Preparation method and application of a passive cooling material based on delignified wood

Abstract

This invention discloses a method for preparing and applying a passive cooling material based on lignin-free wood, and its application in promoting the collection of condensate water in a solar-driven interfacial evaporation system. This method, for the first time, combines a PVDF film with lignin-free wood material. Water is drawn upwards through the capillary action of the tubular cellulose structure in the lignin-free wood substrate, and the water evaporates under sunlight, carrying away heat and achieving evaporative cooling. Simultaneously, the high solar reflectivity (89.338%) and high-mid-infrared emissivity (95.783%) of the PVDF film surface provide excellent radiative cooling. Furthermore, the unique low surface energy rough structure of the PVDF film endows it with hydrophobic self-cleaning properties, protecting the surface of the cooling material from contamination, maintaining a stable cooling effect, and enabling long-term application.

Description

Technical Field

[0001] This invention relates to the field of passive cooling technology, specifically to a method for preparing and applying a passive cooling material based on lignin-free wood. Background Technology

[0002] Currently, global greenhouse gas emissions are still soaring. If urgent action is not taken to reduce greenhouse gas emissions, some parts of the world will be virtually uninhabitable by the end of this century. Today, more than 10% of greenhouse gas emissions come from traditional refrigeration methods. As global warming progresses, the increasing demand for cooling equipment generates even more greenhouse gas emissions and ozone, thus exacerbating the global warming process.

[0003] Passive cooling technology offers an effective way to reduce energy consumption and mitigate global warming. Passive cooling technology refers to utilizing natural energy sources such as solar, wind, and hydropower to remove heat through convection, conduction, and radiation, achieving a cooling effect. Because it requires no additional energy consumption, it has gained widespread research attention in recent years. Passive cooling technologies mainly include evaporative cooling and radiative cooling. Evaporative cooling refers to releasing heat by absorbing heat through the evaporation of moisture from the surface of a material under sunlight. Radiative cooling, on the other hand, is achieved by having an object facing the sky reflect most of the short-wave solar radiation (wavelengths in the range of 0.3 to 2.5 μm) and strongly emit long-wave infrared radiation into the cold universe through an atmospheric transparency window (wavelengths in the range of 8 to 13 μm).

[0004] The core of passive cooling materials lies in their high solar reflectivity. Even with perfect mid-infrared emissivity, absorption of just a few percent of sunlight can negate the heat loss from passive cooling, leading to cooling failure. However, this crucial high reflectivity can degrade significantly after only a few months of exposure to the outdoor environment, resulting in a substantial decrease in cooling performance or even complete failure. Therefore, developing passive cooling materials that maintain stable cooling effects in real-world applications is of great importance for expanding their practical applications. Summary of the Invention

[0005] This invention provides a method for preparing and applying a passive cooling material based on lignin-free wood, and utilizes it to promote the collection of condensate water in a solar-driven interfacial evaporation system. The PVDF film / lignin-free wood composite material absorbs water upwards through the capillary action of the tubular cellulose structure in the lignin-free wood substrate. This water evaporates under sunlight, carrying away heat and achieving evaporative cooling. Simultaneously, the high solar reflectivity (89.338%) and high-mid-infrared emissivity (95.783%) of the PVDF film surface provide excellent radiative cooling. Furthermore, the unique low surface energy rough structure of the PVDF film endows the composite material with hydrophobic and self-cleaning properties, protecting the cooling material surface from contamination, maintaining a stable passive cooling effect, and enabling long-term application.

[0006] A method for preparing a passive cooling material based on lignin-free wood includes the following steps:

[0007] 1) Synthetic Delignified Wood;

[0008] 2) Preparation of PVDF spinning solution;

[0009] 3) Load the PVDF spinning solution prepared in step 2) into the syringe of the electrospinning device. The lignin-free wood obtained in step 1) is covered on the receiving plate. Electrospinning is performed to obtain PVDF film / lignin-free wood material.

[0010] In step 1), the synthesis of lignin-free delignified wood specifically includes:

[0011] NaClO2 was uniformly dispersed in deionized water, and the pH was adjusted to 4.4–4.8 with acetic acid to obtain solution A. Balsa wood was placed in solution A, and the temperature was set to 85–95℃ for 5–7 hours to carry out delignin treatment. The delignin treatment was repeated multiple times. After the delignin treatment was completed, the wood was thoroughly washed with deionized water several times, refrigerated for 8–24 hours, and then dried in a vacuum freeze dryer for 18–30 hours to finally obtain delignin-free wood.

[0012] Step 2) involves preparing the PVDF spinning solution, specifically including:

[0013] PVDF is dispersed in a mixed solvent of N,N-dimethylformamide and acetone, and the temperature is set at 45-55℃ and the rotation speed is 200-300 rpm. The mixture is stirred for 3-5 hours to obtain a PVDF spinning solution.

[0014] In step 3), adjust the distance between the needle connected to the syringe and the receiving plate to 150-250mm.

[0015] In step 3), the receiving plate rotates at 100-200 rpm.

[0016] In step 3), the conditions for electrospinning are:

[0017] The voltage is set to 15–25 kV, the syringe flow rate is 20–40 μL / min, and the temperature is 10–35 °C.

[0018] In step 3), the electrospinning time is 2 to 5 hours.

[0019] The preparation of passive cooling materials based on lignin-free wood includes the following steps:

[0020] 1) Synthesis of lignin-free wood: 12.5 g of NaClO2 (80% purity) was uniformly dispersed in 487.5 mL of deionized water, and the pH was adjusted to 4.6 with acetic acid to obtain solution A (NaClO2 mass fraction 2 wt%). Commercially available balsa wood with dimensions of 10 cm * 10 cm * 1 mm was selected. The balsa wood was placed in solution A and heated at 90℃ for 6 hours to perform lignin removal. The lignin removal process was repeated three times to ensure complete removal of lignin. After the lignin removal process, the obtained wood was thoroughly washed several times with deionized water, refrigerated overnight, and then dried in a vacuum freeze dryer for 24 hours to obtain lignin-free wood.

[0021] 2) Preparation of PVDF spinning solution: Disperse 4.34g of PVDF into a mixed solvent of 10mL DMF and 10mL acetone (PVDF mass fraction is 20wt%), set the temperature to 50℃, the rotation speed to 250rpm, and stir for 4h to obtain spinning solution B by completely dissolving and homogenizing PVDF.

[0022] 3) Electrospinning synthesis of PVDF film on lignin-free wood surface: The spinning solution B obtained in step 2) was loaded into the syringe of the electrospinning apparatus. The distance between the needle connected to the syringe and the receiving plate was adjusted to 200 mm. The lignin-free wood obtained in step 1) was covered on the receiving plate, and the spinning speed was 150 rpm. The voltage of the electrospinning process was set to 20 kV, the syringe flow rate was 30 μL / min, and the environment was room temperature. The entire spinning process lasted for about 3 hours, and finally, PVDF film / lignin-free wood material was obtained.

[0023] The PVDF film / lignin-free wood passive cooling material prepared above can be used to promote the collection of condensate water in solar-driven interfacial evaporation systems.

[0024] Compared with the prior art, the present invention has the following outstanding features and beneficial effects:

[0025] (1) This invention organically combines PVDF and lignin-free wood materials to successfully prepare a passive cooling material with excellent cooling performance and no additional energy consumption. Water is absorbed upward through the capillary action of the tubular cellulose structure in the lignin-free wood substrate, and the water evaporates under sunlight, carrying away heat, thus achieving evaporative cooling; at the same time, radiative cooling is achieved through the optical properties of the PVDF film surface, which has a high solar reflectivity of 89.338% and a high-medium infrared emissivity of 95.783%.

[0026] (2) The special low surface energy rough structure of PVDF film gives the composite material hydrophobic self-cleaning properties, which can protect the surface of the cooling material from contamination, maintain a stable passive cooling effect, achieve long-term application, and broaden the practical significance of the application of cooling materials.

[0027] (3) This invention applies PVDF film / lignin-free passive cooling material with no additional energy consumption to promote the condensate water collection process in a solar-driven interfacial evaporation system. The water collection rate and steam collection efficiency are significantly improved, which proves its excellent cooling performance. Attached Figure Description

[0028] Figure 1 This paper describes the synthesis, structural characterization, and optical properties of a PVDF film / lignin-free wood passive cooling material. Figure a shows a schematic diagram of the synthesis process. Figure b is a photograph of the synthesized PVDF film / lignin-free wood passive cooling material. Figure c shows the Fourier transform infrared (FTIR) spectra of the PVDF film and the lignin-free wood. Figure d is a scanning electron microscope image of the PVDF film surface, with an inset showing the contact angle measurement of the PVDF film surface. Figure e shows the UV-Vis-NIR absorptivity and mid-infrared emissivity spectra of the PVDF film surface.

[0029] Figure 2 This document demonstrates the cooling performance of the PVDF film / lignin-free wood passive cooling material. Figure a shows a schematic diagram of applying the PVDF film / lignin-free wood passive cooling material to promote the condensate water collection process in a solar-driven interfacial evaporation system. Figure b is a photograph of the evaporator in the interfacial evaporation system, specifically the melamine foam loaded with PPy. Figure c is a photograph of the device used to apply this passive cooling material to promote the condensate water collection process in the solar-driven interfacial evaporation system. Figure d shows the effect of applying this material to improve the condensate water collection rate in the solar-driven interfacial evaporation system. Figure e shows the effect of applying this material to improve the condensate steam collection efficiency in the solar-driven interfacial evaporation system.

[0030] Figure 3This document demonstrates the hydrophobic and self-cleaning properties of PVDF film / lignin-free passive cooling materials. Figure a shows the "silver mirror" phenomenon of the hydrophobic surface of the PVDF film in water; the inset is an enlarged view. Figure b shows the hydrophobic morphology of various types of droplets on the PVDF film surface. Figure c is an actual photograph of the process of water flow carrying away dye contamination from the hydrophobic surface of the PVDF film. Figure d is a comparative schematic diagram of water flow passing over contaminated hydrophobic / hydrophilic surfaces. Figure e shows the changes in cooling performance of the cooling material after long-term contamination during application. Detailed Implementation

[0031] The present invention will be further described in detail through the following examples and accompanying drawings.

[0032] In the following examples, unless otherwise specified, all percentages are mass percentages.

[0033] 1. Preparation process of the method of the present invention

[0034] The preparation of passive cooling materials based on lignin-free wood includes the following steps:

[0035] 12.5g of NaClO2 (80% purity, Shanghai Maclean Biochemical Technology Co., Ltd.) was uniformly dispersed in 487.5mL of deionized water, and the pH was adjusted to 4.6 with acetic acid. Commercially purchased balsa wood (from Taobao Zhixing flagship store), measuring 10cm*10cm*1mm, was selected. The balsa wood was placed in the above NaClO2 solution and heated at 90℃ for 6 hours to perform delignin removal. This delignin removal process was repeated three times to ensure complete removal of lignin. After the delignin removal process, the resulting wood was thoroughly washed several times with deionized water, refrigerated overnight for 12 hours, and then dried in a vacuum freeze dryer for 24 hours to obtain delignin-free wood. 4.34 g of PVDF (polyvinylidene fluoride, Shanghai Maclean Biochemical Technology Co., Ltd.) was dispersed in a mixed solvent of 10 mL DMF and 10 mL acetone (PVDF mass fraction 20 wt%). The temperature was set at 50℃, the rotation speed at 250 rpm, and the mixture was stirred for 4 h to ensure complete and homogeneous dissolution of the PVDF, thus obtaining a PVDF spinning solution. A PVDF film was synthesized by electrospinning on the surface of lignin-free wood. The PVDF spinning solution was loaded into the syringe of the electrospinning apparatus. The distance between the needle connected to the syringe and the receiving plate was adjusted to 200 mm. The receiving plate was covered with lignin-free wood. The rotation speed was 150 rpm, the electrospinning voltage was 20 kV, the syringe flow rate was 30 μL / min, and the environment was room temperature (25℃). The entire spinning process lasted for 3 h, ultimately yielding a PVDF film / lignin-free wood passively cooled material.

[0036] 2. Processing procedure of the method of the present invention

[0037] (1) The Fourier transform infrared (FTIR) spectra of the PVDF film were measured to analyze its characteristic chemical bonds. The contact angle of the PVDF film surface was measured to determine its affinity / relativity structure. The microstructure of the upper surface of the prepared PVDF film was observed under a scanning electron microscope. The absorbance of the prepared PVDF film at different wavelengths in the solar spectral range was measured using a UV-Vis-Infrared spectrophotometer with an integrating sphere. The emissivity of the prepared PVDF film at mid-infrared wavelengths was measured using an infrared spectrometer with an integrating sphere.

[0038] (2) The cooling performance of PVDF film / lignin-free wood material was tested by applying PVDF film / lignin-free wood passive cooling material to the condenser end of a solar-driven interfacial evaporation system and recording the water collection rate and steam collection efficiency.

[0039] (3) By placing PVDF film / lignin-free wood material in water, distributing various types of droplets on the material surface, observing the entire process of water flowing through the material surface after it is contaminated by dye, and testing the changes in cooling performance after long-term anti-fouling aging, the hydrophobic self-cleaning properties of the PVDF film / lignin-free wood material surface can be demonstrated.

[0040] 3. Effects achieved by this embodiment of the invention

[0041] This example demonstrates that PVDF film / lignin-free wood materials can achieve excellent passive cooling through the synergistic effect of evaporative cooling and radiative cooling, thereby significantly promoting the condensate collection process in solar-driven interfacial evaporation systems.

[0042] Figure 1 This paper describes the synthesis, structural characterization, and optical properties of a PVDF film / lignin-free wood passive cooling material. Figure a illustrates the synthesis process of the PVDF film / lignin-free wood passive cooling material, which is obtained by electrospinning PVDF film onto balsa wood after lignin removal treatment. Figure b shows a photograph of the PVDF film / lignin-free wood passive cooling material, revealing its ultra-white surface. Figure c shows the Fourier transform infrared (FTIR) spectra of the PVDF film and lignin-free wood, observing the characteristic peaks of distinct -CH2- units, -CF2- units, and CCC-frameworks on the PVDF film surface, as well as the characteristic peaks of CH, OH, C=O, and CO in the lignin-free wood, confirming the successful synthesis of the PVDF film and lignin-free wood. Figure d shows the spinning effect of randomly stacked PVDF film fibers, with an inset confirming the hydrophobic structure of the PVDF film surface and a contact angle of 133.197°. Figure e shows that the PVDF film surface has a high solar reflectance of 89.338% and a high-medium infrared emissivity of 95.783% through integral calculation, which confirms that the material has excellent radiative cooling performance.

[0043] Figure 2 This study demonstrates the cooling performance of the PVDF film / lignin-free passive cooling material. Figures a and c are schematic diagrams and photographs of the apparatus, respectively, showing the application of the PVDF film / lignin-free passive cooling material to promote condensate water collection in a solar-driven interfacial evaporation system. The evaporator is PPy-loaded melamine foam; the condenser wall is a glass cover; the PVDF film / lignin-free passive cooling material is tightly attached to the outside of the glass cover to reduce the condenser wall temperature and thus promote condensate collection; aluminum foil is placed between the glass cover and the PVDF film / lignin-free passive cooling material to accelerate heat exchange; the entire experimental setup is placed under a solar simulator and in a beaker of deionized water. Figure b is an actual photograph of the evaporator, i.e., the PPy-loaded melamine foam, which appears black. Figures d and e illustrate that, compared to the conventional apparatus, applying this material to the condenser end of the solar-driven interfacial evaporation system significantly improves the water collection rate and steam collection efficiency. At 1.2 solar radiation intensities, the water collection rate is increased by 67.7% compared to the conventional apparatus without passive cooling material, reaching 1.0 kg / m³. 2 / h; steam collection efficiency is significantly improved to 72%.

[0044] Figure 3 This document demonstrates the hydrophobic and self-cleaning properties of PVDF film / lignin-free passive cooling materials. Figure a shows the "silver mirror" phenomenon, illustrating the formation of an air layer between water and the material surface, confirming the hydrophobic structure of the PVDF surface. Figure b illustrates that various types of droplets, such as water, dye, milk, and coffee, exhibit near-spherical shapes on the material surface, further confirming the hydrophobic structure of the PVDF surface. Figure c shows a PVDF surface contaminated with dye; water flows across the surface at a certain speed, carrying away the contaminants, confirming the excellent hydrophobic and self-cleaning properties of the PVDF film / lignin-free wood material surface. Figure d schematically illustrates that contaminants on hydrophilic surfaces remain on the surface, while contaminants on hydrophobic surfaces are carried away by water flow, confirming the self-cleaning performance of the hydrophobic structure. Figure e compares the changes in cooling performance of cooling materials with and without PVDF film hydrophobic modification after long-term contamination. After two weeks of simulated outdoor application, the cooling performance of pure lignin-free wood material without PVDF film hydrophobic modification decreased significantly by 33.44%; while the cooling performance of composite passive cooling material with PVDF film hydrophobic modification decreased by only 5.76%, proving that PVDF-DW passive cooling material has a long-term stable cooling effect in practical applications.

Claims

1. A method for preparing a passive cooling material based on lignin-free wood, characterized in that, Includes the following steps: 1) Synthetic lignin-free wood, specifically including: NaClO2 was uniformly dispersed in deionized water, and the pH was adjusted to 4.4-4.8 with acetic acid to obtain solution A. Balsa wood was placed in solution A, and the temperature was set to 85-95℃ for 5-7 h to carry out delignification treatment. The delignification treatment was repeated multiple times. After the delignification treatment was completed, the wood was thoroughly washed with deionized water several times, refrigerated for 8-24 h, and then dried in a vacuum freeze dryer for 18-30 h to finally obtain lignin-free wood. 2) Preparation of PVDF spinning solution; 3) Load the PVDF spinning solution prepared in step 2) into the syringe of the electrospinning device, with the lignin-free wood obtained in step 1) covering the receiving plate, and perform electrospinning to obtain a passive cooling material based on lignin-free wood.

2. The method for preparing a passive cooling material based on lignin-free wood according to claim 1, characterized in that, Step 2) involves preparing the PVDF spinning solution, specifically including: PVDF was dispersed in a mixed solvent of N,N-dimethylformamide and acetone, and stirred for 3-5 h at a temperature of 45-55 °C and a rotation speed of 200-300 rpm to obtain a PVDF spinning solution.

3. The method for preparing a passive cooling material based on lignin-free wood according to claim 1, characterized in that, In step 3), adjust the distance between the needle connected to the syringe and the receiving plate to 150~250 mm.

4. The method for preparing a passive cooling material based on lignin-free wood according to claim 1, characterized in that, In step 3), the receiving plate rotates at 100~200 rpm.

5. The method for preparing a passive cooling material based on lignin-free wood according to claim 1, characterized in that, In step 3), the conditions for electrospinning are: The voltage is set to 15~25 kV, the syringe flow rate is 20~40 µL / min, and the temperature is 10~35℃.

6. The method for preparing a passive cooling material based on lignin-free wood according to claim 1, characterized in that, In step 3), the electrospinning time is 2-5 hours.

7. The application of the passive cooling material based on lignin-free wood prepared by the preparation method according to any one of claims 1 to 6 in the process of promoting the collection of condensate water in a solar-driven interfacial evaporation system.

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