Preparation method and application of kapok-based porous material with radiation cooling function

Abstract

The invention relates to the field of functional materials, in particular to a preparation method and application of a kapok-based porous material with a radiation cooling function. According to the method, natural kapok short fibers are subjected to constant-temperature alkali treatment to peel off wax on the surface layer, a delignification reaction is carried out with an acidic sodium chlorite solution, and the kapok-based porous material with the radiation cooling function is obtained. And the coarse reticular cellulose structure in the material is thoroughly exposed. The treated active fibers and nano cellulose are subjected to high-strength blending in a water phase, nano fibrils are guided to deeply penetrate into gaps of a micron-sized kapok skeleton, and a tightly-meshed composite system is constructed. And dropwise adding an organosiloxane reagent into the acidic liquid phase to complete surface hydrophobic modification, and carrying out controlled low-temperature setting and vacuum freezing sublimation dehydration to form the three-dimensional base material containing a micro-nano dual-pore network. The material can effectively reflect sunlight and emit heat radiation through an atmosphere window, zero-energy-consumption passive radiation cooling is achieved, and meanwhile the excellent heat insulation performance is achieved.

Description

Technical Field

[0001] This invention relates to the field of functional materials technology, specifically to a method for preparing a kapok-based porous material with radiation cooling function and its application. Background Technology

[0002] Cooling is a fundamental need in modern society, but traditional air conditioning and other cooling equipment consume enormous amounts of electricity and create significant environmental pressure. Radiative cooling, a promising passive cooling technology, utilizes the physical mechanism of objects emitting heat radiation into outer space through atmospheric windows, achieving spontaneous cooling without requiring external energy supply. This technology is increasingly valuable in reducing building energy consumption and optimizing personal thermal management.

[0003] Biomass materials, with their abundant sources and green, renewable nature, are gradually becoming popular candidates for preparing radiative cooling media. Kapok fiber, as a typical natural cellulose matrix, possesses a unique high-porosity hollow structure and extremely low density, along with excellent thermal insulation properties, making it widely regarded by academia and industry as an ideal precursor for constructing lightweight porous frameworks. Current research often employs simple physical blending processes to process kapok fiber, attempting to preserve its macroscopic morphology to meet lightweight insulation requirements.

[0004] Natural kapok fibers are inevitably coated with a dense waxy layer, and their interior contains a large amount of heat-absorbing impurities such as lignin. The active groups such as benzene rings and olefins in the lignin molecular structure have extremely high molar absorptivity, exhibiting extremely strong heat absorption characteristics in the solar radiation band. This causes a precipitous drop in the material's solar reflectivity, severely weakening its radiative cooling efficiency. The porous framework formed by conventional simple physical mixing processes has extremely weak inter-linking forces, making it prone to structural collapse under temperature and humidity fluctuations or minor external pressure, resulting in a severe decrease in material porosity and an inability to maintain its optical and thermal insulation performance for a long time. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method for preparing a kapok-based porous material with radiative cooling function and its application, solving the problems of strong heat absorption capacity, poor structural stability, and severely insufficient radiative cooling performance of existing kapok-based materials.

[0006] To achieve the above objectives, the present invention provides a method for preparing a kapok-based porous material with radiation cooling function, comprising the following steps: Step 1: Physically cut the natural kapok fiber to keep the fiber length in the range of 300-600 μm. Immerse the cut kapok fiber in sodium hydroxide solution for alkaline treatment to effectively remove the waxy components attached to the surface. Stir continuously at a constant temperature of 80°C for 4 hours. After the reaction is terminated, collect the obtained fiber through solid-liquid separation and wash it repeatedly with deionized water until it is neutral. Step 2: Transfer the washed fibers to a sodium chlorite solution, add glacial acetic acid to the mixture, and precisely adjust the overall pH value to 4.0-5.0. Then, stir the reaction at 80°C for 1-3 hours to perform delignification treatment, so that the network cellulose structure wrapped inside the kapok fibers can be fully exposed, thereby obtaining delignified kapok fibers. Step 3: The above-prepared lignin-free kapok fibers and nanocellulose are added together into a deionized water system and thoroughly dispersed and mixed to prepare a uniform fiber composite suspension. Step 4: Adjust the pH of the fiber composite suspension to make it acidic, introduce organosiloxane reagent into the suspension to carry out hydrophobic modification reaction, and obtain modified mixed fiber suspension after stirring for a specific time. Step 5: Inject the hydrophobically modified mixed fiber suspension into the molding mold and place it in a low-temperature environment for freezing and shaping. After shaping, transfer it to a vacuum device for freeze-drying to completely remove moisture and finally form a kapok-based porous material with radiation cooling function.

[0007] Preferably, the mass concentration of the sodium hydroxide solution in step 1 is 2.0–16.0 wt%; and the mass concentration of the acidic sodium chlorite solution in step 2 is 1.0–3.0 wt%.

[0008] Preferably, in step 3, the overall solid content of the obtained fiber composite suspension is controlled at 0.2 to 2.0 wt%, and the mass ratio of the lignin-derived kapok fiber to nanocellulose in the suspension system is 1:1.

[0009] Preferably, in step 4, glacial acetic acid or hydrochloric acid is used as an acid regulator to adjust the pH of the fiber composite suspension to 4.0-5.0. The organosiloxane is vinyltrimethoxysilane. The duration of the hydrophobic modification reaction is 3-5 hours, and the mass fraction of the organosiloxane in the fiber composite suspension is 0.1-1.0 wt%.

[0010] Preferably, the cryogenic forming process in step 5 includes two operating conditions: placing it in a freezing device at -30 to -10°C for continuous cooling for 6 to 12 hours or directly immersing it in a liquid nitrogen environment at -196°C for rapid freezing for 10 to 30 minutes.

[0011] Preferably, during the vacuum freeze-drying process in step 5, the temperature inside the drying chamber needs to be maintained at -70 to -50°C, and the operating pressure parameter is set to 0.02 to 0.08 mbar, with a corresponding vacuum sublimation drying time of 24 to 60 hours.

[0012] This invention provides a method for preparing a kapok-based porous material with radiation cooling function and its application. It has the following beneficial effects: 1. This invention utilizes a dual chemical removal method, employing a specific concentration of alkaline solution supplemented by an acidic sodium chlorite solution, to thoroughly clean the dense waxy coating adhering to the surface of kapok fibers and deeply extract and remove a large amount of highly heat-absorbing impurities such as lignin embedded within them. This fundamentally eliminates the inherent heat absorption drawbacks of biomass materials in the solar spectrum. This modification process breaks down the closed surface of natural fibers, allowing the rough and highly active hydroxyl-rich network cellulose structure inside the kapok to be exposed to the greatest extent possible. This creates a highly active reaction substrate for subsequent deep fusion at the nanoscale interface, ensuring that the molded material possesses extremely high solar reflectivity potential.

[0013] 2. This invention uses the exposed network structure after lignin removal as a molecular-level anchor point, and introduces a nanocellulose suspension system for liquid-phase blending. By leveraging the size effect of nanofibers, the nanofibers penetrate deep into the gaps and internal pores of the micron-scale kapok skeleton. Through a dense hydrogen bond network and physical interweaving, deep entanglement and composite formation of micron-fibers and nanofibers are achieved. This specially constructed micro-nano hierarchical dual skeleton system significantly improves the specific surface area and node bonding strength of porous media, completely avoiding the structural disadvantages of conventional purely physically mixed porous materials that are prone to breakage and collapse under environmental stress, thus endowing the material with extremely excellent compression resilience stability and service life.

[0014] 3. This invention utilizes a micro-nano hierarchical cross-linking structure, supplemented by hydrophobic modification with organosiloxanes and vacuum freeze-drying dehydration technology, to precisely shape a complex pore network with characteristic sizes ranging from tens to hundreds of nanometers within the material. The three-dimensional distribution scale of these nanoscale micropores and filaments achieves a perfect size match with the main radiation bands of sunlight, acting as extremely dense and efficient optical scattering centers. Together with the micron-scale coarse framework, it induces a strong multiple Mie scattering effect, greatly extending the transmission path of light within the material and promoting efficient total internal reflection of incident light. Simultaneously, in conjunction with the inherent low thermal conductivity and excellent high-to-medium infrared emissivity of the high-porosity porous material, it achieves an extreme passive cooling performance of up to 11.36℃ in real outdoor strong light irradiation scenarios without energy intervention. Attached Figure Description

[0015] Figure 1 This is a scanning electron microscope image of the kapok-based porous material with radiation cooling function of the present invention; Figure 2This is a partial magnified scanning electron microscope image of the kapok-based porous material with radiation cooling function of the present invention. Figure 3 This is a spectral diagram of the kapok-based porous material with radiation cooling function of the present invention; Figure 4 This is a contact angle diagram of the kapok-based porous material with radiation cooling function of the present invention; Figure 5 This is a diagram showing the thermal conductivity of the kapok-based porous material with radiation cooling function according to the present invention. Figure 6 This is an outdoor radiative cooling test diagram of the kapok-based porous material with radiative cooling function of the present invention; Figure 7 The figure shows the deformation of the kapok-based porous material with radiation cooling function of the present invention after 50 cycles of cyclic compression with a deformation of 40%. Detailed Implementation

[0016] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Example

[0017] This invention provides a method for preparing a kapok-based porous material with radiation cooling function, comprising the following steps: Step 1: Lignin removal treatment of kapok fibers: Commercially available natural kapok fibers were cut to a length of 300 μm using a shearing device. The cut short fibers were then placed in a 4 wt% sodium hydroxide aqueous solution and continuously stirred in a constant temperature water bath set at 80°C for 4 hours. After the predetermined time, solid-liquid separation was performed using a filtration device. The dewaxed fibers were collected and repeatedly rinsed with deionized water until the filtrate was neutral.

[0018] The washed fibers were transferred to a container containing 150 mL of a 1.5 wt% sodium chlorite solution. Glacial acetic acid was added dropwise to the mixture, and the pH was adjusted to a stable range of 4.0–5.0 using a pH meter. The container was placed in an 80°C water bath and stirred at a constant speed for 2 hours. Heating and stirring were immediately stopped when the color of the fibers in the suspension completely changed from light yellow to pure white. After another solid-liquid separation, the product was washed with deionized water until neutral, thus obtaining delignified kapok fibers. This chemical process completely removes a large amount of hidden lignin, fully exposing the interface of the previously encapsulated cellulose network structure.

[0019] Step 2: Preparation of the composite suspension: The delignified kapok fiber suspension prepared in the previous step was mixed thoroughly with nanocellulose (CNF) and an appropriate amount of deionized water. A composite suspension system with a total mass fraction of 1.6 wt% was prepared, in which the dry weight percentage of delignified kapok fiber was 0.8 wt%, and the dry weight percentage of nanocellulose was also 0.8 wt% (i.e., a mass ratio of 1:1). This system was placed at room temperature and vigorously stirred at 800 rpm for 4 hours to obtain a fiber composite suspension with extremely uniform dispersion. The exposed high-roughness kapok cellulose structure provided excellent permeability channels for nanocellulose, promoting deep penetration of nanocellulose into the network structure and forming a tightly interlocked composite dual-skeleton system.

[0020] Step 3: Hydrophobic modification treatment: While maintaining a stirring speed of 800 rpm, slowly add a dilute solution of glacial acetic acid dropwise to the prepared composite suspension, monitoring the pH value in real time and maintaining it between 4.0 and 5.0. Add vinyltrimethoxysilane (VTMO) at a ratio of 0.2 wt% of the total system mass to the acidified suspension. Maintain constant temperature and stirring for 4 hours to ensure that the vinyltrimethoxysilane molecules are fully hydrolyzed in the aqueous phase and uniformly coated on the fiber skeleton surface, completing the deep hydrophobic modification reaction.

[0021] Step 4: Freeze-drying and shaping: A hydrophobically modified mixed composite suspension is uniformly poured into a mold cavity with a specific geometric morphology. The entire mold is then transferred to a -18°C cryogenic chamber and continuously frozen for 12 hours. After freezing and shaping, the solid ice mold is carefully peeled off and quickly transferred into a vacuum freeze-drying chamber. A vacuum is drawn to reduce the absolute pressure inside the drying chamber to below 20 Pa. Under conditions of no liquid phase generation, continuous sublimation drying is performed for 48 hours to completely remove the ice crystal template, ultimately forming a kapok-based porous material with ultra-high porosity and radiation cooling capabilities.

[0022] Performance testing and characterization: The porous material prepared in Example 1 was comprehensively tested and evaluated using high-precision professional testing equipment. Figure 1 and Figure 2 As shown, microscopic morphological observation reveals that the material exhibits an extremely regular and dense three-dimensional honeycomb-like porous network. Figure 4 As shown, the surface wettability test recorded a static contact angle as high as 144.2 ± 0.5°, demonstrating excellent surface hydrophobic protection capabilities. For example... Figure 3As shown, in terms of core optical performance, the material's full-spectrum solar reflectivity jumps to 90.29%, and its mid-infrared emissivity in the atmospheric window band reaches 92.77%. Figure 5 As shown, the thermal conductivity test recorded an extremely low value of 0.0358 W·m. -1 ·K -1 .like Figure 6 As shown, the sample was placed in a real outdoor environment under clear sky and intense sunlight for field verification. Test data showed that the porous material system achieved an astonishing passive cooling range of 7.25℃ on average and a peak temperature of 11.36℃. Figure 7 As shown, mechanical tests verified the high reliability of the structure. After 50 consecutive cyclic compression tests with a set deformation of 40%, the porous material was still able to quickly rebound and recover to its initial macroscopic state. No observable microscopic skeleton fracture or macroscopic collapse occurred, which confirmed the huge enhancement effect of the core process of exposing the network structure after lignin stripping and the deep entanglement of nanocellulose on the overall strength of the material.

[0023] Comparative Example 1: This invention provides a comparative example of a preparation process for a traditional kapok-based porous material (designated AK) that has not undergone delignification treatment. The key difference between this process and Example 1 lies in the omission of several critical chemical processing steps: In step one, only a single alkali treatment (NaOH soaking) for dewaxing was applied to the natural kapok, completely eliminating the acidic sodium chlorite delignin removal process. A large amount of lignin was retained within the original fibers. The nanocellulose composite process and subsequent organosiloxane hydrophobic modification were skipped, and the washed, single-type fibers were directly subjected to freeze-drying and sublimation.

[0024] Performance test results: Tests confirmed that the comparative product exhibited a significant dark yellow hue due to the large amount of lignin residue. This bulk structure, rich in chromophores, caused extremely severe heat absorption and accumulation when the material was exposed to direct sunlight. Field outdoor testing recorded a dramatic temperature increase on the material's surface, with the highest temperature rise compared to the ambient air reaching 7.92°C. This completely negated the original design intent of optical reflection cooling and rendered the required passive radiative cooling effect utterly impossible.

[0025] Comparative Example 2: This invention provides a comparative example of a preparation method for a kapok-based porous material (designated ACK) that incorporates a nanophase but undergoes no pretreatment for lignin removal. The significant difference between this method and Example 1 lies in the following: After completing the basic sodium hydroxide dewaxing operation in step one, the operation chain is directly cut off, omitting the acidic sodium chlorite delignin removal water bath reaction in step two. Subsequent processes are then resumed, namely, the semi-finished kapok fiber retaining lignin is directly added to deionized water at a dry weight ratio of 1:1 with nanocellulose to prepare a mixed suspension with a solid content completely equivalent to that of Example 1, and the hydrophobic modification operation and freeze-drying molding steps with the same parameters as in Example 1 are fully performed.

[0026] Performance test results: Testing revealed that the finished material still maintained a light yellow tint. Even though the forced incorporation of nanocellulose diluted the absolute concentration of color-producing impurities on a certain macroscopic scale and constructed some micro-nano scattering structures in local microscopic areas, the strong light absorption effect caused by the lignin components (containing strongly endothermic groups such as benzene rings) tightly embedded within the kapok body still completely overwhelmed the weak light scattering gain brought by the nanostructure on a physical scale. Actual outdoor exposure data indicated that the porous block still exhibited a significant heat absorption and temperature accumulation trend under sunlight, with the peak surface temperature recorded being 3.99℃ higher than the surrounding ambient temperature. This set of comparative data irrefutably proves that simply filling the biomass material with nano-dimensional cellulose cannot mask or neutralize the inherent heat absorption problem of the biomass material itself; deep chemical exfoliation treatment is necessary to give it a physical basis for cooling.

[0027] Comparative Example 3: This invention provides a comparative example of a kapok-based porous material (denoted as DLK) that has undergone chemical decolorization but lacks a micro / nano hierarchical structure. The process adjustment strategy for Comparative Example 1 is as follows: After completing steps one and two (i.e., alkaline washing and delignination), the obtained deligninated kapok fibers were dispersed separately, completely eliminating the need for the addition and compounding of nanocellulose. A single-component suspension was prepared separately in a deionized water system maintaining the same solids content, and subsequently, VTMO hydrophobic modification and vacuum freeze-drying were performed.

[0028] Performance test results: Optical equipment detected that the solar reflectance of this comparative material dropped to 77.14%, and the mid-infrared emissivity also decreased to 88.44%. Exploring its internal mechanism reveals that while the deep delignification process did eliminate the heat-generating optical absorption groups, the lack of nanoscale filament penetration and entanglement resulted in only micron-sized coarse pores remaining inside the material, failing to construct a complex three-dimensional hierarchical pore network. This extremely simple physical cavity structure severely lacks efficient optical scattering sites with characteristic sizes on the nanometer scale, reducing the frequency of light reflection and refraction within the material. Its overall solar scattering and reflection capability is significantly inferior to the sample in Example 1, which possesses a dual-skeleton system, directly resulting in its inability to meet the stringent optical specifications of a high-performance passive radiative cooling component.

[0029] 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 method for preparing a kapok-based porous material with radiation cooling function, characterized in that, Includes the following steps: Step 1: Physically cut the natural kapok fiber to keep the fiber length in the range of 300-600 μm. Immerse the cut kapok fiber in sodium hydroxide solution for alkaline treatment to effectively remove the waxy components attached to the surface. Stir continuously for 4 hours under constant temperature of 80℃. After the reaction is terminated, collect the obtained fiber through solid-liquid separation and wash it repeatedly with deionized water until it is neutral. Step 2: Transfer the washed fibers to a sodium chlorite solution, add glacial acetic acid to the mixture, and precisely adjust the overall pH value to 4.0-5.

0. Then, stir the reaction at 80°C for 1-3 hours to perform delignification treatment, so that the network cellulose structure wrapped inside the kapok fibers can be fully exposed, thereby obtaining delignified kapok fibers. Step 3: The above-prepared lignin-free kapok fibers and nanocellulose are added together into a deionized water system and thoroughly dispersed and mixed to prepare a uniform fiber composite suspension. Step 4: Adjust the pH of the fiber composite suspension to make it acidic, introduce organosiloxane reagent into the suspension to carry out hydrophobic modification reaction, and obtain modified mixed fiber suspension after stirring for a specific time. Step 5: Inject the hydrophobically modified mixed fiber suspension into the molding mold and place it in a low-temperature environment for freezing and shaping. After shaping, transfer it to a vacuum device for freeze-drying to completely remove moisture and finally form a kapok-based porous material with radiation cooling function.

2. The method for preparing a kapok-based porous material with radiation cooling function according to claim 1, characterized in that, The mass concentration of the sodium hydroxide solution in step 1 is 2.0–16.0 wt%; the mass concentration of the acidic sodium chlorite solution in step 2 is 1.0–3.0 wt%.

3. The method for preparing a kapok-based porous material with radiation cooling function according to claim 1, characterized in that, In step 3, the overall solid content of the obtained fiber composite suspension is controlled at 0.2 to 2.0 wt%, and the mass ratio of the lignin-derived kapok fiber to nanocellulose in the suspension system is 1:

1.

4. The method for preparing a kapok-based porous material with radiation cooling function according to claim 1, characterized in that, In step 4, glacial acetic acid or hydrochloric acid is used as an acid regulator to adjust the pH of the fiber composite suspension to 4.0-5.

0. The organosiloxane is vinyltrimethoxysilane. The duration of the hydrophobic modification reaction is 3-5 hours, and the mass fraction of the organosiloxane in the fiber composite suspension is 0.1-1.0 wt%.

5. The method for preparing a kapok-based porous material with radiation cooling function according to claim 1, characterized in that, The cryogenic forming process in step 5 includes two operating conditions: placing it in a freezing device at -30 to -10°C for continuous cooling for 6 to 12 hours or directly immersing it in a liquid nitrogen environment at -196°C for rapid freezing for 10 to 30 minutes.

6. The method for preparing a kapok-based porous material with radiation cooling function according to claim 1 and its application, characterized in that, During the vacuum freeze-drying process in step 5, the temperature inside the drying chamber needs to be maintained at -70 to -50°C, and the operating pressure parameter should be set to 0.02 to 0.08 mbar, with a corresponding vacuum sublimation drying time of 24 to 60 hours.

7. A kapok-based porous material with radiative cooling function, characterized in that, The porous material is prepared by the preparation method described in any one of claims 1 to 6. The material has a stable micro-nano-level dual porous framework constructed by the deep entanglement of lignin-exposed kapok fibers and nanocellulose, and exhibits hydrophobic properties as well as high reflectivity and high-to-medium infrared emissivity to sunlight.

8. The application of the kapok-based porous material with radiative cooling function as a zero-energy passive radiative cooling and insulation medium in building energy conservation, personal thermal management, outdoor facility protection, and cold chain logistics, according to claim 7.

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