Water-based radiation refrigeration coating, preparation method thereof, composite material and application

Abstract

The invention discloses a water-based radiation refrigeration coating as well as a preparation method, a composite material and application thereof. The coating comprises the following components in percentage by mass: 59.5%-70.4% of organic silicon modified styrene-acrylic polymer emulsion, 2.8%-6.8% of rutile titanium dioxide, 4.8%-5.6% of hydrophilic silicon dioxide and 21.1%-30.8% of a solvent. The coating disclosed by the invention is prepared by adopting a pre-dispersion-gradient charging process, and has excellent water whitening resistance and low water absorption. The deionized water content in the system can be adjusted according to different base material types; when the system is applied to color steel tiles and other hard base materials, the high deionized water content is kept to meet the spraying construction requirement; when applied to flexible substrates such as fabrics and the like, the system viscosity and the fabric surface adhesion performance are improved by properly reducing the content of deionized water. The coating can be sprayed on the surface of a color steel tile or coated on the surface of a fabric to form a radiation refrigeration coating, so that the application of building heat management and personal heat management is realized.

Description

Technical Field

[0001] This invention relates to a cooling coating, its preparation method, composite materials, and applications, and more particularly to a water-based radiation cooling coating, its preparation method, composite materials, and applications. Background Technology

[0002] The continued growth in global energy consumption has led to excessive carbon emissions and exacerbated air pollution, while frequent extreme heat waves have increased the global demand for cooling energy. Passive radiative cooling technology, as an effective and renewable solution, can achieve cooling without any external energy input and has been widely used in energy-efficient buildings, photovoltaic cooling, energy harvesting, and personal thermal management. Radiative cooling technology achieves zero-energy cooling by reflecting visible and near-infrared light from sunlight while utilizing atmospheric windows (8μm-13μm wavelength) to emit heat into outer space as mid-infrared radiation.

[0003] Existing radiative cooling materials are mostly concentrated in solid film materials or solvent-based coatings, which suffer from problems such as complex preparation processes, high emissions of volatile organic compounds, and poor adhesion to substrates. Waterborne coatings have become a research hotspot due to their advantages such as environmental friendliness, safety, and low volatility. However, traditional waterborne acrylic emulsions have drawbacks such as water bleaching, high water absorption, and APEO content that does not meet environmental protection requirements, which limit their application in outdoor radiative cooling. When coatings are exposed to rain and humidity in outdoor environments for extended periods, they are prone to bleaching after absorbing water, leading to decreased reflectivity and reduced radiative cooling effect.

[0004] In addition, existing radiation cooling coatings still need improvement in terms of dispersion stability of functional fillers such as titanium dioxide, coating reflectivity to the solar spectrum, and adhesion to the substrate. Summary of the Invention

[0005] Objective of the invention: The objective of this invention is to provide a water-based radiation cooling coating with excellent radiation cooling effect, good water whitening resistance and low water absorption; the second objective of this invention is to provide a method for preparing the above-mentioned water-based radiation cooling coating.

[0006] A third object of the present invention is to provide a composite material comprising the above-described aqueous radiation cooling coating;

[0007] A fourth objective of this invention is to provide the application of the above-mentioned waterborne radiation cooling coating.

[0008] Technical solution: The water-based radiation cooling coating of the present invention contains the following components by mass percentage: 59.5%-70.4% organosilicon-modified styrene-acrylic polymer emulsion, 2.8%-6.8% rutile titanium dioxide, 4.8%-5.6% hydrophilic silica, and 21.1%-30.8% solvent.

[0009] The silicone-modified styrene-acrylic polymer emulsion is water-resistant, has low water absorption, and is APEO-free. The silicone-modified styrene-acrylic polymer emulsion has a glass transition temperature of 25°C and a solid content of 29-31 wt%. The introduction of silicone segments endows the emulsion with excellent hydrophobicity and weather resistance, while maintaining good adhesion to the substrate.

[0010] The rutile titanium dioxide has a particle size of 0.15-0.25 μm. When applied to rigid substrates such as corrugated steel sheets, the rutile titanium dioxide accounts for 2.5%-6.0% of the coating by mass. When applied to flexible substrates such as fabrics, the rutile titanium dioxide accounts for 2.8%-6.7% of the coating by mass. Different titanium dioxide contents in the coating impart different solar reflectivity and radiative cooling effects to the coating.

[0011] The hydrophilic silica has a particle size of 4.8-5.2 μm. The hydrophilic silica is precipitated silica. The hydrophilic silica is uniformly distributed in the coating, and its Si-O-Si bond vibration absorption peak matches the atmospheric window wavelength, effectively enhancing mid-infrared emissivity.

[0012] The solvent is deionized water, and its content is adjusted according to the type of substrate. When the substrate is a rigid substrate such as color steel tile, the deionized water content is 29.8%-30.8%; when the substrate is a flexible substrate such as fabric, the deionized water content is 20.3%-21.1%, in order to obtain a suitable coating viscosity and reduce the penetration of coating into the fiber.

[0013] The preparation method of the above-mentioned water-based radiation cooling coating includes the following steps:

[0014] (1) Rutile titanium dioxide was added to a solvent for pre-dispersion to obtain a finishing solution;

[0015] (2) Add the organosilicon-modified styrene-acrylic polymer emulsion to the finishing solution and stir;

[0016] (3) Hydrophilic silica is gradually added during the stirring process in step (2). After dispersion, water-based radiation cooling coating is obtained.

[0017] In step (1), the finishing solution is obtained by high-speed dispersion at 400-500 rpm for 10-30 min.

[0018] In step (2), the mixture is stirred at high speed for 20-30 minutes at a speed of 400-500 rpm.

[0019] In step (3), the hydrophilic silica is added gradually over a period of 5-10 minutes. Adding it too quickly can lead to local agglomeration and uneven dispersion; adding it too slowly will prolong the preparation cycle.

[0020] In step (3), the criteria for judging whether the dispersion is complete are that the coating system is uniform and there are no visible particle agglomerates. The dispersion fineness can be tested using a scraper fineness meter, and it is considered qualified if no obvious particles are seen.

[0021] A composite material comprising a substrate and a coating formed by the aforementioned water-based radiation cooling coating adhered to the surface of the substrate.

[0022] The substrate is either a rigid substrate or a flexible substrate; the rigid substrate is preferably a color steel sheet, suitable for outdoor thermal management applications such as building roofs and exterior walls. The flexible substrate is preferably a fabric; the fabric is cotton, polyester, nylon, or a blend thereof.

[0023] The coating is applied to the surface of the substrate by spraying or coating; when the substrate is a color steel tile, a spraying process is used, and when the substrate is a fabric, a coating process is used.

[0024] When the water-based radiative cooling coating is applied to the surface of rigid substrates such as color steel tiles by spraying, a second spray is applied after allowing it to dry, followed by a second drying period to form a radiative cooling coating. Furthermore, the spraying is done using an air spray gun with a 2mm nozzle diameter, a spraying distance controlled at 20-30cm, and a perpendicular spraying angle. Further, after the first spray, the coating is allowed to dry at room temperature before the second spray is applied, with a drying period of 24 hours to ensure complete curing and the formation of a dense and uniform radiative cooling functional layer.

[0025] When the water-based radiation-cooling coating is applied to the surface of a flexible substrate such as fabric by coating method, it is allowed to dry statically to form a radiation-cooling coating. Further, the coating is applied using a coating machine with a scraper. The scraper speed is controlled at 5 mm / s to ensure that the coating is completely coated on the fabric surface.

[0026] Furthermore, the wet film thickness of the radiation cooling coating is approximately 300 μm; if the thickness is too thin, it will affect the radiation cooling effect, and if the thickness is too thick, it will easily lead to coating cracking or decreased adhesion.

[0027] For color steel roofing sheets, the dry film thickness of the radiation cooling coating is preferably 50-100 μm; for fabrics, the dry film thickness of the radiation cooling coating is 40-70 μm.

[0028] The above-mentioned composite materials are used in clothing or building materials.

[0029] Beneficial effects: Compared with the prior art, the present invention achieves the following significant effects:

[0030] (1) This invention uses silicone-modified styrene-acrylic polymer emulsion as the film-forming material. Its water-resistant whitening, low water absorption, and APEO-free properties ensure the long-term stability and environmental friendliness of the coating in outdoor environments. The introduction of silicone segments significantly reduces the surface energy of the emulsion, giving the coating excellent hydrophobic properties. Water is difficult to penetrate the interior of the coating, avoiding the whitening, bubbling, and performance degradation caused by water absorption in traditional acrylic emulsions. At the same time, the absence of APEO complies with increasingly stringent environmental regulations and is suitable for the field of green building materials.

[0031] (2) This invention achieves efficient reflection of the solar spectrum and high emissivity in the mid-infrared band by combining rutile titanium dioxide with hydrophilic silica. Rutile titanium dioxide has a high refractive index, which can effectively reflect the visible and near-infrared bands of the solar spectrum, where energy is most concentrated, thus reducing the absorption of solar radiation by the coating. The Si-O-Si bond vibration absorption peak of hydrophilic silica is highly compatible with the atmospheric window band (8μm-13μm), significantly enhancing the thermal emissivity of the coating in this band, allowing the absorbed heat to be dissipated into outer space through the atmosphere in the form of infrared radiation. The synergistic effect of the two achieves a highly efficient radiative cooling effect, and the surface temperature of the coating is significantly lower than the ambient temperature.

[0032] (3) This invention employs a two-step dispersion process. First, rutile titanium dioxide is rapidly dispersed in deionized water to form a stable slurry, which is then mixed with the emulsion and other components. This effectively solves the problem of easy agglomeration and uneven dispersion of inorganic fillers in the emulsion system. Rutile titanium dioxide has small particle size and high surface energy. Direct addition to the emulsion can easily cause secondary agglomeration, affecting the optical properties and surface smoothness of the coating. Through pre-dispersion treatment, the titanium dioxide particles are fully deagglomerated and stably suspended in the aqueous phase. When mixed with the emulsion, they maintain a uniform dispersion state, ensuring the uniform distribution of functional fillers in the coating and the stability of radiation cooling performance. The gradual addition of hydrophilic silica further avoids agglomeration caused by local over-concentration, ensuring the uniformity of the coating system.

[0033] (4) The coating of this invention is a water-based system. The preparation process does not require organic solvents, and the volatile matter is mainly water. The application process is non-toxic and odorless, which is environmentally friendly and in line with the concepts of green chemistry and sustainable development. The coating has a moderate solid content and adjustable viscosity, making it suitable for both spraying and coating, and can meet the coating needs of different substrates.

[0034] (5) This invention achieves controllable adjustment of radiative cooling performance by regulating the content of rutile titanium dioxide (2.8%-6.8%). Low titanium dioxide content results in coatings with good transparency and flexibility, suitable for applications requiring specific color properties; high titanium dioxide content leads to the highest reflectivity and optimal radiative cooling effect, suitable for applications requiring high cooling efficiency. Coatings with different titanium dioxide contents maintain good water resistance and low water absorption, allowing users to select appropriate ratios based on actual application needs to meet diverse market demands. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of the radiation cooling mechanism of the present invention;

[0036] Figure 2 This is a schematic diagram of the composite material structure of the present invention;

[0037] Figure 3 Hollow SiO2 thermocouple testing;

[0038] Figure 4 Thermocouple testing for different materials in silicone-acrylic emulsion. Detailed Implementation

[0039] The present invention will now be described in further detail.

[0040] Example 1

[0041] A water-based radiative cooling coating comprises, by mass percentage: 61.7% silicone-modified styrene-acrylic polymeric emulsion, 2.5% rutile titanium dioxide (TiO2), 4.9% hydrophilic silica (SiO2), and 30.9% deionized water. The silicone-modified styrene-acrylic polymeric emulsion used in this embodiment is a commercially available product, characterized by water resistance, low water absorption, APEO-free properties, a glass transition temperature of 25°C, and a solid content of 30%. The rutile titanium dioxide has a particle size of 0.2 μm, a content of 94.0%-95.5%, and a density of 4.1 g / cm³. 3 The hydrophilic silica has a particle size of 5 μm; the deionized water conductivity is ≤10 μS / cm.

[0042] Preparation method: Add 0.4g of rutile titanium dioxide to 5g of deionized water and disperse at 500rpm for 20 minutes to obtain a finishing solution; add 10g of organosilicon-modified styrene-acrylic polymer emulsion to the finishing solution and stir at 500rpm for 10 minutes; during the stirring process, gradually add 0.8g of hydrophilic silica over 5 minutes until it is completely dispersed to obtain a water-based radiation cooling coating.

[0043] Figure 1 This is a schematic diagram of the radiative cooling mechanism of the present invention.

[0044] The obtained coating is loaded into an air spray gun and sprayed at a distance of 20-30cm to perform the first spray on the color steel tile substrate; it is then allowed to dry at room temperature; a second spray is then performed; and it is allowed to dry again for 24 hours to form a radiation cooling coating with a dry film thickness of about 300 μm, thus obtaining the composite material. Figure 2 This is a schematic diagram of the composite material structure of the present invention.

[0045] Example 2

[0046] The difference from Example 1 is that the rutile titanium dioxide content is 3.7% by mass, corresponding to 0.6g. The preparation method and application are the same as in Example 1.

[0047] Example 3

[0048] The difference from Example 1 is that the rutile titanium dioxide content is 4.8% by mass, corresponding to a mass of 0.8g. The preparation method and application are the same as in Example 1.

[0049] Example 4

[0050] The difference from Example 1 is that the rutile titanium dioxide content is 6.0% by mass, corresponding to 1.0g. The preparation method and application are the same as in Example 1.

[0051] Comparative Example 1

[0052] A type of corrugated steel sheet commonly used in factory buildings.

[0053] Comparative Example 2

[0054] The difference from Example 1 is that rutile titanium dioxide is not added to the coating, while the other components and preparation method are the same as in Example 1. That is, 10.0g of organosilicon-modified styrene-acrylic polymer emulsion, 0.8g of hydrophilic silica, and 5.0g of deionized water.

[0055] Comparative Example 3

[0056] The difference from Example 1 is that hydrophilic silica is not added to the coating, while the other components and preparation method are the same as in Example 1. That is, 10.0g of organosilicon-modified styrene-acrylic polymer emulsion, 0.4g of rutile titanium dioxide, and 5.0g of deionized water.

[0057] The coatings obtained in Examples 1-4 and Comparative Examples 1-3 were subjected to radiative cooling performance tests under the following conditions: ambient temperature 34℃. The radiative cooling performance test employed an outdoor daytime cooling test. The coated samples were placed in an open outdoor area, and the temperature difference between the coating surface temperature and the initial temperature after 30 minutes of sunlight exposure was measured. A smaller value indicated a better cooling effect. The ambient temperature was also recorded as a reference.

[0058] The adhesion test adopts the cross-cut test method, according to GB / T1720-2024 "Determination of Adhesion of Paint Film", with grade 1 being the best and grade 7 being the worst.

[0059] The test results are shown in Table 1.

[0060] Table 1. Test results of the radiation cooling performance of the coatings obtained in Examples 1-4 and Comparative Examples 1-3.

[0061]

[0062] As shown in Table 1, the hydrophilic silica content remained consistent in the formulation systems of Examples 1-4. The amount of rutile titanium dioxide added gradually increased from 0.4 g to 1.0 g, and the coating temperature difference gradually decreased from 4.3 ℃ to 3.3 ℃. The surface temperature rise of the coating decreased with the increase of titanium dioxide content. Rutile titanium dioxide has a high refractive index and can reflect visible light and near-infrared radiation in sunlight, reducing solar energy absorption. The heat absorbed by the coating under solar radiation conditions is reduced, and the surface temperature change range is reduced.

[0063] Compared to Example 1, the temperature differences in Examples 2, 3, and 4 decreased to 3.8 ℃, 3.5 ℃, and 3.3 ℃, respectively, indicating a gradual decrease in the surface temperature rise of the coating. The increased content of rutile titanium dioxide altered the coating's ability to reflect solar radiation. Higher titanium dioxide content resulted in more scattering interfaces within the coating, leading to multiple scattering and reflection of incident light between particles. This improved the coating's reflectivity to sunlight, reduced the amount of radiant heat absorbed by the substrate, and decreased the surface temperature variation. The adhesion of Examples 1-4 was all rated at grade 1, demonstrating that the silicone-modified styrene-acrylic polymer emulsion can form a stable and continuous film structure on the surface of color steel tiles, indicating a stable interfacial bond between the coating and the substrate.

[0064] Compared to Comparative Example 1, the temperature difference of the coating samples in Examples 1-4 was significantly reduced. The surface of the corrugated steel sheet has a high absorption capacity for solar radiation; solar radiation energy is converted into heat and accumulates on the substrate surface, resulting in a significant increase in surface temperature. In Examples 1-4, after forming a radiation-cooling coating on the substrate surface, the coating reflects and scatters sunlight. Simultaneously, the hydrophilic silica in the coating exhibits vibrational absorption characteristics in the mid-infrared band, releasing the absorbed heat outward through infrared radiation, thus significantly reducing the surface temperature rise of the coating.

[0065] Compared to Example 1, the temperatures of Comparative Examples 2 and 3 were both higher than those of Example 1. Comparative Example 2's formulation system did not contain rutile titanium dioxide, resulting in reduced solar reflectivity and increased absorption of solar radiation energy on the coating surface, leading to a greater surface temperature rise. Comparative Example 3's formulation system did not contain hydrophilic silica, weakening the coating's radiative ability in the mid-infrared band and reducing its ability to radiate absorbed heat outwards. This resulted in heat accumulation on both the coating and substrate surfaces, further increasing the surface temperature rise.

[0066] Example 5

[0067] Aqueous radiative cooling coatings were prepared according to the components and methods described in Example 1, except that the proportion of deionized water was changed. The deionized water mass percentage was 15.1%, corresponding to a mass of 2.0 g.

[0068] Preparation method: Add 0.4g of rutile titanium dioxide to 2g of deionized water and disperse at 500rpm for 20 minutes to obtain a finishing solution; add 10g of organosilicon-modified styrene-acrylic polymer emulsion to the finishing solution and stir at 500rpm for 10 minutes; during the stirring process, gradually add 0.8g of hydrophilic silica over 5 minutes until it is completely dispersed to obtain a water-based radiation cooling coating.

[0069] The prepared water-based radiation-cooling coating was applied to a polyester base fabric using a coating machine to form a radiation-cooling coating with a thickness of 300 μm.

[0070] Example 6

[0071] The difference from Example 2 is that the deionized water content is 14.9%, corresponding to a mass of 2.0g. The preparation method and application are the same as in Example 5.

[0072] Example 7

[0073] The difference from Example 3 is that the deionized water content is 14.7%, corresponding to a mass of 2.0g. The preparation method and application are the same as in Example 5.

[0074] Example 8

[0075] The difference from Example 1 is that the deionized water content is 14.5%, corresponding to a mass of 2.0g. The preparation method and application are the same as in Example 5.

[0076] Comparative Example 4

[0077] A common type of polyester base fabric is polyester nonwoven fabric.

[0078] Comparative Example 5

[0079] The difference from Example 5 is that rutile titanium dioxide is not added to the coating, while the other components and preparation method are the same as in Example 5. That is, 10.0g of organosilicon-modified styrene-acrylic polymer emulsion, 0.8g of hydrophilic silica, and 2.0g of deionized water.

[0080] Comparative Example 6

[0081] The difference from Example 5 is that hydrophilic silica is not added to the coating, while the other components and preparation method are the same as in Example 5. That is, 10.0g of organosilicon-modified styrene-acrylic polymer emulsion, 0.4g of rutile titanium dioxide, and 2.0g of deionized water.

[0082] The coatings obtained in Examples 5-8 and Comparative Examples 4-6 were subjected to radiative cooling performance tests under the following conditions: ambient temperature 34.5℃. The radiative cooling performance test employed an outdoor daytime cooling test. The coated samples were placed in an open outdoor area, and the temperature difference between the coating surface temperature and the initial temperature after 30 minutes of sunlight exposure was measured. A smaller value indicated a better cooling effect. The ambient temperature was also recorded as a reference.

[0083] The results are shown in Table 2.

[0084] Table 2. Test results of the radiation cooling performance of the coatings obtained in Examples 5-8 and Comparative Examples 4-6.

[0085]

[0086] As shown in Table 2, the temperature difference between Examples 5-8 under 30 min of sunlight was 1.8 ℃-3.0 ℃, significantly lower than the 5.4 ℃ of the uncoated polyester base fabric, indicating an effective reduction in fabric surface temperature rise. With increasing rutile titanium dioxide content, the temperature difference decreased from 3.0 ℃ to 1.8 ℃, demonstrating enhanced solar radiation reflectivity of the coating. The comparative samples without rutile titanium dioxide or hydrophilic silica had temperature differences of 5.1 ℃ and 3.1 ℃, respectively, higher than the example samples. These results indicate that the combined effect of rutile titanium dioxide and hydrophilic silica in the coating system reduces fabric surface temperature rise and improves radiative cooling.

[0087] Depend on Figure 3 It can be seen that during the test from 11:00 to 11:30, the temperature curves of the silicone-acrylic emulsion coating and the silicone-acrylic and Al2O3 composite coating were basically the same as or slightly higher than the original samples, and no significant cooling phenomenon was observed. However, the temperature curves of the silicone-acrylic and SiO2 composite coating and the silicone-acrylic and BN coating were slightly lower than the original polyester base fabric of Comparative Example 4, and the effect was far inferior to that of Examples 5-8.

[0088] Depend on Figure 4It can be seen that the temperature of the samples with 0.3g and 0.5g of hollow SiO2 microspheres added is higher than that of the original polyester base fabric of Comparative Example 4. Although the temperature of the sample with 0.8g of hollow SiO2 added is slightly lower than that of the original sample, the cooling effect is not as good as that of Examples 5-8. This is because although hollow SiO2 can scatter some sunlight, its infrared emissivity is only slightly improved. At the same time, the hollow structure introduces a heat insulation effect, causing heat to accumulate on the surface and weakening the radiative heat dissipation effect.

Claims

1. A water-based radiative cooling coating, characterized in that, The product comprises the following components by mass percentage: 59.5%-70.4% silicone-modified styrene-acrylic polymer emulsion, 2.8%-6.8% rutile titanium dioxide, 4.8%-5.6% hydrophilic silica, and 21.1%-30.8% solvent.

2. The water-based radiative cooling coating according to claim 1, characterized in that, The solid content of the organosilicon-modified styrene-acrylic polymer emulsion is 29-31 wt%.

3. The water-based radiative cooling coating according to claim 1, characterized in that, The rutile titanium dioxide has a particle size of 0.15-0.25 μm, and the hydrophilic silica has a particle size of 4.8-5.2 μm.

4. The water-based radiative cooling coating according to claim 1, characterized in that, The solvent is deionized water, and its content is adjusted according to the type of substrate. When the substrate is a rigid substrate such as color steel tile, the deionized water content is 29.8%-30.8%; when the substrate is a flexible substrate such as fabric, the deionized water content is 20.3%-21.1%, in order to obtain a suitable coating viscosity and reduce the penetration of coating into the fiber.

5. A method for preparing the water-based radiation cooling coating according to claim 1, characterized in that, Includes the following steps: (1) Rutile titanium dioxide was added to a solvent for pre-dispersion to obtain a finishing solution; (2) Add the organosilicon-modified styrene-acrylic polymer emulsion to the finishing solution and stir; (3) Hydrophilic silica is gradually added during the stirring process in step (2). After dispersion, water-based radiation cooling coating is obtained.

6. The method for preparing the water-based radiation cooling coating according to claim 5, characterized in that, In step (3), the hydrophilic silica is added gradually over a period of 5-10 minutes.

7. A composite material, characterized in that, It includes a substrate and a coating formed by the water-based radiation cooling coating of claim 1, which is attached to the surface of the substrate.

8. The composite material according to claim 7, characterized in that, The substrate is color steel sheet or fabric; the fabric is cotton, polyester, nylon or a blend thereof.

9. The composite material according to claim 7, characterized in that, The coating is applied to the surface of the substrate by spraying or coating; spraying is used when the substrate is color steel tile, and coating is used when the substrate is fabric.

10. The application of the water-based radiation cooling coating of claim 1 in clothing or building materials.

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