A flexible transparent sound-absorbing soft membrane and a preparation method thereof

Abstract

The application discloses a kind of flexible transparent sound-absorbing soft film and preparation method thereof, belong to sound-absorbing film technical field.The method disperses titanium dioxide nanowire in transparent thermoplastic polyurethane solution, after casting film, make nanowire directional arrangement by direct current electric field parallel to film surface, and gradient temperature solidification is obtained anisotropic polyurethane film;Periodic arrangement of central symmetry multi-petal incision array is cut out on the film by laser, and then bidirectional stretching is applied to make each petal piece uniformly warped outward, and three-dimensional conical through-hole array is formed by self-assembly.The obtained soft film has a visible light transmittance of ≥85%, and an average sound absorption coefficient of greater than 0.6 in the frequency range of 500-4000 Hz, and has high transparency, high flexibility and wideband sound absorption performance.When installed, it is fixed by frame or attached to the surface of glass curtain wall or transparent partition through spacer layer, so as to form an air cavity between the soft film and the substrate.

Description

Technical Field

[0001] This invention relates to the field of sound-absorbing membrane technology, and in particular to a flexible transparent sound-absorbing membrane and its preparation method. Background Technology

[0002] Components in buildings such as glass curtain walls, exterior windows, and transparent partitions have long faced the challenge of balancing light transmission and noise reduction. Common sound-absorbing materials such as glass wool, rock wool, and polyester fiber wool are all opaque and cannot be used in these scenarios.

[0003] In response to the dual needs of light transmission and sound absorption, the industry has carried out relevant technical research, and a small number of sound-absorbing products that can achieve a certain degree of light transmission have appeared on the market. These products are mainly divided into two categories: rigid transparent sound-absorbing structures and flexible transparent sound-absorbing films.

[0004] Rigid transparent sound-absorbing structures are among the very few transparent sound-absorbing solutions that have achieved commercial application, but they face the following problems: (1) The boards are heavy and cannot be rolled up, resulting in high transportation and installation costs; (2) The micro-perforation processing requires high precision, and the cost of the conical hole mold is extremely expensive; (3) A certain amount of back cavity is usually required to achieve effective sound absorption, which occupies building space; (4) Rigid materials are difficult to fit on curved or irregular surfaces of buildings.

[0005] In flexible transparent sound-absorbing film technologies, non-perforated structures primarily employ multilayer composite structures. These structures achieve sound absorption by sandwiching transparent damping layers or viscoelastic material layers between transparent polymer films, utilizing shear friction and damping dissipation at the interlayer interfaces. However, to achieve broadband sound absorption, multiple layers of varying thicknesses are often stacked to match different frequencies, leading to an increase in the number of layers, overall thickness, and a gradual decrease in light transmittance with each layer. Furthermore, the interlayer interfaces are prone to delamination, blistering, and other aging issues during long-term use, affecting service life and appearance. Some solutions introduce opaque fillers or metal coatings to improve low-frequency sound absorption, further sacrificing transparency.

[0006] Therefore, there is an urgent need to develop a transparent sound-absorbing membrane that combines high transparency, high flexibility, wide-band sound absorption, and mass production capability. Summary of the Invention

[0007] The purpose of this invention is to provide a flexible transparent sound-absorbing membrane and its preparation method to solve the problems in the background art.

[0008] To achieve the above objectives, this invention discloses a method for preparing a flexible transparent sound-absorbing membrane, comprising the following steps: (1) Add titanium dioxide nanowires to a transparent thermoplastic polyurethane solution to obtain a uniform transparent dispersion, and cast the transparent dispersion on a substrate to form a wet film. (2) Apply a DC electric field to the wet film to orient the titanium dioxide nanowires along the direction of the electric field, with the DC electric field direction parallel to the film surface; (3) Maintain a DC electric field and cure the wet film to obtain an anisotropic polyurethane film; (4) On an anisotropic polyurethane film, a periodically arranged centrally symmetrical multi-lobed slit array is cut out using a laser. Each slit unit is composed of at least three radial slits converging at the same center point. The center point retains the connection point that is not cut through. The slit width is 30-80 μm and the slit array period is 3-6 mm. (5) Apply biaxial stretching to the cut film with a stretching rate of 20-40%, so that the lobes of each cut unit warp outward uniformly along the connection point under stress, forming a conical through hole with a small end at the center, a large end at the periphery, and a cross section that gradually shrinks from the large end to the small end. Heat set to obtain the stretched polyurethane film. (6) Fix the stretched polyurethane film to the frame, or attach it to the surface of the transparent substrate through a spacer layer, so that an air cavity is formed between the polyurethane film and the transparent substrate, and a flexible transparent sound-absorbing membrane is obtained.

[0009] Preferably, in step (1), the diameter of the titanium dioxide nanowire is 10-20 nm, the length is 500 nm-2 μm, and the aspect ratio is greater than 50; the mass fraction of the titanium dioxide nanowire in the transparent thermoplastic polyurethane solution is 1-3%, and the wet film thickness is 0.5-2 mm.

[0010] Preferably, in step (2), the electric field strength of the DC electric field is 2-5 kV / mm, and the application time is 5-10 min.

[0011] Preferably, in step (3), gradient temperature curing is adopted: first stage: drying at 40-50℃ for 10-15 min; second stage: drying at 70-80℃ for 15-20 min; third stage: annealing at 100-110℃ for 5-10 min.

[0012] Preferably, in step (4), each cut unit consists of 3 or 4 radial cuts converging at the same center point, with each cut having equal length and uniform angle, and a cut width of 30-80 μm. The cut array is arranged in a square or regular hexagonal grid with a period of 3-6 mm.

[0013] Preferably, in step (4), an ultraviolet picosecond laser is used for laser cutting with a laser wavelength of 355 nm and a focused spot diameter of 15-25 μm. The cut is formed by repeated scanning 2-4 times, and the chips are blown away with compressed air or nitrogen. The heat-affected zone at the cut edge is less than 5 μm.

[0014] Preferably, in step (5), the stretching speed of biaxial stretching is 5-20 mm / min, the heat setting temperature is 100-110℃, the time is 5-10 minutes, and the temperature is cooled to room temperature under tension.

[0015] Preferably, in step (6), the spacer layer is a transparent pad or a frame pad, and the depth of the air cavity formed is 3-10cm.

[0016] The present invention also provides a flexible transparent sound-absorbing membrane prepared by the above preparation method, comprising a transparent polyurethane film substrate, wherein the transparent polyurethane film substrate contains titanium dioxide nanowires and a plurality of periodically arranged three-dimensional conical through holes, each three-dimensional conical through hole being formed by a multi-lobed cut under biaxial stretching, the lobes of which warp outward uniformly along the central connection point, having a small end located at the center and a large end located at the periphery, the cross section of the through hole gradually shrinking from the large end to the small end; The visible light transmittance of the flexible transparent sound-absorbing membrane is ≥85%; the average sound absorption coefficient in the frequency range of 500-4000 Hz is greater than 0.6.

[0017] Preferably, the transparent polyurethane film substrate is an anisotropic film with a first direction and a second direction perpendicular to each other in its plane. The Young's modulus of the first direction is greater than that of the second direction, and the anisotropy ratio is 2:1 to 5:1. The titanium dioxide nanowires are oriented along the first direction.

[0018] Preferably, the multi-lobed cut is formed by three or four radial cuts converging at the same center point, with each cut having equal length and uniform angle; The small end diameter of the conical through-hole is 30-80 μm, the large end diameter is determined by the flap warp height, which is 0.15-0.25 mm, the through-hole depth is equal to the film thickness, and the array period is 3-6 mm.

[0019] An air cavity with a depth of 3-10 cm is provided between the flexible transparent sound-absorbing membrane and the transparent substrate.

[0020] In this invention, titanium dioxide nanowires have a dielectric constant much higher than that of the TPU solution matrix. When the wet film is placed in a DC high-voltage electric field, positive and negative charges are induced at both ends of the titanium dioxide nanowires due to the polarization effect, forming an electric dipole moment. This drives the nanowires to rotate around their center of mass until their long axis is parallel to the direction of the electric field, allowing the polyurethane film to obtain differentiated bending stiffness in different directions.

[0021] Subsequently, a planar prefabricated pattern required for the subsequent self-assembly of the aperture is prepared on the film. A centrally symmetrical multi-lobed notch is used, with each unit consisting of at least three radial slits converging at the same central point, leaving an incomplete connection point at the center. When the film is subjected to biaxial stretching, the tensile stress on each lobe is uniform, and a uniform bending moment is formed at the connection point, driving each lobe to warp outward synchronously. All lobes warp with the same amplitude, unfolding symmetrically around the central point to form a channel with a continuously changing cross-section: the opening is largest near the outer lobe tip, gradually narrowing towards the central connection point, ultimately forming a small end at the center, constituting a three-dimensional conical through-hole.

[0022] Therefore, the present invention has the following beneficial effects: 1. This invention utilizes the elastic instability of the thin film itself to achieve the self-assembly of a conical hole. On a polyurethane film with anisotropic properties induced by an electric field, a pre-cut array of centrally symmetrical multi-lobed cuts is made, with each lobe radially converging at the same center point and retaining connection points. When bidirectional tension is applied, the stress on each lobe is uniform, generating a uniform bending moment at the connection points. This drives the lobes to warp outward synchronously along the center point, naturally forming a three-dimensional conical through-hole with a small center, a large periphery, and a cross-section that gradually shrinks from the large end to the small end. The planar precision of 2D laser cutting achieves the spatial geometry of the 3D conical hole, avoiding the industry problems of high processing costs, difficult demolding, and difficulty in controlling micro-tapering in conical hole molds.

[0023] 2. The self-assembled three-dimensional conical through-holes exhibit a gradually shrinking cross-section. Different cross-sectional positions correspond to different acoustic impedances and acoustic masses. Low frequencies are dissipated over a wide range, while high frequencies undergo energy conversion at narrow slits. A single hole can cover a relatively wide frequency band. Hundreds of thousands of conical holes form a parallel acoustic impedance network. Minor geometric deviations between holes ensure smooth overlap of absorption peaks, achieving broadband sound absorption with an average absorption coefficient greater than 0.6 in the 500-4000 Hz range. The resonant frequency of the thin film substrate under pre-stretch tension further supplements low-frequency sound absorption, forming a frequency band complementarity with the mid-to-high frequency frictional sound absorption of the conical holes.

[0024] 3. The sound-absorbing membrane prepared by this invention is fixed on a frame or attached to the surface of a glass curtain wall, building window or transparent partition through a spacer layer, so that an air cavity of 3-10 cm is formed between the membrane and the transparent substrate, providing the acoustic compliance required for low-frequency sound absorption, and is suitable for existing building renovation and space-constrained scenarios.

[0025] 4. The titanium dioxide nanowires used in this invention have a diameter much smaller than the wavelength of visible light, with a filling amount of only 1-3 wt%. After curing, the film transmittance can reach over 90%. Structurally, the C-shaped slit width is only 50 micrometers, and the slit area accounts for less than 2% of the total film area, making it almost invisible to the naked eye. The conical micro-relief only causes slight soft light scattering. The overall transmittance of the finished product is ≥85%, meeting the requirements for building lighting.

[0026] 5. The product of this invention is a fully flexible polymer film with a thickness in the sub-millimeter range. It can be bent and cut arbitrarily, and is suitable for non-planar building surfaces such as curved curtain walls and irregularly shaped partitions.

[0027] The technical solution of the present invention will be further described in detail below through embodiments. Attached Figure Description

[0028] Figure 1 This is a side view of the three-dimensional conical through-hole formed in Example 1; Figure 2 This is a three-dimensional schematic diagram of a single three-dimensional conical through-hole formed in Example 1; Figure 3 The sound absorption coefficient diagrams are shown for the samples prepared in Examples 1-4 and Comparative Examples 1-3. Detailed Implementation

[0029] The technical solution of the present invention will be further described below through embodiments.

[0030] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can be appropriately combined to form other embodiments that can be understood by those skilled in the art. These other embodiments are also covered within the scope of protection of this invention.

[0031] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0032] Unless otherwise specified, the materials, reagents, instruments, and equipment used in this invention are all materials, reagents, instruments, and equipment routinely used by those skilled in the art, and the testing standards all use national or international standards commonly used in the field, without further explanation.

[0033] Example 1 This embodiment provides a flexible, transparent sound-absorbing membrane, and the preparation steps are as follows: (1) Titanium dioxide nanowires with a diameter of 10-20 nm and a length of 500 nm-2 μm were added to an optical-grade transparent thermoplastic polyurethane solution, ultrasonically dispersed for 30 minutes and then subjected to high-speed shear stirring to obtain a transparent dispersion with a nanowire mass fraction of 2%. A wet film with a thickness of 0.8 mm was formed by casting on a substrate with a PET release layer.

[0034] (2) Place the wet film between parallel plate electrodes with an electrode spacing of 5 cm, apply a DC electric field with a field strength of 3 kV / mm, with the electric field direction parallel to the film surface, and continue for 8 minutes to make the titanium dioxide nanowires oriented along the electric field direction.

[0035] (3) Maintain the electric field and perform gradient temperature curing on the wet film: dry at 45℃ for 12 minutes, dry at 75℃ for 18 minutes, and anneal at 105℃ for 8 minutes. After cooling, peel it off from the baseband to obtain an anisotropic polyurethane film with a thickness of 0.2 mm. The Young's modulus in the direction parallel to the electric field is about 98 MPa, the Young's modulus in the direction perpendicular to the electric field is about 24 MPa, the anisotropy ratio is about 4.1:1, and the light transmittance is 91%.

[0036] (4) A picosecond ultraviolet laser with a wavelength of 355 nm, a spot diameter of 18 μm, and a repetition frequency of 350 kHz was used to cut a centrally symmetrical multi-lobed slit array on the thin film. Each slit unit consists of three radial slits converging at the same center point. The slits are of equal length and have an included angle of 120°. The center point retains the connection point that is not completely cut through. The slit width is 50 μm, and the array is arranged in a square grid with a period of 4 mm. The laser scanning speed is 1000 mm / s, and a single slit is completed by scanning three times. Compressed air is used to blow away the chips, and the heat-affected zone at the edge of the slit is less than 5 μm.

[0037] (5) The cut film is subjected to biaxial stretching, with a stretching rate of 30% in each direction and a stretching speed of 10 mm / min. After stretching, the lobes of each cut unit warp outwards uniformly along the central connection point, forming a conical through-hole with a small end at the center, a large end at the periphery, and a cross-section that gradually shrinks from the large end to the small end. The small end is the original cut intersection point, and the large end is formed by the warping of the lobes, with a warping height of approximately 0.18 mm. Maintaining this stretched state, heat-set at 105°C for 8 minutes, then naturally cool to room temperature under tension to permanently fix the geometry of the conical through-hole. Figure 1 and Figure 2 As shown.

[0038] (6) Fix the four sides of the heat-set film to the aluminum alloy frame, and form an air cavity with a depth of 5 cm between the film and the transparent substrate to make a flexible transparent sound-absorbing film.

[0039] Example 2 This embodiment provides a flexible transparent sound-absorbing membrane. The preparation steps are the same as in Example 1, except that the mass fraction of titanium dioxide nanowires in step (1) is adjusted to 1%, and the biaxial stretching ratio in step (5) is adjusted to 35%. The resulting film has a Young's modulus of approximately 55 MPa in the direction parallel to the electric field, approximately 22 MPa in the direction perpendicular to the electric field, an anisotropy ratio of approximately 2.5:1, and a light transmittance of 93%. The warpage height of the conical aperture after stretching is approximately 0.16 mm. The heat setting temperature is 105℃, and the time is 8 minutes.

[0040] Example 3 This embodiment provides a flexible transparent sound-absorbing membrane. The preparation steps are the same as in Embodiment 1, except that the biaxial stretching ratio in step (5) is adjusted to 25%. After stretching, the warpage height of the conical aperture flap is about 0.15 mm. The heat setting temperature is 100℃ and the time is 10 minutes.

[0041] Example 4 This embodiment provides a flexible transparent sound-absorbing membrane. The preparation steps are the same as in Embodiment 1, except that in step (4), the array period is adjusted to 3 mm (square grid), and the unit density is approximately 111,000 units / m². 2 Step (5) Biaxial stretching ratio 30%, flap warping height approximately 0.17 mm. Heat setting temperature 105℃, time 8 minutes.

[0042] Comparative Example 1 This comparative example provides a flexible transparent sound-absorbing membrane. The preparation steps are the same as in Example 1, except that no DC electric field is applied in steps (2) and (3). The titanium dioxide nanowires are randomly distributed in the membrane. After curing, the membrane is isotropic in the plane, and the Young's modulus in each direction is about 55 MPa. Step (5) involves biaxial stretching by 30%, and the heat setting conditions are the same as in Example 1.

[0043] Comparative Example 2 This comparative example provides a flexible transparent sound-absorbing membrane. The preparation steps are the same as in Example 1, except that in step (4), the radial cut intersection of the cut unit is cut through, i.e., no connection point is retained in the center. In step (5), the membrane is stretched by 30% in both directions, and the heat setting conditions are the same as in Example 1.

[0044] Comparative Example 3 This comparative example uses the same TPU substrate and titanium dioxide nanowire content as Example 1, but instead of electric field orientation, laser cutting, and stretching self-assembly, a conical micropore array is prepared using a mold hot pressing method. The mold is a precision-machined stainless steel conical needle array with a cone tip diameter of 50 μm, a cone base diameter of 200 μm, a period of 4 mm, a hot pressing temperature of 120℃, and a holding pressure of 30 seconds.

[0045] The state of the conical holes formed in Examples 1-4 and Comparative Examples 1-3 is shown in Table 1 below: Table 1. Tapered Hole Status

[0046] The warpage height of the conical aperture lobes in Examples 1-4 varied within the range of 0.15-0.18 mm, with the central small end aperture uniformly maintained at 50 μm. Each lobe exhibited uniform warpage, good forming condition, and reliable process robustness. The reduction in nanowire content led to a decrease in the anisotropy ratio of the film, resulting in a corresponding decrease in the lobe warpage height, indicating that the degree of anisotropy directly affects the magnitude of the warpage driving force. A decrease in stretchability resulted in a corresponding decrease in lobe warpage height, demonstrating that stretchability is a key process parameter for controlling the geometry of the conical aperture. Variations in the array period had little effect on the warpage height of a single aperture, but increased unit density improved sound absorption performance.

[0047] Comparative Example 1: Without electric field orientation, the thin film is isotropic. The lobe warpage height is distributed across a wide range of 0.05-0.20 mm, with uneven warpage amplitude among the lobes, making it difficult to form regular conical holes. This directly proves that anisotropy is a necessary condition to ensure uniform lobe warpage. Comparative Example 2: The central connection point of the cut is completely cut through, and the lobe completely detaches after stretching, failing to form any hole structure. Comparative Example 3: Although the mold hot pressing method can obtain regular conical holes, the lobe warpage height is fixed at the mold size of 0.20 mm, making it impossible to flexibly adjust through process parameters. Furthermore, it suffers from engineering defects such as high mold cost and low demolding yield.

[0048] The samples prepared in Examples 1-4 and Comparative Examples 1-3 were tested under the same conditions: the sound absorption coefficient was measured using the impedance tube method, according to GB / T 18696.2 standard, with a test frequency range of 100-5000 Hz. The samples were fixed to the surface of 5 mm thick float glass through a spacer layer, and the air cavity depth was 5 cm. The results are as follows. Figure 3 As shown; transmittance was measured at a wavelength of 550 nm using a UV-Vis spectrophotometer.

[0049] The average sound absorption coefficient of 500-4000 Hz, the peak sound absorption coefficient of 1000 Hz, and the light transmittance results are summarized in Table 2.

[0050] Table 2 Results of sound absorption and light transmission performance

[0051] from Figure 3As shown in Table 2, the sound absorption coefficient curves in the embodiments exhibit a characteristic of first rising and then stabilizing within the 100-5000 Hz range. The average sound absorption coefficient is greater than 0.63 in the 500-4000 Hz range, reaching a peak of 0.81-0.90 near 1000 Hz. Furthermore, the curves on both sides of the peak transition smoothly without any sharp single peak. This indicates that in the low-frequency range of 100-500 Hz, the cavity resonance and the fundamental frequency resonance under the pre-stretch tension of the thin film work together to convert sound energy into membrane vibration damping dissipation, causing the sound absorption coefficient to gradually increase from 0.22-0.25 to 0.55-0.70. In the mid-to-high frequency range of 500-4000 Hz, the parallel acoustic impedance network composed of hundreds of thousands of tapered, gradually varying through-holes plays a dominant role. The tapered cross-section allows sound waves of different frequencies to find impedance matching positions at different depths of the channels, maintaining a stable plateau in the curve, ensuring that the sound absorption coefficient remains at 0.56-0.65 at 4000 Hz.

[0052] In Example 2, the nanowire content was reduced from 2% in Example 1 to 1%, and the warpage height of the conical aperture flaps decreased from 0.18 mm to 0.16 mm. The reduced nanowire content weakened the anisotropy of the film, decreasing the driving force at the flap root during stretching, thus lowering the warpage height. This resulted in a slower contraction of the conical channel cross-section, a more uniform airflow velocity distribution within the channels, and reduced frictional losses. The average sound absorption coefficient from 500 to 4000 Hz decreased from 0.68 to 0.63, and the peak value at 1000 Hz decreased from 0.87 to 0.81. Simultaneously, the reduced nanowire content decreased intrafilm scattering, increasing the final light transmittance from 87% to 89%.

[0053] In Example 3, the elongation was reduced from 30% in Example 1 to 25%, resulting in a decrease in the warpage height of the conical aperture flaps. The reduced elongation lowered the film pretension, shifted the film resonant frequency upwards, and slightly weakened low-frequency sound absorption. In the mid-to-high frequency range, the reduced warpage height weakened the contraction of the conical aperture cross-section, decreasing the average sound absorption coefficient from 0.68 to 0.64 in the 500-4000 Hz range, and the peak value at 1000 Hz from 0.87 to 0.83. Regarding light transmittance, the reduced warpage height decreased the scattering of light by the surface micro-reliefs, resulting in a slight increase in the finished product's light transmittance from 87% to 88%.

[0054] Example 4 reduces the array period from 4 mm in Example 1 to 3 mm, increasing the number of conical holes per unit area by 1.8 times. This significantly increases the perforation rate, making the parallel-operating conical hole network denser and improving the overall mid-to-high frequency sound absorption efficiency. The average sound absorption coefficient from 500 to 4000 Hz increases from 0.68 to 0.71, the highest among all examples. However, the increased perforation rate raises the proportion of the total cut area from approximately 1.1% to approximately 2.0%, increasing the light-blocking area and reducing the light transmittance of the finished product from 87% to 84%.

[0055] Comparative Example 1 has no electric field orientation, the film is isotropic, the petal warp height is distributed in a wide range of 0.05-0.20 mm, the warp amplitude of each petal is uneven, and it is impossible to form a regular conical hole. The sound absorption coefficient curve shows violent fluctuations, and the average sound absorption coefficient of 500-4000 Hz is only 0.38, which is less than 54% of that of Example 1.

[0056] In Comparative Example 2, the central connection point was cut through, and the petals fell off after being stretched, making it impossible to form any porous structure. The sound absorption coefficient across the entire frequency band was less than 0.12, almost completely losing its sound absorption function.

[0057] Comparative Example 3 shows that the effective sound absorption bandwidth of the straight conical hole obtained by the hot pressing method is much narrower than that of the self-assembled gradual conical hole, and it also has engineering disadvantages such as high mold cost and low demolding yield.

[0058] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a flexible transparent sound-absorbing membrane, characterized in that, Includes the following steps: (1) Add titanium dioxide nanowires to a transparent thermoplastic polyurethane solution to obtain a uniform transparent dispersion, and cast the transparent dispersion on a substrate to form a wet film. (2) Apply a DC electric field to the wet film to orient the titanium dioxide nanowires along the direction of the electric field, with the DC electric field direction parallel to the film surface; (3) Maintain a DC electric field and cure the wet film to obtain an anisotropic polyurethane film; (4) On an anisotropic polyurethane film, a periodically arranged centrally symmetrical multi-lobed slit array is cut out using a laser. Each slit unit is composed of at least three radial slits converging at the same center point. The center point retains the connection point that is not cut through. The slit width is 30-80 μm and the slit array period is 3-6 mm. (5) Apply biaxial stretching to the cut film with a stretching rate of 20-40%, so that the lobes of each cut unit warp outward uniformly along the connection point under stress, forming a conical through hole with a small end at the center, a large end at the periphery, and a cross section that gradually shrinks from the large end to the small end. Heat set to obtain the stretched polyurethane film. (6) Fix the stretched polyurethane film to the frame, or attach it to the surface of the transparent substrate through a spacer layer, so that an air cavity is formed between the polyurethane film and the transparent substrate, and a flexible transparent sound-absorbing membrane is obtained.

2. The method for preparing a flexible transparent sound-absorbing membrane according to claim 1, characterized in that, In step (1), the titanium dioxide nanowires have a diameter of 10-20 nm, a length of 500 nm-2 μm, and an aspect ratio greater than 50; the mass fraction of the titanium dioxide nanowires in the transparent thermoplastic polyurethane solution is 1-3%, and the wet film thickness is 0.5-2 mm.

3. The method for preparing a flexible transparent sound-absorbing membrane according to claim 1, characterized in that, In step (2), the electric field strength of the DC electric field is 2-5 kV / mm, and the application time is 5-10 min.

4. The method for preparing a flexible transparent sound-absorbing membrane according to claim 1, characterized in that, In step (3), gradient temperature rise curing is adopted. The first stage is drying at 40-50℃ for 10-15 minutes. Second stage: Dry at 70-80℃ for 15-20 minutes; Third stage: Annealing at 100-110℃ for 5-10 minutes.

5. The method for preparing a flexible transparent sound-absorbing membrane according to claim 1, characterized in that, In step (4), each cut unit consists of 3 or 4 radial cuts converging at the same center point. Each cut has an equal length and a uniform angle. The cut width is 30-80 μm. The cut array is arranged in a square or regular hexagonal grid with a period of 3-6 mm.

6. The method for preparing a flexible transparent sound-absorbing membrane according to claim 5, characterized in that, In step (4), a picosecond ultraviolet laser is used for laser cutting. The laser wavelength is 355 nm and the focused spot diameter is 15-25 μm. The cut is formed by 2-4 repeated scans, and the chips are blown away with compressed air or nitrogen. The heat-affected zone at the cut edge is less than 5 μm.

7. The method for preparing a flexible transparent sound-absorbing membrane according to claim 1, characterized in that, In step (5), the stretching speed of biaxial stretching is 5-20 mm / min, the heat setting temperature is 100-110℃, the time is 5-10 minutes, and it is cooled to room temperature under tension.

8. A flexible transparent sound-absorbing membrane, characterized in that, Prepared by the preparation method according to any one of claims 1-7; comprising a transparent polyurethane film substrate, wherein the transparent polyurethane film substrate contains titanium dioxide nanowires and a plurality of periodically arranged three-dimensional conical through holes, each three-dimensional conical through hole is formed by a multi-lobed cut under biaxial stretching, the lobes of which warp outward uniformly along the central connection point, having a small end located at the center and a large end located at the periphery, the cross section of the through hole gradually shrinks from the large end to the small end; The visible light transmittance of the flexible transparent sound-absorbing membrane is ≥85%; the average sound absorption coefficient in the frequency range of 500-4000 Hz is greater than 0.

6.

9. A flexible transparent sound-absorbing membrane according to claim 8, characterized in that, The transparent polyurethane film substrate is an anisotropic film with a first direction and a second direction that are perpendicular to each other in the plane. The Young's modulus of the first direction is greater than that of the second direction, and the anisotropy ratio is 2:1 to 5:

1. The titanium dioxide nanowires are oriented along the first direction.

10. A flexible transparent sound-absorbing membrane according to claim 8, characterized in that, Multi-lobed cuts are formed by three or four radial cuts converging at the same center point, with each cut having equal length and uniform angle; The small end diameter of the conical through-hole is 30-80 μm, the large end diameter is determined by the flap warp height, which is 0.15-0.25 mm, the through-hole depth is equal to the film thickness, and the array period is 3-6 mm. An air cavity with a depth of 3-10 cm is provided between the flexible transparent sound-absorbing membrane and the transparent substrate.

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