Non-stick coating and method of making the same

Abstract

This invention relates to a non-stick coating for articles, the non-stick coating having at least one adhesion promoter layer (V) disposed on the surface of the article and a cover layer (D) disposed on the adhesion promoter layer, wherein the adhesion promoter layer has a dry film thickness of 5 to 25 μm, and wherein the cover layer has a dry film thickness of 10 to 65 μm. To achieve a combination of non-stick effect and improved abrasion resistance, it is suggested that the cover layer contains 95 to 100% by weight of PTFE and, if necessary, no more than 5% by weight of pigment, based on the total solids content of the cover layer, and is free of fluoropolymers capable of melt processing, and that the PTFE contained comprises 1 to 100% by weight of a polymerically modified PTFE, the modified PTFE comprising less than 1% by weight of perfluoropropyl vinyl ether (PPVE) as a comonomer acting as a modifier and obtained by emulsion polymerization. In a method for applying a non-stick coating, an adhesive layer in liquid form is applied as a first aqueous dispersion to the surface of an article and then thermally dried. A cover layer in the form of a second aqueous dispersion is then applied, and the layers are interconnected and bonded to the surface of the article by subsequent drying and sintering. The second dispersion does not contain fluoropolymers that can be melt-processed and does not contain low-molecular-weight PTFE.

Description

Technical Field

[0001] This invention relates to a PTFE non-stick coating with improved non-stick properties after severe wear, an article having such a non-stick coating, a method for manufacturing the non-stick coating, and the use of the non-stick coating in the manufacture of cooking equipment, frying equipment, kitchen equipment, or other applications requiring non-stick properties and high abrasion resistance. Background Technology

[0002] Organic fluoropolymer coatings exhibit good non-stick properties due to their low surface energy and high heat resistance due to their high CF bonding energy. These coatings are widely used, especially as non-stick coatings for kitchen appliances.

[0003] Non-stick coatings made of fluoropolymers are mostly applied to a substrate in multiple layers. Multilayer fluoropolymer coatings generally include an adhesion promoter layer and a topcoat layer, and optionally include one or more intermediate layers.

[0004] Multilayer non-stick coatings made of fluoropolymers have been known for many years. The adhesion layer of such a system typically consists of one or more heat-resistant organic adhesive resins and one or more fluoropolymers, along with various pigments and fillers. The intermediate layer mainly comprises fluoropolymers, a smaller proportion of effect pigments, fillers, and film-forming aids, while the topcoat layer consists almost entirely of fluoropolymers, particularly high-molecular-weight PTFE obtained by emulsion polymerization of TFE from an aqueous dispersion, or high-molecular-weight PTFE obtained by emulsion polymerization of TFE from an aqueous dispersion with a smaller proportion of melt-processable fluoropolymers, in order to ensure the best possible non-stick properties of the topcoat.

[0005] To improve the wear resistance of multi-layer PTFE non-stick coatings, hard inorganic fillers are often used.

[0006] Thus, for example, US 5,250,356 describes a multilayer system comprising an adsorbent layer composed of PTFE, PFA, and PAI reinforced with fine alumina, an intermediate layer composed of PTFE, PFA, and fine alumina, and a capping layer composed of pure PTFE.

[0007] EP 1016466 describes a multilayer ceramic-reinforced non-stick coating in which the base layer contains ceramic particles, but the ceramic particles are large enough to extend out of the cover layer and thus prevent wear load on the cover layer.

[0008] US 6,761,964 describes a multilayer non-stick coating comprising a base layer made of a fluoropolymer, an adhesion promoter polymer, and larger inorganic hard material particles anchored in the base layer and extending into a subsequent intermediate layer composed of a fluoropolymer and smaller ceramic filler particles, and a capping layer composed solely of a fluoropolymer.

[0009] EP 2001949 describes a three-layer non-stick coating in which the base layer and the top layer are free of ceramic particles >10 μm and the intermediate layer contains at least 3% by weight of ceramic particles >10 μm and a fluoropolymer.

[0010] If these systems are worn, they generally exhibit poor non-stick properties on the worn surfaces because the filler-free cover layer wears away quickly due to its smaller dry film thickness, thus exposing the filler-containing intermediate layer, which exhibits worse non-stick properties than a cover layer typically composed only of fluoropolymers.

[0011] EP 2342279 describes a fluoropolymer coating composition with improved abrasion resistance, applied to an adhesion promoter layer reinforced by alumina or at least one intermediate layer. The described unfilled capping layer consists of acrylate (film-forming aid), cerium ethylhexanoate (catalyst for combustion of the film-forming aid), and a fluoropolymer composition.

[0012] Here, the fluoropolymer composition comprises 30-96% by weight of high molecular weight PTFE (based on the total solids content of the fluoropolymer in the coating), wherein the PTFE is in the form of an aqueous dispersion having a number average molecular weight (Mn) of at least 500,000, wherein the high molecular weight PTFE includes less than 1% of a comonomer that acts as a modifier, and the fluoropolymer composition also includes low molecular weight PTFE in the form of an aqueous dispersion (based on the total solids content of the fluoropolymer in the coating) at a content of 4-70% by weight and having a first melting point of 335°C or lower, and the fluoropolymer composition comprises a fluoropolymer in the form of an aqueous dispersion that can be melt-processed. The disclosed high molecular weight PTFE and the mentioned low molecular weight PTFE are obtained here by emulsion polymerization of TFE.

[0013] The improved wear value of this coating is due to the presence of low-molecular-weight PTFE and fluoropolymers (such as PFA) that can be melt-processed.

[0014] The base layer has a thickness of approximately 8 μm and the capping layer has a thickness of approximately 25 μm.

[0015] WO2012 / 092414 describes aqueous dispersions of fluoropolymers that cannot be melt-processed and coatings made from these aqueous dispersions. Coatings made from the said PTFE dispersions exhibit an increased critical cracking thickness (CCT) before cracking occurs in the layer due to the increased content of nonionic wetting agents and 1-10% by weight of water-soluble alkaline earth metals or 0.1-1% colloidal silica.

[0016] Multilayer non-stick coatings typically have a total dry film thickness of up to 45 μm (adhesive layer: 14-18 μm, intermediate layer: 12-14 μm, capping layer: 10-13 μm). Unfilled (capping layer) aqueous PTFE coating compositions typically exhibit a critical dry film thickness of approximately 12-14 μm before cracking.

[0017] Improved abrasion resistance can also be achieved by increasing the dry film thickness of the intermediate and top coats. However, without the addition of fillers, higher layer thicknesses lead to cracking after the coating is baked.

[0018] For higher dry film thicknesses in intermediate and topcoat layers, crack formation can be minimized or a higher critical dry film thickness can be achieved by adding fillers. However, the added fillers negatively impact the non-stick properties of the worn coating.

[0019] Adding film-forming aids such as acrylates can slightly increase the critical dry film thickness; however, during the coating baking process, the added acrylates burn, and a catalyst must be added to ensure the most complete combustion possible. Unburned acrylate residues also negatively affect the non-stick properties. Summary of the Invention

[0020] Therefore, the technical problem to be solved by the present invention is to provide an improved non-stick coating. Another technical problem to be solved by the present invention is to provide a method for applying the non-stick coating according to the present invention. This technical problem is solved according to the present invention by the non-stick coating as defined in claim 1 or by the method as defined in claim 12.

[0021] The non-stick coating according to the invention comprises at least one adhesion promoter layer (or adhesion facilitator layer) disposed on the surface of an article and a cover layer disposed on the adhesion promoter layer, wherein the adhesion promoter layer has a dry film thickness of 5 to 25 μm, and wherein the cover layer has a dry film thickness of 10 to 65 μm. The cover layer comprises 95 to 100% by weight of PTFE and, if necessary, no more than 5% by weight of pigment, based on the total solids content of the cover layer, and is free of fluoropolymers capable of melt processing. The PTFE comprises 1 to 100% by weight of a high molecular weight modified PTFE, wherein the modified PTFE comprises less than 1% by weight of perfluoropropyl vinyl ether (PPVE) as a comonomer acting as a modifier and is obtained by emulsion polymerization. The remaining portion, which may be no more than 99% by weight, consists substantially of unmodified high molecular weight PTFE.

[0022] The production of modified PTFE by suspension polymerization is known in principle and is especially used to form particulate PTFE with particularly high molecular weight.

[0023] It is also known to produce aqueous dispersions of modified polymeric PTFE by means of emulsion polymerization, which provides very fine aqueous dispersions that can be more easily processed into aqueous coating compositions and have no settling properties in practice compared to modified PTFE powders obtained by suspension polymerization.

[0024] By achieving a higher, crack-free dry film thickness in the coating layer, significantly better abrasion resistance can be achieved compared to existing technologies, and the non-stick properties of the worn layer are significantly more durable because the coating layer does not contain any fillers other than PTFE that might be exposed after the coating layer wears down and thus degrade the non-stick properties. Accordingly, a key feature of the non-stick coating according to the invention is the combination of excellent non-stick performance and excellent abrasion resistance.

[0025] In the method according to the invention, a liquid adhesive layer to be formed is applied as a first aqueous dispersion to the surface of an article and then dried at 60 to 110°C for 5 to 15 minutes. A coating layer in the form of a second aqueous dispersion is then applied to the pre-dried adhesive layer, and the layers are subsequently bonded to each other and to the surface of the article by drying and sintering or melting at 410 to 430°C. The second aqueous dispersion comprises a dispersion of 95 to 100 wt% PTFE based on the total dispersed mass and, if necessary, no more than 5 wt% pigment, and is free of fluoropolymers capable of melt processing. The PTFE comprises 1 to 100 wt% polymeric modified PTFE, which contains less than 1 wt% perfluoropropyl vinyl ether (PPVE) as a comonomer acting as the modifier and is obtained by emulsion polymerization. The remaining portion, which may be no more than 99 wt%, consists substantially of unmodified polymeric PTFE. The second aqueous dispersion does not contain fluoropolymers that can be melt-processed and does not contain low-molecular-weight PTFE.

[0026] Surprisingly, it has been demonstrated that a high dry film thickness of no more than 65 μm can be obtained without cracks in a single coating process by using the method according to the present invention.

[0027] According to claim 10, another aspect of the invention relates to the use of the non-stick coating according to the invention for surface applications requiring high abrasion resistance and good non-stick properties.

[0028] According to claim 11, another aspect of the invention relates to articles, particularly cookware or other household and daily necessities, having a surface provided with a non-stick coating according to the invention.

[0029] Therefore, the non-stick coating according to the present invention has a layer sequence of "article - adhesion promoter layer - cover layer". Optionally, an intermediate layer may be applied between the adhesion promoter layer and the cover layer.

[0030] Advantageous designs of the present invention are described below and defined in the dependent claims.

[0031] According to the preferred design (claims 2 and 14), the cover layer has a dry film thickness of 35 to 40 μm.

[0032] According to other advantageous designs (claims 3 and 15), the PTFE content in the coating layer is 99 to 99.5% by weight. Accordingly, the pigment content is 1 to 0.5% by weight and consists, for example, about 2 parts effect pigment (mica) and about 1 part carbon black pigment.

[0033] In some embodiments, the proportion of polymerically modified PTFE in the capping layer does not exceed 99% by weight, particularly not more than 95% by weight, and especially not more than 75% by weight. Particularly advantageous (claim 4), the proportion of polymerically modified PTFE in the capping layer is 2 to 50% by weight of the total PTFE in the capping layer, preferably 4 to 6% by weight, and especially about 5.5% by weight.

[0034] Furthermore, it is advantageous (claims 5 and 16) that the polymer-modified PTFE has an average particle size of about 200 nm (according to DIN ISO 13321), and particularly about 150 to 250 nm.

[0035] According to the advantageous design (claims 6 and 13), the coating layer is acrylate-free. Unburned acrylate residue negatively impacts non-stick properties and is therefore undesirable.

[0036] The properties of a suitable adhesive layer are known in themselves. In this regard, it has proven advantageous (claim 7) that the adhesive layer consists essentially of a heat-resistant adhesive resin, a fluoropolymer, pigments, and fillers.

[0037] The adhesive resin can, in principle, be selected from a range of known substances, such as, in particular, PES (polyethersulfone), PAI (polyamide-imide), PEK (polyetherketone), PEEK (polyetheretherketone), PPS (polyphenylene sulfide), PI (polyimide), LCP (liquid crystal polymer), and silicone resins, or mixtures thereof. According to an advantageous design (claim 8), the adhesive resin is selected from the group consisting of PES and PAI.

[0038] Furthermore, preferably (claims 9 and 17), the adsorbent layer has a dry film thickness of 10 to 18 μm. Attached Figure Description

[0039] The embodiments of the present invention are described in detail below with reference to the accompanying drawings, in which:

[0040] Figure 1 The layer structure of the non-stick coating according to the invention is shown as a schematic cross-sectional view;

[0041] Figures 2 to 5 An example is shown, after a specific wear time (left) and after an additional milk test and subsequent cleaning (right), i.e., a wear time of 45 minutes ( Figure 2 ), 90 minutes Figure 3 ), 180 minutes Figure 4 ) and 270 minutes ( Figure 5 );

[0042] Figures 6 to 8 Comparative examples are shown, after a specific wear time (left) and after an additional milk test and subsequent cleaning (right), i.e., wear time 45 minutes ( Figure 6 ), 90 minutes Figure 7 ) and 135 minutes ( Figure 8 ). Detailed Implementation

[0043] exist Figure 1 The nonstick coating shown for an article G, such as a cookware, especially a frying pan, has an adhesive layer V disposed on the surface O of the article and a covering layer D disposed on the adhesive layer.

[0044] Example

[0045] A non-stick coating was tested, comprising a capping layer having a dry film thickness of 35 to 40 μm and a base layer having a dry film thickness of approximately 15 μm. In this embodiment, the capping layer comprises approximately 99.3% by weight PTFE based on the total solids content of the capping layer, of which approximately 5.5% by weight is polymerically modified PTFE containing less than 1% by weight of perfluoropropyl vinyl ether (PPVE) as a modifying comonomer and obtained by emulsion polymerization, and having an average particle size of approximately 200 nm. Additionally, it comprises approximately 0.4% by weight of effect pigment (mica) and approximately 0.2% by weight of carbon black pigment based on the total solids content of the capping layer.

[0046] In this embodiment, the adhesion promoter layer consists of approximately 32.5% by weight of alumina, 18.4% by weight of polyamide-imide, 13.7% by weight of silicon carbide, 27.1% by weight of PTFE, 5.5% by weight of PFA, 1.9% by weight of colloidal silica, and 0.9% by weight of carbon black pigment, based on the total solids content of the adhesion promoter layer. The adhesion promoter layer is applied from an aqueous dispersion to a sandblasted substrate (e.g., a frying pan) and then pre-dried at 100°C for 5 minutes. A coating layer is then applied from an aqueous dispersion to the pre-dried adhesion promoter layer and pre-dried at 420°C for 10 minutes.

[0047] Coating performance:

[0048] The baked coating layer according to the invention was examined for cracks using a stereomicroscope at 40x magnification. It was confirmed that the coating layer was free of cracks.

[0049] The abrasion resistance of coated items is tested as follows:

[0050] The coated frying pan was filled with a mixture of refined steel balls (4mm in diameter), corundum (ALODUR EK-P20), and water, and fixed on a KS 501digital circular vibrator (vibration stroke: 30mm) for a specified time at 300 rpm. The amounts of refined steel balls, corundum, and water used were calculated as follows:

[0051] Measure the diameter of the inside of the bottom of the pan and multiply the determined value in cm by a factor of 0.394. The result is the inner diameter of the pan bottom in inches.

[0052] f = (inner diameter of the pan bottom in inches) 2 / 10

[0053] The corresponding quantities of refined steel balls, corundum, and water are calculated using the coefficient f:

[0054] A refined steel ball with a diameter of 4mm weighs 175g.

[0055] ALODUREK-P20 18g·f

[0056] 20 grams of water

[0057] After a determined vibration period, the pan is visually evaluated for visibility of the aluminum substrate. Next, the worn pan is heated to 200±5°C and a pancake is baked using 400g milk (3.0%-4.0% fat), 200g wheat flour, 150g egg, and 4g salt. The method of removing the baked pancake from the pan is then evaluated.

[0058] To further test the non-stick properties, milk was baked until it turned dark brown and began to smoke. The baked milk was then removed from the pan using a water jet, and any residue remained. Results were documented by photographing at different abrasion times.

[0059] Figures 2 to 5 The results obtained by providing an article with a non-stick coating according to the present invention are shown.

[0060] Table 1 shows the results of pancake tests on the article according to the invention after wear and on a comparative example after wear.

[0061] Comparative example :

[0062] The comparative example is a three-layer non-stick coating, wherein the adhesion promoter layer has a dry film thickness of about 18 μm, the intermediate layer has a dry film thickness of about 15 μm and the capping layer has a thickness of about 12 μm.

[0063] The comparative example's adhesive layer consists of approximately 29.9% by weight of alumina, 24.5% by weight of polyamide-imide, 12.7% by weight of silicon carbide, 25.2% by weight of PTFE, 5.1% by weight of PFA, 1.8% by weight of colloidal silica and 0.8% by weight of carbon black pigment, based on the total solids content of the adhesive layer.

[0064] The intermediate layer of the comparative example consists of approximately 5.2% by weight of acrylate, 0.1% by weight of cerium ethylhexanoate, 2.0% by weight of alumina, 12.3% by weight of silicon carbide, 76.7% by weight of PTFE, 1.4% by weight of PFA, 1.9% by weight of effect pigment (mica) and 0.4% by weight of carbon black pigment, based on the total solids content of the intermediate layer.

[0065] The comparative example's coating consists of approximately 3.9% by weight of acrylate, 0.1% by weight of cerium ethylhexanoate, 2.2% by weight of alumina, 91.4% by weight of PTFE, 2.2% by weight of effect pigment (mica) and 0.2% by weight of carbon black pigment, based on the total solids content of the coating.

[0066] Figures 6 to 8 The test results for the comparative examples are shown.

[0067] Table 1: Results of pancake tests after wear according to the embodiments and comparative examples of the present invention.

[0068]

Claims

1. A non-stick coating for articles, wherein, The non-stick coating has at least one adhesion promoter layer (V) disposed on the surface of an article and a cover layer (D) disposed on the adhesion promoter layer, wherein the adhesion promoter layer has a dry film thickness of 5 to 25 μm, and wherein the cover layer has a dry film thickness of 30 to 65 μm, characterized in that, based on the total solids content of the cover layer, the cover layer consists of 99 to 99.5% by weight of PTFE and 1 to 0.5% by weight of pigment, and is free of fluoropolymers capable of melt processing and low molecular weight PTFE, and the PTFE contained comprises 1 to 100% by weight of high molecular weight modified PTFE, wherein the modified PTFE comprises less than 1% by weight of perfluoropropyl vinyl ether (PPVE) as a comonomer acting as a modifier and is obtained by emulsion polymerization, wherein the cover layer is free of acrylates because no acrylates are added during its preparation.

2. The non-stick coating according to claim 1, characterized in that, The coating layer has a dry film thickness of 30 to 42 μm.

3. The non-stick coating according to claim 1 or 2, characterized in that, The proportion of the polymer-modified PTFE in the capping layer is 2 to 50% by weight of the total PTFE in the capping layer.

4. The non-stick coating according to claim 1 or 2, characterized in that, The proportion of polymer-modified PTFE in the capping layer is 4 to 6% by weight of the total PTFE in the capping layer.

5. The non-stick coating according to claim 1 or 2, characterized in that, The proportion of polymer-modified PTFE in the capping layer is 5.5% by weight of the total PTFE in the capping layer.

6. The non-stick coating according to claim 1 or 2, characterized in that, The polymer-modified PTFE has an average particle size of 150 to 250 nm.

7. The non-stick coating according to claim 1 or 2, characterized in that, The adhesive layer is essentially composed of a high-temperature resistant adhesive resin, a fluoropolymer, pigments, and fillers.

8. The non-stick coating according to claim 7, characterized in that, The adhesive resin is selected from the group consisting of PES (polyethersulfone resin) and PAI (polyamide imide).

9. The non-stick coating according to claim 1 or 2, characterized in that, The adhesive layer has a dry film thickness of 10 to 18 μm.

10. Use of the non-stick coating according to any one of claims 1 to 9, wherein the non-stick coating is used in surface applications requiring high abrasion resistance and good non-stick properties.

11. An article having a surface provided with a non-stick coating as described in any one of claims 1 to 9.

12. The article according to claim 11, wherein, The items in question are cooking utensils or other daily necessities.

13. A method for applying a non-stick coating according to any one of claims 1 to 9, wherein, An adhesive layer to be formed in liquid form is applied as a first aqueous dispersion to the surface of an article and then dried at 60 to 110°C for 5 to 15 minutes. A covering layer in the form of a second aqueous dispersion is then applied to the pre-dried adhesive layer, and the layers are interconnected and bonded to the surface of the article by subsequent drying and sintering or melting at 410 to 430°C. The second aqueous dispersion is characterized by being free of fluoropolymers capable of melt processing and free of low-molecular-weight PTFE, and free of acrylates.

14. The method according to claim 13, wherein, The coating layer is applied as a dry film thickness of no more than 30 to 42 μm.

15. The method according to claim 13 or 14, wherein, The proportion of polymer-modified PTFE in the second aqueous dispersion is 2 to 50% by weight of the total PTFE in the second aqueous dispersion.

16. The method according to claim 13 or 14, wherein, The proportion of polymer-modified PTFE in the second aqueous dispersion is 4 to 6% by weight of the total PTFE in the second aqueous dispersion.

17. The method according to claim 13 or 14, wherein, The proportion of polymer-modified PTFE in the second aqueous dispersion is 5.5% by weight of the total PTFE in the second aqueous dispersion.

18. The method according to claim 13 or 14, wherein, The polymer-modified PTFE in the second aqueous dispersion has an average particle size of 150 to 250 nm.

19. The method according to claim 13 or 14, wherein, The adhesive layer is applied as a dry film thickness of no more than 10 to 18 μm.

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