Procedure for the production of a fluorine-free non-stick coating
Patent Information
- Application Number
- ES2020194537T
- Authority / Receiving Office
- ES · ES
- Patent Type
- Patents
- Current Assignee / Owner
- Filing Date
- 2020-09-04
- Publication Date
- 2026-09-08
- Estimated Expiration
- 2040-09-04
Smart Images
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Abstract
Description
Fluoride-free non-stick coating and process for its production Technical field The present invention relates to a fluorine-free non-stick coating, a process for producing the same, and an article provided with the same. State of the art Fluoropolymer-based non-stick coatings have long been known in the prior art, for example, for coating baking molds. Polytetrafluoroethylene (PTFE) is a fluoropolymer widely used in this context. PTFE coatings have excellent non-stick properties due to their low surface energy and high-temperature resistance resulting from high CF bond energies. Baking molds are normally manufactured by a coil coating process followed by forming (deep drawing). In addition to their excellent non-stick properties, PTFE linings generally exhibit low coefficients of friction, which has a beneficial effect on their deep-drawing capabilities. Without the dry lubricating effect of PTFE during the forming process, the sheet metal would tear during deep drawing. However, in addition to the advantages mentioned, fluoropolymer coatings have several disadvantages. PTFE can only be processed at very high temperatures due to its extremely high melt viscosity. The firing temperatures used in this case are around 420 °C. This processing temperature is above the decomposition temperature of PTFE, so toxic and aggressive decomposition products (such as trifluoroacetic acid and fluorophosgene) are released (Nature, Vol. 412, 19, July 2001, pp. 321–324). Furthermore, fluorine-containing wetting agents are often used in the production of fluoropolymers, which can accumulate in the environment due to their lack of biodegradability and can also endanger human health and the environment. The thermoforming process of PTFE linings can also cause undesirable surface abrasion of the lining, which is visible in the article below, especially if the soft PTFE content on the surface is high. Fluoride-free non-stick coatings are also known in the prior art. Silicone-modified polyesters, in which a portion of the polyester is modified by silicones, often exhibit poor temperature resistance because the polyester component burns easily at high temperatures (>230 °C). To achieve sufficient thermoforming capacity in silicone-modified polyesters, the proportion of silicone modification must be kept low to ensure adequate adhesion to the substrate and flexibility for the thermoforming process. This negatively impacts the non-stick properties and temperature stability. For this reason, silicone-modified polyesters are used exclusively in spray applications and not in the much more efficient coil coating processes. Finally, fluorine-free non-stick coatings are known in the prior art, such as those described in EP 2 177 580 B1, which are produced using the sol-gel process. These coatings are highly temperature-resistant, but at the same time, they are very hard and brittle. These non-stick coatings, often also referred to as ceramic coatings, are unsuitable for coil coating processes followed by forming due to their brittleness. EP 2450469 B1 describes a manufacturing process in which a sol-gel coating is applied to a substrate, dried at 70-110 °C for 6 to 8 minutes, formed, and subsequently fully cross-linked. However, the pre-dried coating is not suitable for industrial coil coating processes because it is highly susceptible to mechanical damage and the pre-drying conditions are difficult to control. US20200216669A1 describes a fluorine-free sol-gel coating composition that further comprises a thermoplastic polymer (such as PPS) having a melting point or glass transition temperature of 200 °C or higher. The result is increased impact resistance. However, the flexibility achieved by the coating is too low for a deep-drawing process. EP 2 319 631 B1 describes a coating for a substrate, having a lower layer on the substrate. The lower layer comprises a binder matrix selected from the group consisting of silicone, titanate, zirconate resins and mixtures thereof. The binder matrix contains from 0.5 to 20% by weight, based on the weight of the lower layer, of a thermoplastic having a temperature resistance above 200°C. The lower layer does not contain fluoropolymers and is annealed at a temperature between 150°C and 230°C. An upper layer containing a binder matrix with a lower proportion of a thermoplastic polymer than the lower layer is deposited on the lower layer. In this case, the proportion of thermoplastic in the binder matrix is quite low, up to 20% by weight, and the coating is only annealed to 230°C to prevent decomposition of the silicone resin. US patent 5721 053 A describes a two-layer non-stick coating based on a perfluororesin that is not desirable in the present context. US patent 2017 / 130060 A1 describes a two-layer powder coating produced by dry milling of the ingredients. Both the first and second layers contain a thermoplastic polymer, as well as ceramic particles, which are crucial for the tribological properties of the coatings. Document CH 709779 A2 describes a lacquer dispersion for the production of a non-stick coating that does not contain thermoplastics but is based on polysilazanes. Upon hydrolysis, polysilazanes produce ammonia and highly cross-linked, brittle layers that would not be flexible enough for a thermoforming process. Silicone elastomers and silicone resins used as release agents are also known in the prior art. However, they have the disadvantage of providing only poor adhesion to the substrate and are therefore unsuitable for subsequent forming processes. Temperature-resistant silicone resins are also very brittle and only exhibit sufficient adhesion to mechanically rough surfaces. Presentation of the invention The present invention is based on the task of providing a fluorine-free non-stick coating that overcomes the disadvantages of the prior art. In particular, the coating must allow good adhesion to ECCS (electrolytically coated chromium / chromium oxide steel) and have high flexibility suitable for a subsequent deep-drawing process. The objective is to provide a coating that does not contain perfluorinated or polyfluorinated alkyl compounds and, in particular, does not contain fluoropolymers, especially polytetrafluoroethylene (PTFE). Another object of the invention is to provide a procedure for applying the non-stick coating according to the invention. These tasks are accomplished by means of the non-stick coating defined in claim 1 and by means of the procedure defined in claim 9, respectively. According to claim 15, another aspect of the invention relates to a coated article having a surface provided with a non-stick coating according to the invention. Specifically, this may be a baking mold or other household or utilitarian article. The non-stick coating according to the invention comprises at least a bottom layer baked onto a surface of the article and a top layer baked onto the bottom layer, wherein the bottom layer has a dry film thickness of 1 to 25 µm, and the top layer has a dry film thickness of 1 to 25 µm. The bottom layer contains from 10 to 100% by weight, relative to the weight of the baked bottom layer, of a thermoplastic material having a temperature resistance above 200 °C, and the top layer also contains a thermoplastic having a temperature resistance above 200 °C and optionally a silicone resin. Neither the bottom layer nor the top layer contains perfluorinated or polyfluorinated alkyl compounds.According to the invention, the top layer has a thermoplastic resin content of at least 30% by weight, based on the weight of the baked top layer, and at least 2.5% by weight, based on the weight of the baked top layer, of a silicone oil. The terms "cooked bottom layer" and "cooked top layer" should be understood to mean that a heat treatment corresponding to a temperature between 250 and 440 °C has been carried out to produce said layers. Compared to the prior art, the non-stick coating according to the invention allows for new fluorine-free non-stick coatings with good adhesion to ECCS (electrolytic chromium / chromium oxide coated steel) and very good deep-draw properties, which in cooking tests in combination with dishwasher washing show better cleaning and demolding behavior of the test cake compared to non-stick coatings containing PTFE. In the process according to the invention, the lower layer to be formed is applied as liquid lacquer to the surface of the article and then dried at 250 to 440°C, the upper layer is then applied as liquid lacquer to the pre-dried lower layer and the layers are bonded together and to the surface of the article by a subsequent heat treatment at 250 to 440°C. Surprisingly, it has been shown that the superior coating according to the invention, with only 2.5% by weight of a silicone oil, already provides very good deep-drawing coatings that function without the use of a dry lubricant such as PTFE, without cracking. The drawn surfaces do not show any unwanted surface abrasion of the coating from the forming tools, visible in the subsequent article, as sometimes occurs with non-stick coatings containing PTFE. Furthermore, it has been surprisingly demonstrated that even high cooking temperatures between 250 and 440 °C do not negatively affect the non-stick effect of the silicones used in the non-stick coating according to the invention. The advantageous embodiments of the invention are given in the dependent claims. In principle, various silicone oils are available for the application according to the invention and are also commercially available. In particular, they can be reactive or non-reactive silicone oils. For the intended food applications, the silicone oil contained in the top layer shall have a kinematic viscosity at 20 °C of at least 100 mm²s⁻¹ (claim 2). According to one embodiment, the silicone oil used is a polydimethylsiloxane terminated in α,ω-hydroxy. It is advantageous if the top layer additionally contains one or more silicone resins (claim 3). Suitable silicone resins are, in particular, methyl silicone resins and / or phenyl silicone resins and / or methyl phenyl silicone resins (claim 4). The thermoplastic resin included in the lower and upper layers is independently selected from the group consisting of polyethersulfone (PES), polyphenyleneethersulfone (PPSU), liquid crystalline polymer (LCP), polyarletherketone, polyetherketone (PEK), polyetherketone (PEEK), polyetherketone (PEKK), polyphenylene sulfide (PPS), and mixtures thereof. A particularly advantageous thermoplastic material for the present invention is PES (claim 5). In certain embodiments, the lower and / or upper layer contains at least one additive selected from the group consisting of pigments, fillers, and metallic particles (claim 6). These may be, in particular, aluminum flakes, mica, or soot. However, there are also embodiments in which the lower and / or upper layer is free of pigments and particles. According to an advantageous embodiment, the dry film thickness of the lower layer as well as that of the upper layer is 3 to 4 pm (claim 7). In some embodiments, the same thermoplastic material, for example PES, is used for both the bottom and top layers. However, for certain applications, it may be provided that the top layer contains a different thermoplastic than the bottom layer (claim 8). In one embodiment of the process according to the invention, the lacquer used to form the lower layer or the upper layer contains the thermoplastic resin in dispersion form (claim 10). In another embodiment, the lacquer used to form the lower or upper layer contains the thermoplastic resin in dissolved form (claim 11). In principle, the process according to the invention can be used for articles with different types of surfaces. In an advantageous embodiment, the surface of the article is metallic (claim 12). According to a particularly advantageous embodiment, the lacquers used to form the lower layer and the upper layer are applied by a coil coating procedure (claim 13). In principle, the process according to the invention can be used to coat articles that are already finished. However, in an advantageous embodiment, the article, and therefore also the non-stick coating applied to it, is formed after cooking (claim 14), in particular by deep drawing. Brief description of the figures The following describes in more detail examples of embodiments of the invention with reference to the drawings, which show: Fig. 1 a layered structure of a non-stick coating according to the invention, as a schematic cross-sectional view; Fig. 2 Round shapes of embodiments 1 to 4 and comparative examples 1 and 2, each after a certain number of cooking cycles (cleaning in the dishwasher after cycles 1, 3, 5, 7 and 9). Ways to carry out the invention The non-stick coating shown in Figure 1 for surface O of an item, which is for example a baking mold, has a lower layer G baked onto a surface O and an upper layer D baked onto the lower layer G. Examples According to a particularly preferred embodiment of example 1, a non-stick coating, in particular for baking utensils, has two layers, namely, an upper layer with a dry film thickness of 3 to 4 pm and a lower layer with a dry film thickness of 3 to 4 pm. The lower layer in this particularly preferred embodiment 1 contains 85.1 wt% polyethersulfone, 8 wt% aluminum flakes, 4.3 wt% mica and 2.6 wt% carbon black. The topcoat in this particularly preferred embodiment 1 contains 72.5 wt% of polyethersulfone, 10.4 wt% of a methylphenylsilicone resin, 1.3 wt% of a methylsilicone resin, 3.1 wt% of a silicone oil consisting of an a,w-hydroxy terminated polydimethylsiloxane, 6.9 wt% of aluminum flakes, 3.6 wt% of mica, and 2.2 wt% of carbon black. The base coat is applied in liquid form to a 0.29 mm thick grease-free ECCS sheet using a squeegee and baked at 150 °C for 30 seconds, then at 420 °C for 1 minute, and cooled to room temperature. The top coat is applied to the dried base coat with a squeegee and baked at 150 °C for 30 seconds, then at 320 °C for 1 minute. According to another example of embodiment 2, a non-stick coating, in particular for baking utensils, has two layers, namely, an unpigmented top layer with a dry film thickness of 3 to 4 pm and a bottom layer with a dry film thickness of 3 to 4 pm. The lower layer in this embodiment 2 contains 85.1 wt% polyethersulfone, 8 wt% aluminum flakes, 4.3 wt% mica and 2.6 wt% carbon black. The topcoat in this embodiment 2 comprises 85 wt% of polyethersulfone, 11.5 wt% of a methylphenyl silicone resin and 3.5 wt% of a silicone oil comprising an a, w-hydroxy terminated polydimethylsiloxane. The bottom layer is applied in liquid form to a 0.29 mm thick fat-free ECCS sheet using a scraper and baked at 150 °C for 30 seconds, then at 420 °C for 1 minute, and cooled to room temperature. The top layer is applied to the dried bottom layer with a scraper and baked at 150 °C for 30 seconds, then at 320 °C for 1 minute. According to another embodiment 3, a non-stick coating, in particular for baking utensils, has two layers, namely an upper layer with a dry film thickness of 3 to 4 pm and a lower layer with a dry film thickness of 3 to 4 pm. The bottom layer in this embodiment 3 contains 85.1 wt% polyethersulfone, 8 wt% aluminum flakes, 4.3 wt% mica and 2.6 wt% carbon black. The non-pigmented topcoat in this embodiment 3 comprises 96.1 wt% of polyethersulfone and 3.9 wt% of a silicone oil consisting of an a,w-hydroxy terminated polydimethylsiloxane. The base coat is applied in liquid form to a 0.29 mm thick grease-free ECCS sheet using a squeegee and baked at 150 °C for 30 seconds, then at 420 °C for 1 minute, and cooled to room temperature. The top coat is applied to the dried base coat with a squeegee and baked at 150 °C for 30 seconds, then at 320 °C for 1 minute. According to another embodiment 4, a non-stick coating, in particular for baking utensils, has two layers, namely an upper layer with a dry film thickness of 3 to 4 pm and a lower layer with a dry film thickness of 3 to 4 pm. The bottom layer in this embodiment 4 contains 85.1 wt% polyethersulfone, 8 wt% aluminum flakes, 4.3 wt% mica and 2.6 wt% carbon black. The topcoat in this embodiment 4 comprises 80.9 wt% of polyethersulfone, 7.7 wt% of aluminum flakes, 4.1 wt% of mica, 2.5 wt% of carbon black, 1.4 wt% of a methylsilicone resin and 3.5 wt% of a silicone oil consisting of an a,w-hydroxy terminated polydimethylsiloxane. The base coat is applied in liquid form to a 0.29 mm thick grease-free ECCS sheet using a squeegee and baked at 150 °C for 30 seconds, then at 420 °C for 1 minute, and cooled to room temperature. The top coat is applied to the dried base coat with a squeegee and baked at 150 °C for 30 seconds, then at 320 °C for 1 minute. Comparative example 1 is a single-layer non-stick coating containing fluoropolymer and having the following composition: 77.8 wt% polyethersulfone, 3.9 wt% mica, 2.3 wt% carbon black, 8.6 wt% PTFE, 7.4 wt% aluminum flakes. The coating is applied in liquid form to a grease-free ECCS sheet 0.29 mm thick using a scraper and is burned at 150 °C for 30 seconds and then at 420 °C for 1 minute. Comparative example 2 is a single-layer non-stick coating of a modified silicone polyester having the following composition: 70.6 wt% of a silicone-modified polyester, 2 wt% of a pyrogenic silica, 2 wt% of an organically modified bentonite clay rheological additive, 0.1 wt% of carbon black, 1.5 wt% of an ultramarine pigment, 16.1 wt% of a barium sulfate filler, 4.1 wt% of aluminum flakes, and 3.9 wt% of a non-reactive silicone oil. For the combined cooking and dishwashing test of Comparative Example 2 described below, a round mold made of 0.29 mm thick fat-free ECCS sheet was first pressed and then spray-painted, baked at 150 °C for 30 seconds and then at 420 °C for 1 minute, as this coating cannot be deep-drawn. Properties of the coatings: Using an Erichsen model 212 deep drawing test machine (sheet clamping force 8-9 kN, drawing speed 4-5, drawing punch stroke approx. 60 mm, maximum drawing force 11 kN), square cups measuring approximately 26 x 26 mm (no. 05030132) were drawn from the baked non-stick coatings according to the invention to evaluate their deep drawing capability. The surfaces of the deep drawing test machine that come into contact with the sheet metal during deep drawing were previously degreased with ethyl acetate. No peeling or loss of adhesion was observed with any of the non-stick coatings according to the invention. Furthermore, no surface discoloration or abrasion was observed on any of the non-stick coatings according to the invention after forming. Thus, all the non-stick coatings according to the invention have very good thermoforming properties. Combined cooking and dishwasher washing test: In addition, round molds with a diameter of 22.5 cm and a depth of 4 cm were pressed from the embodiments according to the invention and the comparative examples in order to carry out a cooking test in combination with dishwashing. The following dough recipe was used for the test cake: Cream together 200g butter, 200g sugar, a pinch of vanilla sugar, and a pinch of salt until smooth. Gradually add 4 eggs, 300g flour, and 3 teaspoons of baking powder, stirring constantly. Mix all the ingredients until you have a smooth batter. Preheat a convection oven to 180°C. Pour 200g of cake batter into an ungreased round cake tin and spread it evenly. Bake for 20 minutes. Remove the tin from the oven and let it cool for 8 minutes. Turn the tin upside down and unmold the cake. This may require placing the tin upside down on the table. Observe how easily the cake comes out of the tin. Note the amount of batter remaining. Then, wash the tin by hand with dish soap and dry thoroughly. Evaluate and record how easy it is to clean. A total of 10 cycles are cooked, with dishwasher cleaning after cycles 1, 3, 5, 7, and 9. After each cycle, a photograph is taken of the round mold once the cake has been unmolded. When evaluating cake demolding and round mold cleaning after individual cycles, a maximum of 10 points can be awarded per cycle, resulting in a maximum of 100 points after 10 cycles. Cake demolding is weighted by a factor of 0.6 and round mold cleaning by a factor of 0.4. The cake's release from the mold is evaluated as follows: It doesn't stick, the cake unmolds very easily, 0-5% residue gives 10 points. It sticks slightly, the cake unmolds easily, 5-20% residue gives 7.5 points. It sticks strongly, the cake can be unmolded, 20-40% residue gives 5.0 points. It sticks strongly, the cake can only be unmolded with a dough scraper gives 2.5 points. The cleanliness of the round shape is evaluated as follows: No remainder gives 10 points Light (light pressure) gives 7.5 points moderate (medium pressure) gives 5.0 points Cleaning sponge required gives 2.5 points Table 1 shows the evaluation of the individual cycles of the combined cooking and dishwashing test of the investigated embodiments according to the invention and the comparative examples. As demonstrated in other tests, the presence of at least 2.5% by weight of a silicone oil in relation to the weight of the cooked top layer is necessary to ensure the deep-draw capability of the non-stick coatings according to the invention. Furthermore, Table 1 shows that even without the additional presence of a silicone resin in the top layer (e.g., embodiments 3 and 4), good average results can be achieved in the combined cooking and cleaning test, which are quite comparable to the two comparative examples 1 and 2 of the prior art. The addition of silicone resins in the top layer according to the invention can significantly improve the result of the combined cooking and cleaning test.
Claims
1. A non-stick coating for an article, wherein the non-stick coating comprises at least a lower layer (G) baked onto a surface (O) of the article and an upper layer (D) baked onto the lower layer, wherein the lower layer has a dry film thickness of 1 to 25 µm, and wherein the upper layer has a dry film thickness of 1 to 25 µm, wherein the lower layer contains from 10 to 100% by weight, based on the weight of the baked lower layer, of a thermoplastic resin having a temperature resistance above 200 °C, wherein the upper layer contains a thermoplastic resin having a temperature resistance above 200 °C and, optionally, a silicone resin, and wherein the lower layer and the upper layer are free from per- and polyfluorinated alkyl compounds.wherein the thermoplastic resin contained in the lower and upper layers is independently selected from the group consisting of polyether sulfone (PES), polyphenylene ether sulfone (PPSU), liquid crystalline polymer (LCP), polyaryl ether ketone, polyether ketone (PEK), polyether ether ketone (PEEK), polyether ketone ketone (PEKK), polyphenylene sulfide (PPS), and mixtures thereof, characterized in that the upper layer has a thermoplastic resin content of at least 30% by weight, based on the weight of the baked upper layer, and at least 2.5% by weight, based on the weight of the baked upper layer, of a silicone oil.
2. The non-stick coating according to claim 1, characterized in that the silicone oil contained in the upper layer has a kinematic viscosity at 20°C of at least 100 mmV.
3. The non-stick coating according to any of the preceding claims, characterized in that the top layer additionally contains from 1 to 67,5% by weight, based on the weight of the baked top layer, of silicone resin.
4. The non-stick coating according to claim 3, characterized in that the silicone resin is a methyl silicone resin and / or a phenyl silicone resin and / or a methyl phenyl silicone resin.
5. The non-stick coating according to any one of the preceding claims, characterized in that the thermoplastic resin contained in the bottom layer and in the top layer is polyethersulfone (PES).
6. The non-stick coating according to any one of the preceding claims, characterized in that the bottom layer and / or the top layer contain at least one additive selected from the group consisting of pigments, fillers, and metallic particles.
7. The non-stick coating according to any one of the preceding claims,characterized in that the dry film thickness of the lower layer is 3 to 4 µm and the dry film thickness of the upper layer is 3 to 4 µm.
8. The non-stick coating according to any of the preceding claims, characterized in that the upper layer contains a thermoplastic resin different from that of the lower layer.
9. A method for applying a non-stick coating according to any of the preceding claims, characterized in that the lower layer to be formed is applied as a liquid lacquer to the surface of the article and subsequently dried between 250 and 440°C, and in that the upper layer is subsequently applied as a liquid lacquer to the pre-dried lower layer and the layers are bonded to each other and to the surface of the article by subsequent heat treatment between 250 and 440°C.
10. The method according to claim 9,characterized in that the lacquer used to form the lower or upper layer contains the thermoplastic resin in dispersion form.
11. The process according to claim 9, characterized in that the lacquer used to form the lower or upper layer contains the thermoplastic resin in dissolved form.
12. The process according to any one of claims 9 to 11, characterized in that the surface of the article is metallic.
13. The process according to any one of claims 9 to 12, characterized in that the lacquers used to form the lower and upper layers are applied by a coil coating process.
14. The process according to any one of claims 9 to 13, characterized in that the non-stick coating is subjected to a forming process after heat treatment.
15. A coated article, in particular an oven dish or other household or utilitarian article,whose surface is provided with a non-stick coating according to any one of claims 1 to 8.,