**4. Discussion**

The excellent results achieved by Kapton 200 CR exposed to high temperature could have been, somehow, predicted by a thermogravimetric analysis (TGA). The weight loss of Kapton tapes is negligible below 500°C, as shown in **Figure 13**, confirming the high thermal stability of the tapes.

The Fourier transform infrared (FTIR) spectroscopy of pristine and aged samples is reported in **Figure 14**. Without knowing exactly the chemical structure of the tapes, it is difficult to trace back the FTIR peaks to the corresponding chemical species. Yet, the peak sequence does not appear to be modified in an appreciable way through the series of aging cycles, suggesting that the chemical structure is stable. This is another confirmation that the Kapton 200 CR tapes did not change in a remarkable way their structure during the aging.

**Figure 13.** *TGA test of PI samples.*

*Electrical Endurance of Corona-Resistant Polyimide for Electrical Traction DOI: http://dx.doi.org/10.5772/intechopen.93253*

**Figure 14.**

*FTIR spectroscopy of pristine and aged Kapton samples.*

Both TGA and FTIR suggest that little changed in the PI tapes during changes. Yet, partial discharge endurance halved, with a sharp reduction in stage 3 (300°C). As the chemistry of the material seems to play a marginal role in this phenomenon, it was decided to inspect the morphology of the samples. In a nanostructured dielectric, inorganic nanoparticles are bonded to the polymeric matrix through a coupling agent able to confer a good stability to the resulting structure. Above a critical temperature, the bonds between the host matrix and the nanoparticles can break, enabling the nanoparticles to rearrange. The scanning electron microscope (SEM) imaging was thus used to investigate whether this kind of phenomena was taking place in the sample tapes. To do that, SEM pictures of the surface and of the cross section of both pristine and aged tapes were taken. The pictures of the cross section were obtained by immersing the tapes in liquid nitrogen and then cracking them.

The SEM pictures of pristine and aged sample surfaces at two magnification levels (5000× and 50,000×) are shown in **Figure 15**. The 5000× image does not reveal any specific sign of aging. The 50,000× picture highlights the presence of nanoparticles (white spots) in contrast with the host matrix. No appreciable sign of deterioration can be observed.

**Figure 16** shows the cross section of the samples. At the lowest magnification level (5000×), it can be appreciated that the tapes are probably a three-layer structure. It can be guessed that the central layer is pure PI, conferring elasticity to the tapes (in general, nanostructured materials tend to be less elastic than the host matrix). The outer layer consists of nanostructured PI, to ensure electrical endurance.

At the lowest magnification level (50,000×), signs of reorganization of the nanostructured layers can be observed. Apparently, the nanoparticles appear more

### **Figure 15.**

*SEM images of the surface of samples virgin and at the end of aging stage 3. (a) and (b) are pictures obtained with magnification of 5000× and 50,000×, respectively.*

### **Figure 16.**

*SEM images of the crack surface of samples virgin and at the end of aging stage 3. (a) and (b) are pictures obtained with magnification of 5000× and 50,000×, respectively.*

randomly oriented and with the tendency to form agglomerations. Therefore, regions where the host matrix becomes predominant are created. In these regions, partial discharge erosion can take place more easily leading to a reduction of the partial discharge endurance of the tapes.

These morphological changes could be addressed to the temperatures above 300°C which might have caused the stripping of the nanoparticle/host matrix bonding agent. Besides, the outer layer might have a differential thermal expansion coefficient compared with the central layer. At a microscopical level, the different expansions of the layers might have helped the distortion of the structure (the orientation of the clusters in the picture at 5000× seems to change from horizontal to skewed after aging).
