Juan Martinez-Vega

Dielectric and mechanical properties correlated to physico-chemical characteristics of a polyester-imide resin used in rotating machines insulations

This work is focused on the characterization of dielectric, mechanical and physicochemical properties of a commercially available PEI resin. The complex shear modulus is determined using the Dynamic Mechanical Analysis (DMA). The complex dielectric permittivity is measured using the broadband dielectric spectroscopy. In order to complement the mechanical and dielectric results, Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA) experiments are performed. The dynamic mechanical results show an important and continuous decrease in the storage modulus. The dielectric results present a significant increase in the dielectric losses, especially for the low frequencies. Both of these results show an important diminution in the material viscosity. The increase in dielectric values suggests an increase in the ionic conduction. On the other hand, the decrease in elastic modulus implies a rubber-like behavior of the material. The mechanical and dielectric results are in a good agreement with the DSC and TGA results.

Morphological evolution of polytetrafluoroethylene (PTFE) during thermal-oxidative ageing above and below the melting temperature

The organic electric insulator polytetrafluoroethylene (PTFE) is used in aerospace industry under extreme conditions of temperature and electric field. The melting temperature of PTFE is about 327°C. In our days operating temperature of this kind of insulators could punctually reach 300°C and 350°C for the new generation machines. This last temperature is above the melting temperature of the material. These severe operating conditions can be factors of ageing acceleration and/or degradation of the insulators. In general, when a polymer is exposed to an important thermal stress, chemical degradation reactions may take place, such as the macromolecular chain scissions and the formation of new chemical groups. This may affect significantly the molecular weight, chains arrangement or the size distribution of macromolecules. Hence, all this processes may cause modifications of theirs properties. Our present work is focused on the organic insulator behaviour above and below the melting temperature in order to understand the different mechanisms of thermal-oxidative ageing and degradation in the molten and the solid state. Thin films in PTFE were aged by accelerated method under oxidizing environment (in air) and severe thermal stresses between 250 and 450°C with exposure times from 2 to 300 hours. Before and after each ageing cycle, samples were characterized by weight loss evolution (performed by a microbalance), Fourier transform infrared spectroscopy (FTIR), and Differential scanning calorimetry (DSC).

Dielectric breakdown and morphological evolution of PTFE during thermal-oxidative ageing at temperatures lower and higher than the melting temperature

The organic electric insulator in polytetrafluoroethylene (PTFE) is widely used under extreme conditions of temperature and electric field. Nowadays, operating temperature of this kind of insulators can reach about 350 °C for new generations of machines, which is above than the melting temperature of the material (327 °C). These severe operating conditions can be factors of ageing acceleration and/or degradation of the insulators. Currently, there are very few specifications on the insulators for high temperature and/or high voltage applications, furthermore their ageing and degradation patterns are not well known. Our present work is focused on the organic insulator behavior at high temperature under oxidizing atmosphere in order to characterize the mechanisms of thermal-oxidative ageing and degradation as well as the impact on the dielectric breakdown strength. Thin films in PTFE were aged by severe thermal stresses between 250 and 450 °C under oxidizing environment were characterized by using weight loss measurements, FTIR as well as dielectric breakdown under AC voltage. The experimental results show that irreversible chemical ageing occurs at about 100 °C higher than melting temperature. PTFE losses its thermal stability and electrical performance at 400 °C; no noticeable thermal decomposition was revealed at 340 °C until 300 h of ageing. The degree of crystallinity has tendency to increase at early stage of ageing (340 °C), then it decrease for samples aged at very high temperature during long time (400 °C during 300 h and 450 °C during 50 h).