis of stratification of the two polymers in the final coating. Advantageously, the coating resulting from the application and baking of the PES-SIL powder coating product according to the invention, subjected to laboratory performance tests, showed a hybrid behavior attributable to the combination of the specific characteristics of the two individual polymers:

[0079] - Adhesion to the support, resistance to dishwasher washing cycles, flexibility and impact resistance: characteristics attributable to PES;

[0080] - Thermal resistance, surface hardness and color retention: characteristics attributable to SIL.

[0081] In a preferred embodiment, the amount of the PES polymer inside the binder is between 10% and 40% by weight of the total polymer, more preferably between 14% and 24% by weight.

[0082] The remaining polymer is represented by the silicone polymer. The silicone polymer is therefore in a value of between 60% and 90% by weight with respect to the total polymer, and more preferably between 76% and 86% by weight.

[0083] The powder coating products according to the invention may further include one or more ingredients selected from pigments, fillers, catalysts, fluidizing additives, rheological additives, leveling and degassing additives.

[0084] Pigments have the function of providing the coating with color.

[0085] Examples of pigments that can be used in the present invention, and which are commercially available, are pigments of an inorganic nature with a crystalline structure resistant to high temperatures, such as for example Titanium Dioxide, Iron Oxides, Titanates, Spinels.

[0086] Fillers are inorganic particles that are occasionally included in the composition to improve the structural performance of the final coating, or simply to reduce the cost of the composition by diluting the concentration of the more expensive binder. Examples of fillers that can be used in the present invention, and which are commercially available, are Calcium Carbonate, Barium Sulfate, Silicates of various kinds (Talc, Mica, Kaolin, Quartzite, Cristobalite).

[0087] Catalysts are additives that accelerate the chemical reaction of the binder.

[0088] The addition of additives has the aim of improving or solving issues that may arise during production, application or filming of the powder coating product:

[0089] - fluidizing additives help the production process (examples that can be used in the present invention, and which are commercially available, are: CRYVALLAC PC (COATEX by ARKEMA), RESIFLOW PH 240 (WORLEE); OXYMELT A-2 (ESTRON CHEMICAL));

[0090] - rheological additives intervene in the application phase (examples that can be used in the present invention, and which are commercially available, are: RHEOBYK-7591 (BYK); THIXSEAL 1984, THIXATROL ST, (ELEMENTIS));

[0091] - leveling and degassing additives promote optimal film formation (examples that can be used in the present invention, and which are commercially available, are: BYK 3902 P, BYK 366 P, (BYK); LUVOTIX R400 (LEHVOSS); LUNA-ADD RC961 (DKSH)).

[0092] A powder coating product is stable in storage when the glass transition temperature (Tg) of the binder is high enough to maintain its state under these thermal conditions. At temperatures below the glass transition, the powder particles maintain the solid state, thus preventing softening and melting during transportation and storage operations.

[0093] The minimum preferable Tg for a binder used in these products is above 50°C.

[0094] In a preferred embodiment, said silicone polymer and said polyethersulfone have a glass transition temperature (Tg) higher than 50°C. A further object of the present invention is represented by the use of the powder coating composition according to the invention for the coating of metal articles, preferably for domestic use.

[0095] The present invention is particularly suitable for metal articles subjected to frequent thermal (heating) and / or chemical (washing) stress. More specifically, coatings for the outside of cookware intended for cooking food and compatible with dishwasher washing.

[0096] A further object of the present invention is represented by metal articles externally coated (for example, outside of a pan / pot) with a film obtained by spraying a powder coating composition according to the invention onto said metal article, followed by baking of the article at a temperature between 300 and 380°C.

[0097] Among favorite metal articles, cookware intended for cooking food can be mentioned. A further object of the present invention is represented by the process for producing the powder coating composition according to the invention comprising the steps of: a) mixing a silicone polymer in a solid form and a polyethersulfone in a solid form, at room temperature; b) feeding the solid mixture obtained from step a) into an extruder, at a temperature between 110 and 130°C to obtain a molten mixture; c) cooling the molten mixture obtained from step b) to a temperature equal to or lower than 60°C, preferably between 40 and 50°C; d) crushing and then grinding the solid product obtained from step c) until the powder coating composition is obtained.

[0098] Specifically, raw materials that are solid at room temperature are first amalgamated on a macroscopic level, preferably in a mixer.

[0099] The solid mixture obtained is then fed into an extruder. In particular, the extruder is made of a screw that rotates inside a cylinder with a heating jacket set to a temperature between 110 and 130°C. As the internal screw turns, it transports the material through the extruder.

[0100] The molten compound exiting the extruder should not exceed a temperature of 120°C. Preferably, the exhaust temperature is equal to 100°C.

[0101] The cooling step preferably takes place on a cooling belt.

[0102] The cooled and solidified solid material is then subjected first to crushing (disintegration of the solid material into granules to be sent to grinding) and subsequently to grinding (reduction of the particle size to the desired micrometric levels, preferably through the use of mills).

[0103] Preferably, in the grinding step the particulate is brought to a desired size, where 30% of the particulate has a size smaller than a micron, 60% smaller than 5 microns, and the remaining 10% with particles larger than 5 microns.

[0104] The powder coating product according to the invention may be applied to metal objects by spray application as known in the technical field.

[0105] A further object of the present invention is, therefore, represented by the process for producing a metal article externally coated with a film comprising a spray application step onto said metal article of the powder coating composition obtained according to the process of the present invention.

[0106] An airflow transports the powder particles through the spray gun where they are also electrostatically charged.

[0107] Once they reach the metal surface to be coated, the particles adhere to it due to the difference in electrostatic charge between the powder and the substrate. The object covered with powder is thus obtained. The process for producing metal articles according to the present invention may further comprise a heating step, preferably in a baking oven.

[0108] In the baking oven, the powder coating particles melt to form a uniform, smooth layer. In this melting phase, a critical aspect is determined by the air present among the solid particles, which should be able to escape during the formation of the film so as not to compromise the quality thereof.

[0109] Deaeration can be ensured thanks to the use of degassing agents, introduced as additives in the formulation of the powder coating.

[0110] In a thermosetting system, once the reactive trigger temperature is reached, the chemical reaction of irreversible hardening of the binder (polymerization or crosslinking) takes place.

[0111] In a thermoplastic system, however, the binder melts and relaxes during heating and the film becomes hard as a result of its cooling. In this case, the hardening process is reversible.

[0112] The combination of the two polymers according to the present invention generates a thermoseting system.

[0113] The following examples are intended to further illustrate the invention without however limiting it.

[0114] EXAMPLES

[0115] Below are two typical formulations of an Epoxy and a Polyester powder coating products as a reference (Tables 1-2).

[0116] Table 1

[0117] Table 2

[0118] The following formulation describes instead a reference Polyethersulfonic (PES) powder coating product (Table 3).

[0119] Table 3 These previous formulations were combined with increasing dosages of silicone polymer (type: SILRES 604 Silicone Resins, Wacker Chemie AG).

[0120] The coatings obtained from the application of these dosages were exposed to a thermal shock lasting 5 minutes at a temperature of 350°C in a thermostated oven.

[0121] The results are reported in Table 4 and Figure 1.

[0122] Table 4

[0123] Figure 2 shows the comparison between the dosage corresponding to 80pbw of silicone polymer and 20pbw of organic polymer in the three specific combinations.

[0124] Figure 3 shows the results of the hot hardness test (150°C) at the same dosage.

[0125] The test is carried out by placing the coated sample (panel) on a thermostated plate before carrying out the hardness test using a specific instrument. A 2H pencil was used for the test. Hot hardness is an important performance index in the condition of use as a cooking item (pan / pot etc.).

[0126] The results obtained from the yellowing test at high temperatures and hot hardness test highlight the superior performance of the powder coating composition of the present invention compared to the Epoxy and Polyester comparative compositions. The coating maintains its decorative appearance (color) under thermal stress, while withstanding, at the same time, mechanical stress in critical temperature conditions.

[0127] Tests were carried out on different combinations of the two polymers according to the present invention. The results obtained, shown in Table 5, identify a range where resistance to yellowing, hot hardness and general chemical resistance performances reach an optimal balance. The dosages A, B, C, D, E identify this range.

[0128] Hot yellowing increases as the percentage of PES mixed with the silicone polymer increases (see also graph: AE after heat treatment: 5' at 350°C).

[0129] Dosage C represents a limit within which the AE value after hot yellowing test is still negligible.

[0130] PES intrinsic thermoplasticity affects the hot pencil hardness result of the composition. In the range A, B, C, D, E the hot hardness detected has optimal (high) values. The hot hardness performance decreases beyond dosage A as the % of PES present in the mixture increases.

[0131] Table 5

Claims

Claims A powder coating composition comprising a silicone polymer and a polyethersulfone. The composition according to claim 1 , characterized in that said silicone polymer comprises methyl or phenyl substituents, or a combination thereof. The composition according to claim 2, characterized in that the ratio between the phenyl substituent and the methyl substituent is between phenyl 1 / methyl 0 and phenyl 1 / methyl 1. The composition according to any one of claims 1 to 3, characterized in that said silicone polymer has a molecular weight between 1000 and 4000. The composition according to any one of claims 1 to 4, characterized in that said silicone polymer and said polyethersulfone are in a solid form, preferably in the form of flakes, granules (pellets) or powders. The composition according to any one of claims 1 to 5, characterized in that the polyethersulfone amount is between 10% and 40% by weight with respect to the total polymer consisting of said silicone polymer and said polyethersulfone, more preferably between 14% and 24% by weight, and the silicone polymer amount is between 60% and 90% by weight, more preferably between 76% and 86% by weight. The composition according to any one of claims 1 to 6, characterized in that said silicone polymer and said polyethersulfone have a glass transition temperature (Tg) higher than 50°C. The composition according to any one of claims 1 to 7, characterized in that it further comprises one or more ingredients selected from the group comprising pigments, fillers, catalysts, fluidifying additives, rheological additives, leveling and degassing additives.Use of a powder coating composition according to any one of the preceding claims for the coating of metal articles, preferably for the external coating of metal articles, more preferably for domestic use. Metal articles externally coated with a film obtained by spray application onto said metal article of a powder coating composition according to any one of claims 1 to 8, followed by baking of the article at a temperature between 300 and 380°C. A process for producing a powder coating composition according to any one of claims 1 to 8, comprising the steps of: a) mixing a silicone polymer in a solid form and a polyethersulfone in a solid form, at room temperature; b) feeding the solid mixture obtained from step a) into an extruder, at a temperature between 110 and 130°C to obtain a molten mixture; c) cooling the molten mixture obtained from step b) to a temperature equal to or lower than 60°C, preferably between 40 and 50°C; d) crushing and then grinding the solid product obtained from step c) until the powder coating composition is obtained. A process for producing a metal article externally coated with a film comprising the step of spraying onto said metal article the powder coating composition obtained according to the process of claim 11 . The process according to claim 12, further comprising a heating step, preferably in a baking oven.

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