Frequency Selective Nano-TiO₂ Doped Polyurethane Sound Absorbing Foam and Its Preparation Method and Application

Abstract

The present invention discloses a frequency-selective nano-TiO2 doped polyurethane sound-absorbing foam and its preparation method and application. The sound-absorbing foam comprises the following components: a matrix material, a functional filler, and a foaming agent. Among them, the functional filler is surface-hydroxylated nano-TiO2 with a content of 1 wt%, the foaming agent is water with a content of 0.4 - 0.9 wt%, and the rest is the matrix material. Through the nano-TiO2 doping and gradient foaming process, the present invention successfully develops a polyurethane sound-absorbing foam with frequency selectivity, which has both high-frequency directional noise reduction, low-frequency signal retention, and excellent mechanical properties, and can be widely applied in the fields of transportation, industry, and construction, filling the technical gap of existing sound-absorbing materials in frequency selection characteristics and practicality.

Description

Technical Field

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

[0002] Traditional polyurethane foams rely on porous structures to absorb medium and high-frequency noises (>4000 Hz), but the sound absorption coefficients for low-frequency sound waves (<2000 Hz) are generally lower than 0.4, resulting in signal attenuation in the human voice frequency band (300 - 3400 Hz). Although conventional fillers (such as carbon fibers and glass microspheres) can improve the sound absorption efficiency, they lack frequency selectivity, and broadband absorption interferes with voice communication. In addition, a high porosity (>90%) design is likely to sacrifice the mechanical strength (compressive strength <50 kPa), resulting in a decline in sound absorption performance after compressive 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-mentioned defects or deficiencies in the prior art, it is desirable 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. Among them, the functional filler is surface-hydroxylated nano-TiO₂ with a content of 1 wt%, the foaming agent is water with a content of 0.4 - 0.9 wt%, and the rest is the matrix material. The matrix material comprises the following components: polyoxyethyl glycerol ether: 75 - 78 wt%, toluene diisocyanate: 20 - 22 wt%, triethanolamine: 0.2 - 0.5 wt%, dibutyltin dilaurate: 0.2 - 0.5 wt%, silicone oil: 0.3 - 0.7 wt%.

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

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

[0007] Further, when the thickness of the foam 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 compressive dissipation energy at 60% strain is ≥250 kJ / m³.

[0008] Furthermore, a method for preparing the above sound-absorbing foam is provided, comprising the following steps:

[0009] (a) Preparation of premixed solution: Mix polyoxyethyleneglyceryl ether, triethanolamine, dibutyltin dilaurate, silicone oil, and water, and stir at 600 - 1000 rpm for 20 - 60 minutes;

[0010] (b) Preparation of foaming slurry: Add toluene diisocyanate and nano-TiO₂ to the premixed solution in step (a), stir at a high speed of 1000 - 1400 rpm for 5 - 15 seconds, and control the cell growth by the nucleation effect of nanoparticles;

[0011] (c) Curing and forming: Inject the foaming slurry into a mold and cure at room temperature for 20 - 60 minutes.

[0012] Furthermore, the conditions for room temperature curing are a temperature of 20 - 25 °C and a humidity of ≤60%.

[0013] Furthermore, an application of the above sound-absorbing foam is provided for one of the following scenarios:

[0014] (1) Inside the car cabin: Attach to the car door or ceiling to suppress engine high-frequency noise of 4000 - 6000 Hz;

[0015] (2) High-speed rail sound insulation: As the sandwich material of the seat back panel to absorb the noise of 5000 - 6300 Hz caused by wheel-rail friction;

[0016] (3) Industrial equipment: Wrap the compressor or motor housing to directionally attenuate high-frequency mechanical vibrations of >4000 Hz.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0018] By means of nano-doping, pore size regulation, and process optimization, the problems of low low-frequency efficiency, wide-band absorption interfering with voice communication, and poor mechanical properties of traditional sound-absorbing materials are solved, and it has both high-efficiency frequency-selective sound absorption, mechanical strengthening, and engineering applicability.

[0019] At a thickness of 5 mm, the average sound absorption coefficient of the material for high-frequency noise of 4000 - 6300 Hz is ≥0.9 (the peak value reaches 0.94 @5000 Hz), while the average sound absorption coefficient for the human voice frequency band of 500 - 4000 Hz is ≤0.3, achieving an intelligent frequency-selective effect of "high-frequency absorption - low-frequency transparency".

[0020] The mechanical properties and sound absorption efficiency are improved synergistically. The tensile strength reaches 12.91 kPa, which is 44% higher than that of pure polyurethane foam. Nano-TiO₂ acts as a rigid filler to inhibit crack propagation.

[0021] The compressive dissipation energy reaches 252.85 kJ / m³ at 60% strain (a 110% increase), and the energy dissipation rate remains ≥90% after 5 cycles. The fracture-recombination mechanism of the dynamic hydrogen bond network enhances the anti-fatigue performance.

[0022] Flexible thickness regulation: When the thickness is 10 mm, the low-frequency sound absorption coefficient is increased to 0.5 - 0.6, and the high-frequency remains ≥0.8, meeting the requirements of different scenarios.

[0023] Multi-scale structure optimization, stable acoustic performance, uniform pore size distribution (1.39 ± 0.39 mm), open porosity 77.54% (ASTM D6226 standard), suppressing the diffraction and penetration of low-frequency sound waves.

[0024] Nano-TiO₂ refines the pore size through the nucleation effect, and its surface hydroxyl groups form a hydrogen bond network with the hard segments of polyurethane, enhancing the interfacial bonding strength and the high-frequency sound energy dissipation efficiency.

[0025] The preparation process is simple and environmentally friendly. The gradient foaming process (stirring at 1200 rpm for 7 seconds) and room temperature curing (20 - 25°C, humidity ≤60%) are adopted, without the need for complex equipment or high-temperature conditions. Using water as the foaming agent, it avoids the emission of volatile organic compounds, and the process cycle is short (≤60 minutes).

[0026] The application scenarios are precisely adapted, suitable for scenarios that need to suppress high-frequency noise while retaining the human voice signal, such as in the car cabin (door / ceiling noise reduction), high-speed rail sound insulation (seat back panel sandwich), and industrial equipment (compressor / motor wrapping), and directionally absorbs noise in the range of 4000 - 6300 Hz.

[0027] It should be understood that the content described in the Summary of the Invention section is not intended to limit the key or important features of the embodiments of the present invention, nor 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

[0028] By reading the detailed description of the non-limiting embodiments with reference to the following drawings, other features, objectives, and advantages of the present invention will become more apparent:

[0029] Figure 1 : Optical microscope images (×50 magnification) and pore size distribution histograms of nano-TiO₂@PU foams (compared with pure PU foams). Among them, (a) is the optical microscope image (×50 magnification) and pore size distribution histogram of PU1 foam, (b) is the optical microscope image (×50 magnification) and pore size distribution histogram of PU1-1wt%TiO₂ foam, (c) is the optical microscope image (×50 magnification) and pore size distribution histogram of PU2 foam, and (d) is the optical microscope image (×50 magnification) and pore size distribution histogram of PU2-1wt%TiO₂ foam.

[0030] Figure 2 : (a) The sound absorption coefficients of TiO2@PU foams with a thickness of 10 mm, (b) with a thickness of 5 mm, and (c) with a thickness of 3 mm.

[0031] Figure 3 In (a), it is the average sound absorption coefficients of the TiO2@PU foam with a thickness of 5 mm in the frequency ranges of 500 - 4000 Hz and 4000 - 6300 Hz, which is the result of calculating the average sound absorption coefficients of PU1, PU1 - 1wt% TiO2, PU2, and PU2 - 1wt% TiO2 foams with a thickness of 5 mm in these two frequency ranges.

[0032] Figure 3 In (b), it is the porosity of TiO2@PU, which are the porosities of PU1, PU1 - 1wt% TiO2, PU2, and PU2 - 1wt% TiO2 foams respectively.

[0033] Figure 4 : Compression loading - unloading cycle tests of (a) PU1, (b) PU1 - 1wt% TiO2, (c) PU2, and (d) PU2 - 1wt% TiO2 foams.

[0034] Figure 5 : Curves of (a) storage modulus, (b) loss modulus, and (c) loss factor tanδ of TiO2@PU foam varying with frequency. Detailed implementation manners

[0035] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It can be understood that the specific embodiments described herein are only used to explain the relevant invention and not to limit the invention. Additionally, it should be noted that for the convenience of description, only parts related to the invention are shown in the drawings.

[0036] It should be noted that, without conflict, the embodiments in 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 drawings and embodiments.

[0037] Example 1

[0038] Preparation and performance testing of nano - TiO2@PU foam

[0039] Table 1 Raw material ratio:

[0040]

[0041] 1. Preparation steps:

[0042] First, a polyol system was prepared by mixing polyoxyethylated glycerol ether, triethanolamine, dibutyltin dilaurate, silicone oil, and deionized water in an IL paper cup at 800 rpm according to Table 1 and stirring for 30 minutes. Subsequently, toluene diisocyanate was added to the pre-mixed polyol system and further mixed at 1200 rpm for 7 seconds to form a homogeneous mixture. Then, the PU mixture was quickly poured into a self-made mold (150 mm × 150 mm × 50 mm) and cross-linked and cured at room temperature for 30 minutes. Finally, the foam was taken out of the mold and stored at room temperature for one day before cutting the samples.

[0043] 2. Performance test results:

[0044] Table 2 (Results of PU2 - 1wt% TiO2)

[0045]

[0046] Figure 1 are the optical microscope images and pore size distribution histograms of nano-TiO2@PU foams (compared with pure PU foams). After introducing 1 wt% of nano-TiO2 particles into the prepolymer emulsion, the average pore size of the samples decreased significantly: the average pore sizes of the undoped groups (PU1, PU1 - 1wt% TiO2) were 1.39 ± 0.39 mm and 1.43 ± 0.44 mm, while those of the doped groups (PU2, PU2 - 1wt%TiO2) reached 1.03 ± 0.44 mm and 0.98 ± 0.22 mm respectively, indicating the universality of the nucleation effect of nano-particles in the formulation. Based on the classical nucleation theory, nano-TiO2 promotes heterogeneous nucleation by reducing the Gibbs free energy barrier of the system, and its geometric confinement effect (the particle size - pore size relationship modifies the Young - Laplace model) further inhibits bubble coalescence, ultimately forming a more uniform wide-range pore size distribution. It is worth noting that the standard deviation differences (0.39 mm, 0.44 mm) of the undoped groups (PU1, PU1 - 1wt% TiO2) show that the absence of the nucleating agent nano-TiO2 exacerbates the pore size discretization caused by temperature sensitivity. Combining the prediction of the Bies - Hansen acoustic model, reducing the pore size will increase the flow resistance rate, thereby broadening the sound absorption coefficient of the material in the medium and high frequency bands. The reduction of the average pore size (about 40%) in the doped groups B and D in this example indicates that they may have better performance in high-frequency noise attenuation.

[0047] Figure 2The test results of the sound absorption coefficients of TiO2@PU foams with (a) a thickness of 10 mm, (b) a thickness of 5 mm, and (c) a thickness of 3 mm. In this example, the impedance tube method was used to systematically investigate the sound absorption characteristics of four polyurethane foams, namely PU1, PU1-1wt% TiO2, PU2, and PU2-1wt% TiO2 foams, at three thicknesses of 10 mm, 5 mm, and 3 mm ( Figure 2 ). As can be seen from Figure 2 a, when the thickness is 10 mm, the sound absorption peak frequency of sample PU1 is 2000 Hz, and the maximum sound absorption coefficient is 0.93. The sound absorption peak frequency of sample PU1-1wt%TiO2 is 2240 Hz, and the maximum sound absorption coefficient is 0.99. The sound absorption peak frequency of sample PU2 is 2000 Hz, and the maximum sound absorption coefficient is 0.91. The sound absorption peak frequency of sample PU2-1wt% TiO2 is 1600 Hz, and the maximum sound absorption coefficient is 0.81. In Figure 2 b, when the thickness is 5 mm, the sound absorption peak frequency of sample PU1 is 2800 Hz, and the maximum sound absorption coefficient is 0.70. The sound absorption peak frequency of sample PU1-1wt% TiO2 is 4000 Hz, and the maximum sound absorption coefficient is 0.97. The sound absorption peak frequency of sample PU2 is 3150 Hz, and the maximum sound absorption coefficient is 0.98. The sound absorption peak frequency of sample PU2-1wt% TiO2 is 5000 Hz, and the maximum sound absorption coefficient is 0.94. In Figure 2 c, when the thickness is 3 mm, the sound absorption peak frequency of sample PU1 is 2500 Hz, and the maximum sound absorption coefficient is 0.52. The sound absorption peak frequency of sample PU1-1wt% TiO2 is 3550 Hz, and the maximum sound absorption coefficient is 0.94. The sound absorption peak frequency of sample PU2 is 2800 Hz, and the maximum sound absorption coefficient is 0.96. The sound absorption peak 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 cell pores ( Figure 1 pore size test shows that the pore size of PU1-1wt% TiO2 decreases 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 endows its hard segment microdomains with higher crystallinity, making the formed foam more stable. The hard segment network inhibits pore size shrinkage and deformation, and the hard segment glass transition region generates higher mechanical loss (tan δ max = 1.15) under the dynamic strain of sound waves, dissipating additional energy through chain segment friction during high-frequency vibration.

[0048] However, generally speaking, the doping of nano-TiO2 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 frequencies of all samples shift from the 1600 - 2240 Hz range to the 2800 - 5000 Hz high-frequency range. Meanwhile, 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%.

[0049] The frequency-selective sound absorption performance of PU2-1wt% TiO2 foam is regulated by the dual effects of the thickness and the filler coupling effect. 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 synchronously enhances broadband viscous dissipation. When the material is thinned to the critical thickness (5 mm), the local resonance of nano-TiO2 and the matrix acoustic impedance gradient form a synergistic tuning to achieve targeted suppression of high-frequency noise above 4000 Hz (the maximum sound absorption coefficient >0.94), while avoiding excessive attenuation of the human voice frequency band below 4000 Hz, thus meeting the intelligent noise reduction requirements. When guided by the human voice noise reduction requirements (the main voice frequency is 300 - 3400 Hz), the 5 mm thick PU2-1wt% TiO2 foam realizes an intelligent acoustic response of "low-frequency transparency - high-frequency blocking" through the hard segment-stabilized pore structure + filler resonance trap.

[0050] Table 3: Comparison of Sound Absorption Performance of PU2-1wt% TiO2 Foam with Different Thicknesses

[0051]

[0052] Table 4: Comparison of Mechanical Properties (PU vs. PU2-1wt% TiO2)

[0053]

[0054] Figure 3a is the average sound absorption coefficient of TiO2@PU foam with a thickness of 5 mm in the frequency range of 500 - 4000 Hz and 4000 - 6300 Hz. The average sound absorption coefficients of PU1, PU1-1wt% TiO2, PU2, and PU2-1wt% TiO2 foams with a thickness of 5 mm in the frequency ranges of 500 - 4000 Hz and 4000 - 6300 Hz are calculated. The root cause of the performance differentiation can be traced back to the synergistic regulation of the multi-scale structure of the material by nano-TiO2. Specifically, in the human voice-dominated frequency band 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 systems (PU1: 0.34, PU2: 0.41). However, in the high-frequency noise region (4000 - 6300 Hz), their sound absorption coefficients increase to 0.83 and 0.90 (compared with 0.65 and 0.74 for pure PU). This "low-frequency weakening - high-frequency enhancement" phenomenon is the result of the combined action of pore geometry reconstruction, filler resonance response, and matrix viscoelasticity regulation. First, it is the diffraction penetration of low-frequency sound waves and the structural reverse effect introduced by the filler: in the frequency band of 500 - 4000 Hz, the sound wave wavelength is relatively long (λ≈686~85 mm), and there are significant differences between the filler thickness (5 mm) and the pore structure scale (0.98~1.41 mm). At this time, the sound wave mainly transfers energy through pore diffraction rather than resonance dissipation. Although adding nano-TiO2 refines the pore size through the nucleation effect, the reduction of the pore size leads to negative effects, that is, the decrease of the open porosity ( Figure 3b), the surface open porosity of pure PU2 reaches 82.28%, while for PU2-1wt% TiO2, due to the interference of nanoparticles on the foaming process, the porosity drops to 77.54%. The low open porosity hinders the sound wave from entering the deep area of the complex network through the pore entrances, weakening the potential of sound energy attenuation in the diffraction path. At this time, the low-frequency sound absorption efficiency not only fails to be improved through pore size optimization but deteriorates due to the blockage of the diffraction channels. When the frequency jumps to 4000 - 6300 Hz, the sound wave wavelength shortens (λ≈85 - 54 mm) and matches the material thickness (5 mm), pore size (0.98 mm), and the size of nano-TiO2, 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 between the particles and the polyurethane matrix causes the vibration to concentrate on the surface of nano-TiO2. The vibration energy is converted into heat energy through the friction at the particle-matrix interface; (2) Microcavity resonance coupling: When the sound wave wavelength satisfies λ / 4≈d (cavity depth), where d is the pore depth, but due to the fractal resonance caused by the pores of PU2-1wt% TiO2, the actual resonance frequency is increased to above 4000 Hz due to the extension of the equivalent cavity. At this time, a large number of sub-wavelength microcavities form a superposition of multiple resonance modes, significantly broadening the high-frequency sound absorption bandwidth; (3) Extension of multiple scattering paths: 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 the inhomogeneous structure due to their shorter wavelengths, extending the effective propagation path of sound waves within the limited thickness and strengthening the energy dissipation; (4) Optimization of hard segment network dynamic dissipation: The highly cross-linked hard segments of PU2 provide a rigid support framework for TiO2, and the periodic strain of hard segment microcrystals under high-frequency vibration induces crystal plane slip, which dissipates energy together with the viscoelastic hysteresis of the soft segments. This synergistic mechanism of "hard phase energy consumption + soft phase absorption" enables the PU2-1wt% TiO2 foam to maintain an excellent sound absorption coefficient of 0.90 in the high-frequency band (only 0.74 for pure PU2).

[0055] Experimental data show that although the introduction of nano-TiO2 sacrifices some low-frequency sound absorption performance to achieve "precise suppression of noise above 4000 Hz, it retains the acoustic freedom of the human voice frequency" - this characteristic is particularly applicable to engineering scenarios that require both high-frequency mechanical noise isolation and low-frequency voice communication. It can be summarized as follows: Deliberate retention of the low-frequency acoustic window: Although the suppression of pore size and porosity by TiO2 weakens the low-frequency sound absorption, it avoids the attenuation of human voice signals caused by the efficient absorption of traditional sound-absorbing materials across the entire frequency band (such as the requirement for call clarity in the car cabin); Directional design of high-frequency narrowband absorption: By adjusting the filler mass fraction (1 wt%), the resonance peak is precisely located in the target frequency band (4000 - 6300 Hz), forming an acoustic filtering effect. The frequency-selective sound absorption behavior of 5 mm thick PU2-1 wt% TiO2 foam is essentially the product of "structure-material-acoustic field" triple modulation - nano-TiO2 reconstructs the pore network to suppress the low-frequency diffraction efficiency, while activating high-frequency resonance and scattering dissipation; The high cross-linking network of PU2-1 wt% TiO2 foam maintains high-frequency stability through the optimization of hard-segment dynamics.

[0056] Figure 4 are the test results of the compression loading-unloading cycle of (a) PU1 (b) PU1-1 wt% TiO2 (c) PU2 (d) PU2-1 wt% TiO2 foams. Through quasi-static compression cycle tests (strain rate 0.01 s⁻¹, number of cycles n = 3), the Mullins effect and mechanical strengthening mechanism of four polyurethane foams (PU1, PU1-1 wt% TiO2, PU2, PU2-1 wt% TiO2) were systematically analyzed. The test used a stepped loading program (strain thresholds were 20%, 40%, 60% respectively), and the results showed that the area of the hysteresis loop of all specimens increased exponentially with the increase of compression strain, as Figure 4As shown, the synergistic energy dissipation process of matrix molecular chain slip and filler network reconstruction triggered by strain amplitude was verified. It should be noted that the compressive stresses of PU1-1wt% TiO2 and PU2-1wt% TiO2 foams doped with nano-TiO2 are greater than those of PU1 and PU2 foams without nano-TiO2 under the same compressive strain. 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). This shows that the addition of functional particles helps to improve the mechanical strength of polyurethane foams, which benefits from the hydrogen bond interaction between the hydroxyl groups on the surface of TiO2 and the polyurethane segments. The addition of nanoparticles can effectively increase the matrix modulus. This mechanical strengthening effect can be attributed to the following multi-scale coupling mechanisms: (1) Enhanced interfacial bonding: The hydroxyl groups (-OH) on the surface of TiO2 nanoparticles form a high-density hydrogen bond network with the urethane groups (-NHCOO-) in the hard segments of polyurethane, constructing a tough organic-inorganic interface. (2) Strain hardening and crack blunting: During compression, TiO2 particles, as a rigid support phase, bear the axial load through stress transfer, reducing the local strain concentration in the polyurethane matrix. At the same time, the micro-shear band deflection induced by nanoparticles can blunt the crack tip, significantly delaying the fracture propagation rate. (3) Nanoparticle dispersion effect: The uniform dispersion of TiO2 in the PU matrix can form a percolation-like network.

[0057] As Figure 5 shown, in this example, the frequency-dependent viscoelastic behavior of four polyurethane foams (PU1, PU1-TiO2, PU2, PU2-TiO2) was revealed by dynamic mechanical analysis. As Figure 5 shown in Fig. a, when the test frequency increases from 0.1 Hz to 100 Hz, the storage moduli of PU1, PU1-1wt% TiO2, PU2, and PU2-1wt% TiO2 foams show a positive frequency correlation. Among them, the storage modulus of PU1-1wt% TiO2 reaches 197.44 kPa at 100 Hz, 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; as Figure 5As shown in Fig. b, the loss moduli of the PU1, PU1-1wt% TiO2, PU2, and PU2-1wt% TiO2 foams all increase with the increase in frequency. Among them, the loss modulus of the PU2-1wt% TiO2 foam is much larger than that of the other three foams. The tan δ of the four specimens increases monotonically with frequency, and the PU2-1wt% TiO2 foam is the largest at 100 Hz, with tan δ = 1.15 at this time. This is because the addition of nano-fillers makes the energy dissipation concentrated in a narrower relaxation mode, enhancing the cooperative gain effect of the modulus in the high-frequency region. In addition, the superimposed multi-relaxation processes in the frequency domain further enhance the positive frequency correlation by broadening the effective loss frequency window. When the TiO2 content (1wt%) approaches the percolation threshold, the weakly connected network formed between particles breaks and reconstructs repeatedly under high-frequency stress, significantly amplifying the loss modulus and tan δ values through the percolation hysteresis mechanism. The frequency-dependent modulus growth of the TiO2@PU foam is essentially the result of the dynamic game between "strong rigid constraint" and "multi-mechanism energy dissipation". High-frequency stress promotes the system to respond to the user's vibration energy input demand with a higher modulus through the cooperation of forced chain segment movement and interface network reconstruction. The PU2-1wt% TiO2 foam optimizes the filler anchoring ability through the hard segment enrichment design.

[0058] Example 2: Comparative experiment of different TiO2 contents

[0059] Table 5 Sample groups:

[0060]

[0061] Table 6 Comparison of sound absorption performance:

[0062]

[0063] It can be seen from Table 3 and Table 4 that the 1 wt% TiO2 content is the optimal ratio. Specifically, in the human voice-dominated frequency band 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 systems (PU1: 0.34, PU2: 0.41). However, in the high-frequency noise region (4000 - 6300 Hz), their sound absorption coefficients rise to 0.83 and 0.90 (compared with 0.65 and 0.74 for pure PU).

[0064] The present invention provides a polyurethane sound-absorbing foam with frequency selectivity through nano-TiO2 doping and foaming process, which has high-frequency directional noise reduction, low-frequency signal retention, and excellent mechanical properties, and can be widely applied in the fields of transportation, industry, and construction, filling the technical gap in frequency selection characteristics and practicality of existing sound-absorbing materials.

[0065] The above are only the preferred embodiments of the present application and are not intended to limit the present application. For those skilled in the art, various changes and modifications can be made to the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included within the protection scope of the present application.

Claims

1. A frequency-selective nano-TiO2 doped polyurethane sound-absorbing foam, characterized in that, It comprises the following components: matrix material, functional filler, foaming agent. Among them, the functional filler is surface-hydroxylated nano-TiO₂ with a content of 1 wt%, the foaming agent is water with a content of 0.4 - 0.9 wt%, and the rest is matrix material. The matrix material comprises the following components: polyoxyethyl glycerol ether: 75 - 78 wt%, toluene diisocyanate: 20 - 22 wt%, triethanolamine: 0.2 - 0.5 wt%, dibutyltin dilaurate: 0.2 - 0.5 wt%, silicone oil: 0.3 - 0.7 wt%. When the foam thickness is 5 mm, the average sound absorption coefficient in the frequency band of 4000 - 6300 Hz ≥ 0.9, the average sound absorption coefficient in the frequency band of 500 to 4000 Hz ≤ 0.3, the tensile strength ≥ 12 kPa, and the compression dissipation energy at 60% strain ≥ 250 kJ / m³.

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

3. The sound-absorbing foam according to claim 2, wherein The pore size distribution of the sound-absorbing foam is 1.39 ± 0.39 mm, and the open-cell rate is 75 - 80%.

4. The preparation method of the sound-absorbing foam according to any one of claims 1-3, characterized in that, It includes the following steps: (a) Preparation of premixed liquid: 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 liquid in step (a), and stir at a high speed of 1000 - 1400 rpm for 5 - 15 seconds to control the cell growth by the nucleation effect of nanoparticles; (c) Curing and molding: Inject the foaming slurry into the mold and cure at room temperature for 20 - 60 minutes.

5. The preparation method according to claim 4, characterized in that, The conditions for room-temperature curing are a temperature of 20 - 25 °C and a humidity ≤ 60%.

6. Use of the sound-absorbing foam according to any one of claims 1 to 3, characterized in that, For one of the following scenarios: (1) Inside the car cabin: Affixed to the car door or ceiling to suppress the high-frequency noise of the engine at 4000 - 6000 Hz; (2) Sound insulation for high-speed rail: As the sandwich material of the seat backboard to absorb the noise of wheel-rail friction at 5000 - 6300 Hz; (3) Industrial equipment: Wrapping the compressor or motor housing to directionally attenuate the high-frequency mechanical vibration > 4000 Hz.

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