Preparation method of polymer-based foam with bimodal cellular structure, material prepared from polymer-based foam and application of polymer-based foam

Abstract

The invention discloses a preparation method and application of polymer-based foam with a bimodal cellular structure, and relates to the field of polymer foaming materials. A supercritical fluid foaming technology is mainly adopted, after a polymer or a polymer composite material is formed, controllable preparation of a double-peak foam structure is achieved through a two-step pressure relief process, and the key of two-step pressure relief comprises but is not limited to control over procedures such as the first-time pressure relief speed, the first-time pressure relief amount and the second-time pressure relief speed. The preparation method disclosed by the invention is suitable for preparing various polymer-based foams with the double-peak foam structures, and the obtained polymer-based foams with the double-peak foam structures have enhanced mechanical properties and customized functional application, and have guiding significance for expanding the application of foaming materials.

Description

Technical Field

[0001] The present invention relates to the technical field of polymer materials, and particularly relates to a preparation method of a polymer-based foam with a bimodal cell structure, a foam prepared therefrom, and applications thereof. Background Art

[0002] Polymer foam materials are widely used in fields such as aerospace, automotive manufacturing, and biomedicine due to their light weight and high specific strength. With the continuous increase in the demand for lightweight structural materials, high-performance, multifunctional, and customizable microcellular foaming materials have become a research hotspot. However, the bubbles inside the foam materials result in their performance generally being lower than that of the same type of solid polymer materials, especially in terms of mechanical properties, thermal stability, and functional extensibility. The performance of polymer foams is mainly determined by the properties of their matrix materials and cell structures. To improve the performance of foams, composite foams are usually prepared by introducing inorganic fillers (such as carbon nanotubes, graphene, silica, etc.) to enhance their mechanical strength and improve functions such as thermal conductivity and electromagnetic shielding. The supercritical fluid foaming technology plays an important role in foam preparation. Currently, this technology is mostly used to prepare foams with a uniform single-size cell structure. However, the research on polymer composite foams with a bimodal cell structure is still relatively scarce. The bimodal cell structure includes two different sizes of cells, combining the advantages of low density of large pores and reduced dielectric loss, as well as the characteristics of small pores to enhance the strength and heat insulation performance of the material. Therefore, the bimodal cell structure significantly improves the overall performance of foam materials and provides broad space for the functional expansion of foam materials in multiple fields.

[0003] Although there are already some methods for preparing bimodal foams, the existing technologies still have the following limitations: First, limitations in the preparation method. The existing technologies mainly rely on the double blowing agent method (such as fluorine-containing physical blowing agents or chemical blowing agents), but these methods have environmental pollution problems. In addition, although the polymer blending method can achieve a bimodal cell structure, it is difficult to precisely control the cell size due to the complex phase blending interface. Second, poor universality of the method. Most of the existing methods are only applicable to specific polymers. For example, the blending method requires the use of two incompatible polymers, while the temperature-pressure synergistic method has high requirements for the gas solubility of polymers. Third, insufficient research on composite modification. Currently, bimodal foam materials are mainly limited to a single polymer system, and there is little research on bimodal foams with inorganic nano-filler composites, resulting in the underdevelopment of the functional applications of polymer composite foams with a bimodal cell structure. Summary of the Invention

[0004] The present invention provides a method for preparing a polymer-based foam material with a bimodal cell structure, aiming to solve the problems in the prior art such as limited preparation methods for bimodal cell structures, poor universality, and insufficient research on composite modification. By using supercritical fluid foaming technology combined with a two-step pressure relief process, a polymer-based composite foam material with a bimodal cell structure is successfully prepared, and the size and number of large and small cells can be adjusted by controlling the addition amount of nano-fillers and foaming conditions, endowing the foam with functionality and enhancing its application value in various fields.

[0005] The present invention is achieved through the following scheme:

[0006] A method for preparing a polymer-based foam with a bimodal cell structure, comprising the following steps:

[0007] S1. Molding the polymer or polymer composite; S2. Immersing the molded polymer or polymer composite in a supercritical inert gas fluid, with an immersion pressure of 10 - 60 MPa; S3. Releasing 2 - 52 MPa of pressure within 1 - 3600 seconds and maintaining the pressure within the range of 8 - 58 MPa; S4. Holding the pressure and standing still; S5. Releasing the pressure to below 0.2 MPa within 20 seconds and then cooling and shaping to obtain a polymer-based foam with a bimodal cell structure.

[0008] The molding in S1 can be at least one of melt molding and hot pressing molding, etc.

[0009] In the step S1, if the material is a polymer composite, it includes 0.1 - 25 wt% of nano-fillers and 75 - 99.9 wt% of the polymer.

[0010] The polymer composite can be commercially available or obtained by melt blending or solution blending methods. The preparation process of the solution blending method is not limited to: first, all nano-fillers are solution-blended with a part of the polymer, and then melt-blended with the remaining polymer. Specifically, after solution-blending the polymer and nano-fillers to obtain a polymer masterbatch, the mass fraction of nano-fillers in the polymer composite masterbatch is 15 - 50 wt%, and then the polymer composite masterbatch is melt-blended with the polymer to obtain a polymer composite, and the mass fraction of nano-fillers in the polymer composite is 0.1 - 25 wt%.

[0011] Solution blending is a conventional process in the art, and the specific process is not limited to: dissolving the polymer in a solvent to obtain a polymer solution, dispersing the nano-fillers in the solution under an ultrasonic bath to obtain a nano-filler dispersion; mixing and stirring the polymer solution and the nano-filler dispersion for 1 - 24 hours, then pouring into a mold and heating to 60 - 100 °C to volatilize and shape the solvent to obtain a polymer masterbatch; the solvent is selected from at least one of dichloromethane, chloroform, N,N-dimethylformamide, hexafluoroisopropanol, and benzene.

[0012] The supercritical inert gas fluid described above is selected from one or a mixture of supercritical carbon dioxide fluid and supercritical nitrogen fluid.

[0013] Specifically, the polymer includes but is not limited to polystyrene (PS, glass transition temperature is 90 - 105 °C), high impact polystyrene (HIPS, glass transition temperature is 90 - 105 °C), polymethyl methacrylate (PMMA, glass transition temperature is 105 - 115 °C), polycarbonate (PC, glass transition temperature is 140 - 150 °C), polyvinyl chloride (PVC, glass transition temperature is 75 - 90 °C), acrylonitrile - butadiene - styrene copolymer (ABS, glass transition temperature is 105 - 115 °C), polyvinyl alcohol (PVA, glass transition temperature is 60 - 85 °C), polyurethane (PU, melting point temperature is 120 - 180 °C), etc.

[0014] The melt blending step in step S1 is a conventional process, and a micro twin - screw extruder can be selected, with a rotation speed of 80 - 120 rpm. The temperature is set according to the type of polymer: polystyrene, polymethyl methacrylate, polyvinyl alcohol: 190 - 210 °C; acrylonitrile - butadiene - styrene copolymer: 200 - 240 °C; polyvinyl chloride: 170 - 200 °C; polycarbonate: 260 - 290 °C.

[0015] The inorganic nano - filler described above includes at least one of carbon nanotubes, iron tetroxide, graphene, silica, titanium dioxide, boron nitride, carbon black, or clay.

[0016] In step S2, during the infiltration process, the temperature range is 50 - 150 °C and the pressure holding time is 0.5 - 12 hours, which can reduce cell coalescence.

[0017] Specifically, in step S3, a piston pump (model can be Teledyne ISCO 500D) is selected, which can precisely control the flow of supercritical CO2 and is the key to controlling the pressure reduction rate.

[0018] Preferably, in step S4, the temperature range for static holding under pressure is 50 - 140 °C and the time is 1 - 480 minutes. Conducting the first cell growth under these conditions can effectively make the cell size more uniform.

[0019] In step S5, it is required to release the pressure to below 0.2 MPa, or it can also be directly released to atmospheric pressure, and the atmospheric pressure range can be 0.091 - 0.105 MPa.

[0020] The cooling and shaping can be carried out by cooling in cold water to a temperature below the melting point temperature of the polymer (preferably below the glass transition temperature), and then keeping it warm and static until it is shaped. The temperature of the cold water is 0-5°C.

[0021] Through the process of the present invention, the cell diameter can be controlled within the range of 1-600 μm. Preferably, the polymer-based foam with a bimodal cell structure prepared by the present invention has two sizes of cells. The cells larger than the average diameter are defined as large cells, and the cells smaller than the average diameter are defined as small cells.

[0022] Preferably, the cell density ratio of the small / large cells of the polymer-based foam with a bimodal cell structure is 1000-100000000:1, preferably 1000-1000000:1.

[0023] The density of the polymer-based foam with a bimodal cell structure prepared by the present invention is 0.05-0.9 g / cm 3 .

[0024] The polymer-based foam with a bimodal cell structure prepared by the present invention may contain enhanced nano-fillers, which improve the performance of the foam and endow the foam with functionality.

[0025] The composite foam with a bimodal cell structure prepared by the present invention has the functionality imparted by the nano-fillers and can be widely used in the field of electronic materials, as a conductive or electromagnetic shielding material for 5G communication equipment, a building thermal insulation material, for the thermal insulation layer and buffer protection of energy-saving buildings, or as a lightweight shock-absorbing material for the aerospace and automotive industries.

[0026] It can be seen that in the polymer-based foam material with a bimodal cell structure prepared by the above process, the mass fraction of the nano-fillers is 0.1-25 wt%, the mass fraction of the polymer is 75-99.9 wt%, the density is 0.1-0.8 g / cm 3 , and the cell density ratio of the small and large cells is 1000-100000000:1.

[0027] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0028] 1. By controlling conditions such as the pressure relief speed of the first pressure relief (i.e., the pressure relief duration of S3), the intermediate pressure magnitude, the balance time of the intermediate pressure (i.e., the pressure holding and static time of S4), and the second pressure relief speed (i.e., the pressure relief duration of S5), the present invention can prepare a polymer-based foam with a bimodal cell structure, and the size and quantity ratio of the large and small cells can be precisely regulated according to key factors such as the foaming process.

[0029] 2. The materials of the present invention have a wide applicable system and strong versatility. They can be used for a variety of polymers and nano-fillers.

[0030] 3. The bimodal cell structure polymer-based foam obtained by the present invention optimizes the performance of the foam material and endows the foam with better functionality. Especially when applied to electromagnetic shielding foam materials, the specific small / large cell density ratio bimodal foam of the present invention has better electromagnetic shielding performance. Description of the Drawings

[0031] Figure 1 : Scanning electron microscope (SEM) image of the polystyrene / carbon nanotube foam with a bimodal cell structure obtained in Example 1.

[0032] Figure 2 : SEM image of the polystyrene / carbon nanotube foam with a bimodal cell structure obtained in Example 2.

[0033] Figure 3 : SEM image of the polystyrene / carbon nanotube foam with a bimodal cell structure obtained in Example 3.

[0034] Figure 4 : SEM image of the polystyrene / carbon nanotube foam with a bimodal cell structure obtained in Example 4.

[0035] Figure 5 : SEM image of the polycarbonate / silica foam with a bimodal cell structure obtained in Example 5.

[0036] Figure 6 : SEM image of the polycarbonate / silica foam with a bimodal cell structure obtained in Example 6. Figure 7 : SEM image of the polymethyl methacrylate / graphene foam with a bimodal cell structure obtained in Example 7.

[0037] Figure 8 : SEM image of the polystyrene / carbon nanotube foam with a unimodal cell structure obtained in Comparative Example 1. Detailed Description of the Invention

[0038] The present invention will be described in detail below with reference to specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any form. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can be made. These all belong to the protection scope of the present invention.

[0039] The sources of the raw materials used in the present invention are as follows:

[0040] Polystyrene: Yangzi BASF 158K;

[0041] Polycarbonate: Covestro 6555;

[0042] Polymethyl methacrylate: Chi Mei MC-207 from Taiwan;

[0043] Carbon nanotubes: Nanocyl 7000 (average diameter 9.5 nm, average length 1.5 μm, carbon nanotube content > 90%);

[0044] Silica: Xianfeng Nano (average size: ~100 nm);

[0045] Graphene: Suiheng - multi-layer graphene (>95% purity);

[0046] Micro screw extruder: Model Xplore MC 15-HT.

[0047] High-pressure reactor: Customized, equipped with a Teledyne ISCO piston pump.

[0048] Each test method:

[0049] (1) Density: The density of the foam sample was measured using an electronic densitometer (DH-300, DahoMeter). The measurement principle is to obtain the volume according to Archimedes' drainage method and calculate it together with the weighed mass.

[0050] (2) Cell size, density, distribution, large / small cell density ratio: The cell information of the foam was obtained by counting from SEM images. Specifically: The cell size and cell density of the sample were estimated by Image J software. At least 50 - 100 cells of each foam were counted to obtain the average cell diameter.

[0051] The cell density was estimated as follows:

[0052]

[0053] n is the number of cells counted in the SEM image, A is the area of the SEM image in cm, ρ is the density measured in (1) above.

[0054] The large / small cell density ratio was obtained from the cell density ratio of large cells and small cells.

[0055] (3) Compression modulus: The cyclic compression test of the foam specimen was carried out on a universal testing machine (34TM-30, Instron). The foam specimen with a density of 0.3 g / cm 3 was trimmed into a cuboid of 10*10*5 mm with a razor blade and subjected to vertical compression testing. The deformation rate was 1 mm / min, and the maximum deformation rate was 50%. The maximum stress of the material was obtained at the maximum strain. The compression Young's modulus was determined by the slope of the compression stress-strain curve in the 10% strain range, where there is a linear relationship between stress and strain.

[0056] (4) Electromagnetic wave absorption and reflection loss: On a vector network analyzer (VNA, KEYSIGHT, PNA-L series N5290A), the S-parameters (S11 and S21) of each sample were collected in waveguide mode at 12.4 - 18.0 GHz (Ku band). The samples were cut and trimmed into a rectangular shape of 15.8×7.9×1.85 mm to precisely fit the waveguide fixture. The reflection shielding effectiveness (SE R ) and the total shielding effectiveness (SE T ) can be determined from the S11 and S21 parameters by the following equations, where A, T, and R are the absorption rate, transmittance, and reflectance of the incident electromagnetic wave, respectively.

[0057]

[0058] SE R = -10 log(1 - R)

[0059] SE T = -10 log(T)

[0060]

[0061] Preparation methods of polymer-based foams in examples and comparative examples: S1. When it is a polymer composite: The polymer (specific selection and content are shown in the table) is dissolved in dichloromethane to obtain a polymer solution, and the nanofiller (specific selection and content are shown in the table) is dispersed in dichloromethane under an ultrasonic bath to obtain a nanofiller dispersion; according to the weight percentage of the polymer and the nanofiller in the table, the polymer solution and the nanofiller dispersion are mixed and stirred for 3 hours, then poured into a Teflon mold, and the solvent is allowed to volatilize and solidify to obtain a polymer masterbatch. Then, the polymer composite masterbatch and the polymer (the ratio is shown in the table) are melt-blended through a micro twin-screw extruder to obtain a diluted polymer composite; the above polymer composite is further heat-treated and pressed into a square sheet (size: 15.8×7.9*2 mm); when it is a polymer: it is directly heat-treated and pressed into a square sheet (size: 15.8×7.9*2 mm). S2. Place the polymer composite sheet in a high-pressure foaming container, maintain a specific temperature (the specific temperature is shown in the table), and introduce supercritical CO2 fluid, and the pressure is shown in the table; S3. Perform the first pressure relief, and release the pressure of the high-pressure foaming container by 2 - 52 MPa within a specific time (the specific time is shown in the table), and keep the pressure within a specific range (the specific pressure is shown in the table); S4. Keep the pressure and stand still for a specific time (the specific time is shown in the table), and the temperature during standing is shown in the table; S5. Perform the second pressure reduction, reduce the pressure of the high-pressure foaming container to atmospheric pressure within a specific time (the specific time is shown in the table), use an ice-water bath to lower the polymer below the glass transition temperature, keep it warm and stand still until it is solidified, and then obtain the polymer-based foam.

[0062] Table 1: Process and Results of Double-peak Cell Polymer-based Foams in Examples and Comparative Examples

[0063]

[0064]

[0065] Continued Table 1:

[0066]

[0067]

[0068] It can be seen from Examples 1 / 2 / 3 / 4 / 8 that by adjusting the process parameters of the present application, the small / large cell density can be regulated, and better electromagnetic wave shielding performance can be achieved when the small / large cell density ratio is 1000 - 100000:1.

[0069] It can be seen from Comparative Example 1 that the polymer-based foam obtained by the single foaming method has only a unimodal distribution of pores and bubbles, resulting in insufficient electromagnetic wave shielding performance.

[0070] Continued Table 1:

[0071]

[0072]

[0073] It can be seen from Examples 1 / 2 / 3 / 4 / 8 and Comparative Example 2 that if the pressure is released too much in Step S3 so that the pressure is lower than 8 MPa, it will cause ineffective secondary foaming and form a unimodal pore and bubble structure, resulting in poor electromagnetic wave shielding performance.

[0074] It can be seen from Examples 1 / 2 / 3 / 4 / 8 and Comparative Example 3 that when the secondary pressure release time is too long, the secondary nucleation and foaming cannot be initiated to form small cell pores, but instead the diameter of the first cell pores will increase, forming a unimodal cell pore distribution, resulting in poor electromagnetic wave shielding performance.

[0075] It can be seen from Examples 1 / 2 / 3 / 4 / 8 and Comparative Example 4 that when the pressure release speed of the first pressure release is too slow (pressure release time 70 min), the first nucleation cannot be induced to form large cell pores, and only a unimodal cell pore distribution can be formed, resulting in poor electromagnetic wave shielding performance.

[0076] Furthermore, it can be seen from the comparison of 4 groups of test examples including Comparative Example 1 / Example 2, Comparative Examples 2 - 4 / Example 3 that the density of the above comparative examples is higher than or close to that of the corresponding examples, but the compression modulus of the examples is significantly higher than that of the comparative examples. It can be seen that at the same foaming ratio, the double-peak foam of the present invention can significantly improve the compression modulus.

Claims

1. A method for preparing a bimodal cell structure polymer-based foam, characterized in that: The following steps are involved: S1. Processing a polymer or polymer composite material into a shape; S2. Infiltrating the formed polymer or polymer composite material in a supercritical inert gas fluid with an infiltration pressure of 10-60 MPa; S3. Relieving the pressure of 2-52 MPa within 1-3600 seconds, and maintaining the pressure within the range of 8-58 MPa; S4. Maintaining the pressure and allowing to stand; S5. Relieving the pressure to less than 0.2 MPa within 20 seconds, and then cooling and shaping to obtain a bimodal pore structure polymer-based foam.

2. The method for preparing a bimodal cell structure polymer-based foam according to claim 1, characterized in that: The polymer composite material includes 0.1-25 wt % of the nanofiller and 75-99.9 wt % of the polymer.

3. The method for preparing a bimodal cell structure polymer-based foam according to claim 2, characterized in that: In the step S1, the forming is at least one of melt forming and hot pressing forming.

4. The method for preparing a bimodal cell structure polymer-based foam according to claim 1, characterized in that: The supercritical inert gas fluid is selected from supercritical carbon dioxide fluid, supercritical nitrogen fluid or a mixture of the two; the polymer includes at least one of polystyrene, polymethyl methacrylate, polycarbonate, polyvinyl chloride, acrylonitrile-butadiene-styrene copolymer and polyurethane; the inorganic nanofiller includes at least one of carbon nanotubes, carbon black, ferroferric oxide, graphene, silicon dioxide, titanium dioxide, boron nitride or clay.

5. The method for preparing a bimodal cell structure polymer-based foam according to claim 1, characterized in that: In the step S2, the temperature range during the infiltration process is 50-150° C. and the holding time is 0.5-12 hours.

6. The method for preparing a bimodal cell structure polymer-based foam according to claim 1, characterized in that: In the step S4, the pressure is maintained at a temperature range of 50-140° C. for a time of 1-480 minutes.

7. The method for preparing a bimodal cell structure polymer-based foam according to claim 1, characterized in that: In the step S5, the cooling and shaping is specifically cooling in cold water to a temperature below the melting point of the polymer, preferably below the glass transition temperature, and then keeping the temperature to stand until shaping, and the temperature of the cold water is 0-10°C.

8. A bimodal cellular structure polymer-based foam prepared by the preparation method described in any one of claims 1 to 7.

9. A bimodal cell structure polymer-based foam according to claim 8, characterized in that: The small / large cell density ratio is 1000-100000000:1, preferably 1000-1000000:

1.

10. The use of the bimodal cell structure polymer-based foam according to claim 8, characterized in that: Used for conductive or electromagnetic shielding materials, electromagnetic wave absorption, rebound materials, thermal insulation materials, buffer protection materials or lightweight shock absorbing materials.

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