Peter R. Hondred

Statistical analysis of electrical breakdown behavior of polyimide following degrading processes

Polyimide (PI) tape has been widely used as electrical insulation for wires and cables due to its excellent balance of electrical, mechanical and thermal properties. However, insulation materials are exposed to various environmental degradation mechanisms during service, which can cause deterioration of their insulation properties. In this paper, the effects of thermal exposure and immersion in water on dielectric strength of Kapton® HN PI film are investigated statistically by two-parameter Weibull distribution analysis. The PI samples were heated at 475°C for 1 to 4 h, and at 450 to 480°C for 4 h, to study the influence of thermal exposure. Moreover, to explore the effect of moisture absorption, other PI samples were immersed in distilled water for up to 96 h at room temperature. After the degrading process, breakdown voltage and thickness were measured at 20 different points on each sample. The Weibull cumulative distribution function for each of the degraded samples was plotted, and compared with that of a sample dried by heating at 200°C for 1 h. Significant decreases in both the scale parameter and the shape parameter of the Weibull distribution, obtained through least-squares regression, are observed, indicating a statistical decrease in dielectric strength and a more dense dispersion of electrical weak points in PI. This observed deterioration of insulation capability of PI is attributed to chemical degradation and formation of ionic side groups during heating and immersion in water, respectively.

Degradation kinetics of polyimide film

Through the use of thermogravimetry (TG), this work explores the thermal degradation mechanism and kinetics of Kapton (polyimide) film. Isoconversional kinetic methods, which provide activation energy as a function of degradation, present insight into the development of a five-step competitive and consecutive kinetic model. To evaluate the accuracy and validity of the model, a statistical analysis of the implemented kinetic model is also presented. Coupled TG-mass spectroscopy and TG-Fourier transform infrared analysis were used to investigate the decomposition mechanism and to more fully explain the complex, heating-rate-dependent, decomposition pathway of Kapton.

Degradation kinetics of polytetrafluoroethylene and poly(ethylene-alt-tetrafluoroethylene):

This article studies the degradation kinetics of two fluorocarbon-based polymers, polytetrafluoroethylene (PTFE) and poly(ethylene-alt-tetrafluoroethylene) (ETFE), that are commonly used in coatings, solid lubricants and wiring insulation. During the onset of degradation, these polymers may fail in their desired function, with associated monetary or human cost. This work examines the degradation kinetics of PTFE and ETFE by applying thermogravimetric analysis. Results from an isoconversional method, where activation energy is represented as a function of the extent of degradation, are used to develop kinetic models describing their degradation. A statistical analysis to evaluate the accuracy and validity of the implemented kinetic models is presented and it is found that an nth-order, single-step autocatalytic reaction model best describes the degradation of PTFE, whereas a consecutive three-step autocatalytic reaction model best describes the degradation of ETFE.

Dielectric response of polyimide to thermal and saline degradation

Polyimide (PI) is a widely used wiring insulation material for aircraft. Given the harsh environment of its employment, wiring insulation material for aircraft suffers from various degradation processes which may cause failure of electrical insulation. In this paper, the dielectric response of PI to thermal degradation and to saline exposure is investigated. To investigate the effect of thermal exposure, PI samples were isothermally heated at 400, 425, 450 and 475 °C for up to 5 hr in an isotemp muffle furnace. After heating, the samples were removed from the furnace and allowed to cool at room temperature. To investigate the effect of saline degradation, PI samples were immersed in both distilled water and salt water with salinity of 80 g/l for up to 4 days at room temperature. Measurements of permittivity over the frequency range 1 kHz to 2 MHz at room temperature, obtained using an Agilent E4980A LCR meter, revealed an increase of up to 14% in the measured real relative permittivity of the thermally-exposed samples, compared with a sample completely dried by heating at 200 °C for 1 hr. The frequency-dependence of the dissipation factor was also changed significantly after thermal exposure at 475 °C. The relative permittivity and dissipation factor of Kapton samples immersed in distilled water were increased by approximately 10% and 40%, respectively. The influence of dissolved salt had only a small additional effect, however.