An adaptive radiative cooling coating and method of making the same

Abstract

The application discloses a self-adaptive radiative cooling coating and a preparation method thereof, and aims to solve the problem that the coating cannot keep warm in low-temperature environment after being formed and is not moisture-resistant. In the water-based polyurethane, titanium dioxide, boron nitride and temperature-sensitive color-changing particles are introduced, and then a PDMS / silicon dioxide layer is coated on the surface. The temperature-sensitive material becomes dark to realize efficient sunlight absorption. The absorbed heat can be quickly conducted to the bottom cavity through the heat conduction network formed by the boron nitride, so that the heat accumulation and dissipation are avoided. The application widens the application range of the water-based polyurethane coating, solves the problem of poor moisture resistance of the water-based polyurethane coating, and endows the water-based polyurethane coating with the self-adaptive radiative cooling performance.

Description

Technical Field

[0001] This invention belongs to the field of composite material technology, specifically relating to an adaptive radiation cooling coating and its preparation method. Background Technology

[0002] Passive radiative cooling technology is a zero-energy-consumption technology that radiates heat into outer space through the high emissivity of materials, showing great promise in the field of building energy conservation. Waterborne polyurethane, as an environmentally friendly coating, can be used as a matrix material for radiative cooling coatings. However, its poor adaptability to humid environments and low radiative cooling performance limit its widespread application. Although adding radiative cooling materials can improve the radiative cooling performance of waterborne polyurethane, it still continuously radiates heat into the external space during cold seasons or in low-temperature environments, increasing building heating energy consumption. This "overcooling" problem severely limits its practical application value. Therefore, enhancing the moisture resistance and radiative cooling performance of waterborne polyurethane coatings, while simultaneously endowing them with the functions of radiative cooling in summer (high-temperature environments) and heating in winter (low-temperature environments), can not only improve its practical application value but also enhance the stability of the coating in humid environments. Summary of the Invention

[0003] To address the issue that existing waterborne polyurethane coatings cannot simultaneously provide moisture resistance and insulation in low-temperature environments, this application proposes an adaptive radiative cooling coating and its preparation method. The prepared coating exhibits good moisture resistance and adaptive radiative cooling performance, primarily due to its dual-layer structure design and the synergy of smart materials. The top layer, a hydrophobic coating composed of silica and polydimethylsiloxane, forms a physical barrier, effectively enhancing its moisture resistance. Furthermore, the bottom layer utilizes thermosensitive color-changing particles to achieve adaptive adjustment, reflecting sunlight for cooling at high temperatures and absorbing sunlight for insulation at low temperatures, thus solving the problem of insufficient insulation in low-temperature environments. This combination enables the coating to possess both adaptive temperature control and weather resistance in complex outdoor environments.

[0004] The coating employs a precise synergy between boron nitride, titanium dioxide, and sodium carboxymethyl cellulose to create an adaptive "warm in winter, cool in summer" mechanism that dynamically changes with the seasons. In winter, the color-changing particles turn black to maximize the absorption of solar radiation. At this time, sodium carboxymethyl cellulose plays a crucial role in structural regulation, not only improving the dispersibility of boron nitride but also assisting in building an efficient heat-conducting network to quickly transfer heat generated on the surface to the indoor space. Simultaneously, the black state of the coating naturally suppresses the reflection and radiative cooling properties of titanium dioxide, ensuring that the photothermal conversion effect dominates and achieving efficient heating. As summer approaches, the color-changing particles fade, and the synergistic mechanism switches: the uniformly distributed titanium dioxide becomes the first line of defense, strongly reflecting sunlight and radiating heat, while boron nitride utilizes the specific vibrational mode of its B–N bonds under light to significantly enhance the infrared emissivity of the coating at the atmospheric window, thereby enhancing the passive radiative cooling effect; in this process, sodium carboxymethyl cellulose once again highlights its core value, preventing the aggregation of inorganic fillers, thus avoiding the light scattering failure caused by the aggregation of titanium dioxide, and maintaining the integrity of the boron nitride network, thereby ensuring the maximization of dual cooling performance in summer.

[0005] The preparation method provided by this invention is as follows: (1) Titanium dioxide, boron nitride, thermosensitive color-changing particles and aqueous polyurethane dispersion were ultrasonically mixed to obtain a homogeneous dispersion. (2) Add the hydroxypropyl cellulose solution to the dispersion in step 1 and stir until homogeneous; (3) The dispersion from step 2 is uniformly coated onto the substrate, and a modified waterborne polyurethane coating is prepared by heat treatment. (4) The silica and polydimethylsiloxane precursor solution are uniformly mixed and coated uniformly on the modified waterborne polyurethane coating. The adaptive radiation cooling coating is obtained by heating and curing.

[0006] This coating uses low surface energy polydimethylsiloxane (PDMS) as a matrix and introduces silica particles as rigid fillers to construct a stable micro-nano hierarchical rough structure on the surface. This synergistic effect not only significantly improves the coating's hydrophobicity but also enhances its moisture resistance. In terms of photothermal management, the coating utilizes the photochromic properties of thermosensitive materials to achieve a darker color, thereby efficiently absorbing sunlight. Simultaneously, a highly efficient thermally conductive network constructed through boron nitride rapidly conducts the absorbed heat to the bottom cavity, effectively preventing heat accumulation and loss on the surface. Furthermore, the PDMS / silica composite structure on the surface maintains excellent hydrophobicity and weather resistance while also providing good heat retention, achieving multifunctional synergistic optimization.

[0007] Furthermore, in step (1), the mass ratio of titanium dioxide, boron nitride and thermochromic particles is 1:2:5 ~1:4:5.

[0008] Furthermore, in step (1), the mass ratio of boron nitride to waterborne polyurethane is 1:14 to 2:7.

[0009] Furthermore, in step (2), the volume ratio of hydroxypropyl cellulose solution to aqueous polyurethane dispersion is 3:4 to 3:2.

[0010] Furthermore, in step (4), the mass fraction of the polydimethylsiloxane precursor solution is 10wt% ~ 15wt%.

[0011] Furthermore, in step (4), the mass ratio of silica to polydimethylsiloxane precursor solution is 1:20 to 1:10.

[0012] Furthermore, the curing temperature in step (4) is 80 ~ 100 ℃.

[0013] The beneficial effects of this invention are as follows: This invention broadens the application range of waterborne polyurethane coatings, not only solving the problem of poor moisture resistance of waterborne polyurethane coatings, but also endowing them with adaptive radiative cooling performance. Detailed Implementation

[0014] The following examples are provided to further illustrate the present invention and are intended to explain the invention, not to limit its scope. Unless otherwise specified, all figures are expressed in parts by weight and weight percentages.

[0015] Unless otherwise specified, the raw materials used in this invention are all conventional commercially available products; unless otherwise specified, the methods used in this invention are all conventional methods in the field.

[0016] The embodiments of the present invention will be further described below with reference to several examples.

[0017] It should be understood that the described embodiments are merely some, not all, of the embodiments in this application. All other embodiments obtained by those skilled in the art based on the embodiments in this application without inventive effort are within the scope of protection of this application.

[0018] The terminology used in the embodiments of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application. The singular forms “a,” “the,” and “the” used in the embodiments of this application and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise.

[0019] Example 1 (1) Mix 0.05g titanium dioxide, 0.1g boron nitride, and 0.25g thermochromic particles with 4 mL of aqueous polyurethane dispersion (35wt%) and disperse evenly by ultrasonication; (2) Add 3 mL of hydroxypropyl cellulose solution (1 wt%) homogeneous dispersion to the dispersion in step 1 and stir until homogeneous to obtain a homogeneous dispersion; (3) The dispersion from step 2 is uniformly coated onto the substrate, and a modified waterborne polyurethane coating is prepared by heat treatment. (4) Prepare a 10 wt% polydimethylsiloxane precursor solution, take 10 mL of polydimethylsiloxane precursor solution and 0.5 g of silica powder, mix and sonicate, and coat it on the modified waterborne polyurethane coating of step 3.

[0020] (5) Place the coating from step 4 in a 100 ℃ oven to cure and obtain an adaptive radiation cooling coating.

[0021] Example 2 (1) Mix 0.05g titanium dioxide, 0.2g boron nitride, and 0.25g thermochromic particles with 4 mL of aqueous polyurethane dispersion (35wt%) and disperse evenly by ultrasonication; (2) Add 3 mL of hydroxypropyl cellulose solution (1 wt%) homogeneous dispersion to the dispersion in step 1 and stir until homogeneous to obtain a homogeneous dispersion; (3) The dispersion from step 2 is uniformly coated onto the substrate, and a modified waterborne polyurethane coating is prepared by heat treatment. (4) Prepare a 10 wt% polydimethylsiloxane precursor solution, take 10 mL of polydimethylsiloxane precursor solution and 0.5 g of silica powder, mix and sonicate, and coat it on the modified waterborne polyurethane coating of step 3.

[0022] (5) Place the coating from step 4 in a 100 ℃ oven to cure and obtain an adaptive radiation cooling coating.

[0023] Example 3 (1) Mix 0.05g titanium dioxide, 0.2g boron nitride, 0.25g thermochromic particles with 2mL of aqueous polyurethane dispersion (35wt%) and disperse evenly by ultrasonication; (2) Add 3 mL of hydroxypropyl cellulose solution (1 wt%) homogeneous dispersion to the dispersion in step 1 and stir until homogeneous to obtain a homogeneous dispersion; (3) The dispersion from step 2 is uniformly coated onto the substrate, and a modified waterborne polyurethane coating is prepared by heat treatment. (4) Prepare a 10 wt% polydimethylsiloxane precursor solution, take 10 mL of polydimethylsiloxane precursor solution and 1 g of silica powder, mix and sonicate, and coat it on the modified waterborne polyurethane coating of step 3.

[0024] (5) Place the coating from step 4 in an 80 ℃ oven to cure and obtain an adaptive radiation cooling coating.

[0025] Comparative Example 1 (1) Mix 0.05g of titanium dioxide, 0.1g of boron nitride and 4 mL of aqueous polyurethane dispersion (35wt%) and disperse evenly by ultrasonication; (2) Add 3 mL of hydroxypropyl cellulose solution (1 wt%) homogeneous dispersion to the dispersion in step 1 and stir until homogeneous to obtain a homogeneous dispersion; (3) The dispersion from step 2 is uniformly coated onto the substrate, and a modified waterborne polyurethane coating is prepared by heat treatment.

[0026] Comparative Example 2 (1) Mix 0.05g titanium dioxide, 0.1g boron nitride, and 0.25g thermochromic particles with 4 mL of aqueous polyurethane dispersion (35wt%) and disperse evenly by ultrasonication; (2) Add 3 mL of hydroxypropyl cellulose solution (1 wt%) homogeneous dispersion to the dispersion in step 1 and stir until homogeneous to obtain a homogeneous dispersion; (3) The dispersion from step 2 is uniformly coated onto the substrate, and a modified waterborne polyurethane coating is prepared by heat treatment.

[0027] Table 1 shows the water contact angles of Examples 1, 2, Comparative Example 1, and Comparative Example 2, respectively. As can be seen from the table, the contact angles of Examples 1 and 2 are approximately 109.8° and 112°, respectively, significantly higher than the water contact angles of the coatings in Comparative Example 1 and Comparative Example 2 (approximately 86.9° and 82.9°). This is mainly because polydimethylsiloxane is a low surface energy matrix, and silica particles act as rigid fillers, forming a stable, highly rough structure on the surface of the polydimethylsiloxane. The synergistic effect of these two components enhances the hydrophobicity of the coating and, to some extent, improves its moisture resistance.

[0028] Table 1 Example 1, Example 2, and Comparative Example 1 were placed on a cooling plate (approximately 5 °C) and subjected to a 1 kW m -2After 30 minutes of simulated light source irradiation, the surface temperature rise of Example 1, Example 2, and Comparative Example 1 was approximately 8.2 °C, 4.3 °C, and 2.2 °C, respectively. The temperature rise of Comparative Example 1 was lower than that of the other examples. This is because Comparative Example 1 did not contain thermochromic particles. At low temperatures, the color of Comparative Example 1 remained white, and it could not absorb most photons and generate heat.

[0029] Simultaneously, Example 2, Comparative Example 1, and the uncoated substrate material were placed in a test chamber. The test involved covering the top of an uncovered, sealed cavity with the examples and comparative examples, then illuminating them with a light source. The coating performance was evaluated by recording the temperature inside the cavity. When the outdoor ambient temperature was low (approximately 15 °C), the outdoor sunlight (approximately 0.83 kW m²) was used. -2 After approximately 1 hour of irradiation, the cavity temperature rise in Example 2 was about 1.6 °C and 3.3 °C higher than that in Comparative Example 1 and the uncoated substrate material, respectively. Since Comparative Example 1 does not contain thermosensitive color-changing particles, its temperature rise under sunlight is lower than that in Example 2 and the uncoated substrate material. This is mainly due to the high reflectivity and emissivity of titanium dioxide and boron nitride in the examples and comparative examples, which endow them with radiative cooling properties. At low temperatures, Example 2 achieves efficient solar absorption by changing the color of the thermosensitive material to a darker shade. The absorbed heat can be quickly conducted to the bottom cavity through the thermally conductive network constructed with boron nitride, preventing heat accumulation and loss. The surface PDMS / silica layer maintains hydrophobicity and weather resistance while also ensuring heat retention.

[0030] This invention uses waterborne polyurethane as a matrix and introduces titanium dioxide, boron nitride, thermosensitive color-changing particles, hydroxypropyl cellulose, and polydimethylsiloxane / silica composite to prepare an adaptive radiation cooling coating.

[0031] The above embodiments describe in detail the structure, features, and effects of the present invention. The above description is only a preferred embodiment of the present invention. Any changes made in accordance with the concept of the present invention, or equivalent embodiments modified to have equivalent changes, shall still fall within the scope of protection of the present invention if they do not exceed the scope covered by the specification.

Claims

1. A method for preparing an adaptive radiation cooling coating, characterized in that, It includes the following steps: (1) Titanium dioxide, boron nitride, thermosensitive color-changing particles and aqueous polyurethane dispersion were ultrasonically mixed to obtain a homogeneous dispersion. (2) Add the hydroxypropyl cellulose solution to the dispersion in step 1 and stir until homogeneous; (3) The dispersion from step 2 is uniformly coated onto the substrate, and a modified waterborne polyurethane coating is prepared by heat treatment. (4) The silica and polydimethylsiloxane precursor solution are uniformly mixed and coated uniformly on the modified waterborne polyurethane coating. The adaptive radiation cooling coating is obtained by heating and curing.

2. The method according to claim 1, characterized in that, In step (1), the mass ratio of titanium dioxide, boron nitride and thermochromic particles is 1: (2-4):

5.

3. The method according to claim 1, characterized in that, The mass ratio of boron nitride to waterborne polyurethane is 1:14 to 2:

7.

4. The method according to claim 1, characterized in that, In step (2), the volume ratio of hydroxypropyl cellulose solution to aqueous polyurethane dispersion is 3:4 to 3:2; the concentration of hydroxypropyl cellulose solution is 1wt%; and the concentration of aqueous polyurethane dispersion is 35wt%.

5. The method according to claim 1, characterized in that, In step (4), the mass fraction of the polydimethylsiloxane precursor solution is 10wt% ~ 15wt%.

6. The method according to claim 1, characterized in that, In step (4), the mass ratio of silica to polydimethylsiloxane precursor solution is 1:20 ~ 1:

10.

7. The method according to claim 1, characterized in that, The curing temperature in step (4) is 80 ~ 100 ℃.

8. An adaptive radiation cooling coating prepared by the method of claim 1.

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