Frequency-selective nano TiO2 doped polyurethane sound absorption foam as well as preparation method and application thereof

Abstract

The invention discloses frequency-selective nano TiO2 doped polyurethane sound absorption foam and a preparation method and application thereof, the sound absorption foam comprises the following components: a base material, a functional filler and a foaming agent, the functional filler is surface hydroxylated nano TiO2, the content of the functional filler is 1 wt%, the foaming agent is water, the content of the foaming agent is 0.4-0.9 wt%, and the balance is the base material. The sound-absorbing polyurethane foam with frequency selectivity is successfully developed through a nano TiO2 doping and gradient foaming process, has high-frequency directional noise reduction, low-frequency signal retention and excellent mechanical properties, can be widely applied to the fields of traffic, industry and buildings, and fills the technical blank of an existing sound-absorbing material in the aspects of frequency selection characteristic and practicability.

Description

Technical Field

[0001] The invention relates to the technical field of polymer acoustic materials, and in particular to a frequency-selective nano-TiO2-doped polyurethane sound-absorbing foam, and a preparation method and application thereof. Background Art

[0002] Traditional polyurethane foam relies on porous structures to absorb mid- and high-frequency noise (>4000 Hz), but the sound absorption coefficient for low-frequency sound waves (<2000Hz) is generally less than 0.4, resulting in attenuation of signals in the human voice frequency band (300-3400 Hz). Conventional fillers (such as carbon fiber and glass beads) can improve sound absorption efficiency, but lack frequency selectivity, and broadband absorption interferes with voice communications. In addition, high porosity (>90%) designs are prone to sacrifice mechanical strength (compression strength <50 kPa), resulting in decreased sound absorption performance after compression deformation. Existing nanocomposites (such as carbon nanotubes and silica aerogels) can optimize damping or porosity, but have defects such as electromagnetic interference, high brittleness or poor fatigue resistance. Summary of the invention

[0003] In view of the above defects or deficiencies in the prior art, it is desired to provide a frequency-selective nano-TiO2-doped polyurethane sound-absorbing foam and a preparation method and application thereof to solve the above technical problems.

[0004] A frequency selective nano-TiO2 doped polyurethane sound-absorbing foam comprises the following components: a matrix material, a functional filler, and a foaming agent, wherein the functional filler is surface hydroxylated nano-TiO2, and its content is 1wt%, the foaming agent is water, and its content is 0.4-0.9wt%, and the rest is the matrix material, and the matrix material comprises the following components: polyoxyethyl glycerol ether: 75-78wt%, toluene diisocyanate: 20-22wt%, triethanolamine: 0.2-0.5wt%, dibutyltin dilaurate: 0.2-0.5wt%, and silicone oil: 0.3-0.7wt%.

[0005] Furthermore, the particle size of the surface hydroxylated nano-TiO2 is 50±5 nm.

[0006] Furthermore, the pore size distribution of the nano-TiO2 is 1.39±0.39 mm, and the open porosity is 75-80%.

[0007] Furthermore, when the foam thickness is 5 mm, the average sound absorption coefficient in the frequency band of 4000-6300 Hz is ≥0.9, the average sound absorption coefficient in the frequency band of 500 to 4000 Hz is ≤0.3, the tensile strength is ≥12 kPa, and the 60% strain compression dissipation energy is ≥250 kJ / m³.

[0008] Furthermore, a method for preparing the above-mentioned sound-absorbing foam is provided, comprising the following steps: (a) Preparation of premix: Mix polyoxyethyl glycerol ether, triethanolamine, dibutyltin dilaurate, silicone oil and water, and stir at 600-1000 rpm for 20-60 minutes; (b) Preparation of foaming slurry: Add toluene diisocyanate and nano-TiO2 to the premixed solution of step (a), stir at a high speed of 1000-1400 rpm for 5-15 seconds, and use the nucleation effect of nanoparticles to control the growth of foam cells; (c) Curing and molding: Inject the foaming slurry into the mold and cure it at room temperature for 20-60 minutes.

[0009] Furthermore, the conditions for room temperature curing are a temperature of 20-25° C. and a humidity of ≤60%.

[0010] Furthermore, an application of the above-mentioned sound-absorbing foam is provided, which is used in one of the following scenarios: (1) In the car cabin: attached to the door or ceiling to suppress the high-frequency 4000-6000 Hz noise of the engine; (2) High-speed rail sound insulation: as the interlayer material of seat back panels, absorbing 5000-6300 Hz noise from wheel-rail friction; (3) Industrial equipment: Wrap compressor or motor casing to directionally attenuate high-frequency mechanical vibrations >4000 Hz.

[0011] Compared with the prior art, the beneficial effects of the present invention are: Through nano-doping, pore size control and process optimization, the problems of low low-frequency efficiency, broadband absorption interference with voice communications and poor mechanical properties of traditional sound-absorbing materials have been solved, and it has both high-efficiency frequency-selective sound absorption, mechanical strengthening and engineering applicability.

[0012] At a thickness of 5 mm, the material's average sound absorption coefficient for high-frequency noise of 4000-6300 Hz is ≥0.9 (peak value reaches 0.94@5000 Hz), and the average sound absorption coefficient for the human voice frequency band of 500-4000 Hz is ≤0.3, achieving the intelligent frequency selection effect of "high-frequency absorption-low-frequency transparency".

[0013] The mechanical properties and sound absorption efficiency are improved synergistically, with the tensile strength reaching 12.91 kPa, 44% higher than that of pure polyurethane foam. Nano-TiO2 acts as a rigid filler to inhibit crack propagation.

[0014] The compression dissipation energy can reach 252.85 kJ / m³ (increased by 110%) at 60% strain, and the energy dissipation rate remains ≥90% after 5 cycles. The breakage-reorganization mechanism of the dynamic hydrogen bond network enhances the fatigue resistance.

[0015] Flexible thickness control: When the thickness is 10 mm, the low-frequency sound absorption coefficient is increased to 0.5-0.6, and the high-frequency sound absorption coefficient remains ≥0.8, meeting the needs of different scenarios.

[0016] Multi-scale structural optimization, stable acoustic performance, uniform pore size distribution (1.39±0.39 mm), opening rate of 77.54% (ASTM D6226 standard), suppressing low-frequency sound wave diffraction and penetration.

[0017] Nano-TiO2 refines the pore size through the nucleation effect, and its surface hydroxyl groups form a hydrogen bond network with the polyurethane hard segment, thereby improving the interface bonding strength and high-frequency acoustic energy dissipation efficiency.

[0018] The preparation process is simple and environmentally friendly, using a gradient foaming process (stirring at 1200 rpm for 7 seconds) and room temperature curing (20-25°C, humidity ≤60%), without the need for complex equipment or high temperature conditions. Water is used as the foaming agent to avoid volatile organic compound emissions, and the process cycle is short (≤60 minutes).

[0019] It is precisely adapted to the application scenarios and is suitable for scenarios where high-frequency noise needs to be suppressed while retaining human voice signals, such as car cabins (door / ceiling noise reduction), high-speed rail sound insulation (seat back interlayer) and industrial equipment (compressor / motor wrapping), and can directionally absorb 4000-6300 Hz noise.

[0020] It should be understood that the contents described in the summary of the invention are not intended to limit the key or important features of the embodiments of the present invention, nor are they intended to limit the scope of the present invention. Other features of the present invention will become easily understood through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings: Figure 1 : Optical microscope image (×50 times) and pore size distribution histogram of nano-TiO2@PU foam (compared with pure PU foam). (a) is the optical microscope image (×50 times) and pore size distribution histogram of PU1 foam, (b) is the optical microscope image (×50 times) and pore size distribution histogram of PU1-1wt%TiO2 foam, (c) is the optical microscope image (×50 times) and pore size distribution histogram of PU2 foam, (d) is the optical microscope image (×50 times) and pore size distribution histogram of PU2-1wt% TiO2 foam.

[0022] Figure 2 : Sound absorption coefficient of TiO2@PU foam with (a) thickness of 10 mm, (b) thickness of 5 mm, and (c) thickness of 3 mm.

[0023] Figure 3 (a) is the average sound absorption coefficient of TiO2@PU foam with a thickness of 5 mm at 500~4000 Hz and 4000~6300 Hz. The average sound absorption coefficient of PU1, PU1-1wt% TiO2, PU2, and PU2-1wt% TiO2 foam with a thickness of 5 mm at 500~4000 Hz and 4000~6300 Hz is calculated.

[0024] Figure 3 (b) in the middle is the porosity of TiO2@PU, which are the porosities of PU1, PU1-1wt% TiO2, PU2, and PU2-1wt% TiO2 foams respectively.

[0025] Figure 4 : (a) PU1 (b) PU1-1wt% TiO2 (c) PU2 (d) PU2-1wt% TiO2 foam compression loading and unloading cycle test.

[0026] Figure 5 : Curves of (a) storage modulus (b) loss modulus (c) loss factor tanδ of TiO2@PU foam changing with frequency. DETAILED DESCRIPTION

[0027] The present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It is to be understood that the specific embodiments described herein are only used to explain the relevant invention, rather than to limit the invention. It should also be noted that, for ease of description, only the parts related to the invention are shown in the accompanying drawings.

[0028] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0029] Example 1 Preparation and performance testing of nano-TiO2@PU foam Table 1 Raw material ratio:

[0030] 1. Preparation steps: First, the polyol system was prepared by mixing polyoxyethyl glycerol ether, triethanolamine, dibutyltin dilaurate, silicone oil, and deionized water according to Table 1 in a 1L paper cup at 800 rpm and stirring for 30 min. Subsequently, toluene diisocyanate was added to the premixed polyol system and further mixed at 1200 rpm for 7 seconds to form a uniform mixture. Next, the PU mixture was quickly poured into a homemade mold (150 mm×150 mm×50 mm) and cross-linked and cured at room temperature for 30 min. Finally, the foam was removed from the mold and stored at room temperature for one day before cutting the sample.

[0031] 2. Performance test results: Table 2 (PU2-1wt% TiO2 results)

[0032] Figure 1 The optical microscope image and pore size distribution histogram of nano-TiO2@PU foam (compared with pure PU foam). After introducing 1 wt% nano-TiO2 particles into the prepolymer emulsion, the average pore size of the sample was significantly reduced: the average pore size of the undoped group (PU1, PU1-1wt% TiO2) was 1.39±0.39 mm and 1.43±0.44 mm, while the average pore size of the doped group (PU2, PU2-1wt%TiO2) reached 1.03±0.44 mm and 0.98±0.22 mm, respectively, indicating that the nucleation effect of nanoparticles has formula universality. Based on the classical nucleation theory, nano-TiO2 promotes heterogeneous nucleation by reducing the Gibbs free energy barrier of the system, and its geometric constraint effect (modified Young-Laplace model of particle size-pore size relationship) further inhibits bubble merging, ultimately forming a more uniform wide pore size distribution. It is worth noting that the difference in standard deviation (0.39 mm, 0.44 mm) of the undoped groups (PU1, PU1-1wt% TiO2) indicates that the absence of the nucleating agent nano-TiO2 will aggravate the pore size discretization caused by temperature sensitivity. Combined with the prediction of the Bies-Hansen acoustic model, reducing the pore size will increase the flow resistivity, thereby broadening the sound absorption coefficient of the material in the medium and high frequency bands. In this embodiment, the reduction in the average pore size of doped groups B and D (about 40%) indicates that they may have better performance in high-frequency noise attenuation.

[0033] Figure 2 The sound absorption coefficient test results of TiO2@PU foam with (a) thickness of 10 mm, (b) thickness of 5 mm, and (c) thickness of 3 mm are shown in Figure 1. In this embodiment, the sound absorption characteristics of four polyurethane foams, PU1, PU1-1wt% TiO2, PU2, and PU2-1wt% TiO2 foam, at three thicknesses of 10 mm, 5 mm, and 3 mm are investigated by the impedance tube method system ( Figure 2 ).from Figure 2 a It can be seen that when the thickness is 10 mm, the peak sound absorption frequency of sample PU1 is 2000 Hz, and the maximum sound absorption coefficient is 0.93. The peak sound absorption frequency of sample PU1-1wt% TiO2 is 2240 Hz, and the maximum sound absorption coefficient is 0.99. The peak sound absorption frequency of sample PU2 is 2000 Hz, and the maximum sound absorption coefficient is 0.91. The peak sound absorption frequency of sample PU2-1wt% TiO2 is 1600 Hz, and the maximum sound absorption coefficient is 0.81. Figure 2 In b, when the thickness is 5 mm, the peak sound absorption frequency of sample PU1 is 2800 Hz, and the maximum sound absorption coefficient is 0.70. The peak sound absorption frequency of sample PU1-1wt% TiO2 is 4000 Hz, and the maximum sound absorption coefficient is 0.97. The peak sound absorption frequency of sample PU2 is 3150 Hz, and the maximum sound absorption coefficient is 0.98. The peak sound absorption frequency of sample PU2-1wt% TiO2 is 5000 Hz, and the maximum sound absorption coefficient is 0.94. Figure 2 c, when the thickness is 3 mm, the peak sound absorption frequency of sample PU1 is 2500 Hz, and the maximum sound absorption coefficient is 0.52. The peak sound absorption frequency of sample PU1-1wt% TiO2 is 3550 Hz, and the maximum sound absorption coefficient is 0.94. The peak sound absorption frequency of sample PU2 is 2800 Hz, and the maximum sound absorption coefficient is 0.96. The peak sound absorption frequency of sample PU2-1wt% TiO2 is 4000 Hz, and the maximum sound absorption coefficient is 0.83. The experimental results show that the addition of TiO2 filler has a double-edged sword effect: the rigidity of TiO2 hinders the expansion of the pores ( Figure 1 The pore size test shows that the pore size of PU1-1wt% TiO2 is reduced from 1.39±0.39 mm of pure PU1 to 1.03±0.41 μm, resulting in a sharp drop in the sound absorption coefficient of the PU1-1wt% TiO2 sample after the peak (for example, the sound absorption coefficient of the 5 mm sample drops from 0.97 to 0.69 after 4000 Hz). The high NCO / OH ratio of the PU2 prepolymer gives its hard segment micro-region a higher crystallinity, making the foam more stable after molding. The hard segment network inhibits the shrinkage and deformation of the pore size, and the hard segment glass transition region produces higher mechanical loss (tan δ max =1.15), additional energy is dissipated through chain segment friction during high-frequency vibration.

[0034] However, the doping of nano-TiO2 generally improves the sound absorption coefficient of polyurethane foam. The material thickness has a significant regulatory effect on the sound absorption behavior: as the thickness decreases from 10 mm to 3 mm, the sound absorption peak frequency of all samples shifts from the 1600-2240 Hz range to the high frequency of 2800-5000 Hz, while the sound absorption coefficient in the low frequency band (<1000 Hz) decreases by 36%-58%, while the sound absorption coefficient in the high frequency band (>4000 Hz) increases by 62%-80%.

[0035] The frequency-selective sound absorption performance of PU2-1wt% TiO2 foam is regulated by the coupling effect of thickness and filler. Polyurethane filled with nano-TiO2 exhibits significant frequency-selective sound absorption characteristics in a specific frequency band (>4000 Hz): the hard segment-particle complex captures high-frequency sound energy through standing wave resonance, while the optimized pore structure simultaneously enhances broadband viscous dissipation. When the material is thinned to a critical thickness (5mm), the local resonance of nano-TiO2 and the matrix acoustic impedance gradient form a synergistic tuning to achieve targeted suppression of high-frequency noise >4000 Hz (maximum sound absorption coefficient >0.94), while avoiding excessive attenuation of the human voice frequency band <4000 Hz, thereby meeting the needs of intelligent noise reduction. When guided by the needs of human voice noise reduction (the main frequency of voice is 300-3400 Hz), the 5 mm thick PU2-1wt% TiO2 foam achieves an intelligent acoustic response of "low-frequency transparency-high-frequency blocking" through the hard segment stabilized pore structure + filler resonance trap.

[0036] Table 3: Comparison of sound absorption performance of PU2-1wt% TiO2 foam with different thicknesses

[0037] Table 4: Comparison of mechanical properties (PU vs. PU2-1wt% TiO2)

[0038] Figure 3a is the average sound absorption coefficient of TiO2@PU foam with a thickness of 5 mm at 500~4000 Hz and 4000~6300 Hz, and the average sound absorption coefficient of PU1, PU1-1wt% TiO2, PU2, and PU2-1wt% TiO2 foam with a thickness of 5 mm at 500~4000 Hz and 4000~6300 Hz is calculated. The root of their performance differentiation can be traced back to the synergistic regulation of nano-TiO2 on the multi-scale structure of the material. Specifically, in the dominant frequency band of human voice of 500-4000 Hz, the average sound absorption coefficients of PU1-1wt% TiO2 and PU2-1wt% TiO2 are 0.30 and 0.26, respectively, which are significantly lower than those of pure PU system (PU1: 0.34, PU2: 0.41). However, in the high-frequency noise region (4000-6300 Hz), the sound absorption coefficients rise to 0.83 and 0.90 (compared with 0.65 and 0.74 of pure PU). This phenomenon of "low-frequency weakening-high-frequency enhancement" is the result of the combined effects of pore geometry reconstruction, filler resonance response and matrix viscoelasticity regulation. First, the diffraction penetration of low-frequency sound waves and the structural reverse effect introduced by fillers are the main factors: in the frequency band of 500-4000 Hz, the sound wave wavelength is longer (λ≈686~85 mm), and there are significant differences in material thickness (5 mm) and pore structure scale (0.98~1.41 mm). At this time, sound waves mainly transmit energy through pore diffraction rather than resonance dissipation. Although the addition of nano-TiO2 can refine the pore size through the nucleation effect, the reduction of the pore size leads to a negative effect, namely the reduction of open porosity ( Figure 3b) The surface open porosity of pure PU2 reaches 82.28%, while the porosity of PU2-1wt% TiO2 drops to 77.54% due to the interference of nanoparticles in the foaming process. The low porosity prevents sound waves from entering the deep area of ​​the complex network through the pore entrance, weakening the sound energy attenuation potential of the diffraction path; at this time, the low-frequency sound absorption efficiency not only fails to be improved by pore optimization, but is deteriorated due to the blockage of the diffraction channel. When it jumps to 4000-6300 Hz, the wavelength of the sound wave shortens (λ≈85-54 mm) and matches the material thickness (5 mm), pore size (0.98 mm) and nano-TiO2 size, activating the following four types of enhancement mechanisms: (1) Local resonance energy conversion: high-frequency sound waves (≥4000 Hz) excite the rigid vibration of nano-TiO2 particles, and the high modulus ratio of particles to polyurethane matrix causes the vibration to be concentrated on the surface of nano-TiO2. Vibration energy is converted into heat energy through particle-matrix interface friction; (2) Microcavity resonance coupling: When the wavelength of the sound wave satisfies λ / 4≈d (cavity depth), where d is the pore depth, the actual resonance frequency is increased to above 4000 Hz due to the extension of the equivalent cavity due to the fractal resonance induced by the pores of PU2-1wt% TiO2. At this time, a large number of subwavelength microcavities form a multi-resonance mode superposition, which significantly broadens the high-frequency sound absorption bandwidth; (3) Multiple scattering path extension: The acoustic impedance mismatch between nano-TiO2 particles and the matrix causes strong scattering of sound waves on the particle surface. High-frequency sound waves are more easily scattered by inhomogeneous structures due to their shorter wavelengths, which extends the effective propagation path of sound waves within a limited thickness and enhances energy dissipation; (4) Dynamic dissipation optimization of the hard segment network: The highly cross-linked hard segment of PU2 provides a rigid support framework for TiO2, and the periodic strain of the hard segment microcrystals under high-frequency vibration induces crystal plane slip, which dissipates energy together with the viscoelastic hysteresis of the soft segment. This synergistic mechanism of "hard phase energy consumption + soft phase absorption" enables PU2-1wt% TiO2 foam to maintain an excellent sound absorption coefficient of 0.90 in the high frequency band (pure PU2 is only 0.74).

[0039] Experimental data show that although the introduction of nano-TiO2 sacrifices some low-frequency sound absorption performance to achieve "precise suppression of noise above 4000Hz, but retains the freedom of sound transmission of human voice frequency" - this feature is especially suitable for engineering scenarios that need to take into account both high-frequency mechanical noise blocking and low-frequency voice communication. It can be attributed to: Deliberate retention of low-frequency sound-transmitting windows: Although TiO2's suppression of pore size and porosity weakens low-frequency sound absorption, it avoids the attenuation of human voice signals caused by traditional sound-absorbing materials due to efficient absorption of the entire frequency band (such as the clarity requirements of calls in the car cabin); Directional design of high-frequency narrow-band absorption: By adjusting the filler mass fraction (1wt%), the resonance peak is located exactly in the target frequency band (4000-6300 Hz), forming an acoustic filtering effect. The frequency-selective sound absorption behavior of 5 mm thick PU2-1wt% TiO2 foam is essentially the product of the triple modulation of "structure-material-acoustic field" - nano-TiO2 reconstructs the pore network to suppress low-frequency diffraction efficiency and activates high-frequency resonance and scattering dissipation; while the highly cross-linked network of PU2-1wt% TiO2 foam maintains high-frequency stability through hard segment dynamics optimization.

[0040] Figure 4 The results of compression loading and unloading cycles of (a) PU1 (b) PU1-1wt% TiO2 (c) PU2 (d) PU2-1wt% TiO2 foams were systematically analyzed through quasi-static compression cycle tests (strain rate 0.01 s⁻¹, number of cycles n=3) for the Mullins effect and mechanical strengthening mechanism of four polyurethane foams (PU1, PU1-1wt% TiO2, PU2, PU2-1wt% TiO2). The test adopted a step loading procedure (strain thresholds were 20%, 40%, and 60%, respectively). The results showed that the hysteresis loop area of ​​all samples increased exponentially with the increase of compression strain, such as Figure 4As shown, the synergistic energy consumption process of matrix molecular chain slip and filler network reconstruction triggered by strain amplitude was verified. It is worth noting that the compressive stress of PU1-1wt% TiO2 and PU2-1wt% TiO2 foams doped with nano-TiO2 at the same compressive strain is greater than that of PU1 and PU2 foams without nano-TiO2. For example, when the compressive strain is 60%, the compressive stress of PU1-1wt% TiO2 is 37.35 kPa, which is 14 times higher than that of PU1 (2.4 kPa), and the compressive stress of PU2-1wt% TiO2 foam is 69.55 kPa, which is 22 times higher than that of PU2 (3.06). It shows that the addition of functional particles helps to improve the mechanical strength of polyurethane foam, which is due to the hydrogen bonding between the hydroxyl groups on the surface of TiO2 and the polyurethane segments. The addition of nanoparticles can effectively improve the matrix modulus. This mechanical strengthening effect can be attributed to the following multi-scale coupling mechanisms: (1) Interface bonding enhancement: The hydroxyl groups (-OH) on the surface of TiO2 nanoparticles form a high-density hydrogen bond network with the carbamate groups (-NHCOO-) in the hard segment of polyurethane, constructing a strong organic-inorganic interface. (2) Strain hardening and crack passivation: During the compression process, TiO2 particles act as a rigid supporting phase to carry the axial load through stress transfer, reducing the local strain concentration of the polyurethane matrix. At the same time, the micro-shear band deflection induced by nanoparticles can passivate the crack tip and significantly slow down the fracture propagation rate. (3) Nanoparticle dispersion effect: The uniform dispersion of TiO2 in the PU matrix can form a quasi-percolation network.

[0041] like Figure 5 As shown in FIG. 1 , this embodiment reveals the frequency-dependent viscoelastic behavior of four polyurethane foams (PU1, PU1-TiO2, PU2, and PU2-TiO2) through dynamic mechanical analysis. Figure 5 As shown in Figure a, when the test frequency increases from 0.1 Hz to 100 Hz, the storage modulus of PU1, PU1-1wt% TiO2, PU2, and PU2-1wt% TiO2 foams increases in a frequency-correlated manner. The storage modulus of PU1-1wt% TiO2 at 100 Hz reaches 197.44 kPa, which is 613%, 229%, and 353% higher than that of PU1 (27.74 kPa), PU2 (60.03 kPa), and PU2-1wt% TiO2 (43.58 kPa), respectively. Figure 5As shown in Fig. b, the loss modulus of PU1, PU1-1wt% TiO2, PU2, and PU2-1wt% TiO2 foams all increase with increasing frequency, among which the loss modulus of PU2-1wt% TiO2 foam is much larger than that of the other three foams. The tan δ of the four samples increases monotonically with frequency, and the PU2-1wt% TiO2 foam is the largest at 100 Hz, at which tan δ =1.15. This is because the addition of nanofillers concentrates energy dissipation in a narrower relaxation mode, which enhances the synergistic gain effect of the modulus in the high-frequency region. In addition, the superimposed multi-relaxation process in the frequency domain further enhances the positive frequency correlation by broadening the effective loss frequency window. When the TiO2 content (1wt%) approaches the percolation threshold, the weak connection network formed between the particles repeatedly breaks and rebuilds under high-frequency stress, significantly amplifying the loss modulus and tan δ value through the percolation hysteresis mechanism. The frequency-dependent modulus growth of TiO2@PU foam is essentially the result of the dynamic game between "rigid strong constraint" and "multi-mechanism energy dissipation". High-frequency stress promotes the system to respond to the user's vibration energy input requirements with a higher modulus by forcing the chain segment movement and the reconstruction of the interface network. PU2-1wt% TiO2 foam optimizes the filler anchoring ability through the hard segment enrichment design.

[0042] Example 2: Comparative experiment of different TiO2 contents Table 5 Sample groups:

[0043] Table 6 Sound absorption performance comparison:

[0044] It can be seen from Tables 3 and 4 that 1 wt% TiO2 content is the optimal ratio. Specifically, in the dominant frequency band of human voice of 500-4000 Hz, the average sound absorption coefficients of PU1-1wt% TiO2 and PU2-1wt% TiO2 are 0.30 and 0.26, respectively, which are significantly lower than those of the pure PU system (PU1: 0.34, PU2: 0.41). However, in the high-frequency noise area (4000-6300 Hz), the sound absorption coefficients rise to 0.83 and 0.90 (compared with 0.65 and 0.74 of pure PU).

[0045] The present invention provides a frequency-selective polyurethane sound-absorbing foam through nano-TiO2 doping and foaming process, which has high-frequency directional noise reduction, low-frequency signal retention and excellent mechanical properties. It can be widely used in transportation, industry and construction fields, filling the technical gap in the frequency selection characteristics and practicality of existing sound-absorbing materials.

[0046] The above are only preferred embodiments of the present application and are not intended to limit the present application. For those skilled in the art, the present application may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A frequency selective nano-TiO2 doped polyurethane sound absorbing foam, characterized in that: The invention comprises the following components: a matrix material, a functional filler and a foaming agent, wherein the functional filler is surface hydroxylated nano-TiO2 with a content of 1wt%, the foaming agent is water with a content of 0.4-0.9wt%, and the rest is a matrix material, and the matrix material comprises the following components: polyoxyethyl glycerol ether: 75-78wt%, toluene diisocyanate: 20-22wt%, triethanolamine: 0.2-0.5wt%, dibutyltin dilaurate: 0.2-0.5wt%, and silicone oil: 0.3-0.7wt%.

2. The sound absorbing foam according to claim 1, characterized in that The particle size of surface hydroxylated nano-TiO2 is 50±5 nm.

3. The sound absorbing foam according to claim 2, characterized in that The pore size distribution of the nano-TiO2 is 1.39±0.39 mm, and the open porosity is 75-80%.

4. The sound absorbing foam according to claim 1, characterized in that When the foam thickness is 5 mm, the average sound absorption coefficient in the frequency band of 4000-6300 Hz is ≥0.9, the average sound absorption coefficient in the frequency band of 500 to 4000 Hz is ≤0.3, the tensile strength is ≥12 kPa, and the 60% strain compression dissipation energy is ≥250 kJ / m³.

5. The method for preparing the sound-absorbing foam according to any one of claims 1 to 4, characterized in that: The following steps are involved: (a) Preparation of premix: polyoxyethyl glycerol ether, triethanolamine, dibutyltin dilaurate, silicone oil and water are mixed, and stirred at 600-1000 rpm for 20-60 minutes; (b) Preparation of foaming slurry: toluene diisocyanate and nano-TiO2 are added to the premix of step (a), and stirred at 1000-1400 rpm for 5-15 seconds to control the growth of pores by the nucleation effect of nanoparticles; (c) Curing and molding: the foaming slurry is injected into a mold and cured at room temperature for 20-60 minutes.

6. The preparation method according to claim 5, characterized in that: The room temperature curing conditions are a temperature of 20-25° C. and a humidity of ≤60%.

7. An application of the sound-absorbing foam according to any one of claims 1 to 4, characterized in that: Used in one of the following scenarios: (1) In the car cabin: attached to the door or ceiling to suppress the high-frequency 4000-6000 Hz noise of the engine; (2) High-speed rail sound insulation: as a sandwich material for seat back panels, it absorbs 5000-6300 Hz noise from wheel-rail friction; (3) Industrial equipment: Wrap compressor or motor casing to directionally attenuate high-frequency mechanical vibrations >4000 Hz.

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