Effect of fluorine-containing groups on the corona resistance and dielectricproperties of polyimide
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摘要: 电气设备的高频化趋势对绝缘材料的耐电晕老化性能及介电性能提出了更高的要求。因此，提高绝缘材料的绝缘性能对高频电力系统的发展至关重要。本文以2, 2'-双(三氟甲基)-4, 4'- 二氨基苯基醚(6FODA)、4, 4'-二氨基二苯醚(ODA)、均苯四甲酸二酐(PMDA)为反应单体，设计制备了嵌段共聚型含氟聚酰亚胺(FPI)三层薄膜，并重点研究了材料的耐电晕性能和介电性能。结果表明：不同含氟量嵌段共聚型FPI三层薄膜的耐电晕寿命均高于纯聚酰亚胺(PI)三层薄膜，且随着含氟量的增加，薄膜耐电晕寿命相应增大。当ODA与6FODA摩尔比为1∶9时三层薄膜的耐电晕寿命在常温(20℃)、80 kV/mm下最高可达4.0 h，是纯PI三层薄膜(1.4 h)的2.86倍。随着含氟量的增加，FPI三层薄膜的介电常数呈现先减后增趋势，ODA与6FODA摩尔比为1∶1时三层薄膜的介电常数在1 MHz时最低可降至2.26，介电损耗与电导率呈现先增后减趋势。介电强度随着含氟量的增加有所下降，但均高于纯PI三层薄膜，ODA与6FODA摩尔比为9∶1时，三层薄膜的介电强度高达434 kV/mm。Abstract: The increasing use of electrical equipment at high frequencies poses greater demands on the corona aging resistance and dielectric properties of insulating materials. Therefore, it is crucial to enhance the insulation properties of insulating materials for the development of high-frequency power systems. In this study, we utilized 2, 2'-bis(trifluoromethyl)-4, 4'-diaminodiphenyl ether (6FODA), 4, 4'-diaminodiphenyl ether (ODA), and 1, 2, 4, 5-benzenetetracarboxylic anhydride (PMDA) as reactive monomers to design and synthesize block copolymerized fluoro-polyimide (FPI) three-layer films. The focus was primarily on assessing the corona-resistant and dielectric properties of these materials. Results reveal that the corona-resistant life of the block copolymerized FPI three-layer films, with varying amounts of fluorine, is higher when compared to the pure polyimide (PI) three-layer films. Furthermore, the corona-resistant life of the films increase with an increase in fluorine content. When the molar ratio of ODA to 6FODA is 1∶9, the corona resistance life of the three-layer film can reach up to 4.0 h at room temperature (20℃) and 80 kV/mm, which is 2.86 times higher than that of the pure PI three-layer film (1.4 h). Moreover, with the increase of fluorine content, the dielectric constant of FPI three-layer film shows a trend of decreasing and then increasing, and the dielectric constant of the three-layer film when the molar ratio of ODA and 6FODA is 1∶1 can be reduced to as low as 2.26 at 1 MHz, and the dielectric loss and conductivity show a trend of increasing and then decreasing. The dielectric strength decreases with the increase of fluorine content, but they are all higher than that of the pure PI three-layer film, and the dielectric strength of the three-layer film is as high as 434 kV/mm when the molar ratio of ODA to 6FODA is 9∶1.
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图 1 含氟聚酰亚胺(FPI)三层薄膜制备过程
FPAA—Fluorinated polyamide acid; PI—Polyimide
Figure 1. Fluoro-polyimide (FPI) three-layer film preparation process
图 2 纯PI与FPI薄膜的红外吸收图谱
Figure 2. Infrared absorption spectra of pure PI and FPI thin films
图 4 常温、80 kV/mm场强下纯PI及FPI三层薄膜耐电晕寿命威布尔分布图
Pt—Probability of film breakdown
Figure 4. Weibull distribution of corona-resistant life of pure PI and FPI three-layer films at room temperature and 80 kV/mm field strength
图 5 纯PI、FPI三层薄膜电导电流特性
Figure 5. Conductive current characteristics of pure PI, FPI three-layer films
图 6 FPI三层薄膜耐电晕寿命与受陷载流子密度关系
Figure 6. Corona resistance lifetime versus trapped carrier density for FPI three-layer films
图 7 FPI三层薄膜电晕放电示意图
Figure 7. Schematic diagram of corona discharge of FPI three-layer film
图 8 常温、80 kV/mm电晕击穿SEM图像：(a) FPI-9；(b) 纯PI
Figure 8. SEM images of 80 kV/mm corona breakdown at room temperature: (a) FPI-9; (b) Pure PI
图 9 常温、80 kV/mm电晕1.5 h SEM图像：((a)~(c)) FPI-9；((e)~(g)) 纯PI
Figure 9. SEM images of corona for 1.5 h at room temperature, 80 kV/mm: ((a)-(c)) FPI-9; ((e)-(g)) Pure PI
图 10 纯PI及不同含氟量FPI三层薄膜击穿场强威布尔分布
Figure 10. Weibull distribution for breakdown field strengths of pure PI and FPI three-layer films with different fluorine contents
图 11 纯PI及不同含氟量FPI三层薄膜介电常数
Figure 11. Plot of dielectric constants of pure PI and FPI three-layer films with different fluorine contents
图 12 纯PI及不同含氟量FPI三层薄膜介电损耗图
Figure 12. Dielectric loss diagrams of pure PI and FPI three-layer films with different fluorine contents
图 13 纯PI及不同含氟量FPI三层薄膜电导率图
Figure 13. Conductivity diagrams of pure PI and FPI three-layer films with different fluorine contents
表 1 9种不同含氟量FPI三层薄膜编号
Table 1. Nine different fluorine content FPI three-layer film numbers
Specimen number n(ODA)∶n(6FODA) FPI-1 9∶1 FPI-2 8∶2 FPI-3 7∶3 FPI-4 6∶4 FPI-5 5∶5 FPI-6 4∶6 FPI-7 3∶7 FPI-8 2∶8 FPI-9 1∶9 Notes: n∶n—Molar ratio; ODA—4, 4'-diaminodiphenyl ether; 6FODA—2, 2'-bis (trifluoromethyl)-4, 4'-diaminophenyl ether. 
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表 2 常温、80 kV/mm场强下纯PI及FPI三层薄膜耐电晕寿命威布尔数据表
Table 2. Corona resistance life Weibull data sheet for pure PI and FPI three-layer films at room temperature and80 kV/mm field strength
Specimen number T0/h β Pure PI 1.4 12.04 FPI-1 2.1 4.51 FPI-2 2.3 4.22 FPI-3 2.5 4.11 FPI-4 3.0 4.90 FPI-5 3.0 4.26 FPI-6 3.1 7.18 FPI-7 3.2 7.42 FPI-8 3.7 9.08 FPI-9 4.0 10.16 Notes: β—Shape parameter; T0—Corona resistance life characteristics.
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表 3 FPI三层薄膜受陷载流子密度数据
Table 3. Trapped carrier density data for FPI three-layer films
Specimen number ε/50 Hz VΩ/(kV·mm−1) nt/m−3 FPI-1 2.93 36.61 1.43×1030 FPI-3 2.73 38.83 1.19×1030 FPI-5 2.54 40.26 1.06×1030 FPI-7 2.62 33.57 9.89×1029 FPI-9 2.60 34.17 7.74×1029 Notes: ε—Dielectric constant at working frequency; VΩ—Electrical aging threshold; nt—Trapped carrier density.
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表 4 纯PI及不同含氟量FPI三层薄膜击穿强度数据
Table 4. Breakdown strength data of pure PI and FPI three-layer films with different fluorine contents
Specimen number β E0/(kV·mm−1) PI 9.95 352 FPI-1 10.92 434 FPI-3 25.20 396 FPI-5 13.42 404 FPI-7 25.00 400 FPI-9 13.23 353 Notes: E0—Breakdown field strength eigenvalue.
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