Technical Field

[0001] The invention relates to a geopolymer suspension containing nanoparticles for heat-resistant coatings and a method of the production thereof.

Art

[0002] Inorganic coatings on metallic materials are prepared by a number of physical, electrolytic, or chemical methods. While the hot- or plasma-deposition technologies are highly effective as a top protective layer, the application thereof is highly energy-intensive, and some materials may suffer structural effects due to heating the material during application; therefore, they are not suitable for such materials. In addition, there are sophisticated techniques for coating formation by diffusion processes or vapor-phase deposition of layers. The most common electrolytic methods include the galvanization process. Various technologies of applying aqueous suspensions followed by firing are used for common surface treatments of metallic materials with coatings and enamels.

[0003] The obvious disadvantages of all the above processes include technology- and, especially, energy-intensity. The technological intensity is due to the need for expensive equipment, and the energy intensity is reflected in the price of products, where the profitability of production usually requires large-scale series production.

[0004] The simplest and cheapest method to prepare coatings in a limited range of applications consists of applying them directly by painting or spraying suspensions. Using this method, the organic coatings are mainly prepared; however, they have limited temperature stability and a very short expiry time. However, production and subsequent disposal thereof are highly environmentally and, therefore, financially demanding. It is necessary to take care of protecting the health of persons both during production and especially during application. Inorganic coatings are prepared in this simple method for aesthetic and corrosion protection purposes especially. Inorganic coatings for high- temperature applications are usually prepared on a silicate or silicone base. Siloxane -based silicone paints are easy to apply with typical applications up to a maximum of 400 to 500 °C; however, their cost is high compared to other inorganic coatings. of the Invention [0005] The above shortcomings are largely eliminated by the geopolymer suspension containing nanoparticles for heat-resistant coatings according to the present invention. The summary of the invention is that the suspension contains 20-25% wt. of an aqueous solution of 80-90% wt. phosphoric acid, 25-25% wt. of aluminosilicate, 4, 6-5, 2% wt. of graphite, 5-7% wt. of alumina nanoparticles of a size of up to 90pm, and the remainder is isopropyl alcohol.

[0006] Preferably, the aluminosilicate is washed kaolin and/or metakaolin. In a preferred embodiment, the ratio of the amount of phosphoric acid to the amount of aluminosilicate is 1.1 to 1.35.

[0007] A further object of the invention is a method of preparing geopolymer suspension according to the present invention. The method consists in that the aqueous solution of 80 to 90% phosphoric acid is added to 100 ml of isopropyl alcohol, and aluminosilicate is gradually added while stirring at room temperature for 8 to 15 minutes; and after 9 to 15 minutes, graphite as an additive is added at once, and the resulting mixture is homogenized for another 18 to 25 minutes.

[0008] Thus, the geopolymer suspension consists of an acidic aluminosilicate base matrix (aqueous solution with H ⁺ ), phosphoric acid solution (H3PO4), and a small amount of graphite (additive) and isopropyl alcohol (CsHgO). Then, this mixture is additionally modified with alumina nanoparticles (AI2O3).

[0009] Aqueous suspensions of a mixture of phosphoric acid represent a cheap, environmentally friendly and very simple surface protection application suitable for preparing functional coatings on metallic materials. A wide range of additives can be easily dispersed into the suspensions, providing the coatings with variability in the properties thereof. This flexibility in coating formulation gives the resulting composites new functional properties, particularly in the fields of adhesion and tribology, as well as allows for a broader range of applications. For example, in this case, the alumina nanoparticles in the coatings significantly reduce friction compared to the uncoated surface, increase thermal stability, and reduce wear and tear. The suspension exhibits excellent adhesion to metallic materials and can be applied by painting or spraying without the need for firing at high temperatures. However, after application to the surface, the geopolymer needs to be dried at a specific temperature - 180 to 200 °C, the so-called setting, when the emulsion is thermally stabilized. After setting, the resulting surface is maintenance-free, and the stabilized emulsion can only be removed mechanically.

[0010] The coating stabilized in this way can be used for long-term thermal protection at 500 °C for at least 50 hours and 800 °C for at least 60 minutes. The suspensions are environmentally friendly, long-term stable and affordable. In particular, the coatings on metal substrates allow surface modifications of the resulting surfaces' sliding, tribological and thermal properties, thus expanding their functional, i.e. application possibilities.

[0011] The great advantage of geopolymer emulsion is the long shelf life thereof, which is up to 10-times longer compared to other emulsions. In the case of suspension, there is a small degree of partial separation of the components, but the subsequent homogenization of the mixture, i.e. mixing, is very easy. There is no degradation of the individual components in the mixture, even when stored at room temperature. The guaranteed expiration time is at least 6 months at room temperature when no significant degradation of the mixture has occurred; the total expiration time can be assumed to be much longer. The expiration time of geopolymer suspensions can be further extended through suitable storage at a low temperature of about 4 °C or by forming individual components mixed into the final mixture just before application.

[0012] The great advantage of this suspension consists in the possibility of application on the surface, where the surface does not need to be chemically or mechanically pre-treated, i.e. degreased, soaked, blasted, preheated, etc. Isopropyl alcohol, which is a part of the emulsion, acts as an effective degreaser, and phosphoric acid causes a slight etching of the surface and better adhesion of the coating to the underlying substrate. However, surface pretreatment, e.g. by degreasing using an organic solvent, will positively affect the final surface properties such as adhesion, appearance, etc.

[0013] Before application, the emulsion must be well homogenized by mixing or shaking. In addition, the suspension must be spread thoroughly over the surface when applied by painting, e.g. with a brush, to achieve a very thin layer. It is very preferable as it reduces consumption compared to other types of coatings, such as organic coatings in particular. It has been verified that even with a layer thickness of 40 pm, no surface degradation occurs. The average layer thickness varies between 5 and 20 pm, depending on the application technique during coating. Compared to other common types of coatings, the thickness of the resulting layer is about 10-times thinner. Typically, the organic coatings reach thicknesses of 100 to 200 pm.

[0014] The options for application to the material surface are the same as for conventional organic compounds, i.e. painting with conventional brushes, air spraying with a spray gun or atomizer, etc. After setting, the suspension shows no discoloration and is transparent. The application by spraying will further enhance the aesthetics of the resulting visual surface, but the application by painting achieves perfect aesthetic results of the final surface as well, and such a surface can be used as a visual surface. Another advantage consists of an easy application method by painting. For visual application, the suspension can be further added with color pigments.

[0015] The following advantages include the following facts:

[0016] Preferable composition of the resulting geopolymer suspension. Thermal protection of the machine parts surface and the like up to 800 °C for up to 1 hour. Here, a low layer thickness of 5 to 10 pm can be preferably used, which does not significantly affect the final dimensions of the component. The thermal protection can be further improved by layering individual layers or by the appropriate choice of additives added to the suspension to create a coating with application-specific properties. Long-term thermal protection at 500 °C for at least 50 hours and 800 °C for at least 60 minutes. Reduction in friction and increase in abrasion resistance of machine parts, injection molds, press molds, etc. Fire protection of construction materials in the building industry, e.g. steel beams, etc., in emergency scenarios or machine components up to 800 °C for 0.5 to 1 hour. Improvement of the overall mechanical properties of machine components. The suspension is environmentally friendly, long-term stable for storage - up to 10-times longer than other emulsions, and usable for application. The guaranteed expiration time is at least 6 months at room temperature.

[0017] Geopolymer suspension containing nanoparticles for heat-resistant coatings The geopolymer suspension containing nanoparticles for heat-resistant coatings according to the present invention will be described in more detail using specific example embodiments and the accompanying drawings. The surface can be seen in Fig. 1 at 200x magnification and in detail at 500x magnification in Fig. 2. Fig. 3 shows metallographic cut of the base material without coating in section at 200x magnification; Fig. 4 shows the detail at 500x magnification. The analysis of the suspension surface is shown in Fig. 5 at various magnifications. Fig. 6 shows a detail of a section of the geopolymer layer at 500x magnification. Fig. 7 shows the SEM analysis of the layer; the image on the left shows a detail of each layer, and a similar image with the measured layer thickness is shown on the right. Fig. 8 shows a detail of a thick layer at 500x magnification. The SEM analysis of Fig. 9 confirms the very good bonding between the layer and the underlying substrate. Subsequent linear EDS analysis confirmed the composition of the layers, see Fig. 10. The geopolymer layer can be confirmed mainly by the presence of aluminum and phosphorus. The laser scanning microscope images in Fig. 11 show the mark left by a ball after the Tribolab test. Fig. 12 shows the surface of the sample with a thin layer of geopolymer suspension after thermal loading. 3D scan of the surface is shown in Fig. 13 on the right and detail of the layer section is in Fig. 14. Fig. 15 shows a surface with a thick layer of suspension, and Fig. 16 on the right shows a 3D scan of the surface. Fig. 17 shows the layer section, where the still perfect adhesion of the layer to the underlying substrate is possible to be verified. Fig. 18 shows the surface of the sample with a thin layer of suspension after thermal loading. Fig. 19 on the right represents a 3D scan of the surface. Fig. 20, then, represents the detail of the layer in the section. Fig. 21 shows a surface with a thick layer of suspension; and Fig. 22 on the right represents a 3D scan of the surface. The integrity and adhesion of the layer can also be observed in the detail of the layer section of Fig. 23. Fig. 24 shows the surface of the sample with a thin geopolymer layer, where it can be seen that the layer integrity was not compromised even at very high burner performance and high temperature. Fig. 25 shows the surface of the sample with a thick layer. Fig. 26 shows the detail of the layer section and the 3D scan on the right. Fig. 27 represents the reference sample without coating. Fig. 28 represents a sample with a thin geopolymer layer. Fig. 29 shows a sample with a thick suspension layer. Fig. 30 represents the reference sample after 60 minutes of heat load. Fig. 31 represents a sample with a thin geopolymer layer. Fig. 32 represents a sample with a thick geopolymer layer. Fig. 33 represents a coating after a very long period of heat load at a high temperature. amples of the Invention Embodiments

[0018] Temperature and time stability tests of the suspension, i.e. workability, suspension application and surface treatment, surface analysis of the applied layer, temperature tests, fire tests, and tribometry measurements, are also described. [0019] Two temperature levels were tested to determine the suspension stability and shelf life under normal conditions:

4 °C - refrigerator

22 °C - typical room temperature

[0020] The samples were evaluated after 1, 30, 90 and 180 days. The workability and properties when applied to the underlying substrate 4 were verified for each sample.

[0021] 1) Age of emulsion - 1 day

[0022] The emulsion color is bright white; it has a higher viscosity in the form of a gel and contains no sediments. Application to the underlying substrate 4 is very easy, it spreads well, and a very thin layer can be formed. The contained isopropyl alcohol is very volatile and causes very fast drying of the emulsion within about 5 minutes in combination with a thin layer.

[0023] 2) Age of emulsion - 30 days, i.e. 1 month

[0024] Storage at 4°C

[0025] The emulsion color is bright white; it has the same, i.e. higher viscosity, and contains no sediments. There was a separation of the clear liquid component, which is visible on the surface - about 1/6 of the total volume. The emulsion can be very easily mixed, e.g. Using a glass rod, back to a homogeneous consistency. Application to the underlying substrate 4 is very easy.

[0026] Storage at 22°C

[0027] When the emulsion was stored at room temperature, no visible changes occurred after 30 days compared to storage in the refrigerator, see Fig. 3. The mixing and application are also identical to the sample from the refrigerator.

[0028] 3) Age of emulsion - 90 days, i.e. Approximately 3 months

[0029] Storage at 4°C

[0030] There were no visible changes with the emulsion age compared to previous samples. The emulsion color did not change and remains bright white; it has a higher viscosity and contains no sediments. Again, a separated clear liquid component is visible on the surface - about 1/6 of the total volume. The emulsion can be very easily mixed back to a homogeneous consistency. The application on the underlying substrate 4 is very easy, and, even in this case, the required thin layer can be formed. Storage at 22°C.

[0031] Again, there was no significant change in the emulsion appearance. A separated clear component on the surface is also visible here. Furthermore, a thin layer of sediment appeared at the container bottom. Homogenization and application of the mixture remain easy.

[0032] 4) Age of emulsion - 180 days, i.e. approximately 6 months

[0033] Storage at 4°C

[0034] The emulsion appearance is the same as in the previous cases. There is a separated component that can be very easily mixed into a homogeneous mixture as well. There are no sediments on the bottom, and the viscosity is the same, i.e. a gel. Application and thin layer formation continue to be excellent. Storage at 22°C.

[0035] After 150 days of storage at room temperature, there are visible changes in the emulsion appearance. The separated clear component has increased to about 1/4 to 1/3 of the volume. Again, a thin layer of sediment having a character of very fine sand is visible on the bottom. However, mixing is easy here as well as the subsequent application on the underlaying substrate 4, which can be performed in the desired thickness.

[0036] The suspension expiration is at least 6 months, even when stored at room temperature (22°C). After 6 months of storage in the refrigerator, the samples showed no changes in color, consistency or subsequent application by coating compared to the original freshly prepared mixture. The emulsion stored in this way can be assumed to have a much longer expiration time without changing the resulting properties.

[0037] Moderate changes were observed in the samples stored at 4°C after 6 months - a higher separation of the components by about 20%, thin sediment at the container bottom, but after homogenization of the mixture, which was carried out without any problems, the subsequent application on the underlaying substrate 4 was very easy and did not differ from the application of a freshly mixed sample. Even when stored at room temperature, an expiration time much longer than 6 months can be expected. During the first 6 months, the storage temperature does not affect the final properties of the mixture when applied to the underlaying substrate 4. The emulsion can be stored at room temperature without any problems, which results in a very long expiration time.

[0038] Underlaying substrate 4

[0039] Common structural steel CSN 11375 according to CSN 41 1375 was used as the underlying substrate 4. It is a hot-rolled, non-alloy structural steel suitable for welding, fusion-welded medium- thickness structural and machine components, and is subjected to static and dynamic stresses. The equivalent designation is S235JRG2. This steel was also chosen because of the possible later application thereof as a fire protection for steel beams and fittings in buildings and other infrastructure.

[0040] Surface treatment

[0041] The coating was applied to a mechanically untreated surface, leaving a natural oxidation layer on the surface. This method was chosen to verify the possibility of application on an untreated surface and, therefore, to increase the economic efficiency of the application since mechanical or chemical surface treatments are very time-consuming and costly. Coarse dirt was removed from the substrate surface; the substrate surface was degreased with a common organic solvent, usually acetone. Unless it is heavily soiled with grease, the surface has been proven not to be degreased with organic or other solvents necessarily, as the isopropyl alcohol present in the mixture is able to degrease the surface very well during the application of the suspension itself, which contributes to the economic efficiency of the operation. Mechanical or chemical pretreatment of the surface will further improve the mechanical properties of the coating, i.e. coating adhesion, appearance, etc. In this case, it depends on the final specific usage of the surface/component.

[0042] Application of suspension

[0043] Prior to applying the emulsion, good homogenization of the mixture by mixing, shaking, etc., is necessary to combine any separate components. The suspension is applied with a brush. The suspension is thoroughly spread over the surface by repeated brush strokes to form a uniform thin geopolymer layer on the substrate surface; it is a good idea to dip the brush only a little to avoid forming too thick a layer and later cracking during setting. The difference between clean and coated substrates is hardly noticeable due to the transparency of the geopolymer suspension. The suspension takes about 5 minutes to dry at normal room temperature; however, drying is not a requirement, and the coating can be stabilized immediately after application.

[0044] Suspension setting

[0045] To obtain the final properties of the coating, the suspension is necessary to be thermally stabilized on the substrate surface. The setting takes place for a certain period of time at a given temperature according to the scheme:

[0046] 1) Preheat the dryer/oven to 30°C.

[0047] 2) Insert the coated samples/components into the preheated dryer.

[0048] 3) Increase the temperature to 170°C. The temperature gradient, i.e. the temperature rising rate, must be 5 °C/min as a maximum. During heating to the final temperature, water evaporates from the suspension, and chemical reactions take place, leading to the geopolymerization of the layer and obtaining the final properties. If the temperature rise is too high, the water will not evaporate sufficiently, and the subsequent residual water will interfere with the geopolymerization at a later stage and may lead to deterioration, e.g. lower adhesion, cracking, foaming, of the resulting coating and, thus, to the impaired functionality. Therefore, it is necessary to place the coated components in a slightly preheated oven, and it is not possible to place the components in an oven already heated to a final temperature of 170°C.

[0049] 4) Hold at 170°C for 2 hours.

[0050] 5) Remove the components from the oven, wherein the samples are possible to let cool in the dryer, but this is not a requirement.

[0051] 6) A stabilized geopolymerized coating formed on the component surface.

The layer is insoluble and can only be removed mechanically.

[0052] Due to the transparency of the coating, the presence of the coating is hardly visible, wherein fine brush strokes can be seen.

[0053] To create a multi-layer coating, the setting process must be repeated for each individual layer.

[0054] A LEXT OLS 500 laser microscope and a Tescan Vega 3 scanning electron microscope with an SEM analyzer were used for the detailed analysis of surfaces and layers. Macroscopically, the resulting set layer is compact, without visible defects or cracks, perfectly follows the underlying substrate 4 and is hardly visible due to minimal thickness and transparency thereof.

[0055] Substrate surface analysis 4

[0056] The surface of the underlying substrate 4 is mechanically untreated having irregularities after rolling. The surface can be seen in Fig. 1 at 200x magnification and in detail at 500x magnification in Fig. 2.

[0057] Substrate section analysis

[0058] Fig. 3 shows metallographic cut of the base material without coating in section at 200x magnification. An oxide layer j_, which reaches a thickness of 10 to 20 pm on average, can be seen on the material surface. In detail at 500x magnification, Fig. 4 on the left shows the oxide layer 1 thickness and the corrosion 2 of the underlying substrate 4, which is not in the whole sample area but only in particular points to compare the suspension adhesion to the oxidized and partially corroded surfaces. Fig. 4 on the right shows a detail and thickness of the oxide layer from SEM analysis performed on the electron scanning microscope.

[0059] Layer surface analysis

[0060] The SEM analysis of the suspension surface is shown in Fig. 5 at various magnifications. The resulting layer is visibly whole without damage, flakes, blisters or cracks and perfectly follows the underlying surface.

[0061] Layer section analysis

[0062] For subsequent fire tests and analyses, two different thicknesses of the geopolymer layer 3 were created.

[0063] Thin layer, i.e. layer thickness 5 to 10 pm

[0064] Fig. 6 on the left shows detail of a section of the geopolymer layer 2 at 500x magnification. It can be observed that the layer is compact and perfectly adhered to the underlying substrate. There is the measured layer thickness on the right, which averages 6.8 pm in the drawing. Generally, the thin layer thickness is between 5 pm and 10 pm, in the case of the thin layer. [0065] Fig. 7 shows the SEM analysis of the layer. The drawing on the left shows a detail of each layer - the underlying steel substrate, the oxide layer 1 and the geopolymer layer 3, where the layer structure and its perfect adhesion to the surface can be seen. There is a similar drawing on the right with the measured layer thickness.

[0066] Thick layer, i.e. layer thickness 10 to 30 pm

[0067] In the next drawing, Fig. 8 shows a detail of a thick layer at 500x magnification. Again, good layer adhesion to the substrate and a layer thickness of 26.2 pm can be observed. Generally, the thick layer thickness ranges from 10 pm to 30 pm.

[0068] The SEM analysis of Fig. 9 confirms the very good bonding between the layer and the underlying substrate.

[0069] Subsequent linear EDS analysis confirmed the composition of the layers, see Fig. 10. The geopolymer layer 3 can be confirmed mainly by the presence of aluminum and phosphorus.

[0070] Tribology

[0071] Tribological measurements were performed on the UMT TriboLab from Bruker. The geopolymer was applied to pre-prepared test bodies in the form of a steel disc pre-treated according to the device requirements. A disc without geopolymer coating was chosen as a reference sample for comparison.

[0072] Measurement conditions

[0073] Measuring method: ball on disc

[0074] Total measurement time: 10 minutes

[0075] Speed of the disc rotation: 10 RPM ¹

[0076] Peripheral speed: 0.014 m.s ^-1

[0077] Total measured distance: 8.2 m

[0078] Load: 10 N

[0079] Temperature: 22°C [0080] Measurement results

[0081] Pure steel

[0082] Coefficient of friction: 0.1009 [0083] Standard deviation: 0.0063

[0084] Geopolymer suspension - thin layer

[0085] Coefficient of friction: 0.0482

[0086] Standard deviation: 0.0124 [0087] The friction coefficient course can be seen in Graph 1, where the blue curve indicates the clean, uncoated steel and the black curve is the measured course for the geopolymer suspension coated steel.

[0088] Graph 1 [0089] From the measured values of the friction coefficient, it can be seen that the coated steel has more than 2 times a lower friction coefficient than the clean steel without coating. The laser scanning microscope images in Fig. 11 show the mark left by a ball after the Tribolab test. The width thereof is about 250 pm, and, as can be seen from the images, it is very little noticeable, indicating that there is almost no damage to the coating and no damage to the coating on the substrate, which also indicates resistance to frictional wear.

[0090] The measured friction coefficient values, which are more than 2 times lower for the coated component than for the uncoated component, predetermine the coating for use in applications requiring reduced friction and increased wear resistance, such as moving machine parts. The significant reduction in friction and increase in abrasion resistance for the coating is due to the aluminum oxide nanoparticles added to the compound as an additive.

[0091] Open- flame fire tests were performed on both thin and thick layers to compare them with each other as well as on an uncoated reference sample. A propane -butane burner was used as the fire source. The flame was directed at some distance to the side with the geopolymer coating, and the temperature was measured on the other side, i.e., a K-type measuring thermocouple. The burner performance and flame temperature were controlled according to the specific testing requirement.

[0092] a) Open-flame testing

[0093] Several analyses were performed for different temperatures and performances to determine the heat and fire resistance of the coating:

[0094] 1) Burner performance: low

[0095] Distance between burner and sample: 8 cm; test time: 4 minutes; achieved temperature on the uncoated side: 530°C

[0096] 2) Burner performance: medium

[0097] Distance between burner and sample: 8 cm; test time: 6 minutes; achieved temperature on the uncoated side: 620°C

[0098] 3) Burner performance: high

[0099] Distance between burner and sample: 8 cm; test time: 15 minutes; achieved temperature on the uncoated side: 800°C

[0100] 4) Burner performance: medium

[0101] Distance between burner and sample: 4 cm; test time: 30 minutes; achieved temperature on the uncoated side: 850°C [0102] 5) Burner performance: medium

[0103] Distance between burner and sample: 4 cm; test time: 60 minutes; achieved temperature on the uncoated side: 850°C

[0104] b) Long-term heat load

[0105] As a supplementary test, a long-term heat load in a hardening furnace under the following conditions was chosen: Test time: 48 hours; temperature: 500°C

[0106] 1) Open-flame testing

[0107] B umer performance : low

[0108] Distance between burner and sample: 8 cm; test time: 4 minutes; achieved temperature on the uncoated side: 530°C. The test was chosen to analyze the heat transfer through the geopolymer layer 3 and the steel substrate at a lower heat capacity, i.e. lower thermal load. The burner flame was pointed directly to the coated side, and the temperature reached on the other side of the sample was recorded as a function of time using a thermocouple. As can be seen from the graph, the average temperature reached was lower for the coated sample than for the reference uncoated sample. The resulting temperature reached after 4 minutes was about 530°C for the reference sample. The sample with a thin geopolymer layer reached a temperature of about 527°C. An interesting result was also obtained for the sample having a thicker suspension layer, which retarded the achieved temperature better. The temperature reached after 4 minutes is about 513°C, which is a reduction of 17°C compared to the reference sample. A summary of the average results obtained is shown in Graph 2.

[0109] Graph 2

[0110] Fig. 12 shows the surface of the sample with a thin layer of geopolymer suspension after thermal loading. As can be seen, there was no damage to the layer, flaking or scratching, as evidenced by the 3D scan of the surface in Fig. 13 on the right and detail of the layer section is in Fig. 14. [0111] Fig. 15 shows a surface with a thick layer of suspension, and Fig. 16 on the right shows a 3D scan of the surface. Again, it can be observed that there was no damage to the layer adhesion to the substrate. Fig. 17 shows the layer section, where the still perfect adhesion of the layer to the underlying substrate is possible to be verified. [0112] 1) Burner performance: medium

[0113] Distance between burner and sample: 8 cm

[0114] Test time: 6 minutes

[0115] Achieved temperature on the uncoated side: 620°C

[0116] Applications: heat transfer analysis, basic insulation properties of the coating [0117] As in the previous case, the test was chosen to analyze the heat transfer through the geopolymer layer 3 and the steel substrate at a higher heat load, i.e. higher burner performance.

[0118] A summary of the average results obtained is shown in Graph 3.

[0119] Graph 3

[0120] As can be seen from the graph, the average temperature reached was again lower for the coated sample than for the uncoated reference sample. The resulting temperature reached after 6 minutes was about 619°C for the reference sample. The sample with a thin layer of geopolymer reached a temperature of about 616°C, and the sample with a thicker suspension layer then reached about 614°C, being a temperature reduction by 5°C compared to the reference sample.

[0121] Fig. 18 shows the surface of the sample with a thin layer of suspension after thermal loading. Even at higher heat load and higher temperature, no damage to the layer occurred. Fig. 19 on the right represents a 3D scan of the surface. Fig. 20 represents then the detail of the layer in the section; it can be observed that there was no damage to the layer adhesion to the substrate even in this case.

[0122] Fig. 21 shows a surface with a thick layer of suspension; and Fig. 22 on the right represents a 3D scan of the surface. Again, the layer is not damaged by the direct flame of higher temperature. The integrity and adhesion of the layer can also be observed in the detail of the layer section of Fig. 23.

[0123] 2) Burner performance: high

[0124] Distance between burner and sample: 8 cm

[0125] Test time: 15 minutes

[0126] Achieved temperature on the uncoated side: 800°C

[0127] Applications: fire resistance test

[0128] For the next test, high burner performance was chosen to keep the temperature on the back uncoated side to a minimum of 800°C for 15 minutes for fire resistance testing.

[0129] Fig. 24 shows the surface of the sample with a thin geopolymer layer, and it can be seen that the layer integrity was not compromised even at very high burner performance and high temperature.

[0130] Fig. 25 shows the sample surface having a thick layer, and, as in the previous case, it can be seen that the coating withstood the heat load and no flaking or cracking has occurred, which is demonstrated in detail in the next figure 26 along with the detail of the layer in section and the 3D scan on the right.

[0131] The following tests were chosen to determine the long-term heat resistance to open flame. The distance of the burner from the coating was shorted, thus increasing the flame temperature acting on the coating. The testing time has been increased to 30 minutes and 60 minutes as some fire applications require longer protection times.

[0132] 3) Burner performance: medium

[0133] Distance between burner and sample: 4 cm

[0134] Test time: 30 minutes

[0135] Achieved temperature on the uncoated side: 850°C

[0136] Application: long-term fire resistance test, i.e. higher temperature

[0137] Fig. 27, where the reference sample is uncoated, a highly affected area is visible.

[0138] Next Fig. 28 shows a sample with a thin geopolymer layer. There was no damage to the layer by cracking or flaking; the slight damage in the right part of the sample is due to dirt on the substrate surface during suspension application or the layer evaporation, and the layer is still visible on the sample, wherein fine lines caused by the brush during painting can be observed.

[0139] Fig. 29 shows a sample with a thick suspension layer. The thermally affected area is noticeable. There was no damage to the layer, and it is still clearly visible on the sample - the directed structure after the coating.

[0140] B umer performance : medium

[0141] Distance between burner and sample: 4 cm

[0142] Test time: 60 minutes

[0143] Achieved temperature on the uncoated side: 850°C

[0144] Application: long-term fire resistance test, i.e. higher temperature

[0145] The reference sample after 60 minutes of the heat load is shown in Fig. 30. [0146] Fig. 31 shows a sample with a thin geopolymer layer. Even after 60 minutes, the geopolymer coating was not damaged, and the coating still showed excellent adhesion to the underlying substrate.

[0147] For the sample having a thick geopolymer layer of Fig. 32, the situation is similar and no visible damage can be observed. The layer is still obvious on the sample with excellent adhesion to the substrate surface.

[0148] Fong-term open-flame heat load tests for 30 minutes and 60 minutes confirmed excellent open-flame resistance, wherein no visible destruction of the coating occurred even after 60 minutes. The coating is not cracked, nor has there been any flaking or peeling the coating. Furthermore, even after 60 minutes of exposure to open flame, the coating does not show any loss of adhesion to the underlying substrate. Therefore, the time required to destroy the coating by open flame can be assumed to be many times longer; moreover, a higher temperature would be required for this.

[0149] b) Fong-term heat load

[0150] Test time: 48 hours

[0151] Temperature: 500°C

[0152] Application: long-term heat load resistance test

[0153] The test was carried out in a hardening furnace, where the coated samples having a thin geopolymer layer were heated to 500°C within 2 hours, followed by holding at this temperature for 48 hours. As can be seen from Fig. 33, the coating withstood even a very long period of heat load at high temperature. The coating surface became matted to a certain degree; however, the integrity and adhesion remained unchanged. No cracks or flaking of the coating can be observed.

Industrial

[0154] Surface -treated composites can be used to reduce steel surface adhesion in high- temperature applications, heat and corrosion protection of metal structures against temperature in construction and engineering, etc.

Reference Signs Fist

[0155]