Dielectric properties of some PTFE-reinforced thermosetting resin composites

The dielectric properties of several composites, consisting of thermosetting resins reinforced by woven expanded-poly(tetrafluoroethylene) (e-PTFE) fibers, have been investigated as a function of both frequency (10 Hz to 1 GHz) and temperature (-160°C to 130°C). The resins studied were: an FR-4-type epoxy, a tetrafunctional epoxy, a bismaleimide triazine/epoxy blend (BT-blend), and a cyanate ester. The effect on the dielectric properties of the composite of using an E-glass cofiber to enhance dimensional stability was also considered. The dielectric properties of the composites were found to be well represented by the sum of the properties of each component, weighted by their volume composition. No significant differences were found between the dielectric properties of composites based on the FR-4-type and tetrafunctional epoxy resins. All of the resins investigated showed a β-relaxation, with a maximum in the frequency range 1-100 MHz, at 23°C. The cyanate-ester-based composite displayed the lowest loss at low frequencies and high temperatures. The apparent activation energy for the β-relaxation of the epoxy resins was found to be 51.4 kJ mol.^-1. The addition of 12% (by volume) E-glass cofiber to a composite of the type investigated, while maintaining the proportion of resin, raises the relative permittivity by approximately 0.5