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Research Progress and Challenges of Fluorinated Polyimide Engineering Plastics at China University of Geology

Engineering Plastics Applications 2025-03-26 14:01:22

Polyimide (PI) engineering plastics have found extensive applications in modern industrial fields due to their excellent high and low-temperature resistance, environmental stability, as well as mechanical and electrical properties. According to processing characteristics, PI engineering plastics can be divided into thermoplastic and thermosetting types; according to crystallization characteristics, they can be categorized into amorphous and (semi) crystalline types; based on synthesis processes, they can be classified as "monomer reactant in situ polymerization (PMR)" types; according to reaction principles, they can be divided into condensation and addition types; based on molding processes, they can be classified into compression molding, injection molding, extrusion molding, additive manufacturing (3D printing), and resin transfer molding (RTM) types; according to chemical composition, they can be categorized into pyromellitic dianhydride (PMDA) type (referred to as "PMDA type"), 3,3',4,4'-biphenyltetracarboxylic dianhydride (sBPDA) type (referred to as "biphenyl dianhydride type"), 2,3,3',4'-biphenyltetracarboxylic dianhydride (aBPDA) type (referred to as "isomeric biphenyl dianhydride type"), 3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA) type (referred to as "ketone dianhydride type"), 3,3',4,4'-benzophenone ether tetracarboxylic dianhydride (ODPA) type (referred to as "ether dianhydride type"), and 4,4'-(hexafluoroisopropyl) diphenyl tetracarboxylic dianhydride (6FDA) type (referred to as "fluorinated type"); according to different end groups, they can be classified into nadic anhydride type (referred to as "NA type") and phenylethyne-terminated anhydride type (referred to as "PETI").

Among various types of polyimide (PI) engineering plastics, fluorinated polyimide (FPI) stands out due to its unique high-temperature resistance and thermal oxidative stability, gaining widespread attention in specialized fields such as aerospace and civil high-tech sectors like optoelectronics. Table 1 lists the types of common fluorinated PI engineering plastics along with their typical chemical structures. Fluorination enhances the optical, dielectric, and processing properties of PI materials, making them favored by numerous researchers. Generally speaking, fluorination achieves the following functions: (1) changes in surface characteristics and moisture absorption rates. Fluorination can reduce the surface free energy of PI engineering plastics, enhancing their hydrophobicity and lowering moisture absorption. (2) changes in electrical properties. Fluorination can lower the dielectric constant of PI engineering plastics, especially in high-frequency environments. (3) changes in optical properties. Fluorination can reduce the absorption of PI engineering plastics in the visible and infrared regions, thereby improving their optical transparency and reducing haze. Furthermore, the substitution of C—H bonds with C—F bonds in the molecular structure of PI materials significantly reduces absorption in the infrared communication bands (1.3 μm and 1.55 μm). Fluorination also lowers the refractive index of PI engineering plastics. (4) changes in processing properties. Generally, fluorination can improve the solubility of PI resin in organic solvents, thus endowing it with good processing characteristics. Figure 1 compares the properties of fluorinated PI (FPI) engineering plastic Avimid® (FPI-1, see Table 1) with conventional PI engineering plastics. It is evident that fluorination provides FPI engineering plastics with excellent solubility. The outstanding comprehensive performance has led to the rapid development of FPI engineering plastics in recent years, making them one of the most important members of the specialty engineering plastics family.

Table 1 Typical FPI engineering plastics and their chemical structures

Fig. 1 Processing characteristics of FPI engineering plastics

Recently, there has been a trend in the field of fluorinated polymers internationally, namely the restriction orders on per- and polyfluoroalkyl substances (PFAS). In light of years of research on the harmful effects of PFAS on human health and the environment, organizations such as the European Chemicals Agency (ECHA), the Organisation for Economic Co-operation and Development (OECD), and the United States Environmental Protection Agency (EPA) have begun to issue proposals to restrict the use of PFAS, which are expected to be implemented by 2025. The current mainstream definition of PFAS by ECHA/OECD is "any substance containing at least one carbon atom with a -CF3 or -CF2- moiety (with no H or halogen atoms on that carbon atom)." FPI engineering plastics are typically prepared by polymerizing fluorinated dianhydrides, such as 6FDA (FPI-1 to FPI-4, Table 1) or fluorinated diamines, such as 2,2'-bis(trifluoromethyl) diphenylamine (TFMB) (FPI-5, Table 1) and 1,4-bis[(4-amino-2-trifluoromethyl)phenoxy]benzene (6FAPB) (FPI-6, Table 1) with other types of monomers. According to the current definition of PFAS, commonly used 6FDA dianhydride and diamine monomers like TFMB in FPI engineering plastics will face varying degrees of restrictions. The author briefly reviews the research and development status of FPI engineering plastics under the background of PFAS restriction orders and discusses future development trends.

Research and Development Status of FPI Engineering Plastics

FPI engineering plastics are specifically designed and developed to address the performance deficiencies of conventional PI engineering plastics, such as relatively poor heat and thermal oxidative stability, higher moisture absorption, and unstable dielectric properties. Therefore, the application fields of FPI engineering plastics mainly focus on special applications in aviation, aerospace, and electronics. Based on their structural composition, FPI engineering plastics can be roughly divided into two categories: those based on fluorinated dianhydrides and those based on fluorinated diamines.

1.1 Fluorinated Dianhydride-based FPI Engineering Plastics

Currently, the most widely used fluorinated dianhydride monomer in FPI engineering plastics is 6FDA. In terms of thermosetting FPI engineering plastics, in the 1970s, Garcia et al. from NASA successfully developed PMR-type resins, greatly expanding the application fields of thermosetting PI engineering plastics. The PMR-II (FPI-2, Table 1) material is mainly synthesized by the polymerization of the esterification product of 6FDA (6FDE) with p-phenylenediamine (PDA), using nadic acid anhydride (NA) as a capping agent, with a long-term heat resistance temperature reaching up to 371 ℃, significantly higher than the first generation PMR-15 (BTDA) at 316 ℃. Its synthesis process is shown in Figure 2a. To further improve the processing performance of PMR resin and enhance its thermal oxidative stability, by adjusting the amount of NA capping agent, researchers from the U.S. Air Force further developed the AFR700B (FPI-3, Table 1) engineering plastic. The synthetic route is shown in Figure 2b. The glass transition temperature (Tg) of the cured AFR700B resin exceeded 400 ℃ and was successfully applied to the wing trailing edge structure of the F-117A stealth fighter aircraft. However, during later applications, it was found that although AFR700B had good thermal oxidative stability, its hydrolytic stability was relatively poor. By adjusting the ratio of monomer to capping agent amounts and designing the molecular weight, researchers also successfully developed other engineering plastics such as AFR700A and PMR-II-30. For details, see Table 2.

Fig. 2 Synthesis of thermosetting FPI engineering plastics based on 6FDA.

Tab. 2 Formulations of different PMR PI resins

Note：BTDE is ester of BTDA and methanol。

In response to the performance deficiencies of AFR700B type FPI engineering plastics, researchers at the Air Force Research Laboratory developed a new thermosetting engineering plastic, AFR-PEPA-N (AFR700C, FPI-4, Table 1), based on phenylethyne-terminated phthalic anhydride (PEPA), with the synthesis route shown in Figure 2c. Compared to AFR700B, AFR-PEPA-N exhibits excellent hydrolytic resistance, with only a 3% to 5% decrease in dry Tg after hot and humid aging, while the AFR700B system shows a decline of over 20%. Additionally, the Tg of the AFR-PEPA-N resin system reaches 435 to 455 ℃, demonstrating good mechanical properties and low melt viscosity, allowing for the molding of complex components using resin transfer molding (RTM) processes.

In addition to the aforementioned thermosetting FPI engineering plastics, DuPont has also developed a high-temperature thermoplastic FPI engineering plastic, Avimid® N (FPI-1, Table 1). The synthesis process is shown in Figure 3. This material is copolymerized from the precursor tetracarboxylic compound 4,4'-(hexafluoroisopropylidene) diphenyl tetracarboxylic dianhydride (6FTA), 95% PDA, and 5% m-phenylenediamine (m-PDA). The Tg of this material is 352 °C, and it can maintain high initial performance even after aging for 100 hours at 371 °C in nitrogen or air environments. Avimid® N pure resin exhibits good toughness and excellent high-temperature oxidation stability, with its laminated boards showing significantly better micro-crack resistance compared to the thermosetting PMR-15 system. Its outstanding overall performance makes its molded parts have good application prospects in jet engine structures. However, the main issues in practical applications of this material are high volatile content, difficulty in completely removing solvents, and poor melt flowability.

Fig. 3 Synthesis of thermoplastic FPI engineering plastics based on 6FTA

1.2 Fluorinated Diamines-Based FPI Engineering Plastics

In addition to using fluorinated aromatic dianhydrides to prepare FPI engineering plastics, fluorinated aromatic diamine monomers have also been widely applied in the research and development of FPI engineering plastics. One of the most notable materials is the PETI-375 type developed by NASA's Langley Research Center (LaRC) (FPI-5, Table 1), with its synthetic route shown in Figure 4 [29]. Phenylethynyl-endcapped PI (PEPI) is a type of thermosetting PI engineering plastic developed by researchers at NASA's LaRC in the late 1990s, specifically for the RTM molding process. Compared to traditional thermosetting PIs capped with nadic anhydride, phenylethynyl anhydride, and others, these materials have a wider processing window, meeting the application needs for fiber impregnation. Depending on the Tg, PETI resins can be categorized into various types such as PETI-5 (Tg: 270 °C), PETI-298 (Tg: 298 °C), PETI-330 (Tg: 330 °C), and PETI-375 (Tg: 375 °C). Among them, PETI-375 is synthesized from aBPDA and two diamine monomers [fluorinated diamine TFMB and conventional aromatic diamine 1,3-bis(4-aminophenoxy)benzene], along with the endcapping agent PEPA in N-methyl-2-pyrrolidone (NMP). The isomeric structure of aBPDA endows the PETI-375 resin system with excellent processability and high Tg characteristics, while the incorporation of TFMB provides the resin with outstanding high-temperature oxidation stability. Therefore, PETI-375 resin has promising applications in aerospace, aviation, and mechanical manufacturing fields. Recently, Hong et al. [30] used the fluorinated monomers 6FDA and TFMB to prepare a type of FPI PETI resin, achieving a Tg of 438 °C. Wu et al. [31] conducted copolymerization using BPADA, TFMB, 6FDA, and CBDA, maintaining excellent thermal stability (Tg: 217–242 °C) while also possessing considerable light transmittance (T450: 66%–77%).

Fig. 4 Synthesis of thermosetting FPI engineering plastics based on TFMB

In addition to thermosetting FPI engineering plastics, fluorinated diamines are also widely used in the development of thermoplastic FPI engineering plastics. Yang et al. [16] used ODPA and 1,4-bis[(4-amino-2-trifluoromethyl)phenoxy]benzene (6FAPB) for high-temperature polymerization in NMP, with phthalic anhydride (PA) as the end-capping agent, to prepare KHTPI-2500 type thermoplastic FPI engineering plastic (FPI-6, Table 1). The preparation route is shown in Figure 5. This TPI material exhibits good melt flow properties, with a melt viscosity as low as 2,300 Pa·s at 360 °C and a melt flow rate of 10.1 g/10 min (360 °C). Using this resin as the raw material, thin-walled components with a wall thickness of only 0.2 mm were successfully produced through injection molding.

Fig. 5 Synthesis of thermoplastic FPI engineering plastics based on 6FAPB

Zhi et al. [32] reported a semi-aromatic transparent thermoplastic FPI engineering plastic PI (HBPDA-BDAF) based on the alicyclic dianhydride monomer, hydrogenated biphenyl tetracarboxylic dianhydride (HBPDA), and the fluorinated aromatic diamine monomer 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (BDAF). The synthesis route is shown in Figure 6. The results indicate that this series of resins possesses good melt processing characteristics. The FPI sheets prepared by the compression molding process exhibit a lighter appearance and good transparency. Additionally, the molded products demonstrate excellent mechanical properties and can be further processed using machining, drilling, and other methods. This material also possesses good thermal stability (Tg: 220 °C), low dielectric constant (2.56 at 1 MHz), and low dielectric loss (0.008 at 1 MHz). Recently, Qi et al. [33] prepared FPI-type thermoplastic engineering plastics using BDAF and hydrogenated pyromellitic dianhydride (HPMDA). Compared to HBPDA-BDAF, the resulting PI (HPMDA-BDAF) engineering plastic exhibited superior thermal stability (Tg: 273-274 °C).

Fig. 6 Synthesis of thermoplastic FPI engineering plastics based on BDAF

Challenges Facing the Development of Fluorinated Polyimide Engineering Plastics

In recent years, there has been a growing call internationally to restrict the use of PFAS substances. In addition to small-molecule compounds such as perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS), PFAS substances also include high-molecular-weight compounds such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), and FPI. Due to the excellent comprehensive properties of C-F bonds (high bond energy, high electronegativity, low molar polarizability, low molar refraction, good hydrophobicity, etc.), fluorination can often endow polymer materials with many unique properties. However, fluorinated products, especially those containing PFAS compounds, have high environmental persistence. Some types possess bioaccumulative and toxic characteristics, which can have far-reaching impacts on water environments, soil, and even human health. Dammel et al. from EMD Electronics, a business unit of Merck Group in North America, reviewed the status of PFAS in the semiconductor photolithography field. Common fluorinated chemicals used in the semiconductor photolithography field, including trifluromethanesulfonic acid, perfluorobutane sulfonic acid, fluorine-containing photosensitive polyimide (PSPI), and fluorine-containing photosensitive benzoxazole (PSPBO), are all classified as PFAS substances.

FPI engineering plastics have broad application prospects in national defense modernization and civilian high-tech fields. Currently, the FPI engineering plastics used in the defense sector are unlikely to be affected by the PFAS restrictions in the short term. However, FPI engineering plastics used in civilian high-tech sectors, such as low-dielectric engineering plastics for 5G base station construction, low-dielectric antenna components for smartphones, and various smart modules for infrared transmission, may be impacted by the PFAS restriction order. At present, many uses of materials containing PFAS do not have known substitutes. Furthermore, even if substitute materials are developed, they must undergo rigorous process validation before entering mass production stages, eventually becoming viable alternatives. In some cases, after years of research, it might be found that substitutes without PFAS cannot provide the required chemical functions. Nevertheless, the development of alternative materials for FPI-type engineering plastics in civilian applications should receive close attention from both academia and industry.

3 Conclusion

FPI engineering plastics generally combine the excellent comprehensive properties of PI materials and fluoroplastics, thus after years of development, they have become an important member of the special engineering plastics family. In addition to the increasing application demands in special fields such as aviation, aerospace, and weaponry, many civilian high-tech sectors like telecommunications, optoelectronics, and energy are also increasingly using FPI engineering plastics for their critical components. The future development trends of FPI engineering plastics mainly focus on the following aspects: (1) Integration of structure and function. Fluorination typically endows PI engineering plastics with superior heat and oxidation stability, high insulation, low dielectric constant, and low hygroscopicity. Future research should concentrate on how to design molecular structures to imbue FPI engineering plastics with unique functionalities and how to develop organic-inorganic composite FPI engineering plastics through compositional design. (2) Cost reduction. The fluorine-containing monomers used in preparing FPI engineering plastics often come at a high cost; hence, optimizing the synthesis and purification processes of these monomers to increase their yield and purity is an important research topic for reducing the overall cost of FPI engineering plastics. (3) Environmental friendliness. Although current PFAS restrictions have not yet impacted FPI engineering plastics, it is advisable to start seeking PFAS-free solutions, especially for civilian applications, as a precautionary measure.

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Research Progress and Challenges of Fluorinated Polyimide Engineering Plastics at China University of Geology

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