Microwave Processing and Permittivity Measurement of Thermoplastic Composites at Elevated Temperature

The material properties of greatest importance in microwave processing of a dielectric are the complex relative permittivity ε = ε′ jε′′, and the loss tangent, tan δ = ε′′/ ε′. The real part of the permittivity, ε′, sometimes called the dielectric constant, mostly determines how much of the incident energy is reflected at the air-sample interface, and how much enters the sample. This paper shows that the reflection coefficient of a material, ρ ε ε ≈ − ′ − ′ + ( ) ( ) 1 1 and the depth of penetration of a dielectric, D ≈ ′ 2 0 0 ω μ ε ε δ tan . Therefore the larger the value of the real part of the complex permittivity, the more the incident energy will be reflected by a dielectric but the energy that enters the material will penetrate further than in a dielectric with the same ε′′ but lower ε′. However, the most important property in microwave processing is the dielectric loss, ε′′ which predicts the ability of the material to convert the penetrating energy into heat. Measurements of ε′ and ε′′ are therefore critical in the microwave processing of materials with or without primer. The dielectric constant, ε′, dielectric loss, ε′′, and hence complex relative permittivity, ε and loss tangent, tan δ, of some commonly used thermoplastics have been measured at various temperatures and frequencies. These results may be used to determine whether various fibre-reinforced thermoplastic (FRTP) composites are suitable for microwave processing. This paper describes a convenient laboratory based method to obtain ε′, ε′′ and hence tan δ. The method employs a network analyser together with a waveguide transmission technique chosen because it provides the widest possible frequency range with high accuracy; the hardware and software of the method is also readily available in the electronic laboratory of the University of Southern Queensland. The required data were collected at a range of elevated temperatures and over a band of frequencies.