Radiation cooling fiber braided fabric with double-layer porous structure and preparation method of radiation cooling fiber braided fabric

Abstract

The invention relates to the technical field of polymer processing and forming, in particular to a radiation cooling fiber braided fabric with a double-layer porous structure and a preparation method of the radiation cooling fiber braided fabric. The invention discloses a radiation cooling fiber braided fabric with a double-layer porous structure. The radiation cooling fiber braided fabric is formed by twisting and braiding profiled fibers; after the special-shaped fibers are twisted and woven, different special-shaped fibers are interwoven to form an external porous structure; the profiled fiber comprises a nanofiber porous framework and inorganic photon particles, the inorganic photon particles are coated with the nanofiber porous framework, and an internal porous structure is formed under the supporting action of the inorganic photon particles; the external porous structure and the internal porous structure of the profiled fiber jointly form a double-layer porous structure of the radiation cooling fiber braided fabric. The preparation method comprises the following steps: (1) preparing primary special-shaped gel filaments; (2) a super-stretching stage of the primary special-shaped gel filaments; (3) a special-shaped gel silk extraction stage; and (4) a special-shaped fiber twisting and weaving stage.

Description

Technical Field

[0001] The invention relates to the technical field of polymer processing and molding, and in particular to a radiation cooling fiber braid with a double-layer porous structure and a preparation method thereof. Background Art

[0002] With global warming, about 10% of the world's electricity is currently used for indoor air conditioning each year, and an average of 13 new traditional cooling devices are installed every second. It is expected that by 2050, the demand for air conditioning and refrigeration will triple. This will undoubtedly produce more greenhouse gas emissions and ozone-depleting substances, exacerbating global warming and energy consumption. In order to combat this destructive vicious cycle, Passive Radiative Cooling (PRC) technology with green passive characteristics provides a promising approach.

[0003] PRC is a technology that reflects or scatters most of the solar spectrum (0.25-2.5 μm) and uses the characteristics of the surface atmosphere being transparent in the mid-infrared band (8-13 μm), namely the "atmospheric window", to radiate its own heat through the atmospheric window to the cold universe (~3 K), thereby achieving passive cooling. This requires that the radiation cooling material has good optical properties, namely high solar reflectivity and high and medium infrared emissivity.

[0004] In recent years, a series of new inorganic cooling materials have been designed to achieve radiative cooling, such as multilayer photonic crystals, multilayer materials and metamaterials, which show excellent passive radiative cooling performance. However, these technologies rely heavily on sophisticated equipment and complex preparation processes, and the materials used, such as expensive and highly reflective metal materials such as silver (Ag), limit their large-scale promotion and application.

[0005] In contrast, polymer-based porous materials have developed rapidly in the field of radiation cooling due to their good flexibility, light weight, good optical properties, simple preparation process, and low cost. However, polymer materials lack environmental stability due to their natural properties, and the porous structure introduced to improve optical properties will inevitably reduce the mechanical strength of the material.

[0006] In summary, how to develop a radiative cooling material with high optical performance, high mechanical strength and high environmental stability remains a challenge. Summary of the invention

[0007] The purpose of the present invention is to overcome the defects and shortcomings of the prior art and to provide a high-strength radiation cooling fiber braid with a double-layer porous structure. The braid has a simple structure and has the characteristics of high strength, excellent optical properties and good environmental stability.

[0008] Another object of the present invention is to provide a method for preparing the above-mentioned radiation cooling fiber braid with a double-layer porous structure, which method can realize the controllable structural adjustment and large-scale efficient continuous production of the polymer-based radiation cooling fiber braid, and has high production efficiency and low production cost.

[0009] The purpose of the present invention can be achieved through the following technical solutions: A radiation cooling fiber braid with a double-layer porous structure is formed by twisting and weaving shaped fibers; after the shaped fibers are twisted and weaved, different shaped fibers are intertwined to form an external porous structure; the shaped fibers include a nanofiber porous skeleton and inorganic photon particles, wherein the nanofiber porous skeleton is coated on the outside of the inorganic photon particles, and an internal porous structure is formed under the support of the inorganic photon particles; the external porous structure and the internal porous structure of the shaped fibers together constitute the double-layer porous structure of the radiation cooling fiber braid.

[0010] In the above-mentioned shaped fibers, the internal porous structure uses ultra-high molecular weight polyolefin as the material for the porous skeleton of the nanofibers to provide excellent mechanical support and environmental stability; through the super-stretching process and heat treatment process, the molecular chain orientation and crystallization of the ultra-high molecular weight polyolefin are induced to form a porous skeleton of the nanofibers, thereby greatly improving the mechanical properties and environmental stability of the fibers; the inorganic photonic particles uniformly wrapped in the porous skeleton of the nanofibers provide more interface structures, which not only further improves the solar reflectivity, but also provides excellent mid-infrared emissivity.

[0011] The radiation cooling braid woven from shaped fibers has a special double-layer porous structure. Under the synergistic effect with inorganic photon particles, it can effectively improve the solar reflectivity and mid-infrared emissivity. The main reasons are as follows: First, the inorganic photon particles have strong intrinsic radiation ability in the mid-infrared band and can provide good emissivity; second, the interior of the shaped fiber, filled with ultra-high molecular weight polyolefin material of inorganic photon particles, forms multiple microporous structures under the super-stretching process, which provides sufficient interface for effective solar reflection and exhibits strong light scattering efficiency; third, the exterior of the shaped fiber forms an ideal shaped cross-section under the super-stretching and heat treatment process. After twisting and weaving, the interweaving of different shaped fibers will produce multiple new microporous structures. This microporous structure obtained by stacking the shaped cross-sections of the shaped fibers has a more complex interface, which can further increase the proportion of reflected sunlight.

[0012] The cross-sectional shape of the special-shaped fiber is ○-shaped, Y-shaped, +-shaped, ☆-shaped, trilobal, quadrilobal, pentlobal or fan-shaped.

[0013] The maximum width or equivalent diameter of the shaped fiber is 1-200 μm; in the nanofiber porous skeleton, the pore size of the internal porous structure is 0.1-5 μm, and the porosity is 50-90%. Tests have shown that within this pore size and porosity range, the shaping effect of the shaped fiber can be effectively guaranteed to be good.

[0014] The thickness of the radiation cooling fiber braid is 20-1000 μm, the pore size of the external porous structure is 0.1-20 μm, and the porosity is 20-90%. Experiments have shown that within the pore size and porosity range of the internal porous structure of the above-mentioned shaped fibers and the external porous structure formed by twisting and braiding, the reflectivity of the radiation cooling fiber braid can be effectively guaranteed to meet actual use requirements.

[0015] The main raw material of the nanofiber porous skeleton is an ultra-high molecular weight polyolefin material with a molecular weight of 1.5 to 10 million.

[0016] The ultra-high molecular weight polyolefin material can be one or a combination of ultra-high molecular weight polyethylene (UHMWPE) and ultra-high molecular weight polypropylene (UHMWPP). Ultra-high molecular weight polyolefin has a large entanglement density. On the one hand, it can improve the formability of the shaped fiber under the condition of high load diluent and inorganic photon particles; on the other hand, during the super-stretching and heat treatment process, the lamellar structure of the ultra-high molecular weight polyolefin can be converted into nanofiber crystals to form a highly crystallized nanofiber crystal porous skeleton, which ensures the mechanical strength and environmental stability of the shaped fiber.

[0017] The inorganic photon particles are one or more of silicon monoxide (SiO), silicon dioxide (SiO2), silicon nitride (Si3N4), zinc oxide (ZnO), barium sulfate (BaSO4), fluorides (CaF2 and MgF2), silicon carbide (SiC), chromium oxide (Cr2O3), boron nitride (BN) or titanium dioxide (TiO2).

[0018] In the shaped fibers, the weight ratio of the inorganic photon particles is 10wt% to 80wt%.

[0019] A method for preparing a radiation cooling fiber braid with a double-layer porous structure comprises the following steps: (1) Preparation stage of primary shaped gel filaments: the pre-swollen ultra-high molecular weight polyolefin material, diluent and inorganic photon particles are blended according to a ratio and then melted and plasticized and transported through a blending extrusion device. The mixture is then continuously extruded through a shaped spinneret to form primary shaped gel filaments. (2) Super-stretching stage of primary shaped gel filaments: The primary shaped gel filaments obtained in step (1) are super-stretched to convert the lamellar structure of the ultra-high molecular weight polyolefin material into nanofiber crystals, and the nanofiber porous skeleton is isolated under the condition of high load of inorganic photon particles, thereby obtaining shaped gel filaments; In the step (2), the primary shaped gel filaments may be subjected to wet super-stretching by means of a fiber super-stretching device; or the primary shaped gel filaments after the extractant and the diluent are extracted may be subjected to dry stretching; (3) Shaped gel filament extraction stage: extracting and drying the shaped gel filaments obtained in step (2), and then heat-treating the shaped gel filaments to form shaped fibers with an internal porous structure; (4) Shaped fiber twisting and weaving stage: The shaped fibers obtained in step (3) are twisted and weaved by a fiber weaving machine. The shaped fibers with an internal porous structure form an external porous structure during the twisting and weaving process, thereby producing a radiation cooling fiber braid with a double-layer porous structure.

[0020] In the above-mentioned preparation process, in order to improve the mechanical properties of the radiation cooling fiber braid while improving the radiation cooling performance, in the step (2), after the wet super-stretched shaped gel filaments are extracted and dried, the shaped gel filaments are subjected to dry uniaxial stretching. Experiments and tests have shown that the porous shaped fibers that have been dry-stretched are similar in structure to the shaped fibers that have been wet-super-stretched, and can still maintain a good internal porous structure and external shaped cross-section. At the same time, the external structure of the primary shaped gel filaments will also be deformed to a certain extent due to external forces during the super-stretching process. The difference is that dry stretching can achieve an increase in the internal porosity of the shaped fibers and an increase in the pore size, which is of great benefit to the radiation cooling effect, thereby achieving the purpose of improving the radiation cooling performance.

[0021] In the step (1), the mass of the diluent added to the mixed material accounts for 50-90% of the total mass of the swelling liquid formed after mixing; the diluent is a polymer porogen (such as PVP, PEG, PVA, etc.) or a small molecule porogen (such as mineral oil, vaseline, vegetable oil, decalin, sodium chloride, potassium carbonate, lithium chloride, etc.); when the swelling liquid is blended and melted, the blending extrusion equipment used can be a multi-screw extruder, a single-axis eccentric rotor extruder or a double-axis eccentric rotor extruder, and the plasticizing melting temperature is 170-250°C; the cross-section of the special-shaped spinneret can be designed to be ○-shaped, Y-shaped, +-shaped or ☆-shaped, and the processing temperature of the special-shaped spinneret continuous extrusion is 170-250°C; In the step (2), when the primary shaped gel filaments are subjected to super-multiple stretching, the stretching temperature is 80-130° C., the stretching ratio of the primary shaped gel filaments is 5-40 times, and a single stretching or multiple stretching method is adopted; In the steps (2) and (3), the extraction is performed by static, ultrasonic or stirring extraction, and the extractant is n-hexane, ionic liquid, acetone, xylene, chloroform, water or alcohol or other substances that can remove the diluent; when the special-shaped fibers with a porous structure are heat-treated, the heat treatment equipment used is a vacuum oven, a blast oven or a muffle furnace, the heat treatment temperature is 110-150°C, and the heat treatment time is 1-5 minutes.

[0022] In the above preparation method, ultra-high molecular weight polyolefin is used as the material for preparing the nanofiber porous skeleton of the shaped fiber, which is coated on the outside of the spherical inorganic photon particles. The ultra-high molecular weight polyolefin platelets are converted into nanofiber crystals by super-stretching. At the same time, the nanofiber porous skeleton forms a microporous structure under the support of the inorganic photon particles, and the shaped cross-section of the outside of the shaped fiber can also be controlled and adjusted, so that the obtained braid has an ideal external microporous structure. In addition, through the heat treatment process, the ultra-high molecular weight polyolefin is further crystallized, perfected and coarsened to improve the comprehensive performance of the shaped fiber. The ultra-high molecular weight polyolefin forms a highly oriented and crystallized nanofiber porous skeleton, which makes the mechanical properties of the shaped fiber much higher than those of the traditional functional fiber, so that the finally obtained radiation cooling fiber braid has ultra-high strength and a double-layer porous structure, which greatly improves the optical performance and adapts to a variety of different application scenarios.

[0023] Compared with the prior art, the present invention has the following advantages and beneficial effects: 1. The radiation cooling fiber braid with a double-layer porous structure of the present invention has a simple structure and a wide range of raw materials. The internal porous structure of the special-shaped fibers and the external porous structure formed by the interweaving of special-shaped cross-sections form a double-layer porous structure. Compared with the current radiation cooling porous membrane, it is more controllable and has a richer and more complex interface, so that it exhibits a stronger solar reflectivity. The actual cooling test shows that the radiation cooling fiber braid with a double-layer porous structure has an excellent cooling effect, which can reduce the normal ambient temperature by up to 10.2 °C.

[0024] 2. The shaped fibers of the present invention make full use of the long molecular chains of ultra-high molecular weight polyolefins to form a porous framework of nanofibers, in which inorganic photon particles are embedded and dispersed to provide excellent emissivity. The structure has strong stability, and the inorganic photon particles are not easy to fall off, thereby ensuring the stability of photons in practical applications.

[0025] 3. When ordinary polymers form porous structures, their mechanical strength and durability tend to be significantly reduced, which limits their application in radiation cooling. Compared with ordinary polymers, ultra-high molecular weight polyolefins have extremely high molecular weights, which gives them excellent mechanical strength, wear resistance and impact resistance. The present invention uses a specific processing technology (super-stretching) to form a porous structure of ultra-high molecular weight polyolefin materials. This structure not only does not significantly affect the mechanical properties of the material, but also can further improve its solar reflectivity and mid-infrared emissivity, thereby enhancing the radiation cooling effect.

[0026] The shaped fibers of the present invention make full use of the molecular chain movement and relaxation characteristics of ultra-high molecular weight polyolefins and the stretchable characteristics of fiber products. The stretching ratio of the shaped fibers is increased by super-stretching and further heat-treated, so that the internal and external structures of the shaped fibers can be controllably adjusted and optimized, and the mechanical properties are greatly improved. Furthermore, the radiation cooling fiber braid woven from such high-strength shaped fibers also has stronger mechanical strength and optical properties, and its mechanical strength and optical properties are much higher than the current polymer-based radiation cooling porous membranes. The shaped fibers of the present invention can be adjusted to ultra-high strength and customized structure through super-stretching and heat treatment processes, and are suitable for a variety of different application scenarios and performance requirements.

[0027] 4. The shaped fibers of the present invention can be prepared by a continuous production method. Ultra-high molecular weight polyolefin is used as the matrix of the nanofiber porous skeleton. The material system has low cost, a wide selection of inorganic photon particles, and a simple preparation method, convenient operation, high production efficiency, and is suitable for large-scale industrial production. In addition, this shaped fiber replaces the traditional metal reflective substrate with a polymer matrix, which simplifies the preparation process and cost of the matrix. At the same time, compared with the single structure of the polymer-based radiation cooling porous membrane, the present invention prepares a radiation cooling fiber braid with a double-layer porous structure, which can be structurally adjusted and woven according to demand. In addition, the polymer itself has strong flexibility, which greatly broadens its application scenarios and has a broad market space. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is a schematic diagram of a radial cross section of a shaped fiber according to Example 1 of the present invention; Figure 2 is a schematic axial cross-sectional view of the shaped fiber of Example 1 of the present invention; Figure 3 is an electron micrograph of the internal structure of the shaped fiber of the present invention; Figure 4 is a schematic diagram of a radial cross section of a shaped fiber according to Example 2 of the present invention; Figure 5It is a comparison of the optical path diagrams corresponding to the structures of shaped fibers with different radial cross sections after twisting and the complex interfaces forming the external porous structures; Among them: 1. Nanofiber porous skeleton; 2. Inorganic photon particles. DETAILED DESCRIPTION

[0029] The present invention is further described in detail below in conjunction with embodiments and drawings, but the embodiments of the present invention are not limited thereto.

[0030] Example 1 A radiation cooling fiber braid with a double-layer porous structure is formed by twisting and weaving shaped fibers; after the shaped fibers are twisted and weaved, different shaped fibers are intertwined to form an external porous structure; the shaped fibers include a nanofiber porous skeleton and inorganic photon particles, wherein the nanofiber porous skeleton is coated on the outside of the inorganic photon particles, and an internal porous structure is formed under the support of the inorganic photon particles; the external porous structure and the internal porous structure of the shaped fibers together constitute the double-layer porous structure of the radiation cooling fiber braid.

[0031] like Figures 1 to 3 As shown, in the above-mentioned shaped fibers, the internal porous structure uses ultra-high molecular weight polyolefin as the material for the porous skeleton of the nanofibers to provide excellent mechanical support and environmental stability; through the super-stretching process and the heat treatment process, the molecular chain orientation and crystallization of the ultra-high molecular weight polyolefin are induced to form a porous skeleton of the nanofibers, thereby greatly improving the mechanical properties and environmental stability of the fibers; the inorganic photon particles uniformly wrapped in the porous skeleton of the nanofibers provide more interface structures, which not only further improves the solar reflectivity, but also provides excellent mid-infrared emissivity.

[0032] The radiation cooling braid woven from shaped fibers has a special double-layer porous structure. Under the synergistic effect with inorganic photon particles, it can effectively improve the solar reflectivity and mid-infrared emissivity. The main reasons are as follows: First, the inorganic photon particles have strong intrinsic radiation ability in the mid-infrared band and can provide good emissivity; second, the interior of the shaped fiber, filled with ultra-high molecular weight polyolefin material of inorganic photon particles, forms multiple microporous structures under the super-stretching process, which provides sufficient interface for effective solar reflection and exhibits strong light scattering efficiency; third, the exterior of the shaped fiber forms an ideal shaped cross-section under the super-stretching and heat treatment process. After twisting and weaving, the interweaving of different shaped fibers will produce multiple new microporous structures. This microporous structure obtained by stacking the shaped cross-sections of the shaped fibers has a more complex interface, which can further increase the proportion of reflected sunlight.

[0033] The cross-sectional shape of the shaped fiber can be ○-shaped, Y-shaped, +-shaped, ☆-shaped, trilobal, quadrilobal, pentlobal or fan-shaped. The maximum width or equivalent diameter of the shaped fiber is 1~200μm; the maximum width or equivalent diameter of the shaped fiber is 1~200μm; in the nanofiber porous skeleton, the pore size of the internal porous structure is 0.1~5μm, and the porosity is 50~90%. Experiments have shown that within the pore size and porosity range, the molding effect of the shaped fiber can be effectively guaranteed to be good. The thickness of the radiation cooling fiber braid is 20~1000μm, the pore size of the external porous structure is 0.1~20μm, and the porosity is 20~90%. Experiments have shown that within the pore size and porosity range of the internal porous structure of the shaped fiber and the external porous structure formed by twisting and weaving, the reflectivity of the radiation cooling fiber braid can be effectively guaranteed to meet the actual use requirements. The main raw material of the nanofiber porous skeleton is an ultra-high molecular weight polyolefin material with a molecular weight of 1.5 to 10 million. The ultra-high molecular weight polyolefin material can be one or a combination of ultra-high molecular weight polyethylene (UHMWPE) and ultra-high molecular weight polypropylene (UHMWPP). The inorganic photon particles can be one or more of silicon monoxide (SiO), silicon dioxide (SiO2), silicon nitride (Si3N4), zinc oxide (ZnO), barium sulfate (BaSO4), fluoride (CaF2 and MgF2), silicon carbide (SiC), chromium oxide (Cr2O3), boron nitride (BN) or titanium dioxide (TiO2).

[0034] As a preferred implementation manner, in the shaped fibers, the weight ratio of the inorganic photonic particles is 10wt% to 80wt%.

[0035] The method for preparing the radiation cooling fiber braid having a double-layer porous structure mainly comprises the following steps: (1) Preparation stage of primary shaped gel filaments: the pre-swollen ultra-high molecular weight polyolefin material, diluent and inorganic photon particles are blended according to a ratio and then melted and plasticized and transported through a blending extrusion device. The mixture is then continuously extruded through a shaped spinneret to form primary shaped gel filaments. In step (1) of this embodiment, the ultra-high molecular weight polyolefin material is ultra-high molecular weight polyethylene (UHMWPE), the inorganic photon particles are spherical silicon dioxide (SiO2), the diluent is liquid paraffin (LP), and the raw materials in the primary shaped gel filaments are weighed according to a weight fraction of UHMWPE:SiO2:LP=4:16:80; the shaped spinneret is an ○-shaped spinneret, and the processing temperature of the ○-shaped spinneret for continuous extrusion is 200°C; (2) Super-stretching stage of primary shaped gel filaments: The primary shaped gel filaments obtained in step (1) are super-stretched to convert the lamellar structure of the ultra-high molecular weight polyolefin material into nanofiber crystals, and the nanofiber porous skeleton is isolated under the condition of high load of inorganic photon particles, thereby obtaining shaped gel filaments; In step (2) of the present embodiment, the primary shaped gel filaments are subjected to wet super-stretching by a fiber super-stretching device, the stretching temperature is 110°C, and the stretching is performed twice, with the stretching ratios of the two ends being 5 times and 6 times respectively; during heat setting, the heat setting temperature is 115°C, and the heat setting time is 3 minutes; (3) Shaped gel filament extraction stage: extracting and drying the shaped gel filaments obtained in step (2), and then heat-treating the shaped gel filaments to form shaped fibers with an internal porous structure; In step (3) of this embodiment, ultrasonic extraction is used for extraction, and the extractant used for extraction is n-hexane; a forced air oven is used for heat treatment, the heat treatment temperature is 140° C., and the heat treatment time is 5 min; (4) Shaped fiber twisting and weaving stage: The shaped fibers obtained in step (3) are twisted and weaved by a fiber weaving machine. The shaped fibers with an internal porous structure form an external porous structure during the twisting and weaving process, thereby producing a radiation cooling fiber braid with a double-layer porous structure.

[0036] The UHMWPE-based radiation cooling O-shaped fiber braid prepared by the above method was tested, and the diameter of the obtained O-shaped fiber was about 80μm, and the tensile strength could reach 100MPa; the thickness of the UHMWPE-based radiation cooling O-shaped fiber braid was about 400μm, the solar reflectivity was 93%, and the mid-infrared emissivity was 90%.

[0037] Example 2 This embodiment provides a radiation cooling fiber braid with a double-layer porous structure and a preparation method thereof. The ultra-high molecular weight polyolefin material, inorganic photon particles and diluent still use UHMWPE, SiO2 and LP.

[0038] like Figure 4 As shown, compared with Example 1, the difference is that the special-shaped spinneret adopts a Y-shaped spinneret. The radiation cooling fiber braid with a double-layer porous structure of this embodiment is twisted and woven by Y-shaped fibers with obvious special-shaped characteristics. In this structure, the Y-shaped cross-section of the Y-shaped fiber can greatly improve the solar reflectivity of the radiation cooling fiber braid without affecting the mid-infrared emissivity and intensity. The Y-shaped cross-section is mainly obtained by the cross-section design of the special-shaped spinneret in step (1).

[0039] This radiation cooling fiber braid has excellent optical properties, mainly reflected in its outstanding sunlight reflectivity. The main reasons are as follows: First, the inorganic photon particles have strong intrinsic radiation ability in the mid-infrared band and can provide good emissivity; second, the ultra-high molecular weight polyolefin material filled with inorganic photon particles inside the Y-shaped fiber forms multiple microporous structures under super-stretching, which provides sufficient interfaces for effective sunlight reflection and exhibits strong light scattering efficiency; third, after the Y-shaped fiber is twisted and woven, the microporous structure produced by the interweaving of the Y cross sections outside the Y-shaped fiber has a more complex interface than the ○-shaped fiber, and, as shown in FIG. Figure 5 As shown, under the super-stretching and heat treatment process, the Y-shaped cross-section can further undergo controllable deformation to achieve an ideal cross-sectional shape, which can further increase the proportion of reflected sunlight.

[0040] UHMWPE-based radiation cooling Y-shaped fiber braids with the same thickness and porosity were prepared by the above method. After testing and analysis, the solar reflectivity was increased to 95.4%, the mid-infrared emissivity was 87.6%, and the tensile strength was 105 MPa.

[0041] Example 3 This embodiment provides a radiation cooling fiber braid with a double-layer porous structure and a preparation method thereof. Compared with embodiment 2, the difference is that: In order to further improve the performance of the obtained radiation cooling fiber braid, in step (2), after the Y-shaped gel yarn after wet super-stretching is extracted and dried, before the Y-shaped fiber with internal porous structure is heat-treated in step (3), the special-shaped gel yarn is dry-uniaxially stretched. The specific operation is as follows: the Y-shaped fiber is dry-uniaxially stretched using a fiber stretching device, the stretching chamber temperature is set to 110°C, the sample preheating time is set to 300s, the stretching ratio is 2 times, and the heat setting is performed for 120s after stretching, and the heat setting temperature is 115°C.

[0042] Tests and inspections show that the structure of the shaped fiber after dry uniaxial stretching is similar to that of the shaped fiber after wet super-stretching, and it can still maintain a good internal structure and external shaped cross-section to maintain mechanical properties. The difference is that dry uniaxial stretching can achieve an increase in the internal porosity and expansion of the pore size of the shaped fiber when the internal porous structure has been formed, which is of great benefit to the radiation cooling effect, thereby achieving the purpose of improving the radiation cooling performance. Through testing, the solar reflectivity of the radiation cooling braid of the same thickness is as high as 97.3%, the mid-infrared emissivity is maintained at 91.2%, and the tensile strength is 85MPa.

[0043] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A radiation cooling fiber braid with a double-layer porous structure, characterized in that: It is made of twisted and woven shaped fibers; after twisting and weaving, different shaped fibers are intertwined to form an external porous structure; the shaped fibers include a nanofiber porous skeleton and inorganic photon particles, wherein the nanofiber porous skeleton is coated on the outside of the inorganic photon particles, and under the support of the inorganic photon particles, an internal porous structure is formed; the external porous structure and the internal porous structure of the shaped fibers together constitute a double-layer porous structure of the radiation cooling fiber braid.

2. The radiation cooling fiber braid with a double-layer porous structure according to claim 1, characterized in that: The cross-sectional shape of the special-shaped fiber is ○-shaped, Y-shaped, +-shaped, ☆-shaped, trilobal, quadrilobal, pentlobal or fan-shaped.

3. The radiation cooling fiber braid with a double-layer porous structure according to claim 1, characterized in that: The maximum width or equivalent diameter of the shaped fiber is 1-200 μm; in the nanofiber porous skeleton, the pore size of the internal porous structure is 0.1-5 μm, and the porosity is 50-90%.

4. The radiation cooling fiber braid with a double-layer porous structure according to claim 1, characterized in that: The thickness of the radiation cooling fiber braid is 20-1000 μm, the pore size of the external porous structure is 0.1-20 μm, and the porosity is 20-90%.

5. The radiation cooling fiber braid with a double-layer porous structure according to claim 1, characterized in that: The main raw material of the nanofiber porous skeleton is an ultra-high molecular weight polyolefin material with a molecular weight of 1.5 to 10 million; the ultra-high molecular weight polyolefin material is one or a combination of ultra-high molecular weight polyethylene and ultra-high molecular weight polypropylene.

6. The radiation cooling fiber braid with a double-layer porous structure according to claim 1, characterized in that: The inorganic photon particles are one or more of silicon monoxide, silicon dioxide, silicon nitride, zinc oxide barium sulfate, fluoride, silicon carbide, chromium oxide, boron nitride or titanium dioxide.

7. The radiation cooling fiber braid with a double-layer porous structure according to claim 1, characterized in that: In the shaped fibers, the weight ratio of the inorganic photon particles is 10wt% to 80wt%.

8. A method for preparing a radiation cooling fiber braid with a double-layer porous structure, characterized in that: The following steps are involved: (1) Preparation stage of primary shaped gel filaments: the pre-swollen ultra-high molecular weight polyolefin material, diluent and inorganic photon particles are blended according to a ratio and then melted and plasticized and transported through a blending extrusion device. The mixture is then continuously extruded through a shaped spinneret to form primary shaped gel filaments. (2) Super-stretching stage of primary shaped gel filaments: The primary shaped gel filaments obtained in step (1) are super-stretched to convert the lamellar structure of the ultra-high molecular weight polyolefin material into nanofiber crystals, and the nanofiber porous skeleton is isolated under the condition of high load of inorganic photon particles, thereby obtaining shaped gel filaments; (3) Shaped gel filament extraction stage: extracting and drying the shaped gel filaments obtained in step (2), and then heat-treating the shaped gel filaments to form shaped fibers with an internal porous structure; (4) Shaped fiber twisting and weaving stage: The shaped fibers obtained in step (3) are twisted and weaved by a fiber weaving machine. The shaped fibers with an internal porous structure form an external porous structure during the twisting and weaving process, thereby producing a radiation cooling fiber braid with a double-layer porous structure.

9. The method for preparing a radiation cooling fiber braid with a double-layer porous structure according to claim 8, characterized in that: In the step (2), the primary shaped gel fibers are subjected to wet super-multiple stretching by means of a fiber super-multiple stretching device; or, the primary shaped gel fibers are subjected to dry stretching after the extractant and the diluent are extracted.

10. The method for preparing a radiation cooling fiber braid with a double-layer porous structure according to claim 9, characterized in that: In the step (2), after the wet super-stretched shaped gel filaments are extracted and dried, the shaped gel filaments are subjected to dry uniaxial stretching.

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