Radiation refrigeration coating based on reactive polyurethane and preparation method thereof

Abstract

The invention provides a radiation refrigeration coating based on reactive polyurethane and a preparation method thereof. The radiation refrigeration coating based on reactive polyurethane is prepared from the following components in percentage by weight: 10 to 30 percent of silicon dioxide filler, 30 to 60 percent of macromolecular polyol, 20 to 40 percent of isocyanate, 1 to 10 percent of chain extender and 0.1 to 0.5 percent of catalyst, wherein the silicon dioxide filler is composed of at least two kinds of hollow silicon dioxide with different particle sizes. Through the gradation effect of the multi-particle-size filler, the technical problems of high viscosity and poor dispersity of a system under high filler filling are solved, the coating has high sunlight reflectivity, high infrared emissivity and excellent processing performance, the cooling effect obviously lower than the environment temperature can be achieved in the natural environment, the process is simple, and the coating is suitable for large-scale production.

Description

Technical Field

[0001] This invention relates to the field of daytime radiation cooling coating technology, specifically to a radiation cooling coating based on reactive polyurethane and its preparation method. Background Technology

[0002] Thermal comfort is crucial for human health and quality of life. However, against the backdrop of climate change and accelerated urbanization, extreme heatwaves are becoming increasingly frequent, directly threatening human health and reducing work efficiency. This also leads to a surge in energy consumption for traditional active cooling systems, exacerbating the energy crisis and the urban heat island effect. Therefore, the development of zero-energy passive cooling technologies is urgently needed, with radiative cooling technology attracting particular attention. This technology achieves cooling through the optical properties of the material itself: it fully reflects, refracts, and scatters sunlight in the solar spectrum (0.3~2.5μm), blocking most of the sunlight; simultaneously, the material possesses high emissivity in the atmospheric window band (8~13μm), directly dissipating the absorbed heat into the cold outer space as infrared radiation. This synergistic mechanism of "blocking heat from the outside and dissipating heat into the air" allows for continuous cooling without any external energy input. Radiative cooling functional coatings are one of the most promising pathways for the large-scale, low-cost application of radiative cooling technology. Radiation-cooling coatings can be easily applied to various object surfaces (such as building facades, roofs, storage tanks, textiles, etc.), efficiently performing the dual tasks of solar reflection and infrared radiation heat dissipation within sub-millimeter or even micrometer-scale thicknesses, while also possessing high efficiency, flexibility, and convenient construction. This technology provides a directly deployable solution for addressing global warming and the urban heat island effect, effectively reducing the surface temperature of target objects and decreasing energy consumption.

[0003] Currently, radiation cooling coatings are mainly classified into organic polymer coatings, inorganic coatings, and organic-inorganic composite coatings. Organic-inorganic composite coatings, prepared by mixing organic polymers with inorganic fillers, have gained wider attention due to their combination of the advantages of both substances. Common organic polymer phases in organic-inorganic composite coatings include acrylic resins, epoxy resins, and silicone resins. Polyurethane coatings not only possess excellent film-forming properties and mechanical strength, but also naturally exhibit high infrared emissivity due to the C=O, CO, and NH chemical bonds in their molecular chains, making them one of the optimal choices for the organic phase in radiation cooling coatings. Polyurethane coatings can be further classified into waterborne polyurethane coatings, solvent-based polyurethane coatings, and reactive solvent-free polyurethane coatings. Waterborne polyurethane uses water as the continuous phase, typically has a low solid content, and offers significant advantages such as environmental friendliness, non-toxicity, and non-flammability. However, to achieve good film formation, film-forming aids are often required, and its initial water resistance, drying speed, and final mechanical properties are sometimes inferior to solvent-based systems. In contrast, solvent-based polyurethane coatings use organic solvents (such as DMF, acetone, esters, etc.) as a medium, which can form coating films with higher density, better mechanical properties, stronger adhesion, and faster drying, making them particularly suitable for manufacturing high-performance, high-durability textile coatings; however, their disadvantages include volatile organic compound emissions and certain environmental and safety issues. Reactive solvent-free polyurethane coatings have advantages such as zero VOCs, no solvent toxicity or fire risk, extremely high construction efficiency, and high material utilization, but their high viscosity limits the amount of inorganic fillers that can be added. Common inorganic fillers in organic-inorganic composite coatings include hollow silica, hollow alumina, and hollow titanium dioxide, which enhance the cooling effect by creating multiple scattering and reflection of sunlight between the hollow air layer and the shell. Existing technologies mostly use inorganic fillers with a single particle size, which, although improving scattering ability to some extent, has a limited range of effect and cannot cover a wide range of solar spectra, especially in the visible and near-infrared regions, where the scattering efficiency of single-particle-size fillers is insufficient, limiting further improvement in the overall performance of the material. Meanwhile, the high viscosity of reactive solvent-free polyurethane systems makes it difficult to uniformly disperse high proportions of fillers, resulting in low filler loading and consequently affecting the coating's reflectivity and cooling performance. Therefore, it is urgent to optimize the radiation cooling coating of reactive solvent-free polyurethane to simultaneously possess the superior properties of high solar reflectivity, high infrared emissivity, and excellent application leveling properties. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a radiation cooling coating based on reactive polyurethane and its preparation method, thus solving the problems mentioned in the background section.

[0005] To achieve the above objectives, the present invention provides the following technical solution: According to a first aspect of the present invention, a radiation-cooling coating based on reactive polyurethane and a method for preparing the same are provided. The product comprises, by weight fraction, the following components: 10-30% silica filler, 30-60% macromolecular polyol, 20-40% isocyanate, 1-10% chain extender, and 0.1-0.5% catalyst; The silica filler is composed of at least two types of hollow silica with different particle sizes.

[0006] Preferably, the silica filler is composed of hollow silica with a particle size of 100~300nm, hollow silica with a particle size of 1~2μm, and hollow silica with a particle size of 2~3μm; The mass ratio of hollow silica with a particle size of 100~300nm, hollow silica with a particle size of 1~2μm, and hollow silica with a particle size of 2~3μm is 1~1.5:2~3:1~1.5.

[0007] Existing technologies require a large amount of inorganic filler to achieve high solar reflectivity, which leads to a sharp increase in the viscosity of the reactive polyurethane system and a deterioration in its rheological properties. This, in turn, results in poor coating leveling performance and difficult application. Furthermore, the high filler content can easily lead to a decrease in coating flexibility and an increased risk of cracking. This invention, by selecting multi-sized hollow silica particles to synergistically work with the reactive polyurethane matrix, ensures that the filler is at the optimal distance, maintaining excellent system processing flowability. Simultaneously, it further increases the filler content, fully utilizing the Mie scattering effect to effectively enhance the reflection and refraction of sunlight. This resolves the inherent contradiction between high reflectivity, wide-band scattering, filler dispersion stability, and excellent optical and mechanical properties.

[0008] Preferably, the shell thickness of the hollow silica is 100~300nm.

[0009] Preferably, the molecular weight range of the macromolecular polyol is 1000~3000 g / mol.

[0010] Preferably, the macromolecular polyol is selected from at least one of tetrahydrofuran ether diol, polycaprolactone diol, polycarbonate diol, and highly cis-terminated hydroxyl polybutadiene.

[0011] Preferably, the isocyanate is selected from at least one of toluene diisocyanate, diphenylmethane diisocyanate, isophorone diisocyanate, and hexamethylene diisocyanate; The chain extender is selected from at least one of ethylene glycol, 1,4-butanediol, and 1,2-propanediol; The catalyst is selected from at least one of triethylenediamine, dimethylpyridine, dibutyltin dilaurate, and stannous octoate.

[0012] According to a second aspect of the present invention, a method for preparing a radiation-cooling coating based on reactive polyurethane is provided, comprising the following steps: S1. Mix and stir a portion of macromolecular polyol, chain extender, catalyst and silica filler to obtain a polymer containing capped hydroxyl groups; S2. Under a nitrogen atmosphere, the isocyanate and the remaining macromolecular polyol are stirred and reacted to obtain an isocyanate-terminated prepolymer. S3. The polymer containing hydroxyl groups and the isocyanate-terminated prepolymer are stirred and reacted under vacuum, and then spin-coated onto release paper and heated to cure, to obtain the radiation cooling coating.

[0013] Preferably, in step S1, the mass ratio of the chain extender to the macromolecular polyol is 1:5~7; The stirring temperature is room temperature, the stirring speed is 250~500 rpm, and the stirring time is 30~60 min.

[0014] Preferably, in step S2, the mass ratio of the macromolecular polyol to the isocyanate is 1:2~4. The stirring reaction is carried out at a temperature of 70~90℃, a stirring speed of 250~500rpm, and a time of 1.5~2.5h.

[0015] Preferably, in step S3, the stirring reaction is carried out at room temperature, at a speed of 1500-2000 rpm, and for a time of 30-60 seconds. The thickness of the spin coating is 100~300μm; The heating and curing temperature is 50~80℃, and the time is 1~2h.

[0016] This invention provides a radiation-cooling coating based on reactive polyurethane and its preparation method. It has the following beneficial effects: This solution provides a radiation cooling coating based on reactive polyurethane. By compounding multi-particle-size hollow silica with a polyurethane system, the solar reflectivity can reach over 95%, the infrared emissivity can reach over 94%, it has excellent construction fluidity, and the surface temperature reduction can reach over 6.3℃. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] Unless otherwise specified, all raw materials and equipment used in this invention can be purchased from the market or prepared using existing methods.

[0019] The present invention provides a radiation cooling coating based on reactive polyurethane, comprising the following components by weight fraction: 10-30% silica filler, 30-60% macromolecular polyol, 20-40% isocyanate, 1-10% chain extender and 0.1-0.5% catalyst; The silica filler is composed of at least two types of hollow silica with different particle sizes.

[0020] The silica filler consists of nano-sized hollow silica, submicron-sized hollow silica, and micron-sized hollow silica.

[0021] The present invention provides a method for preparing a radiation-cooling coating based on reactive polyurethane, comprising the following steps: S1. Mix and stir a portion of macromolecular polyol, chain extender, catalyst, and hollow silica to obtain a polymer containing capped hydroxyl groups; S2. Under a nitrogen atmosphere, the isocyanate and the remaining macromolecular polyol are stirred and reacted to obtain an isocyanate-terminated prepolymer. S3. The polymer containing hydroxyl groups is stirred and reacted with the isocyanate-terminated prepolymer and then vacuumed. Finally, it is spin-coated onto release paper and heated to cure, thus obtaining a radiation-cooled coating.

[0022] The technical solution of the present invention will be further described below with reference to specific embodiments, so as to enable those skilled in the art to further understand the present invention.

[0023] Example 1 Preparation of polymers containing capped hydroxyl groups: 36g of polytetrahydrofuran ether diol, 7.2g of 1,4-butanediol, 0.1g of dibutyltin dilaurate, and 11g of multi-particle-size hollow silica (composed of silica particles with particle sizes of 100~300nm, 1~2μm, and 2~3μm in a mass ratio of 1.2:2.5:1.2) were stirred at room temperature and 400 rpm for 45 min to obtain polymers containing capped hydroxyl groups; Preparation of isocyanate-terminated prepolymer: 10g of polytetrahydrofuran ether diol was added to 30g of isophorone diisocyanate and stirred for 2h at 80℃ and 350rpm to form an isocyanate-terminated prepolymer. The polymer containing hydroxyl groups and the isocyanate-terminated prepolymer were added to the reactor and stirred for 40 seconds at room temperature and 1500 rpm. Then, the mixture was vacuumed and poured onto release paper for coating. The coating was then heated and cured at 60°C for 1.5 hours to obtain a radiation-cooling coating with a thickness of 200 μm.

[0024] Example 2 The preparation method of this embodiment is the same as that of Example 1, except that the mass ratio of hollow silica with a particle size of 100~300nm, hollow silica with a particle size of 1~2μm and hollow silica with a particle size of 2~3μm is 1:3:1.

[0025] Example 3 The preparation method of this embodiment is the same as that of Example 1, except that the mass ratio of hollow silica with a particle size of 100~300nm, hollow silica with a particle size of 1~2μm and hollow silica with a particle size of 2~3μm is 1.5:12:1.5.

[0026] Example 4 The preparation method of this embodiment is the same as that of Example 1, except that the mass ratio of polytetrahydrofuran ether diol to isophorone diisocyanate is 1:4 during the preparation of the isocyanate-terminated prepolymer.

[0027] Example 5 The preparation method of this embodiment is the same as that of Example 1, except that the mass ratio of polytetrahydrofuran ether diol to isophorone diisocyanate is 1:2 during the preparation of the isocyanate-terminated prepolymer.

[0028] Example 6 The preparation method of this embodiment is the same as that of Example 1, except that in the preparation process of the polymer containing capped hydroxyl groups, the mass ratio of 1,4-butanediol to polytetrahydrofuran ether diol is 1:6.

[0029] Example 7 The preparation method of this embodiment is the same as that of Example 1, except that in the preparation process of the polymer containing capped hydroxyl groups, the mass ratio of 1,4-butanediol to polytetrahydrofuran ether diol is 1:7.

[0030] Comparative Example 1 The comparative example was prepared using the same method as Example 1, except that no hollow silica was added during the preparation of the polymer containing capped hydroxyl groups.

[0031] Comparative Example 2 The comparative example is prepared using the same method as Example 1, except that hollow silica with a particle size of 1-2 μm is added during the preparation of the polymer containing capped hydroxyl groups.

[0032] Comparative Example 3 The comparative example is prepared using the same method as Example 1, except that the mass ratio of 1,4-butanediol to polytetrahydrofuran ether diol is 1:10 during the preparation of the polymer containing capped hydroxyl groups.

[0033] Comparative Example 4 The comparative example is prepared using the same method as Example 1, except that the mass ratio of polytetrahydrofuran ether diol to isophorone diisocyanate is 1:7 during the preparation of the isocyanate-terminated prepolymer.

[0034] Comparative Example 5 The comparative example was prepared using the same method as Example 1, except that the polymer containing capped hydroxyl groups and the isocyanate-capped prepolymer were added to the reactor and stirred for 2 minutes at room temperature and 1500 rpm before being evacuated.

[0035] Performance testing The solar reflectance (0.3~2.5μm) of the radiation-cooled coating was measured using a UV-Vis-NIR spectrometer. The infrared emissivity (8~13μm) of the radiation-cooled coating was measured using Fourier transform infrared spectroscopy (FTIR). The viscosity of the radiation-cooled coating at 25°C was tested using a rotational rheometer. The surface temperature drop of the coating was tested using an infrared thermal imager under outdoor natural environmental conditions (or indoor temperature measurement under simulated solar irradiation).

[0036] The test results of measuring the solar reflectance, infrared emissivity, viscosity, and surface temperature drop of the reactive polyurethane-based radiation-cooling coating are shown in Table 1.

[0037] Table 1

[0038] In the table above, the data in Comparative Examples 4 and 5 could not be tested for solar reflectivity, infrared emissivity, and surface temperature drop due to excessive viscosity.

[0039] As shown in Table 1, without the addition of fillers, the simple reactive polyurethane coating does not have a radiative cooling function. Although its high infrared emissivity is beneficial for heat dissipation, its extremely low solar reflectivity will cause the overall heat absorption and temperature rise, without any surface cooling effect.

[0040] As demonstrated in Examples 1, 2, and 3, and Comparative Example 2 (single-size filler), the precise blending of nanoscale (100~300nm), submicron (1~2μm), and micron (2~3μm) hollow silica is key to achieving wide-band, high-efficiency solar reflectivity. Single-size fillers can only effectively scatter light in specific wavelengths, while multi-size blended systems, through synergistic effects, can cover the entire solar spectrum from ultraviolet and visible light to near-infrared, thus achieving significantly higher solar reflectivity. Based on Examples 6 and 7 and Comparative Example 3 (excessive macromolecular polyol), it is evident that there is an optimal range (1:5~7) for the mass ratio of chain extender to macromolecular polyol. When the ratio is too high, the viscosity is too high, and the filler cannot be mixed well and evenly, resulting in a decrease in solar reflectivity and infrared emissivity. As shown in Examples 4 and 5 and Comparative Example 4, the mass ratio of polytetrahydrofuran ether diol to isophorone diisocyanate in the preparation of the prepolymer has a significant impact on the viscosity of the coating. In the initial stage of the reaction, when isocyanate is slightly in excess, the molecular weight distribution shifts towards higher molecular weights, which in itself leads to an increase in viscosity. When isocyanate is severely in excess, it directly leads to an infinitely large three-dimensional network of molecules (gel), causing the viscosity to approach infinity, making coating impossible.

[0041] As shown in Comparative Example 5, the polymer containing hydroxyl-terminated polymer and the isocyanate-terminated prepolymer were then added to the reactor. Excessive stirring time at room temperature and 1500 rpm caused a violent reaction, resulting in an exponential increase in viscosity that made coating impossible.

[0042] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A radiation-cooling coating based on reactive polyurethane, characterized in that: The product comprises, by weight fraction, the following components: 10-30% silica filler, 30-60% macromolecular polyol, 20-40% isocyanate, 1-10% chain extender, and 0.1-0.5% catalyst; The silica filler is composed of at least two types of hollow silica with different particle sizes.

2. The radiation-cooling coating based on reactive polyurethane according to claim 1, characterized in that: The silica filler is composed of hollow silica with a particle size of 100~300nm, hollow silica with a particle size of 1~2μm, and hollow silica with a particle size of 2~3μm. The mass ratio of hollow silica with a particle size of 100~300nm, hollow silica with a particle size of 1~2μm, and hollow silica with a particle size of 2~3μm is 1~1.5:2~3:1~1.

5.

3. A radiation-cooling coating based on reactive polyurethane according to claim 1 or 2, characterized in that: The shell thickness of the hollow silica is 100~300nm.

4. The radiation cooling coating based on reactive polyurethane according to claim 1, characterized in that: The molecular weight range of the macromolecular polyol is 1000~3000 g / mol.

5. The radiation-cooling coating based on reactive polyurethane according to claim 4, characterized in that: The macromolecular polyol is selected from at least one of tetrahydrofuran ether diol, polycaprolactone diol, polycarbonate diol, and highly cis-terminated hydroxyl polybutadiene.

6. The radiation-cooling coating based on reactive polyurethane according to claim 1, characterized in that: The isocyanate is selected from at least one of toluene diisocyanate, diphenylmethane diisocyanate, isophorone diisocyanate, and hexamethylene diisocyanate; The chain extender is selected from at least one of ethylene glycol, 1,4-butanediol, and 1,2-propanediol; The catalyst is selected from at least one of triethylenediamine, dimethylpyridine, dibutyltin dilaurate, and stannous octoate.

7. A method for preparing a radiation-cooling coating based on reactive polyurethane according to any one of claims 1 to 6, characterized in that: Includes the following steps: S1. Mix and stir a portion of macromolecular polyol, chain extender, catalyst and silica filler to obtain a polymer containing capped hydroxyl groups; S2. Under a nitrogen atmosphere, the isocyanate and the remaining macromolecular polyol are stirred and reacted to obtain an isocyanate-terminated prepolymer. S3. The polymer containing hydroxyl groups and the isocyanate-terminated prepolymer are stirred and reacted under vacuum, and then spin-coated onto release paper and heated to cure, to obtain the radiation cooling coating.

8. The method for preparing a radiation-cooling coating based on reactive polyurethane according to claim 7, characterized in that: In step S1, the mass ratio of the chain extender to the macromolecular polyol is 1:5~7; The stirring temperature is room temperature, the stirring speed is 250~500 rpm, and the stirring time is 30~60 min.

9. The method for preparing a radiation-cooling coating based on reactive polyurethane according to claim 7, characterized in that: In step S2, the mass ratio of the macromolecular polyol to the isocyanate is 1:2~4. The stirring reaction is carried out at a temperature of 70~90℃, a stirring speed of 250~500rpm, and a time of 1.5~2.5h.

10. The method for preparing a radiation-cooling coating based on reactive polyurethane according to claim 7, characterized in that: In step S3, the stirring reaction is carried out at room temperature, at a speed of 1500~2000 rpm, and for a time of 30~60 s. The thickness of the spin coating is 100~300μm; The heating and curing temperature is 50~80℃, and the time is 1~2h.

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