Sensing performance of dielectric sensor in mixed melting polymer pellets for waste plastic separation

Abstract

Physical value of dielectric properties among relative permittivity, dielectric loss and loss tangent of mixed melting polymer pellets were measured under temperature variance condition and measurement frequency from 0.5MHz to 5MHz in order to develop a real-time non-intrusive measurement technique for high performance waste plastic separation. Various volume fractions of non-polar polyethylene (PE) and polar polyethylene terephthalate (PET) were used as the mixed polymer pellets. As the results, with the volume fractions of PET from 0.05 to 0.22, as the temperature is increased up to melting point temperature and above mold temperature, the relative permittivity is decreased as the temperature is increased. However over melting temperature up to the mold temperature, the relative permittivity is slightly increased as the temperature is increased. On the other hand, the dielectric loss and loss tangent are not affected by the temperature variance, and the dielectric loss and loss tangent are increased as the measurement frequency is increased. As the conclusions, at molding temperature 180°C (453K), the highest sensitivity of dielectric property is the relative permittivity. Meanwhile, the highest linearity relationship between PET volume fraction and predicted relative permittivity is achieved at the measurement frequency of 5MHz. From relative permittivity measurement in these conditions, the performance to distinguish PET volume fraction 0.05 from PE volume fraction 0.95 is achieved in the mixed melting polymer pellets. Therefore, the relative permittivity measurement for waste plastic separation has a possibility to achieve the high purity recycled plastics as compared with conventional methods.