A bacterial cellulose-based sandwich radiant cooling film with self-cleaning ability and a preparation method and application thereof

Abstract

The application discloses a bacterial cellulose-based sandwich radiative cooling film with self-cleaning capability and a preparation method and application thereof, and belongs to the technical field of passive radiative cooling materials.The application forms fiber interlocking between a BC layer and a BC-SiO2 layer through continuous wet-state suction filtration, and forms mechanical anchoring between a PDMS layer and the BC-SiO2 layer through partial infiltration of a PDMS prepolymer, thereby improving interface bonding.Finally, the prepared bacterial cellulose-based sandwich radiative cooling film has good emissivity and reflectivity, and also has good hydrophobicity, water repellency and self-cleaning capability; even if exposed to an outdoor high-humidity environment for a long time, water vapor can be effectively prevented from invading an internal porous structure, and stable radiative cooling performance can be maintained; after a surface is contaminated by dust or liquid, the surface can be easily washed and removed by water flow, and the problems of easy moisture absorption, easy contamination and poor long-term stability of a pure bacterial cellulose-based cooling film are solved.

Description

Technical Field

[0001] This invention relates to a bacterial cellulose-based sandwich radiation cooling membrane with self-cleaning ability, its preparation method and application, belonging to the field of passive radiation cooling materials technology. Background Technology

[0002] Passive radiative cooling is a thermal management strategy that achieves cooling without requiring additional energy input. This technology passively cools object surfaces by reflecting incident solar radiation as much as possible within the solar spectrum (0.3–2.5 μm) and radiating heat into outer space as thermal radiation within the atmospheric window (8–13 μm). This technology holds significant promise for applications in building energy conservation, outdoor equipment protection, cold chain transportation, personal thermal management, and heat dissipation for electronic devices.

[0003] Existing radiation cooling materials typically include inorganic particle-filled coatings, porous polymer membranes, multilayer photonic structure membranes, and inorganic / organic composite membranes. While these materials can achieve high solar reflectivity and infrared emissivity, some solutions suffer from problems such as complex preparation processes, expensive equipment, insufficient flexibility, non-renewable substrates, performance degradation after contamination, or insufficient stability in outdoor humid and hot environments.

[0004] Bacterial cellulose is a type of natural nanocellulose material synthesized by microorganisms, characterized by its renewability, flexibility, low density, high porosity, and abundant hydroxyl groups. Its nanofiber network and porous structure facilitate light scattering, and C–O, C–O–C, and O–H groups also contribute to thermal radiation in the infrared band. Therefore, bacterial cellulose is a favorable platform for constructing bio-based radiation-cooling films. For example, Chinese patent CN115926235A discloses a radiation-cooled bacterial cellulose nanocomposite film with adjustable solar transmittance and its preparation method. This film is prepared by in-situ introducing inorganic particles onto the surface of bacterial cellulose and co-entangled with cellulose nanofibers. It exhibits high reflectivity in the solar range and high emissivity in the atmospheric window (8–13 μm). In addition, Shi et al. reported a scalable bacterial cellulose-based radiation cooling material. By entanglement of silica microspheres with continuously secreted cellulose nanofibers during in-situ bacterial culture, the resulting film achieved a solar reflectance of 95.3% and a mid-infrared emissivity of 93.4% (DOI: 10.1002 / smLl.202301957). Wang et al. prepared a three-dimensional porous composite material M-BC / BaSO4 by combining bacterial cellulose with BaSO4 using a unidirectional cryogenic casting method. Hydrophobicity was achieved through silanization modification, resulting in reflectance and emissivity of 97.75% and 95.74%, respectively (DOI: 10.1016 / j.matlet.2023.135220.).

[0005] However, bacterial cellulose is highly hydrophilic and easily absorbs moisture in humid environments. Its porous surface is also susceptible to dust, liquid water, or contaminants, leading to decreased solar reflectivity and long-term outdoor stability. To address this issue, existing technologies attempt to endow bacterial cellulose-based radiation cooling materials with hydrophobic or amphiphobic properties through methods such as silanization modification. For example, the aforementioned M-BC / BaSO4 composite material, after silanization modification, achieved a surface water contact angle of 154°, exhibiting certain self-cleaning properties. Furthermore, bacterial cellulose-based Janus films have been reported for achieving dual-mode thermal management of radiative cooling and solar heating (DOI: 10.1002 / cjoc.202300282).

[0006] However, the existing technologies mentioned above still have some aspects that urgently need improvement: (1) The preparation process of some methods is complex, involving steps such as unidirectional cryogenic casting, silanization modification, vacuum-assisted filtration, and hot pressing. The process route is long and the operation is cumbersome; (2) Hydrophobic modification increases the complexity of the process. Chemical modification methods such as silanization involve organic solvents and complex reaction conditions, which to some extent affect the green attributes and scalable preparation of the product; (3) Existing methods are difficult to simultaneously meet multiple functional requirements such as high solar reflectivity, high infrared emissivity, surface self-cleaning, long-term outdoor stability, and flexible thermal management in a single material; (4) The synergy between solar transmittance regulation and surface protection has not yet been solved. How to achieve radiation cooling performance while endowing the material with lasting surface anti-pollution ability and environmental stability is still a technical problem that urgently needs to be solved.

[0007] Therefore, there is an urgent need to provide a bacterial cellulose-based radiation cooling membrane that is simple in structure, mild in preparation, and has functions such as solar reflection, infrared emission, surface protection, self-cleaning, and flexible thermal management. Summary of the Invention

[0008] This invention aims to address the problems of bacterial cellulose-based radiative cooling materials being prone to moisture absorption, contamination, and insufficient optical performance stability under humid, polluted, and outdoor conditions. It also aims to improve the limited solar reflectivity of single bacterial cellulose films and the difficulty in simultaneously achieving surface protection and a porous scattering structure. This invention constructs a surface-protective bacterial cellulose-based radiation cooling membrane that does not require complex chemical hydrophobic modification. It proposes a preparation method that combines continuous wet vacuum filtration, freeze drying, PDMS coating, and thermal curing. This method enables the bacterial cellulose support layer and the bacterial cellulose-silica composite scattering layer to form fiber entanglement and hydrogen bonding, and allows PDMS to partially penetrate into the surface pores and then cure to form mechanical anchoring.

[0009] To address the above problems, the present invention provides a bacterial cellulose-based sandwich radiation cooling membrane, comprising a protective layer, a composite scattering layer, and a support layer arranged sequentially from top to bottom; The composite scattering layer is composed of bacterial cellulose nanofibers and silica microspheres, used to form multi-scale refractive index differences and broadband solar scattering; the support layer provides flexibility and structural support; the protective layer provides a hydrophobic or water-repellent surface and participates in thermal radiation through infrared active groups such as Si-O-Si and Si-CH3 in the 8~13 μm atmospheric window, and the protective layer partially enters the surface pores of the composite scattering layer before curing, and forms mechanical anchoring after curing.

[0010] In one embodiment of the present invention, the thickness of the support layer is 0 to 200 μm.

[0011] In one embodiment of the present invention, the thickness of the composite scattering layer is 200–400 μm.

[0012] In one embodiment of the present invention, the thickness of the protective layer is 20~100 μm.

[0013] This invention also provides a method for preparing the aforementioned radiation-cooling membrane. The method employs continuous sequential vacuum filtration. First, a pure bacterial cellulose dispersion is filtered to form a wet bottom layer BC membrane. Then, a bacterial cellulose-silica composite dispersion is further filtered in the same filter / filtering device, causing an upper BC-SiO2 network to deposit on the wet BC layer. This continuous wet filtration process allows the two fiber networks to interpenetrate, entangle, and interlock at the interface. Subsequently, freeze-drying is used to preserve the pore structure, and then a PDMS prepolymer is coated onto the BC-SiO2 side and cured to obtain the final SSBC membrane.

[0014] Specifically as follows: (1) Preparation of bacterial cellulose dispersion; (2) Mix the silica microspheres with the bacterial cellulose dispersion described in (1) to obtain a BC-SiO2 composite dispersion; (3) The bacterial cellulose dispersion described in (1) is filtered to obtain a wet BC layer as a bottom support layer; the BC-SiO2 composite dispersion is applied on top of the wet BC layer and filtered again to form a wet bilayer membrane composed of a bottom support layer and a composite scattering layer. (4) The wet bilayer membrane is dried to obtain a porous SBC membrane; (5) A mixture of hydrophobic polymer and curing agent is coated on the BC-SiO2 side of the porous SBC membrane in (4). After curing, a protective layer is formed, resulting in a bacterial cellulose-based sandwich radiation cooling membrane composed of a bottom support layer, a composite scattering layer and a protective layer.

[0015] In one embodiment of the present invention, the thickness of the bottom support layer is 0~200 μm, the thickness of the composite scattering layer is 200~400 μm, and the thickness of the protective layer is 20~100 μm.

[0016] In one embodiment of the present invention, in step (1), the solid content of the bacterial cellulose dispersion is 0.2~1.5 wt%, preferably 0.8 wt%.

[0017] In one embodiment of the present invention, in step (1), the bacterial cellulose hydrogel is cut into small pieces and added to deionized water, and then subjected to mechanical crushing, homogenization or high-speed dispersion treatment to obtain a uniform bacterial cellulose dispersion.

[0018] In one embodiment of the present invention, the method for preparing the bacterial cellulose hydrogel is as follows: Acetobacter xylinum (… Komagataeibacter xylinus Seed culture was inoculated into the culture medium and incubated at 28-33℃ for 5-15 days to obtain bacterial cellulose hydrogel membranes, which were then treated with alkali and purified.

[0019] In one embodiment of the present invention, the alkaline treatment involves immersing the obtained bacterial cellulose hydrogel membrane in a 0.1 M NaOH solution and treating it at 80°C for 4 h to remove bacterial cells and culture medium residues.

[0020] In one embodiment of the present invention, the purification involves repeatedly washing with deionized water until neutral.

[0021] In one embodiment of the present invention, the Acetobacter xylinum is ATCC 23767 strain.

[0022] In one embodiment of the present invention, in step (2), the average particle size of the silica microspheres is about 0.5~5 μm, preferably 2 μm.

[0023] In one embodiment of the present invention, in step (2), the mass ratio of bacterial cellulose dispersion and silica microspheres in the BC-SiO2 composite dispersion is 1:(0.25~2), preferably 1:(0.5~1.5).

[0024] In one embodiment of the invention, the mixing in step (2) includes stirring and / or ultrasonic dispersion.

[0025] In one embodiment of the present invention, in step (3), the filtration method includes vacuum filtration and negative pressure filtration; preferably, vacuum filtration.

[0026] In one embodiment of the present invention, in step (3), the water content of the wet BC layer is 50%~90%, preferably 75%~80%.

[0027] In one embodiment of the present invention, in step (4), the drying is freeze drying.

[0028] In one embodiment of the present invention, the freeze-drying includes pre-freezing the wet bilayer membrane at -80~-20℃ for 2~12 h, and then freeze-drying it in a freeze dryer for 12~72 h.

[0029] In one embodiment of the present invention, in step (5), the hydrophobic polymer includes PDMS, PVDF-HFP, PVDF, preferably PDMS; the coating method includes any one of spin coating, drop coating, blade coating, spray coating or dip coating, preferably spin coating.

[0030] In one embodiment of the present invention, the PDMS is Sylgard 184 type PDMS prepolymer, the curing agent is a matching curing agent, and the mass ratio of the PDMS prepolymer to the curing agent is 10:1.

[0031] In one embodiment of the present invention, in step (5), the coating thickness of the mixture of hydrophobic polymer and curing agent is 20~100 μm.

[0032] The present invention also provides a method for improving the self-cleaning ability and long-term stability of a radiation cooling film, the method being as follows: (1) Prepare a bacterial cellulose dispersion with a solid content of 0.2~1.5 wt%; (2) Mix the silica microspheres with the bacterial cellulose dispersion described in (1) to obtain a BC-SiO2 composite dispersion, wherein the mass ratio of the bacterial cellulose dispersion to the silica microspheres in the BC-SiO2 composite dispersion is 1:(0.25~2). (3) Vacuum filter the bacterial cellulose dispersion described in (1) to form a wet BC layer with a water content of 50%~90% as a bottom support layer; apply the BC-SiO2 composite dispersion on the wet BC layer and filter again to form a wet double membrane composed of a bottom support layer and a composite scattering layer. (4) The wet bilayer membrane is dried to obtain a porous SBC membrane; (5) A mixture of PDMS and curing agent is coated on the BC-SiO2 side of the porous SBC membrane in (4), and a protective layer is formed after curing, resulting in a bacterial cellulose-based sandwich radiation cooling membrane composed of a bottom support layer, a composite scattering layer and a protective layer.

[0033] In one embodiment of the present invention, the thickness of the bottom support layer is 0~100 μm, the thickness of the composite scattering layer is 200~400 μm, and the thickness of the protective layer is 20~100 μm.

[0034] In one embodiment of the present invention, in step (1), the bacterial cellulose hydrogel is cut into small pieces and added to deionized water, and then subjected to mechanical crushing, homogenization or high-speed dispersion treatment to obtain a uniform bacterial cellulose dispersion.

[0035] In one embodiment of the present invention, the method for preparing the bacterial cellulose hydrogel is as follows: Acetobacter xylinum (… Komagataeibacter xylinus Seed culture was inoculated into the culture medium and incubated at 28-33℃ for 5-15 days to obtain bacterial cellulose hydrogel membranes, which were then treated with alkali and purified.

[0036] In one embodiment of the present invention, the alkaline treatment involves immersing the obtained bacterial cellulose hydrogel membrane in a 0.1 M NaOH solution and treating it at 80°C for 4 h to remove bacterial cells and culture medium residues.

[0037] In one embodiment of the present invention, the purification involves repeatedly washing with deionized water until neutral.

[0038] In one embodiment of the present invention, the Acetobacter xylinum is ATCC 23767 strain.

[0039] In one embodiment of the present invention, in step (2), the silica is silica microspheres, and the average particle size of the silica microspheres is 0.5~5 μm.

[0040] In one embodiment of the invention, the mixing in step (2) includes stirring and / or ultrasonic dispersion.

[0041] In one embodiment of the present invention, in step (3), the water content of the wet BC layer is preferably 75% to 80%.

[0042] In one embodiment of the present invention, in step (4), the drying is freeze drying.

[0043] In one embodiment of the present invention, in step (5), the coating method includes any one of spin coating, drip coating, scraping coating, spraying or dip coating.

[0044] In one embodiment of the present invention, the PDMS is Sylgard 184 type PDMS prepolymer, the curing agent is a matching curing agent, and the mass ratio of the PDMS prepolymer to the curing agent is 10:1.

[0045] The present invention also provides the application of the bacterial cellulose-based sandwich radiation cooling membrane or the bacterial cellulose-based sandwich radiation cooling membrane prepared by the method in the preparation of radiation cooling materials, the application including passive cooling of building roofs, thermal protection of outdoor equipment, thermal management of electronic devices, surface thermal management of cold chain transportation, or personal thermal management.

[0046] Beneficial effects This invention constructs a multi-scale scattering system using BC nanofibers, silica microspheres, and air pores, which can improve the reflectivity in the solar spectral region. A PDMS protective layer is applied to the exposed surface, and continuous wet filtration creates fiber interlocking between the BC and BC-SiO2 layers. Partial infiltration of the PDMS prepolymer mechanically anchors the PDMS and BC-SiO2 layers, improving interfacial bonding. Figure 1 The final bacterial cellulose-based sandwich radiation-cooling film exhibited an integrated solar reflectance of 95.28% in the 0.3–2.5 μm solar spectrum and an average infrared emissivity of 90.46% in the 8–13 μm atmospheric window. The calculated theoretical radiation-cooling power was 101.13 W / m². 2 With a water contact angle of approximately 120°, it exhibits excellent hydrophobic and water-repellent properties and self-cleaning capabilities. Even when exposed to high humidity outdoor environments for extended periods, it effectively prevents moisture from penetrating its internal porous structure, maintaining stable radiative cooling performance. Dust or liquid contamination on the surface can be easily washed away by water flow, solving the problems of hygroscopicity, susceptibility to contamination, and poor long-term stability associated with pure bacterial cellulose-based cooling membranes. The preparation method provided by this invention mainly includes water dispersion, continuous filtration, freeze-drying, and PDMS coating / curing. The process is relatively mild, facilitating laboratory preparation and subsequent scale-up. Attached Figure Description

[0047] Figure 1 : Schematic diagram of the overall structure of the SSBC membrane of this invention; Figure 2 : Morphology, interface structure and composition characterization diagrams of the membrane material of this invention; Figure 3 Solar spectral reflectance and emissivity at the 8–13 μm atmospheric window for films with different samples / SiO2 contents. BC: BC support layer only; SBC: BC-SiO2 / BC bilayer film; SSBC: ... 0.5 The mass ratio of silica to dry BC in the BC-SiO2 composite dispersion is 0.5:1, and the mass ratio of silica to dry BC in the SSBC1 is 1:1. 1.5 The mass ratio of silica to dry BC in the BC-SiO2 composite dispersion is 1.5:1. Figure 4The effect of the mass ratio of silica to dry BC in BC-SiO2 composite dispersion on the reflectivity of SSBC; Figure 5 Temperature change curve of the SSBC membrane under outdoor conditions according to the present invention; Figure 6 : Schematic diagram and temperature change curve of the device for simulating heat dissipation of electronic components using SSBC film of the present invention; Figure 7 The water contact angle, self-cleaning process, and reflectivity test results of the SSBC membrane of this invention before and after contamination / washing are as follows: Figure 8 The difference between membranes formed by discontinuous filtration and membranes formed by continuous filtration; Figure 9 The difference in reflectivity between pure composites and double-layer composites of bacterial cellulose and silica. Detailed Implementation

[0048] The preferred embodiments of the present invention are described below. It should be understood that the embodiments are for better explanation of the present invention and are not intended to limit the present invention.

[0049] The test methods involved in the following embodiments: 1. Reflectance Calculation: Solar reflectance spectra were measured using a UV-Vis-NIR spectrophotometer (Lambda 950, PerkinElmer) equipped with a 300-2500 nm integrating sphere. A PTFE standard white plate was used as a reference. For SSBCs, the incident beam was incident from the PDMS protected side; for SBCs, the incident beam was incident from the BC-SiO2 side. Integrated solar reflectance ( The result was calculated using the AM 1.5G solar spectrum.

[0050] in: It is spectral reflectance. It is AM-1.5G solar irradiance.

[0051] 2. Calculation of Emissivity: Mid-infrared reflectance was measured using an FTIR spectrometer (Nicoleti S10, Thermo Fisher Scientific) equipped with an integrating sphere, with wavelengths exceeding 2.5-14 micrometers. Since the film is nearly opaque in this range, the spectral emissivity was calculated using Kirchhoff's laws:

[0052] in, This is the measured infrared reflectance. The average emissivity of the atmospheric window ( Calculation of )

[0053] in, It is the blackbody radiation intensity at 298 K.

[0054] 3. Calculation of radiative cooling power: Net radiative cooling power is calculated using a heat balance model, comprehensively considering the material's own infrared radiation, atmospheric reflected radiation, solar absorption, and non-radiative heat transfer processes. Its expression is: Where T is the sample temperature. The ambient temperature. The material's own infrared radiation power. It can be obtained by integrating its spectral emissivity with the blackbody radiation spectrum, and can be written as:

[0055] The raw materials used in the following examples: Acetobacter xylophilus culture medium: glucose (20 g / L), peptone (5 g / L), yeast extract (5 g / L), disodium hydrogen phosphate (2.7 g / L), deionized water (to a final volume of 1 L).

[0056] Silica microspheres: purchased from Nantong Zhichuan Microsphere Biotechnology Co., Ltd.

[0057] PDMS prepolymer and curing agent: Sylgard 184 PDMS prepolymer and its matching curing agent, purchased from Dow Corning.

[0058] Example 1: Preparation of SSBC sandwich radiation cooling film This embodiment provides a method for preparing a radiation-cooled SSBC film, wherein the radiation-cooled SSBC film is a PDMS / BC-SiO2 / BC three-layer composite film. The specific steps are as follows: (1) Preparation of BC hydrogel: 1 mL of Acetobacter xylinum ( Komagataeibacter xylinus The seed culture (ATCC 23767) was inoculated into a petri dish containing 50 mL of culture medium and incubated statically at 30°C for 10 days to obtain a bacterial cellulose hydrogel membrane. The obtained BC hydrogel was then immersed in 0.1 M NaOH solution at 80°C for 4 h to remove bacterial cells and culture medium residues. Subsequently, it was repeatedly washed with deionized water until neutral (pH 7.0) to obtain a purified BC hydrogel. (2) Preparation of BC dispersion: The purified BC hydrogel in step (1) was cut into fragments of 1-5 mm in size, deionized water was added, and the mixture was first treated with a high-speed crusher for 10 min, and then homogenized at 12000 rpm for 30 min (the homogenization process was carried out intermittently to avoid overheating of the system). Then it was dried at 60 ℃ to constant weight. According to the measured solid content, it was diluted with deionized water to obtain a BC dispersion with a concentration of 0.8 wt%, which was stored at 4 ℃ for later use. (3) Preparation of BC-SiO2 composite dispersion: 1 mL of SiO2 microsphere suspension with an average particle size of about 2 μm was added to 50 mL of deionized water and sonicated for 10 min; then 3.125 g of the BC dispersion obtained in step (2) (the content of dry BC is 25 mg) was added, and the mixture was mechanically stirred and sonicated for 30 min to form a uniform BC-SiO2 composite dispersion; the mass ratio of silica to dry BC in the BC-SiO2 composite dispersion was 1:1. (4) Preparation of wet SBC by continuous sequential vacuum filtration: Take 3.125 g of the BC dispersion obtained in step (2), dilute it with deionized water to a total volume of 50 mL, stir and ultrasonically disperse it, and then pour it into a vacuum filtration device for vacuum filtration to form a bottom layer-wet BC support layer. At this time, the water content of the wet BC support layer is about 80%. Without removing the wet BC support layer, slowly pour the BC-SiO2 composite dispersion from step (3) into the same filtration device for vacuum filtration. After the filtrate is completely dried, BC-SiO2 is deposited on top of the wet BC layer to obtain a wet BC-SiO2 / BC bilayer membrane, i.e., wet SBC. In this process, adjacent BC networks form sufficient interfacial contact, nanofiber entanglement and hydrogen bonding in the wet state, without the need for additional binders.

[0059] (5) Freeze-drying to form porous SBC: The wet SBC from step (4) is removed from the filter membrane, pre-frozen at -80℃ for 4 h, and then freeze-dried in a freeze dryer for 48 h to remove moisture and retain the porous fiber network, thus obtaining a porous SBC membrane with a BC layer thickness of about 150 μm and a BC-SiO2 layer thickness of about 300 μm.

[0060] (6) PDMS coating and curing: The PDMS prepolymer (Sylgard 184) and curing agent were mixed at a mass ratio of 10:1 and vacuum degassed for 1 h. The porous SBC membrane from step (5) was placed with the BC-SiO2 composite scattering layer facing upwards, and the PDMS mixture was spin-coated onto the surface at 1000 rpm and pre-cured at 60 °C for 10 min. The spin-coating of the PDMS mixture and the pre-curing steps were repeated until the cumulative weight gain of the SBC membrane was about 20 mg. Finally, it was cured at 60 °C for 3 h to obtain a PDMS / BC-SiO2 / BC three-layer SSBC membrane.

[0061] The resulting PDMS protective layer is approximately 50 μm thick, and as... Figure 2 As shown, the PDMS layer and the BC–SiO2 layer are tightly bonded at the interface. Some of the PDMS prepolymer penetrates into the BC–SiO2 porous fiber network and solidifies, forming a distinct embedded mechanical anchoring structure, thereby effectively enhancing the interlayer interface bonding.

[0062] The solar spectral reflectance of the PDMS / BC-SiO2 / BC three-layer SSBC film sample prepared in Example 1 was measured to be 95.28% in the range of 0.3~2.5 μm using a UV-Vis-NIR spectrophotometer.

[0063] The infrared emissivity of the sample was 90.46% in the atmospheric window range of 2.5–14 μm, especially 8–13 μm, using a Fourier transform infrared spectrometer and its matching integrating sphere accessory.

[0064] Based on the measured solar reflectivity and infrared emissivity, the theoretical radiative cooling power of the sample can be calculated to be 101.13 W / m². 2 .

[0065] Comparative Example 1: Pure BC membrane: Referring to Example 1, the difference is that in step (4), only the BC dispersion is filtered and freeze-dried, without adding SiO2 and PDMS, and steps (3) and (6) are omitted.

[0066] The results are as follows Figure 3 , Figure 5 and Figure 6 As shown, its reflectivity and emissivity are significantly lower than those of SBC and SSBC, resulting in a lower cooling effect and an inability to achieve effective radiative cooling.

[0067] Comparative Example 2: SBC bilayer membrane: BC-SiO2 / BC bilayer membranes (SBC membranes) were prepared by continuous filtration and freeze-drying, but without PDMS coating.

[0068] The results are as follows Figure 3 , Figure 5 and Figure 6 As shown, its reflectivity is slightly better than that of SSBC, but its emissivity is lower than that of SSBC. This indicates that the PDMS load has a certain negative effect on the reflectivity of SBC. However, in real outdoor environments, due to the superior emissivity of SSBC, its cooling effect is better than that of SBC.

[0069] The water contact angle is only about 32° when water is dripped, resulting in poor hydrophobicity. It is easily wetted by water, and after absorbing moisture, the internal porous structure is filled with water, which will significantly reduce the solar reflectivity. In addition, dust and dirt easily adhere to the surface, making it difficult to self-clean. Its performance deteriorates significantly with long-term outdoor use.

[0070] Example 2: SSBC films with different silica contents This embodiment, based on Example 1, investigates the effect of different silica contents on the solar reflectance of the PDMS / BC-SiO2 / BC three-layer SSBC film. Details are as follows: Referring to Example 1, the difference lies in adjusting the amount of silica microspheres added to the BC-SiO2 composite dispersion. Specifically, the mass ratio of BC to SiO2 is controlled to be 1:0.25, 1:0.5, 1:0.75, 1:1, 1:1.25, 1:1.5, 1:1.75 or 1:2, while the rest is the same as in Example 1.

[0071] SBC and SSBC films with different silica contents were prepared, and the effect of silica content on solar reflectance was determined by testing the solar spectral reflectance in the range of 0.3–2.5 μm.

[0072] Experimental results are as follows Figure 4 As shown, with the increase of silica content, the solar reflectivity of the film first increases significantly and then tends to stabilize, indicating that an appropriate amount of silica microspheres can increase the scattering centers, but the gain of excessive addition is limited.

[0073] Example 3: Radiative Cooling Performance Test of SSBC Thin Film In an open area, the performance of outdoor passive radiative cooling under direct sunlight was evaluated. During the test, a solar power meter (recording solar irradiance, unit: W / m²) was used. 2 Data was collected using a thermometer (to record ambient temperature), a thermocouple (to record sample temperature), and a hygrometer (to record ambient relative humidity, with an accuracy of ±1% RH).

[0074] Each sample (experimental sample SSBC and control samples BC, SBC, and cotton fabric) was cut into 4 cm diameter discs and placed on a polystyrene foam substrate with the cooling side facing upwards, using a thin polyethylene film as a windproof cover. For the SSBC sample, the PDMS layer faced upwards. The sample chamber was wrapped with aluminum foil to reduce lateral solar radiation heating.

[0075] A thermocouple was attached to the center of the back of each sample to monitor its temperature. Simultaneously, a thermometer was placed 20 cm above the sample, at the same height as the ground, to record the ambient air temperature. The temperature of each sample and the ambient temperature were recorded every 5 seconds, continuously from 10:00 AM to 4:00 PM.

[0076] The results are as follows Figure 5As shown, solar irradiance, ambient temperature, relative humidity, and wind speed continuously changed during the test. In the outdoor radiative cooling performance test, the temperature of the SSBC was lower than that of the blank box, conventional cotton fabric, BC, and SBC, respectively. Among them, the SSBC maintained the lowest sample temperature during the main illumination period; at the peak, the temperature of the cotton fabric was about 6.67℃ higher than that of the SSBC, and the temperature of the blank box was about 9.07℃ higher than that of the SSBC, indicating that the hierarchical scattering of the BC-SiO2 layer can reduce the solar thermal gain, and the PDMS protective layer did not substantially weaken this cooling function.

[0077] Example 4: SSBC used for performance testing of electronic components under simulated sunlight irradiation. Using a xenon lamp to simulate outdoor sunlight, the cooling effect of different samples (experimental sample SSBC and control samples BC, SBC and cotton fabric) on the surface of electronic devices was tested by covering them with different samples. Figure 6 The specific testing process is as follows: A ceramic heating element was connected to an aluminum plate to form a simulated heating platform, which operated at a constant input power of 20 W while being irradiated by a lamp. A sample cut into a 4 cm diameter circular piece was flatly placed over the heating element surface, with a thermocouple placed between them. The PDMS protection side of the SSBC was facing outwards. The blank group was left uncovered. Ensured that the sample was in close contact with the resistive surface, and the xenon lamp was turned on (irradiance set to 1000 W / m²). 2 The K-type thermocouple connected to the multi-channel data logger records the surface temperature of the recording plate. After each group reaches stability, the steady-state temperature is recorded and compared with the blank group to calculate the temperature drop.

[0078] Test results are as follows Figure 6 As shown, the steady-state temperature of the blank group was approximately 83 ℃, while the average temperatures after covering with cotton fabric, BC, SBC, and SSBC were approximately 80.19 ℃, 77.99 ℃, 74.53 ℃, and 75.70 ℃, respectively. SSBC achieved a temperature reduction of approximately 7.3 ℃ compared to the blank group and approximately 4.49 ℃ compared to cotton fabric.

[0079] Example 5: Water contact angle, self-cleaning process, and reflectivity tests before and after contamination / washing of SSBC Water contact angle tests were conducted on SSBC and BC membranes to compare their surface hydrophilic and hydrophobic properties. Subsequently, only the PDMS side of the SSBC was exposed to an outdoor environment prone to dust accumulation, allowing it to receive natural dust accumulation for seven days. Afterward, the SSBC was removed, tilted at approximately 30°, and rinsed with deionized water. The amount of dust residue on the surface was observed, and the change in solar reflectivity of the SSBC before and after water washing was tested to verify its self-cleaning performance.

[0080] Test results are as follows Figure 7As shown, water droplets spread rapidly on the pure BC surface, while on the SSBC surface, water droplets maintained a contact angle of approximately 120° within 30 seconds, indicating that the PDMS protective layer transformed the exposed surface from hydrophilic to hydrophobic. Contaminants reduced the solar reflectance of the SSBC from 95.28% to 74.94%; after water-assisted cleaning, the solar reflectance recovered to 93.92%, equivalent to approximately 98.6% of the initial value.

[0081] This indicates that the PDMS protective layer gives SSBC excellent water-repellent and self-cleaning capabilities. It can basically restore its initial cooling performance with a simple water wash, making it suitable for long-term outdoor use.

[0082] Comparative Example 3: Discontinuous vacuum filtration double-layer membrane Referring to Example 1, the difference is that in step (4), instead of continuous sequential vacuum filtration, wet BC layer and wet BC-SiO2 layer are prepared independently and then directly bonded and stacked, specifically: The wet BC membrane in (2) was vacuum filtered separately to form a membrane; the BC-SiO2 composite dispersion in (3) was vacuum filtered separately to form a membrane; the wet BC membrane and the wet BC-SiO2 membrane were directly laminated and stacked together, and then freeze-dried to obtain a BC-SiO2 / BC bilayer membrane.

[0083] The results are as follows Figure 8 As shown, the interfacial bonding of the bilayer membrane obtained by independently fabricating and then bonding the membranes is weak, and interlayer separation is easily generated during drying or bending. However, the SSBC obtained in Example 1 did not show obvious delamination, warping or structural damage after undergoing 100 bends of more than 90°, indicating that the BC nanofiber entanglement and hydrogen bonding formed by continuous wet filtration can improve the interlayer bonding stability.

[0084] Comparative Example 4: Discontinuous vacuum filtration double-layer membrane Referring to Example 1, the difference is that in step (4), the wet BC membrane in (2) is completely dried first, and then the BC-SiO2 composite dispersion in (3) is poured onto the dried BC membrane and vacuum filtration deposition is performed so that the BC-SiO2 layer is deposited on the surface of the dried BC membrane. After drying, a BC-SiO2 / BC bilayer membrane is obtained.

[0085] The results are as follows Figure 8 As shown, discontinuous filtration membranes are prone to edge delamination and warping on one side after drying, similar to the results of independent membrane bonding in Comparative Example 3. This is because the BC nanofibers in adjacent layers cannot form cross-layer entanglement and cannot form a stable interlayer bonding structure, further verifying the role of continuous sequential wet filtration process in improving the stability of interlayer bonding.

[0086] Comparative Example 5: Uniform BC-SiO2 composite film and SBC composite film: Referring to Example 1, the difference is that in step (4), only the BC-SiO2 composite dispersion is filtered and freeze-dried to obtain a uniform BC-SiO2 composite membrane, without PDMS coating and curing.

[0087] The results are as follows Figure 9 As shown, when using the same amount of bacterial cellulose and silica, the SBC composite film exhibits significantly better reflectivity than the uniform BC-SiO2 composite film. Compared to the uniform BC-SiO2 composite film, the SBC composite film demonstrates higher material utilization.

[0088] Comparative Example 6: Referring to Example 1, the difference is that the BC-SiO2 layer is omitted, and the BC membrane after vacuum filtration and freeze drying is coated and cured with PDMS to obtain a PDMS-coated modified BC monolayer membrane.

[0089] The results showed that, after omitting the BC-SiO2 composite scattering layer, the PDMS-modified BC monolayer film lacked a hierarchical scattering center composed of silica microspheres, bacterial cellulose nanofibers and air pores, and its solar reflectivity was lower than that of SSBC.

[0090] Comparative Example 7: Referring to Example 1, the difference is that the BC layer is omitted, and the BC-SiO2 monolayer membrane after vacuum filtration and freeze drying is coated and cured to obtain a PDMS-coated modified BC-SiO2 monolayer membrane.

[0091] The results, as shown in Table 1, indicate that compared to the PDMS / BC film without the BC–SiO2 scattering layer, the solar reflectance of the PDMS / BC–SiO2 film significantly increased from 81.01% to 93.53%, an increase of 12.52 percentage points. Simultaneously, the emissivity of the 8–13 μm atmospheric window increased from 83.04% to 89.67%, an increase of 6.63 percentage points. This demonstrates that the multi-scale interface formed by BC nanofibers, SiO2 microspheres, and internal pores significantly enhances solar multiple scattering and mid-infrared emission. Furthermore, the addition of a BC support layer to form a complete SSBC structure further increased the solar reflectance to 95.28% and the emissivity to 90.46%, increases of 1.75 and 0.79 percentage points respectively compared to the PDMS / BC–SiO2 film. This suggests that the BC support layer, in addition to providing mechanical support and structural integrity, further improves optical performance by assisting backscattering, reducing light transmission, and increasing infrared optical thickness.

[0092] Table 1

[0093] Comparative Example 8: Referring to Example 1, the difference lies in that, in step (3), the thickness of the BC-SiO2 layer is controlled by adjusting the amount of BC dispersion and SiO2 microsphere suspension added; or, in step (4), the thickness of the BC support layer is controlled by adjusting the mass of the added BC dispersion. Specifically, 3.125g is considered as a BC thickness of 150 μm. (1) In step (3), while keeping the PDMS layer and BC layer unchanged, the amount of BC dispersion and SiO2 microsphere suspension added is changed to 0.521g / 0.167mL, 1.041g / 0.333mL, 2.083g / 0.667mL, 3.125g / 1mL, and 4.167g / 1.333mL, respectively. (2) In step (4), while keeping the PDMS layer and BC-SiO2 layer unchanged, the amount of BC dispersion added to the bottom layer is changed to 0g, 1.041g, 2.083g, 3.125g, and 4.167g respectively.

[0094] The results, as shown in Table 2, indicate that with the increase in the thickness of the BC support layer and the BC–SiO2 scattering layer, both the solar reflectivity and the emissivity of the film within the 8–13 μm atmospheric window generally show an upward trend, but the degree of their influence differs significantly. Specifically, when the thickness of the BC support layer increases from 0 to 200 μm, the solar reflectivity only increases from 93.53% to 95.82%, and the emissivity from 89.67% to 90.63%, indicating that it mainly plays a supporting role in scattering and structural support. In contrast, when the thickness of the BC–SiO2 layer increases from 50 to 400 μm, the solar reflectivity significantly increases from 86.87% to 95.70%, and the emissivity from 85.98% to 90.98%, indicating that this layer is the core scattering layer determining the optical performance of the film. When the thickness of the BC–SiO2 layer exceeds 300 μm and the thickness of the BC layer exceeds 150 μm, the increase in reflectivity and emissivity decreases significantly, indicating that the system gradually reaches optical saturation. Therefore, the combination of a 300 μm BC–SiO2 scattering layer and a 150 μm BC support layer can achieve a good balance between high solar reflectance, high infrared emission, structural stability and material utilization efficiency.

[0095] Table 2

[0096] Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Anyone skilled in the art can make various modifications and alterations without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention should be determined by the claims.

Claims

1. A method for preparing a bacterial cellulose-based sandwich radiation cooling membrane, characterized in that, The preparation method is as follows: (1) Prepare a bacterial cellulose dispersion with a solid content of 0.2~1.5 wt%; (2) Mix the silica microspheres with the bacterial cellulose dispersion described in (1) to obtain a BC-SiO2 composite dispersion, wherein the mass ratio of the bacterial cellulose dispersion to the silica microspheres in the BC-SiO2 composite dispersion is 1:(0.25~2). (3) The bacterial cellulose dispersion described in (1) is filtered to form a wet BC layer with a water content of 50% to 90%, which serves as the bottom support layer; the BC-SiO2 composite dispersion is applied on top of the wet BC layer and filtered again to form a wet bilayer membrane composed of the bottom support layer and the composite scattering layer. (4) The wet bilayer membrane described in (3) is dried to obtain a porous SBC membrane; (5) A mixture of hydrophobic polymer and curing agent is coated on the BC-SiO2 side of the porous SBC membrane in (4). After curing, a protective layer is formed, resulting in a bacterial cellulose-based sandwich radiation cooling membrane composed of a bottom support layer, a composite scattering layer and a protective layer.

2. The preparation method according to claim 1, characterized in that, In step (1), the bacterial cellulose hydrogel is cut into small pieces and added to deionized water. After mechanical crushing, homogenization or high-speed dispersion, a uniform bacterial cellulose dispersion is obtained.

3. The preparation method according to claim 1, characterized in that, In step (2), the average particle size of the silica microspheres is about 0.5~5 μm.

4. The preparation method according to claim 1, characterized in that, In step (5), the hydrophobic polymer includes PDMS, PVDF-HFP, and PVDF.

5. The preparation method according to claim 1, characterized in that, The thickness of the bottom support layer is 0~200 μm, the thickness of the composite scattering layer is 200~400 μm, and the thickness of the protective layer is 20~100 μm.

6. The bacterial cellulose-based sandwich radiation cooling membrane prepared by any one of claims 1 to 5.

7. A method for improving the self-cleaning ability and long-term stability of a radiation-cooling film, characterized in that, The method is as follows: (1) Prepare a bacterial cellulose dispersion with a solid content of 0.2~1.5 wt%; (2) Mix the silica microspheres with the bacterial cellulose dispersion described in (1) to obtain a BC-SiO2 composite dispersion, wherein the mass ratio of the bacterial cellulose dispersion to silica in the BC-SiO2 composite dispersion is 1:(0.25~2). (3) Vacuum filter the bacterial cellulose dispersion described in (1) to form a wet BC layer as a bottom support layer; apply the BC-SiO2 composite dispersion on top of the wet BC layer and filter again to form a wet bilayer membrane composed of a bottom support layer and a composite scattering layer. (4) The wet bilayer membrane is dried to obtain a porous SBC membrane; (5) A mixture of PDMS and curing agent is coated on the BC-SiO2 side of the porous SBC membrane in (4), and a radiation cooling membrane is obtained after curing.

8. The method according to claim 7, characterized in that, The thickness of the bottom support layer is 0~200 μm, the thickness of the composite scattering layer is 200~400 μm, and the thickness of the protective layer is 20~100 μm.

9. The preparation method according to claims 1 to 5 or the application of the bacterial cellulose-based sandwich radiation cooling membrane according to claim 6 in the preparation of radiation cooling materials.

10. The application according to claim 9, characterized in that, The applications include passive cooling of building roofs, thermal protection of outdoor equipment, thermal management of electronic devices, surface thermal management of cold chain transportation, or personal thermal management.

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