Epoxy resin foam with stepped foam structure and preparation method

Abstract

The invention discloses epoxy resin foam with a stepped foam structure and a preparation method thereof. The preparation method comprises the following steps: step 1, mixing epoxy resin and a curing agent, melting and uniformly stirring to obtain a mixed solution; 2, supercritical carbon dioxide is injected into the mixed solution obtained in the step 1, preparation parameters are controlled, the adsorption concentration of the carbon dioxide in the mixed solution meets the set requirement, pressure is quickly relieved after adsorption is completed, and the epoxy resin foam with the stepped foam structure can be obtained after curing; epoxy resin foam with a stepped foam structure is obtained through a supercritical carbon dioxide one-step method and by controlling the adsorption concentration of carbon dioxide in a matrix, the distribution of a nucleating agent in the matrix and the distribution of a temperature field, the wave absorbing performance of the material can be improved, and the sound insulation performance and the impact resistance of the material can also be improved; the preparation method is simple, and the preparation efficiency is improved compared with an existing two-step method.

Description

Technical Field

[0001] The present invention relates to the technical field of foam materials, and particularly to an epoxy resin foam with a stepped pore structure and a preparation method thereof. Background Art

[0002] With the continuous development of technology, the problem of electromagnetic wave pollution has become increasingly serious, posing a potential threat to people's lives and health. Developing efficient electromagnetic wave absorbing materials has become an urgent task. Traditional electromagnetic wave absorbing materials have many defects, such as high density, narrow absorption frequency band, etc., which limit their application scope. Epoxy resin, as a high-performance thermosetting resin, has excellent mechanical properties and chemical corrosion resistance, but it does not have electromagnetic wave absorbing properties itself. Epoxy resin foam can improve its electromagnetic wave absorbing properties to a certain extent, but it is also relatively limited.

[0003] In the field of preparing thermosetting electromagnetic wave absorbing foam materials, traditional methods mostly adopt a two-step method: first, the polymer is pre-cured to form a preform with a certain strength, then it is placed in a supercritical autoclave for gas adsorption, and finally, the foam is formed by rapidly heating in an oil bath after depressurization. Although this two-step method can prepare foam materials with electromagnetic wave absorbing properties, it also has some obvious deficiencies. On the one hand, the pre-curing process requires additional equipment and process control, increasing the production cost and operation complexity. On the other hand, during the gas adsorption of the pre-cured material in the supercritical autoclave and the subsequent heating and foaming process, the problem of uneven gas distribution may occur, resulting in an uneven pore structure of the final material, affecting its electromagnetic wave absorbing properties and mechanical properties. In addition, the production cycle of the two-step method is long, which is not conducive to large-scale production applications.

[0004] Existing preparation methods such as CN113861490A, a method for preparing epoxy microcellular foam based on liquid epoxy resin, disclose a method for preparing epoxy resin foam by the supercritical carbon dioxide method. Although this method is also prepared by a one-step method, the pores obtained are of a uniform morphological structure and do not form a stepped pore structure. Summary of the Invention

[0005] The present invention provides an epoxy resin foam with a stepped pore structure and a preparation method thereof in view of the problems existing in the prior art.

[0006] The technical solution adopted by the present invention is: a preparation method of an epoxy resin foam with a stepped pore structure, comprising the following steps:

[0007] Step 1: Mix epoxy resin and a curing agent, and obtain a mixed solution after melting and stirring evenly;

[0008] Step 2: Inject supercritical carbon dioxide into the mixture obtained in Step 1, control the adsorption conditions to make the adsorption concentration of carbon dioxide in the mixture meet the set requirements, quickly release the pressure after adsorption, and the required epoxy resin foam with a stepped cell structure can be obtained after curing.

[0009] Further, in Step 1, the curing agent is 4,4`-diaminodiphenylmethane, and the mass ratio of epoxy resin to curing agent is 4:1.

[0010] Further, in Step 1, the melting temperature is 50 - 60 °C, the stirring speed is 200 - 400 rpm, and the stirring time is 2 - 10 min.

[0011] Further, in Step 2, the adsorption conditions are: pressure 13 - 18 MPa, temperature 100 - 130 °C, and the pressure holding time is 10 - 40 min.

[0012] Further, in Step 2, the curing temperature is 160 - 180 °C, and the time is 2 - 4 h.

[0013] Further, the mixture in Step 1 further includes a nucleating agent.

[0014] Further, the nucleating agent is carbon nanotubes.

[0015] Further, the nucleating agent accounts for 2.5% of the mass of the epoxy resin.

[0016] Further, in Step 2, the mixture is contained in a gradient heat insulation device.

[0017] An epoxy resin foam with a stepped cell structure, the cell diameter of the epoxy resin foam is 1 - 80 μm, and it has a gradually changing cell structure from the surface to the inside.

[0018] The beneficial effects of the present invention are:

[0019] (1) The present invention prepares an epoxy resin foam with a stepped cell structure by a one-step method of supercritical carbon dioxide, which can not only improve the wave absorption performance of the material, but also improve the sound insulation performance and impact resistance of the material;

[0020] (2) The preparation method of the present invention is simple, and the preparation efficiency is improved compared with the existing two-step method. Description of the Drawings

[0021] Figure 1 It is the SEM image of the epoxy resin foam obtained in Example 1 of the present invention.

[0022] Figure 2 It is the SEM image of the epoxy resin foam obtained in Example 2 of the present invention.

[0023] Figure 3 This is the thermogravimetric analysis (TGA) chart of the epoxy resin foam obtained in Example 1 and Example 2 of the present invention.

[0024] Figure 4 This is the test result of the wave absorption performance of the epoxy resin obtained in Example 2 of the present invention. Detailed implementation manners

[0025] The present invention will be further described below in conjunction with the accompanying drawings and specific implementation manners.

[0026] A preparation method of an epoxy resin foam with a stepped cell structure includes the following steps:

[0027] Step 1: Mix epoxy resin and a curing agent, and obtain a mixed solution after melting and stirring evenly; the curing agent is 4,4`-diaminodiphenylmethane, and the mass ratio of epoxy resin to the curing agent is 4:1. The melting temperature is 50 - 60 °C, the stirring speed is 200 - 400 rpm, and the stirring time is 2 - 10 min. It may also include a nucleating agent (carbon nanotube), and the nucleating agent accounts for 2.5% of the mass of the epoxy resin.

[0028] Step 2: Inject supercritical carbon dioxide into the mixed solution obtained in Step 1, control the adsorption conditions (control the pressure at 13 - 18 MPa, the temperature at 100 - 130 °C, and the pressure holding time at 10 - 40 min), so that the adsorption concentration of carbon dioxide in the mixed solution meets the set requirements. After adsorption is completed, quickly release the pressure. After curing, the required epoxy resin foam with a stepped cell structure can be obtained.

[0029] The preparation method of supercritical carbon dioxide specifically includes the following three cases:

[0030] 1) Pour the epoxy resin mixed solution into a mold, place it in a supercritical autoclave, and inject carbon dioxide fluid. Since supercritical carbon dioxide dissolves into the matrix gradually from the surface, control the pressure at 13 - 18 MPa, the temperature at 100 - 130 °C, and the pressure holding time at 10 - 40 min, control the gas adsorption amount to achieve different gas adsorption amounts in the upper and lower layers, and quickly release the pressure; at a temperature of 160 - 180 °C and a curing time of 2 - 4 h, the required epoxy resin foam with a stepped cell structure can be obtained.

[0031] 2) A nucleating agent can also be added, and the method is as follows: Pour the epoxy resin mixture (the mixture added with the nucleating agent) into a mold and place it in a supercritical autoclave. Because of the different densities of the nucleating agent, the filler with a larger density will deposit towards the bottom due to the lower viscosity of the mixture. Inject carbon dioxide fluid. Since supercritical carbon dioxide penetrates into the matrix gradually from the surface, control the pressure at 13 - 18 MPa, the temperature at 100 - 130 °C, and the pressure holding time at 10 - 40 min to control the gas adsorption amount. Because of the heterogeneous nucleation points during the supercritical carbon dioxide foaming process, quickly release the pressure. At a temperature of 160 - 180 °C and a curing time of 2 - 4 h, the desired epoxy resin foam with a stepped cell structure can be obtained.

[0032] 3) A gradient temperature field can also be formed, and the method is as follows: Pour the epoxy resin mixture into a mold that has been subjected to gradient heat insulation treatment (the gradient heat insulation treatment is set according to the actual situation and can be set to three layers with gradually decreasing heat insulation effects from the outside to the inside, which can be specifically set according to the actual situation), and place it in a supercritical autoclave. Because of the differences in the heat conduction and heat insulation properties of the mold, the polymer in the entire reaction kettle is unevenly heated. Inject carbon dioxide fluid. Since supercritical carbon dioxide penetrates into the matrix gradually from the surface, control the pressure at 13 - 18 MPa, the temperature at 100 - 130 °C, and the pressure holding time at 10 - 40 min to control the gas adsorption amount so as to achieve different gas adsorption amounts in the upper and lower layers. Because the melt strength during the supercritical carbon dioxide foaming process has a direct relationship with the temperature, and different bubble cell morphologies are shown due to the differences in the melt strength at different temperatures, quickly release the pressure. At a temperature of 160 - 180 °C and a curing time of 2 - 4 h, the desired epoxy resin foam with a stepped cell structure can be obtained.

[0033] Example 1

[0034] A method for preparing an epoxy resin foam with a stepped cell structure includes the following steps:

[0035] Step 1: Mix 4 g of epoxy resin E51 and 1 g of 4,4`-diaminodiphenylmethane, stir at 60 °C for 5 min with a stirring speed of 300 rpm to obtain a mixture.

[0036] Step 2: Pour the mixture obtained in Step 1 into a mold, place it in a supercritical autoclave, inject supercritical carbon dioxide, keep it warm at 110 °C and 16 MPa for 26 min. After heat preservation and pressure holding, quickly release the pressure. After cooling, cure it at 180 °C for 3 h, and the desired epoxy resin foam with a stepped cell structure can be obtained.

[0037] Figure 1This is the scanning electron microscope image of the epoxy resin foam obtained in Example 1 of the present invention. It can be seen from the figure that the foam has a closed-cell structure, and the sizes of the left and right pores are inconsistent. The pore diameters in the left half are evenly distributed in the range of 40 - 50 μm, and the pore diameters in the right half are evenly distributed in the range of 70 - 80 μm. The high proportion of micron-sized closed pores provides stable mechanical support and multiple electromagnetic wave scattering paths for the material.

[0038] Example 2

[0039] A preparation method of an epoxy resin foam with a stepped pore structure includes the following steps:

[0040] Step 1: Put 4 g of epoxy resin E51 and 0.102 g of carbon nanotubes into a beaker, stir at 50 °C for 10 min, and the stirring speed is 200 rpm.

[0041] Add 1 g of 4,4`-diaminodiphenylmethane and mix, stir at 50 °C for 10 min, and the stirring speed is 200 rpm to obtain a mixed solution.

[0042] Step 2: Pour the mixed solution obtained in Step 1 into a mold, place it in a supercritical autoclave, inject supercritical carbon dioxide, keep it warm at 130 °C and 13 MPa for 40 min. After heat preservation and pressure holding, quickly release the pressure. After cooling, cure at 160 °C for 4 h to obtain the required epoxy resin foam with a stepped pore structure.

[0043] Figure 2 This is the SEM image of the epoxy resin foam obtained in this example. It can be seen from the figure that the foam has a closed-cell structure. The pore diameters in the upper half are evenly distributed in the range of 60 - 70 μm, and the pore diameters in the lower half are evenly distributed in the range of 10 - 20 μm. The uniform filling property can still be maintained after introducing the nucleating agent.

[0044] Example 3

[0045] A preparation method of an epoxy resin foam with a stepped pore structure includes the following steps:

[0046] Step 1: Mix 4 g of epoxy resin E51 and 1 g of 4,4`-diaminodiphenylmethane, stir at 55 °C for 2 min, and the stirring speed is 200 rpm to obtain a mixed solution.

[0047] Step 2: Pour the mixed solution obtained in Step 1 into a mold with gradient heat insulation, place it in a supercritical autoclave, inject supercritical carbon dioxide, keep it warm at 100 °C and 18 MPa for 40 min. After heat preservation and pressure holding, quickly release the pressure. After cooling, cure at 170 °C for 2 h to obtain the required epoxy resin foam with a stepped pore structure.

[0048] Figure 3For the thermal stability of epoxy resin and its composite with carbon nanotubes in the same proportion. As can be seen from the figure, the weight of pure epoxy resin remains relatively stable before the temperature rises to about 300 °C, and then begins to decrease significantly, indicating the start of its thermal decomposition. Between about 300 °C and 500 °C, the weight decreases rapidly, indicating that thermal decomposition occurs in this temperature range.

[0049] Epoxy resin + 2.5% carbon nanotubes begins to show significant weight loss at a higher temperature (about 350 °C), showing better thermal stability. Between about 350 °C and 600 °C, the weight gradually decreases, but the overall weight loss rate is slower than that of pure epoxy resin, indicating that the addition of carbon nanotubes improves the thermal stability of the material. This improved thermal stability makes the epoxy resin / carbon nanotube composite have a wider application prospect in high-temperature environments (such as aerospace, electronic equipment, etc.).

[0050] Figure 4 It is the radar wave absorption performance diagram of the epoxy resin foam obtained in Example 2 at a thickness of 3.1 mm in the X-band. As can be seen from the figure, there is full absorption in the X-band, and the maximum reflection loss is -23 dB.

[0051] The present invention provides a method for preparing a radar wave absorption foam by a one-step method in view of the two-step method in the preparation of traditional radar wave absorption foams. The liquid mixture is directly put into a supercritical autoclave, kept at a certain temperature and pressure to cure it to a certain extent, and then the pressure is quickly released to foam. This method not only simplifies the process flow, reduces the operation complexity and production cost, but also can obtain a specific foam with a stepped cell structure by setting special preparation process parameters.

[0052] After adding carbon nanotubes, the carbon nanotubes are more evenly dispersed in the epoxy resin, forming a finer and more uniform cell structure, thus significantly improving the radar wave absorption performance and mechanical properties of the material.

[0053] The design of the cell structure has a crucial impact on the performance of the material. Traditional cell materials usually have a uniform or randomly distributed cell structure, but this structure does not meet the requirements of some specific application scenarios. The stepped cell structure obtained in the present invention presents a stepped structure, the pore size changes from 10 μm to 80 μm, and the preparation method of the present invention has a simple and environmentally friendly process, directly foams in one step, and can also introduce electromagnetic functional materials into the foam components to realize the integration of the structure and function of the foam. Applying the supercritical fluid foaming technology to thermosetting materials and realizing stepped cells have never been mentioned in the prior art.

Claims

1. A method for preparing epoxy resin foam having a stepped cell structure, characterized in that: The following steps are involved: Step 1: Mix the epoxy resin and the curing agent, and melt and stir them to obtain a mixed solution; Step 2: inject supercritical carbon dioxide into the mixed liquid obtained in step 1, control the adsorption conditions so that the adsorption concentration of carbon dioxide in the mixed liquid meets the set requirements, release the pressure quickly after the adsorption is completed, and obtain the desired epoxy resin foam with a stepped pore structure after curing.

2. The method for preparing an epoxy resin foam having a stepped cell structure according to claim 1, characterized in that: In the step 1, the curing agent is 4,4'-diaminodiphenylmethane, and the mass ratio of the epoxy resin to the curing agent is 4:

1.

3. The method for preparing an epoxy resin foam having a stepped cell structure according to claim 1, characterized in that: In step 1, the melting temperature is 50-60° C., the stirring speed is 200-400 rpm, and the stirring time is 2-10 min.

4. The method for preparing an epoxy resin foam having a stepped cell structure according to claim 1, characterized in that: The adsorption conditions in step 2 are: pressure 13-18 MPa, temperature 100-130° C., and pressure holding time 10-40 min.

5. The method for preparing an epoxy resin foam having a stepped cell structure according to claim 1, characterized in that: In step 2, the curing temperature is 160-180° C. and the curing time is 2-4 hours.

6. The method for preparing an epoxy resin foam having a stepped cell structure according to claim 1, characterized in that: The mixed solution in step 1 also includes a nucleating agent.

7. The method for preparing an epoxy resin foam having a stepped cell structure according to claim 6, characterized in that: The nucleating agent is carbon nanotube.

8. The method for preparing an epoxy resin foam having a stepped cell structure according to claim 6, characterized in that: The nucleating agent accounts for 2.5% of the epoxy resin mass.

9. The method for preparing an epoxy resin foam having a stepped cell structure according to claim 1, characterized in that: In step 2, the mixed liquid is contained in a gradient insulation device.

10. The epoxy resin foam having a stepped cell structure obtained by the preparation method according to any one of claims 1 to 9, characterized in that: The diameter of the pores of the epoxy resin foam is 1 to 80 μm, and the pore structure changes gradually from the surface to the inside.

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