Fluorine-free release coating and method of making same

Abstract

The invention relates to a release coating for articles, in particular for baking molds or other household or utility articles, having at least one base layer (G) which is baked onto the surface (O) of the article and a cover layer (D) which is baked onto the base layer. The base layer and the cover layer each have a dry film thickness of 1 to 25 µm, wherein the base layer comprises 10 to 100 % by weight of a thermoplastic which has a temperature resistance of more than 200 °C, based on the weight of the baked base layer, and wherein the cover layer comprises a thermoplastic which has a temperature resistance of more than 200 °C and optionally a silicone resin. Neither the base layer nor the cover layer contains perfluoro- and polyfluoroalkyl compounds. Due to the cover layer having a thermoplastic content of at least 30 % by weight, based on the weight of the baked cover layer, and a silicone oil content of at least 2.5 % by weight, based on the weight of the baked cover layer, a novel fluorine-free release coating is obtained which has good substrate adhesion and very good deep drawability. The invention also relates to a process for applying the release coating.

Description

TECHNICAL FIELD

[0001] The present invention relates to a non-fluorine-containing release coating, a method for its production and an article provided with the coating. PRIOR ART

[0002] Release coatings based on fluoropolymers are known in the prior art, for example for coating baking molds. Polytetrafluoroethylene (PTFE) is a very common fluoropolymer in this regard. PTFE coatings have very good release properties due to their low surface energy and high temperature resistance due to the high C-F bond energy.

[0003] Baking molds are mostly produced by a web coating process and subsequent forming (deep drawing).

[0004] In addition to good release properties, PTFE coatings usually have a low coefficient of friction, which has a positive effect on the deep drawability of such coatings. Without the dry lubricating effect of polytetrafluoroethylene, the sheet material would tear during deep drawing.

[0005] In addition to the above-mentioned advantages, fluoropolymer coatings have a number of disadvantages. Due to the very high melt viscosity, PTFE can only be processed at very high temperatures. The baking temperatures used here are approximately 420°C. This processing temperature is above the decomposition temperature of PTFE, whereby toxic and corrosive decomposition products (such as trifluoroacetic acid and fluorophosgene) are released (Nature, Vol. 412, 19, July 2001, pp. 321-324). In the production of fluoropolymers, fluorine-containing wetting agents are also often used, which accumulate in nature due to the lack of biodegradability and also endanger human health and the environment.

[0006] The deep drawing process of polytetrafluoroethylene coatings can also lead to undesirable surface abrasion of the coating, which is visible in the finished article, especially when the surface is soft and has a high polytetrafluoroethylene content.

[0007] Non-fluorine-containing release coatings are also known from the prior art.

[0008] Silicone-modified polyesters, in which part of the polyester is modified with silicone, often exhibit insufficient temperature resistance, since the polyester part is easily flammable at high temperatures (> 230°C). In order to achieve sufficient deep drawability of the silicone-modified polyesters, the proportion of silicone modification must be kept low in order to be able to achieve sufficient substrate adhesion and flexibility for the deep drawing process. This has a negative effect on the release effect and temperature stability. For these reasons, silicone-modified polyesters are only used in spray applications, but not in more efficient web coating processes.

[0009] Finally, fluorine-free release coatings are known from the prior art, for example from EP 2 177 580 B1, which are produced in a sol-gel process. These coatings are very temperature-resistant, but at the same time very hard and brittle. These release coatings, which are also often referred to as ceramic coatings, are therefore also unsuitable for web coating processes with subsequent forming processes due to their brittleness.

[0010] EP 2 450 469 B1 describes a production process in which a sol-gel coating is applied to a substrate, dried at 70 ± 10 °C for 6 to 8 minutes and then shaped, after which it is completely cross-linked. However, the pre-dried coating is not suitable for industrial web coating processes, since the pre-dried coating is very susceptible to mechanical damage and the pre-drying conditions are difficult to control.

[0011] US20200216669A1 describes a fluorine-free sol-gel coating composition which additionally comprises a thermoplastic polymer having a melting point or glass transition temperature of 200 °C or more (e.g. PPS). Improved impact resistance is thereby achieved. However, the flexibility achieved by the coating is too low for deep-drawing processes.

[0012] EP 2 319 631 B1 describes a coating for a substrate, having a base layer on the substrate, wherein the base layer comprises a binding matrix selected from silicone resins, titanates, zirconates and mixtures thereof, wherein the binding matrix contains 0.5 to 20 % by weight of a thermoplastic material having a heat resistance of more than 200 °C, based on the weight of the base layer. The base layer does not contain fluorine-containing polymers and is tempered at a temperature of 150 °C to 230 °C, wherein a cover layer is arranged on the base layer, which contains a binding matrix, the proportion of thermoplastic plastics of which is smaller than in the base layer. The proportion of thermoplastic plastics in the binding matrix is very low here, at a maximum of 20 % by weight, and the coating is tempered at a maximum of 230 °C in order to avoid decomposition of the silicone resin.

[0013] From the prior art, silicone elastomers and silicone resins are also known as release materials. However, they have the disadvantage that they can only form a low adhesion to the substrate and are therefore not suitable for subsequent forming processes. Heat-resistant silicone resins are also very brittle and only exhibit sufficient adhesion on mechanically rough surfaces. SUMMARY

[0014] It is an object of the present application to provide a fluorine-free release coating which overcomes the disadvantages of the prior art. In particular, the coating should allow a good adhesion to ECCS (electrolytically chromium / chromium oxide coated steel) and have a high flexibility which is suitable for downstream deep-drawing processes. Here, a coating is to be provided which does not contain perfluoro- and polyfluoroalkyl compounds and in particular is completely free of fluoropolymers, in particular polytetrafluoroethylene (PTFE).

[0015] It is another object of the present invention to provide a method of applying the release coating according to the present invention.

[0016] These objects are achieved by the release coating according to the definition of the present invention or by the method defined by the present invention.

[0017] Another aspect of the present invention relates to a coated article, the surface of which is provided with a release coating according to the present invention. Herein, in particular, the article can be a baking mold or other household or utility article.

[0018] The release coating according to the present invention comprises at least one base layer fired onto the surface of the article and a cover layer fired onto the base layer, wherein the base layer has a dry film thickness of 1 to 25 pm and wherein the cover layer has a dry film thickness of 1 to 25 pm. The base layer comprises 10 to 100 wt.-% of a thermoplastic having a temperature resistance of more than 200 °C, based on the weight of the fired base layer, and the cover layer likewise comprises a thermoplastic having a temperature resistance of more than 200 °C and optionally a silicone resin. Neither the base layer nor the cover layer contain perfluoro- and polyfluoroalkyl compounds. According to the present invention, the cover layer has a thermoplastic content of at least 30 wt.-%, based on the weight of the fired cover layer, and a silicone oil of at least 2.5 wt.-%, based on the weight of the fired cover layer.

[0019] The terms "fired base layer" and "fired cover layer" are to be understood as a respective heat treatment at a temperature between 250 and 440 °C for the manufacture of the layer.

[0020] In comparison to the prior art, the use of the release coating according to the present invention makes available a new fluorine-free release coating having good adhesion to ECCS (electrolytic chromium / chromium oxide coated steel) and very good deep drawability, which shows improved test cake cleaning and demolding behavior in combination with dishwashing cleaning in the baking test in comparison to PTFE-containing release coatings.

[0021] In the method according to the present invention, the base layer to be formed is applied as a liquid paint (or liquid coating) to the surface of the article and then dried at 250 to 440 °C, whereupon the cover layer is applied as a liquid paint to the pre-dried base layer and the layers are connected to each other and to the surface of the article by the subsequent heat treatment at 250 to 440 °C.

[0022] Surprisingly, it has been shown that a cover layer according to the present invention having only 2.5 wt.-% of silicone oil already provides a very good deep drawable coating, which is possible without the use of dry lubricants such as PTFE, without tearing. The deep-drawn surface shows no unwanted coating surface wear caused by the shaping tool, which is sometimes the case with PTFE-containing release coatings, which is visible on the subsequent article.

[0023] Furthermore, it was surprisingly found that even high baking temperatures between 250 and 440°C do not adversely affect the release effect of the silicone used in the release coating according to the application.

[0024] Advantageous refinements of the application are given below.

[0025] In principle, for the application according to the application various silicone oils are usable and also commercially available. In particular, these can be reactive or non-reactive silicone oils here. For intended applications in the food sector, the kinematic viscosity of the silicone oil contained in the cover layer at 20°C should be at least 100 mm 2 s -1 According to one embodiment, alpha, omega-hydroxy-terminated polydimethylsiloxanes are used as silicone oils.

[0026] It is advantageous if the cover layer additionally comprises one or more silicone resins. Particularly suitable silicone resins are methyl silicone resins and / or phenyl silicone resins and / or methylphenyl silicone resins.

[0027] The thermoplastics contained in the base layer and the cover layer are independently selected from the group consisting of polyether sulfone (PES), polyphenyl ether sulfone (PPSU), liquid crystalline polymers (LCP), polyaryletherketone, polyetherketone (PEK), polyether ether ketone (PEEK), polyether ketone ketone (PEKK), polyphenylene sulfide (PPS) and mixtures thereof. A thermoplastic which is particularly advantageous for the application is PES.

[0028] In certain embodiments, the base layer and / or the cover layer comprises at least one additive selected from the group consisting of pigments, fillers and metal particles. Here, this can be inter alia aluminum flakes (so-called "flakes"), mica or soot. However, there are also embodiments in which the base layer and / or the cover layer are free of pigments and particles.

[0029] According to an advantageous embodiment, the dry film thickness of the base layer as well as the dry film thickness of the cover layer is 3 to 4 pm.

[0030] In some designs, the same thermoplastic material, for example PES, is used for the base layer and the cover layer. However, for certain applications it can be provided that the cover layer comprises a different thermoplastic than the base layer.

[0031] In an embodiment of the method according to the application, the lacquer used to form the base layer or the cover layer comprises the thermoplastic in the form of a dispersion.

[0032] In a further embodiment, the lacquer used to form the base layer or the cover layer comprises the thermoplastic in dissolved form.

[0033] In principle, the method according to the application can be used for articles having different types of surfaces. In an advantageous embodiment, the surface of the article is metallic.

[0034] According to a particularly advantageous embodiment, the lacquer used to form the base layer and the cover layer is applied by a web coating process.

[0035] In principle, the method according to the application can be used to coat articles which have already been made into finished products. In an advantageous embodiment, however, the articles and thus also the release coating applied thereto are shaped after the baking, in particular by deep-drawing. BRIEF DESCRIPTION OF DRAWINGS

[0036] Embodiments of the application are described in more detail below with reference to the drawings, in which:

[0037] Figure 1 The layer structure of the release coating according to the application is shown as a schematic cross-sectional view;

[0038] Figure 2 is a round mold of Examples 1 to 4 and Comparative Examples 1 and 2, each after a certain number of baking cycles (cleaning in a dishwashing machine after cycles 1, 3, 5, 7 and 9). DETAILED DESCRIPTION

[0039] Figure 1 The release coating shown in Figure 1 is used for the surface O of an article, wherein the article is for example a baking mold, which has a base layer G baked onto the surface O and a cover layer D baked onto the base layer G.

[0040] EXAMPLE

[0041] According to a particularly preferred embodiment 1, the release coating, in particular for a baking tray, has two layers, namely a cover layer with a dry film thickness of 3 to 4 pm and a base layer with a dry film thickness of 3 to 4 pm.

[0042] The base layer in this particularly preferred embodiment 1 comprises 85.1 wt.-% polyether sulfone, 8 wt.-% aluminum flakes, 4.3 wt.-% mica and 2.6 wt.-% carbon black.

[0043] The cover layer in this particularly preferred embodiment 1 comprises 72.5 wt.-% polyether sulfone, 10.4 wt.-% methylphenyl silicone resin, 1.3 wt.-% methyl silicone resin, 3.1 wt.-% silicone oil consisting of a, w-hydroxy-terminated polydimethylsiloxane, 6.9 wt.-% aluminum flakes, 3.6 wt.-% mica and 2.2 wt.-% carbon black.

[0044] The base layer was applied in liquid form using a doctor blade to a grease-free ECCS sheet with a thickness of 0.29 mm and baked at 150°C for 30 seconds, then at 420°C for 1 minute and cooled to room temperature. The cover layer was applied to the dry base layer with a doctor blade and baked at 150°C for 30 seconds, then at 320°C for 1 minute.

[0045] According to another embodiment 2, the release coating, in particular for a baking tray, has two layers, namely an uncolored cover layer with a dry film thickness of 3 to 4 pm and a base layer with a dry film thickness of 3 to 4 pm.

[0046] The base layer in this embodiment 2 comprises 85.1 wt.-% polyether sulfone, 8 wt.-% aluminum flakes, 4.3 wt.-% mica and 2.6 wt.-% carbon black.

[0047] The cover layer in this embodiment 2 comprises 85 wt.-% polyether sulfone, 11.5 wt.-% methylphenyl silicone resin and 3.5 wt.-% silicone oil consisting of a, w-hydroxy-terminated polydimethylsiloxane.

[0048] The base layer is applied in liquid form using a doctor blade to a grease-free ECCS sheet with a thickness of 0.29 mm and is baked at 150 °C for 30 seconds, then at 420 °C for 1 minute and cooled to room temperature. The cover layer is applied to the dried base layer with a doctor blade and is baked at 150 °C for 30 seconds, then at 320 °C for 1 minute.

[0049] According to another embodiment 3, the release coating, in particular for a baking tray, has two layers, namely a cover layer with a dry film thickness of 3 to 4 pm and a base layer with a dry film thickness of 3 to 4 pm.

[0050] The base layer in this embodiment 3 comprises 85.1 wt.-% polyether sulfone, 8 wt.-% aluminum flakes, 4.3 wt.-% mica and 2.6 wt.-% carbon black.

[0051] The uncolored cover layer in this embodiment 3 comprises 96.1 wt.-% polyether sulfone and 3.9 wt.-% silicone oil consisting of a, w-hydroxy-terminated polydimethylsiloxane.

[0052] The base layer is applied in liquid form using a doctor blade to a grease-free ECCS sheet with a thickness of 0.29 mm and is baked at 150 °C for 30 seconds, then at 420 °C for 1 minute and cooled to room temperature. The cover layer is applied to the dried base layer with a doctor blade and is baked at 150 °C for 30 seconds, then at 320 °C for 1 minute.

[0053] According to another embodiment 4, the release coating, in particular for a baking tray, has two layers, namely a cover layer with a dry film thickness of 3 to 4 pm and a base layer with a dry film thickness of 3 to 4 pm.

[0054] The base layer in this embodiment 4 comprises 85.1 wt.-% polyether sulfone, 8 wt.-% aluminum flakes, 4.3 wt.-% mica and 2.6 wt.-% carbon black.

[0055] The cover layer in this example 4 comprises 80.9 wt% polyether sulfone, 7.7 wt% aluminum flake, 4.1 wt% mica, 2.5 wt% carbon black, 1.4 wt% methyl silicone resin and 3.5 wt% silicone oil consisting of a, w-hydroxy-terminated polydimethylsiloxane.

[0056] The base layer was applied in liquid form to a grease-free ECCS sheet having a thickness of 0.29 mm using a doctor blade and was baked at 150 °C for 30 seconds, then at 420 °C for 1 minute and cooled to room temperature. The cover layer was applied to the dried base layer with a doctor blade and was baked at 150 °C for 30 seconds, then at 320 °C for 1 minute.

[0057] Comparative example 1 is a single layer release coating comprising a fluoropolymer having the following composition: 77.8 wt% polyether sulfone, 3.9 wt% mica, 2.3 wt% carbon black, 8.6 wt% PTFE, 7.4 wt% aluminum flake.

[0058] The layer was applied in liquid form to a grease-free ECCS sheet having a thickness of 0.29 mm using a doctor blade and was baked at 150 °C for 30 seconds, then at 420 °C for 1 minute.

[0059] Comparative example 2 is a single layer release coating of a modified silicone polyester having the following composition: 70.6 wt% silicone modified polyester, 2 wt% fumed silica, 2 wt% rheological additive made of organically modified bentonite, 0.1 wt% carbon black, 1.5 wt% ultramarine pigment, 16.1 wt% barium sulfate filler, 4.1 wt% aluminum flake and 3.9 wt% non-reactive silicone oil.

[0060] For the combined baking and dishwashing tests described below for comparative example 2, a round mold was first pressed from a grease-free ECCS sheet having a thickness of 0.29 mm, then painted in a spray process, baked at 150 °C for 30 seconds, then at 420 °C for 1 minute, as this coating is not deep drawable.

[0061] Properties of the coating:

[0062] An Erichsen 212 deep draw tester was used to draw a square dish (sheet clamping force 8-9 kN, draw speed 4-5, draw punch stroke about 60 mm, draw force max. 11 kN) of 26 x 26 mm about 60 mm (#05030132) from the baked release coating according to the invention to assess deep drawability. The surface of the deep draw tester which comes into contact with the sheet during the deep draw process was previously degreased with ethyl acetate.

[0063] Here, no delamination or loss of adhesion was found for all the non-stick coatings according to the present application. Furthermore, no change in surface color or scratching was observed on any of the non-stick coatings according to the present application after the modification. Thus, all the non-stick coatings according to the present application are very easy to deep-draw.

[0064] Combined baking and dishwashing test:

[0065] Furthermore, circular molds with a diameter of 22.5 cm and a depth of 4 cm were pressed out of the examples and comparative examples according to the present application in order to be able to carry out the combined baking and dishwashing test.

[0066] For the test cakes the following dough recipe was used:

[0067] 200 g butter, 200 g sugar, 1 pinch of vanilla sugar and 1 pinch of salt were stirred together until creamy. 4 eggs, 300 g flour and 3 teaspoons of baking powder were added step by step under stirring. All ingredients were mixed to a homogeneous dough mixture.

[0068] A convection oven was preheated to 180 °C. 200 g of the cake batter were poured into an ungreased circular mold and distributed evenly in the mold. The dough was baked for 20 minutes. The baking mold was removed from the oven and cooled for 8 minutes. The baking mold was turned over and the cake was removed from the mold. It can be necessary for this purpose to invert the mold on a table. How the cake was released was recorded. Also the amount of residue still present was recorded. Then the baking mold was cleaned by hand with dishwashing detergent and air-dried. Cleanability was assessed and recorded.

[0069] A total of 10 cycles were baked, with a cleaning in the dishwashing machine after the 1st, 3rd, 5th, 7th and 9th cycle. After each cycle, a photo of the circular mold was taken after the cake was released.

[0070] When evaluating the release of the cake and the cleanability of the circular mold after an individual cycle, a maximum of 10 points can be achieved per cycle and a maximum of 100 points after 10 cycles. The release of the cake is weighted with a factor of 0.6 and the cleanability of the circular mold with a factor of 0.4.

[0071] The release of the cake was evaluated as follows:

[0072] No sticking, cake very easy to release, 0-5% residue, 10 points

[0073] Occasionally sticking, cake easy to release, 5-20% residue, 7.5 points

[0074] Strong sticking, cake allowed to release, 20-40% residue, 5.0 points

[0075] Strong sticking, cake only released with a spatula, 2.5 points

[0076] The cleanability of the round moulds was evaluated as follows:

[0077] No residue, 10 points

[0078] Light (light pressure), 7.5 points

[0079] Medium (medium pressure), 5.0 points

[0080] Sponge needed for cleaning, 2.5 points

[0081] Table 1 shows the evaluation of the individual cycles of the combined bake and dishwashing test of the examples and comparative examples according to the present application investigated.

[0082] As shown in further experiments, the presence of at least 2.5 wt.-% silicone oil (based on the weight of the baked-on coating) is necessary to ensure the deep drawability of the anti-stick coating according to the present application.

[0083] Table 1 also shows that even without the additional presence of silicone resin in the coating (Examples 3 and 4), good results can be achieved on average in the combined bake and dishwashing test, which are comparable to the two comparative examples 1 and 2 of the prior art.

[0084] By adding silicone resin to the coating according to the present application, the results of the combined bake and dishwashing test can be significantly improved.

[0085] Table 1

[0086]

Claims

1. Release coating for an article, which is suitable for being applied on the surface of an article by a web coating process and which has a flexibility suitable for a downstream deep-drawing process, wherein the release coating has at least one base layer (G) which is baked onto the surface of the article (O) and a cover layer (D) which is baked on the base layer, wherein the base layer has a dry film thickness of 1 to 4 pm, and wherein the cover layer has a dry film thickness of 1 to 4 pm, wherein the base layer consists of 10 to 100 wt.-%, based on the weight of the baked base layer, of a thermoplastic resin which has a temperature resistance of more than 200°C; optionally at least one additive selected from the group consisting of pigments, fillers and metal particles, wherein the cover layer consists of at least 30 wt.-%, based on the weight of the baked cover layer, of a thermoplastic resin which has a temperature resistance of more than 200°C; at least 2.5 wt.-%, based on the weight of the baked cover layer, of a silicone oil; and optionally a silicone resin; and optionally at least one additive selected from the group consisting of pigments, fillers and metal particles, and wherein neither the base layer nor the cover layer contain perfluoro- and polyfluoroalkyl compounds, And wherein, the thermoplastic resin contained in the base layer and in the cover layer is independently selected from the group consisting of polyether sulfone (PES), polyphenyl sulfone (PPSU), liquid crystalline polymer (LCP), polyaryletherketone, polyetherketone (PEK), polyether ether ketone (PEEK), polyether ketone ketone (PEKK), polyphenylene sulfide (PPS) and mixtures thereof.

2. The release coating according to claim 1, characterized in that, The silicone oil contained in the cover layer has a kinematic viscosity at 20°C of at least 100 mm 2 s -1 .

3. The release coating according to claim 1, wherein The cover layer additionally contains 1 to 67.5 wt.-%, based on the weight of the baked cover layer, of a silicone resin.

4. The release 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 methylphenyl silicone resin.

5. The release coating according to claim 1, wherein The thermoplastic resin contained in the base layer and in the cover layer is a polyether sulfone (PES).

6. The release coating according to claim 1, wherein The dry film thickness of the base layer is 3 to 4 pm and the dry film thickness of the cover layer is 3 to 4 pm.

7. The release coating according to claim 1, wherein The cover layer contains a different thermoplastic resin than the base layer.

8. Process for applying the release coating according to one of claims 1 to 7, characterized in that, The base layer to be formed is applied as a liquid paint to the surface of the article, then dried at 250 to 440°C, then the cover layer is applied as a liquid paint to the pre-dried base layer and the layers are connected to each other and to the surface of the article by subsequent heat treatment at 250 to 440°C.

9. The method of claim 8, wherein, The paint used to form the base layer or the cover layer contains the thermoplastic resin in the form of a dispersion.

10. The method of claim 8, wherein, The paint used to form the base layer or the cover layer contains the thermoplastic resin in dissolved form.

11. The method according to one of claims 8 to 10, characterized in that, The surface of the article is metallic.

12. The method according to one of claims 8 to 10, characterized in that, The paint used to form the base layer and the cover layer is applied by a web coating process.

13. The method according to one of claims 8 to 10, characterized in that, The release coating is formed after baking.

14. Coated article having a surface provided with a release coating according to one of claims 1 to 7.

15. The article of claim 14, wherein, The article is a baking mold or other household item. The article is a baking mold or other household item.

Images