An important step in microwave processing of materials: permittivity measurements of thermoplastic composites at elevated temperatures

Abstract

[Abstract]: The dielectric constant, ε′, dielectric loss, ε″, and loss tangent, tan δ, of some commonly used thermoplastics have been measured at various temperatures and frequencies. They do give some clues to the suitability of microwave processing of certain fibre-reinforced thermoplastic (FRTP) composites but cannot provide definite answers to the problems. Few measurements, if any, of the ε or the tan δ of FRTP composites have been reported in the literature. This paper describes a convenient laboratory based method designed to obtain ε′, ε″ and hence tan δ. The method employs an automatic logic network analyser and is called the waveguide transmission technique or transmission line method and is chosen because it provides the widest possible frequency range with high accuracy and utilised available the hardware and software. The required data were collected at a range of elevated temperatures and over a band of frequencies. The relationship between ε′ and ε″, or the tan δ of the composites and their weldability by microwave energy is also studied and discussed.