Functional radiation refrigeration coating as well as preparation method and application thereof

Abstract

The invention discloses a functional radiation refrigeration coating and a preparation method thereof, and aims to overcome the defects that after a water-based coating is formed, the stability is low under ultraviolet irradiation, the adaptability to a humid environment is limited, and the radiation refrigeration performance is low. The preparation method comprises the following steps that firstly, silane modified zinc oxide dispersion liquid is prepared; mixing the silane modified zinc oxide dispersion liquid with a beta-cyclodextrin solution, and performing ultrasonic uniform dispersion; and then adding the uniform dispersion liquid into the aqueous polyurethane dispersion liquid, continuing to add the poly (N-isopropylacrylamide) dispersion liquid and the thickening agent, and fully stirring. And finally, uniformly coating a substrate with the modified waterborne polyurethane dispersion liquid, and carrying out heating treatment to prepare the multifunctional radiation refrigeration coating. The waterborne polyurethane material obtained by the preparation method can be widely applied to the fields of automobile coatings, energy-saving coatings and the like.

Description

Technical Field

[0001] This invention belongs to the field of polyurethane coating preparation, specifically relating to a functional radiation cooling coating, its preparation method, and its application. Background Technology

[0002] Radiation-cooled coatings achieve cooling through heat radiation exchange between themselves and the external space, without relying on any external energy source. They are suitable for building exteriors, vehicle surfaces, and other applications. Waterborne polyurethane (WPU), using water as the dispersion medium, releases almost no harmful volatile organic compounds (VOCs) such as benzene, toluene, and xylene during production and application, making it an important alternative to traditional solvent-based coatings. It exhibits good film-forming properties, excellent flexibility, and strong adhesion, and is widely used in wood coatings, leather finishing, textile coatings, adhesives, and architectural coatings.

[0003] However, the current application of waterborne polyurethane in the field of radiative cooling faces three major bottlenecks: Waterborne polyurethane has low stability under ultraviolet light, limited adaptability to humid environments, and low radiative cooling performance, which limits its widespread application. Summary of the Invention

[0004] In view of the technical defects of existing waterborne polyurethanes, such as low stability under ultraviolet light irradiation, limited adaptability to humid environments, and low radiation cooling performance, this invention proposes a functional radiation cooling coating, its preparation method, and its application.

[0005] Specifically, this invention significantly improves the radiative cooling performance of a coating by introducing octyltrimethoxysilane-modified zinc oxide and β-cyclodextrin into an aqueous polyurethane. Zinc oxide achieves cooling by efficiently reflecting sunlight in the 250-2500 nm wavelength range and strongly emitting infrared radiation in the 8-13 μm atmospheric window range. Simultaneously, the Si–O–C bonds in octyltrimethoxysilane undergo strong molecular vibrational stretching in the mid-infrared region. When the coating is excited by external radiation, the vibrations of these bonds are further intensified, promoting the transition of high-energy electrons to lower energy levels and releasing photons, ultimately dissipating heat efficiently in the form of mid-infrared electromagnetic waves. The combination of zinc oxide and octyltrimethoxysilane significantly enhances the overall cooling performance.

[0006] Furthermore, by integrating hydrogen bonds, covalent bonds, and hydrophobic interactions, an integrated synergistic structure with multiple interpenetrating networks was constructed. Sodium carboxymethyl cellulose, waterborne polyurethane, poly(N-isoacrylamide), and β-cyclodextrin form a rigid framework network through strong hydrogen bond crosslinking as support. The Si-O-Zn covalent bonds between octyltrimethoxysilane-modified zinc oxide and the simultaneous introduction of hydrophobic interactions between β-cyclodextrin and octyltrimethoxysilane-modified zinc oxide effectively enhance the anchoring effect of inorganic fillers in the organic network, significantly improving the overall anti-peeling performance and durability of the coating.

[0007] One of the technical solutions of the present invention is to provide a method for preparing a functional radiation-cooling coating, comprising the following steps: (1) Add zinc oxide to an octyltrimethoxysilane solution and heat at 55-65 °C for 3-4 h to obtain a silane-modified zinc oxide dispersion; (2) The silane-modified zinc oxide dispersion was mixed with a β-cyclodextrin solution and ultrasonicated to obtain a homogeneous dispersion; the concentration of the β-cyclodextrin solution was 2wt% ~ 3wt%. (3) Add the homogeneous dispersion obtained in step 2 to 35 wt% aqueous polyurethane dispersion and stir to disperse to obtain a uniform composite dispersion. (4) N-isopropylacrylamide, N,N'-methylenebisacrylamide, ammonium persulfate and deionized water were mixed evenly in a mass ratio of 12.5:1.25:1 and heated to 70 ~ 80 °C to obtain poly(N-isopropylacrylamide) dispersion; (5) Add the poly(N-isopropylacrylamide) dispersion to the composite dispersion obtained in step 3, add a thickener, and stir evenly to obtain a modified waterborne polyurethane dispersion. (6) The modified waterborne polyurethane dispersion is uniformly coated on the substrate, and the solvent is removed by heating at 80 ~ 100 ℃ to prepare a functional radiation cooling coating.

[0008] Further, the octyltrimethoxysilane solution in step 1 comprises octyltrimethoxysilane, ethanol, and deionized water in a volume ratio of 3:4:1; the pH of the octyltrimethoxysilane solution is adjusted to 3.5 using hydrochloric acid, and the concentration of hydrochloric acid is 0.1 mol / L.

[0009] In step 1, the mass-to-volume ratio of zinc oxide to water in the octyltrimethoxysilane solution is (0.2-0.8) g: 1 ml.

[0010] Furthermore, the volume ratio of deionized water in the β-cyclodextrin solution to the octyltrimethoxysilane solution is 2:1.

[0011] Furthermore, in step (3), the volume ratio of the aqueous polyurethane dispersion to the β-cyclodextrin solution is 2:1 ~ 2.86:1.

[0012] Furthermore, in step (5), the concentration of the poly(N-isopropylacrylamide) dispersion is 3.8 wt%, and the volume ratio of the dispersion to the β-cyclodextrin solution is 1:1.

[0013] Furthermore, the thickener in step (5) is a 2wt% sodium carboxymethyl cellulose solution with a volume ratio of 1:1 to the β-cyclodextrin solution.

[0014] The second technical solution of the present invention is to provide a functional radiation cooling coating prepared by the above method.

[0015] The third technical solution of the present invention is to provide the application of the above-mentioned functional radiation cooling coating.

[0016] The beneficial effects of this invention are as follows: by adding octyltrimethoxysilane-modified zinc oxide, sodium carboxymethyl cellulose, poly(N-isoacrylamide), and β-cyclodextrin to waterborne polyurethane, the stability, adaptability to humid environments, and radiative cooling performance of waterborne polyurethane are effectively improved. Detailed Implementation

[0017] 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.

[0018] 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.

[0019] The embodiments of the present invention will be further described below with reference to several examples.

[0020] It should be understood that the described embodiments are only 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.

[0021] 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.

[0022] Example 1 (1) Mix 3 mL of octyltrimethoxysilane, 4 mL of ethanol and 1 mL of deionized water, stir for 15 min, add hydrochloric acid to adjust the pH to about 3.5, then add 0.2 g of zinc oxide, stir for 3 h under water bath heating at 55 °C to obtain silane modified zinc oxide dispersion. (2) The obtained silane-modified zinc oxide dispersion was mixed with 2 mL of β-cyclodextrin solution (2 wt%) and sonicated for 5 min to obtain a homogeneous dispersion; (3) Add 12.5g N-isopropylacrylamide, 1.25g N,N'-methylenebisacrylamide and 1g ammonium persulfate to 1000 mL of deionized water and stir for 4 h under water bath heating at 70℃ to obtain poly(N-isopropylacrylamide) dispersion. (4) Pour the homogeneous dispersion from (2) into about 5.71 mL of aqueous polyurethane dispersion (35 wt%), and add 2 mL of poly(N-isopropylacrylamide) dispersion (3.8 wt%) and 2 mL of sodium carboxymethyl cellulose solution (2 wt%). Stir until homogeneous to obtain modified aqueous polyurethane dispersion.

[0023] (5) The modified waterborne polyurethane dispersion in (4) is uniformly coated onto the substrate and heated at 80 °C until the moisture evaporates completely to obtain the coating.

[0024] Example 2 (1) Mix 3 mL of octyltrimethoxysilane, 4 mL of ethanol and 1 mL of deionized water, stir for 15 min, add hydrochloric acid to adjust the pH to about 3.5, then add 0.5 g of zinc oxide, stir for 3 h under water bath heating at 55 °C to obtain silane modified zinc oxide dispersion. (2) The obtained silane-modified zinc oxide dispersion was mixed with 2 mL of β-cyclodextrin solution (2 wt%) and sonicated for 5 min to obtain a homogeneous dispersion; (3) Add 12.5g N-isopropylacrylamide, 1.25g N,N'-methylenebisacrylamide and 1g ammonium persulfate to 1000 mL of deionized water and stir for 4 h under water bath heating at 80℃ to obtain poly(N-isopropylacrylamide) dispersion; (4) Pour the homogeneous dispersion from (2) into about 4.86 mL of aqueous polyurethane dispersion (35 wt%), and add 2 mL of poly(N-isopropylacrylamide) dispersion (3.8 wt%) and 2 mL of sodium carboxymethyl cellulose solution (2 wt%). Stir until homogeneous to obtain modified aqueous polyurethane dispersion.

[0025] (5) The modified waterborne polyurethane dispersion in (4) is uniformly coated onto the substrate and heated at 100 °C until the water evaporates completely to obtain the coating.

[0026] Example 3 (1) Mix 3 mL of octyltrimethoxysilane, 4 mL of ethanol and 1 mL of deionized water, stir for 15 min, add hydrochloric acid to adjust the pH to about 3.5, then add 0.8 g of zinc oxide, stir for 4 h under water bath heating at 55 °C to obtain silane-modified zinc oxide dispersion. (2) The obtained silane-modified zinc oxide dispersion was mixed with 2 mL of β-cyclodextrin solution (2 wt%) and sonicated for 5 min to obtain a homogeneous dispersion; (3) Add 12.5g N-isopropylacrylamide, 1.25g N,N'-methylenebisacrylamide and 1g ammonium persulfate to 1000 mL of deionized water and stir for 4 h under water bath heating at 70℃ to obtain poly(N-isopropylacrylamide) dispersion. (4) Pour the homogeneous dispersion from (2) into about 4 mL of aqueous polyurethane dispersion (35 wt%), and add 2 mL of poly(N-isopropylacrylamide) dispersion (3.8 wt%) and 2 mL of sodium carboxymethyl cellulose solution (2 wt%). Stir until homogeneous to obtain modified aqueous polyurethane dispersion.

[0027] (5) The modified waterborne polyurethane dispersion in (4) is uniformly coated onto the substrate and heated at 80 °C until the moisture evaporates completely to obtain the coating.

[0028] Comparative Example 1 (1) Mix 0.2 g zinc oxide with 2 mL β-cyclodextrin solution (2 wt%) and sonicate for 5 min to obtain a homogeneous dispersion; (2) 12.5g N-isopropylacrylamide, 1.25g N,N'-methylenebisacrylamide and 1g ammonium persulfate were added to 1000 mL of deionized water and stirred for 4 h under water bath heating at 70℃ to obtain poly(N-isopropylacrylamide) dispersion. (3) Pour the homogeneous dispersion from (1) into about 5.71 mL of aqueous polyurethane dispersion (35 wt%), and add 2 mL of poly(N-isopropylacrylamide) dispersion (3.8 wt%) and 2 mL of sodium carboxymethyl cellulose solution (2 wt%). Stir until homogeneous to obtain modified aqueous polyurethane dispersion.

[0029] (4) The modified waterborne polyurethane dispersion in (3) is uniformly coated onto the substrate and heated at 80 °C until the moisture evaporates completely to obtain the coating.

[0030] Comparative Example 2 (1) Apply pure water-based polyurethane coating solution evenly to the substrate and heat at 80°C until dry.

[0031] Table 1 shows the water contact angles of Examples 1, 2, 3, and Comparative Example 1, respectively. It can be seen that the contact angles of Examples 1, 2, and 3 are approximately 105°, 104°, and 108°, respectively, significantly higher than the water contact angle of the coating in Comparative Example 1 (95°). Furthermore, after being placed in a 95% humidity environment for 24 hours, the coatings of Examples 1, 2, and 3 did not exhibit significant swelling or peeling, demonstrating excellent moisture resistance. This is mainly because the long alkyl chains on the surface of the silane-modified zinc oxide effectively repel water molecules through a hydrophobic effect, forming a thin film layer with excellent water resistance. Additionally, β-cyclodextrin may also form a host-guest interaction with the long-chain alkyl groups, potentially enhancing the hydrophobic properties of the film through intermolecular non-covalent interactions. Meanwhile, after placing the coatings of Comparative Example 1, Examples 1, 2, and 3 in a UV chamber for 80 hours, the surface of the coating in Comparative Example 1 became very sticky, and a light touch with a finger could leave an indentation. This phenomenon stems from the lack of UV resistance inherent in the waterborne polyurethane coating itself. Under prolonged ultraviolet (UV) irradiation, the molecular chains of waterborne polyurethane undergo photodegradation, leading to polymer chain breakage and a decrease in molecular weight, resulting in softening and stickiness. However, after UV irradiation under the same conditions, the coatings of Examples 1, 2, and 3 did not show significant stickiness on their surfaces, and no perceptible indentations were produced when pressed with a finger. This excellent UV resistance is primarily attributed to zinc oxide, which enhances the coating's UV resistance and effectively prevents the breakage and degradation of molecular chains within the coating.

[0032] Zinc oxide possesses the ability to effectively reflect ultraviolet and visible light (250-2500 nm band) and release heat through radiation in the far-infrared band (approximately 8-13 μm). Theoretically, coatings containing zinc oxide can be applied in the field of radiative cooling. The coatings of Example 3 and Comparative Example 1 were placed in a self-made test chamber. The test involved covering the top of an open, sealed cavity with the coating, then irradiating the coating (i.e., the top of the cavity) with a simulated light source. The radiative cooling performance of the coating was evaluated by recording the temperature inside the cavity. Table 2 shows that after approximately 1 hour of irradiation with a simulated sunlight intensity, the cavity temperature rise under the coating of Example 3 was approximately 5.8°C lower than that under Comparative Example 1. This is mainly due to the radiative cooling properties imparted by the zinc oxide in the coating of Example 3.

[0033] Table 1

[0034] 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 a functional radiation-cooling coating, characterized in that, Includes the following steps: (1) Add zinc oxide to an octyltrimethoxysilane solution and heat at 55-65 °C for 3-4 h to obtain a silane-modified zinc oxide dispersion; (2) The silane-modified zinc oxide dispersion was mixed with a β-cyclodextrin solution and ultrasonicated to obtain a homogeneous dispersion; the concentration of the β-cyclodextrin solution was 2wt% ~ 3wt%. (3) Add the homogeneous dispersion obtained in step 2 to 35 wt% aqueous polyurethane dispersion and stir to disperse to obtain a uniform composite dispersion. (4) N-isopropylacrylamide, N,N'-methylenebisacrylamide, ammonium persulfate and deionized water were mixed evenly in a mass ratio of 12.5:1.25:1 and heated to 70 ~ 80 °C to obtain poly(N-isopropylacrylamide) dispersion; (5) Add the poly(N-isopropylacrylamide) dispersion to the composite dispersion obtained in step 3, add a thickener, and stir evenly to obtain a modified waterborne polyurethane dispersion. (6) The modified waterborne polyurethane dispersion is uniformly coated on the substrate, and the solvent is removed by heating at 80 ~ 100 ℃ to prepare a functional radiation cooling coating.

2. The method according to claim 1, characterized in that, The octyltrimethoxysilane solution in step 1 comprises octyltrimethoxysilane, ethanol, and deionized water in a volume ratio of 3:4:1; the pH of the octyltrimethoxysilane solution is adjusted to 3.5 using hydrochloric acid; the mass-volume ratio of zinc oxide to water in the octyltrimethoxysilane solution in step 1 is (0.2-0.8) g:1 ml.

3. The method according to claim 1, characterized in that, The volume ratio of deionized water in the β-cyclodextrin solution to the octyltrimethoxysilane solution is 2:

1.

4. The method according to claim 1, characterized in that, In step (3), the volume ratio of the aqueous polyurethane dispersion to the β-cyclodextrin solution is 2:1 ~ 2.86:

1.

5. The method according to claim 1, characterized in that, In step (5), the concentration of poly(N-isopropylacrylamide) dispersion is 3.8 wt%, and the volume ratio of it to β-cyclodextrin solution is 1:

1.

6. The method according to claim 1, characterized in that, The thickener in step (5) is a 2 wt% sodium carboxymethyl cellulose solution with a volume ratio of 1:1 to the β-cyclodextrin solution.

7. A functional radiation-cooling coating prepared by the method described in claim 1.

8. An application of the functional radiation cooling coating as described in claim 7.

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