A method of preparing a material for an electronic product housing

Abstract

The application discloses a preparation method of a material for an electronic product shell and relates to the technical field of polymer preparation, and the preparation method comprises the following steps: 1) Boc-polyamide acid prepolymer preparation; 2) compounding and foaming; and 3) forming and stepwise curing crosslinking. By introducing Boc-lysine and fluorine-containing monomers, the application overcomes the bottleneck of difficult solubility and processing of polyimide, and realizes high solid content processing and foaming. A unique physical and chemical synergistic foaming mechanism and a fiber interface chemical bonding technology are designed and adopted, and a composite material with the characteristics of ultralight weight, high strength, 200 DEG C high-temperature resistance, intrinsic flame retardance and ultralow dielectric constant is successfully prepared.

Description

Technical Field

[0001] This invention relates to the field of polymer preparation technology, and in particular to a method for preparing a material for the casing of electronic products. Background Technology

[0002] With the rapid development of the electronics and information industry towards higher frequencies, higher speeds, thinner designs, and higher reliability, the casings of electronic products no longer merely serve mechanical protection and aesthetic purposes; their impact on the overall performance of the equipment is becoming increasingly prominent. An ideal casing material must simultaneously meet the requirements of lightweight, high strength, excellent heat resistance, outstanding dielectric properties (low dielectric constant and low dielectric loss), and intrinsic flame retardancy. However, existing material systems often excel in only a few properties, making it difficult to meet all requirements and presenting significant technical bottlenecks.

[0003] Currently widely used shell materials, such as acrylonitrile-butadiene-styrene copolymer (ABS), polycarbonate (PC), and their alloys, while possessing good processability and decent mechanical strength, generally have low heat resistance temperatures (typically below 120°C), making them prone to thermal deformation during welding processes or in high-temperature environments inside equipment. Furthermore, these general-purpose plastics contain a large number of polar groups in their molecular chains, resulting in high dielectric constants (Dk > 3.0) and dielectric losses (Df). This leads to significant signal delays, energy losses, and crosstalk in high-frequency, high-speed signal transmission, making it difficult to meet the requirements of advanced electronic devices such as 5G communications and high-end servers.

[0004] In pursuit of lightweight design, polyurethane (PU) and other foam materials have been tried as shells. However, their mechanical strength, heat resistance, and long-term dimensional stability are poor, and most PUs are flammable, requiring the addition of flame retardants, which may worsen their other properties.

[0005] Polyimide (PI) is one of the best-performing polymers known, possessing extremely high heat resistance (decomposes at >400℃), excellent mechanical strength, outstanding chemical stability, and intrinsic flame retardancy, making it a theoretically ideal candidate material for high-performance shells. However, traditional polyimides, due to their rigid backbone and strong intermolecular forces, exhibit extremely poor solubility and non-melting properties in most organic solvents, presenting insurmountable processing challenges. They can typically only be processed as low-solids polyamic acid (PAA) precursor solutions, followed by high-temperature imidization molding. This severely limits their application to preparing films or simple castings, preventing complex injection molding or foaming processes. Even attempts to prepare polyimide foams using external blowing agents suffer from problems such as poor compatibility between the blowing agent and the matrix, decomposition residues affecting performance, and uneven cell structure.

[0006] Regarding dielectric properties, although polyimide (PI) is superior to most plastics, there is still room for reducing the dielectric constant (3.2-3.5) of traditional PI to meet future higher-frequency applications. Common methods for reducing the dielectric constant, such as introducing porous structures or adding low-dielectric fillers, often negatively impact the material's mechanical properties. Especially when reinforcing agents such as glass fibers are added, poor interfacial bonding can easily introduce defects and polarity at the interface, leading to increased dielectric loss and ultimately proving counterproductive.

[0007] Therefore, the electronics industry urgently needs a material solution that can simultaneously overcome multiple technical barriers such as processability, lightweight, high strength, high heat resistance, and low dielectric constant. Summary of the Invention

[0008] The purpose of this invention is to overcome the shortcomings of the prior art and to propose a method for preparing a material for the casing of electronic products.

[0009] This invention first proposes a method for preparing a material for the casing of electronic products, comprising the following steps: (1) Boc-polyamic acid prepolymer: Add Nα-Boc-lysine and dimethylacetamide to the reactor, turn on the mechanical stirrer, introduce nitrogen gas for protection, and stir until completely dissolved; Cool the mixture to 0-5℃ in an ice-water bath, add 4,4'-(hexafluoroisopropyl)diphthalic anhydride, and keep the reaction temperature below 10℃ throughout the reaction, and react for 1 hour. After dissolving 2,2'-bis(trifluoromethyl)benzidine in dimethylacetamide, the solution was slowly added dropwise to the reaction vessel, and the reaction was continued for 1 hour while maintaining a low temperature. Add an equal amount of 4,4'-(hexafluoroisopropyl)diphthalic anhydride as before, and maintain the reaction at low temperature for 2 hours; Remove the ice bath and allow the reaction system to naturally warm to room temperature. Continue the reaction for 12-20 hours to obtain Boc-polyamic acid prepolymer. The anhydride group is a highly active electrophilic group. The -NH2 of 2,2'-bis(trifluoromethyl)benzidine and Nα-Boc-lysine attacks the carbonyl carbon of the anhydride, causing the anhydride ring to open and generate an amide bond. Low temperature slows down the polycondensation rate and avoids local overheating that leads to uneven molecular weight distribution or gelation. The polycondensation reaction is more complete at room temperature, allowing the molecular weight of the prepolymer to reach the design range and forming a stable Boc-polyamic acid prepolymer; Polyamic acid is a soluble precursor of polyimide. Through the polycondensation reaction of diamine with dianhydride and amino with carboxyl groups, polyamic acid prepolymers containing Boc side chains are generated.

[0010] (2) Composite and foaming: Modified glass fibers were added to the Boc-polyamic acid prepolymer and dispersed at high speed for 30-60 minutes using a high-speed shear disperser until the fibers were uniformly dispersed, forming an extremely viscous paste slurry. Use a mechanical mixer to vigorously stir the paste slurry for 5-10 minutes to incorporate a large amount of air until the volume expands to 3-5 times its original size, forming a stable foam slurry. Add a foam stabilizer and set aside. After KH-560 treatment, the surface of the glass fiber changes from inert to active epoxy groups (-O-CH2). When it is subsequently compounded with polyamic acid, the epoxy groups can undergo ring-opening reactions with the -COOH of the prepolymer main chain and the amino groups generated after the deprotection of Boc, thereby enhancing the interfacial bonding force and fixing the glass fiber in the polyimide polymer. (3) Molding and stepped curing crosslinking: The foam slurry is injected into a mold pre-coated with a release agent. The mold is vibrated to remove large air bubbles. The mold is then placed in an oven for programmed heating to complete molding and step-by-step curing and cross-linking. After the program is completed, the oven is turned off, and the sample is allowed to cool to below room temperature before being removed from the mold to obtain modified polyimide.

[0011] Preferably, in step (1), the molar ratio of the total amount of 2,2'-bis(trifluoromethyl)benzidine and Nα-Boc-lysine to the total amount of 4,4'-(hexafluoroisopropyl)phthalic anhydride added is 1:1.1, wherein the molar ratio of 2,2'-bis(trifluoromethyl)benzidine to Nα-Boc-lysine is 3-8:2; the solid content of the Boc-polyamic acid prepolymer is controlled at 20%. The preparation process of the Nα-Boc-lysine includes the following steps: After dissolving lysine hydrochloride in deionized water with stirring, NaOH was slowly added to adjust the pH to 10-11. The NaCl precipitate was then separated by centrifugation to obtain free lysine base. The free base of lysine was transferred into the reaction vessel, tetrahydrofuran was added, and the temperature was lowered to 0℃; a tetrahydrofuran solution of Boc2O was added dropwise, and the reaction was carried out at 0-5℃ for 2 hours, and then the temperature was raised to 25℃ and the reaction was continued for 4 hours. Add concentrated hydrochloric acid until the pH reaches 2-3. Wash the precipitated white precipitate with ice water to obtain Nα-Boc-lysine.

[0012] The α-amino group is selectively protected. Due to its large steric hindrance and low reactivity, the ε-amino group does not react with Boc2O and remains free, so that Nα-Boc-lysine contains only 1 free amino group. Preferably, in the preparation of Nα-Boc-lysine, the molar ratio of NaOH, lysine hydrochloride and Boc2O is 2:1:1.

[0013] Preferably, in step (2), the mass of the modified glass fiber is 15-25% of the solid mass of the Boc-polyamic acid prepolymer; the foam stabilizer is a modified polysiloxane foam stabilizer with a viscosity (25°C) of 500-1500 mPa·s, and the addition amount is 0.1-0.5% of the mass of the foam slurry; The preparation process of modified glass fiber includes the following steps: Mix deionized water and ethanol at a volume ratio of 1:9, and adjust the pH to 4.5-5.5 with glacial acetic acid; slowly add 2% of the total weight of silane coupling agent KH-560, and hydrolyze with magnetic stirring at 40°C for 30 minutes; add short-cut glass fibers, and ensure complete impregnation at 60°C; remove the fibers, rinse lightly with ethanol, vacuum dry, and seal for later use.

[0014] Preferably, in step (3), the heating process is as follows: Heat to 80℃ and keep warm for 2 hours; The polyamic acid backbone undergoes intramolecular dehydration to form a preliminary imide ring, and a small number of Boc groups begin to deprotect. Heat to 120℃ and keep warm for 1 hour; With increased imidization, the density of imide rings increases, and the material begins to exhibit high-temperature resistance. A large number of Boc groups are deprotected to generate free amino groups.

[0015] Heat to 170℃ and keep warm for 2 hours; The imidization reaction is complete, forming a stable polyimide. The deprotected -NH2 reacts with the epoxy groups on the surface of the glass fiber, and at the same time reacts with the prepolymer end anhydride to form a cross-linked network.

[0016] Heat to 200℃ and keep warm for 1 hour; The residual -NH2, epoxy, and anhydride end groups are completely reacted, maximizing the crosslinking density; the residual small molecules such as DMAc, tert-butanol, and H2O in the system are removed, resulting in a denser material structure.

[0017] A material for the casing of electronic products, prepared by the aforementioned method, is a smooth, rigid, closed-cell material with a density of 0.2-0.8 g / cm³. 3 The long-term operating temperature is 250℃.

[0018] The beneficial effects of this invention are: 1. High-performance polymers typically exhibit extremely poor solubility in organic solvents due to their rigid backbone and strong intermolecular forces, making them difficult to process. They often require processing with low-solids polyamic acid precursor solutions followed by imidization at high temperatures. This limits their applications, making it difficult to fabricate thick-walled or complex structural components and hindering secondary processing such as foaming. Even when foaming is possible, external foaming agents are required, potentially leading to compatibility and dispersibility issues.

[0019] This invention innovatively introduces Nα-Boc-lysine as a comonomer during the processing stage. The large Boc group acts as a powerful "temporary" solubilizing group, effectively disrupting the regular arrangement of polyamic acid molecular chains through steric hindrance. This greatly enhances the solubility of the prepolymer in dimethylacetamide (DMAC) solvent, allowing the prepolymer solution to reach a high solids content of 20%, thus forming the extremely viscous paste required for subsequent foaming. During the curing stage, the Boc group is removed through thermal decomposition, restoring the inherent high strength and high heat resistance of polyimide. The active amino groups exposed after decomposition further form a cross-linking network, further enhancing the performance of the final product.

[0020] 2. Traditional physical or chemical foaming may have problems such as uneven bubble distribution, difficulty in controlling the size, poor compatibility between the foaming agent and the matrix, and decomposition residues affecting performance.

[0021] This invention incorporates air into a high-viscosity prepolymer through mechanical stirring, forming initial, relatively stable bubble nuclei. The density and pore size of the foam can be initially controlled by adjusting the stirring speed and time. During step-curing to 170°C, the Boc groups decompose in situ to generate gas. At this point, the polymer matrix has begun to initially solidify, increasing viscosity and strength. The newly generated gas cannot escape and can only expand secondary within the existing physical bubble structure or form finer pores. This synergistic foaming mechanism, primarily physical with secondary chemical action, produces a finer, more uniform foam with a more stable pore structure. The gas in the chemical foaming stage originates from the polymer molecules themselves, leaving no residue and exhibiting excellent compatibility, avoiding the drawbacks of added foaming agents. The final material is a microporous foam composite material that combines lightweight and high strength.

[0022] 3. In the existing technology, the heat resistance temperature of general plastic shells such as ABS and PC is generally lower than 120℃. They are prone to softening and deformation in high temperature environments. Polyurethane foam has even worse heat resistance and is prone to heat aging.

[0023] The final cured product of this invention is a highly aromatic polyimide, whose molecular chain consists of a strong aromatic ring and an imide ring with high bond energy and a thermal decomposition temperature far exceeding 400°C. The heating process ensures complete solvent evaporation, complete Boc decomposition, and thorough imidization. The resulting cross-linked network further restricts the movement of the molecular chains at high temperatures. Therefore, this invention exhibits extremely high thermal stability, with a long-term operating temperature exceeding 200°C. During soldering, such as SMT reflow soldering, in high-temperature environments, or when the equipment generates internal heat, the casing will not soften, deform, or release harmful gases, ensuring the reliability and lifespan of electronic products.

[0024] 4. Common engineering plastics such as PC, ABS, Nylon or epoxy resin contain a large number of polar groups in their molecular chains, such as ester groups, amide groups and hydroxyl groups. These groups will undergo orientation polarization under alternating electric fields, which greatly contributes to the dielectric constant and loss, and their Dk is usually above 3.0.

[0025] The material of this invention can achieve an ultra-low and stable dielectric constant and extremely low dielectric loss; This invention introduces a large amount of trifluoromethyl groups and, through mechanical stirring and Boc decomposition to generate gas, forms numerous micron- and nano-sized closed pores within the material. Air, with a dielectric constant infinitely close to 1, is the lowest of all substances. By controlling the degree of foaming, the final dielectric constant of the material can be precisely controlled; the lower the density and the higher the porosity, the lower the dielectric constant, easily achieving below 2.0, even approaching 1.5. The fluorinated groups and the imidized cross-linked structure give the material extremely strong hydrophobicity, making it resistant to moisture absorption. The intrusion of water (Dk approximately 80) drastically increases the material's dielectric constant and loss. The glass fiber is covalently modified with KH-560 silane coupling agent, resulting in a strong and dense chemical bond between the fiber and the polyimide matrix. Even with the addition of 25% inorganic fiber, due to proper interface treatment, almost no additional dielectric loss mechanism is introduced. The material maintains extremely low dielectric loss across a wide frequency band, significantly reducing heat loss of signal energy. Attached Figure Description

[0026] Figure 1 The 1H NMR spectrum of Nα-Boc-lysine prepared in this invention. Detailed Implementation

[0027] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0028] The purity and manufacturers of the various drugs used in the experiment are shown in Table 1. Table 1. Raw Material Drug Information

[0029] Example 1: Prepared according to the following steps: (1) Boc-polyamic acid prepolymer Add 2,2'-bis(trifluoromethyl)benzidine and dimethylacetamide to the reactor, turn on the mechanical stirrer, introduce nitrogen gas for protection, and stir until completely dissolved; Cool the mixture to 0-5℃ in an ice-water bath, add 4,4'-(hexafluoroisopropyl)diphthalic anhydride, and keep the reaction temperature below 10℃ throughout the reaction, and react for 1 hour. After dissolving Nα-Boc-lysine in dimethylacetamide, the solution was slowly added dropwise to the reaction vessel, and the reaction was continued for 1 hour while maintaining a low temperature. Add an equal amount of 4,4'-(hexafluoroisopropyl)diphthalic anhydride as before, and maintain the reaction at low temperature for 2 hours; Remove the ice bath and allow the reaction system to naturally warm to room temperature. Continue the reaction for 16 hours to obtain Boc-polyamic acid prepolymer. (2) Composite and foaming Modified glass fibers were added to the Boc-polyamic acid prepolymer and dispersed at high speed for 45 minutes using a high-speed shear disperser until the fibers were uniformly dispersed, forming an extremely viscous paste slurry. Vigorously stir the paste using a mechanical mixer to incorporate a large amount of air until the volume expands to 3 times its original size, forming a stable foam paste. Add a foam stabilizer (silicone oil, viscosity (25℃) of 500-1500 mPa·s) and set aside. (3) Molding and step-by-step curing and crosslinking Foam slurry is injected into a mold pre-coated with a release agent. The mold is vibrated to remove large air bubbles. The mold is then placed in an oven for programmed heating to complete molding and step-by-step curing and cross-linking. After the program is completed, the oven is turned off, and the sample is allowed to cool to below room temperature before being removed from the mold to obtain modified polyimide, which can be used for electronic product casings.

[0030] In (1), the molar ratio of the total amount of 2,2'-bis(trifluoromethyl)benzidine and Nα-Boc-lysine to the total amount of 4,4'-(hexafluoroisopropyl)phthalic anhydride added is 1:1.1, wherein the molar ratio of 2,2'-bis(trifluoromethyl)benzidine to Nα-Boc-lysine is 6:4; the solid content of the Boc-polyamic acid prepolymer is controlled at 20%. The preparation process of the Nα-Boc-lysine includes the following steps: After dissolving lysine hydrochloride in deionized water with stirring, NaOH was slowly added to adjust the pH to 10.5. The NaCl precipitate was then separated by centrifugation to obtain free lysine base. The free base of lysine was transferred into the reaction vessel, tetrahydrofuran was added, and the temperature was lowered to 0℃; a tetrahydrofuran solution of Boc2O was added dropwise, and the reaction was carried out at 0-5℃ for 2 hours, and then the temperature was raised to 25℃ and the reaction was continued for 4 hours. Add concentrated hydrochloric acid until pH=3, and wash the precipitated white precipitate with ice water to obtain Nα-Boc-lysine.

[0031] In the preparation of Nα-Boc-lysine, the molar ratio of NaOH, lysine hydrochloride, and Boc2O is 2:1:1.

[0032] Nα-Boc-lysine was taken, purified, and then subjected to 1H NMR spectroscopy. The results are as follows: Figure 1 As shown; The singlet at 1.4 ppm is attributed to the nine equivalent hydrogens of the tert-butyl group in the Boc group; the multiplets at 1.2-1.8 ppm are attributed to the methylene hydrogen in the aliphatic chain; the peak at 2.2-2.4 ppm is attributed to the hydrogen on the α-carbon attached to the Boc; and the small peak at 6.5 ppm is attributed to the NH hydrogen in the α-amino group protected by the Boc. Each peak in the 1H NMR spectrum corresponds one-to-one with the hydrogen atoms in different chemical environments in the Nα-Boc-lysine molecule, especially the characteristic singlet of the tert-butyl group in the Boc group, which directly proves the successful introduction of the Boc protecting group. The peak shape and shift of the aliphatic chain and the α-carbon hydrogen also conform to the structural rules of the target molecule, indicating that the product is Nα-Boc-lysine.

[0033] In (2), the modified glass fiber accounts for 15% of the solid mass of the Boc-polyamic acid prepolymer; the foam stabilizer is a modified polysiloxane foam stabilizer, and the amount added is 0.5% of the mass of the foam slurry; The preparation process of modified glass fiber includes the following steps: Mix deionized water and ethanol at a volume ratio of 1:9, and adjust the pH to 5 with glacial acetic acid; slowly add 2% of the total weight of silane coupling agent KH-560, and hydrolyze with magnetic stirring at 40°C for 30 minutes; add short-cut glass fibers, and ensure complete impregnation at 60°C; remove the fibers, rinse lightly with ethanol, vacuum dry, and seal for later use.

[0034] In (3), the heating procedure is as follows: Heat to 80℃ and hold for 2 hours; heat to 120℃ and hold for 1 hour; heat to 170℃ and hold for 2 hours; heat to 200℃ and hold for 1 hour.

[0035] Example 2: The implementation method is the same as in Example 1, but the foaming ratio is 4 times; The molar ratio of 2,2'-bis(trifluoromethyl)benzidine to Nα-Boc-lysine is 7:3; In (2), the modified glass fiber accounts for 20% of the solid mass of the Boc-polyamic acid prepolymer; the foam stabilizer is a modified polysiloxane foam stabilizer, and the amount added is 0.35% of the mass of the foam slurry.

[0036] Example 3: The implementation method is the same as in Example 1, but the foaming ratio is 5 times; The molar ratio of 2,2'-bis(trifluoromethyl)benzidine to Nα-Boc-lysine is 7:3; In (2), the modified glass fiber accounts for 25% of the solid mass of the Boc-polyamic acid prepolymer; the foam stabilizer is a modified polysiloxane foam stabilizer, and the amount added is 0.1% of the mass of the foam slurry.

[0037] In addition to the examples, the following comparative experiments were designed: Comparative Example 1: The formulation and experimental method were the same as in Example 2, but lysine was used directly without protection by a protecting group; Comparative Example 2: The formulation and experimental method were the same as in Example 2, but the foaming ratio was 1. Comparative Example 3: The formulation and experimental method were the same as in Example 2, but the foaming ratio was 8 times; Comparative Example 4: The formulation and experimental method were the same as in Example 2, but the molar ratio of 2,2'-bis(trifluoromethyl)benzidine to Nα-Boc-lysine was 1:1; Comparative Example 5: The formulation and experimental method were the same as in Example 2, but the molar ratio of 2,2'-bis(trifluoromethyl)benzidine to Nα-Boc-lysine was 6:1; Comparative Example 6: Same formulation and experimental method as Example 2, but modified glass fiber accounts for 5% of the solid mass of Boc-polyamic acid prepolymer; Comparative Example 7: Same formulation and experimental method as Example 2, but modified glass fiber accounts for 30% of the solid mass of Boc-polyamic acid prepolymer; Comparative Example 8: The formulation and experimental methods were the same as in Example 2, but without the addition of modified glass fiber.

[0038] According to ISO 527, GB / T 19466.2, IEC 62631-2-2:2022, GB / T 31838.8-2024, ASTM D257, GB / T 1733, and ISO 62, its density, glass transition temperature, thermal decomposition temperature, flexural strength, dielectric constant, dielectric loss, volume resistivity, chemical resistance, and hygroscopicity were tested. The corresponding results are summarized in Table 2. Table 2. Test data of various properties of modified polyimide

[0039] Data Analysis: The slurry in the embodiment has a porous structure with uniform and regular pores; after molding, the surface is smooth, the texture is uniform, and there is no fiber agglomeration, so fine fiber textures are visible.

[0040] Comparative Example 1: The prepolymer synthesis process was out of control, resulting in uneven slurry with agglomerates. After molding, the texture was messy, with obvious cracks and unfoamed hard lumps, and it stuck to the mold during demolding. Comparative Example 2: After curing, it was a dense solid with no obvious pores; it had the highest density, a hard and heavy texture, no porous texture on the surface, and a smooth feel but poor toughness. Comparative Example 3: The slurry had large and uneven pores, with collapsed bubbles and cracks in some areas; it was loose and brittle; it had poor moldability, deformed after demolding, and had an uneven surface. Comparative Example 4: The texture was relatively soft, easily deformed, and had poor resilience. Comparative Example 5: It was brittle and hard, easily broken when bent, and stress cracks appeared on the surface. Comparative Example 6: The surface was smooth without fiber texture, and it was easily deformed. Comparative Example 7: Due to uneven fiber dispersion, there were obvious agglomerated particles and exposed fibers on the surface; it broke at the fiber agglomeration points when stretched. Comparative Example 8: The texture was soft, with poor toughness, and easily bent and deformed.

[0041] By comparing Example 2 and Comparative Example 1, it can be seen that: In Example 2, the α-amino group is protected due to the presence of Boc during the formation of the prepolymer, and only the carboxyl group participates in the polymerization reaction, forming a controllable linear or mildly cross-linked structure. In Comparative Example 1, unprotected lysine is used, and lysine participates in the reaction in multiple directions, resulting in high cross-linking, irregular structure, and the formation of a brittle and dense material. It can be clearly seen from the performance data table that all key properties of Example 2 are significantly better than those of Comparative Example 1.

[0042] By comparing Example 2, Comparative Example 2, and Comparative Example 3, it can be seen that: Comparative Example 2 was foamed 1 times. Foaming 1 times means that almost no air bubbles were introduced. The material is a solid and dense solid. Its mechanical strength is mainly borne by the continuous polyimide matrix and the glass fibers uniformly dispersed in it. It has the largest effective cross-sectional area to withstand stress. Therefore, it has extremely high flexural strength, but at the cost of the highest density and the material is bulky. Also, because the material has the highest solid content, polar molecular chain segments and groups, such as -CF3, C=O, -CONH-, play a dominant role in polarization, so it has the highest dielectric constant. Comparative Example 3 shows a foaming ratio of 8. An excessively high foaming ratio means the material is filled with a large number of air bubbles, resulting in very thin cell walls. This makes the cell structure highly susceptible to buckling, deformation, and collapse under load. Although the glass fiber still reinforces the matrix, the excessively thin cell walls cannot effectively transfer and disperse stress, becoming weak points in mechanical design. Therefore, its density is the lowest, but its mechanical strength also drops sharply, making the material brittle. The material is filled with a large amount of air with a low dielectric constant, significantly lowering the overall dielectric constant. Because the conduction path is interrupted by numerous air bubbles, the dielectric loss is also the lowest.

[0043] By comparing Example 2, Comparative Example 4, and Comparative Example 5, it can be seen that: The molar ratio of diamine monomer to Boc-lysine determines the structure, rigidity, crosslinking density, and number of functional groups of the polymer chain. In Comparative Example 4, the molar ratio is 1:1, and the proportion of Boc-lysine is very high. During the curing process, after the Boc protecting group is removed, a large amount of free amino groups (-NH2) are released. These amino groups can react with the imide rings on adjacent polyimide chains to form a very high crosslinking density. The extremely high crosslinking density severely restricts the movement of polymer molecular chain segments, thus requiring higher temperatures to undergo glass transition; decomposition requires higher energy, and the thermal stability of the crosslinking network is improved; high crosslinking density restricts the orientation polarization of polar groups and has poor chain segment mobility, reducing dielectric loss. However, high crosslinking density makes the molecular chains more compact, resulting in high melt strength and poor elasticity during foaming, which is not conducive to the full expansion and stability of bubbles. In Comparative Example 5, the molar ratio was 6:1, and the proportion of Boc-lysine was very low. After curing, there were very few amino sites available for crosslinking in the material, mainly forming linear or slightly crosslinked polymer structures. There was a lack of sufficient crosslinking point binding between molecular chains, and the chain segments moved relatively easily. Therefore, the glass transition temperature was low, and the thermal stability of the linear structure was not as good as that of the crosslinked network, resulting in a lower thermal decomposition temperature. When the material was subjected to stress, there was a lack of an effective crosslinked network to transfer stress, and the molecular chains were prone to relative slippage, which manifested as low strength, flexibility but easy deformation. By comparing Example 2, Comparative Example 6, and Comparative Example 7, it can be seen that: The amount of modified glass fiber added determines the composite structure, stress transfer efficiency, and stability of the cell structure of the material. In Comparative Example 6, the fiber content is too low, making it impossible to form an effective continuous reinforcing network in the polymer matrix. The fibers are separated by a large amount of foam matrix, becoming isolated from each other, and have little effect on the molecular chain movement and macromolecular structure of the matrix resin. Therefore, the thermal properties do not change significantly. When the fiber content is too low, the load is mainly borne by the relatively low-strength polyimide foam matrix, and the reinforcing effect of the fibers cannot be fully utilized, resulting in low flexural strength. The dielectric properties of the material are mainly determined by the low-dielectric polyimide matrix and air cells, thus exhibiting the best performance. In Comparative Example 7, the excessive fiber content easily leads to fiber agglomeration, requiring extremely high interface modification effects and making it more prone to interface defects, stress concentration points, and a decrease in flexural strength instead of an increase. At the same time, glass fiber is a polar inorganic material, and its dielectric constant and loss are higher than those of the polyimide matrix. When the content is too high, its negative contribution to the overall dielectric properties begins to dominate, leading to an increase in values. In addition, the possible presence of micro-interface defects may also introduce additional polarization mechanisms, increasing dielectric loss.

[0044] By comparing Example 2 and Comparative Example 8, it can be seen that: Comparative Example 8 is a pure polyimide foam, whose mechanical properties are entirely determined by the polymer matrix itself after foaming. When the foam material is subjected to bending load, the stress is mainly concentrated on the cell walls. The cell walls of pure polymer are relatively thin and have inherent brittleness. Under external force, they are prone to buckling, crack initiation and rapid propagation, leading to failure. This manifests as low bending strength and brittleness. Although pure polyimide itself has good dielectric properties, the absence of fibers during the foaming process may result in slightly poor uniformity of the cell structure. This structural inhomogeneity can cause local polarization differences and energy loss under an electric field.

[0045] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing a material for the casing of electronic products, characterized in that, Includes the following steps: (1) Boc-polyamic acid prepolymer: Add 2,2'-bis(trifluoromethyl)benzidine and dimethylacetamide to the reactor, turn on the mechanical stirrer, introduce nitrogen gas for protection, and stir until completely dissolved; Cool the mixture to 0-5℃ in an ice-water bath, add 4,4'-(hexafluoroisopropyl)diphthalic anhydride, and keep the reaction temperature below 10℃ throughout the reaction for 1 hour. After dissolving Nα-Boc-lysine in dimethylacetamide, the solution was slowly added dropwise to the reaction vessel, and the reaction was continued for 1 hour while maintaining a low temperature. Add an equal amount of 4,4'-(hexafluoroisopropyl)diphthalic anhydride as before, and maintain the reaction at low temperature for 2 hours; Remove the ice bath and allow the reaction system to naturally warm to room temperature. Continue the reaction for 12-20 hours to obtain Boc-polyamic acid prepolymer. (2) Composite and foaming: Modified glass fibers were added to the Boc-polyamic acid prepolymer and dispersed at high speed for 30-60 minutes using a high-speed shear disperser until the fibers were uniformly dispersed, forming an extremely viscous paste slurry. Use a mechanical mixer to vigorously stir the paste slurry for 5-10 minutes to incorporate a large amount of air until the volume expands to 3-5 times its original size, forming a stable foam slurry. Add a foam stabilizer and set aside. (3) Molding and stepped curing crosslinking: The foam slurry is injected into a mold pre-coated with a release agent. The mold is vibrated to remove large air bubbles. The mold is then placed in an oven for programmed heating to complete molding and step-by-step curing and cross-linking. After the program is completed, the oven is turned off, and the sample is allowed to cool to below room temperature before being removed from the mold to obtain modified polyimide.

2. The method for preparing a material for the casing of electronic products according to claim 1, characterized in that, In (1), the molar ratio of the total amount of 2,2'-bis(trifluoromethyl)benzidine and Nα-Boc-lysine to the total amount of 4,4'-(hexafluoroisopropyl)phthalic anhydride added is 1:1.1, wherein the molar ratio of 2,2'-bis(trifluoromethyl)benzidine to Nα-Boc-lysine is 3-8:2; the solid content of the Boc-polyamic acid prepolymer is controlled at 20%; The preparation process of the Nα-Boc-lysine includes the following steps: After dissolving lysine hydrochloride in deionized water with stirring, NaOH was slowly added to adjust the pH to 10-11. The NaCl precipitate was then separated by centrifugation to obtain free lysine base. The free base of lysine was transferred into the reaction vessel, tetrahydrofuran was added, and the temperature was lowered to 0℃; a tetrahydrofuran solution of Boc2O was added dropwise, and the reaction was carried out at 0-5℃ for 2 hours, and then the temperature was raised to 25℃ and the reaction was continued for 4 hours. Add concentrated hydrochloric acid until the pH reaches 2-3. Wash the precipitated white precipitate with ice water to obtain Nα-Boc-lysine.

3. A method for preparing a material for the casing of electronic products according to claim 2, characterized in that, In the preparation of Nα-Boc-lysine, the molar ratio of NaOH, lysine hydrochloride, and Boc2O is 2:1:

1.

4. The method for preparing a material for the casing of electronic products according to claim 1, characterized in that, In step (2), the mass of the modified glass fiber is 15-25% of the solid mass of the Boc-polyamic acid prepolymer; the foam stabilizer is a modified polysiloxane foam stabilizer, and the amount added is 0.1-0.5% of the mass of the foam slurry. The preparation process of modified glass fiber includes the following steps: Mix deionized water and ethanol at a volume ratio of 1:9, and adjust the pH to 4.5-5.5 with glacial acetic acid; slowly add 2% of the total weight of silane coupling agent KH-560, and hydrolyze with magnetic stirring at 40°C for 30 minutes; add short-cut glass fibers, and ensure complete impregnation at 60°C; remove the fibers, rinse lightly with ethanol, vacuum dry, and seal for later use.

5. A method for preparing a material for the casing of electronic products according to claim 1, characterized in that, In (3), the heating procedure is as follows: Heat to 80℃ and hold for 2 hours; heat to 120℃ and hold for 1 hour; heat to 170℃ and hold for 2 hours; heat to 200℃ and hold for 1 hour.

6. A material for the casing of electronic products prepared by the preparation method according to any one of claims 1-5, characterized in that, Modified polyimide is a smooth, rigid, closed-cell material with a density of 0.2-0.8 g / cm³. 3 The long-term operating temperature is 250℃.

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