A gradient pore melamine sound-absorbing foam and a preparation method thereof

Abstract

The application discloses a kind of gradient hole melamine sound-absorbing foams and its preparation method, belong to high polymer foaming material technical field.Melamine formaldehyde precondensation resin, foaming agent, curing agent and surfactant are mixed into foaming slurry, continuously spread on lower heating conveyor and control thickness by upper thickness limiting press belt;By microwave radiation, the material body phase is energized, the first surface is heated by the lower heating conveyor, the second surface is actively cooled by the upper thickness limiting press belt, the heating rate decreases from the first surface to the second surface, the high heating rate side is crosslinked and solidified first, and the smaller pore size is locked, a continuous pore size gradient without interlayer interface is formed in a single continuous matrix;And according to the target sound absorption frequency band, the target pore size is determined, the characteristic attenuation length is inversely solved by the calibrated heating rate-pore size fitting relationship, and then the temperature difference, microwave power, cooling strength and linear velocity and other process parameters are set, and the pore orientation degree gradient can also be superimposed, and smooth excellent sound absorption performance is obtained in a wide frequency range.

Description

Technical Field

[0001] This invention relates to the field of polymer foaming materials technology, specifically to a gradient-pore melamine sound-absorbing foam and its preparation method. Background Technology

[0002] Melamine foam (also known as melamine sponge) is a polymer foam material made from melamine-formaldehyde prepolymer resin through foaming, cross-linking, and curing. It features a high porosity, low density, and a three-dimensional interconnected pore structure, making it a widely used porous sound-absorbing material in the field of acoustic sound absorption and noise reduction. The sound absorption performance of porous sound-absorbing materials is closely related to structural parameters such as pore size, pore size distribution, porosity, and material thickness: structures with smaller pore sizes and higher flow resistance have higher absorption efficiency for high-frequency sound waves, while structures with larger pore sizes and lower flow resistance are more conducive to the dissipation of mid- and low-frequency sound waves. Melamine foam with a uniform pore size structure often only achieves a relatively narrow sound absorption coefficient at a specific thickness, making it difficult to achieve broadband noise reduction, especially for high-frequency noise with a wide frequency spectrum, such as the noise from the drive motors of new energy vehicles (typically in the 1-8kHz band).

[0003] To broaden the effective sound absorption frequency band of melamine foam, existing technologies have proposed multi-layer composite structure solutions. For example, patent document CN209063642U discloses a composite sound insulation material based on melamine sound-absorbing material, which consists of a metal plate, a mesh support layer, and a melamine foam layer stacked sequentially, with the sound absorption frequency adjusted by changing the thickness of each layer. However, this solution is a discrete layered stacked structure, with assembly interfaces between the layers. The pore size of the foam layer itself remains uniform, making it impossible to achieve a continuous pore size gradient transition. There is acoustic impedance mismatch at the interlayer interfaces, which easily generates reflections and sound absorption coefficient traps at specific frequencies. Furthermore, the multi-layer and multi-material stacking increases production complexity and cost.

[0004] On the other hand, in the field of general polymer foam materials, patent document CN110204778A discloses a method for preparing a polymer foam material with a gradient pore structure. This method involves injection molding a preform combined with high-pressure gas physical foaming, utilizing the difference in shear and cooling rates between the surface and interior of the preform to form a gradient pore structure with small surface pores, large interior pores, and a continuous, interface-free structure. However, this method relies on the injection molding and high-pressure gas physical foaming mechanism of thermoplastic polymers. Melamine-formaldehyde prepolymer is a thermosetting system that relies on chemical foaming agents for simultaneous decomposition, foaming, and cross-linking curing under microwave radiation. Therefore, injection molding or high-pressure gas physical foaming processes cannot be used, and the aforementioned techniques cannot be applied. Furthermore, this document does not address the design and optimization of acoustic absorption performance, nor does it disclose how to customize the pore size gradient curve according to the target sound absorption frequency band.

[0005] Furthermore, although it is known in the art that the heating rate and foaming temperature affect the cell size, existing technologies do not provide specific, feasible technical means for quantitatively, controllably, and repeatedly establishing the required pore size difference between the first and second surfaces within a single continuous matrix in a melamine-formaldehyde thermosetting, microwave-cured, and continuous production system, and for customizing the pore size gradient curve according to the target sound absorption frequency band. Therefore, how to provide a gradient-pore melamine sound-absorbing foam capable of quantitatively controlling the pore size gradient within a single continuous matrix and customizing it according to the target sound absorption frequency band, and its preparation method, is a technical problem urgently needing to be solved in this field. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a gradient-pore melamine sound-absorbing foam and its preparation method. This invention quantitatively and controllably forms a continuous pore size gradient without interfaces within a single continuous melamine-formaldehyde thermosetting matrix, solving the technical problems of existing melamine foam sound-absorbing materials relying on discrete layering and superposition, the sound absorption spectrum being prone to notch waves, and the lack of specific means to quantitatively control the pore size gradient in a thermosetting microwave continuous system.

[0007] In a first aspect, the present invention provides a method for preparing gradient-pore melamine sound-absorbing foam, comprising: mixing melamine-formaldehyde prepolymer resin, foaming agent, curing agent and surfactant in proportion to obtain foaming slurry; continuously spreading the foaming slurry on a lower heating conveyor belt and controlling the thickness by an upper thickness-limiting pressure belt to form a continuous material belt having a first surface and a second surface; supplying energy to the bulk phase of the material by microwave radiation, and heating the first surface by contact with the lower heating conveyor belt and cooling the second surface by the upper thickness-limiting pressure belt, so that the heating rate of the first surface is higher than that of the second surface; the first surface side with a higher heating rate first crosslinks and cures irreversibly to lock the smaller pore size, while the second surface side with a lower heating rate is cured and locked to form a larger pore size after longer bubble growth, thereby forming a continuous pore size gradient without interlayer interfaces in a single continuous matrix.

[0008] Furthermore, this invention provides a quantitative control method for the pore size gradient: using β(z) = β1·exp(-z / δ) and d(z) = d1·exp(n·z / δ) as calibrated engineering fitting models, the heating rate distribution and average pore size distribution in the thickness direction are characterized, respectively; after determining the target average pore sizes d1 and d2 according to the target sound absorption frequency band, the characteristic attenuation length δ = n·H / lnR required to achieve the target pore size ratio is solved by inversely solving the expression of d(z); then, based on the pre-calibrated growth index n and the correspondence between δ and temperature difference, microwave volumetric power density, cooling heat transfer coefficient, and operating speed, the process parameters are set to make the obtained pore size distribution approach the target distribution, thereby extending this method to products of different thicknesses; the calibration relationship can be established through a limited number of conventional experiments, and the specific establishment method is described in the specific implementation method. This invention also provides a specific implementation method for applying gradient mechanical constraints to form a pore orientation gradient and determining the target pore size according to the target sound absorption frequency band.

[0009] Secondly, the present invention provides a gradient-pore melamine sound-absorbing foam, comprising a single continuous melamine-formaldehyde resin open-cell foam matrix. The matrix has a first surface and a second surface opposite each other along the thickness direction. The pores do not contain any filling second phase material. It has a continuous and monotonically varying pore size gradient along the thickness direction. The average pore size near the first surface is smaller than the average pore size near the second surface. The two surfaces are part of the same continuous matrix, with no interlayer bonding interface or identifiable phase interface. The open-cell ratio is not less than 95%. In some embodiments, the average pore size near the second surface is 2 to 15 times the average pore size near the first surface. In some embodiments, the matrix also has a continuously varying pore orientation gradient along the thickness direction. The pores near the first surface are stretched and oriented parallel to the board surface with an aspect ratio of 1.5 to 2.2, while the pores near the second surface have an aspect ratio close to 1.0. Furthermore, the sound-absorbing foam has a sound absorption coefficient of not less than 0.7 in the 2 to 8 kHz frequency band.

[0010] Compared with existing technologies, the advantages of this invention are as follows: First, it directly forms a continuous pore size gradient without a second phase or bonding interface within a single continuous melamine-formaldehyde thermosetting matrix, avoiding sound absorption trapping caused by interlayer reflection due to discrete layering. Second, it provides a quantitative relationship between heating rate and pore size, along with the design formula δ=n·H / lnR, and a calibration method, transforming the well-known understanding that "heating rate affects pore size" into a specific control method that can quantitatively determine the process and replicate across thicknesses in a thermosetting microwave continuous system. Third, by superimposing the pore orientation gradient through gradient mechanical constraints and customizing the pore size gradient curve according to the target sound absorption frequency band, it achieves smooth and excellent sound absorption performance over a wider frequency range. Fourth, it can be achieved by adapting the microwave heating inherent in continuous melamine foam production and conventional equipment, without the need for additional equipment such as injection molding or high-pressure gas foaming. Attached Figure Description

[0011] Figure 1 This is a schematic diagram of the differential temperature gradient constrained continuous forming device provided in an embodiment of the present invention; Figure 2 A flowchart of a method for preparing gradient-pore melamine sound-absorbing foam provided in an embodiment of the present invention; Figure 3 This is an enlarged cross-sectional schematic diagram of gradient-pore melamine sound-absorbing foam provided in an embodiment of the present invention; Figure 4 This is a schematic diagram illustrating the relationship between the thickness-direction temperature field distribution and the aperture gradient provided in an embodiment of the present invention. Figure 5 This is a schematic diagram comparing the sound absorption coefficient-frequency curves of the three structures provided in the embodiments of the present invention. Detailed Implementation

[0012] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. In the present invention, "near the first surface" refers to the surface region extending along the thickness direction from the first surface, with a thickness that is the smaller of 20% of the total thickness H of the finished product and 5 mm; "near the second surface" refers to the surface region extending along the thickness direction from the second surface, with a thickness that is the smaller of 20% of the total thickness H of the finished product and 5 mm. "Average pore size" refers to the arithmetic mean of the equivalent diameter of the bubbles in the thickness direction section of the above-mentioned region, statistically calculated based on scanning electron microscopy (SEM) images; "length-to-diameter ratio" refers to the statistical average of the ratio of the longest dimension of the bubbles in the SEM image to the dimension perpendicular to that direction; the porosity can be determined with reference to GB / T 10799. Unless otherwise stated, "connection" includes direct connection and indirect connection through intermediate components.

[0013] (a) Establishment of the temperature difference field between the production unit and the thickness direction (in conjunction with) Figure 1 ) like Figure 1As shown, the differential temperature gradient constrained continuous molding apparatus provided by the present invention includes, in sequence along the material running direction, a raw material mixing and batching unit 100, a fabric forming unit 200, and a microwave radiation unit 300. The raw material mixing and batching unit 100 includes a melamine-formaldehyde prepolymerization resin storage tank 101, a foaming agent metering pump 102, a curing agent metering pump 103, a surfactant metering pump 104, and a static mixer 105. Exemplarily, an aqueous solution of melamine-formaldehyde prepolymerization resin (solid content of about 70-80 wt%), a foaming agent (such as pentane, 5-30 parts by weight of the resin), a curing agent (such as acidic curing agents such as p-toluenesulfonic acid, 1-10 parts by weight of the resin), and a surfactant (nonionic, 0.5-5 parts by weight of the resin) are metered and mixed evenly in the static mixer 105 to obtain a foaming slurry. The above-mentioned types and proportions of raw materials are conventional choices for preparing melamine foam in this field and belong to the prior art. This invention does not impose any special limitations on them. The improvement of this invention lies in the subsequent differential temperature curing in the thickness direction and the quantitative control of the pore size gradient.

[0014] The fabric forming unit 200 includes a lower heating conveyor belt 201 and an upper thickness-limiting pressure belt 203. Foaming slurry is continuously and uniformly spread onto the lower heating conveyor belt 201 via a slit-type fabric spreading device 202, and its thickness is controlled by the gaps in the upper thickness-limiting pressure belt 203, forming a continuous material belt. The continuous material belt has a first surface in contact with the lower heating conveyor belt 201 and a second surface facing the upper thickness-limiting pressure belt 203. A differential temperature control unit 400 is connected to the heating element of the lower heating conveyor belt 201 and the cooling medium circuit or heat dissipation structure of the upper thickness-limiting pressure belt 203, respectively, for setting and adjusting the first surface temperature T1, the equivalent temperature T2 on the second surface side, and the temperature difference ΔT between them. The device may also be optionally equipped with an online detection module 600 and a data processing and feedback adjustment unit 700. Figure 1 (Shown in dashed lines) is used for online detection of the pore size gradient distribution in the thickness direction of a continuous material strip, and the detection results are fed back to the differential temperature control unit 400 (see steps S170~S180 in Section (VI) for details). The product obtained after curing and shaping is the gradient pore melamine sound-absorbing foam 500. The gradual roller gap adjustment mechanism 204 is an optional configuration used to apply gradient mechanical constraints, see Section (III) for details.

[0015] Regarding the establishment of the thickness-direction temperature difference field, it is necessary to explain in detail: the microwave radiation unit 300 provides bulk energy for the foaming and cross-linking curing of the continuous material strip. After foaming and expansion, the melamine-formaldehyde resin / water system has low density and low dielectric loss. The penetration depth of the 2.45GHz microwave in this system is much greater than the thickness of the continuous material strip. Even in the early stages of slurry spreading when the water content is high, the penetration depth is comparable to the material thickness. Therefore, the microwave provides approximately uniform bulk energy along the thickness direction, which is insufficient to establish a significant temperature rise rate difference in the thickness direction. In fact, the microwave application from above may slightly benefit the heating of the second surface. The thickness-direction temperature rise rate difference in this invention does not depend on the incident direction of the microwave, but is actively established by the boundary thermal conditions of the fabric forming unit—that is, by contact heating of the first surface by the lower heating conveyor belt 201 and active cooling / heat dissipation of the second surface by the upper thickness-limiting pressure belt 203. Specifically, the lower heating conveyor belt 201 has built-in zoned electric heating tubes or heat transfer oil circulation channels to maintain the first surface at a set temperature T1 (e.g., 110~140℃); the upper thickness-limiting pressure belt 203 has built-in circulating cooling medium channels (such as circulating water or circulating heat transfer oil) or is equipped with air-cooled or cold roller heat dissipation structures to actively maintain the equivalent temperature of the second surface side at T2 (e.g., 70~100℃), and its cooling heat transfer coefficient h_c for the second surface can be taken as 50~500W / (m). 2 A temperature difference ΔT = T1 - T2 is used to counteract the heating of the second surface by microwaves and reduce the heating rate on that side; the temperature difference ΔT = T1 - T2 can be adjusted within the range of 10~60℃. Because melamine foam has a very low thermal conductivity (approximately 0.03~0.04 W / (m·K)), the aforementioned "hot bottom-cold top" boundary forms a steep and stable temperature gradient in the thickness direction, resulting in a higher heating rate on the side furthest from the microwave source (the first surface, in contact with the hot conveyor belt); even when microwaves are applied from above, the first surface reaches the gel curing temperature first. The foaming agent decomposes upon heating, producing gas that forms bubble nuclei and grows. Simultaneously, the acidic curing agent catalyzes the cross-linking of the melamine-formaldehyde resin, and the cross-linked network gradually and irreversibly "locks" the bubble structure.

[0016] (II) Formation Mechanism and Quantitative Control of Aperture Gradient (Combined with) Figure 4 ) The formation mechanism and quantitative control method of the aperture gradient are as follows. Assume the thickness direction coordinate z points from the first surface (z=0) to the second surface (z=H). The temperature field T(z,t) within the continuous material band approximately satisfies the one-dimensional unsteady-state heat conduction equation: ρc·∂T / ∂t=k·∂ 2 T / ∂z 2+q_v, where ρ, c, and k are the density, specific heat capacity, and thermal conductivity of the material, respectively, and q_v is the microwave volumetric power density (approximately uniform along the thickness); the boundary conditions are: at z=0, contact heating is provided by the lower heating conveyor belt (T(0,t)≈T1), and at z=H, active cooling is provided by the upper thickness-limiting pressure belt (-k·∂T / ∂z|(z=H)=h_c·(T-T2)). Under the combined effect of the "hot bottom-cold top" boundary and low thermal conductivity, the heating rate β(z)=∂T / ∂t at each position in the thickness direction monotonically decreases from the first surface to the second surface, which can be approximately expressed as: β(z)=β1·exp(-z / δ) (Formula 1) In the formula, β1 is the heating rate at the first surface (determined by T1, microwave power, and residence time), and δ is the characteristic decay length of the heating rate along the thickness. δ is determined by the relative strength of microwave bulk heating (which tends to homogenize and increases δ) and boundary temperature difference / cooling (which makes the gradient steeper and decreases δ), denoted as δ=δ(ΔT, q_v, h_c): the larger ΔT and the stronger the cooling h_c, the smaller δ (the steeper the gradient); the larger the microwave power q_v, the larger δ (the more uniform). The pores at each thickness location are locked by the cross-linked network when they reach the local gel solidification temperature T_gel, and the locking time is approximately t_lock(z)≈(T_gel-T0) / β(z). Since the pore size depends on the growth time window before it is locked, the final average pore diameter d(z) increases with increasing t_lock(z), which can be approximately expressed as: d(z)=d1·exp(n·z / δ) (Formula 2) In the formula, d1 is the average pore size (minimum) near the first surface, and n is the growth index related to the formulation (especially the crosslinking temperature sensitivity determined by the amount of curing agent), determined by calibration (exemplarily n≈0.5~1.0). From Equation 2, the pore size ratio near the first surface to the second surface is: R=d(H) / d(0)=d2 / d1=exp(n·H / δ) (Equation 3) Based on this, a process determination method for the target aperture gradient is given: given the target apertures d1 and d2 and the finished thickness H determined by acoustic design (see Section (IV)), the required characteristic attenuation length can be solved by inverse equation 3: δ=n·H / ln(d2 / d1) (Equation 4) Based on the pre-established calibration relationship δ=δ(ΔT, q_v, h_c), the lower-to-upper-layer temperature difference ΔT, microwave volumetric power density q_v, and upper-layer cooling heat transfer coefficient h_c are selected to achieve the required δ. Simultaneously, β1 is determined by T1 and the residence time t_res=L / v (where L is the length of the microwave action zone and v is the linear velocity), thereby determining the absolute level of d1. For products with different thicknesses H, only δ needs to be adjusted accordingly according to Equation 4 (i.e., adjusting ΔT, q_v, h_c, or v) to obtain the same aperture gradient ratio R, thus extending this method to different specifications. It should be noted that Equations 1 and 2 are calibrated engineering fitting models used to determine process parameters, and do not require the actual distribution to be strictly exponential: the target average pore diameters d1 and d2 in the formula are design inputs given by acoustic design, and the actual pore diameter distribution of the sound-absorbing foam obtained is close to the target distribution; if the measured deviation exceeds the allowable range, the temperature difference and microwave power can be further corrected by combining the online detection feedback of steps S170~S180 in Section (VI).

[0017] The calibration relationship is established as follows: Samples are prepared under several known conditions (T1, T2, q_v, v). SEM observations are performed on slices along the thickness direction. The average pore size at each depth is statistically analyzed. Equations 1 and 2 are fitted to obtain δ, n, and the correspondence between δ and (ΔT, q_v, h_c) for this formulation, thus obtaining a reusable process-pore size gradient calibration curve / model. Calibration example: For the formulation used in Example 1, the microwave volumetric power density q_v, linear velocity v = 1.0 m / min, and upper-layer cooling heat transfer coefficient h_c ≈ 200 W / (m²) are fixed. 2 Samples with a thickness of 30 mm were prepared under three conditions: ΔT = 20℃, 40℃, and 60℃. Slices were taken every 3 mm along the thickness direction, and the average pore size at each depth was statistically analyzed using SEM. The values ​​of δ obtained by fitting the samples according to Equation 2 were approximately 13.8 mm, 11.2 mm, and 9.9 mm, respectively. The growth indices obtained from the three fittings were consistent, with n ≈ 0.75, thus yielding the δ-ΔT calibration curve for this formulation under q_v and h_c. By changing q_v and h_c and repeating the above experiment, the complete δ = δ(ΔT, q_v, h_c) calibration model was obtained. The above calibration is a routine experiment; it only needs to be performed once for the same formulation and device to support process settings for products with different thicknesses and target gradients. Figure 4 The relationship between the temperature distribution curve along the thickness direction (continuously decreasing) and the average pore size distribution curve (continuously increasing according to Equation 2) obtained according to the above model is shown.

[0018] (III) Realization of Gradient Mechanical Constraints and Cell Orientation Gradient (Combined with...) Figure 1 ) The gradient roll gap adjustment mechanism 204 is used to apply gradient mechanical constraints. It can be achieved by a gradient roll gap pressure roller with a gradually decreasing roll gap along the running direction, or by a group of multiple (e.g., 3-8) segmented pressure rollers arranged along the running direction with independently adjustable linear pressure, positioned in the foaming rise zone (i.e., the section where the material has foamed to near the target thickness, but the first surface has not yet fully cross-linked and cured). The timing of applying compression is selected when the apparent viscosity of the resin on the first surface side is 10. 4 ~10 6 Pa·s (close to gel but still plastically deformable), while the apparent viscosity on the second surface side remains low (e.g., less than 10). 3 Within a window of Pa·s; at this time, a compression ratio ε=(H0-H1) / H0 (e.g., 10%~35%, where H0 and H1 are the thicknesses before and after compression, respectively) is applied to the continuous material strip along the thickness direction. Due to the high viscosity of the first surface side, it is immediately cross-linked and cured, and the growing cells are flattened and stretched and oriented along the direction parallel to the plate surface, and do not recover, forming oriented cells with an aspect ratio (major axis / minor axis) of 1.5~2.2; the viscosity of the second surface side is low, and viscoelastic recovery occurs after the pressure roller passes through, and the cells basically remain nearly spherical (aspect ratio ≈1.0). Thus, a cell orientation gradient that continuously varies from high to low along the thickness direction from the first surface to the second surface is superimposed on the pore size gradient.

[0019] Method for determining the pressure window: The apparent viscosity-time curve of the formulation was measured using a rotational rheometer following a heating process consistent with the production line. Combined with the linear velocity v, the viscosity was set at 10... 4 ~10 6 The time window of Pa·s is mapped to a position interval along the running direction, within which the pressure roller or pressure roller group is installed. During production, the foaming height can also be monitored by a non-contact thickness / distance sensor, with the section where the foaming height reaches 90%~98% of the target thickness designated as the pressure application window. The linear pressure of each segmented pressure roller is set to increase progressively along the running direction, allowing the compression ratio to gradually reach the set value within the window, avoiding instantaneous overpressure that could cause bubble collapse. The magnitude of the orientation gradient is quantitatively controlled by the compression ratio ε, the timing of pressure application (corresponding to the viscosity on the first surface side, which can be adjusted via β1 and the pressure roller position), and the pressure distribution of the segmented pressure rollers.

[0020] (iv) Design method for customizing aperture gradient curves according to target sound absorption frequency band (combined with) Figure 2 , Figure 5 ) The method for custom designing the pore size gradient curve for the target sound absorption frequency band is as follows. The finished product thickness H is discretized into N (e.g., 20-50) sub-layers along the thickness direction. The average pore size d_i = d(z_i) of the i-th sub-layer is given by the pore size gradient curve (Equation 2). Each sub-layer is considered as a porous layer satisfying the Johnson-Champoux-Allard (JCA) equivalent fluid model, and its static flow resistance σ_i is approximately related to the pore size as follows: σ_i=A·μ / d_i 2 (Equation 5) In the formula, μ is the aerodynamic viscosity (approximately 1.8 × 10⁻⁶). -5 Pa·s), where A is a dimensionless structural constant related to the open-cell foam skeleton structure (determined by calibration, exemplarily A≈20~60). Other JCA parameters are taken according to empirical relationships of open-cell foam and corrected by calibration: porosity φ is the measured open-cell ratio (≥0.95), tortuosity τ∞≈1 / √φ, thermal characteristic length Λ′≈0.5d_i, viscous characteristic length Λ≈0.5Λ′; A and Λ, Λ′ can be corrected by one-time calibration using measured flow resistance and sound absorption curves of 2~3 samples with known pore sizes and uniform pore size. Based on this, the characteristic acoustic impedance and propagation constant of each sublayer are calculated. The transfer matrix method is used to cascade layer by layer with a rigid wall as a backing to obtain the material surface acoustic impedance Z_s(f), which is then calculated using α(f)=1-|(Z_s-ρ0c0) / (Z_s+ρ0c0)| 2 The sound absorption coefficient-frequency curve is obtained (ρ0c0 is the characteristic impedance of air). With the goal of maximizing the average sound absorption coefficient within the target sound absorption frequency band [f_low, f_high], and using d1, d2, and n / δ in Equation 2 as optimization variables, within the manufacturable aperture range (e.g., 30~800μm), and constraining the aperture to monotonically change along the thickness, the aperture gradient curve is optimized (using grid search or conventional optimization algorithms) to obtain the optimal aperture gradient curve. Finally, according to Equation 4 and the calibration relationship, this optimal curve is mapped to specific process parameters (ΔT, microwave power, cooling intensity, linear velocity). As can be seen from Equation 5, the smaller the aperture, the higher the flow resistance. Therefore, the fine pore region near the first surface mainly contributes to high-frequency absorption, while the coarse pore region near the second surface mainly contributes to mid-to-low-frequency absorption. The aperture gradient makes the sound absorption coefficient smooth and excellent across a wide frequency range (see...). Figure 5 ).

[0021] (v) Realization of bidirectional aperture gradient As an extended embodiment of the present invention, when it is necessary to simultaneously enhance the sound absorption of two different frequency bands, or when sound waves may be incident on both sides of the installation scene, symmetrical boundary thermal conditions can be established in the thickness direction, so that the heating rate is symmetrically distributed along the thickness as "fast at both ends and slow in the middle" or "slow at both ends and fast in the middle", thereby forming a bidirectional pore size gradient of "fine-coarse-fine" or "coarse-fine-coarse". Taking "fine-coarse-fine" as an example: both the lower conveyor belt 201 and the upper thickness-limiting pressure belt 203 are used as heating surfaces (both surfaces maintain a high temperature, for example, both are 130°C), and energy is mainly supplied to the middle layer of the material by microwaves; since the thermal conductivity of melamine foam is low, the two surfaces have a high heating rate due to contact heating, and cross-linking and curing first form fine pore regions on both sides, while the middle layer has the lowest heating rate and is cured last to form a coarse pore region, resulting in a "fine-coarse-fine" distribution, where the fine pore regions on both sides contribute to high-frequency absorption and the middle coarse pore region contributes to mid-frequency absorption. Conversely, by configuring active cooling on both surfaces (with the same cooling configuration as the upper thickness-limiting pressure band 203) and setting a high-power microwave action zone in the middle layer, a "coarse-fine-coarse" distribution can be obtained.

[0022] Quantitative design of bidirectional gradient: Under symmetrical boundary conditions, the heating rate in the thickness direction can be approximately expressed as β(z) = βs·[exp(-z / δs) + exp(-(Hz) / δs)], with its minimum value located at z = H / 2 in the middle layer. Applying Equation 2 to the distance to the nearest heated surface ζ = min(z, Hz), the average aperture satisfies d(z) ≈ ds·exp(n·ζ / δs), where βs and ds are the heating rate and average aperture at the surface, respectively. Given the target middle-to-surface aperture ratio R′ = d(H / 2) / ds, the required δs is determined according to δs = n·(H / 2) / lnR′, and the temperatures (or cooling intensities) on both sides and the microwave power in the middle layer are set according to the calibration relationship in Section (II). The high-power microwave action zone in the middle layer can be achieved by densifying the magnetron arrangement in this section or by using a focusing waveguide / slot antenna array, and its volumetric power density can be 1.5 to 3 times that of the other sections. The location and amplitude of the temperature reversal zone are quantitatively determined by the heating / cooling intensity on both sides, the microwave power in the middle layer, and the material thickness according to the above model.

[0023] (vi) Methods and Procedures (in combination) Figure 2 ) like Figure 2As shown, the preparation method of the present invention includes: step S100 (optional), target frequency band design - the required aperture gradient curve and corresponding process parameters are determined by back-calculation according to the method in section (iv); step S110, material mixing; step S120, material forming; step S130, establishing a differential temperature field - the first surface is heated by the lower heating conveyor belt and the second surface is cooled by the upper thickness limiting pressure belt, and the differential temperature and heating rate distribution along the thickness is established according to T1, T2, h_c determined in sections (i) and (ii); step S140, microwave radiation synergistic foaming and curing - microwaves supply energy to the bulk phase of the material, and the material in the thickness direction is successively heated due to the difference in heating rate. Foaming and cross-linking curing and locking; Step S150, continuous formation of pore size gradient -- first curing and locking near the first surface to form a fine pore area, then curing and locking near the second surface to form a coarse pore area, the pore size transitions continuously along the thickness according to Formula 2; Step S160 (optional), apply gradient mechanical constraint (according to Section (III)); Step S170 (optional), online detection of pore size gradient distribution in the thickness direction; Step S180 (optional), determine whether the pore size gradient meets the target range, if not, feedback adjust the temperature difference between the first surface and the second surface and return to step S130; Step S190, cooling and shaping; Step S200, slitting and shaping.

[0024] (vii) Product hole structure (combined) Figure 3 ) like Figure 3 As shown, the gradient-pore melamine sound-absorbing foam 500 of the present invention exhibits a continuous pore size gradient along its thickness from the first surface (high-temperature side T1) to the second surface (low-temperature side T2): the area near the first surface is a fine-pore region 501, with smaller pore sizes, and contains pores with tensile orientation parallel to the plate surface when gradient mechanical constraints are applied; the area near the second surface is a coarse-pore region 502, with larger pore sizes and approximately spherical shapes. There are no identifiable delamination interfaces or adhesive lines between the fine-pore region 501 and the coarse-pore region 502, and the entire matrix is ​​a continuous single solid along its thickness. Figure 3 (H in the middle indicates the total thickness of the finished product). The cell size transitions continuously with the thickness position, which is the essential difference between the present invention and the discrete layered structure formed by multi-layer assembly in terms of microstructure.

[0025] (viii) Acoustic performance testing and implementation examples In this embodiment of the invention, the sound absorption coefficient is tested using the standing wave tube / impedance tube method (refer to GB / T 18696.2 or ISO10534-2), with a test frequency of 500Hz~8000Hz; the porosity is determined according to GB / T 10799; the apparent viscosity of the resin is determined using a rotational rheometer; and the pore size and aspect ratio are statistically analyzed by SEM slices.

[0026] Example 1: Preparation of standard gradient-pore sound-absorbing foam with a thickness H=30mm. The foaming slurry was prepared by mixing according to the above conventional proportions and spread into a continuous material strip with an initial thickness of approximately 35mm. This strip was then passed through microwave radiation unit 300 (2.45GHz) at a linear velocity of approximately 1.0m / min. The lower heating conveyor belt 201 had a temperature T1=130℃, while the upper thickness-limiting pressure belt 203 was circulated with cooling water, with an equivalent temperature T2=90℃ (cooling heat transfer coefficient h_c approximately 200W / (m²)). 2 ·K), temperature difference ΔT=40℃. According to the calibration example in Section (II), under these conditions, n≈0.75, δ≈11.2mm; based on the target pore size ratio R≈7.5, H=30mm, according to Equation 4, δ=n·H / lnR≈0.75×30 / ln7.5≈11.2mm, which is consistent with the calibration value. Therefore, the above ΔT, microwave power and linear velocity are confirmed. After curing and shaping, SEM sectioning shows that: the average pore size near the first surface d1≈80μm, the average pore size near the second surface d2≈600μm, and the pore size ratio is about 7.5, which is consistent with the prediction of Equation 3; the first surface and the second surface are a continuous single matrix, and no interlayer bonding interface or delamination traces are seen; according to GB / T 10799, the open porosity is about 99%. Sound absorption tests were conducted on the gradient-pore sound-absorbing foam, the uniform pore size structure of the same thickness and density (Comparative Example 1), and the discrete layered stacked structure prepared according to the method described in CN209063642U (Comparative Example 2). The results are as follows: Figure 5 As shown: In this embodiment, the sound absorption coefficient remains at 0.72~0.85 in the 2000~8000Hz range, reaching 0.82~0.85 in the 3000~5000Hz range; Comparative Example 1 shows a single peak only in the 3000~4000Hz range (approximately 0.75~0.78), decreasing to approximately 0.25 and 0.30 in the 500Hz and 8000Hz ranges, respectively; Comparative Example 2 shows significant sound absorption trapping (decreasing to 0.40~0.45) in the 4000~5000Hz range due to interlayer interface reflection, verifying the advantage of the continuous aperture gradient of the present invention in broadband sound absorption. Furthermore, after enabling the online detection feedback in steps S170~S180, the batch fluctuation of the aperture ratio R in continuous production narrowed from approximately ±15% to approximately ±5%, significantly improving the consistency of the pore structure.

[0027] Example 2: Based on Example 1, the gradual roll gap adjustment mechanism 204 is used to apply gradient mechanical constraints. Following the method in Section (III), the pressure window is determined using the rheological curve combined with the linear velocity, with a viscosity of approximately 10 on the first surface side. 5Applying a compression ratio ε≈20% within a Pa·s window, the aspect ratio of the foam cells near the first surface reaches 1.6~2.0, while the foam cells near the second surface remain nearly spherical, forming a foam orientation gradient. Tests show that after introducing the orientation gradient, the sound absorption coefficient of this sound-absorbing foam is further improved by about 0.05~0.08 in the mid-to-low frequency (500~2000Hz), while the compressive strength in the direction perpendicular to the board surface is increased by about 12%.

[0028] Example 3: Explanation of customizing aperture gradient curves according to the target frequency band. For the high-frequency noise (main frequency band 3~6kHz) of a new energy vehicle drive motor, following the method in Section (IV): the 30mm thickness is discretized into 30 sub-layers. The flow resistivity of each sub-layer is calculated using Equation 5 and optimized using the transfer matrix method, yielding the optimal aperture curves d1≈50μm and d2≈450μm (R≈9). From Equation 4, δ≈0.75×30 / ln9≈10.2mm is obtained. The required temperature difference is approximately 55℃, obtained by interpolation of the δ-ΔT calibration curve in Section (II). Based on this, T1, T2, microwave power, and linear velocity are set. The sound-absorbing foam prepared with these parameters achieves a measured sound absorption coefficient of 0.80~0.90 in the 3~6kHz range, meeting the design specifications; the uniform aperture structure of the same thickness only achieves 0.55~0.65 in this frequency band. This embodiment also verifies the bidirectional gradient: by maintaining both surfaces at approximately 130°C according to Section (V) and relying on microwaves to power the middle layer, a "fine-coarse-fine" distribution is obtained, with d≈60μm on both sides and d≈500μm in the middle layer (R′≈8.3, corresponding to δs=n·(H / 2) / lnR′≈0.75×15 / ln8.3≈5.3mm, and the microwave power density in the middle layer is approximately twice that of the other sections). This can simultaneously improve the sound absorption performance of the 500~1500Hz and 4000~6000Hz frequency bands, and is suitable for installation scenarios where both sides may be subject to sound wave incident.

[0029] It should be noted that the specific values ​​in the above embodiments (such as temperature difference, linear velocity, pore size, values ​​of δ and n, compression ratio, etc.) are merely examples to facilitate understanding of the present invention and should not be construed as limiting the scope of protection of the present invention; Equations 1 to 5 are approximate descriptions of the physical mechanism, and the characteristic parameters (δ, n, A, etc.) therein are based on the calibration results for specific formulations and devices. Various modifications, equivalent substitutions, and improvements that can be made by those skilled in the art based on the technical solutions disclosed in this invention without creative effort should all be covered within the scope of protection of this invention.

Claims

1. A method for preparing gradient-pore melamine sound-absorbing foam, characterized in that, Includes the following steps: Melamine-formaldehyde prepolymer resin, foaming agent, curing agent and surfactant are mixed in proportion to obtain foaming slurry; The foamed slurry is continuously spread on the lower heating conveyor belt and the thickness is controlled by the upper thickness limiting pressure belt to form a continuous material belt. The continuous material belt has a first surface and a second surface that are opposite each other along the thickness direction. The first surface is in contact with the lower heating conveyor belt and the second surface faces the upper thickness limiting pressure belt. Microwave radiation provides the energy required for foaming and cross-linking curing to the continuous material belt phase, and the lower heating conveyor belt heats the first surface in contact while the upper thickness-limiting pressure belt cools the second surface, so that the heating rate of the first surface is higher than that of the second surface. The melamine-formaldehyde resin on the first surface side with a higher heating rate reaches the cross-linking and curing temperature first. It is irreversibly locked by the cross-linking network during the nucleation and early growth stages of foaming, resulting in a smaller average pore size. The second surface side with a lower heating rate undergoes a longer bubble growth time window before being cross-linked, cured, and locked, resulting in a larger average pore size. This creates a continuously varying pore size gradient in the thickness direction of the continuous material strip, and the average pore size near the first surface is smaller than the average pore size near the second surface. Wherein, the first surface and the second surface are a continuous single substrate, and there is no interlayer bonding interface.

2. The method according to claim 1, characterized in that, The lower heating conveyor belt has built-in electric heating tubes or heat transfer oil circulation channels to maintain the first surface at temperature T1; the upper thickness-limiting pressure belt has built-in circulating cooling medium channels or is equipped with air-cooled or cold roller heat dissipation structures to actively maintain the equivalent temperature of the second surface at T2; the heat transfer coefficient of the upper thickness-limiting pressure belt to the second surface is 50~500W / (m²). 2 ·K), the difference between T1 and T2 is 10~60℃.

3. The method according to claim 1, characterized in that, It also includes the step of determining process parameters based on the target aperture: The heating rate distribution at each position along the thickness direction of the continuous material strip is characterized by fitting β(z) = β1·exp(-z / δ), and the average pore size distribution along the thickness direction of the prepared sound-absorbing foam is characterized by fitting d(z) = d1·exp(n·z / δ), where z is the thickness direction coordinate from the first surface, β1 is the heating rate at the first surface, δ is the characteristic attenuation length of the heating rate along the thickness, d1 is the average pore size near the first surface, n is the growth index, and the correspondence between n and δ and the process parameters is obtained by pre-calibration. The target average aperture d1 near the first surface and the target average aperture d2 near the second surface are determined based on the target sound absorption frequency band. The characteristic attenuation length δ=n·H / lnR required to achieve the target aperture ratio R=d2 / d1 is obtained by inverse solving the expression of d(z), where H is the finished product thickness and n is the growth index. Based on the pre-calibrated correspondence, at least one process parameter is set among the temperature difference between the first surface and the second surface, the volume power density of the microwave radiation, the cooling heat transfer coefficient of the upper thickness-limited pressure belt, and the running speed of the continuous material belt, so that the average pore size distribution of the sound-absorbing foam along the thickness direction approaches the target distribution determined by the target average pore sizes d1 and d2.

4. The method according to claim 1, characterized in that, This also includes the area where the apparent viscosity of the resin on the first surface side is 10 in the foaming rise zone. 4 ~10 6 During a time period of Pa·s, the continuous material belt is compressed at a rate of 10% to 35% along the thickness direction by pressure rollers with gradually varying gaps along the running direction or by multiple segmented pressure roller groups with independently adjustable linear pressure. This causes the pores near the first surface to be stretched and oriented in a direction parallel to the surface of the continuous material belt, with an aspect ratio of 1.5 to 2.

2. Meanwhile, the pores near the second surface remain nearly spherical due to viscoelastic recovery. Thus, a pore orientation gradient that continuously varies along the thickness direction is superimposed on the pore size gradient.

5. The method according to claim 3, characterized in that, The target average pore sizes d1 and d2 are determined according to the target sound absorption frequency band using the following steps: the finished product thickness is discretized into multiple sub-layers along the thickness direction, and the average pore size of each sub-layer is given by the expression for d(z); each sub-layer is considered as a porous layer satisfying the Johnson-Champoux-Allard equivalent fluid model, and its static flow resistance σ satisfies σ=A·μ / d with the average pore size d of that sub-layer. 2 , where μ is the aerodynamic viscosity and A is a structural constant related to the open-cell foam skeleton determined by calibration; the sound absorption coefficient-frequency curve of the material under rigid backing is calculated by the transfer matrix method, and the optimal pore size gradient curve containing the target average pore size d1 and d2 is obtained under the constraint of the maximum average sound absorption coefficient in the target sound absorption frequency band and the constraint of monotonically changing pore size along the thickness.

6. The method according to claim 3, characterized in that, The calibration is performed as follows: under several known temperature differences, microwave volumetric power density, cooling heat transfer coefficient and operating speed conditions, test samples are prepared, the samples are sliced ​​along the thickness direction and the average pore size at each depth position is statistically analyzed, and the growth index n and the characteristic attenuation length δ under each set of conditions are obtained by fitting the expression of d(z), thereby establishing the correspondence between n and δ and process parameters.

7. The method according to claim 1, characterized in that, It also includes online detection of the aperture gradient distribution in the thickness direction of the continuous material strip, and adjustment of the temperature difference between the first surface and the second surface and / or the power of the microwave radiation based on the detection results.

8. A gradient-pore melamine sound-absorbing foam, characterized in that, The material is composed of a single, continuous melamine-formaldehyde resin open-cell foam matrix. The matrix has a first surface and a second surface opposite each other along the thickness direction. The foam cells of the matrix do not contain any filling second phase material. The matrix has a continuous and monotonically varying pore size gradient along the thickness direction. The average pore size near the first surface is smaller than the average pore size near the second surface, and the average pore size near the second surface is 2 to 15 times the average pore size near the first surface. The first surface and the second surface are part of the same continuous matrix, and there are no interlayer bonding interfaces or identifiable phase interfaces. The open-cell ratio of the matrix is ​​not less than 95%. The area near the first surface refers to the surface region extending along the thickness direction from the first surface, with a thickness that is the smaller of 20% and 5 mm of the total thickness of the finished product. The area near the second surface refers to the surface region extending along the thickness direction from the second surface, with a thickness that is the smaller of 20% and 5 mm of the total thickness of the finished product.

9. The gradient-pore melamine sound-absorbing foam according to claim 8, characterized in that, The substrate also has a continuously varying cell orientation gradient along the thickness direction: the cells near the first surface are stretched and oriented in a direction parallel to the substrate plate surface, with an aspect ratio of 1.5 to 2.2, and the cells near the second surface have an aspect ratio close to 1.

0.

10. The gradient-pore melamine sound-absorbing foam according to claim 8 or 9, characterized in that, The sound-absorbing foam has a sound absorption coefficient of not less than 0.7 in the 2-8kHz frequency band.

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