Nanofiber thermal insulation sponge and preparation method thereof

Abstract

The application discloses a kind of nanofiber heat preservation and heat insulation sponges and preparation methods thereof, belong to nanofiber material and heat preservation and heat insulation technical field.The sponge is formed by polymer nanofiber three-dimensional intercommunication porous network, and the fiber lap joint point is connected by anchoring material to form connecting point.The preparation method comprises: blending polymer and anchoring material to carry out electrospinning to obtain two-dimensional nanofiber film;Using sodium borohydride solution to carry out gas foaming, using in-situ hydrogen production to make two-dimensional film expand into three-dimensional sponge;After washing and drying, anchoring treatment is carried out, and the product is obtained.The application adopts "premixing-foaming-anchoring" synergistic technical route, and the process is simple and green, and it is operated at normal temperature and pressure, and no toxic and harmful gas is generated.The prepared sponge has high porosity, low thermal conductivity and excellent compression resilience, solves the problem that traditional foaming sponge is difficult to consider mechanics and thermal insulation performance, and can be widely applied in building, aerospace and civil warming fields.

Description

Technical Field

[0001] This invention belongs to the field of nanofiber materials and thermal insulation technology, and particularly relates to a nanofiber thermal insulation sponge and its preparation method. Background Technology

[0002] Thermal insulation materials are widely used in various fields such as construction, aerospace, new energy, and civil insulation. Their core performance requirements typically include low thermal conductivity, lightweight properties, stable structure, and a simple and environmentally friendly manufacturing process. Currently, commonly used thermal insulation materials include traditional rock wool and glass wool, as well as novel materials such as nanofibers and aerogels. Among these materials, nanofibers, due to their outstanding advantages of high specific surface area and high porosity, have shown promising application prospects in the field of thermal insulation.

[0003] Electrospinning is a common method for preparing nanofiber membranes. This technology can produce two-dimensional nanofiber membranes with uniform thickness and controllable fiber diameter. However, these two-dimensional nanofiber membranes have inherent defects such as low porosity and lack of three-dimensional structure, resulting in a limited volume of static air inside that can restrict heat conduction. This limits their thermal insulation performance and makes it difficult to meet the comprehensive performance requirements of thermal insulation materials in some high-end applications.

[0004] To address the aforementioned issues, various methods for preparing three-dimensional nanofiber sponges have been developed in existing technologies, primarily including sol-gel methods, freeze-drying methods, template methods, and gas foaming methods. However, these methods still have their own shortcomings in practical applications. For example, the sol-gel method is typically time-consuming, and the resulting sponge has relatively low porosity, limiting the improvement of thermal insulation performance. While the freeze-drying method can obtain high-porosity structures, its long preparation cycle and high energy consumption are significant drawbacks, hindering large-scale and low-cost production. The template method requires the use of additional template agents, and the subsequent template removal process is not only complex but also prone to environmental pollution due to solvent use and emissions. Traditional gas foaming methods often use chemical foaming agents, which can easily generate toxic and harmful gases during the foaming process, and the foam pore structure is difficult to control precisely. These factors ultimately affect the thermal insulation performance and long-term structural stability of the sponge.

[0005] Sodium borohydride, a substance capable of reacting in situ with water or protic solvents and gently releasing hydrogen gas, offers a novel option for gas foaming technology. This reaction process is mild, produces non-toxic inorganic salts as byproducts, and does not generate any toxic or harmful gases. Furthermore, the generated hydrogen gas can expand in situ within a dense two-dimensional nanofiber membrane, potentially forming a uniform porous structure and enabling a direct and efficient conversion from two-dimensional fiber membranes to three-dimensional sponges. However, to date, there is no systematic and mature method for applying sodium borohydride gas foaming technology to electrospun nanofiber membranes to prepare three-dimensional sponges with high elasticity, structural stability, and excellent thermal insulation properties. Therefore, developing a novel nanofiber sponge with a simple, environmentally friendly process and excellent thermal insulation properties, along with its preparation method, has significant practical application value. Summary of the Invention

[0006] In view of the above-mentioned shortcomings of the prior art, the purpose of the present invention is to provide a nanofiber thermal insulation sponge and its preparation method, so as to solve the problems of complex preparation process, high energy consumption, poor environmental protection of existing three-dimensional nanofiber sponge, and insufficient thermal insulation performance of two-dimensional electrospun nanofiber membrane due to low porosity and lack of three-dimensional structure.

[0007] To achieve the above and other related objectives, the present invention provides the following technical solution.

[0008] This invention provides a nanofiber thermal insulation sponge, which is composed of a three-dimensional interconnected porous network of polymer nanofibers, with anchoring materials forming connection points at the overlaps of the polymer nanofibers. The sponge has a porosity of 80%~95% and a thermal conductivity of 30~35 mW / (m·K). The preferred density of the sponge is 8~20 mg / cm³. 3 .

[0009] In the sponge of the present invention, the anchoring material is either a thermoplastic anchoring material or a reactive anchoring material. The thermoplastic anchoring material melts and flows when heated above its melting temperature, converging at the fiber overlap points, and then solidifies physically upon cooling to form connection points. Such materials can be selected from one or more of polycaprolactone, thermoplastic polyurethane, polylactic acid, poly(lactic-co-glycolic acid copolymer), polyethylene oxide, and polyvinylpyrrolidone. The reactive anchoring material undergoes a chemical cross-linking reaction through the active functional groups on its molecular chain when heated above its high reactivity temperature, forming permanent chemical connection points at the fiber overlap points. Such materials can be selected from one or more of epoxy resin precursors, unsaturated imides, phenolic resin oligomers, and polydimethylsiloxane precursors.

[0010] The polymer constituting the main body of the sponge skeleton of this invention can be selected from one or more of polyacrylonitrile, polyamide, cellulose acetate, polyvinylidene fluoride, polyvinyl chloride, polyvinylidene chloride, polyethylene terephthalate, polyurethane, polyamic acid, and polyhydroxyalkanoates. These polymers all have good spinnability and film-forming properties, and can form uniform and continuous nanofibers through electrospinning.

[0011] In the three-dimensional interconnected porous network of the sponge of this invention, the connection points are formed in situ at the fiber overlaps by anchoring materials, rather than by adding external adhesives or cross-linking agents. This in-situ anchoring method ensures that the connection points exist only at the fiber overlap locations, preventing a reduction in porosity due to excessive adhesive filling the pores. This achieves synergistic optimization of mechanical reinforcement and thermal insulation performance, giving the sponge high porosity, low thermal conductivity, good compression resilience, and structural dimensional stability.

[0012] Another aspect of the present invention provides a method for preparing the above-mentioned nanofiber thermal insulation sponge, comprising the following steps:

[0013] (1) Dissolve the polymer and anchoring material in a good solvent to prepare a homogeneous electrospinning solution, and prepare a two-dimensional nanofiber membrane by electrospinning;

[0014] (2) The two-dimensional nanofiber membrane is completely immersed in a foaming solution containing sodium borohydride to perform gas foaming, so that the two-dimensional nanofiber membrane expands to form a three-dimensional nanofiber sponge.

[0015] (3) The three-dimensional nanofiber sponge obtained in step (2) is washed with deionized water and dried to obtain a dry three-dimensional nanofiber sponge;

[0016] (4) The dry three-dimensional nanofiber sponge obtained in step (3) is anchored, so that the anchoring material melts or cross-links at the fiber overlap point to form a connection point, and the nanofiber thermal insulation sponge is obtained.

[0017] The core concept of this invention lies in: first, premixing the anchoring material into the polymer fibers; then, using a gas foaming technology that generates hydrogen in situ with sodium borohydride to expand the dense two-dimensional nanofiber membrane into a three-dimensional sponge; and finally, activating the anchoring material inside the fibers through heat treatment, causing it to form connection points in situ at the fiber overlaps. This three-step synergistic technical route of "premixing-foaming-anchoring" achieves a direct and green transformation from a two-dimensional membrane to a three-dimensional highly elastic sponge. Specifically, sodium borohydride reacts in situ with protic solvents such as water or alcohols, gently releasing hydrogen gas. This gas accumulates and expands in the interlayer and fiber gaps of the two-dimensional membrane, uniformly expanding the dense structure to form a sponge with three-dimensional interconnected channels. This gas foaming process is mild, does not produce toxic or harmful gases, and the byproduct is a soluble inorganic salt (sodium metaborate), which can be completely removed by a simple water washing step, demonstrating the green and environmentally friendly process characteristics. Subsequently, through the anchoring process, the anchoring material dispersed within the fibers is thermally activated, selectively migrating and accumulating at the fiber overlap points, forming stable connection nodes in situ through physical curing or chemical cross-linking. This anchoring mechanism transforms the originally loose fiber network, which relied solely on weak physical entanglement and van der Waals forces between fibers, into a holistic structure with a robust skeleton, greatly improving the sponge's compression resilience and dimensional stability.

[0018] In step (1), the content of the anchoring material is a key parameter for controlling the final performance of the sponge. When using a hot-melt anchoring material, its content is 0.1 wt% to 90 wt% of the total solid content of the polymer and anchoring material; when using a reactive anchoring material, its content is 0.1 wt% to 75 wt% of the total solid content of the polymer and anchoring material. If the anchoring material content is too low, it will be difficult to form a sufficient number of effective connection points at the fiber overlap points, and the mechanical reinforcement effect will not be significant; if the content is too high, it may affect the spinnability of the main polymer, or lead to excessive sponge density and blockage of the pore structure, thereby impairing the thermal insulation performance. By controlling the anchoring material content within the above range, a flexible balance between the mechanical properties and thermal insulation performance of the sponge can be achieved. The preferred solvent is selected based on the solubility characteristics of the chosen host polymer and anchoring material, and may be selected from one or more of N,N-dimethylacetamide, N,N-dimethylformamide, acetone, chloroform, dichloromethane, carbon tetrachloride, formic acid, acetic acid, 1,1,1,3,3,3-hexafluoro-2-propanol, tetrahydrofuran, 2,2,2-trifluoroacetic acid, and 1,4-dioxane.

[0019] In the gas foaming process of step (2), the concentration of sodium borohydride in the foaming solution is 0.001 M to 14.5 M. The concentration of sodium borohydride directly determines the hydrogen generation rate and the total gas production, and is the core process parameter for controlling the expansion ratio and porosity of the sponge. If the concentration is too low, the gas production will be insufficient, and the fiber membrane will be difficult to expand fully; if the concentration is too high, the reaction will be violent, which may lead to membrane rupture or uneven pore structure. The solvent of the foaming solution consists of component A and component B, where component A is a mixture of deionized water and methanol, and the volume percentage of deionized water in component A is 0 to 100 vol%; component B is selected from one of ethanol, n-propanol, isopropanol, n-butanol, tert-butanol, ethylene glycol, glycerol, and acetone; component A accounts for 20 vol% to 100 vol% of the total volume of the solvent. By adjusting the ratio of deionized water to methanol in component A, the hydrolysis rate of sodium borohydride can be precisely controlled, thereby regulating the kinetics of hydrogen release and achieving precise control over the sponge's pore structure (such as pore size, uniformity of distribution, and open area). When the deionized water content is high, the hydrolysis rate of sodium borohydride is relatively slow, and the foaming process is gentler, which is beneficial for preparing a sponge structure with more uniform and fine pores. When the methanol content is high, the reaction is relatively faster, and the foaming process is more rapid. The foaming time is determined comprehensively based on the thickness of the nanofiber membrane, the target degree of expansion, and the composition of the foaming solution, typically ranging from 10 seconds to 24 hours, to ensure that hydrogen fully penetrates and uniformly expands all layers of the fiber membrane.

[0020] In step (3), the washing process involves rinsing repeatedly with deionized water at least five times to thoroughly remove residual sodium borohydride, sodium metaborate and other inorganic salt byproducts generated during the reaction, as well as organic solvents. If the washing is insufficient, residual inorganic salts will precipitate during subsequent drying and use, affecting the pore structure and thermal insulation performance of the sponge. The drying temperature is controlled between 2℃ and 40℃, and the air pressure is between 0.1 MPa (atmospheric pressure) and −0.1 MPa (negative pressure / vacuum). The drying time varies from 2 hours to 72 hours depending on the sponge thickness and drying conditions. Low-temperature drying or vacuum drying can effectively slow down the drying rate, preventing the sponge from shrinking or collapsing due to capillary forces during rapid solvent evaporation, and maximizing the preservation of the high porosity obtained from foaming.

[0021] In the anchoring process of step (4), different process parameters are used depending on the type of anchoring material. When the anchoring material is thermoplastic, the anchoring process is carried out in a vacuum or air atmosphere. The anchoring temperature is set above the melting temperature of the thermoplastic anchoring material but below the melting temperature or thermal decomposition temperature of the main polymer, and the processing time is 0.1 hours to 24 hours. When the anchoring material is reactive, the anchoring process is carried out in a vacuum or air atmosphere. The anchoring temperature is set above the high reactivity temperature of the reactive anchoring material but below the melting temperature or thermal decomposition temperature of the main polymer, and the processing time is 1 hour to 72 hours. Controlling the anchoring temperature below the polymer melting temperature is crucial. This ensures that only the anchoring material is selectively activated during the anchoring process, while the morphology and three-dimensional network skeleton of the main polymer fibers are fully maintained, avoiding the overall melting and collapse of the sponge. Through the above anchoring process, the anchoring material inside the fiber migrates to the fiber overlap point and forms a stable connection, transforming the originally mechanically fragile fluffy fiber network into an integral structure with a strong skeleton, thereby giving the sponge excellent structural stability and compression resilience.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] (1) The preparation process is simple and environmentally friendly. The sodium borohydride gas foaming technology can realize the transformation of two-dimensional membrane into three-dimensional sponge at room temperature and pressure without the need for high temperature and high pressure equipment; the foaming process does not produce toxic and harmful gases, and the by-products are soluble inorganic salts that can be removed by washing with water, without environmental pollution; compared with the freeze drying method and the template method, the preparation cycle of this invention is short and the energy consumption is low, making it suitable for large-scale production.

[0024] (2) The sponge structure is controllable and has excellent thermal insulation performance. By adjusting the composition of the foaming solution and the concentration of sodium borohydride, the porosity, expansion ratio and pore structure of the sponge can be flexibly controlled; the prepared sponge has a high porosity of 80%~95% and a low thermal conductivity of 30~35 mW / (m·K), and its thermal insulation performance is significantly better than that of traditional thermal insulation materials and ordinary two-dimensional nanofiber membranes.

[0025] (3) Synergistic optimization of mechanical and thermal insulation properties. By selectively constructing connection points at fiber overlaps using in-situ anchoring technology, the sponge's compression resilience and dimensional stability are significantly enhanced while its high porosity and low thermal conductivity are largely unaffected, resolving the contradiction between the mechanical and thermal insulation properties of traditional foamed sponges. As shown in the comparative example, pure polymer sponges without anchoring treatment undergo irreversible structural collapse during drying and compression, making it impossible to obtain a practically valuable thermal insulation sponge, fully demonstrating the indispensability of the anchoring step.

[0026] In summary, this invention effectively overcomes the various shortcomings of the prior art. The nanofiber thermal insulation sponge provided has the comprehensive advantages of being lightweight, highly elastic, and having low thermal conductivity. It can be widely used in building insulation, aerospace thermal protection, new energy thermal insulation, and civilian thermal clothing, and has extremely high practical application value. Attached Figure Description

[0027] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1 This is a scanning electron microscope (SEM) image of the nanofiber thermal insulation sponge prepared in Example 1 of the present invention.

[0029] Figure 2 The image shows the compressive stress-strain curve of the nanofiber thermal insulation sponge prepared in Example 1 of this invention.

[0030] Figure 3 The image shows the compressive stress-strain curve of the nanofiber thermal insulation sponge prepared in Example 2 of this invention.

[0031] Figure 4 An optical photograph of the nanofiber thermal insulation sponge prepared in Example 3 of this invention.

[0032] Figure 5 This is a comparison chart of the thermal conductivity of the two-dimensional nanofiber membranes used in Examples 2, 3, and 4 of this invention and the final nanofiber-based thermal insulation sponge. Detailed Implementation

[0033] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0034] All raw materials used in this invention are not particularly limited in their source; they can be purchased from the market or prepared using conventional methods known to those skilled in the art.

[0035] There are no particular restrictions on the purity of any of the raw materials used in this invention. However, this invention preferably uses raw materials of analytical grade or purity commonly used in the field of chemical synthesis.

[0036] This invention provides a method for preparing a nanofiber thermal insulation sponge, comprising the following steps: First, a polymer serving as the main skeleton and an anchoring material with anchoring function are dissolved in a good solvent in a certain proportion to prepare a homogeneous electrospinning solution, and a two-dimensional nanofiber membrane is prepared by electrospinning technology; Second, the above two-dimensional nanofiber membrane is subjected to gas foaming treatment using sodium borohydride solution, utilizing the hydrogen gas gently released by the in-situ reaction of sodium borohydride with a protic solvent to expand the dense two-dimensional membrane into a nanofiber sponge with a three-dimensional interconnected porous structure; Subsequently, the foamed three-dimensional sponge is thoroughly washed and dried to remove residual reaction byproducts and solvents; Finally, the dried three-dimensional sponge is subjected to anchoring treatment, and the anchoring material inside the fiber is melted or cross-linked at the fiber overlap points through thermal activation, thereby constructing strong connection points between the fibers, significantly enhancing the overall mechanical strength, compression resilience, and structural dimensional stability of the sponge material.

[0037] In the electrospinning step of this invention, the polymer may be selected from one or more of polyacrylonitrile (PAN), polyamide (PA), cellulose acetate (CA), polyvinylidene fluoride (PVDF), polyvinyl chloride (PVC), polyvinylidene chloride (PVDC), polyethylene terephthalate (PET), polyurethane (PU), polyamic acid (PAA), and polyhydroxyalkanoates (PHA). A good solvent for dissolving the above polymer may be selected from one or more of N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), acetone, chloroform, dichloromethane, carbon tetrachloride, formic acid, acetic acid, 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), tetrahydrofuran (THF), 2,2,2-trifluoroacetic acid (TFA), and 1,4-dioxane.

[0038] In this invention, the anchoring material is a key component for achieving a strong connection between fibers and imparting excellent mechanical properties to the sponge. Anchoring materials can be broadly classified into two categories: thermoplastic anchoring materials and reactive anchoring materials. Thermoplastic anchoring materials melt and flow when heated above their melting temperature, converging at fiber overlaps and solidifying upon cooling to form physical anchoring points. These materials can be selected from one or more of polycaprolactone (PCL), thermoplastic polyurethane (TPU), polylactic acid (PLA), poly(lactic-co-glycolic acid copolymer) (PLGA), polyethylene oxide (PEO), and polyvinylpyrrolidone (PVP). Reactive anchoring materials, when heated above their high reactivity temperature, undergo chemical cross-linking reactions between active functional groups on their molecular chains or with the host polymer, forming permanent chemical connection points. These materials can be selected from one or more of epoxy resin precursors, unsaturated imides, phenolic resin oligomers, and polydimethylsiloxane precursors. In electrospinning solutions, the content of anchoring material needs to be controlled according to its type and target performance: when using hot-melt anchoring material, its content is 0.1 wt% to 90 wt% of the total solid content of the polymer in the solution; when using reactive anchoring material, its content is 0.1 wt% to 75 wt% of the total solid content of the polymer in the solution. If the anchoring material content is too low, it will be difficult to form sufficient effective connection points between fibers, and the mechanical reinforcement effect will not be significant; if the content is too high, it may affect the spinnability of the main polymer or lead to excessive sponge density, impairing the thermal insulation performance.

[0039] In the gas foaming process of this invention, the foaming solution consists of sodium borohydride and a solvent. Sodium borohydride reacts in situ with a protic solvent (such as water or alcohol) in the solution, gently releasing hydrogen gas. This gas accumulates and expands in the interlayer and interfiber spaces of the two-dimensional nanofiber membrane, expanding the dense structure and forming a three-dimensional porous sponge. The concentration of sodium borohydride is a key parameter controlling the foaming rate and degree of expansion, ranging from 0.001 M to 14.5 M. The foaming solvent consists of component A and component B. Component A is a mixture of deionized water and methanol, with deionized water accounting for 0–100 vol% of the volume in component A; component B is one of ethanol, n-propanol, isopropanol, n-butanol, tert-butanol, ethylene glycol, glycerol, or acetone; component A accounts for 20 vol%–100 vol% of the total solvent volume. By adjusting the water / alcohol ratio in component A, the ratio of component A to component B, and the concentration of sodium borohydride, the reaction rate of sodium borohydride and the amount of hydrogen released can be flexibly adjusted, thereby precisely controlling the porosity, expansion factor, and pore structure of the sponge. The foaming treatment time depends on the thickness of the nanofiber membrane, the target degree of expansion, and the solution composition, typically ranging from 10 seconds to 24 hours. During the foaming process, the two-dimensional nanofiber membrane is completely immersed in the aforementioned sodium borohydride solution, and the gradual expansion of the fiber membrane can be clearly observed, transforming from a dense thin film state into a fluffy three-dimensional porous sponge structure.

[0040] After foaming, the sponge needs to be removed from the solution and thoroughly washed and dried. Washing typically involves repeated rinsing with deionized water at least five times to ensure complete removal of residual sodium borohydride, inorganic salt byproducts such as sodium metaborate generated during the reaction, and organic solvents. The drying temperature is controlled between 2°C and 40°C, and the pressure conditions can range from atmospheric pressure (0.1 MPa) to negative pressure (−0.1 MPa). The drying time varies from 2 hours to 72 hours depending on the sponge thickness and drying conditions. Low-temperature or vacuum drying helps prevent structural shrinkage or collapse of the sponge during the drying process, maximizing the preservation of its high porosity.

[0041] The resulting three-dimensional sponge after drying requires anchoring treatment to ultimately obtain a thermal insulation sponge with both high elasticity and structural stability. For sponges containing heat-melting anchoring materials, the anchoring treatment is carried out under vacuum or air atmosphere, with the anchoring temperature set above the melting temperature of the selected anchoring material but below the melting or decomposition temperature of the host polymer. The treatment time ranges from 0.1 hours to 24 hours. For sponges containing reactive anchoring materials, the anchoring treatment is carried out under vacuum or air atmosphere, with the anchoring temperature set above the high reactivity temperature of the anchoring material but below the melting or decomposition temperature of the host polymer. The treatment time ranges from 1 hour to 72 hours. During the anchoring process, the anchoring material components inside the fibers are thermally activated, migrate and accumulate at the overlap points between fibers, and form stable connection nodes through physical melting and solidification or chemical cross-linking reactions. This transforms the fiber network, which was originally maintained only by physical entanglement and weak van der Waals forces, into an integral structure with a strong skeleton, thereby greatly improving the compression resilience and dimensional stability of the sponge. This solves the key problem that unanchored pure polymer nanofiber sponges are prone to irreversible deformation under stress.

[0042] The present invention will be further described below through specific embodiments.

[0043] Example 1

[0044] This embodiment demonstrates a method for preparing nanofiber thermal insulation sponge using polycaprolactone (PCL) as a hot-melt anchoring material and polyacrylonitrile (PAN) as the main polymer, and its structure and properties are described in conjunction with the accompanying drawings.

[0045] Step 1: Prepare the electrospinning solution. Weigh 1.6 g of PAN powder and 0.0016 g of PCL particles, add them to 15 g of N,N-dimethylformamide (DMF), and magnetically stir at 40°C for 6 hours to obtain a homogeneous, transparent spinning solution with a total polymer mass fraction of 10 wt%. In this solution, the content of PCL anchoring material accounts for 0.1 wt% of the total solid content of PAN and PCL. Inject this solution into an electrospinning apparatus for spinning to obtain a two-dimensional PAN / PCL nanofiber membrane with uniform thickness.

[0046] Step 2: Preparation of a three-dimensional sponge via gas foaming. A 14 M sodium borohydride solution was prepared by weighing 26.48 g of sodium borohydride and slowly adding it to 50 mL of deionized water, stirring until completely dissolved. The two-dimensional nanofiber membrane obtained in Step 1 was cut into sheets of appropriate size and completely immersed in the sodium borohydride solution. The immersion time was approximately 10 seconds. Within this very short time, the fiber membrane was observed to rapidly expand, transforming from a dense film into a three-dimensional sponge with a visible, fluffy, porous structure.

[0047] Step 3: Washing and Drying. Remove the expanded sponge from the solution and immediately rinse it repeatedly with sufficient deionized water at least 5 times to ensure complete removal of any residual sodium borohydride and sodium metaborate. After washing, place the sponge in an environment at 2°C and normal pressure (0.1 MPa) to slowly dry for 72 hours, obtaining a dry PCL / PAN nanofiber sponge.

[0048] Step 4: Anchoring Treatment. The dry sponge is placed in a vacuum oven and heated to 95°C under a vacuum atmosphere for heat anchoring treatment. At this temperature, the PCL component in the fiber fully melts and flows to the overlap points of the PAN fibers. The treatment time is 0.1 hours (6 minutes). Subsequently, it is naturally cooled to room temperature, and the PCL re-solidifies at the fiber nodes, forming physical connection points, ultimately resulting in a structurally stable nanofiber thermal insulation sponge.

[0049] The microstructure of the prepared sponge cross section is as follows Figure 1 The scanning electron microscope (SEM) image is shown. From Figure 1 It can be clearly observed that the sponge exhibits a three-dimensional interconnected porous network structure composed of interwoven nanofibers, with pore sizes ranging from tens to hundreds of micrometers. The pore walls are composed of fine nanofibers, and there are obvious fused bonding nodes between the fibers. The density of the sponge prepared in this embodiment was measured to be 10.31 mg / cm³. 3 The thermal conductivity, measured by steady-state heat flow method at room temperature and pressure, is as low as 30.8 mW / (m·K), demonstrating excellent thermal insulation performance.

[0050] The compressive mechanical properties of the sponge in this embodiment are as follows: Figure 2 The compressive stress-strain curve is shown. Testing revealed that the compressive stress at 50% compressive strain reached 2.83 kPa, and after one load-unload cycle, the plastic deformation was only 10%. Furthermore, the compressed sample maintained its intact appearance, without cracking or permanent collapse. This indicates that only a very small amount (0.1 wt%) of PCL anchoring material is needed to significantly improve the elasticity problem of traditional pure PAN nanofiber sponges, which cannot rebound after compression and are prone to structural collapse. In contrast, pure PAN sponges prepared under identical conditions but without any anchoring material (Comparative Example 1) can form a three-dimensional structure after foaming, but they are extremely prone to irreversible collapse during drying and subsequent compression. They cannot withstand 50% strain compression and undergo complete plastic deformation after compression without recovery, lacking the mechanical integrity required for practical applications.

[0051] Example 2

[0052] This embodiment demonstrates a method for preparing nanofiber thermal insulation sponge using thermoplastic polyurethane (TPU) as a hot-melt anchoring material and cellulose acetate (CA) as the main polymer.

[0053] Step 1: Prepare the electrospinning solution. Weigh 0.44 g of CA powder and 1.76 g of TPU particles, and add them to a mixed solvent of 10 g of N,N-dimethylformamide (DMF) and acetone, wherein the mass ratio of DMF to acetone is 7:3. Stir magnetically at 25°C for 6 hours to obtain a homogeneous, transparent spinning solution with a total polymer mass fraction of 18 wt%. In this solution, the TPU anchoring material content accounts for 90 wt% of the total solid content of CA and TPU. Electrospin this solution to obtain a two-dimensional CA / TPU nanofiber membrane.

[0054] Step 2: Preparation of a three-dimensional sponge via gas foaming. A 0.001 M sodium borohydride solution was prepared by weighing 0.00189 g of sodium borohydride and adding it to a 50 mL mixed solvent consisting of 30 mL methanol and 20 mL n-propanol, stirring until completely dissolved. The two-dimensional nanofiber membrane obtained in Step 1 was then completely immersed in this foaming solution for 24 hours. During this slow and continuous hydrogen production process, the fiber membrane was uniformly expanded and stretched, ultimately forming a three-dimensional porous sponge structure.

[0055] Step 3: Washing and Drying. Remove the foamed sponge and rinse it repeatedly with deionized water at least 5 times. After washing, place the sponge in an environment with a temperature of 40℃ and an air pressure of −0.1 MPa (vacuum) to dry for 24 hours to obtain dry CA / TPU nanofiber sponge.

[0056] Step 4: Anchoring Treatment. The dry sponge is placed in a vacuum oven and heated to 120°C under a vacuum atmosphere for 24 hours for heat anchoring treatment. At this temperature, the TPU component in the fiber matrix melts and accumulates at the CA fiber overlap points. After cooling, a strong physical connection is formed, ultimately yielding a nanofiber thermal insulation sponge.

[0057] The density of the sponge prepared in this embodiment was tested to be 9.51 mg / cm³. 3 Its thermal conductivity is as low as 31.1 mW / (m·K). Its compressive mechanical properties are as follows: Figure 3As shown in the compressive stress-strain curve, the compressive stress at 50% compressive strain is 1.76 kPa, the plastic deformation is 12%, and the sample maintains its shape after compression, confirming its good compressive resilience. This example demonstrates that by using a high content of hot-melt anchoring material (90 wt%) in combination with a low concentration of spinning solution in the main polymer, it is still possible to successfully prepare a sponge with high porosity and low thermal conductivity, and the anchoring effect is significant. In contrast, a pure CA sponge prepared under the same conditions but without TPU anchoring material (Comparative Example 2) has an extremely fragile structure after foaming, and it undergoes severe shrinkage and structural collapse during washing and drying, making it impossible to obtain a complete dry three-dimensional sponge. This further verifies the indispensability of anchoring materials for maintaining the three-dimensional structure.

[0058] Example 3

[0059] This embodiment demonstrates a method for preparing nanofiber thermal insulation sponge using polylactic acid (PLA) as a hot-melt anchoring material and polyethylene terephthalate (PET) as the main polymer, and shows a macroscopic optical photograph of the sponge.

[0060] Step 1: Prepare the electrospinning solution. Weigh 0.88 g of PET and 0.88 g of PLA, and add them together to 15 g of hexafluoroisopropanol (HFIP) solvent. Stir magnetically at 25°C for 7 hours to obtain a homogeneous, transparent spinning solution with a total polymer mass fraction of 15 wt%. In this solution, the PLA anchoring material content accounts for 50 wt% of the total solid content of PET and PLA. Electrospin this solution to obtain a two-dimensional PET / PLA nanofiber membrane.

[0061] Step 2: Preparation of a three-dimensional sponge via gas foaming. Prepare a 0.5 M sodium borohydride solution. Specifically, weigh 0.95 g of sodium borohydride and add it to a 50 mL mixed solvent consisting of 10 mL of deionized water and 40 mL of ethanol, stirring until dissolved. Completely immerse the two-dimensional nanofiber membrane in this solution for 1 hour. The membrane gradually expands to form a three-dimensional porous sponge.

[0062] Step 3: Washing and drying. After removing the sponge, rinse it repeatedly with deionized water at least 5 times, and then dry it for 72 hours at a temperature of 25°C and a normal pressure (0.1 MPa) to obtain a dry PET / PLA nanofiber sponge.

[0063] Step 4: Anchoring Treatment. The dry sponge is placed in a vacuum oven and heated to 136°C under a vacuum atmosphere for 1 hour for heat anchoring treatment. At this temperature, the PLA component fully melts and bonds at the fiber nodes. After cooling, the nanofiber thermal insulation sponge is obtained.

[0064] The optical photograph of the sponge obtained in this embodiment is as follows: Figure 4 As shown. From Figure 4 As can be clearly seen, the sponge has a pure white appearance and a fluffy, soft block shape. There are no cracks or obvious defects on the surface or inside. It can easily withstand bending and moderate pressure without breaking, demonstrating good integrity and flexibility. Quantitative testing shows that the sponge's density is 10.59 mg / cm³. 3 The thermal conductivity is 32.49 mW / (m·K). It has good compressive mechanical properties, with a compressive stress of 1.24 kPa at 50% strain, a plastic deformation of 15%, and the sample recovers well after compression.

[0065] Example 4

[0066] This embodiment demonstrates a method for preparing nanofiber thermal insulation sponge using phenolic resin oligomers as reactive anchoring materials and nylon 6 (PA6) as the main polymer.

[0067] Step 1: Prepare the electrospinning solution. Weigh 2 g of PA6 and 0.002 g of phenolic resin precursor (phenolic resin oligomer), add them to 10 mL of 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), and magnetically stir at 25 °C for 8 hours to obtain a homogeneous, transparent spinning solution with a concentration of 20% (w / v). In this solution, the content of phenolic resin precursor accounts for 0.1 wt% of the total solid content of PA6 and phenolic resin precursor. Electrospin this solution to obtain a two-dimensional PA6 / phenolic resin nanofiber membrane.

[0068] Step 2: Preparation of a three-dimensional sponge by gas foaming. Prepare a 0.5 M sodium borohydride solution by weighing 0.95 g of sodium borohydride and adding it to 50 mL of methanol, stirring to dissolve. Completely immerse the two-dimensional nanofiber membrane in the sodium borohydride methanol solution for 20 minutes. The membrane gradually expands to form a three-dimensional porous sponge structure.

[0069] Step 3: Washing and drying. After removing the sponge, rinse it repeatedly with deionized water at least 5 times, and then dry it at 25°C and normal pressure (0.1 MPa) for 72 hours to obtain dry PA6 / phenolic resin nanofiber sponge.

[0070] Step 4: Anchoring Treatment. The dry sponge is placed in a vacuum oven and heated to 136°C under a vacuum atmosphere for anchoring treatment for 1 hour. At this temperature, the phenolic resin oligomers dispersed in the PA6 fibers are thermally activated and undergo condensation cross-linking reactions, forming irreversible covalent bonds at the fiber overlap points, ultimately resulting in a structurally stable nanofiber thermal insulation sponge.

[0071] The density of the sponge in this embodiment was tested to be 10.18 mg / cm³. 3The thermal conductivity is 33.4 mW / (m·K). Compression tests showed a compressive stress of 3.27 kPa at 50% strain, with a plastic deformation of only 10%, and the sample remained intact after compression. Compared to the aforementioned thermoplastic anchoring materials, the reactive anchoring material used in this embodiment, although present in extremely low concentrations (only 0.1 wt%), imparts high compressive strength and good elastic recovery to the sponge through the formation of chemical cross-linking nodes, effectively solving the problem of poor mechanical properties in single-component polymer nanofiber sponges.

[0072] Figure 5 This paper presents a comparison of the thermal conductivity of the original two-dimensional nanofiber membranes used in Examples 2, 3, and 4, and the nanofiber-based thermal insulation sponges finally prepared after foaming and anchoring treatment. Figure 5 It is clear that the two-dimensional nanofiber membranes in all embodiments exhibit high thermal conductivity. This is because the dense two-dimensional membrane structure has low porosity, resulting in a limited volume of static air available to confine heat conduction. However, after the gas foaming and anchoring treatment of this invention, the thermal conductivity of the resulting three-dimensional sponge is significantly reduced. This data strongly demonstrates that this invention, through sodium borohydride gas foaming technology, transforms a dense two-dimensional nanofiber membrane into a three-dimensional porous sponge structure with high porosity, successfully introducing a large number of interconnected micropores that effectively hinder heat conduction and achieve a significant improvement in thermal insulation performance.

[0073] Example 5

[0074] This embodiment demonstrates a method for preparing nanofiber thermal insulation sponge using epoxy resin precursor as reactive anchoring material and polyvinylidene fluoride (PVDF) as the main polymer.

[0075] Step 1: Prepare the electrospinning solution. Weigh 0.7 g of PVDF and 2.1 g of epoxy resin precursor, add them to 15 g of DMF solvent, and stir magnetically at 50 °C for 8 hours to obtain a homogeneous, transparent spinning solution with a total polymer mass fraction of 16 wt%. In this solution, the epoxy resin precursor accounts for 75 wt% of the total solid content of PVDF and epoxy resin precursor. Electrospin this solution to obtain a two-dimensional PVDF / epoxy resin nanofiber membrane.

[0076] Step 2: Preparation of three-dimensional sponge by gas foaming. Prepare a 1.5 M sodium borohydride solution. Weigh 2.8 g of sodium borohydride and add it to a 50 mL mixed solvent consisting of 10 mL deionized water and 40 mL isopropanol. Stir to dissolve. Completely immerse the two-dimensional nanofiber membrane in the foaming solution for 10 minutes. Rapid expansion of the fiber membrane is observed, forming a three-dimensional porous sponge.

[0077] Step 3: Washing and drying. After removing the sponge, rinse it repeatedly with deionized water at least 5 times, and then place it in an environment with a temperature of 35°C and an air pressure of −0.1 MPa (vacuum) for rapid drying for 2 hours to obtain dry PVDF / epoxy resin nanofiber sponge.

[0078] Step 4: Anchoring Treatment. The dry sponge is placed in a vacuum oven and heated to 100°C under a vacuum atmosphere for 5 hours for anchoring treatment. At this temperature, the epoxy resin precursor undergoes a thermal cross-linking reaction, forming stable chemical bonding points at the fiber overlap points through self-curing, resulting in a nanofiber thermal insulation sponge.

[0079] The density of the sponge in this embodiment was tested to be 10.59 mg / cm³. 3 The thermal conductivity is 30.49 mW / (m·K). Under 50% compressive strain, its compressive stress is 2.34 kPa, and the plastic deformation is 10%. The sample structure remains intact after compression. This example demonstrates that using a high content of reactive anchoring material (75 wt%) combined with a rapid drying process can also achieve high porosity and low thermal conductivity, with excellent anchoring effect.

[0080] Example 6

[0081] This embodiment further demonstrates the influence of the preferred range of anchoring material content on sponge performance, supplementing the effect verification in Example 1 when the PCL content was only 0.1 wt%.

[0082] The preparation steps in this embodiment are basically the same as those in Example 1, except that in step 1, the PCL content is adjusted to 15 wt% of the total solid content of PAN and PCL when preparing the spinning solution, that is, 1.36 g of PAN and 0.24 g of PCL are weighed and dissolved in 15 g of DMF. In step 2, the foaming treatment time is adjusted to 5 minutes, and in step 4, the anchoring treatment time is adjusted to 2 hours to accommodate the higher PCL content.

[0083] The density of the sponge prepared in this embodiment was tested to be 13.62 mg / cm³. 3 The thermal conductivity is 33.1 mW / (m·K). Under 50% compressive strain, its compressive stress increases to 8.75 kPa, and its plastic deformation significantly decreases to 3.2%, exhibiting excellent compressive resilience close to perfect elasticity. Compared with Example 1, it can be seen that appropriately increasing the anchoring material content can effectively enhance the density and strength of the fiber connection points. Thus, while significantly improving the mechanical strength and elasticity of the sponge, it only slightly increases the density and thermal conductivity. This verifies that the anchoring material content can be adjusted within the range of 0.1 wt% to 90 wt% to achieve a flexible balance between mechanical and thermal insulation properties.

[0084] Example 7

[0085] This embodiment demonstrates the implementation effect of using a hybrid anchoring material system, that is, using both thermoplastic and reactive anchoring materials.

[0086] Step 1: Prepare the electrospinning solution. Weigh 1.2 g PAN, 0.3 g PCL (hot-melt type), and 0.3 g phenolic resin oligomer (reactive type), add them to 15 g DMF, and magnetically stir at 50°C for 8 hours to obtain a homogeneous spinning solution. Step 2: Foaming treatment using a 1.0 M sodium borohydride solution (solvent is a 1:1 volume ratio of deionized water and ethanol) for 30 minutes. Step 3: After washing, rinse 5 times with deionized water and dry at 25°C and −0.05 MPa for 48 hours. Step 4: Anchoring treatment is performed under vacuum at 145°C for 6 hours. At this temperature, PCL melts and flows to form immediate physical bonds, while the phenolic resin undergoes a cross-linking reaction to form permanent chemical bonds.

[0087] The sponge density in this embodiment was tested to be 11.23 mg / cm³. 3 The thermal conductivity is 31.8 mW / (m·K), the compressive stress at 50% strain is 10.35 kPa, and the plastic deformation is only 2.1%. This dual anchoring mechanism enables the sponge to have both high elasticity and high strength, demonstrating the potential of anchoring material systems in synergistic enhancement.

[0088] Comparative Example 1

[0089] To visually compare the decisive influence of anchoring treatment on the mechanical properties of sponges, this comparative example prepared pure PAN nanofiber sponges without any anchoring materials. The preparation conditions were the same as in Example 1, except that PCL was not added to the spinning solution and the anchoring treatment step in step 4 was omitted. After foaming, the sponges were washed, dried, and tested directly.

[0090] The results showed that although pure PAN fiber membranes could temporarily form a three-dimensional expanded structure after being foamed with sodium borohydride, this structure underwent severe shrinkage, cracking, and collapse during washing and drying, making it impossible to obtain a complete dry three-dimensional sponge. Even small dried samples obtained under extremely careful handling exhibited extremely poor mechanical properties; irreversible collapse occurred with even slight finger pressure, making standard compressive stress-strain testing impossible. This comparative example strongly demonstrates that the weak physical entanglement and friction between fibers alone cannot support the integrity of the three-dimensional sponge structure during drying and use; the anchoring step is an indispensable key technical step in obtaining a stable three-dimensional sponge.

[0091] Comparative Example 2

[0092] This comparative example corresponds to Example 2, and a pure CA nanofiber sponge without TPU was prepared. Except for not adding TPU and omitting the anchoring treatment step, the other conditions were exactly the same as in Example 2.

[0093] The results show that pure CA fiber membranes can temporarily form a three-dimensional structure after foaming, but the samples shrink severely after drying, losing their three-dimensional morphology and making effective testing impossible. This further confirms that different host polymer systems struggle to maintain a stable three-dimensional sponge structure without anchoring.

[0094] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A nanofiber thermal insulation sponge, characterized in that, The sponge is composed of a three-dimensional interconnected porous network of polymer nanofibers, with connection points formed by anchoring materials at the overlap points of the polymer nanofibers; the porosity of the sponge is 80%~95%, and the thermal conductivity is 30~35 mW / (m·K).

2. The nanofiber thermal insulation sponge according to claim 1, characterized in that, The anchoring material is a hot-melt anchoring material or a reactive anchoring material; the hot-melt anchoring material is selected from one or more of polycaprolactone, thermoplastic polyurethane, polylactic acid, poly(lactic acid-glycolic acid copolymer), polyethylene oxide, and polyvinylpyrrolidone; the reactive anchoring material is selected from one or more of epoxy resin precursors, unsaturated imides, phenolic resin oligomers, and polydimethylsiloxane precursors.

3. The nanofiber thermal insulation sponge according to claim 1, characterized in that, The polymer is selected from one or more of polyacrylonitrile, polyamide, cellulose acetate, polyvinylidene fluoride, polyvinyl chloride, polyvinylidene chloride, polyethylene terephthalate, polyurethane, polyamic acid, and polyhydroxyalkanoates.

4. A method for preparing a nanofiber thermal insulation sponge as described in any one of claims 1 to 3, characterized in that, Includes the following steps: (1) Dissolve the polymer and anchoring material in a good solvent to prepare a homogeneous electrospinning solution, and prepare a two-dimensional nanofiber membrane by electrospinning; (2) The two-dimensional nanofiber membrane is completely immersed in a foaming solution containing sodium borohydride to perform gas foaming, so that the two-dimensional nanofiber membrane expands to form a three-dimensional nanofiber sponge. (3) The three-dimensional nanofiber sponge obtained in step (2) is washed with deionized water and dried to obtain a dry three-dimensional nanofiber sponge; (4) The dry three-dimensional nanofiber sponge obtained in step (3) is anchored, so that the anchoring material melts or cross-links at the fiber overlap point to form a connection point, and the nanofiber thermal insulation sponge is obtained.

5. The preparation method according to claim 4, characterized in that, In step (1), the anchoring material is a hot-melt anchoring material or a reactive anchoring material; when a hot-melt anchoring material is used, its content is 0.1 wt% to 90 wt% of the total solid content of the polymer and the anchoring material; when a reactive anchoring material is used, its content is 0.1 wt% to 75 wt% of the total solid content of the polymer and the anchoring material.

6. The preparation method according to claim 4, characterized in that, In step (2), the concentration of sodium borohydride in the foaming solution is 0.001 M to 14.5 M; the solvent of the foaming solution is composed of component A and component B, wherein component A is a mixture of deionized water and methanol, and the volume percentage of deionized water in component A is 0 to 100 vol%; component B is selected from one of ethanol, n-propanol, isopropanol, n-butanol, tert-butanol, ethylene glycol, glycerol, and acetone; component A accounts for 20 vol% to 100 vol% of the total volume of the solvent; and the foaming treatment time is 10 s to 24 h.

7. The preparation method according to claim 4, characterized in that, In step (3), the washing is rinsing with deionized water at least 5 times; the drying temperature is 2℃~40℃, the air pressure is 0.1 MPa~−0.1 MPa, and the drying time is 2 h~72 h.

8. The preparation method according to claim 4, characterized in that, In step (4), when the anchoring material is hot-melt type, the anchoring treatment is carried out in a vacuum or air atmosphere, the anchoring temperature is above the melting temperature of the hot-melt anchoring material and below the melting temperature of the polymer, and the treatment time is 0.1 h to 24 h; when the anchoring material is reactive type, the anchoring treatment is carried out in a vacuum or air atmosphere, the anchoring temperature is above the high reactivity temperature of the reactive anchoring material and below the melting temperature of the polymer, and the treatment time is 1 h to 72 h.

9. The preparation method according to claim 4, characterized in that, In step (1), the polymer is selected from one or more of polyacrylonitrile, polyamide, cellulose acetate, polyvinylidene fluoride, polyvinyl chloride, polyvinylidene chloride, polyethylene terephthalate, polyurethane, polyamic acid, and polyhydroxy fatty acid ester.

10. The preparation method according to claim 4, characterized in that, In step (1), the good solvent is selected from one or more of N,N-dimethylacetamide, N,N-dimethylformamide, acetone, chloroform, dichloromethane, carbon tetrachloride, formic acid, acetic acid, 1,1,1,3,3,3-hexafluoro-2-propanol, tetrahydrofuran, 2,2,2-trifluoroacetic acid, and 1,4-dioxane.

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