A thermally crosslinkable polyimide, polyimide foam, its preparation method and application

Abstract

This invention belongs to the field of high-performance polymer materials technology, and relates to a thermally crosslinkable polyimide, polyimide foam, its preparation method, and applications. Addressing the problem of poor foaming performance in existing polyimide foam materials due to the difficulty in balancing melt strength and chain segment mobility, this invention proposes to control the number-average molecular weight of the precursor (…). M n The precursor (with a molecular weight of 15-45 kDa) is introduced with thermally crosslinkable end-cap groups to regulate its rheological properties. This precursor, after hot-press crosslinking, forms a preform with a specific elastic network structure. Then, through a two-step method of supercritical fluid saturation depressurization and high-temperature secondary foaming, polyimide foams with high foaming ratio, uniform and fine pores, low density, and excellent heat resistance can be prepared. This invention achieves predictable control from molecular parameters to foaming performance, is applicable to various thermoplastic polyimide systems, and has broad application prospects in aerospace insulation and other fields.

Description

Technical Field

[0001] This invention belongs to the field of polymer material processing technology, specifically relating to a thermally crosslinkable polyimide, polyimide foam, its preparation method, and its application. Background Technology

[0002] Polyimide (PI) is a high-performance polymer with an imide ring structure in its main chain. Due to its excellent high-temperature resistance, superior mechanical properties, and flame-retardant self-extinguishing properties, it is widely used in aerospace, microelectronics, and other fields. Preparing polyimide into foam materials can further endow it with lightweight, thermal insulation, and sound absorption properties, thereby expanding its applications in high-tech fields.

[0003] Current methods for preparing polyimide foam materials mainly involve the following steps: dianhydride monomers (mainly 2,3,3',4'-biphenyltetracarboxylic dianhydride) undergo esterification in the presence of alcohol compounds to form corresponding mono- or diester compounds; subsequently, these ester compounds react with a mixture of aromatic diamines and silane-containing diamines to form a polyimide precursor solution; heating removes small-molecule volatiles, yielding polyimide precursor powder; the polyimide precursor powder is then compressed and molded; finally, it is microwave-heated to 300-500℃ to form thermoplastic polyimide foam. However, the thermoplastic polyimide foam prepared by the above method has a glass transition temperature greater than 250℃, low closed-cell rate (<32%), and low compressive strength at room temperature (0.098-0.84 MPa). It easily softens under heating conditions, resulting in a significant decrease in mechanical properties, failing to meet the diverse application requirements under high-temperature conditions. Furthermore, this microwave foaming process is complex and costly, and its foaming process relies on the volatilization of organic solvents, which is not environmentally friendly and may be toxic.

[0004] Supercritical fluid foaming refers to the process of impregnating a polymer resin with a supercritical fluid under high temperature and pressure conditions, followed by rapid cooling and pressure relief to form micron-sized cells within the resin. Compared to chemical foaming, supercritical fluid foaming offers advantages such as being environmentally friendly, producing uniform cell diameter and distribution, and controlling cell density. It is widely used in the polymer foaming field; for example, high-performance elastomers such as TPEE, TPU, and TPAE can be foamed using supercritical fluid foaming to obtain foam materials with uniform cell structure. However, when supercritical fluid foaming is applied to polyimide, it encounters problems such as surface solidification of the foam material, low expansion ratio, and uneven cell structure. Summary of the Invention

[0005] Based on this, the purpose of the present invention is to provide a thermally crosslinkable polyimide that is suitable for supercritical fluid foaming, and polyimide foam with high expansion ratio and uniform cell size can be obtained by supercritical fluid foaming.

[0006] This invention is achieved through the following technical solution:

[0007] A thermally crosslinkable polyimide, the raw materials for which are synthesized include diamine compounds and dianhydride compounds, wherein the diamine compounds include at least one diamine monomer having a flexible chain segment and / or the dianhydride compounds include at least one dianhydride monomer having a flexible chain segment; the molecular chain ends of the polyimide have end-capping groups derived from end-capping agents, the end-capping groups including groups having crosslinkable end groups; the number-average molecular weight of the polyimide is 15 kDa to 45 kDa.

[0008] Compared with existing technologies, the thermally crosslinkable polyimide provided by this invention can serve as a precursor for polyimide foam. The core feature of this precursor lies in its controlled molecular weight (number-average molecular weight measured by GPC). M n It has a strength of 15kDa to 45kDa and a reactive end-capped structure. The elastic network structure formed by thermal crosslinking of this precursor is suitable for supercritical fluid foaming, and the polyimide foam prepared by supercritical fluid foaming has advantages such as high expansion ratio, uniform and fine cell structure, low density, and excellent heat resistance.

[0009] In one embodiment, the number-average molecular weight (NUMBER) M n The Mn content is 25kDa to 30kDa. When the Mn content of the precursor is precisely controlled within the range of 20kDa to 35kDa, especially within the range of 25kDa to 30kDa, the network structure formed by thermal crosslinking achieves the best balance between melt strength and segment mobility in rheology.

[0010] In one embodiment, the capping agent comprises a monohydric anhydride containing an alkenyl or alkynyl groups; and / or, the molar percentage of the group having crosslinkable end groups is 50-100% of all capping groups.

[0011] This invention provides a method for preparing a thermally crosslinkable polyimide, comprising the following steps:

[0012] A polyamic acid solution with end caps is prepared by reacting dianhydride monomer, diamine monomer, and end capping agent; the polyamic acid solution is then subjected to chemical imidization or one-step thermal cyclization to obtain a polyimide solution; the polyimide solution is then precipitated to obtain the thermally crosslinkable polyimide; wherein the molar ratio of dianhydride monomer, diamine monomer, and end capping agent is m:m+1:2, and the value of m ranges from 3 to 20.

[0013] The key to this method lies in the fact that by adjusting the monomer molar ratio of the polycondensation reaction (formula m:(m+1):2), the polyimide content can be precisely controlled. M nIt falls within the target range. This method is compatible with two mature imidization processes: chemical imidization and one-step thermal cyclization (one-step method).

[0014] In one embodiment, the chemical imidization method involves adding a dehydrating agent and a catalyst to a polyamic acid solution and reacting at room temperature.

[0015] The one-step thermal cyclization method is as follows: the polyamic acid solution is heated to 120℃~200℃, and the thermal cyclization dehydration reaction is carried out directly by stirring in an anhydrous and oxygen-free nitrogen atmosphere.

[0016] The present invention also provides a polyimide foam, which is obtained by sequentially hot pressing, supercritical fluid foaming, and high-temperature foaming of the thermally crosslinkable polyimide.

[0017] In one embodiment, the polyimide foam has a density of 0.1 g / cm³. 3 up to 0.5g / cm 3 .

[0018] This invention also provides a method for preparing polyimide foam, comprising the following steps:

[0019] A polyamic acid solution with end capping is prepared by reacting dianhydride monomer, diamine monomer, and end capping agent; the polyamic acid solution is then subjected to chemical imidization or one-step thermal cyclization to obtain a polyimide solution; the polyimide solution is then precipitated to obtain a polyimide precursor; wherein the molar ratio of dianhydride monomer, diamine monomer, and end capping agent is m:m+1:2, and the value of m ranges from 3 to 20;

[0020] S20. The polyimide precursor is subjected to hot pressing treatment to crosslink the polyimide precursor, thereby obtaining crosslinked polyimide. The crosslinked polyimide is then cured and molded to obtain a polyimide preform.

[0021] S30. The polyimide preform is saturated in a high-temperature and high-pressure supercritical fluid atmosphere, and then depressurized and foamed to obtain a foaming intermediate. The foaming intermediate is heated to above its glass transition temperature for secondary foaming to obtain the polyimide foam.

[0022] This method involves cross-linking a polyimide precursor through thermoforming to form an intermediate product with a micro-crosslinked network structure, defined as a "preform". This preform can be a sheet, plate, profile, or a molded part of a specific shape, serving as an ideal matrix for subsequent foaming. Then, using the preform as raw material, a two-step process of "supercritical fluid saturation depressurization + high-temperature secondary foaming" is employed to fully utilize the rheological properties of the micro-crosslinked network structure, producing high-ratio, fine-pore, uniformly distributed high-temperature resistant foam.

[0023] In one embodiment, in step S30, the supercritical fluid is selected from carbon dioxide, nitrogen, or a mixture thereof; the saturation temperature is 60~200℃, the pressure is 5~40MPa, and the saturation time is 5~48h.

[0024] In one embodiment, in step S30, the temperature of the secondary foaming is 220~300℃ and the time is 50~150s.

[0025] The present invention also provides the application of the polyimide foam in aerospace thermal insulation, electronic device protection or lightweight structural components. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the synthesis of the thermally crosslinkable polyimide in Example 1 of the present invention, wherein the capping agent is maleic anhydride.

[0027] Figure 2 This is a flow chart of the preparation process of the polyimide foam described in this invention.

[0028] Figure 3 These are comparison images of the polyimide foam before and after foaming in Example 3, where (a) is a digital image of the polyimide preform; and (b) is a digital image of the polyimide foam.

[0029] Figure 4 This is a scanning electron microscope (SEM) image of the polyimide foam prepared in Example 4 of the present invention.

[0030] Figure 5 This is a schematic diagram showing the density and expansion ratio of the polyimide foam prepared in Examples 1-8 and Comparative Example 1 of the present invention.

[0031] Figure 6 The images are scanning electron microscope (SEM) images of the cross-sectional structures of polyimide foams formed by foaming polyimide preforms prepared in Examples 3, 9 to 12 of this invention at different temperatures, wherein (a1) and (a2) are at 220°C; (b1) and (b2) are at 240°C; (c1) and (c2) are at 260°C; (d1) and (d2) are at 280°C; and (e1) and (e2) are at 300°C.

[0032] Figure 7 The changes in air intake during the foaming process of the polyimide preform are shown in (a) for the effect of saturation time on the air intake of the polyimide preforms in Examples 1-8 at 13 MPa and 160 °C; and (b) for the effect of wetting temperature on the air intake of the polyimide preforms in Examples 5 and 13 at 13 MPa and 14 h.

[0033] Figure 8 This is a cyclic compression curve of the polyimide foam in Example 4.

[0034] Figure 9 This is a pore size distribution diagram of the polyimide foam in Example 4.

[0035] Figure 10 This is a digital image of the polyimide foam from Example 4 undergoing vertical combustion.

[0036] Figure 11 The graph shows the surface temperature of the polyimide foam in Example 4 when it is placed on a hot table at 200°C; where (a) is the relationship between surface temperature and placement time, (b1) represents a 5 mm thick polyimide foam, and (b2) represents a 12 mm thick polyimide foam. Detailed Implementation

[0037] This invention reveals that the foaming temperature (the temperature during supercritical fluid impregnation) in supercritical fluid foaming is generally 125-190℃. Materials like TPU have relatively low glass transition temperatures, allowing them to maintain temperatures above their glass transition temperature during cooling and pressure relief, thus enabling foaming. However, polyimide is a high-temperature resistant, high-performance polymer with a high glass transition temperature, typically above 250℃. During cooling and pressure relief, polyimide rapidly reverts to a glassy state, causing the material surface to solidify and hindering foaming. While heating can prevent this rapid return to the glassy state, it also reduces the solubility of the supercritical fluid, leading to a decrease in the number of bubble nuclei and consequently, a decrease in bubble density.

[0038] Therefore, the primary objective of this invention is to lower the glass transition temperature of polyimide. Unlike methods that use alcohol compounds to esterify dianhydride monomers, this invention directly prepares a polyamic acid solution by mixing dianhydride and diamine monomers. Then, a polyimide adhesive is obtained through a chemical imidization reaction or a one-step thermal cyclization reaction. The polyimide adhesive is then precipitated to obtain a polyimide precursor. The polyimide precursor is hot-pressed to obtain a polyimide preform. The preform is then saturated in a high-temperature, high-pressure supercritical fluid atmosphere, followed by depressurization and foaming to obtain polyimide foam.

[0039] Based on the above method, this invention screens dianhydride monomers and diamine monomers and finds that polyimide foams prepared by combining dianhydride monomers containing flexible segments (such as ether bonds, isopropyl groups, fluorine-containing groups, etc.) with diamine monomers (such as 4,4'-(4,4'-isopropyldiphenoxy)bis(phthalic anhydride), pyromellitic dianhydride, 4,4'-oxobisphthalic anhydride, etc.) and diamine monomers (such as 2,2'-bis[4-(4-aminophenoxyphenyl)]propane, m-phenylenediamine, etc.) have significantly lower glass transition temperatures. Among them, 4,4'-(4,4'-isopropyldiphenoxy)bis(phthalic anhydride) (BPADA) and 2,2'-bis[4-(4-aminophenoxy)phenyl]propane (BAPP) have the advantages of good flexibility, low glass transition temperature and easy control. The glass transition temperature of polyimide foam prepared by using 4,4'-(4,4'-isopropyldiphenoxy)bis(phthalic anhydride) (BPADA) as dianhydride monomer and 2,2'-bis[4-(4-aminophenoxy)phenyl]propane (BAPP) as diamine monomer is 190~210℃.

[0040] However, during the research process, this invention discovered that the currently prepared polyimide foam still suffers from unsatisfactory foaming performance. Further analysis revealed that the excellent supercritical foaming performance of materials such as TPU and TPEE is not only due to their low glass transition temperature, but more importantly, their outstanding rheological properties. Specifically, TPU and TPEE possess suitable melt strength. During the high-temperature, high-pressure impregnation stage, the supercritical fluid diffuses rapidly and uniformly within the material, forming a large number of uniformly distributed bubble nuclei, resulting in high pore density. During the rapid depressurization stage, the melt strength is sufficient to resist the deformation caused by bubble expansion, preventing bubble merging or collapse, ultimately yielding a pore structure with high closed-cell ratio and uniform size. If the melt strength is too high, gas diffusion resistance increases, reducing bubble nucleation points and decreasing pore density. If the melt strength is too low, bubbles are prone to merging or rupture, leading to uneven pore size. Meanwhile, materials such as TPU and TPEE exhibit non-Newtonian fluid properties, with melt viscosity decreasing as shear rate or temperature increases. During the high-temperature, high-pressure impregnation stage, the material viscosity is relatively high, which is conducive to gas dissolution and the formation of a homogeneous system. During the rapid depressurization stage, the material viscosity decreases rapidly, promoting bubble expansion and cell growth while preventing melt collapse. In addition, the molecular structure of these materials is typically a soft-hard segment phase separation structure. The soft segments provide flexibility, while the hard segments form crystalline microregions or physical cross-linking points, giving the material excellent elasticity. During bubble expansion, the tensile viscosity of the material increases rapidly, resisting deformation caused by gravity and preventing cell collapse or merging.

[0041] In contrast, polyimide has linearly arranged molecular chains and lacks a cross-linked network. Its melt strength is significantly lower than that of TPU and TPEE, making it unable to resist deformation caused by bubble expansion. During depressurization, bubbles are prone to coalescing into large bubbles or collapsing due to insufficient melt strength, resulting in uneven cell size and low closed-cell rate. Polyimide also has extremely high melt viscosity; even near its decomposition temperature, its viscosity remains much higher than that of TPU and TPEE, hindering the diffusion and dissolution of supercritical fluids and making it difficult to form a large number of uniformly distributed bubble nuclei.

[0042] Based on this, the present invention considers the preparation of thermally crosslinkable polyimides, using these polyimides as precursors for thermal crosslinking to form polyimide materials with network structures, thereby improving the elasticity and melt strength of the polyimide materials and making them suitable for supercritical fluid foaming. The present invention discovers that the properties of polyimide materials are mainly affected by the degree of crosslinking of their network structure, which is primarily determined by the molecular weight of the precursor. Therefore, the concept of the present invention is to prepare thermoplastic polyimide materials suitable for foaming and capable of constructing micro-crosslinked network structures by end-capping and controlling the molecular weight.

[0043] Specifically, based on the dianhydride monomers and diamine monomers screened in previous studies, this invention further explored the effect of crosslinking degree on the performance of polyimide foam: when the molecular weight of the polyimide (precursor) is too low or too high, even with crosslinking, the foaming ratio of the polyimide is not ideal. Low crosslinking degree leads to insufficient strength, while excessive crosslinking degree results in high viscosity, hindering chain segment movement and inhibiting foaming. When the molar ratio of the dianhydride monomer, diamine monomer, and end-capping agent is m:m+1:2, with m ranging from 3 to 20, the foaming ratio of the polyimide foam is significantly improved.

[0044] In existing technologies, there are strategies for preparing polyimides by adding end-capping agents containing cross-linking functional groups (such as norbornene), but their application is mainly limited to increasing the molecular weight of the polymer or improving its bulk mechanical strength. While these methods focus on the type and addition of end-capping agents, they seriously neglect the research and precise control of the precursor molecular weight, a fundamental parameter that determines the final cross-linked network topology and rheological properties. Uncontrolled molecular weight can lead to an overly dense (poor chain mobility) or overly sparse (insufficient melt strength) cross-linked network, neither of which can achieve the dynamic balance of cell nucleation, growth, and stability during foaming.

[0045] Therefore, this invention prepares a polyimide preform with a micro-crosslinked structure by controlling the molecular weight and degree of crosslinking. On the one hand, by constructing a micro-crosslinked structure, the melt strength of the material is enhanced, the processing window of the material is widened, and the foaming and compression properties of polyimide are enhanced. On the other hand, by adjusting the distance between the crosslinking points, oligomers with appropriate molecular weights are provided, which not only meet the requirements of bubble nucleation under the classical nucleation theory, but also have a melt strength much stronger than that of linear materials to meet the requirements of high-ratio foaming.

[0046] However, this invention found that while the polyimide preform formed from the polyimide precursor can form uniform bubble nuclei and significantly increase bubble density during supercritical foaming, the expansion ratio of the final polyimide foam remains limited, meaning the bubble nuclei are not fully foamed. Analysis revealed that the glass transition temperature of the polyimide foam formed from the aforementioned polyimide precursor is still within the range of 190-210°C, significantly lower than the glass transition temperature of existing polyimides, but still higher than that of materials such as TPU and TPEE. The supercritical fluid vaporization during the depressurization process cannot fully foam the bubble nuclei.

[0047] Therefore, this invention first uses supercritical fluid foaming technology to foam the polyimide for the first time. The supercritical fluid can be uniformly dispersed in the polyimide under high pressure. When the pressure is released from the reactor, the supercritical fluid rapidly vaporizes, forming a large number of uniform bubble nuclei inside the polyimide. Then, the polyimide is foamed for the second time at high temperature, which provides sufficient energy to the polyimide chain segments so that they can overcome the mobility in the glass state and enter the high elastic state, thereby forming a uniform polyimide foam with a high expansion ratio.

[0048] Based on the above analysis, this invention provides a polyimide precursor, the synthetic raw materials of which include a diamine compound, a dianhydride compound, and a capping agent. The diamine compound includes at least one diamine monomer with a flexible chain segment, and / or the dianhydride compound includes at least one dianhydride monomer with a flexible chain segment. The polyimide molecular chain ends with capping groups derived from the capping agent, and the capping groups include groups with crosslinkable end groups. Introducing diamine monomers and / or dianhydride monomers with flexible chain segments is beneficial for improving the rheological properties of the polyimide precursor.

[0049] The end-capping agent includes a monohydric anhydride containing an alkenyl or alkynyl group. The monohydric anhydride combines with the terminal amino group of polyamic acid (PAA) through an anhydride ring-opening reaction. After imidization, unsaturated double or triple bonds remain at the chain end, forming a group with crosslinkable end groups. Theoretically, any end-capping agent containing thermally reactive functional groups such as alkenyl or alkynyl groups is applicable. Specifically, the crosslinkable end groups are derived from unsaturated anhydrides, such as maleic anhydride, nadic anhydride, itaconic anhydride, etc.

[0050] To ensure the formation of an effective network, the molar percentage of the groups with crosslinkable end groups (i.e., end groups containing crosslinkable functional groups such as unsaturated bonds) should be above 50% of all end groups to ensure sufficient crosslinking point density.

[0051] In some embodiments, the molecular chain of the polyimide precursor is mainly composed of flexible segments, the diamine compound includes only diamine monomers with flexible segments (i.e., flexible diamine monomers), and the dianhydride compound includes only dianhydride monomers with flexible segments (i.e., flexible dianhydride monomers).

[0052] In some embodiments, the molecular chain of the polyimide precursor is composed of flexible segments and rigid segments, the diamine compound and / or the dianhydride compound includes at least one flexible monomer, and the diamine compound and / or the dianhydride compound includes at least one rigid monomer.

[0053] As an example, the diamine compound includes at least one flexible diamine monomer and at least one rigid diamine monomer; and / or, the dianhydride compound includes at least one flexible dianhydride monomer and at least one rigid dianhydride monomer; and / or, the diamine compound includes at least one flexible diamine monomer and the dianhydride compound includes at least one rigid dianhydride monomer; and / or, the diamine compound includes at least one rigid diamine monomer and the dianhydride compound includes at least one flexible dianhydride monomer.

[0054] The molar ratio of the flexible monomer (the sum of flexible diamine monomer and flexible dianhydride monomer) to the rigid monomer (the sum of rigid diamine monomer and rigid dianhydride monomer) is 1~4:1.

[0055] The number-average molecular weight of the polyimide precursor ( M n The value is 15-45 kDa, preferably 20-35 kDa, and more preferably 25-30 kDa.

[0056] The structural formula of the polyimide precursor is:

[0057] ;

[0058] Wherein, R1 is selected from , and any one of them;

[0059] R2 is selected from and At least one of them;

[0060] R3 is selected from , , , as well as One or more of them.

[0061] R4 is selected from , , , as well as One or more of them.

[0062] R3 and R4 can use the same group or different groups.

[0063] Where n is a positive integer representing the degree of aggregation. Preferably, n is 3 to 50.

[0064] like Figure 1 As shown, the preparation method of the above-mentioned polyimide precursor includes the following steps S11 to S13.

[0065] S11. Polyamic acid is prepared by prepolymerization reaction of dianhydride monomer, diamine monomer and end-capping agent under low temperature environment.

[0066] S12. The polyamic acid is subjected to a chemical imidization reaction or a one-step thermal cyclization reaction to obtain a polyimide solution.

[0067] S13. The polyimide solution is precipitated and washed to obtain a polyimide precursor, wherein the polyimide precursor is polyimide fiber or polyimide powder (i.e., PI fiber / powder).

[0068] The dianhydride monomer is one or more of 4,4'-(4,4'-isopropyldiphenoxy)bis(phthalic anhydride) (BPADA), pyromellitic dianhydride (PMDA), and 4,4'-oxobisphthalic anhydride (ODPA).

[0069] The diamine monomer is one or more of 2,2'-bis[4-(4-aminophenoxyphenyl)]propane (BAPP) and m-phenylenediamine (MPD).

[0070] The capping agent is one or more of maleic anhydride (MAH), nadic anhydride (NA), itaconic anhydride (IA), phthalic anhydride (PA), and 4-phenylacetylene phthalic anhydride.

[0071] The molar ratio of the dianhydride monomer, the diamine monomer, and the capping agent is m:m+1:2, where m ranges from 5 to 20.

[0072] The solvent for the pre-condensation reaction is dimethylformamide (DMF), dimethylacetamide (DMAc), or methylpyrrolidone (NMP), and the reaction time is 22-24 h.

[0073] The chemical imidization reaction is as follows: a dehydrating agent and a catalyst are added to polyamic acid, and the reaction is carried out at room temperature. The dehydrating agent is acetic anhydride, and the addition ratio is 3 times the equivalent of the dianhydride monomer; the catalyst is pyridine, and the addition ratio is 1.5 times the equivalent of the dianhydride monomer.

[0074] The chemical imidization reaction takes 12 to 24 hours.

[0075] The solid content of the polyimide adhesive is (15~30)wt%.

[0076] Furthermore, the present invention also provides a method for preparing polyimide foam using the above-mentioned polyimide precursor, such as... Figure 2 As shown, it includes steps S10, preparation of the polyimide precursor, S20, crosslinking, and S30, foaming. Step S10 is consistent with the above-described method for preparing the polyimide precursor. In step S20, the shape of the polyimide preform is determined according to the final shape of the required polyimide foam. This invention uses a polyimide sheet as an example to illustrate the foaming method of this invention.

[0077] S20, Crosslinking: The polyimide precursor is subjected to hot pressing to crosslink it, resulting in crosslinked polyimide. The crosslinked polyimide is then cured and molded to obtain a polyimide preform.

[0078] Specifically, the washed and dried polyimide precursor is placed in a hot press mold, then placed in a vacuum hot press, heated in a stepwise manner and pressurized at a certain temperature to obtain a polyimide preform.

[0079] The hot-pressing temperature of the polyimide preform is 80~280℃. Specifically, the heating steps of the stepped heating are as follows: first gradient, heating to 130~170℃ and holding for 0.5~1.5h; second gradient, heating to 180~220℃ and holding for 0.5~1.5h; third gradient, heating to 245~255℃ and holding for 0.5~1.5h; fourth gradient, heating to 260~280℃ and pressurizing to 20MPa, holding for 10min, then turning off the heating and slowly cooling to room temperature.

[0080] Rheological frequency scanning tests were performed at 250℃ with a strain of 1%. The tanδ of the crosslinked polyimide was 0.1~0.5, preferably 0.1~0.3, within the frequency range of 0.1~500 rad / s. At 0.1 rad / s, the composite viscosity of the crosslinked polyimide was 10. 6 ~10 7 Pa.s, preferably 2 × 10 6 ~4×10 6 Pa.s, preferably 3 × 10⁻⁶ 6 Pa.s.

[0081] S30. Foaming: The polyimide preform is saturated in a high-temperature and high-pressure supercritical fluid atmosphere, and then depressurized to foam, resulting in a foaming intermediate; the foaming intermediate is heated to above its glass transition temperature for secondary foaming to obtain the polyimide foam.

[0082] Specifically, the polyimide preform obtained after hot pressing is placed in a supercritical impregnation device, maintaining a temperature of 60~200℃ and a pressure of 7~40MPa until the polyimide preform is completely impregnated, i.e., the polyimide preform becomes saturated in a high-temperature, high-pressure supercritical fluid atmosphere. The pressure is then released for foaming, yielding a foaming intermediate. The supercritical fluid can be carbon dioxide, nitrogen, or a mixture of carbon dioxide and nitrogen.

[0083] The foaming intermediate was transferred to an oven at 250~300℃ for secondary foaming under normal pressure to obtain polyimide foam.

[0084] The density of the polyimide foam is 0.1 g / cm³. 3 up to 0.5g / cm 3 .

[0085] In step S11, the end-capping agent acts as the end-chain reaction of polyamic acid, introducing crosslinkable functional groups such as unsaturated double bonds (C=C) to provide reaction sites for crosslinking. The unsaturated double bonds and other crosslinkable functional groups introduced by the end-capping agent undergo a crosslinking reaction in step S20, causing the polyimide precursor to form a crosslinked polymer with a network structure, i.e., crosslinked polyimide.

[0086] Based on the above-mentioned methods for preparing polyimide precursors and polyimide foams, this invention synthesizes a series of different... M n A thermally crosslinkable polyimide precursor was obtained, and then the polyimide precursor was foamed to form polyimide foam. The foaming performance of the polyimide foam was tested, and key data for determining the molecular weight window and verifying the effect were obtained. Based on the data, the parameters of the preparation method of polyimide precursor and polyimide foam were determined. The test results showed that the polyimide foam obtained by the above method has low density, high pore size and uniform cell distribution, which solves the problem of low foaming ratio of polyimide foam and difficulty in expanding its application.

[0087] The technical solutions and effects of the present invention will be further explained below with reference to embodiments and comparative examples.

[0088] In the examples, the parameter "number-average molecular weight (NUMBER)" is used. M n "Number average molecular weight" refers to the number average molecular weight of the thermally crosslinkable polyimide precursor, which is actually measured by gel permeation chromatography (GPC).

[0089] Parameter "average molecular weight between cross-linking points ( M c "" refers to the average molecular weight of the chain segments between two adjacent crosslinking points in the three-dimensional network (i.e., the network structure of the polyimide preform) formed after the polyimide precursor is crosslinked by hot pressing.

[0090] It should be noted that the described embodiments are merely some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0091] Example 1

[0092] This embodiment provides a method for preparing polyimide foam, comprising step S10, preparation of a polyimide precursor, and steps S20, crosslinking, and S30, foaming. In this embodiment, the dianhydride monomer is 4,4'-(4,4'-isopropyldiphenoxy)bis(phthalic anhydride) (BPADA); the diamine monomer is 2,2'-bis[4-(4-aminophenoxyphenyl)]propane (BAPP); the solvent is methylpyrrolidone (NMP); the end-capping agent is maleic anhydride (MAH); the dehydrating agent is acetic anhydride; and the catalyst is pyridine.

[0093] See Figure 1 Step S10 includes steps S11 to S13.

[0094] S11. Under low temperature conditions, a polyamic acid solution is prepared by pre-condensation reaction of dianhydride monomer, diamine monomer, and end-capping agent.

[0095] Specifically, 13.1362 g of 2,2-bis[4-(4-aminophenoxy)phenyl]propane (BAPP) was dissolved in 100 mL of methylpyrrolidone solution (NMP solution). Under low temperature conditions, 1.5690 g of maleic anhydride monomer was added to the NMP solution containing BAPP and stirred for 1 h. Then, 12.4918 g of 4,4'-(4,4'-isopropyldiphenoxy)bis(phthalic anhydride) (BPADA) was slowly added to the prepolymer solution in batches and stirred for 23 h for prepolymerization reaction to obtain polyamic acid solution (PAA solution).

[0096] The molar ratio of dianhydride monomer, diamine monomer, and capping agent is 3:4:2.

[0097] S12. The polyamic acid solution is subjected to a chemical imidization reaction or a one-step thermal cyclization reaction to obtain a polyimide adhesive.

[0098] This embodiment uses a chemical imidization reaction to prepare polyimide solution, specifically including: adding 9.0g of acetic anhydride to PAA solution, stirring for 0.5h to ensure uniform dispersion, then adding 3.8g of pyridine, stirring and reacting for 24h to obtain polyimide solution (PI solution).

[0099] S13. The polyimide solution is precipitated and washed to obtain the polyimide precursor, which is the thermally crosslinked polyimide.

[0100] Specifically, the PI adhesive solution is slowly poured into a large amount of ethanol solution, and washed three times with the ethanol solution to obtain polyimide fibers (PI fibers), i.e., polyimide precursors. The PI fibers are then placed in a heating device for drying; in this embodiment, the drying temperature is 80°C and the time is 24 hours.

[0101] The number-average molecular weight of dried PI fibers was measured by gel permeation chromatography (GPC). M n The results are shown in Table 1.

[0102] S20, Crosslinking: The polyimide precursor (PI fiber) is subjected to hot pressing to crosslink the polyimide precursor, resulting in crosslinked polyimide. The crosslinked polyimide is then cured and molded to obtain a polyimide preform, specifically including steps S21 to S23.

[0103] S21. Weigh 12.0000g of washed and dried polyimide precursor and place it into a hot press mold.

[0104] S22. The mold is placed in a vacuum hot press for hot pressing, heated in a stepped manner and pressurized at a certain temperature. The stepped heating and pressurization are as follows: first gradient, heating to 150℃ and holding for 1 hour; second gradient, heating to 200℃ and holding for 1 hour; third gradient, heating to 250℃ and holding for 1 hour; fourth gradient, heating to 260℃ and pressurizing to 20 MPa. During the hot pressing process, the polyimide precursor undergoes crosslinking to obtain crosslinked polyimide.

[0105] S23. After holding at this temperature for 10 minutes, turn off the heat and slowly cool to room temperature. The cross-linked polyimide then cures and forms a polyimide preform. In this embodiment, the polyimide preform is a polyimide sheet (i.e., PI sheet) of a certain thickness.

[0106] Calculate the average molecular weight between crosslinking points of the crosslinked polyimide based on the feed ratio. M c The theoretical values ​​are shown in Table 1.

[0107] S30. Foaming: The polyimide preform is saturated in a high-temperature and high-pressure supercritical fluid atmosphere, and then depressurized and foamed to obtain a foaming intermediate; the foaming intermediate is heated to above its glass transition temperature to allow it to undergo secondary foaming (i.e., heating foaming) to obtain polyimide foam, specifically including steps S31 to S32.

[0108] S31. The polyimide preform obtained after hot pressing is placed in a supercritical carbon dioxide impregnation device and impregnated with supercritical carbon dioxide (sCO2) for 14 hours at a temperature of 160℃ and a pressure of 13MPa. The pressure is then released and the vessel is opened to obtain the foaming intermediate.

[0109] S32. Transfer the foaming intermediate to a 260℃ oven for secondary foaming for 100s to obtain polyimide foam (i.e., PI foam).

[0110] Example 2

[0111] Example 2 provides a method for preparing polyimide foam, which is similar to the preparation method in Example 1 and will not be repeated here. The difference is that the molar ratio of dianhydride monomer, diamine monomer, and end-capping agent in step S11 is 6:7:2, specifically:

[0112] S11. Dissolve 22.9886g of BAPP in 200mL of methylpyrrolidone solution. Under low temperature conditions, add 1.5690g of maleic anhydride monomer to NMP solution containing BAPP and stir for 1h. Then, slowly add 24.9835g of 4,4'-(4,4'-isopropyldiphenoxy)bis(phthalic anhydride) in batches to the prepolymer solution and stir for 23h to obtain PAA adhesive.

[0113] Example 3

[0114] Example 3 provides a method for preparing polyimide foam, which is similar to the preparation method in Example 1 and will not be repeated here. The difference is that the molar ratio of dianhydride monomer, diamine monomer, and end-capping agent in step S11 is 9:10:2, specifically:

[0115] S11. Dissolve 32.8408g of BAPP in 300mL of methylpyrrolidone solution. Under low temperature conditions, add 1.5690g of maleic anhydride monomer to NMP solution containing BAPP and stir for 1h. Then, slowly add 37.4753g of 4,4'-(4,4'-isopropyldiphenoxy)bis(phthalic anhydride) in batches to the prepolymer solution and stir for 23h to obtain PAA adhesive.

[0116] See Figure 3 The volume of the polyimide preform increases significantly after foaming treatment.

[0117] Example 4

[0118] Example 4 provides a method for preparing polyimide foam, which is similar to the preparation method in Example 1 and will not be repeated here. The difference is that the molar ratio of dianhydride monomer, diamine monomer, and end-capping agent in step S11 is 12:13:2, specifically:

[0119] S11. Dissolve 42.6930g of BAPP in 400mL of methylpyrrolidone solution. Under low temperature conditions, add 1.5690g of maleic anhydride monomer to NMP solution containing BAPP and stir for 1h. Then, slowly add 49.9670g of 4,4'-(4,4'-isopropyldiphenoxy)bis(phthalic anhydride) in batches to the prepolymer solution and stir for 23h to obtain PAA adhesive.

[0120] Example 5

[0121] Example 5 provides a method for preparing polyimide foam, which is similar to the preparation method in Example 1 and will not be repeated here. The difference is that the molar ratio of dianhydride monomer, diamine monomer, and end-capping agent in step S11 is 15:16:2, specifically:

[0122] S11. Dissolve 52.5453g of BAPP in 500mL of methylpyrrolidone solution. Under low temperature conditions, add 1.5690g of maleic anhydride monomer to NMP solution containing BAPP and stir for 1h. Then, slowly add 62.4588g of 4,4'-(4,4'-isopropyldiphenoxy)bis(phthalic anhydride) in batches to the prepolymer solution and stir for 23h to obtain PAA adhesive.

[0123] Example 6

[0124] Example 6 provides a method for preparing polyimide foam, which is similar to the preparation method in Example 1 and will not be repeated here. The difference is that the molar ratio of dianhydride monomer, diamine monomer, and end-capping agent in step S11 is 18:19:2, specifically:

[0125] S11. Dissolve 62.3975g of BAPP in 600mL of methylpyrrolidone solution. Under low temperature conditions, add 1.5690g of maleic anhydride monomer to NMP solution containing BAPP and stir for 1h. Then, slowly add 74.9506g of 4,4'-(4,4'-isopropyldiphenoxy)bis(phthalic anhydride) in batches to the prepolymer solution and stir for 23h to obtain PAA adhesive.

[0126] Example 7

[0127] Example 7 provides a method for preparing polyimide foam, which is the same as the preparation method in Example 1 and will not be repeated here. The difference is that the molar ratio of dianhydride monomer, diamine monomer and end-capping agent in step S11 is 21:22:2, specifically:

[0128] S11. Dissolve 7.2250g of BAPP in 70mL of methylpyrrolidone solution. Under low temperature conditions, add 0.1569g of maleic anhydride monomer to NMP solution containing BAPP and stir for 1h. Then, slowly add 8.7442g of 4,4'-(4,4'-isopropyldiphenoxy)bis(phthalic anhydride) in batches to the prepolymer solution and stir for 23h to obtain PAA adhesive.

[0129] Example 8

[0130] Example 8 provides a method for preparing polyimide foam, which is similar to the preparation method in Example 1 and will not be repeated here. The difference is that the molar ratio of dianhydride monomer, diamine monomer, and end-capping agent in step S11 is 24:25:2, specifically:

[0131] S11. Dissolve 8.2102g of BAPP in 80mL of methylpyrrolidone solution. Under low temperature conditions, add 0.1569g of maleic anhydride monomer to NMP solution containing BAPP and stir for 1h. Then, slowly add 9.9934g of 4,4'-(4,4'-isopropyldiphenoxy)bis(phthalic anhydride) in batches to the prepolymer solution and stir for 23h to obtain PAA adhesive.

[0132] Example 9

[0133] Example 9 provides a method for preparing polyimide foam, which is the same as the preparation method in Example 3 and will not be repeated here. The difference is that the foaming temperature in step S32 is 220°C.

[0134] Example 10

[0135] Example 10 provides a method for preparing polyimide foam, which is the same as the preparation method in Example 3 and will not be repeated here. The difference is that the foaming temperature in step S32 is 240°C.

[0136] Example 11

[0137] Example 11 provides a method for preparing polyimide foam, which is the same as the preparation method in Example 3 and will not be repeated here. The difference is that the foaming temperature in step S32 is 280°C.

[0138] Example 12

[0139] Example 12 provides a method for preparing polyimide foam, which is the same as the preparation method in Example 3 and will not be repeated here. The difference is that the foaming temperature in step S32 is 300°C.

[0140] Example 13

[0141] Example 13 provides a method for preparing polyimide foam, which is the same as the preparation method in Example 5 and will not be repeated here. The difference is that the temperature of supercritical carbon dioxide impregnation in step S31 is 60°C.

[0142] Comparative Example 1

[0143] Comparative Example 1 provides a method for preparing polyimide foam, which is similar to the preparation method in Example 1 and will not be repeated here. The difference is that the molar ratio of dianhydride monomer, diamine monomer, and end-capping agent in step S11 is 101:100:0. In Comparative Example 1, the polyimide precursor obtained in steps S11 to S13 is a linear polyimide that is not end-capped.

[0144] Test Example 1

[0145] The polyimide foams obtained in Examples 1 to 8 were subjected to performance testing and analysis, and compared with the polyimide foam prepared in Comparative Example 1.

[0146] Test environment conditions: temperature 15~35℃, relative humidity 40%~60%.

[0147] The testing equipment and conditions are as follows.

[0148] (1) Density and expansion ratio

[0149] Testing equipment: Densitometer.

[0150] Test method: 1) Measure the mass M1 of polyimide foam in air and the mass M2 of polyimide foam submerged in water at an ambient temperature of 25℃; 2) Calculate the density of polyimide foam. ρ =(M1 / (M1-M2))× ρ 0, ρ 0 represents the density of water at 25℃; 3) Calculate the expansion ratio = ( ρ 1 / ρ 2)-1.

[0151] (2) Heat resistance

[0152] Test equipment: Thermogravimetric analyzer (TGA).

[0153] Test method: The above samples (crosslinked polyimide preforms) were tested using Perkinelmer PE Pyrisl TGA. The sample mass was 6~10 mg, the gas flow rate was 40 mL / min, the heating rate was 10℃ / min, the test range was 50~800℃, and the test atmosphere was nitrogen.

[0154] (3) Glass transition temperature

[0155] Test equipment: Differential scanning calorimeter (DSC).

[0156] Test method: The glass transition temperature of the above sample (crosslinked polyimide preform) was obtained by DSC 250 tester from TA. The test conditions were as follows: 50 mL / min N2, heating and cooling rates were both 10℃ / min.

[0157] (4) Pore structure

[0158] Testing equipment: Scanning electron microscope (SEM).

[0159] Test method: The surface morphology of a foam cross-section sample sputtered with 100 Å gold at an accelerating voltage of 10 kV and a current of 10 A was photographed using an S4800 scanning electron microscope. The cross-sectional images were then analyzed using IPP.

[0160] The test results are shown in Table 1.

[0161] Table 1. Test results of polyimide precursors, polyimide preforms, and polyimide foams.

[0162]

[0163] The data shown in Table 1 clearly reveal the pattern of change: the polyimide precursors prepared in Examples 1-8 M n In the range of 15–45 kDa, it is significantly lower than that of the polyimide precursor in Comparative Example 1. M n .

[0164] Combination Figure 5 As can be seen from the expansion ratios in Table 1, the polyimide precursors of Examples 1-8 possess foaming properties. M n Within the optimization range of 20~35kDa (Examples 3~5), especially M n Within the molecular weight range of 25–30 kDa, the expansion ratio reaches its peak (6.89–10.51 times), significantly superior to that of Comparative Example 1, indicating its applicability in lightweight structural components and other fields. In other words, Mn The pre-crosslinked product (polyimide precursor) with a strength of 20-35 kDa, under the action of the micro-crosslinked network provided by maleic anhydride, achieves an optimal balance between melt strength and molecular chain mobility, perfectly meeting the requirements for bubble nucleation and growth; while M n Below or above this range (Examples 1-2, Examples 6-8), performance significantly decreases.

[0165] Test Example 2

[0166] The cross-sectional structure of the polyimide foams prepared in Examples 3 and 9-12 was analyzed by SEM, and the obtained SEM images are shown below. Figure 6 As shown, a fine pore structure is exhibited. In particular, the foaming temperature of 260°C in Example 3 is more conducive to reducing the pore size of the pore structure.

[0167] Test Example 3

[0168] The air intake volume of polyimide preforms from Examples 1-8 and Comparative Example 1 at different saturation times, as well as the air intake volume of polyimide preforms from Examples 5 and 13 at different saturation times, were measured. The results are as follows: Figure 7 As shown, extending the saturation time and lowering the immersion temperature are beneficial to increasing the intake volume, but after saturation for 12-14 hours, further extending the saturation time has little effect on the intake volume.

[0169] Test Example 4

[0170] The cyclic compression curve and pore size distribution of the polyimide foam prepared in Example 4 were determined, and the results are as follows: Figure 8 and Figure 9 As shown, the polyimide foam exhibits a high-density, uniformly distributed distribution of small cells (average diameter approximately 4 μm). Its compressive strength at 10% strain is 6.2 MPa, and its compressive modulus is 21.39 MPa.

[0171] Test Example 5

[0172] The polyimide foam from Example 4 was subjected to a standard vertical burning test (UL-94), and the results are as follows: Figure 10 As shown, according to the UL-94 standard, the vertical flammability rating of this polyimide foam reaches V-0, indicating that the polyimide foam of Example 4 has excellent flame retardancy and meets the requirements of being lightweight, fireproof, and heat-resistant.

[0173] Test Example 6

[0174] The thermal insulation performance of the polyimide foam in Example 4 was tested: the polyimide foam was placed on a heating platform at 200°C, and the temperature data and images of the foam surface were recorded using an infrared thermal imager.

[0175] See Figure 11 The study showcased infrared thermal images of PI foam samples with thicknesses of 5 mm and 12 mm on a 200℃ hot stage. Test results showed that the surface temperature of the 5 mm thick sample fluctuated between 75 and 85℃ within 10 minutes, while the 12 mm thick sample remained stable between 47 and 53℃. This directly demonstrates the excellent thermal insulation performance of this foam material, making it suitable for applications in aerospace thermal protection systems, high-power electronic device encapsulation and thermal insulation, and scenarios requiring lightweight and efficient thermal insulation components, such as robots and drones. It significantly reduces weight while maintaining structural rigidity and provides thermal insulation and flame retardant protection for internal precision electronic components, improving the overall safety and endurance of the equipment.

[0176] Test Example 7

[0177] The polyimide preform from Example 4 was subjected to rheological frequency scanning tests at 260°C and a strain of 1%. The tests showed that the composite viscosity of the crosslinked polyimide preform at 0.1 rad / s with a tanδ of 3 × 10⁻⁶ at frequencies of 0.1–0.3 was 3 × 10⁻⁶. 6 Pa.s.

[0178] Based on the above test results, the present invention further explored the amidation method and the raw material monomer system, and the results are shown in Examples 14 to 20 below.

[0179] Example 14

[0180] Example 14 provides a method for preparing polyimide foam. The similarities with the preparation method in Example 3 will not be repeated here. The difference is that in step S12, the polyamic acid is reacted in one step to obtain polyimide slurry.

[0181] Specifically, the thermal cyclization reaction includes: heating the polyamic acid solution to 120-200°C, and directly carrying out the thermal cyclization dehydration reaction by stirring in an anhydrous, oxygen-free, nitrogen-purified environment to obtain a polyimide adhesive.

[0182] The molecular weight of the polyimide precursor was determined, and the results showed that the number-average molecular weight of the polyimide precursor was in the range of 20-35 kDa.

[0183] The expansion ratio of the polyimide foam was measured, and the results showed that Example 14 successfully prepared a polyimide foam with an expansion ratio of up to 7.50 times, with a uniform cell structure and excellent thermal stability.

[0184] Example 15

[0185] Example 15 provides a method for preparing polyimide foam, which is similar to the preparation method in Example 3 and will not be repeated here. The difference is that the end-capping agents in S11 are phthalic anhydride and maleic anhydride, and the monomer feeding ratio is 3:7, specifically:

[0186] S11. Under low-temperature conditions, the molar ratio of dianhydride monomer, diamine monomer, and capping agent is controlled at 9:10:2. The capping agent is phthalic anhydride:maleic anhydride = 3:7.

[0187] The molecular weight of the polyimide precursor was determined, and the results showed that the number-average molecular weight of the polyimide precursor was in the range of 20-35 kDa.

[0188] The expansion ratio of the polyimide foam was measured, and the results showed that Example 15 successfully prepared a polyimide foam with an expansion ratio as high as 7.50 times, exhibiting a uniform cell structure and excellent thermal stability. This indicates that when phthalic anhydride and maleic anhydride are combined as end-capping agents, a thermally crosslinkable polyimide network can be successfully constructed, resulting in good foaming performance. This example demonstrates that, when composed of a specific molecular weight (M... n Based on the suitable network structure determined by the invention, the type of end-capping agent can be adapted to maintain the material's good foaming ability while further optimizing its processing rheological properties (such as melt flowability), thereby extending to other advanced molding processes such as extrusion and 3D printing, enriching the implementation methods and application scenarios of the invention.

[0189] Example 16

[0190] Example 16 provides a method for preparing polyimide foam, which is similar to the preparation method in Example 14 and will not be repeated here. The difference lies in the end-capping agent monomer system in step S11, specifically:

[0191] S11. Under low-temperature conditions, the molar ratio of dianhydride monomer, diamine monomer, and capping agent is controlled at 9:10:2. The capping agent is phthalic anhydride:4-phenylacetylene phthalic anhydride = 3:7.

[0192] The molecular weight of the polyimide precursor was determined, and the results showed that the number-average molecular weight of the polyimide precursor was in the range of 20-35 kDa.

[0193] The expansion ratio of the polyimide foam was measured, and the results showed that Example 16 successfully prepared a polyimide foam with an expansion ratio as high as 10.17 times, with uniform cell structure and excellent thermal stability.

[0194] Example 17

[0195] Example 17 provides a method for preparing polyimide foam, which is similar to the preparation method in Example 3 and will not be repeated here. The difference lies in the dianhydride and diamine monomer system in step S11, specifically:

[0196] S11. Under low-temperature conditions, the molar ratio of dianhydride monomer, diamine monomer, and end-capping agent is controlled at 9:10:2. The dianhydride monomer is 4,4'-(4,4'-isopropyldiphenoxy)bis(phthalic anhydride) (BPADA) and pyromellitic dianhydride (PMDA), with a molar ratio of BPADA:PMDA = 95:5. The diamine monomer is 2,2'-bis[4-(4-aminophenoxyphenyl)]propane (BAPP) and m-phenylenediamine (MPD), with a molar ratio of BAPP:MPD = 80:20.

[0197] The molecular weight of the polyimide precursor was determined, and the results showed that the number-average molecular weight of the polyimide precursor was in the range of 20-35 kDa.

[0198] The expansion ratio of the polyimide foam was measured, and the results showed that Example 17 successfully prepared a polyimide foam with an expansion ratio as high as 8.63 times, exhibiting a uniform cell structure and excellent thermal stability. This demonstrates that the successful implementation of this control strategy does not depend on a single specific monomer combination and is equally applicable to other monomer systems that can construct thermoplastic polyimides (TPIs). As long as the precursor is controlled according to this invention... M n If the value falls within the effective range (20~35kDa), a micro-crosslinked elastic network suitable for supercritical foaming can be constructed in polyimides with various chemical compositions, thereby verifying the wide applicability of the molecular design principles proposed in this invention.

[0199] Example 18

[0200] Example 18 provides a method for preparing polyimide foam, which is the same as the preparation method in Example 3 and will not be repeated here. The difference is that the dianhydride monomer and diamine monomer in step S11 are BPADA and MPD, respectively.

[0201] The heat resistance and glass transition temperature of the polyimide foam were determined. The results showed that the polyimide foam exhibited good heat resistance and glass transition temperature. T d10% The temperature was 497.38℃. T g It is 211.5℃.

[0202] Example 19

[0203] Example 19 provides a method for preparing polyimide foam, which is the same as the preparation method in Example 3 and will not be repeated here. The difference is that in step S11, the dianhydride monomers are BPADA and PMDA, and their molar ratio is BPADA:PMDA = 75:25. The diamine monomer is MPD.

[0204] The heat resistance and glass transition temperature of the polyimide foam were determined. The results showed that the polyimide foam exhibited good heat resistance and glass transition temperature. T d10% It was 548.16℃. T g The temperature was 238.1℃.

[0205] Example 20

[0206] Example 20 provides a method for preparing polyimide foam, which is the same as the preparation method in Example 3 and will not be repeated here. The difference is that the dianhydride monomer and diamine monomer in step S11 are OPDA and MPD, respectively.

[0207] The heat resistance and glass transition temperature of the polyimide foam were determined. The results showed that the polyimide foam exhibited good heat resistance and glass transition temperature. T d10% It was 611.33℃. T g The temperature was 281.52℃.

[0208] The results of Examples 18-20 show that the BPADA and BAPP system helps to reduce the glass transition temperature of polyimide foam.

[0209] In summary, Examples 1-8 demonstrate how changing the feed ratio m (from 3 to 24) can regulate the system. M n The process of verifying its foaming performance was also described. Examples 9-12 demonstrate that within the preferred molecular weight range, adjusting the foaming temperature can still yield excellent foaming performance, with a foaming temperature of 260°C being more conducive to forming a fine and uniform pore structure. Example 13 demonstrates the effect of wetting temperature on foaming. Example 14 demonstrates that a low-temperature one-step thermal cyclization method can be used to replace the chemical imidization method to prepare the precursor. As long as the synthesis parameters are precisely controlled so that the precursor molecular weight falls within the effective range (25-30 kDa) described in this invention, polyimide materials suitable for high-performance foaming can also be obtained. Examples 15-17 demonstrate that within the preferred molecular weight range (based on a formulation with m=9), changing the end-capping agent combination (Examples 15 and 16) or introducing a third monomer to fine-tune the chain structure (Example 17) can still yield excellent foaming performance, further proving the universality of the strategy of this invention.

[0210] Therefore, in summary, this invention has established, through extensive experiments, the molecular weight window and control method for achieving efficient foaming of polyimide. Any technical solution that prepares thermally crosslinkable polyimide precursors, preforms, and foams using the aforementioned or equivalent strategies within the molecular weight parameters and principles declared in this invention should be considered to fall within the protection scope of this invention.

[0211] Compared to existing technologies, this invention provides a strategy rather than a specific formulation. Any method that achieves polyimide micro-crosslinking and foaming by controlling the crosslinking molecular weight within the stated range and the crosslinking agent dosage within the stated ratio range falls within the scope of protection of this invention, regardless of a specific monomer combination. By adjusting two easily controllable synthesis parameters, the foaming behavior of the final material can be precisely regulated, with good process repeatability. This method can successfully prepare PI foams with high foaming ratio, uniform and fine cell structure, and high closed-cell ratio. The foamed material retains the inherent excellent heat resistance, flame retardancy, and mechanical strength of polyimide. This strategy is applicable to various subsequent processing methods such as molding, extrusion, and 3D printing, combined with supercritical fluid foaming processes.

[0212] This invention is not limited to the above-described embodiments. If any modifications or variations to this invention do not depart from the spirit and scope of this invention, and if such modifications and variations fall within the scope of the claims and equivalent technologies of this invention, then this invention also intends to include such modifications and variations.

Claims

1. A thermally crosslinkable polyimide for preparing polyimide foam, characterized in that: The raw materials for its synthesis include diamine compounds, dianhydride compounds, and capping agents. The diamine compounds include at least one diamine monomer with flexible segments, and the dianhydride compounds include at least one dianhydride monomer with flexible segments. The polyimide has capping groups derived from the capping agent at the ends of its molecular chains, and these capping groups include groups with crosslinkable end groups. The polyimide has a number-average molecular weight of 25 kDa to 30 kDa. The diamine monomer with flexible segments is 2,2-diamine. The synthesis material is '-bis[4-(4-aminophenoxyphenyl)]propane, wherein the dianhydride monomer with flexible segments is 4,4'-(4,4'-isopropyldiphenoxy)bis(phthalic anhydride); in the synthetic raw materials, the molar ratio of diamine compound, dianhydride compound and end-capping agent is m:m+1:2, where m ranges from 9 to 15; the end-capping agent is a monohydric anhydride containing alkenyl or alkynyl groups; and the molar percentage of the group with crosslinkable end groups among all end-capping groups is 50 to 100%.

2. A method for preparing a thermally crosslinkable polyimide for preparing polyimide foam as described in claim 1, characterized in that, Includes the following steps: A polyamic acid solution with end caps is prepared by reacting a diamine compound, a dianhydride compound, and an end capping agent; the polyamic acid solution is then subjected to a chemical imidization method or a one-step thermal cyclization method to obtain a polyimide solution; the polyimide solution is then precipitated to obtain the thermally crosslinkable polyimide.

3. The method for preparing thermally crosslinkable polyimide for preparing polyimide foam according to claim 2, characterized in that: The chemical imidization method involves adding a dehydrating agent and a catalyst to a polyamic acid solution and reacting at room temperature. The one-step thermal cyclization method is as follows: the polyamic acid solution is heated to 120℃~200℃, and the thermal cyclization dehydration reaction is carried out directly by stirring in an anhydrous and oxygen-free nitrogen atmosphere.

4. A polyimide foam, characterized in that, The thermally crosslinkable polyimide prepared by the method described in claim 1 or any one of claims 2 to 3 is obtained by sequentially performing thermal crosslinking treatment, supercritical fluid foaming, and high-temperature foaming.

5. The polyimide foam according to claim 4, characterized in that, The density of the polyimide foam is 0.1 g / cm³. 3 Up to 0.5g / cm 3 .

6. A method for preparing polyimide foam, characterized in that, Includes the following steps: S10, diamine compounds, dianhydride compounds, and a capping agent are reacted to obtain a capped polyamic acid solution; the polyamic acid solution is subjected to chemical imidization or one-step thermal cyclization to obtain a polyimide solution; the polyimide solution is precipitated to obtain the thermally crosslinkable polyimide according to claim 1; S20. The thermally crosslinkable polyimide is used as a precursor and subjected to hot pressing to crosslink the precursor, thereby obtaining a crosslinked polyimide. The crosslinked polyimide is then cured and molded to obtain a polyimide preform. S30. The polyimide preform is saturated in a high-temperature and high-pressure supercritical fluid atmosphere, and then depressurized and foamed to obtain a foaming intermediate. The foaming intermediate is transferred to a high-temperature environment for secondary foaming to obtain the polyimide foam. The temperature of the high-temperature environment is higher than the glass transition temperature of the polyimide preform. The temperature of the secondary foaming is 220~300℃, and the time is 50~150s. The polyimide foam has an expansion ratio greater than or equal to 6.89 and a glass transition temperature less than or equal to 238.1℃.

7. The application of the polyimide foam according to claim 4 or 5, or the polyimide foam prepared by the preparation method according to claim 6, in aerospace thermal insulation, electronic device protection, or lightweight structural components.

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