**3. Commercial polyimides evaluated for capacitor applications**

In the early 1990s, the lack of suitable high temperature polymer film capacitors prompted a concerted effort funded by NASA and the U.S. Air Force to develop new capacitor dielectrics based on commercially available heat-resistant polymers with operating temperatures above 200°C. While Kapton® polyimide has been used extensively since the early 1980s as wire and cable insulation for aircraft (continuous operating temperature of 300–350°C [16–18]), it has never been used as a capacitor dielectric due in part to its previous inability to be manufactured in thin films. Nevertheless, it is a common benchmark for development of new dielectrics. One study reported that the dissipation factor of Kapton® at the film level increased from 0.1% at 25°C and 1 kHz to 6% at 300°C, while that the dielectric constant decreased from 3.1 at 25°C and 1 kHz to 2.8 at 300°C [64, 65]. At 10 kHz, Kapton® showed a similar decreasing trend with increasing temperature for the dielectric constant while the dissipation factor remained constant at 0.1% at temperature up to 300°C, as shown in **Figure 7** [36]. Another recent study showed that Kapton® at 1 kHz in a film-foil capacitor configuration exhibited a gradual decrease in capacitance from 25 up to 225°C, followed by an abrupt increase at higher temperature, as illustrated in **Figure 8**. In this configuration, the dissipation factor of Kapton® remained constant at 0.1% up to 200°C and then increased sharply as temperature was further increased [66].

Another aromatic polyimide is SIXEF-44™ (**Figure 9**), which is manufactured by Hoechst Celanese based on 2,2-bis(3,4dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) and 2,2-bis(4-aminophenyl) hexafluoropropane diamine (4,4′-6F diamine). This fluorinated polyimide has a glass-transition temperature (Tg) of 323°C [67] and a dielectric constant of 2.8 at 1 kHz with <10% change across temperature range from −55 to 300°C. The dissipation factor is around 0.1% over the temperature range of 25–250°C from 100 Hz to 10 kHz, as illustrated in

### **Figure 7.**

*Dielectric constant and dissipation factor at 10 kHz for Kapton® film as a function of temperature. Data adapted from [36].*

### **Figure 8.**

*Normalized capacitance and dissipation factor at 1 kHz as a function of temperature for an 8-μm Kapton® HN-30 film capacitor wound with 12-μm copper foil. Data adapted from [66].*

**Figure 9.** *Chemical structure of SIXEF-44™.*

**Figure 10** [68]. The reduced dielectric constant in SIXEF-44™ relative to nonfluorinated counterparts was attributed to the symmetry of the fluorine atoms on the polymer backbone [67, 69, 70].

### **Figure 10.**

*Dissipation factor of SIXEF-44™ as a function of temperature at various frequencies. Data adapted from [68].*

Other aromatic polyimides investigated include perfluoropolyimide (PFPI) (**Figure 11**) and Upilex-S® (**Figure 12**). PFPI was developed by TRW, Inc., and is a perfluoroisopropylidene diamine of 2,2-bis(4,4-aminophenoxy)-phenylhexafluoropropane (4-BDAF) and pyromellitic dianhydride (PMDA) [71]. Upilex-S® was originally synthesized by ICI Americas, Inc. from monomers of 3,3′,4,4′-biphenyl tetracarboxylic dianhydride (BPDA) and p-phenylene diamine (p-PDA). The Tg for PFPI is above 390°C [72] while that of Upilex-S® is 355°C [73]. The dielectric constant of PFPI is 3.1 at 25°C but decreases to 2.9 at 300°C [64, 65], while that of Upilex-S® is 3.3 for 1 kHz and temperatures from 25 to 300°C. The dissipation factor for both PFPI and Upilex-S® at 1 kHz is about 0.1% at both 25 and 300°C.

**Figure 11.** *Chemical structure of PFPI.*

**Figure 12.** *Chemical structure of Upilex-S®.*

*Polyimides as High Temperature Capacitor Dielectrics DOI: http://dx.doi.org/10.5772/intechopen.92643*

**Figure 13.** *Chemical structure of PEI.*

**Figure 14.** *Dielectric constant and dissipation factor at 10 kHz for PEI film as a function of temperature. Data adapted from [36, 57].*

Most aromatic polyimides suffer from the same processing issues as a result of the high degree of aromaticity, which led researchers to develop modified systems with flexible moieties such as ether linkages and alkyl groups in the polymer backbone. One example is Ultem™, which is poly(ether imide) (PEI) (**Figure 13**) that is synthesized from the disodium salt of bisphenol A and 1,3-bis(4-nitrophthalimido)benzene [74]. Subsequent development efforts focused on melt-extrusion and stretching of PEI films have enabled film thicknesses as thin as 5 μm [31]. The Tg of PEI is ~215°C, which is considerably lower than many of the wholly aromatic polyimides due to the increase in flexibility imparted by the ether linkages and alkyl groups [17]. The dielectric constant and dissipation factor of PEI are about 3.1 and 0.2%, respectively, at frequencies from 100 Hz to 10 kHz and temperatures from 25 to 200°C; however, they increase sharply as temperature approaches the Tg, as shown in **Figure 14** [36, 57]. Such temperature dependence for the dielectric constant and dissipation factor is a characteristic for polymers with molecular dipoles as discussed in Section 2.4.
