Heat-resistant, degradable porous cooling film material and preparation method and application thereof

Abstract

The application discloses a heat-resistant and degradable porous cooling film material and a preparation method and application thereof, and belongs to the technical field of radiation cooling materials. The film material takes a blend of poly-L-lactic acid and poly-D-lactic acid as a base body, and a stereocomplex crystal is formed in the base body to improve heat resistance. The film material has a bimodal pore structure formed by removing a spherical calcium carbonate template with protrusions on the surface through acid pickling. The structure is composed of large pores with a size of 1-13 microns and small pores with a size of 0.1-0.8 microns distributed on the inner wall of the large pores, and the porosity is 55-70%. The structure enables the film material to achieve excellent radiation cooling effect with solar reflectivity and atmospheric window emissivity both greater than 0.9 without a metal plating layer, and the film material has good hydrophobicity. The preparation method is simple, environment-friendly, easy to industrialize and low in cost. The film material can be widely applied to outdoor passive cooling fields such as car clothes and building film pasting.

Description

Technical Field

[0001] This invention relates to the field of radiative cooling technology, and in particular to a heat-resistant, biodegradable porous cooling membrane material, its preparation method, and its application. Background Technology

[0002] Against the backdrop of a growing global energy crisis and increasingly severe environmental problems, passive daytime radiative cooling (PDRC) technology, as a zero-energy cooling method, has become a research hotspot in fields such as building and industrial energy conservation. PDRC materials are a class of materials that can efficiently reflect solar radiation in open-air environments and effectively dissipate their own heat into the external space in the form of thermal radiation, thereby reducing their surface temperature to below ambient temperature without the need for external energy input. These materials typically have high reflectivity (i.e., low solar absorptivity) in the 0.3–2.5 micrometer solar radiation band and high emissivity in the 8–13 micrometer atmospheric transparency window band; both values ​​are generally greater than 0.9.

[0003] In recent years, researchers have developed PDRC materials with different structural types, including multilayer structures (such as nano-multilayer photonic structures and micro / nano particle-doped structures), metamaterial structures, and polymer structures (such as porous polymer structures and randomly distributed particle structures based on polymer matrices). Among these, polymer materials have advantages in scalability and cost, and therefore hold great promise.

[0004] Several major polymer materials currently under research, such as polymethylpentene (TPX), polyethylene terephthalate (PET), polydimethylsiloxane (PDMS), acrylic resin, and fluorocarbon resin, are not biodegradable, and their long-term and large-scale application will exacerbate environmental pollution.

[0005] Therefore, we turn our attention to polylactic acid (PLA), an ideal biodegradable polymer. PLA exists in stereoisomers, such as poly-L-lactic acid (PLLA), poly-D-lactic acid (PDLA), and poly-DL-lactic acid (PDLLA). Among them, the most common are the enantiomeric forms PLLA and PDLA. PLA is available in large quantities at a reasonable price and has been widely used in biomedicine, spinning, packaging, agriculture, 3D printing, and other fields.

[0006] Existing polylactic acid (PLA)-related radiation cooling materials are mostly fiber composites or fabrics, with complex preparation processes that generally require large amounts of volatile organic solvents, easily polluting the environment and exhibiting instability. Even slight changes in preparation conditions can significantly alter their internal structure, affecting the stability of their radiation cooling performance. Furthermore, these materials typically use PLLA as the matrix, exhibiting poor heat resistance, usually undergoing significant deformation at around 60 degrees Celsius, greatly limiting the application range of PLA radiation cooling materials.

[0007] Chinese invention patent CN202010172445.3 discloses a heat-resistant and biodegradable polylactic acid radiation cooling film, but it requires the application of magnetron sputtering coating technology to achieve high reflectivity, which has a high process cost and does not optimize the hydrophobic properties of the material surface, thus limiting its long-term applicability in humid outdoor environments. Summary of the Invention

[0008] The purpose of this invention is to provide a heat-resistant, biodegradable porous cooling membrane material that is fully biodegradable, highly heat-resistant, has excellent solar reflectivity, infrared emissivity and good hydrophobicity, and has a simple production process, is easy to industrialize and has a low cost.

[0009] To achieve the above objectives, the solution of the present invention is: a heat-resistant, biodegradable porous cooling membrane material, wherein the matrix of the membrane material is a polylactic acid blend containing PLLA, PDLA and a selective nucleating agent, wherein PLLA and PDLA form stereocomposite crystals in the matrix;

[0010] The matrix has a bimodal porous structure formed after removing the calcium carbonate template with protrusions on the surface; the bimodal porous structure comprises a composite porous unit consisting of large pores with a size of 1 to 13 micrometers and small pores with a size of 0.1 to 0.8 micrometers distributed on the inner wall of the large pores, with an overall porosity of 55 to 70%.

[0011] Furthermore, the membrane material has a solar reflectivity greater than 0.9 in the 0.3~2.5 micrometer wavelength band and an infrared emissivity greater than 0.9 in the 8~13 micrometer wavelength band; in addition, the water contact angle of the membrane material is greater than 100°.

[0012] Furthermore, the mass ratio of PLLA to PDLA in the matrix is ​​1 to 4:1, and PDLA accounts for more than 20% of the total mass of polylactic acid.

[0013] Furthermore, in the matrix, the selective nucleating agent accounts for 0.1% to 0.5% of the total mass of polylactic acid, and the selective nucleating agent is selected from at least one of the following: aluminum complexes of hydrotalcite-containing phosphate esters, polylactic acid grafted with multi-walled carbon nanotubes, zinc phenyl phosphate, p-xylene dialkyl urea, or commercial TMB-5 nucleating agent.

[0014] Furthermore, the template for forming the bimodal porous structure is a spherical calcium carbonate particle with nanoscale protrusions on its surface, with a particle size of 1-13 micrometers and a protrusion size of 0.1-0.8 micrometers.

[0015] Furthermore, the mass ratio of the calcium carbonate particles to the total mass of polylactic acid is 2.5 to 5:1.

[0016] A method for preparing a heat-resistant, biodegradable porous cooling membrane material includes the following steps:

[0017] S1. PLLA, PDLA, selective nucleating agent and calcium carbonate particles with a particle size of 1~13 micrometers and nanoscale protrusions on the surface are dried and mixed to obtain blended particles, wherein the mass ratio of calcium carbonate to total polylactic acid is 2.5~5:1.

[0018] S2. The blended granules are then kneaded, pulverized, and molded to obtain a pre-made film containing a calcium carbonate template.

[0019] S3. Use an acidic solution to decompose and dissolve the calcium carbonate particles in the pre-made membrane, so that a bimodal pore structure is formed in the membrane, and a porous membrane material is obtained.

[0020] S4. The porous membrane material is dried and heat-treated to obtain a heat-resistant, biodegradable porous cooling membrane material.

[0021] Further, in step S1, the raw material granules are stirred and blended at 60~70°C until the moisture content is below 0.1% and then discharged to obtain blended granules;

[0022] In step S2, the blended particles are intensively mixed at 200~230°C and 10~30 rad / min for 2~5 minutes, then pulverized and molded at 220~240°C to obtain a pre-made film;

[0023] In step S3, the pre-made membrane is placed in an acidic solution with a concentration of 0.01~0.5 mol / L and soaked at 40~60°C for 4~8 hours to completely dissolve and leach out the calcium carbonate particles, thereby forming the bimodal pore structure in the membrane.

[0024] In step S4, the acid-washed porous membrane material is heat-treated at 90~120°C for 1~2 hours.

[0025] Furthermore, in step S3, the acidic solution is an aqueous solution of hydrochloric acid, citric acid, or acetic acid.

[0026] Applications of a heat-resistant, biodegradable porous cooling membrane material: for use in car covers, seat covers, transportation mobile equipment films, outdoor substation films, building films, cable sheathing layers, base stations, or outdoor tents.

[0027] After adopting the above solution, the beneficial effects of the present invention are as follows:

[0028] This invention uses a specific ratio of PLLA and PDLA blended together, supplemented with a selective nucleating agent, to form a large number of high-melting-point stereocomposite crystals in the matrix, significantly increasing the heat distortion temperature of the material from about 60°C for pure PLLA to over 90°C, thus improving the heat resistance of the cooling film material.

[0029] This invention uses spherical calcium carbonate with nano-protrusions on its surface as a template. During the dissolution process, the calcium carbonate particles are removed to form large pores (1~13 micrometers), while the nano-protrusions on its surface induce the formation of tiny pores (0.1~0.8 micrometers) on the inner wall, i.e., a bimodal pore structure.

[0030] The large aperture has a certain scattering ability for the entire solar spectrum, while the small apertures on the inner wall further enhance the scattering of high-energy ultraviolet and visible light. The two work together to achieve efficient reflection of the entire solar spectrum.

[0031] This invention employs an acid washing method to remove the calcium carbonate template, enabling precise replication of calcium carbonate templates with unique nano-protrusion morphology. This one-step process forms a spatially interconnected bilevel structure of "macropores + micropores on the inner wall," providing the membrane material with excellent light scattering centers and a hydrophobic surface. Simultaneously, the acid solution exhibits low corrosiveness to the stereocomposite crystal matrix formed by PLLA and PDLA, achieving a porosity as high as 55-70% while perfectly maintaining the heat resistance and mechanical integrity of the matrix. Furthermore, the entire process uses water as a medium, without the use of any volatile organic solvents. The calcium carbonate raw material is inexpensive and readily available, and the process employs melt processing, making it simple, environmentally friendly, and easily industrialized. Attached Figure Description

[0032] Figure 1 This is a scanning electron microscope image of the heat-resistant, biodegradable porous cooling membrane material in Embodiment 1 of the present invention;

[0033] Figure 2 This is a scanning electron microscope image of the calcium carbonate particles of the present invention;

[0034] Figure 3 This is a scanning electron microscope image of the heat-resistant, biodegradable porous cooling membrane material in Embodiment 2 of the present invention;

[0035] Figure 4 This is a scanning electron microscope image of the heat-resistant, biodegradable porous cooling membrane material in Embodiment 3 of the present invention;

[0036] Figure 5 This is a scanning electron microscope image of the heat-resistant, biodegradable porous cooling membrane material in Embodiment 4 of the present invention;

[0037] Figure 6 This is a scanning electron microscope image of the membrane material in Comparative Example 2 of the present invention;

[0038] Figure 7 This is a scanning electron microscope image of the membrane material in Comparative Example 3 of the present invention;

[0039] Figure 8 This is a scanning electron microscope image of the membrane material in Comparative Example 4 of the present invention. Detailed Implementation

[0040] This invention provides a heat-resistant, biodegradable porous cooling membrane material. The matrix of this membrane material is a polylactic acid blend containing poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA), with PLLA and PDLA forming a stereocomposite crystal within the matrix. The matrix exhibits a bimodal pore structure formed after the removal of the calcium carbonate template. This bimodal pore structure comprises composite pore units consisting of large pores (1-13 micrometers in size) and small pores (0.1-0.8 micrometers in size) distributed on the inner walls of the large pores, with an overall porosity of 55-70%.

[0041] The heat-resistant, biodegradable porous cooling membrane has a solar reflectivity greater than 0.9 in the 0.3-2.5 micrometer wavelength range and an infrared emissivity greater than 0.9 in the 8-13 micrometer wavelength range; in addition, the water contact angle of the membrane is greater than 100°.

[0042] The preparation method of the above-mentioned heat-resistant, biodegradable porous cooling membrane material includes the following steps: drying granules, mixing, internal mixing, pulverizing, molding, pickling, drying, and heat treatment. The specific operation of each step is as follows:

[0043] (1) Drying the granules: Place PLLA, PDLA, selective nucleating agent and calcium carbonate granules in an oven at 80°C and dry for 4-8 hours. Drying is very important for the processing of polylactic acid. Even a small amount of moisture can cause polylactic acid to degrade and turn yellow during processing. Therefore, it is important to ensure that polylactic acid is dried properly throughout the entire process.

[0044] (2) Mixing: Mix the above-mentioned granules in proportion at 60~70°C until the moisture content is less than 0.1% and then discharge to obtain the blended granules;

[0045] (3) Internal mixing: Add the above-mentioned blended granules to an internal mixer. Set the internal mixing temperature to 200~230°C, the internal mixer speed to 10~30 rad / min, and the internal mixing time to 2~5 minutes to obtain the internally mixed sample. The internal mixing temperature is related to the PDLA content. The lower the PDLA content, the lower the internal mixing temperature can be. For example, when the PDLA content is 30% of the total mass of polylactic acid, the internal mixing temperature is 210~220°C; when the PDLA content is 50% of the total mass of polylactic acid, the internal mixing temperature is 220~230°C.

[0046] (4) Crushing: Crush the above-mentioned mixed sample with a crusher and dry the crushed sample.

[0047] Samples obtained from an internal mixer are typically in block form. Directly placing them into a molding press can easily lead to uneven distribution of the molten material across the flat surface. One important reason is that the relatively high calcium carbonate content tends to form a gel structure, making it difficult to level. Therefore, a pre-crushing step is necessary to break up the blocks, facilitating even placement during subsequent molding, improving processing efficiency, and resulting in a smooth film sample.

[0048] (5) Molding: Weigh an appropriate amount of the above-mentioned crushed sample, spread it evenly on the mold plate, place it in the molding machine, set the temperature of the molding machine to 220~240 degrees, the molding time to 2~5 minutes, then remove it from the mold and put it into the cold water tank to cool and obtain the film sample.

[0049] The molding temperature is related to the nucleating agent content; a lower nucleating agent content allows for a correspondingly lower molding temperature. For example, if the nucleating agent content is 0.1% of the total polylactic acid (PLA) mass, the preferred molding temperature is 220-230°C; if the nucleating agent content is 0.5% of the total PLA mass, the preferred molding temperature is 230-240°C. Furthermore, the molding temperature should be slightly higher than the mixing temperature. This is because some highly efficient and selective nucleating agents require higher temperatures to fully dissolve in the PLA matrix and remain within the PLA matrix during subsequent processing. This allows for further enhancement of the SC crystal content during heat treatment, improving the heat resistance of the film. Stereoscopic composite crystals can also enhance the matrix stability during acid hydrolysis.

[0050] (6) Acid washing: Treat the membrane sample with an acidic solution with a concentration of 0.01~0.5 mol / L. The solution temperature is set to 40~60°C. The acidic solution can be hydrochloric acid, citric acid, or acetic acid. A 0.1 mol / L citric acid solution is preferred, and the solution temperature is 50°C. Higher temperatures are beneficial for the acid to quickly pass through the polylactic acid membrane and react with calcium carbonate, but excessively high temperatures may increase the operational risks of the acid solution.

[0051] Specifically: The membrane sample is immersed and fixed in an acidic solution. The fixer can be an acid-resistant steel mesh frame. The immersion time is 4 to 8 hours to fully acid-dissolve the calcium carbonate particles in the membrane sample. Then the membrane sample is taken out and rinsed several times with clean water to obtain a porous membrane material.

[0052] (7) Drying and heat treatment: Place the above porous membrane material in an oven at 80°C for 1-3 hours to dry, then place it in a flat plate fixture, which can be made of stainless steel or glass, and heat treat it in an oven at 90-120°C for 1-2 hours to obtain a heat-resistant, biodegradable porous cooling membrane material. This heat treatment step helps to improve the heat resistance of the membrane material.

[0053] The PLLA used can be of grades such as 4032D and 2003D from Natureworks LLC (USA), or products from companies such as Zhejiang Hisun Biomaterials Co., Ltd. Its levorotatory optical purity is not less than 96%, and its weight-average molecular weight is 80,000-400,000 Daltons. The PDLA used is sourced from Purac (Netherlands), with a dextrorotatory optical purity of not less than 98% and a weight-average molecular weight of 100,000-300,000 Daltons. PLLA and PDLA can form stereochemical composite crystals (SC crystals) with a melting point of 230°C, significantly improving the heat resistance of the polylactic acid matrix. The mass ratio of PLLA to PDLA is 1-4:1. Since PDLA is more expensive, its usage can be reduced from an economic perspective, but it should not be less than 20% of the total polylactic acid content; otherwise, it will affect the formation of SC crystals, thus affecting the improvement of heat resistance.

[0054] The selective nucleating agent is selected from at least one of the following: aluminum complexes containing hydrotalcite phosphate esters, polylactic acid grafted with multi-walled carbon nanotubes, zinc phenyl phosphate, p-xylene dialkyl urea, or the commercially available TMB-5 nucleating agent. The content of the selective nucleating agent is 0.1% to 0.5% of the total mass of polylactic acid. This content range effectively promotes SC crystal formation while avoiding excessive addition that could lead to excessively high processing temperatures or pickling failure. Too low a content will not effectively increase the SC crystal content, while too high a content requires higher temperatures to dissolve the nucleating agent in the polylactic acid melt, which is detrimental to polylactic acid molding and processing. Undissolved nucleating agents are prone to inactivation due to interaction with acid during subsequent pickling.

[0055] The template for forming the bimodal porous structure is a spherical calcium carbonate particle with nanoscale protrusions on its surface.

[0056] The calcium carbonate particles are nearly spherical, with a particle size ranging from 1 to 13 micrometers, and the protrusions on their surfaces are 0.1 to 0.8 micrometers in size. Large pores in the matrix are formed by removing these spherical particles, while small pores are induced to form from the protrusions on the particle surface. The large pores can reflect solar radiation across all wavelengths, while the small pores can reflect solar radiation from the ultraviolet to visible light bands. The large and small pores work synergistically to enhance the reflectivity and radiative cooling performance of the membrane material.

[0057] Furthermore, introducing pores enhances the hydrophobicity of polylactic acid (PLA) membranes, and the pore size further influences hydrophobicity. Generally, larger pores, due to their micro-nano rough surface structure, can trap more air and significantly reduce the solid-liquid contact area, resulting in a higher water contact angle and better hydrophobicity, thus improving the membrane's hydrophobic performance. Smaller pores, on the other hand, improve the membrane's reflectivity and radiative cooling performance, but exhibit slightly lower hydrophobicity (see Examples 1-4, Experiment 4, and the contact angle test results). Regardless of the particle size of the calcium carbonate particles (all within the 1-13 micrometer range), generally good radiative cooling performance can be achieved.

[0058] In the preparation process of the membrane material of the present invention, the ratio of the mass of calcium carbonate to the total mass of polylactic acid is controlled at 2.5 to 5:1, as shown in Examples 1 to 4.

[0059] If the calcium carbonate content is too low, it will adversely affect the subsequent pickling process, as shown in Comparative Example 2. Due to insufficient packing density of calcium carbonate particles in the polylactic acid (PLA) matrix, individual calcium carbonate particles are relatively well-encapsulated by PLA. Although PLA has some permeability to water vapor and acid, this permeation process is kinetically controlled. A well-encapsulated PLA layer prolongs the effective time for acid to enter the membrane layer, resulting in faster dissolution of calcium carbonate on the membrane surface, while the calcium carbonate in the core layer is difficult to completely remove. To completely remove the core layer template, the pickling time needs to be extended several times, which not only severely reduces production efficiency and increases energy consumption and cost, but also inevitably induces hydrolysis and degradation of the PLA matrix during prolonged acidic immersion, damaging the mechanical integrity and final properties of the material. Therefore, the samples obtained in this way often form incomplete or uneven porous structures due to residual pore-forming agents in the core, and their solar reflectivity and radiative cooling performance are usually poor. Conversely, an excessively high calcium carbonate content will lead to excessively high membrane porosity and poor mechanical properties after pickling.

[0060] Existing technologies often use microparticles such as salt, sugar, polymethyl methacrylate (PMMA), silica, and sodium bicarbonate as sacrificial templates, or eutectic porogens such as polyethylene oxide, 2-methylimidazolium zinc salt, polyvinyl alcohol, paraffin, and polyvinylpyrrolidone as porogens. These traditional porogens are often difficult to use to obtain cost-effective radiative cooling materials due to problems such as difficulty in controlling template size, high cost, and complex processes (e.g., requiring a freeze-drying step). See Comparative Example 3.

[0061] The calcium carbonate used in this invention has the advantages of high industrialization, mature preparation process, and precise control over particle size and morphology. Using it as a pore-forming agent results in more stable product quality and a relatively low price. Furthermore, this invention uses a mild acid washing aqueous solution (such as citric acid or dilute hydrochloric acid) as the removal medium. This process completely avoids the use of any volatile organic solvents (VOCs), has mild reaction conditions, low energy consumption, and produces soluble calcium salts (such as calcium chloride and calcium citrate) and carbon dioxide as byproducts. The calcium salts can be recycled as chemical raw materials or safely discharged after simple neutralization, achieving a green and environmentally friendly process throughout. More importantly, the morphology and particle size of calcium carbonate are highly controllable, and its interfacial interaction with polylactic acid is stable. This makes the final bilevel porous structure less affected by fluctuations in processing parameters such as mixing and hot pressing, which is beneficial for improving product quality stability.

[0062] The thickness of the membrane material is controlled between 200-1000 micrometers. If the membrane material is too thin, more solar radiation will pass through, resulting in low solar reflectivity and reduced overall radiative cooling performance. If the membrane material is too thick, the pickling time will be too long and it will not be economical.

[0063] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0064] Example 1:

[0065] This embodiment provides a heat-resistant, biodegradable porous cooling membrane material, which is prepared using the following method:

[0066] PLLA, PDLA, selective nucleating agent and calcium carbonate particles were dried in an oven at 80°C for 4 to 8 hours. Calcium carbonate with a particle size of 10 micrometers was selected, and the particle surface had densely distributed protrusions of 0.1 to 0.8 micrometers. The mass ratio of calcium carbonate to total polylactic acid was 5:1.

[0067] The dried raw materials were mixed in proportion (70 parts PLLA, 30 parts PDLA, 0.1 parts selective nucleating agent and 500 parts calcium carbonate) at 60~70°C until the moisture content was less than 0.1% and then discharged to obtain the blended granules.

[0068] Add the blended granules into an internal mixer, set the mixing temperature to 210~220°C, the internal mixer speed to 10~30 rad / min, and the mixing time to 2~5 minutes to obtain a mixed sample; pulverize the mixed sample with a pulverizer and further dry the pulverized sample;

[0069] Weigh an appropriate amount of pulverized sample, spread it evenly on a mold plate, place it in a molding machine, set the molding machine temperature to 220~230°C, and the molding time to 2~5 minutes. Then remove it from the mold and put it into a cold water tank to cool and obtain a film sample.

[0070] The membrane sample was immersed in a 0.1 mol / L citric acid solution at a temperature of 50°C for 4 to 8 hours to fully acidify the calcium carbonate particles in the membrane sample. The sample was then removed and rinsed several times with water to obtain a porous membrane material.

[0071] Finally, the porous membrane material is dried in an oven at 80°C for 1-3 hours, and then placed in a glass plate clamp and heat-treated in an oven at 90-120°C for 1-2 hours to obtain a heat-resistant, biodegradable porous cooling membrane material.

[0072] Such as scanning electron microscope Figure 1As shown, after pickling, a bimodal pore structure is formed, with both the large and small pores being nearly circular. The diameter of the large pores is about 10 micrometers, and the inner wall of the large pores is covered with densely distributed micropores with a diameter of 0.1 to 0.8 micrometers.

[0073] By scanning electron microscope Figure 2 It can be seen that there are protrusions on the surface of large calcium carbonate particles. It is the presence of these protrusions that leads to the formation of many tiny pores, thus forming a bimodal pore structure in the system.

[0074] Example 2:

[0075] This embodiment provides a heat-resistant, biodegradable porous cooling membrane material, which differs from Embodiment 1 in that it uses calcium carbonate with mixed particle sizes (including 13 micrometers, 5 micrometers, and 1 micrometer), and the particle surface has many protrusions of 0.1 to 0.8 micrometers. The remaining preparation methods and parameters are the same as in Embodiment 1.

[0076] Such as scanning electron microscope Figure 3 As shown, after pickling, a bimodal pore structure is formed, with both large and small pores being nearly circular. The diameter of the large pores is 1-13 micrometers, and the inner wall of the large pores is covered with densely distributed micropores with a diameter of 0.1-0.8 micrometers.

[0077] Example 3:

[0078] This embodiment provides a heat-resistant, biodegradable porous cooling membrane material, which differs from Embodiment 1 in that it uses calcium carbonate with a particle size of 6 micrometers, and the particle surface has many protrusions of 0.1 to 0.8 micrometers. The remaining preparation methods and parameters are the same as in Embodiment 1.

[0079] Such as scanning electron microscope Figure 4 As shown, after pickling, a bimodal pore structure is formed, with both the large and small pores being nearly circular. The diameter of the large pore is about 6 micrometers, and the inner wall of the large pore is covered with densely distributed micropores with a diameter of 0.1 to 0.8 micrometers.

[0080] Example 4:

[0081] This embodiment provides a heat-resistant, biodegradable porous cooling membrane material, which differs from Embodiment 1 in that: calcium carbonate with a particle size of 2 micrometers is selected, and the particle surface has many protrusions of 0.1~0.8 micrometers. The mass ratio of calcium carbonate to total polylactic acid is 3:1. The proportions of each raw material are: 70 parts PLLA, 30 parts PDLA, 0.1 parts selective nucleating agent, and 300 parts calcium carbonate. The remaining preparation methods and parameters are the same as in Embodiment 1.

[0082] Such as scanning electron microscope Figure 5As shown, after pickling, a bimodal pore structure is formed, with both the large and small pores being nearly circular. The diameter of the large pores is about 2 micrometers, and the inner wall of the large pores is covered with densely distributed micropores with a diameter of 0.1 to 0.8 micrometers.

[0083] Comparative Example 1:

[0084] This comparative example provides a cooling membrane material, which differs from Example 1 in that the polylactic acid matrix is ​​pure PLLA. The remaining preparation methods and parameters are the same as in Example 1.

[0085] Since the sacrificial template used in this comparative example is the same as that in Example 1, the scanning electron microscope image of the resulting film is identical to that in Example 1. Figure 1 They are similar, but the difference lies in the poor heat resistance of this membrane material, see Table 1.

[0086] Comparative Example 2:

[0087] This comparative example provides a cooling membrane material, which differs from Example 2 in that the ratio of calcium carbonate mass to total polylactic acid mass is 2:1. The remaining preparation methods and parameters are the same as in Example 2.

[0088] Such as scanning electron microscope Figure 6 As shown, after acid washing, the calcium carbonate particles on the upper and lower surfaces have been washed away and disappeared, but a large number of unwashed calcium carbonate particles remain in the core layer. This indicates that when the calcium carbonate content is low, this core layer is more difficult to wash clean.

[0089] The present invention specifies a calcium carbonate to polylactic acid (PLA) mass ratio of 2.5 to 5:1. Insufficient calcium carbonate leads to complete decomposition of the membrane surface layer but poor decomposition of the core layer during subsequent acid washing. Complete decomposition requires significantly longer washing times, which is uneconomical and inefficient. Prolonged immersion also carries the risk of premature degradation. This is because at lower concentrations, the calcium carbonate in the core is more thoroughly coated with PLA. Although PLA has good permeability to water vapor, this permeation is still a kinetically controlled process. A well-coated PLA layer prolongs the effective time for acid to enter the membrane, making it difficult for the calcium carbonate in the core layer to decompose. Such samples typically exhibit poor radiative cooling performance. Conversely, excessive calcium carbonate content results in excessively high membrane porosity and poor mechanical properties.

[0090] Comparative Example 3:

[0091] This comparative example provides a cooling membrane material, which differs from Example 1 in that calcium carbonate is replaced with glucose, the size of which is 1-80 micrometers, and the ratio of glucose to polylactic acid is 2:1. Since glucose is easily degraded at high temperatures, this comparative example is prepared by solution method (using dichloroethane as solvent), without the mixing, crushing and molding processes.

[0092] Such as scanning electron microscope Figure 7 As shown, the membrane material has many large pores of varying sizes, which is due to the large size of glucose itself, making it difficult to control. As can be seen from Table 1, the cooling capacity of this membrane material is poor, far inferior to that of Example 1.

[0093] Comparative Example 4:

[0094] This embodiment provides a cooling membrane material, which differs from Embodiment 2 in that the calcium carbonate particles have no protrusions on their surface. The remaining preparation methods and parameters are the same as in Embodiment 2.

[0095] Such as scanning electron microscope Figure 8 As shown, pickling creates many pores. The inner walls of the large pores are smooth, and there are no micropores.

[0096] Comparative Example 5:

[0097] This embodiment provides a cooling membrane material, which differs from Embodiment 1 in that calcium carbonate particles are not added, resulting in a smooth membrane material with a non-porous structure. The remaining preparation methods and parameters are the same as in Embodiment 1. This sample is not subjected to subsequent radiative cooling performance testing.

[0098] The above embodiments and comparative examples were subjected to the following performance tests. The test methods and results are as follows:

[0099] Experimental Example 1:

[0100] Scanning electron microscopy (SEM) testing, the testing method is as follows:

[0101] The surface morphology of the membrane cross-section was characterized using a ZEISS EV018 scanning electron microscope. The membrane was cut into strips and placed in liquid nitrogen for 30 seconds before being broken by external force. The sample was then fixed flat on the sample preparation stage with conductive adhesive. Because the coating has poor conductivity, the sample needed to be sputter-coated with gold before scanning electron microscopy. During the test, the accelerating voltage was adjusted to 5-15 kV.

[0102] Experimental Example 2:

[0103] Reflectivity and emissivity are tested using the following methods:

[0104] The reflectance of the samples was measured and analyzed using a Cary 5000 integrating sphere spectrometer from Agilent Technologies (China) Co., Ltd., obtaining solar reflectance data ranging from 0.3 to 2.5 micrometers, with a standard white plate used as a reference. Emissivity was measured using an FTIR spectrometer (Nicolet IS50, Thermo Fisher, USA), equipped with a gold-plated integrating sphere, obtaining atmospheric emissivity data ranging from 8 to 13 micrometers.

[0105] Experimental Example 3:

[0106] Outdoor testing, the testing method is as follows:

[0107] The test samples were cut into 3cm diameter circular pieces using a laser cutter. The samples were placed on a self-made radiative cooling test sample stage and fixed to a support. A silver film was used to cover the sample stage to reduce the influence of solar radiation. The temperature of each sample was recorded in real time using a digital thermometer (AT4208) with a probe, and ambient temperature, humidity, and wind intensity were recorded every minute using a 4G mini ultrasonic integrated weather station. The weather station was at the same height as the samples, approximately 1.5 m above the ground. Using a high-performance commercial radiative cooling film (FR1H300, Ningbo Ruiling New Energy Technology Co., Ltd., with reflectivity and emissivity of approximately 0.92 and 0.92 respectively) as a reference, the average temperature difference between 11:00 and 13:00 on a sunny outdoor day was selected as the evaluation index, as shown in Table 1. In the column for temperature difference (°C) with the commercial film, a negative value indicates that the radiative cooling performance is superior to that of the commercial radiative cooling film.

[0108] Experimental Example 4:

[0109] Contact Angle Test (WCA), the test method is as follows:

[0110] The water contact angle of the samples was measured using an OCA 15EC optical contact angle meter from Dataphysics GmbH, Germany. The ultrapure water injection rate and droplet volume were 1 uL / s and 6 uL, respectively.

[0111] Experimental Example 5:

[0112] The heat resistance temperature is tested using the following method:

[0113] Cut a 100mm × 100mm membrane material sample and lay it flat on a smooth steel plate. Place a transparent glass plate parallel to the steel plate above the membrane material, using a metal spacer that does not contact the membrane material to maintain a distance of 3 mm between the glass and the steel plate. Place the sample flat in an oven and heat it from room temperature at a rate of 5 °C / min. Stop heating when any part of the membrane material touches the glass, and record the oven temperature at this point as the heat resistance temperature of the membrane material (test five groups of samples and take the average value) to qualitatively evaluate the heat resistance performance of the membrane material.

[0114] Experimental Example 6:

[0115] Degradation performance was tested using the following methods:

[0116] The degradation rate of the membrane material was characterized by soil burial experiments. The membrane sample was cut into 10cm × 10cm pieces and dried to constant weight in a vacuum oven at 35℃. The initial mass m1 of the sample was recorded. The membrane sample was wrapped in a single layer of gauze and buried in the soil at a depth of approximately 20cm in the natural environment of Xiamen. After 360 days, the sample was removed and its surface was washed successively with tap water, 75% (volume fraction) ethanol, and distilled water. It was then dried to constant weight in a vacuum oven at 35℃, and the mass m2 of the degraded sample was recorded. Then:

[0117] Sample degradation rate = [(m1-m2) / m1] 100%.

[0118] The test results for the above experimental cases are shown in the table below:

[0119]

[0120] Based on the above performance tests and results, the membrane material prepared by this invention has at least the following characteristics:

[0121] 1. Reflectivity and Emissivity

[0122] Combined with appendix Figures 1 to 4 As shown in Table 1, Examples 1 to 4 all formed a bimodal structure with large pores and small pores on the inner wall. This structure effectively enhances light scattering and improves reflectivity and emissivity. The performance of Comparative Example 4 (without protruding template) and Comparative Example 5 (without pores) decreased significantly, with Comparative Example 5 even less than 20%, indicating that the bimodal pore structure has a significant impact on the optical properties of the film material.

[0123] 2. Excellent radiative cooling performance

[0124] All embodiments exhibited solar reflectance exceeding 90% in the 0.3–2.5 μm wavelength range and infrared emissivity exceeding 0.91 in the 8–13 μm wavelength range, meeting the basic requirements for efficient passive radiation cooling materials. In particular, Embodiment 4 demonstrated excellent cooling capability with a reflectance as high as 97% and the best temperature difference performance (–1.8°C). In contrast, Comparative Embodiments 2 to 5 showed reflectance levels below 90% and poorer temperature difference performance.

[0125] 3. Good hydrophobicity and degradability

[0126] All embodiments exhibited water contact angles exceeding 100°, demonstrating good hydrophobicity, which is beneficial for use in humid outdoor environments. Furthermore, their degradation rates were all above 80%, making them environmentally friendly and biodegradable. Comparative Example 3 (glucose template) and Comparative Example 5 (non-porous) had contact angles below 100°.

[0127] 4. Heat resistance

[0128] The heat resistance temperatures of Examples 1 to 4 all reached above 89°C, which is significantly higher than that of Comparative Example 1 (PLLA only), indicating that the formation of stereocomposite crystals by blending PLLA / PDLA can effectively improve the thermal stability of the material.

[0129] 5. Template proportions and structural integrity

[0130] Comparative Example 2 shows that an excessively low calcium carbonate ratio leads to an unclean core layer, incomplete pores, and decreased reflectivity; while Comparative Example 3, which uses a glucose template, results in inconsistent pore sizes and uncontrolled structure, further illustrating the importance of calcium carbonate template ratio and morphology control for structure replication.

[0131] In summary, this invention successfully prepared a heat-resistant, biodegradable porous cooling membrane with a bimodal porous structure by using a spherical calcium carbonate template with nano-protrusions on the surface of a PLLA / PDLA blend. This material exhibits excellent solar reflectivity and infrared emissivity without the need for a metal coating, while also possessing good hydrophobicity and biodegradability.

[0132] To further illustrate the various embodiments, the present invention provides accompanying drawings. These drawings are part of the disclosure of the present invention, primarily used to illustrate the embodiments, and can be used in conjunction with the relevant descriptions in the specification to explain the operating principles of the embodiments. With reference to these drawings, those skilled in the art should be able to understand other possible implementations and the advantages of the present invention.

[0133] The above description is only a preferred embodiment of the present invention and is not intended to limit the design of this case. All equivalent changes made based on the key design features of this case shall fall within the protection scope of this case.

Claims

1. A heat-resistant, biodegradable porous cooling membrane material, characterized in that: The matrix of the membrane material is a polylactic acid blend containing PLLA and PDLA, in which PLLA and PDLA form a stereocomposite crystal in the matrix; The mass ratio of PLLA to PDLA in the matrix is ​​1~4:1, and PDLA accounts for more than 20% of the total mass of polylactic acid; The matrix has a bimodal porous structure formed after the removal of the calcium carbonate template; the bimodal porous structure comprises a composite porous unit consisting of large pores with a size of 1-13 micrometers and small pores with a size of 0.1-0.8 micrometers distributed on the inner wall of the large pores, with an overall porosity of 55-70%; The template for forming the bimodal porous structure is a spherical calcium carbonate particle with protrusions on its surface; the mass ratio of the calcium carbonate particle to the total mass of polylactic acid is 2.5~5:1; The matrix also contains a selective nucleating agent comprising 0.1% to 0.5% of the total mass of polylactic acid.

2. The heat-resistant, biodegradable porous cooling membrane material as described in claim 1, characterized in that: The membrane material has a solar reflectance greater than 0.9 in the 0.3-2.5 micrometer wavelength range and an infrared emissivity greater than 0.9 in the 8-13 micrometer wavelength range; in addition, the water contact angle of the membrane material is greater than 100°.

3. The heat-resistant, biodegradable porous cooling membrane material as described in claim 1, characterized in that: The selective nucleating agent is selected from at least one of the following: aluminum complexes of hydrotalcite-containing phosphate esters, polylactic acid grafted with multi-walled carbon nanotubes, zinc phenyl phosphate, p-xylene dialkyl urea, or commercial TMB-5 nucleating agent.

4. The heat-resistant, biodegradable porous cooling membrane material as described in claim 1, characterized in that: The spherical calcium carbonate particles with protrusions on their surface have a particle size of 1 to 13 micrometers, and the size of the protrusions is 0.1 to 0.8 micrometers.

5. A method for preparing a heat-resistant, biodegradable porous cooling membrane material, characterized in that, The method for preparing the heat-resistant, biodegradable porous cooling membrane material as described in any one of claims 1 to 4 includes the following steps: S1. PLLA, PDLA, selective nucleating agent and calcium carbonate particles with a particle size of 1~13 micrometers and protrusions on the surface are dried and mixed to obtain blended particles; S2. The blended granules are then kneaded, pulverized, and molded to obtain a pre-made film containing a calcium carbonate template. S3. Use an acidic solution to decompose and dissolve the calcium carbonate particles in the pre-made membrane, so that a bimodal pore structure is formed in the pre-made membrane, and a porous membrane material is obtained. S4. The porous membrane material is dried and heat-treated to obtain a heat-resistant, biodegradable porous cooling membrane material.

6. The method for preparing the heat-resistant, biodegradable porous cooling membrane material as described in claim 5, characterized in that: In step S1, the raw material particles are stirred and blended at 60~70°C until the moisture content is below 0.1% and then discharged to obtain blended particles; In step S2, the blended particles are intensively mixed at 200~230°C and 10~30 rad / min for 2~5 minutes, then pulverized and molded at 220~240°C to obtain a pre-made film; In step S3, the pre-made membrane is placed in an acidic solution with a concentration of 0.01~0.5 mol / L and soaked at 40~60°C for 4~8 hours to completely dissolve and leach out the calcium carbonate particles, thereby forming the bimodal pore structure in the membrane. In step S4, the acid-washed porous membrane material is heat-treated at 90~120°C for 1~2 hours.

7. The method for preparing the heat-resistant, biodegradable porous cooling membrane material as described in claim 5, characterized in that: In step S3, the acidic solution is an aqueous solution of hydrochloric acid, citric acid, or acetic acid.

8. The use of the heat-resistant, biodegradable porous cooling membrane material as described in any one of claims 1 to 4, characterized in that: Used for car covers, seat covers, film for transportation mobile equipment, film for outdoor substations, building film, cable sheathing, base stations, or outdoor tents.

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