A method for preparing a thermally self-expanding epoxy foam having electromagnetic shielding properties

Abstract

The invention discloses a method for preparing a heat-expanding epoxy foam with electromagnetic shielding properties. In the process of processing the heat-expanding epoxy foam with epoxy resin as a substrate, before curing and foaming treatment, a conductive filler with heat-expanding microspheres grafted on the surface is added to the system. After the processing is completed, the heat-expanding epoxy foam with electromagnetic shielding properties is obtained; the minimum foaming temperature of the heat-expanding microspheres is 110-120°C, and the maximum foaming temperature is 140-150°C; the temperature of the curing and foaming treatment is 15-25°C higher than the minimum foaming temperature of the heat-expanding microspheres; during the curing and foaming treatment, the viscosity of the epoxy resin is less than 1Pa·s. The method of the invention solves the problem that the conductive filler is difficult to disperse in the resin matrix innovatively by introducing heat-expanding microspheres on the surface of the conductive filler. The obtained heat-expanding epoxy foam with electromagnetic shielding properties has good electromagnetic shielding performance and high use value and economic benefits.

Description

Technical Field

[0001] The invention belongs to the field of electromagnetic shielding composite materials, and in particular relates to a method for preparing a thermal self-expanding epoxy foam with electromagnetic shielding properties. Background Art

[0002] With the rapid development of communication technology, electronic components and communication equipment have been greatly popularized and applied. The widespread use of these electronic devices has brought great convenience to people on the one hand, but also generated a large amount of electromagnetic wave pollution on the other hand, which not only seriously interfered with the normal operation of mobile phones, computers, drones, aircraft and other equipment, but also posed an unignorable threat to human health and safety. To this end, a variety of electromagnetic shielding composite materials have been developed, but most electromagnetic shielding materials face the disadvantages of low shielding efficiency or high cost of shielding materials for large-scale application. At the same time, emerging fields such as 5G communication technology, high-power electronic equipment, and precision electronic components on aircraft and ships not only require high anti-interference capabilities of signals, but also require lightweight electromagnetic shielding composite materials and save energy consumption during use. In the future, lightweight electromagnetic shielding composite materials with both high shielding efficiency and high absorption capacity will inevitably be in great demand.

[0003] For traditional electromagnetic shielding composite materials, the shielding effectiveness is determined by the conductivity of the material and the degree of perfection of the conductive network, which in turn depends on the conductive properties and addition amount of the filler. In order to obtain good conductive properties, more conductive fillers (such as metal fibers, carbon nanotubes, graphene, carbon black, etc.) are required. However, the addition of a large amount of conductive fillers will not only cause serious agglomeration inside the composite material, similar to the "sea-island" structure, but will not significantly improve the conductive network construction of the material, and a large number of "sea-island" structures will cause the mechanical properties of the material to decay seriously. Therefore, in order to improve the electromagnetic shielding performance of the material at present, on the one hand, it is necessary to increase the conductivity value of the filler itself, and on the other hand, it is necessary to solve the problem of the dispersion of the filler inside the material. The latter is the focus of current research.

[0004] In order to solve the above technical problems, patent CN115181340A discloses an electromagnetic shielding natural rubber and its preparation for effectively constructing a three-dimensional conductive network structure. The patent first forms a polydopamine functional layer on the skeleton surface of the foam template through self-polymerization of dopamine, and then immerses it in a graphene oxide dispersion. Through hydrogen bonds and electrostatic interactions, a dense graphene oxide layer is wrapped on the surface of the foam template, the foam template is carbonized, and the graphene oxide is reduced at the same time, and then backfilled with natural rubber, and finally a three-dimensional conductive network structure is formed in the matrix. This method effectively improves the dispersibility of graphene oxide, thereby enhancing the conductivity and electromagnetic shielding performance of the natural rubber composite material, while maintaining good mechanical properties of the material. However, the preparation method is slightly cumbersome and requires high costs. Summary of the invention

[0005] The purpose of the present invention is to solve the problems existing in the prior art and provide a method for preparing a thermal self-expanding epoxy foam with electromagnetic shielding properties.

[0006] To achieve the above object, the scheme adopted by the present invention is as follows:

[0007] A method for preparing a heat-expanding epoxy foam with electromagnetic shielding properties. In the process of processing the heat-expanding epoxy foam with epoxy resin as a base material, before curing and foaming, a conductive filler with heat-expanding microspheres grafted on the surface is added to the system. After the processing is completed, the heat-expanding epoxy foam with electromagnetic shielding properties is obtained.

[0008] The lowest foaming temperature of the heat-expandable microspheres is 110-120°C, and the highest foaming temperature is 140-150°C; the temperature of the curing foaming treatment is 15-25°C higher than the lowest foaming temperature of the heat-expandable microspheres; during the curing foaming treatment, the viscosity of the epoxy resin is below 1Pa·s;

[0009] The conductive filler is a granular structure with an average particle size of less than 500 μm, the number of thermally expandable microspheres grafted on the surface is greater than or equal to 1, and the grafting rate is greater than 50%;

[0010] Alternatively, the conductive filler is a granular structure with an average particle size of 500-800 μm, the number of thermally expandable microspheres grafted on the surface is 2-5, and the grafting rate is above 70%;

[0011] Alternatively, the conductive filler is a fibrous structure with an average length of 1-8 mm, the number of thermally expandable microspheres grafted on the surface is 5-20, and the grafting rate is above 70%.

[0012] The present invention grafts heat-expandable microspheres onto the surface of a conductive filler. After the conductive filler with the heat-expandable microspheres grafted onto the surface is added to an epoxy resin, during a curing and foaming process, the heat-expandable microspheres grafted onto the surface of the conductive filler begin to expand due to heat and become 30-50 times of their own volume. At this time, a relatively large gap is generated between the heat-expandable microspheres. At the same time, the epoxy resin has relatively high fluidity at this stage. On the one hand, the heat-expandable microspheres can easily move in the epoxy resin. On the other hand, the epoxy resin can be filled into the gap between the heat-expandable microspheres, so that the distance between the heat-expandable microspheres is fixed and cannot be restored to an initial state. Since the distance between the heat-expandable microspheres increases, the conductive filler is connected to the heat-expandable microspheres, so that the distance between the conductive filler increases. The conductive filler in the finally obtained thermal self-expanding epoxy foam is relatively evenly distributed, which is conducive to better building a conductive network in the resin matrix and is of great help in improving the electromagnetic shielding performance of the foam.

[0013] Among them, the foaming temperature of the heat-expandable microspheres and the temperature of the curing and foaming treatment jointly determine the volume expansion multiple of the heat-expandable microspheres; the minimum foaming temperature of the heat-expandable microspheres is about 120°C. Below this temperature, the volume expansion multiple of the foaming agent is small and it is difficult to fully expand. The maximum foaming temperature is about 140°C. Above this temperature, the foaming agent may be damaged; the curing and foaming treatment temperature is about 20°C higher than the minimum foaming temperature of the heat-expandable microspheres. Meeting this condition can make the epoxy resin release some heat during the curing process, further increasing the system temperature. High temperature can make the heat-expandable microspheres have more sufficient expansion force and higher volume expansion multiple;

[0014] At the same time, the present invention ensures that the conductive filler is grafted to the heat-expandable microspheres as much as possible, which is more helpful for the dispersion of the conductive filler in the resin matrix at a later stage; for granular conductive fillers, it is necessary to ensure that at least one microsphere is grafted on its surface, and the grafting rate cannot be too low, otherwise the effect of building a conductive network is poor, thereby affecting the electromagnetic shielding performance of the material; for fibrous conductive fillers, it should be ensured that multiple heat-expandable microspheres are grafted to its surface at the same time, so that individual fibers can be better dispersed in the resin matrix to avoid the phenomenon of a large number of fibers being entangled together, which will make the electromagnetic shielding performance of the material unevenly distributed and reduce the mechanical properties of the material.

[0015] As the preferred technical solution:

[0016] The preparation method of the heat-expanding epoxy foam with electromagnetic shielding properties as described above, the preparation process of the conductive filler with heat-expanding microspheres grafted on the surface is: the conductive filler, heat-expanding microspheres, coupling agent, and solvent are evenly mixed for grafting reaction, and the conductive filler with heat-expanding microspheres grafted on the surface is obtained by post-treatment (repeatedly rinsing with deionized water, removing the solvent not involved in the reaction, and drying the product).

[0017] A method for preparing a thermal self-expanding epoxy foam having electromagnetic shielding properties as described above, wherein the conductive filler is one or more of chopped carbon fiber, nickel-plated chopped carbon fiber, carbon fiber cloth, conductive carbon black, graphene, and carbon nanotubes;

[0018] The average particle size of the heat-expandable microspheres is 10-30μm, the outer shell is thermoplastic, and the interior is a hydrocarbon gas that can expand when heated. The initial volume of the heat-expandable microspheres grafted onto the surface of the filler is very small, much smaller than the particle size of the general filler. Therefore, the heat-expandable microspheres can be easily grafted onto the surface of the filler under the action of the coupling agent, and have a high grafting rate.

[0019] The coupling agent is a silane coupling agent (such as methyltrichlorosilane, vinyltrichlorosilane) or a titanate coupling agent (such as tri(2,3-epoxypropyl)propyltrimethoxysilane titanate, di(γ-methacryloyloxy)propyltriisobutoxysilane titanate);

[0020] The solvent is one or more of benzene, toluene, xylene, pentane, dichlorobenzene, dichloromethane, methanol, ethanol and water.

[0021] As described above, in the preparation method of a heat-expanding epoxy foam with electromagnetic shielding properties, the addition amounts of each component in the preparation process of a conductive filler with heat-expanding microspheres grafted on the surface are: 5-15 parts of conductive filler, 5-20 parts of heat-expanding microspheres, 5-12 parts of coupling agent, and 100 parts of solvent; the temperature of the grafting reaction is 40-60°C, and the time is 2-4h.

[0022] In the method for preparing a heat-expanding epoxy foam having electromagnetic shielding properties as described above, the conductive filler is a pretreated conductive filler, and the pretreatment is sequentially performed by washing impurities, strong acid treatment (to increase surface active sites), and drying.

[0023] In the method for preparing a thermal self-expanding epoxy foam having electromagnetic shielding properties as described above, the epoxy resin is a mixture of liquid bisphenol A epoxy resin, solid bisphenol A epoxy resin and novolac epoxy resin 638S.

[0024] In the method for preparing a thermal self-expanding epoxy foam with electromagnetic shielding properties as described above, the liquid bisphenol A epoxy resin is bisphenol A epoxy resin E51, epoxy resin E44 or epoxy resin E42; the solid bisphenol A epoxy resin is bisphenol A epoxy resin E20, epoxy resin E12 or epoxy resin E14.

[0025] The method for preparing a heat-expanding epoxy foam having electromagnetic shielding properties as described above comprises: (1) heating a liquid bisphenol A epoxy resin to 80-100° C. and keeping the temperature for 10-20 minutes to sharply reduce the viscosity of the resin, adding a conductive filler having heat-expanding microspheres grafted on the surface, and using a physical stirring method to fully and evenly disperse the conductive filler having heat-expanding microspheres grafted on the surface to obtain a mixture, and then heating a solid bisphenol A epoxy resin and a novolac epoxy resin 638S to 100-120° C. and keeping the temperature for 60-80 minutes to sharply reduce the viscosity of the resin, and then blending with the mixture and stirring evenly (stirring speed is 300-500 r / min, stirring time is 10 minutes) to obtain material A;

[0026] The present invention simultaneously adds three epoxy resins, namely, liquid bisphenol A epoxy resin, solid bisphenol A epoxy resin, and novolac epoxy resin 638S, because:

[0027] Liquid bisphenol A epoxy resin is liquid at room temperature. Its low viscosity can reduce the viscosity of the system and facilitate the mixing of curing agents, accelerators and various fillers.

[0028] Phenolic epoxy resin 638S has a trifunctional structure, which can improve the connection strength between molecular chains during the curing process of the resin. More cross-linking points can form a complete cross-linking network structure during the curing process, improve the high temperature resistance and strength of the resin, and provide the support required by the heat-expandable microspheres after expansion;

[0029] Solid bisphenol A epoxy resin is a solid resin. Its existence is to maintain the resin system in a solid state at room temperature, making the resin system non-sticky. It can be made into resin sheets for later molding and processing.

[0030] The present invention adds a conductive filler with thermally expandable microspheres grafted on the surface to a liquid bisphenol A epoxy resin with a relatively low viscosity in advance, and then blends it with two other resins with relatively high viscosities (solid bisphenol A epoxy resin and novolac epoxy resin 638S). This treatment method can more evenly disperse the conductive filler with thermally expandable microspheres grafted on the surface in the epoxy resin matrix.

[0031] (2) keeping material A at 70-90° C., adding a curing agent, an accelerator, an auxiliary curing agent, heat-expandable microspheres and a surfactant thereto, and stirring evenly (stirring speed is 400-700 r / min, stirring time is 30-60 min), to obtain material B; the type of heat-expandable microspheres is the same as the heat-expandable microspheres grafted onto the surface of the conductive filler;

[0032] The role of the curing agent is to cure the resin. The epoxy groups in the epoxy resin react under the action of the curing agent to form a cross-linked structure, which converts the initial linear molecular chain into a three-dimensional molecular chain structure with a three-dimensional cross-linked network, so that the molecular chains are mutually attracted and cannot move at will under heating conditions, forming a stable cross-linked structure;

[0033] The role of the accelerator is to reduce the activation energy of the reaction between the curing agent and the resin matrix. It takes a high amount of energy to break the original chemical bonds between the epoxy groups and form new chemical bonds. The role of the accelerator is to reduce the energy required for the reaction, that is, to reduce the curing reaction temperature and better match the expansion temperature required by the foaming agent.

[0034] The function of the auxiliary curing agent is to improve the compatibility of the curing agent with the epoxy resin system and to react with the epoxy group at a lower temperature;

[0035] The role of surfactant is to reduce the surface tension of heat-expandable microspheres, improve the compatibility between heat-expandable microspheres and resin, and obtain more uniform pores;

[0036] Thermal expansion microspheres have a good control effect on the pore size and uniform distribution of epoxy foam. Through the thermal expansion of microspheres, dense pores and uniform pore distribution range can be obtained, providing a structural basis for the lightweight and high-strength performance of the foam;

[0037] The reason why the present invention prepares material A and then adds other additives to prepare material B is that: considering that the viscosity of phenolic epoxy resin 638S and solid bisphenol A epoxy resin is too high, if the conductive filler is directly added thereto, the thermal expansion microspheres grafted on the surface of the filler may fall off easily, and there is a phenomenon of difficulty in dispersion; therefore, the conductive filler is first mixed in the liquid bisphenol A epoxy resin with very low viscosity, and then the liquid bisphenol A epoxy resin is added to the phenolic epoxy 638S and solid bisphenol A epoxy resin. With the addition of the liquid bisphenol A epoxy resin, the viscosity of the three mixed resin systems begins to decrease. At this time, a series of additives such as a curing agent, a accelerator, an auxiliary curing agent, and a foaming agent can be well dispersed in the resin system;

[0038] (3) Extruding material B through a double-roller machine into a resin sheet C with a thickness of 0.5-2 mm, and covering the surface with release paper;

[0039] Since the resin sheet C contains heat-expandable microspheres, the resin sheet C can be foamed;

[0040] Since latent curing agent is added to the resin sheet C, the activation energy required for the reaction of this curing agent with epoxy groups is relatively high, that is, the breaking and recombination of covalent bonds can only occur under high temperature conditions, and the introduction of solid bisphenol A epoxy resin will cause the resin system to change from liquid to solid form. The mobility of molecular chains and chain segments in the solid resin system is greatly weakened, reducing the possibility of reaction. Therefore, the resin sheet C can be stored at room temperature for a certain period of time. The significance of storage at room temperature is that the produced resin sheet can be mass-produced and stored, and the resin sheet can be cut and processed according to the actual sample requirements, and can be used together with prepreg, simplifying the process, reducing production costs, and improving production efficiency.

[0041] (4) removing the release paper, placing the resin sheet C in a mold for curing and foaming, and then cooling it to room temperature;

[0042] Alternatively, the release paper is removed, and the resin sheet C and the carbon fiber prepreg are alternately stacked and placed in a mold for curing and foaming treatment, and then placed at room temperature to cool to room temperature;

[0043] The curing and foaming treatment time is 1-2 hours, and the room temperature is 15-25°C.

[0044] The method for preparing a heat-expanding epoxy foam having electromagnetic shielding properties as described above, in steps (1) to (2), the added amounts of the components are as follows, by weight: 10-30 parts (preferably 15-25 parts) of liquid bisphenol A epoxy resin, 1-10 parts (preferably 5-8 parts) of conductive filler with heat-expanding microspheres grafted on the surface, 10-40 parts (preferably 15-30 parts) of solid bisphenol A epoxy resin, 40-80 parts (preferably 50-70 parts) of novolac epoxy resin 638S, 1-15 parts (preferably 7-12 parts) of curing agent, 0.1-5 parts (preferably 1-2 parts) of accelerator, 0.1-5 parts (preferably 3-4 parts) of auxiliary curing agent, 1-15 parts (preferably 6-10 parts) of heat-expanding microspheres, and 0.1-5 parts (preferably 2-4 parts) of surfactant.

[0045] In the method for preparing a thermal self-expanding epoxy foam having electromagnetic shielding properties as described above, in step (4), the carbon fiber prepreg has a total of 0-10 layers (preferably 0-6 layers).

[0046] In the method for preparing a heat-expanding epoxy foam having electromagnetic shielding properties as described above, in step (2), the curing agent is a fatty amine curing agent (such as diethylenetriamine, triethylenetetramine, tetraethylenepentamine), an ester ring amine curing agent (such as menthane diamine, isophorone diamine), an aromatic amine curing agent (such as m-phenylenediamine, diaminodiphenyl sulfone, diaminodiphenylmethane) or a latent curing agent (such as dicyandiamide);

[0047] The accelerator is one or more of 2,4-toluenebisdimethylurea-isomer grade, 4,4-methylenebis(phenyldimethylurea) isomer grade, and phenyldimethylurea;

[0048] The auxiliary curing agent is one or more of maleimide, maleic anhydride and polyetheramine;

[0049] The surfactant is a nonionic surfactant (such as a polyoxyethylene surfactant, an alkyl alcohol amide surfactant, an alkyl amine oxide surfactant) or a cationic surfactant (such as an ammonium salt surfactant and a quaternary ammonium salt surfactant).

[0050] In the preparation method of the thermal self-expanding epoxy foam with electromagnetic shielding properties as described above, the thermal expansion pressure of the resin sheet C is 0.05-2MPa; the thermal self-expanding epoxy foam with electromagnetic shielding properties has a shielding effectiveness of 30-80dB at a frequency of 8-12GHz and a compressive strength of 5.2-9.5MPa.

[0051] Beneficial Effects

[0052] (1) The method for preparing the heat-expanding epoxy foam with electromagnetic shielding properties of the present invention innovatively solves the problem that the conductive filler is difficult to disperse in the resin matrix by introducing heat-expanding microspheres on the surface of the conductive filler; and adopts a method of compounding three epoxy resins to more evenly disperse the conductive filler with the heat-expanding microspheres grafted on the surface in the epoxy resin matrix, thereby obtaining the heat-expanding epoxy foam with electromagnetic shielding properties that can fully expand under heating conditions;

[0053] (2) The heat-expanding epoxy foam with electromagnetic shielding properties prepared by the present invention not only has good shielding performance, heat resistance and mechanical properties, but also can avoid the problem of complex process manufacturing of special-shaped materials in the processing process, thereby reducing material waste; the heat-expanding epoxy foam with electromagnetic shielding properties prepared by the present invention can reflect electromagnetic waves and prevent electromagnetic wave interference, and can be applied to medical equipment, household appliances, electronic products, high-rise buildings, communication equipment, measuring equipment, etc.; it can also be used to protect precision instruments inside aircraft, and has broad application prospects in civil or military fields; the heat-expanding epoxy foam with electromagnetic shielding properties prepared by the present invention has good electromagnetic shielding performance and has high use value and economic benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Figure 1 is a curve showing the relationship between the shielding effectiveness and the frequency of the thermal self-expanding epoxy foam with electromagnetic shielding properties of Example 1;

[0055] Figure 2 is a curve showing the relationship between the shielding effectiveness and the frequency of the thermal self-expanding epoxy foam with electromagnetic shielding properties of Example 2;

[0056] Figure 3 is a curve showing the relationship between the shielding effectiveness and the frequency of the thermal self-expanding epoxy foam with electromagnetic shielding properties of Example 3;

[0057] Figure 4 The graph is a curve showing the relationship between the shielding effectiveness and the frequency of the thermal self-expanding epoxy foam with electromagnetic shielding properties of Example 4. DETAILED DESCRIPTION

[0058] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall within the scope limited by the appended claims of the application equally.

[0059] The testing methods of some performance indicators in the following embodiments are as follows:

[0060] Shielding effectiveness: The electromagnetic shielding performance of the sample (i.e., thermal self-expanding epoxy foam with electromagnetic shielding properties) was tested using a vector network analyzer (VNA-ZNB20) and a waveguide method, with a test frequency range of 8-12 GHz;

[0061] Compression strength: The test is carried out according to the standard GB / T 8813-2020. By applying compressive stress to a sample with a size of 50×50mm, the displacement loading speed is controlled to 1mm / min until the relative deformation of the sample reaches 10% and then stops; the ratio of the maximum stress of the sample within 10% to the compression area is taken as the "compressive strength" of the sample.

[0062] The average particle size and conductivity of the conductive filler in the following embodiments have a great influence on the electromagnetic shielding performance of the thermal self-expanding epoxy foam with electromagnetic shielding properties. A conductive filler with higher conductivity can improve the electromagnetic shielding performance of the thermal self-expanding epoxy foam with electromagnetic shielding properties. Experiments show that other parameters of the conductive filler have little effect on the electromagnetic shielding performance of the thermal self-expanding epoxy foam with electromagnetic shielding properties, so they will not be described in detail below.

[0063] The type of liquid bisphenol A epoxy resin in the following embodiments has no effect on the electromagnetic shielding performance of the thermal self-expanding epoxy foam with electromagnetic shielding properties. Its presence is mainly to adjust the viscosity of the resin matrix to facilitate the dispersion of various conductive fillers inside.

[0064] The type of solid bisphenol A epoxy resin in the following embodiments has no effect on the electromagnetic shielding performance of the thermal self-expanding epoxy foam with electromagnetic shielding properties. Its existence is mainly to allow the resin system to maintain a solid form at room temperature, to make the resin system non-sticky, and to be made into a resin sheet for later molding and processing.

[0065] The parameters of the phenolic epoxy resin 638S in the following embodiments have no effect on the electromagnetic shielding properties of the thermal self-expanding epoxy foam with electromagnetic shielding properties. Its existence is mainly to help the molding of the thermal self-expanding epoxy foam with electromagnetic shielding properties and to improve the mechanical properties and temperature resistance of the thermal self-expanding epoxy foam with electromagnetic shielding properties.

[0066] The type of accelerator in the following examples has no effect on the electromagnetic shielding performance of the thermal self-expanding epoxy foam having electromagnetic shielding properties. Its presence is mainly to reduce the curing reaction temperature of the epoxy resin.

[0067] The type of auxiliary curing agent in the following embodiments has no effect on the electromagnetic shielding performance of the thermal self-expanding epoxy foam with electromagnetic shielding properties. Its presence is mainly to improve the compatibility of the curing agent with the resin and to have the ability to react with the epoxy resin.

[0068] The type of surfactant in the following embodiments has no effect on the electromagnetic shielding performance of the thermal self-expanding epoxy foam with electromagnetic shielding properties. Its presence is mainly to reduce surface tension and make the expanded microspheres and epoxy resin more compatible.

[0069] Example 1

[0070] A method for preparing a thermal self-expanding epoxy foam having electromagnetic shielding properties, the specific steps are as follows:

[0071] (1) Preparation of raw materials:

[0072] Conductive filler: pre-treated conductive carbon black, with an average particle size of 0.8 μm and a conductivity of 1500 S / m. The pre-treatment includes washing impurities, strong acid treatment, and drying.

[0073] Heat-expandable microspheres: The manufacturer is Nouryon, the brand is 461DU20, the average particle size is 15μm, the minimum foaming temperature is 110℃, and the maximum foaming temperature is 140℃;

[0074] Coupling agent: silane coupling agent KH550;

[0075] Solvent: anhydrous ethanol;

[0076] Liquid bisphenol A epoxy resin: bisphenol A epoxy resin E51;

[0077] Solid bisphenol A epoxy resin: bisphenol A epoxy resin E20;

[0078] Novolac epoxy resin 638S;

[0079] Curing agent: dicyandiamide;

[0080] Accelerator: 2,4-toluene dimethyl urea-isomer grade;

[0081] Auxiliary curing agent: polyetheramine D400;

[0082] Surfactant: polyoxyethylene sorbitan monooleate;

[0083] (2) By weight, 7 parts of conductive filler, 8 parts of heat-expandable microspheres, 5 parts of coupling agent, and 100 parts of solvent were mixed uniformly at a temperature of 40° C. and subjected to grafting reaction for 2 hours. After the reaction, the mixture was repeatedly rinsed with deionized water to remove the solvent not involved in the reaction, and the product was dried to obtain a conductive filler with heat-expandable microspheres grafted on the surface; wherein the number of heat-expandable microspheres grafted on the surface was 1, and the grafting rate was 50%;

[0084] (3) After heating 20 parts by weight of liquid bisphenol A epoxy resin to 80° C. and keeping the temperature for 10 minutes, 10 parts of conductive filler with thermally expandable microspheres grafted on the surface are added and stirred to obtain a mixture, and then 20 parts of solid bisphenol A epoxy resin and 60 parts of novolac epoxy resin 638S are heated to 120° C. and kept for 60 minutes, and then blended with the above mixture and stirred to obtain material A;

[0085] (4) In parts by weight, material A was kept warm at 90° C., 8 parts of a curing agent, 1.7 parts of an accelerator, 3.5 parts of an auxiliary curing agent, 6 parts of heat-expandable microspheres, and 3 parts of a surfactant were added thereto, and the mixture was stirred uniformly to obtain material B;

[0086] (5) Material B is extruded through a double-roll mill into a resin sheet C with a thickness of 1 mm, and the surface of the resin sheet C is covered with a release paper wrapper; the thermal expansion pressure of the resin sheet C is 0.5 MPa;

[0087] (6) removing the release paper, placing the resin sheet C in a mold for curing and foaming at 130° C. for 2 h, and then cooling it to room temperature at 15° C. to obtain a thermal self-expanding epoxy foam with electromagnetic shielding properties;

[0088] During the curing and foaming process, the viscosity of the epoxy resin is 1 Pa·s.

[0089] like Figure 1 As shown, the finally prepared thermal self-expanding epoxy foam with electromagnetic shielding properties has a shielding effectiveness of 45-60 dB at a frequency of 8-12 GHz and a compressive strength of 5.2 MPa.

[0090] Comparative Example 1

[0091] A method for preparing a heat-expanding epoxy foam having electromagnetic shielding properties is basically the same as that in Example 1, except that: there is no step (2), and in step (3), the conductive filler with heat-expanding microspheres grafted on the surface is replaced by a conductive filler of equal mass (the same as in Example 1).

[0092] The finally prepared thermal self-expanding epoxy foam with electromagnetic shielding properties has a shielding effectiveness of 23-30 dB at a frequency of 8-12 GHz and a compressive strength of 4.0 MPa.

[0093] By comparing Example 1 with Comparative Example 1, it can be seen that the mass of the conductive filler in Comparative Example 1 is equal to the mass of the conductive filler with heat-expandable microspheres grafted on the surface in Example 1, and the mass of the conductive filler with heat-expandable microspheres grafted on the surface in Example 1 = the mass of the conductive filler + the mass of the grafted heat-expandable microspheres, that is, the mass of the conductive filler in Comparative Example 1 is greater than the mass of the conductive filler in Comparative Example 1 in Example 1. However, the electromagnetic shielding performance and compressive strength of the thermal self-expanding epoxy foam with electromagnetic shielding properties in Comparative Example 1 are significantly lower than those in Example 1. This is because when the conductive filler is added to Comparative Example 1, the dispersion of the conductive filler inside it mainly relies on mechanical stirring. During the heating process, the volume of the heat-expandable microspheres will continue to increase, squeezing the conductive filler and causing local agglomeration. For the entire foam system, the conductive network density constructed by the conductive filler is poor, the electromagnetic shielding performance will also be reduced, and the local agglomeration phenomenon will deteriorate the mechanical properties of the foam.

[0094] Comparative Example 2

[0095] A method for preparing a thermally self-expanding epoxy foam having electromagnetic shielding properties is basically the same as that of Example 1, except that in step (6), the temperature of the curing and foaming treatment is 122°C.

[0096] The finally prepared thermal self-expanding epoxy foam with electromagnetic shielding properties has a shielding effectiveness of 26-32dB at a frequency of 8-12GHz and a compressive strength of 4.5MPa.

[0097] By comparing Example 1 with Comparative Example 2, it can be seen that the electromagnetic shielding performance of the heat-expanding epoxy foam with electromagnetic shielding characteristics of Comparative Example 2 is significantly lower than that of Example 1. This is because the difference between the temperature of the curing foaming treatment and the lowest foaming temperature of the heat-expanding microspheres is too low, resulting in a low expansion ratio of the heat-expanding microspheres, and the conductive filler is attached to the surface of the heat-expanding microspheres and is fully dispersed along with the expansion of the heat-expanding microspheres. Therefore, the dispersion effect of the conductive filler is not good, which in turn has an adverse effect on the electromagnetic shielding performance of the foam.

[0098] Comparative Example 3

[0099] A method for preparing a thermal self-expanding epoxy foam with electromagnetic shielding properties is basically the same as that in Example 1, except that in step (6), the temperature of the curing and foaming treatment is 138°C.

[0100] The finally prepared thermal self-expanding epoxy foam with electromagnetic shielding properties has a shielding effectiveness of 27-36 dB at a frequency of 8-12 GHz and a compressive strength of 0.3 MPa.

[0101] By comparing Example 1 with Comparative Example 3, it can be seen that the electromagnetic shielding performance and compressive strength of the heat-expanding epoxy foam with electromagnetic shielding properties of Comparative Example 3 are significantly lower than those of Example 1. This is because the difference between the temperature of the curing foaming treatment and the lowest foaming temperature of the heat-expanding microspheres is too high, which causes the heat-expanding microspheres to be damaged and the conductive filler may fall off, which will also affect the construction of the conductive network, and affect the electromagnetic shielding performance and compressive strength of the foam.

[0102] Example 2

[0103] A method for preparing a thermal self-expanding epoxy foam having electromagnetic shielding properties, the specific steps are as follows:

[0104] (1) Preparation of raw materials:

[0105] Conductive filler: pre-treated chopped carbon fiber, with an average length of 3mm and a conductivity of 200S / m. The pre-treatment is washing impurities, strong acid treatment, and drying in sequence;

[0106] Heat-expandable microspheres: The manufacturer is Nouryon, the brand is 043DU80, the average particle size is 10 μm, the minimum foaming temperature is 115°C, and the maximum foaming temperature is 145°C;

[0107] Coupling agent: titanate coupling agent 201;

[0108] Solvent: anhydrous ethanol;

[0109] Liquid bisphenol A epoxy resin: bisphenol A epoxy resin E51;

[0110] Solid bisphenol A epoxy resin: bisphenol A epoxy resin E20;

[0111] Novolac epoxy resin 638S;

[0112] Curing agent: dicyandiamide;

[0113] Accelerator: 4,4-methylenebis(phenyldimethylurea) isomer grade;

[0114] Auxiliary curing agent: polyetheramine D2000;

[0115] Surfactant: polyoxyethylene sorbitan monooleate;

[0116] (2) by weight, 5 parts of a conductive filler, 10 parts of heat-expandable microspheres, 5 parts of a coupling agent, and 100 parts of a solvent were mixed uniformly at a temperature of 45° C. and subjected to a grafting reaction for 2 hours. After the reaction was completed, the mixture was repeatedly rinsed with deionized water to remove the solvent that did not participate in the reaction, and the product was dried to obtain a conductive filler with heat-expandable microspheres grafted on the surface; wherein the number of heat-expandable microspheres grafted on the surface was 5, and the grafting rate was 70%;

[0117] (3) After heating 20 parts by weight of liquid bisphenol A epoxy resin to 80° C. and keeping the temperature for 10 minutes, 3 parts of conductive filler with thermally expandable microspheres grafted on the surface are added and stirred to obtain a mixture, and then 20 parts of solid bisphenol A epoxy resin and 60 parts of novolac epoxy resin 638S are heated to 120° C. and kept for 70 minutes, and then blended with the above mixture and stirred to obtain material A;

[0118] (4) In parts by weight, material A is kept at 90° C., 8 parts of a curing agent, 1.7 parts of an accelerator, 3.5 parts of an auxiliary curing agent, 6 parts of heat-expandable microspheres, and 4 parts of a surfactant are added thereto, and the mixture is stirred uniformly to obtain material B;

[0119] (5) Material B is extruded into a resin sheet C with a thickness of 1 mm through a double-roll mill, and the surface of the resin sheet C is covered with a release paper wrapper; the thermal expansion pressure of the resin sheet C is 0.4 MPa;

[0120] (6) removing the release paper, placing the resin sheet C in a mold for curing and foaming at 130° C. for 2 h, and then cooling it to room temperature at 20° C. to obtain a thermal self-expanding epoxy foam with electromagnetic shielding properties;

[0121] During the curing and foaming process, the viscosity of the epoxy resin was 0.8 Pa·s.

[0122] like Figure 2As shown, the finally prepared thermal self-expanding epoxy foam with electromagnetic shielding properties has a shielding effectiveness of 39-43 dB at a frequency of 8-12 GHz and a compressive strength of 7.5 MPa.

[0123] Comparative Example 4

[0124] A method for preparing a heat-expanding epoxy foam with electromagnetic shielding properties is basically the same as Example 2, except that: in step (2), the heat-expanding microspheres are 5 parts, the grafting reaction time is 30 minutes, the number of heat-expanding microspheres grafted on the surface of the conductive filler with the heat-expanding microspheres is 3, and the grafting rate is 50%.

[0125] The finally prepared thermal self-expanding epoxy foam with electromagnetic shielding properties has a shielding effectiveness of 28-36dB at a frequency of 8-12GHz and a compressive strength of 6.8MPa.

[0126] By comparing Example 2 with Comparative Example 4, it can be seen that the electromagnetic shielding performance of the heat-expanding epoxy foam with electromagnetic shielding properties of Comparative Example 4 is significantly lower than that of Example 2. This is because in the conductive filler with heat-expanding microspheres grafted on the surface, the number and grafting rate of the heat-expanding microspheres grafted on the surface are too low, which makes it easy for the fibers to get entangled during the thermal expansion process. There are not enough heat-expanding microspheres to disperse the chopped fibers, resulting in an incomplete conductive network of the foam system, thereby reducing the electromagnetic shielding performance of the foam.

[0127] Comparative Example 5

[0128] A method for preparing a heat-expanding epoxy foam having electromagnetic shielding properties is basically the same as that in Example 2, except that: in step (3), 10 parts of a conductive filler having heat-expanding microspheres grafted on its surface are directly added to a mixture of 30 parts by mass of epoxy resin E20 and 70 parts by mass of novolac epoxy resin 638S, without adding liquid bisphenol A epoxy resin.

[0129] The finally prepared thermal self-expanding epoxy foam with electromagnetic shielding properties has a shielding effectiveness of 30-39 dB at a frequency of 8-12 GHz and a compressive strength of 7.8 MPa.

[0130] By comparing Example 2 with Comparative Example 5, it can be seen that the electromagnetic shielding performance of the heat-expanding epoxy foam with electromagnetic shielding properties of Comparative Example 5 is significantly lower than that of Example 2. This is because the viscosity of the epoxy resin system is relatively high at this time, and the conductive filler may fall off with the expanded microspheres during the dispersion process therein, resulting in a low grafting rate of the final product, which affects the construction of the conductive network in the foam system and reduces the electromagnetic shielding performance of the foam.

[0131] Example 3

[0132] A method for preparing a thermal self-expanding epoxy foam having electromagnetic shielding properties, the specific steps are as follows:

[0133] (1) Preparation of raw materials:

[0134] Conductive filler: pre-treated nickel-plated chopped carbon fiber with a conductivity of 2000S / m. The pre-treatment is washing impurities, strong acid treatment, and drying in sequence;

[0135] Heat-expandable microspheres: The manufacturer is Nouryon, the brand is 043DU80, the average particle size is 10 μm, the minimum foaming temperature is 115°C, and the maximum foaming temperature is 145°C;

[0136] Coupling agent: silane coupling agent KH550;

[0137] Solvent: anhydrous ethanol;

[0138] Liquid bisphenol A epoxy resin: bisphenol A epoxy resin E51;

[0139] Solid bisphenol A epoxy resin: bisphenol A epoxy resin E20;

[0140] Novolac epoxy resin 638S;

[0141] Curing agent: dicyandiamide;

[0142] Accelerator: phenyl dimethyl urea;

[0143] Auxiliary curing agent: polyetheramine D2000;

[0144] Surfactant: polyoxyethylene sorbitan monooleate;

[0145] (2) by weight, 5 parts of a conductive filler, 8 parts of heat-expandable microspheres, 5 parts of a coupling agent, and 100 parts of a solvent were mixed uniformly at a temperature of 40° C. and subjected to a grafting reaction for 3 hours. After the reaction was completed, the mixture was repeatedly rinsed with deionized water to remove the solvent that did not participate in the reaction, and the product was dried to obtain a conductive filler having a surface grafted with heat-expandable microspheres; wherein the number of heat-expandable microspheres grafted on the surface was 15, and the grafting rate was 70%;

[0146] (3) After heating 20 parts of liquid bisphenol A epoxy resin to 80° C. and keeping the temperature for 10 minutes, 10 parts of conductive filler with thermally expandable microspheres grafted on the surface were added and stirred to obtain a mixture, and then 20 parts of solid bisphenol A epoxy resin and 60 parts of novolac epoxy resin 638S were heated to 120° C. and kept the temperature for 70 minutes, and then blended with the above mixture and stirred to obtain material A;

[0147] (4) In parts by weight, material A is kept at 90° C., 8 parts of a curing agent, 1.7 parts of an accelerator, 3.5 parts of an auxiliary curing agent, 6 parts of heat-expandable microspheres, and 4 parts of a surfactant are added thereto, and the mixture is stirred uniformly to obtain material B;

[0148] (5) Material B is extruded into a resin sheet C with a thickness of 1 mm through a double-roll mill, and the surface of the resin sheet C is covered with a release paper wrapper; the thermal expansion pressure of the resin sheet C is 0.6 MPa;

[0149] (6) removing the release paper, placing the resin sheet C in a mold for curing and foaming at 130° C. for 2 h, and then cooling it to room temperature at 20° C. to obtain a thermal self-expanding epoxy foam with electromagnetic shielding properties;

[0150] During the curing and foaming process, the viscosity of the epoxy resin is 1 Pa·s.

[0151] like Figure 3 As shown, the finally prepared thermal self-expanding epoxy foam with electromagnetic shielding properties has a shielding effectiveness of 50-56 dB at a frequency of 8-12 GHz and a compressive strength of 6.2 MPa.

[0152] Example 4

[0153] A method for preparing a thermally self-expanding epoxy foam with electromagnetic shielding properties is basically the same as that in Example 3, except that in step (5), the thickness of the resin sheet C is 1.5 mm, and the thermal expansion pressure of the resin sheet C is 1.2 MPa; in step (6), the resin sheet C and carbon fiber prepreg (the carbon fiber content in the carbon fiber prepreg is 60 wt%) are alternately stacked and then placed in a mold for curing and foaming treatment, the resin sheet C has 2 layers and the carbon fiber prepreg has 3 layers.

[0154] like Figure 4 As shown, the finally prepared thermal self-expanding epoxy foam with electromagnetic shielding properties has a shielding effectiveness of 65-74 dB at a frequency of 8-12 GHz and a compressive strength of 7.5 MPa.

[0155] Example 5

[0156] A method for preparing a thermal self-expanding epoxy foam having electromagnetic shielding properties, the specific steps are as follows:

[0157] (1) Preparation of raw materials:

[0158] Conductive filler: pre-treated carbon fiber cloth, conductivity is 1000S / s, pre-treatment is washing impurities, strong acid treatment, drying;

[0159] Thermal expansion microspheres: The manufacturer is POLYCHEM, USA, the brand is 180DU25, the average particle size is 20μm, the minimum foaming temperature is 120℃, and the maximum foaming temperature is 150℃;

[0160] Coupling agent: silane coupling agent KH550;

[0161] Solvent: anhydrous ethanol;

[0162] Liquid bisphenol A epoxy resin: bisphenol A epoxy resin E51;

[0163] Solid bisphenol A epoxy resin: bisphenol A epoxy resin E20;

[0164] Novolac epoxy resin 638S;

[0165] Curing agent: dicyandiamide;

[0166] Accelerator: 4,4-methylenebis(phenyldimethylurea) isomer grade;

[0167] Auxiliary curing agent: polyetheramine D2000;

[0168] Surfactant: polyoxyethylene sorbitan monooleate;

[0169] (2) By weight, 10 parts of conductive filler, 20 parts of heat-expandable microspheres, 5 parts of coupling agent, and 100 parts of solvent were mixed uniformly at 40° C. for grafting reaction for 3 hours. After the reaction, the product was repeatedly rinsed with deionized water to remove the solvent that did not participate in the reaction, and the product was dried to obtain a conductive filler with heat-expandable microspheres grafted on the surface; wherein the number of heat-expandable microspheres grafted on the surface was 20, and the grafting rate was 70%;

[0170] (3) heating 20 parts of liquid bisphenol A epoxy resin to 80° C. and keeping the temperature for 10 minutes, and then heating 20 parts of solid bisphenol A epoxy resin and 60 parts of novolac epoxy resin 638S to 120° C. and keeping the temperature for 60 minutes, respectively, and blending with the above mixture, stirring evenly, to obtain material A;

[0171] (4) In parts by weight, material A is kept at 90° C., 8 parts of a curing agent, 1.7 parts of an accelerator, 3.5 parts of an auxiliary curing agent, 6 parts of heat-expandable microspheres, and 4 parts of a surfactant are added thereto, and the mixture is stirred uniformly to obtain material B;

[0172] (5) Material B is compounded with a conductive filler grafted with heat-expandable microspheres through a prepreg device to prepare a resin sheet C, and the thickness of the prepreg is controlled to be 1 mm, and the surface is covered with release paper; the thermal expansion pressure of the resin sheet C is 0.5 MPa;

[0173] (6) removing the release paper, placing the resin sheet C in a mold and performing a curing and foaming treatment at 135° C. for 2 h, and then cooling it to room temperature at 20° C. to obtain a thermal self-expanding epoxy foam having electromagnetic shielding properties;

[0174] During the curing and foaming process, the viscosity of the epoxy resin was 0.8 Pa·s.

[0175] The finally prepared thermal self-expanding epoxy foam with electromagnetic shielding properties has a shielding effectiveness of 65-70 dB at a frequency of 8-12 GHz and a compressive strength of 8.1 MPa.

[0176] Example 6

[0177] A method for preparing a thermally self-expanding epoxy foam having electromagnetic shielding properties is basically the same as that of Example 5, except that in step (6), a resin sheet C and a carbon fiber prepreg (the carbon fiber content in the carbon fiber prepreg is 60wt%) are alternately stacked and then placed in a mold for curing and foaming treatment, the resin sheet C has a total of 2 layers and the carbon fiber prepreg has a total of 3 layers.

[0178] The finally prepared thermal self-expanding epoxy foam with electromagnetic shielding properties has a shielding effectiveness of 74-80 dB at a frequency of 8-12 GHz and a compressive strength of 9.5 MPa.

[0179] Example 7

[0180] A method for preparing a thermal self-expanding epoxy foam having electromagnetic shielding properties, the specific steps are as follows:

[0181] (1) Preparation of raw materials:

[0182] Conductive filler: pre-treated graphene, with an average particle size of 500 μm and a conductivity of 6000 S / m. The pre-treatment is to wash away impurities, treat with strong acid, and dry in sequence;

[0183] Thermal expansion microspheres: The manufacturer is POLYCHEM, USA, the brand is 180DU25, the average particle size is 20μm, the minimum foaming temperature is 120℃, and the maximum foaming temperature is 150℃;

[0184] Coupling agent: a mixture of titanate coupling agent TC-50 and silane coupling agent KH550 in a mass ratio of 1:1;

[0185] Solvent: a mixture of benzene and toluene in a volume ratio of 1:1;

[0186] Liquid bisphenol A type epoxy resin: epoxy resin E44;

[0187] Solid bisphenol A type epoxy resin: epoxy resin E12;

[0188] Novolac epoxy resin 638S;

[0189] Curing agent: diethylenetriamine;

[0190] Accelerator: 4,4-methylenebis(phenyldimethylurea) isomer grade;

[0191] Auxiliary curing agent: a mixture of maleimide and maleic anhydride in a mass ratio of 1:1;

[0192] Surfactant: hexadecyltrimethylammonium chloride;

[0193] (2) By weight, 10 parts of conductive filler, 15 parts of heat-expandable microspheres, 12 parts of coupling agent, and 100 parts of solvent were mixed uniformly at 60° C. for grafting reaction for 2 hours. After the reaction, the mixture was repeatedly rinsed with deionized water to remove the solvent not involved in the reaction, and the product was dried to obtain a conductive filler with heat-expandable microspheres grafted on the surface; wherein the number of heat-expandable microspheres grafted on the surface was 5, and the grafting rate was 80%;

[0194] (3) After heating 15 parts of liquid bisphenol A epoxy resin to 100° C. and keeping the temperature for 20 minutes, 5 parts of conductive filler with thermally expandable microspheres grafted on the surface were added and stirred to obtain a mixture, and then 15 parts of solid bisphenol A epoxy resin and 50 parts of novolac epoxy resin 638S were heated to 100° C. and kept the temperature for 80 minutes, and then blended with the above mixture and stirred to obtain material A;

[0195] (4) In parts by weight, material A is kept warm at 70° C., 7 parts of a curing agent, 1 part of an accelerator, 3 parts of an auxiliary curing agent, 6 parts of heat-expandable microspheres, and 2 parts of a surfactant are added thereto, and the mixture is stirred uniformly to obtain material B;

[0196] (5) Material B is extruded through a double-roll mill into a resin sheet C with a thickness of 0.5 mm, and the surface of the resin sheet C is covered with a release paper wrapper; the thermal expansion pressure of the resin sheet C is 0.3 MPa;

[0197] (6) removing the release paper, placing the resin sheet C in a mold for curing and foaming at 135° C. for 1 h, and then cooling it to room temperature at 25° C. to obtain a thermal self-expanding epoxy foam with electromagnetic shielding properties;

[0198] During the curing and foaming process, the viscosity of the epoxy resin was 0.5 Pa·s.

[0199] The finally prepared thermal self-expanding epoxy foam with electromagnetic shielding properties has a shielding effectiveness of 62-70 dB at a frequency of 8-12 GHz and a compressive strength of 6.8 MPa.

[0200] Example 8

[0201] A method for preparing a thermal self-expanding epoxy foam having electromagnetic shielding properties, the specific steps are as follows:

[0202] (1) Preparation of raw materials:

[0203] Conductive filler: pre-treated chopped carbon fiber, with an average length of 5mm and a conductivity of 250S / m. The pre-treatment is washing impurities, strong acid treatment, and drying in sequence;

[0204] Heat-expandable microspheres: The manufacturer is Nouryon, the brand is 461DU20, the average particle size is 15μm, the minimum foaming temperature is 110℃, and the maximum foaming temperature is 140℃;

[0205] Coupling agent: titanate coupling agent TC-20;

[0206] Solvent: a mixture of dichlorobenzene and dichloromethane in a volume ratio of 1:1;

[0207] Liquid bisphenol A type epoxy resin: epoxy resin E44;

[0208] Solid bisphenol A type epoxy resin: epoxy resin E12;

[0209] Novolac epoxy resin 638S;

[0210] Curing agent: dicyandiamide

[0211] Accelerator: 4,4-methylenebis(phenyldimethylurea) isomer grade;

[0212] Auxiliary curing agent: a mixture of maleimide and maleic anhydride in a mass ratio of 1:1;

[0213] Surfactant: hexadecyltrimethylammonium chloride;

[0214] (2) By weight, 8 parts of conductive filler, 10 parts of heat-expandable microspheres, 8 parts of coupling agent, and 100 parts of solvent were mixed uniformly at 60° C. for grafting reaction for 4 hours. After the reaction, the mixture was repeatedly rinsed with deionized water to remove the solvent that did not participate in the reaction, and the product was dried to obtain a conductive filler with heat-expandable microspheres grafted on the surface; wherein the number of heat-expandable microspheres grafted on the surface was 10, and the grafting rate was 70%;

[0215] (3) After heating 25 parts of liquid bisphenol A epoxy resin to 110° C. and keeping the temperature for 10 minutes, 10 parts of conductive filler with thermally expandable microspheres grafted on the surface were added and stirred to obtain a mixture, and then 30 parts of solid bisphenol A epoxy resin and 70 parts of novolac epoxy resin 638S were heated to 110° C. and kept the temperature for 60 minutes, and then blended with the above mixture and stirred to obtain material A;

[0216] (4) In parts by weight, material A is kept at 90° C., 12 parts of a curing agent, 2 parts of an accelerator, 4 parts of an auxiliary curing agent, 10 parts of heat-expandable microspheres, and 4 parts of a surfactant are added thereto, and the mixture is stirred uniformly to obtain material B;

[0217] (5) Material B is extruded into a resin sheet C with a thickness of 2 mm through a double-roll mill, and the surface of the resin sheet C is covered with a release paper wrapper; the thermal expansion pressure of the resin sheet C is 0.95 MPa;

[0218] (6) removing the release paper, placing the resin sheet C in a mold for curing and foaming at 130° C. for 1 h, and then cooling it to room temperature at 25° C. to obtain a thermal self-expanding epoxy foam with electromagnetic shielding properties;

[0219] During the curing and foaming process, the viscosity of the epoxy resin is 1 Pa·s.

[0220] The finally prepared thermal self-expanding epoxy foam with electromagnetic shielding properties has a shielding effectiveness of 46-53dB at a frequency of 8-12GHz and a compressive strength of 7.6MPa.

[0221] Example 9

[0222] A method for preparing a thermally self-expanding epoxy foam having electromagnetic shielding properties is basically the same as that of Example 8, except that the conductive filler in step (1) is not pretreated.

[0223] The finally prepared thermal self-expanding epoxy foam with electromagnetic shielding properties has a shielding effectiveness of 41-47 dB at a frequency of 8-12 GHz and a compressive strength of 6.5 MPa.

Claims

1. A method for preparing a thermal self-expanding epoxy foam having electromagnetic shielding properties, characterized in that: The specific steps are as follows: (1) After heating liquid bisphenol A epoxy resin to 80-100°C and keeping the temperature for 10-20 minutes, adding a conductive filler with thermally expandable microspheres grafted on the surface, stirring evenly to obtain a mixture, and then heating solid bisphenol A epoxy resin and novolac epoxy resin 638S to 100-120°C and keeping the temperature for 60-80 minutes, blending with the mixture, stirring evenly, and obtaining material A; The minimum foaming temperature of heat-expandable microspheres is 110-120°C, and the maximum foaming temperature is 140-150°C; The conductive filler is a granular structure with an average particle size of less than 500 μm, the number of thermally expandable microspheres grafted on the surface is greater than or equal to 1, and the grafting rate is greater than 50%; Alternatively, the conductive filler is a granular structure with an average particle size of 500-800 μm, the number of thermally expandable microspheres grafted on the surface is 2-5, and the grafting rate is above 70%; Alternatively, the conductive filler is a fibrous structure with an average length of 1-8 mm, the number of thermally expandable microspheres grafted on the surface is 5-20, and the grafting rate is above 70%; (2) Material A is kept at 70-90° C., a curing agent, an accelerator, an auxiliary curing agent, heat-expandable microspheres and a surfactant are added thereto, and the mixture is stirred evenly to obtain material B; The types of heat-expandable microspheres are the same as those grafted onto the surface of the conductive filler; (3) Material B is extruded through a double-roller machine into a resin sheet C with a thickness of 0.5-2 mm, and the surface is covered with release paper; (4) removing the release paper, placing the resin sheet C in a mold for curing and foaming, and then cooling it to room temperature; Alternatively, the release paper is removed, and the resin sheet C and the carbon fiber prepreg are alternately stacked and placed in a mold for curing and foaming treatment, and then placed at room temperature to cool to room temperature; The temperature of the curing and foaming treatment is 15-25°C higher than the lowest foaming temperature of the heat-expandable microspheres; the curing and foaming treatment time is 1-2h, and the room temperature is 15-25°C; during the curing and foaming treatment, the viscosity of the epoxy resin is below 1Pa·s.

2. The method for preparing a thermal self-expanding epoxy foam having electromagnetic shielding properties according to claim 1, characterized in that: The preparation process of the conductive filler with thermal expansion microspheres grafted on the surface is as follows: the conductive filler, thermal expansion microspheres, coupling agent and solvent are uniformly mixed for grafting reaction, and the conductive filler with thermal expansion microspheres grafted on the surface is obtained through post-treatment.

3. The method for preparing a thermal self-expanding epoxy foam having electromagnetic shielding properties according to claim 2, characterized in that: The conductive filler is one or more of chopped carbon fiber, carbon fiber cloth, conductive carbon black, graphene, and carbon nanotubes; The average particle size of the heat-expandable microspheres is 10-30 μm, the outer shell is thermoplastic, and the interior is a hydrocarbon gas that expands when heated; The coupling agent is a silane coupling agent or a titanate coupling agent.

4. The method for preparing a thermal self-expanding epoxy foam having electromagnetic shielding properties according to claim 3, characterized in that: In terms of weight, the amount of each component added during the preparation of the conductive filler with thermally expandable microspheres grafted on the surface is: 5-15 parts of conductive filler, 5-20 parts of thermally expandable microspheres, 5-12 parts of coupling agent, and 100 parts of solvent; the grafting reaction temperature is 40-60°C and the time is 2-4h.

5. The method for preparing a thermal self-expanding epoxy foam having electromagnetic shielding properties according to claim 2, characterized in that: The conductive filler is a pre-treated conductive filler, and the pre-treatment is to wash away impurities, perform strong acid treatment, and dry in sequence.

6. The method for preparing a thermal self-expanding epoxy foam having electromagnetic shielding properties according to claim 1, characterized in that: In steps (1) to (2), the added amount of each component is calculated by weight: 10-30 parts of liquid bisphenol A epoxy resin, 1-10 parts of conductive filler with thermal expansion microspheres grafted on the surface, 10-40 parts of solid bisphenol A epoxy resin, 40-80 parts of novolac epoxy resin 638S, 1-15 parts of curing agent, 0.1-5 parts of accelerator, 0.1-5 parts of auxiliary curing agent, 1-15 parts of thermal expansion microspheres, and 0.1-5 parts of surfactant.

7. The method for preparing a thermal self-expanding epoxy foam having electromagnetic shielding properties according to claim 1, characterized in that: In step (2), the curing agent is a fatty amine curing agent, alicyclic amine curing agent, aromatic amine curing agent or a latent curing agent; The accelerator is one or more of 2,4-toluenebisdimethylurea-isomer grade, 4,4-methylenebis(phenyldimethylurea) isomer grade, and phenyldimethylurea; The auxiliary curing agent is one or more of maleimide, maleic anhydride and polyetheramine; The surfactant is a nonionic surfactant or a cationic surfactant.

8. The method for preparing a thermal self-expanding epoxy foam having electromagnetic shielding properties according to claim 1, characterized in that: The thermal expansion pressure of the resin sheet C is 0.05-2MPa; the thermal self-expanding epoxy foam with electromagnetic shielding properties has a shielding effectiveness of 30-80dB at a frequency of 8-12GHz and a compression strength of 5.2-9.5MPa.

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