Powder mixture for Teflon-free non-stick coating

Abstract

The present invention relates to a powder mixture, either dry or as a dispersion in a liquid, comprising powder particles of at least one polyaryletherketone (PAEK) polymer and, optionally, a polyphenylene sulfide (PPS) polymer, which can be produced without the addition of fluorine or fluorine-containing compounds. Preferably, the powder mixture contains a total fluoride content of up to 1000 ppb, preferably up to 100 ppb, and particularly preferably up to 25 ppb, as measured by combustion ion chromatography (TOF-CIC). Furthermore, the present invention relates to a method for coating articles, particularly cookware and bakeware, using the powder mixture; the use of the powder mixture to obtain a technical effect selected from non-stick and corrosion protection; and articles coated by the method.

Description

[Technical Field]

[0001] The present invention relates to a powder mixture for Teflon-free non-stick coatings, a method for coating articles, particularly cookware and bakeware, using this powder mixture, the use of the powder mixture to obtain a non-stick and slip effect on the surface of articles, particularly cookware and bakeware, and articles, particularly cookware and bakeware, coated by said method. [Background technology]

[0002] Prior art PTFE coatings exist, which, for high-quality cookware, are typically applied as a triple coating, i.e., in three layers, and are typically baked at least twice. First, a base layer, typically consisting of a binder resin dissolved in a solvent, such as PAI dissolved in NMP, is wet-sprayed, dried, and pre-crosslinked. Subsequently, a transition layer and a cover layer are wet-in-wet sprayed, with the cover layer essentially containing a fluoropolymer, typically PTFE. The entire coating is then baked at 420°C for approximately 10 minutes. These coatings may be modified with pigments and various fillers. Disadvantages include the use of fluoropolymers and the fact that the baking temperature is significantly higher than the decomposition temperature of PTFE, which can lead to the formation of potentially harmful substances (PFAS). Furthermore, these coatings have the disadvantage of releasing toxic substances (PFAS) when used at high temperatures.

[0003] Furthermore, so-called ceramic sol-gel coatings are known which contain silicone to obtain a non-stick effect and which are generally very brittle and not durable.

[0004] According to EU regulations, the limit values ​​for PFAS in the EU are to be lowered from 2024 onwards, which, from today's perspective, will require the replacement of PTFE coatings ("Teflon") as non-stick coatings. Summary of the Invention

[0005] The object of the present invention is a universally usable coating for articles, in particular cookware and bakeware, which is free of fluoropolymers and free of fluorine additives.

[0006] The above-mentioned object is achieved by providing a powder mixture according to claim 1, a method according to claim 8, a use according to claim 10, and articles, in particular cookware and bakeware, according to claims 12 or 13. Preferred embodiments are described in the dependent claims. According to the invention, the term cookware and bakeware includes any article that, in terms of its shape, is suitable for containing food and for the subsequent cooking and baking of food. Thus, not only frying pans but also baking trays, baking molds, etc., as well as grill pans, grill containers, grill grates, etc., are included in the above-mentioned term.

[0007] In a first aspect according to the present invention there is provided a powder mixture, either as a dry powder mixture or as a dispersion in a liquid, comprising powder particles consisting of at least one polyaryletherketone (PAEK) polymer and optionally a polyphenylene sulfide (PPS) polymer.

[0008] The powder mixture according to the invention can be produced without adding fluorine or fluorine-containing compounds. This is particularly true if the powder mixture according to the invention does not exceed a certain limit for total fluoride after combustion at 900-1000°C in a moist oxygen-rich atmosphere. Preferably, the powder mixture therefore contains a total fluoride content of up to 1000 ppb, preferably up to 100 ppb, particularly preferably up to 25 ppb, as measured by combustion ion chromatography (TOF-CIC).

[0009] Specifically, according to the present invention, to perform the combustion ion chromatography (TOF-CIC) measurement method, a sample of the powder mixture is loaded into a ceramic boat and introduced into a furnace, where it is subjected to pyrohydrolysis at 900-1000 °C in a humid, O2-rich environment. The sample is oxidized under these conditions, breaking the strong carbon-fluorine bonds, and the vapor is passed through an absorption solution containing Ar. HF, produced during the combustion of the organic fluorine, dissociates in the absorption solution, forming H+ and F- ions. A sample of the absorption solution, which also contains an internal standard for calibrating the analytical results, is then transferred to an ion chromatograph for analysis, and fluoride is measured.

[0010] Surprisingly, the powder mixture according to the invention is particularly suitable for coating articles, in particular cookware and bakeware. The non-stick effect of the coating produced using the powder mixture according to the invention (tested using standard pancakes in accordance with DIN EN 60350-2) was in line with the expectations for a non-stick effect for cookware, without the addition of oil and without any fluorine components. In particular, when using the powder mixture according to the invention to produce a coating, no fluorosurfactants for flow promotion are required.

[0011] A further advantage is that only one, relatively thin layer is needed to meet all the requirements for a nonstick coating for food contact. According to the prior art, three-layer abrasion-resistant polytetrafluoroethylene (PTFE) coatings with a layer thickness of approximately 50-60 μm are currently common. A similar coating with a thickness of approximately 20-30 μm can significantly reduce material usage, volatile organic compounds (VOCs), and energy consumption, in addition to completely avoiding perfluorinated or polyfluorinated alkyl esters (PFAS).

[0012] Preferably, the powder mixture comprises at least two polymers, and the powder particles comprising the first polymer have a mass particle size distribution D50 of at most 70 μm, preferably at most 60 μm, more preferably at most 50 μm, even more preferably at most 40 μm, and particularly at most 30 μm, as measured by static image analysis in accordance with ISO 13322-1:2014. The powder particles comprising the second polymer have a mass particle size distribution D50 of at most 30 μm, preferably at most 25 μm, more preferably at most 20 μm, even more preferably at most 15 μm, and particularly preferably at most 10 μm, as measured by static image analysis in accordance with ISO 13322-1:2014. This enhances mechanical durability. This can be achieved by adding PEK and / or PEKK. A wear-reducing effect has already been demonstrated with lower mass concentrations of the second polymer, of at least 2 wt. %, preferably at least 5 wt. %, and particularly preferably at least 10 wt. %.

[0013] Conversely, by adding PPS in the order of 80%, 90% or even 95% by weight, a further reduction in the baking temperature during the production of the coating and a further improvement in the non-stick effect of the produced coating could be measured.

[0014] Preferably the powder mixture is present as a dispersion and preferably the liquid in the dispersion comprises water.

[0015] Preferably, the powder mixture comprises at least two PAEK polymers or at least one PAEK polymer and one PPS polymer, the PAEK polymers preferably being selected from the group consisting of polyetheretherketone (PEEK), polyetherketone (PEK), polyetherketoneketone (PEKK) and polyetherketoneetherketone (PEKKEK). Preferably, the polymers each have different melting temperatures and / or preferably different melt viscosities.

[0016] Preferably, the average melt viscosity of the at least one polymer is at most 150 Pa·s, preferably at most 120, more preferably at most 100 Pa·s, or particularly preferably at most 90 Pa·s, when measured at 400°C according to ISO 11443. This allows for a significantly smoother flow when heated for polycondensation on the surface to be coated. This improves the non-stick effect of the coated surface. This also applies when the particles are large relative to the layer thickness (e.g., a particle size D50 of 25 μm for a layer thickness of 25 μm).

[0017] Preferably, the powder mixture further comprises a component from the group consisting of polyamideimide (PAI), polyimide (PI), graphite, MoS2 and mixtures thereof, which can further improve the wear resistance.

[0018] In a second aspect, the present invention provides a method for coating articles, particularly cookware and bakeware, comprising: applying the powder mixture according to the invention onto an article, in particular cookware and bakeware, the application being preferably carried out electrostatically, in a fluidized bed sintering process or using a dispersion comprising a liquid, in particular water; When using a dispersion, completely removing the liquid; heating the powder mixture to form a polycondensate; and Baking the powder mixture at a temperature above the melting temperature of the polymer Includes.

[0019] Preferably, the baking temperature (substrate temperature) exceeds the liquidus temperature of the polycondensate by at most 60°C, preferably at most 50°C, more preferably at most 40°C, even more preferably at most 30°C, particularly preferably at most 20°C.

[0020] Experiments showed that the non-stick effect improved when the baking temperature was lowered, or deteriorated too much when the baking temperature was too high.

[0021] Preferably, the process is carried out without the use of a fluorosurfactant, especially in the step of heating the powder mixture to form the polycondensate.

[0022] In a third aspect, the present invention provides the use of a powder mixture for coating articles, in particular cookware and bakeware, to obtain a specific technical effect selected from the group consisting of a non-stick effect as tested using standard pancakes (DIN EN 60350-2), an electrical insulation of the coated inner surface of said articles, in particular said cookware and bakeware, measured in a 5% salt solution of at least 1 V, preferably at least 2 V, more preferably at least 3 V, even more preferably at least 4 V, and particularly preferably at least 5 V, ensuring sufficient corrosion protection of the metal substrate of the article, in particular the cookware and bakeware, and combinations thereof.

[0023] In a fourth aspect, the present invention provides articles, particularly cookware and bakeware, having a coating produced by a method according to the second aspect of the present invention. Preferably, the coating has an average thickness of at most 50 μm, preferably at most 40 μm, more preferably at most 25 μm, and especially at most 25 μm.

[0024] The invention will be explained in more detail below on the basis of examples and the corresponding figures.

[0025] The drawings are merely used for a better understanding of the invention, are merely schematic and are not drawn to scale, and the invention is not limited to the embodiments. Identical or identically acting parts are provided with identical reference numerals. [Brief explanation of the drawings]

[0026] [Figure 1] 1 is a schematic cross-sectional view of a cooking and baking utensil having a dispersion layer applied thereto. [Figure 2]1A-1D show cookware and bakeware having a coating according to the present invention. [Figure 3A] FIG. 2 shows the method sequence during coating according to different method steps. [Figure 3B] Same as above [Figure 3C] Same as above [Figure 4] 1 is a flowchart of coating using a dispersion method. DETAILED DESCRIPTION OF THE INVENTION

[0027] The article to be coated, or the area to be coated of the cookware and bakeware, is illustrated by the substrate SU to be coated. The surface to be coated can first be subjected to a chemical and / or mechanical activation treatment. For this purpose, mechanical roughening, such as with a sandblaster, or etching with acids or alkalis, or plasma or laser treatment can be used. The correspondingly pretreated surface has additional chemical / physical bonding sites, is clean and non-greasy, and, in the case of roughening, has a larger surface area, which leads to better adhesion of the coating to be applied.

[0028] Subsequently, a layer of dispersion DS is applied to this surface, which contains all the components of the coating dispersed in a solvent or solvent mixture with a fine and as homogeneous particle size distribution as possible. A suitable application method is selected to produce the desired layer thickness. The average particle size (D50) of the solids contained in the dispersion corresponds at most to the desired layer thickness of the coating, but is preferably selected to be smaller. Figure 1 shows the coated substrate SU with the dispersion layer DS applied thereto.

[0029] After carrying out a temperature program in which the substrate or cookware / bakeware with the dispersion layer DS is heated to a temperature above the melting point of the thermoplastic resin contained in the dispersion, a homogeneous coating BS is obtained that is pore-free and therefore dense, and has good mechanical cohesion and good adhesion on the substrate SU. Figure 2 shows the finished cookware / bakeware.

[0030] In this case, it is possible to coat only a portion of the surface. In this case, the remaining areas that do not need to be coated can be covered, or a dispersion application method that can distinguish between different surface areas, such as painting or printing, can be selected. During the spraying of the dispersion, the coating can be performed with a shadow mask. This shadow mask can be formed in the form of a film that is applied to the surface of the substrate SU and leaves the surface area that is to be coated empty. After the application of the dispersion layer DS, the film can be removed, for example, peeled off, and the area of ​​the dispersion layer DS that was applied thereto is peeled off as well.

[0031] FIG. 3 shows a modified version of this method, which allows for higher layer thicknesses to be achieved despite the smaller particle diameter of the solids contained in the dispersion. To this end, as shown in FIG. 3A, after the application of the first dispersion layer DS1, at least the solvent is removed, or the first dispersion layer may even be correspondingly pre-compressed by a temperature treatment. In a second step, the dispersion coating is repeated, and a second dispersion layer DS2 is applied. If necessary, this layer can also be pre-compressed, and the coating process repeated again. Finally, as shown in FIG. 3C, the dispersion layer structure consisting of multiple sub-layers is heated to a temperature above the melting point of the thermoplastic resin in a final step, resulting in a completely compressed, pore-free, closed coating BS on the substrate SU.

[0032] Compositions suitable for application using dispersion techniques and lubricated according to the invention include, for example: 100 weight percent PEEK The solids content is 100% by weight.

[0033] If desired, color additives may be added. One or more layers may be applied.

[0034] The solids are dispersed, optionally with an auxiliary, in a solvent, which may be water or, preferably, is miscible or mixed with water, such as an alcohol, especially isopropanol, and the dispersion mixture contains about 30 percent by weight of the above-listed solids.

[0035] 4, the method flow just described is more clearly illustrated once again by means of a flow chart. The method comprises, as step 1, the production and preparation of a powder mixture. For this purpose, ingredients selected from thermoplastic polymers, fillers and dry lubricants are preferably brought to a suitable particle size by grinding and / or subsequent sieving according to the desired particle size distribution as narrow as possible.

[0036] In parallel with this, in step 2, a solvent is prepared, preferably environmentally and health-friendly, preferably water-based, in particular a mixture of alcohol and water, such as isopropanol and water. An advantageous solvent composition contains, for example, 25 to 75 weight percent isopropanol in water. A solvent having about 25 to 50 weight percent isopropanol in water is particularly preferred.

[0037] In step 3, a dispersion is prepared by adding a solvent to the prepared powder mixture, and the solid content is preferably maintained at 20 to 50 weight percent. To improve the dispersion stability, a small amount of a known dispersing aid may be added.

[0038] In step 4, the surface of the article is coated, for example by spraying, dipping, painting, printing or spin coating, aiming for a dispersion layer with as homogeneous a layer thickness as possible, and, if necessary, leaving voids in the coating in the surface areas that should not be coated.

[0039] In step 5, the solvent is removed, preferably by evaporation, which can be optionally assisted by negative pressure or elevated temperatures such as 80°C.

[0040] In the next step 6, the cookware and bakeware with the dried dispersion layer deposited thereon is converted into a homogeneous coating by heating and melting the thermoplastic resin, followed by cooling the cookware and bakeware again.

[0041] Following this step 6, at point 7, the finished coating can be obtained.

[0042] According to method variant V1, steps 4 to 7 can be carried out once again immediately after step 5.

[0043] The second modification V2 follows step 6 by applying a new dispersion layer (step 4) after melting of the first dispersion layer and compressing it accordingly (steps 5-7).

[0044] According to a third variant V3 of the method, after the production of the first coating according to step 6, a second partial layer of a coating different from the first coating is applied. For this purpose, a further dispersion is produced according to method steps 1 to 3, and the cookware and bakeware are coated with this according to steps 4 to 6. Here too, the method can be modified according to variants V1 and V2 by repeating individual method steps or individual method step sequences to achieve the desired layer thickness.

[0045] Particularly homogeneous coatings can be obtained when using fine particles, especially for dispersions, whereby multiple coatings are advantageous or necessary due to the small particle diameter.

[0046] The present invention has been described based on only a few examples, but the present invention is not limited thereto. In particular, potential variations are possible through the appropriate selection of fillers and, if necessary, through the mixing of different fillers. The ratio of the amounts of the components of the coating used is selected depending on the desired load on the coating. The same applies to the layer thickness, which is not limited to the examples described. Advantageously, the coating is applied to a metal surface, but the coating can also be applied to other surfaces, such as ceramic, glass, or suitable plastics. [Example]

[0047] The frying pan coating was prepared as follows. An aqueous dispersion with a 25% solvent content and a 30% solids content was prepared. The solid consisted of Victrex PEEK with a mass-average particle diameter D50 of 10 μm and a specified average melt viscosity of 150 Pa·s (120–180 Pa·s) measured at 400°C according to ISO 11443. The substrate in the form of an aluminum frying pan was cleaned and degreased, followed by blasting. The Rz was approximately 20 μm after blasting (measured by scanning or non-contact optical roughness measurement). The aqueous PEEK dispersion was sprayed with a conventional spray gun, dried at approximately 50°C, and subsequently baked in an oven at 400°C. This produced an optically closed, pore-free surface with a roughness of approximately Rz 20 μm (measured by scanning or non-contact optical roughness measurement).

[0048] However, subsequent migration tests carried out to prove the legal limits to be met showed that the coating was too thin to adequately cover the metal substrate, making it impossible to meet the prescribed limits. In subsequent experiments, the baking temperature was increased, thereby improving flow by reducing the melt viscosity. Surprisingly, it was found that the non-stick effect decreased with increasing baking temperature. Conversely, the non-stick effect improved with decreasing baking temperature.

[0049] Surprisingly, further experiments showed that adding a PEEK type with a lower melt viscosity of 90 Pa·s resulted in an unimpeded, smoother flow and improved non-stick effect, even with a particle size D50 of 25 μm, i.e., the same size as the layer thickness to be achieved. In particular, it was possible to reduce the baking temperature by 30°C, i.e., to an oven temperature of 370°C and a maximum substrate temperature of 360°C.

[0050] The non-stick effect was tested using standard pancakes (DIN EN 60350-2). The non-stick effect of the coatings thus produced was in line with the expectations for non-stick effect of cookware, without the addition of oil or any fluorine components. In particular, the coatings of the present invention do not require fluorosurfactants to promote flow.

[0051] Another advantage is that only one, relatively thin layer is needed to meet all the requirements for a nonstick coating for food contact. Prior art abrasion-resistant PTFE coatings currently consist of three layers, with a thickness of approximately 50-60 μm. A coating of approximately 20-30 μm can significantly reduce material, VOC, and energy consumption, while completely avoiding PFAS.

[0052] In further experiments, it was desired to increase the mechanical durability. This could be achieved by adding PEK and / or PEKK. Already at lower concentrations of 10, 5 or 2%, a wear-reducing effect was shown.

[0053] The wear resistance could be further improved by adding elements from the group consisting of polyamideimide (PAI), polyimide (PI), graphite, MoS2 and mixtures thereof.

[0054] Conversely, by adding PPS in the order of 80, 90, up to 95%, a further reduction in the baking temperature and a further improvement in the non-stick effect could be measured.

[0055] Surprisingly, during the experiment, it was found that mixing different polymer types after baking shifted the melting temperature upward. This ensured that the coating on the frying pan would not become liquid. While frying pans can reach surface temperatures of over 300°C during use, the melting temperature of the coating on the reference frying pan was, for example, 340°C. That is, it was not between the temperatures of the two polymers involved, depending on their concentrations, but rather at or even above the temperature of the polymer with the higher melting point. In particular, it was determined that the coating of the present invention, consisting of PEEK and PEK, did not become liquid until 400°C or higher. However, a coating consisting of PEEK and PPS with a PPS concentration of 90% also did not become liquid until 380°C, despite the melting points of the individual polymers being 285 and 340°C.

Claims

1. 1. A powder mixture, either dry or as a dispersion in a liquid, comprising powder particles consisting of at least one polyaryletherketone (PAEK) polymer and optionally a polyphenylene sulfide (PPS) polymer, said powder mixture being producible without the addition of fluorine or fluorine-containing compounds.

2. 2. The powder mixture according to claim 1, wherein the powder mixture contains a total fluoride content of at most 1000 ppb, preferably at most 100 ppb, particularly preferably at most 25 ppb, as determined by combustion ion chromatography (TOF-CIC).

3. 3. The powder mixture according to claim 1, wherein the powder mixture comprises at least two polymers, and wherein the powder particles comprising a first polymer have a mass particle size distribution D50, measured by static image analysis in accordance with ISO 13322-1:2014, of at most 70 μm, preferably at most 60 μm, more preferably at most 50 μm, even more preferably at most 40 μm, and in particular at most 30 μm, and the powder particles comprising a second polymer have a mass particle size distribution D50, measured by static image analysis in accordance with ISO 13322-1:2014, of at most 30 μm, preferably at most 25 μm, more preferably at most 20 μm, even more preferably at most 15 μm, and in particular at most 10 μm.

4. 4. The powder mixture of claim 1, wherein the powder mixture is present as a dispersion and the liquid comprises water.

5. 5. The powder mixture according to any one of claims 1 to 4, wherein the powder mixture comprises at least two PAEK polymers or at least one PAEK polymer and a PPS polymer, the PAEK polymers being preferably selected from the group consisting of polyetheretherketone (PEEK), polyetherketone (PEK), polyetherketoneketone (PEKK) and polyetherketoneetherketone (PEKKEK).

6. 6. The powder mixture of claim 5, wherein the polymers each have a different melting temperature.

7. 7. The powder mixture according to claim 5 or 6, wherein the PAEK polymers have different melt viscosities, measured according to ISO 11443 at 400°C.

8. 8. The powder mixture according to claim 1, wherein the at least one polymer has an average melt viscosity of at most 150 Pa s, preferably at most 120, more preferably at most 100 Pa s, or particularly preferably at most 90 Pa s, measured at 400°C according to ISO 11443.

9. The powder mixture is composed of polyamideimide (PAI), polyimide (PI), graphite, MoS ２ 9. The powder mixture of any one of claims 1 to 8, further comprising an ingredient selected from the group consisting of: and mixtures thereof.

10. 1. A method for coating articles, particularly cookware and bakeware, comprising:

10. A step of applying the powder mixture according to any one of claims 1 to 9 onto the article, in particular the cookware and bakeware, said application being preferably carried out electrostatically, in a fluidized bed sintering process or using a dispersion comprising a liquid, in particular water, When using said dispersion, completely removing said liquid; heating the powder mixture to form a polycondensate; and baking the powder mixture at a temperature above the melting temperature of the polymer A method comprising:

11. 11. The method according to claim 10, wherein the baking temperature (substrate temperature) is at most 60°C, preferably at most 50°C, more preferably at most 40°C, even more preferably at most 30°C, particularly preferably at most 20°C higher than the liquidus temperature of the polycondensate.

12. 12. The method according to claim 10 or 11, wherein the method, in particular the step of heating the powder mixture to form the polycondensate, is carried out without the use of a fluorosurfactant.

13. Use of a powder mixture according to any one of claims 1 to 9 for coating articles, in particular cookware and bakeware, comprising: Non-stick effect tested using standard pancakes (DIN EN 60350-2), ensuring sufficient corrosion protection of the metal substrate of the article, in particular the cookware and bakeware, such that the electrical insulation of the coated inner surface of the article, in particular the cookware and bakeware, measured in a 5% salt solution is at least 1 V, preferably at least 2 V, more preferably at least 3 V, even more preferably at least 4 V, and particularly preferably at least 5 V; and combinations of these Use for obtaining a technical effect selected from the group consisting of:

14. Articles, particularly cookware and bakeware, having a coating produced by the method of any one of claims 10 to 12.

15. 15. An article, in particular cookware and bakeware, according to claim 14, wherein the coating has a thickness of at most 50 μm, preferably at most 40 μm, more preferably at most 30 μm, and especially preferably at most 25 μm.

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