Dielectric Constant and Dielectric Loss of Plastics as Related to Their Composition

Abstract

Data are presented for the frequency variation of the dielectric constant and dielectric loss factor of various plastics over a broad frequency band extending from 1 kc. to 35 mc. The extremely low loss of polystyrene compared to that of polar plastics confirms the theory that a hydrocarbon is inherently more satisfactory from a dielectric point of view. Of the several possible mechanisms of dielectric loss which might account for the high‐frequency dielectric absorption observed in polar plastics, the rotation of polar units in the chain and of polar side groups appears most probable. The fact that the loss factor maxima of phenol fibers, phenol fabrics, and phenol or urea formaldehyde molding compounds containing cellulosic fillers occur at essentially the same frequency is viewed as evidence that this dielectric absorption is an intrinsic property of cellulose. Substitution of mineral fillers for cellulose reduces the high‐frequency loss to that residing in the polar resin binder. Furthermore, the dielectric loss of mineral‐filled molding compounds is less moisture‐sensitive. The large increase in dielectric loss at low frequency always found in materials of relatively high free‐ion conductivity manifests itself in Duprene, and the humidified phenolic plastics containing cellulose fillers or laminations.

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